Akio Ishii

Dislocation pipe diffusion seems to be a well-established phenomenon. Here we demonstrate an unexpected effect, that the migration of interstitials such as carbon in iron may be accelerated not in the dislocation line direction ξ, but in a conjugate diffusion direction. This accelerated random walk arises from a simple crystallographic channeling effect. c is a function of the Burgers vector b, but not ξ, thus a dislocation loop possesses the same everywhere. Using molecular dynamics and acceler

We energetically predicted the morphology of Zr hydride precipitates in a hexagonal close-packed (HCP) Zr matrix. Considering Zr hydride precipitates as ellipsoids, we used Eshelby’s ellipsoidal inclusions to calculate the elastic energy increment due to the presence of Zr hydride precipitates in the Zr matrix, in which the elastic anisotropy and inhomogeneity of the elastic constants between Zr and Zr hydride were considered. We compared the difference in the elastic energy increment between th

We perform a molecular dynamics (MD) stress relaxation simulation for Zr50Cu40Al10 metallic glass to confirm that the time dependency of stress relaxation conforms with the Kohlrausch–Williams–Watts (KWW) equation, and to derive the temperature dependency of the Kohlrausch exponent βKWW. We also calculate local plastic deformation based on atomic strain, then discuss the morphology of relaxation and calculate the probability density of stress relaxation with respect to the characteristic time of

The existence of the B33 phase in TiNi alloys, which was reported to be a stable phase using density functional theory calculations but not confirmed experimentally, is controversial. Using Eshelby’s ellipsoidal inclusion, which was atomistically informed by density functional theory calculations, we investigated the existence of the B33 phase in the TiNi shape memory alloy. The calculated total strains of the heterogeneously nucleated B33 phase were similar to the eigenstrains of the B19’ phase

We propose an atomistically informed Eshelby’s inclusion analysis to investigate the morphology of secondary phases, which elastically interacted with each other through their respective local strain fields. Using the proposed method, we predict the morphology of δ-hydride precipitates and cracks, which interacted in the α–Zr matrix. Planar cracks nucleate along the basal-normal δ-hydride disk. And at the crack tip, the prismatic-normal δ-hydride disk also nucleates depending on the stress condi

We have constructed an atomistically informed phase-field model for the quantitative energetic analysis of phase transformations. In our model, to describe the general phase transformation with a non-linear correlation between displacive and diffusive modes, we have defined two order parameters, γ and ϕ, which describe the lattice distortion (displacive mode) and shuffling (diffusive mode), respectively. Our method provides a way to introduce the energetics from atomistic simulations to the phas

We energetically predict the morphology of Pd, Ag, Au, and Pt nanoparticles on (0001) sapphire substrates, using density functional theory (DFT) simulations and the well-known Young–Dupre equation. In all cases, the contact angles exceed 90°, indicating that the nanoparticles are spherical. Notably, Au nanoparticles exhibit a higher contact angle than those of their counterparts. The validity of the proposed abinitio nanoparticle morphology prediction approach based on DFT simulations was assess

To address the embrittlement challenges posed by gas blisters in anisotropic materials, the stable shape of constant-pressure blisters in anisotropic materials (hexagonal, tetragonal, and rhombohedral) was energetically investigated based on continuum theory (micromechanics), considering the blister as Eshelby’s ellipsoidal inclusion. The non-negligible change in the blister shape was confirmed in terms of the anisotropic factor η ≡ C3333/C1111. Although the spherical shape of the blister is pre

We have developed an accelerated molecular dynamics (MD) method to model atomic-scale rare events. In this method, a smooth histogram of collective variables is first estimated by canonical ensemble molecular dynamics calculations, and then a temperature-dependent boost potential is iteratively constructed to accelerate the MD simulation. This method not only allows us to observe the rare events but also to evaluate the profile of free energy and trial frequency along the reaction coordinate. We

Considering the nucleation process of Zr hydrides as phase transformation from hexagonal closed-packed (HCP) to face-centered tetragonal (FCT) structure, we calculated the activation energy of the homogeneous nucleation process of Zr hydrides and atomic rearrangement during nucleation for Zr4H, Zr2H, ZrH and ZrH2 using density functional theory calculations and minimum energy path detection. At 0 K limit, although ZrH and ZrH2 have lower chemical potentials and are more energetically stable than

メディエーター型酵素修飾電極の設計では，酵素利用効率の向上，酵素固定化膜内電子移動，膜での基質の物質輸送などを念頭においた酵素修飾膜厚の制御が非常に重要な因子となっている．本研究では，酵素固定化膜の膜厚を操作回数で簡便に制御できる静電的相互作用を利用したレイヤーバイレイヤー法に注目し酵素電極を作製し，評価方法の検討を行った．アニオン性であるκ-カラギーナンとカチオン性であるピロロキノリンキノン依存性グルコースデヒドロゲナーゼとメディエーターであるオスミウム修飾ポリマーから成る酵素•メディエーター積層修飾電極を作製した．バイオエレクトロカタリシス反応の膜厚依存性を触媒定常電流により評価した．膜厚が増えるにつれて触媒定常電流値は増加し，やがて限界値に達した．この膜厚依存性挙動を，酵素反応速度パラメーター，メディエーター及び酵素濃度，メディエーター拡散係数などをパラメーターとする反応層理論に基づいた理論式で解析できることを実証した．

The properties of nanoparticles depend on their sizes, and these size effects in face-centered-cubic (FCC) nanoparticles are attributed to the edge and vertex effects. However, the effects of edges and vertices on the properties of nanoparticles have not yet been explicitly investigated. In this study, we propose a method to evaluate the edge and vertex effects in FCC nanoparticles using density functional theory atomistic simulations. Pd and Au FCC nanoparticles are modeled as conventional trun