Granularity impact on hotspot formation and local chemistry in shocked nanostructured RDX

The nanostructuration of energetic materials results in interesting properties. In particular, the detonation of carbon-rich explosives leads to the formation of nanodiamonds, the size of which is linked to the initial size of the explosive nanograins. This correlation could come from the role played by the granularity in the shock properties, especially the local temperature, which could be enhanced when the shock front crosses the various interfaces and nanoporosities of the material. More generally, the granularity-dependent reactivity also concerns some aspects of the sensitivity of energetic materials and subsequent inhibition or safety issues. In order to investigate this hypothesis, we perform classical and reactive molecular dynamics shock-simulations on cyclotrimethylene trinitramine (RDX), a common high-explosive. We design various nanogranular structures using the body-centered-cubic stacking of nanoparticles shaped as Kelvin cells (truncated octahedra) with slightly-bumped faces. The nanograin size and the bump radius allow to independently control the porosity value and its nanostructure. We show that the global shock properties, especially the temperature, are sensitive to the porosity value but not to the nanograin size. The porosity nanostructure has a local impact, enhancing the temperature heterogeneities between the inter- and intra-grain regions by a few hundred degrees and slowing down the thermal homogenization. For a given porosity, the larger the nanograins, the larger and the hotter the hotspots. In such hotspots, the local chemistry is significantly modified, resulting in a larger reactivity with a quicker formation of some final products. We suggest that the quicker consumption of heteroatoms (namely, H, O, and N) along with higher local temperatures is likely to impact the formation process of solid carbonaceous phases.