Papers by Author: Katsushi Tanaka

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Authors: Veronique Pierron-Bohnes, R.V.P. Montsouka, Christine Goyhenex, T. Mehaddene, Leila Messad, H. Bouzar, Hiroshi Numakura, Katsushi Tanaka, B. Hennion
Abstract: Ferromagnetic L10 ordered alloys are extensively studied nowadays as good candidates for high density magnetic storage media due to their high magnetic anisotropy, related to their chemical order anisotropy. Epitaxial thin bilayers NiPt/FePt/MgO(001) have been grown at 700 K and annealed at 800 K and 900 K. At 800 K, the L10 long-range order increases without measurable interdiffusion. At 900 K, the interdiffusion takes place without destroying the L10 long-range order. This surprising observation can be explained by different diffusion mechanisms that are energetically compared using molecular dynamics simulations in CoPt in the second moment tight binding approximation. In addition, the frequencies of the normal modes of vibration have been measured in FePd, CoPt and FePt single crystals using inelastic neutron scattering. The measurements were performed in the L10 ordered structure at 300 K. From a Born-von Karman fit, we have calculated the phonon densities of states. The migration energies in the 3 systems have been estimated using the model developed by Schober et al. (1981). The phonon densities of states have also been used to calculate several thermodynamic quantities as the vibration entropy and the Debye temperature.
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Authors: Satoshi Tanaka, Yoshiyuki Saito, Katsushi Tanaka, Nozomu Uchida, Keizo Uematsu
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Authors: S. Harada, Katsushi Tanaka, Kyosuke Kishida, Haruyuki Inui
Abstract: Effect of the ternary element of Mo on the crystal structure and thermoelectric properties of ReSi1.75 has been investigated. The crystal structure of Mo-containing ReSi1.75 has shear structure up to 1.5 at%, the structure changes into adaptive structure at larger Mo contents. The concentration of Si vacancies estimated from the crystal structures determined decreases with increasing Mo content. Thermoelectric properties of Mo-containing ReSi1.75 indicate that the character changes from p- to n-type semiconductor upon alloying with Mo, which is explained by the decrease in Si vacancy concentration.
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Authors: Tatsuya Koyama, Akira Ishida, Kyosuke Kishida, Katsushi Tanaka, Haruyuki Inui
Abstract: Manganese-alloyed Ru2Si3-based alloys with various alloy compositions have been prepared and the phase relationships of these alloys have been investigated as a function of the Mn concentration using X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. A series of Ru1-xMnxSiy chimney-ladder phases is confirmed to be formed over a wide compositional range (x ≥0.12). These chimney-ladder phases are considered to be formed to stabilize the HT-Ru2Si3 chimney-ladder phase through the substitution of Ru with Mn. The compositions of the chimney-ladder phases are, however, significantly deviated from the idealized composition satisfying the valence electron concentration rule: VEC=14.
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Authors: Haruyuki Inui, Katsushi Tanaka, Kyosuke Kishida, S. Harada
Abstract: The changes in microstructure and defect structure of two different semiconducting transition-metal silicides, ReSi1.75 and Ru2Si3 with ternary alloying of substitutional elements with a valence electron number different from that of the constituent metal have been investigated in order to see if the crystal and defect structures of these silicides and thereby their physical properties can be controlled through defect engineering according to the valence electron counting rule. The Si vacancy concentration and its arrangement can be successfully controlled in ReSi1.75 while the relative magnitude of the metal and silicon subcell dimensions in the chimney-ladder structures can be successfully controlled in Ru2Si3.
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Authors: Shinichi Ninomiya, Kiyoshi Suzuki, Tetsutaro Uematsu, Manabu Iwai, Katsushi Tanaka
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Authors: Ryuji Matsumoto, Katsushi Tanaka, Kyosuke Kishida, Haruyuki Inui
Abstract: The effect of hydrostatic pressure mechanically applied to hydride on the equilibrium hydrogen gas pressure and temperature have been examined theoretically. From free energy calculations where elastic energy is taken into account, equilibrium gas pressure and temperature increases and decreases with increasing applied compressive stress, respectively. In the case of magnesium hydride, equilibrium temperature decreases to 63 °C at the hydrogen gas pressure being 1 atom when volume expansion at hydriding is perfectly suppressed by an external compressive stress. The temperature is remarkably lower than that obtained by alloying various elements.
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Authors: Toru Inoue, Katsushi Tanaka, Hiroki Adachi, Kyosuke Kishida, Haruyuki Inui
Abstract: The crystallographic orientation distribution, and its change as a function of creep deformation in Ni-based single crystal superalloys have been investigated by X-ray diffractometry. The distribution of the crystallographic orientation has significantly broadened by creep deformations. Directional broadening of the distribution agrees with creep dislocations having the burgers vector of 1/2<101>. High temperature creep strain of superalloys can be estimated by a non-destractive test where the width of rocking curve of a diffraction peak is measured.
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Authors: Katsushi Tanaka, Wataro Hashimoto, Toru Inoue, Haruyuki Inui
Abstract: The effect of elastic driving force on the microstructural change of superalloys in the secondary creep stage is evaluated by elastic energy calculations with the concept of effective eigen strain where both lattice mismatch and creep strain are taken into account The elastic energy calculations indicates that the elastic state in the secondary creep stage is totally different to that in the initial one where the lattice misfit between γ and γ' phases is over accommodated along the [100] and [010] directions by creep deformation in the γ phase. The excess creep dislocations for the over accommodation are required so as to develop an internal stress field to prevent further creep deformations. The planer raft structure with the plane normal oriented to the [001] direction is unstable in the over accommodated state. The γ/γ' lamellar interfaces will be inclined to make a wavy raft structure of which elastic energy is lower than the ideal 001 planer raft structure.
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Authors: Kiyoshi Suzuki, Y. Tanaka, Manabu Iwai, Tetsutaro Uematsu, Katsushi Tanaka
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