Impact of surface energy on the shock properties of granular explosives

Improved predictions of thermomechanical properties of molecular crystals from energy and dispersion corrected DFT

Thermal stability and pressure-dependent changes are key to molecular crystals and their properties. The determination of their thermal properties from ab initio methods is, however, a challenging task. While the low-frequency phonon spectrum related to intermolecular vibrations remains difficult to describe, the Quasi-Harmonic Approximation (QHA) also induces for molecular crystals a significant volume deviation, which makes their thermal behavior ill-determined. To overcome these difficulties, we consider a pragmatic energy correction (EC) that has long been used for atomic crystals, and we presently report the first ever use for molecular crystals. Applying the QHA in dispersion-corrected density functional theory (DFT-D) calculations with an ab initio parameterized EC, the resulting model can simultaneously and accurately derive thermal and mechanical properties of high-explosive molecular crystals. When compared to experiments, the mean absolute percent error of previous DFT-based thermomechanical models is 12% for mechanical and 31% for thermal properties. Our model performs significantly better and reduces these uncertainties to 4.1% and 9.8%, respectively. In particular, the agreement between our model and experiments for the thermal properties is three times better. This significant improvement greatly benefits the determination of thermomechanical properties such as the Grüneisen parameter and the shock properties. The method has been successfully applied to molecular crystals showing a large diversity of weak intermolecular interactions (β-1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), α-1,1-diamino-2,2-dinitroethylene (FOX-7), Triaminotrinitrobenzene (TATB), ε-Hexanitrohexaazaisowurtzitane (CL20), and Pentaerythritol tetranitrate (PETN)-I). Due to its accuracy and transferability, our model is expected to work for a large class of computationally designed molecular crystals and co-crystals, providing a basis for a predictive framework.

The solid phase thermal decomposition and nanocrystal effect of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) via ReaxFF large-scale molecular dynamics simulation

The solid phase thermal decomposition and nanocrystal effect are extremely important to understand the ignition, combustion, reaction growth and buildup to detonation under shock wave action.

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.