Masahito Uchikoshi

Fast, vacancy-free climb of prismatic dislocation loops in bcc metals

Vacancy-mediated climb models cannot account for the fast, direct coalescence of dislocation loops seen experimentally. An alternative mechanism, self climb, allows prismatic dislocation loops to move away from their glide surface via pipe diffusion around the loop perimeter, independent of any vacancy atmosphere. Despite the known importance of self climb, theoretical models require a typically unknown activation energy, hindering implementation in materials modeling. Here, extensive molecular statics calculations of pipe diffusion processes around irregular prismatic loops are used to map the energy landscape for self climb in iron and tungsten, finding a simple, material independent energy model after normalizing by the vacancy migration barrier. Kinetic Monte Carlo simulations yield a self climb activation energy of 2 (2.5) times the vacancy migration barrier for 1/2〈111〉 (〈100〉) dislocation loops. Dislocation dynamics simulations allowing self climb and glide show quantitative agreement with transmission electron microscopy observations of climbing prismatic loops in iron and tungsten, confirming that this novel form of vacancy-free climb is many orders of magnitude faster than what is predicted by traditional climb models. Self climb significantly influences the coarsening rate of defect networks, with important implications for post-irradiation annealing.

Crystal growth and characterization of Mg2Si for IR-detectors and thermoelectric applications

We have investigated the melt growth of Mg2Si crystal and its electrical and optical properties. Progress in Mg source purity and stoichiometric control during the growth enabled the development of a high purity Mg2Si crystal with low carrier density and a high stable Mg2Si with good doping controllability. The Mg2Si crystal grown by the pressure controlled Bridgman method using 5N purity or 6N purity of Mg source and purified PG crucible showed low electron density (~1015 cm−3) and high electron mobility (485 cm2 V−1 s−1 at 300 K and 21900 cm2 V−1 s−1 at 40 K). Silver doping in the high purity crystals performed the low-hole density of p-type Mg2Si (~3 × 1016 cm−3). Ionization energy of residual Al donor in the high purity crystal and Ag acceptor in the Ag doped crystals was determined as 8–9 meV and 26 meV, respectively. Indirect band gap energy Eg of approximately 0.61 eV at 300 K and 0.69 eV at 4 K were estimated by the optical transmission measurements on the high purity crystals. It is also found that the Sb-doped melt grown crystal had good power factor around room temperature (26 µW cm−1 K−2 at 270 K).

Preparation of high-purity cobalt

A process consisting of anion exchange in a hydrochloric acid solution, electrowinning, plasma arc melting and electron beam melting of electrolytic cobalt has been developed for preparing high-purity cobalt. Glow discharge mass spectrometry (GDMS) and residual resistivity ratio (RRR) were applied for analysis and purity evaluation of the produced cobalt. Almost all the metallic impurities except for copper were removed efficiently and reduced to lower than 1 mass ppm by the anion exchange step. Electron beam melting at a final step was very useful for removal of copper by vaporization from the molten cobalt and the copper concentration could be lowered to below 1 mass ppm. As a result, high-purity cobalt with above 99.9995% in purity (RRR=210) was produced by this practical purification process.

Noncontact Laser Modulation Calorimetry for High-Purity Liquid Iron

The heat capacity and thermal conductivity of liquid iron were measured the using recently developed method of noncontact laser modulation calorimetry. An iron sample was levitated using an electromagnetic levitator. Then the convection in the levitated droplet was suppressed to measure the thermal conductivity by the application of a dc magnetic field. High-purity iron (99.9972 mass %) prepared using an ion exchange method was used for measurements. The molar heat capacity of liquid iron at constant pressure was measured to be 45.4 ±3.2 Jmol-1K-1 (1848–1992 K) in low dc magnetic fields because a semi-adiabatic condition was achieved, assisted by the remaining convection in the liquid. The apparent thermal conductivity of liquid iron decreased concomitantly with the increasing dc magnetic field. It finally converged to 39.1 ±2.5 Wm-1K-1 (1794–2050 K) at 9 T or higher. The experimental uncertainties in the molar heat capacity and thermal conductivity are double the standard deviation.

Purification of CuCl2 by Anion Exchange Separation using Multi-column Method

The demand for high purity Cu is growing because it has an advantage to reduce the resistivity of the interconnect in ultra large-scale integration. Anion-exchange separation with the multi-column method was proposed and its separation efficiency was investigated in this work. The recoveries of anion-exchange separation for Cu purification in the previous studies are relatively low. Whole separation efficiency would be improved if the yield could be increased. Prior to the separation tests, the target impurities were determined from thermodynamic consideration of the process for preparation of high purity Cu. As a result, anion-exchange separation with the multi-column method proposed in this paper successfully eliminated the possible impurities, which should be removed in order to obtain a ultra high purity Cu. Thus, the purified CuCl 2 -HCl solution is ready for preparation of the ultra high purity Cu. *To whom correspondence should be addressed +IMRAM, Tohoku University, Japan J Graduate Student, Tohoku University, Japan ^College of Engineering, Ibaraki University, Japan

Solution growth and low-temperature thermoelectric properties of single crystalline β-FeSi 2

We have measured the Seebeck coefficient and power factor of single crystalline beta-FeSi₂ grown by a temperature gradient solution growth (TGSG) method using Ga and Zn solvent. Typical resistivity and carrier density at 300 K were 0.03 Omegacm and 2times10¹⁹ cm^-3 for p-type and 0.2 Omegacm and 5times10¹⁸ cm^-3 for n-type crystals, respectively. The Seebeck coefficient measured along [011] direction was approximately 350 muV/K(p-type) and -700 mu;V/K(n-type) at 300 K and showed a maximum value of 500 mu;V/K(p-type, T=~25 K) and 2100 mu;V/K(n-type, T=~70 K). The maximum power factor was 4.2 times 10^-6 Wcm^-1K^-2 (p-type, T=170 K) and 23 times 10^-6 Wcm^-1K^-2 (n-type, T=100 K). The value was more than one order of magnitude larger than that of previously reported.

Thermal Conductivity Measurement of Molten Cu-Co Alloy Using an Electromagnetic Levitator Superimposed with a Static Magnetic Field

The thermal conductivity of molten Cu-Co alloy with different compositions around the liquidus line temperature was measured by the periodic laser-heating method using an electromagnetic levitator superimposed with a static magnetic field to suppress convection in a levitated droplet sample. During the measurement, a static magnetic field of 10 T was applied to the levitated droplet. To confirm that the strength of the static magnetic field was sufficient to suppress convection in the droplet, numerical simulations were performed for the flow and thermal fields in an electromagnetically levitated droplet under a static magnetic field, and moreover, for the periodic laser-heating method to determine the thermal conductivity. It was found that the thermal conductivity of molten Cu-Co alloy increased gradually with increasing Cu composition up to 80 at. pct, beyond which it increased markedly and reached that of pure Cu. In addition, it was found that the composition dependence of the thermal conductivity can be explainable by the Wiedemann–Franz law.

Anion-exchange Behavior of Mo(V) and W(VI) in HCl Solutions

The distribution coefficients of Mo(V) and W(VI) on anion-exchange resin from hydrochloric acid solutions, the separation factor of W(VI) over Mo(V), and adsorption isotherm of Mo(V) at 30°C were investigated as the fundamentals of chemical separation process of Mo and W. Prior to the acquiring the equilibrium distribution coefficients, the stability of Mo(V) oxy-chloro complex were examined using spectrophotometer and the solubility of WCl6 in HCl solution was determined. Molybdenum(V) adsorbs on anion-exchange resin strongly in> 4 M HCl and slightly in< 2 M HCl and the distribution coefficients of Mo(V) are always higher than those of W(VI). Taking both the distribution coefficients and the separation factors into consideration predicts that the optimum condition for the separation of Mo(V) and W(Vl) is in 2 M HCl at 30°C. In addition, adsorption isotherm of Mo(V) shows that the distribution coefficients in 2 M HCl is independent of the concentration of Mo(V) in the solution phase.

Determination of structures of cupric-chloro complexes in hydrochloric acid solutions by UV-Vis and X-ray absorption spectroscopy

Ultrahigh-purity metals are indispensable to understanding the nature of materials, but the purity of contemporary metals is insufficient for determining their intrinsic properties. The fundamentals of the anion-exchange reaction must be clear in order to increase the refining efficiency of anion-exchange separation for purification of metals. The thermodynamic analysis of the anion-exchange reaction needs to take account of the distributions of metal-chloro complexes in the solution phase in addition to those between the resin and the solution phase. The structures of metal-chloro complexes in the solution phase must be determined as the first step in determining what species is adsorbed on the resin. Copper is one of the base metals most commonly used in modern society, and the anion-exchange behavior of cupric species is representative. The distribution and the molar attenuation coefficients of cupric-chloro complexes in hydrochloric acid solutions were obtained employing factor analysis followed by fitting a thermodynamic model to ultraviolet-visible absorption spectra. The cumulative formation constants were determined as follows: log10β1=0.599

In situ spectroscopic studies of the one-pot synthesis of composition-controlled Cu–Ni nanowires with enhanced catalytic activity

\log _{10}\beta _{1} = 0.599