Effect of synthetic method on the structure of pyrochlore- and zirconolite-type oxides

Abstract

Ceramic materials such as pyrochlore- and zirconolite-type oxides have been studied for several decades due to their applications in catalysis, ferromagnetism, luminescence, ionic conductivity, and radioactive waste sequestration. The studies presented in this thesis investigated the effects of solution-based synthesis methods (coprecipitation and sol-gel) on the long- and short-range structures of pyrochlore- and zirconolite-type oxides annealed at various temperatures and the structural stability of these materials upon irradiation. Powder X-ray diffraction (XRD) was used to confirm the formation of the desired phases at all temperatures studied (pyrochlore: 700 to 1400 °C; zirconolite: 700 to 1400 °C). Changes in the long-range structure of the materials were evaluated by Rietveld refinement of the diffraction patterns. Scanning electron microscopy (SEM) showed an increase in grain size with annealing temperature. X-ray absorption near edge spectroscopy (XANES) was used to probe the local structure of Ti and Zr within the materials. Examination of the Ti K- and Zr K-edge XANES spectra showed no significant change in the structure of the pyrochlore- and zirconolite-type materials annealed at temperatures as low as 800 and 900 °C, respectively. Pellets of pyrochlore- and zirconolite-type materials prepared by the three synthesis methods (ceramic, coprecipitation, and sol-gel) were implanted with high energy Au- ions to simulate radiation-induced structural damage. Ti K-edge glancing angle XANES showed no change in the radiation resistance of zirconolite prepared by any of the synthesis methods. The pyrochlore-type materials exhibited reduced radiation resistance when the synthesis method was changed from ceramic or coprecipitation to sol-gel. This effect could be due to an increase in the porosity of the material. Therefore, it has been shown that pyrochlore- and zirconolite-type materials can be synthesized at temperatures as low as 800 and 900 °C, respectively, by using a coprecipitation or sol-gel method, due to the atomic scale mixing of reactants which occurs in solution. However, use of these synthesis methods could decrease the ability of the materials to withstand radiation-induced structural damage. The reduction of the annealing temperatures required to form pyrochlore- and zirconolite-type materials results in more cost-efficient processing which is ideal for industrial scaling.