Session A25: Focus Session: Simulation of Matter at Extreme Conditions - Energetic Materials

Instability of planar detonation front in energetic materials

Detonation wave propagation in solid energetic materials (EMs), as described by the standard AB model, was studied using a novel moving window molecular dynamics (MW-MD) technique. Parameters of the AB model were modified to investigate the mechanisms of detonation propagation in EMs as a function of the activation barrier for the chemical reaction AB+B -$>$ A+BB + 3 eV. For barriers below 0.2 eV, the detonation front structure remained planar irregardless of the cross-section of the sample. For higher activation barriers, the one-dimensional planar detonation evolves into a cellular detonation upon increase of one of the transverse dimensions of the sample. The cellular detonation transforms into a stable three-dimensional turbulent-like detonation upon simultaneous increase of both transverse dimensions of the sample. These various instabilities of the planar detonation front in solid EMs observed in our MW-MD simulations mirror the major regimes of gas-phase detonation, thus confirming the universal nature of detonation phenomena.   

Shock-induced chemical reactions in organic materials and explosives

Interrogating chemical reactions behind a shock front is immensely difficult and, as a result, the details of shock-induced chemistry remain poorly understood. Previous research has shown that dimerizations, polymerizations, ring-opening and decomposition reactions can occur under shock compression, depending on molecular structure. Questions regarding the thresholds for incipient reaction, the nature of first and subsequent reaction steps, and the influence of shock input conditions on reaction kinetics remain to be answered. Here, we have applied \textit{in-situ} electromagnetic gauging at multiple Lagrangian positions to elucidate the evolution of multiple-wave structures associated with shock-induced reactions of several simple functional groups: carbon-carbon double (-C=C-) and triple bonds, and nitriles. The relative order of group reactivity under single shock conditions for these simple molecules is discussed. From measurements of the reactive flow, we have obtained detailed information about the temporal evolution of the waves, and global kinetic rates associated with transformation(s) between partially- and fully-reacted states. Near the reactive thresholds, evolution in particle velocities point to reaction timescales on the order of tens-to-hundreds of nanoseconds. We further compare evidence of reaction from gas gun-driven experiments to recent results using laser-driven shocks. Spectroscopic details will be presented from both types of experiments.   

Molecular dissociation under extreme conditions

Molecular dissociation under extreme temperatures and pressures is the first step towards thermal or shock initiated decomposition of energetic materials. Fast dissociation rates are challenging to measure, but amenable to first principles calculations. We combine transition-state theory with molecular dynamics simulations based on density-functional theory to predict the temperature-dependent dissociation rates in the gas and the condensed phase. Current applications focus on gas-, solution-, and liquid-phase thermal dissociation of nitramines. These studies will be discussed in the context of developing mesoscale models of initiation of energetic materials.   

Thermodynamic Properties of energetic materials from density functional theory with van der Waals corrections

The calculation of thermodynamic properties for energetic materials from first-principles offers the promise to provide key parameters for mesoscopic and continuum-level simulations of explosives performance for a wide range of pressures and temperatures. While density functional theory with empirical van der Waals corrections, together with corrections for temperature and zero-point effects, can give excellent agreement between calculated and experimentally determined equations of state, quantities such as heat capacities and coefficients of thermal expansion suffer from inaccuracies in the lower frequencies of the calculated vibration spectrum. Additional approaches are discussed to account for the lowest intermolecular modes to increase the accuracy in prediction of thermal properties.   

Relating polymorphism and decomposition of RDX under static and dynamic compression

Knowledge of the reactive behavior of energetic crystals at static high pressures and high temperatures (HP-HT) is an important step toward understanding the shock wave initiation of these crystals. Vibrational spectroscopy in a diamond anvil cell was used to examine the behavior of RDX crystals at the pressures and temperatures relevant to shock wave initiation. Phase boundaries between three RDX polymorphs ($\alpha $, $\gamma $, and $\varepsilon )$ were determined up to 12 GPa and 600 K. Decomposition kinetics for the $\varepsilon $- and $\gamma $-phases were examined at various pressures and temperatures, and were found to have positive volumes of activation. CO$_{2}$, N$_{2}$O and H$_{2}$O were identified as the main decomposition species. Static HP-HT results were used to identify and understand the following processes in shocked RDX: $\alpha -\gamma $ phase transition, identification of the crystal phase at decomposition, and the role of pressure and temperature in accelerating the RDX decomposition under shock compression. This work demonstrated that static HP-HT results provide an important complementary route to elucidate the physical and chemical processes in shocked RDX crystals.   

Atomistic Simulations of Orientation and Shock Velocity Dependences on Pentaerythritol Tetranitrate Detonation

Predicting the behavior of energetic materials requires a detailed description of how chemical reaction, energy and pressure fronts propagate during initial stages of detonation. In this talk, classical molecular dynamics (MD) simulations are used to examine orientation and shock velocity dependences in single crystal pentaerythritol tetranitrate (PETN). This work utilizes an empirical, variable charge reactive force field (ReaxFF) that is implemented in the LAMMPS package with a time-averaged bond-order method for on-the-fly chemical species identification. The accuracy of ReaxFF is validated by comparisons of activation barriers for dissociation of a single PETN molecule along various dissociation channels with higher-fidelity, but more expensive, density functional theory (DFT) calculations. The response of single-crystal PETN to shock compression is simulated using the multi-scale shock technique (MSST) along the insensitive (100) directions, as well as the sensitive (001) and (110) directions, at steady shock velocities ranging from 6-10 km/s. Hugoniot curves, particle velocities of shocked molecules, and evolution of reaction products with time from MD simulations with ReaxFF will be discussed and compared to that from DFT calculations.   

Atomic-Scale Theoretical Studies of Energy Transfer, Inelastic Deformation, and Void Collapse in Molecular Crystals and Polymers

Recent atomic-scale theoretical studies of shock waves in polyatomic molecular crystals and polymers will be presented, with an emphasis on the results and interpretation of molecular dynamics simulations for pentaerythritol tetranitrate (PETN), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), nitromethane, and poly(butadiene) (PBD). The effects of structural and mechanical anisotropy on the material response are of particular interest. Among the topics to be discussed are orientation dependent energy transfer pathways and inelastic deformation mechanisms subsequent to shock wave passage in initially defect-free nitromethane and PETN crystals, shock-induced collapse of variously shaped voids in crystalline RDX, and details of shock wave propagation and energy localization in bulk PBD and at the PBD/RDX interface.   

Explosive initiation of pentaerythritol tetranitrate (PETN) by laser irradiation

Understanding of explosive decomposition of energetic materials triggered by laser irradiation is of great importance for design of new economical formulations with high performance and tunable sensitivity. Earlier, laser irradiation was only considered as a source of heat. Nowadays, it is realized that optical excitation may set off initiation of rapid chemical reactions and govern further decomposition in energetic materials. However, mechanisms of this phenomenon are yet to be established. We present quantum-chemical calculations of the electronic structure of molecular and crystalline PETN to explore the effect of common impurities on its optical properties. We found that charged or excited PETN molecules exhibit significantly different electronic, optical, and chemical behavior. For example, new decomposition pathways that were not available in the ground state become favorable in the charged state of PETN. Three calculated lowest excitation energies of ionized PETN require 0.6, 0.7 and 1.13 eV, which is considerably lower than those for equilibrium PETN. The activation energy of the rate limiting decomposition stage is within 9-12 kcal/mole, while it is 35 kcal/mole in the ground state. We discuss possible ways that originate charge transfer in PETN and presented results in the context of recent experimental data.   

Condensation of carbon during high explosive detonation

The formation of nano-carbon clusters is believed to be responsible for the non-ideal detonation behavior of carbon-rich explosives, such as triamino-trinitrobenzene (TATB). We have developed a new model of carbon formation during detonation. The model is based on the assumption that carbon cluster growth has features of both activated Arrhenius kinetics and diffusion controlled kinetics. In our model the variation of temperature, density, and viscosity throughout the high explosive reaction zone and expansion is calculated using a thermochemical model linked to a hydrodynamic code. We compare our model to new experimental results on the size scaling of detonations in TATB-based explosives.   

Surface-induced effect on sensitivity of beta and delta HMX crystals

It is accepted that sensitivity of energetic materials depends on many factors, including presence of defects, surfaces, interfaces, or voids. However, details of atomistic mechanisms that govern sensitivity to initiation of detonation and correlations between structure, morphology, and degradation of chemical bonds are far from being understood. In this talk, we present quantum chemical calculations combined with transition state theory to analyze chemical decomposition reactions in beta and delta HMX crystals. We calculate the activation barriers and reaction rates in the ideal crystals and materials containing internal surfaces, vacancies, and voids. We show that N-NO$_2$ homolysis is the most favorable decomposition reaction in all cases. We discuss whether a large space available in the vicinity of voids facilitates the N-NO$_2$ break in comparisons to an ideal crystal, and if this effect is enhanced in the delta phase in comparison to beta phase. The conclusions and revealed trends are presented in the context of experimental data.   

1,1-Diamino-2,2-Dinitroethylene Under High-Pressure-High-Temperature

1,1-Diamino-2,2-dinitroethylene (FOX-7) is an insensitive high explosive (IHE) which shows promise for use in low vulnerability ammunitions. With performance comparable to RDX and HMX, there is a growing interest in understanding the behavior under denotation conditions. Through the use of diamond anvil cell (DAC) technology and electrical resistive heating, the vibrational behavior of FOX-7, in both the mid and far-IR, were recorded at multiple isotherms under elevated pressure-temperature (PT). Energy-dispersive x-ray diffraction (XRD) was also employed along with a multi-anvil press for further investigating pressure-temperature phase space. Future planned experiments will focus on using high-resolution angular-dispersive XRD and neutron diffraction techniques to resolve high pressure-temperature structural information and obtain P-V-T data. The experiments on FOX-7 have revealed previously uninvestigated knowledge on the elevated-PT decomposition and phase boundaries allowing for a more developed basis for the behavior of FOX-7 under detonation conditions.   

Study on the deflagration-to-detonation transition course of porous energetic material

The deflagration-to-detonation transition (DDT) course of energetic material with different porosity ratio was studied utilizing a one-dimensional two-phase flow code. The equations were numerically solved by space-time conservation element and solution element (CE/SE) method. The distribution of physical quantities such as pressure and temperature were obtained together with their evolution history. The physical rules before detonation were mainly analyzed and the effect of convection on the chemical reaction of energetic material was emphasized on.