Advanced Flexible Ceramics: Design, Properties, Manufacturing, and Emerging Applications 0323988245, 9780323988247

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Advanced Flexible Ceramics

Elsevier Series in Advanced Ceramic Materials

Advanced Flexible Ceramics Design, Properties, Manufacturing, and Emerging Applications

Edited by

Ram K. Gupta Department of Chemistry, National Institute for Materials Advancement, Pittsburg State University, Pittsburg, KS, United States

Ajit Behera Deparment of Metallurgical & Materials Engineering, National Institute of Technology, Rourkela, India

Siamak Farhad Department of Mechanical Engineering, Director of Center for Precision Manufacturing, University of Akron, Akron, OH, United States

Tuan Anh Nguyen Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-98824-7 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Gwen Jones Editorial Project Manager: Emily Thomson Production Project Manager: Anitha Sivaraj Cover Designer: Vicky Esser Pearson Typeset by MPS Limited, Chennai, India

List of contributors

Srinivasan Alagappan Hydrogen Division, High Energy Batteries (India) Limited, Mathur, Tamil Nadu, India Kevin V. Alex Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, India S. Angitha Department of Science and Humanities, Indian Institute of Information Technology, Tiruchirappalli, Tamil Nadu, India Elsa Antunes College of Science and Engineering, James Cook University, Townsville, QLD, Australia Rajendran Suresh Babu Laboratory of Experimental and Applied Physics, Centro Federal de Educac¸a˜o Tecnolo´gica Celso suckow da Fonesca, Av. Maracana˜ Campus, Rio de Janeiro, Brazil Daneshwaran Balaji Department of Chemistry, School of Advanced Sciences, VIT-AP University, Amaravati, Andhra Pradesh, India Payel Bandyopadhyay Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India Ajit Behera Deparment of Metallurgical & Materials Engineering, National Institute of Technology, Rourkela, Odisha, India Nilotpala Bej School of Mechanical Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar, Odisha, India Vijaykumar S. Bhamare Department of Chemistry, Center for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Belagavi, Karnataka, India Surendra Kumar Biswal Tirupati Graphene and Mintech Research Centre, Bhubaneswar, Odisha, India Ganesh R. Chate Mechanical Engineering Department, KLS Gogte Institute of Technology, Belagavi, Karnataka, India; Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Belagavi, Karnataka, India

xviii

List of contributors

Vaibhav R. Chate Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Belagavi, Karnataka, India; Civil Engineering Department, KLS Gogte Institute of Technology, Belagavi, Karnataka, India Suman Chatterjee Department of Mechanical Engineering, National Institute of Technology Rourkela, Rourkela, Odisha, India; Department of Mechanical and Automotive Engineering, Kongju National University, Cheonan, South Korea Jaeyeop Choi Smart Gym-based Translational Research Center for Active Senior’s Healthcare, Pukyong National University, Busan, South Korea Bian Da College of Mechanical Engineering, Jiangnan University, Wuxi, P.R. China; Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi, P.R. China Satyabati Das Department of Mechanical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh, India Biswajit Dash Modern Institute of Technology and Management (MITM), Bhubaneswar, Odisha, India Tapan Dash Tirupati Graphene and Mintech Research Centre, Bhubaneswar, Odisha, India Ding Ding School of Architecture and Civil Engineering, Xihua University, Chengdu, P.R. China Mohammad Reza Golobostanfard Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin-Film Electronics Laboratory (PV-Lab), Neuchaˆtel, Switzerland Ram K. Gupta Deparment of Metallurgical & Materials Engineering, National Institute of Technology, Rourkela, Odisha, India Vitaly Gurylev National Synchrotron Radiation Research Center, Hsinchu Science Park, Hsinchu, Taiwan T.V. Huynh Department of Mechanical Engineering, Faculty of Mechanical Engineering, Ho Chi Minh City University of Technology and Education, Ho Chi Minh City, Vietnam Ebenezer Olubunmi Ige Department of Mechanical and Mechatronic Engineering, Afe Babalola University, Ado-Ekiti, Nigeria; Department of Biomedical Engineering, Afe Babalola University, Ado-Ekiti, Nigeria

List of contributors

xix

Adijat Omowumi Inyang Ansyl Technologies Limited, Claremont, Cape Town, South Africa A.R. Jayakrishnan Centre for Nanoscience and Engineering, Indian Institute of Science, Bengaluru, Karnataka, India Sambedan Jena School of Nano Science and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Li Jiahong College of Mechanical Engineering, Jiangnan University, Wuxi, P.R. China K. Kamakshi Department of Science and Humanities, Indian Institute of Information Technology, Tiruchirappalli, Tamil Nadu, India Farkhondeh Khodabandeh School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran Hee-Je Kim Department of Electrical and Computer Engineering, Pusan National University, Busan, Geumjeong-gu, South Korea N.S. Kiran Kumar Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, India Tejas Koushik College of Science and Engineering, James Cook University, Townsville, QLD, Australia Raviraj M. Kulkarni Department of Chemistry, Center for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Belagavi, Karnataka, India Sathasivam Pratheep Kumar Department of Chemistry, School of Advanced Sciences, VIT-AP University, Amaravati, Andhra Pradesh, India P. Maiti Advanced Mechanical and Materials Characterization Division, CSIR— Central Glass and Ceramic Research Institute, Kolkata, West Bengal, India Manila Mallik Department of Metallurgical and Materials Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India Adhirath Mandal Department of Mechanical and Automotive Engineering, Kongju National University, Cheonan, South Korea Triveni Rajashekhar Mandlimath Department of Chemistry, School Advanced Sciences, VIT-AP University, Amaravati, Andhra Pradesh, India

of

xx

List of contributors

Subash Chandra Mishra Department of Metallurgical and Materials Engineering, National Institute of Technology Rourkela, Rourkela, Odisha, India Sudip Mondal BK21 FOUR ‘New-senior’ Oriented Smart Health Care Education, Pukyong National University, Busan, South Korea Anoop K. Mukhopadhyay Department of Physics, Sharda School of Basic Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh, India; Advanced Mechanical and Materials Characterization Division, CSIR—Central Glass and Ceramic Research Institute, Kolkata, West Bengal, India Tuan Anh Nguyen Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam Junghwan Oh Industry 4.0 Convergence Bionics Engineering, Pukyong National University, Busan, South Korea; BK21 FOUR ‘New-senior’ Oriented Smart Health Care Education, Pukyong National University, Busan, South Korea; Biomedical Engineering, Pukyong National University, Busan, South Korea; Ohlabs Corporation, Busan, South Korea Takeo Oku Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, Japan Ayodele James Oyejide Department of Biomedical Engineering, Afe Babalola University, Ado-Ekiti, Nigeria; Department of Biomedical Engineering, University of Ibadan, Ibadan, Nigeria Kalpana Parida Department of Physics, Siksha O Anusandan University, Bhubaneswar, Odisha, India Sumin Park Industry 4.0 Convergence Bionics Engineering, Pukyong National University, Busan, South Korea; BK21 FOUR ‘New-senior’ Oriented Smart Health Care Education, Pukyong National University, Busan, South Korea M. Pereira Laboratory of Physics for Materials and Emergent Technologies, LapMET, University of Minho, Braga, Portugal Thuy Dung Nguyen Pham Industry 4.0 Convergence Bionics Engineering, Pukyong National University, Busan, South Korea; BK21 FOUR ‘New-senior’ Oriented Smart Health Care Education, Pukyong National University, Busan, South Korea Swastik Pradhan Lovely Professional University, Phagwara, Punjab, India Manisha Priyadarshini Centurion University of Technology and Management, Bhubaneswar, Odisha, India

List of contributors

xxi

G. Sudha Priyanga Department of Physics, Research Institute for Natural Science, and Institute for High Pressure at Hanyang University, Hanyang University, Seoul, Republic of Korea Nikhil Rangaswamy School of Mechanical Engineering, REVA University, Bangalore, Karnataka, India S. Tanuja Rani Centurion Bhubaneswar, Odisha, India

University

of

Technology

and

Managemen,

Desigan Ravi Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India Ramya Ravichandran Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India R. Rugmini Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, India Santosh Kumar Sahu Department of Mechanical Engineering, VSSUT, Burla, Sambalpur, Odisha, India Rajashree Samantray Department of Metallurgical and Materials Engineering, National Institute of Technology Rourkela, Rourkela, Odisha, India Santosh Sampath Department of Mechanical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Chennai, Tamil Nadu, India S. Sarapure Department of Mechanical Engineering, Sree Vidyanikethan Engineering College, Tirupati, Andhra Pradesh, India K.C. Sekhar Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, India Qian Shanhua College of Mechanical Engineering, Jiangnan University, Wuxi, P.R. China; Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi, P.R. China Manjunath Shettar Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, Karnataka, India J.P.B. Silva Physics Center of Minho and Porto Universities (CF-UM-UP), University of Minho, Campus de Gualtar, Braga, Portugal; Laboratory of Physics for Materials and Emergent Technologies, LapMET, University of Minho, Braga, Portugal

xxii

List of contributors

S.A. Srinivasan R&D - NP High Energy Batteries India Ltd, Mathur, Pudukottai, Tamil Nadu, India Sathish Sugumaran Department of Physics, MVJ College of Engineering, Bengaluru, Karnataka, India M. Tasneem Department of Science and Humanities, Indian Institute of Information Technology, Tiruchirappalli, Tamil Nadu, India Phan Duc Tri Industry 4.0 Convergence Bionics Engineering, Pukyong National University, Busan, South Korea; BK21 FOUR ‘New-senior’ Oriented Smart Health Care Education, Pukyong National University, Busan, South Korea Keerthi Valsalan Department of Chemistry, School of Advanced Sciences, VITAP University, Amaravati, Andhra Pradesh, India Rajangam Vinodh Department of Electronics Engineering, Pusan National University, Busan, Geumjeong-gu, South Korea Chen Yi Suzhou Nilfisk R&D Co., Ltd, Suzhou, P.R. China Moonsuk Yi Department of Electronics Engineering, Pusan National University, Busan, Geumjeong-gu, South Korea Wang Yongguang College of Mechanical Engineering, Soochow University, Suzhou, P.R. China Zhao Yongwu College of Mechanical Engineering, Jiangnan University, Wuxi, P.R. China; Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi, P.R. China Ni Zifeng College of Mechanical Engineering, Jiangnan University, Wuxi, P.R. China; Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi, P.R. China

Contents

List of contributors

Part 1 1

2

3

xvii

Introduction and characterisations

Flexible ceramics: an introduction S.A. Srinivasan and Santosh Sampath 1.1 Introduction 1.2 Methods for fabrication of flexible ceramics 1.2.1 Cofired combustion methods 1.2.2 Printing method 1.2.3 Tape casting process 1.2.4 Roll-to-roll processing method 1.3 Applications and challenges References Further reading Shape memory ceramics Santosh Sampath, Srinivasan Alagappan, G. Sudha Priyanga, Ram K. Gupta, Ajit Behera and Tuan Anh Nguyen 2.1 Introduction 2.2 Smart ceramics 2.3 Mechanism of shape recovery in smart ceramics 2.4 Methods for fabrication 2.5 Electrical and electronic applications of smart ceramics 2.6 Biomedical applications of smart ceramics 2.7 Industrial application of smart ceramic References Further reading Characterization of flexible ceramics Sathasivam Pratheep Kumar, Daneshwaran Balaji and Triveni Rajashekhar Mandlimath 3.1 Introduction 3.2 Characterization techniques 3.2.1 Electron microscopy 3.2.2 Scanning probe microscopy

3 3 4 4 5 5 7 8 10 10 13

13 14 15 16 17 19 22 22 24 25

25 25 25 28

vi

Contents

3.2.3 3.2.4

X-ray diffraction Fourier transform infrared spectroscopy and Raman spectroscopy 3.2.5 Electron diffraction 3.2.6 Energy dispersive X-ray analysis 3.2.7 X-ray photoelectron spectroscopy 3.2.8 Thermogravimetric and differential thermal analysis 3.3 Conclusion References 4

Microstructural characteristics of flexible ceramics Satyabati Das, Kalpana Parida, Nilotpala Bej and Manila Mallik 4.1 Introduction 4.2 Design strategies and microstructures 4.2.1 Itacolumite 4.2.2 Aluminum titanate ceramics 4.2.3 Al2O3/Mo 4.2.4 Al2O3 TiO2 MgO 4.2.5 KZr2(PO4)3 KAlSi2O6 4.2.6 Al2O3/Al/Al2O3 hybrid composite 4.2.7 One-dimensional flexible ceramics 4.2.8 Three-dimensional flexible ceramics 4.2.9 ZnO tetrapods 4.2.10 Metal-alloyed ZnO tetrapod 4.3 Conclusive remark Acknowledgment References

Part 2 5

31 32 34 34 35 38 39 39 45 45 45 46 47 48 49 51 51 53 54 54 54 59 59 59

Mechanical, electrical and optical properties

Mechanical properties of flexible ceramics Desigan Ravi, Ramya Ravichandran, Payel Bandyopadhyay and Anoop K. Mukhopadhyay 5.1 Introduction 5.2 Mechanical properties of conventional ceramics 5.3 Mechanical properties of flexible ceramics materials 5.4 Mechanism of flexibility 5.4.1 Modifying the microstructure 5.4.2 Addition of other materials 5.4.3 By changing the shape 5.5 Conclusion Acknowledgment References

63

63 63 66 71 72 72 72 72 73 73

Contents

vii

6

75

7

Electrical properties of flexible ceramics N.S. Kiran Kumar, A.R. Jayakrishnan, R. Rugmini, J.P.B. Silva, M. Pereira, Sathish Sugumaran and K.C. Sekhar 6.1 Introduction 6.2 Electrical properties of flexible ceramics 6.2.1 Dielectric properties 6.2.2 Piezoelectric properties 6.2.3 Pyroelectric properties 6.2.4 Ferroelectric properties 6.2.5 Electrochemical properties 6.2.6 Current voltage characteristics 6.3 Electrical properties-based applications of flexible ceramic films 6.3.1 Energy storage devices 6.3.2 Energy harvesting 6.3.3 Memory 6.4 Conclusions Acknowledgments References

Optical properties of flexible ceramic films S. Angitha, Kevin V. Alex, J.P.B. Silva, K.C. Sekhar, M. Tasneem and K. Kamakshi 7.1 Introduction 7.2 Concept and fundamentals of optical properties 7.2.1 Interaction of electromagnetic wave with ceramics 7.2.2 Luminescence properties 7.3 Flexible ceramic films and their optical properties 7.3.1 Transmittance of flexible ceramics 7.3.2 Refractive index of flexible ceramic films 7.3.3 Photoluminescence of flexible ceramic films 7.3.4 Electroluminescence of flexible ceramic films 7.3.5 Mechanoluminescence of flexible ceramic films 7.4 Flexible ceramic film-based optical device applications 7.4.1 Photodetectors (photosensors) 7.4.2 Solar cells 7.4.3 Optical memories 7.4.4 Optical (or phosphor) thermometry 7.4.5 Photocatalysis 7.4.6 Light-emitting diodes 7.4.7 Other applications 7.5 Conclusions and future prospects References

75 76 77 81 82 84 88 92 95 95 103 113 119 119 119

129

129 130 131 135 138 139 141 143 146 147 148 148 151 154 158 159 161 162 164 164

viii

Contents

Part 3 8

9

10

Manufacturing

Chemical vapor deposition processing and its relevance to build flexible ceramics materials Vitaly Gurylev 8.1 Introduction 8.2 Chemical vapor deposition: principles and fundamentals 8.2.1 Basic understanding of chemical vapor deposition 8.2.2 Reaction mechanism of chemical vapor deposition and its relation with a substrate 8.2.3 Atomic layer deposition: a special type of chemical layer deposition 8.3 Chemical vapor deposition processing to build flexible ceramics 8.3.1 Current status 8.3.2 Development of film-like structures on the flexible substrates 8.3.3 Development of one-dimensional nanostructures with different geometries and morphologies References Ceramic three-dimensional printing Tejas Koushik and Elsa Antunes 9.1 Introduction 9.2 Classification of three-dimensional printing processes 9.2.1 Slurry-based processes 9.2.2 Powder-based processes 9.2.3 Bulk solid materials 9.3 Process parameters 9.3.1 Slurry-based processes 9.4 Quality control techniques 9.5 Guidelines for technology selection 9.6 Applications of Ceramics 9.7 Conclusion and perspectives References Methods for fabrication of ceramic coatings Vijaykumar S. Bhamare and Raviraj M. Kulkarni 10.1 Introduction 10.2 Ceramic coating materials for fabrication 10.2.1 Different types of oxide ceramic coatings 10.2.2 Different types of nonoxide ceramic coatings 10.3 Methods for fabrication of ceramic coating on metallic materials 10.3.1 Sol gel method 10.3.2 Microarc oxidation 10.4 Liquid phase deposition method

171 171 173 173 174 175 178 178 180 182 188 193 193 194 194 200 203 205 206 207 209 210 211 212 215 215 216 216 217 218 218 220 221

Contents

11

10.5

Atomic layer deposition method 10.5.1 Electrochemical method 10.5.2 Plasma treatment 10.5.3 Magnetron sputtering 10.5.4 Solution immersion process 10.5.5 Laser-cladding method 10.5.6 Chemical vapor deposition method 10.5.7 Dip-coating method 10.6 Conclusions 10.7 Future scope References

222 223 224 225 226 227 228 228 229 231 231

Methods for ceramic machining Manisha Priyadarshini, Swastik Pradhan and Rajashree Samantray 11.1 Introduction 11.2 Traditional machining 11.3 Nontraditional machining 11.4 Hybrid machining 11.5 Comparative studies 11.6 Conclusion References

243

Part 4 12

13

ix

243 244 246 251 252 252 252

Emerging applications

Advanced flexible electronic devices for biomedical application Phan Duc Tri, Thuy Dung Nguyen Pham, Sumin Park, Jaeyeop Choi, Sudip Mondal and Junghwan Oh 12.1 Introduction 12.2 Flexible electronics 12.2.1 Fabrication strategies and materials 12.2.2 Physical, chemical, and biosensors-based flexible ceramics 12.2.3 Advanced flexible electronic for wound healing 12.3 Summary and conclusions Acknowledgments References Transition metal oxide ceramic nanocomposites for flexible supercapacitors Sambedan Jena and Manila Mallik 13.1 Introduction 13.2 Supercapacitor overview: types and components 13.2.1 Electrical double-layer capacitors versus pseudocapacitors 13.2.2 Use of ceramics as supercapacitor electrodes

261

261 262 262 263 268 271 272 272

277 277 279 279 281

x

Contents

13.2.3

Current collectors/substrates for preparing supercapacitor electrodes 13.2.4 Electrolytes 13.3 Recently developed ceramic electrodes for flexible supercapacitors 13.3.1 Metal oxide/conductive polymer composites 13.3.2 Metal sulfides/conductive polymer composites 13.3.3 Metalloid nitrides/carbides ceramics 13.3.4 Metal hydroxide ceramics 13.3.5 Spinel oxide ceramics 13.4 Conclusions and future prospects References 14

15

Metal organic framework and MXene-based flexible supercapacitors Rajangam Vinodh, Rajendran Suresh Babu, Hee-Je Kim and Moonsuk Yi 14.1 Introduction 14.2 Types of flexible supercapacitor 14.2.1 Metal organic frameworks-based flexible supercapacitors 14.2.2 MXene-based flexible supercapacitor 14.3 Summary and conclusion Acknowledgment References Flexible solar cells Farkhondeh Khodabandeh and Mohammad Reza Golobostanfard 15.1 Introduction 15.2 Material properties for flexible substrates 15.2.1 Stability against oxygen and moisture 15.2.2 Thermal properties 15.2.3 Optical properties 15.2.4 Chemical properties 15.3 Flexible substrates 15.3.1 Metals 15.3.2 Ceramics 15.3.3 Polymers 15.4 Flexible absorbers and flexible solar cells 15.4.1 a-Si:H solar cells 15.4.2 CdTe solar cells 15.4.3 Cu(In,Ga)(S,Se)2 solar cells 15.4.4 Organic solar cells 15.4.5 Perovskite solar cells

281 282 282 282 283 285 288 288 292 294

299

299 301 301 308 316 319 319 325 325 326 326 327 328 328 329 330 330 331 331 332 334 336 338 342

Contents

16

17

xi

15.5

Fexible electrodes 15.5.1 Metals 15.5.2 Carbon 15.5.3 Polymers 15.6 Conclusion References

345 346 347 347 348 348

Emerging applications of ceramics in flexible supercapacitors Rajashree Samantray and Subash Chandra Mishra 16.1 Introduction 16.2 Electrode materials 16.2.1 Ruthenium oxide 16.2.2 Manganese oxide 16.2.3 Cobalt oxide 16.2.4 Iron oxides 16.2.5 Vanadium oxides 16.2.6 Tin oxide 16.2.7 Vanadium nitride 16.2.8 Titanium nitride 16.3 Summary References

353

Flexible ceramics for microfluidics-mediated biomedical devices Ebenezer Olubunmi Ige, Ayodele James Oyejide and Adijat Omowumi Inyang 17.1 Introduction 17.2 Flexible ceramics in microfluidics 17.3 Fabrication protocols for flexible ceramics in microfluidics 17.4 Tailoring ceramics for application in medical-related microdevices 17.5 Integration of microelectronic in flexible ceramic-based microfluidics 17.6 General applications of functional and flexible bioceramics in medical technology 17.7 Emerging technologies in bioceramics for medical devices 17.8 Ceramic-based medical devices 17.9 Emerging technologies for bioceramics in the medical device application 17.9.1 Electroceramics 17.9.2 Green state machining 17.9.3 Three-dimensional printing 17.9.4 Bone cancer treatment from bioceramic scaffolds 17.9.5 Sol gel technique 17.10 Prospects of flexible bioceramics in post-COVID era

363

353 353 354 354 355 355 356 356 357 358 359 359

363 364 367 368 369 370 371 375 376 376 377 378 378 378 379

xii

Contents

17.11

Current roles of flexible bioceramics in tackling COVID-19 and expectations in post-COVID-19 era 17.11.1 Silicon nitride bioceramics 17.11.2 Graphitic carbon nitride 17.11.3 Ventilator design References 18

19

20

Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors for damage-resistant applications Anoop K. Mukhopadhyay, S. Sarapure and P. Maiti 18.1 Introduction 18.2 Fabrication of multilayer composites 18.3 Microstructure and properties of multilayer composites 18.3.1 Mechanical properties of multilayer systems 18.3.2 Aspect of toughness improvement in multilayer composite systems 18.4 Summary and conclusions References Flexible ceramics for environmental remediation Triveni Rajashekhar Mandlimath, Keerthi Valsalan and Sathasivam Pratheep Kumar 19.1 Introduction 19.2 Flexible ceramics for environmental remediation 19.2.1 Removal of heavy metals 19.2.2 Air filtration 19.2.3 Adsorption of dyes 19.2.4 Removal of pathogens 19.2.5 Photodegradation of dyes 19.3 Conclusions References Ceramic-based coatings for solar energy collection Ding Ding 20.1 Background 20.2 State-of-art 20.2.1 Normal ceramic collectors 20.2.2 Vanadium titanium black ceramic collectors 20.3 Heat-transfer mechanism 20.4 Building application methods 20.4.1 Module patterns 20.4.2 Integration patterns 20.5 Application cases 20.5.1 Conceptual architecture 20.5.2 Rural residence

380 381 381 382 383

391 391 398 398 398 407 408 408 411

411 411 412 414 416 417 419 421 422 425 425 425 426 427 432 433 433 433 436 436 436

Contents

21

22

23

xiii

20.5.3 Urban high-rise residence 20.5.4 Public building 20.5.5 Agricultural construction 20.6 Future directions References

436 440 442 446 446

Advanced ceramics in the defense and security Suman Chatterjee, Santosh Kumar Sahu, Adhirath Mandal and T.V. Huynh 21.1 Introduction to ceramics in defense and security 21.2 Market report on ceramic coating used in defense and security 21.3 Ceramic coating materials for defense and security industry 21.3.1 Alumina titania ceramic powders 21.3.2 Aluminum oxide powders 21.3.3 Chromium oxide powders 21.4 Ceramic coating in various parts 21.4.1 Submarines 21.4.2 Surface ships 21.4.3 Aircraft 21.4.4 Helicopters 21.4.5 Helicopter rotors 21.5 Various advantages and limitations of ceramic coatings 21.6 Conclusion References

449

Advanced ceramics for anticorrosion and antiwear ceramic coatings Bian Da, Li Jiahong, Chen Yi, Ni Zifeng, Qian Shanhua, Zhao Yongwu and Wang Yongguang 22.1 Introduction 22.2 Anticorrosion ceramic coatings 22.2.1 Solution corrosion 22.2.2 Hot corrosion 22.2.3 Nanocrystalline ceramic coatings 22.3 Antiwear ceramic coatings 22.3.1 Microarc oxidation 22.3.2 Laser cladding 22.3.3 Thermal spraying 22.3.4 Sol gel method 22.4 Conclusions References Crystal structures for flexible photovoltaic application Takeo Oku 23.1 Introduction

449 452 453 453 455 456 457 458 459 459 459 460 460 463 463

469

469 469 469 472 474 475 475 479 481 485 487 488 493 493

xiv

Contents

23.2

Estimation of structural stability of metal organic framework by tolerance factors 23.3 Double perovskites and low-dimensional perovskites 23.4 Grain growth and defects in the metal organic frameworks 23.5 Rietveld refinement of crystal structures for solar cell configuration 23.6 High-temperature annealing and abnormal improvement of conversion efficiencies 23.7 Conclusion Acknowledgments References 24

25

Ceramic materials for coatings: an introduction and future aspects Ganesh R. Chate, Nikhil R., Manjunath Shettar, Vaibhav R. Chate and Raviraj M. Kulkarni 24.1 Introduction 24.2 Ceramic coating material selection 24.3 Ceramic coating materials 24.3.1 Aluminum oxide 24.3.2 Silicon carbide 24.3.3 Yttrium aluminum garnet 24.3.4 Rare-earth cerates and zirconocerate 24.3.5 Silicon nitride 24.3.6 Aluminum nitride 24.3.7 Titanium nitride 24.3.8 Barium titanate 24.4 Coating methods 24.5 Future aspects in ceramics 24.6 Conclusions References Development of an advanced flexible ceramic material from graphene-incorporated alumina nanocomposite Tapan Dash, S. Tanuja Rani, Biswajit Dash and Surendra Kumar Biswal 25.1 Introduction 25.2 Ceramics 25.2.1 Properties of ceramics 25.2.2 Application of ceramics 25.3 Flexible ceramics or flexiramics 25.4 Graphene-incorporated alumina flexible nanocomposites 25.5 Conclusion References

496 497 504 507 509 513 514 514

527

527 529 529 529 531 531 531 531 532 532 532 535 535 536 536

541

541 541 542 543 544 545 547 547

Contents

26

Carbon fiber reinforced ceramics: a flexible material for sophisticated applications Payel Bandyopadhyay, Desigan Ravi, Ramya Ravichandran and Anoop K. Mukhopadhyay 26.1 Introduction 26.2 Fabrication and characterization of carbon fiber-reinforced ceramics 26.3 Microstructure and properties of carbon fiber reinforced ceramics 26.3.1 Microstructural studies 26.3.2 Nanomechanical studies on plan section of C/C composites 26.3.3 Statistical analysis of plan section nanomechanical properties of C/C composites by Weibull model 26.3.4 The nanomechanical studies on cross-section of C/C composites 26.3.5 The nanomechanical studies on carbon fiber 26.3.6 The tensile strength and failure studies on carbon fiber 26.4 Conclusion Acknowledgments References

Index

xv

551

551 553 556 556 557 558 561 563 564 565 565 565 567

Flexible ceramics: an introduction

1

S.A. Srinivasan1 and Santosh Sampath2 1 R&D - NP High Energy Batteries India Ltd, Mathur, Pudukottai, Tamil Nadu, India, 2 Department of Mechanical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Chennai, Tamil Nadu, India

1.1

Introduction

The exponential growth in engineering, particularly related to communication and biofields, is under rapid progress. To effectively deal with and cater to the need aroused, miniaturization of the engineering components is being evolved. Ceramics is the key engineering material having its’ footprints in almost every engineering field, be it domestic, esthetic, or applications-oriented. The exceptional thermophysical properties and their readiness to tailor them are key factors why several researchers swivel around it. Before proceeding with the discussions over ceramics, it will be interesting to know about the crystallography of ceramics that inherits exceptional properties. Ceramics is not a metal, neither a polymer nor a semiconductor, whereas we can find them employed in several applications involving metal/polymer. The atomic arrangement differentiates the ceramics from a longrange ordered single crystal to a short-range ordered ceramics. The periodic arrangement of grains makes these polycrystalline ceramics offer a better property aided by the ionic and covalent sharing of atomic bonds. The history of ceramics dates back to the development of human civilizations through the invention of pottery. In this case, the pace was on the metallic and polymer systems. Still, current demands again leave us to turn the system towards ceramics, and it is worthwhile to mention that ceramics is the head and tail of engineering applications. Apart from incorporating ceramics as particulate additions or heat insulations, structurally tailored ceramics are recently being utilized in aerospace to nuclear engineering. However, they are being utilized in several critical applications challenge lies with their nonductile behavior. Researchers developed polycrystalline nanolaminates in the late 1990s and early 2000, which can be identified as the foundation stones for developing flexible ceramics (FCs). The development was made so that the nanolaminates possess mechanical and thermophysical properties attributed to and compared with the metallic silicates, carbides, or nitrides. Though the ceramics are easily machinable and soft compared to the metallic systems, the readiness of the basal atomic plane to get detached/fractured during a mechanical operation incorporates more brittleness in them. The development of the nanolaminates mainly focused on this shortcoming to make them act ductile and Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00001-4 © 2023 Elsevier Ltd. All rights reserved.

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Advanced Flexible Ceramics

further progressed over the development of multilayer ceramic substrate technology in short—FLEXIBLE CERAMICS or FLEXIBLE OXIDE CERAMICS. On application of an external force/heat on the sandwiched layer (Fig. 1.1), the amorphous ceramics and transitional elements share bonds between their atoms, forming the FC. The crystalline nature of the ceramics makes them harder, and the sandwich structure makes them viable to be employed for engineering applications. The atomic dislocations, mainly misalignments, and slides impart flexibility without compromising properties. Few studies on developing FCs at high temperatures on alumina and zirconia were conducted. The technological development paved the way to incorporate them with binding agents, expanding their applicability.

1.2

Methods for fabrication of flexible ceramics

FCs are effectively utilized in electronics and communications due to their ease in fabrication, miniaturization, etc., due to exponential growth in communications. Though officially, the “FC” terminologies were not in proceedings, by the late 1980s, IBM can be identified as one of the key developers who employed ceramics in developing multilayer ceramic substrate technology. The multilayer lamination of ceramics concepts then evolved as the development of mainframe PCs happened, and at present, we are in the era of miniaturization (Fig. 1.2).

1.2.1 Cofired combustion methods The earlier development of FCs (identified as ceramic laminates) was made using two techniques, high-temperature cofired combustion and low-temperature cofired combustion synthesis. The demarcation between both these techniques was made based on the processing temperature. The process is a self-propagating, facile, and economical technique to prepare many advanced materials, such as ceramics intermetallics, composites, and functionally graded material. Ceramics and metals (Ag, Au, Cu, Pt) are the constituents of which metallic wires are used for conducting and ceramics for semiconducting or insulating, sintered at desired temperatures, and finally, the components were fabricated. The recent advancement overruled

Figure 1.1 Illustration of flexural ceramics (flexiramics).

Flexible ceramics: an introduction

5

Figure 1.2 Multilayered ceramic capacitor. Source: Adapted from N. Blattau, D. Barker, C. Hillman, Lead-free solder and flex cracking failures in ceramic capacitors, in: CARTS 2004: 24th Annual Capacitor and Resistor Technology Symposium, March 29 April 1, 2004.

employing metallic strips or wires over ceramic fiber sheets and employed the ceramic slurries with the structured arrangement of homogenous and nonhomogenous ceramic particulates. The tailoring capability got improved tremendously due to the improvement in the field of manufacturing and fabrication techniques. Silicates, resistors, insulators, oxides, gas and liquid microchannels, amplifiers, and transducers are components fabricated with the CFC synthesis technique.

1.2.2 Printing method Printing techniques are used in several engineering applications over a longer period based on the ink transfer over an elastomeric strip. Modern science adapts the conventional lithographic technique in developing FCs. The lithographic technique transfers the nano-sized ceramics over the thin metallic strip. Depending on the material type, size, and technique adapted, several terminologies are available to the lithographic technique adapted—soft, micro, nano, etc. Nanoimprint lithography (NIL) is one of the most widely applied FC fabrication techniques in developing energy storage strips, data storage devices, photonics, and biotechnology. The NIL is carried out on a solid mold substrate over which nanoimprinting is carried out, and the mold is finally ejected. Polymer-based materials are used as molds in this technique, and the imprints consist of a mixture of ceramic with metallic/polymeric substrates. Organic light-emitting diodes (OLEDs) and micro transistors are the most fabricated components using this technique. Thermo-module generators (TMGs) are also fabricated using the lithographic technique. The P and N types of semiconductors are sandwiched and connected in series over the ceramic substrates employed in heating and cooling applications. Alumina and polyamide are the widely used ceramic substrates to fabricate TMG. In automotive engineering, CO2 and NO2 sensors are fabricated using the lithographic technique with gold and platinum deposited over the alumina substrate. Recent developments made them explore the development of microfluidics for bio-medical engineering (Table 1.1).

1.2.3 Tape casting process Tape casting is a wet synthesis technique that employs the composite ceramic particles made into a slurry, mixing them with the organic and inorganic solvents and

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Advanced Flexible Ceramics

Table 1.1 Comparison of different flexible ceramic fabrication techniques. Methods

Material types

Pros

Cons

References

Tape casting

Slurry

Simple operation

[1,2]

Roll-to-roll bonding

Melt tapes

Centrifugal spinning

Melt slurry

Low flexibility

[4]

Electrospinning

Slurry

thickness reduction possible to ,1 mm High-efficient and lowcost fabrication Compromising

Flexibility depends on shear stress available at blade entry Dispersement of ceramics is ununiform

[5,6]

Solution blow spinning

Slurry

High

Lithography

Slurry, melt, fiber sheets Gas

Ease in fabrication, economic

Nonuniforminity in dispersion Binder and polymer removal is challenging Bond strength is lesser, and processes involved are more Very low efficiency

Sophisticated instrumentation is needed and the process not framed More skill required, and process optimization has not evolved

[10 12]

Chemical vapor deposition

3D printing/ additive manufacturing

Slurry

Plasma electrolytic oxidation and laser-assisted processing

Fibers, melt, slurry

Small components can be made readily A wide spectrum of materials High efficient, better fabrication

[3]

[7,8]

[2]

[9]

[13,14]

subsequently were continuously cast. The slurry was laid into a thin layer over the moving bed and heat-treated, which removes the solvent present, finally forming a thin tape roll to be utilized for desired applications. The mentioned process is also referred to as the paper casting process. Another tape casting technique used is the waterfall casting technique. The slurry mixture containing ceramic particles/fibers with organic and inorganic solvents is made to lay over the flat surface by pouring the slurry over it. The thickness of the tapes is designated, and the moving knife

Flexible ceramics: an introduction

7

blade maintains the slurry layer. This technique’s major advantage is that the thickness can be tailored as desired from millimeters to a few centimeters, and tapes are made after evaporating the solvents by exposure to temperature. The quality of FCs by tape casting technique depends upon particle size, morphology, atomic configuration, packing density, rheological behavior, dispersibility of slurry, and sintering conditions (Fig. 1.3). Dielectric capacitors and fuel cells are a few common engineering applications where the waterfall cast method is employed. As polymeric and metallic-based FCs can be successfully fabricated with tape casting, the microstructural arrangement depends on the ceramic particulate/fiber arrangement, cast shrinkage during drying, and blade shear stress. As the stress generated on the slurry when they pass across the blade is one of the key factors which determines the property of FCs, this process is also referred to as the doctor blading technique in a few contexts. Also, the binders and additives influence the blends’ rheological properties, eventually directly affecting the casting quality.

1.2.4 Roll-to-roll processing method Another commonly employed FC fabrication technique is roll-to-roll (RTR) processing (Fig. 1.4), a.k.a tape calendering process. The raw materials consisting of ceramic particulates/fibers are mixed and blended, followed by deposition, patterning, and packaging as the major steps to fabricate FCs using the RTR process. The FCs made by this process consist of a flexible substrate employed using patterning Mineral oxides powders

Mixing

Organic binder and addives

Homogenisaon Pump

Take up reel Evaporated solvent

Drying

Doctor blade Slurry container Flexible tape

Dry tape

Support table Peeling

Figure 1.3 Typical tape casting process. Source: Adapted from Encyclopedia Britannica.

Carrier film

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Advanced Flexible Ceramics

Figure 1.4 Roll-to-roll processing. Source: Adapted from A. Gregg, L. York, M. Strnad, Roll-to-roll manufacturing of flexible displays, in: G.P. Crawford (Ed.), Flexible Flat Panel Displays, Wiley, 2005, pp. 410 445.

techniques and robust package layers, and the sandwich provides necessary properties incorporated. The raw materials are allowed to deposit over the thin foil and are roll pressed to maintain the thickness during the deposition process. Followed by it will be the insulating film, the metallic source layer as desired based on the application. The sandwich is packaged to prevent degradation due to environmental contacts in the final packaging state. A clear choice of the substrate material and the ceramics designates the property inhibition of the RTR FCs. RTR process is more widely adopted as the FCs fabricated with this technique provide high dimensional stability under thermal loads. Semiconductor devices, micro-batteries, battery strips, and web connect wearables are some of the most common FC items fabricated by the RTR process.

1.3

Applications and challenges

Fig. 1.5 presents the emerging applications of FCs with their methods for fabrication. Beyond electronics and telecommunications, the wings of ceramics have recently expanded towards acoustics engineering in the structural and automotive fields with the development of three-dimensional (3D) printing and additive manufacturing. Conventional ceramics are employed in noise suppression over a long period, and 3D printed FC nanosponges have been utilized as noise absorbents. A top-down 3D fabrication technique involves a multiple-step fabrication process involving time and is also not economically feasible. In the bottom-up approach, structures are assembled from atomic units to nanostructures transforming them into microscopic and macroscopic levels. Tetrapodal and multimodal architectures are the most efficient bottom-up approaches. Flame transport synthesis is one of the most successful synthesizing tetrapodal FCs. As per the 3D geometry under consideration, the interconnected network can be made and synthesized. The 3D printing technique incorporates annealing of the FC substrate, development of pod arms, and reheating process to the formation of pod arms incorporating necessary properties. The high

Flexible ceramics: an introduction

9

Figure 1.5 Applications of flexible ceramics.

structural stability and better mechanical and thermal endurance are the key features of 3D printed FCs. Micro electro mechanical systems (MEMS), Nano electro mechanical systems (NEMS), and radio frequency (RF) resonators are a few successful engineering applications explored using this technique but are incubating. The versatility of this technique lies not only with the development of FCs but also in its process flexibility in manufacturing complex geometry, reduced process time, economic, and mass production. Photopolymerization, laser-assisted sheet lamination, sheet extrusion, and directed energy deposition are a few fabrication techniques employed in 3D printing. Another process in the incubation state is FC by plasma electrolytic oxidation. The ceramic layer is allowed to grow over the thin metallic foil, thus utilizing the flexibility of the metallic substrate and incorporating the properties of ceramic reinforcement over it. Generally, the sandwich structure is adapted in this technique by allowing the β ceramic layer to grow and form a γ phase by adapting the hierarchical fabrication process. In a nutshell, the view of ceramics by researchers across the globe has evolved to develop high-performance FCs from the conventional utilization of ceramics. Though few potential applications were already developed, high-performance material development is in the fray. Several efforts are being carried out in finding a suitable fabrication technique for the manufacture of FCs, whereas the module is yet to be developed. The challenge lies with the property homogenization, binding and additives mainly to overcome these shortcomings. Several pieces of research are prolonged in the areas of metallic incorporations also intriguing on thin ceramic sheets. Organics, perovskites, hybrid materials, and inorganic materials are common materials utilized in developing the FCs. A few promising fabrications were also made to employ FC as liquid electrolytes in energy storage applications. A few key

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Advanced Flexible Ceramics

synthesis techniques were briefly discussed to be a preamble for the subsequent chapters, which deal in detail with several materials and processing techniques evolved.

References [1] J. Grottrup, et al., Three-dimensional flexible ceramics based on inter connected network of highly porous pure and metal alloyed ZnO tetrapods, Ceram. Int. 42 (7) (2016) 8664 8676. [2] Y. Ichikawa, et al., Flexible ceramics in the system KZr2(PO4)3 KAlSi2O6 prepared by mimicking the microstructure of itacolumite, J. Am. Ceram. Soc. 91 (2) (2008) 607 610. [3] L. Gregg A, York, M. Strnad, Roll-to-roll manufacturing of flexible displays, in: G.P. Crawford (Ed.), Flexible Flat Panel Displays, Wiley, 2005, pp. 410 445. [4] B.N. Tepekiran, et al., Centrifugally spun silica (SiO2) nanofbers for high-temperature air filtration, Aerosol Sci. Technol. 53 (2019) 921 932. [5] X.L. Song, et al., Morphology, microstructure and mechanical properties of electrospun alumina nanofibers prepared using different polymer templates: a comparative study, J. Alloy. Compd. 829 (2020) 154502. [6] X. Wang, et al., C2H2 gas sensor based on Ni-doped ZnO electrospun nanofibers, Ceram. Int. 39 (2013) 2883 2887. [7] Z. Huang, et al., Solution blowing synthesis of liconductive ceramic nanofibers, ACS Appl. Mater. Interfaces 12 (2020) 16200 16208. [8] L. Li, et al., Research progress of ultrafne alumina fber prepared by solgel method: a review, J. Chem. Eng. 421 (2021) 127744. [9] J. Chao, et al., Flexible ceramic fibers: recent development in preparation and application, Adv. Fiber Mater. (2021). Available from: https://doi.org/10.1007/s42765-02200133-y. [10] Z. Chen, et al., 3D printing of ceramics: a review, J. Eur. Ceram. Soc. 39 (4) (2019) 667 687. [11] W.L. Huo, et al., In situ synthesis of three-dimensional nanofber-knitted ceramic foams via reactive sintering silicon foams, J. Am. Ceram. Soc. 102 (2019) 2245 2250. [12] H. Wang, et al., Tin oxide nanofiber and 3D sponge structure by blow spinning, IOP Conf. Ser. Earth Environ. Sci. 358 (2019) 052015. [13] K. Pan, et al., A flexible ceramic/polymer hybrid solid electrolyte for solid-state lithium metal batteries, Adv. Mater. (2020). Available from: https://doi.org/10.1002/ adma.202000399. [14] Z. Wang, Henry Hu, Xueyuan Nie, Preparation and characterization of highly flexible al2o3/al/al2o3 hybrid composite, J. Nanomater. 412071 (2015) 1 8.

Further reading A. Altecor, Q. Li, K. Lozano, Y. Mao, Mixed-valent VOx/polymer nanohybrid fibers for flexible energy storage materials, Ceram. Int. 40 (2014) 5073 5077.

Flexible ceramics: an introduction

11

M. Aryal, Stephen W. Allison, Kathy Olenick, Firouzeh Sabri, Flexible thin-film ceramics for high-temperature thermal sensing applications, Optical Mater. 100 (109656) (2020) 1 11. W. Chen, et al., The influence of different additives on microstructure and mechanical properties of aluminum titanate ceramics, Ceram. Int. 47 (1) (2021) 1169 1176. X. Li, et al., Review and perspective of materials for flexible solar cells, Mater. Reports: Energy 1 (1) (2021) 100001. C.S. Martı´nez, et al., Miniaturized total analysis systems: integration of electronics and fluidics using low-temperature co-fired ceramics, Anal. Chem. 79 (21) (2007) 8376 8380. D.A. Pardo, G.E. Jabbour, N. Peyghambarian, Application of screen printing in the fabrication of organic light-emitting devices, Adv. Mater. 12 (17) (2000) 1249 1252. B. Thorstensen, et al., High volume production of ceramic thick sheet materials, Proc. 8th Int. Conf. Multi-Material Micro Manufacture (2011) 272 274. J.C. Wang, H. Dommati, S.J. Hsieh, Review of additive manufacturing methods for highperformance ceramic materials, Int. J. Adv. Manuf. Technol. 103 (2019) 2627 2647. X. Xu, et al., Elastic ceramic aerogels for thermal superinsulation under extreme conditions, Mater. Today 42 (2021) 162 177. Y. Zhao, et al., Elastic and well-aligned ceramic LLZO nanofiber based electrolytes for solid-state lithium batteries, Energy Storage Mater. 23 (2019) 306 313. L.X. Zhou, et al., La2O2CN2:Yb31/Tm31 nanofibers and nanobelts: novel fabrication technique, structure and upconversion luminescence, J. Mater. Sci.: Mater Electron. 28 (2017) 16282 16291.

Shape memory ceramics

2

Santosh Sampath1, Srinivasan Alagappan2, G. Sudha Priyanga3, Ram K. Gupta4, Ajit Behera5 and Tuan Anh Nguyen6 1 Department of Mechanical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Chennai, Tamil Nadu, India, 2Hydrogen Division, High Energy Batteries (India) Limited, Mathur, Tamil Nadu, India, 3Department of Physics, Research Institute for Natural Science, and Institute for High Pressure at Hanyang University, Hanyang University, Seoul, Republic of Korea, 4Department of Chemistry, National Institute for Materials Advancement, Pittsburg State University, Pittsburg, KS, United States, 5 Deparment of Metallurgical & Materials Engineering, National Institute of Technology, Rourkela, Odisha, India, 6Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

2.1

Introduction

In electronic equipments and high-temperature applications, there is a huge problem in the failure of miniaturized materials due to low fusing temperature. To enhance the working temperature of those materials we need ceramic materials. But again we are facing problems due to brittle fractures and catastrophic failure. In recent age, it is possible to develop the toughness of the ceramic structure. This book chapter gives the advanced property and applications of structural and functional ceramics. Materials-dependent ceramic flexibility is discussed here. Microstructure and phases present in flexible ceramic are the most important parameters that give the new direction for various applications. The involvement of influencing parameters is discussed to know the operational life of the flexible ceramic. How smart behavior like shape memory behavior and self-healing behavior of the ceramic will develop by incorporation of specific flexible materials are discussed. Novel thermal, physical, mechanical, electrical, and optical properties of flexible ceramic have been discussed. Various processing routes such as powder metallurgy route, physical vapor deposition, chemical vapor deposition, sol gel, three-dimensional (3D) printing, and roll-to-roll processing are explained here with their specific applications in micro electro mechanical systems (MEMS), and semiconductor industries, energy generation, and storage industries. Based on the properties of flexible ceramics how one can apply flexible ceramics in various advanced equipment has been discussed for nano electro mechanical systems (NEMS), electronics, automobiles, aviation, and healthcare industries.

Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00002-6 © 2023 Elsevier Ltd. All rights reserved.

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2.2

Advanced Flexible Ceramics

Smart ceramics

The ceramic materials made up of ultrafine particles attract several researchers across the globe as they can change the length scale in the nanoscale, and tailor the unexpected material properties. The property of smart ceramics is primarily driven by the particle size, surface energy exertion, and degree of aggregation. Nanotechnology advancements have cleared the door for the creation of new smart materials. Because of their exceptional multidimensional qualities, bioceramics play an essential role in the biomedical area with evolving developments in smart electronics. Metal oxide nanoparticles are the preferred materials of choice compared to the other ceramic oxides. Porous nanostructured ceramics on the other hand has evolved with several applications in the broad spectrum field of bio, chemical, and electronic engineering. Refractory ceramics (Al2O3, ZrO2, MgO, SiC) are under the active consideration of research for several years and they were scaled down to nano-level research too a decade earlier. Zirconia smart ceramics were developed by a few researchers as their properties go in hand with the shape memory alloys (SMAs) [1 3]. Coping with the technological advancements, recent advancements are toward bioceramics like hydroxyapatite for drug delivery mechanisms, yttriumbased superconductive ceramics, titania, and alumina coupled molecular membranes are a few identified smart ceramics developed recently. With a wide synthesis technique available, the range of compositions, homogenous mixing, and temperature flexibility has made the researchers focus on the sol gel method primarily to synthesize the smart ceramics. The solid-state technique was also employed further but is limited to the size of the product. Laserassisted 3D printing and additive manufacturing (AM) are identified as the few most promising techniques available in the new era of manufacturing for making smart ceramics. Though several manufacturing techniques aid in obtaining desired characteristics of smart ceramics, the most prominent property which makes them more distinct from bulk is their higher surface energy. The lower coordination number and presence of unsaturated bonds on the surface make them more accommodative with the additives/second phase materials leading to an extremely high surface-to-volume ratio eventually possessing high surface energy. Few materials are designed for sensing, actuating and also have the ability to self-learn by mutable property coefficients as per the environment. They are technically termed as “very smart materials”. The criteria which distinguish smart ceramics apart from intelligent materials is the ability of the material to automatically modify one or more of its property coefficients. They exhibit active and passive smartness according to environmental exposure. Active smartness is in analogy to the human body which senses, control, actuates, and reiterates based on the working conditions. The feedback system incorporation becomes functionable based on the intrinsic physical property of the smart ceramics. A passive smartness of ceramics differs from that of an actively smart material in which no external fields or forces or feedback systems are used to enhance its behavior. Switchability and sensitivity self-diagnosis are a few passive smartness properties that are being observed before

Shape memory ceramics

15

terming a ceramic as a smart ceramic. A few key applications of these smart ceramics are presented in the next sections.

2.3

Mechanism of shape recovery in smart ceramics

Shape memory has been seen in a range of materials, including alloys that may endure inelastic strain to a certain extent due to reversible martensitic phase shift and then revert to their original shape when thermal loads, such as heating or cooling, are applied (thermally induced phase transformation) [4,5]. Superelasticity (also known as pseudoelasticity) is the ability of a material to change phases when mechanical loads are applied (stress-induced phase transformation). Zirconia is the only ceramic known to exhibit both superelastic and shape memory responses when subjected to mechanical pressures (compressive stress) (heat treatment). Phase transition between two phases, the motion of twin borders, and reorientation all contribute to superelastic (pseudoelastic) and form memory behavior in ceramics (Fig. 2.1, [6]). Functional ceramic materials with phase-changing properties have recently attracted interest for certain applications due to several advantages over metallic

Figure 2.1 Mechanism of shape memory effect in (A) shape memory alloys and (B) shape memory ceramics a comparison. Source: Adapted from K. Uchino, Antiferroelectric shape memory ceramics, Actuators 5 (11) (2016). https://doi.org/10.3390/act5020011.

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Advanced Flexible Ceramics

alloys, such as high strength, high-temperature endurance, and low thermal coefficient of expansion. Piezoelectric ceramics are a type of smart material with form memory that creates a voltage when stressed or the opposite when not. By doping other elements as acceptor or donor dopants in relaxor ceramic compositions like BaTiO3, PbTiO3, and SrTiO3, many attempts have been made to obtain adequate electro-strain generated by electric field-driven ion shifts. Ceramics can be used with alloys or polymers to create new capabilities and boost shape memory responsiveness in specialized high-tech applications. Shape memory ceramics are a relatively new research topic. When materials are stretched past their elastic limit, they display the shape memory effect. The effect is characterized by a micro deformation generated by external forces such as electric or magnetic fields that can be partially or completely reversed by removing the external fields or by heat treatment. The reversible martensiteaustenite transformation, which is temperature-driven in the shape memory effect and stress-driven in superelasticity, is the essential basis for shape memory materials in general. The increase in surface roughness in polycrystals caused by partial martensite-austenite transformation during cycling (rather than complete transformation) is thought to be the cause of crack formation and material failure. The most significant challenge restricting the application of shape memory materials is fatigue, whether structural or functional (change in material functional property and reversibility). Olander discovered the form memory effect in 1932 while researching Au Cd alloys. Shape memory materials attracted a lot of attention after Buehler et al. discovered a 50% N 50% Ti alloy [4,5]. He found that straightening a bent wire at room temperature and then heating it to a higher temperature led it to return to its original shape (the bent shape).

2.4

Methods for fabrication

There are various methods of ceramic production. Some of the widely used methods are portrayed in Fig. 2.2. While each method has its own advantages and disadvantages, the right choice of the method is arrived based on the properties required and the intended application. Evaporation, precipitation, drying, and decomposition occur in a distributed manner in the spray pyrolysis process, which is based on the ultrasonic creation of micrometric-size aerosol droplets at temperatures between 400 C and 600 C. The expanding use of chemistry in ceramic synthesis necessitates the development of colloid science. Colloidal dispersions and other synthesis procedures, such as the sol gel process, are used to engage in the nucleation and increase of the particle size. Sol is a crystalline or amorphous stable dispersion of colloidal particles or polymers in a solvent. A gel encircles a liquid phase and is made up of a continuous 3D network. The system is made up of colloidal particles that have clumped together in a gel. In most cases, hydrogen bonds or van der Waals forces will interact with the sol particles. If there are other interactions, the gelation process can be reversible. For the preparation of metal alkoxide powders, the sol gel method has been widely employed.

Shape memory ceramics

17

Figure 2.2 Widely used fabrication methods of shape memory ceramics.

In the creation of ceramic materials, the electrochemical approach offers significant and distinctive advantages. The electrochemical approach is concerned with processes that take place using electrical energy. The ions flow through a solution using an electric field in the electrochemical deposition process. The voltage is measured between the two electrodes of the electrochemical cell, which consists of a cathode and an anode. Different electron transfer processes between the electrode and electroactive species in the solution are involved in the electrochemical deposition. Ceramic oxides can be deposited electrochemically from alkaline solutions under both oxidizing and reducing conditions. Combustion synthesis is a quick and easy way to make a wide range of highquality ceramics, intermetallics, and composites. An aqueous solution containing metal salts and some organic fuel is gelled and burned in this process. Exothermal redox reactions occur during the combustion process. The reaction caused the overinflated precursor to disintegrate, resulting in a significant volume of gases and a nanocrystalline powder after calcination. The size of the container and the mass of the mixture were used to control the method of combustion.

2.5

Electrical and electronic applications of smart ceramics

Smart ceramics incorporate several functions such as sensing, control, and actuation in both predictive and adaptive techniques thereby performing several smart actions. The early investigation on smart ceramics was made in the late 1980s. One

18

Advanced Flexible Ceramics

earlier study was carried out on utilizing the piezoelectric capability of the ceramics. The smart PbZrO3-PbTiO2-PbSnO3 ternary system material was utilized for sensor applications that possess readability and repeatability properties [7,8]. Yttrium stabilized zirconia was also examined for translucency [9 11] by varying their aging characteristics. They were identified as the development over the second-gen zirconia ceramics which falls under micro ceramics. Though few reports were identified as micro-sized ceramics as smart ceramics, it was widely accepted by the scientific community that nanoceramics could be the better one for the particular mentioned applications. The distortion levels at the test conditions for photo luminence and translucency are identified to be controlled by employing nano-sized particles. The inherent material property and the tailored conditions are attributes toward the property levels. The shape memory effect of a few of these ceramics gives them another notion as shape memory ceramics (SMC) over which few contributions are being made. The physical property applicability to make them employable in extreme and exceptional environments attracted a few of the several researchers. Pan chen [7] and his group has contributed to developing Na0.5Bi0.5TiO3-based smart ceramics for ferroelectric applications. The role of the residual stress restricts the applications of the material mainly as SMC and the ferroelectric smart ceramics exhibited lesser critical residual stresses which makes them more employable in the place of zirconia-based smart ceramics. Shape memory has been demonstrated in perovskite lead lanthanum zirconate titanate (PLZT) ceramics, an important ferroelectric ferroelastic because of the tremendous potential for applications due to the formation of microdomains smaller than the wavelength of light. The AM and the 3D printing technology were also incorporated in the fabrication of smart ceramics. Though few successful trials were made, there exists a challenge in the designing of the manufacturing process flow and a few limitations do exist with process parameters too mainly thermal management [11]. Once again zirconia being one of the superior ceramic materials, is being experimented for making them as smart sensors with AM and 3D manufacturing techniques. Silver wire strengthened zirconia by laser-assisted synthesis is used in prosthesis implants [12] a novel approach for fabrication with intrinsic capacities. The ceramic-nanocomposite technique with bimodal particle size packing and a pressure-less sintering process was examined and investigated to obtain lead magnesium niobate-lead titanate smart ceramic [13]. The study showed that the perovskite phase formation, microstructures, and dielectric broadening properties of ceramic-nanocomposites inherit the smart properties into it. Newnham et al. [14] reported the use of smart ceramics in ferroelectric sensors and actuators. For shape memory actuators, 6.5/65/35 PLZT helix and (Pb, Nb)(Zr, Sn, Ti)O3 ceramics have been presented. For the fully tunable transducer, the elastic nonlinearity and piezoelectric nonlinearity can be applied in the case of the Pb (Mg0.3Nb0.6Ti0.1)O3 ceramics. The incorporation of thin rubber layers in an electrostrictive transducer offers some interesting features, which can be optimized with bias fields and bias stresses. The author indicated that these ceramics-based sensors and actuators can be prepared as multilayer ceramic packages, which consisted of

Shape memory ceramics

19

low permittivity dielectric layers with metal circuitry printed on each layer and interconnected through metalized holes between layers. These 3D packages incorporated the buried capacitors/resistors and other components, such as the smart sensors, adaptive actuators, display panels, and thermistors/ varistors (to protect against current and voltage overloads). Some of the promising smart ceramics are shown in Fig. 2.3. Recent research trends involve fabricating an oligocrystalline pillar with smart ceramics [15]. These materials are said to respond differently for different diameters owing to size effects. This is presented in Fig. 2.4. These ceramics were able to undergo deformation without cracks and recover upto 7% strain. They also have a very high working temperature of 600 C. As a result, they can be a plausible replacement for high-temperature SMAs such as CuAlNi, NiTiPd, etc.

2.6

Biomedical applications of smart ceramics

Oxide ceramics of alumina and zirconia find their extensive application in several bio and medical applications as implants, prosthesis, etc. Porcelain fused metallic ceramics were the early days’ bio-ceramic utilized in dentistry and were identified as the only way possing strong and reliable reinforcement for the patients. The evolution of zirconia was identified as one of the forefront runs in the bio-applications which now moves toward nanozirconia biosmart ceramics having their application in numerous applications ranging from external implants to internal implants and drug delivery. Zirconia smart ceramics in continuations to the traditional usage of the oxides find their employability in several other intricate applications mainly due

Figure 2.3 Promising multifunctional smart ceramics.

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Advanced Flexible Ceramics

Figure 2.4 Schematic representation of the microstructures as a function of pillar diameter (Dia) and the grain size (d) (A C), along with typical SEM images and load displacement curves for pillars of corresponding relative dimensions; #1 (D, G), #2 (E, H) and #3 (F, I). Fc in (G I) indicates the critical load for initiating the martensitic transformation. Source: Reprinted from Z. Du, M. Z. Xiao, L. Qing, L. Alan, A. Shahrouz, M. Ali, et al., Size effects and shape memory properties in ZrO2 ceramic micro- and nano-pillars, Scr. Mater. 101 (2015) 40 43. https://doi.org/10.1016/j.scriptamat.2015.01.013 with permission from Elsevier.

to their meta stability in nature. The low valence oxides of rare earth such as cerium, yttrium are incorporated with the zirconia to maintain tetragonality in structure mainly as dopants in a few cases. The translucent property of these doped structures becomes one of the major studies that was carried out to evaluate the smartness of these ceramics. Few significant successful trials were made on utilizing the yttrium stabilized zirconia smart ceramics with the aid of computer-assisted manufacturing techniques in the field of prosthesis. The evolution of nanotechnology should be greatly acknowledged at this juncture which eases the practicality in applications. Prosthondic and orthopedic applications of doped zirconia are a significant development in biosmart materials. Though the sensitivity to low- temperature degradation is one of the biggest challenges that lies within particularly during the sterilization process. Zirconia doped with hafnium oxide was examined in a few prosthondic applications as they have proven high aging stability.

Shape memory ceramics

21

The boron nitride nanoplatelets (BNNPs) were experimented recently for their application in dentistry. The Homogenously dispersed BNNP reinforced with the zirconia composite exhibited several enhanced properties compared with their zirconia counterpart mainly nontoxic in nature [16]. In dentistry, to replace stainless steel or metal bridges, CERCON has been developed as the first all-ceramic teeth bridge, which is based on a process that enabled the direct machining of ceramic teeth and bridges. This zirconia-based all ceramic bridge is not baked in layers on the metal but it is created from one unit without metal. Its overall advantages are metal-free, biocompatible, and restoration with strength, which may help to resist crack formation [17,18]. For bone tissue engineering, calcium phosphate ceramics [19] and hydroxyapatite ceramics [20] have been developed. Trombetta et al. [21] reported the 3D printing of calcium phosphate ceramics for bone tissue engineering and drug delivery. For drug delivery applications, various ceramics-based drug delivery carriers have been reported [22,23]. As reported, these common bioceramics are betatricalcium phosphate (β-TCP), hydroxyapatite, mesoporous silica, and zirconia hydroxyapatite composite. The advantages of ceramic-based drug systems are (1) adjustable size and structure for effective loading nanoscale drugs, (2) low toxicity, good biocompatibility, biodegradability, and biological stability, and (3) smart release drugs by external or internal stimuli (response to light, pH, magnetism or heat, etc.) [22]. In addition, Yang et al. [24] indicated that nanophase ceramics/ ceramic nanoparticles can be used to improve drug delivery effectiveness. The overall biomedical applications are summarized in Fig. 2.5.

Figure 2.5 Biomedical applications of smart ceramics.

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Advanced Flexible Ceramics

Industrial application of smart ceramic

In the operation of coal-fired power plants with carbon capture and storage, novel ceramic membrane technologies, such as the molecular sieving silica and perovskite membranes, offer the opportunity to reduce efficiency losses by separating gases at high temperatures and pressures [25]. Smart et al. [26] reported the development of porous ceramic membranes and their application as membrane reactors for both gas and liquid phase reaction and separation. For oxygen production, mixed conducting perovskite-type ceramic membranes have been developed for the air separation [27]. This dense ceramic perovskite membranes can replace effectively the cryogenic air separation and reduce O2 production costs (by more than 35%), which can significantly reduce the energy penalty by 50% in case that it is integrated into the oxy-fuel power plant for CO2 capture. Similarly, Diniz da Costa et al. [28] developed dense ceramic perovskite membranes, such as barium strontium cobalt iron mixed oxides, which exhibit fluxes on the order of 10 15 mL/min/cm2. On the other side, Baumann et al. [29] introduced asymmetric ceramic membranes for the efficient separation of oxygen from the air. The authors introduced different types of ceramic membranes, such as La0.6Sr0.4Co0.2Fe0.8O32δ, La0.5Sr0.5CoO32δ, Ba0.5Sr0.5Co0.8Fe0.2O32δ, La0.58Sr0.4Co0.2Fe0.8O32δ, SrCo0.4Fe0.5Zr0.1O32δ, La0.6Ca0.4CoO32δ, which focused on mixed ionic electronic conductors for separating oxygen from air [29]. Therefore the SMC are highly potential materials that have proven their applications in many areas and also have plausible applications in niche areas, owing to their multifunctional and smart characteristics.

References [1] M. Asle Zaeem, N. Zhang, M. Mamivand, A review of computational modeling techniques in study and design of shape memory ceramics, Comput. Mater. Sci. 160 (2019) 120 136. Available from: https://doi.org/10.1016/j.commatsci.2018.12.062. [2] R.H. Hannink, P.M. Kelly, B.C. Muddle, Transformation toughening zirconia containing ceramics, J. Am. Ceram. Soc. 83 (2000) 461 487. [3] P.M. Kelly, L.R. Francis Rose, The martensitic transformation in ceramics—its role in transformation toughening, Prog. Mater. Sci. 47 (2002) 463 557. Available from: https://doi.org/10.1016/S0079-6425(00)00005-0. [4] S. Santosh, V. Sampath, R.R. Mouliswar, Hot deformation characteristics of NiTiV shape memory alloy and modeling using constitutive equations and artificial neural networks, J. Alloy. Compd. 901 (2022) 163451. Available from: https://doi.org/10.1016/j. jallcom.2021.163451. [5] S. Santosh, R. Praveen, V. Sampath, Influence of cobalt on the hot deformation characteristics of an NiTi shape memory alloy, Trans. Indian. Inst. Met. 72 (2019) 1465 1468. Available from: https://doi.org/10.1007/s12666-019-01591-6. [6] K. Uchino, Antiferroelectric shape memory ceramics, Actuators 5 (2016) 11. Available from: https://doi.org/10.3390/act5020011.

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[7] P. Chen, H. Wang, et al., Shape memory effect in Na0.5Bi0.5TiO3-based ferroelectric ceramics, Acta Mater. 223 (2022) 117479. [8] R.E. Newnham, Q.C. Xu, S. Kumar, L.E. Cross, Smart ceramics, Ferroelectrics 102 (1) (1990) 259 266. Available from: https://doi.org/10.1080/00150199008221486. [9] I. Sailer, J. Gottnerb, S. Kanelb, C.H. Hammerle, Randomized controlled clinical trial of zirconia-ceramic and metal-ceramic posterior fixed dental prostheses: a 3-year follow-up, Int. J. Prosthodont. 22 (6) (2009) 553 560. [10] G.K.R. Pereira, et al., Mechanical reliability, fatigue strength and survival analysis of new polycrystalline translucent zirconia ceramics for monolithic restorations, J. Mech. Behav. Biomed. Mater. 85 (2018) 57 65. [11] R. Huang, A. Urban, J. Jiao, J. Zhe, J.-W. Choi, Inductive proximity sensors within a ceramic package manufactured by material extrusion of binder-coated zirconia, Sens. Actuators A: Phys. 338 (1) (2022) 113497. [12] C.G. Moura, et al., Laser printing of silver-based micro-wires in ZrO2 substrate for smart implant applications, Opt. & Laser Technol. 131 (2020) 106416. [13] R. Wongmaneerung, et al., Potential of nanocomposite technique for fabrication of smart ceramics, Chiang Mai J. Sci. 36 (2) (2009) 179 187. [14] R.E. Newnham, Ferroelectric sensors and actuators: smart ceramics, in: N. Setter, E.L. Colla (Eds.), Ferroelectric Ceramics. Monte Verita` (Proceedings of the Centro Stefano Franscini, Ascona), Birkh¨auser, Basel, 1993. Available from: https://doi.org/10.1007/ 978-3-0348-7551-6_13. [15] D. Zehui, X.M. Zeng, Q. Liu, A. Lai, S. Amini, A. Miserez, et al., Size effects and shape memory properties in ZrO2 ceramic micro- and nano-pillars, Scr. Mater. 101 (2015) 40 43. Available from: https://doi.org/10.1016/j.scriptamat.2015.01.013. [16] L. Bin, et al., Boron nitride nanoplatelets as reinforcement material for dental ceramics, Dental Mater. 36 (6) (2020) 744 754. [17] http://www.cercon-smart-ceramics.com. [18] P. Daniela, C. Mariana, N. Marius, C. Liviu, S.C. Radu, Cercon smart ceramics: from theory to practice, Int. J. Med. Dent. 19(1) (2015) 31 36. [19] S. Satyavrata, A.R. Whittington, A.S. Goldstein, Calcium phosphate ceramics in bone tissue engineering: a review of properties and their influence on cell behavior, Acta Biomater. 9 (9) (2013) 8037 8045. Available from: https://doi.org/10.1016/j. actbio.2013.06.014. [20] H. Yoshikawa, A. Myoui, Bone tissue engineering with porous hydroxyapatite ceramics, J. Artif. Organs 8 (2005) 131 136. Available from: https://doi.org/10.1007/ s10047-005-0292-1. [21] R. Trombetta, J.A. Inzana, E.M. Schwarz, et al., 3D printing of calcium phosphate ceramics for bone tissue engineering and drug delivery, Ann. Biomed. Eng. 45 (2017) 23 44. Available from: https://doi.org/10.1007/s10439-016-1678-3. [22] S. Zang, S. Chang, M.B. Shahzad, X. Sun, X. Jiang, H. Yang, Ceramics-based drug delivery system: a review and outlook, Rev. Adv. Mater. Sci. 58 (1) (2019) 82 97. Available from: https://doi.org/10.1515/rams-2019-0010. [23] W. Paul, C.P. Sharma, Ceramic drug delivery: a perspective’, J. Biomater. Appl. 17 (4) (2003) 253 264. Available from: https://doi.org/10.1177/0885328203017004001. [24] L. Yang, B.W. Sheldon, T.J. Webster, Nanophase ceramics for improved drug delivery: current opportunities and challenges, Am. Ceram. Soc. Bull. 89 (2) (2010) 24 32. [25] S. Smart, C.X.C. Lin, L. Ding, K. Thambimuthu, J.C. Diniz da Costa, Ceramic membranes for gas processing in coal gasification, Energy & Environ. Sci. 3 (3) (2010) 268. Available from: https://doi.org/10.1039/b924327e.

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[26] S. Smart, S. Liu, J.M. Serra, J.C. Diniz da Costa, A. Iulianelli, A. Basile, Chapter 8 Poous ceramic membranes for mrembrane reactors, in: A. Basile (Ed.), Woodhead Publishing Series in Energy, Handbook of Membrane Reactors, Volume 1, Woodhead Publishing, 2013, pp. 298 336. Available from: https://doi.org/10.1533/9780857097330. 2.298. [27] K. Zhang, J. Sunarso, Z. Shao, W. Zhou, C. Sun, S. Wang, et al., Research progress and materials selection guidelines on mixed conducting perovskite-type ceramic membranes for oxygen production, RSC Adv. 1 (9) (2011) 1661. Available from: https://doi. org/10.1039/c1ra00419k10.1039/C1RA00419K. [28] J. Diniz da Costa, S. Smart, J. Motuzas, S. Liu, D. Zhang, State art. (SOTA) Rep. Dense Ceram. Membr. Oxyg. Sep. Air (2013) 1 13. Available from: http://www.globalccsinstitute.com/publications/state-art-sota-report-dense-ceramicmembranes-oxygenseparation-air. [29] S. Baumann, W.A. Meulenberg, H.P. Buchkremer, Manufacturing strategies for asymmetric ceramic membranes for efficient separation of oxygen from air, J. Eur. Ceram. Soc. 33 (7) (2013) 1251 1261. Available from: https://doi.org/10.1016/j.jeurceramsoc. 2012.12.005.

Further reading J. Chevalier, What future for zirconia as a biomaterial, Biomaterials 27 (2006) 535 543. Available from: https://doi.org/10.1016/j.biomaterials.2005.07.034. R.H.J. Hannink, P.M. Kelly, B.C. Muddle, Transformation toughening in zirconiacontaining ceramics, J. Am. Ceram. Soc. 83 (2000) 461 487. Y. Zhang, Making yttria-stabilized tetragonal zirconia translucent, Dental Mater. 30 (2014) 1195 1203. Available from: https://doi.org/10.1016/j.dental.2014.08.375. S. Kumar, A.S. Bhalla, L.E. Cross, Smart ceramics for broadband vibration control, J. Intell. Mater. Syst. Struct. 5.5 (1994) 673 677. W.M. Novicoff, A. Manaswi, M.V. Hogan, S.M. Brubaker, W.M. Mihalko, K.J. Saleh, Crit. Anal. Evid. Curr. Technol. Bone Healing Repair. 90 (2008) 85 91.

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3

Sathasivam Pratheep Kumar, Daneshwaran Balaji and Triveni Rajashekhar Mandlimath Department of Chemistry, School of Advanced Sciences, VIT-AP University, Amaravati, Andhra Pradesh, India

3.1

Introduction

Ceramics and their composites find extensive application in household utensils to space vehicles. Advanced ceramics are gaining special attention over traditional ceramics for their fascinating properties such as less brittleness, high thermal and mechanical stability, corrosion and abrasion resistance, and good rigidity [1 3]. These properties support the ceramics and their composites to function in extreme environmental conditions. Advanced ceramics find their suitability in automobile, electronic, and medical industries. Recently, several researchers have shown the possibility of making stretchable and flexible sheets, ribbons, and strips [4 8]. In particular, Yttria-stabilized zirconia and zinc oxide flexible ceramics have made their impact in several applications, which include power generation in the energy sector [9 14]. Similarly, polymer composites of flexible ceramics such as lanthanum aluminum titanate (LTO), SnO2, Al2O3, Al2TiO5, Li6.40La3Zr1.40Ta0.60O12 (LLZTO), Ba0.6Sr0.4TiO3 (BST), lead zirconate titanate (PbZr0.52Ti0.48O3, PZT) have been fabricated for flexible electronic devices [15 24]. Thus flexible ceramics are emerging as next-generation materials. In this chapter, the characterization of flexible ceramics by various analytical techniques is discussed with their instrumentation, sample preparations, interpretations, and applications.

3.2

Characterization techniques

3.2.1 Electron microscopy Electron microscopes are used to scan the internal and external structure of materials. High energetic electrons from an electron beam are being used as a source. This study yields surface morphology, topography, chemical composition, and crystallographic information of a solid material. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are the two important techniques being used to study the internal and external structure of materials. Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00003-8 © 2023 Elsevier Ltd. All rights reserved.

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Advanced Flexible Ceramics

3.2.1.1 Field-emission scanning electron microscopy A stream of high-energy electrons from the electron gun is accelerated downwards through the series of high-power magnetic lenses to focus the electron on a narrow path. The focused electron beam moves back and forth over the specimen. When the electron beam strikes the sample, the secondary electrons are ejected from the surface of the specimen. The ejected electrons are captured by the detector and generate the signals to an amplifier which gets converted into an image [25]. The schematic representation of field emission scanning electron microscopy (FESEM) is shown in Fig. 3.1. The surface morphology, porosity, size and shape of the particles, surface defects, and quantitative elemental information of flexible ceramics can be obtained from SEM and SEM attached with energy dispersive X-ray spectroscopy (SEMEDX). Elemental mapping gives the distribution of elements precisely. Further, it is possible to determine the number of phases present in the materials and structural features when SEM is combined with the backscattered detector and electron backscattering diffraction, respectively. Sample preparation does not require any specific conditions. Generally, samples are coated on the carbon strip and gold or platinum sputtering is used to make the nonconducting samples to conduct. Low- and ultravacuum mode is optional for nonconducting samples [26]. SEM can be used in two different modes such as imaging and diffraction. The imaging method helps to study the morphology and topography of the material and diffraction gives structural information. The sample SEM images of nanofibers are shown in Fig. 3.2. Fig. 3.2A shows the nanofibres of alumina prepared by the electrospun method and Fig. 3.2B represents an assemblage of nanofibres of langbeinite phosphosilicate. The higher magnified images are shown in Fig. 3.2C and D. The

Figure 3.1 Schematic representation of field emission scanning electron microscopy.

Characterization of flexible ceramics

27

Figure 3.2 Scanning electron microscopy images of (A) alumina and (B) (D) langbeinite phosphosilicate nanofibers.

size of nanofibers is found to be in the range of 90 100 nm. Several flexible ceramics have been characterized by FESEM in order to find the surface morphology. For instance, sandwiches like Al2O3/Al/Al2O3 flexible nanosheet coating on the substrate [27], microcracks in KZr2(PO4)3 and KAlSi2O6 composite [28], and the influence of ZrO2 on Al2TiO5 microstructure have been studied [29].

3.2.1.2 Transmission electron microscopy In TEM, a stream of monochromatic electron beams is transformed into a thin coherent by the condenser lenses [30]. The lenses determine spot size and change into a dispersed spot or pinpoint beam. The transmitted condenser aperture restricts the electron beam from knocking out the high-angle electrons. The focused beam by the objective lens strikes the specimen. Optional objective and selected area apertures enhance the contrast by hindering high-angle diffracted electrons and examining the

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periodic diffraction of electrons, respectively. The transmitted electron beam then strikes the phosphor screen to create an image. Fig. 3.3 represents the working process of TEM. The TEM image of strontium peroxide nanofiber prepared by the precipitation method is shown in Fig. 3.4. A cluster of nanofibers can be seen from the image. The surface morphology, uneven distribution of microbubbles on aluminacoated thin film [27], the grain size of SnO2 nanobelts [31], and microcracks, grains interlocking in aluminum titanate [32] have been identified by using TEM.

3.2.2 Scanning probe microscopy Scanning probe microscopy (SPM) is mainly used to determine the surface structure of bulk and nanomaterials. The major SPM techniques include scanning tunneling microscopy (STM) and atomic force microscopy.

3.2.2.1 Scanning tunneling microscopy STM is the first SPM method used for surface investigation. It works based on the probe technique to generate a highly magnified image. It is widely used to study the surface morphology and electronic structure of bulk to nanomaterials in various fields. The electronic structures of conducting materials such as graphite, SiC, SiO2, and other metal oxides have been studied by STM [33,34]. STM principle is based on the quantum tunneling effect. The metallic tip and sam˚ ) [35]. Applying voltage to ple surface are separated by an angstrom level (d , 10 A

Figure 3.3 Schematic representation of transmission electron microscopy.

Characterization of flexible ceramics

29

Figure 3.4 Transmission electron microscopy image of SrO2 nanofiber.

Figure 3.5 Working principle of scanning tunneling microscopy.

the tip and the sample, an electron tunnel is generated and scanning occurs by the movement of the tip from the x y plane perpendicular to the sample surface. By continuing the measurement of the tunneling current, a three-dimensional (3D) image can be obtained [36]. Fig. 3.5 shows the schematic representation of STM. STM

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Advanced Flexible Ceramics

works on two different modes namely, constant height and constant current mode for analyzing uniform surface and nonuniform surface distribution, respectively. The resolution depth of 0.1 2 0.01 nm can be achieved by this method.

3.2.2.2 Atomic force microscopy Atomic force microscopy (AFM) is an ideal scanning probe technique that provides surface information at the nano level with a higher resolution than SEM. It is used to measure surface ordering, adhesion strength, texture, morphology, magnetic force, friction, and physical properties such as size, shape, and sample distribution. Both nano and bulk materials can be analyzed by this method. It does not require any sample treatments. Unlike STM, the samples need not be conductive and the surface structure of ceramics, glass, polymers, composites, and bio-materials can be determined by using AFM [37]. AFM consists of a sharp triangle metal tip attached to the cantilever made up of silicon or silicon nitride. A diamond-sharp tip is also used for wear resistance study and for stable imaging. Cantilever is attached with a piezoelectric arm for the reflection of the laser beam. By bringing the tip near the surface of the sample, a repulsive force generates between the sample and tip. During the scanning process, the tip moves according to the sample surface interaction and measures the diffracted laser beam by a photodiode (Fig. 3.6). AFM works in two different modes namely, tapping and contact mode, however, the tapping method is commonly used [38]. 3D images can be generated through AFM with high accuracy and rapid scanning [39]. Fig. 3.7 represents the AFM image of LiB3O5. The particles are found to be in the micron

Figure 3.6 Working principle of atomic force microscopy.

Characterization of flexible ceramics

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Figure 3.7 Atomic force microscopy image of LiB3O5.

scale. Measurement of average grain size, particle distribution, surface roughness, and thickness have been performed through this technique [40,41].

3.2.3 X-ray diffraction X-ray diffraction (XRD) is one of the most important techniques to identify the phase, crystal structure, composition, and specific properties like stress, strain, density, roughness, and texture of solid materials. It is a nondestructive technique where a beam of X-rays generated from the X-ray tube is passed through the periodic arrangement of atoms, gets diffracted, and generates a particular pattern. X-rays are used to probe the crystal structures as the interatomic spacing of a crys˚ ). talline material and X-rays possess the same order of wavelength (B1 2 A Diffraction occurs when the periodic array of atoms scatters radiation coherently and generates constructive interferences. The diffraction pattern is a result of scattering from different planes of atoms, when the distance traveled by the parallel Xrays is an integer of the wavelength [42]. Fig. 3.8 represents the stack of reflecting lattice planes of crystalline material. The monochromatic X-rays fall on the plane and are reflected from successive planes. When the reflected rays path difference is equal to an integral multiple of wavelength (λ), constructive interference occurs and fulfills the Bragg’s condition; nλ 5 2d sinθ, where n is an integer, d is interplanar spacing and θ is the glancing angle. Fig. 3.9 shows a sample powder XRD pattern (nano-CuO). Phase formation and quantification, strain determination and crystalline size measurement of several flexible ceramics have been studied by the powder XRD technique [29,43]. Interpretation of powder XRD patterns yields several information about the material. The peak positions and relative intensities help to identify phases present in the material. The crystallographic information of inorganic materials can be obtained from the International Center for Diffraction Data, USA [44]. The database includes the patterns of inorganic, organic, minerals, alloys, and metals. The

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Figure 3.8 Bragg’s law.

Intensity (counts)

3000

2500

2000

1500

1000

500

20

30

40

50

60

70

2 theta, Cu K

Figure 3.9 Powder X-ray diffraction pattern of nano-CuO.

data provides information on d spacing and relative intensities and miller indices (hkl). By measuring the intensity of the diffraction lines and comparing them with standards, phase quantification of a crystalline mixture is also possible. Single crystal XRD is being used for the structural analysis of crystals.

3.2.4 Fourier transform infrared spectroscopy and Raman spectroscopy In Fourier transform infrared spectroscopy (FTIR) spectroscopy, the molecules get excited to the higher energy state by absorbing IR radiation and vibrating the

Characterization of flexible ceramics

33

molecule to produce characteristic vibrational bands. These stretching and bending frequencies are compared with the standard functional group bands for structural interpretations. The bonding natures of C H, C O, N H, N O, metal oxide, etc. can be identified by using this method. The typical ranges for OH, C H group (  C H, 5 C H, C H), C  C, C  N, and C 5 O are 3600/cm, 3000 3500/ cm, 2000 2500/cm, and 1500 1600/cm, respectively [45]. In flexible ceramics, metal oxide (Zn O, Sn O, etc.) bands generally appear in the lower frequency region of 400 500/cm [46]. Similarly, the stretching and bending vibrational modes of anionic groups such as PO4, SiO4, and MoO4 fall between 450 and 1220/ cm [47,48]. The working principle of FTIR is shown in the schematic diagram (Fig. 3.10). Typical flexible ceramic materials have been analyzed by FTIR spectroscopy to confirm the presence of hydrophilic and carbonyl groups [45]. When the beam of infrared radiation passed on the samples, the absorbance of IR light by the sample at different wavelengths is measured to find the molecular structure. Two different modes KBr disk and attenuated total reflection (ATR) are being used for the functional group analysis. In the KBr disk method, the powder samples are mixed with KBr and pressed into a thin transparent disk whereas the ATR method does not require KBr. For hard solid samples, ATR accessory is used. Fig. 3.11 depicts a typical example of the FTIR spectrum. Raman spectroscopy is another important technique to study the molecular structure from molecular vibrations. In particular, thermo-Raman spectroscopy where spectra are measured as a function of temperature is used to study the phase transitions and compositional changes of solid materials. Though FTIR and Raman provide structural information with chemical bonding, both vary in principle. FTIR and Raman are active when there is a change in dipole moment and change in

Figure 3.10 Working principle of Fourier transform infrared spectroscopy.

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Advanced Flexible Ceramics

M -O 100

% Transmittance

O -H s

C -H s

50

P -O/Si-O bending

0 4000

P -O/Si-O stretching 3500

3000

2500

2000

Wavenumber

1500

1000

500

(cm-1)

Figure 3.11 Fourier transform infrared spectroscopy spectra of apatite phosphate.

polarizability of a molecule respectively [49]. Unlike FTIR, Raman does not require any sample preparation (disk form).

3.2.5 Electron diffraction Electron diffraction (ED) is another diffraction technique used to find the crystallographic structural information more precisely by using fast-moving electron beam. A beam of light (electrons) passes through the substance leading to diffraction. The diffracted beams are captured by the fluorescent screen through filtration grids. ED can be used in TEM and SEM where an electron beam or gun is the primary source [50]. A thin layer of sample (electron transparent) has to be used for ED studies. Highspeed electrons are produced from an electron gun with the shortest distance with the known wavelength. A carbon target is placed in between the pathway to allow electrons and fall on the sample. The diffracted electron beam from the sample falls on the fluorescent screen as shown in Fig. 3.12. Fig. 3.13 shows the ED pattern of nano-CuO.

3.2.6 Energy dispersive X-ray analysis Energy dispersive X-ray (EDX) analysis is a microanalytical technique for the quantitative analysis of elements that uses a characteristic spectrum of X-rays. In principle, when an electron beam impinges on a sample it ejects the secondary electrons from an inner shell and the vacancy generated gets filled up by the higher shell electrons.

Characterization of flexible ceramics

35

Figure 3.12 Schematic representation of electron diffraction.

Figure 3.13 Electron diffraction pattern of nano-CuO.

The excess energy is released in the form of X-rays which are not only characteristic of the element but also depend on the shell from which electrons move into the holes [51]. The composition of a material can be identified from the EDX data. Fig. 3.14 shows the block diagram of EDX describing the working principle. EDX analysis of K2Zr2P2SiO12 reveals the presence of K, Zr, P, Si, and O as can be seen in Fig. 3.15. The peak at 0.525 and 3.32 KeV correspond to O and K, respectively. The narrow peak at approximately 2.1 KeV is attributed to Zr, P, and Si. EDX has been performed by several researchers for quantitative elemental analysis of flexible ceramics [28,52].

3.2.7 X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) is a primary spectroscopic technique to determine the elemental distribution, oxidation states, and bonding nature of all the

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Advanced Flexible Ceramics

Figure 3.14 Block diagram of energy dispersive X-ray analysis principle. K

Spectrum 1 Si P Zr

O

K

Zr 0 10 5 Full Scale 2629 cts Cursor: 0.000

15

20 keV

Figure 3.15 Energy dispersive X-ray analysis spectra of K2Zr2P2SiO12.

elements except hydrogen and helium. It also provides the empirical formula and electronic state of the elements. All types of solid samples (conductor insulator) can be analyzed by this technique. XPS works based on Einstein’s photoelectric effect, when light hits a particle it emits electrons (Fig. 3.16). The sample is irradiated by the beam of X-rays and it releases electrons as kinetic energy. The released energy passes through the hemispherical analyzer and is converted into signals by the detector. Ultrahigh vacuum mode (#1029 Torr) is to be maintained for the analysis. The difference between the photon energy (hν) and the kinetic energy of the excited electron (KE) can measure the binding energy of the electron. The following formula is used to find the binding energy of the electron [53]: EB 5 hν 2 KE 2 φspec Fig. 3.17 shows the XPS spectrum of rare earth-substituted phosphomolybdate ceramic. The binding energies at 903 and 884 eV are responsible for Ce31, 859 and

Characterization of flexible ceramics

37

4x10

5

3x10

5

2x10

5

1x10

5

133.25 106.25

5

284.75

5x10

233.25

5

398.75 360.75

6x10

851.25 834.75

5

975.25

7x10

530.75

Intensity

Figure 3.16 Schematic representation of X-ray photoelectron spectroscopy.

0 1000

800

600

400

200

Binding energy (eV) Figure 3.17 X-ray photoelectron spectroscopy spectrum of rare earth-substituted phosphomolybdate ceramic.

0

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Advanced Flexible Ceramics

842 eV for La31, 284 eV for C41, 236 and 233 eV for Mo61, and 140 eV for P51. XPS has been performed on several flexible ceramics in order to find the oxidation states and elemental compositions of materials [2,12,24,54].

3.2.8 Thermogravimetric and differential thermal analysis Thermal analytical techniques such as thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry are useful tools to detect the physical changes of a material which include mass loss, thermal decomposition, and phase changes as a function of temperature and time. In general, thermal analysis provide thermal stability and phase stability of solid materials like nanomaterials, nanocomposites, and nanofibers. Thermal analyzer mainly consists of sample and reference holders which are insulated from each other, furnace, thermobalance, and data acquisition system (Fig. 3.18). In TGA, when the sample is heated at a chosen rate dehydration, decomposition, evaporation, absorption, desorption, oxidation, and reduction processes can be determined. Similarly, from DTA, endothermic or exothermic changes due to crystallization, melting, vaporization, sublimation, and solid-state reactions are obtained [55]. The TGA and DTA curves of the polymer-ceramic composite are shown in Fig. 3.19. The TGA curve clearly shows that dehydration and depolymerization at 105 C and 390 C, respectively. The depolymerization is reflected as an exothermic peak in DTA.

Figure 3.18 Block diagram of thermogravimetric and differential thermal analysis principle.

Characterization of flexible ceramics

39 8

TGA

100

105.07°C 95.87%

269.01°C 93.38% 389.67°C 484.40°C 85.72%

6 80

445.73°C

Weight (%)

4 60

387.90°C

440.81°C 46.26%

2

40 259.73°C

464.42°C 30.19%

595.84°C

DTA

20

0 554.92°C 7.590%

899.84°C 7.024%

1088.76°C

1088.70°C 6.443%

0 0

Temperature Difference (°C)

423.76°C 80.13%

300

Exo Up

600 Temperature (°C)

900

Residue: 4.318% (0.1833mg)

-2 1200 Universal V4.7A TA Instruments

Figure 3.19 Thermogravimetric analysis and differential thermal analysis curves of polymer-ceramic composite.

3.3

Conclusion

The demand for flexible ceramics in the current decade is increasing due to their attractive properties. Physicochemical properties investigation of these nanostructured materials is important for the potential applications. In order to analyze the materials, characterization by various techniques is necessary. In this chapter, the most significant characterization methods of flexible ceramics such as FESEM, TEM, STM, AFM, XRD, FTIR, Raman, ED, EDX, XPS, and TG-DTA are discussed. The importance, instrumentation, process, advantages, and applications of each method are briefed with suitable examples. Application of these microscopic, spectroscopic, and diffraction characterization techniques on flexible ceramics, information on the physical properties of these ceramics can be obtained. Even though the other characterization methods ultraviolet-visible spectroscopy, small-angle X-ray scattering, electron probe microanalysis, and inductively coupled plasma spectroscopy have not been discussed in this chapter, the major techniques to characterize flexible ceramics are covered.

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Microstructural characteristics of flexible ceramics

4

Satyabati Das1, Kalpana Parida2, Nilotpala Bej 3 and Manila Mallik4 1 Department of Mechanical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh, India, 2Department of Physics, Siksha O Anusandan University, Bhubaneswar, Odisha, India, 3School of Mechanical Engineering, Kalinga Institute of Industrial Technology, Bhubaneswar, Odisha, India, 4Department of Metallurgical and Materials Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India

4.1

Introduction

Ceramics are very important types of materials for regular applications in our dayto-day life from ordinary household objects to advanced industrial applications. Generally, ceramics are categorized as mixed-phase materials with two or more components or in the form of hybrid mixed metallic oxides [1]. According to the various areas of applications, different ceramic materials are used accordingly in the desired way. Ceramics are fairly widespread for industrial applications, but the brittle behavior limits its proper deployments in future advanced devices. Flexible ceramics have therefore become a demanding field of research. The flexibility of a material refers to (1) the ability of materials to bend without plastic deformation, (2) to change shape permanently, and (3) to get elastically stretched. In this chapter, the focus is to discuss the microstructural aspect of flexible ceramics. Microstructural characterization is the name used for structural characteristics inside a material at the microscopic level. The properties of any solid material depend on the arrangement of atoms as well as the bonding between the constituent atoms. A large group of atomic arrangements combined together forms the microstructure. The microstructural parameters that regulate the material properties are grain size, phases, shape or morphology, and size distribution. Additionally, defects such as grain boundaries, interfaces at two or more phases, dislocations, etc. also play a vital role in the control of resultant properties. The number of microcracks plays a key role in the enhancement of flexibility and hence these parameters can be tailored by various methods.

4.2

Design strategies and microstructures

One of the traditional approaches to improving flexibility is to stimulate the deflection of cracks by particle or fiber reinforcement [2]. Research on itacolumite Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00004-X © 2023 Elsevier Ltd. All rights reserved.

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showed substantial flexibility due to the slight movement of quartz grains in the narrow space provided by the microcracks [3]. This special microstructure paved the way to design other flexible ceramics mimicking itacolumite microstructure such as in Al2O3/Mo [2], Aluminum titanate (Al2TiO3) [4], KZr2(PO4)3 KAlSi2O6, etc. [5]. Metal oxides with one-dimensional (1D) nanostructures exhibit high flexibility as compared to bulk form due to the high surface-to-volume ratio. Nevertheless, these nanostructures are finding difficulties in deployment. Subsequently, new ceramics with three-dimensional (3D) interconnected networks have been recently explored.

4.2.1 Itacolumite A well-known natural flexible sandstone called itacolumite was first discovered in the 18th century. It is a natural flexible rock that can expand spontaneously within a limit when cut into small strips [6,7]. The flexibility of this sandstone is endorsed by its actual microstructure of loosely bound quartz grains and 3D cracking. This cracking forms a jigsaw-like structure and occupies several percentages of the material volume [3,8]. The detailed study of microstructural properties of the flexible sandstone itacolumite can be observed from a scanning electron microscope as shown in Fig. 4.1A. The microstructure of this flexible natural sandstone/itacolumite showed that each quartz has an irregular shape and is divided by a thin space from neighboring grains. A very small protuberance of one grain is connected with the small depression of adjacent grains. Hence, the grains are well interlocked but

Figure 4.1 (A) Typical itacolumite showing bending or flexibility [11] (B) Micrographs of cross-sectional view cut along (a) x y plane and (b) y z plane. (C) Schematic of the bending structure of itacolumite [8].

Microstructural characteristics of flexible ceramics

47

the grain boundaries are not sealed [9]. The quantitative microstructure investigation of itacolumite confirms the loose grain model. In this model, the flexibility of the sample is shown to be due to the interlocking of irregular quartz grains bounded by regular intergranular spaces. This can also be visualized from the schematic as shown in Fig. 4.1B. The origin of the formation of such a unique structure of itacolumite has been investigated by many scientists all over the world. In some articles, it is reported that the existence of mica quartz in itacolumite leads to flexibility. Later on, there were reports about the negligent effect of the mica on the flexibility of this stone. Although the origin of the microstructure remained unresolved for many years, some of the investigations revealed ideas that eventually opened up the path for further exploration. Devries et al. [10] believed that the quartz-mica itacolumite is formed from the recrystallization and grain growth of quartz sand having clay. This quartz sand containing clay under adequate pressure may form mica along with enclosed grain boundaries. Later on, at lower pressures, some of the grain boundary mica or clay might get removed. Thus itacolumite structure, which constitutes of quartz grains with large intergranular decohesions, is formed. The flexibility of itacolumite is however very low because quartz grain has a very low value of tensile strength. So, in order to have a higher flexible material, ceramics with higher tensile strength were considered later to form the jigsaw puzzle-like microstructure.

4.2.2 Aluminum titanate ceramics Aluminum titanate ceramics (Al2TiO5) have many interesting properties such as high thermal shock resistance, low thermal expansion, low thermal conductivity, and good chemical resistance for which it finds widespread technological applications such as in foundry (crucibles, nozzles, etc.), automobile industries (motor converters), glass industries (molds), etc. It is synthesized by stoichiometric mixing of alumina and titania and then heat-treating at 1350 C in atmospheric pressure. A large no of microcracks is observed in the microstructure of aluminum titanate. The size of these microcracks plays a vital role in controlling the main properties of the material. The granulated Al2TiO5 industrial powder was sintered at 1350 C and then cooled [11]. During cooling, the anisotropy in the thermal expansion coefficient produced several microcracks along the grain boundary. This microstructure was similar to that of itacolumite that leads to the flexibility in AT ceramics. The flexibility was further enhanced by increasing the temperature for sintering and also increasing the time of holding. The microstructure for different sintering temperatures is shown in Fig. 4.2A. It has been reported that the optimum temperature for sintering and holding time is 1550 C and 12 h. The notations used for the three specimens of AT are (1) nonflexible (NF), (2) flexible (F), and (3) very flexible (VF). An extensive linear elastic region is shown by the NF specimen which is typical to that of ceramic materials. On the other hand, a nonlinear plastic region is shown by F and VF specimens. This behavior is due to the network of grain boundary microcracks and the interlocking of elongated grains. Moreover, a very large distortion like plasticity with flexure stress was observed in the F and VF sample which is

48

Advanced Flexible Ceramics

due to the weak bonding of the grain boundaries. The VF sample showed more flexibility than the F sample due to its wider microcracks. Chen et al. [4] illustrated in their research work that the additives such as Fe2O3 and MgO in Al2TiO5 help in the development of elongated grains that get interlock with each other thereby restricting the propagation of cracks. Additionally, the gaps provided by the microcracks aid in the movement of grains leading to specimen flexibility. This mechanism has been clearly described through a schematic diagram along with supporting SEM micrographs as shown in Fig. 4.2B D.

4.2.3 Al2O3/Mo In this system, Al2O3 is the ceramic matrix and Mo is the metallic inclusion of nanometer size and an attempt has been made to mimic the microstructure of the itacolumite. The inclusion has been chosen as molybdenum due to its refractory property as a metal and for its lower coefficient of thermal expansion (CTE) than Al2O3. A conventional powder metallurgy route has been adopted to synthesize the nanocomposite [2]. The microstructure revealed elongated Mo layers surrounding a part of Al2O3 grains. In between the interfaces of Al2O3 grains and elongated Mo, branching of cracks occurred for a smaller volume fraction of Mo (,5%). This is

Figure 4.2 (A) Micrographs of flexible AT ceramics sintered at (a) 1350 C, (b) 1400 , (c) 1450 C, (d) 1500 C, and (e) 1550 C [11]. (B) Micrographs of 56 wt.% Al2O3 44 wt.% TiO2 (AT) with additives (a) 0 wt.% (AT), (b) 5 wt.% SiO2 (ATS), (c) 4 wt.% MgO (ATM), and (d) 4 wt.% Fe2O3 (ATF) [4]. (C) Micrographs of 56 wt.% Al2O3 44 wt.% TiO2 (AT) with mixture of additives (a) 4 wt.% MgO and 4 wt.% Fe2O3 (ATMF), (b) 5 wt.% SiO2 and 4 wt.% Fe2O3 (ATSF), (c) 5 wt.% SiO2 and 4 wt.% MgO (ATSM), and (d) 5 wt.% SiO2, 4 wt.% Fe2O3 and 4 wt.% MgO (ATSFM) [4]. (D) A schematic representation of flexibility at the microscopic level of AT ceramics with additives (a) AT, (b) ATS, (c) ATMF/ATSFM, and (d) ATSM/ATSF/ATSFM [4].

Microstructural characteristics of flexible ceramics

49

Figure 4.2 (Continued).

attributed to an apparent higher value of toughness in the 5 vol.% Mo/Al2O3 composite than Al2O3. Further, the toughness value of 20 vol.% Mo Composite enhanced to 7.6 MPa/m-1/2, which is 1.8 times higher than the value of Al2O3. This was evident from the microstructure that revealed crack penetration through the elongated Mo particle and bridging of cracks by the metallic phase as shown in Fig. 4.3.

4.2.4 Al2O3 TiO2 MgO Alumina titania magnesia (ATM) system has been used as high damping materials or ceramics. The itacolumite-like microstructure has been replicated in this ceramic system [12]. This is synthesized by the sintering process during which the discontinuity in grain growth and the anisotropic thermal expansion in the cooling process occur. This leads to the formation of microcracks that were similar to the microstructure of itacolumite.

50

Figure 4.2 (Continued).

Advanced Flexible Ceramics

Microstructural characteristics of flexible ceramics

51

Figure 4.3 Micrographs displaying crack propagation in Al2O3/15 vol.% Mo composites compressed at temperatures (A) 1400 C, (B) 1500 C, (C) 1600 C, and (D) 1700 C [2].

4.2.5 KZr2(PO4)3 KAlSi2O6 KZr2(PO4)3 KAlSi2O6 (KZP KAS) is a flexible composite synthesized by sintering of granulated KZP with powdered KAS and can have applications such as earthquake-proof material due to its stress relaxation ability. Itacolumite-like microstructure with extensive cracks has been mimicked in this flexible ceramic KZP KAS system [5]. The mechanism has been explained evidently with the help of schematic figures and microstructure. The CTE in KZP is low while that of KZS is high. This difference in the coefficients of thermal expansion in KZP and KAS leads to the formation of cracks with itacolumite-like microstructure. The flexibility of the system is however lower as compared to itacolumite. Nevertheless, this strategy along with optimization of processing methods and choosing of initial materials could probably increase the cracking thereby improving the flexibility.

4.2.6 Al2O3/Al/Al2O3 hybrid composite Hybrid composites made up of alumina (Al2O3) and aluminum (Al) has been synthesized by many methods such as welding method, chemical vapour deposition

52

Advanced Flexible Ceramics

(CVD), physical vapour deposition (PVD), etc. Wang et al. [13] fabricated Al2O3/ Al/Al2O3 composite by plasma electrolytic oxidation (PEO) method and studied its flexibility behavior. The composite exhibits very good tensile behavior and flexibility. The strain of the film was determined using the following equation between the film strain (ε), thickness (t), and diameter of curvature (d) [14] ε5

t d

The calculated strain of Al2O3 hybrid composite film was 0.33% for film thickness of 4.5 μm and bending diameter of 1.5 mm. This is greater than the strains exhibited by Alumina ceramics that is reported to be within 0.1% 0.5%. Additionally, this is higher than the previously reported flexible ceramics such as itacolumite, ATM, and KZP KAS ceramics whose maximum strain has been reported to be 0.25% [15]. Microstructural investigations have revealed a hierarchical structure and are believed to provide exceptional tensile behavior as well as flexibility. Fig. 4.4 shows the microstructure of the composite. The cross-sectional view reveals the sandwich structure of aluminum within alumina ceramic. The shape of the interface was observed to be very wavy and uneven. However, there is no typical discontinuity at the interface and hence a good interface bonding is formed. The average grain size of alumina was found to be 17 nm and two phases of α-Al2O3 and γ-Al2O3 were present. The volume fraction of grain boundaries was very high than the conventional Al2O3 composites with coarse grains. All of these contributed to greater resistance to dislocation motion. Therefore the stress required for plastic deformation could be higher. It has been reported by some researchers that the cracks propagate in intergranular mode. Inside the Al2O3 nanofilm, the nanocracks might undergo many deflections and branching that in turn inhibit the crack propagation. Additionally, a large number of nanobubbles were observed mainly around the cracks. These were supposed to be induced by deformation and impede the path of crack propagation. These nanocrystalline structures, shear band formation, and nanobubbles attribute to prolong crack path, rotation of grain, and deformation which thereby lead to high flexibility.

Figure 4.4 (A) TEM image (shear bands), (B) SEM image, and (C) fracture surface of Al2O3/Al/Al2O3 composite [13].

Microstructural characteristics of flexible ceramics

53

4.2.7 One-dimensional flexible ceramics 1D nanoparticles have at least one dimension larger than nanoscales and the other dimensions within the nanorange. Nanofibers, nanorods, and nanotubes are examples of this type of nanoparticle. The intrinsic anisotropy and high surface-to-volume ratios lead to enhanced properties as compared to the bulk ones. This has led to significant growth in the interest of the scientific community to look for its potential applications in the multifunctional field. They are considered as the building blocks for LEDs, field-effect-transistors (FETs), UV films, smart cards, gas sensors, etc. Flexible 1D nanostructures unveil additional mechanical bending features along with a significant change in the properties as compared to the bulk counterpart. These flexible 1D nanostructures have therefore appealed to remarkable attention in the last decade for advanced functional applications. 1D nanostructures display great prospective for nanodevice applications. These are extensively used in gas sensors, solar cells, UV protection films, room-temperature lasers, ultraviolet, smart cards, flexible transparent- FETs and LEDs. Specifically, Cd-doped ZnO nanowires are utilized in sensing nanodevices, LEDs, and thin film transistors [15]. Also, various nano-ZnO built H 2 sensors such as SnO 2 -coated ZnO nanorods, pure and Pt-coated multiple ZnO nanorods, single nanorod/nanowire ZnO and ZnO nanorod arrays, and networks. The microstructural studies of ZnO nanowires on the flexible substrate were done by scanning electron microscope. Fig. 4.5A and B shows the morphology of ZnO nanowires on the ITO/PET substrate. From the figure, it has been observed that the ZnO nanowires were homogenously spread over the entire section of the substrate. These nanowires

Figure 4.5 Micrograph of ZnO nanowires: (A) low magnification and (B) high magnification (inset) [16,17].

54

Advanced Flexible Ceramics

were of a mean length of 2 mm and a mean radius of 80 100 nm. Hence, the aspect ratio was about 10 12. The ZnO nanowires deposited on the flexible substrate were bent without any noticeable damage to the nanowires [17].

4.2.8 Three-dimensional flexible ceramics The 3D flexible ceramics synthesized from inorganic metal oxides with nano and microstructures are candid materials for forthcoming nanotechnologies. A thorough investigation of the microstructure of 3D materials can be highly beneficial for synthesizing newer nano- and micro-3D flexible materials for multifunctional applications such as in sensors, photonic energy conversion, solar cells, etc. Recently, some interesting studies have been reported on the evolution of morphology in 3D porous ceramics with an interconnected network. Attempts have been made by researchers to demonstrate the phenomenon in porous ZnO and metal-alloyed ZnO tetrapods (ZnO-T). A detailed discussion on ZnO and metal alloyed is done in the following section.

4.2.9 ZnO tetrapods The morphology of tetrapods has four spatial 1D arms interconnected with a central part at an angle of 110 C. This unique nanostructure when assembled inhibits close packing thereby leading to high porosities. Among the various metal oxides, ZnO due to its hexagonal-wurtzite crystal structure and growth direction in the (0001) facet forms tetrapods. Flame transport synthesis is a widely adaptable technique used for the synthesis of nano- and micro-ZnO tetrapods. The interpenetration of ZnO-T arms fully, partially, or slightly leads to an interconnected network structure. This type of interconnected network of porous ZnO-T micro- or nanostructure gives rise to flexibility in 3D ceramics [18]. The mechanism for such behavior has been ascribed to the phenomenon that the load is first experienced by a small amount of tetrapods at the surface and subsequently more and more surface tetrapods get in contact leading to tight packing (Fig. 4.6).

4.2.10 Metal-alloyed ZnO tetrapod Metal-alloyed ZnO-T networks show high flexibility. In the case of Cu alloyed ZnO-T networks, a distinct change in the microstructure is revealed that is correlated with a higher Young’s modulus and elastic limit as compared to ZnO-T. With the addition of Cu microparticles, the shape of the base structure of ZnO changes from hexagonal to roundish. Additionally, spherical structures of 100 μm size are observed along with the tetrapod structures. Sn- and Alalloyed ZnO-T network have very less difference in the microstructure and the mechanical properties get reduced. The SEM images show the distribution of

Microstructural characteristics of flexible ceramics

55

Figure 4.6 Micrographs of ZnO-T tetrapods showing 3D network of interconnected tetrapod having densities (A) and (B) 0.2 g/cm3, (C) and (D) 0.4 g/cm3, (E) and (F) 0.6 g/cm3, and (G) (H) 0.8 g/cm3 at lower and higher magnification [18].

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Advanced Flexible Ceramics

microparticles of size ranging from 5 to100 μm randomly along the network and agglomerates with indistinguishable morphology. Also, no interconnection is observed between microparticles and the ZnO-T. In Al- alloyed ZnO-T networks, spherical morphology with sizes ranging between 1 and 20 μm are dispersed along the network that is firmly connected to the ZnO structures (Fig. 4.7).The summary of the design strategies and corresponding microstructures are presented in Table 4.1.

Figure 4.7 Micrographs of Al-alloyed ZnO tetrapods showing 3D network of interconnected tetrapod having densities (A) and (B) 0.2 g/cm3, (C) and (D) 0.3 g/cm3, (E) and (F) 0.4 g/cm3 at lower and higher magnification [17].

Table 4.1 Summary of design strategies and corresponding microstructure. Sl. no

Material

Microstructure

Mechanism

Design strategies

References

1.

Itacolumite

Movement of grains in the narrow space provided by the microcracks

Naturally formed

[6]

2.

Al2O3/Mo nanocomposites Aluminum titanate ceramics (Al2TiO5) -AT

3D cracking and fine-grained quartz Itacolumite-like structure Itacolumite-like microstructure.

Deflection of cracks

Particle or fiber reinforcement Solid phase sintering of the granulated industrial AT powder Adding Fe2O3, MgO, etc. to AT

[2]

Sintering of ceramics with different CTE Sintering of ceramics with different CTE

[12]

Plasma electrolytic oxidation (PEO) method

[13]

3.

4.

Aluminum titanate ceramics 1 Fe2O3, MgO, etc.

AT microstructures with elongated grains and wider microcracks

5.

Al2O3 TiO2 MgO (ATM)

Itacolumite-like microstructure

6.

KZr2(PO4)3 KAlSi2O6 (KZP KAS)

7.

Al2O3/Al/Al2O3 hybrid composite

Itacolumite-like microstructure with extensive cracks Hierarchy structure

Grain boundary microcracks network and interlocked grains

Gaps provided by microcracks aid in grain movement. Grains get elongated and get interlocked thereby restricting the propagation of cracks Microcracks provide the gap for the movement of grains Nanocrystalline structure, shear band formation, and nanobubbles attribute to prolong crack path, rotation of grain, and deformation leading to flexibility Nanostructure, formation of the shear band, and nanobubbles prolong the path of crack, rotation of grain and deformation

[11]

[4]

[15]

(Continued)

Table 4.1 (Continued) Sl. no

Material

Microstructure

Mechanism

Design strategies

References

8.

1D nanostructures

High surface-to-volume ratios

Electrodeposition

[17]

9.

ZnO nanowires deposited on flexible substrate ZnO-T

Load is first experienced by a small amount of tetrapods at the surface and subsequently transmitted to others.

Flame transport synthesis (FTS) process

[18]

10.

Metal-alloyed ZnO-T

3D nanostructures Tetrapods structure with porous 3D interconnected networks 3D nanostructures Base structure of ZnO-T changes

Tuning of ZnO network

FTS process

[18]

Microstructural characteristics of flexible ceramics

4.3

59

Conclusive remark

Researchers all over the world have made a tremendous effort to overcome the brittle behavior of ceramics that limits their application. The change in the internal structure is the main cause of the change in the properties of any materials. This chapter evidently illustrates the microstructural correlations with ceramics that lead to flexibility in ceramics. Flexible ceramic can be fabricated by manipulation of the microstructure to get the desired property. The key microstructural parameters that regulate the flexibility property of ceramics are discussed in the following section. G

G

G

G

G

G

Microcracks play a vital role in the improvement of flexibility by providing space to the grains for movement. Therefore one of the key strategies to improve flexibility is through the formation of microcracks. Additionally, the number of microcracks plays a key role in flexibility. Hence, this is a helpful way to control and manipulate the flexibility in ceramics. The grain shape affects the direction of propagation of cracks such as elongated grains tend to form an interlocked structure that hiders the propagation of cracks. Therefore it is a strategic parameter for the formation of flexible ceramics. The nanostructured particle shape of 1D and 3D is a significant basis for the fabrication of new generation flexible ceramics. The porosity is also one of the important considerations in microstructural characteristics that greatly influence the flexibility of ceramics. Additionally, defects such as grain boundaries, interfaces at two or more phases, dislocations, etc. also play a vital role in the control of flexibility of ceramics.

Acknowledgment Dr. Satyabati Das acknowledges “Department of Science and Technology”, India for their financial support under the Women Scientist Scheme WOS-A project Grant-No-SR/WOS-A/ ET-125/2018.

References [1] W.D. Kingery, Ceramic materials science in society, Annu. Rev. Mater. Sci. 19 (1989) 1 21. [2] M. Nawa, T. Sekino, K. Niihara, Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites, J. Mater. Sci. 29 (1994) 3185 3192. [3] H.C. Kerbey, Itacolumite, flexible sandstone and flexible quartzite a review, Proc. Geologists’ Assoc. 122 (2011) 16 24. [4] W. Chen, A. Shui, Q. Shan, J. Lian, C. Wang, J. Li, The influence of different additives on microstructure and mechanical properties of aluminum titanate ceramics, Ceram. Int. 47 (2021) 1169 1176. [5] I. Sato, Y. Ichikawa, J. Sakanoue, M. Mizutani, N. Adachi, T. Ota, Flexible ceramics in the system KZr2(PO4)S KAlSi2O6 prepared by mimicking the microstructure of itacolumite, J. Am. Ceram. Soc. 91 (2008) 607 610.

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Advanced Flexible Ceramics

[6] M.B. Dusseault, Itacolumites: the flexible sandstones, Q. J. Eng. Geol. Hydrogeol. 13 (1980) 119 128. [7] H. Suzuki, D. Shimizu, Petrography of Indian, Brazilian and Appalachian itacolumites, J. Geol. Soc. Jpn. 99 (1993) 391 401. [8] A. Doncieux, D. Stagnol, M. Huger, T. Chotard, C. Gault, T. Ota, et al., Thermo-elastic behaviour of a natural quartzite: itacolumite, J. Mater. Sci. 43 (2008) 4167 4174. [9] H. Suzuki, T. Yokoyama, M. Nishihara, et al., Scanning electron microscope and acoustic emission studies of itacolumites, J. Geol. Soc. Jpn. 99 (1993) 443 456. [10] R.C. Devries, Structure-property relation in flexible sandstone, J. Am. Ceram. Soc. 51 (1968) 387 390. [11] W. Chen, A. Shui, C. Wang, J. Li, J. Ma, W Tian, X. Ota, X. Xi, Preparation of aluminum titanate flexible ceramic by solid-phase sintering and its mechanical behavior, J. Alloy. Compd. 777 (2019) 119 126. [12] T. Shimazu, M. Miura, N. Isu, T. Ogawa, A. Ichikawa, E.H. Ishida, I.C.M.T.A. General Research Institute of Technology. (2006). Itacolumite like high. damping ceramics in the system. Al2O3-TiO2-MgO, 833. [13] Z. Wang, H. Hu, X. Nie, Preparation and characterization of highly flexible Al2O3Al/ Al2O3 hybrid composite, J. Nanomaterials 2015 (2015) 412071. [14] J. Rodgers, J. Edel, J. Rivera, O. Englander, Bending of nanowire flexible substrate assemblies integrated via direct synthesis methods, Phys. status solidi (a) 208 (2011) 2443 2449. [15] I. Sato, Y. Ichikawa, J. Sakanoue, M. Mizutani, N. Adachi, T. Ota, I. Sato, Y. Ichikawa, J. Sakanoue, M. Mizutani, N. Adachi, T. Ota, Flexible Ceramics in the System KZr2(PO4)S KAlSi2O6 Prepared by Mimicking the Microstructure of Itacolumite, J. Am. Ceram. Soc. 91 (2008) 607 610. [16] O. Lupan, L. Chow, T. Pauporte´, L. Ono K., B. Cuenya R., G. Chai, Highly sensitive and selective hydrogen single-nanowire nanosensor, Sens. Actuators B 173 (2012) 772 780. [17] O. Lupan, T. Pauporte´, et al., Hydrothermal treatment for the marked structural and optical quality improvement of ZnO nanowire arrays deposited on lightweight flexible substrates, J. Cryst. Growth 312 (2010) 2454 2458. [18] J. Gro¨ttrup, I. Paulowicz, A. Schuchardt, V. Kaidas, S. Kaps, O. Lupan, R. Adelung, Y. K. Mishra, Three-dimensional flexible ceramics based on interconnected network of highly porous pure and metal alloyed ZnO tetrapods, Ceram. Int. 42 (2016) 8664 8676.

Mechanical properties of flexible ceramics

5

Desigan Ravi1, Ramya Ravichandran1, Payel Bandyopadhyay1 and Anoop K. Mukhopadhyay2,3 1 Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India, 2Department of Physics, Sharda School of Basic Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh, India, 3 Advanced Mechanical and Materials Characterization Division, CSIR—Central Glass and Ceramic Research Institute, Kolkata, West Bengal, India

5.1

Introduction

Ceramics materials are used for centuries in different forms in civilization. Presently the global market size of ceramic materials is 267.7 billion. The ceramics materials are hard and heat resistant up to a very high temperature which is the reason for the widespread application of ceramics materials. Nowadays ceramics materials are even used in electronics due to their high heat resistance. The fundamental brittleness of ceramic materials limits their applications of the ceramic materials. Now there is a huge need for electronic devices to be small as well as flexible. The flexible electronics components demand ceramic materials to be flexible. The present need for flexible and miniature electronic devices created a boost in research in the area of flexible ceramics.

5.2

Mechanical properties of conventional ceramics

Conventional ceramics are highly brittle showing linear elastic behavior. Ceramic materials have a large fraction of covalent or ionic bonds. At normal temperature, dislocation movement is practically impossible for ceramics due to ionic or covalent bonding and dislocation-induced plasticity is also not present in ceramic materials [1]. The fracture behavior of ceramics depends on many factors like the microstructure, chemistry of the surface, toughness, and R-curve behavior. At low and ambient temperatures, the fracture of ceramics is always brittle. The fracture generally originates from a critical flaw in brittle ceramics. This critical flaw propagates perpendicular to the normal stress as a crack [1]. The origin of the fracture has three different zones, mirror, mist, and hackle. The size of these fracture zones is proportional to the normal stress at the fracture origin. The brittle fractures of ceramics are occurring without any significant plastic deformation [1]. The crack expand steadily with the application of load unless it Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00005-1 © 2023 Elsevier Ltd. All rights reserved.

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Advanced Flexible Ceramics

reaches a certain velocity and sudden failure occurs. There are a few mechanisms identified by the researchers which are responsible for the failure of brittle materials [2]. Those mechanisms can be listed as follows [1,2]: G

G

G

G

Sudden, catastrophic failure subcritical crack growth fatigue creep

The common fracture mechanism is the sudden, catastrophic failure in brittle ceramics [3 6], and can be identified as the final active mechanism as well before failure. If the applied load is more than the critical load, the crack growth rate will be very high, immediate, and uncontrollable. This critical flaw generates mostly during preparation due to thermal expansion mismatch or during the machining of ceramics. Stress corrosion cracking in metals [7] is similar to subcritical crack growth [2,7 9]. The breaking of bonds at the crack tip is the main reason for the subcritical crack growth [2]. The corrosive action of some polar molecules (e.g., by water or water vapor) enhances the possibility of subcritical crack growth. The quick loading may avoid the subcritical crack growth but generally, before all catastrophic failures, there is some extent of subcritical crack growth. The delayed fracture is one of the most common effects of subcritical crack growth in brittle ceramics. The fatigue is caused by repeated cyclic loading. Before the publication of the findings of a group of researchers [10], it was thought that there was no evidence of cyclic fatigue for brittle ceramics. The crack growth due to fatigue can occur because of the breaking of the crack bridge during the part of crack closure [11,12]. Fatigue is the reason for delayed fracture also and sometimes can initiate catastrophic failure as well like the subcritical crack growth. Creep deformation in brittle ceramic is not as prominent as in metal or polymer creep as the activation energy for plastic deformation is much higher in ceramics [13]. At sufficiently higher temperatures, comparable to half of the melting temperature, the activation energy can be achieved and creep deformation is feasible [13,14]. In general, creep damage is caused by the creation, expansion, and coalescence of pores. Creep damage is not a localized damage behavior like crack growth and may occur throughout a component [15]. Depending on the temperature, the brittle (ambient temperature) or ductile (higher temperature) deformation occurs in ceramics. A group of researchers compared the brittleness parameters for a wide range of materials with respect to the critical load to check the brittle and quasiplastic deformation regions. The materials are listed in Table 5.1. Fig. 5.1 shows the variation of brittleness parameters with critical load [16]. They have identified the brittle and quasiplastic regions depending on the brittleness parameters they have found from experimental data. This table is a good reference for the typical Young’s modulus and hardness data for ceramics, metals, and polymer materials. From Table 5.1 it is clear that the glass or glass ceramic materials have moderately high Young’s modulus but comparatively lower hardness. Conventional ceramics materials are having very high Young’s modulus and high hardness. Metal and alloys

Mechanical properties of flexible ceramics

65

Table 5.1 The list of a wide variety of materials and their mechanical properties [16]. Samples

Young’s modulus (GPa)

Soda-lime silica glass Porcelain Glass-ceramic (De´cor) Glass-ceramic (Macor) F-MGC M-MGC C-MGC Al2O3 ZrO2 Si3N4 Diamond Silicon Mild steel Polycarbonate

Hardness (GPa)

70

5.2

0.67

68 69

6.2 3.8

0.92 1.2

63

2.5

1.9

71 67 50 271 390 205 315 335 1000 170 210 2.3

3.8 3.2 2.5 12.3 17.5 12 15.8 21 80 14 1.1 0.14

10 -1

10 -2

10 0

1.0 1.3 1.7 2.7 3.1 5.4 3.9 6 4 0.7 50 2

10 1

10 8

Brittleness parameter, (H/E')(H/T) 2

Toughness (MPa
m1/2)

10 2

Diamond Silicon

10 7

Quasiplastic Glass Porcelain F-Si3N4

Al2O3(AD999)

10 6

F-MGC Al2O3(Inf) Dicor M-Si3N4 M-MGC C-Si3N4 Y-TZP WC

Brittle

10 5 PY = PC

C-MGC Macor

10 4

10 -2

10 -1

10 0

r' = 3.18 mm 10 1

10 2

Critical load ratio, PY / PC

Figure 5.1 The variation of brittleness parameters with the critical load. Source: Added with permission from Y-W. Rhee, H-W. Kim, Y. Deng, B.R. Lawn, Brittle fracture versus quasi plasticity in ceramics: a simple predictive index, J. Am. Ceram. Soc. 84 (2001) 561 565.

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Advanced Flexible Ceramics

are having very high Young’s modulus but low hardness. Polymer materials are having low Young’s modulus and hardness compared to both ceramics and metals. Fig. 5.1 shows the variation of brittleness parameters with a critical load. The materials placed right-hand side of the line PY 5 PC are considered brittle and the materials on the left side are considered quasiplastic. The indentation loads form cone cracks beneath the indentation in brittle solids which expands with the load and lead to fracture [17 20]. The cone crack formation is a very common fracture mechanism for brittle fracture [17 20]. Researchers used several techniques to make ceramics “toughened.” Researchers are trying for a long time to achieve less brittle ceramics [17]. A group of researchers has developed a silicon carbide ceramic with elongated grains. They have compared the indentation stress strain relation of this ceramic with conventional monophase silicon carbide. They have found that the indentation stress strain relationship becomes nonlinear for heterostructure material which they have mentioned as more of a “ductile” behavior. The interesting observation is the change in the deformation mechanism. A “distributed microfracture damage” is noticed by the researchers instead of cone crack which is the typical fracture mechanism for a brittle material.

5.3

Mechanical properties of flexible ceramics materials

Conventional ceramic materials are hard and brittle. The stress strain plot is linear up to the elastic limit after which the material will fail. For flexible ceramics in general the stress strain plots are nonlinear and the deflection at the highest stress is comparably high. The mechanical properties of some flexible ceramics from a few literatures are listed in Table 5.2. A flexible sandstone, called itacolumite quartzite; first discovered in Brazil in the 18th century flexes freely within a limited arc, Fig. 5.2 [21]. Itacolumite is flexible due to its particular microstructure with interlocking grains and microcracks all over the structure [21]. Researchers tried to mimic the structure of itacolumite [21,22] to achieve flexibility for ceramics materials. Potassium zirconium phosphate and potassium aluminum silicate are mixed in different proportions to prepare KZP/KAS composites. The KZP/KAS composites showed ductile fracture behavior instead of a brittle fracture, Fig. 5.3. The maximum strength is close to 1 MPa and a deflection is 0.1 0.2 mm (Fig. 5.3). Microcracks of a few micrometer widths are formed all over the structure of the KZP/KAS composites due to sintering of two raw materials with different thermal expansions. The resultant structure is similar to itacolumite but the flexibility is comparatively low. Another group of researchers developed Al2O3 TiO2 MgO ceramics with microcracks in the structure to mimic the microstructure of the itacolumite [22]. The bending strain is increasing with the addition of MgO up to 5% then decreases [22]. Aluminum titanate-based flexible ceramics are also reported by many researchers [23,24]. The microstructures of different aluminum titanate ceramics in

Table 5.2 Mechanical properties of flexible ceramics. Sr. no.

Material

Mechanical properties

Mechanism for flexibility

References

1.

Itacolumite, potassium zirconium phosphate and potassium aluminum silicate composite mixed in 2:8 and 4:6 ratio

Grain boundary microcracking

[21]

2

Al2O3 TiO2 MgO ceramics with five different weight ratios of Al2O3, TiO2, and MgO Three different samples of aluminum titanate sintered at three different temperatures with three different cooling and dwell time Different aluminum titanate ceramics with different weight percent of SiO2, Fe2O3, MgO additive Three different graphene platelet-reinforced alumina ceramic composites with different vol.% of graphene platelet

Nonlinear stress-strain behavior, Elastic modulus of itacolumite: 75 MPa, Elastic modulus of KZP granule/KAS powder: 890 and 450 MPa for 2:8 and 4:6 ratios respectively Bending strength ( . 1.5%) and internal friction maximum for 5% addition of MgO

Grain boundary microcracking

[22]

Young’s modulus 11.7, 7.1, 3.6 GPa respectively for three different materials Strength 18, 16, and 13 MPa respectively for three different materials Deflection maximum (0.55 mm) for addition of Fe2O3, MgO with 4 wt.% each

Grain boundary microcracking

[23]

Gran boundary microcracking

[24]

Fracture toughness is maximum (4.49 MPa
m1/ 2 ) for 0.38 vol.% of Graphene platelet

Crack deflection due to addition of graphene platelet Addition of polymer

[25]

Addition of Metal

[27]

3

4

5

6

feldspar ceramic, cross-linking polymer resin mixture and initiator

7

Cu, Sn and Al-alloyed ZnO-tetrapod

Flexural strength is 131 160 MPa hardness is 1.05 and 2.10 GPa and elastic modulus is 16.4 and 28.1 GPa, strain at failure (0.5% 1%) Young’s modulus of Sn-alloyed ZnO-T: 0.21 MPa and 0.90 MPa Young’s modulus of Al-alloyed ZnO-T: 0.15 MPa and B0.81

[26]

68

Advanced Flexible Ceramics

Figure 5.2 (A) The flexibility of itacolumite and (B) stress strain curve of itacolumite. Source: Added with permission from I. Sato, Y. Ichikawa, J. Sakanoue, M. Mizutani, N. Adachi, T. Ota, Flexible ceramics in the system KZr2(PO4)3 KAlSi2O6 prepared by mimicking the microstructure of itacolumite, J. Am. Ceram. Soc. 91 (2008) 607 610.

Figure 5.3 Stress versus strain curve of KZP/KAS. Source: Published with permission from I. Sato, Y. Ichikawa, J. Sakanoue, M. Mizutani, N. Adachi, T. Ota, Flexible ceramics in the system KZr2(PO4)3 KAlSi2O6 prepared by mimicking the microstructure of itacolumite, J. Am. Ceram. Soc. 91 (2008) 607 610.

terms of grain size, microcracking width, and grain boundary aspect are controlled by different sintering temperatures, dwell time, and cooling rates [23]. Researchers have tried to create grain boundary microcracks which is the mechanism for achieving flexibility. The sintering temperature and cooling rate after sintering have an important effect on microstructure and flexibility. It is evident from Table 5.3 and Fig. 5.4 that the sintering temperature of 1600 C followed by a dwell time of 16 h and cooling time of 12 h results in the most flexible structure with a microcrack width of approximately 500 nm [23]. The lowest magnitudes of Young’s modulus are obtained for the most flexible structure.

Mechanical properties of flexible ceramics

69

Table 5.3 The preparation and mechanical properties of aluminum titanate flexible ceramics sintered at different temperatures [23]. Different sintering conditions and properties 

Sintering temperatures ( C) Dwell time (h) Cooling time (h) Grain Size (µm) Strength (MPa) Deflection at rupture (mm) Young’s modulus (GPa) Microcracks width (nm)

Brittle

Flexible

Very flexible

1500 1 4 , 20 18 0.27 11.7 , 100

1600 8 4 , 150 16 0.7 7.8 , 150

1600 16 12 , 200 13 1 3.6 , 500

20 Non Flexible (NF)

Stress (MPa)

Flexible (F)

15

Very Flexible (VF)

E = 11.1 GPa E = 7.1 GPa E = 3.6 GPa

10

5

0 0

0.4

0.8

1.2

1.6

Deflection (mm) Figure 5.4 The stress strain relationship of aluminum titanate-based flexible ceramics with different sintering temperatures. Source: Published with permission from C. Babelot, A. Guignard, M. Huger, C. Gault, T. Chotard and T. Ota, Preparation and thermomechanical characterisation of aluminum titanate flexible ceramics, J. Mater. Sci. 46 (2011) 1211 1219.

Another group of researchers analyzed the effect of SiO2, MgO, and Fe2O3 additives on the flexibility of aluminum titanate ceramics [24]. They have found that the addition of MgO, Fe2O3, and their mixture is responsible for the formation of elongated grains and grain boundary microcracks. The elongated grains with a particular shape can interlock together. This resists crack propagation. The microcracks in the grain boundaries provide more free space for grains to move. It provide flexibility to the structure [24]. The nonlinear stress strain curves confirm the flexible nature of all the samples. The samples with MgO and Fe2O3 additives show a

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maximum deflection of 0.55 mm [24]. Alumina ceramics are fundamentally brittle ceramics materials. But additives can improve the flexibility of alumina. As an example, the addition of only 0.38 vol.% graphene platelets to alumina increases the flexural strength and fracture toughness by 30.75% and 27.20%, respectively [25]. Though the further increase in graphene platelets percentage does not improve the flexibility further. The improvement of flexural strength and fracture toughness can be attributed to the crack deflection by graphene platelets [25]. The crack deflection creates a longer path for the cracks to release stress and improve the fracture toughness [25]. The addition of polymers in the porous network of ceramics can improve flexibility [26]. Polymer infiltrated ceramics networks (PICN) with 59% and 63% ceramic density was prepared by the researchers [26]. Maximum flexural strength of approximately160 MPa is achieved for PICN with 59% ceramic density [26]. Further increase or decrease of ceramic density will result in a decrease in the flexural strength for PICN [26]. The deflection of the cracks near the polymer region plays a major role in creating a longer crack path for releasing stress and hence achieving flexibility. The strain (%) at failure is maximum but Young’s modulus and the hardness are minimum for 59% PICN as compared to those of the other PICNs [26]. A group of researchers tried to achieve a flexible structure by alloying the ZnO tetrapod (T) structure with Cu, Sn, and Al [27]. Metal-doped highly porous networks (upto 98%) have been synthesized by tuning the concentration of the doped metal and the ZnO. The stress strain curves are nonlinear in nature, especially in lower densities [27]. The ZnO-T networks are highly flexible with values of Young’s modulus in the kPa region. For all the materials, at higher density, the elastic limit as well as Young’s modulus increases, but stress strain curves become almost linear. The Cu alloyed ZnO-T structure showed the highest stress (Fig. 5.5) [27]. The maximum stress increases with the loading cycle due to work hardening in each metal-doped ZnO structure[27]. Researchers developed Al2O3/Al/Al2O3 hybrid composite [28]. The stress strain curve for all the layered composites are nonlinear (Fig. 5.5). The nanocrystalline structure, shear band formation, nanosized circle bubbles prolonging the crack path, grain rotation, and deformation play important role in the high flexibility of the hybrid composite compared to the previously reported ceramics. Excellent stiffness and high elongation of the fabricated Al2O3/Al/Al2O3 hybrid composite extend the potential application of the material [28]. Some researchers showed that the zirconium alloying of WB2 allows a change in the elastic properties from brittle to ductile while maintaining superb hardness and incompressibility [29]. The addition of zirconium in a suitable amount in the base material can effectively control not only its mechanical properties but also its microstructure. Using of alloying process results in the formation of different structures, which expand from high to low values of Ts/Tm with an increasing amount of added atoms [29]. Researchers tried different shapes and crystal structures for ceramics materials to achieve flexibility [30 32]. A group of researchers prepared alumina ceramics in different sintering temperatures with different crystal structures [30]. The work of fracture as well as the compressive stress varies with the sintering temperature and the crystal structure. The work of fracture also varies with the shape of the pore for 3D printed

Mechanical properties of flexible ceramics 35

(A)

Stress [kPa]

4 3 2

30

ρ = 0.2 g/cm3

Stress [kPa]

5

71

3rd cycle 2nd cycle 1st cycle

1 0 0,00

25

(B) ρ = 0.2 g/cm3

20 15

3rd cycle 2nd cycle 1st cycle

10 5

0,02

0,04

0,06

0,08

0 0,00

0,10

0,02

Compression [GGL/L]

Stress [kPa]

1,5

0,04

0,06

0,08

Compression [GL/L]

ρ = 0.2 g/cm3 3rd cycle 2nd cycle 1st cycle

1,0

0,5

(C)

0,0 0,00

0,01

0,02

0,03

0,04

Compression [GL/L]

Figure 5.5 The nonlinear stress strain curve of (A) tetrapod ZnO and ZnO-T alloyed with (B) Cu and (C) Sn. Source: Published with permission from J. Gro¨ttrupa, I. Paulowicz, A. Schuchardt, V. Kaidas, S. Kapsa, O. Lupana, et al., Three-dimensional flexible ceramics based on interconnected network of highly porous pure and metal alloyed ZnO tetrapods, Ceram. Int. 42 (2016) 8664 8676.

alumina structure [30]. The work of fracture is maximum for the alumina sintered at 1400 C. For the cubic crystal structure, both the work of fracture as well as compressive stress are maximum. The shape of the pore for a hollow structure also plays an important role in the flexibility of porous structure [30]. Preparing the ceramics in a particular shape can improve flexibility. SiCN nanoware shows superior flexibility [31]. Researchers developed a film with a ratio of 70%:30% (ZrO2: PVdF HFP). It was found to be very flexible and free-standing [32].

5.4

Mechanism of flexibility

From the literature data, few mechanisms can be identified which are responsible for flexibility. There are broadly three ways to achieve flexibility for a ceramic material.

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Figure 5.6 The mechanisms of achieving flexibility (A) engineering the microstructure, (B) adding nanoparticle/fiber or different phases.

5.4.1 Modifying the microstructure The special shape and interlocking of grains can lead to flexibility. The grain boundary microcracks also help in creating void space for grain movement which improves flexibility. In the schematic, that is, Fig. 5.6A, the grain boundary microcracks are shown in white color. These cracks can be developed and tuned artificially by changing the raw materials. More void space in microstructure creates space for grain movement and hence, it improves flexibility.

5.4.2 Addition of other materials The addition of other materials like nanoparticles, nanofibers, or more compliant materials, like, polymer leads to flexible structures. The phase other than ceramic material generally deflects cracks and creates a longer path for the crack to release energy, Fig. 5.6B. In Fig. 5.6B, the nanoparticles which are shown as yellow, deflect the cracks in the structure. Sometimes doping of metal particles in brittle ceramic structures leads to ductile fracture.

5.4.3 By changing the shape The flexible structure can achieve by changing the shape as well. Ceramic membranes, thin films, thin sheets, or nanorods are more flexible than the corresponding bulk structure. Different crystal structures also lead to higher flexibility. From the literature, it is noticed that the different pore shapes also lead to flexibility.

5.5

Conclusion

The major application of flexible ceramics is in flexible and portable electronics. Other than that, the energy sectors like batteries, supercapacitors are also another

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field for application. There is a huge demand for flexible ceramics in antivibration systems and refractory applications. The wide possibility of application makes flexible ceramics a new area of recent research. Researchers have been trying to make “toughened” or “ductile” ceramics since the 1990s. Still, the research is ongoing in the present field. Generally, three approaches are identified for achieving flexibility: (1) engineering the grain and grain boundaries, (2) incorporating metal, polymer or other nanoparticles; (3) synthesizing different crystal structures or making thin films, rods, wires, or porous structure with different shape and sizes of the pores.

Acknowledgment The support received from SRM Institute of Science and Technology, Kattankulathur is deeply acknowledged by Payel Bandyopadhyay, Desigan Ravi, and Ramya Ravichandran. Anoop K. Mukhopadhyay deeply appreciates the support of Sharda University, Gautam Buddha Nagar, Greater Noida, Uttar Pradesh, India.

References [1] R. Danzer, T. Lube, P. Supancic, R. Damani, Fracture of ceramics, Adv. Eng. Mater. 10 (2008) 275 298. [2] R. Danzer, in: D. Bloor, R.J. Brook, M.C. Flemings, S. Mahajan, R.W. Cahn (Eds.), The Encyclopedia of Advanced Materials, Pergamon, 1994, p. 385. [3] J.B. Wachtman, Mechanical Properties of Ceramics, Wiley-Interscience, New York, Chichester, 1996. [4] D. Munz, T. Fett, Ceramics, Springer, Berlin, Heidelberg, 1999. [5] A.C. Fischer-Cripps, The Hertzian contact surface, J. Mater. Sci. 3 (1999) 129 137. [6] ISO 14704:2000, International Organization for Standardization Central Secretariat Chemin de Blandonnet 8CP 401 - 1214 Vernier, Geneva, Switzerland, 2000. [7] S.M. Wiederhorn, Influence of water vapor on crack propagation in soda-lime glass, J. Am. Ceram. Soc. 50 (1967) 407 414. [8] Erik Bitzek, James R. Kermode, Peter Gumbsch, Atomistic aspects of fracture, Int. J. Fract. 191 (2015) 13 30. [9] P.A. Bertrand, Effects of perfluoropolyalkyl ethers and perfluorinated acids on crack growth in soda-lime glass, Tribol. Lett. 14 (2003) 245 249. [10] C.J. Gilbert, R.O. Ritchie, Mechanisms of cyclic fatigue-crack propagation in a finegrained alumina ceramic: role of crack closure, Fatigue Fract. Eng. Mater. Struct. 20 (1997) 1453 1466. [11] R.O. Ritchie, Mechanisms of fatigue-crack propagation in ductile and brittle solids, Int. J. Fract. 100 (1999) 55 83. [12] Renato Chaves Souzaa, Claudinei dos Santosa, Miguel Justino Ribeiro Barbozaa, Carlos Antonio Reis Pereira Baptistaa, Kurt Streckerb, Carlos Nelson Eliasc, Performance of 3Y-TZP bioceramics under cyclic fatigue loading, Mater. Res. 11 (2008) 89 92.

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[13] A.C.F. Cocks, M.F. Ashby, On creep fracture by void growth, Prog. Mater. Sci. 27 (1982) 189 244. [14] H.J. Frost, M.F. Ashby, Deformation Mechanism Maps, Pergamon Press, Oxford, 1982. [15] H. Riedel, Fracture at High Temperatures, Springer-Velag, Berlin, 1987. [16] Young-Woo Rhee, Hae-Won Kim, Yan Deng, Brian R. Lawn, Brittle fracture versus quasi plasticity in ceramics: a simple predictive index, J. Am. Ceram. Soc. 84 (2001) 561 565. [17] Brian R. Lawn, Nitin P. Padture, Hongda Caitand, Fernando Guiberteau, Making ceramics ductile, Science 264 (1993) 1114 1116. [18] Brian Lawn, Rodney Wilshaw, Indentation fracture: principles and applications, J. Mater. Sci. 10 (1975) 1049 1081. [19] B.J. Hockey, B.R. Lawn, Electron microscopy of microcracking about indentations in aluminium oxide and silicon carbide, J. Mater. Sci. 10 (1975) 1275 1284. [20] Brian Lawn, Physics of fracture, J. Am. Ceram. Soc. 66 (1983) 83 91. [21] Isshu Sato, Yoshitaka Ichikawa, Junji Sakanoue, Mamoru Mizutani, Nobuyasu Adachi, Toshitaka Ota, Flexible ceramics in the system KZr2(PO4)3 KAlSi2O6 prepared by mimicking the microstructure of itacolumite, J. Am. Ceram. Soc 91 (2008) 607 610. [22] T. Shimazu, M. Miura, N. Isu, T. Ogawa, A. Ichikawa, E.H. Ishida, Itacolumite like high damping ceramics in the system Al2O3-TiO2-MgO, AIP Conf. Proc. 833 (2006) 69. [23] C. Babelot, A. Guignard, M. Huger, C. Gault, T. Chotard, T. Ota, Preparation and thermomechanical characterisation of aluminum titanate flexible ceramics, J. Mater. Sci. 46 (2011) 1211 1219. [24] Weiwei Chen, Anze Shui, Qin Shan, J. Lian, Cong Wang, Jianqiao Li, The influence of different additives on microstructure and mechanical properties of aluminum titanate ceramics, Ceram. Int. 47 (2021) 1169 1176. [25] Jian Liu, Haixue Yan, Kyle Jiang, Mechanical properties of graphene plateletreinforced alumina ceramic composites, Ceram. Int. 39 (2013) 6215 6221. [26] Andrea Coldea, Michael V. Swain, Norbert Thiel, Mechanical properties of polymerinfiltrated-ceramic-network materials, Dental Mater. 29 (2013) 419 426. [27] Jorit Gro¨ttrupa, Ingo Paulowicz, Arnim Schuchardt, Victor Kaidas, S.o¨ren Kapsa, Oleg Lupana, et al., Three-dimensional flexible ceramics based on interconnected network of highly porous pure and metal alloyed ZnO tetrapods, Ceram. Int. 42 (2016) 8664 8676. [28] Zhijiang Wang, Henry Hu, Xueyuan Nie, Preparation and characterization of highly flexible Al2O3/Al/Al2O3 hybrid composite, J. Nanomater. (2015) 2015. [29] Rafał. Psiuka, Michał. Milczarek, Piotr Jenczyk, Piotr Denis, Dariusz M. Jarza˛bek, Piotr Bazarnik, et al., Improved mechanical properties of W-Zr-B coatings deposited by hybrid RF magnetron PLD method, Appl. Surf. Sci. 570 (2021) 151239. [30] Hui Mei, Yuanfu Tan, Weizhao Huang, Peng Chang, Yuntian Fan, Laifei Cheng, Structure design influencing the mechanical performance of 3D printing porous ceramics, Ceram. Int. 47 (2021) 8389 8397. [31] Peng Wang, Laifei Cheng, Litong Zhang, Lightweight, flexible SiCN ceramic nanowires applied as effective microwave absorbers in high frequency, Chem. Eng. J. 338 (2018) 248 260. [32] Shruti Suriyakumar, M. Raja, N. Angulakshmi, Kee Suk Nahm, A. Manuel Stephan, A flexible zirconium oxide based-ceramic membrane as a separator for lithium-ion batteries, RSC Adv. 6 (2016) 92020 92027.

Electrical properties of flexible ceramics

6

N.S. Kiran Kumar1, A.R. Jayakrishnan2, R. Rugmini1, J.P.B. Silva3, M. Pereira4, Sathish Sugumaran5 and K.C. Sekhar1 1 Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, India, 2Centre for Nanoscience and Engineering, Indian Institute of Science, Bengaluru, Karnataka, India, 3Physics Center of Minho and Porto Universities (CF-UM-UP), University of Minho, Campus de Gualtar, Braga, Portugal, 4 Laboratory of Physics for Materials and Emergent Technologies, LapMET, University of Minho, Braga, Portugal, 5Department of Physics, MVJ College of Engineering, Bengaluru, Karnataka, India

6.1

Introduction

In recent times, the focus has been shifted toward the advancement of humanfriendly technology, which includes skin-like sensors, interactive electronics with surrounding environments, wearable medical gadgets, fitness outfits, clothes integrated with electronic displays, portable chemicals and biosensors, etc. [1
5]. But the traditional electronics, which are rooted in rigid platforms and planar integrated circuits could not meet the needs of future technology. Rigidity is the major hindrance to traditional electronics conforming to the desired shape or getting entwined with irregular and fluctuating surfaces like human skin and clothes. Thus the advancement of novel flexible electroceramics is imperative to obtain flexible and stretchable electronics [6]. Flexible electronics or soft electronics is a technology that incorporates flexibility in electronic devices, which can be bent, stretched, rolled, or twisted while maintaining their electrical properties unaffected [7,8]. From flexible electronic displays and electronic textiles, extending to biomedical instruments, active antennas, and even e-skin in robotics, flexibility exposes its vigor and inevitability [9]. However, the degree of flexibility varies from device to device based on its utility. Therefore the selection of materials, their fabrication, and knowledge of their electrical properties are very critical to designing and implementing them in various applications like energy storage capacitors, memory, photovoltaic devices, etc. [9]. Therefore this chapter discusses a contemporary investigation of the electrical properties such as dielectric, piezoelectric, pyroelectric, ferroelectric, electrochemical, and current
voltage characteristics of flexible electroceramic thin films. Also, their novel potential applications in next-generation soft electronics in various fields such as harvesting and storage of energy, sensing, and memory applications are Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00006-3 © 2023 Elsevier Ltd. All rights reserved.

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Figure 6.1 Electrical properties and application of flexible ceramics.

discussed. Fig. 6.1 shows the various applications based on the different electrical properties of flexible ceramic films.

6.2

Electrical properties of flexible ceramics

The exploration of electrical properties revolutionized the journey of ceramics from the mere concept of “pottery” to the most investigated materials in today’s world. The specialty of ceramics is that its electrical conductivity can be tuned with external stimuli. Most ceramic materials are poor conductors of electricity since the ionic and covalent bonds in ceramics do not provide free electrons for conduction, whereas their conductivity gets altered when subjected to an external electric field, temperature, mechanical stress, etc. This versatile nature of the electrical properties of ceramics could span a variety of fields in the electronic industry. As the trend moves forward to next-generation flexible devices, the integration of flexibility with electroceramics needs significant attention. A thorough analysis of the variations in electrical behavior of ceramics as a function of various mechanical stimuli such as bending, stretching, etc., is inevitable to achieve the best-suited material for appropriate applications. The major electrical properties of the ceramics and their response to flexibility are discussed in detail in the following sections.

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Figure 6.2 Schematic diagram of flexible ceramic film in the metal
insulator
metal configuration under (A) flat state and (B) under compressive and tensile stress.

The electrical properties of ceramics are generally studied by sandwiching the material between two electrodes in a metal
insulator
metal (MIM) configuration [10,11]. When flexibility comes into this realm, a flexible substrate becomes an integral part of the configuration in most investigations. Schematically a flexible device can be represented as shown in Fig. 6.2A. Compressive or tensile stress is employed on the device as shown in Fig. 6.2B, to inspect the flexibility effect such as bending radius (r) and bending angle (θ) as defined in Fig. 6.2B on the electrical properties of flexible ceramics and devices.

6.2.1 Dielectric properties A dielectric material is an insulating material that gets polarized when an electric field is applied. When the dielectric material is placed in between two parallel conducting plates, a dielectric capacitor is formed. The capacitance can be quantified in terms of the dielectric constant of the material inserted. The dielectric constant is very sensitive to the electric field, frequency, temperature, etc. [12]. Jiang et al. developed a lead (Pb)-based dielectric capacitor with PbZr0.2Ti0.8O3 (PZT) ferroelectric layer on a mica substrate and studied the field-dependent dielectric properties with different bending radii [13]. The device shows the butterfly features of the dielectric constant versus the electric field plot at different bending radii as depicted in Fig. 6.3A. It confirms that the dielectric nature of PZT film is retained in the device under different bending conditions. The dielectric constant and the specific capacitance (capacitance/area) under compressive and tensile bending conditions at

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Figure 6.3 (A) Variation of the dielectric constant with electric field under different bending conditions and (B) the dielectric constant and the specific capacitance with different bending radii of PZT/mica. Source: Reprinted with permission from J. Jiang, Y. Bitla, C-W. Huang, T.H. Do, H-J. Liu, Y-H. Hsieh, et al. Flexible ferroelectric element based on van der Waals heteroepitaxy, Sci. Adv. 3 (2017) e1700121. https://doi.org/10.1126/sciadv.1700121.

varying radii are given in Fig. 6.3B. It is observed that there is not much difference in the dielectric values under flat and bending states. This is due to the fact that the polarization switching kinetics of PZT/mica are resistant to mechanical bending as the smooth surface of the mica sheet contributes to weak Van der Waals interaction at the interface of the PZT film-substrate. Further, the ferroelectric PZT film has multidomain features and thereby, constant electrical properties [13]. Recently, Gao et al. fabricated a flexible capacitor based on mica/SrRuO3/ Ba0.67Sr0.33TiO3/Pt using the pulsed laser deposition method and studied the electrical properties and mechanical strength under both tensile and compressive bending [14]. A photograph of the experimental arrangement is given in Fig. 6.4A. The capacitor Ba0.67Sr0.33TiO3 (BST) exhibited a large relative permittivity (εr0 ) of 1210 and a small loss tangent (tan δ) of 0.16 under initial flat condition, which varied monotonously with frequency as shown in Fig. 6.4B and C. The same values were retained under the sequential bending at radii 20, 15, 10, and 5 mm. Then the capacitor was reflattened, yet the properties were observed to be unchanged. However, a further decrease in the bending radius to 2 mm causes a noticeable decrease in εr0 (B930) and an increase in tan δ (B0.47) and was not recoverable. It is due to the development of microcracks, which leads to a leak path and consequently increases the dielectric loss. Further, the effect of electric field on εr0 was studied in terms of a parameter called tunability, expressed as follows: 0

0

εr ð0Þ 2 εr ðEÞ Tunability ð%Þ 5 εr 0 ð0Þ

(6.1)

Electrical properties of flexible ceramics

79

Figure 6.4 (A) Photograph of the mica/SRO/BST/Pt device under compressive stress. (B) εr0 and (C) tan (δ) as a function of frequency under different bending conditions. (D) tan (δ) as a function of frequency after different bending cycles. (E) The dielectric properties of different concentrations of SR-CNT composites with bending cycles. Source: Parts (A)
(D) reprinted with permission from D. Gao, Z. Tan, Z. Fan, M. Guo, Z. Hou, D. Chen, et al., All-inorganic flexible Ba0.67Sr0.33TiO3 thin films with excellent dielectric properties over a wide range of frequencies, ACS Appl. Mater. Interfaces 11 (2019) 27088
27097. https://doi.org/10.1021/acsami.9b08712. Part (E) from L.K. Namitha, M.T. Sebastian, High permittivity ceramics loaded silicone elastomer composites for flexible electronics applications, Ceram. Int. 43 (2017) 2994
3003. https://doi.org/10.1016/j. ceramint.2016.11.080.

where εr0 (0) is the zero field permittivity and εr0 (E) is the field-dependent dielectric constant. The tunability was found to be 67% for the initial flat state and retained a stable tunability up to a 5 mm bending radius. However, the dielectric loss hardly varied up to 12,000 bending cycles during the endurance test at a bending radius of 5 mm as displayed in Fig. 6.4D. More importantly, the device retained a tunability of 63% even after 20,000 cycles, suggesting excellent mechanical flexibility and durability. The additional bending into a 2 mm radius resulted in the degradation of values and is explained by the formation of microcracks. Thus the introduction of the mica/SRO/BST/Pt device can be a new insight for the development of ecofriendly flexible and tunable electronic devices [14]. On the other hand, the ceramic
polymer composite layers with 0
3 connectivity are particularly attracting attention as flexible films. Vrejoiu et al. prepared the free-standing composite films of thickness less than 10 μm by combining Pb-free BaTiO3 (BTO) ceramics and perfluorocyclobutene (poly 1,1,1-triphenyl ethane perfluorocyclobutyl ether or PFCB) polymer [15]. The εr0 and tan δ of composites with filling factor, f # 0.5 were estimated at 20 C and 1 kHz with bending cycles up to

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50. It is observed that properties were retained at low ceramic concentration and the samples with the higher ceramic concentration became brittle and were broken after a few cycles. For the composites with lower ceramic concentrations, the properties of the polymer matrix dominate and this enables easy nondestructive mechanical deformations. But at higher concentrations, the effect of ceramic dominates the polymer and thus makes the composite brittle and easily breakable [15]. Recently, it has been observed that elastomer
ceramic composites exhibit better stretchability and hence, they have been considered the best option for flexible device applications [16,17]. Silicone rubber is one of the widely used dielectric elastomers with desirable dielectric characteristics, such as elasticity, biocompatibility, permeability, and is easily available. In view of this, Namitha et al. examined the dielectric properties of flexible ceramic composites based on high permittivity ceramic fillers, say, BaTiO3 (BTO), SrTiO3 (STO), and Ca(12x)Nd(2x/3)TiO3 (CNT) and dimethyl end-blocked silicone elastomer as the matrix [18]. The physical and electrical properties of the above-mentioned ceramics composites are given in Table 6.1. Further, the silicone rubber (SR) ceramic was created using dicumyl peroxide as a vulcanizing agent by the sigma blend method. The composites based on SR as matrix and dielectric ceramics such as BTO, STO, and CNT with different ceramic concentrations were prepared using a specific amount of ceramic fillers [18]. The effect of flexibility on microwave dielectric properties with different filler amounts (SR-BTO, SR-STO, and SR-CNT) at 5 and 15 GHz, respectively, were studied. It is observed that dielectric properties were strongly dependent on ceramic concentration and it is quite obvious that permittivity increases with increasing filler concentration. Further, the flexibility and the fluctuations of the dielectric properties of SR-BTO, SR-STO, and SR-CNT composites as a function of bending have been studied. Fig. 6.4E shows the dielectric properties of SR-CNT composites with bending cycles. This reveals that the relative permittivity is resilient with the bending cycles. Though the composites with higher filler concentrations show slight variation in dielectric loss due to particle agglomeration, the composites with lower filler concentrations are unaffected by bending cycles [18]. The robust dielectric characteristics of the composites with bending cycles make them appropriate for flexible capacitor applications. Table 6.1 Physical and dielectric parameters of composite films based on dimethyl end blocked silicone elastomer
ceramics [18]. Material

Density (g/cm3)

Relative permittivity (εr0 )

Dielectric loss (tan δ)

Coefficient of thermal expansion (ppm/ C)

Thermal conductivity (W/mK)

BaTiO3 SrTiO3 Ca(12x)Nd(2x/3)TiO3

6.01 5.12 4.57

1250 (1 MHz) 290 (1 GHz) 98 (7 GHz)

1022 (1 MHz) 1023 (1 GHz) 1023 (7 GHz)

5.4 9.4 10.3

2.6 12.0 2.4

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6.2.2 Piezoelectric properties Piezoelectric materials convert mechanical energy into electrical energy which is termed the piezoelectric effect. It is an intrinsic property of several materials by which electrical energy is generated when subjected to mechanical stress. Piezoelectricity is exhibited by materials with an asymmetric crystal structure. When these crystals are disrupted by mechanical stress, a charge imbalance is developed in the crystal, and thereby, the charge carriers transfer the energy in the form of current [19]. However, the ability of piezoelectric material to induce piezoelectric current strongly relies on the piezoelectric coefficient (d) and is determined using the equation given below: d5

P k

(6.2)

where P and k denote polarization and stress experienced by the material, respectively. Based on the crystal orientation, the d value will be different for different materials. Table 6.2 shows the piezoelectric coefficient of different piezoelectric materials. However, PZTis one of the predominantly investigated ceramic materials for electronic applications. Babu et al. reported a flexible piezoelectric composite comprising PZT nanoparticle and poly-dimethylsiloxane (PDMS) prepared using the solution cast technique [26]. In this work, all the d33 measurements were performed at a force of 1 N and are shown in Fig. 6.5A. The work noted a d33 value of 25 pC/ N and piezoelectric stress coefficient (g33) of 75 mVm/N for 50 vol.% PZT added PZT-PDMS composite. The high piezoelectricity in 50 vol.% PZT added PZTPDMS composite is attributed to the decreased stress-strain state due to the diminished rubbery effect contributed by the PDMS layer. And recently, Gupta et al., compared the piezoelectric coefficient in poly(vinylidene fluoride-trifluoroethylene) or P(VDF-TrFE) and a nanocomposite structure comprising P(VDF-TrFE) and 10% niobium-doped Pb(Zr,Ti)O3 (NPZT) nanoparticles [27]. The NPZT/PVDF-TrFE exhibited a high d33 value (B 2 101 pC/N) as illustrated in Fig. 6.5B. This is associated with the increased ferroelectric polarization due to enhanced electromechanical Table 6.2 Most used piezoelectric ceramic materials and their corresponding piezoelectric coefficient. Materials

GaN AlN ZnO BTO PZT LiNiO2

Piezoelectric coefficients

References

d33 (pC/N)

d31 (pC/N)

d15 (pC/N)

3.7 5 12.4 149 289 6

2 1.9 22 2 5.0 2 58 2 123 21

3.1 3.6 2 8.3 242 495 69

[20,21] [20,21] [22] [23] [23] [24,25]

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Figure 6.5 (A) Dependence of d33 and g33 of the PZT-PDMS composites with volume % of PZT. (B) Variation of displacement versus electric field for NPZT with and without filler. Source: Part (A) reprinted with permission from I. Babu, G. de With, Highly flexible piezoelectric 0
3 PZT
PDMS composites with high filler content, Compos. Sci. Technol 91 (2014) 91
97. https://doi.org/10.1016/j.compscitech.2013.11.027. Part (B) from S. Gupta, R. Bhunia, B. Fatma, D. Maurya, D. Singh, Prateek, et al., Multifunctional and flexible polymeric nanocomposite films with improved ferroelectric and piezoelectric properties for energy generation devices, ACS Appl. Energy Mater. 2 (2019) 6364
6374. https://doi.org/ 10.1021/acsaem.9b01000.

coupling constituted by the NPZT nanofiller. Such a high ferroelectric polarization in NPZT/PVDF-TrFE is ascribed to the interaction between the NPZT nanoparticles and the pristine PVDF-TrFE, which in turn develops an additional compression during the application of an external electric field. Consequently, an increased d33 value. However, Pb is a toxic element and an alternative material with comparable piezoelectric properties is essential for fabricating eco-friendly electronic devices. Pb-free Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) is a very attractive candidate due to its comparable electrical properties with PZT. Very recently, Liu et al. reported a very good d33 value in BCZT-based flexible thin film on flexible mica substrate [28]. The d33 value was calculated using a laser vibrometer and is found to be 150 pC/N, suggesting the material as an open door to flexible piezoelectric electronic applications.

6.2.3 Pyroelectric properties The pyroelectric effect involves the conversion of temperature change in a material into electrical energy, and the materials that exhibit the pyroelectric effect are known as pyroelectric materials. These materials have spontaneous polarization (Ps) and there exists a charge presence at each surface of the material due to the Ps. Subsequently, free charges like ions or electrons will be attracted toward the pyroelectric material’s surface. At thermal equilibrium, the Ps will be constant and no current is obtained in the external circuit. But when this material is subjected to an increase in temperature, there occurs a decrease in Ps because the dipoles fail to retain their orientation due to the wiggling of atoms. To regain the equilibrium,

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a redistribution of charges takes place [29]. As a result, the pyroelectric current flows through the external circuit as shown in Fig. 6.6. Similarly, as the temperature decreases, Ps level will be significantly enhanced due to reduced wiggling and consequently, a current flow occurs. The equation for the pyroelectric coefficient (p) is given by [29]: pð T Þ 5

I A dT dt

(6.3)

Here, A denotes the area of the electrode and dT=dt denotes the rate of change of temperate with respect to time. The phenomenon described above is known as the primary pyroelectric effect. A piezoelectric material under thermal deformation subjected to constant stress generates electric current due to piezoelectricity induced electric displacement. This phenomenon is assigned as the secondary pyroelectric effect. In such cases, both the mechanical and thermal changes (hybrid effect) in a single material can be utilized for energy harvesting applications [29]. Table 6.3 shows the comparison of the pyroelectric coefficient of different pyroelectric materials. Pb-based ceramic materials have been widely used in pyroelectric application due its high pyroelectric coefficient. Yan et al. created a composite film based on the PbTiO3 (PT) nanowires (NW) and P(VDF-TrFE) polymer [32]. The preparation of the film proceeded via the solution-casting method and hot-pressing are used.

Figure 6.6 Schematic representation of pyroelectric current generation.

Table 6.3 The comparison of the pyroelectric coefficient of different materials. Materials

ZnO

Li2B4O7

LiNbO3

LiTaO3

BaTiO3

SrBaNb

PZT

Pyroelectric coefficient (μC/m2 K1) Reference

29.4

30

283.3

2176

2200

2550

2380

[19]

[22]

[30]

[30]

[30]

[31]

[31]

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The work reported that the value of p for the 50 wt.% PT-NW/P(VDF-TrFE) composite film (B72.8 μC/m2K) is higher with respect to the pure P(VDF-TrFE) which has a p-value of 52.7 μC/m2K. This increases in the p value is ascribed to the small dielectric permittivity and dielectric loss of the two-stage treated sintering process of the PT-NW/P(VDF-TrFE) composite film. Luo et al. reported a Pb-free potassium sodium niobate (KNN)/P(VDF-TrFE) composite film on ITO substrate [33]. The KNN/P(VDF-TrFE) composite sintered at 850 C showed a comparatively good p-value of 63 μC/m2K than that of PZT/P(VDF-TrFE) composites. But the p value of the composite KNN decreases for the higher sintering temperature of 950 C. It is due to the volatilization of the Na and K elements in the KNN for the high sintering temperature. Nanofillers with highly organized porosity structure and tiny size are combined with suitable polymers to achieve the desired flexibility. Sagar et al. described a composite based on the nanofiller BaZrO3 and BaTiO3 with PVDF/PVC matrix [34]. The work reported a p value of 153.04 μC/m2K and 351.4 μC/m2K for PVDF/PVC/BaZrO3 and PVDF/PVC/BaTiO3, which were high compared to the pure PVDF/PVC (88.58 μC/m2K). This variation in the p-value is due to the presence of a nanofiller with high pore density.

6.2.4 Ferroelectric properties Ferroelectric materials exhibit spontaneous polarization below the Curie temperature (Tc) and transform to paraelectric above Tc. The ferroelectric polarization varies nonlinearly and hysterically with the applied electric field as portrayed in Fig. 6.7. The “S”-shaped polarization
electric field (P
E) loop is exhibited by typical ferroelectric materials. The characteristic parameters that define the ferroelectric properties are remnant polarization (Pr), saturation polarization (Ps) and coercive field (Ec) [35].

Figure 6.7 Typical P
E hysteresis loop of a ferroelectric material.

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First time, J. Valasek discovered the ferroelectricity in Rochelle salt (NaKC4H4O6 4H2O), in 1921, during investigations of its anomalous dielectric properties [36]. KH2PO4 is the second ferroelectric material found in 1935 and followed by some of its isomorphs [37,38]. BTO was the first synthesized polycrystalline ceramic which exhibited ferroelectricity and then several ferroelectric ceramics like PZT were synthesized in the family of perovskites having ABO3 structures, where A is monovalent, divalent, or trivalent metals and B is pentavalent, tetravalent or trivalent metals [37]. Since 1952, Pb-based ceramic materials are mostly used in ferroelectric devices and recently, Pb-free materials are encouraged due to environmental concerns. Thus researchers are turning to the synthesis of several Pbfree ferroelectric materials like BTO, BiNaTiO3 (BNT), etc., which can compete with PZT materials [39]. Further, to meet the requirements of electronic devices, the research focus is shifted to fabricate ferroelectric ceramics on flexible substrates like mica, polymers, metal foils, etc. [40
42]. Among flexible substrates, mica is mostly chosen to develop flexible devices due to its availability and low cost. Mica has a stacked-layer structure. The weak Van der Waals forces in it act as a connection between the layers which makes it easier to peel the mica into thin layers with a thickness of a few tens of micrometers and thus make it ideal flexible substrates for the construction of soft electronic devices [40]. Fig. 6.8 shows the comparative Pr of recently developed flexible ceramics on mica substrate under compressive and tensile bending at different radii [43
50]. In 2019 Yang et al. examined the effect of flexibility on ferroelectric performance of mica/Pt/PZT/Au capacitor by applying compressive and tensile stress to bend the device in a semicircular shape with bending radii 12
2 mm [43]. The resulting P
E loops at the bending radii are given in Fig. 6.9A. The study revealed G

Figure 6.8 Comparison of Pr of recently developed flexible ceramics on mica substrate at different bending radii.

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Advanced Flexible Ceramics

Figure 6.9 (A) P
E loop of mica/Pt/PZT/Au capacitor at different compressive and tensile bending conditions and (B) Ps, Pr, and Ec as functions of bending radii. (C) Variation of P
E loops of Mn:NBT
BTO
BFO capacitor under different compressive and tensile bending radii and (D) Pr, Pmax, and Pmax
Pr of Mn:NBT
BTO
BFO capacitor as a function of bending radii. (E) P
E loop of Mn-doped BFO capacitor before, during, and after bending. Source: Parts (A) and (B) reprinted with permission from C. Yang, Y. Han, J. Qian, Z. Cheng, Flexible, temperature-stable, and fatigue-endurable PbZr0.52Ti0.48O3 ferroelectric film for nonvolatile memory, Adv. Electron. Mater. 5 (2019) 1900443. https://doi.org/10.1002/ aelm.201900443. Parts (C) and (D) from C. Yang, P. Lv, J. Qian, Y. Han, J. Ouyang, X. Lin, et al., Fatigue-free and bending-endurable flexible Mn-doped Na0.5Bi0.5TiO3-BaTiO3BiFeO3 film capacitor with an ultrahigh energy storage performance, Adv. Energy Mater. 9 (2019)1803949. https://doi.org/10.1002/aenm.201803949. Part (E) from B. Yang, C. Li, M. Liu, R. Wei, X. Tang, L. Hu, et al., Design of flexible inorganic BiFe0.93Mn0.07O3 ferroelectric thin films for nonvolatile memory, J. Materiomics 6 (2020) 600
606. https:// doi.org/10.1016/j.jmat.2020.04.010.

that the values of Pr, Ps, and Ec change within 4.7%, 2.3%, and 10.1%, respectively, compared to the flat state values as shown in Fig. 6.9B. Further, the retention and fatigue measurements of the flat state show no loss up to a retention time of 105 s and switching cycles of 107. But the device shows a loss of 21% when the fatigue cycles were increased to 109. Even at a minimum radius of 2 mm, the change in the retention and fatigue characteristics was insignificant, which ensured good reliability. Further, to check the feasibility of the mica/Pt/PZT/Au device on the ferroelectric properties, retention, and fatigue characteristics, the device was bent and flattened repeatedly up to 103 times, and the response suggested high feasibility of the device. It is known that the oxide electrodes suppress the fatigue of ferroelectric films by acting as sinks for oxygen vacancies. Thus Gao et al. coated SrRuO3 (SRO) which can act both as a buffer layer and bottom electrode on the mica substrate,

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and fabricated mica/SRO/PZT/Pt devices [51]. This device shows that there is not much variation in polarization as the device is bent to small radii of 2.2 and 1.4 mm. Further, the device exhibited enhanced resistance to fatigue, that is, even after 1010 polarization switching cycles, a large Ps value of 38 μC/cm2 was retained from the original value of 60 μC/cm2 irrespective of whether the device is in its flat or bent state with bending radius 1.4 mm. The reliability test also revealed no significant decay in ferroelectric polarization (Pr 5 37 μC/cm2) even after 104 cycles of bending. Recently, Ren et al. introduced a layer of CoFe2O4 (CFO) in between mica and SRO layer and fabricated a mica/CFO/SRO/PZT/Pt heterostructure [52]. The P
E studies revealed a Pr of 49 μC/cm2 and a Ps of 93 μC/cm2 under a flat state. Further, the device showed similar values under different bending radii ranging from 10 to 4 mm suggesting remarkable flexibility. The robustness of the flexible device was investigated with bending cycles increasing from 100 to 500 at bending radius 6 mm. The mica/CFO/SRO/PZT/Pt heterostructure exhibited constant ferroelectric characteristics during the entire testing range of bending cycles. Although Pb-based materials are excellent candidates for soft electronics, the environmental hazards of these materials provoked the scientific community to find alternatives. Since the early 2000s, research on Pb-free materials dragged huge attention and plenty of alternate materials have emerged which could replace Pb-based materials. Bismuth ferrate (BiFeO3 or BFO) was one of the most extensively investigated multiferroic Pb-free materials [53,54]. The lone pair electrons in the 6 s orbital of Bismuth introduces ferroelectricity in BFO. But the pure BFO materials suffer a leakage current due to oxygen vacancies [55]. It is shown that Mn, Ti co-doping can boost the ferroelectric property of BFO by reducing the oxygen vacancies. Thus Yang et al. fabricated mica/Pt/Bi(Fe0.93Mn0.05Ti0.02)O3/Au devices and obtained a maximum Ps of 93 μC/cm2 and Pr of 66 μC/cm2 [44]. A compressive and tensile bending to a radius from 12 to 2 mm was applied to the device and there was no variation in the polarization values. Also, the devices exhibited an endurance of 103 bending cycles. Further, Mn was doped in NaBiTiO3
BaTiO3
BFO (NBT
BT
BFO) to enhance the electrical resistance and the ferroelectric properties were investigated in mica/Pt/Mn:NBT-BT-BFO/Au. The device exhibited excellent ferroelectric characteristics with Pr 5 14.9 μC/cm2 and Ps 5 112.7 μC/cm2 as illustrated in Fig. 6.9C and the device retained the same polarization under bending states of radii 12
2 mm as given in Fig. 6.9D. The endurance test suggested that the values of Ps and Pr slightly changed by 2.7% and 18% up to 109 cycles. Further, the device shows good stability up to 103 bending cycles [46]. In 2020 Yang et al. fabricated Mn-doped BFO film on fluorphlogopite mica substrate with Au and lithium nickel dioxide (LNO) as the top and bottom electrodes [45]. This capacitor exhibited minor variations at the different bendable states. After the continuous bending, the Pr value has diminished from 69 to 64 μC/cm2 which is shown in Fig. 6.9E and is attributed to the dislocation or defects. But during the tensile bending, the Pr value was increased from 69 to 72 μC/cm2. This increase in the Pr value is due to the flexoelectric phenomenon, in which uneven stresses under external tension cause charges to separate and create displacement polarization [45].

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The other flexible substrates like metal foils and polymers are used to examine the impact of their flexibility on the ferroelectric properties of ceramics. Metal foils, as substrates, have the advantage of electrical conductivity hence, can itself act as the bottom electrode. In the year 2021 Ningning Sun and his co-workers used nickel foil as a substrate to study the properties of Mn-doped Bi0.5Na0.5TiO3
BiNi0.5Zr0.5O3 based thin film capacitor [56]. The flexibility of the device was analyzed at different bending radii, 10, 7.5, and 5 mm, and found that there is not many changes in the Pr and Ps values as compared with the values of the flat stage which are 14 μC/cm2 and 73.7 μC/cm2, respectively. After 104 cycles of compressive and tensile bending, the P
E loop obtained was identical to that of the initial stage. Moreover, the time delay stability of the film was found to be distinctive since the same endurance was obtained when the same test was conducted even after 12 months. Chen et al. coated HfZrO2 (HZO) thin film, a new generation of ferroelectric materials, on a polyimide (PI) substrate [57]. The PI/Al2O3/TiN/HZO/TiN device exhibited good ferroelectricity for different bending radii. The device attained a polarization value of 21 μC/cm2 for a minimum bending radius of 2 mm. The buffer layer Al2O3 improves the smoothness between the PI substrate and HZO film, which prevents the bottom electrode from breaking while bending. Moreover, the amorphous structure of Al2O3 also helps to resist breaking, as stress cannot be applied directly to the grains in an amorphous structure. Rho et al. tested the ferroelectric response of the PZT ceramic film coated on a plastic substrate by varying the bending radius of the flexible device from 16 to 8 mm [58]. The device exhibited a Pr value of 17 μC/cm2 at a bending radius 8 mm which is almost comparable with the value at the flat state. As a result, the use of plastic substrate in the manufacturing of flexible electronics has proven to be successful [58]. The properties of different devices under flat and bent states are summarized in Table 6.4.

6.2.5 Electrochemical properties Currently, batteries play a crucial role in sustainable energy technology and Li-ion batteries (LIBs) are unavoidable due to their low-cost, superior energy storage density, and stable performance. Batteries works on the principle of converting chemical energy into electrical energy. A battery mainly consists of two electrodes (namely, cathode and anode) and an electrolyte. During the discharge, electrons flow through the electrolyte due to the chemical reaction at the electrodes and thereby, developing an electric current in the external circuit. However, recent investigations on LIB are particularly focused on designing flexible electrodes and flexible electrolytes with exceptional mechanical properties as well as electrochemical performance. For instance, a traditional electrode in a LIB is usually formed on a metal foil current collector comprising an active material, PVDF binder, and carbon black additives. However, the weak mechanical and poor binding tenacity separate the active material during repeated bendings. As a result, the performance of the LIB was deprived. To overcome this barrier, in 2020, Sang et al. proposed a graphene-modified flexible silicon oxycarbide (SiOC) ceramics-based electrode for LIB fabrication [62]. SiOC ceramics possess disordered carbon nanoclusters endowing

Table 6.4 Ferroelectric properties of different ceramics deposited on mica substrate under flat and bending states. Material

Electrodes

Minimum bending radius (mm)

Ps

Pr

Under flat state (µC/cm2)

PZT

SRO,Pt

1.4

60

37

BFOMnTi

Pt,Au

2

93

66

BiLaTi

Ag-ITO, ITO

2.2

20

10

BiLaTi

SRO,Pt

1.4







Pb(ZrTi/ZnO)

Pt,Au

4

93

49

4




HfZrO

Al,Ta/TiN

Ps

12

Pr

Bending cycles

References

103 at 2.2 mm

[57]

103 at 4 mm

[50]

After bending state (µC/cm2) 38


No change No change

20


No change







4

[59]

3

10 at 1.4 mm

[60]

500 at 6 mm

[58]

10 at 3 mm

3

10 at 6 mm

[61]

90

Advanced Flexible Ceramics

a high theoretical capacity ( . 1350 mAh/g). Further, the nanopores in SiOC enhance cycling durability by minimizing the volume change at the time of Li-ion intercalation. Sang et al. recorded a super flexible anode for LIBs using a SiOC fiber cloth modified with a 3D graphene (3D-GNS/SiOCf). The flexible SiOC nanofiber cloth is synthesized using SiOC ceramics and polysiloxane via the electrospinning method. Afterward, the SiOC cloth is modified using interconnected 3D graphene frameworks (GNS). The role of SiOC fibers (SiOCf) is to improve the capacity of the material to store Li-ions and the GNS framework promotes the electrochemical process via efficient transfer of electrons. Furthermore, the tightly encircled individual fibers aid the mechanical stability and thereby, increase the life of the LIB. Here, the electrochemical performance test on 3D-GNS/SiOCf showed a very high coulombic efficiency of approximately 78.5%, which was found to be higher than those in SiOCf (58.4%) and GNS (53.8%) at current density 0.1 A/g as given in Fig. 6.10A. The 3D-GNS/SiOCf was analyzed for different cycles and show a high and reversible capacity of 686 mAh/g (at 0.5 A/g) for almost 500 cycles. In fact, the battery maintained a 100% reversible capacity for almost 100 cycles. Thus the device is suitable for superior energy storage applications with exceptional cycling stability. More importantly, a flexible prototype of LIB comprising 3D-GNS/SiOCf anode and LiFePO4 cathode arrangement was used for the bending test. The LIB lighted six light-emitting diodes (LEDs) and the output power remained unchanged at different bending angles ranging from 45 to 180 degrees as displayed in Fig. 6.10B and C [62]. However, a chief issue associated with solid-state lithium batteries is the catastrophic fire due to electrolyte leakage. The formation of solid polymer electrolytes (SPEs) with integrated ceramic fillers is a prominent solution to circumvent such problems and thereby, realize high ionic conductivity as well as mechanical properties at the same time. The incorporation of ceramic fillers favors Li-ion transportation by providing more active sites for Li-ion insertion. Very recently, Fan et al. introduced a versatile strategy using a combination of salt affluent poly(ethylene oxide) (PEO) with Li6.75La3Zr1.75Ta0.25O12 (LLZTO) ceramic fillers to improve the ionic conductivity near room temperature [63]. The Li7La3Zr2O12 constitutes ultrafast Li1 conductors that improve the ionic conductivity of SPEs. In addition, these nanostructured fillers interact significantly with the polymer due to their large surface area. This is critical for preventing polymer chains from recrystallizing and, as a result, enhancing the ionic conductivity of the electrolyte. Further, the salt affluent (lithium salt) introduced in the polymer matrix form internal ionic clusters beneficial for enhancing the capacity of the SPEs to transport Li-ions. Here, Fan et al. fabricated a flexible SPE by inserting 1D LLZTO nanofibers (LLZTO NFs) into a lithium salt affluent PEO matrix. The 1D LLZTO NFs were synthesized by simple electrospinning and calcination processes. Afterward, different weight percentages of LLZTO NFs were dispersed in the salt affluent PEO to form the LLZTO NF reinforced polymer-in-salt (LLZTO NF reinforced PISE) SPEs. Then, the electrochemical stability of both PISE and LLZTO NF reinforced PISE are determined with electrochemical impedance spectroscopy (EIS) profile as presented in Fig. 6.11A. The incorporation of 10% LLZTO NFs provided a stable voltage (B4.9 V), which is two times higher than the PISE electrolyte (2 V). Besides, LLZTO nanofillers increased the electrochemical stability by suppressing

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91

Figure 6.10 (A) Charge/discharge plots of 3D-GNS/SiOCf after different numbers of cycles at 0.1 A/g and (B, C) photograph of 3D-GNS/SiOCf lighting up six light-emitting diodes at different bending angles. Source: Reprinted with permission from Z. Sang, X. Yan, L. Wen, D. Su, Z. Zhao, Y. Liu, et al., A graphene-modified flexible SiOC ceramic cloth for high-performance lithium storage, Energy Storage Mater. 25 (2020) 876
884. https://doi.org/10.1016/j. ensm.2019.11.014.

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Figure 6.11 (A) Electrochemical stability of the fabricated PISE and LLZTO NF reinforced PISE electrolytes. (B) Image of PISE-10% LZONF at different bending states. (C) EIS profiles of PISE-10% LZONF at different bending states. (D) Images of pouch cell formed with PISE-10% LZONF, demonstration of the cell as punch cell, wearable gadget, under burning, cutting, punching and hammering tests. Source: Reprinted with permission from R. Fan, C. Liu, K. He, S. Ho-Sum Cheng, D. Chen, C. Liao, et al., Versatile strategy for realizing flexible room-temperature all-solid-state battery through a synergistic combination of salt affluent PEO and Li6.75La3Zr1.75Ta0.25O12 nanofibers, ACS Appl. Mater. Interfaces 12 (2020) 7222
7231. https://doi.org/10.1021/acsami.9b20104.

oxidation. This is attributed to the fact that the combination of lithium salt and LLZTO NFs immobilizes the anion clusters and provides a continuous path for the Li-ion rapid movement. Subsequently, a very high ionic conductivity with compatible and stable interface with Li electrode at ambient temperature. To understand the flexibility of the LLZTO NF reinforced PISE in the EIS profile, a bending test on Al/ LLZTO NF reinforced PISE/Al model was performed as depicted in Fig. 6.11B. It revealed a higher ionic conductivity in LLZTO NF reinforced PISE at various bending states as portrayed in Fig. 6.11C. Thus, the incorporation of LLZTO NF in the PEO matrix not only enhanced the ionic conductivity but also facilitated superior flexibility. At ambient temperature, the developed electrolyte has an outstanding ionic conductivity of 2.13 3 104 S/cm, a high Li1 transference number (B0.57), and electrochemical stability (B4.9 V). In fact, the built LiFePO4/PISE-10% LLZTO NF/Li all-solid-state batteries revealed a superior initial capacity of approximately 125.8 mAh/g and a retention capacity of 94.9% even after 60 cycles. More importantly, the PISE-10% LLZTO NF-based solid-state battery lighted LED lamps after bending multiple times as shown in Fig. 6.11D [63].

6.2.6 Current
voltage characteristics The knowledge of current
voltage (I
V) characteristics of ceramic films is crucial to understand the transport phenomena and its origin to implement them for the fabrication of the devices. The transport phenomena in ceramic films are mainly governed by

Electrical properties of flexible ceramics

93

space-charge-limited conduction (SCLC), Schottky, and Poole
Frenkel (PF) conduction mechanisms [59] and are briefly discussed as follows.

6.2.6.1 Space-charge-limited conduction mechanism For the SCLC conduction mechanism, the I
V curve exhibits a relation in the form I α Vm. The parameter m describes the behavior of the conduction mechanism. If m 5 1, the mechanism is completely governed by Ohm’s law and usually occurs at the low voltage region, while m 5 2 represents Child’s law region and transpires at a higher field. In the SCLC mechanism, the current is determined by the mobility of the charge carriers. Therefore a current
voltage measurement can be used to determine charge mobility.

6.2.6.2 Schottky and Poole
Frenkel conduction mechanisms In the Schottky conduction mechanism, the electrons gain sufficient energy from thermal activation and, cross the energy barrier at the metal
dielectric interface. The lowering of energy barrier height at the metal
dielectric interface due to image force is called the Schottky effect and the electron emission from the metal to the dielectric is called Schottky emission or thermionic emission. Thus, metal
dielectric interface governs the conduction mechanism. If the current is governed by Schottky emission, then the I
V relation is expressed as [64] ffi3 2qffiffiffiffiffiffiffiffiffiffiffi e3 4πε0 εr d pffiffiffi 5 v LnðIÞα4 kT

(6.4)

where I is the current, e is the electronic charge, ε0 is the electric permittivity of free space, εr is the permittivity of film, d is the film thickness, k is the Boltzmann’s constant, T is the room temperature, and V is applied voltage. The PF emission is analogous to the Schottky emission, in which thermally excited electrons radiate from traps into the dielectric’s conduction band. As a result, sometimes the PF emission is known as internal Schottky emission. In PF mechanism, the electrons tunnel through the trap states in the dielectric layer during PF emission, which is a bulk restricted process. The probability of tunneling is increased by an applied electric field. The field-enhanced thermal emission of trapped electrons from defect states into the conduction band is described widely by PF conduction model [59,64,65]. If the current is governed by the PF mechanism then the I
V relation can be expressed as 2qffiffiffiffiffiffiffiffiffiffi 3   e3 I πε0 εr d pffiffiffi 5 v Ln α4 V rkT

(6.5)

where r is a constant having values from 1 to 2, depending on the Fermi level’s exact position. The conduction process is known as the regular PF mechanism for r 5 1 and the trap-modified PF effect for r 5 2.

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Advanced Flexible Ceramics

Recently, Zhang et al. studied the current
voltage properties of flexible Ag/ZnObased Schottky diodes on polyimide substrate at a bending curvature of 30 mm radius at room temperature [60]. Based on the thermionic emission theory, the device gives an ideality factor (n) of 2.8 and an effective barrier height (φb) of 0.5 eV at zero bias under a flat state. During the bending state, the device showed an increase in the n value toward 3 and a decrease in the φb value toward 0.42 eV. One reason for the increase in the n value is the tunneling process. The lack of uniformity in the interfacial charge distribution, uneven thickness of the film, interface states, and series resistance also contribute to it. On the other hand, the variation in φb depends on a few factors such as the image force, generation and recombination currents in the space charge zone, a thin depletion region in the films, and a higher internal electric field on the application of voltage. Gao et al. suggested that the I
V characteristics were governed by SCLC in a flexible Ba0.67Sr0.33TiO3 (BST) film deposited on a mica substrate [14]. Fig. 6.12A displays the I
V characteristics of BST film for the flat and bent states at bending radius of 2 mm in a log-log scale (labeled as test 1) and Fig. 12B shows that in the flat state and after 20,000 bending cycles at 5 mm radius (labeled as test 2). These plots describe three linear regions with different slopes as shown. Region 1 represents a typical ohmic conduction behavior at | V| , B0.4 V with a slope of approximately 1. Further, the electron density (ne) of the capacitor under test 1 is increased to 2.2 3 1015 cm23, and ne of the capacitor under test 2 is increased to 2.1 3 1015 cm23 as compared to their initial states (5.6 3 1014 cm23 and 1.1 3 1014 cm23, respectively). It is due to the microcracks providing more free electrons. Further, I
V in Region 2 (B0.4 V , |V| , B2 V)

Figure 6.12 Log I
log V plots of flexible BST capacitor (A) at the flat state and bend state of bending radius 2 mm (B) after 20,000 bending cycles at 5 mm radius. Source: Reprinted with permission from D. Gao, Z. Tan, Z. Fan, M. Guo, Z. Hou, D. Chen, et al. All-inorganic flexible Ba0.67Sr0.33TiO3 thin films with excellent dielectric properties over a wide range of frequencies, ACS Appl. Mater. Interfaces 11 (2019) 27088
27097. https://doi.org/10.1021/acsami.9b08712.

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95

follows a modified child’s law. I
V in Region 3 (B2 V , |V| , 3 V) shows a slope exceeding 2, indicating a trap-filled SCLC mechanism. With the help of trap-filled limit voltage, the trap density (Nt) can be calculated. The value of Nt grows from 3.9 3 1017 cm23 to 4.4 3 1017 cm23 and from 4.0 3 1017 cm23 to 4.4 3 1017 cm23 for the capacitor under test 1 and test 2, respectively. After bending, the microcracks are formed and the dangling bonds in microcracks can act as a rich source of trap states and hence the Nt is increased [14].

6.3

Electrical properties-based applications of flexible ceramic films

6.3.1 Energy storage devices To meet our energy needs, it is essential to store electrical energy, the devices like batteries, capacitors, etc. are considered energy storage devices [66,67]. Recently, dielectric-based capacitors are vastly explored for energy storage due to their fast charging and discharging times, low energy loss, immense power density, and easy maintenance [67]. The criteria that govern the energy storage performance of a dielectric capacitor are energy storage density (W) and storage efficiency (η). Generally, the dielectric capacitor’s energy storage density can be accessed via two methods; dynamic and static methods.

6.3.1.1 Dynamic method In the dynamic mode, the parameters that influence the energy storage performance are dielectric permittivity (Ɛr ), polarization (P), and applied electric field (E). Generally, the energy storage density of a linear dielectric capacitor with negligible energy dissipation can be determined using the following equation [67] Energy storage density; W 5

1 Ɛ0 Ɛr E2 2

(6.6)

where Ɛ0 is the permittivity of the vacuum. On the other hand, nonlinear dielectric materials like ferroelectrics (FEs), antiferroelectrics (AFEs) and relaxor ferroelectrics (RFEs) experience certain energy loss as hysteresis loss during depolarization as illustrated in Fig. 6.13A. The recoverable energy density (Wr) can be calculated from the P
E loops using the expression [35], W5

ð Ps E

dPðchargingÞ

(6.7)

E

dPðdischargingÞ

(6.8)

0

Wr 5

ð Ps Pr

96

Advanced Flexible Ceramics

Figure 6.13 (A) Schematic diagram showing the evaluation of energy storage properties. (B) Circuit for measuring energy density by a static method.

The energy storage efficiency (η) of FE, AFE, and RFE materials is a function of Wr and the energy loss (Wl) which is governed by the expression Efficiency; η 5

Wr Wr 1 Wl

(6.9)

6.3.1.2 Static method In the static method, the energy storage density is calculated using a circuit consisting of load resistance (RL), a high vacuum switch, and an external bias as shown in Fig. 6.13B. Here, the sample capacitor is used to determine the energy storage density. The high vacuum switch completes the circuit by switching it on and off. The sample capacitor is initially charged using the external bias. Afterward, a portion of the energy is discharged through the load resistance RL. Consequently, a short-lived current develops in the circuit. Using this time-dependent current, the Wr can be calculated using the formula given as follows: ð Wr 5

I 2 RL dt

(6.10)

where the load resistance and discharge time are represented by RL and t, respectively [67]. However, flexible energy storage devices or power sources are inevitable for the operation of flexible electronic devices. Recent developments in energy storage technology grabbed the interest toward flexible energy storage capacitors to fulfill the requirements such as their miniaturization and integration into pulsed power systems, hybrid electric vehicles, defense equipment, etc. This indeed transformed the existing bulk ceramic-based capacitors into flexible ceramic-based thin film capacitors. In 2018, Lee et al. fabricated a flexible ceramic thin film-based energy storage dielectric capacitor with La-doped PbZrO3 (PLZO) as the dielectric layer and

Electrical properties of flexible ceramics

97

nickel
chromium (Ni
Cr) based stainless steel as the substrate using the chemical solution method [68]. The Ni
Cr stainless steel substrate favors large compressive strain and is attributed to the high thermal expansion coefficient (18.5 3 1026/K) compared to PLZO films (9.1 3 1026/K), consequently, an enhancement in the spontaneous polarization. Fig. 6.14A shows the P
E loops of PLZO capacitor at a flat state and after 1000 bending cycles at a radius of 2.5 mm. The energy storage performance of the PLZO thin film capacitor with respect to the bending cycles is shown in Fig. 6.14B. No observable changes in both plots indicate appreciable flexibility of the PLZO capacitor. Later, Shen et al. reported another Pb-based flexible thick film ceramic capacitor based on Pb0.91La0.09 (Zr0.65Ti0.35)0.9775O3 (PLZT 9/ 65/35) on LaNiO3/F-mica substrate for energy storage properties studies [69]. The thick film capacitor shows a desirable Wr (B40.2 J/cm3) and η (B62%) at

Figure 6.14 (A) P
E hysteresis loops before and after 1000 bending cycles at 2.5 mm radius (inset: photograph of flexible PLZO). (B) Energy storage performance of the PLZO capacitor as a function of bending cycles. (C) W, Wr, and η at different tensile and compressive bending radii. (D) Variation of Wr and η with increasing bending cycles at 4.5 mm bending radius. Source: Parts (A and B) reprinted with permission from H.J. Lee, S.S. Won, K.H. Cho, C.K. Han, N. Mostovych, A.I. Kingon, et al., Flexible high energy density capacitors using Ladoped PbZrO3 anti-ferroelectric thin films, Appl. Phys. Lett. 112 (2018) 092901. https://doi. org/10.1063/1.5018003. Parts (C and D) from Y. Zhang, Y. Li, X. Hao, H. Jiang, J. Zhai, Flexible antiferroelectric thick film deposited on nickel foils for high energy-storage capacitor, J. Am. Ceram. Soc. 102 (2019) 6107
6114. https://doi.org/10.1111/jace.16496.

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1998 kV/cm. The bending test is carried out on Au/PLZT/LNO/F-mica capacitor under different bending states (at bending radii 10, 7.5, and 5 mm) as shown in Fig. 6.14C. Bending at various bending radii resulted in only a small change in the energy storage performance of PLZT 9/65/35 thick film compared to the flat state. Further, the endurance test at a bending radius of 4.5 mm given in Fig. 6.14D exhibited almost stable Wr and η up to 2000 cycles. In the same year, Zang et al. introduced a thick film-based flexible energy storage dielectric capacitor using Pb0.94La0.04Zr0.97Ti0.03O3 (PLZT) forged on Ni foil by sol
gel treatment. The device reported Wr of B15.8 J/cm3 at 1400 kV/cm with a discharge time of 250 ns [70]. Yang et al. in 2019 achieved a Wr of 81.9 J/cm3 and η of 64.4% in a Mn-doped 0.97(0.93Na0.5Bi0.5TiO3 2 0.07BaTiO3) 2 0.03BiFeO3 (NBT-BT-BFO) film capacitor [46]. The NBT-BT-BFO was built on a mica substrate using a chemical solution method followed by a rapid annealing technique. The top and bottom electrodes were Au and Pt, respectively. The energy storage characteristics remained almost unchanged under compressive and tensile bending cycles of 103 at a bending radius of 4 mm and as shown in Fig. 6.15A. Further, the stability of the film at flattenedbent-reflattened states at the same bending radius was analyzed and there was no variation up to 106 cycles. However, Wr and Wl decreased by 1% and 3%, respectively, for further increases in the switching cycles which can be observed in Fig. 6.15B. As the number of bending cycles was increased from 106 to 109, the energy storage efficiency was fatigue-free with an η value of approximately 65 %. Liang et al. in 2019 created a Pt/BaZr0.35Ti0.65O3 (BZT)/ITO/F-mica capacitor by pulsed lased deposition (PLD) and radio-frequency magnetron sputtering technique [71]. The mechanical flexibility was studied using homemade semicircular-shaped molds which are shown in Fig. 6.15C. Besides, the energy storage performance of the device with various bending radii was noteworthy and is shown in Fig. 6.15D. The Pt/BZT/ITO/F-mica capacitor achieved a Wr of 40.6 J/cm3 and η of 68.9 % at a 4-mm bending radius and the energy storage performance was stable up to 104 bending cycles. Very recently, in 2021 Sun et al. developed the Mn-doped Bi0.5Na0.5TiO3
BiNi0.5Zr0.5O3 (BNT
BNZ) capacitor on flexible nickel (Ni) foil adopting the sol
gel method [56]. Ni foil is viable to attain any form similar to paper and is ductile compared to mica substrate. In this work, LaNiO3 and Au are the bottom electrode and top electrode, respectively. The flexible ceramic capacitor achieved a Wr approximately 60.4 J/cm3 with η of 63.2% at 800 kV/cm. Fig. 6.16A
D shows the process of mechanical deformation and the corresponding performance of the device up to 104 bending cycles. No noticeable changes were observed in the Wr and η, suggesting a steady operation of the capacitor at different bending states which is evident from Fig. 6.16D. Further, the flexible ceramic capacitor exhibited almost stable energy storage performance compared to the flat state when subjected to different shapes as demonstrated in Fig. 6.16E
I, indicating the superior ductility of the device. Most importantly, the flexible capacitor remained unaffected without changing its energy storage properties and flexibility over a period of 1 year in the presence of air. An additional list of a variety of flexible energy storage devices and their properties are given in Table 6.5.

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Figure 6.15 (A) Energy storage characteristics of Mn-doped NBT-BT-BFO at 2 mm radius. (B) Variation of Wr, Wl, and η as a function of the switching cycles. (C) Homemade semicircular shaped molds to study the mechanical flexibility of Pt/BZT/ITO/F-mica film. (D) Energy storage performance of BZT capacitor with bending radii. Source: Parts (A and B) reprinted with permission from C. Yang, P. Lv, J. Qian, Y. Han, J. Ouyang, X. Lin, et al., Fatigue-free and bending-endurable flexible Mn-doped Na0.5Bi0.5TiO3-aTiO3-BiFeO3 film capacitor with an ultrahigh energy storage performance, Adv. Energy Mater. 9 (2019) 1803949, https://doi.org/10.1002/aenm.201803949. Parts (C and D) from Z. Liang, M. Liu, L. Shen, L. Lu, C. Ma, X. Lu, et al., All-inorganic flexible embedded thin-film capacitors for dielectric energy storage with high performance, ACS Appl. Mater. Interfaces 11 (2019) 5247
5255. https://doi.org/10.1021/acsami.8b18429.

Supercapacitors are another class of energy storage devices that store electrical energy using chemical reactions. Usually, these devices are made of porous carbon electrodes with a large surface area and the electrolyte used will be ionically conductive. Flexible electrode material in a supercapacitor is a hot topic in the scientific world to design flexible, stretchable, and lightweight supercapacitors with promising electrochemical characteristics. In the year of 2020, Cao et al. chose A2B2O7 type lanthanum titanate (LTO) as the electrode material for flexible supercapacitor fabrication [74]. This is due to the fact that LTO possesses an oxygen vacancy structure, which acts as the charge storage site and thereby, increases the capacitance. The flexible self-supporting LTO film was prepared using a combined technique involving electrospinning and calcination process. The LTO film was

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Figure 6.16 (A) Images of the flat and bend BNT-BNZ film, (B) P
E loops of BNT-BNZ film capacitor with different bending states. Variation of (C) Pr, Pmax, and Pmax-Pr and (D) Wr and η with bending cycle. P
E curves of BNT-BNZ film folded into (E) triangle, (F) square, (G) wave, (H) spiral shapes (inset: images of the corresponding folded states), (I) Wr and η of BNT-BNZ film at different folded states. Source: Reprinted with permission from N. Sun, Y. Li, X. Hao, High energy-storage allinorganic Mn-doped Bi0.5Na0.5TiO3-BiNi0.5Zr0.5O3 film capacitor with characteristics of flexibility and plasticity, J. Alloy. Compd. 879 (2021) 160506. https://doi.org/10.1016/j. jallcom.2021.160506.

Table 6.5 Comparison of energy storage parameters of different flexible devices. Material

Substrate

Wr (J/cm3)

η (%)

References

PLZO BZT Mn:BNT-BT-BFO BFMO-SBT BNT-ET BNT-BNZ

LNO/Ni-Cr ITO/F-mica Pt/F-mica Pt/F-mica Pt/F-mica LNO/Ni

15.2 65.1 81.9 61 65.4 60.4

63.2 72.9 64 68 52 63.2

[68] [72] [72] [46] [73] [56]

connected to a carbon fabric, which acts as the working electrode. Then, the appropriate electrolyte for the supercapacitor studies was selected from one molar (1 M) H2SO4, Na2SO4, and KOH solutions using EIS. The LTO supercapacitor exhibited optimum potential range in 1 M Na2SO4 aqueous electrolyte, when compared to the other two electrolytes which can be concluded from Fig. 6.17A. Further, the galvanostatic charge
discharge (GCD) characteristics of the LTO electrode suggested that the areal capacitance of 1 M Na2SO4 is very much higher compared to 1 M H2SO4 and 1 M KOH solutions. Thus, a two-electrode system

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Figure 6.17 (A) Cyclic voltammograms of LTO electrodes in 1 M aqueous H2SO4, aqueous Na2SO4, and aqueous KOH solutions. (B) Cyclic voltammograms of LTO//LTO device at sweep rates of 5
100 mV/s. (C) Cycling performance of LTO//LTO device at current density 25 mA/cm2. (D) Galvanostatic charge-discharge characteristic of LTO//LTO device at the 1th, 1000th, and 10,000th cycle. (E) Digital photographs, (F) areal capacitance, and (G) cyclic voltammograms of the LTO//LTO device bent at angles 0
180 degrees. (H) Two serially connected LTO//LTO devices bent at 180 degrees illuminating a red LED. Source: Reprinted with permission from Y. Cao, P. Tang, Y. Han, W. Qiu., Synthesis of La2Ti2O7 flexible self-supporting film and its application in flexible energy storage device, J. Alloy. Compd. 842 (2020)155581. https://doi.org/10.1016/j.jallcom.2020.155581.

consisting of two symmetrical LTO electrodes, a semipermeable membrane, and Na2SO4 as the electrolyte (LTO//LTO) was used to test the LTO//LTO flexible supercapacitor device. Fig. 6.17B shows the C
V curve of the LTO//LTO flexible supercapacitor studied using cyclic voltammetry under different scan rates. The C
V curves retained almost the same rectangular shape indicating stable chargedischarge performance with a very high voltage approximately 1.2 V. Besides, the areal capacitance measured using GCD studies revealed a maximum capacitance approximately 84.5 mF/cm2 at a current density of 25 mA/cm2. Further, the LTO// LTO flexible supercapacitor displayed an endurance of 103 cycles at 25 mA/cm2 and the GCD curve retained a similar shape after 104 cycles as presented in Fig. 6.17C and D. More importantly, the LTO//LTO flexible supercapacitor achieved a higher energy density of 19.2 Wh/kg and power density of 9.7 kW/kg. In addition, the electrochemical properties with respect to the flexibility of LTO// LTO flexible supercapacitor are determined at bending angles 0
180 degrees as reported in Fig. 6.12E. At different bending angles, the C
V (at 50 mV/s) curves exhibited negligible shape change indicating the potential of LTO//LTO flexible

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ceramics as promising supercapacitor devices as represented in Fig. 6.17F. Even though the areal capacitance decreased with increased bending angle, LTO//LTO flexible supercapacitor almost retained 90% areal capacitance at 180 degrees bending angle as illustrated in Fig. 6.17G. In fact, two LTO//LTO flexible supercapacitors bent at 180 degrees serially connected powered an LED light as shown in Fig. 6.17H. Thus, LTO//LTO flexible supercapacitors are ideal for flexible device fabrication owing to their excellent mechanical flexibility and bending strength [74]. Zhao et al. reported a flexible supercapacitor comprising PVA-KCl-BTO as the electrolyte and Co
Fe2O3 loaded activated carbon cloth (Co
Fe2O3@ACC) as the electrodes [75]. The electrodes were fabricated using a facile hydrothermal method on ACC, while the electrolyte is synthesized using PVA-KCl aqueous solution and then mixed with polarized BTO under constant stirring. The electrochemical, as well as the self-charging capabilities of the flexible supercapacitor are evaluated with bending angles which is shown in Fig. 6.18A and B. When performed at

Figure 6.18 Electro chemical and self-charging behavior of the PVA-KCl-BTO-based selfcharging supercapacitor (SCSC) at bending angles 0 2 180 degrees. (A) C
V curves, (B) GCD profile, (C) aerial capacitance versus current density, (D) self-charging curve at 60 degrees and 1 Hz, (E) self-charging curve at 90 degrees and 1 Hz (f) self-charging curve at 180 degrees and 1 Hz (inset: SCSC illuminating an LED). Source: Reprinted with permission from D. Zhou, F. Wang, J. Yang, L. Fan, Flexible solidstate self-charging supercapacitor based on symmetric electrodes and piezo-electrolyte, Chem. Eng. J. 406 (2021) 126825. https://doi.org/10.1016/j.cej.2020.126825.

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different bending angles such as 60, 90, and 180 degrees, the device exhibited equivalent C
V and GCD properties compared to the normal state which is evident from Fig. 6.18C. This in turn reveals an enhanced mechanical and electrochemical performance of the supercapacitor. Evidently, the self-charging voltage increased from 0 to 92 mV, 15 to 100 mV, and 6 to 120 mV at 60, 90, and 180 degrees, respectively, by repeated bending for 7 min at 1 Hz. Which is displayed in Fig. 6.18D
F. Thus, this work can be a solution to the charging problem associated with supercapacitors and a pathway for self-powered flexibility [75].

6.3.2 Energy harvesting 6.3.2.1 Piezoelectric nanogenerators For more than five decades, lead-based materials have been ruling the electronic industry due to their excellent piezoelectricity. Wang et al. achieved a giant piezoelectric coefficient (d33 5 1753 pm/V) in 0.55Pb(Ni1/3Nb2/3)O3 2 0.135PbZrO3 2 0.315PbTiO3 (PNN-PZT) ceramics. Yet, the brittle nature of PNN-PZT ceramics restrained them from flexible electronic applications such as energy harvesters [76]. Gupta et al. in 2019, introduced flexible piezoelectric composite film comprising niobium-doped PZT nanofillers and P(VDF-TrFE) matrix (NPZT/P(VDF-TrFE) composites by spin coating [27]. The piezoelectric coefficient (d33) of the composite film was determined by applying a triangular AC voltage and the nonswitching part of the butterfly-shaped loop is utilized. It is determined that the d33 for NPZT/P(VDF-TrFE) is 101 pC/N, which is almost 125% higher than pristine P(VDF-TrFE) films (45 pC/N). Fig. 6.19A illustrates the output performance of the device under mechanical bending. The increased bending angle caused a rise in voltage and is ascribed to the direct relation between the bending stress and the output voltage. The improved piezoelectricity is attributed to the high crystallinity in the nanocomposite film due to enhanced molecular order arising from the long-range dipole-dipole interactions. Later on, He et al. fabricated a super flexible PZT piezoelectric composite film on a glass fiber fabric (GFF) substrate via a simple dipping method [77]. The composite films exhibited a d33 value in the range 289
460.7 pC/N and are comparable with morphotropic phase boundary (MPB) owned PZT ceramics (160
600 pC/N). The flexible device output performance was tested using both bending and pressing modes. The bending/flattening motion in PZT-GFF was executed using a custom-designed clamp with a stepper motor as shown in Fig. 6.19B1. Using the motor displacement, a bending angle variation from nearly 62 to 163.6 degrees is created in the central region of PZT-GFF composite and the strain variation is equivalent to 0.68% to 1.66%, respectively, as reflected Fig. 6.19B2. Consequently, the upper portion suffers a tensile strain, while the lower part experiences a compressive strain. As the strain increases, the output voltage as well as output current increase from 1.7 to 4.0 V, and 35 to 120 nA, respectively. Similarly, the output voltage and the output current increased as a function of increased loads as given in Fig. 6.19B3. The device generated a piezoelectric voltage of approximately 30 V and a current of approximately 410 nA at 10 kg load. In 2020 Wang et al. fabricated PNNPZT ceramic polymer composite using the method of direct 3D printing. At first, a

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Figure 6.19 (A) The output performance of the NPZT/P(VDF-TrFE) device with increases in bending angle. (B1) Photo of the test setup of the stress distribution, (B2) the output current and voltage as a function of strain and bending angle (inset: when strain 5 1.66%), and (B3) as a function of increasing load (inset: when load 5 10 kg). (C1) Photographs of printed Ag/PNN-PZT polymer composite, (C2) drop-weight impact test, (C3) the output voltage for different drop weight impact, and (C4) the pictorial representation of cyclic impact force on Ag/PNN-PZT polymer composite. Source: Part (A) reprinted with permission from S. Gupta, R. Bhunia, B. Fatma, D. Maurya, D. Singh, Prateek, et al., Multifunctional and flexible polymeric nanocomposite films with improved ferroelectric and piezoelectric properties for energy generation devices, ACS Appl. Energy Mater. 2 (2019) 6364
6374. https://doi.org/10.1021/acsaem.9b01000. Parts (B1 2 B3) from S. He, W. Dong, Y. Guo, L. Guan, H. Xiao, H. Liu, Piezoelectric thin film on glass fiber fabric with structural hierarchy: an approach to high-performance, superflexible, cost-effective, and large-scale nanogenerators, Nano Energy 59 (2019) 745
753. https://doi.org/10.1016/j.nanoen.2019.03.025. Parts (C1 2 C4) from Z. Wang, X. Yuan, J. Yang, Y. Huan, X. Gao, Z. Li, et al., 3D-printed flexible, Ag-coated PNN-PZT ceramic-polymer grid-composite for electromechanical energy conversion, Nano Energy 73 (2020) 104737. https://doi.org/10.1016/j.nanoen.2020.104737.

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polymer ink was formed using polydimethylsiloxane (PDMS) [76]. Then, silver (Ag) coated PNN-PZT ceramic powder and multi-walled carbon nanotubes (MW-CNTs) were subsequently mixed with the PDMS ink. The mixture consisting of PDMS, Ag/ PNN-PZT, and MW-CNTs were vigorously stirred and centrifuged to remove the air bubbles. Afterward, the ink was transferred to a double nozzle 3D printer and the ceramic polymer composite ink was printed onto a glass slide as reflected in Fig. 6.19C1. Finally, the specimen is subjected to vacuum drying and then thermal curing to obtain the flexible ceramics. The elasticity of the ceramic
polymer composite and PDMS was tested using a compression test. It is observed that the PDMS exhibited a compressive strength of 25 Mpa, while it was only 12.8 Mpa for ceramic-polymer composite at 80% strain. However, both the samples recovered quickly to their original form after the removal of the external loads. Besides, the low compressive strength in the polymer
ceramic composite is attributed to the micro-pores in the ceramic domain which makes the PDMS matrix more stressful under external load. The ceramic polymer composite was subjected to a force-induced d33 test. The polarized Ag/PNN-PZT is exposed to a periodic impact force of 1 N and the corresponding piezoelectric coefficient is measured. The ceramic polymer composite attained a d33 value equal to 58 pC/ N. Such a low piezoelectric value in the ceramic polymer composite is attributed to the dominating effect of the dielectric PDMS matrix. Even though the ceramic polymer composite has a low d33 value, its piezoelectric stress constant (g33) is very high and is given by g33 5

d33 ε33

(6.11)

where ε33 is the dielectric constant and the subscript “33” suggests the applied force and the induced voltage in all three directions. The measured value of g33 is found to be 400 mVm/N, which is large compared to that of PNN-PZT ceramics. Further, the Ag-coated ceramic polymer composite generated a high peak-peak output voltage of approximately 550 mV. The large electromechanical response in the devices arises from the effortless flow of free charges provided by the Ag/PNN-PZT heterojunction. Moreover, the ceramic-polymer film is sensitive to small deformations (from 20 to 100 g) as shown in Fig. 6.19C2 suggesting a linear behavior of the piezoelectricity as a function of weight (Fig. 6.19C3). This points to the potential applications of this device in the biomedical field. Most importantly, the high g33 valued ceramic-polymer composite is tested as a piezoelectric nanogenerator for energy harvesting applications. Wang et al. used a hammer to give a cyclic impact force on the 3D printed piezoelectric ceramic polymer composite to generate piezoelectric current as shown in Fig. 6.19C4. The maximum peak-peak open-circuit output voltage generated by the hammer’s cyclic impact was around 14 V. The piezoelectric ceramic polymer composite PNG was used in a rectifier bridge to drive twenty commercial red LEDs. The excellent flexibility and stretching capability of Ag/PNN-PZT polymer composite make them a good substitute for brittle piezoceramics in wearable electronics and energy harvesting applications. Recently, Pb-free BaTiO3 (BTO) is a highly studied piezoelectric material owing to its

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outstanding mechanical properties, inherent piezoelectricity, and low cost. In 2020 Yang et al. fabricated a flexible piezoelectric BTO-based film for pressure sensing applications [78]. In this work, polyamide (PDA) was used as a surface modifier on BTO before being combined with a PVDF matrix to get a PDA@BTO/PVDF composite film. This is attributed to the fact that the introduction of PDA reduces the cracks between the BTO and PVDF matrix. A facial solution-casting technique was adopted to prepare PDA@BTO/PVDF composite film. The piezoelectric output voltage of the composite film is evaluated by applying an impact force varying from 12 to 243 N. Fig. 6.20A1 shows a linear increase in the output voltage with increased external impact force. To further understand the output voltage of the flexible film, PDA@BTO/PVDF composite film was subjected to different bending angles as shown in Fig. 6.20A2. The device generated output voltages of 0.176, 0.34, 0.539, and 0.49 V at bending angles 36, 46, 55, and 63 degrees, respectively. The high piezoelectric signals in PDA@BTO/PVDF composite film arise from the reduced interface defects as PDA modification suppresses the accumulated trap charges via decreasing the cracks. Consequently, a very high output voltage is generated [78]. Zhou et al. studied the piezoelectric properties of a composite film composed of carbon-encapsulated BTO nanoparticle (core
shell structure) fillers and (P(VDFTrFE)) matrix (BTO@C/P(VDF-TrFE)) for energy harvester applications [79]. The effect of the bending angle on the output voltage of BTO@C/P(VDF-TrFE) is shown in Fig. 6.20B1 and B2. As the bending angle is increased from 23.6 to 76.5 degrees, the output voltage continuously increases up to 60 degrees suggesting a linear relationship. However, the voltage is found to be decreased after 60 degrees indicating that the BTO@C/P(VDF-TrFE) is unable to absorb enough mechanical energy at larger bending angles. Further, Zhau et al. observed that the effective strain remained unchanged after the bending angle 60 degrees. In addition, the BTO@C/P(VDF-TrFE) was tested 1000 times in the bending-flattening modes and generated a stable operating voltage of 15 6 1.5 V. This reveals the flexibility and stability of the BTO composite film. Very recently, another core
shell structured BTO-based piezoelectric composite film was reported by Shi et al. The poly(methyl methacrylate) (PMMA) modulated BTO nanofillers were dispersed in P(VDFTrFE) matrix (PMMA@BTO/P(VDF-TrFE)) and electrospun to fabricate piezoelectric nanocomposite film. The PMMA@BTO/P(VDF-TrFE) was subjected to an arm bending angle test by varying the bending angle from 0 to 150 degrees using human motion as shown in Fig. 6.20C1. The increase in biomechanical energy due to arm bending enhanced the piezoelectric output voltage that can be useful to drive smart wearable electronic devices as represented in Fig. 6.20C2 [80]. However, the BTO solid solution doped with calcium and zirconium (BCZT) is excellent piezoelectric material having comparable piezoelectric coefficients with PZT materials. Playing with the Ca21/Ba21 and Zr41/Ti21 ratios in the A and B-sites, respectively, provides exceptional piezoelectric coefficient due to the formation of MPB. The MPB diminishes the polarization anisotropy energy and thereby, allows the domain wall motion easier w.r.t the external force. In 2021 Lu et al. fabricated a piezoelectric composite film using BCZT nanofillers and silane rubber via the solution casting

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Figure 6.20 The output voltages of PDA@BTO/PVDF (A1) with increasing contents of PDA@BTO and (A2) with different bending angles Output voltages of BT@C/P(VDF-TrFE) PENGs. (B1) with different mass fractions at bending angle of 60 degrees and (B2) at different bending angles (inset: schematic diagram of the bending). (C1) Schematic diagram of human body with BT@C/P(VDF-TrFE) PENGs and (C2) the corresponding output signals of arm bending. Source: Parts (A1, A2) reprinted with permission from Y. Yang, H. Pan, G. Xie, Y. Jiang, C. Chen, Y. Su, et al., Flexible piezoelectric pressure sensor based on polydopaminemodified BaTiO3/PVDF composite film for human motion monitoring, Sens. Actuators Phys. 301 (2020) 111789. https://doi.org/10.1016/j.sna.2019.111789. Parts (B1, B2) from Z. Zhou, Z. Zhang, Q. Zhang, H. Yang, Y. Zhu, Y. Wang, et al., Controllable core
shell BaTiO3 @carbon nanoparticle-enabled P(VDF-TrFE) composites: a cost-effective approach to highperformance piezoelectric nanogenerators, ACS Appl. Mater. Interfaces 12 (2020) 1567
1576. https://doi.org/10.1021/acsami.9b18780. Parts (C1, C2) from K. Shi, B. Chai, H. Zou, P. Shen, B. Sun, P. Jiang, et al., Interface induced performance enhancement in flexible BaTiO3/PVDF-TrFE based piezoelectric nanogenerators, Nano Energy 80 (2021) 105515. https://doi.org/10.1016/j.nanoen.2020.105515.

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method [81]. The piezoelectric voltage generated by the composite film was tested using human motion such as walking and finger bending. The biomechanical energy resulting from walking delivered an output voltage of 7.4 V, while the finger bending produced an output voltage equal to 9.9 V. Liu et al. reported a BCZT-based flexible nanogenerator with d33 approximately 150 pC/N. Here, the BCZT is sputtered onto a mica sheet and this was attached to a thick polyethylene terephthalate (PET) sheet [28]. Finally, it is covered with polydimethylsiloxane (PDMS) for protection from humidity and corrosion. Afterward, the BCZT-based thin film is studied under different bending angles ranging from 30 to 90 degrees. The releasing from different bending angles generated the output voltages of 2.5, 4.9, and 6.7 V for 30, 60, and 90 degrees, respectively. Another work on the piezoelectric composite film was based on the inorganic Pb-free (Na,K) NbO3-based perovskite. The (Na,K)(Nb,Sb)O3
BaZnO3
(Bi,K)ZrO3 (NKNS
BZ
BKZ) nanoparticles were entwined in a P(VDF-TrFE) matrix using electrospinning method. The material was exposed to a cyclic bend test and delivered a constant output voltage of approximately 0.75 V even after 10,000 cycles at a bending radius 15 mm [82]. Thus, piezoelectric ceramics mark their signature in the energy harvesting technology to revolutionize the flexible electronics industry.

6.3.2.2 Pyroelectric nanogenerators Though the idea of pyroelectricity is aged to 314 BC [83], the most modernized version of this idea, that is, the pyroelectric nanogenerators (PENGs) came to light when Zhong Lin Wang and his co-workers developed a self-powered temperature sensor using PENGs [84]. In pyroelectric nanogenerators, nanostructured pyroelectric materials are subjected to a temperature change giving out a spontaneous polarization change and thereby, a potential developed at the ends of the crystal. Flexibility needs to be incorporated for smart pyroelectric devices. Therefore the idea of exploring the mechanical strength of pyroelectric materials along with the temperature change came into the picture. In such cases, both the mechanical and thermal changes (hybrid effect) in a single material are utilized for energy harvesting applications. In other words, the nanogenerators utilizing the primary pyroelectric effect and the hybrid devices utilizing secondary pyroelectric together form a class of energy harvesting devices. Even though the piezoelectric ceramics with high pyroelectric coefficients are active candidates, the brittle nature of these materials forces the researchers to combine them with flexible substrates such as polymers and metal foils to fulfill the need for flexible PENGs. In 2016, a piezoelectric
pyroelectric hybrid device based on PZT film was developed by Ko et al. using Ni
Cr foil as the flexible substrate [85]. Pt and LaNiO3 (LNO) were the top and bottom electrodes, respectively. A high pyroelectric coefficient of approximately 50 nC/cm2K was achieved along with an appreciable remnant polarization of approximately 28 μC/cm2 and a very good piezoelectric coefficient of approximately 140 pC/N. The working of this nanogenerator was explained using different sources, one of which was the human finger touch and the other was the high-temperature and low-temperature winds at

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109

ambient pressure. In the first case, the film was elongated and flattened by touching it under a thermal charge. The elongated film due to bending produced more surface charges and thereby, generated a higher electric polarization in the poling direction. At the same time, the film is cooled using a cold wind (ΔT , 0) and this reduces the thermal fluctuations. As a consequence, the polarization increases. On the other hand, the film in the flattened state resulted in the electron flow in the reverse direction due to reduced polarization arising from the reduced number of surface charges. At this stage, a hot wind (ΔT . 0) is applied which decreases the polarization further and subsequently, an increased electron flow in the reverse direction. The combined mechanical and thermal treatment resulted in a prominent flexible piezo-pyroelectric nanogenerator with a peak output voltage of 0.34 V and a current of 0.34 VμA/cm2. The variation in the velocity of the wind and the rate of thermal change is related linearly to the output performance of the device. It was successfully demonstrated to obtain currents from a crook, wrist, and finger using this device, and the durability and adaptability of the device were successfully tested in harsh conditions such as elevated temperatures (100 C), on an exhaust pipeline in a car, etc. [85]. Recently, another hybrid device that can work on the basis of piezo-pyro-tribophotoelectric effects was fabricated by Zhang et al. [86]. In this device, PZT act as the base layer which is piezo-pyro active with Ag as the bottom electrode. The Ag nanowires/PDMS film with ITO top electrode was the second layer and the triboactive fluorinated ethelene-propelene (FEP) along with nylon (polyamide), which acts as a flexible vibrating film was the top layer. The excellently executed multifunctional device could charge a 10 μF capacitor to 5.1 V in 90 s. Initially, the randomly oscillating dipoles in PZT at room temperature cannot induce an electron flow to the external circuit, and hence no current is obtained. But whenever there is a change in the thermal environment of these dipoles, say, during heating, the equilibrium is disturbed which can initiate an electron flow from the top to the bottom electrode. During cooling, an opposite flow occurs due to negative pyroelectric potential. The PZT layer also has the advantage that it can absorb photons of a particular frequency from visible light to exhibit the photoelectric effect which is utilized in this device. The air-driven vibrations of polyamide transmit a compressive strain to FEP when it is in contact with the polyamide. This subsequently triggers a piezoelectric effect in the PZT layer and electron flow occurs from the bottom to the top electrode. At this stage, the polyamide and FEP are in electrostatic equilibrium. On the other hand, when polyamide and FEP are separated, the triboelectric effect is triggered and the charges on FEP induce charges on ITO thus, an opposite current is obtained. In effect, the multifunctional device could gain a peak current of approximately 5 μA and a peak voltage of approximately 80 V. Chen et al. designed micropatterned single-crystal (1 2 x)Pb(Mg,Nb)O3-xPbTiO3 (PMN-PT) ribbons which work as a flexible hybrid piezoelectric
pyroelectric hybrid generator [87]. It was demonstrated that the device perceives mechanical strain from the movement of human body parts, wind, sound waves, and thermal fluctuations from the warm or cool water and the illumination of light and converts it into electrical responses. The interesting fact about the device is that it can be

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easily laminated to human skin, like a watch on the wrist. The designed device finds great application in self-powered health-monitoring systems. In 2012 Yang et al. investigated lead-free material based on flexible pyroelectric nanogenerators [88]. The basic material was KNbO3 and the flexibility was introduced by mixing the single crystalline nanowires of KNbO3 with PDMS polymer in 3:7 ratio. The use of PDMS enables the composite to survive a bending of approximately 180 degrees. The composite was sandwiched between top Ag and bottom ITO electrodes. The active material is composed of multidomain. When the temperature increases, the dipoles in the material gain energy from the energized lattice thermal vibrations. This can increase the magnitude of the rotation of electric dipoles about their axes. This disorderness can reduce the total average spontaneous polarization which results in the flow of electrons. It was experimentally demonstrated that the fabricated NG can obtain a maximum peak voltage of 10 mV and a peak current of 120 pA. Even under the thermal change induced by sunlight, the device was found to exhibit the peak voltage and current as 2.5 mV and 25 pA, respectively. As a first lead-free candidate, the NG served its purpose very well [89]. Following the trend, Isakov et al. proposed nanofibers based on 1,4-diazabicyclo [2.2.2]-octane perrhenate (dabcoHReO4) that can be used for pyroelectric NG applications [88]. The electrospinning method was used for fabrication with an aqueous solution of dabcoHReO4 crystals and PVA in 1:1 wt.%. The dabcoHReO4 crystals possess NH
N hydrogen bond networks which cause the polarization. A thermal change induces spontaneous polarization in dabcoHReO4 nanofiber array by which the surface layer gets accumulated with free charges. The electric field developed by this can initiate the charge flow. A peak output current of 200 pA and a pyroelectric coefficient of 8.5 μC/m2K was obtained using this nanofibre array. Besides the low pyroelectric coefficient, the main advantage of such structures is that they can be easily adapted for the fabrication of electronic wearables [88]. Zhong Lin Wang and his coworkers developed a flexible hybrid energy cell that can simultaneously harvest thermal, mechanical, and solar energies [90]. The cell is divided into two parts; the bottom portion which works as the nanogenerator consists of a PVDF film sandwiched between two Ag layers, whereas the top portion which works as a solar cell is formed by arranging the ZnO nanowire array and a P3HT film in the middle of the bottom electrode Ag and a transparent top electrode ITO. A very large pyroelectric coefficient of 244 μC/m2K was obtained. An increase in temperature results in a negative output voltage and a compressive strain results in a positive output voltage. The combined result of the hybrid pyroelectric and piezoelectric device is as shown in Fig. 6.21A. Noticeably, this output could drive an LCD directly. Simultaneously, the ZnO nanowire arrays were used to harvest solar energy. With a sequential connection of six cells, an output voltage and current of 2 V and 6 μA, respectively, were achieved. The device could be successfully employed to harvest thermal, mechanical, and solar energies simultaneously and the combined result is given in Fig. 6.21B [90]. The applications of flexible pyroelectricity find its peak at pyroelectric nanogenerators. As the scientific community moves toward green technology, these investigations are becoming more and more relevant and hence are addressed with serious concern.

Electrical properties of flexible ceramics

Hybrid NG s

Pyroelectric N G

Piezoelectric NG

3 Pyroelectric NG

1.5 0.0 -1.5 -3.0 0

100

200

300

400

500

Time (s)

Curremt (nA)

(B)

3.0

60

Voltage (V)

Voltage (V)

(A)

111

Solar Cells

2 Pyroelectric NG + Solar Cells

1

40 20

0

0

0

150

300

450

600

750

900

-20 0

100

200

300

400

500

Time (s)

Time (s)

Figure 6.21 (A) The output voltage and current of ZnO-PVDF film-based hybrid pyroelectric-piezoelectric nanogenerator and the (B) output voltage of ZnO-PVDF film-based hybrid pyroelectric (after rectification) and solar cells. Source: Reprinted with permission from Y. Yang, H. Zhang, G. Zhu, S. Lee, Z-H. Lin, Z.L. Wang, Flexible hybrid energy cell for simultaneously harvesting thermal, mechanical, and solar energies, ACS Nano 7 (2013) 785
790. https://doi.org/10.1021/nn305247x.

6.3.2.3 Sensors In the modernized world, life is made easier day by day with several automatic electronic gadgets, health monitoring systems, and even security systems. Thermal sensors play a key role in many of these automatic systems where human presence is detected from the infrared rays emitted by the human body, fitness is monitored from the rise in temperature of the body during exercise and other biomedical or chemical applications. These devices not only serve its original purpose but also save a great amount of energy. The sensitivity of a pyroelectric sensor can be interpreted in terms figures of merit for high current sensitivity (FI), voltage responsivity (FV), and detectivity (FD) as given by the following expressions: FI 5

pðTÞ C

(6.12)

FV 5

pðTÞ Cε

(6.13)

FD 5

pðTÞ pffiffiffi C ε

(6.14)

where p(T) is the pyroelectric coefficient, C is the specific heat capacity, and ε is the dielectric constant. In 2015 Jayalakshmy et al. synthesized LiNbO3/PVDF (LN/PVDF) polymer
ceramic nanocomposite by using the solvent-cast technique in which different volume fractions of LN were dispersed in PVDF matrix [91]. The pyroelectric

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coefficient, thermal conductivity and specific heat capacity (C) of the samples were analyzed with different amounts LN and an increase in p(T) and thermal conductivity and a decrease in specific heat capacity were observed with increasing amounts of LN in the composite, which is suitable for sensing application. The FI, FV, and FD were calculated and all these parameters were enhanced with increase in LN fraction in the composite. The flexibility of the material is measured in terms of Shore D hardness, a parameter that vary inversely with the flexibility of the materials, specifically, elastomers or plastics. Unfortunately, the flexibility decreases with an increase in LN fraction and it is concluded that the sensitivity can be enhanced at the expense of flexibility. The work reported a pyroelectric sensing material, the sensitivity of which can be tunable with ceramic filler loading [91]. Another way in which the pyroelectric property can be used for sensing applications is with the help of a thermistor, a device having temperature-dependent resistance. The sensitivity can be analyzed by how fast the thermistor responds to the change in the temperature around it. It is measured in terms of a parameter called the time constant, τ. Low values of τ indicate higher sensitivity. Recently, Tomohiko Nakajima and Tetsuo Tsuchiya fabricated a thermistor based on Sr and Ni-doped SmMnO3 (SSMN) grown on a highly flexible polyimide substrate with Ag nanoparticles and nanowire integrated carbon microcone (CMC) array as bottom electrode [92]. The resistance of the device (R) was measured and translated in terms of the monitoring temperature, T using the expression R 5 R0 eðB=TÞ

(6.15)

where R0 is the resistance at infinite temperature and B is the thermal constant of the thermistor. The sensitivity test was conducted by sliding the thermistor into a chamber of 40 C and sliding out of it while monitoring the temperature of the device. An immediate response of a time constant 680 ns was observed. The rapidity in response was sustained for a temperature range from room temperature to 80 C when the temperature was increased and decreased. The in
out movement of the thermistor was conducted at a heat source of 80 C for 1000 cycles and a stable response was reported. Moreover, the same device was employed to sense the temperature variations introduced by UV irradiation. A fast response was recorded with a time constant of 106 ms. The resistance of the SSMN thermistor was studied by bending the device up to an angle 180 degrees and at a bending radius of 360
500 μm and the variations in resistance (ΔR) and temperature (ΔT) were evaluated. Even at an angle of 180 degrees, the ΔT was only 0.3 C which indicated excellent flexibility. Also, the device showed remarkable durability with 0.01 C to 20.1 C deviation in temperature when subjected to 1000 bending cycles at an angle of 60 degrees. It is noteworthy that the device was successfully tested for real-time monitoring of the temperature of the human body by attaching it to the arm during exercise and the result with ΔT within 0.5 C
0.7 C was achieved. The device portrays the best way to utilize the pyroelectric effect for flexible electronics [92].

Electrical properties of flexible ceramics

113

6.3.3 Memory Memory devices or data storage devices are vital components of electronic systems since any equipment that uses a processor needs a memory integrated with it. There are two types of memories; one is volatile, in which the stored data cannot be retained after the removal of the external field whereas the second one is nonvolatile, in which the stored data continue to persist even when the external field is removed. Most types of computer storage devices like internal and external hard drive, optical disks, memory cards in mobile phones, etc. needs long-term persistence of memory. Fast working time, extended service life, low power consumption, low cost and high storage performance are the demands to be addressed by the nonvolatile memories. For these needs, nonvolatile random access memories (RAM) are emerging as the best-suited ones. The innovations of flexible electronics like smart cards, e-skin, wearable health monitoring systems, etc. demand a highperformance nonvolatile memory to be flexible. The two important flexible memories are 1. Resistive random access memory (RRAM), 2. Ferroelectric random access memory (FRAM).

6.3.3.1 Resistive random access memory Recently RRAMs are considered next-generation memory devices due to their unique properties like multi-bit capability, simple device structure, as well as low power consumption. RRAM consists of a dielectric/semiconductor material sandwiched between two metal electrodes in MIM configuration. When a high voltage is applied, a dielectric breakdown occurs and thus conductive filaments (CFs) or a path for charge transport is formed between the top and bottom electrodes. This set the device in the low resistance state (LRS) or “set” state. By altering the voltage to a low voltage of the same polarity (unipolar) or to a voltage of opposite polarity (bipolar), CFs can be ruptured, and consequently the device is in a high resistance state (HRS) or “reset” state. These two states are used to store the binary “0”s and “1”s. Recently researchers explored the effect of flexibility in HRS and LRS states of RRAM for developing flexible memories [93]. Chungwan Gu and his coworkers demonstrated a flexible RRAM using a halide perovskite (HP), CH3NH3PbI3 material deposited on an ITO-coated PET substrate with Au as the top electrode. The device showed an on/off ratio of 104 at flat state [94]. Fig. 6.22A presents the output characteristics of the device at two bending states of tensile and compressive stress with a bending radius 1.5 mm. It is observed that there is a slight difference in the LRS and HRS of the currents in the negative bias region. Moreover, the mechanical flexibility test up to 100 bending cycles also showed a constant switching ratio of 102. It is because of the uniform perovskite layer formed by solvent engineering. The homogeneous perovskite layers with big grains reduce the contact resistance at the electrode-film interface which results in improved electrical properties [94]. Table 6.6 shows a comparison of flexible memory devices of HP materials deposited on PET substrates.

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Advanced Flexible Ceramics

(C)

SET RESET

10-5 10

-6

10

-7

W/O Bending Tensile Bending Compressive Bending

-1.0

-0.5

10-2

4

106

2

105

0

104

-2

103

0.0 0.5 Voltage (V)

(E)

Set Reset

107

0°

1.0

15° 30° Angle (°)

Reset

-5

10

Vreadout = -0.2 V HRS

10-5

20 cycles 40 cycles 60 cycles 80 cycles 100 cycles

-2

Current (A)

10

Current (A)

Current (A)

Set

10-4

-4

10-5

10-7

LRS

10-3

10

Set

-4

10-6

(F)

10-3

Reset

10-3

-4

45°

(D)

Self-compliance

10-2

Current (A)

10

-4

(B)

Icompliance= 1 mA

Threshold Voltage (V)

10

-3

ON/OFF ratio

Current (A)

(A)

-1

0 1 2 Voltage (V)

3

8x10-4-4 ON current 6x10 4x10-4 2x10-4 20 40 8x10-6 OFF current

60

80

100

6x10-6

10-6

4x10-6 10-7 -2

-1

0

1

2

3

0

5

20

10 15 20 25 30 35 40 45 50

Voltage (V)

40 60 80 100 Bending cycles

Cycles

Figure 6.22 (A) I 2 V characteristics of Au/HP/ITO/PET device without and with bending stress. (B) On/off ratio of MAPbI3 based RRAM at different bending angles. (C) I
V characteristics of RRAM (inset: photograph of bent state). (D) HRS and LRS states as a function of bending cycles. (E) I 2 V characteristics. (F) Bending stability of CsPbBr3/ PEDOT:PSS-based RRAM with increasing bending cycles. Source: Part (A) reprinted with permission from C. Gu, J-S. Lee, Flexible hybrid organic
inorganic perovskite memory, ACS Nano 10 (2016) 5413
5418. https://doi.org/ 10.1021/acsnano.6b01643. Part (B) from P. Shu, X. Cao, Y. Du, J. Zhou, J. Zhou, S. Xu, et al., Resistive switching performance of fibrous crosspoint memories based on an organic
inorganic halide perovskite, J. Mater. Chem. C. 8 (2020) 12865
12875. https://doi. org/10.1039/D0TC02579H. Parts (C
F) from D. Liu, Q. Lin, Z. Zang, M. Wang, P. Wangyang, X. Tang, et al., Flexible all-inorganic perovskite CsPbBr3 nonvolatile memory device, ACS Appl. Mater. Interfaces 9 (2017) 6171
6176. https://doi.org/10.1021/ acsami.6b15149.

Table 6.6 A comparison of flexible memory devices of halide perovskite materials in PET substrates. Device structure

Set/reset voltage (V)

Bending cycles

Bending radius (mm)

On/off current ratio

References

Au/CH3NH3PbI3/ITO Al/CsPbBr3/PEDOT:PSS/ITO Ag/PVOH-ZnSnO3/Ag Cu/NpWo3/ITO

0.7/ 2 0.5 1.7/ 2 0.6 1.5/ 2 1.5 101/ 2 1.1

100 100 1500 1000

1.5
7.5 5.5

10 102 102 105

[94] [95] [96] [97]

Electrical properties of flexible ceramics

115

In 2012 Shu et al. suggested a low-temperature solution process to develop organic
inorganic halide perovskite-based fibrous RRAM architecture [98]. Besides the planar RRAM Devices, the fiber-shaped perovskite RRAMs have potential application in crossbar RRAM devices which ensures high-density storage. Crossbar RRAMs are those which are assembled using crossed functional fibers. In this work, highly malleable Al fibers were used as electrodes and methyl ammonium lead iodide (MAPbI3) was used as the functional material and the Al@MAPbI3 was crossed with metallic Al to obtain a configuration of Al@MAPbI3/Al. The resistive switching (RS) performance of the device was analyzed using a voltage sweep from 0 V to 6 4 V. The device switches from HRS to LRS at 1.66 V and from LRS to HRS at 20.47 V. A switching ratio of 106 and retention time of approximately 104 s were achieved with identical RS behavior in the forward and reverse voltage sweeps. The device at varying bending angles, 0, 15, 30, and 45 degrees shows the same on
off ratio with no considerable deviation as shown in Fig. 6.22B. However, when the bending angle was raised to 180 degrees, an unstable RS behavior was observed with significant damage in the morphology due to the brittleness of MAPbI3. Evidently, the fibrous crossbar perovskite RRAM (FCPeRRAM) proves to be a promising candidate for flexible wearable electronics. In 2016 Liu et al. fabricated Al/CsPbBr3/PEDOT:PSS/ITO/PET-based nonvolatile memory device of thickness 500
600 nm [95]. PEDOT:PSS combination as a charge transfer layer shows a fair adhesion between the substrate and active layer, lack of which leads to cracking and destruction of flexible devices under continuous bending. HRS to LRS and LRS to HRS switching was obtained at potentials 20.6 and 1.7 V with a fairly stable endurance up to 50 cycles as given in Fig. 6.22C and D, where the write-erase voltage was 22.0/3.0 V and the read voltage was 20.2 V. The device was then subjected to a complete cycle of bending at angles starting from 0 degree and proceeding to 60, 180, and 360 degrees. With a slight change in the HRS and LRS current in the negative bias region, the bent states resulted in a robust ION/IOFF ratio of 102. Successfully, this ratio was maintained up to 100 bending cycles which ensured great mechanical stability as shown in Fig. 6.22E and F. Recently, a flexible and transparent memory device was fabricated by Shang et al. by sandwiching hafnium oxide (HfO) in between two ITO electrodes on a PET substrate [99]. The prepared HfO film has a thickness of 10 nm with a mixed amorphous-nanocrystalline structure. The device exhibits a switching ratio of 40 with a set/reset voltage of 0.4 V/ 2 0.2 V. Moreover, 103 sweeping cycles introduce only negligible change in the set/reset voltage and the HRS and LRS are highly stable with a retention time of 105 s. The mechanical flexibility was examined vividly. For this, a tensile stress was introduced to the device to bend it to different radii from 12 to 2 mm sequentially and 300 cycles of bending and flattening were performed consecutively at each radius. It was observed that up to a bending radius 5 mm the RS behavior was almost not affected. Beyond this, the resistance of the on-state increases abruptly due to increased crack density. The switching ratio also decreases and the mechanical failure threshold was marked at a bending radius of 3 mm where the switching states can be distinguished with a ratio 8.1 and survived up to 1200 bending cycles. But at a 2 mm radius, the reset failure occurs and the

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Table 6.7 Comparison of memory characteristics of different ferroelectric materials. Material

Substrate

Bending radius (mm)

Switching ratio

Endurance (number of cycles)

References

h-BN/BiI3 Al2O3 ZrN Ga2O TaO/AlN SiO2 ZnO ZnO ZnO ZnO

Cu foil PET PET PI PEN PEN PET PI PET Plastic

8.75 5 4 5 2 1.5
20 8


107 105 102 105 102 105 10 10 10 10

2500
107 1400 107 106 200 100 2400 105

[100] [101] [102] [103] [104] [105] [106] [107] [108] [109]

device can no longer be used for its purpose. However, the device disclosed remarkable retention of 105 s and stability over 1200 cycles at a bending radius 6 mm. The exquisite mechanical stability of the device is governed by the amorphous-nanocrystalline nature of the HfOx layer. Usually in polycrystalline materials, the bending stress applied is distributed to the components which are aligned parallel to the stress. These components break easily whereas, in the amorphous nanocrystalline layer, the pseudo-straight conductive filaments are aligned perpendicular to the stress and are hardly affected. In effect, a flexible transparent memory device having highly robust RS behavior was proposed by using an amorphous nanocrystalline active layer [99]. Apart from the HP materials, there are plenty of memory devices (RRAM) based on ferroelectric material. Table 6.7 shows the comparison of memory characteristics of different RRAM fabricated on different flexible substrates.

6.3.3.2 Ferroelectric random access memory The FRAM and its storage mechanism are based on the principle of electrically switchable polarization. When a ferroelectric material is exposed to an electric field, it polarizes and this polarization persists even if the external field is removed. The two switchable (positive and negative) remnant polarization (Pr) states are translated to binary “0”s and “1”s to store the information. For flexible memory applications, a ferroelectric material should be endowed with a high Pr value, low coercive field (Ec), and good antifatigue properties which do not alter with mechanical stress applied [110]. Lead-based PbZrxTi1-xO3 is a widely investigated candidate among the ferroelectric perovskite oxides for memory devices. Ghoneim et al. fabricated Pb1.1Zr0.48Ti0.52O3 (PZT) based FRAM on silicon and it displayed a 1 Pr of 13.5 μC/cm2 and a
Pr of 211.5 μC/cm2 and the 1 Ec and
Ec of 50 and 260 kV/cm, respectively. The memory window, which is the difference between

Electrical properties of flexible ceramics

117

the voltages corresponding to the switching and nonswitching current was found to be 10 V. An operation speed of 0.5
1 μs was obtained. The device exhibited a long retention time of 105 s from room temperature to 150 C. However, data loss occurred when the device’s temperature was $ 150 C. The device shows only a feeble fluctuation in Pr even for a minimum bending radius of 5 mm. The retention ability of the device was found to retain till 1300 bending cycles at this minimum bending radius. The device exhibited an uncompromised endurance up to 106 read/ erase cycles, but beyond this, there is a 20% decrease in polarization performance due to fatigue [111]. It is known that graphene has the capacity to hold high mechanical strain. This property of graphene is utilized by Lee et al. to develop a ferroelectric field effect transistor (FFET) consisting of PbZr0.35Ti0.65O3 (PZT) and graphene layers. A typical FFET configuration is pictured in Fig. 6.23A. The PZT layer on Pt/Ti/SiO2 was coated by sol
gel and spin coating methods [112]. Further, PZT/Pt/Ti/SiO2 ribbons were extracted using photolithography on which the graphene active layer was patterned using chemical vapor deposition (CVD). The drain current (ID) versus gate voltage (VG) characteristics were studied and are shown in Fig. 6.23B
D. The memory window was found to be 6 V during the voltage sweep of 6 10 V at a

(A)

(B) Source

Gate

Drain

(C)

2.8

1.4 200 μm

0.7

On Off

2.0 ID (mA)

ID (mA)

2.1

Oxide layer

2.5

VG (sweep) = 2V VG (sweep) = 6V VG (sweep) = 10V

1.5 1.0

SD

Substrate

0.5

G

0.0

-10

-5 0 VG (V)

5

10

0

50

100 150 Time (s)

200

Strain (%) On Off

(F) 5 RC= ∞ 21 mm 14 mm 11 mm 9 mm Return

2.4 ID (mA)

ID (mA)

2.0 1.5 1.0

1.6

0

0.06 0.09 0.11 0.13 0

1.6

4 3

I / I0

(E)

2.5

ID (mA)

(D)

On

On Off

1.2

Off

0.8 0.4

∞ 21 14 11 9 ∞

RC (mm)

2

0.8 1

0.5

RC

0.0 0

500 Cycle

1000

-10

-5

0

0 VG (V)

5

10

∞

21

14

11

9

∞

RC (mm)

Figure 6.23 (A) The schematic diagram of a typical FFET configuration. The output performance of PZT/Pt/Ti/SiO2 FFET. (B) ID
VG characteristics at different drain voltages. (C) Time retention properties at 0.1 V. (D) Endurance test of on and off currents at a drain voltage of 0.1 V as a function of bending cycles. (E) ID
VG characteristics at different bending radii. (F) Variation of on/off states with bending radius. ¨ zyilmaz, Source: Reprinted with permission from W. Lee, O. Kahya, C.T. Toh, B. O J.-H. Ahn, Flexible graphene
PZT ferroelectric nonvolatile memory, Nanotechnology 24 (2013) 475202. https://doi.org/10.1088/0957-4484/24/47/475202.

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drain voltage of 0.1 V. The extent of the memory window in terms of the on/off current ratio was estimated as 6.7. FFET showed a retention capability of 200 ns for on current while the device fails to preserve off current as it deviates from the beginning (Fig. 6.23E and F) and Pr 30 mC/cm2, respectively and the device is resilient after 200 bending cycles at a tiny bending radius of 9 mm. To attain the desirable flexibility, plastic substrates are used by many research groups for the fabrication of devices. In view of this, Yoon et al. utilized the high thermal expansion coefficient and chemical resistance of PEN substrate to fabricate ZnO-P(VDF-TrFE)-based FFET [113]. In this device, the ZnO layer act as the channel layer, P(VDF-TrFE) act as the gate insulator (GI), Au act as the gate electrode, and Ti/Au/Ti act as the source and drain electrodes. The memory window of the device was found to be 7.8 V at sweep voltage from 214 to 12 V, the higher VG values. However, the memory window was remarkable (4.7 V) even at low voltage sweeps of 28 to 6 V. In the bending durability test with bending radius of9.7 mm, the device showed a variation of only 0.7 V in the memory window. Bending cycles of 20,000 at a radius of 23.5 mm could not introduce many variations. Moreover, the on/off ratio of 6.6 3 105 was decreased to 130 after 15,000 s, which described an appreciable retention capacity [113]. In a similar manner, in the place of ZnO as the oxide layer, amorphous InGaZnO (IGZO), having high carrier mobility (10 cm2/Vs) is used. Luisa et al. fabricated indium gallium zinc oxide (InGaZnO or IGZO) and P(VDF-TrFE)-based FFET on a polyimide substrate. The obtained ID
VG characteristics yield a memory window of 3.2 V and an on/off ratio of 1.5 3 106 when the voltage sweep was 6 6 V. The device displayed notable stability under compressive and tensile bending at a bending radius of 5 mm [114]. In 2015 Jung et al. fabricated InGaZnO or IGZO and P(VDF-TrFE) based FFET on polydimethylsiloxane (PDMS) elastomer for flexible memory applications [115]. Choosing IGZO in the ratio In:Ga:Zn as 2:1:2 for the active layer, P(VDF-TrFE) as gate insulator, aluminum as gate electrodes, and Mo as the source and drain electrodes, the Al/P(VDF-TrFE)/IGZO/Mo configuration of the device was obtained. ID
VG characteristics of the device measured at a drain voltage of 5 V exhibited a large on/off ratio of 107 and the memory window was analyzed with different gate sweep voltages. Even at a voltage sweep of 6 5 V, the device’s memory window was obtained as 4 V which is appreciable. The on/off ratio of the device was found to be 5 3 104 and gradually decreased to 102 and was retained up to 3600 s. The ID
VG characteristics at VD 5 5 V and VG 5 0 V under different bending radii from infinity (flat state) to a minimum of 10 mm were studied. The effective working of the device was not affected by bending as it showed negligible variation in the memory window even at 10 mm bending radius. This suggested the incorporation of hybrid organic
inorganic devices as a new arena in flexible memory applications. A comparison of the above-mentioned devices in terms of their performance is codified in Table 6.8. Evidently, the incorporation of ceramic with flexible ferroelectric materials appears to be a hopeful stagey for the newly emerging flexible nonvolatile memory devices.

Electrical properties of flexible ceramics

119

Table 6.8 Comparison of memory characteristics of flexible ferroelectric random access memorys. Material

Memory window (V)

Memory on/off ratio

Bending radius (mm)

Retention time (s)

References

ZnO/P(VDF-TrFE) IGZO/ P(VDF-TrFE) IGZO/ P(VDF-TrFE)

7.8 3.2 13

104 1.5 3 106 107

9.7 5.5 10

15,000 100 3600

[113] [114] [115]

6.4

Conclusions

Flexible devices are substantial in energy harvesting, energy storage, and memory applications. This chapter encounters specific materials employed for flexible device applications with a focus on flexible ceramics as a viable alternative for bulk ceramics. A detailed investigation of the fundamental electrical properties of various flexible ceramic films is presented. This chapter provided a comprehensive overview of the dielectric, piezoelectric, pyroelectric, ferroelectric, and electrochemical properties along with the current
voltage characteristics as a function of mechanical flexibility, and their performance in various applications. Flexible ceramics are more adaptable due to their resilient and lightweight nature. Hence, this technology has the potential to replace bulk ceramics and rigid devices in the future. Therefore significant consideration should be given to material selection, device designing, compatibility, and permanence of the flexible ceramics to achieve high-performance flexible devices. Though flexible devices cannot replace fossil fuels or renewable energy sources, they can play a crucial role in low-power electronic devices that require battery replacement on a regular basis, ensuring an uninterrupted electrical output.

Acknowledgments This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding Contract UIDB/04650/2020.

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5362. Available from: https://doi.org/10.1002/adma.201201414. Y. Yang, H. Zhang, G. Zhu, S. Lee, Z.-H. Lin, Z.L. Wang, Flexible hybrid energy cell for simultaneously harvesting thermal, mechanical, and solar energies, ACS Nano 7 (2013) 785
790. Available from: https://doi.org/10.1021/nn305247x. M.S. Jayalakshmy, J. Philip, Enhancement in pyroelectric detection sensitivity for flexible LiNbO3/PVDF nanocomposite films by inclusion content control, J. Polym. Res. 22 (2015) 42. Available from: https://doi.org/10.1007/s10965-015-0688-4. T. Nakajima, T. Tsuchiya, Ultrathin highly flexible featherweight ceramic temperature sensor arrays, ACS Appl. Mater. Interfaces 12 (2020) 36600
36608. Available from: https://doi.org/10.1021/acsami.0c08718. J. Zhang, W. Li, Perovskite materials for resistive random access memories, in: H. Tian (Ed.), Perovskite Mater. Devices Integr, IntechOpen, 2020. Available from: https://doi.org/10.5772/intechopen.86849. C. Gu, J.-S. Lee, Flexible hybrid organic
inorganic perovskite memory, ACS Nano 10 (2016) 5413
5418. Available from: https://doi.org/10.1021/acsnano.6b01643. D. Liu, Q. Lin, Z. Zang, M. Wang, P. Wangyang, X. Tang, et al., Flexible allinorganic perovskite CsPbBr3 nonvolatile memory device, ACS Appl. Mater. Interfaces 9 (2017) 6171
6176. Available from: https://doi.org/10.1021/acsami.6b15149. G.U. Siddiqui, M.M. Rehman, K.H. Choi, Enhanced resistive switching in all-printed, hybrid and flexible memory device based on perovskite ZnSnO3 via PVOH polymer, Polymer 100 (2016) 102
110. Available from: https://doi.org/10.1016/j.polymer. 2016.07.081. Y. Ji, Y. Yang, S.-K. Lee, G. Ruan, T.-W. Kim, H. Fei, et al., Flexible nanoporous WO3
x nonvolatile memory device, ACS Nano 10 (2016) 7598
7603. Available from: https://doi.org/10.1021/acsnano.6b02711. P. Shu, X. Cao, Y. Du, J. Zhou, J. Zhou, S. Xu, et al., Resistive switching performance of fibrous crosspoint memories based on an organic
inorganic halide perovskite, J. Mater. Chem. C. 8 (2020) 12865
12875. Available from: https://doi.org/ 10.1039/D0TC02579H. J. Shang, W. Xue, Z. Ji, G. Liu, X. Niu, X. Yi, et al., Highly flexible resistive switching memory based on amorphous-nanocrystalline hafnium oxide films, Nanoscale 9 (2017) 7037
7046. Available from: https://doi.org/10.1039/C6NR08687J. C. Li, S. Kuo, Y. Wu, F. Fu, I. Ni, M. Chen, et al., Forming-Free, nonvolatile, and flexible resistive random-access memory using bismuth iodide/van der waals materials heterostructures, Adv. Mater. Interfaces 7 (2020) 2001146. Available from: https://doi. org/10.1002/admi.202001146. M. Kim, K.C. Choi, Transparent and flexible resistive random access memory based on Al2O3 film with multilayer electrodes, IEEE Trans. Electron. Devices 64 (2017) 3508
3510. Available from: https://doi.org/10.1109/TED.2017.2716831. D. Kumar, U. Chand, L.W. Siang, T.-Y. Tseng, ZrN-based flexible resistive switching memory, IEEE Electron. Device Lett. 41 (2020) 705
708. Available from: https://doi. org/10.1109/LED.2020.2981529. K.-J. Gan, P.-T. Liu, T.-C. Chien, D.-B. Ruan, S.M. Sze, Highly durable and flexible gallium-based oxide conductive-bridging random access memory, Sci. Rep. 9 (2019) 14141. Available from: https://doi.org/10.1038/s41598-019-50816-7.

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[104] S. Rajasekaran, F.M. Simanjuntak, D. Panda, S. Chandrasekaran, R. Aluguri, A. Saleem, et al., Fast, highly flexible, and transparent TaOx-based environmentally robust memristors for wearable and aerospace applications, ACS Appl. Electron. Mater. 2 (2020) 3131
3140. Available from: https://doi.org/10.1021/acsaelm. 0c00441. [105] C. Papakonstantinopoulos, P. Bousoulas, M. Tsigkourakos, D. Sakellaropoulos, L. Sygellou, D. Tsoukalas, Highly flexible artificial synapses from SiO2-based conductive bridge memristors and Pt nanoparticles through a crack suppression technique, ACS Appl. Electron. Mater. 3 (2021) 2729
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24. Available from: https://doi.org/ 10.1016/j.jcis.2018.03.001. [107] S. Park, J.H. Lee, H.-D. Kim, S.M. Hong, H.-M. An, T.G. Kim, Resistive switching characteristics of sol-gel based ZnO nanorods fabricated on flexible substrates: Resistive switching characteristics of sol-gel based ZnO nanorods fabricated on flexible substrates, Phys. Status Solidi RRL - Rapid Res. Lett. 7 (2013) 493
496. Available from: https://doi.org/10.1002/pssr.201307187. [108] X. Wu, Z. Xu, Z. Yu, T. Zhang, F. Zhao, T. Sun, et al., Resistive switching behavior of photochemical activation solution-processed thin films at low temperatures for flexible memristor applications, J. Phys. Appl. Phys 48 (2015) 115101. Available from: https://doi.org/10.1088/0022-3727/48/11/115101. [109] S. Kim, H. Moon, D. Gupta, S. Yoo, Y.-K. Choi, Resistive switching characteristics of sol
gel zinc oxide films for flexible memory applications, IEEE Trans. Electron. Devices 56 (2009) 696
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7627. Available from: https://doi.org/10.1166/jnn.2012.6651. [111] M.T. Ghoneim, M.A. Zidan, M.Y. Alnassar, A.N. Hanna, J. Kosel, K.N. Salama, et al., Thin PZT-based ferroelectric capacitors on flexible silicon for nonvolatile memory applications, Adv. Electron. Mater. 1 (2015) 1500045. Available from: https://doi. org/10.1002/aelm.201500045. ¨ zyilmaz, J.-H. Ahn, Flexible graphene
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Optical properties of flexible ceramic films

7

S. Angitha1, Kevin V. Alex2, J.P.B. Silva3,4, K.C. Sekhar2, M. Tasneem1 and K. Kamakshi1 1 Department of Science and Humanities, Indian Institute of Information Technology, Tiruchirappalli, Tamil Nadu, India, 2Department of Physics, School of Basic and Applied Sciences, Central University of Tamil Nadu, Thiruvarur, Tamil Nadu, India, 3Physics Center of Minho and Porto Universities (CF-UM-UP), University of Minho, Campus de Gualtar, Braga, Portugal, 4Laboratory of Physics for Materials and Emergent Technologies, LapMET, University of Minho, Braga, Portugal

7.1

Introduction

Recently, the development of optical ceramic films gained much interest as they have prospective applications in optoelectronic devices [1]. The electronic structure and wide bandgap make the ceramics convenient for different optical applications as optical properties can be tuned by changing their processing conditions [2]. To use optical ceramics in various device applications, gaining knowledge about optical constants like transmittance or reflectivity at a specified wavelength, the refractive index and the absorption coefficient is essential. “The optical constants of material are numbers that describe how a plane electromagnetic wave progresses through the material” [1]. For each frequency, there will be a pair of readings of the constants that measures the attenuation as well as the speed of the wave. Further, the fundamental properties of a material are also described by these constants. This frequency depending nature of the optical constants allows us to study the ceramic film’s physical properties [3]. Generally, the optical properties of ceramics involve light-matter interactions that result in diverse behaviors of light, such as transmission, reflection, absorption, as well as scattering. This will be helpful in explaining the material’s surface structure [4]. Based on optical properties, ceramics were categorized as opaque, translucent, and transparent. As per this categorization, glass is considered as a popular transparent material. Another important characteristic, color in ceramics that occurs due to the interaction of light with matter, may be altered either by the incorporation of additives or dopants or by varying the number of point defects in the ceramic structure. The optical properties of ceramics have drawn great attention recently, as they are important in advancing light-emitting materials like

Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00007-5 © 2023 Elsevier Ltd. All rights reserved.

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Optical Memories

Photovoltaic Devices

LEDs

Transparent & Flexible Ceramic Films Chromatic Devices

Photocatalytic Devices

Opto-thermal Sensors

Figure 7.1 Various applications based on transparent and flexible ceramic films.

luminescent ceramics [5]. The various technologies, where the optical properties like transmission, light emission, and color of ceramic films play a major role are shown in Fig. 7.1.

7.2

Concept and fundamentals of optical properties

The optical properties of flexible ceramic films are highly important in understanding the electronic structure of ceramic films and are hence of great practical importance. Optical parameters describe the progression of an electromagnetic wave in a ceramic film and are related to the constitution of ceramics. The particles in the ceramic materials are electric in nature and the EM wave is a probe to perturbate the material and therefore gives information about its physical nature. The optical constants depend on the frequency of the incoming radiation which provides valuable information related to their physical properties. The optical response of solid material is characterized by Maxwell equations and the complex refractive index in a medium. The complex refractive index can be expressed in terms of two optical constants: the dielectric constant ε and the optical absorption coefficient α [6].

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If I0 is the intensity of light, that enters normally through a ceramic film of thickness “t”, the intensity of light emerging out of the film is given by the following equation [7]: I 5 I0

e2~t

I 5 e2~t I0 I0 5 eαt I
 I0 ln 5 αt I
 1 I0 α 5 ln t I

(7.1) (7.2)

(7.3)

(7.4)

(7.5)

  By measuring II0 , α can be determined. Similarly, by measuring optical constants as a function of frequency, we could know about the frequency dependence of ε and conductivity σ. This will enable us to probe into the microscopic nature of the ceramic film [7].

7.2.1 Interaction of electromagnetic wave with ceramics When an electromagnetic wave interacts with ceramic films, a part of it will be absorbed by the ceramics, while some parts will be transmitted and the rest will be reflected as shown in Fig. 7.2. The sum of the intensity of transmitted light (IT),

Figure 7.2 Schematic representation of fundamental phenomena when light incident on the surface of ceramic films.

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absorbed light (IA), and reflected light (IR) should be equal to the intensity of incident light (I0) which is given by the following equation [7]: Io 5 IT 1 IA 1 IR IT IA IR 1 1 51 I0 I0 I0

(7.6) (7.7)

The fractions in Eq. (7.7) are called transmissivity (IIT0 ), absorptivity (IIA0 ), and reflectivity (IIR0 ). Since their sum is equal to 1, we can say that all the incident light on the surface of the ceramic film is either transmitted, absorbed, or reflected. If the ceramic film transmits light with very little absorption and reflection, then the ceramics are said to be transparent. If the light gets scattered within the interior of the material such that, the objects can’t be seen clearly when looked through it, then it is said to be translucent. The material which does not transmit light is called opaque. All of these phenomena are caused by electromagnetic radiation interacting with atoms, ions, or electrons. Electronic polarization and electron energy transfer are the most important interactions responsible for optical phenomena. Absorption of radiation and retardation in its velocity as it passes through the medium are two major consequences of electronic polarization. Retardation in the velocity of light inside the material is manifested as refraction [7].

7.2.1.1 Absorption Nonmetallic ceramics can be either opaque or transparent to visible light. In regular, transparent materials appear colored. Two mechanisms are responsible for the absorption of light
electronic polarization and transitions between the conduction band and valance bands of ceramic materials. Electronic polarization exists only at frequencies that are in the neighborhood of the relaxation frequency of the constituent atoms while the band-to-band transition depends on the energy of the incoming radiation as well as on the energy band structure [8]. When a photon is absorbed, an electron from the valence band jumps into an empty state available in the conduction band. If “hν” is the energy of photon absorbed by the material and the excitation energy is ΔE, then, ΔE 5 hυ

(7.8)

If hν $ Eg, the absorption of radiation occurs where Eg is the energy bandgap of the material [8].

7.2.1.2 Transmission When a light incident on the surface of ceramic films, a certain amount of light transmits through it. The fraction of incident light passed through the medium is

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denoted as transmittance, which can be given as the ratio of the transmitted light intensity (I) and incident light intensity (I0) [9]. Mathematically, T ð%Þ 5

I 3 100 I0

(7.9)

Transmittance can be categorized into two-regular transmittance (Treg) and diffuse transmittance (Tdiffuse). The transmittance is said to be regular if the incoming light travels through the material in such a way that the exit angle can be anticipated from the entering angle using Snell’s law of refraction [9,10]. The transmittance is referred to as diffuse when the incident light is scattered during its journey through the material due to surface roughness or volume scattering, and hence Snell’s law becomes irrelevant [10].

7.2.1.3 Reflection Even though both media are transparent, when the light goes from one medium to another with differing refractive indices, part of the light will be scattered at the boundary. The reflectivity (R) incident light that is reflected at the interface between two mediums is [8] R5

IR I0

(7.10)

where IR and Io are the reflected and incident beam intensities, respectively. For the normal incidence of light,


n2 2n1 R5 n2 1n1

2 (7.11)

where n1 and n2 are refractive indices of the first and second medium, respectively. If the light ray incidents at an angle to the interface, then reflectivity (R) depends on the incident angle. If the light travels from either air (n1 5 1Þ or vacuum into a solid, the greater will be its reflectivity. As n 5 cv, the refractive index of solid ns depends on the wavelength (λ) of incident light. Hence reflectivity (R) depends on wavelength [8].

7.2.1.4 Refraction The change in direction of a wave traveling from one medium to another due to its change in speed is known as refraction. There will be a reduction in velocity when the light ray enters the solid (transparent) and as a result, it bends at the boundary. The refractive index (n) or the index of refraction is defined as [8] n5

c V

(7.12)

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Here, “c” is the light velocity in a vacuum and “v” is the light velocity in the medium which is given by the equation 1 v 5 pffiffiffiffiffiffi εμ

(7.13)

where ε and μ are the permittivity and permeability of the material, respectively. We know that, 1 c 5 pffiffiffiffiffiffiffiffiffi ε0 μ0

(7.14)

Substituting Eqs. (7.13) and (7.14) in Eq. (7.12) gives pffiffiffiffiffiffi εμ pffiffiffiffiffiffiffiffiffi n 5 pffiffiffiffiffiffiffiffiffi 5 εr μr ε0 μ0

(15)

where εr is dielectric constant and μr is the relative magnetic permeability, respecpffiffiffiffi tively. Since most of the substances are only magnetic, μr 5 1 and hence n 5 εr . Since refraction is correlated to electronic polarization, measuring “n” aids in determining the electronic component of the dielectric constant. [10]. Electronic polarization causes the EM radiation to be retarded in a material. Therefore, the magnitude of this effect is significantly influenced by the size of the constituent atoms or ions. In general, the larger an atom or ion, the greater the electronic polarization, the slower the velocity, and the higher the index of refraction [11]. The refractive index is isotropic and independent of crystallographic directions for crystals having cubic structures. For noncubic crystals, n is anisotropic. It has the highest value in the direction in which the density of atoms is high [12]. Various theoretical models such as Cauchy, Briot, Hartmann, Conrady, Sellmeier, and fixed index dispersion formulae are used to describe the refractive index of transparent materials in terms of the wavelength of light and to determine the imaginary as well as real parts of the complex refractive index. These theoretical models provide a mathematical relation known as the “dispersion formula” to estimate the optical properties of a material. The most commonly used dispersion formula “Cauchy relation” is given by the following equation [13]: nð λ Þ 5 A 1

B 1 λ2

C λ4

(7.16)

where “A”, “B”, and “C” are material-dependent coefficients and “λ” is the wavelength of the light. This dispersion formula works better in the visible spectral range of the materials with the least optical absorption and exhibits normal dispersion.

Optical properties of flexible ceramic films

135

Sellmeier dispersion relation is a modified form of the Cauchy relation which can be useful to a wide spectral range and is given as follows [14]: λ2 n ðλÞ 5 A 1 B 2 λ 2 λ20

!

2

(7.17)

where “A”, “B”, and “λ0” are material-dependent Sellmeier coefficients.

7.2.1.5 Scattering This is one of the phenomena which occurs when light strikes a material. At the microscopic level, the particles and pores will have different sizes. Depending on the roughness of the material, the direction of light will change. Scattering occurs when some amount of incident light reflects by the localized nonuniformities present in the ceramic materials. Shorter wavelengths are more likely to scatter. All atoms in the material will have a definite vibrational frequency, termed natural frequency. If the incident light of the same natural frequency interacts with these atoms, the energy of the light will be absorbed, causing a heating effect [15].

7.2.2 Luminescence properties Luminescence is defined as a phenomenon in which an external energy source excites the electronic states of ceramics, and then, liberates the excited energy from ceramics as wavelength-dependent light emission. It is also referred to as cold emission since the emitted light energy from electronically excited states doesn’t involve heat. The term “light” here refers to visible light (400
800 nm) as well as the nearby near-ultraviolet and near-infrared zones on both ends of the spectrum. The energy gap between the lower and higher energy levels determines the frequency of the generated radiation [8]. The luminescence intensity is calculated by using the formula, I 5 I0 exp

 2 t τ

(7.18)

Here, I0 is the luminescence initial intensity, I is the fraction of luminescence at time t, and τ is relaxation time which should be a constant for a given material. Several categories of luminescence depending on what the source of energy is, or what triggers the luminescence are shown in Fig. 7.3 such as cathodoluminescence (CL)—excitation takes place either by cathode rays or by electron beams; thermoluminescence (TL)—activated thermally after initial irradiation by other means, for example, α, β, γ, UV or X-rays; photoluminescence (PL)—excitation takes place by electromagnetic radiation/photon; Mechanoluminescence (ML)— excitation due to mechanical forces; finally, electroluminescence (EL)—excitation will be done by electric influences [16].

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Figure 7.3 Categories of luminescence phenomena based on different sources of excitation.

7.2.2.1 Photoluminescence PL is the phenomena of emission of short wavelength light (commonly UV or visible) as a result of the excitation of electrons into higher energy states from which radiative decay is feasible. For allowed transitions, typical decay times are of the order of a few nanoseconds, but they can be significantly longer in other instances. The duration of emission or decay time allows us to subclassify the PL into phosphorescence and fluorescence. Fluorescence is defined as the emission of light, with a short decay time (,1028 s) that occurs simultaneously with the absorption of light and is halted instantly when the excitation source is withdrawn. Phosphorescence, on the other hand, is characterized by a long decay time ( . 1028 s) that continues to emit even after the excitation source has been withdrawn. Fluorescence, is essentially temperature independent, whereas phosphorescence has a strong temperature dependency [16,17]. The mechanism of fluorescence and phosphorescence phenomena is schematically represented in Fig. 7.4.

7.2.2.2 Cathodoluminescence Photon emission stimulated by an electron beam is known as CL. It is light or electromagnetic radiation in the UV/VIS/NIR spectral regime produced by fast electrons (cathode rays). CL can be used in semiconductor physics to study key characteristics like dislocations and grain boundaries, distribution of nonradiative recombination centers, and the presence and type of defects including stress fields and compositional fluctuations. CL has enabled imaging of the electrical and optical

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Figure 7.4 Schematic mechanism of fluorescence and phosphorescence phenomena.

characteristics of semiconductor structures having a paramount resolution of about 20 nm [17].

7.2.2.3 Electroluminescence Light emission triggered by electric influences is known as EL. It is a process in which excess electron
hole pairs are formed by an electric current driven by an externally imposed bias, resulting in the generation of photons. When the electroluminescent materials are subjected to strong electric fields or the passing of an electrical current, the impact of high-energy electrons in the luminescent centers of the electroluminescent material causes the emission of light. The applied electric field can either be alternating current or direct current. In semiconductor devices like a light-emitting diode (LED), internal current flow across a p
n junction can generate electron
hole pairs that, when recombine, gives luminescence [17]. The basic electroluminescent structure generally consists of either two inorganic or organic electroluminescent materials, one capable of emitting electrons and the other capable of emitting holes, and two electrodes, in which the top electrode must be transparent [18,19]. Organic or inorganic materials can be used as electroluminescent materials. For electroluminescent panels, indium tin oxide (ITO) mounted on glass or plastic is frequently used as a top electrode due to its good transparency and high conductivity. In most cases, the bottom electrode is a reflective metallic coating. Then this electroluminescent structure is attached to mechanical support, either rigid or flexible, for the intended device application [20]. EL is a well-known phenomenon that has immense applications in rigid as well as flexible electronics and in various flexible textile structures. Electroluminescent

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materials have advanced dramatically in recent years for application in new and developing technologies that have the potential to revolutionize lighting and flatscreen display technology. Electroluminescent materials can be used to make electroluminescent textile devices by combining them with conductive textiles [19].

7.2.2.4 Thermoluminescence TL is a stimulated emission process where a material emits light with temperature. The name TL is deceptive because it does not refer to the excited system’s thermal generation of the excited system. Thermal-stimulated luminescence is a better description of the phenomena. TL occurs when the material emits thermally excited light after the energy absorption from ionizing radiation. The absorbed energy by the crystal, allows electrons to travel freely across the crystal lattice, though some may be trapped in the crystal imperfections. Further crystal heating, results in the emission of light by the release of these trapped electrons [16,19].

7.2.2.5 Mechanoluminescence The phenomenon of ML is also known by various other names, like stress-activated luminescence, piezoluminescence, deformation luminescence, tren-nugslicht, and triboluminescence [19]. ML is a variant of luminescence, which occurs by the application of mechanical forces to solids. ML can be stimulated by different mechanical processes such as cutting, crushing, grinding, rubbing, shaking, cleaving, scraping, and compressing particles. Further, ML can also be stimulated by thermal shocks generated by severe heating or cooling materials along with shock waves which are produced when the samples are exposed to intense laser pulses. ML can also be observed through the growth of some crystals or phase transitions and due to the separation of two dissimilar materials in contact. Because the word “mechano” is associated with a variety of mechanical concepts, such as cutting, grinding, crushing, rubbing, cleaving, compressing, stress, deformation, elastico, piezo, plastico, tribo, and fracto, “ML” has become the preferred terminology [17,19].

7.3

Flexible ceramic films and their optical properties

Next-generation devices won’t be just efficient, but durable enough to bend and stretch as scientists attempt to integrate them around human beings. Thus, researchers shifted focus toward the development of flexible ceramics as flexibility is inevitable in next-generation optical devices. Science is innovating ceramic materials with new qualities via different strategies. Ceramics are traditionally known to be rigid and brittle solids. But researchers have shown that by using special processing methods or by fashioning their nanostructures, ceramics can become flexible and can bend without breaking, which makes them useful for flexible optical device applications. The mechanical flexibility and optical properties of the ceramics can

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be modified either by fashioning its nanostructure into nanoribbons/nanobelts (NBs), coating over flexible substrates, doping with suitable materials or by forming ceramic
polymer composite [21]. Various researchers have studied the optical properties of flexible ceramic films which are discussed in detail in the following sections.

7.3.1 Transmittance of flexible ceramics Transparency which is also known as diaphaneity or pellucidity is currently the most important optical determinant of each material. The transmittance property has a crucial role in various transparent and flexible electronic device applications. The morphology of the ceramic materials can be tuned for more flexibility through various preparation methods like solution blow spinning, electrospinning, atomic layer deposition, self-assembly, centrifugal spinning, etc. The ceramic nanostructures can be transformed into more flexible nanoribbons or NBs with high optical transparency. Huang et al. have shown that excellent flexibility can be achieved by fabricating a network of intertwined SnO2 NBs in conventionally fragile SnO2 by employing the electrospinning method [21]. The SnO2 NB network exhibited an optical transmittance of 80% with low scattering and absorption cross-sections as in Fig. 7.5A. Furthermore, the NB structures exhibited well mechanical flexibility without any detectable fracture till the bending radius was equal to 3 mm. So, this ceramic NB network becomes a potential candidate for developing flexible and highly transparent optical devices. Further, the fabrication of ceramic materials over flexible substrates such as mica, metal foils, etc. can be employed as a strategy to develop flexible ceramic thin films and nanostructures. Gao et al. studied the transmittance of the mica/AgITO/Bi3.25La0.75Ti3O12 (BLT)/ITO structure to develop transparent and flexible optical-read ferroelectric nonvolatile memories [22]. The mica/Ag-ITO/BLT/ITO structure showed a maximum transmittance of 80% in the region of visible light. Furthermore, this structure exhibited well mechanical flexibility up to a bending radius of 3 mm for 104 cycles. Yang et al. used a 10-μm thick mica substrate to fabricate a ceramic heterostructure based on BaTi0.95Co0.05O3 (BTCO) film with a SrRuO3 (SRO) buffering layer and studied their transmittance behavior [23]. The mica/SRO/BTCO structure and the pure mica substrate exhibited transmittance of 50% and 80%, respectively, in the visible light wavelength region of 500
800 nm. The low value of transmittance of the mica/SRO/BTCO structure is due to the absorption of the majority of incident light by the buffered layer SRO as the bottom electrode since BTCO has a bandgap of 3.2 eV. For a minimum bending radius of 1.4 mm with a tolerance of 104 bending cycles, the mica/SRO/BTCO structure exhibited high flexibility. Thus, this work suggested that the large-scale flexible and semitransparent mica/SRO/BTCO structures are helpful for flexible electronic applications. Ren et al. fabricated highly flexible and transparent Pb(Zr0.1Ti0.9)O3 (PZT) ferroelectric film over ITO/YSZ/mica substrate (where “YSZ” is yttrium stabilized zirconia) [24]. In the visible region, the ITO/PZT/ITO/YSZ/mica capacitor exhibited a

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Figure 7.5 Transmittance spectra of (A) SnO2 NB network on quartz under flat condition [21], (B) mica substrate, ITO/YSZ/mica, ITOPZT/ITO/YSZ/mica under flat condition, (C) Au(8 nm)/AZO (20 nm)/mica structure as a function of bending cycles, and (D) PDMS-SiC NWsF-PDMS composite as a function of tensile strain. Source: (B) Reprinted with permission from C. Ren, C. Tan, L. Gong, M. Tang, M. Liao, Y. Tang Y, et al., Highly transparent, all-oxide, heteroepitaxy ferroelectric thin film for flexible electronic devices, Appl. Surf. Sci. 458 (2018) 540
545; (C) from J. Xie, Y. Bi, M. Ye, Z. Rao, L. Shu, P. Lin, et al., Epitaxial ultrathin Au films on transparent mica with oxide wetting layer applied to organic light-emitting devices, Appl. Phys. Lett. 114 (8) (2019) 081902; (D) from B. Sun, Y. Sun, C. Wang, Flexible transparent and free-standing SiC nanowires fabric: stretchable UV absorber and fast-response UV-A detector, Small 14 (12) (2018) 1703391.

maximum transmittance of about 73% as shown in Fig. 7.5B, and remarkable mechanical flexibility up to 500 bending cycles at a bending radius equal to 4 mm with an estimated strain of 0.439%. Xie et al. fabricated Au/AZO films over mica substrate which exhibited a maximum transmittance of about 75% in the wavelength region of 500
700 nm [25]. Furthermore, Fig. 7.5C shows that the transmittance remained almost constant up to 103 bending cycles at a bending radius equal to 5 mm for Au/AZO film. Another strategy that can be employed for the preparation of flexible and transparent ceramics is the integration of ceramic as well as polymer materials into a hybrid structure for exploring both of their individual properties. Sun et al.

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developed a stretchable UV absorber based on transparent, flexible as well as selfstanding SiC nanowires fabric (NWsF) composite with a structural configuration PDMS-SiC NWsF-PDMS; where “PDMS” denotes polydimethylsiloxane [26]. The ceramic
polymer composite structure displayed a high UV absorbance and a considerable visible light transmittance. Furthermore, the PDMS-SiC NWsF-PDMS composite exhibited a slight increase in its transmittance as the tensile strain increased from 0% to 50% as shown in Fig. 7.5D. Chen et al. fabricated a highly transparent and flexible ceramic
polymer hybrid coating of zirconium
siloxane (HP-ZPx where “x” denoted the silane content) [27]. The HP-ZP hybrid coating exhibited a combined characteristic of both ceramic as well as a polymer with superior optical transparency of about 99.5% and high mechanical flexibility with a bending diameter of # 10 mm. The mechanical flexibility tests have shown that as the silane (x) content increases from 0 to 15, the bending diameter reduces from 10 to 1 mm, exhibiting an increase in its flexibility. The transmittance and flexibility properties of different transparent-flexible ceramic films are compared and tabulated in Table 7.1.

7.3.2 Refractive index of flexible ceramic films Different ceramic materials can be treated with a wide range of compositional flexibility, allowing them to be tailored in terms of the transmission window, physical qualities, and optical properties. Therefore, it is possible to achieve a tunable refractive index in certain ceramic materials by changing the composition and microstructure [28]. Liou et al. developed a synthesis route to produce highly transparent polyimide-nanocrystalline titania composite optical films with different titania content from the soluble polyimide 6F-poly(p-hydroxyimide) (6FPI) [29]. The composite films displayed a good tunable refractive index, surface planarity, great optical transparency, thermal stability, and low color. Fig. 7.6A shows the refractive index of the polymer
ceramic composite film at 633 nm with the variation of titania content. It is found that the refractive index increased linearly as titania content is increased. This may be due to the formation of Ti
O
Ti structures by the progressive condensation of the Ti
OH groups in the hydrolyzed precursors. Similarly, Tsai and Liou synthesized zirconia-containing polyimides (PI/ZrO2) that are thermally stable and show tremendous optical characteristics [30]. The flexible PI/ZrO2 hybrid films show greater optical transparency of about 96%, with adjustable Abbe number and refractive index. They studied and compared the optical properties of PI/ZrO2 hybrid films with PI/TiO2 hybrid films for different Zr and Ti content as shown in Fig. 7.6B and C. The PI/ZrO2 hybrid films have higher transparency and Abbe number in the visible region in comparison to the PI/TiO2 system, due to a large energy band gap of ZrO2. It is observed that the ZrO2 hybrid system when compared to titania, enhances both the refractive index and the Abbe number which makes it attractive for various optical device applications.

Table 7.1 Comparison of different flexible and transparent ceramic films. S. no.

1. 2. 3. 4. 5. 6.

Material

SnO2 nanobelts BLT BTCO PZT/ITO Au/AZO HP-ZPx

Substrate

Quartz Mica Mica Mica Mica Glass

Transmittance (%)

80 80 50 73 75 99.5

Flexibility

References

Minimum bending radius (mm)

Bending cycles

2 3 1.4 4 5 0.5

103 104 104 500 103 20

[21] [22] [23] [24] [25] [27]

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Figure 7.6 (A) Refractive index of polyimide-nanocrystalline titania composite films as a function of titanium content [29], refractive index and Abbe number of (B) PI/ZrO2 hybrid film as a function of zirconium content [30], (C) PI/TiO2 hybrid film as a function of titanium content [30].

7.3.3 Photoluminescence of flexible ceramic films The development of luminous inorganic materials has emerged as a hot topic of intense research in current years as these materials are stable, have fascinating features, and can be mass-produced in huge quantities. These superior features make them ideal for use in devices that produce light artificially. Luminescence is greatly affected by several factors, including crystallinity, active centers, host composition, and their interactions [31]. External stimuli like strain, temperature, and electric field control the luminescence properties reversibly which helps in designing highperformance optical devices [32]. The flexibility and PL of ceramic materials will be improved through the fabrication of ceramic-polymer composite structures. Kim et al. fabricated the CsPbBr3/Al2O3PTFE ceramic-polymer composite film (where “PTFE” is polytetrafluoroethylene) on

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PET substrate via aerosol deposition with an excellent endurance towards a high flexural force [33]. The mechanical durability and flexibility of the ceramic
polymer composite film on PET substrate were examined using an automatic bending test and exhibited an excellent tolerance of 105 bending cycles without any surface degradation or structural cracks. The CsPbBr3/Al2O3-PTFE composite
polymer film has exhibited a higher PL intensity than that of the CsPbBr3/Al2O3 ceramic composite film synthesized under helium and nitrogen conditions as shown in Fig. 7.7. Another strategy that can be employed for improving the luminescence and flexibility of ceramic materials is to fabricate them over flexible substrates. Zheng et al. showed that the PL response of Pr-doped Ba0.85Ca0.15Ti0.9Zr0.1O3 epitaxial thin film can noticeably alter in a repeatable and stable manner with a mechanical strain induced by bending in a flexible mica substrate [32]. The PL performance of the host film is strongly sensitive to mechanical bending due to the local crystal field around the Pr31 and alteration due to induced strain in the lattice symmetry of the host film. They evaluated the relative change in the intensity of PL (ΔI/I) of the strongest emission peak with the radius of curvature (ROC) or bending radius. The (ΔI/I) value gradually decreased as the ROC decreases from N (flat state) to 5 mm and showed a steady increase after ROC # 5 mm as in Fig. 7.8A. The relative intensity was observed to be minimum at ROC 5 5 mm due to the minimum effective in-plane strain which leads to a higher structural symmetry and lesser PL emission. The PL intensity was also studied as a function of bending cycles under both flat as well as bent states and observed to maintain more than 95% of their starting values after 104 cycles of bending as shown in Fig. 7.8B. Furthermore, the retention studies of the film as shown in the left inset of Fig. 7.8B exhibited no

Figure 7.7 PL spectra corresponding to the CsPbBr3/Al2O3 composite films using He and N2 carrier gases and addition of PTFE under N2 carrier gas. Source: Reprinted with permission from S. Kim, M.Y. Cho, I.S. Kim, W.J. Kim, S.H. Park, S. Baek, et al., Solvent-free aerosol deposition for highly luminescent and thermally stable perovskite-ceramic nanocomposite film, Adv. Mater. Interfaces 6(13) (2019) 1900359.

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Figure 7.8 The relative PL intensity of BCTZ:Pr/mica structure as a function of (A) bending radius, (B) bending cycles under flat and bending conditions. The insets in (B) show the retention measurement of the PL intensity under flat as well as bent states (left) and a schematic of the cyclic bending process (right) [32].

significant decay in the intensities of PL which demonstrates the BCTZ: Pr film exhibits robust functionality against mechanical bending. These results clearly show the BCTZ: Pr/mica structure’s switching endurance and long-term durability. These studies are useful for the development of next-generation lightweight, reconfigurable, allinorganic, flexible passive luminous, and transparent, devices and for establishing PL characteristics that are controlled mechanically in lattice-sensitive ferroelectric oxides. The high-grade heteroepitaxy on mica is projected to create a novel and easy method to enhance the database of the materials for future soft technologies. Similarly, Zheng et al. fabricated transparent and highly flexible Eu-doped 0.94Bi0.5Na0.5TiO3
0.06BTiO3 (BNTBT:Eu) epitaxial thin films on mica substrates using van der Waals epitaxy [34]. The variation in the PL characteristics of the flexible BNTBT:Eu film with respect to the applied mechanical strain is examined. The compression and the mechanical strains in tension can be induced in the mica substrate that would subsequently propagate to the BNTBT:Eu film, by attaching the flexible BNTBT: Eu/mica structure to the predesigned polyester concave and convex molds under inward and outward bending modes respectively. Each mold has a different ROC to modulate the bending strain of the structure moderately. Fig. 7.9A represents the variation of the relative intensity of PL (ΔI/I) of the strongest emission peak with bending radius (ROC) and observed that the ΔI/I value increases with ROC both under inward bending (ROC 5 22 mm to N) as well as outward bending (ROC 5 N to 12 mm). Furthermore, the relative intensity of PL is plotted with the variation of bending strain in the inset of Fig. 7.9A. It is clearly observed the enhancement in the relative PL intensity is more dominant in the case of outward bending (tensile strain) than that in inward bending (compressive strain). Further, it is observed that the variation in relative PL intensity is only less than 5% after 104 cycles under flat and bending conditions. The retention

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Figure 7.9 (A) The relative PL intensity of BNTBT:Eu/mica structure as a function of bending radius and bending strain (inset), (B) Retention measurement under unbent (ROC 5 N), inward bending (ROC 5 22 mm) and outward bending (ROC 5 12 mm) states. Source: Reprinted with permission from M. Zheng, X.Y. Li, H. Ni H, X.M. Li, J. Gao, van der Waals epitaxy for highly tunable all-inorganic transparent flexible ferroelectric luminescent films, J. Mater. Chem. C. 7(27) (2019) 8310
8315.

studies revealed that there is no significant decay in the relative PL intensity up to 104 s under flat as well as inward and outward bending conditions as shown in Fig. 7.9B. This suggests that the BNTBT: Eu/mica structure has excellent endurance due to the resistant behavior of PL switching to mechanical bending. Thus, the flexible BNTBT: Eu/mica structure is a viable candidate for next-generation flexible luminescent memory devices because of its remarkable endurance and stability of the PL characteristics.

7.3.4 Electroluminescence of flexible ceramic films Ceramic materials with strong luminescent properties can be employed as an electroluminescent layer in various flexible device applications. Straue et al. fabricated a flexible EL lamp with a device structure of PET/ITO/ZnS:Cu/BaTiO3/Ag/PET in which the ITO functioned as the front transparent electrode, ZnS:Cu acted as the luminescent layer while BaTiO3 (BTO) as the reflective dielectric layer and Ag as the bottom electrode with PET as the substrate [35]. The EL lamp exhibited a strong blue-greenish luminescence at the testing conditions of 60 V and 600 Hz under flat as well as bent states as illustrated in Fig. 7.10A and He et al. developed a humidity sensor built on alternating current electroluminescent devices (ACELs) with a device structure Ni/ Filter paper/ZnS:Cu/AgNWs (where “NWs” represent nanowires) [36]. Here, the ZnS: Cu functioned as a humidity sensing element as well as the phosphor layer while Ni and AgNWs acted as the bottom and top electrodes respectively with filter paper as the substrate. The ACEL exhibited a strong blue-green emission at an EL excitation

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Figure 7.10 Performance test of ZnS: Cu-based electroluminescent lamp at 60 V and 600 Hz (A) flat state, (B) bent state, (C) electroluminescent properties of ZnS: Cu-based ACEL device under different bending conditions [36]. Source: (A, B) Reprinted with permission from N. Straue, M. Rauscher, M. Dressler, A. Roosen, Tape casting of ITO green tapes for flexible electroluminescent lamps, J. Am. Ceram. Soc. 95 (2) (2012) 684
689.

wavelength of 506 nm. Moreover, the ACEL showcased a stable blue-green emission at various bending conditions as shown in Fig. 7.10C. Jun et al. employed a new strategy to develop a novel and highly flexible phosphorbased ACEL device without a substrate as well as a dielectric layer which led to the miniaturization of the device [37]. They have fabricated a free-standing ACEL device based on ceramic-polymer composite with a device architecture AgNWs/ZnS:Cu-PVB/ AgNWs (where “PVB” represents polyvinyl butyral). The ACEL device has exhibited only a minor reduction of 12.4% in its EL intensity (from 45.53 to 39.88 cd/m2) after 104 bending cycles at ROC of 0.5 mm. A significant EL emission was observed even at a minimum bending radius of 0.1 mm demonstrating the outstanding flexibility of the ceramic
polymer composite-based ACEL device.

7.3.5 Mechanoluminescence of flexible ceramic films It is known that the luminescence intensity of ML materials linearly increases with the applied pressure which leads to the fact that with the increase of applied

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pressure, the device output signal should also increase [38]. Shohag et al. have indirectly established that the increase of applied pressure on ZnS: Cu-based pressure sensor enhances its ML emission intensity and hence the electrical output of the pressure sensor is increased [39]. The application of mechanical energy on ZnS:Cu stimulated the release of trapped electrons and thus produced a light emission of 543 nm which is, in turn, converted into electrical energy by the integrated metal halide perovskite layer CH3NH3Pb (Br0.1I0.9)3. The sensor also exhibited a stable performance up to 103 cycles under a 3point bending test illustrating its excellent flexibility [39]. Mechanoluminescent visual sensing is an effective method to visualize the strain distribution which was originally invisible. Mechanoluminescent ceramic materials are used as the key element in ML sensors due to their high light emission upon mechanical stress application [40]. Terasaki et al. studied the ML visual sensing properties of a commercial SrAl2O4:Eu21 (SAOE) paint coated on a PEN substrate [40]. The SAOE exhibited a strong green emission with a wavelength of 520 nm. Moreover, the SAOE film displayed no significant variation in the ML intensity at different load cycles ranging from 10 to 104.

7.4

Flexible ceramic film-based optical device applications

The unique transparent features of flexible ceramic films come from a microstructure of closely stacked crystalline grains that allow light propagation with minimal scattering. Transparent and flexible ceramic films have found immense applications in various smart and advanced technologies, including photovoltaic devices, visual memory systems, photocatalytic devices, LEDs, EM wave absorbers, chromatic devices, opto-thermal devices, optical memories, etc. [41].

7.4.1 Photodetectors (photosensors) Photodetector basically converts light into electric signals and has immense applications in optical communications, sensors, missile detectors, etc. Flexible photodetectors can be employed in various portable and wearable optoelectronic systems. The applications of new flexible photodetectors, like oximeters, UV radiation skin sensors, flexible cameras, and e-eyes have begun to use ceramic materials layers to increase the performance of the optical sensors under flexure in current years. Highly transparent, photoresponsive, and flexible materials are essential for the fabrication of flexible photodetector devices [42]. To determine the performance of photodetectors, the crucial factors are photosensitivity and photoresponsivity. Photosensitivity or photoresponse factor (S) is given by the ratio of photocurrent (Iph) to dark current (Idark) as specified in Eq. (7.19) [43], PhotosensitivityðSÞ 5

Iph Idark

(7.19)

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Photodetector responsivity is a measurement of a photodetector’s optical-toelectrical conversion efficiency, commonly given in milliamperes (mA) of photocurrent produced per milliwatt (mW) of the optical signal. Theoretically, the value of responsivity should be constant, but in practice, it varies with both the signal optical power and the signal wavelength [44]. The photodetector’s responsivity (R) at a frequency (f) is given by [45] R 5 ηgP

q hf

(7.20)

where h represents Planck’s constant, η represents total quantum efficiency, and gp represents photoconductive gain [45]. Transparent and flexible ceramic materials are promising candidates to be used as photosensing elements [42]. Kim and Leem fabricated highly transparent and flexible ZnO nanorods (NRs) for UV photodetector applications [46]. With the ZnO NRs-based photodetector, optical transmittance of about 80%, photoresponsivity of 55.6 μA/W, and photosensitivity of 30 have been achieved. The flexibility of the photodetector was evaluated using the mechanical bending test. A steady decrease in photoresponsivity is observed as the bending angle decreased from 180 degrees (flat state) to 30 degrees at a bias voltage 1 V and power density 0.375 mW/cm2, suggesting degradation of its photovoltaic performance as shown in Fig. 7.11. Similarly, Huang et al. showed that SnO2 NB network-based transparent photodetector exhibited an optical transmittance of about 80%, UV photoresponse with excellent sensitivity, reproducibility, and reversibility [21]. The bending test results

Figure 7.11 Photoresponsivity of ZnO nanorods as a function of bending angle. Source: Reprinted with permission from D. Kim, J.Y. Leem, Transparent and flexible ZnO nanorods induced by thermal dissipation annealing without polymer substrate deformation for next-generation wearable devices, RSC Adv. 11 (29) (2021) 17538
17546.

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showed that the NB network may be bent to a bending radius of a minimum value 1 mm as in Fig. 7.12A and may function up to 103 bending cycles at a radius equal to 2 mm as in Fig. 7.12B with a marginal increase in electrical resistance when compared with SnO2 film. The SnO2 NB photodetector exhibited an excellent performance without a significant degradation up to a minimum bending radius of 1 mm as depicted in Fig. 7.12C and the photosensitivity is estimated to be around 102 even after bending to 1 mm. Further, the UV photoresponse of sputtered SnO2 thin film exhibited low sensitivity of approximately 10. Moreover, electrical conductivity decreases rapidly by 90% under dark and UV illumination when the device is subjected to bending with a radius of 1 mm. With the attractive advantages of NB structures like flexible optoelectronics, the free-standing NB network can be easily integrated and transferred into flexible functional electronics [21] in a scalable and cost-effective manner.

Figure 7.12 Sheet resistance of SnO2 nanobelt and film as a function of (A) bending radius, (B) bending cycles at a bending radius of 2 mm, (C) time-dependent UV photoresponse of SnO2 nanobelt networks at different bending radii [21].

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Yalagala et al. fabricated a broadband photodetector based on V2O5 on cellulose paper [47]. It is observed that the V2O5-based photodetector exhibited an enhanced responsivity (31.5 mA/W) under UV illumination when compared with that under visible light (20.52 mA/W) since the band gap of V2O5 lies in the UV region. This enables the generation of more charge carriers under UV illumination. The mechanical flexibility study of the photodetector demonstrated that the responsivity under UV as well as visible light illumination was stable up to 500 bending cycles as shown in Fig. 7.13. This enables us to develop UV photodetectors that are both conformable and invisible on a range of flexible curved substrates. Multishelled hollow structures are found to demonstrate excellent photoelectric properties and hence have immense application in the development of photosensors. Wang et al. fabricated a highly efficient and flexible photosensor based on ZnO@TiO2 cable-like nanoarchitecture photosensitivity of 1.27 3 105 under a UV illumination of 254 nm [48]. The photoresponsivity increased from 4000 to 12,000 A/W as the UV illumination power decreased from 1000 to 80 μW/cm2. The photosensor also displayed outstanding mechanical flexibility with a stable photoresponsivity up to 103 bending cycles with 5 mm bending radius. Furthermore, the ZnO@TiO2 photosensor retained 85% of its initial responsivity value even at a smaller bending radius equal to 0.5 mm. The comparison of photoresponsivity and photosensitivity of various flexible photodetectors is given in Table 7.2.

7.4.2 Solar cells Another promising area of the practical application of transparent and flexible ceramics is flexible solar cells (FSCs). Kim et al. fabricated a highly flexible perovskite solar cell (FPSC) based on TiOx which exhibited a power conversion efficiency

Figure 7.13 Responsivity of V2O5-based photodetector at different bending cycles. Source: Reprinted with permission from B.P. Yalagala, P. Sahatiya, C.S. Kolli, S. Khandelwal, V. Mattela, S. Badhulika, V2O5 nanosheets for flexible memristors and broadband photodetectors, ACS Appl. Nano Mater. 2 (2) (2019) 937
947.

Table. 7.2 Comparison of sensitivity and flexibility parameters of various photodetectors. Material

ZnO nanorods SnO2 nanobelt (NB) network V2O5 ZnO@TiO2 cable-like nanoarchitecture

Sensor parameters

Flexibility parameters

References

Photosensitivity

Photoresponsivity (A/W)

Minimum bending radius (mm)

Bending angle (degree)

Bending cycles

30 102

55.6 3 1026



1

180



103

[46] [21]


1.27 3 105

31.5 3 1023 12,000


5





500 103

[47] [48]

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(PCE) of 12.2% and excellent mechanical flexibility [49]. The PCE of the PEN/ ITO/TiOx/perovskite multilayer solar cell was reduced to only 7% at 1 mm of bending radius as depicted in Fig. 7.14A. Moreover, the PCE showed only a decrease of 5% after 103 bending cycles at 10 mm bending radius as depicted in Fig. 7.14B. Therefore, the PEN/ITO/TiOx/perovskite multilayer device structure can be used as practical wearable solar cells. Kim et al. fabricated highly flexible and transparent W-doped In2O3 (IWO) electrodes as a substitute for the commonly used ITO electrodes in FPSCs [50]. The IWO film exhibited 96% of high optical transmittance in the visible region and a minimum bending radius of 5 mm. In addition, the outer bending test of the IWO/ PET structure showed a nearly constant resistance till the outer bending radius is 5 mm and then the resistance abruptly increases with a further decrease in bending radius as given in Fig. 7.15. Similarly, the results of the inner bending test of the IWO/PET structure showed a critical radius of 2 mm as depicted in Fig. 7.15. Since the sufficient value of critical bending radius is 5 mm for practical device applications, the IWO can be employed as an effective electrode in FPSCs. Moreover, the PCE of the IWO/PET-based FPSCS is found to be 11.33% with a fill factor of 72.54%.

Figure 7.14 Power conversion efficiency of TiOx-based solar cell as a function of (A) bending radius, (B) bending cycles at a bending radius of 10 mm. Source: Reprinted with permission from B.J. Kim, D.H. Kim, Y.Y. Lee, H.W. Shin, G.S. Han, J.S. Hong, et al., Highly efficient and bending durable perovskite solar cells: toward a wearable power source, Energy & Environ. Sci. 8 (3) (2015) 916
921.

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Figure 7.15 Variation in the resistance of IWO/PET and ITO/PET structures as a function of bending radius under outer as well as inward bending tests [50].

7.4.3 Optical memories Optically driven memory devices known as optical memristors consist of memory elements that can be switched by the application of electromagnetic radiations. They have recently become a subject of great interest in the fields of artificial intelligence as well as neuromorphic computing systems [49]. Flexible optical memristors are foreseen as high potential components in next-generation portable as well as wearable optoelectronics devices. Cai et al. fabricated memristors based on Au/PMMA/Ag/MoO3/P3HT:PCBM/ZnO/ITO structures for optical communications [51]. The hybrid structure exhibited a resistive switching (on/off) ratio of about 103 at various bending radii as depicted in Fig. 7.16A. The hybrid structure also showed a stable performance up to 1000 bending cycles at a bending radius of 5 mm as in Fig. 7.16B. It is observed that the SET voltage critically depends on the intensity of light which is found to decrease when the illumination intensity increases. Moreover, it is shown that the hybrid memristor structure can act as an AND gate under the application of both electrical as well as optical signals as shown in Table 7.3. The switching ratio  103 was found to be stable up to 15 input cycles under an optical pulse of 0.172 mW/cm2, an electric pulse of 1 V, and a pulse duration of 1 ms. Because of their low power consumption and ultra-high operational speed, optical memristors can be employed in optically tuneable neural networks and can play a pivotal role in the fabrication of highly advanced information storage and

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Figure 7.16 Electric current of Au/PMMA/Ag/MoO3/P3HT:PCBM/ZnO/ITO memristor as a function of (A) bending radius, (B) bending cycle at a bending radius of 5 mm. Source: Reprinted with permission from S.Y. Cai, C.Y. Tzou, Y.R. Liou, D.R. Chen, C.Y. Jiang, J.M. Ma, et al., Hybrid optical/electric memristor for light-based logic and communication, ACS Appl. Mater. Interfaces 11 (4) (2019) 4649
4653. Table 7.3 Truth table for the Au/PMMA/Ag/MoO3/P3HT:PCBM/ZnO/ITO optical memristor [51]. Input Electricity

Light

Output

0 1 0 1

0 0 1 1

0 0 0 1

artificial intelligence devices. Chen and coworkers fabricated a UV-activated memristor-based artificial flexible visual memory device on a polyimide substrate [52]. It comprises a UV image sensor and a resistive switching memristor as shown in Fig. 7.17A. In2O3 semiconductor micrometer-sized wires are used as the photosensitive materials in the image sensors due to their wide band gap, effective charge transport, and excellent photosensitivity. The memristor was based on Al2O3 with top electrode Ni and the bottom electrode Au as in Fig. 7.17B. Externally applied UV light can be used to modulate the resistance state of the image sensor which showed an on/off ratio (Ilight/Idark) of about 102 at 1 V under 350 nm UV illumination with an intensity of 0.528 mW/cm2. On the other hand, the switching mechanism of the memristor will be controlled by resetting and setting voltages and showcasing a switching ratio of about 104 at 1 V under dark conditions. The UV illumination decreases the resistance of the image sensor which makes the memristor switch from OFF state to the ON state. Therefore, the memristor stores the information. The memory window (Iset/Ireset) of the visual memory system is estimated to be 102 at 1 V with excellent retention of about 1600 s and stability up to 50 cycles. The flexibility of the visual memory unit has illustrated that the memory

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Figure 7.17 (A) Schematic representation of human visual memory system. (B) The architecture of the UV-motivated visual memory device is based on Al2O3 memristor and In2O3 semiconductor micrometer-sized wires image sensor. (C) Bending test of the visual memory device as a function of bending angle. Source: Reprinted with permission from S. Chen, Z. Lou, D. Chen, G. Shen, An artificial flexible visual memory system based on an UV-motivated memristor, Adv. Mater. 30 (7) (2018) 1705400.

window was stable at different bending angles ranging from the flat state (0 degree) to 150 degrees as shown in Fig. 7.17C. Optical (or photonic) artificial synapses are the pivotal components in artificial intelligence (AI) devices that execute the neuromorphic computing process and hence highly transparent and flexible photonic synapses are essential for the fabrication of AI devices. The optically triggered variation in conductivity which is also known as “photo-gating” was employed in transistor-based flash memories in past years. Yang et al. fabricated a highly flexible and transparent optically triggered artificial synapse having a vertical device architecture of ITO/SnO2/CsPbCl3/ TAPC/TAPC:MoO3/MoO3/Ag/MoO3, where perovskite CsPbCl3 acted as the UV absorbed layer and 4,40 -Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] abbreviated as “TAPC” acted as the hole transporter layer [53]. Performance

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157

parameters of the fabricated optical synapses are attributed to the UV light-induced charge carrier trapping/de-trapping at the interface of SnO2/CsPbCl3 heterostructure. The heterostructure band alignment between SnO2 and CsPbCl3 leads to the separation of photogenerated charge carriers in the CsPbCl3-based AI device. The band alignment occurs during the UV light illumination as well as reverse bias and the electrons transit from CsPbCl3 through the SnO2 layer [53]. Furthermore, the current decreases rapidly when the UV illumination is turned off and the electrons trapped at the SnO2/CsPbCl3 interface results in a change in the conductance level. The CsPbCl3-based artificial optical synapse exhibits good bending capability on flexible PET and Parylene-C substrates. Similarly, Wang and coworkers developed a wearable three-dimensional artificial neural network (3D ANN) having a crossbar structure of Pt/HfAlOx/TaN on a flexible PET substrate [54]. This 3D ANN is fabricated as three-layer crossbar arrays in which each layer represents an artificial synapse. The postsynaptic current of three artificial layers was studied as a function of pulse number at a flat state as well as a bent state at a 5 mm bending radius as shown in Fig. 7.18. There were 50 conductance states each in long-term depression (LTD) and long-term potentiation (LTP). All the layers of the 3D ANN exhibited a stable LTP and LTD with 104 spikes demonstrating its stable operation and excellent flexibility to be employed in wearable neuromorphic computing devices.

Figure 7.18 The bending test of different layers of the 3D crossbar memristors based on HfAlOx under flat and bending conditions. Source: Reprinted with permission from T. Y. Wang, J.L. Meng, M.Y. Rao, Z.Y. He, L. Chen, H. Zhu, et al., Three-dimensional nanoscale flexible memristor networks with ultralow power for information transmission and processing application, Nano Lett. 20 (6) (2020) 4111
4120.

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7.4.4 Optical (or phosphor) thermometry Optical or phosphor thermometry is an accurate and versatile noncontact measurement for temperature measurements. Ceramic materials can function even at very high-temperature environments exceeding 1000 C and hence can be employed as phosphor elements as well as an effective thermal shield in numerous device applications. Aryal et al. fabricated flexible YSZ ceramic strips for optical thermometry applications [55]. YSZ can act as a protective shield to the embedded thermographic phosphor composites under extreme thermal conditions. It is found that the YSZ ceramic strips offered no interference to the temperature-dependent emission characteristics of the embedded phosphor composites and only provided a protective role for them. Moreover, the flexural strength of the ceramic strips was also evaluated as a function of the distance between the two supporting pins (d) using a 3-point loading technique at different temperatures as shown in Fig. 7.19. This suggests that YSZ ceramics exhibit good flexibility. Similarly, Regmi and coworkers fabricated dysprosium (Dy) doped yttrium aluminum garnet Y3Al5O12 (YAG:Dy) phosphor coating on YSZ ceramic ribbons for high-temperature sensing applications [56]. The temperature-dependent luminescence decay time measurements of the YAG:Dy (1%) phosphor-coated YSZ ribbons exhibited a stable decay of 900 μs up to 850 C and the decay time gradually reduces to 439 μs as the temperature increases to 1200 C as in Fig. 7.20. The flexural strength of the phosphor-coated, as well as uncoated ceramic ribbons, was studied under strain and stress conditions. No significant variation in flexural strength

Figure 7.19 Flexural strength (F) of the YSZ ceramic strips as a function of the distance between the two supporting pins (d) at different temperatures. Source: Reprinted with permission from M. Aryal, S.W. Allison, K. Olenick, F. Sabri, Flexible thin film ceramics for high temperature thermal sensing applications, Optical Mater. 100 (2020) 109656.

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Figure 7.20 Luminescence decay time of YAG:Dy phosphor-coated YSZ ceramic ribbons as a function of temperature. Source: Re-printed with permission from A.R. Regmi, S.W. Allison, K. Olenick, F. Sabri, High temperature phosphor thermometry with YAG:Dy and LED excitation on flexible YSZ ceramic ribbons, MRS Commun. 11 (3) (2021) 322
329.

was observed for both the phosphor-coated and uncoated YSZ nanoribbons under stress conditions. But under the strain condition, a decrease in the flexural strength is evidenced for the phosphor-coated ceramic nanoribbons. The effect of heating cum cooling cycles with N 5 100 for the phosphor-coated YSZ ribbons has also been studied and no marginal variations are observed.

7.4.5 Photocatalysis Flexible ceramic fibers (FCFs) have potential applications in the area of photocatalysis also, due to their high surface area, excellent mechanical flexibility, and superior thermal as well as chemical stability. FCFs are lightweight fibrous refractory materials that can be fabricated through various techniques such as electrospinning, centrifugal spinning, atomic layer deposition, chemical vapor deposition, selfassembly, polymer conversion, etc. [57]. The FCFs can be employed in photocatalysis in two different ways. Ceramic materials with excellent catalytic activity can be processed into FCFs and can be directly used as a photocatalyst [57]. Song et al. fabricated Zr-doped TiO2 (TZ) nanofibrous membranes and studied the effect of Zr doping (mol.%) photocatalytic activity and mechanical flexibility [58]. The TZ nanofibrous membrane with a Zr content of 10 mol.% (TZ-10) exhibited an enhanced photocatalytic activity of 95.4% under a UV illumination time of 30 min by using methylene blue (MB) dye as the template and a reusability up to 5 cycles. Furthermore, the TZ-10 membranes also showcased a superior mechanical property with Young’s modulus of 120.7 MPa, a tensile strength of 1.32 MPa, bending rigidity of 28 mN, and excellent flexibility up to 200 bending cycles at a 2-mm bending radius.

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On the other hand, FCFs can be employed as catalyst carriers in photocatalytic processes [57]. Zhang et al. integrated TiO2 nanoparticles (NPs) into the TiO2 nanofibrous (NFs) membrane and thus developed an efficient as well as stable nanoparticles-nanofibers composite structure (TiNF-NP) for photocatalytic application [59]. The TiNF-NP composite with NPs concentration of 2 wt.% exhibited 100% photocatalytic activity under a UV illumination of 30 min by using MB as the model dye as in Fig. 7.21A. It is observed that the photocatalytic performance and bending rigidity of TiNF-NP composite structures vary as a linear function of TiO2 NPs concentration (wt.%) as shown in Fig. 7.21B. Highly flexible and catalytically active ceramic nanofiber composite structures can also be fabricated by incorporating different ceramic fiber materials with distinct individual properties. Wang et al. evaluated the photocatalytic performance of a self-cleaning flexible TiO2-ZrO2 (TZ) composite nanofiber-based moisture electric generator (MEG) using rhodamine B dye solution as the analyte [60]. The TZbased MEG exhibited an excellent photocatalytic activity of about 95% under a UV illumination of 200 min. Furthermore, the TZ-based MEG demonstrated excellent flexibility till 103 bending cycles at 15 mm bending radius [60]. Huang et al. fabricated a heterojunction photocatalyst by incorporating N-doped WO3 nanobundles with Ce2S3 nanodots (N-WO3/Ce2S3) on a carbon textile support for the degradation of formaldehyde (HCHO) gas and exhibited 100% conversion efficiency within 80 min under visible light illumination [61]. The performance of the heterojunction photocatalyst remained almost constant under normal as well as bending conditions as demonstrated in Fig. 7.22 and thus showcased its excellent mechanical flexibility.

Figure 7.21 (A) Photocatalytic activity and (B) rate constant and bending rigidity of TiNFNP composite structures as a function of Ti NPs concentration [59]. Source: (A) Reprinted with permission from R. Zhang, X. Wang, J. Song, Y. Si, X. Zhuang, J. Yu, et al., In situ synthesis of flexible hierarchical TiO2 nanofibrous membranes with enhanced photocatalytic activity, J. Mater. Chem. A. 3 (44) (2015) 22136
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Figure 7.22 Photocatalytic activity of N-WO3/ Ce2S3 heterojunction photocatalyst under normal and bending conditions. Source: Reprinted with permission from Y. Huang, Z. Guo, H. Liu, S. Zhang, P. Wang, J. Lu, et al., Heterojunction architecture of N-doped WO3 nanobundles with Ce2S3 nanodots hybridized on a carbon textile enables a highly efficient flexible photocatalyst, Adv. Funct. Mater. 29 (45) (2019) 1903490.

7.4.6 Light-emitting diodes LEDs are having potential applications in the field of solid-state lighting and flexible displays. Flexible ceramic materials can be used as substrates in the fabrication of LEDs with flip-chip technology for improved thermal management and operational efficiency. Kim et al. compared the performance of GaN-based flip-chip LEDs (FC-LEDs) on a flexible YSZ substrate and a polymeric substrate respectively [62]. The emission intensities of LEDs on YSZ and polyimide substrates are approximately identical at low currents (I , 10 mA). On the other hand, at the higher currents ( . 10 mA), the emission intensities of FC-LEDs on the YSZ substrate become higher when compared to those on the polyimide substrate. Furthermore, negligible spectral shift (1 nm) is observed for YSZ substrate while a redshift of 6 nm is observed for the polyimide substrate by increasing the current from 1 to 100 mA. More importantly, it is observed that there is no degradation in luminescence intensity or significant redshift of peak emission for the FC-LED on flexible YSZ in comparison to that on polyimide substrate suggesting its better ability in transferring the heat produced from the chip to the substrate. So, the thermal simulation studies confirm the efficacy of YSZ to be employed as a potential heat dissipation substrate in LED applications. Therefore, the flexible ceramic substrate is favorable for achieving flexible high-power LED applications. Rare-earth atoms doped ceramic materials are known to exhibit enhanced luminescence properties and can be employed as the phosphor component in LED devices. Hua and Yu synthesized a flexible reddish-orange emitting CGN:0.5Sm31PDMS phosphor film (where CGN denotes Ca2GdNbO6 and PDMS represents polydimethylsiloxane) for LED application [63]. The CGN:0.5Sm31-PDMS film

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exhibited a reddish-orange emission at a chromatic coordinate (0.622, 0.376) under a near-ultraviolet (NUV) illumination of 405 nm as depicted in Fig. 7.23B. Mechanical bending of the phosphor film exerted no significant effect on its luminescent properties as shown in Fig. 7.23C demonstrating its excellent flexibility. They have also fabricated a flexible white LED device by incorporating a redemitting CGN:0.5Sm31 phosphor, green-emitting BSS:Eu21, and blue-emitting BAM:Eu21 in a silica matrix with a NUV chip of 405 nm. The packaged LED device demonstrated a strong white light emission with a correlated color temperature (CCT) value of 4896 K, chromatic coordinate (0.350, 0.368), and excellent color rendering index (CRI) of 92.43.

7.4.7 Other applications Other optical applications of flexible ceramics include EM wave absorber, photochromism, acousto-optic modulator, etc. P. Wang and coworkers developed flexible SiC ceramic nanofibers which can be employed as high-frequency EM wave absorbers [64]. The nanofibers exhibited excellent mechanical flexibility without any fracture up to a bending angle of 142.6 degrees and outstanding absorption properties having an effective absorption of about 90%, reflection loss (RL) , 2 10 dB, and bandwidth of 4
18 GHz. Moreover, they demonstrated good hydrophobicity, high-temperature stability as well as corrosion resistance in an alkali environment. Luminescent inorganic photochromic materials especially ceramic materials such as metal oxides, organic
inorganic hybrids, etc. exhibit excellent mechanical strength, thermal stability, and corrosion resistance and hence have numerous applications in 3D optical memory devices. Zhu et al. illustrated an effective strategy to enhance the luminescence switching contrast (ΔRt) of BaMgSiO4:Eu21 (BMS:Eu) photochromic materials by $ 300% via site-selective occupancy engineering [65]. The flexible film was made by incorporating the BMS particles into the silica gel matrix. The composite film which appeared to be white with chromaticity coordinates (L 5 80.82, a 5 2.65, b 5 4.10) turns into pink color with chromaticity coordinates (L 5 62.95, a 5 23.09, b 5 212.45) under a 405 nm illumination for

Figure 7.23 CGN:0.5Sm31-PDMS phosphor film under (A) daylight, (B) NUV illumination (flat state), and (C) NUV illumination (bended state). Source: Reprinted with permission from Y. Hua, J.S. Yu, Synthesis and luminescence properties of reddish-orange-emitting Ca2GdNbO6:Sm31 phosphors with good thermal stability for high CRI white applications, Ceram. Int. 47 (5) (2021) 6059
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30 s. The initial state of the photochromic material can be recovered either by thermal stimulus or light illumination as illustrated in Fig. 7.24A. Here, the photochromic material recovered its initial color by a thermal stimulus of 180 C or by 532nm irradiation for 120 s. The BMS:Eu-silica composite film (8 cm long) exhibited excellent flexibility and can be bent multidirectionally without any fractures. The writing pattern remained clearly visible even at an elongation of 200%. Moreover, the composite also showcased excellent hydrophobicity with high contact angle (CA) of 109.85 degrees under the flat state and an enhanced CA of 115.4 under 200% elongation as illustrated in Fig. 7.24B. Another promising application of transparent ceramic materials is their usage in acoustic-optic modulators. Liu et al. fabricated piezoelectric nanogenerators based on Ba0.85 Ca0.15Zr0.1Ti0.9O3(BCZT) thin films coated on flexible mica substrates [66]. The mica/BZCT has high optical transmittance of more than 70% in the visible light range. It is observed that the short-circuit current and open-circuit voltage is found to increase from 108 to 463 nA and 2.5 to 6.7 V as releasing angle increases from 30 to 90 degrees. The tapping simulations of the BZCT-based PENG exhibited that the average output voltage increases from 0.9 to 2.3 V as the applied force increases from 1 to 9 N. Moreover, the BZCT-based PENG exhibited a steady output voltage of 1.8 V even after 20,000 tapping cycles at 5 N force showcasing its enhanced durability. Hence, the BCZT films can be considered as a promising candidate for the fabrication of flexible acoustic-optic modulators [66]. He et al. fabricated a humidity sensor based on ZnS:Cu ACEL with a strong bluegreen light emission for postharvest preservation of vegetables and fruits [36]. The ACEL can thus simultaneously offer the functions of postharvest preservation as well as humidity sensing. The static bending properties (normal, concaved, and convexed conditions) of the ACEL device under different humidity conditions

Figure 7.24 (A) The changes in the writing pattern and (B) variation in contact angle of BMS:Eu composite film under 200% elongation [65]. Source: Reprinted with permission from Y. Zhu, H. Sun, Q. Jia, L. Guan, D. Peng, Q. Zhang, et al., Site-selective occupancy of Eu21 toward high luminescence switching contrast in BaMgSiO4-based photochromic materials, Adv. Opt. Mater. 9 (6) (2021) 2001626.

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Table 7.4 Sensitivity of humidity sensor based on ZnS:Cu ACEL device at various static bending conditions [36]. Bending condition 

Normal (0 ) Concaved (290 ) Convexed (90 )

Sensitivity (µA/RH) 6.50 6.31 6.36

demonstrated a stable performance with the sensitivity remaining almost constant under the different bending conditions as shown in Table 7.4 [36].

7.5

Conclusions and future prospects

This chapter highlighted the concepts and fundamentals of optical properties in general and the effect of flexibility on different optical properties of various flexible ceramic films, composites, and nanostructures in detail. Various flexible ceramic thin films that are potential candidates for various such as photodetectors, solar cells, optical memristors, opto-thermometry, photocatalysis, LEDs, photochromism, etc. have been explored and the effect of flexibility on the device performance is also discussed. The research on multicomponent flexible ceramic materials has to be boosted as they can enhance the overall device efficiency as well as mechanical flexibility than single-component ceramics. The development of highly efficient FSCs and other electronic devices based on transparent flexible ceramic materials can effectively tackle the issue of space utilization to a great extent. Highly flexible ceramic composite nanostructures with superior photocatalytic activity can be employed for the degradation of various pollutants, air purification, water-splitting process, hydrogen production, etc. in the near future. Flexible ceramic materials can also find potential application in the automotive industry, especially in the manufacture of electric vehicles (EVs) as they can be utilized in the development of advanced EV powertrains because of their improved thermal management. Furthermore, transparent flexible ceramics can be employed in the development of various human
machine interface technologies and other portable as well as wearable devices.

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4689.

Chemical vapor deposition processing and its relevance to build flexible ceramics materials

8

Vitaly Gurylev National Synchrotron Radiation Research Center, Hsinchu Science Park, Hsinchu, Taiwan

8.1

Introduction

Ceramics are identified as one of the most important and utilized classes of materials for technical and industrial applications and their utilization ranges from solar cells and batteries to electronic devices [1 3]. In general, ceramics are regarded as materials that have the combined presence of two or more chemical components which are presented in the form of a single or unified composition. It has a wide representation in terms of utilized chemical elements and can be identified in various electronic and geometrical configurations that include, for example, mixed metal oxides or transition-metal nitrides [4,5]. Following it, one might conclude that the diversity of existing ceramics determines which concrete types of them are supposed to be used with regard to the proposed application and what specific features and characteristics they should have in order to demonstrate the required level of performance, stability or other related parameters that are regarded as the most optimal pathways to reach the desirable and designated outcome. Ceramics originally are considered as heavy, bulky, and brittle materials which cannot sustain any bents or sudden mechanical impact and simply break down in order to compensate the use of external force that is applied to change its morphological and structural composition. Thus, despite demonstrating highly advanced and attractive properties and characteristics that can provide great benefits compared with other compounds and materials systems during their expected employment in various emerging devices and components, the capability to keep the original structure and to deliver certain resistance in terms of the employed stress is appeared to be one of the most crucial and challenging drawbacks that leads to certain limitations in concrete and designated applicability of these materials. In this regard, it was demonstrated that utilization of nanoscale engineering can easily solve this problem as it allows to shape the ceramics into a designated form that allows to increases greatly their flexibility and elasticity thus making them become more sustainable for the changes in shape and form. For example, nanowires, nanorods, and other related one dimensional (1D) nanostructures that have high surface-to-volume ratio and also possess large curvature enable to deliver an unprecedented level of bendability, in the same time preserving or even strengthening further their exceptional physical, Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00008-7 © 2023 Elsevier Ltd. All rights reserved.

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chemical, electrical, electronic, optical, etc. properties [6 8]. Furthermore, it is also possible to use these 1D nanostructures as building blocks to create more complex and advanced morphological compositions and networks that are based, for example, on the implementation of 3D design and features. In turn, one should also look at the synthesis of 2D structures where a thin layer of ceramics presented within a thickness of several nanometers or even angstroms allows reaching certain relaxation of its mechanism bitterness due to the atomically thin features that guarantee the minimized cross-sectional area under similar material length. Thus, it can hold the extremely low stiffness (being “soft”) associated with a certain and highly desirable value of the Yong coefficient. It is necessary to mention that nanoscale composition also allows to greatly increase specific characteristics associated with the surface which can contribute to the great enhancement in the applicability of some ceramic materials in devices and processes where the surface plays a crucial and highly distinguished role. For instance, it is possible to introduce porosity or make a hollow-like structure that positively influences a specific surface area. Following this thought, Gro¨ttrup et al. [9] noticed that “macroscopically porous materials in form of interconnected 3D networks made from the building of nanoscaled blocks exhibit all the interesting nanoscale physical and chemical properties suitable for most advanced applications, and in addition, their large sizes (equivalent to bulk) and mechanical flexibility. . . allow their easy device integrations and appropriate utilizations”. In addition, the same authors stated that the porous 3D interconnected networks based on metal oxides presented in the form of “flexible ceramic networks (FCNs)” [9] attracted numerous attention in recent years that were reflected in advancing its specifications and searching for more intensive applications. However, even though flexible nanoscaled ceramic networks are highly attractive candidates to be utilized in currently existing technologies and related proceedings following those great properties that they possess and enable to show under extreme conditions within the most demanded and desirable range, their fabrication is still a complicated, challenging and somewhat hardly reachable task at a current moment. Various strategies have been extensively employed and investigated to realize this step and advance it to the level that is comparable with other types of materials that show relatively similar chemical compositions. Following it, a certain success which in some terms could be considered as highly impressive and promising has been reached so far [10,11]. But one can say for sure that this field, that is, the field associated with synthesis and formation of flexible nanostructured ceramics is still in great need of more thorough research which means that the search to identify the existence of simple and versatile nanoscaled procedure that allows reaching great adaptability in terms of controlling the specification of produced ceramic materials should be pushed further. Moreover, accessibility for mass production and consequent implementation into industrial processing should also be considered which increases the requirements for the fabrication process and put more requirements on already known and applied experimental protocols. As to approach this topic in more precise terms, and make the present discussion concerning it go deeper into detail, it is necessary to notice that generally for the current there exist two main strategies to create nanoscale compounds that are called as top-down and bottom-up approaches [12]. The top-down approach is less attractive

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given that it suffers from various drawbacks such as the inability to precisely control the feature of produced materials, extreme cost, complications. For example, special lithography techniques have a demand for multistep fabrication processes which turn out to be quite complicated and expensive. In contrast to this, the bottom-up recipe makes the building of nanostructured units become assembled together from atoms to grow nanostructures in the fashion that is designated by the parameters of this processing. One of the examples of this approach is chemical vapor deposition (CVD) where the formation of materials occurs via a chemical reaction between certain species called precursors or source compounds. The intensiveness of this reaction, the time at which it is allowed to proceed, or the types of chemical species that are introduced into the chamber influences greatly the finalized outcome in terms of its structural, chemical, and other related characteristics. Moving closer to the main concept of producing flexible ceramics, the form of 1D nanostructures can be considered in more detail and as a model to deepen the understanding. It can be realized either by direct synthesis when their formation is occurred due to a reaction or by using certain polymer or other type templates which are covered with a thin film of designated material and then these templates are removed. In this regard, it also becomes obvious that the formation of 2D via this experimental setup also could be considered as a perspective given that even in absence of a designated template, yet being formed on the flexible substrate, it can deliver the required level of mechanical properties that in perspective enable to satisfy the requirements toward further application. However, CVD is not a simple process and the particularities of its utilization as well the design of the instrumentation and the type of used chemical reaction are also identified by the nature and of materials that are going to be deposited. To be more clear, different experimental protocols are supposed to be used for the formation of ZrO nanorods or Al2O3 thin film even though in general terms they have a lot of similarities as both could represent relatively the same class of flexible ceramics [13,14]. Thus, in order to be properly used and applied, this approach is required to have a discussion in highly precise and concrete terms that allow reaching a complete understanding of the processes that occurred not only within the reaction chamber when required species are introduced but also with regard to produced nanostructures in terms of particularities, especially the one that represents morphology and surface geometry that they could show. Thus, the goal of the present chapter is to fulfill this gap and to provide necessary information on the application of CVD to create flexible ceramics in the form of various nanoscale compositions and structures in such a term that it might become a highly useful source of knowledge that allow advancing this process with regard to current achievements.

8.2

Chemical vapor deposition: principles and fundamentals

8.2.1 Basic understanding of chemical vapor deposition CVD is a widely used processing technology to fabricate nanostructures with various morphologies on top of the heated substrate by means of a chemical reaction

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that can be proceeded in the form of thermal decomposition or chemical reduction between precursors and the availability of the latter ones is determined by their presence in the form of gas-based compounds. Thermal decomposition most frequently involves the use of organometallic compounds but can also be applied to halides and other related compounds while chemical reduction is obtained via using a reducing agent that is usually identified as hydrogen or metal vapors [15,16]. CVD in a large part is used to produce thin film-related composition but under specific conditions and certain parameters, it is also possible to realize other geometries such as 1D and 0D materials. In contrast to physical vapor deposition methods, such as evaporation and sputtering, CVD offers a clear advantage as it enables to proceed with tunable deposition rates as well as high-quality products with excellent conformality and uniform thickness [17]. Another important feature is that it allows the utilization of greatly localized and selective deposition following the morphological and structural features of the utilized substrate thus creating a patterned composition. Furthermore, by playing with details of the experimental protocol that include the composition of the reaction gas mixture, its pressure, distance to the substrate, temperature and vacuum level of the reaction chamber as well as the duration of the overall processing it is possible to adjust a wide range of physical, tribological, chemical, and structural properties within the deposited materials following very precise accuracy [18]. Overall, the CVD approach currently has almost no analogies in semiconductor processing and it is regarded as the most advanced method to prepare nanoscale compounds allowing to reach only the designated limit of deviation from ideal compositions. There exist various types of CVD processing that include atmospheric-pressure CVD, low-pressure CVD, ultrahigh vacuum CVD, plasma-enhanced CVD, microwave plasma-assisted hot filament CVD, metal-organic CVD, photo-initiated CVD, atomic layer deposition (ALD), spray pyrolysis, liquid-phase, epitaxy [15]. For example, thermal CVD uses higher temperatures to provide the energy necessary for a chemical reaction that results in the formation of designated materials on the substrate while for plasma-assisted CVD the reaction is activated by the presence of a plasma source that transfers its energy to one of the precursors that enhancing the rate of realized reaction. It is often the case that employed CVD can be classified as mixed types since it possesses characteristics of several different CVD processing. As an illustration, this description could be attributed to the atmosphericpressure thermal CVD processing, where the gaseous reactants are carried by an inert gas such as nitrogen into the already heated reactor chamber.

8.2.2 Reaction mechanism of chemical vapor deposition and its relation with a substrate CVD processing is promoted by nucleation and consequent growth mechanisms and thus these two separate steps should be discussed in detailed and concrete terms as it allows to identify how the fabrication of the desired composition is usually realized and which factors have the higher affection. Nucleation is associated with the

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adsorption of atoms on the surface of the substrate and their further diffusion in order to fill energetically the most favorable sites. As it was stated by Bryant [19], modern nucleation theory [20] has been discovered to have great applicability toward an explanation of CVD [21,22] even though some complications might arise due to the presence of a multicomponent system. At the beginning of CVD processing, the formation of nuclei which are defined as the accumulation of several atoms occurs at the most energetically preferable sites on the surface of a substrate. An increase of their dimension following vertical and horizontal expansions is realized via adsorption of an individual or combination into small groups of atoms which arrive is proceeded via the accomplishment of reaction between precursors as a result of diffusion due to their positioning on the surface of the substrate in the form of neighboring species. It is necessary to notice that the growth of materials via the assembling of atoms usually occurs by following the appearance of structural formations that have the lowest energy such as steps or kinks. In this case, the atom becomes faceted which means that the resulting composition usually is presented in the form of crystallites. Their existence becomes especially relevant given that reaction chambers often are subjected to very specific heating which reaches several hundreds of degrees. These crystallites can be presented as single crystals or polycrystalline as it is mostly dependent on how the assembling of atoms occurs in terms of their orientations. In certain instances, these crystallites are very thin and at the beginning of deposition usually present separate islands which later become combined due to consequent coalescence. In this regard, the growth is considered as taking the form of 2D processing that reliability is determined by layered appearance with gradually grown thickness. Thus the conditions of the substrate in terms of the existence of various imperfections and defects determine the existence of preferable sites for nucleation thus serving as a tool to increase or decrease their density. In combination with the appearance or absence of supersaturation, it can serve as a tool to control the appearance of epitaxial growth. In turn, it is important to state that the surface energy of the substrate as well as the structure of its surface layer in terms of crystal orientation or presence of incomplete atomic orders can serve as identification whether the deposition can be realized and in which form. In this regard, it becomes evident that changes in any parameters associated with the presence and processing of precursors in the reaction chambers become only considered as serving a secondary role due to the apparent absence of the nucleation process or its presence in the limited form that cannot be pushed over the designated limit if only to change the substrate.

8.2.3 Atomic layer deposition: a special type of chemical layer deposition ALD is a gas-phase technique capable of producing thin films of a variety of materials. It is regarded as a type of CVD processing since both of them have similar chemistry, yet the way that reaction is realized has significant differences. Particularly, ALD is based on a sequential protocol where each precursor is

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introduced separately into a chamber separated by the purge of inert gas and thus their interactions are only limited once they become adsorbed to the surface of the substrate [23,24], while in CVD all precursors are continuously and simultaneously supplied without interruption and their reactions become occur at above the substrate at any point of the chamber. Following it, ALD enables to show self-limiting mechanism, which is described in more detail below, and it allows to reach exceptional characteristics of deposited films such as high conformality on high-aspectratio structures, thickness control at the angstrom level, and tunable film composition [25]. In this regard, it is considered a more preferential and optimal choice than CVD to create nanostructured formations. As for the more concrete discussion, the growth of a single monolayer by ALD can be described by the following four steps as it is shown in Fig. 8.1: (1) the first precursor exposure, (2) purge or evacuation of the reaction chamber to remove excess precursors and volatile reaction by-products, (3) the second precursor exposure, and (4) purge or evacuation of the chamber [23]. Steps 1 and 3 are ascribed as half-reaction of a single ALD reaction cycle. The reaction parameters such as substrate temperature, reactant vapor pressure, length of exposure of precursor, and the length of the purging periods are crucial to obtain the deposits with desirable thickness [26]. Typically, each full ALD cycle results in the growth of a single monolayer. To produce a film with the desired thickness, the ALD reaction cycle is repeated as much as it is necessary. Since each ALD cycle has a dimension within the subnanometer scale, the overall thickness of the deposited compound can be Precursor

Step 1: Precursor Exposure

Step 2: Purge

1 Cycle Reactant

Byproduct

Step 4: Purge

Step 3: Reactant Exposure

Figure 8.1 Schematics showing the growth process of atomic layer deposition. Source: Reprinted from H. Kim, H.-B.-R. Lee, W.-J. Maeng, Applications of atomic layer deposition to nanofabrication and emerging nanodevices, Thin Solid Films 517 (2009) 2563 2580. https://doi.org/10.1016/j.tsf.2008.09.007 with permission from Elsevier. Copyright 2009.

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adjusted with super precision and its spread would be uniform which means that the existence of unexpected bumps and crackers would be in most cases avoided. It is interesting to notice that ALD processing due to its specifications can be realized at temperatures not only as low as 100 C [27] but even approaching room temperature meaning that it is possible to use substrate or templates such as the one that is identified as polymer-contained that cannot sustain extreme conditions. It is a very important and crucial advantage compared with other related vapor-based processing that requires the use of temperatures over 300 C. Several words need to be said about the self-limiting mechanism since it is considered as the main advantage of ALD that allows producing pin-hole free and very conformal films and also make it usable to the substrate that has a high aspect ratio. It is described as follows. Once the precursors are introduced into the reaction chamber it tends to interact with reactive sites on the surface of the substrate to become adsorbed and then transformed into nucleation spots that initiate the growth of the film. In this regard, if the reactive sites cover the surface of the substrate uniformly, the distribution of precursors with regard to their adsorption on these sites also would follow the same pattern. Thus, once they are fully occupied and there is an excess of precursor, it is unable to find a vacant site to be adsorbed and hence, become swept away by the following purge step. Then, the second precursor arrives and it reacts only with already adsorbed and nucleated atoms or molecules from the first precursor continuing the same pattern associated with the uniform coverage of the substrate surface. Thus, the reaction becomes self-limited as it is only controlled by the availability of the reactive sites on the surface. In this regard, once those sites become fully occupied by the precursors, the presence of the latter usually is considered as reaching status which means that increasing the time of precursor’s exposure would not increase the number of nucleated spots, not their availability, and simply would be considered as wasting the chemicals since all-nonreacted compounds are supposed to be removed. Thus, it might be said that saturation of precursor is identified as a necessary feature and thus evidence of ALD processing, while nonsaturation is a clear sign that CVD takes place. Overall, there is an existing so-called ALD window, which determines what the range of pressure, temperature, parameters of precursor exposure, purge time, etc., could be used to keep the growth process to be within the frame of saturation thus fulfilling the self-limiting mechanism. Beyond this window, ALD eventually could be transformed into CVD where each cycle failed to be similar to others and thus fabrication of film occurs following the quantitative presence of precursors without showing any sign of its limitation regarding its use. As ALD generally is considered to be a dry process, the surface of the substrate should be attractive for the precursor to becoming adsorbed otherwise it would be just removed by the next purge step. Simply to say, the presence of reactive sites on the surface of the substrate is a necessary and highly important requirement that allows the introduced atoms and molecules to nucleate and grow film. These reaction sites are usually based on the existence of structural defects or the presence of functional groups that mostly contain OH-related bonds. For example, Hsueh et al. [28] demonstrated that commercial pristine carbon nanotubes (CNTs) which are

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determined to be almost ideal materials have an extremely high ratio of carbon and absence of sufficient defects on their surface. As a result, Pt nanoparticles (NPs) deposited by ALD simply avoided residing on it as there are no reactive sites for it. Once the oxygen plasma treatment was applied, and CNTs became more defective and demonstrated higher availability of oxygen, Pt NPs became fully occupy them [28]. The same principle was applied by these authors in their other study but instead of oxygen plasma treatment, the acid treatment was used to create OHgroups on the surface of CNTs [29]. Definitely, some substrates and materials already have a great presence of oxygen-based functional groups on their surface such as Si wafer as it is covered with native oxide and thus ALD processing can be realized without preliminary treatment. Similar to CVD, ALD also can be available in various configurations that include not only the original thermal setup, but also the one equipped with plasma. In this case, it is called as plasma-enhanced atomic layer deposition (PEALD) and plasma takes the role of second nonmetal precursor [30]. Apart from it, it also could serve the purpose of creating the functional groups or atomic defects on the surface of the substrate prior to actual deposition thus providing a one-step process for deposition of the thin film without the usage of any intermediate surface treatment protocol. It is considered as a high advantage compare with conventional thermal ALD, where the adaption of substrate for the materials fabrication is proceeded separately and remotely thus creating additional difficulties in optimization. Also, PEALD allows to reach a higher growth rate of materials, that is, thicker monolayer can be deposited per cycle which means that under similar conditions of exposing the precursor, this technique has a great possibility to decrease the levels of its employment by simply increasing the duration of its use. Given that price of precursors for ALD is extremely high and often is provided in the form of homemade substances that can only be obtained via specialized vendors, this benefit is highly important.

8.3

Chemical vapor deposition processing to build flexible ceramics

8.3.1 Current status CVD is identified as one of the best and most utilized approaches to prepare flexible ceramics as it allows not only to achieve target geometries following designated nanoscale dimensionality and size but also provides a great capability in adjusting highly specific morphological and structural aspects. Thus, choosing the correct type of deposition, that is, whether it should represent a thermally activated, plasma-assisted, or radical-supported process, and also controlling its parameters and specifications including the choice of employed substrate and the type of utilized precursors, it is possible to greatly influence the chemical and structural compositions of obtained ceramics. In this regard, the formation and further

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development of some very important properties and characteristics can be realized following a specific direction and narrative. For example, it is possible to define whether the obtained ceramics is presented in fully or only partially crystalline forms. In turn, the presence of vacancies and other lattice-related imperfections also could be managed. Following this thought, one can say that CVD can be used to proceed with the manipulation of both inner and outer structures along with changes in the accompanying features. It can be realized in following an independent or simultaneous manner. In the latter case, these changes enable to resonance with each other in such a way as to allow the material to reach the best outcome in terms of demonstrated performance. As an illustration, decreasing the grain size that positively influences the flexibility [31] also greatly affects the surface smoothness thus decreasing the level of its distortion and also allowing to obtain better conformability of the deposited film. Overall, the applicability of CVD to create flexible nanoscale ceramics is defined by the type of desirable geometry that it is supposed to have. Simply to say, it can be divided into two main categories that include 1D and 2D based compositions. Definitely, one should also notice that the existence of 3D compositions cannot be excluded, yet the discussion for these structures with regard to their development by applying the CVD approach does not only represent a highly complex topic but also practically is unreliable given that only limited number of references are available for it. Returning to our discussion, and leaving aside 2D nanostructures, the formation of 1D nanostructures by CVD could also in turn be divided into two directions that are ascribed as follows. Firstly, it is a direct deposition which means that the designated structure in the form of nanorods, nanowires, nanobelts, etc. could be synthesized following the formation of defined chemical composition where the flat substrate is used as a residence for their localized formation and consequent growth. It is very similar to the synthesis approaches that are used to create thin film-like flexible ceramic. Secondly, it is a template-based approach when the realization of these structures is proceeded via using certain models that usually represent polymer or other compounds. To be more precise, these models become covered with thin layers of designated ceramic materials and then are subjected to elimination whether via chemical proceeding or by applying thermally activated steps. Definitely, in this case, the outer layers should not suffer any damage or unexpected changes in their compositions and structure. Comparing both mentioned approaches that are associated with the formation of 1D flexible ceramic nanostructures and leaving aside the discussion about the formation of 2D nanostructures that are referred to as thin films, one can notice evidently that the former approach is more attractive as it proceeds usually via a single-step experimental protocol which greatly economizes the time and materials resources for its realization which means the decreasing of the overall cost. Furthermore, it also places fewer requirements for the utilized equipment and instrumentation as the only CVD apparatus itself is necessary which makes it more accessible for further implementation. Yet, the second approach allows reaching better control over the morphological features of synthesized ceramics as its changes with regard to shape and morphology fully follow the specifications of the

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template. In this case, for example, the hollow-like structure or multiwalled compositions could be easily created while for the former approach it is a more challenging task. Nevertheless, below each of these strategies including the formation of thin films is described in specific detail as to provide a certain understanding of their main principles and basics. It is also shown what is the difference in terms of demonstrated flexibility and the existence of other mechanical characteristics that nanostructures synthesized by both approaches could demonstrate. As a final word, one should notice that CVD also could be used to extend the mechanical characteristics of already available and formed nanostructures serving as a substrate to deposit an additional layer that covers their surface thus protecting it from extensive mechanical impacts such as the one obtained from using applied stress or extreme bending. In this case, the formation of so-called core shell composition occurs where the core represents the original structure while the shell is produced by CVD. The control over thickness with regard to the chemical and structural compositions of the deposited layer defines its characteristics and also greatly assists in creating the correct synergy between it and the core materials. It is interesting that recently this strategy was discovered to be highly applicable toward materials systems where both core and shell demonstrate similar chemical composition with regard to the presence and arrangement of identical elements. More details are provided below.

8.3.2 Development of film-like structures on the flexible substrates To make the description of this approach more visible and accessible, one can imagine a piece of toast that can be bent and pressed without breakage of its original form and shape. Then this piece becomes covered with a layer of alumina foil that follows its particularities and structural changes with precise accuracy. In this regard, once the toast is subjected to bending again, the alumina foil would follow this movement without the appearance of noticeable cracks or distortion that negatively influence the features of its surface. Definitely, if alumina is taken as a bulk, its thickness is extremely huge, and it is impossible to make any changes in its morphology and geometry without damaging it. A similar method is applied to ceramics. Simply to say, firstly it is defined which flexible substrate is supposed to be used and in the following step, the choice and characteristics of deposited ceramics are identified. Thus, CVD takes the role of the tool that is used to cover the substrate. However, one cannot say that there is an ultimate and union settlement for all ceramics materials in terms of using specific substrates as its applicability toward each certain investigated material system is determined by various factors and should be considered in terms of its own uniqueness and nonsimilarity. For example, Ruoho et al. [32] demonstrated that ZnO and Al2O3 deposited on polyamide enable to show great sustainability with regard to using mechanical stress as it was revealed that only employment of sufficient strain up to 12% enables to make the appearance of cracks and distortions on the surfaces, Fig. 8.2A and B. In turn,

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Figure 8.2 Scanning electron microscope (SEM) overview of the fractured samples after tensile straining up to 12% showing the channel cracks and the transverse buckles/crack owing to lateral contraction. (A) ZnO on polyimide from a 55 degrees angle. (B) Al2O3 deposited at 155 C on polyimide. Optical microscope images of the fragmentation process of the 46-nm thick Y2O3 film grown directly on the polyimide substrate. The strain was applied horizontally in the field of view of the image. (C) Unstrained Y2O3 specimen. (D) At 0.7% tensile strain, the first cracks were observed. Source: Reprinted from M. Ruoho, J.-P. Niemel¨a, C. Guerra-Nunez, N. Tarasiuk, G. Robertson, A.A. Taylor, et al., Thin-film engineering of mechanical fragmentation properties of atomic-layer-deposited metal oxides, Nanomaterials 10 (2020) 558. https://doi.org/ 10.3390/nano10030558. Copyright MDPI 2020. Available under the Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0/).

once the deposited materials were replaced with Y2O3, a different picture could be observed. Utilization of even minimal mechanical stress within the tensile strain of 0.7% could induce the development of cracks. It represents very low flexibility and the inability to substance any bending, Fig. 8.2B and C [32]. Definitely, it is also important to consider the thickness of the deposited layer. It would be logically correct to suggest that following its increase in dimension, the chances that breakage of the surface would occur with the usage of a very low level of applied stress become rapidly enhanced in geometrical progression. In this case, it might be realized that the thinner the film, the more intensive bending and mechanical influence it can sustain. This chain of thinking might perfectly satisfy the ideal system where the absence of any other parameters that influence the features of the film is suspended. However, in reality, it is different. Experimental pieces of evidence clearly demonstrate that thicker film sometimes enables to demonstrate better flexibility than its thinner counterpart. It is explained simply by the fact that apart from mechanism characteristics, other features of the deposited film also influence greatly its capability to become adaptive to the applied stress. For example, Lee et al. [33] investigated mechanical features of Ti-doped ZnO prepared

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via ALD with thickness ranging from 10 to 50 nm and revealed that the best fracture characteristics were reached once the thickness become 30 nm. It was proposed that a larger grain size resulted in a weak interface between grains and would increase crack propagation rates and thereby decrease the strength of the thin film. In turn, at lower thickness, the healing effect is less evident which increases the appearance of cracks.

8.3.3 Development of one-dimensional nanostructures with different geometries and morphologies 8.3.3.1 Direct deposition It is obvious that playing with geometries and morphological features of produced ceramics nanostructures could greatly assist in extending their mechanical and related characteristics making them to reach great and unprecedented advancement compared with their bulk counterparts. Following the above discussion about thin film-like flexible ceramics, it was discovered that 1D nanostructures possess great resistivity to the applied stress and can sustain original structure and morphology even under the most extreme conditions due to the capability of being twisted and sustaining elastic which is also called as fully reversible deformation. Thus, they can be growing on various flexible structures that could easily become subjected to mechanical bending and rolling during their consequent applications. Furthermore, the great diversity of CVD techniques allows the growing of these 1D nanostructures with various morphologies thus extending their mechanical characteristics or making them to direct toward a particular goal. It is also of particular interest that such superior mechanical features that these 1D ceramic nanostructures potentially have could greatly and most importantly positively influence the preservation of their other important characteristics that include, for example, resistivity or optical absorption. To be clearer, once being subjected to bending, 1D ceramics mostly have very small changes in the original properties and can be greatly operatable under these extreme conditions. It is different for example, from thin film-like compositions where even a slight chance in the original geometry with regard to applied stress increase resistivity in tens or even hundreds of times. Thus, applying CVD, this approach has great potential to be implemented in advanced processing where flexibility is appeared to be the main factor that determines the applicability of used devices. However, one should notice that there are not so many reporters available in the literature which demonstrate the employment of this synthesis approach with regard to analyzing the diversity of some properties once the target material system becomes the subject to the changes in mechanical features. Following it, the required knowledge for the present about its relevance could only be taken from reports that used different methodologies and experimental protocols to prepare flexible 1D ceramics and to analyze the dependency of their various characteristics with changes in mechanical properties. Thus, the demonstrated experience with a certain and highly expected level of success could be projected toward realizing via a CVD-based approach as in this case

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more advanced synthesis could be realized. For example, it was reported that the SnO2 nanobelt network created via sol-gel-based electrospinning and subsequent heat treatment could be subjected to bending at different radii. In this regard, its electrical conductivity only proceeds to change within 10%, which has very different from that of a thin-film-like structure where the changes in electrical conductivity have reached over 4500% [34].

8.3.3.2 Template-based deposition Employment of template to prepare the required ceramic nanostructures usually is determined by the requirements to proceed its morphology in the form of 1D associated geometry and this narrative is appeared to have the dominant recognition in terms of its utilization. Especially it is highly applicable toward a synthesis of hollow compositions such as nanotubes. Definitely, it was also reported that the formation of 0D based nanostructures is also possible by means of template-based processing [35]. Yet with regard to the purpose of this book which is dedicated to the investigation of flexible ceramics and methods to create them, utilization of 0D nanostructures possesses less interest in this case due to their inability to show the required level of bending or to sustain the applied stress by generating the elastic deformation that allows them to keep the original geometry and shape accompanied by the absence of any observable changes in other related characteristics. In this regard, 1D nanostructures are more attractive candidates for the discussion toward their realization by using a template-based approach given the advantages that they could deliver following the existence of specific mechanical features associated with their unique morphologies and relevant particularities. A template that is used for CVD approaches (by mentioning this type of vaporbased synthesis, it also means that the ALD technique is included in this discussion) can proceed in various forms, shapes, chemical compositions, etc. For instance, they include not only well-known and widely used polymer-related structures and compositions but also the materials of other chemical structures and composition that morphology could be easily controlled yet they cannot sustain high temperature or are vulnerable to the treatment with certain chemicals that is nonactive to the ceramic layer that covers them. In this regard, it is possible to create more complex formations where the templates are used as building blocks that contribute to each other structure within various geometrical formations that include, for example, the increase of layerness or making definite 3D patterns. In addition, once the additional treatment to remove these templates is avoided, it is possible to realize the appearance of core shell composition where the inner part represents flexible polymer or another type of designated material that can include organic or other related composition, covered with a layer of designated ceramic. Thus, as the polymer itself, for instance, can sustain bending, squeezing, and twisting, the ceramic layer would evidently follow these changes after becoming the subject of external mechanical impact. Once the favorable positioning of its morphology would be reached with regard to the thickness of this layer and also its chemical composition, the appearance of cracks or any negative changes within the overall structure of

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this composition would not be observed which determines its great perspectives in an application. One should become aware that this approach is highly similar to that where the deposition of thin film proceeded on the flexible structure. Yet, certain and very minor differences exist between these two approaches, and they are described in detail below. As an illustration, Chang et al. [36] demonstrated that by using anodic aluminum oxide (AAO) in the role of template, it was possible to create multiwalled TiO2 nanotubes by applying ALD processing, Fig. 8.3. The overall experimental setup is described in very simple terms and is as follows. Firstly, a homemade AAO template was fabricated by two-step anodization that included the initial deposition of Al film with a thickness of 800 nm on a Si wafer and its consequent transformation into AAO by applying the specific voltage and using oxalic acid as an electrolyte. Secondly, the TiO2 thin layer was deposited by ALD on the formed AAO with a designated number of cycles which should not be too high as in this case the all features become nondistinguishable and it would block the existence of pores. It is worth noticing that by adjusting the parameters of ALD processing, it is possible to control the thickness of walls and the coverage level as well since it might represent the geometry of a particle-like film or a thin film as well. Once both steps are completed, the finalized structures become subjected to the chemical treatment by NaOH to remove the AAO via an etching reaction. As a result, the obtained composition represented vertically aligned TiO2 nanotubes with perfect morphology and

Figure 8.3 SEM images of TiO2 MWNTAs. (A) Single-walled TiO2 nanotubes, (B) and (C) double-walled TiO2 nanotubes with gap spans of 10.0 and 20.0 nm, respectively, (D) and (E) triple-walled TiO2 nanotubes with gap spans of 10.0 and 20.0 nm, respectively, and (F) cross-section view of triple-walled TiO2 nanotubes. Source: Adapted from W.-T. Chang, Y.-C. Hsueh, S.-H. Huang, K.-I. Liu, C.-C. Kei, T.-P. Perng, Fabrication of Ag-loaded multi-walled TiO2 nanotube arrays and their photocatalytic activity, J. Mater. Chem. A. 1 (2013) 1987 1991. https://doi.org/10.1039/C2TA00806H with permission from The Royal Chemical Society. Copyright 2013.

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excellent uniformity. By repeating Steps 1 and 2, it becomes possible to realize the formation of TiO2 nanotubes with multiwalled structures, and the distance between the wall could be adjusted simply by depositing Al with a designated thickness. For instance, in Fig. 8.3, these distances called gap spans were demonstrated to be equal to 10 and 20 nm. It could be different with each newly appeared wall or all of these walls could follow the same pattern. Furthermore, it was also possible to create other forms and shapes of 1D nanostructures with this approach. To be clearer, the additional depositions of Al and TiO2 layers above already crated TiO2 nanotubes could fully seal their top thus eliminating the concept of hollow compositions. To avoid it, gentle polishing by silica solution was applied. Without it, the resulting composition was identified to represent the nanorod-like structures, as it was noticed by Chang et al. [36]. One should notice that ALD of TiO2 almost always results in the appearance of an amorphous structure and thus the thermal treatment step is a highly required measure to make it become transformed into a crystalline form. In turn, it is also possible to use soft organic compounds as a template and cover them with mentioned above Al2O3 which is vulnerable to chemical treatment yet sustains certain thermal supported processing. As an illustration, Lee et al. [37] created Al2O3 nanotubes and nanorods fabricated by coating and filling multiwalled carbon nanotubes (MWNTs) with atomic-layer deposition. It was reported that “ALD has an excellent capability to coat and fill any 3D shape of MWNTs conformably without producing any crystallites.” The physical sizes of the Al2O3 nanotubes and nanorods may be controlled by the selection of the inner diameters of the MWNTs, the positions of the inner comparting walls inside the MWNTs, and the number of the ALD cycles. As for the mechanical properties, it was stated specifically by the authors of Ref. [38] where relatively similar core shell geometry, that is, Al2O3/CNTs (foams) was used that it enables to “exhibit unprecedented elastic recovery following 50% compression, and possess values for strength and Young’s moduli which exceed those of aerogels with similar densities” [38]. In fact, various types of ceramic materials could be prepared by this method and their existence is mostly dependent on the availability of the proper precursors and their capability to be in contact with designated templates without producing any negative outcome. By adjusting the geometry of these templates with regard to their lengths or other related characteristics, it is possible to precisely adjust the newly formed ceramics-based composition. Thus, great flexibility in controlling morphological features could be achieved. Also, by choosing the correct template, it is possible to prove the desirable method for its elimination as well as leaving it without any additional proceeding thus making it serves the role of the core that is covered with a designated ceramic shell. For instance, Wang et al. [39] demonstrated the use of GaQ3 organic nanowires prepared via thermal evaporation as a template for realizing the deposition of Al2O3, and the latter step was proceeded via applying the AZO and water as precursors. It is very notorious that the temperature of deposition was designated as 25 C. In turn, to remove the template, the thermal treatment at 350 C in toluene was used. As GaQ3 should have some outlets to be able to proceed escape, the formed Al2O3 1D that includes nanotubes and nanowires

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demonstrated the existence of open-ended tips. As produced nanostructures showed the amorphous composition given the low temperature of deposition, additional thermal treatment is required to make them become fully crystallized. It is very particulate that even after applying the temperature that reaches approximately 900 C, the original geometry was preserved and only very minor changes were observed [39]. However, after looking at the positive sides of this strategy, one should also look at the negative end too as to fully understand its applicability with regard to current reality. Definitely, to realize the scenario that was used in the abovementioned reference, a highly specific experimental protocol is required which means that the designated template has a highly specific chemical and morphological structure, and proceeding only possible following the availability of certain instrumentation and external resources. Thus, its proper implementation is highly challenging. In this regard, Su et al. [40,41] demonstrated the use of commercial polysulphone membranes to prepare Zn-doped and Al-doped TiO2 nanotubes at approximately 100 C temperature which allows creating of a core shell structure, Fig. 8.4A D. The conditions perfectly matched the ones used during the ALD process as does not require the application of harsh requirements to prepare the nanostructured ceramics film, yet for employment in CVD it is a different story. As an illustration, the synthesis of mentioned Al2O3 by this process usually occurs at a temperature that exceeds 400 C and can reach in some specific experiential setup

Figure. 8.4 (A) Schematic illustration for growth of Zn-doped TiO2 nanotubes using PC membrane as a template. (B) Photograph of the PC membrane coated with ZnO/TiO2 by 400 cycles of ALD. (C) SEM image of a cross-section view of the ZnO/TiO2 nanotube array after removal of PC template by heat treatment. (D) SEM image of free-standing ZnO/TiO2 nanotubes. (E) Atomic ratios of Zn in annealed Zn-doped TiO2 nanotubes versus precursor cycle ratio measured by XPS and ICP-MS. The dashed line represents theoretical values calculated by the rule of mixtures formula. The inset shows the atomic ratios of the asdeposited samples measured by XPS. Source: Adapted from C.-Y. Su, C.-C. Wang, Y.-C. Hsueh, V. Gurylev, C.-C. Kei, T.-P. Perng, Enabling high solubility of ZnO in TiO2 by nanolamination of atomic layer deposition, Nanoscale 7 (2015) 19222 19230. https://doi.org/10.1039/C5NR06264K with permission from The Royal Chemical Society. Copyright 2015.

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the value up to 1000 C [42]. Returning back to the study of Su et al. [40,41], the thermal treatment at 450 C was used to remove the polymer membrane and also to activate more active diffusion of Al and ZnO atoms in TiO2 thus making their spread to be more uniform. The obtained nanotubes-like structure demonstrates perfect morphology and great mechanical stability while handling them during the analysis and consequent transportation causes almost no change in the overall geometry and overall compositions. Definitely, more detailed and precise measurement of their flexibility and capability to be bent and twisted as it was realized for the core shell structure should be done in the future yet even within the available data one can notice that this approach demonstrates its great applicability to create doped TiO2 nanotubes via using the very simple and straightforward pathway. Furthermore, changing the ratio of used precursors with regard to the time of their exposure, not only allows to reach certain control over the level of doping but also could be used to rich very accurate tuning over its localization by selective increasing the presence of foreign atoms whether in the surface or in bulk layers, Fig. 8.4E. It is expected that by realizing this scenario, the advancement of certain and highly specific mechanical characteristics could be in perspective archived. In addition, following this model, it is possible to create various types and forms of flexible ceramics based on 1D geometry and include not only Al2O3 or TiO2 as it was said above, but also ZnO, Ta2O5, etc. Moreover, the existence of the core shell or sandwiches-like compounds consisting of two or more ceramics layers combined together could also become the full-scaled reality as its successful realization would only depend on the desired outcome and synergy of these layers with respect to each other. The obtained composition in perspective enables to show the combination of the positive effect associated with assembling of utilized materials, and demonstrate the advanced geometry thus adapting the applied stress in the form of elastic degradation that enables to keep the structure and morphology unchanged rather make it become the source of unexpected breakage and formation of damaging cracks. Another positive feature of using the core shell structure composite where both of the representatives are available as ceramics is that it could demonstrate the accumulation of positive features associated with creating an interface that provides this leading path toward enhanced electrical, electronic, and other related features [43]. Thus, apart from flexibility, various important properties also could be enhanced in perspective. Yet, up to now, there are almost no reports if any which demonstrate the successful realization of this strategy, and thus more data is required to acquire as to reach the required understanding. One should aware that mentioned the above core shell structure is related to composition when both ceramic layers are produced by CVD processing whether in the form of a traditional approach or novel-based ALD technique and the substrate for their accommodation is served to be a specific template that later is eliminated leaving aside the nanostructured formation of desirable morphology. This specification is necessary with regard to further discussion. In turn, already synthesized and fully functional ceramic-based compositions can be advanced further by covering them with a layer of specific ceramic materials via the CVD approach as to enhance their functionality since the synergy created in

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this case leads to the appearance of some important and critical phenomena that serve as a basis for the further advancement of newly created nanocomposite [44]. It is different from the above-mentioned core shell formation as the core for the present strategy is identified as being created and synthesized by other than CVD technique or methodology and thus has higher flexibility in this adjustment. Following it, the additional ceramic layer serves only a supplementary role and is used for the primary goal to enhance the feature of materials that is covered but not to create a novel and independent structural formation that is regarded as a nanocomposite similar to the above-mentioned CNTs covered with Al2O3. Interestingly, this supplementary layer could represent the same or absolutely different materials. It is also referred to their chemical composition given the presence of specific nonidealities that are available in the form of nonstoichiometric features or the presence of extrinsic deficiencies such as metal and nonmetal doping. In addition, playing with crystal structure could also be considered as one of the methods to improve the mechanical prospects of obtained compositions keeping in mind that the stiffness and hardness of materials directly follow the presence of amorphous domains. Overall, this strategy could find great potential to advance the mechanical properties of ceramics thus extending their capability to be used in processing where flexibility plays a crucial role.

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Tejas Koushik and Elsa Antunes College of Science and Engineering, James Cook University, Townsville, QLD, Australia

9.1

Introduction

Ceramics have been one of the oldest materials used by mankind, starting from the tools used by early man to building blocks of megastructures. Their brittle nature and extremely hard surfaces make them very challenging to work with, and consequently, they have been replaced in several applications by metals and alloys that are considerably easier to form. Development in manufacturing technologies has introduced new processes such as injection molding, slip casting, tape casting, cold pressing, etc. While these methods did allow for easier processing of ceramics, they still have the limitation of being expensive and limited to relatively simple shapes. 3D printing (3DP) which was first developed in the 1980s primarily for polymers, has been recently applied to the manufacturing of ceramics [1,2]. The freedom of design and quick turnaround times offered by these processes have led to new applications such as biomedical scaffolds, custom cutting tools and hightemperature impellers. This chapter describes and discusses the available technologies for ceramic 3DP. It also reviews the feedstock requirements and processing parameters for each 3DP. This will function as an effective guide for 3DP technology selection. 3DP processes can be described as additive manufacturing (AM) processes; involving the repeated addition of two-dimensional layers to form a three-dimensional object [3]. This layer-by-layer approach is the cornerstone of the 3DP process, breaking down complex structures into a series of simple layers or images. The slicing process is usually performed with the assistance of software, information from the software is extracted either as a sequence of machine positions or images depending on the 3DP technology used. The resolution achievable at every layer depends on the technology used hence making the choice of technology a very critical decision that needs to take into consideration the engineering requirements, cost, product size and infrastructure available. Fig. 9.1 illustrates the general workflow used for 3DP, from the understanding of the engineering requirements to the final part. Ceramic 3DP was first reported by Marcus et al. [1] and Sachs et al. [2] in the 1990s, when 3DP processes used for processing polymers were adapted to manufacturing ceramic components. While previously the only available technique was binder jetting, which showed considerable anisotropic shrinkage after post-processing [4]. Recent developments in pre-processing ceramics for 3DP has opened other techniques such as Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00009-9 © 2023 Elsevier Ltd. All rights reserved.

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Figure 9.1 General workflow of three-dimensional printing process.

stereolithography (SLA), digital light processing (DLP), robocasting, and fused filament fabrication (FFF) [5]. Unlike metals and polymers where the final component produced by 3DP processes can be used with almost no post-processing, ceramics parts need to undergo sintering to obtain the desired strength.

9.2

Classification of three-dimensional printing processes

Ceramic 3DP processes could be classified into three types based on the nature of feedstock: (1) slurry-based processes, (2) powder-based processes, and (3) bulk solid materials (Fig. 9.2) [5].

9.2.1 Slurry-based processes Slurry-based processes involve the use of fine ceramic particles dispersed in a liquid medium containing suitable additives, to form inks or pastes depending on the required solid loading. The choices of these additives are based on the rheological requirements and nature of the 3DP process. The main steps of ceramic 3DP using slurry-based processes are shown in Fig. 9.3. This section includes the requirements of the feedstock for slurry-based processes, processing parameters and their influence on ceramic parts fabrication, and in-situ and ex-situ quality control (QC) techniques as well as energy consumption of each technique.

9.2.1.1 Process description SLA requires the incorporation of ceramic particles within photocurable polymeric resins. The mixture is selectively exposed to a suitable light source using either a digital micromirror device or scanning galvanometers [6]. The light source used for SLA-based processes includes lasers or light projection-based systems. The major difference between these processes lies in the light source used, laser-based systems have small exposure areas and hence are slower while light projection-based systems are able to fully cover the build area. While the process of curing the polymeric resin remains the same, it can be classified into two types based on the position of the light source as top-down and bottom-up approaches. Top-down approaches require the light source to be directly above the top surface of the printed object. During this printing process, the printed object moves down into the slurry tank, exposing a fresh layer of slurry for the

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Figure 9.2 Classification of three-dimensional printing processes.

Figure 9.3 Workflow of slurry-based processes.

subsequent reaction [7]. This approach allows for using larger build areas and eliminating the need for parts to be lifted off from the base to accommodate subsequent layers. However, printing defects can occur due to inconsistent layer formation on the top surface. Bottom-up approach requires the light source to be below the top layer of the printed object. During the process, the printed object is lifted off from the platform to expose a new layer of ceramic slurry [7]. Further, additional systems are required to ensure an even layer of slurry is coated over the platform, making this a relatively slow process in comparison to top-down approaches. Specific to the bottomup approach, slurries are required to be redistributed along the surface using an additional tool. Slurry viscosity has to be carefully optimized to ensure a uniform layer is coated. Commercially, the bottom-up approach has been widely explored considering some of the key benefits of the top-down. This includes consistency in the slurry layer exposed to the source of irradiation, slurries with increased solid loading can be used to limit shrinkage during post-processing. Lastly, to produce final

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components, these ceramics are debinded to burn-off the binders followed by densification to obtain the desired strength. Robocasting is a versatile tool for ceramic fabrication, with the ability to process multiple ceramics at different length scales. Further, with low binder contents during the slurry preparation process, high densities are achievable after postprocessing. As the nozzle, which is connected to the feeding, traverses its defined path, slurry is carefully deposited along every layer. The simplicity of the setup enables the use of multiple ceramic slurries using multiple nozzles and feeding systems [8]. The rheology of the slurry must be optimized to ensure that does not undergo deformation due to the stacking of multiple layers. Near-net-shape manufactured green parts produced from robocasting require a sintering step that provides strength to the final part. Direct-ink writing is an AM technique for processing ceramics that uses polymerbased ink containing ceramic particles. The delivery of the ink onto the building surface can be achieved using (1) a droplet-based writing system and (2) filament-based writing system [9]. Droplet-based writing systems use the principle of ink-jet printers to carefully deposit ceramic ink layer-by-layer. These can be further divided into dropon-demand systems or continuous-jet methods. Drop-on-demand systems eject inks only when required, simplifying the design of the ink dispenser and preventing ink contamination [9]. A major limitation of this system is the high viscosity of ceramic inks, but recent advances in ink dispenser designs and understanding of printing parameters have enabled the printing of high-viscosity ceramic inks [10].

9.2.1.2 Feedstock requirements Feedstock requirements for slurry-based AM are unique to each process. Some of these requirements include solid loading (vol%), binder quantity, dispersant quantity, rheology, and stability. The slurry rheology is affected by feedstock preparation, factors such as solid loading, particle size, binder quantity, dispersant quantity, and milling time are critical [11]. Further, these factors are correlated, thus making a list of specific individual requirements is challenging. However, ranges of rheological parameters or critical characteristics of ceramic slurry for different materials can be identified from the literature and are summarized in Table 9.1. Solid loading represents the volume occupied by the ceramic particles in the ceramic slurry. Higher solid loading produces dense ceramic objects postsintering; however, increasing solid loading negatively impacts its performance during plastic forming which is essential for slurry-based AM processes. Therefore, the concept of critical powder volume concentration (CPVC) described in previous studies represents the volume limit of ceramic particles added to produce a paste or slurry that remains stable while being processed [13]. This CPVC value depends on the nature of the processing technique used, for example, robocasting and direct-ink writing processes require stiff ceramic pastes that are able to resist any residual stresses and minimize drying shrinkage [14,15]. Although solid loading greater than 50 vol% is preferred to obtain fully dense structures, thus rheology and particle size requirements of the 3DP process need to be considered.

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Table 9.1 Key rheological requirements of ceramic slurries for different slurry-based additive manufacturing processes. Additive manufacturing processes Robocasting or direct-ink writing

Key requirements of slurry/raw material

G

G

G

G

G

G

G

Stereolithography

G

G

G

G

G

Viscoelastic property must be described by the Herschel
Bulkley model Particle agglomerates must not exceed extrusion nozzle diameter Reversible shear thinning behavior is required at viscosities of 10
100 Pa.s at high shear rates Storage modulus (G0 ) must be greater than 200 Pa Organic additive quantity should be less than 3 vol% Powder particles sizes should be within 1
100 μm Slurry must possess pseudo-plastic behavior Must show thixotropic behavior Viscosity of the slurry must be in the range of 0
10 Pa.s Curing thickness of the resin mixture must be 5
10 μm higher than the required layer thickness Particle size needs to be carefully considered during slurry preparation Solid loading greater than 55 vol% is required to achieve good densification postsintering

Ref.

[9,12]

[11]

Binders and dispersants are added to the slurry formulation to increase and decrease its viscosity, respectively. Further, they could be added to introduce critical features required for the processing technique. For example, stereolithographic processes require photocurable polymers to process the ceramic layers [16]. Some commonly used binders and dispersants are presented in Table 9.2, their addition and content depend on the required rheology. These binders and dispersants affect particle
particle interactions, inducing attractive or repulsive forces and causing the formation of aggregated or well-dispersed slurries, respectively. Dominant mechanisms that induce these responses are classified into three types (1) electrostatic, (2) steric, and (3) combined electrosteric stabilizations [12]. Uniform dispersion of ceramic particles and polymer binders prevents particle segregation prior to sintering [5]. Zeta potential measurements can accurately determine the dispersion of ceramic particles in slurries, but can be challenging in the case of highly solid-loaded slurries mixed in nonpolar resins due to weak electrostatic interaction [22]. Therefore, viscosity, thermogravimetric and Fouriertransform infrared spectroscopy measurements are usually required to fully understand the influence of dispersants on ceramic slurries. Another relatively simple technique to assess the dispersion of the ceramic particle is using sedimentation

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Table 9.2 Some commonly used binders and dispersants to prepare ceramic slurries for additive manufacturing processes. Binders

Dispersant

Additive manufacturing process

Ref.

SP-RC700 (epoxy acrylates, acryloylmorpholines and trimethylolpropane triacrylate) 1,6-Hexanediol diacrylate (HDDA) and acrylated polyetherpolyol Carboxy methylcellulose

DISPERBYK-103, TEGO-685

Stereolithography

[17]

Solsperse 41000, Melpers 4350

Stereolithography

[18]

Darvan C-N

Robocasting/direct-ink writing Robocasting/direct-ink writing

[19]

Direct-ink writing

[21]

WB4101 (Aqueous acrylicbased binder) and PL008 (plasticizer) Polyvinylpyrrolidone (PVP) and nanosized MgO particles

DS001

[20]

tests, where dispersion can be compared by measuring the ratio of the height of clear solution (HT) postsedimentation to the initial height of the slurry (Ho). The particle dispersion state can be understood with hydrostatic pressure measurements taken at the bottom of a cylinder containing the slurry. The decreasing slopes of the hydrostatic pressure vs time plots dictate the state of the slurry, smaller slopes indicate uniform dispersions [23]. Dispersion of ceramic particles is a function of particle size, solid loading, and quantity of additives, which determines its feasibility to be processed using slurry-based AM techniques. Slurries used in SLA-based processes require the inclusion of photocurable polymers and their selection depends on the excitation source available in the 3DP system. Other optical characteristics such as diffraction and absorption must be also considered [24
26]. Specifically, in case of direct-ink writing, studies have shown a strong correlation between slurry rheology and printability. The dimensionless number introduced by M’Barki et al. captures the effects of capillary and gravitational forces acting on the printed layers, which is fundamental to designing ceramic inks or slurries that provide consistently dense materials [27].

9.2.1.3 Energy consumption Energy consumption occurs in three different steps: (1) pre-processing step, (2) inprocessing step, and (3) post-processing step. Pre-processing step includes energy consumed in preparing ceramic slurries, but does not include ceramics powder production processes because these processes are significantly different for each type of ceramic.

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Slurry preparations require uniformly mixing ceramic powders with suitable binders and dispersants. Energy consumed during these processes depends on the mixing time and quantity, larger quantities require higher-capacity mixers that consume more power. Lab scale high shear mixers used for preparing uniform dispersion are known to have an operating capacity of 1 mL
50 L with 750 W motors. Increasing the capacity of these mixers to 1
8000 gallons for industrial applications, increasing their power consumption up to 150 kW. Energy consumption during the operation of a ceramic 3D printer is highly dependent on the type of ceramic AM technique utilized. Energy sources to cure ceramic slurries differ based on the energy source which could be either a laser or digital light projection system. In digital light projector systems, the wattage of the projector bulb determines its maximum energy consumption. Similarly, in laserbased systems energy consumption will depend on the wattage of the laser used. Curing of ceramic slurries requires a specific critical energy, which changes depending on processing parameters. Similarly, an ultraviolet-based system or a laser-based system will consume energy based on processing parameters as described in Table 9.3. Stereolithographic techniques require an additional recoating system to ensure a uniform layer of slurry is available during processing. However, the energy consumed by motor drivers is significantly lower than the energy consumed by the laser source. Specifically, in robocasting processes, the lack of usage of any lasers limits the energy consumption (during the process) to the energy consumed by the slurry pump and motor drivers. Energy consumption of these pumps can range between 0.36 and 216 W depending on pump flow rates and pressure requirements at the nozzle end. Sintering and debinding are critical steps toward producing fully dense ceramics. The sintering process is a thermally driven process that usually occurs at temperatures greater than 1200 C. Sintering is carried out in high-temperature furnaces and energy consumption depends on peak temperature, hold time and heating rate. Both peak temperature and hold time depend on the extent of densification required and the type of ceramic material. The power consumption of lab-scale sintering furnaces usually lies between 5 and 20 kW.

Table 9.3 Energy consumption during vat polymerization processes. Process type

Light used

Wavelength/pixels

Maximum electrical power consumption

STL DLP DLP STL DLP

Ultraviolet Ultraviolet LED Ultraviolet Ultraviolet

405 nm 2560 3 1600 pixels 405 nm 355 nm —

2 kW

1.9 kW 500 W

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9.2.2 Powder-based processes Powder-based AM processes utilize the unique binding system to either bond or fuse ceramic particles together. Most commonly used are either a CO2 or Nd:YAG laser; however, the power of these lasers depends on the binding mechanism required, which can be classified under selective laser melting (SLM) and selective laser sintering (SLS). While the above-mentioned processes utilize heat as their primary mode of binding particles, binder jetting process utilizes a resin to selectively adhere ceramic particles [6]. The selection of the AM process will impact postprocessing steps.

9.2.2.1 Process description SLM requires a high-power laser to heat the ceramic particles to a temperature between Tm/2 and Tm (melting temperature), causing the ceramic particle to fuse. The laser source is selectively scanning across the powder bed to fuse ceramic particles on the powder layer. Once completed, the build plate is lowered to the height of the layer thickness and a fresh powder layer is deposited over it. The process is repeated for each subsequent layer to form the threedimensional part. High melting temperatures of ceramics make them extremely hard to process, to minimize the heat required low temperature liquid phases are commonly used to favor the sintering process (liquid phase assisted sintering). The liquid phase mixed within the ceramic raw material melts prior and binds the ceramic particles forming a dense ceramic part [28]. Melting of ceramic powders might produce cracks and heterogeneous microstructure, therefore is not the preferred technique for ceramics processing [28]. SLS of ceramics can be performed directly or indirectly. The direct technique involves heating the ceramic powders and partially fusing them together [6,28]. A combination of particle sizes is required to obtain a high packing density of powders. The indirect method uses low-temperature matrix phases such as polymers [6,28]. The laser source causes these sacrificial polymers to bind, forming a green part that has to be sintered to obtain the final component with the desired strength. Binder jetting is an AM process that utilizes a suitable binding agent to adhere ceramic particles forming a green body. The binding agent is selectively applied across the surface of the powders to produce a single layer, the build plate is then lowered to allow a new layer of the powder to be deposited. This process is repeated several times to produce the final three-dimensional component. This can be achieved in two ways: (1) binders are mixed with ceramic particles and the dispensed liquid triggers the reaction between the ceramic particles and binder and (2) ceramic particles are precoated with a solid binder, and liquid dispensed triggers the chemical reaction to bind the ceramic powders [4]. The produced green part is heated to temperatures close to 200 C, causing the binder to undergo cross-linking and toughen the green ceramic body. Finally, the organics are completely removed through a debinding step and then sintered to obtain ceramic parts with the desired strength [4].

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9.2.2.2 Feedstock requirements Ceramic raw materials are required to be prepared prior to use. This preparation step depends on the selected process. SLM process requires no preparation as powders are melted during the process, causing the particles to fuse. SLS process requires that a low-temperature phase to be coated or mixed with the ceramic particles. Some commonly used low-temperature phases or binders include NH4H2PO4, B2O3, aluminium, epoxy, and phenolic resins (shown in Table 9.4) [29]. The quantity of these binders depends on the required green strength, low amounts are insufficient to create a stable bond between particles; however, excess addition of polymers will increase porosity during the debinding stage. Another key factor during the addition of polymers is flowability to create a uniform layer of powder to be deposited across the build plate. Technologies used for the preparation of coated ceramic powders include mechanical mixing and spray drying. Mechanical mixing involves agitating the mixture of ceramic particles and binders to produce a uniform mixture that is often combined with spray drying to produce spherical ceramic agglomerates. During the preparation of the slurry, suitable binders are added and will serve as the lowtemperature phase for the SLS process. Selection of binders must be based on the nature of interaction during the debinding phase, thus binders inducing changes in the chemistry of the ceramic phase must not be selected. The ceramic loading strongly influences the homogeneity and viscosity of the slurry. For example, large viscous forces present in the slurry will require more energy to form small droplets resulting in larger particle sizes [32,33]. Flowability of these powders is critical for all powder-based processes that require a uniform powder bed to ensure consistency among layer features. Measurement of powder flowability is based on the Hausner ratio, which is defined as the tapped density divided by the apparent density [31]. The powder Hausner ratio can be altered based on the flowability and packing requirements, as these two properties tend to contradict one another. However, a balance can be achieved by carefully choosing powder particle size distribution and its volumetric content [31,34]. Particle shape impacts on flowability and packing of powder particles. Spherical powders reduce packaging density, but have good flowability at high loadings because of their ability to roll relative to one another. On the other hand, irregular particles tend to agglomerate due to the increased friction between their surfaces,

Table 9.4 Resins commonly used as low-temperature phases for ceramic powders using selective laser sintering. Materials description

Quantity

Type of resin

Ref.

Al2O3 SiC Iron oxide doped Al2O3

4
6 wt.% 3 wt.% 2 wt.%

Epoxy resin Epoxy resin Polyethylene glycol (PEG)

[29] [30] [31]

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allowing for higher packaging densities and green strength, but reduced flowability [4]. Therefore, powder-based processes will favor the use of multimodal distribution of powders consisting of various shapes and sizes to produce fully dense ceramic products. Wettability is another key factor specifically in case of the binder jetting process. Binder dispensed from the extruder will penetrate through microscopic pores between ceramic particles ensuring adhesion to its previous layer [4]. Wettability is influenced by multiple factors such as powder shape, size and viscosity of the binder. It can be measured by analyzing the contact angle created between the binder and particles, low contact angles indicate high wettability while high contact angles indicate low wettability [4]. The viscosity of the binder determines its ability to penetrate through the powder surface, low viscosity binders are preferred. However, the resolution of 3DP is closely tied to its viscosity, very low viscosities of the binder will cause it to spread uncontrollably leading to a loss in resolution [4]. Once the binder is in contact with the ceramic particles, the reaction between the powder particles and the binder solution ensures adhesion to its previous layer. Previous studies demonstrated that the size distribution of voids has a greater impact as opposed to surface contact area [4]. Absorption of laser energy is critical in processes such as SLM and SLS; energy from the laser needs to be effectively converted to heat for the ceramic particles to fuse. Studies have indicated that oxide-based ceramics show poor absorptivity to near-infrared lasers [31,35]. The addition of dopants such as carbon, iron oxide, graphite, and boron carbide can increase the absorbance energy of near-infrared lasers, improving laser
powder interactions [31,35
37].

9.2.2.3 Energy consumption Energy consumption during a process could be a critical criterion when considering its limited availability. The energy consumed during the processing of ceramic raw material to the final product is restricted to the energy consumed by the 3DP machine, intermediate steps required in preparing the raw material and postprocessing steps. This approach to measuring energy consumption assumes ceramic powders prepared using various methods are of the desired quality, thereby allowing for a fair comparison between different AM processes. Energy consumption during powder-based processes such as SLM and SLS primarily arises from the laser source, the difference in the energy consumption lies in the wattage of the lasers used in these processes (Table 9.5). Further energy consumption also lies in the operation of auxiliary components such are motor drivers, hydraulics and powder dispensing systems. Energy consumption during these processes changes based on the scale of the machine and available build area. Table 9.5 provides ranges of total power consumption of these machines including small prototype machines to large machines with multiple work heads. Ceramics produced using 3DP systems are termed as green components as they are not fully dense. Production of the fully dense components requires additional post-processing steps such as debinding and sintering. These post-processing steps

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Table 9.5 Wattage of laser systems used in selective laser melting and selective laser sintering-based processes. Powder-based process

Laser type

Laser power

Total power consumption

SLM SLS

CO2 laser or Nd:YAG laser Ytterbium fiber laser, diode lasers and CO2 laser N/A

200
1000 W 10
100 W

1.7
50 kW 1.5
26 kW

N/A

1.3
27 kW

Binder jetting

depend on the powder-based process selected. SLM does not require any postprocessing and is able to produce fully dense parts. SLS and binder jetting systems require a debinding and sintering stage to produce fully dense parts. Debinding performed for these green components, involves thermal treatment at temperatures within the range of 200 C
700 C [38
40]. Furnaces used for debinding ceramics have lower maximum operating temperatures (up to 1200 C), as the temperature requirements are considerably lower. Power consumption of these lab-scale furnaces lies in the range of 16.5
27 kW, depending on the volume of the heating chamber. Sintering process as previously mentioned is essential to produce fully dense parts and remains common to all ceramic-based 3DP techniques. Energy consumed during this process has been previously described in Section 9.2.1.3.

9.2.3 Bulk solid materials The bulk solid materials approach to AM of ceramics uses a material extrusion process consisting of a solid feedstock in the form of filament. These filaments consist of over 45 vol% of ceramic powders that are mixed within suitable binders to ensure they can be extruded into continuous filament. This process is very similar to ceramic injection molding and can produce a wide range of part sizes [41].

9.2.3.1 Process description FFF for ceramics involves the extrusion of a highly viscous mixture of polymers and ceramic particles through a heated nozzle. Polymers exposed to the heated nozzle transition from a brittle to a viscoelastic state allowing it to flow. This viscous solid material containing ceramic particles is selectively deposited according to the intended design. This process is repeated layer-by-layer until the complete green part is formed.

9.2.3.2 Feedstock requirements Feedstock preparation for bulk solid-based ceramic 3DP requires thermoplastic polymer binders that provide sufficient green strength to the formed component. Polymers commonly used are poly-lactic acid, acrylonitrile butadiene styrene, and

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thermoplastic polyurethane. In addition, polymer binders must ensure the feedstock maintains a low viscosity while being extruded from the hot-end nozzle. These polymers must undergo a reduction in viscosity at the nozzle hot-end allowing them to be extruded; the viscosity must be carefully controlled to ensure void-free parts. Ceramic loads of 45
60 vol% are required to ensure minimum warpage after debinding and sintering steps. Particle size of the ceramic powders can play a critical role in the achievable resolution in FFF printing. Firstly, particles must be smaller than the extrusion nozzle to prevent blockage during the extrusion process. The resolution of structural features during the FFF method is strongly dependent on the nozzle diameter [6]; however, when utilizing filaments loaded with ceramic particles, the resolution is also dependent on the size of ceramic particles. During debinding polymers will burn off, leaving behind the loaded ceramics particles as shown in Fig. 9.4, where the white outline represents the required part resolution and the particles in red represent excessively the large particles that contribute to the loss in part resolution. Thus, as a rule, the particle size must be within 10% of the required resolution [41].

9.2.3.3 Energy consumption Energy consumption during FFF process arises from the pre-processing, processing and post-processing stages. While energy consumed during the post-processing stage is similar for all ceramic-based AM processes, energy consumed during preprocessing and processing stages differ. Energy consumed during the production of ceramic powder is not considered in this analysis as discussed before.

Figure 9.4 Influence of ceramics particle size on the achievable resolution in fused filament fabrication.

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Table 9.6 Energy consumption in fused filament fabrication process as a function of build volumes and extruders. Build volumes (l)

Number of extruders

Energy rating

4.9
18.7 4.4
31.5

1 2

150
350 W 320
600 W

Pre-processing stage or preparation of solid filament: ceramic powders and polymer binders are first mixed and pelletized prior to being extruded. Ceramic powders are initially mixed with polymer binders using a mixing stage, energy consumed during this stage has been described in Section 9.2.1.3. A uniformly distributed mixture is then fed into a pelletizer, which converts the mixture into a pellet of fixed sizes. Energy consumed during this stage can run anywhere between 0.375 and 4 kW, depending on its processing capacity. Pellets are then extruded using a filament extruder, the processing capability of these machines can vary from 1 to 20 kg/h. Power requirements for these machines also vary between 1.1 and 15 kW depending on the scale and processing capacity. Energy consumption during the FFF process depends on the build volume size and number of extruders. Larger build areas require more heating elements necessary to heat up the build plate, thereby consuming more energy. Multiextruder printers used in FFF processing consume more energy only during the simultaneous operation of both extruders. Table 9.6 indicates how the power consumption increases with an increase in build volumes as well as the number of extruders. Energy consumption during post-processing stage is mainly from the debinding and sintering steps, which have been previously described in Section 9.2.1.3.

9.3

Process parameters

Process parameters described here refer to the machine parameters influencing the green part produced by 3DP. Layer-by-layer processing is a common feature in all 3DP processes, process parameters common to all the described processes include layer height, part orientation, nozzle diameter, and laser spot size. Slicing is a critical step during the preparation of parts prior to being additively manufactured, involving the division of parts into several two-dimensional layers parallel to the build surface. Increasing the number of layers reduces the thickness of layers and vice-versa. Having an impact on the attainable resolution perpendicular to the build surface, thus increasing the number of layers produces ceramic parts with greater resolution but significantly increases build time [6]. However, the capabilities of each individual process must also be considered, for example, FFF process has minimum layer heights within the range of 50
350 μm whereas SLA techniques can produce layer thickness within the range of 25
100 μm.

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Part orientation is the part position with respect to the 3D printer build plate, which decides the necessary support structures required to ensure sufficient contact with the build plate. Processes such as FFF, SLM, and SLA processes require supporting structures when overhangs are angled greater than 60 degrees [6]. However, some SLS, BJ, and direct writing technologies do not require support due nature of these processes. Selection of suitable part orientation will depend on the available build area and a number of parts to be produced in case of ceramic-based 3DP processes. The resolution of ceramic parts in case of FFF and powder-based techniques depends on nozzle diameter and laser spot size, respectively. Nozzle diameter can be easily changed in the FFF process and they usually range between 0.1 and 2 mm in diameter. Larger nozzle sizes can deposit a higher quantity of material, thereby reducing build times but at the cost of resolution. Spot sizes in powder-based system work in a similar function, increasing spot sizes causes a loss in resolution in the XY direction, but increases productivity. The above-described processing parameters remain similar for all AM processes, but certain processes have special conditions that need to be met before deciding these processing parameters.

9.3.1 Slurry-based processes SLA-based AM techniques utilized to process ceramics rely on selective curing of the photosensitive polymer. The interaction between light and the slurry is chemical in nature governed by Beer
Lambert law shown in Eq. (9.1). Incident light causes epoxy or acrylate resins to initiate a chemical reaction causing them to polymerize. Cure depth (Cd) indicates the depth of this reaction, Dp indicates the ability of the light to penetrate the slurry. Factors such as laser power (PL), scan speed (Vs), Wo (radius of laser beam), and Ec (critical exposure) are the processing parameters that can be adjusted for all slurry-based 3DP processes [6]. Cd 5

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 PL DP ln πWo VS EC

(9.1)

Laser power (PL) is only relevant in case of laser-based systems, the incident power of the beam must exceed the critical energy (Ec) value of the slurry to initiate the chemical reaction. Solid particles in the ceramic slurry cause scattering of the incident light, limiting the light to initiate chemical reactions [42]. Further, solid particles have also been shown to behave as heat sinks, limiting the energy available for the propagation of the chemical reaction. Scan speed (Vs) describes the speed at which the laser rasters across the build surface, faster scan speeds result in higher build rates, but can reduce the energy received at a single point. A combination of these factors largely influences the cure depth, thereby affecting the final part produced [6]. The light projection-based system as previously described uses a light projector that causes a photopolymerization reaction as opposed to using a laser.

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This simplifies the relationship of cure depth (C d) and incident energy to Eq. (9.2) [6]. Cd 5

2H:T

Dp e Ec

(9.2)

where cure depth (Cd) depends on incident irradiance (H) and exposure time (T), limiting the processing variables compared to laser-based systems. Layer thickness must be chosen according to the identified cure depth (Cd), it must be within 60% of the achievable cure depth to ensure there is sufficient adhesion to previous layers [6]. The rate of deposition is a critical parameter in case of robocasting and directink writing processes. In case of direct-ink writing, uncontrolled increase in the rate of deposition will lead to droplet formation or damaged nozzle tips. In case of robocasting, the flow rate (Q) is limited by the maximum shear rate (γ max) of the nozzle as given in Eq. (9.3) [12]. Deposition rates lying between 5 and 50 mm/s are usually providing good shape tolerance [9]; however, individual slurry and nozzle properties must be considered. Further, the robocasting process has been known to use larger nozzle sizes in the range of 100
1000 μm to prevent clogging. ϒmax 5

9.4

4Q πr 3

(9.3)

Quality control techniques

Quality control (QC) is an essential step to ensure three-dimensional printed components meet the design requirements and consistency during production. Fig. 9.5 shows the general steps performed during the development of the 3DP process. The development phase allows the user to identify critical parameters, optimization techniques, process-effect interrelations and degree of compliance with user requirements. Once compliance has been encountered, the process can be utilized for production activities. QC during the production stage identifies dimensional accuracy, defects and features that do not comply with the specified requirements. Thereby, limiting rejection rates of parts further down the production line. The dimensional accuracy of produced parts are checked using manual measurements and standard gauges fabricated to meet the specified user requirements. The complexity of three-dimensional printed parts make conventional measurement techniques unsuitable, therefore automated systems such as co-ordinate measuring machines (CMMs) are used for this purpose instead. Touch probes present on the CMM map out all relevant dimensions and identifies deviation from specified drawings [43]. However, this process is extremely slow and cannot be used during inline QC procedures. Instead, standard gauges provide a quick and effective screening technique to separate defective parts [44,45]. However, multiple gauges might be required in case of complex three-dimensional printed designs, thereby behaving as a bottleneck in the production process.

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Figure 9.5 Description of tasks during the development phase and their relationship to production.

Porosity in ceramics parts impacts significantly on mechanical performance, densification process is unable to eliminate large pore sizes. Therefore, QC process must ensure they remain within a specified limit after 3DP ceramics. Ultrasonic non-destructive techniques are used to detect pores and their sizes, screening out defective parts produced [46]. However, this technique is slow and applied after the sintering process. With the development of new technologies, high-speed cameras or scanners mounted inside machine build chambers can detect and measure the porosity to high accuracy. These new technologies allow the compliance check of the three-dimensional printed part to specified requirements before parts move further down the production line [47,48]. Similar process can be used to estimate parameters such as surface roughness layer adhesion and density of three-dimensional printed ceramic parts [49,50]. Multimaterial ceramics require switching nozzles or slurry vats, causing contamination of layers. Low volumes of this contamination might not be detected. Previous studies have shown the effectiveness of automatic cleaning or solvent purging systems being used before processing the next material to prevent contamination in slurry-based processes [51,52]. In situ techniques on the other hand use computer vision coupled with artificial neural networks to modify processing parameters during the process to produce parts meeting the user requirements (Fig. 9.6). This will assist in reducing defective parts [53], especially in cases where production quantities are large and stopping production is more expensive than the impact of defective products. However, artificial neural networks require large quantities of historical data to make such decisions. While this is ideal for defects within specified limits of the artificial neural network, larger defects require redevelopment. The development phase is typically a lengthy process, involving the use of long statistical experiment designs. However, the recent improvement in

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Figure 9.6 Closed-loop feedback system for in situ quality control during additive manufacturing processes.

technology has enabled faster development of optimum parameters. For example, predictive porosity models are able to utilize data from machine setting parameters and interpret into physical effects during the process, thereby increasing the efficiency of process optimization [54]. Most ceramic 3DP techniques require the use of low temperature polymer phases, polymerization of these phases during 3DP releases heat causing distortions in part dimensions. Studies have shown that multi-physics modeling tools currently available can correlate specific machine parameters to part features observed prior to 3DP, thereby speeding the development process [42].

9.5

Guidelines for technology selection

Each of the ceramic 3D printing techniques discussed in previous sections seems to have unique advantages and limitations, therefore selecting a technique for ceramic part production can be a multifaceted process. The pre-processing complexity, processing speed, build size and post-processing steps widely differ based on the 3D printing technique selected. Understanding the features of each 3DP process and relating them to the design and production requirements of the ceramic parts is critical for technology selection. The map below classifies the relative complexity, significance and capability of all the ceramic 3D printing techniques discussed. In conclusion, this map can be used as a guide for technology selection to 3D printing ceramic materials.

210

9.6

Advanced Flexible Ceramics

Applications of Ceramics

Ceramics are characterized by high mechanical strength and hardness, good thermal and chemical stability. These properties make ceramics ideal for a wide range of applications such as the chemical industry, catalyst support, electronics, aerospace, and biomedical engineering. Some of these applications require highly complex shapes, high dimension accuracy and lightweight ceramics that cannot be produced by conventional ceramic processing techniques [5]. Therefore, 3DP has become very attractive allowing the production of ceramics with complex shapes, porosity and multimaterials. Highly porous ceramics characterized by low density and good mechanical properties can be used for many value-added applications such as filtration membranes, catalyst supports, lightweight materials and tissue-engineering scaffolds. 3DP is a suitable technique to produce customized shapes with hierarchical structures. The structure and type of pores (close or open pores) impact on mechanical and thermal properties of ceramic parts. Ceramic specimens with an open porosity of 83%
94% have shown a compressive strength between 3 and 16 MPa. Another study demonstrated the production of hierarchical scaffolds with a porosity between 4 nm and 400 μm [55]. 3DP has revolutionized the manufacturing industry and is suitable for the preparation of ceramics with hierarchical and honeycomb structures, but the control of porosity is still challenging [55]. Biomedical scaffolds have to be precisely designed to meet the requirements of the living host such as bioactivity, biocompatibility, biodegradability, mechanical properties, structure, and architecture. The size of the pores in the scaffold architecture is fundamental for all the cellular steps: cellular adhesion, migration, proliferation, differentiation and colonization of cells. However, the architectural design

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must also take into consideration the mechanical properties required for the specific anatomical site. 3DP has a great potential to tailor-made biomedical products to specific patients, then significantly reduce the adaption phase to the new product. With the need of customizing to the patient, 3DP is a suitable and cost-effective technique to produce biomedical ceramics compared with conventional manufacturing methods. 3DP technology is suitable for the production of complex and highly porous hierarchical structures required for biomedical scaffolds, but obtaining ceramics with high mechanical properties with simultaneously high bioactivity, biodegradability and biocompatibility is still under development [56]. Aerospace is one of the most promising markets for AM due to the need for components with specific characteristics: complex geometry, customized production, materials difficult-to-machine and high buy-to-fly ratio, on-demand manufacturing and high-performance to weight ratio [57]. AM has the capability to produce components when needed and reduces waste to 10%
20%, these advantages must be considered for the economics. The main limitations of AM for the aerospace industry are the size of the parts, scalability, and limited and expensive materials. Future research will be focused on the production of lightweight and strong components that are simultaneously high-temperature resistant to meet space flight requirements. Further, the development of multifunctional structures and the combination of ceramics with other materials will be also a key research area [57].

9.7

Conclusion and perspectives

3DP has been an area of great interest mainly due to its ability to process a wide range of material classes and provide the unrestricted designing ability. Utilizing this technology for the processing of ceramics has vastly increased their scope of usage in different applications such as biomedical scaffolds, chemical filtration and high-performance aerospace components. Even with significant achievements around 3DP technology, its adoption for processing ceramics is still very slow. This has arisen primarily due to the uncertainty in feedstock material requirements for each process and the influence of processing parameters on the final part. In this chapter, a summary of the feedstock requirements for each ceramic 3DP process has been provided and key processing parameters influencing these processes are briefly discussed. This was followed by a brief comparison of the energy consumption of each process which is a crucial consideration for commercial applications. Based on this information, a technology selection guide has been developed that can assist in understanding the demands of each process and their relative difficulty. This information is crucial to narrowing down suitable 3DP technologies for specific ceramic applications. Technological advances in 3DP are making this technology more reliable and affordable for processing technical ceramics. While the challenge to scale-up build sizes in these cases still requires attention from the broader research community. More recently, sustainability becoming a driving factor in various industries, and

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energy consumption of these processes from raw materials to fully sintered parts must be studied to understand the full impact of 3DP adoption. Finally, optimization of material composition, 3DP processing parameters and sintering conditions are required to not only increase the available technical ceramics that can be processed using 3DP technology, but also to make the production of fully dense ceramics using 3DP more versatile and efficient.

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204. [3] M. McFarland, E. Antunes, Small-scale static fire tests of 3D printing hybrid rocket fuel grains produced from different materials, Aerospace 6 (7) (2019) 81. [4] X. Lv, et al., Binder jetting of ceramics: powders, binders, printing parameters, equipment, and post-treatment, Ceram. Int. 45 (10) (2019) 12609
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Vijaykumar S. Bhamare and Raviraj M. Kulkarni Department of Chemistry, Center for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Belagavi, Karnataka, India

10.1

Introduction

Generally, coatings offer a required appearance to a substrate. It has several functions in different fields. The coating offers protection to the substrate. Paints are widely used to provide protection by forming a film on the surface of the substrate. Polymer resins are generally used to form a protective layer. These coatings cannot withstand drastic conditions. Therefore inorganic ceramic coatings are preferred due to their salient features as compared to organic coatings using polymers. Ceramic coatings with desirable features can be developed using the appropriate composition of elements. Ceramic coatings are applied on metallic materials to offer a special layer that improves the properties of the substrate such as anticorrosion, wear resistance, thermal resistance, friction, etc. [1]. There are many types of coatings and fabrication methods reported in the literature to decorate metallic substrates with ceramic coatings [2 4]. Different types of methods are selected to get desirable coatings with thickness varying from nm to μm scale. Today, different types of ceramic coatings with nanosized particles are preferred to decorate metallic materials with higher thermal stability and better mechanical features [2,5 8]. The advanced methods of ceramic coatings utilize nanoscience and technology approaches to develop protective coatings on metallic materials [9]. Ceramic coatings improve the durability of material under drastic environmental conditions [10]. It was reported that ceramic coatings cannot be repaired easily since these coatings are hard and brittle in nature. At drastic conditions of temperature, these coatings suffer from cracks and debonding [3]. There are many research papers published in the last decade on fabrication methods of ceramic coatings as per the Science Direct record (Fig. 10.1). There are several metals like magnesium, aluminum, titanium, copper, iron, and their alloys are widely utilized in different fields due to their superior physical and mechanical features [11 16]. However, these metals and their alloys suffer from corrosion, fouling, durability, thermal stability, etc. at environmental conditions. Therefore the aforementioned metals are decorated with ceramic coatings to enhance features like hydrophobicity, antifouling, anticorrosion, wear resistance, bioactivity, etc. [17 22]. Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00010-5 © 2023 Elsevier Ltd. All rights reserved.

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Number of research publications

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The present chapter demonstrates different methods of fabrications utilized to form effective and efficient ceramic coatings on metallic materials, and are discussed in this chapter systematically.

10.2

Ceramic coating materials for fabrication

Ceramic materials are broadly divided into two categories such as oxide and nonoxide ceramic materials based on compositions. Most of the ceramic materials are from the category of oxide ceramics utilized for fabrication. A literature survey revealed that many metallic oxides such as titanium dioxide, aluminum dioxide, manganese dioxide, etc. are widely utilized to decorate metallic materials to enhance mechanical features. A literature survey also revealed that there are many nonoxide ceramic materials like silicon carbide, aluminum nitride, hydroxyapatite, etc. utilized to decorate the surface of metallic materials.

10.2.1 Different types of oxide ceramic coatings Titanium dioxide coatings on metallic substrates are preferred due to their unique features such as anticorrosion, nanostructure, antibiocompatibility, etc. under environmental conditions. Shen and coworkers in 2005 decorated titanium dioxide nanofilm on 316 L stainless steel. This coating was found to be anticorrosive, uniform as well as dense in nature [23]. The superhydrophobic titanium dioxide coatings displayed superior anticorrosion and hemocompatibility features of coated

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Ti 6Al 4V materials. This study highlighted that coated samples were totally free from platelets [24,25]. Aluminum dioxide coatings are found to be thermally stable, anticorrosive, and display better mechanical features than metallic materials. Gao and coworkers in 2007 decorated aluminum dioxide layers on magnesium material using the laser cladding technique successfully. The experimental results indicated that aluminum dioxide coatings offer better surface hardness, anticorrosion, and wear resistance features than magnesium material [26]. Silicon dioxide is utilized for decorating metallic substrates and offers better anticorrosion and wear resistance features. Hofman and coworkers reported that silicon dioxide coatings on substrates enhance mechanical features under environmental conditions [27]. Silicon dioxide coatings enhance surface hardness and anticorrosion features of Ni P nano-SiO2 and Ni W SiO2 nanocomposite coatings on metallic materials [28,29]. Manganese dioxide is another important material used for ceramic coatings on metallic substrates. The self-cleaning feature keeps the surface of the substrate-free from any kind of contamination and is useful for applications in different fields [30 35]. Superhydrophilic materials can be cleaned by sprinkling or showering water like rainfall [36,37]. Superhydrophobic materials have a water contact angle greater than 150 degrees and display a self-cleaning feature by removing contamination from their surface [38 40]. This is “lotus effect” in the ceramic coating which shows very less attraction for water [41 43]. Manganese dioxide coatings which are having different textures and mechanical features could be fabricated as per demand for different applications [44,45]. Zhang and coworkers in 2020 fabricated first-time manganese dioxide coatings on AZ31B magnesium material. It displays the lotus flower with superhydrophobic nature. The manganese dioxide coating was treated with stearic acid to obtain the superhydrophobic feature. It was highlighted that superhydrophobic manganese dioxide layers on magnesium have a self-cleaning feature in air and oil. This fabricated material also displays better mechanical features with an excellent lifetime [46].

10.2.2 Different types of nonoxide ceramic coatings Hydroxyapatite is nonoxide ceramic that shows biocompatibility. It is a calcium phosphate that strongly forms bonding with bone [47]. Hiromoto and coinvestigators in 2011 applied this coating on AZ31 Mg material. This coating offers a better anticorrosion feature which was evaluated in sodium chloride solution. This coating contains two layers namely, the inner layer which is dome-shape and dense while the outer layer is coarse and rod-like crystals. It was reported that corrosion current density decreased in the case of coated samples. Magnesium ion-release was also decreased in the case of coated samples [48]. Parsapour and coworkers in 2013 fabricated hydroxyapatite nonoxide ceramic coatings on 316 L stainless steel to understand the anticorrosion and biocompatibility features. The experimental results showed a decline in the corrosion current density of coated samples. This study suggested that these coated samples with treated surfaces could be utilized as human

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body implants [49]. Surmeneva and coworkers in 2016 fabricated titanium-doped hydroxyapatite coatings on Ti 6Al 4V titanium material. The experimental data indicated that titanium doped coatings are better in performance as compared to undoped coated samples due to higher contact angle. This study highlighted that titanium-doped coated samples are anticorrosive in nature at a temperature of 37 C [50]. SiC, AlN, and Si3N4 are nonoxide ceramics and are widely used for different applications due to their unique features such as longer lifetime and better thermal stability [51,52]. Pazo and coworkers reported that silicate glass coatings exhibit better adhesion to a metallic substrate. This coated material enhances the durability of the implants. These coatings are found to be bioactive in nature [53]. All these aforementioned types of ceramic coatings play a crucial role to improve the physical and mechanical features of metallic substrates and are widely utilized in different fields.

10.3

Methods for fabrication of ceramic coating on metallic materials

There are different methods or technologies used for the fabrication of ceramic coatings on the surface of the substrate as shown in Fig. 10.2. These methods are used to get desirable properties to substrates. As a result, ceramic decorated materials are used in different fields.

10.3.1 Sol gel method This is a very simple method for depositing desirable ceramic materials on the surface of metallic materials. It is a cheaper method and provides a strong linkage through a chemical bond. This coating needs low temperature to carry out a chemical reaction. It can utilize zirconia, silicon dioxide, aluminum dioxide, etc., to form ceramic coatings on metallic materials. This method utilizes ceramic materials in combination with other suitable materials to enhance the physical and mechanical features of coatings. It improves anticorrosion and thermal features of ceramic coatings [54]. This technique is easily applied in factories as it is very convenient to use. This method can be used to decorate a large number of substrates and does not depend on the size and geometry of the substrate [55]. These features make it a prominent method for ceramic coating on a metallic substrate. Wang and Bierwagen in 2009 decorated alumina on the surface of stainless steel and different metallic substrates using sol gel method. This study highlighted that there was a formation of a passive layer on the surfaces of substrates. It was reported that there was an enhancement in the pitting resistance of stainless steel during this study [56]. Ruhi and coworkers in 2009 decorated alumina on the treated surface of mild steel by using sol gel technique. During this surface pretreatment acidic zinc

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Microarc oxidaon Liquid phase deposion

Dip coang

Chemical vapour deposion

Atomic layer deposion

Fabricaon methods of

Lasercladding

ceramic

Sol-gel

coangs

Soluon immersion

Electrochemical

Magnetron spuering

Plasma treatment

Figure 10.2 Fabrication methods of ceramic coatings for metallic substrates.

phosphating solution was used to enhance the strength of bonding between alumina and mild steel. In this investigation, resistance to corrosion features was investigated and compared for different surfaces such as decorated, undecorated, and surface pretreated. The analysis of resistance to corrosion was performed by different electrochemical measurements. SEM and XRD techniques were used to study the microstructure as well as the composition of sol gel alumina coating on the surface of mild steel. The experimental data indicated that there is an enhancement in the resistant to corrosion property of decorated surfaces than undecorated and pretreated surfaces of substrates. The SEM analysis indicated that the decorated surface is compact with a thickness of 20 μ. The XRD analysis showed the α-alumina phase in alumina coating on mild steel [57]. Villatte and coworkers in 2015 fabricated titanium dioxide on the surface of fixation-pins with the help of sol gel method. In this technique, firstly titanium dioxide is decorated on the surface of fixation pins. Substrates are degreased and dipped in the sol for a period of 60 s. In order to achieve a proper thickness of the

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ceramic coating, the withdrawing speed was maintained as 31 cm/min. This is referred as cleaning and dip coating of metallic surfaces. Thereafter, annealing of titanium dioxide coated fixation-pins was performed in the furnace for the period of 1 h at a higher temperature 500 C. Subsequently, it was kept for some time without disturbing to attain room temperature [25]. Malecka and Krzak-Ros in 2013 decorated SiO2 material on the surface of γ-TiAl alloy using sol gel method. This ceramic coating was performed to enhance corrosion resistance in drastic conditions such as higher temperatures. The experimental results showed that SiO2 coating on γ-TiAl alloy could not withstand very high temperatures that is, 900 C when exposed for a longer duration [58]. Tiwari and coworkers in 2011 decorated alumina on the surface of mild steel in order to enhance anticorrosion feature of mild steel by using sol gel method. In this investigation, conversion coating was applied on the surface of mild steel. The fabricated ceramic coating surfaces were characterized with the help of different sophisticated techniques [59].

10.3.2 Microarc oxidation Ti-6Al-4V alloy was extensively utilized in aerospace and medical fields due to its exceptional resistance to corrosion and better mechanical properties. This alloy has very low wear resistance as well as surface hardness. As a result, this alloy could not be used in different fields where higher surface hardness, as well as wear resistance is required [60]. Hence, there was a demand to enhance the wear resistance of this alloy so that it can be widely utilized in different fields. There were different methods or techniques available to enhance the surface properties of this alloy. Nevertheless, the wear resistance property of Ti 6Al 4V alloy could not be enhanced using the available methods. Therefore micro-arc oxidation method was utilized for the surface modification of Ti 6Al 4V alloy. This method was superior to traditional anodic oxidation. It offers higher surface hardness as well as thickness. This method offers a strong linkage between ceramic coatings and titanium alloy. This is due to in situ growth behavior. Yizhou and coinvestigators in 2017 fabricated titania alumina composite ceramic coatings on substrate Ti 6Al 4V alloy in a suitable electrolyte using the microarc oxidation method. The surface morphology of ceramic coating was studied and analyzed by sophisticated techniques. The wear resistance property of composite ceramic coating was also investigated and found to be 9.5 times more than Ti 6Al 4V alloy [61]. Moreover, Wu and coworkers in 2014 fabricated antibacterial titanium dioxide ceramic coatings on the surface of titanium using the micro-arc oxidation method. In this study, different concentrations of copper-titanium dioxide materials were utilized. The influence of different concentrations of copper in copper titanium films on the micro-arc oxidation technique was studied thoroughly. The copper titanium dioxide coatings were characterized and analyzed using different sophisticated techniques. The potentiodynamic polarization technique was utilized to determine resistance to corrosion of fabricated copper titanium dioxide ceramic coatings.

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The antibacterial property of fabricated copper titanium dioxide ceramic coatings was analyzed using different methods. This study highlighted that ceramic coatings by micro-arc oxidation are porous in nature. These coatings have anatase phase, rutile phase, and nonoxidized titanium. This study revealed that copper titanium films undergo full oxidation. The experimental data revealed that copper exists as copper oxide in titanium dioxide ceramic coatings. It was reported that copper titanium dioxide ceramic coatings fabricated through the micro-arc oxidation technique display outstanding antibacterial activities and are found to be enhanced with an increase in the concentration of copper. There was an enhancement in resistance to corrosion property of copper titanium dioxide ceramic coatings fabricated through micro-arc oxidation [62].

10.4

Liquid phase deposition method

Flexible poly(vinyl alcohol-co-ethylene) nanofibers are used in aerogels due to a large number of OH groups. A large number of hydroxyl groups is responsible for the formation of stable suspensions. The hydroxyl group offers active sites to modify the surface of nanofibers. These nanofibers have weak resistance to heat. Due to this, these nanofibers undergo fast decomposition at drastic conditions of temperature. Hence, there was a demand for surface modification of these nanofibers to enhance the heat resistance and wear resistance properties [63,64]. The liquid phase deposition (LPD) method was used to modify the surface of the substrate and enhance mechanical properties. Sai and coworkers in 2015 used the LPD method for modifying the surface of cellulose aerogels. In this study, the surface of the 3D web-like skeleton of nanofibers was modified to improve the mechanical properties. During this investigation, blocky bacterial cellulose aerogel was dipped in an organic solvent. This work highlighted that hydrophobic bacterial cellulose aerogels are having density # 6.77 mg/cm3 which indicates that they are light in weight. The porosity of these hydrophobic aerogels was found to be approximately 99.6% which indicates that these cellulose fibers are extremely porous in nature. The surface area of these hydrophobic aerogels was found to be 169.1 m2/g. Furthermore, these nanofibers exhibit exceptional oleophilic and hydrophobic features which makes them very selective for oil adsorption from water [65]. Huang and coinvestigators in 2015 used LPD method and fabricated sponges. There were two steps involved in this polyphenol chemistry approach. During this work, it was observed that fabricated sponges show superhydrophobic and outstanding mechanical features. In this study, sponges were fabricated when liquid-filled samples dried properly. Nevertheless, porosity and other mechanical properties were influenced due to addition and elimination of liquids. It was also noticed that LPD might leave liquid chemical waste during the drying process. This could cause undesirable effects on our environment [66].

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Advanced Flexible Ceramics

Atomic layer deposition method

Chemical vapor deposition (CVD) was also utilized to modify the surface of the substrate. This method was carried out in drastic conditions of temperature. Due to this, CVD could not be utilized for polymeric aerogels [67]. Thereafter, the atomic layer deposition (ALD) method was applied to decorate polymeric aerogels using inorganic species. ALD forms a homogeneous coating of inorganic metal oxide on the surface of polymeric aerogels. ALD technique modifies the inner and outer area of polymeric aerogels. ALD offers excellent resistance to heat and surface hardness features to polymeric aerogels [68,69]. ALD involves surface modification in vapor phase at low temperatures. ALD provides tailored functionalities to the substrate. Titanium dioxide could be decorated on polymeric materials to fabricate flexible humidity sensors [70]. Alumina coating could be considered as good barrier material [71]. Zinc oxide formed antibacterial layers on fibers [72]. Lu and coinvestigators in 2020 decorated inorganic metal oxide on the surface of nanofibrous aerogels by the ALD method. This ceramic-coated material by the ALD method was found to be lighter and porous in nature. The change in the number of ALD cycles improved the surface wettability property of fabricated aerogels. This method was utilized to fabricate materials with enhancement in superhydrophobicity and oleophilicity. These polymeric aerogels are found to be promising materials for the elimination of oil contaminants from environmental waters. During this investigation, different features such as surface hardness and resistance to heat of polymeric aerogels were evaluated and analyzed [73]. There are many methods or techniques available to decorate titanium dioxide layers on the surface of substrates [74 77]. ALD is considered as a prominent process due to its unique features such as fine coating, optimum reaction conditions, good quality, resistance to corrosion, and wear resistance. A literature survey revealed that titanium dioxide layers are decorated on the surfaces of silicon, metals, and polymers [78,79]. Huang and coworkers in 2017 decorated titanium dioxide on cobalt chromium substrate effectively using the ALD method under suitable reaction conditions. It was acting as a deactivation layer to protect the metal substrate. The structure and surface morphology of coatings were studied and analyzed using sophisticated techniques. The characterization study indicated that there is a formation of a highly crystalline titanium dioxide layer on the surface of cobalt chromium substrate. It was observed that the anatase titanium dioxide phase as dominating phase under given reaction conditions. The association curve and scratch morphology indicated strong adhesion of the titanium dioxide layer on the surface of the cobalt chromium alloy. The water contact angles were studied and analyzed for decorated and undecorated cobalt chromium alloy. The experimental data indicated that decorating titanium dioxide material on the surface of cobalt chromium substrate has modified the hydrophobic surface into hydrophilic in nature. This study also revealed the outstanding antifungal property of decorated titanium dioxide on cobalt chromium in the presence of UV light as compared to undecorated cobalt chromium alloy [80].

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Abdulagatov and coworkers in 2011 decorated alumina and titanium dioxide on a copper substrate using the ALD process to protect the metal from corrosion in the presence of water. This coating has a higher corrosion resistance feature. During this investigation, it was found that alumina ALD film on the surface of copper could not protect it from water corrosion. Similarly, titanium dioxide ALD film was decorated on the surface of copper using titanium tetrachloride and water at a temperature of 120 C and found inadequate to protect it from water corrosion. However, the combination of alumina films with titanium dioxide films on the surface of copper was found to be very effective and efficient to protect it from water corrosion [81].

10.5.1 Electrochemical method This method is generally utilized to decorate oxide ceramics on the surface of metal substrates. It does not depend upon the shape and size of the metallic materials. Narayanan et al. in 2006 reported that electrochemical cathodic deposition is an inexpensive method for the deposition of calcium phosphate or hydroxy apatite on the surface of Ti 6Al 4V alloy. Calcium nitrate was utilized as a calcium source for electrochemical deposition. Hydroxy apatite was directly deposited on Ti 6Al 4V alloy by adjusting the pH and molar ratio of the reaction mixture. Electrochemically decorated coatings were studied and analyzed using different sophisticated techniques such as FTIR, TEM, and XRD. The experimental data of the corrosion test revealed that hydroxyapatite coatings are more stable and possess rough surfaces [82]. Song and coinvestigators in 2008 electrodeposited hydroxyapatite on the surface of magnesium alloy. It was reported that electrochemical and immersion tests were performed to check the biodegradable feature of hydroxyapatite decorated AZ91D magnesium substrate. The experimental data indicated an improvement in the biodegradable feature of metallic materials [83]. Charlot and coworkers in 2014 fabricated silicon dioxide coatings using electrophoretic deposition and investigated the influence of different factors on the thickness of silicon dioxide submicron coatings. During this study, low voltages were applied for controlling the submicron silicon dioxide coatings. In order to understand the effect of the nature of substrate, [silicon dioxide] was taken as 3 wt.%. The deposition time was kept as 60 min. X-film software was used to study the thickness and composition of submicron silicon dioxide coatings. It was observed that there was an increase in thickness with an increase in deposition time. There was an increase in resistance with long deposition times. This restricts the movement of particles [84]. Vengatesh and coinvestigators in 2015 fabricated superhydrophobic aluminum material by a combination of methods. In this study, the micro-texture coating was obtained by anodizing electrochemically and subsequently by the grafting method. The influence of an increase in the roughness of coating was investigated thoroughly with higher fatty acids. The experimental data indicated superhydrophobicity depends on the hierarchical roughness and also low surface energy. FTIR-ATR

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and XPS characterization of fabricated materials indicated the covalent bonding between molecules and substrate which decides the stability and durability of coatings. Tafel polarization method was used to evaluate resistance to corrosion of electrochemically anodized coatings. A dry tribology study was used to understand wear resistance and frictional properties of coated materials. This study concluded that the combination of electrochemical anodization and grafting method is an excellent way to fabricate superhydrophobic aluminum material which is more durable, eco-friendly, effective, and efficient [85].

10.5.2 Plasma treatment This technology offers firm adhesion of ceramic materials on the surface of the metal substrate. It is considered as an easy and efficient technique for ceramic coating on the substrate. The plasma electrolytic oxidation (PEO) method utilizes eco-friendly electrolytes to form oxide coatings on the surface of Mg, Ti, and Al materials [86,87]. This method shows higher surface hardness, more thickness, and dense coatings on metallic materials. It was also found that the higher thickness (10 100 μ) is responsible for higher resistance to corrosion, higher wear resistance, and better thermal barrier features. A literature survey revealed that PEO coatings techniques have been utilized in different fields. PEO technique is preferred due to its unique features over other traditional surface treatment methods [88]. Sluginov studied the plasma electrolytic discharge process and analyzed it [89]. Thereafter, Guntherschulze and coworker studied it in 1920 for the development of electrolytic capacitors [90]. Subsequently, Brown et al. studied this process in 1970 thoroughly and applied it to decorate ceramic coatings on aluminum metallic materials in basic electrolytes [91]. It was then called as anodic spark deposition method. Thereafter, this process was modified and applied for applications in different fields and called as PEO coating method [92 95]. In PEO coating, three operations are taking place simultaneously. These operations produce a three-layer coating in which the internal layer is thin and dense, the middle layer is dense and the external layer is porous. This technique is considered as superior to the anodization method due to its unique features such as higher thickness and denser ceramic coatings. It offers a variety of colors and textures to surfaces. As a result, ceramic coatings by the PEO method exhibit higher resistance to corrosion, improved wear resistance, excellent bioactivity, and better thermal stability [96]. PEO technique was extensively utilized for Al [97 101] and Mg materials [102 106]. PEO technique is also applied to other metals and their alloys such as Ti [107 111], Ta [112 116], Zr [117 121] and Nb [122 126]. PEO has been utilized for different metals and their alloys such as Ti 6Al 4V [127], Ti 6Al7 Nb [128], Ti 48Al 2Cr 2Nb [129], Ti1 3Nb 13Zr [130], Ni Ti [131], Ta [132], Mg Ca [133], AZ31 [134], AZ80 [135], AZ91 [136], AM50 [137], AM60 [138], ACM522 [139], 2024 Al [140], 7075 Al [141], 6082 Al [142], and 356 Al [143]. All these metals and their alloys are extremely utilized in different fields such as biomedical, aerospace, automobile, electronics, and marine.

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PEO method has been applied to decorate the surface of Al 7075 alloy by many researchers across the globe. Wang and coworkers in 2020 studied the anticorrosion feature of scratched oxide material. It was found that there is an enhancement in the anticorrosion feature of coated sample due to an increase in current density [144]. Bahramian and coinvestigators in 2015 highlighted the influence of titanium dioxide nanoparticles on AI7075 material substrate [145]. This work reported a higher resistance to corrosion and mechanical features. It may be due to a lower micro-porous structure. Arunnellaiappan and coworkers in 2018 studied and analyzed the influence of mixing alumina and zirconia into electrolytes during the coating process. This paper reported higher resistance to corrosion by PEO coatings using nanoparticles [146]. Sobolev and coworkers in 2020 fabricated ceramic coatings on Al7075 material via the PEO process in molten form. SEM analysis of these fabricated samples indicated the micro-porous structure with two oxide layers of coatings. The characterization of samples by XRD and EDS indicated that γ-Al2O3 as the internal layer while α-Al2O3 as the external layer. The anticorrosion feature for undecorated and decorated samples was studied and analyzed. This analysis showed the decorated ceramic coatings on the substrate exhibit higher resistance to corrosion as compared to undecorated AI7075 material. It was also observed that the current frequency of this process affected the surface morphology of coated material. Due to this, the anticorrosion feature was also affected. The resistance to corrosion for decorated material was found to be higher due to higher current frequency [147]. Magnesium alloys are considered as a potential candidate for their applications in medical fields due to their unique features such as lower density and higher specific strength. Magnesium shows less resistance to corrosion which limits its ability to use in medical fields. Therefore Gao and coworkers in 2018 utilized plasma treatment using hydroxyapatite on AZ91HP magnesium material to check its bioactivity as well as resistance to corrosion. The experimental data indicated that there was an enhancement in resistance to corrosion and bioactivity of magnesium material due to the plasma spraying technique. This hydroxyapatite coating enhanced the hydrophilic nature of magnesium material. This study revealed that hydroxyapatite coating on magnesium material using plasma treatment has excellent anticoagulant activity [148]. Sun and coinvestigators in 2010 decorated titanium oxide layers on the surface of titanium materials by the PEO process successfully. In this study, sodium silicate was utilized as an electrolyte. The influences of different parameters on the porosity and compositions of coated samples were studied and analyzed. The experimental result indicated the major existence of anatase titanium dioxide phase in coated samples. There was an increase in corrosion potential up to 0.13 V and a decline in corrosion current density. This study highlighted an enhancement in resistance to corrosion feature of titanium due to titanium dioxide layers formed on its surface by the PEO process [149].

10.5.3 Magnetron sputtering Magnetron sputtering (MS) is an effective technique utilized to decorate ceramic coatings on metallic substrates. This is a high-rate vacuum depositing method.

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In this technique, the magnetic field is utilized. This method offers higher thickness on metallic substrates. This is considered as a superior technique due to its unique features such as higher deposition rates, strong adhesion, and better purity of coatings. This process is basically a physical vapor deposition. Siva Rama Krishna and Sun in 2005 deposited a titanium layer on the surface of stainless steel using the MS technique and following thermal oxidation of decorated 316 L material. The experimental results revealed better resistance to corrosion and tribological features. The hybrid structure was observed in decorated samples. Three types of structural zones were noticed during this study. This type of hybrid structured deposition has enhanced surface hardness and antiwear features. The presence of rutile titanium dioxide phase on 316 L material is without nitrogen. This clearly indicates that there is no considerable influence of nitrogen on anticorrosion and frictional features. [150].

10.5.4 Solution immersion process Superhydrophobic surfaces exhibit unique features and have fascinated the research community to focus on the fabrication, features, and applications of these surfaces. Among many alloys, magnesium alloys are considered as very useful materials due to their unique features and are widely investigated by the research community to fabricate superhydrophobic magnesium material surfaces using different methods [151 160]. These investigations reported many challenges like toxic materials, expensive methods, lower efficiency, inconvenient device, etc., to fabricate these surfaces. In order to overcome these abovementioned demerits, Song and coinvestigators in 2012 fabricated superhydrophobic magnesium materials surface using the solution immersion method which is a very convenient, efficient, and cheaper method [161,162], During this investigation, it was observed that magnesium materials consist of two major phases such as magnesium-rich α-phase and aluminumrich β-phase [163]. The value of electrode potential of β-phase was found higher as compared to α-phase [164]. The experimental data indicated that β-phase is less corroded while α-phase is more corroded by a hydrogen ion. As a result, α-phase shows higher corrosion velocity. The stable nature of these surfaces was examined in the air. The superhydrophobic nature of materials could be preserved for a minimum period of eight months if stored in air. The hydrophobic materials were modified using stearic acid and showed found to be very stable in air. Hiromoto and Tomozawa in 2011 fabricated hydroxyapatite coatings on the surface of magnesium metal and its alloy. This study was performed at fixed pH (8.9) in an aqueous medium for a period ranging from 2 to 6 h. The anticorrosion features of decorated samples were evaluated using immersion and polarization tests. The corrosion current density was found to be less in hydroxyapatite coatings [48]. The hydroxyapatite materials used in this investigation are eco-friendly, thermally stable, and biocompatible. Xun and coworkers in 2019 reported that a superhydrophobic surface enhances anticorrosion and antibioadhesion features of the AZ31B magnesium coatings.

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In this work, a two-step immersion method was utilized followed by postmodification. The strength of adhesion was found higher between the superhydrophobic surface and AZ31B magnesium material. The stability of coating was found to be higher due to strong interfacial chemical bonding between superhydrophobic surface and AZ31B magnesium material. This study was performed at room temperature without using any drastic conditions. There was a formation of a gradient layer which forms a strong coating on the surface of the substrate. This study concluded that superhydrophobic AZ31B magnesium could be utilized in biomedical fields [165].

10.5.5 Laser-cladding method Laser-cladding can be effectively and conveniently utilized for the fabrication of ceramic coatings on the surface of the metallic substrate due to the very high energy of the laser which increases the rate of metal oxidation. The coatings fabricated by Laser-cladding indicated better anticorrosion and antiwear resistance features [166]. Boinovich and coinvestigators in 2015 decorated a superhydrophobic oxidized surface on a metallic substrate. This coating showed better anticorrosion properties in NaCl solutions. This coating was found very useful in medical, construction, and automobile fields. In this study, nanosecond laser treatment was applied for the fabrication of coating on aluminum alloys. This coating exhibits better barrier properties. The electrochemical and wetting processes indicated an enhancement of protection of coatings due to the synergic effect [167]. Li and coworkers in 2019 reported that the laser etching technique played a crucial role to fabricate hydrophobic surfaces. This research team fabricated a wettability gradient surface on nickel titanium alloy. During this investigation, an ultra-short pulse laser etching technique was applied to prepare a hydrophobic surface. The proper decoration of stearic acid on gradient surface was indicated by XPS analysis. The characterization of decorated samples using SEM and CLSM indicated a structurally gradient surface. The experimental data indicated that decorated nickel titanium alloy exhibits good antiadhesion features, low flow resistance, and good hemocompatibility which is responsible to reduce thrombosis. The laser-cladding method is found to be an effective and easy method. This method is very useful to fabricate coatings that exhibit antithrombotic surfaces [168]. Laser cladding in situ technique has unique features. This technique has resolved the wetting problem. It is found to be a very efficient and effective coating due to better performance [169 175]. Yuxin and coworkers in 2018 decorated Ti/TiBCN ceramic coatings on the surface of 7075 aluminum material using the laser cladding technique. SEM and XRD were utilized to study and analyze the effect of TiBCN content on different features of coatings. TiBCN content was found to be 15 wt.%. The average hardness of the fabricated sample was found to be 750 HV0.2. This average hardness is fivefold more in coated samples as compared to the uncoated substrate. The experimental results revealed that the anticorrosion property increases to the maximum value as

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the TiBCN content increase to 15 wt.%. It was observed that the average friction coefficient declines due to an increase in TiBCN weight percentage [176]. Li and coinvestigators in 2019 used the laser cladding technique to decorate Al2O3 TiB2 TiC layers on carbon steel material. XRD, SEM, and EDS data were used to analyze microstructures of coatings. The performance of coating was found better when alumina reaches 30%. The laser coating was found to be uniform and dense. The experimental data indicated higher anticorrosion, better wear resistance, and higher surface hardness of coated samples as compared to carbon steel material [168].

10.5.6 Chemical vapor deposition method Sun and coworkers in 2021 reported that CVD is a very useful and effective method to decorate ceramic coatings. These fabricated coatings possess proper surface topography [177]. This method involved thermally induced reactions that are taking place on the substrate surface. Hofman and coworkers in 1996 utilized FTIR and XPS techniques to study and understand the effect of humidity on the structure of fabricated ceramic coatings. The mixing of water solvent improves the protection of substrate decorated by noncrystalline silicon dioxide. It was highlighted that the anticorrosion feature and thickness of coatings increase with an increase in the concentration of water. This study reported that silanol groups decline the viscosity of fabricated samples and enhance the efficiency of coatings. The composition of ceramic-coated materials was found to be stoichiometric (SiO2.0) as determined by XPS [27].

10.5.7 Dip-coating method This method is utilized to decorate homogeneous coating on the surface of the substrate. It is also referred as the impregnation method. The substrate material is immersed in a container that contains less viscous material. It is easy, cheaper, and convenient approach of coating as compared to other techniques. It involves five steps as shown in Fig. 10.3. The first step is immersion which involves immersing of pretreated substrate material into a container that contains a coating solution. The second step is a start-up in which the substrate material is kept in a container for a particular time period. The third step is deposition in which the substrate material is taken out from the solution bath at a suitable speed to adjust the thickness of the coated layer. The slower speed of removing the substrate from the solution is responsible for the small thickness of the ceramic coating. The fourth step is referred to as drainage in which surplus liquid is squeezed out from the surface. The final step is referred to as evaporation which involves the evaporation of solvent to get a thin coating on the surface. The quality of dip coating depends on different experimental conditions. Dip coating is a very simple approach and hence coating may be of poor class [178]. Procopio and coworkers in 2020 reported the dip coating technique which is a convenient, cheaper, and simplistic approach for decorating cerium oxide (ceria) on

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Figure 10.3 Graphical representation of dip-coating method.

the surface of titanium dioxide. SEM and EDS techniques were utilized to understand the morphology of coated samples up to ten depositions [179]. Naveas and his team in 2020 reported that dip and spin coating methods are easy, convenient, inexpensive, and versatile to produce functional coatings. This study fabricated nanoporous silicon silver composite coatings. It was found that the cyclic dip-coating technique produced a mixture of flake-like and granular silver structures while the cyclic spin-coating technique produced flake-like structures [180]. Yu and coworkers in 2017 fabricated a surface coating of sodium silicate/alumina composite on the 304 stainless steel surface successfully with desirable features. This study revealed that sodium silicate enters into nanopores of aluminum oxide layer using dip-coating method [181].

10.6

Conclusions

The present chapter demonstrates different methods for the fabrication of oxide and nonoxide ceramic coatings on metallic substrates to offer required features such as anticorrosion, wear resistance, durability, self-cleaning, biocompatibility, hydrophobicity, thermal stability, antifouling, tribological features, antifungal, antibacterial, surface hardness, abrasion resistance, etc. The advanced methods of ceramic coatings utilize nanoscience and technology approaches to develop protective coatings on metallic materials successfully.

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Sol gel method is a very easy process for depositing desirable ceramic materials on the surface of metallic materials. This method utilizes ceramic materials in combination with other suitable materials to enhance the physical and mechanical features of coatings. It improves anticorrosion and thermal features of ceramic coatings. This technique can be easily applied in factories as it is very convenient to use. Microarc oxidation method was utilized for the surface modification of metallic alloy. It was found to be superior to traditional anodic oxidation. It offers higher surface hardness as well as thickness. The LPD method can be utilized for surface modification of nanofibers to enhance the heat resistance and wear resistance properties. ALD method was applied to decorate polymeric aerogels using inorganic species. It forms a homogeneous coating of inorganic metal oxide on the surface of polymeric aerogels. This technique modifies the inner and outer areas of polymeric aerogels. It offers excellent resistance to heat and surface hardness features to polymeric aerogels. The electrochemical method can be utilized to decorate oxide ceramics on the surface of the metal substrate. It does not depend upon the shape and size of the metallic materials. It is concluded that the combination of electrochemical anodization and grafting method is an excellent way to fabricate superhydrophobic aluminum material which is more durable, eco-friendly, effective, and efficient. PEO method utilizes eco-friendly electrolytes to form oxide coatings on the surface of Mg, Ti, and Al materials. This method shows higher surface hardness, more thickness, and dense coatings on metallic materials. MS is an effective technique utilized to decorate ceramic coatings on metallic substrates. This is considered as superior technique due to its unique features such as higher deposition rates, strong adhesion, and better purity of coatings. The experimental results revealed better resistance to corrosion and tribological features. Superhydrophobic surfaces fabricated using the solution immersion process exhibit unique features and fascinated the research community to focus on the fabrication, features, and applications of these surfaces. This coating was found to be eco-friendly, thermally stable, and biocompatible. There was a formation of a gradient layer which forms a strong coating on the surface of the substrate. This study concluded that superhydrophobic AZ31B magnesium could be utilized in biomedical fields. Laser-cladding can be effectively and conveniently utilized for the fabrication of ceramic coatings on the surface of the metallic substrate due to the highly energetic laser which increases the rate of metal oxidation. These coatings fabricated by laser-cladding were found higher in anticorrosion and antiwear resistance features. This method is very useful to fabricate coatings that exhibit antithrombotic surfaces. The laser cladding in situ technique has unique features. This technique resolved the wetting problem. CVD method is very useful and effective to decorate ceramic coatings. These fabricated coatings possess proper surface topography. This method involved thermally induced reactions that are taking place on the substrate surface. It was highlighted that anticorrosion features and thickness of coatings enhance with an increase in the concentration of water. This study reported that silanol groups decline the viscosity of fabricated samples and enhance the efficiency of coatings.

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Dip-coating method forms a homogeneous coating on the surface of the substrate. The substrate material is immersed in a container that contains less viscous material. It is easy, cheaper, and convenient approach of coating as compared to other techniques. The quality of dip coating depends on different experimental conditions.

10.7

Future scope

Ceramic coatings fabricated by different methods depend on many factors such as temperature, pressure, composition, nature of substrate, nature of the electrolyte, the concentration of electrolyte, electric current, etc. All these factors decide the physical and mechanical features of different coatings fabricated on the surface of metallic substrates. So, there is further scope to fabricate ceramic coatings with desirable physical and mechanical features by using suitable experimental conditions. PEO coatings are found to be brittle which can be improved by maintaining suitable reaction conditions. There is a need to perform more research in this field to modify the coatings. It is also suggested that interdisciplinary fields may play a significant role in the development of ceramic coatings. There is a lot of scope for research in plasma electrolytic oxidation.

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[157] J. Wang, D.D. Li, Q. Liu, X. Yin, Y. Zhang, X.Y. Jing, et al., Fabrication of hydrophobic surface with hierarchical structure on Mg alloy and its corrosion resistance, Electrochim. Acta 55 (2010) 6897 6906. [158] J. Wang, D.D. Li, R. Gao, Q. Liu, X.Y. Jing, Y.L. Wang, et al., Construction of superhydrophobic hydromagnesite films on the Mg alloy, Mater. Chem. Phys. 12 (9) (2011) 154 160. [159] W.J. Xu, J.L. Song, J. Sun, Y. Lu, Z.Y. Yu, Rapid fabrication of large-area, corrosion resistant superhydrophobic Mg alloy surfaces, ACS Appl. Mater. Interfaces 3 (2011) 4404 4414. [160] Z.W. Wang, Q. Li, Z.X. She, F.N. Chen, L.Q. Li, Low-cost and large-scale fabrication method for an environmentally-friendly superhydrophobic coating on magnesium alloy, J. Mater. Chem. 22 (2012) 4097 4105. [161] J. Song, Y. Lu, S. Huang, et al., A simple immersion approach for fabricating superhydrophobic Mg alloy surfaces, Appl. Surf. Sci. 266 (2013) 445 450. Available from: https://doi.org/10.1016/j.apsusc.2012.12.063. [162] M. Qu, B. Zhang, S. Song, et al., Fabrication of superhydrophobic surfaces on engineering materials by a solution-immersion process, Adv. Funct. Mater. 17 (2007) 593 596. Available from: https://doi.org/10.1002/adfm.200600472. [163] G. Ballerini, U. Bardi, R. Bignucolo, G. Ceraolo, About some corrosion mechanisms of AZ91D magnesium alloy, Corros. Sci. 47 (2005) 2173 2184. [164] G. Galicia, N. Pe´be`re, B. Tribollet, V. Vivier, Local and global electrochemical impedances applied to the corrosion behaviour of an AZ91 magnesium alloy, Corros. Sci. 51 (2009) 1789 1794. [165] X. Xun, Y. Wan, Q. Zhang, et al., Low adhesion superhydrophobic AZ31B magnesium alloy surface with corrosion resistant and anti-bioadhesion properties, Appl. Surf. Sci. 505 (2019) 144566. Available from: https://doi.org/10.1016/j. apsusc.2019.144566. [166] L.B. Boinovich, E.B. Modin, A.R. Sayfutdinova, et al., Combination of functional nanoengineering and nanosecond laser texturing for design of superhydrophobic aluminium alloy with exceptional mechanical and chemical properties, ACS Nano 11 (2017) 10113 10123. Available from: https://doi.org/10.1021/acsnano.7b04634. [167] L.B. Boinovich, A.M. Emelyanenko, A.D. Modestov, et al., Synergistic effect of superhydrophobicity and oxidized layers on corrosion resistance of aluminium alloy surface textured by nanosecond laser treatment, ACS Appl. Mater. Interfaces 7 (2015) 19500 19508. Available from: https://doi.org/10.1021/acsami.5b06217. [168] Z. Li, M. Wei, K. Xiao, et al., Microhardness and wear resistance of Al2O3-TiB2-TiC ceramic coatings on carbon steel fabricated by laser cladding, Ceram. Int. 45 (2019) 115 121. Available from: https://doi.org/10.1016/j.msec.2020.110847. [169] C.G. Li, Z.S. Yu, Y.F. Zhang, et al., Microstructure evolution of laser remelted Al2O3 13 wt%TiO2 coatings [J], J. Alloy Compd. 576 (2013) 187 194. [170] G.X. Sun, H.C. Liao, et al., Fabrication processes and control of interfacial reaction of particulate reinforced metal matrix composites, Spec. Cast. Nonferrous Alloy 4 (1998) 12 15. [171] G.Y. Liang, J.Y. Su, The microstructure and tribological characteristics of laser-clad Ni-Cr-Al coatings on aluminium alloy, Mater. Sci. Eng. A290 (2000) 207 212. [172] G.F. Guo, F.R. Chen, L.H. Li, et al., Application of laser cladding technology in surface modification of titanium alloys, Surf. Technol. 35 (1) (2006) 66 69. [173] C. Gao, B. Xu, et al., Advances in laser cladding ceramic particle reinforced metal matrix composite coating technology, Surf. Technol. 37 (4) (2008) 63 66.

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[174] A. Emamian, S.F. Corbin, A. Khajepour, The effect of powder composition on the morphology of in situ TiC composite coating deposited by laser-assisted powder deposition (LAPD), Appl. Surf. Sci. 261 (2012) 201 208. [175] S.K. Mishra, S.K. Das, V. Sherbacov, Fabrication of Al2O3 ZrB2 in situ composite by SHS dynamic compaction: a novel approach, Compos. Sci. Technol. 67 (11 12) (2007) 2447 2453. [176] Y. Li, P. Zhang, P. Bai, et al., Microstructure and properties of Ti/TiBCN coating on 7075 aluminum alloy by laser cladding, Surf. Coat. Technol. (2017) 334. [177] L. Sun, G. Yuan, L. Gao, et al., Chemical vapour deposition, Nat. Rev. Methods Prim. 1 (2021) 5. Available from: https://doi.org/10.1038/s43586-020-00005-y. [178] K. Kakaei, [Interface science and technology] graphene surfaces - particles and catalysts Volume 27 ||, Graphene Anticorrosive Prop. (2019) 303 337. Available from: https://doi.org/10.1016/B978-0-12-814523-4.00008-3. [179] A.M.S. Procopio, J.D.L. Carvalho, T.H.R. Silveira, et al., CeO2 thin film supported on TiO2 porous ceramics, Mater. Lett. 276 (2020) 128224. Available from: https://doi. org/10.1016/j.matlet.2020.128224. [180] N. Naveas, M. Manso-Silvan, R. Pulido, et al., Fabrication and characterization of nanostructured porous silicon-silver composite layers by cyclic deposition: dip-coating vs spin-coating, Nanotechnology 31 (2020) 365704. Available from: https://doi.org/ 10.1088/1361-6528/ab96e5. [181] J. Yu, S. Liu, F. Li, et al., Na2SiO3/Al2O3 composite coatings on 304 stainless steels for enhanced high temperature oxidation inhibition and chloride-induced corrosion resistance, Surf. Coat. Technol. 309 (2017) 1089 1098. Available from: https://doi. org/10.1016/j.surfcoat.2016.10.003.

Methods for ceramic machining

11

Manisha Priyadarshini1, Swastik Pradhan2 and Rajashree Samantray3 1 Centurion University of Technology and Management, Bhubaneswar, Odisha, India, 2 Lovely Professional University, Phagwara, Punjab, India, 3Department of Metallurgical and Materials Engineering, National Institute of Technology Rourkela, Rourkela, Odisha, India

11.1

Introduction

Ceramics production has turned into a significant modern material and is generally utilized in various fields for beating innovative impediments. Ceramics are by and large inorganic translucent materials (glass being an exemption) that are made out of metals and nonmetals involving ionic or covalent bonds. In light of synthetic synthesis, modern ceramic production is delegated oxides, borides, silicates, nitrides, carbides, and glass ceramic. In contrast with metals and polymers, the capacity of ceramic production to endure elevated working temperatures makes them a main designing material. Its strength is equivalent to metals and is for the most part artificially lifeless. A large portion of the ceramics are encasings of hotness and power and are impervious to unfavorable conditions. There has generally been trouble in cutting/machining of modern ceramic production which limits their applications because of their outrageous properties. Additionally, because of its higher hardness and fragility, this material is more prone to crack. Thus need for additional handling is required whether through regular or nontraditional machining strategies. Grinding and drilling are the most generally involved regular strategies for machining ceramic production and its composite. Machining ceramic production and its composite by customary techniques is very troublesome and tedious. So to foster financially savvy machining techniques for the chemistry, manufacturing and controls (CMCs) industry changed to present-day machining strategies that incorporate ultrasonic machining (USM), Abrasive Jet machining (AJM), Abrasive Water Jet Machining (AWJM), laser machining, Electric Discharge Machining (EDM), and so on. Material erosion peculiarity in present-day machining strategies joins mechanical scraped area, compound disintegration, dissolving or dissipation, and electrochemical disintegration [1 3]. This chapter summarizes the different handling and machining techniques suitable for ceramics and its composites. The manufacturing of ceramic and its composite by utilizing different sintering techniques can’t deliver precise complex shapes because of shrinkage and other manufacturing constraints [4,5]. Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00011-7 © 2023 Elsevier Ltd. All rights reserved.

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Machining removes the material from the workpiece commonly in various types of abrasives or chips. In traditional machining, the material is taken out with the shear mechanisms like turning the machine and furthermore, there should be a contact in the middle of hardware and workpiece, whereas in current machining material is eliminated by the activity of various energies. The machining of ceramics can be categorized into traditional, nontraditional, and hybrid methodologies.

11.2

Traditional machining

The most well-known traditional machining techniques utilized for fired and its composites are drilling and grinding. Electrolytic in process dressing is originally utilized for electrochemical grinding [6]. Huang and Liu reported the use of a deep grinding methodology for the machining of different ceramic materials [7]. Cryogenic environment for grinding ceramics with decreasing power, energy, and surface texture when contrasted for grinding with dry atmosphere. The grease characteristics of nitrogen jets and temperature control diminish the surface roughness [8]. Koshy et al. utilized electrical sparks for solidified carbides through electric sparks. Grinding surface measures are improved with wear resistivity at low release energy because of the enhancement of the hardness of the material [9]. Material-removing techniques, surface texture, and typical grinding powers typically rely on the kind of ceramic material utilized for high-speed grinding [10]. Many comparisons are performed on high velocity and traditional grinding by Yin et al. for alumina and its alloys [11]. The material-removing techniques and surface textures for both the materials are found similar. There is an impressive lessening in the particular typical powers, extraneous powers, and their power proportions in comparison with traditional grinding [12]. There has generally been trouble in the cutting of modern ceramics which limits their applications because of their outrageous properties. Also, because of higher hardness, these materials are prone to breakage. A lot of cutting powers are required which instigates cracks on a superficial level and edge chipping [13]. An infused diamond bit is considered by Chao and Juntang in drilling aluminum oxide armors [14]. The elective cutting strategies, for example, wire-electrical discharge machining, laser machining, abrasive water stream machining, and hybrid machining have likewise been applied by different scientists worldwide to cut ceramic production. Not many of these procedures are fruitful in delivering top-caliber and exact cutting surfaces without corrupting the parent material. These advanced strategies can possibly defeat the hindrances in manufacturing of designing parts from various ceramic materials. This strategy is powerful for machining hard-to-cut materials with infused diamond blades. It is feasible to deliver a tight kerf-width. However, it is beyond the realm of possibilities to expect to accomplish cutting precision attributable to lower firmness. The redirection and wearing of the sharp edge during machining impact

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compromise the surface integrity and mathematical exactness. Soni et al. [8] concentrated on progress in cutting of material involving diamond wheels of less width. Likewise, many studies were performed for analysis of wear mechanisms coated diamond wheels [15 17]. This is an affordable technique that contains two stages, that is, scribing and breaking. It is generally utilized to cut glasses, Liquid Crystal Display (LCD) boards and silicon wafers. An engraved line is generated. At the point when the break doesn’t drive adequately profound into the material thickness, higher breaking forces are expected in its breakage. Advancement of interaction boundaries is expected to forestall the arrangement of sidelong breaks which might spread and bring about absconds. However, the machining has evolved such that the isolating of glass is being done by moving the defocussed laser bar along the scribed line [18]. Tsai and Huang carried out experiments with diamond scribing and laser isolating of glass plate and laid out a connection within the depth of the cracks, scribe’s depth, and scribing force with the machining speed. The texture of the surface machined by laser is concluded to be better as compared to the mechanically machined one [19]. A wire saw is a more common strategy for cutting ceramic products in comparison with a diamond saw cutting, as this produced slender kerfs [20]. At first, tempered steel wire supported by coolant with added abrasives was utilized to cut ceramics such as Silicon Carbide (SiC). The coolant is a blend of oil/ water/polyethylene glycol and the abrasives (SiC and/or diamond). This prearranged coolant gives lower machining rates, and its reusing is an issue. Additionally, the temperature changes the density of the coolant. For avoiding the above issues, diamond-infused wire is generally preferred. Its material erosion system involves the interface of abrasive particles, moving wire, and substrate. For the most part, in fixed diamond wire cutting, the wire stays unaffected after the material erosion process. Wu et al. directed the crack strength of Si wafers cut by diamond wire. They detailed that the prevalent flexural strength of wafers is accomplished with diamond wire cutting [21]. Tests directed by Wantanabe et al. concluded that the polycrystalline Si wafers are utilized in both the cutting cycles. The profundity of scratch is a lot of lower in the diamond wire. Abrasives slurry cutting creates miniature breaking while fixed grating diamond wire cutting produces smooth notches. The strong and the messed-up lines depict the profundity of scratch and inside harm separately [22]. For cutting various examples or ingots all the while, multiwire slurry is presented. Wu et al. explored the impact of the movement of the wire on the cut surface nature of Si wafers utilized in solar-powered cells [23]. Meng et al. investigated fixed rough diamond interminable wire cutting of aluminum oxide/titanium carbide [24]. They announced that high wire speed decreases surface harshness, wire wear, and cutting powers yet with sped up, wear of the wire, the unpleasantness of the delivered surface increments. Comparable perceptions were accounted for by Hardin et al. while playing out a bunch of examinations for cutting 75 mm breadth SiC wafers utilizing fixed rough diamond wire [25]. The utilization of little measured diamond coarseness can lessen the miniature breaks, stagnation depressions, and grinding on the machined surface and upgrade

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the surface respectability. Clark et al. performed fixed abrasives diamond wire slicing of froth ceramic to examine the wear and life of diamond wire [20]. The diamond corn meal was presented and exposed to crack while really cutting the froth ceramic production. A little length of contact between the wire and the example is kept up continually all through the cutting system. The shaking wire movement system is intended to keep in touch the length among the workpiece and wire and improve the cut quality. The significant models for foreseeing and making sense of the cutting and material erosion components in wire cutting incorporate the rolling indenting model [26] and the scratching indenting model [27]. Johnsen et al. [28] concentrated on the hotness move and temperature appropriation tentatively and computationally to explore the impact of temperature minor departure from the cutting exhibition. The gooey dispersal decreased the adequacy of slurry conveying grating particles in the cutting channel. As diamonds and silicon carbide are exorbitant abrasives; subsequently, the wire saw cutting turns out to be over the top expensive. These materials represent 25% 30% of the absolute wafer cutting expense. In this way, the traditional slicing can create correspondingly precise surfaces, yet it’s anything but a financially savvy process. The legitimate removal of the preowned slurry involving substrate particles (silicon) with abrasives is compulsory to forestall natural concerns. As the silicon carbide particles are costly, thus numerous original procedures were created to segregate substrate particles and silicon carbide to be reused [29].

11.3

Nontraditional machining

USM is a cutting-edge machining strategy in which the material is eliminated by the mechanically scraped area of slurry (combination of liquid and abrasives) within the ultrasonic frequency workpiece. The USM is appropriate for hard material having a hardness above 40 Rockwell hardness and regardless of its material property [30]. USM is the most useful and adaptable technique in making intricate designs [31]. In USM harder materials give lower productivity, higher wear, and a lower surface [32]. Lee and Chan explained the fundamental system of material erosion [33]. It is observed that material erosion for the most part happened because of the spread and convergence of middle and outspread breaks. For a miniature fragile break to happen stunning power should be more noteworthy than a basic burden and furthermore expansion in material erosion rate and surface harshness relies upon the plentifulness of hardware, the static burden on the device, and the coarseness size of grating grains. Material erosion rate diminishes as the static burden expansions in the event of USM [34]. Rajurkar et al. outlined the mechanics of the USM of alumina with its material erosion rate. The test demonstrates that in case of low speed, the material is eroded because of primary breaking down and molecule disengagements and though high impacts elimination of the material takes place by the growth of intergranular micro-cracks [35]. Jianxin and Taichiu detailed the variety in the material erosion

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rate, bulk property, and surface texture of a composite regarding stubble direction [4]. Laser beam machining (LBM) is an elective technique to machine hard-to-cut and fragile materials. The test strategy crack-machining component utilized by Tsai and Chen for laser processing of cavity in the material is appropriate for edge processing as well as be utilized for focus processing with less power prerequisite [36]. To upgrade the machinability of the workpiece, more energy of the laser is utilized [37]. The machinability of materials in laser-assisted manufacturing may be expanded by reducing their mechanical strength [38]. Dubey and Yadava [39] inspected and investigated the potential outcomes of LBM for a variety of materials. LBM for thicker materials and micro-parts needs good research. LBM execution relies on laser boundaries, material boundaries, and cycle boundaries [40]. Laser slicing may be utilized silicon nitride-based composite materials to create a cantilever structure. Sola and Pena examined the impact of various manufacturing conditions for the Al composite close to infrared beam laser [41]. Various elements which influence the laser cooperation process were substrate temperature, the plasma safeguard, the beat number, the example position concerning the central plane, and the functioning recurrence. Yan et al. [42] referenced the advantages of submerged machining involving CO2 laser for profound holes in alumina. Submerged machining involving CO2 aids in decreasing the substrate deserts like recast layer, dross, heat harms, and breaking. Venkatesan et al. [43] in finishing up comments evaluated the future chance of various other machining cycles, for example, processing, grinding, and penetrating by involving laser innovation as past examination fundamentally centered around laser helped turning as it were. Laser scribing is another inventive method for exact cutting of glass, quartz glass, and weak ceramic production. At the point when the workpiece is warmed by the laser pillar, compressive anxieties are created in the surface layers. The coolant is applied quickly following the laser pillar attributable to which tractable burdens are produced. Whenever created pliable anxieties surpass a definitive strength of the material, it brings about miniature breaks arrangement. Yamamoto et al. directed the laser scribing method on soft drink lime glass ceramic production and explored the impact of the thickness of the glass on break profundity and recorder condition [44]. Saman et al. utilized different cutting strategies to cut silicon wafers, where, the laser pillar is illuminated on a little region making compressive pressure. This technique wipes out chips arrangement and creates great surface completion contrasted with mechanical mountings [45]. A synchronous prescoring way to deal with cut 2.54 mm thick alumina ceramic is by utilizing CO2 laser. An altered pillar conveyance framework (double bar yield with a solitary source) is utilized to cut the groove on a superficial level and direct the break way forward of cutting [46]. A significant hindrance that influences the cutting quality is the initiated warm pressure. This pressure brings about break proliferation. Various methods, for example, multipass cutting, submerged cutting, water stream helped cutting, double

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bar game plan, and substrate preheating are done to limit the break harm [46 50]. The disposal of untimely cracks while cutting thick ceramic production is additionally a test. It very well may be constrained by bringing down the hotness develop and warm weight on the substrate. So numerous pass cutting or intruded on slicing is polished to bring down the warm info and high temperature develop. Be that as it may, it diminishes the interaction speed. The crack components were dissected utilizing fractographic perceptions and stress examination. It is seen that the breaking surface showed four unmistakable locales described by dissipation district, columnar grain area, intergranular crack locale, and transgranular crack district separately through and through. The dissipation district is shaped because of hotness fixation with minute breaks. Significant breaks were framed in the columnar grain district because of the hardening of dissolved material. Because of a decline in temperature, tractable pressure is initiated which stretched out the break to the intergranular area and caused shakiness which brought about the arrangement of transgranular crack district. It brought about a decrease of unpleasantness and dross-free slice because of productive erosion of liquid materials. Quintero et al. [51] likewise planned three sorts of gas discharge frameworks and investigated the component of material erosion in ceramics by beat laser combination cutting. Research concentrates on have shown that laser tendency point adversely affects the cut quality. Mullick et al. [52] detailed that a slanted bar impacts the assimilation capacity of the material, striation profundity, power conveyance, and shear pressure appropriation. The failures of the traditional slicing strategies have prompted the improvement of current procedures to slice the ceramic production with fewer cracks. Wire-EDM (WEDM) is an exceptionally encouraging innovation, which can create multifaceted profiles. A consistent progression of dielectric liquid (as a coolant) coaxial with the wire is kept up with all through the cutting system for proficient erosion of liquid material from the flash zone as trash. Throbbing voltage is utilized to make a flash between the cathodes. At the point when the provided voltage arrives at a basic worth, a plasma channel is created, and a flash is started [53,54]. They additionally featured the impact of heartbeat length and profundity of cut on yield factors, for example, kerf width, cutting rate, and instrument wear. A base kerf width of 100 µm is accomplished utilizing the foil terminal technique. Zhao et al. [55] used running super meager multifoil instrument cathode for cutting SiC ingots into wafers. The normal region cutting velocity is improved because of the bigger release region in multicutting contrasted with single cutting. The wire breakage issue in multi-WEDM is likewise tackled utilizing multifoil cutting technique. Zhao et al. [56] detailed a relative report on material erosion rate, kerf width, surface harshness, and wire cathode breakage of SiC and SKD11. Thus, material erosion in EDMed SiC occurred because of softening and disintegrating and furthermore because of breaking and cracking. Pramanick et al. [57] directed probes conductive boron carbide and inspected the ideal unpleasantness and cutting rate at beat on the schedule of 30 µs, beat off-season of 52 µs, dielectric strain of 8 kg/ cm2, and feed pace of 2100 mm/min. A relapse model is formed for surface harshness and machining speed. Cheng et al. [58] performed machining on ZrB2

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ceramics and ZrB2 Cu composite utilizing bite the dust sinking-EDM, wire-EDM, and diamond saw. A recreation framework is created by Han et al. [59] which is effective in duplicating the release peculiarities in WEDM. Liu and Esterling [60] planned a strong displaying strategy to address WEDM cut math. Hou et al. [61] introduced a recreation of warm pressure and temperature field appropriation of Si3N4 protecting fired while machining with WEDM. A twofold layer structure model is intended for reenacting the release condition utilizing FEM. Another WEDM strategy to cut SiC ingots is created by Kimura et al. [62]. A multiwire EDM with track molded area terminal wire is utilized to cut the clay with an intent to diminish kerf width and breaks on the machined surface. This track molded segment metal cathode offered high strain stacking contrasted with roundabout segment wire and subsequently limited the wire vibration. This procedure gave multipoint cutting of ceramic at a specific time in an exceptionally productive way. Lok and Lee [63] estimated the flexural strength of sialon and Al2O3 TiC ceramic utilizing three-point and four-point quarter twisting tests. It is seen that mean flexural strength decreases by 32% and 67% separately because of the development of surface breaks on the machined surface. To get a profoundly precise surface completion is very troublesome because of wire vibration and wire static deserting (wire slack) [64]. In this way, barely any examinations have likewise been directed for understanding the wire’s mechanical way of behaving while at the same time machining. Many control procedures to forestall wire breakage have likewise been created by different analysts [58,65]. EDM is one of the advanced machining strategies utilized as an accuracy machining strategy in which the electrically conductive hard material is eliminated by thermoelectric peculiarity between the workpiece and instrument terminal. In EDM for the most part a flash is produced between the bury terminal hole to eliminate the material from the workpiece. Due to the noncontact machining technique, it is broadly utilized for instrument and bite the dust-making processes, parts for aviation, the car industry, and careful parts [53,66]. Lauwers et al. [47] done a definite examination of the material erosion component of electrical conductive ceramics and tracked down that as well as dissolving/vanishing and spalling other material erosion instruments, for example, oxidation and disintegration of base metal may likewise occur. Various scientists, for example, Azam et al. [46], Shrivastva and Dubey [48] and Gupta and Jain [49] examined the ebb and flow improvements in EDM completed by specialists in view of the pattern and their advantage. Liu et al. talked about the cycle capacities of miniature EDM and by utilizing this strategy they made two and three-layered miniature parts like a miniature blower and little gas turbine impeller [50]. In their exploratory review, Liu and Huang performed micro-electric release machining of Si3N4 supported with various weight levels of TiN [67]. Schubert et al. in their survey introduced the flow situation and past examinations done on miniature electric release machining for ceramics. They enrolled the different benefits of miniature EDM as far as noncontact innovation, close

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resistances, low impact on material, and so forth[68]. Expansion in the release energy during miniature EDM prompts unfortunate surface qualities concerning breaks and micro-pores [69]. The fundamental point of utilizing dry EDM is by and large used to lessen the contamination brought about by various dielectric liquids which bring about the arrangement of fumes during machining [70]. Tzeng and Lee recognized the primary powder qualities which can influence EDM execution as-molecule size, molecule fixation, molecule thickness, electrical resistivity, and warm conductivity [71]. Yu et al. affirm from their trial examination that the dry EDM processing is a superior strategy when contrasted with oil EDM processing and oil bite the dust sinking-EDM concerning three-layered processing of established carbides [72]. In a new report, Dhakkar et al. looked at the changes close to dielectric mediums utilized in electric release machining. Glycerin air blend is viewed as reasonable as far as high material erosion rate and furthermore instrument wear rate is diminished when contrasted with customary EDM [73]. The test examination done by Jahan et al. principally centered around the enhancement of surface property of solidified WC by utilizing dielectric blended in with graphite nanopowder in sinking and processing miniature EDM [74]. Graphite nanopowder assists with working on a superficial level get done with a uniform dispersion of sparkles in both sinking and processing miniature EDM. Albeit higher grouping of powder builds the cathode wear rate. In a new examination, Kolli and Kumar streamlined the centralization of surfactant and graphite powder in dielectric liquid to further develop material erosion rate, lessen surface unpleasantness, and less recast layer [75]. Mai et al. [53] stated the advantages of Carbon Nano Tubes (CNT) blended in dielectric liquid throughout machining time, cathode wear rate, and surface unpleasantness. A comparative examination by Sari et al. concluded that important machining exhibitions utilizing CNT blended dielectric in EDM in various machining boundaries. Dielectric blended in with CNT makes the EDM more effective while the machining boundaries are set at low heartbeat energy. Because of enormous hotness assimilation, the thickness of the reworked layer is diminished [54]. Liew et al. found the adhering of carbon nanofibres to the workpiece surface which then again upgrades the electric release machinability [76]. Bonny et al. [77] recommend that the auxiliary electro-conductive stage in ZrO2 artistic composites significantly affects the frictional way of behaving, mechanical properties, and electrical release machinability, and the material erosion component for ZrO2-WC is simply dissolving and vanishing [78]. Chiang [79] from his trial concentrate on suggested that release current and obligation factor were the critical variables that impact Material Removal Rate (MRR) where concerning cathode wear rate and surface harshness the powerful boundaries were released current and heartbeat on schedule. Though Lin et al. [80] proposed machining extremity and pinnacle current were the huge boundaries to further develop MRR and anode wear rate and for surface unpleasantness, the top current is the most persuasive boundary. Patel et al. [81] distinguished the principle machining boundaries engaged with EDM as, beat on schedule, obligation cycle, release current, and hole voltage. Their

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outcomes show that the beat on schedule and obligation cycle is the primary boundary for working on a superficial level of unpleasantness. In another work, Zhang [82] observed that the expansion in happy level of TiN and number of force semiconductors prompts higher material erosion rate and high surface harshness values. In a new investigation by Melk et al. [83], it is observed that high-temperature age it were answerable for the harmed and scattered CNT’s to during electrical releases’.

11.4

Hybrid machining

Atefi et al. affirmed the improvement of machining execution as far as surface unpleasantness of AISI D2 steel within the sight of dielectric in EDM [84]. In the exploratory investigation of Bai et al. [85] a new machining strategy for powder blended close to dry-EDM is utilized. Gaseous tension and apparatus rotational speed were the most unhuge boundaries which impact MRR. For machining of ceramics by EDM, they should have the least electrical conductivity and homogenous construction regardless of their hardness and strength [86]. Mohari et al. [87] utilized helped anode machining techniques for protecting ceramics with the assistance of sinking-EDM and wire-EDM. Comparatively, Muttamara et al. [88] attempted to track down the chance of micro-machining of Si3N4 by helping cathode machining in EDM. Assisting Electrode Method (AEM) is a most ideal technique for machining three-layered complex shapes by WEDM [89]. Deng and Lee utilized ultrasonic and rough impact surface completing techniques to work on a superficial level uprightness of artistic composites and observed that both strategies were compelling to work on a superficial level nature of WEDM surface [90]. AWJM is a variation of in which abrasives are blended in with high-speed water fly for cutting activity [1 3]. AWJM is multiple times more remarkable than water fly [3]. AWJM is administered by fundamental five boundaries which are waterdriven boundaries, grating boundaries, blending chamber, navigate boundaries, and material properties [91]. Hocheng and Chang [92] did the material erosion examination of clay plates by AWJM cutting and reasoned that the measure of material erosion is fundamentally represented by two principle factors, for example, water-powered strain and grating stream rate. Cross speed additionally influences the kerf width and tightens proportion. Srinivasu et al. concentrated because of kinematic boundaries, for example, stream impingent point and fly feed rate on kerf math of silicon carbide ceramic production in AWJM [93]. Maniadaki et al. proposed a multimolecule reproduction strategy to assess the impact of effect point and stream speed in pit circularity [65]. Surface harshness of ceramic materials if there should be an occurrence of AWJM relies on functional boundaries, for example, water pressure, rough mass stream rate, spout stalemate distance, and navigate speed [66]. In separate works, Azarhoushang and Tawakoli, and Li et al. known as ultrasonic assisted grinding

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(UAG) for clay composites which bring about diminished grinding power and improvement in surface quality. UAG is a viable strategy to machine hard and weak material with working on cutting proficiency by ultrasonic vibrations [67 93].

11.5

Comparative studies

Hanaoka et al. [94] thought about the two unique cycles that are EDM involving AEM and typical EDM for MRR, surface harshness, and terminal wear proportion of Si3N4, Si3N4-CNT, and Si3N4-graphene nanoplatelet. By looking at the aftereffects of the two cycles it is demonstrated that AEM is a decent strategy for getting better machining execution in EDM. Khodke et al. suggested a scientific model to track down material erosion in AJM for fragile materials. They recommended that sway speed, material properties, calculation, and rough particles properties influence the MRR fundamentally [95]. Wakuda et al. [96] observed that the smooth bend is acquired in the event of AJM when contrasted with LBM and the strength of laser machined surface additionally crumbles. Different specialists have performed rotational USM and enroll the benefits as far as its better material erosion rate, diminished cutting powers, and diminished apparatus wear rate when contrasted with the ordinary machining process.

11.6

Conclusion

The significant difficulties that are experienced by various scientists while manufacturing processes are studied and mentioned in this chapter. The compound techniques utilized by different scientists for the creation of fired and its composites are utilized to deliver close to net shape with low handling time and temperature yet then again the composite framed by response holding strategy is viewed as more permeable which prompts decay of mechanical properties. Uniform circulation of support is one more significant difficulty experienced by different specialists. Traditional sintering of recalcitrant materials at high temperatures with a long splashing period prompts strange grain development, poor mechanical properties, and disintegration of the optional stage. Hot squeezing and flash plasma sintering strategies are restricted to just the development of straightforward mathematical shapes with restricted size. Further upgrades in the handling and machining procedures are expected to foster a ceramic composite that can improve its applications.

References [1] P.C. Pandey, H.S. Shan, Modern Machining Processes, Tata McGraw-Hill Education, 1980. [2] P.K. Mishra, Nonconventional Machining, Narosa publishing house, New Delhi, 1997.

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Phan Duc Tri1,2, Thuy Dung Nguyen Pham1,2, Sumin Park1,2, Jaeyeop Choi3, Sudip Mondal2 and Junghwan Oh1,2,4,5 1 Industry 4.0 Convergence Bionics Engineering, Pukyong National University, Busan, South Korea, 2BK21 FOUR ‘New-senior’ Oriented Smart Health Care Education, Pukyong National University, Busan, South Korea, 3Smart Gym-based Translational Research Center for Active Senior’s Healthcare, Pukyong National University, Busan, South Korea, 4 Biomedical Engineering, Pukyong National University, Busan, South Korea, 5Ohlabs Corporation, Busan, South Korea

12.1

Introduction

Flexible electronics have attracted much attention for several decades [1,2]. The fabrication of flexible electronics is based on the integration of electronic components, and circuits on the flexible plastic substrate [3]. The concept of flexible technology was introduced in the 1960s when the first flexible solar cell arrays were designed with thin layers of 100 nm of crystal silicon wafer cells on a plastic substrate [4]. In the following decades, the success in the application of conductive polymers [5], organic semiconductors, and amorphous silicon [6] inflexible technology opened tremendous opportunities for the development of flexible electronic devices. With several advantages of flexibility and stability, flexible electronics have been widely developed in varieties of applications including artificial e-skin [7,8], flexible touch sensors [9], energy harvesting [10], robotics [11,12], medical devices [13,14], and so forth. To improve the mechanical deformability and functionality of flexible electronics, new materials and fabrication technologies have been developed, which is important to break the limitations of conventional methodologies [15,16]. Flexible electronics can be integrated with various living organisms existing in nature to overcome their challenges and open new functionalities [17,18]. With excellent mechanical stability, the flexible device can be bent, and wrapped in curvilinear surfaces, which is a promising technique for the development of wearable and skinattachable sensors [19,20]. Innovative flexible electronic devices have been applied for some applications such as energy generation [21], signal sensing [22], or biosignal monitoring [23,24]. Furthermore, flexible electronics show various advantages in skin treatments and phototherapy applications. The incorporation of stability, flexibility, and high power light density in flexible light-emitting diode

Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00012-9 © 2023 Elsevier Ltd. All rights reserved.

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(LED) devices would provide powerful functionalities for human attachment and different important application [25,26]. This book chapter will introduce an overview and in-depth analysis of current advances, and application of flexible electronics in sensing technology and phototherapy application. Also, this chapter provides key strategies of microfabrication and properties of ceramic materials for the development of physical, chemical and biological sensors with their practical application. This book chapter finally shows a perspective on the fabrication of flexible electronics and provides insight into the development of flexible electronic devices in biomedical engineering.

12.2

Flexible electronics

12.2.1 Fabrication strategies and materials The components of flexible electronic devices such as semiconductors and encapsulated substrates are deformed in an arbitrary manner by stretching, bending, compression and twisting. Thanks to a wide range of unique properties, ceramics are considered a potential material for producing advanced electronic systems. One of the fabrication strategies for flexible electronic devices involves manufacturing the circuits on a carrier substrate either by using chemical etching or lithography-based patterning methods and subsequently loading them onto a flexible substrate. For example, fabrication of silicon nanomembrane was performed by (1) manufacturing on silicon-on-insulator wafers using reactive ion etching and (2) transferring to a prestrained polydimethylsiloxane substrate. The structure flexibility can further be improved in the configuration of noncoplanar mesh, noncoplanar serpentine, or coiled spring. However, this strategy faces the challenges of high production cost, limited surface areas, and mechanical sensitivity. The flexible capacitors with three dielectric layers have been printed with the support of ceramic and a composite ink made of poly(ethylene glycol) diacrylate (PEG-DA) and surface-modified Ba0.6Sr0.4TiO3 (BST) particles [27,28]. In the presence of the silver ink, the dielectric layers of the printed capacitors were of high homogeneity, thin, and increasing capacitance. Upon the application of capillary-assisted electrochemical delamination approach, the speed of delaminating ultrathin electronic foil from silicon wafers was accelerated up to 1.66 mm/s at a voltage of 20 V and NaCl concentration of 2 M [29]. Furthermore, the delaminated foil was immune from stress-induced damage, chemical impairment, and was applicable to construct a high-performance system of carbon nanotube (CNT)-based transistors on parylene-C film, as well as poly(methyl methacrylate), and styrene ethylene butylene styrene (SEBS), thereby showing robust potentials for electronic system design [30]. The performance of lead-free piezoelectric nanofibers was reported to be the best at 1500 rpm of collector rotation speed [31]. The microscale structure, orientation, and perovskite state of the system were evaluated by field emission scanning electron microscope (FE-SEM), fast Fourier transform (FFT), and X-ray diffraction

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(XRD) analysis, respectively. In addition, the high alignment and piezoelectric properties were confirmed by piezoresponse force microscopy (PFM) and polarization
electric field (P
E) loops. The flexible ceramic membrane developed from manganese dioxide (MnO2) nanowires and silicon nitride (Si3N4) particles exhibited high flexibility and interface stability, which could significantly improve and extend the application of ceramic materials [32]. Fig. 12.1 presents SEM images of flexible ceramic membranes with different magnification. The composites of ceramics and polymer, which are feasibly applied in pyroelectric and piezoelectric devices, transducers and capacitors have been developed by combining the electrical and mechanical properties of ceramics and polymers, respectively [33]. Such multiphase materials further display Maxwell
Wagner relaxations and are affected by a wide variety of distinctive factors

12.2.2 Physical, chemical, and biosensors-based flexible ceramics 12.2.2.1 Physical sensor 12.2.2.1.1 Temperature sensor As temperature variation is considered an important indicator for human health, it requires to be accurately and precisely measure by a well-developed temperature sensor [34]. In this regard, a system of multiple ultra-thin ceramic strips (40 μm) has been developed by Makunda Aryal et al to act as thermal protection, shield for phosphor composites without altering their emission properties. This design has improved the limited application of ceramics as well as enabled temperature monitoring to at least 120 μm in depth. Another temperature sensor has recently been fabricated from silicoaluminum carbonitride (SiAlCN) ceramics, which is wellknown as a potential material for high-temperature sensors [35]. Results have

Figure 12.1 (A
C) Scanning electron microscopic images of a flexible ceramic membrane with different magnification. (D) Mapping images of flexible ceramic membrane [32].

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shown that the device exhibited an excellent capability in measuring the temperature of up to 830 C with high repeatability and accuracy [36]. The temperature sensor developed from Er31 doped lead lanthanum zirconate titanate ceramics was capable of measuring temperature (from 313 to 573 K) through fluorescence intensity ratios (FIRs), absorption coefficient, light transmittance and reflectivity [37]. Fig. 12.2 shows absorption spectra of 3/2/1 mol% Er31 doped PLZT transparent ceramics for temperature sensor. Within the tested range of temperature, the FIR (at 2H11/2 and 4S3/2 to ground states) and absorption coefficient of the sample excited by a 980-nm light exhibited a linear relationship with temperature. A similar linear relationship was recorded between the light transmittance/reflectivity and temperature of the sample excited by a 632.8-nm light. Therefore, the developed sensor showed great improvement in terms of reliability and sensitivity. Meanwhile, theα-SiAlON ceramic material doped with Er has been proposed as a promising material for temperature sensors [38]. α-SiAlON ceramic has been recognized for its significant stability [39]. Therefore, by doping with Er, the

Figure 12.2 (A) Absorption spectra of 3/2/1 mol% Er31:PLZT; (B) dielectric temperature spectrum of 3 mol% Er31:PLZT; (C) Raman spectrum of PLZT; (D) X-ray diffraction spectrum of 3 mol% Er31:PLZT ceramics [37].

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α-SiAlON sensor has displayed a stable structure with O atoms, Er31 ions, and the N:O ratio of 5:2, as shown by density functional theory and neutron diffraction calculations [40] [41]. As shown by the FIR method at two levels of excitation (i.e., 980 and 793 nm), the prepared sensor was able to measure the temperature that exceeded 1200 K [42].

12.2.2.1.2 Strain sensor Strain sensors are the devices which developed to measure the translocation of a moving object. For instance, the piezoresistive properties of indium tin oxide (ITO) strain sensors that were prepared by sputtering have been investigated by heat treatment using nitrogen and air at a temperature of 1200 C [43]. Results have shown that as compared to the air-treated ITO, the resistance stability and gauge factor of nitrogen-treated ITO strain sensors were higher and more stable, respectively. The reason for this was proposed as the increasing Sn41 content and photon scattering after nitrogen incorporation into ITO. A previous study by Li et al. reported a flexible strain sensor by employing surface acoustic waves and PMN-PT thick film [44]. The newly developed sensor displayed a significant improvement in sensitivity (up to 1243.49 Hz/με), measurement range ( 6 1000 με), flexibility and repeatability at a small size (6.58 mm 3 4.19 mm 3 63 μm) [44]. Therefore, it can potentially be used for small strain measurement and various related fields.

12.2.2.1.3 Pressure sensors Flexible pressure sensors are used to sense the pressure applied to the natural human skin [45
47]. Fig. 12.3 presents the application of pressure sensors for electronic skin application with a circuit schematic. The piezoelectric composite structure of lead zirconate titanate and polydimethylsiloxane was developed by using freeze-casting. The developed pressure sensor with a size of 14 mm2 displayed a high longitudinal piezoelectric coefficient (d33 ) and complex architectures at a low

Figure 12.3 (A) The 5 3 5 pressure-sensor array placed on the arm. (B) Circuit schematic of the matrix device. (C, E, G) Top view of the metal letters “C”, “A”, “S” positioned over the pressure sensor array. (D, F, H) Current mapping of pressure distributions [45].

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cost and simplified fabrication [28]. Thus, the freeze-casting method can be applied to manufacture various types of self-powered sensors. Electronic sensors could be integrated with power sources and communication components to design a robust system that could be useful for comfortable skin contact and signals measurement from the human body, thereby gathering information about long-term health as well as providing potentials for the development of new sensor devices.

12.2.2.2 Chemical and biological sensors The design of flexible and miniaturized bio- and chemical sensors aims to determine a wide range of biological parameters in various application aspects. Among the materials that are used as electrodes for flexible sensors design [e.g., polyethylene naphthalate (PEN), polyethylene terephthalate (PET), parylene, mylar, and Kapton], graphene exhibited several advantageous properties, including high conductivity, deformability, transparency, and sensitivity. Furthermore, its surface can be functionalized with biomolecules for biosensing applications.

12.2.2.2.1 pH sensors pH is a crucial biochemical factor that affects reactions in the human body and indicates chronic infections. Conventional pH sensors with glass-based electrodes have been shown to be unsuitable for in vivo and wearable uses [48]. Therefore, in a study by Sardarinejad et al. (2015) [49], a pH sensor system has been developed from radio frequency sputtered thin-film ruthenium (IV) oxide (RuO2) sensing and Ag/AgCl reference electrodes [50]. Results have shown that the sensors of the sensitivity corresponded proportionally with the solution temperature and a coefficient of temperature sensitivity of 0.38 mV/pH C [51]. Furthermore, the sensor showed a larger hysteresis width for the acid-first cycle than alkaline-first cycle. The sensor response time was within 3.5 s, which can be considered as fast at the tested pH [52]. Similarly, Ag/AgCl/KCl reference electrode was also used to develop a miniaturized solid-state pH sensor with a thick film of ruthenium (IV) oxide tantalum pentoxide (RuO2
Ta2O5) and RuO2 sensing electrode, using low temperature cofired ceramic technology [53]. The fabricated sensor also exhibited stability and sensitivity in a wide range of pH within a short amount of response time. Therefore, its combination with a wireless system has shown promising results in further applications in online solution monitoring. Recently, La2Ti2O7 and lanthanum (III) oxide (La2O3) membranes were fabricated onto the electrolyte
insulator
semiconductor (EIS) pH sensors by using the spin-coating method. It was clearly shown that the sensors made of La2Ti2O7 membrane displayed a higher sensitivity and lower hysteresis voltage and drift rate than those made of La2O3 membrane [54,55]. The schematic of low-temperature cofired ceramic (LTCC) is present in Fig. 12.4.

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Ag conducting layer

(A) LTCC substrate

Sensitive electrode Insulative resin

KCI-glass layer

Hydration port

Ag conducting layer

(B) LTCC laminate

Sensitive electrode

Insulative resin Reference electrode Hydration port

Figure 12.4 Schematic representation of LTXX-based pH sensor: (A) type 1 and (B) type 2 [55].

12.2.2.2.2 Glucose sensors For diabetic patients, glucose sensors play a central role in (1) monitoring the blood glucose level in a long term and (2) reducing the blood test frequencies as well as (3) minimizing the associated discomfort.

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The composite of silica-graphite ceramic (SiO2/C-graphite) and copper oxide (CuO) nanostructure was fabricated onto a pressed disk electrode to design a nonenzymatic glucose sensor. Results have shown that the SiO2/C/CuO sensor displayed high sensitivity (0.06 μmol/ L), chemical stability, repeatability, and no interference by blood components with a shortened response time. The sensor was subjected to microscopic examination by SEM and energy dispersive X-ray spectroscopy (EDS) mapping, which revealed a highly compact and homogenous structure. Thus, it can be used as a potential device for future nonenzymatic glucose sensors [56].

12.2.3 Advanced flexible electronic for wound healing Wound healing is a complex process with a series of cellular and molecular events to repair affected tissue structures [57]. Chronic nonhealing wounds are a major problem for the aging population and cause significant health and economic burdens for patients and the healthcare system. The global wound care market shows significant development and is now expected to earn US$24.55 billion in 2027 [58]. Thus, it is necessary to develop novel therapeutic methods to accelerate skin wounds and optimize the efficiency of wound treatment. The common methods for wound healing management vary from covering the wound using antimicrobial dressing to using topical [59], and systemic retinoids [60] to prevent bacterial and fungi growth. With the development of advanced flexible electronics, flexible medical devices are attractive as a new technology to improve the efficiency of medical treatment and reduce healthcare costs [61]. With advanced noninvasive, flexible, and safe, medical devices based on LED technology have been studied in a variety of skin treatments and inflammation relief applications [62]. In addition, regular and frequent diagnosis and responses in dermatology treatment are necessary to produce good clinical results. Thus, some flexible organic light-emitting diode (OLED) phototherapy devices have been developed to accelerate the wound healing process with an enhanced antiinflammatory response. Young-min Jeon et al. presented a novel flexible photobiomodulation (PBM) patch using a parallel-stacked OLED (PAOLED) with different peak wavelength regions (600
700 nm) and high power densities levels (100 mW/cm22) [63,64]. The design of wearable PBM based on deformable OLED is shown in Fig. 12.5. The flexible PBM patch was designed to be lightweight (1 g) and thin (1 mm) for skin attachment with different body patches and skin types. The PBM patch can operate for more than 300 h making it capable of long-time operation. The mechanical and thermal test indicates the flexible properties and high thermal stability of the patches. These findings indicated that OLEDs have positive biologic effects on fibroblast proliferation (over B58% of control) and fibroblast migration (over B46% of control) [64]. Furthermore, most photo medical devices have been developed with rigid LED or rigid QLED light sources. The advantages of conformable OLEDs and QLEDs have been investigated in many photobiomodulation studies for dermatological applications. Yongmin Jeon et al presented free-form OLEDSs with hyper-connectivity and

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Figure 12.5 Design of wearable photobiomodulation (PBM) patch: (A) Schematic illustration of the red PAOLED-based wearable PDT system. (B) Schematic illustration of the white PAOLED-based wearable display system. (C) Realization of high power by changing the organic thickness of the ITO reference OLED. (D) Realization of high-power using the optical resonance effect of a microcavity reference OLED. (E) Realization of high power through the PAOLED structural design that electrically stacks an ITO reference OLED and a microcavity reference OLED. (F) Realization of high power through a PAOLED structural design that electrically stacks N OLEDs [63].

sandwich structure for wound healing photomedicine [65]. For wearable applications, the optoelectronic device was designed with a sandwich structure (10 μm) and ultrathin transferable barrier (4.8 μm). The research found that the sandwich-structure transferable OLED (STOLED) shows similar properties as cylindrical-shaped materials. The mechanical performances indicated the high reliability of the transferable OLEDs in both folding and washing reliability tests, which can be explained by using a sandwich structure to provide a free form on shapes and a neutral axis. Furthermore, the photomedicine device exhibited a long operating life ( . 150 h), with low heat generation, and light uniformity. It highlighted the ability of the OLEDs for wearable use with a real photomedicine application. The photo medical tests were investigated using normal human fibroblasts to evaluate the biological

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effects of STOLED. The irradiation of red STOLED light increased the proliferation and migration of human keratinocytes by nearly 26% and 32%, respectively. In an artificial skin model test, the thickness of the epidermis shows a remarkable recovery with more than 39% compared to nontreatment groups. In addition, in an organ culture test on the rat model, due to the positive effects, the irradiated groups showed a 21% reepithelialization in comparison with nonirradiated groups. The STOLED is expected to promote keratinocyte proliferation, migration, and reepithelialization. Which are important factors for accelerating the wound healing process. Han Eol Lee et al. introduce flexible thin-film vertical light-emitting diodes (f-VLEDs) for alopecia treatment. With advances in monolithic fabrication, the maximum power LED power density of the f-VLEDs can be achieved to approximately 30 mW/mm22 with a low forward voltage of approximately 2.8 V [66]. The

Figure 12.6 (A) Schematic illustration of the fabrication procedure and trichogenic photostimulation by monolithic flexible AlGaInP vertical LEDs. (B) Optical image of monolithic f-VLED with the red word “KAIST” under bending state. The upper inset is a cross-sectional SEM image of monolithic red f-VLED. The lower inset image exhibits a magnified microscopic image of 30 3 30 f-VLEDs with a 50 3 50-μm2 chip. (C) Photograph of red f-VLED array affixed on the surface of the human wrist. The inset is a magnified picture of the 30 3 30 LED array [66].

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fabrication process of trichogenic photostimulation with monolithic flexible vertical AlGalnP is shown in Fig. 12.6. The monolithic f-VLEDs exhibited a deeper optical penetrability (2 mm under the dermal surface) and lower energy consumption (50 mW for irradiance of 1 mW/mm22) in comparison with the conventional LED therapy devices. The flexible f-VLEDs were made with 3 μm thick active layers using a monolithic vertical interconnection of top n- and bottom p electrodes. For human attachment applicability, the mechanical and stable properties off-VLEDs were confirmed with harsh periodic bending/unbending motions. The variation of Vf during bending/unbending motions was examined to test the mechanical stability of the proposed optical device. With advances in the stress-free region and excellent mechanical properties, the flexible f-vVLEDs can be a potential candidate for wearable patch photo stimulators. In thermal stability tests, the monolithic f-VLED showed a stable skin temperature (,40 C) with LED power density of 1
5 mW/mm2, which is an acceptable temperature without dermal damage or low-temperature burns. Furthermore, to confirm the technogenic effects of the flexible f-VLED in Alopecia treatment, the hair growth experiments were conducted in 12
14 weeks old females for 20 consecutive days. The groups of mice with f-VLEDs treatment showed a higher hairgrowth rate and wider hair regrowth area (59.76% in 1 3 1 cm2) compared to the control groups. The immune fluorescent and histological tests were carried out to verify the hair growth application of proposed f-LEDs. With excellent mechanical, electrical, and thermal properties, the red f-VLED was confirmed as a potential phototherapy device for not only hair-loss problems but also for other wound healing and skin treatments.

12.3

Summary and conclusions

This chapter highlights the advances and opportunities offered by flexible electronic technology. These directions emerge from the advanced properties, microfabrication to tremendous potentials application in healthcare, and other biomedical applications. The novel microfabrication combined with the unique characteristics of ceramic materials provides wide opportunities for the development of flexible electronic devices, physical and chemical sensors with soft and irregular surface attachment. The combinations of sensors with flexible electronic devices and communication are essential for prolonging the monitoring and analysis of biological signals. Furthermore, another crucial element in developing flexible electronic devices for human application is to ensure long-term use and stable human body attachments that do not cause discomfort and skin irritation. In the future, more advanced flexible sensors will be developed to implant directly on an internal organ, or inner body part for monitoring biosignals and detecting biomarkers. Another development of flexible electronic device is the application for LED therapy devices. Flexible LED therapy devices can also be wearable for wound healing and skin treatment over the long term without the patient’s inconvenience.

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These flexible LED therapy devices are important for improving the quality of public health and contribute to the development of a smart healthcare system in the future. In conclusion, flexible electronics are emerging as a great technology for a wide range of biomedical applications, advanced therapies, and diagnostic. However, to achieve high mechanical, and electrical properties, advances in fabrication methods, substrate materials, and manufacturing technologies are considered as key elements for the development of flexible electronics.

Acknowledgments This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. 2022R1A5A8023404).

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Sambedan Jena1 and Manila Mallik2 1 School of Nano Science and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India, 2Department of Metallurgical and Materials Engineering, Veer Surendra Sai University of Technology, Burla, Odisha, India

13.1

Introduction

Developing and designing novel flexible electrochemical energy storage devices has been at the forefront of numerous research labs all over the world. This field is becoming a major thrust area as it complements other technological developments like portable electronics, wearable electronics, etc. In this area, flexible supercapacitors have a huge advantage compared to their counterparts in terms of power density, cyclability, and structural integrity allowing them to be compatible with devices of various sizes and shapes suited for consumer applications. A considerable amount of research effort has been put into developing various ceramic-based materials for use as electrode materials in supercapacitor technology. Their eligibility lies in their unique set of characteristics such as good capacitive performance, cheaper cost, and environmental friendliness. Their ability to store large amounts of charge and the mechanism of storage/release fall in the pseudocapacitive domain. Thus this chapter aims to provide an overview of the recent advancements in the field of ceramic-based flexible supercapacitors. Additional emphasis is given to the structure property correlations viz., the relationship of particle morphology of the electrode-active ceramic materials with the corresponding electrochemical performance as well as the effect of various electrode components on the mechanical flexibility and stability of the supercapacitor devices. Safety, along with a certain degree of flexibility, is the two key factors that need to be considered. The properties like the speed of reversible energy storage, good power density values, and quick charge/discharge rates need to go hand-in-hand with mechanical properties like strength, flexibility as well as environmental friendliness [1,2]. The other important properties to look out for are low maintenance costs, longer life, and cyclability [3,4]. Depending on the mechanism of charge storage, supercapacitor devices can be classified into three broad categories viz., electrical double-layer capacitors, pseudocapacitors, and hybrid capacitors. Electric double-layer capacitors work by physical, reversible adsorption of ions in the electrode/electrolyte interfaces that allow very fast charge-transfer kinetics and ultra-long cycle life [5]. Different allotropes Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00013-0 © 2023 Elsevier Ltd. All rights reserved.

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of carbon such as carbon nanotubes, graphene, and its derivatives, nanostructured hard carbons, and carbonaceous aerogels generally show this kind of behavior [6]. On the other hand, pseudocapacitors tend to store charge through chemical reactions. However, these chemical reactions also occur at very high rates allowing a quick charge storage mechanism. As a result, they offer significantly higher capacitive values compared to the electric double-layer variants [7]. Most of the transition metal oxides (ceramics) and other conducting polymeric compounds show this kind of pseudocapacitive behavior [8]. Other novel functional materials such as hydroxides, carbides, metal-organic frameworks, and MXenes also show supercapacitive properties [9 12]. Researchers went one step further and decided to develop a new class of supercapacitors having both the properties of electric double-layer capacitors and pseudocapacitors. This ultimately led to the discovery of hybrid capacitors which pushed the boundaries even further. The role of ceramic compounds in the manufacturing of these hybrid capacitors in a flexible form factor is what this chapter focuses on. The ultimate target is to develop flexible supercapacitor devices which can be used in flexible/wearable devices that require a source of electrical energy to work. The whole idea behind this area of research is to complement the worldwide approach for miniaturizing daily-use objects. Recent progress has revealed that the limiting factor in designing miniaturized electronic devices lies in the energy-storage component design. In response, researchers started venturing into developing micro-supercapacitors with unique electrode configurations. These configurations allow the inclusion of these microcapacitors into flexible electronics like chips and integrated circuits. Even though their overall capacitive values remain low, these micro-capacitors can be used in specific applications that require more power density and cycling reversibility instead of high charge storing capacities as these small devices hardly require large volumes of charge to work [13]. So future electronics development requires a constant R&D in designing simple, cheap, adaptable, and environmental-friendly production technologies for supercapacitors with novel form factors. Novel manufacturing technologies like three-dimensional (3D) printing allow the manufacturing of these supercapacitor components (current collectors, capacitors casings, etc.) [14]. As far as the electrode-active materials are concerned, transition metal oxide ceramics allow easy integration into these manufacturing protocols allowing the fabrication of miniaturized and flexible supercapacitor components. Ceramic compounds show promise as supercapacitor electrodes. However, several drawbacks such as low specific surface area and surface activity properties retard their electrochemical performance. In general, such supercapacitive electrode-active materials’ performances are primarily measured by comparing their specific capacitance values and operating potentials which, in turn, determine their energy and power densities. Therefore understanding the complete electrochemical characteristics of any supercapacitor material is primarily determined by a detailed investigation of the molecular structure of the material, the use of a compatible electrolyte, and its working potential range. Achieving a thorough understanding of the ceramic compounds, addressing each of these drawbacks through experimental optimization, and introducing structural engineering steps to enable adequate electrical

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conductivity and electron transport abilities are vital. Hence, this chapter mostly discusses the recent trends and advancements made in ceramic-based supercapacitor electrodes. Topics ranging from the structural design of the recently reported supercapacitors and the corresponding electrochemical performance of the constituent ceramic-based electrodes are explored in depth. In the end, prospects are also discussed with emphasis on how to push the supercapacitor limits further.

13.2

Supercapacitor overview: types and components

Supercapacitors are known to deliver high power densities with ultra-long cyclability either through nonfaradaic (electric double-layer capacitors) and/or faradaic (pseudocapacitors) processes (Fig. 13.1). All the recently reported supercapacitor technologies use novel materials comprising ceramic nanostructures coupled with nanocarbons, carbonaceous materials, and other thin-film materials.

13.2.1 Electrical double-layer capacitors versus pseudocapacitors Electric double-layer capacitors use nonfaradaic processes to store energy in the Helmholtz double-layer. This double layer forms due to the accumulation of electrolyte ions at the electrolyte/electrode interface. Hence, very high power densities can be derived from these classes of supercapacitors as the charge storage and release kinetics is quite fast. On the other hand, the charge storage capacity is limited based on the double-layer thickness which is indirectly associated with the effective surface area of the electrodes. Therefore electrical double-layer capacitors provide low energy densities and can store energy for short-term durations. A general approach to improve the energy storing capacity of electrical double-layer capacitors is to increase the electrolyte/electrode interface area by tuning the surface microstructure of the electrodes. Hence, researchers have explored various nanocarbon materials like 2D carbon sheets (graphene), nanotubes, fibers, and carbonaceous materials for the above purpose as they offer high mechanical strength,

Figure 13.1 Different types of supercapacitors.

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good electronic conductivity, and allow control over tuning the effective surface area [15,16]. Honing on the surface-area tunability property, introducing porous areas in the carbonaceous particles has been the best possible approach to increase the energy density of the electrodes. Several porous polymorphs such as foams, sponges, and aerogels of graphene have been reported [17]. In addition to introducing porosity, creating nanopores on the surface of the carbon lattice improves the capacitance values even further. For example, introducing micro, meso, and macropores into bamboo-like carbon strands forming a hierarchical structure delivers an optimum combination of flexibility and capacitance values [18]. However, the only drawback of this approach is that introducing porosity in carbonaceous materials decreases the volumetric energy density of the electrodes due to the low density of carbon. This mandates that a large amount of material needs to be put in a limited space to achieve the target capacitance value. The use of gel electrolytes also assists in developing flexible solid-state supercapacitors with an acceptable value of capacitance. Mixing polyvinyl alcohol with ionic materials such as phosphoric acid, potassium hydroxide, and other ionic liquids results in a gel electrolyte formulation that can be used in solid-state electric double-layer supercapacitors. The use of polyvinyl alcohol imparts strength, stretchability, and allows control over the thickness to the supercapacitor device. Further advantages include the prevention of electrolyte leakage which allows any wearable capacitor devices to be safely cut and deformed [19]. Researchers also tried filling the pores of the electrode with gel electrolyte using a bottom-up infill approach. They showed that a carbon nanotube electrode infilled with a polyvinyl alcohol gel electrolyte survived extension roll tests indicating exemplary mechanical integrity and flexibility. The electrode was able to retain 95% of its capacitance (2662 mF/ cm2 over 5000 bending cycles). Compared to electric double-layer capacitors, pseudocapacitors provide higher energy density values by applying reversible faradaic reactions in addition to the nonfaradaic adsorption of ions. Here, the choice of electrode material is vital as it should tend to partake in reversible redox reactions which are observed in certain conducting polymeric compounds and transition metal compounds. Several conducting polymers such as polypyrrole, polyaniline (PANI), and poly-3,4-ethylene dioxythiophene are most commonly used in conjunction with transition metal oxides to yield pseudocapacitor electrodes. This is vital as using only metal oxide electrodes is a poor choice as they have insufficient flexibility and low conductivity. At the same time, using only conducting polymers is also not feasible as they offer poor capacitance values, have low density, and offer reduced cyclability due to their tendency to dissolve in organic electrolytes. Hence, combining the two initiates a trade-off between the total capacitance offered by the composite electrode and its flexibility. Furthermore, it is also quite challenging to increase the metal oxide content (decreasing polymer content) in the electrodes while maintaining the mechanical integrity and flexibility of the hybrid electrodes. Researchers recently have also reported certain novel protocols for loading metal oxides onto 3D metallic current collectors without the use of carbonaceous materials and/or conducting polymeric compounds. These current collectors impart flexibility

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as well as mechanical strength to the electrodes. For instance, nickel-iron oxyfluoride free-standing porous films were prepared via an electrochemical route. Intermetallic nickel-iron alloy phase filaments grew over the porous nickel-iron oxyfluoride films which provided flexibility. As a result, a free-standing electrode was obtained without the need for any binders or additives. The porous structure of the films provided additional surface area which delivered a capacitance of 670 F/cm3 [20].

13.2.2 Use of ceramics as supercapacitor electrodes Ceramic compounds are a versatile class of advanced materials that see profound use in several technological areas. Common properties like high mechanical and compressive strength, good thermal and chemical stabilities can be observed in most ceramics. Several unique ones like electrical, magnetic, and optical properties render them useful in various fields such as energy conversion and storage (battery, supercapacitor, solar cell, and fuel cell) [21 23]. Other areas such as catalysis (electrocatalysis, photocatalysis, and heterogeneous catalysis) also find tremendous applications of ceramic compounds [24 26]. The use of ceramics in energy applications has drawn significant attention over the years as nonrenewable energy resources are declining globally leading to the energy crisis. Hence, researchers worldwide have devoted their efforts toward the development of advanced functional materials to achieve maximum energy efficiency and reduce greenhouse emissions leading to sustainability. Few additional properties such as corrosion, radiation, temperature, and thermal shock resistances found in ceramic compounds increase their attractiveness as electrodes for supercapacitors that can be used in extreme environments. Several researchers have reported that modifying the size, shape, and morphology of the ceramic particles modify their specific surface area values thereby amplifying their capacitive nature while simultaneously improving their ion diffusion and electron transport properties.

13.2.3 Current collectors/substrates for preparing supercapacitor electrodes Current collectors/substrates play a critical role in designing flexible supercapacitors. Several parameters such as mechanical flexibility, thinness, wettability, cost, thermal resistance, and material transparency need to be evaluated while choosing a suitable candidate for use as current collectors/substrates. Polymeric substrates show promise as they have the best flexibility among all relevant classes of materials. However, issues like high-temperature stability come into the picture as heating/sintering steps generally accompany the electrode fabrication processes in supercapacitor manufacturing. Furthermore, adequate mechanical strength is also mandatory to allow the substrates to survive various roll-to-roll printing processes [27]. Metallic foils, carbon nanofibers, polyethene terephthalate, polydimethylsiloxane, etc., are some common current collectors/substrates used in flexible supercapacitors. Each of the substrates has its advantages and disadvantages. Metallic foils

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show high electrical conductivity and have adequate mechanical strength and hence are a popular choice for current collectors. Metallic foils of copper, aluminum, nickel, and gold are commonly used as supercapacitor substrates. Plastic substrates are generally chosen due to their low cost, lighter weight, and sustain significant deformation. However, compared to metallic foils, plastic substrates lack adequate electronic conductivity. When transparency is required, polyethene terephthalate films are most popularly used due to abundant availability, good moisture retardation, and tolerance as well as high optical transparency properties.

13.2.4 Electrolytes For use as electrolytes, several electrochemical properties need to be evaluated such as voltage window, rate capability tolerance, and cycling stability. Hence, electrolytes are a significant component of supercapacitors. Several electrolyte formulations can be distinguished in terms of their working temperature range, ionic conductivity, potential window, etc. Gel and solid-state electrolytes are more popularly used in flexible supercapacitor devices compared to their liquid counterparts. This is because gel/solid-state electrolytes allow easier fabrication and assembly of supercapacitor cells, use cheaper packaging, and reduce leaking issues. Gel electrolytes generally deliver high thermal and electrochemical stability along with adequate ionic conductivity and mechanical flexibility. Several gel-type electrolytes have been reported in the literature such as ionic liquid-based gel electrolytes, organic polymer gel electrolytes, and aqueous gel polymer electrolytes. Also, using solid-state electrolytes removes the need for separator materials thereby reducing the weight even further which is essential for micro-supercapacitor fabrication. Hence, it is vital to choose the correct gel or solid-state electrolyte to obtain highperforming flexible supercapacitors.

13.3

Recently developed ceramic electrodes for flexible supercapacitors

13.3.1 Metal oxide/conductive polymer composites Transitional metal oxides and conductive polymeric compounds show a synergistic effect when used in combination with supercapacitor electrodes. This allows the fabrication of flexible electrodes with adequate electrical conductivity with minimal sacrifice of pseudocapacitive properties. Hence, metal oxide/conductive polymer composites achieve improved conductivity and charge-storage kinetics in highperformance supercapacitors [28]. Fabrication and design both depend on the electrical conductivity, mechanical integrity, and flexibility of the conducting polymers used. However, low overall strength and stability reduce their practical applicability. Improving upon these drawbacks requires additives such as graphene, mesoporous carbon, and carbon nanotubes. These forms of carbon have supercapacitor

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properties which add to the overall capacitive content of the composites. Jose et al. proposed a ternary system comprising of electrodeposited PdO-polypyrrole-reduced graphene oxide composite. Here, the reduced graphene oxide sheets are draped over the surface of the polypyrrole functionalized PdO particles. Upon electrodepositing this nanocomposite over stainless steel substrate, a specific capacitance of 595 F/g is obtained at 1 A/g specific current in a 1 M sulfuric acid electrolyte. The supercapacitor cell retains 88% of its specific capacity after 5000 charge discharge cycles at a high specific current of 5 A/g [29]. Metal oxide electrodes are widely known to employ several charge storage mechanisms such as reversible coulombic proton/ metal ion adsorption, surface adsorption, and surface faradaic redox reactions with anions [30 32]. Such transition metal oxides can reversibly store large volumes of charge via quick surface reactions. Lang and coworkers prepared a manganese dioxide embedded vanadium oxide ceramic composite which was able to deliver a very high power density of 422 W/cm3. The molecular framework of the composite showed metallic properties which generated adequate electrical conductivity and allowed quicker charge transfer across the interface between the two phases. Hence, a high rate capability was also obtained owing to the pseudocapacitive properties [33]. Jose et al. used electrospinning to fabricate very thin cobalt oxide nanowires with a high degree of crystallinity. The as-prepared electrodes were tested as asymmetric supercapacitors using carbon black as the counter electrode. The resulting supercapacitor delivered specific energy of 47.6 Wh/kg and specific power of 1400 W/kg (Fig. 13.2) [34]. Similarly, nickel oxide nanowires were also prepared using the electrospinning technique. The nanowires also showed a high degree of crystallinity, good cyclability, and coulombic efficiency during electrochemical testing. The resulting supercapacitor devices delivered a specific capacitance of 670 F/g and a low resistance of 0.76 Ω [35]. Ata and coworkers used the electrophoretic deposition technique to prepare porous manganese dioxide films. They prevented agglomeration during the deposition process by using a catechol-type surfactant. The electrodes also delivered good electrochemical performance [36]. Li et al. demonstrated that micro-supercapacitors can be easily prepared using alumina plates by laser-assisted pattern etching process [37]. Post patterning, graphene ink was added which selectively coated the exposed surfaces due to wettability difference. The resulting device delivered an areal capacitance of 5.5 mF/cm2 with good retention.

13.3.2 Metal sulfides/conductive polymer composites For use as supercapacitor electrodes, metal sulfides also show promise owing to their large surface area and the presence of multioxidation states of their transition metal cores. These properties enable them to accumulate and store significant quantities of charge through a complex process comprising of electric double layer and pseudocapacitive charge storage kinetics [38,39]. Metal sulfides generally form thin nanosheets which provide a large surface area. Their transition metal cores offer multiple oxidation states which also allows them to store electrons by intercalating them in between the sheets. Thus the presence of rapid ion intercalation pathways

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Figure 13.2 The FESEM images of (A) as-prepared polymeric nanofibers and (B) calcined Co3O4 nanowires. (C, D) Bright-field image of a typical nanowire, (E) selected area electron diffraction pattern, and (F) high-resolution lattice image of a typical particle in the TEM sample. (G) Dependence of the discharge CS and the columbic efficiency as a function of charge discharge cycle numbers were performed at 5 A/g in 6 M KOH aqueous solution, (H) dependence of the discharge CS as a function of charge discharge cycle numbers at progressively varying current densities, (I) Nyquist plot for both AC//AC and Co3O4//AC (Continued)

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results in high capacitance and energy densities [40]. Some metal sulfides possess anisotropic crystal lattice in 2D geometry which contains edge planes that show high reactivity towards charge storage reactions [41]. But metal sulfides, like ceramic compounds, suffer from major drawbacks like poor electronic conductivity and they are susceptible to sheet re-stacking like graphene. Hence, the main challenges in this class of compounds lie in achieving high exfoliation efficiency during synthesis, preserving the thinness of the nanosheets, and maximizing reaction active-site concentration on the sheets. Zhao et al. designed a hierarchical MnCoS5 with pinecone-type morphology that have sufficient space to accommodate electrode expansion and contraction occurring during cycling [42]. The resulting high surface area and microporous structure of the particles allow facile redox reactions to occur [43]. The MnCoS5 electrodes deliver a specific capacitance of 992 F/g at a specific current of 10 A/g with 102% retention after 55,000 cycles. Similarly, Liu et al. produced carbon fibers coated with Cu9S8 particles and encased them in polypyrrole via the electrodeposition route. The resulting nanocomposite delivers a specific capacitance of 270 F/g at 10 mV/s scan rate. They also retain 80% of their initial specific capacitance after 3000 cycles due to their high electrical conductivity values and microporous structure [44].

13.3.3 Metalloid nitrides/carbides ceramics

L

Nitrides and carbides of Si and B also offer several advantages such as hightemperature tolerance, excellent thermal stability, lighter weight, and adequate oxidation/corrosion resistance. This renders them as suitable electrode-active materials for use under extreme environmental conditions [45,46]. Chang et al. fabricated boron carbide nanoparticles with a carbon coating on the surface to impart electronic conductivity. The high fracture toughness of the ceramic particles allowed them to sustain the volumetric fluctuation stresses that occur during cycling. To impart flexibility to the electrodes, few-layer graphene sheets were added to the particles via a mask-assisted vacuum filtration method. The nanocomposite delivers a specific capacitance of 7.33 mF/cm2 at a 1 mV/s scan rate [47]. The electrodes also showed good cycling stability and the ability to function between 25 C and 75 C. Chang and his coworkers also investigated the mechanical aspects of the prepared electrodes. 99.4% of the initial specific capacitance was maintained even after 1200 cycles of electrode deformation proving the concept behind adding the graphene nanosheets in the form of a composite. In separate work, Kaskel and codevices at open circuit potential. The insets show the expanded high-frequency regions, and (J) comparative Ragone plots of the symmetric and asymmetric supercapacitors. FESEM, Field emission scanning electron microscopy; TEM, transmission electron microscopy. Source: Reproduced with permission from B. Vidyadharan, R.A. Aziz, I.I. Misnon, G.M. Anil Kumar, J. Ismail, M.M. Yusoff, et al., High energy and power density asymmetric supercapacitors using electrospun cobalt oxide nanowire anode, J. Power Sources 270 (2014) 526 535.

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workers reported the synthesis of SiOC polymer-based ceramics as supercapacitor electrodes. In this work, polyphenylsilsesquioxane precursor was prepared via the sol gel route. Subsequently, pyrolysis and chlorination treatment were incorporated to reduce the precursor into SiOC-derived carbon composite with a microporous structure. The composite prepared at 1000 C calcination temperature delivers a specific capacitance of 86 F/g at a specific current of 30 A/g [48]. Gu et al. prepared silicon carbide-nickel hydroxide core shell nanoparticles and coated them onto flexible carbon fiber cloth [49]. First nanowires of silicon carbide were grown via chemical vapor deposition (CVD) and subsequently, a nickel hydroxide layer was deposited on the nanowires via electrodeposition. This nickel hydroxide layer was introduced to improve the charge-storage kinetics and to form a core shell structure. By tuning the thickness of the nickel hydroxide layer, a high specific capacitance of 1724 F/g was obtained at a 2 A/g specific current. Kim and coworkers prepared nanoneedles of silicon carbide-manganese dioxide doped with nitrogen. Sequential growth of manganese dioxide nanoneedles over silicon carbide particles was carried out which resulted in an increase in the surface area and shortened diffusion pathways. When tested as an asymmetric supercapacitor with an activated carbon counter electrode, the resulting device delivered a specific capacitance of 59.9 F/g at a scan rate of 2 mV/s [50]. In separate work, silicon carbide nanowires were grown on flexible carbon fiber cloth fabrics by CVD using a vapor liquid solid protocol. The materials delivered a specific capacitance of 23 mF/cm2 at a scan rate of 50 mV/s. The electrodes also survived elevated temperature (60 C) testing and retained 90% of their initial capacitance after 100,000 cycles (Fig. 13.3) [51]. Silicon nitride ceramics also show excellent property combinations such as low thermal coefficient of expansion, wear resistance, mechanical toughness, and corrosion resistance [52,53]. Silicon carbonitride nanowires were prepared using a highenergy ball milling technique followed by calcination. Ultra-long nanowires (23 37 nm) with a high aspect ratio were obtained by tuning the processing parameters. The as-prepared nanowires delivered a specific capacitance of 188 mF/cm2 at a current density of 2 mA/cm2 [54]. Similarly, Yao et al. deposited cobalt nanoparticles on amorphous particulates of silicon nitride which were prepared by a prolonged high-energy ball milling process. The resulting composite delivered a specific capacitance of 403 mAh/g [55]. Xiao et al. reported the fabrication of a ternary silicon-silicon nitride-carbon composite with egg-like morphology. The particles have a silicon core and a silicon nitride intermediate layer covered by a carbon shell. While the silicon core contributed to the high capacity of the sample, the silicon nitride intermediate layer provided mechanical buffering to the volumetric fluctuation stresses and facilitated ionic transfer. The carbon shell was added to the electrical conductivity of the samples [56,57]. Park and coworkers further modified the above composition by anchoring the silicon silicon nitride core shell nanoparticles over carbon nanofibers to obtain a flexible electrode. The electrode delivered a high capacity of 665 mAh/g at a specific current of 10 A/g [58]. MXenes also consist of metal carbide/nitride ceramic phases with a 2D microstructure [59]. These compounds have a general formula of Mn11XTx

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Figure 13.3 (A) A photograph of the carbon fabric with SiC NWs grown in the middle region, (B, C) low-magnification SEM images of carbon fabric before and after SiC NWs growth, (D) higher-magnification SEM image of SiC NWs grown on carbon fabric, (E, F) low-magnification TEM images of SiC nanowire before HNO3 and HF treating, (G, H) lowmagnification and lattice-resolution TEM images of SiC nanowire after HNO3 and HF treating, the inset in (H) is the corresponding SAED pattern, (I) EDS spectrum taken on the nanowire and the inset is the corresponding EDS mapping results, (J) CV curves of SiC NWs on CF with a scan rate of 0.2 V/s at different temperatures (0 C 60 C), (K) discharge curves of SiC NWs on CF with a current density of 5 mA/cm2, (L) the plot of specific capacitance vs inverse temperature with the current density of 5 mA/cm2, (M) Nyquist plot of the supercapacitor electrodes at different temperatures, (N) equivalent circuit of Nyquist spectrum, and (O) cycling performance of SiC NWs on CF at 20 C and 60 C with a current density of 40 mA/cm2 (one point per 1000 cycles). Source: Reproduced with permission from L. Gu, Y. Wang, Y. Fang, R. Lu, J. Sha, Performance characteristics of supercapacitor electrodes made of silicon carbide nanowires grown on carbon fabric, J. Power Sources 243 (2013) 648 653.

(M 5 transition metal, X 5 carbon/nitrogen and T 1 O, F, OH) and are prepared by layer-by-layer selectively etching of the carbide/nitride phase [60]. MXenes are also popularly used as supercapacitor electrodes due to their high electrical conductivity and layered structure. Gogotski and co-workers reported that 2D MXenes can sustain high rates of charge storage and release kinetics that are comparable to those of electric double-layer capacitors but with high areal and volumetric specific capacities. They proposed two ways in which MXene architecture allows improved

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ionic access to the redox-active sites. They demonstrated the above concept by preparing a titanium carbide film that delivered 210 F/g at a scan rate of 10 mV/s [61]. Similarly, Tian and coworkers combined titanium MXenes with cellulose nanofibrils to prepare a flexible, multifunctional nanocomposite with good mechanical integrity. The nanocomposite delivered a specific capacitance of 324 F/g at a scan rate of 2 mV/s [62]. Jiang et al. also reported a combination of ruthenium oxide particles with titanium MXenes to fabricate an asymmetric supercapacitor. The titanium MXenes were prepared by layer exfoliation method and subsequently loaded onto flexible carbon fiber fabric. In the counter electrode, ruthenium oxide particles were loaded onto flexible carbon fiber fabric to assemble the asymmetric cell which delivered a specific capacitance of 401 F/g at a scan rate of 10 mV/s [63].

13.3.4 Metal hydroxide ceramics For fabricating supercapacitors with good mechanical strength and high capacitance, metal hydroxide-based ceramics have also been used. Copper hydroxidebased sea-urchin nanostructures were prepared using 3D printing technique followed by electroless plating of copper to obtain a copper hydroxide-copper-ceramic lattice substrate electrode. A symmetric supercapacitor prepared using the above electrode delivered a high capacitance of 6.28 F/cm3 at a specific current of 5 mA/cm3 and retained 60% of the initial capacitance at a high specific current of 200 A/cm3 [64]. An asymmetric supercapacitor prepared using the above electrode showed further improvement in the energy and power densities due to the 3D design with a high surface area and ion transfer pathway. Nickel oxide film-based supercapacitor electrodes were prepared by Gonzalez and coworkers. Initially, nickel hydroxide nanoplatelets were prepared by the sol gel route, and subsequently, a mixed polymeric layer comprising of polyethyleneimine and polyacrylic acid was added to produce a core shell structure. The prepared nanoparticles were then deposited onto nickel foils via electrophoresis followed by sintering which introduced macropores onto the final electrode. The electrode delivered 400 F/g specific capacitance and showed almost 100% retention after 1000 cycles [65]. Similarly, Milne et al. reported the fabrication of iron oxyhydroxide ceramic particles using octyl gallate as a catalyst. The adsorption of octyl gallate molecules over the iron oxyhydroxide particles by ligand chelation allowed an efficient extraction process. The flexible electrodes prepared by combining the above particles with multiwalled carbon nanotubes delivered a specific capacitance of 2.4 F/cm2 at a scan rate of 2 mV/s [66].

13.3.5 Spinel oxide ceramics Numerous advantages are offered by nickel oxide-based spinel compounds such as low cost, environmental abundance, and friendliness. Also, the unique crystal structure of spinel provides improved electrical conductivity than conventional oxides. Furthermore, very high electrochemical activity has also been reported for nickel cobaltite spinel [67]. Additional tuning of surface reactivity and conductivity of

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these spinel classes of compounds is possible by doping other multivalent transition metal cores to introduce additional redox couples to enhance the supercapacitor performance [68]. Nickel cobaltite (NiCo2O4) shows superior supercapacitive properties compared to the constituting binary oxides such as cobalt oxide and nickel oxide. Nickel cobaltite particles of various morphologies have been successfully synthesized by researchers using several synthetic methods allowing the optimization of the surface area of the active material. Flexible electrodes comprising of nickel cobaltite can be prepared by chemical bath deposition. Lei and coworkers prepared nickel cobaltite coated carbon fibers for use as hybrid capacitor electrodes where the carbon fibers were derived from electrospinning lignin-polyacrylonitrile copolymers. The deposition of nickel cobaltite over these fibers was enhanced by providing mild hydrothermal treatment. The final architecture delivered a specific capacitance of 1757 F/g at a current density of 2 mA/cm2 with 138% retention after 5000 cycles. An asymmetric supercapacitor prepared by coupling the above composite electrode with a free-standing reduced graphene oxide electrode delivered a specific capacitance of 134.3 F/g at a specific current of 1 A/g [69]. In separate work, Guan et al. prepared hollow nickel cobaltite nanowall arrays directly on a carbon fiber fabric to obtain a free-standing flexible electrode. The synthesis process involved two steps where the self-assembly of 2D nanowalls of nickel cobaltite phase was promoted via ion exchange and etching steps. Then, a calcination treatment was conducted to impart porosity in the sample and increase the number of reaction sites while shortening the ionic diffusion path length. The final electrode delivered good supercapacitor performance with long cyclability and improved rate capability [70]. Li and coworkers reported a flexible electrode comprising of nickel cobalt nanoparticles grown over carbon cobalt composite nanofibers. The carbon cobalt nanofibers were prepared by electrospinning followed by annealing in an inert atmosphere. The nickel cobaltite particles were prepared by hydrothermal treatment in the presence of the fibers to facilitate a coating process. The resulting flexible electrode was used as-prepared without the need for current collectors. The final electrode delivered a specific capacitance of 1710 F/g at a scan rate of 5 mV/s. It was also able to retain 70% of its initial capacitance after 1000 cycles at a specific current of 1 A/g [71]. Researchers have also reported the fabrication of flexible solid-state supercapacitor hybrids. For instance, Hong et al. combined nickel cobaltite and nickel molybdate ceramic powders to prepare the electrode-active material. The use of ammonium fluoride allowed the fabrication of the final electrode that showed excellent tolerance to bending stresses. The electrode showed no capacitance decay even while bending it repeatedly at an angle of 150 degrees (Fig. 13.4) [72]. Wang and coworkers improved upon the above rationale by coating PANI fibers with a bimetallic oxide layer of NiCo2O4 and NiMoO4. The resulting electrode architecture comprised of NiCo2O4 nanowires and NiMoO4 nanoplates encasing PANI nanofibers. The final flexible cloth-based electrodes delivered an areal capacitance of 2.38 F/cm2 at a current density of 1 mA/cm2 with a 92.36% retention after 5000 cycles [73]. This proved that futuristic flexible supercapacitors must use binary metal oxide ceramics coated onto carbon-fiber fabrics. As a result, Sun et al. prepared carbon nanofiber fabrics from polyacrylonitrile-co-methyl hydrogen

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Figure 13.4 The SEM images for NiCo2O4 were prepared using (A) urea, (B) HMT, and (C) NH4F on Ni foam and prepared using (D) urea, (E) HMT, and (F) NH4F on carbon cloth. The SEM images for NiCo2O4@NiMoO4 were prepared using (G) urea, (H) HMT, and (I) NH4F on Ni foam and prepared using (J) urea, (K) HMT, and (L) NH4F on carbon cloth. The CV curves at 40 mV/s for NiCo2O4@NiMoO4/Ni foam were prepared using (M) urea, (N) HMT, and (O) NH4F under different bending angles; the CV curves at 40 mV/s for NiCo2O4@NiMoO4/Carbon cloth prepared using (P) urea, (Q) HMT, and (R) NH4F under different bending angles. (S) The first and last four GC/D curves and (T) the relation of CF retention and Coulombic efficiency to the cycle number for the SBS with the NiCo2O4@NiMoO4/Ni foam positive electrode prepared using NH4F. The inserted figure in (T) is the picture for the SBS. The current density is 60 mA/cm2. Source: Reproduced with permission from W.-L. Hong, L.-Y. Lin, Design of nickel cobalt oxide and nickel cobalt oxide@nickel molybdenum oxide battery-type materials for flexible solid-state battery supercapacitor hybrids, J. Power Sources 435 (2019).

itaconate copolymer via electrospinning protocol. Subsequently, these nanofibers were coated with nickel cobaltite nanosheets. The final electrode delivered a specific capacitance of 620.23 F/g at a specific current of 500 mA/g and good bending tolerance even at 180 degrees [74]. Waghmode and coworkers reported the effect of urea while preparing nickel cobaltite nanostructures. By changing the additive concentration, different forms of NiCo2O4 microstructures were obtained (nanoflowers, nanoflakes, etc.) via the chemical bath deposition route. The shape of the nanostructures determined the overall supercapacitive performance of the electrodes. The nanoflowers with nanorods-like mixed morphology particles delivered a specific capacitance of 702 F/g. When a solid-state symmetric supercapacitor device was fabricated using the above sample and a polyvinyl alcohol-lithium chlorate gel electrolyte, a specific capacitance of 132 F/g was obtained (Fig. 13.5) [75]. Spongy barium titanate-based polymer-ceramic composites have been reported to possess high specific capacitance values (B300 F/g) and low electrical resistance (B40 Ω). These spongy barium titanate films can be prepared by mixing polypropylene and polyvinylidene fluoride with metal salt precursors and applying

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Figure 13.5 SEM images (resolution of 5 and 1 μm) of U1 (A, B), U2 (C, D), U3 (E, F), U4 (G, H), and U5 (I, J). (K) CV curves of PGE-1 at the scan rates of 5, 10, 20, 40, 60, 80, and 100 mV/s; (L) CV curves of PGE-2 at the scan rates of 5, 10, 30, 50, 80, and 100 mV/s; GCD curves of PGE-1 (M), PGE-2 (N) at the constant current of 2, 4, 6, 8, and 10 mA; (O) Ragone plot of PGE-1 and PGE-2; (P) Nyquist plots of PGE-1 and PGE-2 before and after 1000 cycling (inset: magnified view of high-frequency region). Source: Reproduced with permission from R.B. Waghmode, N.C. Maile, D.S. Lee, A.P. Torane, Chemical bath synthesis of NiCO2O4 nanoflowers with nanorods like thin film for flexible supercapacitor application-effect of urea concentration on structural conversion, Electrochim. Acta. 350 (2020).

solvent-precipitation technique followed by annealing. During the synthesis step, lithium chloride is added to induce sponginess in the resulting material [76]. Gu and coworkers used the above concept to prepare a barium titante-based polymer/ ceramic composite using a combination of thermal curing and roll-coating steps. In the process, micron-sized barium titanate grains were obtained [77]. Converting these particles into nanowire-type morphology improved the energy density further. Other particle morphologies like cubic, sea-urchin, and snowflakes were also obtained by changing the hydrothermal treatment parameters. Additionally, surface

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modification of the barium titanate nanowires was also carried out by introducing amine functional groups which delivered an energy density of 10.48 J/cm3 at an applied electric field of 300 MV/m [78]. Chung et al. used advanced spark plasma sintering processes to fabricate silicon dioxide-coated barium titanate grains with a core shell type arrangement. The use of spark plasma sintering allowed faster heating and cooling rates which caused proper densification. It even allowed the outer silicon dioxide coating to glassify and exist in an amorphous state. This glassy silica coating acts as an oxidation-prevention barrier as well as a dielectric barrier. The prepared electrodes delivered good supercapacitor behavior with low dielectric losses. The electrodes also showed strong tolerance to high temperatures and high accumulation of dielectric properties [79].

13.4

Conclusions and future prospects

Even though progress in the domain of flexible supercapacitor technology has been significant so far, still several challenges remain that need to be dealt with. The solutions to these challenges will surely yield futuristic supercapacitor devices/configurations with superior electrochemical performance and low cost. For example, using the concept of 3D core shell in fabricating ceramic nanostructures with different morphologies still remains an elusive feat. Furthermore, having simultaneous control over the properties while tuning the particle morphology adds to its difficulty. Several reports exist in the literature on the direct preparation of composite-type electrodes for fabricating flexible electrodes. They propose the use of a free-standing composite film without any additives/binders. The applicability of this proposed solution depends on several factors such as conductivity of the material, mechanical rigidity, and tolerance to bending/twisting. The best-case scenario involves the use of graphene sheets as one of the components that somehow solves the mechanical drawbacks. However, repeated expansion contraction of the electrode materials during reversible charge storage often generates significant amounts of mechanical stresses which cause additional problems like delamination and loss of electrical contact. The majority of the solid-state supercapacitor cells show cycling stability of a minimum of 1000 to a maximum of 5000 cycles. Hence, it is quite obvious that using a ternary system is more suitable as the presence of a third component such as carbon nanotubes or metal oxides, aids in stabilizing the mechanical framework of the electrode in addition to contributing to the overall capacitance. Keeping the above arguments in view, we have presented in this chapter, some of the recently reported advanced ceramic-based electrodes for flexible supercapacitor devices. These include metal oxides, metal sulfides, metalloid carbides and nitrides, metal hydroxides, and spinel oxides-based ceramic materials in conjunction with suitable conductive polymers. Some selected examples outline the underlying concepts that are being followed like the synergy between the structural and the compositional aspects of the proposed ceramic electrodes with proper emphasis on

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the corresponding electrochemical performance. The development of several approaches like tuning the internal structure/shape/size of the particles and choosing the correct phase composition has successfully delivered high specific surface area as well as porosity. These approaches have also improved the ion diffusion and electron transport mechanisms significantly. Yet some issues still remain which might attract the attention of the readers and expose them to areas where the scope of further improvement remains. 1. Using advanced sintering protocols: High temperature and longer duration sintering steps are quite energy-intensive which contribute to increased cost of fabrication. Higher manufacturing costs will definitely limit the commercial applicability of the designed supercapacitors. High temperatures also reduce the specific surface of the electrodes as well as the porosity values which, in turn, hinders ionic diffusion through the electrodes. The risk of losing particle morphology is also associated with conventional sintering processes which lack adequate control. Hence, advanced sintering protocols such as cold sintering and ultra-fast high-temperature calcination need to be explored. These protocols will not only reduce manufacturing costs but also will improve the electrochemical performance of the electrodes. 2. Electrode flexibility and size reduction: The major market for flexible supercapacitors is the wearable device domain. Therefore lightweight and comfort are the two major aspects to which flexible supercapacitor devices must adhere. Futuristic possibilities such as flexible/foldable displays, electronic skin materials, implant devices, etc., will need energy sources in the form of supercapacitors that can complement the shape/form factor of the devices along with weight and cost. These micro/small and flexible supercapacitors should get fully charged in a few seconds and should possess high areal power density. These devices also need to have longer lifespans of around 10,000 cycles. The brittle nature of ceramic materials, however, affects the applicability of supercapacitors in the above regions of interest. This is due to the fact that ceramic particles show low toughness and high hardness values which makes the fabrication of thinner electrodes extremely challenging. So, the design and development of miniaturized supercapacitor devices still need much attention. 3. Pushing the electrochemical performance limits: It is quite obvious that the overall electrochemical properties of these ceramic materials need to be pushed further. Properties such as power density, energy density, and specific capacitance of these materials are currently way below their theoretical limits. Various strategies such as nanostructuring, controlled morphology variation, and heteroatom doping are some of the few that are known to alter and improve the electrochemical performance of ceramic-type materials. Futuristic materials with properties like self-healing and binder-free versions will make supercapacitors cheaper and more efficient at the same time. Choosing correct electrolyte formulations with high potential windows (solid-state, gel-type) will provide an option to increase the working potential of the devices thereby increasing g the energy and power densities. Gel, ionic liquid, and solid-state electrolytes are also preferred due to several advantages such as leak resistance, moisture tolerance, and safety. For large-scale implementation in the wearable devices segment, there is a need for lighter, costeffective, and simple supercapacitors. 4. Inventing novel ceramic configurations and supercapacitor types: Presently, understanding the fundamental chemistry behind the energy storage mechanisms offered by these ceramic materials and using the established ideas to formulate new classes of

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materials should be the goal of the hour. These new classes of ceramics are expected to provide novel ways of capacitor charge-storage concepts like a combination of intrinsic and extrinsic pseudocapacitance. Current literature suggests that research is more focused on improving the performance of some common ceramic options that are available today. We need to develop new ceramic electrodes with unique molecular chemistry that will open a new form of charge storage while pushing the supercapacitor limits even further.

There is still a long way to go to obtain a variety of supercapacitor technologies that will cater to the rising needs of allied technological advancements. We believe that addressing the concerns presented in this chapter and directing research efforts to the outlined key areas will definitely help in realizing the dream of nextgeneration flexible supercapacitors.

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Rajangam Vinodh1, Rajendran Suresh Babu2, Hee-Je Kim3 and Moonsuk Yi1 1 Department of Electronics Engineering, Pusan National University, Busan, Geumjeonggu, South Korea, 2Laboratory of Experimental and Applied Physics, Centro Federal de Educac¸a˜o Tecnolo´gica Celso suckow da Fonesca, Av. Maracana˜ Campus, Rio de Janeiro, Brazil, 3Department of Electrical and Computer Engineering, Pusan National University, Busan, Geumjeong-gu, South Korea

14.1

Introduction

Energy is important for the endurance and progress of human development. The increasing demand for energy, as well as the severe conservational contamination caused by fossil fuel burning, has led to universal efforts to investigate different renewable and hygienic energy sources as a primary source of energy in our day-today life. Electric energy, as an energy transporter, can be created by zero-emission sources that do not pollute the atmosphere such as solar energy from sunlight, tidal from sea waves, or wind from the air, and could meet our long-term energy needs. Supercapacitors (SCs), fuel cells, and rechargeable batteries are evolving as efficient energy storage systems in this regard. Owing to their extraordinary specific energy, extreme specific power (greater than 10,000 W/kg), rapid charge discharge capabilities, risk- free, as well as outstanding cyclic stability. SCs have become a type of reliable and well-organized electrical energy storage (EES) device have focused numerous considerations in the research sectors [1 3]. With an increase in applications in many arenas, such as nonnatural electronic skins [4,5], wearable/ implantable medical equipment [6,7], wearable electronics [8], flexible energy storage devices such as stretchable SCs [9], bendable zinc-carbon batteries, flexible alkaline batteries, flexible Li-ion batteries [10], polymer Li-metal batteries and all-polymer batteries have gained a considerable consideration in modern days. Flexible supercapacitors (FSCs) are considered as one of the most capable flexible EES equipment owing to their high specific power, excellent cyclic stability, eco-friendly, etc., among other SCs [11]. Furthermore, FSCs are more facile to pack, not harmful, cheap in price, adjustable, lightweight, high specific power, and have an extensive working temperature from ambient 25 C to 70 C. The FSCs equipment generally consists of pair of stretchable electrode materials, electrolyte (solid gel), the divider analogous Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00014-2 © 2023 Elsevier Ltd. All rights reserved.

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to conventional SCs, and stretchy wrapping materials. The present investigation on FSCs primarily emphasizes on the mechanical flexibility of the equipment’s, with the goal of maintaining superior electrochemical characteristics during repeated mechanical distortion, such as bending, folding, stretching, and twisting [12]. Many materials with extensive application potential have been proven using this method, including carbon-derived electrodes such as activated carbon (AC), carbon nanotubes (CNTs), graphene, transition metal oxides/hydroxides, conducting polymers, etc. [13 15]. Nevertheless, each of the electrode materials listed above has its own set of restrictions. For example, transition metal oxides/hydroxides have superior specific capacitance, but show meager conductivity. On the contrary, carbonaceous materials demonstrate outstanding durability and electrical conductivity but offer lower specific capacitance. Conducting polymers have the merits of higher capacitances, and excellent mechanical strength/conductivity but low cycle life. Because of these aspects make it difficult for FSCs to be commercialized, researchers are being advised to look for novel electrode materials for high-performance SC applications [16]. As a prominent electrode material for SCs, metal organic frameworks (MOFs) and MXene-derived materials have been tested by various investigators for use in FSCs. As illustrated in Fig. 14.1, this book chapter summarizes recent advances on MOF and MXene resultant electrode materials as FSCs, along with the critical supremacies that MOFs and MXene-derived samples provide in FSCs electrode materials. Our main goal is to encourage knowledge transfer from previously published research to generate new ideas for the faster progress of MOFs and MXenes-derived electrode materials in FSCs.

Figure 14.1 Applications of flexible supercapacitors in the future world.

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14.2

301

Types of flexible supercapacitor

14.2.1 Metal organic frameworks-based flexible supercapacitors MOFs with tunable size and void shape is achieved by transition metal ions or metal clusters gathered with organic linkers [17,18]. The supremacies of huge specific surface area and tunable pores enriched architecture offer MOFs extraordinary material in the field of electrochemistry [19,20]. The current swift development of MOFs-derived SCs has grasped the stage of meeting the demand for portable and wearable electronics. In this book chapter, we focus on the latest developments toward MOFs-based FSCs. Few bare MOFs employed for FSCs have been described, but this is generally constrained by its poor conductivity behavior. MOFs have three approaches to enhance the conductivity of MOF-based materials: using novel bare conductive MOFs, emerging the diverse multiple functional MOFs derived hybrid materials, and procurement of the MOFs-based resources. In this section of the book chapter, the MOFs-based electrode material for FSCs, along with the distinctive construction methods for MOFs-derived flexible equipment’s, are completely conferred. The synopsis of the complete electrochemical behavior and recent progress of the existing MOFs-derived sample fabricated equipment’s are offered progressively to envisage the forthcoming propensity to the realization of an eventual behavior of MOFs derived FSCs. Recently, Qiufan Wang et al. reported a facile and effective methodology to fabricate a new CoSe2 twodimensional nanosheets (2D NSs) on Co-based MOFs derived CNT films via an in situ method. Owing to the synergistic characteristics of self-assembled CoSe2 2D NSs and the extreme progression of lD CNTs, the prepared nanocomposite material offers an effective transmission and rapid pathways for the diffusion of ions and electrons [21]. The fabricated flexible asymmetric supercapacitor (ASC) from CoSe2 2D NSs and FeSe2 nanorod as positive and negative electrode materials, respectively provides an ultra-high areal specific energy of 0.25 mWh/cm2 with extraordinary mechanical steadiness and excellent cycle life. Only a few 2D conductive MOF-based SC electrode materials have been published to date. However, all these SCs were constructed from powders of MOF via a catalysts slurry coating technique, which damagingly exaggerated their capacitance behavior. Jia et al. established an in situ anodic electrodeposition (AED) method to construct an evenly distributed 2D conductive MOF on NF and employ it straightly as electrode materials for SCs with no binders/additives (NiPc-MOFAED@NF) [22]. The chemical structure of precursor, NiPC-NH2 and NiPC-MOF was exhibited in Fig. 14.2A. Fig. 14.2B and C represents the electrochemical properties of NiPCMOFAED@NF SC in an aqueous electrolyte (PVA/LiClO4), and organic system (TEABF4/acetonitrile), respectively. The excellent electrochemical behavior of the NiPc-MOFAED@NF SC, comprising superior areal capacitances of 11.5 and 22.1 mF/cm2 for aqueous system and organic system, respectively. In addition, outstanding areal specific power of 1.35 mW/cm2 and specific energy of 22.4 μWh/cm in organic electrolytes, robust durability along with amazing mechanical integrity

Figure 14.2 (A) The structure of precursor, NiPC-NH2 and NiPC-MOF; (B) Electrochemical properties of NiPC-MOFAED@NF in aqueous electrolyte. (C) Electrochemical properties of the prepared electrode in the organic electrolyte (a f illustrate CVs with different scan rate, GCDs with varying current densities, capacitance versus current density, capacitance retention versus cycle number, CVs with different bending angles, and LED-powered SC, respectively). GCD, Galvanostatic charge discharge. Source: Reproduced with permission from H.X. Jia, S. Lu, S.H.R. Shin, M.L. Sushko, X.P. Tao, M. Hummel, et al. In situ anodic electrodeposition of two-dimensional conductive metal-organic framework@nickel foam for high-performance flexible supercapacitor, J. Power Sour. 526 (2022) 231163.

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were further unveiled by electrochemical measurements. Conversely, stretchability is an alternative crucial factor for movable and small electronic devices. To demonstrate the bending characteristics of the fabricated electrode, NiPc-MOFAED@NF SC (aqueous and organic electrolyte), they verified its voltammogram profiles at various twisting angles displayed in Fig. 14.2B(e) and 14.2C(e) and the attained results are well matched with no substantial change in the CV shapes, illustrating its outstanding mechanical stretchability. Furthermore, to instinctively evident the excellent behavior of the prepared electrode material, NiPc-MOFAED@NF aqueous/organic SC was also verified to initiate in commercially available light-emitting diodes (LEDs). A red color LED was effectively lighted by three series-linked equipments (Fig. 14.2B(f) and C(f)), highlighting the potential of the NiPc-MOFAED@NF SC in near future. Wang et al. used conducting ink, PEDOT (poly(3,4-ethylenedioxythiophene)) to improve the electrical conductivity of the MOF, while electron sponge, [PMo12O40]32 (polyoxometalate; PMo12) with huge electronic transfer ability was employed as the capacity donor. Conclusively, MOFs (PCN-224) represented as the host of this hybrid that delivered the EDLC and a PCN-224@PEDOT/PMo12 CCII hierarchical hollow micro-vesicle nanoarchitecture was attained via a facile single-step electro-co deposition method [23]. The micro-vesicle composite was intermingled in MOF hosts and the pictorial illustration was depicted in Fig. 14.3A (a). As depicted in Fig. 14.3A(b) carbon cloth (CC) fibers showed a smooth surface. After the reaction of solvothermal, the piece of black color CC changed into brown-red (refer Fig. 14.3A(c) inset) and the fibers shifted to less smooth compared with the CC fibers, which demonstrated that CC was completely enclosed by PCN224 (Fig. 14.3A(c)). After the codeposition of PEDOT/PMo12, the micro-vesicles enclosed the surface of the fibers and the roughness of the fiber continuous budding, which showed that PCN-224-CC were further covered and intertwined by the unchanging hollow micro-vesicles (Fig. 14.3A(d)). Meantime, the color of the electrode materials was turned into dim blue, which comes from the color of PEDOT and reduced PMo12 (inset of Fig. 14.3A(d)). The formation mechanism of PEDOT/ PMo12 was depicted in Fig. 14.3B and it may be due to the following features: (1) Although EDOT monomer and PMo12 are consistently distributed in the precipitate solution, PMo12 and PEDOT do not enter PCN-224-CC at the same time through the process of electrodeposition. When the voltage shifted from 11 to 21 V, PMo12 was stimulated to the surface and entered to the pores of PCN-224-CC to receive electrons. Subsequently, the matrix showed a negative surface charge. The negative surface charges are not incessant dispersed on the surface but inaccessible by the PMo12 nanocluster. This was the base to form the hollow vesicles. (2) In turn, when the voltage shifted from 21 to 11 V, the positively charged PEDOT (S1) would be adsorbed by the negatively charged PMo12 to form new positive surfaces. (3) Under the voltammogram method, the exteriors nurtured consecutively, lead to a multiporous vesicle architecture. With the advantages of new architecture and the combining effect of multiple constituents, the ideal areal capacitance of the PCN-224@PEDOT/PMo12 CC-II electrode was 4077.8 mF/cm2 at a scan rate of 5 mA/cm2, which is approximately 33 times higher than that of virgin PCN-224 (123.6 mF/cm2). To validate the probable usage of PCN-224@PEDOT/PMo12 CC-II

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Figure 14.3 (A): (a) pictorial illustration of in situ growth of the PCN-224@PEDOT/ PMo12 CC-II, SEM images of (b) CC, (c) PNC-224-CC, and (d) PCN-224@PEDOT/ PMo12 CC-II. (B) Schematic sketch of PEDOT/PMo12 micro-vesicle formation mechanism. (C): (a) PCN-224@PEDOT/PMo12 CC-II flexible SSC pictorial illustration, (b) GCDs at various current densities, (c) Ragone plots of the SSC device, (d) capacitance retention versus cycle number (inset, an LED light), (e) the stability test after bending measurements in 500 times. The inset depicts the photocopy images and voltammograms at various bending stages, and (f) digital photos of LED-linked devices in series starting up one mini fan. Source: Reproduced with permission from B. Wang, S. Liu, L. Liu, W.W. Song, Y. Zhang, S.M. Wang, et al., MOF/PEDOT/HPMo-based polycomponent hierarchical hollow micro-vesicles for high performance flexible supercapacitors, J. Mater. Chem. A 9 (2021) 2948 2958.

in SCs, they constructed an symmetric supercapacitor (SSC) device. The identical 2 parts of PCN-224@PEDOT/PMo12 CC-II electrodes were parallelly positioned and PVA/H2SO4 gel was employed as the separator (Fig. 14.3C(a)). Furthermore, the SSC electrochemical performance was evaluated from galvanostatic charge discharge (GCD) techniques with varying the current density and it was depicted in Fig. 14.3C(b). The SSC exhibited decreased specific capacitance when the current density was increased. The areal-specific capacitances of the SSC device were found to be 215.7 143.8 mF/cm2 at varying current densities from 0.5 to 5 mA/cm2. The supreme specific energies of 0.0297 0.0192 mW h/cm2 were attained at the specific powers of 0.324 5.128 W/cm2, which exposed that the SSC had prominent usage in energy storage devices (Fig. 14.3C(c)). Specifically, the assembled SSC retains 84.59% of its initial capacitance over 10,000 GCD

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cycles at 5 mA/cm2 illustrating the extraordinary cyclic stability (Fig. 14.3C(d)). Deceptively, no significant shape transformation was noticed in the voltammogram plots exposed at angles of 0, 30, and 90 degrees (inset of Fig. 14.3C(e)); at the same time the SSC was exposed to nonstop twisting analysis for 500 cycles and only 8.2% capacitance loss (Fig. 14.3C(e)), which showed the steady mechanical characteristics and prominent consumption as a flexible energy-harvesting device. Furthermore, a three-series device can efficaciously start up with a mini fan (Fig. 14.3C(f)) and powered up one blue LED (inset of Fig. 14.3C(d)), which exhibited a prodigious probable for the real usage of FSC devices. Since cobalt (Co) derived electrode materials have outstanding conductivity and durability whereas nickel (Ni) derived electrode materials have ultra-high theoretical specific capacitance, a sequence of bimetallic Ni/Co MOF electrode with tunable components were prepared via a simple single phase solvothermal technique [24]. In addition, owing to the similarity of the atomic radius and chemical valence state between cobalt and nickel, the in situ replacement was accomplished by altering the initial quantity of Co21 to Ni21 during the synthesis condition. The ideal model not only engaged the stability and electrical conductivity of Co derived MOF materials, but also established excellent electrochemical behavior with a supreme specific capacitance of 1230.3 F/g at 1 A/g and moderate cyclic durability (80% of its initial capacitance retains over 4000 GCD cycles). Further, the constructed ASC showed ultra-high specific energy of 116 Wh/kg at a specific power of 0.795 kW/kg and an extraordinary cyclic life of 92.1% capacitance retention over 6000 GCD cycles. Carbon-related electrode materials are extremely required to create FSC electrodes. Nevertheless, owing to the restrictions of the carbon electrode architecture comprising the poor availability of active surface area, unsuitable porous assembly, nonideal surface chemical states, and low stretchability, their electrochemical behavior may not always be as anticipated in FSCs. Yuan Yue et al. developed a freestanding nitrogen-anchored porous carbon (NPC) fabric electrode by pyrolyzing a ZIF-8/cotton hybrid textile without chemical activation/etching processes [25]. The pictorial representation of the ZIF-8/cotton hybrid and its resultant NPC film was exhibited in Fig. 14.4A. To explore their property-structure association of the cotton, ZIF-8/cotton hybrid, carbon fiber (CF), and NPC film were initially studied by SEM and TEM techniques. Fig. 14.4B(a) discloses that loads of cotton packages are existing in the cotton fabric and are knitted with each other, creating well-organized textile architecture. The interspace among the cotton bundles and fibers fabricates a macroporous architecture. Fig. 14.4B(b) and (c) display that the principal cotton fiber has an average diameter of 10 μm and its external boundary is comparatively flat. Fig. 14.4B(d) and (e) depict that the fiber surface became rough when these cotton fibers were thoroughly shielded with a ZIF-8 particle layer. The hexagonalshaped ZIF-8 particles could be obviously seen the fiber surface, which is reliable with the previously reported works (Fig. 14.4B(f) and its inset) [26]. After carbonization, the ZIF-8/cotton hybrid fabric, the well-organized textile architecture, and the stretchable properties of the NPC film are well reserved compared with the cotton fabric and CF film (Fig. 14.4B(a) (c), (g) (o)). Fig. 14.4B(k) and (l)

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Figure 14.4 (A) pictorial representation of the ZIF-8/cotton composite fabric and its derived NPC film synthesis; (B) SEM and TEM photographs: (a c) cotton fabric, (d f) ZIF-8/ cotton composite fabric, (g i) CF film, (j l and inset in (l)) NPC film, (m o) digital images of cotton fabric, CF film, and NPC film; (C) (a and b) voltammograms of SSC at various bending angles. Digital images of SSC powering a calculator and red LED (c and d). Source: Reproduced with permission from Y. Yue, Y.L. Huang, S.W. Bian, Nitrogen-doped hierarchical porous carbon films derived from metal-organic framework/cotton composite fabrics as freestanding electrodes for flexible supercapacitors, ACS Appl. Electron. Mater. 3 (2021) 2178 2186.

authorizes the existence of a compact ZIF-8-based carbon particle layer on the surface of each CF. No disconnected carbon particle is noticed, illustrating the robust durability of the prepared ZIF-8-based carbon particle layer on the CF. The ZIF-8-based carbon particles are not solid and have disordered pores (inset in Fig. 14.4B(l)). Their porous architecture assistances the upsurge of specific surface area and interface of electrode/electrolyte leading to an improved electrochemical behavior [27,28]. The digital images of the cotton fabric, CF film, and NPC film are also exhibited in Fig. 14.4B(m) (o). The black NPC film displays a 2D film morphology and flexible feature. Advantageous from the multiporous nature, huge

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surface area, nitrogen-anchored surface, and well-organized electrode structure, the ensuing electrode material exhibits a supreme specific capacitance of 375.2 F/g at 2 mV/s, excellent cyclic stability, and outstanding flexibility. Further, the fabricated SSC delivered a specific energy of 17.0 Wh/kg. The SSC was further verified to assess its application prospect. Overlay voltammogram plots are noticed in Fig. 14.4C(a) and (b), when analyzing the SSC at various bending angles and cycles. This discloses that the SSC could stand the stress without abolishing the electrode and SSC structure, displaying a steady electrochemical behavior. A few small electronics (calculator and red LED) were selected to assess the possibility of the SSC as a power source. As shown in Fig. 14.4C(c), the mini calculator could efficaciously work for 60 s and even efficiently show after 120 s when charging the SSC for 30 s. Two identical SSCs linked in series could also light up the red LED for 60 s. Haiyang Zhang et al. proposed a noncalcined approach to prepare nanowire sphere-shaped Co-BTC (CBNWM) [29]. The construction of MOF into a 1D nanowire structure can efficiently enhance its active moieties and the rate of ion transfer. After the MOF nanowires are tangled and unite into microspheres, their mechanical characteristic and chemical stability are enhanced. So, the electrochemical properties of CBNWM have been pointedly enhanced when compared with Co-BTC crystal (CBC). The specific capacity of CBNWM attains 657 F/g at 0.5 A/g with a capacity retention of 81.4% after 3000 GCD cycles. Afterward, a constructed flexible ASC device, CBNWM//AC displayed the highest energy density of 34.4 Wh/kg at a power density of 375.3 W/kg. Yongquan Du et al. prepared NiCo-MOF electrodes by a facile hydrothermal process [30]. The flower-like NiCo-MOF electrode displayed an electrifying cut-off voltage window of 1.2 V and an outstanding specific capacitance of 927.1 F/g at a current density of 1 A/g. The NiCo-MOF//AC (ASC) device exhibited high specific energy of 28.5 Wh/kg at a specific power of 400.5 W/kg and robust long cycle life of 95.4% of its original capacitance remains over 5000 GCD cycles. Elif Erc¸arıkcı et al. studied a new, facile, and cheap-cost technique to construct a flexible bimetallic MOF-anchored graphene sponge (GS) self-supported electrode materials (ZnCo-MOF/GS) [31]. The electrochemical performance of the ZnCoMOF/GS was explored via GCD, EIS, and CV. ZnCo-MOF/GS displayed a supreme specific capacitance of 695 F/g at 1.0 A/g and outstanding durability. In addition, a ZnCo-MOF/GS derived SSC was constructed, and it showed a supreme capacitance of 302 F/g at 1.0 A/g along with excellent energy density (108 Wh/kg) and power density (5037 W/kg). In addition, the assembled FSCs exhibited outstanding performance at various bending modes. Yin et al. reported MOF-derived hollow NiCo2O4 nanocages (NCs) were threaded by interwoven SiC nanowires (NWs) network on CC, forming a unique sugar gourd-like core-shell structure, which was constructed via a multistep method comprising of chemical vapor deposition, solution reaction of MOF templates, ion exchanging/etching and successive heat treatment [32]. Profiting from the exclusive architecture merits, such as multiple porous textures with plentiful active moieties for electrochemical reactions and interwoven conductive networks for electron transport, the developed core-shelled CC/SiCNWs@NiCo2O4 NCs as a binder SC electrode displays outstanding electrochemical behavior with a large

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capacitance of 1377.6 F/g at 1 A/g, decent rate property (r68.8% capacitance retention at a current density of 20 A/g) and exceptional cyclic durability (88.3% capacitance retention over 6000 GCD cycles). In addition, the ASC constructed from CC/ SiCNWs@NiCo2O4 NCs (positive electrode) and AC (negative electrode), not only carries high specific energy of 46.58 Wh/kg at the specific power of 800 W/kg, but also holds good stretchability with high capacitance retention, showing the application potential in the field of flexible electronics. Wu et al. developed heteronanostructured black phosphorus/MOF hybrids formed by P O Co covalent bonding based on a planned droplet microfluidic approach comprising of confined and ultra-fast microdroplet reactions [33]. The resulting hybrid exhibits a large capacitance of 1347 F/g owing to its high surface area of 632.47 m2/g, well-developed micro-porosity of 0.38 cm3/g, and engineered electroactivity. Consequently, the made-up FSCs delivered an ultra-high volumetric energy density of 109.8 mWh/cm3, a large capacitance of 506 F/cm3, and good long-term stability of 12,000 cycles. In short, the electrode materials derived from MOFs is the promising candidate for flexible SC applications. Engineering the morphology, architecture, and characteristics of MOF materials can help to enhance their electrochemical behavior. Continuing research and development of multifunctional flexible material that can be stretched, folded, and bent simultaneously for practical FSCs application. Furthermore, the reference to the basic algorithms of MOFs-based FSSCs material should proceed to another significant point. The summary of key features of typical flexible MOF-derived SCs is listed in Table 14.1.

14.2.2 MXene-based flexible supercapacitor MXene, a novel emerging lineage of 2D nanomaterials with exceptional electrical conductivity, plentiful terminal groups, a distinctive layered structure, a high surface area, and hydrophilicity, makes it a promising material for energy storage applications in flexible and wearable devices [34]. Several ground-breaking studies have been dedicated to the progress of flexible MXene and its based materials with a variety of functions and tailored architectures. As a result, this section summarizes the current development of flexible MXene-based materials for smart devices, with an emphasis on preparation procedures, operational protocols, performances, and applications in SCs. The freestanding films of MXene-based materials have been produced for use in flexible SCs so far [35 37]. For instance, Yang reported high volumetric performance flexible symmetric SCs based on free-standing pristine MXene film via filtration and pressing for [38]. Zhou fabricated the stretchable SCs based on composites of Ti3C2Tx (MXene)-reduced graphene oxide (rGO) films [39]. However, because of inadequate neighboring contacts between MXene NSs, relatively pristine MXene films often cracked or ruptured during deformation, limiting their applicability. Furthermore, the layer-by-layer restacking of MXene NSs were limited access to electrolyte ions, preventing complete exploitation of their surfaces and leading to unsatisfactory characteristics of electrochemical performances [40]. The improvement of mechanical property of MXene-based films by introducing different nanomaterials, for example, conducting polymers (e.g., polypyrrole, polyvinyl alcohol) [41,42] and

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Table 14.1 The summary of key features of metal organic frameworks-derived flexible supercapacitors. S. no.

Electrode

Specific capacitance

Energy and power density

Life cycle

Flexibility

Ref.

1

MOF-CoSe2/ CNT

0.25 mWh/cm2 @ 53.06 mW/cm2

[21]

NiPcMOFAED@NF

Bending

[22]

3

PCN224@PEDOT/ PMo12 CC-II Ni/Co-MOF

0.0297 mWh/cm2 @ 0.324 μW/ cm2 116 Wh/kg @ 0.795 kW/kg

Bending

[23]

375.2 F/g @ 2 mV/s (SSC)

17.0 Wh/kg @ 94.6 W/kg

110.0 F/g @ 0.5 A/g (ASC)

34.4 Wh/kg @ 375.3 W/kg

85.29% after 4000 cycles 91.7% after 10,000 cycles 84.59% after 10,000 cycles 92.1% after 6000 cycles 93% after 6000 cycles 120.5% after 5000 cycles 95.4% after 5000 cycles 77% after 7500 cycles 88.3% after 6000 cycles 89.3% after 12,000 cycles

Bending

2

593.5 mF/cm2 @ 5 mA/cm2 (ASC) 22.1 mF/cm2 @ 0.1 mA/cm2 (SSC) 215.7 mF/cm2 @ 0.5 mA/cm2 (SSC) 328 F/g @ 1 A/g (SSC)

4

5

1.03 μ Wh/cm @ 160 μW/cm2

6

ZIF-8/Cotton composite (NPC) Co-BTC

7

NiCo-MOF

80.3 F/g @ 0.5 A/ g (ASC)

28.5 Wh/kg @ 400.5 W/kg

8

ZnCo-MOF/GS

302 F/g @ 1.0 A/g (SSC)

108 Wh/kg @ 5037 W/kg

9

MOFNiCo2O4@SiC

131 F/g @ 1.0 A/g (ASC)

46.58 Wh/kg @ 800 W/kg

10

P-MOF

506 F/cm3 @ 0.5 A/cm3

109.8 mWh/cm3 @ 0.65 W/cm2

[24]

Bending

[25]

Bending

[29]

Bending

[30]

Bending

[31]

Bending

[32]

Bending

[33]

carbon-based nanomaterials (e.g., carbon nanotubes, graphene) [43] to combine with MXenes to facilitate exceptional mechanical robustness or properties of energy storage which was one of the efficient methods. Zhang et al. constructed the hybrid electrode of 3D Ti3C2Tx MXene/rGO/carbon (MGC-500) using the facile template method. The melamine foam (MF) is used as a template, to develop the porous architecture and introduces the nitrogen heteroatom during the sintering procedure [44]. The fabricated MGC-500 electrode exhibits a high specific capacitance of 276 F/g at 0.5 A/g in a 3-electrode system. The practical utility of MGC-500 electrodes is constructed into symmetric-type SC; it demonstrates a constant electrochemical behavior at various compressive strains. Especially, they found that the PVA/H2SO4 gel electrolyte can be coated into MGC-500 foam electrode surface and compressed into a flexible film. The assembled SC devices also have stable capacitance during various kinds of deformation, for instance, twisting and bending. The proposed method

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provides an effortless approach for fabricating (free-standing) Ti3C2Tx MXene electrodes for energy storage systems which can tolerate a variety of deformations and can be applied to other MXene family members. Cellulose nanofiber (CNF) derived from timber exhibited exceptional characteristics, for example, a well-connected nanofiber network, extraordinary mechanical strength, and flexible, assembling them a perfect building block to construct flexible electrodes for next-generation [9,22]. Though it is regarded as a capable approach to constructing extraordinary performance composite electrodes, research on how to achieve instantaneous improvements in mechanical characteristics and promising energy harvesting properties is in high demand. Lately, Cai et al. developed freestanding MXene nacre (encouraged by the nacre mortar and brick structure) through a self-assembly method for geometrical flexibility and mechanical robustness, with its significant improvements of photothermal conversion capability and the efficiently curbed MXene NSs restacking [45]. In particular, 1D cellulose nanofibrils and 2D SnS2 were used as building blocks for assembling structure-layered 3D architecture as well as efficient intercalators for suppressing intractable of NSs restacking. Furthermore, the in situ grown SnS2 served as additional H1 transport pathways and higher photothermal promotor. The resulting MXene “nacre” demonstrated outstanding mechanical strength (78.3 MPa) with no sacrificing flexibility, good specific capacitance of 190 F/g in 1 M H2SO4 at a sweep rate of 10 mV/s, and high retention of 87.4% after 5000 cycles. The nacre-inspired all-solid-state SC achieved a high energy density of 6.7 μWh/cm2 with capacitance retention of 91.5% after 4000 cycles and remarkable cycle stability over 90% capacitance retention after 500 times of folding/unfolding. Porous carbon (PC) can efficiently improve the characteristic self-stacking phenomenon of 2D MXene-based films as a spacer and can easily customizing their porous structure. However, the contact between 3D PC and 2D MXene flakes commonly exists in a point-to-point form due to the irregular shape of the PC, leading to low efficiency of electron delivery and stress concentration with a fragile characteristic in the resulting films. Yang et al. introduced 1D CNT to fabricate an extremely conductive net architecture, strongly attaching PC on to MXene fragments, thereby confirming the rapid electron distribution by swelling the connection between PC and MXene. Furthermore, the intertwined CNTs bond the horizontal MXene fragments, creating interior architecture more vital, thus improving the stretchability [46]. Fig. 14.5A illustrates the preparation route for Ti3C2Tx/CNT/PC (TCP) films. In brief, CNTs (60 mg) and polyvinylpyrrolidone (30 mL) were mixed with subsequent ultra-sonication for 60 min to attain a firm suspension. Then, a few mg of PC was distributed in DI water followed by ultra-sonication for 600 s to get a suspension of PC. Afterward, the prepared suspensions from PC, CNTs, and delaminated Ti3C2Tx MXenes were completely mixed with a certain weight percentage and sonicated for 0.5 h. Then, the resultant suspension was vacuum-assisted and filtered with a membrane filter. Finally, the Ti3C2Tx/ CNT/PC films were peeled off and kept in a vacuum oven at ambient temperature for 6 h. The as-prepared Ti3C2Tx film shows a trimly loaded lamellar architecture with no obvious pores by the naked eye (Fig. 14.5B(c)). On the contrary, the neighboring Ti3C2Tx fragments are efficiently disconnected by PC as depicted in Fig. 14.5B(d), thereby producing many ion-manageable surface areas on Ti3C2Tx fragments by

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Figure 14.5 (A) Synthesis way of the Ti3C2Tx/CNT/PC film. (B) Morphology characterization of Ti3C2Tx, TP, and TCP film: structure design of (a) Ti3C2Tx, TP, and (b) TCP; SEM pictures of (c) Ti3C2Tx, (d) TP, and (e) TCP; (f) HR-TEM pictures of TCP; (g) zoomed view of the lattice fringes; (h) amorphous carbon of PC; (i) SAED pattern. (C) Electrochemical characteristics of the quasisolid-state supercapacitor: (a) schematic diagram of the supercapacitor; photographic displays representing (b) ultra-thin thickness; (c) stretchability; (d) voltammograms at a scan rate of 50 mV/s with the bending angle varied from 0 to 180 degrees; (e) voltammograms with different scan rate (50 1000 mV/s); (f) GCD plots with different current densities; (g) EIS plot; (h) Ragone plot; (i) plot of cycle number versus capacitance retention (inset: CVs of before and after GCD cyclic test). Source: Reproduced with permission from K. Yang, M. Luo, D. Zhang, C. Liu, Z. Li, L. Wang, et al., Ti3C2Tx/carbon nanotube/porous carbon film for flexible supercapacitor, Chem. Eng. J. 427 (2022) 132002.

easing their self-stacking phenomenon. After introducing CNT, the intertwined CNTs make a network architecture on the surface of PC and Ti3C2Tx as exhibited in Fig. 14.5B(e). This peculiar architecture not only decorated PC onto Ti3C2Tx fragments, enhancing their interaction area but also ties with the horizontal neighboring Ti3C2Tx fragments. The net architecture fabricated by intertwined CNTs further confirms the clarification about the enhanced stretchability of Ti3C2Tx/CNT/PC film as exhibited in Fig. 14.5B(b). TEM photographs of Ti3C2Tx/CNT/PC film display the existence of CNT with bent form (Fig. 14.5B(f)), Ti3C2Tx with a lattice spacing of approximately 0.26 nm (Fig. 14.5B(g)), and PC with amorphous carbon (Fig. 14.5B (h)). These annotations are well matched with the obtained selected area diffraction (SAED) pattern (Fig. 14.5B(i)). Subsequently, the TCP film has the tendency to tolerate a large scan rate of 1 V/s and displays a high areal specific capacitance of 364.8 mF/cm2 at a current density of 0.5 mA/cm2 which leftovers above 80% even at

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a high current density of 50 mA/cm2. The schematic look of the constructed FSC was exhibited in Fig. 14.5C(a). Fig. 14.5C(b) and (c) illustrate the constructed FSC with approximately 0.34 mm thickness and it could bend at a large angle. Furthermore, the voltammogram of FSC (Fig. 14.5C(d)) shows virtually no difference while the bending angle increases from 0 to 180 degrees at a scan rate of 50 mV/s, ensuring its excellent flexibility and outstanding stability. The FSC could bear a supreme high scan rate of 1 V/s (Fig. 14.5C(e)) and exhibit a necessary areal-specific capacitance of 212 mF/ cm2 (Fig. 14.5C(f)) at a current density of 0.1 mA/cm2. In addition, the specific capacitance remains above 65% even increasing the current density to 10 mA/cm2. Such a high rate capability can be accredited to the excellent conductivity (212.6 S/cm1) of Ti3C2Tx/CNT/PC 0.5 film, which is additionally confirmed by the low equivalent series resistance (B0.52 Ω) of the FSC as displayed in the EIS plots of Fig. 14.5C(g). Furthermore, the prepared FSC demonstrates high specific energy of 10.5 μWh/cm2 at 29.8 μW/cm2 as established in the Ragone plot (Fig. 14.5C(h)). The prime specific energy of the FSC could remain above 75% while the specific power increases to approximately 100-folds. Moreover, the fabricated FSC retains approximately 90% of its initial specific capacitance over 15,000 GCD cycles (Fig. 14.5C(i)), signifying exceptional cycling stability. Pure CNF materials have inadequate capacities and deprived behavior owing to their architectural characteristics, including thicker fiber diameters and loosely bounded designs between CNF; these can control the charge accretion in the EDLC and electrolyte ion kinetics during the process of charging and discharging. To overcome these difficulties, highly capacitive or conductive materials such as MnO2, RuO2, and MXene have been considered as hybrid materials [47 52]. Hyewon Hwang et al. prepared self-supported Ti3C2Tx/CNF electrodes via electrospinning of polyacrylonitrile (PAN) and followed by annealing treatment for FSC applications (Fig. 14.6A) [53]. As-electrospun nanofibers (ENFs) remained in straight form (Fig. 14.6B). Though, after pyrolysis treatment, the ENFs contracted, as shown in the inset of Fig. 14.6C, because of the decomposition of PAN and the creation of ring compounds and pyridinoid assemblies [54]. As exposed in Fig. 14.6D, the Ti3C2Tx/CNF rug was effortlessly enfolded around the glass bar, providing self-supported electrode materials for FSC devices. MXene/CNF electrodes displayed a specific capacitance of 90 F/g at a high scan rate of 300 mV/s, which is approximately 2.3 times larger than that of a bare PCNF (PAN derived CNF; 38 F/g at 300 mV/s) while displaying comparable capacitance values of around 120 F/g at the low scan rate of 2 mV/s. A compact FSC device was constructed from MXene/CNF film, and it was depicted in Fig. 14.6E. P-CNF displayed excellent mechanical strength without adding any binder materials, so the MXene/CNF hybrid film could also be stretchable/bendable. The voltammogram of the MXene/CNF FSC exposed stable device operation with a specific capacitance of 76 F/g at 100 mV/s (Fig. 14.6F) even under bending, while the value was slightly lower than that of MXene/CNF operated in sulfuric acid as an aqueous electrolyte. Furthermore, even after 500 bending cycles, the FSC device retains almost 98% of its original specific capacitance in the flat position (Fig. 14.6G). The aforementioned outcomes revealed that the MXene/CNF have a great probability for applications in flexible and multipurpose electronic equipments.

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Figure 14.6 (A) Graphic illustration of electrospinning procedure; (B) electrospun nanofibers; (C) carbonized electrospun nanofibers along with TEM image in the inset; (D) photocopy view of stretchable electrode packaging around a glass bar; (E) MXene/CNF hybrid film-based FSC and photocopy pictures of the constructed flexible prototype equipment; (F) CVs of the FSCs at 100 mV/s under bending position; (G) mechanical stretchability of the prototype equipment over 500 bending cycles. Source: Reproduced with permission from H. Hwang, S. Byun, S. Yuk, S. Kim, S.H. Song, D. Lee, High-rate electrospun Ti3C2Tx MXene/carbon nanofiber electrodes for flexible supercapacitors, Appl. Sur Sci. 556 (2021) 149710.

Incorporation of heteroatom (sulfur, nitrogen, and phosphorous) via pyrolysis treatment under an argon/hydrogen atmosphere should be an effective methodology to enhance the durability and its electrochemical characteristics of as-prepared Ti3C2Tx materials. Another drawback that deters the usage of Ti3C2Tx sheets is the feeble interlayer contacts among adjacent Ti3C2Tx NSs which make them have deprived mechanical characteristics, and tensile strength [55,56]. Furthermore, though incorporation of heteroatom via pyrolysis technique can enhance the characteristics of Ti3C2Tx, the elimination of surface functional moieties during the pyrolysis treatment outcomes on a hydrophobic surface and make it more tedious to attain selfsupporting Ti3C2Tx film. Hence, it is necessary to progress Ti3C2Tx-based sheets with eco-friendly nature, excellent mechanical robustness, outstanding cyclic life, greater electrochemical characteristics, and large-scale construction development.

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Sun et al. prepared new ordered pores enriched “skin/skeleton”-like Ti3C2Tx/biomass-based carbon fibers (MXene/CF) heterostructure by single step annealing, which effectively deteriorates the assembling of Ti3C2Tx NSs [57]. Furthermore, MXene/CF has a well-organized multiporous architecture, thus enabling electrolyte incorporation and offering effective and steady pathways for quick dispersion of ions to the electrode and making functional Ti3C2Tx-derived electrode materials. When Ti3C2Tx/CF heterostructure is employed as a free-standing electrode for SCs, the electrode exhibits supreme volumetric capacitance of 7.14 F/cm3, excellent rate properties (63.9% from 0.5 to 100 A/g), and outstanding durability (99.8% over 5000 cycles). Furthermore, all SSCs derived from Ti3C2Tx/CF electrodes are also fabricated, which not only shows excellent capacitance and rate behavior, but also possesses excellent stretchability and outstanding cycle life. The constructed SSCs still remains organizational integrity and stable capacitance even after 2500 cycles at various bending positions. Wu et al. developed an extremely flexible and conductive CNTs/Ti3C2Tx-TPU hybrid fiber electrodes with pores enriched architecture by the wet spinning method [58]. The mechanical and electrical characteristics of the fiber electrodes continued stable during the tensile procedure of 10 wt.% CNTs/Ti3C2Tx. In the meantime, the electrodes also exhibited a supreme volumetric specific capacitance of 3.9 F/cm3 even at a high scan rate of 100 mV/s. The all-solid-state fibrous SCs derived from CNTs/Ti3C2Tx-TPU also displayed higher electrochemical characteristics and possesses excellent flexibility with 50% strain. The specific capacitance of the all-solid-state fibrous SCs altered with the tensile strain and stretched 3.1 F/cm3 while the stain was 50%. From the above findings, the authors demonstrated that the fibrous SCs were further employed in flexible strain sensors, which can change action signals to electrochemical signals quickly. Liao et al. developed incorporation of heteroatom methodology to tailor the surface functionalities of Ti3C2Tx with subsequent doping of rGO as a conductive additive to attain a scalable production of S, N-MXene/rGO (SNMG-40) composite film with high mechanical robustness (B45 MPa) and energy storage characteristics (698.5 F/cm3) [59]. Remarkably, the SNMG-40 film also establishes excellent cycle life (approximately 98% capacitance retention over 30,000 GCD cycles), which can be sustained under room temperature or soaked in sulfuric acid electrolyte for more than one hundred days. The ASC (aMGSC) derived from SNMG-40 film exhibits ultra-high specific energy of 22.3 Wh/kg, the obtained specific energy value was far better than formerly described Ti3C2Tx-derived electrode materials. Furthermore, the aMGSC also offers outstanding mechanical robustness under various deformation positions. Therefore the present methodology assembles Ti3C2Tx materials more viable for real-life usages such as flexible electronics and electromagnetic interference shielding. To improve the energy harvesting behavior of Ti3C2Tx-derived electrode, both incorporation of heteroatom and introducing electroactive “spacers” are demonstrated to be efficient methodologies. Here, Yu et al. explored a facemask shielding hydrothermal technique to prepare N2 anchored pores enriched MXene/TiO2 heterostructure in one pot, which enables a well-preserved conductivity of porous N-doped Ti3C2Tx and controlled in situ production of homogenously distributed electroactive titania (TiO2) spacers [60]. This distinct composite architecture gives a fortuitous to combine various chemical and physical merits in a balancing facile route. As an outcome, the constructed freestanding film

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electrode, N-incorporated porous Ti3C2Tx/TiO2 heterogeneous layers reveals outstanding energy harvesting behavior with an excellent capacitance of 2194.33 mF/cm2 (918.69 F/g), which beats most of the heteroatom-anchored Ti3C2Tx-derived electrode materials described earlier. Further, the Ti3C2Tx/TiO2 electrode exhibited marvelous cycling stability with a 74.39% capacitance retention over 10,000 cycles. In addition, the constructed FSCs exhibit nearly no deviations in its capacitive behavior when exposed to mechanical deformations, signifying their outstanding stretchability and durability. Energy harvesting miniaturization is yet another research topic, and it has become an important necessity of the period after the quick expansion in wearable and smart electronic types of equipment such as microsensors, nanorobots, and micro-electromechanical systems which depend on the weightless, stretchable, and miniatured power source [61 65]. As one of the most popular miniaturized energy harvesting equipment, mini-batteries have established supreme specific energy but are grieved from inferior specific power and partial life cycle [66,67]. In contradictory, progressive micro-supercapacitors (MSCs) hold viable specific energy and excellent specific power, and outstanding life cycle which are prominent applicants of power supply for tiny-sized electronic equipment [67 71]. Area capacitance is an important consideration as it should increase the energy storage of MSCs over a short operating area [72 74]. Recently, Wang et al. established a novel type of onchip MSC employing MXene/CNTs hybrid as the electrode material via photolithography method and followed by vacuum-filtration method [75]. The hybrid CNTs/Ti3C2Tx primes to the creation of LBL assembly, where CNTs serve as the spacer to avoid the piling and accumulation of MXene NSs throughout the assembly development. Thus the final CNTs/MXene material provides supple layered pathways and well-organized electronic architectures for quick dispersion of electrolyte ions, which allows a noteworthy enhancement in the specific capacitive behavior. The constructed micro-supercapacitor (Ti CNTs/Ti3C2Tx LBL) displayed a maximum areal capacitance of 61.38 mF/cm2 at 0.5 mA/cm2, beating most of Ti3C2Tx or carbon materials derived MSCs. Moreover, it further validates outstanding mechanical stretchability. The voltammogram plots have remained the same while bent from 0 to 180 degrees. Over 200 cycles of bending-discharging at 180 degrees, 93.4% of its original capacitance is remained. Such flexibility can be ascribed to the mechanical strength and durability of stacking CNTs/Ti3C2Tx, whose layered architecture ruins unbroken under huge deformation. Hybrid zinc ion SCs are employed as prominent energy harvesting devices due to their excellent specific energy related to conventional SCs. MXene (Ti3C2Tx) cathode with excellent conductivity, distinct lamellar architecture, and outstanding mechanical stretchability has been established prominent candidate in the fabrication of zinc ion SCs but attaining extended durability and extreme rate stability is still an immense task. Li et al. projected a simple laser script tactic to construct patterned MXene-zinc ion-based MSCs, followed by the in situ annealing of the constructed MSCs to enhance extended cyclic life, which retains 80% of its initial specific capacitance over 50,000 GCD cycles and higher rate capability [76]. The authors studied the impact of the thickness of the cathode materials on the electrochemical evaluation

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of the MSC. The highest areal capacitance of 72.02 mF/cm2 was achieved at 10 mV/s when the cathode thickness of 0.851 μm, which is 1.77-fold superior to that of 0.329 μm cathode thickness (35.6 mF/cm2). Furthermore, the constructed MXene-derived zinc ion MSCs possess outstanding stretchability, a digital clock can be driven by a single device even under a bending position, flexible LED displayer of the “TiC” symbol also can be simply lighted by the MC displays under twisting, crimping, and winding conditions, representing the ascendable construction and usage of the assembled MSCs in movable devices. Wire-shaped SCs (WSCs) are crucial machineries of wearable technology owing to their geometric resemblance with woven fiber. One possible technique for producing WSC devices is the layer-by-layer (LBL) construction method, which is a bottom-up approach for electrode construction. WSCs need adaptive and adhesive coatings of the active substrate to the wire-shaped material, which makes it difficult to obtain other processing techniques such as vacuum separation or spray-coating. Nevertheless, the LBL fabrication method offers flexible and strong coverings that can be deposited on a diversity of substrates and shapes, including wires. Junyeong Yun et al. reported WSCs produced from LBL construction of alternating layers of positively charged rGO functionalized with poly-(diallyldimethylammonium chloride) (PDDA) and negatively charged Ti3C2Tx MXene NSs conformally deposited on activated carbon yarns (ACYs) [77]. The schematic illustration was depicted in Fig. 14.7A. The positively charged rGO-PDDA was obtained by incorporating PDDA and hydrazine into a graphene oxide dispersion. The ACYs were submerged in the rGO-PDDA and MXene dispersions alternately, subsequently by drying and then more cycles of LBL assembly to attain the preferred number of layer pairs (LPs), ensuing in LBL-coated ACYs (LACYs). In this fabrication, the LBL film increases specific capacitance, specific energy, and specific power by 240%, 227%, and 109%, respectively, over without ACY coating, ensuing high specific capacitances (237 F/g) and volumetric capacitances (2193 F/cm3). Fig. 14.7B reveals the snapshot of 40 LP LACY wire-shaped SCs in flat and bending positions. There was no obvious alteration in the voltammogram or the GCD profiles at various bending ranges for the 40 LP LACY wire-shaped SCs (Fig. 14.7C), illustrating that the assembled wire-shaped SCs were mechanically stable up to a bending radius of at least 3 mm. Fig. 14.7D depicts the capacitance retention of a wire-shaped SCs alternating between flat and bending positions (8 mm bending radius), which corresponds to a maximum axial strain of approximately 1.1%, in which 90% of the original capacitance was retained after 200 bending tests. Table 14.2 shows the summary of the main characteristics of various flexible MXene-based SCs.

14.3

Summary and conclusion

FSCs ensure substantial consideration is being paid to energy storing systems with a variety of applications including portable electronics, flexible screens, wearable smart clothing in the military, skin electronics, and compatible medical gadgets. The present

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Figure 14.7 (A) pictorial representation of the fabrication procedure for wire-shaped rGOPDDA/MXene SC; (B) Photocopy pictures of 40 LP LACY wire-shaped supercapacitors in flat and bending positions; (C) CVs at different bending ranges; (D) Number of bending cycles versus capacitance retention (%); Inset of (D): GCD plots of first and 200th bending cycles. Source: Reproduced with permission from J. Yun, I. Echols, P. Flouda, Y. Chen, S. Wang, X. Zhao, et al., Layer-by-layer assembly of reduced graphene oxide and MXene nanosheets for wire-shaped flexible supercapacitors, ACS Appl. Mater. Interfaces 13(12) (2021) 14068 14076.

book chapter provided the latest advancement of various flexible current collectors for the development of different materials of cathode and anode, and new fabrication strategies of FSCs. So far, many scientists have focused on developing different novel electroactive materials in cell arrangements (symmetric or asymmetric, or hybrid) for FSCs. Furthermore, the most recent research innovations of flexible reinforced novel electrode materials with controlled morphology and structure have promised excellent improvement in electrochemical performances in terms of higher specific energy, and specific power with long cycle life. The FSCs efficiency is normally uttered in terms of volumetric and gravimetric dimensions, which promotes the novel electrode design improvement, that is, flexible substrate-support and free-standing designs. It is important to create flexible, lightweight, and efficient use of the whole electrode material to achieve higher energy storing efficiency, which results in excluding the additional mass of current collectors and additives/binders, which are smoother and harder to handle. In the current era, research and development of MOFs-derived FSCs have been progressing quickly. There have been several efforts to use MOFs-based electrode material for SCs as a novel step for energy storage in flexible devices. This book

Table 14.2 The summary of main features of MXene-based flexible supercapacitors. S. no.

Electrode

Specific capacitance

Energy and power density

1

Ti3C2Tx/rGO/carbon

138 F/g @ 0.5 A/g (SSC)

2

CNF/MXene@SnS2

3

Ti3C2Tx/CNT/PC

4 5

Ti3C2Tx/CNF MXene/CF

226 mF/cm2 @ 0.8 mA/ cm2 (SSC) 212 mF/cm2 @ 0.1 mA/ cm2 (SSC) 120 F/g @ 2 mV/s (SSC) 76 F/g @ 0.5 A/g (SSC)

6

CNT/MXene-TPU

7

S, N-MXene/rGO

8

N-MXene/TiO2

9

Ti3C2Tx/CNT LBL

10

Ti3C2Tx-based Znion MSCs rGO-PDDA/ MXene/ACY

9.38 Wh/kg @ 346.8 W/kg 6.7 μWh/cm2 @ 1206 μW/cm2 10.5 μWh/cm2 @ 29.8 μW/ cm2 11 Wh/kg @ 120 W/kg 10.6 Wh/kg @ 125.5 W/kg 1.16 mWh/cm3 @ 0.16 W/cm3 22.3 Wh/kg @ 748.88 W/kg 37.91 μWh/cm2 @ 0.20 mW/cm2 5.46 mWh/cm3 @ 0.20 W/cm3 0.02 mWh/cm2 @ 0.50 mW/cm2

11

7.3 F/cm @ 10 mV/s (SSC) 71.4 F/g @ 1 A/g (ASC) 426.5 mF/ cm2 @ 0.5 mA/cm2 (SSC) 61.38 mF/ cm2 @ 0.5 mA/cm2 72.02 mF/ cm2 @ 10 mV/ s (SSC) 237 F/g

Stability (capacitance retention/cycles)

Flexibility

Ref. [44]

91.5%/4000 cycles

Bending/ twisting Bending

90%/15,000 cycles

Folding

[46]

98%/10,000 cycles 99.8%/5000 cycles

Bending Bending

[53] [57]

Stretchable

[58]

98%/30,000 cycles

Bending

[59]

74.39%/10,000 cycles

Bending

[60]

87.7%/1000 cycles

Bending

[75]

80%/50,000 cycles

Bending

[76]

Bending

[77]

[45]

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chapter mainly describes the recent growths involving some efficacious and potential research works that can attain higher electrochemical behavior constructed on MOFs derived FSCs. The peculiar morphology and architectures of MOFs can offer additional lively reaction moieties while efficiently shortening the ion, communication pathway of electricity load, resulting in high-performance FSCs. In addition to electrodes, a few scientists have started to emphasize their investigation of other machineries outside of electrodes, such as electrolytes, separators, binders, and substrate materials (current collectors). Therefore the priority is how to make sure that the material with the electrode has the most excellent on the whole performance. In summary, many challenges and obstacles require to be addressed to understand the bunch production and realistic application of MOFs in FSCs. Flexible energy storage systems perform on the superhighway, driven by stretchable devices, which provide a landmark for the fast progress of MXene-derived electrode products. The growing demand for extraordinary specific energy promotes the integration of capacitance-type MXenes with other electrode materials of extraordinary specific capacitance. Furthermore, in situ loading of MXenes into flexible substrates without the use of binders creates the best combination between active materials and current collectors, creating easy procedures, improved mechanical and electrochemical durability, and enlarged energy densities. It promises to predict the faster growth of MXene-derived nanomaterials in flexible electronic devices. Furthermore, to understand greater flexibility and better electrochemical behavior, other test results such as the electrolyte and substrate selection also influence the electrochemical properties of 2D materials, MXene. In fact, how to deposit the electrode materials effectively in a plastic flexible substrate with ohmic contact remains a major encounter. Although gel electrolytes, for example, polyvinyl alcohol mixed with LiCl or H3PO4 or KOH, and others work well in FSCs, more research output is needed to mature novel kinds of polymer electrolytes with superior ionic/electronic conductivity, allowing the fabrication of FSCs with excellent electrochemical behavior.

Acknowledgment The authors gratefully acknowledge the financial support of BK21 Plus Creative Human Resource Education and Research Programs for ICT Convergence in the 4th Industrial Revolution, Pusan National University, Busan, South Korea.

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[53] H. Hwang, S. Byun, S. Yuk, S. Kim, S.H. Song, D. Lee, High-rate electrospun Ti3C2Tx MXene/carbon nanofiber electrodes for flexible supercapacitors, Appl. Sur Sci. 556 (2021) 149710. [54] J. Zhao, L. Wang, X. He, C. Wan, C. Jiang, A Si-SnSb/pyrolytic PAN composite anode for lithium-ion batteries, Electrochim. Acta 53 (24) (2008) 7048 7053. [55] Z. Ling, C.E. Ren, M.Q. Zhao, J. Yang, J.M. Giammarco, J.S. Qiu, et al., Flexible and conductive MXene films and nanocomposites with high capacitance, Proc. Natl Acad. Sci. 111 (47) (2014) 16676 16681. [56] S. Zhao, H.B. Zhang, J.Q. Luo, Q.W. Wang, B. Xu, S. Hong, et al., Highly electrically conductive three-dimensional Ti3C2Tx MXene/reduced graphene oxide hybrid aerogels with excellent electromagnetic interference shielding performances, ACS Nano 12 (11) (2018) 11193 11202. [57] L. Sun, Q. Fu, C. Pan, Hierarchical porous “skin/skeleton”-like MXene/biomass derived carbon fibers heterostructure for self-supporting, flexible all solid-state supercapacitors, J. Hazard. Mater. 410 (2021) 124565. [58] G. Wu, Z. Yang, Z. Zhang, B. Ji, C. Hou, Y. Li, et al., High performance stretchable fibrous supercapacitors and flexible strain sensors based on CNTs/MXene-TPU hybrid fibers, Electrochim. Acta 395 (2021) 139141. [59] L. Liao, D. Jiang, K. Zheng, M. Zhang, J. Liu, Industry-scale and environmentally stable Ti3C2Tx MXene based film for flexible energy storage devices, Adv. Funct. Mater. 31 (35) (2021) 2103960. [60] J. Yu, M. Zeng, J. Zhou, H. Chen, G. Cong, H. Liu, et al., A one-pot synthesis of nitrogen doped porous MXene/TiO2 heterogeneous film for high-performance flexible energy storage, Chem. Eng. J. 426 (2021) 130765. [61] Z.S. Wu, Y. Zheng, S. Zheng, S. Wang, C. Sun, K. Parvez, et al., Stacked-layer heterostructure films of 2D thiophene nanosheets and graphene for high-rate all-solid-state pseudocapacitors with enhanced volumetric capacitance, Adv. Mater. 29 (2016) 1602960 1602966. [62] X. Shi, Z.S. Wu, J. Qin, S. Zheng, S. Wang, F. Zhou, et al., Graphene-based linear tandem micro-supercapacitors with metal-free current collectors and high-voltage output, Adv. Mater. 29 (2017) 1703034 1703042. [63] L. Li, Z. Wu, S. Yuan, X.B. Zhang, Advances and challenges for flexible energy storage and conversion devices and systems, Energy Env. Sci. 7 (2014) 2101 2122. [64] T.Q. Trung, N.E. Lee, Materials and devices for transparent stretchable electronics, J. Mater. Chem. C. 5 (2017) 2202 2222. [65] H. Jiang, Z. Wang, Q. Yang, M. Hanif, Z. Wang, L. Dong, et al., A novel MnO2/ Ti3C2TX MXene nanocomposite as high performance electrode materials for flexible supercapacitors, Electrochim. Acta 290 (2018) 695 703. [66] J.W. Long, B. Dunn, D.R. Rolison, H.S. White, Three-dimensional battery architectures, Chem. Rev. 104 (2004) 4463 4492. [67] M. Beidaghi, Y. Gogotsi, Capacitive energy storage in micro-scale devices: recent advances in design and fabrication of micro-supercapacitors, Energy Env. Sci. 7 (2014) 867 884. [68] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, A review of electrolyte materials and compositions for electrochemical supercapacitors, Chem. Soc. Rev. 44 (2015) 7484 7539. [69] A. Hossain, P. Bandyopadhyay, P.S. Guin, S. Roy, Recent developed different structural nanomaterials and their performance for supercapacitor application, Appl. Mater. Today 9 (2017) 300 313.

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[70] D. Qi, Y. Liu, Z. Liu, L. Zhang, X. Chen, Design of architectures and materials in inplane micro-supercapacitors: current status and future challenges, Adv. Mater. 29 (5) (2017) 1602802. [71] C. Yang, W. Que, X. Yin, Y. Tian, Y. Yang, M. Que, Improved capacitance of nitrogen-doped delaminated two-dimensional titanium carbide by urea-assisted synthesis, Electrochim. Acta 225 (2017) 416 424. [72] N.A. Kyeremateng, T. Brousse, D. Pech, Microsupercapacitors as miniaturized energystorage components for on-chip electronics, Nat. Nanotechnol. 12 (2017) 7 15. [73] C. Lethien, J.L. Bideau, T. Brousse, Challenges and prospects of 3D microsupercapacitors for powering the internet of things, Energy Env. Sci. 12 (2019) 96 115. [74] A. Ferris, B. Reig, A. Eddarir, J.F. Pierson, S. Garbarino, D. Guay, et al., A typical properties of FIB-patterned RuOx nanosupercapacitors, ACS Energy Lett. 2 (8) (2017) 1734 1739. [75] R. Wang, S. Luo, C. Xiao, Z. Chen, H. Li, M. Asif, et al., MXene-carbon nanotubes layer-by-layer assembly based on-chip micro-supercapacitor with improved capacitive performance, Electrochim. Acta 386 (138420) (2021) 1 8. [76] L. Li, W. Liu, K. Jiang, D. Chen, F. Qu, G. Shen, In-situ annealed Ti3C2Tx MXene based all-solid-state flexible Zn-ion hybrid micro supercapacitor array with enhanced stability, Nano-Micro Lett. 13 (100) (2021) 1 11. [77] J. Yun, I. Echols, P. Flouda, Y. Chen, S. Wang, X. Zhao, et al., Layer-by-layer assembly of reduced graphene oxide and MXene nanosheets for wire-shaped flexible supercapacitors, ACS Appl. Mater. Interfaces 13 (12) (2021) 14068 14076.

Flexible solar cells

15

Farkhondeh Khodabandeh1 and Mohammad Reza Golobostanfard2 1 School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran, 2Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin-Film Electronics Laboratory (PV-Lab), ˆ Neuchatel, Switzerland

15.1

Introduction

Air pollution and global warming are among the most significant issues threatening the environment and are both related to the widespread application of limited fossil fuels. Accordingly, the accessibility of a renewable, abundant, eternal, and costeffective energy source is one of the prime concerns of mankind in this century. In the field of environmentally friendly energy sources, solar energy has the highest potential to satisfy these requirements and its less geographical dependence makes it an ideal option to replace carbon-based fuels. Traditionally, silicon wafers with efficiency values exceeding 25% have gained so much interest and they are still predominant in the photovoltaic (PV) market, while second-generation solar cells like hydrogenated amorphous silicon (a-Si:H), CdTe, and Cu(In,Ga)(S,Se)2 (CIGS) stand in the next place. With the ever-increasing advancement of flexible and portable electronics, a great demand for flexible power supplies is felt more than ever. However, the crux of the matter is that the rigidity and high weight of mentioned solar cells highly restrict their application to some specific and flat surfaces. Fabrication of thin-film second-generation of solar cells on flexible substrates has opened a new way toward energy supply on curvilinear surfaces like buildings, vehicles, the human body, and spaceships. Flexible substrates like plastics and foils reduce the weight of flexible cells in comparison with their rigid counterparts which can be quite desirable for low-strength surfaces or even space systems. In this regard, flexible solar cells (FSCs) can be molded into desired shapes and sizes and are predicted to be integrated with a variety of applications from foldable cell phones, wearable systems, medical implants, and self-powered electronics to solar cars as it is shown in Fig. 15.1A
C. More importantly, FSCs are usually fabricated through lowtemperature and cost-effective methods compared with rigid cells and they are compatible with roll-to-roll manufacturing techniques that is all helpful for large-scale production. Furthermore, FSCs are easily transported and installed. Subsequently, as the PV market thrives, the FSCs allocate a larger share to them. Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00015-4 © 2023 Elsevier Ltd. All rights reserved.

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Figure 15.1 Emerging applications of FSCs (A) a foldable tablet, (B) a solar backpack, and (C) a solar car [1
3]. Source: Reprinted from https://www.smartprix.com/bytes/best-foldable-phones/, https://www. joom.com/en/products/5c3ff6ac8b2c370101670f58, and https://www.eqmagpro.com/ lightyear-one-is-a-solar-car-with-a-range-of-450-miles/.

As the central part of FSCs, active layers play a determining role in power conversion efficiency (PCE). The most significant criteria to choose absorber materials for FSCs are less consumption and lower cost of raw materials, a bandgap around 1.4 eV with proper band edges, a high absorption coefficient for a wide range of the solar spectrum, and stability under the operating conditions. Recently, some emerging materials like small organic molecules, conjugated polymers, and organometal halide perovskites have also been implemented as the active layer of these cells in addition to those mentioned above labeled as the third-generation of solar cells. It is so disappointing to note that FSCs lose efficiency in exchange for these dramatic advantages. Therefore their performance should improve by the employment of enhancement strategies and material optimization techniques to make them competitive with rigid PV technologies. In addition, mechanical robustness and environmental stability under operating conditions are some ongoing issues to be addressed about FSCs since bending stresses and penetration of humidity highly decrease their lifetime and degrade their performance. Moreover, materials involving FSCs are normally toxic and they are not biodegradable which means they may damage the environment despite expectations. Although FSCs have serious limitations and their commercialization is not still possible, the broad range of their applications is certainly worth more investigation.

15.2

Material properties for flexible substrates

Depending on the application of an FSC, its substrate should have variable degrees of the following properties:

15.2.1 Stability against oxygen and moisture Materials have varying levels of permeability to both moisture and oxygen due to their different atomic packing density. Accordingly, metals and ceramics with a

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cohesive and dense lattice are more impenetrable to humidity. On the contrary, cross-linking between monomer chains creates convenient paths for the diffusion of gas and water molecules in polymers. Nevertheless, the rate of penetration also differs in polymers considering their water absorption behavior. The water vapor transmittance rate (WVTR) of some more common polymer substrates for FSCs is represented in Fig. 15.2. Accordingly, equipping organic substrates to improve their performance, especially under aggressive conditions by some protective coatings like SiOx, Al2O3, SiCx, and SiNx is considered crucial. Atomic layer deposition of Al2O3 (25 nm) coating on 125 μm thick polyethylene naphthalate (PEN) reduced WVTR from 0.5 to 5 3 1026 g/m2.day, as a consequence, the CIGS solar cell deposited on it was estimated to work properly for at least 20 years [4].

15.2.2 Thermal properties A substrate should tolerate the highest temperature during the fabrication process of a solar cell, thus the low thermal stability of the substrate material limits the whole manufacturing process. Generally speaking, metals and especially ceramics can undergo high-temperature treatments whereas polymers almost degrade above their glass transition temperature (Tg). Tg and melting temperature (Tm) values for some common polymeric substrates are listed in Table 15.1 and polyimide. (PI) with the highest Tg is known to be the most efficient organic substrate. Furthermore, the substrate should have a thermal expansion near to the active layer to promise appropriate adhesion between them. In the case of CIGS solar cells, the 1000 EVA (2.67mm) PET (0.178mm) PEN (0.102mm) Clear PVC (0.305mm) Polyimide (0.127mm) Polyvinyl butyral (1.44mm) Polyolefin (1.68mm) TPU (2.5mm) Polyacrylate (0.23mm) PDMS (3.3mm) Polycarbonate (0.1778mm)

WVTR (g/m2/day)

100

10

1

0.1

0.01 0.00286

0.00308

0.00330

0.00352

1/Temperature (1/K)

Figure 15.2 Water vapor transmission rate of common polymer substrates [5]. Source: Reprinted from J. Zhang, W. Zhang, H.-M. Cheng, S.R.P. Silva, Critical review of recent progress of flexible perovskite solar cells, Mater. Today 39 (2020) 66
88.

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Table 15.1 Glass transition and melting temperature of some common polymer substrates. Polymer

Tg ( C, calc.)

Tg ( C, exp.)

Tm ( C)

PET PC PI PEN PU PDMS PMMA

76
88 134
158 190
385 117
122
60.3 to
19
123 to
127 82
105

60
85 137
154



104
105

245
265 255
267 340
408 261
290 141
157
35 to
55 105
160

substrate has to tolerate temperatures up to 550 C and have a thermal expansion coefficient (CTE) of 8
11 3 1026/K [6]. Cooling perovskites with a CTE of 50 3 1026/K deposited on rigid glass substrates with CTE 5 10 3 1026/K after the annealing process creates a high tensile strain between the active layers since glass prevents perovskites from volume reduction. Replacing the glass substrate with a polymer like polyethylene terephthalate (PET) (CTE 5 20 3 1026/K) or polycarbonate (PC) (CTE 5 65 3 1026/ K) decreases the CTE mismatch and guarantees the FSC performance.

15.2.3 Optical properties If a solar cell is going to be fabricated with a superstrate structure (when the substrate serves as the top layer), then the substrate should be extremely transparent to the solar spectrum, hence, ceramics like thin glass or some polymers [e.g., PET, PC, and PI] can be ideal options. A PCE of 10.41% was obtained by PI/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/perovskite/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/PEI/Ag cell due to its flexible transparent PI substrate with surface-confined Ag nanoparticles that created a light scattering effect and localized plasmon resonance [7]. Nanocellulose paper (NCP) with acrylic resin coating was developed as a biodegradable, low-cost, and transparent substrate for NCP/doped PEDOT: PSS/PEDOT:PSS/perovskite/PCBM/Al solar cell. This flexible and green solar cell reached an efficiency of 4.25% and retained .80% of it after bending for 50 times [8].

15.2.4 Chemical properties As a rule of thumb, the more inert the substrate material, the longer the durability of a cell. For a better performance and stability in chemically aggressive environments, the substrate should not react with elements present in the operation condition and similarly should not release any contaminant gas. In other words, it should behave as a barrier against chemicals for the whole cell. In this case, metals are preeminent, for instance, Ti is resistant to acid and base attacks by forming a TiO2 passive layer. Comparable with metals, ceramics are known as inert materials and are highly stable in many environments except for some acids like HF. On the contrary, polymers easily fail in harsh environments especially when they are exposed to UV radiation.

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15.3

329

Flexible substrates

Flexible substrates can be classified based on their material type to metals, ceramics, and polymers. Common flexible substrates for FSCs are summarized in Fig. 15.3 and some of them are shown in Fig. 15.4. Most important properties and examples of each kind will be discussed in the following sections:

Figure 15.3 Chronological chart of widely used flexible substrates for flexible solar cells [9]. Source: Reprinted from https://www.alibaba.com/product-detail/Stainless-Steel-PrecisionStrip-and-Foil_1605137043.html.

Figure 15.4 Examples of flexible substrates: (A) stainless steel foil, (B) corning Willow glass, and (C) PI foil [10
11]. Source: Reprinted from https://www.corning.com/worldwide/en/innovation/corningemerging-innovations/corning-willow-glass/willow-glass-resources.html, G. Cui, Z. Bi, R. Zhang, J. Liu, X. Yu, Z. Li, A comprehensive review on graphene-based anti-corrosive coatings, Chem. Eng. J. 373 (2019) 104
121, https://www.corning.com/worldwide/en/ innovation/corning-emerging-innovations/corning-willow-glass/willow-glass-resources.html.

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15.3.1 Metals Thinning metal foils to a thickness less than 125 μm make them appropriate for substrate applications in FSCs. Metal foils are more resistant to heat than other substrates for example, stainless steel, Ti, and Al can tolerate temperatures up to 1510 C, 1670 C, and 660 C, respectively which is dramatically higher than Tg of many polymers. Employing Ti foil as the substrate of a copper zinc tin sulfide (CZTS)-based solar cell enhanced the cell performance compared to the other substrates like PI and Mo since Ti can endure more annealing temperatures than others which is a critical step for better grain growth of CZTS [12]. Metal foils (especially stainless steel due to their Mo content) degrade at a slower rate than other materials owing to their higher corrosion resistance and permeability against moisture and oxygen. In special cases, the substrate needs to be coated by a barrier layer to prevent the diffusion of metal atoms to the absorber layer, otherwise, the cell composition and morphology disturb. The deposition of CIGS on mild steel leads to a corrosion-resistant solar cell with an efficiency of 18%. For further improvement of the environmental stability of the cell, a double layer of Ni/Cr was introduced between the substrate and the absorber layer. Interestingly, this bi-layer acted as a barrier and prevented the diffusion of iron atoms to the active layer [13]. To compare the influence of different metal substrates for a solar cell, CIGS thin films were grown on both Ti and Ni with varying thicknesses. Ti-based cells represented a superior performance due to the more contribution of the substrate to grain growth. In addition, the efficiency loss after the bending test was lower in 50 μm thick Ti sample than in the 100 μm one on account of more compatibility of thinner substrates to aggressive conditions [14]. Despite all of the excellent properties of Ti, its high cost limits its application for mass production while stainless steel is considered as a promising candidate. However, roll-to-roll processing of metals is not as straightforward as polymers due to their stiffer nature, and some annealing operations are considered crucial if they are work-hardened during the fabrication process. Moreover, their opaqueness limits their unique applications in FSCs with superstrate configuration.

15.3.2 Ceramics Although ceramic substrates are known as some rigid and brittle materials, reducing their thickness like metals to less than 100 μm makes them flexible enough for FSCs applications. Thin glass, as a very common ceramic substrate, has a birefringence transparency of more than 90% which is an exceptional property in comparison with other materials. A conductive transparent fluorinated tin oxide glass substrate improved the performance of a wearable dye-sensitized solar cell (DSSC) by its appreciable transparency and conductivity [15]. Glass is an electrical insulator, thus there is no need for any further insulating coating, it can undergo heat treatments up to~approximately 600 C, and is inert to chemically aggressive environments. A DSSC deposited on woven glass fiber textile represented 0.4% efficiency. Although other textiles like cotton could also be used as the substrate, none of them could tolerate

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331

the 500 C temperature required for TiO2 calcination [16]. Unlike polymers, glass is dimensionally stable and impenetrable to water and air. A CdTe solar cell fabricated on a cesium-doped ultra-thin glass (UTG) substrate produced an efficiency of 14.7%. Bending the cell with a radius of 32 mm for 168 h made no significant change in its J-V characteristic and no residual strain was recognized [17]. Willows glass with a thickness of 100 μm is a fantastic example of thin glass substrates as it can tolerate temperatures up to 700 C and has a WVTR of less than 7 3 1026 g/m2.day. A perovskite solar cell (PSC) with MgF2/Willow glass/ITO/ SnO2/FAMACs/Spiro/MoOx/Al structure reached 18.1% efficiency due to the enhanced stability of the cell in comparison with polymeric substrates [18]. After all, glass is not a tough material, hence, crack propagation should be prohibited by applying some coatings like polymers for better performance. In addition to glass, zirconia is another common ceramic used as the substrate of FSCs.

15.3.3 Polymers Polymers are the most flexible yet economical substrates among other materials and have attracted great attention in FSC applications. However, their low heat resistance is their major disadvantage that limits their application, for example, for CIGS solar cells [6]. Furthermore, a barrier coating for water and oxygen diffusion prevention is considered necessary for their better performance and durability. CTE mismatch is also a problem while using polymeric substrates because the created thermal stress highly damages the cell on account of low elastic modules of polymers. PET is the most popular polymeric substrate owing to its high transparency, chemical inertness, and lower WVTR. Solution processing of an organic solar cell (OSC) on PET substrate resulted in an efficiency of 0.016% [19]. PI is the most thermally stable polymer among conventional plastic substrates for FSCs. Fabrication of a-Si:H solar cells on both PI and Willow glass substrates with p-i-n configuration exhibited that increasing the thickness of the intrinsic layer enhanced the cell efficiency from 3.51% to 5.02% and from 3.38% to 4.38% for glass and PI, respectively [20]. Ecoflex (PBAT) is another plastic substrate used for FSCs. A graphene oxide-based solar cell grown on PBAT was more uniform than samples prepared on glass [21]. Polydimethylsiloxane (PDMS) is a silicon-based polymer that is popular for its high stretchability and transparency. An a-Si:H-based solar cell was fabricated on a rigid glass substrate and then transferred to PDMS via the dissolving sacrificial NaCl layer. As a consequence, the PCE increased to 4.07% owing to increased transparency and electrical conductivity [22].

15.4

Flexible absorbers and flexible solar cells

Several materials have been used as the active layer of FSCs and their highest efficiency values in recent years are depicted in Fig. 15.5. FSCs have made the most progress among other PV technologies in the past year and have set striking

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Advanced Flexible Ceramics

Figure 15.5 Highest efficiency values of FSCs recorded for various absorbers as a function of the bandgap [23]. Source: Reprinted from O. Almora, et al., Device performance of emerging photovoltaic materials (version 2), Adv. Energy Mater. 11(48) (2021) 2102526.

records. In this regard, the performance of GaAs/InGaP and all-perovskite tandem FSCs have surpassed single junction cells. Properties, performance, and scale-up challenges of some of the most conventional active layers are discussed in the following sections:

15.4.1 a-Si:H solar cells Hydrogenated amorphous silicon, labeled as a-Si:H, has the widest variety of applications among other inorganic and flexible thin films like CdTe and CIGS, from biosensors to textiles in mobile chargers. Plasma-enhanced chemical vapor deposition (PECVD) and chemical vapor deposition (CVD) are the two most common methods used for their deposition process. Although the hydrogenation process reduces the number of dangling bonds and makes the bandgap energy tunable, the conversion efficiency highly decreases in exchange. There are some structural reasons associated with this performance decay in comparison with their crystalline counterparts [24] like higher recombination rates in a-Si:H at remaining defects that act as recombination centers. a-Si:H solar cells are fabricated in both multi and single-junction structures and have different cell configurations; substrate configuration (n-i-p) for opaque substrates and superstrate configuration (p-i-n) for transparent ones. Nevertheless, the p-i-n configuration has attracted more attention in which the charge carriers are generated in the intrinsic section and then migrate to the n- and p-type layers. The highest recorded efficiency of a flexible single-junction a-Si:H solar cell is 8.8%

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333

[25], which is by no means comparable with other competitors in this market, thus multijunction structures are much more preferred. More advancements are achieved by the introduction of some Si alloys in multijunction solar cells. Bifacial sextuplejunction a-SiOx:H/a-SiOx:H/a-Si:H/a-Si:H/a-SiOx:H/a-SiOx:H FSC produced an open-circuit voltage of 3.5 and 4.1 V under 100 and 3000 lux. Surprisingly enough, this structure performed well for a self-powered internet of things (IoT) device (these systems need to work efficiently under low illuminance) [26]. These opencircuit voltage values could never be achieved by a single-junction a-Si:H solar cell. Another important application of multijunction a-Si
H solar cells is in miniaturized wearable power sources owing to their functionality under low to high illuminance, flexibility, low cost, lightweight, and comfort. A quintuple-junction aSiC:H/a-Si:H/a-SiGe:H/μc-Si:H solar cell produced an open-circuit voltage of 3 V corresponding to 15.03% efficiency (Fig. 15.6) [27]. To enhance the performance of a-Si:H solar cells and make them comparable with other FSCs, so many optimization techniques especially for p-i-n configuration have been performed. There are several enhancement strategies like increasing the quality of the interface between window and intrinsic layer, applying a-SiC:H as a buffer layer [28], bandgap tuning, reducing reflection, and so forth. If the a-Si:H solar cells are fabricated on polymeric substrates, they may degrade due to water and oxygen diffusion or UV illumination. One of the most practical solutions would be employing organic/inorganic multilayers configuration for these cells. Roll-toroll processing of silica-like bilayer encapsulation films was conducted on PET substrate through the PECVD technique. The WVTR of final cell was nearly 6.9 3 1024 g/m2.day which is a very good sign of proper cell encapsulation [28].

Figure 15.6 Schematic illustration of quintuple-junction a-SiC:H/a-Si:H/a-SiGe:H/μc-Si:H solar cell. Source: Reprinted from B. Liu, et al., High efficiency and high open-circuit voltage quadruple-junction silicon thin film solar cells for future electronic applications, Energy Environ. Sci. 10(5) (2017) 1134
1141.

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These FSCs have the potential to be fabricated in large scales due to their fewer efficiency losses, for instance, the project of 2.5 km of multijunction a-Si:H solar panels in three lines made by uni-solar is among the most successful solar businesses in the world. Nevertheless, the encapsulation process and cost, developing more efficient multijunction cells, and so many other issues still need to be overcome for economical and mass production.

15.4.2 CdTe solar cells CdTe-based solar cells are broadly used in the FSCs industry owing to their high reproducibility, simple stoichiometry, and low cost with exceeding 10% efficiencies depending on their thickness. The bandgap energy of polycrystalline CdTe is around 1.45 eV, which is ideal for light absorption in the visible region that leads to high open-circuit voltages and short-circuit current densities. Although a thick absorber layer of CdTe performs better, the scarcity of Te and toxicity of Cd have raised the demand for using thin films of CdTe. Vapor transport deposition and close-spaced sublimation (CSS) methods are more frequent high-temperature techniques for cell fabrication. As a rule of thumb, low-temperature processing reduces the performance of a CdTe solar cell but instead, various flexible substrates like polymers can be employed. Mainly, the cell structure incorporates a substrate, absorber layer, window layer, and back and front contacts. In principle, due to the p-type nature of the CdTe absorber layer, an n-type window layer should be introduced to the structure to form a p-n heterojunction and prevent surface recombination. The suitable band alignment of n-type CdS with CdTe highly satisfies these demands. Highly regular and three-dimensional nanopillar arrays of CdS embedded in CdTe thin films improved light absorption and charge collection. This approach enables both flexible and rigid solar modules with very high efficiencies [29]. In addition to CdS, CdS:O and MgxZn1
xO (MZO) are two other competitive options. Oxygenated CdS or CdS:O has a wider bandgap, is transparent and surpasses the parasitic absorption of CdS. MZO also has a wider bandgap and lower affinity to electron extraction. At a higher level, the MgZnO/CdS:O/CdTe cell structure has attracted more attention for mass production. Furthermore, some treatments are considered critical in the fabrication process of a CdTe-based solar cell for a better output. As-synthesized CdTe thin films should be annealed under a chloride atmosphere so as to create an active p-n junction with a suitable band position and enhanced grain growth. Annealing usually occurs under CdCl2 vapor and to a lower extent under some alternatives like HCl, elemental Cl2, ZnCl2, MgCl2 gas, and so forth. It is investigated that CdCl2 annealing of CSS-grown CdTe on UTG substrate enabled more controlled morphology, better grain growth, and less deviation from stoichiometry. The annealing treatment in the optimum temperature range of 390 C
405 C changed the bandgap energy range to 1.39
1.46 eV and carrier concentration and resistivity were 1013/cm3 and 104 Ω cm, respectively [30]. To find the most efficient CdTe solar cell, these cells have been fabricated in both substrate and superstrate configurations with different structures (Fig. 15.7A and B). Mo foil has represented perfect properties for the former cell, especially as

Flexible solar cells

335

Figure 15.7 Different configurations of CdTe solar cells: (A) superstrate and (B) substrate, (C) schematic representation of SU-8/Cu/Au/CdTe/CdS/ITO FSC [32,33]. Source: Reprinted from X. Wen, et al., Epitaxial CdTe thin films on mica by vapor transport deposition for flexible solar cells, ACS Appl. Energy Mater. 3(5) (2020) 4589
4599 and A. Romeo, et al., High-efficiency flexible CdTe solar cells on polymer substrates, Sol. energy Mater. Sol. Cell 90(18
19) (2006) 3407
3415.

the back contact due to its high thermal resistance, electrical conductivity, and good contact with CdTe. Roll-to-roll manufacturing of flexible CdTe solar cells on Mo back contact with evaporated submonolayer amounts of Cu for controlled doping of the CdTe layer followed by an annealing step provided 13.6% efficiency [31]. Generally, Mo creates a small charge energy barrier in CdTe cells, hence, usually, an interlayer between Mo and CdTe or CdTe doping is implemented for better cell performance. It should be noted that direct deposition of CdTe films on metal foils needs an extra structure inverting step which restricts the manufacturing process and reduces the efficiency. Interestingly enough, CdTe solar cells can be fabricated on some rigid substrates to achieve higher qualities and then be transferred to flexible substrates. High-quality CdTe thin film was epitaxially deposited on a single-crystal mica substrate and then transferred onto flexible SU-8 thanks to low interface interaction forces and a schematic representation of it as it is shown in Fig. 15.7C. The conversion efficiency of 9.59% for the cell indicates the potential of this method for further developments in the future [32]. Although researchers have gone through a tremendous length to optimize substrate CdTe solar cells, superstrate configuration is more preferred for their fabrication and the highest recorded PCE for a flexible CdTe solar cell equal to 16.4% has been achieved with it [34]. As a consequence, a transparent yet stable substrate is considered crucial for more improvements. In an experiment, several flexible CdTe/ CdS cell structures were manufactured both in substrate and superstrate configurations with three different back contacts including Mo, Cu, and Ag foils. All investigated materials performed satisfactory for the latter configuration, though only Mo was functional for the substrate design. In the case of Cu back contact, the whole cell structure in substrate configuration was disintegrated after the recrystallization process due to the high CTE difference of Cu and CdTe. As expected, the higher CTE difference between Ag and CdTe materials excludes further investigations of

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this configuration [35]. Historically, transparent polymers like PI are the first substrates used for this configuration, but they are almost becoming replaced with thin and alkali-free glass like Willow glass to achieve higher thermal and radiation resistivities. CdTe thin films were grown on ultra-thin Schott D263T glass (for the first time) in the temperatures range of 500 C
600 C led to faster growth, larger grains, higher charge mobility and concentration, and a higher-quality film that can never be achieved by polymeric substrates [36]. In the case of environmental stability issues, there is no serious concern for CdTe-based solar cells as they are so robust and stable if an appropriate encapsulation is applied. More importantly, FSCs should maintain their performance under bending, compressive, and stretching conditions; specifically for module production for which roll-to-roll processing is essential. As mentioned in Section 2.2, the UTG substrate perfectly satisfies this requirement. The efficiency of the CdTe solar cell on UTG remained almost the same after bending with 40 mm radius and 168 h of static bending with 32 mm radius. The XRD results illustrated that no strain remained within the film after the bending test [17]. After all, it is undeniable that CdTe-based solar cells are predominant in the global terrestrial PV market in comparison with other FSCs. Even though they cannot still compete with silicon cells in terms of cost and environmental issues, their simple stoichiometry and uniform cell is a great driving force for further investigations.

15.4.3 Cu(In,Ga)(S,Se)2 solar cells Satisfactory performance and lower cost of flexible CIGS solar cells offer an advantage over its other counterparts. However, high-efficiency GIGS cells need heat treatments up to at least 550 C under chalcogen atmosphere, accordingly, rigid substrates including inexpensive soda-lime glass (SLG) have exhibited superior performance. Unlike CdTe-based cells, CIGS solar cells are usually fabricated with substrate configuration and SLG/Mo/CIGS/CdS/ZnO/AZO/MgF2 structure. The CIGS absorber layer can be deposited by both vacuum-based methods like evaporation and sputtering and nonvacuum solution-based routes like electrodeposition and screen-printing and then they are annealed under sulfur or selenium atmosphere. The bandgap energies of CIGS compounds vary in the range of 1.0
1.6 eV for CIS and CGS, respectively. However, Ga content should be adjusted so that it is between 1.25 and 1.45 eV (optimum values for better light absorption). Nonetheless, flexible substrates, especially PI and stainless steel, have shown dramatic improvements in recent years and the small (1%
2%) gap between efficiency of rigid and flexible CIGS solar cells can easily be narrowed by further technical improvements. Furthermore, there are four elements present in this compound, thus formation of several defects and deviation from stoichiometry are highly expected, but it is not a serious concern and can be surpassed by some passivation methods. In addition to annealing, another critical postdeposition treatment (PDT) after CIGS growth would be the incorporation of alkali elements like sodium to passivate defects and improve efficiency. Although SLG substrate

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provides the required amount of Na by diffusion, flexible substrates, especially stainless steel in the case of CIGS solar cells, cannot supply it. Consequently, a postdeposition treatment via NaF evaporation under lower temperatures is considered essential. Moreover, Na is traditionally the most common alkali element for this treatment, but the introduction of heavier elements including potassium, cesium, and rubidium has paved the way toward efficiencies of more than 20%. Sequential treatment of the CIGS layer by Na and KF increased the cell efficiency to 20.4% which is the highest PCE reported for a flexible CIGS solar cell to date [37]. To compare the effect of Rb and K in PV performance of CIGS solar cells, the CIGS thin films were treated by RbF and KF separately after PDT by NaF. Both samples exhibited almost identical VOC and FF values and both elements improved the performance by a similar mechanism, they accumulated at the Cudeficient CIGS surface and diffused into its structure improving grain growth and passivating defects [38]. In this regard, the main challenges limiting more strategic applications of flexible CIGS solar cells are attributed to flexible substrates in terms of thermal resistance and purity. However, these are not unsolvable problems so that employing innovative synthesis methods for polyimide and barrier coatings for steel are some beneficial solutions. In addition to the development of low-temperature fabrication technologies, there are some lift-off processes that can be employed and a schematic representation of them is illustrated in Fig. 15.8. The latest route called the chemical mechanical polishing lift-off process includes the complete transfer of solar cell from SLG to a flexible substrate by dissolving glass into HF acid, polishing it to remove the Mo electrode, and heating it up to 145 C to melt the supporting quartz substrate. Reducing the thickness and improving adhesion between different layers hinder formation of probable cracks. However, these methods are somehow complicated and cells still have low efficiencies, so, they should be simplified before industrialization [39].

Figure 15.8 Three types of lift-off processes for Cu(In,Ga)(S,Se)2 solar cells. Source: Reprinted from Y.-C. Wang, T.-T. Wu, Y.-L. Chueh, A critical review on flexible Cu (In, Ga) Se2 (CIGS) solar cells, Mater. Chem. Phys. 234 (2019) 329
344.

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Regardless of the addition of alkali elements, another approach to improve CIGS cells is associated with changing the CdS buffer layer with more environmentally friendly and inexpensive materials like Zn-based compounds. In a simulation study, wide bandgap ZnS increased the PCE to 23.54% while it was around 22.6% for CdS [40]. Another promising enhancement strategy of CIGS solar cells is creating a V-shaped Ga gradient during the deposition process. This technique extremely decreases recombination at the CIGS/CdS interface and improves light absorption by CIGS film. Ga gradient is traditionally created by a low-temperature three-stage coevaporation method but the steep gradient produced in this case prevents sufficient charge transport and reduces PCE. Overlapping between the first two stages reduces In and Ga diffusion and guarantees a great back gradient. In this respect, conversion efficiency of 16.7% was achieved for 8 min of overlapping time [41]. Although toxicity of CIGS and scarcity of In are important obstacles, the maturity, high efficiency, and stability of flexible CIGS FSCs make them an ideal candidate for replacing Si solar cells in the PV market. Investigations in this area are highly focused on discovering alternative ways for the introduction of alkali elements like using alkali-containing targets and developing low-temperature processes for sulphurization and selenization of PI substrate to make large-scale production come true.

15.4.4 Organic solar cells Solar cells employing organic absorber layers have achieved multitudes of achievements in recent decades so that they are now considered competitive with inorganic-based solar cells or even superior in some specific applications. Typically, OSCs consist of an active layer sandwiched between two electrodes wholly deposited on a substrate as illustrated in Fig. 15.9A. In the case of OSCs, substrates are supposed to have a high Tg and low thermal expansion coefficient, in

Figure 15.9 Schematic representation of (A) a flexible organic solar cell with planar structure and (B) PEN/AgNWs/PEDOT:PSS/PEI-Zn/PBDB-T-2F:Y6 solar cell [43,44]. Source: Reprinted from C. Liu, C. Xiao, C. Xie, W. Li, Flexible organic solar cells: Materials, large-area fabrication techniques and potential applications, Nano Energy, 89 (2021) 106399 and F. Qin et al., Robust metal ion-chelated polymer interfacial layer for ultraflexible non-fullerene organic solar cells, Nat. Commun., 11(1) (2020) 1
8.

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addition, they should be transparent and stable in the operating conditions. Four OSCs based on poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylvinylene] (MEHPPV) with graphene/PEDOT:PSS/PET, graphene/PEDOT:PSS/PEDOT:PSS/PET, graphene/MEH-PPV/PEDOT:PSS/PET, and graphene/MEH-PPV/PEDOT:PSS/ PEDOT:PSS/PET structures were fabricated on PET substrate [19]. The last configuration reached a conversion efficiency of 0.016% which is extremely lower than that of rigid substrates due to the lower annealing temperatures that created some cracks within the cell. While a thermally stable OSC with PI-integrated graphene electrode with 92% transparency and sheet resistance of 83 Ω/sq reached a high PCE of around 15.2% [42]. As the substrate is the thickest part of an OSC, implementing ultrathin substrates highly contributes to mechanical stability under bending and stretching conditions. The absorber layers in OSCs are commonly donor and acceptor organic semiconductors behaving like p- and n-type semiconductors, respectively, and the chemical structures of some conventional organic semiconductors are demonstrated in Fig. 15.10. Despite the type of conductivity, active layers should have high adhesion and low modulus to avoid delamination. After the interaction of light with organic semiconductors, electron
hole pairs known as excitons are created. As the excitons binding energy is so high in organic semiconductors, another photoactive material is used to separate these excitons and create free charge carriers. Therefore, excitons diffuse to the interface of donor and acceptor and dissociate into free charge carriers and accumulate in electrodes. The simplest heterojunction between donors and acceptors is a planner heterojunction and thickening the active layer to more than the excitons diffusion length, contributes to more photon absorption, but it prevents efficient separation of charge carriers. Thus a bulk heterojunction comprising of a phase blend of acceptor small molecules (close to excitons diffusion length) distributed in a donor matrix of a conjugated polymer is much more preferred. As a consequence, excitons generated everywhere will be close enough to an interface for separation before going through recombination. Inverted bulk heterojunction OSCs with poly(3-hexylthiophene-2,5-diyl) (P3HT): PCBM phase blend active layer was spin-coated on both PET and glass substrates. According to the higher shunt resistance and superior interlayer connections of the nonflexible cell, it reached an efficiency of 2.52% while it was around 0.67% for the flexible one. In addition, the rigid and flexible cells maintained 51% and 19% of their initial performance after 4 weeks, respectively [45]. Polymers have been widely used as donor materials in flexible OSCs thanks to their higher film quality and morphology. Regardless of these advantages, polymers are not reproducible and cannot simply be purified, thus nonpolymeric diketopyrrolopyrrole-based donor small molecule was chosen to be used in the active layer of an OSC. To have an acceptable material distribution in the phase blend, it was thermally annealed and this solution-processed cell produced 4.4% 6 0.4% efficiency [46]. Even though fullerene-based acceptor molecules are traditionally more common in OSCs, they are almost replaced with nonfullerene ones due to their low stability and high cost in addition to the necessity of a purification process. The introduction of nonfullerene acceptor small molecules paved the way for OSCs to reach efficiencies

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Figure 15.10 Chemical structures of some common (A) donor and (B) acceptor organic semiconductors. Source: Reprinted from C. Liu, C. Xiao, C. Xie, W. Li, Flexible organic solar cells: materials, large-area fabrication techniques and potential applications, Nano Energy 89(2021) 106399.

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exceeding 10%. Solution-processed nonfullerene PET/PEDOT:PSS/PEDOT:PSS/ PM6:Y6/PDINN/Al OSC produced an efficiency of 16.61% after doping the anode by CF3SO3H which is the highest reported PCE for a flexible OSC [47]. To estimate the PV performance of a novel polymer with low bandgap called P1 made out of thiazolothiazole, benzothia diazole and thiophene units, two OSCs with IT-4F ((9-bis(2-me thylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2’,3’-d’]-s-indaceno [1,2-b:5,6-b’] dithiophene)) and PCBM active layers were fabricated. Both cells showed optimistic results while the nonfullerene one exhibited 0.6% higher efficiency. The superior performance of the nonfullerene cell was associated with its enhanced charge mobility and suppression of charge recombination [48]. A still remaining challenge of ultra-flexible OSCs with nonfullerene active layers is the chemical compatibility of interlayers with them and their mechanical robustness. In an investigation, Zn21chelated polyethylenimine (PEI-Zn) as an interlayer of PEN/AgNWs/PEDOT:PSS/ PEI-Zn/PBDB-T-2F:Y6 cell (Fig. 15.9B) increased its efficiency to 15.0%. Surprisingly, this ultra-flexible cell persevered nearly 100% of its efficiency after 100 cycles of compression-flat test with 45% compression ratio [44]. From another point of view, utilizing all-polymer OSCs not only contributes to stability but also enables printing processing under ambient conditions, which is totally favorable for their roll-to-roll fabrication. Moreover, the declined toughness caused by small molecule acceptors will highly be compensated. The final crucial issue to be discussed in the field of active layers in OSCs is the development of ternary cells. This strategy involves the addition of a third phase to the existing binary phase blend to improve their efficiency by harvesting more sunlight. A 1 cm2 flexible OSC based on PTB7-Th:COi8DFIC:PCBM phase blend produced an efficiency of 12.16% as compared to 12.37% for a rigid cell with 0.04 cm2 area. Over 99% of the initial efficiency was maintained after bending the cell for 1000 cycles with 10 mm radius [49]. In PET/PH1000/PEDOT:PSS/PM6:Y6:PC71BM/PDINO/Al ternary OSC, PC71BM enhanced the light absorption in the range of 300
500 nm and optimized the morphology of the blend film as the third component [50]. However, selection of the third component has still remained a challenge so that the homogeneous phase of the two acceptor constituents in this case enhanced the performance, otherwise, it would affect oppositely [51]. The photomorphogenesis phenomenon in plants occurs only under illumination with specific wavelengths, thus greenhouse glass should satisfy this requirement by filtering unnecessary light spectra. Replacing greenhouse glass with three different semitransparent OSCs such as a ternary one consisting of FTAZ:IEICO-4F:PC71BM enabled controlled red leaf lettuce growth and power generation simultaneously [52]. Electrodes in OSCs need to have a smooth surface while maintaining their adhesion to the substrates to avoid delamination and failure of the whole system under mechanical tests. In addition to these properties, the bottom electrode (whether anode or cathode) should be transparent as well. Furthermore, it is so important to note that the high sheet resistance of electrodes prevents further increases in the size of OSCs. A transparent and conductive Ag/Cu composite grid with a sheet resistance of less than 1 Ω/sq and a transparency more than 84% was obtained as

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the electrode of PET/Ag/Cu/E100/ZnO/PBDB-T:ITIC OSC. This cell with 1 cm2 area produced 12.26% PCE, but increasing the area to 9 cm2 declined it to 7.35% which was highly greater than that of ITO conventional electrodes [53]. To further increase transparency and mechanical stability of electrodes, nanowires are considered as good options so that AgNWs are known to be one of the main research concentrations in this area. A flexible large-area OSC with PET/Ag grid/AgNWs: PEI-Zn composite electrode reached an efficiency of 13.1% and 12.6% for 6 and 10 cm2 areas, respectively. A great fraction of this enhanced performance was attributed to the low surface roughness, low sheet resistance, and suitable optoelectronic and mechanical properties of the Ag-based transparent electrode [54]. In addition to basic essential components in an OSC, some interfacial layers such as electron and hole transport layers (ETLs and HTLs) are also considered crucial. These extra constituents quench large energy offsets between absorber layers, enhance charge separation, and contribute to adhesion. Bismuth oxychloride nanoplates as an ultralow-cost HTL were used in three OSCs based on P3HT: PC61BM, PTB7-Th:PC71BM, and PM6:Y6 active layers without any hightemperature posttreatment. The efficiencies increased from 3.62%, 8.78%, and 15.63% to 4.24%, 9.92%, and 16.11%, respectively, after the introduction of the novel HTL instead of PEDOT:PSS due to enough oxygen vacancies, transmittance, higher interfacial contact, and hydrophobicity [55]. After so many years of investigation, OSCs have reached a point that they can enter the PV industry on a large scale and take the market with their glomorous properties. For instance, various kinds of solar cells have been employed in wearable electronic and IoT applications but not all of them are mechanically stable and washable like OSCs. Despite multiple applications of flexible OSCs, the market demand is not serious yet. Maybe developing inexpensive and stable substrates, conjugated polymers, small molecules, transport layers, and interface materials can provide more opportunities for their commercialization.

15.4.5 Perovskite solar cells Outstanding properties such as tunable bandgap (1.24
3.55 eV), lower excitons binding energy (wannier type exciton), high charge mobility, high absorption coefficient, and ambipolar charge transport capacity of perovskite materials have made PSCs a second to none option among flexible PV technologies. These materials have the crystal structure of CaTiO3 mineral with ABX3 general formula (Fig. 15.11A) in which B is a divalent cation usually Pb21, Sn21, or Ge21 and X is a halogen in halide perovskites. Finally, A can be whether a monovalent alkali or an organic cation (CH3NH31 or CH3CH2NH31) in alkali halide perovskites and organic-inorganic halide perovskite materials, respectively. A totally cubic structure for perovskite materials is more attractive and beneficial for solar cell application. To determine the degree of distortion from this ideal state, t 5 (rA 1 rX)/O2(rB 1 rX) and µ 5 rB/rA factors (rA, rB, and rX are the atomic radii of corresponding ions) are useful criteria. In total, 0.88 , t , 1.1 and 0.45 , µ , 0.89 are postulated as the most reliable ranges to get a less distorted structure. Perovskite materials have been widely used in rigid

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Figure 15.11 Schematic representation of (A) perovskite structure, (B) working mechanism of PSCs with a cascade band alignment between components, and (C) MgF2/Willow glass/ TCO/SnO2/FAMACs/Spiro-MeOTAD/MoOx/Al cell [9,18,57]. Source: Reprinted from https://www.alibaba.com/product-detail/Stainless-Steel-PrecisionStrip-and-Foil_1605137043.html, B. Dou, et al., High-performance flexible perovskite solar cells on ultrathin glass: implications of the TCO, J. Phys. Chem. Lett. 8(19) (2017) 4960
4966, X. Li, P. Li, Z. Wu, D. Luo, H.-Y. Yu, and Z.-H. Lu, “Review and perspective of materials for flexible solar cells,” Mater. Reports Energy, vol. 1, no. 1, p. 100001, 2021, and P. Roy, N. K. Sinha, S. Tiwari, A. Khare, A review on perovskite solar cells: evolution of architecture, fabrication techniques, commercialization issues and status, Sol. Energy 198 (2020) 665
688.

solar cells with various architectures, but the first attempt to employ them as an active layer in FSCs goes back to 2013 [56]. Mesostructures used for PSCs require high fabricarion temperatures, subsequently, a plannar structure same as inorganic FSCs is more desirable. Planner PSCs regularly consist of a hole (p-type) and an electron (n-type) transport layer, a metallic anode, and a TCO cathode in addition to the perovskite-based active layer so that they maintain a cascade band alignment as it is shown in Fig. 15.11B. This architecture may have whether an n-i-p configuration or an inverted one like the others with reverse order of HTL and ETL deposition. HTLs in PSCs should withdraw holes from the active layer and transfer them to the cathode while blocking electrons simultaneously. Both organic and inorganic materials have been used as HTLs in flexible PSCs. Transparent PEDOT:PSS is a well-known HTL for these cells due to its high hole conductivity, proper band edges, and low cost. PEDOT:PSS as HTL in rigid and flexible (glass or PET)/ PEDOT:PSS/CH3NH3PbI3/PCBM/Ag cell produced 11% and 8% efficiency, respectively [58]. After all, its acidic behavior degrades perovskite and cathode layer so that the cell may need further modifications. Doped Spiro-OMeTAD, PTAA, and P3HT are some other efficient promising candidates for flexible PSCs. In the same manner, ETLs should transfer electrons to the anode and suppress the diffusion of holes. The suitable position of band edges and high stability of TiO2 are extremely favorable for PSCs, but not for flexible cells due to the necessity of high-temperature treatments. Therefore other oxides including ZnO, SnO2, and Nb2O3 are considered as some probable inorganic alternatives. A flexible PSC with SnO2 ETL fabricated below 100oC reached an efficiency of 18.1% and preserved % [59]. The introduction of a TCO layer between 85% of it after 1000 bending cycles

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the substrate and SnO2 ETL enhanced the cell performance by lowering the sheet resistance in the case of indium zinc oxide (IZO) and increasing the current density after the addition of ITO (Fig. 15.11C) [18]. Among organic candidates, fullerene and its derivatives like PCBM with its good conductivity are more frequently used than other materials. For future investigations, hybrid ETLs comprising of both organic and inorganic materials may be better options so that the low conductivity of inorganic ETLs is compensated with the organic ones. Porous Zn2SnO4/SnO2 ETL with a great charge collection behavior reached 20.8% PCE which is the highest recorded efficiency for a flexible PSC [60]. Among various perovskite materials, MAPbI3 has been widely used as the active layer. However, the low thermal stability and efficiency of MAPbI3 have led to its slight replacement with FAPbI3 with a narrower bandgap. Although FAPbI3 performed better than MAPbI3, high-temperature postheating treatments were a limiting factor for flexible cells. To overcome these barriers, mixed-ion systems with high efficiency and thermal stability were introduced. Quintile monovalent cations of Rb, K, Cs, MA, and FA reduced recombination of charge carriers and produced 19.11% efficiency in PEN/ITO/SnO2/perovskite/Spiro-OMeTAD/Au cell [61]. However, as lead-based perovskites are more common than other compounds for PSCs, hence, toxicity and environmental risks are among the challenges for mass production. Tin is supposed to be an appropriate replacement for lead since their halides almost have identical optoelectronic properties. In spike contrast, the PV performance of tin-based PSCs is weaker in line with its lower chemical stability and unsuitable band alignment with other components. After all, employing innovative optimization techniques guarantees the replacement of more environmentally friendly PSCs with their lead-based counterparts. Flexible PSCs have improved with an unprecedented speed in less than a decade but stability challenges hinder their further improvements and mass production. The diffusion of water and oxygen molecules to solar cells is an integrated part of terrestrial applications and flexible PSCs are not an exception. A great number of solutions have been reported to fortify the environmental stability of these cells including the introduction of dopants, buffer layers, passivation layers, or employment of encapsulation methods. Utilizing a carbon-based back contact electrode and Cr buffer layer for carbon/Cr/PCBM:PMMA/perovskite/PTAA/IZO/PET with 1 cm2 area prolonged its stability to 1000 h under operation conditions [62]. In addition, illumination, high temperature, and electrical bias may degrade perovskite materials from another respect. When PSCs are exposed to illumination, the central ion, usually MA, can migrate through the structure due to its low activation energy converting the 3D structure of the layer to a 2D one and exacerbating recombination while degrading PSCs PV performance. Some very common solutions would be replacing the central cation with a larger one so that it has the size of the octahedral site or employing mixed-ion materials. However, ionic defects with high mobility contribute to structure decomposition and phase separation in this case. Regulating the A cation position to get a tolerance factor around 1 by partially substituting FA with ethylammonium cation increased the efficiency to 13% by reducing trap states to more than 1 order of magnitude [63].

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Low-temperature processing and extremely high-efficiency values have paved the way for flexible PSCs in the flexible PV market, but their pragmatic application is way too far. Despite their significant breakthroughs and widespread applications, limitations like mechanical robustness, environmental stability, toxicity, and high cost of charge transport layers need to be overcome for their scale-up production.

15.5

Fexible electrodes

As is already stated, electrodes are an integral part of the solar cells fabrication process in most cases. Normally, the first electrode deposited on the substrate is a transparent and conductive layer while the second electrode on the active layer is purposed to be a reflective layer, although an inverted structure is also possible depending on the cell configuration. Obviously, electrodes in flexible devices should be mechanically flexible to ensure roll-to-roll processing, but widely used electrodes like TCOs fall short of this property. In this case, polymers and to a lower extent thin metals and nanocarbons offer superior performance over TCOs and some of their physical properties are compared in Fig. 15.12. Nonetheless, these flexible electrodes suffer a lack of transparency necessary for an electrode.

Figure 15.12 Relative comparison of five common flexible electrodes in terms of their physical properties (5 5 poor and 10 5 excellent). Source: Reprinted from https://www.alibaba.com/product-detail/Stainless-Steel-PrecisionStrip-and-Foil_1605137043.html.

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Thining metals and the incorporation of some specific elements highly influence their transparency, but decreasing thickness to less than a critical limit reduces PV performance. Fortunately, transparency is not a serious problem for carbon-based electrodes, for instance, graphene provides more than 95% light transmittance. From another perspective, reducing the thickness degrades materials conductivity which is a key feature, especially for bottom electrodes. In a nutshell, the trade-off between flexibility, transparency, and conductivity has still remained an issue to be addressed. Managing phase separation in PEDOT:PSS as a conductive polymeric electrode by doping it with a fluorosurfactant enabled higher mechanical flexibility and increased conductivity and transparency to .4000 S/cm and 80%, respectively in PET/PEDOT:PSS/HTL/Peroveskite/PCBM/top electrode. PCEs recorded for this device were 19% and 10.9% for 0.1 and 25 cm2 areas, respectively, which were in the order of rigid electrodes. These two cells maintained more than 80% of their initial efficiency after a bending test with 5000 cycles and 3 mm radius [64]. More importantly, electrodes should have these properties and maintain them under various conditions including high annealing temperatures, diffusion of oxygen and water molecules, and UV radiation the same as other components to ensure longterm effective performance. Generally speaking, metals weather thin metals or nanowires are more stable under these conditions especially when exposed to high temperatures. However, modifying these materials by doping or other methods to improve their optical properties may degrade their stability to some extent. Coating transparent AgNWs by amorphous AZO layer without any pinholes instead of doping improved the electrode stability against chemical attacks. If the nanowires were not completely covered, they would disappear through the diffusion mechanism, but a sufficient coverage (AZO/AgNW/AZO composite) protected the electrode properly with 88.6% transparency [65].

15.5.1 Metals Metals have been widely employed as electrodes in FSCs both as thin films and nanowires due to their semitransparency and high conductivity. AgNWs networks with excellent conductivity and transparency as well as enough flexibility are a decisive option. To surpass the localized short circuit at the metal/silicon interface in an organic-inorganic hybrid nanowire solar cell with PEDOT:PSS/Si nanowire (SiNW) structure, an Ag/SiO2 electrode was inserted at PEDOT:PSS/Si interface. This newly positioned microgrid electrode prevented optical losses and improved the PCE to 16.1% [66]. To estimate the PV performance of the ZnO fiberreinforced composite, two AgNW and AgNW/ZnO composite electrodes were spray-coated on top of a PSC. In the case of Ag nanowire, sheet resistance and transmittance decreased after a while due to its higher density leading to an efficiency of 5.35%. Coating the AgNW electrode with ZnO nanoparticles contributed to its conductivity by filling the gaps between nanowires and enhanced light transmittance due to the scattering effect of these particles, thus the efficiency increased to 8.18% for a cell with 1 cm2 area [67]. However, the short length (1
50 μm) of Ag nanowires threatens their integrity as a film. In addition to nanowires, metal

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grids and mesh are also common types of metallic electrodes because of their improved light scattering and absorption, especially for OSCs. A photolithographyprocessed nickel/gold (Ni/Au) hexagonal mesh was employed as the electrode of glass/Ni-Au mesh/PH1000/PEDOT:PSS/MAyFA12yPbIxCl32x/PCBM/BCP/Ag PSC. Au improved the overall conductivity and the mesh transmittance was around 80%. This PSC with 0.09 cm2 area obtained 13.88% efficiency comparable with 15.7% for ITO and had no significant current leakage. This lower value of efficiency was attributed to higher recombination and lower charge extraction rates of metal mesh electrodes which can easily be solved by improving the interface and perovskite film quality [68].

15.5.2 Carbon The fascinating optical and mechanical properties of carbon nanomaterials, specifically single-wall carbon nanotubes (SWNTs) and graphene, along with their hydrophobicity and earth-abundance have motivated scientists to modify them for PV applications. However, the shallow work function and low reflective coefficient of carbon nanotubes (CNTs) prevent further improvements in their performance. Utilizing a novel doping process like ex situ vapor-assisted methods by employing trifluoromethanesulfonic acid as the dopant has opened a new horizon toward better PV performance of CNTs by decreasing the sheet resistance by 21.3%. A FACsPbI3-based PSC with this doped CNT-based electrode produced 17.6% efficiency and 24.2 mA/cm2 photocurrent density [69]. Direct drop-casting of triflic acid in nonpolar o-dichlorobenzene solvent had a great doping effect on CNTs and improved their stability. Implementing these doped and optimized top electrodes for a PSC increased the efficiency from 18.4% for a metal-based electrode to 18.8% [70]. To compare the performance and mechanical stability of SWNTs with graphene, they were separately used as the bottom electrode of an inverted PSC. Both carbon-based cells had lower efficiency values than that of inverted cells with an ITO electrode; PCE decreased from 17.8% to 12.8% and 14.2% for nanotube and graphene, respectively [71].

15.5.3 Polymers Polymers with enough transparency and high flexibility including solution-processed PEDOT:PSS have recently been used as electrodes in PV applications. To overcome low conductivity of polymers as electrodes, employing various doping methods or producing composites from them are known to be some effective solutions. Although polar molecules and acids are some strong doping options, they are postulated to be detrimental to the environment. An environmentally friendly approach is utilizing oxygen plasma treatment along with some posttreatments and the addition of solvent blends. This method decreased the resistance of PEDOT:PSS film from 85 to 15 Ω/sq corresponding to 5012 S/cm conductivity [72]. A composite electrode consisting of graphene dots and PEDOT:PSS on paper in a DSSC improved the conductivity and carrier transport by filling the porous structure of paper [73]. A 2D metal-organic

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material based on Cu-BHT (BHT 5 benzenehexathiol) with more than 2500 S/cm conductivity and 82% transparency led to an almost equal performance to an ITO electrode for a PSC and an OSC [74].

15.6

Conclusion

Adequate exciting breakthroughs of FSCs have put them among the most likely candidates for flexible power systems. This chapter provides a comprehensive assessment of key components of FSCs. In the substrates section, various flexible materials suitable for this application each with an outstanding feature were reviewed, though stainless steel and PI seem to be more realistic options. In the case of absorber layers, CIGS cells with their high-efficiency values and dramatic stability under operation conditions are comparable to silicon solar cells. However, high-efficiency PSCs are in the limelight owing to their low-temperature fabrication techniques. The unstoppable trend of the progress of flexible OSCs and PSCs promises a bright future for their real-life applications. In the last section, more conventional electrode materials were introduced and it is predicted that carbon-based electrodes will dominate the PV market if their synthesis process is optimized. As it is mentioned, it is undeniable that challenges like efficiency, stability, and toxicity hinder large-scale production of FSCs and they all need to be addressed to narrow the gap between flexible and rigid solar cells.

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381.

Emerging applications of ceramics in flexible supercapacitors

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Rajashree Samantray and Subash Chandra Mishra Department of Metallurgical and Materials Engineering, National Institute of Technology Rourkela, Rourkela, Odisha, India

16.1

Introduction

The present energy demand has engraved a major concern among people that the requirement is now doubled. Fossil fuels and other conventional energy sources are causing fast depletion, leading to the devastation of bionetworks and surroundings. Over a few decades, a primary concern has been elevated to work on renewable energy sources such as solar energy, wind, biofuel, and electrochemical energy storage systems including batteries, supercapacitors and so on. With fast energy storage and delivery rate in a short span, supercapacitors are categorized as electric doublelayer capacitors (EDLCs) and pseudocapacitors. Carbon materials are in the former type while transition metal oxides or conducting polymers are major electrodes in the latter type [1]. The commercial supercapacitor available on market has delivered a specific energy density of up to 10 Wh/kg. However, a specific energy density of 35
40 Wh/kg is achieved for batteries. Metal oxides or ceramic oxides stand eligible for high-performance supercapacitors due to their significant specific capacitance in a low resistance, outstanding stability and high energy as well as power density. The ceramics show a pseudocapacitance, due to reversible redox reactions in the electric double-layer storage system. A wide range of metal oxides, nitrides, hydroxides, carbides, and sulfides are attempted till date.

16.2

Electrode materials

The construction of a supercapacitor mainly comprises electrode material. Some of the key considerations for an electrode in supercapacitors containing, less toxic, environmentally safe, economic, wide availability on earth, lightweight and so on. The electrode materials are differentiated based on the type of supercapacitor used. For EDLC supercapacitors, carbon electrodes are mostly used, while metal oxides or ceramics are applicable in pseudocapacitors. The performance of a supercapacitor is determined by energy density and power density, which depends on capacitance, equivalent series resistance and cyclic voltage. The major factors viz. electrode material, cycle stability and operating voltage are the most important parameters determining electrochemical Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00016-6 © 2023 Elsevier Ltd. All rights reserved.

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performance. An extensive range of electrode materials like metals, polymers, ceramics, etc., are gathered till date. However, ceramic materials are often ignored due to trifling setbacks of low specific surface area and low surface activity. Yet, they possess good corrosion resistance, high-temperature resistance along with radiation and thermal shock resistance, making them appropriate for electrochemical applications. Ceramic oxides or transition metal oxides work as promising materials for electrodes in the energy storage system since they possess high specific capacitance and energy density. Surface redox reaction works as a working principle here, which causes better electrochemical performance. Some of the widely used oxides are RuO2, IrO2, etc., show good conductivity and capacitive properties, that are hindered in supercapacitor applications due to their high cost and harmfulness. Yet, few oxides viz. MnO2, NiO, Co3O4 depict cost-efficient, environment friendly as well as significant electrochemical properties. Apart from the advantages, low cyclability, weak surface area and depleted power density are the factors causing weaker oxide electrodes. A few oxides used till date are presented as follows.

16.2.1 Ruthenium oxide Ruthenium oxide (RuO2) [2,3] is also known as the first-generation electrode material, resulting in a faradic charge transport reaction. It works well as an electrode material due to high specific capacitance, low resistance [4], high rate capability, broad electrochemical window and long cyclic efficiency. RuO2 has three oxidation states within an operating voltage of 1.2 V. The specific capacitance of the electrode material is usually smaller than that of theoretical value due to high crystallinity and confined power. The electrochemical performance of the RuO2 electrode depends on annealing temperature, crystalline nature and particle sizing. The annealed temperature as well as particle size results in supercapacitor performance in terms of ion transport and high charge
discharge rate capability [5]. Even with possessing certain advantages, RuO2 is limited in electrochemical applications due to its high cost and rare source.

16.2.2 Manganese oxide Manganese oxide (MnO2) [6] have gathered noticeable research to overcome setbacks in RuO2, that is, high cost. Significant theoretical specific capacitance (1370 F/g), low cost, less toxicity, ample accessibility, and environment friendliness are some of the acknowledged advantages of MnO2 over other oxides. It has been in use in a wide range of applications including supercapacitors [7], Li-ion batteries [8], sensors [9], etc. The first work on MnO2 electrodes for supercapacitor applications was discussed in 1999, by Lee and Goodenough. The faradic reactions on the electrode surface are considered as a major mechanism here [10]. The electrochemical performance of MnO2 has several crystal structures (α-, β-, γ-, δ- and λ-). The allotropes play a major role in specific capacitance determination as it decreases in an order of α  δ . γ . λ . β. The pseudo-capacitance in MnO2 is mainly due to

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reversible redox reaction, involving conversation of protons and/or cations with the electrolyte and evolution between various oxidation states. There are two charge storage mechanisms. Firstly, the surface faradic process that includes electrolyte cations C1 5 H1, Li1, K1, Na1 due to adsorption/desorption of electrolytes. ðMnO2 ÞSurface 1 C1 1 e2 2ðMnOOCÞSurface

(16.1)

While the faradic process comprises surface adsorptions of electrolyte cations on the electrode. MnO2 1 C1 1 e2 2MnOOC

(16.2)

16.2.3 Cobalt oxide Cobalt oxide has a cubic structure with lattice constant a 5 0.808 nm (JCPDS-42-1467). They have achieved popularity because of their huge surface area, outstanding pseudocapacitance properties, high theoretical capacitance (3560 F/g) and long-term cyclic stability. Yet, they possess a low potential window, which limits practical application. The redox reaction of Co3O4, can be described as Co3 O4 1OH2 1H2 O23CoOOH1e2

(16.3)

CoOOH1OH2 2CoO2 1 H2 O1e2

(16.4)

Furthermore, the synthesis of meso and microporous nanostructured Co3O4 has been done with chemical bath deposition method [11], hydrothermal method [12], solvothermal synthesis method [13] and combustion synthesis method [14], etc. The hierarchical porous Co3O4 on the rGO nanosheet demonstrated a specific capacitance of 688 F/g at 0.5 mA current. The excellent electrochemical performance was due to ion transfer and agglomerated Co3O4 on rGO surface [15]. Composites of Co3O4 and N-doped MWCNT/poly pyrrole were fabricated by Ramesh et al. [16] and acquired capacitance of 872 F/g at 0.5 A/g current density with 96.8% capacitance retention at 10000 cycles. A hydrothermal route was followed to prepare Co3O4 @Ni material and performed 97% of electrochemical stability after 10000 cycles in 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide ionic liquid electrolyte [17].

16.2.4 Iron oxides Iron oxides are earthly abundant, nontoxic, and possess high physical and chemical stability. The oxides are usually comprised of three types of valence states viz. Fe21, Fe31, and Fe41. The widespread element shows high theoretical capacitance (B3625 F/g), wide operational voltage window, abundant availability, chemical stability and environmentally safe, making it an ideal candidate for supercapacitors [18].

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However, low surface area, depleted conductivity, and inferior cyclability make the electrode challenging for rate performance and practical applications. In recent years, iron oxide-based carbon composites have been prepared and performed in electrochemical systems. Zhu et al. [19] reported Fe3O4
30 glycerol nanorods showed high specific capacitance up to 426.4 F/g at a current density of 1 A/g and 79.5% cycle stability at 5000 cycles. Hsiao et al. [18] incorporated rGO, oyster, and iron oxide (Fe2O3) composite electrode and reported a specific capacitance of 473.9 F/g at 5 mV/s in an aqueous 1 M Na2SO3 electrolyte. Lignin-based nanofibers with Fe3O4 nanoparticles were synthesized by Butnoi et al. [20]. The composite exhibited a specific capacitance of 216 F/g at 0.1 A/g current density with excellent energy density up to 43 Wh/kg. The study involves a composite of iron oxide with carbon composites to make a suitable electrode material for economic and robust energy storage application.

16.2.5 Vanadium oxides Vanadium oxides involve in high electrochemical properties due to high power density, abundancy, high theoretical capacitance up to 2120 F/g and multiple oxygenation compounds such as; vanadium monoxide (VO), vanadium trioxide (V2O3), vanadium dioxide (VO2), vanadium pentoxide (V2O5), V3O7, V6O13, V4O9, etc. [21]. Among them, V2O5 and VO2 are two types of vanadium oxides that are determined as two potentially active cathode materials for electrochemically storage material. High capacitance, high operating voltage, and superior cycle stability are major factors responsible for highperformance storage applications [22]. The sulfur-doped cobalt vanadium nanosheets (S-Co3V2O8) were synthesized by the hydrothermal method. A specific capacity of 410 mAh/g was accomplished at the current density of 2 A/g in an aqueous alkaline electrolyte. A cyclic ability of 94.2% was recorded at 4000 cycles at 5 A/g. Likewise, in the nonaqueous electrolyte, S-Co3V2O5 displayed a specific capacity of 994 mAh/g with 828 Wh/kg energy density at 1 A/g. The electrochemical analysis of S-Co3V2O5 electrode in nonaqueous electrolyte is recited in Fig. 16.1 [23]. The composites of V2O5 with vertically aligned carbon nanotube arrays (VACNTs) equipped with CO2 incorporation, followed by an annealing process, the electrode prepared, that is, V2O5/VACNTs displayed a specific capacitance of 284 F/g at 2 A/g in aqueous Na2SO4 electrolyte [24]. The vanadium oxide composite exhibited outstanding energy storage performance and stands promising for supercapacitor applications.

16.2.6 Tin oxide Among all the transition metal oxides, tin oxide is believed to be a strong electrode material for supercapacitors. These oxides in particular show a high-performance capacitive behavior due to advantageous properties like low cost, less toxicity, high thermal stability. Although, low electric conductivity, and poor rate capability are the challenges that restrict its commercial productivity. Tin oxides have shown noteworthy specific capacitance of 31.50 mAh/g at 0.1 A/g. A good rate capability and high specific

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Figure 16.1 Electrochemical classification of electrode S-Co3V2O8 in a nonaqueous medium. (A) CV images, (B) GCD images at varied scan rates, (C) specific capacity as a function of specific currents, (D) rate capability test, (E) Ragone plot, and (F) stability analysis and Coulombic efficiency (inset: FESEM image of S-Co3V2O8).

capacitance were noted for Ni
Sn alloys, with unique alloyed structure along with high cyclability making it an apt candidate for future supercapacitor applications [25]. Tin oxide has been fabricated with several nanomaterials such as CNTs [26], carbon cloth [27], carbon nanofiber [28], graphene oxide [29] etc. with an aim of achieving significant conductivity and high rate capability. The earlier reports have been evidenced by several authors on tin oxide. Hong et al. [27] described a functionalized carbon cloth with SnO2 fabrication and accomplished capacitance of 197.7 F/g at 1 A/g with 95.5% capacitance retention at 5000 cycles. Babu et al. [30] demonstrated SnO2 quantum dots under N2 annealing environment and gathered a specific capacitance of 79.13 F/g at 1 mA current with 99.4% cyclability at 5000 cycles. Besides, Cao et al. [28] prepared a nanocomposite of porous carbon nanofibers with tin oxide and attained specific capacitance up to 229 F/g at 0.2 A/g. The elevated electrochemical performance was ascribed to high specific surface area and pseudocapacitance of SnOx.

16.2.7 Vanadium nitride In addition to the oxides, nitrides and other compounds are also debated so far in supercapacitor applications. Vanadium nitride is the most popular composite due to its favorable properties like electrical conductivity (1.67 3 106 S/m), superior specific capacitance (1340 F/g), high operating voltage window and inexpensive nature. High specific capacitance is achieved due to pseudocapacitance contribution from the electrode material [31]. Vanadium possesses various oxidation states, which as a result responsible for high specific capacitance and charge
discharge rate. However, poor cycle stability and less cycle life are the major drawbacks of the compound [32].

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Figure 16.2 (A) CV images at a scan rate of 500 mV/s. (B) CV curves at different voltage windows. (C) CV curves at different scan rates. (D) GCD curves at various current densities (inset: Coulombic efficiency vs current density). (E) Cycling behavior and Coulombic efficiency at a current density of 2.0 mA/cm. (F) Energy density vs. power density of the VN and TiNbN electrodes compared with earlier reported metal nitrides in literature.

The composite of niobium titanium nitride (TiNbn) with vanadium nitride (VN) was fabricated and the electrochemical performance of the device was recorded. A high operating voltage window of 1.6 V with an energy density of up to 74.9 mWh/cm3 and power density of 8.8 W/cm3 was documented (Fig. 16.2). The high-performance supercapacitor not only proved vanadium nitride as a suitable material for supercapacitors but also showed excellent electrochemical properties [33].

16.2.8 Titanium nitride Titanium nitride (TiN) is a promising candidate for electrode material for supercapacitor application. Low cost, thermal stability, good electrical conductivity, high cycle life and capacitance are the advantages of the compound. TiN was used as electrode material in the charging
discharging of supercapacitors due to its high conductivity. Shi et al. [34] prepared silicon nanowires/Titanium nitride (SiNWs/ TiN) electrode showed excellent cycle stability. The mechanism of TiN/Ni in charge/discharge, surface potential mapping was conducted. The intercalation/deintercalation mechanism of K 1 in fabricated supercapacitors resulted in obvious changes before and after CV cycles [35]. The mechanism to produce pseudocapacitance of TiN, which is more complicated in vanadium nitride, and is attributed to the charge storage process. In TiO2 and NiO, the oxygen vacancies have been formed as an important participant to stimulate capacitive behavior along with the electrochemical performance of TiN [36]. The electrochemical performance of the electrodes is optimized to improve cyclic performance and enhance capacitive behavior. With a selection of proper electrolytes, electrode material promotes remarkable capacitive performance and cycling stability.

Emerging applications of ceramics in flexible supercapacitors

16.3

359

Summary

Supercapacitors are considered as a powerful tool in energy storage systems. Preparation and selection of electrode material with proper electrolytes and synthesis process are key challenges to develop supercapacitors. Ceramics are proven to be entirely promising electrode candidates to achieve the high-rate capability and high capacitive properties. Herein, the various ceramics used till date and progress made with the design, fabrication and modification of nanostructured transition metal oxides and nitrides as well as a mechanism in different electrodes are discussed. Self-supporting, binder-free, high surface area ceramic electrodes will be a pattern in future advancement since this incredibly simplifies the device, making it productive. Simultaneously, surface activation of electrode material and a good and productive electrolyte implementation is believed to be important criterion.

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[11] X.H. Xia, J.P. Tu, J. Zhang, X.H. Huang, X.L. Wang, W.K. Zhang, et al., Enhanced electrochromics of nanoporous cobalt oxide thin film prepared by a facile chemical bath deposition, Electrochem. Commun. 10 (2008) 1815
1818. Available from: https:// doi.org/10.1016/J.ELECOM.2008.09.025. [12] G. Wang, X. Shen, J. Horvat, B. Wang, H. Liu, D. Wexler, et al., Hydrothermal synthesis and optical, magnetic, and supercapacitance properties of nanoporous cobalt oxide nanorods, J. Phys. Chem. C 113 (2009) 4357
4361. Available from: https://doi.org/ 10.1021/JP8106149. [13] M. Pang, G. Long, S. Jiang, Y. Ji, W. Han, B. Wang, et al., Ethanol-assisted solvothermal synthesis of porous nanostructured cobalt oxides (CoO/Co3O4) for highperformance supercapacitors, Chem. Eng. J. 280 (2015) 377
384. Available from: https://doi.org/10.1016/J.CEJ.2015.06.053. [14] W. Wen, J.M. Wu, J.P. Tu, A novel solution combustion synthesis of cobalt oxide nanoparticles as negative-electrode materials for lithium ion batteries, J. Alloy. Compd. 513 (2012) 592
596. Available from: https://doi.org/10.1016/J.JALLCOM.2011.11.019. [15] S. Jadhav, R.S. Kalubarme, N. Suzuki, C. Terashima, B. Kale, S.W. Gosavi, et al., Probing electrochemical charge storage of 3D porous hierarchical cobalt oxide decorated rGO in ultra-high-performance supercapacitor, Surf. Coat. Technol. 419 (2021) 127287. Available from: https://doi.org/10.1016/J.SURFCOAT.2021.127287. [16] S. Ramesh, K. Karuppasamy, Y. Haldorai, A. Sivasamy, H.S. Kim, H.S. Kim, Hexagonal nanostructured cobalt oxide @ nitrogen doped multiwalled carbon nanotubes/polypyyrole composite for supercapacitor and electrochemical glucose sensor, Colloids Surf. B Biointerfaces 205 (2021) 111840. Available from: https://doi.org/10.1016/J.COLSURFB.2021.111840. [17] M. Marzouki, R. Zarrougui, O. Ghodbane, Application of aprotic ionic liquids based on bis(trifluoromethylsulfonyl)imide anion as polymer gel electrolytes for cobalt oxide symmetric supercapacitors, J. Energy Storage 40 (2021) 102761. Available from: https://doi.org/10.1016/J.EST.2021.102761. [18] C. Hsiao, C. Lee, N. Tai, Reduced graphene oxide/oyster shell powers/iron oxide composite electrode for high performance supercapacitors, Electrochim. Acta 391 (2021) 138868. Available from: https://doi.org/10.1016/J.ELECTACTA.2021.138868. [19] M. Zhu, Q. Luo, Q. Chen, W. Wei, Q. Zhang, S. Li, Glycerol-assisted tuning of the phase and morphology of iron oxide nanostructures for supercapacitor electrode materials, Mater. Chem. Front. 5 (2021) 2758
2770. Available from: https://doi.org/10.1039/ D0QM00900H. [20] P. Butnoi, A. Pangon, R. Berger, H.J. Butt, V. Intasanta, Electrospun nanocomposite fibers from lignin and iron oxide as supercapacitor material, J. Mater. Res. Technol. 12 (2021) 2153
2167. Available from: https://doi.org/10.1016/J.JMRT.2021.04.017. [21] R. Velayutham, R. Manikandan, C.J. Raj, A.M. Kale, C. Kaya, K. Palanisamy, et al., Electrodeposition of vanadium pentoxide on carbon fiber cloth as a binder-free electrode for high-performance asymmetric supercapacitor, J. Alloy Compd. 863 (2021) 158332. Available from: https://doi.org/10.1016/J.JALLCOM.2020.158332. [22] R.S. Kate, S.A. Khalate, R.J. Deokate, Overview of nanostructured metal oxides and pure nickel oxide (NiO) electrodes for supercapacitors: a review, J. Alloy Compd. 734 (2018) 89
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[24] G. Sun, H. Ren, Z. Shi, L. Zhang, Z. Wang, K. Zhan, et al., V2O5/vertically-aligned carbon nanotubes as negative electrode for asymmetric supercapacitor in neutral aqueous electrolyte, J. Colloid Interface Sci. 588 (2021) 847
856. Available from: https:// doi.org/10.1016/J.JCIS.2020.11.126. [25] Y. Tian, Q. Wang, Z. Peng, S. Guan, X. Fu, Ni foam-supported tin oxide nanowall array: an integrated supercapacitor anode, Molecules 26 (2021) 4517. Available from: https://doi.org/10.3390/MOLECULES26154517. [26] A. Abdulhameed, M.N. Mohtar, M.N. Hamidon, I. Mansor, I.A. Halin, The role of the AC signal on the dielectrophoretic assembly of carbon nanotubes across indium tin oxide electrodes, Microelectron. Eng. 247 (2021) 111597. Available from: https://doi. org/10.1016/J.MEE.2021.111597. [27] X. Hong, S. Li, R. Wang, J. Fu, Hierarchical SnO2 nanoclusters wrapped functionalized carbonized cotton cloth for symmetrical supercapacitor, J. Alloy Compd. 775 (2019) 15
21. Available from: https://doi.org/10.1016/J.JALLCOM.2018.10.099. [28] M. Cao, D. Wang, J. Lu, W. Cheng, G. Han, J. Zhou, Electrospun porous carbon nanofibers @ SnOx nanocomposites for high-performance supercapacitors: microstructures and electrochemical properties, Compos. Part A: Appl. Sci. Manuf 143 (2021) 106278. Available from: https://doi.org/10.1016/J.COMPOSITESA.2021.106278. [29] T. Kim, E.P. Samuel, C. Park, Y. Il Kim, A. Aldalbahi, F. Alotaibi, et al., Wearable fabric supercapacitors using supersonically sprayed reduced graphene and tin oxide, J. Alloy Compd. 856 (2021) 157902. Available from: https://doi.org/10.1016/J.JALLCOM.2020.157902. [30] B. Babu, B. Talluri, T.R. Gurugubelli, J. Kim, K. Yoo, Effect of annealing environment on the photoelectrochemical water oxidation and electrochemical supercapacitor performance of SnO2 quantum dots, Chemosphere 286 (2022) 131577. Available from: https://doi.org/10.1016/J.CHEMOSPHERE.2021.131577. [31] M. Balogun, W. Qiu, W. Wang, P. Fang, X. Lu, Y. Tong, et al., Recent advances in metal nitrides as high-performance electrode materials for energy storage devices, J. Mater. Chem. A (2014). Available from: https://doi.org/10.1039/C4TA05565A. [32] V.V.A. Thampi, U. Nithiyanantham, A.K. Nanda Kumar, P. Martin, A. Bendavid, B. Subramanian, Fabrication of Sputtered Titanium Vanadium Nitride (TiVN) Thin Films for Micro-Supercapacitors 29 (2018) 12457
12465. Available from: https://doi.org/ 10.1007/s10854-018-9364-x. [33] B. Wei, F. Ming, H. Liang, Z. Qi, W. Hu, Z. Wang, All nitride asymmetric supercapacitors of niobium titanium nitride-vanadium nitride, J. Power Sources 481 (2021) 228842. Available from: https://doi.org/10.1016/j.jpowsour.2020.228842. [34] J. Shi, et al., Sputtered titanium nitride films on nanowires Si substrate as pseudocapacitive electrode for supercapacitors, Ceramics Int. (2021). Available from: https://doi. org/10.1016/j.ceramint.2021.06.084. [35] A.S. Qadeer, et al., A multifunctional TiN/Ni electrode for wearable supercapacitor and sensor with an insight into charge storage mechanism, App. Sur. Sci. 555 (2021) 149718. Available from: https://doi.org/10.1016/J.APSUSC.2021.149718. [36] Y. Zhou, A review on transition metal nitrides as electrode materials for supercapacitors, Ceramics Int. 45 (17) (2019) 21062
21076. Available from: https://doi.org/ 10.1016/j.ceramint.2019.07.151.

Flexible ceramics for microfluidicsmediated biomedical devices

17

Ebenezer Olubunmi Ige1,2, Ayodele James Oyejide2,3 and Adijat Omowumi Inyang4 1 Department of Mechanical and Mechatronic Engineering, Afe Babalola University, AdoEkiti, Nigeria, 2Department of Biomedical Engineering, Afe Babalola University, Ado-Ekiti, Nigeria, 3Department of Biomedical Engineering, University of Ibadan, Ibadan, Nigeria, 4 Ansyl Technologies Limited, Claremont, Cape Town, South Africa

17.1

Introduction

Native ceramics are known for their characteristics of rigidity for which it has attained remarkable use across a wide spectrum of industrial applications, especially in microelectronics. However, with redefining protocol in manufacturing and a shift in fabrication and material procession technology, innovations in ceramic technology have drifted markedly to the adoption of the term “flexible ceramics.” In ceramics, being flexible implied the ability to bend, fold and shape processed ceramics to suit production requirements. In addition to the present capability to produce foldable, stretchable thin-film ceramics from natively rigid ceramic-based materials, flexible ceramics could also be impacted with desired properties to match required biocompatibility requirements in the biological environment [1]. Thus a flexible and biocompatible ceramicbased material could become a candidate material for application for microfluidic devices in biomedical domains. Hence, one could ascertain that flexible ceramic materials used in microfluidic industries are tailored by certain microfabrication protocols to meet the desired conditions in microenvironments for which the miniaturized device is intended to be used. Thereafter, the flexibility impacted on ceramics by fabrication and postfabrication treatments can bestow a versatile functionality on native ceramics with previously monotonic applications. In addition, the possibility to enact integrated lowvoltage control architecture onto a flexible microfluidics platform further increases the acceptability and versatility of thin-sheet ceramic-based microdevices. Flexible ceramics allow the integration of microelectronic control onto fluidicbased devices, resulting in seamless operation and comprehensive functionality in compact microdevices (see Fig. 17.1). Furthermore, when all this multifunctionality is embedded onto a microdevice, one would posit that flexible ceramics pave the way for internet-of-things-based microfluidics in the near decade. The need for such highly compact microdevices may lead to the development of new technology in organ-assisted microdevices (eg., pacemaker, microrenal, and implantable lungs, among others), where data transmission, cloud computing, and cybermedicine can Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00017-8 © 2023 Elsevier Ltd. All rights reserved.

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Advanced Flexible Ceramics

(B) (A)

Microfluidic Channel

Holder Interconnect Contact Pad

Outlet

CMOS Chip Inlet

Metal SOG Metal

CMOS Chip

Holder Interconnect

(C)

Embedded Chip

Interface

Carrier Fluid In/Outlets 1 mm Top

Bottom

(D)

(E) Metal a-Si:H(n-Type) a-SiGe:H(i-Type) a-Si:H(i-Type) a-SiC:H(i-Type) a-SiC:H(p-Type)

pi3n a-Si:H Color Detector Reservoir Glass

TCO

Platinum Gold

Channel Polymer

Chromium TiN

SiNx Metal

Tungsten Plug

SiOx

Via MOSFET Source

Drain

Silicon

Figure 17.1 A full electronic component in a flexible ceramic microfluidics system. (A) Illustration of the system. (B) SEM image showing interconnects from the IC. (C) A side view of the packaged integrated CMOS microfluidic device. (D) a-Si:H-based sensors on glass substrate, and (E) cross-section of the device showing vias connecting ASIC with fluidic network [2,3]. Source: Reproduced with permission from [2,3] Copyright 2011, IEEE.

be used in real-time. From an industrial standpoint, the use of flexible ceramics in minute-scaled microfluidics devices could hasten the realization of the current industrial revolution’s goals. (I4.0). This chapter explicitly defines the acceptability and usability of thin-film, flexible, biocompatible ceramics. While the chapter focuses on microfluidics-mediated devices in biomedical concerns, it considers the paradigm shift caused by the nCOVID-19 (Coronavirus) pandemic as a prospect that could shape the crusade for innovation in flexible bioceramics and biomaterials in the near future.

17.2

Flexible ceramics in microfluidics

Microfluidics has gained widespread acceptance in virtually every field of science and technology, owing to the significant advantages it provides. Characteristics

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365

features in microfluidics devices are high throughput chemical processing, rapid detection, and economy of reagents. Microfluidic devices are being utilized to replace classical instrumentation laboratories where the need for analytical testing is scarce or expensive. In recent decade, microscale devices have been employed in the medical field for the diagnosis and detection of biological samples, as well as the detection of illnesses. The nomenclature for microfluidic devices varies depending on the target application of the industry; initially, the name micro-total analysis system (μTAS) was popular in research fields. Following that, further application of microfluidic devices in critical areas of biosciences translated the name from μTAS to lab-on-a-chip (LoC). The LoC capabilities enable the construction of the entire analytical laboratory on a minute-scaled chip without sacrificing functionality. The LoC devices were adjourned to perform with credible sensitivity and exactitude in critical bioscience research and field applications [4,5]. These testimonials provided impetus for increased innovation around the LoC technology in both research and clinical domains. One of the most recent developments in microfluidics-oriented technologies is the emergence of organ-on-a-chip; this microscaled technology ensures the complete replication of all features on a complex biological organ on a chip [6]. One of the most recent developments in microfluidics-oriented technologies is the emergence of organ-on-a-chip; this microscaled technology ensures the complete replication of all features on a complex biological organ on a chip [6]. The incredible capability of the organ-on-a-chip device is the ability to perform an intact physiological function of the organ-on-a-chip in a near-perfect resemblance to the native organ. Materials technology is at the heart of microfluidics devices. The functionality of meticulously fabricated microfluidics devices (either μTAS, LoC, or organ on a chip) is linked to the tailored properties impacted on certain materials as required for device operation. While biocompatibility is one of the most important properties for the fabrication of components used in microdevices, a special emphasis is placed on microfluidics used in in vivo applications. The biocompatibility criterion thus determines the suitability of microfluidic devices for use in medical domains, either in vivo or in vitro scenarios. During the early days of microfluidic technology, LoC and μTAS devices were fabricated using micromachining protocols that used silicon as the base material. Emerging fabrication protocols that sought to reduce fabrication costs while emphasizing tailorable materials have considered the use of polymers [7,8], polymeric gels like hydrogel [9], and chitosan [10]. Because of the peculiar functionality demand of microdevices at such a small length scale, certain properties such as chemical inertness, thermal conductivity, and thermal stability are primarily considered in the selection of base materials in the construction of microfluidic devices. Silicon-based materials were popular in the early days of microfluidics due to their stable electro-osmotic stability, good thermal conductivity, and resistance to organic solvents. However, certain conditions such as hardness, difficulty in microfabrication of fluidic propulsion devices, and high cost limited the use of silicon as demand for microfluidic devices increased. In the early 2000s, the Whitesides research group promoted the use of polydimetthylsiloxane (PDMS) in microfluidics

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[11]. The crusade gained sufficient credibility as a pioneering material in the emerging field of microfluidics, which opens a slew of opportunities in the technical and biomedical sciences [12,13]. In addition, PDMS is cherished for its flexibility, ease of fabrication, and cost-effectiveness in microfluidics. Thus, credit for the many significant successes recorded in microfluidics overlast two decades goes to PDMS as an easily tailored polymeric soft matter. With the increasing demand for microscale devices, there has been a shift in material selection towards ceramics, especially the extraction known as low-temperature cofired ceramics (LTCC). The ability of LTCC to be fabricated into flexible thin films is a critical novelty that makes a superior choice over existing microfluidics materials (Fig. 17.2). Recently, material innovation based on thin film technology has resulted in the reintroduction of ceramic-based materials as a topical candidate material for microfluidic devices. The ability to produce flexible and fold sheets of ceramic-based materials with minute thickness is thought to be the driving force behind the continued development of LTCC, a novel class of ceramic materials. This special breed of unique materials for the microfluidics platform is considered to be the result of a merger between LTCC microelectronic technology and thin film materials technology. The ability of LTCC microfluidics to quickly achieve interoperability between microfluidics-based devices and well-established microelectronics is a significant achievement in the drive for complete automation in microfluidics. Multiplexing serial layers of LTCC microthin sheets comprising microfluidics components (such as microconduits, valves, fluidic chambers, etc.) could improve the compactness of the miniaturized device. To accelerate the functionality of LTCC-based ceramicbase materials in microfluidics, a number of reports in the literature have identified an obvious that certain functionality should be impacted onto LTCC substrates. This surface modification may result in specific physiochemical changes, allowing this flexible ceramic to be used in diagnostics, biosensing, and related applications [15]. While conjugating the LTCC base surface with functional groups is an

Figure 17.2 Flexible ceramic-based microfluidic enzymatic bioreactor chip [14]. Source: From K. Malecha, E. Remiszewska, D.G. Pijanowska, Technology and application of the LTCC-based microfluidic module for urea determination, Microelectron Int. 32(3) (2015) 126 132. https://doi.org/10.1108/MI-01-2015-0010/FULL/XML.

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important procedure for achieving the desired criterion for target applications, reinforcement of the thin-film LTCC substrate is important for achieving device structural rigidity [15]. Microfluidics base materials like PDMS could be bonded with LTCC substrate to improve structural rigidity or surface modification with functional groups. Plasma bonding of LTCC base materials and PDMS is possible using an argon-oxygen plasma ignited in a dielectric barrier discharge device.

17.3

Fabrication protocols for flexible ceramics in microfluidics

For decades, LTCC base materials have been a household name in the microelectronics industry. This material allows multilayer and multiplexing of different strata with the component features required for building electronic components and systems. Materials processing and fabrication of LTCC substrates could be easily adopted in numerical microscale devices using this established technology, especially when strength and rigidity are required. In its unfired state, ceramic exists in its green state. The preparation of materials prior to the actual processing is done on the green ceramic (see Figs. 17.3 and 17.4). The blanking process could involve a combination of established manufacturing protocols as well as some emerging techniques as entailed in advanced and flexible manufacturing technologies. Microconduits and microchannels, as well as the construction of active microfluidic components can be achieved on the blanked ceramic substrate which is usually ,120 μm thickness [16]. Malecha [17] employed photo-patterning to create predefined recesses on the ceramic

Figure 17.3 Multilayer protocol for low-temperature cofired ceramics base materials: (A) blanching and registration holes cutting; (B) printing, laser cutting, filling, and forming vias; (C) stocking together in proper order; (D) lamination and cutting; (E) post-treatment (e.g., mounting of surface-mount device components). Source: From K. Malecha, et al., LTCC microfluidic systems for biochemical diagnosis, Biocybern. Biomed. Eng. 31(4) (2011) 31 41. https://doi.org/10.1016/S0208-5216(11) 70024-9.

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Figure 17.4 Protocol design of multilayer low-temperature cofired ceramics microfluidic bioreactor chip. (A) Fluidic channel with connected reacting chamber; (B) microreactor threshold cut; (C) microreactor bottom layer; (D) thermal heating sheet; (E) temperature sensor; (F) temperature sensor cut; (G) vias for heater and temperature sensor cut. Source: From K. Malecha, et al. LTCC microreactor for urea determination in biological fluids, Sens. Actuators B: Chem. 141(1) (2009) 301 308. https://doi.org/10.1016/J. SNB.2009.06.026.

substrate for conduits and components. Photo-patterning is a subtractive technology that involves guided etching on the substrate via photo exposure using a computeraided draft. Ceramics being light-permeable admits high wavelength penetration of radiation to penetrate the structure of their substrate to enact etching in a photochemical manner, leaving behind a subtracted two-dimensional or three-dimensional (3D) structure that could serve as fluidic assays or active components in the microfluidic platform. Multiplexing of thin-sheet layers of the photo-etched ceramic substrate is achievable on flexible ceramics by serial overlying of photo-chemically etched ceramic micro sheets. This multilayer and serial cascading is accompanied by UV laser-guided lamination of each micro sheet onto another. Thereafter, cofiring of the ceramic base materials is implemented under a control thermal furnace at the sintering temperature below 1000 C. In practice, most ceramic cofiring to achieve the LTCC status is done in a confined furnace at temperatures ranging from700 C and 900 C.

17.4

Tailoring ceramics for application in medicalrelated microdevices

The need to tailor desired properties onto the ceramic substrate is considered as an important aspect of the fabrication process in the microfluidic domain. Surface properties such as biocompatibility, hydrophilicity/hydrophobicity, roughness,

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369

surface charge, and surface reactivity can be altered using physical, biological, and chemical methods during surface modification. The surface phase acting on the substrate could interact with the base material and the chemical functional groups to improve the bulk properties of the ceramic base materials. G

G

Plasma treatment: Plasma coating has long been considered a viable method for facilitating dissimilar bonding between ceramic base materials and adjoining substrates strengthened by the interfacial bond. The integration of an LTCC substrate with adjacent materials in the microfluidic structure has been well established in several literatures [18 21]. Theoretically, the surface temperature of the base ceramics facilitates plasma deposition to initiate the bonding process, while optimal bonding is achieved at a critical bonding temperature [22]. Coating materials such as Al2O3, TiO3 AlO3, Cr2O3, and Y2O3 ZrO2 are used to strengthen the interfacial bond. For instance, [23] developed a novel reversible protocol for LTCC-PDMS bonding. The report showed that the adhesion of PDMS-LTCC pair changed depending on the plasma exposure. Following that, the same team of researchers attempted to use the dielectric barrier discharge plasma technique to bond LTCC and PDMS in a fluorescent sensor built on a microfluidic platform [24]. Surface modification: The primary purpose of surface modification is to achieve the desired optimal preconditioning of the microenvironment, certain activities such as hydration and salinization. For instance, the report of Malecha et al. (2014)’s paper showed how glutaraldehyde modification of ceramic results in a change in the surface hydration of LTCC-based ceramic, which was used to immobilize an enzyme in a bioreactor built on a microfluidic substrate. Since the immobilization process, which comes after surface hydration during biomodification, is often blamed for shortcomings, demonstrated that area-specific bilayer deposition on LTCC-type microchannel substrate can enhance the immobilization process, [25] demonstrated that area-specific deposition of bilayer on LTCC-type microchannel substrate can improve the immobilization process.

17.5

Integration of microelectronic in flexible ceramicbased microfluidics

A remarkable advantage of ceramic-mediated microfluidics is the possibility to incorporate microelectronics in the miniature device because LTCC base material is well-known in the field of microelectronics. Structurally, inherent dielectric properties of ceramic base materials support the integration of electronic components, facilitating the realization of full automation in a microfluidic platform [26]. That is fluidic movement, parameter measurement, reaction dynamics, and biochemical processes in the microenvironment in real time. It could be said that implementing complete automation could also create a pathway for in situ harvesting of process data during reaction in microfluidic devices (see Fig. 17.5). Flexible control in the microenvironment is feasible with the incorporation of electronic components in ceramic base material while power harvesting within the system can be envisaged for robust functionality in microfluidic platforms [18]. Self-powering of microfluidics components could enhance the utility of miniaturized devices in remote locations and low-resource applications [27]. Optofluidics is a class of miniaturized

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Figure 17.5 Wireless low-temperature cofired ceramic (LTCC) based microfluidics biosensor chip (A) fabricated through the LTCC process, (B) the sensor mounted with connectors [29]. Source: From Y. Liang, et al., An LC wireless microfluidic sensor based on low temperature co-fired ceramic (LTCC) technology, Sensors 19(5) (2019) 1189. https://doi.org/10.3390/ S19051189.

fluidics devices that takes advantages of electronic components in microfluidics. The use of radiofrequency and microwave technology to manipulate microdroplets for biosensing has been identified as defining a paradigm shift in point-of-care devices in life sciences and biotechnology applications [28]. The incorporation of optical fiber onto ceramic LTCC-type ceramic base materials in a microfluidic channel was shown by [26], revealing its capability for the creatinine detection in a Y-type microfluidic channel. Recently, [30] reported the utility of novel dense structure materials (BaMnV2O7) with the microwave electrodes capability in a miniature device. Malecha [31] also showed that a relationship can be established for fluidic systems and electronic components for selected microwave frequencies, demonstrating the sensitivity of onboard control architecture in an LTCC base microfluidic system. The same research group reports compressive application areas of microwave-mediated thermal techniques in ceramic materials in a related report [32]. Bioplastic sol gel antibodies has been described [33] as a trimethoprim measurement and sensory cum detection device in an LTCC-mediated ceramic integrated in a biomicrofluidics platform. Martı´nez-Cisneros [34] uses a three-dimensional multilayer technique to integrate actuators onto ceramic microfluidics for thermal control in fluidic and enzymatic systems in a Wireless control in fluid miniaturized devices offers hope.

17.6

General applications of functional and flexible bioceramics in medical technology

Bioceramics have been used for a wide range of applications in medicine and associated fields because of their high mechanical stiffness, hardness and fracture

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toughness, biological affinity, corrosion, friction, and wear resistance [35]. They have found use in a variety of applications, including bone fillers, artificial joints including hip, knee and elbow joints, bone graft substitutes, and artificial valves [36]. Several functional bioceramics have been used in medical technology to repair and replace hard tissues, especially bone and teeth. Aalumina and zirconia are bioinert bioceramics with excellent mechanical properties, friction, and wear resistance, making them suitable implant materials for joints [37]. Hydroxyapatites have been used as coatings on metallic prosthetics to enhance implant component osteointegration [38]. Owing to their analogous similarity to the chemical properties of bone proving excellent biocompatibility and high mechanical strength, bioceramics, particularly calcium phosphates such as hydroxyapatite and tricalcium phosphate, have a wide range of applications in bone regeneration. In addition, when bioceramics are implanted in the body, they exhibit osteogenic features with hard tissues [39]. In spite of their widespread use in dental and orthopedic surgery, bioceramics’ clinical use for tissue engineering has been restricted due to their brittleness and shape rigidity when implanted; as a result flexible bioceramics are now being investigated and developed [40]. A description of functional and flexible bioceramics and their biomedical applications are summarized in Table 17.1.

17.7

Emerging technologies in bioceramics for medical devices

Emerging technologies are taking advantage of material science and engineering to develop bioceramic-based medical devices from available materials that are easy to sterilize, reproduce, safe, and cost-effective [56]. Perhaps, bioceramics may have received more attention in biomedical applications than other biomaterials due to the “unlimited” possibilities for manipulating the material alone or as a composite to meet mechanical, clinical, and production requirements. For instance, the past four decades have witnessed drastic transitions in the development of medical devices and materials for regenerative medicine with the use of bioceramics in arthroplasty (hip arthroplasty), arthrodesis device (silicon nitride), spinal fusion, prostheses (bioceramic coatings such as zirconium nitride), orthopedics (bioglass products), dental prosthesis (zirconia with alumina), and Otolaryngology (middle ear implants) [57 61]. However, as a consequence of the ever-dynamic and induced changes in the human body that influence cellular, molecular, and tissue reactions, researchers are compelled to look into producing novel biomaterial, in this case, bioceramics, with effective approaches and techniques to tackle loss of cells, tissues and organs of the human body [62]. Basically, these new techniques may involve modifying tissue responses through controlled delivery using implants or medical devices [63]. Here, we focused on current bioceramics applications in medical devices and discussed some of the emerging technologies in its application.

Table 17.1 Functional and flexible bioceramics and their application. Bioceramic material and compositions

Fabrication technique

Unique properties/distinctive characteristics

Application

References

Calcium polyphosphate

Additive manufacturing techniques. Electrospinning technique.

Flexible, biodegradable scaffold compliant with complex structural bony system. Fibrous, up to 10 mm in length and 10 30 μm in diameter, and the hydroxyapatite grain size was approximately1 μm.

Bone tissue scaffolding.

Zachary et al. [41] Wu et al. [42]

(α-TCP)-based scaffold

Electrohydrodynamic printing process.

Bioglass/β-TCP scaffolds

Micro extrusion based 3D printing. Cryogenic 3D printing followed by sintering.

3D fibrous ceramic-based structure, exhibited high metabolic activity and mineralization. Bone fillers.

Reinforcement for bone substitutes for example, in polymeric hydroxyapatite and as a bony mesh for filtration. Bone tissue regeneration.

Improved cellular responses. Bone cell culture.

Seidenstuecker et al. [44] Song et al. [45]

Dual bioactivities for osteochondral interface reconstruction.

Chen et al. [46]

Hydroxyapatite (Ca10(PO4)6 (OH)2) fibers

Porous hydroxyapatite scaffolds

Lithium-calcium-silicate crystal bioscaffold

3D printing, followed by postsintering.

Hierarchically porous structure (interconnective macro pores and micro holes on strut surface) and superior compressive strength. High mechanical strength, provides support for both osteogenic differentiation of mesenchymal stem cells and chondrogenic differentiation of chondrocytes in vitro and in vivo.

Kim et al. [43]

Outstanding hyperelastic recoverable properties, provides platform for efficientcellular activities. Good flexibility, excellent biomineralization action, and acceptable osteoblast cell biocompatibility.

Hard tissue engineering.

Kim, Yun and Kim, [47]

For implants and drugloading biomaterials for bone tissue regeneration.

Chen et al. [48]

Excellent flexibility, demonstrated bioactive indications for osteogenic differentiation of human mesenchymal stromal cells. Flexible bipolar, fibrous membrane. High stiffness and good ultimate load-to failure.

Bone tissue regeneration.

Gazquez et al. [49]

For improving gradient microstructure in tendon-to-bone healing. Tissue engineering and drug delivery applications. Tissue engineering.

Li et al. [50]

Gelatin/ hydroxyapatite biocomposites scaffolds

Novel bioink printing process.

Poly(butylene succinate)/ polydimethysiloxanemodified bioactive glass/ nanohydroxyapatite (PBSu/ PDMS-BG/nHA) hybrid bioceramic Yttrium-stabilized zirconia nanofibers

Sol gel process.

Nanohydroxyapatite-poly-Llactic acid (nHA-PLLA)

Electrospinning.

Tricalcium phosphate (TCP) Ca3(PO4)2

Electrospinning.

Solid and microporous nanofibers.

Titanium dioxide TiO2

Electrospinning calcination at 700 C Electrospinning, calcination at 500 C reinforced withTiO2 metal oxide nanofibers.

Provides osteoblast capability.

Titanium dioxide TiO2 nanofibres

Electrospinning.

Greater densification and better mechanical characteristics. Increased bulk density, compressive strength, and microhardness. Lowered porosity and water adsorption capacity. Preservation of in vitro bioactivity.

Bone substitute in high load-bearing sites.

Dai and Shivkumar, [51] Wang et al. [52] Aly et al. [53]

(Continued)

Table 17.1 (Continued) Bioceramic material and compositions

Fabrication technique

Unique properties/distinctive characteristics

Application

References

Boron nitride (BN) reinforced gelatin ESMs

Electrospinning followed by glutaraldehyde cross-linking. Electrospinning, calcination at 1300 C.

Increased Young’s modulus nontoxic, biodegradable, highly biocompatible.

For orthopedic applications.

Nagarajan et al. [54]

High shape-memory effect with a recoverable strain of up to approximately 5% and short recovery time (0.16 s). Can lift up to 87 times and the stroke is approximately 3.9 mm. An output stress of 14.5 2 22.6 MPa, a work density of approximately15 2 20 kJ//m3, and a tensile strength of approximately100 2 200 Mpa.

Potential for use as artificial muscles at elevated temperatures.

Du et al. [55]

Nanofiber-based coiled yarns 8CeO2 2 0.5Y2O3 2 ZrO2

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17.8

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Ceramic-based medical devices

Medical devices can be invasive or noninvasive. To be considered by humans, the device must meet some mechanical requirements in its material selection and design in both cases. Simply put, such device must be both biocompatible and bifunctional. Ceramics have been linked to better biocompatibility when used for medical devices than any other biomaterials, so extensive development is in progress to expand their applications and improve their performance and reliability [64]. Although there are challenges to using ceramics as a medical device, it can be said that the collective efforts of material scientists and engineers, such as biomedical engineers, haveyielded advanced biomaterials, particularly bioceramics, using available technologies over the years. The application of bioceramics for medical devices has recently sparked a of interest. In medicine, ceramic-based medical devices can be used to replace damaged or diseased human organs or tissues with minimal risk of failure [65]. Some of the most common applications of bioceramics in dental and orthopedic implants are listed in Table 17.2. Bioceramics were typically used for repairs, implant and reconstruction in joint, bones and dental applications, and the two most popular were alumina (alpha-Al2O3) and zirconia (Y-TZP, Mg-PSZ), used as components of hip and knee replacement bearings [67]; however, technically, there are four types of ceramics namely, oxide ceramics, silicate ceramics, non-oxide ceramics, and piezoceramics [61]. According to [68,69], “The medical device industry continues to find new and expanded applications for this ceramic, particularly as implantable, as engineers gain experience developing ceramic products.” Truly, the distinct properties and possibilities of ceramics are leading to new materials, novel methods of manufacturing, and diverse Table 17.2 Bioceramics applications [66]. Device

Function

Knee, shoulder, elbow, artificial total hip wrist Screws, wires, bone plates Intramedullary nails Harrington rods Permanently implanted artificial limbs Orthodontic anchors

Fractured joints reconstruct arthritic

End osseous tooth replacement implants Andibular reconstruction, alveolar bone replacements Spinal fusion Vertebrae spacers and extensors

Repair fractures Align fractures Correct chronic spinal curvature Replace missing extremities Provide posts for stress application required to change deformities Damaged or loosened teeth Restore the alveolar ridge to improve denture fit Immobilize vertebrae to protect the spinal cord Correct congenital deformity

Source: Adopted from T. Thamaraiselvi, S. Rajeswari, Biological evaluation of bioceramic materials-a review, Carbon 24(31) (2004) 172.

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applications, especially for medical devices and implants. Some of these emerging technologies are discussed further below.

17.9

Emerging technologies for bioceramics in the medical device application

The global market for technical ceramics is projected to reach US$161 billion by 2027 against US$89.4 billion in 2020. This estimation is hinged on the growing demand for customized, patient-specific, longer-lasting and smaller-sized, yet highperforming medical devices. Medical devices manufacturers and material scientists are now investigating innovative techniques and technologies for utilizing ceramics for medical implant applications, especially implants with higher efficiency than the currently available one-size-fits-all products. To this end, technical ceramics is gaining popularity because advanced/technical ceramic materials have recently enabled applications that were previously unthinkable. Technical ceramics are considered to be one of the most efficient materials in the medical and engineering industries due to their unique material properties [57]. These emerging technologies alter previously established design approaches, material selection processes, and bioceramics manufacturing techniques of bioceramics used for medical devices. Here, we discussed a few of the advancements in the use of bioceramics in medical applications, specifically implants, drug delivery, and cancer treatment. Fig. 17.6 summarizes some advanced bioceramics technologies used in medical applications.

17.9.1 Electroceramics Technical ceramics are currently gaining popularity in implantable electronic devices, also known as electroceramics. Some well-known electroceramics applications include hearing devices, cardiac pacemakers, feed-thru, and defibrillators. With newer techniques emerging in the medical device industry, the application of electroceramics in medicine has grown over the years. One such emerging ceramics technique is the maximization of lead zirconate titanate to form piezoelectric ceramics [70]. For instance, some medical implants, such as actuators, convert control signals into mechanical motions. Materials for this function should have a precise of range of inert, electrical, magnetic, and optical properties [65]. This combined property is greatly enhanced by the electroceramics technique, making it of particular interest in thin small sensory components built into body process monitoring devices or devices. Single-crystal piezoelectric materials used in ultrasonic applications are a good example. Multilayer ceramics and piezo composites are newer categorieswith good electrical and mechanical properties [71]. Generally, these novel ceramics-based technologies improve acoustic characteristics in medical devices. With this technology, newer materials unquestionably on the horizon.

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Figure 17.6 Summary of some advanced technologies used for bioceramics in medical applications.

17.9.2 Green state machining The application of biomaterials in medical devices relies to some extent on manufacturing and production techniques. Because ceramics are generally brittle, their machinability is poor. A number of studies have reported different crack behaviors that influence the overall mechanical properties of ceramics after subtractive processes, such as the hard and white machining, which is commonly used in dental applications [72,73]. White machining, hard machining, and green state machining are popular ceramic manufacturing processes [74]. However, while machining biomaterials in the green state is not a new technique, it is yet to be explored by many researchers and manufacturers, possibly due to the difficult process, especially in the presintered stage [74]. Recent studies have shown that there are significant benefits to using green state machining process for ceramics used in medical applications, and the manufacturing process is becoming increasingly popular in modern industries due to the possibility of manufacturing the ceramic parts in small and medium series. This advanced process is simply carried out on a green body, a process that, when compared to the other two manufacturing processes, reduces cracks in the product (especially microceramic products) to a greater extent [75]. The booming field of microelectromechanical systems and drug delivery systems involving the novel use of ceramics are two examples of medical fields taking advantage of green state manufacturing [76].

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17.9.3 Three-dimensional printing With the advent of three-dimensional printing, the use of materials in general has become fascinating. This new technology offers biomedical engineers and material scientists the possibility of developing and producing complex biomaterial shapes that can sometimes be patient-specific [63]. According to the US food and drug administration, three-dimensional printing is a form of additive manufacturing used in the printing of medical devices. A whole structure, device, or organ can be built using this advanced technology by connecting successive layers of raw materials one after the other. The flexibility to manipulate material in a single process, the ability to produce the patient-specific devices, prototyping for comparing other materials and cost savings are all advantages of three-dimensional technology [77]. Cranial plates or hip joints and hand prostheses, orthopedic and cranial implants, surgical instruments, dental restorations such as crowns, and external prosthetics developed from realistic images on CT scans are among the many three-dimensional printed medical devices available [57]. Importantly, the method has spawned new bioceramic applications, especially for nano-device development, such as the fabrication of array biosensors [56]. The planar device is an example of an array biosensor. This fabrication technique is helpful in studies of micro fluids and nano or microtechnology because it is possible to fabricate several testing models. Of course, with researchers’ unwavering efforts to manufacture living organs through three-dimensional printing, the well-appreciated bioceramic-based medical implants will remain the unrivaled choice of manufacturers very soon [78].

17.9.4 Bone cancer treatment from bioceramic scaffolds According to the World Health Organization (WHO), there will be nearly 10 million cancer cases in 2020, with breast, bone, and lung cancers being the leading causes of death [75]. Due to their biocompatibility, bioceramics such as calcium silicate (CaSi) have been used in the development of solutions to these global health issues. Calcium silicate (Ca-Si) has emerged as a novel photothermal material for osteosarcoma therapy and drug delivery enhancement in the treatment of bone cancer [79]. Although efforts are being made to functionalize the material, such as the use of borocarbonitrides nanomaterials [80], reliable manufacturing techniques are required due to the brittle nature of ceramics to advance in the exploration of bioceramics to achieve osteogenesis [81]. Another novel bioceramics-based treatment for hyperthermia involves the application of a microporous scaffold with controlled size and shape of pores that is doped with spinel-like MgFe2O4 magnetic nanoparticles [77]. Recent studies have included the examination of ceramic particles and microspheres, as well as the investigation into the magnetic composite found in bone cancers [82]. This new line of research is expected to advance cancer radiotherapy in the near future.

17.9.5 Sol gel technique One of the areas of concern in biomedical applications is the effect of corrosion on medical devices. Ideally, materials used in medical devices should have appropriate

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structural and mechanical properties to promote a healing response without causing adverse immune reactions. To achieve this outcome, medical device designers are currently using various surface treatments such as coating to enhance or modify bioceramic properties, leading to lubricity, hydrophobicity, functionalization, and biocompatibility [83]. Sol gel technology offers an alternative technique for producing bioactive surfaces for these applications. The sol gel microencapsulation technology and its wide range of applications are now well-known. Sol gel coatings applied to biomaterials include hydroxyapatite (HAp), titanium dioxide (TiO2), bioactive glass coatings, and organic inorganic composite hybrid coatings [73]. This chemistry opens up a new production route for the production of ceramics and composites, which have a wide variety of applications in medicine, biology, and biochemistry. One of the motivations for the sol gel technique is the need to prevent corrosion in medical devices because mechanical failures caused by corrosion or wear can compromise a material’s integrity—the biocompatibility and bioactivity [84]. Preventing such mechanical failure can be accomplished through the process of material surface modification that results in the desired structural and microstructural properties [85] which provides a wide range of methods for producing a coating that prevents corrosion in bioceramic applications to a large extent. The synthesis of hybrid organic inorganic materials [86] is a major technique that has been in use for a while but has promising potential for flexible use of bioceramics, especially bioglass. The combination of both forms of materials is facilitating the development of novel materials that will offer diverse properties for a wide range of applications. For example, in tissue engineering, sol gel has recently been used to develop bioglasses that are used as a cover over metallic prosthetic implants [85]. The porous bioglass derived from this same method now has increased surface areas of material, which is useful in drug delivery, allowing natural tissues to replace the material over a period of time in the body, especially in bone regeneration [87]. It is worth noting that the potential and promising features of sol gel in the medical field, for instance, sol gel nanocoatings for implant materials, are rapidly expanding due to the possibility of manipulating the molecular scale to produce a homogeneous and pure coating. Because of the nanoparticle sizes and high surface areas, it necessitates lower firing temperatures [88].

17.10

Prospects of flexible bioceramics in post-COVID era

The current global outbreak of Coronavirus disease 2019 (COVID-19) has prompted governments, private sectors, and researchers to invest in a perceived solution to the pandemic. Some measures of demand in research institutes and industries currently rely on material scientists and engineers to investigate and develop materials for the current phase and post-COVID era. This responsibility has been placed primarily on material scientists and engineers as a result of

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materials’ ability to provide a wide range of properties necessary for solving medical and health challenges. Materials used for this purpose are biomaterials; selected and processed materials for the development of disease models—such as scaffolds —diagnostic and therapeutic aids, and even the production of vaccines, which is critical to mitigating the effect of the airborne virus that has killed millions and led to the paralyzed the global economy. When the world finally overcomes COVID-19 pandemic, there will be, as expected, a surge in the general demand for biomaterials to cover the limited production of products caused by a shortage of manpower and unavailability of manufacturing. Bioceramicsare expected to gain more attention in the electrical and electronics industries, notably for solid-state batteries, sensors, power pack systems in ventilators, and flexible thin ceramic circuits [89].Bioceramics such as tubular ceramic microfiltration membranes element, the popular hydroxyapatite used in tissue engineering, silicon nitride, and graphitic carbon nitride will also gain attention in the healthcare industry [90]. Driven by the rapidly evolving need to keep the environment disinfected and make medical equipment available, material scientists, researchers, and engineers will continue to develop more small but sophisticated, long-lasting ceramics allowing for greater flexibility in the modification of ceramics used in the industry and on biological systems.

17.11

Current roles of flexible bioceramics in tackling COVID-19 and expectations in post-COVID-19 era

As the world continues to devise means to curb the COVID-19 pandemic, several biomaterials have been considered for studying and predicting the mechanism of coronavirus interaction in the biological system and the physical environment. In this case, the need for biomaterials, is ultimately for disinfecting the environment, protective equipment, and contact surfaces. Materials for this mission would primarily be antiviral, with smart surface chemistry and the mechanical capability to inactivate viral organisms [91]. Generally, biomaterials have been employed to study, evaluate, and develop models for controlling the the mechanics of the COVID-19 virus. Some of the notable contributions include; modeling human adaptive immune responses with tonsil organoids [92], the use of coronavirus antigen microarray for analysis of SARS-CoV-2 antibodies in COVID-19 convalescent blood [93], study of SARSCoV-2 pathogenesis through the deployment of bioengineered in vitro tissue models and therapeutic validation [94] and the posibility of extensive study of SARS-CoV2 with regards to choroid plexus organoid, ocular organoid, lung organoid, and heart organoid [93,95,96], among others. Bioceramics, among the many biomaterials, play some key roles in the current solution-oriented research and methods aimed at reducing the negative impact of COVID-19 on the world at large. Such contributions include the use of bioactive and antibacterial ceramics capable of reducing bacteria levels through a series of

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chemical and biological reactions [97]. However, as of now, two identified flexible bioceramics have received attention for engineering novel disinfectants that inactivate virus transmission, especially in the environment. The bioceramics—silicon nitride and graphitic carbon nitride, are briefly discussed subsequently.

17.11.1 Silicon nitride bioceramics For over a decade, silicon nitride (Si3N4) has remained a top-used biocompatible ceramics in the human body, with no literature or experiment indicating any adverse effects [98,99]. In a controlled environment, the material has the potential to disinfect SARS-CoV-2 and sanitize allergenic compounds when used as micronsized bioceramic powder. This is made possible by the surface chemistry of the material and the inert antiviral property exerted by reactive nitrogen species in moist environments [100]. An earlier study [101] showed that when SARS-CoV-2 was in contact with silicon nitride bioceramic powder in dilute aqueous suspension, the coronavirus was instantly inactivated. Another study [102] found that Si3N4 antiviral surface chemistry makes it suitable for use in facemasks (as micrometric powder), protective polymeric coatings (as dispersoids), surgical fabrics and drapes, and as loading particles (in disinfectant sprays). Already, the COVID-19 pandemic has made people more conscious of their environment; of their interaction with materials, animals, and humans. And, while sanitizers are almost universally available, it is more cost effective to have an environment made of materials that can inhibit virus survival, which bioceramics provide. As a result, silicon nitride bioceramic markets will expand in the postCOVID-19 era, particularly as loading particles in disinfectant spray.

17.11.2 Graphitic carbon nitride Another bioceramics with peculiar catalytic behavior that has been analyzed with the potential for antiviral applications is graphitic carbon nitride (g-C3N4) [103]. Similar to the surface characteristics of Si3N4, the surface chemistry of g-C3N4 has also been reported to possess the ability to instantantly inactivate viruses upon irradiation exposure. Due to its nontoxicity, biocompatibility, metal-free, and visible-light-active nature, the material is rapidly emerging as a sustainable virus eradication technique [104]. A recent study [91] gave insights into the previous use of graphitic carbon nitride to degrade pollution in an aqueous and nonaqueous environments. In another study, Hasija et al. [104] established the use of photocatalysts formulated on graphitic carbon nitride photocatalytic for the inactivation of the coronavirus, like every other virus, with the aim of evaluating the virucidal performance and mechanism of virus deactivation. Although more studies is needed to validate the outcome of the current limited research on the g-C3N4 capability to deactivate coronavirus mechanism, the current findings are indicative of the potential of bioceramics in tackling similar airborne viruses in the post-COVID-19 era.

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17.11.3 Ventilator design Acute COVID syndrome has been reported to disrupt normal respiration in coronavirus patients due to lack of oxygen supply. Currently, respiratory support, specifically the use of mechanical ventilators to increase the supply of oxygen, is a universal means of helping patients critically affected by infection [102]. As the demand for ventilators continues to rise due to new virus variants, so will the the demand for ceramics. This is because ceramic capacitors are still replaceable due to their flexibility, forming one of the important components in the ventilator’s power supply system which powers the motors, pump, control logic, and displays [105 107]. With regards to direct oxygen supply; prior to the coronavirus, there were various dimensions of studies relating to oxygen generation from ceramic sources, such as studying the efficacy of electricity-driven ceramic membrane separation oxygen generation technology for onboard conditions [108,109]. Regarding the current pandemic, [110] discussed that it is possible to use blood oxygen infusion (direct administration of dissolved oxygen into the cardiovascular system) as an alternative method for mechanical ventilators [111]. The authors claim that the method is a simple way to improve the oxygenation of tissues independent of alveolar gas exchange, based on findings from previous studies that suggest that, medical oxygen gas can be modified into nanobubble water or saline solutions that can be administered into the human system using physical means such as ceramic nanofiltration membranes [112,113]. Although no clinical trials have been conducted to contradict the therapy’s potential risk to human health, such as intolerance to the large fluid loads required to deliver enough oxygen, there is clear evidence that bioceramics (ceramic nanofiltration membranes) could in the future safe patients with respiratory challenges. Since microfluidic solutions such as lung-on-a-chip have been demonstrated, then a complete microfluidic-mediated device as a ventilator is feasible in the near future [114]. That is, by manipulating fluidics and intricate interconnected via-shaped conduits, robust and complicated physiological functions can be replicated in the microenvironment without loss of sensitivity [115]. Recently, Duke University researchers showed that from the mechanical cues, assisted ventilation devices mimicking the native process of the native lung can be developed on a microfluidic platform [116]. As a result of ceramic microfluidics’ onboard monitoring and control capability, complete automation of mechanical ventilation procedures could be a solution insight for cases of respiratory-related disease. Finally, flexible bioceramics are multifunctional materials that exhibit functionality, adaptability, and survivability for a wide range of engineered applications. As people strive to recover from the financial instability caused by the pandemic, they will invariably prefer to go for practical and affordable means of manufacturing, which currently produced flexible ceramics offer, eventually making the material one of the key factors that bolster the growth of the health, mechanical, and electrical industries [117].

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Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors for damage-resistant applications

18

Anoop K. Mukhopadhyay1,2, S. Sarapure3 and P. Maiti2 1 Department of Physics, Sharda School of Basic Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh, India, 2Advanced Mechanical and Materials Characterization Division, CSIR—Central Glass and Ceramic Research Institute, Kolkata, West Bengal, India, 3Department of Mechanical Engineering, Sree Vidyanikethan Engineering College, Tirupati, Andhra Pradesh, India

18.1

Introduction

A good deal of the possible application scenario of flexible ceramics involves tape cast multilayer thin ceramics and composites for a wide variety of applications [1–10]. However, the challenge is to overcome the characteristic brittleness of ceramics so that it could be strong, reasonably hard as well as tough [11–13]. This has led researchers to study a variety of processing techniques and the characterization of the multilayered composites (MLCs) systems to examine if the desired improvements in properties have been achieved or not [14–68]. It is represented in Table 18.1. In such laminar structures the aim is to tailor the microstructure through appropriate control of the interface such that; the typical brittle fractures nature can be at least partially avoided and pseudo ductility (to induce the ability to exhibit inelastic and/or slightly ductile behavior) introduced into the ceramic system. While the literature, more often than not, is flooded with conventional synthesis and characterizations [14–68], the total quantum of effort directed toward evaluation of damage-resistant material developments is far from significant. Moreover, there is a clear dearth of the design philosophy approach for the development of advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors for damage-resistant applications. This is where the present chapter derives its scope and hence, focuses. So, the objective of the present chapter is to design the interface with brittle/brittle, tough/tough, brittle/ tough, tough/brittle, tough/weak, etc. different combinations in advanced tape cast multilayer thin ceramics and composites for significant enhancement in damage resistance with a wide variety of inelastic failure behavior. In addition, the implications of such development for damage and particularly sudden impact Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00019-1 © 2023 Elsevier Ltd. All rights reserved.

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Table 18.1 Literature survey on multilayer composite systems. System

Route

Observations

References

Al2O3–Al2O3 MLC

Slip cast, sintering (1500˚C) Tape cast, sintering (1550˚C)

σ of MLC .. σ of Al2O3 monolith K1c(A/A) MLC–4.8 MPam0.5 K1c(A/B) MLC–5.3 MPam0.5 K1c(B/C) MLC–5.7 MPam0.5 all more than K1c of alumina– 3.7 MPam0.5 σ (MLC) 5 1.4 σ monolith Processing of MLC

[14]

K1c MLC (20 MPam0.5) .. K1c Ce-TZP (5 MPam0.5) K1c MLC (6 MPam0.5) .. K1c LAS Glass (1.5 Mpa m0.5), also steep rising R-curve Behavior (KR 5 40 MPam0.5 at Δa 5 3 mm) MLC have high strength, toughness and flaw tolerance

[18]

K1c MLC (18 MPam0.5) .. K1c SiC (3.6 MPam0.5) K1c (MLC) . K1c (Al2O3) for SiC platelet (15 V%) Theoretical model to predict load– displacement behavior of MLC in three-point bend tests

[21]

Al2O3/Al2O3–(A/A) MLC Al2O3/Al2O3 1 5 V% ZrO2–(A/B) MLC, Al2O3 1 5 V% ZrO2/ Al2O3 1 10 V% ZrO2–(B/C) MLC

Al2O3–ZrO2 MLC

PM

Al2O3/Al2O3–m-Zr (Mg)O2, 4 V %/Al2O3 MLC Ce–TZP–Al2O3 MLC

Slip cast, sintering (1625˚C) Wet chemical

LAS/Nicalon MLC

Symmetric arrangement of (0˚/90˚ cross plies)

Al2O3- Al2TiO5 (homogeneous)/ Al2O3-Al2TiO5 (inhomogeneous) SiC/graphite/SiC MLC

Tape casting & sintering (1600˚C)

Al2O3/Al2O3-15 V% SiC(pl.) MLC SiC (150 m)/graphite (7 m)/SiC (150 m) MLC

Extrusion of SiC, 10 m coating of graphite, sintering (2040˚C) Tape cast, vacuum sintering & H I P in argon (1550˚C) Extrusion of SiC, 10 m Coating of graphite, sintering (2040˚C)

[15]

[16] [17]

[19]

[20]

[22]

[23]

(Continued)

Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors

393

Table 18.1 (Continued) System

Route

Observations

References

ZTA/Mullite MLC

Sinter (1200˚C) & HP (1550˚C) Tape cast, sintering (1450˚C–1525˚C) Lamination, hot pressing (175˚C, 350 kPa) or (135˚C, 350 kPa)

Not well characterized

[24]

Processing of MLC

[25]

Step like nonbrittle stress–strain response and enhanced failure strain of MLC as compared to the brittle glass/alumina Role of biaxial, residual compressive stress vis-a-vis tensile stress in MLC to affect surface cracking σ, K1c, wear resistance of MLC .. those of the Al2O3 Matrix

[26]

HP (1850˚C)

σ, K1c MLC . σ, K1c Si3N4

[29]

Sintering (1600˚C)

K1c MLC (15 MPam0.5) .. K1c ZTA (4 MPam0.5)

[30]

Sintering (1500˚C)

Role of critical layer thickness and residual tensile stress in affecting crack growth along layer interface Contact damage resistance (CDR) MLC .. CDR of monolithic SiC K1c MLC .. K1c Al2O3

[31]

Al2O3/Al2O3–5 V% ZrO2 MLC Glass-epoxy MLC, glass-carbon fiber reinforced epoxy (CFRE) MLC, Al2O3-(CFRE) MLC t-Zr(3Y)O2/Al2O3-t-Zr (3Y)O2/t-Zr(3Y)O2

Sequential centrifugation, sintering (1500˚C)

Al2O3-TiC(p) 5-layer MLC, Al2O3-SiC (W) 7-layer MLC cutting tool Si3N4/Amph Si3N4 or Amph SiNC or SiC (Pl
) MLC Al2O3/3Y-TZP/Al2O3 MLC ZTA/3Y-TZP/ZTA MLC ZTA (85 wt.% Al2O3) [Matrix] Al2O3-t-Zr(3Y)O2 or c-Zr(8Y)O2/t-Zr (3Y)O2/Al2O3-t-Zr (3Y)O2 or c-Zr(8Y) O2 MLC

Tape cast, hot pressing (1750˚C)

σ-SiC-β SiC MLC

Sintering

Al2O3/Ce-TZP Al2O3/ Al2O3

Tape casting, lamination, sintering (1530˚C)

[27]

[28]

[32]

[33]

(Continued)

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Table 18.1 (Continued) System

Route

Observations

References

#

Infiltration & Sintering (1600˚C)

[34]

Al2O3/Al2O3-CA6#

Tape casting & sinteriNg (1650˚C)

β-sialon/Si3N4 MLC

Hot-pressing (1900˚C)

Si3N4/BN MLC

HP (1800˚C)

Al2O3–Al2O3 MLC

Tape cast, gluelamination with double sided adhesive tapes, sintering (1600˚C)

Al2O3–Al2O3 MLC

Tape cast, sintering (1540˚C)

TZ[t-Zr(3Y)O2] 1 5 V % Al2O3 (2 mm)/TZ [Zr(3Y)O2]-35– 100 V% m-ZrO2 (5– 150 m)/TZ[t-Zr(3Y) O2] 1 5 V%Al2O3 (2 mm) MLC Ce-TZP/Al2O3/CeTZP MLC Y-TZP/ Al2O3/Y-TZP MLC Ce-TZP/Al2O3–ZrO2/ Ce-TZP MLC Al2O3–TiN MLC

Sequential slip cast, sintering (1500˚C)

K1c MLC (7 MPam0.5) .. K1c Al2O3 (4 MPam0.5) Post indentation strength of MLC . . post indentation strength of Al2O3 σ MLC increased as the β-sialon layer thickness decreased Crack deflection at interface Presence of a low viscosity polymer melt helps as a flux for rearrangement of ceramic particles to affect better joining of the individual components in the laminate Density (ρ) of green laminated MLC 5 f (Pr., Temp., Time) of lamination σ of MLC 6¼ f(Inner thin layer characteristics) but depends on flaw population in the thicker outer layers

Sequential centrifugal casting, sintering (1600˚C)

Significant crack deflection in inner barrier layer due to residual compressive stress

[41]

Not mentioned

Theoretical model of MLC

[42]

Al2O3/Al2O3-CA6

[35]

[36]

[37] [38]

[39]

[40]

(Continued)

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395

Table 18.1 (Continued) System

Route

Theoretical model of single phase MLC with various statistical distribution of grain sizes 3.4YPSZ/Al2O3, 3.4YPSZ /Al2O3– 3.4YPSZ (50/50 v/v), 12Ce-PSZ/ Al2O3, 12 Ce-PSZ/ Al2O3–12Ce-PSZ (50/50 v/v) MLC

Sequential centrifuging, sintering 1600˚C

Al2O3/Al2O3–37 wt.% m-ZrO2 MLC

Tape casting sintering 1550˚C, 90 min

Al2O3–25 wt.%TiC/ SiC/C MLC

Tape casting Hot pressing, 1700˚C, 30 MPa

Si3N4 with alihmned Si3N4 whisker seeds (S)/Si3N4 without alihmned Si3N4 whisker seeds (N) with N–S–N design of MLC

Tape casting gas pressure sintering, 1875˚C, 4 h, 2 MPa N2 pressure

Observations

References

Theoretical model predicts the ratio of maximum to average grain size as the critical parameter of failure for the MLC Significant increase in σ, γ, and Kic was observed only for MLC with barrier layers made of a pure Al2O3. A crack deflection in Al2O3 layer was main mechanism for property enhancement MLC where the cracks are deflected reliably at the interfaces show a much higher resistance to the penetration of a crack in thermal shock in comparison to the performance of a control monolithic alumina ceramic σm—762 MPa, σMLC—605 MPa Kic (m) –5.65 MPam0.5, Kic (MLC)– 5.78 MPam0.5 Kic values on the casting surface of N layer of sample with a N–S–N laminate structure showed anisotropy

[43]

[44]

[45]

[46]

[47]

(Continued)

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Table 18.1 (Continued) System

Route

Observations

References

Mullite/AlPO4 MLC

Tape Casting Sintering, 16,000˚C, 10 h. Slip casting sintering 1400– 1600˚C, grain size 0.3–2.2 m Gel precipitation, sintering, 1050˚C, 7h Coprecipitation, sintering 1150˚C, 4 h 1200˚C for 20 min Suspension

All MLC showed serrated load vs displacement plots σm—129–339 MPa, σMLC—620– 674 MPa both with 49 N indents Powder primary crystallite size— 8 nm. G-60 nm Activation energy calculated. G- , 100 nm

[48]

Surface charge and particle dispersion study in aqueous suspension using various dispersants σ 5 416 MPa, K1c 5 1.61± 0.21 MPam0.5 (DT),1.75 ±0.18 MPam0.5 (Indentation) H 5 10.5–13.7 GPa and K1c 5 4.43– 4.80 MPam0.5 G , 500 nm, H 5 13 GPa, K1c56 MPam0.5

[52]

G , 500 nm

[56]

H 5 15.2 GPa, K1c511.9 MPam0.5

[57]

G-830 nm, K1c (2Y-TZP-0.3 mol% NiO) –9.97 MPam0.5 K1c (2 YTZP)-6.34 MPam0.5

[58]

Y-TZP/Ce-TZP MLC (3 L, 5 L)

Nano 3Y-TZP

Nano 3Y-TZP

Nano 3Y-TZP suspension

8YSZ

Tape cast and sintered sheets were supplied by Siemens

3-YTZP dope with CuO

Coprecipitation and sintering at 1300– 1325˚C, 12 min 4 h Coprecipitation, uniaxial compaction (100 MPa), sintering 1500˚C, 1h Sintering,1300– 1500˚C, 2–6 h Gel casting, Dry pressing, CIP, sintering? Uniaxial pressing, CIP (200 MPa), sintering 1500˚C, 24 h

3YTZP

2–3.5 YTZP 3Y-TZP

2Y-TZP (B)

[49]

[50]

[51]

[53]

[54]

[55]

(Continued)

Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors

397

Table 18.1 (Continued) System

Route

Observations

References

3.3Y-TZP (B)

Slip casting, sintering 1450˚C, 2 h

[59]

2.5 Y-TZP (B)

Sintering.1650˚C, 2 h

3-YTZP 3-YTZP

Coating Sintering

3-YTZP

Colloidal processing, sintering,1300˚C, 2h Hot pressing, 1350– 1850˚C, 1 h, 30 MPa Ar pressure, annealing 1300˚C, 10 min to eliminate m-ZrO2 Colloidal processing, sintering 1150˚C, 2–50 h.

2-Phophonobutane -1,2,4-tricarboxylic acid as dispersant σf–1076 MPa, K1c–4.3 MPam0.5 G-80 nm Ionic conductivity influenced by grain boundary High strain rate superplasticity  600% achieved K1c 5 6.5 MPam0.5

Bulk 3 mol% Y-TZP G-112 nm, H – 12.2 GPa, K1c –9.3 MPam0.5 Al2O3/ZrO2 composite G-94 nm H-16.2 GPa K1c  4.5– 5.9 MPam0.5

[65]

Pore size optimization.

[67]

3Y-TZP-G-120 nm and composite G-20–60 nm

[68]

3-YTZP/SiC nanocomposite

Y-TZP and Al2O3/YTZP nanocomposite
(B)

ZTA (B)

8Y-TZP

(T) 3YTZP-(0.5–1.5 wt.%) Pt nanocomposite

Powder mixing, colloidal processing, uniaxial pressing at (20 MPa), CIP (200 MPa) Sintering 1550˚C, 2 h Tape casting, sintering 1550˚C, 4 h Colloidal processing, sintering1150˚C, 30 h

[60] [61] [62]

[63]

[64]

[66]

σ - Strebgth, K1C - Fracture Toughness, G- Grain Size, TZP - Tetragonal Zirconia Polycrystal, Y-TZP - Yittria Stabilized Tetragonal Zirconia Polycrystal, Ce-TZP- Ceria Stabilized Tetragonal Zirconia Polycrystal, L - Layer, 3L-3 layers, 5 L - 5 Layers, DT-Double Torsion. H-Hardness, σm-Strength of the Matrix Layer, σMLC-Strength of the Multilayer Composite, K1C(m) - Fracture Toughness of the Matrix Layer, K1C(MLC) - Fracture Toughness of the Multilayer Composite. # Calcium hexa-aluminate.

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shock-resistant applications, for example, as in armor materials will be discussed. At the end, the future research directions to blend such development with the newly emerging design philosophy of microstructurally engineered ceramics (MIECs) development will be hinted upon.

18.2

Fabrication of multilayer composites

The current work focused on nanoalumina (NA) powder and nanozirconia (NZ) powder for the fabrication of alumina, zirconia, and zirconia toughened alumina (ZTA) tapes by a tape casting process using a doctor’s blade technique [14,68–70]. The same green tapes were used for the preparation of the laminated NA/NA multilayer composites (NA/NA MLCs) [71]. Similarly, tapes made from NZ powder were used for preparation of the laminated NZ/NZ multilayer composites (NZ/NZ MLCs) [72]. Some zirconia/ZTA, alumina/ZTA, and zirconia/lanthanum phosphate MLC samples were also fabricated utilizing a dip coating technique. The green tapes (3030 mm) and laminates (three layers–20 layers) were all sintered at 1200˚ C–1600˚C in air. The density of as-sintered tapes and laminates were all measured by water displacement technique using Archimedes’s principle. Biaxial flexural strength tests and failure energy were measured by using a Instron universal testing machine. The details of the machine are discussed by Ghosh et al. [71]. Fracture surfaces of the MLC samples were examined by scanning electron microscope.

18.3

Microstructure and properties of multilayer composites

18.3.1 Mechanical properties of multilayer systems 18.3.1.1 Nanoalumina/nanoalumina multilayer composite The load–displacement behavior of MLCs (NA/NA 10-L and 20-L MLC) are shown in Fig. 18.1A. Similar data of monolithic tapes are included for the purpose of comparison. It is observed that the 10-L and 20-L NA MLCs have shown a noncatastrophic serrated failure behavior, reflecting higher magnitude of failure energy, while the single tapes exhibit catastrophic brittle failure behavior. The biaxial strength of NA single tape is measured to be about 282.85 MPa. The biaxial strength of NA 10 layers MLC is measured to be 201.5±28.2 MPa and that of 20layered MLC is 213.41±16.4 MPa. Thus, the biaxial strength slightly decreases but the failure energy increases with increase in the number of layers. It is observed from Fig. 18.1B that the failure energy is minimum for a sintered single alumina tape and the failure energy increases with an increase in the number of layers. The highest failure energy of 65.16±16.57 kJ/m3 is measured for NA 20-layered MLC.

Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors

399

Figure 18.1 (A) Load–displacement behavior and (B) failure energy of nanoalumina single tape and nanoalumina/nanoalumina multilayer composites.

For 10-L MLC samples the failure energy is 41.18±7.42 kJ/m3. The lowest failure energy of 14.56±1.8 kJ/m3 is measured for NA single tape. The grain size of NA single tape is 1.48 m while that of NA/NA MLCs is approximately 400 nm. This finer microstructure would involve more grain boundaries for the crack to negotiate and thereby it can increase the failure energy. The lower strength of MLC can be linked to the presence of pores in the microstructure (the microstructures are not shown here for the sake of brevity).

18.3.1.2 Nanozirconia/nanozirconia multilayer composite The load–displacement data plots of 4-L NZ (nanozirconia)/NZ MLC and a NZ single tape have been compared in Fig. 18.2A. Here also, the NZ/NZ 4-L MLC shows noncatastrophic failure behavior while the single tapes fail in a brittle fashion. The data on the biaxial flexural strength and failure energy of these MLCs are also shown in Fig. 18.2B and C, respectively. The nomenclature “1-L” signifies the monolithic tape data and “3-L” signifies the MLC with three layers and so on. The data of Fig. 18.2B confirm that the values of biaxial strength of NZ/NZ MLC samples were in the range of (e.g., 400–630 MPa) which is reasonably good but it is not as high as that of the NZ single tapes (e.g., 789.77 MPa). The 20L NZ/NZ MLC exhibited the highest biaxial flexural strength (B632.60 MPa) which is approximately 25% lower than that of NZ single tape. Sanchez-Herencia et al. [40] have reported crack bifurcation in the thin layer in zirconia–zirconia MLCs consisting of alternating layers of thin and thick layers. The strength of the MLC containing 50% monoclinic zirconia in the thin layer is approximately 780 MPa [40]. The most commonly observed trend in MLC material is that the strength of the MLC is considerably degraded although the toughness is somewhat improved. However, the variation in the biaxial strength of these MLC samples is not so systematic in terms of the variation in the number of layers. The data represented in Fig. 18.2C show that the NZ/NZ MLC with a higher number of layers (e.g., .8 layers) have failure energy significantly higher than that of the single tapes (e.g., 96.48 kJ/m3). The highest failure energy data (e.g., 694 kJ/ m3) is obtained for the 30-L composite. This is more than six times as high as that

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Figure 18.2 Load–displacement behavior of nanozirconia (NZ) single tape and NZ/NZ multilayer composites (MLCs): (A) 4L and (D) 40L. (B) Biaxial strength and (C) failure energy of NZ single tape and NZ/NZ MLCs.

of the monolithic NZ tape. This also has the highest density of all MLC samples (density 97.46% of theoretical). However, the load–displacement plots of a singlelayer sintered NZ tape and a 40-L nZ/nZ MLC have been compared in Fig. 18.2D. It is observed that the failure energy of the 40-L MLC has failure energy much higher than that of a single tape. Microstructural and fractographic investigations are presently being conducted to understand in detail the reason behind such significant improvement in failure energy of the MLCs. Fig. 18.3A shows the layered structure in the 30-L NZ/NZ MLC. In Fig. 18.3B it is observed that the crack after coming out of the indent gets deflected at the interface. A porous NZ layer may have formed in the interface. The porous interface is weak and when approached by the crack causes the crack to deflect along the porous, weak interface. This may help in raising the toughness of the MLC. The fracture surfaces of a single zirconia tape, a 10-L NZ/NZ and 20L NZ/NZ composite are shown in Fig. 18.4A–C, respectively. The fracture surfaces of 10-L and 20-L NZ/NZ MLC showing crack blunting in microstructure, are represented in Fig. 18.5A and B, respectively. Insets of Fig. 18.5A and B show the exploded view of the marked portion. The enhancement of toughness and failure energy as well as retainment of high biaxial flexural strength in the NZ tape cast laminates is related to the presence of a nanometric grain sized microstructure and the occurrence of a crack blunting mechanism (Figs. 18.4 and 18.5).

Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors

401

Figure 18.3 (A) Layer structure in a 30-L nanozirconia/nanozirconia multilayer composites and (B) crack blunting at interface.

Figure 18.4 Fracture surface of (A) single nanozirconia tape, (B) 10-layer NZ/NZ MLC, (C) 20-layer NZ/NZ MLC. Source: Reused from S. Ghosh, A. Guha, K. M. Krishna, A. K. Mukhopadhyay, H. S. Maiti, Mater. Manuf. Process. 21 (2006) 662–668.

18.3.1.3 Nanozirconia/lanthanum phosphate 20-layer multilayer composite A crack in an MLC deflects at the interface and causes an increase in the crack path which causes an increase in the dissipation of energy and hence, causes toughening.

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Figure 18.5 Fracture surface of (A) 10-layer nanozirconia/nanozirconia multilayer compite (NZ/NZ MLC) and (B) 20-L NZ/NZ MLC, showing crack blunting in microstructure. Source: Reused from S. Ghosh, A. Guha, K. M. Krishna, A. K. Mukhopadhyay, H. S. Maiti, Mater. Manuf. Process. 21 (2006) 662–668.

The interfaces of MLCs need to be properly designed in order to facilitate crack propagation at the interface. A weaker interface would conveniently allow crack deflection at the interface and cause toughening. A material having the strength and failure energy sufficiently weaker than the matrix material may be used as an interface material to assist crack deflection. Recently Morgan and Marshall [73] reported LaPO4 to be compatible with alumina and zirconia. Morgan and Marshall [73] also show promise in deflecting cracks in composites made up of alumina/lanthanum phosphate and zirconia/lanthanum phosphate. Besides crack deflection at the interface other phenomena like layer sliding and residual stresses also play a role. A compressive residual stress in the inner layer makes it difficult for the crack to propagate and causes toughening while during fiber sliding the energy dissipated to overcome friction causes toughening. In our study MLCs of zirconia with lanthanum phosphate (LaP) as weak interface material has been studied. The load–displacement behavior of NZ MLC with lanthanum phosphate (NZ/ LaP, 20 NZ layer with 19 number of interspersed lanthanum phosphate layer) as interface material is shown in Fig. 18.6. It is observed that the NZ/LaP MLC shows load–reload behavior. The load–displacement plot of monolithic NZ tape with a relative density of 75% is also shown in the same figure for the purpose of comparison. The biaxial strength of NZ/LaP (16.15 MPa) is degraded as compared to that of the single NZ tape. However, the failure energy is found to be more than 2 times more than that of monolithic tape. The failure energy of NZ/LaP is found to be approximately 282.38 kJ/m3 and that of monolithic NZ tape is about 96.48 kJ/m3 (Fig. 18.7). Kim et al. [74] reported the bending strength of Mullite layers with aluminum phosphate interphases to be about 154 MPa. It is observed that in all MLCs, there is a gain in failure energy but there is a drop in the biaxial flexural strength. The lower strength of the interface layer may be due to the porosity in the interface layer. The entire coating may act as a flaw and drastically reduce the strength. Also due to the poor adhesion at the interface flaws may develop and reduce the strength.

Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors

403

Figure 18.6 Load–displacement plots of nanozirconia/monazite 20-layer multilayer composite and NZ single tape.

Figure 18.7 High failure energy of nanozirconia/monazite 20-layer multilayer composite.

There is tensile residual stress in the zirconia layers and compressive residual stress in the interface LaP layers. Thus, in zirconia layer with tensile residual stress, the crack propagates in a direction perpendicular to the layers. Several concurrent processes of energy dissipation are observed in the NZ/LaP MLC. For instance, it is observed that the crack after interacting with a pore in the interface does not propagate anymore, Fig. 18.8A. Interaction of the main crack

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Figure 18.8 (A) Crack stops propagating after interacting with a pore in the interface layer. (B) Crack causing extensive delamination of the interface layer and propagating parallel to the interface.

with the weak interface causes extensive cracking of the LaP interface and a subsequently developed crack runs parallel to the interface, causing more energy dissipation, Fig. 18.8B. Both delamination cracks and matrix cracking are observed at the weak LaP interface of the MLC sample, Fig. 18.8B. Crack deflection at the interface increases the crack path length and can cause toughening. A crack after entering the interface layer, which is stressed in a compressive manner, gets deflected and propagates parallel to the interface. The propagation of the crack through the layer having compressive stress dissipates more energy and also the crack deflections at the interface cause an increase in the crack path. Similar observations were made by Chartier et al. in the case of alumina/3Y-TZP MLC samples [75]. The aforesaid phenomena together can contribute to toughening and hence to a hike in failure energy. However, in the case of nano-SiC dispersed alumina/3YTZP MLC system, Adachi et al. [76] noted that indentation cracks propagate at an angle of 45 degrees to the layer direction and related the same to the presence of local residual stress in the microstructure.

18.3.1.4 Nanoalumina/5 zirconia toughened alumina multilayer composite Fig. 18.9A shows the load–displacement plot of NA/5ZTA 5-L and 10-L composite. 5 ZTA means a layer with 5 vol.% NZ in an alumina matrix. A 10-L NA/5 ZTA/ NA MLC means here that 10 layers of NA with nine interspersed layers of a porous 5 ZTA coating obtained by a dip coating technique. For this the slurry viscosity was properly optimized. The idea is to introduce a porous interface in between brittle layers to nullify the tensile residual stress which would be developed in the composite during cooling after processing at a higher temperature. The pores would help to accommodate the tensile stress in the ZTA layer. Tensile stress in the inner 5 ZTA layer is detrimental to both the strength and failure energy of the composite because it helps the crack to propagate and cause catastrophic failure. Fig. 18.9B

Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors

405

Figure 18.9 (A) Load–displacement plots of nano alumina/nano 5 ZTA (5-L, 10-L) multilayer composites. (B) Failure energy and (C) biaxial strength of NA single tape and NA/nano 5 ZTA MLCs.

shows that the failure energy of the composites increases with an increase in the number of layers. The biaxial strength data of NA MLC with 5 ZTA as an interface material is shown in Fig. 18.9C. For comparison purpose, the failure energy and biaxial strength of sintered single NA tape are included. The biaxial strength of NA/5 ZTA 5-L and NA/5 ZTA 10-L are found to be 133.76 and 93.66 MPa, respectively. Although, the strength of NA/5 ZTA MLCs is slightly degraded compared to that of NA/NA MLC, the failure energy increases by many folds. The highest value of failure energy is 262.15 kJ/m3 for NA/5 ZTA 10-L MLCs. Zeng et al. [77] reported the bend strength of alumina 25wt.% titanium carbide monolithic and MLC with SiC/C weak interface, prepared by tape casting to be 762 and 605 MPa, respectively. The fracture toughness of the same monolithic and MLC were 5.65 and 5.75 MPa m0.5, respectively. The failure energy of the alumina–titanium carbide MLC was reported to be 584 J/m2 and that of monolithic was 52.4 J/m2. The failure energy of NA/5 ZTA 10-L is measured to be 743.98 J/m2.

18.3.1.5 Nanozirconia/5 zirconia toughened alumina multilayer composite The load–displacement behavior of NZ with 5ZTA as interface material, prepared by dip coating, as discussed earlier, is shown in Fig. 18.10A. In this case also

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Figure 18.10 (A) Load–displacement plot, (B) biaxial strength, and (C) failure energy of nanozirconia (NZ) single tape and NZ/5 ZTA (20-L) multilayer composites.

degradation in the strength of NZ/5 ZTA MLC (22.96 MPa) compared to monolithic tape is observed, Fig. 18.10B. However the failure energy is measured to be more than 3 times as that of the monolithic zirconia tape, Fig. 18.10C. The failure energy of NZ/5 ZTA is 370 kJ/m3. This high value of failure energy may be explained in terms of the fact that the interfacial porous ZTA layer would have compressive stress. In presence of compressive stress, the crack has a chance to be arrested and has a tendency to align itself parallel to the interfacial layer [40] (Fig. 18.11). This suggestion can qualitatively explain the high value of failure energy measured for Z/5 ZTA MLC. Moon et al. [78] reported that MLCs of alumina and zirconia showed R-curve behavior. The SEVNB fracture toughness increased from 3.6 MPam0.5 from initial crack initiation from V-notch and increased with increase in crack length. The fracture toughness plateau was at a value of 4.5 MPam0.5. It is seen from the coated heterogeneous MLC systems, the failure energy is increased drastically compared to the homogeneous MLC systems. As in case of the particulate composite of ZTA ceramics, it is understood that the role of residual stress might play an important role in improving the mechanical properties. Hence, for MLC systems also the study on residual stresses in future is considered to be important as the MLC systems with the different architectures have shown an increase in the mechanical behavior.

Advanced tape cast multilayer thin ceramics and composites with inelastic failure behaviors

407

Figure 18.11 Optical micrograph of indentation crack propagation in a nanozirconia/5 ZTA multilayer composite.

18.3.2 Aspect of toughness improvement in multilayer composite systems Estimated fracture toughness data (KIC) for the present MLC samples are given in Table 18.2. The (KIC) data are estimated from  0:5 KIC 5 2E=ð1v2 Þ

(18.1)

The toughness of NA tape is only 1.38 MPam0.5, however in NA/NA tape cast MLC it rises to about 7 MPam0.5. Further improvement of NA toughness can be obtained in the NA/5 ZTA/NA (B/T/B combination) system. For instance, NA/5 ZTA 10-L MLC gives a value of about 24 MPam0.5. Similarly, in the case of NZ, the toughness is 2 MPam0.5. For NZ/NZ MLC (30-L), the toughness is enhanced to about 25 MPam0.5. A similar level of toughness improvement can be attained in the case of a 20-L NA/5 ZTA MLC. A significant improvement of toughness is obtained in NZ/LP MLC which shows toughness about 7 times that of NZ single tape, suggesting that a T/W/T combination can be nearly as good in terms of effectiveness as far as failure resistance is concerned. So, it seems plausible to argue that higher magnitude of failure energy of the laminates than that of single tape depends on several factors like (a) relatively fine grain size of laminates, (b) production of more energy dissipation for the “load– unload–reload” pattern of load versus displacement behavior, (c) crack deflection at the interface, and (d) crack blunting. Based on designing the interface with brittle/brittle, tough/tough, brittle/tough, tough/brittle, tough/weak, etc. different combinations in advanced tape cast multilayer thin ceramics and composites can lead to significant enhancement in damage resistance with a wide variety of inelastic failure behavior. All these MLCs

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Table 18.2 Estimated fracture toughness of various multilayer composite systems. MLC system

Failure energy () (kJ/m3)

Fracture toughness (KIC) (MPam0.5)

257.61

14.56

1.38

315.28 308.54 335.91

41.18 65.16 70

4.68 6.96 8.26

345.79

262.15

23.43

139.11

96.48

2.00

171.96 174.74 205.45 192.46 186.06 121.75

170 210 694 300 250 370

7.06 8.37 24.02 15.12 8.49 24.65

138.75

282.38

14.00

Young’s modulus (E) (GPa)

NA single tape NA 10-L NA 20-L NA/5 ZTA 5-L NA/5 ZTA 10-L NZ single tape NZ 10-L NZ 20-L NZ 30-L NZ 40-L NZ 50-L NZ/5 ZTA 20-L NZ/LaP 20-L

(a newly emerging design of MIECs) may be very useful for damage and particularly sudden impact shock-resistant applications, for example, as in armor material.

18.4

Summary and conclusions

• Brittle/brittle, tough/tough, brittle/tough, tough/brittle, tough/weak, etc. different combinations in advanced tape cast multilayer thin ceramics and composites are developed. • The most commonly observed trend in MLC material is that the biaxial strength of the MLC is considerably degraded although the toughness is considerably improved. • Finally, among all MLC, the failure energy of 30-L NZ/NZ MLC is found to be highest at about 694 kJ/m3, which is more than seven times that of the monolithic NZ tape (e.g., 96.48 kJ/m3).

References [1] S. Pfeiffer, K. Florio, D. Puccio, M. Grasso, B.M. Colosimo, C.G. Aneziris, et al., J. Euro. Ceramic Soc. 41 (2021) 60876114. [2] C. Ojalvo, M. Ayllon, A.L. Ortiz, R. Moreno, J. Euro. Ceram. Soc. 41 (2021) 54575465.

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Flexible ceramics for environmental remediation

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Triveni Rajashekhar Mandlimath, Keerthi Valsalan and Sathasivam Pratheep Kumar Department of Chemistry, School of Advanced Sciences, VIT-AP University, Amaravati, Andhra Pradesh, India

19.1

Introduction

Worldwide rapid industrialization and other human activities have induced environmental pollution extensively. The poor air and water quality increases the mortality rate which in turn affects the economic development of the country. The world is facing water and air pollution as major problems. Air pollutants are caused by harmful gases, particulate matter (PM), and infectious pathogens. The water is contaminated by dye molecules, herbicides, pesticides, insecticides, pharmaceutical wastes, oil spillage, heavy metal ions, and harmful microbes. The presence of such contaminants in air and water leads to chronic, respiratory, and cardiovascular diseases, renal failure and so on [1]. Hence, the purification of air and water is essential. Globally, tremendous efforts have been made to save the environment. Among such efforts, direct removal of contaminants from air and water is one of the most effective ways. Conventional bulk materials for the removal of toxic chemicals from air and water have few limitations such as low adsorption efficiency, less durability, small capacity, and non-reusability. Electrospun ceramic polymer nanofibrous composites are one-dimensional nanomaterials that possess a high surface area to volume ratio, high porosity, and flexibility, extraordinary length, low density, high thermo-chemical/mechanical, and good sensing properties. In general, the ceramic nanofibers are classified into oxide and nonoxide nanofibers, namely TiO2, α-Al2O3, BaTiO3 [2 4], and SiC and B4C, respectively [5,6]. These flexible ceramics are suitable for the removal of toxic molecules from water as well as air due to their unique properties. They act as efficient adsorbents for pathogens, heavy metal ions, dyes, insecticides, and pesticides and also degrade the dye molecules present in an aqueous solution. In this chapter, the various flexible ceramics that are being tested for environment remedial applications are briefly described.

19.2

Flexible ceramics for environmental remediation

Flexible ceramics have been employed to achieve a cleaner environment. These ceramics nanofibrous membranes are utilized in the removal of heavy metal ions Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00021-X © 2023 Elsevier Ltd. All rights reserved.

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from water, filtration of harmful matters from the air, adsorption of dyes, inhibition of pathogens, and photodegradation of dyes.

19.2.1 Removal of heavy metals The discharge of effluents from various industries like mining, oil and petroleum refineries, pharmaceuticals, fertilizers, electronics, textiles, and leather tanneries comprises an enormous amount of heavy metal ions. The major heavy metal ions dumped into water bodies include Zn (II), Ni (II), Cr (VI), Pb (II), Cd (II), Hg (II), and As (V) ions are nonbiodegradable and easily bio-accumulable [7]. As per the Environmental Protection Agency and World Health Organization, the permissible limit of heavy metals in water is less than 2.0 mg/L [8]. The consumption or absorption of heavy metal ions by living organisms causes multiple health risks such as liver and kidney damage, heart and lung disease, and cancer [9,10]. Removal of heavy metal ions is generally done by many conventional methods, namely chemical precipitation, chemical adsorption, membrane filtration, ionexchange, electrolysis, coagulation, electrochemical treatments, bio-flocculation, and phytoremediation [11,12]. Among these approaches, chemical adsorption is the most convenient, reliable, economical, and effective approach to remove heavy metal ions from wastewater. This is because of its ease of operation, lowcost, feasibility, selection of diverse adsorbents, simplicity in design, high efficiency, and good reversibility. The heavy metal removal efficiency is mainly depending on the selection of adsorbent. An adsorbent with a large surface area and pore volume, and a greater number of active sites on its surface exhibit high adsorption capacity and adsorption rate. Along with these criteria, a good adsorbent must be easily recoverable and reusable. Flexible ceramic nanofibrous offer high adsorption and filtration of heavy metal ions from water due to their large surface area and porosity. The surface functional groups of the flexible ceramics undergo electrostatic interaction with heavy metal ions. The adsorption studies for most of the flexible ceramics are generally studied by Langmuir isotherm, Freundlich isotherm, and the pseudo-order model. MgFe2O4 flexible fibers with a high aspect ratio are excellent Pb (II) adsorbents from an aqueous solution. The selectivity of heavy metal ions by the magnetic fibers is in the order of Pb (II) . Cu (II) . Zn (II) . Ni (II) . Cd (II). Pb (II) ion strongly binds with the fibers because of its strong acidic nature and low first hydrolysis constant. The adsorption capacity of MgFe2O4 fibers with a specific surface area of 57.88 m2/g synthesized using 8% CTAB surfactant is maximum (283.13 mg/g). At low pH, low adsorption efficiency is because of the weak interaction of Pb with MgFe2O4 [13]. Mesoporous SiO2 MgO ceramic fibers with a high specific surface area of 179.56 m2/g are prepared by electrospinning using the citric acid assisted sol gel method. The surface basicity and mesoporous nature of the fibers are responsible for the enhanced adsorption of Pb (II) and Cu (II) under basic conditions and the mechanism of adsorption involves ion exchange and microprecipitation. These recyclable and reusable fibers are capable of adsorbing methylene blue and fulvic acid [14].

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Zirconium metal organic framework-polyacrylonitrile (MOF-808-PAN) nanofibrous membrane consists of two layers. The top MOF-808 layer is responsible for the adsorption of Pb (II) ions and the bottom layer comprised of PVDF nanofibrous membrane controls the flux due to hydrophobicity. The top layer acts as an adsorbent filter for Pb (II) ion, whereas, Pb (II) ion gets adsorbed and removed on the bottom membrane. The presence of background ions reduces the adsorption capacity of the membrane to 18% 37% [15]. The electrospun mesoporous MgO nanofibers with a high surface area of 232.34 m2/g and pore volume of 0.560 cm3/g shows a high adsorption capacity of 2983.4 mg/g and 1824.0 mg/g for Pb (II) and Cd (II) ions, respectively. The removal efficiency of Pb (II) ions during the first period is fast and becomes slow in the second period. This is because, in the first period, diffusion of Pb (II) ions toward MgO fiber surface occurs instantaneously. At all pH values, 95% of the Pb (II) ions are removed [16]. The hollow ceramic fiber membrane developed from rice husk ash is a good adsorbent and filters the heavy metal ions, namely Zn (II), Fe (II), and Ni (II) ions. Ash from the rice husk is composed of 0.34% Fe2O3, 96.19% silica, and a small quantity of carbon. Being asymmetric, the inner membrane’s surface is constituted by 20% finger-like structure, whereas, the outer surface is constituted by 80% sponge-like structure with uniform porosity. The adsorption takes place on the outer sponge-like surface and all the heavy metal ions are removed completely. [17]. Magnetic Fe3O4 fibers remove Pb (II) from an aqueous solution via adsorption. Fe3O4 fibers show better Pb (II) adsorption efficiency compared to Fe3O4 powders [18]. The fluffy cotton-like structured SiO2 MgO nanofibers exhibit a large specific surface area of 485.3 m2/g with bimodal micropores removing the heavy metal ions in the order of Pb (II) . Cu (II) . Ni (II), Cd (II) and Zn (II) at pH more than 5. The adsorbent shows maximum adsorption capacities of 787.9 and 493.0 mg/g for Pb (II) and Cu (II), respectively. These fibers are regenerated [19]. In MoS2 functionalized SiO2/TiO2 hybrid nanofibers, the sulfide ions strongly adsorb Pb (II) ions via complexation. The porous nanofibers provide a large surface area of 505 m2/g for the removal of Pb (II) ions up to 96% [20]. Flexible 3D titania monolith is the network like structures prepared by the stacking of 2D titanium sheets. The monolith is an excellent adsorbent for Pb (II), Cu (II), Cr (III), and Fe (II). Because of the interlayer electrostatic interaction as well as the large surface area, monolith removes the heavy metals efficiently [21]. Highly porous and flexible zeolite imidazole framework-8 (ZIF-8) on bacteria cellulose composite sponges remove toxic Cd (II) and Pb (II) ions from water. Nonporous Fe2O3 Al2O3 composite nanofibers decontaminate the water by removing Pb (II), Cu (II), Ni (II), and Hg (II) ions. The regeneration efficiency of the fibers is 90% [22]. Several polymer ceramic composite nanofiber membranes are good adsorbents in the process of decontaminating the water: Polystyrene/TiO2 nanofibers remove Cu (II) ions [23], PAN/TiO2 shows excellent adsorption capacity for Pb(II) and Cd (II) ions [24], 3-mercaptopropyltrimethoxysilane (TMPTMS) functionalized PVA/TiO2/ZnO is a good adsorbent for Th (II) ions [25], TiO2/chitosan composite removes Cu (II) and Pb (II) ions [26], PAN/γ-AlOOH composite is an efficient adsorbent for Cu (II), Pb (II), and Cd (II) ion [27], chitosan/PVA nanofibers are the defect free

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adsorbents for the removal of Cr (III), Fe (III), and Ni (III) ions. The adsorption process follows the Langmuir adsorption isotherm [28].

19.2.2 Air filtration Increase in anthropogenic activities such as industrial development, inappropriate agricultural activities, fossil fuel consumption, and construction activities cause serious air pollution. Among the air pollutants, PM is considered as a major threat as it causes harmful effects on the respiratory and cardiovascular systems of human beings [29]. PM consists of solid and liquid particles like nitrates, sulfates, polycyclic aromatic hydrocarbons, endotoxin, and metals like Fe, Cu, Ni, Zn, and V [30]. It is classified based on the size of the particles and they are PM0.1 (ultrafine particles with diameter ,0.1 μm), PM2.5 (fine particles with diameter ,2.5 μm) and PM10 (coarse particles with diameter ,10 μm) [31]. Fine and ultrafine particles cause serious respiratory and cardiovascular health issues whereas coarse particles will cause nasal blockage [32,33]. The potential and efficient technique to protect from such pollutants is air filtration. Many filters like glass fibers, nanofibrous filters are being used for air filtration. The filtration mechanism is followed by physical sieving, interception, diffusion, inertial separation, and electrostatic attraction and thus all particles are captured on the surface and pores of the nanofibers. Pathogens are microorganisms that cause infectious diseases. Most such pathogens are highly contagious. Infectious microbes transmit diseases through air and water. Airborne pathogens cause high-risk infections in animals, birds, and human beings. The most common airborne pathogens are bacteria, fungi, and viruses. Influenza, mumps, chickenpox, tuberculosis, anthrax, COVID-19, and meningitis are a few examples of airborne diseases [34]. Filtration of pathogens from the air via the adsorption process is the most simple and inexpensive technique. Flexible ceramic nanofibers with a high surface area are employed for pathogen removal due to their high removal efficiency. Flexible yttrium stabilized zirconia (YSZ)/silica nanofibers possess a large surface area and high pore diameter are employed for efficient air filtration. The ceramic nanofibers show excellent permeability and filtration efficiency due to the pore size, interconnectivity, and complex nanostructures. The addition of 8 wt.% PVP polymer binder results in a high separation ratio for air/water filtration and a high-quality factor for air filtration. Compared to bulk membranes these ceramic nanofibers with less polymeric binder concentration show excellent air/water filtration [35]. PAN/montmorillonite nanofibrous nanocomposite membrane is an efficient filter to remove atmospheric PM2.5. The stacked structure of montmorillonite, the large specific surface area of the membrane, and the presence of reactive functional groups on the surface allow the filtration of PM2.5 effectively. The pore diameter of 206 nm of the nanofibrous membrane favors PM2.5 filtration. Since the membrane is thermally stable up to 200 C, it is suitable for PM2.5 filtration which comes from exhaustion of vehicles (30 C 80 C), cement plants (160 C 320 C), and coal exhaust furnaces (80 C 180 C). Filtration efficiency of 98.7% by capturing 0.97 mg/m3 s of PM2.5 is achieved by this membrane material. The addition of

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montmorillonite to PAN nanofibers decreased the pressure drop from 96.5 to 46.8 Pa [36]. Al2O3 stabilized ZrO2 filter paper is flexible and foldable. Its thermal stability is high and remains intact up to 1500 C. It exhibits .99% filtration efficiency and is utilized for high-temperature gas exhausts particles removal [37]. Porous and lightweight YSZ nanofiber sponge possesses high mechanical, thermal, and chemical stability. These properties offer efficient air filtration at variable temperatures. The flexible 3D nano sponge exhibits 99.5% and 99.7% filtration efficiency for PM0.32.5 at room temperature and at high temperature under the airflow velocities of 4.8 cm/s and 10 cm/s, respectively. The nanofibrous sponge also captures more than 98% of vehicle exhaust particles. Thus, YSZ nanofiber sponge is an excellent air filter to remove PM0.3- 2.5 [38]. Ag nanoparticles decorated on SiO2 TiO2 nanofibers (Ag@ STPNM) is proven to be highly efficient air filter membrane. Ag@STPNM nanofibers have excellent flexibility and porosity with a pore diameter 12 nm. They filter 98.84% PM2.5 and 97.26% PM10 particles efficiently. Because of high thermal stability, the nanoporous membrane is easily recyclable. The nanofibrous membrane has the potential to purify the air of the indoor environment or personal air when incorporated in masks or devices. [39]. The introduction of Pd in SiOC nanofibers presents high mechanical strength, excellent flexibility, and great softness to SiOC. SiOC membrane has low mechanical strength due to its nanoporous and amorphous structure. The SiOC-xPd nanofibers possess an average diameter of 550 nm. The air filtration efficiency of 99.6% for smaller grain size particles and 100% for 5 μm grain size particles is obtained by the membrane. SiOC-xPd membrane is useful in harsh environments to filter particles [40]. Robust flexible and heat resistant ribbons of YSZ nanofibrous membrane are produced using 5.5% PVP. The average diameter of the thin nanofibers is 126 nm and exhibits excellent mechanical properties. The 220 μm thickness membrane consists of 576 layers of YSZ nanofibers in which the particles are trapped. The filtration efficiency for NaCl particles is 99.96% and hence they can be used in industries. Major advantages of the membrane are the regeneration and recyclability by simple air back blowing and mechanical shaking. This membrane shows high thermal stability up to 1000 C. Thus, YSZ nanofibrous membrane can be utilized to filter PM [41]. TiO2/PAN nanofiber composite membrane show excellent air filtration capacity for NaCl particles. The complex pore structure with high TiO2 content reaches high filtration efficiency of 97%. The TiO2/PAN composite membrane is useful in filtering PM [42]. SiO2 nanoparticles reinforced in electrospun polyimide (PI) nanofibers are utilized for efficient filtration of PM0.3 1.0 with high mechanical strength. SiO2@PI membrane has good thermal stability at low and medium temperatures. The pore size of PI increased from 1.37 μm to 1.7 5.06 μm upon introducing SiO2 which in turn increased the filtration efficiency. Polyphenylene sulfide (PPS) 1.5% SiO2@PI results higher filtration performance of 94.378% for PM1.0. The hybrid nanofiber is reusable after 20 cycles. These hybrid nanofibers show high filtration efficiency to filter PM [43]. Electrospun PAN nanofibers embedded with TiO2, ZnO, and Ag nanoparticles are good for air filtration. The filtration efficiency

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of the nanofibers follows the order TiO2/PAN (100%) .Ag/PAN ( . 98%) .ZnO/ PAN (95%) with decreasing air pressure drop. This is because TiO2/PAN agglomerates whereas Ag/PAN possess narrow pore sizes compared to ZnO/PAN nanofibers. These nanofibers show excellent air filtration efficiency for PM0.1-0.4. PAN and TiO2/PAN nanofibers show ISO class 3 standard whereas Ag/PAN and ZnO/PAN come under class 4 standard as per ISO cleanroom standards. It is suggested that Ag/PAN nanofibers can be used in masks, clean rooms, and indoor air purification for air filtration and antibacterial activity [44]. Flexible air filter paper using hydroxyapatite (HAP) nanowires with cotton (CT) fibers filters PM2.5 and PM10 from air. The HAP nanowires are ultralong, highly porous, and exhibits a large surface area. The surface of HAP consists of Ca21 ions, PO43- ions, and OH- ions abundantly in order to electrostatically interact with fine particles including removal of pathogens. The BET-specific surface area of HAP/CT air filter paper is 36.8 m2/g and 17.4 nm pore size. Compared to HAP, the specific surface is reduced with an increase in pore size. The high porosity and mesoporous structure of HAP/CT made the filtration of air easy and increased the removal efficiency for PM2.5 and PM10 compared to HAP and CT. The HAP/CT air filter paper shows higher filtration efficiency compared to the commercial breathing mask. Furthermore, Ag nanoparticles on the HAP surface of HAP/CT exhibit antibacterial activity [45].

19.2.3 Adsorption of dyes Industrial discharge of complex organic dyes and oil emulsions into water bodies causes serious water pollution. Organic dyes with carcinogenic effects induce health issues in humans and also affect the biological activity of aquatic life. Removal of such toxic dyes using flexible ceramics for dye adsorption has been the emerging solution recently. These materials have a higher tendency to adsorb organic dyes and oils due to their large surface area, higher flexibility, and porosity. Herein, the applications of flexible ceramic materials for adsorption purposes are discussed. Electrospun lead zirconate titanate perovskite nanofibrous membrane (PZT) with excellent flexibility and foldability is utilized for the adsorption of congo red in an aqueous solution. The nanofibrous membrane adsorbs the dye under ultrasonication. It exhibits a maximum adsorption capacity of 61.78 mg/g. The membrane gradually loses its adsorption capacity with an increase in grain size. PZT follows the quasisecond order kinetic model and Langmuir adsorption isotherm model [46]. Aluminasilica nanofibrous membrane fabricated by electrospinning method is a potential adsorbent of Reactive Red-120 (RR-120) dye in water. The nanofibers with an average diameter of 95 nm form a net-like porous membrane that provides higher flexibility, thermal stability, and free-standing properties. The surface charge of the adsorbent is zero, positive and negative under neutral, acidic, and basic conditions, respectively. The membrane is easily separable from the solution. The membrane exhibits a maximum adsorption capacity of 884.5 mg/g when the aqueous solution is in the pH range of 2 3. This is because of the strong electrostatic attraction between the negatively charged species of the anionic dye and positively charged

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nanofibers. The adsorption capacity reduces with an increase in pH from 98% to 10%. The removal capacity remains constant in the pH range 7 11 due to physical adsorption of the dye with the nanofibers through weak vander waal’s force of attraction and hydrogen bonding occurring in the porous structure of the nanofibrous membrane. Also in this pH range, the large number of negatively charged species promotes the desorption of anionic dye molecules by electrostatic repulsion. Thus alumina-silica nanofibrous membranes are the potential filters for the removal of dyes [47]. NiFe2O4@SiO2 nanofibrous ferromagnetic membrane with high flexibility and porosity is employed in dye and emulsion removal applications. The mesoporous nanofibrous membrane exhibits a BET surface area of 42.87 m2/g, average pore width of 7.88 nm, and high pore volume of 0.086 cm3/g. NiFe2O4@SiO2 nanofibrous membrane adsorbs methylene blue dye from aqueous solution completely. The membrane is easily separated from the solution using an external magnet. The porous structure provides easy water permeability through the membrane whereas oil is impermeable. The high energy surface hydroxyl group of NiFe2O4@SiO2 shows higher adsorption capacity for MB and nonpermeable nature toward oil [48].

19.2.4 Removal of pathogens Waterborne pathogens are categorized into viruses, parasites, and bacteria. The most common diseases due to water contamination are typhoid, cholera, dysentery, hepatitis A and so on [49]. The disinfection of water and air has gained a lot of attention in the research fraternity. The utilization of flexible ceramic nanofibers possessing high surface area and surface functionality is a simple and economic way to disinfect the water. Ag decorated on LiNbO3 nanoparticles embedded into polyvinylidene difluoride (PVDF) film by solvent casting method has an excellent bacterial disinfectant property toward Escherichia coli and Staphylococcus aureus bacteria. Ag/LiNbO3PVDF composite film shows the highest inactivation of E. coli bacteria (99.999%) compared to PVDF and LiNbO3-PVDF. This is due to the presence of Ag in the composite film. Also, the surface charge on E. coli and close contact of the bacteria with the film via electrostatic interaction is responsible for this disinfection [50]. The eco-friendly and reusable quaternary ammonium salts (QAC) immobilized on h-boron nitride (hBN) with polyethylene (PP) nanofibrous membranes are used in developing excellent antibacterial face masks to protect from COVID-19. The filter efficiency and air permeability of the PP fibers depend on its pore size and pore diameter distribution. PP ultrafine fibers nonwovens exhibit an average pore diameter of 1.89 μm and high porosity. The removal efficiency of PM, PM2.5 for commercial PP fibers is 92.1%. The QAC/h-BN/PP nanocomposite fibrous membrane shows 99.1% removal efficiency toward PM2.5 and it is increased with increasing the concentration of QAC/h-BN nanoparticles. The composite nanomembrane exhibit excellent antibacterial property against E. coli and S. aureus of 99.3% and 96.1%, respectively. QAC/h-BN/PP composite nanofibrous membrane is used as the core layer in the facial mask. The positive charge of QAC/h-BN/PP fibrous

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membrane and negative bacteria cell were attracted via electrostatic interaction. The inactivation of bacteria occurred by disrupting the cell membranes by the long chains of QAC. The nanofibrous membrane allows a contact-killing mode for inactivating the bacteria. Thus, the facial mask kills the bacteria when it comes in contact with QAC/h-BN/PP composite fibers [51]. The porous membrane consisting of SiO2 TiO2 nanofibers embedded with Ag nanoparticles (Ag@STPNM) is proven as a highly efficient and potential air filter flexible membrane. The nanofibers owe excellent flexibility that can be bent without any cracks. The membrane has excellent antibacterial removal efficiency of 95.8% and the bacteriostatic toward E. coli bacteria. The Ag1 ions in Ag@STPNM generate the reactive oxygen species by interacting with thiol groups of biological enzymes and thereby inhibit the respiratory chain of the bacteria by inducing oxidative stress and cell damage. The membrane is recyclable and durable and can be incorporated in masks or devices for purifying air both in the indoor and personal environment [39]. Zirconium-doped SrTiO3 flexible nanofibrous membrane (ZSTO) can be twisted and bent without deformation when the Zr content is 20%. The introduction of Ag3PO4 in the ZSTO (AZSTO) membrane increases the antibacterial activity to 83.3% and 84.5% against S. aureus and E. coli under dark conditions, respectively. Under visible light, the inhibition rate of AZSTO increased to .99.99, and the inhibition zone width of greater than 3 mm toward both the bacteria. Visible light irradiation induces the generation of free radicals and charge carriers. The destruction of bacterial cells is channeled by the holes and superoxide radicals generated by AZSTO. The excellent antibacterial effect is due to the synergistic effect of Ag and free radicals under visible light [52]. Electrospun PAN nanofibers embedded with different nanoparticles of TiO2, ZnO, and Ag were studied for air filtration. In TiO2/PAN nanofibers, agglomeration of large particles of TiO2 is observed when compared to ZnO/PAN, whereas, in Ag/PAN, due to the small particle size of Ag nanoparticles, Ag was distributed uniformly on PAN nanofibers. Ag/PAN showed a narrow pore size and ZnO/PAN showed a large pore size. Ag/PAN nanofibers show the highest antibacterial activity against E. coli bacteria due to the spherical particle nature of Ag nanoparticles. Other materials also exhibit good antibacterial activity against E. coli. It is suggested that Ag/PAN nanofibers can be used in masks, clean rooms, and indoor air purification for air filtration and antibacterial activity [44]. Flexible and highly porous HAP/CT filter paper possesses a large surface area and acts as air filter paper. The presence of a large number of Ca21, PO43-, and hydroxyl ions on the HAP surface induce the electrostatic interaction with bacteria to disrupt the bacterial cells. HAP/CT filter paper possesses 95% filtration efficiency for PM2.5 and PM10. Further, Ag nanoparticles immobilized on the surface of HAP/CT membrane are a promising air filter and show good antibacterial activity [45]. Flexible ZnO/HAP bio-ceramics are active against S. aureus and E. coli bacteria. The antibacterial activity of the material depends on the quantity of the ZnO adhered to HAP nanowires. When ZnO nanoparticles on HAP nanowires come in contact with bacteria, it damages the proteins and lipids parts [53]. TiO2 nanoparticles loaded on norland optical adhesive and the resulting antifouling shark skin patterns are etched

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on polyethylene terephthalate by the soft nanoimprint lithography technique. The incorporation of 10 wt.% TiO2 NPs inactivates more than 95% E. coli and up to 80% S. aureus in the presence of UV irradiation [54]. Ag nanoparticles decorated on polydopamine@SiO2 composite nanofibrous membranes are excellent in surface-enhanced Raman scattering (SERS) for the direct detection of bacteria and work against both gram-positive S. aureus bacteria as well as gram-negative E. coli bacteria [55]. Size-tunable Ag nanoparticles decorated SiO2 nanofibers sense the SERS label-free S. aureus and E. coli bacteria. They also possess excellent antibacterial property [56]. The electrospun Ag and TiO2 modified polycaprolactone nanofiber mats destroy S. aureus and E. coli. This material is not only inhibiting the bacteria but also acts as SERS and photocatalyst [57].

19.2.5 Photodegradation of dyes Organic compounds such as synthetic organic dyes, pharmaceuticals, and personal care products are the main reasons for water pollution. Among them, dyes are toxic and chemically stable organic complex compounds that are difficult to remove from water. Such dyes are being deposited into water bodies by the textile, paper, and paint industries. Organic dyes cause carcinogenic and mutagenic issues to human beings and aquatic life. Researchers are working on smart materials like flexible ceramics which can degrade dyes in water via photocatalysis. The flexible ceramics can be recovered and recycled in simple steps. CuFe2O4 loaded on polydopamine pretreated flexible ceramic/polyester fabric (CPF) shows excellent photocatalytic activity against methylene blue dye under visible light. CuFe2O4 improves the thermal stability of CPF. Exposure of more active sites on the flexible fabric surface with reduction of agglomeration by CuFe2O4 on CPF enhanced the photocatalytic activity. The apparent rate constant of 0.0235 is obtained from Langmuir-Hinshelwood pseudo-first-order model studies. The material is recyclable and durable after three cycles. The electrons generated by CuFe2O4 react with oxygen to form superoxide radicals and holes react with water to form hydroxyl radicals on the surface of the photocatalyst. Also, the decomposition of H2O2 generates hydroxyl radicals on the surface of CuFe2O4. Photo-Fentonlike nature of catalyst CuFe2O4 contributes the free radicals for the degradation of MB. Therefore, CPF/CuFe2O4 acts as an excellent photocatalyst for the degradation of organic pollutants in water [58]. Versatile 1D PVDF/ZnO doped with lanthanide (Sm, La, Er) is used for the photodegradation of methylene blue and rhodamine B dyes under visible light. The fibrous membrane exhibits a large surface area, high photocatalytic activity, and recovery properties. The PVDF/ZnO: La (37%) shows high photocatalytic degradation of 96.33% for MB and 93.36% for RhB. The removal of color was visible using naked eyes. The material shows a removal efficiency of 97.56% even after 10 cycles. Thus, this eco-friendly composite fibrous membrane is useful for industrial wastewater treatment [59]. TiO2 nanotube arrays (NTAs) sensitized by CdS and PbS are found to be a photocatalyst for degrading rhodamine B and methylene blue dyes under visible light irradiation. The band gap energy of 2.6 and 1.5 eV is

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obtained for CdS and PbS sensitized TiO2 NTAs, respectively. The photocatalytic efficiency of 55.31% and 88.43% for RhB and MB is provided by CdS TiO2 whereas PbS TiO2 obtained 29.31% for RhB degradation under the Xe lamp. When light is irradiated, the photoexcited electrons are transferred into the CB of TiO2 NTAs and the holes in the VB of TiO2 NTAs are transferred into the VB of CdS, which efficiently inhibits the charge carrier recombination. The electrons generate superoxide radicals causing the degradation of the dyes. The functional membrane material gave a high performance in wastewater treatment in removing organic pollutants [60]. The core-sheath structure of ZnO-PVP (polyvinylpyrrolidone) covered on PAN acts as an efficient photocatalyst for the degradation of methylene blue under UV light. The photodegradation follows the pseudo-first-order kinetic model [61]. TiO2/SiO2/C (TSC) is a nanofiber mat (NFM) composed of TiO2 nanocrystals, amorphous SiO2, and carbon. The mat is flexible and consists of meso- and macropores interconnected to each other. The NFM shows photocatalytic degradation of rhodamine B dye in an aqueous solution under UV light. SiO2 gives flexibility and C enhances the photocatalytic activity of TiO2 by the inhibition of electron hole pair recombination to obtain excellent degradation of the dye under UV light. The photogenerated electrons and holes generate superoxide radicals and hydroxyl radicals which degrade RhB by producing CO2 and H2O [62]. BiOX (X 5 Cl, Br and I) immobilized on Al2O3 ceramic fiber sheet is a visible driven photocatalyst for the degradation of methyl orange dye in water. The ceramic fiber sheet consists of BiOX nanoflowers structure which is made of a large number of nanoplates which makes it to efficiently use the incident light. The fiber sheet is flexible, chemically, and thermally stable. BiOX inhibits the recombination of charge carriers and improves photocatalytic activity. This structure makes the material to use the incident light efficiently for the reaction. The large surface area provides sufficient adsorption sites for higher adsorption of the dye and efficiently degrades it under visible light irradiation. The material shows 100% degradation of MO at pH 5 2. BiOX ceramic fiber sheet shows good reusability, easy recovery, and stability [63]. Biogenic silica is used to modify the surface of polyacrylonitrile (PAN) nanofibers to form composites and it is employed for the photodegradation of Malachite green dye under visible light. The nanofiber composite exhibits an average diameter of 148 210 nm. The enhancement of degradation of the dye is caused by the large surface area and exposure to a greater number of active sites due to the porous structure. Diatomite silica/PAN nanofibrous membrane achieves 98% MG degradation. There is an increase in the removal efficiency of the membrane in the pH range 2 7. The formation of superoxide radicals, hydroxyl radicals by the photocatalysts degraded the dye from the aqueous solution by releasing CO2 and water. The membrane is reusable and photodegradation efficiency is greater than 90% after five cycles. The higher photocatalytic activity and flexibility of the membrane make the PAN/biogenic silica to use for potential applications of dye removal from wastewater [64]. Flexible and highly porous three-dimensional ZnO ceramic nano- and microtetrapods degrade methylene blue dye in water under UV light. The photocatalytic

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efficiency of ZnO T Bi2O3, ZnO T Zn2SnO4, ZnO T CeO2, and pure ZnO T networks were 93%, 87%, 86%, and 86%, respectively, for MB degradation under UV light. The enhancement of photocatalytic activity is because of the formation of type-II heterojunctions in the hybrid networks of ZnO T leading to the effective separation of photoexcitons. Thus ZnO T ceramic tetrapods with hybrid networks are useful for water purification [65]. Zinc nanorod arrays (ZNRAs) deposited on flexible stainless-steel mesh is precoated with Al-doped ZnO (AZO) degrades rhodamine B dye under UV light. The flexible material ceramic shows 93.42% of RhB dye degradation under UV light. The synergistic effect of larger surface area and narrower band gap enhanced the photocatalytic activity of ZNRAs-AZO. The material possesses high chemical stability and reusability. Thus, this flexible ceramic material can be used for environmental remediation [66]. Au nanoparticles decorated SiO2 nanofibers/TiO2 nanoparticles composite mats photocatalytically degrade methylene blue dye in water under simulated solar irradiation excellently. The composite is free-standing, mechanically flexible, and durable at high temperatures. The surface plasmon resonance of Au nanoparticles enhances the photocatalytic behavior by efficient separation of electron hole pairs. The material is recyclable after 10 cycles without losing the photocatalytic activity [67]. Double-layered N-doped Zn2SnO4 film on woven metal wires is used for the photocatalytic degradation of Rhodamine B dye under simulated sunlight. The bottom layer slows down the charge carrier recombination and fast transfer of electrons whereas the top layer improves the surface area. These features enhanced the dye adsorption to efficiently degrade the dye under simulated sunlight. The composite material is flexible and recyclable [68]. Photodegradation of methylene blue dye in an aqueous solution is carried out by flexible graphene composites with Al2O3: Eu31 (GCAl) and SrAl2O4:Bi31 (GCSr) under solar irradiation. The flexible graphene composites show 66% and 44% for the MB degradation by GCA1 and GCSr, respectively, due to low sheet resistance [69]. TiO2-loaded PET (polyethylene) nonwoven sheets degrade methylene blue (MB) under solar irradiation. The complete mineralization of dye occurred within 5 h. The material shows high durability and high performance after 7 cycles. This makes the TiO2/PET sheets a highly efficient and durable material for the photodegradation of MB in an aqueous solution [70].

19.3

Conclusions

Flexible ceramic nanomembranes are the new class of advanced ceramic materials with fascinating properties such as high thermal, chemical, and mechanical stability, large surface-to-volume ratio, and high porosity. Fabrication of these ceramic materials is mainly achieved by the electrospinning method prior to the other synthetic methods. Heavy metals, dyes, pathogens, and PM are removed efficiently from water and air, respectively, using flexible nanomembranes by adsorption mechanism. Strong interaction between the surface functional groups of the membrane

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and toxic molecules induces effective and efficient adsorption and filtration. Flexible ceramics exhibit narrow band gaps and degrade the hazardous dye molecules under UV or visible light. The inhibition of charge-carrier recombination and the generation of highly reactive hydroxyl and/or superoxide radicals have proven that these materials are suitable for dye removal to a greater extent. The application of these nanofibrous membranes in environmental remediation is a low-cost, convenient, and highly efficient approach.

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Ceramic-based coatings for solar energy collection

20

Ding Ding School of Architecture and Civil Engineering, Xihua University, Chengdu, P.R. China

20.1

Background

Scholars have sought the use of renewable and sustainable energy in buildings to achieve carbon neutrality [1]. Solar energy is considered the alternative energy source with the greatest potential for replacing conventional fossil fuel energy [2,3]. Current applications of conventional solar collectors, namely, evacuated tube collectors and metal absorber collectors [4], have three main problems. First, the combination of collectors and buildings is still not reasonable in terms of visual esthetics and structure [5,6]. Second, the service life of a conventional collector is only 15
20 years, which hardly matches the life of a building [7]. The primary effects determining the life of an evacuated tube collector are the destruction of the vacuum space and damage to the glass, but efficiency attenuation for a metal absorber collector is usually caused by deterioration of the absorptive coating [8,9]. Third, because the main disadvantage of solar energy collection is related to dispersion, large-scale applications of solar energy remain expensive. Therefore, it is difficult to reduce the price of conventional collectors. Ceramics are inexpensive civil engineering materials that have been widely used in the electronics and construction industries for a long time. Such materials possess good thermal properties and temperature-stress stability, which makes ceramics suitable raw materials for solar collectors and absorbers. To overcome the aforementioned shortcomings of conventional collectors, a vanadium
titanium black ceramic (VTBC) solar collector (Fig. 20.1) was invented [10]. The body of the allceramic collector is a normal white ceramic, and the coating is VTBC recycled from vanadium tailings.

20.2

State-of-art

This section reviews research on ceramic-based coatings for solar energy collection and considers normal ceramic collectors and VTBC collectors. In this section, normal ceramic collectors refer to hollow ceramic panels painted or sprayed black with simple painting or enameling [11]. Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00022-1 © 2023 Elsevier Ltd. All rights reserved.

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Figure 20.1 Appearance of typical vanadium
titanium black ceramic solar collector.

20.2.1 Normal ceramic collectors The ceramic solar collector was first studied in the 1970s in China. However, the research was suspended due to technical problems [12]. In the 1980s, a United States patent titled “Ceramic Solar Collector” described a specific method for making a dual-effect (using liquid or gas as the working medium) flat solar collector using ceramic as the raw material [13]. As shown in Fig. 20.2, collector module 1 has male
female mating configurations on opposite sides, while collector module 1 has a female
female mating configuration. Each of the collector modules includes a module body (3) having a range of openings and channels (4) provided therein and a generally flat absorber surface. The female portion of the collector module is in the form of a recessed area (5). The male portion of the male
female collectors is in the form of an outwardly projecting tongue (6) which is sized to fit within the recess. Each of the manifold modules (2) includes an elongated open body portion (7) and an outwardly projecting tongue (8) sized to fit recessively into the female recessed area. A tube (9) with a plug (10) is provided on one end of each of the manifold bodies.

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Figure 20.2 Exploded perspective view of the ceramic solar collector (proposed in [13]). Note: 1 and 10 —collector module, 2—manifold module, 3—module body, 4—channel, 5—recessed area, 6—outwardly projecting tongue, 7—elongated open body portion, 8—inwardly projecting tongue, 9—tube, 10—tube plug.

In 1982, a project was conducted to identify the solar absorption properties of ceramic materials that are appropriate for flat plate solar collectors [14]. For purposes of comparison, 35 different coatings were prepared on 56 ceramic tiles. A black-coated copper sheet was also tested concurrently with the ceramic materials. The results suggested that an economically viable ceramic solar collector could be constructed if it was engineered to overcome the relatively low thermal conductivity of clay. In 1989, a new water-trickle ceramic collector was investigated [15]. The idea was to use the ceramic as an absorbing surface in a flat plate collector. Ceramic was selected as the absorbing surface because it is usually inexpensive, locally available in developing countries, and easy to manufacture in those countries. In this type of collector, water flowed atop the absorbing surface and was exposed to sun rays (Fig. 20.3). The black ceramic surface presented a good solution for several problems observed with traditional water-trickle steel collectors and white ceramic collectors. Experimental findings showed that the instantaneous efficiency of the ceramic collector was approximately 5%
17% higher than that of a certain fine-tube conventional collector.

20.2.2 Vanadium
titanium black ceramic collectors The starting point for research on VTBC collectors was the invention of a process for extracting vanadium titanium black ceramic from vanadium tailings, which was proposed by Cao [16]. When using industrial waste, the production of VTBC is affordable and sustainable. VTBC collectors contain VTBC as an absorption coating with a porous three-dimensional network structure, which forms the so-called

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Figure 20.3 Water-trickle ceramic collector (proposed in [15]).

Figure 20.4 Macroimage of the vanadium
titanium black ceramic coating.

“sunlight trap” (Fig. 20.4) [17]. The coating has a stable solar absorptance within the range of 0.93
0.97 [18]. In 1987, prototypes of VTBC solar panels and tiles were proposed [19]. In 1990, the first VTBC flat solar collector was invented, produced, and tested. Experimental results showed that the average daily efficiency of the VTBC collector was 46.1% [20]. In 2006, Cao invented the hollow-plate all-ceramic solar collector by using VTBC as the absorptive coating. The collector manufacturing process consists of four basic stages: preparation of raw materials, shaping, drying, and calcining (Fig. 20.5) [7,21]. First, ordinary ceramic raw materials were mixed with a suitable amount of water and ground in a ball mill to form powders finer than 120 mesh. Second, biscuits were shaped with plaster molds. Third, the biscuits were dried and sprayed with VTBC coating. Fourth, the biscuits were calcined in a roller kiln at a high temperature of 1210 C. The appearance of the VTBC collector was normally square, with multiple specifications (Fig. 20.6). The current market price

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Figure 20.5 Production line for the vanadium
titanium black ceramic collector (proposed in [7,21]).

Figure 20.6 Specifications of typical vanadium
titanium black ceramic collectors (proposed in [7,21]).

of the VTBC collector is approximately $28/m2. If produced on a larger scale, the price is expected to be as low as $8/m2. In 2014, the VTBC collector was developed from a water single-effect system into a water
air dual-effect system (Fig. 20.7) [22]. Experimental testing showed the following results: (1) the appropriate air gap between the absorber panel and the transparent cover is 100 mm; (2) when working as a water single-effect heater, the collection efficiency was 39%. When working as an air single-effect heater, the average temperature rise exceeded 30 C. When working as a water
air dual effect

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Figure 20.7 System diagram of the water
air dual-effect vanadium
titanium black ceramic system (proposed in [22]).

Figure 20.8 Appearance and partial section of the nonparallel enamel-passage vanadium
titanium black ceramic plate.

heater, the total collection efficiency exceeded 90%; (3) the air blower had no obvious effect on the water outlet temperature, while the production of hot water reduced the air outlet temperature by approximately 10 C. In 2015, experimental studies illustrated that the light transmittance of the cover plate, the gap between the absorber and the cover, and the insulation of the collector affected the efficiency of the VTBC collector [23]. On the other hand, numerical simulations showed that optimization of the flow path and structure of the VTBC plate provided greater heat transfer efficiency and little increase in resistance loss [24]. In 2016, a new type of nonparallel enamel-passage VTBC plate was invented (Fig. 20.8). The lower edge of the plate was oblique, ensuring that the outlet was

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431

located in the lowest position during fast construction. Moreover, the enamel surface was added inside the fluid passage, reducing the flow resistance and the possibility of scale formation. VTBC solar collection technology can be combined with thermoelectric power generation technology [25]. In 2017, researchers from Poland investigated VTBC collectors [26]. Their experimental study showed that the maximum value of this parameter did not exceed 7.5 C. A solar radiation flux density of 1000 W/m2 gave a good energy conversion efficiency of 65%. The straight line corresponding to this characteristic is described by a zero-loss collector efficiency coefficient equal to 0.8332. Analyses of the test results indicated that despite some disadvantages, ceramic collectors were competitive with traditional solar panels currently available on the market. In 2020, scholars sought to calculate the instantaneous thermal efficiency of the VTBC collection system more accurately and considered the temperatures of water in the collector, the water storage tank, and pipes and proposed a method for calculating the transient temperature of the collection system [27]. These results showed that the collector efficiency decreased sharply with increasing water temperature. Wind speed had little effect on the thermal performance of the collector in the daytime but had a greater effect at night. The overall thermal performance of the system increased gradually with increasing working fluid volumetric flow rate. When the azimuth was positive in the south, the thermal performance of the collector was the highest on sunny days. VTBC is roughly equivalent to other solar collectors in terms of collection efficiency. Efficiency values obtained under laboratory conditions ranged from 39% to 65% [20,22,26,28]. However, the disadvantages of VTBC solar collectors are also obvious. On the one hand, the emissivity of the absorber coating, made from vanadium extraction tailings, is as high as approximately 90% [17]. On the other hand, the thermal conductivity of ceramic is as low as approximately 1.3 W/mK, a value which is only approximately 1/300 of that for copper [29]. These features have a negative impact on the performance of VTBC solar collectors. Therefore, in 2021, Ding analyzed the heat transfer process and thermal performance of VTBC solar collectors theoretically and experimentally and optimized the collector design according to the influence of different parameters [1]: (1) in terms of heat transfer, the performance of VTBC solar collectors can be evaluated by factors such as the efficiency factor, the heat removal factor, and the instantaneous efficiency of the collector. Although the assessment theories are similar to those for traditional metal flat plate solar collectors, ceramic materials have unique characteristics. On the one hand, the VTBC coating has both high absorptance (0.94) and emissivity (90%). On the other hand, the thermal conductivity of VTBC is very low (1.256 W/mK); (2) in terms of optimization, a method for calculating thermal performance under steadystate conditions and the corresponding computer program was set up. From experimental tests, the reference model (η 5 0.89
2.20Tm ) was proposed. Then, the study analyzed the design of fins and transparent covers on the collector and optimized the parameters such as the heat removal factor, the effective transmittance
absorptance product for the absorber, and the total heat loss coefficient of the collector; the resulting optimization model was (η 5 0.92
2.20Tm ).

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Heat-transfer mechanism

The working principle of a VTBC solar collector is similar to that of a metal plate collector: (1) part of the solar energy radiates to the absorber through the transparent cover, while the other part is absorbed by the cover or reflected back to the sky; (2) part of the solar energy reaching the absorber is absorbed and converted into thermal energy, while the other part is reflected by the plate back to the cover; (3) the heat transfer fluid flows into the flow passages from the inlet of the collector and is heated by the thermal energy; (4) after the temperature rises, the medium flows out through the outlet of the collector with useful thermal energy. In such a heat transfer cycle, the incident solar radiation energy is gradually stored in heat storage equipment; additionally, the cover, end walls, and backplate of the collector continuously lose heat to the ambient environment. This cycle continues until the absorber temperature reaches a steady-state [30]. According to the law of energy conservation, the internal energy increment of the collector per unit time (Qs) is equal to the difference between the solar radiant energy projected on the aperture surface of the collector (IAa) and the optical loss (Ql,o), heat loss (Ql,h), and useful energy output by the collector (Qu) (Fig. 20.9A). Transmittance (τ) is related to the solar radiation energy passing through the transparent cover to the surface of the absorber; the absorptance (α) is related to the radiation energy absorbed by the absorber. According to Fig. 20.9B [31], transmission, absorption, and reflection of radiation are repeated infinitely in the transparent coverabsorber system. The solar radiant energy absorbed by the absorber in this process can be summed, and the transmittance
absorptance product for the absorber [(τα)] can be calculated. In the calculation, the α value of the VTBC solar collector is approximately 0.94 [32]. The reflectivity of the transparent cover to scattered radiation (ρd) is related to the extinction coefficient (K) and the travel length (L) of solar radiation.

Figure 20.9 (A) Energy conservation relationship of vanadium
titanium black ceramic solar collectors. (B) The process of radiation transmission, absorption, and reflection in the transparent cover-absorber system.

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20.4

433

Building application methods

VTBC solar absorbers have been applied in building construction in China since 2010. In general, there are two main types of ceramic solar collectors: module patterns and integration patterns. Both patterns can be installed on a balcony fence [7,17], pitched roof [33], flat roof [34], wall, or ground [35].

20.4.1 Module patterns The structure of the VTBC collector module is similar to that of the traditional metal flat plate collector and consists of an absorber panel, a supporting framework, a transparent cover, an insulation system, a backboard, and relevant accessories (Fig. 20.10). On roofs, the VTBC modules may be embedded (Fig. 20.11) or mounted overhead (Fig. 20.12); on walls, the methods include the adhered type and the fence type (Fig. 20.13).

20.4.2 Integration patterns Compared with module patterns, ceramic solar absorbers adopting integration patterns can be installed on and share the structural layer, waterproof layer, and insulation layer of the building. This pattern is better suited for medium-to-large solar heating systems with limited investment. On roofs, the integration methods include the anchor-supporting type, the preembedded anchor type, and the metal shape type; on walls, integration mainly refers to the embedded type (Fig. 20.14).

Figure 20.10 Cross-sections of a typical vanadium
titanium black ceramic collector module. (A) Horizontal view; (B) vertical view.

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Figure 20.11 Roof-embedded vanadium
titanium black ceramic collector modules. (A) Tile roof (insulated); (B) tile roof (partial noninsulated); (C) profiled sheet roof (insulated); (D) profiled sheet roof (noninsulated).

Figure 20.12 Roof overhead vanadium
titanium black ceramic collector modules. (A) Tile roof (high, insulated); (B) tile roof (low, noninsulated); (C) tile roof (noninsulated); (D) profiled sheet roof (insulated); (E) profiled sheet roof (noninsulated).

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435

Figure 20.13 (A) Wall-adhered and (B) fence-type vanadium
titanium black ceramic collector modules.

Figure 20.14 Integration patterns of vanadium
titanium black ceramic collector modules. (A) roof anchor-supporting type; (B) roof preembedded anchor type; (C) roof metal shape type; (D) wall embedded type.

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Application cases

The authors investigated some VTBC building projects by interviewing experts and users and by undertaking a literature review (Table 20.1). Due to the incompleteness of oral descriptions and physical samples of project materials, the authors also conducted field investigations on some of the projects (indicated by gray shading lines in Table 20.1).

20.5.1 Conceptual architecture Sunbloc (Fig. 20.15) is a competition proposed by Team Heliomet (London Metropolitan University and Guangzhou Academy of Fine Arts) for the 2013 Solar Decathlon held in Datong, China [35]. As an inexpensive house prototype, this work involved more than 10 VTBC absorber plates embedded in the ground of a courtyard for domestic water heating.

20.5.2 Rural residence Most building application projects using VTBC collection technology are rural residences. The project in Wupingfang Village, Shandong, involved the integration of the VTBC solar system on buildings, which means that the collectors were placed directly on the building roof [18] (Fig. 20.16). A typical household in this project was a three-story townhouse with a southern single-slope roof. The building area was 171.05 m2, and the conditioned area was 129.26 m2. However, this building was built using normal bricks without insulation. By adopting 102 VTBC absorber plates measuring 720 3 720 mm VTBC absorber plates, the solar system provided a solar collection area of 52.88 m2 (Fig. 20.17). An experimental study showed that the average daily useful heat gain under a daily solar insolation value of 17 MJ/m2 was 8.61 MJ/ m2 in winter (Table 20.2), and the average efficiency was 50.63% [36]. Therefore, this system provided a relatively warm indoor environment for the rural family. Cui House in Hebei is another example of using the VTBC integration mode in a rural residence (Fig. 20.18A). The most prominent feature of this project is the building material: expanded polystyrene (EPS) hollow blocks and slabs (Fig. 20.18B). Because of the insulation provided, 100 m2 of VTBC absorbers installed on the single-slope southern roof provided an acceptable room temperature for approximately 200 m2 of indoor room area in cold winters.

20.5.3 Urban high-rise residence Some high-rise residences in large cities also adopt VTBC modules installed on balcony fences to provide domestic hot water (Fig. 20.19). These projects comprise distributed-collection and distributed-storage systems. The VTBC fence module usually includes 3
5 absorber plates connected to a water tank (100
120 L). The module can be opened indoors, which facilitates necessary maintenance and cleaning.

Table 20.1 Building projects adopting vanadium
titanium black ceramic solar technology in China. Na ( )

Project

Beijing

Shandong

Build areab (m2)

Envelope

Collect areac (m2)

Installation patternd Balcony fence, M Balcony fence, M Balcony fence, M Slope roof, I

A townhouse

39.90 —

—

2.58

Building 38 and 39, No. 8 Fucheng Rd. Yanbao Longquan Jiayuan

39.92 —

—

2.52

39.96 —

—

1.92

Huimin Kindergarten, Xiguanshi Village, Yangfang Town

40.14 1180

Brick wall, 80 mm XPS external insulation

316.8

Comprehensive waste utilization project, Modern Urban Agriculture Boutique Park, Jinan A greenhouse, Modern Urban Agriculture Boutique Park, Jinan Xiying Town, Jinan A greenhouse, Zibo Academy of Agricultural Sciences Rural residences, Wupingfang Village, Taoyuan Town, Juye County, Heze

36.59 —

—

30

36.59 —

—

—

36.51 187.05 370 mm Brick wall 36.80 — Plastic

45 —

35.27 238

52

240 mm Brick wall, rubber powder polystyrene particles insulation

Application

Domestic hot water Domestic hot water Domestic hot water Floor radiation 1 domestic hot water Slope roof, I Biogas heating

Auxiliary energy — Electricity Electricity Electricity

None

Flat roof, Domestic hot — overhead, water M Slope roof, I — — Wall, I — — None Slope roof, I Floor radiation 1 radiator (Continued)

Table 20.1 (Continued) Na ( )

Project

Hebei

Shanxi

Jilin Ningxia

Tibet

Build areab (m2)

Application

Auxiliary energy

Envelope

Collect areac (m2)

Installation patternd

39.72 200

EPS module

100

Slope roof, I Floor radiation

Electricity

39.48 118

370 mm Brick wall

—

Electricity

38.66 136

370 mm Brick wall

—

37.72 82.61

—

18.2

40.10 —

EPS 3D module

—

Changchun Institute of Engineering Rural residences, Yongli New Village, Ningdong Town, Lingwu County-level City, Yinchuan Tibetan herdsmen settlement, Sangzhuzi Area, Shigatse

43.86 —

—

—

Floor Flat roof, radiation overhead, M Flat roof, Floor overhead, radiation M Slope roof, I Floor radiation Ground, I Domestic hot water Roof, M —

38.17 157

65% Energy-saving

—

Slope roof, overhead, M

29.27 70

240 mm Brick wall

—

A greenhouse, Bange County, Naqu

—

Plastic

—

Flat roof, Radiator overhead, M Slope roof, I —

Cui House, Fanzhuang Village, Luanzhou Town, Luan County, Tangshan A rural residence, Menggezhuang Village, Fengnan Area, Tangshan A rural residence, Xiguxian Village, Chuan Town, Renqiu County-level City, Changzhou Rural residences, Jinyuan area, Taiyuan SunBloc, Datong

—

Floor radiation

Coal

Electricity Electricity — Natural gas

None

—

Qinghai

Rural residences, Xuelihe Village, Qinglin Township, Datong County, Xining A greenhouse, Lijiashan Town, Huangzhong County, Xining

37.10 135

Brick wall, stove ash insulation

35

Slope roof, I Floor radiation

Electricity

36.80 —

Plastic

—

Floor radiation

—

36.08 30

—

16

Ground, overhead, M —

Air heating

Coal

36.28 —

—

2,000

Boiler preheating

None

Dabancheng Middle School, Urumchi

43.37 —

—

—

—

—

A greenhouse, 9 Division, Tacheng A rural residence, Qiqiao Village, Liyang Town, He County, Maanshan Natatorium, Panzhihua College, Panzhihua

45.20 375

Plastic

90

31.62 186

240 mm, Brick wall

—

26.57 —

—

1.500

24.78 —

—

26.34

Domestic hot Air source water heat pump Slope roof, I None

23.02 —

—

24.01

Slope roof, I —

—

23.36 —

—

—

—

—

Rural residences, Laxiwa Town, Guide County, Hainan Sifang Heating Power Company, Hainan Xinjiang

Anhui

Sichuan

Fujian

Huatai Company, Yangwei Industrial Area, Cizao Town, Jinjiang County-level City, Quanzhou Guangdong Huashengchang Ceramic Company, Foshan Yunnan Honghe a

N —North latitude. Build area—building area. If the project comprises more than one household, the building area only refers to a typical house type. Collect area—collection area. d M—module pattern, I—integration pattern. b c

Flat roof, overhead, M Ground, overhead, M Wall, I

Floor radiation Slope roof, I Floor radiation

None Electricity

Roof

—

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Figure 20.15 Images from Sunbloc. (A) Sunbloc; (B) vanadium
titanium black ceramic absorbers embedded on the ground.

Figure 20.16 Images of the construction progress of vanadium
titanium black ceramic (VTBC) systems on the roofs of Wupingfang Village. (A) Building the single-slope roof; (B) paving the insulation layer; (C) paving the VTBC absorbers; (D) paving the glass cover layer.

20.5.4 Public building In addition to residential buildings, VTBC absorbers were used in public buildings. Huimin Kindergarten in a village in Beijing is a brick-concrete structure building with 80 mm extruded polystyrene exterior insulation (Fig. 20.20). The VTBC collecting system integrated on the southern-facing roof provided room heat in the

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441

Figure 20.17 Absorber arrangement on the roof of a typical townhouse in Wupingfang Village.

Table 20.2 Test results for vanadium
titanium black ceramic systems on roofs in Wupingfang Village. Date

Inlet water temperature ( C)

Outlet temperature ( C)

H (MJ/m2)

Outdoor temperature ( C)

Indoor temperature ( C)

Jan 9, 2015 Jan10, 2015 Jan 11, 2015

9.3

59.1

11.927

6.6
13.4

12.0

7.7

58.9

12.477

3.9
15.5

14.5

19.2

70.4

11.457

7.4
14.9

16.3

winter and hot water throughout the year. This was a direct and temperature difference-forced system containing 660 pieces of VTBC absorber plates. The tilt angle was 25 degrees, and the collection area was 316.8 m2. To ensure effectiveness on cloudy days, this system also uses three 90 kW electric heaters as an auxiliary heat source. To examine system performance, this project was tested during heating days. The results showed that the collection efficiency was 41%, and the solar fraction was 63% (Table 20.3).

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Figure 20.18 Cui House. (A) Image of Cui house; (B) installation of a vanadium
titanium black ceramic module on an expanded polystyrene hollow slab.

Figure 20.19 Building 38 and 39, No. 8 Fucheng Rd, Beijing. (A) Building appearance; (B) vanadium
titanium black ceramic (VTBC) balcony fence; (C) detailed drawings of the VTBC module.

20.5.5 Agricultural construction Most agricultural applications of VTBC technology are backwall heating systems in greenhouses. These systems have two operation modes: “absorber radiation mode” (collection loop only) and “absorber 1 basal radiation mode” (collection and heating loops) (Fig. 20.22). In the absorber radiation mode, the floor heating loop is closed. The absorbers collect solar heat, convey most of the heat into the water tank, and release the remaining heat into the greenhouse by absorber radiation. The heat collected from the sun circulates between the absorbers and the tank beneath the backwall to increase the indoor air temperature indirectly. In the floor radiation mode, both the collecting loop and heating loop are manipulated to heat the water in the tank and the soil in the greenhouse. The solar energy collected by the absorbers heats the indoor air and uses floor coils to heat the soil matrix. This mode is controlled by both schedule and temperature.

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Figure 20.20 Huimin Kindergarten.

To confirm the instantaneous heat collection and storage performance of the greenhouse, a triple-day test with absorber radiation mode was conducted in a greenhouse in the Tacheng Basin (Fig. 20.21) from January 23 to 25, 2019 [37]. The solar isolation values on an outdoor horizontal surface were 8.46, 8.63, and 8.61 MJ/m2, respectively, on these days. The outdoor air temperature on these days ranged from approximately
31.0 C to
16.5 C. The ceramic solar collecting loop worked from 12:00 to 18:00, while the floor coil heating loop was not operated. On the one hand, the indoor air temperature of the tested greenhouse varied from
0.1 C to 25.0 C, which was 4.2 C higher than that of the unoccupied reference greenhouse, on average. However, from 10:00 to 12:00 on January 24, when the solar radiation peaked for three days, the air temperature in the tested greenhouse was no higher than that in the reference greenhouse. On the other hand, the collecting loop increased the indoor air temperature by approximately 7.7 C
11.0 C in an hour. During the days, the tank temperatures were initially 11 C (tap water), 23 C, and 26 C (water in the tank) and ended at 30 C, 35 C, and 35 C, respectively. Consequently, the effect of solar warming on the water tank was clear, but it was not obvious for indoor temperatures. Additionally, absorber 1 basal radiation mode experiments were conducted from January 24 to February 1, 2019 [37]. To analyze the performance of the greenhouse-integrated ceramic solar heating system, the test data obtained on the

Table 20.3 Test results for Huimin Kindergarten. Date

Ambient temperature ( C)

Solar radiation (MJ/m2)

Collection system heat gain (MJ)

Conventional energy consumption (MJ)

Collection efficiency (%)

Solar fraction (%)

1 2 3 4

3.1 0.2 5.4 5.9

7.76 11.56 15.25 20.62

803.2 1273.8 2373.8 3047.1

2049.9 1699.4 404.3 0

32.7 34.8 49.1 46.6

28.2 42.8 85.4 100.0

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Figure 20.21 Vanadium
titanium black ceramic integrated greenhouse in the Tacheng Basin. (A) Image of the greenhouse; (B) explosive diagram of testing points in the greenhouse.

Figure 20.22 Diagrams of the vanadium
titanium black ceramic solar collection and basal release system. (A) Absorber radiation mode; (B) absorber 1 basal radiation mode.

last three days (January 30 to February 1) were used. This is because, after 1 week of operation, the test data were stable on these 3 days. The solar isolation values on outdoor horizontal surfaces were 10.12, 9.10, and 7.94 MJ/m2, respectively, on these days. First, the air temperature increment ranged from 0.9-22.4 C. The increment tended to be less notable when the solar irradiation was lower. Second, the indoor air temperature increased approximately 8.4 C
10.0 C in a half-hour when the collecting loop began operating, and this rate was approximately two times faster than that of the absorber radiation mode. Third, during the days, the tank temperature began at 21 C and ended at 41 C. Although the inlet water temperature rose according to the outdoor temperature in this mode, the fluid temperature increment was equivalent to that of the absorber radiation mode. Consequently, the effect of solar heating on the water tank was as clear as that on the indoor temperature. Experimental tests showed that the system efficiency of the absorber 1 basal radiation mode was better than that of the absorber radiation mode (Fig. 20.22). The average daily useful heat gain under a daily solar insolation value of 17 MJ/m2

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Figure 20.23 Comprehensive waste utilization project in Shandong. (A) System installation; (B) image of the building.

was 13.8 MJ, and the mean value of the collection efficiency was 0.81. These values are relatively high. Furthermore, the payback time for the project (7 years) is short, which is principally due to the low cost of ceramic materials and the financial savings for the shared construction components (e.g., transparent cover, metal frame, extra insulation). In addition to greenhouse heating, VTBC solar collection technology can be used for biogas heating in agricultural constructions. A project in Shandong (Fig. 20.23), which had a total volume of 210 m3, provided 9000 m3 of biogas and 280 t of biofertilizer per year by managing 300 t of vegetable wastes. In this project, the 30 m2 VTBC absorber plates accelerated the conversion speed.

20.6

Future directions

1. Ceramic materials: Ceramics have high emissivity (B90%) and low thermal conductivity (1.3 W/mK). In addition, previous studies showed that a ceramic collector without a transparent covering could not meet the heating requirements. Therefore, future studies can focus on reducing the emissivity and enhancing the thermal conductivity of ceramic materials. 2. Collector structure: From theoretical and experimental perspectives, the heat transfer process and thermal performance of VTBC solar collectors can be optimized according to the influence of various parameters. 3. Building application: VTBC technology can be applied to more types of buildings in more creative and accurate ways. This will provide an affordable and feasible pathway toward carbon neutrality, especially in developing areas.

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[22] W. He, Thermal Performance of Dual-Effect Ceramic Solar Collector and Integrated Building Design (in Chinese), Tianjin University, Tianjin, 2014. [23] G. Li, The Preparation and Application of Black Porcelain Solar Heat Utilization of Vanadium Titanium Slag System (in Chinese), Xihua University, Chengdu, 2015. [24] G. Zhou, Y.M. Lin, C.H. Liu, Study on the heat transfer mechanism of ceramic solar collector, Adv. Mater. Res. 1070
1072 (2014) 39
43. Available from: https://doi.org/ 10.4028/www.scientific.net/amr.1070-1072.39. [25] L. Ma, Z. Xie, Design and test of household thermoelectric vanadium titanium black porcelain solar energy devices (in Chinese), Educ. Teach. Forum no. 23 (2016) 100
101. [26] M. Zukowski, G. Woroniak, Experimental testing of ceramic solar collectors, Sol. Energy 146 (2017) 532
542. Available from: https://doi.org/10.1016/j. solener.2017.03.022. [27] R. Ma, D. Ma, E. Long, “Experimental study on the dynamic thermal performance of V-Ti black ceramic solar collector under multiple factors, Sol. Energy 201 (December 2019) (2020) 615
620. Available from: https://doi.org/10.1016/j.solener.2020.03.045. [28] D. Xiu, S. Cao, J. Xu, B. Cai, Q. Wang, Y. Yang, Application of ceramic solar plate heating system (in Chinese), Shandong Sci. 26 (2) (2013) 72
77. [29] D. Zheng, D. Wan, S. Yi, Determining the thermal conductivity of ceramic coatings by relative method, Int. J. Appl. Ceram. Technol. 16 (June) (2019). Available from: https://doi.org/10.1111/ijac.13313. [30] T. Gao, Heat Transfer Analysis and Design Optimization of Flat Solar Collector (in Chinese), Tianjin University, Tianjin, 2011. [31] I.L. Wong, P. Eames, A method for calculating the solar transmittance, absorptance and reflectance of a transparent insulation system, Sol. Energy 111 (2015) 418
425. Available from: https://doi.org/10.1016/j.solener.2014.09.028. [32] Y. Yang, S. Cao, J. Xu, B. Cai, All-ceramic solar collectors, Ceram. Int. 39 (5) (2013) 6009
6012. Available from: https://doi.org/10.1016/j.ceramint.2013.01.011. [33] J. Xu, Y. Yang, B. Cai, Q. Wang, D. Xiu, All-ceramic solar collector and all-ceramic solar roof, J. Energy Inst. 87 (1) (2014) 43
47. Available from: https://doi.org/ 10.1016/j.joei.2014.02.005. [34] R. Ma, R. Ma, Application of air source heat pump with V-Ti black porcelain solar collector to swimming pool, China Water Wastewater 30 (16) (2014) 53
57. [35] C. Wang, D. Ding, Roundup of solar decathlon China 2013 (in Chinese), Architectural J. 11 (2013) 110
114. [36] D. Ding, Mechanism and Optimization Design of Vi-Ti Black Ceramic Solar Collecting Technologies Utilized in Rural Residence (in Chinese), Shandong Jianzhu University, Jinan, 2018. [37] D. Ding, C. Liu, Q. Wang, Z. Zhao, J. Xu, S. Cao, Experimental testing of greenhouseintegrated vanadium-titanium black ceramic solar absorbers, J. Therm. Sci. (2022).

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Suman Chatterjee1,3, Santosh Kumar Sahu2, Adhirath Mandal3 and T.V. Huynh4 1 Department of Mechanical Engineering, National Institute of Technology Rourkela, Rourkela, Odisha, India, 2Department of Mechanical Engineering, VSSUT, Burla, Sambalpur, Odisha, India, 3Department of Mechanical and Automotive Engineering, Kongju National University, Cheonan, South Korea, 4Department of Mechanical Engineering, Faculty of Mechanical Engineering, Ho Chi Minh City University of Technology and Education, Ho Chi Minh City, Vietnam

21.1

Introduction to ceramics in defense and security

In recent years, research in the nanoscale realm has dominated global science and technology, particularly in the exploration of novel materials with unique features attributable to their nanosize regimes. Metals (gold, silver, copper, etc.), organic and inorganic materials (metaloxides, polymers), carbon [graphene, carbon nanotubes (CNTs), etc.] are among the most studied materials, both in their pure and composite forms. Polymers are important in this sector because they are utilized to create polymer-based nanocomposites, which are employed in textiles, pharmaceuticals, chemicals, instrumentation, aerospace, aeronautical, and mechanical engineering. The sensors, on the other hand, are a domain that has attracted the most attention from these nanomaterials. Microwave materials have been employed as dielectric resonators, radomes, multilayer packages, electromagnetic shields, and other components in defense and aerospace applications. Because these materials and the gadgets manufactured by them must withstand harsh weather conditions, the supply of suitable materials is limited. Microwave materials are employed in military and aerospace applications for signal propagation as well as shielding harmful signals, depending on their qualities. Very low relative permittivity, low dielectric loss, a minimal temperature change of relative permittivity/resonant frequency, and a low coefficient of thermal expansion are all crucial material properties for signal propagation applications. Substrates, foams, inks, bulk resonators, high-temperature co-fired ceramics, low-temperature co-fired ceramics, printed circuit boards, and other materials are utilized in these applications. For electromagnetic interference (EMI) shielding applications, the materials should absorb or reflect microwaves. The study provides an overview of microwave/composite material requirements, qualities, and applications in military and aerospace antennas, filters, and oscillators. Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00023-3 © 2023 Elsevier Ltd. All rights reserved.

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Polymer ceramic composites, particularly type 0-3, are a class of materials that combine the electrical capabilities of ceramics with the mechanical flexibility, chemical stability, and processing characteristics of polymers, making them a viable group of materials for functional packages. High-frequency compatibility, low dielectric losses, moderate dielectric constant, low coefficient of thermal expansion, high thermal conductivity, low-temperature variation of relative permittivity, low or no moisture absorption, and high dimensional stability are all important characteristics of microwave substrates used in packaging technology. When constructing circuits below 1 GHz, the dielectric loss tangent of base dielectric substrates is less important. As a result, while building a microwave circuit for various electronic devices utilized in defense, space, and information technology, the substrate material chosen is critical (IT). The microwave component business currently uses a variety of substrate materials, including hard and soft substrates. Ceramic substrates offer the advantage of being able to endure localized heat generated during the wire-bonding process. Hard ceramic substrates are generally extremely isotropic, having a high operating temperature, high thermal conductivity (k), low coefficient of thermal expansion (CTE), low or high permittivity, and low dielectric loss. However, they are brittle, difficult to machine, and the expense of a chemically and thermally suitable conducting layer is relatively high. The polymers have a low dielectric constant and loss tangent, but they have a high CTE, a high-temperature fluctuation in relative permittivity, very low thermal conductivity, and strong mechanical characteristics. As a result, utilizing the composite technique, one can obtain reasonably excellent characteristics for microelectronics applications. Soft polymer-ceramic substrates constructed of thermoset and thermoplastic materials provide superior machinability, improved shock resistance, a low-cost conduction/metallization layer, tailor-made characteristics, and beneficial electrical properties. The importance of the connection between the phases of composite materials in getting the required properties is well established. The mechanical and electrical properties, as well as the heat fluxes between the phases, are controlled by the interspatial interactions, or connectedness, in a multiphase material. The size, shape, interfacial characteristics, percolation level, and porosity of the ceramic filler particles have a significant impact on the composite properties [1,2]. The nature of the matrix employed, that is, thermoset or thermoplastic can categorize polymer matrix composites into two categories. The melt temperature of thermoplastic materials relates to them, but the glass transition temperature of thermosets is associated with their dimensional behavior (Tg). Thermoset matrix composites are favored for low-end applications, while thermoplastic composites and applications are favored for high-end applications. Thermoplastic materials have a higher chemical inertness and service temperature than thermoset materials. In flexible microwave substrates, various thermoplastics with low dielectric losses are employed as the matrix. The research on thermoplastic polymers like polyimide, polydimethylsiloxane, polystyrene, polyethylene (POE), benzocyclobutene, polypropylene, and polyurethane copolymers were primarily focused on thermoplastic polymers like polyimide, polydimethylsiloxane, polystyrene, polyethylene, benzocyclobutene, polypropylene, and polyurethane cop The polymers in these

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composites are frequently reinforced with glass or ceramic. Due to its outstanding dielectric qualities, such as low permittivity and exceptionally low loss tangent, polytetrafluoroethylene (PTFE) is also the most favored host matrix. It is also very stable over a wide frequency range. Chaudhary et al. recently developed a promising EMI shielding material for the defense and aerospace industries based on a multiwall carbon nanotube (MWCNT)-mesocarbon microbead (MCMB) composite paper that has high conductivity, is lightweight, and easily foldable using a simple, efficient, and cost-effective strategy. It is credited with being the first effective EMI shielding [3]. In the 8.4 18 GHz frequency range, Joseph et al. found that a polyvinylidene fluoride-graphite flake composite with a thickness of 1 mm made by simple mixing and hot pressing has an EMI SE of 55 57 dB [4]. Provides an overview of research on carbon-based high EMI shielding materials that could be useful in military and aerospace applications. Conducting polymers, whose conductivity is derived from phonon-assisted hopping between randomly dispersed localized states [5,6], are another viable choice for effective shielding materials. Conducting polymers don’t have a lot of physical qualities, so they’re usually combined with other materials like latex, fibers, or polymer blends. Conducting polymers’ permittivity and permeability may be adjusted synthetically, making them appeal to electrical materials [5 7]. The microstructure and shielding properties of polyaniline nanofiber and its graphite composite are shown in. Although electric magnetic materials are the greatest options for narrow broadband absorption, their attenuation is insufficient for military and aerospace applications [8,9]. To provide the appropriate shielding for these applications, dielectrics are frequently combined with metal or carbon particles. Carbonyl iron and ferrites are the only magnetic materials available, and both are prone to corrosion [10]. It is more practical to employ the EMI material as a paint, although this alternative received less attention [11]. To make them usable for military and aeronautical purposes, more study is needed in this field. Ceramic radome materials for hypervelocity ( . Mach 5) missiles are currently a major research priority for various countries for surveillance and military objectives. Ceramic materials with low and steady dielectric properties, as well as those with low and stable dielectric properties against frequency and temperature variations, are particularly significant [12]. The radome property requirements for surface-to-air, air-to-surface, and air-to-air missiles differ significantly. Moistureabsorbing materials, despite having desirable dielectric and thermal properties, are not suited for radome applications because water has a very high dielectric constant (80.4). There has yet to be identified a single material that can meet all of the requirements of a high-speed radome application. The benefits and drawbacks of different ceramic radome materials have been presented and discussed. In this work, information helps to learn about radome design, including wall thickness versus radar frequency (RF) signals, bore-sight error, and the importance of the ogive shaped radome, as well as the generation concept. Slip-cast fused silica (SCFS) has been recognized as the superior material for hypervelocity radome applications among the numerous materials studied thus far. Aqueous colloidal suspensions can also be used to build SCFS radomes with a near-net shape. SCFS radomes, on the

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other hand, have low mechanical strength and limited rain and abrasion resistance qualities, despite having a large internal porosity (up to 18%). In this chapter, the various strategies used to improve the properties of SCFS radomes necessary for hypervelocity applications have been discussed, with all relevant references included. The nitroxyceram (SiO2 BN Si3N4 composite) offers the best combination of characteristics necessary for radome applications among the many fusedsilica composites, and it can be consolidated and densified using industry-standard powder processing processes.

21.2

Market report on ceramic coating used in defense and security

The products developed using ceramic components, and ceramic-coated components have huge potential. There are several reports stating the potential development and their role in day-to-life, security, automobiles and in the defense sectors. The Ceramic Coating Market was valued at US$8.30 billion in 2020, according to a report by Global Market Insights Inc., and is expected to reach US$13.54 billion by 2027, with a CAGR of 7.3% from 2021 to 2027. The research report examines drivers and prospects, market size and forecasts, top winning strategies, swaying market trends, the competitive landscape, and important investment areas in depth. Ceramic coatings have been observed to endure temperatures of more than 1200 F (650 C), with particularly engineered composite ceramics capable of 1600 F (870 C). Product demand is likely to be driven by rising demand for hightemperature and corrosion-resistant coatings. Ceramic coatings’ emerging applications in the healthcare and aerospace industries, owing to their excellent corrosion and adhesion resistance, are expected to drive the ceramic coating market to enormous growth throughout the forecast period. The wear resistance of carbide coating against erosion, galling, abrasion, and fretting has improved product usage in a variety of applications. The carbide coating type category was valued at over US$1.6 billion in 2020 and is predicted to rise at a CAGR of nearly 7% to over US$2.6 billion by 2027. From a technology standpoint, the thermal spray category is expected to grow at a CAGR of roughly 7.5% throughout the assessment period, surpassing a valuation of US$ 8 billion by 2027. The growing use of thermal sprays in ceramic coatings, which offer various advantages such as improved corrosion resistance and better wear resistance, is expected to boost segmental growth. Some of the factors which play a major role in favoring ceramic products are as follows: G

G

G

G

The aviation industry’s product utilization is increasing. Extensive use of the product in automotive applications. Increased product usage in the healthcare business. Rising in popularity as an antiviral coating for Coronavirus disease-2019 (COVID-19).

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The ‘aerospace and defense’ category is expected to maintain its dominance in 2027, according to a report forecasted by 2027, the aerospace and defense category is expected to hold a 17% revenue share in the global ceramic coating market. The expanding aviation industry has boosted product uptake, particularly in rising markets such as China, Japan, and Europe. Furthermore, rising vehicle production in developing markets such as India, China, and Brazil is expected to boost revenue in the ceramic coating industry. While, on a regional scale, the ceramic coating market in North America is expected to account for about 23% of total industry revenue by 2027. Product demand is expected to be boosted by the developing aerospace sector in the United States. Furthermore, the rising demand for ceramic coatings to protect automobile exteriors from UV radiation is likely to aid future regional market expansion. COVID has also played a huge impact on the demand of ceramic products. Following the COVID-19 epidemic, the government and healthcare organizations looked for effective coating solutions to limit the spread of the novel coronavirus in high-traffic areas such as hospitals, nursing homes, and public transportation. To that end, in May 2020, the Green-Surface Engineering for Advanced Manufacturing Network, based at Concordia University in Canada, launched a campaign to promote antiviral metallic and ceramic coatings to inhibit the spread of the virus in such environments. As a result, increased research into the use of ceramic coatings as antiviral coatings to protect individuals, particularly frontline workers, against COVID-19 has aided industrial growth during the epidemic.

21.3

Ceramic coating materials for defense and security industry

21.3.1 Alumina titania ceramic powders Air plasma spraying (APS) is commonly used to create thick alumina titania coatings with highly stratified microstructures. The principal plasma gas in the APS process is nitrogen or argon. The amount of titania (TiO2) present in alumina titania coatings affects the microstructural properties of the coatings. Coatings made from nanostructured agglomerated alumina titania powders, on the other hand, have outperformed conventional coatings in terms of wear resistance. The characteristics of traditional alumina titania coatings can also be altered by adding fillers such as graphene, CNTs, and other nanomaterials. Significant research effort has been under progress on both the industry and academic sides for the past decade, and there is still a lot of room for future research on this topic [13]. Alumina titania coatings are ideal candidates for abrasive wear and hightemperature erosion wear prevention. These coatings are used in electrical insulation and anti-wear applications, such as on-sleeve shafts, thermocouple jackets, pump shafts, and so on. These coatings are also resistant to wear, corrosion, and thermal shock. To reduce the porosity of the sprayed coating, alumina titania

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powder is vacuum sprayed onto graphite. Wear resistance and adhesion strength increase with the addition of titania, while hardness reduces [14]. With decreased porosity, the coatings deposited by high-velocity oxygen fuel (HVOF) are substantially harder and tougher, with abrasion resistance two to three times higher. Bead width and thickness rise as the hydrogen fraction in the plasma gas grows, and bead symmetry increases as the argon gas fraction increases. Coating bond strength, coating thickness, microhardness, and porosity are all affected by gas enthalpy or H2. Titania particles melt at lower power levels, whereas alumina particles stay unmelted. The aluminum oxide content of the coating grows when the power is raised, and the coating composition gradually approaches that of the feedstock powder. The torch power also increases hardness and wear resistance [15]. Literature suggests that alumina titania ceramic coatings made from conventional and nanostructured powders have emerged as a viable alternative to traditional metallic coatings. Various features of coatings, such as increased microhardness, corrosion resistance, and wear resistance, among others, contribute to their use in tribological applications. Various researchers have evaluated the suitability of alumina titania coatings for various applications by using suitable surface treatments such as heat treatment and reinforcement of binders such as CNTs, graphite, reduced graphene oxide, and CeO2, among others. In comparison to coatings made from conventional powders, the alumina titania coatings made from nanostructured powders were primarily used for wear, thermomechanical, and high-temperature applications. Yang et al. [16] have performed a study on the influence of parameters on alumina/titania-coated ceramic composites. In the study, ball-milling, spray drying, and heat treatment were used to create nanostructured alumina/titania composite powders from nanosized alumina and titania doped with nanosized zirconia and ceria. The nanostructured reconstituted powders were then cool isostatically pressed into bulk ceramic composites and sintered under low pressure. X-ray diffractometer and scanning electron microscope were used to evaluate the phase constitution and microstructures of as-prepared ceramic composites. Vickers hardness, flexural strength, and fracture toughness tests were used to assess the mechanical properties of the ceramic composites in this work. Nanodopants were discovered to have the effects of decreasing the sintering temperature, speeding up densification, strengthening, and toughening the composites. Bian et al. [17] have carried out work on nanostructured Al2O3 with 13 wt.% TiO2 composite powders that were successfully produced by spray drying, heat treatment, and plasma treatment. The composite powders were characterized as they were created using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. To assess the tap density and flowability, a tap density instrument and a hall flowmeter were employed. The best slurry for spray drying had a minimum water content of 47%, and the resulting spray-dried powders had the highest density and flowability. The spherical and porous powders were made using the spray drying technique. The density of the powders increased, while the sphericity of the particles and the pores in the particles decreased after heat treatment.

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21.3.2 Aluminum oxide powders Many applications for transparent polycrystalline aluminum oxide have evolved since its synthesis in 1960 [18,19] and the investigation of its optical and mechanical properties. In the near and mid-infrared regions of the spectrum, Al2O3 is now the most common ceramic material for high-temperature applications. In addition to strong strength (up to 600 800 MPa), thermal conductivity, thermal expansion, and high melting point [20], fairly good light transmission equivalent to MgAl2O4 [21 23], and extraordinary corrosion resistance [24], fine-grained ceramics constructed of Al2O3 have exceptional corrosion resistance. The hardest of all transparent impact-resistant materials is Al2O3-ceramics with submicron grain size, with an HV 10 of .20 GPa [18,25]. Different processes are utilized to produce transparent polycrystalline Al2O3 ceramics, including hot pressing, hot isostatic pressing, spark plasma sintering, and combinations of these procedures. Sintering at temperatures exceeding 1700 C in a hydrogen environment [26,27] is the standard method for producing translucent Al2O3 ceramics. MgO, La2O3, and Y2O3 additives [18] are used to minimize porosity. All the above features are dominated by Al2O3, which is also the most cost-effective to produce. Al2O3-based products for particular applications, such as transparent armor, missile fairings, and supersonic-guided missile shells, are achievable thanks to the combination of these features. Al2O3 powder coatings are commonly used to protect surfaces subjected to harsh operating conditions at high temperatures. Alumina coatings are widely used due to their appealing service qualities and low cost [28 30]. Aluminum oxide ceramics have also been shown to have sufficient heat-resistant characteristics in the range of operating temperatures for C/C composites [31]. When applying the aluminum oxide coating, keep in mind that it may be used in thermo-cyclic conditions, which places significant demands on the adhesive strength and toughness of the coating. The goal of the research performed by Sirota et al. [28] is to figure out how ceramic aluminum oxide coatings form on the surface of C/C composites. The microstructure of the resulting coating as well as the deposition properties were studied. Standard powders of aluminum oxide ceramics were heated and accelerated in a cumulative detonation sprayer to form coatings. The researchers [28] created a new multi-chamber cumulative detonation sprayer (MCDS) device for coating deposition. The MCDS differs from the detonation one in that it adds the energy of detonation combustion products of fuel gas mixtures from many precisely designed detonation chambers. The rate and temperature of combustion products are solely determined by the parameters of fuel gas mixture loading in each chamber. Aluminum oxide (Al2O3) ceramic, often known as corundum ceramic, is a form of ceramic material with a-Al2O3 as its primary phase. In aluminum oxide ceramics, the quantity of Al2O3 phase is usually between 75% and 99%. The proportion of Al2O3 phase in the raw materials is commonly used to classify aluminum oxide ceramics; for example, “75 ceramic,” “85 ceramic,” “90 ceramic,” and “99 ceramic” comprise roughly 75%, 85%, 90%, and 99% A12O3, respectively [32]. Aluminum oxide ceramics have great qualities such as high strength at room temperature, high hardness, chemical corrosion resistance, and chemical stability, as

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well as a large number of raw material sources and a reasonable cost. As a result, it is widely employed in the domains of machinery, chemical industry, environmental protection, electronics, metallurgy, and composite materials [33]. Aluminum oxide ceramics have a high melting point of up to 2025 C. This results in a relatively high sintering temperature, which necessitates a lot of energy and puts a lot of strain on sintering equipment [34]. According to the relevant research, lowering the sintering temperature from 1600 1650 C to 1450 1500 C can reduce the sintering energy consumption of aluminum oxide ceramics by 25.5% [32,35]. Studying the low-temperature sintering technology of Al2O3 has become a crucial research subject in terms of conserving resources, lowering energy consumption, and lowering costs. Li [32] in his research work fabricated alumina, calcium oxide, zirconium dioxide, titanium dioxide, and silica sand powders that were used as the main raw materials to create several composite systems (CaO SiO2 ZrO2, CaO SiO2, and CaO SiO2 TiO2), and aluminum oxide ceramics were made using a semi-dry pressing process. To determine the best composite system, researchers looked at the effects of several composite systems on the sintering properties of aluminum oxide ceramics. This is critical for lowering aluminum oxide ceramics production costs and enhancing their characteristics. Lu et al. [36] in their study proposed a process in which aqueous nanotube dispersions and aluminum ions were molecularly mixed to create boron nitride nanotube (BNNT)/Al2O3 composite powders. Spark plasma sintering was used to consolidate the composite powder, with the ceramic phase undergoing phase transformation to Al2O3 (amorphous) while attaching molecularly mixed BNNTs. The addition of 1.5 vol% BNNTs reduced the Al2O3 particle size by 50% while increasing microhardness by 22%. High load (100 N) indentation revealed that the residual indention area dropped by 65% as the BNNTs content increased from 0 to 1.5 vol %, and the morphology changed from very brittle fracture to no visible fissures. As a result, adding BNNTs to Al2O3 improves its strength and toughness at the same time. The main toughening mechanisms include crack bridging, BNNT pull-out, and crack deflection. In-situ and homogeneously dispersed BNNTs, which are chemically associated with the Al2O3 matrix at the molecular level, are credited with grain refinement of the Al2O3 matrix and effective load sharing.

21.3.3 Chromium oxide powders From a scientific as well as a practical standpoint, crystalline and amorphous materials based on pure and doped zirconium dioxide are of great interest [37]. The potential for such materials to be used as oxygen exchange membranes in fuel cells, sensors, and other similar devices, as well as structural nonmetallic wear-resistant materials, determines their importance [38 42]. Due to their high melting temperature (about 2900 F), such materials can be utilized to make structural elements that can withstand strong mechanical loads, chemically aggressive environments, increased temperatures, and the absence of lubrication, among other things. Highprecision processing of diverse materials (metal, wood, glass, crystals, etc.) and high-quality medical devices with cutting-edge thickness values up to 100 nm for

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cardiac thoracic neurosurgery, vascular, maxillofacial surgery, and ophthalmology are both done with zirconia-based materials [43]. This material’s bio-inertness makes it a good candidate for dental implants [44,45]. The unique ability to modify the characteristics of the material by introducing a number of activating impurities, combined with the noticeable optical and mechanical properties of zirconia crystals and ceramics, opens up a lot of possibilities for using stabilized zirconia as active and passive laser elements [46,47]. Iskamov et al. [37] have investigated the properties of zirconium oxide, Raman scattering, luminescence spectroscopy, and quantum-chemical calculations were used to investigate the origin of luminescence centers in ZrO2 crystals. An oxygen vacancy is connected with the 2.7 eV luminescence band and the 5.2 eV absorption/luminescence excitation band. In order to improve the specific properties of TiNi-based SMAs, the effects of various ternary and quaternary alloying additions have been examined before [48 51]. Certain ternary and quaternary additions have been proven to improve thermomechanical behavior, cyclic stability, creep resistance, strength [52 54], and corrosion resistance [55 57]. TiNi-based composites with a range of reinforcements, such as titanium carbide (TiC) [58], nanoalumina (Al2O3) [59], nanohydroxyapatite (HAP) [60], Ti3SiC2, and Ti2AlC [61], have previously been explored. Zirconia (ZrO2) is a popular material for high-temperature and biomedical applications [62,63]. However, according to the literature, the effect of ZrO2 addition on the properties of TiNi shape memory alloys has not yet been thoroughly investigated. As a result, in the study Raza et al. [48], nanoscaled ZrO2 was employed as a reinforcement in a TiNi-based shape memory alloy to explore its impact on mechanical properties and corrosion resistance. In composites made by pressurefree sintering, the comparison reveals that nanoscaled reinforcements have shown superior characteristics to microsized reinforcements [64].

21.4

Ceramic coating in various parts

Corrosion necessitates a corrosive environment for the reasons stated previously. Coatings function as barriers between a steel surface and a corrosive environment by preventing water, ions, and oxygen from getting to the steel. The level of corrosion and coating disintegration is determined by the barrier’s efficiency. The projected lifetime (assuming no mechanical damage to the coating and/or strain imparted to the steel substrate owing to structural behavior) would probably exceed that of the vessel for a perfectly undamaged coating placed to a perfectly clean surface with adequate surface preparation. The coating lifespan is jeopardized by deviations from perfection. The majority of coatings used in the marine industry are poor barriers to gas diffusion. Coatings allow more than enough oxygen to pass through the bulk of the coating to maintain any corrosion reaction occurring at the metal’s interface. Water, in its gaseous state, has a similar ability to penetrate coatings as oxygen. Water vapor condenses at the metal interface and condenses back to liquid water, causing

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osmotic and cold wall blistering. When water vapor penetrates through the covering and condenses on the underlying cold metal, the latter occurs. Because liquid water cannot pass through the covering, blisters form, which usually contains neutral pH fluid. Water vapor diffusion rates inward are often higher than outward diffusion rates. As a result, the corrosion process is rarely hampered by a shortage of water. Coatings not only convey water vapor but also liquid water, which is absorbed into the bulk of the coating and can cause it to swell, not-bonding from the metal, or leach soluble components over time, depending on the type of coating. Cyclic impacts can eventually weaken the coating’s barrier characteristics. The rate of corrosion is usually determined by the presence of aggressive ions such as chlorides or sulfates at the contact. The network structure of cross-linked coatings like epoxies and polyurethanes prevents these ions from passing through the bulk of the coating. Only pores and imperfections in the covering allow ions to reach the interface.

21.4.1 Submarines With the application of ceramics increasing in day-to-day life ranges from automobiles to defense sectors. A new covering is being investigated by the Navy. It would save fuel expenses for ships and submarines by allowing them to glide more freely across the ocean. The coating’s properties are described as “superhydrophobic.” As a result of this, the Navy is expected to save millions of dollars in fuel costs. Millions of air pockets are trapped beneath the covering. The air film generated by tiny pockets of air allows water to glide off a surface. As a result, there is substantially less drag and friction. There is a strange and difficult situation to sustain in the ocean environment. The texture of these coatings must be just correct in order for them to work in this application in the ocean. There was not enough drag created if the pores were too small. Because the water was able to enter the pores if they were too large, drag was increased. Hundreds of chemical combinations were tried before finding the perfect one. The goal is to create a coating that will last for many years. It must also be applied in the same way as the paint is applied to the hull of a ship. It reminds me of a can of spray paint. Because the coating needed to be textured to produce air pockets, it ended up with a rough white surface. From the report published in bulletins in popular mechanics [65]. The University of Michigan is working on a novel water-repellent coating that could help US Navy ships save money on fuel by making it easier for them to cut through water. The new covering has the potential to make ships, particularly submarines, faster and quieter. The Office of Naval Research is funding research at the University of Michigan to develop long-lasting “superhydrophobic” water coatings for ship hulls. Because water has less friction when traveling over air bubbles than it does when passing over a ship’s hull, the solution is to cover the hull in millions of small air bubbles. This lowers drag, lowering the amount of energy required to propel a ship. This means that warships will be more fuel efficient and have greater ranges. The goal of the Experimental Beryllium Oxide Reactor was to make beryllium oxide work as a neutron moderator in high-temperature, gas-cooled reactors.

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21.4.2 Surface ships Hydraulic components used in the maritime industry are frequently subjected to harsh environmental conditions, which can result in faster deterioration, reduced operation, equipment failure, and costly repairs. Maritime hydraulic machine components, such as exposed shafts, splines, and valves, are frequently wetted and dried or completely immersed in seawater. Biofouling and rust are two of the most common reasons for maritime hydraulic failure. This can lead to biofouling and corrosion issues on these important components, necessitating costly repairs to get the equipment back into working order. The adoption of an engineered coating layer that provides high wear resistance as well as biofouling and corrosion protection could be one answer to this problem. An APS coating of ceramic, such as alumina-titania, is one of the current standard coatings used for hydraulic actuator piston rods in marine vessels (Al2O3 TiO2). Because it is an inert oxide, APS Al2O3 TiO2 coatings provide a physical barrier that protects the underlying substrate from corrosion. Ceramic coatings are brittle and can delaminate due to poor coating adherence, physical abrasion, and mechanical damage caused by removing accumulated hard biofouling. Furthermore, postapplication sealants are frequently necessary to close off the intrinsic porosity of APS coatings, which is another processing variable that can contribute to premature coating failure if not done appropriately [65]. Piola et al. [66] in their study, the protection of hydraulic actuators, researchers compared the antifouling performance of three possible replacement HVOF coatings to the current baseline APS ceramic coating.

21.4.3 Aircraft Ceramic coatings are not only designed to protect vehicles, yachts, boats, and residences; they are also ideal for protecting airplanes and helicopters. The coatings will protect air vessels from corrosion and icing while also lowering maintenance expenses. All sorts of air containers can benefit from ceramic coatings. Both for the exterior and the interior. It will add extra luster and appeal, as well as a self-cleaning effect. Ceramic’s hydrophobic characteristics will improve visibility and add to the safety of flights in the rain or snow. Corrosion, pollutants, and unintentional damage to the outside can all be avoided. Because ceramic coatings are nonorganic, acids, bases, and solvents cannot dissolve them. It will stay on a ship indefinitely and keep its brand-new appearance for years. The NASA Environmentally Responsible Aviation (ERA) Project is testing CMC components and environmental barrier coatings for aircraft turbine engine applications. The ERA Project’s purpose is to investigate and develop alternative aircraft designs and technologies that have the potential to reduce noise, pollutants, and fuel consumption in order to meet the National Aeronautics Research & Development Plan’s midterm goals [67].

21.4.4 Helicopters Helicopters and other low-altitude rotorcraft are exposed to evaporated salt, UV rays, moisture, temperature extremes, and pollution compounded by elements such as oil, fuel, exhaust fumes, and bird/insect pollutants, which can cause damage to

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your helicopter’s performance and look [68]. The Detail Port (DP) ceramic coating is a semi-permanent coating that transforms your helicopter’s paintwork into a useful surface. You acquire a coating that cannot be washed off like a wax or sealant by building an inseparable molecular bond with your paintwork. The DP ceramic coating functions as a second transparent layer, providing additional weather protection. This new clear coat is glossier, more chemically resistant, and more durable than the previous one. Saltwater, washing detergents, tiny scratches, swirl marks, acid rain, bird droppings, and UV rays are all protected against this product. Unlike other products, this coating does not simply form a sacrificial layer of protection. Instead, it forms a semi-permanent shell by bonding to your paintwork or composite surface. This shell has a heat resistance of .763 and is a simple-to-clean barrier against contamination adhesion. Many customers prefer to use our ceramic coating near difficult-to-clean exhaust openings. Boeing, Sikorsky, Bombardier, and many more manufacturers have certified DP ceramic coating. SGS lab tested and certified that this meets or exceeds the rigorous Boeing D6-17487 criteria [68].

21.4.5 Helicopter rotors A spherical plain bearing with a ceramic-coated inner ring, which is utilized in helicopter main rotor applications, has undergone comprehensive testing to validate improved operating life and, as a result, decreased maintenance costs. The self-lubricating and maintenance-free spherical plain bearings are made of stainless and corrosion-resistant steel. They are utilized in primary and secondary flight-control systems because they are small and have a high load-carrying capacity-to-weight ratio [69]. Typically, these are made of a mono-ball construction. The inner ring is toughened all the way through, and the spherical surface is ceramic coated utilizing thermal spray technology. The outer ring has a woven PTFE/glass composite liner connected to its inner spherical surface and is cold-formed around the inner ring [69]. The PTFE liner surface wears as a function of pressure and sliding distance at the interface; each angular motion of the operation under load wears the liner relatively little. The liner wear process is divided into four stages [69] as follows: G

G

G

G

There is a period known as the bedding-in period. A PTFE transfer shear film (the lubricant) begins to form at this point. The wear rate of the PTFE transfer film decreases as it becomes more established. Once the liner is fully set, it wears at a fairly consistent rate. After the liner has worn sufficiently, the liner enters the final phase of fast wear. Because flight safety is vital, the bearings should never be permitted to reach this condition on the aircraft.

21.5

Various advantages and limitations of ceramic coatings

Ceramic coatings on metals and alloys can provide high-performance oxide layers to tackle corrosion, wear, heat, insulation, and friction problems. Thermal spray

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coating, plasma spray coating, sputter coating, dry-film lubricants, and various wet chemical and electrochemical coatings are examples of ceramic coatings. Depending on the use and coating techniques, ceramic films can have thicknesses ranging from 50 nm to several micrometers. Due to their superior thermomechanical properties, novel nanoscale ceramic coatings such as Si3N4, silicon carbide, a diamond-like coating, boron nitride, and cerium oxide have recently been studied in metal and alloy coatings to generate promising high-temperature structural materials. Ceramic coatings have a number of benefits, including increasing part lifetime, preventing corrosion, reducing heat on high-temperature components, reducing friction, preventing thermal and acidic corrosion, and improving surface attractiveness. Ceramic coatings have a number of benefits, but they also have some drawbacks: they are extremely brittle and difficult to repair; de-bonding can occur during expansion and shrinkage; corrosion forms easily at cracks; they are heavier than organic coatings; and the coating requires additional equipment, supplies, and labor. The MAO (magnesium alloy ceramic coating) technique in alkaline solution in the presence of nano-additive NaF or Al2O3 was used to produce novel ceramic coatings on rare-earth-containing magnesium alloy AMS4429 substrates. The MgO crystal phase dominates the coating composition. The additional doping has essentially no effect on the ceramic coating’s crystal phase. The coatings’ topography is altered by the additives. MAO coatings with finer uniform nodules than Al2O3 nanoparticles occur from NaF doping, resulting in a successful protective MAO coating. The use of nanoadditives enhances the corrosion resistance of MAO coatings substantially. Fluoride doping of MAO coatings outperforms aluminate nanoparticles in terms of corrosion resistance to the magnesium substrate. The majority of ceramic coatings have been done to improve the oxidation resistance of carbon materials at high temperatures. Impregnation, chemical vapour deposition, or dipping into precursor sols have all been used to coat the surfaces. Boron oxide was used to coat carbon materials using a variety of processes, the main goal of which was to build a glass layer on the surface and avoid oxygen gas contact with carbon at high temperatures, as in the case of the boron carbide/carbon composites discussed in the previous section. Antioxidation was efficient in the temperature range of 600 C 1000 C, however there was no oxidation resistance at low temperatures below 600 C or high temperatures above 1000 C due to the lack of a glass layer and boron oxide vaporization, respectively. The inclusion of additional oxides, such as silica, which affect the property of the glass phase, was tried in order to achieve a high performance of the boron oxide coating. It was also reported that some oxides were coated to be used as frits for porcelains. Different methods for depositing silicon carbide SiC on the surface of carbon materials were tried, and its efficiency in terms of oxidation resistance was reported. Different methods for depositing silicon carbide SiC on the surface of carbon materials were tried, and its efficiency in terms of oxidation resistance was reported. As in the case of boron carbide, the outermost layer converts to silica at the start of oxidation to create a glass phase. A straightforward procedure for forming a SiC concentration gradient in the carbon matrix was recently established,

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which involved dipping carbon materials into molten silicon at 1450 C and determining a suitable ratio of silicon to the carbon material’s physical surface area. The development of a SiC concentration gradient in carbon materials was combined with an overcoating of zircon (ZrSiO4) on their surfaces, resulting in good oxidation resistance at temperatures as high as 1400 C. Because of their durability and resistance to direct impact, abrasion, and corrosion, ceramic coatings are ideal for military applications such as weapon parts, helmets, and machine bodies. Cerakote is a durable coating that is also thin, so it won’t change the thickness of your weapon and hence won’t hinder its capacity to function. DECO (decoration) carries a MIL SPEC (military specifications) color spectrum designed exclusively for military use. In other applications a chemical polymer solution that is placed on the exterior of vehicles and any products to protect it against external paint damage is known as an industry-grade ceramic coating. It’s usually applied by hand and mixes in with your car’s paint, adding an extra layer of hydrophobic protection. The factory paint job on the car is unaffected by this chemical bonding and the formation of a new layer. While many enthusiasts and even detailers believe ceramic coating is a substitute for a clear bra (paint protection film), it is actually, a substitute for waxing. The major goal is to keep dirt, filth, and stain marks off the paint job, which would harm the clear coat. Depending on the coating and kind of polymer employed, ceramic coating, also known as nanoceramic coating, is a permanent or semipermanent answer to your issues. It does not degrade in regular air circumstances such as rain or summer because of its chemically inherent features. With some advantages the coating also presents some limitations. Pores and other flaws may be present in all coatings. These are frequently caused by air entrapment, solvent boiling, pigment segregation, or other related events during the application of a coating. The thicker the coating, the lower the defect concentration and the less likely pores will reach the steel through the surface. The risk of a pore penetrating a single-coat system is larger than a double or multi-coat system of the same overall thickness. Pores can penetrate both layers because the lower layer’s pore can act as a nucleation location for the higher layer’s pore. Over-thickness in a coating, on the other hand, might result in internal tensions and cracking. As a result, a thicker coating isn’t always preferable. Dry film thicknesses (DFTs) and coat numbers should be attained according to the manufacturer’s recommendations. Stripe coatings are often advised for welds and edges before spray spraying to obtain specified DFTs with good adherence to substrate steel. They become standard procedures for new construction. They are also used in the maintenance and repair/refurbishment of equipment. Spray application procedures, as well as the intrinsic nature of liquid paints, cause the coating to draw back from sharp edges, resulting in the creation of a thin film at the edges. Sharp edge grinding and irregular weld grinding are required by ISO standards as a result of the sharp edges. After grinding and weld treatment to ISO 8501-3, specifies a minimum of 2 mm round edge (P2). Furthermore, ISO standards mandate at least two stripe coatings, with the second stripe coat being decreased only if the total DFT fulfills the standard for weld seams. The striped coat is used to add extra coating thickness around sensitive

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locations such as cut edges, welds, and drain holes. Stripe coats are applied during periodic maintenance on board the vessel to extend the life of the paint scheme during repair work.

21.6

Conclusion

The present study described the scope and application of ceramics in defense sectors. Finally, a literature review of several types of sensors for diverse uses in the defense sector is conducted. Chemical and biological warfare diagnostics, radar electromagnetic shielding applications, low- and high-field electric/magnetic field sensing, and low-frequency (SONAR) sensing and other applications are among the primary sectors. The study also shows the future prospects of the challenges associated with ceramic components in various sectors.

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[65] R. Piola, A.S. Ang, M. Leigh, S.A. Wade, A comparison of the antifouling performance of air plasma spray (APS) ceramic and high velocity oxygen fuel (HVOF) coatings for use in marine hydraulic applications, Biofouling 34 (5) (2018) 479 491. [66] National Aeronautics Research and Development Plan, Executive Office of the President, National Science and Technology Council, Washington, DC 20502, February 2, 2010. [67] http://www.detailport.com/en/helicopter-paint-protection-ceramic-coating#:B: text 5 DP%20ceramic%20coating%20is%20a,like%20a%20wax%20or%20sealant. [68] https://evolution.skf.com/helicopter-application-puts-ceramic-coated-spherical-plainbearings-through-their-paces/. [69] https://www.bmwofescondido.com/cilajet-ceramic/.

Advanced ceramics for anticorrosion and antiwear ceramic coatings

22

Bian Da1,2, Li Jiahong1, Chen Yi3, Ni Zifeng1,2, Qian Shanhua1,2, Zhao Yongwu1,2 and Wang Yongguang4 1 College of Mechanical Engineering, Jiangnan University, Wuxi, P.R. China, 2Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, Wuxi, P.R. China, 3Suzhou Nilfisk R&D Co., Ltd, Suzhou, P.R. China, 4College of Mechanical Engineering, Soochow University, Suzhou, P.R. China

22.1

Introduction

Metals and alloys play an important role in the development of modern society. With the development of society, the service environment of metals and alloys is increasingly harsh. This brings wear damage and corrosion damage, etc. to metals and alloys [1 3]. It is significant to introduce functional coatings on metals and alloys to ensure their service life in harsh environment [4 8]. Ceramic coating is an ideal functional coating because of its excellent antiwear resistance and anticorrosion resistance [9,10]. This present chapter reviews the applications of ceramic coating in antiwear resistance and anticorrosion resistance.

22.2

Anticorrosion ceramic coatings

The application of the coating is an effective way to overcome the corrosion damage to metal substrates. Ceramic coatings have received significant attention because of their strength in high-temperature resistance, chemical stability, and antiaging. This section mainly introduces applications of ceramic coatings in solution corrosion and hot corrosion.

22.2.1 Solution corrosion Sol gel technology is the main method to prepare the anticorrosion ceramic coating. Compared with other methods, ceramic coating prepared by the sol gel method has the following characteristics: 1. Simple process equipment without vacuum conditions or expensive equipment; 2. Low curing temperature, which is particularly important for light metal protection; Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00024-5 © 2023 Elsevier Ltd. All rights reserved.

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3. Without requirements for the substrate. The sol gel ceramic coating can be prepared on substrates of different shapes and materials in a large area, and even coated on the surface of powder materials; 4. Easy to prepare uniform multicomponent oxide coating, and effectively control the composition and microstructure of the coating.

The sol gel process can be described as the creation of an oxide network by progressive condensation reactions of molecular precursors in a liquid medium. Generally, there are two methods to prepare coatings by the sol gel process. One method is the inorganic method, which involves the evolution of networks through the formation of a colloidal suspension (usually oxides) and gelation of the sol (colloidal suspension of very small particles, 1 100 nm) to form a network in the continuous liquid phase. The other method is the organic approach, which generally starts with a solution of monomeric metal or metalloid alkoxide precursors M(OR)n in an alcohol or other low-molecular-weight organic solvent. Here, M represents a network-forming element, such as Si, Ti, Zr, Al, Fe, B, etc.; and R is typically an alkyl group (CxH2x11). Sol gel SiO2, Al2O3, ZrO2, TiO2, CeO2, etc., ceramic coatings can provide effective corrosion protection to stainless steel, carbon steel, aluminum alloy, copper alloy, and other metal substrates. Sol gel SiO2 coating can improve the acidic corrosion resistance of the metal substrate. Sanctis et al. [11] prepared a sol gel SiO2 coating on AISI 304, 310, and 316 using the dip-coating technique. It was found that the sol gel SiO2 coating can improve the corrosion resistance of substrates in a 65% solution of boiling nitric acid. Vasconcelos et al. [12] also found that the sol gel SiO2 coating can improve the acidic corrosion resistance of metal substrate due to the formation of a transition layer between the steel substrate and SiO2 layer. The size of SiO2 has an effect on the compactness of the coating, which affects the corrosion resistance of the coating. Two or more oxide coatings can overcome the limitation of single oxide coatings described above, which can broaden their application areas. Abdollahi et al. [13] reported SiO2 ZrO2 acting very efficiently as corrosion protectors of carbon steel 178 in a neutral 3.5% NaCl solution at room temperature. The coating performs better in terms of corrosion resistance compared with SiO2 coating. The corrosion resistance of SiO2 ZrO2 coating could be increased significantly by approximately three and seven times that of SiO2 coating and carbon steel 178, respectively. The coatings by the sol gel process can be prepared to the substrate through various techniques, such as dip-coating and spin-coating, spraying [14], and electrodeposition [15]. However, whatever technique is used to prepare the sol gel ceramic coatings, during the curing of the coating, a substantial volume contraction and internal stress accumulation occur because of the evaporation of solvents and water, which easily leads to cracks in the coating. Usually, the thickness of sol gel coatings is limited. To overcome the limitation of pure inorganic sol gel ceramic coatings, such as brittleness and limited thickness, much work has been done to form the

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organic inorganic hybrid sol gel coatings. The organic inorganic hybrid sol gel coatings research has attracted much attention in the last decade [16,17]. There are three main approaches to prepare the organic inorganic hybrid sol gel coatings: (1) Blending organic components with inorganic components, and there is no chemical reaction between them. (2) Introducing alkyl through molecule structure design of precursors. (3) Utilize functional groups within the polymeric/oligomeric species to react with the hydrolyzed of inorganic precursors, thus introducing chemical bonding between them. El Hadad and coworkers [18] have made new organic inorganic hybrid nanocomposite coatings on Ti 6Al 4V alloys substrates based on tetramethylorthosilicate and gamma-ethacryloxypropyltrimethoxysilane as organofunctional alkoxysilanes precursors and dimethyltrimethylsilylphosphite as a phosphorus precursor. The extent of intermolecular condensation and cross-linking are increased in the organic inorganic hybrid nanocomposite coatings, and the corrosion resistance is also improved. Organic inorganic sol gel coatings are also used to protect the light metal. For example, Lv and Liu [19] has used a saline with aniline substituted group N-(3-(trimethoxysilyl)propyl)aniline and tetraethyl orthosilicate (TEOS) to make sol gel protective coatings on aluminum substrates. And a high corrosion protection efficiency of 99.3% is achieved. Yu and Tai [20] prepared an organic inorganic sol gel coating on Al Zn alloy with TEOS and (3-glycidyloxypropyl) trimethoxysilane. The gel content and swell ratio test results indicated that the degree of crosslinking increased with increasing TEOS content due to the introduction of more silanol groups into the system, which increases the corrosion resistance of the organic inorganic coating. To overcome the thickness limitation of pure inorganic sol gel ceramic coatings, adding one or more ceramic powders, such as SiO2, TiO2, ZrO2, Al2O3, etc. into the sol is also an effective way, which is usually called composite sol gel method. In this case, the gel phase plays the role of a strong binder between the ceramic grains, making it less likely that the film will crack during processing. Moreover, due to the great amount of ceramic powder, the amount of sol gel in the film is decreased and less shrinkage occurs when the film is processed. The coatings obtained by this method present thickness that can be greater than 50 μm without any crack. For example, Touzin and Be´clin [21] prepared Al2O3 SiO2 ceramic coating using this composite sol gel method with the thickness of 30 μm. And the coating is free of cracks. Our group, Bian et al. has done a lot of research in this field [22 25]. The ceramic powders in the sol of our research are ZrO2, Al2O3. With this method, the thickness of sol gel ceramic coating is increased, which is beneficial for the corrosion resistance improvement of the coating. The electrode ingresses into the coating through the holes and cracks in the coating leading to substrate corrosion. The thick coating makes the crack longer, and the ingression of the electrode through the cracks become difficult. According to the ingression way of the electrode, graphene oxide, graphene, carbon fiber, and carbon nanotubes are selected as reinforcement in ceramic coatings. The result shows that these reinforcements can improve the corrosion resistance of ceramic coating because they can bridge cracks to prevent crack width growth. In addition, when

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the crack tip in the direction of the crack propagation meets with these reinforcements, the crack cannot pass through them because of their considerable strength, so the crack can only change its direction along with the reinforcements. This direction change can consume the fracture energy and make the crack more tortuous leading to a difficult diffusion of the electrode, improving the corrosion resistance. Although nanocarbon materials, such as carbon nanotubes, graphene oxide, and graphene, can improve the corrosion resistance of coatings by blocking and changing the direction of the crack propagation due to their special strength, the interfacial adhesion between nanocarbon materials and coatings is very poor because of their smooth surface and chemical inert. The poor interfacial adhesion limits the ability of nanocarbon materials to block and change the direction of the crack propagation. If the interfacial adhesion can be improved, the ability of nanocarbon materials to block and change the direction of the crack propagation can be highly enhanced, which would lead to higher corrosion resistance. From the interfacial design, carbon nanotubes and graphene oxide are modified TiO2, Al2O3, and ZnO. After the modification, the GO surface becomes rough. The rough surface can improve the interfacial adhesion by providing more contact points and increasing mechanical interlocking between carbon nanotubes (or graphene oxide) and ceramic coating. And there is no obvious gap between carbon nanotubes (or graphene oxide) and ceramic coating. when the crack tip in the direction of the crack propagation meets with modified carbon nanotubes or graphene oxide, the crack changes its direction along them, which needs to consume much fracture energy to break the good interfacial adhesion between carbon nanotubes (or graphene oxide) and ceramic coating. This means that the large interfacial adhesion between carbon nanotubes (or graphene oxide) and ceramic coating can consume more fracture energy to stop the crack propagation leading to high corrosion resistance.

22.2.2 Hot corrosion Ceramic coating is also an ideal coating to prevent the metal substrate from hot corrosion damage. There are three main ceramic coatings used in this field: diffusion coatings, overlay coatings, and thermal barrier coatings (TBCs).

22.2.2.1 Diffusion coatings The basic principle of diffusion coatings is, using the concentration difference of chemical elements in the coating under high-temperature conditions, element, such as Al, is driven to diffuse along the coating to the metal substrate to form an Alrich region, and Al2O3 protective film is gradually formed to prevent the metal substrate from the hot corrosion damage. The development of aluminide diffusion coating has experienced two stages: the simple Al-diffusion coating and multialuminides coating. Simple Al-diffusion coating was developed in the 1950s, initially applied to Co-based turbine guide vanes, and began to be widely used on Ni-based guide vanes after the 1960s [26]. Multialuminides coating is the simple Al-diffusion coating by modifications with silicon, chromium, platinum, etc. Ni Al

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coating is a typical representative of diffusion coatings. Kourtidou et al. [27] prepared Ni Al coating on low carbon steel metal substrate formed by the electrochemical deposition of an initial Ni layer followed by an Al top layer deposited by pack cementation. It is found that pack aluminization of the Ni plated substrates resulted in the formation of Ni2Al3 and Ni3Al phases and a Ni Fe transition zone was formed at the steel Ni interface. These metal diffusions contribute to the high hot corrosion resistance. The common methods for preparing diffusion coatings are supersonic flame spraying (HVOF), physical vapor deposition and chemical vapor deposition, magnetron sputtering, arc spraying, etc. Among them, HVOF is widely used because it is easy to prepare, and its spraying flame speed is very fast, which can reduce the interaction between flame and spraying particles. In addition, the lower flame temperature [compared with plasma spraying (PS)] can also prevent grain growth and powder decomposition. However, the coating prepared by HVOF often contains interconnected holes, which provide an invasion channel for corrosive media causing damage to the metal substrate and reducing the corrosion resistance of the coating. Moreover, the equipment required by HVOF is complex and expensive. It is not cost-effective for spraying samples with low thicknesses and small sizes. In contrast, pack cementation has a simple process and low requirements for equipment.

22.2.2.2 Overlay coatings The typical representative of overlay coatings is MCrAlY (M 5 Ni, Co, Ni 1 Co) [28,29]. Unlike diffusion coatings, the metal particles directly form a hightemperature oxide film during the spraying process of MCrAlY, which can prevent the substrate from hot corrosion damage. MCrAlY coating not only has good antioxidation and hot corrosion resistance but also maintains good bonding properties and a similar thermal expansion coefficient with the substrate material because it has similar elements with the substrate. Therefore the composition of alloy elements can be flexibly selected according to the type of substrate and service environment, so that the coating has good working condition adaptability. Each element in MCrAlY plays its own role. For example, Ni and Al in MCrAlY can form Ni-Al compounds leading to high oxidation resistance. Co in the coating is a good thermal corrosion inhibiting element. Esmaeil and coworkers [30] prepared NiCrMo coatings thermally sprayed by high-velocity air-fuel technique. Mo supported the formation of the protective oxide scale rich in Cr, which reduced the formation of volatile CrCl3 resulting in high hot corrosion resistance.

22.2.2.3 Thermal barrier coatings The thermal barrier coating is the continuous development of MCrAlY, which is one of the most advanced coatings for hot corrosion protection presently. TBCs usually take MCrAlY, Ni Al and Pt Al, etc. as bond-coatings with a thickness of 100 150 μm, and apply ceramic coatings (Y2O3, ZrO2) as top-coatings with a

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thickness of 100 400 μm [31]. This structure not only has good high-temperature oxidation and corrosion resistance but also can reduce the working temperature of metal substrates because of the good thermal insulation performance of the ceramic layer. TBCs has become an important protective material for a new generation of aeroengine and gas turbine. The research shows that the hot corrosion resistance of Yttria-stabilized zirconia (YSZ) coating prepared by electron beam physical vapor deposition (EB-PVD) process is much larger than that prepared by PS process, but the preparation cost and thermal conductivity of YSZ coating are higher. The hot corrosion resistance of YSZ mainly comes from dense Al2O3, in which Cr, Ta, and Y can stabilize the formation of Al2O3 improving the hot corrosion resistance of the coating. The external Na, V, and S will react with Y2O3 in YSZ to form YVO4, leading to the degradation of YSZ coating. In the marine environment, chloride is the main factor of hot corrosion of YSZ coating, resulting in a large number of unprotected oxides on the top of the top-coating and between top-coating and bondcoating. The common methods for preparing TBCs include PS, EB-PVD, laser cladding, flame spraying, cold spraying, and so on. At present, PS and EB-PVD technologies are widely used. PS has the advantages of easy preparation, high deposition rate, and low cost. The coating and substrate are mainly mechanically combined. The coating structure is a typical layered structure. The equipment required by EB-PVD is expensive and the deposition rate is not high. The coating and substrate are mainly chemically combined. The coating structure is a columnar crystal structure, which has high strain tolerance and can greatly reduce the stress inside the coating. The outer layers of TBCs are ceramic coatings with low thermal conductivity, which can reduce the surface temperature of the substrate and improve the hot corrosion resistance of the substrate. At present, many modifications of bond-coatings and top-coatings in TBCs have been studied, but there are still some common problems to be solved. The thermal stress caused by the mismatch of thermal expansion coefficient between bond-coatings and top-coatings in TBCs and the formation and growth of thermal oxidation layer in the coating will lead to spalling and failure of TBCs. Therefore many researchers optimize the structure of TBCs and introduce multilayer coating systems to reduce the thermal stress caused by the mismatch of the thermal expansion coefficient. In addition, TBCs with gradient change in composition and structure have also become a research hotspot in this field. Through gradient design, TBCs can effectively reduce the large change of thermal expansion coefficient, alleviate the internal thermal stress and improve the bonding strength of the coating. However, the above coating structure also needs a complex preparation process, and there are still some problems in the preparation process to be solved for industry application.

22.2.3 Nanocrystalline ceramic coatings In order to overcome the mismatch of thermal expansion coefficient in TBCs, the research team led by Yang et al. [32,33] prepared a new nanocrystalline ceramic coating with the same composition as the substrate alloy by magnetron sputtering.

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Due to a large number of grain boundaries in the nanocrystalline ceramic coatings, the oxide film can be formed on the top of the coating rapidly during hightemperature service by the element diffusion, which improves the hot corrosion resistance of the coating. The chemical composition of nanocrystalline ceramic coating is consistent with that of substrate alloy, so there is almost no concentration gradient. This means nanocrystalline ceramic coating and substrate have good chemical compatibility and good bonding strength. However, the multiple grain boundaries in nanocrystalline coatings provide invasion channels for Cl2 and other elements, which leads to hot corrosion. multilayer nanocrystalline coating was designed to avoid the shortcoming. Yang et al. [34] designed a new duplex nanocrystalline coating with nanocrystalline coating as boned coating and NiCrAlY coating as a top coating. the duplex nanocrystalline coating integrated the advantages of both the conventional NiCrAlY coating and the late-model sputtering nanocrystalline coating while avoids their shortcomings.

22.3

Antiwear ceramic coatings

The application of ceramic coatings is also an effective way to prevent the metal substrate from wear damage because of their high hardness and good performance in wear resistance. Antiwear ceramic coatings are usually prepared by microarc oxidation (MAO), laser cladding, thermal spraying, and sol gel method.

22.3.1 Microarc oxidation MAO is a type of surface treatment technology with a simple process, high efficiency, and environmental protection. It is through the combination of electrolyte and corresponding parameters, on the surface of magnesium, aluminum, titanium, and other nonferrous metals and their alloys, relying on the instantaneous high temperature and high pressure generated by arc discharge, in situ growth of a ceramic film mainly composed of matrix metal oxides. Its application on marine vessels and aviation components has attracted widespread attention. The influencing factors of MAO ceramic coating mainly include electrolyte composition and concentration, power supply parameters, and additives. The composition of the electrolyte is a key factor in obtaining a qualified film. The electrolyte used in the MAO process is usually divided into two categories: acid electrolyte and alkaline electrolyte. However, because acidic electrolytes have a certain degree of pollution to the environment, most of the electrolytes used in the current MAO technology are alkaline electrolytes, such as silicates, phosphates, and borates. In addition, there are many cases of using compound salts [35]. Fig. 22.1 shows the microscopic morphologies of ceramic coatings prepared on 6063 aluminum alloy in different electrolyte systems [36]. It can be seen from the figure that when the electrolyte is a silicate, the porosity on the surface of the membrane is small, and the alumina melts together in a large area to form a ceramic

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Figure 22.1 Morphologies of microarc oxidation ceramic coatings prepared in various ectrolytestan (A) silicate electrolyte, (B) borate electrolyte, and (C) aluminate electrolyte [36].

membrane (Fig. 22.1A). When the electrolyte is aluminate, the surface porosity of the film is higher, and the aluminum oxide is not melted in a large area, but the surface is smoother (Fig. 22.1C). Fig. 22.2 shows the friction factors of ceramic coatings prepared in different electrolyte systems. It can be seen that when the electrolyte is silicate and aluminate, the friction factor is relatively high during the first short time of the friction. As the wear time increases, the friction factor continues to decrease, and the film layer becomes smoother. But for borate, the friction coefficient curve is very irregular because the film is worn through. The influence of electrical parameters on MAO is very large. So far, many researchers have done research on the influence of electrical parameters. However, when each researcher studies electrical parameters, the matrix materials, the electrolyte formulations, and the power supply used are different. Taking into account the influence of the electrical parameters and the interaction of the electrolyte, the influence of electrical parameters on MAO should be considered comprehensively. Me´cuson et al. [37] found that the coating thickness and the MAO time increased linearly within a certain period of time, but beyond the range, the relationship between them was nonlinear. Fig. 22.3 shows the microscopic morphologies of ceramic coatings prepared on 6063 aluminum alloy under different current density conditions. When the current density is 10 A/dm2 , the film is smoother, but the film is thinner, not dense enough, and there are larger holes (Fig. 22.3A); when the current density increases to 15 A/ dm2 , the film thickness increases and becomes denser, and a large area of the surface ceramic layer melts and solidifies together (Fig. 22.3B); when the current density exceeds 20 A/dm2 , the film becomes rougher, and microcracks are also increased a lot (Fig. 22.3C and D). It can be explained that as the current density increases, the thickness of the film increases and becomes denser. But when the current density is too large, the MAO reaction becomes violent, making the film rough or even cracked. To improve the wear resistance of the MAO coating, it is a common method to add other elements to the electrolyte and the substrate. Fig. 22.4 shows that TiO2 additives can reduce porosity on the coating surface. The results show that the addition of this compound can improve the quality of the coating by reducing the number of pores. The gradual increase in the concentration

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Figure 22.2 Friction coefficient curves of microarc oxidation ceramic coatings prepared in various electrolytes [36].

Figure 22.3 Morphologies of microarc oxidation ceramic coatings prepared under various current densities (A) 10 A/dm2 , (B) 15 A/dm2 , (C) 20 A/dm2 , and (D) 25 A/dm2 [36].

of additives has a positive impact on the quality of the coating, making the coating denser and denser. However, it should be noted that in high-concentration solutions, the morphology of the coating changes to coarser grains, and the development of large grains can be detected on the surface. Compared with the coating prepared by adding TiO2 to the silicate and aluminate solutions, the coating prepared with the

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Figure 22.4 Surface morphologies of PEO coatings obtained with different concentrations of titania powers in the electrolytes. (A) (D) Morphologies of the coating obtained in aluminateelectrolyte and (E) (H) morphologies of the coating obtained silicate in electrolyte [38].

silicate solution is slightly looser [38]. The surface morphology of the coating changes significantly with the concentration of graphite particles in the solution. Increasing the concentration of graphite in the solution will produce a denser, smoother surface. However, when the concentration is as high as 8 g/L, many spherical particles appear on the surface [39]. In addition, another study used Al nanoparticles (NPs) as additives to modify surface features. It has been reported that the molten material blocked the micropores on the coating surface because the micropores are the way for Al NPs to induce electric field forces [40]. After adding

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ZrO2 particles, these phenomena have also been reported, in which the micro defect rate has a significant mutation. The positive effect of incorporating ZrO2 particles into the coating is significant because they play a key role in plugging and plugging micropores [41,42].

22.3.2 Laser cladding Laser cladding has been widely used in improving the wear resistance of alloys. By choosing appropriate materials and parameters, a coating with excellent wear resistance can be obtained. Ceramics and ceramic reinforced metal matrix composites are widely used in harsh working environments due to their excellent wear resistance, chemical inertness, and performance at high temperatures. For laser cladding, compatibility is the most important principle. The coating material and the substrate should have similar physical properties, such as melting point (Tm), thermal expansion coefficient (γ), and elastic modulus (E). If the difference is too large, it is difficult to achieve the metallurgical bond between the coating and the substrate; similarly, If the difference between γ and E between the two materials is too large, the residual stress will be too high, exceeding the range that any one material can withstand, resulting in cracks and even peeling of the coating. Commonly used laser cladding ceramic materials include nitrides, carbides, and oxides. Generally speaking, the cladding material system on metal materials develops from a single material to multiple materials. In addition, intermetallic compounds are introduced into the laser cladding metal. However, currently, the most popular research focuses on ceramic metal composites (MMCs). MMC mainly refers to ceramic-reinforced metal matrix composite coating. Harder materials such as TiC, TiB, TiB2, TiN, SiC, and WC, are often used as reinforcing materials. The metal or alloy in the composite system plays the role of transition and buffering. That is, they are used as the bonding phase between the ceramic reinforcing phase (whether unmelted or synthesized in situ) and the matrix, effectively reducing the residual stress and cracking tendency. For example, the TiB/Ti 6Al 4V composite coating prepared by laser cladding has achieved significant improvement in sliding wear resistance and friction performance. The coating was found to have strong adhesion. This excellent bonding is attributed to the excellent interfacial bonding between the in situ formed TiB and Ti 6Al 4V matrix [43]. As the name suggests, intermetallic matrix composite (IMC) is a coating based on intermetallic compounds, such as Ti Al, Ti Ni, and Ti Co. As shown in Fig. 22.5, the flower-like TiB-TiC eutectic ceramics are uniformly distributed in the TiNi Ti2Ni dual-phase intermetallic compound matrix. TiC, TiN, and SiC reinforced Ti3Al IMC coatings were in situ synthesized on a pure Ti substrate by laser cladding with Ti 1 Al 1 TiC, Ti 1 Al 1 TiN, and Ti 1 Al 1 SiC, respectively, in an argon protective atmosphere [44]. Li et al. [45] obtained Ti3Al/TiAl IMC coating with TiC particles dispersed on Ti 6Al 4V alloy. During the laser cladding process, Ti3Al, TiAl, and Al3Ti were synthesized in situ, which have high stiffness and wear resistance, greatly improving the tribological properties of the coating (Fig. 22.6).

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Figure 22.5 SEM images of (A) and (B) typical microstructures of laser clad coating, (C) electron back-scattering micrograph, and (D) detailed SEM image of TiB TiC eutectic [44].

Figure 22.6 Wear mass loss of the Ti3Al/TiAl 1 TiC ceramic layers and the Ti 6Al 4V alloy [45].

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It is well known that rare earth (RE) elements, such as Y, Ce, and La, have an effective effect on the properties of molten alloys due to their special physical and chemical properties. A lot of research has been carried out to discuss the effect of RE on the properties of different alloys [46 48]. In recent years, RE elements have been increasingly used in surface modification of metal materials, including laser cladding. Elements such as Y, Ce, La, or their oxides are all introduced into laser cladding [49 51]. Li et al. [52] studied the effect of Y2O3 on the coating and concluded that due to the addition of Y2O3, the cracking sensitivity of the deposited layer was reduced. One of the main reasons is that the Y element accelerates the spheroidization of the primary phase, thereby refining the microstructure. In addition, the Y element also reduces the activity of carbon, preventing carbon from entering and passing through the primary phase interface. Li et al. [53] deposited a NbC-enhanced FMC layer on the substrate through a laser cladding process and proved that the addition of CeO2 can help reduce internal defects and cracks. Wu et al. [54] deposited a carbideenhanced FMC layer on the metal substrate and proved that in addition to Y2O3 and CeO2, the addition of other types of rare earth element oxides (including La2O3, Pr6O11, and Nd2O3) is also beneficial to reduce cracks in the deposited layer. In addition, some other additives have also been proven to improve the performance of the laser cladding layer. In the study of Li et al. [55], the effect of Cu on the structure and properties of TiC/TiB/TiN reinforced composite coatings was studied in detail. When an appropriate amount of Cu is added, the coating structure is dense, without microcracks and pores, and the wear volume loss is about seven to eight times that of the substrate. In addition, WS2 and CaF2 were also used in laser cladding on different substrates to obtain self-lubricating composite coatings [56 58]. The self-lubricating CaF2/Al2O3 ceramic coating was prepared by the laser cladding process [59]. The scanning electron microscope image of the deposited layer (Fig. 22.7) showed that the spherical calcium fluoride particles were uniformly dispersed in the interplate area of the alumina matrix. The friction coefficient of the solid lubricating phase of calcium fluoride is significantly lower. Compared with the alumina substrate, its self-lubricity and wear resistance are significantly improved. In the near future, more and more additives will be introduced into the laser cladding layer to improve the performance of the coating.

22.3.3 Thermal spraying Thermal spraying technology uses a heat source to heat raw materials such as powder, wire, bar, etc. to a molten, semimelted, or plastically softened state and simultaneously accelerates the jet through high-speed airflow to hit the workpiece surface to form coatings. Compared with other surface engineering technologies, thermal spraying technology is easy to achieve the production of large-area coatings at a high deposition rate, and the spraying process can be automated with the help of robots. Thermal spraying technology has small limitations and a wide range of

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Figure 22.7 A SEM image showing the dispersion of spherical CaF2, in interplate regions of Al2O3 matrix [59].

spraying materials, covering metals, alloys, ceramics, metal ceramics, polymer materials, etc. The most widely used ceramic coating by thermal spraying is the Al2O3 ceramic series. The ceramic coating is brittle and poor in quality, and TiO2 needs to be added to improve the performance of the coating. Michalak et al. [60] prepared Al2O3 1 13 wt.% TiO2 coating and conducted experimental research on different spraying parameters. The research results show that in order to obtain a good coating, the initial properties of the powder must be considered, as well as the spraying parameters. When the shortest spraying distance is 80 mm, the particles will impact the substrate in a completely molten state to form a uniform coating and produce a better frit. The fracture toughness of the coating largely depends on the spraying parameters. The coating structure is uniform, the porosity is relatively low (less than 7% each), and satisfactory wear resistance results are obtained. The largest abrasion volume is observed in the coating with a spraying distance of 100 mm, in which microcracks, grooves, and loosely bonded particles are dominant in the abrasion test. The wear resistance of the shorter spraying distance is about 20% higher than that of the longer spraying distance (Fig. 22.8). Sahab et al. [61] increased the spraying distance from 75 to 90 mm, and found that the bonding strength of the coating decreased and the hardness was improved. A too short spraying distance increased the internal stress of the coating due to overheating, resulting in lower bonding strength. Increasing the powder flow rate will increase the bonding strength and deposition rate of the coating, thereby increasing the hardness of the coating. By increasing the current setting, the particle melting effect is promoted, thereby increasing the hardness of the coating. The selection of materials with different specifications and different crystal types will also cause significant differences in coating quality and wear resistance. Zhao

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Figure 22.8 The results of the ball-on-Diskdisc test for AT13 coatings [60].

Figure 22.9 Wear morphology of the nano Al2O3 13wt.%TiO2, coatings at different temperatures [63]. (A) Room temperature, (B) 200 C, and (C) 500 C.

et al. [62] prepared nanoscale (n-AT13) and micron-scale (m-AT13) Al2O3 13 wt. % TiO2 coatings by PS technology, and applied liquid paraffin to the coating to lubricate reciprocating sliding wear, and found that n-AT13, the columnar particles are finer, and the friction factor and the amount of wear increase with the increase of load or speed. The wear mechanism is that the n-AT13 coating gradually changes from polishing to grain wear and deformation, and the m-AT13 coating is pulled out by grains and microcrack-induced fracture. Lu et al. [63] used atmospheric PS to prepare nanocoatings and found that under dry grinding sliding conditions. As shown in Fig. 22.9, smooth and dense areas appear at both 200 C and 500 C, which increase with temperature. This area can play a role in reducing the friction factor of the coating and can passivate and bifurcate cracks, thereby improving the wear resistance of the coating. The WC phase has high hardness and high wear resistance and is also called a superhard alloy. However, WC is relatively brittle, usually adding Co on this basis

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can be used as a binder phase to make it obtain a certain degree of toughness. Different specifications of spray particles not only make the mechanical properties of the coating different but also affect the wear resistance of the coating. Chen et al. [64] used nano and ultra-fine WC Co coatings prepared by PS and conducted dry sliding wear tests. The friction coefficient of the nanocoatings fluctuates smoothly, and there are only some shallow pits on the surface. Ceramic coatings using PS have high hardness and greater brittleness, and usually need to add a lubricating phase to improve the quality of the coating. He et al. [65] prepared nanostructured TiO2-carbon nanotube (CNT) coatings by PS. CNTs have the effect of fine-grain strengthening and have good wettability at the interface with TiO2, which can absorb external heat, reduce internal stress, and bridge CNTs. This phenomenon improves the cohesive strength of the coating and inhibits the initiation and propagation of cracks [66]. The dry friction and wear process of the coating on the zirconia ball-disk sliding is shown in Fig. 22.10. The CNT strengthens the structure through the formation of a good bonding state, which enhances the toughness of the coating; the CNTs protruding on the surface are gradually cut to the vertical. When the sliding direction rotates, the friction redirection occurs, and the friction factor decreases; a large number of CNTs accumulate and combine to form a carbonaceous lubricating film, which reduces wear; the lubricating film is gradually degraded after being sustained by shear force and plastic deformation [67]. Adding CNT ceramic coating, the reason why the cracks are not easy to expand and the surface damage is lighter is the internal bridging connection phenomenon, which improves the fracture toughness, as shown in Fig. 22.11 [68]. CNTs are evenly distributed in the coating, which improves the mechanical

Figure 22.10 Schematic illustration with TEM, SEM, and Raman map images to reveal the carbon nanotubes (CNTs) induced wear mechanism for plasma sprayed ceramic coating: (A) structure strengthening, (B) tribo-protruding of CNTs,(C) tribo-reorientation of CNTs, (D) tribo-film of CNTs, and (E) tribo-degradation of CNTs [67].

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Figure 22.11 Bridging morphology of CNT cracks [68].

properties of the coating and keeps the CNTs intact, ensuring that the coating has good wear resistance [69,70]. The lubrication effect of CNT reduces the friction factor of the coating, and the toughening effect strengthens the nanostructure, thereby hindering the growth and polymerization of grain boundary cracks, and further improving the wear resistance of the coating on the original basis [71]. In short, adding a hard phase to a multiphase composite coating can achieve the effect of reducing wear by increasing the hardness of the coating, while adding a binder phase improves the toughness of the coating and reduces the friction factor. Although the addition of a single lubricating phase can reduce wear, it is limited by the temperature range. The addition of multiple sets of lubricating phases can not only ensure the lubrication effect in each temperature range but also exhibit synergistic lubrication and improve the wear of the coating; the toughening effect of the lubricating phase is passed. Internal bridging connection prevents the initiation and propagation of cracks.

22.3.4 Sol gel method The sol gel method has the advantages of simplicity, good reproducibility, and low cost. It is a coating technology widely used in the preparation of metal oxide films and is becoming more and more popular in research and industrial production. There are two main reactions in sol gel preparation, namely, the hydrolysis process of the precursor in an acidic or alkaline solution and the polycondensation reaction of the hydrolyzed product. The sol gel solution needs to be used in conjunction with coating techniques such as dip coating, spray coating, and spin coating. After high-temperature curing and dry treatment, a new composite ceramic phase will appear on the surface. Ceramic coatings are divided into two categories: (1) oxide and (2) nonoxide. Typical oxides are aluminum oxide and zirconium oxide, while nonoxides are usually carbides, borides, nitrides, and silicides. Liu et al. [72,73] mainly conducted research on the relationship between the curing temperature of phosphate-bonded ceramic coatings, the types of inorganic curing agents, and the thickness of the coatings with the surface morphology and wear resistance of the coatings. It is found that as the curing temperature increases, the

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main curing reaction equations are different, resulting in different final main products. When the curing temperature increases, the main product changes from the adhesive phase of aluminum trihydrogen phosphate hydrate (AlH3(PO4)2
3H2O) to the adhesive phase of aluminum phosphate (AlPO4). The bonding phase AlPO4 can provide a strong bonding force between ceramic particles, thereby increasing the strength of the coating. By studying the bonding ceramic coatings containing different inorganic curing agents, it is found that the wear rates of the bonding ceramic coatings containing calcium oxide curing agents are 13.68, 3.87, and 1.34 times that of the coatings containing magnesium oxide, aluminum oxide, and zinc oxide, respectively. The wear scar width of the magnesium oxide coating is significantly smaller than that of other coatings. By studying the friction and wear behavior of bonded ceramic coatings with different thicknesses, the influence mechanism of coating thickness on the wear resistance of bonded ceramic coatings is revealed. The study found that with the extension of friction and wear time, the friction coefficients of coatings of different thicknesses increase, but with the increase of coating thickness, the friction coefficient increases slowly in the initial stage, and the steady-state friction coefficient and wear rate show a downward trend. According to the friction and wear test results, the wear rate of the 100 μm coating is about 1.34 and 1.48 times that of the 200 and 300 μm coatings, respectively. Among them, the width of the wear scar of the coating with a thickness of 300 μm is significantly smaller than that of other coatings. Pietrzyk et al. [74] reported the use of graphene as an additive to alumina sol gel. There are two types of graphene oxides, G1 and G2, with lateral dimensions below 4500 and 2000 nm, respectively. Alumina and graphene sols (1% and 2% sol suspensions) were prepared respectively, and then heat-treated at 300 C and 500 C for 15 min. The Al2O3 and Al2O3 1 graphene coatings were tested for friction using a ball-disk tribometer (Fig. 22.12). It can be found that after the heat treatment, a large amount of graphene still remains in the coating. The coating of 2% G2 graphene heat-treated at 300 C is stable and has a lower COF during the entire test. The tribological efficiency of G2 graphene in Al2O3 1 G2 coating is affected by its size and oxidation sensitivity during heat treatment. An increase in temperature (over 400 C) will cause the lubricity of multilayer graphene to decrease, and the reduced graphene will undergo high-temperature oxidation [75]. In addition, the smaller G2 graphene size increases the susceptibility of graphene flake edge oxidation and hinders the formation of the friction film, resulting in the tribological performance of the Al2O3 1 G2 coating being worse than that of the Al2O3 1 G1 coating. In short, the sol gel method of alumina ceramic coating, in which graphene is used as a solid lubricant, significantly reduces friction. Simonenko et al. [76] changed its distribution in the SiC framework by sol gel synthesis of finely divided TiC matrix in the pore space of the SiC framework. The results show that as the content of TiC increases, the compressive strength increases and the specific surface area decreases (determined by low-temperature nitrogen adsorption and mercury intrusion method). X-ray microtomography confirmed the composition gradient of SiC TiC ceramics over the entire depth: the total porosity of the ceramics near the surface area differed by 2.9 times.

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Figure 22.12 The ball-on-disk test of (A)Al2O3, (B)Al2O3 1 1% G1 graphene and Al2O3 1 2% G1 graphene, and (C) Al2O3 1 1% G2 graphene coatings and Al2O3 1 2% G2 graphene at 300 C and 500 C [75].

Liu et al. [77] used the sol gel method to prepare ZrB2 ZrC SiC powders with different SiC contents. The effects of ceramization process and SiC content on the microstructure evolution of ceramic powders were studied. In the ceramization process, the carbon content plays an important role in the adjustment of the ZrB2 ZrC SiC ceramic composition by promoting the growth of ZrC and SiC. Due to the spatial effect of ZrB2 ZrC SiC, as the carbon content increases, a finer ceramic phase can be obtained, and the ceramic particle size distribution is narrow. With the increase of SiC content, the grain size of each phase gradually decreases. The introduction of SiC inhibited the growth of ZrB2, but promoted the growth of ZrC. When the SiC content increases from about 7 to 50 wt.%, the particle size distribution of the powder is significantly reduced, and the average particle size is reduced from 610 to 200 nm. With the increase of SiC content, the synergistic effect of SiC and carbon space and grain boundary pinning is enhanced, resulting in uniform and fine ZrB2, ZrC, and SiC ceramic particles with an average particle size of 200 nm.

22.4

Conclusions

Materials science and engineering are playing a very important role in our day-today life, especially in the development of industry. The development of materials

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science and engineering reduces a lot of economic losses caused by wear and corrosion. Ceramic coatings, as the ideal material, play an important role in the protection of wear and corrosion. To obtain much better antiwear and anticorrosion ceramic coatings by the new development of ceramic coatings is a long-term development goal of surface technology. At present, intelligent ceramic coatings have been a research hotspot in this field. The intelligent ceramic coating can realize self-repair when damaged by corrosion and wear. For example, the coating can repair the corrosion performance by releasing related corrosion inhibitors stored in the microcapsule.

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Crystal structures for flexible photovoltaic application

23

Takeo Oku Department of Materials Science, The University of Shiga Prefecture, Hikone, Shiga, Japan

23.1

Introduction

In the present era, the most serious problem in natural environment on our Earth is global warming, which has been caused by generation of carbon dioxides (CO2). Therefore developments of new clean and efficient energy resources to suppress conventional fossil fuels such as oil, coal, and natural gas have become the very important issue to achieve Sustainable Development Goals proposed by United Nations. Since nuclear fusion produces high energy density and forms no CO2, the various types of nuclear fusion reactors have been investigated and studied [1
3]. On the other hands, solar cells are the most promising and usable energy device, and the merits of solar cells are as follows: (1) usage of a resource that is almost infinite and free, (2) environmentally clean, (3) no noise without moving parts, (4) unattended operation, (5) easy maintenance, (6) long lifetime (B30 years), and (7) multiple use of lands [4]. However, there are several demerits such as high cost, power dependence on sunlight irradiation, and small energy density. The largest barrier for spread of the solar cells is the price and sunlight dependence. Since the price for solar power generation is B4 times more expensive compared with thermal and nuclear power generation, reducing the cost of the present silicon solar cells is mandatory. Varioustype new solar cell materials such as poly-crystalline Si, thin-film Si, CuInSe2, dyesensitized TiO2, organic thin films, and perovskites have been developed. The perovskite structure has been considered as the single most versatile ceramic host. By appropriate modification in composition, we can change its important electro-ceramic dielectric phase into metallic conductors, superconductors or into the high-pressure phase [5]. For the flexible perovskite photovoltaics, combination of perovskite compounds and metal
organic frameworks (MOFs) is the new approach, which is expected as the next-generation photovoltaic materials. Since the first application of methylammonium lead triiodide (CH3NH3PbI3) perovskite compound to solar cell materials [6], the perovskite solar cells have been extensively produced and studied [7]. This CH3NH3PbI3 has MOFs, and these perovskite solar cells provide high photoconversion efficiencies and easy fabrication process that are comparable to organic solar cells [8
11]. Subsequently to achievement of conversion efficiency of B15% [12], higher conversion efficiencies have been reported for a number of varied devices and perovskite halide crystals Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00025-7 © 2023 Elsevier Ltd. All rights reserved.

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[13
16], and efficiencies over 20% were achieved [17
23]. Detailed crystal systems and lattice constants of CH3NH3PbI3 are summarized, and several structural models such as the α-CH3NH3PbI3 phase have been reported, as listed in Table 23.1 [30]. For the one model, the methylammonium ion (CH3NH31) has polarity and C3v point group symmetry, which could provide a pseudo-cubic structure [31]. The pseudo-cubic has nearly cubic symmetry with an a/c ratio of B1. The CH3NH31 is arranged along the C2 axis with 12 equivalent orientations in the unit cell. Hydrogen (H) atoms of the CH3NH31 provide two structural configurations on the C2 axis. Accordingly, the degrees of freedom of the MA1 ion are 24 [31]. The performances of photovoltaic devices are strongly dependent on the MOFs and elemental compositions of the perovskite halide crystals [30]. Doping some elements such as tin (Sn) [32
35], antimony (Sb) [36
39], copper (Cu) [40
45], arsenic (As) [46], germanium (Ge) [47
49], zinc (Zn) [49
52], manganese (Mn) [52
54], yttrium (Y) [54], strontium (Sr) [55], indium (In) [48], cobalt (Co) [56
58], europium (Eu) [59], thallium (Tl) [48], or bismuth (Bi) [60] at the lead (Pb) site has been attempted and investigated. The photoconversion ranges of the perovskite devices have been expanded by introducing Sn or Tl [32,48]. Introducing cesium (Cs) [61
63], rubidium (Rb) [21,64,65], potassium (K) [66
68], sodium (Na) [44
46], formamidinium (HC(NH2)2, FA) [18,69
71], ethylammonium (CH3CH2NH3, EA) [72,73], or guanidinium (C(NH2)3, GA) [74
76] at the methylammonium (CH3NH3, MA) site could also affect the electronic states of the perovskite MOFs and enhance the photoconversion efficiencies. Numerous researches on halogen doping such as bromine (Br) [18,77,78] or chlorine (Cl) [78
81] at the iodine (I) positions of the perovskite MOFs have been reported. Cl doping lengthens the diffusion length, which improves the photoconversion efficiencies [9,15,78
81]. Numerous studies reported that the substitutions with atom or molecule at the MA, Pb, and/or I site of the perovskite halides influence the device performances and microstructures of the photovoltaic devices. Device performances are also dependent on the microstructures of thin film configurations, and the morphology can be controlled by additives such as poly(methyl methacrylate) [82
84], phthalocyanines [85,86], or polysilanes [87
89]. A large contact area at the TiO2/perovskite interface can enhance the probability of carrier separation, which improves the short-circuit current density. To reduce the grain boundaries [90], homogeneous and smooth interfaces and surfaces of the perovskite layers could improve the fill factors and open-circuit voltages [91]. Improvement of hole transport layers [92,93] and electron transport layers [94,95] are also important for the carrier transport in the actual cell. In addition, it is necessary to fabricate larger cell area for the commercial use and flexible [96,97]. The photovoltaic performances of the perovskite solar cells depend on the perovskite MOF structures, electron transport layers, hole transport layers, nanoporous scaffold layers, and their interfacial structures. Particularly, MOF structures of the perovskite crystals have effects upon energy gaps, conduction band minimum, valence band maximum, and carrier mobility, which should be analyzed in detail. In this review, the atomic structures of various types of perovskite MOF crystals such as basic CH3NH3PbI3 (MAPbI3), HC(NH2)2PbI3 (FAPbI3), element-substituted

Table 23.1 Reported crystal systems and lattice constants of CH3NH3PbI3. Structure type

Crystal system

Space group

˚) Lattice constant (A

V ˚ 3) (A

Z

V/Z ˚ 3) (A

Analysis method

Temperature (K)

References

α α α α

Pm3̅ m Pm3̅ m Pm3̅ m P4mm

1 1 1 1

251.8 252.1 261.0 251.6

SCND NPD SCXRD SCXRD

350 352 338 400

[24] [25] [26] [27]

995.6

4

248.9

SCND

295

[24]

β

Tetragonal

I4cm

990.0

4

247.5

SCXRD

293

[27]

β

Tetragonal

I4/mcm

982.3

4

245.6

SCXRD

220

[28]

β

Tetragonal

I4/mcm

985.9

4

246.5

NPD

180

[25]

γ

Orthorhombic

pnma

960.3

4

240.0

NPD

100

[25]

γ

Orthorhombic

pnma

951.0

4

237.8

SCXRD

100

[10]

γ

Orthorhombic

pnma

a 5 6.315(3) a 5 6.31728(27) a 5 6.391(1) a 5 6.3115(2), c 5 6.3161(2) a 5 8.8796(6), c 5 12.6266(18) a 5 8.849(2), c 5 12.642(2) a 5 8.800(9), c 5 12.685(7) a 5 8.80625(28), c 5 12.7127(5) a 5 8.86574(30), b 5 12.6293(4), c 5 8.57689(31) a 5 8.8362(11), b 5 12.5804(15), c 5 8.5551(10) a 5 8.8155(4), b 5 12.5980(6), c 5 8.5637(5)

251.8 252.1 261.0 251.6

β

Cubic Cubic Cubic Tetragonal (Pseudocubic) Tetragonal

951.1

4

237.8

NPD

4

[29]

I4/mcm

NPD, Neutron powder diffraction; SCND, single-crystal neutron diffraction; SCXRD, single-crystal X-ray diffraction.

496

Advanced Flexible Ceramics

perovskites, double perovskites, and low-dimensional perovskites, which are expected to be usable as photovoltaic device materials, are summarized and described. Since these perovskite MOFs often have varied nanoscopic structures for the photovoltaic devices, a summary of information on the perovskite MOFs could be mandatory for the structural analysis and the developments of the new perovskite compounds. The nanoscopic structures of the perovskite crystals for the photovoltaic devices can be analyzed using transmission electron microscopy and X-ray diffraction (XRD) by investigating various diffraction conditions. Although single-crystal XRD is suitable for atomic structure determination, actual solar cells have thin film configurations and microcrystalline structures, and Rietveld refinements are one of the good analysis methods to determine the crystal structure of the perovskite compounds in the actual solar cell device configurations. Electron diffraction and high-resolution transmission electron microscopy are also effective instruments to analyze nanostructures of the solar cell materials [98
101] and crystal structures of the perovskite compounds [102,103]. Because one of the serious problems of the perovskite MOFs is stability, high-temperature annealing which is believed to be necessary for realization of stable perovskite solar cells was also described.

23.2

Estimation of structural stability of metal
organic framework by tolerance factors

The perovskite structures used for solar cell devices often have a chemical composition of ABX3 MOF, where A is generally a monovalent cation, B is a divalent cation, and X is a monovalent halogen anion. A basic structural model of ABX3 perovskite MOF indicates that the B cation forms BX6 octahedron. For the CH3NH3PbI3 crystal, A1 is CH3NH31, B21 is Pb21, and X2 is I2. For actual application for solar cells, the basic CH3NH3PbI3 perovskite compounds have two major problems, the toxicity of Pb and the lack of durability. The MAPbI3 compounds are generally unstable at room temperature in air because of the MOFs, and improvement of the stability is one of the most significant issue [104,105]. One approach to stabilize the MAPbI3 is to introduce several elements to the perovskite halide structures. Regarding to the toxicity of Pb, elemental substitution at the Pb sites in the MAPbI3 compounds is one of the solutions. To estimate the structural stability of these perovskite halide materials, an indicator called tolerance factor (t) has been computed and used [30,106
108]. This tolerance factor is calculated as follows: t 5 pffiffi2rðrA 11rXr Þ, where rA, rB, and rX are the B X ionic radii of the A, B, and X ions, respectively, in the ABX3 perovskite halide MOFs. The ionic radii of elements constituting ABX3 perovskites were reported [6,109,110], where the coordination numbers of A, B, and X ions are 12, 6, and 6, respectively. In addition, the ionic radii of CH3NH31 (MA1), HC(NH2)21 (FA1), ˚, CH3CH2NH31 (EA1), and C(NH2)31 (GA1) are 2.17, 2.53, 2.74, and 2.78 A respectively [111]. When the t-value is 1, the perovskite compound has a stable crystal structure with cubic symmetry. Another approach to estimate the structural

Crystal structures for flexible photovoltaic application

497

stabilities of the perovskite is to examine the octahedral factor [107,112]. The octahedral factor (μ) can be expressed as 5 rrXB . The t- and μ-factors are experiential guidelines that do not consider the ionic interactions in the perovskite crystal. Thus stability ranges of the ABX3 perovskite halides change being affected by the elemental and ionic properties. Based on the many previous studies on halide perovskites, the perovskite structure could be experientially formed in the range of 0.813 # t # 1.107 and 0.442 # μ # 0.895 [110]. When μ increases from 0.414 to 0.592, 7-coordinated octahedra would be more suitable [113], and μ values below 0.592 would be better to stabilize the BX6 octahedra. The t- and μ-factors of the ABX3 perovskite halide compounds were calculated and summarized in Table 23.2 and Ref. [30]. The t-factor of MAPbI3 is calculated to be 0.912, which indicates that the MAPbI3 may be somewhat unstable. One approach to stabilize the perovskite structure is to substitute organic ions or other elements in the MAPbI3 crystal. The octahedral factor of MAPbI3 is fixed at 0.541, which is within the range of the perovskite formation. The t- and μ-factors of FAPbI3 are 0.987 and 0.541, respectively, which indicates that the FAPbI3 crystal would be more stable than MAPbI3. Possible compositions (0.813 # t # 1.107, 0.442 # μ # 0.541) for the perovskite structures were indicated in bold fonts in Table 23.2, and candidates of low-toxicity perovskite compounds are indicated by bold letters. The t- and μ-factors can be computed by assuming that ions have rigid spheres and that the ionic radii are constant. However, the t- and μ-factors are a simple and helpful guideline to estimate the structural stability of the perovskite halide MOFs, and a new tolerance factor was also proposed [114]. First-principles calculations based on density functional theory also represent a powerful method to predict the stability and properties of the crystals and doped clusters were also reported [115
118]. Two most standard MOF structures are CH3NH3PbI3 and HC(NH2)2PbI3, and elemental substitution is possible for ABX3-type perovskite crystals, which provide various compositions, structures and electronic properties. Numerous kinds of elemental substituted perovskite MOFs have been reported, which are summarized and listed in Table 23.3. Partial elemental substitutions are often introduced for these MAPbI3 and FAPbI3 perovskite crystals to control the optoelectronic properties. For example, iodine atoms (X site) can be replaced by Br and Cl atoms; MA or FA (A site) can be substituted with Cs and Rb; and Pb (B site) can be substituted with Sn, Ge, Sb, Cu, and other elements.

23.3

Double perovskites and low-dimensional perovskites

In addition to the ordinary elemental substitution, atomic orderings of the substituted elements have also been achieved and reported, which is called double perovskite or elpasolite. The general formula is A2BB’X6, and the ionic valence of B/B’ is 11/31 or 21/21. Two of the examples of the double perovskite structure are

Table 23.2 t- and μ-factors of MABX3, FABX3, and GABX3. Compound

t

μ

Compound

t

μ

Compound

t

μ

MAPbI3 MAPbBr3 MAPbCl3 MASnI3 MASnBr3 MASnCl3 MAGeI3 MAGeBr3 MAGeCl3 MAHgI3 MAHgBr3 MAHgCl3 MACdI3 MACdBr3 MACdCl3 MAZnI3 MAZnBr3 MAZnCl3 MABaI3 MABaBr3 MABaCl3 MASrI3 MASrBr3 MASrCl3 MACaI3 MACaBr3 MACaCl3 MAMgI3 MAMgBr3 MAMgCl3

0.912 0.927 0.938 0.987 1.011 1.027 1.055 1.086 1.108 0.912 0.927 0.938 0.981 1.004 1.020 1.051 1.082 1.104 0.868 0.880 0.888 0.914 0.930 0.941 0.966 0.987 1.002 1.058 1.090 1.112

0.541 0.607 0.657 0.423 0.474 0.514 0.332 0.372 0.403 0.541 0.607 0.657 0.432 0.485 0.525 0.336 0.378 0.409 0.618 0.694 0.751 0.536 0.602 0.652 0.455 0.510 0.552 0.327 0.367 0.398

FAPbI3 FAPbBr3 FAPbCl3 FASnI3 FASnBr3 FASnCl3 FAGeI3 FAGeBr3 FAGeCl3 FAHgI3 FAHgBr3 FAHgCl3 FACdI3 FACdBr3 FACdCl3 FAZnI3 FAZnBr3 FAZnCl3 FABaI3 FABaBr3 FABaCl3 FASrI3 FASrBr3 FASrCl3 FACaI3 FACaBr3 FACaCl3 FAMgI3 FAMgBr3 FAMgCl3

0.987 1.008 1.023 1.069 1.099 1.120 1.142 1.180 1.208 0.987 1.008 1.023 1.062 1.091 1.112 1.138 1.176 1.203 0.939 0.956 0.968 0.990 1.011 1.026 1.045 1.073 1.092 1.145 1.185 1.213

0.541 0.607 0.657 0.423 0.474 0.514 0.332 0.372 0.403 0.541 0.607 0.657 0.432 0.485 0.525 0.336 0.378 0.409 0.618 0.694 0.751 0.536 0.602 0.652 0.455 0.510 0.552 0.327 0.367 0.398

GAPbI3 GAPbBr3 GAPbCl3 GASnI3 GASnBr3 GASnCl3 GAGeI3 GAGeBr3 GAGeCl3 GAHgI3 GAHgBr3 GAHgCl3 GACdI3 GACdBr3 GACdCl3 GAZnI3 GAZnBr3 GAZnCl3 GABaI3 GABaBr3 GABaCl3 GASrI3 GASrBr3 GASrCl3 GACaI3 GACaBr3 GACaCl3 GAMgI3 GAMgBr3 GAMgCl3

1.039 1.064 1.082 1.125 1.160 1.185 1.202 1.246 1.278 1.039 1.064 1.082 1.118 1.152 1.176 1.198 1.241 1.273 0.989 1.010 1.024 1.042 1.067 1.085 1.100 1.132 1.155 1.206 1.251 1.283

0.541 0.607 0.657 0.423 0.474 0.514 0.332 0.372 0.403 0.541 0.607 0.657 0.432 0.485 0.525 0.336 0.378 0.409 0.618 0.694 0.751 0.536 0.602 0.652 0.455 0.510 0.552 0.327 0.367 0.398

MABeI3 MABeBr3 MABeCl3 MAScI3 MAScBr3 MAScCl3 MATiI3 MATiBr3 MATiCl3 MAVI3 MAVBr3 MAVCl3 MACrI3 MACrBr3 MACrCl3 MAMnI3 MAMnBr3 MAMnCl3 MAFeI3 MAFeBr3 MAFeCl3 MACoI3 MACoBr3 MACoCl3 MANiI3 MANiBr3 MANiCl3 MACuI3 MACuBr3 MACuCl3

1.166 1.212 1.245 1.049 1.080 1.101 1.010 1.036 1.054 1.033 1.062 1.082 1.030 1.058 1.078 1.020 1.047 1.066 1.037 1.066 1.087 1.049 1.080 1.101 1.069 1.102 1.126 1.055 1.086 1.108

0.205 0.230 0.249 0.339 0.380 0.412 0.391 0.439 0.475 0.359 0.403 0.436 0.364 0.408 0.442 0.377 0.423 0.459 0.355 0.398 0.431 0.339 0.380 0.412 0.314 0.352 0.381 0.332 0.372 0.403

FABeI3 FABeBr3 FABeCl3 FAScI3 FAScBr3 FAScCl3 FATiI3 FATiBr3 FATiCl3 FAVI3 FAVBr3 FAVCl3 FACrI3 FACrBr3 FACrCl3 FAMnI3 FAMnBr3 FAMnCl3 FAFeI3 FAFeBr3 FAFeCl3 FACoI3 FACoBr3 FACoCl3 FANiI3 FANiBr3 FANiCl3 FACuI3 FACuBr3 FACuCl3

1.262 1.317 1.358 1.136 1.174 1.201 1.093 1.126 1.149 1.119 1.155 1.180 1.115 1.150 1.176 1.104 1.138 1.162 1.122 1.159 1.185 1.136 1.174 1.201 1.157 1.198 1.228 1.142 1.180 1.208

0.205 0.230 0.249 0.339 0.380 0.412 0.391 0.439 0.475 0.359 0.403 0.436 0.364 0.408 0.442 0.377 0.423 0.459 0.355 0.398 0.431 0.339 0.380 0.412 0.314 0.352 0.381 0.332 0.372 0.403

GABeI3 GABeBr3 GABeCl3 GAScI3 GAScBr3 GAScCl3 GATiI3 GATiBr3 GATiCl3 GAVI3 GAVBr3 GAVCl3 GACrI3 GACrBr3 GACrCl3 GAMnI3 GAMnBr3 GAMnCl3 GAFeI3 GAFeBr3 GAFeCl3 GACoI3 GACoBr3 GACoCl3 GANiI3 GANiBr3 GANiCl3 GACuI3 GACuBr3 GACuCl3

1.329 1.391 1.436 1.196 1.239 1.270 1.151 1.189 1.216 1.178 1.219 1.248 1.174 1.214 1.244 1.162 1.201 1.229 1.182 1.223 1.253 1.196 1.239 1.270 1.218 1.265 1.298 1.202 1.246 1.278

0.205 0.230 0.249 0.339 0.380 0.412 0.391 0.439 0.475 0.359 0.403 0.436 0.364 0.408 0.442 0.377 0.423 0.459 0.355 0.398 0.431 0.339 0.380 0.412 0.314 0.352 0.381 0.332 0.372 0.403

Table 23.3 Crystal systems of various perovskite metal
organic framework compounds. Compound

Crystal system

Space group

˚ ) and degree Lattice parameters (A

Z

Temperature (K)

References

CH3NH3PbBr3

Cubic Tetragonal Tetragonal Orthorhombic Cubic Tetragonal Orthorhombic Tetragonal Tetragonal Tetragonal Cubic Rhombohedral Orthorhombic Monoclinic

Pm3̅ m I4/mcm P4/mmm Pna21 Pm3̅ m P4/mmm P2221 P4mm I4cm P4mm Pm3̅ m R3m Pnma P21/n

1 4 1 4 1 2 2 1 4 1 1 1 4 4

298 220 150
155 ,145 200 173
179 ,173 293 200
475 370 250 2

[26] [119] [120] [120] [26] [120] [120] [27] [27] [32] [121] [121] [121] [121]

Cubic Rhombohedral Monoclinic

Pm3̅ m R3m P1c1

1 1 2

478 350 318

[122] [122] [122]

Triclinic

P1

2

297

[122]

Trigonal Cubic Tetragonal Orthorhombic Cubic Tetragonal Orthorhombic Orthorhombic

P3 Pm3̅ m P4/mbm Pnma Pm3̅ m P4/mbm Pnma Amm2

a 5 5.933(2) a 5 8.3381(3), c 5 11.8587(5) a 55 5.894(2), c 5 5.861(2) a 5 7.979(1), b 5 8.580(2), c 5 11.849(2) a 5 5.666(2) a 5 5.656, c 5 5.630 a 5 5.673, b 5 5.628, c 5 11.182 a 5 6.2302(10), c 5 6.2316(11) a 5 8.7577(15), c 5 12.429(3) a 5 5.837(1), c 5 5.853(4) a 5 5.6917(2) a 5 5.6784(1), α 5 90.945(1) a 5 11.1567(1), b 5 7.3601(1), c 5 8.2936(1) a 5 10.9973(1), b 5 7.22043(1), c 5 8.2911(1), α 5 90.470(1) a 5 5.760(1) a 5 5.734(1), α 5 91.90(1) a 5 5.718(1), b 5 8.236(1), c 5 7.938(1), β 5 91.90(1) a 5 5.726, b 5 8.227, c 5 7.910, α 5 90.40 , β 5 93.08 , γ 5 90.15 a 5 17.7914(8), c 5 10.9016(6) a 5 5.98618(2) a 5 8.41525(5), c 5 5.94735(8) a 5 8.37433(9), b 5 11.8609(1), c 5 8.38073(9) a 5 6.30961(1) a 5 8.86227(2), c 5 6.24892(2) a 5 8.81749(8), b 5 12.41641(7), c 5 8.8578(1) a 5 6.3286(10), b 5 8.9554(11), c 5 8.9463(11)

12 1 2 4 1 2 4 2

150 275 175 100 275 175 100 340

[27] [123] [123] [123] [123] [123] [123] [27]

CH3NH3PbCl3

CH3NH3SnI3 CH3NH3SnI3 CH3NH3SnBr3 CD3ND3GeCl3

CH3NH3SnCl3

HC(NH2)2PbI3 HC(NH2)2PbBr3

HC(NH2)2SnI3

HC(NH2)2SnI3

Crystal structures for flexible photovoltaic application

501

Cs2AgBiBr6 and (CH3NH3)2AgBiBr6, as listed in Table 23.4 [30]. AgBr6 and BiBr6 octahedra are alternately ordered in the perovskite crystal. Various double perovskite MOF compounds such as (CH3NH3)2AgBiBr6, (CH3NH3)2KBiCl6, and others have been reported, as listed in Table 23.4, which are all Pb-free compounds. Since the MA2KBiCl6 double perovskite structures have a rhombohedral symmetry, the diffraction patterns are completely different from other double perovskite halides with cubic symmetry. Some of these double perovskite elpasolite compounds are expected to apply to Pb-free solar cells [133
135], and the energy gaps have been reported [124,133]. Application of the double perovskite elpasolites are also be expected for thermal neutron scintillator materials [136,137]. Other types of double perovskite compounds such as vacancy-ordered double perovskites and two-dimensional (2D) double perovskites have also been reported, which will be described as low-dimensional perovskites. Normal perovskite halide compounds, as described above, consist of octahedra sharing all vertices with the neighboring octahedra three-dimensionally. In addition to the common three-dimensional (3D) perovskites, various perovskite compounds with lower dimensional structures have been reported [133,138
140], as summarized and listed in Table 23.4. Like 2D superconducting copper oxide perovskites [102,103], the derivative structures with lower dimensionality could provide finer tunability of the electronic properties [141,142]. Other types of 2D layered perovskite MOFs were also reported, which is called the Dion
Jacobson (DJ) structure [127]. The lead iodides with DJ perovskite structures have the standard formula of A(MA)n21PbnI3n11. C6N2H16PbI4, (C6N2H16) (CH3NH3)Pb2I7, and (C6N2H16)(CH3NH3)3Pb4I13. The organic molecules isolate the PbI6 octahedra, and the 2D layered structure is formed. In the XRD patterns for C6N2H16PbI4, (C6N2H16)(CH3NH3)Pb2I7, and (C6N2H16)(CH3NH3)3Pb4I13, several diffraction reflections are observed at 2θ angles lower than 10 degrees. This indicates that 2D DJ perovskite compounds have larger lattice constants with long periodicity compared with the standard perovskite MOFs. Other 2D perovskites with the Ruddlesden
Popper MOF structure were also reported [137]. These perovskite compounds consist of inorganic perovskite layers inserted with butylammonium cations, and they have the general formula (CH3(CH2)3NH3)2(CH3NH3)n21PbnI3n11 (n 5 1, 2, 3, 4, N). Comparing the structural models of the 2D DJ perovskite structures, PbI6 octahedra in the Ruddlesden
Popper 2D structure are observed to have an antiphase arrangement at the sides of the inserted spacer. In addition to the above 2D perovskites, 2D double perovskite halides were synthesized by incorporating organic spacer cations such as propylammonium (PA), octylammonium (OCA), and 1,4-butyldiammonium (BDA) into standard 3D double perovskites [126]. (NH3(CH2)4NH3)2AgBiBr8 (BDA2AgBiBr8), (CH3(CH2)7NH3)4AgBiBr8 (OCA4AgBiBr8), (CH3(CH2)2NH3)2CsAgBiBr7 (PA2CsAgBiBr7), and (CH3(CH2)2NH3)4AgInCl8 (PA4AgInCl8) with 2D double perovskite structures were reported, as listed in Table 23.4. The general formulas of the single-layered Ruddlesden
Popper type and single-layered DJ type are A4BB’X8 (A 5 PA or OCA) and A2BB’X8, respectively. The bandgap energy can

Table 23.4 Crystal systems of various double perovskite and low-dimensional perovskite metal
organic framework crystals. Compound

Dimensionality

Crystal system

Space group

˚ ) and degree Lattice parameters (A

Z

References

(CH3NH3)2AgBiBr6

3 1 double perovskite 3 1 double perovskite 2 1 double perovskite

Cubic

Fm3̅ m

a 5 11.6370(1)

4

[124]

Rhombohedral

R3̅ m

a 5 7.8372(2), c 5 20.9938(2)

3

[125]

Triclinic

P_1

2

[126]

2 1 double perovskite 2 1 double perovskite 2 1 double perovskite

Monoclinic

C2/m

2

[126]

Monoclinic

C2/m

2

[126]

Triclinic

P_1

1

[126]

Monoclinic

P2/m

1

[126]

Monoclinic

P21/m

2

[126]

C6N2H16PbI4

2 1 double perovskite 2 1 double perovskite 2

Monoclinic

Pc

4

[127]

(C6N2H16)(CH3NH3)Pb2I7

2

Monoclinic

Cc

4

[127]

(C6N2H16)(CH3NH3)3Pb4I13

2

Monoclinic

Cc

4

[127]

(CH3(CH2)3NH3)2(CH3NH3)Pb2I7

2

Orthorhombic

Cmcm

a 5 7.454(3), b 5 7.866(3), c 5 24.629(8), α 5 90.141(10) , β 5 89.992(10) , γ 5 90.049(11) a 5 24.5633(13), b 5 7.7838(4), c 5 8.2501(4), β 5 90.091(2) a 5 24.503(7), b 5 7.964(2), c 5 8.384(2), β 5 90.045(9) a 5 8.1018(3), b 5 8.2433(3), c 5 9.6813(4), α 5 101.1960(10) , β 5 92.055(2) , γ 5 90.3960(10) a 5 8.2024(7), b 5 8.1797(6), c 5 21.1217(18), β 5 101.195(3) a 5 8.053(4), b 5 7.997(3), c 5 18.536(8), β 5 102.503(11) a 5 10.4999(13), b 5 12.5429(9), c 5 12.5289(13), β 5 89.984(9) a 5 23.1333(7), b 5 8.8365(3), c 5 8.8354(3), β 5 90 a 5 8.8587(18), b 5 8.8571(18), c 5 58.915(12), β 5 90 a 5 8.9470(4), b 5 39.347(2), c 5 8.8589(6)

4

[127]

(CH3NH3)2KBiCl6 (CH3(CH2)2NH3)4AgInCl8

(CH3(CH2)2NH3)4AgInBr8 (CH3(CH2)2NH3)4AgBiBr8 (NH3(CH2)4NH3)2AgBiBr8

(CH3(CH2)7NH3)4AgBiBr8 (CH3(CH2)2NH3)2CsAgBiBr7

(CH3(CH2)3NH3)2(CH3NH3)2Pb3I10

2

Orthorhombic

Aba2

(CH3(CH2)3NH3)2(CH3NH3)3Pb4I13

2

Orthorhombic

Ama2

(CH3NH3)2CuCl4

2

Monoclinic

P2/c

(CH3NH3)2CuCl2Br2

2

Orthorhombic

Cmca

(C6H5(CH2)2NH3)2CdCl4

2

Orthorhombic

Aba2

C24H35S8N4PbI5

1

Monoclinic

P21/c

(NH4)2PtI6

0

Cubic

Fm3̅ m

a 5 8.9275(6), b 5 51.959(4), c 5 8.8777(6), a 5 8.9274(4), b 5 64.383(4), c 5 8.8816(4) a 5 7.2574(8), b 5 7.3504(1), c 5 9.9688(5), β 5 111.20 a 5 7.3194(4), b 5 7.3281(4), c 5 19.1344(1) a 5 7.4444(2), b 5 38.8965(3), c 5 7.3737(2) a 5 17.553(2), b 5 8.710(1), c 5 27.122(5), β 5 97.57(2) a 5 11.158(8)

4

[128]

4

[128]

1

[129]

2

[129]

4

[130]

4

[131]

4

[132]

504

Advanced Flexible Ceramics

be tuned by selecting the spacer layer thickness, and the bandgaps of PA4AgBiBr8 and PA4AgInCl8 were reported to be B2.4 and 4.0 eV [126]. Most of the reported bandgap values would be related with the structural dimensionality [133], and the perovskite crystals with lower dimensionality tend to have wider bandgaps. Twodimensional copper perovskite MOF structures such as MA2CuClxBr4-x was also synthesized as Pb-free light harvesters [143,144]. Vertex Cl atoms of CuCl6 octahedra are continuously connected in a 2D manner. In addition to the 2D perovskite MOFs, one-dimensional continuously connected octahedra exist in (C24H35S8N4)PbI5, observed along three main directions. Two octahedra are arranged sharing their vertex atoms, and these octahedral pairs are one-dimensionally arranged along the observation direction, and the connected chain is infinitely continuous. For the 0-dimensional (0D) perovskite, all BX6 octahedra are isolated in the perovskite crystal. For the (NH4)2PtI6 compound in Table 23.4, there are insufficient Pt atoms to form (NH4)Pt0.5I3, and the PtI6 octahedra are isolated in the crystal with A-site cations occupying the cuboctahedral voids. From the viewpoint of double perovskites, elements with tetravalent cations are incorporated to form 41/0 double perovskites. This is called vacancy-ordered double perovskites with the general formula of A2BvX6, where the v means vacant positions corresponding to the B’ site for the A2BB’X6 double perovskites. Despite the isolated octahedral BX6 units, the close-packed iodide lattice provides electronic dispersion, and Cs2SnI6 and other perovskites were applied to solar cells [145
147]. Pb-free solar cells such as FA4GeSbCl12 have been reported [148], in which the double elements were selected to replace Pb. Cs2TiIxBr62x vacancy-ordered double perovskite compounds were also reported to have stability and bandgaps between 1.0 and 1.8 eV [149]. Although the (NH4)2PtI6 is called vacancy-ordered double perovskites, the XRD pattern is dissimilar to other double perovskites, which is due to the lack of B’ site atoms for the A2BB’X6 double perovskite structure.

23.4

Grain growth and defects in the metal
organic frameworks

In the previous sections, crystal structures of various perovskite halide MOF compounds were described, which directly influence the semiconductor and photovoltaic properties. On the other hand, the morphology of perovskite thin films also strongly affects the photovoltaic properties [15,78,150
153]. XRD patterns of standard CH3NH3PbI3 thin films on the glass substrate before and after heating were different [30,154]. The diffraction reflections were indexed by tetragonal and cubic structures for the as-prepared and heated films, respectively. Although only diffraction peaks due to the perovskite halide phase were observed for the as-deposited film, a wider diffraction peak owing to PbI2 formation by heating was observed. A small amount of PbI2 sometimes contribute to the improvement of the photovoltaic properties [105,151,153,155
157], which would have roles of electron blocking

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505

and protecting the perovskite phase against the air at the surface of the perovskite layer. Split diffraction reflections of 002 1 110 and 004 1 220 for the as-prepared specimen varied to 100 and 200 reflections after heating, which showed a structure transformation from the tetragonal to cubic symmetry. The standard CH3NH3PbI3 perovskite crystals provide structurally transition from the tetragonal to cubic structure at B330K [26,29,158]. The XRD profiles indicated the structural transformation of the CH3NH3PbI3 compound and the partial desorption of CH3NH31 to form PbI2 during annealing [30,154]. One should be careful that the structural transformation presented here could be different from the isolated single crystals, and the nanocrystals of perovskite halides with cubic symmetry might be “frozen” both in and on the mesoporous TiO2 layers. A very weak reflection corresponding to 211 of the tetragonal symmetry might appear at the left side of the cubic 111 reflection for the annealed CH3NH3PbI3. To be accurate, it might be better to refer to the cubic phase as a “pseudo-cubic” phase. The pseudo-cubic defined here is a crystal structure that has nearly cubic symmetry with a/c of B1, and a weak 211 reflection of the tetragonal symmetry appears at B23.5 degrees by violation of extinction rule of the cubic symmetry [30]. Wide interfacial areas between n-type TiO2 electron transport layers and the perovskite layers can promote charge separation, which contributes to the increase of the short-circuit current density. Homogeneous and smooth surface/interface structures of the perovskite layers would improve the fill factors and open-circuit voltages [91]. Crystal growth and crystallization of the perovskite compounds during annealing are also important and have been investigated [159
161]. Effects of adding ammonium chloride (NH4Cl) to perovskite CH3NH3PbI3(Cl) solar cells fabricated by an air blowing method are described here [151]. NH4Cl has a role of surfactant, which would facilitate forming homogeneous perovskite surface structures [78,150,162]. In addition, the carrier diffusion length in the perovskite halides would be improved by doping Cl [9,80]. The Cl-added perovskite halide compound is designated as CH3NH3PbI3(Cl). XRD patterns of the CH3NH3PbI3(Cl) devices showed that diffraction intensities of 100 and 200 reflections were enormously enhanced to more than 100 times by introducing NH4Cl and air blow. The diffraction intensity ratio I100/I210 were measured and summarized as listed in Table 23.5 [30]. If the CH3NH3PbI3 perovskite grains are randomly oriented, the I100/I210 value should be 2.08, as shown in Table 23.5. The orientation index I100/I210 is 69 for the cell fabricated using air blow and without NH4Cl, which indicates that the perovskite grains are preferentially (100)-aligned against the cell substrate. The orientation index I100/I210 was further enhanced to 3600 by the addition of NH4Cl, which is 1700 times higher than that of randomly aligned perovskite grains. The cell fabricated with NH4Cl and without PbCl2 or air blow provided an orientation index I100/I210 of 2.8, which indicates that most of the perovskite grains are randomly aligned in the thin film configuration. Two formation mechanisms were proposed for the highly (100)-aligned perovskite thin films on the mesoporous TiO2. The first mechanism is air blow-driven

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Table 23.5 Ratios of 100 diffraction intensities (I100) to 210 diffraction intensities (I210) and FTO substrate diffraction intensities (IFTO), for the perovskite crystals. Compositions of the solar cells as measured by EDS. NH4Cl (mg)

I100/I210

I100/IFTO

Pb (at.%)

I (at.%)

Cl (at.%)

C:N (at.%)

Calculationa 0 1 3 5 5b

2.08 69 510 3400 3600 2.8


51 170 280 270 0.40


24.3 24.0 24.4 24.1 24.0


71.7 71.4 71.1 70.2 71.2


4.0 4.7 4.6 5.7 4.8


61.9:38.1 61.7:38.3 61.8:38.2 61.8:38.2 52.0:48.0

a

Calculated from randomly oriented cubic CH3NH3PbI3 crystals. No air blowing or PbCl2.

b

crystal growth of the perovskite grains. When the precursor solution crystalizes into the perovskite grains, rapid heating using air blowing promotes oriented crystallization of the perovskite grains on the mesoporous TiO2. The (100) of the cubic perovskite structure has low surface energy, which facilitates the crystal growth of (100)-aligned grains. The highly aligned grains reduce the area of high-angle grain boundaries, which induces decrease of the series resistance and increase of the open-circuit voltage. Another formation mechanism is a surfactant effect of NH4Cl, which enhanced the construction of homogeneous crystal-aligned microstructures during heating. Network-like microstructures that connect perovskite grains with nanowire-like crystals were also constructed in the perovskite layer, as reported in Ref. [161]. Then, the surface covering ratio and carrier transport efficiency were improved, which resulted in the increase of the fill factor and short-circuit current density. Improvement of the photoconversion efficiencies could be understood by these formation models. Cl substitution at iodine sites also improves the carrier diffusion in the perovskite halide compounds. Excess CH3NH3Cl is vaporized from the initial stoichiometry of 3(CH3NH3I) 1 PbCl2 [161,163
165], and a little remained Cl is substituted at the I site of the CH3NH3PbI3 structure. The doped-Cl lengthens the exciton diffusion length [9,15,80], which improves the short-circuit current density [ ]. As a result, the constructed (100)-aligned perovskite grains improved the photovoltaic properties. The present air blowing method combined with NH4Cl addition is an effective method to form highly crystal-aligned perovskite thin films in the device configuration. For the formation of the perovskite compounds in thin film configurations, grain growth due to diffusion limitation by solute elements could be the main factor. Optical transmission microscope image of HC(NH2)2PbI3, HC(NH2)2PbI2.85Br0.15, HC(NH2)2PbIBr2, and HC(NH2)2Pb0.95Sb0.05I3 [30,39,166] was reported, which are FAPbI3 doped with Br and Sb at the I and Pb sites, respectively. By adding a small amount of Br and Sb, perovskite crystals with dendritic structures grew densely, and the surface coverage of the perovskite grains on the cell substrate increased.

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Due to these dendritic structures, short-circuit current densities and photoconversion efficiencies increased. Gibbs
Thomson coefficients and liquidus line gradients were estimated, and the dendrites would be formed by satisfying neutral stable conditions on the grain growth rate by increasing the kinds of solute elements. Fractal dimensions of the dendrite structures were calculated to be B2.8 by using a boxcounting method [30]. These dendrite structures would contribute to increase of the surface coverage and interfacial area at the interpenetrating pn junction, which provided the higher current densities and conversion efficiencies. The driving force of grain growth is grain boundary energy, which might be caused by the defects at the grain boundary. Since Pb and Sb could be more stable in the perovskite structure, it is believed that the grain boundary energy would be induced by the diffusion of HC(NH2)2, I, and Br on the surface of the perovskite grain. The grain boundary energy (Eb) with a scalar form has a same dimension as the interfacial tension (σ) with a vector form and can be calculated from the equation: ΔGV 5 2EDb Vm , where ΔGV, Eb, D, and Vm are the Gibbs free energy, grain boundary energy, grain size, and molar volume, respectively. When the CH3NH3PbI3 decomposes into PbI2 and CH3NH3I, the ΔrGo was calculated to % perpendicular to be 10.2 (kJ/mol) [30]. Grain sizes of perovskite crystals observed the substrates are measured to be B5 μm [30], and the grain boundary energies were estimated to be B170 J/m2. On the other hand, crystallite sizes of these thin films measured parallel to the substrates by XRD were B50 nm, and the grain boundary energies were estimated to be B1.7 J/m2. Practically, only the surface of the perovskite grains might decompose, and the actual Eb would be smaller than that. Numbers of the Schottky defects (n) are also expressed by the following equation: n 5 N exp(-Ev/kBT), where N is the number of atoms per volume, Ev is the activation energy for the formation energy of defects, kB is the Boltzmann constant, and T is the temperature (298K). Thus the number for the defects in the MAPbI3(Cl) crystal and the energy for defect formation was calculated as 1.4 3 1020 cm23 and 0.087 eV, respectively [167]. Energies for defect generation for MA, I, and Pb were calculated as 0.58, 0.84, and 2.31 eV, respectively [168,169]. The estimated energy for defect generation in the present work is significantly smaller than that for ordinary ionic crystals, indicating that MA may be easily desorbed from microcrystalline thin films during rapid annealing and cooling.

23.5

Rietveld refinement of crystal structures for solar cell configuration

Single-crystal XRD and neutron diffraction would be the best methods to reveal the accurate crystal structures of perovskite compounds. Although large single crystals such as 5
10 mm have been obtained for the perovskite structure analysis [24,170], the actual microstructures of the perovskite compounds in the solar cell device configuration must be different from those of the single crystals; this is because of the various parameters including formation of microcrystals during rapid annealing,

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coexistence with TiO2 layers and hole transport layers and other fabrication conditions. To understand the actual microstructures of the perovskite compounds in the device configuration, the actual cells should be measured and analyzed. The structural transition from tetragonal to cubic structures in the actual CH3NH3PbI3 thin films was reported [154], and the nanocrystals with cubic symmetry might be restricted and frozen both in and on the mesoporous TiO2 layers. A very weak reflection corresponding to the tetragonal symmetry may appear, and it might be better to refer to the cubic phase as a “pseudo-cubic” phase [30]. The Rietveld analysis method has been often utilized to examine the crystal structures of microcrystalline materials [171]. Nevertheless, this method has rarely been applied for crystal structures in actual perovskite solar cell devices [172]. The Rietveld refinement technique can be applied to investigate the crystal structure of the perovskite halides, and to determine the accurate atomic position and site occupancy of each atom constituting CH3NH3PbI3 in the solar cell configuration [173]. Crystal structures of CH3NH3PbI3 and CH3NH3Pb0.85Sb0.15I3 perovskite films for the solar cell conformation were investigated and analyzed by the Rietveld analysis. This method could be also useful for the Pb-deficient perovskite compounds [174]. Using the Rietveld program RIETAN-2000 [175], a computed XRD pattern was refined to match the measured XRD pattern of the perovskite crystal in the solar cell configuration [173]. Extra peaks from PbI2, TiO2, Au, and F-dope SnO2 in the measured XRD patterns were excluded for the optimization process, and the charge-neutral conditions were considered and applied for the refinement. First, the positions of MA and I ions in CH3NH3PbI3 were investigated and determined. The reported basic crystal structure [26] was used to calculate the site occupancies for CH3NH3PbI3, and the residual factor (R-factor, Rwp) was determined to be 3.38%. XRD profiles agreed well with the computed data by optimization of the atomic coordinates and site occupancies in the unit cell, as listed in Table 23.6. The refined ratio of the composition elements of the perovskite was Pb: MA: I 5 1: 1: 3. For the Sb-added perovskite crystal, elemental concentrations of MA: Pb: Sb: I were determined to be 0.85:0.81:0.19:3. The reduction of MA occupancy would be due to the compensation effect by substituting Pb21 with Sb31, which compensates the deficiency of the CH3NH3 groups during the formation of a CH3NH3PbI3 film [36]. Conversion efficiencies of the photovoltaic devices were improved by adding a small amount of Sb [36,37], and these values monotonically decreased for the excess Sb concentration (z . 0.03). The improvement of conversion efficiencies would be due to the suppression of PbI2 formation by the Sb addition, which would improve the short-circuit current density and fill factor by the electron blocking effect. Addition of Sb would also reduce the lattice constant of CH3NH3Pb12zSbzI3 because of the smaller cationic radius of Sb31 compared to the Pb cation. The reduction of the lattice constant of the perovskite would increase the energy gap, which results in the increase of the open-circuit voltage. On the contrary, the decrease in conversion efficiencies was observed for the devices with high Sb content. From the Rietveld analysis, CH3NH3 vacancies were found to be introduced in the CH3NH3Pb12zSbzI3, and the MA vacancies could promote the recombination of

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509

Table 23.6 Optimized crystal structures and occupancy parameters for cubic CH3NH3PbI3 and CH3NH3Pb0.81Sb0.19I3 in the solar cells. ˚) Cubic CH3NH3PbI3 (space group Pm3̅ m, a 5 6.2883 A Atom

Wyckoff site

x

y

z

Occupancy

Pb I N C

1a 12h 12j 12j

0 0 0.413 0.578

0 0.0435 0.413 0.578

0 0.5 0.5 0.5

1 0.25 0.085 0.085

˚) Cubic CH3NH3Pb0.81Sb0.19I3 (space group Pm3̅ m, a 5 6.2805 A Atom

Wyckoff site

x

y

z

Occupancy

Pb Sb I N C

1a 1a 12h 12j 12j

0 0 0 0.413 0.578

0 0 0.0435 0.413 0.578

0 0 0.5 0.5 0.5

0.81 0.19 0.25 0.071 0.071

Source: Copyright (2018) The Japan Society of Applied Physics.

an electron and a hole. Such Rietveld refinement could be an effective method to determine the actual crystal structures for actual device configurations.

23.6

High-temperature annealing and abnormal improvement of conversion efficiencies

The stability of perovskite solar cells requires improvement before they can be put into practical use [52,176]. In addition to the substitution of MA by alkali metals, incorporating polymeric materials has been studied to improve the stability of perovskite solar cells [177
179], and the polymeric materials showed improvement of stability and promotion of crystal growth when introduced into the perovskite devices [178]. For example, by depositing a poly(methyl methacrylate) (PMMA) layer on the perovskite layer [19,84], the PMMA formed a compact layer via a cross-linked network and protected the device from oxygen and moisture. Poly(propylene carbonate) was also similarly used and the resulting solar cell showed stability in various environments. In this case, large crystals were formed by cross-linking the perovskite particles and polymer which suppressed defects [180]. Polysilane derivatives are also expected as other candidates for the improvement of the perovskite solar cells. Unlike ordinary polymer materials, polysilane derivatives have two advantages. The first is that the polysilane derivatives are a p-type semiconductor, and this facilitates hole transfer and rectification at the pn junction. The second is that the polysilane derivatives have high stability and are expected to act as a protective layer on the photoactive layer. Polysilanes are more stable at

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elevated temperatures above 300 C than ordinary organic materials. Polysilanes may function as a protective layer when deposited on perovskite compounds. Hence, polysilanes, such as decaphenylcyclopentasilane (DPPS), have been applied as hole transport layers (HTLs) [181] and as additives in the photoactive layer [87] of MAPbI3 perovskite devices. DPPS has been found to promote a uniform perovskite morphology, which increases device power conversion efficiencies. However, chlorobenzene is typically used to dissolve and deposit DPPS by solution processing. Chlorobenzene (CB) can also have effects on device performance, which have not been investigated separately from its use as a solvent for DPPS [88,89,182]. Thus there is a need to separately investigate the effects of DPPS and chlorobenzene in detail. Here, the photovoltaic properties and stabilities of perovskite photovoltaic devices treated with a DPPS layer are described. The cells were treated by high-temperature annealing in ambient air. The effects of treating devices with DPPS in chlorobenzene on the photovoltaic properties and microstructures were investigated. When only the DPPS is used as the HTL, the obtained conversion efficiencies is not enough, and then, the DPPS/spiro-OMeTAD bilayer was applied in the present work. The chlorobenzene-treated devices were also compared to investigate the effect of the chlorobenzene. To increase the conversion efficiency by raising the fabrication temperatures of the devices, the device preparation time was shortened. The perovskite precursor solutions were normally spin-coated during the first coating. During the second and third spin-coatings, an air blow method was applied [161]. Then, a standard device was annealed at 140 C for 10 min to form the perovskite compound [183]. When the starting material is CH3NH3I 1 PbI2, the suitable annealing temperature was 100 C, which was confirmed in the previous work [154], whereas the suitable temperature for the starting materials of 3CH3NH3I 1 PbCl2 was B140 C [183], and CH3NH3PbI3 with 2CH3NH3Cl (gas) were formed during the reaction. DPPS (Osaka Gas Chemicals, OGSOL SI-30-15, 10 mg) solutions were prepared using chlorobenzene (Fujifilm Wako Pure Chemical Corporation, 0.5 mL). The DPPS solution was dropped onto the perovskite layer during the last 15 s of spin-coating the perovskite precursor solutions. The devices with DPPS layers were annealed at temperatures in the range of 140 C
260 C for 1
10 min. The summarized values of these cells are listed in Table 23.7. The device prepared only with CB showed a photoconversion efficiency (η) of 3.87% after annealing at 140 C. To compare this CB-treated device with the DPPS added devices, the CBtreated device was also annealed at 190 C. Averaged efficiencies of four cells in a single substrate can provide a description of the uniformity of the films in the substrate. The device prepared with CB 1 DPPS annealed at 190 C showed an η of 9.40%, and the device with DPPS fabricated at 220 C provided a higher η value of 10.04%. All the cell parameters were improved for the CB 1 DPPS-treated devices, and the shortcircuit current density (JSC) and fill factor (FF) were particularly increased as compared with those of the CB-treated devices. Series resistance (RS), shunt resistance (RSh), and averaged efficiency of four cells (ηave) were also improved. Stabilities of the photovoltaic parameters after preparation in ambient air were measured for the cells [184]. After 255 days, the highest conversion efficiency of 12.4% was obtained for the DPPS device prepared at 190 C. The DPPS device

Table 23.7 Photovoltaic parameters of perovskite photovoltaic devices. Solution a

CB CB CB 1 DPPS CB 1 DPPS CB 1 DPPS CB 1 DPPS CB 1 DPPS

Annealing ( C, min)

JSC (mA/cm2)

VOC (V)

FF

Rs (Ω cm2)

Rsh (Ω cm2)

η (%)

ηave (%)

100, 15 140, 12 190, 6 140, 60 190, 30 220, 2 250, 1.5 260, 1

15.8 15.4 12.6 12.6 22.5 22.4 20.5 13.5

0.800 0.674 0.708 0.903 0.753 0.808 0.875 0.884

0.550 0.373 0.298 0.660 0.554 0.554 0.524 0.483

8.89 12.35 18.87 8.13 11.92 9.33 9.90 24.37

525 72 71 916 5670 831 268 709

6.94 3.87 2.66 7.51 9.40 10.04 9.40 5.75

6.76 3.65 2.30 6.06 8.99 9.40 8.71 5.42

140, 12 190, 6 190, 30 220, 2

13.3 10.6 22.2 19.3

0.643 0.620 0.884 0.849

0.543 0.311 0.634 0.618

8.35 23.71 7.35 8.03

330 86 5830 2700

4.64 2.05 12.44 10.15

4.54 1.70 11.84 9.39

After 255 days CB CB CB 1 DPPS CB 1 DPPS a

Prepared from CH3NH3I:PbI2 5 1:1 and without air blowing.

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Advanced Flexible Ceramics

prepared at 220 C showed the good stability over the extended period time. Although the conversion efficiency of the CB-treated device prepared at 190 C decreased, η of the device prepared with CB 1 DPPS at 190 C increased, which shows the DPPS is effective to increase the photovoltaic properties by the hightemperature annealing. Although FF values increased for all the devices, opencircuit voltage (VOC) decreased for the CB-treated device after 255 days, while the VOC for the DPPS-added devices increased. The parameters of the DPPS devices were stable or increased, which would be due to reduction of the influence of moisture, oxygen, and spiro-OMeTAD and the DPPS protected the perovskite layer. Shunt resistances were high for DPPS-added devices, which would be due to the effect of hole transport and electron blocking by DPPS [185]. In this study, the DPPS was dissolved in chlorobenzene, which is often used as an antisolvent to promote grain growth and form smooth surface structures on perovskite films, resulting increased current densities [186
188]. Although temperatures around B100 C are commonly used to fabricate perovskite devices, high temperatures above B180 C are required to improve the efficiencies of DPPStreated cells. Thus, DPPS affects the morphology and photoelectronic properties by a different mechanism from that of chlorobenzene. The DPPS layer suppresses MA desorption and DPPS is also a p-type semiconductor, which has hole transporting properties that inhibit hole and electron recombination. The measured photovoltaic parameters of the best DPPS-treated device with the highest conversion efficiency in that work are listed in Table 23.8 [184]. The device was annealed at 190 C for 5 min. Although the conversion efficiency of this asprepared device was lower than that prepared at 190 C for 30 min, its efficiency increased to B15% after 66 days. Changes of the (100) XRD reflections showed that the perovskite crystallites were randomly aligned after 10 days, and the intensity ratio of I100/I210 increased from 1.9 to 2.6 after 66 days, which indicates that the (100) planes were comparatively well aligned. In addition, the crystallite ˚ after 66 days. This indicates that the increase of size increased from 486 to 617 A the conversion efficiencies would be caused by the crystal growth of the perovskite compounds during room temperature aging. This crystallization mechanism even after the annealing at the high temperature of 190 C would be explained by the DPPS treatment, which might slow the diffusion of ions and crystal growth during Table 23.8 Photovoltaic parameters of the device, treated with DPPS in chlorobenzene and annealed at 190 C for 5 min. Time (day)

JSC (mA cm22)

VOC (V)

FF

Rs (Ω cm2)

Rsh (Ω cm2)

η (%)

ηave (%)

0 10 24 58 66

17.7 19.4 21.6 22.3 22.4

0.839 0.850 0.918 0.930 0.923

0.558 0.593 0.699 0.724 0.727

4.41 3.49 3.96 3.66 3.72

908 1440 1610 2120 3070

8.30 9.80 13.82 15.03 15.10

6.58 8.81 12.48 13.31 13.38

Crystal structures for flexible photovoltaic application

513

annealing. Then, the remained noncrystalized phase might contribute to the crystal growth during the aging. An energy level diagram of the present DPPS-treated perovskite cells was proposed considering the interfacial electronic state [189
191]. When the device was irradiated from the FTO glass side, carriers (holes and electrons) separate at the interfaces. Holes separated in the perovskite layer are carried through the PbI2, DPPS, and spiro-OMeTAD to the gold electrode. Conversely, electrons are transported through titanium dioxide to the FTO. By inserting a DPPS layer between the photoactive layer and the HTL, holes are effectively transported from the valence band maximum of the MAPbI3 to the Fermi level of Au. High shunt resistances were obtained for the DPPS-treated devices, which is attributed to the hole transporting and formation of smoother surface morphology by DPPS. Efficient carrier transport is likely caused by the specific arrangement of the phenyl group around the cyclopentasilane in the DPPS [184,185]. A small PbI2 layer might be formed by MA desorption at the perovskite/DPPS interface at high temperatures. If this thin PbI2 layer forms at the perovskite/DPPS interface during or after annealing, PbI2 may act as a p-type semiconductor and an HTL [105,153]. Activation energies of ion migration of MA1, I2, and Pb21 in the MAPbI3 were reported to be 0.84, 0.58, and 2.31 eV, respectively [167]. Since the activation energy of Pb21 migration is higher than those of other ions, the formed PbI2 layer may remain around the surface of the perovskite. The increased efficiency of the DPPS-treated devices might also be related to crystallization of amorphous grains. During the spin-coating of DPPS, a composite layer of DPPS and amorphous preperovskite compounds forms, which provides a solid interface for room temperature aging. Because DPPS can also function as a hole transport material [185], holes are efficiently transported at the interface, to improve the Rsh and VOC. Since all the processes in the present work were performed in the ambient air, further improvement of photovoltaic properties is expected by controlling the environmental conditions. In summary, conversion efficiencies improved by inserting the DPPS layer during spin-coating of MAPbI3 and annealing above 190 C. A cell fabricated at 220 C had the highest photoconversion efficiency among the as-fabricated cells and the conversion efficiencies of all devices remained stable over more than 8 months in air. In addition, a device fabricated at 190 C had the highest efficiency following room temperature aging. The DPPS layer acts as both a protective layer for the perovskite and as an HTL. The DPPS treatment promoted fewer lattice defects and grain boundaries, which suppressed the leakage current and increased the JSC. These findings indicate that high-temperature annealing of devices treated with DPPS in chlorobenzene is an effective and easy method to improve photoconversion efficiencies and stability of MAPbI3 solar cells.

23.7

Conclusion

Crystal structures of various types of perovskite MOF compounds possibly used for flexible solar cells were reviewed and summarized. The stabilities of the perovskite

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Advanced Flexible Ceramics

MOF structure could be examined by calculating the t- and μ-factors, and several candidates of low-toxicity perovskite compounds can be proposed from this kind of survey. In addition to the standard CH3NH3PbI3 and HC(NH2)2PbI3 crystals, various types of element-substituted perovskite and double perovskite halides were described. Low-dimensional perovskite compounds with 2-, 1-, or 0-dimensionality and 2-dimensional double perovskites were also summarized and described, which will provide the further diversity of these perovskite MOFs. The perovskite layer containing dense grains with high (100)-orientation could be obtained by NH4Cl addition and air blowing method. Dendritic perovskite crystals were also obtained by Br and Sb doping for FAPbI3, which would be effective to control the morphology. Crystal structures of perovskite CH3NH3Pb12xSbxI3 films in the actual device configuration were examined using Rietveld analysis. XRD profiles agreed well with the computed data by optimization of the atomic positions and site occupancies in the unit cell. Even for the single crystal of MAPbI3, some amounts of defects such as CH3NH3 could exist, and the electronically neutral conditions may be maintained by the iodine defects or mixed cation valences of Pb21 and Pb41. This kind of tolerance for defects and nonstoichiometry would provide a wide processing window for these perovskite thin films. Conversion efficiencies and stabilities were also improved by inserting the DPPS layer during spin-coating of MAPbI3 and annealing above 190 C. The DPPS layer acts as both a protective layer for the perovskite and as an HTL. The DPPS treatment promoted fewer lattice defects and grain boundaries in the perovskite MOFs, which suppressed the leakage current and increased the JSC. Understanding of these basic structures and fundamental processes would lead to development of new perovskite MOF compounds and device architectures, and further researches will be continued and carried out for realization of the perovskite solar cells.

Acknowledgments The author would like to acknowledge many colleagues for experimental help and useful discussion as follows: Atsushi Suzuki, Masahito Zushi, Yuya Ohishi, Yuji Ando, Hiroki Tanaka, Masaya Taguchi, Kaede Kitagawa, Yugo Asakawa, Kohei Suzuki, Taisuke Matsumoto, Satsuki Kandori, Naoki Ueoka, Yuri Umemoto, Jun Yamanouchi, Masanobu Okita, Satoshi Minami, Sakiko Fukunishi, and Tomoharu Tachikawa. A part of this work was supported by Satellite Cluster Program of the Japan Science and Technology and a Grant-in-Aid for Scientific Research (C) 21K04809.

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39.

Ceramic materials for coatings: an introduction and future aspects

24

Ganesh R. Chate1,2, Nikhil Rangaswamy3, Manjunath Shettar4, Vaibhav R. Chate2,5 and Raviraj M. Kulkarni2,6 1 Mechanical Engineering Department, KLS Gogte Institute of Technology, Belagavi, Karnataka, India, 2Centre for Nanoscience and Nanotechnology, KLS Gogte Institute of Technology, Belagavi, Karnataka, India, 3School of Mechanical Engineering, REVA University, Bangalore, Karnataka, India, 4Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal Academy of Higher Education, Manipal, Karnataka, India, 5Civil Engineering Department, KLS Gogte Institute of Technology, Belagavi, Karnataka, India, 6Department of Chemistry, KLS Gogte Institute of Technology, Belagavi, Karnataka, India

24.1

Introduction

Materials play a very important role in everyday activities and are not restricted to engineering applications. The study of materials becomes very important in order to develop high-quality components. Advances in engineering and technology have led to the classification of four different types of materials [1] and is shown in Fig. 24.1. Materials are classified into four categories, namely, metals and alloys, ceramics, composite materials, and polymers. The scope of this book is limited to ceramics, so only ceramics is detailed in this section. Ceramics are inorganic, nonmetallic materials that are noncorrosive, thermally and electrically resistant, chemically inert with good hot hardness, good refractoriness, and can be shaped [2]. Ceramics have been used since ancient ages and hence have a very long history, which started with the use of clay (bentonite and kaolinite) to modern-day advanced and fine ceramics [3]. Its application ranges from a simple kitchenware to aerospace components. The properties of ceramics are mainly influenced by the covalent bond and ionic bond between the atoms. Few properties of ceramics that are used as a coating material are listed as follows [4 6]: G

G

They are hard and brittle, and they are used in making abrasives, cutting tools [7], mugs, and floorings. They have good wear resistance and are used in making dies for drawing and extrusion, kitchen wear, etc.

Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00026-9 © 2023 Elsevier Ltd. All rights reserved.

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Advanced Flexible Ceramics

Materials

Metals and Alloys

Ceramics

1.Ferrous 2.Non-Ferrous

Based on Application 1.Glasses 2.Clay Products. 3.Refractories. 4.Advanced ceramics. 5.Abrassives. 6.Ceramics

Composite Materials

1.Metal Matrix 2.Polymer Matrix

Polymers

1.Natural 2.Synthetic

Based on Composition 1.Oxides. 2.Sulfides. 3.Nitrides 4.Florides. 5.Carbides. 6.Silicates.

Figure 24.1 Classification of materials.

G

G

G

G

G

They have good refractory property and thus can be used as a coating material in furnaces. They have good optical properties and find applications in cathode ray tubes, artificial ruby crystal (aluminum oxide doped with chromium). They are not a good conductor of heat; they are used to make filters used in gating system in sand casting to filter molten metal. They are bad conductors of electricity; they find application in parts of power lines, panels, and other electrical and electronic devices. They are chemically stable and thus find application in chemical industries.

Coating is a thin layer deposition of materials on the other material surface up to 100 microns. Coating is done to improve the material properties such as hardness, wear resistance, corrosion resistance, thermal stability, chemical stability, and esthetics [8]. As ceramics have the above said properties, they act as a good coating material for different applications such as coating on cutting tool materials, Magnetic applications of ceramics include portable electrochemical sensors, photocatalysts, and magnetic storage devices [9]. They can also be used as a coating for the aerospace and power plant components, internal combustion engine parts working at high temperature [10,11], structural applications [12], medical applications [13], corrosion, oxidation, and erosion resistance layers in textile industries, nuclear power plants, aerospace components, electrical insulation coatings on very large scale integration (VLSI), and diffusion barrier coatings [14]. This chapter focuses only on ceramic materials used for coating and is dealt in detail in this section.

Ceramic materials for coatings: an introduction and future aspects

24.2

529

Ceramic coating material selection

The selection of a suitable ceramic material for coating depends upon various aspects, which are discussed as follows: G

G

G

G

Functional requirements: The necessity of coating on the base material. Before coating, one must know the conditions and the purpose for which it is done. For example, for corrosion resistance surface of Q235 carbon steel, one can coat FeAl2O4 Fe3O4 using plasma electrolytic oxidation [15] to improve hardness Economy is a very important aspect while selecting the type of a ceramic as a coating material. The approach adopted for coating, the type of coating material, properties of the base material, and properties required after coating decide the financial aspect. The market of some certain products is more driven by this factor. However, economy is not an important factor in defense applications as functionality is very important over cost. Economical aspects and availability of materials are interrelated, so the selection of ceramic material for coating is also dependent on availability. Material science, that is, properties of the material to be coated: All materials are not same in terms of their microstructure, macrostructure, and properties (mechanical, chemical, electrical, magnetic) and hence cannot be coated with the same ceramic material. Hence, this demands different coating methods and materials. Coating ceramics on a metal matrix composite is different than coating an alloy steel, natural fiber composites [16]. Quality attributes of the product: Selection of the coating material is dependent on the quality of the product after coating. The definition of quality, which says “fitness for purpose” [17], can be considered while selecting the ceramic coating material. Some products may require a good surface finish and mechanical properties may not be important, while in other cases, surface finish along with dimensional accuracy and mechanical properties are important. In such situations, selection of ceramic materials for coating is purely based on the quality aspect while keeping manufacturing cost in mind [18].

24.3

Ceramic coating materials

There are different types of ceramic materials coated on the substrate based on application. A simple example for traditional ceramics is porcelain articles, tiles, etc. These are made of clay, silica, feldspar, calcite, etc. [19], and ceramic coatings can further improve their properties [20]. There are many engineering ceramics that are used as coating materials and are discussed below.

24.3.1 Aluminum oxide Aluminum oxide (Al2O3) is a compound of oxygen and aluminum and is commonly known as alumina. It is white in color and has an amorphous structure. It is an odorless material obtained by refining bauxite using Bayer’s process [21]. It has a density of 3.9 g/cm3, compression strength of 2 4 GPa, tensile strength up to 630 MPa, and melting point of 20172 C [22,23]. Al2O3 is chemically inert and

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Advanced Flexible Ceramics

finds its applications such as kilns (clay ware). Because of its hardness, it is used to make abrasives for sandpapers. Because of its high melting point and high thermal resistance, it is used to make furnace insulations, coating on spark plugs, insulating heat sinks, etc. As Al2O3 has good strength-to-weight ratio, it is used in making protective equipment such as body armor and bullet-proof glass. Owing to its good hardness and chemical inertness, it works as a good coating on biomedical implants and lab equipment such as furnaces and labware. It is coated on lithium particles as a positive electrode material for lithium ion batteries [24]. Researchers have used different methods to coat Al2O3 on different substrates. Jobin et al. coated Al2O3 on stainless steel substrate using plasma spray process. Significant improvement was observed in the coated components in terms of leak tightness and wear resistance. L9 orthogonal array of the Taguchi technique was used to optimize the process parameters to get the best results [25]. Bodino et al. successfully coated Al2O3 on polyethylene terephthalate using radio-frequency magnetron sputtering up to 200 nm [26]. Thirumalai Kumara Samy et al. studied the effect of process parameters in plasma spraying of Al2O3 on AZ31B magnesium alloy using response surface methodology [27]. Veena et al. coated aluminum alloy by Al2O3 using spray coating method. The coated component exhibited good vibration damping properties in addition to anticorrosive behavior [28]. Inconel 600 collar was coated with Al2O3 using plasma spray. Furthermore, this coated Inconel collar was also coated with Ni-50cr and NiCrAlY bond deposited by high-velocity oxygen fuel technique. Thus, coated components showed improvement in resistance up to 1011 at room temperature and 108 Ω at 649 C. These components are used in fission chamber neutron detectors as they possess high electrical resistance at a high temperature [29]. Al2O3 mixed with other ceramic materials have been successfully coated to improve the hardness of Al6061, which is used in piston and cylinder liners of IC engines. Two different ceramic coating mixtures (group A and group B) were prepared and coated on Al6061. In group A, Al2O3 was mixed with chromium oxide (CrO2) and silicon carbide (SiC) in different weight proportions and used as a coating material. In group B, Al2O3 was mixed with zirconium dioxide (ZrO2) and titanium oxide (TiO2) in different weight percentage combinations and used as a coating material, where coating was done using plasma spraying. A significant improvement was observed in the wear resistance for group B coating at 60% Al2O3 1 20% ZrO2 1 20% TiO2, and wear rate was about 0.0489 mm3/Nm and 94 HRB. NiCrAlY has been used as a bonding layer between Al6061 and the blended ceramic powders [30]. Kennedy et al. coated Al2O3 on silvered polyethylene terephthalate using ion-beam-assisted physical vapor deposition technique for making solar reflectors. The coated components were found to have better durability and showed solar-weighted hemispherical reflectance of 95% [31]. Lux et al. explained the advantage of Al2O3 coating on carbide tools for steel cutting by using chemical vapor deposition. The coating helped to increase the life of tool by providing good wear resistance, thus improving tool life [32]. Jiyai et al. studied corrosion and wear behavior of gray cast iron coated with Al2O3 using plasma electrolytic aluminating technique [33].

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24.3.2 Silicon carbide SiC, also known as carborundum, is a compound of silicon and carbon. It has good strength, low thermal expansion, high thermal conductivity, and good chemical resistance. It is interesting to note that SiC cannot be coated using thermal spray as it decomposes near to its melting point, and hence, a mixture of ceramic materials [like SiC and zirconium diboride (ZrB2)] is used to coat through plasma spraying [34]. Costa et al. studied the effect of SiC coating on tungsten carbide tools. Coating was done using radio-frequency magnetron sputtering. It was found that wear rates were reduced to 50% and the coated products retained their hardness up to 1100 C [35]. Quian et al. coated carbon composite material using SiC by means of two-step pack cementation technique. This coating prevents oxidation at elevated temperature, thus retaining the strength of the composite material. Carbon/carbon composites have been used in aerospace applications. Without coating, these composites can withstand up to 453 C, and after coating they can withstand up to 1499 C [36]. Cheng et al. coated Sic on carbon/carbon composites substrate using low-pressure chemical vapor deposition [37], and laser-induced chemical decomposition was also used to coat the carbon composites [38].

24.3.3 Yttrium aluminum garnet Yttrium aluminum garnet (YAG, Y3Al5O12)is a synthetic, crystalline and colorless material with a melting point of 1970 C and density of 4.6 g/cm3 [39]. It has good hardness (Vickers hardness up to 17 GPa) [40]. The coatings of Y3Al5O12 are used in gas turbine applications as thermal barrier on stainless steel and Ni-alloy substrate [41].

24.3.4 Rare-earth cerates and zirconocerate Rare-earth cerates have good chemical resistance and thermal stability; they possess ionic conductivity and a very high melting point. These are used as thermal barrier coatings in aircraft engines and gas turbines [42]. Rare-earth oxide powders (La2O3, ZrO2, and CeO2) were bond coated using MCrAlY by arc ion-plating, and zirconate and cerate coatings were deposited via electron beam physical vapor deposition [43].

24.3.5 Silicon nitride Silicon nitride (Si3N4) has good antioxidation property along with high strength, thermal stability, and creep resistance. It has a number of applications such as bearing, cutting tools, engine parts in aircraft, turbine components, and biomedical applications. Russel et al. coated Si3N4 on titanium using plasma-immersion implantation and deposition method. The coated component prevented oxidation and had biomedical applications [44].

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24.3.6 Aluminum nitride Aluminum nitride (AlN) has got good thermal conductivity and piezoelectric properties. It produces anticorrosive and wear-resistant coating on the substrate. AlN melts at 2200 C. It is used to coat surface of acoustic wave sensors, bearings, parts of machine tools. Ionescu et al. successfully coated AlN on stainless steel S3400 and Inconel substrate using reactive radio-frequency magnetron sputtering. It was found that thus coated components had good tribological properties [45]. Muktepavela et al. investigated the AlN and TiN coatings on tungsten carbide and stainless steel substrates using reactive sputtering technique. There was significant improvement in fracture toughness [46].

24.3.7 Titanium nitride Titanium nitride (TiN) is a very hard ceramic material with Vickers hardness of 2100 and chemical stability with a density of 5.21 g/cm3 and melting point of 2947 C [47]. TiN coatings are well suited for substrates such as different grades of steel, aluminum and its alloys, and titanium alloys. Owing to its chemical stability, TiN is used to coat implants. The color of TiN is golden, and hence, it is used for decorative purposes. Nguyen et al. studied the effect of coating TiN on aluminum casting alloy LM25 and pure aluminum by ion-assisted deposition. It was observed that TiN coating has strong bonding with LM25 compared to pure aluminum, and there was improvement in wear resistance, corrosion resistance, and esthetics [48]. It is also coated on rifle parts as it is smooth, which makes it easy for maintenance. TiN can be coated on many substrates such as HSS, WC, AISI D2, and AISI4140. TiN can be coated using chemical vapor deposition, physical vapor deposition, thermal spraying techniques, arc ion plating, cathodic arc evaporation, multiarc ion plating, magnetic arc ion plating, reactive arc evaporation, and magnetron sputtering [49].

24.3.8 Barium titanate There are other ceramic materials with different properties coated on materials for different applications such as surgical instruments, labware, chemical storage tanks, etc. and cordierite (2MgO 2Al2O3 5SiO2), lead zirconium titanate, TiO2, titanium boride (TiB2), uranium oxide (UO2), Y3Al5O12, Yb2O3, multicomponent rare-earth cerate and zirconocerate ceramics, Si MoSi2 ceramic, and bioactive silicate-based ceramics, among other ceramic coating materials. The combination or blending of ceramic materials can also improve the properties. The highlights of some research work on ceramic coating material, substrate, coating process/technique used and the relevant application is shown in Table 24.1.

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Table 24.1 Ceramic coating material, substrate, coating process, and application of coated material. Ceramic material

Substrate

Coating process

Application

Al2O3

Stainless steel

Plasma spray process

Al2O3

Polyethylene terephthalate

Al2O3

AZ31B magnesium alloy

Radio-frequency magnetron sputtering Plasma spraying

Structural applications and automotive parts [25] Automobiles [26]

Al2O3

Inconel 600

Al2O3 1 ZrO2 1 TiO2 and Al2O3 1 CrO2 1 SiC

Aluminum alloy Al6061

Al2O3

Polyethylene terephthalate

Plasma spraying of Al2O3 and intermediate bonding of Ni50cr and NiCrAlY using high-velocity oxygen fuel technique Plasma spraying of blended ceramic mixture on the Al6061 substrate with a NiCrAlY as a bonding layer between ceramic mixture and substrate The coating is deposited by an ion-beamassisted physical vapor deposition technique

Aircraft components, cases of electronic products, such as laptop, mobiles, IC engine parts, and structural components [27] Fission chambers for neutron detectors [29]

Automotive parts and structural applications [30]

Solar reflectors [31]

(Continued)

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Table 24.1 (Continued) Ceramic material

Substrate

Coating process

Application

Al2O3

Carbide tools

Cutting tools [32]

Al2O3

Gray cast iron

SiC

Tungsten carbide and silicon

SiC 1 ZrB2 SiC

Graphite Carbon/carbon composites

SiC

Carbon/carbon composites

Chemical vapor deposition Plasma electrolytic aluminating Radio-frequency magnetron sputtering Plasma spraying Two-step pack cementation technique Low-pressure chemical vapor deposition

SiC

Carbon/carbon composites

YAG/Y3Al5O12

Stainless steel and Ni-alloy

Thermal spray

Zirconate and cerate coatings

Rare-earth oxide powders (La2O3, ZrO2, and CeO2) and was bond coat using MCrAlY by arc ion-plating Titanium

Electron beam physical vapor deposition

Si3N4

AlN

TiN

AlN

Tungsten carbide and stainless steel Tungsten carbide and stainless steel Stainless steel S3400 and Inconel

Plasma-immersion implantation and deposition method Reactive sputtering technique Reactive sputtering technique Reactive radiofrequency magnetron sputtering

Machine parts [33] Cutting tools [35]

Tools [34] Aerospace applications [36] Aerospace applications [37] Aerospace applications [38] Thermal barrier in gas turbine engines [41] Thermal barrier coatings [42,43]

Biomedical applications as oxidation resistance layer [44] Cutting tools [46]

Cutting tools [46]

Ball bearings, bearings, rotating axes, cutting tools, etc. [45] (Continued)

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Table 24.1 (Continued) Ceramic material

Substrate

Coating process

Application

TiN

Aluminum casting alloy LM25 and pure aluminum Aluminum and its alloys

Ion-assisted deposition

Automotive parts [48]

Micro-arc discharge plasma

Aerospace components, electronic equipment, structures of the machine [50]

Al2O3

24.4

Coating methods

In this section, different methods adopted for coating these ceramic materials is discussed along with the applications, advantages, and challenges of the material and processes. The coating methods are chosen based on the following aspects: 1. 2. 3. 4.

Properties of the substrate; Properties of the ceramic material to be coated; Properties required after coating; Economy.

There are different ceramic coating methods used for different substrates and for different applications. These methods are further classified and are shown in Fig. 24.2. Sometimes, a simple hand layup coating can also achieve the intended purpose. For example, clay coating in cupola melting furnace, which helps to reduce heat transfer by its refractoriness, thus improving the efficiency of the furnace. Other ceramics that are termed as thermal ceramic coatings improve the quality of the melt and reduced energy consumption [53]. In some cases, the ceramic coating along with solid lubricants are coated on aluminum to improve tribological properties [50].

24.5

Future aspects in ceramics

Present-day ceramics have limited properties, which restricts the application of ceramics. In future, ceramics can be used in different applications by using different blends of ceramics or as composites normally termed as hybrid ceramics [54]. Especially in dental applications, hybrid ceramics are going to play a vital role due to their ability to enhance mechanical properties and biocompatible requirements [55]. The sandwitching of different ceramic layers may lead to good mechanical properties and chemical stability. Further advancement in ceramic coating on fiber material is required so

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Figure 24.2 Ceramic coating methods [51,52].

that automobile weight can be reduced with better body strength, thus reducing the fuel consumption. The ceramic coating technology can be further exploited in military equipment, such as lightweight bulletproof jackets, coating on armored vehicles, and stealth technology. In medical field, surgical instruments are generally coated with ceramics, which are nontoxic and oxidation resistant; further research in coating may improve the life of these surgical instruments. In the near future, some amount of metallic equipment can be completely replaced by ceramics and also ceramic coating. Coating ceramics on different ceramic substrate can also be an interesting field to explore new properties and also improve the exiting material properties.

24.6

Conclusions

Ceramics are the oldest but most essential material in any field. A ceramic coating offers beneficial properties and enhances the efficiency of the system where those components are used. Many researchers have successfully coated different ceramic materials on different types of substrates, and their experiments and results are discussed in this chapter. The applications of ceramic coatings are endless, and thus, the scope of research in the field of ceramics is tremendous.

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Development of an advanced flexible ceramic material from graphene-incorporated alumina nanocomposite

25

Tapan Dash1, S. Tanuja Rani2, Biswajit Dash3 and Surendra Kumar Biswal1 1 Tirupati Graphene and Mintech Research Centre, Bhubaneswar, Odisha, India, 2 Centurion University of Technology and Managemen, Bhubaneswar, Odisha, India, 3 Modern Institute of Technology and Management (MITM), Bhubaneswar, Odisha, India

25.1

Introduction

Since the human race for an extremely smart and digital way of leading life never ends, the need to develop or change everything that is necessary for a better way never stops. This kind of urge opens the search for better materials with better properties to give better results. This chapter is precisely about the ways of achieving advanced flexible ceramics especially from graphene-incorporated alumina nanocomposites. Ceramics are mostly used materials from time immemorial in many fields. But the concept of flexible ceramics is what makes it more attractive. These can make a different world on earth where one can imagine of using a hard stone as paper. The research for the search of making such ceramics is going at a fast pace.

25.2

Ceramics

Ceramics are a type of material with low density, high melting point, very high elastic modulus, nonreactive and brittle. Due to these unique properties, they are used in our day-to-day life and we find them around us in different forms. In short, ceramics are defined as inorganic and nonmetallic materials made up of either metal or nonmetal compounds that have been shaped and then hardened by heating to high temperatures. In general, they are hard, corrosion-resistant and brittle. These materials are strong in compression and weak in shearing and tension and can withstand chemical erosion that occurs in an acidic or caustic environment. An average of 1600oC also can be tolerated by these ceramic materials except for the inorganic materials which do not include oxygen like silicon carbide. Ceramics find a very large application due to their properties. Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00027-0 © 2023 Elsevier Ltd. All rights reserved.

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Some important examples of ceramics include silica (silicon dioxide, SiO2), the main ingredient in most glass products, alumina (aluminum oxide, Al2O3) used in various applications from abrasives to artificial bones, and more complex compounds such as aluminum silicate (Al2Si2O5(OH)4), the main ingredient in most clay products, etc. In general, ceramic materials growth has divided into mainly two groups traditional ceramics and modern ceramics or advanced ceramics. Traditional ceramic raw materials include clay minerals such as kaolinite, and more recent materials include aluminum oxide, more commonly known as alumina. Modern ceramic materials, which are classified as advanced ceramics, include silicon carbide and tungsten carbide. Both are valued for their abrasion resistance, and hence find use in applications such as the wear plates of crushing equipment in mining operations. Advanced ceramics are also used in the medicine, electrical and electronics industries [1 3].

25.2.1 Properties of ceramics The properties of ceramics are what make it widely used material. These materials have high melting points so they are used as heat-resistant materials. These materials are considered durable so they are long-lasting and hard-wearing. These possess low electrical and thermal conductivity so they are good insulators too. Ceramics are chemically inert so they are unreactive with other materials. This makes them act as corrosion-resistant and antirust material. Apart from all these the mechanical properties of ceramics are very desirable and very necessary in industrial fields. If we separate the properties of ceramics into different categories then we will have the physical properties, mechanical properties, chemical properties, thermal properties, electrical properties, and optical properties [3 7]. The physical properties of ceramics can be found in their crystal structure and also in their chemical composition. Some common properties can be color, odor, and form (solid, liquid, gas). There are also other properties like melting point, boiling point, heat capacity, porosity, etc. Ceramics can be in crystalline or noncrystalline form but the density of crystalline materials is higher compared to noncrystalline materials. The density of ceramic materials lies between polymers and metals. The mechanical properties of ceramics tell the strength of these materials under the required stress and strain. In general, ceramic materials are brittle and ductile. If the stress on ceramics is increased, then the material will undergo elastic deformation as well as plastic deformation. Though ceramics have many good properties their nature of being brittle makes the necessity to modify these materials. That is why the run for removing this brittle nature and making them flexible is going at a fast pace in materials science research. Ceramics are made up of atoms which are bonded by either ionic or covalent bonds. These bonds are very strong leading to a high melting point, but the thermal expansion is very low. The ceramic materials are less soluble in liquids (acidic or alkali solutions), for example, alumina and silicon carbide. Because of this nature, they are resistant to corrosion so they are widely used for industrial applications. In order to increase the chemical resistance of ceramics to the desired level, one can add silica to them.

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Factors like internal porosity, grain boundaries and impurities affect the thermal conductivity of material substances. The same applies to ceramic materials also. Depending on the values of these factors some ceramic materials possess high thermal conductivity and some low thermal conductivity. For example, aluminum carbide and silicon carbide have a high level of thermal conductivity whereas zircon blocks have low thermal conductivity. Accordingly, these ceramics are used depending on the thermal conductivity values in the respective areas. The electrical properties of the ceramics vary depending on the material’s composition. That means some ceramics can act as semiconductors and some can act as insulators. Since the atoms of ceramics are bound by ionic bonds, they don’t allow the free electrons to flow. If changes are made by adding conducting materials in the synthesis of ceramics, one can get semiconducting ceramics. When it comes to optical properties, ceramics are usually amorphous crystals lacking fine particles. This creates microscopic grain boundaries, and microscopic pores and makes it difficult to transmit the light. Since advanced ceramics contain single crystals which lack grain boundaries and pores, so light can transmit easily through such ceramics. It is observed that advanced ceramics have fine particles and ferromagnetic and semiconducting properties. But major ceramics have translucent optical properties. Alumina is a transparent enough material and it is used in high-pressure sodium street lamps. For the past 20 years, various types of transparent ceramics have been developed for many applications. Clays are generally termed as hydro alumina silicate. It combines one or more minerals with traces of silica, organic impurities, and metal oxides.

25.2.2 Application of ceramics There are a wide variety of applications of ceramics. Every single necessity picks up its own suitable property from ceramic material possessions that is why ceramic materials are found almost in all different kinds of fields. The research on these materials continues to fulfill the new ideas of developing any field making the ceramics applications even more. Talking about structural applications, it mostly involves the use of monolithic ceramics. Some examples of structural applications of ceramic materials are bearings, seals, armors, liners, nozzles, and cutting tools [8 32]. Due to their current high cost, ceramic bearings and journals are used only for precision systems. ShN4 balls are used in spindle bearings for cutting tools, turbo molecular pumps, dental drills, and specialty instrumentation bearings. Boron carbide and single-crystal sapphire are used in bearings and seals. Silicon nitride and SiAlON are being considered for gas turbine bearings. The ceramics are also used for military applications for making weapons, defense systems, missile guidance systems, military ground vehicles, aircraft, sensors, military maneuvers, rocket nozzles, etc. These materials are even used as cutting tools and abrasives. This is mainly because of their hardness and also being inert and nonreactive. Such applications involve the use of cemented carbides which are hard carbides like WC, TiC, TaC, and NbC cemented together. These are used for cutting many hard materials in perfect order. Their cutting performance can also be increased by coating

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Advanced Flexible Ceramics

additional materials like TiN, TiC, and AhO3. Especially the cutting tools made from AhO3 have been in industrial use for many years. As abrasives, these materials are even used for polishing and shaping hard materials. For example, SiAlON is used in the rough cutting of Ni-based superalloys for aircraft turbine engines and Ni-based equipment used for corrosive applications in the oil and gas industry, TiC is used to semifinish and finish turning of ductile cast iron and high-speed turning of cast iron with coolant and many more. Automotive applications of ceramics include turbocharger rotors, exhaust port liners, honeycombs for catalytic converters, spark plugs, glow plugs, and sensors. Since ceramics can withstand high temperatures, so mainly used as refractory materials. Some of the refractory applications of ceramics involve dense, high-technology ceramics, including SiC, ShN4, BN, AlN, and pure oxides. Ceramics even find its applications in nuclear physics, for example, B4C is used as control rods and neutron absorbers and ceramic sensors are used to monitor oxygen levels in the liquid sodium cooling systems of liquid metal-cooled fast (neutron) breeder reactors (LMFBRs) and still many more applications are there. Ceramics are also used in the biotechnology field. They are used as especially three different types depending on the area they are used. They are used to strengthen the tissues. There are three types of ceramic materials used in biotechnology. The first type of material enables the ingrowth of tissue into the pores and provides good mechanical bonding. The second types are those which degrade gradually and become replaced by the surrounding natural tissue. The third type of ceramics is bioactive, that is, they interact with the bone and form chemical bonds. Ceramics also find a large application in the fields of electrical, electronic and magnetic fields. Here is the tabulation of its different applications in different areas.

25.3

Flexible ceramics or flexiramics

It is always good to dream of things which we really can’t see happening in real. What about imagining a land where you see that your vehicles can be carried out in a small bag by folding them? what about thinking of a lightweight cloth yet very hard that it cannot let hundreds of bullets pass through it? All these imaginations can be true by making one possibility which is making ceramics as flexible. This is not new in the science field, as there is already research going on in making such materials. The people of Eurekite BV, The Netherlands are working on it and they named flexible ceramics as flexiramics. They have made a sheet of paper made with ceramics as nonflammable. This gives an assurance that one can imagine being in a world where things have been made even simpler with flexiramics. Such thought of having them gives us a lot of ideas. For example, in the military field, the weapons will be lightweight and easy to carry to any hill area, in industry field heavy instruments transport will be very easy, in an electronic field all gadgets can be foldable and lightweight [33 41]. Thus we can observe how fascinating it is to construct a such world of flexiramics.

Development of an advanced flexible ceramic material

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Ceramic oxide particles are mostly used for traditional flexible ceramic materials. Their application is limited due to the inherent brittleness involved. They possess superior mechanical flexibility, high-temperature resistance, and excellent chemical stability [36 41]. Various methods have been developed for preparing flexible ceramic materials, with different benefits and characteristics. These materials usually possess low density, low thermal conductivity, large specific surface area, and many other excellent properties. Because of having outstanding properties, flexible ceramic materials are widely used in the fields of thermal insulation air filtration, water treatment, sound absorption, etc. Flexible ceramic materials can be divided into the basis of oxide, carbide, nitride, and according to their chemical composition. Oxide-based materials usually have high mechanical strength, low thermal conductivity, good electrical insulation and chemical stability, and can remain stable in the oxygen atmosphere. Oxide ceramics such as silica (SiO2), alumina (Al2O3), zirconia (ZrO2), titania (TiO2), zinc oxide (ZnO), mullite, cerium oxide (CeO2), nickel oxide (NiO), cobalt oxide (Co3O4), copper oxide (CuO), stannic oxide (SnO2), etc. can be used for producing flexible ceramic materials. Some hetero-elements and complex oxide ceramic are used including BaTiO3 fibers, Li0.33La0.56TiO3 (LLTO) fibers, Li7La3Zr2O12 (LLZO) fibers, CaCu3Ti4O12 fibers, LiNi0.5Mn1.5O4 fibers, yttrium aluminumgarnet (YAG) fibers, indium tin oxide (ITO) fibers, YBCO fibers, MFe2O4 (M 5 Cu, Co, Ni) fibers, and etc. Among the carbides, silicon carbide (SiC) fibers are the most common carbide fiber. Due to the presence of a strong covalent Si C bond, SiC fibers show a high mechanical resistivity and excellent chemical stability. They also well perform against oxidation, at temperatures higher than 1500 C. Zirconium carbide (ZrC) fibers, Zr-doped SiC fbers, SiOC fbers, SiCN fibers, and Al-doped SiC fibers are other carbide ceramics which can be used as fibers. The introduction of heteroatoms, such as oxygen, nitrogen, zirconium, aluminum into SiC fbers can improve the properties of ceramic fibers. At high temperatures, advanced fiber materials, pure SiC fibers, Zr-doped SiC fibers exhibit a better oxidation resistance. Boron nitride (BN) is largely used in fiber form. Silicon nitride (Si3N4) is widely applied in fiber application. In semiconductor devices, GaN fiber is used as an important functional fiber. Different advanced flexible ceramics such as centrifugal spinning, electrospinning, solution blow spinning, self-assembly, chemical vapor deposition, atomic layer deposition, polymer conversion, etc. are used for the production of flexible ceramic composites.

25.4

Graphene-incorporated alumina flexible nanocomposites

Graphene shows the addition of as little as 0.22% of graphene to alumina made it 50% more resistant to the propagation of cracks under strain. Other mechanical properties stayed on par with untouched alumina, while electrical conductivity was

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found to be increased by a factor of a hundred million. Ceramics such as alumina are widely used in many industries, including aerospace, automotive, medical, thermal management, and semiconductor processing. The very short propagation of cracks in ceramics is one of the most desired behavior of this class of materials. In the words of Alba Centeno, the lead author of the paper, the main advantage of graphene incorporation, at very low loadings, to an Al2O3 matrix, is that graphene makes Al2O3 electroconductive and, at the same time, improves mechanical properties and toughness. This is very important because sometimes when a second phase is incorporated in order to improve one specific property, the other properties are adversely affected.

Epoxy-based composites are generally used material for the structural, aerospace, and coating industries, due to their better tensile strength, high corrosion resistance, chemical resistance, and flexible ceramics. Alumina is resistant to chemical attack by a wide range of chemicals even at elevated temperatures, resistant to abrasion; possesses high hardness, high compression strength, high thermal conductivity, and is also resistant to thermal shock. These materials have a high degree of refractoriness, high dielectric strength, high electrical resistivity even at elevated temperatures, transparent to microwave radio frequencies and low neutron cross-section capture area. Especially the raw material to make alumina is readily available and its price is not subject to violent fluctuation. Some of the properties with their average value are listed out in Table 25.1. These properties can be even enhanced by different methods according to the desired result. A two-dimensional monoatomic layer of graphite is called graphene. It is an allotrope of the carbon family. When Sp3 hybridized graphite is converted to Sp2 hybridized C C bonded with a pie bond electron cloud, it forms graphene. It is a two-dimensional material of one atom thickness. It has drawn attention in research, academic and industries due to its unique properties such as high mechanical strength, current density, high electron mobility, ballistic transport, chemical Table 25.1 Some properties of alumina. Property

Value

Melting point Boiling point Hardness Electrical resistivity Mechanical strength Compressive strength Thermal conductivity Molecular mass Density Appearance

2072 C (3,762  F; 2,345 K) 2977 C (5,391  F; 3250 K) 15 19 GPa (9 on the Mohs scale) 1012 1013 Ωm 300 630 MPa 2000 4000 MPa 20 30 W/mK 101.96 g/mol 3.95 g/cm3 Solid

Development of an advanced flexible ceramic material

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inertness, high thermal conductivity, electrical conductivity, biosensor, energy field, display field superhydrophobicity. Graphene is reinforced to Al Al2O3 to prepare a very hard, tough, and flexible material. Studies involving graphene reinforcements have shown that density, flexibility, hardness, and wear resistance of Al Al2O3 composite increase with increasing alumina content [42 46]; however, it is also observed that the higher volume fraction of the Al2O3 reinforcement particles is reported to decrease the density of the composites. This discrepancy can be attributed to a difference in the shape and size of the reinforcement particles carbon materials such as carbon fibers, graphite carbon nanotubes (CNTs), graphene (G), and graphene oxide (GO) have been used as reinforcements in Al-MMCs [46 53]. In graphene-reinforced alumina aluminum matrix nanocomposites, an improvement of 65 79%, 20 49%, and 30 44% in yield strength, ultimate strength, and Vickers hardness have been reported with 1 wt.% GO addition, and the increase in GO content has led to grain refinement of the composite.

25.5

Conclusion

It has been observed that flexible ceramics is one of the frontier research areas in the field of material science. Especially the recent research on graphene reinforced alumina composite has great scope in the development of flexible ceramics material with extraordinary properties.

References [1] M. Bengisu, Engineering Ceramics, Springer Science & Business Media, 2013. [2] I. Gibson, D. Rosen, B. Stucker, Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing, Springer, 2014. [3] H. Schneider, R.X. Fischer, J. Schreuer, J. Am. Ceram. Soc. 10 (2015) 2948 2967. Available from: https://doi.org/10.1111/jace.13817. [4] T.E. Steyer, Shaping the future of ceramics for aerospace applications, Int. J. Appl. Ceram. Technol. 10 (3) (2013) 389 394. [5] T. Nozawa, T. Hinoki, A. Hasegawa, et al., Recent advances and issues in development of silicon carbide composites for fusion applications, J. Nucl. Mater. 386 388 (2009) 622 627. vol. [6] X.-M. Yu, W.-C. Zhou, F. Luo, W.-J. Zheng, D.-M. Zhu, Effect of fabrication atmosphere on dielectric properties of SiC/SiC composites, J. Alloy Compd. 479 (1-2) (2009) L1 L3. [7] X.-H. Shi, J.-H. Huo, J.-L. Zhu, et al., Ablation resistance of SiC ZrC coating prepared by a simple two-step method on carbon fiber reinforced composites, Corros. Sci. 88 (2014) 49 55. [8] R. Riedel, E. Ionescu, I.-W. Chen, Modern trends in advanced ceramics, Ceramics Science and Technology, Wiley-VCH, Weinheim, Germany, 2008, pp. 3 38. [9] J. Luyten, S. Mullens, I. Thijs, Designing with pores synthesis and applications, Kona 28 (2010) 131 142.

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[10] P. Colombo, Conventional and novel processing methods for cellular ceramics, Philos. Trans. R. Soc. A 364 (2006) 109 124. [11] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, Processing routes to macroporous ceramics: a review, J. Am. Ceram. Soc. 89 (2006) 1771 1789. [12] F.L. Riley, Silicon nitride and related materials, J. Am. Ceram. Soc. 83 (2000) 245 265. [13] J.F. Collins, R.W. Gerby, New refractory uses for silicon nitride reported, J. Met. 7 (1955) 612 615. [14] N.L. Parr, G.F. Martin, E.R.W. May, in: P. Popper (Ed.), Preparation, Microstructure, and Mechanical Properties of Silicon Nitride, in Special Ceramics 1, Heywood, London, 1960, pp. 102 135. [15] G.G. Deeley, J.M. Herbert, N.C. Moore, Dense silicon nitride, Powder Metall. 8 (1961) 145 151. [16] R.F. Coe, R.J. Lumby, M.F. Pawson, Some properties and applications of hot-pressed silicon nitride, in P. Popper (Ed.), Special Ceramics 5, British Ceramics Association, Stoke-on-Trent, UK, 1972, pp. 361 376. [17] B. Bihari Palei, T. Dash, S. Kumar Biswal, R. Saktivel, A novel alumina-magnesium oxide-reduced graphene composite: synthesis and characterizations, Int. J. Innov. Res. Phys. 1 (3) (2020) 37 47. [18] R.K. Sahu, V. Mukherjee, T. Dash, S.K. Padhan, B.B. Nayak, Vibrational and electronic properties of (5, 0) zigzag and (5, 5) armchair carbon and SiC nanotubes using density functional theory, Phys. B: Condens. Matter 615 (15) (2021) 1 14. 413074. [19] N. Nayak, T. Dash, D. Debasish, B.B. Palei, T.K. Rout, S. Bajpai, et al., A novel WC W2C composite synthesis by arc plasma melt cast technique: microstructural and mechanical studies, SN Appl. Sci. 3 (380) (2021) 1 8. [20] Tapan Dash, Binod Bihari Palei, Nibedita Mohanty, Sushree Subhadarshinee Mohapatra, Raj Kishore Mishra, Susanta Kumar Biswal, et al., Study on microstructural influence of graphene on synthesis of BaTiO3”, Mater. Today: Proc. 43 (1) (2021) 447 450. [21] Tapan Dash, Ananta Kumar Sahoo, Raj Kishore Mishra, Binod Bihari Palei, Retaining graphene structure in the synthesis of its composite with BiFeO3, Mater. Today: Proc. 43 (1) (2021) 216 219. [22] Tapan Dash, Tapan Kumar Rout, Binod Bihari Palei, Shubhra Bajpai, Saurabh Kundu, Amar N. Bhagat, et al., Synthesis of α Al2O3 graphene composite: a novel product to provide multi functionalities on steel strip surface, SN Appl. Sci. 2 (1147) (2020) 1 9. [23] R.K. Sahua, T. Dash, V. Mukherjeea, S.K. Pradhan, B.B. Nayak, “Spheroidal growth of graphite in arc plasma treatment”, Chem. Phys. Lett., I. 136986, V. 739 (2020) 1 5. [24] S. Dhar, T. Dash, B.B. Palei, T.K. Rout, S.K. Biswal, A. Mitra, et al., Silicon-graphene composite synthesis: microstructural, spectroscopic and electrical conductivity characterizations, Mater. Today: Proc., V. 33 (2020) 5136 5142. [25] Soumen Dash, Tapan Dash, Tapan Kumar Rout, Preparation of graphene oxide by dry planetary ball milling technique under oxygen atmosphere, IOP Conf. Ser.: Mater. Sci. Eng. 872 (012180) (2020) 1 6. [26] Binod Bihari Palei, Tapan Dash, Susanta Kumar Biswal, Reduced graphene oxide synthesis by dry planetary ball milling technique under hydrogen atmosphere, IOP Conf. Ser.: Mater. Sci. Eng. 872 (012158) (2020) 1 6. [27] Binod Bihari Palei, Tapan Dash, Susanta Kumar Biswal, Successful synthesis of graphene-aluminum composite with improved microhardness, Int. J. Eng. Adv. Technol. (IJEAT) 9 (3) (2020) 2218 2221.

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[28] T. Dash, B.B. Nayak, Tungsten carbide-titanium carbide composite preparation by arc plasma melting and its characterization, Ceram. Int. 45 (4) (2019) 4771 4780. [29] I. Gibson, D. Rosen, B. Stucker, Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing, Springer, 2014. [30] H.L. Marcus, J.J. Beaman, J.W. Barlow, D.L. Bourell, Solid freeform fabricationpowder processing, Am. Ceram. Soc. Bull. 69 (6) (1990) 1030 1031. [31] B. Lee, S. Komarneni, Chemical Processing of Ceramics, Second ed., CRC Press, Boca Raton, 2005. [32] T. Ritter, G. Hagen, J. Lattus, R. Moos, Solid state mixed-potential sensors as direct conversion sensors for automotive catalysts, Sens. Actuat. B 255 (2018) 3025 3032. Available from: https://doi.org/10.1016/j.snb.2017.09.126. [33] Y. Si, X. Wang, L. Dou, J. Yu, B. Ding, Ultralight and fre-resistant ceramic nanofbrous aerogels with temperature-invariant superelasticity, Sci. Adv. 4 (2018) eaas8925. [34] H. Wang, X. Zhang, N. Wang, Y. Li, X. Feng, Y. Huang, et al., Ultralight, scalable, and high-temperature-resilient ceramic nanofber sponges, SciAdv 3 (2017) e1603170. [35] C. Jia, L. Li, Y. Liu, B. Fang, H. Ding, J. Song, et al., Highly compressible and anisotropic lamellar ceramic sponges with superior thermal insulation and acoustic absorption performances, Nat. Commun. 11 (2020) 3732. [36] C. Jia, Y. Liu, L. Li, J. Song, H. Wang, Z. Liu, et al., A foldable all-ceramic air flter paper with high efciency and high-temperature resistance, Nano Lett. 20 (2020) 4993 5000. [37] H. Wang, S. Lin, S. Yang, X. Yang, J. Song, D. Wang, et al., High-temperature particulate matter fltration with resilient yttria-stabilized ZrO2 nanofber sponge, Small 14 (2018) e1800258. [38] L. Su, H. Wang, M. Niu, X. Fan, M. Ma, Z. Shi, et al., Ultralight, recoverable, and high-temperature-resistant SiC nanowire aerogel, ACS Nano 12 (2018) 3103 3111. [39] B. Ren, J. Liu, Y. Rong, L. Wang, Y. Lu, X. Xi, et al., Nanofbrous aerogel bulk assembled by cross-linked SiC/SiOx coreshell nanofbers with multifunctionality and temperature-invariant hyperelasticity, ACS Nano 13 (2019) 11603 11612. [40] Y.M. Xue, P.C. Dai, M. Zhou, X. Wang, A. Pakdel, C. Zhang, et al., Multifunctional superelastic foam-like boron nitride nanotubular cellular-network architectures, ACS Nano 11 (2017) 558 568. [41] Y. Si, J. Yu, X. Tang, J. Ge, B. Ding, Ultralight nanofbre-assembled cellular aerogels with superelasticity and multifunctionality, Nat. Commun. 5 (2014) 5802. [42] H.R. Ezatpour, M. Torabi-parizi, S.A. Sajjadi, Microstructure and mechanical properties of extruded Al/Al2O3 composites fabricated by stir-casting process, Trans. Nonferrous Met. Soc. China 23 (2013) 1262 1268. [43] G.H. Zahid, T. Azhar, M. Musaddiq, S.S. Rizvi, M. Ashraf, N. Hussain, et al., In situ processing and aging behaviour of an aluminium/Al2O3 composite, Mater. Des. 32 (2011) 1630 1635. [44] S.S. Razavi-Tousi, R. Yazdani-Rad, S.A. Manafi, Effect of volume fraction and particle size of alumina reinforcement on compaction and densification behavior of Al Al2O3 nanocomposites, Mater. Sci. Eng. A 528 (2011) 1105 1110. [45] X. Jiang, N. Wang, D. Zhu, Friction and wear properties of in-situ synthesized Al2O3 reinforced aluminum composites, Trans. Nonferrous Met. Soc. China 24 (2014) 2352 2358. [46] M. Rahimian, N. Parvin, N. Ehsani, The effect of production parameters on microstructure and wear resistance of powder metallurgy Al Al2O3 composite, Mater. Des. 32 (2011) 1031 1038.

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[47] M. Jagannatham, P. Chandran, S. Sankaran, P. Haridoss, N. Nayan, S.R. Bakshi, Tensile properties of carbon nanotubes reinforced aluminum matrix composites: a review, Carbon N. Y. 160 (2020) 14 44. [48] I. Sridhar, K.R. Narayanan, Processing and characterization of MWCNT reinforced aluminum matrix composites, J. Mater. Sci. 44 (2009) 1750 1756. [49] K. Munir, C. Wen, Y. Li, Graphene nanoplatelets-reinforced magnesium metal matrix nanocomposites with superior mechanical and corrosion performance for biomedical applications, J. Magnes. Alloy 8 (2020) 269 290. [50] N. Seyed Pourmand, H. Asgharzadeh, Aluminum matrix composites reinforced with graphene: a review on production, microstructure, and properties, Crit. Rev. Solid State Mater. Sci. (2019) 1 49 [CrossRef]. [51] W. Lian, Y. Mai, J. Wang, L. Zhang, C. Liu, X. Jie, Fabrication of graphene oxideTi3AlC2 synergistically reinforced copper matrix composites with enhanced tribological performance, Ceram. Int. 45 (2019) 18592 18598. [52] U. Aybarc, D. Dispinar, M.O. Seydibeyoglu, Aluminum metal matrix composites with SiC, Al2O3 Graphene-Rev. Arch. Foundry Eng. 18 (2018) 5 10. [53] L.A. Elshina, R.V. Muradymov, A.G. Kvashnichev, D.I. Vichuzhanin, N.G. Molchanova, A.A. Pankratov, Synthesis of new metal-matrix Al Al2O3 graphene composite materials, Russ. Met. 8 (2017) 631 641.

Carbon fiber reinforced ceramics: a flexible material for sophisticated applications

26

Payel Bandyopadhyay1, Desigan Ravi1, Ramya Ravichandran1 and Anoop K. Mukhopadhyay2,3 1 Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur, Tamil Nadu, India, 2Department of Physics, Sharda School of Basic Sciences and Research, Sharda University, Greater Noida, Uttar Pradesh, India, 3 Advanced Mechanical and Materials Characterization Division, CSIR—Central Glass and Ceramic Research Institute, Kolkata, West Bengal, India

26.1

Introduction

Carbon-carbon composite materials have a spectrum of applications in high technology sectors due to their excellent properties. Carbon/carbon (C/C) composites have many applications ranging from high-performance braking and thermal protection systems in aircraft, to biomedical applications such as heart valves, bones, tendons, growth scaffolds, tumor drugs, and biosensors [1
10]. Various researchers reported the excellent mechanical, thermal, and electrical properties [1
21] of C/C composites which makes the materials worthy of application in various fields. Some researchers reported a nanoindentation study of five types of carbon fibers namely, high strength polyacrylonitrile (PAN)-based (T800SC and T700SC), high modulus PAN-based (M60JB), high modulus pitch-based (K13D), and high ductility pitch-based (XN05) carbon fibers in longitudinal (0 degrees), 45 degrees, and transversal (90 degrees) direction of the fiber with respect to the fiber axis [3]. The indentation modulus is 70
80 GPa in the longitudinal direction and 10
30 GPa in the transverse direction for different fibers. Irrespective of the type of fibers a strong mechanical anisotropy has been observed by the researchers [3]. Another group of researchers has reported similar observations on four different types of carbon fibers namely, UTS50, IM2C, HR40 and the last one is MPP like K637 (meso-phase pitch) at an angle 0 degrees
90 degrees [4]. They have observed that the values of Young’s modulus is 45
80 GPa parallel to the fiber axis and 15
30 GPa perpendicular to the fiber axis depending on the type of the fiber [4]. The mechanical anisotropy of carbon fibers in SiC was studied using nanoindentation and theoretical modeling and strong mechanical anisotropy is observed [5]. The values of fracture toughness are improved by adding carbon nanofiber to SiC without much change in the hardness of the materials [6]. Combined study of Advanced Flexible Ceramics. DOI: https://doi.org/10.1016/B978-0-323-98824-7.00028-2 © 2023 Elsevier Ltd. All rights reserved.

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nanoindentation with a flat punch indenter and Raman spectroscopy have established that inelastic energy dissipation during indentation is a direct consequence of interfacial graphitization [7]. It is found that the size of the carbon fibers reinforced has an important contribution in determining the mechanical and electrical properties of the C/C composite [8]. The short fiber reinforced composites have elastic modulus 50% higher than that of the reference sample [8]. A group of researchers reported that incorporating short carbon fiber in the composite can improve the mechanical properties significantly [9]. They have noticed about 50% and 25% increase in modulus of elasticity and flexural strength respectively for the sample with 12 wt.% of the shortest carbon fibers [9]. Researchers reported the highest characteristic Young’s modulus of 15.7 GPa for the C/C
SiC composite in the loading direction parallel to the fiber axis compared to that of the C/C composite. The value of Young’s modulus in the direction perpendicular to the fiber axis is reported to be 10 GPa which is 55% higher compared to those reported for other directions [10]. So, for C/C-SiC composite also strong mechanical anisotropy is observed. Based on pertinent literature survey [9
22] for both carbon matrix and carbon fibers the magnitudes of Young’s modulus and hardness are consolidated in Tables 26.1 and 26.2, respectively. It is easily inferred from the data presented in Table 26.1 that depending on the preparation route and the type of precursor type used in the fabrication of the C/C Composite the nanohardness of graphitic matrix can vary from 0.05 to 3.32 GPa and the Young’s modulus can vary from 1.97 to 30.69 GPa. Similarly, it is evident from the data presented in Table 26.2 that depending on preparation route and the type of precursor type for example, PAN-based or Pitch-based carbon fiber used in Table 26.1 Literature data on nanomechanical properties of carbon matrix of C/C composites. Sample type

Charred resin Isotropic graphite CVI,graphitized Pyrolitic graphite ACVD carbon ICVD carbon Pyrolytic graphite Pyrolytic graphite

C-matrix nanohardness H (GPa)

Young’s modulus E (GPa)

References

1.22 0.05


10.70

[11] [16]

0.04 -

1.97 8.75

[16] [22]

3.12 3.32 -

29.92 30.69 7.50

[12] [12] [10]

0.43

5.46

[9]

CVI, chemical vapour infiltration; ACVD, atmospheric chemical vapour deposition; CVD, initiated chemical vapour deposition.

Carbon fiber reinforced ceramics: a flexible material for sophisticated applications

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Table 26.2 Literature data on nanomechanical properties of carbon fiber of C/C composites in parallel (!) and perpendicular (|) directions. Sample type

2D pitch based PAN based PAN based (!) PAN based (|) PAN based (//) Pitch based (!) Pitch based (|) PAN based (|) PAN based

C-fiber nanohardness H (GPa)

References Young’s modulus E (GPa)

2 0.7


2.23 1.56


11.99 24.90 13.36 30.31 15.30 15.80

[11] [16] [22] [22] [22] [12] [12]

1.43 2.70

39.00 20.7

[12] [9]

!Parallel to fiber direction. |Perpendicular to fiber direction. //Inclined fibers.

the fabrication of the C/C Composite the nanohardness of carbon fiber can vary typically from 0.7 to 2.7 GPa with some exceptions and the Young’s modulus can vary from 11.99 to 40 GPa with some exceptions. Recently a group of researchers reported detailed finite element analysis on carbon nanotube (CNT)-reinforced C/C composite [23]. They have suggested from simulation of experiments conducted with the spherical indentation that with the addition of 1.3% single walled carbon nano tube (SWCNT) in epoxy resin can improve the hardness by 17% of the nanocomposite for nonbonded interface. This value is reported to be about 4% lesser than that for a perfectly bonded interface [23]. This research group has also predicted the minimum wall thickness of CNTs to be around 0.2 nm for an outer diameter of 1 nm in order to achieve the maximum magnitudes for the composite properties. They have suggested that if the wall thickness is lesser than 0.05 nm, it can reduce the hardness and modulus of elasticity of CNT-reinforced composites as compared to that of pure matrix [20].

26.2

Fabrication and characterization of carbon fiberreinforced ceramics

In general, the carbon fibers are PAN-based or PITCH-based [1
24]. Typically, a carbon pre-peg lay-up is cured at a specified temperature. This step is followed by curing and carbonization followed by repeated pitch impregnation. Next, the pitch impregnated sample is again put through carbonization followed by intermediate and final heat treatments. Generally, carbonization is done at 1000 C
1500 C and the heat treatment is usually done from 1500 C to 2500 C [2].

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Nanoindentation techniques are utilized to obtain the Young’s modulus and hardness. Very well-known Oliver Pharr (O-P) method [24] is used to determine Young’s modulus and hardness from load-depth plots. The typical P-h data plot (Fig. 26.1) obtained from the nanoindentation [24] experiment gives the peak load (Pmax), maximum depth (hmax), final depth (hf), and contact stiffness (S). According to the (O-P) [21] model H is expressed as: H 5 Pmax =Acr

(26.1)

where, the real contact area, Acr is given by [24]: 1=4 1=128 Acr 5 24:56h2c 1 C1 hc 1 C2 h1=2 c 1 C3 hc 1 . . . 1 C8 hc

(26.2)

where, hc is the true penetration depth and constants C1
C8 are determined by standard calibration method. Now, hc is related to the unloading stiffness (S) as [24]: hc 5 hmax 2 ε

Pmax S

(26.3)

where, hmax is the corrected plastic depth and for a Berkovich indenter ε  0.74 [24] The contact stiffness is given by [24]:
 pffiffiffiffiffiffiffi dP 5 βCA Eeff Acr S5 dh h5hmax

Figure 26.1 Typical nanoindentation load-depth plot.

(26.4)

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where, β is a geometrical coefficient B 1.034 and CA 5 p2ffiffiπ for a Berkovich indenter [24]. From Eq. (26.4), the effective Young’s modulus (Eeff ) is given by [24]: Eeff 5

pffiffiffi π εPmax pffiffiffiffiffiffiffi 2β ðhmax 2 hc Þ Acr

(26.5)

But (Eeff ) is also given by [24]: 1 1 2 ϑ2 1 2 ϑ2i 1 5 Eeff E Ei

(26.6)

where, ϑ and ϑi are the Poisson’s ratios, and E and Ei are Young’s moduli of the test material and the diamond indenter in correspondence. Furthermore, the mean contact pressure, pm, as a representative parameter for the indentation nanohardness is given by [24]: pm 5

Pmax Aca

(26.7)

where, Aca is the apparent projected area of contact and can be expressed as [24]: pffiffiffi Aca 5 3 3ðtanαÞ2 ðhmax Þ2

(26.8)

where, α is approximately 65.30 degrees for a Berkovich tip. Thus, from Eq. (26.8) above: Aca 5 24:56ðhmax Þ2

(26.9)

So, pm is expressed as: pm 5

Pmax Pmax 5 Aca 24:56ðhmax Þ2

(26.10)

Similarly, the relative stiffness (Smax/hmax) of the material is given by: Smax 5 5:52Eeff hmax

rffiffiffiffiffiffiffi Acr Aca

(26.11)

where, Acr/Aca 5 1 (for fully plastic material e.g., metals), ,1 (for elastoplastic materials e.g., ceramics). The contact pressure physically conveys the significance of stress endured at contact just prior to permanent plastic deformation. The relative stiffness physically represents

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the lack of compliance or the presence of it in a given sample. Similarly, the relative spring-back of the material is given by the quantity [(hmax-hf)/hmax]. Basically, this parameter provides the extent of elastic recovery undergone by the material during unloading.

26.3

Microstructure and properties of carbon fiber reinforced ceramics

26.3.1 Microstructural studies The scanning electron microscope (SEM) image of the polished surface of the C/C composite reveals different regions of the composite. The SEM photomicrograph given in Fig. 26.2A shows the region with fibers oriented parallel to the surface and perpendicular to the loading axis. Thus the nomenclature parallel fiber refers to the fibers oriented parallel to the surface. Fig. 26.2B shows the fibers oriented perpendicular to the surface. The matrix region (Fig. 26.2C) is continuous with few void places in between. Fig. 26.2D shows the fracture surface of the C/C composite. The diameter of the carbon fibers is around 8
10 μm, Fig. 26.2D. The structure of the graphite matrix around the fiber is layered, like an onion cell.

Figure 26.2 The SEM microstructure of C/C composite. (A) The fiber oriented parallel to the loading direction, (B) the fiber oriented perpendicular to the loading direction, (C) the matrix region, and (D) the fracture surface of C/C composite showing the fibers.

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From the SEM micrographs, four regions are clearly visible on the surface. These are the perpendicular, parallel and inclined fiber regions, and the matrix region.

26.3.2 Nanomechanical studies on plan section of C/C composites The load depth plots of the different regions are shown in Fig. 26.3. From the load depth plots, Fig. 26.3A
C it is evident that the carbon fibers show extraordinary recovery characteristics. The parallel fiber region has an average nanohardness of about 0.75 6 0.09 GPa and average Young’s modulus of about 12.44 6 1.56 GPa. Further, the perpendicular fiber region has an average nanohardness of about 1.05 6 0.10 GPa and an average Young’s modulus of about 18.63 6 1.61 GPa. Furthermore, the inclined fiber region has an average nanohardness of approximately 0.88 6 0.06 GPa and average Young’s modulus of about 15.6 6 0.88 GPa. The nanomechanical properties of the perpendicular fiber region are the highest and those of the parallel fiber region are the smallest while those of the inclined fiber region is of intermediate magnitude (Table 26.3). However, the matrix region

Figure 26.3 The load depth plots obtained at all four regions in a C/C composite: (A) perpendicular fiber region, (B) parallel fiber region, (C) inclined fiber region, and (D) matrix region.

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(Fig. 26.2D) is less elastic compared to the fiber region. The porous region of the matrix has an average nanohardness of about 0.45 6 0.10 GPa and an average Young’s modulus of about 8.15 6 1.88 GPa. On the other hand, the relatively denser region of the matrix has a higher average nanohardness of about 0.69 6 0.07 GPa and a relatively higher average Young’s modulus of about 12.34 6 1.37 GPa. The data of Young’s modulus, hardness, contact pressure, relative stiffness (RS), and relative spring back (RSB) are given in Table 26.4 for a comparative view of the properties of various regions of the fiber and the matrix. The results presented in Table 26.3 show that the perpendicular fiber region is the stiffest and the parallel fiber region is the least stiff. Accordingly, the relative ability to spring back is the highest in the parallel fiber region and the lowest in the perpendicular fiber region. Similarly, the dense part of the graphitic matrix is the stiffest. On the other hand, the porous part of the graphitic matrix is the least stiff. The amount of energy spent in elastic deformation process during nanoindentation is the least in the perpendicular fiber region and the highest in the parallel fiber region. These results also identify two vital aspects. The first one is the fact that local anisotropy due to fiber orientation gives rise to directional dependence of nanomechanical properties in the plan section itself. The second one is the fact that residual porosity can also exert significant influence on the nanomechanical properties of the C/C Composites. The distribution of data is given in Fig. 26.4A and B for Young’s modulus and hardness, in correspondence. It is evident that there is a large scatter in the data. But, the scatter is characteristic that is, intrinsic in nature. Hence, its treatment deserves special attention. Thus in the present work, the scatter in the nanoindentation data is treated in terms of the Weibull statistics analysis as the microstructures of all the components of a single cell are highly heterogeneous and, in some cases, highly porous. For this purpose, the two-parameter Weibull distribution function is utilized. This Weibull approach is described below. This approach of treatment of the data is done over and above the conventional evaluation of mean and standard deviation of the experimental data as is usually done assuming that the evaluated data and the associated systematic intrinsic error are all distributed following a Gaussian distribution. The characteristics values of all the parameters are calculated using a twoparameter Weibull approach.

26.3.3 Statistical analysis of plan section nanomechanical properties of C/C composites by Weibull model The two-parameter Weibull model provides the survival probability, p, of occurrence for a given parameter, x. It is expressed as,  p512e

m

2xx

0

(26.12)

Table 26.3 Nanomechanical properties of C/C composite. Region

E (GPa)

H (GPa)

We (nJ)

Wp (nJ)

Relative spring back (RSB)

Relative stiffness, RS (GPa)

18.63 6 1.61 15.6 6 0.88 12.44 6 1.56

1.05 6 0.10 0.88 6 0.06 0.75 6 0.09

1.70 6 0.27 2.03 6 0.2 2.34 6 0.3

0.15 6 0.07 0.06 6 0.14 0.14 6 0.10

0.84 6 0.22 0.96 6 0.27 0.9 6 0.2

53.38 6 10.41 44.72 6 7.14 38.33 6 4.76

12.34 6 1.37 8.15 6 1.88

0.69 6 0.07 0.45 6 0.10

2.07 6 0.27 2.59 6 0.56

0.23 6 0.11 0.36 6 0.2

0.85 6 0.16 0.8 6 0.1

45.46 6 7.79 26.9 6 8.0

Fiber Perpendicular Inclined fiber Parallel fiber Matrix Dense Porous

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Figure 26.4 The distribution of (A) Young’s modulus and (B) hardness.

In Eq. (26.12), xo is the scale parameter and “m” is the Weibull modulus. The parameter “m” in Eq. (26.12) is a dimensionless quantity and indicates the degree of scatter in the data. The magnitude of “m” increases with a decrease in the degree of scatter in the relevant experimental data. When the data are arranged in ascending order, the survival probability (p) of the ith observation is given by [6,8
10]: p5

ði 2 0:5Þ N

(26.13)

In Eq. (26.13), N stands for the total number of observations. For both sides of Eq. (26.12) taking natural logarithm for two consecutive times gives:  ln ln

1 ð 1 2 pÞ



5 m½lnln

ðxÞ

2 lnðx0 Þ

(26.14)

It follows from Eq. (26.14) that the values of m and xo can be obtained by fitting the experimental data by the conventional linear least square regression technique. For instance, the slope of the fitted straight line will give the value of m and the Y-intercept ofhthen fittedostraight line will give the value of xo. Finally, i equating the quantity ln ln

1 ð1 2 pÞ

to zero and placing the values of m and xo

in Eq. (26.14), the characteristic value of the related parameter, x can be calculated. In the present study, x will be nanohardness (H), Young’s modulus (E), contact pressure (pm), relative stiffness (Smax/hmax) and relative spring
back [(hmax-hf)/hmax]; in correspondence. The corresponding plots are given in Fig. 26.5. The relevant data on the various characteristics values are given in Table 26.4. For comparison, the average values of the corresponding parameters are also included in Table 26.4.

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Figure 26.5 The Weibull plots for calculating characteristic values of (A) nanohardness, (B) Young’s modulus, (C) relative spring back, and (D) relative stiffness.

Table 26.4 The characteristic (ch) and average (av) values of hardness, Young’s modulus, relative spring back, and relative stiffness of the parallel fiber regions of the C/C composite. Hch (GPa)

Hav (GPa)

Ech (GPa)

Eav (GPa)

RSBch

RSBav

RSch (GPa)

Rsav (GPa)

0.88

0.77

14.81

13.3

0.99

0.9

46.22

41.64

26.3.4 The nanomechanical studies on cross-section of C/C composites The optical photomicrographs of a cross-section of C/C composite (Fig. 26.6) shows the presence of three distinct regions, namely, parallel and perpendicular fibers, and matrix. The corresponding nanoindentation-derived typical load depth plot of the parallel fiber region is shown in Fig. 26.7A. The average nanohardness is about 0.91 6 0.20 GPa, and the average Young’s modulus is about 22.47 6 0.25 GPa. Similarly, the nanoindentation derived typical load depth plot of the

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Figure 26.6 The optical photomicrograph of the cross-section of C/C composite.

Figure 26.7 The load depth plot of (A) perpendicular fiber, (B) parallel fiber, and (C) matrix region of the cross-section of the C/C composite.

perpendicular fiber region is shown in Fig. 26.7B. The perpendicular fiber region shows an average nanohardness of about 2.04 6 0.37 GPa and an average Young’s modulus of about 59 6 12 GPa. Thus the nanomechanical properties of the perpendicular fiber region are much higher than those of the parallel fiber region in the polished cross-section of the C/C composite. The matrix region

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563

shows an average nanohardness of about 0.49 6 0.11 GPa and an average Young’s modulus of about 12.69 6 1.99 GPa, Fig. 26.7C. Thus, the nanomechanical properties are smaller than those of the parallel fiber region in crosssection of C/C composite (Table 26.5). A comparison of the average nanohardness (H), Young’s modulus (E), RSB, and RS from the perpendicular and parallel fiber regions and matrix region of the crosssectional surface of C/C composite is shown in Table 26.6. The data presented in Table 26.6 confirm that the average nanohardness (H), Young’s modulus (E), RSB, and RS of the perpendicular fiber region are the highest while the average nanohardness (H), Young’s modulus (E), RSB, and RS of the parallel fiber region are the second best. Further, the average nanohardness (H), Young’s modulus (E), and RS of the matrix region are the smallest. From the data presented in Table 26.5, it is very clear that the mechanical anisotropy is prominent for the C/C composite when probing at two different sections of the same composite.

26.3.5 The nanomechanical studies on carbon fiber The nanoindentation technique is also used in the present work to measure the Young’s modulus and Hardness of different types of individual carbon fiber, for example, Sample 1, Sample 2, and Sample 3. The nomenclature is changed due to strategic reasons and as advised by the Sponsor. For this purpose, the carbon fibers are cold embedded in epoxy and carefully polished after that. The results of nanomechanical properties evaluation directly on the cross sections of the three different types of single carbon fibers are given in Table 26.6. Table 26.5 Nanomechanical properties of the cross-section of C/C composite. Region

H (GPa)

E (GPa)

RSB

RS (GPa)

0.91 6 0.2 2.04 6 0.37

22.47 6 4.25 59 6 12

0.84 6 0.05 0.84 6 0.03

82.77 6 14.39 224.26 6 49.33

0.49 6 0.11

12.69 6 1.99

0.85 6 0.03

49.80 6 7.19

Fiber Parallel Perpendicular Matrix Matrix

Table 26.6 The nanomechanical properties on cross sections for given carbon fiber samples. Sample ID

Young’s modulus (GPa)

Hardness(GPa)

1 2 3

65.85 6 11.58 36.5 6 10.9 52.00 6 4.00

2.4 6 0.4 1.32 6 0.50 1.90 6 0.17

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26.3.6 The tensile strength and failure studies on carbon fiber The tensile test is carried out in a conventional universal testing machine (UTM) to measure the mechanical properties of the individual carbon fibers. The average failure strength and failure strain of Sample 1 is about 4200 6 45 MPa and about 1.3% with a range of 1.04%
1.64%. The average tensile Young’s modulus of Sample 1 carbon fibers is about 323 6 37 GPa. The average failure strength and failure strain of Sample 2 are about 3224 6 41 MPa and about 1.3% with a range of 0.95%
1.57%. The average tensile Young’s modulus of Sample 2 carbon fibers is about 248 6 49 GPa. The average failure strength and failure strain of Sample 3 are about 6270 6 73 MPa and about 1.4% with a range of 1.33%
1.65%. The average tensile Young’s modulus of Sample 1 carbon fibers is about 418 6 43 GPa. These data are comparable to literature data [25]. Also, these data are slightly higher than those evaluated for the C/C composites evaluated by the present author. So, in the tensile test, the values of Young’s modulus for all type of fibers are very high, in the range of a few hundred GPa but when they are in composite their Young’s modulus values drop to a few tens of GPa (Tables 26.4 and 26.6). There may be a few reasons for such discrepancy. It is yet unclear as to why the Young’s modulus measured by the nanoindentation technique tends to hit the lower bounds not only in present work but also in global literature [1
25]. It is argued that it does not represent the Voigt upper bound. This bound is also called the isostrain average as it gives the ratio of average stress to average strain under the assumption that all crystallites experience the same strain. It is argued further that for an isostrain condition to be preserved, the loading must be in a direction that is parallel to both the soft and the hard constructional components in the composite. On the other hand, the Reuss average is called the lower bound. This bound is also called the isostress average as it gives the ratio of average stress to average strain under the assumption that all crystallites experience the same stress. It is thus well known that for an isostress condition to be preserved, the loading must be in a direction that is perpendicular to both the soft and the hard constructional components in the composite. It is argued [2
25] therefore that as most of the basal planes of the graphite crystallites are aligned along the fiber direction, the stressing condition around the nanoindenter inclines to provide an isostress condition along the transverse direction, particularly during nanoindentation along the longitudinal direction of the fibers. Consequently, the Young’s modulus by the nanoindentation technique should be closer to the lower bound. Such a picture explains why the nanoindentation derived Young’s modulus data are a bit on the lower side compared to what is measured in classical tensile tests. There can be a few other possibilities for such discrepancies. These are as follows. When the carbon fiber composites are processed there may be some degradation of their individual properties as individual carbon fibers. Further, some surface defects or slightly subsurface defects might generate in the C/C composite samples during processing. The inevitable interaction of such defects with the penetrating nanoindenter may be reflected in the slightly lower values of Young’s modulus and nanohardness in comparison to those of the individual carbon fibers. However, all these measurements are done in local compression as during the nanoindentation experiments the zone under the nanoindenter is under localized

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565

hydrostatic compression. On the other hand in conventional uniaxial testing as is done in a UTM the entire length of the sample is in a state of tension. It also needs to be recognized that per se these defects for example, small cracks, pores, and weak interfacial regions between the carbon fiber and the graphitic matrix, are the major attributes of the damage and strain-tolerant microstructure that makes the C/ C composites a unique choice for damage tolerant applications.

26.4

Conclusion

The C/C composite materials are highly elastic and mechanically anisotropic. The elastic recovery is so high that there is no observable plastic impression of the nanoindentation. The mechanical anisotropy is there on the same plane within different regions as well as in different planes of loading. Depending on the relative degree of fiber orientation to the loading direction the Young’s modulus and hardness can reduce even by 75% or more. In general, the effect of anisotropy is more in Young’s modulus than in nanohardness.

Acknowledgments All the authors gratefully acknowledge the fund received from ISRO Respond Project scheme. PB especially acknowledges the financial support (Junior Research Fellowship) received from ISRO Respond Project. The support received from SRM IST, KTR is deeply acknowledged by Payal Bandyopadhyay, Desigan Ravi, and Ramya Ravichandran. Anoop K. Mukhopadhyay deeply appreciates the support of the authorities of the Sharda University, Greater Noida, Uttar Pradesh, India. All the authors acknowledge the contribution of Dr. G. Krishnakumar, Dr. J. Paul, Dr. B. M. Pillai, and Dr. S. Mahendran from VSSC, Thiruvananthapuram, Kerala, India. Also, all authors acknowledge the kind support of Director, CSIR—Central Glass and Ceramic Research Institute, Kolkata 70032, India in execution of the ISRO Respond Project.

References [1] Z. Tan, X. Zhang, J. Ruan, J. Liao, F. Yu, L. Xia, et al., Synthesis, structure, and properties of carbon/carbon composites artificial rib for chest wall reconstruction, Sci. Rep. 11 (2021) 11285. [2] L.M.Manocha, High performance carbon
carbon composites, Sadhana 28 (2003) 349. [3] K. Shirasua, K. Goto, K. Naito, Microstructure-elastic property relationships in carbon fibres: a nanoindentation study, Compos. Part. B: Eng. 200 (2020) 108342. [4] T.S. Guruprasad, V. Keryvina, L. Charleux, J.-P. Guinc, O. Arnould, On the determination of the elastic constants of carbon fibres by nanoindentation tests, Carbon 173 (2020) 572. [5] T. Csana´di, D. Ne´meth, C. Zhang, J. Dusz, Nanoindentation derived elastic constants of carbon fibres and their nanostructural based predictions, Carbon 119 (2017) 314.

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[6] S. Mubina, P.S. Phani, A.K. Khanra, B.P. Saha, A nanoindentation based study to evaluate the effect of carbon nanofibres on the mechanical properties of SiC composites, Compos. Interfaces 4 (2020) 363. [7] A.S.K. Mohammed, H. Sehitoglu, R. Rateick, Interface graphitization of carbon-carbon composites by nanoindentation, Carbon 150 (2019) 425. [8] M. Zambrzycki, J. Tomala, A. Fraczek-Szczypta, Electrical and mechanical properties of granular-fibrous carbon-carbon composites with recycled carbon fibres, Ceram. Int. 44 (2018) 19282. [9] S. Sarkar, A. Dey, P.K. Das, A.K. Mukhopadhyay, Evaluation of micromechanical properties of carbon/carbon and carbon/carbon
silicon carbide composites at ultralow load, Int. J. Appl. Ceram. Technol. 8 (2011) 282. [10] N. Iwashita, J.S. Field, M.V. Swain, Indentation hysteresis of glassy carbon materials, Philos. Mag. A 82 (2002) 1873. [11] S. Ozcan, J. Tezcan, P. Filip, Microstructure and elastic properties of individual components of C/C composites, Carbon 47 (2009) 3403. [12] D.T. Marx, L. Riester, Mechanical properties of carbon—carbon composite components determined using nanoindentation, Carbon 37 (1999) 1679. [13] Z. Ling, J. Hou, A nanoindentation analysis of the effects of microstructure on elastic properties of Al2O3/SiC composites, Compos. Sci. Technol. 67 (2007) 3121. [14] S. GuiC/Ciardi, D. Sciti, C. Melandri, A. Bellosi, Nanoindentation characterization of submicro- and nano-sized liquid-phase-sintered SiC ceramics, J. Am. Ceram. Soc. 87 (2004) 2101. [15] S. GuiC/Ciardi, A. Balbo, D. Sciti, C. Melandri, G. Pezzotti, J. Eur. Ceram. Soc. 27 (2007) 1399. [16] M. Kanari, K. Tanaka, S. Baba, M. Eto, Nanoindentation behavior of a twodimensional carbon-carbon composite for nuclear applications, Carbon 35 (1997) 1429. [17] S. Guo, Y. Kagawa, Effect of thermal exposure on hardness and Young’s modulus of EBPVD yttria-partially-stabilized zirconia thermal barrier coatings, Ceram. Int. 32 (2006) 263. [18] A. Waloddi Weibull, Statistical distribution function of wide applicability, J. Appl. Mech. Trans. 18 (1951) 293. [19] A. Bolshakov, G.M. Pharr, Influences of pileup on the measurement of mechanical properties by load and depth sensing indentation techniques, J. Mater. Res. 13 (1998) 1049. [20] W.J.J. Walker, J.S. Reed, S.K. Verma, Influence of granule character on strength and weibull modulus of sintered alumina, J. Am. Ceram. Soc. 82 (1999) 50. [21] Lippo V.J. Lassila, A. Tezvergil, Scott R. Dyer, Pekka K. Vallittu, The bond strength of particulate-filler composite to differently oriented fibre-reinforced composite substrate, J. Prosthodont. 16 (2007) 10. [22] P. Diss, J. Lamon, L. Carpentier, J.L. Loubet, P. Kapsa, Carbon 40 (2002) 2567. [23] K.S. Ahmed, I. Ibrahim, A. Kok Keng, Advanced nanoindentation simulations for carbon nanotube reinforced nanocomposites, Heliyon 6 (2020) 1. [24] W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564. [25] A. Leatherbarrow, H. Wu, Mechanical behaviour of the constituents inside carbonfibre/carbon-silicon carbide composites characterised by nano-indentation, J. Eur. Ceram. Soc. 32 (2012) 579.

Index

Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively. A a-Si:H solar cells, 332 334 AAO. See Anodic aluminum oxide (AAO) Abbe number, 141 Abrasive Jet Machining (AJM), 243 Abrasive Water Jet Machining (AWJM), 243, 251 Abrasive water stream machining, 244 Abrasives slurry cutting, 245 246 Absorber radiation mode, 442 Absorption, 132 Absorption coefficient, 264 Absorptivity, 132 Activated carbon (AC), 299 300 Activated carbon yarns (ACYs), 315 316 Additive manufacturing (AM), 8 9, 14, 193 Adsorption of dyes, 416 417 AFM. See Atomic force microscopy (AFM) Agricultural construction, 442 446 Air filtration, 414 416 Air plasma spraying (APS), 453 Air pollutants, 411 Air pollution, 325 Aircraft, 459 Alkali solutions, 542 Alternating current electroluminescent devices (ACELs), 146 147 Alumina (Al2O3), 529 530, 542, 546, 546t Alumina titania ceramic powders, 453 454 Aluminum dioxide coatings, 217 Aluminum nitride (AlN), 532 Aluminum oxide (Al2O3), 48 49, 57t, 455 456, 529 530 Aluminum oxide powders, 455 456 Aluminum phosphate (AlPO4), 485 486 Aluminum titanate ceramics (Al2TiO5), 47 48, 57t AM. See Additive manufacturing (AM)

Ammonium chloride (NH4Cl), 505 Amorphous silicon, 261 Anodic aluminum oxide (AAO), 184 185 Anodic electro deposition (AED) method, 301 303 Anodic spark deposition method, 224 Anticorrosion ceramic coatings, 469 475 antiwear ceramic coatings, 475 487 laser cladding, 479 481 microarc oxidation, 475 479 thermal spraying, 481 485 hot corrosion, 472 474 diffusion coatings, 472 473 overlay coatings, 473 thermal barrier coatings, 473 474 nanocrystalline ceramic coatings, 474 475 sol gel method, 485 487 solution corrosion, 469 472 Antiferroelectrics (AFEs), 95 96 Antimony (Sb), 494 Antiwear ceramic coatings, 475 487 laser cladding, 479 481 microarc oxidation, 475 479 thermal spraying, 481 485 Aqueous colloidal suspensions, 451 452 Arsenic (As), 494 Artificial intelligence (AI), 156 157 ASC. See Asymmetric super capacitor (ASC) Asymmetric super capacitor (ASC), 301 Atomic force microscopy (AFM), 30 31, 30f, 31f Atomic layer deposition (ALD), 174 178, 222 229, 326 327 chemical vapor deposition method, 228 dip-coating method, 228 229 electrochemical method, 223 224

568

Atomic layer deposition (ALD) (Continued) growth process of, 176f laser-cladding method, 227 228 magnetron sputtering, 225 226 plasma treatment, 224 225 single monolayer by, 176 177 solution immersion process, 226 227 window, 177 ATR. See Attenuated total reflection (ATR) Attenuated total reflection (ATR), 33 AZ31B magnesium material, 226 227 B Ball milling technique, 286, 454 Ball-on-Diskdisc test, 483f Barium titanate, 532 534 Bayer’s process, 529 530 Bedding-in period, 460 Beer Lambert law, 206 Benzenehexathiol (BHT), 347 348 Benzocyclobutene, 450 451 Berkovich tip, 555 Beta-tricalcium phosphate (β-TCP), 21 Biaxial flexural strength tests, 398 Binder jetting, 200 Bioceramics, 370 371 applications, 375t emerging technologies for bioceramics in, 376 379 bone cancer treatment from bioceramic scaffolds, 378 electroceramics, 376 green state machining, 377 sol gel technique, 378 379 three-dimensional printing, 378 flexible bioceramics in tackling COVID19 and expectations in post-COVID19 era, 380 382 prospects of flexible bioceramics in postCOVID era, 379 380 Biofouling, 459 Biogenic silica, 420 Biological sensors, 266 268 Bismuth (Bi), 494 Bismuth ferrate (BiFeO3), 87 BN. See Boron nitride (BN) BNNPs. See Boron nitride nanoplatelets (BNNPs) Boltzmann’s constant, 93

Index

Bone cancer treatment from bioceramic scaffolds, 378 Bone regeneration, 371 Boron carbide (B4C), 543 544 Boron nitride (BN), 545 Boron nitride nanoplatelets (BNNPs), 21 Boron nitride nanotube (BNNT), 456 Bottom-up approach, 8 9, 171 172, 194 195 Bragg’s law, 32f Bromine (Br), 494 Building application methods, 433 435, 446 integration patterns, 433 435 module patterns, 433 Bulk solid materials, 203 205 energy consumption, 204 205 feedstock requirements, 203 204 process description, 203 Butyldiammonium (BDA), 501 504 C CA. See Contact angle (CA) Calcium nitrate, 223 Calcium silicate (Ca-Si), 378 Carbon, 347 Carbon cloth (CC) fibers, 303 305 Carbon dioxides (CO2), 493 Carbon fiber (CF), 305 307 Carbon fiber reinforced ceramics fabrication and characterization, 553 556 microstructure and properties, 556 565 microstructural studies, 556 557 nanomechanical studies on carbon fiber, 563 nanomechanical studies on crosssection of, 561 563 nanomechanical studies on plan section, 557 558 statistical analysis of plan section nanomechanical properties, 558 560 tensile strength and failure studies on, 564 565 Carbon microcone (CMC), 112 Carbon nanofiber (CNF), 281 282, 312 Carbon nanotubes (CNTs), 177 178, 262, 277 278, 299 300, 347, 484f, 546 547 bridging morphology of, 485f Carbon-related electrode materials, 305 307

Index

Carbon/carbon (C/C) composite materials, 551 Carbonyl iron, 451 Cathodoluminescence (CL), 135 137 Cauchy relation, 134 135 CdTe solar cells, 334 336 Cellulose nanofibers (CNFs), 308 312 Ceramic coating advantages and limitations of, 460 463 aircraft, 459 in defense and security, 449 452 for defense and security industry, 453 457 alumina titania ceramic powders, 453 454 aluminum oxide powders, 455 456 chromium oxide powders, 456 457 helicopter rotors, 460 helicopters, 459 460 market report on ceramic coating used in defense and security, 452 453 submarines, 458 surface ships, 459 Ceramic Coating Market, 452 Ceramic coating material selection, 529 economy, 529 functional requirements, 529 material science, 529 quality attributes of the product, 529 Ceramic coating materials, 529 534 aluminum nitride, 532 aluminum oxide, 529 530 barium titanate, 532 534 rare-earth cerates, 531 silicon carbide, 531 silicon nitride, 531 titanium nitride, 532 yttrium aluminum garnet, 531 zirconocerate, 531 Ceramic coatings, 215, 216f atomic layer deposition method, 222 229 chemical vapor deposition method, 228 dip-coating method, 228 229 electrochemical method, 223 224 laser-cladding method, 227 228 magnetron sputtering, 225 226 plasma treatment, 224 225 solution immersion process, 226 227 for fabrication, 216 218

569

nonoxide ceramic coatings, 217 218 oxide ceramic coatings, 216 217 liquid phase deposition method, 221 on metallic materials, 218 221 microarc oxidation, 220 221 sol gel method, 218 220 Ceramic machining comparative studies, 252 hybrid machining, 251 252 nontraditional machining, 246 251 traditional machining, 244 246 Ceramic materials, 446 Ceramic oxides, 17, 354, 545 Ceramic raw materials, 201 Ceramic Solar Collector, 426 Ceramic-based coatings application cases, 436 446 agricultural construction, 442 446 conceptual architecture, 436 public building, 440 441 rural residence, 436 urban high-rise residence, 436 439 background, 425 building application methods, 433 435 integration patterns, 433 435 module patterns, 433 heat-transfer mechanism, 432 state-of-art, 425 431 normal ceramic collectors, 426 427 vanadium titanium black ceramic collectors, 427 431 Ceramic-based medical devices, 375 376 Ceramic-nanocomposite technique, 18 Ceramic/polyester fabric (CPF), 419 Ceramics, 3, 425, 527, 541 544 application of, 210 211, 543 544 flexible ceramics, 544 545 future aspects in, 535 536 graphene-incorporated alumina flexible nanocomposites, 545 547 history of, 3 production, 243 properties of, 542 543 Cesium (Cs), 494 CFs. See Conductive filaments (CFs) Characterization techniques, 25 38 electron microscopy, 25 28 field-emission scanning electron microscopy, 26 27, 26f

570

Characterization techniques (Continued) transmission electron microscopy, 27 28, 28f scanning probe microscopy, 28 31 atomic force microscopy, 30 31, 30f differential thermal analysis, 38, 38f electron diffraction, 34, 35f energy dispersive X-ray analysis, 34 35, 36f Fourier transform infrared spectroscopy, 32 34, 33f Raman spectroscopy, 32 34 scanning tunneling microscopy, 28 30, 29f thermogravimetric analysis, 38, 38f X-ray diffraction, 31 32, 32f X-ray photoelectron spectroscopy, 35 38, 37f Chemical mechanical polishing lift-off process, 337 Chemical properties, 328 Chemical sensors, 266 268 Chemical vapor deposition (CVD), 51 52, 117 118, 172 173, 228, 285 286, 332, 461 atomic layer deposition, 175 178 basic understanding of, 173 174 to build flexible ceramics, 178 188 current status, 178 180 film-like structures on flexible substrates, 180 182 one-dimensional nanostructures, 182 188 reaction mechanism of, 174 175 Chlorine (Cl), 494 Chlorobenzene (CB), 509 510 Chromium oxide (CrO2), 530 Chromium oxide powders, 456 457 CL. See Cathodoluminescence (CL) Close-spaced sublimation (CSS) methods, 334 Closed-loop feedback system, 209f CMMs. See Co-ordinate measuring machines (CMMs) CNTs. See Carbon nanotubes (CNTs) Co-ordinate measuring machines (CMMs), 207 Coating methods, 535 Cobalt (Co), 305, 494

Index

Cobalt oxide, 355 Cobalt chromium substrate, 222 Coefficient of thermal expansion (CTE), 48 49, 450 Cofired combustion methods, 4 5 Collector structure, 446 Color rendering index (CRI), 161 162 Conceptual architecture, 436 Conducting polymers, 280, 299 300, 451 Conductive filaments (CFs), 113 Contact angle (CA), 162 163 Conventional ceramics, 63 66 Coolant, 245 246 Copper (Cu), 494 Copper oxide (CuO), 268 Copper zinc tin sulfide (CZTS), 330 Core shell composition, 180 Coronavirus disease 2019 (COVID-19), 379 380 Correlated color temperature (CCT), 161 162 CPVC. See Critical powder volume concentration (CPVC) Creep damage, 64 deformation, 64 CRI. See Color rendering index (CRI) Critical powder volume concentration (CPVC), 196 Crystalline, 456 457 CTE. See Coefficient of thermal expansion (CTE) Cu(In,Ga)(S,Se)2 solar cells, 336 338 Curie temperature, 84 Current-voltage characteristics, 92 95 Schottky and Poole Frenkel conduction mechanisms, 93 95 space-charge-limited conduction mechanism, 93 Cutting-edge machining strategy, 246 CVD. See Chemical vapor deposition (CVD) Cyclic dip-coating technique, 229 Cyclic voltammograms, 101f D Decaphenylcyclopentasilane (DPPS), 509 510 Detail Port (DP), 459 460 Diaphaneity or pellucidity, 139

Index

Dielectric capacitors, 7 constant, 77 78, 78f, 103 106 layer, 96 98 properties, 77 80 Differential thermal analysis (DTA), 38, 38f Diffusion barrier coatings, 528 Digital light processing (DLP), 193 194 Dimethyltrimethylsilylphosphite, 471 Dion Jacobson (DJ) structure, 501 Dip-coating method, 228 229 graphical representation of, 229f Direct deposition, 182 183 Direct-ink writing, 197t Dispersion formula, 134 135 Distributed microfracture damage, 66 DLP. See Digital light processing (DLP) Double perovskites, 497 504 Drilling, 243 Droplet-based writing system, 196 Dry process, 177 178 DTA. See Differential thermal analysis (DTA) Dye-sensitized solar cell (DSSC), 330 331 Dynamic method, 95 96 E Ecoflex (PBAT), 331 EDM. See Electric Discharge Machining (EDM) EDS. See Energy dispersive X-ray spectroscopy (EDS) EES. See Electrical energy storage (EES) Einstein’s photoelectric effect, 36 EL. See Electroluminescence (EL) Electric Discharge Machining (EDM), 243, 249 250, 252 Electric double-layer capacitors (EDLCs), 277 278, 280, 353 Electric double-layer variants, 277 278 Electric vehicles (EVs), 164 Electrical conductivity, 76 Electrical double-layer capacitors versus pseudocapacitors, 279 281 Electrical energy storage (EES), 299 300 Electrical properties of flexible ceramics, 76 95 applications of flexible ceramic films, 95 118

571

energy harvesting, 103 112 energy storage devices, 95 103 memory, 113 118 current voltage characteristics, 92 95 Schottky and Poole Frenkel conduction mechanisms, 93 95 space-charge-limited conduction mechanism, 93 dielectric properties, 77 80 electrochemical properties, 88 92 ferroelectric properties, 84 88 piezoelectric properties, 81 82 pyroelectric properties, 82 84 Electroceramics, 376 Electrochemical impedance spectroscopy (EIS), 90 92 Electrochemical method, 223 224 Electrochemical properties, 88 92 Electrode flexibility, 293 Electrode materials, 353 358 cobalt oxide, 355 iron oxides, 355 356 manganese oxide, 354 355 ruthenium oxide, 354 tin oxide, 356 357 titanium nitride, 358 vanadium nitride, 357 358 vanadium oxides, 356 Electroluminescence (EL), 135, 137 138, 146 147 Electrolyte insulator semiconductor (EIS), 266 Electrolytes, 282 Electromagnetic interference (EMI), 449 Electron beam physical vapor deposition (EB-PVD) process, 473 474 Electron diffraction, 34, 35f, 494 496 Electron energy transfer, 132 Electron microscopy, 25 28 field-emission scanning electron microscopy, 26 27, 26f transmission electron microscopy, 27 28, 28f Electron transport layers (HTLs), 339 341 Electronic polarization, 132, 134 Electronic sensors, 266 Electrospinning method, 88, 139, 282 283 Electrospun nanofibers (ENFs), 312 Electrospun PAN nanofibers, 418 419

572

Energy consumption, 198 199, 199t, 202 205 Energy dispersive X-ray (EDX) analysis, 34 35, 36f Energy dispersive X-ray spectroscopy (EDS), 268 Energy harvesting, 103 112 miniaturization, 315 316 piezoelectric nanogenerators, 103 108 pyroelectric nanogenerators, 108 110 sensors, 111 112 Energy storage devices, 95 103 dynamic method, 95 96 static method, 96 103 Environmental Protection Agency, 412 Environmental remediation flexible ceramics for, 411 421 adsorption of dyes, 416 417 air filtration, 414 416 heavy metals, 412 414 photodegradation of dyes, 419 421 removal of pathogens, 417 419 Environmentally Responsible Aviation (ERA) Project, 459 Epitaxy, 174 Epoxy-based composites, 546 Escherichia coli, 417 418 Europium (Eu), 494 Excitons, 339 341 Exothermal redox reactions, 17 Expanded polystyrene (EPS), 436 Experimental Beryllium Oxide Reactor, 458 F Facemask shielding hydrothermal technique, 314 315 Fast Fourier transform (FFT), 262 263 Fatigue, 64 FCFs. See Flexible ceramic fibers (FCFs) FCNs. See Flexible ceramic networks (FCNs) FCs. See Flexible ceramics (FCs) FEP. See Fluorinated ethelene-propelene (FEP) Ferrites, 451 Ferroelectric field effect transistor (FFET), 117 118 Ferroelectric properties, 84 88, 89t

Index

Ferroelectric random access memory (FRAM), 113, 116 118 Ferroelectrics (FEs), 95 96 FESEM. See Field emission scanning electron microscopy (FESEM) FETs. See Field-effect-transistors (FETs) FFET. See Ferroelectric field effect transistor (FFET) FFF. See Fused filament fabrication (FFF) FFT. See Fast Fourier transform (FFT) Fibrous crossbar perovskite RRAM (FCPeRRAM), 115 Field emission scanning electron microscope (FE-SEM), 26 27, 26f, 262 263 Field-effect-transistors (FETs), 53 54 Filament-based writing system, 196 FIRs. See Fluorescence intensity ratios (FIRs) First-generation electrode material, 354 ruthenium oxide, 354 Flame transport synthesis, 8 9, 54 Flexible absorbers, 331 345 a-Si:H solar cells, 332 334 CdTe solar cells, 334 336 Cu(In,Ga)(S,Se)2 solar cells, 336 338 organic solar cells, 338 342 perovskite solar cells, 342 345 Flexible ceramic fibers (FCFs), 159 Flexible ceramic films, 138 148 based optical device applications, 148 164 applications, 162 164 light-emitting diodes, 161 162 optical (or phosphor) thermometry, 158 159 optical memories, 154 157 photocatalysis, 159 160 photodetectors, 148 151 solar cells, 151 153 electroluminescence of, 146 147 mechanoluminescence of, 147 148 photoluminescence of, 143 146 refractive index of, 141 142 transmittance of, 139 141 Flexible ceramic materials, 545 Flexible ceramic networks (FCNs), 171 172 Flexible ceramics (FCs), 3 4, 4f, 363, 544 545 fabrication of, 4 8

Index

applications and challenges, 8 10, 9f cofired combustion methods, 4 5 printing method, 5 roll-to-roll processing method, 7 8 tape casting process, 5 7 Flexible electronics, 75, 261 262 fabrication strategies and materials, 262 263 physical sensor, 263 266 wound healing, 268 271 Flexible energy storage devices, 96 Flexible perovskite solar cell (FPSC), 151 153, 344 Flexible solar cells (FSCs), 151 153, 325 emerging applications of, 326f flexible absorbers, 331 345 a-Si:H solar cells, 332 334 CdTe solar cells, 334 336 Cu(In,Ga)(S,Se)2 solar cells, 336 338 organic solar cells, 338 342 perovskite solar cells, 342 345 flexible electrodes, 345 348 carbon, 347 metals, 346 347 polymers, 347 348 material properties for, 326 328 chemical properties, 328 optical properties, 328 stability against oxygen and moisture, 326 327 thermal properties, 327 328 Flexible substrates, 325, 329 331 ceramics, 330 331 polymers, 331 Flexible supercapacitor, 301 316 metal organic frameworks, 301 308 MXene, 308 316 Flexible supercapacitors (FSCs), 299 300, 316 317 Flexible thin-film vertical light-emitting diodes (f-VLEDs), 270 271 Fluorescence intensity ratios (FIRs), 264 Fluorinated ethelene-propelene (FEP), 109 Foams, 279 280 Fossil fuels, 353 Fourier transform infrared spectroscopy (FTIR), 32 34, 33f, 197 198 FPSC. See Flexible perovskite solar cell (FPSC)

573

Freeze-casting method, 265 266 Freundlich isotherm, 412 Friction coefficient curves, 477f FSCs. See Flexible solar cells (FSCs) Fuel cells, 7, 299 300 Fully reversible deformation, 182 Functional ceramic materials, 15 16 Fused filament fabrication (FFF), 193 194 G Galvanostatic charge discharge (GCD), 99 102 Gamma-ethacryloxypropyltrimethoxysilane, 471 Gel electrolytes, 282 Germanium (Ge), 494 Gibbs Thomson coefficients, 506 507 Glass, 330 331 Glass fiber fabric (GFF), 103 106 Global warming, 325 Glucose sensors, 267 268 Grafting method, 223 224 Graphene, 117 118, 277 278, 299 300, 546 547 Graphene oxide (GO), 546 547 Graphene sponge (GS), 307 308 Graphene-incorporated alumina flexible nanocomposites, 545 547 Graphene-modified flexible silicon oxycarbide (SiOC), 88 Graphite, 250 Graphitic carbon nitride (g-C3N4), 381 Green state machining, 377 Green-Surface Engineering for Advanced Manufacturing Network, 453 Grinding, 243 H h-boron nitride (hBN), 417 418 Hafnium oxide (HfO), 115 116 Halide perovskite (HP), 113 Heat-transfer mechanism, 432 Heavy metals, 412 414 Helicopter rotors, 460 Helicopters, 459 460 Herschel Bulkley model, 197t High resistance state (HRS), 113 High-rate vacuum depositing method, 225 226

574

High-resolution transmission electron microscopy, 494 496 High-velocity oxygen fuel (HVOF), 453 454 Hole transport layers (HTLs), 339 341, 509 510 Hot corrosion, 472 474 diffusion coatings, 472 473 overlay coatings, 473 thermal barrier coatings, 473 474 HRS. See High resistance state (HRS) Huimin Kindergarten, 443f, 444t Hybrid machining, 244, 251 252 Hydroxyapatite (HAp), 217 218, 370 371, 379, 416 I Impregnation method, 228 In situ techniques, 208 Indium (In), 494 Indium tin oxide (ITO), 137, 265 Indium zinc oxide (IZO), 343 344 Industry-grade ceramic coating, 462 Inert materials, 328 Integration patterns, 433 435 Interaction of electromagnetic wave with ceramics, 131 135 absorption, 132 reflection, 133 refraction, 133 135 scattering, 135 transmission, 132 133 Intermetallic matrix composite (IMC), 479 Internal Schottky emission, 93 Internet of things (IoT), 332 333 Inventing novel ceramic configurations, 293 294 Iodine (I), 494 Iron oxide (Fe2O3), 355 356 Isostrain average, 564 Isostress average, 564 Itacolumite, 46 47, 57t, 68f Itacolumite quartzite, 66 K KZr2(PO4)3 KAlSi2O6 (KZP KAS), 51, 57t

Index

L Lab-on-a-chip (LoC), 364 365 Langmuir adsorption isotherm model, 416 417 Langmuir isotherm, 412 Langmuir-Hinshelwood pseudo-first-order model, 419 Lanthanum titanate (LTO), 99 102 Laser beam machining (LBM), 247 Laser cladding, 217, 227, 479 481 Laser etching technique, 227 Laser machining, 243 244 Laser slicing, 247 Laser-assisted 3D printing, 14 Laser-cladding method, 227 228 Layer exfoliation method, 286 288 Layer pairs (LPs), 315 316 Layer-by-layer (LBL) construction method, 193, 315 316 LBM. See Laser beam machining (LBM) Lead (Pb), 77 78, 494 Lead zirconate titanate, 265 266 LEDs. See Light-emitting diodes (LEDs) Li-ion batteries (LIBs), 88 Light projection-based systems, 194 Light-emitting diodes (LEDs), 88 90, 137, 161 162, 261 262, 301 303 Liquid metal-cooled fast breeder reactors (LMFBRs), 543 544 Liquid phase deposition (LPD), 221 Liquid-phase, 174 Lithium nickel dioxide (LNO), 87 Lithographic technique, 5, 172 173 Load resistance (RL), 96 Long-term depression (LTD), 157 Long-term potentiation (LTP), 157 Lotus effect, 217 Low resistance state (LRS), 113 Low-dimensional perovskites, 497 504 Low-temperature cofired ceramics (LTCC), 266, 365 367, 370f LPD. See Liquid phase deposition (LPD) Luminescence properties, 135 138 cathodoluminescence, 136 137 electroluminescence, 137 138 mechanoluminescence, 138 photoluminescence, 136 thermoluminescence, 138 Luminescence spectroscopy, 456 457

Index

M Macroscopically porous materials, 171 172 Magnesium alloys, 225 Magnetron sputtering (MS), 225 226 Malachite green, 420 Manganese (Mn), 494 Manganese dioxide (MnO2), 217, 262 263 Manganese oxide (MnO), 354 355 Maritime hydraulic machine components, 459 Material-removing techniques, 244 Materials science, 487 488 Maxwell equations, 130 Maxwell Wagner relaxations, 263 MB. See Methylene blue (MB) Mechanical properties of conventional ceramics, 63 66 of flexible ceramics materials, 66 71, 67t mechanism of flexibility, 71 72 addition of other materials, 72 by changing shape, 72 microstructure, 72 Mechanoluminescence (ML), 135, 138, 147 148 Medical devices, 375 Melamine foam (MF), 308 312 Memory devices, 113 118 ferroelectric random access memory, 116 118 resistive random access memory, 113 116 MEMS. See Micro electro mechanical systems (MEMS) Mesocarbon microbead (MCMB), 450 451 Metal doping, 187 188 Metal foils, 88 Metal hydroxide ceramics, 288 Metal oxide/conductive polymer composites, 282 283 Metal sulfides/conductive polymer composites, 283 285 Metal-alloyed ZnO tetrapod, 54 58, 57t Metal insulator metal (MIM), 77 Metal organic frameworks (MOFs), 300, 308, 309t, 493 crystal systems of, 500t double perovskites, 497 504 grain growth and defects in, 504 507

575

high-temperature annealing and abnormal improvement of conversion efficiencies, 509 513 low-dimensional perovskites, 497 504 Rietveld refinement of crystal structures for solar cell configuration, 507 509 structural stability of, 496 497 Metallic foils, 281 282 Metalloid nitrides/carbides ceramics, 285 288 Metaloxide nanoparticles, 14 Metals, 346 347 Methylene blue (MB), 159, 421 Mica, 85 Micro electro mechanical systems (MEMS), 8 9 Micro-pores, 103 106 Micro-supercapacitors (MSCs), 315 316 Microarc oxidation (MAO), 220 221, 475 479 Microcracks, 66 Microfluidics, 364 367 ceramic-based medical devices, 375 376 emerging technologies in bioceramics for medical devices, 371 374 flexible ceramics, 364 367 fabrication protocols for, 367 368 functional and flexible bioceramics in medical technology, 370 371 integration of microelectronic in flexible ceramic-based, 369 370 tailoring ceramics for application in medical-related microdevices, 368 369 plasma treatment, 369 surface modification, 369 Microstructural characterization, 45 design strategies, 45 58 Al2O3/Al/Al2O3 hybrid composite, 51 52 Al2O3/Mo, 48 49 alumina titania magnesia, 49 50 aluminum titanate ceramics, 47 48 itacolumite, 46 47 KZP KAS, 51 metal-alloyed ZnO tetrapod, 54 58 one-dimensional flexible ceramics, 53 54 three-dimensional flexible ceramics, 54

576

Microstructural characterization (Continued) ZnO tetrapods, 54 Microstructural studies, 556 557 Microstructurally engineered ceramics (MIECs), 391 398 Microwave materials, 449 ML. See Mechanoluminescence (ML) Modern nucleation theory, 174 175 Module patterns, 433 MOFs. See Metal organic frameworks (MOFs) Moisture absorbing materials, 451 452 Moisture electric generator (MEG), 160 Morphotropic phase boundary (MPB), 103 106 MS. See Magnetron sputtering (MS) Multi-chamber cumulative detonation sprayer (MCDS), 455 Multi-walled carbon nanotubes (MWCNTs), 103 106 Multialuminides coating, 472 473 Multilayered ceramic capacitor, 5f Multilayered composites (MLCs), 391, 392t aspect of toughness improvement in, 407 408 fabrication of, 398 microstructure and properties of, 398 408 nanoalumina/5 zirconia toughened alumina, 404 405 nanoalumina/nanoalumina, 398 406 nanozirconia/5 zirconia toughened alumina, 405 406 nanozirconia/lanthanum phosphate 20layer, 399 400 nanozirconia/nanozirconia, 398 399 Multimaterial ceramics, 208 Multiwalled carbon nanotubes (MWNTs), 185, 450 451 MXene nacre, 308 312 MXenes, 277 278, 286 288, 300 N Nano electro mechanical systems (NEMS), 8 9 Nanoalumina (NA) powder, 398 Nanoalumina/nanoalumina, 398 406 Nanocellulose paper (NCP), 328 Nanoceramic coating, 462 Nanocrystalline ceramic coatings, 474 475

Index

Nanodopants, 454 Nanofiber mat (NFM), 420 Nanofibers, 53 54 Nanofibrous (NFs) membrane, 160 Nanoimprint lithography (NIL), 5 Nanoindentation techniques, 554 Nanoparticles (NPs), 476 479 Nanoribbons/nanobelts (NBs), 138 139 Nanorods, 53 54 Nanotube arrays (NTAs), 419 420 Nanotubes, 53 54 Nanozirconia (NZ) powder, 398 Nanozirconia/nanozirconia, 398 399, 400f Near-ultraviolet (NUV), 161 162 Nickel (Ni), 305 Nickel cobaltite (NiCo2O4), 288 290 Nickel chromium (Ni Cr)-based stainless steel, 96 98 Niobium titanium nitride (TiNbn), 357 358 Nitrogen-anchored porous carbon (NPC), 305 307 Nonmetal doping, 187 188 Nonmetallic ceramics, 132 Nonoxide ceramic coatings, 217 218 Nontraditional machining, 246 251 Normal ceramic collectors, 426 427 Nucleation, 174 175 O Octylammonium (OCA), 501 504 Office of Naval Research, 457 Ohm’s law, 93 OLEDs. See Organic light-emitting diodes (OLEDs) Oleophilicity, 222 Oliver Pharr (O-P) method, 554 One dimensional (1D) nanostructures, 171 172 direct deposition, 182 183 template-based deposition, 183 188 One-dimensional flexible ceramics, 53 54 One-dimensional nanomaterials, 411 One-dimensional nanostructures, 182 188 direct deposition, 182 183 Opaque, 132 Ophthalmology, 456 457 Optical (or phosphor) thermometry, 158 159 Optical absorption, 182 183

Index

Optical memories, 154 157 Optical memristors, 154 Optical parameters, 130 Optical properties, 328 flexible ceramic film-based optical device applications, 148 164 applications, 162 164 light-emitting diodes, 161 162 optical (or phosphor) thermometry, 158 159 optical memories, 154 157 photocatalysis, 159 160 photodetectors, 148 151 solar cells, 151 153 flexible ceramic films, 138 148 electroluminescence of, 146 147 mechanoluminescence of, 147 148 photoluminescence of, 143 146 refractive index of, 141 142 transmittance of, 139 141 interaction of electromagnetic wave with ceramics, 131 135 absorption, 132 reflection, 133 refraction, 133 135 scattering, 135 transmission, 132 133 luminescence properties, 135 138 cathodoluminescence, 136 137 electroluminescence, 137 138 mechanoluminescence, 138 photoluminescence, 136 thermoluminescence, 138 Optofluidics, 369 370 Organic compounds, 419 Organic dyes, 416, 419 Organic light-emitting diodes (OLEDs), 5, 268 Organic or inorganic materials, 137 Organic semiconductors, 261 Organic solar cell (OSC), 331, 338 342 Oxide ceramic coatings, 216 217 Oxide electrodes, 86 87 P P1 made out of thiazolothiazole, 339 341 PAN. See Polyacrylonitrile (PAN) PANI. See Polyaniline (PANI) Parallel-stacked OLED (PAOLED), 268

577

Particle size, 204 Particulate matter (PM), 411 Pathogens, 414 PDMS. See Polydimethylsiloxane (PDMS) PEALD. See Plasma-enhanced atomic layer deposition (PEALD) PEDOT. See Poly(3,4ethylenedioxythiophene) (PEDOT) PEG-DA. See Poly(ethylene glycol) diacrylate (PEG-DA) Perfluorocyclobutene (poly 1,1,1-triphenyl ethane perfluorocyclobutyl ether) (PFCB), 79 80 Perovskite nanofibrous membrane (PZT), 416 417 Perovskite solar cell (PSC), 331, 342 345 Perovskite structure, 493, 496 PFM. See Piezoresponse force microscopy (PFM) pH sensors, 266 Phase transformation, 15 Phosphoric acid, 280 Photo-gating, 156 157 Photobiomodulation (PBM), 268, 269f Photocatalysis, 159 160 Photodegradation of dyes, 419 421 Photodetectors, 148 151 Photoluminescence (PL), 135 136, 143 146 Photopolymerization, 8 9 Physical sensor, 263 266 biological sensors, 266 268 chemical sensors, 266 268 pressure sensors, 265 266 strain sensor, 265 temperature sensor, 263 265 Physical vapour deposition (PVD), 51 52, 173 174 PICN. See Polymer infiltrated ceramics networks (PICN) Piezoelectric nanogenerators, 103 108 Piezoelectric properties, 81 82 Piezoresponse force microscopy (PFM), 262 263 Planck’s constant, 149 Plasma electrolytic discharge process, 224 Plasma electrolytic oxidation (PEO) method, 51 52, 224 225 coating method, 224

578

Plasma spraying (PS), 473 Plasma treatment, 177 178, 224 225, 369 Plasma-enhanced atomic layer deposition (PEALD), 178 Plasma-enhanced chemical vapor deposition (PECVD), 332 Poisson’s ratios, 555 Poly-(diallyldimethylammonium chloride) (PDDA), 315 316 Poly-3,4-ethylene dioxythiophene, 280 Poly-dimethylsiloxane (PDMS), 81 82 Poly(3-hexylthiophene-2,5-diyl) (P3HT), 339 341 Poly(3,4-ethylenedioxythiophene) (PEDOT), 303 305 Poly(ethylene glycol) diacrylate (PEG-DA), 262 Poly(ethylene oxide) (PEO), 90 92 Poly(methyl methacrylate) (PMMA), 106 108, 262, 494, 509 Poly(vinyl alcohol-co-ethylene), 221 Poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE), 81 82 Polyacrylic acid, 288 Polyacrylonitrile (PAN), 312, 420, 551 552 Polyamide (PDA), 103 106 Polyaniline (PANI), 280 Polycarbonate (PC), 327 328 Polycrystalline, 174 175 Polydimethylsiloxane (PDMS), 103 108, 118, 265 266, 281 282, 331, 365 366, 450 451 Polyethene terephthalate, 281 282 Polyethylene (PE), 417 418, 450 451 Polyethylene naphthalate (PEN), 326 327 Polyethylene terephthalate (PET), 327 328 Polyethyleneimine, 288 Polyimide (PI), 88, 415 416 Polymer infiltrated ceramics networks (PICN), 69 70 Polymer resins, 215 Polymer-based materials, 5 Polymeric resin, 194 195 Polymeric substrates, 281 282 Polymers, 347 348, 449 Polyphenylene sulfide (PPS), 415 416 Polyphenylsilsesquioxane precursor, 285 286 Polypropylene, 450 451

Index

Polypyrrole, 280 Polysilane derivatives, 509 510 Polystyrene, 450 451 Polytetrafluoroethylene (PTFE), 143 144, 450 451 Polyurethane, 450 451 Polyvinyl alcohol, 280 Polyvinylidene difluoride (PVDF), 417 418 Porosity, 208 Porous carbon (PC), 308 312 Postdeposition treatment (PDT), 336 337 Potassium (K), 494 Potassium aluminum silicate, 66 Potassium hydroxide, 280 Potassium zirconium phosphate, 66 Potential material for high-temperature sensors, 263 264 Potentiodynamic polarization technique, 220 221 Pottery, 76 Powder-based processes, 200 203 energy consumption, 202 203 feedstock requirements, 201 202 process description, 200 Power conversion efficiency (PCE), 325 326 Pressure sensors, 265 266 Primary pyroelectric effect, 83 Printed object, 195 Printing method, 5 Process description, 194 196 Process parameters, 205 207 slurry-based processes, 206 207 Product demand, 452 453 Propylammonium (PA), 501 504 Pseudo-cubic phase, 505 Pseudocapacitors, 277 278, 280 Pseudoelasticity, 15 Pt nanoparticles (NPs), 177 178 Public building, 440 441 Pulsed lased deposition (PLD), 98 Pushing electrochemical performance limits, 293 Pyroelectric coefficient, 82 83, 83t, 111 Pyroelectric materials, 82 83 Pyroelectric nanogenerators (PENGs), 108 110 Pyroelectric properties, 82 84

Index

Q Quality control (QC), 207 techniques, 194 Quaternary ammonium salts (QAC), 417 418 Quintile monovalent cations, 344 R Radar frequency, 451 452 Radio frequency (RF), 8 9 Radio-frequency magnetron sputtering technique, 98 Radius of curvature (ROC), 144 145 Raman scattering, 456 457 Raman spectroscopy, 32 34, 551 552 Rapid annealing technique, 98 Rare earth (RE) elements, 481 Rare-earth cerates, 531 Raw materials, 7 8 Reactive Red-120 (RR-120), 416 417 Reduced graphene oxide (rGO), 308 312 Reflection, 133 Reflection loss (RL), 162 Reflectivity, 132 Refraction, 133 135 Refractive index, 141 142 Regular PF mechanism, 93 Relative spring back (RSB), 557 558 Relaxor ferroelectrics (RFEs), 95 96 Removal of pathogens, 417 419 Resistive random access memory (RRAM), 113 116 Resistive switching (RS), 115 Resistivity, 182 183 Rhodamine B dyes, 419 421 Rietveld refinement of crystal structures, 508 Rigidity, 75 Robocasting, 193 194, 196, 197t Roll-to-roll (RTR) processing, 7 8, 8f Roll-to-roll manufacturing techniques, 325 RS. See Resistive switching (RS) RSB. See Relative spring back (RSB) Rubidium (Rb), 494 Rural residence, 436 Ruthenium oxide (RuO2), 354 S Sandwich-structure transferable OLED (STOLED), 268 270

579

Scanning electron microscope (SEM), 25, 27f, 181f, 454, 556 Scanning probe microscopy (SPM), 28 31 atomic force microscopy, 30 31, 30f differential thermal analysis, 38, 38f electron diffraction, 34, 35f energy dispersive X-ray analysis, 34 35, 36f Fourier transform infrared spectroscopy, 32 34, 33f Raman spectroscopy, 32 34 scanning tunneling microscopy, 28 30, 29f thermogravimetric analysis, 38, 38f X-ray diffraction, 31 32, 32f X-ray photoelectron spectroscopy, 35 38, 37f Scanning tunneling microscopy (STM), 28, 29f Scattering, 135 Scholars, 425 Schottky and Poole-Frenkel conduction mechanisms, 93 95 Schottky defects, 507 Schottky effect, 93 Schottky emission, 93 Schottky, and Poole Frenkel (PF) conduction mechanisms, 92 93 Scratching indenting model, 245 246 SEBS. See Styrene ethylene butylene styrene (SEBS) Selective laser melting (SLM), 200 Selective laser sintering (SLS), 200 Self-limiting mechanism, 175 177 Self-lubricating, 460 Self-propagating, 4 5 Sellmeier dispersion relation, 134 135 SEM. See Scanning electron microscope (SEM) Sensors, 111 112 Series resistance, 510 Shape memory alloys (SMAs), 14 Shape memory ceramics (SMC), 18 Shunt resistances, 510 512 Sigma blend method, 80 Silica nanofibers, 414 415 Silicoaluminum carbonitride (SiAlCN), 263 264 Silicon carbide (SiC), 530 531

580

Silicon dioxide, 217 Silicon nitride (Si3N4), 262 263, 381, 531, 543 544 Silicon rubber (SR), 80 Simple dipping method, 103 106 Single crystals, 174 175 Single phase solvothermal technique, 305 Single walled carbon nanotube (SWCNT), 347, 552 553 Sintering, 199, 293 SiOC fibers (SiOCf), 88 90 Size reduction, 293 SLA. See Stereolithography (SLA) Slicing, 205 Slip-cast fused silica (SCFS), 451 452 SLM. See Selective laser melting (SLM) SLS. See Selective laser sintering (SLS) Slurry, 5 7 Slurry coating technique, 301 303 Slurry vats, 208 Slurry viscosity, 195 Slurry-based processes, 194 199, 206 207 direct-ink writing, 196 energy consumption, 198 199 feedstock requirements, 196 198 robocasting, 196 workflow of, 195f Smart ceramics, 13 15 biomedical applications of, 19 21, 21f electrical and electronic applications of, 17 19 fabrication methods, 16 17, 17f industrial application of, 22 multifunctional, 19f shape recovery in, 15 16 SMAs. See Shape memory alloys (SMAs) SMC. See Shape memory ceramics (SMC) Soda-lime glass (SLG), 336 Sodium (Na), 494 Soft, 171 172 Soft electronics, 75 Soft polymer-ceramic substrates, 450 451 Sol-gel technique, 378 379 Sol gel coatings, 379 Sol gel method, 98, 218 220, 485 487 Sol gel process, 16, 470 Solar cells, 151 153, 171, 328 Solar energy, 425 Solid loading, 196

Index

Solid polymer electrolytes (SPEs), 90 92 Solid-state electrolytes, 282 Solid-state lithium batteries, 90 92 Solid-state technique, 14 Solution corrosion, 469 472 Solution immersion process, 226 227 Sp3 hybridized graphite, 546 547 Space-charge-limited conduction (SCLC) mechanism, 92 93 Spinel oxide ceramics, 288 292 SPM. See Scanning probe microscopy (SPM) Sponges, 279 280 Spongy barium titanate, 290 292 Spray drying technique, 454 Spray pyrolysis, 174 Staphylococcus aureus, 417 418 State-of-art, 425 431 normal ceramic collectors, 426 427 vanadium titanium black ceramic collectors, 427 431 Static method, 96 103 Stereolithographic techniques, 199 Stereolithography (SLA), 193 194, 197t STM. See Scanning tunneling microscopy (STM) Strain sensor, 265 Stress versus strain curve, 68f Stress-induced phase transformation, 15 Stripe coatings, 462 463 Strontium (Sr), 494 Styrene ethylene butylene styrene (SEBS), 262 Submarines, 458 Supercapacitors (SCs), 99 102, 279 282, 299 300 current collectors/substrates for, 281 282 electrical double-layer capacitors versus pseudocapacitors, 279 281 electrolytes, 282 recently developed ceramic electrodes for, 282 292 metal hydroxide ceramics, 288 metal oxide/conductive polymer composites, 282 283 metal sulfides/conductive polymer composites, 283 285 metalloid nitrides/carbides ceramics, 285 288

Index

spinel oxide ceramics, 288 292 types of, 279f use of ceramics as, 281 Superelasticity, 15 Superhard alloy, 483 484 Superhydrophilic materials, 217 Superhydrophobic surfaces, 226 Surface modification, 369 Surface ships, 459 Surface-enhanced Raman scattering (SERS), 418 419 Sustainable Development Goals, 493 Sustainable energy technology, 88 SWCNT. See Single walled carbon nanotube (SWCNT) Switching nozzles, 208 Synergistic effect, 282 283 T Tafel polarization method, 223 224 Taguchi technique, 530 Tape casting process, 5 7 TEM. See Transmission electron microscopy (TEM) Temperature sensor, 263 265 Template-based deposition, 183 188 Tensile strain, 103 106 Tensile stress, 77 Tetraethyl orthosilicate (TEOS), 471 Tetrapodal, 8 9 TGA. See Thermogravimetric analysis (TGA) Thallium (Tl), 494 Thermal barrier coatings (TBCs), 472 Thermal conductivity, 542 Thermal equilibrium, 82 83 Thermal properties, 327 328 Thermal sensors, 111 Thermal spraying, 481 485 Thermally induced phase transformation, 15 Thermionic emission, 93 Thermo-module generators (TMGs), 5 Thermogravimetric analysis (TGA), 38, 38f Thermoluminescence (TL), 135, 138 Thermoplastic materials, 450 451 Three-dimensional (3D) perovskites, 501 Three-dimensional artificial neural network (3D ANN), 157 Three-dimensional flexible ceramics, 54

581

3DP. See Three-dimensional printing (3DP) Three-dimensional printing (3DP), 8 9, 193, 194f, 278 279, 378 classification of, 194 205, 195f bulk solid materials, 203 205 powder-based processes, 200 203 process description, 194 196 slurry-based processes, 194 199 pre-processing ceramics for, 193 194 process parameters, 205 207 quality control techniques, 207 209 for technology selection, 209 210 3-mercaptopropyltrimethoxysilane (TMPTMS), 413 414 Throbbing voltage, 248 Ti-6Al-4V alloy, 220, 223 Time constant, 93 Tin oxide, 356 357 Tissue engineering, 379 Titanium carbide (TiC), 457 Titanium dioxide (TiO2), 379 coatings, 216 217 double-walled nanotubes, 184f MWNTAs, 184f single-walled nanotubes, 184f triple-walled nanotubes, 184f Titanium nitride (TiN), 358, 532 Titanium oxide (TiO), 530 TL. See Thermoluminescence (TL) TMGs. See Thermo-module generators (TMGs) Top-down approaches, 171 172, 194 195 Traditional machining, 244 246 Transition metal oxides/hydroxides, 282 283, 299 300 Transition-metal nitrides, 171 Transmission, 132 133 Transmission electron microscopy (TEM), 25, 27 28, 28f, 29f, 454, 494 496 Transmissivity, 132 Transmittance, 133, 139 141, 432 Trifluoromethanesulfonic acid, 347 Tunability, 78 79 Tunneling process, 93 Two-dimensional (2D) double perovskites, 501 Two-dimensional nanosheets (2D NSs), 301

582

U Ultrasonic helped grinding (UAG), 251 252 Ultrasonic machining (USM), 243 Ultrasonic non-destructive techniques, 208 Universal testing machine (UTM), 564 Urban high-rise residence, 436 439 UTM. See Universal testing machine (UTM) V Vacancy-ordered double perovskites, 504 Van der Waals forces, 85 Van der Waals interaction, 77 78 Vanadium nitride (VN), 357 358 Vanadium oxides (V2O5), 356 Vanadium titanium black ceramic (VTBC), 425, 427 431, 429f Ventilator design, 382 Vertically aligned carbon nanotube arrays (VACNTs), 356 Very large scale integration (VLSI), 528 Very smart materials, 14 15 Vickers hardness, 454 VOCs. See Volatile organic compounds (VOCs) Voigt upper bound, 564 Voltammogram method, 303 305 W Water vapor transmittance rate (WVTR), 326 327 Waterborne pathogens, 417 Wavelength, 34 Wear resistance, 453 454 Wearable electronics, 299 300 Weibull distribution function, 558 Weibull model, 558 560 Weibull statistics analysis, 558 Welding method, 51 52 Wet synthesis technique, 5 7 Wettability, 202 Wetting processes, 227 Wire-electrical discharge machining, 244

Index

Wire-shaped SCs (WSCs), 315 316 World health organization (WHO), 378, 412 Wound healing, 268 271 X X-film software, 223 X-ray diffraction (XRD), 31 32, 32f, 262 263, 454, 494 496 X-ray diffractometer, 454 X-ray microtomography, 486 X-ray photoelectron spectroscopy (XPS), 35 38, 37f XPS. See X-ray photoelectron spectroscopy (XPS) XRD. See X-ray diffraction (XRD) Y Young’s modulus, 64 66, 564 Yttrium (Y), 494 Yttrium aluminum garnet (YAG), 531 Yttrium stabilized zirconia (YSZ), 414 415 Z Zeolite imidazole framework-8 (ZIF-8), 413 414 0-dimensional (0D) perovskite, 504 Zero field permittivity, 78 79 Zeta potential measurement, 197 198 Zinc (Zn), 494 Zinc nanorod arrays (ZNRAs), 420 421 Zinc oxide, 222 Zircon (ZrSiO4), 461 462 Zirconia (ZrO2), 3 4, 331, 457 Zirconia smart ceramics, 14 Zirconia toughened alumina (ZTA), 398 Zirconium carbide (ZrC) fibers, 545 Zirconium metal organic frameworkpolyacrylonitrile nanofibrous membrane, 413 Zirconium oxide, 456 457 Zirconocerate, 531