Session A39: Focus Session: Materials in Extremes: Theory and Simulations

Aluminum/water reactions under extreme conditions

We discuss mechanisms that may control the reaction of aluminum and water under extreme conditions. We are particularly interested in the high-temperature, high-strain regime where the native oxide layer is destroyed and fresh aluminum is initially in direct contact with liquid or supercritical water. Disparate experimental data over the years have suggested rapid oxidation of aluminum is possible in such situations, but no coherent picture has emerged as to the basic oxidation mechanism or the physical processes that govern the extent of reaction. We present theoretical and computational analysis of traditional metal/water reaction mechanisms that treat diffusion through a dynamic oxide layer or reaction limited by surface kinetics. Diffusion through a fresh solid oxide layer is shown to be far too slow to have any effect on the millisecond timescale (even at high temperatures). Quantum molecular dynamics simulations of liquid Al and water surface reactions show rapid water decomposition at the interface, catalyzed by adjacent water molecules in a Grotthus-like relay mechanism. The surface reaction barriers are far too low for this to be rate-limiting in any way. With these straightforward mechanisms ruled out, we investigate two more complex possibilities for the rate-limiting factor; first, we explore the possibility that newly formed oxide remains a metastable liquid well below its freezing point, allowing for diffusion-limited reactions through the oxide shell but on a much faster timescale. The extent of reaction would then be controlled by the solidification kinetics of alumina. Second, we discuss preliminary analysis on surface erosion and turbulent mixing, which may play a prominent role during hypervelocity penetration of solid aluminum projectiles into water.   

Development of a Reactive Force Field for Shock-Induced Chemistry in Ti/B Nanocomposite

A ReaxFF reactive force field is under development for describing the physics and chemistry of Ti/B mixtures under shock compression. In this presentation, we will summarize the parameterization of the force field for the reactants and the most stable product of the reaction, TiB$_{2}$ in the P6/mmm space group. We will describe the behavior of crystalline TiB$_{2}$ under uniaxial and hydrostatic compression and the structure of the crystal with varying void densities as calculated with periodic DFT. In addition, we will compare the results obtained for these properties and others ($e.g.$, lattice constants, elastic constants, bulk modulus) with the newly developed ReaxFF force field. The force field developed in this work for TiB$_{2}$ is combined with Ti and B ReaxFF force fields developed previously to yield a force field suitable for describing shock-induced reactions of Ti and B. Preliminary molecular dynamics studies will also be detailed.   

Rotational defects and plastic deformation in molecular crystal RDX

Defects in molecular crystals differ in many aspects from their atomic counterparts. Molecules in the crystal lattice can undergo conformational changes or twist and rotate into various configurations during deformation. These processes play an important role in the mechanics at a larger scale by controlling critical parameters like dislocation mobility. We present a computational study of such processes in cyclo-trimethylene-trinitramine (RDX), an energetic molecular crystal. Conformational changes, rotational defects and their role in the deformation mechanics of RDX is investigated using molecular dynamics simulations. Structure and mobility of dislocations are also presented and role of conformational and rotational defects in dislocation mobility is discussed.   

Electronic stopping power from ab-initio Ehrenfest molecular dynamics

Many materials are exposed to particle radiation: Metal walls of nuclear reactors in fission systems are subject to ion bombardment. Solar cells and semiconductor components in satellites are damaged by ions from cosmic rays. In order to achieve high radiation tolerance, it is essential to comprehend the interaction of fast projectiles with the ionic and electronic system of the target at a fundamental level. Based on the real-time propagation of time-dependent Kohn-Sham equations we developed a highly parallel plane-wave implementation of non-adiabatic Ehrenfest molecular dynamics, overcoming the adiabatic Born-Oppenheimer approximation. Thanks to the excellent scalability of our explicit integration scheme on supercomputers, it allows for the parameter-free computation of electronic stopping with hundreds of atoms in the calculation. We summarize our approach with some attention to important computational details. The influence of different charge states of H, He, and Li projectiles penetrating an Al target will be outlined. While we find good agreement with experiment up to the maximum of electronic stopping, deviations for high velocities are discussed in the light of the theoretical framework and off-channeling effects. Prepared by LLNL under Contract DE-AC52-07NA27344.   

Simulations of nonequilibrium warm dense gold produced by ultrafast heating

The interaction of femtosecond laser pulses with metals produces nonequilibrium states consisting of hot electrons and cold ions. These can last for many picoseconds before relaxing to a thermodynamic equilibrium. Recent experiments using a chirped pulse probe technique provided AC conductivity data of gold at a sufficient time resolution to observe this relaxation process. We developed an ab-initio model that characterizes thermodynamic properties of warm dense matter states in nonequilibrium. Our theoretical scheme combines a standard two temperature model with temperature dependent material parameters and an energy transfer rate that are obtained by means of ab-initio simulations. This enables us to give a prediction for the temperature evolution during the relaxation process. Additionally, we derive the AC conductivity of the nonequilibrium states from our simulations using the Kubo-Greenwood formula. It is used to test our model against measurements. We observe agreement with experiment using an energy relaxation rate, that is smaller than predicted, giving us reason to revisit its determination. We can furthermore provide thermodynamical and structural data of nonequilibrium warm dense gold which are not accessible in experiment.   

Finite-temperature orbital-free GGA molecular dynamics for warm dense hydrogen

The computational description of warm dense matter (WDM) by means of a combination of the Kohn-Sham (KS) finite-temperature density functional theory (DFT) for the electrons and classical molecular dynamics (MD) for the ions becomes an intractable task at high $T$ (typically a few hundred kK). Finite-temperature orbital free DFT (OF-DFT) is a less expensive alternative. Only two non-interacting free-energy functionals for OF-DFT had been published and used until recently: the finite-temperature Thomas-Fermi (ftTF) model (Feynman {\it et al.}, 1949) and ftTF with second-order gradient corrections (ftSGA) (Perrot, 1979). Here we report first results of OF-DFT MD simulations for warm dense H with a pair of newly developed ftGGA free energy functionals [1] for the non-interacting kinetic energy and entropy. The equation of state from these new functionals shows much better agreement with the reference KS MD results than results from the ftTF and ftSGA models. Other issues, {\it e.g.} convergence of the OF self-consistent procedure, also will be discussed.\\[4pt] [1]. V.V.\ Karasiev, T.\ Sjostrom and S.B.\ Trickey, Phys.\ Rev.\ B {\bf 86}, 115101 (2012).   

Temperature Dependence of the Kinetic Energy of the Correlated Electron Plasma by Restricted Path-Integral Molecular Dynamics

Recent progress in orbital-free Density Functional Theory (OF-DFT), particularly with regard to temperature dependent functionals, has promise for the simulation of warm dense matter (WDM) systems. WDM includes systems with densities of an order of magnitude beyond ambient or more and temperatures measured in kilokelvin. A challenge for the development of temperature dependent OF-DFT functionals is the lack of benchmark information with temperature and pressure dependence on simple models under WDM conditions. We present an approach to fill this critical gap using the restricted path-integral molecular dynamics (rPIMD) method. Electrons are described as harmonic necklaces within the discrete path integral representation while quantum exchange takes the form of cross linking between electron necklaces. A molecular dynamics algorithm is used to sample phase space and the fermion sign problem is addressed by restricting the density matrix to positive values. The temperature dependence of kinetic energies for the strongly coupled electron plasma is presented for a number of Wigner-Seitz radii in terms of a fourth order Sommerfeld expansion.   

Path Integral Simulations of Heavy, Warm Dense Matter

We develop an all-electron path integral Monte Carlo (PIMC) method for warm dense matter composed of elements with core electrons. For several second- and third-row elements, PIMC pressures, internal energies, and pair-correlation functions compare well with density functional theory molecular dynamics (DFT-MD) at low temperatures and enable the construction of coherent equations of state over a wide range of temperatures and densities. Details of the method and results will be discussed.   

Atomistic and first principles studies of Si nanoparticles under pressure

In this talk, we will discuss the structural, optical and electronic properties of silicon nanoparticles under high pressure, obtained using a combination of classical molecular dynamics and first principles density functional theory calculations. The results will be corroborated with experimental findings.   

Anharmonic Phonons in Complex Systems: Application to MgSiO3-Perovskite

We propose a strategy to capture phonon frequency renormalization due to phonon-phonon interactions included in molecular dynamics simulations (self-consistent phonons). This strategy is effective irrespective of crystal structure complexity and facilitates the Fourier interpolation of anharmonic frequencies throughout the Brillouin zone. Calculation of anharmonic frequency shifts in MgSiO$_{3}$-perovskite validates the method by reproducing well irregular thermal shifts measured by Raman spectroscopy at ambient conditions. \textit{Research supported by NSF/EAR}   

Phase stability in pulsar and magnetar crusts

The outermost several hundred meters of a neutron star crust is similar to a white dwarf interior, consisting of nuclei screened by a relativistic, degenerate electron gas. Free neutrons don't appear until a density of $4\times10^{11}$ g/cc. Below a depth of several tens of meters, corresponding to $10^6$-$10^8$ g/cc, the nuclei are thought to crystallize. Unlike white dwarfs, most observed neutron stars have enormous magnetic fields. On the surface of a typical pulsar, the field is $\sim 10^{12}$ gauss, while for magnetars it is several orders of magnitude stronger. Sub-surface fields are likely to be of a similar or greater strength. Quantum ab-initio methods for this regime are still in a state of infancy. In this talk we describe a solution of the nonlinear Thomas-Fermi PDE for completely degenerate, super-strongly magnetized electrons, using a domain decomposition technique with boundary conditions appropriate to close-packed lattices of nuclei. Excited Landau levels are included in the model. Our numerical method makes use of Hypre multigrid-preconditioned solvers. Equation of state and phase diagram calculations will be presented, and implications for astrophysical observations discussed.   

Magnetic Evolution of the \textless 100\textgreater\ Interstitial Loop Formation Process in bcc Iron

Interstitial loops are a signature of radiation damage in materials and are only observed in systems far from equilibrium state due to their high formation energies (approximately 4eV). Unlike other bcc metals, in which the interstitial loops are almost exclusively $\frac{1}{2}$ \textless 111\textgreater\ type, two types of loops, \textless 100\textgreater and $\frac{1}{2}$ \textless 111\textgreater\, are identified in bcc iron. Although $\frac{1}{2}$ \textless 111\textgreater\ loops can be formed directly by atomic displacment cascades, the mechanism of \textless 100\textgreater\ loop formation had remained undetermined since they were observed fifty years ago. Recently, the formation mechanism has been discovered using self-evolving atomistic kinetic Monte Carlo (SEAKMC) simulations. Here we describe the influence of magnetism in the corresponding loop formation process using the \textit{ab initio} locally self-consistent multiple-scattering (LSMS) method. Significant magnetic moment changes during the loop formation process are observed and their effect on the loop stability are evaluated. In addition, the effects of \textless 100\textgreater\ loop formation on the microstructural evolution and material properties will be discussed.   

Using numeric simulations to inform experimental data analysis: A new method to account for characteristic bending under dynamic loading and release

Dynamic high pressure experiments are often subject to unwanted wave interactions such as reflections from window interfaces or free surfaces where there is an impedance mismatch. In ramp loading experiments or under shock loading of materials resulting in a complex wave structure, these wave interactions can result in changes to the observed wave speeds. This effect, known as characteristic bending, can lead to significant errors in the measured material properties if not properly accounted for. Several approaches exist to correct for characteristic bending, however, they are limited to a one-to-one material response. New methodology has been developed based on control system theory to correct for characteristic bending without this limitation. By comparing simulated \textit{in-situ} and window (or free surface) data, a transfer function is defined which captures the effects due to wave interactions. Application of this function to the experimental data results in \textit{in-situ} profiles free from perturbations due to wave interactions. Experimental data, both with and without strong characteristic bending present, will be presented to illustrate the utility of this new approach.