M. Terzieva

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.

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 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.

Calcium-substituted lanthanum-magnesium alloys for hydrogen storage

Abstract Alloys with the general formula La 2− x Ca x Mg 17 (0.1 ⩽ x ⩽ 1.0) are prepared. At x values of 0.1, single-phase products are synthesized whose X-ray spectrum is identical with that of the alloy La 2 Mg 17 , whereas at x >0.8 the substance obtained is a mixture of La 2 Mg 17 , CaMg 2 and Mg. At x values ranging from 0.2 to 0.6, the X-ray spectra of the alloys show, in addition to the main phase La 2 Mg 17 , weak peaks of an unstable phase of the type Ce 5 Mg 41 . The absorption and desorption characteristics towards hydrogen of the La 1.8 Ca 0.2 Mg 17 and La 1.6 Ca 0.4 Mg 17 alloys are determined. It is found that under the same hydriding conditions these alloys absorb smaller hydrogen amounts than does the pure La 2 Mg 17 alloy, but hydrogen desorption from them proceeds with a considerably higher rate than from La 2 Mg 17 . The probable causes of these effects are discussed.

Multiphase alloys La2−xCaxMg17 for hydrogen storage

Abstract Hydrogen storage by alloys with composition La2−xCaxMg17 (1.4 ⩽ x ⩽ 1.8) representing a mixture of the phases La2Mg17, CaMg2 and magnesium in various ratios has been investigated. It is found that these alloys have good absorption-desorption characteristics and exhibit the highest hydrogen capacity at the lowest hydriding pressure (5 bar). Prolonged cycling with the alloy La0.4Ca1.6Mg17 has shown an initial decrease in its absorption capacity till the 80–100th cycle, after which it remained constant and fairly high till the 250th hydriding-dehydriding cycle. The desorption characteristics of the alloys under investigation are close to those of single-phase alloys with the same general formula and a high lanthanum content (x

On the hydriding kinetics of the alloys La2Mg17 and La2−xCaxMg17

Abstract The hydriding kinetics of the alloys La2Mg17, La1.8Ca0.2Mg17 and La1.6Ca0.4Mg17 are studied using experimental data from previous papers [Khrussanova et al., J. Less-Common Metals86, 153 (1982); Khrussanova et al., Int. J. Hydrogen Energy10, 591 (1985)] and the kinetic equations proposed by Park and Lee [J. Less-Common Metals83, 39 (1982)] and Song and Lee [Int. J. Hydrogen Energy8, 363 (1983)]. It is established that at low values of the reacted fraction (0

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.

Dehydriding kinetics of mechanically alloyed mixtures of magnesium with some 3d transition metal oxides

The dehydriding kinetics of mechanically alloyed mixtures of 90 wt% Mg and 10 wt% of Fe 2 O 3 or MnO 2 , respectively, have been investigated. It is estabished that with both mixtures the dehydring rate is limited by an intrinsic process. In the case of Mg-Fe 2 O 3 , when the reaction transformed function F<0.4, this process is the surface conversion β→α, the desorption being hindered probably by the formation of Mg 2 FeH x . Depending on temperature and pressure, the rate controlling step of dehydriding for mixtures of 90 wt% Mg+10 wt% MnO 2 and the same F values can be either the β→α phase transition or the hydrogen diffusion through the α phase layer

A Mössbauer study of a hydrided mechanically alloyed mixture of magnesium and iron(III) oxide

Abstract It has been shown by Mossbauer spectroscopy that when a mechanically alloyed mixture of 90 wt.% Mg and 10 wt.% Fe203 is hydrided at 350°C and a pressure of 20 bar, the reaction products contain, in addition to magnesium hydride, a ternary hydride of magnesium and iron, Mg 2 FeH 6 . The latter is formed during interaction between hydrogen, magnesium and the iron obtained in the system by reduction of Fe 2 O 3 . The higher stability of Mg 2 FeH 6 in comparison with MgH 2 is the reason for the slower hydrogen desorption from the hydrided mixture than that observed for similar mixtures of magnesium and oxides of other 3d metals.