Structure of nanocomposite metal hydrides

It has recently been shown that MgH2-(V, Nb) nanocomposite has very fast hydrogen sorption kinetics. This could be explained by the presence of vanadium which eases hydrogen penetration into the material and by the particular microstructure of this nanocomposite. In order to have a better understanding of the nature and hydrogen desorption mechanism, a systematic structural study has been undertaken. The powder morphology and chemical phase distribution were observed by SEM. X-ray diffraction under hydrogen pressure was performed at different temperatures in order to see the effect of absorption/desorption process on the crystal structure. Crystallite size was evaluated by X-ray powder diffraction peak broadening. Real time X-ray investigations of hydrogen desorption in MgH2-Nb nanocomposite were performed using synchrotron radiation. For the first time, we were able to get direct evidence of the dehydrogenation mechanism. A metastable new niobium hydride phase associated with the desorption process was detected. This metastable hydride is probably the result of long-range ordering of hydrogen in the niobium. hydride lattice in order to facilitate the hydrogen flow. (C) 2002 Elsevier Science B.V. All rights reserved.