SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF NONCRYSTALLINE MULLITE PRECURSORS

Two different types of non-crystalline mullite precursor with identical bulk composition (72 wt% Al2O3, 28 wt% SiO2) were prepared from tetraethoxysilane and silicon chloride, respectively, and aluminium sec-butoxide, by using different methods of hydrolysis. The precursors, designated as type I and III, display different crystallization processes above approximate to 900 degrees C: type I precursors directly form mullite, while type III precursors yield crystallization of transient gamma-alumina. Infrared (IR) spectroscopy, large angle X-ray scattering (LAXS) and Al-27 nuclear magnetic resonance spectroscopic studies, and Si-29 nuclear magnetic resonance (NMR) literature data give evidence for a high degree of structural mixing in type I precursors and for a beginning of segregation into Al2O3-rich domains in type III precursors prior to crystallization (less than or equal to 900 degrees C). Both precursors are composed of (SiO) tetrahedra and of (AlO) octahedra, tetrahedra and pentahedra although pentahedra are dominant in type I while octahedra occur more frequently in type III precursors. The driving force for mullitization (type I) and gamma-alumina formation (type III) taking place at the same temperature is believed to be the instability of pentahedrally coordinated Al above approximate to 900 degrees C. The sudden disappearance of Al pentahedra probably depends on the formation of reactive network centers during dehydroxylation. This hypothesis is derived from the observation that dehydroxylation and condensation strongly take place in a similar temperature range prior to crystallization.