Use of alkoxides in the preparation of low-expansion reconstructed TiO2-SiO2 glasses

Abstract

Reconstructed TiO2-SiO2 glasses were prepared by impregnating porous glass with solutions containing titanium tetraisopropoxide Ti(OC3H7)4, converting the alkoxide in the pores to titania, and consolidating the porous structure. The final glasses contained from about 6 to 11 wt% TiO2. On subjecting them to treatments in an oxy/gas flame (term for pure oxygen combustion) or a high-temperature furnace it was possible to obtain clear glasses with thermal expansion coefficients ranging from -0.2 to +0.3 · 10^-6/K. Reconstructed glasses containing about 7 wt% TiO2 were thermally stable, showing no significant change in expansion after 10 h heating in air at 970 °C. However, glasses with higher TiO2 contents were not thermally stable when reheated. X-ray analyses revealed that they contained rutile, a high-expansion phase that readily accounts for the increase in expansion observed after reheating. Water permeation studies of alkoxide-impregnated porous glass showed that the rate of penetration of water, and hence the rate of hydrolysis of the alkoxide, is diffusion-controlled. Reactions that are likely to occur in the impregnated porous glass on hydrolysis and on heating are discussed.