DETERMINATION OF CRITICAL CONCENTRATIONS OF SILICA AND OR CALCIA FOR ABNORMAL GRAIN-GROWTH IN ALUMINA

Minimum amounts of SiO2 and CaO required for inducing abnormal grain growth in alumina were determined using ultrapure alumina (>99.999%) and sintering at 1900-degrees-C for 1 h in a contamination-free condition. The critical concentrations of silicon in cationic mole fractions in alumina were 300 ppm without calcium, 200 ppm with 10 ppm calcium, and 150 ppm with 20 ppm calcium. The critical concentration of calcium alone was 30 ppm. These concentrations seemed to match approximately the solubility limits reported in the literature, which suggested that the abnormal grain growth in commercially pure alumina was indeed related with the formation of a small amount of liquid phase during sintering.