CRYSTALLIZATION OF MULLITE FROM AMORPHOUS MATERIALS

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Amorphous materials which have a stoichiometric composition of mullite as an oxide were obtained by hydrolysis of mixture of aluminum isopropoxide and methylsilicate. When the hydrolysis was treated rapidly, prepared material crystallized to spinel phase at about 980ﾟC, accompanied with the exothermic reaction. When the hydrolysis was treated slowly, the material crystallized to mullite and spinel phase at 980ﾟC. These spinel phases disappeared at about 1250ﾟC and well crystallized mullite grew. On the other hand, the amorphous material prepared by spray-pyrolysis of ethanol-water solution of aluminum nitrate and methylsilicate at 600ﾟC, crystallized to mullite directly at 980ﾟC. This material crystallized to a small amount of mullite and not to spinel phase after heating at 920ﾟC for 48 hours. Whether mullite or spinel phase appears at 980ﾟC, depends on homogeneity of distribution of aluminum and silicon components in amorphous materials. Aluminum isopropoxide is hydrolyzed much more … More rapidly than methylsilicate. When alcohol solution of these mixture is treated rapidly to hydrolyze, aluminum isopropoxide is hydrolyzed preferentially and aluminum component is concentrated partly. gamma - Al_2O_3 (spinel phase) would crystallize in these portions. Spray-pyrolyzed material has an atomically homogeneous distribution of aluminum and silicon components and crystallizes to mullite directly. Mullite crystallized at lower temperature has a large value of a。 axis and this means high alumina content in mullite. This value decreases with elevating firing temperature and reaches to normal value of stoichiometric mullite.A compact of these amorphous materials shrank drastically at about 980ﾟC, accompanied with the exothermic crystallization reaction. The shrinkage was large for large exothermic content of which material crystallizes to mullite directly. Mullite ceramics with excess silica content which were fired above cristobalite- mullite eutectic temperature were composed of mullite grains and glass phase. The glass phase were crystallized to cristobalite after reheating at 1500ﾟC , below the eutectic temperature. The crystallization to cristobalite occurred from the surface of compact to the inner part. Mullite-cristobalite composite after the crystallization had superior mechanical properties to mullite-glass ceramics before the crystallization. Less