Session Z01: Shocks in Energetic Materials

Explosive Hot Spots: Self-Similarity and the Importance of 3D Effects

Shock-induced collapse of voids with diameters in the 100-1000 nm range is thought to govern explosive initiation, but details of the hot spot formation process at this scale remain elusive to "full-physics" treatments based in all-atom molecular dynamics (MD). Using large-scale quasi-2D MD simulations, we predict the initial characteristics of hot spots formed in a model crystalline molecular explosive on multi-micron computational domains. Comparing a range of pore diameters and shock loading orientations shows a high degree of consistency in the of hot spots formed at a given shock strength. Statistical tests are developed that reveal rapid convergence and self-similarity in the predicted hot spot temperature fields with increasing pore diameter for a given orientation and shock strength. Assessments of quasi-2D and full 3D MD simulations of hot spot formation give direct evidence that widely adopted 2D void geometries significantly underestimate the peak temperature under shock initiation conditions. 

 

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Approved for unlimited release, LLNL-ABS-843928.

    

Hotspots vs “Coldspots”:Extemporaneous Mechanochemistry and its Relative Efficiency to Thermal Reactions

The formation of hotspots, via different mechanisms such as pore collapse, shear band formation, and interfacial friction, not only leads to localized heating, but an additional localization of potential energy, which manifests from intra-molecular strains. These large molecular strains and distortions are known to influence chemical reactions by lowering activation barriers and alter the reaction pathways undergone via a phenomenon known as mechanochemistry.

However, the overall mechanisms that form these large intra-molecular strains under shock loading is not properly understood, as well as how the changes to local kinetics can affect hotspot criticality and overall ignition. Mechanochemistry is known to accelerate kinetics, but it also increases the total work done on the system to reach said kinetics.

Therefore, in this talk, I will explore the resultant localizations of intra-molecular strain for various microstructural conditions using large scale, non-reactive molecular dynamics (MD) simulations, assess how this alters hotspot reaction kinetics using large scale reactive MD, and utilize a novel steered MD approach to extract trends in kinetics and reaction paths for specific atomic deformations found commonly in hotspots. Significant focus will be placed on using the steered MD to assess an efficiency of mechanochemistry in hotspot reactions, assessing if, under a total energy budget, if it is advantageous for all energy to be in temperature, or if some distribution of thermal and mechanical strain energy leads to faster reaction kinetics. A theoretical model that is an extension of Arrhenius kinetics is derived and compared to the MD results. This model is then used to explore a wider range of mechanochemical conditions relevant to shock induced initiation and ignition.