Kimio Itagaki

Low temperature crystal structure of Ni–Mn–Ga alloys

Transmission electron microscope (TEM) observation at room temperature of the low temperature phase of Ni2.14Mn0.86Ga shows stress-induced 5 periodicity modulation along a pseudo-[111] cubic zone axis. On cooling to 173 K, the periodicity changes to 7. In stress-free areas, a [001] zone axis with new lattice parameters for the tetragonal phase was observed. The new lattice parameters are smaller by a factor of √2 than the expected lattice parameters. A new model for the tetragonal compound Ni2MnGa and the non-stoichiometric phases was developed with the help of TEM data, X-ray powder diffraction measurements, and a computational simulation based on the X-ray powder diffraction patterns.

High temperature heat contents of III-V semiconductor systems

Abstract As part of calorimetric studies on metals and alloys at high temperature, heat contents of the III-V compounds of AlN, GaN, AlP, GaP, InP, AlAs, GaAs, InAs, AlSb, GaSb and InSb were measured over the temperature range 650 to 1550K using a drop calorimeter. The heat content and the heat capacity equations for the solid compounds were derived by the use of the Shomate function. Heat contents of the Ga-As, In-As, Ga-P and In-P binary alloys were also measured to derive the thermodynamic quantities of these liquid alloys by the use of a therniodynamic analysis method, and the vapour pressure-temperature-composition diagrams were constructed.

Thermodynamic analysis of the In-P, Ga-As, In-As and Al-Sb systems

Abstract Thermodynamic and phase diagram data are used to optimize the thermodynamic model parameter values of the liquid phases in four III-V binary systems: In-P, Ga-As, In-As and Al-Sb. An associated solution model is used to describe the thermodynamic properties of the liquid phases and the III-V compound semiconductors are taken to be stoichiometric (or line) compounds. For each of the binary systems, a set of thermodynamic parameters for the liquid and compound semiconductors are obtained. Using these values, the calculated phase diagrams are in agreement with the experimentally determined ones. Moreover, the model-calculated thermodynamic values also in agreement with the experimentally determined values.

Evaluation of the phase diagram and thermochemistry of the CU-Y system

Abstract Experimental information on the phase equilibria and thermochemistry of the Cu-Y system were combined with the help of a computer optimization program to achieve a self-consistent analytical description of the thermodynamic properties of Cu-Y alloys. The resulting set of coefficients allows calculation of thermodynamic functions for the pure components, stoichiometric phases or solution phases in the system, as well as calculation of the equilibria between the phases.

Heat content and heat of formation measurements of RNi5 ± x alloys (R ≡ La, Ce, Pr or Nd) and heat balance in a reduction-diffusion process

Abstract The heat contents of the LaNi 5.25 , CeNi 5.13 , PrNi 4.65 and NdNi 5.85 alloys were measured over the temperature range 800–1400 K using a drop calorimeter. The heat content and heat capacity equations were derived using the Shomate function. The heats of formation of these alloys were determined at 773 K from the heats of dissolution of these compounds and their components in a liquid tin solvent using a Calvet-type solution calorimeter. Heat balance was calculated for a reduction-diffusion process to make the RNi 5 powders directly for use as hydrogen storage media by reducing the rare earth resources with metallic calcium, magnesium and lithium.

Magnetic domain structure in Ni53.6Mn23.4Ga23.0 shape memory alloy films studied by electron holography and Lorentz microscopy

Abstract Magnetic domain structure in Ni 53.6 Mn 23.4 Ga 23.0 films containing nonmagnetic MnO precipitates was examined by electron holography and Lorentz microscopy. Close experiments discovered peculiar zig-zag-shaped lines of magnetic flux, which are presumably typical configuration in the martensite accompanied by many twin plates. Near the edge of the films, this configuration was modified into a distinct form that was favorable to the reduction of magnetic charges. Inside the martensite plates as crystallographic domains, the lines of magnetic flux were wavy due to the electromagnetic interaction with the nonmagnetic MnO precipitates. The observations have promoted an understanding of the magnetic domain structure in ferromagnetic shape memory alloy films, and also provided essential information for developments of magnetic field-driven microactuators using these films.

Formation of RExNiy (RE = La, Ce, Pr or ND) compounds by calciothermic reduction of La2O3, CeO2, Pr6O11 and Nd2O3

LaNi5, CeNi5, PrNi5 and NdNi5 powders were prepared from the coorresponding oxides by a reduction-diffusion method using calcium as reductant and the morphology of the product and the mechanism of the reaction were examined. It was clarified that CaxNiy compounds formed during the reaciton had a significant role in the formation of RExNiy. Instead of a common direct reaction of xRE + yNi = RExNiy, an indirect substitution reaction of aCaxNiy + bRE =cRExNiy + dCa prevailed. This enhanced the rate of formation of RENi5 significantly.

Calciothermic reduction of mischmetal oxide and formation of RmNi5

Abstract A lanthanum-base or cerium-base mischmetal oxide was reduced by metallic calcium at temperatures around 1273 K in the presence of nickel particles and the reduced mischmetal elements diffused into the particles to form an RmNi5 compound directly. A thin layer of CaNi5 was also formed between the RmNi5 layer and the unreacted nickel core, indicating that an exchange reaction, CaNi5 + Rm = RmNi5 + Ca, occurred. It was considered that the rate of formation of RmNi5 was enhanced by this reaction. A hydride of the lanthanum-base RmNi5 presented a plateau region in the PCT diagram while small hydrogen absorbability was shown for the cerium-base RmNi5. The absorbability was increased significantly by replacing some nickel sites in the lattice of the cerium-base RmNi5 by cobalt, aluminium or manganese.

A thermodynamic evaluation of the Ba-Cu-Y System

Abstract Experimental thermodynamic and phase equilibrium measurements in the binary systems Ba-Cu, Ba-Y and Cu-Y, together with experimental information on the miscibility gap in the ternary system Ba-Cu-Y, have been evaluated with the aid of the ‘Lukas optimization program’. The resulting set of coefficients allow phase equilibria and thermodynamic values to be reproduced within satisfactory limits.

Two-way Shape Memory Behavior of Sputter-deposited Ni-rich Ni2MnGa Films

The properties of the Ni-rich Ni2MnGa films and their shape memory behavior are investigated with respect to various sputtering conditions. The films are deposited on a poly-vinyl alcohol substrate with a radio-frequency magnetron sputtering apparatus using Ni50Mn25Ga25 and Ni52Mn24Ga24 targets. The sputtering power is in the range of 50–400 W. After separating from the substrate, the films are vacuum-annealed at 1073 K for 3.6 ks. The chemical composition of the films depends on the sputtering power. The martensitic transformation temperature increases with the valence electron density of the films, while the Curie temperature slightly decreases. The film obtained with 50 W using the Ni50Mn25Ga25 target after heat treatment at 1073 K shows a two-way shape memory effect through the martensitic transformation and its reversion.