Characterization of low temperature mullitization

Anomalously low temperature mullitization was observed in the precursor prepared by the Nishio and Fujiki's method using aluminum nitrate nonahydrate (ANN), aluminum iso-propoxide (AIP), and silicon ethoxide (TEOS) and its mechanism was examined by various methods. The low temperature mullitization was found to occur only when the precursor was once heat-treated at 250 degrees C before heating up to crystallization. The heat-treated precursor showed two-step mullitization with small amounts of mullitization at 450 degrees C and large amount of mullitization at 900 degrees C. On the other hand the as-prepared precursor showed one-step mullitization at around 900 degrees C. The presence of two small regions with slightly different chemical composition was found in the particles of the heat-treated precursor samples by Si-29 nuclear magnetic resonance (NMR) spectra, small angle X-ray scattering (SAXS) and X-ray Rietveld analysis. The low temperature mullitization was considered to occur at the interfaces of the two small regions because they acted as the heterogeneous nucleation sites.