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Top 20 Most Read Articles
May 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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Developments in AMTEC Devices, Components and Performance AIP Conf. Proc. 301, pp. 1495-1500; doi:http://dx.doi.org/10.1063/1.2950169 (6 pages) Online Publication Date: 9 July 2008
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Improvement of the performance of an AMTEC device requires improvement and development of components as well as of device geometry and construction. The research and development effort at JPL includes studies which address both overall device construction and studies of components. This paper discusses recent studies on components and devices which have been carried out at JPL. Components investigated include the electrode materials titanium nitride (TiN) and rhodium‐tungsten (RhW) and the electrolyte materials sodium β“‐alumina and potassium β” ‐alumina. We have studied the mechanical characteristics of sodium and potassium β“‐alumina ceramic and conditions for fabrication of potassium β”‐alumina. Device studies include fabrication and operation of a wick fed cell using a graded, sintered wick, a higher voltage vapor‐vapor multicell which includes three “ subcells” which are internally series connected, and an AMTEC which uses potassium as the working fluid. |
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Fractional Charge from Topology in Polyacetylene and Graphene AIP Conf. Proc. 939, pp. 341-350; doi:http://dx.doi.org/10.1063/1.2803825 (10 pages) Online Publication Date: 12 October 2007
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We review the old story of charge fractionalization in polyacetylene, and its newer realization in graphene. |
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AIP Conf. Proc. 1353, pp. 5-10; doi:http://dx.doi.org/10.1063/1.3589483 (6 pages) Online Publication Date: 25 April 2011
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In this paper a simulation based optimization procedure, based on the combination of the Simulated Annealing and the Nelder Mead algorithms, for the time reduction of the thermal cure cycle of composite laminates, has been proposed and discussed. The cycle time has been assumed as the objective function to be minimized, taking into account technological limitations by the means of a penalty function. Several simulations have been performed to test the proposed method and investigate the influence of the optimization algorithm parameters on the quality of the solution and the convergence rate of the algorithm, evidencing the capability of the procedure to converge to an optimal process window for cure cycle design. |
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New methods for probing the opacity and optical properties of dense low‐temperature plasmas AIP Conf. Proc. 381, pp. 279-286; doi:http://dx.doi.org/10.1063/1.51320 (8 pages) Online Publication Date: 12 May 2008
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We have demonstrated new techniques by which well characterized dense (1019−1020 cm−3), cold (1−15 eV), and strongly coupled (Γ∼1) plasmas are produced by laser vaporization and ionization of thin metallic films. By limiting the plasmas to small (r∼100–500μm) but unconfined volumes it is possible to create fully accessible plasmas for diagnostic inquiry. The plasmas are very absorptive but, as a result of their small size, their optical depths are typically less than one. Laser interferometry, absorption probing, and spectroscopy are used to characterize the plasmas. Detailed measurements of the plasma density, temperature, opacity and spectra are compared to theory in the regime were the photon energies are of the same order as the average inter‐particle energies of the plasma. © 1996 American Institute of Physics. |
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Size Selective Precipitation of CdSe Colloidal Quantum Dots AIP Conf. Proc. 1199, pp. 309-310; doi:http://dx.doi.org/10.1063/1.3295425 (2 pages) Online Publication Date: 15 January 2010
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Cadmium Selenide (CdSe) nanoparticles were synthesized through the colloidal chemistry approach. Dynamic light scattering experiments (DLS) have been performed in CdSe aqueous dispersions in order to determine the nanoparticle hydrodynamic radii. The DLS data together with photoluminescence (PL) measurements allowed for size analyses throughout the synthetic and post‐preparative procedures. We report here a study of size‐selective precipitation (SSP). DLS technique provided fast results, yielding values for the hydrodynamic radii in several size separation steps. Efficiency of the SSP procedure could thus be estimated. |
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AIP Conf. Proc. 1353, pp. 1-1; doi:http://dx.doi.org/10.1063/1.3589482 (1 page) Online Publication Date: 25 April 2011
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AIP Conf. Proc. 1355, pp. 1-3; doi:http://dx.doi.org/10.1063/1.3584038 (3 pages) Online Publication Date: 19 May 2011
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Logarithmic decay in a two‐component model AIP Conf. Proc. 708, pp. 559-564; doi:http://dx.doi.org/10.1063/1.1764224 (6 pages) Online Publication Date: 14 June 2004
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The correlation functions near higher‐order glass‐transition singularities are discussed for a schematic two‐component model within the mode‐coupling theory for ideal glass‐transitions. The correlators decay in leading order like −ln(t/τ) and the leading correction introduces characteristic convex and concave patterns in the decay curves. The time scale τ follows a Vogel‐Fulcher type law close to the higher‐order singularities. © 2004 American Institute of Physics |
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Implementation Of Fuzzy Automated Brake Controller Using TSK Algorithm AIP Conf. Proc. 1324, pp. 1-3; doi:http://dx.doi.org/10.1063/1.3526193 (3 pages) Online Publication Date: 3 December 2010
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In this paper an application of Fuzzy Logic for Automatic Braking system is proposed. Anti‐blocking system (ABS) brake controllers pose unique challenges to the designer: a) For optimal performance, the controller must operate at an unstable equilibrium point, b) Depending on road conditions, the maximum braking torque may vary over a wide range, c) The tire slippage measurement signal, crucial for controller performance, is both highly uncertain and noisy. A digital controller design was chosen which combines a fuzzy logic element and a decision logic network. The controller identifies the current road condition and generates a command braking pressure signal Depending upon the speed and distance of train. This paper describes design criteria, and the decision and rule structure of the control system. The simulation results present the system’s performance depending upon the varying speed and distance of the train. |
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Fluid modeling of intense laser-plasma interactions AIP Conf. Proc. 569, pp. 154-162; doi:http://dx.doi.org/10.1063/1.1384346 (9 pages)
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We discuss various aspects of implementing numerical solutions to cold fluid models of laser-plasma interactions. Using the conservative formulation of the fluid model allows us to apply standard computational fluid dynamics methods. We discuss some of the details of this process showing the complications that can arise, and the trade-offs between performance and accuracy involved. We also discuss some results in one- and two-dimensions, showing pump-depletion effects and short-pulse generated nonlinear wake fields. Additionally, we describe work-in-progress on the numerical analysis of the algorithms and the different forms for the model equations. © 2001 American Institute of Physics. |
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Characterization of ultra-thin oxides using electrical C-V and I-V measurements AIP Conf. Proc. 449, pp. 235-239; doi:http://dx.doi.org/10.1063/1.56801 (5 pages) Online Publication Date: 27 March 2008
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The measurement of electrical parameters from capacitance-voltage (C-V) and current-voltage (I-V) curves provides a fast means of characterizing oxides in MOS capacitors or transistor structures. For ultra-thin oxides (<2 nm), conventional, well-established techniques must be reconsidered and modified due to several increasingly important physical effects including polysilicon depletion and surface quantum mechanical effects. In this work these effects have been incorporated into a rapid analysis program for extracting ultra-thin oxide parameters from measured C-V and I-V data. The technique uses a physically based model of structure charge and potential combined with a non-linear least squares fitting technique to extract device parameters. © 1998 American Institute of Physics. |
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Preface for the RGD Proceedings AIP Conf. Proc. 1333, pp. 9-11; doi:http://dx.doi.org/10.1063/1.3567952 (3 pages) Online Publication Date: 23 May 2011
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Electrical conductivity in inhomogeneous media AIP Conf. Proc. 40, pp. 2-45; doi:http://dx.doi.org/10.1063/1.31150 (44 pages) Online Publication Date: 9 July 2008
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The history of this field is reviewed, with emphasis on the relationship to the development of molecular field concepts in dielectric theory, in the last century, and with emphasis on the relationship to the study of disordered structures, in recent decades. A few of the many methods for calculating effective conductivities will be presented and discussed. One of these is based on the direct macroscopic application of the Clausius‐Mossotti relationship. In that connection we emphasize the shortcomings of the commonly accepted Lorentz derivation for the internal field and restate a less well known existing alternative derivation. The symmetrical and unsymmetrical effective medium theories of Bruggeman are presented. Connection is made to transport in randomly chosen resistor networks, to percolation threshold problems, and to transport in magnetic fields in the presence of inhomogeneities. Two more specialized topics are also discussed. One of these is the variability in field effect transistor thresholds arising from the limited size of the samples in which threshold is determined by the onset of percolation. The other specialized topic: The occurrence of strong spatial inhomogeneities in fields and currents in metals, in the presence of lattice defects, even though the mean free path is large compared to the extent of the defect. |
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Period Variations of Delta Scuti Stars AIP Conf. Proc. 1170, pp. 410-414; doi:http://dx.doi.org/10.1063/1.3246530 (5 pages) Online Publication Date: 23 September 2009
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In most Delta Scuti stars, the measured period changes are considerably larger than those expected from stellar evolution. In order to study these period and amplitude changes, a few selected stars are measured photometrically for hundreds of nights with a dedicated telescope. These measurements cover several years or decades. The Delta Scuti stars provide more information (than classical pulsators) since a number of simultaneously excited radial and nonradial modes can be studied. The present results indicate the presence of at least two effects: beating of independent modes with close frequencies and stellar cycles. For period and amplitude changes with time scales less than one year, we confirm the beating hypothesis in three stars. This is shown by the correctly correlated relationship between amplitude and phase changes as well as the repetitions of these cycles. However, the observed period variations with longer time scales are not due to simple beating between two close frequencies. For the star 4 CVn we can derive accurate annual frequency values for at least seven radial and nonradial modes. The annual phases are in excellent agreement with predictions from nearby years, thereby confirming the values and their observed long‐term changes. For prograde and retrograde modes, the period variations are of identical size, but with opposite signs. The radial mode shows no (or little) changes. Furthermore, all period variations show a reversal around 1990. These results suggest long‐term, regular cycles affecting individual modes differently with some common systematic behavior. |
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An Overview of Integrated Sensing and Processing using Coded Aperture Imaging Systems AIP Conf. Proc. 860, pp. 95-106; doi:http://dx.doi.org/10.1063/1.2361210 (12 pages)
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The concept of Integrated Sensing and Processing (ISP) suggests that a sensor should collect data in a manner that is consistent with the end objective. Thus ISP seeks to minimize the collection of redundant data, reduce processing time and improve overall performance. A traditional approach designs the “best” sensor in terms of SNR, resolution, data rates, integration time and so forth, while traditional algorithms seek to optimize metrics such as probability of detection, false alarm rate, and class separability. The goal of ISP is to change this disjoint “sensing then processing” approach by allowing the algorithms to control the sensing parameters to collect the “best” information in order that the algorithm performs optimally. At Lockheed Martin, we are experimenting with an ISP system which utilizes a near Infrared (NIR) Hadamard multiplexing imaging sensor, built by PlainSight Systems. This prototype sensor incorporates a digital mirror array (DMA) device in order to realize a Hadamard multiplexed imaging system. Specific Hadamard codes can be encoded on the sensor aperture to measure their inner products with the underlying scene rather than the scene itself. The developed ISP algorithm uses an automatic target recognition (ATR) metric to send particular codes to the sensor in order to collect only the information relevant to the ATR problem. The result is a variable resolution hyperspectral cube with full resolution where targets are present and less than full resolution where there are no targets. This approach greatly improves the sensing process by reducing the overall volume of data and the time required to collect it. Several examples are also presented for illustrative purposes. © 2006 American Institute of Physics |
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Robust optimization of metal forming processes using a metamodel‐based strategy AIP Conf. Proc. 1353, pp. 11-16; doi:http://dx.doi.org/10.1063/1.3589484 (6 pages) Online Publication Date: 25 April 2011
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Robustness, optimization and Finite Element (FE) simulations are of major importance for achieving better products and cost reductions in the metal forming industry. In this paper, a metamodel‐based robust optimization strategy is proposed for metal forming processes. The applicability of the strategy is demonstrated by application to an analytical test function and an industrial V‐bending process. The results of both applications underline the importance of including uncertainty and robustness explicitly in the optimization procedure. |
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From the Kinetic Theory of Gases to Continuum Mechanics AIP Conf. Proc. 1333, pp. 15-27; doi:http://dx.doi.org/10.1063/1.3562621 (13 pages) Online Publication Date: 23 May 2011
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Recent results on the fluid dynamic limits of the Boltzmann equation based on the DiPerna‐Lions theory of renormalized solutions are reviewed in this paper, with an emphasis on regimes where the velocity field behaves to leading order like that of an incompressible fluid with constant density. |
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Fundamental processes in the interacting boson model: 0νββ decay AIP Conf. Proc. 1355, pp. 7-14; doi:http://dx.doi.org/10.1063/1.3584039 (8 pages) Online Publication Date: 19 May 2011
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A program to calculate nuclear matrix elements for fundamental processes in the interacting boson model has been initiated. Results for the nuclear matrix elements in neutrinoless double beta decay 0νββ are presented. |
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On Lorentz Transformations in Symplectic Deformations AIP Conf. Proc. 1256, pp. 183-187; doi:http://dx.doi.org/10.1063/1.3473851 (5 pages) Online Publication Date: 20 July 2010
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In this paper we study noncommutative Lorentz transformations using symplectic deformations. In this framework we define an infinitesimal line element that is invariant under this noncommutative Lorentz transformations. Using the symplectic geometry formalism, we find that noncommutative Lorentz transformations intertwine the canonical momentums with canonical position coordinates. |
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Research on Topology Optimization of Truss Structures Based on the Improved Group Search Optimizer AIP Conf. Proc. 1233, pp. 707-712; doi:http://dx.doi.org/10.1063/1.3452262 (6 pages) Online Publication Date: 28 May 2010
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In this paper, a novel optimization algorithm, named group search optimizer (GSO), is used to truss structure topology optimization. The group search optimizer is improved in two aspects which including using harmony memory and adhering to the boundary. Two topology methods, such as heuristic topology and discretization of topology variables, are incorporated with GSO to make sure that the topology optimization works well. In the end of the paper, two numerical examples were used to test the improved GSO. Calculation results show that the improved GSO is feasible and robust for truss topology optimization. |
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Sandia National Laboratories
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