| Research Abstract |
In the process of forming cerium oxide solid solution by heating aqueous solution, it was found that particles formed by microwave heating at particular condition were different from those formed by conventional heating. It was also found that when these particles were calcined at 900℃, the density of sintered samples was high. After sintering and heat treatment, cerium oxide solid solutions were examined by x-ray diffraction and densities of them were measured. For CeO_2 doped with Nd, Gd, and Y, solid solutions were in fluorite structure in the concentration of about 30 % or less and they were in rare earth C-type structure in the higher concentration. As these dopants were added into CeO_2, the change in lattice parameter depended on the dopant species : the lattice parameter increased continuously (CeO_2 doped with Nd), decreased continuously (CeO_2 doped with Y), or first increased and then decreased (CeO_2 doped with Gd). The predominant defect in these solid solutions was anion vacancy for the region of fluorite structure. Although in the rare earth C-type structure region the predominant defect was anion vacancy for CeO_2 doped with Nd, it changed to cation vacancy based on YO_<1.5> for CeO_2 doped with Y. For CeO_2 doped with Gd, there was an indication that cation interstitial might be present at 10 % GdO_<1.5>. Further investigation is needed to confirm this. Solubility of NdO_<1.5> into CeO_2 was found to be large in the temperature range of 900-1400℃. At lower temperature (500℃) the existence of two-phase region was identified at 70 % NdO_<1.5>, suggesting a slight temperature dependence of solubility. The oxygen diffusion coefficient in CeO_2 doped with Nd at 400-550℃ was found to be very large and were on the line extrapolated from higher temperatures (750℃ or higher).
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