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Yamagata-shi, Japan

Park I.,University of Tokyo | Terashita N.,Japan Metals and Chemicals Co. | Abe E.,University of Tokyo
Journal of Alloys and Compounds | Year: 2013

Using transmission electron microscopy (TEM), we systematically investigate hydrogenation-induced microstructural changes of pseudo-binary (Pr xMg1-x)Ni2 Laves compounds varying x from 0.3 to 1.0, which lead to the averaged constituent atomic-size ratio RA/RB ranging 1.34-1.47 of the supposed AB2 compound. It is empirically known for the AB2 Laves compounds that hydrogen-induced-amorphization (HIA) takes place when the RA/R B exceeds 1.37. We find that, based on careful analyses of electron diffraction patterns and TEM images, the hydrogenation-processed microstructures of the (PrxMg1-x)Ni2 compounds exceeding the critical ratio are not pure amorphous but composed of Ni nano-crystals embedded in an amorphous matrix of hydride PrH2. This provides a direct evidence of hydrogenation-induced micro-phase separation (HIMPS), and accordingly it is suggested that HIA believed so far should be attributed to HIMPS phenomenon. © 2013 Elsevier B.V. All rights reserved. Source


Iwase K.,Ibaraki University | Terashita N.,Japan Metals and Chemicals Co. | Mori K.,Kyoto University | Yokota H.,Ibaraki University | Suzuki T.,Ibaraki University
Inorganic Chemistry | Year: 2013

The hydrogen absorption-desorption property and the crystal structure of Pr4MgNi19 was investigated by pressure-composition isotherm measurement and X-ray diffraction (XRD). Pr4MgNi 19 consisted of two phases: 52.9% Ce5Co19-type structure (3R) and 47.0% Gd2Co7-type structure (3R). Sm5Co19-type structure (2H) and Ce2Ni 7-type structure (2H) were not observed in the XRD profile. The Mg atoms substituted at the Pr sites in a MgZn2-type cell. The maximum hydrogen capacity reached 1.14 H/M (1.6 mass%) at 2 MPa. The hysteresis factor, Hf = ln(Pabs/Pdes), was 1.50. The cyclic hydrogenation property of Pr4MgNi19 was investigated up to 1000 absorption-desorption cycles. After 250, 500, 750, and 1000 cycles, the retention rates of hydrogen were reduced to 94%, 92%, 91%, and 90%, respectively. These properties were superior to those of Pr2MgNi 9 and Pr3MgNi14. © 2013 American Chemical Society. Source


Kim H.,Japan National Institute of Advanced Industrial Science and Technology | Sakaki K.,Japan National Institute of Advanced Industrial Science and Technology | Ogawa H.,Japan National Institute of Advanced Industrial Science and Technology | Nakamura Y.,Japan National Institute of Advanced Industrial Science and Technology | And 5 more authors.
Journal of Physical Chemistry C | Year: 2013

Reduction in reversible hydrogen storage capacity with increasing hydrogenation and dehydrogenation cycle number is observed in numerous hydrogen storage materials, but the mechanism behind this unfavorable change has not been elucidated yet. In this study, we have investigated the development of structural defects or disorders in V1-xTixH2, x = 0, 0.2, and 0.5, during the first 15 hydrogen absorption and desorption cycles using the atomic pair distribution function (PDF) analysis of synchrotron X-ray total scattering data to find out the possible structural origin of the poor cyclic stability of V1-xTix alloys. While pure vanadium shows no significant change in the PDF, alloy samples subject to several hydrogenation and dehydrogenation cycles display fast decaying of the PDF profile due to a progressive increase in the PDF peak width with increasing r. This r-dependent PDF peak broadening effect becomes stronger with cycle number. Molecular dynamics (MD) simulations demonstrated that dislocation defects explain characteristic features in our experimental PDFs very well and suggested that a large number of dislocations are formed during hydrogen cycling. We found there is a close relation between the reduced amount of the reversible hydrogen content of V0.8Ti0.2 and the amount of generated dislocations. On the basis of the PDF analysis results, a possible mechanism behind degradation in the reversible hydrogen storage capacity of V1-xTix is discussed. © 2013 American Chemical Society. Source


Iwase K.,Ibaraki University | Terashita N.,Japan Metals and Chemicals Co. | Mori K.,Kyoto University | Ishigaki T.,Ibaraki University
Inorganic Chemistry | Year: 2012

Structural parameters of Pr 3MgNi 14 after a cyclic hydrogen absorption-desorption process were investigated by X-ray diffraction. Pr 3MgNi 14 consisted of two phases: 80% Gd 2Co 7-type structure and 20% PuNi 3-type structure. The pressure-composition (P-C) isotherm of Pr 3MgNi 14 indicates a maximum hydrogen capacity of 1.12 H/M (1.61 mass%) at 298 K. The cyclic property of Pr 3MgNi 14 up to 1000 cycles was measured at 313 K. The retention rate of the sample was 87.5% at 1000 cycles, which compares favorably with that of LaNi 5. After 1000 cycles, the expansions of lattice parameters a and c and the lengths along the c-axes of the PrNi 5 and PrMgNi 4 cells of the Gd 2Co 7-type structures were 0.20%, 1.26%, 0.47%, and 3.68%, respectively. The metal sublattice expanded anisotropically after the cyclic test. The isotropic and anisotropic lattice strains can be refined by Rietveld analysis. The anisotropic and isotropic lattice strains were almost saturated at the first activation process and reached values of 0.2% and 0.1%, respectively, after 1000 cycles. These values are smaller by 1 order of magnitude than those of LaNi 5. © 2012 American Chemical Society. Source


Sakaki K.,Japan National Institute of Advanced Industrial Science and Technology | Terashita N.,Japan Metals and Chemicals Co. | Tsunokake S.,Japan Metals and Chemicals Co. | Nakamura Y.,Japan National Institute of Advanced Industrial Science and Technology | And 2 more authors.
Journal of Physical Chemistry C | Year: 2012

The effect of the rare earth elements and alloy composition on the hydrogenation properties and crystal structures of hydrides in Mg 2-xRE xNi 4 (RE = La, Pr, Nd, Sm, and Gd; x = 0.6 and 1.0) was investigated. All Mg 2-xRE xNi 4 alloys had a C15b Laves phase before hydrogenation. Mg 1.4RE 0.6Ni 4 (RE = Pr, Sm, and Gd) alloys were hydrogenated through one plateau to form Mg 1.4RE 0.6Ni 4H ∼3.6 while maintaining the C15b structure. Mg 1.0RE 1.0Ni 4 (RE = La, Pr, and Nd) alloys were hydrogenated to ∼1.0 H/M proceeding through two plateaus, and Mg 1.0RE 1.0Ni 4 (RE = Sm and Gd) alloys were hydrogenated to 0.6-0.7 H/M through one plateau. Mg 1.0RE 1.0Ni 4 alloys initially transformed into Mg 1.0RE 1.0Ni 4H ∼4 with an orthorhombic structure. In addition it was experimentally confirmed that Mg 1.0RE 1.0Ni 4H ∼4 with La, Pr, and Nd transformed into Mg 1.0RE 1.0Ni 4H ∼6 with a C15b structure, while no formation of Mg 1.0RE 1.0Ni 4H ∼6 (RE = Sm and Gd) was observed at 40 MPa at 250 K. Theoretical calculations suggest that Mg 1.0RE 1.0Ni 4H ∼4 with Sm and Gd also transform to Mg 1.0RE 1.0Ni 4H ∼6 at higher pressures than those used in our experiments (264 MPa for Mg 1.0Sm 1.0Ni 4 and 8.5 GPa for Mg 1.0Gd 1.0Ni 4 at 253 K). It was found that the hydrogenation properties and crystal structure of the hydrides in Mg 2-xRE xNi 4 are dependent on the alloy composition, i.e., the ratio of Mg to RE in the alloy phase, but independent of the choice of rare earth element. © 2012 American Chemical Society. Source

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