P. Keblinski

Exact method for the simulation of Coulombic systems by spherically truncated, pairwise r−1 summation

Based on a recent result showing that the net Coulomb potential in condensed ionic systems is rather short ranged, an exact and physically transparent method permitting the evaluation of the Coulomb potential by direct summation over the r−1 Coulomb pair potential is presented. The key observation is that the problems encountered in determining the Coulomb energy by pairwise, spherically truncated r−1 summation are a direct consequence of the fact that the system summed over is practically never neutral. A simple method is developed that achieves charge neutralization wherever the r−1 pair potential is truncated. This enables the extraction of the Coulomb energy, forces, and stresses from a spherically truncated, usually charged environment in a manner that is independent of the grouping of the pair terms. The close connection of our approach with the Ewald method is demonstrated and exploited, providing an efficient method for the simulation of even highly disordered ionic systems by direct, pairwise r−1...

Amorphous structure of grain boundaries and grain junctions in nanocrystalline silicon by molecular-dynamics simulation

Molecular-dynamics simulations using the Stillinger-Weber three-body potential are used to synthesize fully dense nanocrystalline silicon with a grain size up to 7.3 nm by crystallization from the melt. The structures of the highly-constrained grain boundaries, triple lines and point grain junctions are found to be highly disordered and similar to the structure of amorphous silicon. These results suggest that nanocrystalline silicon may be treated as a two-phase system, namely, an ordered crystalline phase in the grain interiors connected by an amorphous, intergranular glue-like phase.

Structure of grain boundaries in nanocrystalline palladium by molecular dynamics simulation

The atomic structures of the grain boundaries (GBs) in nanocrystalline materials and their effect on properties have been the subject of extensive discussion ever since the first ultrafine-grained polycrystals were synthesized by consolidation of small clusters formed via gas condensation. A key question that has emerged is whether and under what conditions the atomic structure of the GBs in nanocrystalline materials can be extrapolated from those of coarse-grained polycrystalline materials and bicrystals. To address this question directly, in recent years computer simulation methods capable of providing structural information on the GBs in nanocrystalline microstructures have been developed; this type of local information is not readily available from experiments, such as x-ray diffraction studies, which provide only average structural information on these highly inhomogeneous materials. One such method uses molecular dynamics (MD) simulations to grow fully dense nanocrystalline microstructures from a melt into which small crystalline seeds with more or less random orientations are inserted. For the case of silicon as a model material, these simulations have enabled the authors to elucidate the connection between the GBs present in the nanocrystalline microstructure and the structure of bicrystalline GBs. In this paper, the authors present the results of a similar comparison formorexa0» the case of face-centered cubic (fcc) metals. By contrast with their earlier simulations involving a generic fcc-metal (Lennard-Jones) interatomic potential, here they study palladium as a model fcc metal, with atomic interactions described by an embedded-atom-method (EAM) potential. Their choice of Pd is motivated by the significant amount of experimental data on the structure, mechanical and thermodynamic properties of nanocrystalline Pd, including self-diffusion and phonon properties. As in Si, these results suggest that a nanocrystalline microstructure with random grain orientations contains only high-energy GBs.«xa0less

Self-diffusion in high-angle fcc metal grain boundaries by molecular dynamics simulation

Abstract Recent molecular dynamics simulations of high-energy high-angle twist grain boundaries (GBs) in Si revealed a universal liquid-like high-temperature structure which, at lower temperatures, undergoes a reversible structural and dynamical transition from a confined liquid to a solid; low-energy boundaries, by contrast, were found to remain solid all the way up to the melting point. Here we demonstrate for the case of palladium that fcc metal GBs behave in much the same manner. Remarkably, at high temperatures the few representative high-energy high-angle (tilt or twist) boundaries examined here exhibit the same, rather low self-diffusion activation energy and an isotropic liquid-like diffusion mechanism that is independent of the boundary misorientation. These observations are in qualitative agreement with recent GB self- and impurity-diffusion experiments by Budke et al. on Cu. Our simulations demonstrate that the decrease in the activation energy at elevated temperatures is caused by a structural...

Molecular-Dynamics Simulation of Grain-Boundary Diffusion Creep

Molecular-dynamics (MD) simulations are used, for the first time, to study grain-boundary diffusion creep of a model polycrystalline silicon microstructure. Our fully dense model microstructures, with a grain size of up to 7.5 nm, were grown by MD simulations of a melt into which small, randomly oriented crystalline seeds were inserted. In order to prevent grain growth and thus to enable steady-state diffusion creep to be observed on a time scale accessible to MD simulations (of typically 10-9s), our input microstructures were tailored to (i) have a uniform grain shape and a uniform grain size of nm dimensions and (ii) contain only high-energy grain boundaries which are known to exhibit rather fast, liquid-like self-diffusion. Our simulations reveal that under relatively high tensile stresses these microstructures, indeed, exhibit steady-state diffusion creep that is homogenous (i.e., involving no grain sliding), with a strain rate that agrees quantitatively with that given by the Coble-creep formula.

ROLE OF BONDING AND COORDINATION IN THE ATOMIC STRUCTURE AND ENERGY OF DIAMOND AND SILICON GRAIN BOUNDARIES

The high-temperature equilibrated atomic structures and energies of large-unit-cell grain boundaries (GB{close_quote}s) in diamond and silicon are determined by means of Monte-Carlo simulations using Tersoff{close_quote}s potentials for the two materials. Silicon provides a relatively simple basis for understanding GB structural disorder in a purely sp{sup 3} bonded material against which the greater bond stiffness in diamond combined with its ability to change hybridization in a defected environment from sp{sup 3} to sp{sup 2} can be elucidated. We find that due to the purely sp{sup 3}-type bonding in Si, even in highly disordered, high-energy GB{close_quote}s at least 80{percent} of the atoms are fourfold coordinated in a rather dense confined amorphous structure. By contrast, in diamond even relatively small bond distortions exact a considerable price in energy that favors a change to sp{sup 2}-type local bonding; these competing effects translate into considerably more ordered diamond GB{close_quote}s; however, at the price of as many as 80{percent} of the atoms being only threefold coordinated. Structural disorder in the Si GB{close_quote}s is therefore partially replaced by coordination disorder in the diamond GB{close_quote}s. In spite of these large fractions of three-coordinated GB carbon atoms, however, the three-coordinated atoms are rather poorly connected amongst themselves, thus likelymorexa0» preventing any type of graphite-like electrical conduction through the GB{close_quote}s. {copyright} {ital 1998 Materials Research Society.}«xa0less

On the nature of grain boundaries in nanocrystalline diamond

Abstract The atomic structures of a few representative large-unit-cell grain boundaries thought to largely determine the behavior of nanocrystalline diamond are determined via Monte-Carlo simulation. In these highly disordered grain boundaries up to 80% of the C atoms exhibit local sp2 bonding. However, because the three-coordinated C atoms are poorly connected to each-other, graphite-like electrical conduction through the grain boundaries is unlikely without “bridging” impurities. Surprisingly, based on their fracture energies, the high-energy, large-unit-cell boundaries are more stable against brittle decohesion into free surfaces than low-energy ones and perhaps even the perfect crystal.

Relationship between nanocrystalline and amorphous microstructures by molecular dynamics simulation

Abstract A recently developed molecular-dynamics simulation method for the growth of fully dense nanocrystalline materials by crystallization from the melt was used together with the Stillinger-Weber three-body potential to synthesize nanocrystalline silicon with a grain size up to 75A. The structures of the highly-constrained grain boundaries (GBs), triple lines and point grain junctions were found to be highly disordered and similar to the structure of amorphous silicon. These and our earlier results for fcc metals suggest that a nanocrystalline microstructure may be viewed as a two-phase system, namely an ordered crystalline phase in the grain interiors connected by an amorphous, intergranular, glue-like phase. The analysis of the structures of bicrystalline GBs in the same materials reveals the presence of an amorphous intergranular equilibrium phase only in the high-energy but not the low-energy GBs, suggesting that only high-energy boundaries are present in nanocrystalline microstructures.

Synthesis and Characterization of a Polycrystalline Ionic Thin Film by Large-Scale Molecular-Dynamics Simulation

A simulation methodology for the synthesis of polycrystalline, ionic thin films is developed. The method involves the preparation of a polycrystalline substrate onto which a thin film is subsequently grown by crystallization from the melt. A detailed structural analysis of a textured sixteen-grain FeO film, with a grain size of approximately 4.7 nm, shows that the interiors of the grains are almost perfect single crystals with only a very few vacancies and no interstitials. The grains are delineated by 〈001〉 tilt grain boundaries; as expected, the low-angle grain boundaries in the film consist of arrays of dislocations, while the high-angle grain boundaries are relatively narrow and well ordered.

Thermodynamically stable amorphous intergranular films in nanocrystalline silicon

Abstract Molecular-dynamics simulations were used to synthesize fully dense nanocrystalline silicon by crystallization from the melt. The equilibrium structures of the highly constrained grain boundaries and grain junctions in these metastable microstructures are found to be highly disordered and similar to the structure of amorphous silicon. These results demonstrate that in thermodynamic equilibrium nanocrystalline silicon may be treated as a two phase system composed of crystalline grain interiors that are connected by an amorphous intergranular phase.