ROLE OF HYDROLYSIS WATER-ALCOHOL MIXTURE ON MULLITIZATION OF AL2O3SIO2 MONOPHASIC GELS

Two kinds of monophasic gels were synthesized from silicon ethyl ester (TEOS) and aluminium nitrate mixture by varying the water-alcohol content of the system. The mullitization reaction paths of the two gels were studied by differential thermal analysis (DTA) and X-ray diffraction techniques. The first kind of monophasic gel exhibited cubic mullite (Si-Al spinel) on the 980 degrees C exotherm and a further two more broad exotherms at 1150 and 1243 degrees C. This phenomenon resembles the DTA thermogram of a kaolinitic clay. The later two exotherms have been explained as due to the formation of mullite by two different reaction paths from the two phases appearing in the 980 degrees C exotherm. Nucleation and crystallization of mullite from amorphous alumino silicate phase takes place at similar to 1150 degrees C, and transformation of cubic mullite to orthorhombic mullite takes place at similar to 1245 degrees C. The second kind of monophasic gel exhibited only one exotherm and produced exclusively tetragonal mullite at similar to 980 degrees C. The results suggest that pure mullite formation is possible when water induced hydrolysis of TEOS is avoided during the gelation process.