Structural Investigation and Energetics of Mullite Formation from Sol-Gel Precursors

The influence of homogeneity of sol-gel precursors on mullite (Al₆Si₂O₁₃) formation was examined with a combination of structural and thermodynamic characterization by NMR, X-ray diffraction, and drop solution calorimetry. The most homogeneous gels crystallize to mullite at 980 °C. Gels with distinct silica and alumina phases crystallize to mullite above 1250 °C. Crystallization at 980 °C and the nature of the dominant crystalline phase, spinel or mullite, were related to the abundance of pentacoordinated aluminum atoms in the amorphous phase at 900 °C. The enthalpy of crystallization to mullite from the gels varies from −112 kJ/mol- (Al₆Si₂O₁₃) for complete mullite formation at 980 °C from a homogeneous gel formed by controlled hydrolysis of TEOS and aluminum isopropoxide, to −33 kJ/mol for mullitization at 1250 °C from a heterogeneous gel made of boehmite in hydrolyzed TEOS. Gels with intermediate mixing formed an alumina-rich spinel phase at 980 °C, which contained 7 wt % SiO₂ as shown by ²⁹Si MAS NMR. Mullite formed at 980 °C is aluminum-rich with an Al/Si ratio of 5, with the excess silica phase separating as amorphous silica. Between 950 and 1200 °C the silica and mullite react increasing the silicon content in mullite crystals. The enthalpy of the substitution reaction, Al³⁺ + 1/2 vacancy → Si⁴⁺ + ½O²⁻, was determined to be −52 kJ/mol. The enthalpy of formation of mullite from mixtures of α-alumina and quartz varied with the Al/Si ratio and was a minimum at Al/Si = 3.2.