Evolution of Crystal Orientation in Obliquely Deposited Magnesium Nanostructures for Hydrogen Storage Applications
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Evolution of Crystal Orientation in Obliquely Deposited Magnesium Nanostructures for Hydrogen Storage Applications Mehmet F. Cansizoglu1, Fumiya Watanabe2, Pei-I Wang3, and Tansel Karabacak1 1 Department of Applied Science, University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, AR, 72204 2 Arkansas Nanotechnology Center, University of Arkansas at Little Rock, Little Rock, AR, 72204 3 Center of Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NC, 12180 ABSTRACT Crystal orientation (texture) is an important parameter in the hydrogen absorption and desorption properties of various materials. In this study, we investigate the formation of magnesium nanorod arrays with crystal orientations that are not normally observed in conventional Mg thin films. Mg nanorods are produced using an oblique angle deposition technique through a physical self-assembly process. In this study sputtering and thermal evaporation systems are utilized for the growth of Mg nanorods and thin films on silicon wafer pieces. We present a detailed X-ray diffraction and scanning electron microscopy analysis. It is discussed that under oblique incidence, evolution of crystal orientations with lower adatom mobility are promoted due to the shadowing effect. INTRODUCTION Due to increasing considerations about the current fossil based energy sources in the world, a switch towards cleaner and sustainable energy media has been a necessity. Among various candidates, hydrogen is an ideal element for storage, transport, and conversion of energy. Hydrogen storage in advanced materials has been an intense area of research due to many advantages it offers such as reduced gas pressure requirements allowing lower container weights. Among metal hydrides for hydrogen storage, magnesium dihydride (MgH2) combines a H2 capacity of 7.6 wt % with the benefit of the low cost of production and abundance. The main difficulties for direct usage of pure MgH2 are slow absorption/desorption kinetics, a high thermodynamic stability, and a high reactivity toward air and oxygen, which are also common issues in most lightweight metal hydrides.1-2 There has been intensive research on nanostructured magnesium through various fabrication methods, especially derivatives of ball-milling and mechanical alloying techniques.3,4,5 Nanostructured magnesium can be beneficial in some aspects: It is proposed that, both magnesium and magnesium hydride become less stable with decreasing cluster size and thus the hydrogen desorption energy decreases significantly when the crystal grain size becomes smaller. Thus nano-sized material size can affect the hydrogen absorption and desorption properties of metal hydride materials.1-2 Nanostructuring of magnesium may also contribute to absorption-desorption rates by increased surface area of interaction and decreased diffusion lengths of hydrogen into Mg crystal.1-2 The crystal structure and the transformation of crystal structure during the Hydrogenationdehydrogenation processes are also thought to be
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