Controlling Hysteresis of Metal-hydride Transformations in Epitaxial Thin Films
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Controlling Hysteresis of Metal-hydride Transformations in Epitaxial Thin Films Brad M. Boyerinas1, Hugh A. Bruck1, and Alexander L. Roytburd2 1
Department of Mechanical Engineering, University of Maryland, College Park, MD
20742, USA 2
Department of Materials Science and Engineering, University of Maryland, College Park,
MD 20742, USA
ABSTRACT Metal hydrides present a feasible means of energy storage and hydrogen sensing but have several performance criteria that must be addressed, including the hysteresis effect during hydrogen loading and unloading. We present the results of a theoretical and experimental study which demonstrates the possibility to control or eliminate hysteresis during metalhydride transformation in epitaxial Pd thin films. Theoretical analysis predicts stabilization of two-phase metal-hydride state in film due to its elastic interaction with the substrate. It is shown, by atomic force and scanning electron microscopy, that transformation in 100nm thick epitaxial Pd films on Al2O3 substrate proceeds by the formation of transversely modulated two-phase nanostructure. Morphology and crystallographic orientation of the metal-hydride interface corresponds to the theoretically predicted characteristics of coherent phases. INTRODUCTION Metal hydride transformations are usually accompanied by hysteresis [1]. There are two primary sources that cause this hysteresis: extrinsic and intrinsic. Extrinsic hysteresis is the result of plastic deformation which relaxes internal stresses arising in the metalhydride mixture. Extrinsic hysteresis is diminished in the case of purely coherent metalhydride transformations. Coherent phase transformations are more preferable in high quality epitaxial films, where relaxation of internal stress through plastic deformation and fracture can be prevented or at least minimized. However, even during coherent phase transformation, there is intrinsic thermodynamic hysteresis due to elastic interaction between the hydride and metal phases. We show that in epitaxial films this intrinsic hysteresis can be minimized or eliminated as a result of constraint by the substrate. As an example, we consider theoretically and experimentally metal-hydride transformation in epitaxial Pd film. Pd is considered due to its applicability to H storage and sensing systems [2]. Plate morphology of coherent hydride-metal phases in Pd foil has been observed in [3]. Hydride-metal phase coherency has been studied in multi-oriented Pd thin films [4], but morphology has not been characterized. In this study, 100nm thick epitaxial Pd films are hydrogenated under high temperature and high pressure conditions. Scanning electron
microscopy and atomic force microscopy were used to image and characterize ordered nanostructures on the film surface after hydrogenation. To theoretically demonstrate the stabilizing effect of external constraint, we consider coherent transformation in a single crystalline film mechanically constrained by a substrate. Metal-hydride transformations proceed by movement of inters
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