INSTITUTO DE FISICA LA PLATA
Unidad Ejecutora - UE
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Complex Mg-based hydrides obtained by mechanosynthesis: characterization and formation kinetics
LORENA BAUM; MARCOS MEYER; LUIS MENDOZA ZELIS
Simposio; International Symposium on Metastable, Mechanically Alloyed and Nanocrystalline Materials; 2008
Mg and its alloys are nominally among the best candidates for hydrogen storage in the form of solid hydrides due to their high volumetric and gravimetric capacity (110 kg H2/m3 for MgH2) but their sorption kinetics must be considerable improved in order to become of practical use. The reduction of grain sizes to the nanometer scale and the addition of small quantities of transition metals as catalysts, have demonstrated to be useful for that purpose. Furthermore, Mg combines with hydrogen and some transition metals like Fe, Co and Ni to form complex ternary hydrides with very high volumetric hydrogen content. A noteworthy case is that of Mg2FeH6, containing 150 kg H2/m3, although Mg and Fe are practically immiscible and do not form any compound in the absence of a third element. When Fe-catalyzed Mg is cyclically H2-charged and discharged, the ternary hydride could form, affecting not only the net charge capacity but also the system microstructure and thus the sorption kinetics. Mechanical milling is being much used, both to refine the Mg matrix and to disperse the selected metallic catalyst. When Mg-Fe mixtures are milled in H2 atmosphere the formation of the ternary compound was also observed, even for minute Fe addition. By selecting the milling time and the starting powder mixture composition, the relative fraction of MgH2, Fe and Mg2FeH6 as well as the sample microstructure can be varied, together with the associated sorption properties. Similar results are expected for Mg-Co and Mg-Ni mixtures that give rise respectively to MgH2/Mg2CoH5 and MgH2/Mg2NiH4 systems. On the other hand, Mössbauer spectroscopy on 57Fe nuclei allows the investigation of atomic configurations in the neighborhood of Fe atoms and may help to understand the role of Fe in the catalytic process and the conditions for the formation of ternary hydrides. We present here our results on a set of Mg samples milled in H2 with a varying amount of Fe, from a few atomic percent, where Fe acts as a catalyst for H2 absorption, to the higher concentration range where it reacts to form Mg2FeH6. We studied the mechanically assisted hydriding process in Mg1-xFex powder mixtures with x = 0.02; 0.03; 0.10; 0:20 and 0.33, following the hydriding kinetics up to saturation from the hydrogen absorption during milling. All the steady state samples and some partially hydrided ones, with x = 0.03 and x = 0.33, were characterized by X ray diffraction and Mössbauer spectroscopy. The results were analyzed in the framework of possible mechanisms for the transition metal role in the thermally activated H2 sorption in Mg based systems and the influence of complex hydride formation in such processes.