Gianguido Baldinozzi CEA-CNRS-ECP
Yanwen Zhang Pacific Northwest National Laboratory
Katherine L. Smith Embassy of Australia
Kazuhiro Yasuda Kyushu University
V1: Radiation Effects
Monday AM, November 30, 2009
Room 207 (Hynes)
9:30 AM - **V1.1
Effects of Ionization on Irradiation Damage Evolution and Thermal Recovery in Ceramics.
William Weber 1 , Yanwen Zhang 2 , Ram Devanathan 1 Show Abstract
1 Fundamental & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington, United States, 2 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington, United States
Irradiation with energetic electrons and ions results in the transfer of energy to both atomic nuclei and the electronic structure. Kinetic energy transfer to atomic nuclei results in energetic atomic displacements and the production of atomic-level defects, while ionization energy loss to the electronic structure generates electron-hole pairs and localized electronic excitations. The understanding and modeling of atomic collision cascades and their role in irradiation damage evolution is well advanced. The effects of ionization are less understood. In ceramics, the localized electronic excitations can result in localized charge at defects and interfaces, rupture or change in nature of covalent and ionic bonds, enhanced defect and atomic diffusion, and changes in phase transformation dynamics, which affect the dynamics of atomic processes and the interpretation of the results from ion and electron irradiation experiments. Under irradiation with different ions, the ratio of electronic to nuclear stopping powers varies locally for both the primary ion and the secondary recoils produced. It will be shown that the critical temperature for ion-beam induced amorphization can exhibit a strong dependence on the ratio of electronic to nuclear stopping, which demonstrates that the local rate of in-cascade ionization has a significant effect on the dynamic recovery processes that determine the critical temperatures. Simultaneous electron and ion irradiation are shown to dramatically affect the dynamics of damage accumulation. In post-irradiation studies of ion-irradiated materials, ionization-enhanced recovery and recrystallization due to electron beam irradiation are observed, and the kinetics of the enhanced recovery processes has been determined. In the case of high-energy heavy ions (~0.1 to 2 GeV), such as fission fragments or swift-heavy ions, the intense energy deposition into the electronic structure produces a thermal spike. Computer simulations of thermal spikes in a range of materials demonstrate that the damage produced can range from the production of isolated point defects and defect clusters to the formation of tracks with fine structure.
10:00 AM - V1.2
The Need for Quantum Mechanics in Large-scale Atomistic Simulations of Radiation Damage in Metals.
C. Race 1 , D. Mason 1 , M. Finnis 1 2 , W. Foulkes 1 , A. Horsfield 2 , T. Todorov 3 , A. Sutton 1 Show Abstract
1 Department of Physics, Imperial College London, London United Kingdom, 2 Department of Materials, Imperial College London, London United Kingdom, 3 School of Mathematics and Physics, Queen’s University Belfast, Belfast United Kingdom
It has long been recognised that electronic excitations caused by high velocity particles in metals are central to understanding how these particles are slowed down. Quantum mechanics has played a key role in modelling such processes in idealized free electron gases (jellium models). The imperative now is to develop quantum mechanical treatments of metals with real atomic structures for large-scale atomistic simulation of radiation damage. In this paper we present an example of such large-scale simulation applied to the phenomenon of channelling.When a particle with a high kinetic energy enters a crystalline solid it may travel large distances along channels in the crystal structure. This process is called channelling. It plays a central role in determining the depth of irradiation damage suffered by materials exposed to high energy incident particles in nuclear reactors and in ion implantation. A key question centres on the mechanisms by which such a high energy particle loses its energy as it rattles down a channel in the crystal. It is known that at very high energies the principal mechanism is electronic, that is the channelling particle creates electronic excitations and gradually loses its energy until it has slowed sufficiently to create a cascade of atomic displacements. We present a simulation of this process based on solving the time-dependent Schrodinger equation for the electrons in a crystal as an interstitial particle of high kinetic energy channels through it. Unlike many previous simulations we consider the real atomic structure of the metal, and not a free electron gas. We find good agreement with previous models predicting a stopping force linear in projectile velocity. We also find a new mechanism of electronic excitation arising from the discrete atomic structure of the metal. This mechanism is absent in the earlier free electron models and results in a resonance in the ion charge at low channelling velocity.
V2: Complex Microstructures
Monday AM, November 30, 2009
Room 207 (Hynes)
11:15 AM - **V2.1
Can We Describe Phase Transition under Irradiation in Insulators within the Random Phase Approximation Framework?
David Simeone 1 2 , Gianguido Baldinozzi 2 1 , Dominique Gosset 1 2 , Laurence Luneville 1 2 Show Abstract
1 CEA/DEN/DANS/DMN/SRMA/LA2M-MFE, CEA, Gif sur yvette France, 2 CNRS-ECP/SPMS-MFE UMR 8580, CNRS-ECP, Chatenay Malabry France
The renewed interest in nuclear energy production and the environmental impact of energy are bringing about a renaissance in materials sciences. The compelling need for valid predictive models and accurate data are needed to forecast the radiation effects and long-term degradation of reactor components and radioactive waste hosts are expected to become increasingly critical over the next decade. The radiation tolerance of insulating ceramics for fusion energy systems and of nuclear fuel for fission systems is also a matter of great concern. The Random Phase Approximation seems to give a valuable framework to understand microstructural transformations induced by radiation damages in metals and alloys. Based on experimental evidences, the aim of this talk is to analyze phase transitions triggered by irradiation damages in two different oxides, pure zirconia and magnesium spinels, within this framework pointing out limitations of this approach.
11:45 AM - V2.2
Phase-Field Simulation of Void and Fission-Gas Bubble Evolution in Irradiated Polycrystalline Materials.
Paul Millett 1 , Anter El-Azab 2 , Michael Tonks 1 , Srujan Rokkam 2 , Dieter Wolf 1 Show Abstract
1 Nuclear Fuels and Materials, Idaho National Laboratory, Idaho Falls, ID 83415, Idaho, United States, 2 Scientific Computing, Florida State University, Tallahassee, FL 32310, Florida, United States
The interactive evolution of both polycrystalline microstructure and irradiation-induced defects such as voids and fission gas-filled bubbles in nuclear fuels and structural alloys is complex and critically important to the long-term performance of fission reactors. Here, the phase-field technique is used to model the evolution of multiple point-defect species (vacancies, self-interstitials, and gas atoms), generated randomly in space and time to represent collision cascade events, thus allowing spatially-resolved simulations of void and gas bubble nucleation and growth both within grain interiors and at grain boundary interfaces (which are shown to be heterogeneous nucleation sites). Illustrative results including the formation of void denuded zones and void peak zones adjacent to grain boundaries, the interlinkage of intergranular gas bubbles leading to fission gas release, and the effects of temperature and stress gradients will be presented. This work was supported by the DOE-BES Computational Materials Science Network (CMSN).
12:00 PM - V2.3
Phase Field Modeling of Void Nucleation and Growth in Irradiated Metals.
Srujan Rokkam 1 , Santosh Dubey 2 , Anter El-Azab 2 , Paul Millett 3 , Dieter Wolf 3 Show Abstract
1 Department of Mechanical Engineering, Florida State University, Tallahassee, Florida, United States, 2 Department of Scientific Computing, Florida State University, Tallahassee, Florida, United States, 3 Nuclear Fuels and Materials, Idaho National Laboratory, Idaho Falls, Idaho, United States
Irradiation of materials by energetic particle (e.g., neutrons in nuclear reactors) is accompanied by excessive point defect generation by atomic collision cascades. The diffusion and interaction of these point defects with each other and with pre-existing defects results in microstructure evolution. An important aspect of this evolution is the nucleation and growth of voids, which causes swelling and dimensional instabilities which are detrimental to the structure. Here, we present a phase field model for void nucleation and growth in irradiated metals. The formalism developed herein thus treats both the nucleation and growth processes simultaneously in a spatially resolved fashion. The material is described in terms of free energy functional obtained from the enthalpic and entropic (configurational and vibrational) contributions. Point defect fluxes and defect densities are obtained using a Cahn-Hilliard type description for the vacancy and interstitial concentration fields. The dynamics of void growth are obtained in terms of the evolution of a non-conserved order parameter field, whose evolution is prescribed by a phenomenological Allen-Cahn type equation. Using the case of pure metals as an example, we illustrate model capabilities with regards to void nucleation and growth in the presence of interacting point-defects, and defects interacting with lattice sinks. The effects of vibrational entropy on the defect dynamics and void evolution are investigated. In addition, void nucleation is studied as a function of thermal fluctuations and cascade damage. Furthermore, we use the concept of stochastic point process in space and time to model the generation of point defects due to cascades. Finally, the effect of spatially resolved point defect sinks (such as dislocations) on void nucleation and growth is investigated.This work was supported by DOE-BES Computational Materials Science Network(CMSN)
12:15 PM - V2.4
HRTEM Studies of Nano-Particles in an ODS Steel.
Luke Hsiung 1 , Jeffery Aguiar 1 , Nigel Browning 1 , Michael Fluss 1 , Akihiko Kimura 2 Show Abstract
1 Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Institute of Advanced Energy, Kyoto University, Kyoto Japan
Many key issues remain unsolved for developing ODS steels for fission and fusion applications including incomplete understanding of the effect of irradiation on low-temperature fracture properties, the role of fusion relevant helium and hydrogen transmutation gases on the deformation and fracture of irradiated material at low and high temperatures, and mechanisms of swelling suppression in ODS steels. In preparation for ion-beam experiments, we are currently performing HRTEM and STEM studies of a 16Cr-5Al-2W-0.3Ti-0.4Y2O3 ODS steel with an emphasis on the crystal and interfacial structures of the nanoscale oxide particles and their coherency with respect to the Fe (Cr) matrix. We will report on some of the studies and will address the critical features which may illuminate the influence of thermodynamics and kinetics on the growth and refinement of the nano-particles. We will also point to those features that may be of interest with respect to the suppression of radiation-induced dimensional changes due possibly to the nano-dispersoids. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DEAC5207NA27344.
12:30 PM - **V2.5
Atomic-scale Analysis of Irradiation-induced Structural Change in Magnesium Aluminate Spinel Compound.
Syo Matsumura 1 , Tomokazu Yamamoto 1 , Kazuhiro Yasuda 1 Show Abstract
1 Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University, Fukuoka Japan
The present talk will give an overview of our recent results on irradiation-induced structural change in magnesium aluminate spinel compound, which is known as a radiation tolerant oxide, especially to volumetric swelling. Magnesium aluminate spinel of MgO-nAl2O3 with n=1.1, was irradiated with swift heavy ions of 200 MeV Xe14+ (Se=24 keV/nm) and 350 MeV Au28+ (Se=34 keV/nm) at a Tandem ion-accelerator. Transmission electron microscopy techniques of high-resolution (HR) imaging, STEM dark-field imaging as well as high angular resolution electron channeling x-ray spectroscopy (HARECXS) were employed in quantitative analysis of irradiation-induced structural change. Dark spotty contrast appears at ion-tracks formed by swift-heavy irradiation in STEM dark-field imaging, indicating lower density inside the ion-tracks. Clear lattice fringes are observed in HR images even inside the ion tracks in both Xe14+ and Au irradiated specimens. However, the fringe pattern inside the tracks is different from that appearing in the matrix, being indicative of formation of a defective NaCl structure. Molecular dynamics (MD) simulations have shown that the spinel structure becomes unstable by accumulation of displaced interstitials and a defective NaCl structure is formed after preferential evacuation of cations from the tetrahedral positions. Quantitative HARECXS analysis showed that cation disordering progresses successively with ion fluence. It was revealed that the disordered regions are extended over about 12 nm in diameter along the ion-tracks, which is much wider than the defective volume detected by HR images. The present study was supported in part by Grant-in-Aid for Scientific Research (A) (#18206068) and for the Junior Scientist from JSPS.
V3: Metallic Materials I
Monday PM, November 30, 2009
Room 207 (Hynes)
3:00 PM - V3.2
Irradiation-Induced Point Defects in Nanocrystalline Molybdenum by Molecular-Dynamics Simulation.
Dilpuneet Aidhy 1 , Paul Millett 2 , Simon Phillpot 3 , Alex Chernatynskiy 3 , Dieter Wolf 2 Show Abstract
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Nuclear Fuels and Materials, Idaho National Laboratory, Idaho Falls, ID 83415, Idaho, United States, 3 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Evolution of irradiation-induced point defects in the presence of grain boundaries (GBs) is studied in bcc Molybdenum (Mo) using molecular dynamics (MD) simulation. Point defects created due to the radiation events can annihilate primarily by two mechanisms: mutual recombination of interstitials and vacancies (bulk), and by elimination at the GBs. By calculating the source/sink strength of the GBs, in accord with the rate-theory model, the dominant point-defect annihilation mechanism is predicted. At high temperatures, their high diffusivity leads to mutual annihilation in the bulk. In contrast, at low temperatures, because they less often recombine in the bulk, they annihilate predominantly at the GBs. It is further found that the defect concentration also dictates the annihilation mechanism. At low concentrations annihilation takes place at GBs, while conversely, at high concentrations annihilation takes place in the bulk. Finally, the annihilation mechanism also depends upon the grain size, with GB mechanism prevalent at smaller grain sizes. As the grain size increases, a crossover between the two mechanisms is observed. At ~ 300 K, the critical grain size is tens of nanometer, indicating that the GB mechanism dominates at nanometer grain-size materials. This work was funded by DOE-NERI Awards DE-FC07-07ID14833, and by the DOE-BES Computational Materials Science Network (CMSN).
3:15 PM - V3.3
TEM Characterization of Neutron- and Ion-irradiated Nano-structured Ferritic Alloys.
James Bentley 1 , D. Hoelzer 1 Show Abstract
1 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Mechanically alloyed (MA) nano-structured ferritic alloys (NFA) have the potential to be highly resistant to radiation damage in fission and fusion environments. High concentrations (>1023 m-3) of small (<5 nm) Ti-Y-O nano-clusters (NC) not only result in outstanding mechanical properties, but also are expected to promote point-defect recombination and trap transmutation-produced He in small bubbles. Early results of microstructural characterization of NFA irradiated with neutrons and ions are encouraging: several publications indicate that NC in NFA with 9 and 14%Cr are not detectably changed by irradiation at ~500°C with light ions, heavy ions or neutrons and no bubbles/cavities larger than 2 nm form in MA957 neutron irradiated at 500°C to 9 displacements per atom (dpa) with ~380 appm He. Characterization by conventional transmission electron microscopy (TEM) is supplemented by energy-filtered TEM (EFTEM) methods such as thickness and elemental (Fe-M, O, Ti-L, Cr-L) mapping. Importantly, Fe-M jump-ratio images reliably reveal NC as small as 2 nm diameter for sufficiently thin regions (<50 nm) and are insensitive to surface oxide films or modest surface contamination. Specimens of 12YWT and MA957 have been neutron irradiated to 9 dpa at ~500°C; microstructural characterization is in progress. Unless radical changes in size or concentration are induced, studies of the effects of irradiation on NC are hampered by the highly heterogeneous NC distributions. The dominance of TEM-specimen surfaces as point-defect sinks notwithstanding, we are pursuing the use of in-situ ion irradiation of 14YWT to study the effects of irradiation on NC (and of NC on the development of damage structure). In-situ heavy-ion irradiation at ~25 and ~500°C at the JANNuS facility in France will allow NC to be imaged by EFTEM as a function of ion dose. Experiments are also in progress using an alternative approach that involves characterization, including EFTEM imaging of NC at Oak Ridge National Laboratory (ORNL), of selected regions of 14YWT TEM specimens before and after in-situ ion irradiation at the IVEM-Tandem Facility of Argonne National Laboratory (ANL). The vacuum quality at the specimen during elevated-temperature in-situ irradiation is of great importance because of potential interstitial-impurity (e.g. O, C or N) pick-up or even oxidation, especially since NC imaging by EFTEM is limited to such thin regions. Even without irradiation, in-situ annealing of 14YWT at 500°C for 1 h at ~2 x 10-7 Torr resulted in severe specimen degradation. Research supported by the Division of Materials Sciences and Engineering, and at the ORNL SHaRE User Facility by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Special thanks to D. Kaoumi and A.T. Motta (Penn State), and M.A. Kirk (ANL) for exploratory in-situ irradiations at ANL.
3:30 PM - **V3.4
Multiscale Modelling of High Electric Field Effect on Metal Surfaces.
Flyura Djurabekova 1 4 , Helga Timko 2 , Aarne Pohjonen 1 , Leila Costelle 4 , Kai Nordlind 1 4 , Konstantin Matyash 3 , Ralf Schneider 3 , Sergio Calatroni 2 , Walter Wuensch 2 Show Abstract
1 , Helsinki institute of Physics, Helsinki Finland, 4 Department of Physics, University of Helsinki, Helsinki Finland, 2 , CERN, Geneve Switzerland, 3 , Max-Planck-Institut fur Plasmaphysik, Greifswald Germany
Sparks near metal surfaces cause a considerable damage to metal parts in devices employing high gradient electric fields. The next generation of high-end particle accelerators, needed to unravel the fundamental structure of matter in the universe, willbe linear colliders. The design of future accelerators such as the Compact LInear Collider (CLIC) involves very high gradient electric fields (~ 100 MV/m). Unfortunately, the upper energy limit of the beams is strongly restricted by the significant probability of electrical breakdowns inside of rf-structures, known as sparking. In the same time, fusion reactors, that involve high electric and magnetic field gradients, also experience problemsrelated to sparking phenomena.The trigger of sparking is a matter of long-standing debate, nonetheless, it still remains absolutely unclear. Despite the fact that the surfaces of the inner parts of rf-structures are thoroughly treated before use and operated under ultra-high vacuum conditions, the probability of sparks is still significant. Insight into the triggering of sparking, can help in managing sparking and arcing problems occurring both in the particle accelerators and in fusion reactors. As a successive process to the breakdown triggering, the formation of a near-surface plasma must be considered, where ions can be accelerated towards the surface and cause further surface damage by sputtering.We are developing a three-step multiscale modelling scheme to simulate the onset, plasma buildup and surface damage aspects of sparking.For the onset, we have developed a novel hybrid Electrodynamics-Molecular Dynamics (ED-MD) code on a base of the parcas MD code, which allows simulating the evolution of surfaces under high electric fields. We have tested the model in the regime of dc electric field evaporation (10 GV/m) and clearly observed single atoms being dragged out of the surface. For the plasma buildup, we employ Particle-in-Cell simulations with Monte Carlo collisions (PIC MCC). These show that under conditions relevant to linear colliders, a sheath potential forms in the plasma, which accelerates Cu ions towards the surface with fluxes of the order of 10^25 ions/cm^2/s with an energy distribution peaked around 10 keV. For the surface damage, we use MD simulations taking as input the flux and energy distribution from the PIC simulations.The MD simulations show that the formation of spark craters is due to multiple overlapping heat spikes producedby the ions accelerated in the plasma sheath.
V4: Ceramic Materials and Wasteforms I
Monday PM, November 30, 2009
Room 207 (Hynes)
4:30 PM - **V4.1
Mechanisms of Radiation Damage and Properties of Nuclear Materials.
Gregory Lumpkin 1 , Katherine Smith 1 , Karl Whittle 1 , Bronwyn Thomas 1 , Nigel Marks 2 Show Abstract
1 Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, Menai, New South Wales, Australia, 2 Nanochemistry Research Institute, Curtin University of Technology, Perth, Western Australia, Australia
A wide range of materials are currently under consideration for use in advanced nuclear fuel cycle applications. The effects of radiation on these materials by exposure to external neutron irradiation and internal alpha and beta decay processes may have significant effects on the physical and chemical properties. This is especially true for materials that are subject to hundreds of displacements per atom during their service life. In this paper, we explore some of the radiation damage mechanisms prevalent in oxide based materials, including mathematical models and other concepts of amorphization (e.g., percolation), the role of defects on picosecond time scales, and longer term effects such as diffusion and recrystallization. As radiation "tolerance" or the ability of a material to maintain crystallinity under intense irradiation is a key issue for many fuel cycle applications, we will briefly review and comment on some of the underlying factors that have been identified as important in driving the short-term damage recovery. These include aspects of the structure (e.g., connectivity, polyhedral distortion), bonding, energetics of defect formation and migration, and melting point and similar criteria. The primary materials of interest here are those under development as special purpose nuclear waste forms, novel materials for separations, inert matrix fuels, and transmutation targets. In this context, we will illustrate the behavior of simple oxides and several more complex oxides such as perovskite, multicomponent fluorite systems, and related derivative structures (e.g., pyrochlore and zirconolite). The damage mechanisms in these materials are briefly compared with those of intermetallic and metallic materials.
5:00 PM - V4.2
Comparison of Microstructural Changes in ZnAl2O4 Spinel Under Ion Irradiation in the Electronic and in the Nuclear Energy Loss Regime.
Alexis Quentin 1 , Isabelle Monnet 1 , Dominique Gosset 2 , David Simeone 2 , Christina Trautmann 3 , Laurence Herve 4 , Serge Bouffard 1 Show Abstract
1 , CIMAP-CIRIL, Caen France, 2 , CEA/DEN/DMN/SRMA/LA2M, Gif-sur-Yvette France, 3 , GSI, Darmstadt Germany, 4 , CRISMAT, Caen France
ZnAl2O4 is a typical ternary compound spinel that belongs to the space group Fd-3m where the anion sublattice is arranged in a cubic close-packed network and the cations are distributed in one-eighth of the tetrahedral sites and in half of the octahedral sites. This structure is known for exhibiting cation exchange versus temperature, and the space group remains Fd-3m over a broad temperature range. Under irradiation, ZnAl2O4 undergoes additional structural change. Whatever the nature and the energy of the incident particles, a crystal-crystal transition occurs at room temperature for different fluence values [1,2,3]. In the nuclear energy loss regime, the irradiation with 4 MeV Au ions transformed part of the initial phase into a random phase characterized by cations randomly occupying octahedral and tetrahedral sites. With increasing fluence, the volumic fraction of this beam-induced random phase follows an S-like shape and finally saturates at 80% . In the electronic energy loss regime, high energy ions can also induce cation inversion in spinels , and in addition amorphisation by defect accumulation . For 91-MeV Xe ions, the amorphous phase appears above a critical fluence of 4×1012 cm-2 and grows with increasing fluence. Using X-ray diffraction in combination with Rietveld analysis and transmission electron microscopy, the inversion parameter, the amorphous fraction, and the size of diffracting domains were analyzed for polycrystalline samples irradiated with different swift heavy ions such as 83-MeV Kr, o91-MeV, Xe, 740-MeV Zn , and, 2-GeV Au. provided at GANIL and GSI. D. Simeone, C. Dodane-Thiriet, D. Gosset, P. Daniel, M. Beauvy, Journal of Nuclear Materials 300 (2002) 151 G. Baldinozzi, D. Simeone, D. Gosset, M. Dolle, L. Thomé, L. Mazérolles , Nuclear Instruments and Methods in Physics Research B 250 (2006) 119 G. Baldinozzi, D. Simeone, D. Gosset, S. Surblé, L. Mazérolles, L. Thomé, Nuclear Instruments and Methods in Physics Research B, 266 (2008) 2848 K. Yasuda T. Yamamoto, M. Shimada, S. Matsumura, Y. Chimi, N. Ishikawa , Nuclear Instruments and Methods in Physics Research B, 250 (2006) 238 A. Quentin, I. Monnet, D. Gosset, B. Lefrançois, S. Bouffard, Nuclear Instruments and Methods in Physics Research B, 267 (2009) 980
5:15 PM - V4.3
Transmission Electron Microscopy Observations of Alpha-Al2O3 Irradiated at High Temperature with 10 MeV Au Ions.
Jonghan Won 1 , Igor Usov 1 , Kurt Sickafus 1 Show Abstract
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Alpha-alumina (α-Al2O3) is a widely-used industrial ceramic that is also being considered for application in certain radiation environments. There has been significant prior work regarding radiation damage behavior of alumina under neutron, electron, and ion irradiation conditions. However, the behavior of α-Al2O3 under high-energy, high-mass, high-temperature ion irradiation conditions, has not been studied to date. We report here on such a study in which high-temperature ion irradiation damage evolution in α-Al2O3, due to 10 MeV Au ions, was analyzed using cross-sectional transmission electron microscopy (TEM).The pristine α-Al2O3 samples used for this study included both polycrystalline alumina (commercially-available sintered alumina from Coors Tek) and single-crystalline sapphire (c-cut, (0001) sapphire from Union Carbide). We irradiated these samples with 10 MeV Au3+ ions at elevated substrate temperatures (up to 1273 K). Irradiations were performed to an ion fluence of 5x1015 Au/cm2. The ballistic damage profile for these irradiation conditions (estimated using the Monte Carlo code SRIM) indicates that the peak displacement dose is approximately 12 displacements per atom (dpa) at fluence 5x1015 Au/cm2, and this peak occurs approximately 2 μm beneath the sample surface. Grazing incidence X-ray diffraction (GIXRD) measurements indicate that there is no phase transition following ion irradiation. However, cross-sectional TEM observations of polycrystalline alumina samples revealed irradiation-induced dislocation loops at a depth of ~2.4 μm from the surface, and a high density of voids closer to the free surface. The size of these voids was found to range from 1 to 10 nm in diameter. These voids seem to differ from those observed in previous neutron and ion irradiation experiments. These voids seem to be randomly oriented and increase in size closer to the free surface and to pre-existing pores.
5:30 PM - **V4.4
Molecular Dynamics Simulation of Radiation Damage Accumulation in Pyrochlores.
Ram Devanathan 1 , William Weber 1 Show Abstract
1 Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, Richland, Washington, United States
We have used molecular dynamics simulations to examine fundamental mechanisms of radiation damage accumulation and phase transformation in pyrochlores. In the present study, high energy recoils were simulated in Gd2Ti2O7 and Gd2Zr2O7 using rigid ion potentials. In addition, the accumulation of cation and anion sublattice defects was also studied. Our results show that the high mobility of defects, such as oxygen vacancies, plays an important role in defect annihilation. Moreover, recoil energy is dissipated by replacement collision sequences in pyrochlore, which reduces damage accumulation. In gadolinium zirconate pyrochlore, there are mechanisms for the accommodation of radiation damage, which result in minimal volume expansion and energy increase. As a result, there are considerable differences in the evolution of mechanical properties in Gd2Ti2O7 and Gd2Zr2O7. These results provide valuable insights into experimental observations of radiation damage in pyrochlores and will be discussed in light of experimental findings.
Gianguido Baldinozzi CEA-CNRS-ECP
Yanwen Zhang Pacific Northwest National Laboratory
Katherine L. Smith Embassy of Australia
Kazuhiro Yasuda Kyushu University
V5: Modelling Complex Materials I
Tuesday AM, December 01, 2009
Room 207 (Hynes)
9:30 AM - **V5.1
Irradiation Studies in Non-metallic Materials : Impact of ab-initio Calculations.
Yves Limoge 1 , Layla Martin-Samos 2 , Guido Roma 1 Show Abstract
1 DMN/SRMP, C.E.A. FRANCE, Gif sur Yvette France, 2 Democritos and Sissa, CNR-IFNM , Trieste Italy
The study of points defects in solids, and their contribution to matter transport and related properties, has been renewed in the last ten years or so by the use of the so called ab-initio methods. Being based on the determination of the properties of the defects using a fully quantum treatment of the bonding, and involving a very small number of adjustable parameters, if not zero, these techniques have proved to be able to allow the determination of the base parameters of defects with a great precision. In non-metallic materials these capabilities however are for a part impeded by several problems, either of a computational nature, or in a deeper way linked to the approximations used for solving the Schrödinger equation. Among the first source of errors is the so called ~Simages interaction problem~T, linked to the periodic boundary conditions frequently used as a model of an infinite body. The last kind of difficulties is due to the well kown problems suffered by most of the ab-initio methods in handling the band gap of non-metallic systems. As is well known the DFT approach of the electronic structure is tailored for ground state studies, so it underestimates generally the width of the forbidden band. These errors have many detrimental consequencies for the defects having deep levels in the gap, which indeed can as a consequence be also badly described. This drawback is particularly severe for the charged states of these defects, the formation energy of which is prone to be given in error by several eV. In this work we will show on a few simple systems how the two kinds of errors can be solved in a more satisfactorily manner using state of the art electronic structure tools, in particular the GW method. We will also discuss the materials conditions leading to such a catastrofic failure of the stand
10:00 AM - V5.2
First-principles Modeling of Nuclear Fuel Materials with High Efficiency and Accuracy.
Fei Zhou 1 , Vidvuds Ozolins 1 Show Abstract
1 Materials Science & Engineering, UCLA, Los Angeles, California, United States
Actinide compounds present serious challenge to moderndensity-functional theory (DFT) based electronic-structure techniques due to strong electron correlations and orbital ordering phenomena of the localized f-electrons. While advanced quantum-mechanical methods such as self-interaction-corrected local-density approximation (SIC-LDA), hybrid functional and dynamical mean-field theory (DMFT) have been used to investigate actinide compounds, these methods are usually computationally expensive and limited to small system. The LDA+U method, which combines the efficiency of LDA with an explicit treatment of correlation for the f-electrons, has received much interest for studying actinide compounds. High computational efficiency means that relatively large and complicated systems can be modeled with this method.We have identified a critical problem with the currently available versions of LDA+U: although the method was invented to remove self-interaction of localized electrons, significant orbital-dependent self-interactions remain. These aspherical self-interaction errors are up to 0.4 eV per electron, leading to erroneous orbital ground states in many cases. An alternative scheme that improves upon the original LDA+U is proposed as a remedy. We show that our method reproduces the expected degeneracy of $f^1$ and $f^2$ states in free ions and the correct ground states in the PrO2 and UO2 solids. In particular, the Gamma 5 orbital state of UO2 is confirmed as the ground state. As a first application, the crystal-field excitation energies to the Gamma 3, 4 and 1 states of UO2, between 0.1 to 0.2 eV, are reproduced with good accuracy compared to experiment.This work was supported by the U.S. Department of Energy, Nuclear Energy Research Initiative Consortium (NERI-C).
10:15 AM - V5.3
First-principles Calculations of Lattice Defects in γ-Uranium.
Daniel Aberg 1 , Paul Erhart 1 , Babak Sadigh 1 Show Abstract
1 Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California, United States
Uranium has a polymorphic phase diagram and undergoes several structural transitions as a function of temperature. The low- and high-temperature phases are related to the α-U and γ-U structures, respectively. To predict the microstructural evolution under irradiation and thermal gradients using KMC or PFM simulations, point defect information is needed. As these quantities must be obtained as a function of temperature we have performed ab-initio MD simulations of γ-U at elevated temperatures to study the stabilization of γ-U, point defect formation energies and diffusion. For the ideal material, we show that at temperatures near the α-γ transition the cubic phase is stabilized by anharmonic vibrations and that the short-range order is α-like whereas the long-range order is γ-like. We also obtain vacancy and interstitial formation energies on the order of 1 eV and predict that the barriers for their migration are very small. Prepared by LLNL under Contract DE-AC52-07NA27344.
10:30 AM - V5.4
Activation Energies for Xe Transport in UO2±x From Density Functional Theory Calculations.
David Andersson 1 , Pankaj Nerikar 1 , Blas Uberuaga 1 , Christopher Stanek 1 Show Abstract
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
From a thermodynamic perspective most fission gases have low solubility in the fuel matrix and as a result there is a significant driving force for segregation of gas atoms to heterogeneities such as grain boundaries and subsequently for nucleation of gas bubbles. Under this assumption one of the controlling steps for evolution of fission gas micro-structures is diffusion of individual gas atoms through the fuel matrix to existing bubbles or grain boundaries (sinks). This process is largely governed by the activation energy for bulk diffusion of gas atoms, the driving force for segregation to existing sinks (bubbles or grain boundaries) and their saturation limit. Here we have studied the bulk diffusion mechanisms of Xe, which is one of the most important fission gases, by calculating the corresponding activation energies as function of the UO2±x stoichiometry using density functional theory (DFT) methods. In the present study we assume Xe diffusion to occur via uranium vacancies that bind to the stable Xe trap sites. Estimating the activation energy involves determining Xe migration barriers as well as thermodynamics of Xe trap sites in UO2±x and their interactions with Uranium vacancies that enable Xe in trap sites to move. We present results for all these components of the activation energy and discuss the importance of appropriately treating charge-compensation for defects in UO2±x in order to best reproduce experimental data. Since diffusion of Xe atoms is closely connected to diffusion of Uranium vacancies, we have also analyzed the stoichiometry dependent activation energy for diffusion of uranium ions via vacancy mechanisms. Due to the complex nature of the point defects and the clusters of point defects that interact with Xe atoms, the DFT based modeling is inevitably associated with uncertainties that in some cases may be rather significant. In order to mitigate this issue, we have tried to identify systematic errors, after which we categorize the most probably diffusion mechanisms. In order to achieve self-consistency and assess the accuracy of our conclusions, this exercise is complimented by re-analysis of key experimental data within the framework of proposed diffusion models.
11:15 AM - **V5.5
Chemical Evolution through Radioactive Decay: A Case Study in Sr-90.
Nigel Marks 1 , Ashley Lawler 1 , Damien Carter 1 , Chao Jiang 2 , Chris Stanek 2 , Kurt Sickafus 2 , Blas Uberuaga 2
1 Nanochemistry Research Institute, Curtin University of Technology, Perth, Western Australia,