Solid State Phenomena Vol. 258

Abstract: We present new results of molecular dynamic (MD) simulations in 3D bcc iron crystals with edge cracks (001)[010] and (-110)[110] loaded in mode I. Different sample geometries of SEN type were tested with negative and positive values of T-stress according to continuum prediction by Fett.

Abstract: We show a correlation between nanoscale damage and fragmentation length scale through atomistic simulations. We simulated homogeneously expanding perfect, single crystal copper at rates ranging from 1E+08 to 3E+10 s^-1 and temperatures from 200 to 1000 K. Damage was quantified in terms of void number density, average void volume, and void volume fraction. We quantified fragmentation size in terms of a length scale parameter, the solid volume per void surface area. A-1⁄2 power law relationship between the fragment length scale and strain rate was observed following the predictions of Mott. The fragmentation length scale and the maximum void number density are strongly correlated for this damage mechanism. We can scale up the relationships between damage and fragmentation observed in the molecular dynamics simulations to motivate a continuum scale fragmentation model.

Abstract: The tensile response under displacement controlled loading of nanosized single crystal Cu beams, solid or holding square shaped through-the thickness voids, have been investigated through 3D molecular dynamics simulations using free-ware LAMMPS [1]. For the same beam size and void height, the void width along the beam length axis was varied. Two different crystallographic orientations were considered. It was found that, under some circumstances, voids were able to close and heal the beam cross section, causing final failure through necking in the region of the initial void. For other cases instead the void split in two, smaller voids that both eventually healed. A third scenario was that the void widened, splitting the beam in two ligaments that each necked individually. As expected, both defect geometry and crystal orientation influences the mechanical behavior.

Abstract: A molecular dynamics simulation of bi-component nanoparticle formation under synchronous electric explosion of Cu and Ni wires was carried out. The approximation of the embedded atom method for a description of the interatomic interactions was used. The simulated nanowires had a cylindrical shape. Periodic boundary conditions were used along the cylinder axis, while in the other directions a free surface was simulated. Heating of the nanowires was performed by scaling of the atomic velocities following a linear law while maintaining a Maxwell distribution. It was shown that as a result of the synchronous electrical dispersion of metal wires the bi-component nanoparticles having a block structure may be formed. The basic mechanism of particle synthesis was the agglomeration of smaller clusters, and the minor one was the deposition of atoms from the gas phase on the particle surfaces. It was found that the distribution of chemical elements was non-uniform over the cross section of the synthesized particles. The concentration of Cu atoms in the subsurface region was higher than in the particle volume. It was noted that the method of molecular dynamics can effectively be used to select the optimal technological mode of producing nanoparticles with a block structure using electric explosion of metal wires.

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: The validity of the molecular dynamics (MD) simulation is highly dependent on the accuracy or reproducibility of interatomic potentials used in the MD simulation. The neural-network (NN) interatomic potential is one of promising interatomic potentials based on machine-learning method. However, there are some parameters that should be determined heuristically before making the NN potential, such as the shape and number of basis functions. We have developed a new approach to select only relevant basis functions from a lot of candidates systematically and less heuristically without loosing the accuracy of the potential. The present NN potential for Si system shows very good agreements with the results obtained using ab-initio calculations.

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: Thermoelectric properties were simulated for low-dimensional atomistic model structures based on first-principles calculation. New methodology about the first-principles simulation on Seebeck coefficient at arbitrary temperature and carrier concentration is presented. Dependence of Seebeck coefficient on scale, temperature, and carrier concentration has been demonstrated for silicon and beta silicon carbide nanowire models. Compared with the corresponding bulk models, a significant increase of the absolute value of Seebeck coefficient can be observed owing to quantum confinement by dimensional reduction. By the simulation with considering the energy dependence of the relaxation time, the Seebeck coefficient from the viewpoint of first principles can be evaluated as a range determined by the scattering constants peculiar to particular scattering processes.

Abstract: A density functional theory based method was used to model dynamics of high frequency delocalized normal mode and discrete breathers in graphene at T = 0 K. For the normal mode the comparison of results was made with modeling by means of classical molecular dynamics. Discrete breathers have been found only in presence of uniaxial strain applied in “zigzag” direction. The oscillations of breather core atoms appeared to be polarized along “arm-chair” direction. In the case of “arm-chair” uniaxial strain there were no breathers found. The frequency on the amplitude dependency of DBs in graphene corresponds to the soft nonlinearity type that is due to the soft nonlinearity type of the high frequency normal mode on which breather is constructed.