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Ultrafast Chemistry under Nonequilibrium Conditions and the Shock to Deflagration Transition at the Nanoscale

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Prose below summarizes the J. Phys. Chem. C article identified by doi and pdf_path. It is not new primary claims by this wiki.

Summary

Energetic materials often initiate from localized “hot spots” where mechanical work concentrates into nanoscale regions of extreme pressure and temperature. This J. Phys. Chem. C article uses large-scale reactive molecular dynamics to follow chemistry after shock-induced collapse of a cylindrical nanopore in crystalline RDX, focusing on a shock with particle velocity ~2 km/s. The central scientific question is how nonequilibrium molecular collisions during pore collapse couple to exothermic chemistry on picosecond time scales, and whether that chemistry can seed a self-sustaining reaction front rather than merely heating and quenching a nanoscale hot spot. The framing contrasts dynamical loading with static thermal hot spots of comparable size and thermodynamic conditions, because continuum-style initiation models frequently bake in assumptions about local equilibration that may fail when gradients are ultrasteep and chemistry is collisionally driven.

Methods

LAMMPS ReaxFF (papers/ReaxFF_others/Wood_Cherukara_Strachan_RDX_JPCC_2015.pdf, §2.2) merges nitramine and combustion ReaxFF lines with RDX-reaction training data from prior work. α-RDX crystals contain one cylindrical void (10–40 nm diameter) from replicated 8-molecule cells; the 40 nm example extends ~243 nm along the shock axis with the pore 60 nm from the impact face. 3D PBC and a piston (momentum mirror) launch shocks. Preparation: minimization, 5 ps NPT (isobaric–isothermal) equilibration at 300 K with a barostat active, then 10 ps NVT (isochoric–isothermal) equilibration at 300 K; production is NVE after piston impact with 0.1 fs timestep and self-consistent ReaxFF charges (conjugate-gradient to 10⁻⁶ tolerance each step). No thermostat during NVE shock propagation. U_p ≈ 2 km s⁻¹ is tied to ~11 GPa shock pressure in the defect-free reference discussion. A static hot spot at matched thermodynamic state but without shock history serves as a control (§2). No electric field or enhanced sampling is used.

Force-field training: N/A — merged parametrization is taken from cited prior training literature.

Static QM / DFT: N/A — not the reported primary modality beyond validation references in the ReaxFF section.

Findings

For U_p ≈ 2 km s⁻¹, collapse of a 40 nm pore drives deflagration-like behavior: collisional, multistep chemistry on picosecond scales yields exothermic products that sustain heating rather than immediate quench. By ~30 ps the authors report a ~0.25 km s⁻¹ reaction front ~5 nm thick emanating from the hot spot (abstract-level numbers). The static control does not reproduce that deflagration wave, underscoring nonequilibrium shock loading versus purely thermal hot-spot pictures. Pore size and U_p control localization versus sustained fronts. Discussion notes simplification of real microstructures and ReaxFF limits on quantitative barriers. External Purdue / LANL work—reference for shock + energetic-material reactive MD.

Limitations

Nanoscale single-pore models simplify real microstructures (defect statistics, polycrystallinity, mesoscale transport). ReaxFF chemistry for energetic materials requires independent validation for quantitative barrier and product distributions.

Relevance to group

Reference for shock and EM reactive MD adjacent to combustion and initiation mechanics in the wider corpus.

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