Session D2: MD-1: Molecular Dynamics 1

Phase diagram and thermodynamic properties of nanocarbons in detonation conditions from atomistic simulations using the LCBOPII potential

Several earlier studies showed that the detonation of oxygen deficient explosives produces substantial amounts of nanometre-sized carbon residues. The presence of these carbon nanoparticles needs to be accounted for in thermochemical models to obtain accurate estimations of the thermodynamic properties of the detonation products. Thus the determination of the thermodynamic properties of nanocarbons, and in particular of the size-dependence of their phase diagram, is highly desirable for pressure and temperature ranges close to those achieved at the Chapman-Jouguet point. In this communication we will present the carbon phase diagram obtained by Monte Carlo simulations with the LCBOPII potential, an empirical potential for carbon developed by Los et al. which is known to give a good description of bulk carbon phases under high pressure and temperature [1]. In particular we will emphasize on the region of the phase diagram of interest for detonation products (close to the CJ point) and extrapolations of the coexistence lines to the nanometer-sized carbon clusters will be provided using a simple model. Our results will then be compared to the structure and phase transitions of nanocarbons obtained by Molecular Dynamics simulations. \\[3pt] [1] Phys. Rev. B 72, 214102 (2005)   

Molecular dynamics simulation of shock-induced phase transition in Germanium

Results from shock-wave and ramp-wave uniaxial loading of Germanium will be presented. Germanium is known to transition from ambient cubic diamond (cd) phase to the high-pressure body-centered tetragonal (bct) or $\beta$-tin phase at pressures between 10 and 12 GPa. Large-scale molecular dynamics (MD) simulations were used to study the phase transition in single-crystal Germanium under uniaxial compression along several different crystal axes. We observed that the transition from the cd phase to the bct phase nucleates through shear banding and advances to relieve uniaxial strain. The macroscopic properties are compared with experimental results for both the Modified Embedded Atom Method (MEAM) and Tersoff potentials. Simulation techniques included standard non-equilibrium MD, as well as alternative computational methods, such as the Continuous Hugoniot Method and homogeneous uniaxial ramp methods. \\[4pt] This work is supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.   

An abundance of mechanisms for plastic flow in an extremely brittle material: dislocations and phase transformations in RDX

Orientation-dependent anisotropies in the initiation sensitivity of PETN can be rationalized elegantly by the availability and activity of dislocation slip systems for a given crystallographic orientation of the shock propagation direction. The true power of the resulting steric hindrance model is that it provides a framework for predicting anisotropies in the initiation sensitivity of any pristine energetic molecular crystal once its slip systems have been identified and characterized rigorously. I will present a review of recent molecular dynamics simulations and experiments that demonstrate that the energetic molecular crystal RDX is a rather plastic material under compression owing to a surprising number of mechanisms for plastic flow and stress relaxation. I will focus on the homogeneous nucleation of dislocations under shock compression and surface indentation, complex patterns of spatially localized melting and flow during void collapse, and the discussion of two shock-induced phase transformations in oriented RDX single crystals.   

Ab Initio Studies of Shock-Induced Chemical Reactions of Inter-Metallics

Shock-induced and shock assisted chemical reactions of intermetallic mixtures are studied by many researchers, using both experimental and theoretical techniques. The theoretical studies are primarily at continuum scales. The model frameworks include mixture theories and meso-scale models of grains of porous mixtures. The reaction models vary from equilibrium thermodynamic model to several non-equilibrium thermodynamic models. The shock-effects are primarily studied using appropriate conservation equations and numerical techniques to integrate the equations. All these models require material constants from experiments and estimates of transition states. Thus, the objective of this paper is to present studies based on ab initio techniques. The ab inito studies, to date, use ab inito molecular dynamics. This paper presents a study that uses shock pressures, and associated temperatures as starting variables. Then intermetallic mixtures are modeled as slabs. The required shock stresses are created by straining the lattice. Then, ab initio binding energy calculations are used to examine the stability of the reactions. Binding energies are obtained for different strain components super imposed on uniform compression and finite temperatures. Then, vibrational frequencies and nudge elastic band techniques are used to study reactivity and transition states. Examples include Ni and Al.   

Molecular Dynamics and Hydrodynamics Simulations of Detonation Wave Refraction at the Boundary of TATB-like HE and Beryllium

Here we present results of investigations of the process of detonation wave refraction on the border with inert material. The effects of broad reaction zone in TATB-like HE and high sound speed in Be were of particular interest. Molecular Dynamics (MD) was chosen as an instrument of the investigation. An atomistic approach to the contrast of HydroDynamics (HD) does not use any phenomenological models for physical processes but intreatomic potentials. Therefore MD allows for the direct and explicit simulation of such phenomena as detonation kinetics, elastic-plastic transition mechanism and shear stress relaxation kinetics from the microscopic point of view. Nevertheless it was very interesting and important to compare results of MD and HD approaches to the same problem. To make possible hydrodynamics modeling the parameters of the models used in HD were determined from MD simulations. In the course, we used MD results to choose parameters for Be and TATB-like HE equations of state and to evaluate parameters of elastic- plastic transition models for these materials. HD and MD results have been compared and analyzed.