Whisker formation in tin (Sn) plating in small electronic devices, which can cause a short circuit resulting in fatal functional damage, has been an important issue in the engineering field. With the recent requirement of lead-free materials, doping of Pb as a measure to reduce the whisker growth is not a practical solution any longer. Although the mechanism of the whisker formation and growth is quite complicated, stress migration is suspected to play an important role. Thus, it is impending to reveal the mechanism of diffusion of atoms in Sn under stress and to find a way to suppress it. In this study, we examine the effect of impurity atoms on the vacancy migration by ab initio density functional theory (DFT) calculation.

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To understand the nature of ferroelectric instabilities under finite electric fields, e.g., domain switching and polarization reversal, as an origin of critical failure of ferroelectric devices, it is essential to evaluate critical electrical and/or mechanical conditions where the ferroelectric state becomes unstable. However, the instability criterion for such a multi-physics system where the mechanical and electric properties interact each other has not yet been proposed. In this paper, we developed an analytical method to rigorously describe the multi-physics instability criterion for arbitrary atomic structures under finite electric field and/or mechanical load. According to the proposed method, the instability starts when the minimum eigenvalue of the Hessian matrix of the potential energy reaches zero. The corresponding eigenvector indicates the displacement of atoms at the instability. We applied the method to 180° domain walls in ferroelectric PbTiOs under external electric fields, and our criterion can successfully describe the onset of instability, namely, the domain switching, as well as their displacement pattern at the switching. This clearly indicates the validity of our multi-physics instability criterion. This success opens the door to investigate the intrinsic mechanism of more complicated ferroelectric instabilities, such as a vortex polarization reversal in nano-structured ferroelectrics and a pinning effect of vacancies of the domain switching.

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Based on the ab initio (first-principles) density-functional theory calculations within the local density approximation, we investigated the ferroelectricity as well as the atomic and electronic structures at Σ5(001) twist boundaries in PbTiO_3. The twist boundary structure with the coincidence site lattice (CSL) was found to be energetically favorable. We found the coexistence of both the rectilinear polarization and the vortex in-plane polarization at the twist boundary. An applied tensile strain tends to increase the rectilinear polarization, especially at the twist boundary. The vortex polarization on the PbO layer is also enhanced with the increase of tensile strain, whereas the vortex polarization on the TiO_2 layer decreases and finally disappears at a critical strain.

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In this study, we investigated thermal and mechanical properties of a compositional graded SiGe nano thin-film using molecular dynamics techniques. Stillinger-Weber potentials were employed to calculate interatomic interactions between Si and Ge atoms, and combined models with different atomic compositions were constructed in order to estimate influences of a compositional gradient on thermal and mechanical properties. Our results show that a large gradient of atomic compositions induces a large misfit energy and stress regardless of boundary conditions. Moreover, the misfit stress brings about changes in elastic properties, and thus elastic constants misfits of combined models become diminished. DOS analyses represent a change of vibration mode around an interface of combined models, implying a change in thermal properties.

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In the present paper, we propose the numerical scheme in which the Cahn-Hilliard (CH) equation is used to track the interface; the temperature recovery method is introduced to represent the phase change. In the present scheme, phase change can be easily taken into consideration by using CH equation involving the additional source term. To improve heat flux computation, we consider the boundary condition including contact angle and introduce microlayer model. As a practical problem, the result of the two dimensional pool boiling is reported.

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[in Japanese]

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This paper describes a phase-field model of solid-liquid phase transitions with inhomogeneous temperature in one-component systems. Our model includes fluid flow and non-linear elasticity in solids: plastic deformations take place at large elastic strains. We present two numerical results of melting in two dimensions. First, a solid domain is placed in a warmer liquid under shear flow. The solid melts gradually, preserving circular shape due to elasticity. Cooling of the surrounding liquid is accelerated by external shear flow. Second, a solid rod is under uniaxial compression in liquid. Slips appear from the solid-liquid interface, leading to plastic deformation. Melting starts in the slips, leading to the fracture of the rod into pieces.

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The improvement of the atomization of liquid jets is required in several engineering applications. Although the breakup process of liquid jets has been studied so far, the mechanism of second breakup contributing to the performance of atomization has not been well explained. Hence, in the present paper the temporal numerical simulation, which is a predictably effective method to analyze the breakup process in detail, is proposed. As remarkable features, the Cahn-Hilliard equation is employed to trace the interface between the gas-liquid two-phase flow. In order to validate and examine the possibilities of the proposed method, we compare the present results with both experimental and DNS results.

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[in Japanese]

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