Unsupervised Learning-Based Multiscale Model of Thermochemistry in 1,3,5-Trinitro-1,3,5-triazinane (RDX)

Abstract

The response of high-energy-density materials to thermal or mechanical insults involves coupled thermal, mechanical, and chemical processes with disparate temporal and spatial scales that no single model can capture. Therefore, we developed a multiscale model for 1,3,5-trinitro-1,3,5triazinane, RDX, where a continuum description is informed by reactive and nonreactive molecular dynamics (MD) simulations to describe chemical reactions and thermal transport. Reactive MD simulations under homogeneous isothermal and adiabatic conditions are used to develop a reduced-order chemical kinetics model. Coarse graining is done using unsupervised learning via non-negative matrix factorization. Importantly, the components resulting from the analysis can be interpreted as reactants, intermediates, and products, which allows us to write kinetics equations for their evolution. The kinetics parameters are obtained from isothermal MD simulations over a wide temperature range, 1200−3000 K, and the heat evolved is calibrated from adiabatic simulations. We validate the continuum model against MD simulations by comparing the evolution of a cylindrical hotspot 10 nm in diameter. We find excellent agreement in the time evolution of the hotspot temperature fields both in cases where quenching is observed and at higher temperatures for which the hotspot transitions into a deflagration wave. The validated continuum model is then used to assess the criticality of hotspots involving scales beyond the reach of atomistic simulations that are relevant to detonation initiation.

Predicted Melt Curve and Liquid Shear Viscosity of RDX up to 30 GPa

“…[a] M. P. Kroonblawd length and time scales has recently been used to calibrate and validate grain scale model terms through direct comparison [25][26][27][28][29][30][31]. Despite the promise of a one-and-done approach to calibration through scalebridging simulations, it is often more advantageous to gradually increase grain scale model complexity and independently determine physics terms due to the highly coupled and nonlinear nature of dynamic material response [24,28,29].…”

Mode-Selective Vibrational Energy Transfer Dynamics in 1,3,5-Trinitroperhydro-1,3,5-Triazine (RDX) Thin Films

“…If we assume that the VET timescales and pathways are reversible (phonon up-pumping versus vibrational relaxation), then the results herein of ~200 ps, phonon-mediated VET, point away from the "doorway mode" concept for shock initiation 1,2 in favor of direct coupling to high frequency reporter modes 4,27 which lie along commonly recognized reaction pathways. 90,91 Indeed, our observations of ℴ(100 ps) dynamics in the three energetic materials we have presently or previously investigated-RDX, PETN, 13 and TATB 14 -suggest that the direct coupling of phonons to reactive high frequency modes, and the breakdown of the rigid molecule approximation, may be a general feature of energetic molecular crystals. The observed mode-selectivity in these longest dynamics, coupled with DFT simulations showing compression orientation-dependent peak shifts, may provide key clues to the observed crystallographic orientation dependence on shock sensitivity.…”

Mode-Selective Vibrational Energy Transfer Dynamics in 1,3,5-Trinitroperhydro-1,3,5-Triazine (RDX) Thin Films

“…If we assume that the VET timescales and pathways are reversible (phonon up-pumping versus vibrational relaxation), then the results herein of ~200 ps, phonon-mediated VET, point away from the "doorway mode" concept for shock initiation 1,2 in favor of direct coupling to high frequency reporter modes 4,27 which lie along commonly recognized reaction pathways. 90,91 Indeed, our observations of ℴ(100 ps) dynamics in the three energetic materials we have presently or previously investigated-RDX, PETN, 13 and TATB 14 -suggest that the direct coupling of phonons to reactive high frequency modes, and the breakdown of the rigid molecule approximation, may be a general feature of energetic molecular crystals. The observed mode-selectivity in these longest dynamics, coupled with DFT simulations showing compression orientation-dependent peak shifts, may provide key clues to the observed crystallographic orientation dependence on shock sensitivity.…”