Mechanism Of Hydrogen-induced Amorphization Research Articles

Computer Simulation of Structural Stability by Hydrogenation in Hydrogen Storage Materials

ABSTRACTMicroscopic mechanism of Hydrogen-Induced Amorphization (HIA) in AB2 C15 Laves phase compound is studied. Experimentally, compounds in which A atoms are contracted and B atoms are expanded in Laves phase show HIA. It suggests that the relative atomic size is the controlling factor. We investigate the role of the size effect by molecular Dynamics (MD) methods using Lennard-Jones pair-wise 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. The relaxation of B-atom positions by hydrogenation gives the fluctuation contribution. The fluctuation effect is essential in the elastic instability, although the energy change is small.

Formation of crystalline and amorphous hydrides by hydrogenation of C15 Laves phase YFe 2

Formation conditions of crystalline (c-) and amorphous (a-) YFe 2H x hydrides during thermal analysis of C15 Laves phase YFe 2 in a hydrogen pressure (0.1–5.0 MPa) at heating rates of 0.17 and 0.33 K/s have been investigated by XRD, Ar-DSC, TEM and hydrogen analysis. Hydrogen absorption, hydrogen-induced amorphization (HIA), the precipitation of BiF 3-type YH 3 and the decomposition of the remaining amorphous hydride into α-Fe + YH 3 occur exothermally with increasing temperature. The peak temperature T p for hydrogen absorption, HIA, and the decomposition of the remaining amorphous hydride show the negative pressure dependence, but that for the precipitation of the hydride shows the positive one. As a consequence of such pressure dependence, HIA overlaps with the precipitation of YH 3 at 0.1 MPa H 2 and 0.33 K/s. Formation conditions of c- and a-YFe 2H x are compared with those of the other C15 Laves phases RFe 2 and the mechanism of HIA in C15 Laves phases RFe 2 is discussed.

Process and mechanism of hydrogen-induced amorphization in C15 Laves phases RFe 2

The pressure dependence of structural changes in C15 Laves phase DyFe 2 was investigated using a pressure differential scanning calorimeter (PDSC) at 0.1–5.0 MPa H 2 and compared with that of ErFe 2 and CeFe 2. By thermal analysis of DyFe 2, four exothermic peaks of hydrogen absorption, hydrogen-induced amorphization (HIA), the precipitation of BiF 3-type DyH 3 and the decomposition of the amorphous hydride were observed above 0.5 MPa H 2, although tri-hydride DyH 3 takes usually the HoH 3-type structure. HIA overlapped with the precipitation of DyH 3 at 0.2 MPa H 2, but c-DyFe 2H x decomposed directly into α -Fe and DyH 3at 0.1 MPa H 2. Hydrogen absorption, HIA, and the decomposition of a-DyFe 2H x showed a negative pressure dependence, but the precipitation of DyH 3 showed a positive one. As a result of such dependence, HIA does not occur at lower hydrogen pressure. The structural changes of ErFe 2were similar to that of DyFe 2. On the other hand, hydrogen absorption and HIA occurred simultaneously for every hydrogen pressure in CeFe 2. The mechanism of HIA in the C15 Laves phases RFe 2 was discussed on the basis of the experimental results.

Hydrogen-Induced Amorphization in C15 Laves Phase DyCo<sub>2</sub> Studied by Pressure Calorimetry

Structural changes in C15 Laves phase DyCo2 on heating using a pressure differential scanning calorimeter (PDSC) in a hydrogen atmosphere between 0.1 and 5.0 MPa were investigated by a powder X-ray diffractometer (XRD), a differential scanning calorimeter under an argon flow atmosphere (Ar-DSC), a transmission electron microscope (TEM) and a hydrogen analyzer. As the temperature of DyCo2 increases, the reactions such as hydrogen absorption in a crystalline state, HIA (hydrogen-induced amorphization), precipitation of DyH3 and decomposition of the remaining amorphous phase into β-Co + DyH3 occurred exothermically for every hydrogen pressure. The mechanism of HIA in DyCo2 is discussed on the basis of the experimental results.

Pressure dependence of hydrogen-induced transformations in C15 Laves phase DyFe 2 studied by pressure differential scanning calorimetry

By thermal analysis of DyFe 2: (1) hydrogen absorption; (2) hydrogen-induced amorphization (HIA); (3) the precipitation of BiF 3-type DyH 3; and (4) the decomposition of the remaining amorphous hydride occur exothermically with increasing temperature at 1.0 MPa H 2. T p/ T m (the peak temperature/the melting temperature of DyFe 2) for hydrogen absorption, HIA, the precipitation of DyH 3 and the decomposition of the amorphous hydride are 0.28, 0.36, 0.43 and 0.48, respectively, which are closely related with kinetics of the transformations. The peak temperature T p for HIA shows a large and negative pressure dependence, but that for the precipitation of DyH 3 shows a small and positive one. As a consequence of such pressure dependence, HIA overlaps with the precipitation of DyH 3 at 0.2 MPa H 2, while the crystalline hydride decomposes directly into α-Fe and DyH 3 at 0.1 MPa H 2. The activation energy E A for hydrogen absorption, HIA, the precipitation of DyH 3 and the decomposition of the amorphous hydride are calculated to be 56, 75, 308 and 162 kJ/mol Dy, respectively, which are closely related with the mechanism of HIA. The mechanism of HIA is discussed on the basis of the experimental results.

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.

Amorphization of C15 Laves RFe2 compounds by hydrogen absorption

ABSTRACTC15 Laves RFe2 (R= Y, Ce, Sm, Gd, Tb, Dy, Ho and Er) compounds were thermally analyzed to elucidate conditions of hydrogen-induced amorphization (HIA), i.e. amorphization of intermetallics by hydrogen absorption, using a pressure scanning differential calorimeter (PDSC) in a hydrogen atmosphere of 1.0 MPa. As the temperature increases, hydrogen absorption in the crystalline state, HIA, precipitation of RH2 in the amorphous phase and decomposition of the remaining amorphous phase into RH2+oc-Fe occur exothermally for RFe2 (R=Y, Sm, Gd, Tb, Dy and Ho). HIA and precipitation of ErH2 occur simultaneously in ErFe2. On the contrary, hydrogen absorption of CeFe2 always leads to HIA, i.e. no hydrogen absorbed crystalline phase is formed. The mechanism of HIA in the C15 Laves phases RFe2 is discussed on the basis of the experimental results.

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.

水素誘起アモルファス化における副格子の弾性論的不安定化の役割

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.

Amorphization of RAl intermetallic compounds by hydrogenation

Structural changes of RAl compounds (RLa, Ce, Pr, Nd, Sm, Gd, Dy, Tb, Ho, Er) induced by hydrogenation were investigated by X-ray diffractometry (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Hydrogen-induced amorphization (HIA) has been observed in R 3Al, R 2Al and a part of RAl, but not been observed in C 15 Laves RAl 2 compounds. The HIA behavior of the RAl compounds and the crystallization behavior of the resulting amorphous alloys depend not only on the rare earth metals and their compositions, but also on the hydrogen pressure and the hydrogenation temperature. These behaviors of the RAl compounds are compared with those of the C 15 Laves compounds RM 2 (MFe, Co, Ni) and liquid quenched amorphous alloys of RAl. In addition, structures of amorphous alloys prepared by HIA and liquid quenching were measured by XRD. The mechanism of HIA was discussed on the basis of the structural data and thermodynamic considerations.

Hydrogen-induced amorphization of intermetallics

This paper describes our investigations on hydrogen-induced amorphization (HIA) in intermetallics caused by hydrogenation. The crystal structures and chemical compositions of amorphizing intermetallics are presented at first. In addition, the formation conditions and the formation processes of amorphous alloys, the mechanism of HIA, the factors controlling the occurrence of HIA and the relation between the structures of the hydrogen-induced amorphous alloys and the stability of C15 Laves phases are given.

The mechanism of hydrogen-induced amorphization in intermetallic compounds

In this article, the amorphization mechanisms of intermetallic compounds by hydrogen absorption are discussed on the basis of reported experimental results. It is found that there are two different modes for amorphization. One is a gradual and thermally activated process, while the other is a strain-induced reaction during hydrogen absorption. As factors affecting the amorphization modes, the hydrogen concentration and the elastic property of the compound are discussed. Of these two factors the elastic property of the compound plays a key role on the amorphization. When an elastic strain exceeding the elastic limit was imposed on the compound, the crystal lattice was not deformed plastically but collapsed into the amorphous state because the brittleness of the compound did not permit plastic deformation.

A differential scanning calorimetry study of hydrogen-induced amorphizatiqn in GdMn 2, PrMn 2 and NdMn 2 laves phases

Comparative studies on hydrogen-induced amorphization in the GdMn 2, PrMn 2 and NdMn 2 Laves phases were performed using differential scanning calorimetry (DSC), X-ray diffraction and transmission electron microscopy measurements in order to understand hydrogen-induced amorphization phenomena. In all of the RMn 2 ( R ≡ Gd, Pr, Nd) Laves phases, two exothermic reaction peaks, which are related to crystalline hydride formation and the formation of microcrystals of RH 2 and α-Mn, are observed in the DSC trace performed in a H 2 atmosphere. Only GdMn 2 shows the phenomenon of hydrogeninduced amorphization when hydrogenated isothermally at an elevated temperature and a high hydrogen pressure. Hydrogen-induced amorphization was not observed in the PrMn 2 and NdMn 2 Laves phases. PrH 2, NdH 2 and α-Mn grow in crystalline hydride phases in the PrMn 2 and NdMn 2 phases. The activation energies for the formation of microcrystalline phases in GdMn 2, PrMn 2 and NdMn 2, determined by Kissinger's method, were found to be 21.6, 42.7 and 39.4 kcal mol −1 respectively. From the present results, the mechanism of hydrogen-induced amorphization and the atomic environments around hydrogen atoms in the amorphous state are discussed. The mechanism of amorphization would be related to the motion of manganese atoms within short range distances. The surroundings of the hydrogen atoms seem to be similar to the configurations of hydrogen atoms in GdH 2.