SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF NONCRYSTALLINE MULLITE PRECURSORS

Two different types of non-crystalline mullite precursor with identica l bulk composition (72 wt% Al2O3, 28 wt% SiO2) were prepared from tetr aethoxysilane and silicon chloride, respectively, and aluminium sec-bu toxide, by using different methods of hydrolysis. The precursors, desi gnated as type I and III, display different crystallization processes above approximate to 900 degrees C: type I precursors directly form mu llite, while type III precursors yield crystallization of transient ga mma-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 precursor s prior to crystallization (less than or equal to 900 degrees C). Both precursors are composed of (SiO) tetrahedra and of (AlO) octahedra, t etrahedra and pentahedra although pentahedra are dominant in type I wh ile octahedra occur more frequently in type III precursors. The drivin g force for mullitization (type I) and gamma-alumina formation (type I II) taking place at the same temperature is believed to be the instabi lity of pentahedrally coordinated Al above approximate to 900 degrees C. The sudden disappearance of Al pentahedra probably depends on the f ormation of reactive network centers during dehydroxylation. This hypo thesis is derived from the observation that dehydroxylation and conden sation strongly take place in a similar temperature range prior to cry stallization.