Effects of Hydrolysis on the Kinetics of High-Temperature Transformations in Aluminosilicate Gels

The reaction kinetics for the formation of mullite (3Al₂O₃· 2SiO₂) from sol-gel-derived aluminosilicate gels prepared under various hydrolysis conditions were studied using dynamic X-ray diffraction (DXRD) and differential thermal analysis (DTA). The DXRD experiments showed that the apparent single-phase gels aluminosilicate gels were not completely single-phase gels but a composite of a single-phase gel (which has molecular-scale mixing) and a diphasic gel (which has nanometer-scale mixing). Mullite formation from these composite gels exhibits a two-stage conversion, the first at about 980°C and the second at about 1220°C. Gels prepared by a slower hydrolysis rate tend to have a higher conversion after the first stage, therefore, better molecular-scale mixing. Simultaneous formation of Al-Si spinel and mullite was also observed at 980°C. This coincidence of mullite and spinel formation could explain some of the controversy in the literature concerning the 980°C thermal event.