T. Nambu

Development of low hydrogen overpotential electrodes utilizing metal ultra-fine particles

Cathode materials with high catalytic activity for the hydrogen evolution reaction were developed by using metal ultra-fine particles (UFPs) of Ni and Mo. Ni UFPs sintered at about 573 K were extremely porous and had an enormously large surface area. The measured hydrogen overpotential of the Ni UFPs in sulfuric acid solution was shown to be as low as that of Pt. Further decrease in hydrogen overpotential was observed when Mo UFPs were mixed with the Ni UFPs.

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.

Alloying effect of 3d transition elements on the ductility of chromium

Chromium and its alloys have good corrosion resistance in corrosive environments and good oxidation resistance at high temperatures. In addition, they exhibit an excellent combination of low density and high creep strength. However, there is still a large barrier to the practical use because of their poor ductility at room temperature. According to recent investigations, an environmental effect was found on the ductility of high purity polycrystalline chromium. In this study, in order to find a way to improve the ductility of chromium at room temperature, the alloying effect on the ductility of chromium was investigated experimentally in several test environments.

A New Concept for Alloy Design of Nb-Based Hydrogen Permeable Alloys with High Hydrogen Permeability and Strong Resistance to Hydrogen Embrittlement

A concept for alloy design of Nb-based hydrogen permeable alloys has been proposed based on the mechanical properties of niobium in hydrogen atmosphere and also on the hydrogen chemical potential in metal membrane. Following this concept, Nb-based alloys are designed and developed that possess excellent hydrogen permeability without showing any hydrogen embrittlement.

Effect of surface imperfections on the ductility of pure chromium

The fracture behaviour of high-purity polycrystalline chromium was investigated using bend tests at room temperature in air. It was found that plasticity was dependent largely on surface imperfections. For example, brittle fracture took place in a roughly polished specimen containing a large number of small cracks on the surface. However, once the specimen was polished finely or electropolished, it was able to be bent by more than 90‡ without showing any brittle fracture. Such a surface effect on the fracture mode is very similar to the observations in glasses and in alkali halide crystals (for example, NaCl and LiF).

Hydrogen Solubility and Resistance to Hydrogen Embrittlement of Nb-Pd Based Alloys for Hydrogen Permeable Membrane

The alloying effects of Pd on the hydrogen solubility and the resistance to hydrogen embrittlement are investigated for Nb-xmol%Pd-ymol%Zr (x=0~19; y=0, 1) alloys. The hydrogen solubility at 673 K is found to decrease with increasing Pd content in the alloys. Both pure Nb and Nb-Pd alloys possessed ductility in vacuum at 673 K. However, severe hydrogen embrittlement occurs in pure Nb when it is tested under the hydrogen pressure even as low as 0.01 MPa. In view of the small punch (SP) absorption energy, the addition of Pd into Nb improves the resistance to hydrogen embrittlement by decreasing the hydrogen solubility in the alloy, although brittle fracture is still observed in the Nb-15mol%Pd alloy tested under a hydrogen pressure of 0.015 MPa at 673 K.

Alloying effects on the electronic structure of chromium

Alloying effects on the electronic structure of Cr metal were investigated in order to obtain useful information for alloy design. The electronic structures were calculated by the DV - molecular orbital method, and two alloying parameters were determined theoretically. One was the d-orbital energy level Md and the other was the bond order Bo for various alloying elements M in Cr. By using these parameters, alloying behaviour was elucidated successfully. For example, the solidus temperature of binary Cr - M alloys showed a positive correlation with the bond order Bo. Also, the solubility limits of alloying elements in the Cr solid solution at 873 K were associated with Md and Bo. Furthermore, the appearance of intermetallic compounds, e.g. the -phase and the Laves phase in the Cr binary system, was predictable with these alloying parameters. The present results were shown to be useful as a guide for designing high-performance chromium-based alloys.

Correlation between electronic structure and phase stability of metal hydrides

Abstract Local electronic structures around hydrogen in a small octahedral cluster are simulated by the DV-Xα molecular orbital method. It is shown that the ionicity of hydrogen in the cluster is a good indicator of the stability of hydrides, as long as the charge transfer from metal to hydrogen atoms is responsible for the hydride stability. For example, the heat of formation of pure metal dihydrides, ΔH, correlates well with the calculated ionicity of hydrogen. A similar correlation is observed in binary vanadium alloys and ternary V–Ti–Ni alloys.

Environmental effects on the ductility of pure chromium

Chromium and its alloys exhibit an excellent combination of low density, high creep strength and good oxidation resistance at high temperatures. However, there is still a large barrier to its practical use because of poor ductility at room temperature. Recently, high-purity chromium has been developed due to the progress of refining technology. As a result, there is an increasing possibility of improving the ductility of chromium even at room temperature, and hence increasing the expectation for the practical use of chromium. A similar situation is also seen in intermetallic compounds such as TiAl and FeAl, which are some of the promising high temperature materials. Recently, it has been reported that the ductility of the intermetallic compounds is influenced strongly by the environment in which specimens are exposed during mechanical tests. A similar environmental effect will occur in pure chromium because of the resemblance in the mechanical properties between them. The purpose of the present paper is to show the first result of the environmental effect on the ductility of high-purity polycrystalline chromium.

Estimation of liquidus temperature of Sn-based alloys and its application to the design of Pb-free solder

An interesting relation between the melting temperature and the alloy composition was discovered in Sn-based alloys through the evaluation of a series of experimental data on the liquidus temperatures (LTs) for 134 types of multi-component Sn alloys. In these Sn alloys, the degree of liquidus temperature drop with alloying was not affected by the kind of individual elements but by their total atomic fraction alone. The compositional dependence on the LT could be expressed by the equation, LT(K)=499.79−1.799X, where X was the total mole percentage of alloying elements. It was demonstrated that the use of this equation would make it possible to develop Pb-free solder alloys more efficiently.