Session Q26: Focus Session: Materials in Extremes: Reactive Chemistry

Fast Quantum Molecular Dynamics Simulations of Shock-induced Chemistry in Organic Liquids

The responses of liquid formic acid and phenylacetylene to shock compression have been investigated via quantum-based molecular dynamics simulations with the self-consistent tight-binding code LATTE. Microcanonical Born-Oppenheimer trajectories with precise conservation of the total energy were computed without relying on an iterative self-consistent field optimization of the electronic degrees of freedom at each time step via the Fast Quantum Mechanical Molecular Dynamics formalism [A. M. N. Niklasson and M. J. Cawkwell, Phys. Rev. B, \textbf{86}, 174308 (2012)]. The conservation of the total energy in our trajectories was pivotal for the capture of adiabatic shock heating as well as temperature changes arising from endo- or exothermic chemistry. Our self-consistent tight-binding parameterizations yielded very good predictions for the gas-phase geometries of formic acid and phenylacetylene molecules and the principal Hugoniots of the liquids. In accord with recent flyer-plate impact experiments, our simulations revealed i) that formic acid reacts at relatively low impact pressures but with no change in volume between products and reactants, and ii) a two-step polymerization process for phenylacetylene. Furthermore, the evolution of the HOMO-LUMO gap tracked on-the-fly during our simulations could be correlated with changes transient absorption measured during laser-driven shock compression experiments on these liquids.   

Slow N-O chemistry in detonating oxygen balanced mixtures

The chemical evolution and states of matter of energetic materials under detonation conditions remains an open question despite decades of research. Reaction zones for many energetic materials are inferred from hydrodynamic measurements to be anywhere between nanosecond to microsecond time scales. However, the molecular level processes that govern these reaction zone lengths are poorly understood for many organic materials and composites. To this end, we have conducted quantum molecular dynamics simulations of zero and positive oxygen balance mixtures of hydrogen peroxide/nitromethane under detonation conditions to close to equilibrium time scales. We observe the formation of metastable nitrogen oxide intermediates that effectively act as an oxygen ``trap'' by directly slowing the formation of the equilibrium products CO$_{2}$ and N$_{2}$. This is in sharp contrast to the decomposition mechanism of carbon-rich, negative oxygen balanced energetic materials, where N-O chemistry equilibrates extremely rapidly, and carbon condensation and carbon-oxygen bond chemistry are the rate limiting steps to achieving chemical equilibrium. Further work is underway to fully determine the kinetic parameters for N-O chemistry under these conditions for possible use in hydrocode models.   

Shock compression of glow discharge polymer (GDP): density functional theory (DFT) simulations and experiments on Sandia's Z machine

Glow discharge polymer (GDP) is used extensively as capsule/ablation material in inertial confinement fusion (ICF) capsules. Accurate knowledge of the equation of state (EOS) under shock and release is particularly important for high-fidelity design, analysis, and optimization of ICF experiments since the capsule material is subject to several converging shocks as well as release towards the cryogenic fuel. We performed Density Functional Theory (DFT) based quantum molecular dynamics (QMD) simulations, to gain knowledge of the behavior of GDP - for example regarding the role of chemical dissociation during shock compression, we find that the dissociation regime along the Hugoniot extends from 50 GPa to 250 GPa. The shock pressures calculated from DFT are compared experimental data taken at Sandia's Z-machine. The GDP samples were grown in a planar geometry to improve the sample quality and maintained in a nitrogen atmosphere following manufacturing, thus allowing for a direct comparison to the DFT/QMD simulations.   

Modeling of amorphous poly-CO structure with N and He

Density functional theory simulations of amorphous poly-CO structure were performed with addition of N or He atoms to crystalline delta phase of CO. For the CO-N mixtures the concentration of N was varied in the range from 6.25 {\%} to 50{\%} with different distribution of N atoms in the unit cell. For all studied concentrations and initial configurations, isotropic compression led to polymerization beginning at a pressure of 11 GPa. For the CO-He mixtures, the concentration of He atoms in delta phase of CO was 6.25{\%}. Formation of random networks begins at 9 GPa and at 11 GPa all CO molecules have formed a combination of closed rings and chain type structures without isolated CO molecules with a density of 2.40 g/cm3. He atoms facilitate complete formation of the random structure at lower pressure than that for pure poly-CO, which isn't completely polymerized until compressed to a pressure of 18 GPa. He atoms also help stabilize the structure while lowering the pressure down to 100 Bar with only few CO molecules detaching in the process. Without He atoms at the same pressure there are approximately ten times the number CO molecules occupying voids in the random network.   

Vibrational Energy Relaxation in Common High Energy Density Materials Through Reactive Molecular Dynamics Simulations

We use molecular dynamics with the reactive force field ReaxFF to study the decomposition and subsequent reactions of the nitramine HMX under induced by electric fields and temperature. We find that electric fields of appropriately chosen frequencies can trigger chemical decomposition for total energy input significantly smaller than thermally excited systems. In addition, the energy barriers associated with exothermic chemical reactions are also dependent on the character of the excitation for electric field driven samples. We are able to characterize the frequency-dependent energy input and subsequent equilibration using the power spectra of atomic velocities and we find that the non-equilibrium nature of the energy distribution obtained via electric field excitations is responsible for the dependence of energy threshold for decomposition on type of perturbation. Timescales and decay pathways for vibron energy are discussed for HMX and other energetic materials.   

Shock response of Ni/Al reactive inter-metallic composites

Intermolecular reactive composites find diverse applications in defense, microelectronics and medicine, where strong, localized sources of heat are required. Motivated by experimental work which has shown that high-energy ball milling can significantly improve the reactivity as well as the ease of ignition of Ni/Al inter-metallic composites, we present large scale ($\sim$41 million atom) molecular dynamics simulations of shock-induced chemistry in porous, polycrystalline, lamellar Ni/Al nano-composites, which are designed to capture the microstructure that is obtained post milling. Shock propagation in these porous, lamellar materials is observed to be extremely diffuse, leading to substantial inhomogeneity in the local stress states of the material. We describe the importance of pores as sites of initiation, where local temperatures can rise to several thousands of degrees, and chemical mixing is accelerated by vortex formation and jetting in the pore. We also follow the evolution of the chemistry after the shock passage by allowing the sample to ``cook'' under the shock induced pressures and temperatures for up to 0.5 ns. Multiple ``tendril-like'' reaction fronts, born in the cauldron of the pores, propagate rapidly through the sample, consuming it within a nanosecond.   

The influence of mesostructural properties in concentric Ni-Al laminates on the accommodation of large plastic strain during high strain rate dynamic loading

Ni-Al laminates have been shown to be good candidates for use in reactive material systems due to their ability to release chemical energy via an intermetallic reaction initiated by thermal or mechanical stimuli. The Thick Walled Cylinder (TWC) technique allows for the testing and complete recovery of samples during a tunable high strain/strain rate plastic deformation process under plane strain conditions. Ni-Al laminates constructed from alternating concentric Ni (25.4 micron thickness) and Al (38.1 micron thickness) layers demonstrated that the cooperative buckling of layers was the dominant mode of plastic strain accommodation. Intermetallic reaction spots were observed in many of the apexes of these buckles. Alterations to mesoscale properties in these laminates using the TWC method help to understand the role of the mesostructural properties on the accommodation of plastic strain and possible development of intermetallic reaction. Finite element simulations show good agreement with the TWC experiments and provided insights into the evolution of the mesostructure during the collapse. These insights may be used to tailor the mesostructure to enhance the reactivity in the Ni-Al laminates.   

Atomistic Calculations of Nanosecond Timescale Kinetics of Shock-Compressed SiO2

Unraveling the behavior of SiO2 under high dynamic pressures is important for understanding meteor impacts, laser-induced damage of optics, and interpretation of shock compression experiments in a geophysical context. This behavior is made complicated by the presence of several high pressure phases and complex kinetic processes that yield long-lived metastable states. Quantitative understanding of the kinetics is nearly as important as the thermodynamics for this material. Here we make the first atomistic calculations of kinetic processes in SiO2 shocked to pressures near the stishovite and melt boundary. We perform atomistic calculations with up to 1 million atoms for timescales of 10 nanoseconds to elucidate the nucleation and growth and subsequent grain size evolution of the stishovite phase. These calculations are enabled by the multi-scale shock technique (MSST) implemented in LAMMPS. We further study the role of the quantum nature of the nuclei at 1 million atom scales using a fast semiclassical variation of MSST called QBMSST.   

A Gibbs Formulation for Reactive Condensed Phase Materials with Phase Change

A large class of applications have pure, condensed phase constituents that come into contact, chemically react and simultaneously undergo phase change. Phase change in a given molecular material has often been considered to be separate from chemical reaction. Continuum modelers of phase change often use a phase field model whereby an indicator function is allowed to change from one value to another in regions of phase change, governed by evolutionary (Ginzburg-Landau) equations. Whereas classic chemical kinetics literally count species concentrations and derive evolution equations based on species mass transport. We argue that the latter is fundamental and is the same as the former, if all species, phase or chemical are treated as distinct chemical species. A self consistent continuum-thermomechanical model, must account for all significant energetic quantities and have correct molecular and continuum limits in the mixture. The use of Gibbs potentials for all relevant species, chemical and phase does this neatly and allows the use of classical potentials, while allowing for modeling of phase interaction terms and reaction rate and phase change kinetics. The phase field quantities are the mass fractions. One has a single stress tensor, and a single temperature.   

Physical Properties of Supercritical Silica

Supercritical states of silica -- states lying above the vapor dome -- are produced on release from strong shocks applied to solid quartz, as may have occurred in a planetary impact that formed the moon. These states are also reached in shocks applied to highly porous aerogels. In the supercritical region, ion-ion correlations and thermal excitation of electrons are significant and non-trivial. Models used in wide-ranging equations of state such as Sesame generally have simplified physics in this region. We present results from extensive quantum molecular dynamics simulations for the equation of state and optical properties of supercritical silica. The simulations are found to reproduce the aerogel Hugoniot very well. We examine the nature of the ion correlations and electronic spectrum with an eye toward improving equation of state models in this difficult region.   

Developing a Reactive Potential for Hydrocarbons Under Extreme Deformation

In traditional molecular dynamics, interaction potentials define and maintain a fixed topology of chemical bonds. The development of reactive cluster potentials, such as the AIREBO and the REAXFF, have allowed modelers to explore dynamic bonding processes at the expense of more complex many-particle interactions. The complexity of these many-particle interactions makes parameterization of such models difficult; consequentially, current cluster potentials quickly lose fidelity outside a limited range of ambient pressures and temperatures, corresponding to near equilibrium states of the bonds described. In order to accurately describe the dynamics of extreme loading and failure, existing reactive cluster potentials must be modified to improve the description of highly strained bond configurations. Here we present a modified AIREBO potential for hydrocarbons with accuracy extended to highly strained intra (and inter) molecular configurations. Implementing recently developed techniques of bond-screening and a reparameterization of van der Waal's interactions based on second-order Moller-Plesset perturbation theory, we explore the equilibrium states of condensed phase hydrocarbons under high pressure and yield under extreme loads.