EFFECT OF PH ON 980-DEGREES-C SPINEL PHASE-MULLITE FORMATION OF AL2O3-SIO2 GELS

Different reaction paths of mullite formation via sol-gel processing techniques are reviewed. These variations are due to differences in hydrolysis/gelation behaviours of the silica and alumina components used. Variations of pH during processing without altering other variables follow three different routes of mullite formation. In the highly acidic region (pH less than or equal to 1), the gel does not exhibit a 980 degrees C exotherm but forms gamma-Al2O3. Mullite forms at high temperature by diminution of alpha-Al(2)O(3)pH range of 3-4.5, gels exhibit a 980 degrees C exotherm and develop only mullite. In the highly alkaline region (pH similar to 14), the gel produces a Si-Al spinel phase at the 980 degrees C exotherm and mullite formation at the similar to 1330 degrees C exotherm takes place from the intermediate Si-Al spinel phase.