Development of SEM Metallography for the Study of the Mg-MgH2 Phase Transformation
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Development of SEM Metallography for the Study of the Mg-MgH2 Phase Transformation Marco Vittori Antisari1, Amelia Montone2, Nadica Abazovic3, Annalisa Aurora2, Milica Drvendzija4, Rita Mancini1, Daniele Mirabile Gattia1, and Luciano Pilloni1 1 Materials and Technology, C.R. ENEA Casaccia, Rome, 00061, Italy 2 Materials and Tecnology, C.R. ENEA Casaccia, Rome, 00061, Italy 3 Laboratory of Radiation chemistry and physics, Vinca Institute of Nuclear Sciences, Belgrade, 11000, Yugoslavia 4 Laboratory of Materials Sciences, Vinca Institute of Nuclear Sciences, Belgrade, 11000, Yugoslavia ABSTRACT The study of sorption reactions of metal hydrides is the subject of numerous researches in connection with the development of a safe technology for hydrogen storage, and MgH2, in particular, is one of the most promising materials. Several efforts have been already carried out in order to understand the kinetic mechanisms involved in the MgH2 decomposition which is at the basis of H2 release. However, the role of additives and induced structural defects on the sorption cycles is not clear yet. With the purpose of supporting the reaction analysis we have developed an experimental protocol for the metallographic examination at high spatial resolution of partially desorbed MgH2 powders. In particular, this procedure allows cross-sectional analysis of powders embedded into conductive matrix, while the observation method produces a different contrast among metallic Mg, MgH2 and additive particles made of heavier materials. INTRODUCTION Among the alternative solutions to carbon-containing fuels, H2 is widely considered as a promising candidate due to its high energy content (142 MJ/kg) [1] and to its full harmless for the environment. Among the technological problems connected with the hydrogen as energy carrier, ways of hydrogen storing are not completely assessed and conventional methods (liquefied gas and pressurized containers) still exhibit several technological and safety drawbacks [2]. As a consequence, a great attention is focused to alternative solutions among which metal hydrides are attractive. In particular, MgH2 shows interesting properties such as high H2 gravimetric storage capacity (7.6 wt%), low cost and high abundance. However, this material displays too high temperatures of decomposition, mainly owing to high thermodynamic stability and slow decomposition kinetics [3], so that several routes have been proposed in order to fasten the reaction and to reduce the decomposition temperature. Most of these processes are based on high-energy ball milling, that is able to introduce a high density of crystal defects [4,5], and a fine dispersion of second phase particles with catalytic effects [6]. These phase-transformation studies are generally supported by thermal analyses and by X-Ray diffraction, while a microscopic approach is seldom used, even if the metallography of partially transformed powders can be a powerful way to identifying features of the process, like nucleation sites and structure of the inter-p
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