Colloidal sol/gel processing of ultra-low expansion TiO₂/SiO₂ glasses

A series of titania-silica glasses with 0-9% TiO₂ were fabricated using a sol/gel process. The sol was prepared by dispersing colloidal silica fume in an aqueous solution of titania which was synthesized through the acid-catalyzed hydrolysis of titanium isopropoxide. The sols gelled in 2-4 days, and then were dried for 6-8 days. The dry gels were sintered at 1450-1500°C to produce clear, dense, microstructure-free glasses. The gels underwent a total shrinkage of ∼50% to yield glass rods about 50 mm long and 5 mm in diameter, or glass discs about 4 cm in diameter and 5 mm thick. The drying step was most critical in the production of crack-free specimens. In the gel, the transmission electron microscope (TEM) revealed the presence of 1-5 nm rutile microcrystallites uniformly distributed within a network of colloidal silica particles. After sintering to 1450-1500°C, though, a dense, transparent, microstructure-free glass was created. Fourier transform infrared spectroscopy (FTIR) verified the formation of an amorphous solid-solution of titania and silica after sintering. The thermal expansion of the glasses was measured using a differential dilatometer. The average linear coefficients of thermal expansion (CTE @ 25-675°C) varied between +5 × 10^-7 and -0.2 × 10^-7°C^-1 in the range 0 to 9% TiO₂. The glass with 7.2% TiO₂ exhibited a zero thermal expansion coefficient at 150-210°C. The hysteresis in CTE on heating and cooling was of the order of 0.01-0.02 ppm.