Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q24: Focus Session: Multiscale Modeling: Heterogeneous Systems and Interfaces |
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Sponsoring Units: DCOMP DMP Chair: Srinivasan Srivilliputhur, University of North Texas Room: D167 |
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q24.00001: Kinetic Monte Carlo with fields: diffusion in heterogeneous systems Invited Speaker: It is commonly perceived that to achieve breakthrough scientific discoveries in the 21$^{st}$ century an integration of world leading experimental capabilities with theory, computational modeling and high performance computer simulations is necessary. Lying between the atomic and the macro scales, the meso scale is crucial for advancing materials research. Deterministic methods result computationally too heavy to cover length and time scales relevant for this scale. Therefore, stochastic approaches are one of the options of choice. In this talk I will describe recent progress in efficient parallelization schemes for Metropolis and kinetic Monte Carlo [1-2], and the combination of these ideas into a new hybrid Molecular Dynamics-kinetic Monte Carlo algorithm developed to study the basic mechanisms taking place in diffusion in concentrated alloys under the action of chemical and stress fields, incorporating in this way the actual driving force emerging from chemical potential gradients. Applications are shown on precipitation and segregation in nanostructured materials. Work in collaboration with E. Martinez, LANL, and with B. Sadigh, P. Erhart and A. Stukowsky, LLNL. Supported by the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy (Award {\#} 2008LANL1026) at Los Alamos National Laboratory \\[4pt] [1] B. Sadigh et al. to be published \newline [2] E. Martinez et al. J. Comp. Phys. 227 (2008) 3804-3823 [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q24.00002: On the atomic-scale design of metal-metal heterointerfaces Invited Speaker: I will describe a multiscale modeling effort to understand and control the properties of heterointerfaces in metal-metal nanocomposites, using their effect on radiation response as an example. For selected model interfaces, atomistic simulations are used to characterize interface structure and to determine the mechanisms of interface-point defects interactions, including trapping, diffusion, and defect reactions. This information is then incorporated into mesoscale dislocation-based and continuum approaches to investigate the steady-state interface response to radiation-induced defect fluxes. With insights gained from studying this ``forward'' problem of predicting radiation response of selected model interfaces, one may attempt to solve the ``inverse'' problem of determining what interfaces will yield desired radiation response. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q24.00003: Atomistic Mechanism of Kinking in the Vapor-Liquid-Solid Growth of Silicon Nanowires Seunghwa Ryu, Wei Cai Understanding the atomistic growth mechanism of semiconductor nanowires from the catalytic droplet is important for better control of the shape and orientation of nanowires deposited through the Vapor-Liquid-Solid (VLS) process. Kinking is a frequently observed event, in which the nanowire suddenly changes the growth orientation. This behavior is usually undesirable, but can also be explored to grow nanowires of complex shapes if it can be controlled. Unfortunately, the atomistic origin of kinking is not well understood. We employ advanced sampling methods to compute the probability of the orientation change during VLS growth. Several growth directions and nanowire diameters are simulated at 1000 K. The simulation uses a recently developed Au-Si inter-atomic potential fitted to the experimental binary phase diagram. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q24.00004: Prediction of the anisotropic properties of energetic materials at elevated pressures and temperatures Oscar Ojeda, Tahir Cagin Localization of strain and changes under extreme conditions in energetic materials (EM) can cause runaway reactions and unexpected initiation. A clear understanding of the mechanical properties is a perquisite in understanding the interplay between mechanical, chemical and thermodynamic properties that relate sensitivity and EM's before they undergo initiation. We have conducted first principles ground state studies, complemented by atomistic calculations at elevated temperatures and pressures, for energetic commonly used secondary EM's with varying sensitivities. Chemical information found from ab intio methods, and from compression at elevated temperatures show that external conditions relevant to impact and shock behavior can have different effects on the studied systems. These range from changes in local conformation, changes in the hydrogen-bonding network, and more drastically to a full crystallographic transition in which the symmetry of the system undergoes a transformation. Due to the chemical, mechanical and thermodynamic level information that provides, multiscale modeling methods, can then be applied to the understanding of other type of systems and give a clearer understanding of the molecular processes that undergo energetic materials, prior to initiation. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q24.00005: Migration energies of native defects and fission products in uranium dioxide Alexander Thompson, Chris Wolverton Despite the importance of fission products like Xe in nuclear fuels, the mechanism of how these atoms diffuse in the lattice is not known. In an effort to identify this mechanism, we have used density functional theory as well as a variety of different classical potentials for to study the migration energies of a variety of atomic steps in UO2, with and without Xe impurities and native defects. We find that the classical potential of Basak gives results which compare favorably with density functional theory for the diffusion of a Schottky defect cluster. We observe a new path for xenon-tetravacancy (a UO2 Schottky defect plus an additional U vacancy) motion using molecular dynamics. This path has a lower energy barrier than previously reported xenon-tetravacancy paths. We examine the possibility of a uranium vacancy dissociating from the xenon-tetravacancy cluster and find that large barriers for this dissociation. We also calculate xenon-double Schottky defect migration and find it has a slightly larger barrier than xenon-tetravacancy motion with the oxygen vacancies being weakly bound to the defect. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q24.00006: Interface Mediated Nucleation and Growth of Dislocations in fcc-bcc nanocomposite Ruifeng Zhang, Jian Wang, Irene J. Beyerlein, Timothy C. Germann Heterophase interfaces play a crucial role in determining material strength for nanostructured materials because they can block, store, nucleate, and remove dislocations, the essential defects that enable plastic deformation. Much recent theoretical and experimental effort has been conducted on nanostructured Cu-Nb multilayer composites that exhibited extraordinarily high strength, ductility, and resistance to radiation and mechanical loading. In decreasing layer thicknesses to the order of a few tens of nanometers or less, the deformation behavior of such composites is mainly controlled by the Cu/Nb interface. In this work, we focus on the cooperative mechanisms of dislocation nucleation and growth from Cu/Nb interfaces, and their interaction with interface. Two types of experimentally observed Cu/Nb incoherent interfaces are comparatively studied. We found that the preferred dislocation nucleation sites are closely related to atomic interface structure, which in turn, depend on the orientation relationship. The activation stress and energies for an isolated Shockley dislocation loop of different sizes from specific interface sites depend strongly on dislocation size, atomic interface pattern, and loading conditions. Such findings provide important insight into the mechanical response of a wide range of fcc/bcc metallic nanocomposites via atomic interface design. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q24.00007: Atomic and Surface Interactions of Electrode Metals with a p-Type Organometallic Conductor Bhaskar Chilukuri, Thomas Cundari A computational study of the interaction of high and low work function electrode metal atoms (M' = Al, Au, Cu, La, Ni, Pd, Pt, Ru, Ni) used in electronic devices with cyclo-[Au($\mu $-Pz)]$_{3}$ trimer (T) (Pz = pyrazolate ligand), a p-type organometallic semiconductor is presented. Metal (M'$_{M})$ and ligand (M'$_{L})$ sites of the gold trimer are investigated as the possible sites of deposition for the metal atoms. Examination of metal binding, geometric and electronic properties suggest that these metal-based, p-type conductors will form stable interfaces with good electron transfer with typical source/drain electrode metals. Encouraged by the molecular simulation results, we performed periodic interface calculations of metal (001) and (111) surfaces with a monolayer of cyclo-[Au($\mu $-Pz)]$_{3}$ trimer using a plane-wave DFT approach. Structural and electronic properties of metal-trimer interfaces and implications for interface stability and electron transfer will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q24.00008: Structure of charge trapping in cerium-doped aluminophosphate and phosphosilicate glasses: combining molecular dynamics simulations and \textit{ab initio} DFT calculations Leopold Kokou, Yun Li, Jincheng Du Cerium doping glasses find wide applications in optical and photonic devices. Both Ce$^{3+}$ and Ce$^{4+}$ can be present in oxide glasses, and their ratio depends on the glass composition, heat history and melting environment. In either oxidation state, the environments of cerium ions are important to the optical absorption and emission properties. In this paper, we present classical molecular dynamic simulations of cerium-containing aluminosilicate and phosphosilicate glasses using newly developed potential models containing cerium ions. The local environments around Ce$^{3+}$ and Ce$^{4+}$ are studied, and the bond length and coordination of cerium ions are determined. Small samples of the glasses are simulated using MD and then further relaxed with Density Functional Theory (DFT) calculations. Comparison of the structure of glasses from MD and after DFT relaxation is made, and the two are found to be in reasonable agreement. It is found that Ce$^{3+}$ has a longer bond distance and higher coordination number of oxygen. Most interestingly, cerium ions are found to be preferentially coordinated by phosphorus ions in the second coordination shell in the glasses. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q24.00009: Surface Structure and Work Function of ZnO Based on First Principle DFT Calculations Yun Li, JinCheng Du Zinc Oxide is a well known n-type wide band gap semiconductor material and remains actively as a strategic material for various photonic applications. The fabricate ZnO, is effectively used as a sensor in various applications, Because of its high infrared reflectance and high visible transmittance. Due to that fact, its electron property plays vital role and attract our attention. Via simulation method, their electron properties were studied through density function theory. Based on first principle theory, their structures with distinct cleaved planes were obtained and completed relaxed in DFT based methods. Depending on cleaved planes, there were Oxygen or Zinc atoms terminated along (001) direction and both of them locating on the cleaved surface (110). Work function and other electron properties will be discussed in detail for all of them and compared with the experimental values, the difference and prediction will be made. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q24.00010: Coupling Fluctuating Hydrodynamics with Molecular Dynamics at the Nanoscale Nikolaos Voulgarakis, Jhih-Wei Chu Hydrodynamic fluctuations and solvation interactions are essential driving forces of transport phenomena in the micrometer to nanometer regime, including inter- and intra-cellular flows and flows in nanofabricated devices. Although all-atom molecular dynamics (MD) simulations can be used to model molecular fluids, the accessible time- and length-scales are severely limited. Since most of computational cost for MD simulations comes from the represetion of solvent molecules, a possible solution to this limitation is to model fluids with fluctuating hydrodynamics (FHD). While this approach reduces the computational time of MD simulations by three orders of magnitude, an accurate protocol to couple FHD with MD is still necessary. In this work we present a new methodology that couples FHD with MD by allowing the fluctuating fields to directly interact with particles through repulsive, attractive, and dissipating/fluctuating forces without introducing new degrees of freedom or boundary conditions. Numerical results show that solvation energy and diffusion dynamics are correctly described within our framework. Simulations on the collapse of two hydrophobic particles are also presented. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q24.00011: Multiscale Modeling of Solutions Olayinka Olatunji-Ojo, Sandra Boetcher, Thomas Cundari The~sequestration of carbon dioxide is one proposed solution to alleviate the~growing problem of increased atmospheric CO$_{2}$ concentration, and its resulting effect on global climate. However, the efficacy of such methods has yet to be demonstrated. Improved CO$_{2}$ sequestration methods are needed and this can be achieved through a better understanding of the chemical and physical consequences of CO$_{2}$ encapsulation through multiscale modeling. Multiscale modeling is an effective tool for combining different methods thereby creating an efficient way of modeling diverse chemical and physical phenomena. The goal of this research is to model carbon dioxide interactions in solutions from the quantum to continuum level. This is achieved through a combination of DFT calculations, molecular modeling (mesoscale) and computational fluid dynamics (continuum) simulations on CO$_{2}$ + H$_{2}$O. Interaction energies and interatomic distances are obtained from DFT calculations, which are used to derive a Lennard-Jones potential, from which one may obtain continuum properties such as viscosity via reverse non-equilibrium molecular dynamics (RNEMD) simulations. [Preview Abstract] |
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