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Los Alamos, NM, United States

McCafferty E.,Research Applications Corporation
Electrochimica Acta

Expressions are derived to calculate the equilibrium oxidation-reduction potentials for the Al+3/Al, Cu+2/Cu, and Zn+2/Zn systems in small-scale electrolytes. The geometrical system consists of a droplet of electrolyte resting on a flat metal plate, and the metal is considered to be immersed in a solution of its own ions. When the radius of the drop is allowed to vary, both the size of the electrolyte and the size of the active metal beneath the droplet change simultaneously. The total free energy change for the system consists of both electrochemical and surface chemical contributions. The interfacial free energy for the solid/liquid interface has been estimated from the Girifalco-Good expression or from spreading pressure considerations. When the droplet becomes sufficiently small in radius, the surface chemical contributions become significant, and the calculated redox potential changes from its normal value to more negative values as the size of the system decreases. The magnitude of this effect depends on the particular system. For 2 M Cu+2, the calculated redox potential for a 0.8 nm radius droplet is 0.259 V more negative than for the bulk electrolyte. The effect is much smaller for aluminum and zinc. In all three systems, calculated redox potentials approach values for the bulk solution for droplet radii of about 10 nm. © 2010 Elsevier Ltd. All rights reserved. Source

McCafferty E.,Research Applications Corporation
Journal of Applied Electrochemistry

The free energy change is calculated for the interaction of 19 different oxyanions (metalates) with iron (steel) or aluminum surfaces. The oxyanions considered here are those of the transition metals in the fourth through sixth periods of the periodic table. The oxyanions which produce more negative values of ΔG o (per mole of oxyanion) than that of chromate (CrO 4-2) are permanganate (MnO4-), nickelate (NiO4-2), ruthenate (RuO4- or RuO4-2), and rhodate (RhO4-2). The oxyanions which produce values of ΔG o (per mole of oxyanion) similar to CrO4-2 are osmate (OsO 4-2), and iridate (IrO4-2). © 2009 Naval Research Laboratory, Washington DC. Source

McCafferty E.,Research Applications Corporation
Journal of the Electrochemical Society

The pitting corrosion of aluminum is modeled as a queueing process in which chloride ions (customers or units) enter the passive film and are serviced (form an emerging pit) at certain sites (servers) at the oxide/metal interface. X-ray photoelectron spectroscopy (XPS) data show that chloride ions penetrate the passive film on aluminum and are considered to lie in a queue. The servicing (or processing) stage includes chloride-assisted localized dissolution of aluminum, development of local acidity within the emerging pit, and buildup of chloride ions within the developing pit. The arrival rate (λ) of chloride ions is determined from quantitative XPS intensity data. The servicing rate (μ) has been determined from anodic polarization curves and also from Scully's criterion for stable pit growth. The following queueing characteristic parameters have been calculated: the number of units (Lq) in the queue, the number of units (L) in the system (queue plus server), the average time (Wq) a unit spends in the queue, and the average time (W) a unit spends in the system (queue and server). This is done for the following queueing systems: single queue with a single server, single queue with multiple servers, and multiple queues with single or multiple servers. A long time (W) in the system corresponds to a low rate for the ongoing corrosion reaction. The time (W) that a unit spends in the system is maximized for (i) a decrease in the number of servers, (ii) a decrease in the servicing rate μ, (iii) a decrease in the number of queues, or (iv) an increase in the arrival rate (up to λ=μ). © 2010 The Electrochemical Society. Source

Lewis A.C.,United States Naval Research Laboratory | Qidwai S.M.,United States Naval Research Laboratory | Geltmacher A.B.,Research Applications Corporation
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Abstract: To determine statistically relevant microstructure-yield correlations in three-dimensional (3-D) microstructures, large volumes comprised of many grains must be studied. With the aim of limiting computational loads without reducing the fidelity of the volume being simulated, this work investigates the use of reduced constitutive parameters, specifically the number of available slip systems, to analyze initial plastic flow in the microstructure. This is performed by embedding a 3-D reconstruction of a single-phase beta-Ti microstructure in a finite element (FE) computational model and subjecting it to a number of loading conditions. Three separate singlecrystal plasticity formulations were used for each loading: reduced 12 slip systems (<111>{110} family), reduced 24 slip systems (<111>{110}+<111>{112} families), and full 48 slip systems (<111>{110}+<111>{112}+<111>{123} families). The analysis results show that the 24-slipsystem model accurately predicts the global stress-strain behavior and locations of initial yield under all loadings, with no more than 10 pct error in the spatial description of local state variables compared to the full 48-slip-system model. The 12-slip-system model generally follows the full model predictions and provides an even better cost improvement, but with errors in excess of 40 pct in local descriptions. Computational cost and data reduction are improved by 26 and 53 pct, respectively. © The Minerals, Metals & Materials Society and ASM International 2010. Source

Van Laethem F.,U.S. National Institutes of Health | Tikhonova A.N.,U.S. National Institutes of Health | Tikhonova A.N.,New York University | Pobezinsky L.A.,U.S. National Institutes of Health | And 9 more authors.

Thymic selection requires signaling by the protein tyrosine kinase Lck to generate T cells expressing αβ T cell antigen receptors (TCR). For reasons not understood, the thymus selects only αβTCR that are restricted by major histocompatibility complex (MHC)-encoded determinants. Here, we report that Lck proteins that were coreceptor associated promoted thymic selection of conventionally MHC-restricted TCR, but Lck proteins that were coreceptor free promoted thymic selection of MHC-independent TCR. Transgenic TCR with MHC-independent specificity for CD155 utilized coreceptor-free Lck to signal thymic selection in the absence of MHC, unlike any transgenic TCR previously described. Thus, the thymus can select either MHC-restricted or MHC-independent αβTCR depending on whether Lck is coreceptor associated or coreceptor free. We conclude that the intracellular state of Lck determines the specificity of thymic selection and that Lck association with coreceptor proteins during thymic selection is the mechanism by which MHC restriction is imposed on a randomly generated αβTCR repertoire. © 2013 Elsevier Inc. Source

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