ABSTRACT
This chapter examines the physics and chemistry of sol-gel processing and the ways in which these relate to the practical fabrication of ceramics and glasses. Silica gel can be formed by hydrolysis and condensation of aqueous silicates such as silicic acid, but, more commonly, it is formed by hydrolysis and condensation of silicon alkoxides. Metal alkoxides are the most common precursors used in sol–gel processing, because they react readily with water. The most commonly used precursors for the sol–gel processing of silica are tetraethoxysilane (TEOS), which is also referred to as tetraethylorthosilicate or silicon tetraethoxide, and tetramethoxysilane (TMOS), which is also referred to as tetramethylorthosilicate or silicon tetramethoxide. The preparation techniques for the production of simple oxides are fairly straightforward. Gel compositions with one type of metal cation yield simple oxides on pyrolysis and are referred to as single-component gels. Multicomponent gels have compositions with more than one type of metal cation and yield complex oxides on pyrolysis.
