Surface-Bulk Electrochemical Coupling and off-Stoichiometry in Uranium Dioxide

• Other Authors: Hassan, Abdel-Rahman (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Materials science
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-4896

Irradiation alters the local stoichiometry of oxides significantly. The resulting stoichiometric changes play a critical role in the dynamics of defects and microstructure evolution in oxides under irradiation. Stoichiometry in oxides is also sensitive to the surrounding oxygen environment. Motivated to study the equilibrium state of UO2, this thesis investigates a theoretical approach to model spatial distribution of defects and charge carriers. In general, the levels of point defects and electronic charge carriers in an oxide are sensitive to the oxygen partial pressure in contact with the oxide and temperature. The objective of this research is twofolded. First, a detailed point defect model based on density functional theory results is devised. The model takes into account multiple charge states for each defect type as well as all the dependencies of the formation thereof. Second, a space-charge analysis is used to find the effect of surface and environment on the spatial variation of concentrations at equilibrium. A surface charge is known to form on the surface as a result of its interaction with the environment. This interaction is explained by the theory of ionosorption. The resulting effect is found by coupling bulk concentrations from the point defect model with the surface charge through solving the electrochemical sytem. The results found by the point defect model are shown to match the experimental data on UO2±x . As for the space-charge analysis, the concentrations of individual defects showed an order of magnitude variation in the subsurface region. This implies the importance of the space-charge effect in any kinetic study of the system since it is controlled by the defect composition at the interface. === A Thesis submitted to the Materials Science and Engineering Program in partial fulfillment of the requirements for the degree of Master of Science. === Fall Semester, 2011. === November 7, 2011. === Includes bibliographical references. === Anter El-Azab, Professor Directing Thesis; Petru Andrei, Committee Member; Milen Kostov, Committee Member.