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-degrees-C. Gels with distinct silica and alumina phases crystallize to mullite above 1250-degrees-C. Crystallization at 980-degrees-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-degrees-C. The enthalpy of crystallization to mullite from the gels varies from -112 kJ/mol-(Al6Si2O13) for complete mullite formation at 980-degrees-C from a homogeneous gel formed by controlled hydrolysis of TEOS and aluminum isopropoxide, to -33 kJ/mol for mullitization at 1250-degrees-C from a heterogeneous gel made of boehmite in hydrolyzed TEOS. Gels with intermediate mixing formed an alumina-rich spinel phase at 980-degrees-C, which contained 7 wt % SiO2 as shown by Si-29 MAS NMR. Mullite formed at 980-degrees-C is aluminum-rich with an Al/Si ratio of 5, with the excess silica phase separating as amorphous silica. Between 950 and 1200-degrees-C the silica and mullite react increasing the silicon content in mullite crystals. The enthalpy of the substitution reaction, Al3+ + 1/2 vacancy --> Si4+ + O-2(1/2), was determined to be -52 kJ/mol. The enthalpy of formation of mullite from mixtures of alpha-alumina and quartz varied with the Al/Si ratio and was a minimum at Al/Si = 3.2.