Preparation and characterisation of double doped-CeO₂ systems
Torrens, R. (2002). Preparation and characterisation of double doped-CeO₂ systems (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14101
Permanent Research Commons link: https://hdl.handle.net/10289/14101
Potential Solid Oxide Fuel Cell (SOFC) electrolyte materials of the composition Ce₀.₈Gd₀.₂₋ₓMₓO₂₋δ (M = Pr or Sm, x = 0.005 - 0.05) were prepared by two wet-chemical methods: oxalic acid co-precipitation and a variant of the Pechini method. X-ray diffraction of the oxide powders indicates that they are single-phase cubic fluorite materials. High temperature x-ray diffraction studies show that there is no evolution of secondary phases up to the maximum temperature of testing, 1273 K. Thermal expansion coefficients, calculated from lattice parameters measured at different temperatures, reveal the praseodymia containing samples to have a higher coefficient of thermal expansion, 1.3 x 10⁻⁵ K⁻¹ as opposed to 1.0 x 10⁻⁵ K⁻¹ for Ce₀.₈Gd₀.₂O₂δ. There was little variation in particle size (measured by laser diffraction) or surface area (measured by gas adsorption) between the powder prepared by the different preparation techniques and no systematic variation with double dopant concentration. Scanning electron microscopy (SEM) of the powders showed the preparation techniques produced particles with different morphologies; co-precipitated particles were elongated crystals composed of primary crystallites, while Pechini method particles appeared to be porous agglomerates of primary crystallites. Powders produced by the Pechini method proved to be more difficult to fabricate into pellets, with problems arising during pressing and sintering, resulting in a high rejection rate. X-ray microscopy of pressed and sintered pellets indicates that the use of a binder/lubricant reduces intra-pellet density variations. Sintering studies indicate that, while the Pechini method powders had rapid initial densification, the co-precipitated powders were more able to obtain closed porosity (96% of the theoretical density) at lower temperatures. Praseodymia containing samples exhibited enhanced sintering compared to the other samples, achieving 98-99% of theoretical density after sintering for 10 hours at 1400 ℃. SEM investigation of sintered pellets indicates that both of the double dopant species acted to retard grain growth. Impedance spectroscopy results show the praseodymia double doped materials to have higher conductivities than the base Ce₀.₈Gd₀.₂O₂δ material. The praseodymia containing composition showing the highest conductivity was Ce₀.₈Gd₀.₁₈₅Pr ₀.₀₁₅O₂δ with a conductivity of 1.3 x 10⁻² S cm⁻¹ 1 (at 600 ℃, in air). Samaria double doping also increased conductivity, though not to the same extent as praseodymia double doping. The highest conductivity obtained for a samaria containing sample was 1.0 x 10⁻² S cm⁻¹ (at 600 ℃, in air) for the composition Ce₀.₈Gd₀.₁₉Sm₀.₀₁O₂δ. The samples exhibiting the highest conductivities also had the lowest activation energies for conduction. Impedance spectroscopy results at low oxygen partial pressures indicate no difference in ionic domain boundaries. The thesis concludes with a list of future work needed to carry on these studies.
The University of Waikato
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