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

Starting powders containing 72 wt% Al/sub 2/O/sub 3/ and 28 wt% SiO/sub 2/ were prepared by sol-gel methods classified as colloidal and polymeric. Compacts fired at 1700/sup 0/C showed significant differences in microstructure. The specimens formed with the colloidal powder had mullite grains of prismatic shape and 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 Al/sub 2/O/sub 3/ content, resulting in an excess of SiO/sub 2/ which is the base for the liquid phase. In the second case, the mullite grains have the same Al/sub 2/O/sub 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.