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High pressure phase transformation in iron under fast compression

Description: We present experimental results on the solid-solid, {alpha} to {epsilon} phase transformation kinetics of iron under high pressure dynamic compression. We observe kinetic features - velocity loops - similar with the ones recently reported to occur when water is frozen into its ice VII phase under comparable experimental conditions. We analyze this behavior in terms of general ideas coupling the steady sample compression with phase nucleation and growth with a pressure dependent phase interface velocity. The model is used to predict the response of iron when steadily driven across the {alpha} - {epsilon} phase boundary on very short time scales, including those envisioned to be achieved in ultra-fast laser experiments.
Date: July 7, 2009
Creator: Bastea, M; Bastea, S & Becker, R
Partner: UNT Libraries Government Documents Department

Aggregation kinetics in a model colloidal suspension

Description: The authors present molecular dynamics simulations of aggregation kinetics in a colloidal suspension modeled as a highly asymmetric binary mixture. Starting from a configuration with largely uncorrelated colloidal particles the system relaxes by coagulation-fragmentation dynamics to a structured state of low-dimensionality clusters with an exponential size distribution. The results show that short range repulsive interactions alone can give rise to so-called cluster phases. For the present model and probably other, more common colloids, the observed clusters appear to be equilibrium phase fluctuations induced by the entropic inter-colloidal attractions.
Date: August 8, 2005
Creator: Bastea, S
Partner: UNT Libraries Government Documents Department

Transport in a highly asymmetric binary fluid mixture

Description: We present molecular dynamics calculations of the thermal conductivity and viscosities of a model colloidal suspension with colloidal particles roughly one order of magnitude larger than the suspending liquid molecules. The results are compared with estimates based on the Enskog transport theory and effective medium theories for thermal and viscous transport. We also discuss the consequences of these results to some proposed mechanisms for thermal conduction in nanocolloidal suspensions.
Date: October 13, 2006
Creator: Bastea, S
Partner: UNT Libraries Government Documents Department

Conductivity maximum in a charged colloidal suspension

Description: Molecular dynamics simulations of a charged colloidal suspension in the salt-free regime show that the system exhibits an electrical conductivity maximum as a function of colloid charge. We attribute this behavior to two main competing effects: colloid effective charge saturation due to counterion 'condensation' and diffusion slowdown due to the relaxation effect. In agreement with previous observations, we also find that the effective transported charge is larger than the one determined by the Stern layer and suggest that it corresponds to the boundary fluid layer at the surface of the colloidal particles.
Date: January 27, 2009
Creator: Bastea, S
Partner: UNT Libraries Government Documents Department

Freezing Kinetics in Overcompressed Water

Description: We report high pressure dynamic compression experiments of liquid water along a quasi-adiabatic path leading to the formation of ice VII. We observe dynamic features resembling Van der Waals loops and find that liquid water is compacted to a metastable state close to the ice density before the onset of crystallization. By analyzing the characteristic kinetic time scale involved we estimate the nucleation barrier and conclude that liquid water has been compressed to a high pressure state close to its thermodynamic stability limit.
Date: September 27, 2006
Creator: Bastea, M; Bastea, S; Reaugh, J & Reisman, D
Partner: UNT Libraries Government Documents Department

Exp6-polar thermodynamics of dense supercritical water

Description: We introduce a simple polar fluid model for the thermodynamics of dense supercritical water based on a Buckingham (exp-6) core and point dipole representation of the water molecule. The proposed exp6-polar thermodynamics, based on ideas originally applied to dipolar hard spheres, performs very well when tested against molecular dynamics simulations. Comparisons of the model predictions with experimental data available for supercritical water yield excellent agreement for the shock Hugoniot, isotherms and sound speeds, and are also quite good for the self-diffusion constant and relative dielectric constant. We expect the present approach to be also useful for other small polar molecules and their mixtures.
Date: December 13, 2007
Creator: Bastea, S & Fried, L E
Partner: UNT Libraries Government Documents Department

Near-equilibrium polymorphic phase transformations in Praseodymium under dynamic compression

Description: We report the first experimental observation of sequential, multiple polymorphic phase transformations occurring in Praseodymium dynamically compressed using a ramp wave. The experiments also display the signatures of reverse transformations occuring upon pressure release and reveal the presence of small hysteresys loops. The results are in very good agreement with equilibrium hydrodynamic calculations performed using a thermodynamically consistent, multi-phase equation of state for Praseodymium, suggesting a near-equilibrium transformation behavior.
Date: February 12, 2007
Creator: Bastea, M & Reisman, D
Partner: UNT Libraries Government Documents Department

Major Effects in the Thermodynamics of Detonation Products: Phase Segregation versus Ionic Dissociation

Description: Water (H{sub 2}O) and nitrogen (N{sub 2}) are major detonation products of high explosives and it has long been conjectured that they may phase segregate at high enough temperatures and pressures to influence detonation properties of common explosives. We analyze the phase diagram of H{sub 2}O-N{sub 2} mixtures using a thermodynamic theory for polar-nonpolar mixtures and find that phase segregation is unlikely to occur above approximately 1600K. Therefore, H{sub 2}O-N{sub 2} immiscibility is not likely to be relevant for detonation predictions. We propose instead that the high pressure ionic dissociation of water plays an important role in detonation, and model it using a new ionic thermodynamics. We employ this model in chemical equilibrium calculations of standard high explosives, e.g. PETN, HMX and RDX, and find that it performs very well under a wide range of conditions. Thus, although it may require further development, it is likely that explicitly ionic thermodynamics will become a standard tool for explosives modeling.
Date: March 9, 2010
Creator: Bastea, S & Fried, L E
Partner: UNT Libraries Government Documents Department

Phase segregation via Vlasov-Boltzmann particle dynamics

Description: In order to better understand and model the phase segregation of binary fluids we opted for a mesoscopic description that proves to be simplifying both conceptually and computationally. The system that we studied is a mixture of two kinds of particles. All particles interact with each other through strong short-range interactions modeled by hard spheres with the same mass and diameter. There is also a smooth long-range repulsion between particles of different kinds. At low overall densities and weak enough repulsion the natural dynamical description for this system is given in terms of two coupled, energy and momentum conserving Vlasov- Boltzmann equations, making it what we call a dynamical mean-field model. The computational scheme that we used is a combination of direct sim- ulation Monte Carlo (DSMC) and particle-in-the-cell (PIC) evolution, that inherits the efficiency and robustness of these two algorithms. The DSMC is a stochastic algorithm due to Bird that consistently incorporates the as- sumptions behind the Boltzmann equation into the particle dynamics. The method is essentially the following: the physical space is divided into a net- work of cells containing typically tens of particles and the free flow of the particles over a small time interval {Delta}t is followed by representative collisions among pairs of particles sharing the same cell. The typical linear dimension of a cell is a fraction of the mean free path between collisions. The PIC method for integrating the equations of motion was first used to deal with the l/r potential in plasma physics. It takes advantage of the simple form of the Vlasov potential, which is a product in Fourier space, by calculating the densities on a grid through some weighting, then the potentials and forces on the same grid, and finally interpolating the forces at the position of each particle. These two ...
Date: January 19, 1999
Creator: Bastea, S.
Partner: UNT Libraries Government Documents Department

Phase separation in H2O:N2 mixture - molecular dynamics simulations using atomistic force fields

Description: A class II atomistic force field with Lennard-Jones 6-9 nonbond interactions is used to investigate equations of state (EOS) for important high explosive detonation products N{sub 2} and H{sub 2}O in the temperature range 700-2500 K and pressure range 0.1-10 GPa. A standard 6th order parameter-mixing scheme is then employed to study a 2:1 (molar) H{sub 2}O:N{sub 2} mixture, to investigate in particular the possibility of phase-separation under detonation conditions. The simulations demonstrate several important results, including: (1) the accuracy of computed EOS for both N{sub 2} and H{sub 2}O over the entire range of temperature and pressure considered; (2) accurate mixing-demixing phase boundary as compared to experimental data; and (3) the departure of mixing free energy from that predicted by ideal mixing law. The results provide comparison and guidance to state-of-the-art chemical kinetic models.
Date: September 25, 2006
Creator: Maiti, A; Gee, R; Bastea, S & Fried, L
Partner: UNT Libraries Government Documents Department

Equation of state for high explosives detonation products with explicit polar and ionic species

Description: We introduce a new thermodynamic theory for detonation products that includes polar and ionic species. The new formalism extends the domain of validity of the previously developed EXP6 equation of state library and opens the possibility of new applications. We illustrate the scope of the new approach on PETN detonation properties and water ionization models.
Date: June 28, 2006
Creator: Bastea, S; Glaesemann, K R & Fried, L E
Partner: UNT Libraries Government Documents Department

Observation of off-Hugoniot shocked states with ultrafast time resolution

Description: We apply ultrafast single shot interferometry to determine the pressure and density of argon shocked from up to 7.8 GPa static initial pressure in a diamond anvil cell. This method enables the observation of thermodynamic states distinct from those observed in either single shock or isothermal compression experiments, and the observation of ultrafast dynamics in shocked materials. We also present a straightforward method for interpreting ultrafast shock wave data which determines the index of refraction at the shock front, and the particle and shock velocities for shock waves in transparent materials. Based on these methods, we observe shocked thermodynamic states between the room temperature isotherm of argon and the shock adiabat of cryogenic argon at final shock pressures up to 28 GPa.
Date: February 23, 2010
Creator: Armstrong, M; Crowhurst, J; Bastea, S & Zaug, J
Partner: UNT Libraries Government Documents Department

Isentropic Compression with a Rectangular Configuration for Tungstene and Tantalum, Computations and Comparison with Experiments

Description: Isentropic compression experiments and numerical simulations on metals are performed at Z accelerator facility from Sandia National Laboratory and at Lawrence Livermore National Laboratory in order to study the isentrope, associated Hugoniot and phase changes of these metals [1]. 3D configurations have been calculated here to benchmark the new beta version of the electromagnetism package coupled with the dynamics in Ls-Dyna and compared with the ICE Z shots 1511 and 1555. The electromagnetism module is being developed in the general-purpose explicit and implicit finite element program LS-DYNA{reg_sign} in order to perform coupled mechanical/thermal/electromagnetism simulations. The Maxwell equations are solved using a Finite Element Method (FEM) for the solid conductors coupled with a Boundary Element Method (BEM) for the surrounding air (or vacuum). More details can be read in the reference [2], [3].
Date: February 13, 2006
Creator: Lefran├žois, A.; Reisman, D. B.; Bastea, M.; L'Eplattenier, P. & Burger, M.
Partner: UNT Libraries Government Documents Department

Reactive Flow Modeling of Liquid Explosives via ALE3D/Cheetah Simulations

Description: We carried out reactive flow simulations of liquid explosives such as nitromethane using the hydrodynamic code ALE3D coupled with equations of state and reaction kinetics modeled by the thermochemical code Cheetah. The simulation set-up was chosen to mimic cylinder experiments. For pure unconfined nitromethane we find that the failure diameter and detonation velocity dependence on charge diameter are in agreement with available experimental results. Such simulations are likely to be useful for determining detonability and failure behavior for a wide range of experimental conditions and explosive compounds.
Date: March 10, 2010
Creator: Kuo, I W; Bastea, S & Fried, L E
Partner: UNT Libraries Government Documents Department

Photoacoustically Measured Speeds of Sound and the Equation of State of HBO2: On Understanding Detonation with Boron Fuel

Description: Elucidation of geodynamic, geochemical, and shock induced processes is limited by challenges to accurately determine molecular fluid equations of state (EOS). High pressure liquid state reactions of carbon species underlie physiochemical mechanisms such as differentiation of planetary interiors, deep carbon sequestration, propellant deflagration, and shock chemistry. In this proceedings paper we introduce a versatile photoacoustic technique developed to measure accurate and precise speeds of sound (SoS) of high pressure molecular fluids and fluid mixtures. SoS of an intermediate boron oxide, HBO{sub 2} are measured up to 0.5 GPa along the 277 C isotherm. A polarized exponential-6 interatomic potential form, parameterized using our SoS data, enables EOS determinations and corresponding semi-empirical evaluations of >2000 C thermodynamic states including energy release from bororganic formulations. Our thermochemical model propitiously predicts boronated hydrocarbon shock Hugoniot results.
Date: March 9, 2010
Creator: Zaug, J M; Bastea, S; Crowhurst, J; Armstrong, M; Fried, L & Teslich, N
Partner: UNT Libraries Government Documents Department

Photoacoustically Measured Speeds of Sound of Liquid HBO2: On Unlocking the Fuel Potential of Boron

Description: Elucidation of geodynamic, geochemical, and shock induced processes is often limited by challenges to accurately determine molecular fluid equations of state (EOS). High pressure liquid state reactions of carbon species underlie physiochemical mechanisms such as differentiation of planetary interiors, deep carbon sequestration, propellant deflagration, and shock chemistry. Here we introduce a versatile photoacoustic technique developed to measure accurate and precise speeds of sound (SoS) of high pressure molecular fluids and fluid mixtures. SoS of an intermediate boron oxide, HBO{sub 2} are measured up to 0.5 GPa along the 277 C isotherm. A polarized Exponential-6 interatomic potential form, parameterized using our SoS data, enables EOS determinations and corresponding semi-empirical evaluations of > 2000 C thermodynamic states including energy release from bororganic formulations. Our thermochemical model propitiously predicts boronated hydrocarbon shock Hugoniot results.
Date: March 24, 2010
Creator: Bastea, S.; Crowhurst, J.; Armstrong, M. & Teslich, Nick, Jr.
Partner: UNT Libraries Government Documents Department

Equations of state of nonspherical fluids by spherical intermolecular potentials

Description: The equilibrium properties of anisotropic molecular fluids can be in principle calculated in a statistical mechanics framework, but the theory is generally too cumbersome for many practical applications. Fortunately, at high densities and temperatures the anisotropy can be averaged-out by means of a density and temperature independent potential (the median) that produces reliable thermodynamics [1,2]. The proposal of Shaw and Johnson [1], which turns out to be the so-called median potential [2], is very successful in predicting the thermodynamics of simple fluids such as N{sub 2} and CO{sub 2} at reasonable high pressures and temperatures [3]. Lebowitz and Percus [2] pointed out some time ago that the success of this approximation could perhaps be understood in terms of a simple theory that treats the asphericity as a perturbation. The median appears to be the best choice for hard nonspherical potential [4], which may explain its success for fluids at high densities, where the hard core contribution is known to be dominant.
Date: August 16, 1999
Creator: Bastea, S & Ree, F H
Partner: UNT Libraries Government Documents Department

Thermodynamics of anisotropic fluids using isotropic potentials

Description: We study the effectiveness and limitations of the median potential recipe for mixtures such as N{sub 2} + O{sub 2} and N{sub 2} + CO{sub 2}, that are important in detonation applications. Conversely, we treat effective spherical potentials extracted from Hugoniot experiments (e.g., N{sub 2} and O{sub 2}) as median potentials and invert them to extract atom-atom potentials. The resulting non-spherical potentials compare remarkably well with the atom - atom potentials used in studies of solid state properties. Finally, we propose a method to improve the median potential for stronger anisotropic fluids such as CO{sub 2} and its mixtures.
Date: August 16, 1999
Creator: Bastea, S & Ree, F H
Partner: UNT Libraries Government Documents Department

Shock compression of precompressed deuterium

Description: Here we report quasi-isentropic dynamic compression and thermodynamic characterization of solid, precompressed deuterium over an ultrafast time scale (< 100 ps) and a microscopic length scale (< 1 {micro}m). We further report a fast transition in shock wave compressed solid deuterium that is consistent with the ramp to shock transition, with a time scale of less than 10 ps. These results suggest that high-density dynamic compression of hydrogen may be possible on microscopic length scales.
Date: July 31, 2011
Creator: Armstrong, M R; Crowhurst, J C; Zaug, J M; Bastea, S; Goncharov, A F & Militzer, B
Partner: UNT Libraries Government Documents Department

Solid-Density Plasma characterization with X-ray scattering on the 200-J Janus Laser

Description: We present collective x-ray scattering (CXS) measurements using a Chlorine He-{alpha} x-ray source pumped with less than 200 J of laser energy. The experimental scattering spectra show plasmon resonances from shocked samples. These experiments use only 10{sup 12} x-ray photons at the sample of which 10{sup -5} have been scattered and detected with a highly efficient curved crystal spectrometer. Our results demonstrate that x-ray scattering is a viable technique on smaller laser facilities making CXS measurements accessible to a broad scientific community.
Date: April 25, 2006
Creator: Neumayer, P B; Gregori, G; Ravasio, A; Price, D; Bastea, M; Landen, O L et al.
Partner: UNT Libraries Government Documents Department

Electrical conductivity of fluid oxygen at high pressures

Description: Electrical conductivities of fluid oxygen were measured between 30 and 80 GPa at a few 1000 K. These conditions were achieved with a reverberating shock wave technique. The measured conductivities were several orders of magnitude lower than measured previously on the single shock Hugoniot because of lower temperatures achieved under shock reverberation. Extrapolation of these data suggests that the minimum metallic conductivity of a metal will be reached near 100 GPa.
Date: August 20, 1999
Creator: Bastea, M; Mitchell, A C & Nellis, W J
Partner: UNT Libraries Government Documents Department