Low-Temperature Mullitization in Boehmite-Tetraethoxysilane Gel Precursor Containing γ-Alumina and Mullite Nucleating Seeds

The formation of mullite from diphasic mullite gels of stoichiometric composition 3Al2O3·2SiO2 through a mixture of seed nuclei has been studied. The precursor gels are prepared from boehmite (AlOOH) sol and tetraethoxysilane (TEOS) seeded with mullite (2 wt %) and γ-Al2O3 (2 wt %) individually and as a mixture consisting of γ-Al2O3 (1 wt %) and mullite (1 wt %). The phase formation in such gels has been compared with sample containing no seeds. The presence of mullite is observed as early as 1150 °C in the mixed seeded sample, while in the unseeded one, this is observed only at 1250 °C, as reported generally. On the otherhand, the mullitization reaction has been initiated at a temperature of 1200 °C in gels seeded separately with 2 wt % γ-Al2O3 and 2 wt % mullite. The results suggest that the mechanism of mullite formation in seeded gel is similar to that of diphasic gels in which the transitional alumina−amorphous silica reaction predominates to form mullite. Mixed seeding appears to be appreciably favorable in reducing the transformation temperature of mullite compared to the individually seeded counterparts and also results in a finely grained dense microstructure.