E1: Diffusion I
Session Chairs
Monday PM, November 26, 2007
Constitution B (Sheraton)
9:00 AM - **E1.1
Diffusion in Intermetallics Using On-the-fly Kinetic Monte Carlo.
Murray Daw 1 , Erdi Bleda 1 , Xing Gao 1
1 Dept of Physics, Clemson, Clemson, South Carolina, United States
Show AbstractWe have calculated the vacancy-assisted tracer diffusivities in these binary intermetallic alloys using on-the-fly kinetic monte carlo: Cu-Au, Au-Ag, and Cu-Ni. This set encompases behavior from weak segregation to moderately strong ordering. The energetics are based on the Embedded Atom Method, and the saddlepoints are found on-the-fly (that is, beginning from scratch with each valley.) Using this technique we have calculated the tracer diffusivities over a wide range of temperature and composition. We evaluate the strengths and weaknesses of the approach. Where possible we compare to experimental data.
9:30 AM - E1.2
Kinetic Monte Carlo Simulations of Defect Diffusion in hcp alpha-Zirconium.
Cristina Arevalo 1 3 , Jose Perlado 1 , Maria Caturla 2
1 , Fusion Nuclear Institute, madrid Spain, 3 , Escuela Técnica Superior de Ingenieros, Sevilla Spain, 2 , Universidad de Alicante, Alicante Spain
Show AbstractThe study of point defect clustering in hexagonal-close-packed (hcp) metals is dominated by a consideration of the geometry of the hcp lattice and lattice parameters ratio (c/a). Because of this crystallographic anisotropy, defect anisotropic diffusion is expected (jump distances and jump rates depend on jump directions). This study has focused on hcp alpha-Zirconium (c/a 1.594, lower than ideal 1.633 and similar to Titanium, c/a 1.587). We have created a new and original model for the understanding of the microscopic evolution (defect diffusion) in hcp metals, using a kinetic Monte Carlo (kMC) simulation technique. This technique allows us to understand the evolution of damage accumulation, due to either neutron or electron irradiation, for long times (hours-months). Several multiscale modelling simulations steps have been used in order to understand the microscopical fission reactor cladding behaviour. We have focused on zirconium alloys claddings (Zircaloy-4 and Zr-2.5%Nb). The first step we have simulated has been the neutron spectra. We have obtained spectra for current pressure water reactors (PWR) and high burn-up advanced reactors. We have also obtained neutron spectra in several burnt steps and we have represented the isotopic variation in the cladding (looking for Helium and Hydrogen apparition in the system). Taking these results as input data we have reproduced the Primary Knock-on Atom (PKA) spectra. From those data a systematic analysis of primary damage has been obtained using binary collisions code SRIM for high energy recoils, in order to get distribution of cascades and subcascades for these recoil energies. With these data we have studied the evolution of the microstructure during irradiation under environment conditions of 600K, dose rate 1e-6 dpa/s and final dose of 0.5 dpa. We have selected the molecular dynamics (MD) simulations cascades that fit best to PKA spectra and we have used them as input data on defect energetics and cascade damage. We have considered isotropic motion for vacancies and we have studied how the accumulation of damage is affected considering from one dimension to three dimension movement for interstitials.
9:45 AM - E1.3
Kinetic Monte-Carlo Simulation of the Effect of Strain Field on Hydrogen Diffusion in hcp Zr.
Kenichi Nakashima 1 , Naoki Soneda 1 , Akiyoshi Nomoto 1 , Misako Iwasawa 1
1 Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, Komae, Tokyo, Japan
Show AbstractHydride formation and associated embrittlement is very important for the integrity of the nuclear fuel cladding and the safety of the nuclear power plants. However, the mechanism of the hydride formation ahead of a crack tip of the cladding has not been well understood especially at the atomic level. In the fuel cladding, relatively strong strain field is produced by not only the hoop stress due to internal pressure but also the hydrides or the stress concentration at the crack tip. Such a strain field should affect the diffusion of the point defects produced by irradiation and solute atoms such as hydrogen, and thus, the macroscopic phenomena such as hydride formation might be affected by the results of cumulative effect of migration energy changes.In this paper, diffusion of a hydrogen atom in a strain field in hcp zirconium crystal is studied by means of kinetic Monte Carlo (kMC) computer simulation method. In order to consider the strain effect, we first calculate migration energies of a hydrogen atom under a wide range of given uniform strain field using molecular dynamics. The computation box of the kMC simulations is subdivided into small (1nm cube) cells, and the strain field of the box is modeled by assigning an appropriate constant strain field to each of the small cells. Then, effect of strain field to hydrogen atom diffusion is considered by choosing appropriate migration energy for the hydrogen jump depending on the strain field of the cell in which the hydrogen is located.In the present simulations, we place edge dislocations as a source of strong strain field in the computation box where hydrogen atoms are also uniformly distributed in the box as an initial condition. Then the crystal is heated up to a given temperature (600K) to let the hydrogen atoms diffuse in the strain field in the crystal. No interaction between the hydrogen atoms is considered in the present simulations. General tendency of the hydrogen diffusion is that hydrogen atoms agglomerate in the tension strain region, and the population of the hydrogen in the compression region decreases. Steady state is reached after 1ms. This result clearly demonstrates that the strain fields ahead of crack tips or hydrides provide locations where hydrogen atoms prefer to agglomerate.
10:00 AM - **E1.4
The Lattice Monte Carlo Method for Addressing Mass and Thermal Transport Problems in Materials.
Irina Belova 1 , Graeme Murch 1
1 School of Engineering, The University of Newcastle, Callaghan, New South Wales, Australia
Show Abstract10:30 AM - E1.5
In-diffusion and Out-diffusion of Oxygen in Ag-MgO Composites: Analysis with Finite Element and Monte Carlo Methods.
Irina Belova 1 , Andreas Oechsner 2 1 , Graeme Murch 1
1 School of Engineering, The University of Newcastle, Callaghan, New South Wales, Australia, 2 Department of Applied Mechanics, Technical University of Malaysia, Skudai, Johor , Malaysia
Show AbstractThe in-diffusion of oxygen during the formation of Ag-MgO composites results in oxygen segregation to the metal-ceramic interface and subsequent weakening of the interface bonding. Removal of oxygen by out-diffusion can be achieved at high temperatures. These diffusion processes are simulated using finite element and Lattice Monte Carlo methods.
E2: Diffusion II
Session Chairs
Monday PM, November 26, 2007
Constitution B (Sheraton)
11:15 AM - E2.1
Quantitative Modeling of Self-Interstitial Diffusion in Silicon.
N. Modine 1
1 1132, Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractPredictive modeling of the early-time transient annealing of radiation damage in electronic devices requires a detailed, quantitative understanding of the behavior of the fundamental defects in the device material. The isolated self-interstitial in silicon is extremely difficult to observe experimentally, and therefore accurate theoretical results should be very valuable. We apply electronic structure calculations based on the Kohn-Sham Density Functional Theory (DFT) in concert with Kinetic Monte-Carlo (KMC) techniques to study diffusion of the silicon self-interstitial as a function of the majority and minority carrier populations. We perform carefully converged DFT calculations to identify the structures that are locally stable (stable or metastable) for each charge state and the reaction pathways and transition states for transformation between these configurations. The resulting transition barriers are incorporated into a KMC model to determine the diffusion rate as a function of temperature and carrier concentrations. For non-equilibrium carrier concentrations, fluctuations in the charge state of the defect lead to non-Arrhenius behavior via a modified Bourgoin-Corbett mechanism. The rather complex behavior of the calculated diffusion coefficient will be explained in terms of fundamental physical mechanisms. The effects of transient trapping of the silicon self-interstitial in a complex with the oxygen impurity will also 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.
11:30 AM - E2.2
Diffusion of H and C through Fe Alloys.
Donald Johnson 1 , Emily Carter 2 3
1 Chemistry, Princeton University, Princeton, New Jersey, United States, 2 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States, 3 Program in Applied and Computaitonal Mathematics, Princeton University, Princeton, New Jersey, United States
Show AbstractMany industrial applications, such as automotive engines or high-pressure vessels for oil recovery, require strong materials to withstand high temperature or high pressure environments. Under these harsh conditions, corrosion of steel by various reactive agents is a great concern, even under reducing atmospheres. For example, attack by H2S leads to incorporation of hydrogen into steel and subsequent embrittlement. Similarly, at high temperature, attack by CO and subsequent incorporation of C into steel leads to metal dusting and carburization. In this work, we investigate whether formation of an Fe alloy thin film on steel can curb corrosion in reducing atmospheres by limiting C and H diffusion into and through steel. In particular, we employ density functional theory to calculate rate constants and energy barriers for the diffusion of H and C through bulk FeAl and bulk Fe3Si alloys. We also investigate the reaction pathways of H/C diffusing from the alloy surface to subsurface layers, in order to determine the overall kinetics of hydrogen and carbon incorporation into these Fe alloys.
11:45 AM - **E2.3
Determination of Multicomponent Interdiffusion Coefficients and its Applications.
Yongho Sohn 1
1 AMPAC-MMAE, University of Central Florida, Orlando, Florida, United States
Show AbstractSolid-state diffusion is a subject of great interest for its intellectual merit and practical applications in materials and coatings for advanced energy generation systems such as gas turbines and nuclear reactors. This talk will highlight the importance of multicomponent-multiphase interdiffusion with specific examples from materials and coatings used in gas turbine engines and metallic nuclear fuels. Results and analysis from both laboratory experiments and field applications are presented to emphasize the cross-fertilization of science and applications. Examples will include, for gas turbine applications, ternary interdiffusion in Ni3Al (L12) with Ir or Ta ternary alloying additions, ternary and quaternary interdiffusion in austenitic NiCr and FeNiCr alloys with Al, Si, Ge or Pd alloying additions for improved oxidation resistance, and interdiffusion between oxidation resistant coatings and superalloy substrates. Diffusion controlled degradation of metallic nuclear fuels due to interdiffusion with cladding and thermotransport will be presented with appropriate analysis for the determination of critical thermo-kinetic parameters that in turn are used for fuel performance prediction. This work was financially supported by CAREER award from National Science Foundation (NSF-CAREER DMR-0238356), U.S. Department of Energy (DE-FC26-02NT41431) subcontract through Clemson University (No. 01-01-SR103), and U.S. Department of Energy (DE-AC07-05ID14517) subcontract through Idaho National Laboratory (0051953) with technical assistance from Oak Ridge National Laboratory and National Institute of Materials Science (NIMS) of Japan.
12:15 PM - E2.4
Oxygen Diffusion in δ-Bi2O3: A Molecular Dynamics and Density Functional Theory Approach.
Dilpuneet Aidhy 1 , Susan Sinnott 1 , Juan Nino 1 , Eric Wachsman 1 , Simon Phillpot 1
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractAmong all the fluorite-based electrolytes proposed for use in solid oxide fuel cells, δ-Bi2O3 has the highest oxygen ion conductivity. To achieve this fluorite crystal structure at relatively lower temperatures, δ-Bi2O3 is doped with different lanthanides [1]. However, doping decreases the oxygen conductivity; a direct proportional dependence of oxygen conductivity on atomic polarizability and radius are observed to be the two contributing factors [1, 2]. Here, the effects of ionic polarizability and ionic radius on oxygen conductivity are separated using molecular dynamics (MD) simulations. Using MD simulations, artificial cations are generated to separate the two factors’ effects by keeping one constant and varying the other. In the pure δ-Bi2O3, we find that high bismuth polarizability is important to achieve high oxygen conductivity; the ionic radius is not important. We have further observed that neither the oxygen radius nor its polarizability plays a role in its high diffusion. In doped δ-Bi2O3, elastic stresses are developed which effect the oxygen diffusion. Using both MD and density functional theory, we also present a detailed structural and oxygen diffusion mechanism comparison between the pure and doped δ-Bi2O3. This work is supported by NASA under the grant NAG3–2930 and by DOE through the High Temperature Electrochemistry Center (HiTeC) at the University of Florida, Contract No. DE-AC05-76RL01830. 1.Jiang N., Wachsman E. D., J. Am. Cer. Soc., 82 (11) (1999), 3057.2.Wachsman E. D., Ionics, 7 (2001), 1.
12:30 PM - E2.5
Ab initio Study of Surface and Surface Oxygen Diffusion Properties of LaMnO3.
Yueh-Lin Lee 1 , Dane Morgan 1
1 Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show Abstract(La,Sr)MnO3 (LSM) is the cathode catalyst generally used in commercial solid oxide fuel cells (SOFCs). However, SOFCs are significantly limited by the oxygen reduction reaction (ORR) kinetics on this material, particularly at lower temperatures. To understand the relationship between surface structure and ORR in LSM, we adopt an atomistic modeling approach based on ab initio density functional theory (DFT) simulations. Initial work has focused on understanding the energetics of surfaces, near surface vacancies, and oxygen transport in LaMnO3 (LMO). The energies for LMO surfaces obtained by simply truncating the bulk orthorhombic phase structure are calculated to be 64 meV/Å2 (100), 76 meV/Å2 (110), and 75 meV/Å2 (111). Vacancy formation energies associated with the most stable (100) surface are found to deviate dramatically from their bulk value near the surface. This will lead to orders of magnitude differences in vacancy concentrations near the surfaces, potentially strongly impacting the ORR mechanisms. The electronic correlation effects in LMO were explored with GGA+U and the relative vacancy formation energies were found to be largely insensitive to the choice of U. The oxygen vacancy and binding energetics have been incorporated into a thermokinetic model to predict the oxygen coverage on the surface, near surface vacancy concentrations, and oxygen diffusivity on the surface and in near surface layers.
12:45 PM - E2.6
Cation Migration Processes in Magnesium Aluminate Spinel.
Samuel Murphy 1 , Jonathan Ball 1 , Grimes Robin 1 , Blas Uberuaga 2 , Kurt Sickafus 2 , Roger Smith 3
1 Materials, Imperial College London, London United Kingdom, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Department of Mathematical Science, Loughborough University, Loughborough United Kingdom
Show AbstractMagnsium aluminate spinel has demonstrated a significant resistance to degradation of its mechanical properties under neutron irradiation. As a consequence, spinel has been considered for numerous applications within the nuclear industry. Spinel’s high tolerance to some types of radiation is currently believed to be a result of a high interstitial – vacancy recombination rate coupled with its ability to tolerate a significant amount of disorder between its cation sublattices. Here we report on atomistic simulations, based on pair potentials, to investigate the behaviour of residual point defects within the spinel matrix. Isolated interstitial defects of all species in spinel were found to form complex split interstitial structures. For example, the oxygen interstitial rather than being a simple Oi defect, forms an Oi-VMg-Oi structure aligned along the 〈1-10〉direction. Both the Al3+ and Mg2+ interstitial ions split across the tetrahedral magnesium site, aligned along〈110〉.Migration is an important process to consider when investigating the interstitial – vacancy recombination rate as the interstitials and vacancies may have to migrate over significant distances within the lattice in order to recombine. The present studies suggest an interesting new concept; Al3+ prefers to migrate via a mechanism rather than via a vacancy mechanism on its own sublattice or via an interstitial mechanism. Aluminium interstitial migration is limited as the Al3+ interstitial ion collapses to form an AlMg antisite specie and a split Mgi-VMg-Mgi. The predicted activstion energies for Mg2+ to be transported via a vacancy mechanism, on its native sublattice, or via an interstitial mechanism are found to be essentially the same.Previous experimental and theoretical investigations of radiation damage in spinel have indicated that the number of antisite defects are increased after interaction with radiation. As mentioned previously spinel can tolerate a significant amount of disorder on the cation sublattices, however, the barriers to formation as well as annihilation of antisite defects via a vacancy mechanism are found to be very high. Effectively decoupling the two sublattices and ensuring these processes are not promoted as a result of thermal activation.
E3: Microstructural Evolution
Session Chairs
Monday PM, November 26, 2007
Constitution B (Sheraton)
2:30 PM - **E3.1
Nucleation and Growth of Thin Films and Minerals.
John Harding 1
1 Engineering Materials, University of Sheffield, Sheffield United Kingdom
Show AbstractThe modelling of nucleation and growth remains a basic problem in materials. It is common to a range of fields: from biomineralization and nanotechnology to corrosion and scale formation in boilers. Crystal growth is a multiscale problem in both length-scales and time-scales. It is therefore necessary to use a range of simulation methods from full-scale ab initio methods to coarse-grained or mean field techniques to capture the important physical phenomena. Frequently, the substrate can control the phase, shape and properties of the material that is grown on it. It is possible to design substrates that induce unusual phases and shapes because these are the most stable option at an early stage of the growth and thereafter become “frozen in” because the activation barrier to a thermodynamically stable state is prohibitively high. Unusual diffusion mechanisms, particularly cooperative mechanisms [1] are often present at surfaces and interfaces and can determine the evolution of the system. We will use a number of examples from the growth of biominerals [2], thin films (in particular wurtzite structures) [3] and growth of metals on oxides [4] to illustrate how a range of simulation methods can be used to understand these phenomena[1] M.Y. Lavrentiev, N.L. Allan, J.H. Harding, D.J. Harris and J.A. Purton, Comp. Mater. Sci. 36 (2006) 54-59[2] J.H. Harding and D.M. Duffy; J. Mater. Chem. 16 (2006) 1105-1112[3] C.L. Freeman, F. Claeyssens, N.L. Allan and J.H. Harding, Phys. Rev. Lett. 96 (2006) 066102.[4] J.A. Venables, L. Giordano and J.H. Harding; J. Phys. Cond.Mat.18 (2006) S411-S427
3:00 PM - E3.2
Heterogeneous Nucleation in Alloys using MCCask, a Parallel Monte Carlo Code in the Off-lattice Transmutation Ensemble.
Babak Sadigh 1 , Enrique Martinez 1 , Alfredo Caro 1 , Luis Zepeda-Ruiz 1 , Magdalena Caro 1 , Edmundo Lopasso 2
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 , Centro Atomico Bariloche, 8400 Bariloche Argentina
Show AbstractWe present a massively parallel off-lattice Monte Carlo code in the transmutation ensemble. Besides the usual constrains for the Semi-Grand Canonical ensemble a new one as been introduced to allow simulations within the miscibility gap in a segregating binary solution. The second order moment of the concentration, i. e. the dispersion around the average concentration, has been added so that the concentration can be fixed even within the miscibility gap. We derive the transition matrix for this algorithm which drives the system towards equilibrium while maintaining ergodicity and macroscopic eversibility. By implementing a subdivision of each processor's domain we solve the problem of the parallelizing algorithm in a way that minimizes communications. The code follows a route from an initial state towards a state closer to thermodynamic equilibrium, as defined by the minimum of free energy for the model considered and the state variable specifications. Heterogeneous precipitation in the FeCu system is used as example of its applications. This code opens the possibility to study dirty interfaces by placing the impurities at locations dictated by free energy criteria with no approximations.
3:15 PM - E3.3
Kinetics of Precipitation in Fe-Cu Alloys: a Multiscale Modelling.
Frederic Soisson 1 , Chu Chun Fu 1
1 , CEA Saclay, Gif-sur-Yvette France
Show AbstractThe kinetics of copper precipitation in α-iron, both during thermal ageing and under irradiation, strongly depend on the details of point defects diffusion properties. They are modelled here by Monte Carlo simulations based on a diffusion model by vacancy and interstitial jumps, which takes into the dependence of point defect concentrations and migration barriers on the local atomic environment. These parameters are fitted on ab initio data, calculated within the Density Functional Theory.For thermal ageing, the simulated precipitation kinetics are in good agreement with experimental ones. The Fe-Cu system is characterized by a low mutual solubility, which results in the formation of almost pure copper precipitates; and by a large difference between the vacancy formation in bcc iron and metastable bcc copper, which leads to a strong trapping of vacancies by the precipitates. As a result, precipitates containing up to several tens of copper atoms can be much more mobile than individual copper atoms. This result is analysed with a simple model of cluster diffusion, which suggests that a similar behaviour could be observed in other alloys.Under irradiation, the evolution of the system can be modified by the coupling between point defects and copper fluxes: we discuss the effects of these phenomena on the precipitation kinetics.
3:45 PM - E3.5
Molecular Theory of Ostwald Ripening.
Victor Burlakov 1
1 Materials, University of Oxford, Oxford United Kingdom
Show AbstractMicrostructure coarsening, or Ostwald ripening (OR), is a phenomenon often observed at the late stages of many first order phase transformations in two-phase mixtures, binary alloys, clusters on surfaces, oil-water emulsions, and during epitaxial growth and synthesis of nanoparticles. During OR small clusters of atoms/molecules dissolve transferring their mass to bigger clusters. In contrast to the classical theories of OR [1-2], which are based on Gibbs-Thomson effect, we analyze the mass exchange between spherical molecular clusters considering emitted and absorbed molecular fluxes and taking into account that some of the emitted molecules return to the cluster of origin. The return probability for molecules is found to be determined by the regime of molecular transport, and is higher for bigger clusters suggesting that the return process itself may account for a new, purely kinetic ripening mechanism in the absence of the Gibbs-Thomson effect. We show that in the limit characteristic to Wagner theory [2] and for molecular mean free path small compared to cluster radii the ripening process is entirely due to the kinetic mechanism. The developed molecular theory reproduces major features of the classical theories and identifies parameter regions of their applicability. It also identifies an intermediate regime [3] with respect to the Lifshitz-Slyozov [1] and Wagner [2] theories, when the time dependence of average cluster size and the cluster size distribution are entirely different from those predicted by the classical theories. Implications of the obtained results are discussed.References[1] M. Lifshitz, V. V. Slezov., J. Phys. Chem. Solids, 19, 35 (1961).[2] C. Z. Wagner, Electrochem. 65, 581 (1961).[3] V. M. Burlakov, Phys. Rev. Lett. 97, 155703 (2006).
E4: Evolution of Microstructure and Properties
Session Chairs
Monday PM, November 26, 2007
Constitution B (Sheraton)
4:30 PM - E4.1
Quantitative Phase Field Model for Diffusion-controlled Microstructural Evolutions in the Solid State.
Arnoldo Badillo 1 , Robert Averback 1 , Pascal Bellon 1
1 Materials Science and Engineering, UIUC, Urbana, Illinois, United States
Show AbstractCurrent phase field models for diffusion-controlled evolutions in the solid state are based on phenomenological kinetic equations, such the Cahn-Hilliard diffusion equation for conserved order parameters, and as a result, simulations relying on these equations lack absolute time and space scales. We propose here a new derivation of such equations, starting from an atomistic description and using a coarse graining in space and time to define the continuous variables for the phase field model. For a binary alloy where atoms interact through pairwise interactions, this approach makes it possible to derive the coarse-grained free energy of an inhomogeneous system, as well as the kinetic equation describing the evolution of its composition field under isothermal condition. It is shown that physical parameters of the coarse grained model, including the gradient energy coefficient and the atomic mobility, display dependence with the size of the cells used for coarse graining. These dependences, however, produce the desirable result that, within a given mean-field approximation, measurable quantities such as concentration profiles at equilibrium and kinetics of phase separation are independent of the coarse graining cell size. This is at variance with past results where coarse graining was applied in the space but not in the time domain. Simulations based on this new coarse graining possess physical length and time scales, which are directly inherited from the underlying microscopic model.Coarse-grained static and kinetic equations are here derived in the point and pair mean-field approximation, and their extension to higher order approximations is outlined. Examples of the application of the model are given for spinodal decomposition in three dimensions with either a direct atom-exchange mechanism or with a vacancy assisted mechanism for atomic diffusion. We also demonstrate that this model can be applied to the evolution of defect clusters in irradiated solids by including a source term for the production of point defects and point-defect clusters, a recombination term for vacancy-interstitial annihilation, and a sink term for the elimination of point defects on clusters or on extended defects such as dislocation. The presence of an absolute space scale in our simulations makes it possible to study recoil spectrum effects, that is, to compare irradiation conditions with the same average rate of production of defects but with different size distribution of defects clusters.
4:45 PM - E4.2
Combined Experimental and Computational Investigation of Microstructural Plasticity.
Luke Brewer 1 , Corbett Battaile 3 , W. Counts 2 , Thomas Buchheit 4
1 Materials Characterization, Sandia National Laboratories, Albuquerque, New Mexico, United States, 3 Computational Materials Science and Engineering, Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Microstructure Physics and Metal Forming, Max-Planck-Institut für Eisenforschung GmbH , Dusseldorf Germany, 4 Microsystem Materials, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractA great deal is known about the evolution of plastic deformation fields at the continuum level; however, there is a lack of combined theoretical and experimental investigation of plasticity at the microstructural length scale. This study examines the evolution of plastic deformation in a given microstructural field of view as a function of macroscale strain. Well-annealed tensile bars of high purity Ni were strained in steps of 0, 1%, 5%, and 10% strain. At each strain increment, the distribution of plastic deformation was mapped using electron backscatter diffraction (EBSD) orientation and misorientation maps. The misorientation maps from this data were compared to misorientation maps calculated using both a local and a novel non-local crystal plasticity-based finite element method (FEM). The degree of agreement between experiment and simulation differed upon the length scale of the comparison. The distribution of intragranular crystal rotation was qualitatively better captured by the local FEM model, while the non-local model had closer agreement with experiment in the total amount of intragranular crystal rotation. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United Stated Department of Energy (DOE) under contract DE-AC0494AL85000.
5:00 PM - E4.3
Modeling the Effect of Particle Size Distribution on Platinum Surface Area Loss in PEMFC Cathodes.
Edward Holby 1 , Dane Morgan 1 , Wenchao Sheng 2 , Yang Shao-Horn 2
1 Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe long-term durability of platinum catalysts in proton exchange membrane fuel cell cathodes is an important issue in achieving the Department of Energy’s 5000 hour PEMFC lifetime goal. Fuel cell efficiency is decreased as electrochemically active surface area (ECASA) of platinum is lost. It is important to better understand the mechanisms of platinum surface area loss in the cathode and if they can be mitigated in order to meet lifetime requirements. We have used a kinetic model to explore the influence of Particle Size Distribution (PSD) on the loss of ECASA under both constant potential holds and cycled potentials for carbon supported Pt catalysts (Pt/C). This model extends recent work from Darling and Meyers1, and includes the effects of platinum dissolution into the ionomer, platinum reprecipitation both on and off the carbon support and surface oxidation and passivation. All of these strongly couple to particle size, due to the Gibbs-Thomson effect. It is found that the mean diameter and functional form of the initial PSD are important considerations in the long-term loss of cathodic platinum ECASA in PEMFCs. Also, we confirm that changes in the PSD upon heat treatment help to explain the improved durability associated with heat-treating Pt/C catalysts.1. Darling, R.M. and J.P. Meyers, Kinetic Model of Platinum Dissolution in PEMFCs. Journal of the Electrochemical Society, 2003 150(11): p. A1523-A1527.
5:15 PM - E4.4
Relaxation of Biaxial Strain in Ultra-Thin Films of Face-centered Cubic Metals through Ductile Void Growth, Grain Nucleation, and Structural Phase Transitions.
Kedarnath Kolluri 1 , M. Rauf Gungor 1 , Dimitrios Maroudas 1
1 Chemical Engineering, University of massachusetts, Amherst, Massachusetts, United States
Show AbstractNanometer-scale-thick metallic films are used increasingly in modern technologies, from microelectronics to various areas of nanofabrication. Processing of such materials generates voids and structural defects and may induce structural transformations. The structural evolution of these materials during the relaxation of electro- and thermo-mechanical stresses underlies many mechanical reliability problems. Improvement of the reliability, functionality, and performance of nano-scale devices requires a fundamental understanding of the atomistic mechanisms of strain relaxation in thin films and the associated structural changes in order to establish links between structural evolution and mechanical behavior.In this presentation, we report a computational analysis of atomistic mechanisms of relaxation of biaxially applied tensile strain over a broad range of strain levels in free standing ultra-thin copper films with the film surface oriented normal to the [110] crystallographic direction; the films studied contained voids extending throughout the thickness of the film with the void axes oriented along [110]. Our study is based on isothermal-isostrain large-scale molecular-dynamics simulations, using an embedded-atom-method parameterization to describe the interatomic interactions. Our analysis reveals four regimes of structural response as the applied biaxial strain level, ε, increases. In the first regime (ε < 2%), the films respond elastically. In the second regime (2% < ε < 8%), strain relaxation is governed by ductile void growth that is mediated by dislocation emission from the surface of the void and formation of a plastic zone around the void. Dislocation glide and dislocation-dislocation interactions lead to formation of multi-layered microtwin boundaries and Lomer-Cottrell locks in the thin-film material. In addition, dislocation arrays are nucleated at the surface of the void and lead eventually to lattice rotations and nanometer-sized grain formation in the thin film. In the third regime (8% ≤ ε < 12%), dislocations and dislocation arrays are nucleated at the surface of the thin film in addition to those that are nucleated at the void surface. The fourth regime (ε ≥ 12%) is characterized by the nucleation of a high density of dislocation arrays, leading to a phase transformation of the thin film from a face-centered cubic (fcc) lattice structure to a hexagonal close-packed (hcp) lattice structure. Throughout the strain range examined, we characterize the surface roughness of the strain-relaxed films and calculate their elastic properties. Furthermore, we examine the validity of static lattice stability criteria in capturing the phase transformation onsets in the metallic thin films and construct the underlying free-energy landscapes in order to determine accurately these transformation onsets.
5:30 PM - E4.5
Simulation of the Columnar-to-Equiaxed Transition in Alloy Solidification -- the Effect of Alloy Solidification Range, Nucleation Undercooling and Density of Nuclei on the Transition.
Huijuan Dai 1 , Hongbiao Dong 1 2 , Helen Atkinson 1 , Peter Lee 3
1 Department of Engineering, University of Leicester, Leicester United Kingdom, 2 Precision Casting Facility, Rolls-Royce plc., Derby United Kingdom, 3 Department of Materials, Imperial College London, London United Kingdom
Show Abstract5:45 PM - E4.6
Abnormal Grain Growth of Fe-3%Si Steel Approached by Solid-State Wetting Mechanism.
Kyung-Jun Ko 1 , Pil-Ryung Cha 2 , Jong-Tae Park 3 , Jae-Kwan Kim 3 , Nong-Moon Hwang 1
1 Material Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Advanced Materials Engineering, Kook-Min University, Seoul Korea (the Republic of), 3 POSCO Technical Research Laboratories, POSCO, Pohang Korea (the Republic of)
Show Abstract Abnormal grain growth (AGG) which is also called secondary recrystallization takes place in many metallic systems especially after recrystallization of deformed polycrystals. A famous example is the evolution of the strong Goss texture after secondary recrystallization in Fe-3%Si steel. Extensive efforts have been made to identify the growth advantage of Goss grains due to its scientific and technological interests. However, the Goss secondary recrystallization or AGG still remains as a puzzling phenomenon since its first report by Goss in 1935.In this study, we suggest a new approach to the growth advantage of AGG by solid-state wetting, where a grain wets or penetrates the grain boundary or the triple junction of neighboring grains just as the liquid phase wets along the grain boundary or the triple junction. If the energy sum of the two grain boundaries is lower than the energy of the other grain boundary in contact at the triple junction, the high energy grain boundary will be replaced by the two low energy grain boundaries through the wetting process. Once the solid-state wetting occurs, the triple junction, which tends to be a rate-determining step in grain growth, migrates much faster than the grain boundaries in contact and therefore, the triple junction constraint in grain growth disappears. According to the solid-state wetting mechanism, the sub-grain boundaries which have very low energy can play a critical role in inducing AGG, because they provide grains with the exclusive growth advantage by increasing the wetting probabilityAGG was approached based on the solid-state wetting mechanism by both experiments and the 3-dimensional grain growth simulation using Phase-Field model. The effect of solid-state wetting on AGG by phase field model (PFM) simulation is compared the microstructural feature of AGG such as the formation of numerous island and peninsular grains observed experimentally in Fe-3%Si alloy. The misorientation angles between island or peninsular grains and the abnormally-growing Goss grains were mostly low angles less than 15o with some high angle boundary of coincidence lattice relationship, which indicates that they have the low grain boundary energy. Very high frequency of island and peninsular grains formed at or near the growth front of abnormally growing grains could be best explained by solid-state wetting. 3-sided or 4-sided grains with negative curvatures, which are formed near the growth front of abnormally growing grains, are the two-dimensional section vertical to the triple junction wetting in three-dimensional polycrystalline structure. These 3-sided or 4-sided grains were observed near the abnormally-growing Goss grains and were shown to be connected to the abnormally-growing grain by sequential polishing and observation.
E5/T5: Joint Session: Modeling Defects in Nuclear Materials I
Session Chairs
Tuesday AM, November 27, 2007
Constitution B (Sheraton)
9:30 AM - **E5.1/T5.1
Defects and Impurities in Nuclear Materials from First Principles.
Chu Chun Fu 1 , F. Willaime 1
1 DEN/DMN, SRMP-CEA, Sacaly, Gif sur Yvette France
Show AbstractFerritic steels play an central role in metallurgical and nuclear technology, in particularas structural materials for fission and future fusion nuclear reactors.Although their mechanical properties have been extensively investigated, little is knownabout the structural, electronic and magnetic properties at the origin of their macroscopicbehavior. First principles calculations within the Density Functional Theory (DFT) provide suchinformation at atomic scale, which is not directly accessible through experiments.However, the application of first principles studies in this field is rather new compared withother Solid State Physics and Material Science disciplines.The objective of this talk is to report key contributions of recent DFT studies which allowto reconcile theory and experiments:The determination of the 3D migration of self-interstitial atoms (SIAs) -- elementary defectscreated by irradiation -- induced an overall revision of the widely accepted picture of damageaccumulation under irradiation predicted by empirical potentials [1]. The coupled ab initio andmesoscopic kinetic Monte Carlo simulation provided strong evidence to clarify long-standingcontroversial interpretations of electrical resistivity recovery experiments concerning themobility of vacancies, SIAs, and their clusters [2]. The behavior of Carbon, one of theessential alloying element in steels, and of impurities such as Phosphorus which are responsibleof reactor pressure vessel (RPV) steels embrittlement will also be discussed in detail [3].[1] C. C. Fu, F. Willaime and P. Ordejon, Phys. Rev. Lett. 92, 175503 (2004)[2] C. C. Fu et al. Nature Mater. 4, 68 (2005)[3] E. Meslin et al. Phys. Rev. B 75, 094303 (2007)
10:00 AM - **E5.2/T5.2
Computer Simulation of Defect Properties in Irradiated Metals.
David Bacon 1 , Alexander Barashev 1 , Andrew Calder 1 , Yuri Osetsky 2
1 Engineering, University of Liverpool, Liverpool United Kingdom, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show Abstract10:30 AM - E5.3/T5.3
Ab initio-based Radiation-induced Segregation Modeling in Fe-Ni-Cr Alloys.
Julie Tucker 1 , Todd Allen 1 , Dane Morgan 2
1 Nuclear Engineering & Engineering Physics, University of Wisconsin - Madison, Madison, Wisconsin, United States, 2 Materials Science and Engineering, Univerisity of Wisconsin - Madison, Madison, Wisconsin, United States
Show AbstractHigh concentrations of point defects, such as those created in radiation environments, can cause severe material degradation as they migrate and cluster. Radiation induced segregation (RIS), the process by which the local composition of an alloy is altered near point defect sinks, is a phenomenon that has concerned the nuclear industry for decades. While substantial progress has been made in the area of RIS prediction by empirical fitting, many questions remain about the diffusion mechanisms of point defects and how they are affected by local environment changes in multi-component alloys. This research uses ab initio methods to determine diffusion coefficients associated with both vacancy and interstitial migration in Ni rich fcc Fe-Ni-Cr alloys. We find that the alloy kinetics differs significantly from that predicted by simple extrapolation of empirical fits to high-temperature data. The calculated diffusion coefficients are used to parameterize a rate theory type RIS model.
10:45 AM - E5.4/T5.4
Microstructural Evolution Under High Flux Irradiation of Dilute Fe-CuNiMnSi Alloys Studied by Atomic Kinetic Monte Carlo Model – Effect of the Self Interstitials.
Christophe Domain 1 2 , Edwige Vincent 1 2 , Raoul Ngayam Happy 2 1 , Charlotte Becquart 2
1 MMC, EDF R&D, Moret sur Loing France, 2 Laboratoire de Métallurgie Physique et Génie des Matériaux, UMR 8517, Villeneuve d'Ascq France
Show AbstractE6/T6: Joint Session: Modeling Defects in Nuclear Materials II
Session Chairs
Tuesday PM, November 27, 2007
Constitution B (Sheraton)
11:30 AM - **E6.1/T6.1
Multi-Scale Simulations of Ion-Solid Interactions in SiC and GaN.
Fei Gao 1 , William Weber 1 , Haiyan Xiao 2 3 , Zhouwen Rong 1 , Yanwen Zhang 1 , Ram Devanathan 1 , Lumin Wang 2 , Xiaotao Zu 3
1 MS K8-93, Pacific Northwest National Laboratory, Richland, Washington, United States, 2 Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan, United States, 3 Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, Sichuan, China
Show AbstractRecent progress in multi-scale simulations of fundamental ion-solid interactions in SiC and GaN is discussed. Large-scale ab initio simulation methods (up to a few thousand atoms) have been developed for the study of ion-solid interactions in materials, and these methods are employed to investigate the properties of defects and defect clusters. Atomic structures, formation energies and binding energies of these small clusters are determined, and their relative stabilities are described. Furthermore, ab initio molecular dynamics methods have been used to calculate the threshold displacement energy surface and to simulate the primary damage states for the PKA (primary knock-on atom) energies up to 1 keV in SiC and GaN. These simulations provide significant insights into electronic effects on ion-solid interaction processes. Molecular dynamics (MD) have been used to simulate the high-energy ion-solid interactions with energies up to 50 keV, while kinetic Monte Carlo methods have been employed to investigate the recovery of defects during annealing, with input parameters determined by ab initito calculations and empirical potential MD simulations. A large number of 10 keV displacement cascades are randomly generated in a model crystal to simulate multiple ion-solid interactions and damage accumulation, as well as the mechanisms controlling the crystalline-to-amorphous transition. The amorphous-to-crystalline (a-c) transition has also been studied using MD methods, with simulation times of up to a few hundred ns, in 4H-SiC at temperatures between 1000 and 2000 K. Based on a model developed in the previous annealing simulations of 3C-SiC, the activation energy spectra for recrystallization along the three directions have been determined. In general, the activation spectra show that there are a number of activation energy peaks associated with different recrystallization processes. These activation energy values for full recrystallization are in the range of from 1.2 to 1.7 eV for the amorphous layers with the a-c interfaces along [-12-10] and [-1010] directions, and 1.1 to 2.3 eV for the amorphous layer with the a-c interfaces along [0001] direction.
12:00 PM - E6.2/T6.2
Defect Structure and Stability in Uranium and Zirconium Nitrides.
Robin Grimes 1 , Eugene Kotomin 2
1 materials, Imperial College London, London United Kingdom, 2 Joint Research Centre, Institute for Transuranic Elements, Karlsruhe Germany
Show AbstractActinide nitrides exhibit higher thermal conductivity and metal density than their corresponding oxides. Consequently they may be preferred as fuels in certain circumstances. Unfortunately, the basic defect behaviour of nitrides is not nearly as well understood as that of oxides. Since fuel performance depends on defect mediated processes this presents a problem in our being able to establish a predictive capability for nitride systems. Here we will consider the basic defect properties of uranium and zirconium nitrides as predicted by atomic scale computer simulations. In all cases, quantum mechanical simulation was employed, based on the density functional codes CASTEP and VASP. The atomic and electronic structures of basic vacancy and interstitial defects were studied and these were used to understand basic defect processes associated with disorder and nonstoichiometry. Predictions were successfully compared with experimental data were possible.
12:15 PM - E6.3/T6.3
Insights into Radiation Tolerance of Ceramics from Large Scale Molecular Dynamics Simulations.
Ram Devanathan 1 , William Weber 1
1 Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractRadiation tolerant ceramics are needed to advance the utilization of nuclear energy to meet rising global energy demand. They have potential to meet the demands of radiation environments in applications as nuclear fuel and waste forms. Their discovery and development are hampered by a lack of fundamental understanding of the physics underlying radiation tolerance of ceramics. Several theories have been advanced in the literature based on structure, radius ratio of cations and ionicity or covalency of bonds. Most of these theories focus on defect production and accumulation processes only, but neglect in-cascade and thermal annihilation of defects. We have performed large scale molecular dynamics simulations of 30 keV U and Zr recoils in zircon (ZrSiO4), zirconia (ZrO2) and yttria-stabilized zirconia (YSZ) to understand the atomic-level mechanisms that contribute to radiation tolerance, particularly fast annihilation processes. Zircon is amorphized by irradiation, while zirconia and YSZ do not undergo radiation-induced amorphization. Our results reveal that dynamic defect annihilation is very effective at controlling defect accumulation in radiation tolerant materials. Based on our results, we will discuss a strategy for improving the radiation tolerance of ceramics.
12:30 PM - **E6.4/T6.4
Molecular Dynamics Simulation of Irradiation Induced Phase Transition in MgAl2O4.
Alain Chartier 1 , Tomokazu Yamamoto 2 1 , Kazuhiro Yasuda 2 , Constantin Meis 3 , Kenichi Shiiyama 2 , Syo Matsumura 2
1 , CEA-Saclay, Gif-Sur-Yvette France, 2 , Kyushu University, Fukuoka Japan, 3 , INSTN, Gif-Sur-Yvette France
Show AbstractSpinel MgAl2O4 is known to be highly radiation resistant: it may loose its crystalline structure (i.e. become amorphous) at cryogenic temperatures and at very high doses. The eventual amorphization is preceded by a phase transition towards a crystalline state, whose structure is analyzed whether to be of rock-salt type or a disordered spinel, depending on authors. Finally, the amorphous state may be re-crystallized when submitted to electron irradiations, both at the amorphous / crystalline interface by epitaxial growth and in the amorphous region by nucleation and growth.In the present study, the objective is to reveal this kinetics of phase transitions under radiation of MgAl2O4 using molecular dynamics (MD) simulation of Frenkel pair (FP) accumulation, starting whether from the normal (N) spinel or from the amorphous (A) spinel. Such a FP accumulation is designed to mimic the point defects accumulations resulting from displacements cascades in N spinel, or will be considered for mimicking the electron irradiation of the A spinel. Starting from N-spinel, the structure transits directly towards the rock-salt (NaCl) spinel for temperature lower than 600K. This transition is preceded by an intermediate disordered (D) spinel for higher temperature than 600K Amorphization has not been observed with increasing the dose up to 68 dpc (at 30K). The critical dose for NaCl-spinel transition increased as a function of temperature. It relies on spontaneous recombination mechanisms as the simulation time is too short for thermal diffusion to occur.Starting from the A-spinel, the FP accumulation induced a re-crystallization towards the NaCl spinel, with few defective atoms. The re-crystallization is not induced by any local heating since the target temperature for each simulation is strictly maintained and since the amorphous spinel is stable in temperature. The atomic displacements induced by electron irradiation can thus be considered as the driving mechanism that kinetically induces the re-crystallization. The super-cell used being small, such a re-crystallization can be consid-ered as homogeneous. As expected, the doses needed for the armorphous spinel to transit towards the rock-salt structure have been observed to decrease as a function of temperature. At 30K, a dose of around 6 dpc is needed for the re-crystallization to occur, while less than 1 dpc is needed at 1800 K.Finally, starting whether from N-spinel or A-spinel, FP accumulation drove the spinel towards the NaCl spinel structure (the steady state under irradiations), rather than the D-spinel. The transitions were driven by Frenkel-pairs recombination, as already observed in pyrochlore. We also demonstrated that non diffusive point defects (FPs, mimicking the electron irradiation) may induce temperature dependant homogeneous crystallization of A-spinel.
E7/T7: Joint Session: Modeling Microstructural Evolution in Irradiated Materials I
Session Chairs
Tuesday PM, November 27, 2007
Constitution B (Sheraton)
2:30 PM - **E7.1/T7.1
Atomistic Simulations and Continuum Modeling of Microstructural Evolutions Driven by Irradiation.
Pascal Bellon 1 , Pavel Krasnochtchekov 1 , Arnoldo Badillo 1 , Charles Enloe 1 , Robert Averback 1
1 Materials Science and Engineering, university of Illinois, Urbana, Illinois, United States
Show AbstractIrradiation drives materials into nonequilibrium states, resulting into forced phase transformations, microstructural evolutions, and dimensional changes. In advanced nuclear reactors, such as GEN IV fission reactors and fusion rectors, materials will be subjected to untested irradiation environments, in particular large doses and high irradiation temperatures. Due to time constraints, it is not possible to perform experiments in the actual service conditions these materials would be subjected to. Simulations and modeling will thus be employed to overcome this limitation, either by allowing a safe extrapolation of results obtained in accelerated tests, or even by brute force multi-scale modeling. As a first example, we will show how atomistic simulations (MD, KMC) and quantitative continuum modeling can be used to study the stability of pre-existing precipitates, or the formation of nonequilibrium precipitates during the irradiation of metallic alloys. We will show how the characteristic length scale of the atomic replacements forced by nuclear collision play a role on precipitate stability, and on the possible self-organization of these precipitates in nanoscale patterns. We will also show that, when interstitials dominate the transport of solute atoms, the resulting morphology of the precipitates is quite different from the case where solute transport is dominated by vacancies. In a second example, we will show how quantitative phase field modeling can be employed to simulate the evolution of point defect clusters under irradiation. For making safe predictions as discussed above, it is imperative that the primary state of damage be taken into account. This information, however, is absent from most existing models since it relates to the amplitude of the fluctuations of the external forcing. We have introduced a mixed discrete-continuum approach to overcome this problem, and we will show how the primary state of damage affects the size and number density of defect clusters. The results will be compared to existing simulation results obtained by MD+KMC and by cluster dynamics.
3:00 PM - E7.2/T7.2
Multi-Physics Simulation of Grain Restructuring in Fast Reactor Nuclear Fuels.
Veena Tikare 1 , Timothy Bartel 1 , Mark Lusk 2 , Steven Wright 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractFast reactor oxide nuclear fuels are cylindrical pellets that are approximately 1 inch in height by 1/4 inch in diameter. Their initial microstructure is homogeneous with equi-axed grains of 10 to 20 μm and uniformly distributed porosity of 10 to 15 vol%. During service in fast reactors, these fuel pellets experience high service temperatures with very large radial temperature gradients, which drive tremendous restructuring of the microstructure by grain growth and pore migration. Furthermore, volumetric strain, called swelling, by creep occurs to accommodate the addition atoms generated by fission. The restructuring, fission product release and migration modeling is required to accurately predict the performance behavior of fuel-cladding mechanical and chemical interactions that control the integrity of the fuel pin and its ability to operate as designed without releasing fission products. We will extend a current Material Point Method, MPM, model to incorporate kinetic Monte Carlo, kMC, algorithms to simulate fuel restructuring due to curvature driven grain growth and pore migration. We will also simulate swelling by creep. This multi-physics model will be presented, it application to simulating fuel restructuring will be demonstrated and finally the limitations of the model will be discussed.This work was done 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.
3:15 PM - E7.3/T7.3
Computer Simulation of Helium Gas Bubbles in Uranium Dioxide.
David Parfitt 1 , Robin Grimes 1 , Kaajal Desai 1
1 Department of Materials, Imperial College London, London United Kingdom
Show AbstractHelium is formed in nuclear fuel as a product of the α-decay of actinides during long-term storage and normal operation. The high heat of solution of this helium drives the precipitation of the atoms into gas bubbles; these act as reservoirs, absorbing helium from the lattice, and through thermal and radiation-enhanced resolution, returning it back to the lattice. Understanding the effect of these bubbles upon the thermal and mechanical properties of uranium dioxide is important for safe and economical storage and operation. Here we present molecular dynamics simulations of helium gas bubbles and their interaction with displacement cascades within the uranium dioxide lattice. A simple two-body effective potential has been used to successfully model the dynamics of several different bubbles sizes and morphologies with a range of internal gas pressures. The interaction of these bubbles with recoil cascades, representative of radiation damage due to alpha-decay and nuclear fission, has been examined with initial recoil energies of up to 30 keV. Our results show that, as expected, the thermal resolution of helium is almost non-existent at typical fuel storage and operating temperatures. In contrast to previous models however, the dominant mechanism of radiation-enhanced resolution is not the ballistic recoil of helium atoms, rather it results from the rapid and concerted incorporation of the helium into the disordered regions adjacent to the bubble. Once absorbed into the disordered region, this helium acts to prolong its lifetime far beyond that normally observed in such radiation damage simulations. We propose that these new mechanisms will significantly impact upon models of the gas resolution rate and the mobility of bubbles in uranium dioxide. In particular, the combination of re-absorption of helium into the disordered regions of the lattice and the transfer of uranium and oxygen atoms across the bubble will lead to the net migration of these bubbles along any anisotropy in the radiation field.
3:30 PM - E7.4/T7.4
Reactor Transient Material Instability Models for High Temperature and High Burn-up Nuclear Fuel.
Ray Stout 1
1 , Rho Beta Sigma Affaires, Livermore, California, United States
Show AbstractThe designs of next generation nuclear reactors that operate efficiently and perform safely at higher localized fuel temperatures and fuel burn-ups will require an increased mechanistic understanding of materials’ responses to develop and evaluate ceramic nuclear fuel performance. At higher temperatures, any given non-elastic/plastic deformation rate will occur at a lower stress state and the transport rate of fission gas atoms to bubbles will also be increased. The higher burn-ups, measured as fissions per unit volume of ceramic nuclear fuel, will locally increase the number density of fission gas atoms that will be transported to existing porosity and/or additionally created fuel bubbles. Based on these facts, pressurized bubble density in nuclear fuels and porosity swelling strains of future reactor designs, as measured by the number of bubbles per size attributes(radius and radius rate) and per fission gas(atom content), will evolve significantly different from the pressurized bubbles densities observed in the existing light-water reactors. The quasi-steady evolution of bubble density and bubble density dependent fuel deformation has been formulated for nuclear fuel response models[1] in terms of: (1). A bubble density field equation to describe the time evolution of the discrete bubble species of different size(radius) bubbles and different gas content bubbles; (2). finite deformation and finite strain dependence on fission gas bubble density; and(3). stress/bubble-pressure equilibrium dependence on fission gas bubble density. This system of equations is “quasi-rate unstable” because the material stiffness response decreases as the bubble density evolves. For reactor quasi-steady operation, this is a reactor life-time constraint. However, for any operational excursion from an existing fuel state (fuel state is characterized by fuel temperature and fuel burn-up), the safety of reactor performance is a decreasingly limited functional of fuel temperature and/or fuel burn-up excursion increments.[1]. Stout, R.B.[2006]: Stochastic Deformations and Bubble Density Evolution in Nuclear Materials, rbsA-Rpt0016, Jun06.
3:45 PM - E7.5/T7.5
Phase Field Modeling of the Effect of Irradiation Damage on Thermal Conductivity at the Microstructural Scale.
Paul Millett 1 , Michael Pernice 2 , Tapan Desai 1 , Dieter Wolf 1
1 Material Sciences Dept., Idaho National Laboratory, Idaho Falls, Idaho, United States, 2 Center for Advanced Modeling and Simulation, Idaho National Laboratory, Idaho Falls, Idaho, United States
Show AbstractE8/T8: Joint Session: Modeling Microstructural Evolution in Irradiated Materials II
Session Chairs
Tuesday PM, November 27, 2007
Constitution B (Sheraton)
4:15 PM - **E8.1/T8.1
Effect of Strain Field on the Microstructural Evolution in Irradiated Fe: Kinetic Monte Carlo Study.
Naoki Soneda 1 , Kenichi Nakashima 1 , Akiyoshi Nomoto 1 , Akiyuki Takahashi 2 , Toshiharu Ohnuma 1 , Kenji Nishida 1 , Kenji Dohi 1
1 Materials Science Research Laboratory, CRIEPI, Komae, Tokyo, Japan, 2 Department of Mechanical Engineering, Tokyo University of Science, Noda, Chiba, Japan
Show AbstractMicrostructural features in irradiated metals, such as dislocations, dislocation loops, voids and solute clusters / precipitates, cause strain fields in the crystal, which affect the diffusion of point defects and solute atoms. Understanding the effect of such strain field is very important to develop models for point defect cluster formation, dislocation decoration with SIA loops, solute segregation to grain boundaries and heterogeneous solute clustering. Kinetic (Lattice) Monte Carlo simulation is a powerful technique to describe the microstructural evolution such as point defect and solute clustering under irradiation. The conventional K(L)MC, however, does not consider the effect of strain field explicitly. Rather it employs the concept of capture radius for the interaction between particles, within which the interaction occurs spontaneously. Therefore it is likely that this conventional K(L)MC approach may not be appropriate to perform the simulations of especially the interaction of defects with dislocations and the effect of high doses where a lot of well-developed irradiation features exist.In this paper, we will present our recent results to consider the effect of strain field in the KMC simulations. In our approach, computation box is subdivided into small cells, and each cell is assigned an appropriate constant strain. Migration and formation energies of point defects and clusters are calculated as a function of strain, and the jump probability of a particle is calculated using the migration and formation energies corresponding to the strain value of the cell of the particle. Results of this new KMC simulation will be presented for the microstructural evolution in irradiated bcc Fe.
4:45 PM - **E8.2/T8.2
On the Role of Helical Turns in the Formation of Clear Bands in Irradiated Metals.
Thomas Nogaret 1 , Marc Fivel 1 , David Rodney 1
1 SIMAP, INP Grenoble, Saint Martin d Heres France
Show Abstract5:15 PM - E8.3/T8.3
Structure and Defect Stability of Calcium Phosphate Minerals.
Emily Michie 1 , Robin Grimes 1
1 Materials, Imperial College London, London United Kingdom
Show AbstractThe choice of material for radioactive waste immobilization depends both upon the chemistry of the waste type and the physical environment in which the waste form is to be stored. Natural mineral phases are attractive due to their proven long time scale stability. Consequently, apatite is of interest because (i) it is the most abundant phosphatic mineral, found in almost all igneous, some sedimentary and metamorphic rocks and (ii) it exhibits a large chemical variability.
Indeed, due to the difficulty of incorporating halides in conventional materials, apatites are being considered as potential hosts for radioactive waste streams containing both actinides and halides. Apatite can demonstrate considerable non-stoichiometry, providing significant compositional flexibility, ideal for incorporating a range of waste species. It is therefore imperative to better understand the stability of apatites and the relative solubility of different ions.
Atomistic scale computer simulation is used to examine the apatite structure and the defect mechanisms associated with incorporating both alkali halide and various cation species. Initially, the structures for various fluorapatites were established by comparing quantum based calculations (CASTEP) and experimental values.
Of particular interest was the substitution of Sr2+ onto the two non-equivalent Ca2+ cation sites in fluoroapatite. Previous experimental studies are unclear as to which site Sr2+ substitutions are most likely to occur. Here it is found that there is no energetic difference between substituting Sr2+ onto either site. Zn2+, Mg2+, and Ba2+ substitutions were also investigated, however these showed a significant difference between the cation sites.
5:30 PM - E8.4/T8.4
Dynamics of Irradiation-induced Nano-fiber Formation in Germanium.
Kun-Dar Li 1 , Wei Lu 3 , Lumin Wang 1 2
1 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Nuclear Engineering & Radiological Science, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractIt was initially thought that amorphous materials would be ideal for use in a radiation environment to avoid the additional disorder. However, irradiation induced catastrophic swelling has been observed in several amorphous materials, e.g. amorphous germanium, by experiments with ion beam irradiation in the past two decades [1-2]. A common ion irradiation effect in these materials is the development of a high density of porosity separated by nanofibers of ~10-20 nm in diameter. The mechanisms for the formation of this porous structure in the amorphous phase are still unclear. In this study, we develop a theoretical model that considers the kinetics of defect build-up during irradiation, including the recombination effect of defect and the dynamic diffusion process. A phase field approach is adopted in this model. Pores are treated as high vacancy concentration regions. The local change in defect concentration can be written as the net result of local production rate, reaction rates and divergence of diffusion flow. Dynamic processes, such as pores coalescence, are captured by updating the concentration profile over time. We incorporate the free energy of mixing and interfacial energy into the driving force for vacancy diffusion (based on Cahn-Hilliard equation) while the production and annihilation of vacancy that are due to the ion irradiation and the reactions with interstitials and sinks, respectively (based on Sizmann’s equation). The application of a diffuse interface allows pores to emerge or dissolve naturally, and the system can form whatever lattice it favors. A series of simulations are performed for dynamic porous formation and evolution by this model. It has been observed that initially spherical voids are formed and their size increases over time. Continuous observation of a set of voids in the simulation reveals void coalescence events during the early stage of pore growth. Eventually they become enlarged leading to a sponge-like structure. With increasing the irradiation dose, the structure continues to evolve into a network of fiber-like structure with nearly uniform diameters of fibers. These calculated evolutions of the porous structure are consistent well with all the experimental observations in different studies [2-4]. Our model provides a distinct numerical approach to investigate the mechanism of microstructural/morphological instability in irradiated germanium. The approach may also be applied to other irradiated materials such as GaSb and InSb.[1] B. R. Appleton, O. W. Holland, J. Narayan, O. E. Schow III, J. S. Williams, K. T. Short and E. M. Lawson, Appl. Phys. Lett. 41 (1982) 711.[2] L. M. Wang and R. C. Birtcher, Appl. Phys. Lett. 55 (24) (1989) 2494.[3] S. M. Kluth, J. D. Fitz Gerald and M. C. Ridgway, Appl. Phys. Lett. 86 (2005) 131920.[4] S. M. Kluth, David Llewellyn and M. C. Ridgway, Nuclear Instruments and Methods in Physics Research B 242 (2006) 640.
5:45 PM - E8.5/T8.5
Physics Mechanisms Involved in the Formation and Recrystallization of Amorphous Regions in Si through ion Irradiation.
Ivan Santos 1 , Luis Marques 1 , Lourdes Pelaz 1 , Pedro Lopez 1 , Maria Aboy 1
1 Departamento de Electronica, University of Valladolid, Valladolid, Valladolid, Spain
Show AbstractIon implantation and annealing are process traditionally used for the fabrication of Si devices. The introduction of energetic ions during the implantation step generates a large number of defects in the Si lattice. These defects may also diffuse, interact among them and with dopants, annihilate at interfaces, etc. resulting in a final dopant and defect configuration that determines the device performance. It is necessary have a good understanding of all these mechanism to optimize the device. Multi-scale modeling is required to capture the detailed physics involved in the mechanisms and to access to scales directly comparable with experiments. We focus this work on multi-scale modeling of the physics involved in ion beam induced amorphization and recrystallization in Si, but this scheme can be applied to other materials. We use molecular dynamics (MD) to study the formation mechanisms of amorphous regions. We have observed that along with energetic ballistic collisions that generate Frenkel pairs (energy transfers above the displacement energy in Si), low energy interactions can cause amorphous damage through the melting and quenching of local regions where energy is deposited. We will show that the competition between melting and heat diffusion define the conditions for the formation of local amorphous regions at the low energy regime (around and below the displacement energy). This mechanism of damage formation is very relevant for heavy and molecular ions.By quantifying the results obtained with MD, we have developed a model that complements the damage generation in the binary collision approximation (BCA). This model reproduces the characteristics of damage of MD cascades: amount and morphology of generated defects. Complex damage structures appear during the cascade generation in agreement with MD. However, this model has a much lower computational cost than MD simulations, which allows us to simulate thousand of cascades. We have successfully applied our model to B18 implantation.We have used MD results related to the recrystallization behavior of local amorphous regions to define the energetic of defects in a computationally efficient Kinetic Monte Carlo (KMC) code. The combination of all these tools, MD (fundamental studies of damage formation and recrystallization), improved BCA (including ballistic and melting-related damage) and KMC (for efficient defect kinetics modeling during the implantation and the subsequent annealing) allow us to model the effect of ion mass, beam current, implant temperature on the amount and morphology of residual defects in Si.
E9: Electronic and Atomistic Methods
Session Chairs
Wednesday AM, November 28, 2007
Constitution B (Sheraton)
9:30 AM - E9.1
A Density Functional Theory Assessment of the Synergy between He and H in Tungsten.
Charlotte Becquart 1 , Christophe Domain 2 1
1 UMR 8517, Laboratoire de Métallurgie Physique et Génie des Matériaux, Villeneuve d'Ascq Cédex France, 2 MMC, EDF, Moret sur Loing France
Show AbstractIn the near surface of plasma facing materials, high concentrations of hydrogen, hydrogen isotopes and helium can build up, which both will interact with the point defects resulting from the bombardment of the surface as well as with the impurities of the materials. He and H most stable site in interstitial configuration is the same: the tetrahedral site but their diffusion properties or their tendency to form clusters are completely different. Because both elements will be present at the same time in the material, it is important to assess the influence of one light element on the properties of the other, as they can be drastically changed. We have used density functional theory based ab initio calculations and the VASP code to determine the interactions of He and H with vacancy complexes and vacancy clusters as well as self interstitials in W. The role of impurities such as carbon will also be presented. These results will be discussed in the light of experimental data.
9:45 AM - E9.2
The Effect of Sr and Co Substitutions on the Performance of LaFeO3 for Solid Oxide Fuel Cell Cathodes: Predictions of Density Functional Theory Calculations.
Chanwoo Lee 1 , Eric Wachsman 1 , Simon Phillpot 1 , Ram Devanathan 2 , Susan Sinnott 1
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractFirst-principles electronic structure calculations are performed to understand the complex conduction mechanisms that are known to occur in La1-xSrxCoyFe1-yO3 (LSCF), which is being considered for use as a solid oxide fuel cell (SOFC) cathode material. In particular, density-functional theory within the generalized gradient approximation is used to model the LSCF system. The influence of substituting Sr for La and Co for Fe on the LaFeO3 (010) surface energy is determined. In addition, the effect of these substitutions on the adsorption and absorption behavior of oxygen, an important process for the operation of SOFCs, is investigated. Specifically, the adsorption and absorption energies at different sites on the doped and pristine (010) surface are determined with and without oxygen vacancies, and the oxygen migration energies are calculated using the nudged-elastic band method. Lastly, the effects of Sr and Co dopants on oxygen conduction in bulk LSCF are quantitatively investigated and compared to what happens at the surface. This work is supported by DOE through the High Temperature Electrochemistry Center (HiTEC) at the University of Florida, Contract No. DE-AC05-76RL01830.
10:00 AM - E9.3
Valence-Dependent Bond-Order Potentials for Modelling Ni-based Superalloys.
Thomas Hammerschmidt 1 , Ralf Drautz 1 , David Pettifor 1
1 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractThe creep properties of Ni-based superalloys degrade with time due to precipitation of topologically close-packed (tcp) phases that contain refractory elements. We have compiled a structure map of the occurrence of tcp phases for binary transition-metal compounds from experimental databases. The structure map highlights the well-known role of the average d-band filling for the stability of tcp phases. Atomistic modelling of tcp stability requires to go beyond the second-moment approximation to the electronic density of states by including up to at least the sixth moment [1]. We have developed an analytic bond-order potential (BOP) that systematically takes into account higher moment contributions to the density of states and depends explicitly on the valence of the transition metal elements [2]. We will briefly summarize the derivation of the bond-order potential from the tight-binding electronic structure and show that it contains the Finnis-Sinclair potential as the lowest-order approximation. By including up to sixth-moment terms the analytic BOP is able to reproduce the structural trend from hcp to bcc to hcp to fcc stability across the non-magnetic 4d and 5d transition metal series.For the parameterization of the new BOP, we performed extensive density functional theory (DFT) calculations of the elemental and binary compound phases of Ni, the technologically important alloying element Cr, and the refractory metals Mo, Re, and W. In addition to more than 40 ordered and disordered fcc and bcc based structures, we investigated the tcp phases A15, C14, C15, C36, μ, σ, and χ in all possible occupations of the inequivalent sites for each binary compound Ni-Cr, Re-W, Mo-Re, and Mo-W. We will discuss the structural trends of the DFT calculations and compare to the predictions of the analytic BOP within the canonical d-band model. Furthermore, we will discuss our progress on the parameterization of the analytic BOP for the Re-W system.[1] P.E.A. Turchi, Mat. Res. Soc. Symp. Proc. 206, 265 (1991).[2] R. Drautz and D.G. Pettifor, Phys. Rev. B 74, 174117 (2006).
10:15 AM - E9.4
Valence-dependent Analytic Bond-order Potential for Magnetic Transition Metals.
Ralf Drautz 1 , David Pettifor 1
1 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractWe recently derived an analytic interatomic bond-order potential (BOP) that depends explicitly on the valence of the transition metal element [1]. This analytic potential predicts the structural trend from hcp to bcc to hcp to fcc that is observed across the non-magnetic 4d and 5dtransition metal series. In this talk we discuss how the analytic BOP may be extended to include magnetic contributions to the binding energy. We show that the magnetic potential describes the different ferromagnetic moments of the alpha (bcc), gamma (fcc) and epsilon (hcp) phase of the 3d transition metal iron, the difference between the ferromagnetic and anti-ferromagnetic states as well as non-collinear spin configurations.[1] R. Drautz and D.G. Pettifor, Phys. Rev. B 74, 174117 (2006).
10:30 AM - **E9.5
Understanding the Electronic Properties of Si(111)-7x7.
Xin Xu 1
1 Chemistry, State Key Lab for Phys Chem of Solid Surf, Xiamen, Fujian, China
Show AbstractIt is well established that the reconstructed Si(111)-7×7 adopts the so-called dimer-adatom-stacking-fault (DAS) model, which contains 9 subsurface dimers, 12 adatoms (Sia), 6 rest atoms (Sir), 1 corner hole (Sih) and a stacking fault in a half unit cell. DAS model reduces the number of dangling bonds within the surface unit cell from 49 to 19. Currently, there exist two different views for the understanding of the electronic property of the surface. On one hand, it is generally believed that 19 dangling bond electrons are redistributed such that dangling bonds on the 6 rest atoms and 1 corner hole are doubly occupied, consuming 14 electrons, while the 12 adatoms only possess 5 electrons. We may prescribe such a point of view an ionic picture, as the formal charge on a rest atom is -1 and that on an adatom is +7/12. On the other hand, one may take a radical picture that each surface atom (Sia, Sir and Sih) processes one electron of 19 dangling bonds. Hence an adjacent adatom-rest atom pair may be viewed as a di-radical pair. In this work, we use cluster models, combined with the unrestricted spin density functional theory in the flavor of B3LYP to explore the reaction pathways for CH3OH, NH3 and NO dissociation over an adatom-rest atom pair site on Si(111)-7x7. We find that an adatom-rest atom pair site should be best regarded as a di-radical and that adsorbate lone-pair electrons can stimulate efficiently the charge transfer from Si adatoms to Si rest atoms and vice versa. From the ionic point of view that Sia is positively charged and Sir is negatively charged, it is nature to assume that the adsorbate molecule approaches the Sia site, instead of the Sir site, with its lone-pair electrons, as the Sir initiating mechanism is counter-intuitive. This is, however, not in agreement with the recent experimental results from the N 1s scanned-energy mode photoelectron diffraction study, which demonstrated that the NH2 species (almost) exclusively adopt the rest atom sites for NH3 dissociation on Si(111)-7x7. Here we show that the radical point of view can provide a reasonable understanding of the experimental observation that rest atoms are more active than adatoms towards the dissociation of a lone-pair electron donor adsorbate.Based on our thoeretical modeling, we predict that in a Si(111)-7x7 unit cell, no more than three adatoms can be involved in a (dissociative) adsorption of a lone-pair-electron-donor adsorbate, because the adsorbate lone-pair stimulated charge transfer mechanism is limited by the available three rest atoms. We predict charge transfer is ineffective between Sia…Sia, thus dissociation reaction cannot occur across half unit cells. On the other hand, we predict that radical-electron-donor adsorbate such as NO is able to adsorb on all 12 Si adatoms in a Si(111)-7x7 unit cell.
E10: Atomistic Methods I
Session Chairs
Wednesday PM, November 28, 2007
Constitution B (Sheraton)
11:30 AM - **E10.1
Extending the Size Scale in Accelerated Molecular Dynamics Methods.
Arthur Voter 1
1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractMany important processes in materials science take place on time scales that vastly exceed the nanoseconds accessible to molecular dynamics simulation. Examples relevant to this symposium are the diffusion and annihilation events that occur in the aftermath of a radiation damage cascade or, more generally, the microstructural evolution of a material under the influence of repeated cascades. Over the last ten years, we have been developing a new class of methods, accelerated molecular dynamics, designed to simulate the long-time dynamical evolution of infrequent-event systems like these. These methods (hyperdynamics, parallel replica dynamics, and temperature accelerated dynamics) exploit the known characteristics of infrequent-event systems to make the successive activated events take place more quickly, in a dynamically correct way. For certain processes, this approach has been remarkably successful, offering an atomistic view of complex dynamical evolution on time scales of microseconds, milliseconds, and sometimes even much longer. Examples include metallic surface diffusion and growth, radiation damage annealing in ceramics, and carbon nanotube dynamics. A characteristic of all of these methods, however, is that the computational work scales superlinearly with the number of atoms. This has restricted their use to systems smaller than a few thousand atoms, while it is often desirable to treat much larger systems. After a brief introduction to these methods, I will present our recent progress on extending the methods so that they can treat much larger systems with a minimum number of new approximations. We are achieving this both through direct modification to change the intrinsic scaling and through spatial parallelization.
12:00 PM - E10.2
Specifying Markov Chains for Efficient Kinetic Monte Carlo Acceleration.
Brian Puchala 1 , Michael Falk 1 , Krishna Garikipati 2
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe Kinetic Monte Carlo (KMC) simulation method is efficient because the time step adjusts as necessary to resolve the quickest event. KMC simulations of discrete state systems can become trapped when the energy landscape consists of relatively deep basins containing states separated by low energy barriers. The system can be accelerated out of a trapping basin by calculating escape probabilities and times, either approximately by assuming the system equilibriates in the basin or exactly using Markov chain analysis. Even in simple systems the huge number of possible atomic configurations can prevent identifying trapping basins before they are encountered. Here we present effective methods for on-the-fly identification of states to include in the Markov chain, allowing us to simulate vacancy-mediated arsenic diffusion in silicon on much longer time scales than would otherwise be possible. The first method adds states to the Markov chain as the system reaches them. The second method includes in the Markov chain those states considered most likely to be visited. The first method prevents unnecessarily calculating the rates for a state the system never visits, while the second reduces redudant Markov chain calculations. The preferred method depends on the energy landscape and the relative time needed for calculating rates or performing the Markov chain analysis. If the Markov chain defines a sufficiently deep equilibriating basin, that approximation can be used without loss of accuracy. Observed acceleration is many orders of magnitude, depending on the energy landscape and temperature.
12:15 PM - E10.3
An Event-Driven Hybrid MD/DSMC Algorithm: Hydrodynamics of Polymer Chains in Shear Flow.
Aleksandar Donev 1 , Alejandro Garcia 2 , Berni Alder 3
1 Lawrence Postdoctoral Fellow, Lawrence Livermore National Labs, Livermore, California, United States, 2 Dept. Physics & Astronomy, San Jose State Univ., San Jose, California, United States, 3 , Lawrence Livermore National Labs, Livermore, California, United States
Show AbstractWe present a novel asynchronous event-driven (AED) variant of the Direct Simulation Monte Carlo (DSMC) algorithm and combine it with event-driven hard-particle molecular dynamics (MD). Unlike the classical time-driven DSMC, the AED algorithm does not suffer from discretization errors due to finite step sizes, and is also more efficient at low collision rates. More importantly, the algorithm is easily combined with classical hard-sphere molecular dynamics to yield a powerful hybrid algorithm. We apply this algorithm to the challenging problem of simulating the long-time dynamics of a polymer chain in complex micro and nano flows. The polymer solute is represented via a hard-sphere-tether model similar to commonly-used bead-spring FENE models. The solvent is represented with atomistic detail as a hard-sphere fluid, and the collisions between solute-solute and solvent-solute particles are processed exactly using AED MD. The novel feature of our approach is that the interactions between solvent particles are not treated exactly using MD; instead, collisions between solvent particles are treated using DSMC. Furthermore, only the solvent close to a polymer chain is treated with the AED DSMC algorithm, while the remainder of the solvent particles are treated using the classical time-driven DSMC algorithm. This greatly improves the speed of simulation at realistically high solvent densities. We have implemented a variety of boundary conditions including open boundaries appropriate for future hybrid MD/DSMC/Continuum. As an illustrative example we study the fluctuation motion of a polymer chain tethered to a hard wall and subject to a simple shear flow, and we compare to results obtained using purely MD simulations. The new algorithm is found to yield a 10-25 speedup over hard-sphere MD while still providing a satisfactory representation of the fluctuating and coupled nano-flow.
12:30 PM - E10.4
Precipitation, Segregation, and the Origin of Corrosion Resistance of FeCr Alloys.
Magdalena Caro 1 , Paul Erhart 1 , Alfredo Caro 1
1 , LLNL, Livermore, California, United States
Show AbstractE11: Atomistic Methods II
Session Chairs
Wednesday PM, November 28, 2007
Constitution B (Sheraton)
2:30 PM - **E11.1
Short-range Order and Precipitation in Fe-rich Fe–Cr Alloys.
Paul Erhart 1 , Alfredo Caro 1 , Magdalena Serrano de Caro 1 , Babak Sadigh 1
1 Chemistry, Materials and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractWednesday, Nov 28Transferred E3.4 to *E11.1 @ 1:30 PMShort-range Order and Precipitation in Fe-rich Fe–Cr Alloys. Paul Erhard
3:00 PM - **E11.2
Atomic-scale Modelling of Materials: A Prerequisite to Any Multi-scale Approach to Structural and Dynamical Properties.
Carlo Massobrio 1
1 , IPCMS, Strasbourg France
Show Abstract3:30 PM - E11.3
Molecular Dynamics Simulations of the Intercalation of Li Ions into a Graphite Anode Material.
Ibrahim Abou Hamad 1 , Mark Novotny 1 2
1 HPC<sup>2</sup>, Center for Computational Sciences, Mississippi State University, Starkville, Mississippi, United States, 2 Department of Physics & Astronomy, Mississippi State University, Starkville, Mississippi, United States
Show AbstractWe present large-scale molecular dynamics simulations of the anode half-cell in a lithium-ion battery. The system is composed of a stack of graphite sheets representing the anode, ethylene carbonate and propylene carbonate molecules as the electrolyte, and lithium and hexafluorophosphate ions. The simulations are done in the NVT ensemble and at room temperature.We explore different charging schemes. In particular, we start with the lithium and hexafluorophosphate counter-ions in different intial positions and velocities, and calculate the time for the lithium ions to intercalate into the spaces between the graphite sheets. These intercalation simulations have been run to many hundreds of nanoseconds in time. Charging schemes investigated include normal charging in which intercalation is enhanced by electric charges on the graphitic sheets, as well as simulations of the same system undergoing charging with additional external forces. The simulations were performed on 2.6GHz Opteron processors, using 160 processors at a time. The experimental implementation of our forced charging schemes is very feasible. Our simulation results show an improvement in the intercalation time of the lithium ions, and hence presumably in the charging time for lithium-ion batteries, when our proposed charging schemes with external forces are used.
3:45 PM - E11.4
Coarse-Grained Molecular Dynamics Simulations of Nano-Particle Internalization into Bilayer Membranes.
Sulin Zhang 1 , Ju Li 2 , George Lykotrafitis 3 , Subra Suresh 3
1 Department of Engineering Science and Mechanics, Pennsylvania State University, University Park, Pennsylvania, United States, 2 Department of Materials Science and Engineering, Ohio State University, Columbus, Ohio, United States, 3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe present a coarse-grained model for the efficient simulation of generic bilayer membranes. While this coarse-grained model is well suited to study many such biophysical processes as membrane self-assembly, fusion, and phase separation, with significantly improved computational affordability as compared to other existing membrane models, our present focus will be put on internalization of colloidal particles into vesicles. Specifically, the degree of wrapping and the geometrical transition of the vesicle are investigated as a function of particle size and specific/non-specific particle-membrane adhesion energies. For multiple particles adsorbed onto the vesicle, we show wrapping and internalization are governed by curvature-mediated interactions. The model can conveniently be extended to simulate receptor-mediated particle internalization, for which we show that the competition for receptors among particles governs their uptake rate, exhibiting a source-limited process. The simulations provide an improved understanding of the energetics and kinetics of endo/exocytosis.
E12: Atomistic Methods III
Session Chairs
Wednesday PM, November 28, 2007
Constitution B (Sheraton)
4:30 PM - E12.1
A Molecular Dynamics Study of Model Energetic Crystals under Shock Loading.
Yunfeng Shi 1 , Donald Brenner 1
1 Department of Materials Science and Engineering, North Carolina State Univ., Raleigh, North Carolina, United States
Show AbstractMaterials in extreme conditions, such as under shock loading, can exhibit many distinct physical processes, such as plastic deformation, phase transformation, chemical reactions or the combination of the above. Particularly, the understanding of the interaction between mechanical perturbations and defects in materials, such as heterogeneous interfaces, voids and shear bands, is crucial in elucidating the underlying mechanisms, such as hotspot formation in detonation, at the molecular level. Such complex phenomena are difficult to probe experimentally and many new insights have been gained in atomic scale simulations. In this work, by using an efficient reactive force field, we investigated threshold detonation behaviors at the presence of heterogeneous interfaces and rectangular voids by large-scale molecular dynamics simulations. We show that both types of defects enhance shock chemistry and reduce the detonation threshold. For heterogeneous interfaces, shock focusing and preferential compression control the intensity and location of hotspot formation. For voids, jetting due to compression in the direction normal to the shock causes hotspot formation during void collapsing. In both cases, the defect geometry determines the amount of reduction of the detonation threshold.
4:45 PM - E12.2
A Molecular Dynamics Study of the Deposition of SrTiO3 Thin Films.
Jennifer Wohlwend 1 , Rakesh Behera 1 , Cosima Boswell 1 , Simon Phillpot 1 , Susan Sinnott 1
1 Materials Science and Engineering , University of Florida, Gainesville, Florida, United States
Show AbstractThin film deposition of SrTiO3 is important for producing electronic devices. Pulsed laser deposition (PLD) is an effective deposition process yielding dense, homogeneous thin films. Here, classical molecular dynamics simulations are used to determine the mechanisms involved in PLD. In particular, these simulations consider the deposition of SrO and TiO2 molecules and stoichiometric SrTiO3 clusters with a kinetic energy between 0.1 and 1 eV/atom on a (001) surface of SrTiO3. The simulations show how collisions between incident particles and the substrate induce chemical reactions. They also indicate how the composition and size of the depositing particles influence film growth. The effects of impact energy, orientation of the incident particles, cluster size and surface termination layer (SrO vs. TiO2) are also examined. The simulation predictions are compared to existing experimental data, and new insights into the growth of SrTiO3 thin films are discussed. This work is supported by the National Science Foundation (DMR-0426870).
5:00 PM - E12.3
Oxygen in Al Grain Boundaries: A Molecular Dynamics Study.
Andreas Elsener 1 , Olivier Politano 2 , Peter Derlet 1 , Helena Van Swygenhoven 1
1 , Paul Scherrer Institute, Villigen Switzerland, 2 , 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. Molecular dynamics simulations are performed to investigate the presence of oxygen in grain boundaries on the internal stress distribution and the resulting plasticity mechanism. A modified variable charge method based on the Streitz and Mintmire (PRB 50, 11996 (1994)) approach that incorporates a local chemical potential is used to simulate oxidation in a predominantly metallic Al environment. Simulations are performed using a new interaction potential for Al/Al, Al/O and O/O respectively based on Mishin and Farkas EAM-Al-potential (MRS Symp. Proc. Vol 538, 535 (1999)). In particular, the spatial extent of the charge transfer in the oxidation region, the resulting changes in local stress distribution and the influence of an applied stress on grain boundary mobility and grain boundary sliding will be presented.
5:15 PM - E12.4
Molecular Dynamics Simulation of Phase Transformations and Diffusion in Pure Zr.
Mikhail Mendelev 1 , Graeme Ackland 2
1 MEP, Ames Laboratory, Ames, Iowa, United States, 2 School of Physics, University of Edinburgh, Edinburgh, Scotland, United Kingdom
Show AbstractIn recent years there have been some 30 published studies of molecular dynamics (MD) in Zr primarily of its twinning deformation and response to radiation damage: an application for which its low thermal neutron absorption makes it uniquely suited. Surprisingly, current interatomic potentials used do not capture the unusual structural properties of Zr: high stacking fault energy, anomolous diffusion, melting and phase transformation under temperature and pressure (or alloying). Ab initio calculations have shown deficiencies in the description of point defects, both vacancies and interstitials, by existing interatomic potentials, which can now be rectified by refitting. In this presentation, we show how to include calculation of phase transitions self - consistently in fitting and present a potential for Zr which correctly reproduces energetics of our own extended database of ab initio configurations and high temperature phase transitions. The potential has an analytic many - body form, making it suitable for existing large - scale MD codes. We also present a best - fit potential for hcp and its defects. As a first test, we have applied the potential to study self - diffusion in hcp and bcc, finding that the anomalously high diffusivity is associated with self - interstitial migration in hcp, and intrinsic defect formation in bcc.
5:30 PM - E12.5
Al2O3/Ti(C,N) – Interface and Growth Study from ab initio Density Functional Calculations.
Jochen Rohrer 1 , Carlo Ruberto 2 , Per Hyldgaard 1
1 BioNano Systems Laboratory, Dep. of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg Sweden, 2 Materials and Surface Theory Group, Dep. of Applied Physics, Chalmers University of Technology, Göteborg Sweden
Show Abstract
E13: Evolution of Nanostructures
Session Chairs
Thursday AM, November 29, 2007
Constitution B (Sheraton)
9:15 AM - E13.1
Formation of Y-Ti-O Nanoclusters in Nanostructured Ferritic Alloys by Kinetic Monte Carlo Simulations.
Celine Hin 1 , Brian Wirth 1 , Hyon-Jee Lee 1
1 Nuclear engineering, UC Berkeley, Berkeley, California, United States
Show AbstractThe nanoscale oxide dispersion strengthened (ODS) ferritic alloys currently being developed are attractive for fusion reactor application because of their improved creep resistance at high temperature and potential to trap transmutant helium and thus prevent the formation of embrittling grain boundary helium bubbles. Many questions currently exist to understand the formation kinetics of nanoscale Y-Ti-O clusters, and their thermal and irradiation stability. Kinetic Monte Carlo simulations of Y-Ti-O nanoclusters precipitation are being developed to provide atomic-scale knowledge of the formation kinetics. The simulation are performed on a rigid body-centered cubic lattice, with oxygen atoms placed on the octahedral sites and Fe, Ti and Y atoms placed on substitutional sites. Oxygen atoms diffuse by interstitial mechanism and the Fe, Ti and Y atoms diffuse by a vacancy mechanism. The simulation box includes a vacancy source and sink, which maintains a steady-state concentration. The kinetic Monte Carlo algorithm has been parameterized with input from thermodynamic and kinetic experimental data and ab-initio calculations. The Monte Carlo results provide the kinetic path of precipitation, the nucleation rate and the composition of the nanoclusters as a function of the time. A comparison with experimental results will be performed.
9:30 AM - E13.2
Multiscale Modeling of Nafion Membrane Nanostructure and Its Effect on Molecular Transport.
Ram Devanathan 1 , Arun Venkatnathan 1 , Michel Dupuis 1
1 Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractPolymer electrolyte membrane fuel cells can generate electricity efficiently and with minimal pollution. Existing membranes are not cost effective, perform poorly at low hydration, let water crossover, and have low service life. Optimization of membrane structure and chemistry based on an understanding of dynamics in polymer membranes requires multiscale modeling, because proton transfer, molecular transport and membrane dynamics take place on different length and time scales. We have modeled membrane nanostructure and proton and molecular transport in the widely used membrane NafionTM (Du Pont Inc.) in an effort to understand the relation between membrane nanostructure and transport properties in hydrated polymer membranes. Our work has employed the quantum hopping (Q-HOP) method [1] to study proton transport and classical molecular dynamics to study water transport and membrane nanostructure [2,3]. Our results reveal that increasing membrane hydration causes the sulfonate groups to move apart. At low hydration, most of the water molecules and hydronium ions are bound to the sulfonate groups. The strong binding between hydronium ions and sulfonate groups prevents vehicular transport of protons. In addition, multiple sulfonate groups surrounding the hydronium ion at low hydration hinder structural diffusion of protons. We will present our results in light of recent neutron scattering and infrared spectroscopy experiments and ab initio calculations.
References:
1.Lill M. A.; Helms V. J. Chem. Phys. 2001, 115, 7985; ibid. 115, 7993.
2.Venkatnathan, A.; Devanathan, R.; Dupuis, M. J. Phys. Chem. B 2007 (in press).
3.Devanathan, R.; Venkatnathan, A.; Dupuis, M. J. Phys. Chem. B 2007 (in press).
9:45 AM - E13.3
Growth of sp-sp2 Nanostructures in a Carbon Plasma.
Luciano Colombo 1 , Yasutaka Yamaguchi 2 , Paolo Milani 3 , Luca Ravagnan 3 , Paolo Piseri 3
1 Department of Physics, University of Cagliari, Monserrato (Ca) Italy, 2 Dept. Mech. Eng., University of Osaka, Osaka Japan, 3 Dept. of Physics, University of Milano, Milano Italy
Show AbstractThe growth of sp-sp2 nanostructures in a carbon plasma is simulated by tight-binding molecular dynamics (TBMD) [1], at various temperatures and mass densities. The thermodynamical conditions are selected so as to mimic pulsed microplasma cluster source (PMCS) experiments,[2] which indeed allow to growth nanostructured carbon films containing both sp and sp2 stable complexes.The formation of the sp-sp2 nanostructures in the carbon plasma is a very long process, occuring over the nanosecond time scale and, therefore, TBMD simulations of unprecedented duration have been performed. In order to age atomic trajectories for such a long time, we got a substantial reduction of the computational workload by exploiting the disconnected topology of the simulated carbon plasma. In fact, we implemented a divide-and-diagonalize procedure, which boosted the calculation as much as (about) 300 times, in the most favourable case.Present TBMD simulations prove that several structures can be formed in PMCS experiments (namely, graphite-like sheets, fullerene-like cages, chains, and rings), depending on the plasma temperature and density. A thorough characterization of the observed topologies, as well as their evolution (caused both by thermal annealing and by cluster ripening) is provided.[3]This work was partly supported by Japan Society for the Promotion of Science, by Forschungszentrum Karlsruhe and by MiUR PON-CyberSar project.[1] L. Colombo, Rivista Nuovo Cimento, volo.28(10), 1 (2005)[2] E. Barborini et al., J. Phys. D: Appl. Phys., vol.32, L105 (1999)[3] Y. Yamaguchi et al., submitted for publication (2007)
10:00 AM - **E13.4
Transport Properties of Atomic and Molecular Wires: First Principles Calculations.
Hiroshi Mizuseki 1 , Yoshiyuki Kawazoe 1
1 Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan
Show Abstract10:30 AM - **E13.5
Multiscale Computational Study of [2]rotaxane and [2]Catenane Molecular Electronic Switches.
Yong-Hoon Kim 1 , William Goddard 2
1 Materials Sciences and Engineering, University of Seoul, Seoul Korea (the Republic of), 2 Materials and Process Simulation Center, California Institute of Technology, Pasadena, California, United States
Show AbstractMechanically interlocked supramolecular complexes are important elements of the bottom-up approach toward nanotechnology and are also promising candidates of molecular electronics. In this talk, I will report on our copmputational investigation of the electronic switches based on bistable [2]catenane and [2]rotaxane molecules. Discussed topics will include the configuration of molecular monolayers, mechanism of electrical switching, reliability of switching signal, structure-property correlation, effect of metal atom penetration. Our multiscale computational approach based on density-functional and matrix-Green’s function calculations combined with force-field molecular dynamics simulation will be also briefly summarized. Acknowledgments: This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (Grant No. KRF-2005-041-C00125).
E14: Structure and Properties of Nanostructured Materials I
Session Chairs
Thursday PM, November 29, 2007
Constitution B (Sheraton)
11:30 AM - **E14.1
Quantum Simulations of Atomic Layer Deposition.
Charles Musgrave 1 , Atashi Mukhopadhyay 1 , Javier Sanz 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractWe have used a combination of theoretical techniques to explore the atomic layer deposition of metal oxides and nitrides on various substrates. Our main focus has been on the mechanisms involved in the ALD of HfO2, which we will use to illustrate how simulation can provide insight into ALD and the materials characteristics of deposited films and their interfaces. We have employed density functional theory using both the cluster and periodic supercell models of the reacting surface to determine the detailed atomistic mechanisms of ALD. We have also used DFT to predict ab initio thermodynamic phase diagrams of H2O on HfO2 and to simulate the behavior of the surfaces under various conditions using Born-Oppenheimer molecular dynamics. We have also simulated the evolution of the interfaces and the deposited films using classical reactive molecular dynamics. Finally, we have used DFT to predict the effect of interface structure on the work function of Ru-HfO2 interfaces.
12:00 PM - **E14.2
Computational Design of the Transition-metal-decorated Polymers and Molecules for Hydrogen-storage Media.
Jisoon Ihm 1 , Seung Hoon Ji
1 Physics, Seoul National University, Seoul Korea (the Republic of)
Show AbstractThursday, Nov 29New Presenter *E14.2 @ 11:00 AMComputational Design of the Transition-metal-decorated Polymers and Molecules for Hydrogen-storage Media. Seung Hoon Ji.
12:30 PM - E14.3
Methodologies for Size and Temperature Dependent Materials Properties.
Mingxia Gu 1 , Changqing Sun 1 , Shangzhong Wang 2
1 EEE, Nanyang Technological University, Singapore, Singapore, Singapore, 2 Design Centre, ST Microelectronics, Singapore, Singapore, Singapore
Show AbstractWith the miniaturization of a solid down to nanometers scale, the thermal, electrical and mechanical properties are different from their corresponding bulk counterparts. The trends of relative changes of these quantities also depend on temperature of measurement and the nature of chemical bonds involved. A systematic understanding of the atomic origin of the unusual behavior of mechanical and thermal properties of a nanosolid is presented here towards the predictions for design and controllable growth of nanosctructured materials. The bond-order-length-strength (BOLS) correlation mechanism in size and temperature domain has been developed, which enables the tunability of various measurable properties, such as elastic constants, optical phonon frequency shift. The BOLS correlation mechanism indicates that the bond order loss or the coordination number (CN) imperfection of an atom at sites surrounding the surface skin or at the location of defects causes the remaining bonds of the lower-coordinated atoms to contract spontaneously. The spontaneous bond contraction is associated with bond-strength gain or atomic potential well depression, which localizes electrons and hence the density of charge, mass, and energy at the surface skin or defects locations change. Furthermore, the BOLS correlation mechanism indicates that any measurable parameters can be related to four bond related quantities: bond length, bond strength, bond nature and coordination number (number of bonds associated with an atom in consideration) and change of these bond quantities when the external stimulus, such as size or temperature, are applied. A set of analytical expressions are presented herewith showing that the observed size and temperature dependent change of material properties could be reproduced by considering the size and temperature effects on these four bond related quantities. Agreement between predictions and observations reveal that the shortened and strengthened surface bonds dictate the observed size dependent change of material properties, while the temperature induced bond expansion and weakening are the physical origin for the temperature dependence of measurable material properties.
12:45 PM - E14.4
Tensile Behavior of Nanocrystalline Tantalum Using Molecular Dynamics Simulation.
Zhiliang Pan 1 , Qiuming Wei 1 , Yulong Li 2
1 Mechanical Engineering and Engineering Science, University of North Carolina at Charlotte, Charlotte, North Carolina, United States, 2 Graduate School, Northwestern Polytech University, Xi'an, Shannxi, China
Show AbstractSo far the majority of research efforts on the understanding of the mechanical behaviors of nanocrystalline metals (grain size < 100 nm) have been focused on those with face-centered cubic (fcc) structure such as Cu, Ni, Au, Al, etc. This is true both for experimental results and theoretical investigations using molecular dynamics simulations. Little has been known for body-centered cubic (bcc) metals such as Fe, Ta, W, etc. One reason for this unbalanced efforts lies in the difficulty in producing fully-dense nanocrystalline bcc metals due to their refractoriness. In this work, we present some interesting molecular dynamics simulation results on the tensile behavior of nanocrystalline tantalum (melting point ~3290K, density ~16.23 gm/cc) with grain size from ~6.5 nm to ~13.0 nm. We observed much enhanced strain rate sensitivity, with the accompanying activation volume close to 1 b^3, indicating a change in plastic deformation mechanism. Also observed are stress induced phase transformations which are reversible upon unloading. Other phenomena are also observed associated with the tensile loading of the nanocrystalline tantalum sample. Some of the results have been substantiated by experimental results.
E15: Structure and Properties of Nanostructured Materials II
Session Chairs
Thursday PM, November 29, 2007
Constitution B (Sheraton)
2:30 PM - E15.1
Ab-initio Calculation for the Study of Nano Scale Silicon-based Structure.
Sudip Chakraborty 1 , G. Shashidhar 2 , S. Ghaisas 1
1 Department of Electronic Science, University of Pune,Pune,India, Pune, Maharashtra, India, 2 School of Physics, University of Hyderabad, Hyderabad India
Show AbstractMotivated by the possibility of scaling down of various electronic devices in nanolevel,we have chosen a simple p-n junction like structure in Silicon.An aggregation of 128 Silicon atoms,passivated by the Oxygen is considered.We compute the electronic structure of such a cluster using Density Functional approach.We find the the density of states(DOS) and the optical spectra(OS) for this aggregate.We modify this aggregate by substituting a Phosphorus and a Boron atom within this cluster of Silicon atoms on both sides in order to make a p-n junction like structure.A further variant of this p-n junction like structure is studied.Here we introduce a slab of oxygen between the Phosphorus and Boron regions.DOS and OS for the modified Silicon Clusters are also obtained using the same method and the results are compared for these cases.The comparison of the electronic structures of all these three systems reveal interesting properties.
2:45 PM - E15.2
Atomistic Modeling and Optimization of Thermoelectric Properties of SiGe Nanowires.
Maria Chan 1 , Ying Meng 1 , Tim Mueller 1 , Gerbrand Ceder 1 , John Reed 2 , Trinh Vo 2 , Andrew Williamson 2 , Giulia Galli 3 2
1 , MIT, Cambridge, Massachusetts, United States, 2 , Lawrence Livermore National Laboratory, Livermore , California, United States, 3 , University of California, Davis, Davis, California, United States
Show AbstractNano-structured thermoelectric materials have been shown experimentally to have superior figure of merit compared to bulk materials. It is of interest to develop physically accurate models in order to understand the origin of this superiority and be able to design systems with optimal thermoelectric properties. We developed such a model including aspects of electron and phonon transport in SiGe nanowires.For electronic transport, we work in the diffusive regime with Boltzmann transport, combining ab initio density functional theory calculations with a perturbative treatment of electronic scattering to obtain the energy- and momentum-dependent relaxation times and hence electrical conductivity and Seebeck coefficient,. Both quantum confinement effects and scattering processes are treated explicitly without employing bulk parameters. The phonon contribution to thermal conductivity is obtained from classical equilibrium molecular dynamics simulations using the Green-Kubo formalism. We then employed cluster expansion techniques to model the transport coefficients and figure of merit so obtained in order to efficiently design materials with optimal thermoelectric properties.
3:00 PM - **E15.3
Multiscale Modeling of Silicon Nanowire Electronics.
Kyeongjae Cho 1
1 Physics, UT Dallas, Richardson, Texas, United States
Show AbstractSilicon nanowires (SiNWs) are promising nanoelectronic device material which may be used to continue device scaling beyond sub 24 nm node. Silicon nanowires would be easier to be integrated into slicon-based microdevice fabrication process compared to another promising nanoelectronic device material, carbon nanotubes. For 10 – 20 nm channel length device, gate electrode control would require a small nanowire diameters of about 1 nm. Such a small diameter silicon nanowires would have very large surface area surrounding relatively small bulk atoms within the wire. We have investigated the effects of nanowire surface on the electronic band structure of SiNWs: surface chemical passivation on the electronic structure changes of SiNWs [1], and surface roughness on the electronic structure and quantum transport through the nanowires [2]. More recently, we have studied the effects of strain on silicon nanowires which show a strong electromechanical coupling [3]. We investigate <110> and <111> SiNWs with diameters of 0.7 - 2 nm for mechanical properties such as Young’s Modulus, Poisson’s ratio, band gap, effective mass, work function, and deformation potential. Finally, we use a tight binding approach coupled with non equilibrium Green’s function method to calculate the ballistic transport through a <110> and <111> Si NWs as a function of strain.[1] P. W. Leu, B. Shan, and K. Cho, “Surface Chemical Control of the Electronic Structure of Silicon Nanowires: Density Functional Calculations,” Phys. Rev. B 73, 195320 (2006).[2] A. Svizhenko, P. Leu, and K. Cho, “The effect of growth orientation and surface roughness on electron transport in silicon nanowires,” Phys. Rev. B 75, 125417 (2007).[3] P. Leu, A. Svizhenko, and K. Cho (unpublished).
3:30 PM - E15.4
Electronic Upconversion in Nanodevices.
Karel Kral 1
1 , Institute of Physics, ASCR, v.v.i., Prague 8 Czech Republic
Show Abstract3:45 PM - E15.5
Negative Differential Resistance in Nanojunctions.
Keivan Esfarjani 1 2 , Hossein Cheraghchi 2
1 Physics, UC Santa Cruz, Santa Cruz, California, United States, 2 Physics, Sharif University of Technology, Tehran Iran (the Islamic Republic of)
Show AbstractThe phenomenon of negative differential resistance (NDR) was observed in recent experiments on stretched nanotubes and atomic wires.We have investigated this phenomenon using non-equilibrium Green's function (NEGF) technique applied to junctions between two carbon nanotubes, graphene ribbons, and chains.The basic requirement for such phenomenon, which has a different mechanism than the one proposed by Esaki in the context of superlattices and heterostructures, is electron confinement at the two ends of the junction.Using simple arguments, supported with realistic NEGF model calculations, we have deduced the necessary conditions for occurence of NDR.One such condition is localization of the electronic statesat the junctions with energies in the integration window of width equal to the applied bias.This can happen at large enough applied bias provided that there is a sharp voltage drop across the nanocontact.Thus the potential profile together with the existence of localized states, which depends on the atomic configuration of the contact end, determine whether or not NDR will occur.We will also report the observed even-odd effect for short atomic chains bridging the two contacts, and will discuss the dependence of the NDR threshold voltage on the chain length.
4:00 PM - E15.6
Ab initio Study on Interface and Electronic Structures of Atomic Switch Composed of Mixed Conductors.
Tomofumi Tada 1 , Zhongchang Wang 1 , Tingkun Gu 1 , Satoshi Watanabe 1
1 Department of Materials Engineering, The University of Tokyo, Tokyo Japan
Show AbstractA novel atomic switch [1] composed of a mixed ionic and electronic conductor such as Ag2S and Cu2S has attracted much attention because of its stability and reliability at room temperature. It is believed that OFF-to-ON (ON-to-OFF) switching originates from the appearance (disappearance) of a metallic bridge in the mixed conductor. However, the microscopic understanding of the switching mechanism is still unclear. In particular, physical and chemical processes at the metal/mixed conductor interface have to be clarified to promote further development of the atomic switch. In this study, as an initial step to clarify its switch mechanism, we have examined interface structure, electronic states and electron transport properties of Ag/Ag2S/Ag and Au/Cu2S/Cu systems from ab initio non-equilibrium Green's function [2] calculations. In the case of the Ag/Ag2S/Ag, we adopted two types of interface structures having large and small mismatch at the Ag/Ag2S interface. We found that a zigzag arrangement of Ag atoms is formed in Ag2S after structure optimization and that the Ag/Ag2S/Ag system shows metallic characteristics when the interface mismatch is large [3]. This is the first report on the spontaneous metallization of Ag2S at the Ag/Ag2S interface. On the other hand, such a metallization of Ag2S does not occur in the small mismatch case. However, we found that the addition of certain amount of Ag into the Ag2S layer makes the system metallic in this case. The Cu2S system shows similar behavior. [1] K. Terabe, T. Hasegawa, T. Nakayama, and M. Aono, Nature 433, 47 (2005).[2] M. Brandbyge et al., Phys. Rev. B 65, 165401 (2002).[3] Z. Wang, T. Kadohira, T. Tada, and S. Watanabe, submitted for publication.
E16: Properties of Defects
Session Chairs
Thursday PM, November 29, 2007
Constitution B (Sheraton)
4:30 PM - E16.1
Ab initio Study of Solute - Defect Interactions in Fe.
Par Olsson 1 , Christophe Domain 1
1 MMC, EDF R&D, Moret sur Loing France
Show AbstractFe based alloys are used to great extent in structural components in nuclear facilities of today and will certainly play an equally important role in those of tomorrow. In most interesting Fe based alloys, there are significant levels of solute transition metal impurities. We have performed a systematic study of the interaction of these elements with point defects in Fe using density functional theory. Solute-solute interactions, solute-vacancy interactions and solute-self interstitial interactions have been characterised. The trends here presented convey an important message to designers of steel compositions as well as to explain experimentally observed phenomena. It is shown that the role of magnetism clearly is significant and dominates over that of elastic effects.
4:45 PM - E16.2
Point Defects and Diffusion Mechanisms near Tilt Grain Boundaries in Ni3Al Intermetallide.
Mikhail Starostenkov 1 , Dmitry Sinyaev 1 , Roman Rakitin 1
1 General Physics, Altay State Technical University, Barnaul Russian Federation
Show Abstract5:00 PM - E16.3
Kinetic Consequences of Point Defect Energetics and Deformation Mechanisms in Metastable Alloys.
Timothy Lau 1 , Mukul Kabir 1 , Sidney Yip 1 2 , Krystyn Van Vliet 1
1 Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Nuclear Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractMany technologically critical metallic alloys are intentionally processed as metastable microstructures comprising a supersaturation of crystal defects. Although the interaction among these multidimensional defects is known to control mechanical properties and deformation mechanisms, the local distribution and kinetics of defects in such complex materials are challenging to resolve experimentally. Hardened steels, body-centered cubic iron supersaturated in both carbon (1 wt%) and vacancies, are an important example of alloys in which deformation behavior is intrinsically coupled to the lattice defects. Here, we determine the energetic properties of point defect microstructures and their concentrations in metastable Fe-C alloys via ab initio, density functional theory. From these energetics and our predicted defect phase diagrams, we develop a new many-body classical potential which correctly reproduces the physical properties of such point defects/defect clusters. From this potential, we rapidly survey key defect interactions and apply the nudged elastic band method to calculate migration barriers of statistically abundant defect complexes, as well as the effective self-diffusivity of iron as a function of local chemical composition and mechanical stress. For this metastable alloy, we find that the self diffusivity and the efficiency of creep deformation is controlled by the concentration of carbon-free vacancy clusters.
5:15 PM - E16.4
Magnetic Bond-order Potential: Application to Defect Behavior.
Matous Mrovec 2 1 , Duc Nguyen-Manh 3 , Christian Elsaesser 1 2 , Peter Gumbsch 1 2 , David Pettifor 4
2 IZBS, University of Karlsruhe, Karlsruhe Germany, 1 , Fraunhofer Institute for Mechanics of Materials, Freiburg Germany, 3 UKAEA Culham Division, Culham Science Centre, Abingdon United Kingdom, 4 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractDevelopment of reliable interatomic potentials for simulations of structural defects in iron and iron-based materials presents a significant challenge. Energetics and structural stability of magnetic materials is strongly influenced by magnetic effects and details of the Fermi surface. These materials are therefore described rather poorly by the embedded-atom-method or Finnis-Sinclair-type potentials, which are only density dependent. We present a bond-order potential (BOP) for iron, which is based on a tight-binding bond representation. The model is able to capture the directional character of bonds present in transition metals and includes a description of magnetic effects within the Stoner model of itinerant magnetism. The constructed BOP is applied in studies of point defects, dislocations, and grain boundaries in ferromagnetic iron.
5:30 PM - E16.5
Direct Transformation of Vacancy Voids to Stacking Tault Tetrahedra.
Blas Uberuaga 1 , Richard Hoagland 1 , Arthur Voter 1 , Steven Valone 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractDefect accumulation is the principle factor leading to the swelling and embrittlement of materials during irradiation. It is commonly assumed that, once defect clusters nucleate, their structure essentially remains constant while they grow in size. Here, we describe a new mechanism, discovered during accelerated molecular dynamics simulations of vacancy clusters in FCC metals, that involves the direct transformation of a vacancy void to a stacking fault tetrahedron (SFT) through a series of 3D structures. This mechanism is in contrast to the collapse to a 2D Frank loop which then transforms to a SFT.The potential energy barrier for this transformation is very large: about 2.5 eV for 20-vacancy voids and 4 eV for 45-vacancy voids. Assuming a standard rate prefactor of 1e13/s, these processes would take millions of years at temperatures of 400 K. However, the kinetics of this mechanism are characterized by an extremely large rate prefactor, tens of orders of magnitude larger than is typical of other atomic processes in FCC metals. This large prefactor gives rates that are on the order of nanoseconds for the 20-vacancy void at 400 K. This process, then, is entropically driven.In addition to presenting this transformation process, we explain the origin of the large entropy involved via the volume change associated with the collapse of the void. We find that this simple model gives entropy changes that agree qualitatively with the calculated ones. We also compare the stability of voids versus SFTs in other FCC metals, identify metals in which this transformation might occur even faster. Finally, we discuss the role of He on the rate of transformation of voids to SFTs.
5:45 PM - E16.6
First-principles Study of Molecular Point Defects in Proton-disordered Hexagonal Ice.
Maurice deKoning 1 , Alex Antonelli 1 , Antonio Jose Roque da Silva 2 , Adalberto Fazzio 2
1 Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, Campinas, SP, Brazil, 2 Instituto de Física, Universidade de Sao Paulo, Sao Paulo, SP, Brazil
Show AbstractE17: Poster Session
Session Chairs
Friday AM, November 30, 2007
Exhibition Hall D (Hynes)
9:00 PM - E17.1
Dependence of the Strain Rate Sensitivity of Solid Solutions on the Solute Distribution.
Catalin Picu 1 , Zhijie Xu 1
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractThe strength and strain rate sensitivity of metallic solid solutions are usually described in terms of the mean solute concentration. In this study we investigate the role of higher moments of the distribution function of the solute concentration on these parameters. It is shown that solute heterogeneous distribution controls to a large extent the strain rate sensitivity of the material, while the effect on the strength (critical resolved shear stress) is smaller. Specifically, heterogeneous solute distributions lead to lower (positive) strain rate sensitivity compared with homogeneous/random distributions. This is important in the quest for designing alloys with improved strength and formability; relevant implications will be discussed.
9:00 PM - E17.10
Aggregatization of Interstitial Atoms in Ni3Al Intermetallide.
Mikhail Starostenkov 1 , Nikolay Medvedev 2 , Olga Pozhidaeva 1
1 General Physics, Altay State Technical University, Barnaul Russian Federation, 2 Physics, Biysk Pedagogical State University, Biysk Russian Federation
Show Abstract9:00 PM - E17.11
Ab Initio Calculations of Defect Formation Energies in TiO2.
Patrick Chiu 1 , Jun He 1 , Susan Sinnott 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractWhile it is well known that grain boundaries in polycrystalline materials affect their electronic and vibrational properties, the exact nature of this effect is not well understood. Transition metal oxides, such as rutile TiO2, have properties that are important for various applications such as catalysis, electronic devices, and electrochemical cells. Here, defect formation energies in TiO2 grain boundaries are calculated using density functional theory in combination with thermodynamics, and are compared to the corresponding defect formation energies in bulk TiO2. The specific defects that are examined are point defects, including interstitials and vacancies of both titanium and oxygen. Defect complexes are also considered, including Schottky, Frenkel, and anti-Frenkel defects. The influence of temperature and oxygen partial pressure on the results is discussed. This work is supported by the National Science Foundation through grant number DMR-0303279.
9:00 PM - E17.12
Density Functional Theory (DFT) Studies on the Substituents Effects of Optical Characteristics for OLED Materials.
Soojin Park 1 , Dae-Yup Shin 1 , Jae Il Kim 1 , Hyea dong Kim 1 , Ho Kyoon Chung 1
1 R&D Center, Samsung SDI, Yongin-si Gyeonggi-do Korea (the Republic of)
Show Abstract9:00 PM - E17.13
Symmetry Breaking in Diffusion-limited Two-dimensional Star-branched Polymers.
Carlos Mendoza 1 , Guillermo Ramirez-Santiago 2
1 Polimeros, IIM, Universidad Nacional Autonoma de Mexico, Mexico, Distrito Federal, Mexico, 2 Fisica-Quimica, IF, Universidad Nacional Autonoma de Mexico, Mexico, Distrito Federal, Mexico
Show AbstractWe present results for the structural properties of two-dimensional star-branched polymers with up to 100 arms and 50000 monomers grown by diffusion from a central colloidal particle. We show that the diffusive process used to build up the branched structures, induces polydispersity in the length of the arms. It also gives rise to a symmetry breaking consisting in that only a small number of arms (around five) define the final structure, at relatively large distances, independently of the initial number of arms. We calculate the fractal dimension of the aggregates and characterize their structure by means of the particle-particle correlation function.Acknowledgment: this work was partly supported by the grants DGAPA-PAPIIT numbers IN-110103 and IN-107607 and CONACYT 43596-F.Keywords: Symmetry breaking, diffusion-limited aggregation, star-polymers.
9:00 PM - E17.15
First-Principles Calculation and Monte Carlo Simulation of FePt Alloy.
Yuji Misumi 1 , Satoru Masatsuji 1 , Soh Ishii 1 , Kaoru Ohno 1
1 , Yokohama National University, Yokohama Japan
Show AbstractIntroduction; FePt alloy has become one of the most intensely investigated nanostructured materials. It is well known that FePt alloy which has the L10 structure indicates large magnetic anisotropy. However, unlike the extensive studies made on the magnetic properties of FePt alloy, little attention has been paid to the atomic configuration of FePt alloy clusters whose size is the order of nm. The purposes of this study are 1.To calculate the transition temperature including the effect of lattice vibrations, and 2. To examine the atomic configuration and aggregation process of FePt alloy clusters in the vacuum. 2.To this end, we carry out a first-principles calculation and a Monte Carlo (MC) simulation, and evaluate the long range order (LRO) parameter of the clusters. Method; First of all, using a first-principles approach, we evaluate the total energy of a simple cubic unit cell in which four atoms exist at the tetrahedral sites. Here, we adopt the Vienna ab-initio simulation package (VASP) [1]. The method uses the first-principles Vanderbilt ultrasoft pseudopotential and plane waves within the local-density approximation (LDA) of the density functional theory (DFT). When calculating the transition temperature, we use the potential renormalization theory to include the effect of lattice vibrations [2]. That is using a 2×2×2 supercell including 32 atoms, we calculate the energy deviation in which just one atom is moved a little from the perfect crystal position. Calculating altogether 20 configurations, we take the local trace of the Boltzmann distribution to determine the renormalized potential which depends on the temperature. This potential includes the effect of lattice vibrations when we treat the perfect crystal in which all atoms are fixed at the lattice points. Next, using these potentials, we perform fcc-lattice MC simulations. MC simulations are carried out by means of the Metropolis algorithm for the exchange of atoms, together with 3D periodic boundary conditions. Firstly, we start from a random or ordered initial configuration. Then, by quenching the system with keeping the temperature constant, we observe the time evolution of the system. In addition, we evaluate the LRO at all times. We judge that the system reaches equilibrium when the LRO converged to the steady value. Results; According to the result of MC simulations and the LRO, 1. We found that the transition temperature is around 1800 K. This is closer to the experimental value of 1650 K than the value without the effect of lattice vibrations 1950 K [3]. That is, we confirmed the validity of the potential renormalization theory, and2. We found that FePt alloy cluster with diameters greater than 3.5nm in vacuum can keep the ordered phase. The other results will be explained in the presentation. Reference; [1] G. Kresse et al, Comput. Mat. Sci. 6, 15 (1996).[2] K. Ohno, Trans. Mat. Res. Soc. Jpn. 29, 3787 (2004).[3] S. Masatsuji, Master Thesis, YNU(2006)
9:00 PM - E17.16
Effects of Supports on Hydrogen Adsorption on Pt Clusters.
Kazuyuki Okazaki-Maeda 1 , Yoshitada Morikawa 2 1 , Shingo Tanaka 3 , Masanori Kohyama 3 1
1 CREST, Japan Science and Technology Agency, Kawaguchi Japan, 2 ISIR, Osaka University, Ibaraki Japan, 3 UBEQEN, National Institute of Advanced Industrial Science and Technology, Ikeda Japan
Show AbstractIn a proton-exchange membrane fuel cell, Pt nano-particles supported on carbon materials are used as electrode catalysts, because Pt particles have excellent activity for both hydrogen-dissociation and hydrogen-oxidation at low temperature. However, experimentally, nano-scale structures of Pt-C electrodes such as contacts between Pt and carbon materials and morphology of carbon surfaces as well as sizes and dispersion of Pt particles have serious effects on the electrode performance. It is of great interest to examine the effects of carbon materials on the catalytic activity of Pt. Thus we investigated adsorption energies of hydrogen atoms on Pt clusters supported or unsupported on graphene sheets, using the first-principles calculations based on the density functional theory, and discussed the dependence of the Pt catalytic activity on the Pt-graphite interactions. For an unsupported Pt atom, a hydrogen atom is adsorbed with the energy gain of 1.65 eV. For the systems of a Pt atom deposited on a graphene sheet, we examined the following four sites; the hollow (H) site around six carbon atoms, the top (T) site above one carbon atom, the bridge (B) site between the two nearest carbon atoms, and the vacant (V) site removing one carbon atom. The adsorption energies of a Pt atom on graphene are 1.45, 1.97, 2.17, and 8.01 eV on the H, T, B, and V sites, respectively. The adsorption energies of a hydrogen atom on a Pt atom supported on graphene are 0.89, 0.80, 0.53, and 0.18 eV for each Pt atoms on the H, T, B, and V sites, respectively. It can be said that the interaction between hydrogen and a Pt atom becomes weaker if the Pt-graphene interaction exists. The present results indicate the possibility that the Pt-C interactions may reduce the catalytic activity of Pt-C cathode where the hydrogen-atom adsorption on Pt is mainly involved in, although the examination of Pt clusters on graphite is necessary. In this paper, we present further examinations of the hydrogen adsorption on various sites on several kinds of Pt clusters on graphite.
9:00 PM - E17.17
Near Grain Boundary Coarsening in Mg-based Alloys Strengthened by Precipitation Hardening.
Alexander Katsman 1
1 Materials Engineering, Technion - Israel Institute of Technology, Haifa Israel
Show AbstractThe worldwide consumption of Mg alloys for the automotive industry has grown very rapidly over the last decade. Extensive research work was devoted to Mg-based alloys strengthened by precipitation hardening. However, due to overaging, these alloys exhibit poor structural stability at elevated temperatures. Increasing the aging time leads to the appearance of zones depleted of precipitates near grain- and sub-grain boundaries. The formation of precipitate depleted (PD)-zones is explained by near-grain boundary (NGB) coarsening. Evolution of PD-zones was considered on the basis of the model taking into account diffusional fluxes between adjacent precipitates. The set of equations was solved numerically by using a fourth-order Runge-Kutta method for different initial size of precipitates and density of precipitate layers near grain boundaries. Dissolution of precipitates in the NGB-zones is initially provided by diffusion from them to large precipitates at the grain boundary, and then also by diffusion from these decreased precipitates to the larger precipitates at the outer border of PD-zone. As a result, outer borders of the depleted zones are adjoined by bands of enlarged precipitates forming a "crust" of the PD-zone. Being a diffusion controlled process, the depleted zones are widened with temperature and aging time. Experimental investigation of PD-zones' evolution was conducted by SEM and TEM on the Mg-Zn-Sn-alloys aged at different temperatures for different times. Comparison of the calculated results with experimental data allowed the evaluation of the model parameters and physical parameters of the system (diffusion coefficients and interface energy of the precipitated phase MgZn2).
9:00 PM - E17.18
Effects of Grain Boundary Anisotropy on Nucleation of Ni3Al Precipitates in Ni-Al Alloys.
Celine Hin 1 , Brian Wirth 1
1 Nuclear engineering, UC Berkeley, Berkeley, California, United States
Show AbstractSuper-alloys that are widely used as structural materials for aeronautics applications are currently designed with very approximate models for interfacial precipitation. The simplest of these is the Ni-Al system, where aluminium induces the formation of small precipitates that increase the tensile strength and inhibit recrystallisation. However, current material processes are designed with the assumption that each interface behaves identically, even though it is well-known that interfacial anisotropy is common in super-alloys. The optimization of material properties and processes requires knowledge of the role of interfacial anisotropy and thermodynamics in the precipitation kinetics. Kinetic Monte Carlo (KMC) simulations of Ni3Al precipitation at a sigma 5 grain boundary (GB) in Ni-Al have been performed. KMC is based on an atomic description of the main parameters which control the kinetic pathway: a GB with a real geometry that reproduces the equilibrium segregation properties of Al in Ni, a realistic diffusional behavior of Ni and Al atoms at the GB and in the bulk and a point defect source which drives the vacancy concentration towards its equilibrium value.Depending on the material and processing conditions, KMC simulations predict different kinetic behaviors, including Al segregation to the boundary, wetting phenomena and competition between homogeneous and heterogeneous Ni3Al precipitation. The KMC results are compared with available experimental data.
9:00 PM - E17.19
First-principles Calculations of Au-Pd Core-shell Nanoparticles and Slabs with Adsorbates.
Shingo Tanaka 1 , Noboru Taguchi 2 , Tomoki Akita 1 , Fuminobu Hori 2 , Masanori Kohyama 1
1 MATSCI, UBIQEN, AIST, Ikeda, Osaka Japan, 2 Dept. of Materials Science, Osaka Pref. Univ., Sakai, Osaka Japan
Show AbstractFirst-principles calculations of Au-Pd core-shell nanoparticles and slabs with adsorbatesNoble metal nanoparticles with core-shell structure play important roles for nano-scale catalysts [1-5]. Recently, the Au-Pd bimetallic nanoparticles prepared by sonochemical technique show the Au-core and Pd-shell (Au@Pd) structures confirmed by various analyses [1,2,6] and have higher catalytic activities in comparison with the pure Au or Pd system. However, there have not yet clarified the fine atomic and electronic structures of the core-shell interface and the catalytic reactivity of adsorbates of the Pd-shell surface. In this paper, we have performed the first-principles calculations of Au@Pd nanoparticles and corresponding Au-Pd slab models using the projector augmented-wave (PAW) program code [7] and reveal the atomic and electronic structures and the local reactivity of the atomic hydrogen as adsorbates for various coverage ratios. In the adsorption energy analyses of the slab models, the atomic hydrogen on the hollow or bridge site of Pd overlayers is more stable than that on the top site. The adsorbed hydrogen forms the rather strong covalent bond to the Pd-shell atoms and the charge transfer from Pd to hydrogen occurs. The Pd atoms interacted with the adsorbates show large displacements toward the adsorbates which change proportional to the lattice strain of the slabs. The above tendency is similar to the other first-principles studies [8]. We deal with the Au@Pd nanoparticles (N=13 and 55) with the adsorbates. The Pd-shell atoms have a large atomic displacement and a strong change of the charge density distribution near the corner atoms. This work was partially supported by the Japan Society for the Promotion of Science (JSPS) Research (Grant-in-Aid for Scientific Research B, No.17360314). [1] Y. Mizukoshi et al., J. Phys. Chem. B 104 6028 (2000). [2] H. Takatani et al., Mater. Sci. Forum 445-6, 192 (2004). [3] J. K. Edwards et al., J. Catal. 236, 69 (2005) [4] D. I. Enache et al., Science 311, 362 (2006). [5] B. E. Solsona et al., Chem. Mater. 16, 2689 (2006). [6] T. Akita et al., Catal. Today (2007) in print. [7] Quantum MAterials Simulator (QMAS), S. Ishibashi, T. Tamura, S. Tanaka, M. Kohyama and K. Terakura, in preparation. [8] A. Roudgar and A. Gross, Phys. Rev. B 67, 033409 (2003); J. Electroanal. Chem. 548, 121 (2003).
9:00 PM - E17.2
The Role of Solute Segregation on the Evolution and Strength of Dislocation Junctions.
Qian Chen 1 , Xiang-Yang Liu 1 , Bulent Biner 1
1 , Ames Laboratory, Ames, Iowa, United States
Show AbstractIn this study, the role of solute segregation on the strength and the evolution behavior of dislocation junctions is studied by utilizing kinetic Monte Carlo and 3D dislocation dynamics simulations based on the anisotropic elasticity. The different binding energies of solutes, solute concentration, elastic anisotropy ratio and the character of junctions are all included in the simulations in an effort to make a parametric investigation. The results indicate that the solutes have a profound effect not only on the stress levels required to break the junctions but also on the evolution of equilibrium configuration of junctions. For identical solute and junction parameters, a strong influence of the elastic anisotropy ratio was observed originating from the different segregation behavior of solutes to the junctions. These results were also compared with known simple analytical solutions.
9:00 PM - E17.20
Integration of Thermal Spray Process: From Particle In-Flight to Coating Build-up.
Wei Zhang 1 , Hui Zhang 2 , Lili Zheng 2 , Sanjay Sampath 1
1 Materials Science and Engineering, Stony Brook University, Stony Brook, New York, United States, 2 Mechanical Engineering, Stony Brook University, Stony Brook, New York, United States
Show Abstract9:00 PM - E17.23
Development of 3-D Kinetic Monte Carlo (KMC) Simulations to Directly Correlate with In situ Transmission Electron Microscopy (TEM) Studies of Cu(001) Oxidation.
Chao Fang 1 , Xuetian Han 1 , Alan McGaughey 2 , Susan Sinnott 3 , Simon Phillpot 3 , Judith Yang 1
1 MEMS, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 3 , University of Florida, Gainesville, Florida, United States
Show Abstract9:00 PM - E17.24
Modelling of Elastic Modulus and Molecular Structure Interrelationship of an Oriented Crystalline Polymer.
Ulmas Gafurov 1
1 , Instiute of Nuclear Physics, Tashkent Uzbekistan
Show Abstract9:00 PM - E17.25
Modeling of Local Load Redistribution in Creep of an Oriented Crystalline Polymer.
Ulmas Gafurov 1
1 , Instiute of Nuclear Physics, Tashkent Uzbekistan
Show Abstract9:00 PM - E17.28
First-Principles Investigation of the Role of Mg and Ag in Stabilizing the α-Al/Ω Interface in Al-Cu-Mg-Ag Alloys.
Lipeng Sun 1 , Douglas Irving 1 , Mohammed Zikry 2 , Donald Brenner 1
1 Material Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractThe addition of trace amounts of alloying elements to age hardenable Al-alloys can significantly alter the materials’ mechanical properties. In Al-Cu alloys, Cu atoms aggregate to a disc shape on the Al{001} planes and evolve into the θ’ (CuAl2) phase on the same habit plane. However, the addition of trace amounts of Mg and Ag completely changes the precipitation process: a new phase designated as Ω are observed on the Al{111} planes at the expense of the precipitation of the θ’ phase. The resulting Al-Cu-Mg-Ag alloy exhibits superior strength and creep resistance, which makes it a promising candidate for aerospace and armor applications. The role of Mg and Ag in promoting the precipitation of the Ω phase and enhancing mechanical properties is still not understood completely. In the work reported here, we used density functional theory (DFT) to study the atomic structure and energetics of Mg and Ag at the α-Al/Ω interface and their effects on the interface cohesion. A supercell consisting of 152 atoms is used to model the α-Al/Ω interface. In agreement with experiment, the most stable structure of α-Al/Ω interfaces are found to be connected by Al-Al bonds with the hexagonal Al lattice of the surface of the Ω phase sitting on the hollow sites of the Al {111} matrix planes. Mg and/or Ag are added by substituting the lattice Al atoms. Relative heats of formation calculations show that the segregation of Mg and Ag atoms at the interfacial region can stabilizes the interface. Compared with Mg, Ag has a more significant effect on the interface stabilization. Given the same Mg/Ag ratio, increasing the total amount of Mg and Ag stabilizes the interface more efficiently than varying lattice substitution sites. The most stable interface structure found are two layers of Mg and Ag atoms with full coverage of the surface of the Ω phase by Mg and bulk Al {111} surface by Ag. This is in agreement with the recent experimental observation that there are two layers of Mg and Ag segregation. Electronic structure analysis shows that the Ag-Mg, Ag-Al, and Mg-Al bonds at the interface are weaker than the Al-Al bonds because the Al-Al bond is “directional bonding” while others are “hard-sphere-and-glue” like. The weaker bonding at the interface also implies that the Mg and Ag embrittle the alloy.
9:00 PM - E17.29
Temperature Dependence of Elastic Properties of Transition Metals Using Face-Centered-Cubic Lattice Model with Renormalized Potentials.
Ryoji Sahara 1 , Hiroshi Mizuseki 1 , Kaoru Ohno 2 , Yoshiyuki Kawazoe 1
1 , Institute for Materials Research, Tohoku University, Sendai Japan, 2 , Yokohama National University, Yokohama Japan
Show AbstractLattice models are a simple and fast method for the study of thermodynamic properties. One advantage is that it can treat systems larger both in time scale and in spatial size as compared with atomic-scale molecular dynamics (MD) simulations so that it can treat qualitatively thermodynamic equilibrium or diffusion phase transition phenomena in the solid state. Although it has been pointed out that thermal lattice vibrations (anharmonicity effects) play an essential role, its effect has not yet been properly treated in the lattice model. To overcome the problem of neglecting the internal entropy which originates from thermal vibrations, the potential renormalization technique has been developed (Refs. 1-3). It offers an effective way to map interatomic potentials (for example, classical MD potentials) onto lattice models. The fundamental idea of potential renormalization is to make a new potential function for discretized space without changing the value of the partition function for continuous space. Using the idea of potential renormalization, the temperature dependence of the elastic properties of transition metals are studied by introducing face-centered cubic (FCC) lattice model. In order to treat actual systems as quantitatively as possible, empirical second moment approximation (SMA) potentials proposed by Rosato et al. and by Cleri et al., which have been used widely for molecular dynamics (MD) simulations, are mapped onto the lattice using the potential renormalization. It is found that the temperature dependence of the elastic properties such as the bulk modulus, the elastic constants, the shear modulus, and the Young’s modulus agree well with the results of MD simulations and also the experimental results.References: (1) K. Ohno, The Sci. Rep. Res. Inst. Tohoku Univ. A 43 (1997) 17. (2) R. Sahara, H. Mizuseki, K. Ohno, S. Uda, T. Fukuda and Y. Kawazoe, J. Chem. Phys. 110 (1999) 9608. (3) R. Sahara, H. Ichikawa, H. Mizuseki, K. Ohno, H. Kubo and Y. Kawazoe, J. Chem. Phys. (2004) 9297.
9:00 PM - E17.3
Computing Electrochemical Impedance of Solid Electrolyte from Fluctuations.
Eunseok Lee 1 , Wei Cai 1 , Fritz Prinz 1
1 Mechanical Engineering, Stanford University, Stanford , California, United States
Show AbstractWe present a new method for computing the electrochemical impedance of solid electrolyte through kinetic Monte Carlo (kMC) simulations of ion diffusion. In the conventional approach, the impedance at a given frequency is obtained by a non-equilibrium kMC simulation subjected to an AC voltage at this frequency. Using the fluctuation-dissipation theorem, the impedance at all frequencies can be obtained from the correlation function of microscopic current fluctuations through a single equilibrium simulation. This fast method allows us to systematically examine the effects of temperature, doping, and electrostatic interaction on various aspects of charge diffusion and ionic conductivity in the solid electrolyte. We applied this method to calculate the impedance of yttria-stabilized zirconia (YSZ) and investigated how those effects contribute to the ionic conductivity of YSZ.
9:00 PM - E17.30
Reaction Rate as an Effective Tool for Analysis of Chemical Diffusion in Solids.
Misha Sinder 1 , Zeev Burshtein 1 , Joshua Pelleg 1
1 Materials Engineering, Ben Gurion University of the Negev, Beer Sheva Israel
Show AbstractIn their paper, R. Merkle, J. Maier, K.D. Becker and M. Kreye [1] conducted an experimental study on the chemical diffusion of oxygen in Fe-doped SrTiO3 single crystals driven by large changes in the oxygen ambient partial pressure. The stoichiometry dependence of the chemical diffusion coefficient was derived on the basis of the concept of conservative ensembles for two independent trapping reactions, which then served for calculating the evolution of vacancy profiles. The theoretical predictions were compared to the experimental results. In the framework of the same model, in the present communication, the chemical diffusion of oxygen was analyzed by the concept of a dynamic reaction front [2, 3]. We show, that by using a quasi-chemical reaction rate profile, it is possible to obtain information relating to the position and width of the zone where the reaction takes place. It is indicated, that the reaction rate distribution can be directly calculated from measured concentration profiles of the immobile reactant. [1] R. Merkle, J. Maier, K.D. Becker and M. Kreye, Phys. Chem. Chem. Phys., 6, 3633 (2004)[2] M. Sinder, J. Pelleg, Phys. Rev. E 61, 4935 (2000)[3] Z. Koza, Phys. Rev. E 66, 011103 (2002)
9:00 PM - E17.31
First-principles Calculations of the Atomic and Electronic Structures in Au-Pd Slab Interfaces.
Noboru Taguchi 1 , Shingo Tanaka 2 , Tomoki Akita 2 , Masanori Kohyama 2 , Fuminobu Hori 1
1 Materials Science, Osaka Prefecture University, Sakai, Osaka, Japan, 2 Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka, Japan
Show AbstractIt has been reported that binary Au-Pd nanoparticles, which has Au-core and Pd-shell separated structure, synthesized by sonochemical method have high catalytic activities for hydrogen [1]. The core-shell structure of nanoparticle plays important role for catalytic activities. In the present study, in order to estimate the atomic and electronic structures of the Au-Pd interface, we performed first-principles calculations using the first-principles PAW (the projector augmented-wave) method for Au/Pd slab interface model. The energetic estimation from the calculation of adhesive energy for the thin Pd layers (2-3 layers) stacking on Au (111) and on Au (100) slab with an epitaxial relationship represents that Pd overlayers have a lateral expansion in both case. This trend is in good agreement with other experimental results for Au-Pd core-shell nanoparticles, such as electron microscopy, X-ray diffraction, positron annihilation [2,3]. Contractions of Pd lattice are also found in vertical direction to the interface. In addition, an intermixing configuration near the Au-Pd interface is more stable than the binary separated one. [1]Y.Mizukoshi et al., J Phys. Chem. B, 104, 6028 (2000) [2]T.Akita et al., Catalytic Today, submitted [3]H.Takatani et al., Mater. Sci. Forum 445-6, 192 (2004)
9:00 PM - E17.32
Optical Property of Thermal Barrier Coating at High Temperature.
Geunsik Lim 1 , Aravinda Kar 1
1 Laser-Aided Manufacturing, Materials and Micro-processing Laboratory (LAMMMP), College of Optics and Photonics, Center for Research and Education in Optics and lasers (CREOL), MMAE, University of Central Florida, Orlando, Florida, United States
Show Abstract9:00 PM - E17.33
Ab initio Optical Properties of ZnSe Clusters.
Sachin Nanavati 1 , Vijayaraghavan Sundararajan 1 , Shailaja Mahamuni 3 , Subhash Ghaisas 2
1 Scientific and Engineering Computing Group, Centre for Development of Advanced Computing (C-DAC), Pune, Maharashtra, India, 3 Dept. of Physics, University of Pune, Pune India, 2 Department of Electronic Science, University of Pune, Pune, Maharashtra, India
Show AbstractWe report the optical properties of ZnSe clusters using an adiabatic, time-dependent density functional theory (TD-DFT) formalism, with in the local density approximation (LDA). The calculations were carried out in real space for bare and passivated clusters. The passivation was carried out using partially charged hydrogen atoms to account for the surface dangling bonds. We observed the characteristic blue shift in the calculated optical spectra by TD-DFT method as compared to that of DFT. Further, surface passivation removes the localized states between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital ( LUMO). The variations in the HOMO-LUMO gap, binding energy and bond lengths are studied as a function of number of atoms in the clusters. A comparative study is done with similar sized CdSe clusters.
9:00 PM - E17.34
Analysis of Structural and Dynamical Properties of SiO2 During Cooling: A Molecular-dynamics Study.
Byoung Min Lee 1 , Teruaki Motooka 2 , Shinji Munetoh 2 , Yeo Wan Yun 1
1 Materials Engineering, Korea University of Technology and Education, Cheonan Korea (the Republic of), 2 Materials Science and Engineering, Kyushu University, Fukuoka Japan
Show AbstractThe structural properties of SiO2 liquid at finite temperatures have been investigated by molecular dynamics (MD) simulations utilizing the Tersoff interatomic potential. During cooling process, the SiO2 liquid structure quenched with a cooling rate of 1.0×1011 K/sec shows the traditional properties observed in the experiments. Although the properties and atomic configurations of glass could be reproduced well, they contained structural defects consisting of the fivefold coordination of Si, and the threefold coordination of O atoms. The calculated diffusion coefficients which are calculated by monitoring the mean-square displacement of atoms drop to almost zero below 3000 K (<10-6 cm2/sec) but has a fluctuations at low temperature. The structure properties of SiO2 liquid shows a significant dependence on the temperature during cooling process. Bond-angle distribution at around 120° originate from the O and Si atoms consisting of the over-coordinated O atoms. A detailed analysis of two-body and three-body correaltion functions exhibits a high degree of consistency and support the understanding of the dynamical properties of SiO2 glass. The splitting of peak at high frequencies near 132.7 and 147.5 meV was not observed in the phonon density of states due to the ignorance of Coulombic interaction
9:00 PM - E17.35
Diffusion Coefficients of Fluids in Confined Porous Matrices.
Hector Dominguez 1 , Jose Salinas 1
1 Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Mexico D.F Mexico
Show AbstractSeries of Molecular Dynamics simulations to study diffusion of a fluid in a matrix confined by a slit-pore were investigated. The matrices were prepared by two different methods; in the first method, simulations of a fluid at a fix density were conducted and the last configuration of its particles was taken as the matrix configuration. In the second method a binary mixture was simulated where one of the components served as a template material and the final porous matrix configuration was obtained by removing template particles from the mixture. In both methods the matrices were confined by two parallel walls (slit-pore) modeled by continuous solid surfaces. The results showed that diffusion inside the matrices did not decrease with the fluid density, it presented a maximum at some values of the fluid density. On the other hand, the results also showed that the matrix structure and porosity were affected by the way the porous matrices were prepared.
9:00 PM - E17.36
Molecular Dynamics Simulation of Conductivity in Ionic Conducting Membranes.
Huang Tien-Jung 1 , Chang Jian-Chuang 1 , Yeh Jyi-Tyan 1 , Chiu Yu-Tsung 1
1 Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractThe main objective of this research is to study the ionic conductivity properties of main chain polymer conducting membrane, Poly(arylene ether sulfone) copolymer(BPSH), Polysulfone(PS) and side chain polymer conducting membrane, Nafion117, Polybezimidazole(PBI) by molecular dynamics simulation. Furthermore, the hydronium ion conductivity at different swelling values for different polymer system is calculated. The two types of membrane were found to exhibit different properties due to the difference in microstructure. The conductivity of main chain polymer conducting membrane is 10-3Scm-1and side chain polymer conducting membrane is 10-2Scm-1.Via the analysis of a free volume theory, we find the side chain polymer conducting membrane swelling of water molecule so that the polymer conformation becomes looser. Moreover, medium to large cavities are formed, therefore, these factors are believed to be responsible for the ionic diffusion of side chain polymer structure.
9:00 PM - E17.37
Efficient Atomistic Simulations through Smart Particle Labelling.
Luciano Colombo 1 , Simone Meloni 2 , Mario Rosati 2
1 Department of Physics, University of Cagliari, Monserrato (Ca) Italy, 2 Dept. Mat. Sci., CASPUR, Rome Italy
Show AbstractThe computational cost of MD simulations can effectively be reduced by taking advantage of the short range character of the underlying atomic cohesion model. Although this is not a universal feature, it nevertheless applies to quite a few organic and inorganic materials as well as to several biosystems. As a matter of fact, algorithms based on suitable interaction lists (like, e.g., the Verlet list method) are based on the possibility to cut off atomic interactions beyond some threshold and are ubiquitously used in computational materials science.While the search for ever more advanced algorithms is always ongoing, little attention is reserved to investigate whether such interaction-list-based methods are well suited to take full advantage of modern computer archtectures. In fact, they often are not, as proved in this work.In MD simulations atoms are identified with indices - hereafter referred to as atomic labels - and atomic data (e.g. postions, forces) are arranged in memory according to these labels. In diffusive or disordered systems, the distance between labels corresponding to two interacting particles might be very large. Therefore, data relative to interacting atoms might be far away in memory. This fact is unfortunate since it violates both spatial and temporal locality principles that modern scalar computers are based on.We argue that reordering atomic labels - so that data belonging to particles close in physical space would also be close in memory - offers an intriguing perspective for a better use of computer memory (especially, cache one). In this work we develop the concept of reordering within the MD framework and we show that it is possible to greatly benefit from it, as much as to reduce down to one-third the computing time needed in a typical MD simulation. Examples will be provided, based on atomic cohesion models largely used in materials science.Finally, we show that the present label reordering procedure can be used to devise an efficient parallel one-dimentional decomposition MD scheme.One of us (L.C.) acknowledges support by MiUR under project PON-CyberSar (OR7).
9:00 PM - E17.38
Extracting System Dynamics from Time Series Recurrence Analysis.
Theodoros Karakasidis 1 , A. Fragkou 1 , A. Liakopoulos 1
1 Civil engineering, University of Thessaly, Volos Greece
Show Abstract9:00 PM - E17.39
Chemomechanical Modeling and Simulation of Ligand-Receptor Binding Kinetics.
Emily Walton 1 , Ranjani Krishnan 2 , Mukul Kabir 1 , Krystyn Van Vliet 1
1 Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe reversible binding of molecular receptors on cell surfaces to corresponding molecular ligands on adjacent material surfaces controls key biological processes ranging from cell-material adhesion to drug metabolism. The unbinding trajectories of these molecular pairs provide direct access to the kinetics of ligand-receptor interactions, from which the mechanisms of extracellular load transfer at the cell-material interface can be modeled. As multiple physical and chemical factors influence the unbinding of biomolecular complexes, ranging from temperature to pH to loading history, accurate models of biomolecular interactions in vivo must capture the coupled effects of these parameters on bond lifetime and binding kinetics that span timescales up to tens of seconds. Under current computational resources, no one modeling approach can incorporate these multiple effects over physiologically relevant timescales. Here, we demonstrate the integration of steered molecular dynamics, Monte Carlo methods, and transition state theory that enable us to predict the unbinding pathway and kinetics of such molecular complexes. We present results for biotin-streptavidin, a ligand-receptor pair of widespread use in the development of biomaterial surfaces, and for integrin, a cell surface receptor of adhesive ligands that transfers both chemical and mechanical cues through the cell membrane. Through comparison of our modeling and simulation approach with molecular spectroscopy experiments, we successfully predict kinetic rates of dissociation as a function of local physical environment in vivo. We show that the energy landscape and the binding kinetics of these complexes are strongly altered by the local mechanical compliance of the extracellular ligands and/or the cell membrane.
9:00 PM - E17.4
MD Simulations of Mixed Dislocations Interaction with Glissile SIAs in BCC Iron.
X. Liu 1 , S. Biner 1
1 Materials and Engineering Physics Program, Ames Laboratory (USDOE), Iowa State University, Ames, Iowa, United States
Show Abstract9:00 PM - E17.40
First-principles Calculation of Thermal Conductivity of Bulk Si.
Keivan Esfarjani 1 , Joseph Feldman 2 3 , Harold Stokes 4
1 Physics, UC Santa Cruz, Santa Cruz, California, United States, 2 , Naval Research Labs, Washington DC, District of Columbia, United States, 3 Physics, George Mason University, Fairfax, Virginia, United States, 4 Physics, Brigham Young University, Provo, Utah, United States
Show AbstractVery few calculations of thermal conductivity have been done from first principles. The difficulty lies in the quatitative calculation of the scattering rates.For ideal bulk structures of pure crystals, one can ignore defect scattering and only include umklapp and normal processes.First principles methods allow us to calculate the second and higher order derivatives of the total energy with respect to atomic displacements and extract theharmonic and anharmonic force constants. In this presentation, after briefly introducing a new methodology to extract force constants, we report our results onthe calculation of thermal conductivity of bulk Si using the Green-Kubo formula in which the ensemble average of the heat current autocorrelation is computed from the molecular dynamics simulation. The force field used in MD isconstructed from the Taylor expansion of the total energy about the equilibrium configuration. An alternative way to calculate the thermal conductivity is to use perturbation theory results on scattering rates in the Boltzmann-Peierls formula. The results on the two methods will be compared, and the advantage of each will be discussed.
9:00 PM - E17.41
Surface-reaction-limited Phase Transformation Dynamics in LiFePO4.
Martin Bazant 1 , Gogi Singh 1 , Gerbrand Ceder 2
1 Mathematics, MIT, Cambridge, Massachusetts, United States, 2 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show Abstract9:00 PM - E17.42
Atomistic Simulations of the Aluminum-silicon Interfaces Under Shear Loading.
Alica Noreyan 1 , Vesselin Stoilov 1
1 Mechanical, Automotive and Materials Engineering , University of Windsor , Windsor, Ontario, Canada
Show AbstractIn the present study molecular dynamics simulations were carried out to investigate the deformation of pure FCC aluminum and diamond cubic silicon interfaces under shear stress. Second nearest-neighbor modified embedded atom method was used to describe the interactions between Al-Al, Si-Si and Al-Si atoms. Three Si-Al interfaces were considered: Si<100> || Al<100> and Si<100> || Al<110> alignments between Si(100) and Al(100) surfaces, Si(100) and Al(111), and Si(111)_Al(111) surfaces. Bond angle distribution, pair correlation functions and shortest path ring analysis were used to determine the nature of plastic deformation due to the sliding. It was shown that the deformation is localized at approximately 40 Å thickness of the interface for Al, while for Si it is about 10 Å. Different forces in the range of 0.005 to 1.0 eV/Å were applied to find the threshold of force for initiating sliding at the interface and plastic deformation in base material. When applied force exceeds 0.04eV/Å partial amoprphisation of Al near the interface occurs. It was found that while the threshold forces are almost the same for initiating sliding for two crystallographic alignments ~ 0.0145 to 0.015 eV/Å between Si(001) and Al(001) surfaces, it is significantly smaller for Si(100) and Al(111) interface. Thus it was found that anisotropy of deformation is strongly dependent on interface planes, but has relatively small dependence on interface alignment. It was also illustrated that all interfaces act differently upon further increase of applied force. While for smaller applied forces the region of plastic deformation was concentrated in Al near the interface, further increase of force causes plastic deformation in the form of slip in Al and partial amoprphisation in Si near the region where external forces were applied. The threshold of applied forces for causing plastic deformation in Si was found. In both orientations for Si(100) and Al(100) interface a plastic deformation is also occurring in Si when external forces are 0.45 eV/Å, while for Si(100) and Al(111) interface the threshold force is lower: 0.35 eV/Å. When external forces are larger than 0.6 eV/Å no sliding is observed at the interface while plastic deformation is taking place in base materials near the region of applied forces. The results for Si-Al interface were also compared with all-Al material with different grain orientations, and it was shown that while for specific case of Al (100) monocrystal exhibits better shear strength, in general introducing Si improves it for different grain orientations.
9:00 PM - E17.43
A Perfect Mirror Full of Holes: Simulating New Photonic Crystal Geometries.
Daniel Cogswell 1 , Karlene Maskaly 2 1 , W. Craig Carter 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 Applied Electromagnetics, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractParallelized software was developed for solving Maxwell's equations using the finite-difference time-domain method, and was used to study new 2D and 3D dielectric composites. These composites consist of three dielectric components and may have complete or nearly complete bandgaps in addition to being self-supporting and simple to manufacture. Currently known structures with complete bandgaps, such as spheres or voids arranged on a diamond lattice or various mathematical surfaces, are difficult to manufacture. 1D photonic crystal stacks were arranged in a square array and it was discovered that sizeable bandgaps for both 2D and 3D geometries appear along the principle axes of the structure for different polarizations, effectively producing a perfect mirror full of holes. Furthermore, bandgaps in different directions and polarizations could be made to overlap for reasonably large frequency ranges. Variations in different structural parameters may be used to optimize the 3D structure and produce a complete photonic bandgap. A monte-carlo algorithm was implemented to search for the geometric parameters of the structure that optimize the structure's optical performance and to look for conditions that produce a complete bandgap. Results of this optimization will be presented.
9:00 PM - E17.44
Topological Properties of Microstructures in Nanocrystalline Materials.
Tao Xu 1 , Mo Li 1
1 School of MSE, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractIt is well known that topological properties of microstructures play an important role in physical and mechanical properties in polycrystalline materials. The same has been expected in the nanocrystalline materials. However, the fundamental structure-property relationship still remains largely elusive due to the difficulties in determining and controlling the microstructures in nanocrystalline materials experimentally. In this study, we present newly developed numerical methods and their applications in characterizing different topological properties in nanocrystalline materials. Realistic microstructures with desired topological properties are generated using inverse Monte Carlo method and then relaxed by large-scale molecular dynamic simulation. Topological properties, such as grain boundaries, triple junctions, grain size and shape distribution, and misorientations, are calculated and compared with either experimentally available data or theoretical predictions. The results show that these numerical methods provide a quantitative approach for understanding the fundamental microstructure-property relation in nanocrystalline environment and could serve as an effective tool for designing polycrystalline materials in general.
9:00 PM - E17.45
Morphological Evolution of Intragranular Void by Surface Drift-diffusion Driven by the Capillary, Electromigration and Thermal-stress Gradient Generated by the Steady State Heat Flow in Encapsulated Metallic Films.
Tarik Ogurtani 1 , Oncu Akyildiz 1
1 Metallurgy&Materials Engineering, Middle East Technical University, Ankara Turkey
Show AbstractThe morphological evolution of intragranular voids induced by the surface drift-diffusion under the action of capillary forces combined with the electromigration (EM) and thermal stress gradient (TSG) caused by the inhomogeneous temperature distribution associated with the steady state heat flow is investigated in passivated metallic thin film via computer simulation using the front-tracking method. In the mesoscopic nonequilibrium thermodynamic formulation of the generalized driving forces for the thermal-stress induced surface drift-diffusion not only the customary elastic strain energy density (ESED) contribution but also the elastic dipole tensor interaction (EDTI) between the induced thermal-stress field and the mobile atomic species (mono-vacancies) are also considered. Computer simulations on the initially cylindrical intragranular voids yield two distinct scenarios which are found to be strictly depending upon whether the heat flux crowding occurs due to proximity effects of the insulating boundaries or not. In the case of TSG dominating regime, the void center of gravity exhibits steady and uniform displacement velocity proportional with and opposite to the induced TSG. In both morphological cases however, one also observes two well defined regimes, namely: the EM and TSG dominating regions in the EM versus EDTI parametric space, which may be bounded by a threshold level curve for the EDTI parameter above which an extremely sharp crack-tip nucleation and propagation occurs in the highly localized minima in the triaxial stress regions surrounding void surface layer, and extending along the longitudinal and off-diagonal directions respectively. As far as the device reliability is concerned, the most critical configuration occurs even at low thermal-stresses levels when the specimen width with respect to the void radius is below certain range of values, which also marks the onset of the heat flux crowding as a dominate regime. In the absence of EM this manifests itself by the formation of two symmetrically disposed finger shape extrusions (pitchfork shape slits) on the upper and lower shoulders of the void surface on the windward side, which later stage extend almost inclinations towards the sidewalls, and cause eventually the fatal catastrophic interconnect breakdown by the growth process induced by the condensation of super saturated vacancies in the bulk matrix. This morphology at high thermal stresses is replaced by the fracture mode induced by diffusive-crack formation and propagation. Outside of the heat flux crowding regime below the TSG levels, the void takes an egg shape pointed towards the high temperature edge of the interconnect, and steadily drifts against the heat flow or the upstream direction without causing any transgranular damage. Above the TSG thresholds levels however it is replaced by a sharp-crack formation regime with the accelerated propagation that may eventually cause catastrophic electrical circuit breakdown.
9:00 PM - E17.46
In-Plane Rotated Crystal Structure in Continuous Growth of Bismuth Cuprate Superconducting Film.
Satoru Kaneko 1 , Kensuke Akiyama 1 , Takeshi Ito 1 , Hiroshi Funakubo 2 , Mamoru Yoshimoto 2
1 Kanagawa Industrial Technology Center, Kanagawa Pref. Government, Ebina, Kanagawa, Japan, 2 Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan
Show AbstractIn most case, thin film shows different properties, and/or phenomena than bulk samples. Film samples of bismuth cuprate superconductor shows the lower transition temperature, and slight different crystal structure including supercell; characteristic structure consisting of several unit cells with long incommensurate modulation. How strongly unit cells distorted from original position is dependent on mismatch between BiO and perovskite layers or extra oxygen in BiO block in bismuth cuprates. The size of supercell was varied with external strain induced by lattice mismatch in superlattice structure[1]. To the contrary, as single film continuously grows, the strain will be released and its structure might approach one in bulk sample. In this study, such a thick film was prepared on MgO substrate and investigated to compare with a reference film with thickness of ~1000 Å. As film continuously grew, the size of supercell extended while unit cell rotated by 32° along the surface normal.
Bi2Sr2Ca1Cu2OX (Bi-2212) was deposited on MgO(001) substrate by pulsed laser deposition (PLD) using the fourth harmonics of YAG laser. The substrate temperature and target distance were 780°C and 40 mm, respectively. The energy density was 1.7 mJ/cm2. X-ray diffraction (XRD) of θ-2θ and φ scans verified epitaxial growth of c-axis Bi-2212 on MgO substrate with relation of Bi-2212(100) parallel to MgO(110). Relatively thick Bi-2212 film was prepared to compared with the reference film with film thickness of 1000 Å. Grazing XRD methods such as in-plane and x-ray reflectivity (XRR), and x-ray reciprocal space mapping (XRSM) were employed to investigate the crystal structure in details.
With increase of film thickness, XRD pole figure with Bi-2212(115) peaks showed 12 peaks instead of four peaks on the reference film, indicating that epitaxial Bi-2212 film rotated by by 32° along the surface normal relative to the initial layer of Bi-2212, which also verified by in-plane φ scan using Bi-2212(200) peaks. The angle of 32° can be interpreted by the near coincidence site lattice (CSL) with Σ = 17 and 18. On the reference film, XRSM showed asymmetric intensity distribution around Bi-2212(0020), as previously reported[2]. Interestingly, XRSM showed split peaks on supercell satellite peaks.
[1] S. Kaneko et. al. Phys. Rev. B 74 054503 (2006)
[2] S. Kaneko et. al. Appl. Phys. Lett. 85 2301 (2004)
9:00 PM - E17.47
Coupling of Atomistic and Mesoscopic Schemes Towards the Visualization of Biopolymer Translocation through Narrow Pores.
Maria Fyta 1 , Simone Melchionna 2 , Massimo Bernaschi 3 , Efthimios Kaxiras 1 , Sauro Succi 3
1 Physics and School of Engineering and Applied Sciences, Harvard Univeristy, Cambridge, Massachusetts, United States, 2 Physics - INFM-SOFT, University of Rome, La Sapienza, Rome Italy, 3 Istituto Applicazioni Calcolo, CNR, Rome Italy
Show Abstract9:00 PM - E17.48
Ni Clusters on Pristine and Defective Carbon Nanotubes.
Jung Woo Lee 1 , Yoon Jeong Choi 1 , Hyung Seok Kim 1 , Jeung Ku Kang 1
1 Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of)
Show AbstractNi clustering inclinations on the single-walled carbon nanotubes (CNTs) have been investigated by density functional theory calculations. The adsorption sites, binding energies, and cohesive energies of Ni clusters on both pristine and defective (5, 5) CNTs is demonstrated. For the pristine CNT, the slight difference between binding energies and cohesive energies shows the weak interaction of Ni clusters and CNT surface. However, Ni clusters are adsorbed with relatively high binding energies on the defective CNT. In this respect, the introduction of defect sites could enhance Ni binding as nucleation sites and pristine CNTs seem to be more difficult to disperse transition metal on the surface than defective CNTs. These results could help fundamental understanding of transition metal adsorption on CNTs systemically and suggests the fabrication of nanosized clusters on CNTs for hydrogen storage medium, nanoelectronics, and fuel cells.
9:00 PM - E17.49
Effects of Energy Dispersion of Incident Atoms on the Atomic Structure of ta-C Films : Molecular Dynamics Study.
Kyung Soo Kim 1 2 , Seung-Cheol Lee 1 , Kwang-Ryeol Lee 1 , Pil-Ryung Cha 2
1 Computational Science Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of)
Show Abstract9:00 PM - E17.5
A Model for Fission Track Formation in Fluorapatite Considering Radiation Induced Decomposition.
Weixing Li 1 , Kundar Li 2 , Kai Sun 1 , Lumin Wang 2 1 3 , Rod Ewing 3 1 2
1 Department of Material Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan, United States, 3 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractFission tracks in fluorapatite have been used extensively for mineral dating. Our experiments with advanced transmission electron microscopy (TEM) have suggested that the fission track in fluorapatite contains a nano-scale “hollow” core that might be filled with several gaseous phases instead of an amorphous core as commonly assumed. Based on this observation, we propose a model for fission track formation that involves radiation induced decomposition of the volatile element rich materials under high energy ionizing radiation. The change of energy deposition along the track trajectory has been considered in our model that is in contrast with the Mozumder model. Our calculation based on the new model has shown that the decomposition energy for the sublimation of volatile elements has a significant influence on the track radius. The initial radius of the fission track induced by two fission fragments of 235U fission fragments, i.e. Sr of 97.35 MeV and Xe of 72.5 MeV, has been calculated to be 35.8 Å, and 37.9 Å, respectively that agree well with the TEM observation. With the consideration of changes in the energy deposition along the penetration depth of the fission fragments, our numerical results can then explain why the track radii do have small changes along the track trajectory in the TEM observation.
9:00 PM - E17.51
Random Circuit Breaker Model for the Resistance Switching: Its Applications to the RRAM.
J. Lee 1 , S. Chae 1 2 , T. Noh 1 2 , B. Kahng 1
1 Dept. of Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of), 2 ReCOE, Seoul National University, Seoul Korea (the Republic of)
Show Abstract9:00 PM - E17.6
Propagation of Instability in Dielectric Elastomers.
Jinxiong Zhou 1 2 , Wei Hong 1 , Xuanhe Zhao 1 , Zhigang Suo 1
1 SEAS, Harvard University, Cambridge, Massachusetts, United States, 2 School of Aerospace, Xi’an Jiaotong University, Xi'an, Shaanxi, China
Show AbstractWhen an electric voltage is applied across the thickness of a thin layer of an dielectric elastomer, the layer reduces its thickness and expands its area. This electrically induced deformation can be rapid and large, and is potentially useful as soft actuators in diverse technologies. Recent experimental and theoretical studies have shown that, when the voltage exceeds some critical value, the homogeneous deformation of the layer becomes unstable, and the layer deforms into a mixture of thin and thick regions. Subsequently, as more electric charge is applied, the thin regions enlarge at the expense of the thick regions. On the basis of a recently formulated nonlinear field theory, we develop a meshfree method to simulate numerically this instability.
9:00 PM - E17.7
Modelling and Simulation of the Flow-induced and Meniscus-confined Colloial Crystal Self-assembly.
Qin Li 1 2 , Hongfei Fang 2 , Ulrich Jonas 1
1 , Max Planck Institute for Polymer Research, Mainz Germany, 2 Chemical Engineering, Curtin University of Technology, Perth, Western Australia, Australia
Show AbstractColloidal particles with a narrow size distribution are known to be able to self-assemble into highly ordered structures under both equilibrium and non-equilibrium conditions. Convective self-assembly has been extensively studied experimentally, but a detailed understanding of the underlying mechanisms is still lacking, even though theoretical analysis has provided some rationales. In this study, a simplified model based on the discrete element method is proposed to track particle motions. Simulations are carried out to study this flow-induced, meniscus-confined colloidal self-assembly and elucidate the structure formation mechanisms in more detail. Various contributions, such as hydrodynamic, electrostatic, van der Waals, Brownian motions, and contact mechanic forces are taken into account in the calculation.
9:00 PM - E17.8
Crystal-Melt Interfaces in a Hard Sphere Colloidal System.
Ingo Ramsteiner 1 , David Weitz 1 , Frans Spaepen 1
1 SEAS, Harvard University, Cambridge, Massachusetts, United States
Show AbstractCrystal-Melt Interfaces in a Hard Sphere Colloidal SystemInterfaces between a crystal and its melt are of great importance in physics, chemistry and materials science. Being buried between two dense phases, the atomic structure of the interface eludes real-time in-situ experiments. This has motivated a large number of simulation studies in recent years, focusing mainly on the interfacial free energy IFE). The challenge is that this parameter has to be determined with high accuracy, since the shape and growth direction of dendrites depends crucially on very small (typically 1% in metallic systems) anisotropies. While there are several established methods to extract the IFE from atomic scale data, the capillary fluctuation method has been the most successful one for tracking subtle anisotropies of this quantity. Several groups have recently applied the method to MC [1] and MD [2] simulations of the hard sphere system.We use the same approach on data obtained from a colloidal dispersion of silica particles. Large single crystals are grown by sedimentation on lithographically patterned templates. Compared to atomic systems, the interface fluctuations between the crystalline and fluid phases occur on convenient time and length scales and can be imaged in three dimensions with confocal microscopy. The limitations of system size are less severe than for computer simulations. [1] Mu et al., J.Phys.Chem.B 109 (2005) 6500 [2] Davidchack et al., J.Chem.Phys.125 (2006) 94710
9:00 PM - E17.9
Hydrogen Storage in MgH2 Matrices: A Comparative Study of Mg-MgH2 Interfaces.
Simone Giusepponi 1 , Massimo Celino 1 , Amelia Montone 1 , Fabrizio Cleri 2
1 Materials Science Department, ENEA, Rome Italy, 2 , IEMN, Villeneuve d'Ascq France
Show AbstractThe remarkable hydrogen capacity of magnesium has fostered intense research efforts in the last years, in view of possible its future applications, for which a where light and safe hydrogen-storage media will be are needed.Magnesium can reversibly store about 7.7 wt% hydrogen, and it is both a has light weight and is a low-cost material. However, further research is still needed since pure metallic Mg has a high operation temperature and slow absorption kinetics, that preventing at present its for the moment the use in practical applications.Further Insights in the Mg H-storage mechanisms can be are gained by the atomic-scale study of characterizing and comparing the Mg-MgH2 interfaces, which are supposed to play a major role in the hydrogen diffusion during the absorption and desorption cycles. From an experimental point of view, there is not yet a clear evidence of about which interfaces are involved in the hydrogen diffusion, and which is the atomic dynamics at the interfaces.In this work we studied representative Mg-MgH2 interfaces by means of accurate ab-initio molecular dynamicssimulations based on the density-functional theory with norm-conserving pseudopotentials and plane-wave expansion of the electronic wavefunction some interfaces are reproduce and studied.Extensive electronic structure calculations are used were carried out, in order to characterize the interfacial equilibrium properties, and the behavior of the interfaces in terms of total energy considerations and atomic diffusion.Surfaces with low Miller indexes and good compatibility with a minimal lattice mismatch at the Mg-MgH2 interface are chosen and compared. For the MgH2 [110] surface well agree, in terms of atomic Mg-Mg distance can be made commensurate with the [010] and [1-10] surfaces of Mg, with only a minor residual interfacial strain. After ionic relaxation, the surfaces are free to adapt each other, revealing a modest displacement of one surfaces with respect to the other, and a significant displacement of hydrogen atoms toward the interfaces. The interfacial work of adhesion and interfacial stress of the interfaces was also characterized, in order to find the thermodynamically stable interfaces. Interfacial H-diffusion barriers were obtained along several diffusion paths, to gain insight into the interfacial H kinetics.
E18: Dislocations and Plasticity I
Session Chairs
Friday AM, November 30, 2007
Constitution B (Sheraton)
9:30 AM - **E18.1
Magneto-Elastic High-Temperature Instability of Dislocation Loops in BCC Iron.
Sergei Dudarev 1 , Ron Bullough 1 , Peter Derlet 1
1 Theory and Modelling, UKAEA, Oxfordshire United Kingdom
Show AbstractMagnetic fluctuations at high temperatures erode the stability of the bcc phase of iron, giving rise to the gradual softening of one of the 110 transverse acoustic phonon modes and to the α-γ bcc-fcc phase transition occurring at 912C at normal pressure. The elastic moduli of bcc iron strongly vary as a function of temperature with C’=(C11-C12)/2 decreasing to zero as we approach the α-γ transition point. We show that this decrease has a significant effect on the intrinsic structural stability of dislocations present in such magnetic material. At room temperatures and above, bcc iron is characterised by strong elastic anisotropy and thus the evaluation of accurate self-energies of dislocations in α – iron requires the use of the full anisotropic approximation. Our analysis shows that the elastic self-energies of straight edge dislocations strongly depend on temperature, and the energies of both the 100 [001] and the 111[11-2] edge dislocations sharply decrease as we approach the α-γ transition. The elastic self-energy of the 111 [1-10] edge dislocations on the other hand remains almost independent of temperature up to the point of α-γ transition. In addition, using atomistic simulations and experimental information, we also evaluate the core energies of dislocations, and show that it is likely that competition between temperature-dependent (anisotropic) elastic self-energies of various types of dislocations is responsible for the frequent occurrence of the 100 type interstitial edge dislocation loops in irradiated ferromagnetic bcc iron. This work was supported by the UK Engineering and Physical Sciences Research Council, by EURATOM, and by EXTREMAT integrated project under contract number NMP3-CT-2004-500253.
10:00 AM - E18.2
Electronic Structure Calculations at Macroscopic Scales -- Studies on Prismatic Loop Nucleation.
Vikram Gavini 1 , Kaushik Bhattacharya 1 , Michael Ortiz 1
1 Engineering and Applied Science, California Institute of Technology, Pasadena, California, United States
Show AbstractDensity-functional theory has provided insights into various materials properties in the recent decade. However, its computational complexity has made other aspects, especially those involving defects, beyond reach. Here, we present a seamless coarse-graining scheme for orbital-free density-functional theory (OFDFT), namely quasi-continuum orbital-free density-functional theory (QC-OFDFT) that enables the study of multi-million atom clusters with no spurious physics and at no significant loss of accuracy. The key ideas are (i) a real-space formulation of OFDFT, (ii) a nested finite-element implementation of the formulation, and (iii) a systematic means of adaptive coarse-graining retaining full resolution where necessary and coarsening elsewhere with no patches, assumptions or structure. Fully-resolved OFDFT and finite lattice-elasticity are obtained as special limits of this scheme.This methodological development has enabled OFDFT calculations on large systems, which have revealed interesting physics and have provided new insights into the mechanism of prismatic dislocation loop nucleation --- a topic of great importance in radiation damage in materials. This study, for the first time using electronic structure calculations, establishes vacancy clustering and collapse of these clusters as a possible mechanism for prismatic dislocation loop nucleation. Studies also suggests that prismatic loops as small as those formed from a 7-vacancy cluster are stable, thus shedding new light on the nucleation size of these defects which was hitherto unknown.
10:15 AM - E18.3
Thermally-activated Glide of Dislocations: Atomistic Simulations with Flexible Boundary Conditions.
David Rodney 1
1 SIMAP, INP Grenoble, Saint Martin d Heres France
Show AbstractThe thermally-activated glide of dislocations below their Peierls stress is difficult to simulate at the atomic-scale because when the stress or the temperature decrease, the waiting time between kink-pair nucleation events become rapidly longer than the simulated time achievable by Molecular Dynamics (MD). We analyze here MD simulations at constant strain-rate of the glide of dislocations with the aim to extract the kink-pair activation enthalpy as a function of stress. First, we propose flexible boundary conditions in replacement of the usual rigid boundary conditions because the latter induce spurious forces on the dislocations due to the mismatch between the elastic strain imposed by the rigid conditions and the plastic strain of the dislocation motion. We then present a statistical analysis to rigorously extract enthalpy-stress relations from dynamical simulations. The data thus obtained are then compared to static Nudged Elastic Band method calculations on the same computational model. The good agreement between the dynamical and static results shows that, despite its complexity, the dislocation glide process is well captured by a simple thermally-activated kink-pair formation law.
10:30 AM - E18.4
Modelling Dislocation Climb in Dislocation Dynamics Simulations.
Dan Mordehai 1 , Emmanuel Clouet 1 2 , Marc Fivel 3
1 SRMP, CEA/Saclay, Gif sur Yvette France, 2 LMPGM, Université Lille, Lille France, 3 GPM2, ENSPG, BP 46, 38402 St. Martin d'Hères France
Show AbstractDislocation Dynamics simulations (DD) are one of the computational methods to study dynamic collective evolution of dislocations in a solid under an external loading. In this method, dislocations are considered as entities and the interaction with an external loading and between them is treated according to elasticity theory. I will present a model to couple DD simulations with diffusion theory of vacancies, which allows us to incorporate the diffusional climb of dislocations within the conventional DD method. In this work we make use of a 3-dimensional Discrete Dislocation Dynamics (DDD) simulation, in which each dislocation is represented by pure edge and screw dislocation segments. The DDD was used to study the activation of Bardeen-Herring climb dislocation sources upon the application of an external stress or under superconcentration of vacancies, as well as loop shrinkage and expansion due to vacancies emission or absorption. Our calculations of the loop shrinkage rate and its temperature dependency agree with experimental observations. Additionally, the evolution in the population of dislocation loops was studied in this method, and the expansion of large loops, on the expense of small ones, was observed.
10:45 AM - E18.5
Modeling and Experiments Relating to the Initiation Sensitivity of RDX.
Marc Cawkwell 1 , Thomas Sewell 1 , Kyle Ramos 2 , Daniel Hooks 2
1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Dynamic and Energetic Materials Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe complex interplay between shock-induced plastic deformation and the initiation sensitivity of the secondary explosive RDX has been studied using large-scale molecular dynamics (MD) simulations and flyer plate experiments. The steric-hindrance model for initiation sensitivity, originally developed for the secondary explosive PETN, proposes that initiation sensitivity is greatest when the shock propagation direction is oriented such that there is no resolved shear stress on any dislocation slip system. Since dislocation mediated plasticity is not able to relax shear stresses in these special orientations, molecules become entangled leading to the initial chemical events in molecular decomposition and detonation. In the original work on PETN, experiments performed under quasi-static conditions were used to determine the slip systems of that material. Recent experimental work has shown that the steric-hindrance model exhibits poor transferability to RDX and HMX when the slip systems determined under quasi-static conditions are utilized. MD simulations of the response of oriented RDX single crystals to shock loading performed using an accurate and transferable model for inter and intra molecular bonding have shown a variety of material responses that could not have been extrapolated from those seen under quasi-static conditions. For example, shocks oriented normal to (100), (111), and (010) exhibit shear bands, the homogeneous nucleation of partial dislocation loops, and a phase transformation, respectively. Hence, while the steric-hindrance model is fundamentally sound, for a predictive capability it must be based upon slip systems determined under shock loading conditions. This work highlights both the multi-scale and multi-physics phenomena associated with initiation sensitivity of high-energy materials.
E19: Dislocations and Plasticity II
Session Chairs
Friday PM, November 30, 2007
Constitution B (Sheraton)
11:30 AM - E19.1
Modeling Hydrogen Embrittlement of Metals: A Combined Discrete Crystal Plasticity and Kinetic Monte Carlo Approach.
Ashwin Ramasubramaniam 1 , Mitsuhiro Itakura 2 , Weinan Ee 3 , Michael Ortiz 4 , Emily Carter 5
1 Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey, United States, 2 , Japan Atomic Energy Research Institute, Tokyo Japan, 3 Department of Mathematics and Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey, United States, 4 Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, California, United States, 5 Department of Mechanical and Aerospace Engineering and Program in Applied and Computational Mathematics, Princeton University, Princeton, New Jersey, United States
Show AbstractThe problem of hydrogen diffusion in metals and its implications for stress corrosion cracking are studied from an atomistic perspective. Long-range diffusion of hydrogen within the crystal is modeled using on-the-fly kinetic Monte Carlo (KMC) calculations. Density functional theory (DFT) is used to establish the relevant diffusion pathways within the lattice; additionally, DFT information can also be used to provide a priori rates, thereby providing computational speed-up in regions of small to moderate strains. The host crystal itself is modeled using discrete crystal plasticity. This approach treats dislocations as energy minimizing structures that lead to locally lattice-invariant but globally incompatible eigendeformations. As a first approximation, atomic interactions are treated by a harmonic, albeit non-convex, potential. Results are presented that demonstrate the ability of this model to capture long-range hydrogen diffusion, assisted by stress gradients, over extended periods of time. The model is also shown to capture other relevant phenomena such as trapping of hydrogen at dislocation cores, and segregation to free surfaces, among others. Current limitations and proposed extensions of this work are noted.
11:45 AM - E19.2
Precipitate Shape and Coherency Loss Mechanisms in Au-Rh Alloys.
Peihua Jing 1 , Hyon-Jee Lee 1 , Jae-Hyeok Shim 2 , I. Robertson 3 , Brian Wirth 1
1 Department of Nuclear Engineering, University of California, Berkeley, Berkeley, California, United States, 2 , Korea Institute of Science and Technology, Seoul Korea (the Republic of), 3 Department of Materials Science, University of Illinois, Urbana-Champaign, Illinois, United States
Show AbstractIn precipitate hardened materials, the strength and creep properties are controlled by dislocation interactions, which depend on the precipitate interfacial structure. We present the results of atomistic simulations specifically designed to investigate the precipitate coherency loss and dislocation bypass mechanisms in a Au-Rh alloy with a large (~6%) lattice misfit. Embedded atom method interatomic potentials of the Sutton-Chen type have been fit to the elastic properties of Au and Rh, and ab-initio calculations of the Au-Rh mixing enthalpy and interface energies. The molecular dynamics simulations indicate that coherency loss occurs for precipitates with diameter of about 2 nm on both sides of the phase diagram, although the critical size is sensitive to the stress state and presence of nearby dislocations. Coherency loss of spherical precipitates results in precipitate/matrix structures that possess an octahedral structure composed of high-energy a/3<100> stair rod dislocations and triangular stacking fault facets in the surrounding matrix. At larger sizes, segments of Shockley partial dislocations nucleate from the intersection of the stair-rod dislocations. The interaction and detachment mechanism between gliding dislocations and the non-coherent precipitates are controlled by the interactions between the dissociated dislocation and the interfacial dislocations. Where possible, the simulation results are compared to in-situ TEM deformation studies.
12:00 PM - E19.3
Multi-paradigm Modeling of Chemistry in Mechanical Deformation of Metals.
Dipanjan Sen 1 2 , Markus Buehler 1
1 Laboratory for Atomistic and Molecular Mechanics, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractModeling and prediction of the effect of environmentally embrittling species like water or hydrogen and organic oxidative chemicals on the deformation mechanisms in metals remains a critical challenge in materials modeling. Here we propose a concurrent combination of the first principles based reactive force field, ReaxFF and the embedded atom method (EAM) in a generic multiscale modeling framework, the Computational Materials Design Facility (CMDF) that enables the treatment of large reactive systems within a classical molecular dynamics framework. Our hybrid method is based on coupling multiple Hamiltonians by weighting functions that allow accurate modeling of chemically active sites with the reactive force field, while the other parts of the system are described by the computationally less expensive EAM potential. We validate the mixed Hamiltonian approach between the ReaxFF and EAM potentials by studying propagation behavior of elastic shock waves and dislocations through a single crystal of nickel whose domain is decomposed into ReaxFF and EAM regions, and showing that the handshaking of the different methods doesn’t induce discontinuities in the energy landscape. We apply our hybrid modeling scheme in a study of fracture of a nickel single crystal under the presence of oxygen molecules, where thousands of reactive atoms are used to model the chemical reactions that occur under the presence of oxygen. We observe that the oxide formed on the crack surface produces numerous defects surrounding the crack, including dislocations, grain boundaries and point defects. We show that the mode of crack propagation changes from brittle crack opening at crack tip to void formation ahead of crack and void coalescence for {111}<11-2> orientation of the crack. Our results illustrate the significance of considering oxidative processes in studying deformation of metals, an aspect largely neglected in most modeling work carried out with pure EAM potentials. Our hybrid method allows the study of multiphysics of mechanical deformation in the presence of chemical reactions and constitutes an alternative to existing methods that are based on coupling quantum mechanical methods such as DFT to empirical potentials.
12:15 PM - E19.4
Atomistic Simulations of Tribology at Sliding MoS2 Surfaces.
Tao Liang 1 , Wallace Sawyer 2 , Scott Perry 1 , Simon Phillpot 1 , Susan Sinnott 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractMolybdenum disulphide (MoS2) is the most commonly used solid lubricant coating in aerospace applications. In this work, we examine nano-scale friction between sliding MoS2 surfaces using both density functional theory (DFT) calculations and classical molecular dynamics (MD) simulations using newly developed, empirical many-body potentials. In particular, we use the static potential energy surface from the DFT calculations to estimate the friction energy and the force that must be applied in order to slide a layer of MoS2. In addition, MD simulations of MoS2 interfacial sliding at various loads and temperatures were carried out. The loads and friction forces were extracted to calculate the friction coefficient of the MoS2 as a function of temperature, and the results are compared to experimental pin-on-disk measurements of MoS2 coatings and AFM measurements on single crystal MoS2 surfaces. The results from both the DFT calculations and the MD simulations help us to better understand the origins of the thermo-lubricity that is commonly observed in MoS2 at elevated temperatures. The authors gratefully acknowledge the support of an AFOSR-MURI grant FA9550-04-1-0367.
E20: Interface Behavior
Session Chairs
Friday PM, November 30, 2007
Constitution B (Sheraton)
2:30 PM - E20.1
Off-lattice Grand Canonical Monte Carlo Simulations of Incoherent Heterophase Interfaces.
Michael Demkowicz 1 , Richard Hoagland 1
1 MST-8, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractGrand canonical lattice Monte Carlo simulations have been successfully used before to study coherent interfaces at elevated temperatures. Incoherent interfaces like those seen in Cu-Nb composites, however, exhibit a large variety of complex atomic configurations that can only be accurately described using an off-lattice approach. Unfortunately, the large energy barriers associated with inserting and removing atoms in such high-density solid systems have made efficient off-lattice grand canonical Monte Carlo simulations infeasible. We present a method for circumventing this difficulty by adapting a technique originally developed for simulating dense liquids in which atoms are allowed to exist in a chain of “intermediate” states. Successive intermediate states of a given atom are characterized by increasing strengths of energetic coupling of that atom to its surroundings, ranging from zero (no coupling) to unity (full coupling). Inserting or removing particles is made manageable by spreading out the associated energy penalty over a succession of such intermediate states. This method is applied to determining the equilibrium structure and point defect sink strength of incoherent Cu-Nb interfaces at elevated temperature, modeled using an EAM potential. The high-temperature interface properties are related to the radiation damage resistance of Cu-Nb multilayer composites. This work was supported by the Los Alamos National Laboratory Directed Research and Development Program (LDRD) and the Los Alamos National Laboratory Director’s Fellowship Program.
2:45 PM - E20.2
Interface Energy Trends for Semi-coherent Fe(001)/MX(001) Systems using Ab-initio Calculations.
Dan Fors 1 , Goran Wahnstrom 1
1 , Chalmers University of Technology, Göteborg Sweden
Show AbstractTransition metal (M) carbides and nitrides (X) constitute common equilibrium phases in steels. Their structures range from the more complex M23C6 structure down to the simpler MX nacl structure. The fine dispersion of these precipitates is believed to have a highly beneficial effect on the creep resistance of steels by preventing dislocation movements. The MX nacl structure precipitates are usually found as small thin discs, where the flat part has a semi-coherent interface behavior with respect to the iron matrix whereas the side has large lattice misfit and tends to be an incoherent interface. Using ab initio calculations we investigate interface energies and structures for the semi-coherent Fe(001)/MX(001) interface systems. We apply a continuum approach using the Peierls-Nabarro model in order to account for the elastic displacements arising from the lattice misfit and the periodic misfit dislocations in the interface. The chemical part of the interface energy is obtained by using density functional theory calculations. The resulting γ-surfaces show decreasing trends along the 3d, 4d and 5d element rows. We also find decreasing trends within the Ti and V group for Fe/MN, while the corresponding trend for Fe/MC is missing. The γ-surfaces for the Fe/MC-interfaces also tends to be higher than the Fe/MN-interfaces. The trends and differences are explored using projected density of states and charge density analysis. We also find that the Fe/MX-interfaces have large misfits, which cause that many atoms will be unfavorable positioned and therefore the elastic energy will constitute a significant part of the interface energy.
3:00 PM - E20.3
Phase Field Modeling of the Electrochemical Interface.
Jonathan Guyer 1 , David Saylor 3 , James Warren 1 , William Boettinger 1 , Geoffrey McFadden 2
1 Metallurgy Division, NIST, Gaithersburg, Maryland, United States, 3 Division of Chemistry and Materials Science, FDA, Silver Spring, Maryland, United States, 2 Mathematical and Computational Sciences Division, NIST, Gaithersburg, Maryland, United States
Show AbstractWe previously developed a phase field model of the electrochemical interface [Phys. Rev. E 69, 021603 & 021604 (2004)] and demonstrated that a simple set of assumptions gives rise to a rich set of behaviors, includingelectrocapillary phenomena, differential capacitance curves that resemble experimental measurements, and non-linear kinetics consistent with the empirical Butler-Volmer relation. Despite these successes, numerical constraints limited the applicability of the model to a few nanometers in one dimension. These constraints stem from two sources: a very small length scale (the Debye length) associated with charge separation distance at the electrode-electrolyte interface and very steep gradients in concentration at that interface. The need to resolve both very small distances and very large differences in concentrations placed severe limits on the time steps that could be taken, even with implicit techniques.We have recently applied a variety of numerical techniques to this problem, including moving meshes and different discretizations, that have resulted in simulating larger domains, in higher dimensions, than previously achievable,while retaining the essential physics of the electrochemical interface. We have been able to increase the maximum stable time step by several orders ofmagnitude.We will present the approaches we have taken to make this problem tractable, as well as the results of simulations for a variety of applications, including interconnect fabrication in microelectronics and antibiotic metal nanoparticles.
3:15 PM - E20.4
A Local Semi-Implicit Level-Set Method for Complex Interface Motion under Multiple Energetic Forces.
David Salac 1 , Wei Lu 1
1 Mechanical Engineering, University of Michigan, Ann Arbor, Ann Arbor, Michigan, United States
Show Abstract3:30 PM - E20.5
Interfaces of High Dielectric Constant Gate Oxides and Metals.
John Robertson 1 , Koon yiu Tse 1
1 Engineering, Cambridge University, Cambridge United Kingdom
Show Abstract3:45 PM - E20.6
Atomic Adsorption on Transition-metal Carbides and Nitrides.
Aleksandra Vojvodic 1 , Anders Hellman 1 , Carlo Ruberto 1 , Bengt Lundqvist 1
1 Department of Applied Physics, Chalmers University of Technology, Göteborg Sweden
Show AbstractOn the reactive TiX(111) (X = C, N) surfaces earlier density-functional theory calculations have shown pyramid-like trends in the atomic adsorption energies. These have been accounted for qualitatively within a concerted-coupling model (CCM), in which the adatom interacts with both (i) a Ti3d-derived localized surface resonance and (ii) a number of X2p-derived second-layer surface resonances [1-4]. Here the calculations are extended to a number of 3d-, 4d- and 5d-period early transition-metal carbides and nitrides (TMX's). Characteristic variations in the adsorption energy both with respect to the adsorbate and to the substrate are found. Furthermore, the correlation between the adsorption strength and the electronic structure of the TMX's under investigation is analyzed in terms of an extended CCM [5]. 1. C. Ruberto and B. I. Lundqvist, Physical Review B (in press). 2. A. Vojvodic, C. Ruberto, and B. I. Lundqvist, Surface Science, 600, 1612 (2006). 3. C. Ruberto, A. Vojvodic, and B. I. Lundqvist, Surface Science, 600, 3619 (2006). 4. C. Ruberto, A. Vojvodic, and B. I. Lundqvist, Solid State Communications, 141, 48 (2007). 5. A. Vojvodic, A. Hellman, C. Ruberto, and B. I. Lundqvist, (in preparation).
4:00 PM - E20.7
Gibson Soil is Flaw Tolerant.
Haimin Yao 1 , Huajian Gao 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractThe adhesion strength between two bonded solids is usually weakened by the interfacial flaws arising from the surface roughness, impurities, contaminants and trapped air bubbles, etc. How to suppress the impacts of the interfacial flaws on the adhesion strength becomes a crucial problem frequently encountered in the design of strong adhesion. The optimal scenario that one can expect is to achieve the flaw tolerance state, in which a pre-existing crack does not propagate even as the material is stretched to failure near the theoretical adhesion strength. Here, we demonstrate that the Gibson soil, an incompressible material with linearly graded elastic modulus, can be designed to suppress the growth of crack-like flaws of any length scales. For the more general case involving compressible materials, an asymptotic condition for flaw tolerant adhesion is proposed. These results provide a theoretical foundation for the novel applications of graded materials in engineering.
4:15 PM - E20.8
Mechanical Properties and Impurity Effects of Metallic Grain Boundaries by First-principles Analysis Using Energy and Stress Densities.
Ruzhi Wang 1 , Shingo Tanaka 1 , Masanori Kohyama 1 , Guang-Hong Lu 2 , Tomoyuki Tamura 3 , Shoji Ishibashi 3
1 Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka, Japan, 2 School of Science, Beijing University of Aeronautics and Astronautics, Beijing China, 3 Research Institute for Computational Sciences, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan
Show AbstractGrain boundaries have serious effects on the mechanical properties of metallic materials. Grain boundaries act as barriers for dislocation transmission as well as sinks or sources of dislocations. These points are greatly featured in nano-structures formed by severe plastic deformation [1]. The mechanical properties of grain boundaries also depend on the kinds and concentration of impurities, and especially impurity-induced grain-boundary embrittlement has been a long-standing unsolved problem. This is dominated by multi-physics with different length scales and principles, such as impurity segregation or precipitation, impurity-metal or interfacial bonds, electronic-structure changes, strength and deformation processes of interfaces with and without impurities, and so on. In order to understand the mechanical properties of metallic grain boundaries and the effects of impurities, recent first-principles tensile-testing simulations have made valuable contributions [2]. On the other hand, from the view points of multi-physics, it is desirable to develop new theoretical schemes to deal with both the electronic and mechanical properties more effectively and directly. In this paper, we perform the first-principles analysis using local energy density [3] and local stress density [4] implemented in our original code QMAS [5] for the projector-augmented wave (PAW) method. For coincidence boundaries in Al and Cu with and without impurities such as Si and Mg, we clarify the local energy and local stress distributions in the supercell, which provide great insights into the effects of the structural disorder and the impurities on the stability and mechanical properties of grain boundaries in the framework of the density-functional theory. This work was partly supported by Monbusho Grant (No.18062007). [1] Z. Horita et al., Adv. Mater. 17 (2005) 1599, [2] G.H. Lu et al., Phys. Rev. B 73 (2006) 224115, [3] N. Chetty and R.M. Martin, Phys. Rev. B 45 (1992) 6074, [4] A. Filippetti and V. Fiorentini, Phys. Rev. B 61 (2000) 8433, [5] S. Ishibashi et al., unpublished.
E21: Thermodynamics and Phase Transitions
Session Chairs
Friday PM, November 30, 2007
Constitution B (Sheraton)
4:30 PM - E21.1
A General Examination for the Accuracy for the Supercell Method in Calculating Thermodynamic Properties.
Yi Wang 1 , Hui Zhang 1 , Shunli Shang 1 , Long-Qing Chen 1 , Zi-Kui Liu 1 , Chris Wolverton 2
1 Materials Science & Engineering, Penn State, University Park, Pennsylvania, United States, 2 Materials Science & Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractThe aim of this work is to provide a systematic study on the accuracy of supercell method. We address the convergence of the calculated data against the supercell size. The linear-response calculations for selected cases are also engaged for supporting the results calculated by the supercell method. The examples include the pure elements Al, Cu, Mg, Si, Mo, Ta, and Ni and compounds β-NiAl, γ’-Ni3Al, θ’-Al2Cu, θ-Al2Cu, β”-Mg5Si6, and β-Mg2Si. Totally with the 40 supercell phonon calculations, we have demonstrated that the supercell method can reach the same accuracy as the linear-response method for most of the important thermodynamic properties as long as the employed supercell is large enough and yet within an affordable computational cost.
4:45 PM - E21.2
Entropy Driven Stabilization of Energetically Unstable Crystal Structures Explained from First Principles Theory.
Petros Souvatzis 1 , Olle Eriksson 1 , Mikhail Katsnelson 2 , Sven Rudin 3
1 Physics Department - Theoretical Magnetism, Uppsala University, Uppsala Sweden, 2 Institute for Molecules and Materials, Radboud Univrsity, Nijmegen Netherlands, 3 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show Abstract5:00 PM - E21.3
Oxygen Pressure Dependence of HfO2 Stoichiometry: An ab-initio Thermodynamic Investigation.
Chunguang Tang 1 , Ramamurthy Ramprasad 1
1 , university of connecticut, Storrs, Connecticut, United States
Show Abstract The desire for continued miniaturization of microelectronic devices has spurred considerable research on high dielectric constant (or high-k) materials such as HfO2 and ZrO2 as prospective substitutes for conventional SiO2 gate dielectrics. However, the thermodynamic stability of these high-k materials in contact with Si is reduced by the presence and mobility of oxygen vacancies and interstitials in the oxides. It has been observed that undesirable interfacial phases such as Hf silicides, Hf silicates, and silica can form between the oxide and the Si substrate. An understanding of the relationship between the stoichiometry of the oxides and external factors will be valuable to the control of the concentration of oxygen defects, and consequently in the elimination of the interfacial phases. In this work, the oxygen pressure and temperature dependence of the formation of excess oxygen vacancies and interstitials in monoclinic HfO2 was investigated by performing first principles and thermodynamic calculations. The HfO2 stoichiometry was controlled by considering a range of oxygen defect concentrations in supercells of various sizes. The critical oxygen pressure that would result in the spontaneous creation of vacancies or interstitials of a specific concentration at a given temperature was determined by introducing the oxygen chemical potential. Upper and lower critical oxygen pressures are identified that heavily favor the formation of oxygen interstitials and vacancies, respectively. The ratio of these critical pressures can be specified unambiguously as the sum of the formation energies of O vacancies and interstitials at 0 K obtained from first principles calculations. Our results indicate that processing atmosphere control is a key factor for modulating the HfO2 stoichiometry and the chemistry of Si:HfO2 interfaces.
5:15 PM - E21.4
Surface and Particle-size Effects on the Thermodynamics of LiFePO4 from First-principles Simulations.
Fei Zhou 1 , Lei Wang 1 , Gerbrand Ceder 1
1 Dept. of materials science and engieering, MIT, Cambridge, Massachusetts, United States
Show AbstractIn recent years much research effort has been invested to improve the electrochemical performance of LiFePO4 as the cathode material in rechargeable li-ion batteries, especially through control of its particle size. Experiments indicate that the miscibility gap in LixFePO4 change systematically with particle size in the nanoscale regime and with temperature at a constant particle size, although the origin of such a shift is not completely clear. Changes in the solubility limits between FePO4 and LiFePO4 are determined by the thermodynamics of both the ionic and electronic degrees of freedom. Experimental and theoretical works point towards strong coupling between the electron/hole and lithium/vacancy charge carriers, which affects both the transport properties as well as the solubility limits. Recently we have successfully explained the unexpected phase diagram of LixFePO4 revealed by experiments with the picture of coupled charge carriers, and found that electronic entropy is the dominant factor in the existence of a eutectoid transition at elevated temperature. In the current work, we attempt to use the same ideas of charge carrier coupling to explain how the materials properties are modified by finite size.We calculated surface energies from five surfaces that appear in the Wulff shape of LiFePO4. We find in general that surfaces where the Fe coordination is reduced a lot, have high energy. A reduction of Li coordination. by oxygen on the other hand has no detrimental effect on the surface energy and in several cases can be shown to have a stabilizing effect on the surface. The constructed Wulff shape with the calculated energies is dominated by two low-energy surfaces, (0 1 0) and (2 0 1) facets, which make up almost 85% of the surface area. Similar calculations of surface energies for FePO4 indicate a very low energy for the (0 1 0) surface of FePO4. Surface redox potentials for the extraction/insertion of Li from various surfaces are also calculated. Interestingly, the Li redox potentials at surfaces vary significantly from those in the bulk, but also strongly vary from surface to surface. We will demonstrate how this plays a role in the thermodynamics and kinetics of lithiation and delithiation of the material.We concentrate on the (0 1 0) surface and perform Monte Carlo simulations on the LixFePO4 system at different finite particle sizes. We will also discuss the impact of surface and particle size on the thermodynamic properties.
5:30 PM - E21.5
Phase-Transformation Stability in Coherent Multiphase Lithium-Iron-Phosphate Particles.
Hsiao-Ying Huang 1 , Nonglak Meethong 1 , Ming Tang 1 , Yet-Ming Chiang 1 , W. Carter 1
1 Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractLithium-iron-phosphate (LFP) represents a promising cathode material for use in high capacity lithium ion batteries. In LFP nanoparticles, elastic misfit-strain energy occurs due to the volume mismatch between the lithiated and delithiated phases. The magnitude of this elastic strain energy depends upon factors such as phase transformation morphologies and concentration-dependent equilibrium molar volumes. In the current study we present models illustrating the important role of coherent phase transformations in governing the reliability of lithium-intercalating compounds such as LFP during thermoelectric cycling. The thermodynamics illustrates how elastic stresses should be incorporated in thermodynamic treatments of electrochemistry, and how the effects of cathode particle interface morphology manifest.
5:45 PM - E21.6
Simulation of Surface-enhanced Ordering in Smectic Films.
Nasser Abukhdeir 1 , Alejandro Rey 1
1 Chemical Engineering, McGill University, Montreal, Quebec, Canada
Show Abstract