Chapter 1 Ceramic Fabrication Processes: An Introductory
Overview .............................................. 1
1.1 Introduction ............................................... 1
1.2 Ceramic Fabrication Processes .............................. 2
1.2.1 Gas-Phase Reactions .................................. 2
1.2.1.1 Chemical Vapor Deposition .................... 4
1.2.1.2 Directed Metal Oxidation ..................... 7
1.2.1.3 Reaction Bonding ............................. 8
1.2.1.4 Silicon Nitride .............................. 9
1.2.1.5 Oxides ...................................... 10
1.2.1.6 Silicon Carbide ............................. 11
1.2.2 Liquid Precursor Methods ............................ 12
1.2.2.1 Sol-Gel Processing .......................... 12
1.2.2.2 Polymer Pyrolysis ........................... 15
1.2.2.3 Silicon Carbide ............................. 15
1.2.2.4 Silicon Nitride ............................. 17
1.2.2.5 Boron Nitride and Boron Carbide ............. 18
1.2.3 Fabrication from Powders ............................ 19
1.2.3.1 Melt Casting ................................ 19
1.2.3.2 Sintering of Compacted Powders .............. 20
1.3 Production of Polycrystalline Ceramics from Powders:
An Overview ............................................... 21
1.3.1 Powder Synthesis and Powder Characterization ........ 21
1.3.2 Powder Consolidation ................................ 22
1.3.3 The Sintering Process ............................... 22
1.3.4 Ceramic Microstructures ............................. 25
1.4 Case Study in Processing: Fabrication of TiO2 from
Powders ................................................... 28
1.5 Concluding Remarks ........................................ 30
Problems ....................................................... 30
References ..................................................... 32
Chapter 2 Synthesis of Powders ................................. 37
2.1 Introduction .............................................. 37
2.2 Powder Characteristics .................................... 37
2.3 Powder Synthesis Methods .................................. 38
2.4 Powder Preparation by Mechanical Methods .................. 39
2.4.1 Comminution ......................................... 39
2.4.1.1 High Compression Roller Mills ............... 40
2.4.1.2 Jet Mills ................................... 41
2.4.1.3 Ball Mills .................................. 41
2.4.2 Mechanochemical Synthesis ........................... 45
2.5 Powder Synthesis by Chemical Methods ...................... 46
2.5.1 Solid-State Reactions ............................... 46
2.5.1.1 Decomposition ............................... 47
2.5.1.2 Chemical Reaction between Solids ............ 53
2.5.1.3 Reduction ................................... 57
2.5.2 Liquid Solutions .................................... 58
2.5.2.1 Precipitation from Solution ................. 58
2.5.2.2 Evaporation of the Liquid ................... 74
2.5.2.3 Gel Route ................................... 79
2.5.2.4 Nonaqueous Liquid Reaction .................. 81
2.5.3 Vapor-Phase Reactions ............................... 81
2.5.3.1 Gas-Solid Reaction .......................... 82
2.5.3.2 Reaction between a Liquid and a Gas ......... 84
2.5.3.3 Reaction between Gases ...................... 84
2.6 Special Topic: Synthesis of Nanoscale Ceramic Powders ...... 87
2.6.1 Liquid Solution Techniques .......................... 87
2.6.2 Vapor-Phase Techniques .............................. 88
2.7 Concluding Remarks ........................................ 89
Problems ....................................................... 89
References ..................................................... 91
Chapter 3 Powder Characterization .............................. 97
3.1 Introduction .............................................. 97
3.2 Physical Characterization ................................. 97
3.2.1 Types of Particles .................................. 98
3.2.1.1 Primary Particles ........................... 98
3.2.1.2 Agglomerates ................................ 98
3.2.1.3 Particles ................................... 98
3.2.1.4 Granules .................................... 99
3.2.1.5 Flocs ....................................... 99
3.2.1.6 Colloids .................................... 99
3.2.1.7 Aggregates .................................. 99
3.2.2 Particle Size and Particle Size Distribution ........ 99
3.2.2.1 Definition of Particle Size ................ 100
3.2.2.2 Average Particle Size ...................... 101
3.2.2.3 Representation of Particle Size Data ....... 102
3.2.3 Particle Shape ..................................... 106
3.2.4 Measurement of Particle Size and Size
Distribution ....................................... 106
3.2.4.1 Microscopy ................................. 106
3.2.4.2 Sieving .................................... 107
3.2.4.3 Sedimentation .............................. 109
3.2.4.4 Electrical Sensing Zone Techniques (the
Coulter Counter) ........................... 110
3.2.4.5 Light Scattering ........................... 111
3.2.4.6 X-Ray Line Broadening ...................... 113
3.2.5 Surface Area ....................................... 113
3.2.6 Porosity of Particles .............................. 116
3.2.6.1 Gas Adsorption ............................. 116
3.2.6.2 Mercury Porosimetry ........................ 119
3.2.6.3 Pycnometry ................................. 121
3.3 Chemical Composition ..................................... 122
3.3.1 Optical Atomic Spectroscopy: Atomic Absorption
and Atomic Emission ................................ 122
3.3.2 X-Ray Fluorescence Spectroscopy .................... 124
3.4 Crystal Structure and Phase Composition .................. 125
3.5 Surface Characterization ................................. 127
3.5.1 Surface Structure .................................. 129
3.5.2 Surface Chemistry .................................. 130
3.5.2.1 Auger Electron Spectroscopy (AES) .......... 130
3.5.2.2 X-Ray Photoelectron Spectroscopy (XPS) ..... 132
3.5.2.3 Secondary Ion Mass Spectrometry (SIMS) ..... 133
3.6 Concluding Remarks ....................................... 136
Problems ...................................................... 136
References .................................................... 138
Chapter 4 Science of Colloidal Processing ..................... 141
4.1 Introduction .............................................. 141
4.2 Types of Colloids ......................................... 142
4.3 Attractive Surface Forces ................................. 142
4.3.1 van der Waals Forces between Atoms and Molecules ... 142
4.3.2 van der Waals Forces between Macroscopic Bodies .... 144
4.3.3 The Hamaker Constant ............................... 146
4.3.4 Effect of the Intervening Medium ................... 148
4.4 Stabilization of Colloidal Suspensions .................... 148
4.5 Electrostatic Stabilization ............................... 149
4.5.1 The Development of Charges on Oxide Particles in
Water .............................................. 149
4.5.1.1 Isomorphic Substitution .................... 150
4.5.1.2 Adsorption of Ions from Solution ........... 150
4.5.2 Origins of the Electrical Double Layer ............. 153
4.5.3 Isolated Double Layer .............................. 154
4.5.4 Surface Charge ..................................... 157
4.5.5 Repulsion between Two Double Layers ................ 158
4.5.6 Stability of Electrostatically Stabilized
Colloids ........................................... 160
4.5.7 Kinetics of Flocculation ........................... 161
4.5.8 Electrokinetic Phenomena ........................... 162
4.5.8.1 Microelectrophoretic Technique ............. 163
4.5.8.2 Significance of the ζ Potential ............ 164
4.6 Steric Stabilization ..................................... 164
4.6.1 Adsorption of Polymers from Solution ............... 165
4.6.2 Origins of Steric Stabilization .................... 167
4.6.3 Effects of Solvent and Temperature ................. 168
4.6.4 Stability of Sterically Stabilized Suspensions ..... 170
4.6.5 Stabilization by Polymers in Free Solution ......... 171
4.7 Electrosteric Stabilization .............................. 172
4.7.1 Dissociation of Polyelectrolytes in Solution ....... 172
4.7.2 Adsorption of Polyelectrolytes from Solution ....... 174
4.7.3 Stability of Electrosterically Stabilized
Suspensions ........................................ 175
4.8 Structure of Consolidated Colloids ....................... 176
4.9 Rheology of Colloidal Suspensions ........................ 178
4.9.1 Rheological Properties ............................. 178
4.9.2 Factors Influencing the Rheology of Colloidal
Suspensions ........................................ 180
4.9.2.1 Influence of Interparticle Forces .......... 180
4.9.2.2 Influence of Particle Interactions on the
Viscosity .................................. 183
4.9.2.3 Influence of Particle Characteristics ...... 185
4.10 Concluding Remarks ....................................... 187
Problems ...................................................... 187
References .................................................... 189
Chapter 5 Sol-Gel Processing .................................. 193
5.1 Introduction ............................................. 193
5.2 Types of Gels ............................................ 194
5.2.1 Particulate Gels ................................... 194
5.2.2 Polymeric Gels ..................................... 195
5.3 Sol-Gel Processing of Aqueous Silicates .................. 197
5.3.1 Effect of pH ....................................... 198
5.3.1.1 Polymerization in the pH range of 2 to 7 ... 198
5.3.1.2 Polymerization above pH ≈ 7 ................ 200
5.3.1.3 Polymerization below pH ≈ 2 ................ 200
5.4 Metal Alkoxides .......................................... 201
5.4.1 Preparation of Metal Alkoxides ..................... 201
5.4.1.1 Reactions between Metals and Alcohols ...... 201
5.4.1.2 Reactions Involving Metal Chlorides ........ 202
5.4.1.3 Miscellaneous Methods ...................... 202
5.4.2 Basic Properties ................................... 204
5.4.2.1 Physical Properties ........................ 204
5.4.2.2 Chemical Properties ........................ 205
5.5 Sol-Gel Processing of Silicon Alkoxides .................. 206
5.5.1 Precursors ......................................... 206
5.5.2 Hydrolysis and Condensation ........................ 207
5.5.2.1 Acid-Catalyzed Conditions .................. 208
5.5.2.2 Base-Catalyzed Conditions .................. 209
5.5.3 Polymer Growth ..................................... 209
5.5.3.1 Polymer Growth Models ...................... 210
5.5.3.2 Structural Evolution of Sol-Gel
Silicates .................................. 212
5.5.3.3 Rheological Measurements ................... 212
5.5.4 Gelation ........................................... 215
5.5.5 Aging of Gels ...................................... 216
5.5.6 Drying of Gels ..................................... 216
5.5.6.1 Conventional Drying ........................ 217
5.5.6.2 Supercritical Drying ....................... 227
5.5.6.3 Structural Changes during Drying ........... 229
5.5.7 Gel Densification during Sintering ................. 230
5.6 Sol-Gel Preparation Techniques ........................... 234
5.6.1 Preparation of Particulate Gels .................... 234
5.6.1.1 Single-Component Gels ...................... 234
5.6.1.2 Multicomponent Gels ........................ 236
5.6.2 Preparation of Polymeric Gels ...................... 237
5.6.2.1 Use of Double Alkoxides .................... 237
5.6.2.2 Partial Hydrolysis of the Slowest
Reacting Alkoxide .......................... 238
5.6.2.3 Use of a Mixture of Alkoxides and Metal
Salts ...................................... 239
5.6.2.4 Slow Addition of Small Amounts of Water .... 240
5.6.2.5 Matched Hydrolysis Rates ................... 241
5.7 Applications of Sol-Gel Processing ....................... 241
5.7.1 Thin Films and Coatings ............................ 242
5.7.1.1 Dip Coating ................................ 242
5.7.1.2 Spin Coating ............................... 244
5.7.1.3 Structural Evolution in Films .............. 245
5.7.1.4 Applications of Films ...................... 245
5.7.2 Fibers ............................................. 245
5.7.3 Monoliths .......................................... 246
5.7.4 Porous Materials ................................... 247
5.8 Concluding Remarks ....................................... 247
Problems ...................................................... 247
References .................................................... 249
Chapter 6 Mixing and Packing of Powders ....................... 253
6.1 Introduction ............................................. 253
6.2 Mixing of Particulate Solids ............................. 254
6.2.1 Types of Mixtures .................................. 254
6.2.2 Mixing and Segregation Mechanisms .................. 254
6.2.3 Mixture Composition and Quality .................... 257
6.2.3.1 Statistical Methods ........................ 257
6.2.3.2 Measurement Techniques ..................... 259
6.2.4 Mixing Technology .................................. 260
6.3 Packing of Particles ..................................... 262
6.3.1 Regular Packing of Monosized Spheres ............... 263
6.3.2 Random Packing of Particles ........................ 264
6.3.2.1 Monosized Particles ........................ 265
6.3.2.2 Bimodal Mixtures of Spheres ................ 266
6.3.2.3 Binary Mixtures of Nonspherical
Particles .................................. 269
6.3.2.4 Ternary and Multiple Mixtures .............. 270
6.3.2.5 Continuous Particle Size Distributions ..... 271
6.4 Concluding Remarks ....................................... 274
Problems ...................................................... 274
References .................................................... 276
Chapter 7 Forming of Ceramics ................................. 279
7.1 Introduction ............................................. 279
7.2 Additives in Ceramic Forming ............................. 280
7.2.1 Solvents ........................................... 280
7.2.2 Dispersants ........................................ 282
7.2.2.1 Inorganic Acid Salts ....................... 283
7.2.2.2 Surfactants ................................ 284
7.2.2.3 Low- to Medium-Molecular-Weight Polymers ... 286
7.2.3 Binders ............................................ 287
7.2.4 Plasticizers ....................................... 290
7.2.5 Other Additives .................................... 292
7.3 Forming of Ceramics ...................................... 292
7.3.1 Dry and Semidry Pressing ........................... 292
7.3.1.1 Die Compaction ............................. 292
7.3.1.2 Isostatic Compaction ....................... 301
7.3.2 Casting Methods .................................... 302
7.3.2.1 Slip Casting ............................... 302
7.3.2.2 Pressure Casting ........................... 306
7.3.2.1 Tape Casting ............................... 308
7.3.2.4 Gelcasting ................................. 311
7.3.2.5 Electrophoretic Deposition ................. 313
7.3.3 Plastic-Forming Methods ............................ 316
7.3.3.1 Extrusion .................................. 317
7.3.3.2 Coextrusion ................................ 320
7.3.3.3 Injection Molding .......................... 320
7.4 Solid Freeform Fabrication ............................... 323
7.4.1 Particle-Filled Polymer Methods .................... 324
7.4.1.1 Fused Deposition Modeling .................. 324
7.4.1.2 Laminated Object Manufacturing ............. 325
7.4.2 Powder Methods ..................................... 325
7.4.2.1 Selective Laser Sintering .................. 325
7.4.2.2 Three-Dimensional Printing ................. 326
7.4.3 Suspension Methods ................................. 326
7.4.3.1 Stereolithography .......................... 326
7.4.3.2 Direct Ceramic Ink-Jet Printing ............ 327
7.4.3.3 Robocasting ................................ 328
7.4.3.4 Freeze Casting ............................. 328
7.5 Concluding Remarks ....................................... 330
Problems ...................................................... 330
References .................................................... 333
Chapter 8 Drying, Debinding, and Microstructural
Characterization of Green Bodies .................... 337
8.1 Introduction ............................................. 337
8.2 Drying of Granular Ceramics .............................. 338
8.2.1 Drying of Granular Layers .......................... 338
8.2.2 Drying of Granular Solids .......................... 340
8.2.2.1 Driving Forces for Shrinkage and Moisture
Movement ................................... 341
8.2.2.2 Stages of Drying ........................... 341
8.2.2.3 Moisture Distribution and Movement ......... 347
8.2.2.4 Drying Stresses ............................ 348
8.2.2.5 Warping and Cracking ....................... 349
8.2.2.6 Drying Technology .......................... 351
8.3 Binder Removal (Debinding) ............................... 352
8.3.1 Extraction by Capillary Flow ....................... 352
8.3.2 Solvent Extraction ................................. 352
8.3.3 Thermal Debinding .................................. 353
8.3.3.1 Stages and Mechanisms of Thermal
Debinding .................................. 353
8.3.3.2 Models for Thermal Debinding ............... 354
8.3.3.3 Thermal Debinding in Practice .............. 357
8.4 Green Microstructures and Their Characterizations ........ 358
8.5 Concluding Remarks ....................................... 361
Problems ...................................................... 361
References .................................................... 362
Chapter 9 Sintering and Microstructure Development ............ 365
9.1 Introduction ............................................. 365
9.2 Solid-State Sintering .................................... 366
9.2.1 Driving Force for Sintering ........................ 367
9.2.2 Effects of Surface Curvature ....................... 369
9.2.2.1 Stress on the Atoms under a Curved
Surface .................................... 369
9.2.2.2 Chemical Potential of the Atoms under a
Curved Surface ............................. 370
9.2.2.3 Vacancy Concentration under a Curved
Surface .................................... 371
9.2.2.4 Vapor Pressure over a Curved Surface ....... 372
9.2.3 Grain Boundary Effects ............................. 373
9.2.4 Mechanisms of Sintering ............................ 375
9.2.5 Stages of Sintering ................................ 377
9.2.6 Sintering Models and Their Predictions ............. 379
9.2.6.1 Analytical Models .......................... 379
9.2.6.2 Predictions of the Analytical Models ....... 385
9.2.6.3 Effect of Applied Pressure ................. 387
9.2.6.4 Numerical Simulations of Sintering ......... 387
9.3 Grain Growth and Coarsening .............................. 387
9.3.1 Normal Grain Growth ................................ 389
9.3.2 Abnormal Grain Growth .............................. 394
9.3.3 Ostwald Ripening ................................... 395
9.3.4 Control of Grain Growth ............................ 398
9.3.4.1 Effect of Dopants: Solute Drag ............. 398
9.3.4.2 Effect of Fine, Inert Second-Phase
Particles .......................................... 401
9.3.5 Grain Growth and Pore Evolution in Porous
Compacts ........................................... 403
9.3.6 Interactions between Pores and Grain Boundaries .... 406
9.3.6.1 Pore Mobility .............................. 406
9.3.6.2 Kinetics of Pore-Boundary Interactions ..... 409
9.3.6.3 Microstructural Maps ....................... 410
9.4 Liquid-Phase Sintering ................................... 412
9.4.1 Stages of Liquid-Phase Sintering ................... 414
9.4.2 Kinetic and Thermodynamic Factors .................. 416
9.4.2.1 Contact Angle .............................. 416
9.4.2.2 Dihedral Angle ............................. 418
9.4.2.3 Effects of Gravity ......................... 418
9.4.2.4 Grain Boundary Films ....................... 419
9.4.2.5 Volume Fraction of Liquid Phase ............ 420
9.4.3 Phase Diagrams in Liquid-Phase Sintering ........... 420
9.5 Sintering Practice ....................................... 421
9.5.1 Heating Schedules .................................. 422
9.5.1.1 Isothermal Sintering ....................... 422
9.5.1.2 Constant Heating-Rate Sintering ............ 423
9.5.1.3 Multistage Sintering ....................... 423
9.5.1.4 Rate-Controlled Sintering .................. 424
9.5.1.5 Fast Firing ................................ 425
9.5.1.6 Microwave Sintering ........................ 426
9.5.1.7 Plasma-Assisted Sintering .................. 427
9.5.2 Pressure-Assisted Sintering ........................ 428
9.5.2.1 Hot Pressing ............................... 428
9.5.2.2 Sinter Forging ............................. 430
9.5.2.3 Hot Isostatic Pressing (HIPing) ............ 430
9.5.3 Particle and Green Compact Characteristics ......... 430
9.5.3.1 Particle Size .............................. 430
9.5.3.2 Particle Size Distribution ................. 431
9.5.3.3 Particle Shape and Particle Structure ...... 432
9.5.3.4 Particle Packing ........................... 432
9.5.3.5 Effect of Green Density .................... 434
9.5.4 Chemistry Considerations ........................... 435
9.5.4.1 Gases in Pores ............................. 435
9.5.4.2 Vapor Transport ............................ 437
9.5.4.3 Volatilization and Decomposition ........... 437
9.5.4.4 Oxidation State ............................ 438
9.5.4.5 Defect Chemistry and Stoichiometry ......... 439
9.6 Concluding Remarks ....................................... 440
Problems ...................................................... 441
References .................................................... 446
Appendix A: Physical Constants ................................ 453
Appendix B: SI Units - Names and Symbols ...................... 455
Appendix C: Conversion of Units ............................... 457
Appendix D: Aperture Size of U.S. Standard Wire Mesh Sieves
(ASTM E 11:87) .................................... 459
Appendix E: Densities and Melting Points of Some Elements,
Ceramics, and Minerals ............................ 461
Index ......................................................... 465
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