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 o f TEOS and boehmite, with and without dehydroxylation at 400 degrees C . The precursor samples during the process of phase formation were cha racterized with respect to TGA, DTA, TMA, and X-ray diffraction at var ious temperatures. The result shows that by using a partially dehydrox ylated boehmite, an enhanced transitional alumina phase is found at a lower temperature (similar to 1000 degrees C) along with traces of alp ha-Al2O3. With an increase in soaking time, the intensity of alpha-alu mina is often found to increase. The formation of mullite takes place at 1225 degrees C and the alpha-Al2O3 formed at a lower temperature re mains unaffected even at 1500 degrees C. On the other hand, in the sam ples prepared from boehmite and TEOS, the transitional alumina phase i s observed only at 1225 degrees C and it transforms to phase pure orth orhombic mullite at 1250 degrees C. Thus while the partially dehydroxy lated boehmite appears to be more reactive to transform to transitiona l alumina phase at lower temperature, the mullite formation temperatur e does not occur below 1225 degrees C. The results of investigation ar e presented and discussed. (C) 1998 Elsevier Science Limited. All righ ts reserved.