Session Y5: First-Principles and MD VIII: Reactive Molecular Dynamics

Micron-scale Reactive Atomistic Simulation of Void Collapse and Hotspot Growth in PETN

Material defects and other heterogeneities such as dislocations, micro-porosity, and grain boundaries play key roles in the shock-induced initiation of detonation in energetic materials. We performed non-equilibrium molecular dynamics simulations to explore the effect of nanoscale voids on hotspot growth and initiation in micron-scale pentaerythritol tetranitrate (PETN) crystals under weak shock loading (Up $=$ 1.25 km/s; Us $=$4.5 km/s). We used the ReaxFF potential implemented in LAMMPS. We built a pseudo-2D PETN crystal with dimensions 0.3 $\mu $m x 0.22 $\mu $m x 1.3 nm containing a 20 nm cylindrical void. Once the initial shockwave traversed the entire sample, the shock-front absorbing boundary condition was applied, allowing the simulation to continue beyond 1 nanosecond. Results show an exponentially increasing hotspot growth rate. The hotspot morphology is initially symmetric about the void axis, but strong asymmetry develops at later times, due to strong coupling between exothermic chemistry, temperature, and divergent secondary shockwaves emanating from the collapsing void.   

Shock and Laser Induced Non-Equilibrium Chemistry in Molecular Energetics

In this study, we have used large scale reactive molecular dynamics (MD) simulations to study how contrasting initiation mechanisms from either shock or electromagnetic insults compare to traditional thermal initiation. We will show how insults of equal strength but different character can yield vastly different reaction profiles and thus the evolution of hot-spots. For shocked RDX (U$_{\mathrm{p}} \quad =$ 2km/s), we find that the collapse of a cylindrical 40 nm diameter pore leads to a significant amount of non-equilibrium reactions followed by the formation of a sustained deflagration wave. In contrast, a hot spot that is seeded into a statically compressed crystal with matching size and temperature will quench over the same timescale, highlighting the importance of insult type. Furthermore, MD simulations of electromagnetic insults coupled to intramolecular vibrations have shown, in some cases, mode specific initial chemistry and altered kinetics of the subsequent decomposition. By leveraging spectroscopic and chemical information gathered in our MD simulations, we have been able to identify and track non-equilibrium vibrational states of these materials and correlate them to these observed changes. Implications of insult dependent reactivity and non-equilibrium chemistry will be discussed.   

Atomistic Simulation of Initiation in Hexanitrostilbene

We report on the effect of cylindrical voids on hot spot formation, growth and chemical reaction initiation in hexanitrostilbene (HNS) crystals subjected to shock. Large-scale, reactive molecular dynamics simulations are performed using the reactive force field (ReaxFF) as implemented in the LAMMPS software. The ReaxFF force field description for HNS has been validated previously by comparing the isothermal equation of state to available diamond anvil cell (DAC) measurements and density function theory (DFT) calculations and by comparing the primary dissociation pathway to \textit{ab initio} calculations. Micron-scale molecular dynamics simulations of a supported shockwave propagating through the HNS crystal along the [010] orientation are performed with an impact velocity (or particle velocity) of 1.25 km/s, resulting in shockwave propagation at 4.0 km/s in the bulk material and a bulk shock pressure of $\sim$ 11GPa. The effect of cylindrical void sizes varying from 0.02 to 0.1 $\mu $m on hot spot formation and growth rate has been studied. Interaction between multiple voids in the HNS crystal and its effect on hot spot formation will also be addressed. Results from the micron-scale atomistic simulations are compared with hydrodynamics simulations.   

Kinetic model for thermal decomposition of energetic materials from ReaxFF molecular dynamics

In the present work we perform molecular dynamics simulations of the thermal decomposition of isolated molecules and single crystals of PETN, RDX and HMX. For isolated molecules we use multi-replica approach with different preconditioned atomic velocities to obtain statistics of the decomposition. In this model we only consider the initial stage of the reactions, that shows first order kinetics. In the model of single crystal, we directly observe reaction pathways that result in product formation, as well as the dependences of concentrations of main chemical species on time after heating. Initial temperatures are in the range of 1000 to 2800 K. On the basis of the obtained dependences of concentrations we propose a kinetic model that describes thermal decomposition process. Reaction rate constants are well described by the Arrhenius law. Activation energies for the initial stage appear to be lowered by 30-60 kJ/mole in condensed phase compared to the isolated molecule. We compare these results between different ReaxFF parametrizations and DFT calculations.   

Chemistry away from local equilibrium: shocking high-energy and energy absorbing materials

In this presentation I will describe reactive molecular dynamics and coarse grain simulations of shock induced chemistry. MD simulations of the chemical reactions following the shock-induced collapse of cylindrical pores in the high-energy density material RDX provide the first atomistic picture of the shock to deflagration transition in nanoscale hotspots. We find that energy localization during pore collapse leads to ultra-fast, multi-step chemical reactions that occur under non-equilibrium conditions. The formation of exothermic products during the first few picoseconds of the process prevents the hotspot from quenching, and within 30 ps a deflagration wave develops. Quite surprisingly, an artificial hot-spot matching the shock-induced one in size and thermodynamic conditions quenches; providing strong evidence that the dynamic nature of the loading plays a role in determining the criticality of the hotspot. To achieve time and lengths beyond what is possible in MD we developed a mesoscale model that incorporates chemical reactions at a coarse-grained level. We used this model to explore shock propagation on materials that can undergo volume-reducing, endothermic chemical reactions. The simulations show that such chemical reactions can attenuate the shockwave and characterize how the characteristics of the chemistry affect this behavior. We find that the amount of volume collapse and the activation energy are critical to weaken the shock, whereas the endothermicity of the reactions plays only a minor role. As in the reactive MD simulations, we find that the non-equilibrium state following the shock affects the nucleation of chemistry and, thus, the timescales for equilibration between various degrees of freedom affect the response of the material.