Physical mechanism of anisotropic sensitivity in pentaerythritol tetranitrate from compressive-shear reaction dynamics simulations

Abstract

We propose computational protocol ͑compressive shear reactive dynamics͒ utilizing the ReaxFF reactive force field to study chemical initiation under combined shear and compressive load. We apply it to predict the anisotropic initiation sensitivity observed experimentally for shocked pentaerythritol tetranitrate single crystals. For crystal directions known to be sensitive we find large stress overshoots and fast temperature increase that result in early bond-breaking processes whereas insensitive directions exhibit small stress overshoot, lower temperature increase, and little bond dissociation. These simulations confirm the model of steric hindrance to shear and capture the thermochemical processes dominating the phenomena of shear-induced chemical initiation. Experiments by Bridgman 1 demonstrated that chemical transformations occur far more readily under combined shear and pressure loads. Coupling between mechanical, thermal, and chemical effects is important for initiation of detonation in explosives under mechanical impact. Understanding how the detonation sensitivity depends on formulation and structural properties is a critical issue in explosive technology. A great deal of effort has gone into the development of experimental techniques to study these processes ͑e.g., time resolved emission spectroscopy 2-4 ͒ but they have not yet provided satisfactory resolution of initial steps of detonation. Little is known about how mechanical and chemical processes couple to initiate detonation.An ideal system to examine for hints on the atomistic origin of sensitivity, is single crystal of pentaerythritol tetranitrate ͑PETN, four CH 2 -O-NO 2 chains connected to a central carbon, see inset of Fig. 1͒. showed that shock compression of the single crystal in different directions leads to dramatic differences in the sensitivity. Thus the pressure threshold for detonation along the ͓100͔ direction is at least ϳ4 times that for ͓110͔ direction. 10 Such single crystal experiments eliminate many of the variables that complicate interpretations ͑grain boundaries, voids, and internal defects͒ providing an unambiguous challenge to any proposed mechanisms. Dick et al. 8,9 attributed the orientational anisotropy of PETN sensitivity to steric hindrance to the shear of molecules in the neighboring slip planes. Recently, Plaksin 11 showed clear-cut experimental evidence that initiation of detonation occurs preferentially in directions with the maximum shear stress.Here we use reactive dynamics ͑RD͒ simulations at constant shear rate on uniaxially compressed PETN single crystal to show that the physical and chemical responses depend dramatically on the compression direction and slip system. In contrast, RD simulations of pure uniaxial shock compression in various directions show no correlation of the reactive behavior with anisotropy of sensitivity in experiment. Key to our simulations is the ReaxFF reactive force field, 12,13 whose parameters are trained to match quantum mechanics descriptions of reaction barriers for all plausibl...

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

“…The first-generation ReaxFF implementation of van Duin et al 1 established the utility of the force field in the context of various applications. This serial, fortran-77, implementation was integrated into the publicly available, open-source LAMMPS code 150 by Thompson et al 151 as the Reax package 152 -the first publicly available parallel implementation of ReaxFF. 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.…”

Stabilized silicene within bilayer graphene: A proposal based on molecular dynamics and density-functional tight-binding calculations

“…34 Numerical simulations are carried out using the LAMMPS code 37 which includes the ReaxFF method. 38 In order to have an independent test of our used model we performed extra calculations using DFTB/MD [24][25][26] .…”