Structure-Function Relationships in Metal Hydrides: Origin of Pressure Hysteresis

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Gray, Evan

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Dobson, John

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2017
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Abstract

The hydrogen storage properties of a metal are highly dependent on its structure, including the crystal structures of the metal and the hydride, the defect structures of the metal and the relationship between hydrogen cycling and these properties. The aim of this project was to establish a greater understanding of the link between hysteresis and the structure of the metal, investigated by conducting in-situ diraction experiments on both palladium and LaNi5 and their hydrides. Dislocations have a signicant impact on the pressure hysteresis in metal hydrides, but the exact link between them is poorly understood. A first experiment was performed aiming to increase the understanding of pressure hysteresis by investigating the annealing characteristics of dislocations in palladium hydride. Dislocations are created in the first traversal of the two-phase region in hydrogen cycled palladium but it is not clear what happens to the dislocations subsequent to the first cycle. An experiment was carried out to measure the density of dislocations while annealing in the phase, annealing under vacuum, and hydrogen cycling at increasing and decreasing temperatures. The dislocation density was measured using high resolution in-situ neutron diraction. It was found that annealing under vacuum and annealing in the phase produced the same result, but the dislocation density decreased much faster with temperature when the sample was hydrogen-cycled. Therefore the phase transformation signicantly aided in the removal of dislocations from the sample. It is suggested that the dislocations are gliding at the front of the advancing phase boundary, resulting in the removal of dislocations once the absorption/desorption is complete and the dislocations have reached the edge of a grain. This means that dislocations can be created and removed simultaneously during hydrogen cycling, resulting in a stable dislocation density. Dislocations gliding at the front of the phase boundary can also accommodate the strain of the transformation, explaining the reduced hysteresis on subsequent cycles compared to the first cycle.

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Thesis (PhD Doctorate)

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Doctor of Philosophy (PhD)

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School of Natural Sciences

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The author owns the copyright in this thesis, unless stated otherwise.

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Subject

Hydrogen storage properties

Metal hydrides

Pressure hysteresis

Palladium hydride

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