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

Reconstructed TiO 2 -SiO 2 glasses were prepared by impregnating porous glass with solutions containing titanium tetraisopropoxide Ti(OC 3 H 7 ) 4 , converting the alkoxide in the pores to titania, and consolidating the porous structure. The final glasses contained from about 6 to 11 wt% TiO 2 . 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% TiO 2 were thermally stable, showing no significant change in expansion after 10h heating in air a 970°C. However, glasses with higher TiO 2 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