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

Abstract

The influence of homogeneity of sol-gel precursors on mullite (AleSi2013) 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(A&Si2013) 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 5% Si02 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, AP+ + liZ vacancy - Si4+ + 1/202-, was determined to be -52 kJ/mol. The enthalpy of formation of mullite from mixtures of a-alumina and quartz varied with the Al/Si ratio and was a minimum at Al/Si = 3.2.