Aleaciones base Mg para el almacenamiento de hidrógeno producidas por aleado mecánico

Abstract

Geometry optimization jobs based on Density Functional Theory were performed on given atom arrangements to estimate the enthalpy of formation of the hydrides MgH2 and (Mg-M)H2 (M= Y, Sc, Li, Zn, Ga, Al, Ag) and assess the thermodynamic destabilization brought about by the addition of M. The effect of the dissolution of hydrogen in Mg and Mg-M was also investigated in order to find a more favorable environment for the dissolution of H atoms in the Mg M-substituted matrix. The largest destabilizing effects are found by the alloying additions (Ag and Ga) which produce the smallest reduction of the enthalpy of solution of H. On the other hand, those which favor the dissolution of H the most (Sc and Y) are precisely those which produce the most stable hydrides. Nevertheless, the addition of Ag, Li, Ga, Al or Zn seems to produce both a less stable hydride and a more favorable environment for the dissolution of hydrogen, pointing to the possibility of lower dehydriding temperatures and faster hydriding/dehydriding kinetics. Experimentally, elemental powders of Mg and Mg-M mixtures (M = Al, Ag, Zn, LiH, Fe and Y) were subjected to mechanical alloying (MA) in order to produce binary Mg-M (M = Al, Ag, Zn, LiH, Fe e Y) alloys. Looking up for synergy effects, ternary Mg-M-Fe (M = Ag y Zn) and four-element (Mg-Zn-Ag-Fe) alloys were also produced. The as-milled powders were characterized by X-ray diffraction (XRD), SEM-chemical microanalysis and TEM. Phase quantification by the Rietveld Method was also performed. A crystal size below 50 nm was found in all the as-milled powder alloys after 216 ks of high-energy ball milling. Apparent solid solutions of M in Mg were obtained in the cases of M = Al, Ag and Zn, judging by the presence of only Mg peaks in the corresponding XRD patterns. Reflections from Yttrium hydrides and MgO were found, beside those of Mg, in the XRD pattern of the Mg-Y alloy. Bragg reflections from MgO, Mg and Fe were found in the XRD pattern of the Mg-Li alloy, the iron presence explained by the wearing-off of the milling media. Fe content in this alloy ranged between 3-6 wt. %. The hydriding reaction was carried out in a pressurized-stainless steel vessel using the MA powder products without any activation treatments under 200 °C/0.5 MPa 𝑃𝐻2 y 300 °C/3 MPa 𝑃𝐻2for 10 and 30 minutes. The hydrided powders were characterized by X-ray diffraction (XRD), SEM-chemical microanalysis and TEM. Phase quantification by the Rietveld Method was also performed. Maximum contents of MgH2 of 62, 73 and 81 wt. % (corresponding to 4.8, 5.6 y 6.2 wt. %p of hydrogen) were found for milled Mg and binary Mg-Li and Mg-Fe alloys, respectively after hydriding at 300 °C/3 MPa 𝑃𝐻2for 30 minutes. None of the ternary or four-element alloys showed higher MgH2 contents that the binary Mg-Fe alloy. Selected alloys (Mg, Mg-Li y Mg-Fe) were also subjected to in situ hydriding/dehydriding reactions using an automated thermogravimetric apparatus to obtain kinetic (wt. % H vs. time plots) and thermodynamic data (pressure-composition isotherms). Hydriding (350 °C/3 MPa 𝑃𝐻2) velocities of 0.25, 0.15 and 0.18 wt. %min-1 were found for the Mg-Li, Mg-Fe and Mg samples, respectively. The corresponding values for the dehydriding velocity were 0.14, 0.09 and 0.09 wt. %min-1, respectively. A destabilizing effect was found by the addition of Fe, according to the values of the equilibrium pressure (dehydriding) of 0.38, 0.22 and 0.07 MPa for the Mg-Li, Mg-Fe and Mg samples. The best hydriding/dehydriding properties were those of the Mg-Li alloy (also containing Fe), presumably by the combined catalytic effect of Li and Fe. More detailed studies, both theoretical and experimental, are required to elucidate the catalytic mechanism of these additions on Mg.