Formation of mullite phase in diphasic gels consisting of TEOS and boehmite with and without dehydroxylation

Mullite (3Al(2)O(3) 2SiO(2)) samples have been prepared from mixture of TEOS and boehmite, with and without dehydroxylation at 400 degrees C. The precursor samples during the process of phase formation were characterized with respect to TGA, DTA, TMA, and X-ray diffraction at various temperatures. The result shows that by using a partially dehydroxylated boehmite, an enhanced transitional alumina phase is found at a lower temperature (similar to 1000 degrees C) along with traces of alpha-Al2O3. With an increase in soaking time, the intensity of alpha-alumina is often found to increase. The formation of mullite takes place at 1225 degrees C and the alpha-Al2O3 formed at a lower temperature remains unaffected even at 1500 degrees C. On the other hand, in the samples prepared from boehmite and TEOS, the transitional alumina phase is observed only at 1225 degrees C and it transforms to phase pure orthorhombic mullite at 1250 degrees C. Thus while the partially dehydroxylated boehmite appears to be more reactive to transform to transitional alumina phase at lower temperature, the mullite formation temperature does not occur below 1225 degrees C. The results of investigation are presented and discussed. (C) 1998 Elsevier Science Limited. All rights reserved.