Surface of Silica Glass Reacting with Silicon Melt: Effect of Raw Materials for Silica Crucibles

Abstract

The density of brownish spots formed on the surface of silica glass through the reaction with silicon melt is studied using three types of silica glass, that are made from natural quartz powder, synthetic silica powder, and Al-doped synthetic silica powder. The density of the brownish spots was higher in natural quartz glass (NQG) than in synthetic silica glass (SSG) and in Al-doped synthetic silica glass (ASG). The result is consistent with that of the devitrification of silica glass observed in the interior of the bulk glass after a heat treatment. Aluminum and calcium are detected as aggregates in the devitrification spots of NQG. It is noted that ASG, containing a comparable concentration of well-dispersed aluminum, shows a significantly lower density of brownish spots and internal devitrification spots than NQG. Thus, it is considered that the formation of the brownish spots originates from the devitrification of silica glass caused by aggregates of impurities.