Sol–Gel Technology

Abstract The sol–gel process, which has a potentially high technological value, enables materials having unique structures and properties seldom obtained by other processing methods to be produced. Materials prepared by sol–gel technology are of very high purity and homogeneity, having controlled multiphase microstructures, net‐shape forming, and as‐cast surface features made at lower temperatures than those used for other processes. High purity metal alkoxides are employed as precursors for the synthesis of nanometric entities dispersed in a liquid environment or sol that eventually forms a gel. The nanostructure of the gel can be tailored by controlling the chemical reactions involving the precursors. The steps of sol–gel processes for silica‐ and alumina‐based materials are discussed. Sol–gel technology involves precursor mixing and formation of a colloidal suspension, generally followed by a casting operation in which the low viscosity sol is introduced in molds. Aging provides the required mechanical property for further handling. Following aging, the solvents within the pores are eliminated by drying. A stabilization step is also required to eliminate any residual chemisorbed water. Production of a fully dense body involves a densification step at higher temperatures when the porosity is reduced to zero. Application of the sol–gel process include preparation of porous and dense monoliths, silica and alumina fibers, ceramic thin films, biomedical devices, and organic–inorganic hybrid composites. Many of the materials prepared by sol–gel technology are nanostructed. The majority of these substances are multicomponent. The various compositional possibilities are presented.