Search

ositron annihilation spectroscopy and first-principles calculations have been applied to identify lattice defects introduced in LaNi 5, which is widely used as hydrogen storage materials, during hydrogen absorption–desorption process. Positron lifetime measurement and calculated positron lifetimes revealed that Ni vacancies are formed during hydrogen absorption in LaNi 5. First-principles calculations have been performed in order to estimate the stability of the Ni vacancy in the hydrogen absorption process. The binding energy at the Ni sites is reduced by absorbed hydrogen atoms in LaNi 5 hydrides. In the case of the Ni 2b site surrounded by four hydrogen atoms, the binding energy is lowered by about 0.7 eV. Another contribution to the stability of the Ni vacancy is a decrease in the energy of additional lattice sites occupied by the atoms removed as vacancies. In the hydrogen absorption process, additional hydrogen sites are occupied by hydrogen atoms supplied in the process, which leads to the decrease in the vacancy formation energy. As a result, the formation energy of Ni vacancy is reduced to almost zero in the hydrogen absorption process of LaNi 5. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

To theoretically examine the structural transition of vacancy–solute complexes in Al–Mg–Si alloys, we performed first-principles calculations for layered vacancy–solute complexes with additional Mg atoms. The central Mg atom in the additional Mg layer shifted to the Si layer with the increase in the number of Mg atoms to weaken the repulsive Mg–Mg interaction and to form Mg–Si bonds. When five Mg atoms were added to the layered vacancy–solute complex, the central Mg atom completely shifted to the Si layer, and a Mg vacancy was formed in the Mg layer, which indicated that the β″-eye is formed upon the addition of Mg atoms. We reproduced β″-eye formation from a solid solution with a vacancy using first-principles-based Monte Carlo simulations. Once the β″-eye was formed on the layered vacancy–solute complex, the process can be repeated by the formation of alternate Mg and Si layers along (010) β″. These results clearly indicate that the layered vacancy–solute complex plays an important role in β″-eye formation. [ABSTRACT FROM AUTHOR]

Abstract: The electronic structures and the bonding properties in amorphous Zr67Ni33 and Zr67Cu33 alloys have been calculated by the discrete variational (DV)-Xα molecular orbital method to investigate the chemical bonding in the amorphous structures. The calculations about Zr2Ni and Zr2Cu and β-Zr crystals were also performed for comparison. The broad agreement between the DOSs for the amorphous Zr67Ni33 alloy and the Zr2Ni crystal implies the structure similarity. The calculated DOSs and the bond overlap population analyses suggest that the amorphous structures are significantly influenced by the tight binding pair (Zr–Ni in the Zr–Ni systems and Cu–Cu in the Zr–Cu systems). The glass-formation abilities reflect the bonding properties in the amorphous and crystal structures. [Copyright &y& Elsevier]

Abstract: To clarify the effect of substitution of Sn for Ni of LaNi5 on cyclic durability of hydrogen storage, the behavior of lattice defects during the hydrogenation cycles at 288K in LaNi4.93Sn0.27 was investigated by in situ positron lifetime measurements. Mean positron lifetime increased to 175ps by vacancy formation during initial hydrogenation and then decreased to 135ps by vacancy recovery during dehydrogenation. Vacancies were reversibly introduced and removed at 288K, although quenched-in vacancy cannot migrate below 448K in LaNi4.93Sn0.27. Reversible change of the positron lifetime was observed in subsequent cycles, and dislocation density and vacancy concentration remained almost constant at around 6×109 cm−2 and 10ppm, respectively, which are two orders of magnitude lower than those in binary LaNi5. These results indicate that the degradation of hydrogen capacity during a cycle closely relates to the concentration of lattice defects accumulated with hydrogenation. [Copyright &y& Elsevier]

Abstract: The formation of the vacancy and dislocation by the initial hydrogenation and dehydrogenation in LaNi4.5M0.5 (M=Al, Co, and Ni) was observed by means of the positron lifetime technique. The concentrations of vacancy introduced by these processes were 0.25, 0.13 and 0.01at.% for LaNi5, LaNi4.5Co0.5 and LaNi4.5Al0.5, respectively. Al substitution into LaNi5 significantly prevented from vacancy formation, compared with LaNi5 and LaNi4.5Co0.5. In LaNi4.5Al0.5, the increase of the hardness and the enhancement of the pulverization, i.e. enhancement of the formation of micro cracks compared with LaNi5 were observed while the Co substitution had little effect on pulverization and hardness as well as vacancy formation. These results show that the formation of micro cracks became more active process by Al substitution than the formation of the lattice defects to release the strain energy generated by the hydride formation because of the higher formation energy of the lattice defects in LaNi4.5Al0.5, although both the formation of micro cracks and lattice defects were still observed in all alloys we studied. [Copyright &y& Elsevier]

Abstract: To clarify the hydrogen-related vacancy formation mechanism, positron lifetime measurements were performed for Pd1−x Ag x alloys that were hydrogenated at 296 or 373K. Positron lifetime increased only when the alloys were hydrogenated below the critical temperature for phase transformation of the hydrogenation reaction, while it remained constant when they were hydrogenated above the critical temperature. This strongly suggests that vacancies formed only when phase transformation occurs. Therefore, hydrogen-related vacancy formation must be caused by the strain generated as the result of the phase transformation. [Copyright &y& Elsevier]