Role of hydrolysis water-alcohol mixture on mullitization of Al2O3-SiO2 monophasic gels

Abstract

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 °C exotherm and a further two more broad exotherms at 1150 and 1243 °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 °C exotherm. Nucleation and crystallization of mullite from amorphous alumino silicate phase takes place at ∼ 1150 °C, and transformation of cubic mullite to orthorhombic mullite takes place at ∼ 1245 °C. The second kind of monophasic gel exhibited only one exotherm and produced exclusively tetragonal mullite at ∼ 980 °C. The results suggest that pure mullite formation is possible when water induced hydrolysis of TEOS is avoided during the gelation process.