M. Khrussanova

Hydriding and dehydriding characteristics of Mg-LaNi5 composite materials prepared by mechanical alloying

Abstract The hydriding and dehydriding kinetics of composite materials with the nominal composition Mg– x wt.% LaNi 5 ( x =10, 20, 30) prepared by mechanical alloying of the components in a planetary mill with an acceleration of 12 g have been investigated. It has been shown that the addition of LaNi 5 improves the absorption–desorption characteristics of magnesium towards hydrogen, the best achievements belonging to the composite containing 30 wt.% LaNi 5 . The data obtained on the changes occurring in the phase composition of the samples as a result of hydrogen cycling and available information from the literature have been used to explain the behaviour of the materials during hydriding. Comparison with results on composites with the same nominal composition obtained by other methods shows that mechanical alloying ensures preparation of samples with favourable kinetic characteristics which are hydrided under the softest possible conditions.

Hydriding and dehydriding characteristics of mixtures with a high magnesium content obtained by sintering and mechanical alloying

The hydriding and dehydriding kinetics of mixtures of magnesium and a LaNi5 type MM′5 alloy (where M is a mischmetal consisting of La, Ce, Pr and Nd, and M′ is nickel with Al, Co and Mn impurities) are investigated at various temperatures and pressures. Mixtures with the compositions 70wt% Mg + 30wt% MM′5 are prepared by (i) sintering and (ii) mechanical alloying of the components in a planetary mill. It is shown that addition of the alloy to magnesium improves its absorption-desorption characteristics towards hydrogen, this effect being more pronounced with the mechanically alloyed mixture. It is established that with an acceleration of 12g the best mechanically alloyed samples for hydrogen storage are obtained after 30 min grinding.

Hydrogen sorption properties of an Mg-Ti-V-Fe nanocomposite obtained by mechanical alloying

Abstract The absorption–desorption characteristics with respect to hydrogen of a magnesium-based nanocomposite obtained by high-energy ball milling have been investigated. The composite contains 5 wt.% (∼3at.%) Ti, 10 wt.% (∼5.5 at.%) V and 10 wt.% (∼5 at.%) Fe, of which the former two transition metals only form a binary hydride. It has been shown that at 623 K the composite may be hydrided up to a very high absorption capacity whose values remain appropriate for practical purposes even at much lower hydriding temperatures. Part of the iron present in the composite has been found to interact with magnesium and hydrogen under the hydriding conditions, the ternary hydride Mg 2 FeH 6 being formed. Its presence in the composite-hydrogen system has been assumed to be responsible for the reduced rate of hydrogen desorption from the particle surfaces and for some peculiarities of the composite behaviour during hydriding.

Hydriding of mechanically alloyed mixtures of magnesium with MnO2, Fe2O3 and NiO

The absorption-desorption characteristics towards hydrogen of mechanically alloyed mixtures containing 90 wt.% magnesium and 10 wt.% MnO2, Fe2O3 or NiO, respectively, have been investigated. Differences are established in the hydriding and dehydriding kinetics and the hydrogen capacity of the mixtures depending on the oxide component. It is shown that the highest absorption capacity corresponds to the MnO2-containing mixture. This sample also exhibits the highest hydriding and dehydriding rates. The mechanically alloyed NiO-containing sample has a lower hydrogen capacity, whereas the sample with Fe2O3 is characterized by the poorest desorption properties. The effects observed are explained on the basis of the difference in behaviour of the three oxides under the hydriding conditions.

Hydrogen sorption properties of the nanocomposites Mg–Mg2Ni1−xFex

Abstract The hydrogen sorption properties of the composites 85 wt.% Mg–15 wt.% Mg 2 Ni 1− x Fe x ( x =0, 0.1 and 0.3) prepared by high-energy ball milling were studied. It was shown that the presence of a Mg 2 Ni 1− x Fe x additive significantly improved the kinetics of hydrogen absorption and desorption of magnesium, the absorption capacity remaining sufficiently high even at temperatures between 423 and 573 K. The catalytic effect of the Mg 2 Ni 1− x Fe x intermetallics was explained as due to the formation of a second hydride, Mg 2 NiH 4 , to the appearance during the mechanical alloying of defects facilitating the formation of hydride nuclei and to the presence of Ni and Fe clusters on the surface of nanocomposite particles.

Hydrogen storage characteristics of magnesium mechanically alloyed with YNi5-xAlx (x=0, 1 and 3) intermetallics

The hydrogen sorption properties of the composite materials Mg+10 wt% YNi5−xAlx (x=0, 1 and 3) prepared by ball milling were studied. In the case of YNi2Al3 addition, mixtures with a nominal content of 1 and 50 wt% of the intermetallic compound were also investigated. The crystal structure of the YNi4Al phase was determined (CaCu5 type structure, a=0.4940 nm, c=0.4036 nm). It was established that the presence of YNi4Al or YNi2Al3 in the composites significantly improved the kinetics of hydrogen absorption and desorption, whereas YNi5 did not affect the sorption process. An explanation of the results obtained, based on the catalytic effect of the YNi4Al and YNi2Al3 intermetallics is proposed. It is shown that a higher intermetallic content of the composites is accompanied by: (i) an increase of the initial rate of their hydriding, (ii) a decrease of the maximum hydrogen capacity, and (iii) an increasing of the latent period of dehydriding.

On the hydriding and dehydriding kinetics of magnesium with a titanium dioxide admixture

Abstract The paper deals with the hydriding and dehydriding kinetics at different temperatures and pressures of mixtures consisting of 90 wt% Mg and 10 wt% TiO 2 (rutile) obtained (a) by mechanical alloying in a planetary mill and (b) by the usual procedure of grinding and homogenization. It is shown that the addition of TiO 2 improves the absorption and desorption characteristics of magnesium, this effect being more pronounced in the mechanically alloyed mixture. It is established that at all temperatures the highest absorption capacity of the samples corresponds to pressures close to the equilibrium one. The possible reasons for these effects are discussed.

Surface composition of MgTiO2 mixtures for hydrogen storage, prepared by different methods

Abstract The reasons for the observed improvement of hydrogen storage capacity of a MgTiO 2 mixture prepared by mechanical alloying under vacuum in comparison with the capacity of a mixture with the same composition obtained by the usual procedure of grinding and homogenization have been studied. In this connection, the surface composition of both mixtures has been investigated by X-ray photoelectron spectroscopy (XPS). The results obtained confirm an earlier suggestion concerning the formation of titanium forms of an oxidation degree lower than +4 during mechanical alloying. In addition, mechanical alloying is found to favour the appearance of a clean Mg surface without the usual thin oxide film.

Influence of hydriding on the magnetic properties of Gd3Ni6Al2

Abstract Some details of the synthesis peculiarities of the intermetallic Gd 3 Ni 6 Al 2 compound have been studied. It has been shown that this compound can only be prepared after prolonged annealing of a multiphase product obtained by melting of the constituent metals in a stoichiometric ratio. However, even after this treatment the samples contain some impurities, as proved by X-ray and electron microprobe analysis. As a result of a kinetic study on hydrogen uptake, the formation of a ternary hydride with a maximum content of 8.6 H atoms per formula unit of Gd 3 Ni 6 Al 2 has been confirmed. In this hydride the original cubic Ca 3 Ag 8 -type structure is preserved and only an increase of the unit cell parameter of about 1.45% is observed. Resistivity and low field magnetization measurements have shown the occurrence of ferromagnetic ordering in Gd 3 Ni 6 Al 2 at T c = 118(1) K. The value of M sat = 7.34(4) B = 4.8 T, which is very close to the saturation moment of 7.0 μ B calculated for Gd 3 ion. Hydrogen absorption leads to an important decrease of the Curie temperature ( T c = 69(1) K for Gd 3 Ni 6 Al 2 H 8.6 ). Similarly to the parent compound, a value of 7.0(1) μ B /Gd is obtained for the saturation moment of the hydride at B = 4.8 T. An attempt is made to explain this behaviour by a non-uniform spatial distribution of H atoms in the hydrided sample as well as by a weakening of the GdGd interactions in this sample caused by the increase of the lattice constant.

Effect of the partial replacement of magnesium by nickel on the properties of some lanthanum-magnesium alloys for hydrogen storage

Abstract The absorption-desorption properties of LaMg 11 Ni and La 0.9 Ca 0.1 Mg 11 Ni alloys with respect to hydrogen are investigated. The results obtained on the hydrogen desorption rate from the hydrided alloys are compared with data on this rate found with the isostructural alloys LaMg 12 and La 0.9 Ca 0.1 Mg 12 as well as with the binary alloy La 2 Mg 17 and its analogues La 2 Mg 16 Ni, La 1.8 Ca 0.2 Mg 17 and La 1.8 Ca 0.2 Mg 16 Ni. It is shown that the partial substitution of nickel for magnesium in lanthanum-magnesium alloys causes acceleration of the desorption of hydrogen stored in them. The extent of this effect depends on the number and character of phases formed during hydriding of the alloys.