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

The reaction kinetics for the formation of mullite (3Al 2 O 3 · 2SiO 2 ) 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.