Papers by Author: Mitsuhiro Hasebe

Abstract: A thermodynamic analysis of the Ti-Zr-H ternary system has been performed by combining first-principles calculations with the CALPHAD approach. To enable the thermodynamic description of the binary systems, the results from our previous evaluation were adopted for the Ti-H, Zr-H, and Ti-Zr systems. The ternary compound, Ti2ZrH4, with an Fd3m-type crystal structure, exists over a wide composition range, and the (Ti)2(Zr)1(H,Va)4-type three-sublattice model was applied to describe its thermodynamic properties. Because of the lack of experimental information available, the enthalpy of formation of the Ti2ZrH4 phase was evaluated using the Full Potential Linearized Augmented Plane Wave method, and the estimated values were introduced into a CALPHAD-type thermodynamic analysis with some other experimental information. The calculated phase diagrams and the hydrogen isotherms were in good accordance with previous experimental results. Our calculations revealed that the ternary compound decomposes into a bcc and gas phase in the vicinity of 1270 K.

Abstract: A thermodynamic analysis of the Fe-Mn-P ternary system has been carried out using the CALPHAD method. Among the three binary systems relevant to this ternary phase diagram, the thermodynamic parameters of the Mn-P binary system were evaluated in this study. The enthalpy of formation of the binary phosphides obtained from our first-principles calculations was utilized in the present analysis to compensate for the lack of available experimental data. The thermodynamic descriptions of the Fe-Mn and Fe-P binary systems were taken from previous studies. The phase equilibria in the Fe-Mn-P ternary system were analysed based on the experimental data on the phase boundaries. The calculated phase diagrams are in agreement with the experimental results.

Abstract: A thermodynamic analysis of the Fe−M−P (M = Nb, Ti) ternary system has been performed by combining first-principles calculations with the CALPHAD approach. Because of the lack of experimental information available, thermodynamic properties of orthorhombic anti-PbCl2-type FeMP were evaluated using the Full Potential Linearized Augmented Plane Wave method, and the estimated values were introduced into a CALPHAD-type thermodynamic analysis. Applying this procedure, the phase diagrams of the Fe−M−P ternary phase diagrams whose contents are uncertain so far were calculated with a high degree of probability. Phase diagrams for high-purity ferritic stainless steels obtained following the same procedure are also presented.

Abstract: Thermodynamic assessment of the Al-Cr system has been carried out by incorporating first-principles calculations into the CALPHAD approach. A regular solution approximation was adopted to describe the Gibbs energy of the solution phases. The several phases appearing in the composition range between about 30 and 42 at.%Cr were treated as a single homogeneous γ-phase, based on recent experimental results, and the Gibbs energy of the γ-phase was represented using the four-sublattice model with the formula (Al,Cr)8(Al,Cr)8(Cr)12(Al)24. The calculated results enable the reproduction of experimental results on both the phase equilibria and thermochemical properties. In addition, a B2 ordered bcc phase, which was suggested to form as an equilibrium phase in a previous X-ray diffraction study, is not likely to form in either the stable state or metastable state based on our first-principles calculations.

Abstract: We carried out the growth of single crystalline silicon carbide (SiC) from Si-C-X (X= Co, Fe) ternary solutions. These ternary solutions are expected to show large carbon solubility compared with Si solvent (self-flux) by means of CALPHAD (CALculation of PHAse Diagrams) method. We investigated the growth rate and the polytype of the grown crystal from the ternary solutions. Then we found that the growth rate from the ternary solutions is much larger than that from the self-flux. The growth rate from Si-C-Co (Si-C-Fe) system was about 6mm/hr (12mm/hr) while that from the self-flux was only 2mm/hr. The grown crystal from the ternary solutions is classified into 6H that takes over the seed polytype.

Abstract: The growth of 6H-SiC crystal from Si-Ti-C ternary solution was conducted under the temperature gradient and the crystalline quality evaluations of the grown crystals were carried out. 6H-SiC(0001) on-axis pvt-grown crystal was used as a seed crystal. Micropipes in the seed crystal were terminated during the solution growth and 28mm􀊷28mm self-standing micropipe-free SiC crystals were obtained. The quality of the grown crystals was investigated by SIMS, high-resolution x-ray diffraction and molten KOH etching. The content of residual impurities in the SiC were very low. The X-ray 􀐢-rocking curves of the solution grown SiC showed single peak with high peak intensity ,while that of the seed crystal showed several peaks due to the misoriented domains. Moreover, it was found that the number of etch-pit in the grown crystal is much less than that in the seed crystal and it decreases with the increase of the growth thickness. These results indicate that the crystalline quality of grown crystal was significantly improved during the solution growth.