Introduction to Sol-Gel Processing (eBook)

Preface 7
Background 7
Scope 8
Acknowledgments 9
Contents 10
Chapter 1: General Introduction 23
1.1 Short History 23
1.1.1 Scientific Basis 23
1.1.2 Colloids 23
1.1.3 Gels 24
1.1.4 The Traditional Sol-Gel Processing of Ceramics 24
1.1.5 Recent Chemical Developments 25
1.2 Sols, Gels, and Gelation 26
1.2.1 Sols 26
1.2.2 Gels 27
1.2.3 Gelation 27
1.2.4 Xerogels and Aerogels 28
1.2.5 Gelatinous Precipitates 28
1.2.6 Sol-Gel Processes 28
1.3 Outline of Sol-Gel Processing 29
1.4 Sol-Gel Processing Applications 31
1.4.1 Materials 31
1.4.2 Advantages and Limitations of Sol-Gel Processing 31
1.5 Organization of the Book 33
References 34
Chapter 2: The Sol-Gel Chemistry of Oxides from Metal Salts 36
2.1 Introduction 36
2.2 Solvents 37
2.2.1 Water 37
2.2.2 Nonaqueous Solvents 40
2.3 Basis of Cation Transformations in Solution 42
2.3.1 The Partial Charge Model 42
2.3.2 Transformation Mechanisms of Complexes 46
2.4 Hydrolysis of Cations in Solution 48
2.4.1 Ion Solvation in Water 49
2.4.2 Hydrolysis of Cations in Aqueous Media 50
2.4.2.1 The Formation of Hydroxo Ligands 50
2.4.2.2 Formation of Oxo Ligands 52
2.4.2.3 Application of the Partial Charge Model to the Hydrolysis of Cations 53
2.4.3 Hydrolysis of Hydrated Cations in Organic Solvents 55
2.5 Polymerization by Condensation of Hydrolyzed Cations 56
2.5.1 Condensation by Olation 56
2.5.2 Condensation by Oxolation (Jolivet et al. 1994) 57
2.5.3 Condensation and the Partial Charge Model 59
2.6 Complexation by Anions 61
2.6.1 Complexation by Anions X- and the Partial Charge Model (Jolivet et al. 1994) 62
2.6.1.1 Example: Complexation of [Fe(OH)2(OH2)4]+ by Bidentate Anions (Jolivet et al. 1994) 63
2.6.2 Overall Complexation of a Metal M by Anions 65
2.6.3 Formation of a Solid Phase 66
2.7 Sol-Gel Behavior of Cations as a Function of Their Nature 70
2.7.1 Cations with Valence I 71
2.7.2 Cations with Valence II 71
2.7.3 Cations with Valence III 72
2.7.3.1 Case of Aluminum 72
2.7.3.2 Other Cations 75
2.7.4 Cations with Valence IV 76
2.7.4.1 Case of Zr 76
2.7.4.2 Case of Ti 77
2.7.4.3 Case of Sn(IV) 77
2.7.4.4 Case of Si 78
2.7.5 Cations with Valence V or Higher (Jolivet et al. 1994) 80
2.8 Metal Salt Mixing 82
2.8.1 Modes of Cation Mixing in the Final Solid 82
2.8.2 Complexation with Carboxylic Acids 83
2.8.3 The Pechini Method 83
2.8.3.1 Possible Future Developments Regarding Cation Complexation 84
References 85
Chapter 3: The Sol-Gel Chemistry of Oxides from Alkoxides 89
3.1 Introduction 89
3.2 Structure and Properties of Alkoxides 89
3.2.1 Chemical Nomenclature of Alkoxides 89
3.2.2 Physical and Structural Characteristics of Alkoxides 90
3.2.3 Chemical Characteristics of Alkoxides 92
3.2.4 Silicon Alkoxides 93
3.3 Hydrolysis of Alkoxides 95
3.3.1 The Main Parameters of Alkoxide Hydrolysis 95
3.3.2 Formation of Hydroxo Ligands 95
3.3.3 Formation of Oxo Ligands 97
3.4 Polymerization by Condensation from Hydrolyzed Alkoxides (Stockmayer 1943 Aelion et al. 1950
3.4.1 Condensation by Olation 99
3.4.2 Condensation by Oxolation 99
3.5 SolGel Behavior of a Few Homometallic Alkoxides as a Function of their Cation Nature 101
3.5.1 Boron Alkoxides 101
3.5.1.1 Hydrolysis of Boron Alkoxides 101
3.5.1.2 Condensation of Hydrolyzed Boron Alkoxides 102
3.5.2 Aluminum Alkoxides 103
3.5.2.1 Hydrolysis of Aluminum Alkoxides 103
3.5.2.2 Condensation of Hydrolyzed Aluminum Alkoxides 103
3.5.3 Titanium Alkoxides 104
3.5.4 Zirconium Alkoxides 104
3.5.5 Silicon Alkoxides 106
3.5.5.1 Hydrolysis of Silicon Alkoxides (Brinker et al. 1990) 106
3.5.5.2 Condensation of Silicon Alkoxides 108
3.6 Formation of Solid Phases from Alkoxides 110
3.6.1 Boron Oxides 111
3.6.2 Alumina 111
3.6.3 Titania 112
3.6.4 Zirconia 114
3.6.5 Silica 115
3.6.5.1 The Two-Step Hydrolysis Condensation Process of Silica 117
3.6.5.2 Sonolysis 117
3.6.5.3 Study of Ionic Solvents 118
3.6.5.4 Overall Structural Description of Silica Oligomers 119
3.7 Alkoxysilanes 119
3.7.1 The Diversity of Alkoxysilanes 119
3.7.2 Organotrialkoxysilanes 119
3.7.3 Functionalization of Organotrialkoxysilanes 121
3.7.3.1 Thiol-Ene Click Reactions (Shenoi-Perdoor et al. 2016) 121
3.7.3.2 Cu(I)-Catalyzed Alkyne-Azide Click Cycloaddition (Shernoi-Perdoor et al. 2016) 122
3.7.4 Si Coordination Polyhedral Notation in Materials Derived from Alkoxysilanes 123
3.7.5 Polyhedral Oligomeric Silsesquioxane (POSS) 124
3.8 Other Precursors 125
3.8.1 The Span of Precursors 125
3.8.2 Organometallics 126
3.8.3 Glycol-Modified Silanes (GMS) 127
3.8.4 Polyhedral Oligometallasilsesquioxanes (POMS) and Other Non-Si Clusters 128
3.8.5 Other Metal-Organic Complexes 128
3.9 Precursor Mixing 130
3.9.1 Mixing Two Alkoxides 131
3.9.1.1 Heterometallic Alkoxides 131
3.9.1.2 Simultaneous Hydrolysis of Simple Alkoxides 133
3.9.1.3 Matching the Hydrolysis rate of Different Alkoxides 134
3.9.2 Mixing an Alkoxide with a Metal Salt 136
3.9.3 Mixing with Fine Solid Powders 137
3.9.4 Cations Mixing with Silicon by the Glycol-Modified Silane (GMS) Method 137
3.10 Non-hydrolytic Processes 138
3.10.1 Non-hydrolytic Hydroxylation Reactions 139
3.10.2 Aprotic Reactions 140
References 141
Chapter 4: The Sol-Gel Chemistry of Non-oxides 149
4.1 Introduction 149
4.2 Chalcogenides 149
4.2.1 Deposition from a Chalcogenide Solution 150
4.2.2 Sol-Gel Synthesis from Alkoxides 151
4.2.3 Sol-Gel Synthesis from Organometallics 153
4.2.4 Sol-Gel Synthesis from Inorganic Precursors 154
4.2.4.1 Sol-Gel Synthesis by Linking of Chalcogenide Zintl Clusters 154
4.2.4.2 Colloidal Particle Formation 156
4.2.4.3 Sol-Gel Synthesis by Disulfide Bridging of Chalcogenide Nanoparticles 157
4.2.4.4 Sol-Gel Synthesis by Ion Exchange in Chalcogenide Gels 158
4.3 Fluorides 158
4.3.1 The Hydrolytic Route 159
4.3.2 The Trifluoroacetate Route 159
4.3.3 The Fluorolytic Route 160
4.4 Preceramic Polymers 162
4.4.1 Carbides 162
4.4.2 Nitrides 164
4.5 Organic Gels 165
4.5.1 Melamine-Formaldehyde and Resorcinol-Formaldehyde Gels 166
4.5.2 Cellulosic and Polyurethane-Based Gels 167
4.5.3 Other Synthetic Organic Hydrogels 169
4.5.4 Hydrogels from Biopolymers 171
4.6 Carbon and Graphene Gels 171
4.6.1 Carbon Gels Derived from Organic Gels 171
4.6.2 Graphene and Carbon Nanotube Gels 172
4.6.2.1 Graphene 173
4.6.2.2 Carbon Nanotubes 177
4.6.3 Carbon Nanotube and Graphene Gel Formation 177
References 179
Chapter 5: Nanoparticle Formation 185
5.1 Introduction 185
5.2 Nucleation and Growth Versus Spinodal Decomposition 186
5.2.1 Relationship Between Hydrolysis, Condensation, and Formation of Solid Particles 186
5.2.2 Phase Transformation Modes According to Gibbs 187
5.3 Nucleation of Solid Particles 187
5.3.1 Gibbs Free Energy of a Spherical Particle 187
5.3.2 Gibbs Internal Free Energy Change, per Unit Volume, Due to Phase Transformation 189
5.3.2.1 Derivation of DeltaGv,f for the Growth Stage of Homogeneous Nucleation 189
5.3.2.2 Derivation of DeltaGv,n for the Homogeneous Nucleation Stage 191
5.3.3 Homogeneous Nucleation Rate 194
5.3.4 Heterogeneous Nucleation 196
5.3.5 LaMer Model, for the Growth of Monodisperse Particles 196
5.3.5.1 Thermodynamics of the LaMer Model 197
5.3.5.2 Forced Hydrolysis 198
5.3.5.3 Controlled Release of Anions or Cations 199
5.3.5.4 Modification of the Temperature 199
5.3.5.5 Use of Separate Reactors 200
5.4 Crystalline Growth Mechanisms of Solid Particles 200
5.4.1 Kinetics of Growth Controlled by the Fixation of New Complexes: Mononuclear Regime 201
5.4.2 Polynuclear Growth Regime 202
5.4.3 Kinetics of Growth Controlled by the Diffusion of Complexes in Solution 203
5.4.4 Growth Regime Transition 204
5.4.5 Importance of Crystal Defects: Growth of Amorphous Particles 204
5.4.6 Importance of Thermal Diffusion in the Growth Process 205
5.5 Examples of Solid Particles Made by Nucleation and Growth from Precursor Solutions 205
5.5.1 Particle Shape 205
5.5.2 Monodisperse Particles 206
5.5.3 Growth Termination 209
5.5.4 The Stöber Process 212
5.5.5 Quantum Dots 213
5.6 Other Techniques to Synthesize Solid Particles from a Solution 217
5.6.1 Hydrothermal Processing 217
5.6.2 Electrochemical Precipitation 217
5.6.3 Particle Growth in Aprotic or Non-hydrolytic Sol-Gel Processes 217
5.6.4 Particle Nucleation and Growth Inside a Gel 219
5.6.5 Use of Microemulsions 219
5.6.6 Exfoliation Methods 219
5.7 Solid Particles Synthesized by More Physical Processes 219
5.7.1 Physical and Chemical Vapor Deposition 220
5.7.2 Pyrolysis of Precursors 220
5.7.3 Spray-Drying Techniques 221
5.7.4 Freeze-Drying 222
5.7.5 Liquid Drying 222
5.7.6 Aerosol Hydrolysis 222
5.7.7 Advantages of Particle Synthesis by Sol-Gel Processing 223
References 223
Chapter 6: Peptization of Colloidal Sols 229
6.1 Introduction 229
6.2 Sols 230
6.2.1 Peptization 230
6.2.2 Kinetic Stability of a Sol 230
6.2.3 Main Interactions Involved in the Stability of a Sol 231
6.3 The Classical Derjaguin, Landau, Verwey, Overbeek (DLVO) Stabilization Theory 231
6.3.1 Van der Waals Interaction 231
6.3.1.1 Van der Waals Interactions at the Molecular Level 231
6.3.1.2 Van der Waals Interaction Between Colloidal Particles 232
6.3.2 Adsorption of Ions and Electrical Double Layer 235
6.3.2.1 Zero-Point Charge ``z.p.c.´´ 235
6.3.2.2 Diffuse Layer of Potential ``Non-determining´´ Ions 236
6.3.2.3 Zeta Potential zeta, Isoelectric Point (i.e.p.) and Electrophoretic Mobility 237
6.3.2.4 Surface Electric Potential Psi0 of a Particle as a Function of the pH 239
6.3.3 Gouy-Chapman Model 241
6.3.3.1 Electric Potential Psi(x) at a Distance x from a Planar Surface (Hiemenz 1977 Masliyah 1994)
6.3.4 Debye-Hückel Approximation (Hiemenz 1977 Masliyah 1994)
6.3.5 Stern Model (Hiemenz 1977 Masliyah 1994)
6.3.6 Case of a Charged Spherical Particle (Masliyah 1994) 245
6.3.6.1 Electric Potential Psi(x) Created by a Spherical Particle 245
6.3.6.2 Electrophoretic Mobility of a Spherical Particle (Hiemenz 1977 Masliyah 1994)
6.3.7 Electrostatic Repulsion Force Between Two Charged Surfaces 250
6.3.7.1 Case of Parallel Planar Surfaces (Hiemenz 1977 Masliyah 1994)
6.3.7.2 Electrostatic Interaction Between Spherical Particles (Hiemenz 1977 Masliyah 1994)
6.3.8 The Main Routes to Adjust the Electrostatic Repulsion Between Spherical Particles 257
6.3.9 Total Interaction Energy in Electrostatic Sols (Hiemenz 1977 Masliyah 1994)
6.3.10 Coagulation (Overbeek 1977 Hiemenz 1977
6.3.11 Effect of Ion Solvation 260
6.3.12 Electrostatic Charge Reversal (Masliyah 1994) 262
6.4 Coagulation Kinetics 264
6.4.1 Smoluchowski Derivation of Coagulation Rate 264
6.4.2 Reversibility of Coagulation 265
6.5 The Steric Stabilization Theory 267
6.5.1 Origin of Steric Stabilization 267
6.5.2 Polymer Solutions 267
6.5.2.1 The Flory-Huggins Theory 267
6.5.2.2 The State Equation Theory 269
6.5.3 Steric Interactions Between Colloidal Particles 270
6.5.4 Steric Interaction Energy 271
6.5.5 Mixed Steric and Electrical Interactions: Case of Surfactant Solutions 272
6.5.6 Mixed Steric and Magnetic Interactions: Ferrofluids 273
6.5.7 Bridging Polymer Adsorption 273
6.6 Other Interactions Applying in Extended DLVO Theories 273
6.6.1 Hydration Forces 275
6.6.2 Hydrophobic Forces 275
6.7 Potential of Mean Field (PMF) Simulations 277
6.7.1 Limitations of the DLVO and Steric Theories 278
6.7.2 The PMF Theory for Colloids 278
6.7.2.1 PMF as a Statistical Thermodynamics Approach 278
6.7.2.2 Simplified PMF Simulation Corresponding to the Original DLVO Theory of Colloids 278
6.7.2.3 More Complex PMF Simulations 279
6.7.3 Main Results of the PMF Modelizations 280
6.7.3.1 The Primitive Model 280
6.7.3.2 Particle Shape and Charge Discreteness Modeling 280
6.7.3.3 Other Modeling 281
6.8 Other Phenomena in Sols 282
6.8.1 Sol Demixion 282
6.8.2 Liquid Crystal-Like Sols 283
6.8.3 Aging Evolution of Sols 284
6.8.3.1 Solid-Phase Recrystallization 285
6.8.3.2 Ostwald Ripening 286
References 287
Chapter 7: Gelation 290
7.1 Introduction 290
7.2 Percolation Models of Gelation 290
7.2.1 Flory-Stockmayer Model 290
7.2.1.1 Gel Point 291
7.2.1.2 Characteristics of the Flory-Stockmayer Model 291
7.2.2 Percolation Models 292
7.2.2.1 Site and Bond Percolations 292
7.2.2.2 Percolation Threshold, Critical Exponents, and Scaling Laws 293
7.2.2.3 Mean Field or Effective Medium Theory 296
7.2.2.4 Other Critical Parameters in Percolation 296
7.2.2.5 Other Percolation Models 297
7.3 Growth Models of Gelation 298
7.3.1 Main Differences Between Percolation and Growth Models of Gelation 298
7.3.2 Example of Growth Models 299
7.3.2.1 Polymerization Model by Manneville and de Seze (1981) 299
7.3.2.2 Invasion Percolation Growth Model 299
7.3.2.3 Eden Model and Other Non-mass Fractal Growth Models 299
7.3.2.4 Rikvold Crystallization Model (Rikvold 1982) 300
7.3.2.5 The ``Electric Breakdown´´ Model 301
7.3.2.6 Diffusion-Limited Aggregation Model (DLA Model) (Witten Jr. and Sander 1981 Deutch and Meakin 1983)
7.3.2.7 Onoda and Toner Hierarchical Model 303
7.3.2.8 Other Mathematical Models 303
7.4 Gelation and the DLVO Theory 303
7.4.1 Experimental Critical Electrolyte Concentration for Gelation 304
7.4.2 Electrostatic Conditions of Gelation 304
7.4.3 Example of Gel Structure According to DLVO Theory 306
7.5 Experimental Study of Gelation 306
7.5.1 Rheological Methods 306
7.5.1.1 Steady Flow Curves 306
7.5.1.2 Oscillatory Shear Flow (Winter and Chambon 1987) 308
7.5.2 Vibrational Spectroscopy (Larkin 2011) 311
7.5.2.1 IR Spectroscopy 312
7.5.2.2 Raman Spectroscopy 313
7.5.3 Light Scattering 316
7.5.4 Small-Angle X-Ray Scattering (SAXS) and Small-Angle Neutron Scattering (SANS) (Craievich 2016) 316
7.5.5 Nuclear Magnetic Resonance Spectroscopy (NMR) (Vidal et al. 2016) 318
7.5.6 Microscopic Techniques 320
7.5.7 Other Experimental Techniques 320
7.5.8 Computer Calculation Methods 321
7.5.8.1 DFT Calculations 321
7.5.8.2 Force Field Methods 322
7.5.8.3 Reactive Bond Modeling 323
7.5.8.4 Mesoscale and Coarse-Grained Models 324
7.6 Gelation of Real Sols 324
7.6.1 Gel Shaping 324
7.6.2 Gelation Theories as Frameworks of Experimental Studies 325
7.6.3 Importance of the Cation Nature and Chemistry 325
7.6.4 Irreversible Gelation 326
7.6.5 Reversible Gelation 329
7.6.6 Gelation of Real Materials and the Percolation or Aggregation Models 333
7.6.6.1 Growth Models: Limitation by Transport or by Fixation of New Colloidal Particles 333
7.6.6.2 Gelation and Mixed Percolation and Growth Models 334
7.6.7 Gelation of Multicomponent Systems 336
References 337
Chapter 8: Wet Gels and Their Drying 342
8.1 Introduction 342
8.2 Network Structure and Classification of Gels 342
8.2.1 Originality of Wet Gels as Materials 342
8.2.2 Gel Classifications 343
8.2.2.1 Gel Classification by Flory 343
8.2.2.2 Colloidal Versus Polymeric Gel Classification 345
8.2.2.3 Gel Classification According to Various Wet Medium Evolution, and Drying Technique 345
8.2.2.4 Gel Classification According to their Chemical Nature 345
8.3 Properties of Wet Gels 346
8.3.1 Solid Properties 346
8.3.1.1 Plastic Properties 346
8.3.1.2 Elastic Properties 347
8.3.2 Transport Properties in the Liquid of a Gel 350
8.4 Reversible Swelling of Wet Gels 350
8.4.1 Osmotic Swelling Theory of Covalent Organic Polymeric Gels 350
8.4.2 Swelling of Inorganic Gels 352
8.4.3 Donnan Equilibrium (Donnan 1911 Hiemenz 1997)
8.5 Syneresis of Inorganic Wet Gels 356
8.6 Aging Wet Gels 360
8.6.1 Chemical Evolution During Aging 360
8.6.2 Physical Evolution During Aging 360
8.7 Drying Gels 363
8.7.1 Drying by Evaporation 363
8.7.1.1 The Capillary Mechanism 364
8.7.1.2 Drying by Evaporation and the DLVO Theory 367
8.7.1.3 Stresses Developed in a Gel During Drying by Evaporation 369
8.7.2 Supercritical Drying 371
8.7.3 Ambient Pressure Drying 374
8.7.4 Subcritical Drying 375
8.7.5 Freeze-Drying 376
8.7.6 Drying by Liquid-Liquid Extraction 376
References 377
Chapter 9: Dry Gels 382
9.1 Introduction 382
9.2 Texture of Dry Gels 382
9.2.1 Pore Characterization Techniques 382
9.2.2 Mercury Porosimetry 383
9.2.3 Adsorption Isotherms 384
9.2.3.1 Examples of Nitrogen Adsorption Isotherms 386
9.2.3.2 Determination of the Specific Surface Area 387
9.3 Structure of Dry Gels 390
9.3.1 Gel Fractal Structure (Reichenauer 2011) 391
9.3.2 Gel Crystallographic Structure 391
9.3.3 Gel Surface Structure 393
9.3.3.1 Hydrophobization of Oxide Gels 394
9.4 Oxide Gels 397
9.4.1 Silica Gels 397
9.4.1.1 Simple Alkoxide-Derived Gels 397
9.4.1.2 Silica Gels Made from Functionalized Alkoxides 402
9.4.2 Borate Gels 404
9.4.3 Alumina Gels 405
9.4.4 Titania Gels 408
9.4.5 Zirconia Gels 409
9.4.6 Oxides Made by the Epoxidation Method (See Chap. 2, Sect. 2.4) 410
9.4.7 Vanadium and Tungsten Oxide Gels 410
9.4.8 Mixed Oxide Gels 412
9.4.8.1 Silicate Mixed Oxide Gels 412
9.4.8.2 Titanate Mixed Oxide Gels 414
9.4.9 Oxide Gels Made by Non-hydrolytic Process 414
9.5 Non-oxide Gels 415
9.5.1 Chalcogenide Gels 415
9.5.2 Organic Gels 416
9.5.2.1 The Case of Resorcinol-Formaldehyde (RF) Gels 418
9.5.2.2 Hydrogels 419
9.5.3 Carbon Aerogels 422
9.5.3.1 Carbon Gels Derived from Organic Gels 422
9.5.3.2 Graphene Gels 422
9.5.3.3 Carbon Nanotube Gels 424
9.6 Thermal Conduction and Mechanical Properties of Dry Gels 425
9.6.1 Thermal Conductivity 426
9.6.2 Mechanical Properties 428
References 432
Chapter 10: Hybrid Organic-Inorganic and Composite Materials 440
10.1 Introduction 440
10.2 Classes of Hybrid Organic-Inorganic Sol-Gel Materials (Sanchez and Ribot 1994) 441
10.2.1 Ormosils and Ceramers 441
10.2.2 Class I and Class II Hybrids 441
10.2.3 Hybrid Gel Architectures 443
10.2.4 Hybrid Gels Versus Composite Materials 444
10.3 Examples of Class I Hybrid Architecture 445
10.3.1 Class I Hybrids Made by Entrapment of Various Dispersed Components 445
10.3.2 Class I Hybrid Made by Entrapment of an Organic Dye in Silica Gel 445
10.3.3 Class I Hybrid Made by Entrapment of Short Organic Polymers in an Oxide Gel, or of Oxide Clusters in a Polymer Gel 446
10.3.4 Silica POSS Cluster Class I Hybrids 447
10.3.5 Class I Hybrids Made by Gelation of Two Interpenetrating Gel Networks 448
10.3.5.1 Simultaneous Gelation of Inorganic and Organic Sol-Gel Precursors 448
10.3.5.2 Impregnation of an Oxide Gel with an Organic Precursor Solution 449
10.3.6 Class I Hybrids Made by Polymer Intercalation in Lamellar Inorganic Gels 449
10.4 Examples of Class II Hybrid Architecture 450
10.4.1 Class II Ormosil Hybrids 450
10.4.1.1 Ormosils Based on Silica Gel Carrying Pendant Organic Groups 451
10.4.1.2 Class II Ormosils Based on Polymer-Bridged Silica Clusters 452
10.4.1.3 POSS-Organic Polymer Class II Hybrids 454
10.4.1.4 Hybrids Based on Coupling Silica with Hydrogels 456
10.4.2 Class II CERAMER Hybrids 456
10.4.2.1 Inorganic-Organic Polymer Class II Hybrids Containing Other Cations than Si 456
10.4.2.2 Inorganic-Organic Polymer Class II Hybrids Made by the Pechini Method 458
10.4.2.3 Inorganic-Organic Polymer Class II Hybrids Made by a Polymeric Gel Precursor Method 459
10.5 Sol-Gel Composites 460
10.5.1 Composites Designed by Mixing of Constituents 461
10.5.2 Carbon Nanotube (CNT) Aerogels/Sol-Gel Oxide Composites 463
10.5.3 Composites Made by Spontaneous Phase Separation 464
10.6 Main Properties of Hybrid Gels 464
10.6.1 Mechanical Properties of Hybrids 464
10.6.2 Other Properties of Hybrids 467
10.6.2.1 Specific Surface Area of Hybrids 467
10.6.2.2 Thermal Conductivity of Hybrids 468
References 468
Chapter 11: Surfactant-Templated Sol-Gel Materials 475
11.1 Introduction 475
11.2 Solute Adsorption at a Liquid/Air or an Immiscible Liquid/Liquid Interface 476
11.2.1 Gibbs Adsorption Isotherm (Hiemenz 1976 Hiemenz and Rajagopalan 1997)
11.2.2 Aqueous Solute Classification (Hiemenz 1976 Hiemenz and Rajagopalan 1997)
11.3 Surfactants (Berthod 1983) 480
11.3.1 General Structure of Surfactant Molecules and Behavior Below a Concentration < c.m.c
11.3.2 Families of Surfactant Molecules 481
11.3.2.1 Anionic Surfactants 481
11.3.2.2 Cationic Surfactants 481
11.3.2.3 Nonionic Surfactants 482
11.3.2.4 Amphoteric Surfactants 482
11.3.3 Hydrophilic-Lipophilic Balance (HLB) of a Surfactant (Griffin 1954) 483
11.4 Behavior of Surfactants at a Concentration > c.m.c. (Hiemenz 1976
11.4.1 Micelle Formation 484
11.4.2 Micelle Structure (Berthod 1983 Mittal 1979
11.4.2.1 Spherical Micelles 485
11.4.2.2 Rodlike Micelles 486
11.4.2.3 Lamellar Micelles 486
11.4.2.4 Inverse Micelles in an Organic Liquid 486
11.4.3 Factors Influencing the c.m.c. 487
11.4.3.1 Chemical Composition 487
11.4.3.2 Temperature 489
11.5 Solubilization of Organic Nonpolar Compounds in Water by Micelles (Robb 1982 Berthod 1983)
11.5.1 Micellar Solutions 490
11.5.2 Micelle-Stabilized Microemulsions 490
11.5.3 State Diagrams of Ternary Solution Systems Made with Surfactants 491
11.6 Microparticle Synthesis in Water-in-Oil (W/O) Microemulsions Stabilized by Surfactants 493
11.6.1 Solid Microparticles and Microcapsules 493
11.6.2 Core-Shell Microparticles Made by Stabilization of Colloidal Sols by a Surfactant 497
11.7 Application of Surfactants to Synthesize Ordered Mesoporous Materials (Corma 1997) 497
11.7.1 Importance of Micelles for Templating 497
11.7.2 Surfactant Templating and Sol-Gel Electronic Interactions 499
11.7.3 Surfactant Templating of Sol-Gel Silica 500
11.7.3.1 Mechanism of Formation 500
11.7.3.2 Templating with Polysilsesquioxane Sol-Gel Precursors 502
11.7.4 Templating with Block Copolymer Surfactants 503
11.7.4.1 Other Surfactant-Related Synthesis Techniques and Structures 504
11.7.4.2 Surfactant-Templated Oxides Other than Silica 506
11.8 Some Characteristics of Surfactant-Templated Mesoporous Materials 507
11.8.1 Texture 507
11.8.2 Structure 508
11.8.3 Mechanical Properties 509
References 510
Chapter 12: Phase Transformation 514
12.1 Introduction 514
12.2 Transformations in the Hüttig Range 516
12.2.1 Types of Transformations in the Hüttig Range 516
12.2.2 Gel Dehydration 516
12.2.2.1 Dehydration of Adsorbed Water 518
12.2.2.2 Dehydration of Structural Water 518
12.2.2.3 Example: Structural Water Dehydration of Boehmite Gels 519
12.2.3 Other Chemical Group Elimination 521
12.2.3.1 Residual Organics 521
12.2.3.2 Residual Anions 523
12.2.4 Chemical Transformations of Non-oxide Gels 524
12.2.4.1 Fluorides 524
12.3 Transformations in the Lower Tammann Range 525
12.3.1 Gel Network Transformation 525
12.3.1.1 Network Consolidation 525
12.3.1.2 Pore Texture Evolution 526
12.3.2 Topotactic Crystallization 527
12.3.3 The Topotactic Formation of Transition Aluminas 528
12.3.3.1 TTT Diagram of the Most Common Transition Aluminas 528
12.3.3.2 Mechanism of the Topotactic Transformations in Transition Aluminas 529
12.3.4 Diversity of Topotactic Transition Phases in Sol-Gel Materials 530
12.3.4.1 Importance of the Sol-Gel Chemistry 530
12.3.4.2 Example of Zirconia 530
12.3.5 Topotactic Phase Transformations in Multicomponent Oxides 531
12.3.6 Gels Made by Polymerizable Complex Processing 534
12.4 Glass Formation 535
12.4.1 Glasses, a Class of Amorphous Materials 535
12.4.2 Experimental Characterization of the Glassy State 536
12.4.2.1 The Traditional Experimental Study of Glass Formation 536
12.4.2.2 Experimental Values of a Few Tg for Melt-Quenched Glasses 537
12.4.2.3 TTT Diagrams for Glass Forming from the Melt 538
12.4.2.4 X-ray Diffraction During the Sol-Gel-Derived Glass Transition 540
12.4.3 Glass Formation Mechanism from Polymeric Gels 541
12.4.4 Glass Formation from Gels Above Tg 542
12.4.5 Glass Compositions Studied by Sol-Gel 543
12.4.6 Non-oxide Sol-Gel Glasses 545
12.4.7 Differences Between Melt-Quenched Glasses and Sol-Gel Silica Glasses 545
12.5 Phase Transformations in the Upper Tammann Range 546
12.5.1 Spinodal Decomposition 547
12.5.2 Formation of the Stable Thermodynamics Phases by Nucleation and Growth 550
12.5.3 Variation of the Specific Surface Area During Nucleation and Growth 551
12.5.4 Crystallization of Glasses 552
12.6 Conversion to Non-oxides by Chemical Reactions 554
12.6.1 Carbon Aerogels 554
12.6.2 Carbides 554
12.6.2.1 Reaction of Oxide Gels with a Carbon Source 554
12.6.2.2 Transformation of Silane Precursors 555
12.6.3 Oxynitrides and Nitrides 556
12.6.3.1 High-Temperature Reaction of Oxides with Nitrogen or Ammonia 556
12.6.3.2 Silazanes 557
12.6.4 Borides 558
12.6.5 Sulfides 558
References 558
Chapter 13: Sintering Sol-Gel Ceramics 567
13.1 Introduction 567
13.2 Possible Texture Evolution 568
13.2.1 Thermodynamics 568
13.2.2 Textural Transformation Kinetics 569
13.2.2.1 Herring´s Scaling Laws: Principle 569
13.2.2.2 Example of Scaling Law 570
13.2.2.3 Comparison of a Few Mechanisms 571
13.2.2.4 Influence of the Texture Scale 571
13.2.3 Competition Between Grain Growth and Sintering 572
13.2.4 Gel Network Transformation During Sintering 573
13.2.4.1 Effect of Unsymmetrical Particles 574
13.2.4.2 Evolution of Strings of Particles 574
13.2.4.3 Effect of Particle Segregation by Size 575
13.3 Atomic Transport Mechanisms Operating During Sintering 576
13.3.1 Atomic Diffusion 576
13.3.1.1 Case of Ionic Solids 576
13.3.1.2 Atomic Diffusion in Sol-Gel Materials 577
13.3.1.3 Sintering and Crystallization in Sol-Gel Ceramics 578
13.3.2 Viscous Flow Sintering 580
13.3.2.1 General Description of Viscous Flow Sintering 581
13.3.2.2 Sintering Models 581
13.3.2.3 Densification of Gels Depending on Their Structure 583
13.4 Grain Growth 584
13.4.1 Basic Mechanism 584
13.4.2 Grain Growth Models 584
13.4.3 Grain Boundaries Pinning by Impurities 585
13.5 Interaction of Pores with the Sintering Process 586
13.5.1 Possible Pore Transformations 587
13.5.1.1 Kinetic Stability of a Pore 587
13.5.1.2 Mobility of a Pore 587
13.5.1.3 Pore Coarsening 588
13.5.2 Action of Pores on the Grain Boundary Mobility (Brook 1969) 589
13.5.3 Abnormal Grain Growth (Brook 1969) 591
13.5.3.1 Pore Separation from Grain Boundaries 592
13.5.3.2 Pores Catching Up Grain Boundary 593
13.5.3.3 Sintering Maps 594
13.5.3.4 Prevention of Abnormal Grain Growth 595
13.5.3.5 Abnormal Growth in Sol-Gel Ceramics 596
13.5.4 Pores Due to Initial Powder Packing 597
13.5.4.1 Case of Agglomerates 597
13.5.4.2 Monodispersed Powder Packing 598
13.5.4.3 Polydisperse Powder Packing 599
13.6 Hot Pressing 600
13.7 Sintering Under an Electric Field 603
13.7.1 Microwave-Assisted Thermal Treatments 603
13.7.2 Fast and Flash Sintering 604
References 609
Chapter 14: Applications of Sol-Gel Processing 613
14.1 Introduction 613
14.2 Health Hazards 614
14.3 Applications in the Sol or in the Gel State 614
14.3.1 Sols 614
14.3.2 Gels 615
14.3.2.1 Wet Gels 615
14.3.2.2 Dry Xerogels and Aerogels 616
14.4 Coatings and Thin Films 616
14.4.1 Functions of Sol-Gel Coatings 616
14.4.2 Fabrication Techniques 619
14.4.3 Free-Standing Films 621
14.5 Fibers 622
14.5.1 Main Compositions 622
14.5.2 Fabrication Techniques 623
14.6 Monoliths 625
14.6.1 Gel Monoliths and Derived Ceramic Monoliths 625
14.6.2 Monoliths from Hybrids 626
14.6.3 Ambigel and Aerogel Monoliths 627
14.6.4 Monoliths from Sintered Sol-Gel Powder 628
14.6.4.1 The Case of Complex Titanate Synthesis from Sol-Gel Powders 628
14.6.4.2 Other Complex Ceramics Synthesized from Sol-Gel Powders 630
14.7 Filtration Membranes 630
14.7.1 Porous Membranes 631
14.7.2 Ceramic Membranes 632
14.7.3 Sol-Gel Ceramic Membranes 634
14.7.3.1 Self-Supported Sol-Gel Ceramic Membranes 634
14.7.3.2 Supported Sol-Gel Ceramic Membranes 635
14.7.3.3 Recent Sol-Gel Ceramic Membranes Studied 637
14.7.3.4 Catalytic Sol-Gel Membranes 637
14.8 Thermal and Acoustic Insulation 639
14.8.1 Thermal Insulation 640
14.8.2 Acoustic Insulation 643
14.9 Optical Applications 643
14.9.1 Optical Transparency of Silica Gels 643
14.9.2 Cherenkov Counters 644
14.9.3 Luminescent Materials 646
14.9.4 Optical Coatings 647
14.9.5 Nonlinear Optics 652
14.10 Electrical, Dielectrical, and Other Electromagnetic Applications 653
14.10.1 Electrical Conduction Applications 653
14.10.2 Electrodes and Batteries 654
14.10.3 Superconductors 655
14.10.3.1 Dielectric Applications 655
14.10.3.2 Piezoelectric Applications 655
14.11 Applications as Immobilization Medium 657
14.11.1 Confinement Applications 657
14.11.2 Environment Remediation Applications 658
14.11.3 Capture of CO2 Gas 659
14.11.4 Other Immobilization Applications 660
14.12 Sol-Gel Catalysts 661
14.12.1 The Catalytic Process (Satterfield 1990) 661
14.12.1.1 Activity and Selectivity of a Catalyst 661
14.12.1.2 Oxide Active Sites 662
14.12.1.3 Special Characteristics of Sol-Gel Oxides 664
14.12.2 Synthesis of High-Value Organic Compounds 665
14.12.2.1 Types of Chemical Reactions Catalyzed 665
14.12.3 Protection of the Environment 666
14.12.4 Recent Sol-Gel-Made Catalysts 668
14.12.4.1 Aerogel Catalysts 668
14.12.4.2 Catalysts Made by Non-hydrolytic Sol-Gel Process 669
14.12.4.3 Ordered Mesoporous Catalysts 670
14.12.4.4 Application of POSS in Catalysis 672
14.12.4.5 Sol-Gel Fluoride Catalysts 672
14.12.5 Photocatalysis 673
14.12.6 Sol-Gel Biocatalysts 674
14.12.7 Sensors 675
14.13 Medical Applications and Biomaterials 677
14.13.1 Biomaterials (Pierre 2016) 677
14.13.2 Drug Carriers 680
References 680
Index 702