Phase Transformation from Mg to MgH 2 Studied by SEM Metallography
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Phase Transformation from Mg to MgH2 Studied by SEM Metallography Amelia Montone, Annalisa Aurora, Daniele Mirabile Gattia and Marco Vittori Antisari ENEA MATCOMP Research Center of Casaccia, Via Anguillarese, 301 00123 Rome, ITALY ABSTRACT Metallographic information has been used to support the kinetic analysis and the relative experimental findings in the phase transformation from Mg to MgH2, by absorption of hydrogen gas. In particular the method provides to be able to provide detailed information on the role of localized features like catalyst particles and defects present in the sample, which is obtained from ball milled MgH2 enriched with 5wt% Fe. In this work we have compared the kinetics of the hydrogenation reaction carried out at two different temperatures while keeping constant the thermodynamic force driving the reaction. This last was achieved by carefully controlling the hydrogen gas pressure. Despite this fact, the sample microstructure shows a marked effect of the temperature on the nucleation mechanism. In particular we have noticed that the density of sites active for nucleation is higher at lower temperature. Instant nucleation deduced by the kinetic analysis was confirmed by comparing the microstructure of samples at different reaction stages. INTRODUCTION The dehydrogenation and hydrogenation reaction of MgH2 is widely studied owing to the potentiality of this material as reversible storage medium for hydrogen. Fast reaction kinetics have been obtained by intensive plastic deformation and catalyst introduction, generally performed by ball milling, that are able to overtake the reaction rate limiting steps present in the pristine material. The ability of the material to a fast uptake and release of hydrogen is generally measured by the classical kinetic analysis performed by comparing the reaction rate with what expected from the available models. This approach provides information on the rate limiting step of the reaction. However, the information is averaged on a large sample volume and the hypothesis that are at the basis of the models are not always fully satisfied, mainly when a powder sample characterized by a broad particle size distribution function is considered. We have already proposed [1] a method able to provide information on the role of localized features like free surface, catalyst particles and defects on the hydrogen sorption process in MgH2-Mg system. It allows to integrate the analytical approach with information concerning the microstructure of partially reacted samples, obtained by high spatial resolution metallography. In fact, the spatial distribution of the different phases can provide important issues in the phase transformation like nucleation site and nucleation mechanisms which are often difficult to assess by indirect methods. The method is based on Low Voltage Scanning Electron Microscopy (LV SEM) observation of cross sectional samples of partially transformed powders, so that a mixture of Mg and MgH2 constitutes the sample. In the low voltage SEM obser
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