'Hysteresis' in Interaction of Nanocrystalline Magnesium with Hydrogen

摘要

In recent years the kinetics of hydrogenation/dehydrogenation of magnesium have been significantly improved by the introduction of new methods of fabrication of nanocrystalline magnesium-catalyst composites. However, the equilibrium hydrogen pressure for decomposition of nanocrystalline MgH_2 was found to be somewhat lower than for conventional magnesium hydride. Moreover, the essential difference in equilibrium hydrogen pressure for absorption and for desorption of hydrogen by nanocrystalline magnesium was reported by many authors. This difference called "hysteresis" is a common phenomenon for hydrides, but it is not observed for magnesium powders with an ordinary particle size (more than 1 μm). The aim of the present work was to elucidate why the hysteresis arises in magnesium-hydrogen system with decreasing the particles size. It is shown, that the "hysteresis" observed in nanocrystalline magnesium is a seeming phenomenon, which is due to a hindered nucleation during hydriding at pressures closed to equilibrium. The pressures measured for desorption represent the real equilibrium. The plateau pressure of Mg + H_2 ←→ MgH_2 equilibrium is lower for nanocrystalline magnesium than that for conventional magnesium. This result is explained in terms of smaller surface energy of magnesium hydride in comparison with magnesium.