Etsuo Akiba

Study of Mg-M (M = Co, Ni and Fe) mixture elaborated by reactive mechanical alloying: hydrogen sorption properties

Abstract Reactive Mechanical Alloying (MA in H2) of magnesium powder allows to obtain partial conversion of Mg to MgH2. The ratio MgH2/Mg can be improved by increasing the milling time or by adding a 3d-element (with the same initial particle size). X-ray diffraction was applied successfully to determine the quantity of MgH2 formed. The RMA allows to suppress the activation procedure generally requested for magnesium. The hydriding is determined to be a two step process: first nucleation and then diffusion. A direct relationship exists between the nucleation duration and the specific surface. It is demonstrated that RMA for short time (2 h) is an effective way to strongly improve the hydrogen storage properties of both magnesium and ( Mg +10 wt % M ) mixtures.

Hydrogen absorption and modulated structure in Ti–V–Mn alloys

Abstract In order to apply hydrogen-absorbing alloys to new energy carrier uses, such as hydrogen storage tanks or Ni–MH batteries, a drastic increase in the hydrogen capacity of these alloys is required. For years, intermetallic compounds or single-phase alloys have been studied as hydrogen-absorbing materials. New approaches to alloy design, relating to multiphase alloys, are proposed and verified. These approaches help to identify a new structure on a multiphase alloy. As a result, a new hydrogen-absorbing alloy, consisting of a nano-structure having a large hydrogen capacity and good desorbing properties, was found.

The hydrogen storage characteristics of Ti–Cr–V alloys

Abstract The crystal structures, the lattice parameters and the characteristics of hydrogen storage at 303 K have been investigated in ternary alloys of the Ti–Cr–V system. All of these alloys, in the range of this study, have shown a BCC structure. The hydrogen storage capacities and the effective hydrogen storage capacities of the alloys were strongly dependent on the composition ratio of Ti/Cr, showing their maximum values at a Ti/Cr ratio of about 0.75. It was also found that the lattice parameters of the alloys increased linearly with increasing Ti/Cr ratio. The differences in affinities to hydrogen and the atomic radius of each element could explain the Ti/Cr ratio dependence of the lattice parameter and hydrogen storage capacity of the alloys.

Direct synthesis of Mg2FeH6 by mechanical alloying

Abstract The hydride Mg 2 FeH 6 was synthesized by high-energy ball milling of MgH 2 and Fe under argon atmosphere without subsequent sintering. After 60 h of milling, 56% wt. of Mg 2 FeH 6 was synthesized. This yield was deduced from Rieltveld analysis of the X-ray powder measurements and confirmed by pressured differential scanning calorimeter (PDSC). Hydrogen capacity measurements indicated that the loss of capacity with cycling is minimal.

Preparation of the hydrides Mg2FeH6 and Mg2CoH5 by mechanical alloying followed by sintering

Mg2CoH5 and Mg2FeH6 were prepared by mechanical alloying (MA) and followed by sintering under mild conditions. The effect of milling atmosphere on the end products was also studied. The mixtures 2Mg + Fe and 2Mg + Co were milled in a planetary ball mill under hydrogen and argon atmospheres. After milling, the mixtures were sintered at 623 K for 1 day under 50 bars of hydrogen. X-ray powder diffraction analysis was used not only for identification of phases formed but also for crystal structural refinement and mass fraction calculation using the Rietveld method. We found that, under the above sintering conditions, the hydrides Mg2FeH6 and Mg2CoH5 were produced only in samples that were milled prior to sintering. For 2Mg + Fe mixture, milling under hydrogen prior to sintering produced 84% Mg2FeH6 in the product excluding oxide. The mixture 2Mg+Co was milled and sintered producing MgH2 and Mg2CoH5, but the mass fraction of Mg2CoH5 was independent of the milling atmosphere.

Kinetics of the reaction between MgNi alloys and H2

The kinetics of the hydriding and dehydriding reaction of Mg2Ni and Mg-10 wt % Ni was investigated. The reaction rate at various hydride compositions was measured by the pressure change in a constant volume. In the case of Mg2Ni, at the beginning of the reaction the amount of absorbed/desorbed hydrogen can be expressed by n = k(p0 – peq) (ns – n0), where n is the ratio of H to Mg in the solid, p0 and peq are the initial hydrogen pressure of the system and the equilibrium pressure, and n0 and ns are the values of n at the start of the reaction and a constant around 2, respectively. After a transient period of approximately 1 min, a diffusion-controlled stage took place as dn/dt = k′(p - peq)/t, where p is the pressure at time t. In the case of Mg-10 wt % Ni, throughout a run the absorption/desorption rate was expressed by dn/dt = k″ (p - peq)1.25.

1H, 7Li, and 19F nuclear magnetic resonance and ionic conductivity studies for liquid electrolytes composed of glymes and polyetheneglycol dimethyl ethers of CH3O(CH2CH2O)nCH3 (n=3–50) doped with LiN(SO2CF3)2

Solid polyethyleneoxides (PEO) are in effect “polymer liquids” due to their flexibility and high solubility for alkaline salts. To clarify the role of PEO in electrolyte systems, electrolytes composed of members of the “glyme family” [i.e., diglyme(DG), triglyme(TG), tetraglyme(TeG), pentaglyme(PG), and polyethyleneglycol dimethyl ethers(PEGDM)] doped with LiN(SO2CF3)2 were investigated. PEGDMs form a series of low molecular weight PEO-like homologues with molecular weights between 400 and 2500. Electrolytes of the glymes and PEGDMs were prepared for two salt concentrations (ether oxygen: lithium; O:Li) 20:1 and 10:1. The ionic conductivities and the self-diffusion coefficients of the solvent, anion and lithium ions in the electrolytes were measured using pulsed-field gradient spin-echo (PGSE) 1H, 19F, and 7Li NMR, respectively. From the spin-lattice relaxation times (T1) it was found that the segmental motions of the CH2CH2O moiety and the lithium hopping motions are correlated and that the rate of the s...

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.

H2 Absorbing-desorbing characterization of the TiVFe alloy system

Abstract Twenty six kinds of alloys with compositions Ti(33–47 mol%)-V(42–67 mol%)-Fe(0–14 mol%) were tested to find the most suitable composition for hydrogen absorbtion (including tritium recovery and storage). The best alloy composition was found to be Ti 43.5 V 49.0 Fe 7.5 , and this alloy was able to absorb hydrogen up to 3.90 wt% (H/M= 1.90) at 253 K. The hydrogen capacity was estimated to be 2.4 wt% when hydrogen was absorbed at 253 K and desorbed at 573 K at 1 atm hydrogen pressure. The hydrogen absorption rate at 253 K was faster than that of the LaNi 5 H 2 system at 293 K. Conspicuous changes in pressure-composition ( p-c ) isotherms were not found after 50 cycles of hydrogen absorption and desorption. All dehydrided alloys are bcc, dihydrides of the alloy are fcc. Among the series of Ti V X alloys (X = Fe, Co, Ni, Cr and Pd), iron was found to be the best substituent element for X. Two kinds of industrial ferrovanadium alloys containing about 4 wt% of impurities (Al, Si, etc.) were tested to prepare the alloys. Remarkable differences in the p-c isotherms were observed which may be due to the effect of impurities in the ferrovanadium.

New approach for synthesizing Mg-based alloys

Abstract Mg 2 Ni, Mg 2 Cu and MgZn 2 were synthesized by means of ball-milling plus solid-state reactions. Firstly, the mixtures of the two elemental powders, corresponding to the compositions of Mg 2 Ni, Mg 2 Cu and MgZn 2 , were subject to ball-milling for 20 min. Then, they were pressed into pellets and sintered at temperatures below the melting points of the components. X-ray diffraction experiments show that the yields of the desired products are above 97 wt.%, higher than those prepared by the conventional metallurgical method. The hydrogenation properties of the Mg 2 Ni sample synthesized by solid-state reaction are in agreement with the previously published results on alloys prepared by the conventional methods.