Ryuichiro Oshima

Giant magnetostriction in an ordered Fe3Pt single crystal exhibiting a martensitic transformation

Magnetostriction measurements have been made in an ordered Fe3Pt single crystal with degree of order of about 0.8, which exhibits a cubic-tetragonal martensitic transformation at 97 K. The specimen was cooled down to 4.2 K without magnetic field, and then a magnetic field of 4 T is applied to the specimen along 〈001〉 at 4.2 K and removed. As a result, a reversible giant magnetostriction of about 0.5% is observed. This reversible magnetostriction will be caused by the rearrangement of crystallographic domains, being three times as large as that of Terfenol-D (Fe2DyxTb1−x: typical magnetostrictive materials).

Preparation of platinum nanoparticles by sonochemical reduction of the Pt(IV) ions: role of surfactants

Sonochemical reduction processes of Pt(IV) ions in water have been investigated in the presence of various kinds of surfactants such as sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (DBS) as anionic surfactants, and polyethylene glycol monostearate (PEG-MS) as non-ionic, dodecyltrimethylammonium chloride (DTAC) and bromide (DTAB) as cationic surfactants. An improved colorimetric determination reveals that Pt(IV) ion is reduced to zero valent metal in two steps: step (1)--Pt(IV) ion to Pt(II) ion, and step (2)--Pt(II) ion to Pt(0), and after the completion of step (1), step (2) sets in. It appears that rapid scrambling reactions among platinum ions and/or atoms, that is, Pt(I) + Pt(IV)-->Pt(II) + Pt(III), etc. take place. In the sonolysis of aqueous solutions of SDS, DBS or PEG-MS, two kinds of organic reducing radicals, R(ab) and R(py), are proposed to contribute to the reduction. Radical R(ab) is formed from the reaction of the surfactants with primary radicals such as hydroxyl radicals and hydrogen atoms originated from the sonolysis of water, and radical R(py) is formed from the direct thermal decomposition of surfactants in the interfacial region between the collapsing cavities and the bulk water. R(ab) is effective for both the reduction steps, whereas R(py) is involved only in the reduction step (1). This fact coincides with the previous reported sonochemical reduction of Pt(II) ions. Hydrogen atoms themselves scarcely participate in the reduction. The average diameter (1.0 nm) of platinum particles prepared from the system of PEG-MS is smaller than those from the aqueous solution of anionic surfactant SDS (3.0 nm) and DBS (3.0 nm).

Sonolytical preparation of various types of metal nanoparticles in aqueous solution

Composite nanoparticles of Au, Pd and Pt, and nanoparticles of transition metal oxides, MnO2, were prepared by the sonochemical reduction of corresponding ions (Au(III), Pd(II), Pt(II), Pt(IV) and Mn(VII)) in an aqueous system in the presence of surfactants. The prepared noble metal particles were stable and the sizes were of the order of nanometer with narrow distribution. Surfactants were found to be stabilizers for the noble metal nanoparticles as well as important sources of reducing species for the metal ions. In binary metallic particles, which were prepared from aqueous mixed solutions of noble metal ions, three possible constitutional forms were obtained. MnO2 particles were reduce to water soluble Mn(II) ions by further sonication.

A study of vacancy-type defects in the B2-phase region of the Fe–Al system by positron annihilation method

Abstract Vacancy-type defects of B2-type intermetallic compound FeAl have been investigated as functions of composition ranging from 41.2 to 50.7 at.% Al and quenching temperature ranging from 610 to 1373 K by means of positron annihilation technique. At quenching temperatures lower than 873 K, both the mean positron lifetimes and S-parameters increase with increase in Al content. With increasing quenching temperature from 873 K, the S-parameters for 41.2, 49.0 and 50.7 at.% Al certainly increase. The results are discussed by considering changes in the defect type and the atom configuration around the vacancies. The composition dependence at lower quenching temperatures is explained partly as a change in the fractions of Fe-vacancies, VFe, and the di-vacancies on the Fe-sites, 2VFe, and partly as a change in the fraction of antisite Fe atoms, FeAl. The temperature dependence of the S-parameter is explained by change in defect type from di-vacancies, 2VFe, to different type of di-vacancies, VFeVAl, sitting on the nearest neighboring Fe and Al sites.

Electron microscopy of noble metal alloy nanoparticles prepared by sonochemical methods

Abstract Argon-saturated aqueous solutions of NaAuCl4 and PdCl2 or K2PtCl4 were reduced simultaneously by ultrasound irradiation to prepare noble metal alloy nanoparticles. Droplets recovered from colloidal dispersions were placed on carbon-supported copper grids, and dried in a vacuum. They were examined by TEM, HRTEM, and nano-area energy dispersion X-ray spectroscopy(EDX). The Au-Pd nanoparticles exhibited monodispersive distribution (8 nm), and consisted of a gold core and a palladium shell. Au-Pt alloy nano-particles could not be produced from NaAuCl4 and K2PtCl4 aqueous solutions either by simultaneous or successive reduction.

Effect of solute atoms on swelling in Ni alloys and pure Ni under He+ ion irradiation

The effects of solute atoms on microstructural evolutions have been investigated using Ni alloys under 25 keV He irradiation at 500 °C. The specimens used were pure Ni, Ni -Si, Ni-Co, Ni-Cu, Ni-Mn and Ni-Pd alloys with different volume size factors. The high number densities of dislocation loops about 1.5 × 10 22 m 3 were formed in the specimens irradiated to 1 x 10 19 ions/m 2 , and they were approximately equivalent, except for Ni-Si. The mean size of loops tended to increase with the volume size factor of solute atoms. In a dose of 4 × 10 20 ions/m 2 , the swelling was changed from 0.2% to 4.5%, depending on the volume size factors. The number densities of bubbles tended to increase with the absolute values of the volume size factor, and the swelling increased with the volume size factors. This suggests that the mobility of helium and vacancy atoms may be influenced by the interaction of solute atoms with them.

In-situ HRTEM observation of the melting-crystallization process of silicon

Abstract Behaviors in the melting-solidification process of float-zone silicon (Fz-Si) were examined by in-situ high-resolution transmission electron microscopy (HRTEM). Point defect clusters were observed to form at high temperatures. {1xa01xa01} liquid–solid interfaces were confirmed to proceed by the lateral movement of a pair of steps having several atomic heights, while {1xa00xa00} liquid–solid interfaces were flat and moved between both end facets. A crystallized {1xa01xa01} liquid–solid interface was observed at about 20 layers. A twin band was observed at an interface close to silicide formed by surface diffusion.

Oxygen tracer diffusion in the YBa2Cu3Oy superconductor

Abstract The oxygen diffusion coefficient of YBa 2 Cu 3 O y (YBCO) single crystals was determined by secondary ion mass spectrometry in a wide temperature range from 628 to 1073 K. It was found that the slope of the Arrhenius plot of the diffusion coefficient changes at the transition temperature from the orthorhombic to the tetragonal phase (TP). The activation energies of oxygen in the TP and the orthorhombic phase (OP) are 89.1±1.4 and 114.6±1 kJ mol −1 in the ab plane, and 284±2.8 and 126.9±0.5 kJ mol −1 in the c axis direction, respectively. In the OP, the activation energies in the ab plane and the c axis direction are almost the same, but the diffusion coefficients in the ab plane are larger than that in the c axis direction by more than three orders of magnitude. At higher temperatures of the TP, the slope of the Arrhenius plot in the c axis direction becomes larger and the diffusion coefficient of the c axis becomes closer to that of the ab plane towards the incongruent melting point of YBCO compounds.

Superconductivity in Bi-2212 thin films of Bi2Ln0.3Ca2.7Cu2Oz (Ln=Pr, Nd, Sm, Eu and Gd) prepared by laser ablation method

Abstract The Bi-2212 thin films of the Bi2Ln0.3Ca2.7Cu2Oz system (Ln=Pr, Nd, Sm, Eu and Gd) have been prepared by the laser ablation method. The films are of metastable Bi-2212 phase, which has not been synthesized by the solid state reaction method. The highest zero resistivity temperature (Tcz) of 57 K has been found for the film with Ln=Pr, and the valence of Pr determined by XPS measurement was trivalent. Further, each of the films with various Ln elements had a Tcz value higher by 12–45 K than the corresponding sample of the bulk Bi2Ln0.5Ca2.5Cu2Oz system.

Defect structures and phase transitions of FeRh alloys deformed at high speed deformation

Abstract Fe–Rh alloys of Rh concentrations ranging from 40 to 50at.% and of B2 phase were deformed by use of a compression machine capable of running tests at high speeds of impact. Induced complicated substructures and phases were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM), and positron annihilation spectroscopy (PAS). A characteristic arrangement of L1 0 domains was observed, along with very small transformed A1 domains and dispersed in the residual B2 matrix. On the basis of the experimental results, we suggest a coupling of a pair of shears along {112}〈11−1〉 B2 for the transformation mechanisms from B2 to L1 0 and A1 phases.