E1: Diffusion I
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 Show Abstract
1 Dept of Physics, Clemson, Clemson, South Carolina, United States
We 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 Show Abstract
1 , Fusion Nuclear Institute, madrid Spain, 3 , Escuela Técnica Superior de Ingenieros, Sevilla Spain, 2 , Universidad de Alicante, Alicante Spain
The 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 Show Abstract
1 Materials Science Research Laboratory, Central Research Institute of Electric Power Industry, Komae, Tokyo, Japan
Hydride 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 Show Abstract
1 School of Engineering, The University of Newcastle, Callaghan, New South Wales, Australia
10:30 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 Show Abstract
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
The 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
Monday PM, November 26, 2007
Constitution B (Sheraton)
11:15 AM - E2.1
Quantitative Modeling of Self-Interstitial Diffusion in Silicon.
N. Modine 1 Show Abstract
1 1132, Sandia National Labs, Albuquerque, New Mexico, United States
Predictive 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 Show Abstract
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
Many 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 Show Abstract
1 AMPAC-MMAE, University of Central Florida, Orlando, Florida, United States
Solid-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 Show Abstract
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Among 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 . 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 Show Abstract
1 Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States
(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 Show Abstract
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
Magnsium 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
Monday PM, November 26, 2007
Constitution B (Sheraton)
2:30 PM - **E3.1
Nucleation and Growth of Thin Films and Minerals.
John Harding 1 Show Abstract
1 Engineering Materials, University of Sheffield, Sheffield United Kingdom
The 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  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 , thin films (in particular wurtzite structures)  and growth of metals on oxides  to illustrate how a range of simulation methods can be used to understand these phenomena M.Y. Lavrentiev, N.L. Allan, J.H. Harding, D.J. Harris and J.A. Purton, Comp. Mater. Sci. 36 (2006) 54-59 J.H. Harding and D.M. Duffy; J. Mater. Chem. 16 (2006) 1105-1112 C.L. Freeman, F. Claeyssens, N.L. Allan and J.H. Harding, Phys. Rev. Lett. 96 (2006) 066102. 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 Show Abstract
1 , Lawrence Livermore National Laboratory, Livermore, California, United States, 2 , Centro Atomico Bariloche, 8400 Bariloche Argentina
We 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 Show Abstract
1 , CEA Saclay, Gif-sur-Yvette France
The 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 Show Abstract
1 Materials, University of Oxford, Oxford United Kingdom
Microstructure 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  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  with respect to the Lifshitz-Slyozov  and Wagner  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 M. Lifshitz, V. V. Slezov., J. Phys. Chem. Solids, 19, 35 (1961). C. Z. Wagner, Electrochem. 65, 581 (1961). V. M. Burlakov, Phys. Rev. Lett. 97, 155703 (2006).
E4: Evolution of Microstructure and Properties
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 Show Abstract
1 Materials Science and Engineering, UIUC, Urbana, Illinois, United States
Current 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 Show Abstract
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
A 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 Show Abstract
1 Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
The 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 Show Abstract
1 Chemical Engineering, University of massachusetts, Amherst, Massachusetts, United States
Nanometer-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  crystallographic direction; the films studied contained voids extending throughout the thickness of the film with the void axes oriented along . 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 Show Abstract
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
5:45 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 Show Abstract
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)
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
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 Show Abstract
1 DEN/DMN, SRMP-CEA, Sacaly, Gif sur Yvette France
Ferritic 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 . 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 . 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 . C. C. Fu, F. Willaime and P. Ordejon, Phys. Rev. Lett. 92, 175503 (2004) C. C. Fu et al. Nature Mater. 4, 68 (2005) 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 Show Abstract
1 Engineering, University of Liverpool, Liverpool United Kingdom, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
10:30 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 Show Abstract
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
High 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 Show Abstract
1 MMC, EDF R&D, Moret sur Loing France, 2 Laboratoire de Métallurgie Physique et Génie des Matériaux, UMR 8517, Villeneuve d'Ascq France
E6/T6: Joint Session: Modeling Defects in Nuclear Materials II
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 Show Abstract
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
Recent 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  direction.
12:00 PM - E6.2/T6.2
Defect Structure and Stability in Uranium and Zirconium Nitrides.
Robin Grimes 1 , Eugene Kotomin 2 Show Abstract
1 materials, Imperial College London, London United Kingdom, 2 Joint Research Centre, Institute for Transuranic Elements, Karlsruhe Germany
Actinide 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 Show Abstract
1 Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, Washington, United States
Radiation 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 Show Abstract
1 , CEA-Saclay, Gif-Sur-Yvette France, 2 , Kyushu University, Fukuoka Japan, 3 , INSTN, Gif-Sur-Yvette France
Spinel 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
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 Show Abstract
1 Materials Science and Engineering, university of Illinois, Urbana, Illinois, United States
Irradiation 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 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Colorado School of Mines, Golden, Colorado, United States
Fast 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 Show Abstract
1 Department of Materials, Imperial College London, London United Kingdom
Helium 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 Show Abstract
1 , Rho Beta Sigma Affaires, Livermore, California, United States
The 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 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.. Stout, R.B.: 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 Show Abstract
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
E8/T8: Joint Session: Modeling Microstructural Evolution in Irradiated Materials II
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 Show Abstract
1 Materials Science Research Laboratory, CRIEPI, Komae, Tokyo, Japan, 2 Department of Mechanical Engineering, Tokyo University of Science, Noda, Chiba, Japan
Microstructural 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 Show Abstract
1 SIMAP, INP Grenoble, Saint Martin d Heres France
5:15 PM - E8.3/T8.3
Structure and Defect Stability of Calcium Phosphate Minerals.
Emily Michie 1 , Robin Grimes 1 Show Abstract
1 Materials, Imperial College London, London United Kingdom
The 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 Show Abstract
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
It 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. 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. L. M. Wang and R. C. Birtcher, Appl. Phys. Lett. 55 (24) (1989) 2494. S. M. Kluth, J. D. Fitz Gerald and M. C. Ridgway, Appl. Phys. Lett. 86 (2005) 131920. 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 Show Abstract
1 Departamento de Electronica, University of Valladolid, Valladolid, Valladolid, Spain
Ion 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
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 Show Abstract
1 UMR 8517, Laboratoire de Métallurgie Physique et Génie des Matériaux, Villeneuve d'Ascq Cédex France, 2 MMC, EDF, Moret sur Loing France
In 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 Show Abstract
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
First-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 Show Abstract
1 Department of Materials, University of Oxford, Oxford United Kingdom
The 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 . 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 . 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. P.E.A. Turchi, Mat. Res. Soc. Symp. Proc. 206, 265 (1991). 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 Show Abstract
1 Department of Materials, University of Oxford, Oxford United Kingdom
We recently derived an analytic interatomic bond-order potential (BOP) that depends explicitly on the valence of the transition metal element . 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. 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 Show Abstract
1 Chemistry, State Key Lab for Phys Chem of Solid Surf, Xiamen, Fujian, China
It 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
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 Show Abstract
1 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Many 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 Show Abstract
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
The 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 Show Abstract
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
We 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 Show Abstract
1 , LLNL, Livermore, California, United States
E11: Atomistic Methods II
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 Show Abstract
1 Chemistry, Materials and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States
Wednesday, 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 Show Abstract
1 , IPCMS, Strasbourg France
3:30 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 Show Abstract
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
We 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 Show Abstract
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
We 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
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 Show Abstract
1 Department of Materials Science and Engineering, North Carolina State Univ., Raleigh, North Carolina, United States
Materials 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 Show Abstract
1 Materials Science and Engineering , University of Florida, Gainesville, Florida, United States
Thin 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 Show Abstract
1 , Paul Scherrer Institute, Villigen Switzerland, 2 , Université de Bourgogne, Dijon France
One 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 Show Abstract
1 MEP, Ames Laboratory, Ames, Iowa, United States, 2 School of Physics, University of Edinburgh, Edinburgh, Scotland, United Kingdom
In 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 Show Abstract
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
E13: Evolution of Nanostructures
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 Show Abstract
1 Nuclear engineering, UC Berkeley, Berkeley, California, United States
The 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 Show Abstract
1 Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, Washington, United States
Polymer 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  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.
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 Show Abstract
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
The growth of sp-sp2 nanostructures in a carbon plasma is simulated by tight-binding molecular dynamics (TBMD) , at various temperatures and mass densities. The thermodynamical conditions are selected so as to mimic pulsed microplasma cluster source (PMCS) experiments, 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.This work was partly supported by Japan Society for the Promotion of Science, by Forschungszentrum Karlsruhe and by MiUR PON-CyberSar project. L. Colombo, Rivista Nuovo Cimento, volo.28(10), 1 (2005) E. Barborini et al., J. Phys. D: Appl. Phys., vol.32, L105 (1999) 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 Show Abstract
1 Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan
10:30 AM - **E13.5
Multiscale Computational Study of rotaxane and Catenane Molecular Electronic Switches.
Yong-Hoon Kim 1 , William Goddard 2 Show Abstract
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
Mechanically 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 catenane and 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
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 Show Abstract
1 Chemical Engineering, Stanford University, Stanford, California, United States
We 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 Show Abstract
1 Physics, Seoul National University, Seoul Korea (the Republic of)
Thursday, 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 Show Abstract
1 EEE, Nanyang Technological University, Singapore, Singapore, Singapore, 2 Design Centre, ST Microelectronics, Singapore, Singapore, Singapore
With 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 Show Abstract
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
So 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
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 Show Abstract
1 Department of Electronic Science, University of Pune,Pune,India, Pune, Maharashtra, India, 2 School of Physics, University of Hyderabad, Hyderabad India
Motivated 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 Show Abstract
1 , MIT, Cambridge, Massachusetts, United States, 2 , Lawrence Livermore National Laboratory, Livermore , California, United States, 3 , University of California, Davis, Davis, California, United States
Nano-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 Show Abstract
1 Physics, UT Dallas, Richardson, Texas, United States
Silicon 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 , and surface roughness on the electronic structure and quantum transport through the nanowires . More recently, we have studied the effects of strain on silicon nanowires which show a strong electromechanical coupling . 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. 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). 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). P. Leu, A. Svizhenko, and K. Cho (unpublished).
3:30 PM - E15.4
Electronic Upconversion in Nanodevices.
Karel Kral 1 Show Abstract
1 , Institute of Physics, ASCR, v.v.i., Prague 8 Czech Republic
3:45 PM - E15.5
Negative Differential Resistance in Nanojunctions.
Keivan Esfarjani 1 2 , Hossein Cheraghchi 2 Show Abstract
1 Physics, UC Santa Cruz, Santa Cruz, California, United States, 2 Physics, Sharif University of Technology, Tehran Iran (the Islamic Republic of)
The 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 Show Abstract
1 Department of Materials Engineering, The University of Tokyo, Tokyo Japan
A novel atomic switch  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  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 . 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.  K. Terabe, T. Hasegawa, T. Nakayama, and M. Aono, Nature 433, 47 (2005). M. Brandbyge et al., Phys. Rev. B 65, 165401 (2002). Z. Wang, T. Kadohira, T. Tada, and S. Watanabe, submitted for publication.
E16: Properties of Defects
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 Show Abstract
1 MMC, EDF R&D, Moret sur Loing France
Fe 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 Show Abstract
1 General Physics, Altay State Technical University, Barnaul Russian Federation
5:00 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 Show Abstract
1 Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Nuclear Science and Engineering, MIT, Cambridge, Massachusetts, United States
Many 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 Show Abstract
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
Development 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 Show Abstract
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Defect 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 characte