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Title: Aspects of the surface science of lithium hydride and uranium
Author: Tonks, James P.
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2017
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This thesis contains work on three distinct but related themes, the commissioning of an integrated surface science instrument, the ageing of LiH and the surface reactivity of uranium with H2O, H2 and O2. The integrated system contains a variety of surface science techniques which have been used extensively in the investigations of LiH and uranium presented here. These techniques are: scanning electron microscopy (SEM), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), secondary ion mass spectrometry (SIMS), temperature programmed decomposition (TPD), low energy electron diffraction (LEED) and molecular beam scattering (MBS). LiH ageing has been split into two regimes, wet and dry ageing. Wet ageing explicitly requires the presence of an external source of H2O, while dry ageing does not. This work has primarily looked at dry ageing, in which reactions are thermally driven, on both bulk and thin film LiH. Thin films of lithium were deposited in-situ on Ni(100) and were converted to LiH through exposure to H2 at 1 mbar. Four reactions were found to occur in the dry ageing process. The rate-limiting mechanisms were found to be different in thin films compared to bulk. The rate-limiting mechanism for the decomposition of LiOH was found to be the rate of nucleation in bulk samples, whereas it was the rate of growth of nuclei in thin films. For the solid state reaction (LiH + LiOH → Li2O + H2), the same mechanism was found to be rate limiting in both cases (i.e. 3D diffusion). However, the activation energy was determined to be significantly higher in a thin film, thought to be a result of a decrease in defect concentration and the number of grain boundaries in the thin film. The Li2O layer formed through the solid state reaction exhibited a high defect concentration and poor crystallinity, attributed to the large lattice mismatch between itself and LiH. This was shown to impact on its subsequent reactivity. On exposure of a LiH thin film to H2O, a chemisorbed H2O state was observed. High purity polycrystalline uranium metal sample was prepared in-situ by ambient temperature Ar+ sputtering. Annealing of the clean uranium sample caused segregation of UO2-x and UOxCy to the surface, similar to that observed in UO2 samples. H2 and H2O have been shown to dissociatively adsorb onto a uranium metal surface, with the latter causing partial surface oxidation. O2 exposure caused irreversible surface oxidation of uranium metal, which could be described by a precursor state model whereby adsorption directly onto the surface is more probable than adsorption mediated by a physisorbed second layer. A high purity UO2 surface was formed by heating the uranium metal in an O2 atmosphere. The interaction of D2O with UO2 at sub-ambient temperatures exhibited the formation of transient ice multilayers. The dynamics of D2 on the oxide surface suggested the occurrence of trap-desorption, rather than rotationally inelastic scattering.
Supervisor: Watts, John ; Baker, Mark ; Galloway, Ewan Sponsor: Engineering and Physical Sciences Research Council ; AWE plc
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available