Symposium Organizers
Yue Qi General Motors R&D and Planning
H. Eliot Fang Sandia National Laboratories
Nick Reynolds Accelrys
Zi-Kui Liu The Pennsylvania State University
W1: New Approaches Toward Multiscale Matierals Design
Session Chairs
Monday PM, December 01, 2008
Constitution B (Sheraton)
9:30 AM - **W1.1
Multiscale Modeling in Multilevel Materials Design.
David McDowell 1
1 School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show Abstract10:00 AM - **W1.2
The Prediction of Crystal Structure with Knowledge Methods as a Crucial Ingredient for Computational Materials Design.
Gerbrand Ceder 1 , Geoffroy Hautier 1 , Chris Fischer 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractThe prediction of structure is a key problem in computational materials science that forms the platform on which rational materials design can be performed. Without detailed structure information the prediction of properties rapidly becomes irrelevant. We will present an ab initio approach that rapidly finds the stable crystal structure of materials with > 95% of success. The premise of the approach is that for many materials chemistries standard computational quantum mechanics is highly accurate in selecting the true ground state of a system from a small set of candidate structures, though notable exceptions exist. Finding ground states by traditional optimization methods on quantum mechanical energy models is difficult due to the complexity and high dimensionality of the coordinate space. An unusual, but efficient solution to this problem can be obtained by merging ideas from heuristic approaches and ab initio methods: In the same way that scientist build empirical rules by observation of experimental trends, we have developed machine learning approaches that extract knowledge from a large set of experimental information and a database of over 20,000 first principles computations, and used these to rapidly direct accurate quantum mechanical techniques to the lowest energy crystal structure of a material. Knowledge is captured in a Bayesian probability network that relates the probability to find particular crystal structure at a given composition to structure and energy information at other compositions. We show that this approach is highly efficient in finding the ground states of binary metallic alloys and can be easily generalized to more complex systems. We have already used this approach to identify several hundred new compounds
10:30 AM - W1.3
From Molecular Grand-canonical Density Functional Theory towards the Rational Multiscale Design of Chemical Compounds from First Principles.
Anatole Lilienfeld 1
1 Multiscale Dynamic Material Modeling Department, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThe fundamental challenge of rational compound design, i.e. the reverse engineering of chemical compounds with predefined specific properties, originates in the high-dimensional combinatorial nature of chemical space. Chemical space is the hyper-space of a given set of molecular observables that is spanned by the molecular grand-canonical variables (elementary particle densities of electrons and nuclei) which define chemical composition. A working definition of chemical space has been given within the notion of a molecular grand-canonical ensemble multi-component density functional theory framework [1]. I will discuss this approach, as well as numerical results for controlling molecular properties, described within various levels of theory on the multiscale hierarchy, through variation of chemical composition. Properties include electronic molecular eigenvalues, intermolecular energies, or drug-enzyme affinity [2-5]. Eventually, the effects of erroneous many-body interatomic potentials on cohesive energies, or of details in the pseudopotential approximation on band-gap estimates, shall exemplify the important issue of ensuring sufficient accuracy across the multiple scales in the context of multiscale materials design [6,7].[1] von Lilienfeld and Tuckerman, J Chem Phys, 125, 154104 (2006)[2] von Lilienfeld et al, Phys Rev Lett, 95, 153002 (2005)[3] von Lilienfeld and Tuckerman, J Chem Theory Comput, 3, 1083 (2007)[4] Marcon et al, J Chem Phys, 127, 064305 (2007)[5] von Lilienfeld et al, (in preparation)[6] von Lilienfeld and Schultz, Phys Rev B, 77, 115202 (2008)[7] von Lilienfeld and Tkatchenko, Phys Rev B, accepted (2008)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.
10:45 AM - W1.4
Green's Function Molecular Dynamics: Applications to Tribology and Contact Mechanics.
Ling Ti Kong 1 , Carlos Campana 1 2 , Colin Denniston 1 , Martin Muser 1
1 Applied Mathematics, University of Western Ontario, London, Ontario, Canada, 2 , CANMET-Materials Technology Laboratory, Natural Resources Canada, Ottawa, Ontario, Canada
Show Abstract11:00 AM - W1: Multi1
BREAK
11:30 AM - W1.5
Enhancing Molecular Dynamics to Capture Electronic Effects.
Normand Modine 1 , Reese Jones 1 , David Olmsted 1 , Jeremy Templeton 1 , Gregory Wagner 1 , Ryan Hatcher 2 , Matthew Beck 3
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Lockheed Martin Advanced Technology Laboratories, Cherry Hill, New Jersey, United States, 3 , Vanderbilt University, Nashville, Tennessee, United States
Show AbstractIn modeling non-equilibrium thermal transport in solids, classical molecular dynamics (MD) has the primary strength of explicitly representing phonon modes and the defects that scatter phonons. On the other hand, electrons and their role in energy transport are missing. These effects are vital in applications such as laser processing, thermoelectrics, and current induced thermal failure. In nanoscale and nanostructured systems, the behavior of the system is complicated further by phonon-confinement, ballistic transport, and discrete defect scattering effects. These effects are absent in phenomenological models of heat transport, but naturally captured by MD. Our goal is to couple a MD treatment of the ionic subsystem with a partial differential equation (PDE)-based model of the electronic subsystem in order to accurately capture the aggregate behavior of the coupled electron-ion system. Along these lines, we have enhanced the LAMMPS MD package by coupling the ionic motions to a finite element (FE) based representation of electronic charge and heat transport. The coupling between the subsystems occurs via a local version of the two-temperature model that allows the ionic and electronic subsystems to exchange energy and eventually come into equilibrium. The rate of equilibration between the ionic and electronic temperatures is calculated for a representative system from first principles using a Time Dependent Density Functional Theory (TDDFT) simulation. The ions are initially in thermal motion, while the electrons are initially in their ground state. During the TDDFT simulation, the electrons leave the Born-Oppenheimer surface and gain energy to equilibrate with the ions. Our approach is intrinsically multiscale and multiphysics. Furthermore, the tight coupling between the MD and FE paradigms utilizes the inherent strengths of each. Initial demonstrations of our approach and capabilities have focused on heat transport in representative carbon nanotubes, and these results will be discussed.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.
11:45 AM - W1.6
Coupled Quantum Mechanics and Finite-Element Simulations of Mechanical Properties of Defects.
Noam Bernstein 1 , Viacheslav Sorkin 2 , Ellad Tadmor 2 , Gabor Csanyi 3
1 Center for Computational Materials Science, Naval Research Laboratory, Washington, District of Columbia, United States, 2 Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, Minnesota, United States, 3 Engineering Laboratory, Cambridge University, Cambridge United Kingdom
Show Abstract12:15 PM - W1.8
Phase-field Modeling of Reactive Fluid Flow with Solute Precipitation and Dissolution.
Zhijie Xu 1 , Paul Meakin 1
1 , Idaho National Lab, Idaho Falls, Idaho, United States
Show AbstractA phase-field approach to the dynamics of liquid-solid interfaces that evolve due to solute precipitation and/or dissolution is presented. In contrast to solidification processes controlled by a temperature field that is continuous across the solid/liquid interface (with a discontinuous temperature gradient), precipitation/dissolution is controlled by a solute concentration field that is discontinuous at the solid/liquid interface. The Gibbs-Thomson effect on the interface dynamics was included in the phase-field model, and a sharp-interface asymptotic analysis of the phase-field equations was performed for precipitation/dissolution processes to demonstrate that the phase-field equations converge to the proper sharp-interface limit. The mathematical model was validated by numerically solving the coupled phase-field equations for a one- and two- dimensional precipitation/ dissolution problems and by comparison with the analytical solutions.
12:30 PM - W1.9
Recursive Coarse-Grained Particle Method for Inhomogeneous Materials.
Takahide Nakamura 1 2 , Ryo Kobayashi 1 2 , Shuji Ogata 1 2
1 Department of Scientific and Engineering Simulation, Nagoya Institute of Technology, Nagoya Japan, 2 CREST, Japan Science and Technology Agency, Saitama Japan
Show AbstractA coarse-graining method has been proposed [1] for a crystalline system of atoms to describe the propagation of relatively long-wavelength waves, in which the inter-particle interaction is obtained through coarse-graining of the partition function of the atomic Hamiltonian in the harmonic approximation.Though the method has attractive features such as its natural incorporation of atomistic phonons and its potential suitableness to connection to both atomistic and continuum simulation methods bridging the wide scale-gap, the original formulation limits its application to periodic systems without surfaces.In this paper, we advance the method to be applicable to realistic systems of meso-scales with various shapes under stressed conditions at relatively low computation costs. The points of advancement include: (i) recursive coarse-graining procedure [2] for both homogeneous and inhomogeneous systems, (ii) application to pseudo-2D systems such as the graphene sheet and the carbon nanotube. Accuracy of the present method is analyzed through the phonon spectra in homogenous systems, the propagation and scattering of elastic waves in both homogenous and inhomogenous systems, the elastic properties, and so on. Also we will demonstrate its concurrent hybridization with both the molecular dynamics for atoms and the lattice Boltzmann method for fluids, for multiscale simulation of interesting systems.[1] R.E. Rudd and J.Q. Broughton, Phys. Rev. B 58, R5893 (1998).[2] R. Kobayashi and S. Ogata, Mat. Trans. (2008), in press.
12:45 PM - W1.10
Computational Materials Design with the Configurational Forces Concept.
Otmar Kolednik 1 5 , Jozef Predan 2 , Narendra Simha 3 , Dieter Fischer 4 5
1 , Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria, 5 , Materials Center Leoben Forschung GmbH, Leoben Austria, 2 Faculty of Mechanical Engineering, University of Maribor, Maribor Slovenia, 3 Department of Orthopaedic Surgery, University of Minnesota, Minneapolis, Minnesota, United States, 4 Institute of Mechanics, Montanuniversität Leoben, Leoben Austria
Show AbstractThe concept of configurational forces is a powerful computational tool for the quantitative description of the behavior of defects in materials and structural components. It enables us to(1)evaluate the crack driving force in arbitrary micro- or macroscopically inhomogeneous materials and components, (2)take into account the influences of eigenstrains and residual stresses,(3)estimate the crack growth direction using the criterion of maximum dissipation,(4)assess the shielding and anti-shielding effects of near-tip and remote plasticity. In this presentation, first a short overview shall be given about theory and computational aspects. Then specific applications are shown, e.g. coated steels, ceramic multilayered materials, or biological materials. These examples demonstrate that the concept of configurational forces - applied in combination with sophisticated experimental methods to determine the spatial variations of local material properties and residual strains - opens new prospects for the design of future damage resistant materials and components.
W2: Materials in Energy Applications
Session Chairs
Monday PM, December 01, 2008
Constitution B (Sheraton)
2:30 PM - W2.1
Phase-Field Modeling of Intercalation Processes in Rechargeable Batteries.
Damian Burch 1 , Martin Bazant 2 1
1 Mathematics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, Stanford University, Stanford, California, United States
Show Abstract2:45 PM - W2.2
Simulations Lithium Intercalation into a LixFePO4 Battery Electrode During Recharging with Kinetic Monte Carlo Method.
Celine Hin 1 , W. Craig Carter 1 , Yet Chiang 1
1 Materials Science and Engineering , MIT, Cambridge, Massachusetts, United States
Show AbstractThe demand for reliable and high energy density batteries for use in hybrid and plug-in hybrid vehicles requires improvements of currently available lithium batteries systems. One strategy is to use fine-particle electrode materials that provide large surface area for lithium ion intercalation in electrochemical processes. Thus, the role of interfaces in the transport and reaction behaviour becomes more important for informed battery-microstructure design. These roles have been studied experimentally [1-5] and by modelling [6-8]. More particularly, we will show how battery modelling can account for complex and different reactions that takes place at electrode interfaces for different experimental conditions, such as charging rate.A kinetic Monte Carlo algorithm based on rigid lattice was used to study kinetic of lithium intercalation into a LixFePO4 electrode during a galvanostatic discharge. This algorithm takes into account: (i) the structure of both phases in presence (e.g. Li(alpha)FePO4 and Li(1-β)FePO4), (ii) the solubility limit of Li in FePO4, (iii) the role of the Volmer-Butler relation on lithium ion interface transport and (iiii) the configuration dependent activation barriers within the lattice. The results provide insight into the role atomistic behavior in microstructure development and phase transitions during recharging.References:[1] Chiang Y.M., J. Electroceram. , 1997, 1, 205.[2] Chung S.Y., Bloking J.T., Chiang Y.M., Nat. Mater., 2002, 1,123.[3] Chung S.Y., Bloking J.T., Chiang Y.M., Nat. Mater., 2003, 2,702. [4] Meethong N., Shadow Huang H.-Y., Speakman S.A., Carter W.C., Chiang Y.-M., Adv. Funct. Mater. 2007.[5] Meethong N., Shadow Huang H.-Y., Carter W.C., Chiang Y.-M., Electrochem. Solid-State Lett. 2007, 10(5), A134-A138.[6] Doyle M., Fuller T. F., Newmann J., Journal of the Electrochemical Society 1993, 140 (6), 1526.[7] Garcia R.E., Chiang Y.-M., Carter W.C., Limthongkul P., Bishop C. M., Journal of the Electrochemical Society, 2005, 152 (1), A255-263.[8] Garcia R.E., Chiang Y.-M., Journal of the Electrochemical Society 2007, 154 (9), A856-A864.
3:00 PM - **W2.3
Thermoelectric Materials by Design.
Jihui Yang 1
1 Materials and Processes Lab, GM R&D Center, Warren, Michigan, United States
Show Abstract Increasing awareness and concern for energy resources and the environment has prompted the recent worldwide effort on thermoelectric (TE) materials research and technology development. Advances in TE technology can have a significant impact on the automotive industry in terms of fuel economy improvements by generating electricity from waste heat and augmenting air conditioning efficiency. A critical component of developing automotive TE technologies is the discovery and development of highly efficient TE materials. From the fundamental materials research perspective, this is motivated by the more than half-century-long challenge of simultaneously enhancing the electrical conductivity and thermopower, and lowering the thermal conductivity of materials; and of understanding the underlying mechanisms. In this talk I will highlight some of our recent work on void filling limits, effects of misch-metal filling, and dual-frequency resonant phonon scattering in filled skutterudites; and show how we are able to improve the thermoelectric performance of these materials and predicting new materials through experimental effort and first principle-based computational modeling.
3:30 PM - W2.4
A General Strategy For Screening Hydrogen Purification Membranes by Modeling Amorphous Metals.
Shiqiang Hao 1 , David Sholl 2
1 Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 School of Chemical &
Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractThe increasing demand for clean and efficient energy has resulted in an increased global willingness to embrace the proposed ``hydrogen economy''. The use of amorphous metal films as membranes to purify hydrogen has potential to overcome at least some of the disadvantages of existing crystalline metal membranes. We introduce a general strategy combining density functional theory and statistical mechanics to quantitatively predict solubility, diffusivity and permeability of interstitial H in amorphous metals. These calculations make it possible for the first time to quantitatively evaluate the performance of amorphous metal films as hydrogen purification membranes. These methods are introduced by examining amorphous Fe3B and a crystalline analogue with the same composition. A membrane made from the amorphous material is predicted to have a hydrogen permeability 1.5-2 orders of magnitude higher than a crystalline membrane. The methods we introduce here will be useful in accelerating the development of amorphous membranes for practical applications.
3:45 PM - W2.5
Metal Decorated Periodic Mesoporous Organosilicas (PMOs) for Hydrogen Storage.
Liping Huang 1 2 3 , Erik Santiso 1 4 , Keith Gubbins 1 , Marco Buongiorno Nardelli 2 5
1 Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Physics, North Carolina State University, Raleigh, North Carolina, United States, 3 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 4 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 5 CSMD, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractPeriodic mesoporous organosilicas (PMOs) have hexagonal pore arrangement in the mesoscale and crystalline pore wall with alternating hydrophilic silicate layers and hydrophobic organic layers. The silicate layers provide the rigidity to the materials and the organic layers can be functionalized for potential applications including hydrogen separation and storage. By using first-principles density functional theory calculations, we demonstrated that the interaction between H2 and PMOs can be modified by metal atoms. Metal decorated PMOs have been designed for efficient hydrogen storage. Our studies show that the bonding between metal and hydrogen is of a combination of chemical and physical adsorption, which is essential for reversible hydrogen uptake/release. Car-Parrinello molecular dynamics simulations demonstrate that these systems are stable and exhibit associative desorption of H2 upon heating without breaking the bond between PMOs and metal. This fulfills another requirement for reversible hydrogen storage.
4:00 PM - W2: Engy1
BREAK
4:30 PM - **W2.6
Multi-scale Simulation Approach for Designing New Polymeric Materials in Fuel Cell Technology.
Seung Soon Jang 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractIn order to develop new polymeric materials for fuel cell applications, we applied multi-scale simulation approach. First, we will summarize our previous studies on Nafion, sulfonated PEEK and Dendrion in which we investigated the effect of various molecular variables such as monomeric sequence and polymer chain architectures on the nanophase-segregation and proton transport. In these studies, we used full atomistic simulation techniques to predict the most probable structures in the presence of water. To evaluate the characteristics of the structures, we analyzed structure factor profiles, solvation of water and sulfonate group, and intermolecular structure in water phase. From these studies, we found that the nanophase-segregation can be controlled as a function of the monomeric sequence and chain architecture determine and that the well-developed nanophase-segregation facilitates the proton transport. And then, we will present 1) the parameterization of the properties of our interest for meso-scale simulations and 2) the results of the meso-scale simulations for the systems above-mentioned. From the results obtained these studies, we believe that the multi-scale modeling protocol is generic for any polymeric materials for fuel cell membrane application.
5:00 PM - W2.7
Energetics and Electronic Properties of TiO2 Nanotubes by DFT Calculations.
Jian-jie Liang 1 , Alexander Goldberg 1 , Mathew Halls 1 , Paul Kung 1
1 , Accelrys Inc, San Diego, California, United States
Show AbstractOrdered heterojunctions templated by semiconducting TiO2 nanostructures had been identified as a key to the future success of highly-efficient polymer-based solar cells. To decipher the structure-energy-property relationships of the TiO2 nanostructures, single-walled TiO2 nanotubes of ca. 3 nm were constructed based on 6-, 5-, and 4-coordinations of the Ti atoms, respectively. Calculations based on Density Functional Theory (DFT) were used to obtain optimized structures, calculate binding energies, and derive electronic band structures of the nanotubes. The binding energies of the nanotubes of all configurations studied were found to be substantially smaller than those of the bulk phases (rutile, anatase, and brookite), and became incrementally smaller as the diameter of the nanotubes grew. The bandgaps of the nanotubes increased by ca. 50% compared to the bulk. The 6-coordinated nanotubes were determined to have direct bandgaps, whereas the 5- and 4-coordinated nanotubes have indirect bandgaps.
5:15 PM - W2.8
Toward the Nanoscale Design of Catalysts for Fuel Cells: A Computational Approach.
Guofeng Wang 1
1 Department of Mechanical Engineering, Indiana University Purdue University Indianapolis, Indianapolis, Indiana, United States
Show AbstractMonday 12/1New Presentation Time/Paper NumberW2.9 @ 4:30 PM to W2.8 @ 4:15Toward the Nanoscale Design of Catalysts for Fuel Cells: A Computational Approach. Guofeng Wang
Symposium Organizers
Yue Qi General Motors R&D and Planning
H. Eliot Fang Sandia National Laboratories
Nick Reynolds Accelrys
Zi-Kui Liu The Pennsylvania State University
W3: Nano Technology & Devices
Session Chairs
Nick Reynolds
Guofeng Wang
Tuesday AM, December 02, 2008
Constitution B (Sheraton)
9:30 AM - **W3.1
First Principles-based Atomistic Modeling for the Synthesis and Structure of Silicon Nanocrystals Embedded in Silica.
Gyeong Hwang 1
1 Chemical Engineering, The University of Texas at Austin, Austin, Texas, United States
Show Abstract10:00 AM - W3.2
Point Defects and Interfacial Phases in Si:HfO2 Heterojunctions: A First principles Study.
Chunguang Tang 1 , Rampi Ramprasad 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show AbstractDriven by a need for device miniaturization in the microelectronic industry, Hf-based high-permittivity materials, such as HfO2, have gained interests for their potential application as gate dielectrics. However, between HfO2 and Si substrate undesirable interfacial phases such as Hf silicides, silicates and silica are known to form in nanometer sizes and degrade the performance of devices. It has been postulated that these interfacial phases are related to the segregation of point defects (such as O vacancy or interstitial) to the interface. In this work we have examined the effects of point defects on the evolution of interfacial phases by performing first principles computations on the point defects. Firstly the relationships between the stoichiometry of HfO2 and the processing conditions (oxygen pressure and temperature) have been studied by calculating the formation energies of point defects with various concentrations. Corresponding to different processing conditions, extensive calculations of the formation and migration energies have been performed for O vacancies, O interstitials, and Hf vacancies at various distances from Si:HfO2 interface. The results indicate that there exist strong thermodynamic and kinetic driving forces for these defects to segregate from bulk HfO2 part to the interface, which can result in the formation of interfacial Hf silicides, silica, and Hf silicates, respectively. Then the growth of the interfacial phases has been modeled by an accumulation process of the point defects at the interface. It is found that the thermodynamic driving force for the segregation of defects is well maintained as defect density at the interface increases, which indicates that the growth of interfacial phases is energetically favorable. Finally, by combining thermodynamics with our defect accumulation data, we have predicted interface morphologies as a function of temperatures and oxygen chemical potentials. The results show that as temperature increases, the clean Si:HfO2 interface, which is stable only for a small range of oxygen chemical potentials, tends to decompose into either O rich interface, such as SiOx (upon high chemical potentials), or O deficient interface, such as Hf silicides (upon low chemical potentials). The conclusions agree well with experiments. Our work deepens the atomic-scale understanding of the formation of interfacial phases in Si:HfO2 heterojunctions.
10:30 AM - W3.4
Modeling Facet Nucleation and Growth of Hut Clusters on Ge/Si(001).
John Venables 1 2 , Michael McKay 3 , Jeff Drucker 1
1 Physics, Arizona State University, Tempe, Arizona, United States, 2 LCN, University College, London United Kingdom, 3 , Lawrence Semiconductor Research Laboratory, Inc., Tempe, Arizona, United States
Show AbstractRecent STM observations of homogenous distributions of pyramid and hut clusters on Ge/Si(001) have shown that these clusters grow extremely slowly during annealing at intermediate temperatures, T ~ 450 oC, when there is a super-saturation of mobile ad-particles above and within the wetting layer. Data has been obtained on the absolute length of the clusters as a function of time L(t), and thereby the evolution of the growth rate, over periods of order 100 hours [1]. We model this slow growth as a layer by layer (2D) facet nucleation and growth problem, in the presence of strain-induced energies both on and around the facets. All of these energies can markedly influence the nucleation rate of new facets. First, they justify the observation that facet nucleation occurs from the apex of the hut, as has been observed in several other studies [2, 3]. Second, they indicate a substantial slowing down of the nucleation rate, by many orders of magnitude, relative to the case when such energies are not present. Finally the need to undo the stable reconstruction on the {105} facets as each new layer is formed contributes an extra energy of order 0.5 eV [4] to the energy of the critical nucleus. Inclusion of these effects, with experimental values of the Ge diffusion coefficient, provides a quantitative fit to the L(t) data, and sets bounds on step and facet energies appropriate to hut clusters. 1. M.R. McKay, J.A. Venables and J. Drucker, submitted to Phys. Rev. (2008);2. F. Montalenti et al., Phys. Rev. Lett., 93, 216102 (2004);3. S. Cereda, F. Montalenti and L. Miglio, Surface Sci. 591, 23-31 (2005);4. S. Cereda and F. Montalenti, Phys. Rev B 75, 195321 (2007).
10:45 AM - W3.5
Modeling of Self-organized Pattern Formation on Semiconductor Surfaces under Ion Sputtering.
Kun-Dar Li 1 2 , Qiangmin Wei 1 , Lumin Wang 1 2
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Nuclear Engineering and Radiological Science, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractLow-energy ion sputtering on surfaces of various semiconductors to achieve self-organized nanostructures has attracted attention due to their potential applications in optical and electronic devices. Depending on the material properties, temperature, ion energy, angle of incidence, and other experimental parameters, one may observe the development of rough or cellular surfaces, or, most importantly, the more or less regular ripple or nanodot patterns under oblique ion incidence. For example, well-ordered patterns of nanodots formed on Si, GaSb, InP, and InAs surfaces by low-energy ion-beam sputtering at normal incidence. This pattern formation is related to the instability between sputtering that roughens the surface and smoothing by different surface relaxation mechanisms. In contrast, at an off-normal incidence, periodic pattern of highly ordered hexagonal nanodots of Ga were produced on irradiated GaAs surface. A mass of experimental studies on ion sputtered surfaces have motivated extensive theoretical work aiming to reveal the mechanism responsible for such nanoscale pattern formation. In this study, a phase field approach was adopted to model self-organized pattern formation on sputtered surfaces. The microstructure of the system was described by a composition field, and pores were treated as high vacancy concentration regions which represent the pseudo-phase of voids. The effect of tilted ion incidence is included in the phase field model which integrated the production and elimination rates of vacancies with the free energy of mixing and interfacial energy as the driving force for vacancy diffusion. The theoretical model successfully explains the dynamic processes of formation and evolution of partially and highly ordered patterns of nanodots. Our theoretical simulation, supported by the experiments, suggests that one of the main dominating factors for the formation of the hexagonally ordered pattern of nanodots is the preferential migration direction under tilted ion beam. The influence of temperature, material property, vacancy production and recombination rate are all taken into consideration in the model for the formation of surface morphology. The morphology and length scale of our calculation results are consistent well with many experimental observations.
11:00 AM - W3: device
BREAK
11:30 AM - **W3.6
Working in Relativity: Material Application Development Through the Use of Molecular Modeling.
Nancy Iwamoto 1
1 , Honeywell Specialty Materials, Ramona, California, United States
Show Abstract The role of molecular modeling often takes on an investigative rather than an exploratory quality regardless of whether it is being used to develop a new material, predict how a material might fail in application, or diagnose why a material has failed at a customer site. This investigative quality is due in large part to the problem of multiple performance requirements that must be simultaneously satisfied when developing a material for a specific application. From this aspect, molecular modeling can be used to determine whether the molecular structure has the inherent characteristics to suspect success in an application, or to suspect that it could be involved during failure. When dealing with profiling inherent character, a relative predictive metric often becomes more useful than an absolute property prediction because of the complications of scale and the fact that a material in use at the customer is usually used in conjunction with other materials that are not necessarily divulged to the material developer. That is, because the molecular model can only sample a small atomistic area and because most performance issues are due to interactions with other materials, the molecular model can be most efficient when predictions are made in a relative manner by examining key interactions while surveying the different possible chemistries in-play. In doing so, it has been found that the use of molecular modeling provides important bridges in understanding between measurement and performance. This is especially true for areas such as electronic materials development where multiple interfaces must be considered in multi-layer constructions or composites and where the interfaces may not be chemically well-defined. In this manner the performance may be traced to issues surrounding the chemical structure and provide possible new chemistries, structural ranges or directions that may be used in the experimental work. This paper will relay different practical examples of the use of relative molecular modeling for material development in various applications such as stationary phases used in micro (MEMS) GCs, thin films used in integrated circuits (IC), cleaners used in lithography, and adhesives used in electronic packaging. For instance, for the micro-GC application relative molecular modeling was used to help select stationary phases by simulating relative separation performance and adsorption leading to a better general understanding of materials needed for separation versus concentration. In an example of the use of molecular modeling for thin films, relative modulus and adhesion trends were developed for interlayer dielectrics in order to understand fundamental performance limitations of the structures under development. And for the electronic packaging adhesives, relative surface energies, adhesion, and stress cycling trends were used to determine chemistries used for new materials designed for increased robustness to moisture.
12:00 PM - W3.7
Multi-scale Simulation of Metal/metal Interfacial Degradation under High Electromagnetic Stress.
Douglas Irving 1 , Clifford Padgett 2 , Donald Brenner 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Department of Chemistry and Physics, Armstrong-Atlantic State University, Savannah, Georgia, United States
Show AbstractNumerous macroscopic-level models of electro-magnetic launchers (EMLs) have been carried out that include coupled electro-magnetics and mechanics of the rail and armature. Although these studies provide a clear picture of engineering principles for EMLs, the problem of rail degradation remains unsolved. Extensive experimental studies continue to search for materials that dramatically extend rail lifetimes. In contrast to macroscopic modeling, there have been no published atomic simulations of rail-armature contact that include the influence of current flow. Given that rail degradation remains a major hurdle to multiple shot EMLs, the insights from this type of simulation can have a major impact on EML technology. In this talk we present results from a novel multi-scale modeling approach we have developed in which a molecular dynamics simulation of metal-metal contact is coupled to a finite difference solution of heat and current flow equations. We will describe the results of a series of simulations that use this methodology to explore the effect of applied voltage, applied load, sliding velocity, and the formation of molten interface layers on the atomic level dynamics of bare metal single asperity Al-Cu contact. Under an applied normal load localized melting of the aluminum is seen. If the voltage is held constant, Al continues to melt while the Cu asperity undergoes a surface disordering and intermixing with the localized molten Al. If the applied voltage changes to maintain a constant current, the initial melt recrystallizes and yields plastically under the applied normal load. In the sliding simulations heat is generated not only from the electric current but also from frictional heat, which results in extensive damage and material transfer. The authors acknowledge support from the U.S. Office of Naval Research through a Multi-University Research Initiative.
12:15 PM - W3.8
Terthiophene on ZnO Surfaces: First-principles Calculations and Experiments.
Guowen Peng 1 , Peerasak Paoprasert 2 , John Uhlrich 1 , Soonjoo Seo 2 , Padma Gopala 2 , Paul Evans 2 , Thomas Kuech 1 , Manos Mavrikakis 1
1 Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractFunctionalizing ZnO with small organic molecules such as terthiophene is of great interest in hybrid organic/inorganic electronic devices. For that purpose, a clear microscopic understanding of the terthiophene/ZnO interfaces is highly desirable. Terthiophene is a small molecule similar to thiophene polymers that can be linked to the surface using siloxane chemistry. Here, we investigate the structural and electronic properties of sub-monolayer coverages of terthiophene on both polar ZnO(000\bar{1}) and non-polar ZnO(10\bar{1}0) surfaces using first-principles methods. We show that an electric dipole is developed at the terthiophene/ZnO heterojunction. The surface dipole is sensitive to the molecular coverage, the molecular orientation relative to the surface, and the surface facet. The dependence of the electronic properties of the terthiophene/ZnO interfaces on the induced surface dipole is discussed and compared with experimental data obtained by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and Fourier transform infrared absorption spectroscopy (FTIR).
12:30 PM - W3.9
Effective Work Functions and Phase Equilibria of TaCxN1-x and Pt in Contact with HfO2 Through ab Initio Thermodynamics.
Hong Zhu 1 , Rampi Ramprasad 1
1 Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States
Show AbstractNext generation electronic devices will use a HfO2 based dielectric layer (instead of the currently used silica layer), and a metal gate on top of HfO2 (instead of polycrystalline Si). The choice of the metal on top of HfO2 is determined by the metal work function (i.e., the alignment of the metal Fermi level with the band edges of the Si substrate beneath hafnia). TaCxN1-x is a promising gate metal due to its resistance to O diffusion and due to the variation in the work function that can be accomplished by composition changes, while Pt is a stable, well-studied, model metal gate. There is very strong evidence that shifts in the metal work function occurs depending on the nature of the interface between HfO2 and the metal. Thus, controlling the composition of the gate metal and the metal:HfO2 interface structure are seen as approaches to controlling the “effective” work function of the metal gate. In this work, we will first present the work function of several surfaces of pure Pt and TaCxN1-x (with x spanning from 0 to 1) through first principles calculations. Our computed results for these pure systems are in good agreement with available experimental data. We will then present our results concerning metal work function shifts (resulting in an “effective” work function) when an interface between the metal and HfO2 is created. We find that the effective work function is a strong function of interfacial O coverage. In order to determine the most probable interfacial O coverage for a given temperature and O pressure during deposition, we use density functional theory total energy results within a statistical thermodynamics model. We are thus able to relate experimental processing conditions to interface morphology (for a given composition), and consequently to the effective work function. This approach thus offers a method for guiding the right choice of the metal electrode composition and metal:HfO2 interface morphologies so as to result in a metal gate with the desired effective work function.
12:45 PM - W3.10
Multi-Scale Transport Calculations for Molecular Wires and Carbon Nanotubes.
Kenji Hirose 1 , Hiroyuki Ishii 2 , Nobuhiko Kobayashi 3
1 , NEC, Tsukuba Japan, 2 , AIST, Tsukuba Japan, 3 , Univ. of Tsukuba, Tsukuba Japan
Show AbstractWe present multi-scale calculation methods for the electric transport of molecular wires and carbon nanotubes (CNT) bridged between electrodes. Here we present three kinds of calculation methods. The first is the recursion-transfer-matrix (RTM) method, which is a reliable tool to calculate scattering waves in plane-wave expansions. Combined with the non-equilibrium Green’s function (NEGF) method and density-functional formalism, we perform calculations of transport properties of single molecules and molecular wires attached to metallic electrodes. We show the exponential behaviour of conductance as a function of molecular wires due to tunnelling process determined by the HOMO-LUMO energy gap and detailed data of electronic states of molecular wires at contacts under strong electric fields. We will also discuss the inelastic electron-phonon coupling effect to transport for the local heating problems.The second method is the time-dependent wave-packet approach. Based on the linear-response Kubo formula with tight-binding approximation, we perform O(N) calculations for the transport of huge system up to several μm length. We apply the method for the CNT-FET device and will show the mobility of CNT-FET under electron-phonon scatterings. Also we will present the transitions of transport behaviors of CNT device from ballistic to diffusive regimes.Finally, we present the third method using NEGF with atomically localized basis sets within density-functional formalism. This method enables us to treat fairly large system (up to several 10 nm) without any experimentally determined parameters. We apply this method to the contact problems of CNT to metallic electrodes. We show the effects of electrodes on conductance of metallic and semi-conducting CNT, especially taking various kinds of electrode materials into account.
W4: Hetrogenous Materials
Session Chairs
Seung Soon Jang
Nick Reynolds
Tuesday PM, December 02, 2008
Constitution B (Sheraton)
2:30 PM - **W4.1
Computational Thermodynamics Based Design of Cermets for Long Term Microstructural Stability.
Shiun Ling 1 , Thirumalai Neeraj 1 , ChangMin Chun 1 , Rao Bangaru 1 , Zi-Kui Liu 2
1 Corporate Strategic Research, ExxonMobil Research & Engineering Co., Annandale, New Jersey, United States, 2 Department of Materials Science and Engineering, Pennsylvania State University, State College, Pennsylvania, United States
Show AbstractCeramic-metal composites (Cermets) are good candidate materials for wear and erosion applications that require a combination of high hardness and toughness. Existing commercial cermets, for example, WC-Co are mostly used in ambient temperature applications such as drill bits, cutting tools etc. Applications in the petroleum and chemical industries, however, expose the cermets to high temperature (>400C) for prolonged periods of time. Under these conditions the cermet microstructure can change and degrade over time affecting its performance.In this investigation, we have employed the CALPHAD based computational thermodynamics methodology to understand the long term microstructural stability of two multicomponent cermet systems: chromium carbide in Fe-Ni-Cr alloy binder matrix, and titanium diboride in Fe-Ni-Cr alloy binder matrix. The phase stability calculations performed were validated with microstructural observations of specimens subjected to long-term elevated temperature aging up to 1000 hours in lab experiments. The science insights generated in this study were then used to guide the design of cermet systems with stable microstructures for long term service at high temperature.
3:00 PM - W4.2
Auxetic to Non-auxetic Transition in Porous Carbon Networks.
Vitor Coluci 1 , Pedro Autreto 2 , Douglas Galvao 2 , Ray Baughman 3
1 , Center for Higher Education on Technology, University of Campinas, Limeira, Sao Paulo, Brazil, 2 Applied Physics, State University of Campinas, Campinas, Sao Paulo, Brazil, 3 , Alan MacDiarmid Nanotech Institute, Richardson, Texas, United States
Show AbstractMost materials shrink laterally and become less dense when stretched. Materials that laterally expand (contract) when stretched (compressed) are called auxetic. This unusual property can be quantified by the Poisson’s ratio (PR) values. PR is defined as the ratio of percent lateral contraction to percent applied tensile elongation. While conventional materials present positive PR values, auxetics present negative ones. An important characteristic of many materials exhibiting negative PR is the presence of pores (void spaces) in their structure. The pores allow structural changes that can be responsible for the appearance of the auxetic property. Thus, it would be important to investigate how the filling of these pores would affect the material mechanical properties, in particular under which conditions the auxetic behavior can be suppressed. A few auxetic crystalline phases are known. In 1993 Baughman and Galvao [1] proposed hypothetical auxetic carbon-based crystalline networks with polydiacetylene motifs. In order to investigate the pore filling effect in auxetic materials we selected one of these hypothetical allotropic carbon phases – the hinged polydiacetyilene. This is a very convenient structure to our study since it presents the two characteristics we are interested here, negative PR and pores. We used He atoms to fill the pores in the hinged structure. We then calculated the PR values as a function of the number of He atoms randomly inserted in a supercell (3x3x3 unit cells). For each He concentration the supercell is reoptimized (cell parameters and atomic positions) and the elastic constants calculated using the second derivative procedure. The interactions among He and C atoms and C-C atoms were described using the well-known universal molecular force. Our results show that after a critical limit of number of He atoms (~2% mass fraction) a transition from negative to positive PR was observed, that is, the auxetic behavior is eliminated. These results confirm that the porosity plays a fundamental role defining the auxetic behavior. Although the hinged polydiacetylene is a hypothetical structure we believed that the general trends obtained here should be valid for other porous auxetic structures. In fact a transition of this kind (from negative to positive PR) was recently reported [2] for a porous material composed of mixed single and multiwalled carbon nanotubes (CNTs). [1] R. H. Baughman and D. S. Galvao, Nature 365, 735 (1993).[2] L. H. Hall, V. R. Coluci, D. S. Galvao, M. E. Kozlov, M. Zheng, S. O. Dantas, and R. H. Baughman, Science 320, 504 (2008).
3:15 PM - W4.3
Multiscale Modeling in Polymer Clay Nanocomposites for Simulations Based Design.
Dinesh Katti 1 , Kalpana Katti 1
1 Civil Engineering, North Dakota State University, Fargo, North Dakota, United States
Show Abstract3:30 PM - W4.4
Interfacial Friction of Carbon Nanotubes in Diamond Matrix Nanocomposites.
Lili Li 1 , Zhenhai Xia 1 , William Curtin 2
1 Department of Mechanical Engineering, University of Akron, Akron, Ohio, United States, 2 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractInterfaces play an essential role in toughening in ceramic composite systems. In the failure process of the composites, fiber pullout from the matrix is one of the major sources of energy dissipation. In ceramic nanocomposites the interface roughness approaches atomic length scale due to the extremely small size of the reinforcement and larger interfacial area is available compared with microscale composites. A fundamental understanding of the origins of friction and its role in determining the toughening would be a key to design a new class of tough nanocomposites. In this work, the interfacial friction in carbon nanotube (CNT) reinforced diamond composites were analyzed by using molecular dynamics. The preliminary results demonstrate that multiwall CNTs with interwall sp3 bonding show significant enhancement in interfacial load transfer by more than 2 times over single-wall CNTs. The interfacial frictional stress increases with an increase of normal stress on the CNTs, and shows a relatively high value even when the normal stress is in tension due to van der Waals interaction between nanotube and matrix.
3:45 PM - W4.5
To What Extent Can Current Interatomic Potentials Accurately Describe Grain Boundaries in Strontium Titanate?
Nicole Benedek 1 2 , Alvin Chua 2 , Christian Elsasser 3 , Adrian Sutton 2 , Mike Finnis 1 2
1 Materials, Imperial College London, London United Kingdom, 2 Physics, Imperial College London, London United Kingdom, 3 , Fraunhofer Institute for Mechanics of Materials, Freiburg Germany
Show AbstractMultiscale simulation methods are currently at the forefront of materials modelling research. Interatomic potentials are often at the heart of such approaches, because they provide the bridge between the very detailed electronic level and the coarser-grained mesoscopic level. For example, an interatomic potential model fitted to first-principles data was recently used in large-scale molecular dynamics (MD) simulations of domain wall motion in ferroelectric lead titanate. However, systematic studies on the reliability of these potentials, particularly for ionic oxides, are sorely lacking. Strontium titanate (STO) is a perovskite-structured oxide commonly used to make barrier layer capacitors. The technological importance of STO is directly linked to its interfacial and grain boundary properties, which are at present poorly understood. A complete understanding (including links with experiment) requires information from many length scales, including electronic and atomistic. We have tested the ability of a number of interatomic potentials from the literature to accurately describe the structures and energetics of some simple grain boundaries in STO. Our aim was to identify a reliable potential for use in molecular dynamics simulations of complex, nonstoichiometric STO grain boundaries. The potentials we have tested are of three types: Born model, shell model and partial charge model. We have also performed a detailed first-principles Density Functional Theory (DFT) study of these boundaries and used this data to validate the interatomic potentials. None of our chosen potentials can reproduce the energy ordering of the boundaries predicted by the DFT calculations. Some of the boundary structures produced by the potentials agree reasonably well with the DFT structures. We discuss the implications of our findings for STO grain boundary research and their relevance to other titanate perovskite systems.
W5: Polymer & Biomaterials
Session Chairs
Seung Soon Jang
Nick Reynolds
Tuesday PM, December 02, 2008
Constitution B (Sheraton)
4:30 PM - **W5.1
Investigation of Epoxy Molecular Structural Features for Increased Polymer Deviatoric Deformation.
Stephen Christensen 1
1 Phantom Works, Boeing, Seattle, Washington, United States
Show AbstractRecently developed continuum level structural analysis indicates that the composite polymeric matrix deviatoric deformation is key to maximizing composite ultimate properties. Distortion is a dissipative process controlled chiefly by torsional rearrangements of the polymer structure. Using computational methods coupled with experiment we have established a methodology for new composite matrix formulation that takes advantage of relations we have established between desired bulk properties and the molecular structure. Our hierarchical multi-scale approach required the development of several new techniques. A method to simulate fully dense and equilibrated high glass transition temperature and high cross-link density epoxy resins while maintaining proper stoichiometry and a means for extracting a value for the critical dilatation and distortion that relates to our bulk property measurement. Results of our simulations and experiments will show that improved composite performance is achievable through proper selection of the organic moiety of the amine and epoxy components of the resin system.
5:00 PM - W5.2
Morphologies of Perfloroalkylethylacrylated Co-Oligomers and Water Dynamics at Their Interface.
Gokhan Kacar 1 , Alimet Sema Ozen 2 , Canan Atilgan 3
1 Materials Science and Engineering, Sabanci University, Istanbul Turkey, 2 Materials Science and Engineering, Sabanci University, Istanbul Turkey, 3 Materials Science and Engineering, Sabanci University, Istanbul Turkey
Show AbstractBlock co-oligomers are known as self-organizing synthetic materials. We investigate the equilibrium and dynamical properties of two such systems which are known to form superhydrophobic surfaces, [1,2] (i) styrene-co-perfloroalkylethylacrylate (styrene-co-PFA) and (ii) methylmethacrylate-co-perfloroalkylethylacrylate (MMA-co-PFA). We first study the morphologies of each system in the solvent tetrahydrofuran (THF) using Dissipative Particle Dynamics method.[3] These fluorocarbon oligomer systems form different micellar or lamellar morphologies at various concentrations, which are studied in the range of 5 – 100 mol %. The model systems consist of A10BD7 oligomers, where the coarse grained beads carry the characteristics of the atomic details only through the interaction parameter: A is the styrene or the MMA monomer, B is the linker H2C=(CH)–(C=O)–O–(CH2)2 and D is a CF2 (or CF3) unit. Spherical micelles form at 10 % styrene-co-PFA, and a phase transition to the cylindrical micellar structure is obtained at 80 % concentration. On the other hand, 10 % and 20 % MMA-co-PFA form cylindrical micellar and lamellar structures, respectively. To understand the origins of the interactions between the beads, the global – local hardness and polarizability of the beads are calculated using quantum mechanical methods. Finally, to study the dynamics of water on these surfaces, atomic details of the oligomers are reverse mapped on selected morphologies and detailed molecular dynamics simulations are performed. The relaxation times and ordering of water molecules at the interface of the self-organized units are deduced, and the differences from the bulk water properties are related to the superhydrophobic nature of these materials.References:1. Simsek, E. Wettability of smooth and rough surfaces of perfluoroacrylate copolymers. M.Sc. Thesis, Sabanci University, Tuzla, Istanbul, 2006. 2. Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Angew. Chem. Int. Ed. 2004, 43, 5210 – 5213.3. Ozen, A.; Sen, U.; Atilgan, C. J. Chem. Phys. 2006, 124, 64905 – 64913.
5:15 PM - W5.3
Phase Transition and Morphology of Polydispersed ABA’ triblock copolymers - Determined by Continuous and Discrete Simulations.
Armand Soldera 1 , Yue Qi 2 , Weston Capehart 2
1 Chemistry, Universite de Sherbrooke, Sherbrooke, Quebec, Canada, 2 R&D, General Motors, Warren, Michigan, United States
Show AbstractDifferent paths exist to improve performance of hydrocarbon-based block copolymer proton exchange membrane. A triblock copolymer ABA’ architecture, constituted of hydrophilic (A and A’) and hydrophobic (B) strands, offers special advantages regarding to random copolymers: improved connectivity, preserving mechanical properties when swollen by water. A certain order of the two phases has to be maintained inside the system for high proton conductivity and mechanical stability. However, the polycondensation of the different blocks results in a polydispersity in the weight of the different strands, which greatly affects the order to disorder transition (ODT) of block copolymers. The purpose of the presentation is then to consider polydispersity in triblock copolymer by supplementing calculations using the random phase approximation (RPA) by a discrete approach performed at the mesoscale level to further capture morphology details. In the latter calculations, the Edwards mean field equation is solved in a self-consistent way using the SUSHI code. To carry out this compatible and complementary procedure, a discretization process of polymer length distribution is exposed. Both methods can predict ODT accurately. Effects of individual chain lengths on morphology, phase segregation and mechanical properties of PEM are specifically regarded using the mesoscale modeling.
5:30 PM - W5.4
Forces Between Functionalized Silica Nanoparticles.
Ahmed Ismail 1 , Matthew Lane 2 , Michael Chandross 3 , Kelly Anderson 4 , Chris Lorenz 5 , Gary Grest 2
1 Performance Assessment and Decisions Analysis, Carlsbad Programs Group, Sandia National Laboratories, Carlsbad, New Mexico, United States, 2 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States, 3 Computational Materials Science and Engineering, Sandia National Laboratories, Albuquerque, New Mexico, United States, 4 Modeling and Simulation, Proctor and Gamble, Cincinnati, Ohio, United States, 5 Division of Engineering, King's College, London, London United Kingdom
Show AbstractPolymer-coated nanoparticles have a wide variety of applications including drug delivery, adhesives, coatings, and magnetics. Although their complexity precludes atomistic simulations of large numbers of nanoparticles in solution, using large-scale molecular dynamics simulations, it is possible to study the interaction between pairs of nanoparticles in an explicit solvent and express these interactions in terms of empirical forces such as solvation, depletion, and lubrication forces. From these simulations, we can compute the forces exerted on nanoparticles treated with explicit-atom models, which can be used in coarse-grained simulations at larger length and time scales. In particular, we present results between two bare or polymer-grafted silica nanoparticles as a function of chain length, core size, and approach velocity. We show the work required to bring together two bare silica nanoparticles in water, alkylsilane-coated nanoparticles in decane, or poly(ethylene oxide)-coated nanoparticles in water, and compare them to the expected behaviors of nanoparticles in solution. We also examine the “equilibrium” forces on a pair of stationary nanoparticles, as well as the work required to retract the particles following an approach.
5:45 PM - W5.5
Computational Investigations of Assembly and Stabilization of RNA Nanoparticles.
Yaroslava Yingling 1 , Andrey Semichaevsky 1 , Abhishek Singh 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractBiomolecular nanostructures hold a tremendous promise for effective use in such applications as drug delivery, nanoelectromechanical systems, molecular sensors, and molecular lithography. RNA molecules are especially appealing molecules for nanodesign since they can carry genetic information, posses catalytic properties and can naturally fold or can be programmed to self-assemble into complex structures. Moreover, protein-free RNA nanoparticles do not induce a detectable immune response which is crucial for use in medical applications and drug delivery. RNA molecules can make up effective nanoparticles, nanotubes and scaffolds. However, these structures are not determined by base pairing alone and unpaired residues play a critical role in nanodesign and super-assembly. Yet there is a limited understanding of the rules of formation of RNA superstructures. For successful design we need to understand and control the intermolecular associations, base on natural tendency and favorability and various physical components. We use molecular modeling to understand self-assembly processes of natural and synthetic RNA. The most common motifs found in nature and used in bionanotechnology are hairpin loops which consist of a helical part and a loop with unpaired residues. The unpaired residues in these elements can lead to further super-assembly of RNA structures via formation of the loop-loop interactions. These loop-loop interactions regulate biological functions in both prokaryotic and eukaryotic organisms and are also actively used in bionanotechnology for self-assembly of RNA building blocks into novel nanostructures. It has been observed that the super-assembly of RNA directly depends on the presence and specific concentration of ions. In order to understand the role of ions in loop-loop formation and stability, we conducted a series of explicit solvent atomistic molecular dynamics simulations, electrostatics simulations, and normal mode analysis of ten distinct kissing loops elements taken from various organisms. We discovered that in most organisms the loop-loop assembly process depends on the presence of electronegative and hydration channel. The size of this channel and RNA sequence determines the stability, the hydrogen bonding interactions and the angle of the distinct kink between stems. Using these angles we can engineer the assembly of RNA building blocks via kissing loops motifs into nanostructures of a predefined geometry. We will show how we used the specific RNAI/RNAII loop-loop motif to design a novel RNA hexagonal nanoparticle and RNA fabric, the assembly of which was verified by experiment.
W6: Poster Session I: Computational Material Design
Session Chairs
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - W6.1
An efficient Method to Study Ordering in Low-symmetry Materials.
Tim Mueller 1 , Gerbrand Ceder 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMany material properties can be modeled through the use a cluster expansion, in which the property is expressed as a linear combination of localized interactions. Cluster expansions are commonly used to develop effective Hamiltonians of systems with substitutional disorder, and they have been successfully used to identify material ground states, calculate phase diagrams, and predict other material properties. The coefficients of the linear expansion are typically fit to a set of training data generated using ab-initio methods. For highly symmetric systems such as simple bulk crystals, symmetry can be used to greatly reduce the number of significant independent coefficients. For such systems, a small set of training data is required to generate sufficiently accurate cluster expansions. However, for low-symmetry systems such as nanoparticles, a much larger number of unique coefficient values must be determined, requiring a larger set of training data. In addition, the cost of performing ab-initio calculations to generate the training data is greatly increased due to the low symmetry of the system. For these reasons, it has been computationally impractical to use the cluster expansion to study multi-component nano-structured materials with the same level of accuracy as bulk materials. We have addressed this problem by developing a Bayesian framework for determining cluster expansion coefficients, and we present new methods developed using this framework. Our methods explicitly combine physical insight with the training data to significantly reduce the prediction error of a cluster expansion for a given training set size. The new methods also allow for more accurate estimation of the average prediction error for a given cluster expansion, resulting in generally higher quality cluster expansions. We demonstrate how our methods can be used to generate expansions for low-symmetry structures such as nanoparticles at a fraction of the current computational cost. This approach makes it computationally feasible to study atomic ordering and surface segregation in multi-component nanostructures at a nearly ab-initio level of accuracy.
9:00 PM - W6.10
Knowledge-based Approach to Gas Sorption in Glassy Polymers by Combining Experimental and Molecular Simulation Techniques.
Matthias Heuchel 1 , Ole Hoelck 2 3 , Martin Boehning 2 , Martin Siegert 1 , Dieter Hofmann 1
1 Center for Biomaterial Development, Institute of Polymer Research, GKSS Research Center, Teltow Germany, 2 , Federal Institute for Materials Research and Testing (BAM), Berlin Germany, 3 Micro Materials Center, Fraunhofer Institute for Reliability and Microintegration IZM, Berlin Germany
Show Abstract9:00 PM - W6.11
Modeling Equilibrium Concentrations of Bjerrum and Molecular Point Defects and their Complexes in Ice Ih.
Maurice de Koning 1 , Alex Antonelli 1
1 Instituto de Física, Universidade Estadual de Campinas, Campinas Brazil
Show AbstractWe present a density-function theory (DFT) study of Bjerrum-defect trapping centers involving the molecular vacancy and interstitial in ice Ih. As a first step, we compute the intrinsic migration barrier to D-defect motion using the nudged elastic band (NEB) method and find them to be of the same order of magnitude as the energy barriers involving intrinsic L-defect motion. This finding suggests that intrinsic mobility factors cannot explain the experimentally observed inactivity of D defects, supporting the idea that D defects are trapped at other lattice-defect sites. Next we study the defect complexes formed by the combination of isolated D and L defects with a molecular vacancy. The corresponding geometries show that the formation of these aggregates significantly reduces elastic distortions that are present in isolated Bjerrum defects, leading to a significant binding energy. In contrast to the complexes involving the molecular vacancy, however, the results suggest that the molecular interstitial binds preferentially to the D-type Bjerrum defect. Using both theoretical binding and formation free energies as well as available experimental data we find that the preferential binding and the substantial presence of the interstitial as the predominant point defect in ice Ih may lead to conditions in which the number of free D defects becomes considerably smaller than that of free L defects. Such a scenario could possibly be involved in the experimentally observed inactivity of D-type Bjerrum defects in the electrical properties of ice Ih.An analysis of the energetics involved in the formation of both defect complexes reveals a significant binding energy, indicating that the molecular vacancy represents a strong trapping center for Bjerrum defects. On the other hand, the fact that there is no difference between the absolute values of the binding energies for both D and L defects suggests that the vacancy affects both species of Bjerrum defects in a similar fashion, possibly ruling out the vacancy trapping centers as an explanation for the experimentally observed inactivity of D defects.
9:00 PM - W6.12
Modeling of Memorial Effects in Spinodal Decomposition of a Binary Systems.
Nicolas Lecoq 1 , Helena Zapolsky 2 , Peter Galenko 3
1 Groupe de Physique des Materiaux, University of Rouen, Saint Etienne du Rouvray France, 2 Groupe de Physique des Matériaux, University of Rouen, Saint Etienne du Rouvray France, 3 Institue of Materials Physics in Space , German Aerospace Center, Cologne Germany
Show Abstract Considerable recent interest has focused on spinodal decomposition in a wide variety of physical systems. Theoretical efforts thus far have mainly concentrated on understanding the linear stage of this phase separation. This stage of spinodal decomposition has been theoretically described by Cahn-Hilliard (CH) equation [1,2] which is considered as a diffusion-reaction type of equation. This equation is valid if the time of relaxation of system to local equilibrium is shorter than the characteristic time for dynamics of spinodal decomposition. However, if the propagation speed of wave, which characterise a spinodal decomposition, is of the order of the atomic diffusion speed the memory effect in the diffusion field can not be ignored [3]. We consider the modification of the Cahn–Hilliard equation when a time delay process through a memory function is taken into account [4]. As a result, a causality diffusion equation has a similar form to the telegraph equation with the decomposition delay described by the term τd ∂2 c / ∂t2 , where τdis the relaxation time for the atomic diffusion flux with memory effects. Using this model, we study the process of spinodal decomposition in fast phase transitions by computational methods. Finite-time memory effects are seen to affect the dynamics of phase transition at short times and have the effect of delaying, in a significant way, the process of rapid growth of the concentration that follows a quench into the spinodal region. Results of our modeling demonstrate that these effects are important in earlier stage of spinodal decomposition.[1] J.W. Cahn and J.E. Hilliard, J. Chem. Phys. 28, 258 (1958).[2] J.W. Cahn, Acta Metall. 9, 795 (1961).[3] P. Galenko, Phys. Lett. A 287, 190 (2001). [4] P. Galenko and D. Jou, Phys. Rev. E 71, 046125 (2005).
9:00 PM - W6.13
Calculation of Protein Dielectric Response using Molecular Dynamics Simulations.
George Patargias 2 , Sarah Harris 2 , John Harding 1
2 Department of Physics and Astronomy, University of Leeds, Leeds United Kingdom, 1 Engineering Materials , Univ. of Sheffield , Sheffield United Kingdom
Show AbstractProteins contain many ionizable groups spread throughout the structure. It is possible to measure the ionization of the protein experimentally, but difficult to ascribe this to individual groups within the molecule. In principle, it is possible to calculate the ionization state of groups individually but for large proteins, it is not practical to perform the calculations for all the possible combinations of ionized groups coupled with the conformations of the protein. Therefore simple models based on classical dielectric continuum theory have been used. Such models often give acceptable results, but they can fail for no apparent reason. We have performed a series of molecular dynamics calculations on a candidate protein, Hen Egg White Lysozyme, to investigate the ionization of a number of groups and use these results to test the continuum theory. The importance of electrostatic interactions between these ionizable groups in determining protein structure and function is well established. However, the dielectric properties which determine the magnitude of charge-charge interactions within biomolecules have received relatively less attention and remain poorly understood theoretically. The dielectric response of different regions of Hen Egg White Lysozyme has been mapped using molecular dynamics simulations. The dielectric constant for a particular region of the protein is obtained via the following approach. A charged probe is inserted in the region in question and the protein's reorganization energy is calculated through a free energy perturbation method. A value for the dielectric constant is then obtained by relating this reorganization energy to the Born solvation energy of the same charged probe in a continuum dielectric medium. We discuss the effective dielectric constants in the light of the changes in the molecular conformations predicted by the simulations. High values of effective dielectric constants of the protein correlate strongly with large relaxations of the protein. Moreover, our results show that the assumption of a homogeneous dielectric constant for the protein is not valid.
9:00 PM - W6.14
Multiscale Computer Simulation of Tensile Failure in Polymer-Coated Silica Aerogels.
Brian Good 1
1 Materials and Structures Division, NASA GRC, Cleveland, Ohio, United States
Show AbstractThe low thermal conductivities of silica aerogels have made them of interest to the aerospace community for applications such as cryotank insulation. Recent advances in the application of conformal polymer coatings to these gels have made them significantly stronger, and potentially useful as lightweight materials for impact absorption as well. In this work, we perform multiscale atomistic computer simulations to investigate the tensile strength and failure behavior of silica and polymer-coated silica aerogels.The gels' nanostructure is simulated via a Diffusion Limited Cluster Aggregation (DLCA) procedure, modified to introduce loops that increase the connectivity of the gel network and make the model gel more rigid. The procedure produces fractal aggregates that exhibit fractal dimensions similar to those observed in real aerogels. The largest distinct feature of the clusters is the so-called secondary particle, typically tens of nm in diameter, which are composed of primary particles of amorphous silica an order of magnitude smaller. The secondary particles are connected by amorphous silica bridges that are typically smaller in diameter than the particles they connect.We investigate tensile failure via the application of a uniaxial tensile strain to the DLCA clusters. In computing the energetics of tensile strain, the detailed structure of the secondary particles is ignored, and the interaction among secondary particles is described by Morse pair potentials, representing the strain energetics of the silica gel and the polymer coating, parameterized such that the potential ranges are much smaller than the secondary particle size. The Morse parameters are obtained by separate atomistic simulation of models of the interparticle bridges and polymer coatings, with the tensile behavior of these bridges modeled via molecular statics.We consider the energetics of tensile strain and tensile failure, and compare qualitative features of low-and high-density gel failure.
9:00 PM - W6.15
Computational Study on the Morphological Instability of a Nematic/Isotropic Interface in a Temperature Gradient.
Ezequiel Soule 1 2 , Nasser Abukhdeir 1 , Alejandro Rey 1
1 Chemical Engineering, Mcgill University, Montreal, Quebec, Canada, 2 INTEMA, University of Mar del Plata - CONICET, Mar del Plata, Buenos Aires, Argentina
Show AbstractThe morphological stability of the nematic/isotropic interface of 5CB (pentyl-cyanobiphenyl) in a temperature gradient is studied via computational 2-D simulations. A Landau-de Gennes type quadrupolar tensor order parameter model for the first-order isotropic/nematic transition is used. An energy balance, taking anisotropy into account, was derived and incorporated into the time-dependent model.The evolution of a small amplitude sinusoidal interface, representing a planar interface with a small perturbation to the planar shape, was analyzed for different wavelengths of the perturbation and in the presence of different temperature gradients. Dispersion diagrams (growth/shrinkage velocity of the perturbation vs. wavelength), are constructed in the small-amplitude regime and compared with a sharp-interface linear stability theory. It was found that, besides the shape instability, there is a texturing process. When the director is initially homeotropic at the nematic/isotropic interface, a defect forms and then is shed from the interface into the bulk nematic phase, giving rise to a region in the interface where the orientation becomes planar. This can be explained taking into account that the planar orientation is energetically more favorable than the homeotropic orientation. A similar defect formation and shedding mechanism has been observed in simulation of nematic spherulite growth [Wincure, B. M. and Rey, A. D.; Nano Letters, 2007, 7, 1474], due to the presence of strong orientation gradients along the interface. When the director is initially planar and in the same plane that the perturbation, a second nematic region is formed, also with a planar orientation but perpendicular to the plane of the perturbation. This second region is nucleated at the interface and then grows into the bulk. This can be explained considering that the homeotropic orientation parallel to the perturbation implies a distortion in the director as it follows the shape of the interface, while if the orientation is perpendicular to the plane of the perturbation there is no distortion. This type of phenomena has been observed in directional growth experiments [Bechhoefer, J. in “Pattern Formation in Liquid Crystals”, eds. Buka, A. and Kramer L., Springer, N.Y., 1996], although it was related to an anchoring conflict between the nematic isotropic interface and the plates, while in the present case is related with the distortion energy.
9:00 PM - W6.16
Exfoliation of a Layer of Platelets in Polymer Matrix: Effect of Entropic Constraints and Solvent Quality by a Monte Carlo Simulation.
Ras Pandey 1 , Barry Farmer 2
1 Physics and Astronomy, University of Southern Mississippi, Hattiesburg, Mississippi, United States, 2 , Air Force Research Laboratory, Dayton, Ohio, United States
Show AbstractDispersion of layered sheets (a model for clay platelets) in a polymer solvent matrix is studied by an efficient Monte Carlo simulation on a discrete cubic lattice. A layer of four sheets with a small inter-layer distance is placed at the center of the lattice embedded in a random distribution of polymer chains. Sheets and polymer chains are modeled by tethered nodes with fluctuating bonds in planar and linear chain configurations respectively with appropriate molecular weight. Chains and platelets interact with a short range interaction and execute their stochastic movement with the Metropolis algorithm. At low molecular weight of the polymer matrix, the repulsive interaction between polymer and sheets enhances exfoliation while attractive interaction suppresses it. Increasing the molecular weight of the polymer chains leads to entropic trapping in layered structure in an effective cage due to entanglement. Effects of molecular weight of the polymer solvent and its quality on the dispersion will be discussed.
9:00 PM - W6.17
Phase-field Modelling of Partially Molten Grain Systems.
Frank Wendler 1 , Britta Nestler 1
1 Institute of Computational Engineering, Karlsruhe University of Applied Sciences, Karlsruhe Germany
Show AbstractMany material properties of alloys as well as ceramics are to a large extend determined by their specific morphology on the grain scale. As compared to grain growth in completely solid samples, polycrystalline compounds in contact with a liquid phase show a completely different coarsening dynamics, often accompanied by the formation of a liquid pore network. The clarification of the process is important to improve metal forming methods as thixoforming and liquid-phase sintering, and has a long-ranging impact on understanding partial melt rock formation in structural geology [2].To model the microstructure evolution in partially molten 2D and 3D systems, we apply the formerly introduced multi-phase-field model of Allen-Cahn type [1]. Grains of different orientations as well as liquid phase regions are thereby characterized by a set of continuous phase-field parameters. Fist, volume constraints for the melt phase are introduced to establish an equilibrium situation between solid and liquid state, using a method given in [3]. Based on the results of large scale simulations, the key parameters determining the coarsening rate are found to be the solid-solid-liquid dihedral angle (studying values from 10° to 90°), the liquid fraction and distribution of liquid in the grain structure. A comparison with the evolution of an experimental analoque system (norcamphor-alcohol) is presented. As in the experiments, partially wetted grain boundaries form in the course of the simulations, but modify the coarsening dynamics only sligthly. Furthermore, the pore network formation for dihedral angles < 60° and the transition to complete wetting in 3D grain systems is studied. For many polycrystalline minerals and ceramics, the solid-liquid surface energy anisotropy is very large. We present the results of a phase-field study on grain boundary evolution and grain growth when using a hexagonal, facet-forming anisotropy.[1] B. Nestler, H. Garcke and B. Stinner, Phys. Rev. E 71 (2005) 041609-1[2] F. Wendler, J.K. Becker, B. Nestler, P.D. Bons and N.P. Walte, submitted to Computers & Geosciences[3] H. Garcke, B. Nestler, B. Stinner and F. Wendler, Math. Mod. Meth. Appl. Sci. 18 (2008), in press
9:00 PM - W6.20
Ab-initio Molecular Dynamics Simulations of Polyaromatic Hydrocarbons with Palladium (II) Acetylacetonate.
Samir Mushrif 1 , Gilles Peslherbe 2 , Alejandro Rey 1
1 Chemical Engineering, McGill University, Montreal, Quebec, Canada, 2 Centre for Research in Molecular Modeling and Dept. of Chemistry & Biochemistry, Concordia University, Montreal, Quebec, Canada
Show AbstractThe effect of addition of an organometallic salt on the characteristics and applications of activated carbon adsorbents is widely studied in the literature. It is well known that the presence of metal species affects the pore size and surface area generation of the adsorbent and thus controls its end application and performance. However, a limited investigation has been done to gain the fundamental knowledge of the chemistry of organometallic salts with the carbon matrix and their interactions, under different processing conditions. This research focuses on developing an unambiguous understanding of the metal-salt and carbon matrix interactions using ab-initio molecular dynamics simulations. We simulate the carbon matrix using polyaromatic hydrocarbons and palladium (II) acetylacetonate is chosen as the metal salt of interest due to its role in enhancing hydrogen adsorption capacity of active carbon adsorbents. Ab-initio calculations are performed using the planewave-pseudopotential implementation of the density functional theory and the Car-Parrinello scheme is used for molecular dynamics. Simulations are carried out at 500, 800 and 1200 K and the effect of temperature on the organometallic interactions is also studied.
9:00 PM - W6.21
The Effect of Extensional Flow on the Phase Diagram and Orientational Structure of Carbonaceous Mesophase Mixtures.
Mojdeh Golmohammadi 1 , Alejandro Rey 1
1 , McGill University, Montreal, Quebec, Canada
Show AbstractA Maier-Saupe model for binary mixtures of uniaxial discotic nematogens under extensional flow is formulated to describe the phase ordering in carbonacoues mesophases, differing only in molecular weight. The orientational structure and the thermodynamic phase diagram depend on (i) the intrinsic properties of the mixture, i.e. molecular weight difference and the interaction parameter, (ii) the operating properties, i.e. concentration and temperature and (iii) the processing condition, i.e. flow rate. The results obtained from our pervious study [Golmohammadi et al., JCP, 2008, submitted] shows that depending on the intrinsic properties of the system, two types of uniaxial nematic mixtures arise: (i) non-ideal mixtures with a minimum in its Nemtic to Isotropic (NI) transition as a function of the concentration, and (ii) ideal mixtures with a monotonic trend of the NI transition temperature as a function of concentration. In the current work we study the effect of extensional flow on the type of the mixture and the value of the concentration corresponding to the minimum transition temperature. The results can be used to design the processing flow rate to form a desired fiber structure.
9:00 PM - W6.22
Design Active Nanostructures: Simulated Nanorockets Propelled by Catalytic Chemical Reactions.
Yunfeng Shi 1
1 Department of Materials Science and Engineering, North Carolina State Univ., Raleigh, North Carolina, United States
Show AbstractThis talk details a molecular-level design of an autonomous energy transducer that converts chemical energy to mechanical work by means of molecular dynamics simulations. The propulsion comes from asymmetric catalytic chemical reactions. This design is motivated both by the catalytic molecular motors in hydrogen peroxide solutions and by conventional rocket engines. However, due to the differences in size, speed and viscosity of the environment, the nanomotor using the current design works in a different hydrodynamic regime than the other two cases. Thermodynamic efficiency of the nanomotor is measured by imposing various external loads. Moreover, the maximum efficiency is shown to be proportional to a dimensionless number, which can be served as a guideline to design future functional nanostructures for energy conversion. One final goal of this work is to encourage experimental efforts on catalytic molecular motors in reactive gas phases.[1] Y. F. Shi and D. W. Brenner, NanoLetter, under review (2008).*New address: Department of Materials Science and EngineeringRensselaer Polytechnic Institute, Troy, NY 12180
9:00 PM - W6.23
The Design and Calculation of Photonic Bandgaps in Two-Dimensional Photonic Quasi-crystals by FDTD Simulation Method.
Jong-Bin Yeo 1 2 , Sang-Don Yun 1 , Hyun-Yong Lee 2
1 Department of Functional Nano Fine Chemicals, Chonnam National University, Gwangju Korea (the Republic of), 2 Faculty of Applied Chemical Engineering, Chonnam National University, Gwangju Korea (the Republic of)
Show AbstractIn the this present work, we design and calculate photonic bandgaps (PBGs) of 2 dimensional photonic quasicrystal (2D PQCs) structures. After making the rotation symmetrical PQCs model, we establish the FDTD method for model by the operator conversion. We prove that 2D PQCs is good model of the possibility for wide omni-photonic bandgaps. Photonic crystals (PCs) are high index contrast periodic arrays characterized by photonic bandgaps (PBGs) or electromagnetic stop bands. And photonic quasicrystals (PQCs) are aperiodic structure having characteristic of rotation symmetric property. PQCs have omni-PBGs region at low index contrast array. PBGs have a functional similarity to the energy gap that electrons experience in semiconductor crystals. That is PQCs can prohibit the propagation of electro-magnetic waves (or photons) at frequencies which exist within the PBGs. PQCs are expected to be used in many applications in optoelectronics and optical communications. Because It is able to widely perfect omni-PBGs. In order to realize functional PCs devices such as selective transmission filters, optical waveguides, and microcavities, various defect structures have to be inserted into the PCs. So PQCs devices can applied various optical component. We come to the conclusion that 2D PQCs have widely omni-PBGs. So we believe that 2D PQCs are able to applied very many optical component. And multi exposure holo-lithography system is a suitable tool to realize the pattern generation method for 2D PQCs. Futher studies to realize 2D PQCs by phase changing of amorphous chalcogenide films.
9:00 PM - W6.24
Structural and Electronic Properties of a Buckycatcher.
Marcelo Flores 1 , Fernando Sato 1 , Douglas Galvao 1
1 Department of Applied Physics, University of Campinas - UNICAMP , Campinas, Sao Paulo, Brazil
Show AbstractRecently, functional molecular structures have been object of investigation by several groups [1]. As an example of these new nanostructures, Sygula et. al. [2] reported the synthesis and crystal assembly of stable carbon-based molecule, consisting of two corannulene subunits linked by a carbon-based joint forming a chalice-like configuration for its most stable structure. Due to its ability of acting as fullerene receptor, such molecule was named by them as buckycatcher (BCKY). In the present work we present the dynamics, configuration and electronic properties calculated using the universal force field (UFF) and three different density functional theory (DFT) functionals, with both local (PWC) and non-local (BLYP and PBE) corrections for the BCKY with and without incorporating C60. All the simulations were carried out using Cerius2 computer package for classical and quantum mechanical simulations. Our conformational searches for the isolated BCKY (without the presence of C60) showed the existence of three very stable different conformations, with small relative difference energies among them. Molecular dynamics simulations (up to 600 K) showed that the interconversion barrier is high enough to preclude conformational transitions. However when the C60 is ‘captured’ the barriers are smaller and conformational transitions can occur. It has been suggested [2] that the interaction between the BCKY and the C60 is of pi-pi nature. Our DFT calculations produced conflicting results depending on the used functional (significant pi-pi coupling was observed only when PWC functional was employed, while both GGA functionals failed to produce this behavior). The well-known poor description of van der Waals interactions within DFT scheme may account for this. However, all DFT single-point (using as input the UFF optimized geometry calculations, where the van der Waals interactions are better described) calculations produced results compatible with the proposed pi-pi nature of the buckycatcher-C60 interactions. The molecular dynamics simulations of the ‘catching’ processes showed that in vacuum this occurs only when there is a good alignment between the BCKY ‘mouth’ and the C60. Otherwise, the thermal fluctuations and high torsion barriers render the process unlikely to occur. It is expected that in solution these processes will be facilitated and more effective. [1] Chen, C. W. and Whitlock Jr., H. W. J. Am. Chem. Soc. 1978, 100, 4921.[2] Sygula, A. et al. J. Am. Chem Soc. 2007, 129, 3842.
9:00 PM - W6.25
Transmission and Refraction of Electromagnetic Waves in Transmission Lines, Photonic Crystals and Left Handed Materials: Localized States in the Photonic Gap.
John Vassiliou 1 , Kevin Creedon 1
1 Physics, Villanova University, Villanova, Pennsylvania, United States
Show AbstractUsing multiscale modeling, the Maxwell’s equations are solved for different electromagnetic systems including transmission filters, photonic crystals and left-handed materials (metamaterials). The transmitted electromagnetic power is calculated relative to the input power for Transverse Electric (TE) and Transverse Magnetic (TM) modes. The program inputs include the boundary conditions of the system and the index of refraction of the materials involved. The dielectric constant and the permitivity of the materials involved are given as a function of wavelength or as discreet values. The systems studied include transmission filters and photonic crystals configured as pillars of Gas or Si in air or pillars of air (holes) in GaAs or Si. For several of the systems modeled, the transmitted spectrum of photonic crystals shows clearly photonic gaps for various ranges of wavelengths. When disorder is introduced in the photonic crystal localized states appear in the gap which eventually lead to the emergence of additional gaps or the disappearance of the gaps in case of strong disorder. The ratio of width to the middle gap wavelength is studied as a function of the geometric characteristic of the constituents of the photonic crystal. In case of metamaterials, the refraction of the E&M waves from a wedge of an orthogonal prism is clearly demonstrated in agreement with the Snell’s law for negative index of refraction materials. The focusing of the refracted beam from the metamaterial is demonstrated and is compared to the equivalent behavior of a refracted beam by a normal material for the same geometric setting. The above result demonstrates the possibility of construction of a “perfect lens”. For certain combinations of the dielectric constant and permittivity of the left handed material, the electromagnetic wave is totally dissipated rendering the metamaterial impenetrable to the electromagnetic radiation. In particular geometries where a metamaterial is cloaking a region of a normal material a beam of electromagnetic wave is rerouted around the normal material, thus concealing the normal material and rendering it invisible to electromagnetic radiation.
9:00 PM - W6.26
Amorphous SiO2/SiC Interface Defect Structure Generated with First-principles Molecular Dynamics Simulation.
Atsumi Miyashita 1 , Toshiharu Ohnuma 2 , Misako Iwasawa 2 , Hidekazu Tsuchida 2 , Masahito Yoshikawa 1
1 Quantum Beam Science Dirctirate, Japan Atomic Energy Agency, Takasaki, Gunma, Japan, 2 Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, Komae, Tokyo, Japan
Show AbstractSilicon carbide (SiC) semiconductor devices are expected to be used in severe environments such as outer space and/or nuclear power plants. The performance of SiC metal-oxide-semiconductor (MOS) devices to date is low as compared with the ideal performance. This is considered to be attributed to defects generated at the interface between amorphous SiO2 (a-SiO2) layers and 4H-SiC substrates of MOS devices. However, it is not clear at the present what kind of defects affect the degradation of electrical characteristics. In this work, a large-scale slab model was prepared on the basis of an atomic network of beta quartz (β-SiO2) on 4H-SiC crystal and theoretical structures simulating the interface in the actual SiC MOS devices were successfully obtained on a super computer. A basic model was made on the basis of an atomic network of β-SiO2 on 4H-SiC crystal. The atomic networks composed of twelve layers were employed for the models of β-SiO2 and 4H-SiC crystal, respectively. By using 3×3 super-cell composed of the basic structure, a large-scale slab model with the number of 1017 atoms was prepared. The first principles molecular dynamics calculation using Vienna ab initio simulation package (VASP) [1] was performed for the large-scale slab model by heating and quenching method to form theoretical structures which simulate the a-SiO2/4H-SiC interface of actual SiC MOS devices. The heating of slab model was done at 4000 K for 2 ps followed by quenching to room temperature with the cooling speed of -2000 K/s. The obtained SiO2 layers have amorphous like characteristics on the 4H-SiC substrate. It is found that the theoretical structure at the SiO2/SiC interface consists of three different types of defect-free atomic networks, two different types of atomic networks with defect and a dangling bond of Silicon atom involved in a 4H-SiC substrate. The atomic networks with defect contain a Si-Si bond, respectively. Some atomic networks, which were not proposed in the theoretical structure model in the reference [2,3], were found at the interface in our simulation. Reference: [1] G. Kresse, and J. Hafner: Phys.Rev. B 47, 558 (1993); ibid. 49, 14251 (1994). [2] F. Devynck, F. Giustino, P. Broqvist, and A. Pasquarello: Phys.Rev. B 76, 075351 (2007). [3] J.M. Knaup, P. Deák, Th. Frauenheim, A. Gali, Z. Hajnal, and W.J. Choyke: Phys.Rev. B 71, 235321 (2005).
9:00 PM - W6.29
Quasiparticle Properties of Semiconductors from GW-Wannier Method.
Xiaofeng Qian 1 , Nicola Marzari 1 , Paolo Umari 2
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Theory at Elettra Group, CNR-INFM Democritos, Basovizza (Trieste) Italy
Show AbstractDensity functional theory (DFT) has been extensively applied to calculate the ground-state properties of materials successfully. However, the excited-state properties, including band gap, optical absorption spectrum, and electron energy loss spectrum, are less investigated at the ab initio level, which is very important for better theoretical understanding and systematic device design of modern electronics and photonics. Currently there are two major state-of-the-art approaches available for studying excited states. One is many-body perturbation theory such as Hedin's GW approximation [1,2]. The other is time-dependent density functional theory. However, both approaches involve much more intensive computation effort compared to DFT calculations. To study the excited-state properties of materials in larger scale recently we have developed an efficient and accurate ab initio method based on Hedin's quasiparticle GW method and maximally localized Wannier functions on top of plane-wave DFT method [3]. As benchmarks, we have first applied this method to calculate band gaps of various semiconductors and insulators with both norm-conserving and ultrasoft pseudopotentials [4]. It is shown that our GW-Wannier (GWW) method largely increases the DFT band gap by including dynamical electron correlation effect beyond DFT into the self-energy operator. The nice quantitative agreement between the calculated band gaps and experimental data demonstrates the reliability of the GWW method. Meanwhile, the localization property of the maximally localized Wannier functions enables us to largely reduce the total number of polarization function basis, thus greatly improves the computation efficiency of full GW calculations without losing the accuracy.[1] L. Hedin, Phys. Rev. 139, A796 (1965)[2] M. S. Hybertsen and S. G. Louie, Phys. Rev. B 34, 5390 (1986)[3] P. Umari, to be submitted to Phys. Rev. Lett (2008)[4] X. Qian, N. Marzari and P. Umari, to be submitted to Phys. Rev. B (2008)
9:00 PM - W6.30
Surface Stability and Electronic Structure of Hydrogen and Fluorine Terminated Diamond Surfaces: a First Principles Investigation.
Fatih Sen 1 , Yue Qi 2 , Ahmet Alpas 1
1 Department of Mechanical, Automotive and Materials Engineering, University of Windsor, Windsor, Ontario, Canada, 2 Materials and Process Lab, GM Research and Development Center MC 480-106-224, Mound Road, Warren, Michigan, United States
Show AbstractFirst principles calculations were used to investigate the surface properties of fluorine modified DLC and diamond coatings in order to design tool surfaces with improved environmental stability for aluminum machining and forming. The stability and electronic structure of fully H or F terminated and mixed H and F terminated diamond (111) surfaces were studied. It was found that both fluorine and hydrogen caused surface carbons to make sp3 type bonding, which results in a more stable 1x1 construction rather than the p-bonded 2x1 construction. The surface energies corresponding to each H and F concentration were calculated and a surface phase diagram showing the predominance of stable surface composition region was constructed as a function of chemical potentials of H and F sources. The diagram shows that there is no 3F1H surface termination. Considering common gas sources of fluorine and hydrogen, fluorine doping results in a decrease in the surface energy. The F terminated surface is more stable and chemically more inert due to the strong ionic CF bonding and the close packing of the large F atoms. Although the fluorine atoms are larger, they seemed to develop attractive rather than repulsive forces. Consequently, they will hinder the interaction of carbon atoms with the environment, which results in a chemically more inert surface.
9:00 PM - W6.31
A Molecular Dynamics Study of the Rotational Dynamics and Polymerization of C60 in C60-Cubane Crystals.
Vitor Coluci 1 , Fernando Sato 2 , Scheila Braga 2 , Munir Skaf 3 , Douglas Galvao 2
1 , Center for High Education on Technology, University of Campinas , Limeira, SP, Brazil, 2 , Applied Physics Department, Institute of Physics, University of Campinas, Campinas, SP, Brazil, 3 , Institute of Chemistry, University of Campinas , Campinas, SP, Brazil
Show Abstract9:00 PM - W6.32
Adaptive Hybridization of Density-Functional Theory and Molecular Dynamics: Reaction of Pressurized Water Molecule Trapped in Between Nano-Structured Diamond and Silicon.
Shuji Ogata 1 2 , Yuya Abe 1 2 , Ryo Kobayashi 1 2
1 Department of Scientific and Engineering Simulation, Nagoya Institute of Technology, Nagoya Japan, 2 CREST, Japan Science and Technology Agency, Saitama Japan
Show AbstractWe have been developing a hybrid simulation scheme by combining the electronic-density-functional-theory (DFT) and the classical molecular dynamics (MD) method using the empirical inter-atomic potential [1-3]. In the hybridization scheme, multiple DFT regions each of which is composed of a relatively small number of atoms, are embedded in a system of classical MD atoms. For robust coupling of the DFT and the MD regions with reasonable mechanical accuracy, the buffered cluster method has been proposed, which requires no link-atoms and is applicable to a wide range of materials and settings. During a hybrid DFT-MD simulation run, the sizes and number of the DFT regions change adaptively following a simple rule to trace the chemical reaction with reduced computation cost as compared to the full DFT calculation. Recently interesting observation has been reported in the experiments about the chemical reaction in the nano-indentation: when the diamond-tip was pressed repeatedly to the Si surface in moisture environment the resulting indentation mark shrank gradually, while it enlarged if the pressing was done in vacuum as expected [4]. The fact suggests that local oxidation due to the pressing occured in the case of the moisture environment. Motivated by this, in the present paper, we perform the hybrid DFT-MD simulation of pushing and sliding of H-terminated diamond tip on H-terminated Si(100)2x1 surface with a water molecule inserted in between. We thereby find that, depending on the various settings including the sharpness of the diamond-tip, the water molecule can be pushed into a native dimple of the H-terminated Si surface, which then decomposes and oxidizes the Si to form the Si-O-Si structure.[1] S. Ogata, et al., Comp. Phys. Commun. 149, 30 (2002).[2] S. Ogata, Phys. Rev. B 72, 45348 (2005).[3] T. Kouno and S. Ogata, J. Phys. Soc. Jpn 77, 54708 (2008).[4] I. Zarudi, L.C. Zhang, and M.V. Swain, Key Eng. Mat. 233-236, 609 (2003).
9:00 PM - W6.33
Accelerated Kinetic Monte Carlo Simulations of Vacancy-Mediated Arsenic Diffusion and Clustering in Silicon.
Brian Puchala 1 , Michael Falk 3 1 , Krishna Garikipati 2
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractDuring semiconductor device fabrication, ion implantation of dopants creates large populations of defects, vacancies and interstitials, which mediate dopant diffusion. Experiments have shown large changes in dopant diffusivity in silicon as a function of annealing time and dopant concentration. We perform kinetic Monte Carlo (KMC) simulations of vacancy-mediated arsenic diffusion in silicon to investigate the effect of dopant concentration, distribution and clustering on diffusivity. In order to follow the diffusion and breakup of clusters, on the order of minutes, our KMC simulations are accelerated using absorbing Markov chain analysis on states intelligently chosen on-the-fly to fill trapping basins in the local energy landscape. At lower dopant concentrations, we calculate the diffusivity and breakup rates of different cluster types and a mean field approach can be used to describe the overall cluster population evolution and dopant diffusivity. Above a critical concentration this mean field description fails as dopants become close enough to form a percolating structure throughout the material. At all concentrations, diffusivity decreases significantly over time as larger, less mobile clusters form.
9:00 PM - W6.37
A First Principles Study of Si and Y Doping of HfO2.
Chunguang Tang 1 , Rampi Ramprasad 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show Abstract9:00 PM - W6.38
Heteroepitaxial Modeling on Strained Surfaces using a Mean Field Approach.
James DeGraffenreid 1 , Ramon Grima 3 , John Venables 1 2
1 Physics, Arizona State University, Tempe, Arizona, United States, 3 Systems Biology, University of Edinburgh, Edinburgh United Kingdom, 2 LCN, University College, London United Kingdom
Show Abstract9:00 PM - W6.4
Phase Diagram of Silicon Using a DFT-based Neural Network Potential.
Oliviero Andreussi 1 , Jörg Behler 3 , Michele Parrinello 2
1 DMSE, MIT, Cambridge, Massachusetts, United States, 3 , Ruhr-Universität, Bochum Germany, 2 , ETH , Zürich Switzerland
Show Abstract9:00 PM - W6.6
Renormalization Method for the Multiscale Modeling of Aperiodic Materials.
Vicenta Sanchez 1 , Chumin Wang 2
1 Departamento de Fisica, Facultad de Ciencias, Universidad Nacional Autonoma de Mexico, Mexico D.F. Mexico, 2 Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Mexico D.F. Mexico
Show AbstractCrystalline solids are traditionally studied by taking the advantage of the reciprocal space and the Bloch theorem. However, these methods become inadequate when extended defects are present, such as in amorphous or quasicrystalline materials, where cluster calculations with a limited number of atoms are usually employed. In fact, for mesoscopic periodic systems a real space approach would be most desirable. In this work, we report a real-space renormalization method to address multi-scale aperiodic systems. The frequency-dependent electrical conductivity of periodic and quasiperiodic systems is investigated by means of the Kubo-Greenwood formula within the tight-binding formalism. For the periodic case, electronic transport is ballistic, i.e., the electrons can go from one end to the other without being scattered. An analytical expression of its ac conductivity at low frequency region is obtained for periodic chains of N atoms connected to two semi-infinite periodic leads [1]. The dc conductivity of quasiperiodic chains with Fibonacci bond disorder, also connected to two periodic semi-infinite leads, is always smaller than the periodic case, except for the transparent states which have the same conductivity of periodic chains. However, when an oscillating electric field is applied, we observe that the ac conductivity of quasiperiodic systems at certain resonant states can be larger than the periodic one. Analysis of this behavior in systems with mirror symmetry [2] and in nanowires is also presented.[1] V. Sanchez and C. Wang, Phys. Rev. B 70, 144207 (2004). [2] V. Sanchez and C. Wang, J. Phys. A 39, 8173 (2006).
9:00 PM - W6.8
Adsorption in flexible metal organic framework materials : Molecular simulations compared to experiments
Aziz Ghoufi 1 , Nilton Rosenbach 1 , Fabrice Salles 1 , Philip Llewellyn 2 , Sandrine Bourrelly 2 , Christian Serre 3 , Gérard Férey 3 , Guillaume Maurin 1
1 UMR CNRS 5253 UM2, Institut Charles Gerhardt Montpellier, Montpellier France, 2 UMR CNRS 6264, Laboratoire Chimie Provence, Marseille France, 3 UMR CNRS 8637, Institut Lavoisier, Versailles France
Show Abstract9:00 PM - W6.9
Mechanical Testing and Material Modeling of Thermoplastics:Polycarbonate, Polypropylene and Acrylonitrile-Butadiene-Styrene.
Jean-Luc Bouvard 1 , H. Brown 1 , Esteban Marin 1 , P. Wang 1 , Mark Horstemeyer 1
1 Center for Advanced Vehicular Systems, Mississippi State University, Starkville, Mississippi, United States
Show AbstractTuesday, 12/2New Presenter - Poster W6.9Mechanical Testing and Material Modeling of Thermoplastics:Polycarbonate, Polypropylene and Acrylonitrile-Butadiene-Styrene. H.R. Brown
Symposium Organizers
Yue Qi General Motors R&D and Planning
H. Eliot Fang Sandia National Laboratories
Nick Reynolds Accelrys
Zi-Kui Liu The Pennsylvania State University
W7: Metals and Alloys: From Atom to Microstructure I
Session Chairs
Long-Qing Chen
Zi-Kui Liu
Wednesday AM, December 03, 2008
Constitution B (Sheraton)
9:30 AM - **W7.1
First Principles Design of Metal Alloys and Oxides To Limit Chemical Degradation and Creep.
Emily Carter 1
1 , Princeton University, Princeton, New Jersey, United States
Show AbstractWe employ density functional theory within periodic boundary conditions, the generalized gradient approximation to exchange-correlation, and the projector augmented wave method to explore how best to dope metals and oxides to limit hydrogen embrittlement and carburization of metals and creep of oxides under harsh operating conditions. The materials we will discuss are iron (ferrite, as a model for low-carbon steel) and iron alloys, alumina, and possibly tungsten.
10:00 AM - W7.2
Modeling the Early Stage of Copper Oxidation by ab Initio Calculation.
Minyoung Lee 1 , Alan J.H. McGaughey 1 , Susan Sinnott 2 , Simon Phillpot 2 , Judith C. Yang 3
1 Mechanical Engineering, Carnegie Mellon University, Pittsburgh , Pennsylvania, United States, 2 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 3 Mechanical Engineering & Material Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractCopper (Cu) is one of the most widely used and studied materials for preventing corrosion and enhancing catalysis. Based on experimental and theoretical results, it has been suggested that the missing row reconstruction of the Cu(100) surface is the intermediate state between a clean Cu(100) surface and copper II oxide (cuprite) formation. The missing row reconstruction on Cu(100), however, is not the only possible state on a Cu(100) surface during oxidation. For example, the oxygen-induced c(2x2) structure with 0.25 monolayer (ML) disordered Cu vacancies is observed on the Cu(100) surface between temperatures of 473 and 1000 K for oxygen partial pressure between 10-10 and 10-4 Torr.To identify the transition between a Cu(100) surface and copper II oxide (cuprite) formation we are performing density functional theory (DFT) calculations with the plane wave package VASP. Structural relaxations, single point energy calculations, and nudged elastic band (NEB) calculations are performed. A Cu(100) slab five layers thick is used for all calculations. The bottom layer is fixed and all others can move. We investigate the following Cu(100) surface structures: p(2x2) at 0.75 ML coverage, the missing row reconstructed surface, and the c(2x2) structure with 0.25 ML Cu vacancies.We compared the reconstructed (missing row) Cu(100) surface to the unreconstructed c(2x2) surface at 0.5 ML coverage after structural relaxation. We find that the topmost Cu layer of the reconstructed surface moves 0.17 A° further upward and the adsorbed oxygen atoms move 0.53 A° further downward than for the case of unreconstructed surface. Also, we compared the distance between the adsorbed oxygen atom and each Cu layer for the reconstructed surface to the experimentally measured results and they match well. We then calculated the atomic oxygen adsorption energy for both surfaces and found that the reconstructed surface is energetically more stable, consistent with experimental results at low temperature.In future work, we will relax the p(2x2) and missing row reconstructed surfaces. We will put oxygen molecules a few A° above the surface and relax the system. Different initial positions and numbers of oxygen molecules will be considered. We will then investigate the relation between initial conditions (number of oxygen molecules and their position) and the structural change of Cu(100) surface in order to find the intermediate state during the early stages of Cu(100) oxidation. During oxide formation and growth on a Cu surface, the copper oxide grows horizontally and vertically. We are therefore also performing NEB calculations for oxygen embedded into the Cu(100) surface and will compare the energy barriers to those for surface diffusion.
10:15 AM - W7.3
Solid Solution Strengthening using the First-principles Results of Misfit Strain in Al-based Alloys.
Tokuteru Uesugi 1 , Kenji Higashi 1
1 Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai Japan
Show Abstract10:30 AM - W7.4
Validation of ReaxFF Cobalt Parameters Through the Study of Crystal Defect Behavior.
Matthew LaBrosse 1 2 , Karl Johnson 1 2
1 Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 , National Energy Technology Laboratory, Pittsburgh, Pennsylvania, United States
Show AbstractFully periodic plane-wave Density Functional Theory (DFT) calculations have been performed on a multitude of pure cobalt systems as a means to parameterize ReaxFF, a reactive force field code. The parameter training set from the DFT calculations includes equations of state, elastic moduli, vacancy formation energies, surface formation energies, small Co clusters, and amorphous melts, all of which were performed on many bulk Co crystalline phases. This extensive training set was used to optimize Co parameters for the ReaxFF molecular dynamics code. ReaxFF differs from traditional force fields in that the bonds are not defined explicitly, but rather defined as a function of interatomic distance which allows for change in bond order. After optimization, the Co parameters were evaluated for accuracy to comparing ReaxFF results to a variety of additional DFT calculations as well as results using a semi-empirical embedded atom potential. We use large-scale molecular dynamics simulations to investigate vacancy-mediated diffusion and vacancy coalescence in bulk Co. We compare our simulations with experiments where possible.
11:00 AM - W7:Metal
BREAK
11:30 AM - **W7.6
Overcoming Barriers to the Agreement between Microstructural Simulation and Experiment.
Luke Brewer 1 , Corbett Battaile 1 , Remi Dingreville 1 , Timothy Bartel 2
1 Computational Materials Science and Engineering, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Advanced Nuclear Fuel Cycle Technology, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show Abstract12:00 PM - W7.7
Multiscale Prediction of Polycrystal Elastic Properties of Ultra-light Weight Mg-Li Alloys using Ab Initio and FEM Approaches.
A. Counts 1 , M. Friak 1 , C. Battaile 2 , Dierk Raabe 1 , J. Neugebauer 1
1 , Max-Planck-Institut fuer Eisenforschung, Duesseldorf Germany, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractBCC magnesium-lithium alloys are a promising class of ultra light-weight structural materials. As a first step in a theoretically guided materials design strategy single crystal elastic constants for bcc magnesium-lithium alloys with different compositions were computed using ab initio methods. These single crystal elastic constants were then used to predict the corresponding polycrystalline elastic constants using various analytic homogenization techniques (Voigt, Reuss, self-consistent approach) as well as the finite element method. The Voigt and Reuss bounds form the upper and lower bounds on the polycrystalline elastic constants, which the predicted values of the self-consistent approach and finite element fall in between. The results are compared to experimental data.
12:15 PM - W7.8
Three-Dimensional Image-Based Finite Element Modeling in beta-Ti Microstructures.
Alexis Lewis 1 , Muhammad Qidwai 2 1 , Andrew Geltmacher 1
1 Multifunctional Materials Branch, Naval Research Laboratory, Washington, District of Columbia, United States, 2 , SAIC, Washington, District of Columbia, United States
Show AbstractThe three-dimensional microstructure of a single-phase beta Titanium alloy was measured and reconstructed using serial sectioning with optical microscopy and Electron Backscatter Diffraction. The reconstructed microstructure was used as input for image-based finite element modeling of mechanical response. This quantitative analysis of experimentally-derived 3D data in conjunction with finite element simulation results allows for the investigation of correlations of mechanical behavior with microstructure and crystallographic features on multiple scales. Using anisotropic elasticity and crystal plasticity constitutive models, high strains were observed within particular grains, as well as at particular grain boundaries and junctions. The microstructural, morphological, and crystallographic features which correlate with these high strains were examined on multiple scales, from the crystallography of individual junctions to the geometric relationships between nearest neighbor and nest-nearest neighbor grains. The results are analyzed in the context of those material parameters (such as texture and grain geometry) which can be altered through processing, with the ultimate goal of accurate and efficient simulation of the effects of real 3D microstructure, geometry, and crystallography on mechanical response.
12:30 PM - **W7.9
Measuring and using Grain Boundary Properties: From the Atomic to the Mesoscale.
Elizabeth Holm 1 , David Olmsted 1 , Stephen Foiles 1
1 Materials and Process Modeling Department, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractBecause mesoscale materials models are not typically derived from first principles, they require a physical model and physical inputs. Atomic scale simulations can help inform both the materials model and the property data it requires.We have utilized novel molecular dynamics methods to construct a large catalog of grain boundaries and measure their energies and mobilities. Results of boundary mobility calculations are used to develop models for grain boundary motion that are integrated in mesoscale simulations of polycrystalline grain evolution. For example, atomic simulations show a wide range of boundary roughening temperatures, with associated abrupt mobility changes, as well as inhibition of boundary motion when shear is suppressed. These effects are included in the mesoscale grain boundary motion model, and can significantly alter polycrystalline evolution kinetics and morphology.Thermodynamic properties, such as grain boundary energy, are used directly as inputs in mesoscale simulations. However, in the vast five-dimensional space of grain boundary crystallography, we cannot measure all relevant energies. Instead, we must extrapolate between measured energies – a nontrivial challenge. We have developed a metric that permits interpolation of energies; when included in a mesoscale simulation, we observe changes in microstructural topology and geometry consistent with the energy variation.When the results of atomic scale grain boundary simulations are integrated into a mesoscale model for microstructural evolution, we realize a deeper understanding of materials structure and properties than either length scale can provide alone.This work was performed in part at Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.
W8: Metals and Alloys: From Atom to Microstructure II
Session Chairs
Wednesday PM, December 03, 2008
Constitution B (Sheraton)
2:30 PM - **W8.1
Flying Cybersteels: Green Materials by Design.
Greg Olson 1
1 MSE, Northwestern University, Evanston, Illinois, United States
Show AbstractA systems approach integrates processing/structure/property/performance relations in the conceptual design of multilevel-structured materials. Using examples of high performance alloys, numerical implementation of materials science principles provides a hierarchy of computational models defining subsystem design parameters which are integrated via computational thermodynamics in the comprehensive design of materials as interactive systems. Recent initiatives integrate materials science with quantum physics and applied mechanics, and address the acceleration of the full materials design, development and qualification cycle. The ongoing ONR/DARPA "D3D" Digital Structure Consortium initiative combines a suite of multiscale 3D tomographic characterization tools supporting higher fidelity 3D microstructural simulators for greater accuracy of predictive-science-based design. A corrosion resistant high-strength steel, specifically designed to eliminate the need for cadmium plating in aircraft landing gear, is in the final stages of flight qualification.
3:00 PM - W8.2
Multi-scale Modelling of the Structure and Mobility of Small Defect Clusters in Iron.
Mihai-Cosmin Marinica 1 , François Willaime 1
1 DEN/DMN Service de Recherches de Métallurgie Physique, CEA Saclay, Gif s/ Yvette France
Show AbstractThe mobilities of self-interstitial atoms (SIA) and their clusters in metals, especially body-centered cubic (bcc) metals, are one of the main issues in multiscale models for the prediction of the microstructure evolution that these materials undergo under irradiation. In iron, where dumbbells in clusters may have <110>, <111> or <100> orientations, this question is particularly challenging because the number of possible configurations increases rapidly with the number of defects in the cluster. Configurations made of non-parallel dumbbells and with a reduced mobility have been recently identified from high temperature molecular dynamics simulations. Their stability has been confirmed by DFT calculations. These results showed that non-conventional configurations and finite temperature effects must be taken into account. We propose to address these two points more thoroughly using on the one hand the activation relaxation technique nouveau (ARTn), an eigenvector following method for systematic search of saddle points and transition pathways on a given potential energy surface, and on the other hand lattice dynamics calculations. Using the Ackland-Mendelev EAM potential for iron, we have determined the formation energies of all bonded configurations of clusters containing up to 5 SIAs. For the most stable ones, we have identified their migration mechanism. This shows in particular that some configurations with low saddle point energies have to be considered in the kinetics of the system, although they are not the most stable ones. Lattice dynamics calculations show that at high temperature configurations with <111> dumbbells and/or non-parallel dumbbells are favoured. The low frequency modes at the origin of this stabilisation driven by vibrational entropy are analyzed. DFT calculations performed using the SIESTA code are used to assess the main findings on new low energy configurations and on the low frequency modes.
3:30 PM - W8.4
Application of Spectral Methods for Anisotropy Design of Ti-Nb Polycrystals for Biomedical Applications based on ab Initio Elastic Single Crystal Constants and Fast Fourier Homogenization.
Marko Knezevic 1 , D. Ma 2 , Dierk Raabe 2 , Surya Kalidindi 1 , M. Friak 2 , J. Neugebauer 2
1 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States, 2 , Max-Planck-Institut für Eisenforschung, Düsseldorf Germany
Show Abstract3:45 PM - W8.5
A Multiscale Model of First and Second Order Phase Transformations in Shape Memory Alloys.
Vesselin Stoilov 1 , Jun Su 1
1 Mechanical Automotive and Materials Engineering, University of Windsor , Windsor, Ontario, Canada
Show AbstractThis work aims to connect an atomistic model with continuum theory of phase transformations in shape memory alloys (SMAs). A formulation of the Helmholtz free energy potential based on Einstein potential has been developed. The atomic potential was used to describe the interatomic interactions in a biatomic crystal of NiTi. The microscopic expressions of the instantaneous mechanical (continuum) variables of mass, momentum, internal energy, and temperature have been derived in terms of the atomic variables. The developed Helmholtz thermodynamic potential is used in the context of the sharp phase front-based continuum framework proposed by Stoilov and Bhattacharyya [Acta Mater., 50 (2002), pp. 4939–4952] to study the micro-macro transition during the thermomechanical response of NiTi crystals. The developed model has been successfully used to predict the response of a one-dimensional single crystal system.
W9: Thin Film and Coating
Session Chairs
Wednesday PM, December 03, 2008
Constitution B (Sheraton)
4:30 PM - **W9.1
Computational Design of Phase Transitions, Domain Structures and Properties of Ferroelectric Thin Films.
Long-Qing Chen 1
1 , Penn State University, University Park, Pennsylvania, United States
Show AbstractThis presentation will discuss the applications of computational modeling to design phase transitions, domain structures, properties of ferroelectric thin films. While many computational approaches, ranging from electronic structures calculations to continuum theories, have been developed and applied to ferroelectric thin films, the focus of this presentation will be on phenomenological thermodynamic theories and the mesoscale phase-field method. In particular, it will be shown that one can use computational modeling to not only help interpreting experimental observations but also provide guidance to achieve desirable transition temperatures, specific domain states and domain wall orientations by choosing appropriate substrates and temperatures. It is demonstrated that strain may be used to tune the coercive field and dramatically enhance piezoelectric responses of ferroelectric thin films. Examples to be discussed result from recent collaborations between the presenter’s group and seveal experimental groups. These include a number of important oxide systems, BaTiO3, PbZrxTi1-xO3, BiFeO3 and BaTiO3/SrTiO3 superlattices.
5:00 PM - W9.2
Controlling the Intrinsic Stress in Nano-crystalline Diamond through Deposition Temperature – the Role of Hydrogen at Grain Boundaries.
Yue Qi 1 , Haibo Guo 2 , Brian Sheldon 3 , Xingcheng Xiao 1
1 Materials & Processes Lab, General Motors, Warren, Michigan, United States, 2 Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina, United States, 3 Engineering Division, Brown University , Providence, Rhode Island, United States
Show AbstractThe intrinsic stress in Nano-crystalline diamond (NCD) coatings is typically tensile at growth temperatures above 700oC. However it gradually decreases then changes to compressive with decreasing deposition temperatures down to 400oC. The main contribution for the stress evolution is due to the chemistry at the grain boundaries. The hydrogen coverage on diamond surfaces should increase with decreasing growth temperature, as is predicted by the kinetics of the absorption and desportion of hydrogen atoms during NCD growth and evidenced by Raman spectra and elastic recoil detection. The coalescence of two diamond grains in forming a grain boundary is modeled by two diamond (100)-2×1 surfaces with different hydrogen concentrations approaching each other via density functional theory (DFT). The interaction between the two surfaces is attractive when hydrogen coverage less than 75% and is repulsive when all the surface diamond bonds are terminated by hydrogen (100% coverage). Thus the hydrogen concentration is responsible to the evolution of the intrinsic stress at different deposition temperatures. The consistent experimental results and theoretical simulation provide the guidance in controlling stress levels in NCD coatings for a variety of different applications.
5:15 PM - W9.3
Lateral Alloy Segregation in Thin Heteroepitaxial Films.
Christian Ratsch 1 , Jason Reich 1 , Xiaobin Niu 1 , Russel Caflisch 1
1 Deapartment of Mathematics, UCLA, Los Angeles, California, United States
Show AbstractWe have studied the segregation and alloy formation of thin heteroepitaxial films. We use an atomistic strain model that has a cubic geometry and includes nearest neighbor bonds, next nearest neighbor bonds, and bond bending terms. Our motivation is the well established fact that for many heteroepitaxial systems growth proceeds in the Stranski-Krastanov growth mode, where islands form after the formation of a wetting layer. Recent results indicate that intermixing and thus vertical variations of the alloy concentration are a crucial factor in controlling the formation and thickness of the wetting layer. Our results suggest that in addition to vertical segregation there is also lateral segregation before island formation. Thermodynamically, the system prefers to have one big feature of the epilayer material that is embedded in the substrate but is near the surface. In practise, there will be a typical separation distance of these subsurface features because of kinetic limitations. We use and island dynamics model that is based on the level set method to show that such sub-surface structures control the placement of newly nucleated islands.
5:30 PM - W9.4
Classical Molecular Dynamics Simulations of Heterogeneous Interfacial Systems with COMB Potentials.
Tzu-Ray Shan 1 , Jianguo Yu 2 1 , Simon Phillpot 1 , Susan Sinnott 1
1 Materials Science & Engineering, University of Florida, Gainesville, Florida, United States, 2 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractMany high-performance devices consist of multilayers of materials that form heterogeneous interfaces with significant changes in bonding as one crosses from one side of the interface to the other. For example, electronic junctions consist of silicon/silica/metal interfaces. Traditionally, computational studies of these interfacial systems have relied on electronic structure methods, such as density functional theory (DFT), because of the difficulty in describing the changes in bonding environment with empirical approaches. While effective, DFT calculations are limited to structures that are too small to model many aspects of the interface. Here, empirical, charge optimized many-body (COMB) potentials are used in classical molecular dynamics simulations to model Si/SiO2/Cu and Si/SiO2/Cu2O/Cu interfaces. The results are compared to experimental data and to the results of DFT calculations. The COMB potentials allow for dynamic charge transfer between atoms and across interfaces, and do a good job of describing metallic, covalent, and ionic bonding in these various materials. This work is supported by the National Science Foundation (DMR-0426870) and by the Department of Energy (DE-FG02-07ER46446).
5:45 PM - W9.5
Sintering and Microstructure Evolution in Columnar Thermal Barrier Coatings.
Ramanathan Krishnamurthy 1 2 , David Srolovitz 2 3
1 Materials Modeling Division, Technical Center, Caterpillar Inc., Mossville, Illinois, United States, 2 Department of Mechanical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Yeshiva College, Yeshiva University, New York, New York, United States
Show AbstractSintering of thermal barrier coatings changes their key properties such as thermal conductivity and thermal shock resistance, thus adversely impacting their reliability. We present a novel multi-scale modeling approach to study the evolution of coating structure during the sintering of topcoat columns. We model the sintering of individual topcoat columns using a thermodynamic principle, and incorporate the center-to-center approach rates for the columns calculated using this principle in a larger scale discrete dynamics model for the evolution of a large number of columns. Surface energies, grain boundary energies and strain energies associated with the deformation of the columns are all included in this framework, while sintering is assumed to occur by the concerted action of surface and grain boundary diffusion. Two sets of initial conditions corresponding to different extents of pre-sintering among neighboring columns are considered. When the extent of pre-sintering is small, we observe that small clusters containing 5-20 columns are formed. In contrast, where a larger amount of pre-sintering exists, we observe, especially at large column densities, that clusters containing 50-100 columns separated by large inter-cluster pores/channels that appear to organize themselves into a network, are formed. We extract statistical measures, such as the pair correlation function, column coordination number, and in-plane porosity and pore size distribution from the coating microstructures and study their variation as a function of coating system parameters. The variables that have the greatest effect on these measures are the column density and the extent of the ‘feathery’ structure of the coating. At early times, low column densities, and where the ‘feathery’ protrusions extend only to a small distance, small clusters containing mainly neighboring columns are formed. On the contrary, when these variables assume large values, clusters many columns--wide are formed. All these observations are in good agreement with experimental observations reported by Lughi et al in a recent publication. Quantitative estimates of in-plane pore sizes and cluster sizes also compare well with the width and the spacing of the ‘mud-crack’ like coating structures seen in that publication. We discuss the implications of these results for thermal barrier coating processing, reliability and failure.
Symposium Organizers
Yue Qi General Motors R&D and Planning
H. Eliot Fang Sandia National Laboratories
Nick Reynolds Accelrys
Zi-Kui Liu The Pennsylvania State University
W10/EE8: Joint Session: Computational Nanomechanics I - Dislocations & Radiation Effects
Session Chairs
Thursday AM, December 04, 2008
Constitution B (Sheraton)
9:15 AM - **W10.1/EE8.1
Surface Controlled Dislocation Multiplication in Metal Micro-pillars.
Wei Cai 1 , Christopher Weinberger 1
1 Mechanical Engineering, Stanford University, Stanford, California, United States
Show AbstractUnderstanding plasticity and strength of crystalline materials in terms of the dynamics of microscopic defects has been a goal of materials research in the last seventy years. The size-dependent yield stress observed in recent experiments of sub-micrometer metallic pillars provides a unique opportunity to test our theoretical models, allowing the predictions from defect dynamics simulations to be directly compared with mechanical strength measurements. While easy escape of dislocations from sub-micrometer pillars is expected and provides a plausible explanation of the observed size-effect, we predict the opposite to be true in body-centered-cubic (BCC) pillars through a series of Molecular Dynamics and Dislocation Dynamics simulations. Under the combined effects from the image stress and the atomistic core, a dislocation nucleated from the surface of a BCC pillar generates one or more dislocations moving in the opposite direction before it exits from the surface. The process is repeatable so that a single nucleation event is able to produce a much larger amount of plastic deformation than that in face-centered-cubic (FCC) pillars. This self-replication mechanism calls for a different explanation of the size-dependence of yield stress in FCC and BCC pillars.
9:45 AM - **W10.2/EE8.2
Mechanisms of Size-dependent Crystal Flow Gleaned from Three-dimensional Discrete Dislocation Simulations.
Satish Rao 1 , Dennis Dimiduk 2 , Michael Uchic 2 , Triplicane Parthasarathy 1 , Christopher Woodward 2
1 Materials and processes Division, UES Inc., Dayton, Ohio, United States, 2 Materials and Manufacturing Directorate, Wright-patterson Air Force labs, WPAFB, Ohio, United States
Show AbstractRecent experimental studies discovered that micrometer-scale face-centered cubic crystals show strong strengthening effects, even at high initial dislocation densities. We use large-scale 3-D discrete dislocation simulations (DDS) to explicitly model the deformation behavior of FCC Ni microcrystals in the size range 0.25 – 20 micron under both single-slip and multi-slip conditions. The study shows that two size-sensitive athermal hardening processes, beyond forest hardening, are sufficient to develop the dimensional scaling of the flow stress, stochastic stress variation, flow intermittency and, high initial strain-hardening rates, similar to experimental observations for various materials. One mechanism, source-truncation hardening, is especially potent in micrometer-scale volumes. A second mechanism, termed exhaustion hardening, results from a break-down of the mean-field conditions for forest hardening in small volumes, thus biasing the statistics of ordinary dislocation processes. Effects of thermally activated cross-slip of screw-oriented dislocations on the stress-strain behavior of microcrystals of Ni is also discussed.
10:15 AM - W10.3/EE8.3
Anisotropic Diffusion of Point Defects in Metals under a Biaxial Stress Field.
Wai Lun Chan 1 , Robert Averback 1 , Yinon Ashkenazy 2
1 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem Israel
Show AbstractWe study the diffusion anisotropy (DA) of point defects in fcc and bcc metals induced by a biaxial stress field. The study shows that the DA strongly depends on the crystal structure and the crystallographic direction in which the stress is applied. For example, interstitials in fcc metals diffuse faster in the plane of the stress than normal to it when the stress is applied to the (001) plane, but they diffuse slower when the stress is applied in the (111) plane. In contrast, applied biaxial stress in the (001) plane of a bcc metal has no effect on the DA. These results can be explained by considering the interaction of the defects in their saddle point configurations with the external field together with the constraints imposed by the crystal structure on the defect jump directions. Our calculations show that the DA can be significant in a number of practical situations where large numbers of non-equilibrium defects and high stress is presence, e.g. irradiation-induced creep, solute segregation in irradiated alloys, and stress creation during ion bombardment.
10:30 AM - W10.4/EE8.4
Atomistic Simulation Studies of Indentation into Cu-Ni and Cu-Nb Multi-layers.
Sergey Medyanik 1 , Shuai Shao 1
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractNanoscale multilayered metallic structures often exhibit very high strength levels. This strengthening has been attributed to the presence of interfaces between dissimilar materials that serve as barriers to dislocation propagation. In this work, we present atomistic simulation studies of dislocation nucleation and propagation in nanoscale multilayered metallic systems (Cu-Ni and Cu-Nb). Nanoindentation model is used to generate dislocations at and near the surface. Interaction of the propagating dislocations with coherent and incoherent types of interfaces is analyzed. In the case of coherent Cu-Ni interface dislocations that initiate in Cu layer propagate through the interface into Ni. However, the interface acts as an obstacle for dislocation propagation and leads to a higher dislocation density near the interface. In the case of incoherent Cu-Nb interface dislocations that initiate in Cu do not propagate into Nb even at very high indentation depths and tend to accumulate in copper near the interface. We provide further analysis of the results focusing on the mechanisms for strengthening in the nanoscale multilayered metallic systems due to the presence of interfaces.
10:45 AM - W10.5/EE8.5
Heterogeneous Deformation and Dislocation Dynamics in Cu Single Crystal Micropillars under Compression.
Sreekanth Akarapu 1 , Hussein Zbib 1 , David Bahr 1
1 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractThe size dependent deformation of sub-micron Cu single crystals with thickness ranging from 0.2 to 2.5 microns subjected to uniaxial compression was studied using a Multi-scale Discrete Dislocation Plasticity (MDDP) approach. MDDP is a hybrid elasto-visco plastic simulation model which couples 3D discrete dislocation dynamics at the micro-scale with macroscopic plastic deformation. The stress-strain behavior exhibited plastic yielding in discrete strain bursts conforming qualitatively to experimental observations. An explanation to the observed macroscopic strain bursts is given by investigating the associated dislocation mechanisms. The operation of dislocation arm was identified as the prominent mechanism causing plastic deformation. The critical stress to bow an average minimum dislocation arm length is responsible for the observed size dependent response of the single crystals. Hardening rates, similar to that shown experimentally, are shown to occur under relatively constant dislocation densities, and are shown to be linked to the bowing of arms and the addition of pinning sites, and not through dislocation starvation mechanisms. Crystal rotation during compression was predicted in the simulation, and is in accord with published results observed in electron backscatter diffraction experiments
11:30 AM - **W10.6/EE8.6
Designing Heterophase Interfaces for Radiation Damage Resistance.
Michael Demkowicz 1 , Richard Hoagland 1 , John Hirth 2 , Amit Misra 2
1 MST-8: Structure-Property Relations Group, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 MPA-CINT: Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractWe couple atomistic and continuum modeling with experiments to understand the connection between the structure of heterophase interfaces and the role they play as sinks for radiation-induced point defects. The insights we gain allow us to construct a general model of the effect of interfaces on radiation damage reduction and to propose strategies for the informed, interface structure-driven design of radiation tolerant nanocomposites.We acknowledge the support of the LANL Directed Research and Development program, a LANL Director's fellowship, and the DOE Office of Basic Energy Sciences.
12:00 PM - W10.7/EE8.7
Effects of Grain Size on Defect Evolution.
Yongfeng Zhang 1 , Hanchen Huang 1
1 Mechannical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractGrain boundaries play a critical role to and evolution of radiation-produced defects. Several groups have examined the production and evolution of cascades in nanograins at the atomic level. In an effort to avoid the random production of cascades, particularly when only few cascades are produced in each simulation, we here examine the evolution of uniformly produced vacancies and interstitials in nanograins. Using classical molecular dynamics simulations, our results show that grain boundaries absorb more interstitials than vacancies, leaving the grains interiors to be vacancy rich. For crystals with small grain size, grain boundary absorption dominates defect annihilation. When grain size is smaller than 20 nm, most interstitials end up at grain boundaries and no interstitial clusters exist inside the grain after 100 ps. Beyond 20 nm, interstitial clustering becomes important.
12:15 PM - W10.8/EE8.8
Accelerating Copper Dissociated Dislocations to Transonic and Supersonic Speeds.
Paulo Branicio 1 , Hélio Tsuzuki 2 , José Rino 2
1 Materials Theory and Simulation Laboratory, Institute of High Performance Computing, Singapore Singapore, 2 Departamento de Física, Universidade Federal de São Carlos, São Carlos, São Paulo, Brazil
Show Abstract12:30 PM - W10.9/EE8.9
Atomic Scale Study of Effect of Nano-voids on Mechanical Properties of Nanocrystalline Metals.
Avinash Dongare 1 , Arunachalam Rajendran 2 1 , D. Brenner 3 , Mohammed Zikry 1
1 Mechanical Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 , U. S. Army Research Office, Raleigh, North Carolina, United States, 3 Materials Science, North Carolina State University, Raleigh, North Carolina, United States
Show Abstract12:45 PM - W10.10/EE8.10
Multi-physics Modeling for Dislocation and Hydrogen Coupled Evolution in BCC Iron.
Hideki Mori 1 , Hajime Kimizuka 1 , Shigenobu Ogata 1
1 Mechanical Science and Bioengineering, Osaka University, Osaka Japan
Show AbstractWe construct a numerical model of the coupled evolution of hydrogen-concentration and defect fields in iron based on a phase-field (PF) microelasticity theory, with coupling of the long-range elastic interactions and short-range chemical interactions that control hydrogen and dislocation motion. To obtain the physical parameters included in the PF free-energy functional, the interaction energy between a hydrogen atom and dislocation core, and the hydrogen-concentration dependence of misfit energy and eigenstrains are quantitatively determined using an embedded-atom-method (EAM) and density-functional-theory (DFT) calculations. Based on these data, we investigate an evolution of the hydrogen-dislocation interactions, and also a hydrogen diffusion and concentration around piled-up dislocations under applied stresses at various temperatures. It is clearly observed that the hydrogen is significantly localized and concentrated around dislocation cores, so that the remarkable difference exists in hydrogen concentration between in the bulk region and in the vicinity of dislocation cores, ranging from several weight ppm to several thousands weight ppm. Also, the spatial profile of trapped hydrogen around dislocations strongly depends on the stress field produced by dislocations. With increasing temperature, the trapped hydrogen escapes from dislocations and hydrogen concentration around dislocation cores steadily decreases. From our EAM and DFT results, the misfit energy of iron is remarkably lowered by the hydrogen impurity at high concentration. This fact brings the result that the distribution of hydrogen concentration affects the dislocation configurations mutually, and the width of dislocation core becomes broader due to the trapped hydrogen.
W11/EE9: Computational Nanomechanics II - Nanocrystals & Nanowires
Session Chairs
Thursday PM, December 04, 2008
Constitution B (Sheraton)
2:30 PM - **W11.1/EE9.1
The Limits of Strength in Materials at the Nanoscale: A Quantitative Description of Plastic Deformation in Computer Generated Nano-crystalline Cu.
Nhon Vo 1 , Robert Averback 1 , Pascal Bellon 1 , Alfredo Caro 2
1 Materials Science and Engineering, University of Illinois, Urbana-Champaign, Illinois, United States, 2 Chemistry, Materials, and Life Sciences, LLNL, Livermore, California, United States
Show AbstractUsing the concept of Richardson’s pairs developed to study turbulent flow, we construct an algorithm to quantify the contribution to plastic deformation in computer simulations of plasticity in nanophase metals originated in grain boundaries, in perfect and partial dislocations, and in formation of twins. We conclude that the competition between these mechanisms depends on strain rate and grain size. Contrary to the often-reported findings from computer simulation that metals soften as their grain sizes fall below 10-15 nm, we find an absence of such softening when the grain boundaries are suitably relaxed. Rather than “inverse” Hall Petch behavior, our calculations show that by thermally annealing the specimens prior to deformation, flow stresses either remain constant below 10 nm or even continue to increase as the grain size falls below even ~ 6 nm. These results provide a rationalization for why some experiments find an inverse Hall-Petch relationship at grain sizes below 10-20 nm while others do not, and they provide a key to resolving the long standing controversy concerning the limits of strength in materials at the nanoscale.
3:00 PM - W11.2/EE9.2
Annealing and Mechanical Response of Nanocrystalline Cu with and without Fe Impurities.
Diana Farkas 1 , Alfredo Caro 2 , Eduardo Bringa 2
1 Materials Science, Virginia Tech, Blacksburg, Virginia, United States, 2 Chemistry, Materials and Life Sciences, Lawrence Livermore National Lab, Livermore, California, United States
Show AbstractWe report fully three dimensional atomistic molecular dynamics studies of grain growth kinetics in nanocrystalline Cu of 5 nm average grain size. We observe the formation of annealing twins, including five fold twins as part of the grain growth process. The grain size and energy evolution was monitored as a function of time for various temperatures, yielding activation energy for the process. Annealing twins are formed during the grain growth process controlled by the emission of Shockley partial dislocations from the moving boundaries. We also report on the role played by Fe impurities in nanocrystalline Cu. We found a strong decrease in grain boundary mobility resulting in an enhancement of the stability of nanophase grain boundaries against annealing. Virtual tensile tests of samples with and without impurities performed using molecular dynamics techniques revealed a hardness that is unaffected by the presence of the Fe impurities.
3:15 PM - W11.3/EE9.3
Dislocation Activity Within Nanocrystalline Metals: A Molecular Dynamics Study.
Christian Brandl 1 , Erik Bitzek 1 , Peter Derlet 1 , Helena Van Swygenhoven 1
1 ASQ/NUM – Materials Science & Simulation, Paul Scherrer Institut, Villigen PSI Switzerland
Show AbstractThe use of large scale molecular dynamics to study the mechanical properties of FCC nanocrystalline (nc) metals provides a detailed picture of the atomic-scale processes during plastic deformation at room temperature. Simulations have revealed that grain boundaries can act as both sources and sinks for partial or full dislocations and that the surrounding grain boundary environment can significantly affect the motion of a dislocation as it propagates through the grain (Acta Mater. 54, 1975 (2006)). Simulations have recently revealed that cross-slip via the Fleischer mechanism occurs in nc-Al (Phys. Rev. Lett. 100, 235501 (2008)), and that the grain boundary structure is found to strongly influence when and where cross-slip occurs. It is found that cross-slip allows a dislocation to avoid local stress concentrations that would otherwise act as strong pinning sites for dislocation propagation. A statistical analysis of dislocation activity as a function of strain up to 9% total strain has also been performed (Acta. Mater. in press (2008)) revealing that (1) significant slip activity is only observed beyond the maximum flow stress whereas dislocation nucleation occurs at lower stresses indicating that propagation is the rate limiting process in simulation, (2) the resolved stress at which a slip event takes place can be correlated with the underlying dislocation process and (3) there is a distribution of critical resolved shear stresses at which slip is initiated. These results are discussed in the framework of realistic micromechanic models for nanocrystal plasticity.
3:30 PM - W11.4/EE9.4
Grain Growth of Metallic Nanocrystals: Insights from Molecular Dynamics.
Stephen Foiles 1
1 Computational Materials Science and Engineering Department, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractStructural evolution of nanograin metals critically impacts both the processing of these materials as well as their stability during use. This talk describes the evolution of the grain structure of nanograined Ni determined by molecular dynamics simulations of fully three-dimensional random grain structures with initial grain sizes ranging from 5 to 15 nm. These simulations provide important information for use in higher length scale models. In particular, we will discuss the mechanisms of twin boundary formation and the role of grain rotation in these systems.Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC0494AL85000.
3:45 PM - W11.5/EE9.5
Atomistic Modeling of the Interaction of Glide Dislocations with ‘Weak’ Interfaces.
Jian Wang 1 , Richard Hoagland 1 , John Hirth 1 , Amit Misra 1
1 , LANL, Los Alamos, New Mexico, United States
Show AbstractUsing atomistic modeling and anisotropic elastic theory, we have explored the interaction of glide dislocations with interfaces in a model Cu-Nb system. The incoherent Cu-Nb interfaces have relatively low shear strength and are referred to as ‘weak’ interfaces. Our work shows that such interfaces are very strong traps for glide dislocations and thus, effective barriers for slip transmission. The key aspects of the glide dislocation-interface interactions are as follows. (i) The weak interface is readily sheared under the stress field of an impinging glide dislocation. (ii) The sheared interface generates an attractive force on the glide dislocation, leading to the absorption of dislocation in the interface. (iii) Upon entering the interface, the glide dislocation core readily spreads into an intricate pattern within the interface. Consequently, the glide dislocations in both Cu and Nb crystals are energetically favored to enter the interface when they are located within 1.5 nm from the interface. Besides the trapping of dislocations in weak interfaces, we also discuss geometric factors such as the crystallographic discontinuity of slip systems across the Cu/Nb interfaces, which contribute to the difficulty of dislocation transmission across an interface. The implications of these findings to the unusually high strengths experimentally measured in Cu/Nb nanolayered composites are discussed.
4:30 PM - **W11.6/EE9.6
Incipient Plasticity and Creep of Nanowires, Nanopillars and Nanoparticles.
Eugen Rabkin 1 , Dan Mordehai 1 , Leonid Klinger 1 , David Srolovitz 2
1 Department of Materials Engineering, Technion, Haifa Israel, 2 Department of Physics, Yeshiva University, New York, New York, United States
Show AbstractWe report on a series of molecular dynamics simulations of the uniaxial compression of gold nanopillars and nanoparticles of various shapes and sizes. The yield stress of nanopillars observed in simulations is either a linear or parabolic function of temperature, depending on the choice of interatomic potential, nanopillar cross-section and/or nanopillar size. We suggest a simple yield nucleation criterion in which the nucleation of the first Shockley partial at the surface of a nanopillar occurs at a critical strain at the surface, which includes contributions from thermal vibration, elastic loading and thermal expansion. We demonstrate that the yield condition correctly describes the temperature dependence of the yield stress and locations of the surface nucleation sites of the Shockley partials observed in the full set of computer simulations. In the simulations of nanoparticles fixed on rigid substrate and compressed by a rigid punch, the first Shockley partials nucleate at the facet corners and at the topmost surface steps for faceted and rounded particles, respectively. The stacking faults produced by leading Shockley partial propagate toward the substrate and then spread along it. We demonstrate that plasticity is generated during the jump-in adhesive contact between the punch and the nanoparticle and discuss the geometry of the contact in terms of macroscopic theories of adhesion. We also consider the diffusional creep deformation mechanisms of nanowires and nanoparticles at the stresses below their yield stress taking into account stress-driven diffusion along the grain boundaries and punch-particle interface. Diffusion creep represents a viable deformation mechanism of gold nanoparticles at the temperatures above 500 K.
5:00 PM - W11.7/EE9.7
Predicting the Elastic Modulus of Nanowires from First-principles Density Functional Theory Calculations on Their Surface and Bulk Materials.
Guofeng Wang 1 , Xiaodong Li 2
1 Department of Mechanical Engineering, Indiana University Purdue University Indianapolis, Indianapolis, Indiana, United States, 2 Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina, United States
Show AbstractNano-devices employ nanowires as their active components to generate, transmit, and convert powers and motions. Hence, the dependence of their mechanical properties on their geometric size is a very important factor in determining the performance of nanowires in those devices. So far, several different fashions of the size dependence of the elastic properties of nanowires have been revealed: (1) elastic modulus increases with the decreasing size, for examples, in Ag and Pd nanowires; (2) elastic modulus decreases with the decreasing size, for examples, in ZnO and GaN nanowires; and (3) elastic modulus shows little dependence of the size of the nanomaterials such as Au nanowires. In this work, we combined first-principles density functional theory calculations with linear elasticity theory. Using the concept of surface stress, we developed a model that is able to predict the elastic modulus of the nanowires as a function of their diameters based on the calculated properties of its surface and bulk materials. Furthermore, we applied this computation approach to Ag, Au, and ZnO nanowires. For both Ag and Au nanowires, our prediction results agree excellently with the experimental data in the literature. For ZnO nanowires, our predictions are qualitatively consistent with some of experimental data for ZnO nanobelts. Therefore, we found that surface stress plays a very important role in determining the elastic modulus of nanowires. Our finding suggests that the elastic properties of nanowires could be engineered by altering the surface stress through rational control of the adsorptions, charges, structure, and impurities in the surfaces.
5:15 PM - W11.8/EE9.8
Structural Study of the Formation of Linear Atomic Suspended Chains from Platinum Nanowires Stretching.
Pedro Autreto 1 , Fernando Sato 1 , Pablo Coura 3 , Socrates Dantas 3 , Varlei Rodrigues 1 , Daniel Ugarte 1 2 , Douglas Galvao 1
1 , State University of Campinas, Campinas/SP, São Paulo, Brazil, 3 , UFJF, Juiz de Fora, MG, Brazil, 2 , LNLS, Campinas, SP, Brazil
Show AbstractIn the last years a considerable number of experimental and theoretical studies has been devoted to metallic nanowires (NWs) and suspended atomic chains (LACs) [1]. NWs and LACs have attracted a great interest due to observation of very interesting physical phenomena, such as spin filters, quantized conductance, etc., with possible technological applications in diverse areas of nanotechnology. Atomic-size NMs generated by stretching can provide a wealth of information on the elasticity of metallic nanostructures.In this work we report results from the study of the atomistic aspects of the elongation and rupture of Pt NWs using real-time atomic-resolution transmission electron microscopy (dynamical HRTEM) and molecular dynamics (MD) simulations. We used tight-binding molecular dynamics (TB-MD) techniques with second-moment approximation (SMA), a methodology that has been proved to be very effective to study pure and alloy metallic nanostructures [2,3].We have carried out a systematic study of the structural properties of NWs and LACs formed from Pt nanostructures under simulated mechanical stretching. We have considered different crystallographic orientations ([100], [110] and [111]) for the pulling directions. Diverse parameters such as temperature, cluster size (up to ~ 400 atoms), and speed of pulling were varied in order to determine their relative importance to the LAC formation.Our results are in good agreement with the structural information data from HRTEM experiments. For defectless structures the LAC formation is statistically favored for [110], followed by [100] and [111], respectively. One interesting result is that when we have mismatched boundary grains (structures with different crystallographic orientations) this favors the LAC formation for all cases investigated. The disorder increases the probability of LAC formation and structures type “triple helix” in general appears before the final stages prior to LAC formation. It remains to be investigated whether this is a specific feature of Pt NWs or a general behavior for other metallic NWs.Work supported in part by the Brazilian Agencies CAPES, CNPq, FAPEMIG, and FAPESP.[1] N. Agrait et al, Phys. Rep. 377, 81 (2003).[2] V. Rodrigues, F. Sato, D. S. Galvão, and D. Ugarte, Phys. Rev. Lett. 99, 255501 (2007).[3] J. Bettini, F. Sato, P. Z. Coura, S. O. Dantas, D. S. Galvão, and D. Ugarte, Nature Nanotechonology 1, 182 (2006).
5:30 PM - W11.9/EE9.9
Torsion and Bending Simulations of Metallic Nanowires.
Christopher Weinberger 1 , Wei Cai 1 , William Fong 1 , Erich Elsen 1
1 Mechanical Engineering, Stanford University, Stanford, California, United States
Show Abstract5:45 PM - W11.10/EE9.10
Mechanisms of Dislocation Depletion in Small-volume Structures of FCC Metals.
Kedarnath Kolluri 1 , M. Rauf Gungor 1 , Dimitrios Maroudas 1
1 Department of Chemical Engineering, University of Massachusetts, Amherst, Amherst, Massachusetts, United States
Show AbstractRecent experimental studies on nanometer-scale face-centered cubic (fcc) metals have shown that the strength of such fcc small-volume structures increases with decreasing characteristic lengths and that the dislocation density always decreases during the application of the strain. The nearly defect-free crystal then deforms elastically until new dislocations nucleate. Furthermore, comparative studies of nanometer-scale pillars of fcc and body-centered cubic (bcc) metals showed that dislocation depletion and ultra-high strength is almost exclusive to fcc metals. This suggests that the differences in the fundamental processes of dislocation motion and dislocation-dislocation interactions between nanoscale fcc and bcc metals play an important role in determining their mechanical behavior. These fundamental atomic-scale mechanisms, however, are difficult to observe and analyze experimentally.In this presentation, we report an atomic-scale analysis of the mechanisms of dislocation depletion in small-volume structures of fcc metals, focusing on free-standing ultra-thin copper films that are subjected to biaxial tensile strain. Our study is based on large-scale molecular-dynamics simulations at constant temperature and high strain rate, using an embedded-atom-method parameterization to describe the interatomic interactions. Our analysis of the films' mechanical response to the applied tensile strain reveals three stages of deformation. During the first stage, most of the dislocations in the material are unpinned; these dislocations glide under the application of biaxial strain in such a direction that they unzip the stacking faults they bound and the dislocation networks that they are a part of. With the stacking fault area reduced, there are fewer barriers to dislocation glide in the thin films. Consequently, the remaining dislocations glide faster and farther. During the second stage, gliding dislocations interact with the stacking faults formed by other dislocations. These interactions lead to dislocation dissociation and cross-slip and they aid in dislocation annihilation. We have identified three classes of dislocation-stacking fault interactions where the stacking faults act as barriers to dislocation glide and as sources for dislocation cross-slip. As the dislocation density decreases, the thin film's strength increases significantly and its mechanical behavior is observed to be closer to that of an elastic solid. In the third deformation stage, continued application of strain leads to an increase in the film's stress and, eventually, to nucleation of new dislocations in the thin film. Dislocation nucleation and depletion through dislocation-stacking fault interactions in the thin film continue in cycles until the failure of the film.
W12: Poster Session II: Computational Material Design
Session Chairs
Friday AM, December 05, 2008
Exhibition Hall D (Hynes)
9:00 PM - W12.1
Computational Study of the Deposition of SrTiO3 Thin Films.
Jennifer Wohlwend 1 , Cosima Boswell 1 , Simon Phillpot 1 , Susan Sinnott 1
1 Materials Science and Engineering , University of Florida, Gainesville, Florida, United States
Show AbstractClassical molecular dynamics simulations are used to examine the growth of SrO, TiO2, and SrTiO3 (STO) thin films on STO. The simulations consider the deposition of SrO and TiO2 molecules with varying beam composition and stoichiometric STO clusters at incident energies ranging from 0.1 to 1.0 eV/atom onto the (001) surface of STO. Along with the size of the particles and their incident energy, the role of STO surface termination layer (SrO vs. TiO2) is considered. In the case of SrO thin film deposition, smooth, ordered films are produced for all of the incident energies considered and for both surface terminations. In contrast, in the case of TiO2 deposition, three-dimensional islands are formed under all conditions. These predictions are in agreement with experimental data, and the simulations explain why these differing morphologies are produced for SrO and TiO2 deposition. Alternating monolayer deposition produces more ordered films when compared to alternating particle deposition and both deposition methods yield superior layer segregation and structural order.
9:00 PM - W12.10
Modelling the Glide of Dislocations in High Peierls Stress Crystals.
David Rodney 1 , Laurent Proville 2
1 SIMAP, INP Grenoble, Saint Martin d Heres France, 2 SRMP, CEA, Gif sur Yvette France
Show Abstract9:00 PM - W12.11
Internal Elastic Fields and Dislocation Density Tensor in Deformed fcc Crystals: Computational Modeling and Experiments.
Jie Deng 1 , Anter El-Azab 2 , Ben Larson 3
1 Mechanical Engineering, Florida State University, Tallahassee, Florida, United States, 2 School of Computational Science, Florida State University, Tallahassee, Florida, United States, 3 Materials Science and Technology Division, Oak Ridge National Lab, Oak Ridge, Tennessee, United States
Show AbstractThe statistical character of dislocation density and associated internal elastic fields in deformed crystals play an important role in the plastic deformation of crystals. A theoretical investigation of the statistics of dislocation systems has been initiated to understand the spatial distribution characteristics of internal elastic fields in deformed crystals. In particular, the elastic strain and lattice rotation fields have been computed using an exact formulation of the elastic boundary value problem of dislocation fields in representative volume elements of an fcc crystal with high density of dislocations. The dislocation distributions were computed using the method of dislocation dynamics simulation. Owing to the discrete nature of dislocations in crystals, these internal fields exhibit a statistical character. We analyze the statistics of the stress, the elastic strain, and the lattice rotation in terms of spatial patterns by performing local averaging at various pixel sizes to reveal characteristic wavelengths in the field patterns. Since the elastic strain and lattice rotation fields are directly related to the lattice curvature and dislocation density tensor fields, we have analyzed the spatial features and probability density functions for the lattice curvature and dislocation density tensor as well. The statistical properties and spatial patterns obtained in the simulations will be compared with preliminary submicron-resolution 3D x-ray microscopy measurements of deformation microstructure in axially strained copper single crystals. Comparison of simulation results with experimental data provides a direct evaluation of theoretical plasticity model predictions.The research at Florida State University and ORNL was supported by the DOE Office of Basic Energy Sciences Division of Materials Sciences and Engineering.
9:00 PM - W12.13
The Dependence of Strength on the Nature and Distribution of Dislocations at the Interfaces in Nanoscale Metallic Multilayered (NMM) Composites.
Cory Overman 1 , Hussein Zbib 1 , Firas Akasheh 2 , David Bahr 1
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States, 2 Mechanical Engineering, Tuskegee University, Tuskegee, Alabama, United States
Show AbstractIn a previously published work (Akasheh et al., JAP, 101, 084314, 2007), we investigated the effect on the channeling strength of a single interfacial dislocation intersecting the path of a layer-confined glide dislocation in the Cu/Ni cube-on-cube system. In spite of the enhanced predictions of the model compared to the isolated glide model, it was not able to capture the experimentally observed dependence of the strength of NMM composites on their individual layer thickness in the few nanometers thickness range. In this work, we use dislocation dynamics (DD) analysis to examine the influence of networks of interfacial dislocations whose nature and distribution are commensurate with the level of relaxation and loading of the structure. Misfit and pre-deposited interfacial dislocation arrays, as well as realistic combinations of both, are studied and the dependence of strength on layer thickness is reported. DD analysis not only captures the effect of long-range stresses on dislocation motion but also the effect of short-range interactions which proved to be crucial to understand the strength and dislocation structures and mechanisms observed in real systems.
9:00 PM - W12.14
Breakdown of Self-Similar Hardening Behavior in Au Nanopillar Microplasticity.
Jaime Marian 1 , Jaroslaw Knap 2
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 , Army Research Laboratory, Aberdeen, Maryland, United States
Show Abstract9:00 PM - W12.15
Phase-field Model for Deposition Process of Platinum Nanoparticles on Carbon Substrate.
Shunsuke Yamakawa 1 , Kazuyuki Okazaki-Maeda 2 , Masanori Kohyama 3 , Shi-aki Hyodo 1
1 , Toyota Central R&D Labs., Inc., Nagakute Japan, 2 Department of Mechanical Engineering, Osaka University, 2-1 Yamadaoka, Suita Japan, 3 Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, 1-8-31 Midorigaoka, Ikeda Japan
Show AbstractPlatinum (Pt) supported on a carbon carrier is widely used as a catalyst for polymer electrolyte membrane fuel cells. The catalytic activity is significantly affected by the size distribution and morphologies of the Pt particles. Therefore, the formation process of Pt particles is of great interest. The phase-field method has recently attracted considerable attention as a possible approach to understand nanoscale phenomena. Initially, we extended the phase-field approach [1] to describe the formation process of Pt particles by combining it with first-principles calculations. Secondly, we investigated several aspects of Pt particles deposited on carbon black. The microstructural evolution of a nanoparticle was described by the temporal evolution of the field variables related to the concentration of Pt, long-range crystallographic ordering, and the phase transition. First-principles calculations utilizing the STATE program [2] were performed in order to estimate the interaction energies between several different types of Pt clusters and a graphene sheet. The Pt density profile concentrated over the substrate surface led to the formation of three-dimensional islands, which was in accordance with the Volmer-Weber mode of growth. The overall size of the Pt particles was found to be less than 10 nm. The particle shape was nearly spherical or hemispherical. The size distributions of the Pt particles were sensitive to the heterogeneity of the substrate surface and to the competitive nucleation and growth processes. At a glance, it appears that our results are consistent with experimental results such as those of high-resolution transmission electron microscope (TEM) images. These results imply that the phase-field method provides a reasonable microstructural evolution of the Pt nanoparticles. This work was supported by a grant from Core Research for Evolutional Science and Technology (CREST) by the Japan Science and Technology Agency (JST), Japan. [1] Yamakawa S, Okazaki-Maeda K, Kohyama M, and Hyodo S 2008 J. Phys.: Conf. Ser. 100 072042. [2] Okazaki-Maeda K, Yamakawa S, Morikawa Y, Akita T, Tanaka S, Hyodo S, and Kohyama M 2008 J. Phys.: Conf. Ser. 100 072044.
9:00 PM - W12.16
A Molecular Dynamics Study of Fcc-bcc Phase Transformation Kinetics of Iron.
Shinji Tateyama 1 , Yasushi Shibuta 1 , Toshio Suzuki 1
1 Department of Materials Engineering, the University of Tokyo, Tokyo Japan
Show AbstractThe kinetics of the fcc-bcc interface during the fcc-bcc phase transformation of iron was investigated by molecular dynamics simulations using a Finnis-Sinclair potential. The kinetics of interfaces with Nishiyama-Wassermann (N-W) and Kurdjumov-Sachs (K-S) relations, which are the experimentally commonly-observed orientations, was examined. Planar propagation of the fcc-bcc heterointerface was observed for the N-W interface, whereas needle-like growth along a particular orientation was observed for the K-S interface. This difference was due to the pattern difference of the matching area between the fcc and bcc lattice at the interface, since the transformation started from the matching area along the Bain deformation path. The ratio of the matching area to the total area of the N-W relation is higher than that of the K-S relation, and the matching area appears in a reticular pattern in the case of the N-W relation, whereas the matching area appears as elongated regions in the case of the K-S relation. The reticular distribution of matching area in the N-W relation induced the planar fcc-bcc transformation homogeneously. The elongated regions of matching area in the K-S relation induced a propagation velocity bias in the early stages, and this bias was gradually enhanced during transformation, inducing needle-like growth.
9:00 PM - W12.18
Variable Charge Studies on the Oxidation of Polycristalline Aluminum.
Aurelien Perron 1 , Olivier Politano 1 , Gurcan Aral 2 , Sebastien Garruchet 1 , Vincent Vignal 1
1 ICB, UMR 5209 CNRS, Universite de Bourgogne, Dijon France, 2 Department of Physics, Izmir Institute of Technology, Gulbahce Campus, Izmir Turkey
Show AbstractWe investigate the oxidation of aluminum polycrystalline surfaces using molecular dynamics (MD) simulations with the variable charge model that allows charge dynamically transfer among atoms. The interaction potential energy between atoms is described by the variable-charge scheme, called electrostatic plus (Es+) potential model, which is composed of an embedded atom method potential and an electrostatic term. In our earlier studies, the variable-charge scheme was used to study the oxidation of aluminum single-crystals under an oxygen pressure of 10 atm and lead to obtain the growth of an oxide film of 3nm thick [1-3]. The present simulations were performed on polycrystalline aluminum surfaces with a mean grain size of 5 nanometers. We have mainly concentrate on the effect of the temperature parameter on the oxidation process. In particular, we investigated the relation between the oxide-scale growth kinetics and the evolution of the chemical composition in the oxidation process, by studying the structure of the developing oxide film under a constant molecular oxygen density (1 atm) as a function of the temperature (from 300 to 750K).The results show that, beyond a transition regime, the growth kinetics follow a direct logarithmic law and present a limiting thickness of ~3nm. The present work permits to characterize the growth of a crystalline oxide layer. We will show how both the lower oxygen pressure and the substrate microstructure play a major role on the development of a crystalline oxide structure as amorphous oxide film was obtained by oxidizing aluminum single-crystals. We will also show a correlation of the oxide microstructure with the temperature as the oxide films becomes more crystalline when increasing the temperature. The role played by an external applied stress (up to 7%) on the oxidation kinetics and oxide microstructure was also studied.References :[1] A. Hasnaoui, O. Politano, J.M. Salazar, G. Aral, R. Kalia, A. Nakano and P. Vashishta, Surf. Sci. 579 (2005) 47.[2] A. Hasnaoui, O. Politano, J.M. Salazar and G. Aral, Phys. Rev. B 73 (2006) 035427.[3] T.J. Campbell, G. Aral, S. Ogata, R. Kalia, A. Nakano, P. Vashishta, Phys. Rev. B 71 (2005) 205413.
9:00 PM - W12.19
Nanomechanical Study of the Production of Bimetallic Clusters by Coalescence.
S. Mizuno 1 , K. Shintani 1
1 Department of Mechanical Engineering and Intelligent Systems, University of Electro-Communications, Chofu, Tokyo, Japan
Show AbstractMetallic clusters have attracted much attention of researchers because they are good catalysts in fuel cell electrode reactions. Their excellent performance as catalysts is due to the high surface-to-volume ratio. Magnetic clusters have also been intensely studied in recent years. They are applicable to magnetic recording media. These magnetic clusters are ordinarily alloyed clusters. On the other hand, coasening of Pt clusters on carbon has been experimentally reported, and whether the coasening is caused either by coalescence of Pt clusters via migration or by Ostwald ripening via Pt dissolution and redeposition is an open question. In addition, coalescence must be one of the growth modes of clusters in an inert-gas aggregation source. Bimetallic clusters have either alloyed or core-shell structure, and they are usually synthesized by using laser vaporization sources or from organometallic compounds. In this paper, however, we suggest a method of synthesis of bimetallic clusters via coalescence of two clusters of different elements based on molecular-dynamics analysis. All pair combinations of the elements Ni, Cu, Au, Ag, Pt, and Pd are considered. After coalescence, the original surfaces of the two clusters decrease, and the surface energy is transformed into the kinetic energy. Consequently, the temperature of the united cluster rises. If this temperature is higher than the melting temperature, melting and local alloying at the interface occur. If alloying spreads into the united cluster, an alloyed bimetallic cluster is synthesized. If melting occurs only in one of the two clusters, and the atoms in liquid phase gradually cover the surface of the other cluster, a core-shell cluster appears. Furthermore, if part of the atoms in liquid phase diffuse into the core, a three-shell onion-like cluster is created. Finally, if the temperature of the united cluster is lower than the melting temperature, an epitaxially-joined cluster appears. The morphological evolutions in the three modes of coalescence are followed, and under what conditions each mode of coalescence occurs is discussed. The results show that alloyed, core-shell, and epitaxially-joined clusters appear depending on the differences between the surface energies, lattice constants, and diffusivities of the elements of the original clusters.
9:00 PM - W12.2
Growth Instabilities on Cu Vicinals: Role of Impurities.*
Rajesh Sathiyanarayanan 1 , Ajmi BHadj Hammouda 1 , T. Einstein 1
1 Department of Physics, University of Maryland, College Park, Maryland, United States
Show AbstractThe study of growth instabilities during molecular beam epitaxy (MBE) is important in order to improve the fabrication of surfaces with desired morphology. Cu(100) and its vicinal surfaces are ideally suited for such studies because they do not reconstruct. Experimental results show that during growth, Cu(100) surfaces develop a mounding instability and its vicinal surfaces undergo meandering instability.1 The meandering wavelength (λm) is found to scale with deposition rate (F) as λm ~ F-γ with an exponent γ ≈ 0.19. When deposition is continued for at least 10 ML, small pyramids appear on the surface. The smaller value of γ rules out Bales-Zangwill instability (with an expected value of γ = 0.5) as a possible mechanism for the observed meandering instability. Though other mechanisms such as Kink Ehrlich-Schwoebel barrier Effect (KESE) and Unhindered Step Edge Diffusion (USED) predict exponents closer to the observed value, they fail to explain the formation of small pyramids.
A. Ben-Hammouda et al. showed that impurities coadsorbed on the surface during deposition could explain the discrepancy between the experimental observations and the known instability mechanisms.2 Using kinetic Monte Carlo simulations on a solid-on-solid (SOS) model, they could reproduce the λm–F scaling behavior and the formation of small pyramids under the following conditions: (i) the bond between an impurity atom and a Cu adatom is stronger than the Cu-Cu bond and (ii) the impurities diffuse slowly compared to Cu adatoms. Due to their stronger bonds with Cu adatoms, impurity atoms slow down Cu adatom diffusion, thereby shortening the diffusion length and making λm less sensitive to F. Also, impurity atoms act as nucleation centers for the formation of small pyramids. Some common impurities include oxygen, sulfur, CO and water vapor; Monte Carlo simulations alone cannot identify which of these impurities (or another) is responsible for the observed instabilities. Hence, we conduct a systematic evaluation of surface energies and barriers for all likely impurity species.
Density-functional theory (DFT)-based software packages have been used successfully to calculate surface energies and diffusion barriers. Using the Vienna Ab initio Simulation Package (VASP), we compute the adsorption energies and diffusion barriers for the likely impurity atoms. This talk discusses the specific results of these calculations and the trends they imply, along with the role of impurities in growth instabilities in similar systems.
*Supported by NSF MRSEC Grant DMR 05-20471, computational resources from NSF MRAC grant TG-DMR 070003N
1N. Néel
et al., J. Phys.: Condensed Matter
15, S3227 (2003)
2A. Ben-Hammouda
et al., Phys. Rev. B
77, 245430 (2008)
9:00 PM - W12.20
Homogenization of Polycrystal Elastic Properties using Ab Initio Simulations, Fast Fourier Transforms, and Crystal Elasticity Finite Element Schemes.
D. Ma 1 , Dierk Raabe 1 , R. Lebensohn 2 , M. Friak 1 , J. Neugebauer 1
1 , Max-Planck-Institut fuer Eisenforschung, Duesseldorf Germany, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show Abstract9:00 PM - W12.21
Microstructure Modelling and Performance Evaluation of Mixed Conducting (MIEC) Nanoscale Cathodes for SOFC Application.
Bernd Ruger 1 , Jochen Joos 1 , Andre Weber 1 , Ellen Ivers-Tiffee 1
1 , Institut fuer Werkstoffe der Elektrotechnik IWE, Universitaet Karlsruhe (TH), Karlsruhe Germany
Show Abstract9:00 PM - W12.22
Using Density Functional Theory to Design Ternary MgLiX Alloys.
William Counts 1 , Martin Friak 1 , Dierk Raabe 1 , Jorg Neugebauer 1
1 , Max Planck Institute for Iron Research, Dusseldorf Germany
Show AbstractBCC magnesium-lithium alloys are a promising class of ultra light-weight structural materials whose mechanical properties can be further improved by alloying with a third element. In order to screen for potentially beneficial alloying elements, the elastic properties of 5 different MgLiX (X = Al, Si, Zn, Ca, or Cu) were determined using density functional theory. Based on these DFT determined properties, engineering parameters like the ratio of bulk (compressive) modulus over shear modulus (B/G) and the ratio of Young’s modulus over mass density (Y/ρ) were calculated. These engineering parameters were then used to identify alloys that have optimal mechanical properties of a light-weight structural material.
9:00 PM - W12.23
Effects of Strain Rates and Temperatures on the Mechanical Properties of Thin Metallic Nanowires: A Combined Molecular Dynamics and Accelerated Molecular Dynamics Simulations Study.
Chun-Wei Pao 1 , Danny Perez 1 , Sriram Swaminarayan 1 , Arthur Voter 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe electronic and mechanical properties of metallic nanowires and contacts have drawn increasing interest due to shrinking device sizes and various novel potential applications, such as atomic switches and shape memory nanowires. Recent high-resolution transmission electron microscopy (HRTEM) experiments provide fascinating real-time images of the formation of metallic nanocontacts, nanowires and even single atomic chains, by putting two metallic tips together for a while and then slowly pulling them apart. Pursuing the retraction process further then provides a wealth of information on the elastic and plastic properties of the wires. However, HRTEM can only provide an integrated profile view of the system with a time resolution limited to hundredths of seconds. Moreover, some experimental conditions are not amenable to HRTEM imaging. Atomistic scale simulations such as molecular dynamics (MD) simulations and more recently developed accelerated molecular dynamics (AMD) methods can provide atomistic scale details of dynamical evolution of the contact formation and the processes taking place during retraction. In this study, we simulate the retraction processes of thin metallic nanowires under a wide range of strain rates (from 1m/s to 1μm/s) and temperatures (300 K and 77K). The extra-slow strain rate is achieved by applying accelerated molecular dynamics simulations. We will present simulation results on how the mechanical properties of thin metallic nanowires are affected by different strain rates and temperatures.
9:00 PM - W12.24
Oxygen in Grain Boundaries of Aluminum: A Molecular Dynamics Study.
Andreas Elsener 1 , Olivier Politano 2 , Peter Derlet 1 , Helena Van Swygenhoven 2
1 ASQ/NUM – Materials Science & Simulation, Paul Scherrer Institut, Villigen PSI Switzerland, 2 Institut Carnot de Bourgogne, Université de Bourgogne, Dijon France
Show AbstractOne of the important differences between simulation and experiments in grain boundary dominated metallic structures is the lack of impurities such as oxygen in computational samples. A modified variable charge method is presented, based on the Streitz and Mintmire approach (Phys. Rev. B 50, 11996 (1994)) incorporating local chemical potentials to efficiently simulate oxidation in a metallic Al environment (Modell. Simul. Mater. Sci. Eng. 16, 025006 (2008)). The present work reports on the application of this method to investigate aluminum samples with a dilute amount of oxygen under load. In particular, using aluminum bicrystals with symmetrical tilt grain boundaries, the influence of the presence of Oxygen on the coupled grain boundary migration is investigated. It is found that the stick-slip process identified by Cahn and Mishin (Acta Mat. 54, 4953 (2006)), that is associated with the grain boundary migration, requires a higher applied shear to activate when Oxygen atoms exist within the grain boundary. This result is rationalized in terms of the stress signature of the Oxygen within the boundary and the associated atomistic mechanism for coupled grain boundary motion to occur.
9:00 PM - W12.25
Simulation of Initial Growth Process of Pt Clusters on Carbon Materials – First-Principles Calculations.
Kazuyuki Okazaki-Maeda 1 , Shunsuke Yamakawa 2 , Yoshitada Morikawa 3 , Shiaki Hyodo 2 , Tomoki Akita 4 , Yasushi Maeda 4 , Shingo Tanaka 4 , Masanori Kohyama 4
1 Dept. Mechanical Engineering, Osaka University, Suita Japan, 2 , Toyota Central R&D Labs., Inc., Nagakute Japan, 3 ISIR, Osaka University, Ibaraki Japan, 4 UBIQEN, National Institute of Advanced Industrial Science and Technology, Ikeda Japan
Show AbstractPlatinum (Pt) nano-particles supported on carbon materials are used as electrode catalysts for a proton exchange membrane fuel cell (PEMFC). The performance is affected by the size distribution of Pt nano-particles and the condition at the interface between Pt nano-particles and carbon materials. Therefore, it is important to investigate the growth process of Pt nano-particles on carbon materials. We previously investigated the interaction between small Ptn clusters (n=1-4) and graphene without any defects by the first-principles calculations [1]. Yamakawa et al. have also simulated the growth process of Pt nano-particles on carbon supports by the phase-field method [2], where the first-principles results were referred as the interaction energy between Pt and carbon supports. In the present study, we firstly examined the interaction energy between Ptn clusters (n=1-13) and a graphene sheet in order to enhance the simulation accuracy. When the Pt cluster is small (n<7), it is found that the planar (2D) clusters vertically adsorbed on graphene are rather stable than the three-dimensional (3D) clusters. As increasing the size of Pt clusters, the 3D clusters are stably adsorbed on graphene. Stability of the Ptn/graphene system is dominated by the cohesive energy of the cluster (Ecoh) and the interaction energy between the cluster and graphene (Eint). For the smaller clusters, Eint is dominant, because the edge of the cluster slightly stronger interacts with garphene than the plane of the cluster. On the other hand, Ecoh is dominant for the larger clusters, because the difference in Ecoh between the 2D and 3D clusters becomes larger than the difference in Eint between the 2D and 3D clusters. Second in the present study, we examined the interaction energy between a 3D Pt13 cluster and graphene including atomic vacancies, in order to investigate the effects of defects. As the result, one isolated Pt atom is stably adsorbed on the vacancy site on graphene and the interaction energy is much larger than that between the Pt atom and ideal graphene [1, 3]. One surface Pt atom of the Pt13 cluster is adsorbed on the vacancy site and the interaction energy between the cluster and graphene with atomic vacancies is much larger than that between the cluster and ideal graphene. The present results suggest that the existence of some defects is important to adsorb the Pt nano-particles on carbon materials with stability. Here we note that the actual “defect” means not only vacancies but also impurities. We also considered the effects of B or N impurities on the interaction between Pt13 clusters and graphene. The data of the interaction between the cluster and graphene including vacancies or impurities is the useful information for the mesoscopic simulation such as the phase-field method.[1] K. Okazaki-Maeda et al., J. Phys.: Conf. Ser. 100 072044. [2] S. Yamakawa et al., J. Phys.: Conf. Ser. 100 072042. [3] K. -J. Kong et al., Mater. Sci. Eng. C 26 (2006) 127.
9:00 PM - W12.26
Plastic Deformation Behavior of Single Crystal and Nanocrystalline Copper Doped with Antimony.
Rahul Rajgarhia 1 , Douglas Spearot 1 , Ashok Saxena 1
1 Mechanical Engineering, University of Arkansas, Fayetteville, Fayetteville, Arkansas, United States
Show AbstractMolecular dynamics simulations have been used to study dislocation nucleation under constant tensile strain in single crystal and nanocrystalline copper with varying concentrations of antimony (0.0-2.0 at.%). In low concentrations, antimony does not form clusters or precipitates in copper and completely segregate to grain boundaries. Therefore for the initial configuration, antimony atoms were randomly distributed in the single crystal model. It was observed that the strained regions around the antimony atoms act as sources for partial dislocations and the maximum stress decreased with the increasing concentration of antimony. In the nanocrystalline model, antimony atoms were preferentially located at grain boundaries and the effect of grain size and antimony concentration will be discussed. The ultimate goal is to understand the underlying mechanisms involved with plasticity in Cu-Sb to fabricate a Cu-Sb nanocrystalline alloy with superior mechanical properties.
9:00 PM - W12.28
Topological Refinement of k-Space Interactions in Alloys.
Volodymyr Bugaev 1 , Oleg Shchyglo 2 , Ruslan Kurta 1 , Alejandro Diaz-Ortiz 1 , Helmut Dosch 1
1 Metastable and Low-Dimensional Materials, Max Planck Institute for Metals Research, Stuttgart Germany, 2 Laboratoire D'etude des Microstructures, ONERA, Chatillon France
Show Abstract9:00 PM - W12.30
Effects of Cyclic Loading on Materials Response to Nanoindentation: Simulations and Experiments.
Arun Nair 1 , Megan Cordill 2 , Diana Farkas 1 , William Gerberich 2
1 Materials Science, Virginia Tech, Blacksburg, Virginia, United States, 2 Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractDynamic nanoindentation of single crystal Ni film of thickness 20 nm oriented in the [111] direction was carried out. Two different indenter frequencies showed a significant effect in the overall mechanical response to indentation, as compared to a static monotonic case. These results are consistent with experiments performed for the same films thicknesses and similar indenter sizes. . It has been found out that in the dynamic indentation case the dislocations formed in one cycle of indentation can act as sources for further dislocation emission, resulting in a softer material response. A simple model is utilized to represent this phenomena and to compare with the experimental results, bridging the timescales of the experiment and the simulation. Consistency is maintained due to plasticity being controlled by dislocation nucleation rather than by dislocation velocity.
9:00 PM - W12.31
Plastic Deformation of Trimetallic Cu-Ni-Nb Nano Composites.
Ioannis Mastorakos 1 , Hussein Zbib 1 , David Bahr 1
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractNanometalic material composites (NMC) represent a novel class of advanced engineering materials whose scientific significance and technological potential as high performance materials is just beginning to be explored. NMC were found to exhibit near-theoretical strength, high ductility and morphological stability, making them uniquely multifunctional materials. Currently, NMC’s are made of bimetallic systems and are typically classified into two types based on the physics of interface strengthening: coherent systems and incoherent systems. In coherent systems, the two metals have the same crystal structure and a small lattice parameter mismatch such that atomic arrangement and slip systems are continuous across the interface, e.g., the fcc/fcc Cu/Ni system with cube-on-cube orientation. In incoherent systems, slip systems are not continuous due to difference in lattice structure and or large lattice parameter mismatch, e.g., the fcc/bcc Cu/Nb system with Kurdjumov-Sachs orientation. While incoherent systems are generally stronger, coherent systems are more ductility. Incoherent interfaces are opaque to dislocations and acts to trap them, which in turn leads to the shearing of the interface. In the present work, we use Molecular Dynamic simulations to investigate the plastic behavior of trimetallic NMC’s under various loading conditions. The potentials used are based on the Embedded Atom Method theory, and are a combination of functions for fcc-fcc materials (for the case of Cu-Ni interface) and fcc-bcc (for the case of Ni-Nb interface).
9:00 PM - W12.32
Thermal Transport Across a Nanojunction from First-Principles.
Keivan Esfarjani 1 , Natalio Mingo 2 1
1 , UC Santa Cruz, Santa Cruz, California, United States, 2 , CEA, Grenoble France
Show Abstract9:00 PM - W12.33
Rippling Morphology and Elastic Scaling Law of Multi-Walled Carbon Nanotubes: Multiscale Simulations.
Sulin Zhang 1 , Xu Huang 1 , Jian Zou 1
1 Engineering Science and Mechanics, Penn State University, University Park, Pennsylvania, United States
Show Abstract9:00 PM - W12.35
Thermal Properties of Al/SiC Metal Matrix Composite using Finite Element Method.
Eusun Yu 1 , Jeong-Yun Sun 1 , Hee-Suk Chung 1 , Kyu Hwan Oh 1
1 , Seoul National Univ., Seoul Korea (the Republic of)
Show Abstract9:00 PM - W12.36
The Effect of Surface Roughness on the Intensity Distribution of Electromagnetic Fields Around Silver Nanoparticles.
Shuzhou Li 1 , George Schatz 1
1 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
Show Abstract9:00 PM - W12.4
Dynamics of Confined Smectic Films and Filaments with Weak Surface Ordering.
Nasser Abukhdeir 1 , Alejandro Rey 1
1 Department of Chemical Engineering, McGill University, Montreal, Quebec, Canada
Show Abstract9:00 PM - W12.7
Strain Sensors based on Metamaterials.
Niwat Angkawisittpan 1 , Alkim Akyurtlu 1
1 Electrical Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractIn this work, metamaterials-based strain sensors are discussed. The electrical properties of composite metamaterials depend on the resonant structures from which they are composed will be affected by the applied strain. Therefore, their properties, including resonant frequencies, will change according to the applied strain. 3D isotropic metamaterials will detect the strain isotropically. These 3D metamaterials will be fabricated using template transfer methods (Ming Wei et al, Macromolecular rapid communications, 2006) and can be embedded in structures.In order to obtain the aforementioned metamaterial design, we will apply Finite-Difference Time-Domain (FDTD) simulations as a tool to study the effects of strain to the resonant frequency of metamaterials. The results of our strain sensing metamaterials will predict the Transmission behaviors of the metamaterials when the strain is applied to. To illustrate our work, we will design the metamaterials for strain sensing, and optimize the geometries of metamaterials to give us the maximum sensitivity to the strain. Also mechanical properties of materials i.e. Young‘s Modulus, and Poisson’s ratio will be taken into account.
9:00 PM - W12.8
Dislocation Evolution Functionals for Deformation and Non-equilibrium Thermodynamics in Metals During Work Hardening and/or Annealing.
Ray Stout 1
1 , RhoBetaSigma Affaires, Livermore, California, United States
Show AbstractDuring work hardening/annealing of metals, the density of dislocations changes significantly from an initial dislocation density around 108 to a work hardened density of around 1012; and then returns to a dislocation density around 108 during annealing[1]. Each incremental dislocation density change has an associated incremental deformation and internal energy change, part of the latter is elastic strain energy from the local strain and stress fields of dislocations. In continuum plasticity and creep response models, the evolution of dislocation density is not explicitly represented as contributions to deformation and internal energy integral functionals. Furthermore, the evolution in dislocation density, dislocation strain, and dislocation strain energy developed with classical elasticity models are definitively coupled, but are not explicitly described in continuum mechanics models. Reasons for this are complex, and are partly due to accepting an additive separation of elastic strain and plastic strain tensors; which is a common and well accepted phenomenological conjecture to model classical elastic plus constant volume plastic material responses[2]. In some neo-classical models, conceptually similar separation metrics for deformation gradients are defined from conjectured independent elastic deformations and plastic deformations[3]; and from conjectured independent recoverable displacements and non-recoverable dislocation density dependent displacement functionals[4]. However, these decompositions [2, 3, 4] for strain metrics are not consistent with available experimental data[1]. However, by reconsidering deformations from dislocation creation kinetics[4] as part of recoverable displacements rather than non-recoverable displacements, which is a revision in the dislocation density dependent deformation functional[4], the kinematics to define a recoverable strain tensor functional are also revised to contain dislocation density dependent functional terms. This revision results in sub-component changes in the thermodynamic internal energy during work hardening of metals that are partly due to recoverable strain energy components that are stored as dislocation(point defects are neglected) density evolves and partly due to irreversible work lost during dislocation creation and dislocation transport. Some internal energy changes due to dislocation density changes during work hardening are partially recoverable during annealing, but transport of dislocations is always an irreversible energy lost process. [1]. Honeycombe, R.W.K.[1984]; The Plastic Deformation of Metals, 2nd Edition(pbk), Edward Arnold, Ltd, London.[2]. Hill, R.[1950]; The Mathematical Theory of Plasticity, Clarendon Press, Oxford.[3]. Lee, E.H.[1969]; Elastic-Plastic Deformation at Finite Strains, J. Appl. Mech, V-36. [4]. Stout, R. B.[1981]; Modelling the deformations and thermodynamics for materials involving a dislocation kinetics, Crys Lat Defs, Vol. 9, pp 65-91.
Symposium Organizers
Yue Qi General Motors R&D and Planning
H. Eliot Fang Sandia National Laboratories
Nick Reynolds Accelrys
Zi-Kui Liu The Pennsylvania State University
W13: Computational Mechanics
Session Chairs
Friday AM, December 05, 2008
Constitution B (Sheraton)
9:30 AM - W13.1
Multiscale Dislocation Dynamics (DD) and Continuum Analysis of Strength in Heterogeneous Nanoscale Metallic (NMM) Multilayered Composites.
Firas Akasheh 1 , Hussein Zbib 2 , Sreekanth Akarapu 2 , David Bhar 2
1 mechanical engineering, Tuskegee University, Tuskegee, Alabama, United States, 2 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractTypically, DD analysis of plasticity in crystalline materials assumes homogenous material properties in the RVE and hence does not account for Koheler image forces due to elastic properties mismatch. Such forces become increasingly significant in the case of NMM composites, affecting the strength and dislocation interaction among themselves and with the interfaces. Within the frame work of DD-continuum multiscale modeling, we developed and implemented a methodology based on the concepts of eigenstrain and superposition to account for such effects. Using an existing analytical solution for the stress fields of an infinite straight dislocation near the interface between two semi-infinite crystals, the developed methodology is verified. Furthermore the Peach-Koheler force due to image forces near the interface are calculated and shown to result in the repulsion/attraction to the interface of a nearby dislocation if the dislocation is in the more compliant/stiff crystal, as expected. Finally, the channeling strength of layer-confined glide dislocations in Cu-Ni cube-on-cube system is shown to increase by approx. 10% when the heterogeneity effect is considered and that this effect increases as the individual layer thickness decreases. Such effect is quantified for other systems as well.
9:45 AM - W13.2
Low-angle Grain Boundary Migration and Mobility in the Presence of Extrinsic Dislocations.
Adele Lim 1 , Anthony Rollett 2 , Mikko Haataja 1 , David Srolovitz 3
1 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States, 2 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 3 Physics, Yeshiva University, New York, New York, United States
Show AbstractGrain boundary migration facilitates many important microstructural evolution processes such as recrystallization and grain growth. From a theoretical perspective, developing predictive models for the mobility of an arbitrary grain boundary (GB) is very challenging due to the collective nature of GB migration. In this work, we focus on the behavior of low-angle GBs in the presence of extrinsic dislocations that can interact with and pin the boundary. The goal of this work is to develop a mechanistic understanding of how low-angle GBs migrate under an applied stress in the presence of extrinsic dislocations.
The pinning/depinning behavior of a low-angle grain boundary by extrinsic dislocations was studied as follows. We began by constructing a symmetric tilt boundary from an array of parallel edge dislocations with Burgers vector b1 = [ 1 1 0] and line direction ξ = [1 -1 2] in the presence of parallel extrinsic dislocations with Burgers vectors b2 = ± [-1 1 0]. The boundary plane normal is parallel to [1 1 0] and the misorientation θ between the grains is inversely proportional to the spacing between intrinsic dislocations in the array D given by θ = b1/D as usual. A shear stress is then applied on the (1 -1 -1) plane along the [1 1 0] direction so that the boundary migrates when the intrinsic dislocations glide under an applied stress. Since the slip plane of the extrinsic dislocation is perpendicular to those of the intrinsic dislocations, extrinsic dislocations can only follow the migrating boundary by climbing.
The following dynamics was assigned to the dislocations. First, the Peach-Kohler force f on every dislocation was evaluated using isotropic linear elasticity. Second, we assumed that the dislocation motion is overdamped and is a linear function of the force on the dislocation, i.e., v = M.f, where M is a tensor to account for glide and climb motion of the dislocation. Finally, we numerically integrated the equation of motions for all dislocations present in the system.
Our results indicate that there is a threshold for the applied stress to depin the boundary from extrinsic dislocations. At zero climb mobility, the GB remains immobile until the threshold stress is reached, after which it migrates at a velocity dictated by the product of glide mobility and external stress. For a finite climb mobility, on the other hand, the GB migrates at a non-zero, climb mobility-dependent velocity below the threshold. Furthermore, the threshold stress is a linear function of the ratio of climb mobility to glide mobility. The threshold stress also increases linearly with misorientation θ if the linear density of extrinsic dislocations to intrinsic dislocations is held constant.
10:00 AM - **W13.3
A Theory for Stress-Driven Interfacial Voiding upon Cationic Selective Oxidation of Alloys.
Haitham El Kadiri 1
1 CAVS, MSU, Starkville, Mississippi, United States
Show AbstractImagine a cavity at an interface; separating an outwardly growing oxide and a substitutional solid solution of two metallic Elements A and B. Assume the metal interface oxidizes, but the cavity free surface does not. Interdiffusion inside the metal, and misfit dislocation activities at the oxidizing interface, both generate a stress-free strain rate field. The compositional and material constraints in the presence of a non-oxidizing cavity give rise to a multi-axial tensile stress field, while a viscoplastic strain field arises to relax stress. The tensile stress at the interface enforces a concave curvature near the cavity tip through the continuity condition of the chemical potential. Atoms interflow along the cavity surface under the combined action of curvature, stress and composition gradients. They enter the metal/oxide interface and flow under the action of local stress, curvature and composition fields. The cavity grows. The stress at the interface relaxes, and the interface recedes partially and non-uniformly. Interfacial voiding upon cationic-selective oxidation is a long standing topic in the world of thermal barrier coating systems. Governing equations are developed, within the alloy, for stress generation upon composition evolution and induced plastic strain. Governing equations at the interface and the cavity surface are next formulated to describe a moving boundary problem that accounts for the simultaneous local cavity extension and interface recession rates. These governing equations are boundary conditions for the bulk formulation.
10:30 AM - W13.4
Nucleation and Propagation of Twins in Hexagonal Materials.
Jian Wang 1 , Richard Hoagland 1 , John Hirth 1 , Carlos Tome 1
1 , LANL, Los Alamos, New Mexico, United States
Show AbstractTwinning plays an important role in accommodating plastic deformation in these materials. However, twinning activity, such as nucleation and propagation, was not understood well in hcp materials. To develop a fundamental grasp of twinning mechanisms will be very meaningful advance in understanding the deformation of hexagonals. We use atomistic calculation based on molecular dynamics (MD) and/or the density function theory (DFT). DFT simulations are used to calculate the formation energy of planar defects and kinetic barrier associated with forming such planar defects. MD simulations are carried out to reveal mechanisms of nucleation and propagation of deformation twins. It is shown that the twinning dislocation associated with the growth of (10-12) twin has a very small Burgers vector of 0.0488 nm in Mg, however the formation of (10-12) twin nuclei does not only rely on this twinning dislocation.
10:45 AM - W13.5
Computational Modeling of Magnesium Dislocation Core Structures.
Joseph Yasi 1 , Louis Hector 2 , Dallas Trinkle 3
1 Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 , General Motors Technical Center, Warren, Michigan, United States, 3 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractMagnesium alloys are a material of interest in the automotive industry due to their high strength to weight ratio. Predictive modeling of the strength and ductility of new alloys requires accurate electronic-structure based methods due to complex solute-dislocation interactions. Density functional theory (DFT) coupled with flexible boundary condition methods allow for the computationally efficient relaxation of isolated dislocation cores. An ultrasoft pseudopotential for Mg with PW91 GGA accurately reproduces lattice constants, elastic constants, phonon spectra and stacking fault energies from experiment and other DFT calculations. We present the first DFT calculation of isolated basal and prismatic screw and edge dislocation core structures in hcp Mg with Al and Zn solutes in the core. Nye tensor density maps quantify the core geometry and partial splitting. We calculate Peierls stresses for each geometry, local electronic density of states and solute binding energies for dislocation-solution interactions. Ab-initio calculations for basal and prismatic slip systems provide necessary information for large-scale dislocation dynamics and constitutive models for the design of new alloys with improved ductility and strength.
11:30 AM - **W13.6
Origin of Dynamic Strain Aging in Metals:Solute Aging of both Mobile and Forest Dislocations.
William Curtin 1 , Monica Soare 1
1 , Brown University, Providence, Rhode Island, United States
Show AbstractA full rate-dependent constitutive theory for dynamic strain aging is developed based on two key ideas. The first idea is that both solute strengthening and forest strengthening must exist and must exhibit aging phenomena. A full rate theory clearly shows that neither alone can yield negative strain-rate-sensitivity under normal assumptions. The second idea is that a single physical aging mechanism, cross-core diffusion within a dislocation core, controls the aging of both the solute and forest strengthening mechanisms. All of the material parameters in the model, aside from forest dislocation density evolution parameters, are derivable from atomistic-scale studies so that the theory contains essentially no adjustable parameters. In application to a variety of Al-Mg alloys, the model predicts the steady-state stress/strain/strain-rate/temperature/concentration dependent material response, including negative strain rate sensitivity, in qualitative and quantitative agreement with available experiments. With no additional assumptions, the model also reveals the origin of non-additivity of solute and forest strengthening and explains the observed transient stress behavior in strain-rate jump tests. This mechanistic theory thus captures essentially all aspects of the dynamic aging phenomenon.
12:00 PM - W13.7
Hydrogen Trapping and Diffusion in Palladium Dislocation Cores.
Dallas Trinkle 1
1 Materials Science and Engineering, Univ. of Illinois, Urbana-Champaign, Urbana, Illinois, United States
Show AbstractPd has a high H solubility for a metal, and a high diffusivity due to low binding energy in the bulk. However, experiments have shown that additional binding sites are available in single-crystal Pd with much higher binding energy, effectively storing residual H in the crystal after removal from high pressure H. The storage of H is believed to occur in dislocation cores, which acting as nanoscale H traps. Electronic-structure calculations of an isolated Pd dislocation core using flexible boundary conditions—to accurately couple to the long-range elasticity solution—determine the binding energy of H to a dislocation core, the changes in local geometry and electronic structure. Local vibrational modes of H give information about dynamics and compare with neutron scattering measurements; together with energy barrier calculations, H pipe diffusion is compared with bulk diffusivity. These calculations help elucidate the physical ingredients to design more energetically favorable hydrogen storage traps in materials.
12:15 PM - W13.8
Elastic Interactions between Solutes and Dislocations in Structural Materials.
Yuranan Hanlumyuang 1 , Peter Gordon 2 , Neeraj Thirumalai 2 , Daryl Chrzan 1
1 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 2 , ExxonMobil Research and Engineering, Annandale , New Jersey, United States
Show AbstractMany important properties of structural materials such as yield point, toughness, and the Portevin-Le Chatelier effect are influenced by interactions between solute atoms and dislocations. Here we have developed the means to calculate the interaction energies between a carbon solute atom and dislocations in BCC iron. The method employs only small calculation cells, obviating the need for the expensive supercell calculations. Our results yield excellent agreement with total energy calculation based Fe-C empirical potentials for a distance of two Burgers vector or greater from the core. The method is based on a simple continuum theory yielding an analytical expression that can be easily transferred to other alloying systems. Thus the approach has the potential to serve as a basis for multi-scale modeling of the interactions of solutes with dislocations in common structural materials. This research is supported by ExxonMobil Research and Engineering.
12:30 PM - W13.9
Dislocation Pinning/Depinning by Impurities and Obstacles: A Level Set Simulation Study.
Zi Chen 1 , Kevin Chu 2 , Mikko Haataja 1 , Jeffrey Rickman 3 , David Srolovitz 4
1 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States, 2 , Vitamin D, Inv, Menlo Park, California, United States, 3 Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania, United States, 4 Yeshiva College, Yeshiva University, New York, New York, United States
Show AbstractA basic understanding of the mechanical behavior of advanced metallic systems is essential for the design of robust structures to ensure their performance under a variety of loads and under extreme thermal environments. In particular, in systems that are solution and/or precipitation hardened, such as TiAl and Al-Mg-Cu alloys and reinforced MoSi2, interactions among defects lead to a complex plastic response that is difficult to capture by empirical constitutive relations. In this work, we employ a parallel three-dimensional level-set code to simulate the dynamics of isolated dislocation lines and loops in an obstacle-rich environment. Specifically, we have extracted the effective glide mobility of an initially straight edge dislocation as a function of solute concentration, spatial obstacle distribution, misfit strain, and external stress. The effect of dislocation climb mobility on critical resolved shear stress was also investigated. Interestingly, allowing for climb reduces the effective glide mobility in the presence of misfitting solutes relative to pure glide. The long term goal of these studies is the development of coarse-grained dislocation dynamics models, which provide a microstructurally-informed description of the continuum mechanics of plastically deformed metals.
12:45 PM - W13.10
First Principles Predictions of Mechanical Properties of FeMn-Alloys.
Alexey Dick 1 , Tilmann Hickel 1 , Jörg Neugebauer 1
1 Computational Materials Design, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf Germany
Show AbstractDue to exceptional mechanical characteristics high-Mn-steels are excellent candidates for the next generation of light-weight high-strength steels. The mechanical properties of such steels sensitively depend on the properties/energetics of crystal defects. The stacking faults are of particular importance since their energies define which plasticity mechanism (twinning induced or transformation induced plasticity) prevails in the steel [1]. An atomistic understanding of such structural defects is necessary to explore chemical trends, to deliver parameters for phenomenological models, and to identify new routes to optimize high-Mn-steels. We have, therefore, performed a first principles ab initio study of the stacking fault properties of FeMn-alloys, which is a host structure for realistic high-Mn-steels. The relevant atomic configurations have been indentified by the cluster-expansion method and the concept of special quasirandom structures and have been used as an input for the axial interaction model as well as explicit calculations of stacking fault energy surfaces. The results for stacking fault energies as a function of chemical composition, chemical and magnetic ordering are presented and compared to available experimental data.[1] G. Frommeyer, U. Brüx, and P. Neumann, ISIJ Int. 43, 438 (2003).