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Concentration dependence of hydrogen diffusion in clamped vanadium (001) films
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
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2017 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 4, article id 045402Article in journal (Refereed) Published
Abstract [en]

The chemical diffusion coefficient of hydrogen in a 50 nm thin film of vanadium (0 0 1) is measured as a function of concentration and temperature, well above the known phase boundaries. Arrhenius analysis of the tracer diffusion constants reveal large changes in the activation energy with concentration: from 0.10 at 0.05 in II V-1 to 0.5 eV at 0.2 in II V-1. The results are consistent with a change from tetrahedral to octahedral site occupancy, in that concentration range. The change in site occupancy is argued to be caused by the uniaxial expansion of the film originating from the combined hydrogen induced expansion and the clamping of the film to the substrate.

Place, publisher, year, edition, pages
2017. Vol. 29, no 4, article id 045402
Keyword [en]
diffusion, hydrogen, single crystal
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-312024DOI: 10.1088/1361-648X/29/4/045402ISI: 000389233200001OAI: oai:DiVA.org:uu-312024DiVA, id: diva2:1062396
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2017-01-05 Created: 2017-01-04 Last updated: 2018-05-14Bibliographically approved
In thesis
1. Hydrogen in nano-sized metals: Diffusion and hysteresis effects
Open this publication in new window or tab >>Hydrogen in nano-sized metals: Diffusion and hysteresis effects
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Metal hydrides can be used as hydrogen storage materials for fuel cells and batteries, and as sensors for detecting hydrogen gas. The use of metal hydrides for hydrogen storage can be hindered by poor kinetics and low capacity. Moreover, poor sensitivity, long recovery and response time, limit the applications of metal hydrides as hydrogen sensors. Diffusion is an important factor affecting the hydrogen kinetics and response time. Hysteresis effects accompany the phase transition of hydrogen in metals and can influence the properties of metal hydrides as well. These need to be considered in their applications as storage materials or sensors.

This thesis concerns the possibility of tuning hydrogen diffusion and studies the mechanism of hysteresis effects of hydrogen absorption in metals. In these experiments, nano-sized vanadium is used as the model system for these studies. Hydrogen concentration is determined by the light transmission. By measuring the concentration profiles and isotherms of hydrogen, it is possible to determine the diffusion coefficients and hysteresis effects.

A profound decrease of hydrogen diffusion in Fe/V(001) superlattice has been found, as compared to that in bulk vanadium. This result is interpreted as lower zero-point energy in octahedral site than that in tetrahedral site. Profound isotope effect on diffusion has also been found. Influence of clamping of the substrate on the diffusion of hydrogen with concentration in vanadium thin film is discovered. The diffusion coefficient below c = 0.1 [H/V] is close to that in bulk vanadium and decreases substantially when c > 0.1 [H/V] compared with that in bulk vanadium. This finding is interpreted as the site change from tetrahedral to octahedral occupancy when the hydrogen concentration increases. Large finite size effect on deuterium chemical diffusion is observed, which is concluded to be caused by D-D interaction change that will influence the deuterium chemical diffusion at different thickness of vanadium layers. However, finite size has no effect on hydrogen transport at extremely low hydrogen concentrations in Fe/V (001) superlattices, this illustrates that the interface can not influence the mean free path of hydrogen in any way. This is completely different from electron transport condition in nano-sized metals. Hysteresis effect is observed below critical temperature in Fe/V(001) superlattices; this occurrence confirms the hypothesis that hysteresis effect is caused by coherency strain in coherent  transformation.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 61
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1523
Keyword
Hydrogen, diffusion, hysteresis, optical technique
National Category
Natural Sciences
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-320796 (URN)978-91-554-9928-0 (ISBN)
Public defence
2017-06-13, Ång/4001, Lägerhyddsvägen 1, Uppsala, 13:30 (English)
Opponent
Supervisors
Available from: 2017-05-22 Created: 2017-04-25 Last updated: 2017-06-08
2. The effect of nano-confinement on hydrogen uptake in metallic superlattices
Open this publication in new window or tab >>The effect of nano-confinement on hydrogen uptake in metallic superlattices
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The absorption of hydrogen is exothermic in vanadium whereas it is endothermic in iron and chromium. Investigations of the hydrogen uptake within Fe/V(001) and Cr/V(001) superlattices allow therefore a detailed exploration of finite size effects and the influence of boundaries on hydrogen absorption. Fe/V(001) and Cr/V(001) superlattices can be grown as single crystal structures with a small mosaic spread, as determined by X-ray reflectometry and diffraction. Furthermore when the thickness ratio of the constituents is kept constant the crystal quality can be retained in the range from a few up to 40 monolayer repeat distances (Λ). Neutron reflectometry was used to simultaneously determine the volume expansion and concentration of hydrogen in the vanadium layers. Large differences are found in the expansion of Fe/V(001) and Cr/V(001) superlattices, in good agreement with density functional theory (DFT) calculations. The findings are consistent with tetrahedral and octahedral site occupancy in Cr/V(001) and Fe/V(001) superlattices, respectively. Full fitting of the reflectivity pattern is required to obtainan accurate measure of expansion if the number of repeats is small. Under these conditions, the shift of the first order superlattice peak can be an inaccurate measure of the volume changes. By using a specially designed neutron scattering chamber, allowing simultaneous neutron and optical transmission measurements, it is found that the optical transmission scales linearly with hydrogen concentration. By comparing the experimental results to ab-initio DFT calculations, it is shown that optical transmission scales with electron density changes in the samples, explaining the linearity with concentration. This change is dominated by the hydrogen induced expansion of the lattices and depends therefore strongly on the site occupancy of the hydrogen. Finally, X-ray diffraction was used to address the local strain fields and the α to β phase transition, typically observed in bulk vanadium. Below 448 K the results are consistent with an α to β phase co-existence, separated along the surface normal of the samples.

Place, publisher, year, edition, pages
Uppsala: Uppsala University, 2018. p. 65
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1682
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-350591 (URN)978-91-513-0360-4 (ISBN)
Public defence
2018-06-14, Room 2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2018-05-22 Created: 2018-05-14 Last updated: 2018-05-22

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