D. H. Robertson

Properties of fullerene nanotubules

Abstract Recent reports show that single graphitic tubules with diameters on the order of fullerene diameters have been synthesized. We have examined both the electronic and structural properties of these materials using a range of methods. We have calculated the electronic structure of several graphitic tubules using a first-principles, self-consistent, all-electron Gaussian-orbital based local-density functional (LDF) approach. We have found that the class of high-symmetry tubules we denote as “serpentine” are metallic, with nonzero gaps for other finite radius tubules. With potential electronic and structural applications, these materials appear to be appropriate synthetic targets for the current decade.

Fullerene/Tubule Based Hollow Carbon Nano-Gears

We report the design of several symmetric star-shaped closed carbon cages using ball and stick models. Despite the presence of seven- and eight-membered rings, these hollow carbon sprockets—containing less than 500 carbon atoms—are predicted to be more energetically stable than C 60 . Also, we show that these sprockets can be connected to capped fullerene tubules to form a nanostructure similar to a mechanical gear and shaft. In addition, using molecular dynamics and a reactive hydrocarbon potential, we show that these gears can be turned against each other at high angular velocities without large deformations. These nanogears illustrate some of the possible complex small structures that can be formed by inserting 5-, 7-, and 8-membered rings in an otherwise graphitic network. These nanogears also show surprisingly robust mechanical properties.

SIMULATIONS OF CHEMICALLY-SUSTAINED SHOCK FRONTS IN A MODEL ENERGETIC MATERIAL

Tersoff-like potentials together with molecular dynamics calculations are used to simulate the detonation of an energetic two-dimensional semi-infinite molecular solid. The resulting shock front produced in this system exhibits four separate regions whose distinct interfaces between the regions move at constant and different velocities. The properties of the resulting shock front are independent of the initiation conditions. This model predicts a critical minimum impact velocity for a chemically-sustained shock front.

Molecular dynamics simulations of shock-induced chemistry: application to chemically sustained shock waves

Detonations travel through solid explosives as shock waves accompanied by rapid rises in temperature and pressure that cause the exothermic chemical reactions which sustain them. Because of the short time and length scales involved, molecular dynamics simulations can potentially probe their initiation and subsequent interplay with chemistry at the shock front. Such simulations may also clarify how this discrete shock induced chemistry relates to those properties of detonations that are understood by continuum (hydrodynamic) theories. To be at all convincing, however, these simulations require a model that is sufficiently simple to treat condensed phase systems containing several thousand atoms while incorporating traditional chemical concepts such as concerted reactions and energy release via the formation of product molecules. We have developed such a model for a two-dimensional diatomic energetic molecular solid using empirical bond order potentials. Simulations using this model possess characteristics expected of a detonation while displaying such rich behavior as shock wave splitting caused by a loss of molecular identity at high pressures.

Dissociative phase transitions from hypervelocity impacts

Abstract Molecular dynamics simulations are used to study hypervelocity impacts of an ultrathin flyer plate with a semi-infinite two-dimensional model diatomic molecular solid. These hypervelocity impacts are shown to produce a dissociative phase transition from a molecular to a close-packed solid in the target material. Although this close-packed phase persists for less than 10 picoseconds and is confined to a domain less than 10 nanometers wide it nevertheless behaves in a manner consistent with continuum theory.

Relative Energetics of C 44 Fullerene Isomers

We have carried out an exhaustive search of the energetics of all 87 isomers of the fullerene C44 using an empirical potential function. We find no single structural isomer with exceptional stability, suggesting that the enhanced abundance of this cluster observed in some studies may be due to a mixture of different molecules. We find in general that smaller, more spherical clusters and clusters whose pentagons are more isolated tend to be more stable, although the energy differences are not sufficiently large with our classical potential function to identify a particular criterion that distinguishes the stability of various similar isomers.

Detonation of solid O3: Effects of void collapse

Because of the short length and time scales associated with chemical processes during a detonation these systems are ideal candidates for study by molecular dynamics simulation. To study these systems however, a potential energy surface which allows for a realistic description of chemical reactivity is required. We have developed such a potential for the oxygen system using the REBO formalism. In this work we present a two-dimensional simulation of a chemical detonation of a model crystalline ozone molecular solid. We also study the effects of nanoscale void defects on the initiation process and find that the presence of these defects lowers the initiation threshold of detonation.

Molecular dynamics of void collapse mechanisms in shocked media

We have carried out a series of molecular dynamics simulations on a model system to study the dynamics of void defect collapse during pressure‐wave propagation in condensed‐phase systems. Three‐dimensional molecular‐dynamics methods were used for a model system of identical particles arranged as diatomic molecules aligned with the center of mass of each molecule at fcc lattice sites, using a {111} layering for the two‐dimensional boundary conditions. The diatoms were internally coupled via a harmonic potential; all other interactions were modeled with Morse potentials between all particles other than the immediate diatomic partner. Using this model, we have investigated the effect of a cylindrical void at right angles to the direction of layering (and impact). Depending on the energy density of the incident pressure wave, the void defect can either collapse smoothly and symmetrically (as in a balloon gradually losing air), or asymmetrically and turbulently. In the latter case, we note the transient format...

Molecular dynamics simulations of shock processes

Shock waves in condensed‐phased material occur on a time and length scale that makes them ideal for study using molecular dynamics (MD) techniques. However, the potential energy surface used in these MD simulations limits the shock‐related phenomena that can be simulated. We have developed many‐body potentials to study the coupling of shock waves with varying physical and chemical processes such as polymorphic phase transitions and chemical reactivity. We have shown that MD simulations using these many‐body potentials can model such complex phenomena as shock wave splitting resulting from the presence of a polymorphic phase transition and chemically sustained shock waves. These chemically sustained shock waves can show differing qualitative behavior depending on whether or not a polymorphic phase transition is present.

MOLECULAR DYNAMICS SIMULATIONS OF PRESSURE WAVE EFFECTS AT VOIDS IN A MODEL CONDENSED-PHASE MATERIAL

We have carried out a series of molecular dynamics simulations on a model system to study the effects of voids on hot-spot formation during pressure-wave propagation in condensed-phase systems. Three-dimensional molecular-dynamics methods were used for a model system of identical particles arranged as diatomic molecules aligned with the center of mass of each molecule at fcc lattice sites, using a {111} layering for the two-dimensional boundary conditions. The diatoms were internally coupled via a harmonic potential; all other interactions were modeled with Lennard-Jones potentials between all particles other than the immediate diatomic partner. Using this model, we have investigated the effect of a cylindrical void at right angles to the direction of layering (and impact) and compared these results with those obtained for a pristine (i.e., no void) material. The transient formation (for periods of several hundreds of femtoseconds) of a “hot spot” is noted at the void location both in terms of the local effective temperature and the vibrational energies of the diatoms.