Publications

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Gold nanostructured materials exhibit important size- and shape-dependent properties that enable a wide variety of applications in photocatalysis, nanoelectronics and phototherapy. Here we show the use of superfast dynamic compression to synthesize extended gold nanostructures, such as nanorods, nanowires and nanosheets, with nanosecond coalescence times. Using a pulsed power generator, we ramp compress spherical gold nanoparticle arrays to pressures of tens of GPa, demonstrating pressure-driven assembly beyond the quasi-static regime of the diamond anvil cell. Our dynamic magnetic ramp compression approach produces smooth, shockless (that is, isentropic) one-dimensional loading with low-temperature states suitable for nanostructure synthesis. Transmission electron microscopy clearly establishes that various gold architectures are formed through compressive mesoscale coalescences of spherical gold nanoparticles, which is further confirmed by in-situ synchrotron X-ray studies and large-scale simulation. This nanofabrication approach applies magnetically driven uniaxial ramp compression to mimic established embossing and imprinting processes, but at ultra-short (nanosecond) timescales.

Complementary gas-gun experiments and computational simulations have examined the time-resolved motion and post-mortem deformation of cylindrical metal samples subjected to impact loading. The effect of propagation distance on a compressive waveform generated in a sample by planar impact at one end was determined using a velocity interferometer to track the longitudinal motion at the center of the opposing rear (i.e., free) surface. Samples (25.4-mm diameter) were fabricated from aluminum (types 6061 and 7075), copper (OFHC = oxygen free, high conductivity), stainless steel (type 316), and cobalt alloy L-605 (AMS 5759; also referenced as Haynes®25 alloy). For each material, waveforms obtained for a 25.4-mm long cylinder corresponded to two-dimensional strain at the measurement point. The wave-profile data have been analyzed to (i) establish key dynamic material modeling parameters, (ii) assess the functionality of the Sierra Solid Mechanics-Presto (Sierra/SM) code, and (iii) identify the need for additional testing, material modeling, and/or code development. The results of subsequent simulations have been compared to benchmark recovery experiments that showed the residual plastic deformation incurred by cylinders following end, side, and corner impacts. ∗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 Nuclear Security Administration under contract DE-AC04-94AL85000.

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The capability for statically pre-compressing fluid targets for Hugoniot measurements utilizing gas gun driven flyer plates has been developed. Pre-compression expands the capability for initial condition control, allowing access to thermodynamic states off the principal Hugoniot. Absolute Hugoniot measurements with an uncertainty less than 3% on density and pressure were obtained on statically pre-compressed fluid helium utilizing a two stage light gas gun. Helium is highly compressible; the locus of shock states resulting from dynamic loading of an initially compressed sample at room temperature is significantly denser than the cryogenic fluid Hugoniot even for relatively modest (0.27-0.38 GPa) initial pressures. The dynamic response of pre-compressed helium in the initial density range of 0.21-0.25 g/cm3 at ambient temperature may be described by a linear shock velocity (us) and particle velocity (up) relationship: us = C0 + sup, with C0 = 1.44 ± 0.14 km/s and s = 1.344 ± 0.025.

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We have developed the design of Thor: a pulsed power accelerator that delivers a precisely shaped current pulse with a peak value as high as 7 MA to a strip-line load. The peak magnetic pressure achieved within a 1-cm-wide load is as high as 100 GPa. Thor is powered by as many as 288 decoupled and transit-time isolated bricks. Each brick consists of a single switch and two capacitors connected electrically in series. The bricks can be individually triggered to achieve a high degree of current pulse tailoring. Because the accelerator is impedance matched throughout, capacitor energy is delivered to the strip-line load with an efficiency as high as 50%. We used an iterative finite element method (FEM), circuit, and magnetohydrodynamic simulations to develop an optimized accelerator design. When powered by 96 bricks, Thor delivers as much as 4.1 MA to a load, and achieves peak magnetic pressures as high as 65 GPa. When powered by 288 bricks, Thor delivers as much as 6.9 MA to a load, and achieves magnetic pressures as high as 170 GPa. We have developed an algebraic calculational procedure that uses the single brick basis function to determine the brick-triggering sequence necessary to generate a highly tailored current pulse time history for shockless loading of samples. Thor will drive a wide variety of magnetically driven shockless ramp compression, shockless flyer plate, shock-ramp, equation of state, material strength, phase transition, and other advanced material physics experiments.

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The success of dynamic materials properties research at Sandia National Laboratories has led to research into ultra-low impedance, compact pulsed power systems capable of multi-MA shaped current pulses with rise times ranging from 220-500 ns. The Genesis design consists of two hundred and forty 200 kV, 80 kA modules connected in parallel to a solid dielectric disk transmission line and is capable of producing 280 kbar of magnetic pressure (>500 kbar pressure in high Z materials) in a 1.75 nH, 20 mm wide stripline load. Stripline loads operating under these conditions expand during the experiment resulting in a time-varying load that can impact the performance and lifetime of the system. This paper provides analysis of time-varying stripline loads and the impact of these loads on system performance. Further, an approach to reduce dielectric stress levels through active damping is presented as a means to increase system reliability and lifetime. © 2011 IEEE.

Genesis is a compact pulsed power platform designed by Sandia National Laboratories to generate precision shaped multi-MA current waves with a rise time of 200-500 ns. In this system, two hundred and forty, 200 kV, 80 kA modules are selectively triggered to produce 280 kbar of magnetic pressure (>500 kbar pressure in high Z materials) in a stripline load for dynamic materials properties research. This new capability incorporates the use of solid dielectrics to reduce system inductance and size, programmable current shaping, and gas switches that must perform over a large range of operating conditions. Research has continued on this technology base with a focus on demonstrating the integrated performance of key concepts into a Genesis-like prototype called Protogen. Protogen measures approximately 1.4 m by 1.4 m and is designed to hold twelve Genesis modules. A fixed inductance load will allow rep-rate operation for component reliability and system lifetime experiments at the extreme electric field operating conditions expected in Genesis. © 2011 IEEE.

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Veloce is a medium-voltage, high-current, compact pulsed power generator developed for isentropic and shock compression experiments. Because of its increased availability and ease of operation, Veloce is well suited for studying isentropic compression experiments (ICE) in much greater detail than previously allowed with larger pulsed power machines such as the Z accelerator. Since the compact pulsed power technology used for dynamic material experiments has not been previously used, it is necessary to examine several key issues to ensure that accurate results are obtained. In the present experiments, issues such as panel and sample preparation, uniformity of loading, and edge effects were extensively examined. In addition, magnetohydrodynamic (MHD) simulations using the ALEGRA code were performed to interpret the experimental results and to design improved sample/panel configurations. Examples of recent ICE studies on aluminum are presented.

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