Phase Study and Thermal Expansion of Yttria Stabilized Zirconia Doped with PuO2-x

Abstract

Phase relations between PuO2 and yttria stabilized zirconia (YSZ, Pu/(Pu+YSZ)=0-0.5) were investigated in air, vacuum and H2 atmospheres. All samples prepared in air and vacuum were single-phase solid solutions with the fluorite structure while those prepared in H2 were single-phase either the fluorite (Pu/(Pu+YSZ)<0.3) or the pyrochlore structure (Pu/(Pu+YSZ)>0.3). The observed lattice parameter change of the fluorite solid solution with the Pu content was well explained by an ionic model where oxygen vacancies occurred preferentially to Zr4+ and the coordination number (CN) of Zr4+ reduced to 7 while the CN of the other metal ions remained 8. Thermal expansion of YSZ doped with PuO2 was also studied by high-temperature X-ray diffraction method at temperatures between room temperature and 1273 K in air. Linear thermal expansion coefficient values of the solid solution (Pu/(Pu+YSZ) = 0-0.25) fell in the range 6-12 x 10-6 K-1. Though there was no apparent relationship between thermal expansion coefficients and Pu content, it was seen that thermal expansion coefficients became small with increasing Pu content above 1000 K. The constant-pressure heat capacities of the solid solutions were also calculated from the thermodynamical relation.