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).