Shin-ichi Yamaura

Thermal stabilities and discharge capacities of melt-spun Mg–Ni-based amorphous alloys

Abstract Mg–Ni–M (M=Ca, La or Pd) ternary alloys were synthesized by the melt-spinning technique. All as-solidified alloys possessed an amorphous single phase by the additional effect of the third element, though it was difficult to obtain an amorphous Mg 67 Ni 33 binary alloy by melt-spinning. We examined the thermal stability and electrochemical cyclic life property of the ternary amorphous alloys. The crystallization temperature of the amorphous alloys increases with increasing M content. All the alloys except Mg 67 Ni 28 Pd 5 examined in the present study maintain the amorphous structure even after hydrogen absorption at 373 K for Mg–Ni–Ca and Mg–Ni–Pd and at 423 K for Mg–Ni–La. The crystallization temperature increases by absorbing hydrogen, indicating that the alloys are thermally stabilized by hydrogen absorption. In the electrochemical cyclic life measurements up to five cycles, the Mg–Ni–Pd amorphous alloys exhibit high discharge capacities ranging from 100 to 400 mA h/g as well as small cyclic life degradation tendency, though the Mg–Ni–Ca and Mg–Ni–La amorphous alloys possess small discharge capacities of 10–100 mA h/g with significant cyclic life degradation. It is thus concluded that the good cyclic life property of the amorphous hydrogen storage alloys can be obtained by application of the melt-spinning technique to Mg-based alloys with appropriate alloy compositions.

The effect of grain boundary microstructure on Barkhausen noise in ferromagnetic materials

Abstract The Barkhausen noise (BHN) was measured in ferromagnetic materials to investigate the interaction between the grain boundary microstructure and magnetic domain walls. The grain-size effect of the BHN was studied for pure iron specimens. The Hall–Petch type relationship between the grain-size and the BHN power was found. The effect of the grain-boundary misorientation on the BHN was also studied for silicon steel specimens. The BHN power increased with increasing the grain boundary misorientation angle. It has been suggested that the arrangement of magnetic domains along a grain boundary can be affected by the grain boundary character and that hysteresis of magnetization can be optimized by controlling the grain boundary microstructure in ferromagnetic materials. The importance of understanding the behavior of magnetic domains was discussed in terms of the control of magnetic properties in ferromagnetic materials.

Production and mechanical properties of aluminum alloys with dispersed nanoscale quasicrystalline and amorphous particles

By the dispersion of nanoscale quasicrystalline and amorphous particles in Al phase, new Al-based alloys with good mechanical properties were developed in a high Al concentration range of 93–95 at.% for Al−Cr−Ce−Co, Al−V−Fe, Al−Ti−M and Al−Fe−Cr−Ti alloy systems. The Vickers hardness of a melt-quenched (MQ) Al84.6Cr15.4 alloy with almost a single icosahedral quasicrystalline phase (QC) was 710. The addition of Ce and Co in the Al−Cr binary alloys was effective for the extension of the concentration range of the QC to a lower solute concentration range. The fracture strength (σf) increased to 1340 MPa for the MQ Al94.5Cr3Ce1Co1.5 alloy in which the particle size and volume fraction were approximately 40 nm and 70%, respectively. The σf of the MQ Al94V4Fe2 alloy was 1390 MPa and the particle size and volume fraction were about 10 nm and 50%, respectively. Similarly, σf of the MQ Al93Ti4Fe3 alloy was 1320 MPa and the particle size and volume fraction were about 11 nm and 30%, respectively. Power metallurgy (P/M) Al93Fe3Cr2Fe2 alloy with dispersed nanoscale QC exhibited ultimate tensile strength (σUTS) of 660 MPa, 0.2 % proof stress (σ0.2) of 550 MPa, plastic elongation (εP) of 4.5%, Youngs modulus (E) of 85 GPa, Vickers hardness (Hv) of 192 and specific strength (σUTS/ρ) of 2.20×105 Nm/kg at room temperature and σUTS of 350 MPa, σ0.2 of 330 MPa and εP of 1.5% at 573 K. The QC structure in the P/M Al93Fe3Cr2Ti2 alloy remained almost unchanged even after annealing for 720 ks at 573 K and good wear resistance against S50C steel was also maintained for the extruded alloy tested at sliding velocity of 0.5 to 2 m/sec. These mechanical properties are promising for the future extension of the new Al-based alloys to practical materials.

Coulomb oscillation of a proton in a Ni–Nb–Zr–H glassy alloy with multiple junctions

Electric current-induced voltage oscillation at 500–560kHz was observed in the current-voltage curves of nanoscopic size (∼0.9nm) tunnel junctions arranged in a low-capacitance (∼1aF), multiple-junction configuration of (Ni42Nb28Zr30)100−xHx (5.2⩽x⩽15.2) glassy alloys in the temperature range of 205–6K. This behavior appeared to be derived from Coulomb oscillation resulting from the tunneling of individual protons charging and discharging the vacancy capacitance of Zr–H-◻-H–Zr atomic bond arrays among Zr-tetrahedral clusters, where ◻ is the vacancy barrier, termed the free volume, in the glassy alloys.

Electrode properties of rapidly solidified Mg67Ni23Pd10 amorphous alloy

A Mg67Ni23Pd10 amorphous alloy was synthesized by rapid solidification and its electrode properties were examined. The discharge capacities increased with increasing cycle number and reached as high as 550 and 529 mAh/g at the discharge current densities of 10 and 20 mA/g, respectively. The plateau pressure of electrochemically derived P–C isotherm was about 10−5 MPa.

Hydrogen absorption in Ti–Zr–Ni–Cu amorphous alloy

Abstract The hydrogen absorption behavior of a Ti 60 Zr 15 Ni 15 Cu 10 amorphous alloy was measured by using the conventional pressure–volume–temperature technique. It was found that under same temperature and pressure conditions the amorphous phase shows higher hydrogen absorbing capacity as compared with corresponding crystalline phase. Higher hydrogen to metal ratio H/M up to 1.41H/M can be achieved in the Ti 60 Zr 15 Ni 15 Cu 10 amorphous alloy at room temperature, which is 17.6 times higher than that of the corresponding crystalline phase. The structure of the as-quenched and hydrogenated samples was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is found that hydrogen absorption is accompanied by an expansion of the average nearest neighbor distance.

Damping properties of hydrogen-absorbed rod metallic glasses

Abstract Internal friction of hydrogen-absorbed Zr–Al–Ni–Cu–Pd metallic glasses has been investigated using rod samples. A broad peak is observed around 230 K in the temperature dependence of internal friction. Strain amplitude and frequency dependences of internal friction have been also investigated. The activation energy of hydrogen jumps and pre-factor estimated from the Arrhenius plots on the basis of the frequency dependence are about 0.4 eV and 3×10−13 s, respectively. Effects of hydrogen concentration on the temperature dependence of internal friction have been also clarified. It is found that the peak temperature decreases with increasing hydrogen concentration and that the peak internal friction increases linearly with hydrogen concentration up to 1 at.%.

Hydrogen absorption of nanoscale Pd particles embedded in ZrO 2 matrix prepared from Zr-Pd amorphous alloys

Nanocomposite materials consisting of ZrO 2 and Pd phases were prepared by heating the amorphous Zr 6 5 Pd 3 5 alloy for 24 h at 553 K in air. The maximum hydrogen absorption amount is about 2.4 mass% (H 2 /Pd) at 323 K and2.2 mass% (H 2 /Pd) at 423 K at hydrogen pressure of I MPa. The absorption amount of Pd nanoparticles in the nanocomposite is a few times larger than those for the bulk and powder Pd metals. The remarkable increase in the hydrogen absorption/desorption amounts seems to result from the isolated dispersion state of Pd nanoparticles in the ZrO 2 phase containing a tremendously large interface area in the nanocomposite.

A proton dot tunneling in a Ni-Nb-Zr-H glassy alloy with multiple junctions

Following the discovery of Coulomb oscillation by the proton tunneling in Ni-Nb- Zr-H glassy alloys, we investigated the temperature-dependent hydrogen effect for dc electric conduction of the (Ni36Nb24Zr40)100-xHx (0≤x≤15.7) glassy alloys. Electric current-induced voltage oscillation at 660 kHz and evidence of the Coulomb staircase of I-V characteristics show Coulomb oscillation in nanoscopic size tunnel junctions arranged in a low-capacitance, based on proton resonance among the multiple-junctions of the glassy alloys in the temperature range of 240 K to 6 K.

Discharge capacities of melt-spun Mg–Ni–Pd amorphous alloys

Abstract Mg-rich Mg–Ni–Pd amorphous alloys were synthesized by melt-spinning and their electrode properties were examined. We found that degradation of discharge capacity with increasing cycle number was suppressed in melt-spun amorphous Mg60Ni30Pd10, Mg75Ni15Pd10 and Mg80Ni10Pd10 alloys. The discharge capacity of the melt-spun amorphous Mg80Ni10Pd10 at the first cycle at the discharge current density of 10 mA/g was ∼475 mAh/g.