ABSTRACT
Aqueous Silicates . . . . . . . . . . . . . . . . . . . . . . . . . . 616
Hydrolysis and Condensation of
Silicon Alkoxides . . . . . . . . . . . . . . . . . . . . . . . . . . 617
Gelation and Aging . . . . . . . . . . . . . . . . . . . . . . . . . 620
Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622
Structure and Consolidation of Dried Gels . . . . . . . 624
Surface Structure and Chemistry . . . . . . . . . . . . . . . 627
Fiber Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 628
Particle Formation . . . . . . . . . . . . . . . . . . . . . . . . . . 628
Film Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 629
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 632
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . 632
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 632
The sol-gel process for preparing silica and silicates from metal alkoxide precursors is reviewed and
compared to the processing of aqueous silicates as described by Iler. Sol-gel processing combines
control of composition and microstructure at the molecular level with the ability to shape material in
bulk, powder, fiber, and thin-film form. In sol-gel processing of metal alkoxides, hydrolysis reactions
replace an alkoxide group with a hydroxyl group. Subsequent condensation reactions involving the hydroxyl
groups produce siloxane bonds. The structure of the evolving silicates is a consequence of the successive
polymerization, gelation, aging, drying, and heating steps. Often the structures of polymers, gels, and
dried gels (either xerogels or aerogels) may be characterized on the 1-20-nm length scale by a mass or
surface fractal dimension. On longer length scales, dried gels are micro-or mesoporous, with surface
areas often exceeding 800 m2/g. During heating, these gels undergo continued polymerization, structural relaxation, and viscous sintering; dense amorphous silica essentially indistinguishable from its convention-
ally prepared counterpart ultimately results.
