Calcium in alpha-alumina:: The myth and some EM observations

Changes in the microstructure of alpha alumina with increasing amounts of calcium concentrations in the temperature range between 1400degreesC to 1600degreesC were studied by using various electron microscopy techniques. Variations in the average grain sizes were related to hypothetical excess calcium amount at the grain boundaries (Gamma(Ca)) assuming negligibly small solubility of calcium in bulk alpha-alumina. analytical and high-resolution transmission electron microscopy studies were done on these Ca-doped alumina ceramics. Under all sintering conditions, the grains were uniform in size and equi-axed 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. For the samples that were sintered at 1500degreesC and 1600degreesC, slab like abnormally grown grains appeared for critical calcium excess concentrations of Gamma(Ca) = 4.5-8 Ca-atoms/nm(2). No amorphous triple point pocket and grain boundary phases and/or Films were observed by high resolution electron microscopy. SEM/EDS chemical analysis showed precipitation of calcium hexa-aluminate precipitates that are as large as the matrix alumina grains after certain calcium excess level.