Anisotropic grain growth of mullite in high-energy ball milled powders doped with transition metal oxides

Dense mullite ceramics with anisotropic grains were derived from the high-energy ball milled mixtures of Al2O3 and amorphous silica with the presence of transition metal oxides (FeO1.5, CoO and NiO). The mullitization and grain growth behavior of the unmilled mixture without the addition of the transition metal oxides and the undoped system of Al2O3 and amorphous silica with and without milling were also investigated and compared. The mullitization temperature was about 1200 °C in the milled systems, 100 °C lower than that required by the conventional solid-state reaction process. The lowered mullitization temperature, as well as the anisotropic grain growth, was attributed to the refined structure of the oxide powders, as a result of the high-energy ball milling. The experimental results have been explained by a dissolution-precipitation mechanism.