Dynamic Transition in the Structure of an Energetic Crystal during Chemical Reactions at Shock Front Prior to Detonation

Ken-ichi Nomura, Rajiv K. Kalia, Aiichiro Nakano, Priya Vashishta, Adri C. T. van Duin, and William A. Goddard, III

Phys. Rev. Lett. 99, 148303 – Published 5 October 2007

Abstract

Mechanical stimuli in energetic materials initiate chemical reactions at shock fronts prior to detonation. Shock sensitivity measurements provide widely varying results, and quantum-mechanical calculations are unable to handle systems large enough to describe shock structure. Recent developments in reactive force-field molecular dynamics (ReaxFF-MD) combined with advances in parallel computing have paved the way to accurately simulate reaction pathways along with the structure of shock fronts. Our multimillion-atom ReaxFF-MD simulations of l,3,5-trinitro-l,3,5-triazine (RDX) reveal that detonation is preceded by a transition from a diffuse shock front with well-ordered molecular dipoles behind it to a disordered dipole distribution behind a sharp front.

Received 26 April 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.148303

Authors & Affiliations

Ken-ichi Nomura, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishta

Collaboratory for Advanced Computing and Simulations, Departments of Chemical Engineering & Materials Science, Physics & Astronomy, Computer Science, University of Southern California, Los Angeles, California 90089-0242, USA

Adri C. T. van Duin and William A. Goddard, III

Materials and Process Simulation Center, Beckman Institute (139-74), California Institute of Technology, Pasadena, California 91125, USA

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Issue

Vol. 99, Iss. 14 — 5 October 2007

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Images

Figure 1
(a) An RDX molecule with carbon (yellow), hydrogen (white), oxygen (red), and nitrogen (blue) atoms. (b) The unit cell of an RDX crystal contains 8 RDX molecules, which are colored blue and red depending on whether the ${NO}_{2}$ groups faces away from (group 1) or faces towards (group 2) the shock plane. The ${NO}_{2}$ in group 1 and group 2 molecules are colored in cyan and magenta, respectively. (c) Schematic of a planar shock simulation, where $V_{p}$ and $V_{s}$ are the particle and shock velocities, respectively.Reuse & Permissions
Figure 2
(a) Molecular dynamics (circles with solid lines to guide the eyes for $N = 145, 152$ atoms and squares for $N = 2, 322, 432$ atoms) and experimental (dashed) Hugoniot curves (particle velocity $V_{p}$ vs shock velocity $V_{s}$ ) for the RDX crystal. (b) Number of molecular fragments (for $N = 145, 152$ atoms) produced during 1 ps after shock loading at $V_{p} = 3 km / s$ (blue) and $5 km / s$ (red).Reuse & Permissions
Figure 3
Time variations of the rotational (solid curves) and vibrational (dashed curves) temperatures of RDX molecules in group 1 (a) and group 2 (b) at $V_{p} = 1 km / s$ . The repulsive wall starts to interact with the material immediately after the simulation begins at time $t = 0$ . (a) Group 1 molecules exhibit a transition from rotational to vibrational motion with a 0.3 ps time delay. (b) Group 2 molecules exhibit direct excitation of vibrational modes with the arrival of the shock wave.Reuse & Permissions
Figure 4
Snapshots of an RDX molecule from group 1 (top row) and group 2 (bottom row) before shock loading and 1 ps after the arrival of the shock front with particle velocities $V_{p} = 1$ , 3, and $4 km / s$ (from left to right). Yellow, white, red, and blue spheres are carbon, hydrogen, oxygen, and nitrogen atoms, respectively. The arrows indicate the directions of molecular dipole moments and the colors represent the magnitude of the dipole moment. Group 1 molecules rotate so that the $C_{3} N_{3}$ ring is oriented perpendicular to the direction of shock propagation, whereas group 2 molecules deform and are flattened, which reduces the dipole moment.Reuse & Permissions
Figure 5
Molecular dipole moments near the shock front at $V_{p} = 1 km / s$ (a) and $3 km / s$ (b). The shock front is diffuse and the dipole distribution is well ordered behind the shock at $V_{p} = 1 km / s$ (a). In contrast, a sharp shock front and disordered molecular dipole distribution are observed at $V_{p} = 3 km / s$ (b). Also shown are the polar and azimuthal angles of the dipole moments of a single group 1 molecule at $V_{p} = 1 km / s$ (c) and $3 km / s$ (d). Continuous variation of angles at $V_{p} = 1 km / s$ becomes abrupt at $V_{p} = 3 km / s$ .Reuse & Permissions

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