DTA characterisation of three types of Al2O3-SiO2 gels made from TEOS-Al(OBu)(3) mixture with variation of water

Mixtures of TEOS and Al (OBu)(3) had been hydrolysed by varying the water contents at a fixed acid concentration. This led to the formation of three different kinds of gels. Diphasic gel formed when excess of water was used, which transformed to mullite at 1320 degrees C in exothermic reaction. When the amount of water content was reduced, the resultant monophasic gel completed its mullitization at lower temperatures from an amorphous aluminosilicate phase and cubic mullite by two paths at 1150 degrees C and 1250 degrees C exothermic reactions, respectively. When less water is used, the monophasic gel converted directly to tetragonal mullite at 980 degrees C in exothermic reaction. Thus, the variation of water contents during the gelation caused great variations in the course of mullitization processes. (C) 1996 Elsevier Science Limited and Techna S.r.l