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

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 forcefield 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 multimillionatom 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. [12 -14]. A major unsolved problem in this area of mechanochemistry is how strains trigger chemical reactions that cause detonation. Recently, the role of a mechanical stimulus, such as bond bending, in initiating reaction at a shock front has attracted much attention. Manna et al. have shown on the basis of density functional calculation of a TATB crystal that bond-bending of nitro group in TATB molecule closes the HOMO-LUMO gap and have speculated that reaction may proceed at supersonic speed [15]. Atomistic-level understanding of the structure of shock fronts at the onset of detonation requires multimillion-atom simulations with reaction chemistry under high strain rate deformations.We have performed a series of molecular dynamics (MD) simulations of planar shock on a slab of an l,3,5-trinitro-l,3,5-triazine (RDX) [16,17] crystal. Each RDX molecule consists of 21 atoms [ Fig. 1(a)], and the unit cell of the RDX crystal contains 8 RDX molecules [ Fig. 1(b)]. The x, y, and z axes are aligned with the [100], [010], and [001] crystallographic orientations, respectively. We apply periodic boundary conditions in all directions after adding 2 nm of vacuum layers on both sides of the slab along the x axis. We have simulated a system of N 2 322 432 atoms with dimensions 318:48 284:08 271:76 A 3 and also a system of 145 152 atoms to confirm that major results do not change with the system size. Initially, the system is relaxed for 1 ps at 5 K and then quenched to 0 K.Simulations reported here are based on a scalable implementation of a reactive force-field (REAXFF) MD [18,19] on massively parallel computers [20]. The REAXFF potential energy function consists of bonding and nonbonding interactions. Bonding interactions comprise coordination energy, two-body stretching, three-body bending and fourbody torsion energy functions. Nonbonding interactions consists of van der Waals and Coulomb energies [21], where charge transfer is included by an electronegativity FIG. 1 (color). (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. Th...

Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked materials

“…Thus ReaxFF provides the possibility of realistic simulations to probe the atomistic mechanism controlling detonation, [5][6][7][8][9][10][11][12] and leads to an accurate description of the complex chemistry of cyclotrimethylene trinitramine (RDX) under shock-loading conditions, 6 and similar calculations on polyethylene (PE) and poly(4-methyl-1-pentene) polymer lead to good agreement with the experimental Hugoniot. 9 It is generally accepted that detonation of PBXs is initiated at hot spots, but the mechanism responsible for hot-spot ignition is not clear.…”

ReaxFF reactive molecular dynamics on silicon pentaerythritol tetranitrate crystal validates the mechanism for the colossal sensitivity

“…This is because similar ReaxFF reactive force fields for other materials have been validated to predict accurately both the reactivity of bonds and mechanical properties of condensed phases. 16,[22][23][24][25][26][27][28][29][30][31] The studies of anisotropic sensitivity of PETN and HMX, 16,24 thermal decomposition of HMX (cyclotetramethylene tetranitramine), TATB (triamino-trinitrobenzene), and RDX, [25][26][27] shock dynamics of RDX and PBX (plastic bonded explosives), [28][29][30][31] and so forth using ReaxFF-RMD lead to the results in accordance with available experimental data, making it practical to describe chemical reactions occurring under various conditions during the large scale dynamical processes involving millions of atoms with currently available computational facilities.…”

The ReaxFF reactive force-field: development, applications and future directions

“…Nomura et al have developed a parallel ReaxFF implementation, which has been used in a number of largescale simulations, including high-energy materials, metal grain boundary decohesion, water bubbles and surface chemistry. [153][154][155][156][157][158][159] This Nomura et al code is not publicly available. Figure 6.…”