Abnormal grain growth in alumina

Abnormal grain growth of alumina is not one of intrinsic properties but an extrinsic property controlled by certain impurities introduced during powder synthesis, processing or sintering. When small amounts of glass-forming impurities are introduced, some portion beyond their solubility limit would accumulate at grain boundaries at the final stage of densification, form thin intergranular glass films of thermodynamically stable thickness; and induce sudden appearance of abnormal grains by increasing abruptly mobility of grain boundaries. The proposition was tested experimentally with small amounts of SiO2 in ultrapure alumina (> 99.999%) in a contamination-free sintering condition. Average grain sizes for the appearance of abnormal grain growth was indeed inversely related with the concentration of SiO2. Thus, the critical concentration of SiO2 per unit area of grain boundary is uniquely defined at the onset of abnormal grain growth.