The effect of microstructure and impurities on transport in actinide oxide films

Abstract

Materials are never perfect. In a perfect world all materials would be single stoichiometric crystals but in reality they are not. In reality materials contain non stoichiometric defects, such as Frenkel and Schottky defects, chemical defects, such as foreign anions and cations substituted at lattice sites or free in the lattice and structural defects, such as surfaces, grain boundaries and dislocations. Furthermore, these different types of defects can combine, giving rise to further complexity. Predictions based upon stoichiometric materials are valid but without taking into account the atomistic effect of defects they neglect the reality of the state of a material. The main aim of this work is to utilise computational techniques to gain an understanding on how defects and combinations of defects influence and define the chemistry of UO2 and CeO2, and to ascertain methods for improving the function these materials. This thesis investigates the structure and dynamics of three structural defects (Surfaces, Grain boundaries and Surface-Grain boundary junctions) and how they are modified when the material is doped with foreign cations.

This thesis begins with an investigation of the surfaces of CeO2 and M3+ doped CeO2 surfaces in the presence of water and carbon dioxide adsorbants. The results demonstrate that M3+ dopants have a significant effect on the adsorption properties of water and carbon dioxide and alter the particle morphology of CeO2 under certain conditions of temperature and pressure.

Analysis of UO2 and CeO2 grain boundaries has also been conducted and when doped with M3+ grain boundaries have significantly altered the structure, segregation and transport properties

Finally, grain boundary-surface junctions have been investigated and M3+ dopant segregation explored. Dopants are predicted to segregate preferentially to the surface grain boundary junctions followed by the surface and grain boundary.

A combination of DFT and potential based atomistic computational modelling has been used. Adsorption of water and carbon dioxide at the surface has been studied with density functional theory energy minimisation calculations, while potential based molecular dynamics simulations were used to evaluate transport properties in grain boundaries and defect segregation has been studied with potential based Monte Carlo simulations.

The results presented in this thesis provide insight into the effect of these different types of defects and allows experimental and further computational work to be carried out with a greater understanding of the effect of defects.

Date of Award	13 May 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Michael Hill (Supervisor) & Steve Parker (Supervisor)