Publications

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The Sandia lightning simulator at Sandia National Laboratories can provide up to 200 kA for a simulated single lightning stroke, 100 kA for a subsequent stroke, and hundreds of Amperes of continuing current. It has recently been recommissioned after a decade of inactivity and the single-stroke capability demonstrated. The simulator capabilities, basic design components, upgrades, and diagnostic capabilities are discussed in this paper.

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Traditional reactive flow modeling provides a computational representation of shock initiation of energetic materials. Most reactive flow models require ad hoc assumptions to obtain robust simulations, assumptions that result from partitioning energy and volume change between constituents in a reactive mixture. For example, most models assume pressure and/or temperature equilibrium for the mixture. Many mechanical insults to energetic materials violate these approximations. Careful analysis is required to ensure that the model assumptions and limitations are not exceeded. One limitation is that the shock to detonation transition is replicated only for strong planar shocks. Many models require different parameters to match data from thin pulse, ramp wave, or multidimensional loading, an approach that fails for complex loading. To accurately simulate reaction under non-planar shock impact scenarios a new formalism is required. The continuum mixture theory developed by Baer and Nunziato is used to eliminate ad hoc assumptions and limitations of current reactive flow models. This modeling paradigm represents the multiphase nature of reacting condensed/gas mixtures. Comparisons between simulations and data are presented.

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Recent advances in magnetic loading techniques have permitted quasi-isentropes to be measured to unprecedented levels. However, the relevant equations for planar waves provide no information about transverse stresses, leaving the deviatoric (strength) behavior of an isentropically loaded material unknown. Because materials are much cooler under isentropic loading than under shock loading, they can remain solid and thus retain strength to very high pressures. Thus, to improve our ability to model material behavior under isentropic loading, techniques to measure strength are needed. In this paper, existing techniques for determining high-pressure strength will be discussed along with their limitations. A technique for assessing the strength of isentropically loaded materials will be presented and used to determine the strength of an aluminum alloy using data from the Z machine and gas gun experiments. These results will be compared to existing models for material strength. Finally, limitations of the technique and future work needed will be discussed.

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In gas chromatography, a chemical sample separates into its constituent components as it travels along a long thin column. As the component chemicals exit the column they are detected and identified, allowing the chemical makeup of the sample to be determined. For correct identification of the component chemicals, the distribution of the concentration of each chemical along the length of the column must be nearly symmetric. The prediction and control of asymmetries in gas chromatography has been an active research area since the advent of the technique. In this paper, we develop from first principles a general model for isothermal linear chromatography. We use this model to develop closed-form expressions for terms related to the first, second, and third moments of the distribution of the concentration, which determines the velocity, diffusion rate, and asymmetry of the distribution. We show that for all practical experimental situations, only fronting peaks are predicted by this model, suggesting that a nonlinear chromatography model is required to predict tailing peaks. For situations where asymmetries arise, we analyze the rate at which the concentration distribution returns to a normal distribution. Numerical examples are also provided.

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The propagation of shock waves through target materials is strongly influenced by the presence of small-scale structure, fractures, physical and chemical heterogeneities. Reverberations behind the shock from the presence of physical heterogeneity have been proposed as a mechanism for transient weakening of target materials as are localized shock effects seen in some meteorites. Pre-existing fractures can also affect melt generation. Recent mesoscale studies in computational hydrodynamics have attempted to bridge the gap in numerical modeling between the microscale and the continuum,. Methods are being devised using shock physics hydrocodes such as CTH and Monte-Carlo-type methods to investigate the shock properties of heterogeneous materials and to compare the results with experiments. Recent numerical experiments at the mesoscale using these statistical methods suggest that heterogeneity at the micro-scale plays a substantial and statistically quantifiable role in the effective shear and fracture strength of rocks. This paper will describe the methodology we are using to determine the strength of heterogeneous geologic and planetary materials.

Wild strains of Bacillus subtilis are capable of forming architecturally complex communities of cells known as biofilms. Critical to biofilm formation is the eps operon, which is believed to be responsible for the biosynthesis of an exopolysaccharide that binds chains of cells together in bundles. We report that transcription of eps is under the negative regulation of SinR, a repressor that was found to bind to multiple sites in the regulatory region of the operon. Mutations in sinR bypassed the requirement in biofilm formation of two genes of unknown function, ylbF and ymcA, and sinI, which is known to encode an antagonist of SinR. We propose that these genes are members of a pathway that is responsible for counteracting SinR-mediated repression. We further propose that SinR is a master regulator that governs the transition between a planktonic state in which the bacteria swim as single cells in liquid or swarm in small groups over surfaces, and a sessile state in which the bacteria adhere to each other to form bundled chains and assemble into multicellular communities.

Detailed studies of the properties of ceramic CaCu{sub 3}Ti{sub 4}O{sub 12} (CCTO) have clarified the physics of this interesting material and revealed several features not reported before. The dielectric relaxational properties of CCTO are explained in terms of a capacitive-layer model, as for an inhomogeneous semiconductor, consisting of semiconducting grains and insulating grain boundaries as also concluded by others. The kinetics of the main [low-temperature (T)] relaxation reveal that two different thermally activated processes in CCTO grains control the dynamics. A likely candidate defect responsible for the two processes is the oxygen vacancy which is a double donor. A higher-T relaxation is determined by grain boundary conduction. Both Nb and Fe doping lowered both the apparent dielectric constant {var_epsilon}{prime} and the dielectric loss, but increased Fe doping led to more dramatic effects. At 3 at.% Fe doping, the anomalous {var_epsilon}{prime}(T) response was removed, making the CCTO an intrinsic, very-low-loss dielectric. The intrinsic {var_epsilon}{prime}({approx}75) and its T dependence are measured and shown to be largely determined by a low-lying soft TO phonon. At low T, cubic CCTO transforms into an antiferromagnetic phase at T{sub N} = 25 K. T{sub N} is essentially independent of Nb doping (up to 4 at.%) and of hydrostatic pressure (up to {approx}7 kbar), but decreases significantly with Fe doping. Analysis of the high-T dependence of the magnetic susceptibility provided insight into the role of Fe as a dopant. Finally, an {var_epsilon}{prime}(T) anomaly associated with the onset of antiferromagnetic order has been discovered, providing evidence for coupling between the polarization and sublattice magnetization. The possible origin of this coupling is discussed.

The influences of hydrostatic pressure and biasing electric field on the dielectric properties and phase behavior of a single crystal of the perovskite compound Pb(Sc{sub 0.5}Nb{sub 0.5})O{sub 3}, (PSN) have been investigated. On cooling from high temperatures, the crystal first enters a relaxor (R) state and then spontaneously transforms to a ferroelectric (FE) phase at a temperature, T{sub c}, substantially below the peak temperature, T{sub m}, in the dielectric susceptibility. Based on earlier work on ceramic samples, this behavior suggests substantial chemical (Sc and Nb) disorder at the B sites. Pressure enhances the R state with strong indications that the FE phase should vanish at a pressure somewhat higher than the highest pressure reached in the experiments, making the R state the ground state of the crystal at reduced volume. A significant feature of the temperature (T)-pressure (P) phase diagram is the finding that the T{sub c}(P) phase line should terminate at a pressure between 10 and 15 kbar in a manner akin to a critical point; however, in the case of PSN this feature represents a FE-to-R crossover. Such behavior suggests that a path can be defined that takes the crystal from the FE phase to the R state without crossing a phase boundary. A biasing electric field favors the FE phase over the R state, and the results indicate that the R state vanishes at 5 kV/cm. The magnitudes of both the high T Curie-Weiss constant, C, and the change in entropy (or latent heat) at T{sub c} are found to be comparable to those of simple displacive perovskite oxides such as BaTiO{sub 3} and PbTiO{sub 3}.