Preparation and Thermal Evolution of the Microstructure of Sol-gel-derived Cordierite and Cordierite-Zirconia Powders

Pure cordierite and a cordierite-$ZrO_2$ (20 wt%) composite have been prepared by the sol-gel technique using both acid and base catalysts. The evolution of crystalline phases and the microstructures have been studied using X-ray diffractometry, IR spectroscopy, and transmission electron microscopy. The effect of acid/base catalysis on the nature of the reaction has been carefully examined. Formation of observed products have been rationalized keeping in view the various acid-base reactions between the oxides and the catalyst in the sol. The molecular electronegativity differences of the component oxides have been used to understand the observed reactions. In all cases, the formation of a-cordierite has been observed at temperatures $>12OO^oC$ only. Zircon is formed in the composites and $ZrO_2$ is mostly present as particulates with c/t structure. Evidence has been found for the possible solid solution formation of cordierite and mullite. $ZrO_2$ has been stabilized in the c/t phase by reaction with MgO, but at higher temperatures the exsolution of MgO from $ZrO_2$, has been evidenced through microscopic studies