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 m, for which the ultraviolet absorption coefficient, , 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.