Print Email Facebook Twitter Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films Title Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films Author Eijt, S.W.H. Kind, R. Singh, S. Schut, H. Legerstee, W.J. Hendrikx, R.W.A. Svetchnikov, V.L. Westerwaal, R.J. Dam, B. Faculty Applied Sciences Department Radiation, Radionuclides and Reactors Date 2009-02-24 Abstract We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and Mg2Ni films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480–600 K, leads to a clear Pd–Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the ? 5?nm thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg–Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that Mg2Si formation is fast relative to MgH2. The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to Mg2Si for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of Mg2Si after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of Mg2Si is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed Mg2Si. Subject dissociationelectronic structuregrain boundariesheat treatmenthydrogenhydrogenationmagnesiummagnesium compoundsmetal-insulator transitionpositron annihilationpositroniumsorptionsurface chemistrythin filmstransmission electron microscopyX-ray diffraction To reference this document use: http://resolver.tudelft.nl/uuid:bb2cac60-49e2-4df4-9462-a13e22cc4645 DOI https://doi.org/10.1063/1.3075762 Publisher American Institute of Physics ISSN 0021-8979 Source http://link.aip.org/link/JAPIAU/v105/i4/p043514/s1 Source Journal of Applied Physics, 105 (4), 2009 Part of collection Institutional Repository Document type journal article Rights (c) 2009 The Author(s); American Institute of Physics Files PDF Eijt2_2009.pdf 1.38 MB Close viewer /islandora/object/uuid:bb2cac60-49e2-4df4-9462-a13e22cc4645/datastream/OBJ/view