Center for Materials Science and Nanotechnology

Oslo, Norway

Center for Materials Science and Nanotechnology

Oslo, Norway
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Jensen I.J.T.,University of Oslo | Lovvik O.M.,University of Oslo | Lovvik O.M.,Sintef | Schreuders H.,Technical University of Delft | And 3 more authors.
Surface and Interface Analysis | Year: 2012

X-ray photoelectron spectroscopy (XPS) was employed to investigate Mg 80Ti 20 thin film samples prepared by magnetron sputtering and density functional theory (DFT) calculations were performed on atomistic models with similar stoichiometry. As Ti is known to be immiscible in Mg, the microstructure and atomic distribution of Mg-Ti thin films are not fully understood. In this work, it was shown by DFT calculations that the density of states (DOS) depends strongly on whether Ti is arranged in nano-clusters or if it is distributed quasi-randomly. The calculated DOS was compared to valence band spectra measured by XPS, as a new way of indirectly probing short-range order of such thin films. The XPS results of Mg 80Ti 20 were found to correspond best with the DOS calculated for the nano-cluster model, supporting the view that Ti forms small clusters in such sputtered thin films. Copyright © 2012 John Wiley & Sons, Ltd. Copyright © 2012 John Wiley & Sons, Ltd.

Jensen I.J.T.,University of Oslo | Jensen I.J.T.,Sintef | Thogersen A.,Sintef | Lovvik O.M.,Sintef | And 4 more authors.
International Journal of Hydrogen Energy | Year: 2013

Thin film samples of Mg80Ti20 (Mg-Ti) and Mg, both with and without H, were investigated in a series of X-ray photoelectron spectroscopy (XPS) measurements. The samples were covered with a thin protective layer of Pd, which was removed by Ar+ sputtering prior to data acquisition. This sputtering was found to reduce both oxides and hydrides. A distinct, previously unknown peak was revealed in the Mg KLL spectrum of the Mg-Ti-H samples, located between the metallic and the MgO component. This peak was attributed to trapping of H in very stable interstitial sites at the interface between Ti nano-clusters and the Mg matrix, based on earlier density functional theory calculations and supported by so-called Bader analysis. The latter was performed in order to study the theoretical charge distribution between Mg, Ti and H, establishing a link between the position of the previously unknown peak and the effect of H on the valence state of Mg. The composition of the samples was studied both by energy dispersive spectroscopy using transmission electron microscopy and by quantitative XPS analysis. Final state Auger parameters (AP) were obtained for metallic Mg, MgO and MgH2, as well as Mg affected by trapped H. No difference between the AP values from the metallic components was found between the Mg and the Mg-Ti samples. The AP values for MgO and MgH2 were consistent with previous reports in literature; several eV lower than the metallic value. Mg in the vicinity of trapped hydrogen, on the other hand, showed a more metallic character, with its corresponding AP value less than 1 eV below the AP for pure Mg. Copyright © 2013, Hydrogen Energy Publications, LLC.

Jensen I.J.T.,University of Oslo | Diplas S.,Sintef | Diplas S.,Center for Materials Science and Nanotechnology | Lovvik O.M.,University of Oslo | Lovvik O.M.,Sintef
Applied Physics Letters | Year: 2012

The hydrogenation of Mg 0.8125Ti 0.1875 was investigated by density functional calculations, using a model where Ti was segregated into nano-clusters. Introducing small amounts of hydrogen resulted in significant stabilization, with the mixing enthalpy (cohesive energy relative to standard state elements) becoming negative for hydrogen contents exceeding 0.07 H per metal. H prefers sites on the interface between Mg and Ti, with hydrogenation energies down to -115 kJ/(mol H 2). Trapping of H on these very stable sites is proposed as an alternative explanation to why the reversibility of Mg-Ti thin films, which are initially meta-stable, can be preserved over many cycles of hydrogenation. © 2012 American Institute of Physics.

Jensen I.J.T.,University of Oslo | Diplas S.,Sintef | Diplas S.,Center for Materials Science and Nanotechnology | Lovvik O.M.,University of Oslo | And 5 more authors.
Surface and Interface Analysis | Year: 2010

In this work the indepth compositional variations of MgH2 thin films grown directly by reactive sputtering in H2 gas were studied over time. XPS was performed using a Thermo Scientific Theta Probe with monochromatic Al Kα radiation (hυ = 1486.6eV). Measurements at different sample depths were obtained by Ar+ ion sputtering, and high-resolution Mg 2p and O 1s spectra were used to quantify the data. Indications of differential charging effects were observed in the O 1s spectra. At the unsputtered surface a layer of Mg(OH)2 was found to grow with air exposure time. Underneath this layer, both MgH2 and MgO were present in near equal amounts, and deeper in the film there was an increasing amount of metallic Mg. Apart from the growth of Mg(OH)2 the film appeared to be stable in air over the time period of the experiment. Copyright © 2010 John Wiley & Sons, Ltd.

Jensen I.J.T.,University of Oslo | Diplas S.,Sintef | Diplas S.,Center for Materials Science and Nanotechnology | Lovvik O.M.,University of Oslo | Lovvik O.M.,Sintef
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Mg-Ti-H thin films exhibit interesting optical and electrical properties, offering a wide range of possible applications from coatings on solar collectors and smart windows to optical hydrogen sensors and semiconductor devices. However, Ti is known to be immiscible in Mg, and the microstructure of Mg-Ti thin films is not fully understood. In this work density functional theory calculations were used to investigate the Mg100-y Tiy system with 1.56≤y≤98.44. The crystal structure, mixing enthalpy, and electronic structure were compared for two different distributions of Ti: quasirandom and segregated. It was found that although the crystal structures did not differ significantly, the formation enthalpy per Ti atom was lowered by up to ∼0.5 eV when Ti was arranged in nanoclusters. This gives support to previous experimental findings of chemical short-range order in Mg-Ti thin films. As a consequence of the decrease in the mixing enthalpy upon clustering the occurrence of short-range chemical order in all reported metastable Mg-Ti alloys with extended solubility is proposed. Further inquires into the influence of the size of the clusters revealed that the mixing enthalpy reaches a minimum after which further growth makes little difference, indicating that the phenomenon of nanoscale clustering must be understood separately from the larger scale phase separation occurring in an equilibrium process. The relaxed crystal structures were compared to experimental values from several sources and lattice-parameter variations as well as deviations from Vegard's law were discussed. Dependence of the size and shape of the nanoclusters on synthesis method is offered as an explanation for the large variation among the experimental lattice parameters. Local density of states calculations illustrated how segregated Ti forms a local environment resembling pure Ti while it was necessary to perform Bader analysis in order to obtain the correct picture of the charge transfer between Mg and Ti. Randomly distributed Ti atoms affect the charge distribution severely and the further the Ti atoms are apart the larger is the charge transfer from Mg. It was shown that a very limited amount of Ti nearest neighbors is sufficient for Ti to experience an imitation of elemental state, removing the driving force for further nucleation. © 2010 The American Physical Society.

Knott S.,University of Vienna | Li Z.,Center for Materials Science and Nanotechnology | Wang C.-H.,National Tsing Hua University | Mikula A.,University of Vienna
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2010

The thermodynamic properties of the ternary Bi-In-Zn system were determined with the electromotive force (EMF) method using a liquid electrolyte. Four different cross sections with constant In/Bi ratios of 1:2, 1:1, 2:1, and 9:1 were applied to measure the thermodynamic properties of the ternary system in the temperature range between the liquidus temperature of the alloys and 973 K (700 °C). Zinc was added in steps of 5 at. pct from 5 to 90 pct. The partial free energies of Zn in liquid Bi-In-Zn alloys were determined as a function of concentration and temperature. The integral Gibbs free energy and the integral enthalpy of the ternary system at 873 K (600 °C) were calculated by Gibbs-Duhem integration. The ternary interaction parameters were evaluated using the Redlich-Kister-Muggianu polynomials. © 2010 The Minerals, Metals & Materials Society and ASM International.

Cebasek N.,Center for Materials Science and Nanotechnology | Haugsrud R.,Center for Materials Science and Nanotechnology | Li Z.,Center for Materials Science and Nanotechnology | Norby T.,Center for Materials Science and Nanotechnology
Journal of the American Ceramic Society | Year: 2013

Chemical (impurity) tracer diffusion of Pr, Nd, and Co into polycrystalline La2NiO4+δ was done at 950°C-1350°C in air, argon, and intermediate pO2 (5.5 × 10-3 atm O 2), and diffusion coefficients were extracted from depth profiles determined by Secondary Ion Mass Spectrometry (SIMS). The Pr and Nd profiles have only one broad region, corresponding to bulk diffusion, whereas the Co tracer depth profile has two distinct regions with different slopes, where the outer shallow region represents bulk diffusion and the inner region with deep penetration depths represents grain-boundary diffusion. It is thus concluded that the diffusivity on the Ni-site is enhanced by grain-boundary diffusion. The bulk diffusion was evaluated using the solution of Fick's second law for thin-film source, and the grain-boundary diffusion was evaluated according to Whipple-Le Claire's equation. The average apparent activation energies for Pr and Nd bulk diffusion are 165 ± 15 kJ/mol, for Co bulk diffusion 295 ± 15 kJ/mol, and for Co grain-boundary diffusion 380 ± 20 kJ/mol. Qualitatively, the diffusivities and activation energies follow levels and trends in agreement with those from other experimental techniques. The apparent lack of - in fact reverse - correlation between activation energy and level of diffusivity is discussed in terms of a possibility that the faster species (Ni) reach equilibrium defect concentrations while the slower (La) is in effect frozen in. © 2012 The American Ceramic Society.

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