Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

We propose a method to automatically defeature a CAD model by detecting irrelevant features using a geometry-based size field and a method to remove the irrelevant features via facet-based operations on a discrete representation. A discrete B-Rep model is first created by obtaining a faceted representation of the CAD entities. The candidate facet entities are then marked for reduction using a geometry-based size field. This is accomplished by estimating local mesh sizes based on geometric criteria. If the field value at a facet entity goes below a user-specified threshold value then it is identified as an irrelevant feature and is marked for reduction. The reduction of marked facet entities is performed using various facet operators. Care is taken to retain a valid geometry and topology of the discrete model throughout the procedure. The original model is not altered as the defeaturing is performed on a separate discrete model. Associativity between the entities of the discrete model and that of original CAD model is maintained in order to decode the attributes and boundary conditions applied on the original CAD entities onto the mesh via the entities of the discrete model. Example models are presented to illustrate the effectiveness of the proposed approach. © Springer-Verlag London Limited 2012.

The Domain Name System Security Extensions (DNSSEC) add an element of authentication to the DNS, which is a foundational component of the Internet. However, the maintenance of a DNSSEC deployment is more complex than that of its insecure counterpart. This paper discusses some specific misconfigurations that impact DNSSEC deployments, analyzes their prevalence via an extended survey of production DNS zones implementing DNSSEC, and assesses the maintenance and corrective actions. Our survey indicated that more than one-half of the zones analyzed were affected by misconfigurations. Also, the survey revealed a significant number of repeat occurrences and average correction times of up to two weeks. This paper summarizes the survey findings and suggests approaches for improving the quality of DNSSEC deployments. © 2012 Elsevier B.V.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Earlier synchrotron photoionization mass spectrometry experiments suggested a prominent ring-opening channel in the OH-initiated oxidation of cyclohexene, based on comparison of product photoionization spectra with calculated spectra of possible isomers. The present work re-examines the OH + cyclohexene reaction, measuring the isomeric products of OH-initiated oxidation of partially and fully deuterated cyclohexene. In particular, the directly measured photoionization spectrum of 2-cyclohexen-1-ol differs substantially from the previously calculated Franck-Condon envelope, and the product spectrum can be fit with no contribution from ring-opening. Measurements of H 2O 2 photolysis in the presence of C 6D 10 establish that the addition-elimination product incorporates the hydrogen atom from the hydroxyl radical reactant and loses a hydrogen (a D atom in this case) from the ring. Investigation of OH + cyclohexene-4,4,5,5-d 4 confirms this result and allows mass discrimination of different abstraction pathways. Products of 2-hydroxycyclohexyl-d 10 reaction with O 2 are observed upon adding a large excess of O 2 to the OH + C 6D 10 system. © 2012 American Chemical Society.

This paper describes recent results from the Extremely High Temperature Photonic Crystal System Technology (XTEMPS) technology program. The XTEMPS program has developed a Photonic Crystal (PhC) based high efficiency IR emitter array for use in the emerging generation of wide field of view high performance scene projectors. Cyan's approach provides high dynamic range, multispectral emission from SWIR to LWIR and is uniquely capable of accurately simulating very realistic system spectral signatures. The PhC array is fabricated from refractory materials to provide high radiance and long service lifetime. Cyan is teamed with Sandia National Laboratories for design and fabrication of the emitter and with Nova sensors to utilize their advanced Read In Integrated Circuit (RIIC). PhC based emitters show improved inband output power efficiency when compared to broad band "graybody" emitters due to the absence of out-of-band emission. Less electrical power is required to achieve high operating temperature, and non-Lambertian emission pattern puts a large fraction of the emitted energy into a straight ahead beam. Both effects significantly boost effective radiance output. Cyan has demonstrated pixel designs compatible with Nova's medium format RIIC, which ensures high apparent output temperatures with modest drive currents and low operating voltages of less than five volts. Unit cell pixel structures for high radiative efficiency have been demonstrated and arrays using PhC optimized for up to four spectral bands have been successfully patterned and fabricated into high yield wafers. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

In this study, a novel approach was developed to formulate surrogate fuels having characteristics that are representative of diesel fuels produced from real-world refinery streams. Because diesel fuels typically consist of hundreds of compounds, it is difficult to conclusively determine the effects of fuel composition on combustion properties. Surrogate fuels, being simpler representations of these practical fuels, are of interest because they can provide a better understanding of fundamental fuel-composition and property effects on combustion and emissions-formation processes in internal-combustion engines. In addition, the application of surrogate fuels in numerical simulations with accurate vaporization, mixing, and combustion models could revolutionize future engine designs by enabling computational optimization for evolving real fuels. Dependable computational design would not only improve engine function, it would do so at significant cost savings relative to current optimization strategies that rely on physical testing of hardware prototypes. The approach in this study utilized the state-of-the-art techniques of 13C and 1H nuclear magnetic resonance spectroscopy and the advanced distillation curve to characterize fuel composition and volatility, respectively. The ignition quality was quantified by the derived cetane number. Two well-characterized, ultra-low-sulfur #2 diesel reference fuels produced from refinery streams were used as target fuels: a 2007 emissions certification fuel and a Coordinating Research Council (CRC) Fuels for Advanced Combustion Engines (FACE) diesel fuel. A surrogate was created for each target fuel by blending eight pure compounds. The known carbon bond types within the pure compounds, as well as models for the ignition qualities and volatilities of their mixtures, were used in a multiproperty regression algorithm to determine optimal surrogate formulations. The predicted and measured surrogate-fuel properties were quantitatively compared to the measured target-fuel properties, and good agreement was found. © 2012 American Chemical Society.

A high-performance computer was used to simulate ninety-six years of operation of a five megawatt wind turbine. Over five million aero-elastic simulations were performed, with each simulation consisting of wind turbine operation for a ten minute period in turbulent wind conditions. These simulations have produced a large database of wind turbine loads, including ten minute extreme loads as well as fatigue cycles on various turbine components. In this paper, the extreme load probability distributions are presented. The long total simulation time has enabled good estimation of the tails of the distributions down to probabilities associated with twenty-year (and longer) return events. The database can serve in the future as a truth model against which design-oriented load extrapolation techniques can be tested. The simulations also allow for detailed examination of the simulations leading to the largest loads, as demonstrated for two representative cases.

Metal Organic Frameworks (MOFs) are a rapidly developing class of nanoporous materials with numerous applications in diverse fields such as chemical detection, hazardous gas detection, and carbon capture. Even though numerous articles have been written emphasizing the adsorption properties of these MOFs, their compatibility with respect to the sensing device has not been explored. While there are numerous types of sensing devices that could benefit from the use of MOF-based coatings to enhance sensitivity and selectivity, we are particularly interested in microcantilevers because of the high sensitivity they can provide within a compact, lower-power architecture. In this paper, we address this need by analyzing the effect of the mechanical properties of MOFs on the sensor response. In particular, we are interested in the structural flexibility of MOFs, because this unique guest-induced property can be used for strain-induced sensing attribute of the microcantilever. In this regard we examined the effects of important MOF mechanical properties such as the Young's Modulus, Poisson's ratio, and density on the sensor response for a range of values representative of the MOFs available in the literature. From our analysis we determined that increasing the Young's Modulus and Poisson's ratio improve the response, while the density of the MOF has a negligible effect on the cantilever response. In addition, we also examined the influence on cantilever response of the intermediate layer used to bind the MOF, from which we observe that SiO 2 provides the best sensor response for a given MOF layer. © 2012 Elsevier B.V.

We discuss recent experiments and modeling for the chirped-probe-pulse generation of single-laser-shot femtosecond pure-rotational CARS/CSRS spectra from room-temperature gases. A pure-rotational Raman coherence is impulsively generated using near-transform-limited femtosecond pump/Stokes excitation, and the coherence is probed by stretching a nominally 100-fs near-transform-limited probe beam to approximately 1.7 ps via the refractive-index dispersion in a 30-cm long flint-glass rod. The linearly chirped probe spectrum and phase beat against the time-dependent Raman polarization to generate complex spectra. Chirped-probe-pulse rotational CARS/CSRS offers an interesting alternative to hybrid fs/ps rotational CARS, in which a band-limited pulse of limited energy is used, because all of the available probe pulse energy can be retained in a chirped-probe-pulse experiment. Our early chirped-probe spectra are presented and the details of our initial model calculations are provided. The temperature sensitivity of the chirped-probe results is illustrated using calculated spectra.

An efficient, stability-preserving model reduction technique for non-linear initial boundary value problems whose solutions exhibit inherently non-linear dynamics such as metastability and periodic regimes (limit cycles) is developed. The approach is based on the 'continuous' Galerkin projection approach in which the continuous governing equations are projected onto the reduced basis modes in a continuous inner product. The reduced order model (ROM) basis is constructed via a proper orthogonal decomposition (POD). In general, POD basis modes will not satisfy the boundary conditions of the problem. A weak implementation of the boundary conditions in the ROM based on the penalty method is developed. Asymptotic stability of the ROM with penalty-enforced boundary conditions is examined using the energy method, following linearization and localization of the governing equations in the vicinity of a stable steady solution. This analysis, enabled by the fact that a continuous representation of the reduced basis is employed, leads to a model reduction method with an a priori stability guarantee. The approach is applied to two non-linear problems: the Allen-Cahn (or 'bistable') equation and a convection-diffusion-reaction system representing a tubular reactor. For each of these problems, bounds on the penalty parameters that ensure asymptotic stability of the ROM solutions are derived. The non-linear terms in the equations are handled efficiently using the 'best points' interpolation method proposed recently by Peraire, Nguyen et al. Numerical experiments reveal that the POD/Galerkin ROMs with stability-preserving penalty boundary treatment for the two problems considered, both without as well as with interpolation, remain stable in a way that is consistent with the solutions to the governing continuous equations and capture the correct non-linear dynamics exhibited by the exact solutions to these problems. Published 2012. This article is a US Government work and is in the public domain in the USA. © 2012 John Wiley & Sons, Ltd.

The development of Pb-free solutions for the highreliability electronics community necessitates the consideration of hybrid microcircuit (HMC) products. This study used a test vehicle that included both plastic and ceramic packages as well as leaded and area-array solder joints on an alumina substrate. The conductor was a Ag-Pd thick film layer. The shear strength was measured for interconnections made with 63Sn-37Pb (wt.%, abbreviated Sn-Pb) and 95.5Sn-3.0Ag-0.5Cu (Sn-Ag-Cu) solders as a function of isothermal aging, thermal cycling, and thermal shock environments. The area-array packages indicated that solder joint fatigue was not altered significantly in a forward compatibility situation (i.e., Sn-Pb balls and a Sn-Ag-Cu assembly process). Local CTE mismatch fatigue strains are important for solder joints connecting ceramic area array packages to ceramic substrates. The gull-wing lead, SOT plastic package solder joints assembled with the Sn-Ag-Cu solder exhibit a greater strength loss under temperature cycling than did the corresponding Sn-Pb interconnections. Thermal shock is more detrimental to Sn-Pb HMC solder joints than are the equivalent number of thermal cycles. Copyright 2012 ASM International® All rights reserved.

Similar to other refractory metals, commercially pure niobium is difficult attach using soldering processes without first plating with nickel-gold, nickel-tin or similar materials that are directly solderable. Currently used procedures require the aforementioned plating process or a step-brazing process in which copper substrates are brazed at a lower temperature onto the niobium surfaces eliminating the plating requirements. A solder-dipping process is then used to pre-tin the exposed copper surfaces, preparing them for next-assembly soldering steps. As part of a product development effort to reduce or eliminate entire processes or processing steps, a project was initiated to replace commercially pure niobium sheet material with explosively bonded niobium-copper sheet. The exposed copper surfaces could then be subsequently coated using a solder dipping procedure. To simulate the component brazement geometry, explosively bonded niobium and copper metal sheets were actively brazed to 94% alumina ceramic test specimens. The thickness of the explosively bonded substrates was 0.5 mm and the thickness of the niobium metal approximately twice that of the copper. ASTM F19 tensile buttons were fabricated using the explosively bonded niobium-copper material as the interlayers. The test samples were active brazed using a commercially available gold-based active brazing filler metal of the composition 35Au-62Cu-2Ti-1Ni (wt %). Brazing peak temperatures and soak times at peak temperatures were varied to assess the process robustness. Finite element analysis (FEA) simulations were performed to determine the theoretical residual stresses in the braze samples. Helium mass spectrometer leak detection data, brazed sample tensile strengths and scanning electron microscope image analysis of the niobium-copper, niobium-alumina and copper-alumina interfaces will be presented. Copyright 2012 ASM International® All rights reserved.

The high strength and low density characteristics of fiber reinforced composite materials have made them applicable to a large variety of applications. As these applications grow, their performance in high strain rate shock environments has increased. The modeling and simulation of such materials is difficult due to their anisotropic behavior and complex internal geometries. Fiber reinforced composite materials consist of a collection of layers that create a laminate. Each layer is typically transverse isotropic or orthotropic consisting of a fiber and matrix material. One approach is to explicitly model each layer, while accurate, this is often not feasible for full system calculations as the laminate layer count increases in size. Additionally, modeling each layer given the finite thickness proves to be a challenging process and typically a smearing approach is used to represent the laminate response removing the identity and material response of each layer. The creation of a layering capability is a good compromise between the inaccuracy of smearing and the computational cost of explicitly modeling each layer. The layering is done using a sub-grid technique in an individual grid cell. The grid cell is partitioned based on layer location in the laminate and the material deformation. The volume occupied by the given layer is computed and the layer calculates a material response based on the cell strain field. The resulting material stress and state variables are volume weighted with the remaining layers in the given grid cell yielding a cell response. The result is a technique that requires less computation time than modeling each layer while increasing the accuracy over smeared approximations. © 2012 American Institute of Physics.

The use of physical vapor deposition is an attractive technique to produce microenergetic samples to study sub-millimeter explosive behavior. Films of the high explosive PETN (pentaerythritol tetranitrate) were deposited through vacuum thermal sublimation. Deposition conditions were varied to understand the effect of substrate cooling capacity and substrate temperature during deposition. PETN films were characterized with surface profilometry and scanning electron microscopy. Detonation velocity versus PETN film thickness was analyzed using a variation of the standard form for analysis of the diameter effect. Results were compared with previous work conducted on PETN films deposited with lower substrate cooling capacity. Seemingly subtle variations in PETN deposition conditions led to differences in detonation behaviors such as critical thickness for detonation, detonation velocity at "infinite" thickness, and the shape of the critical thickness curves. © 2012 American Institute of Physics.

A new adaptive tabulation scheme for multi-phase equations of state (EOS) is described. Adaptation allows verification that a table represents an EOS model to some desired accuracy at a much lower computational cost than standard tables. Computational efficiency is provided through the use of a quad-tree representation. Using both rectangular and triangular interpolation regions results in accurate descriptions of phase boundaries. The new format is demonstrated on a representative multi-phase EOS model. © 2012 American Institute of Physics.

X-ray momentum coupling coefficients, C M, were determined by measuring stress waveforms in planetary materials subjected to impulsive radiation loading from the SNL Z-machine. Targets were prepared from iron and stone meteorites, dunite (primarily magnesium rich olivine) in solid and powder forms (∼5 - 300 μm grains), and Si, Al, and Fe. All samples were ∼1 mm thick and, except for Si, backed by LiF single-crystal windows. The spectra of the incident x-rays included thermal radiation (blackbody 170 - 237 eV) and line emissions from the pinch material (Cu, Ni, Al, or stainless steel). Target fluences of 0.4 - 1.7 kJ/cm 2 at intensities 43 - 260 GW/cm 2 produced front surface plasma pressures of 2.6 - 12.4 GPa. Stress waves driven into the samples were attenuating due to the short ∼5 ns duration of the drive pulse. CM was determined using the fact that an attenuating wave impulse is constant, and accounted for the mechanical impedance mismatch between samples and window. Values ranged from 0.8 - 3.1 x 10 -5 s/m. CTH hydrocode modeling of x-ray coupling to porous and fully dense silica corroborated experimental results and extrapolations to other materials. © 2012 American Institute of Physics.

Neat pressings of HNS powders have been used in many explosive applications for over 50 years. However, characterization of its crystalline properties has lagged that of other explosives, and the solid stress has been inferred from impact experiments or estimated from mercury porosimetry. This lack of knowledge of the precise crystalline isotherm can contribute to large model uncertainty in the reacted response of pellets to shock impact. At high impact stresses, deflagration-to-detonation transition (DDT) processes initiated by compressive reaction have been interpreted from velocity interferometry at the surface of distended HNS-FP pellets. In particular, the Baer-Nunziato multiphase model in CTH, Sandia's Eulerian, finite volume shock propagation code, was used to predict compressive waves in pellets having approximately a 60% theoretical maximum density (TMD). These calculations were repeated with newly acquired isothermal compression measurements of fineparticle HNS using diamond anvil cells to compress the sample and powder x-ray diffraction to obtain the sample volume at each pressure point. Hence, estimating the model uncertainty provides a simple method for conveying the impact of future model improvements based upon new experimental data. © 2012 American Institute of Physics.

For the past decade, a large, interdisciplinary team at Sandia National Laboratories has been refining the Z Machine (20+ MA and 10+ MGauss) into a mature, robust, and precise platform for material dynamics experiments in the multi-Mbar pressure regime. In particular, significant effort has gone into effectively coupling condensed matter theory, magneto-hydrodynamic simulation, and electromagnetic modeling to produce a fully self-consistent simulation capability able to very accurately predict the performance of the Z machine and various experimental load configurations. This capability has been instrumental in the ability to develop experimental platforms to routinely perform magnetic ramp compression experiments to over 4 Mbar, and magnetically accelerate flyer plates to over 40 km/s, creating over 20 Mbar impact pressures. Furthermore, a strong tie has been developed between the condensed matter theory and the experimental program. This coupling has been proven time and again to be extremely fruitful, with the capability of both theory and experiment being challenged and advanced through this close interrelationship. This paper will provide an overview of the material dynamics platform and discuss several examples of the use of Z to perform extreme material dynamics studies with unprecedented accuracy in support of basic science, planetary astrophysics, inertial confinement fusion, and the emerging field of high energy density laboratory physics. © 2012 American Institute of Physics.

The response of beryllium to dynamic loading has been extensively studied, both experimentally and theoretically, due to its importance in several technological areas. We use a MEAM empirical potential to examine the melt transition. MD simulations of equilibrated two-phase systems were used to calculate the HCP melting curve up to 300 GPa. This was found to agree well with previous ab initio calculations. The Hugoniostat method was used to examine dynamic compression along the two principal orientations of the HCP crystal. In both directions, the melting transition occurred at 230 GPa and 5000 K, consistent with the equilibrium melting curve. Direct NEMD simulations of uniaxial compression show a transition to an amorphous material at shocked states that lie below the equilibrium melt curve. © 2012 American Institute of Physics.

The double shock layered high-velocity flyer plate is one new capability being developed on Sandia's Z machine. With this technique, dynamic material data at high energy densities can be obtained at points in phase space which lie neither on principal Hugoniots nor on quasi-isentropic ramp curves. We discuss the double shock capability development experiments being performed on Z. © 2012 American Institute of Physics.

Hydrocarbon foams are commonly used in high energy-density physics (HEDP) applications, for example as tamper and ablation materials for dynamic materials or inertial confinement fusion (ICF) experiments, and as such are subject to shock compression from tens to hundreds of GPa. Modeling of macro-molecular materials like hydrocarbon foams is challenging due to the heterogeneous character of the polymers and the complexity of voids and large-scale structure. Under shock conditions, these factors contribute to a relatively larger uncertainty of the post-shock state compared to that encountered for homogenous materials; therefore a quantitative understanding of foams under strong dynamic compression is sought. We use Sandia's ALEGRA-MHD code to simulate 3D mesoscale models of poly-(4-methyl-1-pentene) (PMP) foams. We devise models of the initial polymer-void structure of the foam and analyze the statistical properties of the initial and shocked states. We compare the simulations to multi-Mbar shock experiments conducted on Sandia's Z machine at various initial foam densities and flyer impact velocities. Scatter in the experimental data may be a consequence of the initial foam inhomogeneity. We compare the statistical properties of the simulations with the scatter in the experimental data. © 2012 American Institute of Physics.

We describe a technique for measuring the pressure and density of a metallic solid, shocklessly compressed to multi-megabar pressure, through x-ray radiography of a magnetically driven, cylindrical liner implosion. Shockless compression of the liner produces material states that correspond approximately to the principal compression isentrope (quasi-isentrope). This technique is used to determine the principal quasi-isentrope of solid beryllium to a peak pressure of 2.4 Mbar from x-ray images of a high current (20 MA), fast (∼100 ns) liner implosion. © 2012 American Institute of Physics.

Comparisons are made among Molecular Dynamics (MD), Classical Density Functional Theory (c-DFT), and Poisson-Boltzmann (PB) modeling of the electric double layer (EDL) for the nonprimitive three component model (3CM) in which the two ion species and solvent molecules are all of finite size. Unlike previous comparisons between c-DFT and Monte Carlo (MC), the present 3CM incorporates Lennard-Jones interactions rather than hard-sphere and hard-wall repulsions. c-DFT and MD results are compared over normalized surface charges ranging from 0.2 to 1.75 and bulk ion concentrations from 10 mM to 1 M. Agreement between the two, assessed by electric surface potential and ion density profiles, is found to be quite good. Wall potentials predicted by PB begin to depart significantly from c-DFT and MD for charge densities exceeding 0.3. Successive layers are observed to charge in a sequential manner such that the solvent becomes fully excluded from each layer before the onset of the next layer. Ultimately, this layer filling phenomenon results in fluid structures, Debye lengths, and electric surface potentials vastly different from the classical PB predictions. © 2012 American Chemical Society.

Large-scale molecular dynamics simulations are used to simulate a layer of nanoparticles floating on the surface of a liquid. Both a low viscosity liquid, represented by Lennard-Jones monomers, and a high viscosity liquid, represented by linear homopolymers, are studied. The organization and diffusion of the nanoparticles are analyzed as the nanoparticle density and the contact angle between the nanoparticles and liquid are varied. When the interaction between the nanoparticles and liquid is reduced the contact angle increases and the nanoparticles ride higher on the liquid surface, which enables them to diffuse faster. In this case the short-range order is also reduced as seen in the pair correlation function. For the polymeric liquids, the out-of-layer fluctuation is suppressed and the short-range order is slightly enhanced. However, the diffusion becomes much slower and the mean square displacement even shows sub-linear time dependence at large times. The relation between diffusion coefficient and viscosity is found to deviate from that in bulk diffusion. Results are compared to simulations of the identical nanoparticles in 2-dimensions. © 2012 American Institute of Physics.

The transmission simulator method of experimental dynamic substructuring captures the interface forces and motions through a fixture called a transmission simulator. The transmission simulator method avoids the need to measure connection point rotations and enriches the modal basis of the substructure model. The free modes of the experimental substructure mounted to the transmission simulator are measured. The finite element model of the transmission simulator is used to couple the experimental substructure to another substructure and to subtract the transmission simulator. However, in several cases the process of subtracting the transmission simulator has introduced an indefinite mass matrix for the experimental substructure. The authors previously developed metrics that could be used to identify which modes of the experimental model led to the indefinite mass matrix. A method is developed that utilizes those metrics with a sensitivity analysis to adjust the transmission simulator mass matrix so that the subtraction does not produce an indefinite mass matrix. A second method produces a positive definite mass matrix by adding a small amount of mass to the indefinite mass matrix. Both analytical and experimental examples are described. © The Society for Experimental Mechanics, Inc. 2012.

This paper explores methods that can be used to characterize weakly nonlinear systems, whose natural frequencies and damping ratios change with response amplitude. The focus is on high order systems that may have several modes although each with a distinct natural frequency. Interactions between modes are not addressed. This type of analysis may be appropriate, for example, for structural dynamic systems that exhibit damping that depends on the response amplitude due to friction in bolted joints. This causes the free-response of the system to seem to have damping ratios (and to a lesser extent natural frequencies) that change slowly with time. Several techniques have been proposed to characterize such systems. This work compares a few available methods, focusing on their applicability to real measurements from multi-degree-of- freedom systems. A beam with several small links connected by simple bolted joints was used to evaluate the available methods. The system was excited by impulse and the velocity response was measured with a scanning laser Doppler vibrometer. Several state of the art procedures were then used to process the nonlinear free responses and their features were compared. First the Zeroed Early Time FFT technique was used to qualitatively evaluate the responses. Then, the Empirical Mode Decomposition method and a simple approach based on band pass filtering were both employed to obtain mono-component signals from the measured responses. Once mono-component signals had been obtained, they were processed with the Hilbert transform approach, with several enhancements made to minimize the effects of noise. © The Society for Experimental Mechanics, Inc. 2012.

The importance of memory performance and capacity is a growing concern for high performance computing laboratories around the world. It has long been recognized that improvements in processor speed exceed the rate of improvement in dynamic random access memory speed and, as a result, memory access times can be the limiting factor in high performance scientific codes. The use of multi-core processors exacerbates this problem with the rapid growth in the number of cores not being matched by similar improvements in memory capacity, increasing the likelihood of memory contention. In this paper, we present WMTools , a lightweight memory tracing tool and analysis framework for parallel codes, which is able to identify peak memory usage and also analyse per-function memory use over time. An evaluation of WMTools , in terms of its effectiveness and also its overheads, is performed using nine established scientific applications/benchmark codes representing a variety of programming languages and scientific domains. We also show how WMTools can be used to automatically generate a parameterized memory model for one of these applications, a two-dimensional non-linear magnetohydrodynamics application, Lare2D . Through the memory model we are able to identify an unexpected growth term which becomes dominant at scale. With a refined model we are able to predict memory consumption with under 7% error.

Charge trapping and slow (from 10 s to > 1000 s) detrapping in AlGaN/GaN high electron mobility transistors (HEMTs) designed for high breakdown voltages (> 1500 V) is studied through a combination of electrical, thermal, and optical methods to identify the impact of Al molefraction and passivation on trapping. Trapping due to 5-10 V drain bias stress in the on-state (V gs = 0) is found to have significantly slower recovery, compared with trapping in the off-state (V gs < V th, V ds = 0). Two different trapping components, i.e., TG1 (E a = 0.6 eV) and TG2 (with negligible temperature dependence), in AlGaN dominate under gate bias stress in the off-state. Al 0.15 Ga 0.85N shows much more vulnerability to trapping under gate stress in the absence of passivation than does AlGaN with a higher Al mole fraction. Under large drain bias, trapping is dominated by a much deeper trap TD. Detrapping under monochromatic light shows TD to have E a ≈ 1.65 eV. Carbon doping in the buffer is shown to introduce threshold voltage shifts, unlike any of the other traps. © 2012 IEEE.

Ferrites are promising materials for enabling solar-thermochemical cycles. Such cycles utilize solar-thermal energy to reduce the metal oxide, which is then re-oxidized by H2O or CO2, producing H2 or CO, respectively. Mixing ferrites with zirconia or yttria-stabilized zirconia (YSZ) greatly improves their cyclabilities. In order to understand this system, we have studied the behavior of iron oxide/8YSZ (8 mol-% Y2O3 in ZrO2) using in situ X-ray diffraction and thermogravimetric analyses at temperatures up to 1500 °C and under controlled atmosphere. The solubility of iron oxide in 8YSZ measured by XRD at room temperature was 9.4 mol-% Fe. The solubility increased to at least 10.4 mol-% Fe when heated between 800 and 1000 °C under inert atmosphere. Furthermore iron was found to migrate in and out of the 8YSZ phase as the temperature and oxidation state of the iron changed. In samples containing >9.4 mol-% Fe, stepwise heating to 1400 °C under helium caused reduction of Fe2O3 to Fe3O4 to FeO. Exposure of the FeO-containing material to CO2 at 1100 °C re-oxidized FeO to Fe3O4 with evolution of CO. Thermogravimetric analysis during thermochemical cycling of materials with a range of iron contents showed that samples with mostly dissolved iron utilized a greater proportion of the iron atoms present than did samples possessing a greater fraction of un-dissolved iron oxides.© 2012 JCPDS-ICDD.

Abstract not provided.

Sandia National Laboratories has the need to predict the behavior of structures after the occurrence of an initial failure. In some cases determining the extent of failure, beyond initiation, is required, while in a few cases the initial failure is a design feature used to tailor the subsequent load paths. In either case, the ability to numerically simulate the initiation and propagation of failures is a highly desired capability. This document describes one approach to the simulation of failure initiation and propagation.

In order to elucidate how the degradation of individual components affects the state of the photovoltaic inverter as a whole, we have carried out SPICE simulations to investigate the voltage and current ripple on the DC bus. The bus capacitor is generally considered to be among the least reliable components of the system, so we have simulated how the degradation of bus capacitors affects the AC ripple at the terminals of the PV module. Degradation-induced ripple leads to an increased degradation rate in a positive feedback cycle. Additionally, laboratory experiments are being carried out to ascertain the reliability of metallized thin film capacitors. By understanding the degradation mechanisms and their effects on the inverter as a system, steps can be made to more effectively replace marginal components with more reliable ones, increasing the lifetime and efficiency of the inverter and decreasing its cost per watt towards the US Department of Energy goals.