Sol–Gel Processing of Silica

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.