Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R32: Focus Session: Computational Nanoscience V |
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Sponsoring Units: DCOMP DMP Chair: Wenchang Lu, North Carolina State University Room: Baltimore Convention Center 329 |
Wednesday, March 15, 2006 2:30PM - 2:42PM |
R32.00001: Multi-scale Modeling of Thermal-Electromechanical Response of Piezoelectric Thin Film Liming Xiong, James Lee, Youping Chen, Yajie Lei This study aims at understanding and improving the properties of the piezoelectric thin films through numerical simulation. On the basis of first principle calculation, empirical inter-atomic potential is obtained by fitting to the lattice parameters, energy surface and phonon dispersion relation. Both traditional MD simulation and a new atomistic multi-scale field theory are employed to simulate the material behavior of piezoelectric thin films under thermal, electrical and mechanical loading. Results are well agreed with each other between MD simulation and the newly developed field theory. Meanwhile, it is found out that the newly developed field theory is more efficient in studying nonequilibrium phenomenon, high temperature and high pressure working conditions for thin films. Size effects, thermal conductivity, failure process and material behavior in harsh environments for the thin films are investigated, which provide us with scientific information for the design and optimization of industrial application. [Preview Abstract] |
Wednesday, March 15, 2006 2:42PM - 2:54PM |
R32.00002: Multi-scale Simulations of SiO$_2$ Systems I: Classical and Hybrid Simulations Chao Cao, Yao He, Hai-ping Cheng Classical MD simulations show that bulk and nano-wire silica glass exhibit completely different stress-strain curves. A typical bulk sample breaks almost instantaneously while a nano-wire sample usually breaks ``gradually.'' A further simulation on nano-wire sample surrounded by water molecule shows that the stress-strain curve is not affected very much by the presence of water molecules. However, from previous quantum calculations (by Du et. al), one would believe that the water molecules do affect the fracture of silica a lot. Therefore, multi-scale simulations on a simple model SiO$_2$ chain are performed and compared with classical simulations on the same system to investigate mechanisms underlying the inconsistency. The multi-scale simulation is also compared with a full quantum calculation in our second talk. [Preview Abstract] |
Wednesday, March 15, 2006 2:54PM - 3:06PM |
R32.00003: Multiscale Simulation of SiO2 II: Quantum Simulations Yao He, Chao Cao, Hai-Ping Cheng To investigate effects of external stress and the influence of water molecules on the energetic stability of low dimension silica systems, we have performed ab initio molecular-dynamics simulations as well as hybrid classical/quantum simulations of SiO2 chains. These silica chains are formed by edge-sharing two-rings. We present strain dependent interactions of silica-water systems at room temperature. Our investigations provide qualitative and quantitative descriptions of the reaction processes. We have also compared these ab initio results with results obtained from our multiscale simulations method, in which the quantum region is embedded in a classical region. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:42PM |
R32.00004: Dissipation of Mechanical Energy in Carbon Nanotube-based Mechanical Devices Invited Speaker: The creation of functional mechanical devices at the nanoscale is a major goal of nanoscience, and multiwalled carbon nanotubes (MWCNT) are promising materials for constructing such devices. The large disparity between the strengths of intralayer and interlayer interactions in MWCNTS allows for smooth relative motion of concentric tubes. Some simple mechanical elements such as linear and rotational bearings have already been constructed from MWCNTs by exploiting this feature[1]. However, little is known about the dominant mechanisms for dissipation of mechanical energy at the nano scale, and devices based on MWCNTs are both technologically important and useful as test systems for theoretical investigations. In this talk, atomistic simulations of some simple MWCNT-based mechanical systems will be presented. It is shown that reducing the dimensions of a device can have a strong impact on phononic friction. The small masses of nanotubes relative to the forces between them means that relative velocities can be comparable to and larger than atomic thermal velocities. In this regime, theories based on a quasi-adiabatic response of atoms to the relative motion of nanotubes fail and simulations reveal a strong and complex velocity dependence of friction[2]. Large edge to surface ratios mean that edges can play an important role in energy dissipation and for concentric nanotubes of length 10s of nm, the ends of the nanotubes have been shown to dominate sliding friction[2]. These and other important features of friction in nanotube-based devices will be discussed and illustrated with the results of molecular dynamics simulations. This work was supported by the NSF Grant No. DMR04-39768 and U.S. DOE Contract No. DE-AC03-76SF00098. [1] J. Cumings and A. Zettl, Science 289, 602 (2000); A. M. Fennimore et al., Nature 424, 408 (2003). [2] P. Tangney, S. G. Louie, and M. L. Cohen, Phys. Rev. Lett. 93,065503 (2004) ; P. Tangney, M. L. Cohen, and S. G. Louie, to be published. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R32.00005: Elastic properties of SiC nanoscopic wires Maxim Makeev, Madhu Menon, Deepak Srivastava Mechanical properties of crystalline and amorphous SiC nanowires have been investigated using molecular dynamics simulations with the Tersoff bond-order interatomic potential. The crystalline and a-SiC nanowires of different diameters were studied under tension/compression, torsion, and bending. The bending and torsion rigidities are found to be strongly dependent on the wire size. This is unlike the Young's modulus computed from uniaxial loading curves. Atomistic relaxations effects near the thresholds of structural stability are investigated for the four employed load types. The mechanical properties of crystalline SiC nanowires are compared with a-SiC wires of the same radii. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R32.00006: Simulation Studies of Mechanical Properties of Novel Silica Nano-structures Krishna Muralidharan, Joan Torras Costa, Samuel B. Trickey Advances in nanotechnology and the importance of silica as a technological material continue to stimulate computational study of the properties of possible novel silica nanostructures. Thus we have done classical molecular dynamics (MD) and multi-scale quantum mechanical (QM/MD) simulation studies of the mechanical properties of single-wall and multi-wall silica nano-rods of varying dimensions. Such nano-rods have been predicted by Mallik \textit{et al.} to be unusually strong in tensile failure. Here we compare failure mechanisms of such nano-rods under tension, compression, and bending. The concurrent multi-scale QM/MD studies use the general PUPIL system (Torras \textit{et al.}). In this case, PUPIL provides automated interoperation of the MNDO Transfer Hamiltonian QM code (Taylor \textit{et al.}) and a locally written MD code. Embedding of the QM-forces domain is via the scheme of Mallik \textit{et al.} Work supported by NSF ITR award DMR-0325553. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R32.00007: On The Nonlinear Mechanics of Carbon Nanocones Using The Consistent Atomic-scale Finite Element Method Arash Mahdavi, Eric Mockensturm In the present work, a new multiscale modeling technique called the Consistent Atomic-scale Finite Element (CAF\'{E}) method is introduced. Unlike traditional approaches for linking the atomic structure to its equivalent continuum, this method directly connects the atomic degrees of freedom to a reduced set of finite element degrees of freedom without passing through an intermediate homogenized continuum. As a result, there is no need to introduce stress and strain measures at the atomic level. The Tersoff-Brenner interatomic potential is used to calculate the consistent tangent stiffness matrix of the structure. In this finite element formulation, all local and non-local interactions between carbon atoms are taken into account using overlapping finite elements. In addition, a consistent hierarchical finite element modeling technique is developed for adaptively coarsening and refining the mesh over different parts of the model. This process is consistent with the underlying atomic structure and, by refining the mesh, molecular dynamic results will be recovered. In contrast with most other multiscale methods, there is no need to introduce artificial boundaries for coupling atomistic and continuum regions. The applicability of the method is shown with several examples of deformation of carbon nanocones subjected to different loads and boundary conditions. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R32.00008: Nanocrystalline Domain Formation as a Strain Relaxation Mechanism in Ultra-Thin Metallic Films M. Rauf Gungor, Dimitrios Maroudas In this presentation, we report results for the atomistic mechanisms of strain relaxation over a wide range of applied biaxial tensile strain (up to 17{\%}) in free standing Cu thin films based on isothermal-isostrain molecular-dynamics simulations. After an elastic response at low strain ($<$ 2{\%}), plastic deformation occurs through ductile void growth accompanied by emission of screw dislocations from the void surface, as well as emission of threading dislocation loops from the film's surface. At strain levels below 8{\%}, expansion of the plastic zone around the void during void growth is the major strain relaxation mechanism. At higher levels of applied strain ($>$ 8{\%}), a practically uniform distribution of dislocations is generated in the metallic thin film, which mediates the transformation of the initially single-crystalline film structure to a nanocrystalline one. Furthermore, void growth is inhibited as the dislocations emitted from the void surface are pinned by the simultaneously generated network of defects in the nanocrystalline material. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R32.00009: New and exotic self-organized patterns for modulated nanoscale systems Eliana Asciutto, Christopher Roland, Celeste Sagui The self-organized domain patterns of modulated systems are characteristic of a wide variety of chemical and physical systems, and are the result of competing interactions. From a technological point of view, there is considerable interest in these domain patterns, as they form suitable templates for the fabrication of nanostructures. We have analyzed the domains and instabilities that form in modulated systems, and show that a large variety of new and exotic patterns -- based on long-lived metastable or glassy states -- may be formed as a compromise between the required equilibrium modulation period and the strain present in the system. The strain results from topologically constrained trajectories in phase space, the effectively preclude the equilibrium configuration. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R32.00010: Studies of elastic and plastic deformation of nanostructured materials: A continuum approach Mikko Haataja, Peter Stefanovic, Nikolas Provatas Nanostructured materials can have physical and mechanical properties that are strikingly different from their corresponding bulk counterparts. Consequently, unraveling the physical mechanisms that give rise to their behavior at the atomic scale is essential in order to exploit and harness their unique properties. From a theoretical perspective, capturing dynamic phenomena across very long time scales with direct atomistic simulation methods (e.g., Molecular Dynamics [MD]) becomes a very challenging task due to inherent time scale limitations. Here we present a continuum field theory approach for modeling both elastic and plastic deformations, free surfaces, and multiple crystal orientations in systems with both hexagonal and cubic symmetry in two spatial dimensions. The model is based on a free energy for the local, temporally coarse-grained atomic density, which is minimized by spatially periodic structures. Hence, it incorporates, by construction, both elastic phenomena as well as defects in the form of, e.g., vacancies, dislocations, and grain boundaries. Furthermore, its dynamics is constructed such that it incorporates both diffusive and elastic relaxation phenomena. By introducing a variable elastic time scale, we are able to maintain mechanical equilibrium while simulating microstructural evolution on time scales well beyond those accessible by conventional atomistic MD simulation methods. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R32.00011: Van der Waals forces between nanoclusters: importance of many-body effects Hye-Young Kim, Jorge Sofo, Darrell Velegol, Milton Cole, Amand Lucas Van der Waals (VDW) interactions between nanoclusters have been calculated using a self-consistent, coupled dipole method. The method accounts for all multi-body (MB) effects and can be used to calculate dispersion interaction energies between nanoclusters of arbitrary shape. Comparison is made between the exact potential energy, V, and the values obtained with two alternative methods: V$^{(2)}$, a sum of two-body interactions, and V$^{(2) }$+ V$^{(3)}$, the sum of 2-body and 3-body interactions. For some clusters and orientations, V$^{(2)}$ is close to the exact result. In other situations, MB effects can be large. For all cases considered here, the 3-body term alone does \textit{not} accurately represent the MB contributions to V. MB contributions are especially important for shape-anisotropic clusters. [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R32.00012: Average speeds and durations of pulsed plane waves transmitted through chiral sculptured thin films Joseph Geddes, Akhlesh Lakhtakia We computed, with a finite-difference algorithm, the durations and average speeds of pulsed, ultrashort, optical plane waves transmitted through chiral sculptured thin films (STFs). Chiral STFs are assemblages of parallel nanohelixes affixed to a substrate; the helixes possess diameters of 10--300 nm, lengths of micrometers, and pitches that can be engineered during the fabrication via physical vapor deposition. We modeled the chiral STFs as continuously nonhomogeneous, anisotropic dielectric materials which are either linear or exhibit a Kerr-type nonlinearity. We computed the equivalent, root mean square, and correlation durations of the transmitted pulses and found that these quantities tend to increase with decreasing carrier wavelength, though there is an exception to this trend when the linear film exhibits the circular Bragg phenomenon. Increasing nonlinearity also tends to increase the durations of transmitted pulses. We computed the average peak speed, center-of-energy speed, and correlation speed of the pulsed plane wave and found that these speeds tend overall to increase with carrier wavelength but decrease with increased nonlinearity. Our results will have application in the design of optical pulse shaping devices. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R32.00013: Approximate Approach to Multi-Dimensional Tunneling in Solids Mario Ancona For reasons of tractability elastic tunneling within or from solids is typically treated with one-dimensional or quasi-one-dimensional approximations. In this presentation methods for analyzing multi-dimensional solid-state tunneling that maintain physical fidelity yet remain computationally efficient are discussed. After a brief review of the microscopic EBK approach, the discussion focuses on a macroscopic form of quantum transport theory whose equations are most easily understood via analogies with gas dynamics and electron optics. Various analytical implications of the theory are derived and numerical illustrations are provided. [Preview Abstract] |
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