Effect of different calcium doping levels on the microstructure of high‐purity α‐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 (ΓCa) that was calculated assuming zero solubility of calcium in bulk α‐alumina. Under all sintering conditions, grains were uniform in size and equiaxed for low calcium concentrations (<3 Ca atoms/nm2). The grain morphology became elongated when the calcium concentration at the grain boundaries reached calcium excess of ΓCa= 3–3.5 Ca atoms/nm2 in all samples. The average grain sizes of undoped samples were ∼10% 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 1500° and 1600°C, slablike abnormally grown grains appeared for critical calcium excess concentrations of ΓCa= 4.5–8 Ca atoms/nm2. 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 1500° and 1600°C, when the samples sintered at 1400°C, although the hypothetical calcium coverage exceeded ΓCa= 11 Ca atoms/nm2, 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.

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Calcium is always present in alumina systems as an unintentional (or intentional) dopant, and yet the fundamental effect of its incorporation into the aluminas is not well understood, and is further complicated by the presence of Si. The synthesis of powders of two calcium aluminate phases (CaAl 4 O 7 , which is also known as CaO Á 2Al 2 O 3 or CA 2 , and CaAl 12 O 19 , which is also known as CaO Á 6Al 2 O 3 or CA 6 ) has been investigated using lowtemperature chemical-processing techniques. The crystallization of these powders from the amorphous precursor has been examined using various characterization techniques. The precursors for the powders were prepared by mixing stoichiometric proportions of the nitrate salts into a 5 wt% aqueous solution of poly(vinyl alcohol). Conversion of the amorphous precursors to crystalline powders and the subsequent phase transitions were monitored using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and powder X-ray diffractometry (XRD). While powders with CA 2 stoichiometry crystallized directly at 883 C, amorphous powders with CA 6 stoichiometry first crystallized into an intermediate structure without partitioning and then transformed into CA 6 at 1175 C. Fully and partially crystallized powders were analyzed using transmission electron microscopy and electron energy-loss spectroscopy (EELS). Measured near-edge structures (Al-L 2,3 , Ca-L 2,3 and O-K) are presented for the CA 2 , -Al 2 O 3 and CA 6 phases. The intermediate phase, identified as -Al 2 O 3 , was found to accommodate a significant concentration of Ca.

Changes in the microstructure of alpha alumina with increasing amounts of calcium concentrations in the temperature range between 1400°C to 1600°C 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 (Γ Ca ) assuming negligibly small solubility of calcium in bulk α-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 Γ Ca =3-3.5 Ca-atoms/nm 2 in all samples. For the samples that were sintered at 1500°C and 1600°C, slab like abnormally grown grains appeared for critical calcium excess concentrations of Γ 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.