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The effect of nano-confinement on hydrogen uptake in metallic superlattices
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
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: urn:nbn:se:uu:diva-350591ISBN: 978-91-513-0360-4 (print)OAI: oai:DiVA.org:uu-350591DiVA, id: diva2:1205467
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: 2018-09-27
List of papers
1. Crystal perfection by strain engineering: The case of Fe/V (001)
Open this publication in new window or tab >>Crystal perfection by strain engineering: The case of Fe/V (001)
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2017 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 636, p. 608-614Article in journal (Refereed) Published
Abstract [en]

We study the effect of bilayer thickness at fixed volume fraction on the structural quality of Fe/V (001)superlattices. We find that such artificial metallic superlattices can be manufactured with excellent crystalquality and layering up to at least 50 Å in repeat distance (K = LFe +LV). For an intended fixed ratio of theconstituents: LFe/LV= 1/7, out-of-plane coherence lengths comparable to the thicknesses of the sampleswere obtained. We evaluate the strain in- and out-of-plane of both layers as a function of the bilayer thicknessand comment on the growth using the framework of linear elasticity theory. We interpret the stabilityof the superlattice against crystal degradation due to the alternating compressive and tensile strain, yieldingclose to ideal lattice matching to the substrate.

Keywords
Superlattice; Iron/Vanadium; Sputtering; Epitaxy; Reciprocal space mapping; X-ray diffraction; X-ray reflectivity; Linear elasticity
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:uu:diva-332050 (URN)10.1016/j.tsf.2017.07.005 (DOI)000408037800086 ()
Available from: 2017-10-23 Created: 2017-10-23 Last updated: 2018-05-14Bibliographically approved
2. Beating effects in multilayer systems studied with neutron reflectometry
Open this publication in new window or tab >>Beating effects in multilayer systems studied with neutron reflectometry
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We demonstrate the limitations of a partial, phenomenological tting approach comparedto full simulations, including reection and refraction at all interfaces on theexample of hydrogen loading in ultra thin vanadium layers. Fe/V superlattices areloaded with deuterium and the lattice expansion and deuterium concentration isextracted from neutron reectivity data. We nd a noticeable dierence betweenthe direct extraction of concentrations and bilayer thicknesses from the superlatticepeaks and ts of the full density prole using the Parratt formalism. Our results underlinethe importance of carefully considering the limitations of phenomenologicalapproaches, in order to obtain robust results.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-350582 (URN)
Available from: 2018-05-14 Created: 2018-05-14 Last updated: 2018-09-14Bibliographically approved
3. Using interfaces to influence thermodynamic properties of metal hydrides
Open this publication in new window or tab >>Using interfaces to influence thermodynamic properties of metal hydrides
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We report profound proximity effects on the enthalpy of solution and critical temperature ofnano-sized vanadium hydrides. We use single crystalline Cr/V and Fe/V (001) superlattices forthese studies, in which the V layers are under close to identical strain. The thermodynamic propertiesare determined using isothermal pressure and optical transmission measurements, utilisingthe fully reversibel hydrogen uptake and release.The underlying mechanism is argued to be rootedin a radically different volume expansion in the two systems, arising from different site occupancyof hydrogen in Fe/V(001) and Cr/V(001). The optical transmission is found to be linear withconcentration in both types of superlattices, while exhibiting different slopes. The differences areattributed to changes in electron density arising from the volume expansion, rather than a directinfluence from a redistribution of electronic states.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-350589 (URN)
Available from: 2018-05-14 Created: 2018-05-14 Last updated: 2018-09-14Bibliographically approved
4. The influence of site occupancy on diffusion of hydrogen in vanadium
Open this publication in new window or tab >>The influence of site occupancy on diffusion of hydrogen in vanadium
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, article id 064310Article, review/survey (Refereed) Published
Abstract [en]

We investigate the effect of site occupancy on the chemical diffusion of hydrogen in strained vanadium. The diffusion rate is found to decrease substantially, when hydrogen is occupying octahedral sites as compared to tetrahedral sites. Profound isotope effects are observed when comparing the diffusion rate of H and D. The changes in the diffusion rate are found to be strongly influenced by the changes in the potential energy landscape, as deduced from first-principles molecular dynamics calculations.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-275058 (URN)10.1103/PhysRevB.95.064310 (DOI)000395988800002 ()
Note

The manuscript version of this article is part of two PhD theses: http://uu.diva-portal.org/smash/record.jsf?pid=diva2:900624

http://uu.diva-portal.org/smash/record.jsf?pid=diva2:950756

Available from: 2016-01-28 Created: 2016-01-28 Last updated: 2018-05-14Bibliographically approved
5. Finite size effects: deuterium diffusion in nm thick vanadium layers
Open this publication in new window or tab >>Finite size effects: deuterium diffusion in nm thick vanadium layers
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2017 (English)In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 19, article id 123004Article in journal (Refereed) Published
Abstract [en]

We investigate the effect of finite size on the chemical diffusion of deuterium in extremely thin V(001) layers. A five fold increase in the diffusion coefficient is observed at concentrations around 0.2 [D/V], when the thickness of the V is decreased from 28 to 14 atomic layers (approximate to 2.1-4.2 nm). The size dependent deuterium-deuterium interaction energy is argued to be the root of the observed changes as the diffusion rates are similar at low concentrations. The results demonstrate the feasibility of using finite-size effects to enhance the chemical diffusion of light interstitials in solids. We discuss the general applicability of these effects to other systems.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2017
Keywords
superlattices, optical transmission, deuterium diffusion, octahedral sites
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-347665 (URN)10.1088/1367-2630/aa94df (DOI)000424885300004 ()
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2018-04-05 Created: 2018-04-05 Last updated: 2018-05-14Bibliographically approved
6. Concentration dependence of hydrogen diffusion in clamped vanadium (001) films
Open this publication in new window or tab >>Concentration dependence of hydrogen diffusion in clamped vanadium (001) films
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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.

Keywords
diffusion, hydrogen, single crystal
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-312024 (URN)10.1088/1361-648X/29/4/045402 (DOI)000389233200001 ()
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2017-01-05 Created: 2017-01-04 Last updated: 2018-09-14Bibliographically approved
7. Experimental observation of hysteresis in a coherent metal-hydride phase transition
Open this publication in new window or tab >>Experimental observation of hysteresis in a coherent metal-hydride phase transition
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2017 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 49, article id 495701Article in journal (Refereed) Published
Abstract [en]

We investigate the hysteresis obtained in the hydrogen absorption and desorption cycle for a single crystal Pd/V-28 [Fe-4/V-28](11) superlattice. Below the critical temperature, a small but clear hysteresis is observed in the pressure-composition isotherms, while it is absent above. The experimental results thereby prove the relevance of macroscopic energy barriers for obtaining hysteresis in coherent structural transformations. The textured Pd layer exhibits substantially larger hysteresis effects, which can be related to an irreversible energy loss caused by defect generation in Pd.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2017
Keywords
hysteresis, coherent, incoherent, phase transition
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-342651 (URN)10.1088/1361-648X/aa9696 (DOI)000415854900001 ()
Funder
Swedish Energy AgencySwedish Research Council
Available from: 2018-02-28 Created: 2018-02-28 Last updated: 2018-05-14Bibliographically approved

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Droulias, Sotirios A.

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