Crystallization of cristobalite from glass phase in mullite ceramics with excess SiO2 compositions

Abstract

Crystallization of glass phase to cristobalite was investigated in mullite-glass ceramics with excess SiO2 compositions (5.3-16.4 mass%), namely, higher than stoichiometric mullite. Starting powders were prepared by alkoxide hydrolysis method. Compact specimens were sintered at 1600°C above the eutectic temperature (1587°C) for 2 h. Crystallization treatment was carried out at 1500°C, below the eutectic temperature, for times from 4 h to 96 h. Crystallization of glass phase proceeded from the surface of the specimen toward its inner part, in linear dependence on the annealing time. At the polished surface of the specimen, crystallization started in large glass pockets and the crystallized area extended spherically toward the inner part. This phenomenon suggests that nucleation occurs at the minimum parts in the elastic energy generated by the volume change involved in the crystallization of glass phase to cristobalite.