Masahiko Katagiri

MOLECULAR DYNAMICS SIMULATION ON HYDROGEN STORAGE IN METALLIC NANOPARTICLES

Hydrogen storage in a metallic nanoparticle was simulated by classical molecular dynamics. Distribution of hydrogen atoms inside nanoparticle was investigated by changing length and energy parameters of metal–H bonds. Hydrogen atoms diffused into the particle and distributed homogeneously in case of weak metal–H bonds. In case of strong metal–H bonds, a hydrogen-rich surface layer was observed which suppresses the inward diffusion of hydrogen atoms. Structural modification of nanoparticle accompanied by grain boundary formation due to hydrogen loading was also observed. These variations in dynamical and structural features are considered to affect the hydrogen storage properties in nanoparticles.

Hydrogen-induced phase transformation

Hydrogen-induced amorphization (HIA) of intermetallic compounds was simulated by the molecular dynamics (MD) method using pairwise potentials. The microscopic mechanism of HIA in AB2 C15 Laves phase compound is discussed. The hydrogenation causes elastic softening in the bulk modulus and induces elastic instability. A phase transformation by elastic instability follows paths with minimal activation barriers, and nonequilibrium transformations including amorphization can be realized. The HIA can be considered as a case. The key to induce HIA is the expansion of the B atoms in a Laves phase; it facilitates the instability of the sublattice of B atoms. If the amount of hydrogen exceeds a critical value, HIA occurs. The HIA is a potential-driven phase transformation, and the resultant amorphous structure is potentially favored over the hydrogenated crystal. We also report the fracture process by isotropic loading and compare it to HIA.

Atomic-size effect in hydrogen-induced amorphization

The microscopic mechanism of hydrogen-induced amorphization (HIA) in C15 Laves phases of AB2 compounds is studied. Experimentally, compounds in which the AA internuclear distance is reduced and BB internuclear distance expanded compared to pure crystals show hydrogen-induced amorphization which suggests that the relative atomic size is the controlling factor. We investigate the role of the size effect by static and molecular dynamics methods using Lennard–Jones potentials. Our simulations show that in such a compound, the bulk modulus is remarkably reduced by hydrogenation compared to the isotropic tensile load, so that elastic instability is facilitated. This situation is caused by the negative increase of the pressure-fluctuation contribution in the elastic constant. We also report the fracture process under isotropic tensile loading. An elastic analysis at sublattice level shows that one of the sublattices is less stable in the HIA material.

Atomistic Simulation on Hydrogen Storage in Metallic Nanoparticles

Hydrogen storage in metallic nanoparticles was investigated by classical molecular dynamics and parameter physics. We observed phenomenological variation due to the differences in potential parameters of metal-hydrogen pair and crystal lattices. Three patterns of hydrogen distribution in both b.c.c. and f.c.c. nanoparticles were observed: non-absorbing, homogeneously-absorbing and heterogeneously-absorbing. In the last case, hydrogen atoms distribute just beneath the particle surface to form a hydrogen-rich layer. This layer prevents the diffusive motions of hydrogen atoms into the nanoparticle. We also carried out long simulation runs up to 1 nm to observed the structural variation of nanoparticles due to hydrogenation. Generation of grain boundaries was observed in b.c.c nanoparticles with the condition of strong metal–hydrogen interaction. Most of the grain boundaries were symmetric-tilt type and migrated inside the particle to reduce the interface energies. Formation of grain boundary was not observed in f.c.c. nanoparticles.

ELECTRONIC AND CRYSTAL STRUCTURAL CHANGES IN BCC TYPE HYDROGEN STORAGE MATERIALS

To design an ideal hydrogen (H) storage material, the understanding of the interaction between a host material and hydrogen is essential. The strength of the interaction determines the enthalpy and activation energy for H hopping. To obtain those physical quantities by utilizing first principle method, the crystal structure should be elucidated. We focused on Vanadium (V) as a representative of BCC (body-centered-cubic) type of hydrogen storage materials. The feature of BCC type storage material is characterized by the existence of β phase. We investigated the crystal and electronic structural changes with various H concentration in V. Without taking vibrational contribution into account, H atoms prefer to locate at tetrahedral sites. While taking the contribution into account, β phase can be the stable structure at VH composition.

A Molecular-Dynamics Study on Metal-Immersed Hydrogen Fluids

This work is aimed at the interactions between hydrogen atoms contained in high concentrations in metal lattices. Effects of metals on hydrogen interactions are surveyed by carrying out molecular dynamics (MD) simulations on hydrogen fluids containing palladium atoms, from a viewpoint in contrast to previous simulations on low concentrations of hydrogen in metal lattices. Some results of these simulations reveal the dissociation of H 2 molecules to H atoms due to the presence of Pd atoms under densification, and therefore imply the change of attractive H-H interactions in H 2 molecules to repulsive interactions of H atoms in Pd lattices. These repulsive interactions are consistent with an empirical “2-A rule” of hydrogen atoms in metal lattices, and impose limits on hydrogen-storage capacities of metals.

Role of Elastic Instability of Sublattice in Hydrogen-Induced Amorphization

Mechanism of Hydrogen–Induced Amorphization (HIA)was studied by Molecular Dynamics method (MD). C15 Laves AB2 compounds were examined. The HIA phenomenon is experimentally observed for Laves phase with contracted A atoms and expanded B atoms, in comparison with their atomic size in stable pure crystals. Aim of our study is to reveal the atomic size effect in the occurrence of HIA. Our previous MD simulation shows that hydrogenation makes the elastic constants soft. It comes from the nonlinearity of the interatomic potentials during volume expansion by hydrogenation. In HIA, another origin is observed for the softening. We suggested that the latter softening facilitates the bulk–modulus instability and HIA occurs. In present study, MD simulation under isotropic tensile load was performed instead of hydrogenation. We investigated the elastic stability change due to the volume expansion. We defined a stability criterion of sublattice and evaluated the elastic stability in sublattice. In HIA, the bulk–modulus stability of expanded B–atom sublattice is low. Its instability occurs before instability of total lattice. Once hydrogen is incorporated in such AB2 crystal, atoms relax. As a result, the pressure–fluctuation term of elastic constant increases negatively and the bulk modulus is reduced. Such a bulk–modulus instability gives HIA.