Sol-Gel Materials: Chemistry and Applications

Sol-Gel processing methods, first used historically for decorative and constructional materials, were extensively developed in the last century for applications such as glasses, ceramics, catalysts, coatings, composites and fibres. Today they are reaching their full potential, enabling the preparation of new generations of advanced materials not easily accessible by other methods yet using mild, low-energy conditions. The topic is therefore increasingly included in advanced undergraduate, MSc and PhD programmes in the areas of chemistry, physics and materials science. This concise introductory text, written at the advanced undergraduate/first-year postgraduate level, is also suitable as an introduction to the development, mechanisms, chemistry, characterisation methods and applications of the technique. It provides readers with an extensive yet concise grounding in the theory of each area of the subject and details the real and potential applications and the future prospects of sol-gel chemistry.

John D. Wright is Reader in Materials Chemistry at the Centre for Materials Research, School of Physical Sciences, University of Kent, UK.

Preface	p. ix
Introduction	p. 1
Colloid Stability	p. 1
Control of Particle Nucleation	p. 3
The Silicon Alkoxide Sol-Gel Process	p. 3
Advantages of Sol-Gel Synthesis	p. 4
Limitations of Sol-Gel Synthesis	p. 5
Overall Structure of this Book	p. 6
The Historical Development of Sol-Gel Processing	p. 6
Origins	p. 6
The Beginning of Sol-Gel Science	p. 7
The Explosion of Sol-Gel Technology	p. 8
References	p. 14
Silica Sol-Gels: Reaction Mechanisms	p. 15
Hydrolysis	p. 15
Acid and Base Catalysis	p. 15
Precursor Substituent Effects	p. 16
Hydrophobic Effects and Co-Solvents	p. 17
Effect of Water: Alkoxide Ratio (R)	p. 18
Condensation	p. 19
Acid and Base Catalysis	p. 19
Precursor Substituent Effects	p. 20
Overall Kinetics of Hydrolysis and Condensation	p. 20
Non-Hydrolytic Sol-Gel Processing	p. 21
Gelation	p. 22
Models of Gelation	p. 23
Ageing	p. 24
Cross-Linking and Syneresis	p. 24
Coarsening and Ripening	p. 24
Phase Transformations	p. 25
The Significance of Ageing	p. 25
Drying	p. 26
The Constant Rate Period	p. 26
The Critical Point	p. 26
First Falling-Rate Period	p. 26
Second Falling-Rate Period	p. 27
Consequences of Drying	p. 27
Avoiding Cracking	p. 27
Densification	p. 28
Stages of Densification	p. 28
Effects of Heating Rate and Gas Evolution	p. 30
Conclusion	p. 31
References	p. 31
The Chemistry of Sol-Gel Silicates	p. 33
Introduction	p. 33
Chemical Control of the Sol-Gel Process	p. 33
The Hydrolysis and Condensation Reaction	p. 33
Gelation	p. 36
Ageing	p. 36
Additives for Structuring and Processing	p. 37
Drying Control Additives	p. 37
Organic Templates	p. 38
Ormosils	p. 41
Modified Precursors	p. 41
Entrapment of Functional Materials	p. 43
Hybrid Materials	p. 46
Surface Modification	p. 48
Dehydroxylation	p. 48
Surface Functionalisation	p. 50
References	p. 51
Metal Oxide Gels	p. 53
Introduction	p. 53
Hydrolysis and Condensation Reactions of Metal Salt Precursors	p. 54
The Partial Charge Model	p. 54
Equilibrium Species in Aqueous Solutions of Metal Salts	p. 55
Condensation and Polymerisation in Aqueous Solutions of Metal Salts	p. 56
Effects of the Counter-lon	p. 59
Reactions of Metal Alkoxide Precursors	p. 60
Hydrolysis and Condensation	p. 60
Acid and Base Catalysis	p. 61
Steric Factors and Solvent Effects	p. 61
Control of Metal Alkoxide Reactions	p. 62
The Non-Hydrolytic Sol-Gel Method for Metal Oxides	p. 63
Particle Growth and Aggregation	p. 64
Preparation of Monodisperse Particles	p. 66
References	p. 67
The Characterisation of Sol-Gel Materials	p. 69
Introduction	p. 69
Chemical Characterisation	p. 69
Nuclear Magnetic Resonance	p. 69
Vibrational Spectroscopy	p. 71
Physical Characterisation	p. 72
Nitrogen Adsorption Porosimetry	p. 72
Mercury Porosimetry	p. 74
Thermoporosimetry	p. 74
NMR Spin-Spin Relaxation Measurements	p. 76
NMR Spin-Lattice Relaxation Measurements	p. 76
Small-Angle Scattering	p. 77
Other Structural Techniques	p. 80
Indirect Characterisation Methods	p. 82
References	p. 83
Applications of Sol-Gel Silicates	p. 85
Introduction	p. 85
Optical Materials	p. 85
Non-Doped Glasses	p. 85
Doped Glasses	p. 87
Contact Lenses	p. 89
Chemical Sensors	p. 89
Optical Chemical Sensing	p. 89
Biosensors	p. 90
Catalysts	p. 91
Coatings	p. 93
Membranes	p. 94
References	p. 94
Applications of Metal Oxide Sol-Gels	p. 97
Introduction	p. 97
Ceramics	p. 97
Catalysts	p. 99
Electronic Materials	p. 101
Ferroelectric Materials	p. 101
Electrochromic Materials	p. 102
Solid Electrolytes	p. 103
Other Electronic Materials	p. 103
Flammable Gas Sensors	p. 104
References	p. 106
The Future	p. 109
The Players	p. 109
Market Prospects	p. 109
Developments in Characterisation Methods	p. 110
Composite Materials	p. 110
New Precursors	p. 111
New Processing Methods	p. 111
Sol-Gel Supramolecular Chemistry	p. 112
References	p. 114
Index	p. 115
Chemical Substance Index	p. 121
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Sol-Gel Materials: Chemistry and Applications

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