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Top 20 Most Read Articles
April 2011
The 20 articles with the most full-text downloads during the month, in descending order.
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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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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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E‐Waste Management and Challenges AIP Conf. Proc. 1324, pp. 217-220; doi:http://dx.doi.org/10.1063/1.3526198 (4 pages) Online Publication Date: 3 December 2010
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E‐Waste is one of the silent degraders of the environment in the fast‐growing world. This paper explores briefly the ultra‐modern problem of E‐Waste. After enumerating the causes and effects of the E‐Waste, it focuses on management of the E‐waste using modern techniques. The paper also deals with the responsibilities of the governments, industries and citizens in reducing E‐waste. |
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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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A constitutive model for multi‐phase steels AIP Conf. Proc. 1315, pp. 3-8; doi:http://dx.doi.org/10.1063/1.3552477 (6 pages) Online Publication Date: 27 January 2011
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Mean‐Field homogenization algorithms for materials involving two or more elastic‐plastic constituent phases are investigated. The Voigt, Reuss and Self consistent schemes which are directly applicable to multi‐phase systems are implemented. The shortcomings of these schemes are accuracy for the former two and computational efficiency for the latter. A new interpolative model is proposed which is aimed to be both computationally efficient and accurate. The results of the models are studied on the material point level for a prescribed uniaxial tensile deformation. It is observed that the response computed by the proposed scheme closely matches that computed by the Self Consistent approach. |
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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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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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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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A novel criterion for determination of material model parameters AIP Conf. Proc. 1353, pp. 23-28; doi:http://dx.doi.org/10.1063/1.3589486 (6 pages) Online Publication Date: 25 April 2011
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Parameter identification problems have emerged due to the increasing demanding of precision in the numerical results obtained by Finite Element Method (FEM) software. High result precision can only be obtained with confident input data and robust numerical techniques. The determination of parameters should always be performed confronting numerical and experimental results leading to the minimum difference between them. However, the success of this task is dependent of the specification of the cost/objective function, defined as the difference between the experimental and the numerical results. Recently, various objective functions have been formulated to assess the errors between the experimental and computed data (Lin et al., 2002; Cao and Lin, 2008; among others). The objective functions should be able to efficiently lead the optimisation process. An ideal objective function should have the following properties: (i) all the experimental data points on the curve and all experimental curves should have equal opportunity to be optimised; and (ii) different units and/or the number of curves in each sub‐objective should not affect the overall performance of the fitting. These two criteria should be achieved without manually choosing the weighting factors. However, for some non‐analytical specific problems, this is very difficult in practice. Null values of experimental or numerical values also turns the task difficult. In this work, a novel objective function for constitutive model parameter identification is presented. It is a generalization of the work of Cao and Lin and it is suitable for all kinds of constitutive models and mechanical tests, including cyclic tests and Baushinger tests with null values. |
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Transport properties of doped GeSn alloys AIP Conf. Proc. 1199, pp. 57-58; doi:http://dx.doi.org/10.1063/1.3295552 (2 pages) Online Publication Date: 15 January 2010
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We present a systematic study of the transport properties of n‐ and p‐type Ge0.98Sn0.02 alloys using infrared spectroscopic ellipsometry and electrical measurements. We measure the dielectric function of our samples in the infrared range where the response is mainly due to free carrier absorption. In the case of p‐type material, we observe, in addition to the free carrier response, optical transitions between split‐off (SO), light (LH), and heavy‐hole (HH) bands. The electron and hole mobilities for Ge0.98Sn0.02 alloys with carrier concentrations >1018 cm−3 are comparable to those found in Ge samples with similar doping concentrations. The electron and hole effective masses of Ge0.98Sn0.02 alloys are close to that of n‐doped and p‐doped Ge respectively. |
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A Three‐Dimensional Ultraviolet Curable Nanoimprint Lithography (3D UV‐NIL) AIP Conf. Proc. 1151, pp. 114-117; doi:http://dx.doi.org/10.1063/1.3203214 (4 pages) Online Publication Date: 24 July 2009
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This study investigates the use of ultraviolet nanoimprint lithography (UV‐NIL) for patterning three dimensional (3D) structures. Generating the 3D structures is a challenging task especially on an insulating substrate such as quartz. The quartz transparent mold is essential for the UV‐NIL process. The 3D profiles were created on the negative tone photoresist, Microresist ma‐N2403 using a Raith‐ 150 electron beam lithography (EBL) tool in a single step variable dose controlled exposure. The developed 3D resist profiles were subsequently utilised as the 3D masking layer. The 3D patterns were transferred into the quartz mold substrates by a single‐step reactive ion etching (RIE). Surface roughness below 2 nm has been achieved when the RIE process pressure is lower than 6 mTorr. The replication of the 3D mold structure by using the UV‐NIL technique requires a two‐step imprint process. The master mold profile was replicated onto a Microresist Ormocomp US‐S4 resist on the first imprint to become the soft mold. The cured Ormocomp soft mold, was subsequently used as a mold for replicating the 3D pattern structures on the Microresist mr‐UVCur06 resist in the second imprint step to create a positive replica of the original mold. A test pattern of a 3D pyramid‐shaped array with multilevel features was successfully replicated using this technique. This paper reviews our recent work on 3D UVNIL mold making and imprinting processes. |
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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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Relativistic calculations for many electron atoms AIP Conf. Proc. 136, pp. 162-175; doi:http://dx.doi.org/10.1063/1.35499 (14 pages) Online Publication Date: 29 May 2008
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Many improvements have now been introduced in ab initio methods for relativistic atomic structure calculations. After a short description of the different methods, we review the various contributions to energy levels and compare the most recent theoretical and experimental results for few electron heavy ions. |
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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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Fabrication of Metal Particles Embedded Aluminum Matrix Composite by Friction Stir Processing (FSP) AIP Conf. Proc. 1315, pp. 51-56; doi:http://dx.doi.org/10.1063/1.3552499 (6 pages) Online Publication Date: 27 January 2011
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Conventional metal matrix composites (MMCs) suffer from the disadvantage of low ductility. In order to overcome this, reinforcing the metal matrix with metal particles can be taken as an alternative approach. However, processing such composites can pose serious challenges as the metal particles can either go in to solution or form undesirable intermetallics during processing through conventional routes. Friction stir processing (FSP) is emerging as a versatile tool for processing and modification of variety of materials. In the present study, metal particulate reinforced aluminum matrix composite was processed by incorporating nickel particles through friction stir processing (FSP) in one step. The microstructure was characterized by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). SEM observations revealed that particles are uniformly dispersed in the aluminum matrix with excellent interfacial bonding. FSP also lead to grain refinement of the matrix as observed by EBSD and TEM analysis. Moreover, no harmful Al‐Ni intermetallics formed in the matrix. The mechanical properties were determined by tensile tests to evaluate the effect of metal particulate reinforcement. The novel feature of the composite is that it exhibits a threefold increase in the yield stress (0.2% proof stress) while appreciable amount of ductility is retained. |
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Hadron Spectroscopy in Double Pomeron Exchange Processes with the STAR Detector at RHIC AIP Conf. Proc. 1257, pp. 53-58; doi:http://dx.doi.org/10.1063/1.3483389 (6 pages) Online Publication Date: 16 August 2010
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A new hadron spectroscopy program to study particle production at the central region through diffractive processes in polarized p+p collisions at  = 200–500 GeV has been initiated with the STAR detector at RHIC. Staged implementation of multiple Roman Pot stations for tagging forward protons in diffractive processes allows studying for the first time the dynamics of the particle production in the diffractive processes systematically at the RHIC energy regime. The STAR detector system with complete azimuthal coverage at the central region equipped with excellent particle reconstruction capabilities enables studies of the constituent gluonic degree of freedom in double Pomeron exchange processes. |
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PEM Fuel Cells for Transport Applications: State of the Art and Challenges AIP Conf. Proc. 1169, pp. 3-12; doi:http://dx.doi.org/10.1063/1.3243269 (10 pages) Online Publication Date: 17 September 2009
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In order to offer a true alternative to the internal combustion engine, whether fuelled with today’s fossil fuels or with first and second generation biofuels, the fuel cell technology needs to mature to such a level that it meets consumer expectations with respect to vehicle performance, driving range and refueling time, while at the same time vehicle efficiency and well to tank emissions are such that overall emissions can be diminished drastically. The present paper addresses the present state of the art of fuel cell technology (PEMFC—proton exchange membrane fuel cells) for transportation, and the materials issues for both the short term and the long term that need to be addressed to fulfill the expectations. |
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Jacob’s ladder of density functional approximations for the exchange-correlation energy AIP Conf. Proc. 577, pp. 1-20; doi:http://dx.doi.org/10.1063/1.1390175 (20 pages)
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The ground-state energy and density of a many-electron system are often calculated by Kohn-Sham density functional theory. We describe a ladder of approximations for the exchange-correlation energy as a functional of the electron density. At the lowest rung of this ladder, the contribution to the energy from a volume element of 3-dimensional space is determined by the local density there. Higher rungs or levels incorporate increasingly complex ingredients constructed from the density or the Kohn-Sham orbitals in or around this volume element. We identify which additional exact conditions can be satisfied at each level, and discuss the extent to which the functionals at each level may be constructed without empirical input. We also discuss the research that remains to be done at the exact-exchange level, and present our “dreams of a final theory.” “Jacob left Beer-sheba and went toward Haran. He came to a certain place and stayed there for the night, because the sun had set. Taking one of the stones of the place, he put it under his head and lay down in that place. And he dreamed that there was a ladder set up on the earth, the top of it reaching to heaven; and the angels of God were ascending and descending on it.” © 2001 American Institute of Physics. |
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Ductile damage parameters identification for cold metal forming applications AIP Conf. Proc. 1353, pp. 47-52; doi:http://dx.doi.org/10.1063/1.3589490 (6 pages) Online Publication Date: 25 April 2011
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Ductile damage mechanics is essential to predict failure during cold metal forming applications. Several damage models can be found in the literature. These damage models are coupled with the mechanical behavior so as to model the progressive softening of the material due to damage growth. However, the identification of damage parameters remains an issue. In this paper, an inverse analysis approach is set‐up to identify ductile damage parameters, based on different kind of mechanical tests and observables. The Lemaitre damage model is used and damage is coupled with the material behavior. The Efficient Global Optimization (EGO) method is used in a parallel environment. This global algorithm based on kriging meta‐model enables the identification of a set of damage parameters based on experimental observables. Global and local observables are used to identify these parameters and a special attention is paid to the computation of the cost function. Finally, an identification procedure based on displacement field measurements is presented and applied for damage parameters identification. |
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