Hiroshi Yukawa

Effects of dopants and hydrogen on the electrical conductivity of ZnO

The effects of dopants on the electrical conductivity of ZnO were investigated through the ac impedance spectroscopy. The doping of Al increased the electrical conductivity of ZnO greatly, whereas the doping of Li decreased it both in the grain and in the grain boundary. The doping of the 3d transition metals (Co, Mn, and Cu) made the grain boundary more resistive, but the doping effect on the electrical conductivity inside the grain was varied depending on the doping elements. The doping of Co had no significant effects on the electrical conductivity of the grain, and the doping of Mn made the grain a little more resistive. The doping of Cu made the grain much more resistive. In addition, hydrogen was introduced into ZnO by the ion implantation method. The electrical conductivity in the hydrogen-implanted ZnO layer increased by four orders of magnitude. The mechanisms for the doping effects were discussed in this investigation.

Roles of the hydride forming and non-forming elements in hydrogen storage alloys

Abstract The electronic structures of small octahedral model clusters containing hydrogen, 3d, 4d, 5d transition and non-transition elements are investigated by the DV-Xα molecular orbital method. It is found that hydrogen makes a strong chemical bond with the hydride non-forming elements, B, as long as the hydride forming elements, A, exist in the neighborhood. This is a reason why hydrogen is located preferentially near the hydride non-forming elements in many hydrides. Also it is suggested that the ratio of the A–B bond strength to the A–A bond strength lies in a certain range for conventional hydrogen storage A–B alloys.

Electronic structures of hydrogen storage compound, TiFe

Abstract The electronic structures of TiFe hydrogen storage compound containing a variety of alloying elements, M , are investigated by the DV-Xα cluster method in order to understand alloying effects on the hydrogen absorption and desorption characteristics of this compound. It is found that hydrogen atoms make a strong chemical bond with Fe atoms rather than Ti atoms in pure TiFe, despite the larger affinity of Ti atoms for hydrogen than Fe atoms in the binary metal–hydrogen system. It is also shown that the nature of the chemical bond between the constituent atoms determines the stability of TiFe hydrides. For example, the ratio of the Fe( M )–Ti bond order to the Ti–Ti bond order correlates well with the experimental data of the equilibrium plateau pressure of hydrogen.

Electronic structures of lithium manganese oxides for rechargeable lithium battery electrodes

Abstract The modification of electronic structures of manganese oxides due to lithium intercalation has been investigated by the DV-Xα molecular orbital method. In this study the process of lithium intercalation is assumed to be divided into two steps: the lithium occupancy (chemical effect) and the lattice expansion (structural effect). By the lithium occupancy the energy gap lying between the O-2p and the Mn-3d bands increases, and there is attendant increase in the ionic character in the chemical bonding. For example, the ionicity of the oxygen becomes more negative whereas that of the manganese becomes more positive. However, by the lattice expansion, the energy gap decreases and the electronic state recovers to some extent to the original state in MnO 2 . In particular, it is noted that the ionicity of the manganese in LiMn 2 O 4 is fully recovered to the value in MnO 2 , indicating that lithium intercalation reduces the oxygen ion, but not manganese ion.

Chemical bond state and hydride stability of hydrogen storage alloys

Abstract The electronic structures for typical hydrogen storage alloys, LaNi 5 , TiFe, ZrMn 2 , Mg 2 Ni and b.c.c.V, all containing a variety of alloying elements, M, are investigated by the DV-Xα molecular orbital method. It is shown that atomic interactions of controlling the stability of their hydrides, depend strongly on the way how crystal structural evolution takes place in the course of hydrogenation. For example, when the hydride structure is the derivative one of the starting alloy, the metal–metal interaction and its change with hydrogenation will determine the stability of the hydride. LaNi 5 , TiFe and ZrMn 2 belong to this group. On the other hand, if the crystal structure of the hydride is completely different from that of the starting alloy, the importance of the metal–hydrogen interaction increases, and in case of b.c.c.V, only the metal–hydrogen interaction is responsible mainly for the relative stability of alloyed VH 2 . In case of Mg 2 Ni, both the metal–metal and the metal–hydrogen interactions control the stability of the hydride, Mg 2 NiH 4 .

Alloying effects on the stability of vanadium hydrides

Abstract The alloying effects on the stability of the γ phase (VH 2 ) and the β phase (VH or V 2 H) was investigated experimentally in binary V-1mol%M and V-3mol%M alloys, where M represents various alloying elements. It was found that the stability of the γ phase is dependent largely on the alloying elements, and changes systematically following the order of elements in the periodic table. On the other hand, two endothermic peaks are observed in the course of hydrogen desorption from the β phase in the DSC experiments. They are attributable to the onset of the successive transformations of the β phase to the bcc(α) phase containing hydrogen and then of this bcc(α) phase to the hydrogen-free bcc vanadium. Both the peak profile and the peak temperature vary significantly with alloying elements, M, indicating that the hydrogen desorption from the β phase is strongly modified by alloying.

Heterogeneous distributions of Magnesium atoms near the precipitate in Al-Mg based alloys

The Mg concentration profiles were measured in the precipitate free zones (PFZ) and also in the grains of Al-9 mol%Mg based alloys using analytical electron microscopy. There was significant supersaturation of the Mg concentration in PFZ even after prolonged aging. Also, the presence of a steep concentration gradient was observed near the β (Al3 Mg2) precipitate on the grain boundary. This concentration profile was scarcely modified by the Cu addition. However, the Zn addition made the gradient more gentle. The supersaturated state was also observed in the intermediate region between the precipitates inside the grain, even though it depended largely on the interspacing between them. Furthermore, it was suggested that a very low solute-concentration region existing near the β precipitate in the grain-boundaries will cause poor resistance to the stress corrosion cracking of the alloys.

Roles of constituent elements and design of hydrogen storage alloys

Abstract The chemical interactions between atoms in hydrogen storage alloys have been investigated by the DV-Xα molecular orbital method. In view of the nature of the chemical bond between atoms, a completely different interstitial space for the hydrogen occupancy is formed in binary A–B hydrogen storage alloys, where A and B are the hydride forming and non-forming elements, respectively, as compared to the interstitial space in pure A metals. As a result, hydrogen interacts more strongly with the B element than the A element in every hydrogen storage alloy, despite the fact that the A element has a larger affinity for hydrogen than the B element in the binary metal–hydrogen system. In addition, it is shown that the A:B compositional ratio of hydrogen storage alloys is predictable using a simple equation, 2×Bo(A–B)/[Bo(A–A)+Bo(B–B)], where Bo(A–B), Bo(A–A) and Bo(B–B) are the bond orders between atoms given in the parentheses.

Compositional dependence of hydriding properties of vanadium alloys at low hydrogen pressures

Abstract The alloying effects have been investigated experimentally on the hydriding properties of vanadium at low hydrogen pressures. The PCT curves for the β phase (V2H or VH) are measured using an electrochemical method. It is found that the logarithm of the plateau pressure for the V2H and the VH phases change almost linearly with the amount of alloying element in vanadium metal. Also, when the Ti/Cr compositional ratio is fixed at 1, the equilibrium hydrogen pressure increases for the V2H phase but decreases for the VH phase with increasing total content of Ti and Cr. However, both the V2H and the VH phases become unstable with decreasing Ti/Cr compositional ratio, so that the PCT curve shifts towards the lower H/M side and the second plateau region for the VH2 phase existing at high hydrogen pressures spreads to some extent, which leads to the improvement in the effective hydrogen capacity of the alloy.

Electronic structure and hydriding property of titanium compounds with CsCl-type structure

The electronic structures of titanium compounds, TiFe, TiCo and TiNi with CsCl-type structure are investigated by the DV-Xα molecular orbital method. From the results of the calculation, it is found that hydrogen interacts more strongly with constituent elements, X (=Fe, Co, Ni), rather than Ti atoms. The bond strength between Ti and X atoms changes in the sequence, Fe>Co>Ni. This is the same order as the capacity of the hydrogen absorption in these TiX compounds. The characteristics of the hydrogen absorption in these compounds are discussed in view of the nature of the chemical bond between atoms.