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Thin films of the high-temperature superconductor YBa 2 Cu 3 O 7 ؊ ␦ exhibit both a large critical current (the superconducting current density generally lies between 10 11 and 10 12 A m −2 at 4.2 K in zero magnetic field) and a decrease in such currents with magnetic field that point to the importance of strong vortex pinning along extended defects 1,2 . But it has hitherto been unclear which types of defect-dislocations, grain boundaries, surface corrugations and anti-phase boundaries-are responsible. Here we make use of a sequential etching technique to address this question. We find that both edge and screw dislocations, which can be mapped

The standard approach for the search of new hydrogen-storage materials is to synthesize bulk samples and to use volumetric [1,2] or gravimetric [3] techniques to follow their hydrogenation reaction and to record pressure-concentration isotherms (p-c isotherms). The equilibrium pressure of the metal-to-hydride transition is determined from the plateau of the p-c isotherm. The enthalpy of hydride formation is extracted from the temperature dependence of the equilibrium pressure, by means of the Van 't Hoff relation [4] lnwhere DH is the enthalpy of formation in kJ (mol H 2 ) -1 , DS 0 is the entropy of formation in JK -1 (mol H 2 ) -1 at standard pressure, R the gas constant, the absolute temperature, p 0 = 1.013 × 10 5 Pa the standard pressure, and p eq the H 2 equilibrium plateau pressure of the p-c isotherm. The great disadvantage of this approach is that a bulk sample is needed for each investigated chemical composition. Thin films provide an interesting alternative to bulk, as their nanostructure is controlled by the deposition conditions. Because of the small amount of material and large surfaces present, diffusion and local heating issues are minimized, the kinetics are fast, and the measurement time is reduced drastically. [5] Moreover, a large number of different chemical compositions can be deposited on a single substrate in a combinatorial way. The fact that hydrogen absorption in a metal leads to large optical changes [6,7] is the basis of a new combinatorial method that we call hydrogenography. With a straightforward optical setup, hydrogenography makes it possible to monitor hydrogen ab-and desorption simultaneously on thousands of samples under exactly the same experimental conditions. [8][9][10] We show here that hydrogenography is much more than a monitoring technique, as it also provides a high-throughput method to measure quantitatively the key thermodynamic properties (enthalpy and entropy) of hydride formation. We describe the essential ingredients of hydrogenography with the Mg-Ti-H system and demonstrate its combinatorial power with the Mg-Ti-Ni-H system. We show in particular that there is a relatively narrow range of compositions in the ternary Mg-Ti-Ni phase diagram with a remarkable combination of favorable properties for light-weight hydrogen storage. Pure MgH 2 would in principle be an attractive system for hydrogen storage as it can contain as much as 7.6 wt % of hydrogen. However, its large negative enthalpy of formation (-74 kJ (mol H 2 ) -1

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Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. The structural, optical, and electrical transformations induced by hydrogen absorption and/or desorption in Mg-Ti thin films prepared by co-sputtering of Mg and Ti are investigated. Highly reflective in the metallic state, the films become highly absorbing upon H absorption. The reflector-to-absorber transition is fast, robust, and reversible over many cycles. Such a highly absorbing state hints at the coexistence of a metallic and a semiconducting phase. It is, however, not simply a composite material consisting of independent MgH 2 and TiH 2 grains. By continuously monitoring the structure during H uptake, we obtain data that are compatible with a coherent structure. The average structure resembles rutile MgH 2 at high Mg content and is fluorite otherwise. Of crucial importance in preserving the reversibility and the coherence of the system upon hydrogen cycling is the accidental equality of the molar volume of Mg and TiH 2 . The present results point toward a rich and unexpected chemistry of Mg-Ti-H compounds.