Specialty Optical Fibers Handbook

such as Si(OC

2

H

5

)

4

, is reacted with water in the presence of ethanol and an acid

catalyst. The ‘‘sol’’ is cast into cylindrical molds and poly-condensation of the

resulting silanol groups produces a filamentary siloxane gel network. The gel

body is dried and consolidated to form silica glass as films or bulk bodies [39].

Alternatively, commercial colloidal powders obtained from flame hydrolysis,

commonly known as ‘‘fumed silica,’’ are formed into bodies by mechanical

compaction [40], centrifugation [41], or casting/gelation [42]. In this last ap-

proach, the silica particles (generally 0:05–0:5 mm) are dispersed in water to

form a sol. Control of pH or addition of surface active agents is used to promote

electrostatic and steric stabilization to