Junpei T. Okada

Persistence of Covalent Bonding in Liquid Silicon Probed by Inelastic X-Ray Scattering

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.

Atomic and electronic structures of an extremely fragile liquid

The structure of high-temperature liquids is an important topic for understanding the fragility of liquids. Here we report the structure of a high-temperature non-glass-forming oxide liquid, ZrO2, at an atomistic and electronic level. The Bhatia–Thornton number–number structure factor of ZrO2 does not show a first sharp diffraction peak. The atomic structure comprises ZrO5, ZrO6 and ZrO7 polyhedra with a significant contribution of edge sharing of oxygen in addition to corner sharing. The variety of large oxygen coordination and polyhedral connections with short Zr–O bond lifetimes, induced by the relatively large ionic radius of zirconium, disturbs the evolution of intermediate-range ordering, which leads to a reduced electronic band gap and increased delocalization in the ionic Zr–O bonding. The details of the chemical bonding explain the extremely low viscosity of the liquid and the absence of a first sharp diffraction peak, and indicate that liquid ZrO2 is an extremely fragile liquid.

Thermoelectric properties of polygrained icosahedral Al71−xGaxPd20Mn9 (x=0,2,3,4) quasicrystals

The electrical and thermal transport properties of quaternary icosahedral Al71−xGaxPd20Mn9 (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 Al71Pd20Mn9 to 0.26 for Al68Ga3Pd20Mn9 quasicrystal.

Spectral emissivity measurements of liquid refractory metals by spectrometers combined with an electrostatic levitator

A spectral emissivity measurement system combined with an electrostatic levitator was developed for high-temperature melts. The radiation intensity from a high-temperature sample was measured with a multichannel photospectrometer (MCPD) over the 700?1000?nm spectral range, while a Fourier transform infrared spectrometer (FTIR) measured the radiation over the 1.1?6??m interval. These spectrometers were calibrated with a blackbody radiation furnace, and the spectral hemispherical emissivity was calculated. The systems capability was evaluated with molten zirconium samples. The spectral hemispherical emissivity of molten zirconium showed a negative wavelength dependence and an almost constant variation over the 1850?2210?K temperature range. The total hemispherical emissivity of zirconium calculated by integrating the spectral hemispherical emissivity was found to be around 0.32, which showed good agreement with the literature values. The constant pressure heat capacity of molten zirconium at melting temperature was calculated to be 40.9 J mol?1?K?1.

Improvement of thermoelectric properties of icosahedral AlPdRe quasicrystals by Fe substitution for Re

The thermoelectric properties of quaternary icosahedral AlPdReFe quasicrystals obtained by replacing Re with Fe in icosahedral AlPdRe quasicrystals have been studied according to the guiding principle of weakly bonded rigid heavy clusters (WBRHCs). Anisotropy in the microstructures of arc-melted and annealed samples was found, and the electrical and thermal conductivities are larger in the direction perpendicular to the bottom of the arc-melted button than in the parallel case. The dimensionless figure of merit (ZT) in the perpendicular direction was increased 2.6 times from 0.08 to 0.21 by Fe substitution for Re in the icosahedral AlPdRe quasicrystals, which was caused by the increase of the Seebeck coefficient and the decrease of the phonon thermal conductivity, and the increase of the electrical conductivity, expected and not for the WBRHCs, respectively.

Compact electrostatic levitator for diffraction measurements with a two axis diffractometer and a laboratory x-ray source

A compact electrostatic levitator was developed for the structural analysis of high-temperature liquids by x-ray diffraction methods. The size of the levitator was 200 mm in diameter and 200 mm in height and can be set up on a two axis diffractometer with a laboratory x-ray source, which is very convenient in performing structural measurements of high-temperature liquids. In particular, since the laboratory x-ray source allows a great amount of user time, preliminary or challenging experiments can be performed with trial and error, which prepares and complements synchrotron x-ray experiments. The present small apparatus also provides the advantage of portability and facility of setting. To demonstrate the capability of this electrostatic levitator, the static structure factors of alumina and silicon samples in their liquid phases were successfully measured.

Thermal and optical properties of glass and crystalline phases formed in the binary R2O3-Al2O3 (R = La-Lu and Y) system

Rare-earth aluminate (RAlO3, R = La–Lu and Y) glass and crystalline phases were prepared by containerless levitation in an aerodynamic levitation furnace. In the RAlO3 system, La, Nd and Sm aluminum perovskites solidified as glass and Eu–Lu and Y aluminum perovskites solidified as crystalline phases. The glass forming region decreased with decreasing ionic radius of the rare-earth element. Scanning electron microscopy images and x-ray diffraction results revealed the formation of a single RAlO3 phase from the undercooled melt. The glass transition temperature, Tg, and density increased and the molar volume decreased with decreasing rare-earth element ionic radius. The refractive index at 589 nm exceeds 1.85 in each composition and a transparency of approximately 72% was achieved for the LaAlO3 glass.

Thermophysical properties of the melts of AlPdMn icosahedral quasicrystal

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.

Evidence of the Fine Pseudogap at the Fermi Level in Al-based Quasicrystals

Theoretical studies have predicted that quasicrystals (QCs) have conspicuously fine electronic structures near the Fermi level ( E F ); 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 E F 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 E F are observed for these alloys. This is a direct experimental result that confirms the fine electronic structure existing in the vicinity of the E F .

Thermophysical Properties of Platinum Group Metals in their Liquid Undercooled and Superheated Phases

This review briefl y describes the vacuum electrostatic levitation furnace developed by JAXA and the associated non-contact techniques used to measure the density, the surface tension and the viscosity of materials. The paper then presents a summary of the data taken with this facility in the equilibrium liquid and non-equilibrium liquid phases for the six platinum group metals (pgms): platinum, palladium, rhodium, iridium, ruthenium and osmium over wide temperature ranges that include undercooled and superheated phases. The presented data (density, surface tension and viscosity of Pt, Rh, Ir, Ru and Os and density of Pd) are compared with literature values.