Effect of Sol‐Gel Mixing on Mullite Microstructure and Phase Equilibria in the α‐Al2O3‐SiO2 System

Abstract

Starting powders containing 72 wt% Al2O3 and 28 wt% SiO2, were prepared by sol‐gel methods classified as colloidal and polymeric. Compacts fired at 1700°C showed significant differences in microstructure. The specimens formed with the colloidal powder had mullite grains of prismatic shape and a liquid phase; with polymeric powder, mullite grains were granular with no liquid phase present. It is shown that the mullite grains in the first case are higher in AI2O3 content, resulting in an excess of SiO2 which is the base for the liquid phase. In the second case, the mullite grains have the same Al2O3 content as the starting powders. The presence of a liquid phase in the first case is considered to be metastable, resulting from the nature of the starting materials and processing conditions employed.