MgH 2 by Gas Phase Condensation: Nanostructure Morphology and Hydrogen Sorption Behaviour
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1042-S04-08
MgH2 by Gas Phase Condensation: Nanostructure Morphology and Hydrogen Sorption Behaviour Ennio Bonetti1, Elsa Callini1, Amelia Montone2, Luca Pasquini1, Emanuela Piscopiello3, and Marco Vittori Antisari2 1 Department of Physics, Universita' di Bologna and CNISM, viale Berti Pichat 6/2, Bologna, 40127, Italy 2 FIM Department, ENEA, C.R. Casaccia C.P. 2400, Rome, 00123, Italy 3 ENEA, C.R. Brindisi, via Appia km 706, Brindisi, 72100, Italy
ABSTRACT Inert gas condensation was employed to prepare nanoparticles of Mg and MgH2 which morphology, clustering degree and structural stability have been investigated by X-ray diffraction and electron microscopy. Thermodynamic functional properties of the Mg and MgH2 nanostructured samples were investigated by high pressure differential scanning calorimetry. Some specific features of the morphology of the samples prepared by inert gas condensation are compared with powders obtained by ball milling through desorption kinetics behavior. INTRODUCTION A significant research effort was recently devoted to optimize the functional properties of Mg hydrides, trying to overcome major limitations preventing their application: the slow hydrogen sorption kinetics and the relatively high intrinsic thermal stability. Two major issues have been explored to enhance the sorption kinetics and lower the thermodynamic stability, namely: i) the synthesis of nanocrystalline hydrides by ball milling (BM), inert gas condensation (IGC) or thin films vacuum deposition, and ii) the addition of suitable additives such as transition metal and transition metal oxides [1-10]. Phenomenological models have been recently proposed to explain the improved kinetics of hydrogen sorption which take into account several factors such as the grain-particles size, shape and size distribution, the presence of oxide surface layers, the dispersion of metal oxide particles, and the catalytic effect of additives in nanostructured form [ 11-14]. The detailed influence of nanoparticles/film morphology and oxidation state on the hydrogen diffusion at the atomic scale are not still completely clarified. We present here results of a comparative systematic investigation of structural and functional properties of Mg/MgH2 nanopowder prepared by IGC and BM MgH2 powder. SAMPLE PREPARATION AND EXPERIMENTAL High energy BM was carried out on MgH2 powders (95 wt% MgH2 the rest being Mg) with a Spex-8000 Mixer/Mill (ball to powder ratio 10:1), using special steel vials which allow evacuation and gas filling [10,15]. For the synthesis by IGC, 99.8 % pure Mg was Joule-heated up to sublimation in a helium gas atmosphere at different pressures in the range 0.02–2 mbar.
The Mg nanoparticles were collected on a liquid nitrogen cooled rotating cylinder and subsequently subjected to surface passivation by slow admission of oxygen into the synthesis chamber. A small amount of the scraped nanopowder was dispersed in ethanol by sonication and a drop of the suspension was allowed to dry on holey carbon support grid. Transmission electron m
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