Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P40: Surface Effects, Thermodynamics and Fabrication |
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Sponsoring Units: DCMP Chair: David Singh, Oak Ridge National Laboratory Room: Morial Convention Center 232 |
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P40.00001: Surface core-level shifts and atomic coordination at the W(320) surface Xubing Zhou, J.L. Erskine, O. Kizilkaya High resolution 4 $f_{7/2}$ Core-level photoemission spectra are reported from the W(110) and from the related vicinal W(320) surfaces. Curve fittings of the spectra permit tests of core-level binding-energy shift models that relate local atomic coordination to binding -energy differences associated with (for example) terrace and step-edge atoms. A well-resolved shoulder on the W(320) surface peak is attributed to step-edge atoms and contributions from surface atoms having higher atomic coordination are obtained from the curve-fitting exercises. The results are discussed in relation to prior core-level measurements, tight-binding models, and ab-initio calculations of core-level shifts for W(320). [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P40.00002: Secondary electron spectra of Au and Cu under bombardment by very low energy positrons S. Mukherjee, A.H. Weiss, M.P. Nadesalingam, P. Guagliardo, A. Sergeant, J. Williams Measurements of the secondary electron energy spectra resulting from very low energy positron bombardment of a polycrystalline Au and Cu (100) surfaces are presented. The low energy part of the secondary spectra contain significant contributions from two processes: 1. annihilation induced Auger electrons that have lost energy before leaving the surface and 2. secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions ($\sim $3eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P40.00003: Fermi-liquid effects in propagation of low frequency electromagnetic waves through thin metal films Natalya Zimbovskaya, Grigory Zimboskiy In the present work we theoretically analyze the contribution from a transverse Fermi-liquid collective mode to the transmission of electromagnetic waves through a thin film of a clean metal in the presence of a strong external magnetic field. We show that at the appropriate Fermi surface geometry the transverse Fermi-liquid wave may appear in conduction electrons liquid at frequencies $ \omega $ significantly smaller than the cyclotron frequency of charge carriers $ \Omega $ provided that the mean collision frequency $ \tau^{-1}$ is smaller than $\omega. $ Also, we show that in realistic metals size oscillations in the transmission coefficient associated with the Fermi-liquid mode may be observable in experiments. Under certain conditions these oscillations may predominate over the remaining size effects in the transmission coefficient. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P40.00004: Ground-state structure of the hydrogen double vacancy on Pd(111) Sungho Kim, Seong-Gon Kim, Steven Erwin We determine the ground-state structure of a double vacancy in a hydrogen monolayer on the Pd(111) surface. We represent the double vacancy as a triple vacancy containing one additional hydrogen atom. The potential-energy surface for a hydrogen atom moving in the triple vacancy is obtained by density-functional theory, and the wave function of the fully quantum hydrogen atom is obtained by solving the Schr\"odinger equation. We find that an H atom in a divacancy defect experiences significant quantum effects, and that the ground-state wave function is centered at the hcp site rather than the fcc site normally occupied by H atoms on Pd(111). Our results agree well with scanning tunneling microscopy images. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P40.00005: Band Mapping in Higher-Energy X-Ray Photoemission: Phonon Effects and Comparison to One-Step Theory Jan Minar, Lukasz Plucinski, Brian Sell, Juergen Braun, Hubert Ebert, Claus Schneider, Charles Fadley In view of the present interest in more bulk sensitive band mapping via x-ray photoemission, we have studied the temperature dependence of W(110) angle-resolved spectra excited at photon energies of 260, 870 eV, and 1254 eV and between 300K and 780K. Experimental results are compared to both a free-electron final-state model and theoretical one-step model calculations. At 300K, clear band dispersions can be observed in the data. The ratio between direct and non-direct transitions is approximately estimated from a Debye-Waller factor. One-step theoretical calculations reproduce well band dispersions and matrix element effects in the measured spectra at room temperature, but including phonon effects via complex phase shifts does not predict density-of-states related features observed in higher-temperature spectra. We will also discuss the implications of this work for future experiments on other materials and at even higher photon energies up to 10 keV. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P40.00006: Electronic Gr\"{u}neisen Parameter In Paramagnetic Nickel Shouhua Nie, Xuan Wang, Junjie Li, Richard Clinite, Mark Wartenbe, Jianming Cao We have conducted the first measurement of electronic Gr\"{u}neisen parameter \textit{$\gamma $}$_{e}$ in the paramagnetic state of ferromagnetic transition metal nickel by monitoring the laser-induced ultrafast stress dynamics using femtosecond electron diffraction. This method overcomes the restriction of traditional low-temperature methods and offers a unique path to study electronic thermal expansion in magnetic metals. Our measurement indicates that the local magnetic moment that persists in the paramagnetic state of nickel does not contribute significantly to electronic thermal expansion. This result would serve as an important test of current models regarding the magnetism in ferromagnetic transition metals. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P40.00007: Generalized Surface Thermodynamics of Solids with Application to Nucleation Robert Cammarata J.W. Gibbs formulated a general thermodynamics for surfaces in multicomponent fluid systems. For the case of solid-fluid surfaces, he restricted attention to single component solids. Attempts to generalize Gibbs' results for surfaces between multicomponent solids and fluids are problematic owing to the difficulty that the surface chemical potentials in the solid are generally not well defined, and therefore any expressions involving these chemical potentials will also not be well defined. A formulation of a general surface thermodynamics that can take into account capillary effects in systems involving interfaces between multicomponent solids and fluids while avoiding the aforementioned difficulties will be discussed that utilizes the concept of thermodynamic availability. It will be shown how this approach allows Gibbs-Thomson effects for finite size solids, an adsorption equation for a solid surface, and the thermodynamics of nucleation during solidification to be treated in a straightforward manner that avoids all references to ill defined surface quantities. In particular, a derivation will be presented that is the first to properly generalize Gibbs' analysis for the reversible work of nucleation to the case of solidification. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P40.00008: Controlling precipitate growth in aluminum rich alloys via externally applied stress Jack Franklin, Jennifer Lukes The material properties of metallic alloys are directly influenced by their microstructure. The final microstructure of bulk specimens is currently determined through specific heat treatments designed to control the homogenous precipitation of secondary phases from a saturated matrix. This talk will introduce a novel stress-based method of controlling the precipitation and directed growth of secondary phases to create desired microstructures on the surface of an aluminum-copper alloy. Microstructures obtained under different stress conditions will be presented and the mechanisms leading to their formation will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P40.00009: Bulk nanostructured alloys by large strain extrusion machining Wilfredo Moscoso, Christopher Saldana, Jon Madariaga, Ravi Shankar, Srinivasan Chandrasekar, Dale Compton Large strain extrusion machining (LSEM) is presented as a method of severe plastic deformation for the creation of bulk nanostructured materials in a wide range of metal alloys. This method combines inherent advantages afforded by large strain deformation in chip formation by machining, with simultaneous dimensional control of extrusion in a single step of deformation. Bulk nanostructured materials in the form of foils, plates, and bars of controlled dimensions are shown to result by appropriately controlling the geometric parameters of the deformation in large strain extrusion machining. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P40.00010: Engineering materials-design parameters of the Mg-Li Alloy System from ab initio calculations William Counts, Martin Friak, Dierk Raabe, Jorg Neugebauer Ab initio calculations are becoming increasingly useful to engineers interested in designing new alloys because these calculations are able to accurately predict basic material properties only knowing the atomic composition of the material. Fundamental physical properties (like formation energy and elastic constants) of 11 bcc magnesium-lithium alloys were calculated using density-functional theory (DFT) and compared with available experimental data. These DFT determined properties were in turn used to calculate engineering parameters like the bulk modulus/shear modulus (B/G) and Young's modulus/density (E/$\rho )$. From these engineering parameters, alloys with optimal mechanical properties need for a light weight structural material were identified. It was found that the stiffest bcc magnesium-lithium alloys contain about 70 at.{\%} magnesium while the most ductile alloys have 0-20 at.{\%} magnesium. In addition, the specific modulus for alloys with 70 at.{\%} magnesium was found to be equal to that of aluminum-magnesium alloys and slightly lower than that of aluminum-lithium alloys. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P40.00011: Testing the MacPherson-Srolovitz Theory in Simulations of 3D Grain Growth Anthony Rollett, Fatma Uyar, Seth Wilson, Jason Gruber, Sukbin Lee The theory by MacPherson and Srolovitz provides an exact prediction of the growth rate of individual cells or grains in a space-filling network (microstructure). Testing the predictions of the theory requires measurement of mean width and edge lengths where three cells meet at triple line junctions. This is most easily accomplished in networks that are discretized with a mesh. A Moving Finite Element (MFE) model was used to simulate the evolution (grain growth) over short times of a network discretized on a tetrahedral mesh and growth rates. Volumes, mean widths and edge lengths were measured. The growth rates measured from the simulation were found to be in very good agreement with the predictions of the MacPherson-Srolovitz theory. The results from similar measurements in Monte Carlo and Phase Field models of grain growth will also be reported. In this case, measurement of mean width and edge length is complicated by use of a regular grid to discretize the network on a set of points or voxels. A modified algorithm by Ohser and M\"ucklich is used to measure mean width. Edge length measurement along triple lines requires conversion of the voxel image to a surface mesh. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P40.00012: Phase-field Simulation for Analyzing Time Evolution of Grains of Precipitate in Phase Decomposition Processes of Magnetic Alloys Yoshiharu Kanegae The time evolution of the average grain size and the number of grains of precipitate in phase decomposition processes of magnetic alloys in two-dimensional systems was investigated using phase-field simulation. Specifically, the phase decomposition processes of Fe-Cu and Fe-Cr systems were studied. Chemical free energy from a thermodynamic database of phase diagrams was used and magnetic contribution was considered. In appropriate compositions, these systems show spinodal decomposition followed by Ostwald ripening. In the long-time region of these processes, the time evolution of the average grain size and the number of grains of precipitate was evaluated by the power of time. The exponent of the power of time of the average grain size \textit{$\alpha $} was \textit{$\alpha $}$\sim $1/3, consistent with the Lifshitz-Slyozov-Wagner theory. On the other hand, that of the number of grains \textit{$\beta $} was \textit{$\beta $}$\sim $-2/3, inconsistent with the theory, which predicts \textit{$\beta $}$\sim $-1. However, it was shown that in two-dimensional systems these results are reasonable. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P40.00013: Modeling Digestive and Ostwald Ripening of Nanocrystals Michael Tambasco, Sanat Kumar Ostwald and digestive ripening are two diametrically opposite phenomena that dramatically impact nanocrystal polydispersity. Ostwald ripening allows for large nanocrystals to grow at the expense of small ones, while digestive ripening involves the propagation of small nanocrystals at the cost of the larger. A detailed theoretical description of these two processes would aid in developing techniques that control nanocrystal size and polydispersity; however, there currently exists no a-priori means of describing the physics of these two processes. Here, we present an approach that is capable of characterizing both types of phenomena. We apply a mean field theory in order to model the role that ligands play in the ripening processes. We examine the effects of ligand concentration and chain length on the average size and size distribution of gold nanocrystals. We then employ potentials derived from the theoretical results in Monte Carlo simulations of the ripening processes in order to study the physics of the ripening phenomena. [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P40.00014: Mean-field Theory of Multicomponent Phase Coarsening Ke-Gang Wang Study of phase coarsening in multicomponent systems is rare. Morral and Purdy developed a general theoretical frame for phase coarsening in n-component alloys. However, all work considered only the effect of solution thermodynamics, and ignored the kinetic effect from non-zero volume fraction. Therefore, all studies are valid only in the case of vanishing volume fraction. When $V_V$ is not zero, the interactions among precipitates exist. The diffusion screening length is used to describe these interactions, and it is found that the growth rate of particle depends on the volume fraction through diffusion screening length. A mean-field theory of multicomponent phase coarsening will be presented, which includes effects of both multicomponent thermodynamics and kinetics from nonzero volume fraction. [Preview Abstract] |
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