UBC Theses and Dissertations
Development, characterization and creep resistance of cubic zirconia doped with rare earth oxides processed through spark plasma sintering Robles Arellano, Karen Denisse
Development of new technological systems, such as fuel cells or nuclear power generation stations, often relies on the availability of materials whose properties accommodate the operational requirements of the system. Fabrication of creep-resistant ceramic materials is currently receiving worldwide interest due to their potential use in solid oxide fuel cells, thermal barrier coatings and insulators for hostile environments. In this research a novel fabrication process, Spark Plasma Sintering (SPS), was used to prepare cubic Yttria-stabilized Zirconia (YSZ) composites doped with three different rare earth oxides. Specifically, YSZ was doped with 6.4wt%, 12.7 wt% and 18.8 wt% CeO₂; 11.6 wt%, 21.6 wt% and 30.5wt% La₂O₃; and 12wt% and 22 wt% Nd₂O₃. Composites were successfully fabricated using the SPS technique, and the effect of selected parameters on density and grain size was studied. The novel composites were characterized using Scanning Electron Microscope –X-Ray Energy Dispersive Spectroscopy (SEM-XEDS), X-Ray Diffraction (XRD) and Optical Microscopy (OM). Also, compressive creep testing was performed and the active creep deformation mechanisms were studied. The results reveal that increasing the sintering temperature resulted in grain growth in all the composites. Addition of the rare earth oxides had a strong grain refinement effect on YSZ. Further, addition of La₂O₃ and Nd₂O₃ resulted in formation of a bimodal microstructure. In contrast, the effect of variation of dopant wt% on the grain size and relative density of the composites could not be established. Compressive creep tests in the temperature range of 1200-1330°C were carried out for YSZ, 6.4wt% CeO₂–YSZ and 11.6wt% La₂O₃ –YSZ composites. The stress exponent for all composites ranged between 1.2-2.7. The activation energy was ~730 kJ/mol for YSZ, ~490 kJ/mol for 6.4wt% CeO₂–YSZ and ~566 kJ/mol for 11.6wt% La₂O₃ –YSZ. These values of n and Q suggest that creep deformation was primarily accommodated by grain boundary sliding possibly assisted by lattice diffusion. These results were supported by microstructure evaluation, where evidence of grain boundary sliding was observed as crenellated cracks along boundaries, as well as grain boundary cavitation. There was a potential stress-induced phase transformation in the YSZ material, possibly related to the metastable t’ or monoclinic phases.
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