M. Pezat

Amelioration des conditions de synthese de l'hydrure de magnesium a l'aide d'adjuvants

Addition of metals or alloys whose hydrides have a high dissociation tension allows a considerable increase of the hydrogenation rate of magnesium. The influence of the nature of the adjuvant and its specific concentration, of temperature and hydrogen pressure on the reaction rate, have been carefully studied. The pressure-composition isotherms have been determined especially when the adjuvant is LaNi5.

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 ue5fc 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 ue5fc 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.

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.

A study of the decomposition of magnesium hydride by thermal analysis

To relate thermal desorption spectra to reaction kinetics and occupation site, the desorption kinetics of magnesium hydride were studied by the thermal analysis technique. The rate-controlling step for the desorption of magnesium hydride was assumed to be an interfacial chemical reaction and a theoretical rate equation was derived, based on the continuous moving boundary model. The apparent activation energy for the desorption of magnesium hydride is 142 kJ mol−1.

Etude des transformations allotropiques de Mg2NiH4

Abstract Three allotropic varieties of Mg 2 NiH 4 with monoclinic, orthorhombic and cubic symmetry have been identified. The unitcell dimensions are : a = 12.99 A , b = 6.390 A , c = 6.598 A and β = 93.22 A for the monoclinic form. The orthorhombic structure whose parameters are : a = 6.499 A , b = 6.415 A et c = 6.589 A , represents a slight distortion from the cubic one. Under pressure, an irreversible transformation from the monoclinic to the cubic variety has been observed. The transition temperature is 245°C at 1 bar. On the other hand, the orthorhombic form transforms reversibly into the cubic one about 235°C under 1 bar pressure. The orthorhombic-monoclinic transition occurs only at very high pressure.

A kinetic study on the reaction of hydrogen with Mg2Ni

Abstract The kinetics of the reaction of hydrogen with Mg2Ni has been investigated under hydrogen pressures from 3 up to 7 bar at various temperatures (548 ≤ T ≤ 593 K). Hydrogen absorption rates for the hydriding reaction show an inverse temperature dependence under our experimental conditions. The results are analyzed by considering the increase in temperature due to the heat of reaction in the course of hydrogen absorption. The rate-controlling step for hydriding reaction in the initial stage is the dissociative chemisorption on Ni involving heat transfer as “a rate-delaying step.” In later stages, hydrogen absorption rates decrease gradually with the effect of the particle size distribution and the diminution of the unreacted fraction. Other phenomena are also discussed, i.e., hydrogen absorption in α-solid solution region and an incubation period.