Thermally induced chemical and structural changes in alumina-zirconia-silica gels during the formation of ceramic composites

Abstract

Ceramic composites have been prepared from the system Al₂O₃---ZrO₂---SiO₂ using the sol-gel technique. The influence of catalyst acids and bases on the chemistry, structure, and morphology of the products obtained after heat treatment at various temperatures (800-1400°C) have been studied using powder X-ray diffractometry and transmission electron microscopy. Formation of compounds at higher temperatures have been shown to be related to the various acid-base reactions between the oxides and the catalyst acid or base in the sol. Formation of zircon (ZrSiO₄) in the presence of mullite and by direct SiO₂---ZrO₂ reactions in different compositions has been discussed. A new approach based on the molecular electronegativity differences of the component oxides has been made in order to rationalize the observed reactions. Partial charges on oxygen in various oxides have been evaluated using molecular electronegativities and are shown to be consistent both with well-known basicities of component oxides and the observed reactions. The importance of considering electronegativities in oxide reactions is emphasized.