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A nuclear magnetic resonance and quasielastic neutron scattering study of hydrogen diffusion in metal-hydrogen systems
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Barnfather, Karl Jon (1992) A nuclear magnetic resonance and quasielastic neutron scattering study of hydrogen diffusion in metal-hydrogen systems. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3252321~S15
Abstract
The result of work investigating hydrogen diffusion in a variety of Metal- Hydrogen systems using both the nmr and qns techniques is presented.
Chapter 1 provides a description of the uses and general character of MeH systems and is intended as a pragmatic introduction. In addition, diffusion theory is discussed and again general theory relevant to later work is presented regarding the properties of hydrogen diffusion within metal lattices. Chapter 2 gives an exposition on both the fundamental theory of the techniques used and detailed theoretical results which describe the effect of hydrogen diffusion on various observable parameters; primarily the nuclear spin-lattice relaxation time Ti and the neutron scattering function S(Q,ώ). Chapter 3 describes the experimental apparatus and procedures used for both the nmr and neutron scattering studies as well as for sample preparation.
Chapter 4 sets out the results of an nmr study of single crystal samples of β-NbHx, β-VHx and -YHx. Individual theoretical treatments of the expected anisotropy of the hydrogen Knight shift and spin-lattice relaxation rate due to the dipole-dipole interaction are presented for each of the systems. The T1 results obtained for a single domain sample of β-NbHx 75 are in excellent agreement with theory, however it is not possible to determine the exact nature of the diffusion mechanism except to predict that it is likely to be an isotropic mechanism rather than a one dimensional mechanism such as intra-chain. A Ti anisotropy is also observed in two single crystal samples of β-VHx. In the case of a single domain sample the orientational dependence of the results is in good agreement with theory and the anisotropy of T1 [001]//Bo I T1 [001]!Bo=17% is in excellent agreement with the maximum predicted 16%. The results tend to confirm the theoretical methods used and the diffusion mechanism suggested by earlier qns studies. For a single crystal of -YHx0.17, again excellent agreement with theory is observed in the form of the Ti angular dependence. An anisotropy of T1 [001]//Bo / Ti min=85% is observed which greater than predicted by three theoretical diffusion mechanisms. The magnitude of and errors in the results do not enable any determination of the true diffusion mechanism in the -YHx system.
Chapter 5 presents a study of the high temperature T1 anomaly which is an unexpected reduction in the relaxation time at temperatures where the Korringa (or conduction electron) mechanism is expected to dominate. Richards’ model of large anharmonic vibration of hydrogen at neighbouring interstitial sites which wras suggested as an explanation of the anomaly is described. The results of a qns study of Nb0.75V0.25H0.20 and YH1,97 powdered samples, which display the T1 phenomenon, are then presented. The study measures hydrogen diffusion coefficients for both samples in the temperature regime of the anomaly, which finds that hydrogen diffusion in both samples exhibit the usual Arrhenius temperature dependence. For the -phase alloy system Ea -0.16 ± 0.02 eV and D0 -1.76 (±0.4) x 10-4 cms-1, and for the sub-stoichiometric dihydride sample Ea =0.33 ± 0.09 eV and D0 -0.70 (±0.4) x 10-4 cms-1. The conclusion is that Richards’ model does not hold and other possible models are discussed including sub-lattice melting and hydrogen pairing.
| Item Type: | Thesis (PhD) | ||||
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| Subjects: | Q Science > QC Physics | ||||
| Library of Congress Subject Headings (LCSH): | Nuclear magnetic resonance, Quasielastic neutron scattering, Metals -- Hydrogen content, Hydrogen -- Diffusion rate | ||||
| Official Date: | December 1992 | ||||
| Dates: |
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| Institution: | University of Warwick | ||||
| Theses Department: | Department of Physics | ||||
| Thesis Type: | PhD | ||||
| Publication Status: | Unpublished | ||||
| Supervisor(s)/Advisor: | Styles, Graham A. | ||||
| Format of File: | |||||
| Extent: | v, 201 leaves : illustrations | ||||
| Language: | eng |
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