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Title: Crystallography of new materials for clean energy production and the switch to a hydrogen based economy
Author: Dunn, Iain
ISNI:       0000 0004 2735 6875
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2012
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New energy materials have been investigated, including hydrogen storage materials and dilute nitride semiconductors. The two potential hydrogen storage materials to have been investigated are di-sodium amide borohydride (Na2NH2BH4) and lithium sodium borohydride (LiNaBH4). Additionally, the homoepitaxial growth of InNSb and GaNSb across a range of growth temperatures and rates has been examined and the effect of annealing GaNSb layers grown on GaAs has also been studied. It has been shown that Na2NH2BH4 undergoes a first order phase transition between a low temperature orthorhombic phase and a high temperature cubic phase. There is a large coexistence region between the two phases of -10°C to 70°C. The relative percentages of each of these phases in this region are dependent on whether the sample is been heated or cooled and the rate of change of the temperature, leading to a discrepancy in the overall volume of the sample between heating and cooling. It has also been proved that there is a metastable cubic phase of the sample that is seen when this material is first formed and cooled. Phase pure samples of LiNaBH4 have been synthesised from mixtures of sodium borohydride and lithium borohydride, with varying amounts of lithium inclusion. This lithium inclusion has resulted in some disorder in the sodium borohydride structure of the samples up to temperatures of 200°C, which disorder is increased as the amount of lithium increases. The inclusion of lithium has reduced the hydrogen desorption temperature by c.a.10% from 550°C for pure sodium borohydride to 504°C for the sample with the most lithium inclusion. Both InNSb and GaNSb exhibit a linear relationship between growth temperature and amount of nitrogen inclusion, with both more nitrogen being included and a greater maximum growth temperature seen in the GaSb-based material. In both types of material higher growth rates have resulted in less nitrogen inclusion at a given temperature. It has been shown that the increased amount of nitrogen inclusion has improved the quality of the grown layer. Annealing of hetroepitaxially grown GaNSb has increased the amount of substitutional nitrogen in these layers by allowing interstitial nitrogen to diffuse on to the crystallographic B site of the material, at higher temperatures this effect has been reversed. Increased nitrogen incorporation has resulted in a reduction in the crystal quality of these layers, differing from the effects seen in the homoepitaxial layers.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry ; TP Chemical technology