J. T. Okada

Japan Aerospace Exploration Agency, Chōfu, Tokyo-to, Japan

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Publications (15)32.13 Total impact

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    ABSTRACT: In-situ identification of metastable phases formed from the undercooled LuFeO3 melt under controlled oxygen partial pressure Po2 was studied by x-ray diffraction measurements at a synchrotron radiation source. Real-time observation of the formation and growth of individual phases during the single recalescence of Lu3Fe2O7 and LuFe2O4 phases at Po2 of 1 × 103 Pa has been revealed by a high speed imaging system at 1 kHz. The obtained diffraction pattern of the metastable phase in the LuFeO3 system was consistent with that of the metastable and stable phases reported in the Lu-Fe-O system.
    Applied Physics Letters 05/2012; · 3.79 Impact Factor
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    ABSTRACT: Metallic liquid silicon at 1787 K is investigated using x-ray Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Our results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled liquid states. The population of covalent bond pairs in liquid silicon is estimated to be 17% via a maximally localized Wannier function analysis. Compton scattering is shown to be a sensitive probe of bonding effects in the liquid state.
    Physical Review Letters 02/2012; 108(6):067402. · 7.73 Impact Factor
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    ABSTRACT: Thermophysical properties of liquid gadolinium were measured using non-contact diagnostic techniques with an electrostatic levitator. Over the 1585K to 1920K temperature range, the density can be expressed as ρ(T)=7.41×103 − 0.46 (T − T m)(kg · m−3) where T m=1585K, yielding a volume expansion coefficient of 6.2×10−5K−1. In addition, the surface tension data can be fitted as γ(T)=8.22×102 − 0.097(T − T m)(10−3N · m−1) over the 1613K to 1803K span and the viscosity as η(T)=1.7exp[1.4×104/(RT)](10−3Pa · s) over the same temperature range. KeywordsDensity-Gadolinium-Liquid-Surface tension-Viscosity
    International Journal of Thermophysics 01/2010; 31(2):388-398. · 0.57 Impact Factor
  • Y. Takagiwa, J.T. Okada, K. Kimura
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    ABSTRACT: The composition and carrier concentration variations of the thermoelectric properties of the binary narrow-gap compound Ga67−xRu33+x have been investigated. The measured samples were synthesized by a combination of arc-melting and spark plasma sintering (SPS), succeeded in crack-free samples. We report that the temperature dependences of the electrical resistivity and Seebeck coefficient of the Ga67−xRu33+x are strongly affected by nominal Ru concentration. The Seebeck coefficients showed large positive values from 170 to 350 μV K−1 at 373 K. Also, large power factors from 2.2 to 3.0 mW m−1 K−2 were obtained at 773 K. The dimensionless figures of merit ZT beneficially increased with increasing temperature and reached a maximum value of 0.50 at about 773 K.Research highlights▶ Ga67−xRu33+x compounds were synthesized by arc-melting and spark plasma sintering methods. ▶ Large power factors from 2.2 to 3.0 mW m−1 K−2 were obtained at 773 K. ▶ Maximum dimensionless figure of merit was 0.50 at 773 K for Ga66.6Ru33.4 compound.
    Journal of Alloys and Compounds 01/2010; 507(2):364-369. · 2.73 Impact Factor
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    ABSTRACT: The electrical and thermal transport properties of quaternary icosahedral Al <sub>71-x</sub> Ga <sub>x</sub> Pd <sub>20</sub> Mn <sub>9</sub> (x=0,2,3,4) quasicrystals, obtained by replacing Al in icosahedral Al–Pd–Mn quasicrystal with Ga, have been measured in accordance with the guiding principle of “weakly bonded rigid heavy clusters” (WBRHCs). While the electrical conductivity and Seebeck coefficient were not dramatically changed, the thermal conductivity effectively decreased with increasing Ga concentration except for the sample with x=4 . Although the thermoelectric properties do not obey the WBRHCs, the dimensionless figure of merit increased by a factor of 1.4 from 0.18 for Al <sub>71</sub> Pd <sub>20</sub> Mn <sub>9</sub> to 0.26 for Al <sub>68</sub> Ga <sub>3</sub> Pd <sub>20</sub> Mn <sub>9</sub> quasicrystal.
    Journal of Applied Physics 11/2008; · 2.21 Impact Factor
  • Y. Miyazaki, J. T. Okada, E. Abe, K. Kimura
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    ABSTRACT: Two-dimensional structures of decagonal approximant crystals have been studied in a B-Mg-Ru system where decagonal quasicrystals have been predicted. High-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) imaging confirms that four approximant phases, which form at high temperatures, can be described as tessellations of hexagon and boat subunits with the same atomic decorations as B11Mg5Ru13. The images also support the existence of bow-tie flip structures in the all four approximant crystals. Long periodicities in one direction up to 8.5 nm are interpreted to be caused by modulation of rows of boat tiles.
    Philosophical Magazine 01/2008; 88. · 1.60 Impact Factor
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    ABSTRACT: Positron annihilation lifetimes of the stable icosahedral (i) Al–Pd–Mn quasicrystal and its (1/1, 2/1)-AlPdMnSi and 1/0-Al12Re approximant crystals were measured at ambient temperature. The observed single-lifetime of the i-Al–Pd–Mn quasicrystal is very close to that of the (1/1, 2/1)-AlPdMnSi approximant crystals, indicating that the same type of structural vacancies exists in all these specimens. On the other hand, the lifetime of 1/0-Al12Re approximant crystal, which has no structural vacancies, is significantly shorter than the above results. We discuss the relationship between the positron lifetimes and mean valence-electron density.
    Philosophical Magazine 01/2008; 88(13):1929-1934. · 1.60 Impact Factor
  • Y. Miyazaki, J. T. Okada, K. Kimura
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    ABSTRACT: We have discovered four new approximant phases of the B–Mg–Ru decagonal quasicrystal. Six phases were observed in electron diffraction patterns of B–Mg–Ru alloys prepared at 1673 K from B2 x Mg(1- x )Ru2(1- x ) (x = 0.44–0.48). Two of them are known, B4Mg2Ru5 and B11Mg5Ru13, and the other four are new approximant phases. Their lattice parameters range from a = 0.84 nm to 8.5 nm; some of them can be described with the Fibonacci sequence. We suggest tiling models of these approximants using two subunits of B4Mg2Ru5 and B11Mg5Ru13, ‘hexagon’ and ‘boat’.
    Philosophical Magazine A 06/2007; 87(Nos. 18-21):2701-2706.
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    ABSTRACT: Theoretical studies have predicted that quasicrystals (QCs) have conspicuously fine electronic structures near the Fermi level (EF); the density of states (DOS) consists of sharp peaks and deep valleys with several 10 meV intervals. These fine electronic structures are believed to explain the unusual sensitivity of the electrical conductivity to slight changes in the chemical composition of QCs. We have investigated the DOS around the EF on single grains of icosahedral (i-)AlCuFe, i-AlPdMn, and decagonal (d-)AlCuCo QCs by break-junction tunneling spectroscopy at 4.2 K. Well-defined and reproducible pseudogaps at the EF are observed for these alloys. This is a direct experimental result that confirms the fine electronic structure existing in the vicinity of the EF.
    Journal of the Physical Society of Japan 01/2007; 76(3). · 2.09 Impact Factor
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    ABSTRACT: Thermophysical properties of Al72Pd20Mn8 icosahedral quasicrystal melts were measured using the electrostatic levitation (ESL) technique. Viscosity increases substantially with decreasing temperature and increases rapidly just above the melting point. In the solidification process, an under-cooled state and a recalescence were not observed.
    Philosophical Magazine A 01/2007; 87(18):2965-2971.
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    ABSTRACT: The dynamic structure factor S(Q,ω) of the melt of an icosahedral quasicrystal, Al72Pd20Mn8, was measured at 1223K near the melting point, Tm=1140K, for momentum transfers, Q, from 1.5nm−1 to 5.55nm−1 by means of inelastic X-ray scattering technique using synchrotron radiation at SPring-8. The composition of Al72Pd20Mn8 is a special one in AlPdMn ternary alloys, since icosahedral AlPdMn quasicrystal is formed directly from the melt. The acoustic mode was observed in the low Q region, and a sound velocity was estimated to be 4170±100m/s.
    Journal of Non-Crystalline Solids 01/2007; 353(32):3174-3176. · 1.72 Impact Factor
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    ABSTRACT: The electron momentum distributions in icosahedral Al64Cu23Fe13, icosahedral Al63Cu23Ru13 and decagonal Al65Cu15Co20 quasicrystals have been studied using the high-resolution Compton scattering technique. The electron-per-atom ratios (e/a) of the quasicrystals were determined quantitatively for the first time from the Compton profiles. The radii of the Fermi spheres were evaluated from the values of e/a on the basis of the free-electron model. Comparisons between the radius of the Fermi spheres and the size of the quasi-Brillouin zones show that the icosahedral quasicrystals meet the empirical matching condition, while the decagonal quasicrystal does not do this so well. This implies that the Hume-Rothery mechanism works for the formation of the pseudogap near the Fermi level in the icosahedral quasicrystals, although it operates only slightly in the decagonal quasicrystal.
    Journal of Physics Condensed Matter 07/2006; 18(31):7203. · 2.22 Impact Factor
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    ABSTRACT: The dynamic structure factor S(Q,ω) of the melt of an icosahedral quasicrystal, Al72Pd20Mn8, was measured at 1223 K near the melting point, Tm = 1140 K, for momentum transfers, Q, from 1.5 to 27.15 nm−1 by means of an inelastic x-ray scattering technique using synchrotron radiation at SPring-8. The composition of Al72Pd20Mn8 is a special one in AlPdMn ternary alloys, since an icosahedral AlPdMn quasicrystal is formed from the melt. The acoustic mode was observed in the low-Q region, and a substantial broadening of the longitudinal current–current correlation function and the crossover of the effective sound velocity from hydrodynamic to viscoelastic regions were observed around 6 nm−1, which hints at the existence of a cluster with a size of about 1 nm in the melt.
    Journal of Physics Condensed Matter 01/2006; 18(49):L613-L618. · 2.22 Impact Factor
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    ABSTRACT: The electron momentum distribution in icosahedral Cd84Yb16 has been studied using the high-resolution Compton scattering technique with a momentum resolution of 0.16 a.u. The experimental valence-electron Compton profile is decomposed into two components: an inverted parabolalike one and a broad Gaussian-like one. We have found that the Fermi sphere, deduced from the number of electrons under the inverted parabolalike component, just coincides with the quasi-Brillouin zones constructed from the intense (211111) and (221001) reciprocal points. The Gaussian-like part is attributed to the electron occupation of the Yb 5d states. These facts are taken as signature that both the Hume-Rothery mechanism and the sp-d hybridization mechanism contribute to the formation of the pseudogap, stabilizing the icosahedral phase of Cd84Yb16.
    Physical Review B 10/2003; 68(13). · 3.66 Impact Factor
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    ABSTRACT: Tunneling spectroscopy studies of an α-Al9Mn1.8Si2 single crystal were made at 4.2 K with the break-junction technique. A narrow dip at zero bias (V=0) with peaks on both sides was observed in the spectrum, which corresponds to a pseudogap of 25 meV in the density of states. The dense set of sharp spikes predicted by the band calculations for the crystal was not observed.
    Journal of Non-crystalline Solids - J NON-CRYST SOLIDS. 01/2002; 312:513-516.

Publication Stats

25 Citations
32.13 Total Impact Points

Institutions

  • 2010–2012
    • Japan Aerospace Exploration Agency
      • Institute of Space and Astronautical Science (ISAS)
      Chōfu, Tokyo-to, Japan
  • 2003–2008
    • The University of Tokyo
      • • Department of Advanced Materials Science
      • • Institute of Industrial Science
      Tokyo, Tokyo-to, Japan
  • 2006
    • RIKEN
      Вако, Saitama, Japan