Handbook of Liquids-Assisted Laser Processing 0080444989, 9780080444987, 9780080555041

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HANDBOOK OF LIQUIDS-ASSISTED LASER PROCESSING

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HANDBOOK OF LIQUIDS-ASSISTED LASER PROCESSING ARVI KRUUSING Department of Electrical and Information Engineering University of Oulu, Finland

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Elsevier The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam,The Netherlands First edition 2008 Copyright © 2008, Elsevier Ltd. All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN–13: 978-0-08-044498-7 For information on all Elsevier publications visit our web site at books.elsevier.com Printed and bound in Great Britain 08 09 10 10 9 8 7 6 5 4 3 2 1

C ONTENTS

Foreword

ix

1. Introduction

1

1.1 1.2 1.3

LALP Chronology Laser Processing and Analysis of Liquid Systems That Are Not Covered in This Book Inventions in Liquids-Assisted Laser Processing

2. Cleaning 2.1 2.2

2.3

2.4

2.5

Introduction Principles of Liquids-Assisted Laser Cleaning 2.2.1 Particles removal by frontside laser irradiation (steam laser cleaning) 2.2.2 Particles removal by backside laser irradiation 2.2.3 Removal of particles by laser-generated acoustic waves in liquid 2.2.4 Liquid-assisted laser shock cleaning 2.2.5 Removal of particles by bubble collapse induced flow 2.2.6 Removal of surface layers by laser ablation/spallation in liquid 2.2.7 Removal of frozen gas and liquid layers from optical surfaces 2.2.8 Laser-generated shock wave enhanced scale removal 2.2.9 Removal of organic contaminants by water decomposition products 2.2.10 Cleaning of surfaces through contaminants dissolution in laser-generated supercritical solution 2.2.11 Dehydroxylation of a silica glass surface 2.2.12 Ice-assisted laser particles removal Particles on Solid Surfaces 2.3.1 Adhesion phenomena and adhesion forces 2.3.2 Adhesion force theories considering the deformation of the particle and the substrate Experimental Techniques in Laser Wet/Steam Cleaning Research 2.4.1 Preparation of particles covered surfaces 2.4.2 Application of liquid and monitoring the liquid film thickness and condition 2.4.3 Complete cleaning systems 2.4.4 Measuring and monitoring techniques in steam laser cleaning Physics and Phenomenology of Liquids-Assisted Laser Removal of Particles from Surfaces 2.5.1 Detailed description of the standard steam cleaning process 2.5.2 Optical effects 2.5.3 Acceleration and inertial effects 2.5.4 Heating and phase change (absorbing substrate, non-absorbing liquid) 2.5.5 Hydrodynamic effects

4 6 8

11 11 12 12 12 13 13 13 14 15 16 16 16 16 17 17 17 25 30 30 31 33 33 37 37 37 37 39 43 v

vi

Contents

2.5.6 2.5.7

Particles removal threshold and efficiency in steam laser cleaning Effect of capillary condensed water in ‘dry’ laser cleaning

3. Shock Processing 3.1 3.2 3.3

3.4

3.5

Introduction Residual Stresses and Their Measurement Laser Shock Peening 3.3.1 Introduction 3.3.2 Experimental techniques 3.3.3 Shock pressure 3.3.4 Shock propagation and wave phenomena 3.3.5 Shock-induced changes in materials 3.3.6 Mathematical models of laser shock peening 3.3.7 Applications of laser peening Laser Shock Forming and Cladding 3.4.1 Forming 3.4.2 Cladding Densification of Porous Materials

4. Subtractive Processing 4.1

4.2

4.3 4.4

4.5

4.6

Frontside Machining 4.1.1 Introduction 4.1.2 Frontside micromachining 4.1.3 High-power laser underwater and water-assisted cutting Liquid-Jet-Guided Laser Beam Machining 4.2.1 Applications and performance 4.2.2 Molten salt-jet-guided laser beam Water at Backside of an Opaque Material Backside Machining of Transparent Materials 4.4.1 Introduction 4.4.2 Technologies, phenomenology, and etching mechanisms Machining of Liquid-Containing Materials 4.5.1 Rock drilling 4.5.2 Biological materials Laser Cleaving of Crystals in Water and of Water-Containing Crystals 4.6.1 Breaking of single-crystal silicon wafers 4.6.2 Cleaving of protein crystals

5. Generation and Modification of Particles 5.1 5.2 5.3 5.4

Introduction Optical Properties of Small Particles Experimental Techniques of Particles Generation Metal Particles 5.4.1 Introduction 5.4.2 Mechanisms determining the particles size 5.4.3 Modification of suspending particles by laser irradiation

44 45

69 69 70 77 77 77 81 82 84 88 103 140 140 140 141

143 143 143 145 167 171 174 174 177 177 177 181 202 202 202 203 203 203

209 209 210 213 214 214 214 217

vii

Contents

5.5 5.6 5.7 5.8

Inorganic Compound Particles 5.5.1 Hydrothermal growth Silicon and Amorphous Carbon Particles Diamond and DLC Particles and Films Organic Particles

6. Surface Modification, Deposition of Thin Films, Welding, and Cladding 6.1

6.2

6.3

Surface Modification 6.1.1 Modification of surfaces of inorganic materials 6.1.2 Modification surfaces of organic materials Deposition and Transfer of Thin Films 6.2.1 Laser ablation deposition in water vapour 6.2.2 Laser ablation deposition using a liquid target 6.2.3 Laser ablation deposition using frozen target 6.2.4 Forward transfer from solution (LIF T, MDW) Welding and Cladding Under Water

7. Physics and Chemistry of Laser–Liquid–Solid Interactions 7.1

7.2

7.3

7.4 7.5

7.6

Laser Beams and Their Propagation 7.1.1 Properties of Gaussian beams 7.1.2 Reflection of light 7.1.3 Propagation of Gaussian beams Phase Change Phenomena 7.2.1 Overall phenomenology 7.2.2 Vaporization from free liquid surfaces 7.2.3 Nucleation of vapour bubbles 7.2.4 Bubble dynamics Optical Breakdown of Liquids and Plasma 7.3.1 Photoionization of a dielectric liquid 7.3.2 Cascade ionization (avalanche ionization) 7.3.3 Photoionization absorption coefficients of atoms 7.3.4 Thermal ionization 7.3.5 Diffusion loss of electrons from the plasma 7.3.6 Recombination loss 7.3.7 Thermal conductivity of the plasma 7.3.8 Rate equation for free electrons 7.3.9 Internal energy density of electrons and particles in plasma 7.3.10 Energy balance equation for electrons 7.3.11 Heat flux conducted from plasma to adjacent matter 7.3.12 Dependence of optical breakdown threshold on laser pulse length 7.3.13 Factors affecting the breakdown threshold in liquids 7.3.14 Temperatures and pressures at laser breakdown and ablation in water Shock Waves in Liquids and Solids Laser-Induced Reactions of Carbon with Organic Solvents and Water 7.5.1 Reactions of carbon with organic solvents 7.5.2 Reactions of carbon with water Behaviour of Oxides in High Temperature Water and Water Vapour

240 240 250 250 258

261 261 261 262 262 262 266 272 273 277

281 281 282 285 287 288 288 289 290 292 295 295 296 297 297 297 298 298 298 299 299 300 300 300 301 302 306 306 308 308

viii

Contents

8. Liquids and Their Properties 8.1 8.2 8.3

Introduction Properties of 100 Selected Liquids Properties of Water

315 315 332 379

References

387

Glossary

423

Subject index

441

Liquids

451

F OREWORD

The aim of this book is to present a reference for the research work done during the last two decades in laser processing assisted by neutral liquids (LALP). At present, the total number of scientific-technical papers dealing with LALP exceeds 700, and of patents 500, which justifies the need for a comprehensive reference. The book does not systematically cover the use of lasers in medicine, despite the fact that organs and tissues contain a significant amount of liquid. Nor does it cover laser etching in reactive liquids and laser deposition form solutions. References to these kinds of processing are given in the introduction (Chapter 1). The four main areas of LALP are: (i) laser peening, where water is used as a safe confining medium conforming with the workpiece; (ii) cutting and drilling, where water is also preferably used, in order to cool the workpiece and to prevent the redeposition of debris; (iii) the generation of colloidal particles in water or in organic solvents; (iv) the removal of microparticle contamination from solid surfaces through laser vaporization of a liquid film (water and alcohols) on the surface. Altogether, about 70 different liquids have been used until now, including liquid metals and liquefied gases. The principles of organising the data in the book are as follows: •

•

• • •

Essential data of research reports about the main four kinds of processing (i) to (iv) are presented in chronological tables, with an accent on the materials processed or achieved. Concise receipts of the processes are presented. As far as possible, the experimental conditions and results are described quantitatively. General principles, experimental techniques, main phenomena, and mechanisms of every kind of processing are described by text and graphics. Related topics, such as residual stress measurement and alternative processing methods are dealt with to some extent in order to help the readers from other areas or students. General topics on the physics and chemistry of laser–liquid–solid interactions are gathered in a special chapter (Chapter 7). A comprehensive table of 61 properties of 100 liquids has been included. In addition to the liquids used in LALP, several common solvents and cryoliquids are added. The book contains a glossary with about 330 terms. It is intended to help the less prepared readers, especially students, who do not have previous experience in this special field.

The book contains material from literature sources originally acquired for the following research projects: Project 0140215s98 (Estonian Ministry of Education), Projects 4512 and 5864 (Estonian Science Foundation). The original figures were drawn by CorelDraw software licensed to the University of Oulu. I am grateful to many researchers, especially toYuji Sano, Stephan Roth,Walter Huber, Boris Luk’yanchuk, Vladimir P. Zharov, Tianqing Jia, and Dongsik Kim who provided me with original figures; to a number of publishers and authors who kindly permitted the use of their material in this book, and to the team Elsevier Science for their patience and good cooperation. Invaluable help in the manuscript preparation was provided by my son Aavo Kruusing and my daughter Airi Männamaa. Arvi Kruusing, Oulu, April 2007

ix

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C H A P T E R

O N E

Introduction

Contents 1.1 LALP Chronology 1.2 Laser Processing and Analysis of Liquid Systems That Are Not Covered in This Book 1.3 Inventions in Liquids-Assisted Laser Processing

4 6 8

There are occasions where the workpiece at laser processing is in contact with liquids (e.g. in natural bodies of water, nuclear reactors, boreholes, etc.); the workpiece may contain liquid in its normal state (e.g. moisture in building materials, wood, paper) or the liquid may be applied to workpiece in order to enhance the processing or to achieve some other useful effect. Very often the liquid present is water as most abundant and safe (see Fig. 1.1).

(a)

(b)

(c)

H2O

(d)

(e)

(f)

Figure 1.1 Examples of liquid presence at laser materials processing: (a) processing in water environment; (b) workpiece/material is immersed into/suspended in liquid; (c) liquid is applied onto surface of workpiece; (d) liquid acts as lightguide; (e) processing in vapour; and (f) material contains capillary or chemically bonded liquid. Handbook of Liquids-Assisted Laser Processing ISBN-13: 978-0-08-044498-7

© 2008 Elsevier Ltd. All rights reserved.

1

2

Handbook of Liquids-Assisted Laser Processing

Table 1.1 Main physical principles of LALP (in ascending order pursuant to increasing laser–liquid interaction intensity). See also Fig. 2.24.

Method

Why used

Light-matter interaction needed

Desired changes in the workpiece

Liquid as waveguide or lens

Concentration of light without solid optical elements

Light reflection/ refraction/selffocusing in liquid

Various

Liquid as photomask; backside etching of transparent materials

Liquid conforms with the surface of workpiece

Light absorption in liquid

Various

Photochemical processes (oxidation, etc.)

Liquid serves as the source of chemical species

Photoactivation of liquid

Photochemical

Removal of particles from surfaces

Lower risk of surface damage

Vaporization of liquid only

No changes

Generation of micro/nanoparticles

Cleaner particles – no extra chemicals used; rapid and simple process

Vaporization of both workpiece and of liquid

Vaporization

Subtractive processing (cutting, drilling, removal of oxide layers, etc.)

Practically no debris redeposition in the work zone; lower thermal load on the workpiece

Vaporization of both workpiece and of liquid

Vaporization

Shock processing (peening, forming, densification)

Shock pressures up to 10 times larger than in gas or in vacuum; simpler and safer than in case of solid confinement media

Vaporization and ionization of liquid only

Plastic deformation

Besides water, about 70 other liquids are used in laser materials processing, mostly organic solvents. In high-energy processing regarding the peening without exception water is used as the safest and cheapest liquid, in microprocessing (micromachining, particle generation) organic solvents are often the choice. Liquids metals (e.g., Hg, Ga) and molten salts (e.g., NaNO3 , KNO3 ) have also been used. The book also refers to some cases where frozen liquid layers on the surfaces are laser ablated (water ice, solid N2 , and CH4 a.o.). Table 1.1 lists the main types of liquids-assisted laser processing (LALP) andTable 1.2 provides a comparison of relevant advantages and disadvantages. Experiments of laser irradiation of liquid–solid interfaces started soon after the invention of lasers in 1960s, but systematic research on LALP began at the end of 1980s (see Figs 1.2–1.4). At the beginning of 1990s four main directions emerged: (i) laser peening, (ii) liquids-assisted laser micromachining, especially the backside etching of optical components, (iii) removal of microparticles from silicon wafers, and (iv) generation of nanoparticles in liquids.

3

Introduction

Table 1.2 Overall advantages and disadvantages of LALP (in comparison with laser processing in vacuum/gas and with alternative kinds of processing; only liquids neutral under normal conditions are considered). Advantages

Disadvantages

• Non-contact (low mechanical load on workpiece) • Flexible and rapid process control • Many process control parameters available in extreme range: laser wavelength, pulse length, fluence, energy density, liquids properties, liquid’s temperature, flow rate, etc. • Can be applied on inclined and curved surfaces (light and liquid conform with sloped and uneven surfaces) • Can be applied inside of tubes, etc. • Can be applied under water (e.g. in nuclear reactors, sea) without the need for local dry zone • High-energetic efficiency if short light pulses are used • Low thermal load on workpiece: narrow HAZ, little damage of biomaterials • Reduced risk of atmosphere contamination by gases and particles • Liquid may serve as a lightguide • Liquid may serve as a source of starting materials (carbon, nitrogen, oxygen), but also of highly reactive species (OH, H2 O2 , F2 , Cl2 ) • At elevated temperatures and pressures the solubility of solids in liquids may increase considerably (dissolution of debris, hydrothermal growth, etc.) • Bubble dynamics and migration generates strong hydrodynamic forces that carry the debris away • Shorter thermal relaxation time than in gas or in vacuum • Laser wavelength is shorter than in vacuum and gases • Self-focusing in liquids may be used for concentration of light.

• Expensive equipment (laser) • Need for auxiliary liquid-handling system • Burn and eye damage hazard by laser light, especially at IR-wavelength • Power loss due to cooling by liquid • Explosion, toxicity, and electronic apparatus damage hazard due to liquid vapours • Explosion hazard due to thermal or photolytical liquid dissociation products (e.g. O2 + H2 ) • Reflection loss at water surface • Light scattering by mist, liquid surface unevenness, thermal gradients, suspended particles and bubbles • Splashes at liquid surface may contaminate the optical components • Light absorption and scattering in liquids is greater than in gases • Corrosion/oxidation (in case of oxygen- or halogen-containing liquids) • Contamination of workpiece with carbon, nitrogen, etc. from liquids • Hydrogen incorporation into workpiece from hydrogen-containing liquids (causes brittleness) • Polymerization of organic liquids • Laser-induced thermal and mechanical shocks are more intense than in gas or vacuum (more dislocations, deformations, or cracking of materials) • Collapse of bubbles may cause surface damage • Process monitoring, modelling, and simulation are more complicated than in gas or vacuum • Lower optical breakdown threshold than in gas (water–air)

80 Number of publications

70 60 50 40 30 20 10

19 7 19 4 7 19 5 7 19 6 7 19 7 7 19 8 7 19 9 8 19 0 8 19 1 8 19 2 8 19 3 8 19 4 85 19 8 19 6 8 19 7 8 19 8 8 19 9 9 19 0 9 19 1 92 19 9 19 3 9 19 4 9 19 5 9 19 6 9 19 7 9 19 8 9 20 9 0 20 0 0 20 1 0 20 2 0 20 3 0 20 4 0 20 5 06

0

Figure 1.2 Development of the number of scientific-technical publications (excl. patents) about LALP. The total number of research reports and reviews referred in this book is about 700.

4

Number of publications

Handbook of Liquids-Assisted Laser Processing

200 180 160 140 120 100 80 60 40 20 0 e

iv

r

bt

Su

Figure 1.3

t ac

o Sh

ck

g

s

le

in

an le C

c rti

Pa

er

th

O

Relative research activity in the four main areas of LALP.

Number of publications

35 30 shock

20

cleaning

15

particles

10

other

5 0 1985

Figure 1.4

subtractive

25

1990

1995

2000

2005

2010

Development of research activities in the main areas of LALP.

1.1 LALP Chronology 1963

G.A. Askar’yan and E.M. Moroz (P. N. Lebedev Physics Institute, Moscow, Russia) propose mechanical momentum generation by laser vaporization on solid targets

1963

R.M. White (General Electric Company, Palo Alto, USA) reports about pressure pulse generated at ruby laser irradiation of aluminium target

1968

Studies of laser peening at Batelle Columbus Laboratories start (Columbus, USA)

1970

Confined ablation-mode laser shock processing reported (N.C. Anderholm – Sandia Laboratories, Albuquerque, USA)

1971

Generation of vacancies in laser-shocked materials reported (S.A. Metz and F.A. Smidt Jr. – Naval Research laboratory,Washington, USA)

1973

Permanent local deformation of laser-shocked metal targets reported (J.D.O’Keefe, C.H. Skeen, and C.M. York – TRW Systems Group and University of California, USA)

1974

Laser shock treatment in water confinement reported (J.A. Fox – US Army Mobility Equipment Research and Development Center, Fort Belvoir, USA)

1974

First laser peening patent issued (P.I. Mallozi and B.P. Fairand – US3850698)

5

Introduction

1975

Laser ablation of various metals in various liquids reported (V.A. Ageev – V.I. Lenin Tadzhik State University, Dushanbe, USSR)

1975

Surface damage of the backside of a glass plate in contact with water due to laser irradiation reported (R.K. Leonov,V.V. Efimov, S.I. Zakharov, N.F. Taurin, and P.A. Yampol’skii – All-Union Scientific-Research Institute of Optophysical Measurements, Moscow, USSR)

1981

Initiation of corrosion pits by laser ablation in electrolyte solution reported (R.K. Ulrich and R.C. Alkire – University of Illinois, Urbana, USA)

1983

Liquid jet–guided laser-enhanced electroplating reported (R.J. von Gutfeld, M.H. Gelchinski, L.T. Romankiw, and D.R. Vigliotti – IBM T. J. Watson Research Center,Yorktown Heights, USA)

1986

Laser cutting of 3-mm thick steel sheet under water reported (R. Schünemann – Universität Hannover, Germany)

1987

Metal ions desorption from silicon surface in water under laser irradiation was reported (E.Yu. Assendel’ft,V.I. Beklemyshev, I.I. Makhonin,Yu. N. Petrov,A.M. Prokhorov, and V.I. Pustovoi – Institute of General Physics, Moscow, Russia)

1988

Start of laser shock processing research in France at Laboratoire pour l’Application des Lasers de Puissance (LALP)

1988

Photo-resist particles removal from solid surfaces due to acoustic wave generated by absorption of the laser light on the free surface of water was reported (E.Yu. Assendel’ft,V.I. Beklemyshev, I.I. Makhonin, Yu. N. Petrov,A.M. Prokhorov, and V.I. Pustovoi – Institute of General Physics, Moscow, Russia)

1989

Backside drilling of holes and channels in fused silica in contact with water solution of NiSO4 (J. Ikeno,A. Kobayashi, and T. Kasai – Japan)

1990

Steam Laser Cleaning – removal of Al2 O3 particles from Si wafer, covered with water film reported (K. Imen, S.J. Lee, and S.D. Allen – Center for Laser Science & Engineering, Iowa City, USA)

1991

Densification of porous materials by laser shock reported (D. Zagouri, J.- P. Romain, B. Dubrujeaud, and M. Jeandin – France)

1992

Formation of diamond particles at laser irradiation of graphite in benzene reported (S.B. Ogale, A.P. Malshe, S.M. Kanetkar, and S.T. Kshirsagar – Poona University, Pune, India)

1993

Water jet–guided laser technology was invented by B. Richerzhagen – Eidgenössische Technische Hochschule Lausanne (ETHL), Switzerland

1993

Generation of colloidal Au and Ni nanoparticles by laser ablation of metal targets in liquids reported (A. Fojtik and A. Henglein – Hahn-Meitner-Insitut, Berlin, Germany)

1995

Conversion of tensile surface residual stresses into compressive by laser peening in water without protective coating using multiple impacts demonstrated (N. Mukai, N. Aoki, M. Obata,A. Ito,Y. Sano, and C. Konagai – Toshiba Corporation,Yokohama, Japan)

1996

Improvement of laser cutting quality of marble by saturating it by water reported (K. Sugimoto,T. Aihara, H. Kamata, and S. Kanaoka – Taisei Corporation and Mitsubishi Electric Corporation, Japan)

1996

Cathodic potential controlled laser ablation of oxide layers in electrolytes reported (R. Oltra, O. Yava¸s, and O. Kerrec – Université de Bourgogne, France)

1996

Reduction of colloidal Ag particles size by laser irradiation reported (A. Takami, H. Yamada, K. Nakano, and S. Koda – University of Tokyo, Japan)

1998

Observation of PbZrTiO3 nanoplatelets growth at laser-irradiated solid–liquid interface (A. Kruusing – Tallinn Technical University, Estonia)

1998

Laser MicroJet® technology was commercialized by Synova S.A. in Lausanne, Switzerland

1998

Conversion of fluorocarbon resin surface from hydrophobic to hydrophilic by laser irradiation under water and aqueous solutions reported (K. Hatao, K. Toyoda, and M. Murahara – Japan)

6

Handbook of Liquids-Assisted Laser Processing

1999

Precise backside laser etching of fused silica in contact with pyrene solution in acetone reported (J. Wang. H. Niino, and A. Yabe – National Institute of Materials and Chemical Research,Tsukuba, Japan)

1999

Laser peening was applied to combat against stress corrosion cracking in Japanese nuclear power reactors

2000

Microscale laser shock processing reported (W. Zhang and Y.L. Yao – Columbia University, New York, USA)

2000

Control of laser-ablation generated colloid size by surfactants reported (F. Mafuné, J. Kohno,Y. Takeda, T. Kondow, and H. Sawabe – Japan)

2000

Generation of conducting polymer particles by laser ablation in water reported (Y. Tamaki,T. Asahi, H. Masuhara, Osaka University – Japan)

2001

Photo-induced transformation of spherical Ag nanoparticles into nanoprisms reported (R. Jin,Y. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, and J.G. Zheng – Northwestern University, Evanston, USA)

2001

MAPLE and MDW/LIFT techniques reported (P.K. Wu, B.R. Ringeisen, J. Callahan, M. Brooks, D.M. Bubb, H.D. Wu,A. Piqué, B. Spargo, R.A. McGill, and D.B. Chrisey – Naval Research Laboratory, USA)

2002

Formation of polyynes by laser irradiation of graphite particles in liquids reported (M. Tsuji,T. Tsuji, S. Kuboyama, S.-H. Yoon,Y. Korai,T. Tsujimoto, K. Kubo,A. Mori, and I. Mochida – Kyushu University, Kasuga, Japan)

2004

Removal of particles from surfaces by laser-induced cavitation bubbles reported (W.D. Song, M.H. Hong, B. Lukyanchuk, and T.C. Chong – Data Storage Institute, Singapore)

2004

Laser backside etching of fused silica using an absorbed layer of toluene reported (K. Zimmer, R. Böhme, and B. Rauschenbach – Leibnitz-Institut für Oberflächenmodifizierung e.V., Leipzig, Germany)

2004

Liquids-assisted laser shock cleaning for nanoscale particles removal reported (Deoksuk Jang and Dongsik Kim – POSTECH, Pohang, Korea)

2006

Observation of ZnSe nanorod growth at laser-irradiated solid–liquid interface (T. Jia, M. Baba, M. Huang, F. Zhao, J. Qiu, X. Wu, M. Ichihara, M. Suzuki, R. Li, Z. Xu, and H. Kuroda – Japan and China)

2006

Laser backside etching of fused silica in contact with gallium and mercury reported (K. Zimmer, R. Böhme, D. Ruthe, and B. Rauschenbach – Leibnitz-Institut für Oberflächenmodifizierung e.V., Leipzig, Germany)

2006

Removal of oil film from metal surfaces by water decomposition products generated by laser cavitation reported (H. Hidai and H. Tokura – Tokyo Institute of Technology, Japan)

2006

Laser-assisted transformation of Hg into Au under laser exposure of Hg suspensions in D2 O reported (G.A. Shafeev, F. Bozon-Verduraz, and M. Robert – A.M. Prokhorov General Physics Institute, Moscow, Russia; Université Paris 7, France)

1.2 Laser Processing and Analysis of Liquid Systems That Are Not Covered in This Book Following publications are recommended for reference of LALP technologies and analytical techniques not covered in this book.

Stereolithography Ready JF, Farson DF, Feeley T, et al., eds. LIA Handbook of laser materials processing. Berlin: Springer-Verlag and Heidelberg GmbH & Co.; July 2001:545–554. Upcraft S, Fletcher R. The rapid prototyping technologies. Assemb Autom 2003; 23(4):318–330. Bertsch A, Jiguet S, Bernhard P, Renaud P. Microstereolithography: A review. Mater Res Soc Symp Proc 2003; 758:3–15.

Introduction

7

Liquid-phase photochemistry Donohue T. Applied laser photochemistry in the liquid phase. Opt Eng (Laser Appl Phys Chem) 1989; 20: 89–172. Eisenthal KB. Ultrafast chemical reactions in the liquid state. Topics Appl Phy (Ultrashort Laser Pulses) 1993; 60:319–356, 461–469.

Laser wet etching in reactive liquids Ogale SB. Laser-induced synthesis, deposition and etching of materials. Bull Mater Sci 1988; 11(2–3):137–157 Bäuerle D. Laser processing and chemistry, 3rd edn. Berlin: Springer; 2001:325–333.

Laser reactive quenching at liquid–solid interface Kanetkar SM, Ogale SB. Pulsed laser reactive quenching at liquid–solid interface. Bull Mater Sci 1988; 11(2–3):167–190.

Laser-assisted liquid-phase deposition and electroplating Ogale SB. Laser-induced synthesis, deposition and etching of materials. Bull Mater Sci 1988; 11(2–3):137–157. Bäuerle D. Laser processing and chemistry, 3rd edn. Berlin: Springer; 2001:449–458.

Laser machining and treatment of biological materials and objects Niemz MH. Laser-tissue interactions: Fundamentals and applications, 2nd edn. Berlin: Springer; 2002. Vogel A, Venugopalan V. Mechanisms of pulsed laser ablation of biological tissues. Chem Rev 2003; 103(2):577–644. Vogel A, Noack J, Hüttman G, Paltauf G. Mechanisms of femtosecond laser nanosurgery of cells and tissues. Appl Phy B: Laser Opt 2005; 81(8):1015–1047.

Laser desorption from solid surfaces З. . Lazneva, Lazerna fotodecopbci (od ped. . . Konopova) L.: Izd-vo LŴU, 1990, 199 c. (E. F. Lazneva, Laser photodesorption. Lenigrad, Leningrad State University Press, 1990).

Matrix-assisted laser desorption (MALDI) Stump MJ,Fleming RC,GongW-H,Jaber AJ,Jones JJ,Surber CW,Wilkins CL. Matrix-assisted laser desorption mass spectrometry. Appl Spectros Rev 2002; 37(3):275–303. Creaser CS, Ratcliffe L. Atmospheric pressure matrix-assisted laser desorption/ionisation mass spectrometry: A review. Curr Anal Chem 2006; 2(1):9–15. MALDI Recipes. www.nist.gov/maldi; http://polymers.msel.nist.gov/maldirecipes/index.cfm

Laser-induced breakdown spectroscopy (LIBS) in liquids and at solid–liquid interfaces Rusak DA,Castle BC,Smith BW,Winefordner JD. Fundamentals and applications of laser-induced breakdown spectroscopy. Crit Rev Anal Chem 1997; 27(4):257–290. Song K, Lee YI, Sneddon J. Applications of laser-induced breakdown spectrometry. Appl Spectros Rev 1997; 32(3):183–235. Schechter I. Laser induced plasma spectroscopy. A review of recent advances. Rev Anal Chem 1997; 16(3):173–298.

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Handbook of Liquids-Assisted Laser Processing

Cremers DA, Radziemski LJ. Handbook of laser-induced breakdown spectroscopy. Chichester: John Wiley; 2006. Miziolek AW, Palleschi V, Schechter I, eds. Laser-induced breakdown spectroscopy (LIBS): Fundamentals and applications. Cambridge: Cambridge University Press; 2006.

1.3 Inventions in Liquids-Assisted Laser Processing Main classes of International Patent Classification (IPC, version 2007.01) regarding the main arts of LALP:

Subtractive processing B23K B23K 26/00 B23K 26/12 B23K 26/14 B23K 26/16

working by laser beam (e.g. welding, cutting, boring) in a special atmosphere (e.g. in an enclosure) using a flow (e.g. a jet of gas, in conjunction with the laser beam) removing of by-products (e.g. particles or vapours produced during treatment of a workpiece) B23K 26/36 removing material B23K 26/38 by boring or cutting B23K 26/40 taking account of the properties of the material involved

Shock processing B22F Working metallic powder; manufacture of articles from metallic powder; making metallic powder B22F 3/087

using high-energy impulses (e.g. magnetic field impulses)

B23K B23K 26/00

working by laser beam (e.g. welding, cutting, boring)

C21 Metallurgy of iron C21D 1/09 C21D 7/00 C21D 10/00

by direct application of electrical or wave energy; by particle radiation modifying the physical properties of iron or steel by deformation modifying the physical properties by methods other than heat treatment or deformation

C22 Metallurgy; ferrous or non-ferrous alloys; treatment of alloys or non-ferrous metals C22F 3/00 changing the physical structure of non-ferrous metals or alloys by special physical methods (e.g. treatment with neutrons)

Introduction

9

F01 Machines or engines in general F01D 5/14 form or construction

Cleaning B08 Cleaning B08B 3/00 cleaning by methods involving the use or presence of liquid or steam B08B 7/00 cleaning by methods not provided for in a single other subclass or a single group in this subclass B08B 3/10 with additional treatment of the liquid or of the object being cleaned (e.g. by heat, by electricity, by vibration)

Generation and modification of particles B22F Working metallic powder; manufacture of articles from metallic powder; making metallic powder B22F 9/00 making metallic powder or suspensions thereof B22F 9/02 using physical processes

B82 Nanotechnology B82B 3/00 manufacture or treatment of nanostructures The number of patents in LALP is around 500, about 50 per cent regarding subtractive processing, and 20 per cent regarding laser peening. Selected inventions are described under corresponding sections of this book.

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C H A P T E R

TW O

Cleaning

Contents 2.1 2.2 2.3 2.4 2.5

Introduction Principles of Liquids-Assisted Laser Cleaning Particles on Solid Surfaces Experimental Techniques in Laser Wet/Steam Cleaning Research Physics and Phenomenology of Liquids-Assisted Laser Removal of Particles from Surfaces

11 12 17 30 37

2.1 Introduction Liquids may facilitate the removal of particles or surface layers from solids in several ways: by reduction of adhesion forces, by providing expanding vapours, or by acting as a medium for acoustic or shock waves. At the presence of liquid, the threshold laser energies/fluences for cleaning and thus the surface damage hazard is lower, as a rule. The most important application of liquids-assisted (wet) laser cleaning has been the removal of particulate contamination from solid surfaces, especially from silicon wafers for semiconductor integrated circuits (IC). Particles on wafer mask light in photolithographic process and cause declinations from the desired geometry, in worst case shortcuts and breaks [1, 2]. According to different sources, the minimum permissible contaminating particle size is 1/10 to 1/4 of the minimum feature size of IC [3, 4]. Today, there is a need to remove particles of diameters down to tens of nanometres. The particles may origin from the ambient atmosphere (SiO2 , Al2 O3 ), from previous processing steps (photoresist residuals, Cu, TEOS, Al-F), from equipment (wear particles), and from humans (textile wear). Regarding other areas, liquid-assisted laser techniques have proved to be effective for removal of small particles from rotating magnetic information storage disc surfaces [5] and from telescope mirrors [6]. Particles on surface are not always contaminants. Konov et al. [7] describe a process where diamond nanoparticles on surface were used as nuclei for diamond film growth. By selective laser removal of these seed particles in a water–alcohol solution, patterned diamond films were achieved in the subsequent diamond growth. Liquids may be beneficial also at laser removal of surface layers from solids, by lowering the thermal load on the materials and preventing the dissipation of debris into the ambient atmosphere. Local removal of oxide layers is needed, for example: • • •

in microelectronics for fabricating openings in passivating the insulating layers for electrical contacts [8]; in mechanical engineering to enable welding or gluing [9]; in corrosion research for initiation of corrosion pits.

Handbook of Liquids-Assisted Laser Processing ISBN-13: 978-0-08-044498-7

© 2008 Elsevier Ltd. All rights reserved.

11

12

Handbook of Liquids-Assisted Laser Processing

Table 2.1

Liquids used at laser cleaning.

Liquids

Additives

Water, ethanol, methanol, IPA, acetone

NaCl, methanol, ethanol, IPA

Laser techniques have been considered appropriate also for removal of radioactive contaminants (watercontaining or under water) in nuclear facilities, and for cleaning of optical surfaces in space systems from frozen water and gases. Table 2.1 lists the liquids and their additives used in wet laser cleaning. Alcohols and alcohol additives to water were used for better wetting (for achieving of continuous liquid film on surface). NaCl additive to water was found to enhance the ‘long-term memory effect’ of acoustic cavitation [10] (see Section 7.2.4). Advantages of liquids-assisted laser cleaning of surfaces (in comparison with dry laser cleaning (DLC) and other cleaning methods): • • • • •

• •

Liquid may considerably lower the adhesion forces (van der Waals and double-layer forces). In liquid the capillary force is absent. The cleaning threshold (minimum laser fluence) is lower in liquids. Smaller particles can be removed. Particles may be removed individually. Lower hazard to damage the surface to be cleaned: the focusing of light at transparent particles can be avoided by using absorbing in the liquid light or by choosing a liquid with index of refraction equal to that of the particles [11]. Consumption of ultra-pure liquids is drastically reduced (in comparison with conventional wet cleaning). Electrical (electrochemical) layer removal control is possible. Disadvantages and hazards of liquids-assisted laser cleaning:

• • •

Laser sources are expensive. Liquid droplets on surface may act as lenses and concentrate the light. Generated vapours may be harmful to optical and electronical systems.

2.2 Principles of Liquids-Assisted Laser Cleaning 2.2.1 Particles removal by frontside laser irradiation (steam laser cleaning) Steam laser cleaning (SLC) is the most important kind of laser removal of particulates from surfaces. Here, a thin liquid film, of thickness up to some micrometres, is condensed from vapour onto the contaminated surface. At laser irradiation, the liquid vaporizes and the pressure and movement of the expanding vapours propels the particles off the surface (Fig. 2.1). Also the displacement of surface due to thermal expansion and acoustic transients may contribute to the removal of particles (see Section 2.5.3). The film may be discontinuous, but it is important that there is liquid in contact with the particles. The liquid may origin also from the humidity in the ambient atmosphere – if the substrate and particle surfaces are hydrophilic, a capillary condensation of the humidity occurs.

2.2.2 Particles removal by backside laser irradiation Particles on transparent to laser light substrates may be effectively removed by heating the liquid through the substrate by absorbing in the liquid light. In case of water, the Er:YAG lasers emitting at water absorption maxima near 2.94 µm is often the choice. In comparison with SLC, the thermal load on particles is greatly reduced; for example, living cells have been safely removed from glass slides (Fig. 2.2).

13

Cleaning

(a)

(b)

(c)

(d)

Figure 2.1 Situations in Steam Laser Cleaning; (a) transparent liquid – transparent substrate – opaque particle; (b) transparent liquid – opaque substrate – transparent particle; (c) transparent liquid – opaque substrate – opaque particle; (d) opaque liquid (after the articles by Tam et al. [3], Oltra and Boquillon [12], and Veiko and Shakhno [13]). Cover slide H2O

Micro-objective Sample cavity

CCD

Lens

Particles or cells Absorbing layer

X, Y, Z stage Fast thermal expansion

Laser beam

Er:YAG 2.94 μm, 400 μs 0,1–100 J/cm2

Figure 2.2 Principle of removal of particles and living cells by backside laser irradiation [14]. © SPIE (2002), reproduced with permission from Ref. [14].

2.2.3 Removal of particles by laser-generated acoustic waves in liquid In the pioneering work about laser particles removal from an immersed into liquid substrate, Assendel’ft et al. [15, 16] used a 100 ns, 0.3 J pulsed CO2 -laser beam focused onto free surface of water. Photo-resist particles of size 1–0.1 µm were effectively removed from Si substrates by laser-induced acoustic transients. Acoustic pressure at particles in the cleaning regime was estimated to be in range from 0.02 to 38 MPa.

2.2.4 Liquid-assisted laser shock cleaning Liquid-assisted laser shock cleaning (LLSC) is a combination of SLC with laser shock cleaning (LSC), where a shock wave is generated by laser breakdown in the gas above the specimen. In LLSC, the surface to be cleaned is first covered by a liquid film and then subjected to laser heating and shock wave simultaneously (Fig. 2.3). The technique has been proved to be effective to remove nanoparticles as small as 20 nm with over 90% efficiency from silicon wafers, thus being superior to any other cleaning method [17].

2.2.5 Removal of particles by bubble collapse induced flow Song et al. describe an experiment [18] where SiO2 and polystyrene particles were removed from Si wafers by laser-generated bubbles collapse induced flow (Fig. 2.4). The bubbles collapse flow near solid surfaces in the cleaning regime was later studied by Ohl et al. [19] using particle image velocimetry (PIV). The tangential to surface flow velocities were highest during the time interval of jet impact (see Section 7.2.4) and exceeded 10 m/s (at bubble max size 2 mm); the high tangential velocities were deemed to be the main reason for particles detachment.

14

Handbook of Liquids-Assisted Laser Processing

Compressed gas

Flow controller

Timinig control unit

Translation stage

Laser for optical brakedown

Sample

Liquid reservoir Lens Heater

Temperature control unit

Thermometer Mirror

Laser for liquidfilm evaporation

Figure 2.3 Scheme of liquid-assisted laser shock cleaning. The substrate to be cleaned is covered with a thin liquid film (condensed vapour). An Nd:YAG laser pulse then induces breakdown of air and a spherical shock wave propagates from the centre of the plasma. An excimer laser pulse is fired at the moment when the shock wave touches the centre of the cleaning zone with the sample moving periodically on a translation stage under multiple number of laser pulse irradiation. Courtesy by D. Kim, POSTECH, Pohang, Korea, © Dongsik Kim, reproduced with permission. Optical system

Laser

Stage

Bubbles Liquid

Substrate

Figure 2.4 Schematics of particles removal by bubble collapse induced flow. © American Institute of Physics, reprinted with permission (2004) from Ref. [18].

2.2.6 Removal of surface layers by laser ablation/spallation in liquid In situ local removal of passive oxide layers from metal surfaces by a focused laser beam was found to be useful in corrosion studies (initiation of corrosion pits). In comparison with mechanical methods like scraping, straining, abrading, shearing, guillotining, and fracturing, laser ablation method provides several advantages: (i) there is no contamination form film removing tools, (ii) uniform and reproducible depassivation is achieved in a few microseconds, (iii) depassivated area is well defined and can be controlled easily by changing the size of the laser beam on the working electrode surface [20, 21]. Interestingly, removal of iron oxide layers by this scheme was found to be enhanced when the specimen was held in an electrolyte solution under proper cathodic potential (e.g. 1.45V/SCE for 40 min) (Fig. 2.5)

15

Cleaning

Laser pulse Electrolyte Oscilloscope

Potentiostat

Transducer

Figure 2.5 Experimental configuration for the laser-induced oxide film removal in a liquid confinement at controlled electrochemical potential [22]. The workpiece is immersed into the liquid and laser irradiation causes melting, vaporization, or spallation of the oxide layer. Here, the laser light is fed to the sample through an optical fibre and the ablated area corresponds to the core diameter of the fibre. © Elsevier. 0.8 0.7

kFe

3O4

0.6 0.5

Before polarization

0.4 0.3 0.2

After 40 min of polarization

0.1 0.0

500

600

700

800

1000

Wavelength (nm)

Figure 2.6 Computed spectra of the imaginary part of the refractive index k of a Fe3 O4 layer before and after cathodic polarization. © SPIE (2000), reproduced with permission from Ref. [25].

[23, 24]. Further studies revealed that at cathodic polarization the transparency of the oxide layer was increased considerably (Fig. 2.6), so that the laser light could penetrate deeper and cause the oxide layer spallation due to thermal stresses. In addition, mechanical effects resulting from H2 incorporation (enbrittling of the material and increase of stresses due to volume increase) might have been contributed to the oxide layer removal as well. In the article by Cortona et al. [26], the removal of porous oxide layer, containing 18 per cent of water, from AlMgSi1 alloy surface by laser ablation is reported. Some investigations directed to laser removal of radioactively contaminated layers from concrete are described in the articles by Savina et al. [27–29] (see Table 4.11, Savina (1998) [27], Savina (2000) [28], and Robinson (2001) [29].

2.2.7 Removal of frozen gas and liquid layers from optical surfaces Orbiting earth spacecrafts optics suffers form contamination by dust, H2 O, CO2 , O2 , and various organic molecules, originating from micrometeorite impacts, from high-energy particles (electrons, oxygen a.o.) irradiation of construction materials (outgassing and offgassing), and from manoeuvring motors. Organic contaminants tend to polymerize under sunlight UV radiation. The condensates form islands at surface defects and degrade the performance of optical components [30]. Different techniques have been proposed for removal of contamination from optical surfaces of orbiting spacecrafts, like electron and ion bombardment. Laser irradiation was found to be a favourable alternative here.

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Handbook of Liquids-Assisted Laser Processing

Laser pulse

Micro phone

HCI solution

Shock Wave front Sample

Figure 2.7 Experimental setup for the removal of oxide scale on low carbon steel enhanced by shock wave generated by laser breakdown on the surface of a aqueous HCl solution [33]. Microphone was used for shock intensity estimation by audible sound level. ‘Fig. 1 of Laser-assisted chemical cleaning for oxide scale removal from carbon steel surfaces’ reproduced with permission from Journal of Laser Applications, February 2004,Volume 16, Issue 1, pp. 25–30, Laser Institute of America, Orlando, Florida. The Laser Institute of America disclaims any responsibility or liability resulting from the placement and use in the described manner. © Laser Institute of America (2004). www.laserinstitute.org. All rights reserved.

Piper et al. [30], Pierce et al. [31] have investigated laser cleaning of cryogenic mirrors (Ni-coatedAl, Au/Nicoated Al, Be) by CO2 and Nd:YAG lasers. The mirrors were contaminated by dust and frozen at 100–140 K components of laboratory air, mainly H2 O and CO2 . It was found that CO2 laser was proper for contaminants removal, because its light was effectively absorbed in the contaminant layer, but 1.06 µm Nd:YAG laser not.

2.2.8 Laser-generated shock wave enhanced scale removal In the articles by Lim et al. [32, 33], an oxide scale on low carbon steel was removed by laser-generated mechanical impact in liquid; but only in case when the workpiece was held at least 10 s in at least 10% HCl solution before laser irradiation. Without laser, the minimum HCl concentration needed for scale removal was 18 per cent (Fig. 2.7).

2.2.9 Removal of organic contaminants by water decomposition products In the article by Hidai et al. [34], a tapping oil contamination was removed from various metal surfaces (Ni, Cu, Zn, SUS304), thereby from the inside of holes, by water decomposition products, generated by a 150 mJ ArF laser beam focused onto water surface. Except Zn, no damage of the metal was observed (Fig. 2.8).

2.2.10 Cleaning of surfaces through contaminants dissolution in laser-generated supercritical solution Dolgaev et al. [35] report about non-diamond carbon layer removal from suspended in HNO3 aqueous solution diamond particles (4 nm) in result of irradiation of the suspension by YSGG:Cr3+ :Yb3+ :Ho3+ laser beam (2.92 µm, ≈130 ns, 1 kHz, 10 J/cm2 ). Contamination removal was ascribed to the solvation of non-diamond carbon in supercritical solution.

2.2.11 Dehydroxylation of a silica glass surface Halfpenny [36] and Fernandes [37] report about dehydroxylation of silica glass surface by laser irradiation (Fig. 2.9). Irradiation of the surface by UV light (255.3 nm = 4.86 eV) led to breaking of OH bonds (ED = 4.436 eV) and removal of the hydroxyl groups. The process was proposed for controlling the particles adherence to silica surfaces.

17

Cleaning

Laser beam Lens F⫽180 Sample

Water surface

L

Figure 2.8 Experimental setup used for cleaning of metal samples from tapping oil by laser-generated water decomposition products [34]. Oil layers were totally removed by 18 000–36 000 laser pulses of energy 150 mJ at 193 nm wavelength. © Elsevier. UV photon H

O

H

H

O

H H

H

H

H

H etc.

O

O

O

O

O

Si

Si

Si

Si

Si

O

O

O

Heat

Bulk silica

O Si

O Si

O

Si O

Si O

Bulk silica

Figure 2.9 Modification of the chemical structure of a silica surface by laser irradiation: an hydrophilic to hydrophobic transition occurs [36]. Reproduced with kind permission of Springer Science and Business Media.

2.2.12 Ice-assisted laser particles removal In patent US2004140298 [38], a water ice layer deposition onto surface to be cleaned before laser irradiation was proposed.

2.3 Particles on Solid Surfaces 2.3.1 Adhesion phenomena and adhesion forces In order to remove a particle from a surface, the adhesion forces need to be overcome. In laser removal of micrometre and nanometre-sized particles from solid surfaces, the adhesion forces to be considered are: van der Waals force, double-layer force, capillary force, and chemical bond force (Fig. 2.10). On ferromagnetic substrates, also magnetic forces may be significant. In comparison with macroscopic systems, the gravitational force is unimportant. The adhesion is greatly affected by the surface roughness and the environment (Fig. 2.11). Much of experimental and theoretical research is done by spherical particles; highly spherical latex, glass, silica, and alumina particles of various sizes are commercially available, also of calibrated size. In real cleaning situations, however, the particles are mostly of irregular shape.

Cohesion energy approach Interaction energy of electrically neutral bodies in vacuum can be expressed by Dupré equation: γ = γ1 + γ2 − γ12 ,

(2.1)

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Handbook of Liquids-Assisted Laser Processing

10⫺5

Force (N)

10⫺6

Capillary force van der Waals force due to 1% deformation

10⫺7

van der Waals force

Electrostatic image force

10⫺8 Electrical doublelayer force

10⫺9 0.1

Gravitational force

1.0

10

100

Partical diameter (␮m)

Figure 2.10 The adhesion forces as a function of the diameter for an Al2 O3 particle on a flat Si substrate [39–41]. In dry ambient, the capillary force may be absent. Compilation of data by Kohli [42]. © Koninklijke Brill NV. Republished with permission. Capillary condensed water

(a)

(b)

(c)

(d)

Figure 2.11 Situations in a particle–substrate system: (a) irregular particle on a rough surface (the real case); (b) model spherical particle on a flat surface; deformation of the substrate by adhesion forces and capillary liquid are shown; (c) immersed into liquid system; and (d) particle–substrate system after long storage (hundreds to thousands of hours).

where, γ is the energy per unit area of the interface (Dupré energy or thermodynamic work of adhesion), γ1 and γ2 are the respective surface energies (surface tensions) of both materials, and γ12 is the interfacial energy. Dupré energy of adhesion corresponds to the work per unit area required to separate the surfaces from contact to infinity. For completely apolar materials. √ γ12 = 2 γ1 γ2 . (2.2) Hamaker constants Aii (see below) are related to γ1 and γ2 as [43]: γi =

A11 Aii → γ1 = , 2 24πl0 24πl02

γ2 =

A22 , 24πl02

where l0 is the ‘practical’ minimum equilibrium distance, l0 = 157 ± 9 pm.

(2.3–2.5)

19

Cleaning

Electrostatic forces In general, the force on a charged particle resting on a conducting substrate in the presence of an applied electric field is given as [39, 42]: Fe = qE −

q2 qEd 3 3 πε0 d 6 E 2 + − , 16πε0 h2 16h3 128 h4

(2.6)

where d is the particle diameter, E the electric field strength, h is the distance of the particle from the surface, and q is the total electrical charge of the particle. The physical meanings of the terms in this formula are: 1st term: Coulomb force, 2nd term: image force exerted by an image charge of −q at position −h from the surface, 3rd term: dielectrophoretic force on the induced dipole caused by the gradient of the field from the image charge, 4th term: polarization force due to interacting of the induced dipole and its image. Image and polarization forces between a particle and a surface are always attractive, the other forces may be whether attractive or repulsive. Formulae for electrostatic forces acting on rough particles are given in the article by Soltani andAhmadi [39]. When the particle and the substrate are materials with different contact potentials, electrons from the material with lower work function are transferred to the material with higher one until the Fermi levels in both materials reach the same level. The potential difference U arising from this levelling can reach values of up to 0.5V. For a sphere on a flat surface, the corresponding force is [44]: Fel = πε0

R (U )2 , h

(2.7)

where h is the distance of the particle from the surface.

Chemical bond forces In adhesion of oxide particles to oxide or oxidized surfaces, the hydrogen bond forces may be considerable or even dominant (Fig 2.12) [45]. Wu et al. [45] estimate the hydrogen bond force as: FHbond =

DSEbond , dbond

(2.8)

where D is the OH group density, S and Ebond are the total interaction area and the hydrogen bonding interaction energy between particle and substrate, respectively, and dbond is on the dissociation length of the hydrogen O H

H O

O H

H

H

O

O

O

Si

Si

Si

Si

Si

O O O

O O O

O O O

O O O

O O O

R H

Figure 2.12 Hydrogen bonding between a SiO2 or oxidized Si surface and a hydrogen-bonded liquid. The dashed lines are hydrogen bonds. ‘R’ may be a hydrogen atom (for water) or a radical (for alcohols). After Wu et al. [45] and Fernandes et al. [37].

20

Handbook of Liquids-Assisted Laser Processing

bond. Ebond depends on the nature of the surfaces, in particular on their degrees of hydroxylation and on the electronic structure of the materials. The average bonding energy of the O—H—O hydrogen bond is about 5 kcal/mole (∼0.22 eV/bond) [46, 47]. The dissociation length of the hydrogen bond in the order of 1 Å [45]. According to Wu et al. [45], the hydrogen bond force for SiO2 and Al2 O3 particles on silicon is an order of magnitude greater than the van der Waals force, both in air and in alcohols.

van der Waals force van der Waals force is an electrical force caused by polarization induced mutually in the particle and in the substrate. For spherical particles, the corresponding interaction energy and force are expressed as: WvdW = −

A123 R , 6h

FvdW =

A123 R , 6h2

(2.9)

where A123 is the effective Hamaker constant, R is the particle radius in sphere–plane interactions, or the reduced particle radius in sphere–sphere interactions, R=

R1 R2 , R1 + R2

(2.10)

and h is the distance between the particle and the substrate. Equation (2.9) is valid if the distance h is less than few per cent of the particle’s radius. At larger separations, the potential retardation can be taken into account by formula [48]:   A123 R 1 WvdW = − (2.11) 6h 1 + 11.12h/λ

where λ is the characteristic wavelength for interaction (distance between atoms in solids ∼90 nm). This expression is a good approximation in case of separation distances smaller than 20 per cent of the particle radius and in particle size range at least 0.1–1 µm. The effective Hamaker constant A123 depends on the materials that are interacting and may be calculated from the individual Hamaker constants Ajj [49, 50], A123 =

√ √  √ √  A11 − A33 · A22 − A33 ,

(2.12)

where A11 and A22 are the Hamaker constants for the particle and the surface, respectively, and A33 is the Hamaker constant for the third medium (gas or liquid). The Hamaker constants Aii for some materials of interest to this book are given in Table 2.2. For interactions across vacuum, A123 reduces to A12 , √ (2.13) A12 = A11 A22 . When inserted into liquid, the van der Waals interaction (adhesion force) may be reduced considerably: for example about 2 times for Au or Ag surfaces, 4 times for polystyrene (PS), and 6 times for MgO surfaces [50]. Equation (2.12) suggests also that effective Hamaker constant may also obtain negative values, thus the force between particles may become repulsive if immersed into a liquid. The condition for repulsion is [50]: A11 < A33 < A22

or

A11 > A33 > A22 .

(2.14)

Photoenhanced van der Waals force According to Kimura [53], the illumination of metal colloids at Mie resonance frequencies (see Section 5.2) can accelerate their coagulation by 100 to 5000 times due to decrease of interparticle potential energy and the corresponding increase of van der Waals force.

21

Cleaning

Table 2.2 Non-retarded (static) Hamaker constants Aii for two identical materials interacting over vacuum (at room temperature if not given else). Material

Aii [J]

Reference −20

Visser [51]

−20

Visser [51]

−20

Visser [51]

Diamond

−20

32.6 × 10

Visser [51]

Si

25.6 × 10−20

Visser [51]

6H−SiC

24.8 × 10−20

Bergström [52]

β-SiC

24.6 × 10−20

Bergström [52]

54.5 × 10

Au

44.6 × 10

Ag

30.6 × 10

Cu

18 × 10

β-Si3 N4

−20

Bergström [52]

−20

Bergström [52]

−20

Bergström [52]

−20

Bergström [52]

−20

Bergström [52]

−20

Bergström [52]

SiO2 (quartz)

−20

8.68 × 10

Bergström [52]

SiO2 (amorphous)

6.50 × 10−20

Bergström [52]

Si3 N4 (amorphous)

16.7 × 10

15.2 × 10

α-Al2 O3 TiO2 (tetragonal)

15.3 × 10

12.1 × 10

MgO (cubic) ZnO (hexagonal)

9.21 × 10

7.3 × 10−20

PS (polystyrene)

Visser [51]

−20

6.7 × 10

Glycerol

van Oss [43]

−20

van Oss [43]

−20

van Oss [43]

−20

van Oss [43]

−20

van Oss [43]

−20

van Oss [43]

−20

van Oss [43]

4.66 × 10

Benzene

4.62 × 10

Water

4.39 × 10

Ethanol ◦

Argon (−188 C) ◦

Nitrogen (−183 C) ◦

Helium (−271.5 C)

2.33 × 10

1.42 × 10

0.0535 × 10

The ratio of interparticle potential energy when illuminated Uirr , to the potential energy in dark Udark is expressed by: Uirr 16α0 E02 k2 =− Udark 27ωM



R DM

2

,

(2.15)

where α0 is static polarizability, E0 is the amplitude of the external field, k is a numerical factor, ωM is Mie resonance frequency, and R is the (reduced) radius of the particles. DM = ReD(ωM ), where D(ω) is the centroid of the surface screening charge. DM roughly coincides with the position of maximum induced electron density. For silver, DM ≈ −0.85 Å and the photoenhanced interaction energy has maximum at particle radius of R ≈ 20 nm.

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Handbook of Liquids-Assisted Laser Processing

Capillary force Capillary force is due to capillary condensed liquid (Fig. 2.13) [54, 55]. It may be the dominant particle adhesion force in humid ambient [54]. Due to increased vapour pressure at a concave interface, Eq. (7.57), capillary condensed water can be very stable – according to Bhattacharya and Mittal [40] even a 24 h baking at 180◦ C did not decrease the adhesion strength of particles to silicon. Capillary force has two components, the capillary pressure force Fcp and the surface tension force Fst [55]. Capillary pressure force: Fcp = 2γ πR (cos 1 + cos 2 ) .

(2.16)

Fst = lγ cos α,

(2.17)

Surface tension force:

where γ is the surface tension, and 1 and 2 are the wetting angles of the particle and of the substrate. The total capillary force becomes: Fc = Fcp + Fst = 2γ πR( cos 1 + cos 2 ) + lγ cos α.

(2.18)

The analysis by Pakarinen et al. [55] showed that the surface tension force is negligible for a 1 µm radius spherical particle, but for a 15 nm radius particle of it can be the largest component of the total capillary force. For a Si—SiO2 system, the capillary force should be taken into account beginning from RH > 20 per cent [55]. The volume of the capillary condensed liquid for complete wetting may is expressed by [56, 57]: Vl ≈ 4πRK2 R,

(2.19)

where RK is the Kelvin radius. For water: RK ≈

μσ  , ρl RG T ln RH −1

or in nanometers 0.52 , RK ≈  ln RH −1

(2.20)

where RG is the universal gas constant, ρℓ is the density of the liquid, μ its molar weight, σ the surface tension coefficient, and RH is the relative humidity. ␣ ␪ 1

R

␪2

Figure 2.13 Capillary liquid at a particle (hydrophilic particle on a hydrophilic substrate). The liquid may originate from capillary condensed vapour of a rest of the bulk liquid. After Pakarinen et al. [55].

23

Cleaning

Double-layer force Double-layer force is an electrostatic force determined by ions distribution at charged surfaces in electrolytes. Solid surfaces may acquire charge in several ways, for example, at SiO2 surface the process goes on the way: −SiOH ↔ −SiO− + H+

(2.21)

Cations from solution absorb on the negatively charged surface and attract in turn negative anions from the solution, thus building up a double ion layer structure [58]. Different double-layer theories use different simplifications of the physical situation; a solution proposed for cleaning situations (spherical particle on a flat surface, low constant potential approximation) is given by [59, 44, 60]:  εR  2 2 · 0 + part Wdl = 4



2 202 part 2 02 + part



1 + exp(−κh) + ln [1 − exp(−2κh)] · ln 1 − exp(−κh) 

(2.22)

where ε is the permittivity of the medium, 0 is the surface potential of the surface, part is the surface potential of the particle, and κ is the Debye–Hückel inverse double-layer thickness (reciprocal length parameter):

κ=



2000e 2 NA I , εkB T

(2.23)

where e is the electronic charge, kB is the Boltzmann’s constant, T is the temperature, NA is the Avogadro constant, and I is the ionic strength of the (bulk of) solution, n

I=

1 Ci Zi2 2 i=1

where Ci is the molarity concentration (mol/l) of ion i, Zi is the charge of that ion, and the sum is taken over all ions in the solution (Table 2.3). Equation (2.22) is a good description of the interaction energy for part and 0 values smaller than 50–60 mV and if the product κR > 5. This means that for an average particle radius of 0.12 µm, the equation holds if the ionic strength is larger than 10−4 M. At lower ionic strengths, as in case of deionized water, the calculated electrostatic interaction energy should, therefore, be considered as indicative [60]. For practical calculations the surface potentials may be approximated by their zeta-potentials (ζ-potential). ζ-potentials are dependent on the nature and concentration of the ions, and on the pH of the solution (Fig. 2.14). However, the isoelectric points of oxide materials (Table 2.4), which are of main interest in laser cleaning, are rather independent on kinds of ions and their concentration, but depend on the structure of the material [61]. Table 2.3 Debye–Hückel inverse double-layer thickness for some electrolytes. [43] Solution

1/κ(nm)

H2 O

1000

10

−5

mol NaCl

100

10

−3

mol NaCl

10

10

−1

mol NaCl

1

24

Handbook of Liquids-Assisted Laser Processing

10 Al2O3

ζ-potential (mV)

20 30

ZnO

40 0

TiO2

⫺40 ⫺30

SiO2 Neutral

⫺20 ⫺10 0

2

4

6

8

10

12 pH

Figure 2.14 Dependence of ζ-potentials of some materials on pH of the solution (schematically after Hunter [61] Hann [62] Vos et al. [60] Kamada et al. [63] and Kalin et al. [64]). Other oxides behave similarly. Table 2.4 Point of zero charge (isoelectric point) of some materials of interest to wet laser cleaning and to laser particles generation [60, 63, 64]. Material

Isoelectric point (pH)

Quartz, SiO2

2–3.7

Silicon carbide

3.2

Cassiterite, SnO2

4.5

Rutile,TiO2

4.7–6

Zirconia

6

Hydroxyapatite, Ca5 (PO4 )3 (OH)

7

Si3 N4

4.6–8.8

Corundum, Al2 O3

8–9

ZnO

∼9.7

Magnesia, MgO

12

Electrostatic double-layer force can be calculated by Hogg–Healy–Fuerstenau (HFF) equations [50]: (1) constant potential approximation:   κe −κh εR  2 201 02 2 −κh 01 + 02 − e , 2 + 2 2 1 − e −2κh 01 02

(2.24)

  κe −κh εR  2 201 02 −κh 2 F(h) = +e , 01 + 02 2 + 2 2 1 − e −2κh 01 02

(2.25)

F(h) = (2) constant charge approximation:

where 01 and 02 are the potentials of the interacting surfaces and κ is the inverse double-layer thickness (Eq. (2.23)).

25

Cleaning

Double-layer force may be considerably reduced by proper choice of the liquid and the solutes. Principally, at high pH values the potential or the charge of the adherents increases, the adhesion lowers and the attraction may change into propulsion [50].

Magnetic force Force F on a magnetized particle in a magnetic field B can be calculated by [65]: F=



(M · B)dS,

(2.26)

S

where M is the magnetization and dS is a vector in the surface normal direction, whose modulus equals to the area dS.

2.3.2 Adhesion force theories considering the deformation of the particle and the substrate Adhesion forces cause deformation of both substrate and of particle leading to an increase of the contact area and this way to an increase of the adhesion force as well. According to Kohli [42], due to the deformations, the adhesion force between polymer particles rises about 100 times and the force between metal or oxide particles about 20 times. In the following, some significant to laser cleaning results of adhesion theories are presented [66–69]. In the formulae below, K and E* are the combined elastic moduli of two spheres (or of a sphere and a plane), given by:   1 3 1 3 1 − ν12 1 − ν22 = · ∗, = − (2.27) K 4 E1 E2 4 E Parameters μ and λ, used as criteria for applicability of different models, are: 32 μ= 3π

 3

2RWa2 πE ∗2 z03

and

λ = 1.16 µ.

(2.28)

Hertz model In Hertz model no adhesion is considered, the deformation is elastic and is caused by the external force F only (Fig. 2.15). The radial distribution of the contact pressure is given by:    r 2  r 2 3Ka 3F p (r) = = , 1− 1 − 2πR a 2πa2 a

(2.29)

where a is the contact radius, a3 = RF/K . Due to deformation, the sphere approaches to the surface (Fig. 2.39) by: F . (2.30) Ka Without external force no deformation occurs. Hertz theory is not applicable directly to the particle adhesion problems; however, it is incorporated into other theories which consider the adhesion forces as well. Hertz pressure distribution is shown in (Fig. 2.16). δ=

26

Handbook of Liquids-Assisted Laser Processing

Bradley’s model This model considers two rigid spheres interacting via Lennard–Jones potentials. Force between two spheres:     −2  8πwR 1 h −8 h F (h) = − . 3 4 h0 h0

(2.31)

The maximum adhesion force (pull-off force) occurs at h = h0 : FBradley = 2πRWa .

(2.32)

DMT model (Derjaguin, Muller, Toporov) The bodies are considered to deform according to Hertz theory, but forces acting outside of the contact region are taken into account. Contact radius:  R 3 a = (F + 2πWA R) . (2.33) K Contact radius at zero external force:  2 3 πWA R a0 = . (2.34) K Pull-off force: FDMT = 2πRWa ,

(2.35)

This model is applicable to small compressible solids where μ < 1.

JKR model (Johnson, Kendall, Roberts) This model neglects long range forces outside the contact area and considers only short range forces inside the contact region. Deformation is assumed to be Hertzian. Contact radius:    3 R 2 a= F + 3πRWa + 6πRWa F + (3πRWa ) , (2.36) K

F R (1) Hertz

y aHertz δ

Figure 2.15

JKR

aJKR

Surface deformations according to Hertz and JKR models [69]. © Elsevier.

27

Cleaning

Contact radius at zero external force: a0 =

 3

6πWa R 2 . K

(2.37)

Pull-off force: 3 πRWa 2 The model is applicable to highly adhesive bodies where μ > 1. FJKR =

(2.38)

MD model (Maugis, Dugdale) Adhesion is considered as a constant additional stress σ0 over an annular region around the contact area up to a maximum separation h0 beyond which it falls to zero, as shown in (Fig 2.16). Adhesion energy is Wa = σ0 h0 . This model applies to all materials, from large rigid spheres with high surface energies to small compliant bodies with low surface energies (Fig 2.17) [69, 70]. The adhesion force in MD theory can be calculated from a set of parametric equations [69, 70]: 4  2 δ = A − Aλ m2 − 1, 3 2  4λ2 A        λA  2 −m + 1 + m2 − 1 arctan m2 − 1 = 1 m − 1 + m2 − 2 arctan m2 − 1 + 2 3 and    3 2 F = A − λA m2 − 1 + m2 arctan m2 − 1 ,

(2.39)

(2.40)

(2.41)

where,

a , A=  3 πWa R 2 /K F=

(2.42)

F , πWa R

R

p

P1 a

(2.43)

p1

a

d Pa c

s 0 h0

pa c

(a) d

a

h0 u (b)

Figure 2.16 (a) The MD traction distribution is made up of two terms: Hertz pressure p1 on area r < a, and adhesive tension pa on area r < c. In the annulus a < r < c the traction is constant (=σ0 ) and the surfaces separate up to a distance h0 . The net load P = P1 − Pa . (b) A liquid meniscus at the edge of a contact gives rise to a Dugdale adhesive tension σ0 = γ/ρ, where γ is the surface tension of the liquid [67]. © Elsevier.

28

Handbook of Liquids-Assisted Laser Processing

104 Hertz

5

p⫽ p 0/

0.0

JKR

102 MD

d

100

10⫺1 10⫺3

⫽

DMT

0.

05

Bradley (rigid)

10⫺2

d0 /h0 ⫽ 20

101

1 /h 0

d0 /h0 ⫽ 0.05

Load P ⫽ P /pwR

103

100

10⫺1

101

102

Elasticity parameter, l ⫽ 1.16 μ

Figure 2.17 Adhesion map for elastic spheres based on the MD model [67]. In the Hertz zone adhesion forces are negligible. The Bradley, DMT, and JKR asymptotic theories may be used in the zones so marked. © Elsevier.

and δ δ=  . 3 2 π Wa2 R/K 2

(2.44)

The material properties are taken into account by a dimensionless parameter λ (cf. Eq. (2.28)): 2.06 λ= h0

 3

RWa2 , πK 2

(2.45)

where h0 is a typical atomic dimension. DMT and JKR models are special cases of MD model (λ → 0 and λ → ∞, correspondingly). According to Johnson and Greenwood [67], MD model exactly reproduces also the adhesion due the capillary forces exerted by the meniscus (Fig. 2.13). In this case the Dugdale stress σ0 is given by the capillary pressure γ/ρ, and the thickness h0 by 2ρ cos θ, where γ is the surface tension of the liquid. The effective work of adhesion is then given by Wa′ = 2γ cos θ.

MP model (Maugis, Pollock) MP model assumes the contact profile of pressure Hertzian, but with the radius of curvature changed due to the plastic deformation [71, 72]. Adhesion force: √ 9 π Wa K √ Fa = F + 2πWa R. (2.46) 8 H 3/2 Contact radius: a=



2WA R , 3Y

where, H is contact hardness, H ≈ 3Y and Y is yield strength of the material in compression.

(2.47)

29

Cleaning

Deformation of the surface can be accounted in the van der Waals force Eq. (2.9) by adding a term derived from the formula for van der Waals force per unit area between two plates [50]: Fdeform =

A , 6πh03

(2.48)

yielding Fa = Fsphere−plane + Fdeform

AR = 2 6h0



a2 1+ Rh0



,

(2.49)

where A is the Hamaker constant, R the particle radius, h0 the distance between particle and substrate (often assumed as 4 Å), and a the contact radius that may result from adhesion-induced deformation, Eqs (2.34), (2.36), (2.37), (2.42), and (2.47).

Numerical solutions Elasticity-adhesion problem for two spheres was solved numerically (iteratively) by Greenwood [73]. Gilabert et al. [74] simulated the adhesion and pull-off force of polystyrene spheres of radia 1–8 nm by molecular dynamics method using Lennard–Jones potential. These calculations demonstrated a pretty good adequacy of analytical models (from Bradley to MP) to particle adhesion problems.

Influence of the surface roughness For particles on dry surfaces the adhesion force decreases considerably if the surface roughness increases (Fig. 2.18). 1.4 1.3 Total pull-off force, Pc ⫽ Pc / N pc

1.2 1.1 0.1 0.9 0.8 0.7 0.6 0.5

JKR

0.4 0.3

DMT

0.2 0.1 0

0

0.5

1.0

1.5 2.0 2.5 Roughness, s/ dc

3.0

3.5

4.0

Figure 2.18 Effect of random roughness on adhesion between nominally flat surfaces having N asperities each of radius R and standard deviation of height σ. Pc = JKR pull-off force (=1.5πωP) and δc = JKR pull-off displacement = (3/4)(π2W2a R/E*2 )1/3 for each aspherity [67]. © Elsevier.

30

Handbook of Liquids-Assisted Laser Processing

(a)

(b)

Figure 2.19 Scanning Electron Microscope (SEM) images of 1.5 µm SiO2 particles: (a) after 17 h storage and (b) after 1350 h storage [76]. Reproduced with kind permission of Springer Science and Business Media.

Long-time stability of particle-surface contact Adhesion forces cause a deformation of both the particle and of the substrate, balanced by elastic forces arising from deformations of both of the particle and the substrate. The elastic deformation tends to relax in time (hundreds and thousands of hours) via creep and migration of the matter (Fig. 2.19), causing the increase of the adhesion force [75, 76].

2.4 Experimental Techniques in Laser Wet/Steam Cleaning Research 2.4.1 Preparation of particles covered surfaces In the research of laser cleaning, there is a need for controlled covering of substrates with particles. The particles density should be high enough to achieve statistically reliable counts over the laser spot and coarse enough to avoid particles aggregation.

‘Dip and tap’ method (Fig. 2.20a) The substrate to be covered is dipped into a large volume of particles or the particles are spooned onto the substrate. The loose particles are then removed by sharp tapping or fast flow of dry gas. The method leads to a medium density of particles, 10–40 per cent coverage by area, on solvent cleaned glass slides, and to lower densities on ultrasonically cleaned slides, 0.1–7 per cent, average 1.8 per cent [77, 78].

Drying of a suspension (Fig. 2.20b) Suspension of particles in an organic solvent (e.g. isopropylalcohol, IPA) is prepared by ultrasonic agitation. A drop of the suspension is applied onto the substrate. The solvent vaporizes but particles remain on the surface. Particles density may be controlled by spinning of the suspension-coated substrate. Higher rotating velocities result in thinner fluid films and lower surface density of particles [79, 80]. A variant of this method is described in the article by Neves et al. [81] (Fig. 2.20c). The substrate to be coated by particles was placed in a 10-cm-diameter Petri dish with ethanol, the whole being placed on a heated vibrating table (≈50◦ C) and a specific amount of metallic particles was placed in the centre of the wafer. After a certain time, the ethanol evaporated leaving the metallic particles uniformly distributed over the surface of the wafer.

31

Cleaning

(a)

(b)

(c)

(d)

Figure 2.20 Some important methods of preparation of particle-covered surfaces. (a) ‘dip and tap’ method, (b) drying of particles suspension, (c) in situ suspension preparation, and (d) laser ablation of a compacted powder target.

xy stages 0.1–3.5 s Valve opening control Substrate Flow meter

Computer

water + 8% alc.

40°C Laser triggering

Nitrogen

z stages

50°C

Heater

fluence 10–200 J/m2, 10 ns

Mirror

Focusing lens Pulsed laser: excimer, Nd:YAG, etc.

Thermometer

Figure 2.21 Schematics of steam laser cleaning system developed at École Polytechnique de Montréal. The wafer is kept face down to avoid resettlement of the removed particles. © SPIE (1999), reproduced with permission from Ref. [85].

Laser ablation deposition (Fig. 2.20d) Particles with high tendency to form aggregates like Al2 O3 may be effectively dispersed by laser ablation [78]. A compacted Al2 O3 powder target was irradiated by a focused XeCl laser beam (308 nm) at fluence 4.4–12.3 J/cm2 . Prepared by this method samples had 1 µm alumina particle densities of 0.01–1.5 per cent, in average 0.6 per cent. Particles may be deposited also electrophoretically [82], be dusted or sprayed onto surfaces. Fernandes and Kane [83] list the particle deposition methods and give further references. There are industrial devices for controlled deposition of particles onto silicon wafers available as well, for cleaning standards and for research purposes [84]. Figs. 2.21–2.23 present some complete steam laser cleaning systems.

2.4.2 Application of liquid and monitoring the liquid film thickness and condition In steam laser cleaning process, the liquid film is formed by condensation of vapours on the contaminated surface (Figs 2.21–2.23). Vapour is generated by heating the liquid (water–alcohol mixture) in a special vessel and fed to the cleaned surface by pulsed nitrogen flow. The thickness of the liquid film may be controlled by vapour pulse duration. Usually the vapour pulse lasts some seconds yielding a liquid film of thickness

32

Handbook of Liquids-Assisted Laser Processing

Low pressure N2 Liquid film Flow controller

Sample stage controller

Filter

Port Nozzle

Level sensor Liquid supply controller

Stage Servo controller

Liquid supply

Puffer Heaterthermocouple (8% IPA) assembly Heater Mirror

Beam splitter Rotating mirror Energy meter

Lenses Beam homogenizer

Filter

Heater power supply & temperature controller

Thermocouple Reservoir (40% IPA, 60% water)

Lens

Computer

Beam expander

Controls each subcontroller and laser

KrF excimer laser

Mirror

Figure 2.22 Schematics of a high-throughput laser cleaning tool developed at IBM [86]. The beam from an industrial KrF excimer laser (248 nm radiation, 200 Hz repetition rate, 200 W output) is scanned galvanometrically and the wafer by a translation stage; the liquid film is deposited continuously. © Elsevier.

Dry N2

Humidified N2

Process monitoring microscope

Focusing lenses and mirror

OPO Pulsed laser Laser beam

Humidifier

Photodiode

Reflected light

Particle

Dark field illumination laser Suction

Valve ⫹ MFC

Water layer consensed Rotation axis

Narrow gap suction Translation axis

Figure 2.23 Particle removal system for high-volume manufacturing system by Sumitomo Mitsubishi Silicon Group [87]. An image analyzing system detects individual particles which are thereafter removed by local steam deposition, laser irradiation, and suction. Capability of the system to clean 4000 silicon wafers in 2 weeks was demonstrated. © Trans Tech Publications Inc., reproduced with permission.

33

Cleaning

⌬Ip(°C) 104 Abiation

I(MW/cm2)

103

10 4 10 3

102

10 2

10 10

1 10⫺1

10⫺3

Hotter Melting

Deeper

Cleaning

Photothermal sensing

10⫺2

10⫺1

1

10

102

103

104

105

106

t(ns)

Figure 2.24 Parametric space indicating various possible effects when a solid surface (e.g. stainless steel) is irradiated by a laser beam with various intensities, I and pulse widths, τ [86]. © Elsevier.

0.2–10 µm. Liquid film last on the surface some seconds, therefore, the laser pulse is fired about 0.1 s after the vapour pulse. The vapour may be supplied also continuously (Fig. 2.22). The thickness of the liquid film can be monitored interferometrically and its lasting by optical reflectometry (Figs 2.27–2.28).

2.4.3 Choice of laser beam parameters In steam laser cleaning, it is energetically advantageous to use lasers whose wavelength does not absorb in liquid, but in the substrate. Thus, excimer and frequency multiplied Nd-ion lasers are the best choice. For 1.06 µm wavelength form Nd:YAG and similar Nd-ion lasers, the reflectivity of solid surfaces is usually higher that in the UV-VIS region. The use of CO2 laser, whose 10.6 µm light cannot penetrate a common liquids film, but vaporizes the liquid surface only, is rationalized by independence of the cleaning process of the substrate material and by absence of the substrate damage hazard. A discussion about the choice of laser wavelength for steam laser cleaning is presented in the article by Oltra and Boquillon [12]. Figure 2.24 presents a comparison of laser beam parameters in cleaning with these in other kinds of laser processing.

2.4.4 Measuring and monitoring techniques in steam laser cleaning Detection of acoustic emission (sound) Thermal expansion of the laser heated target and rapid vaporization of the liquid induce hearable sound transients, whose intensity can be used for monitoring the laser–matter interaction intensity (Fig. 2.7).

Detection of displacements of interfaces Laser heating caused displacement of the rear side of the substrate can be measured by interferometric or piezoelectric probes (Fig. 2.25) (cf. Figs 3.13 and 3.15). Probing of the backside displacements is needed also for precise determination of vapour film thickness by an interferometric probe at the front side (Fig. 2.26).

34

Handbook of Liquids-Assisted Laser Processing

Laser pulse

Laser pulse

Oxidized metallic sample

Oxidized metallic sample

Interferometric probe

Piezoelectric transducer

(a)

(b)

Figure 2.25 Experimental setups for on-line monitoring of laser-induced oxide film removal process using: (a) interferometric probe and (b) piezoelectric probe [22]. © Elsevier.

Light source Beam splitter I1 I2 Mirror Effective bubble layer

Solid sample

Bubbles

Probe beam (diameter ~1 mm)

Water

Cr Quartz substrate

Figure 2.26

Principle of interferometrical measurement of vapour film thickness [88]. © Elsevier.

Detection of reflected and scattered light Level of surface contamination and the vaporization onset can be monitored by reflected or scattered light (Fig. 2.27). The dynamics of vaporization and ejected particles flow can be probed by deflection/scattering of a probe beam parallel to the surface (Fig. 2.28).

Surface plasmon probe A versatile high-resolution probe, sensitive to refractive index change of a liquid at a solid interface, is the surface plasmon probe (SPP) (Figs 2.29 and 2.30). Because the refractive index of a liquid or a gas depends on the density, pressure, and temperature, these parameters may be measured and mapped by SPP. The time resolution

35

Cleaning

FM BS 40/60

Nd:YAG

Dump BS 50/50

Attenuator

PD3 IF

p–pol. IF PD2

PBS M

s–pol.

NDF

AFR

L4

Cell

L3

L2

L1

PD1 IF

Sample

F

Cover

Heater Ar⫹ laser

M

Figure 2.27 Reflecting/scattering light probe for monitoring of vapour layer state [80]. The response time of the system to bubble nucleation was > λ with refractive index n = 1.5 [96]. Reproduced with kind permission of Springer Science and Business Media. 30

(a)

20

55 61 64 67 68 (mJ/cm2)

10

0

0

200

400 600 Time (ns)

800

1000

Figure 2.32 Displacement of a cleaning target induced by an excimer laser pulse with liquid-film deposition (pure water) on the laser spot (248 nm, 24 ns). Recorded by an interferometric probe at backside of the substrate © American Institute of Physics (2003), reprinted with permission from Ref. [100]. Laser beam m⫻an Particle Fad

Substrate

SAW pulse

Figure 2.33 Interaction of particles with short-pulsed laser-generated surface waves. © American Institute of Physics (1998), reprinted with permission from Ref. [103].

Surface waves Rapid thermal expansion of laser-heated substrate may excite surface waves, which can accelerate the particles also outside of the irradiated area (Fig. 2.33). Detachment of 1–2 µm Al2 O3 particles by laser-generated surface waves have been investigated by Kolomenskii, Mikhalevich et al. [101, 102]. Particle detachment from

39

Cleaning

silicon surfaces occurred at some mJ/cm2 , thus at ≈100 times lower fluences in comparison with direct laser irradiation [103].

Inertial force Grigoropoulos and Kim [98] present a formula for scaling of the inertial force acting on a particle on a rapidly heated surface Fi ∝

4 πR 3 ρβdth T , 3 τ2

(2.50)

where R is the particle’s radius, ρ is the particle’s density, β is the volume expansion coefficient of the substrate, T is the temperature increase, τ is laser pulse length, and dth is thermal penetration depth, √ dth = ατ =



λτ , ρC

where α is the thermal diffusivity, λ is thermal conductivity, ρ is density, and C is the material specific heat.

2.5.4 Heating and phase change (absorbing substrate, non-absorbing liquid) Because the laser spot diameter is usually much larger than the thermal penetration depth of the substrate, heat transfer may be considered 1D.Temperature transients and temperature distributions in laser cleaning situations were calculated by Yavas et al. [104], Park et al. [95] Kim et al. [88] (Figs 2.34 and 2.35). The temperatures in (Figs 2.34 and 2.35) were calculated without taking heat resistance of interfaces into account. For silicon and water/IPA interfaces the heat transfer coefficients were measured by Leiderer et al. [80]: hH2 O = 3 × 107 W/m2 K, hIPA = 1 × 107 W/m2 K. Rapid heating, ∼1010 K/s, drives the liquid into superheated state (see Section 7.2.3). Below critical temperature, the vaporization (bubble nucleation) is a statistical process that depends also on surface properties. Theoretical and experimental determination of the superheating temperature and of vapour dynamics in real situations has been the topic of many investigations (see Table 2.5).

250 at the surface at 0.5 ␮m from the surface at 2.0 ␮m from the surface

Temperature (⬚C)

200

150

100

50

0

0

100

200

300

400

Time (ns)

Figure 2.34 Computed temperature transients at different locations inside a crystalline silicon irradiated with a KrF excimer laser (λ = 248 nm, τ = 16 ns) [95]. Heat transfer to the ambient was neglected. © IEEE (1994), reproduced with permission.

40

Handbook of Liquids-Assisted Laser Processing

Surface temperature (K)

550

(a)

70

40

500 450

50

80

60

(mJ/cm2)

400 350 Laser pulse

300 250

0

200

400 Time (ns)

600

800

60

14 PZT I OPT I PZT II OPT II PZT III OPT III PZT IV OPT IV PZT V OPT V

12 10 8.0 6.0

50 40 30 20

4.0

10

2.0 0.0

Bubble nucleation threshold 0

20

40

60

80

100

Reflectance drop (%)

Maximum pressure (bar) (absolute)

Figure 2.35 Calculated temperature increase at the Cr–water interface heated by an excimer-laser pulse of different fluences [88]. The dotted line shows the temporal shape of the laser-pulse intensity in arbitrary units (triangular pulse with peak intensity at t = 17 ns and width of 48 ns). © Elsevier.

0 ⫺10 120

Fluence (mJ/cm2)

Figure 2.36 The pressure pulse amplitudes (water on chrome) plotted as a function of excimer laser fluence (248 nm, 24 ns). The data produced by piezoelectric transducer are represented by the symbols labelled with ‘PZT’ and the data by the deflection probe are represented by the symbols labelled with ‘OPT’. The experiments were repeated 5 times as indicated by the Roman numerics. The amplitude of the optical specular reflectance drop is also plotted with the dashed line. The bubble nucleation threshold is marked by the arrow. The experimental setup is shown in (Fig. 2.28). © American Institute of Physics (1996), reprinted with permission from Ref. [89].

For nanosecond laser pulses, the following observations have been made (transparent liquid, opaque surface, water of water–alcohol mixtures): • • • • •

Superheated liquid layer thickness is some hundreds of nanometres.[104] Embryonic nucleation starts immediately after the temperature exceeds boiling temperature. [105, 106] Superheating temperatures range up to 250◦ C on atomically smooth silicon surfaces; on rough surfaces the superheating temperatures may be 2 times lower. [97, 80]; Bubble-growth induced pressures reach several MPa [89, 107] (Fig. 2.36). A vapour layer is formed near the heated surface that lifts a liquid disc from the surface (Fig. 2.37).

Both high-speed photography (Fig. 2.37) and molecular dynamics simulations (Figs 2.38 and 7.8) have revealed that in a typical steam laser cleaning process a liquid disc is ejected from the surface.

41

Cleaning

OTISCE T8000.IAX1

Figure 2.37 Image of liquid disc ejected from laser-heated surface. Snapshot was taken 8 µs after laser pulse [108]. © Koninklijke Brill NV, republished with permission.

Figure 2.38 Snapshots of molecular dynamics simulations of a 3.4 nm water film with 6.46 nm diameter particles on a rigid gold substrate, suddenly heated from 48.4 K to 193.6 K [109]. From left to right, the elapsed time is 0.22, 0.44, 0.66, 0.88 ps, respectively. Simulations with identical initial conditions did not result in particles removal in any case: the upper row shows a simulation where there was particle removal, while the lower row shows a simulation, at the same temperature, where particle removal did not occur, cf. Fig. 7.8. Reproduced with kind permission from Springer Science and Business Media.

Lang and Leiderer [107] measured the dynamics of the ejected liquid film by optical reflectivity with high precision (2 nm spatial and 0.2 ns time resolution) (Figs 2.39 and 2.40). Under assumption that the vapour follows the equation of state PV n = constant, where n is the polytropic exponent, and neglecting the compression of the liquid, the following equation of motion of the liquid layer was proposed [107]: d2 P0 d(t) = · dt 2 ρ·h



d0 d(t)

n

−

Patm , ρ·h

(2.51)

where P0 is the initial pressure under the film, Patm is the atmospheric pressure, d0 is the initial distance of the film from the surface after the vapour formation, ρ is the density of liquid, and h is the thickness of the liquid film. For a case of an isopropanol film, the fitting of the experimental data using initial conditions d(t = 7.1 ns) = 8.7 nm and v(t = 7.1 ns) = 0 m/s, yielded P0 = (4.9 ± 0.2) MPa and n = 1.00, indicating an isothermal process.

42

Handbook of Liquids-Assisted Laser Processing

2500

d (nm)

2000 1500 1000 500 0 0

20

40

60

80

110

120

140

160

t (ns)

Figure 2.39 Trajectory of an isopropanol film after laser heating of the substrate [107]. The solid line represents a parabol fit to the data points and corresponds to a constant acceleration of the film. © Institute of Physics, reproduced with permission.

120 100

d (nm)

80 60 40 20 0

6

7

8 t (ns)

9

10

Figure 2.40 Magnification of the first few nanoseconds of the ejection process in Fig. 2.39 [107]. The solid line corresponds to the same fit as in Figure 2.39. After the formation of a vapour layer in the first 700 ps (6.4–7.1 ns), the overlying liquid is accelerated away from the substrate for about 8.6 ns until the expansion causes the pressure under the film to drop below the pressure above. © Institute of Physics, reproduced with permission.

Ejection force Lu et al. [5, 110] estimated the force excerted by expanding vapours on a particle by  Fc = πR 2 2ρc(Pv − P∞ )vf ,

(2.52)

where R is the particle radius, ρ is liquid density, c is transmit speed of the stress wave, Pv is vapour pressure inside the bubble, P∞ is ambient pressure, v is expansion velocity of the vapour, and f is volume fraction of vapour. The assumptions of the model were: (i) bubble generation is an inertia-controlled process; (ii) in the region near the liquid/substrate interface, the vapour layer created by the evaporation of the liquid acts as a plane piston, compressing the adjacent liquid and generating stress waves; (iii) the value of the volume fraction of

43

Cleaning

vapour inside the superheated liquid layer is less than 1; (iv) the expansion velocity of the vapour layer is equal to the growth velocity of the bubbles; and (v) the pressure inside the vapour layer is equal to the saturation vapour pressure of the superheated vapour layer due to the non-uniform temperature distribution in the liquid film (citation from the article by Wu et al. [111]). Wu et al. [111] presented a modified version of Eq. (2.52): Fc = πR

2

 4

8 2 2 ρc f (Pv − P∞ )3 . 3

(2.53)

The same authors presented also a different laser wet cleaning theory, based on a model where the lasergenerated bubble growth in the fluid medium generates an explosive blast wave, and the particle is lifted by the pressure of this wave after reflection from the substrate surface. They found for upper limit of the particle removal force due to bubble generation:

where Prefl =

Fc = πR 2 Prefl ,

(2.54)

Pshock (8Pshock − P∞ ) ; Pshock + 4P∞

(2.55)

with notations: Prefl is reflected from the substrate surface overpressure and Pshock is shock-generated pressure. The other assumptions of the model were: (1) the shock-generated pressure is approximately equal to the vapour pressure in the vapour layer at the water/substrate interface, i.e. Pshock ≈ Pv (T ); T is the temperature in the vapour layer; (2) the temperature in the vapour layer is approximately equal to the temperature at the substrate surface; (3) the pressure inside the vapour layer is equal to the saturation vapour pressure of the superheated vapour layer due to the non-uniform temperature distribution in the liquid film and (4) the vapour layer thickness, limited by the thickness of the superheated liquid layer, may exceed the particle radius since the thermal penetration depth in water is of the order of 1 µm (citation from the article by Wu et al. [111]). A discussion of these ejection force models is given in the article by Leiderer et al. [80] It is pointed to, that the high superheating temperatures at smooth silicon surface, the finite temperature jump between the substrate and the liquid, and the thickness of the liquid film should be taken into account in the future models.

2.5.5 Hydrodynamic effects In the pioneering reports about water-assisted laser cleaning by Assendel’ft et al. [15, 16], laser-generated acoustic waves were used for removal of the particles form solid surfaces. Later, in studies of laser-generated bubbles collapse assisted particle removal from surfaces, the high-speed near-surface flow was found to be responsible for the cleaning process [18, 19]. Near-surface flow is also the main factor in megasonic cleaning. Zhang et al. [112] give following criterion for particles detachment from surfaces by a boundary flow (see notations in Fig. 2.41): RM = 1.339 ·

FD R > 1, Fa a

(2.56)

where RM is adhesion resisting moment, FD is the drag force, R is the radius of the particle, Fa is the adhesion force, and a is contact radius. Drag force on a spherical particle in a slow linear shear flow is expressed by: FD = 10.2πμR · U (R)

(2.57)

and for near wall sub-layer flow by: FD =

32μ (Re ∗ )2 , ρ

(2.58)

44

Handbook of Liquids-Assisted Laser Processing

Fel

U

Mr Fd 1.4R

R a

␦ Ma Fa

Figure 2.41 Conditions at a particle in a surface flow. U is the liquid velocity, Fa is the adhesion force, Fel is the elastic force, Fd is the drift force, Ma is the adhesion moment, and Mr is the flow caused rotation moment.

where μ is the fluid viscosity, U (R) is the fluid velocity at a distance of R from the wall, ρ is the fluid density, and Re∗ is the shear Reynolds number and is given by: Re ∗ =

RU ∗ , v

where v is the kinematic viscosity of the fluid and U ∗ is the friction velocity:  2τ , U∗ = ρ

(2.59)

(2.60)

where τ is the shear stress, τ = F/A. A review of particle-wall hydrodynamics is given by Kim and Lawrence [113].

2.5.6 Particles removal threshold and efficiency in steam laser cleaning Threshold fluence Veiko and Shakhno [13] provided the following first-order criteria for particles removal thresholds for different situations in light transmission/absorption.

(a) Absorbing particle at a transparent substrate ρ p c p hp (Tb − Tin ) , εth = Ap

(2.61)

where εth is the cleaning threshold, ρc is density of the particle, cp is specific heat of the particle, hp is the height of the particle, Ap is the average absorption coefficient of the particle, which includes the influence of the angle of incidence, Tb is boiling temperature of the liquid, and Tin is the initial temperature.

(b) Transparent particle at an absorbing substrate Particles detachment occurs when the substrate surface temperature Tm exceeds a critical value Tth , given by relation: √ as τ T m − T b · + R = hmin , (2.62) γ Tm − Tin where as is substrate, τ is laser pulse duration, γ is laser radiation absorbance, R is the height of unevenness of rough surface, and hmin is the bubble critical size.

45

Cleaning

0.1

0.3 1 3 Particle diameter (␮m)

10

0

Surface damage threshold

Cleaning threshold

Particle density

After cleaning

Cleaning efficiency (%)

100 Before cleaning

0

0.1

0.2 0.3 Laser fluence (J/cm2)

(a)

0.4

(b)

Figure 2.42 Schematical dependences of steam laser cleaning efficiency on particle size (a) and on laser fluence (b). (Schematically after Héroux et al.,[82] Meunier et al.,[85] Leiderer et al. [80]). For silicon the surface damage (melting) threshold is ∼275 mJ/cm2 (λ = 532 nm, τ = 8 ns) [115].

(c) Absorbing particle at an absorbing substrate The nature of the particle detachment process is judged by a criterion, ϕ: √ Ap a p ϕ= √ , As a s

(2.63)

where as is thermal diffusivity of the particle and As is absorption coefficient of the substrate. If ϕ > 1, the situation reduces to case (a), if ϕ < 1, to case b). Leiderer, Mosbacher et al. [114, 79, 115] have proved experimentally that the threshold fluence of steam laser cleaning of silicon wafers (110 mJ/cm2 ) is independent of particles material, size, and shape. Such universal threshold indicates that the particle removal forces are far larger than the adhesion forces. The universal threshold for SLC differs from bubble nucleation threshold for bulk water–silicon system, 80 mJ/cm2 (single 1064 nm pulse) [80].

Cleaning efficiency The typical steam laser cleaning efficiency dependences on particle size and on laser fluence for nanosecond laser pulses are given in (Fig. 2.42). The cleaning efficiency starts to decrease for particle size 1.5 J/cm2 fluence optically visible surface damage observed; Al2 O3 particles were harder to remove from obviously due to positive ζ-potential of Al2 O3 while SiO2 surface has negative ζ-potential

Boughaba (1996) [130]

Cr layer (0.15 µm) on 0.35 µm p-Si; quartz substrate

Water, methanol

KrF, 248 nm, 16 ns, 15–82 mJ/cm2

Static pressure up to 100 at%

Transient temperature studies at laser irradiation of solid surface in water, using temperature dependence of p-Si reflectance (from both front and rear side, enabling more precise measurements in comparison with Leung (1992) [90]); embryonic nucleation starts immediately after temperature exceeds boiling temperature; max. superheat temperatures (bubble growth threshold) was measured to be ≈100◦ C (42.2 mJ/cm2 ); vaporized mass in of order 1%

Park (1996) [105, 106],

Ag layer (80 nm) on quartz

Water

KrF, 248 nm, 25 ns, up to 62 mJ/cm2

Arrangement for measuring transient pressures on nanosecond time scale using surface plasmons described; laser irradiation of solid–liquid interface generated pressures 1.8 MPa at 43 mJ/cm2 and 2.8 MPa at 62 mJ/cm2

Schilling (1996) [131], Leiderer (1998) [114]

Ag layer (53 nm)

Water

KrF, 248 nm, 25 ns, up to 60 mJ/cm2 , spot 1×1 cm2

Optical front-and rear-side transient reflection (SPP) studies; bubble nucleation was observed to start at 10.5 mJ/cm2 (superheating 11◦ C); fractional volume of bubbles in the superheated layer was estimated (using Maxwell–Garnett’s theory) to be ≈0.05–0.1; bubble-growth induced pressures were measured to range ≈1–5 MPa with a pressure pulse length ≈40 ns

Yavas (1997) [132]

Target immersed into water, vessel covered by window

(Continued )

Table 2.5

(Continued)

Substrates

Particles

Cr layer (0.2 µm) and Ag layer (53 nm, for SPP)

Liquids

Lasers and beam parameters

Water, IPA, ethanol, methanol, water + IPA

KrF, 248 nm, 16 ns, up to 45 mJ/cm2

Other features of the experiment

Novel features, observed phenomena, comments

References

Bubble nucleation studies by optical reflection, scattering, piezoelectric transducer and SPP; in water, embryonic bubbles nucleate at 9.5 mJ/cm2 (superheating 11◦ C); bubbles growth velocities: 4 m/s (water), 2.2 m/s (alcohols)

Yavas˛ (1997) [133], Leiderer (1998) [114]

Si

1–9 nm

Water

CO2 , 10.6 µm, 0.2 µs, up to 2 J/cm2

Condensed from vapour film on substrate

100% particle removal required 5–9 cleaning cycles; surface optical reflectivity measurement proved to be suitable for monitoring water film and droplets on surface

Allen (1997) [4]

Si wafer

Silica and PS spheres, 800 nm

Water + alcohol

2ω-Nd:YAG, 532 nm, 7 ns, up to 180 mJ/cm2

Condensed from vapour film on substrate

Cleaning threshold 110 mJ/cm2 independent of particles material and size (down to 60 nm); at 170 mJ/cm2 , 90% of particles were removed with the first laser shot; surface damage threshold 320 mJ/cm2

Leiderer (1998) [114]

NiP

Al2 O3 (1 µm)

IPA

KrF, 248 nm, 23 ns

A IPA drop was applied onto surface

Particles removal threshold ≈30 mJ/cm2 ; at 50 mJ/cm2 ≈90% of particles were removed; theoretical estimation of bubble expansion generated force on particles and of cleaning threshold

Lu (1998) [5, 134, 135], (1999) [136], (2000) [137, 138], (2001) [110]

Si wafer

SiO2 (0.3 µm)

Water

Air saturated with moisture

In moisture saturated air ≈88% of particles were removed contra 12% in dry air

DeJule (1998) [139]

Si wafer

Fe2 O3 (0.5–2 µm)

Water, 2 µm layer

KrF, 248 nm, 22 ns, up to 200 mJ/cm2

Condensed from vapour film on substrate

At 200 mJ/cm2 , 90% of ≥0.3 µm particles were removed from surface; without water layer, no considerable cleaning occurred up to laser fluences 350 mJ/cm2

Beaudoin (1998) [140]

Si (100), thickness 380 µm; Fused silica

Al2 O3 (1–10 µm)

Capillary condensed water or opaque liquid layer (0.1 mm)

N2 , 337 nm, 10 ns, up to 50 Hz, 9 mJ, illuminated area 0.15 × 0.8 mm

SAW Rayleigh pulse of wavelength of 100 µm was formed

Theoretical and experimental investigation of surface acoustic waves (SAW) in Si wafers, generated by laser irradiation; accelerations needed for particles removal can be achieved at laser fluences of some mJ/cm2 (≈100 times less fluence needed than at direct irradiation); SAW-assisted cleaning in vacuum and in ambient air compared; in vacuum the cleaning process was more efficient

Kolomenskii (1998) [103]

Glass

Al2 O3 (0,1, 0.3, 1, 3 and 10 µm)

Capillary condensed water obviously present

2ω-Cu-vapour, 255.3 nm, 35 ns, in kHz range, up to 0.5 J/cm2

Focused laser beam, spot Threshold fluences needed for removal of 50% laboratory air

KrF, 248 nm, 12 ns ns

Water, condensed from moisture

XeCl, 308 nm, 8 ns, up to ≈10 J/cm2

Particles were deposited by laser ablation (Kane (2002) [78])

History of steam laser cleaning of surfaces from particulates, with accent on work performed at IBM Research Laboratories; contain also new data about of liquid film dynamics, achieved by high-speed photography; at irradiation of Si/water-alcohol interface by KrF laser (180 mJ/cm2 ), a liquid disc of thickness 0,87 µm was formed, departing the substrate with velocity of 37–20 m/s (at distance from substrate’s surface 0–800 µm); liquid film acceleration was ≈4 × 10 m/s2 ; above the disc, an acoustic wavefront was observed, starting with 400 ns delay before laser pulse and propagating with velocity of 370 m/s

Zapka (2002) [94]

A review with 73 refs. about experimental research on both dry and wet laser cleaning of particles from surfaces; experiment conditions and main results tabulated

Kane (2002) [77]

Cleaning results did not depend considerably on the properties of two very different glasses in contrast with dry cleaning theory, obviously the cleaning was enhanced by capillary water and hydrocarbons from laboratory air; cleaning efficiencies 95–100% were achieved using a single laser pulse (CVL, ≈0.5 J/cm2 ); cleaning thresholds were ≈100 mJ/cm2 for CVL and 330–400 mJ/cm2 for excimer lasers, obviously due to coherence length difference

Kane (2002) [77]

A technique for deposition of microparticles by laser ablation of packed particles target in air was developed; the agglomeration of particles due to capillary forces was avoided

Kane (2002) [77]

Threshold fluence was measured ≈90 mJ/cm2 independent on particle surface density (effects of agglomerated particles were reduced)

Fernandes (2002) [154]

Si wafers

Diamond (5–20 nm and 5–7 nm)

Water + alcohol spot 0.1 × 0.1 mm, 0.8 J/cm2 50 pulses

KrF, 248 nm, 20 ns, 50 Hz,

NiP

Al2 O3

Water + IPA (30–50% vol), film thickness a few micrometer

Nd:YAG, 355 and 1064 nm, 6 ns

KrF, 248 nm, 24 ns

Incident angle of laser beam 40◦ , condensed from vapour film on substrate

A short review of the use of laser cleaning for removal of moulding flash on IC packages heat sinks (dry process), and of particles from MR head sliders (dry and steam assisted); steam cleaning efficiency was higher than of dry cleaning

Song (2002) [155]

Cleaning in liquid was more efficient and did not cause surface damage

Konov (2002) [7]

In addition to presented earlier results (She (1999) [144], Park (1996) [89], Kim (2001) [88]), a diagram of adhesion forces between 0.1–100 µm Al2 O3 particles and Ni substrate is presented and significance of inertial forces is highlighted: inertial forces on micrometre-sized particles should be taken into account also in laser wet cleaning

Grigoropoulos (2002) [98]

A review (56 pp., 27 figs., 79 refs.) of steam laser cleaning with accent on the research done at University Konstanz (see Leiderer,Yavas˛, Mosbacher, Schilling above); reflectivity of p-polarized probe light is much more sensitive than of s-polarized light for nucleated bubbles detection; bubble growth velocity for non-adiabatic case calculated; superheating temperatures for smooth and structured surfaced determined (smooth Si/water: 250◦ C; Si with nano-holes/water: 160◦ C, smooth Si/IPA: 116◦ C); heat transfer coefficients between Si and water/IPA determined: ξH2O = 3 × 107 W/m2 K, ξIPA = 1 × 107 W/m2 K; a critical comparison of wet laser cleaning theories presented

Leiderer (2002) [80]

An analytical expression for an attached to heated surface stable bubble shape obtained, taking into account temperature gradient in the liquid; wet cleaning threshold and superheating temperature are calculated from condition that critical bubble diameter equals to the heated up to boiling temperature liquid layer thickness (transparent particles on absorptive surface, water and ethanol); surface roughness is taken into account by roughness-dependent surface tension of liquids

Veiko (2002) [156]

(Continued )

Table 2.5

(Continued)

Substrates

Particles

Liquids

Lasers and beam parameters

Other features of the experiment

Novel features, observed phenomena, comments

References

A review (30 pp., 8 figs.) of physical mechanisms of dry and wet laser cleaning of particles and solid films from surfaces, see Veiko [150, 151, 156]

Veiko (2002) [13]

A general review (8 pp., 3 figs., 34 refs.) of laser removal of particles from surfaces, both dry and wet

Allen (2002) [157]

A review (13 pp., 3 figs., 59 refs.) of laser removal of particles from surfaces, mostly of dry process; with accent on adhesion and removal forces theory and light enhancement near particles

Lu (2002) [158]

A review (19 pp., 9 figs., 44 refs.) of laser removal of particles from Si wafers, both dry and wet, with accent on experimental work

Mosbacher (2002) [159]

A review (13 pp., 9 figs., 17 refs.) of laser removal of particles from Si wafers and Si membrane stencil masks

Zapka (2002) [108]

Si wafer, Ag film (50 nm) on glass

PS Water (bulk (0.11–4.1 µm) and capillary condensed)

Nd:YAG, 532 nm, 8 ns, up to ≈300 mJ/cm2

Experiments performed in water, vacuum and air (RH 30–40%)

Cleaning threshold Fth depends on particles radius r roughly as Fth (r) ≈ 1/r k , k = 1–2; the actual dependence Fth (r) included an oscillating component due to Mie resonances of light field; these resonances may enhance the light intensity at particles ≈30 times (computed) causing local surface damage; in air the cleaning threshold was lower for particles smaller than 800 nm up to ≈30%; at atomically smooth Si surface the water superheating temperature was close to theoretical value (250◦ C), on holy Si and rough Ag the superheating temperatures were 160 and 130◦ C

Mosbacher (2002) [97]

Steel, glass, Al, marble

Kaolin

Water

XeCl, 308 nm, 20 ns, 130–200 mJ

Condensed from vapour water film on surface

Optoacoustic transient 4–88 mm above the target’s surface recorded using probe beam (HeNe laser); evolution of signal’s amplitude and delay time as a function of number of cleaning cycles presented

Bregar (2002) [160]

Quartz

Red blood cells, PS spheres (10 µm)

Water solution

Er:YAG, 2,94 nm, 400 µs, 0.1–100 J/cm2 2ω-Nd:YAG 532 nm, 10 ns, 1–100 µJ

Laser irradiation from backside (see Fig. 2.2)

Cells and PS particles were successfully removed by backside 2.94 nm irradiation; backside irradiation was chosen in order to avoid cell damage due to overheating (absorption depth of 2.94 nm light in water ≈0.8 µm); in case of 532 nm light, jumping of some PS particles was observed, obviously due to thermal expansion generated forces

Zharov (2002) [14]

Si wafer

SiO2 (0.5 and 1.5 µm)

Water + IPA (10:1 and 4:1)

Cr film (0.3 µm) on quartz, NiP

Rigid Au substrate

Spherical particles 6.46 nm

Lennard–Jones liquid

Si wafer

PS (0.14–1.3 µm)

Water or IPA, (condensed steam)

Si wafer

Monodisp. SiO2 , PS and PMMA, 60–1000 nm

Si wafer

Meniscus growth observed at Si wafer – SiO2 particles contact during ‘storage time’ 100–1350 h observed (see Fig. 2.19); dry cleaning efficiency dependence on fluence and ‘storage time’ presented

Schrems (2003) [76]

Interferometrical and optical reflectance studies of processes at explosive vaporization of liquid; the covered by liquid film substrate performed a normal displacement up to 27 nm (at fluence 68 mJ/cm2 , dry surface 70 nm; in 90–130 nm thickness range particles redeposition occurred, due to light interference in liquid film, the film thickness affects the energy reaching the wafer surface; light field concentration at particles caused surface damage: in water for particles size over 840 nm in any case; in IPA at cleaning threshold damage free particles removal was observed; light focusing by liquid droplets may also contribute to surface damage

Lang (2003) [162]

A short review (2 pp.) of research on dry and steam laser cleaning recently done at University of Konstanz, Inst. de Optica (Madrid) and Univ. Linz (see Leiderer and Mosbacher above)

Leiderer (2003) [163]

Optical reflectivity and vapour plume transmission measurements (0.5–3 mm above the surface); initial velocity of plume expansion ranges 10–20 m/s; the initial velocity decreased with laser fluence increase due to the phenomenon, that at higher fluences the liquid in contact with substrate reaches the critical temp. faster and vapour layer hinders the further heat transfer from surface to liquid

Kudryashov (2003) [164]

KrF, 248 nm, 24 ns Nd:YAG, 355 and 1064 nm, 6 ns, up to 0.67 J/cm2

Nd:YAG, 532 nm, 8 ns, up to 250 mJ/cm2

Several sources, from 150 fs to several ns Water droplets on surface (condensed from steam)

KrF, 248 nm, 20 ns, up to 0.76 J/cm2

Laser beam focused by cylindrical lens

(Continued )

Table 2.5

(Continued) Lasers and beam parameters

Other features of the experiment

Novel features, observed phenomena, comments

IPA

KrF, 248 nm, 20 ns, up to 0.7 J/cm2

Condensed from vapour liquid film on surface

Results of photoacoustic investigations of laser irradiation generated IPA plume propagation presented and discussed; explosive vaporization threshold was 0.17 J/cm2

Kudryashov (2004) [164]

Substrates Particles

Liquids

Si wafer

References

Si wafer

SiO2

Water (capillary condensed from moisture)

KrF, 248 nm, 28 ns Nd:YAG, 532 and 1064 nm, 6 ns

RH 94–97%; pressure 31–37 mbar (other gases 4000 wafers were cleaned in 2 weeks

Wachs (2005) [87]

A review of research of dry and steam laser cleaning of particles from Si wafers done at Univ. Konstanz; in part of SLC (see Mosbacher (2000) [79], Lang (2003) [162],(2004) [169])

Graf (2005) [168]

PIV visualization of flow fields using 8 µm fluorescent particles

Liquid flow near a solid surface during laser-induced bubble growth and collapse was investigated by particle image velocimetry (PIV); tangential to surface flow velocity is highest during the time interval of jet impact: ≈10 m/s (bubble max size 2 mm); the high tangential velocity is obviously responsible for removal of particles from surface in cavitation-induced cleaning (see, e.g. Song (2004) [18])

Ohl (2006) [19]

Condensed from vapour IPA film on surface

Dynamics of liquid film at laser-heated surface was recorded by optical reflectivity with 2 nm, 0.2 ns resolution; estimated with aid of temperature calculations initial vapour pressure (at liquid film lift-off ) was ∼5 MPa; ejection velocity of liquid film varied from ∼50 m/s (97 nm film) to ∼40 m/s (227 nm film)

Lang (2006) [107]

Water (obviously)

Si wafer

No

IPA film (97–227 nm)

Nd:YAG, 532 nm, 7 ns, 138 mJ/cm2 , spot several mm

(Continued )

Table 2.5

(Continued)

Substrates

Particles

Liquids

Si wafer

CuO (50 nm aver.),Al2 O3 (50 and 100 nm av)

Water + IPA (10:1)

Lasers and beam parameters

Other features of the experiment

Novel features, observed phenomena, comments

KrF, 248 nm, 25 ns 170 mJ/cm2 (for heating) Nd:YAG, 1064 nm, 60 ns, 520 mJ (for shock generation)

Condensed from vapour liquid film on surface; cleaning was enhanced by shock wave generated in air above the substrate

Cleaning efficiency over 90% for Al2 O3 particles as small as 20 nm was achieved (20 cleaning cycles; scanned substrate); shadowgraph images of shock propagation and liquid film vaporization are presented

Jang (2006) [17]

A updated review of Kane (2002) [77] with 120 refs. about experimental research on both dry and wet laser cleaning of particles from surfaces; experiment conditions and main results tabulated

Fernandes (2006) [83]

A review with 60 refs. about the principles and theory of dry and wet/steam laser cleaning of surfaces from particles; surface nanopattering by particles arrays determined light fields, surface polishing by laser irradiation, and about PTFE surface modification for enhanced biocompatibility

Bäuerle (2006) [57]

References

Reports where only reactive liquids were used, are not refereed here; the entries are mostly in the original style;‘Si’ means single crystalline silicon wafer, as a rule; water was distilled and deionized as rule;Ar+ -lasers are usually CW; the energies and energy densities for pulsed laser beams are for one pulse; the experiments were performed at room temperature if not mentioned else. Notations SC: single crystalline; PS: polystyrene; PI: polyimide; PMMA: polymethylmethacrylate; IPA: isopropylalcohol (isopropanol); PSL: polystyrene latex; SPP: surface plasmon probe; RH: relative humidity; SAW: surface acoustic wave; OPO: optical parametric oscillator; PIV: particle image velocimetry; MR: magnetoresistive; CVL: copper vapour laser.

Table 2.6

Liquids-assisted laser removal of surface layers and related experiments (examples).

Substrate

Layer removed

Laser and beam parameters

Other features of the experiment

Novel features, observed phenomena, comments

Fe

Native oxide

Aqueous solution of 0.1 M Na2 B4 O7 + 0.01 M NaCl

Dye, 15 ns, 20 Hz, 30 µJ, spot 5 µm, 150 J/cm2 , 1010 W/cm2

Process was carried out in situ in an electrolysis cell, 860 mV SCE

Oxide layer was removed by laser ablation in course to study the pit initiation process; diameter of the ablation crater was 5–10 µm, depth 1–2 µm

Ulrich (1981) [20], (1983) [170]

Stainless steel (CrNiMo Fe)

Native oxide

Water solution of NaCl (30 g/l)

Rhodamine 6G dye, 570–625 nm, 15 Hz

Specimen immersed into free-surface solution

Oxide layer was removed in situ by laser ablation in course to study the pit initiation process

Oltra (1986) [171]

Ni-coated Al, Au/Ni-coated Al, Be (mirrors)

Dust from laboratory air; frozen constituents of air, mainly H2 O and CO2 , (∼100–140 K)

No

CO2 , 2 µs, up to 15 J, some pulses Nd:YAG, 10 ns, 10 Hz, up to 1 J, irradiation time up to 240 s

Experiments were performed in vacuum 10−5 –10−6 Torr

A review of space optics (cryogenic optics) contamination problems and cleaning options is presented; CO2 laser was able to remove over 5 µm thick frozen layer from surfaces, but Nd:YAG laser only 0.1 µm

Piper (1990) [30]

Be

SiO2 , up to ∼10 µm Carbon particles, solvent residues

No

308 nm, 0.2 J/cm2 , (0.9 J/cm2 for SiO2 particles)

In air (Au also in vacuum at 35 K)

12 different contaminants were successfully removed from 14 different surfaces

Osiecki (1990) [172]

MgF2 -overcoated Al on quartz

Liquids

References

Au

Carbon particles, frozen water

Au-overcoated Ni on Al (mirror)

Water, CO2 , NH3 , vacuum pump oil, dust

No

CO2 , pulsed up to 25 Hz, 1.5 J

At 34 and 90 K

10 × 10 cm2 areas on mirrors were cleaned successfully in 15 min by a scanned laser beam

Pierce (1990) [31]

Fe

Native oxide

Water solution of HClO4 , pH 1

Nd:YAG, 6 ns, 1–40 MW/cm2

Flowing electrolyte, 1 cm/s, 300 mV SSE

Local depassivation of a Fe electrode by laser ablation for transient electrochemistry studies

Oltra (1993) [173]

(Continued )

Table 2.6

(Continued)

Substrate

Layer removed

Liquids

Laser and beam parameters

Other features of the experiment

Novel features, observed phenomena, comments

References

Au and Au-coated mirrors

Solid H2 O (1–10 µm), O2 and CO2 (≈1 µm)

No

308 nm, 1.06 and 10.6 µm, nanosecond to microsecond pulse length

Substrates temperature ≈25 K

Solid cryofilms removed by laser irradiation; analytical expressions for 1D transient temperature distributions presented, taking into account temp. dependences of materials properties; bulk Au surface damage threshold was estimated to be 37 J/cm2 (10.6 µm, 200 ns) and 120 J/cm2 (10.6 µm, 2 µs)– close to measured values

Jette (1994) [174]

Limestone sculpture

Black crust, 0.05 mm

Water, 0.1 mm layer

Nd:YAG, 1.06 µm, 6 ns, 0.4 and 0.8 J/cm2 , spot 5 mm

Water was brushed onto workpiece

Cleaning efficiency was higher when water was applied – dry cleaning needed 30 laser pulses, wet cleaning only 10 pulses (0.4 J/cm2 ); scattered in removed material plume probe light intensity and acoustic signal were proportional, thus acoustic signal may be used to control the cleaning process

Fe, steel

Native oxides, 0.1 µm

Boric acid, borate buffer and sodium hydroxide solutions

Nd:YAG, 1064 nm, 14 ns, 0.55–0.66 J/cm2 , 20 pulses

Laser light feed trough 1.5 mm optical fibre, distance to sample ≈1 mm

Efficient oxide removal without reoxidation achieved in basic solution at cathodic potential 800◦ C

Oxide scale (≈10 µm), composed of FeO, Fe2 O3 , Fe3 O4 , and machining oil

Water + HCl (5, 10 and 18%); 25 and 80◦ C; 1 mm layer over the workpiece

Nd:YAG, 1064 nm, 6 ns, spot ≈10 µm, up to 0.5 J/cm2

Workpiece immersed horizontally into freesurface solution, laser beam focused onto surface of the solution (Fig. 2.7)

The removal of the scale was enhanced by laser-generated mechanical impact in liquid, but only when the workpiece was held at least 10 s in the HCl solution before laser irradiation; use of laser enabled to remove the scale at HCl concentration of only 10%, instead of 18% in case of purely chemical treatment

Lim (2003) [32], (2004) [33]

(Continued )

Table 2.6

(Continued)

Substrate

Layer removed

Liquids

Laser and beam parameters 3+

Other features of the experiment

Novel features, observed phenomena, comments 3+

References

Diamond particles (4 nm)

Non-diamond carbon layer

Water + HNO3 (≈1 ml for 10 ml solution)

YSGG:Cr : Yb3+ : Ho3+ , 2.92µm, ≈130 ns, 1 kHz, 10 J/cm2 Cu-vapour, 510 nm, 20 ns, 10 kHz, 2–3 J/cm2

Particles suspension irradiated by focused laser beam

Irradiation by YSGG:Cr : Yb3+ :Ho3+ laser removed the non-diamond carbon layer from particles, irradiation by Cu-vapour laser led to increase of the non-diamond carbon amount; the cleaning occurred obviously mainly as result of non-diamond carbon solvation in supercritical solution

Dolgaev (2004) [35]

Ni, Cu, Zn, SUS304

Tapping oil Sumitap super

Water

ArF, 193 nm, 30 Hz, 150 mJ

Laser beam focused on water surface (Fig. 2.8)

Removal of oil film from metal surfaces (plates and hole) was achieved using 18 000–36 000 laser pulses causing the water decomposition; the range of region cleaned was 5 mm around the focal point of the lens; the water decomposition products etched Zn, but not other metals

Hidai (2006) [34]

Fused silica, H2 O 2 cleaned

Hydroxyl groups

No

KrF, 248 nm, 12 ns, 20–200 Hz, 0.1–1.2 J/cm2 , 300–10 000 pulses

The experiment was performed in air

A review (26 pp., 13 figs., 27 refs.) of silica surface structure and laser dehydroxylation experiments is presented; laser dehydroxylation was found to be mainly a thermal process; dependence of SiOH+ /Si+ ratio of dehydroxylated samples at SIMS analysis on laser fluence, pulse repetition rate and total pulse number is presented (see also Halfpenny (1997) [177] and Fernandes (2002) [181] in this table)

Fernandes (2006) [37]

Reports where only reactive liquids were used, are not refereed here. Notations SCE – saturated calomel electrode; SSE – silver–silver chloride reference electrode; SUS304 – a kind of stainless steel; SIMS – secondary ion mass spectrometry.

C H A P T E R

T H R E E

Shock Processing

Contents 3.1 3.2 3.3 3.4 3.5

Introduction Residual Stresses and Their Measurement Laser Shock Peening Laser Shock Forming and Cladding Densification of Porous Materials

69 70 77 140 141

3.1 Introduction In laser shock processing (LSP), the mechanical recoil impulse of rapidly expanding vapour and plasma is utilized for introduction permanent changes in the workpiece (Fig. 3.1). The light power density on the workpiece surface is chosen to be so high (1–100 GW/cm2 ) that optical breakdown occurs and plasma is created. The rapid expansion of high-pressure plasma (velocity ∼1500 m/s and pressures over 2 GPa in water) creates a shock wave that, propagating through the material, creates dislocations and induces plastic deformations. A higher dislocation density results in higher surface hardness and strength, while plastic deformations reduce porosity and can create compressive surface stresses, the latter being responsible for increased fatigue and cavitation strength and stress corrosion resistance of the material. Also an increase of theYoung’s modulus and Poisson’s ratio, and grain refinement due to the shock has been reported. If the surface is covered by a transparent coating, solid or liquid (Table 3.1), the expansion of the vapour and plasma is suppressed and the pressure and impulse on the surface are considerably higher. As confinement Laser pulse Lens

Water

High-pressure plasma

Material

Figure 3.1 Principle of LSP. Sudden expansion of laser-generated plasma creates a pressure pulse that drives a shock wave into the workpiece. © ASME, reproduced with permission from Ref. [185].

Handbook of Liquids-Assisted Laser Processing ISBN-13: 978-0-08-044498-7

© 2008 Elsevier Ltd. All rights reserved.

69

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Table 3.1

Comparison of different confinement media for LSP.

Confining medium

Advantages

Disadvantages

Inorganic solid (glass, quartz)

Highest pressure and impulse (due to highest acoustic impedance)

Not applicable to curved surfaces, glass pieces remain inside the machinery, multiple shocking troublesome

Polymer (acrylic, rubber)

Can be applied to curved surfaces

Multiple shocking is time- and material consuming

Liquid (water)

May be applied to curved surfaces, suits well for multiple shocking

Wet method, lower pulse pressure

41

(a)

(b) 44

A

38 46 42

40

(c)

(d)

(e)

Figure 3.2 Principles of: (a) shot peening; (b) deep rolling; (c) water cavitation peening; (d) ultrasonic shot peening (after Xing and Lu [187]) and (e) ultrasonic peening by strikers (after patent US2002037219 [188]).

medium (tamper layer) glass, water, and some polymeric materials have been used (see also Table 2.2 in the book by Ding and Ye. [186]). In LSP, only a mechanical impact on the workpiece is desired. Heating of the material by laser light is kept minimal by using short laser pulses and protective coatings (ablators). In technology, LSP has been applied for peening, densification, and forming of materials, the peening being of greatest importance. In many aspects similar to laser peening results may be achieved also by shot peening, water cavitation peening, and deep rolling (Fig. 3.2 and Table 3.2). In fundamental research of matter behaviour under shock loads, flyers and explosives are used as well.

3.2 Residual Stresses and Their Measurement Residual stresses play a critical role in fatigue, creep, wear, stress, corrosion, cracking, fracture, buckling, etc. [195]. Conversion of tensile residual stresses into compressive is the main goal of LSP and residual stresses are the most important process parameters of LSP. Research and process control of LSP rely to a great extent on the determination of surface and bulk residual stresses in the material. Harmful tensile surface residual stresses are created by majority of subtractive machining methods, including mechanical and chemical milling, turning, broaching, grinding, electro-discharge machining, and laser cutting.

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Shock processing

Table 3.2 Comparison of some peening methods. There are several examples of SP and DR performance in Table 3.8 along with laser peening results. Method

Important characteristics

References

Shot peening (SP)

Simple, inexpensive; risk of introducing foreign material into the workpiece or its surroundings, roughens the surface

[189]

Laser peening (LSP)

Laser beam can access places non-accessible by accelerated shots; the impacts may be localized (down to micrometers), but also have large area (up to 100 mm2 ); well controllable, rapid, does not cause significant macroscopic deformation of the treated zone; strain rate up to 106 /s achievable, plastically affected zone 5–10 times deeper than in case of SP; does not increase surface roughness considerably; high cost of the equipment

Peyre (1996) [190] Hammersley (2000) [191]

Deep rolling (DR)

Better finish in comparison to LSP at equal plastically affected depth; workpiece geometry restricted, high load on workpiece

Nalla et al. (2003) [192]

Water (jet) cavitation peening (WCP)

Up to 1000 MPa residual compressive stresses were achieved in spring steel SAE 1070

Qin et al. (2006) [193]

Ultrasonic peening

Plastically affected depth from 0.3 mm (using shots) to 1.5 mm (using strikers)

Xing and Lu (2004) [187] Kudrjavtsev (2004) [194]

Macrostresses

Peening

Cold hole expansion

Bending

Welding

Figure 3.3 Some common cases of residual stress formation. © The Institute of Materials, Minerals and Mining, reproduced with permission from Ref. [200].

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Table 3.3

Residual stress measurement techniques [196,201,200,198].

Technique

Restrictions to materials

X-ray diffraction

Penetration

Spatial resolution

Accuracy

Comments

Crystalline

5 µm (Ti), 50 µm (Al)

20 µm depth, 1 mm Lateral

±20 Mpa

Combined often with layer removal for greater depth

Synchrotron diffraction (hard X-rays)

Crystalline

>500 µm, 100 mm for Al

20 µm lateral to incident beam, 1 mm parallel to beam

±10 × 10−6 strain

Triaxial stress, access difficulties

Neutron diffraction

Crystalline

4 mm (Ti), 25 mm (Fe), 200 mm (Al)

500 µm

±50 × 10−6 strain

Triaxial, low data acquisition rate, access difficulties

Curvature/Layer removal

0.1–0.5 of thickness

0.05 of thickness

Hole-drilling

∼1.2 hole diameter

50 µm Depth

Stress field not uniquely determined ±50 Mpa

Flat surface needed (for strain gauges), semi-destructive

Slitting (crack compliance)

Flat surface needed, destructive

Surface contour

Simple and cheap, suits well for welds, destructive

Ultrasonic

Metals, ceramics

>10 cm

5 mm

10%

0.5–10 MHz

Magnetic

Magnetic

10 mm

1 mm

10%

Microstructure sensitive

Raman/ fluorescence/ birefringence

Ceramics, polymers

1). At 1064 nm, the breakdown process was found to be dominated by avalanche ionization whereas at 532 and 355 nm the multiphoton ionization played the dominant role [260].

Zhang, Yao, Noyan Zhang et al. [263] present a laser plasma model for the case of microscale laser impacts (spot size ∼10 µm) under the following conditions: (1) Plasma expands only in the axial direction in the early stage; density, internal energy, and pressure of the plasma are uniform within the plasma volume but can vary in time. (2) Plasma obeys ideal gas laws. (3) Only the coating layer is vaporized, the metal target experiences neglible thermal effects. (4) The coating layer is thin and well coupled with the metal target, thus the shock pressure and the particle velocities of the coating layer and the metal target are equal. The water–plasma target system was divided into six regions: unshocked water, shocked water, plasma, coating layer, shocked solid, and unshocked solid. The shocked and unshocked properties of water were related by mass, energy, momentum-conservation, and shock speed constitutive relations: ρw0 Uw − Uw0 =1− , ρw Dw − Dw0

(3.26)

Pw − Pw0 = ρw0 (Dw − Dw0 )(Uw − Uw0 ),      2  1 1 Uw0 1 Uw2 , − − Ew0 + = (Pw + Pw0 ) Ew + 2 2 2 ρw0 ρw

(3.27)

Dw = Dw0 − Sw Uw ,

(3.29)

(3.28)

where U denotes the particle velocity, D the shock velocity, with subscripts w0 for unshocked and w for shocked water. Mass and energy conservation equations for plasma were used in form: t t ρP (t) (UpL + UpR )dt = (MFw + MFc )dt, 0

0

(3.30)

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where UL is the particle velocity near water, UR is the particle velocity near target, MF w is the mass flow from water into plasma, and MF c is the mass flow from the coating into plasma; and Ept + Wp − EMF =

t

AP × I (t)dt,

(3.31)

0

where Ept is the total energy stored in the plasma, Wp is the work done by the plasma, EMF is energy exchanged through mass flow, and AP is fraction of the energy absorbed by the plasma (determined from experiments). The set of equations was solved numerically, using mass and energy conservation relations at interfaces.

Colvin, Ault, King, Zimmerman Colvin et al. [251] developed a computational model for pressure generation in case of solid dielectric confinement, accounting for the initial absorption of light onto a metal surface, low-intensity photoionization absorption in neutral vapour, collisional ionization, recombination, dielectric breakdown and band gap collapse of the dielectric, electron conductivity, thermal transport, and constitutive properties of the materials. Analytical Quotidian EOS (Eq. 3.64) was used for all of the materials. The model was incorporated into a 2D-radiation–hydrodynamics code LASNEX. No free variables were needed. Simulations showed that most of the laser energy is absorbed in the dielectric tamper (fused silica or sapphire), and a little part in the ablator (Al or Zn). Wu and Shin Wu and Shin [259] presented a self-closed thermal model for LSP under water confinement. The model considered laser ablation of the coating layer, water evaporation, plasma ionization and expansion, energy loss of plasma through radiation and electron conduction, laser absorption by plasma through inverse Bremsstrahlung effect and photoionisation, and reflection of laser beam at the air–water interface and plasma-water interface. No free variables were needed. Assumptions taken in the model are the following: (1) The physical processes were considered 1D. (2) Plasma state variables as temperature, density, etc. are uniform in space, but vary with time. (3) The main mechanisms of laser absorption by plasma are electron-ion and electron-atom inverse Bremsstrahlung absorption and photoionization. (4) All the free electrons in the plasma were assumed to have the same temperature Te , and all the particles (atoms and ions) in it were also assumed to have the same temperature Ti (two temperature model). (5) Water molecules were assumed to be completely dissociated into H and O atoms immediately after evaporation. The receding velocities of coating and water surface due to evaporation were calculated from Hertz–Knudsen equation (Eq. 7.58). The plasma pressure caused moving velocities of the water and the coating surface were calculated separately through the momentum–conservation equation and shock speed constitutive relations, the same way as in (Eqs 3.26–3.27): uw,pre =

P P = , ρw Dw ρw (Dw0 + Sw uw,pre )

(3.32)

P P = , ρc Dc ρc (Dc0 + Sc uc,pre )

(3.33)

uc,pre =

where ρw and ρc are densities, and Dw and Dc are shock velocities of water and coating, respectively. Dw0 , Sw , and Dc0 , Sc are material property constants (cf. Eq. 7.119) for shock velocity calculations. The total receding velocities of the water and the coating surface were taken as a sum of the evaporation and plasma pressure caused boundary velocities. The calculation by this model peak plasma pressure was in good agreement with experiments (less than ±10 per cent difference) in range of laser power densities 1–10 GW/cm2 and for different combinations of

95

Shock processing

pulse shape, wavelength and duration: (Gaussian, 25 ns, 1064 nm), (Gaussian, 25 ns, 532 nm), (Gaussian, 0.6 ns, 1064 nm), (short-rise-time pulse, 30 ns, 1064 nm). At power densities more than 25 GW/cm2 the measured pressure was lower than the calculated one. The calculations demonstrated that the reflection of laser light at water–plasma interface ranges up to ∼35 per cent for 1064 nm light and up to 8 per cent at 532 nm, and that the stable value of α is ∼0.5.

3.3.6.2 Models for residual stresses Nomenclature a – edge of square-shaped impacts r0 – radius of circle-shaped impacts τ – pressure pulse duration P – shock pressure ρ – density of the target λ, μ – Lamé constants v – Poisson’s ratio ε – strain ε – strain tensor, also strain vector εp – plastic strain εp – plastic strain tensor σ – stress σ – stress tensor, also stress vector σ0 – initial residual stress σY ,YS – uniaxial compressive static yield strength (elastic static limit) σsurf – surface (superficial) residual stress PH , Ph, HEL = Hugoniot elastic limit = yield strength under a uniaxial shock condition Lp – plastically affected depth Ce – speed of elastic longitudinal waves Cp – speed of plastic longitudinal waves

Ballard’s model Ballard’s model describes analytically the plastic deformation and the magnitude and depth of induced residual stresses in a laser-shocked body [264, 265, 190, 230]. Assumptions (1) (2) (3) (4) (5)

The shocked body is a elastic-perfectly plastic half-plane. Shock waves are longitudinal and plane. Plastic strain follows a von Mises yielding criterion, |σr − σx | = σ0 + σY . The applied strain is uniform over the impacted area. Duration of the impact is sufficiently small, satisfying the relationship:

τ ≪ r0



ρ (λ + 2μ) 4μ(λ + μ)

(3.34)

In such case, the induced waves can be considered longitudinal and plane. (6) Applied pressure pulse is rectangular (P is constant); the pressure is uniform on the impacted surface. (7) Viscous effects in the material are negligible. This assumption is applicable for steels and aluminium alloys at laser pulse durations greater than 1 ns [265]. (8) Work hardening of the material is ignored. Under these assumptions, the shock may be described by propagating into the depth independent elastic and plastic waves (Fig. 3.33).

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Elastic recoil waves (2HEL amplitude)

Plastic waves Z

Peak pressure

Stress profiles Elastic waves (HEL amplitude)

Figure 3.33

Schematics of waves propagation in the Ballard model, after Peyre et al. [190] © Elsevier.

In cylindrical coordinates r, θ, z, the tensors of applied strain and stress and of induced plastic strain become   0 0 0 (3.35) ε= 0 0 0 , 0 0 ε   σr 0 0 σ = 0 σr 0 , (3.36) 0 0 σz ⎡ ⎤  −εp 2 0 0 εp = ⎣0 (3.37) −εp 2 0 ⎦. 0 0 εp

From generalized Hooke’s formula, the radial stresses are expressed as

σz = (λ + 2μ)ε

σ = λ tr(ε) + 2μ(ε − εp ),

(3.38)

σr = λε (elastic)

(3.39)

Plastic strain induced by LSP, ε p

σz = (λ + 2μ)ε − 2μεp

0

σr = λε + μεp

Elastic PH deformation

Hugoniot limit ⫽ straining condition

(elastic–plastic).

2PH

(3.40)

P Bounding condition Reverse straining with surface release waves

2PH 3l⫹2m ⫽bounded plastic strain

Plastic deformation bounding

Figure 3.34 Surface plastic strain dependence on peak pressure induced by a laser impact. Below PH , no plastification occurs; between PH and 2PH , plastic strain occurs with a purely elastic reverse strain; above 2PH , elastic reverse strain is bounded to 2PH and plastic strain is also bounded to 2PH /(3λ + 2μ). Above 2.5 PH , in reality, surface release waves focus and amplify from the edges of the impacts thus modifying the residual stress field (see Fig. 3.20). Schematically after Ballard [264] and Peyre et al. [190,230].

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Shock processing

From von Mises yielding criterion in a biaxial condition, |σr −σx | = σY − σ0 , the Hugoniot limit PH becomes   λ 1−v PH = 1 + (σY − σ0 ) (3.41) · (σY − σ0 ) = 2μ 1 − 2v Without initial stresses, the introduction of plastic strain in the Ballard’s model can be schematically represented as shown in Figs 3.34 and 3.35. Table 3.6 presents a summary of Ballard’s theory results, and Fig. 3.36 compares the experimental values of HEL with theoretical ones. 2

sx P

ing oad

Plastic loading

unl stic

PH ⫺YS

1

Elastic loading 4

␴r

⫽

YS ⫺

Magnitude of elastic release waves

Ela

Impact pressure

2PH

0

⫺ ␴x

3 Plastic unloading sr

⫺YS

P ⭓ 2PH

von Mises criterion

Figure 3.35 Surface stress excursion during pressure transient at laser peening according to the model by Ballard [230]. Reproduced with kind permission of Springer Science and Business Media. 3 Previous studies This study

2.5

55C1 steel (ferritic)

2 HEL (GPa)

X12CrNi12-2 (martensitic)

1.5 316L stainless steel (austenitic)

1

7075

HEL ⫽ 0.5 0

(1⫺v) s (1⫺2v) Y

AI-12%Si AI-7%Si 0

0.2 0.4 0.6 0.8 Static yield strength sY (GPa)

1

Figure 3.36 Dependence of the Hugoniot elastic limit (HEL) of various materials under laser shock loading on the corresponding static values [239]. American Institute of Physics (1998), reprinted with permission from Ref. [239].

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Table 3.6 Analytical expressions for mechanical effects induced by a fast laser shock impact on an elastic-perfectly plastic material. After Ballard [264], Dubouchet [266], and Peyre et al. [190, 230, 267]. Calculated value Plastic strain condition (Hugoniot elastic limit – HEL) PH

Equation number Comments

Analytical formula   λ (σY − σ0 ) PH = 1 + 2μ

(3.42)

For a pure uniaxial deformation; increases with σ0 < 0

  Peak pressure condition λ P = 2P = 2 1 + (σY − σ0 ) sat H (saturated plastic strain) 2μ

(3.43)

Plastic deformation

(3.44)

Starts at PH , saturates at 2PH and depends linearly on P

(3.45)

Drives εp to saturation

(3.46)

Depends linearly on the pressure duration τ

εp = −

2PH 3λ + 2μ

Optimal pressure

P = 2–2.5 PH

Plastified depth (triangular pressure pulse)

Cel Cpl τ L= Cel − Cpl

Superficial residual stresses (square impact) Superficial residual stresses (circular impact)

σsurf = σ0 − σsurf







P −1 PH



  

P − (σY − σ0 ) 1 + λ 2μ    2σY 1 + λ 2μ

μεp

1+ν + σ0 1−ν



 √  Lp 4 2 1− (1 + ν) π a

1+ν = σ0 − μεp + σ0 1−ν



(3.47)

 √ (3.48) Lp 4 2 (1 + ν) √ 1− π r0 2

Increases with εp Decreases with Lp Increases with σ0 < σ Increases with the size of the impact

Nomenclature a – square-shaped impact edge r0 – circle-shaped impact radius τ – pressure pulse duration (FWHM) P – shock pressure λ, µ – Lamé constants ν – Poisson’s ratio σ0 – initial residual stress (for unshocked material) σY – static yield strength (actually, the dynamic yield strength should be used [229], see Fig. 3.37) σsurf – surface residual stress εp – plastic deformation induced by LSP Lp – plastically affected depth Ce and Cp , the speeds of elastic and plastic longitudinal waves in the target [268].

Cel =



λ + 2µ = ρ



(1 − ν)E 1 · , (1 + ν)(1 − 2ν) ρ

Cpl =



λ + 2μ/3 = ρ



E 1 · 3(1 − 2ν) ρ

(3.49) and (3.50)

Chen, Hua, Cai Chen et al. [269] assume the residual axial stress profile exponential and the same over the shocked area, σz (z) = EkPmax e −bz/E ,

(3.51)

where Pmax is peak shock pressure at the surface, and k and b are empirical constants. It follows for in-plane stress Eν kPmax e −bz/E σx (z) = σy (z) = (3.52) ν−1

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Shock processing

For 35CD4 steel, k = 2.3 × 10−6 MPa−1 and b = 2.16 × 108 MPa/m. The plastically affected depth becomes:  Pmax −9 −6 −1 zpl = 4.63 × 10 · E · ln 2.3 × 10 · (ν − 1) · E · ν · (3.53) σY

Forget, Strudel, Jeandin Forget et al. [245] presented an analytical model of surface residual stress distribution for circular impacts. The main assumptions taken in this model were: (1) (2) (3) (4)

2D-model, planar uniform circular loading. Neither yield strength nor Hugoniot elastic limit were considered. Energy dissipation (friction and plastic deformation losses) was not taken into account. When subjected to a stress loading σ, the material instantaneously reacts according to Hooke’s law: σ = εe ,

(3.54)

where  is the stress tensor. (5) Plastic flow appears progressively with time, εp being proportional of the time and the deviatoric part of the stress tensor:  εp = C s dt, (3.55) where C is a proportionality coefficient depending on the material and s is the deviatoric part of the stress tensor. The matter in the shock area deforms according to ε = εe + εp ,

(3.56)

where the only non-zero value is εzz (planar shock). (6) At the impact boundary, two cylindrical waves are created, one of whose propagates towards the centre of the impact at speed c. The amplitude of the waves was estimated by ur =

w (r − r0 + ct)f (r) c

for r0 − ct < r < r0 ,

(3.57)

ur =

w (r0 − r + ct)f (r) c

for r0 < r < r0 + ct,

(3.58)

where w =

ν 2ρc(1 − ν)

(3.59)

and f (r) =



r0 r

(3.60)

with notations: r0 is radius of the impact,  is plasma pressure, ν is Poisson’s ratio, ρ is density of the target. (7) Kinetic energy during the wave movement was assumed constant (no energy dissipation occurs), therefore the displacement increases as the wave approaches the centre resulting in a wave focusing and residual stress drop formation.

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Yield stress (MPa)

1600

800

Mayer, Tension Mayer, Compression Follansbee, Compression Maiden & Green , Compression LSP analysis model

0 1.E⫺03

1.E⫹00

1.E⫹03

1.E⫹06

Strain rate (per second)

Figure 3.37 Titanium-6Al-4V yield strength strain-rate dependence [270]. © Elsevier.

Despite its simplicity, the model realistically predicted the magnitude and distribution of surface residual stresses in an Astroloy sample.

Numerical models Numerical models enable to take into account dynamic and non-linear phenomena, additional material parameters, real shape of the workpiece, and coupling between mechanical and thermal phenomena, including damping, viscosity, work hardening, thermal stresses, and strain rate effects (Fig. 3.37). Using numerical models, it is possible to study the phenomena inside the workpiece, not accessible to measurements. Equations of state Mie–Grüneisen equation of state (EOS) is frequently used for solids and liquids. It establishes an hydrostatic relationship between pressure P and internal energy E with reference to the material Hugoniot curve:   Ŵη ρ0 C02 η · 1 − + Ŵρ0 (e − e0 ), (3.61) p = p0 (1 − Ŵη) + (1 − sη)2 2 where ρ0 , ρ  ∂T β V Ŵ=− · , = T ∂V S κ · ρ · cV η=1−

(3.62) (3.63)

with notations: ρ0 is the density, C0 is the speed of the sound, Ŵ = Ŵ0 is the dimensionless Grüneisen coefficient in normal state, e − e0 is specific internal energy (per unit mass), s is the linear Hugoniot slope coefficient s = dUs /dup , β is the volumetric thermal expansion coefficient, κ is isothermal compressibility, cV is the heat capacity at constant volume. Mie–Grüneisen EOS is used for example in SHYLAC code and in laser peening simulations by Peyre et al. [229]. Quotidian equation of state (QEOS) Quotidian equation of state is a general-purpose analytical equation of state model for use in hydrodynamic simulation of high-pressure phenomena. Electronic properties are obtained from a modified Thomas–Fermi statistical model, while ion thermal motion is described by a multiphase equation of state combining Debye,

101

Shock processing

Grüneisen, Lindemann, and fluid-scaling laws. The theory gives smooth and usable predictions for ionisation state, pressure, energy, entropy, and Helmholtz free energy. When necessary, the results may be modified by a temperature-dependant pressure multiplier which greatly extends the class of materials that can be treated with reasonable accuracy. (citation from the article by More et al. [271]). The QEOS is applicable for both solid and gaseous states. Quotidian equation of state is presented through Helmholtz free energy per mass unit F(ρ, Te , Ti ) = Fi (ρ, Ti ) + Fe (ρ, Te ) + Fb (ρ, Te ),

(3.64)

where ρ is density, Te is electron temperature, Ti is ion temperature, Fi is ion free energy, Fe is electron free energy, and Fb is a correction for chemical bonding effects that can also represent exchange or other quantum effects. The expressions for the terms of Eq. (3.64) and application examples are given in the op. cit. [271]. Quotidian EOS was used for simulation of laser peening by Colvin et al. [251]. Linear equation of state is given by P = KV ,

(3.65)

where K is the bulk modulus, K = E/3(1 − 2ν) This EOS was used by Braisted and Brockman [270] in a 2D-axisymmetric numeric simulation of laser shock propagation and residual stresses in Ti-6Al-4V and 35CD.

Stress–strain constitutive relations Johnson–Cook law To reproduce the stress–strain dependence at high strain rate, the Johnson–Cook plasticity law with isotropic work hardening was used in FEM-simulations by Peyre et al. [272 , 229] and Fan et al. [273] The Johnson–Cook law enabled to take into account the strain rate dependence of the stress between ε0 = 10−2 /s (quasi-static load) and ε = 106 /s occurring at the laser shock. The stress σ is expressed as σ=

(A + Bεneq )





ε˙ 1 + C ln ε˙ 0





× 1−



T − T0 Tm − T 0

m

,

(3.66)

where A, B, C, and n are material constants (e.g. for pure aluminium A = 120 MPa, B = 300 MPa, n = 0.35, and C = 0.1), εeq is equivalent plastic strain, and ε˙ is strain rate, ε˙ 0 is strain rate under quasi-static loading, T0 is the reference temperature (e.g. 20◦ C), and Tm is the melting temperature. Steinberg–Cochran—Guinan model The constitutive relations for G and Y as functions of ε, P, and T for high ε˙ in this model are [274] 

G = G0 1 +



n

Gp′ G0 



P + √ 3 η



GT′ G0

Yp′



P + √ 3 η

Y = Y0 [1 + β(ε + εi )] × 1 +



Y0





(T − 300) ,



GT′ G0



(3.67)



(T − 300) ,

(3.68)

subject to limitation that Y = Y0 [1 + β(ε + εi )]n ≤ Ymax .

(3.69)

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Notations: G is the shear modulus, Y is the yield strength (in the von Mises sense), P is the pressure, Y0 and G0 are the values of reference state (e.g. T = 300 K, P = 0 Pa, ε = 0), η is the volume compression coefficient, defined as the initial specific volume v0 divided by the specific volume, v, β, and n are the work-hardening parameters, εi is the initial plastic strain, normally equal to zero. Primed parameters are defined as: GP′ =

dG , dP

GT′ =

dG , dT

YP′ =

dY , dP

YP′ G′ ≈ P. Y0 G0

(3.70)

Tabulated values of this model’s parameters for 14 materials are given in the op. cit. [274]. Steinberg–Cochran–Guinan model does not take into account strain rate effects. It was used by Zhang and Yao [275] at modelling of laser shocking by micrometer-sized impacts at pressures above 10 GPa, where rate-dependent effects played a minor role, but the pressure effects were of importance.

Selected numerical LSP modelling codes and cases SHYLAC SHYLAC code (Simulation Hydrodynamique Lagrangienne des Chocs) was developed at Laboratoire de Combustion et de Détonique (LCD), ENSMA, Poitiers, France [241, 276]. It enables 1D-simulation of elasto-plastic and hydrodynamic response of materials under a laser-driven loading. The code includes a Mie– Grüneisen equation of state referenced to the linearized Hugoniot curve, and elasto-plastic behaviour for solid materials. The SHYLAC code can also simulate the spallation process. The data required for the simulations are the laser–matter interaction pressure profile and the mechanical properties of the material: density, yield strength, shear modulus, Mie–Grüneisen coefficient, bulk sound velocity, and linear Hugoniot slope coefficient. Braisted and Brockman Braisted and Brockman [270] performed a 2D-axisymmetric numeric simulation of laser shock propagation and residual stresses using ABAQUS software. The materials (Ti-6Al-4V and 35CD4) were modelled as elastic-perfectly plastic with a yield strength defined by Y = HEL(1 − 2ν)/(1 − ν). Thus, it was assumed that all the plastic deformation occurs at roughly the same high strain rate. From consideration, that the pressure levels induced during LSP are generally less than 3 times the HEL, a linear equation of state was used (see above). The materials were specified by four constants, ν, E, HEL, and ρ, only. Sano, Yoda, Mukai, Obata, Kanno, Shima Sano et al. [277,278] conducted 3D-axisymmetric and spherical FEM simulations of shock propagation and residual stresses at laser peening of SUS304 stainless steel, taking into account adiabatic cooling of the plasma, realistic stress–strain relation (without idealizations), and strain hardening. Ding and Ye Ding andYe [186] considered the target elastic-perfectly plastic, but took damping and materials viscosity into account. The damping was accounted by a ‘damping stress:’ σd = βR Del ε˙ ,

(3.71)

where βR is constant, Del is elastic stiffness, and ε˙ is the strain rate. Viscosity was introduced with purpose to improve the modelling of high strain rate phenomena (to limit numerical oscillations). The ABAQUS/Explicit algorithm contains: (a) Linear bulk viscosity stress σ1 = b1 ρCd L e ε˙ ,

(3.72)

103

Shock processing

where b1 is a damping coefficient, ρ is density, Cd is dilatational wave speed, L e is element characteristic length and (b) Quadratic bulk viscosity σ2 = ρ(b2 L e )2 |εvol | min(0, ε˙ vol ),

(3.73)

where b1 is a damping coefficient and ε˙ vol is the volumetric strain rate (was applied only when the volumetric strain rate was compressive). The simulations were performed by ABAQUS in up to 3D.

Zhang, Yao, Noyan Zhang et al. [263] performed ABAQUS simulations of laser micro-shocking of copper/silicon bilayer structures with copper layer thickness of 1, 1.5, and 3 µm, and silicon thickness of 20 µm. Laser spot diameter was 12 µm. At simulation of tangential sliding at interface, the Coulomb’s friction law was used τ = μσn ,

(3.74)

where τ is the frictional shear stress, μ is the friction coefficient, and σn is the normal (compressive) stress. The plasma pressure model developed in this work was described above.

Fan, Wang, Vukelic, Yao Fan et al. [273] report about an explicit/implicit finite element simulation (using ABAQUS) of microscale materials processing by laser-generated shock waves. Explicit dynamic analysis was implemented for shock wave propagation in strain-rate dependent and elastic–plastic solids, and implicit analysis was applied for relaxation of pressured materials. The Mie–Grüneisen equation of state was implemented; the materials were 5-mm thick Al samples (simulation of peening) and 100-µm thick copper sheets (simulation of forming). Peyre, Chaieb, Braham In the recent work by Peyre, et al. [229] 2D-axisymmetric shock propagation and residual stresses in 12Cr and 316L stainless steels were simulated by ABAQUS/Explicit software (12 000 elements). The materials were assumed to follow the Grüneisen EOS and Johnson–Cook’s plasticity model. The simulation agreed with experiment rather well except a 50–100 µm thick surface region, probably due to the ignorance of initial stresses and surface waves phenomena, or due to inadequacy of X-ray stress measurements.

3.3.7 Applications of laser peening There are two important applications of laser peening: the treatment of aeroplane components, and of nuclear reactor components. In both cases water confinement is used.

Aeroplane components Turbine blades, rotor components, fastener holes, etc. have been treated with laser shocks with purpose to enhance/restore their fatigue strength. A running water curtain on the workpiece has been commonly used (Fig. 3.38) and the productivity reaches 1 m2 /h [191] (see also the book by Ding and Ye [186], pp. 43–44 for a short overview).

Nuclear reactor components Laser peening technology for in situ treatment of nuclear reactor components against stress corrosion cracking (SCC) was developed in 1990s by Toshiba Corporation in Japan and has since been applied to reactor core shrouds and nozzle welds of 10 nuclear power reactors in Japan [280] (Fig. 3.39). LPwC process is applied (Tables 3.7 and 3.8).

104

Handbook of Liquids-Assisted Laser Processing

134

Workpiece

119

Water

Water

20 103

AX2

Laser beam

16 121

05

AX1

102

A1 145

A2 153

Laser beam

152 LD

Figure 3.38

Schematics of simultaneous dual-sided laser shock peening process (after EP1088903 [279]). Optical fiber

Laser system

Controller

Shroud Remote handling system

CRD stub tube

Figure 3.39 Fibre-delivered laser peening system for control rod drive (CRD) stub tube of boiling water reactor (BWR) [281]. © ASME, reproduced with permission from Ref. [282]. Table 3.7 Comparison of laser peening process parameters in aerospace industries and in nuclear reactor maintenance (after Sano et al. [282]). Parameter

Aerospace industry

Nuclear reactor maintenance

Protective coating (ablator)

Yes

No

Laser wavelength

1064 nm

532 nm

Pulse duration

2 GPa were recorded; peak pressure rises nearly linearly with laser power density; paint coating had significant effect on peak pressure only at power densities ≈1 GW/cm2

Fabbro (1990) [233]

CMSX-2,AM1 (both single crystalline), Astroloy (polycrystalline)

Water/black paint

Nd:glass, 1.06 µm, 0.6–40 ns Nd:YAG, 1.06 µm, 25 ns

Combustion turbine blade materials (Ni-based) processed; spot diameter mostly 8 mm; diffraction patterns observed on shocked surface; shock-induced residual surface stresses were ≈3 times lower in the middle of the shocked area (Fig 3.22); central stress drop was avoided by overlapping shocks; surface roughness was practically unaffected by laser shocks; a 2D analytical model for residual stresses explaining also the central stress drop is presented

Forget (1990) [245]

18 Ni(250) maraging steel (4.15 mm), also weld zones

Water (≈3.5 mm)/ black paint (≈0.1 mm)

Nd:YAG, 1.06 µm, 100 ps, 20 mJ, 8 Hz, spot ≈0.1 mm, ∼1012 W/cm2 , scanned beam

LSP results have been compared with shot peening (20 min with chopped steel fibres of 0.5 mm diameter and of 1.5 mm length in a 0.54 MPa air, 100% coverage) results; the modified depth (raised hardness and compressive residual stresses) were 0.05 and 0.25 mm for LSP and SP, respectively, the residual stresses in weldments – 416 and 893 MPa, and the fatigue strength of welded samples (2 × 106 cycles) 380 and 587 MPa, respectively

18 Ni(250) maraging steel

Water (≈3.5 mm)/ black paint (≈0.1 mm)

Nd:YAG, 1.06 µm, 150 ps, 8 Hz, spot ≈0.1 mm, scanning with shot overlapping

Shock processing increased the hardness and fatigue strength (17%) of weldments; plastically affected depth was ≈50 µm, containing reverted austenite phase and increased dislocation density (according to TEM studies)

Bana´s (1990) [316]

Al (foils 10–300 µm)

Glass

108 –1010 W/cm2

Reports about an 1D-code and results of calculation of plasma pressure and velocity of confined laser-shocked foils; the simulation code is based on Mie-Grüneisen equation and elastic–plastic behaviour of materials; best fit with experiment was achieved using α ≈ 0.1 for 15–40 ns laser pulses and α ≈ 0.01 for 3 ns pulses (α – fraction of incident laser energy absorbed by the plasma and transformed into shock energy); calculated pressure and velocity transients presented for Al foils confined by glass

Romain (1990) [317]

35CD4, XC38

Glass or water (2 mm)/ black paint

Nd:glass, 1.06 µm, 2.5, 3, 25 and 30 ns (Gaussian or asymmetrical), spot 8 mm, up to 70 GW/cm2

Residual stress distribution vs. depth for irradiance power densities 1–70 GW/cm2 presented; repeated shocks increase the plastically affected depth but decrease the surface stress (1–6 laser pulses); laser peening increased fatigue strength 40% and did not affect the surface roughness

Fournier (1991) [247]

35CD4 50 HRC steel

Water

30 ns, 8–10 GW/cm2

The theory presented in Ballard (1988) [314] is laid out in more detail; measured residual stresses and plastically affected zone depth agreed fairly with the calculations

Ballard (1991) [265]

304 steel

No/black paint

Nd:glass, 0.6, 2.5 and 25 ns, spot 1 and 4 mm, 0.1–5TW/cm2 up to 55 Gpa

Maximum microhardness (∼400 kg/mm2 ) was achieved at ∼20 GPa (25 ns) respectively ∼40 GPa (0.6 and 2.5 ns); the maximum hardness depended little on laser pulse length; twin density increased with increasing shock pressure (histograms for 1–4 twin systems presented); α-phase embryos were found in material shocked by 25 GPa pulses

Hallouin (1991) [318]

Bana´s (1990) [215]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

Waspaloy (SC with γ ′ -precipitates, 4 mm)

Glass (5 mm)/ Al foil or Sellotape

7075-T6, 2024-T3, and 5456 Al alloys; 1026 and 4340 steels

Novel features, observed phenomena, comments

References

Nd:glass, 1.06 µm, 20–30 ns, 60 J, spot 4, 5 and 8 mm, 0.5–9.5 GW/cm2

Measured peak shock pressure was ∼4 GPa; shearing configurations involving superlattice stacking faults were observed in the shocked zone; the shock propagation was simulated by EFHYD code – the physical model used in EFHYD code is explained in detail

Décamps (1991) [319], Puig (1992) [320]

Mostly water/ black paint

1.06 µm, ≈20 ns, spot 5–10 mm

Data about residual stress distribution, fatigue life and surface hardness of laser-shocked samples presented

Clauer (1991), (1992) [321, 322], Vaccari (1992) [323]

Fe–Ni alloy (TRIP alloy, 30% Ni), 2-mm thick sample

Vacuum

0.53 µm, 1 ns, 4 kJ, spot 4.3 and 25 mm, 1011 and 1013 W/cm2

Martensitic transformation close to the back face of the sample observed, obviously induced by the expansion wave generated at wave reflection from the back face; nearly sinusoidal depth distribution of residual stresses (period ≈150 µm) was observed

Grevey (1992) [324]

AISI 316L

No/black paint

0.6 ns, 80 J, spot 7.2 mm, 0.3TW/cm2 , 18 GPa

LP was compared with explosive shock treatment (1–2 GPa, 1 µs); the formed microstructures and hardness profiles were quite similar; surface mirohardness was raised from 180 HV to ∼300 HV in both cases, the surface hardness of LP processed samples was somewhat higher (345 HV) but the hardness decrease was more faster than in explosively treated material; the residual stresses in both cases were stable during a whole cycling of a plastic fatigue test (constant plastic strain rate 2 × 10−3 s−1 ), contrary to SP processed material

Gerland (1992) [325]

Al (foil 25–150 µm and 1 µm layer on quartz)

Water (2–3 mm), quartz (6 mm)

Nd:glass, 1.06 µm, 3 and 30 ns (Gaussian and short-rise pulses), spot 5–6 mm

Measurements of the pressure induced by the plasma performed; for 1–10 GW/cm2 laser energy density the measured pressure agrees particularly well with an analytical model; at high-power densities (10W/cm2 ), the dielectric breakdown appears to be the main limiting process of the confining method; for shorter laser pulses the breakdown threshold was higher; short-rise-time laser pulse provided greater peak pressure than Gaussian pulse

Devaux (1993) [326]

304 austenitic steel (foil)

No/black paint

Nd:glass, 0.6 ns, spot 3–6 mm,

Surface hardness increased with peak shock pressure up to 25 GPa (0.5TW/cm2 ), then remained rather constant

Gerland (1994) [327]

0.25–1.62TW/cm2 , 15–60 GPa

up to 55 GPa (1.5TW/cm2 ), then decreased at higher pressures; twins were present at any shock pressure, the number of twin sets increased with pressure up to 25 GPa, then decreased, but the mean twin spacing continuously decreased; α-phase embryos were only present in the pressure range 15–25 GPa; histograms of the dependence of twin density on twin spacing for 1–4 twin systems are presented A review (17 pp., 22 ref.) of techniques, theory and applications of laser shock treatment of materials

Peyre (1995) [230]

7075-T7351

Water/Al self-adhesive foil (0.1 mm)

Nd:glass, ∼25 ns, spot 0.5–1 mm (square and circular), 1–7 GW/cm2

Residual stress and hardness profiles, and fatigue resistance were determined for both laser shocked and shot peened (0.6 mm steel beads,Almen intensity F20-23A/F23-27A, coverage rate 125%) samples; LSP was found to provide greater improvement of fatigue limit than SP; combined SP + LSP treatment was found to be beneficial; the experimental results are compared with analytical and numerical (SHYLAC 1D) models predictions

Peyre (1995) [328]

AM1 (SC, [001], [111] and [110]), Inconel 718

Water/black adhesive tape

17–40 ns, spot 8 mm, up to 16.9 GW/cm2

Superficial micro-roughness of laser-shocked samples studied; variation of deformations with distance from centre of the spot is explained by release wave propagation and formation of residual stresses

Forget (1995) [329]

≈8 GW/cm−2 , overlap 66%, 1–3 passes

Virmoux (1996) LSP processed samples were fatigue-contact tested [330] (107 cycles) in cryogenic conditions (−195◦ C); nearly 50% increase in the maximum sustainable contact force (from 8 to 11.5 GPa) and huge reduction of oxidation was achieved; surface hardness of shocked samples was enhanced up to a depth of ≈1 mm (3 passes) (see also Fabbro (1998) [250])

Nd:glass, 1.054 µm, 0.6 ns, 100 J, spot 3–3.5 mm, 2.4×1012 W/cm2

LSP caused extensive formation of ε hexagonal close-packed (hcp) martensite (35 vol%) and caused up to a 130% increase of surface hardness; the LSP strengthening effect was attributed to the combined effects of the partial dislocation/stacking fault arrays and the grain refinement due to the presence of the ε-hcp martensite; a comparison with shot peening is presented; SEM/TEM micrographs of materials structure (11 photos) are presented and discussed

Z100CD17 (martensitic)

Hadfield manganese steel (1%C-14%Mn, 3.3 mm)

Vacuum/black paint (40–50 µm)

Chu (1995) [331]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

0.55% C steel

Water or glass

SUS304 (austenitic, 20 mm)

Novel features, observed phenomena, comments

References

Nd:glass, 1.06 µm, 25 ns, spot 5 mm (circle and square), 1.7–10 GW/cm2 , 25% overlap

Residual compressive stresses down to depth of 1.1 mm exceeding up to –350 MPa on surface were formed; shocked surface depression was up to 7 µm; the surface hardness was not modified; the central stress drop was present in case of circular laser spot, but was absent in case of square spot

Masse (1995) [246]

Air and water (specimen were immersed into water)

Cu-vapour (511 nm) and Nd:YAG (532 nm), 5–50 ns, spot 0.2–1.1 mm, 75–375 J/cm2 , 15–75TW/m2

The specimen were laser peened using scanned beam, coverage 500–8000%; surface compressive residual stresses of 200–400 MPa were built up; the depth of compressive residual stresses was over 200 µm; the peened sample’s surface was oxidized down to a depth of 3 µm; high-speed photographs of plasma radiation in air and in water presented – in water the plasma lasted ∼10 ns; plasma expansion velocity in water was ∼1500 m/s, in air ∼7700 m/s

Mukai (1995) [332]

A356-T6,Al12Si-T6, 7075-T7351 (Al alloys)

Water (2–5 mm)

Nd:glass, 1.06 µm, 25 ns (Gaussian), spot 5–12 mm, 1–8 GW/cm2 ; up to four impacts (square, ellipse or circle, up to 67% overlapping)

Laser shock-induced residual compressive stress field extended to depth more than 1 mm, surface hardening was limited to +10% of the initial value, half of the increase achievable by conventional SP (+22%); in contrast to SP, laser shocking did not affect the surface roughness of the materials; fatigue life (107 cycle tests) of lasershocked specimen exceeded these of shot peened; a review of LSP theory with reference to the theses of Ballard (1991) [264] and Dubouchet (1993) [266] is included

Peyre (1996) [190]

55C1 steel

Water (3–5 mm)/ paint (100 µm)

Nd:glass, 1.06 µm, 20–25 ns, spot 1–2 mm, 1 shot every 1.5 min

It was demonstrated that LSP with 1–2 mm spot-size range could provide at least as beneficial surface effects as larger ones but limited to about 0.8 mm in depth whereas large impacts affected more than 1.2 mm at the same power densities; simulation by SHYLAC demonstrated that the shock wave from small impacts decayed earlier because of spherical attenuation

Peyre (1996) [333]

55C1 steel (10 mm)

Glass/PMn treated surface

Nd:glass, 1014 W/m2 6 GPa

Quenched by 5 kW CO 2 laser in Ar atmosphere sample was shocked by Nd:glass laser; a 50% increase of surface stresses (−280 to −420 MPa) and a suppression of the tensile peak at depth of ∼500 µm was observed; the physical processes and analytical modelling of LSP are reviewed

Peyre (1996) [267]

Al (457 µm foil, optionally supported by BK7 glass plate)

Water (3–4 mm)

Nd:glass, 1.064 µm, 20 ns, ≈40 J, spot 3 mm, up to 28 GW/cm2

Shock pressure dependence on laser fluence determined by comparison the measured by VISAR rear side velocity (exceeds 250 m/s at 2 GW/cm2 ) of the target with theory; as confirmed by high-speed photography of plasma radiation, the pressure saturates at light intensities ≈10 GW/cm2 due to optical breakdown at the water surface; due to the breakdown, the laser pulse length at target shortens; numerical simulations of target velocity by SHYLAC are compared with VISAR measurements

Berthe (1996) [334], (1997) [335]

SUS304

Water

2ω-Nd:YAG, 532 nm, 5 ns, 10 Hz, spot 0.75 mm

Konagai (1996) [336]

Cu-vapour, 511 nm, 60 ns, 4 kHz, spot 0.5 mm

Relatively large pulse frequencies were used resulting in large shot coverage factors of 500–8000%; peak power densities ranged 15–75TW/cm2 ; residual compressive stresses exceeding 400 MPa were developed over 100 µm in depth; in case of 60 ns, 4 kHz laser irradiation, a 10 µm surface layer exhibited tensile stresses

SUS304, Inconel 600

Water

2ω-Nd:YAG, spot 0.75 mm, 230 kJ/m2 , 50TW/m2

Compressive residual stresses over 100 MPa were created Sano (1996) [337] by laser shocking

Al, Cu (20–120 µm foils)

Water (on both sides of the foils)

Nd:glass, 1.06 µm, ≈20 ns, ≈4 GW/cm2

Experimental, numerical, and analytical study results of Romain (1997) [338] the acceleration and deceleration process of thin metallic foils presented; peak velocities reached 650 m/s and shock pressures ≈2.5 GPa

SUS304

Water

2ω-Nd:YAG, 532 nm, 5 ns, spot 0.75 mm, 45TW/m2

Compressive residual stresses over 200 MPa were created Sano (1997), (1998) by laser shocking; plasma radiation photographs at 5, 15, [242, 339] and 25 ns in air and in water presented; plasma expansion velocity was ≈1500 m/s in water and ≈7500 m/s in air; 20% of the plasma energy was estimated to be converted into thermal energy; and plasma pressure to exceed 2 GPa; the measured values were compared with calculated ones (Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

Novel features, observed phenomena, comments

References

Plane and surface waves propagation in lasershocked workpiece and their interaction is discussed in detail; contains also a short review of principles of LSP and its applications

Dubrujeaud (1997) [244]

A review (13 pp., 11 figs.) of LSP with accent to residual stress distribution and fatigue properties; in soft alloys a slight surface depression about 6 μm occurs while laser shocked

Clauer (1997) [340]

AISI 316L (austenitic), Z12 CNDV 12.02 (martensitic)

Water or glass/ metallic paint or foil

Nd:glass, 1.06 µm, 2.3 and 20 ns, spot 6 mm, up to 40 GW/cm2 , up to 12 shots

Evolution of plastically affected depth, surface hardness, residual stresses with the number of laser shots (up to 12 shots, 316L steel) presented; laser impacts overlapping of 66% provided even distribution of residual stresses (Z12 steel); corrosion properties of Z12 steel were advantageously modified in course of LSP: free potentials were shifted to anodic values and passive current densities reduces; when the pressure pulse length was 0.6 ns, the breakdown in water occurred at ∼120 GW/cm2

Scherpereel (1997) [341]

Al (200 µm foil), AISI 316L (200 and 500 µm), 55C1 (360 µm and notched samples), 7075-T7351 (1.6 mm)

Water/Al-based paint or adhesive (90–140 µm)

Up to 8.5 GW/cm2 , spot sizes 1 and 6 mm

VISAR-measurements of back free velocities (BFV) behind foil targets (up to 500 m/s at ≈6 GPa) agreed well with SHYLAC-simulations; paint/adhesive layers enhanced the shock pressure for ≈50%; HEL of materials was determined from BFV at elastic-plastic inflection point (1.2 σY for 7075; 2.3 σY for 316L and 55C1); using small impacts (1 mm, 25% overlap) the achieved residual stresses were higher, but surface waviness was ≈2 times greater (1,3 µm) than in case of 6 mm impacts (50% overlap) and ≈20 times greater than that of untreated material (55C1 steel); fatigue life of 55C1 was improved for ≈30% at R = σ min /σ max = 0.1

Peyre (1997) (1998) [342, 343]

A review (10 pp., 8 figs., 10 refs.) of LSP physics and applications (shock pressure, residual stresses, and fatigue life); comparison of maximum residual stress achieved in different materials presented (Fig. 3.25)

Peyre (1997) [344]

Metal plate

Water

Nd:glass, 1.064 µm, 25–30 ns, spot 3–4 mm, up to 25 GW/cm2

Optical breakdown at the water surface was investigated by transient transmission of a probe beam (514 nm): transmission cut-off occurred at ≈6 GW/cm2 , above 10 GW/cm2 , the power density transmitted by the plasma is limited to 10 GW/cm2 ; the laser pulse transmitted by the plasma corresponds approximately to the part of the incident laser pulse preceding the transmission cut-off

Berthe (1997) (1998) [345, 346]

Ti-6Al-4V (Inconnel)

Not specified

30 ns, 200 J/cm2

Laser shock-induced residual stress profiles for single and dual impacts are presented; construction and operation principles of the LLNL new 100 J, 6 Hz Nd:glass laser are laid out in detail

Dane (1998) [347, 348]

316L (200 µm), 55C1, 12%Cr (martensitic)

Water/Al paint (140 µm) or Al adhesive (100 µm)

0.6, 2.3, and 10–25 ns; spot 1 and 7 mm; up to 40 GW/cm2

For 55C1 steel, small impacts (1 mm) provided 30% greater fatigue limit (490 MPa at R = σmin / σmax = 0.1) than 7 mm impacts; for 12% Cr steel, corrosion tests were performed (10 mM NaCl + 10 mM Na2 SO4 ): LSP reduced the passive current density from 1.2 to 0.5 µA/cm2 , kept the pitting potential almost constant, and prevented anticipated initiations of pits or inclusions at lower potentials; the article also includes a concise review of LSP mechanisms, experimental techniques and applications

Peyre (1998) [349]

X12CrNiMo12-2-2 (martensitic, 0.2–1.29 mm); 316L (austenitic, 0.2–1.25 mm)

Water (3–4 mm)/ Al paint (70 µm) or Al adhesive (70 µm)

Nd:glass, 1.064 µm, 8–10 ns, spot 3–4 mm, 10 GW/cm2 , coverage rate up to 300%

Stress loadings close to 7 GPa, 20 ns were created at the surface of the targets;VISAR-measurements were used for determination of shock wave decay and for estimation, through the determination of elastic precursors, the dynamic yield strengths at strain rates approaching 106 /s; some 50–100% increases could be found between dynamic yield strengths and static plastic flow limits at 10−3 /s; simulation of attenuation of shock waves in depth matched well with experiments for 316 L steel but not for X12CrNiMo12-2-2; it was shown on 316 L that shock decays could be reduced on materials exhibiting a large work-hardening level such as shot-peened surfaces

Peyre (1998) [239]

Al,Ta, Cu, Mo (50 and 250 µm foils)

Air?

Nd:glass, 1.06 µm, 25 ns, spot 1–3 mm, 1010 –5×1011 W/cm2

Precise data concerning the pressure loading induced by pulsed laser irradiation at the front face of a solid material were obtained in the laser intensity range of 10– 500 GW/cm2 ; the pressure amplitudes determined in function of incident laser intensity, reached 60 kbar

Tollier (1998) [241]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

Al, 316L, 55C1, X12CrNiMo12-2-2

Water/Al-based coating (70–130 µm, optionally)

Nd:glass, 1.06 µm, 0.6–20 ns Nd:YAG, 1.06 µm, 6–7 ns, 10 Hz XeCl, 308 nm, 40 ns, 5 Hz

SUS 304

Water (sample immersed into water)

Nd:YAG, 532 nm, 5 ns, 100 mJ, spot 0.75 mm, 230 kJ/m2

Novel features, observed phenomena, comments

References

Shorter laser pulse duration provides greater peak plasma pressures (up to 9.5 GPa at 0.6 ns); shorter wavelength provides somewhat greater pressure, but water breakdown occurs at lower laser intensities;Al-based protective coating enhances plasma pressure ≈50%; surface hardness of laser shock peened surfaces remained lower than of shot peened surfaces; LSP increased the corrosion resistance of X12CrNiMo12-2-2 steel; comparison of results with other steels and Al and Ni-alloys is provided graphically

Peyre (1998) [243]

Experimental investigations of LSP physics: on part of laser pulse length and wavelength dependence of peak plasma pressure, see Ref. [243]; on part of water breakdown, see Ref.[345] and Ref. [346].

Berthe (1998) [350]

In addition to the results presented in Sano (1992) [242], this article presents the results of FEM-simulation (2D cylindrical coordinates) of shock wave propagation and residual stresses

Sano (1998) [339]

A comprehensive up to date review (15 pp., 33 figs., 52 refs.) of LSP

Fabbro (1998) [250]

2024-T62 (2.5 mm)

Glass (4.5 mm)/ black paint (0.1 mm)

Nd:glass, 1.06 µm, 30 ns, spot 7 mm, 0.7–1.75 GW/cm2

2 mm diameter hole area was shock treated from both sides separately; as a result of laser shock treatment the fatigue life of the sample was enhanced for ≈6 times (R = 0.1, 13 Hz); surface roughness lowered from Ra 6.3 µm to 0.1 µm; dislocation density increased and 110 µm deep dip was formed onto surface

Zhang (1999) [252]

Al

Water

1.064 µm, 3 ns, up to ≈50 GW/cm2

Laser beam interaction with water in typical LSP regime has been investigated experimentally by recording reflected laser light and water breakdown plasma radiation; the threshold of generation of confined plasma was 5 GW/cm2 and its absorption near 80%; the thickness of ablated matter was about 2 µm per

Berthe (1999) [351]

pulse at 23 GW/m2 ; the estimated (by comparison with simulations by ACCIC) temperature of plasma was ≈1 eV, electronic density 2 × 1022 / cm3 , and the coupling parameter ≈6 corresponding to strongly coupled plasma Al (150–200 µm foils, with open backside of on 5 mm BK7 glass)

Water

Nd:glass, 1,064, 0.532 and 0.355 µm, 25–30 ns, spot 1–3 mm, 1–20 GW/cm2

Comparison of laser wavelength dependence of plasma peak pressure and pressure pulse duration; shorter wavelengths provided ≈30% larger peak pressures, but water surface breakdown thresholds were lower; it was concluded that shorter wavelengths are advantageous for LSP

Berthe (1999) [258]

AISI 316L

Water/Al adhesive (40 or 80 µm)

3 and 10 ns, spot 12–13 mm, 6 and 20 GW/cm2 , 3 and 7 impacts

Despite no chemical changes at the lasershocked surface were detected (SIMS, EPMA), a corrosion improvement was obtained in NaCl 0.05 M; anodic shifts on pitting potentials nearly +100 mV were observed after treatment, together with increases of repassivating potentials during cathodic polarizations

Peyre (1999) [352]

Ti-6Al-4V (airfoil geometry sample)

Water curtain/paint

Spot 5.6 mm, overlap 30%

The narrower edge of the sample (0.75-mm thick) was laser shock processed from both sides simultaneously; results of fatigue crack growth rate and fractographic investigations are presented

Ruschau (1999) [353]

SAE1010 (ferritic, 1.3 mm)

Vacuum/black paint (40–50 µm)

Nd:glass, 1.054 µm, 0.6 ns, 120 J, spot 3 mm, 2.4 × 1012 W/cm2

LSP caused the surface to be recessed for ≈1.5 µm and resulted in extensive formation of dislocations; modified depth was ≈100 µm; surface hardness increased ≈80%; comparison with shot peening is presented

Chu (1999) [354]

50 ns, spot 8 mm. 2.8–5 GPa

2D-axisymmetric numeric simulation of laser shock propagation and residual stresses using ABAQUS software; pressure pulse was assumed to be whether triangular (for Ti-6Al-4V) or Gaussian (for 35CD4); the model realistically predicted the residual stress distribution, thereby the central stress drop

Braisted (1999) [270]

20 ns, 50 J

Laser shock stress amplitudes on the back of the targets were monitored with VISAR using LiF as the window material; the peak shock stress produced in LiF (titanium thickness zero) was measured to be 16 ± 1 GPa; the laser shock amplitude decays to about 2.7 GPa while propagating through 3-mm thick disk of titanium 6-4.

Brar (2000) [355]

Ti-6Al-4V, 35CD4

Ti-6-4 (0.1–3.05 mm, glued to 6.35 mm LiF)

Water (flowing)/ black paint

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

Novel features, observed phenomena, comments

References

A review (10 pp., 14 figs., 24 refs.) of achievements in LSP during 1995–1999: influence of laser intensity, wavelength and pulse length on LSP, effect of different protective coatings, optical breakdown in water, effect of LSP on materials residual stresses, fatigue, and corrosion properties

Fabbro (2000) [356]

SUS 304 (20% cold worked, 10 mm)

Water (sample immersed into water)

Nd:YAG, 532 nm, 8 ns, 200 mJ, spot 0.8 mm, 36 impacts/mm2

Surface tensile stresses converted to compressive up to depth of ≈1 mm; corrosion tests performed (water, 561 K, O2 8 ppm, 500 h,), LSP totally inhibited the stress corrosion cracking; FEM-simulation results (2D cylindrical coordinates) of shock wave propagation and residual stresses presented; a system for in situ processing of nuclear reactor core shrouds in water is described

Sano (2000) [277; 278]

SUS 304 (0–30% cold worked)

Water/no

Nd:YAG, 532 nm, 8 ns, 10 Hz, 100–300 mJ, spot 0.4–1.2 mm, 1800–5400 impacts/cm2

Residual surface compressive stresses were ∼40% greater in the laser beam scanning direction than in the transverse direction; surface residual stress had a maximum value at spot size 0.8 mm (constant pulse energy 200 mJ, 3600 pulses/cm2 ); at fixed spot size 0.8 mm the surface residual stresses had a minimum value at ∼200 mJ laser pulse; cold working rate had little influence on the residual stresses; the material’s surface was oxidized up to a depth of 1.2 µm

Obata (2000) [357]

SUS 304 (20% cold worked, 10 mm)

Water/no

Nd:YAG, 532 nm, 8 ns, 10 Hz, 200 mJ, spot 0.8 mm, coverage 1800%

Laser peening was applied in order to simulate neutron irradiation hardening; shocking with multiple laser pulses extends the stress-improved depth to ∼1 mm; numerical simulation (3D axisymmetric and spherical) results of shock propagation and residual stresses are presented; the agreement with the experiments was reasonable

Sano (2000) [277]

Al (polished)

Water

Nd:glass, 1.064 µm, 3 and 15 ns, spot 5–6 mm, 1–50 GW/cm2

Laser beam interaction with water in typical LSPregime has been investigated experimentally by recording reflected laser light and water breakdown plasma radiation; depending on laser pulse duration, the absorption of the confined plasma was 80–90%; the ablated thickness was 1.1 µm (3 ns squared pulse,

Berthe (2000) [257]

20 GW/cm2 ) and 0.75 µm (15 ns Gaussian pulse, 1 GW/cm2 ); plasma parameters were estimated with aid of an updated theory as follows: density range of heavy particles 3.4–4.6 × 1021 /cm3 , degree of ionization 0.39– 1.3, temperature 1.7–4.0 eV; coupling parameter of confined plasmas 0.2–1 Al (200, 457 and 1000 µm foils)

Water

XeCl, 0,308 µm, 50 and 150 ns, spot 1 × 4 mm, 0.1–6 GW/cm2

Peak pressures up to 2.5 GPa were generated, limited by optical breakdown in water at >1–2 GW/cm2 ; the plasma thermal to internal energy ratio α was estimated to equal 0.4 (at 1.06 µm, α ≈ 0.25)

Berthe (2000) [358]

AISI 316L (austenitic, 8 mm), G10380 (ferritic, 8 mm), G41400 (martensitic, 8 mm)

Water/Al based coating (0.1 mm)

Nd:YAG, 1.06 µm, 10–20 ns, up to 30 J, spot 6–7 mm, 1 shot/min

Laser-shocked G10380 and G41400 were corrosion tested in an acid HKSO4 -0.3 M solution; only in the case of G41400 martensitic steel was a reduction of the corrosion current observed, depending on the degree of work hardening and the amplitude of compressive stresses; laser shocking of AISI 316L suppressed extensively stress corrosion cracking during 24 h in MgCl2 44%, 153 ◦ C solution

Peyre (2000) [359]

AISI 316L (austenitic, 13 mm

Water (2–5 mm, flowing)/Al adhesive (100 µm)

1.06 µm, 10 ns Gaussian, up to GW/cm2 1.06 µm, 3 ns, 0.2 ns rise time, up to 20 GW/cm2

Structural changes in material are compared with those induced by shot peening (microphographs presented); surface residual stress of laser shocked material reaches ∼−500 MPa (6 GW/cm2 , 12 impacts) and hardness 250 HV (8 GW/cm2 , 6 impacts); laser-shocked surface layer was contaminated by C, O, and H for 0.4 µm in depth; corrosion tests in NaCl (30 g/l) showed an improvement of corrosion behaviour of both shot peened and laser shock processed samples

Peyre (2000) [360]

Al (50 and 100 µm), 316L (75 µm),Al12Si

Water (2–5 mm)/Al paint (12–60 µm)

1.06 µm, 0.6 ns (Gaussian) or 3.2 ns (rise time 0.2 ns), spot 6–10 mm, 0–200 GW/cm2

EMV and PVDF gauges as shock wave sensors were compared; the operation range of EMV gauge was 0–20 GW/cm2 and this of PVDF gauge 0–160 GW/cm2 ; both 0.6 and 3.2 ns laser pulses provided the same ∼9.5 GPa maximum pressure, limited by water breakdown; water breakdown threshold was ∼60–100 J/cm2 (0.6–10 ns) depending sublinearly on laser pulse duration; optimum Al overlay thickness was 12 µm, providing 25% more intense plastification of the target; maximum remanent surface deformation of Al12Si was ∼10 µm independent on laser pulse length

Peyre (2000) [237]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

AISI304 (20% cold-worked, 10 mm)

Water (flowing)

2ω-Nd:YAG, 532 nm, 5 ns, 10 Hz, 100 mJ, spot 0.6–0.7 mm

Novel features, observed phenomena, comments

References

Laser light was fed through silica fibre (core diameter 1.5 mm, length 5 m); tensile near-surface stresses (up to 400 MPa) were converted to compressive stresses (down to −700 MPa, peak ∼30 µm below the surface, irradiation 21 J/cm2 /4.2 GW/cm2 , 10 000 pulses/cm2 )

Schmidt–Uhlig (2000), (2001) [361, 362]

A review (11 pp., 7 figs., 7 refs.) of both shot peening and laser shock peening; procedure of calibration of shot peening machines using ‘Almen strips’ is described; laser and shot peening techniques and performances are compared; achievements in high-power pulsed lasers development at LLNL are overviewed

Hammersley (2000) [191]

Zhang (2000) [275]

Al 1100 (70 µm foil)

Water (3 mm)/Al foil (16 µm) and vacuum grease (∼10 µm)

3ω-Nd:YAG, 355 nm, 50 ns, spot ∼12 µm, 4 GW/cm2

Microscale LSP studies; ∼100–200 µm diameter dents on laser-shocked surface proved that shocking caused plastic deformation of the sample; process variables as shock pressure and plastic strain have been calculated in cylindrical coordinates (3D-simulation) by ABAQUS software, using Steinberg constitutive model taking into (account pressure effects but not strain rate effects)

2024-T62 (2.5 mm)

K9 glass/black paint

Nd:glass, 1,06 µm, 30 ns, 8.1–20.2 J, spot 7 mm, 0.7–1.75 GW/cm2

As result of LSP, the fatigue life of the specimen Tang (2000) [363] increased by 2.2–8.7 times (different specimen, tensile test, R = 0.1), hardness ∼30%, and surface residual compressive stress was ∼−30 MPa; fracture surface micrographs are presented

6061-T6 (6 mm, welded by 5083 and 5356)

Water (1–3 mm)/black paint

1064 nm, ∼40 ns, 6–8 J, 100 and 200 J/cm2 , spot ∼ 1.8 and 2.75 mm, 2.5 and 5 GW/cm2

Laser shocking increased the hardness down to 3 mm in depth; the elastic modulus of material was decreased in the bulk peak-aged material, but increased in the overaged metal in the HAZ and weld zone; the changes in elastic modulus were from −5% to +10% and in hardness from +4% to +76%; measured hardness and elastic modulus profiles are presented

Montross (2000) [364]

Al single crystal (111), 400 atomic layers

A review (22 pp., 23 figs., 36 refs.) of LSP, includes a description of processing equipment at LSP Technologies, Inc.

Clauer (2001) [365]

The transmission of breakdown plasma in water during LSP experiments was investigated theoretically for laser wavelengths from 355 to 1064 nm and pulse length of 25 ns; at 1064 nm the breakdown process was found to be dominated by avalanche ionization, but at 355 and 532 nm by multiphoton ionization

Sollier (2001) [260]

4ω-Nd:YAG, 266 nm, 1.5 and ps, spot 27.8 nm, 100 and 200 GW/cm2

Impact of an intense laser pulse on Al target was simulated by molecular dynamics method; ablation, shock wave formation and stress and dislocations generation has been simulated on ps-timescale

Fukumoto (2001) [366]

6061-T6 (6 mm)

Water (curtain 1–3 mm)/black paint

1.064 µm, ∼40 ns, 100 and 200 J/cm2 , spot 1.8 and 2.75 mm, 2.5 and 5 GW/cm2

A single impact of 2.5 GW/cm2 (3.5 GPa) increased the hardness at the surface from 1.19 to 1.34 GPa to a depth of 1.75 mm, and 5 repetitions did not significantly change the surface hardness or depth of shock wave property modification; a single impact of 5 GW/cm2 (6 GPa) increased the surface hardness to a value greater than that achieved with one or five repetitions with 3.5 GPa shock waves and five repetitions at 6 GPa significantly increased the surface hardness and shock wave property modification depth over one repetition at 6 GPa

Montross (2001) [367]

AISI 316L

Water/Al adhesive (50 µm, in case of 2.5 ns pulses) or bare surface (7 ns pulses)

2.5 ns, 80 J, spot 13 mm, 25 GW/cm2 , 3 or 5 impacts 7 ns, 0.2 J, spot 0.8 mm, 4 GW/cm2 , 16 impacts

Influence of LP and laser surface melting (CW, 954 nm, 1 kW, 25 kW/cm2 ) on pitting corrosion resistance in 50 mM Na+ Cl− was investigated; anodic shifts of pitting potentials were + 100 mV in case of LP and + 220 mV in case of laser surface melting; it was assumed that LP-induced compressive stresses promote the growth kinetics of the passive film (dual oxide and hydroxide composition) thus reducing the sensitivity to pitting; LP without protective coating resulted in cathodic shift in pitting potential

Peyre (2001) [368]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

Novel features, observed phenomena, comments

References

Ti-sapphire, 210 fs and 6 ns, 50 mJ, spot 0.09 mm2 , 260TW/cm2 and 9 GW/cm2 Excimer lasers, 45 and 50 ns, 1–1.5 J, spot ∼2 mm2 , ∼1 GW/cm2

Distinct and long-range hardening was observed only at LSP with nanosecond pulses under confined ablation conditions using a thermoprotective coating; in material modification regime, slip and deformation twinning was observed in all the bcc metals investigated; in Mo and Fe, the laser-shock induced hardening was ascribed to raised dislocation density and not to the formation of deformation twins; the defect density in Mo and Fe saturated after about 6 and 24 impacts, respectively; SEM and TEM micrographs of treated surfaces are presented

Kaspar (2001) [369]

2024-T3 (2 mm)

Water/black paint

Nd:glass, 1.054 µm, 18 ns (∼Gaussian), spot 10 mm, 5 GW/cm2

Specimen with a fastener (5 mm) hole and with 2 or 6 crack stop-holes (1.5 mm) were laser shocked from both sides; fatigue tests were performed at R = 0.1, 10 Hz, σmax = 100 MPa; crack length vs. number of cycles diagrams are presented; due to LSP, the crack initiation life was increased ∼2–6 times, and fatigue life ∼10 times

Yang (2001) [370]

Cu (90 µm and 0.8 mm), Ni (120 µm)

Water (3 mm)/Al foil (25 µm) and vacuum grease (∼10 µm)

3ω-Nd:YAG, 355 nm, 50 ns, 1 kHz, spot 12 µm, 2.83–4.24 GW/cm2

Microstucture studies by orientation imaging microscopy revealed that LSP improved grain size uniformity and increased texture; fatigue test (0.8 mm Cu, axial load 110–220 MPa, 80 Hz) showed a 2 times fatigue lifetime improvement of laser-shocked samples; results of numerical simulation of plasma pressure and stress/strain phenomena in the target are presented

Zhang (2001), [371, 372]

2024-T62 (2.5 mm)

Glass/black coating

Nd:glass, 30 ns, 40 J, spot 7 mm, 0.5–2.3 GW/cm2

Both sides of the sample were shocked successively; ultrasound velocity measurements were used for determination of C11 , λ, and Poisson’s ratio distribution in shocked samples; all these elastic constants had raised values (12–24%) at the centre of the laser impact; yield strength, tensile strength and surface hardness increased (13–117%) as result of LSP, saturating at power laser density ∼1.5 GW/cm2

Zhang (2001) [373, 374]

1.064 µm, 40 ns, spot 1.8 mm, 100 and 200 J/cm2 , 2.5 and 5 GW/cm2 (3.5 and 6 GPa)

Investigation of the effect of various coatings on LSP; self-adhesive Al-foil was superior to other coatings in sense of adherence, withstanding 100 J/cm2 fluence; as result of LSP, the hardness of samples surface increased by 7.5–15%, the elastic module was reduced for 5–12%;

Montross (2001) [375]

SUS304, HT1000, S15C Water/no coating (12 mm)

Nd:YAG, 532 nm, 60 and 200 mJ, spot 0.4–1 mm, coverage 1696–10603%

The LSP-induced residual stress in laser beam scanning direction were lower than in transverse direction; a steep stress gradient with negative sign form surface to inner part of the specimen was discovered; this gradient cannot be detected by using sin2 ψ method only; compressive residual stresses were formed till 500 µm in depth

Yoshioka (2002) [204]

100Cr6 steel (38 mm)

Water

10 ns, 22 J, spot 7 mm, 5.5 GW/cm2 , 4 GPa, overlap 25%

LSP caused an enhancement of surface compressive stresses (up to ∼−400 MPa) and of surface hardness (from 270 to 350 HV), an increase of the surface waviness, a decrease of friction coefficient and of wear rate (rolling–sliding contact, 75 MPa)

Yakimets (2002), (2004) [376, 377]

Alloy 600 (pipes of 1D 15 mm)

Water

2ω-Nd:YAG, 532 nm, 130 mJ, 20–50TW/m2 , spot 0.7 mm, 18–27 impacts/mm2

A system for LSP treatment of pipes inner surface underwater is described, laser beam being fed through a gas-filled tube; LSP experiments on similar conditions with Alloy 600 plates and pipes showed the feasibility of generation of surface compressive stresses up to −800 MPa in up to 1 mm depth

Sano (2002) [378]

Cu (90 µm)

Water (3 mm)/Al foil (16 µm) and vacuum grease (∼10 µm)

3ω-Nd:YAG, 355 nm, 1 kHz, 50 ns, spot 12 µm, 160–240 µJ, 2.83–4.24 GW/cm2

The samples were shocked by up to six laser impacts at each location; dents of depth ∼1 µm and of diameter ∼50 µm were formed into sample’s surface; shock propagation and deformations were calculated by ABAQUS (stress and strain distribution graphs presented), calculated surface compressive residual stresses were up to 165 MPa (single pulse 240 µJ)

Zhang (2002) [378]

2024-T62 (2.5 mm)

Glass (4.5 mm)/black coating (100 µm)

Nd:glass, 30 ns, 30 J, spot 7 mm

Extended version of the report by Zhang (2001) [373]; due to laser impact, a crater of depth 275 µm was formed in the surface of the sample; formulae for calculation of laser shock generated displacement waves are presented; ultrasonic velocity distributions and surface micrographs are presented

Zhang (2002) [380]

2011-T3 (10 mm)

Water (flowing, 1–3 mm)/ automotive primers and black paint (0.1 mm) or Al foil (0.11 mm)

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Al, Cu,Ti, 40C130 steel (1.5 mm)

Confining/absorbing layers or environment

Glass (3 mm)/black paint + vacuum grease

Lasers and beam properties

Nd:YAG, 1.06 µm, 80 ns (Gaussian), 6 J, spot 2 mm, 2.39 GW/cm2

Novel features, observed phenomena, comments

References

A review (16 pp., 15 figs., 87 refs.), covering LSP techniques, residual stress distribution, LSP effect on fatigue and hardness, and applications of LSP

Montross (2002) [381]

Pressure transients for direct and confined ablation for various targets are presented; the peak pressure was ∼0.3 GPa for direct ablation and ∼4 GPa for confined ablation for all the targets used; the surface hardness of the targets was enhanced ∼30–50% by direct and ∼50–120% by confined ablation, the changes were largest in Al

Oros (2002) [236]

A review (13 pp., 12 figs., 24 refs.) of research in LSP during 1996–2000, containing also information from doctoral theses, research project reports, and conference proceedings; the review covers the shock pressure dependence on laser pulse length and wavelength, confined LSP with and without protective coating, influence of LSP on corrosion resistance, fatigue life and wear of steel and aluminium alloys

Peyre (2002) [382]

7049-T73 (9.5 and 25 mm)

Water?

Nd:glass, 12–18 ns, spot 3.2–5 mm, 45 and 60 J/cm2 , overlap 10% and 50%

Residual stress distribution was measured by slitting method using 0.79 mm long strain gauges; effects of 14 different combinations of 2 laser shots of different spot size, fluence, pulse width and location were compared with each other and with shot peening; the results are presented in 10 graphs and discussed extensively

Rankin (2002), (2003) [214, 383]

Stainless steel

Water/Al paint (60 µm)

1064 nm, 3 ns, 10 GW/cm2

Simulation results of plasma pressure and temperature by ACCIC and of residual stresses by ABAQUS are presented and compared with experimental results; graphs presenting plasma pressure, temperature, electron density, impedance, coupling, and degeneracy parameters are given (Fig. 3.32); plasma peak temperature at LSP ranges 4000–7000 K

Sollier (2003) [262]

12% Cr steel,Al, 7075-T7351

Pressure pulse 2.5, 10, or 25 ns

Fe, SS304

2024-T351

Ti-6AL-4V (rod of diameter 7 mm)

Water/Al coating (70 µm)

Water?

Nd:YAG, 0.532 µm, 6–7 ns, ∼1.3 J, spot 2 mm, 10 GW/cm2 (∼5 GPa), 50% overlap

18 ns, 7 GW/cm2 , spot 2.6 × 2.6 mm, to both sides simultaneously coverage 200%

Axisymmetric 3D-FEM-simulation of laser peening induced residual stresses; Johnson–Cook plasticity law with isotropic hardening, taking into account strain rate dependence of the stress was applied; pressure pulse and HEL were taken from experiments; shock propagation and residual stresses distributions are presented for single and multiple laser impacts; simulation describes the central stress drop

Peyre (2003) [272]

Analytical models in cylindrical coordinates for LSP plasma temperature, pressure, and thermal stresses for ramp-up, ramp-down, and rectangular laser pulses, including confined ablation with coating (see Fig. 7.13 for calculated temperatures)

Thorslund (2003) [384]

Residual stress and hardness profiles and fatigue life vs. maximum surface stress for SP (intensity 4A, incident angle 45◦ , coverage 200%) and LSP (2 and 3 passes) processed specimens presented; LSP provided hardness increase and compressive residual stresses to a larger depth; combined SP and LSP was proved to be advantageous for fatigue life extension; fractographic analysis results, and fracture surface microphotographs are presented

Rodopoulos (2003) [385]

A computer program, LSP-1D, for solving by FD-method the elasto-plastic wave propagation in solids, is described; a hypo-elastic rate-dependent plastic deformation is assumed for modelling the material behaviour; examples of calculated shock propagation and stress fields are presented

Arif (2003) [386]

Deep rolling (roll diameter 6.6 mm, 150 bar, feed 0.1125 mm/revol.) is compared with LSP; DR provided up to 40% longer fatigue life than LSP, obviously due to a higher magnitude of induced compressive stresses, a higher degree of work hardening, and significant decrease in the surface roughness

Nalla (2003) [192]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

AISI 304, Ti-6AL-4V (20 mm)

Water?

Ti-6Al-4V (20 mm)

Novel features, observed phenomena, comments

References

See the entry for Nalla (2003) [192]

Fatigue lifetime of deep rolled specimens was investigated up to 600◦ C; in the whole temperature region, the lifetime of rolled specimen was higher than that of untreated one; microstucture of both DR and LSP processed AISI 304 was investigated up to 900◦ C (micrographs presented), the near-surface work-hardened zones were stable up to 650 ◦ C (deep rolled) and ∼800◦ C (laser shocked)

Altenberger (2003) [387]

?

18 ns, spot 2 × 3 mm, 7 GW/cm2 , coverage 200%

Comparison of LSL and deep rolling (roll diameter 6.6 mm, 150 bar, feed 0.1125 mm/revol.); maximum residual stresses were over −400 MPa (LSP) vs. over −900 MPa (DR); DR provided somewhat longer fatigue life than LSP; at fatigue tests at 450◦ C the compressive residual stresses relaxed to about −200 MPa in both cases, however, the fatigue properties remained improved, obviously due to more stable nanocrystalline surface structure formed at DR and LSP

Altenberger (2003) [388]

304 stainless steel and mild steel (both 4 mm)

Glass (0.1 mm)

Nd:YAG, 1.06 µm, 9 ns, 10 Hz, 0.5 J, spot ∼1.5–2 mm

The hardness of lasershocked materials was ∼20% (mild steel), respectively 70% (SS) higher than that of untreated materials; the modified depth was 450 µm; shock pressure and elastic–plastic wave propagation were calculated numerically assuming Maxwell velocity distribution of vapour molecules and linearly elastic, power-law work-hardening plastic stress–strain relationship of the target material

Yilbas (2003) [389]

35 CD4 50 HRC

Water/black paint

30 ns, spot 5 × 5 mm, 8 GW/cm2 , 3 GPa

Numerical simulation of the experiment by Ballard (1991) [264], using a 3D-model in ABAQUS, assuming pressure pulse square in time and uniform over laser spot, and the plastic strain following von Mises yielding criterion with dynamic yield strength HEL(1–2 ν)/(1–ν); calculated dynamic stresses and residual stress profiles are presented for different shock pressures, shock duration, and the number of impacts

Ding (2003) [390]

1Cr18Ni9Ti (1.2 mm), GH30 (1.6 mm)

Quartz (4 mm)/black paint

Nd:glass, 1.06 µm, 20–50 ns, 10–50 J, spot 3–7 mm, ∼4 GW/cm2

As result of LSP, materials hardness was enhanced down to the depth of 0.7 mm;TEM micrographs of treated material are presented: laser shocking induced martensitic transformation in 1Cr18Ni9Ti stainless steel and dense dislocations and twins in GH30 superalloy

Wang (2003) [391]

UNS N06022 (20 and 33 mm, weldments)

Water (1 mm)/Al tape (120 µm)

25 ns, 10 GW/cm2 , up to 20 peening layers

Plastically affected depth up to 12 mm was observed; near-surface residual compressive stresses were larger than −300 MPa and the depth of compressive stresses was up to 7.7 mm

Hill (2003) [392]

LSP technology at LLNL-MIC is described, cases demonstrating the usefulness of LSP for processing of aircraft parts are presented (see also Hill (2003) [392]); the magnitude of residual compressive stresses in Ti-6Al-4V was found to be largely independent of laser beam incident angle in range 0–60◦

Hill (2003) [393]

Ti-6Al-4V (8,7 mm)

?

ns-pulses, 180 J/cm2 , 200% coverage

Compressive residual stresses up to −800 MPa were induced at the specimen surface, balancing tensile residual stresses were located 2 mm deep beneath the material’s surface; residual stress distribution was determined by neutron diffraction

Evans (2003) [394]

Ti-6Al-44V (1,6 mm)

Water?

Nd:glass, 18 ns, spot 2.5 × 2.5 mm, 126 J/cm2 , 3 layers, both sides simultaneously

Residual stress distribution was determined by slitting method – the methodology is laid out in detail, the number and positions of strain gauges were optimized; the near-edge compressive residual stress reached 98% of the material’s yield strength, but extended over 38% of the laser peened area

Rankin (2003) [395]

Cu

Fused silica/Al (50 µm) or sapphire/Zn (0.5 µm)

Nd:glass, 1.06 µm, 20–50 ns, spot 9 mm (super Gaussian) up to 60 J/cm2

A 2D-computational model, incorporated into LASNEX code, is presented; the model accounts for the initial absorption onto a metal surface, low-intensity photoionization absorption in neutral vapour, collisional ionization, recombination, dielectric breakdown, band gap collapse of the tamper, electron conductivity, thermal transport, and constitutive properties of the materials; most of the laser energy is absorbed in the dielectric tamper, not the ablator; the simulations agreed well with the experimental results

Colvin (2003) [251]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties 2

Novel features, observed phenomena, comments

References

BS L65 l, 2024-T351

?

10 ns, 22 J, spot 0.75 cm , 3 GW/cm2 , 67% overlap, 3 GPa

A fractal dimension determination methodology for analysis of fracture surfaces of laser peened and fatigued specimen is described; for fractal fracture mechanics see the review by Cherepanov et al. [397]

Shaniavski (2004) [396]

Alloy 22 (13.5 and 20 mm; welds in 33 mm plate)

Flowing water (1 mm)/Al tape (120 µm)

25 ns, 6 Hz, 20 J, 7–13 GW/cm2 , up to 20 layers

Residual stress distribution was determined by slitting and contour methods; the depth of compressive residual stresses varied from 4.3–7.7 mm

DeWald (2004) [216], Hill (2005) [398]

316 stainless steel (2 mm)

Overlay (0.1 mm)

Nd:YAG, 355 nm, 8 ns, 10 kHz, 450 mJ, spot 2.5 mm, 15 impacts

Laser shocking enhanced the surface hardness of the workpiece for 80%; the dislocation density of treated areas exceeded 2–8 × 1011 /cm2 ; calculated recoil pressure vs. laser intensity, and stress transients are presented; the numerical model is described (see the entry for Yilbas, 2003 [389])

Yilbas (2004) [399]

Ti-6Al-4V (2 mm)

Overlay (0.1 mm)

Nd:YAG, 355 nm, 8 ns, 10 kHz, 450 mJ, spot 2.5 mm, 15 impacts

Yilbas (2004) [400] Laser shocking enhanced the surface hardness of the workpiece for ∼50%; calculated peak surface temperature was over 5000 K; calculated stress profiles in the material at different times are presented; the numerical model is described (see the entry for Yilbas, 2003 [389])

Cu (1 and 3 µm) on Si (111)

Water curtain/Al foil (16 µm) + vacuum grease

Nd:YAG, 355 nm, 50 ns, spot ∼10 µm, (∼209 and 244 µJ, 3.67 and 4.31 GW/cm2

Laser shocking induced stress in Cu layer has been evaluated from structure curvature measurements (∼300 MPa for 3 µm Cu); a improved analytical model for plasma pressure is presented, taking in to account both axial and radial effects (important for small pot size); the stress/strain analysis was performed by ABAQUS software (stress and strain distributions are presented)

Zhang (2004) [263]

Cu (1, 1.5, and 3 µm films on 0.5 mm Si )

Water?/Al foil (16 µm) + vacuum grease

Nd:YAG, 355 nm, 50 ns, spot 12 µm, 174, 209, and 244 µJ; 3.08, 3.67, and 4.31 GW/cm2

Hardness and stress/strain field of lasershocked areas was investigated by X-ray microdiffraction and nanoindentation; hardness of shocked regions was increased by 11%; compressive residual stresses were found in shocked material; simulated (by the model described in Zhang (2004) [263]) strain energy distribution is presented

Zhang (2004) [401]

Al (110), Cu (001), both 5 mm thick

Water (5 mm)/Al foil (16 µm) + vacuum grease

Nd:YAG, 355 nm, 50 ns, 10 kHz, spot 12 µm, ∼4 GW/cm2

Laser shock-induced lattice rotation was measured by electron backscatter diffraction (EBSD) method; maximum in-plane rotation was ∼3◦ in Al and ∼2◦ in Cu sample; the measured rotation distributions are compared with numerical simulation data; the Al sample surface was deformed up to ∼1.3 µm in depth

Chen (2004) [402]

Al (001), Cu (110), both 5 mm thick

(3 mm)/Al foil (16 µm) + vacuum grease

See the entry for Chen (2004) [402]

Microdiffraction measurements (X-ray beam size 5–7 µm) of residual stress in lasershocked samples are described; compressive residual surface stresses up to −100 MPa were found; asymmetry of diffraction peaks indicated dislocation cell structure formation

Chen (2004) [403]

6061-T6 (6.3 mm)

Water

Nd:YAG, 1.064 µm, 8 ns, 10 Hz, 1.2 J, spot 1.5 mm

The specimen were laser shocked by 900, 1350, and 2500 pulses/cm2 ; surface hardness was increased up to 10%, residual compressive stresses up to −280 MPa were formed (below the surface), and fatigue crack growth rate K was reduced by 20 MPa(m)1/2

Rubio–González (2004) [404]

QT700-2 (10 mm)

?/LTV silicone rubber film

Nd:glass, 1.064 µm, 25 ns (∼Gaussian), 10–30 J, spot 7 mm

Yang (2004) [405] The parameters of LP were optimized by an ANN method; the depth of hardened layer was 0.31–1.4 mm (1–4 impacts); the hardness was increased by 45–82%; peak compressive residual surface stresses ranged from −165 to −410 MPa (12–20 J); laser shock treatment-induced dense dislocations and refined grains

AISI 304 (rod, 7 mm in diameter)

Water?

18 ns, spot 2.5 × 2.5 mm (sq.), 10 GW/cm2 , 200% coverage

LSP was compared with deep rolling (spherical rolling element of diameter 6.6 mm, 150 kbar); both treatments produced almost identical fatigue lifetime enhancements in temperature range 25–600◦ C; residual compressive stress relaxation was fastest between RT and 100◦ C, but near-surface work hardening started to anneal out only at over 400◦ C

Nikitin (2004), (2005) [406, 407]

SUS304

Water

Nd:YAG, 532 nm, 8 ns, 10 Hz, spot 0.8, and 1 mm, coverage up to 800%

LSP was performed without protective coating; at successive impacts (at rising coverage rate), the residual tensile surface stresses were gradually converted into compressive (see Fig. 3.26)

Akita (2005) [231]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

Novel features, observed phenomena, comments

References

A mathematical model of pressure generation at water confined LSP is described; the model considers the processes to be 1D, the plasma homogeneous and two-temperature laser beam absorption due to el.-ion and el.-atom IB and photoionization only, and Hertz–Knudsen surface evaporation; the model was in good agreement with experimental data at 532 and 1064 nm, 0.6–25 ns, and 1–10 GW/cm2 ; the calculations give insight into plasma parameters as the density of species, light transmission, thermal to internal energy ratio, water–plasma interface reflectivity, and energy balance

Wu (2005) [259]

Al (5 mm)

Water (3 mm)/Al foil (16 µm) + vacuum grease

Nd:YAG, 355 nm, 50 ns, spot 12 µm, 226 µJ, 4 GW/cm2

Residual compressive stresses up to −80 MPa and dents of depth down to 1.8 µm were created in the surface of specimen; shock propagation and deformation simulation results are presented

Fan (2005) [273]

Ti-6Al-4V (12.7 mm)

Flowing water/black vinyl tape

Nd:glass, 1.054 µm, 20 ns, spot 5.3 mm, 8 GW/cm2

Laser shocks were applied simultaneously to both sides of the samples; shot peening was investigated as well (6–8A using cast steel shots and 6–9 N using glass beads); LSP resulted in ∼−600 MPa surface residual stresses; a linear elastic fracture mechanics analysis of crack growth threshold is presented; LSP proved to be superior over SP

Shepard (2005) [408]

A5083

Water (22◦ C)/no coating

2ω-Nd:YAG, 532 nm, 6–7 ns, 10 Hz, 10–250 mJ, spot 0.4 mm, 12–310 MW/mm2

At a fixed scanning speed 2 mm/s the maximum compressive stress (∼−250 MPa) was achieved at laser power density of 31 MW/mm2 ; at fixed power density of 61 MW/mm2 the same maximum compressive stress was achieved at 1 mm/s; ablated depth was ∼100 µm at 61 MW/mm2 and 0.1 mm/s; at 2 mm/s the average ablated depth did not exceed 6 µm (10–310 MW/mm2 ); surface roughness of laser treated material increased with increase of laser power density and with decrease of scanning speed (scanning speed range was 0.1–15 mm/s), exceeding tens of µm Rz

Kusaka (2005) [253]

SUS304

Water

Nd:YAG, 532 nm, 6–10 ns, 120 and 300 Hz, 60–250 mJ, spot 0.4–1.2 mm, 36 and 70 pulse/mm2

A laser peening system for in situ treatment of boiling water nuclear reactor (BWR) cores is described; the laser beam was transported whether by mirrors or by light guide; optical schemes of the system are presented; focusing system is described in detail

Mukai (2005) [409]

1080 carbon steel

?

?

Four kinds of laser surface modifications were compared: laser shock peened only, laser glazed only, laser glazed then shock peened, and laser shock peened then glazed; the latter provided maximum friction reduction 43% in a dry pin-on-disc test against alumina

Aldajah (2005) [410]

A review (6 pp) of the work at University of Kassel, see Altenberger, Nalla, Nikitin above; LSP is compared with shot peening (SP) and deep rolling (DR); highest fatigue strength can be obtained by thermomechanical surface treatment (SP or DR) at elevated temperatures (300◦ C)

Altenberger (2005) [411]

Ti-6Al-4V (10 and 15 mm)

?

?

Residual stress vs. depth profiles of laser-shocked samples were measured by neutron diffraction; compressive stress from LSP reached 1.25 mm in depth; the maximum of tensile stresses (160 MPa) located at a depth of 2.6 mm

King (2005) [412]

Cu ([011] and [134], 3–5 mm), CuAl ([011] and [134], 3–5 mm)

?

2.5 ns, 40–300 J, spot 3 mm, 15–70 MJ/m2 , 12–40 GPa/cm2

Slip-twinning transition was determined quantitatively and predicted as a function of orientation, temperature, and stacking fault energy; the experimentally determined threshold twinning stress for pure copper in the [0 0 1] orientation was 25 GPa, whereas the one for the [1 3 4] orientation was between 40 and 60 GPa.

Schneider (2005) [413]

Si (SC)

SiO2 -glass/Cu (0.05–0.3 mm)

KrF, 248 nm, 10–50 ns, 1.33–6 GW/cm2

Dislocations structure and dynamics in SC Si during LSP at temperatures 850–1073 K was studied by numerical simulation; it was concluded, that LSP process can generate plastic flow in brittle materials

Cheng (2005) [414]

Al (110) and (001), Cu (110)

Water (3 mm)/Al foil (16 µm) + vacuum grease

3ω-Nd:YAG, 355 nm, 1 kHz, spot 10 µm, 4 GW/cm2

LSP-induced dislocations were studied by synchrotron XRD (beam size 5–7 µm); data about measured distribution of average mosaic size, strain and dislocations density are given and are compared with FEM-simulations; material with (001) orientation and material with higher stack fault energy (Al) showed higher dislocation density under LSP

Chen (2005) [415]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

6061-T6, 2024

Water

Ti-6Al-4V, Al2024

Inconel 600, 316L (5 mm)

Al

Water (3–10 mm)

Water

Novel features, observed phenomena, comments

References

Nd:YAG, 1.064 µm, 8 ns, 10 Hz, spot 0.8 and 1.5 mm

The specimen were laser shocked by 2500 (6061-T6) and 5000 impacts/cm2 (2024); compressive residual stresses up to −1600 MPa were produced in 6061-T6 alloy and up to −1400 MPa in 2024 alloy;

Gomez-Rosas (2005) [416]

Nd:YAG, 10 ns, spot 0.75 mm, 0.3–1 J, 900 impacts/cm2

A 3D-LSP simulation computer program, SHOCKLAS, was developed; examples of simulation the residual stress distribution of given materials at given regime are presented

Morales (2005) [417]

Nd:YAG, 12 ns, 4 Hz, 0.8 J, spot 1 mm, 100 J/cm2 , scanned beam

It was found that the laser spot scanning of the surface during processing resulted in a system of column-like microstructure, which was tilted in the direction of scanning; spherical nanoparticles formation with diameter of ∼60 nm was observed during LSP

Bugayev (2005) [418], Bugayev (2006) [419]

A solution has been presented of a axisymmetric problem about elastic deformation of a flexible plate due to a prescribed distribution of eigenstrains, such as may be generated during shot peening treatment; the spatial variation of residual stresses and strains through the plate thickness, and the deformed plate shape, can be predicted using this technique; the approach may be applied also for case of LSP as demonstrated by Korsunsky (2006) [421]

Korsunsky (2006) [420]

A 1D-Lagrangian hydrodynamic code, HYADES, was adopted for simulation of plasma pressure and of propagation of laser shock in Al;Al was considered as an elastic-perfectly plastic material; the propagation of the shock wave inside the metal was found to be mainly influenced by the laser fluence, not by laser intensity; calculated plasma peak pressure, impulse and shock velocity dependence on laser pulse risetime (1–17 ns) and intensity/fluence are presented

Lee (2006) [261]

1.064 µm, trapezoidal pulse, 8–50 J/cm2 , 2–10 GW/cm2

Inconel 132, Inconel 182, also weldments with SUS304 and Inconel 600

Ti-6Al-4V (BSTOA, 15 mm)

Water

Flowing water/Al tape

2ω-Nd:YAG, 532 nm, 100 mJ, spot 0.6–1 mm, 36–70 impacts/mm2

18 ns, spot 3 mm, 12 GW/cm2

Compressive residual stresses up to −1000 MPa at surface and up to ∼1 mm in depth were formed in the LSP-treated materials; corrosion tests (561 K, dissolved O2 8 ppm, conductivity 0.1 mS/m, 500 h) demonstrated complete suppression of stress corrosion cracking by LSP

Sano (2006) [422]

New empirical formulae for estimation of residual stress distribution in laser-shocked materials is presented (see section 3.3.6.2); the calculations matched well with experimental data for 40Cr and 45# steels

Chen (2006) [269]

Slip steps were observed within grains oriented with their c axis nearly parallel to the specimen surface normal; grains with slip steps had the lowest Taylor factors; all the localized lattice rotations were concentrated about the steps, with almost no orientation variations in between slip steps

El-Dasher (2006) [423]

Wu (2006) [426] Laser beam transmission through water breakdown plasma in LSP regime was simulated by solving an electron rate equation coupled with Maxwell’s wave equation; the calculated with aid of this model transmitted laser peak power density, transmitted laser pulse length, peak pressure, and pressure duration as function of incident power density agreed well with the experimental data from literature 6061-T6 (6.3 mm)

Water

Nd:YAG, 1.064 µm, 8 ns, 10 Hz, spot 1.5 mm

The specimen laser shocked by 900, 2500, and 5000 pulses/cm2 were tested for wear and friction by roll-on-flat tribometer; wear rate was reduced about 68% using 5000 pulse/cm2 ; with LSP the time to reach the same wear depth was increased by as much as 100%, depending on the applied load and pulse density; wear mechanisms included adhesive wear, abrasive wear, and wear due to plastic deformation

Sánchez-Santana (2006) [425]

SUS304, SUS316L

Water/no protective layer

2ω-Nd:YAG, 532 nm, 60–200 mJ, spot 0.4–1 mm, 9.5–100TW/m2

The specimen were laser shocked by 36–135 impacts/mm2 ; the shocked surface of SUS304 was oxidized for ∼1 µm depth and surface roughness Ra was less than 2 µm; the depth of LPPC-induced compressive residual stress exceeded 1 mm from the

Sano (2006) [254]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

Novel features, observed phenomena, comments

References

surface for both materials; LPPC completely prohibited the initiation of SCC and the propagation of small pre-cracks on SUS304 in an environment that accelerates SCC (water 561 K, dissolved O2 8 ppm, 500 h); rotating bending fatigue tests showed that LPPC enhanced the fatigue strength of SUS316L by a factor of 1.4–1.7 at 108 cycles 35CD4 50 HRC

Water/black paint

30 ns, spot 5 × 5 mm, 8 GW/cm2

3D-FEM analysis of single and multiple LSP by LS-DYNA and ANSYS commercial software is described; the target material was assumed to be perfectly elastic–plastic and the plastic strain to follow the von Mises yielding criterion and the dyn dynamic yield strength be σY = HEL(1–2ν)/(1–ν); calculated residual stress distribution (also 3D) and deformed surface profiles are presented for the experimental conditions of Ballard (1991) [264]

Hu (2006) [426]

Ti-6Al-4V (aeroengine fan blade contact surfaces)

Water?

9 GW/cm2 , coverage 200%

In-plane residual stresses of order 700–800 MPa were introduced by laser shock peening near surface, the compressive region extending to a depth of ∼1.5 mm; a tensile peak residual stress of 250 MPa was located at a depth of around 2.5 mm; fretting fatigue loading of Dovetail Biaxial Rig samples treated with combined LSP and SP on their contact surface causes significant stress relaxation extending to a depth of 0.5 mm; FEM has been used to determine the profile of plastic misfit (eigenstrain) introduced by peening responsible for the observed distributions of elastic strain

King (2006) [427]

35CD4 30 HRC (15 mm)

Water

50 ns (Gaussian), spot 8 mm, 2.8 GPa

Laser shock propagation in the specimen and residual stresses were calculated by ABAQUS using a 2D axisymmetric model; calculated residual stress distributions are compared with analytical and experimental results of Peyre (1998) [239] and Ballard (1991) [264] (diagrams presented); the size of residual stress field and the depth of the plastic deformation increases clearly when the diameter of the spot is increased from 2 to 8 mm

Ding (2006) [428]

6061-T6 (5 and 6.3 mm)

Water jet/black paint (13 µm)

Nd:YAG, 1.064 µm, 8 ns, 10 Hz, spot 1.5 mm, 2500 pulse/cm2

Without absorptive coating, the maximum compressive residual stress in samples was reached at a depth about 0.7 mm; the corresponding value for coated samples was between 0.1 and 0.2 mm (stress profiles given); superficial grooves along the scan direction were observed

Rubio–González (2006) [429]

Ti-6Al-4V (8.5 mm)

?

Nd;YAG, 355 nm, 20 ns, spot ∼2 × 3 mm, ∼7 GW/cm2 , coverage 200%

LSP-induced residual elastic strain profiles were measured by synchrotron radiation (∼60 keV) diffraction; microstrain values up to −4000 were measured; plastically affected depth was over 3 mm; difference between hcp α-phase (00.2) and (11.0) peak strains displayed the greatest sensitivity to plastic deformation; strain measurement methodology by the presented method is laid out in detail

Korsunsky (2006) [430]

Ti-6Al-4V (8.5 mm)

?

see Korsunsky (2006) [430]

The measured in Korsunsky (2006) [430] residual elastic strain distributions were modelled using a distribution of laser shock-induced eigenstrain near the surface; and the most likely eigenstrain profile was deduced using a variational matching procedure; the mathematical framework of this approach is presented and discussed

Korsunsky (2006)[421]

35CD4 30HRC, Ti-6Al-4V, 7050-T7451

The first book about LSP (162 pp., 85 figs., 123 refs.); main topics covered: physical and mechanical mechanisms of LSP, simulation methodology, 2D/3D-simulation of single and multiple LSP (examples presented for 35CD4 30HRC), 2D-simulation of two-sided LSP on thin sections (examples presented for Ti-6Al-4V); simulation of LSP on cylindrical surface (examples presented for 7050-T7451); history of mechanical energies and distribution of dynamic and residual stresses in workpiece are presented for various LSP conditions; ABAQUS software was used in all simulations

Ding (2006) [186]

Ti-6Al-4V, 316L

A multi-axial contour method for determination of residual stresses in continuously processed materials is described and applied to laser-shocked specimen

DeWald (2006) [217]

(Continued)

Table 3.8

(Continued)

Processed materials/ targets

Confining/absorbing layers or environment

Lasers and beam properties

SKD61, SUS304, SUS316L,Ti-6Al-4V, AC4CH

Water/no coating

SKD61, SK3, SUS304,AC4CH

Novel features, observed phenomena, comments

References

Nd:YAG, 532 nm, 8 ns, 60–200 mJ, spot 0.6–0.8 mm, up to 200 pulse/mm2

A overview of recent advances in LPwCtechnology: residual stress profiles in laser peened SKD61 and SUS304 are presented; the achieved stress profile in SUS304 was quite stable even at thermal loading up to 673 K; rotating-bending and push–pull fatigue life of SUS316L,Ti-6Al-4V and AC4CH was improved significantly despite of increase of surface roughness; SSC (accelerated tests) in SUS304 was completely eliminated in result of laser peening

Sano (2006) [282]

Water/no coating

Nd:YAG, 532 nm, 8 ns, 70–200 mJ, spot 0.6–0.8 mm, up to 100 pulse/mm2

A overview of recent advances in LPwCtechnology: in addition to the material presented by Sano (2006) [282], LPwC treatment of 9.5 mm inner diameter SK3 steel tubes, and investigation of crack propagation in AC4CH by X-ray micro tomography is described; fibre delivery of laser beam and autofocus system are also described

Sano (2006) [280]

Alloy 600

Water/no coating

Nd:YAG, 532 nm, 80 mJ, spot 0.4 mm, 70 pulse/mm2

Residual stress profiles in laser peened Alloy 600 samples are presented; laser peening operations of welds in PWR nuclear reactors are described in detail

Yoda (2006) [185]

12Cr steel, 316L (4 mm)

Water/Al (70–80 µm, optional)

Pressure duration 6–50 ns, peak pressure 3–5 GPa, impact size 1.6 and 5 mm

FEM calculation (2D axisymmetric, by ABAQUS) of shock propagation and residual stresses; materials were given by hydrodynamic Grüneisen EOS and Johnson–Cook’s plasticity model; effects of impact pressure and diameter, pressure pulse duration, number of impacts and of sacrificial overlay were studied numerically; without sacrificial layer, the heating of workpiece by plasma was found to last several microseconds and thermally affected depth was ∼30 µm (6 ns pressure pulse)

Peyre (2007) [229]

Notations FWHM – full-width half maximum CW – continuous wave (laser) SP – shot peening LSP – laser shock processing, laser shock peening LP – laser peening LPPC, LPwC – laser peening without protective coating SC – single crystalline, single crystal HAZ – heat affected zone PAZ – plastically affected zone SHYLAC – Simulation Hydrodynamique Lagrangienne des Chocs – a computer code for hydrodynamic simulation of fluid motion and shock wave propagation, developed at LCD-ENSMA Poitiers, France TEM – transmission electron microscope SEM – scanning electron microscope SIMS – secondary ion mass spectrometry EPMA – electron probe microanalysis ‘momentum trap’ – a solid plate in contact with the backside of laser-shocked sample, in purpose to avoid wave reflecting from back side of the sample (see Clauer et al. [232]) PE – polyethylene LLNL – Lawrence Livermore National Laboratory MIC – Metal Improvement Company BFV – back-free velocity, the velocity of target’s free back side ACCIC (Auto Consistent Confined Interaction Code) – a code for simulation of laser – confined target interaction, developed at CLEA-LALP,Arcueil, France EMV – electromagnetic displacement gauge, the operation relies on change of magnetic flux due to change of current carrying loop area PVDF – polyvinylidene fluoride, a high performance piezoelectric polymer ABAQUS – a commercial general-purpose finite element program, designed primarily to model the behaviour of solids and structures under externally applied loading ANSYS – a commercial multiphysics finite element simulation program FEM – finite element method, finite element modelling FD – finite difference ANN – artificial neural network ID – inner diameter 1D, 2D, 3D – one-dimensional, two-dimensional, three-dimensional HV – Vickers hardness DR – deep rolling SS – stainless steel HEL – Hugoniot elastic limit (see Glossary) RT – room temperature (∼20–25◦ C) IB – inverse Bremsstrahlung XRD – X-ray diffraction BSTOA – beta solution treated and overaged SCC – stress corrosion cracking PWR – pressurized water reactor EOS – equation of state

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Handbook of Liquids-Assisted Laser Processing

3.4 Laser Shock Forming and Cladding 3.4.1 Forming Deformation of thin metal foils due to laser plasma pressure was observed already by O’Keefe and Skeen [295] and Ageev [431] in the 1970s. Later the process was developed by Dubrujeaud and Jeardin, Zhang et al. Zhou et al. [432–439] who all used solid confinement layers. Water confinement has been applied only recently, by Fan et al. [273] for micro forming. Laser shock forming (also called laser peen forming) has considered as an alternative to other dynamic forming methods like explosive and magnetic forming. It is a convenient tool for introducing microscale deformations into materials (Fig. 3.40) [273].

3.4.2 Cladding Laser cladding is an alternative to explosive cladding, capable to join dissimilar materials with uneven surfaces. Laser process has been implemented using a glass confinement layer only so far. In the work by Dubrujeaud and Jeandin [432], a grooved 2024 aluminium alloy specimen was clad by an 20-µm thick aluminium foil; 6 ns laser pulses of fluence 350 J/cm2 were used, the estimated pressure being 6 GPa (Fig. 3.41). The process was carried out in vacuum. Melted material was found at the bottom of the grooves, obviously due to shock wave focusing. Pulsed laser

Transparent overlay Vapourized opaque material (explosive pressure) Opaque material Shock compressive wave Metal sheet Deformation

fdl

Pulsed laser Top press plate Transparent layer Black paint Metal sheet Bottom base

fdb

Figure 3.40

Schematics of confined laser shock forming [434]. © Elsevier.

Laser Glass Black paint

Metal foil Workpiece

Figure 3.41 Principle of laser shock cladding [432]. The process may principally carried out also in water confinement, but the space between foil and the workpiece should be evacuated.

141

Shock processing

3.5 Densification of Porous Materials Fabrication of machine parts by powder compacting (powder metallurgy) is attractive because ‘near-net’ shaped parts are ready achieved, and because complicated-shaped parts of hard materials like carbides may be easily fabricated. However, the porosity of compacted from powder parts causes a reduction of their strength and an increase of friction and wear. In some cases, however, the porosity is a benefit. Porosity may be reduced by compacting the material by flyers or explosives [440–442]. The laser shock process (Fig. 3.42), is a less dangerous and easier to control alternative here. Some laser process examples are given in Table 3.9. The process has been carried out mostly using glass confinement [230, 244, 443–447], and to a less extent in water confinement [448, 449, 445]. The compacted depths achieved were about 0.5 mm and the surface residual porosities some per cents (Table 3.9). Numerical simulation of shock phenomena and residual porosity profiles are reported in the article by de Rességuier and Romain [450]. A computational model of shock compression of loose (also metal) powders can be found in the work by Benson et al. [451]. Improved wear properties Laser impact

Confining medium (glass)

Shock waves attenuation

Porosities

Densified depth

Figure 3.42 Principle of surface densification of porous materials by confined laser shocks [230]. Reproduced with kind permission of Springer Science and Business Media.

Table 3.9

Laser shock densification of porous materials. Confining/absorbing layers or environment

Lasers and beam properties

Novel features, observed phenomena, comments

Al-alloy (5 wt% Cu, 1 wt% Pb, 1 wt% Mg, 1 wt% Mn, 0.5 wt% Fe, 10-mm thick)

Water/Al adhesive (100 µm)

Nd:glass, 8–10 ns, 0.011 Hz, spot 6 mm, 5 and 8 GW/cm2 , overlapped impacts

LSP rendered the surfaces slightly more homogeneous and smooth, and less porous; but the wear of treated surfaces was greater than of untreated

Schnick (1999) [448]

Al-SiC (50–50%, HVOF-sprayed, ∼400-µm thick)

Al (10-mm thick)

Water/Al foil 100 µm

Nd:glass, 1.06 µm, 5–20 ns, spot 6 mm, 10 GW/cm2

Laser shock treated samples exhibited lower porosity and better contacts between Al and SiC reinforcement; plastic deformation of Al matrix and dislocations generation was observed; sliding wear resistance of shocked surface was somewhat greater that of unshocked one

Podlesak (2000) [449]

Distaloy AE (sintered porous steel, 190–570-µm thick, void volume 15% and 28%)

no

A drop of water

Nd:glass, 1.06 µm, ∼20 ns, ∼20 J, spot 5 mm, 5 GW/cm2

After laser shocking, the porosity near the shocked side was reduced up to 3 times; the depth of porosity reduction was up to hundreds of µm; calculated porosity profiles agreed reasonable with experimental results; cross-section micrographs of the material before and after shock are presented; Hugoniot curves of the materials and rear-side pressure transients are presented

de Rességuier (2001) [450]

Porous material

Substrate

Al-SiC (100–410 µm, thermal sprayed, 15 and 50 wt% SiC, 3.5–45 µm thick)

Notation HVOF – high velocity oxy-fuel

References

C H A P T E R

F O U R

Subtractive Processing

Contents 4.1 4.2 4.3 4.4 4.5 4.6

Frontside Machining Liquid-Jet-Guided Laser Beam Machining Water at Backside of an Opaque Material Backside Machining of Transparent Materials Machining of Liquid-Containing Materials Laser Cleaving of Crystals in Water and of Water-Containing Crystals

143 171 177 177 202 203

4.1 Frontside Machining 4.1.1 Introduction Laser machining (drilling, cutting, carving, etc.) has been recognized to be useful and competitive in case of hard materials, curved surfaces, hard to access places (inside of tubes, etc.), rapid prototyping, carving of complex patterns onto surfaces, micromachining, etc. The process is non-contact, can be carried out at atmospheric pressure, and is easy to control. However, laser machining is a thermal process where the material removal occurs via melting and vaporisation. Looking, for example, at a laser cut in a metal, all typical to thermal processes imperfections like taper, structural changes, recast, debris, and burr can be found (Figs 4.1 and 4.2).

Debris

Taper

Recast

fs/pspulses or in liquid

ns-pulses

Burr

HAZ (a)

(b)

Figure 4.1 Imperfections of a laser cut in a metal (schematically). The cut quality may significantly be improved by presence of a liquid without a need for ultrashort laser pulses or vacuum.

Handbook of Liquids-Assisted Laser Processing ISBN-13: 978-0-08-044498-7

© 2008 Elsevier Ltd. All rights reserved.

143

144

Handbook of Liquids-Assisted Laser Processing

Konventionelle Strukturiering: HE = 9 J/cm2. ff = 20 Hz

200 ␮m

Strukturiering unter Flussigkeit: 200 ␮m HE = 9 J/cm2. ff = 20 H z

(a)

(b)

Figure 4.2 Effect of water film on laser carving results in Si3 N4 ceramics. (a) Process performed in air and (b) a water film was sprayed onto surface. Laser: KrF, 248 nm, energy density 9 J/cm2 , pulse frequency 20 Hz, 500 pulses. The pattern was created by mask projection technique. Courtesy by Stephan Roth, Bayerisches Laser Zentrum GmbH (BLZ), Germany. © Stephan Roth, published with permission. See also the article by Roth and Geiger [452] for a similar experiment with SiC.

Techniques of avoiding redeposition of laser ablation debris and its removal • • • • •

Ablation in vacuum. Purging the ablation zone by a high-speed gas jet. Aftercleaning of the workpiece by solvents, detergent solutions, reactive oxygen plasma or in ultrasonic bath. Using water-soluble protective coatings like polyvinyl alcohol (PVA) [453]. Machining in liquids or having a liquid film on the surface of the workpiece.

Reasons why liquids are applied in laser machining • • • • • • •

Little or no debris and/or recast, less melt, reduced HAZ depth, less taper, and burr. Contamination of the ambient atmosphere by aerosols and gases is extensively avoided. Lower thermal load on heat-sensitive materials, for example, decomposition of heat-sensitive materials like HgCdTe can be avoided [454]. Cracking of brittle materials (e.g. SiN) is reduced or avoided [455]. Graphitization of diamond can be avoided; graphitic layer is electrically conducting and decreases the catalytic activity of diamond in metal deposition [456–458]. Silicon layer formation on an SiC surface can be avoided; silicon layer leads to a reduction of catalytic activity of SiC in metal deposition [459–461]. In water, it was possible to capture and fix latex microparticles to be machined by optical tweezers [462].

Possible disadvantages and hazards of liquids-assisted laser machining • •

Contamination of the workpiece surfaces by liquid dissociation products (oxygen and hydrogen from water, carbon from organic solvents, nitrogen from liquid nitrogen, etc.). Vapours of the liquids and their decomposition products may be harmful for personal and electronic equipment.

For frontside laser machining, non-toxic transparent to laser light liquids have been commonly used. Molten NaCl and NH4 Cl were tested as self-focusing media for laser drilling by Ramanathan and Molian [463], molten NaNO3 and KNO3 jets were proposed for guiding the laser light in DE10238339 [464] (Table 4.1). Addition of salts and bases to water was found to improve the finish [465] and to enhance the etch rate [466] in some cases. Organic additives are used for improving the wetting of thin water film on the surfaces [467]. Soapy additives were tested by Roth and Geiger [452], but no effect on the machining results was observed. Water with a saccharose additive was applied as a micromachining mask [468] (see section 4.1.2.7). The main parameters of lasers used for subtractive processing are given in Tables 4.2, 4.3, 4.6 and 4.9–4.11. For frontside micromachining, nanosecond-pulsed UV–VIS–NIR lasers of wavelengths in liquids transparency

145

Subtractive processing

Table 4.1

Liquids and their additives used at frontside laser machining (for backside machining, see Table 4.9).

Liquids

Additives

Water, heptane, perfluorocarbons, benzene, o-xylol, p-xylol, ethanol, glycerin, ether, DMSO, DMFA, N2 H4 , liquid nitrogen. molten NaCl, NH4 Cl, NaNO3 and KNO3

H2 O2 , NaCl, CaCl2 , NaNO3 , KNO3 , Na2 SO4 , K2 SO4 , CuSO4 , KOH, methanol, ethanol, isopropanol, soapy additives, saccharose

Notations DMFA – dimethylformamide DMSO – dimethylsulfoxide (CH3 )2 SO

window are commonly used. In high-power applications, cheap and energetically effective CO2 lasers are the choice.

4.1.2 Frontside micromachining 4.1.2.1 Experimental arrangements Figure 4.3 schematically depicts the main methods of providing the working zone with liquids. In Figs 4.4 and 4.5 two more sophisticated systems including means for process monitoring and control are shown.

4.1.2.2 Phenomenology and mechanisms: nanosecond-laser pulses Ablation mechanisms Similar to the ablation in gases, in liquids the major ablation mechanisms are melting and vaporization of the material. In addition, some phenomena that are of second order in gas and vacuum become more pronounced in liquids, because the cooling rate and the density of chemically active species is larger in liquids (see also Section 4.1.2.3).

Pressure effects Many times higher vapour and plasma pressures in confined ablation in comparison with ablation in vacuum and in gas are believed to be responsible for high ablation rate in liquids, but the detailed mechanism has not been clarified yet. Fatigue damage At low laser fluences (surface temperature below the melting point of the material), fatigue damage was observed in single-crystal silicon [471, 474, 475]. Xia et al. [476] calculated thermal stresses in laser-heated single-crystalline Si and Ge (below ablation threshold) to be ∼1 GPa (laser: 1.06 µm, 10 ns, 6.4 GW/cm2 ), that is, larger than the stress thresholds for fracture in these materials. Cavitation impact Isselin et al. [477] have studied the surface damage of metals due to bubble collapse (cavitation erosion) in relation to the bubble diameter (0.9–3.8 mm) and the distance of bubbles from the surface. They estimated the peak shock wave pressure to be 120–160 MPa, the microjet impact pressure to be 1–7 MPa, and the pressure influence time to be about 300 ns. Geiger et al. [469] also found evidence of Water

(a)

(b)

(c)

Steam

(d)

(e)

Figure 4.3 Methods for providing liquid into the working zone during laser micro machining. These method were used, for example in: (a) Geiger et al. [469], (b) Sakka et al. [470], (c) Shafeev and Simakin [471], (d) Dupont et al. [472], (e) Geiger et al. [469]. In the cases (d) and (e), laser light not well transmitted by the liquid can be used. The thickness of the liquid layer over the target in the case (a) has typically been 1 mm. Besides focused laser beam, mask projection pattering has been widely used as well.

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Compressed air Timing control unit

Air filter

Flow controller

Translation stage Sample

Nozzle

Aperture

Beamsplitter

Energy meter

Heater

Temperature controller

Nd:YAG laser (λ = 1064 nm FWHM = 6 ns)

Thermocouple

Figure 4.4 Schematics of a steam-assisted laser ablation system. A liquid film is formed on the workpiece surface through vapour condensation [473]. © Elsevier.

Photodetector Interference filter

Lens HeNe laser

Target Nd:YAG laser pulse Microphone Liquid film

Lens Lens Photodetector

HeNe laser

Figure 4.5 Techniques of monitoring of liquid-assisted laser machining process by surface reflectivity, photoacoustic deflection and acoustic emission techniques. Onset and intensity of vaporization, and the velocity of acoustic and shock waves can be determined this way [473]. © Elsevier.

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mechanical damage in A12 O3 ceramics laser etched in water. Shafeev et al. [458] assumed that the microjets (Fig. 7.10) damage the graphite layer at diamond etching in water, resulting in high catalytic activity of the etched diamond for electroless metal deposition.

Dissolution Dissolution of workpiece in laser-generated supercritical water was deemed to contribute to machining of Si3 N4 in the work by Hidai and Tokura [478] and of Al2 O3 in the work by Dolgaev et al. [479]. Physical conditions at laser micromachining in liquids Pressure The pressure dynamics at laser etching of solids in water was investigated by Ageev et al. [480] and Zweig [481]. Ageev et al. [480] recorded, using a piezoelectrical sensor, the oscillations of pressure in the etching vessel (laser: pulse length ∼500 µs, pulse energy some J). They estimated by calculations the peak pressure to be 10 GPa and measured the bubble diameters to be 0.55–0.58 mm (irradiation power density 1.2 × 107 W/cm2 ). Zweig [481] measured, optically, the velocity of the shock wave near the surface of a polyimide sheet target at distances up to 1 mm from the target and calculated the corresponding pressures. He got a pressure value >10 kbar (1 GPa) at the target’s surface at laser fluence of 90 J/cm2 . Shock pressure varied as a square root of the incident laser fluence of up to 90 J/cm2 . Bubble generation started at 8 mJ/cm2 , and pressure waves were detected beginning at 50 mJ/cm2 . The dependence of pressure on fluence was in good agreement with the ideal gas model. Zhu et al. [482] using the theory of Fabbro et al. [233] estimated that the pressures at ablation in water are 5.8 times greater than at ablation in air (4.5 J/cm2 , 23 ns). As reported by Daminelli et al. [483] at machining by nanosecond-laser pulses, 50–70 per cent of the incoming energy may be coupled into photoacoustic phenomena, such as shock wave pressure and cavitation bubbles; for 30-ps pulses the conversion of laser energy into mechanical effects is about 18 per cent and for 100-fs pulses 7 per cent.

Vapour and plasma dynamics Many researchers have photographed the liquid, vapour, and plasma dynamics at laser etching in water, thereby using high-speed cameras [452, 469, 472, 480, 484]. Roth et al. [452] report that in case of a sprayed water film on surface the vapour phase lasts for about 500 µs (laser fluence 25 J/cm2 ), but in case of dry ablation the plasma relaxes in 2.5 µs; Geiger et al. [469] measured for the duration of the vapour phase under water ∼800 µs (20 J/cm2 , water level 10 mm); Dupont et al. [472] photographed the plasma luminescence both in air and in water. The duration of the luminescence was about 80 μs in air, and about 25 µs in case of a vertically flowing water film (laser: 17 J/cm2 , 24 × 109 W/cm2 ). Acoustic emission Roth and Geiger et al. [452, 485] reported that both the brightness of plasma and the emitted sound were significantly lower at laser ablation under water compared to these in air (laser fluence up to 30 J/cm2 , sprayed water film on the workpiece surface). In contrast, Zhu et al. [482] found that sound was 25 per cent stronger at ablation in water compared to the ablation in air (2–5 J/cm2 , 1-mm water film). Mechanisms responsible for debris removal Laser heating generated thermal gradients and bubbles cause a convection of the liquid, whereas drag forces on particles are much larger than in gases. On the other hand, the settling velocity for particles is considerably lower in liquids. Thus, the debris is effectively removed from the working zone, and fabrication of deep trenches and long channels by ablation in a neutral liquid becomes possible (Figs 4.6 to 4.8). Drag force on small spheres in an incompressible viscous fluid is given by Stokes formula [486]: FD = 3πμ dv,

(4.1)

and gravitational settling velocity by: 1 g (ρ1 − ρ) d 2 , (4.2) 18 μ where μ is (dynamic) viscosity of the fluid, d is diameter of the sphere, v is velocity, g is gravitational acceleration, ρ1 is density of the sphere, and ρ is density of the fluid. For example, the viscosity of air is 18.3 µPa s and of water is 0.89 mPa s (25◦ C), thus the drag force is 48 times larger and settling velocity is 48 times lower in water than in air. Despite the settling time of debris is much longer in liquids, a circulation of the liquid is recommended, because the suspending debris scatters and absorbs the laser light. v=

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Laser

Laser

Window Workpiece

(a)

Heated zone

(b)

Figure 4.6 Thermal (a) and bubble-driven (b) convection of liquid in laser irradiated zones. (Schematically after Shafeev et al. [471] and Ohara et al. [484].) When the liquid has a free surface, the Marangoni convection may contribute essentially. In air

a

b

c

100 ␮m

Figure 4.7 Effect of liquid ambient becomes especially pronounced at machining of deep grooves and blind holes. Here, grooves machined in a NdFeB magnet in air and in water are compared [487]. Laser: 1.064 µm, 180 ns, 1.8 mJ, 1 kHz (feed rate in mm/s)/(number of passes): (a) 0.8/6; (b) 0.8/8; (c) 0.08/8; (d) 0.08/2; (e) 0.08/4; (f) 0.08/8. © Elsevier.

Ohara et al. [484] estimate the rate of bubble generation by the formula: E = ρ[(tv − tR )C + Hv ],

(4.3)

where ρ is specific gravity (g/cm3 ), tv is vaporization temperature, tR is initial liquid temperature, C is specific heat, and Hv is latent heat of vaporization; and the discharge rate of produced bubbles by the formula 6η dv = g − 2 v, dt a ρ

(4.4)

where g is the constant of gravity, η is the coefficient of viscosity, and a is the radius of the bubble. For Cu and Al etching in water, EtOH and PFC, the observed etching rates correlated with calculated by Eqs. (4.3) and (4.4) bubble generation and discharge rates (0–10 mJ laser pulses, 10 µm spot size): it was concluded, that the more the bubbles were generated and the faster they were discharged, the faster was the removal of debris and the higher the etching rate. Shafeev et al. [471] presented photographs of the convective flow of a 300-µm-thick horizontal liquid layer between the target and the window, and estimated flow velocity to be about 1 cm/s (water, DMFA, or DMSO, irradiation power densities up to some kW/cm2 ).

Chemical processes at laser ablation in liquids At laser-induced plasma temperatures, thousands of kelvins, and due to plasma UV radiation, liquids molecules may be excited, ionized, and dissociated and thus become chemically active. In many investigations, the formation of oxides in laser plasma [470] and on the workpiece surface [459–461, 471, 472, 475] were observed at laser ablation in water. Shafeev et al. [458] suppose that released from the liquid hydrogen contributes to laser etching of diamond in water and in (CH3 )2 SO.

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0459

6KV

10mm ×400

16mm

Figure 4.8 A trench in oxidized (111) silicon laser cut under still water layer [488]. Laser: Nd:YAG, 1.06 µm, 180 ns, 1 kHz, ∼1,7W average, spot 50 µm, scanning speed 0.1–2 mm/s. No debris is left, but the quality of the cut is poor for this pulse length. © SPIE (1999), republished with permission. 2

1

Ablation rate (␮m/pulse)

Al2O3 1,6

2 In water 3

1,2 0,8 0,4 In air 0

1

10

20 25 30 Laser fluence (J/cm2)

35

Figure 4.9 Dependence of the ablation rate on the laser fluence at laser etching of Al2 O3 in water and in air [469]. Laser wavelength: 308 nm (XeCl laser); pulse duration: 50 ns; pulse frequency 2 Hz. Curves: 1: sprayed water; 2: 2-mm-thick water layer; 3: 10-mm-thick water layer. Al2 O3 exhibits an exceptionally high etching rate in water. Elsevier Science Ltd (1996). + Hidai and Tokura [478] identified hydrothermal reaction products, NH+ 4 , NH4 –N, and silica ion/boric ion after laser ablation of Si3 N4 /cBN in water (Table 4.9, Hidai 2001). Also Dolgaev et al. [479] found evidence that laser ablation of sapphire in water and electrolyte solutions (see Table 4.9, Dolgaev 2001) was assisted by dissolution of the material in supercritical liquid. Ablation rate was dependent on type and concentration of cations, but independent of anions. Miyazawa and Murakawa [489] report about formation of CO, CO2 , CH4 , and misty particles of K2 CO3 at laser cutting of diamond in aqueous KOH solution. Chemical reactions at laser ablation of carbon in organic solvents and water are reported in Section 7.5.

4.1.2.3 Ablation efficiency In most cases the ablation rate by nanosecond-laser pulses has been larger in liquids than in gases (see Fig. 4.9 and Table 4.2). This is explained by the following: •

Increased energy-coupling efficiency by optical matching: since the refractive index of water is greater than that of air, the overall optical absorptivity of air–water–aluminium system was found to be larger than that at the air–aluminium interface [467].

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Ablation rate (volume), ␮m3

70 000 60 000

Dry surface H2O-coated surface

50 000 40 000 30 000 20 000 10 000 0 0.5 0.6 0.7 0.8 0.9 1 2 Normalized laser fluence (F/Fd)

Figure 4.10 Ablation rates of dry and liquid-coated aluminium (100-µm-thick foil) as a function of laser fluence normalized by the ablation threshold of a dry surface Fd . © American Institute of Physics (2001), reprinted with permission from Ref. [467].

•

• • •

• •

The bubbles carry the debris effectively away, thus avoiding the absorption of laser light on debris. High temperature of confined plasma [459, 482]. Liquid prevents the expansion of plasma and thus enhances the action of laser radiation. Mechanical impact of microjets generated at collapse of vapour bubbles [467]. Shock waves, originating from plasma expansion, collapse of gas bubbles and from microjet impacts can destroy passive layers on the target surface: for example, graphite layer on the diamond [456–458]; Si and SiO2 layers from an SiC surface [460, 461]. Shafeev et al. [471] observed that when bubbles were generated, there was always some surface damage of single-crystal silicon. Geiger et al. [469] suppose that shock creates cracks in ceramic workpieces. Dissolution of workpiece and debris in supercritical water [478, 479]. Laser-enhanced electrochemical dissolution in neutral salt solutions: metal dissolution may set in due to localized breakdown/dissolution of the passive film induced by the laser beam [465]. Occasionally, the etch rate may be lower in liquids than in gases/in vacuum, due to:

• • •

reduction of the transparency of the liquid due to accumulation of debris in suspension [459]; scattering of light by bubbles, lowering this way the energy density at workpiece; hardening of the material due to laser shocks (observed e.g. in steel 304 AISI) [472].

Ablation threshold and incubation effect A higher etching threshold in liquids compared to this in air has been commonly observed and explained by larger heat losses in the liquid (Figs 4.9–4.11). In single-crystalline materials, the etching rate (both in gases and liquids) has been observed to increase with the number of laser pulses (Fig. 4.12) [474, 475, 483]. This phenomenon, called incubation, is explained by enhancement of laser–material interaction due to defect accumulation (see also Section 4.4.2.1). According to Jee et al. [490] the threshold fluence Fth decreases with pulse number N as: Fth (N ) = Fth (1) × N ξ−1 ,

(4.5)

where ξ is the incubation coefficient. For example in silicon ablation with 800 nm, 130-fs pulses, ξ = 0.83 ± 0.04 and ξ = 0.82 ± 0.02, for water and air, respectively [483].

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14 000

Ablation depth (nm)

12 000 10 000 8000 6000 4000 2000 0 (a)

0

0.5

1 Fluence (J/cm2)

1.5

2

2.5

0

0.5

1 Fluence (J/cm2)

1.5

2

2.5

14 000

Ablation depth (nm)

12 000 10 000 8000 6000 4000 2000 0 (b)

Figure 4.11 Variation of silicon ablation depth with laser fluence for different number of pulses in air (a) and under water (b) [491]. (△) 5000 pulses (×), 4000 pulses, () 3000 pulses, (×) 2000 pulses, (+) 1000 pulses, and (⋄) 500 pulses. Laser: 248 nm, 25 ns. © Elsevier.

Influence of liquid layer thickness Influence of water and water/IPA film thickness in micrometre range on 6 ns Nd:YAG laser ablation of aluminium was studied by Kim and Lee [467] The ablation rate was found to be strongly dependent on the liquid film thickness and to increase with the film thickness for both liquids. However, once the thickness exceeded a certain critical value, typically few microns, the ablation rate saturated and then decreased slightly as bulk liquid layers of thickness ≈1 mm were applied [467].

4.1.2.4 Surface quality As a rule, the higher ablation rate in liquids is accompanied by increased surface roughness and in some cases also increased porosity (Figs 4.13 and 4.14). Geiger et al. [485] measured the change in the flexure strength of ceramics after etching in water and in air. After processing in water, Si3 N4 was stronger but Al2 O3 was weaker in comparison with specimens processed in air. They also report that the surface roughness of all materials was greater when etched in water compared to etching in air. Shafeev et al. [471] and Kruusing et al. [488] observed pores formation on a single-crystal Si surface laser irradiated in water. Laser-assisted etching of various materials in salt, base, or acid solutions is known to yield smoother surfaces than etching in pure water [465, 489, 493, 494]. Also etching of SiC in N2 H4 resulted in a smoother surface than etching in water [461].

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14 000 12 000 Ablation depth (nm)

10 000 8000 6000 4000 2000 0

0

1000

2000 3000 Number of pulses

4000

5000

0

1000 2000 3000 Number of pulses

4000

5000

(a) 14 000 12 000 Ablation depth (nm)

10 000 8000 6000 4000 2000 0

(b)

Figure 4.12 Variation of silicon ablation depth with the number of pulses at various laser fluences in air (a) and under water (b) [491]. (△) 1.7 J/cm2 , (×) 1.8 J/cm2 , () 1.9 J/cm2 , (×) 2.0 J/cm2 , (+) 2.1 J/cm2 , and (⋄) 2.2 J/cm2 . Laser: 248 nm, 25 ns. © Elsevier.

100 ␮m

Figure 4.13 Rutile target surface etched by laser under water. Laser: 266 nm, spot 40 µm, 100 J/cm2 . © American Chemical Society (2004), reprinted with permission from Ref. [492].

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nm 50

x Profile

nm 600

30

320

10

40

−10 −30 −50

−240 −520 −800

0

79

238 159 micrometer

318

397

x Profile

0

79

20 pulses

(a)

159 238 micrometer

318

397

(b)

Figure 4.14 Cross-sectional profiles of silicon surface etched by laser in air (a) and under water (b) [491]. Laser: 248 nm, 25 ns. © Elsevier.

4.1.2.5 Machining by ps/fs-laser pulses In comparison with nanosecond pulses, ablation with fs/ps-pulses has differences in etch rate and in machined surface morphology.

Ablation rate and crater shape Compared with nanosecond pulses, in case of fs/ps pulses the ablation rate at the same laser fluence is lower, probably due to weaker conversion of incoming energy into mechanical effects [483]. Ablated surfaces are at least at moderate fluences better defined and smoother in case of nanosecond pulses, even in high thermal conductivity materials like gold and silver (Figs 4.15 and 4.16). Formation of ripples and rings on ablated surface Irradiation of single-crystalline silicon by femtosecond laser pulses in known to produce short-period surface ripples [497]. In water, the period of the ripples is smaller, 100 nm vs. 700 nm in air (Fig. 4.17). Katayama et al. [498] observed formation of concentric ring patterns at laser irradiation of silicon surface by 200 fs 15 mJ/cm2 pulses under water (Fig. 4.18). Their formation was explained by bubbles oscillation induced acoustic waves impact on liquid silicon.

4.1.2.6 Laser beam autofocusing in liquid At high laser intensities, some liquids may act as focusing lens, a phenomenon called autofocusing [499]. Ramanathan and Molian [463] used laser beam autofocusing for drilling of micrometre-sized holes in 316 stainless steel. A two-fold decrease in the hole size and reduced taper was achieved in comparison with traditional solid focusing optics. Polarization effects were also substantially reduced (Fig. 4.19). Best results were achieved using carbon disulfide (CS2 ), a well-known optically nonlinear liquid. Selffocusing of a light beam is due to increase of the refractive index (decrease of the speed of light) with laser field intensity: |E|2 , (4.6) 2 where n0 is linear refractive index of the medium, n is refractive index change, n2 is nonlinear refractive index, I is the intensity of the light, and E is the amplitude of the electric field. For carbon disulfide at 1064 nm, n = 9.0 × 106 + 9.6 × 10−12 × |E|2 (in esu units; n2 [esu units] = (c × n0 /40π)γ [SI units], where c is the speed of light in vacuum). Self-focusing length (distance at which the beam shrinkage occurs) is [500]: n = n0 + n = n0 + γI = n0 + n2

l=



n0 D D · = δn 2E 2



n0 , 2n2 I0

(4.7)

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In water (900 shots) 120 fs

Depth (20 ␮m/div) (a)

Diameter (100 ␮m/div)

(b)

Diameter (100 ␮m/div)

In air (450 shots) 8 ns

Depth (20␮m/div)

Depth (20 ␮m/div)

120 fs

(c)

100 ␮m

Depth (20 ␮m/div)

8 ns

Diameter (100 ␮m/div)

(d)

Diameter (100 ␮m/div)

Figure 4.15 SEM (Scanning electron microscope) images and depth profiles of craters in silver targets generated in various ablation conditions [495]. Focusing condition was adjusted for each ablation condition; 900 and 450 pulses were applied to targets in water and those in air, respectively. © Elsevier.

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(a)

(b)

Figure 4.16 Typical craters in a gold target in water after 5000 laser pulses at F = 60 J/cm2 (a) and F = 1000 J/cm2 (b). Laser: 800 nm, 110 fs. Dimensions of the craters/scale of the images were not given. © American Institute of Physics (2003), reprinted with permission from Ref. [496]. Air

Water

2.5 ␮m

2.5 ␮m

(a)

(b) Air

Water

2.5 ␮m

2.5 ␮m

(c)

(d)

Figure 4.17 Femtosecond laser irradiated silicon surface: comparison of ripple periodicities observed in water and air experiments (SEM view) [483]. (a) F = 1.6 J cm−2 (7 × Fth ), N = 100, specimen vertically; (b) F = (0.20 ± 0.02) J cm−2 (1 × Fth ), N = 100; (c) F = 1.5 J cm−2 (8 × Fth ), N = 1000, specimen vertically; (d) F = (0.45 ± 0.06) J cm−2 (4 × Fth ), N = 1000. Laser: 800 nm, 130 fs. © Elsevier.

where D is the initial diameter of the beam and I0 is the maximum intensity. In the work by Ramanathan and Molian [463], l = 40 mm. Critical power needed to self-focus the beam is [501]: Pcr =

π(1.22λ)2 ε0 c , 32n2

(4.8)

where λ is wavelength of the laser beam, ε0 is dielectric permittivity of vacuum, and c is speed of light in vacuum. For carbon disulfide the calculated critical power for self-focusing is 11.32 kW (at 1064 nm wavelength).

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10 ␮m

Figure 4.18 A typical AFM image of a silicon surface after irradiation by a single-laser pulse in water. Laser: 800 nm, 200 fs, 15 mJ/cm2 . © American Institute of Physics (2003), reprinted with permission from Ref. [498]. Laser

Prism and Solid Lens

Liquid optics

Stainless steel X–Y Stage

Figure 4.19 Experimental setup for nonlinear liquid-assisted laser drilling. Laser: 1064 nm, 15 ns, 1 Hz, 400 mJ. © American Institute of Physics (2001), reprinted with permission from Ref. [463].

Besides CS2 , another candidate nonlinear liquid for laser machining is benzene with n = 8.5 × 106 + 5.4 × −12 10 × |E|2 , and critical power of 20.12 kW (at 1064 nm).

The holes drilled by 316 stainless steel with the solid optics [463] were distorted in shape due to the linear polarization of the Nd:YAG beam. Using liquid optics, the polarization effects were considerably reduced. In addition, the number of pulses required to drill through the sample with liquid optics was much less than that required with solid optics (Fig. 4.20).

4.1.2.7 Liquid as micromachining mask In the work by Lapczyna and Stuke [468], a droplet of saturated saccharose solution in water was used as an ablation mask in laser etching of circular structures on the surface of PMMA (Fig. 4.21). Ablation was performed with an F2 laser (157 nm) in vacuum. The mask surface remained smooth even after more than 1 h of pumping at 10−2 mbar. The diameters of the mesas (Fig. 4.22a) were about 10 µm and their height about 6 µm. The surface roughness of the ablated areas was below 100 nm. By injecting an air bubble into the masking fluid (Fig. 4.22b), rings with an outer diameter of 650 µm and an inner diameter of 350 µm were achieved. The high surface tension of solution provided a regular shape of the droplets and thus of ablated areas as well.

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Hole diameter (microns)

180

Entrance 4–8 pulses

160 140 Solid optics

120 100

Entrance 1 pulse

80 60 40

Exit

Liquid optics

20 180

210

240

270 300 Energy (mJ)

330

360

Figure 4.20 Variation of hole diameter in 316 stainless steel with pulse energy for solid and liquid optics (CS2 ). Laser: 1064 nm, 15 ns, 10 Hz. © American Institute of Physics (2001), reprinted with permission from Ref. [463].

Air bubble

Liquid mask Substrate

Result

(a)

(b)

Figure 4.21 Formation of surface relief on PMMA using a liquid droplet as an ablation mask [468]. Reproduced with kind permission of Springer Science and Business Media.

Acc.V Spot Magn WD 5.00 kV 4.0 5000x 8.9

5␮m PMMA. 157nm

(a)

Acc.V Spot Magn WD 5.00 kV 5.0 150x 8.5

200␮m PMMA. 157nm

(b)

Figure 4.22 Circular (a) and annular (b) mesas ablated in PMMA using liquid droplet as ablation mask. The micro trench (vertical line in (b)) was etched through a silicon contact mask prior to the ring. The horizontal line crossing the ring in the lower part of the picture indicates a step ablated subsequent to the ring, again by means of a silicon mask [468]. Reproduced with kind permission of Springer Science and Business Media.

Table 4.2

Liquids-assisted frontside laser micromachining and related experiments. (reports where only reactive liquids were used, are not refereed here).

Materials machined

Liquids/gases in contact with specimen

Laser type and beam parameters

Other features of the experiment

Sn, Pb, Zn, Bi, Mg, Air, water, heptane, benzol, Al, Cu, Mn, Mo, spirit, p-xylol, o-xylol, Ta,W ether, glycerin

Nd: glass, 140 µs, 10 J, 106 –1010 W/cm2

Focused beam, no The etching rate in water was about half of that in air; Ageev (1975) scanning, specimen correlation between thermal parameters of targets and [431] immersed into water formed crater dimensions tabulated for ablation in air and in water

Ar+ , CW

Focused beam, 3.3 × 105 W/cm2 specimen immersed vertically into water

Etch rate up to 4 µm/s (0.5 M NaNO3 ); crystallization of salt observed; etch craters are irregular and having melted surface in water, but of relatively good morphology in salt solutions

Ti sheet 60 µm; Zr Air, water, ethanol, liquid plate 0.5 mm nitrogen (LN2 )

Nd:YAG, 100W

Focused beam, 1–30 mm water layer over the specimen

Arzuov Dependence of through-hole drilling time on laser power and water layer thickness presented; drilling in (1987) [502] liquids is slower than in air and faster in water than in ethanol; bubble generation and contamination of surfaces by oxygen and carbon and observed in water and ethanol; in LN2 intensive boiling and nitride film formation observed

Silicon nitride ceramics

Nd:YAG, 100 ns, up to 50 kHz

Specimen immersed About 100-µm diameter holes drilled in air and in into circulating water; in water with 100-ns pulses below 10 kHz no water, focused beam recast layer and cracks were observed 0.3 mm, up to 7.1× 107 W/cm2

1-ms pulsed laser, 120 Hz

Focused beam Optically tweezered particles of diameters 4–7 µm

Precision tool steel, 304 stainless steel

Water, water solutions of NaNO3 , K2 SO4 , NaCl

Air, water

Pyrene-doped PMMA latex

Water

3ω-Nd:YAG, 355 nm, 7 ns

Mn0.6 Zn0.4 Fe2.3 O4 ferrite

Water, KOH and CaCl2 solutions

Ar+ , 514 nm, 1W peak, Specimen under a CW, pulse modulated liquid layer (2 mm) Cu-vapour, 510.6 nm, 25 ns, 6 kHz

Novel features, observed phenomena, comments

References

Datta (1987) [465]

Morita (1988) [455]

Through-holes of sub-micrometre diameter drilled; the role of water was to enable optical tweezering

Misawa (1990) [462]

Studies of bubble growth in water depending on pulse width and frequency; in KOH solution the bubbles were smaller and did not adhere to sample’s surface; addition of salt did not enhance the etch rate in case of Cu-vapour laser

Hussey (1991) [503]

Hgx Cd1-xTe

Water, DMSO, DMFA, Br-containing etchants

Cu-vapour

Specimen under a steady or flowing liquid layer, scanning by focused beam

Cooling by liquids avoids decomposition of HgCdTe; in comparison with laser etching in reactive solutions the non-irradiated areas are not etched; generation of particles of diameter 10 J/cm

200–300 nm/pulse

>10 nm

Laser ablation in air, fs/ps-pulses [582, 583]

6 J/cm2 (1.2 ps)

20–40 nm/pulse

>10 nm

Laser ablation in air, VUV, nanosecond pulses [580, 582]

1 J/cm2 (157 nm)

20–40 nm/pulse

4–8 nm (r.m.s.)

LIBWE, organic solutions [586]

0.3 J/cm2

5–30 nm/pulse

0.23–10 nm

LIBWE, liquid metals [585]

1.3 J/cm2 (248 nm) 7 J/cm2 (1064 nm)

Up to 600 nm/pulse

1.5–7 nm (r.m.s.)

Halogenated hydrocarbons have reduced liquid decomposition effects (reduced incubation phenomena, less debris) [588]. Liquid metals (Hg, Ga) do not show neither incubation effect nor produce debris, they provide high etch rate and enable the use of longer wavelength lasers [585].

Considerations for the choice of additives • • •

High light absorption at the laser wavelength ensuring that only a micrometre-thick layer of the liquid in contact with the workpiece is heated. Little debris from decomposition and chemical reactions. High photostability and absence of luminescence (was the argument for choice of K2 CrO4 in the work by Paraskevopoulos et al. [589]).

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Figure 4.37 Surface morphology of a calcium fluoride plate etched with 500 pulses of KrF laser at 900 mJ/cm2 , using an acetone solution containing pyrene at a concentration of 0.4 mol/dm3 [574]. © Elsevier. Table 4.8

Liquids and additives used in backside laser machining of transparent materials.

Liquids

Additives

Cyclohexane, tetrachloromethylene, tetrachloroethylene, benzene, toluene, cumene, t-butylbenzene, 1,2,4-trimethylbenzene, chlorobenzene, dichlorobenzene, fluorobenzene, isopropanol (IPA), tetrahydrofuran, methylmethacrylate, methyl benzoate, acetone, mercury, gallium

NiSO4 , CrO3 , KMnO4 , CrO3 , FeCl3 , KMnO4 , KNO3 , K2 CrO4 , carbon particles, pyrene, pyranine, benzil, naphthalene, phenanthrene, anthracene, 9-methyl-anthracene, 9,10-dimethyl-anthracene, 9-phenyl-anthracene, fluoranthrene, Rose Bengal dye, Np(SO3 Na)3

Cheng et al. [590] present a table of etching thresholds, extinction coefficients and fluorescence quantum yields for 8 additives (pyrene, naphthalene, phenanthrene, anthracene, 9-methyl-anthracene, 9,10-dimethylanthracene, 9-phenyl-anthracene, fluoranthrene) to organic solvents.

Choice of the laser Nanosecond lasers are effective for LIBWE in organic solvents only in the UV region of wavelength. Femtosecond lasers are applicable also in NIR region [584]. Cheng et al. [591] succeeded in backside etching of glass with a 532-nm, 15-ns laser in conjunction with Rose Bengal dye solution in acetone. In case of liquid metals as absorbents, low-cost VIS and IR lasers can be applied.

Advantages of liquids-assisted laser backside etching • • • • • • •

Etching threshold may be 10–20 times lower than in gas. Etching occurs well below the optical damage threshold of the materials (for example, the damage threshold of quartz is ≈20 J/cm2 for direct laser irradiation [587, 592]). Low surface roughness in comparison with VUV and fs/ps-laser ablation in air, and with reactive plasma etching [580]. Low debris, no microcracks. One-step method in comparison with lithography. Fabrication of long and bent channels is easier than in gases because of more efficient debris removal by liquid motion and debris dissolution. There is no plasma shielding of laser light at backside etching.

Subtractive processing

181

Disadvantages of liquids-assisted laser backside etching • •

The surface may be contaminated by liquid and solute decomposition/reaction products like carbon, chlorine [593], and chromium oxide [573, 594, 595]. Etching rate in organic solutions may be significantly lower than the etching rate in gas by fs/ps- or VUV lasers (Table 4.7).

However, a low etching rate is an advantage in fabrication of sub micrometre features in optical materials, because the process control is easier.

Backside etching due to laser-generated hydrofluoric acid In the experiments by Murahara [596], backside etching (polishing) of fused silica occurred due to hydrofluoric acid, generated at laser irradiation of fluoroethylenepropylene in water (see Table 4.9, Murahara 2001). The chemical surface reaction is similar to this presented in Fig. 6.1.

4.4.2 Technologies, phenomenology, and etching mechanisms 4.4.2.1 Organic and aqueous solutions at backside of the workpiece Absorption of laser light For efficient coupling of laser light into a workpiece, high absorption coefficient of the liquid at laser wavelength is needed along with short heat diffusion length. Most widely used additives to organic solvents, like pyrene and pyranine start to absorb considerably only in the UV region (Figs 4.38 and 4.39), whereas used in the work by Cheng et al. [591]. Rose Bengal dye has the absorption maximum in yellow (Fig. 4.40). Etching mechanisms Laser etching of inorganic transparent materials with organic or aqueous solutions at backside of the workpiece is supposed to proceed in the following way.

(1) Absorption of light Absorption of light in solute following energy transfer to solvent and to workpiece. In pyrene, there is a strong evidence that multiphotonic absorption is the primary mechanism of absorption [574, 599, 600] (Figs 4.41 and 4.42). Typical absorption coefficients of used hydrocarbon solutions at UV wavelengths are about 104 cm−1 [590, 585], thus the heated by nanosecond-laser pulses depth is about 1 µm [479].

(2) Modifications of the workpiece surface At laser irradiation, the workpiece surface may undergo changes that enhance the light absorption. This leads to so called incubation effect (Figs 4.47 and 4.48), where ablation is absent or ablation rate is low for the first laser pulses and increases thereafter. Zimmer et al. [601] found, that at backside ablation of silica glass in pyrene/toluene solution, a surface layer of ∼30–50 nm became amorphous and its absorption coefficient at 248 nm raised up to 104 –105 cm−1 . Total 10–30 per cent of incident laser energy was absorbed in this modified layer. Organic solvents decompose due to laser heating (e.g. the decomposition temperature of acetone is around 700 K [587]); the decomposition products like carbon adhere on the surface and enhance its absorption (Fig. 4.49). Dolgaev et al. observed thermal decomposition of CrO3 near the solid–liquid interface resulting in formation of water-insoluble of Cr2 O3 suspension [595] and film of Cr2 O3 [594] on the workpiece. Using carbon suspension in toluene, a thin carbon film was deposited on the workpiece, that led to self-modulation of the depth of etching of sapphire by a scanned laser beam (Fig. 4.35). Vass et al. [602] point that naphthalene methacrylate as working liquid may polymerize under action of laser light and form an absorptive layer on the surface. (3) Heat transfer and temperature rise Heat and temperature rise generated in liquid and in the absorptive surface layer is transferred into bulk of the liquid and into the workpiece, causing thermal stresses/softening of the solid, and melting/vaporization of both materials at higher temperature levels.

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Absorption (a.u.)

Handbook of Liquids-Assisted Laser Processing

0 200

0 200

300

400

250

300

500 600 700 Wavelength (nm)

800

900

Figure 4.38 Spectrum of linear absorption of a 0.5 M pyrene/toluene solution [597]. The inset shows a more detailed view of the UV range. © Institute of Physics, reproduced with permission.

Molar absorption coefficient (cm−1dm3 mol−1)

⫻104

SO3Na

3 NAO3S

SO3Na

2 OH

1

0 200

400 Wavelength (nm)

600

Figure 4.39 Optical absorption spectrum of 32 µM aqueous solutions of pyranine (8-hydroxy-1, 3,6-pyrenetrisulfonic acid trisodium salt) before (solid line) and after (broken line) irradiation with 5000 pulses from a KrF laser at fluence of 1.5 J/cm2 [598]. Reproduced with kind permission of Springer Science and Business Media.

Several researchers have calculated the temperature distribution at laser backside etching of fused silica [603– 605]. However, as shown by Zimmer et al. [601] the neglecting of light absorption in the modified surface layer of silica may lead to a considerable underestimation of the peak temperature. Considering absorption, they calculated for maximal interface temperature 6860 and 12 010 K at 950 mJ/cm2 fluence and wavelengths of 351 and 248 nm, respectively; while without absorption the maximum temperature was only 1079 K. For reference, the melting and boiling temperatures of fused silica are Tm = 1983◦ C and Tb = 2250◦ C.

(4) Thermal stresses Dolgaev et al. [595] attributed the ablation of sapphire below the melting threshold to the cracking of the surface due to thermal stresses between the sapphire and formed onto it Cr2 O3 layer. (5) Plasma effects High temperatures in the working zone cause thermal dissociation and ionization of solvent and target vapours. Laser heats the plasma due to inverse Bremsstrahlung absorption, and the heated plasma causes further heating of the sample. Bombardment of the workpiece by ions and electrons from plasma generates impurities, defects, defect-trapped and free electrons. In organic solvents, carbon species in plasma

183

Subtractive processing

Cl

0.6

Cl

Cl O

Absorbance

Cl

C

0.4

ONa l

l

NaO

O

O l

l

0.2

450

500

550 600 650 Wavelength (nm)

700

Figure 4.40 An absorption spectrum of 5 µM Rose Bengal (RB) in acetone [591]. The inset shows the chemical structure of RB. © Institute of Physics, reproduced with permission.

Sn Tm S1 T1

S0

Figure 4.41 Excitation of pyrene and photophysical process: possible mechanism for cyclic multiphotonic absorption [574]. © Elsevier.

[Pyrene]** [Pyrene] Laser (pulse)

Figure 4.42

Cyclic multiphotonic absorption

Rapid internal conversion

Super-heated liquid

[Pyrene]*

Plausible mechanism for LIBWE by cyclic multiphotonic absorption [574]. © Elsevier.

emit light from visible to extreme UV, thus capable to excite the electrons in optical materials like silica to defect or vacuum levels, giving rise to an increase of absorption of laser light by the material [606].

(6) Pressure, bubbles, and shock Rapid heating of liquid by laser generates high-pressure transients and shock waves in both the workpiece and in the liquid. Thereafter a vapour bubble starts to expand, reaching maximum size in ∼100 µs and shrinking then again [607] (Fig. 7.5). During the collapse of the bubble, a microjet forms and strikes the solid surface at speed of 100–200 m/s (Fig. 7.10). Both the pressure inside the bubble and microjet impact are believed to contribute to the modification of the material and to its removal from the workpiece [601].

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0.6 M 0.4 M

Each rate (nm/pulse)

0.8 0.6 0.4 0.2 0.0 0.0

0.4

0.8

1.2

1.6

Fluence (J/cm2)

Figure 4.43 Etch rate dependence on the laser fluence at backside etching of fused silica in aqueous solution of Np(SO3 Na)3 by a 30 ns KrF laser [608]. Reproduced with kind permission of Springer Science and Business Media. 45

Etch rate (nm/pulse)

40 35 30 25 20 15 10 5 0 0.0

0.2

0.4

0.6 0.8 1.0 1.2 Laser Fluence (J/cm2)

1.4

1.6

Figure 4.44 Etch rate vs. laser fluence of a 30 ns KrF laser etching of silica glass: () pure toluene liquid; () pyrene in acetone solution (concentration: 0.4 mol dm−3 ) [607]. © Elsevier.

Ding et al. [608] measured the instant velocity of the jet that formed at the collapse of an R = 0.8 mm bubble to be 200 m/s at a delay time of 100 ns. The impact pressure of the liquid jet was estimated using the formula [609] P = ρCVjet ,

(4.12)

where ρ and C are the density of water and the acoustic velocity in water, respectively. A jet velocity of 200 m/s corresponds to a pressure of 300 MPa. Böhme and Zimmer [610] explained by bubble size the influence of the laser spot size on the etch rate of fused silica in pyrene/toluene. Large bubbles persist a longer time and more solvent is decomposed and deposited onto surface of the workpiece, thus the etch rate should be larger for larger laser spot size.

(7) Dissolution of workpiece in supercritical solution Dolgaev et al. [479] pointed to a possible hydrothermal dissolution mechanism in laser backside etching of sapphire. Vass et al. [605] observed that

185

Subtractive processing

28 60

BaF2 quartz

24 Etch rate (nm/pulse)

Etch rate (nm/pulse)

70

50 40 30 20

(a)

160 140

20

120

16

100

12

80 60

8

40 4

10 0

180

CaF2 Sapphire

0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 Fluence (J/cm2)

0 0.8 (b)

20 0 1.2

1.6 2.0 2.4 2.8 Fluence (J/cm2)

3.2

3.6

Figure 4.45 Etch rates vs. laser fluences at LIBWE (a) for BaF2 and quartz, (b) for CaF2 and sapphire [612]. Solution: 0.4 M pyrene in acetone; laser: XeCl, 25 ns. © Elsevier.

Etch depth after one pulse (nm)

250 200

Fused silica

150

Laser

Carbon layer

Liquid medium

100 50 0 0

1000

2000 3000 4000 Laser fluence (mJ/cm2)

5000

Figure 4.46 Etch depth – laser fluence dependence in case of a predeposited carbon layer on the surface [606]. Liquid: water, carbon layer thickness:  26 nm,  22 nm. Reproduced with kind permission of Springer Science and Business Media.

at lower energy densities (210 mJ/cm2 ) no melted silica droplets were found in the working zone, despite the etching occurred. Dissolution rates of some materials in high-temperature high-pressure water are given in Table 7.4.

Etch rate Etch rate dependence on laser fluence at LIBWE in aqueous and organic solutions is characterized by a twoslope curve (Figs 4.43 to 4.45). Similar dependencies were found also at etching of fused silica in pyrene/acetone [611], BaF2 and quartz in pyrene/acetone [612], and fused silica in naphthalene/methyl-methacrylate [605]. Vass et al. [605] found by calculations that the breaking point corresponds to the onset of melting of silica. Figure 4.46 presents the results of an experiment where a carbon layer was predeposited onto the surface of the workpiece, in order to get support to the liquid decomposition etching mechanism. In cases of metals or a solution of Rose Bengal dye in acetone at the backside of the workpiece [591], the etch depth was found to depend linearly on the laser fluence over all used range of laser fluences.

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Number of incubation pulses

800

C ⫽ 0.4 M (Pyrene/Acetone)

700 B

600 500 400 300 1

200

2

100 0.6

0.8

1.0

1.2 1.4 1.6 1.8 Fluence (J/cm2)

2.0

2.2

Figure 4.47 Number of pulses required to initiate etching of quartz [587]. © SPIE (2004), reproduced with permission from Ref. [587].

Incubation effect A characteristic feature of laser backside etching using organic solutions (both bulk and absorbed layer [592]) is that at low fluences the etching rate tends to increase with time (Figs 4.47 and 4.48). The incubation process can be explained by modification of the etched material due to high temperature and plasma irradiation (e.g. amorphization of fused silica) or by formation of an carbon layer due to decomposition of organic substances (Fig. 4.49). The raised absorption coefficient of interface confines the absorption of the laser energy into a thinner layer, which increases the magnitude of the temperature jump [587, 593, 601]. Similar phenomena have been observed also at laser etching of polymers and fused silica in air [613]. The incubation effect is greatly reduced in case of halogenated organic solvents [586, 606] and does not occur in case of liquid metals.

Surface roughness At LIBWE in organic solutions, three distinct laser fluence regions with different surface relief can be distinguished (Fig. 4.50). In case of quartz, the mechanisms responsible for etch rate and surface profile formation are explained as follows [587]: Region 1: low fluences, low etch rates, high surface roughness. Here the laser softens the material but does not melt it. The mechanical impact of collapsing bubbles and mechanical stresses in case of a carbon deposit are responsible for high surface roughness. Region 2: intermediate fluences, low surface roughness. The melting temperature of quartz (2000 K) is reached. The higher laser-induced temperature will also generate a stronger pressure jump, compared to the lower fluences, which removes the molten material with a single-laser pulse. Region 3: high etch rates, high surface roughness. A further increase of the etch rates and surface roughness in the high fluence range may be due to plasma formation in solution. Kopitkovas et al. [587] observed that at larger pyrene concentrations the surface roughness and incubation time decreased. A possible high pyrene concentration (1.4 mol/l) was found to be beneficial for laser backside etching of quartz. Böhme et al. [614] suppose that more efficient heating of surface peaks in contrast to the valleys results in higher etch rates of first, and leads to a smooth surface this way. The smoothing effect is determined by the thermal diffusion length. On the other side, the described mechanism rounds the corners of small structures, which may be undesirable in some applications. For surface roughness of samples, backside etched in toluene vapours, see Fig. 4.61.

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Subtractive processing

Average each rate (nm/pulse)

30

⌽ ⫽ 650 mJ/cm2 As ⫽ 650 ⫻ 650 ␮m2 As ⫽ 200 ⫻ 200 ␮m2 As ⫽ 100 ⫻ 100 ␮m2 As ⫽ 30 ⫻ 30 ␮m2

25 20

⌽ ⫽ 1000 mJ/cm2 As ⫽ 100 ⫻ 100 ␮m2

15 10 5 0 1

10

(a) 30 Average each rate (nm/pulse)

100

1000

Pulse number

⌽ ⫽ 650 mJ/cm2

⌽ ⫽ 1000 mJ/cm2

As ⫽ 650 ⫻ 650 ␮m2 As ⫽ 200 ⫻ 200 ␮m2 As ⫽ 100 ⫻ 100 ␮m2 As ⫽ 30 ⫻ 30 ␮m2

25 20

As ⫽ 100 ⫻ 100 ␮m2

15 10 5 0 1

(b)

10

100

1000

Pulse number

Figure 4.48 (a) Averaged etch rate in dependence on the applied pulse number for different spot sizes determined from the final depth of the etch pits using 0.5 M pyrene/toluene. (b) Calculated ‘real’ etch rate per laser pulse. (The lines are used to guide the eyes.) Workpiece: fused silica; solution: 0.5 M pyrene in toluene; laser: KrF, 30 ns [610]. © Elsevier. Toulene: C7H8 C•(s)⫹CyHz(g) Acetone: C3H6O

C•(s)⫹CyHz (g)⫹?CO2 (g)

C2Cl4: C2Cl4

C•(s)⫹CyHz (g,l)⫹?Cl-

1␮m (a)

(b)

Figure 4.49 (a) SEM picture of a thin film around the etched area in fused silica as a result of decomposition process and (b) possible decomposition reactions of the solvents used for LIBWE processing [593]. © Elsevier.

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45

C = 0.4 M (Pyrene/acetone)

35 A

30 25 20 15 10

1

3

Etch roughness (nm)

Etch rate (nm/pulse)

40

2

5 0 0.8

1.2

1.6 2.0 2.4 Fluence (J/cm2)

2.8

Figure 4.50 Etch rate and surface roughness of quartz, laser backside etched using 0.4 mol/l pyrene in acetone solutions as etching media. © SPIE (2004), reproduced with permission from Ref. [587].

Application examples (Figs 4.51–4.54)

Figure 4.51 Array of micro-sized blind holes etched in fused silica by LIBWE [611]. Such arrays are useful for microtiter plates. © Elsevier.

Figure 4.52 SEM picture of a cylindrical structure etched into fused silica employing the scanning contour mask technique [611]. The bottom of the structure is very smooth as shown in the inset. © Elsevier.

189

Subtractive processing

0

19.3 ␮m 0 100 ␮m 200 ␮m 300 ␮m 400 ␮m (a)

–2 –4

␮m

500 ␮m 400 ␮m 300 ␮m

Depth (␮m)

18 16 14 12 10 8 6 4 2 0

–6 –8 –10 –12 –14 –16 –18

200 ␮m 100 ␮m

(b)

50 100 150 200 250 300 350 400 450 500 Position (␮m)

Figure 4.53 (a) 3D-profilometer scan of a Fresnel lens etched in CaF2 by LIBWE using a XeCl excimer laser and (b) line scan of etched profile [612]. © Elsevier.

10 ␮m

750 ␮m

750 ␮m

Figure 4.54 Confocal scanning laser microscopic image of an etched grating pattern on the surface of a fused silica plate fabricated by 400 pulses of KrF irradiation at 1.0 J/cm2 and 4 Hz using a solution of pyrene in acetone with a concentration of 0.5 mol/dm3 [615]. © Elsevier.

Deep trenches and channels Effective provision of fresh solution and removal of debris by the motion of bubbles enables etching of deep trenches and channels in transparent materials, useful for example for microfuidic devices (Figs 4.55 and 4.56). The process may be further enhanced by ultrasound agitation (Figs 4.57 and 4.58).

4.4.2.2 Backside etching with an adsorbed liquid layer on the workpiece (LESAL) Using instead of bulk liquid an adsorbed liquid layer on the workpiece (Fig. 4.59), the relaxation time of the system after laser pulse is greatly reduced. Thus, in the experiments by Böhme et al. [616] the etch rate did not depend on the laser pulse repetition rate up to 100 Hz. Another distinct feature is the occurrence of a region where the etch rate does not depend on the laser fluence (Fig. 4.60). It is believed, that in this region, higher laser fluences more intensively desorb the liquid from the surface, so that the amount of absorbed laser energy per unit area remains constant [617]. Otherwise, the etching mechanism and surface properties remain to a great extent similar as at LIBWE (Fig. 4.61).

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Laser-absorbing region

Organic solution

Laser beam

Silica glass Etch front by LIBWE

Figure 4.55 Principle of laser backside deep wet etching of transparent materials [575]. © Institute of Pure and Applied Physics, republished with permission.

180 ␮m

40 ␮m

XY microstage

Figure 4.56 Cross-sectional SEM images of deep trenches on silica glass fabricated by the LIBWE method using 12 000 pulses at 10 Hz for a trench about 9 µm wide. A saturated pyrene/acetone solution was in contact with the silica glass plate, and a KrF excimer laser beam was irradiated at F = 1 J/cm2 per pulse. Before SEM observation, the silica plates were cut perpendicular to the deep trenches [575]. © Institute of Pure and Applied Physics, republished with permission. Glass cuvette

50X Objective NA ⫽ 0.42

fs laser pulses

Z microstage Water Ultrasonic transducer Ultrasonic cleaner

Figure 4.57 Schematics of ultrasound assisted LIBWE [584]. The negative pressure driven by ultrasonic waves generates many tiny cavitation bubbles. As these cavitation bubbles collapse, they release high-frequency energy, which detaches trapped bubbles near the entrance of the machined hole. Furthermore, the frequent growth and collapse of cavitation bubbles cause pressure fluctuations that scrape residual bubble mixtures from the channel cavity. Reproduced with kind permission of Springer Science and Business Media.

191

Subtractive processing

100 μm

Circular bentholes: t1⫽100 ms, t2⫽1 ms, 4 μm/s, 33 μJ pulse energy.

Figure 4.58 Examples of bent channels fabricated in a glass sample in contact with methanol by ultrasound-assisted LIBWE [584]. Magnified views of vertical holes are on the right-hand side of the picture. Etching was performed by 800 nm, 100 fs, 1 kHz laser pulse packets of period t1 and of length t2 . Feed rate was 4 µm/s. Reproduced with kind permission of Springer Science and Business Media. Laser beam

Transparent material

Laser-induced surface modification

Absorbed layer

Vaporized toluene

⫹

Air

Heater Theater ⬎Tvapour ⬎Tsample ⬎Tcondensation

Figure 4.59 Principal experimental set up for LESAL processing [616]. One of the main requirements for LESAL process is that the sample temperature (Tsample ) is higher than temperature for condensation of vapour medium (Tcondensation ). © Elsevier. Etch rate at 75˚C chamber temperature

Each rate (nm/pulse)

100 10

Low fluence region

Saturation region High fluence region

1 0.1 0.01 1E.3 1.0

2.0

3.0

4.0

Laser fluence

5.0

6.0

7.0

8.0

(J/cm2)

Figure 4.60 Etch rates on fused silica in dependence on the laser fluence at a chamber temperature of 75◦ C using an adsorbed toluene layer (the line is used for guide the eyes [592] The splitting into three fluence regions is pointed out. Laser: 248 nm, 30 ns © Elsevier. Similar dependencies were observed also at etching of sapphire and MgF2 [616].

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Chamber temperature 75˚C

1000

100

100

10

10

1

1 0.1

0.1 RMS, interference microscope RMS,AFM Etch rate

0.01 1E.3 0.0

Each rate (nm/pulse)

Roughness, rms (nm)

1000

0.01 1E.3

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

Laser fluence (J/cm2)

Figure 4.61 Surface roughness and etch rate of a LESAL-etched fused silica using an adsorbed toluene layer [592]. Laser: 248 nm, 30 ns. The surface roughness is lowest on the constant etch rate plateau. © Elsevier. 700 30 (a) Each rate (nm/pulse)

600 500

0.5 M pyrene/toluene

20

(b)

10

400 300

0 0.5

1.0

1.5

Gallium

200 100 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 Laser fluence (J/cm2)

Figure 4.62 Etch rate in dependence on the laser fluence at backside etching of fused silica using (a) 0.5 M pyrene/toluene solution and (b) gallium as absorbing metallic liquid [620]. Laser: KrF, 20 ns. © Elsevier.

4.4.2.3 Liquid metals at backside In comparison with etching in organic and aqueous solutions, etching with metals (Hg, Ga) at backside of the workpiece has following distinctive features: no incubation effect, high etching threshold and etching rate and linear dependence of the etching rate on laser fluence (Fig. 4.62). No UV lasers are needed: a 1.06 μm Nd:YAG laser was successfully applied in the experiments by Zimmer et al. [618, 619]. The process occurs essentially the same way as in solutions: laser heats of the metal, heat is transferred by conduction to the workpiece, and molten and/or evaporated material is ejected [620]. High etching threshold was explained by high reflectivity and high thermal conductivity of the metals. For example, the reflectivity of gallium is about 80 per cent and thermal conductivity is ∼300 times higher compared to toluene. Rapid growth of the etching rate at higher fluences may originate from a change in the mechanism of the etching, for instance, due to the beginning of the gallium evaporation. Because the absorption coefficients of mercury and gallium are about 10 times higher (α > 105 cm−1 ) than of hydrocarbon solutions, the possible modification of the absorption of the workpiece has less influence on the process and an incubation phenomenon is not observed [585].

Table 4.9

Laser-induced backside wet etching of transparent materials (LIBWE, LESAL, etc.) and related experiments.

Target or etched material(s)

Liquids

Additives

Laser type and beam parameters

Etch rate

Novel features, observed phenomena, comments 2

Reference(s)

Glass K-8

Water

No

Neodymium, 40 ns

Threshold of surface damage 25–65 J/cm (at spot diameter 1.15–0.32 mm); shock pressure measured and calculated (up to 100 MPa at distance ≤1 mm)

Leonov (1975) [571]

ZnSe

Water

No

CO2 , 10.6 µm, 100 ns, 50 J/cm2

Circular symmetric cracks in ZnSe observed, shock waves in water photographed

Davidson (1980) [572]

Fused silica

Water

NiSO4 , 2 mol/l

Nd:YAG, 1.06 µm, 1 ms

40 µm/pulse (3 J/pulse)

0.2 mm diameter holes drilled into 1.5 mm plate; bent microchannels fabricated

Ikeno (1989) [621]

Sapphire (α-Al2 O3 )

Water

CrO3 (1.5 g/ml); KMnO4 (saturated)

Cu-vapour, 510 nm, 10 ns, 8 kHz, ≈0.5W average

Up to 300 nm/pulse = 2 mm/s

Dolgaev (1996) [593, 594]

Toluene

4–5 nm carbon particles

Drilling, beam scanning, and mask projection machining with spatial resolution of 3 µm; isolated tilted channels formed while scanning; decomposition of toluene and release of gas bubbles of 10–100 µm size observed; C2 O3 correspondingly carbon film on etched surface

Sapphire

Water

CrO3 (6 mol/l); FeCl3 (2 mol/l), KMnO4 (1 mol/l)

Cu-vapour, 10 ns, 8 kHz

Up to 300 nm/pulse

Epitaxial growth of Cr2 O3 , FeO3 and MnO2 on surface observed

Dolgaev (1997) [595]

Glass, fused silica, Al2 O3 , CaF2

Benzene, toluene

Glassy carbon particles (4–5 nm)

Cu-vapour, 510.6 nm, 20 ns, 8 kHz, 0.2–1.5 J/cm2

0.2 Å/pulse (glass), 0.4 Å/pulse (fused silica), 4.5 Å/pulse (CaF2 ), 1.1 Å/pulse (Al2 O3 ) [all at 1.1 J/cm2 ]

Grooves of depth up to 140 nm were etched in glass by 4500 laser pulses (in benzene, 0.2 J/cm2 ); calculated peak temperature during laser pulse was 600 K (sapphire substrate with DLC film, 1.1 J/cm2 )

Simakin (1999) [622]

Glass, fused silica, Toluene, sapphire benzene, cumene

Carbon nanoparticles (3–4 nm)

Cu-vapour, 510.6 nm, 20 ns, 8 kHz, spot 50 µm, up to 1.5 J/cm2

Grooves of depth ∼0.5 µm were etched into glass by a scanned beam (0.8 J/cm2 , 1.2 mm/s)

Lyalin (1999) [623]

510 nm,

(Continued )

Table 4.9

(Continued)

Target or etched material(s)

Liquids

Additives

Laser type and beam parameters

Fused silica

Acetone

Pyrene, 0,4 mol/dm3

PEP

Etch rate

Novel features, observed phenomena, comments

Reference(s)

KrF, 248 nm, 30 ns, 2 Hz

5–25 nm/pulse (0.4–1.3 J/cm2 ; etch threshold 0.24 J/cm2 )

10 µm lines etched by mask projection

Wang (1999) [624, 579], (2000) [574, 625], (2001) [599],Yabe (2001) [600]

Pyrene, Tetrahydrofuran 1 mol/dm3 (THF)

XeCl, 308 nm, 30 ns, up to 0.5 J/cm2

5–20 nm/pulse (0.16– 0.5 J/cm2 , etch threshold 0.1 J/cm2 )

Grooves with wavy surface etched by mask projection; Wang (1999) absorption length of solution at laser wavelength was [626] 0.4 µm

Quartz (c-SiO2 )

Acetone

Pyrene, 0.4 mol/dm3

KrF, 248 nm, 30 ns, 2 Hz

5–25 nm/pulse (0.4–1.3 J/cm2 ; etch threshold 0.24 J/cm2 )

10 µm lines etched by mask projection; etch depth 3.5 µm, no debris or cracks

Wang (1999) [627], (2000) [625],Wang (2001) [599]

CaF2 (single crystal)

Acetone

Pyrene, 0.4 mol/dm3

KrF, 248 nm, 30 ns, 2 Hz

1–15 (0.7–1.3 J/cm2 ; etch threshold 0.74 J/cm2

10 µm lines etched by mask projection

Wang (1999) [579], (2000) [574, 625], (2001) [599], Yabe (2001) [600]

Fused silica

Pyrene, 0.1– Tetrahydrofuran 1 mol/dm3 (THF) Benzil, Acetone 0.8 mol/dm3 MethylNo additives benzoate

KrF, 248 nm, 30 ns

5–34 nm/pulse (0.4– 1.3 J/cm2 )

Estimated light absorption depth 0.48–2.9 µm; estimated maximum liquid temperatures from 1900 K (acetone/pyrene) up to 4410 K (acetone/benzil); at various solute concentrations 0.1–1 mol/dm3

Wang 2000 [603]

Al2 O3 , LiF (single crystal)

Acetone

KrF, 248 nm, 30 ns

No etching up to the fluence 1.5 J/cm2

Pyrene, 0.4 mol/dm3

Wang (2000) [603]

PEP

Tetrahydrofuran

Pyrene, 1.0 mol/dm3

XeCl, 308 nm, 20 ns, 2 Hz

Up to 36 nm/pulse (0.1–0.6 J/cm2 ; etch threshold 0.045 J/cm2 )

10 µm lines etched by mask projection

Wang (2000) [625], (2001) [599],Yabe (2001) [600]

Soda-lime glass, Pyrex, sapphire

Toluene, benzene, cumene, also with addition of glassy carbon particles, 3–5 nm

Cu-vapour, 510.6 nm, 20 ns, 8 kHz, up to >1.5 J/cm2

Etch threshold 0.5 J/cm2

Smooth etched surfaces; etch rate of sapphire was greater than of glass and fused silica

Simakin (2000) [628, 629]

Quartz

Water

K2 CrO4

KrF, 248 nm, 16 ns, up to 1 J/cm2

Transient reflectivity studies of water–quartz interface; reflectivity decreases rapidly when pulse energy density exceeds 1000 J/cm3 , probably due to supercritical state of the solution

Nikiforov (2000) [630]

Fused silica

Water

HF (generated in a photochemical reaction between water and FEP)

ArF, 193 nm, 10 ns, 100 Hz, up to 50 mJ/cm2

Surface roughness 1 nm was achieved at laser fluence 25 mJ/cm2 and processing time 60 min; the chemical reaction where HF is generated, is similar to this in Fig. 6.1

Murahara (2001) [596]

Silica glass

Water

No

Ti:sapphire, 800 nm, 120 fs, 1 kHz, 1–10 µJ/pulse

Bent channels of diameters 4 µm and length up to 200 µm (at 60 J/cm2 ) respectively, diameter 21 µm and length up to 600 µm (at 600 J/cm2 ) etched

Li (2001) [631]

Sample was grind using FEP turntable with a water layer between; laser light irradiated the turntable through sample

(Continued )

Table 4.9

(Continued)

Target or etched material(s)

Liquids

Additives

Laser type and beam parameters

Sapphire

Water

KOH, KCl or Na2 CO3 (up to 7 mol/l)

Fused silica

Acetone

Fused silica

Etch rate

Novel features, observed phenomena, comments

Reference(s)

Cr,Yb,Ho:YSGG, 2.92 µm, 130 ns, 1 Hz, spot 100 µm, 120 J/cm2

Up to 2.4 µm/pulse (KOH, 7 mol/l); 0.24 µm/pulse in pure water

Ablation rate had linear dependence on electrolyte concentration; dissolution of sapphire in supercritical solution obviously contributes to high ablation rate

Dolgaev (2001) [479]

Pyrene, 0.4 mol/dm3

XeCl, 308 nm, 20 ns, 5 Hz, 0.2–1.5 J/cm2

Up to 22 nm/pulse (0.4– 1.5 J/cm2 ); threshold fluence 20 nm), the peak width increases again, because of more inhomogeneous polarization of the larger nanoparticles in the electromagnetic field of the incoming light and due to excitation of a higher number of different multipole modes, the so-called extrinsic size effect [682].

Estimation of the average diameter of nanoparticles from absorbance From Drude theory follows a linear dependence between absorbance and average diameter of nanoparticles [683]: katom = f0 dav ,

(5.7)

where f0 is a constant. The constants w0 and f0 may be determined from direct measurements of particle size with electron or scanning probe microscopes.

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Generation and modification of particles

5.3 Experimental Techniques of Particles Generation Main experimental setups used for laser ablation fabrication of colloids are presented in Figs 5.4–5.7. Glass beaker

Laser beam 1064 nm 3.5 ns 10 Hz

Isopropyl alcohol

Graphite target

Focusing lens

Figure 5.4 Setup with vertical target [684]. For avoiding of crater formation and absorbance increase due to generated suspension, the target my be rotated [685]. © Elsevier. (a) Preparation of colloids 1064 nm

(38 J/cm2)

(b) Modification of colloids 355 nm (4–12 mJ/pulse)

Silver nanoparticles

Water

Silver plate

Figure 5.5 Setup for ablation of an horizontal target in liquid (left) and for irradiation of suspended particles (right). © The Laser Society of Japan, reproduced with permission from Ref. [686].

Laser beam Lens – Quartz cell Distilled water

Target holder

Target Stirring bar

Magnetic stirrer

Figure 5.6

Setup with horizontal target and stirred liquid [687]. © Elsevier.

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Handbook of Liquids–Assisted Laser Processing

Laser beam Lens Splash of suspension

Laser beam

l/2 plate Lens 53° Splash of suspension

Suspension (a)

Suspension (b)

Figure 5.7 Setup with inclined laser beam (b), compared to conventional setup (a). The conventional system suffers from splashes at laser energies over 30 mJ/pulse. System (b) with laser beam at an angle to the surface avoids the splashes reaching the lens. At Brewster angle, also the reflection losses may be avoided. Pulse energies up to 150 mJ were applicable with this optical arrangement [688]. © Elsevier.

5.4 Metal Particles 5.4.1 Introduction Noble metals colloids (Fig. 5.8) are useful in photography, optoelectronics, catalysis, biosensing, labelling of proteins, etc. Due to plasma resonance in visible region (Fig. 5.2), the Raman scattering and other optical nonlinearities of the nanoparticles are greater here by orders of magnitude compared with those of flat surfaces. In comparison with conventional chemical methods of fabrication of noble metal colloids, in laser process the particles are clean, because no other substances but a metal target and a liquid are needed. A recent application-oriented review about photophysical and photochemical properties of metal nanoparticles was published by Kamat [689] and a review of using surface plasma resonance techniques in biomedical sciences by Englebienne et al. [690]. Magnetic colloids are useful in catalytic chemistry, magnetic recording, magnetorheological fluids, etc. In comparison with the common fabrication methods of magnetic particles, such as decomposition of organometallic precursors and mechanical milling, laser ablation is simpler and helps to avoid contamination of particles. However, when ablation is performed in oxygen-containing liquids, the surface of the particles becomes oxidized.

5.4.2 Mechanisms determining the particles size In many applications, particles of same size are of advantage, for example in SERS-based sensors [692]. In the following, the major phenomena controlling the size of small particles during laser irradiation and subsequent growth are characterized.

Dependence of the melting temperature on particles size Because the vapour pressure depends on the surface curvature (Eq. (7.57)), the melting temperature of solid particles decreases with the decrease of their size (Fig. 5.9).

215

Absorbance

Generation and modification of particles

200 300 400 500 600 700 800 900 Wavelength (nm) (a)

(b)

Figure 5.8 (a) Electron micrograph and (b) optical absorption spectrum of platinum nanoparticles with an average diameter of 6 nm produced by laser ablation at 1064 nm of a platinum metal plate in pure water [691]. © Elsevier.

Tr /T0

1.0

0.95

0.9

0

20

40 R, nm

Figure 5.9 Variation of melting temperature Tr with radius for gold particles in vacuum,T0 – melting temperature for macroscopic bodies (after Sambles [693], © Royal Society of London, reproduced with permission).

According to Sambles [693] (with reference to Reiss and Wilson [694] and Curzon [695]), the melting temperature of small particles follows the relation: γsl γl H m ρs (T0 − Tr ) = + 2M T0 r −p r

  ρs , 1− ρl

(5.8)

with notations: Hm – latent heat of fusion, T0 – bulk melting point, Tr – melting point at radius r, ρs – density of solid, ρl – density of liquid, γl – surface energy of liquid, γsl – mean solid–liquid interfacial energy, M – molecular mass, p – relevant skin thickness. Sambles [693] gives to the parameter p an estimate p = 2.2 ± 0.5 nm.

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Handbook of Liquids–Assisted Laser Processing

Ostwald ripening Because the solubility of smaller particles is larger than this of larger ones, mass transfer from smaller particles to larger occurs. Ostwald ripening tends to minimize the total surface area of the particle system.

Heat transfer efficiency Small particles cool faster than larger ones. Inasawa et al. [696] have shown that for a given laser fluence, there exists a critical size over which the temperature of the particle does not reach the melting temperature during the laser pulse length. This phenomenon explains the particle size reduction and narrowing of their size distribution at laser irradiation of suspensions. The time to reach the boiling temperature of the particles, t b is ′ − t b = tm

1 A − B (Tb − Tw ) ln , B A − B (Tm − Tw )

(5.9)

where   N  rn′2 ′ 3FM , 1− 1 − 2 ηε A= 4τρCp r r n=1 B=

3M λ , ρCp r 2

(5.10)

(5.11)

′ is the time for the particle to melt, tm

4πr 3 ρ Hm ′ 3M  tm = tm + ,   ′2 N ! rn ′ F 2 − 4πrλ (Tm − Tw ) 1− 1 − 2 ηε πr τ r n=1

tm is the time for the particle to reach the melting temperature, given by the equation      N  3M λ rn′2 ′ Fr Tm = Tw + t , × 1 − exp − 1− 1 − 2 ηε m 4τλ r ρCp r 2 n=1

(5.12)

(5.13)

with notations: Tw – ambient temperature, Tm – melting point of the particle, Tb – boiling point of the particle, F – laser fluence per pulse, τ – laser pulse width, M – atomic mass of the particle, ρ – density of the particle, Cp – specific heat of the particle, λ – thermal conductivity of the surroundings, r – radius of the particle, rn′ – radius of the nth (111) plane: r ′2 = r 2 − (r − nd)2 , d – distance between (111) planes, N – number of (111) planes included in a particle, N = 2r/d, η – fraction of the area of the plane occupied by metal atoms (η = 0.91 for a gold (111) plane), ε′ – absorption coefficient of a metal atom, ε′ = ε/επr02 NA , ε – mole absorption cross-section of the metal, NA – Avogadro’s number, r0 – bond radius of metal atoms. (111) Planes mean that the particle is modelled as being composed of n layers of equal thickness perpendicular to the laser beam axis. Particle’s size reduction occurs if the particles temperature reaches the boiling temperature, tb ≤ τ, during the laser pulse.

Surfactants Surfactants were found to control efficiently the size of laser ablation formed nanoparticles through a so-called dynamic formation mechanism [697–699]: (1) Immediately after the laser ablation, a dense cloud of metal atoms is built over the laser spot of the metal plate. As the interatomic interaction is much stronger than the interaction between a metal atom and a

217

Generation and modification of particles

surfactant molecule or a solvent molecule, metal atoms are aggregated as much as metal atoms collide mutually. (2) This initial rapid aggregation continues until metal atoms in the close vicinity are consumed almost completely. As a result, an embryonic metal particle forms in a region void of metal atoms (cavity). However, the supply of metal atoms outside the region through diffusion causes the particle to grow slowly even after the rapid growth ceases. (3) This slow growth terminates when the surfaces of the particles are fully covered with surfactant molecules or the free metal atoms are consumed completely in the solution. Full covering of particles by surfactant molecules occurs when the surfactant concentration exceeds the critical micelle concentration. Using this criterion, Mafuné et al. [698] developed a formula for maximum particle radius rs growing in a surfactant solution: rs (t) =



Ns S = 4π



S · 3



k′ ds vs · kVa da va



1 r0 + kVa da va t 4

3

− 3r0 ,

(5.14)

where Ns is the number of surfactant molecules absorbed on particle, S is surface area occupied by one surfactant molecule on the particle, k is attachment coefficient of metal atoms by the particle (attachment cross-section = kπr 2 ), k′ is attachment coefficient of surfactant atoms, ds is density of a surfactant molecules in the solution, vs is velocity of surfactant molecules in the solution, da is the number density of metal atoms in the cloud of the metal atoms, va is diffusion velocity of metal atoms in the vapour, Va is volume of the metal atom, and r0 is the radius of the embryonic particle. Ionic surfactants, but also cyclodextrines were used to control the growth of laser-generated particles, reducing this way their size and size dispersion. Cyclodextrines were chosen due to their biocompatibility [700].

Effect of chlorides Bae et al. [701] found that presence of chlorides in the aqueous medium during laser ablation contributed to the reduction of the average particle size, prevented formation of large particles, and increased the formation efficiency of small nanoparticles thereby. However, the long-term stability of Ag nanoparticles formed in NaCl solution was reduced by enhanced spontaneous aggregation compared to those in neat water.

5.4.3 Modification of suspending particles by laser irradiation 5.4.3.1 Reduction in size and fragmentation Often there is a need to convert larger particles into smaller ones, enlarging this way their overall surface area (in sensing and catalysis) or increasing the density of the particles on a surface (in information storage). Particles exposed to light may loose their mass due to photodissolution and vapourization. In case of ultrashort intense pulse irradiation, particles may decay into fragments in a Coulomb explosion process. Fig. 5.10 presents some situations in particles fragmentation under the action of laser light. Lasers provide a unique possibility to reduce the size of noble metal particles due to their intense plasma resonance in visible region [702]. Figure 5.11 presents the dependence of final size of irradiated in suspension 45-nm size Au particles depending on the laser fluence. The mechanisms controlling the final size of particles were described in Section 5.4.2. Particle size reduction may occur also due to disintegration of aggregated particles, a process going on also below the melting temperature of the material [703].

Modification of particles by ps/fs-laser pulses Shorter pulses melt the particles at lower pulse energy, because the energy losses due to heat transfer from particles to liquid is smaller. According to Hodak et al. [704], the characteristic time of heat transfer from nanoparticles to liquid is about 100–200 ps.

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Handbook of Liquids–Assisted Laser Processing

AuCl4⫺

Laser beam Laser beam

Laser beam

Heated to the b.p by laser pulses (T ⬎b.p.)

T ⬎b.p.

Size reduction of larger particles

Photoreduction and nucleation

Without particle growth

Particle growth and nucleation

Evaporation of gold atoms from the surface and cooling the particle

Aggregation of gold atoms, formation of small particles.

(a)

• Size reduction of larger particles • Particle growth and nucleation

Wide size distribution

(b)

Narrow size and distribution

(c)

Figure 5.10 Schematic of laser-induced size reduction of gold nanoparticles [696]. (a) Heated by laser pulses, gold atoms evaporate from the particle surface when the particle temperature is above the boiling point. Then the particle becomes smaller and evaporated gold atoms aggregate to form small particles. (b) With laser irradiation to gold nanoparticles, fragmented particles cannot grow because of a lack of source material, AuCl4 , which causes a wide size distribution. (c) With laser irradiation into AuCl4 solution, particle growth and laser-induced size reduction occur at the same time. Fragmented particles can grow to the maximum diameter controlled by the irradiated laser fluence, which results in narrow size distribution. © Institute of Pure and Applied Physics, reproduced with permission.

Maximum diameter (nm)

50

40

30

20 m.p.

10

0

102

b.p.

103

104

Absorbed laser energy, Q(J/(g pulse))

Figure 5.11 Dependence of the maximum diameter of Au particles on the absorbed laser energy [702]. Notations: m.p. – melting point of the material reached; b.p. – boiling point of the material reached. Solution: water + citric acid; laser: 532 nm, 7 ns. © American Chemical Society (1999), reprinted with permission from Ref. [702].

Because of smaller mass of electrons, they gain easier energy from laser light, so that the electron temperature may considerably exceed the temperature of ions (Fig. 5.12). If a significant amount of hot electrons leave the particle, the particle may explode due to repulsive forces between the positive ions, a phenomenon called Coulomb explosion (Fig. 5.13).

219

Generation and modification of particles

⫽ 30 ps

Electrons

4000

380 Temperature (K)

Temperature (K)

6000

2000

Electrons

⫽ 5 ns

360 ions

340 320

ions 300 0

200 400 Delay time (ps)

600

0

5

10

15

20 ⫻ 103

Delay time (ps)

Figure 5.12 Temporal evolution of electron temperature Te (dotted line) and ion temperature Ti (full line) calculated for 20-nm Au particles excited with (a) 30 ps and (b) 5 ns laser pulses with Eabs = 2.05 mJ/pulse. © American Chemical Society (2000), reprinted with permission from Ref. [704].

e

e h␯

e e

Ag nanocluster

e

e e

Electron ejection

Ag⫹ e ⫹ e Ag e ⫹ ⫹ e Ag Ag ⫹ e Ag⫹ Ag ⫹ Ag e e e Ag⫹ Transient state

Ag⫹ e Ag⫹ e

Ag⫹ e Ag⫹ e

Fragmentation

Figure 5.13 Fragmentation of a Ag cluster with laser excitation [705]. A transient aggregate formed via the photoejection of electrons is considered to be a precursor for complete fragmentation of the particle. © American Chemical Society (1998), reprinted with permission from Ref. [705].

5.4.3.2 Melting without fragmentation It is possible to modify the shape or/and structure of the particles by melting. The corresponding changes in optical absorption spectra are expected to be applicable for optical information storage [704].

Melting of nanorods At melting, the rod-shaped particles transform into spheres, minimizing this way their surface energy (Fig. 5.14). The changes start at the middle of the rods, what is explained by poorer cooling and thus higher temperature there (Fig. 5.15). Changes in optical absorption spectra during the transformation are shown in Fig. 5.16. Shape transformation at nanoparticles at an exposure to light may occur also without melting. Jin et al. [708] observed conversion of 8-nm-sized spherical Ag nanoparticles into prisms at exposure to a fluorescent lamp light. In another study the same researchers [709] found that it was possible to control the nanoprisms size in range of 30–120 nm by the ratio of the amplitudes of two wavelengths from an Xe-lamp, the first wavelength corresponding to dipole plasmon resonance and the second to quadrupole plasmon resonance of the nanoprisms (see Table 5.4 for experimental details, Jin 2001, and 2003).

Melting of core-shell particles Laser melting of core-shell particles may also cause significant changes in their absorption spectra (Fig. 5.17) having a potential for use in optical information storage.

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Handbook of Liquids–Assisted Laser Processing

Figure 5.14 TEM image of a gold nanorods solution after exposure to 800-nm nanosecond laser pulses [706]. The laser fluence was 0.64 J/cm2 . Nanoparticles having an odd shape (φ-shape) are highlighted in the TEM image by circles. Particles of this particular shape are absent in the original starting solution, and a high abundance of this particular shape is mainly produced by irradiation with low-power nanosecond laser pulses (the length of rods was 44 nm and width 11 nm before laser irradiation). © American Chemical Society (2000), reprinted with permission from Ref. [706].

) (001)

(1

11

11

)

(1

(110)

(a) (110) (1

) (001)

11

11

)

(1

in

(b)

Tw

(11

)

11

1)

(1

1)

)

(001)

(

(1

11

11

(c)

)

11

(1

11

)

in

Tw

(001)

(d)

(1

Figure 5.15 A schematic process for the structural transformation of a gold nanorod to nanodot under laser irradiation [707]. © 2000 American Chemical Society, reprinted with permission from Ref. [707].

5.4.3.3 Enlargement in size and coagulation Growth of particles may occur even at low-level light exposure due to photodissolution and Ostwald ripening. Jin et al. [708] observed a size reduction of 8 nm Ag particles at an exposure to a fluorescent lamp light with subsequent growth into prisms. Mafuné et al. [682] report that growth of gold clusters into nanoparticles continued within 2 h after the pulsed laser for the size reduction was switched off.

221

Generation and modification of particles

0.30

τ ⫽ 7 ns

Absorbance

0.25 0.20 0.15 0.10 0.05 0.00 500

600

700

800

900

1000

900

1000

Wavelength l/nm

(a)

(b)

0.30

τ ⫽ 100 fs

Absorbance

0.25 0.20 0.15 0.10 0.05 0.00 500 (c)

600

700

800

Wavelength l/nm (d)

Figure 5.16 Comparison of the optical absorption data and TEM images for two gold nanorod samples irradiated by laser pulses having the same fluence (0.25 J/cm2 ) but different laser pulse width (7 ns (top: a, b) vs. 100 fs (bottom: c, d)) [706]. Only an optical hole burning at the laser wavelength (800 nm) and a partial melting of the gold nanorods are found when nanosecond pulses are used. Especially a high abundance of φ-shaped particles as shown in Fig. 5.14 is clearly visible. However, a complete melting of the gold nanorods into nanodots and a complete depletion of the nanorods are achieved with femtosecond laser pulses of the same energy (fluence). This result leads to the conclusion that nanosecond laser pulses are less effective in melting the gold nanorods. (The length of rods was 44 nm and width 11 nm before laser irradiation). © American Chemical Society (2000), reprinted with permission from Ref. [706].

Light may stimulate aggregation of particles by increasing van der Waals forces between them (Section 2.3.1). The effect is most pronounced at Mie resonances where interparticle energy may be enhanced by many orders of magnitude. Laser-heated particles may melt together, as shown in Fig. 5.18. Having a mixture of particles of different materials, alloy particles may be achieved. Izgalijev et al. [710] report about AgAu alloy particles formation at irradiation of a mixture Ag and Au colloids by laser light. Chandrasekharan et al. [712] observed laser-stimulated melting together of gold particles, previously aggregated through adsorbed Rhodamine 6G molecules. In some cases, the particles aggregation into nanowires and nanonetworks was observed [713–715] (Fig. 5.19).The mechanisms determining the morphology of the aggregates is not clear. Formation of networks may be controlled by surfactants in the solution (Fig. 5.20).

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Handbook of Liquids–Assisted Laser Processing

1.0

Absorbance (1 cm)

a b c d 0.5

0.0 300

400 500 600 Wavelength (nm)

700

Figure 5.17 Absorption spectra of AucoreAgshell particles (molar ratio Au:Ag = 1:0.5) following photoexcitation with 532 nm, 30 ps laser pulses: (a) non-irradiated; (b)–(e) Eabs = 0.13, 1.16, 4.6, and 6.7 mJ/pulse [704]. Further details of the experiment are given inTable 5.4, Hodak 2000. Reprinted with permission from J. H. Hodak,A. Henglein, M. Giersig, G. V.Hartland, Laser-induced inter-diffusion in AuAg core-shell nanoparticles. J. Phys. Chem. B.; (Article); 2000; 104(49): 11708-11718. © American Chemical Society (2000), Ref. [704].

TiO2

nhν

TiO2

Au Au

Figure 5.18 Schematic diagram illustrating the fusion of TiO2 /Au nanoparticles at laser irradiation [711]. © American Chemical Society (2001), reprinted with permission from Ref. [711].

Atomic transmutations observed at laser irradiation of suspended particles It is well known that even in low-temperature deuterium plasma, free neutrons are generated – laboratory neutron sources use only some kilovolt of excitation.Thus, at laser processing of materials in heavy or semiheavy water, the generation of neutrons in laser plasma or at bubble collapse is expected. The released neutrons can cause nuclear reactions in the surrounding materials. Also extraordinary high-electric fields near small metal particles at Mie resonance (Section 5.2) may contribute to nuclear reactions. Shafeev et al. [716] report about transmutation of mercury into gold in course of irradiation of mercury suspension in heavy water by picosecond Nd:YAG and Ti:Sapphire laser pulses of energy density of ∼1010 W/cm2 . The supposed nuclear reaction was: 196

Hg + n →

197

Hg + γ,

(5.15)

223

Generation and modification of particles

(a)

(b)

Figure 5.19 TEM images of laser-generated Au networks in water [715]. (a) Preparation within an ice-bath and (b) preparation under the room temperature. The scale bar length corresponds to 50 nm. © Elsevier.

Absorbance @ 250 nm

0.9 0.8 0.7

Nanonetworks

0.6 0.5

Small nanoparticles

0.4 0.3 0.2

1

100

10

Concentration (mM)

Figure 5.20 Typical optical absorption spectra of Pt particles irradiation products after laser excitation of the interband at 355 nm in different SDS concentrations [691]. The arrow indicates the critical micelle concentration of SDS. SDS – sodium dodecyl sulphate (C12 H25 OSO3 Na). © Elsevier.

where γ stands for γ-photon. 197 Hg decays within 2.7 days into 197Au through electron capture from its own K shell: 197

Hg (Z = 80) + e − →

197

Au (Z = 79).

(5.16)

After 4 h of irradiation of an Hg suspension by 350 ps, 1.06 μm laser pulses, up to 13 per cent of mercury was converted into gold.

Table 5.4 Metal colloids prepared or modified by laser irradiation, and related research (solids targets in liquids); av – average size; st – standard deviation; λmax wavelength (in vacuo) of main absorption peak. Lasers or other light sources

Particles size, achieved or after treatment

2-propanol, chloroform, acetone, ethanol, n-hexyl alcohol

Hg-lamp, 100W, up to 30 h

Ba, Ca

Superfluid liquid He (1.6 K)

Au particles, 10 nm, in suspension

Targets

Liquids

Novel features, observed phenomena, comments

References

Au particles, 11 nm, in suspension

Aggregated particles

Stable in dark colloids aggregated in course if irradiation in 16 h; at the beginning of the growth Ostwald ripening was observed; coagulation started with formation of particle chains, later particles networks developed

Hasegawa (1991) [717]

Nd:YAG, 1064 and 532 nm, 1 and 0.2 mJ, respectively. 2ω-Nd:YLF,0.2 mJ

Clusters and atoms of Ba and Ca

Laser ablation of metal targets resulted in metal clusters which at further irradiation decomposed into atoms; excitation and emission spectra of the triplet transitions of Ba and Ca in He are presented; growth of tangle thin wires of diameter ∼1 µm was also observed at laser ablation of targets in superfluid He

Fujisaki (1993) [718]

Aqueous solution used for particles preparation by chemical reduction

Ar+ , 0.5W, beam area 1 mm2 , up to 200 min

Aggregated particles

Laser irradiation caused the colloids to aggregate, obviously due to light-enhanced van der Waals forces

Eckstein (1993) [719]

Au film

2-propanol, water, cyclohexane

Ruby, 694 nm, 2.3–27 J/cm2

2–19 nm (Au) 3–4 nm (Ni)

Dependence of suspension absorption spectra on laser fluence presented; λmax = 520 nm (Au)

Fojtik (1993) [720]

Ni film

water + sodium polyacrylate 0.2 mM Calculation of optical absorption spectra for metal colloids, review of physical and chemical properties of small metal particles in solutions

Henglein (1993) [721]

Stable (at least over some months) metal colloids fabricated by ablation in water; SERS spectra of various absorbed on colloids molecules were of high quality; higher pulse energy yield obviously smaller particles; λmax : 399 nm (Ag, water), 414 nm (Ag, methanol), 521 nm (Au, water), 625 nm (Cu, water)

Neddersen (1993) [692]

Ag,Au, Pt, Cu

Water, acetone, methanol

Nd:YAG, 1064 nm, 10 Hz, 55 mJ

10–50 nm, 20 nm av (Ag)

Au colloids, 8 nm

Acetone, ethanol, 2-propanol, chloroform

High-pressure Hg-lamp, 100W with water filter

Ca, Cu,Ag

He II (1.7 K)

Nd:YAG, 532,355, and 266 nm, pulsed, 10 Hz ∼20 mJ

Ag, Mg,Yb,Al, Ga, In

He II (1.7 K)

Ag

Irradiation caused full coagulation of colloid in 20 h (in dark stable for several years); first particle chains, then fractal conglomerates formed; optical absorption spectra changes were explained by Ostwald ripening; the possible mechanisms of coagulation were photon neutralization of Ag particles and/or by surface plasmon oscillation-enhanced van der Waals forces; photocoagulation was observed also in case of Zn colloids

Satoh (1994) [722]

Clusters and µm-sized particles

Larger particles were further dissociated by continuing laser irradiation, the dissociation was more effective at shorter wavelengths; absorption and emission spectra of Ca2 , Cu2 ,Ag2 in UV–VIS region are presented

Persson (1995) [723]

Nd:YAG, 532 and 355 nm, pulsed, 10 and 20 Hz, 10–20 mJ

Neutral atoms, clusters, and particles

Emission and absorption spectra and dynamics of neutral atoms, also residing at microscopic He bubbles were investigated

Hui (1995) [724]

Water

Nd:YAG, 1064 nm, 10 Hz, 55 mJ ≈15 min

20 nm mean

Surface of colloids were modified by I− and Br− ; effect of this modification on plasma resonance frequency was small

Sibbald (1996) [725]

Ag particles, 19 nm mode

Aqueous solution of AgNO3 , NaBH4 , and SDS

3ω-Nd:YAG, 355 nm, 10 ns, 60 mJ/cm2 2ω-Nd:YAG, 532 nm (less effective)

9 nm mode (15 min irradiation with 355 nm)

Irradiation reduces the particle mean size to less than ≈10 nm and changed ζ-potential from −35 to −50 mV; achieved particles were stable at least one week (no aggregation or precipitation); λmax : 400 nm (before irradiation), 450 nm (after 15 min irradiation with 355 nm light)

Takami (1996) [703]

Ag

He II (1.6 K)

Nd:YAG, 532 nm, pulsed, 10 and 20 Hz, 10–20 mJ

Ag atoms, clusters and particles; AgHe2 -exciplexes

Produced by Nd:YAG-laser Ag particles were further dissociated by XeCl-laser (308 nm, 10 Hz, 10 mJ); linear He-Ag-He-exciplexes, trapped in microcavities were found; formation of AgHe2 -exciplexes was confirmed by ab initio calculations

Persson (1996) [726]

(Continued)

Table 5.4

(Continued)

Targets

Liquids

Lasers or other light sources

Particles size, achieved or after treatment

+

Ar -ion, 514 and 488 nm, up to 1.8W, up to 20 h

Novel features, observed phenomena, comments

References

Laser irradiation promotes the coagulation of high-concentration colloids; acetone was detected in irradiated solution, obviously in photochemical process where Au particles get electrons from 2-propanol

Takeuchi (1997) [727]

Colloids prepared in water and NaCl solution were stable at least for a year; prepared in phtalazine solution precipitated within 1 day; SERS-activities of colloids and deposited onto surface particles studied

Procházka (1997) [728]

Au particles 300 nm filtered Nd:YAG, 355 and 532 nm, ≈18 ps, 2–3 mJ Nd:YAG, 355 nm, ≈6 ns

5–20 nm (355 nm, 18 ps, 10 Hz, 1.5 mJ, 3 min)

Laser irradiation of particles by ps-pulses causes photoexcitation of electrons and this way plasmon absorption to bleach; photoejection of electrons leads to particles fragmentation (see Fig. 5.13); longer wavelength (532 nm instead of 255 nm) causes preferentially the fragmentation of larger or irregularly shaped particles

Kamat (1998) [705]

TNA-capped Au particles

Water + citric acid + sodium citrate

2ω-Nd:YAG, 532 nm, 18 ps, 1.5 mJ

About 10 nm particles fused during irradiation (532 nm) for 1 min, and fragmented again during 30 min

Fujiwara (1999) [737]

Au particles 5–50 nm

Water + citric acid, agitated

2ω-Nd:YAG, 532 nm, 10 Hz, 7 ns, up to 800 mJ/cm2

Non-spherical particles of size 20–50 nm changed in some minutes into spherical particles of size less than 10 nm, obviously due to melting and vaporization; thermal radiation measurements indicated that the particles temperature exceeded the Au melting temperature.; shift of λmax from 531.5 to 517 nm during irradiation

Takami (1999) [702]

Au rods, e.g. ≈10 nm diam., ≈50 nm length, also silica-covered and micellestabilized

Electrolyte solution used for preparation of nanorods

Nd:YAG, 532 and 1064 nm, 6 ns, up to 10 Hz, up to 67.4 mJ/cm2

At 532-nm laser irradiation (SPtrans excitation) causes mainly a rod-to-sphere conversion; at 1064 nm (SPlong excitation) an incomplete photoannealing process was observed resulting in φ-shaped along their bent and twisted forms nanostructures, probably representing an early stage of the rod-to-sphere shape transition; the restructuring of the Au nanorods starts from the centre portion of the particle

Chang (1999) [738]

(Continued)

Table 5.4

(Continued) Lasers or other light sources

Targets

Liquids

Au rods, 8 nm diameter, 31 nm length; and 11 nm diameter, 44 nm length

Electrolyte solution used for preparation of nanorods

Ti:sapphire,800 nm, 100 fs, 1 kHz, up to 1 mJ, spot 25 µm OPO, 800 nm, 7 ns, 10 Hz, up to 20 mJ, spot 25 µm

Au rods, (e.g. 10.2 nm diameter, 28.6 nm length)

Electrolyte solution used for preparation of nanorods

Ti:sapphire, 400 nm, 100 fs, 500 Hz, 20 µJ, spot 100 µm, 10 min

Au, Ag suspensions, 5–100 nm

Water, also with I+ and CN− additives

2ω-Nd:YAG, 532 nm, 15 ns, 10 Hz

Ag

Water + 0.003–0.1 M SDS (Cn H2n+1 OSO3 Na, n = 8, 10, 12, 16)

2ω-Nd:YAG, 532 nm, 10 ns, 10 mJ/ pulse for 5 ns pulses; in case of 30 ps pulses at 1 and 4 mJ/pulse, correspondingly; dissipation of energy absorbed in 75 nm particles occurs in 100–200 ps; melting of particles was observed to start at much lower temperatures than the expected melting point (is depending on particle size); at used fluences, the complete alloying of particles needs hundreds of laser pulses

Hodak (2000) [704]

Ag nanorods 44/11 nm

Tetraalkylammonium bromide salts solution

T:sapphire, 800 nm, 100 fs, 1 mJ, 0.2 mJ/cm2 – 10.2 J/cm2 OPO, 800 nm, 7 ns, 0.64–16.7 J/cm2

Laser irradiation caused the nanorods to melt into spherical and φ-shaped particles; for fs-pulses, the energy threshold for particle melting was found 100 times lower as for ns-pulses; fs-pulses provide more homogeneous (in sense particles size and shape) colloidal solution; at ns-pulse irradiation much φ-shaped particles formed at low fluences (Figs 5.14 and 5.16)

Link (2000) [706]

(Continued)

Table 5.4

(Continued)

Targets

Liquids

Au nanorods

Aqueous solution used for particles preparation

Au nanorods, 44/11 nm (average length/diameter)

Lasers or other light sources

Particles size, achieved or after treatment

Novel features, observed phenomena, comments

References

T:sapphire, 800 nm, 100 fs, 1 mJ/cm2 OPO, 800 nm, 7 ns, 250 mJ/cm2

Laser irradiation below the melting threshold induces point and line defects, mostly (multiple) twins and stacking faults, which are the precursor that drives the nanorods to convert their {110} facets into the more stable {100} and {111} facets and hence minimize their surface energy, followed by surface reconstruction and diffusion, leading through φ-shaped particles to spheres (Fig. 5.15)

Link (2000) [707]

Aqueous solution used for particles preparation

T:sapphire, 410 and 820 nm, 100 fs, 1 mJ/cm2 , up to 30 µJ

As estimated from the changes in SPR longitudinal band optical absorption (at 800 nm), energy for melting a single Au nanorod was in average 60 fJ at both laser wavelengths

Link (2001) [745]

Ni, Cu, Nb

Ethylene glycol, diethylene glycol, 2-ethoxy-ethanol

CO2 , 10.64 µm, CW Nd:YAG, 1.064 µm, pulsed

The liquids contained AgNO3 and Ni(NO3 )2 – precursors for Ni and Cu; the power of laser beams was 150–1100W, beam diameter 3 and 6 mm, interaction time 1–3 min; spherical pure Ag particles, but porous dual phase Ni and Ni oxide particles were achieved

Poondi (2000) [746]

Ag Au core-shell particles

Aqueous stirred solution of salts used at particle synthesis

532 nm, 5 ns, 10 Hz, up to 12.8 mJ; 532 nm, 30 ps, 10 Hz, up to 6.7 mJ

Laser heating transformed core-shell particles (up to ≈60 nm in size) after some pulses into homogeneous alloyed particles; the thresholds for alloying and fragmentation are many times lower for 30 ps pulses (1 and 4 mJ) than for 5 ns pulses (5–6 and 12 mJ)

Hodak (2000) [704]

Au and Ag colloids

Aqueous solution used at particle synthesis

532 nm, 0.245 J, up to 25 min

Average size of starting particles was Au: 13.7 nm, Ag: 16.8 nm; at beginning of irradiation (at 5 min) temporarily colloid networks formed

Chen (2001) [747]

Au

Water

2ω-Nd:YAG, 532 nm, 10 Hz, 5 min

Cross-linked networked nanowires and twisted nanorods formed, diameter of wires 6 nm, structure fcc-polycrystalline Au; wires prepared at 0◦ C are thinner; laser melting at 1064 nm affected selectively the twisted nanorods with aspect ratio of 6 (burning a hole into distribution histogram)

Chen (2001) [715]

1–5 nm (Ag) and 0.4–1.2 nm (Ni Ni oxide) spherical particles;Ag Ni nanotubes

≈5 nm av dependent on metals ratio

Au

Water + SDS, 0.1–10 mM

Nd:YAG, 532 and 1064 nm, 10 ns, 80 mJ, 5 J/cm2 elongated particles were formed with aspect ratios 4.2–6.5 (at 10–5 carbon atoms in alkane chain)

Compagnini (2003) [760], (2004) [761]

Au

Water, water + cyclodextrines (α,β,γ-CD) 0.1–10 mM

Ti:sapphire, 800 nm, 110 fs, 1 kHz, 0.8 mJ

2.1–2.3 nm av (10 mM β-CD)

Colloids fabricated in pure water continued to grow and started to precipitate in some days; most small and stable (at least 45 days) colloids were achieved at ablation in 10 mM β-CD solution

Kabashin (2003) [762, 763]

Au

Water in a rotating vessel

Ti:sapphire, 800 nm, 110 fs, 1 kHz, 60–1000 J/cm2

4 nm av (60 J/cm2 ), rises to 125 av (1000 J/cm2 )

Particle formation threshold in water 5 times larger than in vacuum; size distribution function may be decomposed into two Gaussian distributions, pointing to two different mechanisms

Kabashin (2003) [496], (2004) [764]

Ag,Au

Water, ethanol (also with PVP additive), acetone, moving cuvette 1 mm/s

Cu-vapour, 510.6 nm, 20 ns, 15 kHz, up to 35 J/cm2

60 nm maximum abundance (Ag in water)

In water: as fabricated Ag particles were disk shaped and Au particles elongated; at further irradiation (35 J/cm2 ) of the colloid Au particles changed to disk-shaped dav = 20 nm; stability time of Au particles ≈30 days; in acetone and ethanol somewhat smaller and spherical particles were achieved; PVP addition to ethanol reduced the average size down to ≈4 nm; particles generation rate 1012 particle/(cm3 h); theory of optical absorption spectrum evolution presented (only in 2003 publications)

Bozon-Verduraz (2003) [673], Simakin (2003) [765], (2004) [766]

Mixture of Au and Ag colloids

Water, ethanol, ethanol + PVP 0.1 g/l

Cu-vapour, 510.6 nm, 20 ns, 15 kHz, 30–50 µm spot, 9–9.4 J/cm2

Au-Ag-alloy particles formed, ≈10 nm av

Hybrid Au-Ag particles were converted to alloyed particles; PVP 0.1 g/l enhances the alloying rate, but 0.5 g/l inhibits it (the experimental conditions in Simakin (2004) [766] and Izgaliev (2004) [767] were somewhat different)

Simakin (2003) [765], (2004) [766], Izgaliev (2004) [767]

Targets

Liquids

Au

Ag foil, chemical prepared hydrosol

Stirred water under air or Ar, chloroform

Nd:YAG, 532 and 1064 ns, 6 ns, 10 Hz

30 nm av (air, 1064 nm), 49 nm av (Ar, 1064 nm), 6 nm av in chloroform

After fragmentation first by 1064 nm and thereafter by 532 nm the average diameter was reduced about 2 times; air (dissolved atmospheric gases) contributes to reduction of particle size, both at generation and at fragmentation; irradiation of chemical prepcipitate particles by 532 nm reduced their average size from 45 to 15 nm; photodissolution of Ag may be a reason for particle size reduction

Pfleger (2003) [768]

Co

Water in a rotating vessel

Nd:YAG, 532 and 1064 nm, 7 ns, 10 Hz, up to 60 min

17 nm av

Spherical particles formed; 1064 nm is more efficient than 532 nm for particle generation; coercitivity of particles was 230 Oe, much more that of bulk cobalt, obviously due to antiferromagnetic cobalt oxide core

Chen (2003) [769]

Au particles, photo- and chemically reduced

Solutions where the particles were prepared

XeCl, 308 nm, up to 260 mJ/cm2

The size of photoreduced particles was 8.3–17.7 nm av, fluence dependent), size of chemically prepared ones was 36.1 nm av; laser irradiation of chemically prepared particles reduced their size to 7.5–13.3 nm (260–75 mJ/cm2 ); a semi-quantitative theory of laser size reduction presented (Eqs (5.9)–(5.13))

Inasawa (2003) [696]

Ag particles, 4.8 nm av

Solutions where the particles were prepared

Xe-lamp, 150W, 550 nm pass filtered, beam power 50 nm formed

Studies of postirradiation of achieved colloids by 266, 532, 400, and 800 nm laser light for 5 min at fluences 0.1, 0.5, 0.1, and 0.6 J/cm2 correspondingly; size reduction of single particles and transformation of nanodisc agglomerates to nanowire agglomerates observed; optical absorption spectra for all achieved colloids presented and discussed

Tarasenko (2005) [784]

Au

Water

Nd:YAG, 266 nm (5–40 J/cm2 ) and 532 nm (10–250 J/cm2 ), 10 ns, 10 Hz

15 nm av (532 nm, 95 J/cm2 ), ≈7 nm av (266 nm, 16 J/cm2 )

At subsequent irradiation with 532 nm light, 0.3 J/cm2 , both particles fragmentation and growth/chains formation was observed

Tarasenko (2006) [785]

Ag

Water, water + NaCl, KCl, and MgCl2 , 0.2 mM

Nd:YAG 1064 nm, 10 Hz, 36 J/cm2 , 10 min

10–100 nm, spherical (as ablated in water)

Irradiation of achieved colloid by 355 and 532 nm, 6 ns, 50 mJ/cm2 laser light or by fluorescent lamp light in 0.2 mM NaCl solution yielded Ag crystals of prismatic, rod a.o. shape; their growth was explained by photo-oxidation of Ag particles with twin planes by Cl− -ions and following photo-reduction of silver ions, although Ag crystals were found also at irradiation of colloids in pure water

Tsuji (2006) [786]

Inconel 600, 316L

Water

Nd:YAG, 12 ns, 4 Hz, 0.8 J, spot 1 mm, 100 J/cm2 , scanned beam

∼60 nm, spherical

Spherical nanoparticles formation with diameter of ∼60 nm was observed during laser shock peening of the material

Bugayev (2006) [419]

Hg suspensions (dispersed ultrasonically)

D2 O (RT or frozen, −10 ◦ C)

Cu-vapour, 510 and 578 nm Ti:sapphire, 810 or 405 nm Nd:YAG, 1.06 µm, 90 ps Nd:YAG, 1.06 µm, 350 ps

10 nm, Hg →Au transmutation observed

Laser beam parameters: (i) CVL: 20–30 ns, 10 kHz, 100 µJ, 2 × 108 W/cm2 ; (ii) Ti:sapphire: 120 fs, 1 kHz, 900 µJ (at 810 nm), 2 × 1012 W/cm2 ; (iii) Nd:YAG: 90 ps, 40 mJ, 10 Hz, 1013 W/cm2 ; (iv) Nd:YAG: 350 ps, 350 µJ, 300 Hz, 1010 W/cm2 ; transmutation of Hg into Au, obviously due to the generation of thermal neutrons during laser exposure was observed; using Hg of natural isotopic composition, the conversion of Hg into Au is close to the content of 196 Hg (0.15%). In case of 196 Hg-enriched (52%) Hg, the conversion amounts to 13% (350 ps Nd:YAG-laser pulses, 4 h)

Shafeev (2006) [716]

Abbreviations: SDS – sodium dodecyl sulfate (C12 H25 OSO3 Na) SHS – sodium hexadecyl sulphate (C16 H33 NaSO4 ) SDBS – n-dodecylbenzene sulfonate (C12 H25 C6 H4 SO3 Na) SOS – sodium n-octyl sulfonate (C8 H17 SO3 Na) PVP – polyvinylpyrrolidone DT, DDT – dodecanethiol [CH3 (CH2 )11 SH] CD – cyclodextrines bpy – 2,2′ -bipyridine tppz – 2,3,5,6-tetrakis(2’-pyridyl)pyrazine TNA – thionicotinamide SOG – spin on glass PMMA – polymethylmetacrylate PS – polystyrene CTAB – cetyltrimethylammonium bromide (hexadecyltrimethylammonium bromide), C19 H42 BrN Rh6G – Rhodamine 6G CMC – critical micelle concentration LIF – laser-induced fluorescence CVL – copper vapour laser

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Handbook of Liquids–Assisted Laser Processing

5.5 Inorganic Compound Particles Inorganic nanoparticles (Fig. 5.21) are efficient in catalysis and sensors due to their large surface/bulk ratio (TiO2 , SnO2 ). They may be useful also as luminophores (Eu2 O3 ), semiconducting quantum dots (ZnSe, CdSe) and hard, high-temperature conductivity materials (BN). Table 5.7 gives an overview of the related experimental research up to the end of 2006. Laser ablation of zinc and some selenide and oxide materials in water has resulted in growth of differently shaped nanostructures in the ablation zone (Figs 5.22–5.25).There is a strong evidence that the process proceeds through formation of a water solution of the starting or intermediate (ZnO) materials. High temperatures generated by laser are known to enhance the dissolution of many solids in water (Table 7.4). The growth is most intense at the bottom of an ablation groove, obviously because both temperature and solute concentration remain high there for a sufficient time. The growth obviously occurs after the laser pulse, else the fragile structures would be broken by laser- induced shock and flow. At ablation of same or similar materials in air, no growth of such structures has been observed. In comparison with hydrothermal growth under static conditions (Table 5.5) the laser-induced growth is faster by many orders of magnitude.

5.5.1 Hydrothermal growth It is probable that the growth of nanorods and nanoplatelets in laser ablation zone in water occurs via an hydrothermal route: the solid starting material dissolves in laser-heated water and the solute crystallizes thereafter

200 nm

Figure 5.21 The TEM morphologies of the prepared by laser ablation c-BN nanocrystals with diameters of 30–80 nm [787]. Target: h-BN, ambient: acetone; laser: 532 nm, 10 ns. © Elsevier.

1 μm

Figure 5.22 ZnO columnar single crystals, 500–600 nm long and 200 nm wide, formed by pulsed laser ablation of Zn in deionized water at 80◦ C [788]. © Elsevier.

241

Generation and modification of particles

Figure 5.23 SEM image of the ablation crater and ZnSe nanowires. Liquid: water; laser: 800 nm, 150 fs, 220 µJ, 2000 pulses (courtesy by Tianqing Jia, The Institute for Solid State Physics, The University of Tokyo, Japan; and State Key Laboratory of Optoelectronic Materials and Technologies, Zhongshan University, China. Read more in the article by Jia et al. [518].

2 μm (a) Zinc hydroxide layers DS molecules layers

.8

38

Å

26.52 Å

34.5° (b)

Figure 5.24 TEM image and SAED pattern (a) of organic/inorganic nanocomposite produced by laser ablation of Zn target in 0.01 M SDS solution at 100 mJ/pulse [789]. The drawing (b) shows the schematic structure of model of the nanocomposite. Laser: 355 nm, 5–7 ns. SAED – selected area electron diffraction; SDS – sodium dodecyl sulphate (C12 H25 OSO3 Na). © The Laser Society of Japan, reproduced with permission from Ref. [789].

Figure 5.25 PZT platelets grown at laser ablation of PZT ceramics (Pz 26, Ferroperm A/S) under water [790]. Laser: Nd:YAG, 1.064 µm, 180 ns, 1000 Hz; spot diameter about 50 µm, fluence 59 J/cm2 (0.6 GW/cm2 ) scanning speed 0.16 mm/s, number of passes – 4.

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Handbook of Liquids–Assisted Laser Processing

Table 5.5

Hydrothermally fabricated single crystalline platelets (conventional hydrothermal processes).

Material synthesized

Reactants

Reaction time and temperature

Size of platelets

Reference

PbTiO3

H2 O,TiO2 , KOH Pb(CH3 COO)2 · 3H2 O

200◦ C; 15 h

≈10 µm

Peterson (1999) [792]

PbTiO3

H2 O,TiO2 , KOH (or NaOH, or RbOH), Pb(NO3 )2

150◦ C; 24 h

≈0.3 µm

Chien (1999) [791]

SnS

H2 O, SnCl2 · H2 O, Na2 S, thioglyeolic acid

200◦ C; 48 h

≈7 µm

Zhu (2005) [796]

Table 5.6

Examples of materials synthesized or grown by hydrothermal techniques [797].

Material class

Examples

Growth rate

Single oxides

SiO2 ,TiO2 , ZrO2 , HfO2 , Cu2 O, BeO, Bi2 O3 ,Al2 O3 , ZnO, Fe2 O3

2.5 mm per day (SiO2 )

Perovskite type mixed oxides

A(Ti, Zr)O3 , where A = Ca, Sr, Ba, Pb, Bi

Carbonates

CaCO3 , MnCO3 , FeCO3 , CdCO3 , NiCO3 ,

0.2 mm per day (CaCO3 )

Phosphates

AlPO4 (berlinite), GaPO4

0.5 mm per day (AlPO4 )

Hydroxyapatites

A10 (BO4 )6 X2 , where A = Ca, Sr, Ba, Fe, Pb, Cd and 3− 3− 3− 2− BO4 = PO3− 4 ,VO4 , SiO4 ,AsO4 , CO3 ; 2− − − − X = OH , Cl , F , CO3

Silicates, zeolites, germanates, fluorides, sulphides, tungstates, molybdates, titanates, tantalates, neobates, selenides, aluminates, antimonites and antimonates, ferrites Piezoelectric materials

Li2 B4 O7 , Pb(Zr,Ti)O3

Laser hosts

YVO4 , GdVO4

Nonlinear optical crystals

KTiOPO4 (KTP), LiB3 O5 , {K+ } [Ti4+ ]O [P5+ ]O4

Superconductors

YBa2 Cu3 O7−δ , Bi2 Sr2 CaCu2 O8+δ ,

Superionic conductors

Li4 B7 O12 Cl, LiH2 B5 O9

1.8 mm per week (KTP)

into regular-shaped structures. This hypothesis is supported by the fact that, for example, platelet growth has often been observed at solution synthesis of similar materials under static conditions: in hydrothermal synthesis of PbTiO3 [791, 792], in chemical coprecipitation synthesis of Bi4Ti3 O12 [793], and also in molten salt synthesis of Bi4Ti3 O12 [794]. Growth of Al2 O3 platelets was observed at calcination of boehmite in an HF aqueous solution [795] However, the aspect ratio of platelets (up to 50) has remained smaller by 1–2 orders of magnitude compared to this in laser-assisted process (aspect ratios up to 500). Laser-assisted growth is also much faster (∼10 µm/s) than chemical coprecipitation growth (∼10 µm/h) [793] or the hydrothermal growth (∼10 nm/h) [791, 792]. Table 5.5 summarizes the main conditions and results of some hydrothermal synthesis processes of nanoplatelets. Note that the ordinary methods need, as a rule, foreign substances in the solution while laser ablation-induced growth may occur in pure water. Table 5.6 presents further examples of materials, whose growth may accur at laser irradiation of corresponding solids in water. Table 5.7 presents a chronological reference of the research about inorganic compound particles preperation by laser ablation of solids in liquids.

Table 5.7

Inorganic compound particles prepared by laser ablation of solids in liquids.

Targets

Liquids

Lasers

Particles achieved

Size

Novel features, observed phenomena, comments

References

Graphite, polycrystalline

Ammonia solution, 1–2 mm layer

2ω-Nd:YAG, 532 nm, 10 ns, 5 Hz, 1010 W/cm2 , up to 45 min

α-C3 N4 , β-C3 N4 , graphite-C3 N4 , cubic-C3 N4

50 nm av (Yang (2000) [799])

Most part of achieved powder was sheet- and sphere-shape graphite

Yang (2000) [799], Wang (1998) [798]

Poly-B4.3 C

Ethanol

Nd:YAG, 1064 nm, 10–15 ns, 1 Hz, 0.5–1 J

Achieved: encapsulated in boron carbon spherules of size 70–2800 nm, boron grains and carbon fibres

Liu (2001) [800]

Au-capped TiO2 particles, 10–40 nm

Aqueous solution used for particles preparation, flowed

Nd:YAG, 532 nm, 18 ps, 10 Hz, 2–3 mJ, 5 min

TiO2 multicore Au-shell composite

up to 100 nm

Due to laser irradiation fusion the particles volume grows 6–8 times

Dawson (2001) [711]

TiO2 particles, 2 nm and 170–1050 nm

Water or aqueous solution used for particles preparation, stirred

XeCl, 308 nm, 15 ns, 5 Hz, unfocused beam

TiO2 , spherical

≈10 nm av

At laser irradiation the particles in range 2–1050 nm transformed into ≈10 nm diameter spherical particles with a narrow size distribution; the size 10 nm approximately corresponds to both the surface energy/cohesion energy and cooling/collisions balance

Sugiyama (2002) [801]

SC Ni-doped ZnSe, CdS

Water, acetone, isobutanol, diethylene glycole, ethanol, DMSO

Cu-vapour, 510.6 nm, 20 ns, 10 kHz, spot 20–80 µm

ZnSe, CdSe

10–20 nm av

Crystalline nanoparticles were achieved in all liquids (was not proved in case of DMSO)

Anikin (2002) [802]

Eu2 O2

Anhydrous ethanol, under N2 atmosphere

2ω-Nd:YAG, 532 nm, 8 ns, 10 Hz

Non-crystalline europium oxide

1 week, in pure water and 1 mM SDS unstable amorphous particles were formed, but transformed to anatase and grew in size (8 nm av) after 3 h annealing at 500◦ C

Liang (2004) [813]

LiCoO2 powder (3 µm av) suspension

Water, methanol, cyclohexane

3ω-Nd:YAG, 355 nm, 6 ns, 10 Hz, focused, 30 mJ, 60 min

LiCoO2 , Co3 O4

10–200 nm (LiCoO2 ), 1 mmol/dm3 provided highest exciton luminescence and lowest green luminescence, probably due to the occupation of O defects of the surface of ZnO by the O in carboxyl groups of LDA

Usui (2005) [816]

Zn

Water and water solution of SDS, 10 mmol/dm3

3ω-Nd:YAG, 355 nm, 7 ns, 10 Hz, 6.7 J/cm2 , 60 min

In SDS solnution β-Zn(OH)2 /SDS octagonal multilayer plates

In water spherical ZnO particles formed; in SDS, β-Zn(OH)2 /SDS multilayer plates of hexagonal crystal symmetry formed with thickness of inorganic layer 4.6 Å; diffuse reflectance and PL spectra recorded; both particles exhibited UV emission

Usui (2005) [817]

Ti, Sn, Zn

Water and water solution of SDS, 10 mmol/dm3

Nd:YAG, 355 nm

TiO2 (anatase), SnO2 (cassiterite), β-Zn(OH)2 /SDS multilayers

A review (6 pp., 7 figs., 32 refs.) of oxide nanomaterials fabrication at AIST by laser ablation in gases and in liquids; in gas, Fe2 O3 , Co3 O4 , and BaTiO3 particles were prepared

Sasaki (2005) [789]

Co, CoO,and Co3 O4 powder suspensions

Water and hexane, stirred

3ω-Nd:YAG, 355 nm, 10 Hz, 30 mJ, 60 min

Co3 O4 (in water from all materials), Co and CoO in hexane

In water up to micrometers, mostly 2 is smaller than in vapour

Gaumet (1996) [832]

Carbon black, 25 nm in suspension

Water

Nd:YAG, 1.06 µm, 16 ns, 10 Hz, 0.7 J, up to 6000 shots

Carbon

Up to 400 nm

Spherical structures with dense shells and lower density cores formed; tiny bubbles and audible sound generation observed; produced gases detected

Chen (1997) [833]

Graphite

Water

Nd:YAG, pulsed

Carbon

35 nm av

Partly crystallized spherical carbon particles were achieved of size 20–50 nm

Chen (2002) [834]

Si

Water (also with surfactant additives) ethanol, dichloroethane

Cu-vapour, 510.5 nm, 20 ns, 15 kHz, spot 50 µm, 1–2 J/cm2

60–84 nm av

Crystal size was almost independent on laser fluence; in case of PVP additive in water the particle size was about 10% smaller

Dolgaev (2002) [753]

Graphite particles, 75 µm, suspended in liquid

Benzene, toluene, hexane, stirred

Nd:YAG, 355, 532 and 1064 nm, 5–9 ns, 10 Hz, 0.2 J/cm2 Nd:YAG, 1064 nm, 1 ms, both focused or non-focused

Polyynes Cn H2 , n = 10, 12, 14, 16 formed in benzene and toluene; n = 8, 10, 12, 14 in hexane; shorter laser wavelength and starting particle concentration of 4 mg/ml provided greatest effective for polyyenes formation; polyynes formation paths discussed

Tsuji (2002) [835]

Cn H2

(Continued)

Table 5.8

(Continued)

Particles size

Novel Novel features, observed phenomena, comments

References

Targets

Liquids

Lasers

Particles type

C60 suspension

Hexane, methanol (stirred)

Nd:YAG, 266, 355, 532, and 1064 nm, 5–9 ns, 0.2 J/cm2 , non-focused, 60 min

Cn H2

Graphite particles and hydrogen-capped polyynes Cn H2 , n = 8, 10, 12 were formed, C8 being dominant in all cases; C2 radicals produced from C60 are obviously polymerized and hydrogenated to form Cn H2 ; dependence of polyynes yield on various experimental parameters studied

Tsuji (2003) [836]

Diamond particles, 5 nm in diameter

Ethanol

2ω-Nd:YAG, 532 nm, ≈7 ns, 20 Hz, 1.3 J/cm2

Cn H2

Polyynes Cn H2 , n = 8, 10, 12, 14, 16 formed; ablation of graphite particles yielded less and shorter polyynes

Tabata (2004) [837]

Graphite

Water, cyclohexane under Ar, free surface liquids

2ω-Nd:YAG, 532 nm, 10 ns, 10 Hz, spot 0.5 mm, up to 66 J/cm2

Most of particles were of graphite; some of diamond (in both liquids); atomic H in plasma, detected by optical spectroscopic, may be responsible for diamond growth

Pearce (2004) [674]

Glassy (vitreous) carbon

Water, rotating vessel

Nd:YAG, 532 and 1064 nm, 7 ns, 10 Hz, 0.8 J/cm2 , 5 min

15 nm av

Photostable colloids achieved; average graphitic domain size estimated from Raman spectra was 1.56 nm; optical limiting setup ≈0.3 J/cm2 at 532 nm

Chen (2004) [829]

Glassy carbon

Tetrahydrofuran (THF)

Nd:YAG, 532 and 1064 nm, 7 ns, 10 Hz, 1 J/cm2 , 30 min

6.5 nm av

Photostable (at least up to 12 J/cm2 ) and stable in time (over 3 months) colloid achieved; probablyTHF polymerizes onto particles surface;

Chen (2004) [838]

Graphite

Isopropyl alcohol

≈2 µm, ≈20 µm

Nd:YAG, 1064 nm, 3.5 ns, 30 Hz, 1 J/cm2 , 108 W/cm2 , 30 min

Graphite

Carbon black particles suspension, 14 nm – 5 µm av

Water, stirred

Nd:YAG, 355, 532 and 1064 nm, 7 ns, 10 Hz, up to 250 mJ/cm2 , up to 20 min

Graphite, coal or C60 powder in suspension

Benzene, toluene, hexane, cyclohexane, methanol, hexafluorobenzene, perfluorooctane, perfluorodecaline (stirred)

Nd:YAG, 266, 355, 532, and 1064 nm, 5–9 ns, 10 Hz, 40 mJ, 0.2 J/cm2 , non-focused, 60 min

Notation PVP – polyvinylpyrrolidone

C2n H2

Rose-shaped particles of ≈2 µm size and cracknel-shaped particles of ≈20 µm size were formed; FTIR, PL, PLE, and Raman studies

Kitazawa (2005) [684]

Theory of diamond nucleation and growth at laser ablation of graphite in liquids presented; at temperatures up to 5000 K and pressures up to 30 GPa the particles sizes are predicted to be in range 25–250 nm

Wang (2005) [839]

Almost no morphological changes were observed for small particles; 5 µm particles were covered after laser irradiation by nets of short wires composed of small particles; no crystallinity was developed

Miyazaki (2006) [840]

Hydrogen-capped polyynes C2n H2 , n = 4–8 were formed, the measured absorbance, abundance, and Raman spectra are presented for different experimental conditions (starting materials, solvents, and laser wavelengths); the yield of polyynes increased with decreasing laser wavelength; largest distributions of long-chain polyynes were achieved at ablation of graphite in aromatic hydrocarbons; formation mechanisms of polyynes are discussed

Tsuji (2006) [830]

Table 5.9

Diamond and diamond-like carbon (DLC) formation by laser irradiation of liquids and solid–liquid interfaces

Targets

Liquids

Lasers

Particles size

Novel features, observed phenomena, comments

References

W

Cyclohexane, decalin, n-hexane

KrF, 248 nm, 20 ns, 5 Hz, ≈4 J/cm2 , 20 pulses at each point

20–50 nm

In cyclohexane and decalin a mixture of hexagonal polytypes along a small fraction of cubic phase formed; no diamond in hexane, obviously because the structure of molecule does not mach the diamond lattice

Sharma (1993) [848]

Cu, SC (100) and polycrstalline

Benzene (3 mm layer)

XeCl, 308 nm, 30 ns, 1–4 J/cm2

Some nm

Four laser pulses: cubic diamond, 10 pulses: cubic, 2H- and 6H-hexagonal

Singh (1993) [849]

Si (100) ∼600◦ C

Santovac 5 vacuum oil (a polyphenyl ether)

ArF, 193 nm, 10 Hz, 220 mJ, spot 2 × 5 mm

DLC film with a large amount of sp3 bondings was obtained, deposition rate was 0.1 Å/pulse; in comparison with a PMMA target, no particulates were found in the deposited films

Xiao (1995) [842]

Stainless steel (above the liquid surface)

Santovac 5 vacuum oil (a polyphenyl ether)

ArF, 193 nm, 10 Hz, 220 mJ, focused beam

Ablation/deposition performed in O2 /H2 O2 vapour mixture (200 mbar); film deposition rate 0.1 Å/pulse; 120 nm thick film obtained after 12 000 laser pulses; hydroxyl ions OH − promote the diamond growth

Xiao (1995) [841]

Si (100)

Cyclohexane (2–3 mm layer)

KrF, 248 nm, 23 ns, 1 Hz, 1–10 J/cm2 , 2 pulses

Mostly graphitic particulates formed, but also diamond; formation mechanism probably includes preferential breaking of C−H bonds in cyclohexane and atomic hydrogen formation

Lu (1998) [847]

Si, SC

Toluene (≈ 2 mm layer)

Cu-vapour, 510.6 nm, 20 ns, 8 kHz, up to 1 J/cm2 , focused steady or scanned beam

Simultaneous with carbon deposition etching of the substrate and generation of suspended particles observed; the carbon dots and lines had good adhesion to substrate and ohmic contacts; adding organometallic substances (ClAuPPh3 ) to toluene did not result in doped carbon films

Shafeev (1999) [850]

500 nm cubic

Glassy carbon dots and lines

Probability of graphite to diamond transformation Wang (1999) probability as function of temperature and pressure [844] calculated for range 1000–5000 K, 0–20 GPa (see Figs 5.29 and 5.30); formation of nanometre-sized diamond at laser irradiation in liquids is explained by high nucleation rate

Glass, fused silica, Al2 O3 , CaF2

Benzene, toluene, also with glassy carbon or diamond particles (4–5 nm) added

Cu-vapour, 510.6 nm, 20 ns, 8 kHz, 0.2–1.5 J/cm2 ; liquid–solid interface was irradiated through the substrate

DLC film 80– 100 µm

Well adherent and stable in DLC films were achieved; film thickness saturated at 100 nm, despite the ablated depth of the surface increased; calculated peak temperature of the sapphire-film-liquid structure during laser pulse was 600 K

Simakin (1999) [622]

Glass, fused silica, sapphire

Toluene, benzene, cumene, containing carbon particles (3–4 nm)

Cu-vapour, 510.6 nm, 20 ns, 8 kHz, spot 50 µm, up to 1.5 J/cm2 , up to 25 min, scanned beam up to 1.2 mm/s

DLC film ∼100 µm

The sp3 fraction in deposited films amounted to 60–70% depending on the precursor; the films showed excellent adherence, were transparent in the visible and have microhardness of 50-70 GPa

Lyalin (1999) [623]

Soda-lime glass, Pyrex, sapphire

Toluene, benzene, cumene, also with addition of glassy carbon particles, 3–5 nm

Cu-vapour, 510.6 nm, 20 ns, 8 kHz, up to >1.5 J/cm2 , scanned focused beam 0.3–3 mm/s

DLC film 100 µm thick formed on surface, 70% sp3 bonds

Backside of the substrates in contact with liquid irradiated; film deposition took place along the etching of the substrate; no optical breakdown or plasma was observed; film microhardness 50–70 GPa; film thickness saturates at ≈ 100 nm, probably due to heating it by laser beam following graphitization and periodic detachment of the film due to thermal stresses

Simakin (2000) [628, 629]

Glass

Water + dissolved methane (at 350 kPa)

ArF, 193 nm, 23 ns, 30 Hz, 150 mJ, 20 min

Hydrogen DLC film; particles size in film 30 nm

Focused laser beam irradiation of methane solution in water resulted in formation of granular DLC film, containing 7.2 wt% hydrogen; dependence of the transmittance of a focused laser beam by water on the defocus distance was studied as well, five-fold variation of the transmittance was observed

Hidai (2000) [851]

Graphite (polycrystalline)

Water, 1–2 mm layer

Nd:YAG, 532 nm, 10 ns, 5 Hz, 250– 350 mJ

Diamond particles, 300 nm (for example)

Intergrowth diamond crystals achieved with both cubic and hexagonal structure, graphite conversion to diamond occurs via metastable intermediate rhombohedral graphite phase

Yang (2001) [846], Wang (1998) [798]

Graphite (polycrystalline)

Acetone, 1–2 mm layer

Nd:YAG, 532 nm, 10 ns, 5 Hz, 1010 W/cm2 , 45 min

Diamond particles, 30 nm av

Obtained particles consisted of 5% diamonds and 95% graphite; a new Raman line of the irradiated surface, 926/cm, was found, obviously originating from nano-diamonds

Wang (2002) [852]

(Continued)

Table 5.9

(Continued)

Targets

Liquids

Lasers

Particles size

Novel features, observed phenomena, comments

References

Glass

Benzene, toluene (also with Pd(acac)2 additive)

Cu-vapour, 510.6 nm, 20 ns, 8 kHz, 0.5 J/cm2 , scanned focused beam 0.5 mm/s

DLC film ∼100 µm thick on surface

Glass–liquid interface was irradiated through the glass; addition of Pd(acac)2 to the liquid resulted in Pd-doped films which served as seed layer for subsequent CVD copper deposition; efficiency of DLC film as diffusion barrier for Cu was demonstrated

Simakin (2002) [853]

No

Water + dissolved methane (up to 72 mg/l)

ArF, 193 nm, 23 ns, 30 Hz, 40–150 mJ, 20 min

DLC particles, 200–700 nm

The achieved particles consisted of DLC, covered by multiwall carbon nanotubes; in case of laser irradiation of the gas near the liquid surface, DLC particles of diameters 50–200 nm were achieved

Hidai (2002) [854]

W

Benzene

Ar-ion, 514.5 nm, CW, spot 150 µm2 ; 0.72–1.09 MW/cm2

DLC film The tip of a W needle (10 nm in radius) covered (graphitic), cluster by benzene was irradiated by laser light; near-field effect size 34 nm provided enhanced DLC deposition at needle tip; the deposited in liquid films were rougher and thicker than those deposited in benzene vapour by KrF-laser (248 nm, 23 ns, spot 0.1 cm2 , ∼3 J/cm2 )

Graphite

Water

Notations ClAuPPh3 – triphenylphosphine complex of Au Pd(acac)2 – palladium acetylacetonate CVD – chemical vapour deposition

Diamond

Shi (2005) [855]

Thermodynamic calculations of nanodiamond Wang (2005) formation at laser ablation of graphite in water; [856] calculated radia of critical nuclei and graphite–diamond phase transition probabilities are presented at temperatures up to 5000 K and pressures 7–21 GPa; formation of diamond particles of size 3–5 nm was predicted to be most favourable around 12 GPa and 4500 K

257

Generation and modification of particles

Heating wires thermocouples

rotating motor

excimer laser beam liquid target

focusing lens

viewing window

reactive gas

cooling water substrate

vacuum pump

gravity

Figure 5.27 Laser ablation deposition of diamond films using a liquid target [842]. An high viscosity, low-vapour pressure liquid-like vacuum oil is needed for this process (see also Section 6.3.2). © American Institute of Physics (1995), reprinted with permission from Ref. [842]. 50 G

J

Pressure (GPa)

40

Diamond

30

F I

E

20 H

CW D

10

Liquid

B

PLIIR

A

P

HTH

C

Graphite 0

0

1000

2000

3000

4000

5000

6000

Temperature (K)

Figure 5.28 P, T phase and transition diagram for carbon as understood from experimental observations. Solid lines represent equilibrium phase boundaries [843]. A: commercial synthesis of diamond from graphite by catalysis; B: P/T threshold of very fast (less than 1 ms) solid–solid transformation of graphite to diamond; C: P/T threshold of very fast transformation of diamond to graphite; D: single crystal hexagonal graphite transforms to retrievable hexagonal-type diamond (shock-wave synthesis); E: upper ends of shock compression/quench cycles that convert hex-type graphite particles to hex-type diamond; F: upper ends of shock compression/quench cycles that convert hex-type graphite to cubic-type diamond; B, F, G: threshold of fast P/T cycles, however generated, that convert either type of graphite or hexagonal diamond into cubic-type diamond; H, I, J: path along which a single crystal hex-type graphite compressed in the c-direction at room temperature loses some graphite characteristics and acquires properties consistent with a diamond-like polytype, but reverses to graphite upon release of pressure. Notations: SW – shock waves; HTHP – high temperature high pressure; PLIIR – pulsed laser-induced liquid–solid interfacial interaction. © Elsevier.

cyclic and aromatic compounds. Cyclohexane, decaline, and benzene are favourable compounds for diamond synthesis, because only breaking of relatively weak C–H bonds is needed to get free carbon rings. Abstracted atomic hydrogen and formed in solvents OH-radicals have known to contribute to the nucleation of diamond as well [847].

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Handbook of Liquids–Assisted Laser Processing

2.0⫻1010

10 ⫺2

5⫻

10 ⫺

3

⫺3

10 ⫺4 10 5⫻

⫺4

10

1.0⫻1010 ⫺5

10 0 1000

2000

10 ⫺

10 ⫺3 5⫻

2

3

5⫻

10 ⫺3

0.5⫻1010

e

lin

10 ⫺4 10 ⫺4

S B–

10 ⫺

Pressure (Pa)

1.5⫻1010

3000

4000

5000

Temperature (K)

Figure 5.29 Schematic illustration of the probability of phase transformation in the pressure–temperature diagram [844]. The B–S line is the Berman–Simon line. The curves above and below the B–S line are fd and fg , respectively. fd – probability of the transformation from graphite to diamond; fg – probability of the transformation from diamond to graphite. © Institute of Physics, reproduced with permission. 10⫺3 10⫺4 10⫺5 10⫺6

c b a

fd

10⫺7 10⫺8 10⫺9 10⫺10 10⫺11 10⫺12 3500 3000 2500 2000 1500 1000 500 Temperature (K)

Figure 5.30 fd –T curves of diamond formation probability: (a) P = 6 GPa. (b) P = 8 GPa and (c) P = 10 GPa [844]. © Institute of Physics, reproduced with permission.

5.8 Organic Particles Laser ablation of organic materials in liquids has been used for fabrication of phthalocyanine and its metal derivatives particles. These materials possess useful photoconductive and semiconductive properties and are applied for sensors, bioprobes, and organic microdevices. The common methods of fabrication of particles of these materials are evaporation and reprecipitation. Laser ablation of bulk materials in liquids presents a simple way to control the size and molecular aggregation structure of the particles [857]. Nanoparticles of some materials cannot be fabricated another way, for example of quinacridone particles of size below 50 nm [858] (Table 5.10).

Table 5.10

Organic colloids prepared by laser ablation in liquids.

Target materials

Liquids

Lasers

Size

Novel features, observed phenomena, comments

References

VOPc, CuPc, FePc, Water anthracene, perylene, pyrene, abd coronene powders in suspension

XeF, 351 nm, 30 ns, 5 Hz, up to 340 mJ/cm2 , up to 180 min

≈100 nm (VOPc, 340 mJ/cm2 , 180 min)

Transparent colloid solutions stable for at least several months were achieved; phase transitions of Pc due to laser irradiations are likely

Tamaki (2000) [859]

VOPc crystalline powder (few tens of µm) floated in water

Water, stirred

XeF, 351 nm, 30 ns, 5 Hz, up to 68 mJ/cm2

Triangular 60/19 nm hexagonal 49/17 nm (mean width/height)

StableVOPc nanoparticle colloids achieved; threshold fluence Tamaki ≈20 mJ/cm2 ; as fabricated particles were structurally (2002) [857] metastable, a phase transformation occurred in some days

VOPc powder floated in liquid by stirring

Water, methanol, ethanol, 1-propanol, ethyl acetate

XeF, 351 nm, 30 ns, 5 Hz, 30 mJ/cm2 for 10 min

50 nm av

Thermal diffusivity of the liquid determines the size and crystalline phase of the particles

Tamaki (2003) [860]

VOPc, CuPc, FePc powders in suspension

Water, water + (SDS or Igepal CA-630)

3ω-Nd:YAG, 4 ns, 20 Hz, spot 2 × 2 mm, up to 80 mJ/cm2 , up to 80 min

60–100 nm av (VOPc, 0.4–0.01 mM surfactants)

In pure water VOPc nanoparticles associated in some days, but were stable at least 2 months if surfactants were added; lowering the temperature down to 5◦ C increased the efficiency of particles generation; surfactants lowered the threshold fluence of particle generation

Li (2003) [861]

FePc powder in suspension

Water solutions of SDS (1–16.4 mM) and Igepal CA-630 (0.184–0.41 mM)

3ω-Nd:YAG, 20 Hz, spot 2×2 mm, 80 mJ/cm2

60 nm av

At higher concentrations of surfactants the Q-band in optical abstraction spectra is shifted towards longer wavelengths

Li (2004) [862]

50 nm av (355 nm, 98 mJ/cm2 , 20 min); 20 nm av (580 nm, 90 mJ/cm2 , 30 min)

Laser spot size typically 23 mm2 , fluence up to 120 mJ/cm2 , irradiation up to 30 min; thresholds for particle modification: 30 mJ/cm2 at 355 nm and 15 mJ/cm2 at 580 nm; achieved colloid stable at least for 1 month; prepared by 355 nm nanoparticles were of β-form and by 580 nm, >50 mJ/cm2 of γ-form

Sugiyama (2006) [858]

β – quinacridone par- Water, stirred ticles, 0.2 µm and 1– 10 µm in suspension

4 ns,

3ω-Nd:YAG, 355 nm, ns-pulses OPO, 580 nm, 7 ns, 10 Hz

Notations VOPc – vanadyl phthalocyanine, oxo(phthalocyaninato) vanadium FePc – iron phthalocyanine CuPc – copper phthalocyanine SDS – sodium dodecyl suphate (C12 H25 SO3 Na) Igepal CA-630 – octylphenoxy polyethoxy ethanol ((CH3 )3 CCH2 (CH3 )2 CC6 H4 O(CH2 CH2 O)9 H)

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C H A P T E R

S I X

Surface Modification, Deposition of Thin Films, Welding, and Cladding

Contents 6.1 Surface Modification 6.2 Deposition and Transfer of Thin Films 6.3 Welding and Cladding Under Water

261 262 277

6.1 Surface Modification 6.1.1 Modification of surfaces of inorganic materials Laser irradiation may modify the surface of a solid by melting and vaporizing it, or/and by inducing chemical reactions between the solid and the ambient. In a liquid, there are two main differences in comparison with gases or vacuum: (a) Cooling rate of the laser melted zone is faster which may result in metastable phases; (b) The density of chemical species (e.g. oxygen and nitrogen) is greater in liquid than in gas, thus the reaction efficiency is greater. Using lasers, the modification of surfaces can easily be performed locally without a need for masks.

Laser-induced quenching in water Much research has been done in quenching of laser melted silicon in water. In comparison with air ambient, in water the quench rate was ∼30 per cent higher (for 270 nm deep melts, 4 ns laser pulses). After irradiation of single crystalline silicon under water, perfect epitaxy was obtained with no surface oxidation or changes in surface morphology. Si regrowth velocities over 7 m/s were observed, but the critical for formation of amorphous silicon quench rate 15 m/s was not achieved [863–865] (see also Table 6.1, Polman 1988 and 1999). In conventional nanosecond laser melting of solids in gas or vacuum, the solidification velocity v can be estimated by: ∂T λ · , (6.1) v= H ∂z where ∂T /∂z is the temperature gradient in the solid just behind the interface, H is the enthalpy of melting, and λ is the thermal conductivity of the solid [863]. Handbook of Liquids-Assisted Laser Processing ISBN-13: 978-0-08-044498-7

© 2008 Elsevier Ltd. All rights reserved.

261

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Handbook of Liquids-Assisted Laser Processing

A review of laser reactive quenching at liquid–solid interface has been published by Kanetkar and Ogale [866].

Oxidation and nitriding At laser-induced high temperatures, inert under normal conditions liquids like water and liquid nitrogen, dissociate, liberating chemically reactive species. In Table 6.1, some examples of corresponding research are presented (see also the review by Kanetkar and Ogale [866]). In the articles by Imai et al. [867], and Watanabe and Sameshima [868] some examples of aftertreatment of laser irradiated materials by water are presented (see Table 6.1).

6.1.2 Modification of surfaces of organic materials Fluorocarbon polymers like PTFE possess excellent chemical and thermal stability, low wettability, low electrical conductance, small dielectric losses up to very high frequencies, etc., which makes them useful in many applications. On the other hand, high chemical stability hinders, for example, their joining with other materials, needed for example in fabrication of electronic printed boards. Adhesion and biocompatibility of fluorocarbon polymers can be improved by treatment in plasma, but also by irradiation by UV light in water or aqueous solutions (see Table 6.2). Using of UV light, thereby from lasers, avoids the need for a vacuum system and enables selective treatment without masks. Besides lasers and liquids, excimer lamps and nitrogen or ammonia gases have given similar results [874, 55]. The principle of PTFE hydrophilization is shown in Fig. 6.1. Surface irradiation by UV (excimer) laser light at presence of water leads to replacement of surface fluorine atoms by OH-groups, and liberation of hydrofluoric acid (which in turn may be used for etching of silica as described by Murahara [596]) (see Table 4.9, Murahara 2001). H2 O + [CF2 ]n + hν(193 nm) → [CFOH]n + HF.

6.2 Deposition and Transfer of Thin Films 6.2.1 Laser ablation deposition in water vapour Laser ablation deposition in water vapour has found to be beneficial in fabrication of bio-compatible hydroxylapatite coatings on implants, and of TiO2 passivating films on silicon solar cell structures [881] (Table 6.3). A schematic representation of PLD is shown in Fig. 6.2. Focused pulsed laser light (usually from an excimer laser because of short pulse and high absorption) irradiates the target and causes explosive vaporization of the material. The ejected material condenses on a substrate placed some centimetres away. In case of laser pulse length in nanoseconds, the chemical composition of the coating closely resembles the composition of the target [882]. Apatites, in particular hydroxylapatite Ca10 (PO4 )6 (OH)2 (also named hydroxylapatite or HA), have chemical composition and structure similar to the calcium phosphate phase of the bone and the tooth mineral [883]. Hydroxoapatite is the material of choice for biologically compatible coatings on metal substrates, implants and prostheses for orthopaedics, neurosurgery, and dentistry [884]. It is the most stable calcium phosphate in contact with the body fluids [885]. HA coatings may be fabricated by sputtering, plasma and flame spraying, electrophoretic deposition, electrolysis, RF sputtering, ion beam deposition, powder sintering, etc. [886]. The commonly used method is plasma spraying, but it suffers from pores in the coatings [887]. Laser ablation deposition has attracted as a method for achieving pore-free well-crystalline HA coatings. For PLD of HA, the targets were made of compressed HA powder, the laser fluences at the target were 1.5–3.5 J/cm2 , and the substrate was heated up to some hundreds of degrees of Celsius. For the deposition of coatings of some micrometres thick, 10 000–20 000 laser pulses at 193 and 248 nm wavelength were

Table 6.1

Modification of inorganic materials surfaces by laser irradiation under liquids and related research (examples).

Materials processed

Environment

Fe (foil)

Air, water

Fe,W, Fe+Al layer (40 nm), Fe+B layer

Laser or other light source Other features of and beam parameters the experiment

Novel features, observed phenomena, comments

References

Ruby, 694 nm, 30 ns, 10 and 15 J/cm2

Metastable Fe oxide was formed; results of oxide characterization by CEMS, XRD, RBS, and XPS are presented and discussed

Patil (1987) [869]

Water, NH3 , benzene, LN2

Ruby, 694 nm, 30 ns, up to 15 J/cm2

Processed surface studied by CEMS, XRD, RBS, XPS and TEM; transient reflections measurement in cource of process; in Fe:H2 O and Fe:NH3 cases FeO and γ-Fe-N austenite, respectively found, in the W:C6 H6 case a multiphase composite comprised of W3 C, β-W2 C, and WC1−x was observed; laser irradiation of Fe+Al layered structures in LN2 led to dimeric metastable solid solution

Ogale (1987) [870]

Si (100)

Air, water

2ω-Nd:YAG, 532 nm, 4 ns

Light conducted through quartz guide diffusor without focussing; irradiated area ∼6 mm in diameter

Physical phenomena at solid–liquid interface were studied by transient electrical conductivity and optical reflectivity measurements; quench rate may be enhanced by 30% for deep (down to 270 nm) melts if irradiated in water; Si regrowth velocities over 7 m/s were observed

Polman (1988) [863, 864]

Si (100) and SOS

Air, water

2ω-Nd:YAG, 532 nm, 4 ns, up to 28 mJ

See Polman (1988) [863] or Polman (1988) [864]

In addition to the results presented in Polman (1988) [863, 864], calculated reflectivities, TEM-micrographs, and a thorough discussion of physical phenomena at liquid–solid interface are presented; a short review of related previous research with 94 references is also presented

Polman (1989) [865]

Silica, titania and silica-titania sol–gel coatings

The coatings were saturated with water vapour (60–180◦ C, 1–72 h)

Synchrotron, 6–20 eV 4ω-Nd:YAG, 4.7 eV Low pressure Hg-lamp, 4.9 eV, 1.4 mW/cm2

Experiments were performed in vacuum

Irradiation caused densification and crystallization of sol–gel films; subsequent exposure to water vapour was found to accelerate the rearrangement of sol–gel films, leading densification of silica and crystallization of titania and phase separation of silica–titania gel films

Imai (1999) [867]

38HMJ steel

Ar, liquid nitrogen (77 K)

CO2 , 10.6 µm, 1 kW,∼130 kW/cm2

Sample was immersed into LN2 bath and scanned 0.17–2 cm/s

At laser irradiation in LN2 the sample’s surface layer ∼30 µm became enriched by nitrogen; an increase of hardness down to the depth of 270–330 µm was observed (in Ar gas 400–500 µm)

Jendrzejewski (2000) [871]

(Continued)

Table 6.1

(Continued)

Materials processed

Environment

Laser or other light source and beam parameters

Other features of the experiment

Novel features, observed phenomena, comments

References

Poly-Si (25 nm films)

Water vapour

XeCl, 308 nm, 30 ns, 280 mJ/cm2 , 50 shots in vacuum

Aftertreatment in vapour was performed at 260◦ C and 1.3 MPa for 3 h

Amorphous Si layers were crystallized by laser irradiation and then exposed to water vapour; as result of aftertreatment by vapour, the density of defect states in the crystallized silicon films was reduced from 1 × 1014 cm−2 (as crystallized) to 3.2 × 1012 cm−2

Watanabe (2002) [867]

Al (films 10–150 nm) on glass

Water (15 µm)

Ar-ion, 488 nm, up to 30 mW, spot ∼250 nm

Water was covered by glass, scanning rate up to 33 µm/s

Electrically insulating Al oxide lines of width down to 266 nm were obtained, increased oxidation rate was obviously due to increased diffusivity and convection transport mobility of oxygen in water and due to the increased of Al3+ ions in the oxide; grooves formation was also observed

Haefliger (2002) [509]

Al (coating 120 nm) on Si3 N4

Water

Ar-ion, 488 nm, up to 30 mW, spot 10 µm, 10 and 15 s

SNOM tip was immersed into a tips drop of water, illuminated from below by laser

Protrusions of up to 30 nm height and 38 nm in diameter were formed at aluminized Si3 N4 SNOM as result of laser heating destruction of passivating oxide layer followed by local corrosion of the metal

Haefliger (2002) [872]

Al (∼60 nm films on PDMS, SU-8 and glass)

Water

Ar-ion, 488 nm, CW, up to 17 mW, spot 2.5 µm

Water-immersion micro-objective was used for laser beam focusing

Al electrode film was patterned by laserassisted corrosion in water

Haefliger (2003) [873]

Sintered CeO2

Water

Nd:YAG, 5–40 kJ/cm2 (total dose?)

Target immersed into water

At 6–15 kJ/cm2 amorphous layer formed; at 20–25 kJ/cm2 10 µm thick nanocrystalline films formed, crystallite size 50–150 nm

Chen (2005) [818]

Notations CEMS – conversion-electron Mössbauer spectroscopy XRD – X-ray diffraction RBS – Rutherford-backscattering spectrometry XPS – X-ray-photoelectron spectroscopy TEM – transmission electron microscopy SOS – silicon on sapphire SNOM – scanning near-field optical microscope PDMS – poly(dimethylsiloxane) CW – continuous wave SU-8 – a kind of high-viscosity photoresist

Table 6.2

Modification of organic materials surfaces by laser irradiation under liquids and related research (examples).

Materials processed

Environment

Laser or other light source and beam parameters

Other features of the experiment

PTFE (film)

Water solution of B(OH)3 (1.8%, 50 µm layer)

ArF, 193 nm, up to 25 mJ/cm2 , up to 3000 pulses

PTFE, FEP (10–1000 µm)

Vacuum, gaseous NH3 and N2 H4

Fluorocarbon resin

Novel features, observed phenomena, comments

References

Liquid was covered by fused silica plate

As result of laser irradiation, the surface of PTFE became hydrophilic and oleophilic, due to replacement of surface F atoms by CH3 and OH functional groups; the tensile shear strength of modified surface PTFE bonded by epoxy resin to stainless steel was 12 MPa

Murahara (1995) [875, 876]

Excimer lamps Kr2 * (146 nm) and Xe2 * (172 nm); pulse tens of nanoseconds, 10–20 mW/cm2

Irradiation time up to 90 min

Treatment resulted in hydrophilic surface, where abstraction of fluorine atoms and introduction of nitrogen, oxygen, and hydrogen atoms occurred; the modified surface layer showed higher absorption in the UV–VIS spectral region

Heitz (1996) [874]

Water and water solution of B(OH)3 (1.2%)

ArF, 193 nm, 20–30 mJ/cm2 , up to 4000 pulses

Liquid was covered by fused silica plate

Adhesive strength of fluorocarbon resin to epoxy resin was improved 275 times/up to 98 kgf/cm2 (treatment in water), respectively, 490 times/up to 55 kgf/cm2 (treatment in B(OH)3 solution)

Hatao (1997) [877]

PTFE

Water and water solutions of H3 BO3 ,NaOH, CuSO4 , NaAlO2

XeCl, 308 nm, 10 Hz, 10–535 mJ/cm2 , up to 2500 shots

Liquid layer was covered by silica window

Laser irradiation converted the PTFE surface from hydrophobic to hydrophilic (minimum contact angle with water was 28◦ if treated in 1% H3 BO3 ); the bond strength to epoxy resin 509 was increased from 2 to 26.2 kg/cm2 (1% NaAlO2 )

Lou (1998) [878], Huang (1999) [879]

FEP

Water

ArF, 193 nm, 10 ns, 100 Hz, up to 50 mJ/cm2 , up to 4000 pulses

Sample was grind using FEP turntable with a water layer between; laser light irradiated the turntable through the sample

Laser treatment in water (25 mJ/cm2 , 3000 pulses) resulted in hydrophilic surface of FEP; adhesion strength of FEP to epoxy resin was increased due to laser treatment from 0.2 to 110 kg/cm2

Murahara (2001)

PTFE (50 µm)

Air, amino-ethanol, 1,2-diamino-ethane, triethylene-tetramine

ArF, 193 nm, 20 ns, up to ∼8 mJ/cm2 , 1500 pulses

PTFE–liquid interface was laser irradiated through PTFE (transmission 51%)

Laser irradiation in liquids (but not in air) converted the PTFE surface from hydrophobic to hydrophilic (minimum contact angle with water became down to 30◦ if treated in aminoethanol and triethylene-tetramine; the bond strength to epoxy resin Uverapid 20 was increased 100–200 times, up to 9 MPa (when treated in triethylene-tetramine)

Hopp (2003) [880]

Notations FEP – poly(tetrafluoroethylene-co-hexafluoropropylene) PTFE – poly(tetrafluoroethylene)

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Handbook of Liquids-Assisted Laser Processing

H2O H ArF laser

F F

OH

F

C

C

C

C

F

F

F

F

F

F

F

C

C

C

F

F

F

PTFE

Fluorocarbon

Figure 6.1 Principle of the photochemical reaction providing the replacement of F atoms by OH functional groups. © SPIE (2001), reproduced with permission from Ref. [596].

Window

Plasma plume

Rotating target

t° Heater Substrate To vacuum pump

Water vapour

Figure 6.2 Schematics of pulsed laser deposition (abbreviated as PLD or LAD). Because a focused laser beam acts only on a small area of the target, the latter is rotated for even consumption of the material and for avoidance of formation of craters. The substrate may be heated in order to improve the adhesion and crystallinity of the deposited film. The irradiation densities of the target are some joules per centimetre square.

needed [888]. A 0.5 mbar water vapour pressure in the chamber was found to yield the best coatings (highly crystalline) [888]. Regarding other materials, only TiO2 coatings fabrication by PLD in water vapour ambient has been reported [881]. 0.55 mbar vapour pressure was found to yield best passivating films for silicon solar cells.

6.2.2 Laser ablation deposition using a liquid target Pulsed laser ablation deposition has established as a method for fabrication of high purity thin films of compound materials without little declination from the composition of the target. It is also easy to fabricate multilayer films by in situ target changing. However, PLD suffers from target deterioration (craters and cones formation) and from particulates in the deposited film. The particulates originate from droplets ejected from the molten target. Both of these problems can be avoided by using a liquid target. In this way, the focused laser beam ablates always a smooth surface and, hence, target deterioration is completely prevented without target rotation. Further, splashing can be avoided by using a viscous liquid [842]. For example, a liquid GaAl target was used by Willmott et al. [896], Fig. 6.3, for fabrication of AlGa films, from considerations that ablation of solid Al produces easily droplets and because liquid GaAl target has less impurities than a sintered target. (see also Table 6.4)

Table 6.3

Laser ablation deposition in water vapour (examples). Laser or other light source Other features of and beam parameters the experiment

Targets

Substrates

Novel features, observed phenomena, comments

References

HA (sintered pellets)

Ti-6Al-4V, Si (RT–800◦ C)

KrF, 248 nm, 20 Hz, ∼200 mJ, ∼2 J/cm2 , incident angle 45◦

O2 , He,Ar, or Kr was bubbled through a water bath at rate 10 sccm

In water vapour-enriched inert gas environments, deposition of hydroxylapatite was observed at temperatures between 400◦ C and 700◦ C and tetracalcium phosphate at temperatures above 700◦ C

Cotell (1993) [885]

HA and natural apatite

Ti-6Al-4V, Ti (RT)

KrF, 248 nm, 20 ns, 10 Hz, 300 mJ, spot 0.4 × 2.2 mm, 0.5–10 J/cm2

Scanned laser beam, residual gas pressure 2–100 Pa (no gases added)

Coatings of thickness 0.2–1.2 µm were deposited; dependence of the density of macroparticles and chemical composition of films on laser fluence, distance between target and substrate, and residual gas pressure are presented

Bagratashvili (1995) [884]

HA

Ti-6Al-4V, fused silica (200–780◦ C)

KrF, 248 nm, 30 ns, 10 Hz, 3–7 J/cm2

The deposition was performed in Ar/water vapour environment (Ar flow 9–18 sccm, water vapour flow 0.7–10 sccm)

0.4–1 µm HA films were deposited; deposition rate was around 0.1 nm/pulse; the best crystallinity films were achieved at 600–700◦ C and high water ratio; surface micrographs and XRD spectra of the films at various deposition conditions are presented; the adhesion of the films was generally good except if deposited at high temperatures (780◦ C) and low water content ambient

Jelínek (1996) [887]

HA (powder pellet)

No

KrF, 248 nm, 30 ns, 10 Hz, 2.6 J/cm2 , incident angle 45◦

The deposition was performed in vacuum chamber, water vapour pressure 0.1 mbar

Optical emission intensity and spectrum, and temporal Serra (1998) evolution of HA laser ablation plume was investigated; [889] three distinct components were identified: a fast shock wave generating component including Ca and P ions, a intermediate faint component and a slow micrometre-size particulates component

HA (powder pellet)

Ti-6Al-4V (575◦ C)

ArF, 193 nm, 10 Hz, 3.5 J/cm2 KrF, 248 nm, 10 Hz, 3.5 J/cm2

Water vapour pressure in vacuum chamber was 0.15–1.5 mbar; number of laser shots 15 000

HA coatings of thickness of 2–3 µm were deposited; pure HA phase was obtained using ArF laser; in case of KrF laser, best films (highest degree of hydroxylation and best crystalline properties) were achieved at water vapour pressure of 0.5 mbar

FernándezPradas (1998) [888]

(Continued)

Table 6.3

(Continued) Laser or other light source Other features of and beam parameters the experiment

Targets

Substrates

Novel features, observed phenomena, comments

References

HA (sintered)

ArF, 193 nm, 20 ns, Ti-6Al-4V, Si (100); (485◦ C) 10 Hz, 0.8 J/cm2

Pressures of water vapour: 0.15–0.8 mbar

HA coatings of thickness of 0.85 µm were deposited; Arias (1998) the dependence of the coatings composition on water [883] vapour pressure was investigated by FT–IR spectroscopy

TiO2 (SC, rotating target)

p-Si (100), Pyrex glass; (300◦ C)

KrF, 248 nm, up to 6 J/cm2

O2 ,Ar, or water vapour environment

Deposition in 0.55 mbar water vapour was found to Doeswijk provide lowest density of states at TiO2 /Si interface (1999) [881] and the largest lifetime of charge carriers (27.8 µs); the optimal laser fluence at target was 2 J/cm2

HA (powder pellet)

No

KrF, 248 nm, 30 ns, spot 0.8 × 3.1 mm, 2.6 J/cm2 , incident angle 45◦

Ne or water vapour environment (0.1 mbar)

The dynamics of found in Serra (1998) [889] laser plume components was investigated by high-speed photography at different wavelengths: (i) Ca: 520 nm, (ii) atomic oxygen (O): 777 nm, velocity 20 km/s, (iii) Cax Oy : 600 nm, velocity 2.3 km/s

Serra (1999) [890]

HA (powder pellet)

No

Nd;YAG, 355 nm, 10 ns, 1.5 J/cm2

Vacuum, Ne (0.1 mbar) or water vapour (0.1 and 0.2 mbar) environment

Images of laser plume obtained in water vapour revealed that species are confined by the background gas leading to the formation of a planar shock wave at 0.1 mbar and a spherical shock wave at 0.2 mbar; in both cases the presence of chemical reactions with the background atmosphere leads to the formation of calcium oxide radicals that become the dominant emissive species in the plume

Serra (1999) [891]

HA (powder pellet)

Ti-6Al-4V (20–600◦ C)

Nd;YAG, 355 nm, 10 ns, 10 Hz, 73 mJ, 3.1 J/cm2 , incident angle 45◦ , 18 000 shots

Water vapour environment (10–45 Pa)

HA coatings of thickness of 1–4 µm and of surface roughness of 0.4 µm were fabricated; coatings deposited at substrate temperatures under 400◦ C were amorphous; at over 500◦ C and 10 Pa H2 O, the coatings contained crystalline phases rich in calcium, as CaO and Tetra CP; coatings deposited at 45 Pa H2 O were composed of HA and a-TCP; scratch test results are presented as well

FernándezPradas (2000) [892]

HA (sintered)

Si (111)

ArF, 193 nm, 20 ns, 20 Hz, spot 1.4 × 3.2 mm, 1.6 J/cm2

Water vapour environment (45 Pa)

Thickness distributions of deposited coatings was determined at target-substrate distances 9–48 mm; the coatings were more homogeneous at greater distances, while at shorter distances the coatings also contained undesired phases and surface damage

Arias (2002) [893]

HA (sintered)

No

ArF, 193 nm, 20 ns, 20 Hz, spot 1.4 × 3.2 mm, 0.9 J/cm2

Ar, O2 , or water vapour environment (15–80 Pa)

HA ablation rate was measured in dependence of the kind of ambient gas and its pressure; for water vapour the ablation rate sinks linearly from 122 nm/pulse (15 Pa) to 108 nm/pulse (80 Pa); in O2 ambient, the ablation rate was ∼125 nm/pulse and in Ar close to that in H2 O

Arias (2003) [894]

HA (sintered)

Ti (300–460◦ C)

ArF, 193 nm, 20 ns, 20 Hz, 1.2 J/cm2

Water vapour environment (45 Pa, 25 Pa · l/s); DC discharge 0–60 mA

Using electric discharge, crystalline HA coatings could be obtained at lower temperatures (as low as 300◦ C), due to both the higher incorporation of OH− in the coatings (higher H2 O dissociation by the ionization current) and the higher mobility and ionization of the particles on the substrate (provided by the electron bombardment of the coating during its growth)

Jiménez (2004) [895]

Notations HA – hydroxylapatite, hydroxyapatite, Ca10 (PO4 )6 (OH)2 XRD – X-ray diffraction DC – direct current RT – room temperature (∼20–25◦ C) SC – single crystalline FT-IR – Fourier transform infrared spectroscopy CP – Calcium phosphate TCP – tricalcium phosphate, Ca3 (Po4 )2

Table 6.4

Laser ablation deposition using liquid targets and related research (examples).

Targets

Substrates

Laser or other light source and beam parameters 2

Other features of the experiment

Novel features, observed phenomena, comments ◦

References

Molten Ge

Si, CdTe, GaAs, NaCl (25–300◦ C)

CO2 , 100–120 J/cm

Deposition was performed in vacuum

Ge films deposited from molten Ge on 300 C substrates were smooth, single crystalline and epitaxial; at RT dense, low stress, bulk refractive index, and very low optical absorption films were achieved; use of liquid Ge target completely elilminated the generation and ejection of particulates

Sankur (1989) [897]

Santovac 5 vacuum oil (a polyphenyl ether)

Si (100), ∼600◦ C

ArF, 193 nm, 10 Hz, 220 mJ, spot 2 × 5 mm

Experiment was performed in vacuum chamber, O2 flow 3 sccm

DLC film with a large amount of sp3 bondings was obtained, deposition rate was 0.1 Å/pulse; incomparison with a PMMA target, no particulates were found in the deposited films

Xiao (1995) [842]

Santovac 5 vacuum oil

Stainless steel (above the liquid surface)

ArF, 193 nm, 10 Hz, 220 mJ, focused beam

Ablation/deposition performed in O2 /H2 O2 vapour mixture (200 mbar)

120 nm thick film containing cubic diamond particles was obtained after 12 000 laser pulses; film deposition rate was 0.1 Å/pulse; hydroxyl ions (OH− ) obviously promote the diamond growth

Xiao (1995) [841]

In (solid 300 K, and liquid 600 K)

No

KrF, 248 nm, 15 ns (up to 180 mJ/cm2 ) and 0.5 ps (up to 19 mJ/cm2 ), 10 Hz, spot 0.3 mm

Experiment was performed in vacuum

The threshold fluence of liquid In ablation by 15 ns pulses was 30 mJ/cm2 (100 mJ/cm2 for solid In); with 0.5 ps laser pulses the ablation threshold was the same for both solid and liquid metal, 2.5 mJ/cm2 , but above the threshold, the ablation was more efficient for liquid In

Götz (1997) [898]

Si, Ge (molten and solid), Cu (solid)

No

KrF, 248 nm, 25 ns;ArF, 193 nm, 17 ns; 1–8 J/cm2

Experiment was performed in vacuum

Kinetic energy distribution of the ions ejected from the targets was studies by TOF technique; the most probable kinetic energy has values of several tens of electronvolts for singly charged ions, and was larger by a factor exceeding 2 for doubly charged ions

Franghiadakis (1999) [899]

Molten Sn, Bi

No

ArF, 193 nm, 18 ns, spot 0.5 × 1.6 mm, up to 5.5 J/cm2

Experiment was performed in vacuum

High-speed photographs of material ejection from targets are presented; the velocity of the front of the ablated plume was approximately 6 km/s for both Sn and Bi at 5.5 J/cm2 ; laser irradiation excites surface waves (radial velocity ∼1 m/s), in case of Bi also droplets emission; relaxation times of wave processes were ∼0.3 and ∼1.2 s for Sn and Bi, respectively

Tóth (1999) [900]

Molten Ga,Al-Ga

Si (111), 640–740◦ C

KrF, 248 nm, 17 ns, 8 and 12 Hz, spot size 0.1 and 0.15 mm, 3.5–6.4 J/cm2

Pulsed N2 or NH3 ambient, ∼400 µs, 2.5 × 1017 molecules per pulse; non-wetting glass-ceramic crucibles were used for molten targets

GaN (0001) and Al x Ga1−x N (0001) films of thickness up to 3.37 µm (75 000 laser pulses) were achieved, without the need for a GaN or AlN buffer layer; the growth rate ranged Pth :    2Ep 2r 2 . − 2 tth = τe ln 3 2 2 Pth π / τe w (z) w (z)

(7.27)

7.1.2 Reflection of light (normal incidence) Reflectivity of a vacuum–medium interface (n − 1)2 + k2 , (n + 1)2 + k2 where n is the refractive index indicating the phase velocity and k is the extinction coefficient:   1 n2 = · ε21 + ε22 + ε1 , 2   1 k2 = · ε21 + ε22 − ε1 . 2 ε1 and ε1 are the components of the complex dielectric permittivity, R=

ε = ε1 + iε2 .

(7.28)

(7.29)

(7.30)

(7.31)

k is related to the (linear) absorption coefficient a by a=

2ωk . c

(7.32)

Reflectivity and transmittance of an interface between two media When light is propagating from a medium with refractive index n0 to another medium with refractive index n1 , the reflectivity R and transmittance T is given as   Ir n0 − n1 2 R= (7.33) = I0 n0 + n 1 It = 1 − R, (7.34) I0 where I0 is the incident, Ir is the reflected, and It is the transmitted light intensity. Reflectivity of still water surface to visible light at normal conditions and normal incidence is about 2 per cent. T =

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Reflectivity of two parallel interfaces (e.g. air–water–solid) For light with coherence length smaller than that the distance between the interfaces, T =

T 1 T2 τ , 1 − R 1 R2 τ 2

(7.35)

where T1 is the transmittance of the interface 1, R1 is the reflectance of the interface 1,T2 is the transmittance of the interface 2, R2 is the reflectance of the interface 2, and τ is the transmittance of the medium between the interfaces. According to calculations by Kim and Lee [467], a water layer on aluminium increases the overall surface absorptivity from 0.08 to 0.108.

Reflectivity of liquid–plasma interface The reflectivity at the liquid–plasma interface, Rlp , can be calculated in frames of Drude model as [931, 259] 2  2  npl − nl + kpl − kl (7.36) Rlp =  2 , 2  npl + nl + kpl + kl where npl = kpl =



ε′ +

√

ε′2 + ε′′2 , 2

ε′′ , 2npl

ε′ = 1 − ε′′ = γ

(7.38) ωp2

ω2 + γ 2 ωp2



(7.37)

ω ω2 + γ 2  ne e 2 , ωp = me ε 0  8kB Te γ = np σc πme

,

(7.39)

,

(7.40)

(7.41)

(7.42)

where ε′ and ε′′ are the real and imaginary parts of the dielectric function of plasma, ω is the laser frequency, ε0 is the dielectric constant of vacuum, e is the electron charge, ωp is the plasma frequency, npl and kpl are the real and imaginary parts of the refractive index of plasma, nl and kl are those for liquid, respectively, ne and np are electron and particle density, respectively, γ is the electron-particle collision frequency, and σc is electron-particle collision cross-section.

Light pressure Light (or acoustic) pressure on an interface perpendicular to the propagation direction of the light (sound) is given by I p = W (1 + R) = (1 + R) , (7.43) c where W is the energy density, I is the intensity of light (sound), and R is the reflection coefficient.

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Physics and chemistry of laser–liquid–solid interactions

p = ε0 E 2 (1 + R) = μ0 H 2 (1 + R) .

In vacuum:

(7.44)

7.1.3 Propagation of Gaussian beams Transformations of a Gaussian beam in a paraxial linear optical system may conveniently be described by the ABCD-method. Below, an example of finding geometrical relations for a laser beam focused onto a workpiece in liquid is given. If the window is absent, the parameter g should be taken equal to zero (Fig. 7.4). The ABCD-matrix for the interval between the beam waists w0 and wwp (interval a-b-g-h) is given as product of the ABCD-matrices of homogeneous intervals and interfaces [932]: 

A C

B D



=



1 h · 0 1

1 0

0 ng nl

  1 · 0



⎡

1

g ·⎣0 1

⎤  0 1 ⎦· 1 0 ng

 1 b 1 · 1 − F

⎤ g hF g h ab ag ahF b − − a + b + + − − + 1 − ⎢ F ng F nl ng nl F ng F nl ⎥ ⎥. =⎢ ⎦ ⎣ F 1 aF − − nl nl nl Using complex beam parameter q defined for a medium with refractive index n as ⎡

1 1 λ0 = , −i q R (z) nπw 2 (z)

  1 · 0 1 h

a 1



(7.45)

(7.46)

the parameters q0 and qwp at the locations 0 and wp are related as: qwp

Aq0 + B = Cq0 + D

or

1 qwp

   C + D 1 q0   . = A + B 1 q0

(7.47)

Taking into account that in our model at the boundaries of the interval a-b-g-h, R(z) = ∞ (Eq. (7.10)), from (7.46) follows: 1 λ0 = −i 2 , q0 πw0

(7.48)

1 λ0 = −i , 2 qwp nl πwwp

(7.49)

and

n1

Focusing lens

ng

wwp

h

w0

Beam expander (optional)

Laser

F

g

b

a

Figure 7.4 Propagation of a Gaussian beam in a model system of liquids-assisted laser processing. In comparison with focusing in air, in liquid the focus spot lies more away from the laser.

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Handbook of Liquids-Assisted Laser Processing

Using (7.47), 0D C − i λπw 2 λ0 πw02 C − iλ0 D 0 −i = = . λ0 B 2 nl πwwp πw02 A − iλ0 B A − i πw 2

(7.50)

0

The equations for real and imaginary parts of (7.50) are: Re: π2 w04 AC + λ20 BD = 0,

(7.51)

2 (AD − BC) = 0. Im: π2 w04 A2 − λ20 B 2 − π2 nl w02 wwp

(7.52)

Equations (7.45), (7.51), and (7.52) relate the geometrical and material parameters of the model a, b, g, h, w0 , wwp , λ, F, nl , ng .

7.2 Phase Change Phenomena 7.2.1 Overall phenomenology On nanosecond–microsecond time scale, a typical laser-generated transient at a liquid–solid interface looks like in Fig. 7.5. Immediately after laser energy absorption the leading front of excitation may be regarded 1D; later spherical. Decay phase of the bubble is presented in more detail in Fig. 7.6. Nomenclature r1 , r2 – liquid–vapour interface curvatures in two perpendicular planes containing the normal to the interface T – thermodynamic temperature T0 – ambient temperature Tv – temperature of the vapour Tvl – temperature difference across liquid–vapour interface p0 – ambient pressure pg – gaseous phase (vapour) pressure pl – ambient liquid pressure ρ, ρ1 – density of the liquid (a) Silica glass

Laser beam

Toluene loquid Shock wave

(b)

(c) Shock wave

Shock wave Bubble

200 ␮m

(d)

200 ␮m

200 ␮m (f)

(e)

Bubble 200 ␮m

200 ␮m

200 ␮m

Figure 7.5 Time-resolved optical micrographs of laser ablation of toluene liquid through a glass plate at the delay times of (a) 100 ns, (b) 500 ns, (c) 1.2 µs, (d) 10 µs, (e) 50 µs, and (f) 100 µs [607]. Laser: 248 nm, 30 ns, 1.6 J/cm2 pulse−1 . © Elsevier.

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Physics and chemistry of laser–liquid–solid interactions

40 ␮s delay

400 ␮s delay

800 ␮s delay

Figure 7.6

Bubble decay at laser ablation of alumina under water [469]. © Elsevier.

ν1 – special volume of liquid σ – surface tension σˆ – accommodation coefficient, ranges 0.02–0.04 for water and lower alcohols [933] α – vaporization coefficient m – particle (atom or molecule) mass Hv – latent heat (enthalpy) of vaporization per unit mass qi′′ – heat flux across liquid–vapour interface Vlv – change of molecular volume at vaporization, Vlv = Vv − Vl J – nucleation rate (number of nuclei per unit volume and time) αl – thermal diffusivity of the liquid kB – Boltzmann’s constant, kB = 1.3806505(24) × 10−23 J/K Rg – universal gas constant, Rg = 8.3144 kJ/(kg mol K).

7.2.2 Vaporization from free liquid surfaces Equilibrium vapour pressure (saturated vapour pressure) Clausius–Clapeyron equation (defines the slope of the vapour pressure curve):

Saturated vapour pressure:

dp Hv . =  dT T Vg − V l 

Hv m ps (T ) = p0 exp Rg T0



T0 1− T

(7.53)



(7.54)

Dependence of vapour pressure on surface curvature Pressure difference across a curved liquid–vapour interface is given by the Laplace equation (Young–Laplace equation):   1 1 P g − Pl = σ . (7.55) − r1 r2

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Handbook of Liquids-Assisted Laser Processing

Vaporization/condensation rate Hertz–Knudsen equation: j = σˆ



m [ps (TR ) − pv ], 2πRg TR

(7.56)

where j is the intensity of vaporization or condensation (particles per unit area), M is the liquid molar mass, TR is the temperature at the vapour–liquid interface, and ps is the saturation vapour pressure corresponding to the temperature TR .

Velocity of surface recession at vaporization [934] 

∂x ∂t



x=0

≈ αpb exp



Hv m kB



1 1 − Tb T



×

m . √ ρl 2πmkB T

(7.57)

Heat flux to the liquid–vapour interface [554] qi′′

=



2σˆ   2 − σˆ



Hv2 Tv Vlv



m 2πRg Tv

  Pv Vlv 1− Tvl . 2Hv

(7.58)

Heat transfer coefficient of a liquid–vapour interface: hi′′ =

qi′′ . Tvl

(7.59)

7.2.3 Nucleation of vapour bubbles Definitions Homogeneous nucleation – nucleation in the interior of a uniform substance. Heterogeneous nucleation – nucleation at interfaces or inclusions. Critical nucleus size – nuclei of size smaller than critical shrink spontaneously, and of greater size grow spontaneously. Critical nucleation rate Jcr – rate of nucleation of critical nuclei. Binodal (vapour pressure curve) – the line on the phase diagram where the liquid and vapour are the thermodynamically stable phases. Spinodal – locus of states of infinite compressibility (∂p/∂V )T = 0; spinodal is the boundary of unstable and metastable regions on state diagram. Fluctuations in density, however small they are, will grow spontaneously. Kinetic spinodal (cloud line) – locus in the phase diagram, where the lifetime of metastable states becomes shorter than a relaxation time to local equilibrium. If the surface tension is known, the physical boundary of metastable states in this approach is completely determined by the equation of state only, (i.e. by the equilibrium properties of the system). Fisher limit – homogeneous nucleation limit derived by Fisher [935]; depends on the size of the volume under consideration and the duration of the applied stress. Phase explosion (Explosive boiling) – sharp increase of homogeneous nucleation in a superheated liquid.

Homogeneous nucleation Homogeneous nucleation of vapour bubbles occurs if the state of the liquid crosses a certain curve in the pressure–temperature diagram (Fig. 7.7). Different theories predict different nucleation limits; in case of heating by nanosecond and shorter laser pulses, the kinetic spinodal is closest to the observed nucleation onset. Homogeneous nucleation rate (nuclei per time and volume unit) is given by [554]:    16πσ 3 3σ exp − (7.60) J = N0  2 , πm 3kB Tl ηpsat − pl

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Physics and chemistry of laser–liquid–solid interactions

25

50 C.P.

H2O

20

Pressure (MPa)

0

C.P.

Binodal Spinodal Kinetic spinodal Nucleation limit (Skripov) Nucleation limit (Zheng)

15 ⫺50 10 Binodal Kinetic spinodal Spinodal Nucleation limit (Skripov) Nucleation limit (Zheng) Fisher’s theory

⫺100

⫺150

⫺200

300

400

500

5 H2O

0

600

700

⫺5

560

Temperature (K)

580 600 620 Temperature (K)

640

Figure 7.7 Calculated pressure of liquid water along the binodal, spinodal, and kinetic spinodal as a function of temperature [936]. The dotted curve corresponds to the nucleation limit in Fisher’s theory, the circles indicate experimental data of Skripov and Chukanov, and the triangles indicate the experimental data of Zheng recalculated in P −T coordinates with the analytic equation of Soul and Wagner. © Elsevier.

where η ∼ = exp



 νl  pl − psat (Tl ) . RTl

(7.61)

Feder et al. [937] and Dömer and Bostanjoglo [938] presented an improved formula for nucleation rate for phase explosion situations, taking into account the presence of a Knudsen layer at the liquid–gas interface. The liquid, superheated to a temperature T , was assumed to be exposed to the recoil pressure 0.54 ps (T ) of atoms evaporating into a Knudsen layer, with ps (T ) being the saturated vapour pressure at temperature T . The equilibrium temperature TE is then determined by ps (TE ) = 0.54 ps (T ). The vapour was approximated by an ideal gas. Then the stationary homogeneous nucleation rate of critical bubbles becomes

   Hv 16πkB T σ 3 ρ (T ) 2σ (7.62) N˙ = exp − exp −  2 , 0.54 m πm kB T 3 0.54ps (T ) g where Hv (T ) is the atomic evaporation enthalpy, g =



T Tg

   Hv T dT .

(7.63)

However, in laser processing situations, for example in cleaning, the exact value of nucleation rate is of minor influence on the experimentally observable nucleation threshold, because the exponential rise of the nucleation rate with superheating leads to an extremely sharp increase over many orders of magnitude within a narrow temperature interval [80]. In water and lower alcohols under typical circumstances, for example, a temperature increase of 1o C causes the nucleation rate to increase three orders of magnitude [554].

Heterogeneous nucleation Heterogeneous nucleation rate is given by [554]: N 2/3 (1 + cos θ) J= 0 2F



3Fσ πm

1/2



 16πFσ 3 exp −  2 , 3kB Tl ηpsat − pl

(7.64)

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Handbook of Liquids-Assisted Laser Processing

Distance from the Au surface (nm)

20

15

10

5

0 90 ps

115 ps

140 ps

165 ps

Figure 7.8 Snapshots from the molecular dynamic (MD) simulation of 24 water layers on a Au(111) surface suddenly heated to 1000 K. The time between successive frames is 25 ps. © American Chemical Society (2001), Reprinted with permission from Ref. [939].

where F = F (θ) =

 1 2 + 3 cos θ − cos3 θ , 4

(7.65)

2/3

where θ is the contact angle of the liquid at the interface and N0 is the number of molecules per unit area at the interface. In laser cleaning, a nucleation rate Jcr = 1022 m−3 s−1 was measured at Si–water interface [80]. Likely to homogeneous nucleation, also here an exponential increase over many orders of magnitude in a very narrow temperature interval is observed, giving rise to a relatively sharp nucleation threshold [80]. At intense short pulse irradiation of a solid–liquid interface, no vapour bubbles, but a continuous vapour layer formation is observed and predicted by MD-simulations (Figs 2.37, 2.38, and 7.8).

7.2.4 Bubble dynamics Bubble in an infinite space Bubbles created in bulk liquid by laser pulses or by cavitation, expand and shrink periodically as shown in Fig. 7.9. In many liquids, thereby in water and in alcohols, the bubble emits a short light pulse (sonoluminescence) and shock wave every time it collapses. Dynamics of a spherical bubble much smaller than the sound wavelength is given by Rayleigh–Plesset equation [941]:    d2 R 2σ 4η dR 3 dR 2 1 R 2 + pg − P0 − P (t) − − = , (7.66) dt 2 dt ρl R dt R with notations: ρl is the density of the liquid, pg is the pressure in the gas, assumed to be spatially uniform, P0 is the background static pressure (usually 1 bar), P(t) is the pressure in the neighbourhood of the bubble, η is the shear viscosity, and σ is the surface tension of the gas–liquid interface. The bubble growth velocity becomes [554] dR = dt



2 [T0 − Ts (P0 )] Hv ρv · . 3 Ts (P0 ) ρl

(7.67)

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Bubble

Shock wave

R

t

Figure 7.9 Bubble pulsation in a bulk liquid. Every time the bubble collapses, a short light pulse (∼150 ps) and a shock wave are generated in many liquids, thereby in water. After Isselin et al. [477] and Brujan [940].

Equation (7.66) does not consider the energy dissipation through heat conduction, viscosity, etc. Leiderer et al. [80] achieved better match with experiment by d2 R 3 R 2 + dt 2



dR dt

2

     3γ R0 1 t = exp − − p0 , pmax ρl R τ

(7.68)

where γ is the polytropic exponent. The energy loss was accounted by the relaxation time τ; τ = ∞ corresponding to the adiabatic model.

Bubble at a heated surface Carey [554] gives a formula for bubble growth on a heated surface (constant temperature): √ R(t) = 0.470 Ja Pr l−1/6 αl t, where Ja is the Jakob number, Ja =

(7.69)

[T0 − Ts (P0 )]Cpl ρl , ρv H v

(7.70)

ν , α

(7.71)

and Pr is the Prandtl number, Pr =

where ν is the kinematic viscosity and α is the thermal diffusivity. Heat transfer controlled growth of a hemispherical bubble on a heated surface has been analysed numerically by Robinson and Judd [942]. Veiko et al. [156] present a differential equation for the equilibrium shape of a bubble on an heated surface, taking into account the wetting angle.

Bubble decay at interfaces When a bubble collapses at a solid boundary, a liquid jet, directed to the boundary develops (Fig. 7.10). At millimetre-size bubbles the jet velocity ranges up to 200 m/s (depends on bubble radius and on the distance to the wall) and can cause damage even of hard materials (cavitation erosion) [471, 943].

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t

Figure 7.10 Collapse of a gas/vapour bubble near a rigid boundary. Schematically after Blake et al. [946]. The jet diameter is about one-tenth of the bubble initial diameter. As the investigations by Tomita and Shima [947] indicate also hemispherical bubbles generate a liquid jet at solid boundary.

Liquid jet is formed also at bubble (gravitational) detachment from a heated surface [944] and at bubble collapse near a free liquid surface [945]. The impact pressure of the liquid jet is given by the formula [609, 477]: P=

ρl Cl · ρs Cs · vjet , ρl C l + ρ s C s

(7.72)

where (ρl Cl ), (ρs Cs ) are, respectively, the acoustical impedances of water and solid material. For a perfectly rigid wall an assumption (ρl Cl ) ≪ (ρs Cs ) can be made. Thus Eq. (7.72) becomes: P = ρl · Cl · vjet .

(7.73)

Chen et al. [948] measured the microjet impact pressure 320–490 MPa for laser pulse energy in range of 5–22 mJ (iron in water; laser: 1064 nm, 30 ns). Bubbles collapse induced flow near a solid boundary was investigated by Ohl et al. [19]. The tangential to boundary flow velocities were highest during the time interval of jet impact and ranged up to ≈10 m/s at maximum bubble size of 2 mm.

Relict microbubbles After a bubble decays near a solid boundary, many microbubbles with initial radii between 5 and 150 µm remain for hundreds of microseconds [10, 949]. The next laser-induced pressure transient forces these bubbles to collapse, causing a plurality of small cavitation erosion pits over an extended area around the initial bubble epicentre [477]. The lowering of acoustical cavitation threshold by relict microbubbles is called memory effect in cavitation. Antonov et al. [950] observed that also after optical breakdown in bulk water the breakdown threshold for successive pulses remained ∼3 times lower than the initial threshold (Nd:YAG-laser, 15 ns). The initial threshold recovered in a day. According to Bunkin and Bunkin [951], if a liquid with dissolved gas contains small amounts of electrolytes (in concentrations of ∼0.01 ppm), whose ions have surface-active properties, under equilibrium conditions it should contain stable microbubbles of a free gas (called ‘bubbstons’). Thus, after optical breakdown the water decomposition products may form long-live bubbstons that lower the breakdown threshold for successive pulses.

Chemical reactions induced by bubble collapse Temperature in collapsing bubbles is estimated to rise up to 6000–20 000 K [952], which causes the dissociation of the liquid. According to Mason and Peters [953], the following reactions occur at bubble collapse in pure water: H2 O H. + O2 2 HO. 2 HO.2

→ → → →

HO. + H. HO.2 H 2 O2 H2 O2 + O2

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7.3 Optical Breakdown of Liquids and Plasma Nomenclature ne – electron density ρv – electron density in valence band (of the liquid) ρc – electron density in conduction band (of the liquid) ni – ion density na – density of neutral atoms gi – partition function for single ionized atoms ga – partition function for neutral atoms m, me – electron mass, 9.1093826(16) × 10−31 kg kB – Boltzmann’s constant, kB = 1.3806505(24) × 10−23 J/K h – Planck’s constant, h = 6.6260693(11) × 10−34 Js  – Dirac’s constant,  = h/2π = 1.054 571 628(53) × 10−34 J s T – thermodynamic temperature Tp – plasma temperature; it is assumed here Tp = Te = Ti ν – frequency ω – angular frequency ε0 – dielectric permittivity of vacuum, ε0 = 8.8541878176 × 10−12 F/m c – speed of light In dielectric liquids, which are of main interest in laser processing, the ionization (plasma formation) is possible by (1) direct ionization of the liquid by multiphonon or tunnel ionization, and/or by (2) cascade ionization (avalanche ionization) via inverse Bremsstrahlung absorption. The latter mechanism needs one or more ‘seed’ electrons generated by thermal ionization of impurities or by multiphonon ionization, depending on the purity of liquid (after Sollier et al. [260]).

7.3.1 Photoionization of a dielectric liquid For photon energies below the ionization potential (for water, E = 6.5 eV), free electrons have to be generated by multiphoton or tunnel ionization. The time-averaged ionization rate for a field with angular frequency ω and intensity I acting on an electron density ρv − ρc in the ground state is given by Keldysh equations [954] 

3/2

  ˜  2ω 1 + γ 2 mω dρc × Q γ, = dt photo 9π γ  ω    ⎫ ⎧ γ ⎪ ⎪ ( ) K √γ ⎪ ⎪ √ − E ⎨ ⎬ ˜ 1+γ 2 1+γ 2    × (ρv − ρc ) exp −π , +1 × ⎪ ⎪ ω ⎪ ⎪ ⎩ ⎭ E √1

(7.74)

1+γ 2

where

. Q (γ, x) = . . /

π 2K



√1

1+γ 2

  ⎫  ⎧ γ γ ⎪ ⎪ ⎪ ⎪ √ √ − E K ∞ ⎨ ⎬  1+γ 2 1+γ 2 ×   exp −πn ⎪ ⎪ ⎪ ⎪ n=0 ⎩ ⎭ E √1

. . ×. /

1+γ 2

π (2 x + 1 − 2x + n)     1 1 √ √ E 2K 2 2 1+γ

1+γ

(7.75)

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Here x represents the integer part of the number x, K and E denote elliptic integrals of the first and second kinds, and  denotes the Dawson probability integral,  (z) =

z 0

  exp y2 − x2 dy.

(7.76)

At room temperature the initial steady-state free electron density in the conduction band resulting from the Boltzmann distribution is negligible. Thus, the steady-state electron density in the ground state corresponds to the total electron density ρv = 6.68 × 1023 cm−3 [955]. ˜ for creating an electron–hole pair in The Keldysh parameter γ and the effective ionization potential  condensed matter exhibiting a band structure (e.g. water) are given by  ω cε0 m γ= (7.77) e 4I and

 1 2 1 + γ2 ˜ =  E   . π γ 1 + γ2

(7.78)

where I is irradiance and  is bandgap energy.

7.3.2 Cascade ionization (avalanche ionization) As soon as free electrons exist in the interaction volume, they gain kinetic energy through inverse Bremsstrahlung absorption of photons and can generate further free electrons through impact ionization once their energy exceeds the critical energy. The ionization rate per electron participating in the cascade is then given by (case electron-ion inverse Bremsstrahlung) [955]:   e2 τ 1 mc ω2 τ ei , (7.79) ηIB = 2 2    I− ˜ ω τ + 1 cn0 ε0 mc 3 2  M

where τ is the time between collisions, c is the vacuum speed of light, I is irradiance, and n0 is the refractive index of the medium at frequency ω. The masses of the electron and the liquid molecules are m and M , respectively. For large irradiances, the cascade ionization rate is proportional to I (after Vogel et al. [956]). Net absorption coefficient for electron-ion inverse Bremsstrahlung is given by [957, 959]     ne2 e 6 g ω me ei aIB = 3 1 − exp − , (7.80) kB T p 6ε0 cω3 me2 6πkB Tp

where g is average Gaunt factor √     3 ω ω g (ω, Te ) = K0 , exp π 2kB Te 2kB Te

(7.81)

where K0 (x) is the modified Bessel function. ei may be expressed as [958] Alternatively, aIB    ω Z 2 ni ne ei 1 − exp − , ≈ C · λ3  aIB kB T p Tp

√ 2 2e 6 where C ≈ √ √ ≈ 1.37 × 10−35 when λ is in micrometers. 3 3πc 4 me3/2 k B

(7.82)

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Physics and chemistry of laser–liquid–solid interactions

The electron-atom inverse Bremsstrahlung absorption coefficient is given by Wu and Shin [259]  e 2 ne ni σc 8kB Te ea , aIB = πmcv2 πm where c is the speed of light in vacuum and ni is the total number of ions.

(7.83)

7.3.3 Photoionization absorption coefficients of atoms Photoionization absorption coefficients of atoms produced by thermal dissociation of the liquid, may be calculated as [259, 957]

    ∞  −θa,i θi,a 3 2 −22 1    , exp na api = 7.9 × 10 (7.84) Z 2 nhν ga kB T p 1 − 1 n 2 n=n 1

where

  n = integer θi,a hν .

(7.85)

and θa,i is ionisation potential of particle i. Equation (7.85) states that the lower limit in summations is determined from the condition that the photon energy is greater than the binding energy of the electron in the atom. The total absorption coefficient at is the sum of the electron-ion and electron-atom inverse Bremsstrahlung absorption coefficients and of photoionization absorption coefficient, ei ea + aIB + api . at = aIB

(7.86)

7.3.4 Thermal ionization Near laser heated surface, the generation of plasma by thermal ionization is the usual case. Equilibrium electron and ion concentrations in plasma are expressed by Saha’s equation [959]:   3  2gi 2πme kB Tp 2 n e ni θi , (7.87) = exp − na ga h2 k B Tp where θi is ionisation energy of atom i, gi is the electronic partition function of ion, gi = 1, and ga is the electronic partition functions of atom, given by    n∗  θi 1 2 ga = 2n exp − 1− 2 , (7.88) kB T p n n=1

where

n ∼ = ∗

√ Z 3 np a0

,

(7.89)

and a0 is the Bohr radius.

7.3.5 Diffusion loss of electrons from the plasma The diffusion loss of electrons depends on the shape of the plasma region. In bulk liquid, the plasma region may be considered elliptical (Fig. 7.3). In the model of Kennedy [955], the ellipsoid was approximated with a cylindrical volume with radius w0 (beam waist radius) and length zR = πw02 /λ (Rayleigh length of the laser beam). This led to the following expression for the diffusion rate per electron    2  2τεav 1 2.4 2 ηdiff = + . (7.90) 3me ω0 zR The same equation may be applied also in case of irradiation of a solid surface in liquid, using instead zR the actual thickness of the plasma.

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7.3.6 Recombination loss At calculation of optical breakdown in water,Vogel et al. [956] used for the recombination rate an empirical value determined by Docchio through inspection of the decay of the plasma luminescence [960],   dρC = −2 × 10−9 cm3/s × ρc2 . (7.91) dt rec In reality, recombination of free electrons in water is not a one-step process but consists in hydration of the electron within about 300 fs and subsequent decay of the hydrated state that has an average lifetime of ≈300 ns [961].

7.3.7 Thermal conductivity of the plasma The plasma electron conductivity λe can be calculated by the Spitzer–Härm expression [962], λe = δT 20

 3/2 (kB Te )5/2 kB 2 , √ 4 π me Z (ln )

(7.92)

where ln  is the Coulomb logarithm, =

3

,  2e 3 kB3 Te3 πne

(7.93)



Z is the average charge of ion and δT = 0.225 when Z = 1. When  < 1, the Spitzer–Härm expression is not valid, and thermal conductivity can be calculatedas [963] 5 λe = 2



2 kB (kB Te )5/2 √ π me Ze 4 R

where R =



√ −1 52 16 2 + , 15 15 Z

1 η − 1 − ln η · , 2 (1 − η)2 λ=

λD = 

η=

1 , λ2

λD , bc kB T  , 2 Z + Z i

(7.94)

(7.95) (7.96) (7.97)

4πe 2 n

bc =

Ze 2 . 3kB T

(7.98)

7.3.8 Rate equation for free electrons The time evolution of the electron density ρc in the conduction band of a liquid under the influence of the laser light is in generic form given by [964, 956]           dρc dρc dρc dρc dρc dρc = + + + + dt dt photo dt therm dt casc dt diff dt rec =



dρc dt



photo

+



dρc dt



therm

+ ηcasc ne − ηdiff ne − ηrec ne2 .

(7.99)

Physics and chemistry of laser–liquid–solid interactions

299

Definitions of the terms: 1st term: production of free electrons mediated by the strong electric field in the laser focus (photoionization via multiphoton and tunnelling ionization). 2nd term: production of free electrons by thermal ionization. 3rd term: production of free electrons by cascade ionization. 4th term: diffusion loss of free electrons. 5th term: recombination loss of free electrons. The cascade ionization rate ηcasc and the diffusion loss rate ηdiff are proportional to the number of already produced free electrons, while the recombination rate ηrec is proportional to ρc2 , as it involves an interaction between two charged particles (an electron–hole pair) (citation from Vogel et al. [956]). One speaks of optical breakdown when a critical free electron density of 1018 − 1020 cm−3 is exceeded during the laser pulse [260].

7.3.9 Internal energy density of electrons and particles in plasma [965, 259] Ee = Ep =

 3 n e k B Te + ni θi 2

3 np kB Tp + np,0 El,diss 2

(7.100) (7.101)

where summation in (7.100) is over all particles in the plasma, θi is the ionisation energy of atom i in plasma, and El,diss is the total dissociation energy for the molecule of the liquid.

7.3.10 Energy balance equation for electrons Electrons gain energy by absorption of the laser light and loose energy by collision with atoms and ions, via conduction, radiation, and plasma expansion. For a water-confined plasma of thickness L at a solid surface, the energy balance equation for electrons was given by Wu et al. as follows [259]:     d (LUe ) = I 1 − Rwp [1 − exp (−at L)] + I 1 − Rwp exp (−at L) Rc dt    3  ′ σTe4 − kB Te − Tp vtr ne L − qcdc − qcdw − (1 − Rc ) σTe4 − 1 − Rwp 2   − Pe uw,pre + uwev + uc,pre + ucev , vtr =

2me ,   mp,ave np σc 8kB Te πme

(7.102) (7.103)

with notations: L is the thickness of plasma layer, Ue is the energy density for electrons, I is the laser power ′ are the liquid–plasma interface reflectivity to laser and plasma radiation, respectively, density, Rlp and Rlp ′ Rc and Rc are the solid surface reflectivity to laser and plasma radiation, respectively, σ is the Stefan–Boltzmann constant, Pe is the partial pressure of electrons, at is the total absorption coefficient, and qcdc and qcdw are the heat flux conducted from plasma to the solid surface and liquid surface, respectively, vtr is the electron-particle energy transfer frequency, mp,ave is the average particle mass, σc is the electron-particle collision cross section, uw,pre and uwev are the pressure- and evaporation-caused receding velocities of the liquid surface and uc,pre and ucev are the pressure- and evaporation-caused velocities of the solid surface.

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7.3.11 Heat flux conducted from plasma to adjacent matter [966, 967, 259] Wu and Shin [259] used in simlation of laser peening in water confinement the relation   Te − T m kB T e , fne kB Te qc = min λe , 0.5L me 

(7.104)

where Tm is the temperature of the adjacent medium, and f is a dimensionless number ∼0.03–0.1 [966]. The total pressure of plasma P is the sum of the electron partial pressure and the particle partial pressure P = Pe + Pp = kB Te ne + kB Tp np ,

(7.105)

where the subindex p denotes particles. Plasma models used for simulation of laser peening were described in Section 3.3.6.1.

7.3.12 Dependence of optical breakdown threshold on laser pulse length Calculated and measured optical breakdown thresholds for bulk water are presented in Fig. 7.11. Optical breakdown is a stochastic process and it depends on hard to avoid particulate impurities in the liquid; at low laser fluences it may not occur at every laser pulse. The fluence, at which breakdown occurs at every pulse, may be 10 times higher than the minimum fluence at which breakdown becomes possible [968]. Table 7.1 presents a comparison of optical breakdown thresholds of some common solvents.

7.3.13 Factors affecting the breakdown threshold in liquids Optical breakdown thresholds in liquids are lowered by suspending particles [969, 930] and by dissolved gases [970, 971]. Bunkin and Lobeev [968] studied the probability of Nd:YAG-laser breakdown in water in dependence on temperature and on dissolved electrolyte concentration. According to Kennedy et al. [972] the impurities

Breakdown threshold (J/cm2)

104 1013

103 102

1012 101 100

1011

10⫺1 10⫺2

10⫺14

10⫺13

10⫺12

10⫺11

10⫺10

10⫺9

Breakdown threshold (W/cm2)

1014

105

1010 10⫺8

Pulse duration (s)

Figure 7.11 Optical breakdown thresholds for bulk water. The circles are experimental data in W/cm2 . Solid lines are calculated dependencies for 800 nm wavelength using critical electron density ρcr = 1021 cm13 . After Vogel et al. [956].

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Physics and chemistry of laser–liquid–solid interactions

Table 7.1 Relative to water optical breakdown thresholds in various liquids for Nd:YAG-laser pulses (calculated from the data by Bunkin and Lobeev [968]). Liquid

Ith /Ith, water

Water

1.00

Heptane

0.4

Ethanol

0.47

Benzene

0.36

Carbon tetrachloride

0.28

in water affect the breakdown thresholds for pulse lengths greater than 10–100 ps, but not for shorter pulses (1064 nm wavelength). For pure water, the calculations by Vogel et al. [956] showed that laser wavelength starts to influence the breakdown threshold only beginning from 1 to 10 ps pulse length.

7.3.14 Temperatures and pressures at laser breakdown and ablation in water Figures 7.12 and 7.13 and Table 7.6 present some examples of temperatures of laser beakdown and processing plasmas. Data on plasma pressures can be found in Figs 3.16–3.18 and in Table 7.6. Figure 7.14 shows the spatial distribution of luminescence of a laser-induced plasma at a solid–liquid interface. Compared with laser plasmas in air or in vacuum, the confined plasmas by liquids or solids have higher temperature, density, and pressure. The results of some experimental work on laser-generated plasmas at solid–liquid interfaces and in suspensions are summarized in Table 7.6. The observations of Sakka et al. [974, 470] have shown that the typical plasmas occurring at laser processing in liquids are neither thin nor dense – there is a broadened line spectrum with self-absorption reversed dips on a continuous background.

Plasma temperature (K)

20 000

10 000

5000 0.1

1

10

100

1000

Laser pulse energy (mJ)

Figure 7.12 Measured maximum plasma temperature as a function of laser pulse energy at optical breakdown in bulk water. Schematically after Kennedy et al. [972].

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Temperature, T(r,z,t) (K)

15 000 Ramp-up, z ⫽ 0 Ramp-up, z ⫽ 100 nm Pamp-down, z ⫽ 0 Ramp-down, z ⫽ 100 nm Rectangular, z ⫽ 0 Rectangular, z ⫽ 100 nm

10 000

t ⫽ 30 ns

5000

0

0

0.2

0.4 0.6 Radius, r (mm)

0.8

1

Figure 7.13 Calculated by an analytical model radial temperature distributions at a distance z = 0 and z = 100 nm over a laser irradiated iron target in water. Laser: τ = 30 ns, Pave = 50W, spot size r0 = 1 mm; pulse shapes: ramp-up, ramp-down, and rectangular [384]. © Elsevier. 21 ns

40 ns

60 ns

80 ns

100 ns 0.5 mm

Figure 7.14 A series of images of the light-emitting region generated by the irradiation of a pulsed Nd:YAG-laser to a graphite target in water. Exposition time for each frame was 13 ns. A white broken line indicates a rough estimation of the position of the target surface [973]. © Elsevier.

7.4 Shock Waves in Liquids and Solids Nomenclature ρ0 – density ahead the shock front ρ – density behind the shock front m˙ – mass flux of the material passing through the shock wave p0 – pressure ahead the shock front p – pressure behind the shock front e0 – specific internal energy ahead the shock front e– specific internal energy behind the shock front Us – shock velocity up – particle velocity behind the shock front

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u0 – relative to shock front particle velocity ahead the shock front, u0 = −Us u – relative to shock front particle velocity behind the shock front, u = up − Us h – enthalpy ht – total enthalpy v0 – specific volume ahead the shock front v – specific volume behind the shock front εxx – xx-component of the strain tensor σxx – xx-component of the stress tensor μ, λ – Lamé constants. Ŵ – Grüneisen coefficient (Mie-Grüneisen coefficient) Shock waves in liquid-assisted laser processing are commonly considered as a discontinuity of material properties, density, pressure, particle velocity, and internal energy in the space. This is justified by circumstance that the shock front width in liquids and solids is of order of only few angstroms. The properties of matter at both sides of the shock front are related by following conservation relations [975]: Conservation relations Conservation of mass:   ρ0 Us = ρ Us − up = m˙ (7.106) Conservation of linear momentum:

p − p0 = ρ 0 U s u p

Conservation of energy:

pup = ρ0 Us



(7.107)

1 2 u + e − e0 2 p



(7.108)

For solids, the last two equations may be written also (for shock propagating in x-direction) [976] σxx = p0 + ρ0 Us up

(7.109)

1 2 ρ0 Us e − e0 + up = σxx up 2

(7.110)



Rankine–Hugoniot relations (Conservation relations in moving with shock front coordinates) ρu = ρ0 u0

(7.111)

p + ρu2 = p0 + ρ0 u02 p 1 p0 1 + e + u2 = + e0 + u02 ρ 2 ρ0 2

(7.112) (7.113)

Bernoulli’s equation 1 1 h + u2 = h0 + u02 = ht 2 2

(7.114)

Hugoniot equation e − e0 = Rayleigh equations

 1 p − p0 (v0 − v) 2

ρ02 Us2 = ρ02 u02 = ρ2 u2 = −

p − p0 v − v0

(7.115)

(7.116)

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Snay & Rosenbaum (1952) Rice & Walsh (1957) Nagayama et al (2002 Bloom & Keeler (1974) Sound speed at 1pm & 20⬚C Ref. [977] (Flyer impact method) Linear fit of all data

3

12 Shock Velocity (km/s)

Shock velocity (km/s)

4

Water

2

11 10

Linear fit of all data D ⫽ 1471 ⫹ 1.956u

1

0

0.2 0.4 0.6 0.8 Particle velocity (km/s)

9 Iron

8 7 6

1

5 1

(a)

2 3 4 Particle velocity (km/s)

5

(b)

Figure 7.15 (a) Shock velocities as a function of particle velocity for water. © American Institute of Physics (2004), reprinted with permission from Ref. [977]. (b) Shock velocities as a function of particle velocity for iron [978]. Open circles are the reprocessed Los Alamos standards data. Filled circles are the two stage light–gas gun data. Experimental uncertainties lie within the symbol size. Dashed line is linear fit and solid line is quadratic fit. © American Institute of Physics (2000), reprinted with permission from Ref. [978].

Gain in the kinetic energy per unit mass of the material by the passage of the shock wave in the laboratory frame coordinates:  1 1 2 Up = (u − u0 )2 = (7.117) p − p0 (v + v0 ) 2 2 Loss in the kinetic energy per unit mass of the material by the passage of the shock wave in shock front-fixed coordinates:   1 1 2 (7.118) u0 − u2 = p − p0 (v + v0 ) 2 2 Shock impedance Z = ρ0 Us

(7.119)

In liquids and solids, the relation between shock and particles velocity can often be approximated by a linear function (cf. Fig. 7.15 and Table 7.2). Us = C0 + Sup , (7.120) where C0 is speed of the sound ahead the shock wave. Using Eq. (7.120) and jump conditions at the shock front, the Hugoniot pressure and internal energy may be expressed as [273, 976] ρ0 C02 η , (1 − Sη)2 ηp , e = 2ρ0

p =

(7.121) (7.122)

where η=1−

V ρ0 =1− . V0 ρ

(7.123)

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Table 7.2 Relation between shock and particles velocities for some liquids [979, 980]. Liquid

Shock velocity (m/s)

Acetone

Us = 1940 + 1.38 up

Ethanol

Us = 1730 + 1.75 up

Ether

Us = 1700 + 1.46 up

Ethylene glycol

Us = 2150 + 1.55 up

Mercury

Us = 1750 + 1.72 up

Liquid oxygen

Us = 1880 + 1.34 up

Water

Us = 1483 + 1.79 up

(Mie)–Grüneisen equation of state Equation of state for shock-compressed bodies with linear Us (up ) relationship (Eq. 7.120) can be derived from Mie–Grüneisen equation  Ŵ (V )  p (V ) = p0 (V ) + e (V ) − e0 (V ) , (7.124) V using modified Rankine–Hugoniot relation (achieved through elimination of up and Us from Eqs (7.106) and (7.108)),   1 e − e0 = (V0 − V ) p + p0 , (7.125) 2 where V ≡ 1/ρ. Combining Eqs (7.121), (7.124), and (7.125) yields   ρ0 C02 η Ŵη (7.126) + Ŵρ0 (e − e0 ) . · 1 − p = p0 (1 − Ŵη) + 2 (1 − Sη)2 In this expression, it is assumed that the shocked matter is in hydrostatic compression; for solids it means that the shock pressure is much larger than the yield strength of the material. Elastic–plastic shock waves in a solid (propagating in x-direction) σxx = (λ + 2μ) εxx

Elastic shock wave:

σyy = σzz = λεxx If yielding occurs behind the elastic precursor wave, the shock yield stress Y is Y = 2μεxx

(7.127) (7.128) (7.129)

and hydrostatic pressure at the wavefront    2 1 σxx + σyy + σzz = λ + μ εxx . p= 3 3

(7.130)

Leonov et al. [571] calculated the shock pressure at laser irradiation of glass–water interface using the formula by Zaharov [981] 1 Ea r0 (7.131) ln 2   , p (r) = Ŵ Vf r ln r r0

valid if r > df , where r0 = df /2, df is the diameter of the focus spot, Ea is the absorbed laser energy, Vf is the focal volume, and Ŵ ≈ 1.5 is Grüneisen coefficient. The shock speed in adiabatic compression approximation

306

Handbook of Liquids-Assisted Laser Processing

is given by U 2 (r) p (r) = C02 ρ0 C02

1−

ρ0 C02 n

1  p (r) +

 ρ0 C02 −1/n n

,

(7.132)

where the factor n ≈ 7 is valid for water. Models of shock propagation at laser peening were described in the section 3.3.6.2.

7.5 Laser-Induced Reactions of Carbon with Organic Solvents and Water 7.5.1 Reactions of carbon with organic solvents Amongst other possible chemical reactions occurring at liquids-assisted laser processing of solids, the reactions of carbon with organic solvents and water have been studied more extensively. Wakisaka et al. [982] proposed the following reaction schemes between graphite and benzene vapours (A) or liquid benzene (B) at laser irradiation. Reproduced by permission of The Royal Society of Chemistry. The conditions of the experiment are given in Table 7.6:Wakisaka (1993). Condition A H

C1

C1

C2

C2

C CH Intermediate 1

Condition B CH3 C1

H

C1

CH3 Intermediate 2

CH3

CH3

C2H5

CH3

C1

CH3

CH3

Gaumet et al. [983] have identified the following reactions between carbon clusters of different sizes with benzene. The main reaction product was phenylacetylene. Reproduced by permission of The Royal Society of Chemistry. H C1

C2

C2

C Intermediate 1

CH (1)

Phenylactetylene

(A) Cn addition to benzene (linear and cyclic)

C1*

+

*

CH3

C

toluene

CH phenylacetylene

C2*

+

* CH

CH2 styrene

307

Physics and chemistry of laser–liquid–solid interactions C

C3*

C

CH3 1-phenylprop-l-yne

*

+

Indene

C

C

C

H

1-phenylbuta-1,3-diyne

*

+

C4*

C

naphthalene

(B) Reaction between Cn

C12H2

C8H2 H

(C

C

C

C)2

H

H

(C

C

C

C)4

H

(C) Reaction between aromatic rings

biphenylene

biphenyl

McGrath et al. [984] detected a number of gaseous and liquid reaction products generated by laser irradiation of graphite suspensions in toluene and benzene (Table 7.3):

Table 7.3 Reaction products of laser irradiation of graphite in toluene and benzene [984]. A carbon suspension made from 25 nm diameter particles was used: 133 mg/l for benzene and 200 mg/l for toluene. The amount of gaseous products are expressed as a percentage of the total gas amount of moles. Laser: 1064 nm, 10 ns, 650 mJ, 6000 pulses. Graphite + benzene

Graphite + toluene

Gaseous products (%)

Liquid products

Gaseous products (%)

Liquid products

H2 (94.6)

1-Methylene

H2 (92.6)

1,2-Dimethylbenzene

CH4 (2.2)

H-indene

CH4 (4.9)

1-methylene-2propenybenzene

C2 H2 (2.9)

naphthalene

C2 H2 (2.2)

1-propynylbenzene

C2 H4 (0.3)

biphenyl biphenylene acenaphthylene 1-methytriphenylene

C2 H4 (0.2) C2 H6 (0.1)

1-methylnaphthalene naphthalene 2-ethenylnaphthalene biphenyl 1,1′ -methylenebisbenzene 4-methyl-1,1′ -biphenyl biphenylene bibenzyl 2,2′ -dimethylbiphenyl 9H-fluorene

308

Handbook of Liquids-Assisted Laser Processing

7.5.2 Reactions of carbon with water The studies by Chen et al. [833] revealed the main reactions at 1.06-µm laser irradiation of carbon suspension in water: C + H2 O → H2 + CO, CO + H2 O → H2 + CO2 . McGrath et al. [984] identified both gaseous and liquid products generated in a suspension of 25-nm diameter carbon particles in water by a laser beam of 1064 nm, 10 ns, 650 mJ, and 6000 pulses (percentage is of the total molar amount of gas produced). Gaseous products: CO (68%), H2 (25%), C1 (2.0%), C2 (4.4%), C3 (0.2%), and C4 (0.4%). GC-MS was used to detect individual hydrocarbons which were determined to be methane (CH4 ), ethane (C2 H6 ), ethene (C2 H4 ), ethyne (C2 H2 ), propene (C3 H6 ), 1,2-propadiene (C3 H4 ), 1-propyne (C3 H4 ), 1-buten3-yne (C4 H4 ), and 1,3-butadilyne (C4 H2 ). The main product in each hydrocarbon group is highlighted in italics (citation from McGrath et al. [984]). Liquid products: Carboxyl (R-COOH) and ester (R-COO-R) functional groups, arene carbon, alkenes, and alkynes were detected by 13 C NMR technique.

7.6 Behaviour of Oxides in High Temperature Water and Water Vapour It was pointed out by Dolgajev et al. [479] and Hidai and Tokura [478] that at laser ablation in water the hydrothermal dissolution of solids may play an important role. When the temperature and pressure of water rise from normal to supercritical values, the solubility of many oxides, commonly machined by laser, rises several hundred-fold [985, 797] (Table 7.4). Table 7.4 Solubility of some oxides in pure water at 500◦ C and 1000 atm (100 MPa). (After Matson and Smith [986]). Oxide

Solubility (ppm)

UO2

0.2

Al2 O3

1.8

SnO2

3.0

NiO

20

Nb2 O5

28

Ta2 O5

30

Fe2 O3

90

SeO

120

SiO2

2600

GeO2

8700

309

Physics and chemistry of laser–liquid–solid interactions

Table 7.5

Generation and thermodynamic data on some metal and silicon hydroxides [988].

Group

Reaction

(kJ/mol)

( J/mol K)

D◦298 (M—OH) (kJ/mol)

VIII

½Fe2 O3 (s) + ½H2 O(g) = Fe(OH)(g) + ½O2 (g)

653

213

334

Linear

669

229

318

Bent

½Fe2 O3 (s) + H2 O(g) = Fe(OH)2 (g) + ¼O2 (g)

324

102

411

Bent

CuO(s) + ½H2 O(g) = Cu(OH)(g) + ¼O2 (g)

400

145

260

Linear

429

161

230

Bent Bent

IB

r H◦298

r S◦298

Geometry of M—OH bond

IIB

ZnO(s) + H2 O(g) = Zn(OH)2 (g)

201

55

300

IIIA

½Al2 O3 (s) + ½H2 O(g) = Al(OH)(g) + ½O2 (g)

779

199

549

½Al2 O3 (s) + ½H2 O(g) = AlO(OH)(g)

498

134

566

½Al2 O3 (s) + H2 O(g) = Al(OH)2 (g) + ¼O2 (g)

572

121

458

½Al2 O3 (s) + 3/2H2 O(g) = Al(OH)3 (g)

188

−7.3

487

½Ga2 O3 (s) + ½H2 O(g) = Ga(OH)(g) + ½O2 (g)

550

211

428

Linear

570

224

408

Bent

675

190

297

Linear

718

188

254

Bent

260

62

436

Linear

317

64

408

Bent

45

−76

487

Bent

IVA

SiO2 (s) + ½H2 O(g) = SiO(OH)(g) + ¼O2 (g) SiO2 (s) + H2 O(g) = SiO(OH)2 (g)

SiO2 (s) + 2H2 O(g) = Si(OH)4 (g)

Many oxides form volatile hydroxides by reaction with water vapour. Even the moisture in laboratory air could create high volatility hydroxides and oxy-hydroxides during high-temperature exposure [987] (Tables 7.5 and Fig. 7.16). Tables 7.6 and 7.7 present essentials of some selected experimental and theoretical work on laser-liquid-solid interactions, having importance to several kinds of materials processing.

310

Handbook of Liquids-Assisted Laser Processing

Temperature (K) 1800

1600

1400

1200

⫺4 Si(OH)4 A

Log (P, atm)

⫺6

Si(OH)4 K

⫺8 SiO(OH)2 A

SiO(OH)2 K

⫺10 SiO(OH) K ⫺12

⫺14

5.5

6.0

6.5

7.0

7.5

8.0

8.5

10 000/T (K)

Figure 7.16 Calculated vapour pressure of Si–OH species over SiO2 with x(H2 O) = 0.37 and P(total) = 1 bar [988]. The lines labelled K were calculated from thermodynamic functions taken from Krikorian’s estimates based on the pseudo halide behaviour of the hydroxyl group. The lines labelled A were calculated from the thermodynamic functions taken from Allendorf ’s et al. ab initio calculations. The vapour pressure of SiO(OH) (g) from Allendorf ’s calculations was too low to appear on this graph. © Elsevier.

Table 7.6 Some experimental research of physical–chemical processes at laser irradiation of solids in liquids.

Materials irradiated

Liquids/gases in contact with specimen

Graphite

Laser type and beam parameters

Other features of the experiment

Novel features, observed phenomena, comments

Benzene

2ω-Nd:YAG, 532 nm, 10 ns, 10 Hz, 22 mJ

Sample immersed horizontally into free surface liquid, focused laser beam

Reaction products identified by mass chromatography: phenylacetylene, xylene, ethylbenzene, styrene, C6 H6 -C4 , biphenyl, diphenylacetylene

Wakisaka (1993) [982]

Graphite

Benzene (under Ar), benzene vapour

Nd:YAG, 266, 532, and 1064 nm, 6 ns, 10 Hz, 1010 W/cm2

Benzene was used as a reactive molecule for trapping the laser-induced Cn clusters; Cn reactions with phenyl radical are listed: the main reaction product was phenylacetylene; in liquid the yield of Cn , n > 2 was smaller than in vapour (see also Section 7.5)

Gaumet (1996) [832]

Carbon black, 25 nm in suspension

Water

Nd:YAG, 1.06 µm, 16 ns, 10 Hz, 0.7 J,up to 6000 shots

Reaction products identified by gas chromatography: CO, H2 , C2 H2 , CH4 , C2 H4 (concentrations given), O2 , N2 , CO2 , C2 H6 (traces)

Chen (1997) [833]

Graphite, Poly-BN

Water, benzene, n-hexane, carbon tetrachloride

Nd:YAG, 1064 nm, 20 ns, fluence ≈8–9 J/cm2

Workpiece immersed into circulating liquid, covered by window

Optical emission spectra at 29–1000 ns from the laser pulse presented; mass spectrography study of reaction products in liquid; the early stage plasma density is estimated ≈ 1020 cm−3 ; lifetime of line emissions ≈100 ns

Sakka (2000) [470]

Si, Hg

Air, water

Nd:YAG, 532 nm, ∼ 13 ns, spot 5 mm, 0.05–1.4 J/cm2

Target immersed horizontally into water, covered by window, 20◦ C

Fluid dynamics observed by high-speed camera up to 20 Mfps and by reflectance of a probe beam; peak pressures up to 10 MPa were measured by PVDF-sensor; transient reflectance data are compared with theoretical predictions of temperature rise and bubble nucleation

Ueno (2001) [989]

Graphite

Air, water

Nd:YAG, 1064 nm, 20 ns

Target immersed into water, focused laser beam, 10 J/cm2 , up to 0.52 GW/cm2

Plasma emission images at 21–1080 ns from the laser pulse in air and at 21–100 ns in water presented; estimated density of carbon atoms in plasma at 10–20 ns 6.7 × 1021 cm−3 , plasma temperature ≈7500 K, pressure 700–1100 MPa

Saito (2002) [973]

References

(Continued)

Table 7.6

(Continued)

Materials irradiated

Liquids/gases in contact with specimen

Al

Laser type and beam parameters

Other features of the experiment

Novel features, observed phenomena, comments

Water

Nd:YAG, 1064 nm, 20 ns

Target immersed into circulating water, focused laser beam, ≈1 mm spot, 7.2–10.4 J/cm2

Optical emission spectra at 20–80 ns from the laser pulse recorded; 396 nm Al line (2 P–2 S) changes at 40–50 ns from absorption line to emission line; analytical model of plasma transients presented; calculated plasma temperature varies from ≈7000 K (t = 0) to ≈4000 K (t = 100 ns)

Sakka (2002) [990]

Graphite

Water

Nd:YAG, 1064 nm, 20 ns

Target immersed into water, focused laser beam, ≈10 J/cm2

Optical emission spectra in 535–575 nm (C2 Swan band region) recorded at 50–500 ns from laser pulse; vibrational temperature of C2 ≈ 5000 K during the whole time interval; thermal cooling of the gas cavity is slower than the collapse of the cavity

Sakka (2002) [991]

Carbon suspension, 13–75 nm

Water, toluene, benzene

Nd:YAG, 1064 nm, 10 ns, 10 Hz, 650 mJ, beam diameter ≈1 mm

Evolution of tiny gas bubbles observed; in water, H2 and CO were the main reaction products along numerous hydrocarbons ranging from C1 to C4 ; in toluene and benzene H2 was the main gas product with small amounts of C1 to C3 hydrocarbons; the main liquid product in toluene was bibenzyl and in benzene biphenyl, along with numerous polycyclic aromatic hydrocarbons in smaller concentrations (Table 7.3); possible reaction paths discussed

McGrath (2002) [984]

Optical emission spectra in 512–518 nm (C2 Swan band tail region) recorded at 150–1200 ns from laser pulse; rotational temperature of C2 ≈6000 K up to ≈1000 ns; rotational temperature is more reliable for laser ablation plume in liquids than vibrational

Saito (2003) [992]

Nd:YAG, 1064 nm, 100 ps, focused and unfocused Ti:sapphire, 780 nm, 40 fs, unfocused Graphite

Air, water

Nd:YAG, 1064 nm, 20 ns, ≈70 mJ

Target immersed horizontally into free surface water, water layer 15-mm, focused laser beam, spot ≈87 µm, ≈1.2 kJ/cm2

References

Ag,Au, Si

Water

Graphite

Air, water

Si wafer

IPA

Nd:YAG, 1064 nm, 10 ns, 18 and 36 J/cm2

Nd:YAG, 532 nm, 7 ns, 138 mJ/cm2 , spot several mm

Condensed from vapour film (97–227 nm) on surface

Time-resolved imaging of laser ablation process; in first µs a vertical jet 10 km/s observed; bubble growth velocity 400 m/s; bubble lifetime 200 µs (18 J/cm2 ), 300 µs (36 J/cm2 )

Tsuji (2004) [672]

Further analysis of results by Saito 2003 [992]; self-absorption parameter introduced; self-absorption of plasma emission is considerable in water

Sakka (2005) [993]

Dynamics of liquid film at laser-heated surface was recorded by optical reflectivity with 2 nm, 0.2 ns resolution; estimated with aid of temperature calculations initial vapour pressure (at liquid film lift-off) was ∼5 MPa; ejection velocity of liquid film varied from ∼50 m/s (97 nm film) to ∼40 m/s (227 nm film);

Lang (2006) [107]

Table 7.7 Some theoretical research of physical–chemical processes at laser irradiation of solids in liquids.

Targets Tungsten film on glass

Liquids/gases in contact with target Water

Other features of the system under study Water layer over specimen; scanned laser beam

Results, comments

References

3D-numerical calculations of temperature distribution at laser irradiated interface below liquid vaporization threshold presented

Geretovszky (1996) [994]

The transmission of breakdown plasma in water during LSP experiments was investigated theoretically for laser wavelengths from 355 to 1064 nm and pulse length of 25 ns; at 1064 nm the breakdown process was found to be dominated by avalanche ionization, but at 355 and 532 nm by multiphoton ionization

Sollier (2001) [260]

Au (111)

Water (6, 12 or 24 molecular layers)

Molecular dynamics simulation of water on suddenly heated from 0 to 1000 K surface on time internal 0–400 ps; the simulation describes water superheating and film lift-off (Fig. 7.8)

Dou (2001) [939]

Fe, SS304

Water

Analytical models in cylindrical coordinates for LSP plasma temperature, pressure, and thermal stresses for ramp-up, ramp-down, and rectangular laser pulses, including confined ablation with coating (see Fig. 7.13 for calculated temperatures)

Thorslund (2003) [384]

Water

Wu (2005) [259] A mathematical model of pressure generation at water confined LSP is described; the model considers the processes to be 1D, the plasma homogeneous and two-temperatures laser beam absorption due to electron-ion and electron-atom IB and photoionization only, and Hertz–Knudsen surface evaporation; the model was in good agreement with experimental data at 532 and 1064 nm, 0.6–25 ns and 1–10 GW/cm2 ; the calculations give insight into plasma parameters as the density of species, light transmission, thermal to internal energy ratio α, water–plasma interface reflectivity and energy balance

C H A P T E R

E I G H T

Liquids and Their Properties

Contents 8.1 Introduction 8.2 Properties of 100 Selected Liquids 8.3 Properties of Water

315 332 379

8.1 Introduction About 70 different neutral liquids have been used in laser materials processing, Tables 8.1 and 8.2, the most frequently used liquid being water, following with alcohols. Water is also often a constituent of materials. Many materials like oxides have adsorbed water on their surfaces under normal conditions. The codes and synonyms of 100 selected liquids from the first three classes in Table 8.1 are presented in Table 8.6, their molecular structures in Table 8.7, and properties in Table 8.8. The main physical properties of some important to laser processing metals, semiconductors, oxides and other inorganic compounds are also given in this chapter,Tables 8.3 and 8.4. The composition of sea water is given in Table 8.5.

Table 8.1

Classes of liquids used in laser materials processing.

Inorganic liquids H2 O D2 O H4 N2

Organic liquids Hydrocarbons Halocarbons Alcohols Ethers Esters Ketones Amines Carbon disulphide DMSO Silicon oil Vacuum oil

Handbook of Liquids-Assisted Laser Processing ISBN-13: 978-0-08-044498-7

Liquefied or frozen gases

Molten or liquid metals and semi-conductors

Ar He CH4 CO2 NH3 N2 O2 Freones

Bi Ga,Al-Ga Ge Hg In Si Sn

Molten salts KNO3 NaNO3 NaCl NH4 Cl

© 2008 Elsevier Ltd. All rights reserved.

315

316 Table 8.2

Handbook of Liquids-Assisted Laser processing

Liquids used in laser processing classified by the processes.

Process

Liquids

Additives

Cleaning

Water, ethanol, methanol, IPA, acetone

NaCl, methanol, ethanol, IPA

Shock processing

Water

No

Front-side machining

Water, heptane, perfluorocarbons, benzene, o-xylol, p-xylol, ethanol, glycerine, ether, DMSO, DMFA, N2 H4 , liquid nitrogen, molten NaCl, NH4 Cl, NaNO3 and KNO3

H2 O2 , NaCl, CaCl2 , NaNO3 , KNO3 , Na2 SO4 , K2 SO4 , CuSO4 , KOH, methanol, ethanol, isopropanol, soapy additives, saccharose

Back-side machining

Cyclohexane, tetrachloromethylene, tetrachloroethylene, benzene, toluene, cumene, t-butylbenzene, 1,2,4trimethylbenzene, chlorobenzene, dichlorobenzene, fluorobenzene, isopropanol (IPA), tetrahydrofuran, methylmethacrylate, methyl benzoate, acetone, mercury, gallium

NiSO4 , CrO3 , KMnO4 , CrO3 , FeCl3 , KMnO4 , KNO3 , K2 CrO4 , carbon particles, pyrene, pyranine, benzil, naphthalene, phenanthrene, anthracene, 9-methyl-anthracene, 9,10-dimethylanthracene, 9-phenyl anthracene, fluoranthrene, Rose Bengal dye, Np(SO3 Na)3

Generation of metal particles

water, D2 O, pentane, hexane, cyclohexane, heptane, octane, nonane, decane, chloroform, methanol, ethanol, ethylene glycol, diethylene glycol, 1-propanol, 2-propanol (IPA),isobutanol, n-hexanol, 2-ethoxyethanol, acetone, liquid He II

NaCl, KCl, MgCl2 ,AgNO3 , NaBH4 , I+ , CN− , phtalazine, citric acid, sodium citrate, dodecanethiol, gelatine, cyclodextrines, PVP, SDS, SHS, SOS, SDBS, CTAB, sodium polyacrylate, tetraalkyl-ammonium bromide salts

Generation of inorganic compound particles

Water, hexane, dichloroethane, toluene, xylene, ethanol, 2-propanol (IPA), ethylene glycol, diethylene glycole, isobutanol, acetone, DMSO, silicon oil

Ammonia,AgNO3 , SDS, LDA, CTAB

Generation of carbon and silicon particles (not diamond or DLC)

Hexane, cyclohexane, perfluoro-octane, perfluorodecalin, benzene, hexafluorobenzene, toluene, methanol, 2-propanol (IPA), tetrahydrofuran (THF)

Generation of diamond and DLC particles and films

Hexane, cyclohexane, decalin, benzene, toluene, cumene, acetone, vacuum oil (a polyphenyl ether)

Carbon or diamond particles, dissolved methane (in water), Pd(acac)2

Generation of organic particles

Water, methanol, ethanol, 1-propanol, ethyl acetate

SDS, Igepal CA-630

Surface modification

Water, benzene, aminoethanol, 1,2-diaminoethane, triethylenetetramine, NH3 , liquid nitrogen

H3 BO3 , B(OH)3 , NaOH, NaAlO2 , CuSO4

Ablation deposition from liquid targets

Ga,Al-Ga, Ge, In, Sn, Bi, Si, vacuum oil (a polyphenyl ether) (Continued)

317

Liquids and their properties

(Continued)

Table 8.2 Process

Liquids

Additives

Ablation deposition from frozen targets (inorganic compounds)

Acetylene, N2 , CH4 , CO2

Ablation deposition from frozen targets (MAPLE)

Water, chloroform, tert-butanol, glycerole, phosphate buffer

Forward transfer deposition (LIFT)

Water, glycerine, mineral oil

Tris–HCl, EDTA, PBS, SDS

Notations DLC – dry laser cleaning LIFT – laser induced forward transfer Np(SO3 Na)3 – naphthalene-1,3,6-trisulphonic acid trisodium salt PVP – polyvinylpyrrolidone SDS – Cn H2n+1 OSO3 Na SHS – sodium hexadecyl sulphate, C16 H33 NaSO4 SOS – sodium n-octyl sulphonate, C8 H17 SO3 Na SDBS – n-dodecylbenzene sulphonate, C12 H25 C6 H4 SO3 Na CTAB – cetyltrimethylammonium bromide (hexadecyltrimethylammonium bromide), C19 H42 BrN LDA – lauryl dimethylaminoacetic acid betaine, CH3 (CH2 )11 N+ (CH3 )2 CH2 COO− Pd(acac)2 – palladium acetylacetonate Igepal CA-630 – octylphenoxy polyethoxy ethanol (CH3 )3 CCH2 (CH3 )2 CC6 H4 O(CH2 CH2 O)9 H Tris-HCl – 2-Amino-2-(hydroxymethyl)-1,3-propanediol, hydrochloride, C4 H11 NO3 ClH EDTA – ethylenediaminetetraacetic acid, C10 H16 N2 O8 PBS – phosphate buffered saline solution

Table 8.3

Properties of some metals and elemental semiconductors [995–998]

ρ kg/m3

α × 106 K−1

Cp J/kg K

λ W/m K

Tm ◦ C

Tb ◦ C

Hm kJ/kg

Hvap kJ/mol

n (400 nm)

k (400 nm)

R (400 nm)

2519

397

291

0.49

47.86

0.9243

1287

2471

877

292

2.90

3.13

0.537

1907

2671

404

342

1.50

3.62

0.691

1.28

2.14

0.489

1.66

1.94

0.371

1.83

3.04

0.58

Al

2700

23.1

897

237

Be

1850

11.3

1825

200

Cr

7150

4.9

449

Co

8860

13.0

421

100

1495

2927

275

Cu

8960

16.5

385

401

1084.62

2562

208.7

307

Ga

5910

29.76

2204

80.2

270

Au

19 300

14.2

129

1064.18

2856

63.7

343

In

7310

32.1

233

81.6

156.60

2072

28.6

232

Fe

7870

11.8

449

80.2

2861

247.3

340

371

93.7

40.6 317

660.32

1538

(Continued)

318

Handbook of Liquids-Assisted Laser processing

(Continued)

Table 8.3

ρ kg/m3 Mg

1740

Hg

13 533.6

Mo 10 200 Ni

8900

α × 106 Cp λ Tm K−1 J/kg K W/m K ◦ C

◦

Tb C

Hm Hvap n k R kJ/kg kJ/mol (400 nm) (400 nm) (400 nm)

24.8

1090

349

1023 140

4.8

251

13.4

444

156

650

8.34 −38.83 138

356.73

128

11.4 59.1

2623

4639

390.7 590

3.03

3.22

0.550

90.7

1455

2913

298

1.62

2.39

0.479

375

Nb

8570

7.3

265

53.7

2477

4744

323

Pd

12 000

11.8

246

71.8

1554.9

2963

157.3 361

Pt

21 500

8.8

133

71.6

1768.4

3825

113.6 469

1.73

2.85

0.556

Ag

10 500

18.9

235

961.78 2162

104.8 258

0.17

1.95

0.848

Ta

16 400

6.3

140

57.5

3017

5458

Sn

7260

22.0

228

66.6

231.93

26.2

Ti

4510

8.6

523

21.9

1668

3287

295

W

19 300

4.5

132

3422

5555

284.5 824

3.39

2.41

0.464

V

6000

8.4

489

1910

3407

422

Zn

7140

30.2

388

116

419.53

907

112

Si

2329

2.6

700

130

1412

Ge

5323.4

5.9

310

58

937

C

3515

0.8

520

600

3547

429

174 30.7

202.1 59.2 296 426

114

Table 8.4 Properties of some inorganic compounds [999–1001]. SC – single crystalline, subl – sublimes, decp – decomposes, expl – explods. α × 106 K−1

Cp J/kg K

λ W/m K

Tm C

Tb ◦ C

Hm kJ/mol

1465

28.158

decp 520

165.7

1689

23.849

Compound

ρ kg/m3

NaCl

2165

NH4 Cl

1530

Na2 SO4

2680

884

LiNO3

2380

254

NaNO3

2260

310

expl 537

16

KNO3

2109

337

decp 400

12

Al2 O3

3980

6.5

∼25

2047

2980

SiO2 (SC)

2651 (0◦ C)

0.55

1.6 (500◦ C)

1423

2950

◦

801 sublimes

25.5

(Continued)

319

Liquids and their properties

Table 8.4

(Continued) α × 106 K−1

Cp J/kg K

Tb C

Compound Fe2 O3

5240

1565

CuO

6480

1326

11.80

ZnO

5610

4.0

25.2

1975 subl

52.3

TiO2 (rutile)

4250

9.0

9

1867

2500–3000

SnO2 (cassiterite)

6950

1630

subl 1800–1900

Co3 O4

6110

decp 900

ZrO2

5760

MgO

3581

CeO2

7650

Si3 N4

3190

2.5

c-BN

3487

1.2

SiC

3220

5.3

AlN (SC)

3255

5.27

ZnSe (SC)

5420

1517

CdS (SC, hexagonal)

4820

1750

Table 8.5 [1002]

8.0

λ W/m K

Tm C

ρ kg/m3

1.5 60.0 (27◦ C)

◦

2710

∼5000

∼2852

∼3600

2400

∼600 600

◦

17

subl 1900

740

2973

84

2760

285

3000

3500

sublimes

decomposes

subl 980

Major composition of sea water (salinity 35‰)

Constituent

Concentration g/kg

Na+ Mg2+ Ca2+ K+ Sr2+ Cl− SO2− 4 HCO− 3 Br− F− B

10.77 1.29 0.4121 0.399 0.0079 19.354 2.712 0.1424 0.0673 0.0013 0.0045

Hm kJ/mol

78

Nomenclature of 100 selected organic solvents, waters and cryoliquids. Molecular structure is presented in Table 8.7 and properties in Table 8.8.

Number

Halocarbons (not aromatic)

Hydrocarbons

Class

Table 8.6

IUPAC Name

Composition

Linear molecular formula

CAS Reg. No.

Beilstein Reg. No.

EG/EC number

Common synonyms

1

Pentane

C5 H12

CH3 (CH2 )3 CH3

109-66-0

969132

203-692-4

n-Pentane, 1,3-dimethyl propane, diethyl methane

2

2-Methylbutane

C5 H12

CH3 CH2 CH (CH3 )2

78-78-4

1730723

201-142-8

Iso-pentane, isopentane

3

Hexane

C6 H14

CH3 (CH2 )4 CH3

110-54-33

1730733

203-777-6

n-Hexane

4

Heptane

C7 H16

CH3 (CH2 )5 CH3

142-82-5

1730763

205-563-8

n-Heptane, n-dipropylmethane, n-heptylhydride

5

2,2,4-Trimethylpentane

C8 H18

CH3 C(CH3 )2 CH2 540-84-1 CH(CH3 )CH3

1696876

208-759-1

Isobutyltrimethylmethane, isooctane

6

Cyclopentane

C5 H10

C5 H10

287-92-3

1900195

206-016-6

Pentamethylene, cyclopentyl

7

Cyclohexane

C6 H12

C6 H12

110-82-7

1900225

203-806-2

hexahydrobenzene, hexamethylene, naphthene

8

Methylcyclohexane

C7 H14

C6 H11 CH3

108-87-2

203-624-3

Cyclohexylmethane

9

Decalin

C10 H18

C10 H18

mix. 91-17-8 cis 493-01-6 trans 493-02-7

202-046-9

Decahydronaphthalene

10

Petroleum ether*

mixture of hydrocarbons (mostly alkanes)

101316-46-5; 64742-49-0

265-151-9 232-453-7

Petroleum benzin, petroleum spirit, mineral spirits, ligroine, naphtha petroleum

11

Bromoform

CHBr3

CHBr3

75-25-2

1731048

200-854-6

Tribromomethane

12

Dichloromethane

CH2 Cl2

CH2 Cl2

75-09-2

1730800

200-838-9

Methylene chloride, chloromethylene

13

Chloroform

CHCl3

CHCl3

67-66-3

1731042

200-663-8

Methylidyne trichloride, trichloromethane

14

Tetrachloromethane

CCl4

CCl4

56-23-5

1098295

200-262-8

Carbon tetrachloride, carbon tet, Freon 14, CFC-14

15

Fluoroform

CHF3

CHF3

75-46-7

200-872-4

Trifluoromethane, fluoryl, Freon 23, HFC-23

16

1,2-Dichloroethane

C2 H4 Cl2

ClCH2 CH2 Cl

107-06-2

605264

203-458-1

Ethylene chloride, ethylene dichloride, EDC, Freon 150

17

1,1,2-Trichloroethene

C2 HCl3

ClCH CCl2

79-01-6

1736782

201-167-4

Trichloroethylene, ethylene trichloride, trichloroethene,TCE

878165

Halocarbons (not aromatic) Aromatic hydrocarbons, their derivatives

18

1,1,2,2Tetrachloroethene

C2 Cl4

CCl2 CCl2

127-18-4

1361721

204-825-9

Tetrachloroethylene, ethylene tetrachloride, perchloroethylene, tetrachloroethene, PERC, PCE

19

1,1,1,2,2,3,3,4,4,5,5,6,6, 6-Tetradecafluorohexane

C6 F14

C6 F14

355-42-0

1802113

206-585-0

Tetradecafluorohexane, perfluorohexane, perfluoro-n-hexane, perflexane, PP1, FC72

20

1,1,2,2,3,3,4,4,4a,5,5,6, 6,7,7,8,8,8aOctadecafluorodecalin

C10 F18

C10 F18

306-94-5 cis 60433-11-6 trans 6043312-7

2067113

206-192-4

Perfluorodecalin, octadecafluorodecahydronaphthalene, perfluorodecahydronaphthalene

21

Benzene

C 6 H6

C6 H6

71-43-2

969212

200-753-7

Cyclohexatriene, benzol

22

Chlorobenzene

C6 H5 Cl

C6 H5 Cl

108-90-7

605632

203-628-5

Phenyl chloride

23

1,2-Dichlorobenzene

C6 H4 Cl2

C6 H4 Cl2

95-50-1

606078

202-425-9

o-Chlorobenzene

24

1,2,4-Trichlorobenzene

C6 H3 Cl3

C6 H3 Cl3

120-82-1

956819

204-428-0

1,2,4-TCB

25

Fluorobenzene

C 6 H5 F

C 6 H5 F

462-06-6

1236623

207-321-7

Phenyl fluoride, monofluorobenzene

26

1,2,3,4,5,6Hexafluorobenzene

C 6 F6

C6 F6

392-56-3

1683438

206-876-2

Hexafluorobenzene, perfluorobenzene

27

Benzonitrile

C 7 H5 N

C6 H5 CN

100-47-0

506893

202-855-7

Phenyl cyanide

28

Toluene

C 7 H8

C6 H5 CH3

108-88-3

635760

203-625-9

Methylbenzene, toluol

29

Styrene

C 8 H8

C6 H5 CH CH2

100-42-5

1071236

202-851-5

Phenylethylene, vinylbenzene, styrol

30

o-Xylene

C8 H10

C6 H4 (CH3 )2

95-47-6

1815558

202-422-2

1,2-Dimethylbenzene, ortho-xylol

31

m-Xylene

C8 H10

C6 H4 (CH3 )2

108-38-3

605441

203-576-3

1,3-Dimethylbenzene, meta-xylol

32

p-Xylene

C8 H10

C6 H4 (CH3 )2

106-42-3

1901563

203-396-5

1,4-Dimethylbenzene, para-xylol

33

Cumene

C9 H12

C6 H5 CH(CH3 )2

98-82-8

1236613

202-704-5

2-Phenylpropane, isopropylbenzene, isopropylbenzol, cumol

34

1-Chloronaphthalene

C10 H7 Cl

C10 H7 Cl

90-13-1

970836

2019673

α- Chloronaphthalene

35

1-Methylnaphthalene

C11 H10

C10 H7 CH3

90-12-0

506793

201-966-8

α- Methylnaphthalene (Continued)

(Continued)

Number

Alcohols

Class

Table 8.6

IUPAC Name

Composition

Linear molecular formula

CAS Reg. No.

Beilstein Reg. No.

EG/EC number

36

Methanol

CH4 O

CH3 OH

67-56-1

1098229

200-659-6

Hydroxymethane, methyl alcohol, wood alcohol, carbinol, MeOH

37

Ethanol

C 2 H6 O

C2 H5 OH

64-17-5

1718733

200-746-9

Ethyl alcohol, grain alcohol, hydroxyethane, EtOH

38

Ethane-1,2-diol

C 2 H6 O 2

HOCH2 CH2 OH

107-21-1

505945

203-473-3

Ethylene glycol, monoethylene glycol, glycol, MEG

39

2,2,2-Trifluoroethanol

C 2 H3 F3 O

CF3 CH2 OH

75-89-8

1733203

200-913-6

Trifluoroethyl alcohol, β,β,β-trifluoroethyl alcohol

40

2-Aminoethanol

C2 H7 NO

NH2 CH2 CH2 OH 141-43-5

505944

205-483-3

Ethanolamine, 2-aminoethyl alcohol, monoethanolamine, MEA

41

Propan-1-ol

C 3 H8 O

CH3 CH2 CH2 OH 71-23-8

1098242

200-746-9

1-Propanol, propanol, n-propanol, propyl alcohol, n-propyl alcohol

42

Propan-2-ol

C 3 H8 O

(CH3 )2 CHOH

67-63-0

635639

200-661-7

2-Propanol, iso-propanol, isopropanol, isopropyl alcohol, dimethyl carbinol, IPA

43

Propane-1,2,3-triol

C 3 H8 O 3

HOCH2 CH(OH) CH2 OH

56-81-5

635685

200-289-5

Glycerol, glycerine, glycerin, 1,2,3-propanetriol

44

Butan-1-ol

C4 H10 O

CH3 (CH2 )3 OH

71-36-3

969148

200-751-6

1-Butanol, n-butanol, butyl alcohol, n-butyl alcohol

45

Butan-2-ol

C4 H10 O

CH3 CH2 CH(OH) 78-92-2 CH3

773649

201-158-5

2-Butanol, sec-butyl alcohol

46

2-methylpropan-1-ol

C4 H10 O

(CH3 )2 CHCH2 OH

78-83-1

1730878

201-148-0

Isobutyl alcohol, 2-methyl-1-propanol, isobutanol, iso-butanol

47

2-Methylpropan-2-ol

C4 H10 O

(CH3 )3 COH

75-65-0

906698

200-889-7

tert-butanol, 2-methyl-2-propanol, tert-butyl alcohol, trimethyl carbinol

48

2-[Bis(2-hydroxyethyl)amino]ethanol

C6 H15 NO3

(HOCH2 CH2 )3 N

102-71-6

1699263

203-049-8

Triethanolamine, 2,2′ ,2′′ -nitrilotriethanol, tris (2-hydroxyethyl)amine,TEA

49

Phenylmethanol

C 7 H8 O

C6 H5 CH2 OH

100-51-6

878307

202-859-9

Phenyl methanol, benzyl alcohol, benzene methanol, phenyl carbinole

50

Octan-1-ol

C8 H18 O

CH3 (CH2 )7 OH

111-87-5

1697461

203-917-6

1-Octanol, n-octanol, alcohol C-8, capryl alcohol, octyl alcohol

Common synonyms

Ether alcohols Ethers Esters

51

2-Methoxyethanol

C 3 H8 O 2

CH3 OCH2 CH2 OH

109-86-4

1731074

203-713-7

Ethylene glycol monomethyl ether, methyl glycol, methyl cellosolve, 2ME, EGMM

52

2-Ethoxyethanol

C4 H10 O2

C2 H5 OCH2 CH2 OH

110-80-5

1098271

203-804-1

Ethyl glycol, 2EE monoethyl ether, ethyl cellosolve, cellosolve®

53

2-(2Hydroxyethoxy)ethanol

C4 H10 O3

(HOCH2 CH2 )2 O

111-46-6

969209

203-872-2

Diethylene glycol, 2,2′ -dihydroxydiethyl ether, 2,2′ -oxydiethanol, diglycol, bis(2-hydroxyethyl) ether, DEG,TL4N

54

2-Butoxyethanol

C6 H14 O2

CH3 (CH2 )3 OCH2 111-76-2 CH2 OH

1732511

203-905-0

Butyl glycol, ethylene glycol monobutyl ether, ethylene glycol butyl ether, butyl cellosolve

55

Oxolane

C 4 H8 O

C 4 H8 O

109-99-9

102391

203-726-8

Tetrahydrofuran, tetramethylene oxide, 1,4-epoxybutane, oxacyclopentane,THF

56

1,4-Dioxane

C 4 H8 O 2

C4 H8 O2

123-91-1

102551

204-661-8

Diethylene oxide, ethylene dioxide, dioxane, p-dioxane, dioxacylohexane, glycolethylether, 1,4-diethylene dioxide, 1,4-dioxacyclohexane

57

Ethoxyethane

C4 H10 O

(CH3 CH2 )2 O

60-29-7

1696894

200-467-2

Diethyl ether, ether, ethyl ether, 1,1′ -oxybisethane

58

1,2-Dimethoxyethane

C4 H10 O2

CH3 OCH2 CH2 OCH3

110-71-4

1209237

203-794-9

Dimethyl glycol, dimethylglycol, ethylene glycol, dimethyl ether, monoglyme, DME

59

2-Methoxy-2-methylpropane

C5 H12 O

(CH3 )3 COCH3

1634-04-4

1730942

216-653-1

tert-Butyl methyl ether, methyl tert-butyl ether, MTBE, DRIVERON®

60

1-Methoxy-2-(2methoxyethoxy)ethane

C6 H14 O3

(CH3 OCH2 CH2 )2 O

111-96-6

1736101

203-924-4

Diethylene glycol dimethyl ether, 2-methoxyethyl ether, dimethyl diglycol, dimethyldiglycol, bis(2-methoxyethyl) ether, Diglyme

61

Anisole

C 7 H8 O

CH3 OC6 H5

100-66-3

506892

202-876-1

Methoxybenzene, methyl phenyl ether

62

1-Butoxybutane

C8 H18 O

[CH3 (CH2 )3 ]2 O

142-96-1

1732752

205-575-3

Dibutyl ether, butyl ether, di-n-butyl ether

63

Methyl formate

C 2 H4 O 2

HCO2 CH3

107-31-3

1734623

203-481-7

Methyl methanoate, methyl ester, formic acid, formic acid methyl ester

64

Ethyl acetate

C 4 H8 O 2

CH3 COOC2 H5

141-78-6

506104

205-500-4

Acetic acid ethyl ester

65

Methyl 2-methylprop-2enoate

C 5 H8 O 2

CH2 =C(CH3 ) COOCH3

80-62-6

605459

201-297-1

Methyl methacrylate, methyl 2-methylpropenoate, methacrylic acid methyl ester, MMA

66

Butyl acetate

C6 H12 O2

CH3 COO(CH2 )3 CH3

123-86-4

1741921

204-658-1

n-Butyl acetate, acetic acid n-butyl ester

67

Methyl benzoate

C 8 H8 O 2

C6 H5 COOCH3

93-58-3

1072099

202-259-7

Benzoic acid methyl ester, Clorius, Niobe oil (Continued)

(Continued)

Number

Nitrogen compounds

Ketones

Class

Table 8.6

IUPAC Name

Composition

Linear molecular formula

CAS Reg. No.

Beilstein Reg. No.

EG/EC number

Common synonyms

68

Acetone

C 3 H6 O

CH3 COCH3

67-64-1

635680

200-662-2

Propanone, 2-propanone, dimethyl ketone

69

Butan-2-one

C 4 H8 O

C2 H5 COCH3

78-93-3

741880

201-159-0

2-Butanone, methyl ethyl ketone, ethyl methyl ketone, MEK

70

Pentan-2-one

C5 H10 O

CH3 COCH2 CH2 CH3

107-87-9

506058

203-528-1

2-Pentanone, methyl propyl ketone

71

Cyclohexanone

C6 H10 O

C6 H10 (=O)

108-94-1

385735

203-631-1

Pimelic ketone

72

4-Methylpentan-2-one

C6 H12 O

(CH3 )2 CHCH2 COCH3

108-10-1

605399

203-550-1

4-Methyl-2-pentanone, methyl isobutyl ketone, isobutyl methyl ketone, isopropylacetone, MIBK

73

5-Methylhexan-2-one

C7 H14 O

(CH3 )2 CHCH2 CH2 COCH3

110-12-3

506163

203-737-8

5-Methyl-2-hexanone, isobutylacetone, isopentyl methyl ketone, methyl isoamyl ketone, isoamyl methyl ketone, MIAK

74

Formamide

CH3 NO

HCONH2

75-12-7

505995

200-842-0

Formic amide, formic acid amide, methane amide, Amide C1,

75

Nitromethane

CH3 NO2

CH3 NO2

75-52-5

1698205

200-876-6

Mononitromethane, nitrocarbol

76

Acetonitrile

C 2 H3 N

CH3 CN

75-05-8

741857

200-835-2

Methyl cyanide,ACN

77

Ethane-1,2-diamine

C 2 H8 N2

NH2 CH2 CH2 NH2 107-15-3

605263

203-468-6

1,2-Diaminoethane, 1,2-ethanediamine, ethylenediamine

78

N ,N Dimethylmethanamide

C3 H7 NO

HCON(CH3 )2

68-12-2

605365

200-679-5

N ,N -Dimethylformamide, formic acid dimethylamide, DMF, DMFA

79

N ,N Dimethylethanamide

C4 H9 NO

CH3 CON(CH3 )2

127-19-5

1737614

204-826-4

N ,N -Dimethylacetamide, acetic acid dimethylamide, DMAC

80

Pyridine

C 5 H5 N

C5 H5 N

110-86-1

103233

203-809-9

Azabenzene, azine

81

1-Methylpyrrolidin-2one

C5 H9 NO

C5 H9 NO

872-50-4

106420

212-828-1

1-Methyl-2-pyrrolidinone, N -methyl-2pyrrolidinone, m-pyrrole, 1-methyl-2-pyrrolidone, N -methyl-2-pyrrolidone, M-PYROL®, NMP

82

Hydrazine

H 4 N2

NH2 NH2

302-01-2

Diazane, diamide, levoxine

Misc. Water Liquefied gases

83

Methanedithione

CS2

CS2

75-15-0

1098293

200-843-6

Carbon disulphide, carbon bisulphide, Carbon sulphide

84

Methylsulphinylmethane

C2 H6 OS

(CH3 )2 SO

67-68-5

506008

200-664-3

Dimethyl sulphoxide, methylsulphoxide, DMSO

85

Silicone oil*, 5 cSt

[-Si(CH3 )2 O-]n

63148-62-9

EINECS

Polydimethylsiloxane, polysilicone oil

86

Oxidane (water*)

H2 O

7732-18-5

231-791-2

Water, ordinary water, light water, hydrogen oxide

87

Heavy water*

D2 O

7789-20-0

232-148-9

Deuterium oxide, water-d2

88

Sea water*

89

Hydrogen

H2

1333-74-0

215-605-7

90

Deuterium

D2

7782-39-0

91

Helium

He

7440-59-7

231-168-5

92

Nitrogen

N2

7727-37-9

231-783-9

93

Oxygen

O2

7782-44-7

231-956-9

94

Neon

Ne

7440-01-9

231-110-9

95

Argon

Ar

7440-37-1

231-147-0

96

Krypton

Kr

7439-90-9

231-098-5

97

Xenon

Xe

7440-63-3

231-172-7

98

Carbon dioxide

CO2

124-38-9

1900390

204-696-9

99

Methane

CH4

74-82-8

1718732

200-812-7

100 Air* *

Other common name.

N 2 , O2

CH4

2050024

Methyl hydride, biogas, marsh gas

326

Handbook of Liquids-Assisted Laser processing

Molecular structure of 90 liquids, listed in Table 8.6.

Table 8.7

Pentane

2-Methylbutane

Hexane

C5H12 109-66-0

C5H12 78-78-4

C6H14 110-54-3

CH3

CH3

H3C

1

H3C

CH3

H3C 2

CH3

3

Heptane

2,2,4-Trimethylpentane

Cyclopentane

C7H16 142-82-5

C8H18 540-84-1

C5H10 287-92-3

CH3 H3C

CH3

CH3 CH3

H3C CH3

6

5

4 Cyclohexane

Methylcyclohexane

cis-Decalin

C6H12 110-82-7

C7H14 108-87-2

C10H18 493-01-6 H

CH3 H

7

8

9a

trans-Decalin

Bromoform

Dichlromethane

C10H18 493-02-7

CHBr3 75-25-2

CH2Cl2 75-09-2

H

Br Br

Cl Cl

Br

H 9b

11

12

Chloroform

Tetrachloromethane

Fluoroform

CHCl3 67-66-3

CCl4 56-23-5

CHF3 75-46-7

Cl 13

F

Cl

Cl Cl

Cl 14

Cl

F

Cl

F

15 (Continued)

327

Liquids and their properties

Table 8.7

(Continued) 1,1,2-Trichloroethene C2HCl3 79-01-6

1,2-Dichlroethane C2H2Cl2 107-06-2

1,1,2,2-Tetrachloroethene C2Cl4 127-18-4

Cl

Cl

Cl

Cl

Cl

Cl Cl

Cl

Cl

16

17

FF

FF

F

F

FF

F

F F

F

F

FF

19

F F

F

F FF

trans-1,1,2,2,3,3,4,4,4a, 5,5,6,6,7,7,8,8,8aOctadecafluorodecalin C10F18 60433-12-7

cis-1,1,2,2,3,3,4,4,4a, 5,5,6,6,7,7,8,8,8aOctadecafluorodecalin C10F18 60433-11-6

1,1,1,2,2,3,3,4,4,5,5,6,6,6Tetradecafluorohexane C6F14 355-42-0

FF

18

20a Benzene C6H6 71-43-2

F

F F

F

F F

F F F F F F

F F

F

F

F

F

F F

20b

Chlorobenzene C6H5Cl 108-90-7

F

F F

F

F

F F

1,2-Dichlorobenzene C6H4Cl2 95-50-1 Cl Cl

22

21 1,2,4-Trichlorobenzene C6H3Cl3 120-82-1

Cl

23 1,2,3,4,5,6Hexafluorobenzene C6F6 392-56-3

Fluorobenzene C6H5F 462-06-6

Cl

Cl

F F

Cl

24

25

F

26 Toluene C7H8 108-88-3

Benzonitrile C7H5N 100-47-0

F

F

F

F

Styrene C8H8 100-42-5

CH2 CH3

N

27

28

29 (Continued)

328

Handbook of Liquids-Assisted Laser processing

Table 8.7

(Continued) o-Xylene C8H10 95-47-6

m-Xylene C8H10 108-38-3

p-Xylene C8H10 106-42-3

H3C

CH3 CH3 30

H3C

CH3 31

Cumene C9H12 98-82-8

CH3

32 1-Chloronaphthalene C10H7Cl 90-13-1

1-Methylnaphthalene C11H10 90-12-0

CH3

Cl CH3 CH3 33

34

35

Methanol CH4O 67-56-1

Ethanol C2H6O 64-17-5

Ethanol-1,2-diol C2H6O2 107-21-1

H3C

H3C

HO

HO

36

OH

38

37 2,2,2-Trifluoroethanol C2H3F3O 75-89-8

OH

2-Aminoethanol C2H7NO 141-43-5

Propan-1-ol C3H8O 71-23-8

F H2N

OH

F

40

39 Propan-2-ol C3H8O 67-63-0

Propane-1,2,3-triol C3H8O3 56-81-5

Butan-1-ol C4H10O 71-36-3

OH HO

42

OH

41

OH H3C

H3C

OH

H3C

OH

OH

CH3 43

44 (Continued)

329

Liquids and their properties

(Continued)

Table 8.7

Butan-2-ol C4H10O 78-92-2

2-Methylpropan-1-ol C4H10O 78-83-1

OH

OH

CH3 CH3

H3C

2-Methylpropan-2-ol C4H10O 75-65-0

OH

H3C

45

46

47

2-[Bis(2-hydroxyethyl)amino]ethanol C6H15NO3 102-71-6

Phenylmethanol C7H8O 100-51-6

Octan-1-ol C8H18O 111-87-5

OH

OH

OH

CH3 CH3

H3C

H3C

N

OH

OH 48

49

50

2-Methoxyethanol C3H8O2 109-86-4

H3C

2-Eethoxyethanol C4H10O2 110-80-5

OH

O

OH

H3C

51

2-(2-Hydroxyethoxy)ethanol C4H10O3 111-46-6

O

HO

52 2-Butoxyethanol C6H14O2 111-76-2

HO

OH

O

53 Oxolane C4H8O 109-99-9

1,4-Dioxane C4H8O2 123-91-1

O

O

CH3

O

O 54

55 Ethoxyethane C4H10O 60-29-7

56 1,2-Dimethoxyethane C4H10O2 110-71-4

2-Methoxy-2-methylpropane C5H12O 1634-04-4 CH3

H3C

O

CH3

O H3C

O

CH3

O

H3C 57

58

59

CH3 CH3 (Continued)

330

Handbook of Liquids-Assisted Laser processing

Table 8.7

(Continued)

1-Methoxy-2(2-methoxyethoxy)ethane C6H14O3 111-96-6

Anisole C7H8O 100-66-3

1-Butoxybutane C8H18O 142-96-1

CH3 H3C

O

O

CH3

O

60

H3C

O

61 Methyl formate C2H4O2 107-31-3

CH3

O

62 Methyl-2-methylprop2-enoate C5H8O2 80-62-6

Ethyl acetate C4H8O2 141-78-6

O O H

O

O

H3C

O

O OCH3

CH3

67 Butan-2-one C4H8O 78-93-3

Pentan-2-one C5H10O 107-87-9

CH3

Cyclohexanone C6H10O 108-94-1

O CH3

O

CH3 H3C

69

70 4-Methylpentan-2-one C6H12O 108-10-1

H3C

H3C

68

O

H3C

CH3

O

66

H3C

O

Acetone C3H6O 67-64-1

Methyl benzoate C8H8O2 93-58-3

O H3C

H 3C 65

64 Butyl acetate C6H12O2 123-86-4

O

CH3

CH3 63

H2C

71 5-Methylhexan-2-one C7H14O 110-12-3

Formamide CH3NO 75-12-7

CH3

O

O CH3

H4C

CH3

H

NH2

O 72

73

74 (Continued)

331

Liquids and their properties

(Continued)

Table 8.7

Nitromethane CH3NO2 75-52-5

Ethane-1,2-diamine C2H8N2 107-15-3

Acetonitrile C2H3N 75-05-8

O H3C

H3C

N

H2N

N

NH2

O 75

76 N,N-Dimethylmethanamide C3H7NO 68-12-2

77

O H

Pyridine C5H5N 110-86-1

N,N-Dimethylethanamide C4H9NO 127-19-5

O CH3

CH3

H3C

N

N N CH3

CH3 79

78

80 Methanedithione CS5 75-15-0

Hydrazine H4N2 302-01-2

1-Methylpyrrolidin-2-one C5H9NO 872-50-4

CH3 N

H2N

S

NH2

C

S

O 82

81

83 Silicone oil

Methylsulfinylmethane C2H6OS 67-68-5

CH3

O

Si

S H3C

Oxidane (water) H2O 7732-18-5

CH3

O

O

CH3

n

85

84 Heavy water D2O 7789-20-0

H

H

86 Carbon dioxide CO2 124-38-9

Methane CH4 74-82-8

H O 2H

87

O

2H

C

O H

98

99

H H

332

Handbook of Liquids-Assisted Laser processing

8.2 Properties of 100 Selected Liquids Most important physical and chemical properties, and references to optical spectra of 100 liquids, used or of potential importance in laser materials processing, are given in Table 8.8. All properties correspond to liquids of maximum possible purity. In case of many different values for the same property in the same source, one of the middle values was chosen. The properties of sea water are for 35–40 ‰ of salinity. All parameters are given at normal conditions, 298.15 K (25◦ C) and 1.01325 bar (1 atmosphere), unless noted with *. * and ** and *** denote that measurement conditions or composition of the substance are specified at the references below.

Definitions of the properties (More detailed definitions and further explanations for all properties are given in the Glossary.) E

Hazard codes: B Biohazard

Highly Flammable Extremely F+ Flammable

O Oxidizing

C Corrosive

Xn Harmful Xi Irritant

R Radioactive

E Explosive

N

F

Dangerous for the environment

F G H I J K L M N O P Q R

Molar mass Molar volume Density ρ dρ/dT Melting point (K) Melting point (◦ C) Hm Heat capacity Diffusion coefficient Heat conductivity Surface tension Dynamic viscosity, η d(ln η)/dT

S T U V W X Y

Relaxation time Thermal expansion Isothermal Compressibility Adiabatic compressibility Sound velocity Acoustic impedance US absorption

Z AA AB

Acoustic non-linearity Shock velocity Boiling point (K)

T Toxic T+ Very Toxic

Molar mass M (g/mol) Liquid molar volumeVliq (cm3 /mol) Density ρ (kg/m3 ) Temperature coefficient of density dρ/dT (kg/m3 K) Atmospheric (1.01325 bar) freezing/melting point Tm (K) Atmospheric (1.01325 bar) freezing/melting point Tm (◦ C) Enthalpy change of atmospheric melting (kJ/mol) Heat capacity at constant pressure Cp liq (J/mol K) Diffusion coefficient D (10−5 cm2 /s) Heat conductivity λ (J/s m K) Surface tension γ (N/m) Dynamic viscosity η (kg/m s) Temperature coefficient of dynamic viscosity d(ln η)/dT (10−2 K−1 ) Orientational relaxation time τ (ps) Volumetric thermal expansion coefficient β (K−1 ) Isothermal compressibility κT (kPa−1 ) Adiabatic compressibility κS (kPa−1 ) Longitudinal sound velocity vL (m/s) Acoustic impedance Z (Pa s/m); (1 Mrayls = 1 MPa s/m) ultrasound absorption coefficient (10−15 s2 /m), near 25◦ C at 104–107 MHz Acoustic non-linearity parameter B/A Shock velocity Us (m/s) at shock pressures close to 10 GPa Atmospheric (1.01325 bar) boiling point Tb (K)

333

Liquids and their properties

AC AD AE AF AG AH AI AJ AK AL AM AN AO AP AQ AR AS AT AU

Boiling point (◦ C) Superheat temperature Nucleation rate Hb Evaporation rate Vapour density Vapour pressure Antoine equation parameter A Antoine equation parameter B Antoine equation parameter C Saturation concentration Flash point Ignition temperature Explosion range Critical temperature (K) Critical temperature (◦ C) Critical pressure Critical volume Critical compressibility factor

AV AW AX

Pitzer acentric factor Electrical conductivity Dipole moment

AY AZ BA

Polarity parameter Dielectric constant 1000 × d ln ε/dT

BB BC BD

Magnetic susceptibility Index of refraction 1000 × dnD /dT

BE

Kerr coefficient

BF BG BH BI BJ

Scattering coefficient Depolarization factor IR spectrum IR/Raman Spectrum UV–VIS Spectrum

BK BL BM BN BO

UV cut-off point UV 5% absorption Ionization energy Hf (0) Gf (0)

BP BQ BR BS BT BU BV BW

Hildebrandt parameter Oxygen solubility Nitrogen solubility CO2 solubility Solubility in water Riddick reference Marcus reference Poling reference

Atmospheric (1.01325 bar) boiling pointTb (◦ C) Attainable atmospheric superheat temperature Tsh (◦ C) Homogeneous bubble nucleation rate J (cm−3 s−1 ) Enthalpy of vaporization at Tb (kJ/mol) Evaporation rate, ER, BuOAc Vapour density (vs. air) Vapour pressure (kPa) Antoine equation parameter A (SI system of units) Antoine equation parameter B (SI system of units) Antoine equation parameter C (SI system of units) Saturation concentration in air (g/m) Flash point (◦ C) Ignition temperature (◦ C) Vapour explosion range (vol% in air) Vapour/liquid critical temperature Tc (K) Vapour/liquid critical temperature Tc (◦ C) Vapour/liquid critical pressure Pc (bar) Vapour/liquid critical molar volume Vc (cm3 /mol) vapour/liquid critical compressibility factor Zc = Pc · Vc /(R · Tc ) Pitzer acentric factor ω = −log10 (Pvp /Pc )T /Tc = 0.7 Electrical conductivity σ (!−1 cm−1 ) Molecular dipole moment D (Debyes), 1 Debye = 3.162 × 10−25 ( J m3 )1/2 Solvent polarity parameter ETN Dielectric constant relative to vacuum ε Temperature coefficient of dielectric constant, 1000 × d ln ε/dT (K−1 ) Molar magnetic susceptibility χm (10−6 cm3 /mol) Index of refraction nD at 589 nm Temperature coefficient of the index of refraction 1000 × dnD /dT (K−1 ) Kerr coefficient B (10−9 cm−1 esE−2 ); 1 esE = 300 V/cm; Bs = Kerr coefficient of CS2 Light scattering coefficient R90 , relative to benzene Light depolarization factor u × 102 Spectrum number in the Sadtler handbook [1003] Spectrum number in the Raman/IR Atlas [1004] Spectrum number in the Perkampus UV–VIS Atlas [1005] UV cut-off point (nm) UV 5% absorption point (nm) Gas phase ionization energy (eV) Standard state enthalpy of formation Hf (0) (kJ/mol) standard state Gibbs energy of formation for gas Gf (0) (kJ/mol) Hildebrandt solubility parameter δ (MPa1/2 ) Oxygen solubility xg (mole fractions) Nitrogen solubility xg (mole fractions) Carbon dioxide solubility xg (mole fractions) Solubility in water (g/l) Substance number in Riddick handbook [1006] Substance number in Marcus handbook [1007] Substance number in Poling handbook [1008]

334

Handbook of Liquids-Assisted Laser processing

Table 8.8

Properties of 100 selected liquids listed in Table 8.6

Substance

Property code (PC) →

E

F

G

H

No.

Formula or name

CAS Reg. No.

Hazard codes

Molar mass M (g/mol)

Molar volume Vliq (cm3 /mol)

Density ρ (kg/m3 )

1

C5 H12

109-66-0

F+ Xn N

72.150

115.22

621.39

2

C5 H12

78-78-4

F+ Xn N

72.150

116.46

614.2

3

C6 H14

110-54-3

F Xn N

86.177

131.59

654.84

4

C7 H16

142-82-5

F Xn N

100.204

147.47

679.46

5

C8 H18

540-84-1

F Xn N

114.231

166.07

687.81

6

C5 H10

287-92-3

F

70.134

94.73

740.45

7

C6 H12

110-82-7

F XnN

84.161

108.75

773.89

8

C7 H14

108-87-2

F Xn N

98.188

128.35

765.06

9

C10 H18 , mix

91-17-8

CN

138.253

9a

C10 H18 , cis

493-01-6

CN

138.253

154.83

892.88

9b

C10 H18 , trans

493-02-7

CN

138.253

159.66

865.96

10

Petroleum ether

64742-49-0 101316-46-5

F+ Xn N

11

CHBr3

75-25-2

TN

252.73

12

CH2 Cl2

75-09-2

Xn

84.932

64.53

1316.78

13

CHCl3

67-66-3

Xn

119.377

80.68

1479.70

14

CCl4

56-23-5

TN

153.822

97.07

1584.36

15

CHF3

75-46-7

70.014

51.66*

16

C2 H4 Cl2

107-06-2

TF

98.96

79.45

17

C2 HCl3

79-01-6

T

131.39

1451.4*

18

C2 Cl4

127-18-4

Xn N

165.83

1614.32

19

C6 F14

355-42-0

338.044

20

C10 F18 , mix

306-94-5

462.08

20a

C10 F18 , cis

60433-11-6

462.08

20b

C10 F18 , trans

60433-12-7

462.08

21

C6 H 6

71-43-2

FT

78.114

89.41

873.60

22

C6 H5 Cl

108-90-7

Xn N

112.558

102.22

1100.9

23

C6 H4 Cl2

95-50-1

Xn N

147.00

24

C6 H3 Cl3

120-82-1

Xn N

181.45

25

C6 H 5 F

462-06-6

F Xi

96.10

1013.14*

26

C6 F 6

392-56-3

F

186.05

1607.32

0.645–0.665 2877.9

1246.37

198.91* 1.917*

1300.33

(Continued)

335

Liquids and their properties

Table 8.8

(Continued)

Substance

Property code (PC) →

E

F

G

H

Hazard codes

Molar mass M (g/mol)

Molar volume Vliq (cm3 /mol)

Density ρ (kg/m3 )

No.

Formula or name

CAS Reg. No.

27

C7 H 5 N

100-47-0

28

C7 H 8

108-88-3

F Xn

92.141

29

C8 H 8

100-42-5

Xn

104.15

30

C8 H10

95-47-6

Xn

106.167

121.25

875.94

31

C8 H10

108-38-3

Xn

106.167

123.47

860.09

32

C8 H10

106-42-3

Xn

106.167

123.93

856.61

33

C9 H12

98-82-8

Xn N

120.194

140.17

857.43

34

C10 H7 Cl

90-13-1

35

C11 H10

90-12-0

Xn N

142.200

139.37*

1016.76

36

CH4 O

67-56-1

FT

32.042

40.73

786.37

37

C2 H 6 O

64-17-5

F

46.069

58.68

784.93

38

C2 H 6 O2

107-21-1

Xn

62.07

1110.0

39

C 2 H 3 F3 O

75-89-8

Xn

100.04

1373.6*

40

C2 H7 NO

141-43-5

C

61.08

1012.7

41

C3 H 8 O

71-23-8

F Xi

60.096

75.14

799.60

42

C3 H 8 O

67-63-0

F Xi

60.096

76.92

781.26

43

C3 H 8 O3

56-81-5

44

C4 H10 O

71-36-3

Xn

74.123

91.96

805.75

45

C4 H10 O

78-92-2

Xi

74.123

92.35

802.41

46

C4 H10 O

78-83-1

Xi

74.123

92.91

797.8

47

C4 H10 O

75-65-0

F Xn

74.123

94.88

775.45*

48

C6 H15 NO3

102-71-6

49

C7 H 8 O

100-51-6

50

C8 H18 O

51

103.12

1000.6 106.87

862.19 901.22

162.62

1193.82*

92.09

1255.9

149.19

1119.6

Xn

108.140

1041.27

111-87-5

Xi

130.230

C3 H 8 O2

109-86-4

T

76.09

960.24

52

C4 H10 O2

110-80-5

T

90.12

925.20

53

C4 H10 O3

111-46-6

Xn

106.12

1116.4*

54

C6 H14 O2

111-76-2

Xn

118.17

896.25

55

C4 H 8 O

109-99-9

F Xi

72.107

81.71

889.2*

56

C4 H 8 O2

123-91-1

F Xn

88.106

85.29*

1027.97

57

C4 H10 O

60-29-7

F+ Xn

74.123

104.75

707.82

158.37

821.57

(Continued)

336

Handbook of Liquids-Assisted Laser processing

Table 8.8

(Continued)

Substance

Property code (PC) →

E

F

G

H

No.

Formula or name

CAS Reg. No.

Hazard codes

Molar mass M (g/mol)

Molar volume Vliq (cm3 /mol)

Density ρ (kg/m3 )

58

C4 H10 O2

110-71-4

FT

90.126

104.56

863.70

59

C5 H12 O

1634-04-4

F Xi

88.15

60

C6 H14 O3

111-96-6

T

134.17

938.4

61

C7 H 8 O

100-66-3

108.14

989.32

62

C8 H18 O

142-96-1

Xi

130.23

764.1

63

C2 H4 O2

107-31-3

F+ Xn

60.053

62.14

966.4

64

C4 H8 O2

141-78-6

F Xi

88.106

98.55

894.55

65

C5 H8 O2

80-62-6

F Xi

100.12

66

C6 H12 O2

123-86-4

67

C8 H8 O2

93-58-3

Xn

136.15

68

C3 H 6 O

67-64-1

F Xi

58.080

73.94

784.40

69

C4 H 8 O

78-93-3

F Xi

72.107

90.13

799.7

70

C5 H10 O

107-87-9

F

86.134

107.33

801.5

71

C6 H10 O

108-94-1

Xn

98.144

72

C6 H12 O

108-10-1

F Xn

100.161

73

C7 H14 O

110-12-3

Xn

114.19

74

CH3 NO

75-12-7

T

45.04

75

CH3 NO2

75-52-5

Xn

61.040

76

C2 H 3 N

75-05-8

F Xn

41.05

776.49

77

C2 H 8 N 2

107-15-3

C

60.10

893.1

78

C3 H7 NO

68-12-2

T

73.09

943.87

79

C4 H9 NO

127-19-5

T

87.12

936.337

80

C5 H 5 N

110-86-1

F Xn

79.101

81

C5 H9 NO

872-50-4

Xi

99.13

82

H4 N2

302-01-2

83

CS2

75-15-0

84

C2 H6 OS

67-68-5

85

[-Si(CH3 )2 O-]n

63148-62-9

86

H2 O

7732-18-5

18.015

18.07

997.0474

87

D2 O

7789-20-0

20.028

18.13

1104.36

116.160

32.045 FT

943.31* 132.51

876.36 1079.01*

945.2* 125.81

796.3

1129.15 53.96

80.88

1131.28

978.24 1025.9

31.79*

76.14

1255.5

78.13

1095.37

(Continued)

337

Liquids and their properties

Table 8.8

(Continued)

Substance

Property code (PC) →

F

G

H

Hazard codes

Molar mass M (g/mol)

Molar volume Vliq (cm3 /mol)

Density ρ (kg/m3 )

No.

Formula or name

88

Sea water

89

H2

1333-74-0

2.016

28.39*

70.721*

90

D2

7782-39-0

4.0282

24.41*

163.94*

91

He

7440-59-7

4.0026

32.54*

125.01*

92

N2

7727-37-9

28.014

34.84*

807.14*

93

O2

7782-44-7

31.999

27.85*

1141.8*

94

Ne

7440-01-9

20.180

16.76*

1207.7*

95

Ar

7440-37-1

39.948

29.10*

1397.1*

96

Kr

7439-90-9

83.800

34.63*

2416.3*

97

Xe

7440-63-3

131.290

42.91*

2947.2*

98

CO2

124-38-9

44.010

99

CH4

74-82-8

35.54*

422.7*

100

Air

(PC) →

No.

CAS Reg. No.

E

F+

16.0428 28.958

875.99*

I

J

K

L

M

N

O

dρ/d T (kg/m3 K)

Melting point Tm (K)

Melting point Tm (◦ C)

Hm (kJ/mol)

Heat capacity Cp liq ( J/mol K)

Diffision coefficient D (10−5 cm2 /s)

Heat conductivity λ ( J/s m K)

1

−0.975

143.43

−129.72

8.40

167.19

5.62

2

−1.02

113.26

−159.89

5.16

164.80

4.85

3

−0.891

177.84

−95.31

13.07

195.43

4.21

4

−0.840

182.59

−90.56

14.03

224.98

3.11

5

−0.824

165.80

−107.35

9.04

238.55

2.42

6

−0.986

179.28

−93.87

0.61

126.80

7

279.69

6.54

2.63

156.20

8

146.56

−126.59

6.75

184.50

0.123*

0.0967*

1.41

9 9a

−0.760

230.14

−43.01

232.00

9b

−0.749

242.75

−30.40

228.50

140

86

1498

1.494

373.15

100

87

1400

1.546

374.55

101.4

88

1531

89

1098*

11800*

20.345

−252.805

90

876*

10970*

23.264

−249.886

5.25*

(Continued)

348 Table 8.8 (PC) →

Handbook of Liquids-Assisted Laser processing

(Continued) W

X

Y

Z

AA

AB

AC

No.

Sound velocity vL (m/s)

Acoustic impedence Z (Mrayls = MPa s/m)

Ultrasound absorption (10−15 s2 /m)

Acoustic non-linearity B/A

Shock velocity Us (m/s)

Boiling point Tb (K)

Boiling point Tb (◦ C)

91

177*

92

851*

93

905*

94

595*

95

831*

96

691*

119.62

−153.53

97

639*

164.78

−108.37

98

839*

99

1340*

111.51

−161.64

100

865*

78.9

−194.25

(PC) →

AD

AE

AF

AG

AH

AI

AJ

No.

Superheat temperature Tsh (◦ C)

Nucleation rate J (cm−3 s−1 )

Hb (kJ mol−1 )

Evaporation rate BuOAc

Vapour density vs. air

Vapour pressure (kPa)

Antoine equation parameter A

1

145

104 −1018

25.79

2.48

68.33

5.97786

24.69

2.6

91.7

5.92023

∼3

20.17

6.00091

2 3

138 182

−268.928

4.2221 6.6*

5090*

77.237

−195.913

4644*

90.062

−183.088

27.061

−246.089

87.169

−185.981

6052*

10−10

7 20

28.85

18

31.77

3.5

6.09

6.02167

30.79

3.9

6.5

5.92885

100−10 6

10 −10

8.9

4

213.5

5

215.3

6

180

106

27.30

∼2

42.4

6.04584

7

218.5

106

29.97

2.9

13.04

5.96407

8

237.2

31.27

3.4

6.1

5.94790

4.76

9 9a

41.00

0.10

6.00019

9b

40.20

0.164

5.98171

10

2.5

11

8.7

0.79

6.15631

12

179.9

13

173

14

25*

28.06

14.5

2.9

58.10

6.07622

100

29.24

10.45

4.1

25.97

5.96288

1000

29.82

6.0

5.32

15.36

6.10445 (Continued)

349

Liquids and their properties

Table 8.8 (PC) → No.

(Continued) AD

AE

AF

AG

AH

AI

AJ

Superheat temperature Tsh (◦ C)

Nucleation rate J (cm−3 s−1 )

Hb (kJ mol−1 )

Evaporation rate BuOAc

Vapour density vs. air

Vapour pressure (kPa) [mmHg]

Antoine equation parameter A

2.43

15 31.98

4.46

3.4*

11.11*

6.28356

4.46

4.5

6.307

6.15298

2.10

5.83

2.462

6.10170

17.5

[6.6]*

2.77

12.7

6.02232

3.86

1.567

6.30963

5.1

0.171

6.19518

31.20

10.48

6.07698

31.670

10.733*

6.14231

0.1*

5.87121

3.2

3.8036

6.08540

3.6

0.841

6.34792

36.24

3.7

0.88

6.13072

35.66

3.7

1.1

6.13785

32

35.67

3.7

1.2

6.11140

33

37.50

4.1

0.61

6.06588

16 17 18 19

136.6

10

6

78.7

20 20a 20b 21

225.3

100−1018

30.72

22

250

100

35.19

5.1

0.15

23

>6

24 25 26

191.7

10−10

6

27 28

253.5

100

33.18

1.90

29 30 31

235

0.052*

34 46.00

35

0.00895

6.16082

36

186

10–1018

35.21

2.10

1.11

16.937

7.20519

37

190.9

10–104

38.56

1.60

1.59

7.870

7.16879