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Ames, United States

Mendelev M.I.,Ames Laboratory
Modelling and Simulation in Materials Science and Engineering

A specially designed semi-empirical potential of the FinnisSinclair type was used to simulate the phase transformation in a disordered one-component system. The potential provides that the face-centered cubic (fcc) phase is the most stable phase in the system below the melting temperature, T m; however, the potential does not lead to the fcc nucleation during molecular dynamics (MD) simulation, allowing studying the liquidglass transformation. The potential also allows studying the fcc-liquid and fcc-glass interface migration. It was found that the liquidglass transformation described by this potential is of the first order. The WilsonFrenkel theory of the solidliquid interface (SLI) migration satisfactory describes the results of the MD simulation in the temperature interval from 0.55T m to T m while the BroughtonGilmerJackson theory is less accurate in describing the temperature dependence of the SLI velocity in the same temperature interval. Below 0.55T m, the results of the MD simulation strongly depend on how the disordered phase model was prepared and none of the existing theories is capable of reproducing the temperature dependence of the interface velocity. © 2012 IOP Publishing Ltd. Source

Jaynes D.B.,Ames Laboratory
Precision Agriculture

While numerous researchers have computed economically optimal nitrogen rate (EONR) values from measured yield-N rate data, nearly all have neglected to compute or estimate the statistical reliability of these EONR values. In this study, a simple method for computing EONR and its confidence bands is described and demonstrated. The method is illustrated for seven yield response functions, namely, the linear plateau, quadratic, quadratic plateau, square root quadratic, spherical plateau, and exponential and exponential plateau. Only the quadratic and square root quadratic functions are linear in their parameters, with least squares regression yielding parameters that are normally distributed. The other five functions are non-linear and give parameter estimates that are non-normal and biased in their distribution when fit by least squares. Prior to computing EONR distributions, the non-linear functions were reparameterized to give fitted parameters that were nearly unbiased and normally distributed. EONR distributions were computed using a Monte Carlo method to generate 1,000 realizations of EONR based on the fitted response function parameters. From the 1,000 realizations, the expectation, confidence bands, and cumulative probability distributions for EONR were easily computed. Applying the approach to six yield data sets from the literature illustrated that the 68% confidence bands for computed EONR can span several tens of kilograms per hectare and are typically skewed about the expectation. There were considerable differences among the distributions of EONR computed from the seven functions, with the cumulative probability distributions sometimes not overlapping. Given the limited statistical reliability possible for EONR, it is essential that confidence bands always be reported when EONR values are computed from yield data. © 2010 US Government. Source

Kogan V.G.,Ames Laboratory
Physical Review B - Condensed Matter and Materials Physics

It is argued on the basis of the BCS theory that the zero-T penetration depth satisfies λ-2(0)â̂σTc (σ is the normal state dc conductivity) not only in the extreme dirty limit ξ0/ℓâ‰1, but in a broad range of scattering parameters down to ξ0/ℓ∼1 (ξ0 is the zero-T BCS coherence length and ℓ is the mean free path). Hence, the scaling λ-2(0)â̂σTc, suggested as a new universal property of superconductors, finds a natural explanation within the BCS theory. © 2013 American Physical Society. Source

Shaw S.,Iowa State University | Cademartiri L.,Iowa State University | Cademartiri L.,Ames Laboratory
Advanced Materials

Unique properties (e.g., rubber elasticity, viscoelasticity, folding, reptation) determine the utility of polymer molecules and derive from their morphology (i.e., one-dimensional connectivity and large aspect ratios) and flexibility. Crystals do not display similar properties because they have smaller aspect ratios, they are rigid, and they are often too large and heavy to be colloidally stable. We argue, with the support of recent experimental studies, that these limitations are not fundamental and that they might be overcome by growth processes that mimic polymerization. Furthermore, we (i) discuss the similarities between crystallization and polymerization, (ii) critically review the existing experimental evidence of polymer-like growth kinetic and behavior in crystals and nanostructures, and (iii) propose heuristic guidelines for the synthesis of "polymer-like" crystals and assemblies. Understanding these anisotropic materials at the boundary between molecules and solids will determine whether we can confer the unique properties of polymer molecules to crystals, expanding them with topology, dynamics, and information and not just tuning them with size. Unique properties determine the utility of polymer molecules and derive from their morphology and flexibility. Crystals do not display those properties because they have smaller aspect ratios and are stiffer. We argue that these limitations might be overcome by growth processes of controlled aggregation that mimic polymerization. This rising area of interest is discussed and recent experimental evidence reviewed. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

King A.H.,Ames Laboratory
Scripta Materialia

We assess the impact of triple lines in materials preparation and use by considering several examples of materials behavior in which they have identifiable effects. The microstructural roles of triple lines are also considered and some persistent scientific questions are raised. © 2010 Acta Materialia Inc. Source

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