CRYSTALLIZATION OF MULLITE FROM AMORPHOUS MATERIALS

用异丙铝和甲基硅酸盐的混合物水解制得以莫来石为氧化物的非晶态材料。当快速水解处理时，制备的材料在980℃左右结晶为尖晶石相，并伴有放热反应。当缓慢水解时，材料在980℃时结晶为莫来石和尖晶石相。在1250℃左右，这些尖晶石相消失，莫来石生长。而硝酸铝和甲基硅酸盐的乙醇-水溶液在600℃下喷雾热解制备的非晶态材料，在980℃下直接结晶为莫来石。该材料在920℃加热48小时后结晶为少量莫来石，未结晶为尖晶石相。在980℃时出现莫来石相还是尖晶石相取决于非晶材料中铝和硅组分分布的均匀性。异丙醇铝的水解速度比甲基硅酸盐快得多。当对这些混合物的醇溶液进行快速水解时，优先水解异丙醇铝，部分浓缩铝组分。γ - Al_2O_3（尖晶石相）会在这些部分结晶。喷雾热解材料具有铝和硅组分原子均匀分布，并直接结晶成莫来石。在较低温度下结晶的莫来石具有较大的a值。这意味着莫来石中氧化铝含量高。该值随着烧成温度的升高而降低，并达到化学计量莫来石的正常值。这些非晶态材料在980℃左右急剧收缩，并伴有放热结晶反应。放热含量大，材料直接结晶成莫来石，收缩率大。过量硅含量的莫来石陶瓷是由莫来石晶粒和玻璃相组成，在方石-莫来石共晶温度以上烧制而成。在低于共晶温度1500℃下再加热后，玻璃相结晶为方石云石。方石英的结晶过程由压块表面向内部进行。结晶后的莫来石-方石英复合材料的力学性能优于结晶前的莫来石-玻璃陶瓷。少

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