The Sol-Gel Process to Oxide Ceramics

Starting in the 1950s a process was developed that leads from small-molecule silicon alkoxides such as tetraethoxysilane (tetraethyl orthosilicate), to organosiloxane oligomers and low polymers, and eventually to silica via a low temperature synthesis route.14 24 A simplified outline of the basic chemistry is shown in reactions (l)-(4), where R is an ethyl or higher alkyl unit. Any or all of the Si-OR bonds can be 

Insoluble (may swell), infusible, non-volatile, thermally reactive 

The complexity of this reaction mechanism is legendary. The reaction pathways and types of products change with variations in pH, the nature of the alkoxy group OR, the rate at which water and alcohol are removed during heating, the type of solvent, the 

Acidic conditions High concentrations Lower temperatures 

Alkaline conditions High dilutions Higher temperatures 

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