Gel formation in mullite precursors obtained via tetraethylorthosilicate (TEOS) pre-hydrolysis

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Tetraethylorthosilicate (TEOS) and aluminium chloride were taken as sources of SiO2 and Al2O3 to prepare precursors of mullite by pre-hydrolysis of TEOS under refluxing conditions. Gelation was carried out at different pH values and the effect of the pH of gelation on the subsequent temperature-induced phase transformations was characterized by differential thermal analysis, powder X-ray diffraction and 29Si and 27Al solid-state nuclear magnetic resonance spectroscopy. A pH-dependent exothermic peak was observed at ∼980°C. Strong acidic conditions (pH=1.5) were found to be beneficial for improving the mixing scale of the Al–Si components, leading to a mullitization temperature of ∼1200° and a sharp 980°C exothermic peak. In strong basic conditions (pH=11.5), no evident ∼980°C exothermic peak was detected, and the mullitization temperature was as high as 1350°C, probably due to heterogeneity in the mixing scale of the Al–Si components in the precursor system. A gel formation process has been proposed.

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