Hirotoshi Enoki

Phase equilibria in the Fe–Co binary system

Abstract α (A2)/γ (A1) phase equilibria of the Fe–Co system between 400 and 800°C were determined by means of lattice parameter measurement using thin film specimens. Bulk specimens were also analyzed to compare the extent of attainment to the equilibrium. The thin film technique was found to be greatly advantageous for obtaining the phase equilibria at lower temperatures where solid-state reactions are too slow to reach the equilibrium state in the conventional methods using bulk specimens. It was confirmed that the α+γ two-phase region extends below the temperature at which the α (A2)/α′ (B2) transus meets the α/α+γ boundary. Thermodynamic analysis was also conducted by taking the magnetic and chemical ordering contributions of the B2 structure into account, findings of which confirmed the extension of the α+γ two-phase region below the α/α′ ordering temperature.

New approach for synthesizing Mg-based alloys

Abstract Mg 2 Ni, Mg 2 Cu and MgZn 2 were synthesized by means of ball-milling plus solid-state reactions. Firstly, the mixtures of the two elemental powders, corresponding to the compositions of Mg 2 Ni, Mg 2 Cu and MgZn 2 , were subject to ball-milling for 20 min. Then, they were pressed into pellets and sintered at temperatures below the melting points of the components. X-ray diffraction experiments show that the yields of the desired products are above 97 wt.%, higher than those prepared by the conventional metallurgical method. The hydrogenation properties of the Mg 2 Ni sample synthesized by solid-state reaction are in agreement with the previously published results on alloys prepared by the conventional methods.

Effect of Fe addition on hydrogen storage characteristics of Ti0.16Zr0.05Cr0.22V0.57 alloy

Abstract The effect of Fe addition on hydrogen storage characteristics of Ti 0.16 Zr 0.05 Cr 0.22 V 0.57 alloy has been studied at 303 K. The X-ray diffraction (XRD) patterns of the alloy powders showed the typical patterns of BCC structure as a main phase in all of the alloys. With increasing Fe content, the lattice parameters of the BCC phases decreased linearly in accordance with Vegard’s law. With regard to the maximum hydrogen storage capacities of the alloys, there was a noticeable decrease between alloys that contained 3 and 5 at% of Fe. The reason for this can be deduced from three factors: the decrease in the lattice parameter of BCC phase, the decrease of the amount of BCC phase itself in the alloy, and the excess of the electron-to-atom ratio over 5.1 (5.09 at 3 at% Fe and 5.16 at 5 at% Fe) with increasing Fe content. The Fe addition remarkably increased the second plateau pressures in the pressure composition (P–C) isotherms. However, the first plateau was not observed in all the alloys under the conditions of the present study. From the relation between the hydrogen storage capacities and the lattice parameters of the alloys, it was found that controlling the lattice parameter similarly to that of V (3.03 A), i.e. within the range of about 3.02–3.04 A, is of fundamental importance to obtain the largest effective hydrogen storage capacity in this alloy systems under the present conditions.

In situ X-ray diffraction study of hydrogen-induced phase decomposition in LaMg12 and La2Mg17

Abstract In situ X-ray diffraction (XRD) shows that LaMg 12 and La 2 Mg 17 are thermally stable under argon upon heating to 330°C, while under hydrogen they decompose to LaH 3 and MgH 2 at about 290 and 270°C, respectively. The decomposition is believed to result from the large difference in enthalpy between the parent compounds and the resultants. Prior to the decomposition, the lattice parameters of LaMg 12 and La 2 Mg 17 do not change, indicating that hydrogen does not dissolve into them to form solid solution of LaMg 12 H x and La 2 Mg 17 H x . The enhanced mobility of La and/or Mg was observed in the presence of hydrogen, which might be related to the formation of the copious vacancy.

Ball-milling of Mg2Ni under hydrogen

Abstract The intermetallic compound Mg 2 Ni is milled under hydrogen in a high-energy planetary mill. The resulting material is a mixture of heavily deformed Mg 2 Ni, low-temperature Mg 2 NiH 4 , and, possibly, high-temperature Mg 2 NiH 4 structures. The relative amount of each phase depends on the initial hydrogen pressure in the milling vial.

The intermediate compound in the In2O3-SnO2 system

The In2O3-SnO2 binary system between 1473 and 1873 K has been investigated by TEM observations in detail. The intermediate compound has been detected above 1573 K in the composition range from 47.9 to 59.3 mol% SnO2, that crystal structure is long range ordered cubic system similar to In2O3 phase. On the other hand, below 1473 K, the intermediate compound is decomposed into In2O3 and SnO2 phases, according to the eutectoid reaction.

Development of an energy-domain 57Fe-Mössbauer spectrometer using synchrotron radiation and its application to ultrahigh-pressure studies with a diamond anvil cell.

An energy-domain (57)Fe-Mössbauer spectrometer using synchrotron radiation (SR) with a diamond anvil cell (DAC) has been developed for ultrahigh-pressure measurements. The main optical system consists of a single-line pure nuclear Bragg reflection from an oscillating (57)FeBO(3) single crystal near the Néel temperature and an X-ray focusing device. The developed spectrometer can filter the Doppler-shifted single-line (57)Fe-Mössbauer radiation with a narrow bandwidth of neV order from a broadband SR source. The focused incident X-rays make it easy to measure a small specimen in the DAC. The present paper introduces the design and performance of the SR (57)Fe-Mössbauer spectrometer and its demonstrative applications including the newly discovered result of a pressure-induced magnetic phase transition of polycrystalline (57)Fe(3)BO(6) and an unknown high-pressure phase of Gd(57)Fe(2) alloy placed in a DAC under high pressures up to 302 GPa. The achievement of Mössbauer spectroscopy in the multimegabar range is of particular interest to researchers studying the nature of the Earths core.

Phase components and hydriding properties of the sintered Mg-xwt.% LaNi5 (X=20-50) composites

Abstract Powder mixtures of magnesium and LaNi 5 with nominal composition Mg– x wt.% LaNi 5 ( x =20–50) were pressed into pellets and sintered at 973 K for 1 h. After sintering, it was found that when x =20–40 the major phases were pure Mg, Mg 2 Ni and La 2 Mg 17 , including small amounts of MgO. Further increasing x to 50, Mg 2 Ni and La 2 Mg 17 remained the major phases but pure magnesium disappeared. In contrast to previously published results, the LaMg 12 phase was not found in the above composition range. The phase abundance as a function of composition was quantitatively determined by Rietveld analysis. It was also found that changing the sintering temperature from 723 to 973 K did not change the resultants of the sintered Mg–50 wt.% LaNi 5 composite as well as their relative amounts. All samples were easily activated at 623 K, and their pressure–composition isotherms of hydrogenation were measured at 573 K.

Thermal cycling of iridium coatings on isotropic graphite

The surface morphology observed on the iridium-coated isotropic graphite substrate varied widely between 300 and up to 2173 K thermal cycling and heat-treatment testing. The columnar structure was retained after thermal cycling between 300 and 1873 K. At high temperatures between 300 and 1973–2173 K thermal cycling, the columnar grain structure was replaced by dense equiaxed grains and the grain size increased with time and temperature. The structure obtained contains pores going outwards from the coating after thermal cycling between 300 and 1873 K; on the other hand, high-temperature thermal cycling porosities were diminished. Transmission electron microscopy of the specimens showed very little difference in grain size between as-deposited coatings and those thermally cycled between 300 and 1873 K. X-ray diffraction analysis indicated that the preferred orientation of columnar structure was destroyed by thermal cycling. In addition, there was no loss in weight after thermal cycling and heat treatment testing in nitrogen.

Phase equilibria in cobalt-rich portions of the Co-Si and Co-Ge systems

Abstract The cobalt-rich portions of the Co-Si and Co-Ge systems between 700 and 1100°C have been investigated by means of electron probe microanalysis. The equilibrium between α(f.c.c.) and e(c.p.h.) phases has been detected mainly by the diffusion couple technique, and it has been clarified that the c.p.h. phase is stable up to temperatures greater than 1100 °C in both the systems. It has also been found that the Co 3 Ge phase, probably having an A 15-type structure, exists somewhere between about 770 and 636 °C, being in equilibrium with the e and βCo 5 Ge 3 phases.