Introduction to Ceramics; Fabrication, Characterizations, and Applications 9781003470571

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Introduction to Ceramics The field of ceramics has applications in diverse fields including electronic engineering, electrical engineering, biochemical engineering, automobile engineering, and defense sector. This textbook discusses ceramic raw materials, properties of ceramics, fabrication techniques of ceramics, and testing of ceramics. It comprehensively discusses mechanical properties, thermal properties, optical properties, electrical properties, and magnetic properties of ceramics. The text covers structural characteristics, properties, and applications of advanced ceramic materials and examines their difference from conventional ceramics. A separate chapter discusses testing methods of ceramics including testing of raw materials, testing of physical properties, testing of mechanical strength, and testing of electrical properties in depth. This textbook begins by discussing ceramic raw materials, proceeds with conventional ceramics, continues with properties and fabrication techniques of ceramic materials and testing of ceramics, and ends with covering advanced ceramics. This book: • Covers from traditional to advanced ceramics. • Discusses fabrication, characterization, and applications of ceramics in detail. • Examines mechanical, thermal, optical, electrical, and magnetic properties of ceramics in detail. • Covers structural characteristics, properties, and applications of ­carbides, nitrides, oxides, and borides. • Discusses processing techniques including mechanical and magnetic separation of ceramics. It will help serve as ideal study material for senior undergraduate and g­ raduate students in the fields of chemical engineering, materials science and engineering, and ceramic technology.

Introduction to Ceramics

Fabrication, Characterizations, and Applications

Sujoy Bose and Chandan Das

Front cover image Annie Schlechter First edition published 2024 by CRC Press 2385 NW Executive Center Drive, Suite 320, Boca Raton FL 33431 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN CRC Press is an imprint of Taylor & Francis Group, LLC © 2024 selection and editorial matter, Sujoy Bose and Chandan Das; individual chapters, the contributors Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify it in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, ­reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any i­ nformation storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www.­ copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978‑750‑8400. For works that are not available on CCC please contact ­[email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. ISBN: 978‑0‑367‑75057‑2 (hbk) ISBN: 978-1-032-74404‑9 (pbk) ISBN: 978‑1‑003‑47057‑1 (ebk) DOI: 10.1201/9781003470571 Typeset in Sabon by codeMantra

Table of Contents

Preface xvi About the authors xviii Acknowledgments xxi 1 Ceramic raw materials

1

1.1 Introduction 1 1.2 Natural materials  2 1.2.1 Principle of structures  2 1.2.2 Non‑plastic materials  8 1.2.3 Plastic raw materials  11 1.2.4 Fluxing agents  15 1.2.5 Refractory raw materials  16 1.3 Synthetic materials  21 1.3.1 Sol‑gel method  22 1.3.2 Spray pyrolysis  25 1.3.3 Spray drying  25 1.3.4 Freeze drying  27 1.3.5 Zirconia (ZrO2) 28 1.3.6 Titania (TiO2) 29 1.3.7 Ba‑titanate 30 1.3.8 Rice husk ash (RHA)  32 References 33 Short questions  33

2 Principles of ceramic technology 2.1 2.2

Traditional ceramics  34 Ceramic coatings (glaze)  36 2.2.1 Lead (II) oxide  37 2.2.2 Biomimetic route  47

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2.3 Glass 59 2.4 Refractories 61 2.4.1 Refractoriness 61 2.4.2 Porosity and slag permeability  61 2.4.3 Strength 62 2.4.4 Specific gravity  62 2.4.5 Spalling 62 2.4.6 Permanent linear change (PLC) on reheating  62 2.4.7 Thermal conductivity  62 2.4.8 Bulk density  62 2.5 Cements and concrete  63 2.6 Advanced ceramics  63 References 66 Short questions  71

3 Properties of ceramics 3.1

3.2

3.3

Mechanical properties  72 3.1.1 Plastic deformation  72 3.1.2 Elastic deformation  74 3.1.3 Anelasticity 75 3.1.4 Brittle fracture and crack propagation  75 3.1.5 Creep and fatigue  79 3.1.6 Flexural strength  83 3.1.7 Effects of microstructure  84 Thermal properties  85 3.2.1 Heat capacity  85 3.2.2 Density and thermal expansion of crystals, glasses, and composite bodies  86 3.2.3 Thermal conduction  87 3.2.4 Thermal stress  89 3.2.5 Temperature gradients  90 3.2.6 Thermal shock and thermal spalling  90 3.2.7 Thermal tempering and annealing  91 Optical properties  92 3.3.1 Refractive index  92 3.3.2 Dispersion 92 3.3.3 Reflection and refraction  93 3.3.4 Absorption 94 3.3.5 Scattering 95 3.3.6 Polarizability 96 3.3.7 Boundary reflectance  97

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3.3.8 Surface gloss  98 3.3.9 Opacity and translucency  98 3.3.10 Color 99 3.4 Electrical properties  99 3.4.1 Electrical conduction  100 3.4.2 Dielectric conductivity  101 3.4.3 Dielectric strength  101 3.4.4 Piezoelectricity 101 3.5 Magnetic properties  103 3.5.1 Magnetic phenomena  103 3.5.2 Susceptibility 105 3.5.3 Permeability 106 3.5.4 Flux density  107 References 107 Short questions  110

4 Traditional ceramics 4.1 Introduction 111 4.2 Body formulation  112 4.2.1 Porcelain 112 4.2.2 Earthenware 115 4.2.3 Bone china  118 4.2.4 Sanitary ware  120 4.2.5 Hotel China  124 4.2.6 Terracotta 124 4.2.7 Majolica 126 4.2.8 Steatite bodies  126 4.2.9 Cordierite bodies  126 4.2.10 Rutile bodies  127 4.2.11 Titanate bodies  128 4.2.12 Zircon bodies  129 4.2.13 Lava bodies  130 4.3 Whiteware products  131 4.3.1 Manufacturing process and properties  132 4.3.2 Whitewares at home  135 4.3.3 Construction use  145 4.3.4 Electrical use  150 4.3.5 Industrial use  153 4.4 Heavy clayware products  156 4.5 Compositions, properties and applications  156 4.5.1 Face bricks  156

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4.5.2 Paving bricks  157 4.5.3 Hollow bricks  158 4.5.4 Roofing/floor tiles  159 4.5.5 Sewer pipes  162 4.5.6 Stoneware 162 References 164 Short questions  164

5 Advanced structural ceramic materials

166

5.1 Structural characteristics, properties, and applications of carbides  167 5.1.1 Silicon carbide  170 5.1.2 Boron carbide  171 5.1.3 Tungsten carbide  173 5.1.4 Titanium carbide  174 5.2 Structural characteristics, properties, and applications of nitrides  175 5.2.1 Titanium nitride  177 5.2.2 Boron nitride  178 5.2.3 Silicon nitride  180 5.2.4 Aluminum nitride  184 5.3 Structural characteristics, properties and applications of oxides  186 5.3.1 Alumina 186 5.3.2 Zirconia 191 5.4 Structural characteristics, properties, and applications of borides  193 5.4.1 Metal‑rich borides  194 5.4.2 Monoborides 194 5.4.3 Diborides 196 5.4.4 Boron‑rich borides  196 5.4.5 Titanium borides  197 References 199 Short questions  201

6 Processing of ceramic raw materials 6.1 Introduction 202 6.2 Quarrying 203 6.3 Size reduction  203 6.3.1 Laws of size reduction  205

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6.3.2 Mechanism of size reduction  206 6.3.3 Different crushers and grinders  208 6.3.4 Closed‑circuit and open‑circuit grinding  217 6.4 Mechanical separation  217 6.4.1 Screening 218 6.4.2 Equipment 219 6.4.3 Effectiveness of screen  220 6.4.4 Test sieves  220 6.4.5 Filtration 221 6.4.6 Separation based on movement through a fluid  225 6.4.7 Magnetic separation  230 6.5 Mixing 230 6.5.1 Mechanism 230 6.5.2 Types of mixers  230 6.5.3 Liquid mixers  234 6.6 Conveying and storage of materials  235 6.6.1 Solid conveying  235 6.6.2 Liquid conveying  236 6.6.3 Storage of ceramic powders  236 6.6.4 Problems in bin storage  236 References 238 Short questions  238

7 Ceramic fabrication techniques 7.1 7.2

7.3

7.4

Introduction to ceramic processing  240 Slip forming process  241 7.2.1 Introduction 241 7.2.2 Slip 241 7.2.3 Plaster mold  242 7.2.4 Slip casting  243 Plastic forming process  246 7.3.1 Particulate forming  246 7.3.2 Hydroplastic forming  246 7.3.3 Plastic mass preparation  246 7.3.4 Shaping methods  247 Dry forming process  252 7.4.1 Theory of packing  252 7.4.2 Pressing 253 7.4.3 Vibration compaction  256 7.4.4 Isostatic pressing  257 7.4.5 Reactive hot pressing  258

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7.5

Drying and finishing  258 7.5.1 Mechanism of drying  259 7.5.2 Types of dryers  259 7.5.3 Process of drying  261 7.5.4 Drying defects  261 7.6 Firing 262 7.6.1 The stages of a firing  263 7.6.2 Action of heat on ceramic bodies  264 7.6.3 Firing equipment  265 7.6.4 Firing schedules  265 7.6.5 Liquid phase sintering  266 7.6.6 Vitrification 266 7.6.7 Microstructure control  268 References 270 Short questions  273

8 Testing methods of ceramics 8.1

8.2

8.3

8.4

Testing of raw materials  274 8.1.1 Sampling methods  274 8.1.2 Measurement of moisture content by IR moisture balance  275 8.1.3 Particle size analysis  276 8.1.4 Analytical optical microscopy  278 8.1.5 Analytical electron microscopy  282 8.1.6 X‑ray diffractometer  283 8.1.7 Determination of surface area  283 8.1.8 Adsorption 286 Testing of physical properties  289 8.2.1 Plasticity 289 8.2.2 Casting 291 8.2.3 Control of casting slips  292 8.2.4 Contraction 293 8.2.5 Modulus of rupture  293 Testing of mechanical strength  295 8.3.1 Flexural strength  295 8.3.2 Fracture toughness  297 8.3.3 Hardness 298 Testing of electrical properties  298 8.4.1 Electrical conductivity  299 8.4.2 Four‑point probe technique  299 8.4.3 Contactless measurements  301

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8.4.4 Hall effect measurement  302 8.4.5 Impedance spectroscopy  302 8.5 Testing for glaze  303 8.5.1 Measurement of viscosity of glazes at  303 8.5.2 Test for the solubility of lead frit  303 8.6 Testing for refractories  305 8.6.1 Refractoriness 305 8.6.2 Refractoriness under load  305 8.6.3 Cold crushing strength  306 8.6.4 Permanent linear change on reheating  306 8.6.5 Thermal conductivity  307 8.6.6 Creep 307 8.6.7 Thermal shock resistance  308 8.6.8 Hot modulus of rupture  309 8.6.9 Slag resistance test  310 8.7 Quality control  310 8.7.1 Introduction 311 8.7.2 Basic concepts  311 8.7.3 Indian standards for ceramic materials—ISO 9000  312 8.7.4 Zero defects  314 8.7.5 Total quality management (TQM) in ceramic industries  315 References 315 Short questions  317

9 Glass 9.1

318

Principles of glass formation  318 9.1.1 Definition 318 9.1.2 Difference between glass and crystalline material  318 9.1.3 Glass formation  319 9.1.4 Structures of glasses  324 9.1.5 Structural models for silicate glasses  324 9.1.6 Binary alkali silicates  325 9.1.7 Aluminosilicate glasses  325 9.1.8 PbO‑silicate glasses  325 9.2 Raw materials and preparation of glass batch  326 9.2.1 Raw materials  326 9.2.2 Glass formers  326 9.2.3 Intermediates 326 9.2.4 Modifiers  327 9.2.5 Cullet 327

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9.2.6 Refining agents  327 9.2.7 Decolorizers 327 9.2.8 Coloring oxides  327 9.2.9 Selection of glass composition  327 9.2.10 Glass batch calculation  328 9.2.11 Example of a batch calculation:  328 9.3 Glass melting process 329 9.4 Properties of glass  330 9.4.1 Thermodynamic and thermal properties  330 9.4.2 Electrical and transport properties  337 9.4.3 Other properties  337 9.5 Testing and quality control  338 9.5.1 Flat glass defects  338 9.5.2 Container glass defects  338 9.6 Fabrication process  344 9.6.1 Forehearth & feeder  344 9.6.2 Flatware 344 9.6.3 Hollowware 345 9.7 Annealing 345 9.8 Advanced glasses  346 9.8.1 Laminated glass  346 9.8.2 Tempered glass  346 9.8.3 Decorative glasses  346 9.8.4 Vycor & microporous glass  346 9.8.5 Photosensitive glass  346 9.8.6 Glass‑ceramics 347 9.8.7 Glass fibers  347 References 349 Short questions  350

10 Refractories 10.1 Introduction 351 10.1.1 Definition 351 10.1.2 Production 352 10.1.3 Demand and growth of refractories in India  352 10.1.4 Classification of refractory  353 10.1.5 Fundamental properties of refractories  356 10.1.6 Factors for selection and use of refractories  358 10.2 Silica refractories  359 10.2.1 Raw materials and composition  359 10.2.2 Manufacturing process steps  359

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10.2.3 Phase transformation of quartzite  359 10.3 Aluminosilicate refractories  361 10.3.1 Types of raw materials  361 10.3.2 Different alumina‑silicate refractories  361 10.3.3 Manufacturing steps  364 10.4 Basic refractories  364 10.4.1 Manufacturing process  364 10.4.2 Properties and uses of refractories based on  365 10.5 Special refractories  366 10.5.1 Different carbide and nitride refractories  366 10.5.2 Carbon‑ and carbon‑based refractory  369 10.5.3 Zirconia 370 10.5.4 Beryllia 370 10.5.5 Thoria refractory  371 10.5.6 Fused cast refractories  371 10.5.7 Cermets 371 10.5.8 Ceramic fibers  371 10.6 Refractories for iron and steel industry  372 10.6.1 Coke oven  372 10.6.2 Blast furnace  373 10.6.3 Open‑hearth furnace  374 10.6.4 Linz‑Donawitz (LD) converter  374 10.6.5 EAF 374 10.6.6 IF 375 10.6.7 Ladle furnace  375 10.6.8 Slide plate system  375 10.6.9 Nozzle 376 10.6.10 Shroud  376 10.6.11 Continuous casting  376 10.6.12 Monolithic  377 10.7 Refractories for non‑ferrous and non‑metallic industries  378 10.7.1 Refractories in non‑ferrous industries  378 10.7.2 Refractories in non‑metal industries  380 10.8 Refractories for glass and ceramic industry  381 10.8.1 Glass industry  381 10.8.2 Ceramic industry  382 10.9 Refractories for insulation  384 10.9.1 Purpose of insulation  384 10.9.2 Ceramic fiber products  386 10.9.3 Design and installation  386

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10.10 Refractories for space and nuclear applications  387 10.10.1 Ceramics for space  387 10.10.2 Ceramics for nuclear reactors  388 References 391 Short questions  395

11 Cement and concrete 11.1 Cement 396 11.1.1 Raw materials of cement  396 11.1.2 Manufacturing process of cement  397 11.1.3 Composition of cement phases  399 11.1.4 Hydration of cement  400 11.2 Testing and quality control of cement  401 11.2.1 Tests on properties of cement  401 11.2.2 Quality control of cement  404 11.3 Types of cement  404 11.3.1 Hydraulic cement  404 11.3.2 Non‑hydraulic cement  405 11.3.3 Types of Portland cement  405 11.3.4 Blast‑furnace slag cement  406 11.3.5 High alumina cement  407 11.3.6 White and colored cement  407 11.3.7 Oil‑well cement  407 11.3.8 Hydrophobic cement  408 11.3.9 Waterproof cement  408 11.3.10 Super‑sulfate cement  408 11.3.11 Sulfate resisting cement  409 11.4 Concrete 409 11.4.1 Aggregates 410 11.4.2 Admixtures 412 11.4.3 Proportioning of concrete mixtures  415 11.4.4 Recent advances in concretes  419 11.4.5 Types of concrete  420 11.4.6 Significance of concrete  424 11.4.7 Characteristics of concrete  426 11.5 Properties of concrete  427 11.5.1 Strength of concrete  427 11.5.2 Permeability of concrete  429 11.5.3 Creep of concrete  429 11.5.4 Thermal properties of concrete  430 11.5.5 Shrinkage of concrete  432

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11.5.6 Moisture movement in concrete  434 11.5.7 Modular ratio and Poisson’s ratio of concrete  435 11.5.8 Abrasion resistance  436 11.5.9 Fire resistance  437 11.5.10 Freeze‑thaw resistance  438 11.5.11 Electrical properties of concrete  439 References 440 Short questions  442

Index 443

Preface

Ceramics is one of the most ancient industries since the late Paleolithic period (28000  BCE). The journey of ceramics began with pottery from China (18000–17000 BCE), and nowadays, ceramics are essential to our daily lifestyle and found all around us starting from our bedroom to toilet including kitchen. This category of materials includes bricks, tiles, watches, plates, kitchenware, and toilet accessories. Ceramics can also be found everywhere from land to sea to sky in making space shuttles, ships, electri‑ cal and electronic appliances and airplanes, and so on. Ceramic and mate‑ rial engineers are the people who design and develop the process, in which these products can be made, create novel ceramic products, and find differ‑ ent uses of it in daily life. This book is based upon the fundamentals of ceramics from conventional to the advanced including different raw materials, ceramic manufacturing, testing methods, and various applications. Basically, this book is organized and divided into 11 chapters that start with the traditional ceramics, pro‑ ceed with their properties and structures, and end with advanced ceramics and their applications. Although a good number of books are available on the topic, no doubt, many of which are of excellent quality within their scope. This book is a modest effort to provide a balanced blend of fabrication, characteriza‑ tion, and applications of traditional ceramics, extending to present the recent advances in ceramics. This book is mainly aimed at undergraduate and postgraduate students and academics in the field of ceramic engineer‑ ing around the globe. On the other hand, this book is also a guideline to researchers, scientists, and students in chemical engineering. We heartily regret that it was not possible to cover all theoretical and practical concepts of ceramic engineering in a single book. Still, we hope that most of the read‑ ers will agree with us that it is a book that covers almost every portion of ceramic engineering with best of knowledge.

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Preface xvii

We would like to thank IIT Guwahati and IIChE for their support. Moreover, the authors would like to thank their parents and family mem‑ bers and all the well‑wishers for their constant support. Sujoy Bose and Chandan Das Kolkata and Guwahati, February 2024

About the authors

Dr.  Chandan Das is a Professor in the Department of Chemical Engineering at Indian Institute of Technology Guwahati (IITG). He has received his PhD in Chemical Engineering from Indian Institute of Technology, Kharagpur (IITKGP). He has guided, so far, 12 scholars for their doctoral degree and 32 M.Tech. students, and is guiding 9 more doctorate scholars. He has authored 5 books and 11 book chapters. He has three patents in his credit. He has handled nine sponsored projects and seven consultancy projects up to now. He has visited Denmark, Malaysia, Sri Lanka, Japan, Greece, France, and Athens for exchanging ideas. His major research areas include (i) membrane‑based separation technology, (ii) food engineering, (iii) biomedicine, (iv) material science, (v) corrosion, (vi) wastewater treat‑ ment, and (vii) simulation and modeling. Being associated with various research works in the area of water and wastewater treatment, such as treatment of tannery wastewater using membrane separation technology, as well as removal of pollut‑ ants using micellar‑enhanced ultrafiltration, Dr. Das has gained exper‑ tise in membrane separation technology for removing various pollutants from contaminated water and wastewater. His research activity encom‑ passes both an understanding of fundamental principles during filtra‑ tion and the development of technology based on membrane separation. In particular, his research areas are modeling of microfiltration, ultra‑ filtration, nanofiltration, reverse osmosis, treatment of oily wastewater, and tannery effluent using membrane‑based processes. He has explored the detailed quantification of flux decline from fundamentals. As an off‑ shoot of the major research, he has fabricated ceramic membranes using low‑cost precursors as sawdust. A catalyst is coated on the ceramic sup‑ port for manufacturing a catalytic membrane reactor. He is also working on decontamination of chromium‑laden aqueous effluent using Spirulina platensis. He is actively involved in the produc‑ tion of high value‑added products, namely, total phenolics, flavonoids, tocopherol, ,and so on from black rice as well as of 6‑gingerol, vitamin C content, and essential oil content from ginger of North East India. xviii

About the authors  xix

He is a Fellow of the Royal Society of Chemistry, United Kingdom, and a Fellow of the Indian Institute of Chemical Engineers. He is the subject coordinator for PMRF, Govt of India. He is the coordinator for an inter‑ national joint master’s degree program (IMFST) with Gifu University, Japan. He is a liaison from IIT Guwahati for the Ministry of Education, Culture, Sports, Science and Technology (MEXT) project, Japan. Dr.  Sujoy Bose is an Assistant Manager at the Indian Institute of Chemical Engineers, Kolkata, India. Before, he was the Principal of Basantika Institute of Engineering & Technology (Polytechnic), West Bengal, India. He also worked as an Assistant Professor at the Department of Chemical Engineering, Durgapur Institute of Advanced Technology & Management, West Bengal, India. Previously, he was an Assistant Professor at the National Institute of Technology, Calicut, India. He graduated from Durgapur Institute of Advanced Technology & Management under West Bengal University of Technology. He has finished post‑graduation from the National Institute of Technology Durgapur. He has completed PhD from the Indian Institute of Technology Guwahati. Dr.  Bose has a long‑term experience in the field of membrane technology. He is an expert in the area of developing ceramic membranes for different applications. Dr. Bose was able to make a breakthrough in a highly important area of ceramic membrane fabrication and economics, that is, the use of sawdust as a novel cheap raw material for the manufacturing of ceramic membranes. His research interests also include material science, advanced ceramics, and nanofluids. Dr. Bose has published journals in reputed peer‑reviewed international journals such as Materials Letters, Ceramics International, Industrial and Engineering Chemistry Research, Applied Catalysis A: General, Applied Thermal Engineering, and International Journal of Ceramic Engineering and Science. He has published one book entitled Advanced Ceramic Membranes and Applications at CRC Press, Taylor & Francis Group (ISBN: 9781138055407). Dr.  Bose is a Life Associate Member of the prestigious Indian Institute of Chemical Engineers (IIChE). He also has served as an executive committee member of IIChE Guwahati region chapter from 2013 to 2014. He has received prestigious Prof. A. Suryanarayana and Mrs. Vanajakshi Award for the Best Author/(s) of Chemical Engineering Book and/or Book Chapter by Indian Institute of Chemical Engineers for the year 2023 for the book Advanced Ceramic Membranes and Applications. Major research fields: Field 1: Fabrication and Applications of Catalytic Membrane and Membrane Reactors Application: Sulfur recovery from sour gas during the production of petroleum products

xx  About the authors

Field 2: Fabrication and Applications of Low‑Cost Ceramic Membranes Application: Usable as support membrane in UF, MF, NF, and ­catalytic membranes Field 3: Fabrication and Applications of Light‑Weight Ceramic Materials Application: Advanced armor system

Acknowledgments

We would like to express our gratitude to all those who helped and guided us in different ways in completing this book within the period of one year directly or indirectly. First of all, we would like to express our sincere gratitude to Mr.  Gauravjeet Singh Reen, Senior Commissioning Editor‑Engineering, Ergonomics and Human Factors, Occupational Health and Safety CRC Press, Taylor and Francis Group, for enabling us to publish this book. We are indebted to him for his useful suggestions and necessary arrangements made for us throughout the entire period. We are grateful to all those who provided support, talked things over, read, wrote, offered comments, allowed us to quote their remarks, and assisted in the editing, proofreading, and design. We are also grateful to all our colleagues who supported and encouraged us continuously. We are thankful to our doctoral scholars, Mr.  Bharat Bhushan Negi, Mr. Ajay K. Shakya, Ms. Aswani K. Viswanath, and Ms. Mounika Chevula for their valuable inputs in all the chapters of this book. Most of all, we would like to express our deepest sense of gratitude to all our family members, our parents, and all the well‑wishers. Their love, care, sacrifices, and encouragement have made it possible for us to come so far. Chandan Das Sujoy Bose

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Chapter 1

Ceramic raw materials

LEARNING OBJECTIVES • To understand the range and scope of various natural minerals and inorganic nonmetallic materials to be used as raw materials for ceramic products • To know different ideas about the crystalline structures of natural and synthetic materials • To realize the difference between natural and synthetic raw materials 1.1 INTRODUCTION The term “ceramic” is originated from “keramikos” (a Greek word), which means “of pottery”; often it means “for pottery” also. Generally, ceramic materials are available in the form of inorganic and nonmetallic materials. Nowadays oxides, nitrides, or carbides of metals are also used extensively in the fabrication of ceramics. Some elements, such as carbon or silicon, have also been developed and considered ceramics for the electrical, nuclear power, and engineering industries. But the oldest and most natural ceramic raw material is definitely clay. Clay occurs naturally as a fine‑grained powder sourced from either rocks or soil, typically containing one or more clay minerals along with traces of metal oxides and organic material. When combined with water, it forms a cohesive, sticky mass. Clay as a mass can be moldable due to its plasticity, but if dried, it becomes hard and brittle and holds its shape. Furthermore, if fired to redness, it turns into a much harder material without losing its shape and is no longer prone to the action of water. Clay can give the impression in various colors such as white, dull gray, brown, or deep orange‑red based on the soil’s content in which it is found. Clay varies over wide ranges in physical, chemical, and mineralogical charac‑ teristics, but a common characteristic is their crystalline structure that leads to fine grain size and permits the particles to move freely over one another, giving rise to softness, soapy, and foamy feel. The most common DOI: 10.1201/9781003470571-1

1

2  Introduction to Ceramics

clay minerals are kaolinite, and the other compositions are pyrophyllite, halloysite, mica, etc. [1]. Although possibly the earliest ceramic articles were made entirely from clays, nowadays in the pottery industry, the chief raw materials used are different clays such as kaolinite (china clay), bentonite, sedimentary clay, fireclay (flux and silica), and stoneware that consist of different types and amounts of minerals. In the refractory industry, the most commonly used raw materials are alumina, magnesite, dolomite, and chrome ore. Quartz serves as a refractory backbone together with clay and feldspar, an anhy‑ drous aluminosilicate containing K+, Na+, or Ca2+ as a flux that aids in the formation of a glass phase. Dolomite is a solid solution of magnesium and calcium carbonates, used also in the steel industry to make basic brick. Chrome ore is made up of a complex solid solution of spinels, (Mg,Fe) (Al,Cr)2O4 along with various magnesium silicates [1,2]. Talc is an anhydrous magnesium silicate with a layered crystalline struc‑ ture, Mg3(Si2O5)2(OH)2 . It has widespread use in the manufacturing of elec‑ trical and electronic components and tiles. Asbestos fits a set of hydrous magnesium silicates (fibrous structure). Moreover, mineral‑derived substances like soda ash (Na 2CO3), borates including kernite (Na 2B4O7∙4H 2O), and borax (Na 2B 4O7∙10H 2O) are employed as fluxing agents. Fluorspar, CaF2 , is also used as a powerful fluxing agent for making glasses and glazes. Even though most traditional ceramic formulations are based on the use of natural raw materials that are cheap and easily available, an increas‑ ing number of specialized chemically processed ceramic materials are used widely for several applications, called advanced ceramics. Examples are carbides and nitrides used for abrasives; TiO2 in rutile phase used for mak‑ ing ferroelectric materials; steatite or talc for electric insulators; barium and titania as the raw materials for making capacitors; iron oxide for making magnetic ceramics; alumina, zirconia, beryllia, and thoria as refractories and electrical insulators; uranium oxide as a nuclear fuel element.

1.2  NATURAL MATERIALS

1.2.1  Principle of structures Before understanding the occurrence, properties, and uses of raw materials, we must know something about their structure. The fundamentals defined here will help in realizing the structures of some common raw materials. 1.2.1.1  Structure of quartz Usually, quartz (in pure form) is colorless and transparent or translucent, but colored varieties are also available, which include citrine, rose quartz, amethyst, smoky quartz, and milky quartz. Crystal structure of quartz is

Ceramic raw materials  3

a constant framework of silicon‑oxygen tetrahedra (SiO4) in which single oxygen is shared in‑between two tetrahedral (Si–O–Si) silicon atoms to form a spiral chain. According to the crystallinity, quartz polymorphs are classified into two categories – α‑quartz and β‑quartz (an example of chiral crystal structures). Both have helical structures based on the arrangements of SiO4 with the “ridgepole” of one tetrahedron associated with the “keel” of the subsequent tetrahedron in the spiral. Structurally, α‑quartz (trigonal crystal system) is a distorted version of β‑quartz and has threefold screw rotation axes, whereas β‑quartz fits in a hexagonal system with both sixfold and threefold screw rotation axes (Figure 1.1). Chirality of a quartz crystal can be con‑ firmed by detecting the direction of rotation of plane‑polarized light that penetrates through the crystal. Quartz can also show piezoelectric proper‑ ties that can improve an electrical potential, then deform, and vice versa [3]. 1.2.1.2  Structure of the clay minerals There are two main groups of clay mineral, which are the kaolins, Al2Si2O5(OH)4, and montmorillonites, Al2Si4O10(OH)2. The kaolin group comprises kaolinite, nacrite, dickite, and halloysite. Kaolinite belongs to the triclinic crystal struc‑ ture and is the most significant among all kaolin materials, since it is the prin‑ cipal constituent of china clay, fireclay, ballclay, and so on. The basic structural features of all the kaolin materials are the same: they consist of a layer of Si‑O atoms, silica, or tetrahedral layer, joined by common O atoms to a similar layer of Al‑O atoms, gibbsite, or octahedral layer (Figure 1.2a). In the silica or tetrahedral layer, the fourfold Si atoms are in coordination with oxygen atoms to form six‑membered rings. In the gibbsite or octahedral layer, the sixfold Al atoms are in coordination with oxygen or (OH). The montmorillonite group contains the minerals pyrophyllite and talc. In this group, two silica layers are condensed with one gibbsite layer by losing two hydroxyl groups unlike the kaolin group (Figure 1.2b) [4].

(a)

(b)

Figure 1.1  The structure of (a) α‑quartz and (b) β‑quartz (oxygen; silicon).

4  Introduction to Ceramics

Aluminum

(a)

(b)

Figure 1.2 The structure of (a) kaolin group and (b) montmorillonite group.

1.2.1.3  Structure of the micas The micas, or muscovite, KAl2AlSi3O10(OH)2 , so often occur in clays, but they are not clay minerals exactly. Basically, it is derived from talc, Mg3(Si3Al)− O10(OH)2 , or pyrophyllite, Al2(Si2O5)2(OH)2 , by replacing one Al atom as a substitute for four Si atoms. This phenomenon is called isomorphous sub‑ stitution. A potassium ion, K+, is introduced to hold the negatively charged oxygen units together, as shown in Figure 1.3, which is quite similar to pyro‑ phyllite structure, except that there are K+ ions between the silica layers. 1.2.1.4  Structure of chlorites Chlorites are structurally related to the micas, often classified as clay minerals. They may either be available naturally or by combining differ‑ ent clay minerals. Chlorites consist of a substituted talc layer, of formula Mg3(Si3Al−)O10(OH)2 , which carries a negative charge which is neutralized by a substituted magnesium hydroxide (brucite) layer, having the formula, Mg 2Al−(OH)6. This layer is positively charged because of the replacement of Mg by Al. Addition of these talc and brucite layers provides the unit for‑ mula of chlorites, that is, Mg5Si3Al2O10(OH)8. 1.2.1.5  Structure of illites Illite, (K,H 2O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2 ,(H 2O)], is a kind of crystalline mineral and acts as a clay. It is structurally similar to the micas but contains less potassium. Illite is a combination of silica tetrahedron – alumina octa‑ hedron layer (alumino‑silicate) and silica tetrahedron layer (silicate min‑ eral). Illite is classified on the basis of Si:O ratio, for example, sorosilicates (2:7), cyclosilicates (1:3), and inosilicates (1:3) for single chain and 4:11 for double chain and phyllosilicates (2:5) and tectosilicates (1:2) for monoclinic crystal system. Structurally, illite to a certain extent is similar to micas; it

Ceramic raw materials  5

Figure 1.3  The structure of the micas.

contains a little more Si, Mg, Fe, and water and to some extent, less tetra‑ hedral aluminum and interlayer potassium. 1.2.1.6  Structure of andalusite, sillimanite, and kyanite These three minerals all have the empirical formula Al2SiO5 with parallel chains of Al‑O groups, which are linked sideways by Si and Al ions alternatively (Figure 1.4). Andalusite is an aluminum nesosilicate mineral with an ortho‑ rhombic crystal structure. At higher temperatures and pressures, andalusite may convert to sillimanite. Sillimanite is another material that belongs to the group of aluminosilicate polymorphs. Sillimanite crustal structures are ortho‑ rhombic in nature. Kyanite also belongs to the same aluminosilicate series. 1.2.1.7  Structure of mullite and steatite Generally, for fireclay refractories, highly pure primary ingredients such as alumina and silica mixtures are required. Typically, in this mixture, alumina content varies in the range of 25–45 wt%. Mullite is a combination of pure alumina or bauxite and clay or sillimanite; it can also be found naturally but

6  Introduction to Ceramics

Figure 1.4  C  rystal structure of Al2SiO5 polymorphs.

is not very common. Mullite contains 72% aluminum oxide. The structure of mullite is similar to that of sillimanite, in which some Si is replaced by Al. The approximate formula of mullite is Al6Si2O13. Synthetic mullite ceramics can be manufactured via different combinations of starting material (solids, sols, etc.), for example, a mixture of solids, a mixture of sols, or a mixture of sol and salt – each can be used as the starting material. Mullite is also one of the most vital elements of porcelain and has exten‑ sively been used in refractories and glass and steel industries. Mullite has excellent high‑temperature properties with improved thermal shock and thermal stress resistance owing to minimum thermal expansion, maximum durability, and outstanding interconnecting grain structure. It also provides low thermal conductivity and wear resistance to the ceramic. Nowadays, advanced mullite materials are used for making high‑strength infrared transmitting windows, electronic substrates, and protective coatings due to their improved mechanical and physical properties. Steatite is known as soapstone or French chalk as it is soft and has a “soapy” feel. Steatite has a structure similar to that of talc that contains Al, Ca, and Fe, composed of a layer of magnesium hydroxide (brucite) sand‑ wiched between two Si‑type layers. 1.2.1.8  Structure of alumina Decomposition of bayerite, Al(OH)3, and/or boehmite, AlOOH, leads to metastable aluminas before transformation to corundum finally occurs. Alumina (Al 2O3) exits commonly in one principal form, that is, α (hexago‑ nal), and three transition forms have been distinguished, namely, γ (cubic), θ (monoclinic), and δ (tetragonal or orthorhombic) phases. The commonest form of alumina is α‑Al2O3, which is the most stable, also known as corun‑ dum. In corundum, a hexagonal (rhombohedral) crystal system constructed of Al3+ ions (radius 0.053 nm) that are each surrounded by six oxygen atoms (valence of −2 each), are not bonded at the corners of a regular octahedron. Electrical neutrality in corundum is maintained by keeping two Al3+ ions

Ceramic raw materials  7

in between every three O2− ions. As a result, in the lattice of corundum, oxygen atoms form a distorted hexagonal close packing considerably in which two‑thirds of the lattice gaps between the octahedra are occupied by aluminum ions, as presented in Figure 1.5a. The γ phase is a much‑grained alumina, has a structure based on dis‑ torted cubic spinels with a general formula A 3B6O12 (Figure 1.5b), but con‑ verts readily to α phase (between 1,273 and 1,373 K). However, γ‑alumina powder has a specific surface area of about 100 m 2 /g, while by compari‑ son, α‑alumina has about 5 m 2 /g. This difference clarifies quick coarsening of alumina grains when they experience the transformation. Change in phase from the γ to the α phase belongs to exothermic reaction (Q = 20–40 KJ/mol), which drives up the temperature of the system, thus resulting in coarsening and further phase transformation, which perpetuates and makes temperature stabilization in this transformation regime very dif‑ ficult. From a thermodynamic point of view, α‑alumina is the most stable aluminum oxide compared with its transient, metastable counterparts. The strong ionic and covalent chemical bonds between Al3+ and O2− contribute to the characteristic material properties, such as high melting point, low electric and high elastic modulus and hardness, and excellent resistance to the attack of strong inorganic acids such as orthophosphoric and hydrogen fluoride. Upon heating, gamma alumina, one of the poorly crystalline tran‑ sition aluminas, undergoes a transition to delta (between 973 and 1,073 K), to theta (between 1,173 and 1,273 K), and finally to alpha phase. 1.2.1.9  Structure of magnesium oxide and dolomite Magnesium oxide, known as magnesite, is widely used as refractories. The crystal structure of magnesium oxide is simple and is similar to NaCl (rocksalt structure), shown in Figure 1.6a. As shown in the figure, the entire (a)

(b)

Figure 1.5  The structure of (a) corundum and (b) spinel structure of γ‑Al2O3.

8  Introduction to Ceramics

(a)

(b)

Figure 1.6  The crystal structure of (a) magnesite or magnesium oxide, and (b) dolomite.

lattice is built up with cubes containing Mg and O atoms at each corner of the cube. Electro neutrality of the crystal lattice is maintained by the orien‑ tation of Mg atom, which is surrounded by six O atoms, and similarly, each O atom is surrounded by six Mg atoms. The crystal structure of dolomite is almost similar to magnesium oxide and calcium oxide (rocksalt) except for the crystal system. Dolomite belongs to a hexagonal close packing system not cubic like magnesite and calcium oxide (Figure 1.6b) [3,4].

1.2.2  Non‑plastic materials In the introduction, it was mentioned that clays are widely used raw materi‑ als for the fabrication of ceramics; in addition to this, a number of benefi‑ cial materials, such as silica, sillimanite, kyanite, andalusite, mullite, and zircon sand, are present. To gain a clear idea, a thorough study of the properties and uses of those raw materials is required. Here, we will discuss the types, properties, and transformations of silica materials and their uses rather than the details of crystal structures. 1.2.2.1  Silicon dioxide Silicon dioxide (SiO2), also known as silica (from the Latin silex), is a chem‑ ical compound that has been known since prehistoric times. Usually, silica is found in nature as quartz, composed of quartzite rock and ganister, as sand, as flint pebble, as well as in many living organisms. Silica is commonly used in refractory industries and in pottery. Nowadays it is also being used as a raw material for advanced ceramic applications. Pure quartz occurs as transparent and the crystal structure is hexagonal.

Ceramic raw materials  9

SiO2 is available in an amount of different crystalline forms (polymorphs) such as tridymite, cristobalite, and stishovite and coesite (found in high‑pres‑ sure phase) rather than quartz. The structure of tridymite and cristobalite are almost similar and easier to understand. In both cases, the silica tetrahedral form rings containing six Si atoms and six O atoms with a distorted plane; thus, Si atoms are not lying in the same plane. The only difference between the two is the packing of the oxygen atom in the lattice plane. Figure 1.7a shows the atomic arrangement of tridymite. First, the lowest three oxygen atoms form the triangular base of the lower tetrahedron. Second, an apex is formed when the second tetrahedron is joined to the first through a com‑ mon oxygen atom and is inverted with its base uppermost. Next, the three basal oxygen atoms of the upper tetrahedron fall directly below correspond‑ ing oxygen atoms in the base of the lower tetrahedron. This atomic arrange‑ ment of oxygen repeats throughout the structure of tridymite. Figure  1.7b shows the packing of oxygen atoms in cristobalite, and it is clear that the basal oxygen atoms of the respective tetrahedral are no longer linked. Unlike tridymite and cristobalite, the crystal structure of stishovite resembles that of rutile (TiO2). The Si atoms in stishovite are being bounded by six oxides, and belong to octahedral coordination geometry. Coesite is another form of silica which is formed during the application of high pressure (2–3 GPa) and mod‑ erately high temperature (973 K) on quartz. In coesite, four oxygen atoms are attached with a single silicon atom in a tetrahedron and each oxygen atom is then bonded with two Si atoms to form a structure. The structures of quartz, tridymite, and cristobalite differ greatly, and it is somewhat difficult to convert one to another. Conversion of differ‑ ent silica polymorphs only involves the breaking of Si–O bonds. However, if quartz is heated above 1,743 K for a considerable time, it is gradually

(a)

(b)

Figure 1.7 Arrangement of oxygen atoms in (a) tridymite and (b) cristobalite.

10  Introduction to Ceramics

converted to cristobalite in the presence of lime. In addition, if cristobalite is heated in the range of 1,143–1,743 K, it will gradually convert into tridy‑ mite. However, it is very difficult to reconvert these highly stable materials (at room temperature) to quartz, but the conversion from tridymite to cris‑ tobalite is possible. Sometimes certain minor changes in the structure of these crystalline forms of silica do occur, though they are relatively stable at room tempera‑ ture. For example, if quartz (α‑quartz) is heated to 846 K or higher, the atoms become less closely packed and expansion occurs, which attribute to the for‑ mation of a new material, that is, β‑quartz by somewhat straightening the Si–O–Si bonds. This inversion is reversible. Similarly, if cristobalite (α‑phase) is heated at 493–553 K, it is converted to β‑phase of cristobalite. This conver‑ sion causes a sudden expansion (0.7%) in the structure of a silica material (e.g., silica brick), forms macrocracks, and reduces the durability of the mate‑ rial. A series of reversible inversions are observed also for tridymite when it is heated to 390 and 436 K to form tridymite (II) and tridymite (III), respectively. Microscopically, tridymite usually appears as wedge‑shaped crystals and is rarely found in nature. The specific gravity of tridymite is lower −2.27 at 293 K because of loosely packed atoms. Cristobalite is also found rarely in nature similar to tridymite, but unlike tridymite, it is observed as a mass of small crystals if we observe microscopically. In cristobalite, the arrange‑ ment of the atoms is also less dense than in quartz; therefore, the specific gravity of cristobalite is observed as 2.33 at 293 K. 1.2.2.2  Sillimanite, kyanite, and andalusite These three minerals all have the empirical formula Al2SiO5 and contain 63% of alumina. Sillimanite is available in South Africa and India, Kyanite comes from the United States and India, and andalusite is available in the United States and South Africa. Sillimanite, kyanite, and andalusite are non‑plastic aluminosilicate polymorphs. Therefore, it is necessary to process these materi‑ als by fine grinding, with the addition of binder or plasticizer. All three materi‑ als decompose to form mullite and cristobalite when heated at about 1,823 K. This group of materials are widely used in the refractory industry because of their high refractoriness and high resistance to alkaline slags attack. 1.2.2.3  Zircon sand Zircon is a zirconium silicate crystal with a chemical composition of ZrSiO 4, is reddish brown, yellow, green, blue, and gray, and is sometimes colorless. Zircon salts are formed along ancient coastlines where the heavier minerals have been concentrated by wave and wind action. Most zirconium silicate deposits are found in unconsolidated fossil shores away from the present coastline. Generally, elemental zirconium is not available in nature. Rather, zirconium bonds with sodium, calcium, iron, silicon, titanium, thorium,

Ceramic raw materials  11

and oxygen to form a number of different zirconium‑bearing minerals, for example, baddeleyite (ZrO2). Other zirconium‑bearing minerals (large in size) are rare and have not been found in deposits suitable for mining. The general chemical composition of zircon is 67% zirconia and 32.8% silica. Zircon usually contains some hafnium, typically about 1%. Zircon also acts as a good resistant to heat and corrosion. Due to its hardness, durabil‑ ity, and chemical inertness, zircon is used in several ceramic manufacturing units. Because of its low coefficient of thermal expansion, its high melting point, its high refractive index, and its chemical inertness, it is an attractive refractory material. Zircon sand is mainly used as a coating element and provides high thermal conductivity. It has excellent refractory properties, low magnetic susceptibility, and electrical conductivity [5].

1.2.3  Plastic raw materials Clays are plastic raw materials. We have already mentioned the crystal structure of different clay minerals but did not explain the classification, composition, and properties of clay. In this section, therefore, we shall attempt to provide some information regarding those points. According to the geologists, two main types of clay are available: one is residual and the other is sedimentary. Residual clays are those that have not been transported by natural agencies but have been found in their place of origin such as china clay. Residual clays are larger in size and contain no impurities. Sedimentary clay or transported clay is detached from the source and reserved in a new and perhaps distant location. These types of clay contain impurities and are smaller in size. Ball clays, fireclays, and brick clays are examples of sedimentary clays. Normally, residual clays are obtained by surface weathering via three different routes: a. Chemical decay of rocks, such as granite containing silica and alumina b. The solution of rocks, such as limestone containing clay impurities, which, being insoluble, are dumped as clay c. Fragmentation and solution of shale Clay rocks can be recognized by the ultra‑fine grain size of 2 mm diameter), sand (1/16 to 2 mm diameter), and mud (clay is 15% iron; the most common forms are banded iron formations and ironstones. • Phosphatic sedimentary rocks comprise of phosphate minerals that contain more than 6.5% phosphorus. For instance, deposits of phos‑ phate nodules, bone beds, and phosphatic mudrocks.

1.2.3.1  Ball clays The ball clays are examples of sedimentary clays, and it consists of SiO2 (40%–60%), Al 2O3 (25%–40%), Fe2O3 (0.25%–4%), Na2O (0%–0.75%), and K 2O (0.5%–4%). Ball clays are available mostly in the United Kingdom. The method adopted for extracting ball clays is either “open‑pit” or “deep mining.” Selection of these methods is based on the depth of the clay seam, the rigidity of clay, the depth of overburden, and the inclination of the seam to the horizontal. Ball clays can be available in various colors such as blue, black, and ivory and are used in pottery and refractory industries. The pres‑ ence of high organic matter defines the blueness and blackness of the ball clay, whereas the presence of iron oxide presents the ivory ball clay. The properties such as particle size distribution, specific surface area, wet‑to‑dry shrinkage, deflocculation and flocculation, and plasticity are essential to understand the nature of the clay minerals. The grain of ball clays covers a varied range of sizes from 1 μm and consists primarily of coarse‑grained quartz, mica, coaly matter, and other impurities. A property closely related to the particle size is a specific surface area, which is considered to be the total area of the surfaces of all the particles of clay in a unit weight of that clay. The smaller a particle, the greater will be its specific surface area. Specific surface area can be calculated from the particle size distribution approach. During firing, the liquid pres‑ ent inside the clay begins to fuse, causing a decrease in the overall volume of the clay called shrinkage. The wet‑to‑dry shrinkage of ball clay is very high. A ball clay suitable for pottery manufacture should have a linear shrinkage within the range of 12%. The shrinkage in ball clay can be controlled by the

14  Introduction to Ceramics

proportions of colloidal material (Ca2+, Mg2+, Na+, or K+) present in the clay. The greater the proportion of colloidal material in the clay, the greater the shrinkage. Ball clays have a high cation‑exchange capacity as they contain a high percentage of organic matter. Plasticity is a significant term for under‑ standing the properties of a clay mineral. Ball clays are believed to be one of the most plastic clays as they contain most organic matter, being added to poorly plastic bodies to improve their cohesion. 1.2.3.2  China clay Rocks that are rich in kaolinite are known as kaolin. It is a kind of residual clay. English china clay is one of the purest sources of kaolinite. China clay can be available in different forms such as porcelain clay, bone china clay, sanitary play, and earthenware clay. The percentages of SiO2 and Al2O3 are 46% and 38.02%, respectively. The percentages of other constituents such as magnesium oxide and potash present in china clay are 0.27% and 0.02%, respectively. The grain size of china clay does not cover an exten‑ sive range of particles than that of ball clays, causing less plasticity. The less plasticity of china clay indicates its less binding capacity. It has a low cation‑exchange capacity compared with that of ball clays. Due to this, china clays containing Ca2+ require less deflocculant. Generally, less water is required to solidify china clay, and as a result, the chance of shrinkage is less in the case of the china clay body. China clay is used in various indus‑ tries rather than ceramics, for example, • • • • • • •

toothpaste manufacturing industry cosmetics (facial masks, soap) industry paint industry to enhance glossiness rubber industry paper industry chemical industry – adhesive manufacturing chemical and petrochemical industry – act as adsorbents in water and wastewater treatment

1.2.3.3 Fireclays The name “fireclays” suggests a clay that can resist heat, and it belongs to the group of sedimentary clays. Fireclay is largely used for making sanitary fireclays, engineering and refractory bricks, tiles, and so on. Fireclays are extracted by open‑pit methods. Sometimes deep mining method is also used to extract fireclays if the seam is a deep one. The chief minerals present in the fireclays are kaolinite, mica, and quartz along with impurities such as carbonates of calcium, magnesium, iron, organic matter, pyrites, hydrated Fe2O3, and anatase. The average particle size distribution for fireclays lies in the range of 0.1 to 25 μm approximately and is measured either by a method

Ceramic raw materials  15

based on settling under gravity for larger grains or by centrifugal method for smaller grains. The casting of a body containing fireclays can be governed by the amount of deflocculant and the slip density. Slip is a suspension of clay in water and/or other materials used in the production of ceramic ware defloccu‑ lates, for example, sodium silicate which is not a thin sloppy mud‑like slurry. For fireclay product, such as sanitary earthenware, the most commonly used deflocculant is the mixtures of sodium carbonate and sodium silicate. We mentioned earlier that plasticity is a key parameter for clay materials to understand the binding capacity, measured by the amount of organic matter present in it. But in the case of fireclay, underclays, which contain more free silica and other impurities, do contribute to plasticity rather than organic matter. Plasticity or binding power of fireclays is less than that of ball clay due to their high proportion of coarse material. 1.2.3.4  Brick clays The principal constituents of brick clays are kaolinite and chlorite along with quartz, illite, and organic matter. Considerable amounts of iron oxide and CaCO3 are also found in many brick clays. Soluble salts such as CaSO4 are sometimes found in these clays. It is mainly used for the preparation of refractory bricks. Brick clays are commonly extracted in open workings, but if they outcrop on the side of a hill with an excessive overburden, then the most convenient method is tunnel mining. The overburden can be stripped off by skimmers, bulldozers, and power shovels, and the underlying clay is removed with a dragline scrapper or skimmer. The size distribution of par‑ ticles in brick clays ranges from coarse (25 mm) to fine grains ( 90°); (c) hydrophilic surface (θ  150°).

(Figure 2.6b–d). The self‑cleaning coatings are usually ultra‑super hydro‑ phobic as their water contact angle is greater than 150°. Researchers around the globe often draw inspiration from nature to ­create aesthetically pleasing self‑cleaning functional systems. One notable phenomenon in nature is the “lotus effect,” wherein water droplets form on a lotus leaf’s surface owing to its hydrophobic properties. This effect was initially observed and documented by a team of scientists [32]. The mystery behind this mechanism was spread after the reveal of the SEM images of lotus leaves in mid‑1999s (Figure 2.7). The macroscopically smooth surfaces, along with the presence of epicuticular wax crystalloids, make the lotus leaves ultra‑super hydrophobic [33,34]. A super‑hydrophobic surface can be obtained only if the hydrophobic surface is roughened on the micro‑ and nanometer scales. Therefore, the efficiency of a self‑cleaning coating is dependent on the roughness and chemical composition of the surface and dirt particle adhesion to water droplets. These conclusions led the way for the fab‑ rication of various biomimetic super‑hydrophobic surfaces inspired by nature and disclosed that there are two major types of surface microstructures in plant leaves with ultra‑superhydrophobicity: (1) hierarchical micro‑ and nanostructures and (2) unitary micro‑line structures [35–38] (Figure 2.8). Inspired by the super‑hydrophobic properties exhibited by nature, scien‑ tists around the world adopted two techniques to produce hydrophobic and super‑hydrophobic surfaces that can be broadly classified into two catego‑ ries: (1) making a rough surface from a low surface energy material and (2) modifying a rough surface with a material of low surface energy.

Principles of ceramic technology  43

Figure 2.7  SEM images of natural super‑hydrophobic surfaces with hierarchical structures. (a) and (b) are the SEM images of lotus leaf with low and high magnifications, respectively, and the inset of (b) is a water CA on it with a value of about 162°; (c) the SEM image of biomimetic super‑hydrophobic surfaces made by replicating the lotus leaf’s surface structure with poly(dimethylsiloxane) (PDMS), and the inset is a visualization of the advancing and receding CAs on the surface; and (d) a photopolymer replica with UV‑nanoimprint lithography and the inset is the magnified image.

Generally, silicones (PDMS, i.e., polydimethylsiloxane) and fluoro‑ carbons are used extensively for roughening the surface of low surface energy material. The intrinsic deformability and hydrophobic properties of PDMS make it a highly suitable material for producing super‑hydro‑ phobic surfaces [39–41]. Similarly, roughening fluorinated polymers result in super‑hydrophobic surfaces. The super‑hydrophobic property achieved is due to the presence of fibrous crystals with large fractions of void space on the surface [42–44]. Several techniques such as electrospin‑ ning, casting, electrostatic spinning, and spraying are mainly used to pro‑ duce super‑hydrophobic surfaces. A few organic materials, for example, low‑density polyethylene (LDPE) [45], dimethylformamide (DMF) [46], alkylketene [47], polycarbonate [48], and polyamide [49], also exhibit excellent super‑hydrophobic properties. Super‑hydrophobic properties have also been exhibited by a few inorganic materials such as TiO2 and ZnO [50]. Figure 2.9 displays SEM images of surfaces made by different materials and different techniques.

44  Introduction to Ceramics

Figure 2.8  SEM images of natural super‑hydrophobic surfaces with a unitary structure. (a) and (b) are the SEM images of the ramee leaf rear face with low and high magnifications, respectively, and the inset of (b) is a water CA on it with a value of about 164°; (c) and (d) are the SEM images of the Chinese watermelon surface with low and high magnifications, respectively, and the inset of (d) is the water CA on it with a value of about 159°.

Figure 2.9  A SEM image of a (a) PDMS surface treated with a CO2 pulsed laser; (b) PS‑PDMS/PS electrospun fiber mat and the droplets on it; (c) PS‑PDMS surface cast from a 5 mg/mL solution in dimethylformamide (DMF) in humid air; (d) Polystyrene (PS) surface produced by electrostatic spinning and spraying.

Principles of ceramic technology  45

In the recent past, various techniques have been reported to fabricate rough surfaces and subsequently modify the surface chemistry to produce super‑hydrophobic membranes. The various techniques are as follows: • Wet chemical reaction and hydrothermal reaction: Wet chemical reaction is a simple technique that can efficiently control the dimen‑ sionality and morphology of the nanostructures (nanoparticles, nanowires, and mesoporous inorganics) produced. In recent past, this method has been widely used in the fabrication of biomimetic super‑hydrophobic surfaces on metal substrates such as copper, ­aluminum, and steel [51,52]. The hydrothermal technique is a recently developed method that uses a “bottom‑up” route in efficiently fabricating functional materi‑ als with different patterns and morphologies [53–56]. • Electrochemical deposition: Electrochemical deposition is exten‑ sively used to develop biomimetic super‑hydrophobic surfaces since it is a flexible technique to prepare microscale and nanoscale struc‑ tures [57]. • Lithography: Lithography is a conventional technique used to create micro‑ and nanopatterns. Different lithographic techniques that are in practice are (1) photolithography, (2) electron beam lithography, (3) X‑ray lithography, (4) soft lithography, (5) nanosphere lithography, and so on [58–60]. • Self‑assembly and layer‑by‑layer (LBL) methods: The self‑assembly and LBL assembly techniques are based on sequential adsorption of a substrate in solutions of oppositely charged compounds. These tech‑ niques continue to be the most popular and well‑established meth‑ ods for the formation of multilayer thin films. Self‑assembly and LBL deposition are inexpensive techniques in which micro‑ and nanoscale super‑hydrophobic structures can be easily fabricated with finely con‑ trolled surface morphologies [61]. • Electrospinning techniques: Electrospinning is a leading technique for synthesizing fine nanofibers. This technique is broadly used by several scientists to provide sufficient surface roughness for inducing super‑ hydrophobicity [62]. • Etching and chemical vapor deposition (CVD): Plasma etching pro‑ cesses and CVD have been extensively used with polymers to fabricate functional surfaces with different morphologies [63]. CVD is a com‑ petent technique to produce micro‑ and nano‑surface topographies on a macroscopic substrate [64]. • Sol–gel method and polymerization reaction: The sol–gel method can also be used in the fabrication of super‑hydrophobic surfaces in all kinds of solid substrates [65]. Polymerization is another efficient method to modify different surface topographies for the fabrication of super‑hydrophobic surfaces [66].

46  Introduction to Ceramics

Apart from the techniques mentioned above, research groups around the world are working on several other methods such as texturing [67], elec‑ trospraying [68], and sandblasting [69] which were engaged to fabricate super‑hydrophobic surfaces in recent times. The flowchart in Figure 2.10 provides a concise outline of various mate‑ rials and fabrication procedures involved in the fabrication of self‑cleaning coatings. 2.2.1.5  Bioceramic coatings Bioceramics, representing functional ceramics of significant interest, are defined as any substance, other than a drug, or combination of substances, synthetic or natural in origin, which can be used for any period, as a whole or as a part of a system which treats, augments, or replaces any tissue, organ, or function of the body. Bioceramics are used primarily for repair and recon‑ struction of diseased or damaged parts of the musculoskeletal system. Numerous coating techniques have been developed and applied to deposit bioceramic coatings, in particular, osseoconductive calcium phosphate coatings on a variety of substrates including metals such as titanium and magnesium and their alloys, austenitic surgical steels, bio‑inert ceramics, such as alumina and zirconia, as well as polymers such as poly(ethylene), poly(ether ether ketone), poly(methyl methacry‑ late), poly(lactic acid), poly(ε‑caprolacton), carbon nanotubes, and several Materials

Process

Silicones (PDMS)

Etching/casting

Fluorocarbons (Tef lon)

Streching/casting

Organic materials (PS+DMF)

Electrospinning

Inorganic materials (ZnO)

Two step method

Polycrystalline metals (Al/Cu) Composites (Au) Polyelectrolyte + silica nano particles Alkyloxisilane

Etching Lithography LBL and colloidal assembly Sol-gel processing

Roughening the surface of low energy material

Making rough surface and modifying the surface with material of low surface energy

Hydrophobic/ superhydrophobic coatings

Self-cleaning coatings

Titania Titania + dopants Zinc oxide

CVD/ PVD/ Sol-gel/ Spraying

Hydrophilic Photo- catalytic coatings

Titanium oxide

Figure 2.10  A flowchart explaining the summary of various materials and fabrication procedures.

Principles of ceramic technology  47

other materials. Generally, bioceramics coating is classified into two categories – non‑thermal and thermal methods. Some of the important and frequently used non‑thermal and thermal methods from the aforesaid methods are discussed below.

2.2.2  Biomimetic route Biomimetics is a distinctive and quickly rising discipline that provides a deep understanding of the untold account of how nature’s biological path‑ ways work, how they combined with diverse aspects of chemistry, physics, and engineering, and in which way they can be imitated to offer materials and objects useful for various applications including growth of bioceramic films and coatings for clinical uses in orthopedic and dentistry [70]. 2.2.2.1  Sol–gel deposition A sol is a suspension of colloidal particles in a liquid or a solution of polymer molecules while a gel is a semi‑rigid mass formed when the colloidal particles form a network either by crosslinking or interlinking of polymer molecules. The sol–gel processing is used to fabricate ceramic materials that involve the preparation and gelation of sol. Two types of sol–gel processes can be illus‑ trated, depending on whether a sol or solution is used. The two different routes are the particulate gel route and the polymeric gel route [71]. In sol–gel processing, the precursors for the preparation of sol consist of inorganic salt or metal‑organic compounds (metal alkoxides). Metal alkoxides, a class of metal‑organic compounds having the general formula M(OR)X, where M is a metal of valence X and R is an alkyl group, are the most common precursors used in sol–gel method because they react readily with water. Depending upon the electronegativity of the metal, the methods for the preparation of metal alkoxides are divided into two groups: (1) reac‑ tion between metals and alcohols for more electropositive metals and (2) reaction including metal chlorides for relatively less electropositive metals. In addition to this, there are some other processes, such as trans‑esterifica‑ tion, alcohol interchange, and esterification reaction. In a particulate sol, colloidal particles are dispersed within water and peptized using acid or base to generate a sol. This process unfolds in three distinct stages: the precipitation of metal alkoxides, peptization (wherein a precipitate is converted into a colloidal sol by agitation with a dispersion medium in the presence of a small amount of electrolyte), and sintering. Gelation can be attained by (1) removal of water from the sol by evapora‑ tion to reduce its volume or (2) changing the pH to slightly reduce the stabil‑ ity of the sol. The reactions involved in this process are

Precipitation:

Al (OR )3 + H 2O → Al (OH )3

48  Introduction to Ceramics

Peptization:  



Al (OH )3 γ → Al2O3 ⋅ H 2O ( Bohmite ) or δ ‑Al2O3 ⋅ 3H 2O

(Bayenite) Sintering: γ ‑Al2O3 ⋅ H 2O → γ ‑Al2O3 + H 2O

In polymeric sol route, the reactions involved throughout the process are hydro‑ lysis, polymerization, and crosslinking. Polymerization occurs in three stages: 1. Polymerization of monomers to form particles 2. Growth of particles 3. Linking of particles into chains Three reactions involved in this process are Ti (OR )4 + H 2O → Ti (OR )2 (OH )2 + ROH



Hydrolysis:



Polymerization: nTi (OR )2 (OH )2 →  − Ti (OR )2 – O −  + H 2O n



Crosslinking:

 − Ti (OR )2 – O −  n →  − Ti (OH )2 – O − 

Advantages: • • • • •

Highly pure product Good chemical homogeneity with multicomponent system Lower temperature sintering for ceramic fabrication Preparation of ceramics and glasses with novel compositions Ease of fabrication for films and fibers

Disadvantages: • • • • •

Expensive starting material Conventional drying Limited to the fabrication of small articles Drying step leads to long fabrication time Special handling of raw materials usually required

2.2.2.2  Dip coating Dip coating is a simple, economical, dependable, and reproducible method that involves the immersing of a substrate into a tank containing coating solution, removing the specimen from the tank, and allowing it to drain. The coated specimen can then be dried by force‑drying or baking. It is a well‑accepted way of fabricating a thin and uniform coating onto flat or cylindrical substrates.

Principles of ceramic technology  49

Figure 2.11 shows a schematic representation of the processing steps. The dip coating process can be, generally, separated into five stages [72]: I. Immersion: At a constant speed, the substrate is dipped into the coat‑ ing solution. Based on the kind of substrate, a pretreatment process would be carried out before this step. II. Startup: The substrate remains in the solution for a designated time to allow for the coating material to apply itself to the substrate that is also known as dwelling time, and then it starts to be withdrawn. III. Deposition: While the substrate is withdrawn, the thin film coating starts to be deposited on it. The thickness of the coating is directly dependent on the speed by which the substrate is being pulled out. The faster the substrate is withdrawn from the tank, the thicker the coating material that will be applied to the board. IV. Drainage: In this step, excess liquid is drained out from the substrate surface. V. Evaporation: The solvent starts to evaporate from the surface of the substrate to form a thin film. If the solvent is volatile, this step might happen in step III. The critical parameters that can affect the coatings produced by dip coat‑ ing are speed, temperature, and the atmosphere of the solution. In view of chemistry, dilution and solvent are considered to be the most effective parameters that can influence the coatings. Dip coating techniques can be described as a process in which the sub‑ strate to be coated is dipped into a liquid and then pulled out with a definite withdrawal speed under controlled temperature and atmospheric condi‑ tions. The coating thickness is mostly governed by the withdrawal speed, by the solid content, and the viscosity of the liquid. If the withdrawal speed is chosen such that the shear rates keep the system in the Newtonian regime,

Figure 2.11  A schematic representation of the processing steps of dip coating: (a) immersion, (b) startup, (c) deposition, (d) drainage, and (e) evaporation.

50  Introduction to Ceramics

the coating thickness can be calculated by the Landau–Levich equation (Eq. 2.5):

(η ⋅ ν )

2



h = 0.94

1

3

γ LV6 (ρ ⋅ g)

1

2

(2.5)

where h = coating thickness, η = viscosity, γ LV  = liquid–vapor surface ten‑ sion, ρ = density, and g = gravity. Advantages: • Simple and durable process • Ability to provide complete coverage to substrates with challenging geometries • With a dip coat, however, the coating flows over the entire surface, providing uniform coverage with relative ease • Dip coating lines are also highly efficient, with minimal waste systems Disadvantages: • If parts are too light relative to the coating, they may float upon immersion, making it difficult to achieve the desired finish • Film thickness can vary from top to bottom, commonly known as “wedge effect” • Fatty edges form at the base of components as surplus coating drains away. Refluxing caused by the solvent vapors above the tank removes a portion of the coating • Dip coatings are also susceptible to sags and runs if they are not spe‑ cifically formulated to guard against them as a result of gravity during cure time 2.2.2.3  Spin coating Spin coating generally involves the application of a thin film evenly across the surface of a substrate by casting a solution of the desired material in a solvent (e.g., water, ethanol, methanol, and dichlorobenzene) while it is rotating. In this process, a small drop of the coating material is laden onto the center of a substrate, which is then spun at a controlled high speed, and when a solution of a material and a solvent is spun at high speeds, the cen‑ trifugal force and the surface tension of the liquid together create a smooth layer. In the spin coating process, the substrate spins around an axis that is perpendicular to the coating area. The action of spinning causes the coating material to spread out and reach the edge of the substrate, leaving a thin film of coating on the surface.

Principles of ceramic technology  51

Spin coating involves depositing a liquid solution onto a spinning sub‑ strate to produce a thin film of solid material, such as a polymer. The four stages of spin coating are explained as follows with the help of a schematic diagram (Figure 2.12): I. Deposition: The initial step of casting the solution onto the substrate, typically using a pipette. If the substrate is already spinning (dynamic spin coating) or is spun after deposition (static spin coating), the cen‑ trifugal motion will spread the solution across the substrate. II. Spin‑up: The substrate reaches the desired rotation speed  –  either immediately or following a lower‑speed spreading step. At this stage, most of the solution is expelled from the substrate. Initially, the fluid may be spinning at a different rate than the substrate, but eventu‑ ally, the rotation speeds will match up when drag balances rotational accelerations – leading to the fluid becoming level. III. Spin‑off: The fluid now begins to thin, as it is dominated by viscous forces. As the fluid is flung off, often the film will change color due to interference effects. When the color stops changing, this will indicate that the film is mostly dry. Edge effects are sometimes seen because the fluid must form droplets at the edge to be thrown off. IV. Evaporation: Fluid outflow stops, and thinning is dominated by evaporation of the solvent. The rate of solvent evaporation will

(b)

(a)

Substrate

Polymer Solution

Rotating Platform

(c)

Outward liquid flow due to centrifugal force

Outward liquid flow due to centrifugal force

(d)

Evaporation

Angular Velocity Evaporation n Final Polymer Film

Figure 2.12  Schematic representation of the spin coating technique where (a) denotes the deposition stage, (b) illustrates the spin‑up stage, (c) demonstrates the spin‑off stage, and (d) elucidates the final evaporation stage

52  Introduction to Ceramics

depend on the solvent volatility, vapor pressure, and ambient condi‑ tions. Non‑uniformities in evaporation rate, such as at the edge of a ­substrate, will cause corresponding non‑uniformities in the film. Clearly, these two stages (Stages II and IV) describe the nature of the coat‑ ing (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin process such as the rotation speed. As the range of thicknesses is important for the desired material, the range of spin speeds available is also vital. In general, the spin speed range can vary from 500 to 8,000 rpm to produce uniform films, although in some cases, good film quality can also be achieved up to around 12,000 rpm. Commonly, the final film thickness is determined using a simple propor‑ tionality rule, as expressed in Eq. (2.6):

hf ∝

1 (2.6) ω

where ω is the angular velocity/spin speed and hf is the final film thickness. This equation is only valid for predictions of film thickness with experi‑ mental data. Applications of spin coating vary significantly from laboratory to indus‑ try which include advanced ceramics, ceramic membranes, flat panel TVs, photoresists, insulators, organic semiconductors, synthetic metals, nano‑ materials, metal and metal oxide precursors, transparent conductive oxides, and many more materials. Advantages: • • • •

Simple and easy to handle Economical Fast drying due to high spin speeds Quickly and easily produce uniform films

Disadvantages: • • • • •

Large substrate size Single batch, low throughput Wastage of coating material Templates can’t be used several times Lack of material effectiveness

2.2.2.4  Electrochemical deposition Electrochemical deposition is an exclusive technique in which a variety of materials can be processed, including polymers, metals, and even ceramics. In this method, a thin and firmly adherent desired coating of metal, oxide, or

Principles of ceramic technology  53

salt can be deposited onto the surface of a conductor substrate by simple elec‑ trolysis of a solution containing the desired metal ion or its chemical complex. Electrochemical deposition works on the basis of fundamental electro‑ chemical properties. Two electrodes, an anode and a cathode, are dipped into an electrolytic solution. When electricity is passed through the elec‑ trodes and the electrolyte, oxidation and reduction occur. The flow of ions through the electrolyte onto one of the electrodes causes the electrochemi‑ + cal deposition to occur (Figure 2.13). The reduction of metal ions M Z in aqueous solution is represented by

Z+ Msolution + ze → Mlattice (2.7)

This can be accomplished by means of two different processes: (1) an elec‑ tro‑deposition process in which z electrons (e) are provided by an external power supply and (2) an electroless (autocatalytic) deposition process in which a reducing agent in the solution is the electron source (no external power supply is involved). These two processes, electro‑deposition and elec‑ troless deposition, combine the electrochemical deposition. Four types of primary subjects are involved in the process represented by Eq. (2.7): (1) metal–solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice Mlattice, and (4) structure and proper‑ ties of the deposits. The rate of the deposition reaction ν (Eq. 2.7) is defined as the number + of moles of M Z depositing per second and per unit area of the electrode surface:

Figure 2.13  Scheme of electrochemical process.

54  Introduction to Ceramics



+ v = k  M Z  (2.8)  

+

where k is the rate constant of the reduction reaction and [ M Z ] represents + the activity of M Z . The rate constant k of electrochemical processes is interpreted on the basis of the statistical mechanics and is given by the expression

k=

kBT  ∆Ga+  − (2.9) h  RT 

where kB is the Boltzmann constant, T is the absolute temperature, h is the Planck constant, ∆Ga+ is the electrochemical activation energy, and R is the gas constant. The electrochemical activation energy is a function of the elec‑ trode potential E:

∆Ga+ = f ( E) (2.10)

There are two main reasons to use the electrochemical deposition process: Esthetic appeal: Precious metals are expensive. One way to give a cheaper metal the look of a precious metal is to plate it with a thin layer of gold, platinum, or silver. This is very common in the jewelry industry. To give a material a protective coating: A popular example of this is electroplating steel with a zinc coating. Many different automotive components are manufactured from zinc‑plated steel that was coated using the electrochemical deposition process. Advantages: • • • • •

Low‑temperature treatment Low friction High hardness Applicable to a wide range of metal substrate Thick layers possible

Disadvantages: • • • •

Hydrogen embrittlement Not applicable to insulating substrates Possible environmental concerns with plating bath Poor thickness uniformity on complex components

Principles of ceramic technology  55

2.2.2.5  Electrophoretic deposition Electrophoretic deposition (EPD) is a two‑step process, including electro‑ phoresis and deposition. In the first step, charged particles suspended in a liquid medium move toward the oppositely charged electrode under the effect of an externally applied electric field (electrophoresis). In the sec‑ ond step, the particles deposit on the electrode, forming a more or less thick film, depending on the process conditions (concentration of particles in solution, applied electric field, time). The substrate acts as an electrode, and the deposit of particles is the coating (Figure 2.14). The deposit takes the shape imposed by this electrode. Hence, after drying and removal from the electrode, a green shaped ceramic body is obtained. Firing this green body then results in a ceramic component. The only shape limitation is the ­feasibility of removing the deposit from the electrode after deposition. The main requirement to obtain an efficient EPD process is to use suit‑ able suspensions where ceramic particles are well suspended and dispersed. When a ceramic particle is in a liquid medium, it can be charged through four mechanisms: a. selective adsorption of ions onto the solid particle from liquid, b. dissociation of ions from the solid phase to liquid, c. adsorption or orientation of dipolar molecules at the particle surface, and d. electron transfer between the solid and the liquid phase due to the dif‑ ference in work function.

Figure 2.14  A schematic representation illustrates scheme of electrophoretic coating.

56  Introduction to Ceramics

Applications of the EPD process are in a spread number of sectors: bioc‑ eramic coatings, fuel cells, barrier coatings, electronics, catalysis, optical devices, etc. Particularly successful in the use of EPD to produce porous, laminated, and graded ceramic coatings as well as fiber‑reinforced com‑ posites. EPD is mostly advantageous in the deposition of hydroxyapatite [Ca5(PO4)3(OH)] coatings because this technique allows the control of coatings composition, thickness, and microstructure, which are essential to obtain the maximum benefit. Advantages: • • • • •

Highly versatile Cost effective Simple and cheap Can coat objects with a complex shape High deposition rate

Disadvantages: • Need for post‑deposition heat treatment to increase the density of the coating (1,200°C). 2.2.2.6 Electron‑ and ion beam–assisted deposition (EBAD and IBAD) Electron beam evaporation is a physical vapor deposition (PVD) technique whereby a powerful, electron beam is generated from a filament and driven via electric and magnetic fields to hit source material and vaporize it within a vacuum environment. Hence, EBPVD uses a target anode that is bom‑ barded with an electron beam generated by a charged tungsten filament under high vacuum (Figure 2.15). Advantages: • Controlled variations in the structure and composition of condensed material • Multilayer coating with a good surface finish • High deposition rate • Dense coating • Low contamination • High thermal efficiency Disadvantages: • Cannot be used to coat the inner surface of complex geometries • Filament degradation in the electron gun results in a non‑uniform evaporation rate.

Principles of ceramic technology  57

Figure 2.15  Schematic description of a typical vacuum chamber for EBAD and IBAD.

2.2.2.7  Radio frequency (RF) magnetron sputtering Sputtering is the process whereby atoms or molecules of a material are ejected from a target by the bombardment of high‑energy particles. Magnetron sputtering is a widely used PVD technique to fabricate thin films, from a few nanometers to several micrometers in thickness and in large areas relevant for industrial applications. There are a number of different sputter techniques available, including ion beam, reactive, diode, radio frequency, and magnetron sputtering. The most versatile form of sputtering is radio frequency (RF) magnetron sputtering, as shown in Figure  2.16. Earlier,

Figure 2.16  A pictorial view of radio frequency magnetron sputtering.

58  Introduction to Ceramics

this process was used only in the electrical industry but has more recently been used to produce bioactive coatings. The coatings via such process are dense, homogeneous, and adhere well to the substrate, which is normally flat. Again, the coatings are very thin in nature (1–10 μm), with a deposition rate in the order of 1.0–1.5 μm/hour. RF sputtering is the technique involved in alternating the electrical poten‑ tial of the current in the vacuum environment at radio frequencies to avoid a charge building up on certain types of sputtering target materials, which over time can result in arcing into the plasma that spews droplets creating quality control issues on the thin films and can even lead to the complete cessation of the sputtering of atoms terminating the process. It shows several important advantages over other vacuum coating techniques, a property that led to the development of a large number of commercial applications from microelectronic fabrication to bioceramic coatings. Advantages: • • • • • • • •

high deposition rates ease of sputtering any metal, alloy, or compound high‑purity films extremely high adhesion of films excellent coverage of steps and small features ability to coat heat‑sensitive substrates ease of automation excellent uniformity on large‑area substrates, for example, architec‑ tural glass.

Disadvantages: • Deposition rates are very low for some materials in RF sputtering technique. • Application of RF power is not simple. It requires an expensive power supply and an additional impedance‑matching circuitry. • Stray magnetic field leakage from ferromagnetic target disturbs the undergoing sputtering process. To avoid this leakage, sputter guns with strong permanent magnets should be used, which increases the cost of the system. • Most of the incident energy on the target becomes heat energy. This needs to be removed. • It is difficult to deposit uniformly on complex structures, for example, turbine blades. • It is hard to produce high‑performance thick coatings due to higher internal residual stress levels.

Principles of ceramic technology  59

A number of analytical methods exist to assess the chemical, mechanical, micro‑structural, and biological properties of bioceramic coatings that manage their performance in vitro and in vivo. Chemical properties and phase composition of bioceramic coatings can be characterized using conventional characterization techniques including X‑ray diffraction (XRD), vibrational spectroscopy techniques such as Fourier transform infrared (FTIR), Raman spectroscopy, and nuclear magnetic reso‑ nance (NMR) spectroscopy. These methods provide a bunch of information on bulk phase composition, degree of crystallinity, and crystallite size. Some special technique such as cathode‑luminescence helps to expose intrinsic coating properties that cannot be done by conventional analytical techniques. Mechanical properties such as cohesive and adhesive bond strengths, porosity, surface roughness, and extent and sign of residual stresses are basic requirements for suitable coating performance. The adhesion of the coatings to the substrate can be performed with several techniques, for example, the common tensile pull test (ASTMC633‑13), the hardly ever used Ollard‑Sharivker test, the modified peel test (ASTM D3167‑10), and a range of other specialized test methods including shear, tape, scratch, and laser shock adhesion tests. The surface roughness of coatings can often be determined by recording asperities using a diamond‑stylus tester in con‑ junction with electronic filtering methods by RC high‑pass filters. Residual stresses can be assessed by monitoring the deformation of the crystallo‑ graphic lattice of polycrystalline materials subjected to mechanical stress (thermal and quenching stress) and by establishing a relation between the change of the d‑value of the inter‑planar spacing of selected lattice planes {hkl} relative to the stress‑free state (sin2Ψ technique). Biological properties are normally determined to understand the response of living cells and tissue to foreign materials introduced into the human body. Commonly applied in vitro tests for cell viability and proliferation in contact with bioceramic coatings include not only biomimetic immersion tests in simulated body fluids but also determination of alkaline phospha‑ tase activity, expression of non‑collagenous proteins such as osteocalcin and osteopontin, and Alamar BlueR and MTT assays, all designed to indicate the degree of osseointegration as well as the absence of cytotoxicity [70]. 2.3 GLASS Conventionally, glass is formed via three steps – melting, forming, and anneal‑ ing. From the 10th century onwards, glass has been employed in stained glass windows of churches and cathedrals. Demand for glass during the 17th cen‑ tury rose because, in addition to master church builders using glass in church windows, builders of castles and stately townhouses were now discovering how to use glass to enclose spaces as well. Since the 19th century, there has been a revival in many ancient glass‑making techniques including cameo

60  Introduction to Ceramics

glass, achieved for the first time since the Roman Empire and initially mostly used for pieces in a neo‑classical style. In the 20th century, new types of glass such as laminated glass, reinforced glass, and glass bricks have increased the use of glass as a building material and resulted in new applications of glass. Multi‑storied buildings are frequently constructed with curtain walls made almost entirely of glass. In the 21st century, scientists observing the proper‑ ties of ancient stained glass windows, in which suspended nanoparticles pre‑ vent UV light from causing chemical reactions that change image colors, are developing photographic techniques that use similar stained glass to capture true color images of Mars for the 2019 ESA Mars Rover mission. Pliny (Roman historian) claimed that Phoenician sailors cooking on blocks of Natron (alkali salts used for mummification) noticed primitive glass melts formed in beach sands around the cooking fires. Three basic components are • Sand (SiO2) • Natron (Na 2O) • Sea Shells (CaCO3) Development of defect‑free glass passes through a variety of scientific revolutions: A. Broad Sheet (1226): first produced in Sussex. B. Crown glass (1330): for artwork and vessels first produced in Rouen, France C. Glass windows: replacing dark wooden shutters/oiled paper in Europe, 1400s and the development of superior mirrors → heightened aware‑ ness of cleanliness and hygiene D. Mirror and coach plates (1600s): first produced in London E. Optical glass (1500s): microscopes (Huygens) revolutionized biology → telescopes (Galileo) revolutionized astronomy F. Thermometer glasses (1800s): accurate/reproducible m ­ easurement of temperature responsible for experimental underpinnings of thermodynamics G. Tempered glass: is developed by Francois Barthelemy Alfred Royer de la Bastie (1830–1901) of Paris, France by quenching almost molten glass in a heated bath of oil or grease H. Laboratory glass (1800s): chemical revolution (Michael Faraday) I. Float glass (1900s): launched in the United Kingdom. Invented by Sir Alastair Pilkington Today >98% (by wt.) of commercial glasses are from the group of silicates, containing 72% SiO2 , 14% Na2O, 11% CaO, and 3% other and several other minor additives. Very clear and durable quartz glass can be made from pure silica, but the high melting point and very narrow glass transition of quartz make glassblowing and hot working difficult.

Principles of ceramic technology  61

2.4 REFRACTORIES Refractories are inorganic nonmetallic materials that can resist high tempera‑ tures (e.g., up to 2,000°C or more, zirconia) without undergoing physicochem‑ ical changes while remaining in contact with molten slag, metal, and gases. The refractory range incorporates fired, chemical, and carbon‑bonded materials that are made in different combinations and shapes for diversified applications. Some of the important growths since the early days of refractory have been the following: • • • • • • • •

Silica refractories: first made in South Wales in 1842 Chrome refractories: as bricks, since 1896 Magnesite refractories: initially used in Europe and was first used in 1888 Dolomite refractories: initially used in Europe, first commercially used for brick making in 1965 High‑Alumina refractories: in the form of kaolin, since the 20th cen‑ tury, but discovered as bauxite in 1888 Insulating refractories: insulating firebrick was developed in the mid‑1920s Plastics and castables: popularly used for monolithic structures since 1920s Fusion‑cast refractories: alumina‑silica and zirconia are used for the casting of molten refractory materials in blocks, and were developed over the last 50 years

Refractories are produced from natural and synthetic materials, usually nonmetallic, or combinations of compounds and minerals such as alumina, fireclays, bauxite, chromite, dolomite, magnesite, silicon carbide, and zirco‑ nia. Based on chemical composition, refractories can be classified into three categories: acidic, basic, and neutral. Broadly speaking, in physical form, refractory materials are either bricks or monolithic. The properties of various refractory materials are briefly described below.

2.4.1 Refractoriness Refractoriness is a property at which a refractory will deform under its own load. The refractoriness is indicated by PCE (pyrometric cone equiva‑ lent). It should be higher than the application temperatures. Refractoriness decreases when refractory is under load. Therefore, more important is refractoriness under load (RUL) rather than refractoriness.

2.4.2  Porosity and slag permeability Porosity affects chemical attack by molten slag, metal, and gases. Decrease in porosity increases strength and thermal conductivity.

62  Introduction to Ceramics

2.4.3 Strength It is the resistance of the refractory to compressive loads, tension, and shear stresses. In taller furnaces, the refractory has to support a heavy load; hence, strength under the combined effect of temperature and load, that is, refractoriness under load, is important.

2.4.4  Specific gravity Specific gravity of the refractory is important to consider the weight of a brick. Cost of bricks of higher specific gravity is more than that of lower specific gravity. But the strength of bricks of higher specific gravity is greater than one with lower specific gravity.

2.4.5 Spalling Spalling relates to fracture of refractory brick which may occur due to the following reasons: • A temperature gradient in the brick which is caused by sudden heating or cooling • Compression in a structure of refractory due to expansion • Variation in CTE between the surface layer and the body of the brick • Variation in CTE between the surface layer and the body of the brick is due to slag penetration or due to structural change

2.4.6  Permanent linear change (PLC) on reheating In materials, certain permanent changes occur during heating, and these changes may be due to • • • •

Change in the allotropic form Chemical reaction Liquid phase formative Sintering reactions

2.4.7  Thermal conductivity Thermal conductivity of the bricks determines heat losses. Increase in porosity decreases thermal conductivity and at the same time decreases strength too.

2.4.8  Bulk density Decrease in bulk density increases volume stability and heat capacity.

Principles of ceramic technology  63

2.5  CEMENTS AND CONCRETE A hydraulic cement made by fine pulverization of clinker which is produced by calcining a mixture of argillaceous (substance containing SiO2 , Al2O3, and Fe2O3 such as clay, shale, marly clay, that is, mix of calcium carbonate and clay, glass, etc.) and calcareous materials (group of lime such as lime‑ stone, chalk, marble, and lime sand). Portland cement (ASTM C150), the most active ingredient in concrete, is hydraulic cement capable of setting, hardening, and remaining stable under water. It is composed of calcium silicates (di‑ and tri‑), tri‑calcium alumi‑ nate, tetra‑calcium alumino ferrite, and some amount of gypsum. Portland cements are classified into five categories, which are Type I: For general purposes. For the use when the special properties specified for any other types are not required. Type II: For general use, more especially when moderate sulfate resis‑ tance or moderate heat of hydration is desired. Type III: For use when high early strength is desired. Type IV: For use when low heat of hydration is required. Type V: For use when high sulfate resistance is desired. Since 5,000 years, from the time of the Egyptian Pyramids to the present day, decorative concrete has developed gradually. Concrete has been used for many amazing things throughout history, including architecture and infrastructure. In 1836, the first test of tensile and compressive strength took place in Germany, which drove further development in the concrete industry. After that, the cement and concrete industry never look back. Today, concrete is the world’s most widely used building material. Global production is 5 bil‑ lion cubic yards per year (using approximately 1.25 billion tons of cement). Concrete is a mixture of cement (~11%), water (~16%), coarse aggregates (CA) (~41%), fine aggregates (FA) (~26%), and admixtures (~6%). A chain of reaction, known as hydration, is responsible for the concrete manufac‑ turing process. First, when cement powder is mixed with water, it produces cement paste. Then the FA are allowed to mix with the cement paste to form mortar. Finally, concrete is produced by mixing mortar with CA. FA normally called sand, either natural sand or crushed stone, represents par‑ ticles smaller than 3/8 inch. Generally, it accounts for 30%–35% of the mixture. CA are either gravel or crushed stone, comprising particles greater than 1/4 inch. 2.6  ADVANCED CERAMICS Approximately 15,000 years ago marked the emergence of the earli‑ est ceramic products. Fast forward around 250 years ago, and the term “ceramics” predominantly referred to pottery. At that time, ceramic

64  Introduction to Ceramics

products encompassed a range of items such as tableware, roofing tiles, bricks, clay pipes, porcelains, bottle glass, sheet glass, refractory bricks, enamels, cements, lime, gypsum, and abrasives. These ceramics or ceramic products are called traditional ceramics. In recent years, the field of ceram‑ ics has broadened and expanded. Advanced ceramics are categorized on the basis of their properties and applications, as shown in Figure 2.17. Advanced ceramics offer unique and amazingly powerful physical, ther‑ mal, mechanical, and electrical properties that have opened up a whole new world of development opportunities for manufacturers in a wide range of industries. Applications of advanced ceramics are based on and resistant to various properties, such as chemical, biological, electrical, mechanical, optical, physical, structural, and thermal properties. In addition to this, there are various applications, for example, catalysts, sensors, biocompat‑ ible parts, electrical conductors and insulators, thermal conductors and insulators, semiconductors, superconductors, positive and negative tem‑ perature coefficient resistors, magnetic parts, dielectrics, piezoelectrics, ferroelectrics, pyroelectrics, lubricants, carbon fibers, abrasives, heaters, automobile engine parts, cutting tools, and laser oscillators. Materials used for advanced ceramics are carbides, borides, carbons, hydroxides, nitrides, oxides, and so on. Examples of these materials are boron nitride (BN), silicon carbide (SiC), diamond (C), hydroxyapatite (Ca5(PO4)3(OH)), silicon nitride Si3N4, alumina (Al2O3), barium titanate (BaTiO3), mullite (3Al2O3 ‑2SiO2), silicon oxide (SiO2), zirconia bromide (ZrB2), titanium oxide (TiO2), and zirconia (ZrO2). Glass ceramics, another important segment of advanced ceramics in terms of quantity and economics, refer to a material that combines two types of materials to form a product that is in a category of its own between glasses and polycrystalline ceramics. Glass ceramics show practically no physical expansion over a wide temperature range and special optical prop‑ erties. The most vital and commercially popular glass ceramics products are presented below: • Vision® (Corning Inc., United States): domestic products • Keraglass®, Eurokera®, Keralite®, and Eclair® (Corning Inc./St. Gobain, France): cooktop panels and safety‑rated materials • Ceran® (Scott AG, Germany): cooktop panels • Zerodur® (Scott AG, Germany): telescopic mirror Biomorphous ceramics from lignocellulosic materials are of great need for materials of enhanced performance and reliability, which provide better efficiency, specialization, and optimization of their properties. A variety of biomorphous ceramics have been fabricated from fibers, wood tissue, and wood pulp, and fiberboard performs (see Table 2.1).

Principles of ceramic technology  65

Figure 2.17  Categories of advanced ceramics based on their applications and properties.

66  Introduction to Ceramics Table 2.1  E xamples of biomorphous ceramics derived from natural and preprocessed plant performs Template Fibers Wood

Other plants Wood products

Plant Cotton Sisal Beech Balsa, Cypress Maple Oak Pine Walnut Sponge Rattan Wood fibers Fiber papers Wood fiber boards

Material SiC Al2O3, TiO2 SiC, Al/SiC SiC SiC SiC SiC, Al2O3, SrAl2O4 SiC/Si Ca5(PO4)3OH Ca5(PO4)3OH, Si/SiC Si‑Cu/SiC SiC, Al2O3 SiC/Si/C

Nowadays, the use of porous ceramics also changes from filtration, absorption, catalysts, and catalysts supports to lightweight structural components.

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Principles of ceramic technology  67 [8] Gao W., Li Z., Developments in high temperature corrosion and protection of materials. In A Volume in Woodhead Publishing Series in Metals and Surface Engineering (Gao W., Li Z., eds), Woodhead Publishing (2008). ISBN 978‑1‑84569‑219‑3. [9] Cao X.Q., Vassen R., Stoever D., Ceramic materials for thermal barrier coat‑ ings, Journal of the European Ceramic Society, 24(1) (2004) 1–10. [10] Clarke D.R., Phillpot S.R., Thermal barrier coating materials, Materials Today, 8(6) (2005) 22–29. [11] Zhu D., Miller R.A., Thermal conductivity and sintering behavior of advanced thermal barrier coatings, Ceramic Engineering and Science Proceedings, 23(4) (2002) 457–468. 26th Annual Conference on Composites, Advanced Ceramics, Materials, and Structures: B; Cocoa Beach, FL; United States; 13–18 January 2002. [12] Vassen R., Cao X., Tietz F., Basu D., Stöver D., Zirconates as new materials for thermal barrier coatings, Journal of the American Ceramic Society, 83(8) (2000) 2023–2028. [13] Lehman H., Pitzer D., Pracht G., Vassen R., Stöver D., Thermal conductiv‑ ity and thermal expansion coefficients of the lanthanum rare‑earth‑element zirconate system, Journal of the American Ceramic Society, 86(8) (2003) 1338–1344. [14] Smallman R.E., Ngan A.H.W., Chapter 16 ‑ Oxidation, corrosion and sur‑ face engineering. In Modern Physical Metallurgy 8th edition (Smallman R.E., Ngan A.H.W., eds), pp.  617−657. Butterworth‑Heinemann (2014). ISBN 9780080982045, https://doi.org/10.1016/B978‑0‑08‑098204‑5.00016‑X [15] Opila E.J., Hann R., Paralinear oxidation of CVD SiC in water vapor, Journal of the American Ceramic Society, 80(1) (1997) 197–205. [16] Jacobson, N.S., Smialek, J.L., Fox D.S., Molten salt corrosion of SiC and Si3N4. In Handbook of Ceramics and Composites (Cheremisinoff N.S., ed.), 1, pp. 99–135. Marcel Dekker, New York (1990). [17] Lee K.N., Current status of environmental barrier coatings for Si‑based ceramics, Surface and Coating Technology, 133–134 (2000) 1–7. [18] Lee K.N., Key durability issues with mullite‑based environmental barrier coatings for Si‑based ceramics. Journal of Engineering for Gas Turbines and Power, 122 (4) (Oct 2000) 632−636122. https://doi.org/10.1115/1.1287584 [19] Ueno S., Jayaseelan D.D., Ohji T., Development of oxide‑based EBC for silicon nitride, Internation Journal of Applied Ceramic Technology, 1 (2004) 362–373. [20] Krishnamurthy R., Sheldon B.W., Haynes J.A., Stability of mullite protec‑ tive coatings for silicon‑based ceramics, Journal of the American Ceramic Society, 88 (2005) 1099–1107. [21] Schneider H., Schreuer J., Hildmann B., Structure and properties of mullite ‑ a review, Journal of the European Ceramic Society, 28 (2008) 329–344. [22] Ueno S., Ohji T., Lin H.T., Corrosion and recession of mullite in water vapor environment, Journal of the European Ceramic Society, 28 (2008) 431–435. [23] Lee K.N., Miller R.A., Jacobson N.S., New generation of plasma‑sprayed mullite coatings on silicon carbide, Journal of the American Ceramic Society, 26 (1995) 705–710.

68  Introduction to Ceramics [24] Lee K.N., Miller R.A., Oxidation behavior of muilite‑coated SiC and SiC/ SiC composites under thermal cycling between room temperature and 1200−1400°C, Journal of the American Ceramic Society, 79 (1996) 620–626. [25] Lee K.N., Fox D.S., Eldridge J.I., Zhu D., Robinson R.C., Bansal N.P., Miller R.A., Upper temperature limit of environmental barrier coatings based on mull‑ ite and BSAS, Journal of the American Ceramic Society, 86 (2003) 1299–1306. [26] More K.L., Tortorelli P.F., Bhatia T., Linsey G.D., Evaluating the stability of BSAS based EBCs in high water‑vapor pressure environments. Proceedings of the ASME Turbo Expo: Power for Land, Sea, and Air, pp. 377–384. (2004), Volume 2: Turbo Expo 2004. Vienna, Austria. June 14–17, 2004. ASME. https://doi.org/10.1115/GT2004‑53863 [27] Lee K.N., Eldridge J.I., Robinson R.C., Residual stresses and their effects on the durability of environmental barrier coatings for SiC ceramics, Journal of the American Ceramic Society, 88 (2005) 3483–3488. [28] Xu Y., Hu X., Xu F., Li K., Rare earth silicate environmental barrier coatings: present status and prospective, Ceramics International, 43 (2017) 5847–5855. [29] Eaton H.E., Gary D. Linsey, Ellen Y. Sun, Karren L. More, Joshua B. Kimmel, Jeffrey R. Price, Narendernath Miriyala, EBC protection of SiC/SiC compos‑ ites in the gas turbine combustion environment: continuing evaluation and refurbishment considerations. Proceedings of the ASME Turbo Expo: Power for Land, Sea, and Air. Volume 4: Turbo Expo 2001 (June 4–7, 2001).New Orleans, Louisiana, USA, 2001‑GT‑0513, V004T02A010. [30] Lee K.N., Current status of environmental barrier coatings for Si‑based ceramics, Surface and Coating Technology, 133–134 (2000) 1–7. [31] Venkatesan A.G., Kumar H.K., Nair S.A., Ramakrishna S., A review of self‑cleaning coating, Journal of Material Chemistry, 21 (2011) 16304–16322. [32] Dorrer C., Rühe J., Some thoughts on superhydrophobic wetting, Soft Matter, 5(1) (2009) 51–61. [33] Solga A., Cerman Z., Striffler B.F., Spaeth M., Barthlott W., The dream of staying clean: lotus and biomimetic surfaces, Bioinspiration and Biomimetics, 2(4) (2007) S126–S134. [34] Ball P., Engineering shark skin and other solutions, Nature, 400 (1999) 507–509. [35] Luo Z.Z., Zhang Z.Z., Hu L.T., Liu W.M., Guo Z.G., Zhang H.J., Wang W.J., Stable bionic superhydrohphobic coating surface fabricated by a conven‑ tional curing process, Advanced Materials, 20 (2008) 970–974. [36] Mock U., Michel T., Tropea C., Roisman I., Rühe J., Drop impact on chemically structured arrays, Journal of Physics: Condensed Matter, 17 (2005) 595–605. [37] Otten A., Herminghaus S., How plants keep dry: a physicist’s point of view, Langmuir, 20 (2004) 2405–2408. [38] Sun M.H., Luo C.X., Xu L.P., Ji H., Ouyang Q., Yu D.P., Chen Y., Articial lotus leaf by nanocasting, Langmuir, 21(19) (2005) 8978–8981. [39] Khorasani M.T., Mirzadeh H., Kermani Z., Wettability of porous polydimeth‑ ylsiloxane surface: morphology study, Applied Surface Science, 242(3) (2005) 339–345. [40] Jin M., Feng X., Xi J., Zhai J., Cho K., Feng L., Jiang L., Super‑hydrophobic PDMS surface with ultra‑low adhesive force, Macromolecular Rapid Communications, 26 (2005) 1805–1809.

Principles of ceramic technology  69 [41] Ma M., Hill R.M., Lowery J.L., Fridrikh S.V., Rutledge G.C., Electrospun poly(styrene‑block‑dimethylsiloxane) block copolymer fibers exhibiting superhydrophobicity, Langmuir, 21 (2005) 5549–5554. [42] Zhao N., Xie Q., Weng L., Wang S., Zhang X., Xu J., Superhydrophobic surface from vapor‑induced phase separation of copolymer micellar solution, Macromolecules, 38(22) (2005) 8996–8999. [43] Zhang J., Li J., Han Y., Superhydrophobic PTFE surfaces by extension, Macromolecular Rapid Communications, 25(11) (2004) 1105–1108. [44] Shiu J.Y., Kuo C.W., Chen P., Mou C.Y., Fabrication of tunable superhy‑ drophobic surfaces by nanosphere lithography, Chemistry of Materials, 16 (2004) 561–564. [45] Lu X., Zhang C., Han Y., Low‐density polyethylene superhydrophobic surface by control of its crystallization behavior, Macromolecular Rapid Communications, 25 (2004) 1606–1610. [46] Jiang L., Zhao Y., Zhai J., A lotus‑leaf‑like superhydrophobic surface: a porous microsphere/nanofiber composite film prepared by electrohydrody‑ namics, Angewandte Chemie International Edition, 43 (2004) 4338–4341. [47] Mohammadi R., Wassink J., Amirfazli A., Effect of surfactants on wetting of super‑hydrophobic surfaces, Langmuir, 20(22) (2004) 9657–9662. [48] Zhao N., Xu J., Xie Q., Weng L., Guo X., Zhang X., Shi L., Fabrication of biomimetic superhydrophobic coating with a micro‑nano‑binary structure, Macromolecular Rapid Communications, 26(13) (2005) 1075–1080. [49] Zhang J., Lu X., Huang W., Han Y., Reversible superhydrophobicity to super‑ hydrophilicity transition by extending and unloading an elastic polyamide film, Macromolecular Rapid Communications, 26(6) (2005) 477–480. [50] Feng X., Feng L., Jin M., Zhai J., Jiang L., Zhu D., Reversible super‑hydro‑ phobicity to super‑hydrophilicity transition of aligned ZnO nanorod films, Journal of American Chemical Society, 126(1) (2003) 62–63. [51] Feng L., Wang S., Jiang L., One‑step solution‑immersion process for the fab‑ rication of stable bionic superhydrophobic surfaces, Advanced Materials, 18 (2006) 767–770. [52] Qu M., Zhang B., Song S., Chen L., Zhang J., Cao X., Fabrication of superhy‑ drophobic surfaces on engineering materials by a solution‑immersion process, Advanced Functional Materials, 17 (2007) 593–596. [53] Zhu Y., Li J., Wan M., Jiang L., Superhydrophobic 3D microstructures assembled from 1D nanofibers of polyaniline, Macromolecular Rapid Communications, 29(3) (2008) 239–243. [54] Jiang P., Zhou J.‑J., Fang H.‑F., Wang C.‑Y., Wang Z.‑L., Xie S.‑S., Hierarchical shelled ZnO structures made of bunched nanoware arrays, Advanced Functional Materials, 17(8) (2007) 1303–1310. [55] Li H., Bian Z.F., Zhu J., Zhang D.Q., Li G.S., Huo Y.N., Li H., Lu Y.F., Mesoporous titania spheres with tunable chamber structure and enhanced photocatalytic activity, Journal of American Chemical Society, 129(27) (2007) 8406–8407. [56] Srinivasan S., Praveen V.K., Philip R., Ajayaghosh A., Bioinspired superhy‑ drophobic coatings of carbon nanotubes and linear π systems based on the “bottom‐up”, Self‐Assembly Approach, Angewandte Chemie, International Edition, 47(31) (2008) 5750–5754.

70  Introduction to Ceramics [57] Safaee A., Sarkar D.K., Farzaneh M., Superhydrophobic properties of sil‑ ver‑coated films on copper surface by galvanic exchange reaction, Applied Surface Science, 254(8) (2008) 2493–2498. [58] Li Y., Huang X.J., Heo S.H., Li C.C., Choi Y.K., Cai W.P., Cho S.O., Superhydrophobic bionic surfaces with hierarchical microsphere/SWCNT composite arrays, Langmuir, 23(4) (2007) 2169–2174. [59] Li Y., Li C., Cho S.O., Duan G.T., Cai W.P., Silver hierarchical bowl‑like array:  synthesis, superhydrophobicity, and optical properties, Langmuir, 23(19) (2007) 9802–9807. [60] Cai W., Li Y., Cao B., Duan G., Sun F., Li C., Jia L., Two‑dimensional hierar‑ chical porous silica film and its tunable superhydrophobicity, Nanotechnology, 17 (2006) 238–243. [61] Zhu Y., Hu D., Wan M.X., Jiang L., Wei Y., Conducting and superhydro‑ phobic rambutan‑like hollow spheres of polyaniline, Advanced Materials, 19 (2007) 2092–2096. [62] Ma M., Mao Y., Gupta M., Gleason K.K., Rutledge G.C., Superhydrophobic fabrics produced by electrospinning and chemical vapor deposition, Macromolecules, 38(23) (2005) 9742–9748. [63] Liu B., He Y., Fan Y., Wang X., Fabricating super‐hydrophobic lotus‐leaf‐ like surfaces through soft‐lithographic imprinting, Macromolecular Rapid Communications, 27(21) (2006) 1859–1864. [64] Yan X.B., Tay B.K., Yang Y., Po W.Y.K., Fabrication of three‑dimensional ZnO−carbon nanotube (CNT) hybrids using self‑assembled CNT micropa‑ tterns as framework, The Journal of Physical Chemistry C, 111(46) (2007) 17254–17259. [65] Gan W.Y., Lam S.W., Chiang K., Amal R., Zhao H., Brungs M.P., Novel TiO2 thin film with non‑UV activated superwetting and antifogging behav‑ iours, Journal of Materials Chemistry, 17 (2007) 952–954. [66] Garcia N., Benito E., Guzman J., Tiemblo P., Use of p‑toluenesulfonic acid for the controlled grafting of alkoxysilanes onto silanol containing sur‑ faces:  preparation of tunable hydrophilic, hydrophobic, and super‑hydro‑ phobic silica, Journal of the American Chemical Society, 129(16) (2007) 5052–5069. [67] Wang F., Song S., Zhang J., Surface texturing of porous silicon with capillary stress and its superhydrophobicity, Chemical Communications, 2009 (2009) 4239–4241. [68] Hsieh C.‑T., Chen W.‑Y., Wu F.‑L., Hung W.‑M., Superhydrophobicity of a three‑tier roughened texture of microscale carbon fabrics decorated with sil‑ ica spheres and carbon nanotubes, Diamond Related Materials, 19(1) (2010) 26–30. [69] Burkarter E., Saul C.K., Thomazi F., Cruz N.C., Roman L.S., Schreiner W.H., Superhydrophobic electrosprayed PTFE, Surface and Coatings Technology, 202(1) (2007) 194–198. [70] Guo Z., Liang J., Fang J., Guo B., Liu W., A novel approach to the robust Ti6Al4V‐based superhydrophobic surface with crater‐like structure, Advanced Engineering Materials, 9(4) (2007) 316–321.

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SHORT QUESTIONS 1. Classify ceramic and write down the terminologies of ceramics. 2. Define and classify traditional ceramics with appropriate examples. 3. Indicate the name of raw materials used in the preparation of ceramic glazes and discuss their functional activities in making glazes. 4. Describe self‑cleaning coating method with an example. 5. Explain sol–gel technique with a block diagram. Mention pros and cons of this process. 6. Elucidate electrochemical and EPD technique. Cite one difference between them. 7. Describe spin coating method with their advantages and disadvantages. 8. What is glass? Mention different types of glasses in terms of scientific revolution. 9. What is refractory? Categorize refractory materials based on chemical composition. Comment on different properties of refractory materials. 10. Define cement and concrete. Describe several applications of cement and concrete. 11. Write a short note on: a. Glaze b. Thermal barrier coatings c. Environmental carrier coatings d. Advanced ceramic

Chapter 3

Properties of ceramics

LEARNING OBJECTIVES • • • •

To understand different properties of ceramics To realize the basic concepts of mechanical properties of ceramics To know the fundamentals of thermal and optical properties To recognize the essentials of electrical and magnetic properties

The physical properties of any ceramic substance depend upon its crystal‑ line structure and chemical composition. Solid state chemistry exposes the fundamental concept of physical properties of ceramics such as strength, hardness, toughness, thermal stability, electrical conductivity, magnetic phenomena, and polarizability, which are responsible for different ceramic properties such as mechanical strength, thermal properties, dielectric constant, magnetic properties, and optical [1–4]. Hence, in this chapter, we will discuss all the significant properties of ceramics mentioned above. 3.1  MECHANICAL PROPERTIES Generally, ceramics have huge applicability in several fields but based on their mechanical properties, they have limited applications. The main drawback of ceramics is its brittle nature due to catastrophic failure [5–7].

3.1.1  Plastic deformation Although plastic deformation is not a prominent feature in ceramics, this section provides a brief explanation of plastic deformation for contextual understanding. Plastic deformation is defined as the permanent distortion of a material ensued after the release of the load applied on it, due to stable atomic displacements. For crystalline and amorphous ceramics, plastic deformation is different [6,8,9]. In the case of crystalline ceramics (ionic bonding), plastic deforma‑ tion is caused due to slip (dislocation movement) in crystallographic plane of 72

DOI: 10.1201/9781003470571-3

Properties of ceramics  73

ceramics in response to an applied shear stress. On the contrary, for amor‑ phous ceramic (covalent bonding), plastic deformation does not occur by slip as they do not follow regular atomic structure. It had better to say that amor‑ phous ceramics deform due to the effect of viscous flow. In the case of viscous flow, atoms or ions slide past one another by the breaking and reforming of interatomic bonds while a shear stress is applied over a body. Microscopically, the viscous flow of fluid is clarified and established in Figure 3.1 [10,11]. Viscosity is the characteristic property of viscous flow and is a measure of resistance to deformation. In Figure 3.1, a liquid is placed between two flat and large parallel plates with area A, separated by a distance y. Shear stresses are being imposed on the liquid by the plates. The system is initially at rest (t 5%), a given percentage might work well but a slightly higher amount can cause drastic change on the ceramic surface. To avoid this problem, sometimes people do a line blend trying a range of percentages to determine an optimal amount. In glazes, rutile can be quite sensitive to the presence of opacifiers. While an unspecified glaze might appear quite stunning, the addition of a zir‑ con opacifier will alter its appearance significantly because the variegation imparted is dependent on the glaze having depth and transparency or trans‑ lucency. Strangely rutile and tin, another opacifier, can produce some very interesting reactions. In these cases, the tin appears to react in the crystal formation rather than opacify the glaze. Rutile powder, although its color makes it appear to be a very crude ground mineral, normally contains >90% TiO2 . The mineralogy and signif‑ icant other impurities in rutile are a major factor in the way it acts in glazes and are not easily duplicated using a blend of other things. Sometimes the special effects that rutile produce in glazes are also partly a product of a coarser grade. These likewise cannot be easily duplicated by more refined materials. Although rutile will normally stain a glaze brown or yellow, its crystallization effects can significantly lighten the color of iron glazes. Higher amounts of rutile in stoneware glazes will often contribute to glaze imperfections.

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4.2.11  Titanate bodies Titanate ceramics are available in several forms such as barium titanate, alumi‑ num titanate, and lead titanate based on the properties. Ferroelectric ceramics are a special group of minerals that have ferroelectric properties: the strong dependence on the dielectric constant of temperature, electrical field, the pres‑ ence of hysteresis, and others. Barium titanate is one of the examples of fer‑ roelectric ceramics with a photorefractive effect and piezoelectric properties. Barium titanate (BaTiO3) is a white powder and is transparent as larger crystals. It is used in capacitors, electromechanical transducers, and nonlin‑ ear optics. Barium titanate can be synthesized either by sol–hydrothermal method or by heating barium carbonate and titanium dioxide via liquid phase sintering. The solid barium titanate can exist in five phases, list‑ ing from high temperature to low temperature: hexagonal, cubic, tetrago‑ nal, orthorhombic, and rhombohedra crystal structure. The entire phases exhibit the ferroelectric effect except the cubic phase. It is soluble in many acids including sulfuric, hydrochloric, and hydrofluoric. It is insoluble in alkalis and water. In the pure form, it is an electrical insulator. However, when doped with small amounts of metals, most notably scandium, yttrium, neodymium, samarium, and so on, it becomes semiconducting. As a semiconductor, it exhibits positive temperature of coefficient of resistiv‑ ity (PTCR) properties in the polycrystalline form. This means at a certain tem‑ perature, called the Curie temperature, the material will exhibit an increase in resistivity, the increase typically being several orders of magnitude. The Curie temperature can, to some extent, be controlled by the dopant. At the Curie temperature, barium titanate undergoes a phase change from tetrahedral to cubic. It has also been reported that single crystals of barium titanate exhibit negative temperature coefficient of resistivity (NTCR) properties. Aluminum titanate is a ceramic material consisting of a mixture of alu‑ mina (Al2O3) and titania (TiO2) forming a solid solution with stoichiomet‑ ric proportion of the components like Al 2TiO5. It is prepared by heating a mixture of alumina and titania at a temperature above 1,350°C. The powder is then sintered at a temperature in the range of 1,400°C–1,600°C in the air atmosphere. Pure aluminum titanate is unstable at temperatures above 750°C when the solid solution decomposes into two separate phases that are Al2O3 and TiO2 . Aluminum Titanate ceramics are doped with MgO, SiO2 , and ZrO2 to stabilize the solid solution structure. The distinctive property of aluminum titanate ceramics is their high ther‑ mal shock resistance, which is a result of very low coefficient of thermal expansion. The following characteristics are typical for aluminum titanate ceramics: • Low coefficient of thermal expansion • Low modulus of elasticity

Traditional ceramics  129

• • • • •

High thermal shock resistance Low thermal conductivity Low wettability in molten non‑ferrous metals Good chemical resistance Good wear resistance

The disadvantage of aluminum titanate ceramics is the relatively low mechanical strength caused by micro‑cracks formed as a result of anisot‑ ropy of thermal expansion along the three primary axes of the crystal lattice. Aluminum titanate ceramic materials ceramics are used for manufac‑ turing crucibles, launders, nozzles, riser tubes, pouring spouts and ther‑ mocouples for non‑ferrous metallurgy, portliner, and cylindrical liners in automotive engines, master molds in the glass industry, spacing rings of catalytic converters. Lead titanate (PbTiO3) has been considered to be one of the most impor‑ tant members of this family. It has a high Curie temperature, high pyroelec‑ tric coefficient, low dielectric constant, and high spontaneous polarization. PbTiO3 is a ferroelectric ceramic that has not been proven to be a techno‑ logically important material by it but is a significant component material in electronics such as capacitors, ultrasonic transducers, thermistors, and optoelectronics. It is also a promising material for pyroelectric infrared detector applications because of its large pyroelectric coefficient and rela‑ tively low permittivity. PbTiO3 has also been extensively used in a range of piezoelectric applications. At ambient temperature, the material has a strong anisotropy which develops during cooling through the cubic‑tetragonal phase transition of approximately 490°C. The conventional method of synthesizing PbTiO3 relies on the solid‑state reaction between TiO2 and PbCO3 at high temperatures. The conventional solid‑state reaction has a tendency to produce a coarse PbTiO3 powder with compositional homogeneity and a degree of particle agglomeration if the processing parameters are not carefully optimized. Therefore, many chemistry‑based processing routes, including co‑precipitation, sol‑gel syn‑ thesis, hydrothermal, and citrate routes, have been devised for the prepara‑ tion of an ultrafine, sintering‑reactive PbTiO3 powder.

4.2.12  Zircon bodies Zirconia (ZrO2) is a ceramic material with adequate mechanical properties for manufacturing of medical devices. Zirconium oxide occurs as mono‑ clinic, tetragonal, and cubic crystal forms. Densely sintered parts can be manufactured as cubic and/or tetragonal crystal forms. The fine grain size of zirconia enables the material to have sharp edges and very smooth sur‑ faces. To prevent, control, and stabilize structural changes, several different oxides such as magnesium oxide (MgO), calcium oxide (CaO), or yttrium oxide (Y2O3) can be dispersed into the zirconia crystal structure during

130  Introduction to Ceramics

production. Other stabilizers sometimes used are cerium oxide (CeO2), scandium oxide (Sc2O3), or ytterbium oxide (Yb2O3). Zirconium oxide (ZrO2) has gained importance in the last few years due to its • • • • • • • •

high fracture toughness, high density, thermal expansion similar to cast iron, extremely high bending strength and tensile strength, high resistance to wear and corrosion, low thermal conductivity, oxygen ion conductivity, and very good tribological properties.

Zirconia is used in a wide range of applications, such as precision ball valve (seats and balls), valves and impellors, pump seals, oxygen sensors, high‑density grinding media, fuel cell membranes, thread guides, medi‑ cal prostheses, cutting blades, gears, metal forming, radio frequency heat‑ ing susceptors, metrology components, bearings, bushes, and drive shafts. Zirconia‑based ceramics are also used in many other applications. For example, they can be used as auxiliaries in welding processes, as tools for wire forming, as oxygen measurement cells, as insulating rings in thermal processes (Figure 4.5a), and as materials for crowns and bridges in the den‑ tal industry (Figure 4.5b). These ceramics have been developed to such an extent that infinite designs of microstructure are now possible by controlling fabrication route, composition, thermal treatment, and final machining.

4.2.13  Lava bodies Magma is a mixture of molten or semi‑molten rock, volatiles, and solids that is found beneath the surface of the Earth and is expected to exist on

Figure 4.5  Applications of zirconia ceramics: (a) insulating rings and (b) dental framework: crowns and bridges.

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other terrestrial planets and some natural satellites. Besides molten rock, magma may also contain suspended crystals, dissolved gas, and sometimes gas bubbles. Lava is the molten rock expelled by a volcano during an eruption. The two most important properties of lava are its viscosity and the amount of gases dissolved in the liquid rock. Viscosity is a term that describes the flu‑ idity of the lava. The fluidity of lava depends on the pressure; the composi‑ tion, especially the amount of silica (SiO2 or quartz) melted in the lava and nature of the molten rock; and its temperature. Silica molecules have the tendency to stick together to form long chains. These long chains literally get tangled together and make it difficult for molecules in the melt to slide past each other. This slowing of movement at the molecular level makes the whole lava less fluid. A small difference in silica content can make a huge difference in viscosity: lavas with about 70% silica (called rhyolites) are stiffer than lavas with about 50% silica (called basalts). Basaltic lava is used as a flux in ceramic masses and glazes, for the production of glass ceramics (Figure 4.6). 4.3  WHITEWARE PRODUCTS Whiteware is the term used to describe a product that has a white body. The term is rather loosely applied to some wares that are not strictly white but are made of the same kind of raw materials and by the same processes. It is also used for products that have white bodies but are covered with colored glazes. There are two major types of whiteware bodies, vitreous and semi‑vitreous.

Figure 4.6  Volcanic or lava glazes over a traditional bottom glaze.

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Vitreous whiteware is one which has been heated to a point where basi‑ cally all of the pores of the body have been filled with the glassy bond and the sintered body has practically zero absorption, for example, porcelain tableware, china tableware, sanitary ware, and so on. Vitreous whiteware has superior physical properties than that of semi‑vitreous bodies but the former is quite expensive. A semi‑vitreous body is one that has been sintered only to partial ­verification. This kind of whitewares does not provide good strength to the body; moreover, glazing is necessary to make it practical and waterproof. Unlike vitreous bodies, these are cheap, for example, semi‑vitreous porce‑ lain ­tableware and semi‑vitreous china ware. Whiteware is made from white‑burning raw materials such as kaolin, white‑burning ball clays, flint, and quartz. In addition, numerous raw materials of lesser importance are used among which are, dolomite, talc, magnesia, zirconia, calcium fluoride, and barium oxide, and so on. The kaolin is the most important raw material which usually forms the bulk of the body. Feldspar is used as a flux which melts first upon firing and forms a viscous liquid. For a vitreous body, feldspar acts as a solvent and a large portion of other materials dissolve in it whereas for semi‑vitreous body, the feldspar melts but forms a glassy matrix over the body rather than acting as a solvent. Fluxes promote the formation of a glassy bond during vitri‑ fication. Fluxes provide alkaline (Na 2O and K 2O) or alkaline earth (CaO or MgO) to the composition, which promote glass formation and reduce glass viscosity during firing, which serves to enhance vitrification. The level of fluxing components must be optimized to achieve the desired fired property in the selected firing range. Other materials such as spodumene (Li,Al)SiO3, calcined bone ash (Ca(PO 4)2), limestone (CaCO3), dolomitic limestone (Ca,Mg)CO3, wollastonite (CaSiO3), and talc (Mg3Si4O10(OH)2) are also used as fluxes. In vitreous bodies, flint acts as a source of SiO2 and produces silica‑rich glassy bond in the fired ware while flint acts as a relatively inert filler which provides a network to maintain the shape of the ware during the sintering operation. Ball clays are commonly used in whitewares where higher plas‑ ticity is required. Ball clays typically fire to a buff or tan color due to their iron or titanium content.

4.3.1  Manufacturing process and properties Figure 4.7 gives an idea about the steps involved in the preparation of white‑ ware bodies. The most common method of manufacturing whiteware bod‑ ies is to first grind the non‑plastic materials together with a small amount of clay in a ball mill. This grinding is done wet, and it is continued until the particle size of the materials is fine enough to give the desired results. A small amount of clay is then added to the other raw materials such as flint or feldspar during grinding to prevent them from settling from water

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Figure 4.7 Whiteware manufacturing process steps in ceramic industries.

and forming a desired hard shape. Shaping of whiteware bodies can be done either using conventional techniques such as slip casting, plastic, and dust pressing (pressing high‑moisture dust into steel molds at high pressures). Jiggering is also employed in the manufacture of tableware. Jiggering involves the mixing of a plastic mass and turning it on a wheel beneath a template to a specified size and shape. After giving the desired shape to the material, they are usually dried in driers and follow the next step, called fettling. Fettling is the operation of shaving or smoothing the surface of undried clay ware. After that, the material undergoes sintering/firing pro‑ cess to achieve a hard and rigid shape. After the ware has been dipped in the liquid glaze and allowed to dry, the excess of dry glaze is scraped out of the hollows and perforations. The surface is then retouched with a brush in spots for final decoration. The physical properties of fired whiteware ceramics are established by the product requirements, and they can vary widely depending on the spe‑ cific formulation, additives, forming method, and firing. A list of key physi‑ cal properties is given in Table 4.1. One of the vital physical properties of all types of whiteware ceramics is water adsorption or porosity. Low values of water adsorption or porosity indicate a high degree of vitrification. The mechanical strength of a whiteware product is important for several types of applications. For sanitary ware, the product must be able to resist

134  Introduction to Ceramics Table 4.1  Key physical properties of whitewares Product type

Important physical properties

Tile Sanitary ware Tableware Electrical porcelain

Water adsorption (porosity), strength, abrasion resistance, chemical resistance, dimensional tolerance, surface finish, and color Water adsorption (porosity), strength, thermal shock resistance, resistance to staining, and color Impact resistance, abrasion resistance, chemical and staining resistance, resistance to metal marking, resistance to lead and cadmium release and color Mechanical strength, water adsorption (porosity), and electrical resistance

the stresses applied during normal use, while for tile, the product must be able to stand the compressive load applied to the floor. Mechanical prop‑ erties of several whiteware products along with the standard ASTM tests are given in Table 4.2. For electrical porcelain applications, the electrical properties of the whiteware ceramic must be measured, including dielectric strength and electrical resistively.

Table 4.2  Typical mechanical properties of whiteware ceramics

Property Water absorption, % Bulk density, g/ cm3 Compressive strength, MPa Modulus of rupture, MPa Modulus of elasticity, GPa Thermal expansion coefficient, /oC, 20°C–500°C Thermal expansion coefficient, /oC, 20°C–1,000oC

Hard Bone Earthenware porcelain china 6–8 2.20

Normal Hard Hotel electrical electrical China porcelain porcelain Standard tests

0.0–0.5 0.0–1.0 0.1–0.3 2.40

2.70

2.60

400 55–72

39–69 97–111 82–96

55

69–79

96

82

7.3–8.3

8.4

7.3–8.3

5.7

3.5–4.5

0.0

0.0

2.40

2.77

700

700

105

175

6.7

ASTM C 373‑16e1 ASTM C 373‑16e1 ASTM C 733 ASTM C 648, ASTM C 674 ASTM C 623, ASTM C 674 BS EN ISO 10545‑8:2014; ASTM C 372 BS EN ISO 10545‑8:2014; ASTM C 372

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4.3.2  Whitewares at home 4.3.2.1 Tableware

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4.3.2.2 Kitchenware

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4.3.2.3 Flame

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4.3.2.4  Resistant ware

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4.3.2.5  Art ware

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4.3.2.6 Containers

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4.3.3  Construction use 4.3.3.1  Floor tile

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4.3.3.2  Wall tiles

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4.3.3.3  Sanitary ware

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4.3.4  Electrical use 4.3.4.1  Low‑tension insulators

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4.3.4.2  High‑tension insulators

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4.3.4.3  High‑frequency, low‑loss insulators

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4.3.5  Industrial use 4.3.5.1  Abrasion resistance

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4.3.5.2  Chemical resistance

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4.3.5.3  Heat resistance

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4.4  HEAVY CLAYWARE PRODUCTS Heavy clayware products are planned for use in building construction. Typical heavy clayware products are building brick, paving brick, terra‑cotta facing tile, roofing tile, and drainage pipe, and so on. These objects are made from commonly occurring natural materials, which are mixed with water, formed into the desired shape, and fired in a kiln to give the clay mixture a permanent bond. Finished heavy claywares display vital properties such as load‑bearing strength, resistance to wear, resistance to chemical attack, attractive appearance, and an ability to take a decorative finish. Advantages: • • • • • • •

Highly water‑repellent silicone‑resin network Water‑vapor permeability Beading behavior repels dirt and prevents unsightly salt efflorescence Easy to use and process Retarded algal growth Prolonged material durability Flower pots

4.5 COMPOSITIONS, PROPERTIES AND APPLICATIONS A brick is a building material used to make walls, pavements, and other elements in masonry construction. Conventionally, the term brick referred to a unit composed of clay. A brick can be composed of clay‑bearing soil, sand, and lime, or concrete materials. Normally, bricks contain the follow‑ ing ingredients: Silica (sand): 50%–60% by weight Alumina (clay): 20%–30% by weight Lime: 2%–5% by weight FeO (iron oxide): ≤7% by weight Magnesia: