J.-L. Bobet

Addition of nanosized Cr2O3 to magnesium for improvement of the hydrogen sorption properties

Abstract The combination of (i) the catalytic effects of Cr2O3, (ii) reactive mechanical grinding (RMG), and (iii) nanosize particles allow a huge improvement in the sorption properties of magnesium. For short milling duration, the absorption kinetics were already as good as that reported for nanocrystalline (Mg+Cr2O3) mixtures. The desorption process is also improved but not in a similar manner. It is assumed that the RMG of nanosized particles of Cr2O3 results in the formation of some Cr atoms in the mixture which greatly influence the sorption behavior.

Synthesis of magnesium and titanium hydride via reactive mechanical alloying: Influence of 3d-metal addition on MgH2 synthesize

TiH2 and MgH2 have been synthesized by Reactive Mechanical Alloying (MA in H2 or RMA) at room temperature starting from Ti and Mg powders. Ultrafine powder of TiH2 was obtained after short milling time (<1 h for Ti powder and <5 h for Ti sponge). A complete conversion of α-Ti to TiH2 was achieved. RMA of Mg powder only lead to a partial conversion of Mg to MgH2. However the ratio MgH2/Mg can be improved by increasing the milling time or by adding a 3d-element. X-ray diffraction was applied successfully to determine the quantity of MgH2 formed.

Investigations on the synthesis, structural and microstructural characterizations of Mg-based K2PtCl6 type (Mg2FeH6) hydrogen storage material prepared by mechanical alloying

Abstract This paper deals with the formation of new ternary hydride Mg 2 FeH 6 (K 2 PtCl 6 type) in a single-step procedure following the process of mechanical alloying of initial stoichiometric ingredients Mg and Fe under hydrogen. The optimum yield of formation of single phase Mg 2 FeH 6 was achieved by hydrogen (∼10 atm.) milling of constituent elements at a speed of 400 rpm for various milling durations. The structural characterization of the ball-milled (2 Mg+Fe) powder was carried out using Philips X-ray diffractometer by taking samples from the attritor mill at regular intervals of time. It was found that the Mg 2 FeH 6 phase starts forming at a milling duration of 14 h and the optimum Mg 2 FeH 6 phase formation was obtained at 20 h. The proportion of this phase was estimated by employing Rietveld refinement analysis of the X-ray powder diffraction data and it was found to be 63%. This is the highest phase proportion reported so far for the Mg 2 FeH 6 phase when formed from elemental Mg and Fe following the route of mechanical alloying. Together with the Mg 2 FeH 6 phase, some quantity of Fe (about 37%) is also present. Fe, being a magnetic impurity, can be removed leaving the Mg 2 FeH 6 content to be nearly 90–100%. However, such purification was not done in the present investigation. We also investigated synthesis of the material obtained by longer milling durations of 25, 28 and 30 h. The XRD patterns for the 25, 28 and 30 h ball-milled materials revealed that the intensity of Mg 2 FeH 6 peaks is reduced in comparison to the Fe peaks. This implies that beyond 20 h, there is no further increase in the phase proportion and the amorphization starts taking place. The post-sintering process of these mechanically alloyed samples did not improve the Mg 2 FeH 6 phase proportion and yield as evidenced by XRD. The X-ray structural characterizations revealed that the as-milled Mg 2 FeH 6 material (milling duration of 20 h; under H 2 pressure ∼10 atm., speed ∼400 rpm) corresponds to the known face centered cubic structure with lattice parameter a =0.6446(2) nm. The elemental (chemical) compositional analysis was carried out for the mechanically alloyed Mg 2 FeH 6 materials using the EDAX technique. The results confirm the correct stoichiometric ratio of the initial mixture (2Mg+Fe). The surface morphologies of the (2Mg+Fe) mixture before and after mechanical alloying are performed using scanning electron microscopic technique. The SEM explorations reveal the spongy like feature of Mg 2 FeH 6 agglomerates.

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.

Crystallographic and hydrogen sorption properties of TiMn2 based alloys

Microprobe analysis and X-ray powder diffraction were performed for a series of intermetallic compounds Ti0.95Zr0.05Mn1.45M0.5 where M=V, Cr, Mn, Co, Ni and Al. These compounds crystallize in the hexagonal C14-type Laves phase. The study of their hydriding properties at 298K shows that the intermetallics based on V and Cr present the best behavior (insertion of 3 H per formula unit and decrease of the plateau pressure to around 2.2 and 2.6 MPa, respectively). These properties can be rationalized by the crystal structure investigation.

Hydrogen absorption properties of CeNiAl: influence on its crystal structure and magnetic behaviour

CeNiAl absorbs up to 1.93(5) hydrogen per formula unit at room temperature and at a pressure of 1 MPa. This hydride is stable in air. Hydrogenation induces both: (i) a structural transition hexagonal ZrNiAl-type→hexagonal AlB 2 -type; (ii) a valence transition for cerium. At low temperature for T<7 K. the hydride CeNiAlH 1.93 shows a spin fluctuation behaviour.

Improvements of hydrogen storage properties of Mg-based mixtures elaborated by reactive mechanical milling

The influence of mechanical grinding under hydrogen (Reactive Mechanical Grinding=RMG) on the chemical properties (crystallographic and phase composition) and on the hydrogen storage properties of Mg-based mixtures is examined. Different additives were studied such as: Co, YNi, oxides, BN. All these additives lead to an unequal improvement in the hydriding properties. For Cr2O3, the effect of both the morphology and crystallinity were studied by elaborating Cr2O3 by supercritical fluid process.

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.

Influence of the mechanical grinding on the magnetic properties of GdMn2

Abstract The cubic C15 phase GdMn2 was submitted to high-energy ball milling. The resulting products were investigated by X-ray powder diffraction, scanning electron microscopy, ac-magnetic susceptibility and dc-magnetization measurements. With increasing speed of the milling treatment, amorphization of the sample appears, as shown by X-ray diffraction analysis. Furthermore, this treatment induces a magnetic transition from antiferromagnetism (unmilled sample) to ferromagnetism below TC≅105(5) K (milled sample at high speed). This effect is compared with that of the application of hydrostatic pressure on GdMn2.