Density functional computations have been performed to investigate the structure, potential en- ergy surface and reactivity for organic systems closely related to bisphenol-A-polycarbonate(BPA- PC). The results provide the basis for the construction of two different empirical models, the first extending the atomistic simulations into the mesoscopic range (104–105 atoms and 0.01–0.1 μs), the second providing an idealized description of polymerization in BPA-PC. The combination of models and computational techniques focusing on different length and time scales provides a route to determine mechanical and thermal properties of materials without experimental input.

We discuss concurrent multiscale simulations of dynamic and temperature-dependent processes found in nanomechanical systems coupled to larger scale surroundings. We focus on the behavior of sub-micron Micro-Electro-Mechanical Systems (MEMS), especially micro-resonators. The coupling of length scales methodology we have developed for MEMS employs an atomistic description of small but key regions of the system, consisting of millions of atoms, coupled concurrently to a finite element model of the periphery. The result is a model that accurately describes the behavior of the mechanical components of MEMS down to the atomic scale. This paper reviews some of the general issues involved in concurrent multiscale simulation, extends the methodology to metallic systems and describes how it has been used to identify atomistic effects in sub-micron resonators.

Recently a method has been proposed which allows the calculation of complex magnetic structures from a simple d-band tight-binding Hamiltonian including Coulomb and exchange interactions with the help of a molecular-dynamics simulations. In this article an improved version of this ap- proach is suggested which retains the rotational symmetry of the simulated system. The improved algorithm is applied to systems with electron numbers in the range 7 ≤nd ≤ 8.

In this work we use classical molecular dynamics to study strain rate and temperature dependent plasticity of carbon nanotube (CNT) under compressive strain. We focus on two types of defects: sp3 bond formation and bond rotation. Our simulation shows that thermal fluctuations help the strained CNT to overcome the local energy barrier to obtain plastic deformation. The yielding strain of a compressed CNT found to be strain-rate and temperature dependent, and low strain rate limit of the yielding strain is estimated to be less that 6%.

Waves propagate through disordered systems in a variety of regimes. There is a threshold of disorder beyond which waves become localized and transport becomes restricted. The intensity I of the wave transmitted through a system has a dependence on the length L of the sample that is characteristic of the regime. For example, I decays as L−1 in the diffusive regime. It is of current interest to characterize the transport regime of a wave, from statistical studies of the transmittance quantities through it. Studies suggest that the probability distribution of the intensity could be used to characterize the localized regime. There is an ongoing debate on what deviations from the classical Rayleigh distribution are to be expected. In this numerical work, we use scalar waves to obtain the intensity, transmission, and conductance of waves through a disordered system. We calculate the intensity, by setting an incoming plane wave towards the sample from a fixed direction. The outgoing intensity is then calculated at one point in space. This process is repeated for a collection of samples belonging to the same ensemble that characterizes the disorder, and we construct the probability distribution of the intensity. In the case of transmission, we evaluate the field arriving to a series of points distributed in the far field, and repeat the same statistical analysis. For the conductance, we calculate the field at the same series of points for incoming waves in different directions. We analyze the distribution of the transmittance quantities and their change with the degree of disorder.

In this paper we focus on one of the key issues in polycrystalline plasticity: the unit mechanisms involving the interactions between dislocations and grain boundaries (GB). Using a combination of large-scale molecular dynamics simulations based on an embedded atom potential and an analysis in terms of the line-tension model we identify and characterize the geometrical parameters that govern the occurrence of slip transmission, absorption, blockage, etc. in dislocation-GB interactions. The results provide a guideline for the development of quantitative micro-constitutive equations for dislocation-GB interactions to be used in meso-scale simulations of polycrystal plasticity.

In this paper a Monte Carlo method is introduced for determining the diffusivity in the void space of porous media constructed from spheres. We studied ordered and random arrangements of spheres, the latter with different sizes. It is found that the effect of structure on the diffusivity is fairly slight. The empirical curve of Neale and Nader is shown to provide a convenient upper bound. The result of Bruggemann provides a convenient lower bound.

In this paper we present three-dimensional molecular dynamics simulations of dislocation nucleation and propagation during thin film deposition. Aiming to identify mechanisms of dislocation nucleation in polycrystalline thin films, we choose the film material to be the same as the substrate – which is stressed. Tungsten and aluminum are taken as representatives of BCC and FCC metals, respectively, in the molecular dynamics simulations. Our studies show that both glissile and sessile dislocations are nucleated during the deposition, and surface steps are preferential nucleation sites of dislocations. Further, the results indicate that dislocations nucleated on slip systems with large Schmid factors more likely survive and propagate into the film. When a glissile dislocation is nucleated, it propagates much faster horizontally than vertically into the film. The mechanisms and criteria of dislocation nucleation are essential to the implementation of the atomistic simulator ADEPT.

In this paper we introduce a Monte Carlo method for numerical analysis of the problem of tracer diffusion in the presence of isolated grain boundaries with a variable mesh in the direction perpendicular to the grain boundary plane. A number of isoconcentration contour profiles were studied. Two different expressions for the angle between the isoconcentration contours and the grain boundary were derived for the instantaneous tracer source following procedures analogous to Fisher's and Whipple's approximations for the constant tracer source. Comparison with the numerical data was made. Both these approximations overestimate the grain boundary diffusivity.

Nacre is a segmented layered composite containing both nanoscale-thick organic and sub- micrometer thick scale inorganic phases. In addition to controlling the formation of the intricate architecture, the organic plays a significant role in the mechanical properties of the biocomposite. In our previous work, three dimensional finite element models of nacre were constructed to design “brick and mortar” micro-architecture to study effects of nonlinear response of the organic component. Recently, nanomechanical properties such as hardness and elastic moduli of the individual components of nacre have been determined using nanoindentation techniques. In this work, we used these actual properties of the components to perform mesoscale finite element models to quantitatively evaluate nanoscale effects. Specifically, we studied the effect of the solid contacts between the platelets through the organic layer on bulk properties under tensile and compressive loading. In the new 3D finite element model, we also incorporated the pseudo-hexagonal platelet morphology to more accurately represent the nacre microstructure. The multiscale approach in our study involves incorporation of experimentally obtained nanoscale parameters into meso-macroscale numerical models. Our simulations imply that mineral contacts in the aragonitic platelets have marginal impact both on bulk elastic behavior of nacre and yielding. Stress concentration in contact regions were high enough for the contacts to break long before yield started in nacre. These results have important implications in the design of biomimetic segmented-layered composites for improved mechanical properties.

We have used the ab-initio full potential LMTO method to model native defects and chlorine-impurity-related defects in ZnSe and ZnxMg1−xSe. Our results show that there is a strong tendency for formation of a defect complex between a chlorine impurity at the Se site and a vacancy at the neighboring Zn site. The formation energies of this complex and other chlorine related defects decrease in the presence of magnesium. However, the maximum achievable electron concentration in the presence of magnesium is lower because of the increase in the band gap.

In the present study we examine the well-known analysis in which the dislocation pipe diffusivity is determined by means of a grain boundary type analysis of the tail of a tracer concentration depth profile. We use a Monte Carlo grid method for testing the analysis. The results show that the analysis is really only satisfactory when the spacing between the dislocations is roughly twice the diffusion length (Dlt)½ where Dland t are the lattice diffusivity and time respectively.

On the basis of the methods of pseudopotential and large unit cell the research of deep lev- els of point and cluster defects in ZnGeP2 (ZGP) was carried out. It was shown that owing to lowering of the crystal lattice symmetry localised states of point defects in the ternary compound essentially differ from such states in its binary analogue GaP. Small quasicubic antisite defects clusters reduce the value of band gap, but render weak influence on the probability of optical transitions. But both single and the cluster antisite defects cause large modifications in deep levels of another point defects closed to them. On an example of phosphorus vacancy it was shown that intensive optical transitions with small energies can appear and cause an absorption in near IR – range because of antisite defects effect.

In order to study the dislocation density evolution of body centered cubic (bcc) crystals at low temperature by dislocation dynamics (DD) simulations, we investigated carefully three different boundary conditions (BC) for DD, i.e., the quasi-free surface BC, the flux-balanced BC, and the periodic BC. The latter two BCs can account for the dislocation loss from the boundary of the finite simulation box. PBC can also eliminate the influence of surfaces and improve the line connectivity. We have found that the PBC provides a convenient and effective boundary condition for DD simulations and have applied it to the study of dislocation density evolution of bcc metals during stage 0 deformation at low temperature.

Topological changes in microstructure are strictly related to the microscopic evolution of triple junctions (TJ). The three-sided grain disappearance, usually called T2 process, is here investigated via 3D-atomistic modeling. In particular the stability of a three-sided grain insertion in a triple junction in silicon is studied within the framework of Molecular Dynamics simulations. The Stillinger-Weber interatomic potential is adopted and constant-traction border conditions are considered to ensure a proper embedding of the atomistic region in a virtually infinite bulk continuum. Dealing with the T2-event, the critical radius below which the three- sided inner grain become unstable is evaluated to be three to four times the lattice constant of silicon. Moreover, we show that the instability sets in through the amorphization of the central shrinking grain.

Certain classes of sol-gel transition have been modeled as diffusion-limited cluster-cluster aggregation (DLCA), but it produces excessive dangling branches on the resulting network that underestimates the rigidity of gels. The “dangling bond deflection” (DEF) model was developed to simulate spatial fluctuation of the dangling branches under thermal energy. Collision and sticking of two dangling branches within the same cluster turns these branches into a loop. Combination of the DLCA and DEF models creates network that possesses extensive loop structure and negligible dangling mass. The networks are substantially stiffened by the loop structure, and successfully reproduce the empirical scaling relationship between linear elastic modulus and density exhibited by real aerogels. The gel structure can be represented by the “blob-and-link” model, in which blobs refer to dense, rigid collections of particles, interconnected by tenuous links of particle chains. When the network is deformed, only these few weak links contribute to the stiffness, leaving the blobs unstrained. The gel modulus drops significantly as porosity increases because more particles reside in the blobs and fewer particles carry the strain.

We study the structure and formation energy of kinks in 1/2a<111> screw dislocation in metallic Ta Embedded Atom Model Force Field parameterized using quantum mechanical computations. We studied a/3<112> kinks using a simulation cell containing four dislocations in a quadrupole arrangement. We impose periodic boundary conditions in the directions perpendicular to [111] and fixed boundaries in the [111] direction. We find that two, energetically equivalent, core configurations for the 1/2a<111> dislocation lead to 8 distinguishable single kinks and 16 kink pairs. The different mismatches of core configurations along [111] direction cause variations in kink formation energy. The lowest formation energy of a kink pair is determined to be 0.73 eV. The geometric features of such kink pair have been studied with the help of structural analysis of the atomistic model. We also compare the activation energy for dislocation motion via the double kink mechanism with the activation energy for a rigid dislocation motion from a dipole annihilation simulation. We find that the migration energy for dislocation motion via double kink formation is 0.016 eV/b, which is less than the quarter of the migration energy associated with the kink free motion of a straight dislocation, 0.073 eV/b.

Multiscale computational approach that combines different methods to study laser ablation phenomenon is presented. The methods include the molecular dynamics (MD) breathing sphere model for simulation of the initial stage of laser ablation, a combined MD - finite element method (FEM) approach for simulation of propagation of the laser-induced pressure waves out from the MD computational cell, and the direct simulation Monte Carlo (DSMC) method for simulation of the ablation plume expansion. The multiscale approach addresses different processes involved in laser ablation with appropriate resolutions and, at the same time, accounts for the interrelations between the processes. A description of the ablation plume appropriate for making a connection between the MD simulation of laser ablation and the DSMC simulation of the ablation plume expansion is discussed.

We investigate the reactive ion etching of amorphous silicon by halides using a hierarchy of models on different time and length scales. The feature evolution is modeled using a two- dimensional cell based Monte-Carlo feature scale simulator. The fluxes, the energy distributions, and the angular distributions of the wafer-incident particles are provided by a hybrid plasma sheath simulator. The relevant surface reaction rates are calculated by a molecular dynamics simulator using a Stillinger-Weber representation of the interatomic potential. Our investigations show that the surface reaction rates are strongly determined by the particular surface morpho- logy, which, in turn, is strongly influenced by the kinetic properties of the impinging particles. Thus, we link the molecular dynamics simulator into the model as a whole.

As results, we present calculations for the etching of amorphous silicon by fluorine, chlorine, and bromine. A Stillinger-Weber representation of the bromine and the silicon-bromine potential which was not yet available in literature is additionally developed. We discuss the different morphologies of halogenated silicon surfaces as a consequence of the energy distri- bution and the angular distribution of the impinging particles. Comparisons of the sputter yield functions of bare amorphous silicon surfaces and corresponding halogenated surfaces exhibit considerable differences, qualitative as well as quantitative.

The pulsed nature of the irradiation and the high neutron dose are the critical factors in an Inertial Fusion Energy reactor (IFE). The damage that structural materials suffer under these extremes conditions require a careful study and assessment. The goal of our work is to simulate, trough the multiscale modelling approach, the damage accumulation in μ-Fe under conditions relevant to a IFE Reactor. We discuss how the pulse frequency, 1 Hz, 10 Hz, and the dose rate of 10 ⊏2 and 10 ⊏1 dpa/s affect the damage production and accumulation. Results of the damage that this demanding environment can produce on a protected first structural exposed to 150 keV average recoil ion will be presented. A further comparison it has been made with the damage produced by a continuous irradiation at similar average dose.