Molecular dynamics simulations of shock waves in oriented nitromethane single crystals: Plane-specific effects

Abstract

Molecular dynamics simulations of supported shock waves (shock pressure P(s) ∼ 15 GPa) propagating along the [110], [011], [101], and [111] directions in crystalline nitromethane initially at T = 200 K were performed using the nonreactive Sorescu-Rice-Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. These simulations, combined with those from a preceding study of shocks propagating along [100], [010], and [001] directions in nitromethane for similar conditions of temperature and shock pressure [L. He, T. D. Sewell, and D. L. Thompson, J. Chem. Phys. 134, 124506 (2011)], have been used to study the post-shock relaxation phenomena. Shocks along [010] and [101] lead to a crystal-crystal structure transformation. Shocks propagating along [011], [110], [111], [100], and [001] exhibit plane-specific disordering, which was characterized by calculating as functions of time the 1D mean square displacement (MSD), 2D radial distribution function (RDF), and 2D orientation order parameter P(2)(θ) in orthogonal planes mutually perpendicular to the shock plane; and by calculating as functions of distance behind the shock front the Cartesian components of intermolecular, intramolecular, and total kinetic energies. The 2D RDF results show that the structural disordering for shocks along [100], [110], and [111] is strongly plane-specific; whereas for shocks along [001] and [011], the loss of crystal structural order is almost equivalent in the orthogonal planes perpendicular to the shock plane. Based on the entire set of simulations, there is a trend for the most extensive disordering to occur in the (010) and (110) planes, less extensive disordering to occur in the (100) plane, and essentially no disordering to occur in the (001) plane. The 2D P(2)(θ) and 1D MSD profiles show, respectively, that the orientational and translational disordering is plane-specific, which results in the plane-specific structural disordering observed in the 2D RDF. By contrast, the kinetic energy partitioning and redistribution do not exhibit plane specificity, as shown by the similarity of spatial profiles of the Cartesian components of the intermolecular, intramolecular, and total kinetic energies in orthogonal planes perpendicular to the shock plane.

Post-shock relaxation in crystalline nitromethane

“…For example, the strengths and directions of shocks can be tuned to effect specific responses. [1][2][3][4] To make full use of shocks as an experimental technique for rapidly adding energy to solids or liquids, it is important that we come to understand the fundamental behavior of shocks in complicated anisotropic materials and at interfaces. [5][6][7][8] The large anisotropic strains and high strain rates imposed by shock wave passage through a crystal result in molecules containing relatively large amounts of energy that is, in general, non-thermally distributed immediately behind the shock front.…”

“…While there have been numerous reports of "temperature" profiles in molecular dynamics (MD) simulations of shock waves based on the local temperature -usually defined as T = 2E k /(3Nk) and sometimes referred to as mean kinetic energy temperature or simply kinetic temperaturecalculated for thin sections of material perpendicular to the shock direction, 3,4,[13][14][15][16][17][18][19][20][21] to our knowledge, Klimenko and Dremin 13,14 were the first to use MD specifically to study the question of whether the temperature (i.e., the Maxwell velocity distribution or Maxwell-Boltzmann kinetic energy distribution) exists in a shock front. They considered strong shocks in a Lennard-Jones liquid 13 and crystal 14 using parameters appropriate for Ar.…”

“…It was also observed that the translational and rotational degrees of freedom rapidly superheat, while the vibrational energy increases much more slowly. He et al 3,4 studied the structural relaxation of crystalline nitromethane, initially at T = 200 K and shocked to ∼15 GPa along seven different crystal directions. It was observed that the intermolecular (lattice) temperature exhibited large transient excitations, by hundreds of Kelvin for subpicosecond times.…”

“…We have been using MD methods to explore the fundamental molecular-level and corresponding macroscopic (e.g., temperature, pressure, phase, plasticity) changes resulting from shock loading. 3,4,16,17,19,20,26 Although the ultimate goal is to understand how chemistry occurs in crystals, we have chosen to use non-reactive force fields because they can be more accurately formulated than reactive force fields, and thus provide a means of gaining reliable information about the physical processes leading up to chemical reactions. Although we have studied shock waves in other molecular crystals 2,16,17,20,27,28 and metals, 29,30 nitromethane has been the system for which we have performed the most studies.…”

“…In the present study, we build on previous 3,4,19,26 MD simulation studies of supported shock waves in oriented crystalline nitromethane. As before, we use the fully flexible Sorescu-Rice-Thompson 31, 32 (SRT) force field, which has been validated against a wide variety of thermo-physical and spectroscopic information.…”