Kimberly A. Obrey

Strong, Low-Density Nanocomposites by Chemical Vapor Deposition and Polymerization of Cyanoacrylates on Aminated Silica Aerogels

Strong polymer-silica aerogel composites were prepared by chemical vapor deposition of cyanoacrylate monomers onto amine-modified aerogels. Amine-modified silica aerogels were prepared by copolymerizing small amounts of (aminopropyl)triethoxysilane with tetraethoxysilane. After silation of the aminated gels with hexamethyldisilazane, they were dried as aerogels using supercritical carbon dioxide processing. The resulting aerogels had only the amine groups as initiators for the cyanoacrylate polymerizations, resulting in cyanoacrylate macromolecules that were higher in molecular weight than those observed with unmodified silica and that were covalently attached to the silica surface. Starting with aminated silica aerogels that were 0.075 g/cm(3) density, composite aerogels were made with densities up to 0.220 g/cm(3) and up to 31 times stronger (flexural strength) than the precursor aerogel and about 2.3 times stronger than an unmodified silica aerogel of the same density.

Developing Monolithic Nanoporous Gold with Hierarchical Bicontinuity Using Colloidal Bijels.

We report a universal platform for the synthesis of monolithic porous gold materials with hierarchical bicontinuous morphology and combined macro- and mesoporosity using a synergistic combination of nanocasting and chemical dealloying. This robust and accessible approach offers a new design paradigm for the parallel optimization of active surface area and mass transport in porous metal electrodes.

Fluorescent Single-Walled Carbon Nanotube Aerogels in Surfactant-free Environments

A general challenge in generating functional materials from nanoscale components is integrating them into useful composites that retain or enhance their properties of interest. Development of single walled carbon nanotube (SWNT) materials for optoelectronics and sensing has been especially challenging in that SWNT optical and electronic properties are highly sensitive to environmental interactions, which can be particularly severe in composite matrices. Percolation of SWNTs into aqueous silica gels shows promise as an important route for exploiting their properties, but retention of the aqueous and surfactant environment still impacts and limits optical response, while also limiting the range of conditions in which these materials may be applied. Here, we present for the first time an innovative approach to obtain highly fluorescent solution-free SWNT-silica aerogels, which provides access to novel photophysical properties. Strongly blue-shifted spectral features, revelation of new diameter-dependent gas-phase adsorption phenomena, and significant increase (approximately three times that at room temperature) in photoluminescence intensities at cryogenic temperatures all indicate greatly reduced SWNT-matrix interactions consistent with the SWNTs experiencing a surfactant-free environment. The results demonstrate that this solid-state nanomaterial will play an important role in further revealing the true intrinsic SWNT chemical and photophysical behaviors and represent for the first time a promising new solution- and surfactant-free material for advancing SWNT applications in sensing, photonics, and optoelectronics.

Development of a polar direct-drive platform for studying inertial confinement fusion implosion mix on the National Ignition Facilitya)

Experiments were performed to develop a platform for the simultaneous measurement of mix and its effects on fusion burn. Two polar direct drive implosions of all-plastic capsules were conducted for the first time on the National Ignition Facility (NIF). To measure implosion trajectory and symmetry, area image backlighting of these capsules was also employed for the first time on NIF, an advance over previous 1-D slit imaging experiments, providing detailed symmetry data of the capsules as they imploded. The implosion trajectory and low-mode asymmetry seen in the resultant radiographs agreed with pre-shot predictions even though the 700 kJ drive energy produced laser beam intensities exceeding laser-plasma instability thresholds. Post-shot simulations indicate that the capsule yield was reduced by a factor of two compared to pre-shot predictions owing to as-shot laser drive asymmetries. The pre-shot predictions of bang time agreed within 200 ps with the experimental results. The second shot incorporated a narrow groove encircling the equator of the capsule. A predicted yield reduction factor of three was not observed.

Asymmetric directly driven capsule implosions: Modeling and experiments—A requirement for the National Ignition Facility

Direct-drive experiments at the University of Rochesters OMEGA laser [T. R. Boehly, R. L. McCrory, C. P. Verdon et al., Fusion Eng. Des. 44, 35 (1999)] have been performed to prototype eventual campaigns on the National Ignition Facility (NIF) [E. I. Moses and C. R. Wuest, Fusion Sci. Technol. 43, 420 (2003)] to investigate the mixing of target materials. Spherical-implosion targets with equatorial defects have been irradiated with polar direct drive, a requirement for direct-drive experiments at NIF. The physics question addressed by these results is whether simulations can match data on 0th-order hydrodynamics and implosion symmetry, the most basic implosion features, with and without the defect. The successful testing of hydrodynamic simulations leads to better designs for experiments and guides accurate planning for polar-direct-drive-ignition studies on the NIF platform.

Manufacturing complex silica aerogel target components

Abstract Aerogel is a material used in numerous components for inertial confinement fusion and high-energy density physics targets. In the past, these components were molded into the proper shapes. Artifacts left in the parts from the molding process, contour irregularities from shrinkage, and density gradients caused by the skin have caused Los Alamos National Laboratory to pursue machining as a way to make the components. The machining of aerogel is an involved process, and many manufacturing aspects need to be considered including holding the material for machining, achieving the desired surface roughness and the desired dimensional accuracy, conceivably producing a part with enhanced dimensional tolerance and minimal density variations. Therefore, an effort has been established to develop a method to more accurately determine density errors, perform machining experiments, acquire physical property data, and model the machining process.

Designing symmetric polar direct drive implosions on the Omega laser facility

Achieving symmetric capsule implosions with Polar Direct Drive [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004); R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005); F. J. Marshall et al., J. Phys. IV France 133, 153–157 (2006)] has been explored during recent Defect Induced Mix Experiment campaign on the Omega facility at the Laboratory for Laser Energetics. To minimize the implosion asymmetry due to laser drive, optimized laser cone powers, as well as improved beam pointings, were designed using 3D radiation-hydrodynamics code HYDRA [M. M. Marinak et al., Phys. Plasmas 3, 2070 (1996)]. Experimental back-lit radiographic and self-emission images revealed improved polar symmetry and increased neutron yield which were in good agreement with 2D HYDRA simulations. In particular, by reducing the energy in Omegas 21.4° polar rings by 16.75%, while increasing the energy in the 58.9° equatorial rings by 8.25% in such a way as to keep the overall energy to the target at 16 kJ, the second Legendre mode (P2) wa...

Incorporation of Tracer Elements Within Aerogels and CH Foams

Abstract Low-density materials containing tracer elements are an important component of target platforms for high-energy density physics experiments. High-Z elements can be dispersed homogeneously by changing chemistry of the matrix or by simple physical mixing; alternately, tracers can be introduced heterogeneously in the form of ultrathin foils or particles. We have recently focused on how best to manufacture and embed tracer elements into silica aerogels and polystyrene-divinylbenzene (CH) foams. The ability to control dopant concentration and distribution is critical to final shot success. We have produced low-density CH foams doped with chlorine at levels up to 2 at. %. In addition, we have placed metal particles and foils precisely within silica aerogel monoliths.

NONDESTRUCTIVE INVESTIGATIONS OF A COPPER-AND ARGON-DOPED SPUTTERED BERYLLIUM CAPSULE USING X-RAYS IN THREE DIMENSIONS

Abstract Three-dimensional (3-D) computed micro X-ray tomography (micro CT) and 3-D confocal micro X-ray fluorescence (MXRF) combined are very useful nondestructive metrology techniques for determining the unique compositional and morphological information of fusion targets and target materials. Micro CT and confocal MXRF are being used in concert to examine a beryllium ablator capsule that has been sputtered and graded doped with copper and argon. In this manuscript, we will show that two-dimensional (2-D) MXRF imaging in concert with a simple radiograph is very useful for approximating the copper and argon profiles in the x and y dimensions, but because of the lack of signal discrimination in the z direction, image “bleed” from the sample regions where the X-rays are out of focus is prevalent. Data collected using the micro CT and overlapped with the confocal MXRF data produce absorbance and elemental line profiles without the signal bleed. Overlapping the 3-D data from these techniques provides a more accurate picture of the composition of these capsules than 2-D nondestructive techniques.

Utilizing Conventional Machining Tools with Customized Machining Techniques to Manufacture Multifaceted Targets

Abstract Three recent experimental campaigns at Los Alamos National Laboratory have required unique application of traditional machining techniques to manufacture the components. For pRad experiments at Los Alamos Neutron Science Center (LANCE), unique planar targets with varying profiles required unique fixturing: a custom programming software to create concentric rings with a 2-deg taper that had five different sine waves machined across the face. Also, experiments using P8 modulated capsules for Asymmetric Burn Experiment (ABEX) experiments at Omega made use of a water-soluble ultraviolet-curable glue, which was used for holding and locating purposes during machining operations to produce an indicating datum, as well as a custom fixturing system, which allowed the ability to apply the impression gum from behind. Finally, for the milling of a 125-μm-thick silica aerogel for dense-plasma equation-of-state experiments, we used an ultraprecision milling machine with a high-speed spindle and precise positional accuracy that permits micrometer depth of cuts at higher feed rates, which allowed for a reduction in machining time.