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[en] In this work, MgH_2 was used as a starting material. Samples with compositions of 94 wt% MgH_2-6 wt% Ni, 88 wt% MgH_2-12 wt% Ni, 85 wt% MgH_2-15 wt% Ni, and 82 wt% MgH_2-18 wt% Ni were prepared by reactive mechanical grinding. The variations of the hydriding and dehydriding properties at the first hydriding-dehydriding cycle with Ni content were then investigated. MgH_2-12Ni had the highest hydriding rate and the largest quantity of hydrogen absorbed for 60 min, followed in order by MgH_2-6Ni, MgH_2-15Ni, and MgH_2-18Ni. The effects of reactive mechanical grinding were the strongest in the MgH_2-12Ni sample. As the Ni content increased from 6 wt% to 18 wt%, the percentage of the hydrogen quantity desorbed for 30 min to the theoretical capacity increased.
[en] The properties of the M dwarfs have been investigated by spectroscopic and photometric observations of over 150 red, high proper motion stars. The fainter stars are generally all high velocity stars. Members of the group discussed have weak or no TiO bands but instead the spectrum is dominated by the hydride bands CH, MgH and CaH
[en] This work deals with the synthesis of magnesium hydride MgH2 by a reactive grinding of metallic magnesium under hydrogen atmosphere. A vibrating grinding mill has been used in which only one ball makes a regular vertical movement. In these conditions, the final state only depends of a limited number of parameters. After grinding, the powders microstructure has been studied by X-ray diffraction, scanning and transmission electron microscopies, electronic microprobe and their performances have been determined by thermal gravimetric analysis. In the optimal conditions, have been obtained nano-structured powders of magnesium hydride presenting storage capacities per unit of mass of 7%, value which is very near the theoretical capacity. (O.M.)
[en] 90 wt% MgH_2+5 wt% Ni+2.5 wt% Fe+2.5 wt% Ti (called MgH_2+Ni+Fe+Ti), a hydrogen storage and release material, was fabricated by grinding in a hydrogen atmosphere, and then its quantities of stored and released hydrogen as a function of time were examined. A nanocrystalline MgH_2+Ni+Fe+Ti specimen was made by grinding in a hydrogen atmosphere and subsequent hydrogen storage-release cycling. The crystallite size of Mg and the strain of the Mg crystallite after ten hydrogen storage-release cycles, which were obtained using the Williamson-Hall method, were 38.6 (±1.4) nm and 0.025 (±0.0081) %, respectively. The MgH_2+Ni+Fe+Ti sample after the process of grinding in a hydrogen atmosphere was highly reactive with hydrogen. The sample exhibited an available storage capacity of hydrogen (the amount of hydrogen stored during 60 minutes) of about 5.7 wt%. At the first cycle, the MgH2+Ni+Fe+Ti sample stored hydrogen of 5.53 wt% in 5 minutes, 5.66 wt% in 10 minutes and 5.73 wt% in 60 minutes at 573 K and 12 bar of hydrogen. The MgH_2+Ni+Fe+Ti after activation released hydrogen of 0.56 wt% in 5 minutes, 1.26 wt% in 10 minutes, 2.64 wt% in 20 minutes, 3.82 wt% in 30 minutes, and 5.03 wt% in 60 minutes.
[en] 'Full text': Aggregates of hydrogen molecules with small clusters of a light metal are predicted to have H2atM(n) isomers with the dihydrogen trapped inside the metal cage. The H2 core significantly re-structures the surrounding M(n) shell via strong interaction between these oppositely charged components. The molecule steadily stretches with increasing cage size, up to dissociation, due to increasing electron donation from the cage. The dihydrogen may thus be stored inside the metal cluster in either dissociated or non-dissociated form. It should be noted that the weight-efficiency of such storage is rather low (∼1% of hydrogen). Future work will be aimed at increasing the hydrogen content. The system stability to dissociation into H2 + M(n) increases with size from slightly metastable to weakly bound (at ∼0.1 eV scale) systems. The charge-transfer stabilization is thus nearly compensated by the molecule and cage deformations, allowing a relatively easy release of hydrogen, unlike the high-temperature release from bulk MgH2 solid. In fact, the cluster systems may actually be stable only at low temperatures. The closed-shell H2atM(n) species also show stability in terms of electronic excitation, ionization and electron-attachment, which should favour their low chemical reactivity and efficient formation in experiments. Their detection can be assisted by high IR intensities of the vibrational modes associated with relative translations of the oppositely charged dihydrogen and cage. (author)
[en] A metal hydro-borate Zn(BH_4)_2 was prepared by milling ZnCl_2 and NaBH_4 in a planetary ball mill in an Ar atmosphere. This sample contained NaCl. 95 wt% MgH_2-2.5 wt% Zn(BH_4)_2-2.5 wt% Ni samples [named MgH_2-2.5Zn(BH_4)_2-2.5Ni] were then prepared by milling in a planetary ball mill in a hydrogen atmosphere. The hydrogen absorption and release properties of the prepared samples were investigated. In particular, variations in the initial hydriding and dehydriding rates with temperature were examined. MgH_2-2.5Zn(BH_4)_2-2.5Ni dehydrided at the fourth cycle contained Mg, MgO, and small amounts of β-MgH2 and Mg2Ni. The sample after hydriding-dehydriding cycling had a slightly smaller average particle size and a larger BET specific surface area than the sample after milling. Increasing the temperature from 573 K to 623 K led to a decrease in the initial hydriding rate. The initial dehydriding rate increased as the temperature increased from 573 K to 643 K. At 573 K under 12 bar H_2, the sample absorbed 3.85 wt% H for 2.5 min, 4.60 wt% H for 5 min, 4.64 wt% H for 10 min, and 4.80 wt% H for 60 min. The MgH_2-2.5Zn(BH_4)_2-2.5Ni had an effective hydrogen storage capacity (the quantity of hydrogen absorbed for 60 min) of near 5 wt% (4.96 wt% at 593 K).
[en] At the first cycle (n=1), pure MgH_2 absorbed hydrogen extremely slowly at 593 K under 12 bar H_2, absorbing 0.04 wt% H for 60 min. Activation of the pure MgH_2 was completed after five hydridingdehydriding cycles. At the 6"th cycle, the pure MgH_2 absorbed 2.41 wt% H for 5 min, 3.00 wt% H for 10 min, and 4.21 wt% H for 60 min, showing that the activated pure MgH_2 had a much higher hydriding rate than the activated pure Mg. The pure Mg absorbed 0.10 wt% H for 5 min, 0.38 wt% H for 30 min, and 0.51 wt% H for 60 min at the first cycle. The activated pure Mg, whose activation was also completed after five hydriding-dehydriding cycles, absorbed 1.76 wt% H for 5 min, 2.17 wt% H for 10 min and 3.40 wt% H for 60 min. The XRD pattern of the pure MgH_2 after hydriding-dehydriding cycling (n=7) revealed that the sample contained Mg, a small amount of MgO, and a large amount of MgH_2, showing that a large fraction of MgH_2 remains even after dehydriding in vacuum at 623 K for 2 h.
[en] Highlights: •Mg3Ag compound with high-purity was prepared by hydrogen metallurgy. •Mg3Ag is first employed for reversible hydrogen storage with altered thermodynamics. •The enhanced cyclic stability is due to the prevention of MgH2 sintering by MgAg. -- Abstract: For the first time, the compound Mg3Ag was employed as a medium for hydrogen storage. It has been demonstrated that the hydriding/dehydriding process of Mg3Ag is reversible through the reaction Mg3Ag + 2H2 ↔ 2MgH2 + MgAg with obtaining altered thermodynamics. An enhanced cycling stability is also achieved by the capacity retention of 95% after 30 cycles, much higher than 70% for the pure Mg sample, which can be explained that the agglomeration and sintering of the resulting MgH2 are efficiently prevented by the formation of hard and brittle MgAg phase upon multi-cycling