Microstructural evolution of calcium-doped α-alumina

Effect of different calcium doping levels on the microstructure of high-purity a-alumina was studied as a function of sintering time and temperature using scanning electron microscopy (SEM). Microstructural evolution was related to hypothetical calcium excess at the grain boundaries (Gamma(Ca)) that was calculated assuming zero solubility of calcium in bulk alpha-alumina. Under all sintering conditions, grains were uniform in size and equiaxed for low calcium concentrations (<3 Ca atoms/nm(2)). The grain morphology became elongated when the calcium concentration at the grain boundaries reached calcium excess of Gamma(Ca) = 3-3.5 Ca atoms/nm(2) in all samples. The average grain sizes of undoped samples were similar to10% larger than the average grain sizes of low-calcium-doped samples. This decrease is believed to be due to solute drag effect of segregated Ca impurities on the grain boundary mobility. For the samples that were sintered at 1500degrees and 1600degreesC, slablike abnormally grown grains appeared for critical calcium excess concentrations of Gamma(Ca) = 4.5-8 Ca atoms/nm(2). With abnormally grown grains a dramatic increase in average grain size was observed. However, when the calcium concentration was increased further, above certain calcium excess concentration depending on sintering temperature, a significant decrease in grain size was observed. In contrast to samples sintered at 1500degrees and 1600degreesC, when the samples sintered at 1400degreesC, although the hypothetical calcium coverage exceeded Gamma(Ca) = 11 Ca atoms/nm(2), only few grains grew abnormally without significantly affecting the average grain size. Observations clearly indicated that calcium impurities caused elongated (slablike) grain morphology when their excess concentrations reached a critical level at the grain boundaries.