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Aspects of the surface science of lithium hydride and uranium.

Tonks, James P. (2017) Aspects of the surface science of lithium hydride and uranium. Doctoral thesis, University of Surrey.

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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.

Item Type: Thesis (Doctoral)
Divisions : Theses
Authors :
Tonks, James P.
Date : 30 November 2017
Funders : The Engineering and Physcical Sciences Research Council, AWE plc
Grant Title : Industrial Doctorate Centre: Micro- and NanoMaterials and Technologies
Copyright Disclaimer : © British Crown Owned Copyright 2017/AWE Published with permission of the Controller of Her Britannic Majesty's Stationary Office. This document is of United Kingdom origin and contains proprietary information which is the property of the Secretary of State for Defence. It is furnished in confidence and may not be copied, used or disclosed in whole or in part without prior written consent of Defence Intellectual Property Rights DGDCDIPR-PL - Ministry of Defence, Abbey Wood, Bristol, BS34 8JH, England.
Contributors :
ContributionNameEmailORCID, John, Mark, Ewan
Depositing User : James Tonks
Date Deposited : 15 Dec 2017 11:24
Last Modified : 15 Dec 2017 11:24

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