We obtained evidence for the partial chemical segregation of as-deposited and hydrogenated Mg 1−y Ti y films ͑0 Յ y Յ 0.30͒ into nanoscale Ti and Mg domains using positron Doppler-broadening. We exclusively monitor the hydrogenation of Mg domains, owing to the large difference in positron affinity for Mg and Ti. The electron momentum distribution broadens significantly upon transformation to the MgH 2 phase over the whole compositional range. This reveals the similarity of the metal-insulator transition for rutile and fluorite MgH 2 . Positron lifetime studies show the presence of divacancies in the as-deposited and hydrogenated Mg-Ti metal films. In conjunction with the relatively large local lattice relaxations we deduce to be present in fluorite MgH 2 , these may be responsible for the fast hydrogen sorption kinetics in this MgH 2 phase. © 2010 American Institute of Physics. ͓doi:10.1063/1.3368698͔Mg-Ti alloys are promising materials for application as hydrogen storage media, metal hydride rechargeable batteries, hydrogen sensors, and smart solar collectors. 1,2 Hydrogenation of Mg-Ti films with Ti-concentrations larger than ϳ15% ͑Mg 0.85 Ti 0.15 ͒ leads to the formation of a fluorite Mg 1−y Ti y H x phase with substantially faster hydrogenation kinetics than the common rutile MgH 2 phase. At the same time, a high hydrogen storage capacity of up to 6.5 wt % capacity is reached for Mg 0.80 Ti 0.20 . 1 The Mg-Ti͑-H͒ films have an intriguing microstructure. While Ti and Mg are immiscible metals on a macroscopic scale, codeposition of Mg and Ti by magnetron sputtering leads to Mg-Ti films with a coherent structure, as indicated by x-ray and electron diffraction studies, 3-5 and a very high degree of intermixing of the Mg and Ti. Extended x-ray absorption fine structure ͑EXAFS͒ studies, in contrast, indicated that atomically mixed alloys are not formed. Instead, a partial chemical segregation into Mg and Ti domains on a length scale of the order of less than 10 nm was suggested. 4 The stabilization of the cubic fluorite phase is thought to occur via elastic coupling of MgH 2 to the nanoscale TiH 2 domains. This coupling remains effective during the full ͑de-͒hydrogenation cycle because of the similarity of the unit cell volumes of cubic TiH 2 and hexagonal closed packed Mg in the intermediate state. 4 A direct observation of these phase segregated domains, however, is currently lacking.Furthermore, the fast hydrogen kinetics of these types of films-especially in the cubic hydride phase formed at Ti concentrations larger than 15%-is not well understood. The metal hydride lattice contracts upon increased incorporation of Ti, leading to reduced space for hydrogen mobility. 3,4 It was suggested that the unoccupied octahedral positions in the fluorite MgH 2 structure could aid the hydrogen mobility. 6 Ab initio studies indicate that vacancies form a key factor in the hydrogen diffusion of both the rutile and fluorite MgH 2 phases. [7][8][9] Further insight into the occurrence of vacancies and their possible role in t...

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Our aim is the development of high-throughput production technology for high efficiency, amorphous and microcrystalline thin film silicon photovoltaics (PV) on flexible substrates. A roll-to-roll system is developed, with three deposition chambers for the continuous deposition of n-type, intrinsic and p-type Si. Novel MW sources for plasma enhanced chemical vapour deposition (PECVD) are introduced in the roll-to-roll coater as MW-PECVD combines good quality with large area, high rate deposition of intrinsic Si. A comparison is made between Raman and UV-reflection spectroscopy.