On the remarkable optical and structural transformations in Mg-Ti-H thin films

October 16, 2006 - 12:00pm
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In searching for battery electrode materials, Mg-Ti-H thin films triggered much interest with its hydrogen storage capacity of approx. 4 times larger than that of conventional NiMH batteries [1]. Besides this, the same system has also absolutely remarkable optical properties [2]. Highly reflective in the metallic state, the films become highly absorbing upon hydrogen absorption. Mg0.80Ti0.20Hx thin films combine a high absorption (87% of the solar spectrum) with a low thermal emissivity (only 10%), while after removal of hydrogen Mg0.80Ti0.20 absorbs no more than 1/3 of the solar spectrum. This combined with a fast, robust and reversible optical transition highlights the applicability of Mg-Ti-H thin films also as switchable smart coatings for solar collector. Such a highly absorbing state hints to coexistence of a metallic and a semiconducting phase. However, the simplistic approach of a composite material consisting of MgH2 and TiH2 fails in reproducing exactly both the optical and the electrical properties. By continuously monitoring the structure during H uptake, we identify a more subtle distribution of sites. The structural data are compatible with a random alloys with short range order (short range order parameter s=0.85). In this picture, the Ti-containing sites (Ti4, Mg2Ti2, Mg3Ti and MgTi3) will uptake H first but remain metallic whereas in a second step, an optical transition takes place which coincides with filling of the Mg4 sites. This mechanism and the complete reversibility of the system are understood based on the similarities in molar volume between TiH2 and Mg. The present results point toward a rich and unexpected chemistry of Mg-Ti-H compounds. In comparison, results on other Mg-TM-H (TM=V, Cr) systems emphasize the importance of the transition metal on the kinetics, reversibility and structural transformations. 1. P. Vermeulen, R.A.H. Niessen, and P.H.L. Notten, Electrochem. Comm. 8, 27 (2006). 2. D. M. Borsa, A. Baldi, M. Pasturel, H. Schreuders, P. Vermeulen, P.H.L. Notten, B. Dam, R. Griessen, Appl. Phys. Lett. 88, 241910 (2006). For more information about this seminar, please contact: Venkat Srinivasanx2679

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