Thermochemical Processes Principles and Models Thermochemical Processes Principles and Models C.B. Alcock DSc, FRSC OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Avenue, Woburn, MA 01801-2041 A division of Reed Educational and Professional Publishing Ltd First published 2001  C.B. Alcock 2001 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder’s written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data Alcock, C. B. Thermochemical processes:principles and models 1 Thermodynamics 2 Chemical processes 3 Materials at high temperatures I Title 660.2 0 969 Library of Congress Cataloguing in Publication Data Alcock, C. B. Thermochemical processes:principles and models/C.B. Alcock. p. cm. Includes bibliographical references. ISBN 0 7506 5155 5 1 Thermodynamics 2 Chemical processes 3 Materials at high temperatures I Title TP155.2.T45 T47 2000 660 0 .2969–dc21 00-049367 ISBN 0 7506 5155 5 Typeset by Laser Words, Chennai, India Printed in Great Britain Contents Preface xi Part 1: Processes with gaseous reaction control 1 1 Vapour deposition processes 3 Vapour deposition for the preparation of thin films 3 Vapour pressure data for the elements 4 The kinetic theory of a gas in a container 4 Molecular effusion 6 Vapour deposition of elements 6 The deposition rate on a cool substrate 8 Vapour deposition of alloys 8 Vapour deposition of compounds 10 Free evaporation coefficients of solids 11 Other techniques for the preparation of thin films 16 Single and epitaxial films in semiconducting systems 16 Thin film production by the sputtering of metals 17 The production of nanoparticles 20 Coating with thin diamond films 22 Plasma evaporation and pyrolysis of carbon to form Fullerenes 23 Materials science and the formation of thin films 24 The formation of nuclei from the vapour phase 24 The formation of a film from nuclei 28 Grain growth in the initial deposit 30 Point defects in solids 31 Edge and screw dislocations 33 Interfacial energies in solid systems 35 Bibliography 38 Appendix: Vapour pressure data for the elements 38 2 Gaseous reaction kinetics and molecular decomposition 42 Theories of reaction kinetics 42 Thermal energies and the structures of molecules 43 The collision theory of gaseous reactions 45 Transition state theory of gaseous reactions 47 Empirical estimates of the activation energy 49 The order of chemical reactions 50 vi Contents Time dependence of the extent of reaction 52 Chain reactions 52 Combustion chain reactions 53 Chain reactions in the combustion of gaseous fuels 56 Fuel/air mixing in combustion systems 58 The thermal efficiencies of combustion engines 59 Bibliography 62 Molecular dissociation and chain reactions in chemical vapour deposition 62 Thermochemical data for the dissociation of gaseous molecules 63 Bond character in gaseous heteronuclear compounds 64 Hybridization of covalent bonds 66 Bond energies of gaseous polyvalent metal halides 67 Thermal decomposition of hydrides and organometallic compounds 68 Bibliography 71 Radiation and electron decomposition of molecules 72 Photochemical reactions 73 Dissociation cross-sections 75 Substrate heating by transmitted radiation 77 Radiation and convection cooling of the substrate 82 Laser production of thin films 82 Molecular decomposition in plasma systems 84 Bibliography 85 3 Vapour phase transport processes 86 Vapour transport processes 86 Thermodynamics and the optimization of vapour phase transport 86 The direction of vapour transport across a thermal gradient 89 The choice of halogen in transport reactions 91 The vapour phase refining and separation of metals 91 The thermodynamics of the vapour phase transport of compounds 93 Multicomponent thermodynamics in gaseous systems 95 Sintering by vapour phase transport 99 Grain growth by vapour phase transport 100 Vapour transport in flowing systems 102 Transport along a thermal gradient 102 Mass transport across a flowing gas 103 Material deposition from a flowing gas 106 Transport and thermal properties of gases 108 Equations of state for ideal and real gases 112 Molecular interactions and the properties of real gases 114 Bibliography 117 4 Heterogeneous gas –solid surface reactions 118 The zeroth order reaction 118 Adsorption of gases on solids 119 Surface structures of catalytic materials 124 Adsorption and the surface energies of metals 125 Contents vii Bond mechanisms of adsorbed molecules 126 Supported metal catalysts 128 Examples of industrially important catalysts 129 Thermodynamics of the water–gas shift and steam reforming reactions 129 Kinetic factors in steam reforming 132 The Fischer–Tropsch production of organic molecules 134 The production of ammonia from its elements 136 The catalytic converter for automobile exhaust 138 Catalysis by metal oxides 140 Coupling reactions of methane 142 Reactors for catalytic processes 143 Bibliography 145 Part 2: Rate processes in the solid state 147 5 Electrical charge and heat transport in solids 149 The transport of electrons and positive holes 149 Metals and alloys 149 Electromigration in alloys 153 Elemental and compound semiconductors 154 Metal oxides 158 Thermal transport in condensed phases 163 Heat capacities 164 Thermal conductivity 166 Bibliography 169 6 Rate processes in metals and alloys 170 Structure and diffusion-controlled processes in metallic systems 170 The structures of metals 170 Volume diffusion in pure metals 170 Diffusion in inter-metallic compounds 176 Diffusion in alloys 177 Steady state creep in metals 180 Diffusion in interstitial solutions and compounds 181 Phase transformations in alloys 184 The decomposition of Austenite 184 Transformations in substitutional alloys 188 Order–disorder transformation 189 The age-hardening of copper–aluminium alloys 190 Spinodal decomposition of binary alloys 190 Metals and alloys in nuclear power reactors 194 Bibliography 195 Grain boundary and surface-driven properties in metallic systems 195 The measurement of the surface energies of metals 196 Diffusion in grain boundaries and dislocations 197 Surface diffusion on metals 199 viii Contents Powder metallurgy 201 The production of metal powders 201 The sintering of solid metal particles 204 Hot pressing 207 Ostwald ripening 209 Grain growth in polycrystalline metals 213 Processing of powders to form metallic articles 214 Self-propagating combustion reactions 216 Inter-diffusion and interaction in thin film microelectronic structures 219 Bibliography 221 7 Rate processes in non-metallic systems 223 Diffusion in elemental semiconductors 223 Structures and diffusion in metal oxides 224 The measurement of diffusion coefficients in simple oxides 229 Surfaces and surface energies in ionic crystals 232 Sintering of metal oxides 233 The production and applications of ceramic oxide materials 234 Electroceramic oxides 236 Dielectric or ferroelectric oxides 236 Magnetic oxides 237 Solid electrolyte sensors and oxygen pumps 239 Solid oxide fuel cells and membranes 244 Ceramic superconductors 247 The redistribution of fission products in UO 2 nuclear fuels 249 Bibliography 250 8 Gas–solid reactions 251 The oxidation of metals and compounds 251 The parabolic rate law 251 The linear and logarithmic rate laws 252 Oxidation of metals forming more than one oxide 253 The oxidation of nickel: volume and grain boundary diffusion 254 The oxidation of silicon 255 Complex oxide formation in the oxidation of alloys 256 Internal oxidation of alloys 257 The theory of the parabolic oxidation law 260 The carburizing and oxidation of transition metals 262 The oxidation of metallic carbides and silicides 266 The oxidation of silicon carbide and nitride 268 Bibliography 269 9 Laboratory studies of some important industrial reactions 270 The reduction of haematite by hydrogen 270 Erosion reactions of carbon by gases 271 The combustion of coal 273 The oxidation of FeS – parabolic to linear rate law transition 274 Oxidation of complex sulphide ores – competitive oxidation of cations 275 Contents ix The kinetics of sulphation roasting 276 Heat transfer in gas–solid reactions 277 Industrial reactors for iron ore reduction to solid iron 279 The industrial roasting of sulphides 281 The corrosion of metals in multicomponent gases 283 Bibliography 285 Appendix: Thermodynamic data for the Gibbs energy of formation of metal oxides 285 Part 3: Processes involving liquids 289 10 Physical properties and applications of liquid metals 291 The structures and mechanism of diffusion of liquid metals 291 Thermophysical properties of liquid metals 294 Viscosities of liquid metals 294 Surface energies of liquid metals 295 Thermal conductivity and heat capacity 296 The production of metallic glasses 297 Liquid metals in energy conversion 300 Liquid phase sintering of refractory materials 301 Bibliography 304 The production of crystalline semiconductors 304 Zone refining of semiconducting elements 304 11 Physical and chemical properties of glassy and liquid silicates 307 Metal solubilities in silicate glasses 310 The production of silicate glasses and glass-containing materials 310 The production of porcelains 311 Ceramic electrical insulators 313 The production of glass-ceramics 313 Cements 314 Optical fibres 315 Chalcogenide glasses 315 Bibliography 316 12 The structures and thermophysical properties of molten salts 317 Hot corrosion of metals by molten salts 319 Molten carbonate fuel cells 321 Bibliography 322 13 Extraction metallurgy 323 The principles of metal extraction 324 Metal–slag transfer of impurities 324 The electron balance in slag– metal transfer 327 Bubble formation during metal extraction processes 328 The corrosion of refractories by liquid metals and slags 329 Extractive processes 330 The production of lead and zinc 330 Co-production of lead and zinc in a shaft furnace 332 x Contents The ironmaking blast furnace 333 The reduction of stable oxides in carbon arc furnaces 335 Steelmaking and copper production in pneumatic vessels 337 Steel 337 Copper 339 The reduction of oxides and halides by reactive metals 341 Magnesium 341 Chromium 342 Manganese 343 Heat losses in crucible reactions 344 Zirconium 345 Uranium 346 The electrolysis of molten salts 347 Magnesium 347 Sodium 347 Aluminium 348 Refractory metals 349 Bibliography 349 14 The refining of metals 351 The effect of slag composition on impurity transfer 351 The thermodynamics of dilute solutions 354 The refining of lead and zinc 356 The separation of zinc and cadmium by distillation 357 De-oxidation of steels 360 Vacuum refining of steel 361 Refining by liquid salts and the Electroslag process 363 15 Factorial analysis of metal-producing reactions 365 Bibliography 369 Index 371 Preface This book is intended to be a companion to Kubaschewski’s Metallurgical Thermochemistry, and as such deals primarily with the kinetic and transport theory of high temperature chemical reactions. I have chosen the title Thermo- chemical Processes rather than High Temperature Materials Chemistry since many of the important industrial processes which are described hardly deserve the high temperature connotation, and such a title would have implied a larger structural and thermodynamic content than is required for the description of the industrial processing of materials. It will be seen that the book has a significant content from the chemical engineer’s approach, and I feel that this rapprochement with the materials scientist is overdue. The origins of the material contained in this book are to be found in the rapid growth of the scientific description of extractive metallurgical processes which began after World War II. This field was dominated by thermody- namics originally, and the development of kinetic and transport descriptions of these processes followed later. At that time the study of glasses and ceramics was largely confined to phase diagrams of the multicomponent systems, and processes in which gaseous reaction kinetics were rate-controlling were of more interest to the chemist than to the materials scientist, a field which, practically, did not exist in that era. The quantitative description of materials processing has now advanced to the state where most of the processes which are in industrial use can be described within a logical physico-chemical framework. The pace of devel- opment in this field has largely been determined by the rate of improvement of our experimental capabilities in high temperature chemistry; the ab initio theoretical contribution to the building of our present knowledge is growing rapidly under the influence of computer capabilities which simplify the funda- mental basis for apriori calculation. However, the processes and substances with which the materials scientist works are usually complex, and the preci- sion of the information which is required to describe a process accurately is still too high to be calculated theoretically. The practical situation can now be assessed from the substantial results of experimental studies which cover almost every situation to be found on the present industrial scene. The role of the physico-chemical study of materials processing has been consigned to a secondary position of interest by those engaged directly in [...]... species S(s) ! S(g) C S2 (g) and higher polymers is less than unity, and increases with temperature 12 Thermochemical Processes: Principles and Models Oxides such as MgO and Al2 O3 , also have coefficients which are less than unity, between 0.1 and 0.5, depending on the temperature Data for the evaporation mechanisms of these systems can be obtained from mass spectrometry and, as is the case for the... where 2 cm s 1 2 is the density of the gas in particles/cm3 6 Thermochemical Processes: Principles and Models The relationship between the average velocity c and c2 is 1/3 c2 D c D 8p/ /8 c 1/2 2 D 8RT/ M R D 8.314 ð 107 ergs K 1 mol 1/2 1 D 8.314 J K n0 D 1/4 nc D 3.64 ð 103 n T/M 1 mol 1 and therefore 1/2 where T is the absolute temperature and M is the molecular weight of the gas The mass of gas impinging... evaporation and pyrolysis of carbon to form Fullerenes The vapour phase in the evaporation of carbon at high temperatures contains a number of gaseous species C, C2 , C3 and higher polymers Of these the first 24 Thermochemical Processes: Principles and Models three molecules constitute the major species at 4000 K, the relative partial pressures favouring the trimer at 1 atmos pressure, and the monomer... radiation of suitable wavelength and particles, such as electrons and gaseous ions with sufficient kinetic energies, can be used to bring about molecular decomposition It follows that the development of lasers and plasmas has considerably increased the scope and utility of these thermochemical processes These topics will be considered in the later chapters As with all of the processes described, these are... re-evaporation of the atoms comprising the thin film, and hence there is this practical limit on the choice of the substrate temperature during film formation The individual processes which take part in thin film production are thus: 1 The vaporization of elements 2 Formation of nuclei of the condensing substance on a support 4 Thermochemical Processes: Principles and Models 3 Growth of nuclei by surface diffusion... Processes: Principles and Models (111) plane (100) plane (010) plane (a) A B B A A B In-plane atoms A atoms above and below plane (Hexagonal structure) A atoms above and B atoms below (F.C.C structure) (b) Figure 1.4 (a) Close packing of atoms in a cubic structure, showing six in-plane neighbours for each atom; (b) An expanded diagram of the packing of atoms above and below the plane A above and A below represents... gradient in and immediately below the surface This will lower the evaporation rate compared to that which is calculated from the apparent, bulk, temperature of the evaporating sample as observed by optical pyrometry, and thus lead to an apparently low free surface vaporization coefficient This is probably the case in the evaporation of alumina in a vacuum, 16 Thermochemical Processes: Principles and Models. .. the vapour pressure of the pure element at the temperature T, He and Se are the standard heat and entropy of vaporization (See Appendix for current data for many elements.) It will be seen throughout this discussion of thermochemical processes that these require a knowledge of both thermodynamic and kinetic data for their analysis, and while kinetic theory obviously determines the rate at which any... expression for the flux JA D pA 1/2 MA RT 1/2 where MA is the molecular weight of the species A It follows that in the evaporation of a binary alloy, JA /JB D pA /pB MB /MA 1/2 10 Thermochemical Processes: Principles and Models and on substituting for the vapour pressure ratio using the thermodynamic functions, that JA /JB D ° ° A XA pA / B XB pB MB /MA 1/2 Hence, the evaporation rate of each element will... earths and the refractory metals, have a high affinity for oxygen, so vaporization of these elements in a ‘normal’ vacuum of about 10 4 Pa, would lead to the formation of at least a surface layer of oxide on a deposited film The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10 9 Pa 8 Thermochemical Processes: Principles and Models . Thermochemical Processes Principles and Models Thermochemical Processes Principles and Models C.B. Alcock DSc, FRSC OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE. publishers British Library Cataloguing in Publication Data Alcock, C. B. Thermochemical processes: principles and models 1 Thermodynamics 2 Chemical processes 3 Materials at high temperatures I Title 660.2 0 969 Library. Publication Data Alcock, C. B. Thermochemical processes: principles and models/ C.B. Alcock. p. cm. Includes bibliographical references. ISBN 0 7506 5155 5 1 Thermodynamics 2 Chemical processes 3 Materials