The Potential Energy Hotspot: Effects of Impact Velocity, Defect Geometry, and Crystallographic Orientation

Abstract

In energetic materials, the localization of energy into “hotspots” is known to dictate the initiation of chemical reactions and detonation. Recent all-atom simulations have shown that more energy is localized as internal potential energy (PE) than can be inferred from the kinetic energy (KE) alone. The mechanisms associated with pore collapse and hotspot formation are known to depend on pore geometry and dynamic material response such as plasticity. Therefore, we use molecular dynamics (MD) simulations to characterize shock-induced pore collapse and the subsequent formation of hotspots in 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a highly anisotropic molecular crystal, for various defect shapes, shock strengths, and crystallographic orientations. We find that the localization of energy as PE is consistently larger than the KE in cases with significant plastic deformation. An analysis of MD trajectories reveals the underlying molecular- and crystal-level processes that govern the effect of orientation and pore shape on PE localization. We find that the regions of highest PE relate to the areas of maximum plastic deformation, while KE is maximized at the point of impact. Comparisons against octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) reveal less energy localization in TATB, which could be a contributing factor to the latter’s insensitivity.

Systematic Builder for All‐Atom Simulations of Plastically Bonded Explosives

“…These hotspots originate entirely from inter-granular pore collapse as no defects are built within the grains during the building process. A comparison of this temperature map with the initial microstructure, Figure 6(a), indicates that elongated pores result in the highest temperatures, this is consistent with prior MD simulations [11,40].…”

“…The shock compression of materials can drive a variety of events such as phase transformations [1][2][3], plasticity [4][5][6][7], melting [8,9], intra-molecular deformations [10][11][12], and chemical reactions [13][14][15][16]. Most of these phenomena can be exacerbated by the localization of energy due to the interaction of the shockwaves with the materials microstructure and defects.…”

Extemporaneous Mechanochemistry: Shock-Wave-Induced Ultrafast Chemical Reactions Due to Intramolecular Strain Energy

“…Recent MD simulations revealed that the shock-induced collapse of porosity results not just in energy localization in the form of heating but also in significant intramolecular strains 42,43 . The excess potential energy (PE), or intramolecular strain energy 44 , is readily stored in modes relevant to prompt chemical initiation and exceeds the expected value based on the thermal vibrational energy 42 .…”

“…Here, we aim to utilize all-atom molecular dynamics to study the entire extemporaneous event via the shock initiation of a porous sample of TATB (1,3,5-triamino-2,4,6-trinitrobenzene), an insensitive HE material. Our simulation setup is designed with inspiration from previous studies ,, to provide the best opportunity to see an extemporaneous mechanochemical event.…”

“…Recent MD simulations revealed that the shock-induced collapse of porosity results not just in energy localization in the form of heating but also in significant intramolecular strains. , The excess potential energy (PE), or intramolecular strain energy, is readily stored in modes relevant to prompt chemical initiation and exceeds the expected value based on the thermal vibrational energy . Similarly, quantum-chemistry calculations indicate that HOMO–LUMO gap closure arises in TATB when nitro groups are deformed out of plane, correlating mechanochemical effects with torsional (4-body) strains .…”