SYNTHESIS AND PHASE-TRANSFORMATIONS OF MULLITES OBTAINED FROM SIO2-AL2O3 GELS

Synthesis and structural characterization of mullite phases obtained f rom three nominal compositions in the SiO2-Al2O3 system is reported an d comparisons between them are established. Monosized gels with Al/Si atomic ratios of 2:3, 3:1 and 9:1 were obtained and thermally treated at temperatures between 700 and 1500 degrees C. The thermal evolution of amorphous gels was followed by thermal analysis, X-ray diffraction and infra-red spectroscopy. The microstructural changes were studied b y scanning electron microscopy and transmission electron microscopy. T he formation of Al2O3-rich mullite started around 1000 degrees C for a ll samples and was fully developed at 1400 degrees C. The chemical com position of mullites changed as a function of calcining temperature. A n anomalous low value of the lattice parameter a was obtained for the sample with Al/Si atomic ratio of 9:1 compared with the 3:1 sample at 1400 degrees C, which may be due to the slow reaction rate of Al2O3 wi th SiO2-rich glassy phase during mullite formation.