Appearance of a Novel Pressure Plateau in RNi 5 -H (R = Rare Earth) Systems

We investigated the hydrogen storage properties in a HoNi 5 -H system combined with TbNi 5 -H and DyNi 5 -H systems. Pressurecomposition (P-C) isotherms in the HoNi 5 -H system show one reversible pressure plateau during hydrogen absorption and desorption with a hysteretic phase transition. The maximum hydrogen storage capacity was found to be H/HoNi 5 ¼ ca. 2, and no second plateau similar to that observed in other RNi 5 -H (R: rare earth) systems was observed, even at 196 K. Two pressure plateaux found in light rare earth-based R L Ni 5 -H (R L ¼ La, Pr, Nd, Sm and Gd) systems tend to disappear in TbNi 5 -H and DyNi 5 -H systems and are scarcely present in HoNi 5 -H system. On the other hand, a novel pressure plateau appears at low hydrogen content in these systems due to the presence of a new hydride phase, and the plateau region extends from TbNi 5 -H and DyNi 5 -H to HoNi 5 -H systems. Contrary to the other plateaux, the pressures of the novel plateau during both hydrogen absorption and desorption decrease with decreasing unit cell volume of RNi 5 compounds or with increasing atomic number of the R element in RNi 5 -H systems. To clarify the effect of 4f electrons in the R element in RNi 5 -H systems on the novel plateau, we have evaluated the hydrogen storage properties in an YNi 5 -H system. Our results show that YNi 5 compound with lattice constants similar to those of TbNi 5 and DyNi 5 compounds has a similar P-C isotherm, suggesting that 4f electrons has no direct influence on the appearance of the novel plateau

Hydrogen storage properties and corresponding phase transformations of MG/PD laminate composites prepared by a repetitive-rolling method

Mg/Pd laminate composites (Mg/Pd = 6) prepared by a repetitive-rolling method can reversibly absorb and desorb a large amount of hydrogen, up to 1.47 H/M (4 mass%) at 573 K. Pressure-composition isotherms of the Mg/Pd laminate composites show two plateaux, P L ¼ 0:2 MPa and P H ¼ 2 MPa, during hydrogen absorption and desorption. To clarify the correlation between hydrogen storage properties and phase transformations, we investigated structural changes of the Mg 6 Pd intermetallic compound with in-situ XRD. The low-pressure plateau P L corresponds to the decomposition of Mg 6 Pd into Mg 5 Pd 2 and MgH 2 , and the high-pressure plateau P H to the decomposition of Mg 5 Pd 2 into MgPd and MgH 2 . In subsequent dehydrogenation processes, part of the MgH 2 reformed Mg, and the Mg and MgPd form Mg 5 Pd 2 at the highpressure plateau P H and then the remaining MgH 2 reformed Mg, and the Mg and Mg 5 Pd 2 form Mg 6 Pd at the low-pressure plateau P L . According to this mechanism, the Mg 6 Pd can absorb and desorb hydrogen through reversibly disproportionation and recombination processes

Observation of hydrogen absorption/desorption reaction processes in Li-Mg-N-H system by in-situ X-ray diffractmetry

The in-situ XRD measurements on dehydrogenation/rehydrogenation of the Li-Mg-N-H system were performed in this work. The ballmilled mixture of 8LiH and 3Mg(NH2)2 as a hydrogenated phase gradually changed into Li2NH as a dehydrogenated phase during heat-treatment at 200 °C in vacuum for 50 h. Neither Mg-related phases nor other intermediate phases were recognized in the dehydrogenated phase. With respect to the hydrogenation process, the dehydrogenated state gradually returned to the mixed phase of the LiH and Mg(NH2)2 without appearance of any intermediate phases during heat treatment at 200 °C under 5 MPa H2 for 37 h and during slow cooling down to room temperature through 24 h. In the hydrogenation process at 200 °C under 1 MPa H2, however, the growing up of the LiNH2 and LiH phase was observed in the XRD profiles before the 3Mg(NH2)2 and 8LiH phases were formed as the final hydrogenated state. This indicates that the LiNH2 and LiH phase essentially appears as an intermediate state in the Li-Mg-N-H system composed of 3Mg(NH2)2 and 8LiH

The Origin of Highly Crystallized Face-Centered Cubic YH3 High-Pressure Phase when quenched to ambient condition

高圧相であるfcc構造のYH3が序温情圧下に回収される起源を透過型電子顕微鏡、X線回折、放射光その場観察技術により調べた。試料の急冷によって導入される欠陥が常圧相であるhcp構造への相転移を抑制していることが分かった

Hydrogenation and Dehydrogenation Properties of R H Ni 5 (R H = Heavy Rare Earth) Binary Intermetallic Compounds * 1

We systematically investigated the hydrogenation and dehydrogenation properties for heavy rare earth-based binary R H Ni 5 (R H = Gd, Tb and Dy) intermetallic compounds and evaluated the correlations between crystallographic and thermodynamic properties. XRD analysis shows that all R H Ni 5 compounds crystallize in the hexagonal CaCu 5 -type crystal structure. In analogy to the light rare earth-based R L Ni 5 (R L = La, Pr, Nd and Sm) compounds both lattice constants of R H Ni 5 compounds decrease with increasing the atomic number of R H element due to the lanthanide contraction. On the pressure-composition (P-C) isotherms, GdNi 5 -H 2 system shows two well-separated pressure plateaux qualitatively similar to R L Ni 5 -H 2 systems. Looking over from Gd to Dy in the R H Ni 5 compounds, we find three specific dehydrogenation properties on the P-C isotherms: 1. The first plateau pressure (p P1 ) increases in this order (at around H/R H Ni 5 = 2.5) due to less stability of hydrogen in the unit cell by the lanthanide contraction. Linear correlations are also observed between log p P1 and the unit cell volume (V) which fall onto the same lines extrapolated from those observed in case of the R L Ni 5 compounds. 2. The second plateau (P2) tends to disappear because the P-C isotherm goes beyond the critical point of the phase transition. 3. Fairly flat first plateau separates into two parts in which a new plateau (PN) appears at low hydrogen content (H/R H N

Stability of Zirconium-Substituted Face-Centered Cubic Yttrium Hydride

The stability of hydrogen in a zirconium (Zr)-substituted face-centered cubic (FCC) yttrium (Y) hydride (ZSY) phase was investigated experimentally and theoretically. Two possible sites for hydrogen atoms exist in the FCC structure, namely the T- and O-sites, where hydrogen is present at the center of the tetrahedron and the octahedron composed of Y and/or Zr metals. The P-C isotherms revealed that the hydrogen content per metal (H/M) with 33% Zr-substituted YH3−δ was 2.2-2.3, which was lower than the expected value calculated from the starting composition of YH3-33%ZrH2 (Y0.67Zr0.33H2.67) and 2H MAS. An NMR study revealed that the deuterium (hydrogen) at the O-site in ZSY mainly reacted during the deuterium (hydrogen) desorption/ absorption process. Based on a theoretical analysis, when at least two of the six vertices of the octahedron were composed of Zr, its center, i.e., H at the O-site, became unstable. Such unstable O-sites, where more than two Zr is coordinated, nonlinearly increased with an increasing in the Zr content, and when the Zr content was over 50%, almost all of the O-sites were unstable. The theoretical discussion supported the experimental results, and the Zr substitution was confirmed to reduce the stability of the H at the O-site in the FCC YH3 significantly