Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T22: Multiscale Materials (Theory, Modeling and Experiments that Bridge Scales) |
Hide Abstracts |
Sponsoring Units: DCOMP GSCCM Chair: Thomas Sewell, University of Missouri Columbia Room: Portland Ballroom 252 |
Wednesday, March 17, 2010 2:30PM - 3:06PM |
T22.00001: Non-Uniqueness in Energy Minimization of Atomistic and Multiscale Problems: A Branch-Following and Bifurcation Investigation Invited Speaker: Static multiscale and atomistic simulations aim to obtain an equilibrium configuration (local energy minimum) of a body composed of discrete atoms subject to applied loads and/or displacements. Often a system of ``proportional loading'' is considered and the evolution of the body's equilibrium configuration is determined in an incremental fashion as the scalar ``loading parameter'' is varied. At each step, a minimization procedure, such as the conjugate gradient method, is employed using the previous relaxed configuration as an initial guess. A practical problem with such simulations is that due to the highly nonlinear nature of such problems, many equilibrium configurations should be expected. Therefore, the real possibility of multiple competing physical processes is encountered. Unfortunately, the simulation procedure described above provides only one of the possible equilibrium evolutions. Even more troubling is the fact that this one equilibrium evolution will, generally, depend on the numerical energy minimization method employed and the particular values used for its parameters. This work takes a different approach to the exploration of the equilibrium behavior of atomistic and multiscale systems. It performs a \emph{Branch-Following and Bifurcation} (BFB) investigation in order to map out a large number of equilibrium configurations over a wide range of the problem's loading parameter. Once a reasonably complete picture of the system's \emph{possible behaviors} is in hand, it is then possible to interpret these results to draw conclusions about the most likely behavior of the system. To illustrate this novel application of BFB methods to atomistic multiscale problems, some representative problems will be presented including the results for a small ``simple'' atomic slab subjected to axially compressive displacements. The set of possible equilibrium states found is much more complex than first expected and is a vivid illustration of the complex behavior these systems are capable of. These results will be described and some suggestions for ``new'' simulation/interpretation procedures will be discussed. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T22.00002: Electronic structure calculations at macroscopic scales using orbital-free DFT Balachandran Gadaguntla Radhakrishnan, Vikram Gavini Defects play a crucial role in influencing a wide range of material properties and their energetics are determined by the electronic-structure of the core of a defect as well as long-ranged elastic and electrostatic effects. In this work, we present the development of a seamless multi-scale method that enables electronic structure calculations on multi-million atom systems using orbital-free DFT. The key ideas that constitute the method are: (i) a local real-space variational formulation of orbital-free DFT including the recently proposed kernel energies for kinetic energy functionals; (ii) an adaptive coarse-graining of the finite-element basis, which is used to discretize the formulation, retaining full resolution where necessary and coarse-graining elsewhere. We demonstrate the accuracy and effectiveness of the method through studies on mono-vacancy and di-vacancies in aluminum. Our results show remarkable cell-size effects in the energetics of vacancies, and suggest much larger computational domains than those considered previously are necessary in electronic-structure studies on defects. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T22.00003: Quantum Mechanical Simulations of Nanoindentation of Al Thin Film with Mg imurities Qing Peng, Xu Zhang, Chen Huang, Emily A. Carter, Gang Lu QCDFT is a multiscale modeling approach that can simulate multi-million atoms effectively via density functional theory (DFT). The method is based on the framework of quasicontinuum (QC) approach with DFT as its sole energetics formulation. The local QC energy is calculated by DFT with Cauchy-Born hypothesis. The nonlocal QC part is treated by a self-consistent embedding approach, which couples DFT nonlocal atoms to the vertices of finite-elements at the local QC region. The QCDFT method is applied to a nanoindentation study of an Al thin film in the presence and absence of Mg impurities, as well as the cluster of the impurities. The results show that the randomly distributed Mg impurities can significantly increase the ideal and yield strength of the Al thin film, while the Mg impurities clustered in tension field will reduce the yield strength of the Al thin film. [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T22.00004: On calculating the equilibrium structure of molecular crystals Ann E. Mattsson, Ryan R. Wixom, Thomas R. Mattsson The difficulty of calculating the ambient properties of molecular crystals, such as the explosive PETN, has long hampered much needed computational investigations of these materials. One reason for the shortcomings is that the exchange-correlation functionals available for Density Functional Theory (DFT) based calculations do not correctly describe the weak intermolecular van der Waals' forces present in molecular crystals. However, this weak interaction also poses other challenges for the computational schemes used. We will discuss these issues in the context of calculations of lattice constants and structure of PETN with a number of different functionals, and also discuss if these limitations can be circumvented for studies at non-ambient conditions. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T22.00005: Atomistic models of Si nanocrystals embedded in Si-rich nitride and oxide matrices Tianshu Li, Davide Donadio, Francois Gygi, Giulia Galli Several experimental techniques have been recently developed to precipitate Si nano-crystals in nitride and oxide amorphous matrices, and photo-luminescence of these nanoparticles has been reported by several authors. However the role played by the nanocrystal/host interface in determining the optical properties of the nanocrystals is not yet understood and no atomistic model of the interface is available. Using techniques recently developed to study nucleation in supercooled liquid silicon , we derive realistic models of nanocrystal/matrix interfaces; in particular, we simulate the nucleation of crystalline Si nanoparticles from Si-rich nitride and oxide supercooled liquids, by means of molecular dynamics coupled to advanced sampling techniques. Such an approach allows one to closely mimic experimental conditions, without any assumptions on either the shape of nanocrystals or the defect concentration at the interface. The composite structure obtained in our simulations can then be used as input for {\em ab initio} calculations to investigate the opto-electronic properties of the nanocrystals. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T22.00006: A multiphysics numerical technique for THz-frequency carrier transport in semiconductors and metals K. J. Willis, S. C. Hagness, I. Knezevic We present a novel computational tool for THz-frequency characterization of materials with high carrier densities, such as highly-doped semiconductors and metals. The numerical technique tracks carrier-field dynamics by combining, for the first time, the ensemble Monte Carlo (EMC) simulator of carrier dynamics with the finite-difference time-domain (FDTD) technique for Maxwell's equations and the molecular dynamics (MD) technique for close-range Coulomb interactions. While EMC/FDTD has been proven to accurately characterize high-frequency behavior in electronic systems with low carrier densities ($<$10$^{16}$ cm$^{-3}$), the inclusion of short-range Coulomb interactions via MD becomes necessary at higher carrier densities ($>$10$^{18}$ cm$^{-3}$). This three-pronged multiphysics technique captures electromagnetic wave propagation and transport in materials in which carrier dynamics may be strongly impacted by Coulomb interaction between carriers. [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T22.00007: Magnetic Resonance and Electrical Conductivity in Mixed Porosity Systems S. Ryu, Z. Zhang, L. Schwartz, D. Johnson Many composite materials exhibit a wide range of pore sizes. In reservoir rocks one often sees macro-pores that are 20--50 $\mu $m in diameter and micro-pores whose size can be 100 times smaller. Spatially, the macro and micro-pores can be arranged either in series, in parallel or a combination of the two. We present the results of numerical simulations on mixed porosity systems based on the packing of grains and on data derived from x-ray microtomography ($\mu $CT). We begin with ordered packings of overlapping spherical grains. Micro-porosity is introduced in one of two ways -- we can have a micro-porous layer coating each grain or the grains can be comprised of smaller grains. In both cases we take the ratio of macro to micro pore diameters to be in the realistic range between 20 and 100. We will present numerical simulations of electrical conductivity and surface-induced proton spin relaxation. In calculations based on carbonate $\mu $CT data we find that the connected pathways almost always involve transport and diffusion through the micro-pores. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T22.00008: Earthquake Models in the Natural Time Domain John Rundle, James Holliday Earthquake dynamics iare difficult to understand using conventional (``calendar'') time, a result of the strong clustering of events in the system. The instability associated with nucleation and growth of slip has been viewed in various models as a type of avalanche phenomenon, similar to a sandpile. As each site fails, or topples, stress is both reduced and transferred to other sites within the range of interaction. For this reason, the slip events occur in short bursts, widely separated in calendar time. However, if time is counted in event numbers (natural time), events become regularly spaced in time and are easier to analyze. The difficulty, or course, lies in constructing the inverse mapping of natural time to calendar time. Nevertheless, the natural time domain offers significant analytical advantages. In this talk, we discuss methods and problems for earthquake dynamics and forecasting in the natural time domain. We also discuss forecast verification and validation using standard methods adapted from weather and financial forecasting. [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T22.00009: Damage Mechanics Model for Fracture Nucleation and Propagation John Rundle, Gleb Yakovlev, Joseph Gran, Donald Turcotte, William Klein We consider a slider-block model for rupture nucleation and propagation of shear fractures. Time to failure for each sliding block is specified from a Poisson distribution, a model that has been used elsewhere. A new feature is that the hazard rate is assumed to have a power-law dependence on stress. When a block fails, it is removed, and the stress on the block is redistributed uniformly to a specified number of neighboring blocks in a given range of interaction. We solve this problem for a constant applied stress at t = 0. Damage is the fraction of blocks that have failed. Time to failure and modes of rupture propagation are determined a s function of the hazard-rate exponent and the range of interaction. Results are compared with observations. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T22.00010: Atomistic Simulation of Lithiated Mesoporous Manganese Dioxide Phuti E. Ngoepe, Thi X. Sayle, Dean C. Sayle Recent studies have shown that mesoporous manganese dioxide is capable of reversibly storing specific capacity of 284 mAh/g. Atomistic simulated amorphisation recrystallisation technique, involving tens of thousands of atoms, has been successfully used to generate models of various nano-forms of the complex manganese dioxide, which include microstructural details. In the current study, we will apply the method to the study of lithiated mesoporous structures. We observe microstructural features that compare well with the high resolution electron microscopy mirographs. We also use the atomistic simulations to explore the mechanical strength of porous nanomaterials. We show that mesoporous manganese dioxide undergoes a significant volume expansion when Li is fully intercalated, which can only be sustained without structural collapse, if the nano-architecture is symmetrically porous, enabling elastic deformation during intercalation [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T22.00011: Coupling the atomistic and continuum simulation Guowu Ren, Dier Zhang, Xingao Gong A hybrid multiscale method coupled atomistic and continuum model is present. In the continuumr region, an atomic-scale finite element method (AFEM) is proposed and exhibits a perfect coupling with molecular dynamics (MD) method used for the atomistic region. In the framework of hybrid method, utilizing the concept of filter, we also design a new interfacial condition which damps out the spurious reflection of high-frequency phonons at the atomistic/continuum interface without much influence on the low-frequency ones. A series of simulation results show the advantage of our schemes. [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T22.00012: Locally adaptive parallel temperature accelerated dynamics method Yunsic Shim, Jacques G. Amar The recently-developed temperature-accelerated dynamics (TAD) method [M. S{\o}rensen and A.F. Voter, J. Chem. Phys. {\bf 112}, 9599 (2000)] along with the more recently developed parallel TAD (parTAD) method [Y. Shim et al, Phys. Rev. B {\bf 76}, 205439 (2007)] allow one to carry out non-equilibrium simulations over extended time and length scales. The basic idea behind TAD is to speed up transitions by carrying out a high-temperature MD simulation and then use the resulting information to obtain event times at the desired low temperature. In a typical implementation, a fixed high temperature $T_{high}$ is used. However, in general one expects that for each configuration there exists an optimal value of $T_{high}$ which depends on the particular transition pathways and activation energies for that configuration. Here we present a locally adaptive high-temperature TAD method in which instead of using a fixed $T_{high}$ the high temperature is dynamically adjusted in order to maximize simulation efficiency. Preliminary results of the performance obtained from parTAD simulations of Cu/Cu(100) growth using the locally adaptive $T_{high}$ method will also be presented. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700