Session X24: Nicholas Metropolis Award: Materials in Extremes

First-principles Equations of State and Shock Hugoniots of First- and Second-Row Plasmas

A first-principles methodology for studying high energy density physics and warm dense matter is important for the stewardship of plasma science and guiding inertial confinement fusion experiments. In order to address this challenge, we have been developing the capability of path integral Monte Carlo (PIMC) for studying dense plasmas comprised of increasingly heavy elements, including nitrogen, oxygen (J. Chem. Phys., 164507 (2015)), and neon (Phys. Rev. B, 91, 045103 (2015)). In recent work, we have extended PIMC methodology beyond the free-particle node approximation by implementing localized nodal surfaces capable of describing bound plasma states in second-row elements, such as silicon (Phys. Rev. Lett. 115, 176403 (2015)). We combine results from PIMC with results from density functional theory molecular dynamics (DFT-MD) calculations to produce a coherent equation of state that bridges the entire WDM regime. Analysis of pair-correlation functions and the electronic density of states reveals an evolving plasma structure and ionization process that is driven by temperature and pressure. We also compute shock Hugoniot curves for a wide range of initial densities, which generally reveal an increase in compression as the second and first shells are ionized.   

X-Ray Thomson Scattering Without the Chihara Decomposition

X-Ray Thomson Scattering is an important experimental technique used in dynamic compression experiments to measure the properties of warm dense matter. The fundamental property probed in these experiments is the electronic dynamic structure factor that is typically modeled using an empirical three-term decomposition (Chihara, J. Phys. F, 1987). One of the crucial assumptions of this decomposition is that the system's electrons can be either classified as bound to ions or free. This decomposition may not be accurate for materials in the warm dense regime. We present unambiguous first principles calculations of the dynamic structure factor independent of the Chihara decomposition that can be used to benchmark these assumptions. Results are generated using a finite-temperature real-time time-dependent density functional theory applied for the first time in these conditions. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Security Administration under contract DE-AC04-94AL85000.   

Comminution of Ceramic Materials Under High-Shear Dynamic Compaction

The post-failure ``granular flow'' response of high-strength lightweight ceramics has important implications on the materials' effectiveness for ballistic protection. We study the dynamic compaction and shear flow of ceramic fragments and powders using computational and experimental analysis of a collapsing thick-walled cylinder geometry. Using newly developed tools for mesoscale simulation of brittle materials, we study the effect of fracture, comminution, shear-enhanced dilatation, and frictional contact on the continuum compaction response. Simulations are directly validated through particle Doppler velocimetry measurements at the inner surface of the cylindrical powder bed. We characterize the size distribution and morphologies of the initial and compacted material fragments to both validate the computational model and to elucidate the dominant failure processes.   

Real-time dynamics of high-velocity micro-particle impact~

High-velocity micro-particle impact is important for many areas of science and technology, from space exploration to the development of novel drug delivery platforms. We present real-time observations of supersonic micro-particle impacts using multi-frame imaging. In an all optical laser-induced projectile impact test, a monolayer of micro-particles is placed on a transparent substrate coated with a laser absorbing polymer layer. Ablation of a laser-irradiated polymer region accelerates the micro-particles into free space with speeds up to 1.0 km/s. The particles are monitored during the impact on the target with an ultrahigh-speed multi-frame camera that can record up to 16 images with time resolution as short as 3 ns. In particular, we investigated the high-velocity impact deformation response of poly(urethane urea) (PUU) elastomers to further the fundamental understanding of the molecular influence on dynamical behaviors of PUUs. We show the dynamic-stiffening response of the PUUs and demonstrate the significance of segmental dynamics in the response. We also present movies capturing individual particle impact and penetration in gels, and discuss the observed dynamics. The results will provide an impetus for modeling high-velocity microscale impact responses and high strain rate deformation in polymers, gels, and other materials.   

Sensitivity Characterization of Pressed Energetic Materials using Flyer Plate Mesoscale Simulations.

Heterogeneous energetic materials like pressed explosives have complicated microstructure and contain various forms of heterogeneities such as pores, micro-cracks, energetic crystals etc. It is widely accepted that the presence of these heterogeneities can affect the sensitivity of these materials under shock load. The interaction of shock load with the microstructural heterogeneities may leads to the formation of local heated regions known as ``hot spots''. Chemical reaction may trigger at the hot spot regions depending on the hot spot temperature and the duration over which the temperature can be maintained before phenomenon like heat conduction, rarefaction waves withdraws energy from it. There are different mechanisms which can lead to the formation of hot spots including void collapse. The current work is focused towards the sensitivity characterization of two HMX based pressed energetic materials using flyer plate mesoscale simulations. The aim of the current work is to develop mesoscale numerical framework which can perform simulations by replicating the laboratory based flyer plate experiments. The current numerical framework uses an image processing approach to represent the microstructural heterogeneities incorporated in a massively parallel Eulerian code SCIMITAR3D. The chemical decomposition of HMX is modeled using Henson-Smilowitz reaction mechanism. The sensitivity characterization is aimed towards obtaining James initiation threshold curve and comparing it with the experimental results.   

Blow Up Exponents and Deviations from Ideal Taylor Cone Shapes in Ultrathin Liquid Metal Films

We employ a finite element, moving mesh model to investigate the axisymmetric flow of an ultrathin liquid metal film overlay by a thin vacuum layer confined between two circular disks held at a constant potential difference close to field evaporation values. Within nanoseconds, a small Gaussian protrusion centered about the origin evolves into a sharpened cusp elongated by Maxwell stresses and rounded by capillary stresses. Previous analytic studies \footnote{N. M. Zubarev, \textbf{JETP Lett.} 73, 613 (2001)} and numerical simulations based on marker and cell techniques \footnote{V. G. Suvorov, \textbf{Surf. Interface Anal.} 36, 421 (2004)} \footnote{V. G. Suvorov and N. M. Zubarev, \textbf{J. Phys. D: Appl. Phys.} 37, 289 (2004)} have uncovered a self-similar regime in time where the opposing stresses and kinetic energy exhibit blow up behavior with a characteristic exponent of - 2/3, and cusp shapes that deviate from the ideal Taylor cone angle. Our simulations consistently yield exponents in the range -3/4 to -4/5, with values that depend sensitively on the choice of blowup time. We also find that deviations from the ideal Taylor cone angle become significant all along the film interface as the Gaussian amplitude increases beyond fractions of a micron.   

A Study of the Multiferroic State Under High Pressure for Co doped MnWO$_4$

Multiferroic materials are well understood to be sensitive to small perturbations induced through chemical substitution, magnetic and electric fields, or external pressure. These sensitivities can result in rich and complex phase diagrams to explore; one such system is Co doped MnWO$_4$. To gain further insight into this system, high pressure measurements were conducted up to 18 kbars. Results thus far suggest that, in a Co doping range near 13 %, pressure causes a flop of the ferroelectric polarization from the a-axis to the b-axis, also resulting in a sizable increase of the polarization value. This effect is explained by a change from the a-c spiral to the conical spin structure induced by external pressure.   

High Pressure Seebeck Coefficient Measurements Using Paris-Edinburgh Cell

We have developed a new type of sample cell assembly for the Paris-Edinburgh (PE) type large volume press for simultaneous x-ray diffraction, electrical resistance, and thermal measurements at high pressures. We demonstrate the feasibility of performing in situ measurements of the Seebeck coefficient over a broad range of pressure-temperature conditions by observing the well-known solid-solid and solid-melt transitions of bismuth (Bi) up to 3GPa and 450 K. We observed a gradual increase in the Seebeck coefficient which becomes positive during transition to the Bi - II phase. Also, we have performed successful Seebeck coefficient measurements on the thermoelectric material PbTe. This new capability enables us to directly correlate pressure-induced structural phase transitions to electrical and thermal properties.   

Mechanical Model for Dynamic Behavior of Concrete Under Impact Loading

Concrete is a geo-material which is used substantively in the civil building and military safeguard. One coupled model of damage and plasticity to describe the complex behavior of concrete subjected to impact loading is proposed in this research work. The concrete is assumed as homogeneous continuum with pre-existing micro-cracks and micro-voids. Damage to concrete is caused due to micro-crack nucleation, growth and coalescence, and defined as the probability of fracture at a given crack density. It induces a decrease of strength and stiffness of concrete. Compaction of concrete is physically a collapse of the material voids. It produces the plastic strain in the concrete and, at the same time, an increase of the bulk modulus. In terms of crack growth model, micro-cracks are activated, and begin to propagate gradually. When crack density reaches a critical value, concrete takes place the smashing destroy. The model parameters for mortar are determined using plate impact experiment with uni-axial strain state. Comparison with the test results shows that the proposed model can give consistent prediction of the impact behavior of concrete. The proposed model may be used to design and analysis of concrete structures under impact and shock loading.   

Jetting mechanisms of particles under shock wave acceleration: the role of force chains

The particle jetting phenomenon is widely observed in many problems associated with blast/shock dispersal of granular materials, although its origin is still unidentified. We carried out discrete element simulations of the shock dispersal of two-dimensional particle rings in order to extract the particle-scale evolution of the shocked rings in terms of the velocity profile and the force-chain networks. Initially the force chains distribute uniformly along the circumference, but after several dozens of microseconds, they disseminate into a handful of blobs which mainly consist of long linear or branched chains align with the radial direction. These blobs are separated by zones featuring relatively sparse force chains which take forms of short chains or small compact polygons. The radial-like force chains in blobs serves as the channels transferring the momentum from the inner layers to outer layers, resulting in fast moving blocks without appreciable velocity differences. By contrast, the shock energy in the zones with short force chains is largely dissipated among the particle collision. Thus particles in these zones lag behind those bound by strong force chains. The resultant heterogeneous velocity profile acts as the precursor of the ensuing particle jetting.