Influence of Synthesis and Composition on Mullite Crystallization

Gels with nominal compositions Al2O3·SiO2 and 2Al2O3·SiO2 have been prepared using two sol−gel routes, the first one by long gelation time of a solution of TEOS and aluminum nitrate nonahydrate, the second one starting from TEOS and Al alkoxides with different processing conditions. Xerogels with different levels of homogeneity as depicted by their crystallization schemes have been obtained. Whereas mullite with different compositions is obtained after the first 1000 °C exotherm, a similar evolution of the compositions is observed for all samples at higher temperature. The alumina content at 1000 °C has been related to the compositions and relative proportions of local heterogeneities present inside the xerogels.