MULLITE FORMATION FROM NONSTOICHIOMETRIC DIPHASIC PRECURSORS

Diphasic gels with Al/Si atomic ratios of 6/1, 3.1/1, 3/1, 2/1, and 1/1 were used to study the effect of precursorr composition on mullite formation process and the resulting microstructure. Mullite formation initiated at about 1300-degrees-C for all samples, with only some slight differences in temperatures. The mullite formation temperature was a minimum for an Al/Si ratio of 3.1/1 and increased as the Al/Si ratio of the gels increased or decreased. For the 6/1 gel, dissolution of alumina into mullite solid solution was observed after the initial mullite formation. The dissolution process was reversed above 1450-degrees-C with the formation of theta-Al2O3 and then alpha-Al2O3. Change of microstructures from equiaxed to elongated mullite grain structures was found within a narrow range of composition near the nominal Al/Si ratio of 3/1.