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Thermodynamics of Hydrogen in Confined Lattice
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Three of the most important questions concerning hydrogen storage in metals are how much hydrogen can be absorbed, how fast it can be absorbed (or released) and finally how strongly the hydrogen is bonded. In transition metals hydrogen occupies interstitial sites and the absorption as well as desorption of hydrogen can be fast. The enthalpy of the hydride formation is determined by the electronic structure of the absorbing material, which determines the amount of energy released in the hydrogen uptake and the energy needed to release the hydrogen.

This thesis concerns the possibility of tuning hydrogen uptake by changing the extension of the absorbing material and the boundary conditions of extremely thin layers. When working with extremely thin layers, it is possible to alter the strain state of the absorbing material, which is used to influence the site occupancy of hydrogen isotopes. Vanadium is chosen as a model system for these studies. V can be grown in the form of thin films as well as superlattices using MgO as a substrate. Special emphasis are on Fe/V(001) and Cr/V(001) superlattices as these can be grown as high quality single crystals on a routine basis. The use of high quality samples ensured well-defined conditions for all the measurements.

In these experiments the hydrogen concentration is determined by the light transmittance of the thin films.  By changing the temperature and the pressure of the hydrogen gas, it is possible to determine the thermodynamic properties of hydrogen in the samples, from the obtained concentrations.  Measurements of the electrical resistivity is used to increase the accuracy in the measurements at low concentrations as well as to provide information on ordering at intermediate and high hydrogen concentrations. The thermodynamic properties and the electrical resistivity of VH are strongly affected by the choice of boundary layers. For example, when hydrogen is absorbed in V embedded by Fe, Cr or Mo in the form of superlattices, both the thermodynamic properties and the changes in the resistivity are strongly influenced.

The critical temperature and H-H interactions of hydrogen in thin V(001) layers are found to increase with thickness of the thin films and superlattices. The observed finite size effects resemble same scaling with the thickness of the layers as does the magnetic ordering temperature. The results were validated by investigations of isotope effects in the obtained thermodynamic properties. Close to negligible effects are obtained when replacing hydrogen by deuterium, with respect to the thermodynamic properties. These observations are rationalised by an octahedral occupancy in the strained layers, as compared to tetrahedral occupancy in unstrained bulk. The octahedral site occupancy is found to strongly alter the diffusion coefficient of hydrogen in thin V layers.

 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 59 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1344
Keyword [en]
thermodynamics hydrogen superlattice
National Category
Condensed Matter Physics Nano Technology
Identifiers
URN: urn:nbn:se:uu:diva-275629ISBN: 978-91-554-9473-5 (print)OAI: oai:DiVA.org:uu-275629DiVA: diva2:900624
Public defence
2016-03-17, Häggsalen, Ångströmlaboratoriet, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2016-02-22 Created: 2016-02-04 Last updated: 2016-03-09
List of papers
1. Finite-Size Effects: Hydrogen in Fe/V(001) Superlattices
Open this publication in new window or tab >>Finite-Size Effects: Hydrogen in Fe/V(001) Superlattices
2014 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, no 4, 046103- p.Article in journal (Refereed) Published
Abstract [en]

We investigate the effect of finite size on phase boundaries of hydride formation in ultrathin metallic films, using Fe/V(001) superlattices as a model system. The critical temperature is determined to scale linearly with the inverse thickness of the V layers. The decrease of the ordering temperature with decreasing layer thickness arises from the missing H neighbors at the interfaces, analogous to observed finite-size effects in magnetic layers and nanosized ice crystals.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-231118 (URN)10.1103/PhysRevLett.113.046103 (DOI)000339620300012 ()
Available from: 2014-09-05 Created: 2014-09-04 Last updated: 2017-12-05Bibliographically approved
2. Hydrogen in V: site occupancy and isotope effects
Open this publication in new window or tab >>Hydrogen in V: site occupancy and isotope effects
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-275054 (URN)
Available from: 2016-01-28 Created: 2016-01-28 Last updated: 2016-03-09
3. Influence of boundary on hydrogen absorption in V
Open this publication in new window or tab >>Influence of boundary on hydrogen absorption in V
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-275055 (URN)
Available from: 2016-01-28 Created: 2016-01-28 Last updated: 2016-03-09
4. Proximity effects on the H absorption in ultra-thin V layers
Open this publication in new window or tab >>Proximity effects on the H absorption in ultra-thin V layers
Show others...
2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 4, 045420- p.Article in journal (Refereed) Published
Abstract [en]

We discuss proximity effects on the hydrogen uptake in 14 atomic layers of vanadium. The enthalpy and entropy of solution were measured and compared to ab initio density functional calculations. We show that there exists a large difference in the hydrogen uptake of V in Cr/V and Fe/V superlattices, in which the V is under close to identical strain states. The calculations show that neither local strain effects nor charge redistributions can be the cause for the observed effect. This leaves magnetic and long ranged elastic strain fields-neither captured by the current calculations-as possible mechanisms for the observed effects.

National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-140284 (URN)10.1103/PhysRevB.90.045420 (DOI)000339482200006 ()
Available from: 2011-01-04 Created: 2011-01-04 Last updated: 2016-03-09Bibliographically approved
5. 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
Show others...
2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, 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: 2017-11-30Bibliographically approved

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