STRUCTURAL EVOLUTION OF SOL-GEL GLASSES.

Abstract

This review examines the stages of the sol-gel process, including hydrolysis, condensation, gelation, aging, drying, and sintering. The species produced in the sol are polymeric, rather than being dense glass-like colloidal particles. Considerable control over the structure of the polymer is possible through an understanding of the chemistry of hydrolysis and condensation. The properties of the gel and its response to heat treatment are sensitive to the structure created in the sol stage. Solvent must be removed slowly to prevent the high capillary stresses from causing cracking. A model of drying is presented that explains the relationship between cracking, drying rate, gel size, and permeability. Heat treatment causes densification of the solid phase, as well as collapse of the pores. The sintering behavior of gels is complex, because the viscosity of the gel is affected by concurrent structural relaxation and changes in hydroxyl content.