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Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics III.
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2004 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 70, 165411- p.Article in journal (Refereed) Published
Abstract [en]

The structural, thermodynamic and optical properties of Mg2Ni thin films covered with Pd are investigatedupon exposure to hydrogen. Similar to bulk, thin films of metallic Mg2Ni take up 4 hydrogen per formula unitand semiconducting transparent Mg2NiH4−d is formed. The dielectric function e˜ of Mg2Ni and fully loadedMg2NiH4−d is determined from reflection and transmission measurements using a Drude-Lorentz parametrization.Besides the two “normal” optical states of a switchable mirror—metallic reflecting and semiconductingtransparent—Mg2NiHx exhibit a third “black” state at intermediate hydrogen concentrations with low reflectionand essentially zero transmission. This state originates from a subtle interplay of the optical properties of theconstituent materials and a self-organized double layering of the film during loading. Mg2NiH4−d preferentiallynucleates at the film/substrate interface and not—as intuitively expected—close to the catalytic Pd cappinglayer. Using e˜Mg2Ni and e˜Mg2NiH4and this loading sequence, the optical response at all hydrogen concentrationscan be described quantitatively. The uncommon hydrogen loading sequence is confirmed by x-ray diffractionand hydrogen profiling using the resonant nuclear reaction 1Hs15N,agd12C. Pressure-composition isothermssuggest that

Place, publisher, year, edition, pages
2004. Vol. 70, 165411- p.
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-92037DOI: 10.1103/PhysRevB.70.165411OAI: oai:DiVA.org:uu-92037DiVA: diva2:164979
Available from: 2004-09-01 Created: 2004-09-01 Last updated: 2013-06-19Bibliographically approved
In thesis
1. Synthesis and Characterisation of Potential Hydrogen Storage Materials
Open this publication in new window or tab >>Synthesis and Characterisation of Potential Hydrogen Storage Materials
2004 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The dissociative and non-dissociative hydrogen uptake in carbon nanostructures and metallic films were investigated by measurements and analysis of solubility isotherms. The total, non-dissociative, uptake for multi-walled nano-barrels and amorphous nanoporous carbon was determined to be 6.2 and 4.2 wt. % respectively at 77 K and the adsorption energies (at lowest coverage) -7.2 and -4.2 kJ/mol. At 298 K the H-uptake was negligible.

At low concentrations the H-uptake of Nb-films is strongly affected by the film thickness. For thicknesses less then about 31 nm, the absorption energy was found to be temperature dependent. Such changes have not been observed in Nb films before. The presence of multiple absorption energies was shown to limit the possibility to obtain relevant absorption and interaction energies by traditional Sievert's and van 't Hoff analysis.

The Mg1-xNix system (0<0.43) was investigated with respect to the hydrogen uptake. For Mg2Ni the hydrogen uptake, at an external hydrogen pressure of 1 bar, is close to 1.33 H/M (Mg2NiH4). The enthalpy of formation is smaller in the film as compared to bulk material. The changes in the absorption energy are caused by the adhesion to the substrate as well as the nanocrystallinity of the absorbing layers. The optical band gap of Mg2NiH4 was determined to be 2.4 eV.

In Mg1-xYx (0<0.17) it was found that the Y-concentration limits the hydrogen uptake at 1 bar. However, the kinetics of the uptake improves substantially with a minimum of 7 at.% of Y. For Mg-Y the optical band gap (3.6 eV) is independent of Y concentration within the concentration range investigated, while the transmittance decreases with increasing Y content.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2004. 74 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 1005
Physics, Hydrogen, adsorption, absorption, thermodynamics, thin films, carbon nanostructures, Fysik
National Category
Physical Sciences
urn:nbn:se:uu:diva-4509 (URN)91-554-6020-8 (ISBN)
Public defence
2004-09-23, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Polacksbacken, Uppsala, 10:15 (English)
Available from: 2004-09-01 Created: 2004-09-01 Last updated: 2009-04-30Bibliographically approved

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