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Mg/Ti multilayers: Structural and hydrogen absorption properties
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
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2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 22, 224203- p.Article in journal (Refereed) Published
Abstract [en]

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a "spinodal-like" microstructure with a small degree of chemical short-range order in the atomic distribution. In the present study we artificially engineer short-range order by depositing Pd-capped Mg/Ti multilayers with different periodicities. Notwithstanding the large lattice mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. On exposure to H-2 gas a two-step hydrogenation process occurs with the Ti layers forming the hydride before Mg. From in situ measurements of the bilayer thickness Lambda at different hydrogen pressures, we observe large out-of-plane expansions of Mg and Ti layers on hydrogenation, indicating strong plastic deformations in the films and a consequent shortening of the coherence length. On unloading at room temperature in air, hydrogen atoms remain trapped in the Ti layers due to kinetic constraints. Such loading/unloading sequence can be explained in terms of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by diffusion calculations on a model multilayered systems. Absorption isotherms measured by hydrogenography can be interpreted as a result of the elastic clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces.

Place, publisher, year, edition, pages
2010. Vol. 81, no 22, 224203- p.
National Category
Physical Sciences
URN: urn:nbn:se:uu:diva-135537DOI: 10.1103/PhysRevB.81.224203ISI: 000278492600002OAI: oai:DiVA.org:uu-135537DiVA: diva2:377779
Available from: 2010-12-14 Created: 2010-12-07 Last updated: 2012-01-27Bibliographically approved
In thesis
1. Influence of Self-trapping, Clamping and Confinement on Hydrogen Absorption
Open this publication in new window or tab >>Influence of Self-trapping, Clamping and Confinement on Hydrogen Absorption
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The dissociation of hydrogen molecules at surfaces is the first step in the absorption process. If the absorbing material is covered by an oxide, this layer will determine the effective uptake rate of an underlying absorbing material. This effect is illustrated when determining the rate of transport of hydrogen through amorphous aluminium oxide layers. The transport rate was determined to be strongly thickness dependent.

Hydrogen absorbed in a transition metal causes a volume expansion generated by a strain field around the absorbed hydrogen. This strain field causes a self-trapping of the hydrogen and a temperature dependent distribution in the atomic distances. The local strain field generated by the self-trapping process is found to be crucial for understanding both the hydrogen induced volume expansion as well as the diffusion of hydrogen. Ab-initio molecular dynamics simulations were used to reveal the temperature dependence of the unbinding of the hydrogen and the local strain field and its influence on the diffusion rate. The symmetry of the local strain field is also important for phase formation in metallic films and superlattices which are clamped to a substrate. As the thicknesses reduced from 50 to 10 nm thick vanadium films, substantial finite size effects become apparent in the phase diagrams. The volume change associated with the strain field cannot be accurately measured using x-ray diffraction because of its sensitivity to local arrangements of atoms. X-ray and neutron reflectivity were found to be more reliable probes of global effects of the sumof the local strainfields.

Finite size effects in extremely thin V layers were also explored in metallic superlattices composed of iron and vanadium. The co-existence region, composed of a hydrogen gas and a solid-like phase, was found to be suppressed by at least 100 K to below 300 K. The hydrogen-hydrogen interaction can also be influenced by the electronic states in the non hydrogen absorbing layers, as demonstrated when comparing hydrogen absorption in Fe/V and Cr/V superlattices.


Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 59 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 798
hydrogen diffusion, thin films, superlattices, phase transitions, confinement, clamping, self-trapping, finite-size effect
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
urn:nbn:se:uu:diva-140329 (URN)978-91-554-7983-1 (ISBN)
Public defence
2011-02-18, Häggsalen, Ångström laboratory, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 728Available from: 2011-01-31 Created: 2011-01-04 Last updated: 2011-03-21Bibliographically approved

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Pálsson, Gunnar K.Hjörvarsson, Björgvin
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