Titania-silica glasses using a colloidal sol-gel process

A colloidal sol-gel process has been developed to prepare a silica-10% titania glass for low thermal expansion substrates. The method utilizes titanium isopropoxide which is first reacted with ethylene glycol and citric acid at 120°C. This stabilizes the titanium isopropoxide against hydrolysis/precipitation so that it can be mixed with water and then used as the medium for dispersion of colloidal silica. This sol is then cast in tubes, sealed, and heated to 60-70°C to promote gelation. The gels are removed from the tubes after 2-3 days, dried in air for 24-36 h, and then sintered to 1230°C. The decomposition of the Ti-isopropoxide/ethylene glycol resin occurs at 300-350°C at which point the average pore-size increases to about 10.0 nm. The final density of the sintered gel is comparable to that reported for glasses of similar composition prepared by chemical vapor deposition (CVD). The glasses are translucent and TEM examination revales that this is due to the presence of uniformly distributed microcrystallites of anatase (<150 A ̊). The coefficients of thermal expansion are intermediate between those of fused silica and a commercial titania-silica glass (Corning 7971).