Motoyuki Toki

A large-size silica glass produced by a new sol-gel process

Abstract A large-size silica glass was fabricated with a new sol-gel process involving the following procedures: (a) hydrolysis of Si(OC2H5)4 with HCl, (b) addition of silica powders and their high dispersion, (c) adjustment of the pH value by adding ammonia solution, (d) gellation, (e) drying to dry-gel, and (f) sintering to silica glass. With this process a dry-gel plate as large as 520 x 360 mm2 was obtained, which was sintered to a 420 x 290 mm2 sized silica glass. The effect of pH on the gelation time of the sol, and the correlation between the weight percentage of silica powder and porosity of dry-gel were examined. In addition, fiber preforms were produced by this sol-gel process.

Fabrication of high-purity silica glass through the WSPA-sol-gel process

High-purity and large-size silica glass was prepared by a wet process silica and pH adjustment (WSPA)-sol-gel process which involved the following steps: (1) colloidal silica synthesis through the hydrolysis reaction of TEOS catalysed by ammonia solution; (2) sol preparation through the hydrolysis reaction of TEOS by hydrochloric acid; (3) mixing the colloidal silica and sol solution; (4) adjusting the pH values of the mixed solution to 4–6; (5) gelling; (6) drying to a dry gel; (7) heat treatment of the gel to collapse the pores, finally to become a silica glass. The purity of the derived silica glass was examined in relation to the metallic impurities and -OH impurities. The relation between ultraviolet absorption of silica glass and metallic impurities was investigated. Ti addition to the silica glass gave a strong absorption at 200 μm, for which the ultraviolet absorption coefficient, ɛ, was found to be 1.57×104 l mol−1 cm−1. The effects of the properties of colloidal silica on the -OH concentration and also the relations between the sintering conditions and residual -OH concentration in silica glass were examined. Using colloidal silica with a large particle size and heat treatment in vacuum resulted in a silica glass with a low concentration of residual -OH.