Microstructural evolution of calcium doped alpha-Al2O3

Official URL: http://risc01.sabanciuniv.edu/record=b1063520 (Table of Contents)

Effects of different calcium doping levels on the microstructure of high purity [alpha]-alumina was studied as a function of sintering time and temperature using scanning electron microscope (SEM). Samples were prepared from high purity AKP-500, Sumitomo á-alumina powder that contained maximum 13 ppm total cation impurity initially. Extra pure calcium nitrate tetrahydrate (GR for analysis) were used as the calcium source. Alumina powders with calcium concentrations varying from 0 to 1000 ppm (molar ratio of Ca/Al2O3) were dispersed in 2-propanol (analytical reagent) and ball milled for 12 hours with 99.7% pure alumina balls. After drying, powders were pressed first unidirectionally into discs under 28 MPa and then cold isostatically pressed at 250 MPa. Bulk chemical analysis of doped powders were done by ICP-OES. According to ICP results the doped powders contained less than 5 ppm silicon impurity. Sintering of samples were carried out at 1400, 1500 and 16000C for 1 and 12 hours. Microstructural evolution under these conditions were related to calcium excess at the grain boundaries (ÃCa). ÃCa was calculated using a simplified McLean-Langmuir adsorption model. As expected with increasing sintering time and temperature the average grain size increased. Under all sintering conditions, the grains were uniform in size and equiaxed for low calcium concentrations. The grain morphology became elongated when the calcium concentration at the grain boundaries reached calcium excess of ÃCa=3-3.5 calcium atoms/nm2 in all samples. For the samples that were sintered at 15000C and 16000C, slab like abnormally grown grains appeared between a critical calcium excess concentration of ÃCa=4.5-8 calcium 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 15000C and 16000C, when the samples sintered at 14000C, although the calcium coverage exceeded ÃCa=11 calcium atoms/nm2, only few grains grew abnormally without affecting the average grain size. Observations clearly indicated that calcium atoms cause elongated (slab like) grain morphology when their excess concentrations reach a critical level at the grain boundaries. This is most likely due to the preferential segregation of calcium ions to basal plane in á-alumina as previously shown in literature on alumina with calcium and silicon impurities. In this study, it is indisputably shown that calcium is responsible for the elongated grain morphology observed in polycrystalline alumina. Results obtained in this investigation supported the argument that calcium has an influence on abnormal grain growth (AGG) in [alpha]-Al2O3. However, it appears that at least one other impurity may be necessary, most likely silicon, to trigger AGG.