Papers by Author: Yoshitaka Umeno

Abstract: We carried out molecular dynamics (MD) simulation and atomistic instability (ASI) analysis with carbon nanotubes (CNTs) under axial compression to reveal the mechanism of buckling. We investigated the development of instability mode until buckling of structure. For single-walled carbon nanotubes (SWCNTs), Euler-type buckling was found in relatively thin and long nanotubes, while buckling with deformation change of cross-sectional shape (radial buckling) was found in thick and short carbon nanotubes. The crossover between the Euler-type buckling modes and radial buckling modes was clearly seen in the ASI analysis.

Abstract: Coarse-graining molecular dynamics simulations were carried out for deformation and fracture problems in polycarbonate. The effects of temperature and molar mass on deformation behavior were qualitatively investigated. Temperatures below the glass transition point were examined. The temperature effect was found to be consistent with all-atom simulations and experiments. On the investigation of molar mass effect, it was revealed that complex entanglement of massive molecules can prevent voids from growing.

Abstract: A coarse-grained particle (CG) model was developed based on all-atom molecular dynamics simulation results, aiming at applying to deformation and fracture analyses of polycarbonate. After confirming the validity of the model, the developed CG model was applied to deformation analyses to investigate the effects of strain rate and multiaxial tension. The effect of strain rate was found to be consistent with an experiment. Two types of deformation behavior were observed according to the type of multiaxial tension.

Abstract: We carried out the atomistic structural instability (ASI) analysis with an empirical interatomic potential for carbon nanotubes (CNTs) under axial compression with the aim to reveal the mechanism of buckling. We investigated how ‘latent’ instability modes develop until one of them is activated at the structural instability. For pristine single-walled carbon nanotubes (SWCNTs), Euler-type buckling was found in relatively thin nanotubes, while buckling modes corresponding to change in the cross-sectional shape (radial buckling) were found in thick nanotubes. The crossover between the Euler-type buckling and radial buckling modes was clearly seen in the ASI analysis. While the reduction of Hessian eigenvalues in the pristine nanotubes and nanotubes with a vacancy is nearly linear until instability, rapid decrease of eigenvalues just before instability was found in models with Stone-Wales defects. This is due to localization of instability mode vectors around the defects that tends to arise before structural instability.

Abstract: This contribution provides simulated results of cross-sectional deformations observed in carbon nanotubes under high pressure. Molecular dynamics (MD) simulations were performed to explore radial buckling characteristics of multi-walled carbon nanotubes, and confirmed a variety of large-amplitude deformation modes. The energetically stable deformation mode turned out to be strongly dependent on the diameter of the innermost tube and the number of concentric walls. Critical buckling pressure obtained by MD simulations was compared with that estimated from a continuum elastic approximation, by which the validity of the continuum approximation was assessed.

Abstract: There have been a lot of studies dedicated to structural instability in solids. For local instability, theoretical (ideal) strength of crystals has been extensively studied with ab initio calculations. Global instability taking into account the collective motion of atoms involved in deformation has also been investigated. However, these studies have usually been done at 0 K and little has been understood about the effect of temperature. In this study, we demonstrate computational approaches to the effect of temperature on local and global instabilities. Ideal shear strength (ISS) of silicon at finite temperatures is calculated by molecular dynamics (MD) simulations with an empirical potential. ISS is obtained as a function of temperature. Our results imply that, unlike metals, the reduction in ISS by temperature cannot be estimated simply by taking into account thermal expansion of volume. In addition, global instability for dislocation nucleation in a Cu thin film model under tension is investigated. We first evaluated instability modes at 0 K with increasing strain, and then performed MD simulations at 50 K. After the nucleation of a partial dislocation, the second dislocation can be one to create a twin or one to create another partial dislocation. These different deformations can be understood as the competition of latent instability modes that have relatively small eigenvalues.

Abstract: We carry out ab initio density functional theory calculations to investigate the fundamental mechanical properties of stacking faults in 3C-SiC, including the effect of stress and doping atoms (substitution of C by N or Si). Stress induced by stacking fault (SF) formation is quantitatively evaluated. Extrinsic SFs containing double and triple SiC layers are found to be slightly more stable than the single-layer extrinsic SF, supporting experimental observation. Effect of tensile or compressive stress on SF energies is found to be marginal. Neglecting the effect of local strain induced by doping, N doping around an SF obviously increase the SF formation energy, while SFs seem to be easily formed in Si-rich SiC.

Abstract: The problem of whisker formation in tin (Sn) wiring in small electronic devices has become an important issue with the requirement of lead-free wiring, because doping of Pb to reduce whisker formation cannot be applied. It is therefore urged to better understand stress migration in tin, which is suspected to play a key role in whisker growth. We aim to study grain boundary diffusion in tin by atomistic simulation. After constructing an efficient interatomic potential suitable for diffusion of atoms using the genetic algorithm (GA), we perform molecular dynamics (MD) simulation of grain boundary diffusion in Sn under stress. We find that the magnitude of stress effect on diffusion depends on the boundary structure. Moreover, we examine the effect of impurities on vacancy migration by ab initio calculation to find atom doping that has potential to suppress diffusion.

Abstract: Using the first-principles calculation, the elastic constant C44 of Ag/Al multilayers with different modulation periods from 0.43 nm to 2.27 nm has been evaluated in order to examine the effect of atomic and electronic structures on it. With increasing modulation period, C44 decreases and becomes close to that obtained by the conventional mixing rule, however, the difference of 8 % still remains at the modulation period of 2.27 nm. As C44 correlates with the average interplanar spacing, the decrease of C44 can be explained by the decrease of the charge density in the stacking direction due to the increase of the average interplanar spacing. The difference in the electronic structure is included in the effect of atomic structure.

Abstract: The ideal strength of a nano-component, which is the maximum stress of the structure, provides an insight into the mechanical behavior of minute material. We conducted tensile simulations for cylindrical-shaped Cu nano-wires composed of an atomic chain as a core wrapped around by shell(s) with the structure of (111) layers in an fcc crystal. The results are compared with Cu atomic chain and sheet which are components of the nanowire. Young’s moduli and the ideal strengths of the wires are less than a single atomic chain and a sheet. The mechanical strength of the wire is weakened by the following three factors: (A) Change in electron arrangement caused by combining core and shell; (B) Larger interatomic distance (inherent tensile strain) of the outer shell introduced by the mismatch of atomic layers due to the curvature difference; (C) Mismatch between shells due to curvature difference. Factor (A) reduces the bonding strength in the shell(s) that occupy a greater part of the wire. 5-1 wire, which consists of a core and a shell, is weaker than the single atomic chain and the single sheet due to (A) and (B). 10-5-1 wire, consisting of a core and two shells, has less strength than 5-1 wire due to (C) in addition to (A) and (B).