B. Darriet

Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr2O3, Al2O3 and CeO2

Abstract We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H 2 (reactive mechanical grinding) with oxides Cr 2 O 3 , Al 2 O 3 and CeO 2 . The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH 2 during reactive mechanical grinding. The Mg+10wt.%Cr 2 O 3 powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2 O 3 . The Mg+10wt.%Cr 2 O 3 powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . Mg+10wt.%Cr 2 O 3 absorbs 5.87wt.% H at 573 K, 11 bar H 2 during 60 min at the first cycle. The Mg+10wt.%Cr 2 O 3 powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2 O 3 . It desorbs 4.44 wt.% H at 573 K, 0.5 bar H 2 during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr 2 O 3 , Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr 2 O 3 , Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr 2 O 3 in the Mg+10wt.%Cr 2 O 3 powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr 2 O 3 after cycling.

Hydriding and dehydriding characteristics of mechanically alloyed mixtures Mg-xwt.%Ni (x = 5, 10, 25 and 55)☆

Abstract The kinetic characteristics of magnesium have been improved either by adding a catalyst or by using magnesium-based alloys. A comparative study of Mg2Ni and a mechanically alloyed mixture Mg-55wt.%Ni has been recently carried out in the laboratory. The mixture presents some interesting properties and the preparation of this material is easier and less energy consuming owing to the absence of heating. However, while the hydrogen capacity (3.2 wt.%) is close to that of Mg2Ni, it is significantly smaller than that of magnesium (7.6 wt.%). Therefore, to improve this characteristic, several compositions of mechanically alloyed mixtures Mg-xwt.%Ni with a lower nickel weight content have been studied (x = 5, 10 and 25). It was shown that the amount of nickel has an important effect on the hydriding-dehydriding kinetics. The decrease of the particle size and the augmentation of the density of defects are because of hydriding-dehydriding cycling. These effects are enhanced by an increase in the nickel content and increase the rate of the hydriding and dehydriding reactions. In contrast, the hydrogen storage capacity diminishes as the amount of nickel increases. For these reasons, Mg-10wt.%Ni and Mg-25wt.%Ni are the most favourable compositions.

Application of magnesium rich rare-earth alloys to hydrogen storage

The hydriding behaviour of the LnMg12, Ln2Mg17 and Ln5Mg41 alloys (Ln = La, Ce or mischmetal) has been investigated. They decompose during hydriding to give magnesium hydride and the corresponding rare-earth hydride. The study of absorption-desorption hydriding cycles at different temperatures and various pressures shows that such alloys could be used as high performance hydrogen storage materials.

A comparative study of magnesium-rich rare-earth-based alloys for hydrogen storage☆

The absorption of hydrogen by the magnesium-rich alloys LnMg12 (Ln  La, Ce, mischmetal), La2M17 and Ce5Mg41 was investigated. The alloys decompose during hydriding to give magnesium hydride and the corresponding rare earth hydride. A comparative study of the hydriding and dehydriding processes was carried out at two different temperatures. In addition, alloys with compositions corresponding to the formula CeMg11M (M  V, Cr, Mn, Fe, Co, Ni, Cu, Zn) were prepared and their absorption and desorption rates were compared with those of CeMg12 and La2Mg17. The hydriding rate is not changed significantly by the presence of a 3d element but the dehydriding rate is greatly enhanced by the presence of vanadium, chromium, manganese, iron or nickel. These results are discussed.

Hydriding properties of a mechanically alloyed mixture with a composition Mg2Ni

Hydriding properties of a mechanically alloyed 2Mg + Ni mixture have been investigated and compared with those of the Mg2Ni alloy prepared by melting and sintering. The mechanically alloyed 2Mg + Ni mixture was estimated as an excellent hydrogen storage material.

Les alliages terre rare-magnesium riches en magnesium et leur application au stockage de l'hydrogene

The LnMg/sub 12/, Ln/sub 2/Mg/sub 17/ and Ln/sub 5/Mg/sub 41/ alloys (Ln = La, Ce or mischmetal) have been investigated. The composition range and the stability of the various phases have been determined. They decompose during hydriding with formation of magnesium hydride and the corresponding rare-earth hydride. The study of absorption-desorption hydriding cycles at different temperatures and various pressures shows that these alloys could be used as high performance hydrogen storage materials.

Hydrogen sorption of Mg-based mixtures elaborated by reactive mechanical grinding

The use of mechanical grinding (MG) under H2 of magnesium powder improves the hydrogen sorption properties. The hydrogenation of Mg starts in situ during the milling process that allows suppressing the activation procedure generally requested for Mg. The effects of the addition of various elements or compounds have been studied. The hydriding is a two-step process: nucleation and diffusion. A direct relationship exists between the nucleation duration and the specific surface. A critical milling time exists below which the diffusion process is improved and above which no further improvement is observed (the maximum internal stress in the powder is also reached at this critical time). The diffusion is controlled by the number of crystallites per particle that can be reduced by increasing the milling time up to 10 h. The addition of Co (catalyst), YNi (hydrogen pump) or oxides (abrasive element and nucleation centre) leads to an improvement of the hydrogen sorption properties (but a strong dependence upon the milling time is reported). Finally, the sorption properties of our mixtures are comparable with thus reported for MgH2–metal mixtures.

Sur quelques nouveaux pyrochlores des systemes MTO3 WO3 et MTO3 TeO3 (M = K, Rb, Cs, Tl; T = Nb, Ta)

Abstract In the scope of a general investigation of the MTO 3 -WO 3 and MTO 3 -TeO 3 (M = K, Rb, Cs, Tl; T = Nb, Ta) systems, the structures of the pyrochlore related phases RbNbTeO 6 and KTaWO 6 , H 2 O are determined and discussed from the crystal chemical point of view.

The Mg2Ni0.75M0.25 alloys (M=3d element): their application to hydrogen storage

Abstract The solid solutions of Mg 2 Ni 1− x M x (when M = V, Cr, Fe, Co, Cu and Zn) have a Mg 2 Ni-type structure with a large homogeneity range. A comparative study of the action of hydrogen has been carried out on all alloys corresponding to an x = 0.25 formulation. The absorption-desorption process of hydrogen is reversible and after dissociation of the hydride the starting material is regenerated except for copper which has not been examined here. Hydriding leads for all other alloys to formation of quaternary hydrides. The thermal stability is very close to that of Mg 2 NiH 4 stability: partial substitution of nickel by cobalt in Mg 2 Ni leads at given temperature to a lower dehydriding rate.

Relationship between hydrogen sorption properties and crystallography for TiMn2 based alloys

In order to develop suitable materials for a hydrogen storage tank or for electrochemical applications, TiMn2-based C14 Laves phase alloys were prepared. The effect of non-stoechiometry was first studied (TiMn2 exist as a single phase from TiMn1.6 to TiMn2.1 according to the Ti–Mn phase diagram). It was shown that maximum hydrogen storage capacity was obtained for the slightly under stoechiometric Ti0.95Zr0.05Mn1.95. Then, the substitution effect of transition metals such as Cr, V, Fe, Ni, Co (and Al) was also examined. It was found that Ni substitution led to an increase of the plateau pressure, Cr and Co substitutions did almost not influence the hydruration characteristics, Al was suitable to reduce the plateau pressure but also led to a huge decrease of the hydrogen storage capacity. Finally, we found that the best substitution effect was obtained with V which allowed us to decrease the plateau pressure and increase the hydrogen storage capacity. For each compound, a relationship was made with the lattice structures.