Shin-ichi Orimo

Effects of nanometer-scale structure on hydriding properties of MgNi alloys: a review

The materials, which are produced by mechanical grinding/alloying, possess superior hydriding properties depending on their nanometer-scale structures. In this review paper, we present our recent works on the hydriding properties and nanometer-scale structures of some mechanically produced Mgue5f8Ni alloys. The first topic is the properties of some nanostructured Mg2Ni (Mg-33 at%Ni) consisting of nanometer-scale crystallite (intra-grain) and inter-grain regions. In this system, the existence of the inter-grain region gives rise to notable hydriding properties of the nanostructured Mg2Ni such as the enhanced hydrogen dissolution and low-temperature dehydriding. The second topic is the properties of the alloys Mg-33, 38, 43 and 50 at% Ni in which a different amount of the amorphous (a-)MgNi phase depending on Ni content is homogeneously dispersed around nanometer-scale Mg2Ni crystallites. In this system, the total hydrogen content increases with increasing Ni content. This experimental result is well explained by a model based on three nanostructural constituents of these alloys and their hydriding properties.

Hydrogen in mechanically prepared nanostructured h-BN: a critical comparison with that in nanostructured graphite

Nanostructured h-BN was prepared by mechanical milling under hydrogen atmosphere. The hydrogen concentration reaches up to 2.6 mass% after milling for 80 h, and this value corresponds to ca. 35% of that of nanostructured graphite as was previously reported. In addition to the hydrogen desorption starting at about 570 K, nitrogen desorption was also detected at about 700 K. There was no recrystallization phenomenon at least below 1173 K. The dissimilarities on the (de-)hydriding properties between nanostructured h-BN and graphite might be due to the different local electronic structure near the specific defects.

Location of deuterium atoms absorbed in nanocrystalline graphite prepared by mechanical alloying

Abstract Neutron diffraction is an important technique for studying the structure of hydride materials. H/D isotopic substitution was employed to observe the location of deuterium atoms in deuterated nano-graphite because the coherent scattering length of deuterium is large enough to be observed in comparison with that of the metal atoms forming hydrogen absorbing materials. The preparation of the nano-crystalline graphite and the absorption of deuterium atoms in the graphite were carried out simultaneously by mechanical alloying under D 2 gas atmosphere. The transformation from the hexagonal graphite as a starting material into amorphous-like carbon was observed by neutron diffraction. The conformation in the graphite was changed by the creation of dangling bonds and deuterium was absorbed by the solid-gas reaction when the milling proceeded. Two types of the deuterium coordinations were found in radial distribution function, RDF(r) . One is the C–D covalent bond and the other is deuterium located between layers of the graphite.

Hydrogen storage properties in nano-structured magnesium- and carbon-related materials

Abstract We review hydrogen-storage properties of two nano-structured systems; multi-layered Pd/Mg films prepared by the RF sputtering method and mechanically milled graphite under a high hydrogen atmosphere. The former prepared under optimized RF sputtering conditions absorbs hydrogen of ∼5xa0mass% at 373xa0K under 0.1xa0MPa hydrogen pressure, and desorbs all the hydrogen at 360xa0K in vacuum. The latter prepared by milling for 80xa0h absorbs hydrogen up to 7.4xa0mass%. Hydrogen molecules are desorbed by the two processes above at 600 and 950xa0K, respectively.

Hydrogen interaction with carbon nanostructures - current situation and future prospects

Recent research on hydrogen in various carbon nanostructures is reviewed. Based on these research activities, we focus on a defect mediated hydrogen sorption in carbon nanostructures. Mechanically prepared nanostructured graphite has been reported to exhibit a specific interaction with hydrogen, probably due to the partial formation of the defect mediated hydrogen sorption. Current situations and future prospects of carbon nanostructures providing hydrogen storage functions are critically, but still positively, described in this paper.

NMR studies of hydrogen motion in nanostructured hydrogen–graphite systems

Abstract Nanostructured hydrogen–graphite systems, C nano H x ( x =0.24, 0.31, 0.96), have been characterized by first nuclear magnetic resonance (NMR) measurements. The NMR spectrum of C nano H 0.96 is well represented by the sum of a Lorentzian and a Gaussian line, indicating two types of hydrogen coordinations. These two components may be ascribed to hydrogen in graphite interlayers and hydrogen chemisorbed at dangling bonds. Information on the hydrogen hopping frequencies is provided by the spin–lattice relaxation rate Γ 1 . The temperature dependence of Γ 1 yields high hydrogen diffusivities and low activation energies of E a ≈0.1 eV. A change in the Γ 1 data of C nano H x with x =0.24 and 0.31 occurred after the samples had been heated to about 400–430 K. This suggests that in this temperature range hydrogen atoms start to occupy sites with different site energies, resulting in a distribution of the activation energies for hydrogen motion.

Hydrogen diffusion in metallic and nanostructured materials

The diffusion mechanisms of hydrogen in metallic and nanostructured materials have been studied systematically by different nuclear magnetic resonance techniques. The present paper reviews three examples of our recent work: (i) The hydrogen-stabilized Laves-phase compound C15-HfTi2H4, with rather complex mechanisms of hydrogen diffusion. Long-range diffusion and localized motion coexist on different time scales in this compound. (ii) Nanostructured vanadium-hydrides n-VHx, in which the dynamical properties of hydrogen are fundamentally changed compared to that in a crystalline compound. The diffusion parameters of hydrogen in the grain boundary regions could be determined independently of the hydrogen motion inside the crystalline grains. (iii) Hydrogen in nanostructured hydrogen-graphite-systems n-CHx, where the NMR spectra reveal two types of hydrogen coordinations. The relaxation data indicate high hydrogen mobilities at ambient temperatures.

The local structure of hydrogen storage nanocrystalline graphite by neutron scattering

Abstract The total and inelastic neutron scattering measurements were employed in order to get more information on the local structure of nanocrystalline graphite prepared by mechanical milling under D 2 gas atmosphere. In the RDF( r ) for the sample after 50 h of milling, newly grown peak around 0.154 nm was found at the larger r side of the first nearest peak corresponding to the C–C correlation. The distance 0.154 nm of the C–C correlation is attributed to 4-fold bonding. Moreover, the inelastic neutron scattering peak observed in the 160–190 meV region for the samples after 20 h of milling indicates new emergence of sp 3 bonding. The results apparently indicate that terminating D atoms at the edges of the nano-lattice plane of graphite create the new sp 3 bonding of C atoms during the milling process under D 2 gas atmosphere.

Hydriding properties of ordered-/disordered-Mg-based ternary Laves phase structures

Abstract Ternary Laves phase structures with compositions MgYNi 4 , MgCaNi 4 and CaYNi 4 were prepared, and the relationship between the structures and hydriding properties was studied in detail. Only in MgYNi 4 are Mg and Y found to be ordered and a plateau pressure is clearly observed in the P–C isotherm during the dehydriding process. In MgCaNi 4 , however, Mg and Ca are disordered, and hydrogen content of MgCaNi 4 is ∼30% larger than that of MgYNi 4 . Control of their order/disorder in Laves phase structures may provide the hydriding properties with higher hydrogen concentrations and flatter plateau regions.

Structural and hydriding properties of amorphous MgNi with interstitially dissolved carbon

Abstract MgNiC x with an amorphous structure were synthesized up to x =1.31 by mechanical alloying of amorphous (a-)MgNi and graphite as starting materials. The core-level X-ray photoelectron spectroscopy revealed that the original bonding states with metallic character of a-MgNi remained even in the sample with high carbon content although the graphite dissolved as interstitial carbon and it mainly bounded with Mg in a-MgNi. The crystallization temperature of MgNiC x increased with increasing carbon content up to x =0.85, which was accompanied by a drastic change in the crystallization processes. Upon hydrogenation of MgNiC x , the atomic ratio of hydrogen plus carbon to metal (H+C)/M remained a constant value of ≃0.9 through the whole system, indicating that carbon atoms dissolved into the sites that otherwise would have been occupied by hydrogen atoms.