Donald W. Brenner

A second-generation reactive empirical bond order (REBO) potential energy expression for hydrocarbons

A second-generation potential energy function for solid carbon and hydrocarbon molecules that is based on an empirical bond order formalism is presented. This potential allows for covalent bond breaking and forming with associated changes in atomic hybridization within a classical potential, producing a powerful method for modelling complex chemistry in large many-atom systems. This revised potential contains improved analytic functions and an extended database relative to an earlier version (Brenner D W 1990 Phys. Rev. B 42 9458). These lead to a significantly better description of bond energies, lengths, and force constants for hydrocarbon molecules, as well as elastic properties, interstitial defect energies, and surface energies for diamond.

THE STRESS-STRAIN BEHAVIOR OF POLYMER-NANOTUBE COMPOSITES FROM MOLECULAR DYNAMICS SIMULATION

Stress–strain curves of polymer–carbon nanotube composites generated from molecular dynamics simulations of a single-walled carbon nanotube embedded in polyethylene are presented. A comparison is made between the response to mechanical loading of a composite with a long, continuous nanotube (replicated via periodic boundary conditions) and the response of a composite with a short, discontinuous nanotube. Both composites are mechanically loaded in the direction of, and transverse to, the nanotube axis. The long-nanotube composite shows an increase in the stiffness relative to the polymer and behaves anisotropically under the different loading conditions considered. The short-nanotube composite shows no enhancement relative to the polymer, most probably because of its low aspect ratio. The stress–strain curves from molecular dynamics simulations are compared with corresponding rule-of-mixtures predictions. Published by Elsevier Ltd.

Carbon nanostructures for advanced composites

Recent advances in the science and technology of composites utilizing carbon nanostructures are reviewed, including experimental results and modelling studies of composite properties and processing. Carbon nanotubes are emphasized, with other carbon nanostructures such as fullerenes, ultradispersed diamond clusters and diamond nanorods also being discussed.

Mechanical properties of nanotubule fibers and composites determined from theoretical calculations and simulations

Theoretical Youngs moduli have been estimated for carbon fibers composed of single-walled fullerene nanotubules aligned in the direction of the tubule axis. In the limit of infinitely long tubules, the fibers can have a Youngs modulus comparable to that of diamond. Exploiting this property of nanotubule fibers, we investigate a new carbon composite composed of layered nanotubule fibers and diamond. Such a composite is found to be a high-modulus, low-density material that is quite stable to shear and other distortions.

Predicted structure and electronic properties of individual carbon nanocones and nanostructures assembled from nanocones

Calculations using an analytic potential show that carbon nanocones can exhibit conventional cone shapes or can form concentric wave-like metastable structures, depending on the nanocone radius. Single nanocones can be assembled into extended two-dimensional structures arranged in a self-similar fashion with fivefold symmetry as system size is increased. Calculations of the electronic properties of nanocones indicate that a pentagon in the centre of a cone is the most probable spot for emitting tunnelling electrons in the presence of an external field. This implies that nanocone assemblies, if practically accessible, could be used as highly localized electron sources for templating at scales below more traditional lithographies.

Atomistic simulation of the influence of pre-existing stress on the interpretation of nanoindentation data

By using molecular dynamics simulations, we have accurately determined the truecontact area during plastic indentation of materials under an applied in-plane stress.We found that the mean pressure calculated from the true contact area varied slightlywith applied pre-stress with higher values in compression than in tension and that themodulus calculated from the true contact area is essentially independent of thepress-stress level in the substrate. These findings are largely consistent with thefindings of Tsui, Pharr, and Oliver. On the other hand, if the contact area is estimatedfrom approximate formulae, the contact area is underestimated and shows a strongdependence on the pre-stress level. When it is used to determine mean pressure andmodulus, the empirically determined area leads to large errors. Our simulationsdemonstrate that this phenomenon, first reported for macroscale hardnessmeasurements dating back to 1932, also exists at the nanometer-scale contact areas,apparently scaling over 10 orders of magnitude in contact area, from ∼mm

On the disclination-structural unit model of grain boundaries

Abstract The relation between two elastic continuum approaches to grain boundary structure, the dislocation and disclination models, is discussed. It is shown that the disclination model has two advantages: a well-behaved expression for the elastic energy of disclination dipole walls, which describes the elastic energy over a wide interval of misorientations, and a continuous misorientation angle dependence of the elastic energy of grain boundaries in an interval between two delimiting boundaries. The elastic energy of the most general, faceted disclination wall is calculated. For cases in which both the energies of delimiting boundaries and elastic constants are available from atomic simulations (〈001〉 and 〈111〉 tilt boundaries in copper and 〈001〉 and 〈011〉 tilt boundaries in diamond) quantitative agreement between the disclination model and simulation results is obtained.

Atomic scale studies of spall behavior in nanocrystalline Cu

The micromechanisms related to ductile failure during dynamic loading of nanocrystalline Cu are investigated in a series of large-scale molecular dynamics simulations. Void nucleation, growth, and coalescence is studied for a nanocrystalline Cu system with an average grain size of 6 nm under conditions of impact of a shock piston with velocities of 250, 500, 750, and 1000 m/s and compared to that observed in single crystal copper. Higher impact velocities result in higher strain rates and higher values of spall strengths for the metal as well as nucleation of larger number of voids in smaller times. For the same impact velocity, the spall strength of the nanocrystalline metal, however, is lower than that for single crystal copper. The results obtained for void nucleation and growth in nanocrystalline Cu for various impact velocities and for single crystal copper [001] suggests two distinct stages of evolution of voids. The first stage (I) corresponds to the fast nucleation of voids followed by the second ...

Atomistic simulations of the nanometer-scale indentation of amorphous-carbon thin films

Molecular dynamics simulations are used to examine the nanometer-scale indentation of a thin film of amorphous carbon with a nonrigid sp3 bonded carbon tip. The simulations show in detail the atomic-scale mechanism of the indentation process and compare the bonding character of the film before and after indentation. The computationally determined elastic modulus of the amorphous-carbon film is found to be 243 GPa, in good agreement with experiment.Molecular dynamics simulations are used to examine the nanometer-scale indentation of a thin film of amorphous carbon with a nonrigid sp3 bonded carbon tip. The simulations show in detail the atomic-scale mechanism of the indentation process and compare the bonding character of the film before and after indentation. The computationally determined elastic modulus of the amorphous-carbon film is found to be 243 GPa, in good agreement with experiment.

Molecular Dynamics Simulations of Carbon Nanotube Rolling and Sliding on Graphite

Abstract Molecular dynamics simulations were carried out to investigate the origin of friction for carbon nanotubes on graphite substrates. In an initial simulation, a (10,10) nanotube was placed in an ‘in-registry’ starting position where the hexagonal lattice of the substrate matched that of the nanotube. In a second simulation, the substrate was oriented 90 degrees to the nanotube. A uniform force was applied to the nanotubes for 500 fs to set them into motion. The simulation was then run until the nanotubes stopped moving relative to the substrate. Only sliding was observed in the out-of-registry simulation, while periodic sliding and rolling was observed in the in-registry simulation. The latter is a result of the relatively larger surface corrugation for the in-registry case and occurs to avoid direct atomic collisions between nanotube and substrate atoms as the nanotube is moved along the substrate. Analysis of the kinetic energy suggests that the transition between sliding and rolling contributes to enhanced energy dissipation and higher net friction. These results are consistent with preliminary experimental observations by Superfine and coworkers.