Fourier-like Thermal Relaxation of Nanoscale Explosive Hot Spots

Abstract

Hot spots are local regions of high temperature that are widely considered to govern explosive initiation. Hot spot dynamics rests on a delicate balance between heat generation due to chemical reactions and heat loss through thermal conduction, making accurate determinations of the conductivity under extreme conditions a key component of predictive explosive models. We develop here an approach to directly determine the thermal transport properties of explosive hot spots with realistic initial structures through a combination of molecular dynamics (MD) and diffusive heat equation (HEq) modeling. Effective thermal conductivity values are determined by fitting HEq models to MD predictions of long timescale hot spot relaxation. The approach is applied to model hot spots in the molecular crystalline explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) for a range of shock strengths and two limiting cases for impact orientation. Isotropic and anisotropic HEq models yield similar results, despite TATB exhibiting some of the largest and most anisotropic thermal conductivity values for explosive near normal conditions. The conductivity is found to be a strong function of density, which parametrically captures dependence on temperature, pressure, and material state. The associated root-mean-square errors of the fitted HEq models are approximately 5% of MD predicted final equilibrium temperatures. The conductivity values determined here for TATB hot spots are considerably larger than those used in a prior hot spot criticality study, which may significantly impact predictions for critical hot spot sizes. The approach provides a convenient foundation for determining the effective thermal conductivity for hot spot problems in other explosives and directly yields information on reasonable approximations that might be taken in higher-level models for those materials.

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

“…This excess PE is the result of large intramolecular deformations that do not significantly relax on timescales comparable to the onset of exothermic chemistry. Nonreactive, hotspot thermal conduction simulations in TATB showed that hotspots formed from the collapse of 40 nm pores take nearly a full nanosecond to equilibrate with the surrounding material . Assessments on the decay of the PE hotspot show almost no relaxation of intramolecular deformations within ∼200 ps of collapse, well within the typical timescale of exothermic relaxation in similar reactive pore collapse simulations. , …”

Many-Body Mechanochemistry: Intra-molecular Strain in Condensed Matter Chemistry

“…35,39 This energy localization, in the form of intra-molecular potential energy, is persistent well into reaction timescales. The increase in strain energy in TATB is caused primarily by torsional and out-of-plane deformations of the nitro and amino groups, shown in Section S5 of the SI, and incurs a local amorphization of the material that affects kinetics 59 and heat transport 60 .…”

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

“…The initiation of chemical reactions and transition to detonation in composite HE formulations under dynamical loading is dominated by energy localization into hotspots via the interaction of the shockwave with the microstructure 19 . These hotspots are almost exclusively characterized by their temperature fields [20][21][22][23] . However, recent molecular dynamics (MD) simulations indicate a deviation from this traditional picture.…”

“…The initiation of chemical reactions and transition to detonation in composite HE formulations under dynamical loading is dominated by energy localization into hotspots via the interaction of the shock wave with the microstructure . These hotspots are almost exclusively characterized by their temperature fields. − However, recent molecular dynamics (MD) simulations indicate a deviation from this traditional picture. Dynamical hotspots can be significantly more reactive than thermodynamically equivalent ones created under nonshock conditions. , Path-dependent reactivity and mechanochemistry have been proposed to explain these observations.…”