Sol—Gel Processing of Silica

Abstract

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 conventionally prepared counterpart ultimately results.