Crystallization of Fibrous Mullite from Glass Melt

Abstract

Mullite is one of the most outstanding ceramic materials used at a high temperature in these days. We have made a fundamental research to produce ceramics/ceramics composites by dispersing mullite fibers into the silicate glass matrix using a solidification processing. The influence of parents materials, namely SiO₂/mullite systems or SiO₂/Al₂O₃ systems, and melting conditions on the morphology of crystallized mullite were mainly examined.
(1) In both systems the mullite nucleated as primary phase during isothermal treatment in the solid/liquid co-existence range grew into fibrous single crystals by cooling. The shape of the mullite changed from fibrous to granular with increasing Al₂O₃ content in the melt.
(2) Mullite nucleated and grew during isothermal treatment at 1973 or 1923 K when the melt contained more than 14.7 mass%Al₂O₃ in the SiO₂/Al₂O₃ systems and more than 10.7 mass%Al₂O₃ (15 mass% mullite) in the SiO₂/mullite systems.
(3) As the mullite grew, the SiO₂ content in the melt increases. This induced the increase in viscosity of the melt and then the decrease in diffusion coefficient. As the result, the residual melt solidified to glass.
(4) Dendritic mullite fibers could be formed only in the case of 14.7 mass%Al₂O₃ melt for both systems.