Two kinds of xerogels were prepared by the slow and rapid hydrolyses of tetraethoxysilane and aluminum nitrate nonahydrate dissolved in ethanol. Xerogels prepared by slow hydrolysis crystallized mullite directly from the amorphous state on firing whereas those formed by rapid hydrolysis crystallized a spinel phase before mullite formation. The composition of the spinel phase was characterized by various methods to be near SiO 2 ·6Al 2 O 3 . The process of mullite formation is discussed in relation to the structures of the starting materials.
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Okada et al. (1986) studied this question.
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