The Electrical Properties of Pure and Doped Nanocrystalline

Nanocrystalline cerium oxide is an attractive material for catalysis of environmentally hazardous gas-phase species such as sulfur dioxide and carbon monoxide. Oxygen vacancies in cerium oxide are believed to coordinate and aid in the transfer of oxygen ions between the reactive species. We have sought to characterize these defects in nanocrystalline cerium oxide as a function of the heat treatment and grain size of the material. By understanding the fundamental defect thermodynamics of the nanocrystalline material, we have determined the unique behavior of this material which appears to contribute to its role as a catalyst. This has been achieved through the formation of dense (>90% theoretical density) nanocrystalline pellets which were studied via impedance spectroscopy to separate the contributions of the grain boundary and bulk material. This work shows that nanocrystalline cerium oxide exhibits enhanced electrical conductivity and reduced behavior in regimes where the conventional material is not reduced, and we attribute this to the presence of low-energy sites for defect formation at the grain boundaries. It has also been found that annealing the material at moderate temperatures for short times without coarsening leads to an atomic relaxation process which eliminates these low energy sites for reduction. Doped nanocrystalline cerium oxide has also been studied to explore the interaction between the unique character of the nanocrystalline material and the effects of doping on the conductivity. Thesis Supervisor: Professor Yet-Ming Chiang Title: Kyocera Professor of Ceramics Table of