Formation of mullite and other alumina-based ceramics via hydrolytic polycondensation of alkoxides and resultant ultra- and microstructural effects

This manuscript discusses the formation of alumina-based ceramic materials from alkoxide starting materials. Particular attention is given to Al2O3-SiO2 systems, especially mullite. When the oxide network of these materials is formed via chemical polymerization of alkoxides, the ultrastructure of the resultant condensates is fundamentally affected by the nature and kinetics of the condensation reactions. Crystallization of mullite from an amorphous Al6Si2O13 at ∼ 980°C appears to be related to the intimacy and homogeneity of the aluminium-silicon bonding in the precursor networks. Network inhomogeneities due to the different hydrolysis rates of mixed alkoxides diminish by reduction of the chemical encounter rates with water during the hydrolytic polycondensation. Mullite produced from homogeneous and stoichiometric condensate shows a large spontaneous densification at ∼ 980° C. The microstructure of this material shows no indication of a liquid phase at the grain boundaries.