Effect of Sol‐Gel Mixing on Mullite Microstructure and Phase Equilibria in the α‐Al 2 O 3 ‐SiO 2 System

Starting powders containing 72 wt% Al 2 O 3 and 28 wt% SiO 2 , 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 AI 2 O 3 content, resulting in an excess of SiO 2 which is the base for the liquid phase. In the second case, the mullite grains have the same Al 2 O 3 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.