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Heterogenous materials under shock compression can be expected to reach different shock states throughout the material according to local differences in microstructure and the history of wave propagation. Here, a compact, multiple-beam focusing optic assembly is used with high-speed velocimetry to interrogate the shock response of porous tantalum films prepared through thermal-spray deposition. The distribution of particle velocities across a shocked interface is compared to results obtained using a set of defocused interferometric beams that sampled the shock response over larger areas. The two methods produced velocity distributions along the shock plateau with the same mean, while a larger variance was measured with narrower beams. The finding was replicated using three-dimensional, mesoscopically resolved hydrodynamics simulations of solid tantalum with a pore structure mimicking statistical attributes of the material and accounting for radial divergence of the beams, with agreement across several impact velocities. Accounting for pore morphology in the simulations was found to be necessary for replicating the rise time of the shock plateau. The validated simulations were then used to show that while the average velocity along the shock plateau could be determined accurately with only a few interferometric beams, accurately determining the width of the velocity distribution, which here was approximately Gaussian, required a beam dimension much smaller than the spatial correlation lengthscale of the velocity field, here by a factor of ∼30×, with implications for the study of other porous materials. [ABSTRACT FROM AUTHOR]

The equation of state (EOS) of bulk niobium (Nb) was investigated within the framework of density functional theory, with Mermin's generalization to finite temperatures. The shock Hugoniot for fully-dense and porous Nb was obtained from canonical ab initio molecular dynamics simulations with Erpenbeck's approach based on the Rankine-Hugoniot jump conditions. The phase space was sampled along isotherms between 300 and 4000 K, for densities ranging from ρ = 5.5 to 12 g/cm 3. Results from simulations compare favorably with room-temperature multianvil and diamond anvil cell data for fully-dense Nb samples and with a recent tabulated SESAME EOS. The results of this study indicate that, for the application of weak and intermediate shocks, the tabular EOS models are expected to give reliable predictions. [ABSTRACT FROM AUTHOR]

We have developed the Sample Test Array and Recovery (STAR) platform for the National Ignition Facility (NIF) for studying the thermal and hydrodynamic responses of materials in extreme environments. The STAR platform expands the range of obtainable fluences and quadruples the rate that materials experiments can be conducted at the NIF. Example configurations are demonstrated for fluences spanning 0.56–34 J/cm2 with environmental isolation for post-shot material recovery and inspection and up to 1740 J/cm2 without isolation, with surface heating rates of up to 2 × 1014 K/s. An example experiment involving thermally driven shock and spallation of aluminum alloy 7075 is briefly discussed. [ABSTRACT FROM AUTHOR]

We examine the optical activity, birefringence, and transparency of Lanthanum-doped, lead zirconate titanate (PLZT 7/65/35) bulk ceramic wafer sections over visible and near-IR spectra and on heating. Optical transitions are compared to both crystallographic (rhombohedral-cubic) and domain (relaxor-ferroelectric) transitions identified with x-ray diffraction, dielectric, and calorimetry measurements. Optical activity and birefringence are shown to be enhanced for disordered domains near room temperature, to attenuate above the relaxor-ferroelectric transition and to gradually decay above the Curie point regardless of the initial poling state. The results are interpreted in light of the change of crystallographic symmetry due to the local strains induced by ferroelectric architecture. The heterogeneous local strains more strongly influence the optical properties than the macro-scale structure of the polycrystalline PLZT ceramic. This mechanism is significant for understanding optical rotation and birefringence in polycrystalline systems. Finally, the specific rotation (up to 350°/mm) lies among the highest reported for crystalline materials. Along with strong poling contrast and comparatively small dispersion for the unpoled state, these properties are promising for electro-optics applications. [ABSTRACT FROM AUTHOR]