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Submicrometer Si02-A1203 powders with compositions of 46.5 to 76.6 wt % A1203 were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and AI-Si spinel at around 980°C. The compositions of mullite produced at 1400' and 1550°C were richer in A1203 than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO2-AlZO3 system. Particle size decreased with increasing Al2O3 content. The sintered densities depended upon the amount of Si02-rich glassy phase formed during sintering and the green density expressed as a function of particle size. [