Scientific Programme

  • Click on the name to see abstracts or full poster session.  PDF file with Schedule. PDF file with full programme and abstracts (1.4MB)

    Sunday

    June 8

    Time

    Monday

    June 9

    Time

    Tuesday

    June 10

    Time

    Wednesday

    June 11

    Time

    Thursday

    June 12

    Time

    Friday

    June 13

    09:00

    Opening

    09:00

    Oxides & Glasses

    (Chair: R. Kurtz)

    09:00

    Materials Nuclear Applications

    (Chair: M. J. Caturla)

    09:00

    Electronic excitations

    (Chair: Z. Postawa)

    09:00

    Boundaries & Interfaces

    (Chair: K. Nordlund)

    09:20

    New methods

    (Chair: L. Malerba)

    (I3) J-M. Delaye

    (I5) T. Jourdan

    (I6) J. Kohanoff

    (I9) W-S Lai

    (I1) K.A. Fichthorn

    09:30

    (O18) K. Jolley

    09:30

    (O31) D. Terentyev

    09:30

    (O41) W. Weber

    09:30

    (O58) B. Uberuaga

    Hike

    in the mountains

    by the seashore

    Sign up!

    09:50

    (O1) S. D. Kenny

    09:50

    (O19) A. Rivera

    09:50

    (O32) C. Barouh

    09:50

    (O42) M. Dapor

    09:50

    (O59) K. P. Boyle

    10:10

    (O2) I. Martin-Bragado

    10:10

    (O20) A. Prada

    10:10

    (O33 ) L. Messina

    10:10

    (O43) P. de Vera

    10:10

    (O60) P. Zhang

    10:30

    (O3) J-P. Crocombette

    10:30

    (O21) – Peña-Rodríguez

    10:30

    (O34) L. Malerba

    10:30

    (O44) A. Wucher

    10:30

    (O61) C. L. Guerrero

    10:50

    (O4) G. Hobler

    10:50

    (O22) – T. Oda

    10:50

    (O35) E. Martinez

    10:50

    (O45) R. Ullah

    10:50

    (O62) A. Suzuki

    11:10

    Coffee break

    11:10

    Coffee break

    11:10

    Coffee break

    11:10

    Coffee break

    11:10

    Coffee break

    11:30

    (Chair: J. P. Crocombette)

    (O5) G. Martin

    11:30

    Semiconductors

    (Chair: M. Posselt)

    11:30

    (Chair: R. Schäublin)

    (O36) Y. N. Liu

    11:30

    (O46) T. Apostolava

    11:30

    Nanomaterials

    (Chair: B. Uberuaga)

    (O63) E. Bringa

    11:50

    (O6) H.Rouchette

    (I4) L.A. Marques

    11:50

    (O37) D. Nguyen-Manh

     

    11:50

    Graphite & SiC

    (Chair: R. Webb)

    11:50

    (O64) R. Garcia-Molina

    12:10

    (O7) Z. Chang

    12:00

    (O23) L. Pizzagalli

    12:10

    (O38) P. Grigorev

    (I7) N. Marks

    12:10

    (O65) K. Nordlund

    12:30

    (O8) S. Fitzgerald

    12:20

    (O24) M. Prieto-Depedro

    12:30

    (O39) S. Jin

    12:20

    (O47) K. D. Krantzman

    12:30

    (O66) T-H Yen Vu

    12:50

    (O9) R. Schaeublin

    12:40

    (O25) M. W. Ullah

    12:50

    (O40) M. Backman

    12:40

    (O48) J. Xi

    12:50

    (O67) A. Bahm

    13:10

    Lunch

    13:00

    Lunch

    13:10

    Lunch

    13:00

    Lunch

    13:10

    Closing

    14:30

    He & H

    (Chair: R. Smith)

    14:30

    (O26) J. L. Gomez-Selles

    Excursion

    Altea

    14:30

    Nuclear Waste & Fuels

    (Chair: W. Weber)

    (I2) J. Marian

    14:50

    Surfaces

    (Chair: H. Urbassek)

    (O27) Y. Osetsky

    (I8) C. Stanek

    15:00

    (O10) X. Gai

    15:10

    (O28) B. Weidtmann

    15:00

    (O49) P. Olsson

    15:20

    (O11) C. Ortiz

    15:30

    (O29) P. Phillip

    15:20

    (O50) S. T. Murphy

    15:40

    (O12) H-B. Zhou

    15:50

    (O30) Z. Postawa

    15:40

    (O51) C. Scott

    16:00

    Coffee break

    16:10

    Coffee break

    16:00

    Coffee break

    16:20

    (Chair: I. Martín-Bragado)

    (O13) A. De Backer

    16:30

    Poster Session

    (Chairs: C. Denton & A. Rivera)

    16:30

    (O52) P. A. Burr

    16:40

    (O14) R. Kurtz

    16:50

    V, Mo & other metals

    (Chair: T. Yoshiie)

    (O53) V. Jansson

    17:00

    (O15) C. Gonzalez

    17:10

     (O54) D. Xu

    17:20

    (O16) T. Yoshiie

    17:30

    (O55) W. Hu

    Welcome

    reception at

    Melia

    17:40

    (O17) A. Milocco

    17:50

    (O56) A. B. Sivak

    18:30

    18:10

    (O57) A. V. Korchuganov

    Banquet- VOLVO OCEAN RACE

     

    Monday, June 9   9:00 Opening New Computational Methods 9:20h – I1

    ATOMIC-SCALE SIMULATION OF RARE-EVENT KINETICS: ADDRESSING THE SMALL-BARRIER PROBLEM

    Kristen A. Fichthorn

    The Pennsylvania State University, University Park, PA, USA

    A significant challenge in simulating kinetics in solids is dealing with the small-barrier problem, which occurs when groups of recurrent free-energy minima connected by small barriers are separated from the full phase space of the system by high barriers. These groups of states are called superbasins and they can considerably reduce the efficiency of the simulation. We have addressed this problem with both accelerated molecular dynamics (AMD) [1-3] and in kinetic Monte Carlo (KMC) simulations. In this talk, I will highlight our local superbasin KMC (LSKMC) method [4] for identifying local superbasins in KMC simulations containing both superbasins and non-superbasin events. I will demonstrate aspects of our AMD and LSKMC methods in several examples involving thin-film growth and dynamics at surfaces, which highlight the computational efficiency of these algorithms.

    [1] R. A. Miron and K. A. Fichthorn, “Accelerated molecular-dynamics of rare events with the bond-boost method”, J. Chem. Phys. 119, 6210 (2003).

    [2] R. A. Miron and K. A. Fichthorn, “Multiple-time scale accelerated molecular dynamics: Addressing the small-barrier problem”, Phys. Rev. Lett. 93, 128301 (2004).

    [3] Y. Lin and K. A. Fichthorn, “An accelerated molecular dynamics study of the GaAs(001) 2(2×4)/c(2×8) surface”, Phys. Rev. B 86, 165303 (2012).

    [4] K. A. Fichthorn and Y. Lin, “A local superbasin kinetic Monte Carlo method”, J. Chem. Phys. 138, 164104 (2013).

    Top 9:50h – O1

    INFLUENCE OF THE PREFACTOR IN LONG TIME SCALE SIMULATIONS OF DEFECT MOTION

    Steven D Kenny, Roger Smith, Tomas Lazauskas

    Department of Mathematical Sciences, Loughborough University, Loughborough LE11 3TU, United Kingdom

    Due to the increasing computational power, long time-scale simulation techniques, such as Kinetic Monte Carlo (KMC), become more and more popular to study atomic diffusion, after such events as radiation damage. The first successful attempt to perform KMC within the harmonic transition state theory without a predefined event table was carried by Henkelman and Jonsson [1] where systems were characterised by local energy minima, multiple transition search were carried out on-the fly and the rates calculated using pre-exponential constant calculated from the Vineyard equation. Since then, various groups in their KMC techniques tend to use fixed pre-exponential constants varying from 1012 to 1013 s-1, due to the computational time required to estimate this constant.

    We present a study of the influence of the prefactor in the Arrhenius equation for the long time scale motion of defects in α-Fe, simulated by on-the-fly kinetic Monte Carlo technique. We found that calculated prefactors vary widely between different defects, thus it is important to determine them accurately within the simulations. Even calculating the prefactor to one significant figure accuracy made a great impact during the simulations. The results were verified by reproducing many events using a combination of Molecular Dynamics and Temperature- Accelerated Dynamics simulations.

    For the simulations of α-Fe calculating the prefactors was shown to change the relative interstitial-vacancy diffusion rates by an order of magnitude, compared to the assumption of a constant prefactor value. It also changed the ordering of the rate table for the interstitial defect migration mechanisms. In addition, very small prefactor values were observed for clusters consisting of 4 dumbbells with low barrier transitions, thus these mechanisms did not dominate the simulation as would have been predicted with a constant prefactor. It was also observed that when the clusters of dumbbells were close to vacancy clusters that moves that reduced the separation between the clusters typically had a large prefactor, thus enhancing the recombination rate of these defects.

    The otf-KMC results help us to better understand and study the recombination and clustering processes of post-cascade defects that occur on the longer time scales. The results in α-Fe systems show high mobility of interstitial clusters and symmetrical clustering of vacancy defects.

    [1] G. Henkelman, H. Jónsson, Long time scale kinetic Monte Carlo simulations without lattice approximation and predefined event table, The Journal of Chemical Physics 115 (2001) 9657.

    Top

    10:10h – O2

    PARALLELISM IN OBJECT KINETIC MONTECARLO: IMPLEMENTATION AND RESULTS FOR LATTICE AND OFF-LATTICE APPROACHES

    Ignacio Martin-Bragado1, J. Abujas2, P. L. Galindo2, J. Pizarro2

    1IMDEA Materials Institute. Eric Kandel 2, 28906 Getafe, Madrid, Spain

    2Departamento de Ingenieria Informatica. Universidad de Cadiz. Puerto Real, Cadiz, Spain.

    The Kinetic Monte Carlo algorithm is widely used in many fields of computational physics with notable success since its inception. Given its widespread use, is then understandable that a big effort is being done towards the parallelization and sacalability of this technique. This effort tries to overcome one of the main KMC limitations: it is computationally expensive to simulate realistic sizes and times in KMC. Since KMC is highly anisochronous, it is not straightforward to implement a domain decomposition and a time evolution that will work accurately and efficiently.

    To overcome such limitation, a synchronous parallel kinetic Monte Carlo algorithm based in the generalization of the rejection-free n-fold method has been proposed for diffusion reaction systems[1] and 3D Ising systems[2], but a question remains on whether such algorithm can be applied succesfully to a general OKMC or LKMC problem with several objects and reaction and high inhomogeneities in space.

    Thus, in this work we have applied the proposed ideas to an existing and general serial KMC code, MMonCa[3], extended it to use the shared memory parallel paradigm and run realistic simulations in two different fields where results for serial simulations have been recently published: Lattice KMC for recrystallization of amorphized semiconductors[4], and off-lattice KMC for the simulation of isochronous annealing of irradiated alpha-iron[3], allowing us to create a fair comparison and assessment of the current state-of-the-art for parallelism in KMC.

    We will describe the details of our implementation, the resulting speed-up, and the lessons that can be learnt for future applications.

    [1] Martinez et al. “Synchronous parallel kinetic Monte Carlo for continuum diffusion-reaction systems”, J. Computational Physics, 227, 3804, (2008)

    [2] Martinez et al.. “Billion-atom synchronous parallel kinetic Monte Carlo simulations of critical 3D Ising systems”, J. Computational Physics, 230, 1359 (2011)

    [3] Martin-Bragado et al, “MMonCa: an Object Kinetic Monte Carlo simulator for damage irradiation evolution and defect diffusion”, Computer Physics Communication, 184, 2703 (2013)

    [4] Martin-Bragado and Moroz, “Facet formation during solid phase epitaxy regrowth: A lattice kinetic Monte Carlo model”, Appl. Phys. Lett. 95, 123123 (2009)

    Top

    10:30h – O3

    CELL MOLECULAR DYNAMICS FOR CASCADES (CMDC) : A NEW MD TOOL FOR CASCADE SIMULATION

    Jean-Paul Crocombette and Thomas Jourdan

    CEA, DEN, Service de Recherches de Metallurgie Physique (SRMP), Saclay, France

    The evolution of materials under irradiation depends on the damage directly created by the atomic displacement cascades initiated by the fast moving neutrons or ions. The exact nature of this so-called primary state of damage pilots the subsequent number and nature of point and extended defects. Binary Collision Approximation (BCA) calculations are very fast but only approximate. To obtain an accurate description of the primary state of damage, one therefore uses MD simulations. The procedure is then to consider a large (periodically repeated) and to accelerate one atom inside the box. This method however faces some difficulties, among which the size of the simulation box. Because of the periodic repetition of the box and the temperature control on its border, any cascade which comes close to the borders of the box must therefore be put to garbage after calculation. To avoid such situation one may increase the size of the box, but the computer power needed to calculate high energy events becomes prohibitive. All in all this strongly limits the energy of calculated cascades and number of cascades that are performed in a given study, thus giving poor statistics on the nature and amount of damage.

    To circumvent this problem we propose a modified MD simulation of cascades: Cell Molecular Dynamics for Cascades (CMDC). The goal of the method is to accelerate as much as possible the calculation of cascades by MD without loss of accuracy of the results, considering the specificities of the cascade unfolding. It is based on the observation that many parts of the usual MD boxes do not take part in the cascade and are just present in case the cascade would go there. The idea behind CMDC is then to build the box as the cascade develops. More and more atoms are added as the cascade unfolds. Symmetrically, the parts of the materials where the cascade is over, i.e. when the local structure does not evolve anymore on the MD scale (after the ballistic and thermal phases), are removed from the dynamic simulation. The “waking up” and ‘turning off” of cells in the material are based on a local temperature criterion. Additionally, a spatially varying time step, adapted to local atomic velocities, is implemented. With all these features, the CMDC tool allows to calculate displacement cascades much faster than with usual MD. We will illustrate the capabilities of CMDC with two examples.

    First, we calculated the damage created by the implantation of helium in iron at 60 keV. For each energy, one thousand cascades have been calculated, which would have been impossible using standard MD, given the projected range of helium atoms (greater than 200 nm for 60 keV atoms). This large number of calculations provides enough statistics to transfer the description of created defects to simulation codes modeling the kinetic evolution of the damage induced by helium implantation.

    Second, displacement cascades in iron were calculated for 10 energies ranking from 5.8 keV to 1.8 MeV. For each energy, one hundred cascades were simulated. This allows us to discuss the fragmentation of cascades at very high PKA energies which has recently been questioned.

    Top

    10:50h – O4

    COMBINED BINARY COLLISION AND CONTINUUM MECHANICS MODEL APPLIED TO FOCUSED ION BEAM MILLING

    Gerhard Hobler

    Vienna University of Technology, Vienna, Austria

    Dynamic” binary collision simulation is a well-established technique for the prediction of compositional changes and sputtering characteristics of solids due to ion bombardment [1]. In recent years the approach has been extended from 1D to 2D and 3D geometries using different models for the relaxation of the local density changes caused by the implanted ions and relocated atoms [2-4]. Common to all implementations is the assumption that deviations from the equilibrium densities instantaneously fully relax towards an effectively stress-free state. This, however, contradicts the experimental observation of nanowire [5], membrane [6], and wafer bending [7], disagrees with theories of spontaneous pattern formation [8], and is unevaluated in all other applications. In this work we extend our previous 2D model of focused ion beam milling [3] by combining the binary collision (BC) algorithm with finite-element (FE) simulation of the viscoelastic material behavior. In the continuum mechanics model the bulk and shear modulus of elasticity as well as radiation induced viscosity are considered. The latter is modeled as depending on the local damage formation rate [9]. BC and FE simulations are run in turns for certain fluence increments with the BC simulation yielding the local compositional changes and the damage distributions, and the continuum mechanics simulation providing the displacements of the computational grid, driven by the density changes obtained from the BC simulation. The approach is illustrated with a few examples of focused ion beam milling.

    [1] W. Möller, W. Eckstein, and J.P. Biersack, “TRIDYN – Binary collision simulation of atomic collisions and dynamic composition changes in solids,” Comp. Phys. Comm. 51, 355 (1988).

    [2] A. Mutzke and R. Schneider, „SDTRIMSP-2D: Simulation of particles bombarding on a twodimensional target,” Technical Report, Max-Plank-Institut fü Plasmaphysik (2009).

    [3] G. Hobler and D. Kovac, “Dynamic binary collision simulation of focused ion beam milling of deep trenches,” Nucl. Instrum. Meth. B 269, 1609 (2011).

    [4] W. Möller, “TRI3DYN – Collisional computer simulation of the dynamic evolution of 3-dimensional nanostructures under ion irradiation,” Nucl. Instrum. Meth. B 322, 23 (2014).

    [5] W. Li et al., “Three-dimensional nanostructures by focused ion beam techniques: Fabrication

    and characterization,” J. Mater. Res. 28, 3063 (2013).

    [6] W. J. Arora et al., “Membrane folding by helium ion implantation for three-dimensional device fabrication,” J. Vac. Sci. Technol. B 25, 2184 (2007).

    [7] C.A. Volkert, “Stress and plastic flow in silicon during amorphization by ion bombardment,” J. Appl. Phys. 70, 3521 (1991).

    [8] M. Castro et al., “Stress-induced solid flow drives surface nanopatterning of silicon by ionbeam irradiation,” Phys. Rev. B 86, 214107 (2012).

    [9] S.G. Mayr, Y. Shrenazy, K. Albe, and R. S. Averback, “Mechanisms of radiation-induced viscous flow: Role of point defects,” Phys. Rev. Lett. 90, 055505 (2003).

    Top

    11:10h – Coffee Break

    11:30h – O5

    WHAT CAN BE EXPECTED FROM MOLECULAR DYNAMICS SIMULATION IN ADDITION TO THE BINARY COLLISION APPROXIMATION?

    G. Martin1, M. Krack2, S. Maillard1

    1CEA – DEN/DEC/SESC/LLCC, Bât. 352, 13108 Saint-Paul-Lez-Durance Cedex, France

    2Laboratory for Reactor Physics and Systems Behaviour, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

    The binary collision approximation (BCA) is a simple approach emerged in the 50’s [1] to describe the atomic displacements which occur when energetic atoms transfer their energy to matter through a nuclear energy loss regime. It relies on the threshold displacement energy of atoms inside the considered solid. Some modern simulation codes which describe the radiation damage in materials (such as the famous code SRIM [2]) rely on this approximation, and the damage unit derived from the BCA, the displacement per atom or dpa, is still commonly in used in the scientific community.

    Besides, the molecular dynamics simulation technique has been more and more applied concomitantly with the development of supercomputing. It has been used to better estimate the primary radiation damage created under irradiation and also to better understand the damage processes which occur in displacement cascades. For instance in CEA, studies were carried out notably on uranium dioxide (see e.g. [3,4]), but metals have also been extensively studied using this atomistic simulation tool (see for instance [5])

    The results which have been obtained provide an interesting idea of what can be expected from this modern simulation technique. Molecular dynamics provide a physical description of displacement cascades and some BCA concepts such as the threshold displacement energy can indeed be discussed in the light of these studies. Last, the description of the damage is also more accurate, and a new unit of radiation damage is here proposed to better account for the recombination and the nature of the defects created by displacement cascades.

    [1] G. Kinchin and R. Pease, “The displacement of atoms in solids by radiation”, Rep. Prog. Phys., 18, 1 (1955).

    [2] J.F. Ziegler, “SRIM-2003”, Nucl. Instr. And Meth. B, 219, 1027 (2004).

    [3] J.P. Crocombette, “Can thermal spike calculations reproduce displacement cascades?”, Nucl. Instr. And Meth. B, 267, 3152 (2009).

    [4] G. Martin, S. Maillard, L. Van Brutzel, P. Garcia, B. Dorado, C. Valot, “A molecular dynamics study of radiation induced diffusion in uranium dioxide”, J. Nucl. Mater., 385, 351 (2009).

    [5] F. Gao, D.J. Bacon, P.E.J. Flewitt, T.A. Lewis, “The influence of strain on defect generation by displacement cascades in a-iron”, Nucl. Instr. And Meth. B, 180, 187 (2001).

    Top

    11:50h – O6

    DISLOCATION LOOP SINK STRENGTHS CALCULATION IN ELASTICALLY ANISOTROPIC CRYSTALS

    Hadrien Rouchette1, Ludovic Thuinet1,3 , Alexandre Legris1,3, Antoine Ambard2, Christophe Domain1,3

    1Unité Matériaux Et Transformations (UMET), UMR CNRS 8207, Université Lille 1, 59655 Villeneuve D’Ascq, France

    2EDF R&D MMC, Électricité de France, 77810 Moret-sur-Loing, France

    3Laboratoire commun EDF-CNRS Étude et Modélisation des Microstructures pour le Vieillissement des matériaux (EM2VM), France

    Irradiation defect evolution is mainly driven by selective absorption of migrating defects by microstructural sinks, such as voids, dislocations and grain boundaries. Rate theory models generally consider a sink efficiency parameter for each type of migrating defect and sink. This parameter reflects the capacity of the sink to capture migrating point defects and clusters. The sink efficiencies depend on the migrating mechanism, the sink geometry (cylinder, spherical, toroidal) and on the elastic interactions between defects and the sink.

    Most analytical models rely on simplifying assumptions to calculate those sink efficiencies: (i) the sink is generally isolated in a defect-free region with arbitrary geometry, (ii) a concentration is fixed on the outer boundary of the sink-free region instead of considering a uniform point defect (PD) production, (iii) the PDs are considered as pure dilatation centers in an isotropic crystal when elasticity is accounted for. In order to better estimate the sink efficiencies, we developed a 3D phase-field (PF) model that includes the anisotropic microelasticity theory coupled to the diffusion of PDs in any dislocation network.

    Numerical results on benchmark cases have been compared with analytical solutions, when available. This preliminary step allowed to validate the method [1]. Nevertheless, calculations show that for dislocation loops, capture efficiencies have been significantly underestimated by existing models, even in the simplest cases.

    As an application, anisotropic properties of PDs in zirconium have been computed by atomic scale ab initio method [2], and used as input data in the PF model. Thus, the strong elastic anisotropy of SIAs has been taken into account to calculate the sink efficiencies of dislocation loops with different Burgers vectors and habit planes. The results show a preferential SIA flux to prismatic loops due to the strong deformation of SIAs in the basal plane. This effect tends to promote the growth of experimentally observed basal vacancy loops.

    [1] H. Rouchette, L. Thuinet, A. Legris, A. Ambard, C. Domain, “Quantitative phase field model for dislocation sink strength calculations”, Computational Materials Science, 88, 50 (2013)

    [2] G. Vérité, C. Domain, C.-C. Fu, P. Gasca, A. Legris, and F. Willaime, “Self-interstitial defects in hexagonal close packed metals revisited: Evidence for low-symmetry configurations in Ti, Zr, and Hf”, Phys. Rev. B 87, 134108 (2013).

    Top

    12:10h – O7

    MULTISCALE CALCULATIONS ON DISLOCATION BIAS

    Zhongwen Chang1, Pär Olsson1, Nils Sandberg2 , Dmitry Terentyev3

    1Reactor physics: Royal Institute of Technology KTH, Stockholm, Sweden

    2Swedish Radiation Safety Authority, SSM, Solna, Sweden

    3SCK-CEN, Nuclear Materials Science Institute, Mol, Belgium

    Irradiation induced swelling is one of the primary issues in development of new types of nuclear power plants. This effect on structural materials has severely restricted the lifetime of a reactor. It was recognized that the microstructure of the material after the irradiation is the key for understanding the phenomenon [1]. In addition, the microstructural evolution, such as the growth of voids, the formation and migration of dislocation loops, are of importance. Many researches have been done and some models have been developed to explain the variety of phenomena observed. One of the widely recognized parameter that used in those models is dislocation bias. Dislocation bias describes the preference of dislocation on self-interstitials than on vacancies. This bias is regarded as the intrinsic driving force in the standard rate theory model [2] and an integral part of production bias model (PBM) [3]. Nevertheless, the parameter is either obtained by fitting certain experiments with a model or calculated analytically with the first order size interaction between point defects and dislocations.

    In the present work, large-scale atomistic simulations with empirical potentials were applied to map the dislocation – point defects interaction energy in both FCC Cu and BCC Fe model lattices. The interaction energies are then used to numerically obtain the dislocation bias within the frame of finite element method. The importance of the dislocation core region is studied under a typical reactor operating temperature range 573 K to 1173 K and the dislocation densities 1*1013 m-2 and 1*1014 m-2. The results show that the FCC Cu model lattice gives a larger dislocation bias than BCC Fe, which is reasonable given the fcc materials have much more problem in void swelling than bcc ones. The possible reasons are studied and discussed in the paper.

    [1] L.K. Mansur, “Theory and experimental background on dimensional changes in irradiated alloys,” Journal of Nuclear Materials, 216, 97 (1994).

    [2] S.I. Golubov, B.N. Singh, “On recoil-energy-dependent defect accumulation in pure copper Part II. Theoretical treatment,” Philosophical Magazine A, 81, 2533 (2001).

    [3] B.N. Singh, S.I. Golubov, et al. “Aspects of microstructure evolution under cascade damage conditions,” Journal of Nuclear Materials, 251, 107 (1997).

    Top

    12:30h – O8

    CROWDION-SOLUTE INTERACTIONS: ANALYTICAL MODELLING AND STOCHASTIC SIMULATION

    Steve Fitzgerald

    Department of Materials, University of Oxford, UK.

    <111> crowdions are the most stable self-interstitial atomic (SIA) defect in the majority of the bcc transition metals, and are formed in large quantities when these metals, and their alloys, are irradiated [1]. In pure metals, they cluster together to form nanometre-scale prismatic loops that, together with their vacancy-formed counterparts, play an important role in irradiation hardening and embrittlement [2]. It is well known that alloys often fare better than elemental metals where radiation damage is concerned, and this can probably be attributed to the interactions of vacancies and interstitials with solute atoms (in dilute alloys) and precipitates (in concentrated ones). DFT calculations have determined the binding energies for crowdions and solute atoms in various systems [3-5], and it is important to note that whether the interaction is attractive or repulsive, it will impede the agglomeration of the SIAs into prismatic loops (a crowdion could either be bound to an attractive solute, or pinned between two repulsive ones). This process takes place over timescales of seconds or more (depending on the dose rate), and is hence inaccessible to atomistic methods (MD).

    In this paper I will apply the analytical Frenkel-Kontorova model [6,7] to crowdion defects lying in <111> strings containing a solute impurity. This was previously investigated in [8], where impurities having different masses and couplings were considered. These lead to quite different interaction potentials, and will be applied here to both transmutation impurities (same period, different group) and deliberately introduced alloying elements. The functional form of the crowdion-solute interaction potential is determined from the analytical model, and its parameters can then be fitted to the DFT results.

    Once the interaction potentials are known, a coarse-grained simulation methodology such as Langevin dynamics [9] can be developed. This has the advantage of tracking only the “interesting” degrees of freedom (the defects themselves), accelerating the simulations by ~107 cf MD and furthermore, does not require any rates to be calculated a priori cf kinetic Monte Carlo. The method will be introduced and preliminary results presented.

    [1] Nguyen-Manh, D et al, Phys Rev B 73 020101 (2006)

    [2] Dudarev, SL, et al, J. Nucl Mat 386 1 (2009)

    [3] Olsson, P, et al, Phys Rev B 75 014110 (2007)

    [4] Muzyk, M, et al, Phys Rev B 84 104115 (2011)

    [5] Kong, X-S, et al, Acta Mat 66 172-183 (2014)

    [6] Braun, OM and Kivshar, YS, “The Frenkel-Kontorova Model: Concepts, Methods, and Applications”, Springer (2004)

    [7] Fitzgerald, SP et al, Phys Rev Lett 101 115504 (2008)

    [8] Braun, OM et al, Physl Rev B 43 1060 (1991)

    [9] Swinburne, TD, et al, Phys Rev B 87 064108 (2013)

    Top

    12:50h – O9

    TEM OF RADIATION DAMAGE IN FERRITICS: ROLE OF IMAGE FORCES

    R. Schaeublin1, A. Prokhodtseva2, W. Wu3, B. Décamps4

    1Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland

    2Ecole Polytechnique Fédérale de Lausanne (EPFL), Centre de Recherches en Physique des Plasmas, Association Euratom-Confédération Suisse, CH-5232 Villigen PSI

    3Department of Nuclear Energy and Safety, Paul Scherrer Institut, CH-5232 Villigen PSI

    4Centre de Sciences Nucléaires et de Sciences de la Matière (CSNSM), CNRS-IN2P3-Univ. Paris-Sud 11, UMR 8609, Bât. 108, 91405 Orsay, France

    A review of the primary damage induced by irradiation in ultra high purity (UHP) Fe(Cr) alloys investigated by transmission electron microscopy (TEM) is given, with a critical analysis of results with respect to the presence of the free surfaces inherent to the TEM thin foil. Indeed, free surfaces induce so-called “image forces” that can bias the resulting irradiation induced damage. This was soon remarked, first by Masters in 1963 [1], in the study of the type of dislocation nanometric loops induced in pure Fe thin foils at 500-600°C. Since then many studies have confirmed that in such a thin foil the loop population is dominated by a0 <100> loops, counterbalancing the ½ a0 <111> ones observed in bulk irradiated Fe, and even at lower temperatures. Here we review results of UHP Fe(Cr) irradiations at room temperature in situ in a TEM coupled to two ion accelerators, providing simultaneously 500 keV Fe+ and 10 keV He+ ions [2,3]. Single Fe ions and dual Fe and He ions beam experiments were performed up to a dose of 1 dpa and to a He content of up to 1000 appm. Defects appear in TEM bright field imaging in the form of nanometric black dots with sizes between 1 and 5 nm; they stem from nanometric dislocation loops. From these studies it is concluded that the primary loop population is dominated by ½ a0 <111> loops, which in thin foils escape to free surfaces. This is now for the first time quantitatively explained by a proper analysis of the image forces, as we will see. These recent calculations of the image forces in a UHP Fe thin foil, including anistropy [4], indicate that free surfaces have a strong effect on the loop population within the foil, with a much larger and deeper impact within the foil on the ½ a0 <111> relative to the a0 <100> loops. With this new understanding, the effect of He and Cr on the irradiation induced nanometric loops will be discussed.

    [1] B.C. Masters, “Dislocation loops in irradiated iron”, Nature, 200, 254 (1963).

    [2] A. Prokhodtseva, B. Décamps, A. Ramar, R. Schaeublin, “Impact of He and Cr on defect accumulation in ion-irradiated ultrahigh-purity Fe(Cr) alloys”, Acta Materialia, 61, 6958 (2013)

    [3] A. Prokhodtseva, B. Décamps, A. Ramar, R. Schaeublin, “Comparison between bulk and thin foil ion irradiation of ultra high purity Fe”, Journal of Nuclear materials, 442, S786 (2013)

    [4] W. Wu, J. Chen, R. Schaeublin, “General dislocation image stress of anisotropic cubic thin film”, Journal of Applied Physics, 112, 093522 (2012)

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    13:10h – Lunch

    He & H effects

    14:30h – I2

    STOCHASTIC CLUSTER DYNAMICS SIMULATIONS OF MULTIPLE SPECIES IRRADIATION OF FERRITIC SYSTEMS

    Jaime Marian1, Tuan Hoang1, Vasily Bulatov1, Laurent Capolungo2, Enrique Martínez3

    1Lawrence Livermore National Laboratory, Livermore, CA, USA

    2Georgia Tech Lorraine, Metz, France

    3Los Alamos National Laboratory, Los Alamos, NM, USA

    Numerical techniques based on the mean-field approximation for irradiation damage accumulation calculations are computationally efficient for systems with low dimensions in the number of species. However, microstructural processes in irradiated nuclear structural materials involve multiple species, often evolving in heterogeneous environments. While object kinetic Monte Carlo methods are capable of spatial resolution and of treating complex species, they are numerically intensive and unable to reach irradiation doses that are representative of future nuclear systems. We will present the stochastic cluster dynamics (SCD) method for simulating irradiation damage in materials up to relevant doses. SCD is based on the mean field approximation but uses a kinetic Monte Carlo algorithm for evolving integer-valued defect populations in finite volumes. As such, SCD can provide volume fluctuations and treat complex species trivially. We will showcase the method’s capabilities by applying it to simulations of triple-ion (H/He/Fe) irradiation in Fe and FeCr, as well as fast neutron irradiation in W. We also describe a recent extension of the method to include spatially-resolved defect populations. In this fashion, materials such as thin films or TEM discs can be dealt with by using a finite-difference approach to discretize the spatial dependence of the defect population. We identify the conditions under which complex vacancy-He-H clusters accumulate giving rise to increased levels of swelling in TEM specimens.

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    15:00h – O10

    HELIUM BUBBLES IN BCC FE

    Xiao Gai , Roger Smith, Steven Kenny

    Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK

    The presence of transmutation-created helium plays an important role in the microstructural evolution of reactor steels under neutron irradiation where He bubbles have been shown to form in these materials [1]. Small helium-vacancy clusters may play an important role in the nucleation of the He bubbles. However, the atomistic properties of He in metals are difficult to identify experimentally. Thus atomistic simulations such as Molecular Dynamics and Kinetic Monte Carlo provide useful tools to study the formation and the stability of these clusters. Using these techniques, the properties of helium bubbles in a body-centred cubic Fe lattice have been examined using a recently constructed potential [2]. The atomic configurations and formation energies of different He-vacancy complexes have been determined and the evolution of small clusters and the stability of the larger bubbles close to collision cascades were also investigated [3]. The equilibrium configurations, stability and formation mechanisms for small He clusters have also been considered. Isolated interstitials and small clusters can diffuse quickly through the lattice. MD simulations of randomly placed interstitial He atoms at 500K show clustering over the time scale of nanoseconds with He clusters containing up to 4 atoms being mobile. He clusters containing 4 or 5 atoms can eject an Fe dumbbell interstitial which could then detach from the He cluster and diffuse with the remaining He-vacancy complex being effectively immobile. Collision cascades initiated near larger bubbles show that Fe vacancies produced by the cascades readily become part of the He-vacancy complexes. Energy barriers for He to join an existing bubble as a function of the He-vacancy ratio are also calculated. These can be larger than the diffusion barrier in the pristine lattice, but are lower when the bubbles contain excess vacancies, thus giving another indicating that certain optimal bubble sizes may be preferred. The implication is that there are two possible explanations for He bubble growth. The first depends on the addition of Fe vacancies by radiation to a cluster, followed by the subsequent accumulation of mobile He interstitials. The second explanation relies only on the diffusion of He and the fact that at certain optimal bubble sizes, it is preferable for He to diffuse in the lattice rather than join the optimal bubble.

    [1] L. Yang, H.Q. Deng, F. Gao, H.L. Heinisch, R.J. Kurz, S.Y. Hu, Y.L. Li and X.T. Zu. “Atomistic studies of nucleation of He clusters and bubbles in bcc iron,” Nucl. Instrum. and Meth. B 303 pp 68-71 (2013).

     

    [2] F. Gao, Huiqiu Deng, H. L. Heinisch, R. J. Kurtz. “A new Fe-He interatomic potential based on ab initio calculations in α-Fe,” J. Nucl. Mater. 418, pp 115-120 (2011).

    [3] X. Gai, R. Smith and S.D. Kenny. “Helium Bubbles in Fe: equilibrium Configurations and Modification by Radiation,” MRS Proceedings, 1514, pp 21-26 (2013)

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    15:20h – O11

    RATE THEORY SIMULATION OF SIMULTANEOUS EVOLUTION OF He AND H IN IRRADIATED FE

    Christophe J. Ortiz1, Pablo L. Garci­a Muller2, Rafael Vila Vazquez1, Miguel Pruneda3,4

    1Laboratorio Nacional de Fusion por Confinamiento Magnetico – CIEMAT, Madrid, Spain

    2Departamento de Teconologia – CIEMAT, Madrid, Spain

    3Centre d’Investigacio en Nanociencia i Nanotecnologia (ICN2), Bellaterra, Spain

    4Consejo Superior de Investigaciones Cientificas (CSIC) ), Bellaterra, Spain

    In future fusion reactors, a large amount of He is expected to form in the bulk of materials under irradiation due to nuclear transmutation between the lattice atoms and neutrons. He atoms tend to agglomerate with vacancies generated by displacement cascades, leading to the formation of bubbles, which can affect macroscopic properties of the material. It is thus crucial to predict the evolution of He under irradiation conditions. However, H atoms will also form by transmutation and then will evolve along with He. The question is therefore to know whether both gases will interact and whether there will be synergetic effects between H and He. It was observed experimentally by Tanaka et al [1] that when He and H are simultaneously implanted in FeCr model alloys, the swelling observed is significantly higher than when they are implanted separately. Tanaka also observed in that case, that the cavity size of observed bubbles is much larger than when they evolve separately. This strongly suggests that important synergetic effects are to be expected between H and He in Fe.

    In this work we present a model based on a Rate Theory approach that takes into account the simultaneous evolution of He and H in a–Fe and their interaction with defects generated in cascades, in particular, the agglomeration of He, H and V into clusters. Main parameters of this model, the binding energies of H-He-V clusters, were obtained by Molecular Dynamics simulations. We used this model to explore different experimental implantation conditions to determine whether synergistic effects between H and He could occur. Defects generated during displacement cascades were calculated using MARLOWE code based on the Binary Collision Approximation[2]. In particular we explored different conditions such as the implantation of He into Fe containing different initial H concentrations or such as sequential H/He implantations. In the conditions considered here, our simulations did not show any synergetic effects between He and H. This is explained by the fact that the binding energy of H to He-H-V clusters is always much lower than the one of He or V and thus clusters containing H dissolve quickly at room temperature.

    [1] T Tanaka, K Oka, S Ohnuki, S Yamashita, T Suda, S Watanabe, E Wakai, “Synergistic effect of helium and hydrogen for defect evolution under multi-ion irradiation of Fe–Cr ferritic alloys” Journal of Nuclear Materials, 329, 294 (2004)

    [2] M. Hou, C. J. Ortiz, C.S. Becquart, C. Domain, U. Sarkar, A. De Backer, “Microstructure evolution of irradiated tungsten: Crystal effects in He and H implantation as modelled in the Binary Collision Approximation” Journal of Nuclear Materials, 403, 89 (2010)

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    15:40h – O12

    PHYSICAL ORIGIN FOR STRONG ATTRACTION AND SELF-TRAPPING OF HELIUM IN METALS

    Hong-Bo Zhou, Jin-Long Wang, Feng Liu, Yu-Hao Li, Shuo Jin, Ying Zhang, Guang-Hong Lu

    Department of Physics, Beihang University, Beijing 100191, China

    Helium (He) is a typical impurity in metals. The solubility of He in metals is extremely low, yet it can lead to significant changes in microstructure and mechanical properties, such as the high temperature He embrittlement at extremely low concentration [1]. Helium is produced from (n, α) transmutation reactions in both fission and fusion. It is well known that He atoms are energetically favorable clustering with each other, producing point defects, and further resulting in radiation damage and mechanical property degradation of metals [2]. The self-trapping of He should be responsible for the effects of He on the properties of metals. However, the physical intrinsic mechanism for He self-trapping is still unclear. Taking tungsten (W) as an example, we have investigated the He-He interaction in metals using a first-principles method. The stability of He in metals is directly associated with the He-He interaction. We found a single He prefers to occupy the tetrahedral interstitial site in comparison with the octahedral interstitial site in W. To further shed light on the physical mechanism underlying the stability of He in W, it is helpful to decompose the solution energies into two contributions, including the mechanical contribution (the deformation energy induced by the embedded He atom) and the electronic contribution (the electronic effect). Interestingly, it is found that the mechanical contribution dominates in the relative stability of He, while the electronic contribution plays a key role in the poor solubility of He in W. Furthermore, the electronic contribution will decrease linearly with decreasing of electron density. The binding energy between He atoms is calculated to be 1.03 eV with the equilibrium distance of 1.50 Ã… in W, suggesting the strong attractive interaction of He. We further investigate the energies and the atomic configuration in order to explore the origin of the He-He attractive interaction in W. Our first-principles calculations suggest that the electronic contribution of two congregate He atoms is much lower than that of two isolated He atoms, but the mechanical contribution shows a contrary tendency. This can be attributed to that the deformation of W induced by two congregate He atoms is larger than that of two isolated He atoms, leading to the more significant decrease of electron density at the He most stable site. Consequently, the strong He-He attraction in W originates from the decrease of electron density induced by the synergistic effect of He atoms, which further explain the selftrapping of He.

    [1] H. Ullmaier, “The influence of helium on the bulk properties of fusion reactor structural materials,” Nuclear Fusion, 24, 1039 (1984).

    [2] W.D. Wilson, C.L. Bisson, and M.I. Baskes, “Self-trapping of helium in metals,” Physical Review B, 24, 5616 (1981).

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    16:00h – Coffee Break

    16:20h – O13

    MODELING OF HELIUM BUBBLE NUCLEATION AND GROWTH IN AUSTENITIC STAINLESS STEELS USING AN OBJECT KINETIC MONTE CARLO METHOD

    A. De Backer1,2, G. Adjanor3, C. Domain3, S. Jublot-Leclerc4, F. Fortuna4, A. Gentils4, T. Jourdan5, C. J. Ortiz6, A. Souidi7, C. Becquart1

    1 UMET, Universite Lille 1, Villeneuve d’Ascq, France

    2 CCFE, Culham Center for Fusion Energy, Culham, United Kingdom

    3 EDF R &D, MMC Centre des Renardieres, Moret-sur-Loing, France

    4 CSNSM, Universite Paris-Sud and CNRS, Orsay, France

    5 CEA, DEN, Service de Recherche de Metallurgie Physique, Gif-sur-Yvette, France

    6 CIEMAT, Laboratorio Nacional de Fusion por Confinamiento Magnetico, Madrid, Spain

    7 Universite Dr. Tahar Moulay de Saida, Saida, Algeria

    In the internals of current nuclear reactors Helium can be produced due to transmutation reactions. In service condition, the Helium production is small. In order to model He effect some He implantation have been performed with much higher He concentration rate. In these extreme conditions, Helium accumulates on microstructure defects and forms bubbles which impact the mechanical properties of the material. However the real conditions widely vary from one case to another and numerical modelling is necessary, particularly to predict the long term evolution. The models need to be validated using dedicated experiments. For this work, implantation of 10keV Helium in FeNiCr thin foils has been performed in JANNuS facilities and modeled using a multiscale approach. DFT atomistic calculations [HEPB2013] have been used for the properties of He and He-vacancy clusters and an adjusted method for the calculation of the slowing down of He ions [HOU2010]. The processes involved in the homogeneous bubble nucleation and growth of He clusters have been defined and implemented in the Object Kinetic Monte Carlo code, LAKIMOCA. With this multiscale approach, we modeled the formation of bubbles (with size up to nanoscale and thus visible in Transmission Electronic Microscope). Their densities and sizes have been studied as a function of fluence (up to 5×1019 He/m2) at two temperatures (473K and 723K). The effect of the thickness of the sample has been studied (25–250nm). In the investigated conditions, it has been observed that the damage is not simply due to the collision cascades but is strongly controlled by the He accumulation in pressurized bubbles that causes SIA emission. At high temperature a lower density of larger bubbles is obtained compared to the low temperature. The predictions of the model is that the presence of the back surface in thin foil prepared for in-situ TEM observation considerably reduces the bubble density by one order of magnitude in the region where the foil is 50 nm thick compared to the bulk sample. Comparison with the available experimental data has been discussed and a good agreement has been obtained. Furthermore the sensitivity of the model has been investigated which gave rise to improvements of the parameterization and perspectives.

    [HEPB2013] D. J. Hepburn, D. Ferguson, S. Gardner, G. J. Ackland, Phys. Rev. B 88, 024115 (2013).

    [HOU2010] M. Hou, C. Ortiz, C.S. Becquart, C. Domain, U. Sarkar, A. De Backer, Journal of Nuclear Materials, 403, 89 (2010).

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    16:40h – O14

    ATOMISTIC SIMULATIONS OF HELIUM BEHAVIOR TO EXPERIMENTALLY RELEVANT TIME SCALES

    Fei Gao1, Li Yang1,2, Ning Gao1,3, Richard J. Kurtz1

    1Pacific Northwest National Laboratory, Richland, WA 99352, USA

    2University of Electronic Science and Technology of China, Chengdu 610054, China

    3Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China

    It is well known that the nucleation, growth and diffusion of He bubbles will significantly affect the global evolution of microstructures and consequently degrade the mechanical properties of materials. Development of advanced steels for reactor components that are resistant to helium effects requires an understanding of the kinetics of helium atom migration, including the formation of clusters and bubbles, as well as their interactions with microstructural features in materials, which is inherently a multiscale phenomenon. A self-consistent accelerated molecular dynamics (SCAMD) method is developed to model infrequent atomic-scale events. In this method, the three-dimensional probability density of atoms is first determined using a canonical ensemble MD simulation, and then, a boost potential is derived from the probability density each time-step to accelerate the MD simulation. This approach is self-evolving and can be applied to the coupled motion of fast and slow dynamics. We have applied the SCAMD method to investigate the evolution of He-V clusters in times on the order of ~103 s in Fe. A helium-rich He-V cluster migrates by an interstitial-assisted mechanism, which contrasts with the vacancy-assisted migration mechanism found for a vacancy-rich He-V cluster. The corresponding pre-factors and migration energy berries for these He-V clusters are determined. In addition, the SACMD approach is applied to study the growth of a nano He-V cluster from two small clusters, and it is found that Ostwald ripening ism responsible for cluster evolution by mass transport from one cluster to another. These simulations well demonstrate the applicability of the method to rare events on a rough potential energy surface.

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    16:40h – O15

    FIRST PRINCIPLES SIMULATIONS OF HE ATOMS AT CU/NB INTERFACES

    C. Gonzalez, M. A. Cerdeira, S. L. Palacios, D. Fernandez- Pello and R. Iglesias

    Departamento de Fi­sica, Universidad de Oviedo, 33007 Oviedo, Spain

    Cu/Nb interfaces have been recently proposed as interesting candidates for the first wall in the future fusion reactors [1]. Due to the low miscibility of both materials, incoherent metallic nanosized interfaces can be an excellent sink where He atoms, produced in the fusion reaction, can be adsorbed, drastically reducing radiation damage. Experimentally, multilayered Cu/Nb composites with nanodimensional interlayer spacing exhibit excellent resistance to irradiationinduced structural changes [2]. The Orientation Relationships of interest of these bcc/fcc heterophase interfaces are the Kurdjumov-Sachs (KS) and, to a lesser extent the Nishiyama- Wassermann (NW), as given by Demkowicz et al [3]. We present a complete energetic analysis of the point defects at the Cu/Nb interface based on Density Functional Theory (DFT) techniques as implemented in the Vienna Ab initio Simulation Package (VASP) [4]. The results show the preferential sites for He implantation and the subsequent trapping of helium atoms inside the interface due to the relatively high energy barrier they have to overcome. Additionally, the relative stabilities of the monovacancies that will be created as a consequence of a radiation damage event have been studied for both metals. Typically, helium atoms in bulk systems tend to be stabilized inside such monovacancies as compared to their insterstitial positions. According to our energetic analysis, helium would also prefer to occupy a metallic vacancy instead of the main interface.

    [1] M. J. Demkowicz, A. Misra, and A. J. Caro, “The role of interface structure in controlling high helium concentrations” Current Opinion in Solid State and Materials Science 16, 101 (2012)

    [2] A. Misra, M. J. Demkowicz, X. Zhang, and R. G. Hoagland, “The Radiation Damage Tolerance of Ultra-High Strength Nanolayered Composites” JOM 59, 62 (2007)

    [3] M. J. Demkowicz, R. G. Hoagland, and J. P. Hirth, “Interface Structure and Radiation Damage Resistance in Cu-Nb Multilayer Nanocomposites” Physical Review Letters 100, 136102 (2008)

    [4] G. Kresse and J. Hafner, “Ab initio molecular dynamics for open-shell transition metals” Physical Review B 48, 13115 (1993); G. Kresse and J. Furthmüller, “Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set”, Phys. Rev. B 54, 11169

    (1996); G. Kresse and D. Joubert, “From ultrasoft pseudopotentials to the projector augmentedwave method”, Phys. Rev. B 59, 1758 (1999).

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    17:20h – O16

    REACTION KINETIC ANALYSIS OF REACTOR SURVEILLANCE DATA

    T. Yoshiie1, K. Sato1, Q. Xu1, Y. Nagai2

    1Research Reactor Institute, Kyoto University,

    Kumatori-cho, Sennan-gun, Osaka-fu 590-0494, Japan

    2The Oarai Center, Institute for Materials Research, Tohoku University,

    Oarai-cho, Ibaraki-ken 311-1313, Japan

    Nuclear materials property and performance change under high energy particle irradiation. Especially the degradation of structural materials performance and mechanical integrity is important in nuclear power plants. Nuclear reactors in the world are getting old and embrittlement of reactor pressure vessel steels by neutron irradiation is one of the important materials ageing issues. Recently, some surveillance data of reactors near and above 40 years old have indicated higher embrittlement than expected by embrittlement correlation methods. One of features of defect structures in surveillance pieces is low defect cluster densities. Main defect clusters are precipitates and only small number of loops is observed. No evidence of vacancy clusters was detected. From these defect structures, it is impossible to explain the increase of ductile brittle transition temperature. This paper describes the reaction kinetic analysis of defect structures in surveillance data. Authors have simulated the defect structural evolution of pressure vessel model alloys irradiated by Kyoto University Reactor using reaction kinetic analysis [1]. Experimental results of defect structural evolution, the increase of precipitates and microvoids, were well simulated. Using the same methods, and only changing the damage rate and irradiation temperature to power reactor condition, however, the defect structural evolution of surveillance pieces was impossible to simulate. It was clarified that the interaction between point defects and solute atoms in pressure vessel steels was quite different from that in model alloys.

    [1] T. Yoshiie, Q. Xu, K. Sato, ”Reaction kinetic analysis of damage rate effects on defect structural evolution in Fe-Cu”, Nucl. Inst. Meth. B 303, 37 (2013).

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    17:40h – O17

    SIMULATION OF RADIATION DAMAGE PARAMETERS IN THE TEST STATION FOR FUSION MATERIALS AT THE EUROPEAN SPALLATION SOURCE

    Alberto Milocco1, Giuseppe Gorini1, Luca Zanini2

    1Physics Department ’G. Occhialini’, Milano-Bicocca University, Milan, Italy

    2Target Division, European Spallation Source, Lund, Sweden

    Europe is working towards the construction of a 5 MW, 2 GeV spallation neutron source, the European Spallation Source (ESS). The primary goal of the ESS neutronic effort has been to optimisation of the thermal and cold neutron spectra in the beamlines devoted to condensed matter research. Nonetheless, fast neutron and/or proton spectra originating from the spallation process might be utilized parasitically without disturbing the beamlines’ performance, by adding suitably designed irradiation ducts in the monolith to the target or by irradiation of samples close to the target.

    The fusion community needs ad hoc material testing facilities but their construction proves to be cumbersome (as in IFMIF case). Thanks to their high energy neutron/proton fluxes, worldwide spallations source can meet some of the fusion requirements. At the ESS, the feasibility of a test station for fusion materials is being assessed [1].

    A box containing sample materials would be placed close to the target (at the centre of the Target-Moderator-Reflector system) and irradiated for mid-long periods. The radiation damage parameters of interest to the fusion community are the Displacements Per Atom (DPA), the Hydrogen and Helium gas production and their ratio to the DPA. Radiation transport is performed with the code MCNPX. The computational model includes a semi-realistic description of the ESS facility and of the fusion test station. The radiation damage parameters are simulated as reaction rates using radiation damage cross sections. The data files are recent evaluations from KIT and PSI, they extend up to 3 GeV and are based on the NRT model. Up to 16 DPA have been calculated in Iron. Moreover, the test station at ESS would provide a wide range of values for the ratio between gas production and DPA.

    Even though the approach is macroscopic and engineering-based, attention is also paid to the quality of the adopted cross section data. The issue of the uncertainties that might eventually be associated to them is addressed by comparison with different libraries of data and by sensitivity studies to different neutron/proton energies. This is done in order to perform a preliminary assessment of the reliability of present simulations, since experimental data are not available to our knowledge for the the high energy range of the radiation at the ESS.

    The potential for fast neutron/proton irradiation at ESS is quite noticeable. Some advance has been achieved for fusion applications. The simulated values of the radiation damage parameters are surely of interest to the fusion community, but they need to be thoroughly assessed to ascertain that the estimations are also reliable.

    [1] . Milocco, G. Gorini, L.Zanini, F. Mezei, S. Ansell, “Neutronic Design of Fast Neutron Irradiation Ports for the European Spallation Source”, Proc. Eleventh Topical Meeting on Nuclear Applications of Accelerators (AccApp 2013), 5-8 August 2013, Bruges, Belgium.

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    Tuesday, June 10

    09:00h – I3

    BALLISTIC EFFECTS ON STRUCTURE AND MECHANICAL PROPERTIES OF BOROSILICATE GLASSES: A MOLECULAR DYNAMICS STUDY

    J.-M. Delaye 1, D. Kilymis1, L.-H. Kieu1, S. Peuget1

    1CEA Marcoule, DEN, Laboratoire d’Etudes des Matériaux et Procédés Actifs, 30207 Bagnols-sur-Cèze, France

    In France, the long lived radionuclides are isolated in a glassy matrix called R7T7 glass. This is a complex aluminoborosilicate glass containing more than 30 different oxides. This matrix is aimed to be stored in a deep geological repository and, in order to guarantee its long term behaviour, it is important to certify in particular its resistance to internal irradiation.

    Recently, it has been shown experimentally using complex and simplified borosilicate glasses that the ballistic damage associated to the recoil nuclei stemming from α disintegrations is responsible for the evolution of some structural characteristics (swelling, decrease of the glass polymerization, etc.) that can explain some mechanical property changes (hardness and elastic moduli decrease, fracture toughness increase) [1,2]. Moreover, it appears that the fictive temperature of a borosilicate glass submitted to ballistic damage increases. All these effects present analogies with what happens when a glass is fast quenched.

    To have a deeper insight into the atomic processes induced by recoil nuclei, we have simulated series of displacement cascades by classical molecular dynamics in several simplified nuclear glasses prepared at different quenching rates [3]. Seven initial structures (containing SiO2, B2O3 and Na2O) were prepared with quenching rates ranging from 2 1012 K/s to instantaneous quench to dispose of structures characterized by a set of initial disorder and polymerization levels. Then, each glass is subjected to a series of 100 displacement cascades (600eV) to accumulate ballistic damage and the final structures are analyzed. We find effectively analogies between the structural changes observed in the irradiated structures and those induced by accelerating the quenching rate even if some differences remain.

    To complete our understanding of the ballistic effects in borosilicate glasses, we have simulated by classical molecular dynamics in pristine and in pseudo irradiated glasses (i.e. glasses quenched quickly to induce the ballistic effects) fracture propagation and nanoindentation. The dynamical processes, in particular the plastic flows, are different between the pristine and the pseudo irradiated glasses due to differences in density and polymerization level. These modifications can explain why there is an increase of the fracture toughness and a decrease of the hardness in glasses subjected to ballistic effects, in agreement with what is observed experimentally.

    [1] S. Peuget, P.-Y. Noël, J.-L. Loubet, S. Pavan, P. Nivet, A. Chenet, “Effects of deposited nuclear and electronic energy on the hardness of R7T7-type containment glass”, Nuclear Instruments and Methods in Physics Research B, 246, 379 (2006).

    [2] J. de Bonfils, S. Peuget, G. Panczer, D. de Ligny, S. Henry, P.-Y. Noël, A. Chenet, B. Champagnon, “Effect of chemical composition on borosilicate glass behavior under irradiation”, Journal of Non-Crystalline Solids, 356, 388 (2010).

    [3] J.-M. Delaye, S. Peuget, G. Bureau, G. Calas, “Molecular dynamics simulation of radiation damage in glasses”, Journal of Non-Crystalline Solids, 357, 2763 (2011).

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    09:30h – O18

    A STUDY OF BOROSILICATE GLASS INTER-ATOMIC POTENTIALS FOR RADIATION DAMAGE SIMULATIONS

    Kenny Jolley, Roger Smith

    1Mathematical Sciences, University of Loughborough, Loughborough, LE11 3TU, UK

    We compare three borosilicate glass (SiO2-B2O3) potentials from the literature and assess their suitability for use in simulations of radiation damage. We investigated two potentials from J. M. Delaye’s group (Delaye1996 [1] and Delaye2011 [2]) and one potential from M. Rushton (MRushton2008 [3]). For a range of densities, we generate glass structures by quenching at 5×1012 K/s using constant volume Molecular Dynamics. We find the minimum lattice energy occurs at a density of 1.75, 2.75 and 2.9gcm-3 respectively for each potential. These all differ from the experimentally reported value of 2.042gcm-3 [4], furthermore, our result of 2.75gcm-3 for the Delaye2011 potential suggests that the value (1.98gcm-3) stated by original authors [2] is only a local minimum.

    In each case, we also measure the bond lengths, mean bond angles, bulk modulus, melting point and displacement energy thresholds and compare to experimental data. Whereas the bond lengths and mean bond angles are reasonably well predicted, we find that the potentials predict melting temperatures, bulk moduli, and defect energy barriers that are generally higher than experimental data.

    [1] J.M. Delaye, D. Ghaleb, “Molecular dynamics simulation of SiO2 + B2O3 + Na2O + ZrO2 glass”, J. Non-Cryst. Solids 195 239 (1996)

    [2] L.H. Kieu, J.M. Delaye, L. Cormier, C. Stolz, “Development of empirical potentials for sodium borosilicate glass systems”, J. Non-Cryst. Solids 357 3313 (2011)

    [3] M.J.D. Rushton, R.W. Grimes, S.L. Owens, “Predicted Changes to Alkali Concentration Adjacent to Glass–Crystal Interfaces”, J. Am. Ceram. Soc., 91 [5] 1659 (2008)

    [4] C.A. Maynell, G.A. Saunders, S. Scholes, “Ultrasound propagation in glasses in the metastable immiscibility region of the sodium borosilicate system”, J. Non-Cryst. Solids 12 271 (1973)

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    09:50h – O19

    FROM ION IRRADIATION-INDUCED HIGH ELECTRONIC EXCITATION IN SILICA TO PERMANENT MODIFICATION IN STRUCTURAL, OPTICAL AND MECHANICAL PROPERTIES

    A. Rivera1, J. Olivares2,3, A. Prada1, O. Peña-Rodríguez1, P. Diaz1, A.R. Paramo1, M.J. Caturla4, E. Bringa5, T. Apostolova6, F. Sordo1,7, J.M. Perlado1

    1 Instituto de Fusión Nuclear, Universidad Politécnica de Madrid, Spain

    2 Centro de Micro-Análisis de Materiales, Universidad Autónoma de Madrid, Spain

    3 Instituto de Óptica (CSIC), Madrid, Spain

    4 Department of Applied Physics, University of Alicante, Spain

    5 CONICET & Instituto de Ciencias Básicas, U. Nacional de Cuyo, Mendoza, Argentina

    6 Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria

    7 Consorcio ESS-Bilbao, Leioa, Spain

    Silica irradiation with ions exceeding ~0.1 MeV/amu leads to high electronic excitation in nmsized regions around the ion trajectory. Subsequent evolution is determined by the carrier evolution and eventually by the transfer of the deposited energy to the lattice. The kinetic energy of atoms near the ion trajectory is then severely enhanced, which is the origin of a number of often irreversible effects, such as, electronic sputtering, mass transfer and atom disordering followed by fast atom rearrangement. As a result, technologically relevant permanent modifications in long tracks with typical radii of a few nm are observed, e.g., densification, color center generation, enhanced optical absorption, refractive index modification, changes in stoichiometry and stress/strain field generation. A full description requires different approaches to cover the myriad of interconnected processes evolving over very different timescales. In this presentation, we will describe our methodology to follow track formation based on: (I) a quantum kinetic model to describe the electronic evolution upon irradiation; (ii) Molecular dynamics to follow the thermal evolution of the lattice during and after heating due to energy transfer from the electronic system; (iii) a Monte Carlo code to describe track overlapping; (iv) a continuous model based on finite element methods to follow the stress/strain field evolution during ion irradiation; (v) simulations with finite difference time domain codes to explain the optical response to ion irradiation. This approach is useful to explain track formation in silica and makes possible to make predictions on the modification of relevant properties, as shown by comparison to a broad range of experiments. Examples include: track radius, track density, defect concentration, reflectivity variation or continuous layer formation by track overlapping.

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    10:10h – O20

    SIMULATION OF TRACKS IN SILICA WITH EMBEDDED SILVER NANOPARTICLES

    Alejandro Prada1, Eduardo Bringa2, Antonio Rivera1, Ovidio Peña1, J.M. Perlado1

    1Instituto de Fusión Nuclear, Universidad Politécnica de Madrid, C/José Gutiérrez Abascal 2, E-28006 Madrid, Spain

    2CONICET, Instituto de ciencias básicas, Universidad Nacional de Cuyo, Mendoza 5500, Argentina

    Metal nanoparticles such as silver with diameters of tens of nanometers in silica are interest due to their interesting optical properties, mainly related to the strong surface plasmon resonance (SPR) they present. Experiments show that metallic nanoparticles embedded in silica can be dramatically modified by swift ion irradiation, i.e., spheroids can be elongated resulting in ellipsoids with aspects ratios as high as 10. As a result the optical properties are strongly modified [1, 2, 3, 4]. In this paper we describe the elongation process of spheroids by swift heavy ion irradiation (SHI), using atomistic simulations. We carried out molecular dynamics simulations of ion tracks in silver nanoparticles embedded in a silica matrix. We modeled silica with the BKS potential, silica-silver interactions with different pair potentials [5], and silver with an embedded-atom method (EAM). First of all, we studied the stability of nanoparticles without irradiation, and found that small nanoparticles, below a threshold radius became amorphous. In order to study ion irradiation, we imposed an initially hot cylinder, with a radius and temperature correlated to the stopping power of the incident ion, with the purpose of observing the effects of SHI and possible changes in Ag nanoparticle structure which would then result in changed optical properties.

    [1] O. Peña et al., “Determination of the size distribution of metallic nanoparticles by optical extinction spectroscopy” Applied optics, Vol. 48, 3 (January 2009).

    [2] O. Peña et al., “Au-Ag core-shell nanoparticles: efficient all-plasmonic Fano-resonance generators” Nanoscale, Vol. 3, 3609 (2011).

    [3] B. Joseph et al., “Effect of 100 MeV Au irradiation on embedded Au nanoclusters in silica glass” NIMB, Vol. 256, 659 (2007)

    [4] Koichi Awazu et al., “Mechanism of elongation of gold or silver nanoparticles in silica by irradiation with swift heavy ions”, NIMB, Vol. 267, 941 (2009)

    [5] Dirk Timpel et al., “Silver clustering in sodium silicate glasses: a molecular dynamics study” Journal of Non-Crystalline Solids, Vol. 221, 187 (1997).

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    10:30h – O21

    OPTICAL PROPERTIES OF SILICA IRRADIATED WITH SWIFT HEAVY IONS

    Ovidio Peña-Rodríguez1, Pablo Díaz-Núñez1, Antonio Rivera1, Alejandro Prada1, Miguel Crespillo2, José Manuel Perlado1, José Olivares3,4

    1Instituto de Fusión Nuclear, Universidad Politécnica de Madrid, C/ José Gutiérrez Abascal 2, E-28006 Madrid, Spain

    2Dpt. Materials Science & Engineering, 204 TANDEC-Textiles and Nonwovens Development Center, 1321 White Avenue, The University of Tennessee, Knoxville, TN 37996-1950, USA

    3Centro de Micro-Análisis de Materiales, Universidad Autónoma de Madrid, Cantoblanco, E-28049 Madrid, Spain

    4Instituto de Óptica, Consejo Superior de Investigaciones Científicas, C/ Serrano 121, E-28006 Madrid, Spain

    Irradiation with swift heavy ions (SHI), roughly defined as those having atomic masses larger than 15 and energies exceeding 1 MeV/amu, may lead to significant modification of the irradiated material in a nanometric region around the (straight) ion trajectory (i.e., latent tracks). In the case of amorphous silica it has been reported that SHI irradiation originates nano-tracks of either higher density than the virgin material (for low electronic stopping powers, Se < 7 keV/nm) [1] or having a low-density core and a dense shell (Se > 12 keV/nm) [2]. The intermediate region has not been studied in detail but we will show in this work that essentially no changes in density occur in this zone. An interesting effect of the compaction is that the refractive index is increased with respect to that of the surroundings. In the first Se region it is clear that track overlapping leads to continuous amorphous layers that present a significant contrast with respect to the pristine substrate and this has been used to produce optical waveguides. The optical effects of intermediate and high stopping powers, on the other hand, are largely unknown so far. In this work we have studied theoretically (optical calculations using the Finite-Difference Time-Domain – FDTD – method, supported by molecular dynamics simulations) and experimentally (irradiation with SHI and optical characterization) the dependence of the macroscopic optical properties (i.e., the refractive index of the effective medium, nEMA) on the electronic stopping power of the incoming ions. Our results show that the core-shell tracks of the high-Se region produce a quite effective enhancement of nEMA that could prove attractive for the fabrication of optical waveguides at ultralow fluences (as low as 1011 cm-2).

    [1] J. Manzano, J. Olivares, F. Agulló-López, M.L. Crespillo, A. Moroño, E. Hodgson, Optical waveguides obtained by swift-ion irradiation on silica (a-SiO2), Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. At. 268 (2010) 3147–3150.
    doi:10.1016/j.nimb.2010.05.075.

    [2] P. Kluth, C.S. Schnohr, O.H. Pakarinen, F. Djurabekova, D.J. Sprouster, R. Giulian, et al., Fine structure in swift heavy ion tracks in amorphous SiO2, Phys. Rev. Lett. 101 (2008) 175503. doi:10.1103/PhysRevLett.101.175503.

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    10:50h – O22

    EFFECTS OF CHARGE VARIATION IN RECOIL SIMULATION: A STUDY USING DENSITY FUNCTIONAL BASED TIGHT BINDING METHOD

    Taukji Oda

    Department of Nuclear Engineering, Seoul National University, Seoul, Republic of Korea

    Displacement cascade simulations using classical molecular dynamics (MD) method have been widely utilized and have provided many microscopic insights in radiation damage processes. However, there are always some concerns in its accuracy, especially when it is applied to semiconductors and insulators. Specifically, classical MD cannot explicitly deal with charged defects which widely appear in semiconductors and insulators. To improve the accuracy, MD driven with forces determined by ab-initio calculations (AIMD) has been also performed for recoil simulations [1]. However, if we have use AIMD, the system size is inevitably limited and thus another concern on errors due to the system size effect emerges, such as a large temperature rise due to the introduction of energetic atom in a small simulation box. In order to simultaneously reduce these two sorts of errors and then improve the overall accuracy of recoil simulation, application of semi-empirical approach should be a promising choice. Density functional based Tight Binding (DFTB) method [2] coupled with self-consistent charge (SCC) algorithm [3] is a popular computational method among available semi-empirical methods. Because SCC-DFTB can allow charge redistribution, it is expected to improve the description of radiation damage evolution in semiconductors and insulators. In the present paper, the performance of SCC-DFTB recoil simulation is examined focusing on low-energy recoils (~50 eV) around threshold displacement energy. In simulations, DFTB+ code and LAMMPS codes are utilized for DFTB MD and for

    classical MD, respectively. The test material is rutile-TiO2, which has been extensively studied in classical MD regarding the recoil behavior and for which well-constructed DFTB setting is available. A series of recoil simulations are performed at 300 K with changing (1) recoil energy (< 100 eV) and (2) recoil direction. The non-SCC version of DFTB calculation is also performed to clarify the effect of charge variation in recoil simulations. In SCC-DFTB MD simulation, the average Mulliken charges were around +0.92e for Ti and -0.46e for O at 300 K, respectively. The ionic charges were fluctuated by up to ±0.06e for Ti and ±0.04e for O in thermal equilibrium conditions. In recoil simulations, several times larger charge variation was observed in the region where collision events took place. This charge redistribution modified the interatomic forces, and thus brought some differences in atomic trajectories between SCC and non-SCC MD simulations. In most cases, displacement was more easily taken place in SCC-DFTB simulation than non-SCC-DFTB simulation. In presentation, a statistical analysis of the defect formation probability, directional dependences of charge variation behaviors, and cell-size effects of simulation results will be given in comparison among SCC-DFTB, non-SCC-DFTB and classical MD simulation results.

    [1] F. Gao et al., Phys. Rev. Lett. 103 (2009) 027405.

    [2] D. Porezag et al., Phys. Rev. B 51 (1995) 12947.

    [3] G. Seifert et al., J. Quantum Chemistry 58 (1996) 185.

    11:10h – Coffee Break

    11:30h – I4

    ATOMISTIC MODELING OF ION IMPLANTATION TECHNOLOGIES IN SILICON

    Luis A. Marqués , L. Pelaz, I. Santos, P. López, M. Aboy, M. López

    Departamento de Electrónica, Universidad de Valladolid,

    E.T.S.I. de Telecomunicación, 47011 Valladolid, Spain

    Ion implantation is a very well established technique to create junctions in Si for the manufacturing of electronic devices. Apart from the introduction of dopants, the implantation process creates defects in the semiconductor substrate. Their complexity may range from point defects and small defect clusters, to extended defects such as dislocations, or even full amorphous layers, depending on the particular implantation parameters. Usually defects have negative effects on the final device performance, for example, causing the spread of dopant profiles or increasing leakage currents. Understanding how defects are generated during implantation and how they interact afterwards are key factors for defining strategies aimed to minimize such deleterious effects. Atomistic simulation techniques can be really helpful in this task, especially in conditions where experiments are difficult, expensive or just not feasible.

    This work will be focused on the role that implantation-induced defects have on the front-end processing of novel Si logic devices. Requirements for the manufacturing of such devices at the nanometric scale are becoming more and more demanding on each new technology node, driving the need for the fabrication of ultra-shallow junctions and ultra-thin body structures [1]. In particular, we use atomistic simulation techniques to study some of the aspects related to the application of novel implantation strategies, such as cluster and cold implants, aimed to reduce the amount of end-of-range defects through substrate amorphization. We also analyze the role played by surfaces and amorphous-crystal interfaces on damage generation and recombination. We show that, close to surfaces and amorphous-crystal interfaces, damage generation and damage stability are enhanced with respect to bulk. As a result, the threshold dose for amorphization is reduced in ultra-low energy implants, and the regrowth of ultra-thin body devices becomes more difficult.

    [1] International Technology Roadmap for Semiconductors, http://www.itrs.net/

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    12:00h – O23

    MULTISCALE INVESTIGATIONS OF NOBLE GAS BUBBLES FORMATION IN SILICON

    Laurent Pizzagalli1, Marie-Laure David1, Azzam Charaf-Eddin2, Sandrine Brochard1 and Marjorie Bertolus3

    1 P’ Institute, DPMM, UPR 3346 CNRS – University of Poitiers, Poitiers, France

    2CEISAM, CNRS UMR 6230, University of Nantes, Nantes, France

    3 CEA, DEN, DEC/SESC/LLCC, Centre de Cadarache, France

    It is now well established that the incorporation in materials of noble gas atoms inevitably leads to their aggregation and to the formation of bubbles. This situation has been exploited in semiconductors technology applications, such as gettering, smartcut, and strain engineering processes, but is also potentially detrimental to mechanical properties of materials used in fission and fusion domains, through the potential occurrence of swelling, embrittlement, or blistering. Although a large amount of works have been devoted to investigate the properties of these bubbles, the initial steps of the formation remain relatively undetermined, essentially because of the small space and time scales involved, and the difficulty to experimentally probe embedded noble gas atoms.

    In order to improve the current knowledge, we have performed several investigations over different space and time scales. First, the stability and mobility of single noble gas atoms embedded in silicon have been studied using first-principles calculations, taking into account the presence of vacancies [1]. These calculations allowed us for explaining available diffusion measurements. Also, we propose a model combining a repulsive interaction between the host local electronic density and the noble gas atom, and an elastic description of this atom as a deformable spherical inclusion into an homogenenous isotropic medium. Restricting our investigations to helium, aggregation mechanisms have also been investigated using firstprinciples calculations. In particular, we determined the optimal helium fillings in small vacancy clusters. In a second step, the size and time limitations of first-principles approachs were overcome by developing a semi-empirical potential based on the MEAM formalism and then performing classical molecular dynamics simulations [2]. The latter showed a clear separation of time between the aggregation of vacancies and the slower incremental filling of these cavities by single helium atoms. Migration and coalescence mechanisms were also observed during molecular dynamics runs. Finally, combining available experiments and the results of our calculations, an estimation of the pressure inside bubbles is proposed [3].

    [1] L. Pizzagalli et al., “Stability and migration of noble gas atoms in interaction with vacancies in silicon”, submitted to Phys. Rev. B (2014).

    [2] L. Pizzagalli et al., “Molecular dynamics simulation of the initial stages of He bubbles formation in silicon”, Modelling Simul. Mater. Sci. Eng., 21, 065002 (2014).

    [3] A. Charaf-Eddin et al., “First-principles calculations of helium and neon desorption from cavities in silicon”, J. Phys.: Condens. Matter, 24, 175106 (2012).

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    12:20h – O24

    ATOMISTIC SIMULATIONS OF SOLID-PHASE EPITAXY REGROWTH OF SILICON AMORPHIZED VIA ION IMPLANTATION

    M. Prieto-Depedro1, B. Sklenard2, I. Martin-Bragado1

    1IMDEA Materials Institute, Eric Kandel 2, Getafe, Madrid, Spain

    2CEA, LETI, Minatec Campus, 17 rue des Martyrs, 38054, Grenoble France

    Two different atomistic simulation techniques, Molecular Dynamics (MD) and Lattice Kinetic Monte Carlo (LKMC), are used to study solid-phase epitaxy regrowth of silicon amorphized via ion implantation. The amorphous silicon is deposited over a substrate forming an a/c interface. MD simulations are performed using Tersoff (T3) interatomic potential to describe interactions between atoms. For several nanosecond, samples are annealed at 2000 K, below the melting point predicted by T3. The shapes of recrystallization fronts are strongly dependent on the specific orientation of c-Si, where two have been considered: (100) and (110). In the absence of external stresses, defects generation occurs in some regions. The defective regrowth is analysed to determine if defects are generated because of a slowdown in recrystallization or if defects are the reason themselves of reducing the regrowth rate. Finally LKMC simulations are also carried out taking into account both possibilities, having larger sizes of the samples and lower temperatures, more realistic for device applications. Results obtained from both techniques are compared.

    [1] K. L. Saenger et al. “An examination of facet formation during solid-phase epitaxy regrowth of line-shaped amorphized regions in (001) and (110) Si” J. App. Phys., 101, 104908 (2007)

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    12:40h – O25

    CUMULATIVE IRRADIATION OF GaN BY SINGLE AND MOLECULAR IONS

    M. W. Ullah1, A. Kuronen1, K. Nordlund1, F. Djurabekova1, P. A. Karaseov2 and A. I. Titov2

    1 Department of Physics, University of Helsinki, P.O. Box 64, FIN-00014, Helsinki, Finland

    2 St. Petersburg State Polytechnic University, 195251, St. Petersburg, Russia

    We compare the results of Molecular dynamics (MD) simulation and experiments of damage formation in GaN under 0.6 keV/amu atomic and molecular ion irradiation. Simulated irradiations were done using P, F, PF2, PF4 and Ag ions directed towards the surface at an angle tilted 70 off the sample normal to study the effect of different projectiles in (0001) GaN at room temperature. Cumulative simulation of up to 50 ions of each type was done and for comparison between single and molecular ion same fluence was converted into Displacement per atom (DPA). It has been observed in experiment that distributions and the total amount of damage are dramatically different for atomic and molecular ions. In simulation, we do not observe any significant non-linearity with increase of ion mass when total number of defects has been taken into account. Although the total damage production is linear for all the molecules and ions, molecular projectiles produce bigger defect cluster in comparison with single ion. The difference in defect production can be explained as a result of non-linear collision cascade effect. In MD simulation we observed, stable damage formation for molecular ions is a result of effective clustering of point defects in the cascades with higher primary defect density.

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    13:00h – Lunch

    14:30h – O26

    ION-BEAM INDUCED DAMAGE IN GE: AN ATOMISTIC APPROACH

    Jose. L. Gomez-Selles1, Benoit Sklenard2, Ignacio Martin-Bragado1

    1IMDEA Materials Institute, Eric Kandel 2, Parque cientifico tecnologico, 28906 Getafe, Madrid, Spain

    2CEA, LETI, Minatec Campus, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France

    An atomistic kinetic Monte Carlo model of Ge damage accumulation and amorphization by means of ion-beam irradiation is presented. Damage distribution generated by different ions within the implanted target becomes crucial for the dynamic and temperature-dependent effects. The competition between damage creation and dynamic annealing depends on the pre-existing damage, evidencing the importance of channeling effects for different ions and giving rise to different implantation regimes. A comprehensive model based on different energies of recrystallization for amorphous pockets (AP) of different sizes is presented, explaining the mechanisms of damage buildup in Ge. The range of these energies starts at ~0.5 eV for the smallest AP until the 2.17 eV of Solid Phase Epitaxial Regrowth (SPER). The model is validated reproducing experimental results, showing an excellent agreement with the measurements of amorphous/crystalline interfaces and damage distributions for very distinct implantation conditions

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    14:50h – O27

    NEAR-SURFACE DISPLACEMENT CASCADES IN BCC IRON

    Y.N. Osetsky1, A.F. Calder2 and R.E.Stoller1

    1Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA

    2The University of Liverpool, Liverpool, Merseyside, UK

    Recent molecular dynamic simulations of hundreds of surface cascades in iron have revealed that a sub-picosecond shock-front defines the nature of surface and bulk damage under irradiation with energetic ions. A decelerating supersonic shock front produces a volume of destroyed lattice expanding away from the primary recoil event. The interaction between the transonic shock boundary and the free surface soon after the pka event can greatly enhance the later vacancy-type damage in the form of dislocation loops or/and craters from cascades. Similar to the subpicosecond interstitial capture mechanism by low density cores of secondary zones in the bulk study, the free surface can capture the higher density transonic boundary material, resulting in large groups of adatoms or if the cascades is sufficiently close to the surface, a crater with a rim of adatoms. Either way this leaves a high concentration of vacancies from the cascade core which later form 1/2[111] and [100] vacancy loops. Subsequent loss of 1/2[111] vacancy loops by glide to the free surface has also been directly observed.

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    15:10h – O28

    THE ROLE OF THE SURFACE MORPHOLOGY FOR MODELING SECONDARY ION FORMATION AFTER CLUSTER BOMBARDMENT

    B. Weidtmann, A. Duvenbeck, A. Wucher

    University of Duisburg-Essen, Duisburg, Germany

    In the last years, a hybrid computer simulation model for secondary ion formation was established in our workgroup, which is based on molecular dynamics simulations for the particle dynamics combined with a diffusive transport model for the electronic excitation energy generated by the particle kinetics in the collision cascade. The detailed trajectories of the particles leaving the surface due to sputtering in connection with the strongly space and time dependent surface electron temperature profile obtained this way are then employed to calculate an individual ionization probability for each sputtered particle via the substrate excitation model published by Sroubek et al. [1]. These values can then be averaged to predict experimentally measurable quantities like the average ionization probability, emission angle and velocity distributions of emitted secondary ions which are important to understand and interpret data measured in Secondary Ion Mass Spectrometry (SIMS) surface analysis

    In the above model, the surface plays an important role due to the fact that it enters the electron energy transport calculation as a Neumann boundary condition. As a consequence, the detailed fine structure of the surface morphology during the collision cascade initiated by a projectile impact may significantly influence the process of secondary ion formation. While this is not a big problem for events induced by atomic projectiles under linear collision cascade conditions, it evolves into a major problem for events induced by the impact of a cluster, where the crater formation process massively violates the common model assumption of a planar surface. Moreover, the assumption of a homogenous conduction band electron gas is also highly questionable in the crater volume where the density of the sample may momentarily be significantly reduced. Therefore, our computational approach is extended such to treat the detailed dynamics of the surface morphology as a moving boundary value problem employing a surface detection code developed by Sanner et al [2]. In doing so, it turns out that the results may strongly depend on the details regarding the determination of the exact shape of the momentary surface (i.e., the question which atoms are counted as surface atoms) and an emitted particle’s distance to that surface. Different implementation techniques and their influence on calculation results will be discussed.

    [1] Z. Sroubek, Spectrochim Acta B,44, 317 (1988)

    [2] M. Sanner, J.C. Spehner and A. Olsen, Biopolymers 38, 305 (1996)

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    15:30h – O29

    MD SIMULATIONS OF CONTINUOUS CARBON DEPOSITION ON A SILICON SURFACE

    Arindam Jana1,2, Ludovic Briquet1, Tom Wirtz1, Gerard Henrion2, Patrick Philipp1

    1 Department Science and Analysis of Materials (SAM), Centre de Recherche Public – Gabriel Lippmann, 41 rue du Brill, 4422 Belvaux, Luxembourg

    2 Institut Jean Lamour, UMR CNRS – Universite de Lorraine, Department Chemistry and Physics of Solids and Surfaces, Parc de Saurupt, CS 50840, F-54011 Nancy, France

    The continuous deposition of carbon atoms on a Si(100) surface was modelled up to a fluence of 5×1014 atoms/cm2 for energies ranging from 1 eV to 30 eV and incidence angles from 0° to 60° with respect to the surface normal. For this work the reactive force field developed by the group of Kieffer at the University of Michigan [1-3] and modified for irradiation studies [4] was used. In this presentation we will focus on the evolution of the system with carbon fluence and on the influence of the impact energy on the system structure and damage formation. For 1 eV energies, carbon atoms are expected to deposit on the surface or at lower depths than at 30 eV. This is also observed for fluences in the low 1014 at/cm2 range. For higher fluences, 1 eV carbon atoms can be found at implantation sites several monolayers below the surface, i.e. at depths much higher than expected for such a small energy. This is related to the change of the silicon structure with increasing carbon deposition. The mechanisms responsible for this behaviour as well as the carbon implantation and damage formation for the different simulation conditions will be discussed in detail.

    [1] L.P. Huang and J. Kieffer, “Molecular dynamics study of cristobalite silica using a charge transfer three-body potential: Phase transformation and structural disorder”, J. Chem. Phys., 118, 1478 (2003).

    [2] L.P. Huang and J. Kieffer, “Thermomechanical anomalies and polyamorphism in B2O3 glass: A molecular dynamics simulation study”, Phys. Rev. B, 74, 224107 (2006).

    [3] J.H. Zhou and J. Kieffer, “Molecular dynamics simulations of monofunctionalized polyhedral oligomeric silsesquioxane C6H13(H7Si8O12)”, J. Phys. Chem. C, 112, 3473 (2008).

    [4] L.G.V. Briquet, A. Jana, L. Mether, K. Nordlund, G. Henrion, P. Philipp, and T. Wirtz, “Reactive force field potential for carbon deposition on silicon surfaces”, J. Phys.: Condens. Mat., 24, 395004 (2012).

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    15:50h – O30

    COMPUTER SIMULATIONS OF FRAGMENT FORMATION DURING KEV C60 AND AR872 BOMBARDMENT OF OCTANE AND B-CAROTENE

    G. Pałka1, M. Kanski1, D. Maciazek1, M. Mlynek1, B. J. Garrison2 and Z. Postawa1

    (1) Smoluchowski Institute of Physics, Jagiellonian University, ul. Reymonta 4, 30-059 Krakow, Poland

    (2) Department of Chemistry, 104 Chemistry Building, Penn State University, University Park, PA16802 USA

    Molecular dynamics (MD) computer simulations are used to investigate material ejection and fragment formation during keV C60 and Ar872 bombardment of organic solids composed from octane and b-carotene molecules. Both systems are found to sputter efficiently. For the octane system, material removal occurs predominantly by ejection of intact molecules, while fragment emission is a main ejection channel for b-carotene. A difference in the molecular dimensions is proposed to explain this observation. Most of molecular fragments are created very early during C60 bombardment (<1 ps), while fragment formation is much more extended in time for Ar872 projectiles. A difference in the energy deposition pathways of these two projectiles is proposed to explain this observation.

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    16:10h – Coffee Break

    16:30h – POSTER SESSION

    Wednesday, June 11

    Materials for Nuclear Applications

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    09:00h – I5

    OBSERVATION AND SIMULATION OF INTERSTITIAL DISLOCATION LOOP COARSENING IN a-IRON

    S. Moll, T. Jourdan, H. Lefaix-Jeuland

    CEA, DEN, Service de Recherches de Métallurgie Physique, F-91191 Gif-sur-Yvette, France

    The kinetics of particle coarsening in materials by Ostwald ripening provides valuable information on physical parameters, such as the diffusion coefficient of monomers in the matrix [1]. Under irradiation, self-defect clusters such as interstitial dislocation loops are created, which can then evolve by Ostwald ripening upon subsequent annealing. In the case of α-iron, thermal vacancies are much more easily created than interstitials by the various elements of the microstructure, owing to the high formation energy of interstitials. In general, interstitial loop coarsening can therefore occur only by vacancy emission. However, in addition to interstitial loops, vacancy clusters are also created by irradiation. These clusters emit vacancies more easily than interstitial loops, which diffuse in the matrix and make interstitial loops shrink. This phenomenon renders the observation of interstitial loop coarsening very difficult.

    In this work we implanted α-iron with 60 keV helium ions at room temperature up to a high fluence of 1016 He/cm2, in order to create bubbles and dislocation loops that can be observed by transmission electron microscopy (TEM). Additional TEM observations during in-situ isochronal annealings revealed that the mean dislocation loop radius sharply grows at around 850 K. To interpret the results, the implantation and annealing phases were modeled using cluster dynamics [2]. These simulations show that bubbles are highly pressurized after implantation, so that the vacancy emission rate by bubbles is very low. Therefore dislocation loops evolve due to a net emission of vacancies by large loops and a net absorption by small loops. As in experiments, this Ostwald ripening by vacancy emission occurs at a precise temperature in our simulations, which greatly depends on the properties of the mono-vacancy. In particular, it is seen that accounting for entropic contributions to the formation and migration free energies of the vacancy permits to significantly improve the agreement between simulations and experiments [3]

    [1] A. J. Ardell, in Phase Transformations ’87, edited by G. W. Lorimer (Institute of Metals, London, 1988), p. 485.

    [2] T. Jourdan, G. Bencteux and G. Adjanor, “Efficient simulation of kinetics of radiation induced defects: A cluster dynamics approach”, J. Nucl. Mater., 444, 298 (2014).

    [3] S. Moll, T. Jourdan and H. Lefaix-Jeuland, “Direct observation of interstitial dislocation loop coarsening in α-iron”, Phys. Rev. Lett., 111, 015503 (2013).

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    09:30h – O31

    MICROSTRUCTURAL EVOLUTION IN BCC IRON UNDER IRRADIATION: IMPACT OF CARBON

    D. Terentyev1, I.M. Bragado2, Yu. Osetsky3, A. Serra4

    1Nuclear Materials Science Institute, SCK-CEN, Boeretang 200, B-2400, Mol, Belgium

    2IMDEA Materiales, C/ Eric Kandel, 2, Tecnogetafe, 28906 Getafe, Madrid РEspa̱a

    3Materials Science and Technology Division, ORNL, Oak Ridge, TN 37831, USA

    4 Dept. Matemàtica Aplicada III, Universitat Politecnica de Catalunya, Barcelona, Spain

    Understanding of the nano-structural evolution in BCC Fe-Carbon solid solution is an important step towards its assessment in real commercial ferritic steels applied in nuclear industry. Interstitial Carbon atoms dissolved the Iron/steel matrix are known to strongly interact with radiation defects, both: point defects and their clusters. The existence of this interaction implies strong impact of Carbon on the evolution of radiation-induced nano-structure. Another important point is that Carbon has different affinity to self interstitials, vacancies and dislocation loops. Consequently, the impact of Carbon on the evolution of nano-structure depends on irradiation temperature. Here, we discuss (i) different temperature regimes reflecting binding of Carbon with different nano-structural objects and (ii) mechanisms which Carbon affects the evolution through. The discussion concerns a series of recent atomistic studies published in 2010-2014, based on which we formulate the physical model to describe the microstructural evolution in Fe-Carbon system. The model is then applied in the novel modular object Monte Carlo code to provide quantitative assessment of the impact of Carbon on ‘experimentally detectable’ microstructure upon technologically relevant irradiation conditions.

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    09:50h – O32

    VACANCY ASSISTED DIFFUSION AND CLUSTERING OF INTERSTITIAL SOLUTES IN α-FE FROM FIRST PRINCIPLES

    Caroline Barouh, Chu-Chun Fu1 and Thomas Jourdan

    CEA, DEN, Service de Recherches de Métallurgie Physique, F-91191 Gif-sur-Yvette, France

    Under irradiation, a large amount of vacancies (V) are produced. They strongly interact with interstitial solutes (X) such as carbon (C), nitrogen (N) and oxygen (O) atoms, which are always present in steels, either as alloying elements or as impurities. The V-X attraction influences the mobility of both the solutes and the vacancies. On one hand, a decrease of the vacancy mobility has been revealed experimentally in the presence of carbon and nitrogen, most likely due to the trapping of vacancies at small vacancy-solute complexes [1, 2]. On the other hand, however, it is not clear whether vacancies always reduce the mobility of the interstitial elements.

    Density Functional Theory (DFT) calculations have been performed to study the energetic and kinetic properties of VnXm clusters in α-iron. Low-energy configurations of small VnXm have been determined. It has been revealed that vacancies enhance the clustering of solutes. Moreover, a systematic comparison of C, N and O – neighbors in the Periodic Table – shows different behaviors of the solutes in the vicinity of vacancies as a function of the electronic band filling. The mobility of the VnXm clusters has been carefully studied. We especially focused on the VnX clusters as it has been shown that V2 and V3 are even more mobile than a monovacancy in α-Fe [3]. As a result, all the V3X have been found to be very mobile. In particular, some clusters can be as mobile as the isolated solutes. Therefore, vacancies may be efficient to drag the interstitial solutes towards sinks such as grain boundaries, dislocations and free surfaces. Also, the result found on the mobility of small VnN clusters may explain the apparent discrepancy between the resistivity recovery experiments and the DFT data [2]. The interpretation of such experiments may be worth revisiting in the light of the present DFT prediction.

    The obtained DFT data have been used to parameterize a Cluster Dynamics model, based on the Rate Theory, which allows to predict the time evolution of the clusters concentration. The consequences of small highly mobile clusters on the kinetic properties of vacancies and solutes under various irradiation conditions have been explored using this model. This work is supported by the joint program “CPR ODISSEE” funded by AREVA, CEA, CNRS, EDF and Mécachrome under contract n°070551.

    [1] S. Takaki et al., “The resistivity recovery of high purity and carbon doped iron following low temperature electron irradiation,” Rad. Eff., 79, 87 (1983).

    [2] A.L. Nikolaev et al., “On the interaction between radiation-induced defects and foreign interstitial atoms in alpha-iron,” J. Nucl. Mater., 414, 374 (2011).

    [3] C.-C. Fu et al., “Multiscale modeling of defect kinetics in irradiated iron,” Nature Mater., 4, 68 (2005).

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    10:10h – O33

    MOBILITY OF VACANCY-SOLUTE COMPLEXES IN FERRITIC ALLOYS

    Luca Messina1, Pär Olsson1, Lorenzo Malerba2

    1KTH Royal Institute of Technology, Reactor Physics, 106 91 Stockholm, Sweden

    2Structural Materials Group, Institute of Nuclear Materials, SKC•CEN, B-2400 Mol, Belgium

    The formation of embrittling solute-defect nanoclusters in irradiated reactor pressure vessel (RPV) steels is the main phenomenon responsible for the degradation of the RPV mechanical properties. In particular, two classes of precipitates have been identified [1]: Cu-rich and Mn-Nirich precipitates (MNPs), the latter being potentially responsible for a further unexpected shift of the ductile-to-brittle transition temperature. The mechanisms leading to the formation and growth of these precipitates rely on the interaction between solute atoms and defects. It has been recently highlighted [2] that single vacancies and interstitials may act as carriers of solute atoms and drag them towards loops and voids. Therefore, the latter grow acting as solute cluster nucleation sites, their mobility being progressively reduced by their growing size and the presence of solutes. Whereas the role of Cu has been widely investigated, little is known about the synergic interaction between Mn, Ni and defects leading to the formation of MNPs. In particular, vacancy cluster mobility in presence of significant concentration of solutes is a key parameter to understand the causes and mechanisms of formation of these embrittling nanofeatures. Moreover, object kinetic Monte Carlo (KMC) simulations and rate theory models simulating the long-term evolution of such alloys require full knowledge of the vacancy cluster mobility, in presence or absence of solute atoms. Such information is still missing and can be obtained via atomistic KMC simulations. The aim of this work is therefore to investigate the mobility of vacancy clusters in ternary Fe-Mn-Ni alloys as function of their size and impurity content. Diffusion coefficients, mean free path and mean lifetime of the solute-vacancy complexes are calculated by using the KMC code Lakimoca [3], whose most recent parametrization has been improved in order to update it with the latest thermodynamic and kinetic studies on this alloy. The aim of the improved parametrization is to give a more realistic representation of the vacancy-Mn-Ni interaction, providing the ground for more reliable microstructural evolution simulations of RPV alloys. Furthermore, the obtained vacancy cluster mobilities will be employed in object KMC models for longer-term simulations that may help understanding the formation and evolution of these embrittling nanonclusters beyond the currently planned RPV end of life.

    [1] M.K. Miller et al., “Atom probe tomography characterizations of high nickel, low copper surveillance RPV welds irradiated to high fluences”, J. Nucl. Mat., 437, 107 (2013).

    [2] L. Messina et al., “Exact ab initio transport coefficients in bcc Fe-X dilute alloys”, submitted to Phys. Rev. B (2014).

    [3] C.S. Becquart, C. Domain, “Introducing chemistry in atomistic kinetic Monte Carlo simulations of Fe alloys under irradiation”, Phys. Status Solidi B, 247, 9 (2010).

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    10:30h – O34

    NANOSTRUCTURE EVOLUTION UNDER IRRADIATION IN MODEL ALLOYS FOR REACTOR PRESSURE VESSEL STEELS

    Monica Chiapetto1, Lorenzo Malerba1, Charlotte Becquart2, Christophe Domain3

    1Structural Materials Group, Nuclear Materials Science Institute, SCK·CEN, Mol, Belgium

    2UMET, CNRS UMR 8207, Universite de Lille 1, 59655 Villeneuve d’Ascq Cedex,France EDF R&D, Dept. MMC, Les Renardieres, 77818 Moret sur Loing Cedex, France.

    Radiation-induced embrittlement of bainitic steels is one of the most important lifetime limiting factors of existing nuclear light water reactor (LWR) pressure vessels (RPV). The primary mechanism of embrittlement is the obstruction of dislocation motion produced by nanometric defect structures that develop in the bulk of the material due to irradiation. The development of models that describe based on physical mechanisms the nanostructural changes in these types of materials due to neutron irradiation are expected to help to understand better which features are mainly responsible for embrittlement. The chemical elements that are thought to influence most the response under irradiation of low-Cu RPV steels, especially at high fluence, are Ni and Mn, hence there is an interest in modelling the nanostructure evolution in irradiated FeMnNi alloys. As a first step in this direction, we developed sets of parameters for object kinetic Monte Carlo (OKMC) simulations that allow this to be done, under simplifying assumptions, using a “grey alloy” approach that extends the already existing OKMC model for neutron irradiated developed for Fe-C binary alloys [1]. Our models proved to be able to describe the buildup of irradiation defect populations at the operational temperature of LWR (~300 °C), in terms of both density and size distribution of the defect cluster populations in FeMnNi model alloys. Specific reference irradiation experiments were simulated and insight into the physical mechanisms that influence the nanostructural evolution undergone by the materials during irradiation was obtained. The effect of dose-rate and temperature, as well as, in first approximation, solute concentration, was also studied and post-irradiation annealing simulations were performed. The main outcomes of this work will be highlighted in this paper.

    [1] V. Jansson, M. Chiapetto, L. Malerba, J. Nucl. Mater., 442, 341 (2013) “Paper_Title,” Journal_Title, vol, first_page (2012).

    10:50h – O35

    HELIUM SEGREGATION TO SCREW AND EDGE DISLOCATIONS IN A-IRON AND THEIR YIELD STRENGTH

    Enrique Martínez, Daniel Schwen and Alfredo Caro

    Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, 87545 NM, US

    Helium (He) presents one of the mayor concerns in the nuclear materials community as it modifies the mechanical properties of the material withstanding fast neutron spectra. Ferritic/martensitic steels are one of the main candidates as structural materials. It is well-known that He segregates to system heterogeneities promoting embrittlement. In this paper we follow a multiscale approach to study substitutional He segregation to screw and edge dislocations in _-Fe at different temperatures. We observe how He forms precipitates at the dislocation cores with spherical shapes. The yield strength turns out not to be linear in temperature. This effect is rationalized in terms of the competition between the distribution of bubbles at the dislocation core, increasing the number of bubbles per dislocation length with decreasing temperature, and the fact that temperature tends to soften the material. The fact that the diameter and spacing between bubbles vary with temperature invalidates commonly used models for the yield strength temperature dependence. In those models, temperature effects are decoupled from obstacle distribution, which in a general sense, is an incorrect assumption.

    11:10h – Coffee Break

    11:30h – O36

    HYDROGEN INDUCED GROWTH OF VACANCY CLUSTER NEAR TUNGSTEN SURFACE

    Yi-Nan Liu1,2, T. Ahlgren2, L. Bukonte2, K. Nordlund2, Xiaolin Shu1, Yi Yu1, Guang-Hong Lu1

    1School of Physics & Nuclear Energy Engineering, Beihang University, Beijing, 100191, China

    2Association EURATOM-TEKES, University of Helsinki, Helsinki, PO Box 64, 00560, Finland

    Tungsten metal is considered as one promising candidate for the plasma facing material in the future fusion reactors. However, the hydrogen blistering at its surface under plasma irradiation remains a challenging issue that needs to be addressed. It is believed that a vacancy trapping mechanism is responsible for the initial nucleation of hydrogen bubble and our previous work showed that vacancies could be generated by the retained hydrogen atoms in the system [1].

    In this work, we have performed classic molecular dynamic simulations to further investigate the probability of the vacancy cluster growth induced by hydrogen near the tungsten surface and explore the underlying mechanism. Hydrogen atoms are sequentially inserted into a vacancy cluster with different sizes and distances to the tungsten surface at temperature of 300 K. The crowdion formation is observed from the vacancy cluster towards the surface to yield new vacancies and the dynamic process is studied in detail from the aspects of atomic centrosymmetry, potential energy and stress. Especially, the stress release of hydrogen atoms inside the vacancy cluster is detected along with the crowdion formation. The volume of vacancy cluster is obviously increased due to the hydrogen atoms inside, and the cluster tends to grow towards the surface, even connecting with the surface. Furthermore, H2 molecules are detected in the vacancy clusters, which are considered as a critical feature for hydrogen bubble formation.

    [1] Y.-N. Liu, T. Ahlgren, L. Bukonte, K. Nordlund, X. Shu, Y. Yu, X.-C. Li, G.-H. Lu, “Mechanism of vacancy formation induced by hydrogen in tungsten”, AIP Advances, 3, 122111 (2013).
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    11:50h – O37

    TRAPPING OF HE CLUSTERS BY INERT-GAS IMPURITIES IN TUNGSTEN: FIRST-PRINCIPLES PREDICTIONS AND EXPRIMENTAL VALIDATION

    Duc Nguyen-Manh, Sergei L. Dudarev

    Culham Centre for Fusion Energy, Abingdon, Oxfordshire, OX14 3DB, United Kingdom

    Modelling radiation effects formed through the bombardment of W and W alloys by inert gas ions is important because inert gas defects contribute to embrittlement and swelling of structural and plasma-facing materials, an issue of critical significance for fusion power generation technologies. So far, experimental and theoretical effort has been focused primarily on the combined synergetic effects associated with the simultaneous accumulation of helium and hydrogen, and the effect associated with the incorporation of inert gases has not been systematically studied even though the agglomeration of noble gas atoms in metals and alloys is a well-known phenomenon observed in multi-beam ion implantation irradiation experiments. We investigate the structure and properties of defects resulting from the incorporation of noblegas atoms (He, Ne, Ar, Kr, Xe) into all the bcc transition metals, including tungsten, using firstprinciples density functional theory (DFT) calculations. Helium is a relatively small atom and the scale of He defect energies is smaller than that corresponding to other noble gas atoms. The atom size effect changes the relative stability of tetrahedral and octahedral defects for Ne, Ar, Kr and Xe in comparison with He. There is a remarkable trend exhibited by the binding energy associated with interaction between inert-gas atoms and vacancies, where a pronounced and colossal size effect is observed when going from He to Ne, Ar, Kr, Xe. The origin of this trend can be explained by electronic structure calculations that show that p-orbitals play an important part in distinguishing the last four inert-gas elements from helium, which contains only 1s2 electrons in the outer shell. Our predicted binding energies of a helium atom trapped by five different defects (the substitutional He atom defect HeV, Ne, Ar, Kr, Xe) in bcc-W are in an excellent agreements with experimental data obtained from thermal desorption spectroscopy [1]. We have also investigated the attachment of He clusters to inert gas impurity atom traps in tungsten and other bcc transition metals. The results form a trapping energy database required for modeling helium bubble nucleation in W and W alloys. This work was part-funded by the RCUK Energy Programme [grant number EP/I501045] and by the European Union’s Horizon 2020 research and innovation programme. To obtain further information on the data and models underlying this paper please contact PublicationsManager@ccfe. ac.uk. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

    [1] E.V. Kornelsen and A.A. van Gorkum, J. Nucl. Mater., 92, 79 (1980).

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    12:10h – O38

    TRAPPING OF HYDROGEN ON DISLOCATIONS IN BCC TUNGSTEN

    P. Grigorev1,2,5, D. Terentyev2, A. Bakaev1,3,5, V. Dubinko3, G. Van Oost4, E. Zhurkin5

    1 SCK·CEN, Nuclear Materials Science Institute, Boeretang 200, Mol, 2400, Belgium

    2 Department of Applied Physics, Ghent University, St. Pietersnieuwstraat 41,9000 Ghent, Belgium

    3 Center for Molecular Modeling, Department of Physics and Astronomy, Ghent University,

    Technologiepark 903, 9052 Zwijnaarde, Belgium

    4 National Science Center, Kharkov Institute of Physics and Technnology, Kharkov 61108, Ukraine

    5 Department of Experimental Nuclear Physics K-89, Institute of Physics, nanotechnology and

    telecommunications, St.Petersburg State Polytechnical University, 29 Polytekhnicheskaya str., 195251, St.Petersburg, Russia

    A new mechanism for the nucleation and growth of hydrogen bubbles on dislocations under plasma exposure of tungsten is proposed on the basis of direct ab initio calculations. The interaction of H with a screw dislocation (SD), the main microstructural feature of BCC metals including W, is assessed using density functional theory (DFT) calculations. It is demonstrated that H atoms are strongly bound to the SD core and exhibit fast one-dimensional (1D) migration along the dislocation line. An elementary dislocation segment accepts up to six H atoms practically without losing the interaction strength. Once the number of trapped hydrogen atoms exceeds eight, the emission of a jog on dislocation takes place converting a pure H cluster into a HN-V configuration. On the basis of these results the kinetic model was formulated to evaluate the conditions (i.e. range of temperature and flux exposure) for the transformation of pure H clusters into supercritical hydrogen-vacancy clusters attached to the dislocation line. The obtained results allow one to rationalize depth and temperature dependence of the experimentally observed hydrogen deposition after high flux low energy plasma exposure for ITER relevant conditions.

    12:30h – O39

    CRITICAL CONCENTRATION FOR HYDROGEN BUBBLE FORMATION IN TUNGSTEN

    S. Jin1, L. Sun1, H. B. Zhou1, Y. Zhang1, G. H. Lu1, X. L. Shu1, W. Zhang2, H. T. Lee3 and

    Y. Ueda3

    1 School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China

    2 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

    3 Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan

    In the fusion devices, tungsten (W) will be exposed to high fluxes of H isotopes, leading to unexpected surface blistering. The local H concentration is believed to directly affect the formation process of H bubble, and thus play a critical role in determining the property degradation and mechanical integrity of W. Using the energetics of H calculated from first principles as input parameters, we are able to determine the H concentration in metals such as W via the thermodynamic models. For H concentration in intrinsic metals without any defects, such thermodynamic model has been well established as the well-known Sievert’s law. It is more practical, however, to determine the H concentration in metals with defects such as a vacancy. With the consideration of defects in W, we further derive formulations of the thermodynamic model that are more suitable for vacancy and multiple H-vacancy complex. Based on the derived formulations, we calculate the equilibrium H concentration and its dependence on the H pressure and temperature using the first-principles dissolution energies for W as inputs [1]. At a certain temperature, the H concentration exhibits a sharp increase beyond a critical H pressure, which is mostly originated from the increase of H at the mH-V complexes. This indeed corresponds to a critical H concentration associated with the H bubble formation in W. Such critical concentration and pressure are clearly defined as the values when the concentration of H at one certain mH-vacancy complex first equals to that of H at the interstitial, which are 24 ppm/7.3 GPa at 600 K in W. Beyond such critical H concentration, considerable H atoms will accumulate into the vacancy, leading to the formation and rapid growth of the H- vacancy complexes, which is the preliminary stage of the H bubble formation. Consequently, we are able to plot a pressure-temperature phase diagram for W with and without H bubble formation separated by the critical H concentration as a function of temperature. Experimentally, the ion-driven H isotope permeation in W has been investigated using a high flux ion beam test device coupled with a permeation device [2]. This gives clear information whether the H bubble forms for the measured H concentration points based on the Zakharov’s diffusivity. The experimental H concentrations are within the predicted phase region by the thermodynamics model both with and without H bubble formation. This suggests that the predicted critical H concentrations are consistent with experimental observations, and thus may serve as a possible criterion to evaluate the H induced failure of metallic PFMs in the future fusion reactors.

    [1] L. Sun, S. Jin, X.C. Li, Y. Zhang, and G.H. Lu, J. Nucl. Mater. 434 (2013) 395.

    [2] H.Y. Peng, H.T. Lee, Y. Ohtsuka, and Y. Ueda, J. Nucl. Mater. 438 (2013) S1063.

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    12:50h – O40

    DIFFERENTIAL EFFECT OF HELIUM VERSUS NEON IMPLANTATION ON TUNGSTEN SURFACE EVOLUTION

    Marie Backman, Faiza Sefta, Niklas Juslin, Karl D. Hammond, Brian D. Wirth

    Department of Nuclear Engineering, University of Tennessee, Knoxville, Tennessee, USA

    Tungsten is a leading candidate material for the divertor in future nuclear fusion reactors. Previous experiments have demonstrated that surface defects and bubbles form in tungsten when exposed to helium and hydrogen plasmas, even at modest ion energies. In some regimes, between 1000 K and 2000 K, and for He energies below 100 eV, “fuzz” [1] like features form. The mechanisms leading to these surfaces comprised of tungsten “tendrils” which include visible helium bubbles are not currently known. Experiments [2] indicate that tungsten tendrils do not form under other noble gas plasma exposure such as neon or argon. Understanding why tungsten “fuzz” does not form under neon exposure may help isolate the specific role of helium in “fuzz” growth. Molecular dynamics (MD) simulations are well suited to describe the time and length scales associated with initial clustering and bubble formation mechanisms of helium and neon on tungsten. Initial noble gas clusters form and eventually grow to nanometer-sized bubbles. Previous MD simulations [2] have shown that during the bubble formation process, He clusters create self-interstitial defect clusters in W by a trap mutation process, followed by the migration of these defects to the surface that leads to the formation of layers of adatom islands on the tungsten surface. As the helium clusters grow into nanometer-sized bubbles, their proximity to the surface and extremely high gas pressures leads them to rupture the surface thus enabling helium release. Helium bubble bursting induces additional surface damage and tungsten mass loss, which varies depending on the nature of the surface. Here, we compare the effect of helium implantation in tungsten to that of neon. We investigate the cluster formation process resulting from the implantation of a flux of neon ions with an ion energy of 60 eV or 100 eV. We not only analyze the atomic retention and atom depth distribution as a function of time but more importantly the tungsten surface evolution resulting from neon plasma exposure and how it differs from that of helium plasma.

    [1] M.J. Baldwin and R.P. Doerner, “Helium induced nanoscopic morphology on tungsten under fusion relevant plasma conditions”, Nucl. Fusion 48, 035001 (2012).

    [2] M. Yajima et al., “Comparison of damages on tungsten surface exposed to noble gas plasmas”, Plasma Sci. Technol. 15, 282 (2013).

    [3] F. Sefta et al., “Tungsten surface evolution by helium bubble nucleation, growth and rupture”, Nucl. Fusion 53, 073015 (2013).

    13:10h – Lunch

    16:30h – EXCURSION (ALTEA)

    Thursday, June 12

    09:00h – I6

    CAN LOW-ENERGY ELECTRONS PRODUCE DNA STRAND BREAKS?

    Jorge Kohanoff1, Maeve McAllister1, Maeve Smyth1, Gareth Tribello1, Amy Williamson1, Lila Boüessel du Bourg1,2, Alberto Fraile1,3, Bin Gu1,4, and Ilya Fabrikant5

    1Atomistic Simulation Centre, Queen’s University Belfast, Northern Ireland, UK

    2Department of Chemistry, Ecole Normale Supérieur, Paris, France

    3CCQCN, Department of Physics, University of Crete, Heraklion, Greece

    4Department of Physics, NUIST, Nanjing, China

    5Department of Physics and Astronomy, University of Nebraska-Lincoln, NE, USA

    DNA damage caused by irradiation has been studied for many decades. Motivations include assessing the dangers posed by radiation, and understanding how to improve its efficiency in combating cancer. Since the seminal work of Sanche and co-workers [1], low-energy secondary electrons produced by ionization became an important player in the field, together with free radicals. In this presentation I will describe a research programme we are conducting with the goal of understanding, via computer simulation, the role of low-energy electrons in the behaviour of DNA components in a realistic, physiological-like environment. Firstly, we conducted R-matrix calculations for microsolvated nucleobases, which showed an enhancement of the dissociative electron attachment (DEA) cross section [2]. We then ran first-principles molecular dynamics (FPMD) simulations using initial conditions drawn from the DEA distribution, observing interesting differences between gas-phase, microsolvated and fully solvated environments. We then examined the role of excess electrons in the dynamics and thermodynamics of increasingly complex solvated DNA models, from bases to polynucleotides. Dynamical simulations after vertical attachment show a fast localization of the excess electron from a pre-solvated state to a valence bound orbital [3], while in polynucleotides they exhibit a rich pattern of localization and fluctuation between the various bases, which depends on the sequence. The protective role of histones in chromatin was addressed by simulating nucleobases in glycine. Free energy barriers for phosphodiester bond cleavage were calculated by means of constrained FPMD. Barriers of the order of 5-10 kcal/mol suggest that this is a regular feature at 300K [4]. The competition between bond cleavage and protonation was also studied by constrained MD, while the effects of sequencing on bond breaks in polynucleotides were addressed using metadynamics. Finally, we conducted simulations of shock waves in solvated nucleotides, in a first attempt to assess, from first principles, the role of thermo-mechanical effects due to ion irradiation.

    [1] B. Boudaiffa, P. Cloutier, D. Hunting, M.A. Huels, and L. Sanche. “Resonant formation of DNA strand breaks by extremely low energy (3-20 eV) electrons”, Science 287, 1658 (2000).

    [2] M. Smyth, J. Kohanoff and I. I. Fabrikant, “Electron-induced hydrogen loss in uracil in a water cluster environment”, submitted to J. Chem. Phys.

    [3] M. Smyth and J. Kohanoff, “Excess Electron Localization in Solvated DNA Bases”, Phys. Rev. Lett. 106, 238108 (2011).

    [4] M. Smyth and J. Kohanoff, “Excess Electron Interactions with Solvated DNA Nucleotides: Strand Breaks Possible at Room Temperature”, J. Am. Chem. Soc. 134, 9122 (2012).

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    09:30h – O41

    ROLE OF ELECTRONIC ENERGY LOSS ON ION IRRADIATION EFFECTS

    William J. Weber1,2, O. H. Pakarinen2, E. Zarkadoula2, Y. Zhang2,1

    1University of Tennessee, Knoxville, TN 37992, USA

    2Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

    Ion irradiation effects are the direct result of ion energy loss to atomic nuclei and electrons in solids. At low energies, nuclear energy loss dominates, causing damage production via ballistic collision processes, which are readily simulated by molecular dynamics (MD) methods [1]. At high energies typical of swift heavy ions, electronic energy loss dominates, leading to intense local ionization that can cause damage production [2], track formation [3] or damage recovery [4], which are often simulated by an inelastic thermal spike model using MD. At intermediate ion energies, nuclear and electronic energy losses are of similar magnitude and can lead to additive [1,5] or competitive [5,6] processes, which may be simulated by combining ballistic recoils and thermal spikes in a MD environment [7]. We have begun exploring the separate and combined effects of nuclear and electronic energy loss at intermediate ion energies by integrating experimental and computational approaches. The computational results, validated by experiments, demonstrate that electronic energy loss can lead to both additive and synergistic effects on damage production in some materials; while in other materials, the competitive effects of electronic energy loss lead to damage recovery. These results have significant implications for damage accumulation and microstructure evolution under irradiation, ion beam modification of materials, non-thermal recovery of ion implantation damage, and the response of materials to extreme radiation environments.

    This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division.

    [1] E. Zarkadoula et al., “High-energy radiation damage in zirconia: Modeling results,” J. Appl. Phys. 115, 083507 (2014).

    [2] M. Toulemonde et al., “Synergy of nuclear and electronic energy losses in ion-irradiation processes: The case of vitreous silicon dioxide,” Phys. Rev. B 83, 054106 (2011).

    [3] J. Zhang, et al., “Nanoscale Phase Transitions under Extreme Conditions within an Ion Track,” J. Mater. Research 25, 1344 (2010).

    [4] A. Debelle, et al., “Combined experimental and computational study of the recrystallization

    process induced by electronic interactions of swift heavy ions with silicon carbide crystals,” Phys. Rev. B 86, 100102 (2012).

    [5] L. Thomé et al. “Combined effects of nuclear and electronic energy loss in solids irradiated with a dual-ion beam,” Appl. Phys. Lett. 102, 141906 (2013).

    [6] Y. Zhang, et al., “Competing effects of electronic and nuclear energy loss on microstructural evolution in ionic-covalent materials,” Nucl. Instr. Meth. B 327, 33 (2014).

    [7] M. Backman, et al., “Cooperative effect of electronic and nuclear stopping on ion irradiation damage in silica,” J. Physics D: Applied Physics 45, 505305 (2012).

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    09:50h – O42

    ENERGY LOSS OF ELECTRONS IMPINGING UPON GLASSY CARBON, AMORPHOUS CARBON, AND DIAMOND: COMPARISON BETWEEN TWO DIFFERENT DISPERSION LAWS

    Maurizio Dapor1, Lucia Calliari1, Giovanni Garberoglio1

    1Interdisciplinary Laboratory for Computational Science, FBK and University of Trento, via Sommarive 18, I-38123 Povo, Trento, Italy

     The aim of this paper is to compare and discuss calculated inelastic mean free path, stopping power, range, and reflection electron energy loss spectra obtained using two different and popular dispersion laws [1,2]. We will present and discuss, as case studies, the results we obtained investigating the interaction of electron beams impinging upon three allotropic forms of carbon, i.e. solid glassy carbon, amorphous carbon, and diamond. We will compare the numerical results with experimental reflection electron energy loss spectra.

    [1] R.H. Ritchie and A. Howie, “Electron excitation and the optical potential in electron microscopy,” Philosophical Magazine, 36, 463 (1977).

    [2] D. Emfietzoglou, I. Kyriakou, R. Garcia-Molina, and I. Abril, “The effect of static manybody local-field corrections to inelastic electron scattering in condensed media,” Journal of Applied Physics, 114, 144907 (2013).

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    10:10h – O43

    SIMULATION OF THE ENERGY SPECTRA OF SWIFT PROTON BEAMS AFTER TRAVERSING CYLINDRICAL TARGETS

    Pablo de Vera1, Rafael Garcia-Molina2, Isabel Abril1

    1Departament de Física Aplicada, Universitat d’Alacant, E-03080 Alacant, Spain

    2Departamento de Física – Centro de Investigación en Óptica y Nanofísica, Regional Campus of International Excellence “Campus Mare Nostrum”, Universidad de Murcia, E-30100 Murcia, Spain

    We have performed detailed simulations of the energy spectra of proton beams after traversing thin cylindrical targets of different nature: liquid (water and ethanol jets) and solid (aluminium wire). Our simulations take into account the different interactions that each projectile can suffer when moving through the target, such as electronic stopping, nuclear scattering or electron charge-exchange processes. The geometry of the target is also accounted for. By using a suitable description of the electronic excitation spectrum of each one of the targets studied in this work, we find that our simulated energy distributions are in excellent agreement with recent published measurements [1, 2] by only slightly reducing the diameter of the cylindrical targets. Our results can be used to assess the available proton stopping powers, especially the one corresponding to liquid water, which in our case is different from the obtained in Refs. [1, 2], is closer to the Bethe stopping power, and in agreement with recent independent experimental measurements [3].

    [1] M. Shimizu, M. Kaneda, T. Hayakawa, H. Tsuchida, A. Itoh, Nucl. Instr. Meth. B 267, 2667 (2009).

    [2] M. Shimizu, T. Hayakawa, M. Kaneda, H. Tsuchida, A. Itoh, Vacuum 84, 1002 (2010).

    [3] T. Siiskonen, H. Kettunen, K. Peräjärvi, A. Javanainen, M. Rossi, W.H. Trzaska, J.

    Turunen, A. Virtanen, Phys. Med. Biol. 56, 2367 (2011).

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    10:30h – O44

    COMPUTER SIMULATION OF ANISOTROPIC EFFECTS IN ELECTRON TRANSPORT IN SOLIDS

    S. Hanke, P. Kucharzcyk, A. Duvenbeck, A. Wucher

    1Faculty of Physics, University of Duisburg-Essen, Germany

    In recent studies on ion bombarded metal-insulator-metal junctions it has been reported that the measured impact angle dependence of the internal electron emission yield can be qualitatively interpreted in terms of an anisotropic momentum distribution of kinetically excited electrons. [1,2] In order to quantitatively study the role of the aforementioned anisotropy, we are developing a microscopic ballistic transport model of electronic excitation based on the numerical solution of the corresponding Boltzmann transport equation for the distribution function f ( k , r ,t ) r r , where k r denotes the quasi momentum of the electrons, rr the space coordinate and t the time after primary particle impact. The underlying electron-electron scattering is described by a multidimensional collision integral with the scattering matrix elements taken as the Fourier-transform of a screened Coulomb potential. Employing this model it is possible to prepare distinct electronic excitation states in the momentum space as initial system conditions and to follow the time and space evolution of the distribution function f. By studying multiple equi-energetic initial excitation conditions, which, however, differ from each other by their orientational distribution of k-vectors in momentum space, the role of anisotropic excitation will be revealed and discussed in detail. The ultimate goal of the present work is to describe the anisotropic excitation dynamics associated with the impact of a keV particle onto a solid surface which has been detected experimentally [2]. Since the full three-dimensional solution of the Boltzmann equation still appears too computationally intensive to describe the excitation dynamics associated with a collision cascade involving thousands of atoms, we report on a strategy to include anisotropic effects into the simple kinetic excitation and transport model developed earlier in our group, using input parameters taken from the microscopic transport calculations.

    [1] S. Hanke et al., “Computer simulation of internal electron emission in ion-bombarded metals”, Nuclear Instruments and Methods in Physics Research B, 303, 55 (2013).

    [2] C. Heuser et al., “The possible role of anisotropy in kinetic electronic excitation of solids by particle bombardment”, Nuclear Instruments and Methods in Physics Research B, 269, 1090 (2011).

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    10:50h – O45

    ELECTRONIC STOPPING POWER OF H IN Ge FROM FIRST PRINCIPLES SIMULATIONS

    Rafi Ullah1, Fabiano Corsetti1, Daniel Sánchez-Portal2,5, Emilio Artacho1,3,4,5

    1CIC nanoGUNE, Tolosa Hiribidea 76, 20018 Donostia-San Sebastián, Spain

    2Centro de Física de Materiales CSIC-UPV/EHU,

    Paseo Manuel de Lardizabal 5, 20018 San Sebastián, Spain

    3Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom

    4Basque Foundation for Science Ikerbasque, 48011 Bilbao, Spain

    5Donostia International Physics Center, 20018 Donostia-San Sebastián, Spain

    The electronic stopping power (ESP) is the energy lost per unit distance to electronic excitations by a charged projectile shooting through a material. The most successful quantitative approaches for the calculation of the ESP are limited to the homogeneous electron gas model of metals and do not take into account important features like core state excitations and band gaps in case of insulators and semiconductors. Relatively recently time-dependent density-functional theory (TD-DFT) based first-principles calculations [1-3] have been performed for insulators and noble metals to explain the experimental observations [4,5]. TD-DFT calculations have successfully reproduced the expected threshold behavior in wide band gap insulators and the role of delectrons in non-linear behavior in gold. The ESP of small band gap semiconductors has not been studied using real-time first principles methods. We calculate the ESP of H in Ge using TD-DFT. The calculations are carried out in channeling conditions with different impact parameters and in different crystal directions. We also investigate the role of core state excitations as a potentially additional channel of dissipation. The agreement between simulations and experiment [6] is good, showing a well-defined velocity threshold, with a good agreement in its value at 0.05 atomic units of velocity.

    [1] J. M. Pruneda et al., “Electronic Stopping Power in LiF from First Principles”, Phys. Rev. Lett., 99, 235501 (2007)

    [2] M. A. Zeb et al., “Electronic Stopping Power in Gold: The role of d electron and the H/He Anomaly”, Phys. Rev. Lett. 108, 225504 (2012)

    [3] M. A. Zeb et al., “Electronic stopping power of H and He in Al and LiF from first principles” Nucl. Instrum. Meth. B, 303, 59 (2013)

    [4] S. N. Markin et al., “Electronic stopping of low-energy H and He in Cu and Au investigated by time-of-flight low-energy ion scattering”, Phys. Rev. B, 80, 205105 (2009)

    [5] S. N. Markin et al., “Vanishing Electronic Energy Loss of Very Slow Light Ions in Insulators with Large Band Gaps”, Phys. Rev. Lett., 103, 113201 (2009)

    [6] D. Roth et al., “A procedure to determine electronic energy loss from relativemeasurements

    with TOF-LEIS”, Nucl. Instrum. Meth. B, 317, 61 (2013)

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    11:10h – Coffee Break

    11:30h – O46

    ULTRA-INTENSE LASER IRRADIATION INDUCED-HIGH ELECTRONIC EXCITATION IN BANDGAP MATERIALS: A QUANTUM-KINETIC MODEL BASED ON BOLTZMANN EQUATION

    T. Apostolova 1, P. Detistov1, Y. Orieult2, J.M. Perlado2, A. Rivera2

    1 Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria

    2Instituto de Fusión Nuclear, Madrid, Spain

    A quantum kinetic approach based on the Boltzmann equation is employed to describe the response of dielectric and semiconductor materials to high excitation laser irradiation from the initial photo ionization inter-band processes through free carrier absorption inducing additional impact ionization to the final heat up by electron-phonon coupling. Swift thermalization through electron-electron scattering, Auger recombinatation and formation of free excitons, their self-trapping and subsequent decay are included. The energy exchange between the electrons and phonons are given by a separate equation for the lattice temperature where the rate of energy transfer from the electrons to the lattice per unit volume are defined quantum mechanically. As a result of our calculations the electron energy distribution function, average kinetic energy of the electron system and electron density are obtained as a function of laser intensity, laser photon energy (wavelength) and laser pulse duration. Furthermore we obtain the change of lattice temperature. Our calculations show that the resulting damage depends on the excited electron density induced by the irradiation. For low electron densities (<1019 cm-3), processes such as defect generation and densification occur. On the other hand high electron densities (∼ 1022 cm-3) lead to severe consequences for the integrity of the material. Adapting the same theoretical formalism it will be possible to explore a number of aspects related to the modification of plasmonic systems (metallic nanoparticles in dielectric medium) by means of high electronic excitation caused by the intense ultra short laser pulses. Finally, we will discuss the coupling of our kinetic model to a molecular dynamics code in order to follow lattice evolution in a very realistic manner.

    11:50h – I7

    SIMULATION OF RADIATION DAMAGE IN GRAPHITE

    Nigel Marks1, Marc Robinson2, Irene Suarez-Martinez2, Helen Christie3, Daniel Roach3, Keith Ross3, Alice McKenna4, Thomas Trevethan4, Malcolm Heggie4

    1Discipline of Physics & Astronomy, Curtin University, Perth, Australia

    2Nanochemistry Research Institute, Curtin University, Perth, Australia

    3Department of Physics, University of Salford, Manchester, UK

    4Department of Chemistry, University of Surrey, Guildford, UK

    Despite being one of the original nuclear materials, surprisingly few molecular dynamics simulations have been performed to study radiation response in graphite. The large difference between the MD literature for graphite and that of metals and oxides can be traced to the challenges associated with the description of bonding in carbon, in particular the anisotropic interactions which are central to sp2 carbon. Aside from point defect energetics and estimates of threshold displacement energies, little is known from a computational perspective about radiation processes in graphite. In a modern context, understanding of damage in graphite is motivated by lifetime extensions associated with Advanced Gas Reactors in the UK and proposed Gen-IV technologies such as the high-temperature gas-cooled reactor where graphite is a moderator.

    We have performed what we consider to be the first systemic study of radiation response in graphite using molecular dynamics. Chemical bonding is described using the Environment Dependent Interaction Potential (EDIP) for carbon [1], while short-range interactions are modelled using the conventional ZBL approach. Cascade simulations reveal that graphite behaves a manner remarkably distinct from metals and oxides, with the cascade primarily generating point defects, in contrast to connected regions of transient damage as are familiar from metals and oxides. Other unique attributes include exceedingly short cascade lifetimes and fractal-like atomic trajectories which show a remarkable visual similarity to historical models from the literature [2,3]. Unusually for a solid, the binary collision approximation is useful across a wide energy range, and as a consequence atomic displacements and defect production are consistent with the Kinchin-Pease and Norgett-Robinson-Torrens models, respectively. Comparison of defect energetics computed with EDIP against values from density-functional-theory show that the underlying description of defect behaviour is sound. At the level of defect creation itself, the MD simulations quantify threshold displacement energies for which a broad range of values have been reported in the literature.

    [1] N.A. Marks, “Generalizing the environment-dependent potential for carbon,” Phys. Rev. B, 63, 035401 (2001).

    [2] R. E. Nightingale, “Nuclear Graphite”, United States Atomic Energy Commission, Academic Press, 1962

    [3] J. H. W. Simmons, “Radiation Damage in Graphite”, Pergamon Press, 1965

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    12:20h – O47

    UNDERSTANDING THE PROCESS OF CARBON BUILD-UP ON SILICON AND THE FORMATION OF A Si–C MIXING REGION BY MOLECULAR DYNAMICS SIMULATIONS

    Kristin D. Krantzman1, Barbara J. Garrison2

    1Department of Chemistry and Biochemistry, College of Charleston Charleston, SC, USA

    2Department of Chemistry, The Pennsylvania State University, University Park, PA, USA

    Beams of single C+ ions are used for the incorporation of Si in the synthesis of thin films of SiC, which have a wide range of technological applications. We present a theoretical investigation of the use of C60 cluster beams to produce thin films of SiC on a Si substrate, which demonstrate that there are potential advantages to using C60+ cluster ion beams over C+ single ion beams. Molecular dynamics (MD) simulations of the multi-impact bombardmentof Si with 20 keV normal incident C60 projectiles are performed to study the buildup of carbon and the formation of a region of Si–C mixing up to a fluence of 1.6 impacts/nm2 (900 impacts) [1]. The active region of the Si solid is defined as the portion of target that contains almost all of the C atoms, and the height ranges from 3 nm above to 7 nm below the average surface height. The C fraction in the active region is calculated as a function of fluence and a simple model is developed to describe the dependence of the C fraction on fluence. An analytic function from this model is fit to the data from the molecular dynamics simulations and extrapolated to predict the fluence necessary to achieve equilibrium conditions in which the C fraction is constant with fluence. The fraction of C atoms at equilibrium is predicted to be 0.19 and the fluence necessary to achieve 90% of this asymptotic maximum value is equal to 4.0 impacts/nm2. Results with 8 keV C60 projectiles demonstrate that the C composition is highly dependent on the incident kinetic energy of the projectile. Thus, there is the potential to be able to fine tune the near surface C fraction for synthesizing films with different SiC compositions.

    [1] K. D. Krantzman, C. A. Briner, B. J. Garrison, “Investigation of Carbon Build-Up in Simulations of Multi-Impact Bombardment of Si with 20 keV C60 Projectiles” J. Phys. Chem. A Article ASAP, (2014).

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    12:40h – O48

    THE ROLE OF POINT DEFECTS IN THE SWELLING AND ELASTIC MODULUS OF IRRADIATED CUBIC SILICON CARBIDE

    Jianqi Xi, Peng Zhang, Chaohui He, Hang Zang, Daxi Guo, Tao Li

    Department of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an, China

    A molecular dynamics study has been performed to investigate the role of point defects in volumetric swelling and elastic modulus of irradiated 3C-SiC in the low and intermediate temperature regime. It is found that different kinds of point defects have distinctive effects on the swelling and Young’s modulus. The vacancies have the negligible influence on volumetric swelling while significant on the Young’s modulus. However, the value of volumetric swelling and the change in Young’s modulus due to the formation of other point defects vary with defect concentration exponentially which we are interested. Furthermore, it is indicated that the relation of the swelling of 3C-SiC and the contribution of point defects is independent on the temperature. During the tensile test, however, the role of carbon interstitial in Young’s modulus would be enhanced by temperature. Finally, an approximate model to estimate the antisite defect concentrations in irradiated 3C-SiC is presented.

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    13:00h – Lunch

    14:30h – I8

    ACCELERATED CHEMICAL AGING OF NUCLEAR WASTE FORMS

    C.R. Stanek

    Los Alamos National Laboratory, Los Alamos, NM USA

    Selection criteria for nuclear waste form compositions have predominantly focused on that composition’s radiation tolerance and leaching resistance. However, the effect of transmutation of radionuclides, especially “short-lived” 90Sr and 137Cs, to chemically distinct daughter products (Zr and Ba respectively) will also have a significant impact on nuclear waste form stability. Due to the technical challenges associated with this studying problem, the topic of transmutation has received limited attention during the past 30 years of waste form development. In order to develop a predictive capability to design radiation tolerant and chemically robust nuclear waste forms, we must first address a fundament materials science question: What is the impact of daughter product formation on the stability of solids comprised of radioactive isotopes? To answer this question, a multidisciplinary approach integrating density functional theory calculations with the synthesis and characterization of small, highly radioactive surrogate samples has been developed to accelerate the chemical aging process [1].

    The chemical evolution that occurs during the lifetime of a waste form can be simulated by performing DFT calculations to assess phase stability as a function of composition – and therefore time. For example, we predicted that rocksalt BaCl may form via the decay of 137Cs in CsCl where all of the cesium atoms are the radioactive isotope 137Cs, an important, short-lived fission product (half life of 30 years and 137Cs undergoes β-decay to 137Ba) [2]. We termed the phenomenon of the formation of metastable crystalline daughter phases via the transmutation of a radionuclide in the parent phase radioparagenesis. That our first principles calculations predicted the formation of 137BaCl from the decay of 137Cs suggests that in situ daughter product formation may lead to non-intuitive defect structures or phases. That is, based on ionic bonding theory, one would expect the formation of rocksalt BaCl2 upon Cs decay, rather than BaCl, since Ba is a rigidly 2+ cation. This unusual BaCl phase has never been observed, perhaps because it has never been synthesized in this manner.

    In this presentation, details of the accelerated chemical aging approach are discussed as well as recent results for a range of materials systems, including: 109Cd1-xAgxS, 55Fe2-xMnxO3 and 177Lu2-xHfxO3. In addition, implications of in situ transmutation are also discussed, including unconventional defect chemistry, backward design of nuclear waste forms, exploration of novel materials and even the role of transmutation on DNA stability [3].

    [1] C.R. Stanek, B.P. Uberuaga, B.L. Scott, R.K. Feller and N.A. Marks, “Accelerated Chemical Aging of Crystalline Nuclear Waste Forms,” Current Opinion of Solid State and Materials Science, 16, 126 (2012).

    [2] C. Jiang, C.R. Stanek, N.A. Marks, K.E. Sickafus and B.P. Uberuaga, “Predicting from First Principles the Chemical Evolution of Crystalline Compounds Due to Radioactive Decay: The Case of the Transformation from CsCl to BaCl,” Physical Review B, 79, 132110 (2009).

    [3] M. Sassi, D.J. Carter, B.P. Uberuaga, C.R. Stanek and N.A. Marks, “Carbon-14 decay as a source of non-canonical bases in DNA,” Biochim. Biophys. Acta, 1840, 526 (2014).

    15:00h – O49

    STUDY OF GAMMA RADIATION DAMAGE IN SPENT FUEL CANISTERS

    P. Olsson1*, E. Toijer1, N. Sandberg1,2, C. Anghel2

    1KTH Royal Institute of Technology, Reactor Physics, Roslagstullsbacken 21, 106 91 Stockholm, Sweden

    2Swedish Radiation Safety Authority (SSM), Solna Strandväg 96, 171 16 Stockholm, Sweden

    Spent nuclear fuel emits significant fluxes of low energy gammas, notably the 0.66 MeV decay channel of Cs-137 [1]. In the Swedish long-term repository program, the spent fuel bundles will be encased in a cast iron frame protected by a copper shell. It has been noted that the damage induced by these gamma rays could induce hardening and possibly embrittlement of the cast iron matrix through radiation and temperature enhanced copper diffusion and clustering over the course of the first few hundred years of storage [2]. We here revisit the theoretical foundations behind these conclusions, taking into account recent advances in the field of primary damage simulations [3]. The here obtained results are compared to electron irradiation experiments and the predicted consequences for the integrity of the canisters in the long term repository are discussed.

    [1] M.W. Guinan, “Radiation effects in spent nuclear fuel canisters,” SKB report TR-01-32 (2001).

    [2] L. Brissonneau, A. Barbu, J.-L. Bocquet, “Radiation effects on the long-term ageing of spent fuel storage containers,“ RAMTRANS 15 (2004) 121.

    [3] P. Olsson, C. Domain, “Revisiting the threshold displacement energies in iron using ab initio molecular dynamics simulations,” Submitted to Phys. Rev. Lett. (2014)

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    15:20h – O50

    FISSION GAS – DISLOCATION INTERACTIONS IN UO2

    Samuel T. Murphy1, Paul Fossati1, Robin W. Grimes1

    1Centre for Nuclear Engineering, Imperial College London, SW7 2BP, UK

    As the burn-up of a nuclear fuel is increased there is a marked increase in both the dislocation density and the concentration of fission products in the matrix. While it has been shown that fission products may segregate to the dislocations [1,2] there is currently very little understanding of what happens to the fission products once they reach the dislocation core. Shea has proposed that dislocations may provide a pathway for enhanced diffusion of gaseous fission products out of the fuel grain, which may explain the enhanced gas release observed during transients at high temperatures[3].

    Here we perform molecular dynamics simulations, employing empirical pair potentials to investigate the interactions between individual fission gas atoms and bubbles with dislocations. We explore both the influence of the dislocation on fission products by examining the diffusion of Xe in the core region of the dislocation as well as studying how the presence of the gas atoms/bubbles can affect dislocation motion. The simulations show that there is enhanced diffusion of Xe in the vicinity of the dislocation, however, this leads to the formation of small nanobubbles which become immobile thereby reducing the influence of the dislocations on Xe mobility. We also observe a significant increase in the critically resolved shear stress as the concentration of Xe around the dislocations is increased.

    [1] P. V. Nerikar, D. C. Parfitt, L. A. Casillas Trujillo, D. A. Andersson, C. Unal, S. B. Sinnott, R. W. Grimes and C. R. Stanek, “Segregation of xenon to dislocations and grain boundaries in uranium dioxide,” Phys. Rev. B, 84, 174105 (2011).

    [2] A. Guyal, T. Rudzik, B. Deng, M. Hong, A. Chernatynskiy, S. B. Sinnott and S. Phillpot, “Segregation of ruthenium to edge dislocations in uranium dioxide,” J. Nucl. Mater., 441, 96 (2012).

    [2] J. Shea, “An extension to fission gas release modeling at high temperatures,” EHPG Storefjell (2013).

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    15:40h – O51

    H DIFFUSIVITY IN d-PU

    Chris Scott1, Steven D. Kenny1, Mark T. Storr2, Andrew Willets2

    1Loughborough University, Loughborough, LE11 3TU, UK

    2Atomic Weapons Establishment, Aldermaston, Reading, RG7 4PR, UK

    The behaviour of H in Ga stabilised δ-Pu (Pu-Ga) has been investigated using atomistic computer simulation techniques. We have modelled both H diffusivity in a reference, undamaged material, and how H interacts with simple point defects, namely a vacancy and a split interstitial, and the effect the presence of these defects has on H diffusivity. These studies provide a number of insights into the diffusion of H in Pu-Ga systems. We have used the Modified Embedded Atom Method (MEAM) potential to model the interactions between Pu and Ga [1]. H-H self interactions were also modelled by the MEAM potential, which was developed to model H interactions in metals and allows for the formation of a H2 molecule. Pu/Ga-H interactions were modelled using the purely repulsive ZBL screened Coulomb potential. As such there was no possibility for the formation of the hydride and this limits the validity of our results to the solid solution portion of the phase diagram. H diffusivity was calculated in a reference, undamaged material over a range of Ga concentrations, from 1 to 10 at. %, covering the δ-phase stability range [2]. Although the system Ga concentration was fixed, due to the random Ga distribution within the system, there exist localised regions of higher Ga concentration. These localised regions of high Ga concentration were observed to block H diffusion. As the system Ga concentration was increased, the volume of the high concentration Ga regions increased, resulting in lower H diffusivity. This is an important finding that could have important implications, since it is known that grain boundaries are Gapoor [2] and thus could become conduits for H diffusion. Also, if you were able to control Ga concentration within the material then you could potentially control H diffusion. To understand the effect point defects have on H diffusivity we have considered systems containing a H interstitial and either a single Pu interstitial or vacancy. The binding between the H and the vacancy was shown to be very strong when the defects were within 0.4 nm of each other. Once a H atom entered this region it would always become trapped by the vacancy and dissociation was very unlikely. Transition searches have indicated that the barrier for dissociation was approximately 1.8 eV; this is very unlikely to occur at relevant temperatures and time scales; at 350 K this transition will occur approximately once every 260,000 years, although at higher temperatures it can occur more frequently (once per millisecond at 900 K). No binding was observed between the H and the interstitial and, in both cases, no long range interaction was observed.

    [1] S.M. Valone et al, Physical Review B, 73, 214209 (2006)

    [2] S.S. Hecker, Los Alamos Science, 290-335 (2000)

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    16:00h – Coffee Break

    16:30h – O52

    CLUSTERING OF CR AND FE IN IRRADIATED ZR ALLOYS

    Patrick A Burr1,2, Mark R Wenman1, Robin W Grimes1

    1Department of Materials, Imperial College London, UK

    2Institute of Material Engineering, ANSTO, Lucas Heights, NSW, Australia

    Zr alloys used for nuclear fuel cladding commonly contain small amounts of Cr and Fe, which are mostly retained in the form of intermetallic second phase particles. Dissolution of these particles, due to neutron irradiation is a known phenomenon, and the redistribution of the alloying elements may be cause of concern for high burn-up fuels [1]. The current work employs DFT to investigate the solubility, migration and clustering of dissolved Fe and Cr in α-Zr; and their interaction with intrinsic point defects caused by radiation damage. Recent unexplained XRD results show that high levels of Fe and Cr can be forced into solution with negligible lattice expansion [2]. The current work shows that this may be achieved through the accommodation of extrinsic species by substitution and interstitial mechanisms simultaneously. The strain field associated with a point defect of one type lowers the energy of formation for a defect of the other type. It also creates an attractive force in favour of clustering. Various defect clusters were investigated, and it was found that elongated defects present the lowest formation energy, and significant binding is observed even at long defect pair separations. This provides insight into the initial steps towards the formation of nano-sized elongated defects that have been observed in previous atom probe studies [3,4].

    [1] B. Cox, J. Nucl. Mater. 336, 331-368 (2004).

    [2] M. Ivermark, PhD Thesis, University of Manchester, 2009.

    [3] B. Gault, P.J. et al., Mater Lett, 91, 63–66 (2013).

    [4] Y. Dong, A.T. Motta and E.A. Marquis, J Nucl Mater, 442, 270–281 (2013).

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    16:50h – O53

    KINETIC MONTE CARLO MODELLING OF THE NANOSTRUCTURAL EVOLUTION OF FeC and FeCCu UNDER IRRADIATION AT 560 K

    V. Jansson1, M. Chiapetto2, L. Malerba2

    1Helsinki Institute of Physics and Department of Physics, University of Helsinki, Helsinki, Finland
    2NMS, SCK-CEN, Mol, Belgium

    Neutron irradiation induces structural nano-scale changes in steels that in the long term cause degradation of the mechanical properties of the materials. These processes are important to understand to e.g. ensure the integrity of the steel wall of the reactor pressure vessel during the operational life-time of a nuclear power plant. In our previous work, we developed a Object Kinetic Monte Carlo model to study some of these defects and their evolution in the Fe-C model alloy under irradiation at low temperature (~340 K) [1]. In this work we extend this model to the operational temperatures of light water reactors or ~560 K [2] and by introducing Cu solute atoms in the model.

    In the model we take into account the dynamics between small carbon-vacancy (C-V) complexes and large self-interstitial atom (SIA) clusters by introducing generic traps for the latter large clusters. The trapping energy is dependent on the type of C-V complex and also on the size of the trapped SIA cluster, as determined in earlier atomistic studies. The trapping energy is also used to account for the presence of the two different kinds of SIA clusters, 1/2 and SIA clusters. The latter kind is observed in experiments to dominate at 560 K, but our simulations show that the presence of invisibly small clusters of the former kind can not be excluded. The C-V clusters are shown to play an important role in the defect evolution under irradiation, e.g. as nucleation points for promoting the growth of large SIA clusters. We have also introduced Cu solute atoms in the model and we are able to reproduce results with good agreements from positron annihilation, small-angle neutron scattering, atom-probe tomography and transition electron microscopy studies of irradiation experiments at 560 K [3-4].

    The model, which is fully based on physical considerations and only uses a few parameters for calibration, is found to be capable of reproducing the experimental trends, thereby providing insight into the physical mechanisms of importance to determine the type of nanostructural evolution undergone by the material during irradiation.

    [1] V. Jansson and L. Malerba, “Simulation of the nanostructure evolution under irradiation in Fe–C alloys”, Journal of Nuclear Materials 443 (2013) 274–285.

    [2] V. Jansson, M. Chiapetto and L. Malerba, “The nanostructure evolution in Fe–C systems under irradiation at 560 K”, Journal of Nuclear Materials 442 (2013) 341–349

    [3] M. Lambrecht et al., Journal of Nuclear Materials 406 (2010) 84-89.

    [4] E. Meslin et al., Journal of Nuclear Materials 406 (2010) 73-83.
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    17:10h – O54

    DEFECT ACCUMULATION IN ION AND NEUTRON IRRADIATED MOLYBDENUM

    Donghua Xu1, Brian D. Wirth1, Meimei Li2, Marquis A. Kirk3, Gerrit VanCoevering4, Gary Was4

    1Department of Nuclear Engineering, University of Tennessee, Knoxville, TN, U.S.A.

    2Division of Nuclear Engineering, Argonne National Laboratory, Lemont, IL, U.S.A.

    3Division of Materials Science, Argonne National Laboratory, Lemont, IL, U.S.A.

    4Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI, U.S.A

    Both electron microscopy experiments and cluster dynamics modeling are conducted to examine ion and neutron irradiated molybdenum, with the intent to gain insights into fundamental mechanisms involved in irradiation damage in BCC type of metals. With a single set of physical constructs and parameters, the cluster dynamics model is able to reproduce, qualitatively and semi-quantitatively, the main features of defect accumulation experimentally observed in Mo under both thin film ion irradiation and bulk neutron irradiation. Among these features are the dependencies of defect density and size distribution on the dose (fluence), dose rate (flux), and sample thickness. The model is then employed to probe the feasibility of using a temperature shift to compensate for other differences, such as dose rate (flux) and sample geometry, in varying irradiation conditions, towards producing the same defect evolution path. Outstanding issues in the modeling of defect evolution in Mo and potentially other BCC metals are also discussed.

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    17:30h – O55

    MOLECULAR DYNAMICS SIMULATION OF DISPLACEMENT CASCADES IN NI-MO ALLOY

    Nengwen Hu1,2, Guiyan Wu2, Shifang Xiao2, Huiqiu Deng2, Cuilan Ren3, Ping Huai3, Wangyu Hu2,1

    1College of Materials Science and Engineering, Hunan University, Changsha, 410082, China

    2Department of Applied Physics, Hunan University, Changsha, 410082, China

    3Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800, China

    Hastelloys with the predominant compositions of Nickel and Molybdenum atoms are being widely considered as one of the most important candidates of structural material in high-temperature molten-salt reactor (MSR) because of their high fluoride salt melts corrosion resistance and high temperature strength. Nevertheless, neutron irradiation can greatly damage the microstructures of Hastelloys and leads to irradiation swelling, high-temperature strength degradation, hardening and so on, which severely limit the running lifetime of MSR. Moreover, few researches are carried out on the displacement cascades in Hastelloys so far. It is, therefore, of great importance to investigate the irradiation damage in the Ni-Mo alloy, the main composition of Hastelloys. Molecular dynamics methods are used to simulate the displacement cascades in nickel-molybdenum binary alloy. The effects of the radiation temperature, the primary knock-on atom (PKA) energy and the contents of solution molybdenum atoms on the displacement cascades have been systematically taken into consideration. It is found that the PKA energy and temperature can greatly influence the number of Frenkel Pairs. The number of Frenkel Pairs (NFP) increases as the PKA energy and the temperature increase. The size and types of defects and their clusters depend strongly on the concentration of molybdenum atoms. The clustering behavior of defects can also be impacted by the molybdenum atoms.

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    17:50h – O56

    DISLOCATION SINKS EFFICIENCIES IN BCC (Fe, V) AND FCC (Cu) CRYSTALS

    A.B. Sivak1, V.M. Chernov2, V.A. Romanov3, P.A. Sivak1

    1National Research Centre “Kurchatov Institute”, Moscow, Russia

    2JSC “A.A. Bochvar High-technology Research Institute of Inorganic Materials”, Moscow, Russia

    3SSC RF A.I. Leypunsky Institute for Physics and Power Engineering, Obninsk, Russia

    Simulation of self-point defects (SPDs) diffusion in elastic fields of edge, screw and mixed full dislocations in different slip systems was performed by object kinetic Monte Carlo (OKMC) method for BCC (Fe, V) and FCC (Cu) crystals with dislocation density rd = 1×1012 – 3×1014 m–2 at T = 293 – 1000 K and under applied external loads (up to 200 MPa). Elastic interaction of SPDs considered as elastic dipoles (vacancy and self-interstitial atom in stable and saddle-point configurations) with dislocation and external stress fields was calculated by means of the anisotropic theory of elasticity. SPD jump directions were chosen randomly according to probabilities determined by corresponding energy barriers. The dislocation sink (DS) efficiencies were calculated as x = k2/rd, where k –1 is the mean diffusional length of the SPDs before being absorbed at the sink. Model crystallites containing a dislocation in their center were right prisms of infinite length with square footing with periodic boundary conditions applied to their side faces. Starting positions of SPDs were distributed randomly in the model crystallite. The trajectories were calculated until the SPD approached to the distance shorter than 3a from the dislocation and was regarded as absorbed. At least 105 trajectories for each condition were calculated to obtain a statistically reliable value of the sink efficiency (the inaccuracy was less than 1%). DS efficiency weakly depends on a dislocation slip system at a given angle between dislocation line and Burgers vector (scatter within 12% at 293 K). The DS efficiencies decrease with increasing temperature and tend to a limit value for a non-interacting linear sink (NLS). Dislocation bias factor (D = 1 – x-/x+, minus for vacancies, plus for self-interstitial atoms) equals to ~43%, ~21%, ~24% for edge dislocations at T = 1000 K in Fe, V, Cu, respectively. The value of D is several times lower for screw dislocations than for edge dislocations in bcc (Fe, V) crystals. In fcc Cu crystal, the value of D for the screw dislocation is negative. Accounting for elastic interaction between DS and SPDs significantly increases the influence of the dislocation density on the DS efficiency. Obtained analytical expressions allow one to calculate the DS efficiencies at rd < 3×1014 m–2 with good accuracy. Comparison of OKMC-results for DS and NLS efficiencies (x and x0, respectively) under different applied external loads has led to the conclusion that x(sij)/x(0) ≈ x0(sij)/x0(0), where sij external stress tensor. Saralidze’s analytical solution for NLS [1] obtained in the case of a weak anisotropy of SPD diffusivity tensor describes accurately corresponding OKMC-results within the range of external stresses and temperatures of practical interest.

    [1] Z.K. Saralidze “Radiational Growth Due to Diffusional Anisotropy,” Soviet Atomic Energy, 45, 697 (1978).

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    18:10h – O57

    GENERATION OF ELASTIC WAVES IN BCC METALS UNDER IRRADIATION

    Korchuganov A.V.1*, Zolnikov K.P.1,2, Chernov V.M.3, Kryzhevich D.S.1,2, Psakhie S.G.1

    1Institute of Strength Physics and Materials Science of Siberian Branch Russian Academy of Sciences, Tomsk, Russian Federation

    2Tomsk State University, Tomsk, Russian Federation

    3A.A.Bochvar High-technology Research Institute of Inorganic Materials, Moscow, Russian Federation

    Features of elastic wave formation and propagation related to generation of atomic displacement cascades in iron and vanadium crystallites are studied by means of molecular dynamics method. Interaction of elastic waves with point defects and their clusters is investigated. Interatomic interactions in iron are described by pair potential [1]. Many-body potential constructed in terms of Finnis-Sinclair approach is used for vanadium [2]. Simulated crystallites contain from 200 thousand to 84 million atoms. Periodic boundary conditions are applied in all directions. Calculations are carried out for range of specimen temperatures from 10 to 300 K and energies of primary knock-on atom from 15 eV to 100 keV. Parameters of elastic waves generated by atomic displacement cascades have been assessed and their dependence on crystallographic direction of propagation, energy of primary knock-on atom and temperature of specimen has been investigated. It has been shown that elastic wave caused by Frenkel pair formation virtually attenuates on the distance of 20-40 lattice parameters depending on crystallographic direction and type of simulated material. It has been found that elastic waves generated by atomic displacement cascades move with speed of sound. These waves may cause rearrangement of point defect clusters and lead to enhanced point defect diffusion.

    [1] V. A. Romanov, V. M. Chernov, A. B. Sivak, “Energetic, Kinetic and Crystallographic Characteristics of Self-point Defects in Vanadium,” Abstracts of The 8th IEA International Workshop on Vanadium Alloys for Fusion Applications, St. Petersburg, Russia, June 5-6, 2006, p. 29 (2006).

    [2] V. A. Romanov, A. B. Sivak, V. M. Chernov, “Crystallographic, energetic and kinetic properties of self-point defects and their clusters in bcc iron,” Voprosy Atomnoy Nauki I Tekhniki, Ser.: Materialovedenie i novye materialy, 1(66), p.129 (2006).

    20:30h – BANQUET – VOLVO OCEAN RACE

    Friday, June 13

    09:00h – I9

    INFLUENCE OF THERMAL BARRIER OF GRAIN BOUNDARY ON BULK CASCADES IN ALPHA-ZIRCONIUM REVEALED BY MOLECULAR DYNAMICS SIMULATION

    Ya-Nan Jin, Tian-Yu Wu, Wen-Sheng Lai

    Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China

    Nano-structured materials were developed to mitigate radiation-induced damage as it was assumed that the grain boundary was helpful to annihilate radiation-induced defects. The effect of grain boundary (GB) on bulk cascades in nano-structured α-Zr has been studied by molecular dynamic simulations. It turns out that the existence of GB increases the total number of surviving defects in grains, suggesting that the GB may act as a thermal barrier and postpone the annihilation of defects within grains. Such phenomenon is more obvious with increasing the primary knock-on atom (PKA) energy. Thus, the influence of GBs on radiation damage in the nano-structured materials comes from the competition between damage increase in grains and defect annihilation at GBs.

    09:30h – O58

    THE RELATIONSHIP BETWEEN GRAIN BOUNDARY STRUCTURE, DEFECT MOBILITY, AND GRAIN BOUNDARY SINK EFFICIENCY

    Blas Pedro Uberuaga

    Los Alamos National Laboratory, Los Alamos, NM, 87545, USA

    Mass transport is a key materials property, enabling a large variety of applications from fast ion conductors to diffusion barrier coatings. The defect kinetics responsible for mass transport are also central to understanding phenomena such as radiation damage evolution and sintering. It has been long established that grain boundaries, and interfaces more generally, significantly influence mass transport. Thus, nanoscale architectures often exhibit enhanced transport compared to larger scale counterparts. The importance of boundaries for influencing mass transport has lead to a number of theoretical studies of defect mobility at and near grain boundaries. These studies have found that defect mobility at boundaries is complex, with the boundaries exhibiting trap sites that in some cases impede the mobility of defects as compared to far from the boundary. However, in spite of this large body of work, there are still important questions that remain unanswered, particularly relating to the interplay between defects produced in extreme environments, such as irradiation, and grain boundaries. In such conditions, the defect content at the boundaries can be significantly higher such that defects begin to interact and cluster. How this clustering is influenced by grain boundary structure and subsequently influences the overall response of the material is an open question. We present atomistic simulations of the mobility of defects at a set of grain boundaries that differ in their structure and character. We find that the mobility is very sensitive to both grain boundary structure and the size of the defect cluster in question. In particular, as the cluster size increases, the mobility of the cluster decreases substantially such that defect mobility within the boundary plane is significantly slower than in the bulk. Using a lattice kinetic Monte Carlo model, we explore the ramifications of this reduced mobility on the sink efficiency of the boundary. We find that changes in the in-boundary mobility of defects can dramatically change the sink efficiency of the boundary. We propose that one important factor in determining the sink efficiency of a grain boundary is the mobility of the defects within the boundary, which depends not only on the grain boundary character, but also the irradiation conditions.

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    09:50h – O59

    INTERACTION BETWEEN GRAIN BOUNDARIES AND DISPLACEMENT CASCADES IN HCP-ZIRCONIUM

    K. P. Boyle, T. J. Robotham

    CanmetMATERIALS, Natural Resources Canada, Hamilton, Ontario, Canada

    Zirconium alloys are used in current nuclear reactor systems as they offer exceptional neutron transparency and adequate mechanical stability at the operating temperatures of interest. Nevertheless, zirconium and its alloys are susceptible to degradation phenomena, such as irradiation creep and growth. Although a complete understanding is lacking, mechanistic theories generally attribute irradiation-enhanced creep and growth to the interaction between irradiation induced defects and pre-existing crystallographic defects such as dislocations and grain boundaries. Nevertheless, a more fundamental understanding of the mechanisms associated with irradiation creep and growth is desirable in order to improve our ability to assess and predict the life-time of in-core reactor components. In the current work, molecular dynamics simulations are used to study the interaction of displacement cascades with symmetric tilt grain boundaries in hcp-zirconium as a function of distance of the incident recoil atom from the grain boundary plane. Changes in the energetics and structural properties of the grain boundaries are monitored. Further, the number and character of defects produced in the bulk and their tendency to migrate to the grain boundary is quantified. Grain boundaries are found to preferentially absorb self-interstitial defects such that the capacity for absorption is found to scale with grain boundary energy. The findings are discussed with regards to the implications on the mechanisms of irradiation-enhanced creep and growth of incore zirconium components.

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    10:10h – O60

    THE EFFECT OF INTERSTITIAL IMPURITIES AND ALLOYING ELEMENTS ON GRAIN BOUNDARY IN VANADIUM

    Pengbo Zhang1,2, Ruihuan Li2, Jijun Zhao2, Pengfei Zheng3, Jiming, Chen3

    1 Department of physics, Dalian Maritime University, Dalian 116026, China.

    2Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University

    of Technology, Ministry of Education, Dalian 116024, China.

    3Southwestern Institute of Physics, Chengdu 610041, China.

    Vanadium (V) based alloys are considered as the promising structural materials of first wall of fusion reactor. Understanding the effects of interstitial impurities and alloying elements on embrittlement and structural properties of grain boundary is important for the future usage of these structural materials. As a continuation of our previous efforts on the behavior of H/He impurities and additional elements with vacancy in pure V and V-4Cr-4Ti alloy [1-4], in this work, we investigated the effects of interstitial impurities (C, N, O, S, and B) and alloying elements (Ti and Cr) at a Σ3 (111) grain boundary (GB) in V using first-principles methods. Energetically, all interstitial impurities are favorable to locate at the GB with exothermic reaction. Strengthen energy calculations indicate that interstitial C, N, and B strengthen the Σ3 GB cohesion, while interstitial O and S act as embrittlers. We also compared the behaviors of these impurities at V GB with the available results of them at Fe GB. For alloying elements, Cr atom slightly prefers GB center than substitutional site and Ti atom prefers substitutional site energetically. We found that Cr atom acts as strengthening of the Σ3 GB cohesion, while Ti atom acts as embrittler. Moreover, we analyzed charge density distributions and local structural changes after additional elements, giving a reasonable explanation for strengthen and embrittlement behaviors.

    [1] P. B. Zhang, J. J. Zhao*, Y. Qin, B. Wen, Stability and migration property of helium and self defects in vanadium and V–4Cr–4Ti alloy by first-principles, J. Nucl. Mater., 413, 90 (2011).

    [2] P. B. Zhang, R. H. Li, J. J. Zhao*, Synergetic effect of H and He with vacancy in vanadium solid from first-principles simulations, Nucl. Instrum. Methods Phys. Res., Sect. B, 303 74 (2013).

    [3] C. Zhang, P. B. Zhang, R. H. Li, J. J. Zhao*, C. Dong, Stability and migration of vacancy in V–4Cr–4Ti alloy: Effects of Al, Si, Y trace elements, J. Nucl. Mater., 442, 370 (2013).

    [4] R. H. Li, P. B. Zhang, C. Zhang, X. M. Huang, J. J. Zhao*, Vacancy trapping mechanism for multiple helium in monovacancy and small void of vanadium solid, J. Nucl. Mater., 440, 557 (2013).

    10:30h – O61

    AB INITIO SIMULATION STUDY OF INTERFACES IN NANOSTRUCTURED TUNGSTEN

    Carlo L. Guerrero C.1, C. González2, R. Iglesias2, N. Gordillo1, A. Rivera1, R. Gonzalez-Arrabal1 and J.M. Perlado1

    1Universidad Politécnica de Madrid, Instituto de Fusión Nuclear, Jose Abascal 2, Madrid, Spain

    2Universidad de Oviedo, Facultad de Ciencias, c/ Calvo Sotelo s/n Oviedo, Spain

    In both, inertial and magnetic confinement fusion, reactors, the walls have to withstand high thermal loads and radiation fluxes. This work reports on a first principles study of the light atoms behavior (H , H-isotopes and He) in nanostructured tungsten. In order to carry out this work we present the approximation of the necessary foundations for the analysis using the SIESTA code [1]. The results are compared to data [2-3] obtained with a more precise plane wave code as VASP [4]. Moreover, the work is aimed to interpret our experimental results on structural and mechanical properties. The obtained results are valuable for subsequent simulations on a larger scale, such as kinetic Monte Carlo [3] or Molecular Dynamics. This complete analysis allows having a nanoscopic view of the phenomena leading to bubble formation and eventual trapping of light atoms at native defects in the bulk of tungsten, paying special attention to interfaces.

    [1] Soler J. Artacho E. Gale J. Garcia A. Junquera J. Ordejon P. and Sanchez-Portal D., J. Phys. Condens. Matter, 14 (2002) 2745.

    [2] Becquart C. and Domain C., Nucl. Instrum. Meth. Phys. Res. B 255 (2007) 23-26.

    [3] Gonzalo Valles, Antonio Rivera, Cesar González et al (for published)

    [4] Kresse G and Hafner J. Phys. Rev. B 47 (1993) R558 ; Kresse G, Furthmuller J. J, Phys. Rev. B 54 (1996) 11169; Kresse G, Joubert D. Phys. Rev. B 59 (1999) 1758.

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    10:50h – O62

    TIGHT-BINDING QUANTUM CHEMICAL MOLECULAR DYNAMICS ON THE OXYGEN MIGRATION EFFECT IN THE RH-CEO2 AND RH-ZRO2 SURFACES

    Ai Suzuki1, Ryuji Miura1, Nozomu Hatakeyama1, Akira Miyamoto1

    1Toholu University, Aoba, Aramaki, Aoba-ku, Sendai, Miyagi, Japan 980-8579

    The chemical interaction between noble metals and the ceria is a prerequisite for industrially important catalytic reactions, which has been applied in the fuel cells and automobiles. There are two prominent characteristics, which govern the sinterability, durability, and reactivity of catalysts in the noble metals/cerium oxide system. The one is the SMSI (strong metal-support interaction) and the other is the OSC (Oxygen storage and release capacity). This study clarified the relevance of these two fundamental features with insight into the surface atomic mobility in the aim of designing the better practical catalysts.

    Tight-binding quantum chemical molecular dynamics (TB-QCMD) method was applied with first-principles parametrization. Each parameter used in the TB-QCMD was determined by the DFT (Density Functional Theory) calculations.

    Adsorption energies of the 1nm-sized Rh55 cluster to the ceria surfaces, (Ead) were concluded in Fig.1. Ead = E (Rh55/CeO2) – E(Rh55) – E (CeO2) (1), Where E (CeO2), E(Rh55) and E(Rh55/CeO2) are the total binding energies of the bare ceria support, a free Rh55 cluster, and the corresponding ceria support with Rh55 (Rh55/CeO2). A preferable adsorption could only be obtained when the Rh stayed by its square facet. The Rh55 could be adsorbed to the (110) more stably than on the (100) surfaces with any types of its initial orientation. Migration of surface oxygen atoms can be extracted during the simulations using the time-dependent mean square displacement and were shown in Fig.2. The Rh enhanced the mobility of topmost oxygen atoms so much in the CeO2 (110) surfaces as compared with bare CeO2. Increase in the MSD of oxygen atoms in the ceria surface by the Rh can explain the rise in the OSC and the oxygen exchange rate in the presence of Rh.1, 2

    This study elucidated the process of oxygen migration by the possible bond formation, i.e., Rh-O or Rh-Ce bonds which act as trigger for ejecting oxygen atoms as a result of SMSI between Rh metal and CeO2 surfaces. Manipulation of supporting Rh on the CeO2 (110) surface will lead to develop quite reactive catalyst, which has high OSC performance.

    [1] H. C. Yao, Y. F. Y. Yao, “Ceria in Automotive Exhaust Catalysis” J. Cat. 86, 254 (1984).

    [2] D. Martin, D. Duprez, “Mobility of surface species on oxides”, J. Phys. Chem. C, 100, 9429. (1996).

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    11:10h – Coffee Break

    11:30h – O63

    IRRADIATION OF NANOFOAMS AND NANOWIRES

    E. Figueroa1, D. Tramontina2, G. Gutierrez1, S. Davis1, C. Anders3, H. Urbassek3, A. Stukowski4, M. Caro5, A. Caro5, and Eduardo Bringa2

    1Universidad de Chile, Santiago, Chile

    2Instituto de Ciencias Básicas, Universidad Nacional de Cuyo, Mendoza, Argentina

    3Technische Universitat Kaiserslautern, Kaiserslautern, Germany

    4Technische Universitat Darmstadt, Darmstadt, Germany

    5LosAlamos National Laboratory, Los Alamos, New Mexico, USA

    Metallic nanofoams have been shown to be radiation resistant under certain conditions [1,2]. To understand complex foam behavior, nanowires are often considered as simplified, elementary units, for nanofoams. There is an increasing interest in mechanical properties of nanowires and irradiation has the potential to change their mechanical behavior [3]. Atomistic simulations of irradiation of nanofoams and nanowires by keV ions have been carried out, and the radiation-induced microstructure was analyzed during the collision cascade and after relaxation of the samples. Point defects, together with defect clusters, stacking faults and twins are observed. Vacancy clusters can lead to dislocation loops, nearly spherical voids and stacking fault tetrahedra (SFTs). Comparison with bulk damage will be discussed alongside temperature effects in the microstructure, and relevance to recent nanofoam radiation experiments. Studies of changes in the mechanical properties of irradiated foams and pillars will also be presented.

    [1] Kiener et al., Nat. Mat. 10, 608 (2011).

    [2] Bringa et al., Nano Letters 12, 3351 (2012).

    [2] Fu et al., Appl. Phys. Lett. 101, 191607 (2012).

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    11:50h – O64

    ENERGY SPECTRA OF PROTON BEAMS INCIDENT ON MULTI-WALLED CARBON NANOTUBES AT NORMAL INCIDENCE

    Rafael Garcia-Molina1, Isabel Abril2, Carlos Celedón3,4, Rodrigo Segura5, Patricio Vargas3, Néstor R. Arista4, Jorge E. Valdés3,4

    1Departamento de Física – Centro de Investigación en Óptica y Nanofísica, Regional Campus of International Excellence “Campus Mare Nostrum”, Universidad de Murcia, E-30100 Murcia, Spain

    2Departament de Física Aplicada, Universitat d’Alacant, E-03080 Alacant, Spain

    3Departamento de Física-Laboratorio de Colisiones Atómicas, Centro para el Desarrollo de la Nanociencia y la Nanotecnología (CEDENNA) Universidad Técnica Federico Santa María, UTFSM, Valparaíso 2390123, Chile

    4Centro Atómico Bariloche, División Colisiones Atómicas, RA-8400 S.C. de Bariloche, Argentina

    5Departamento de Química y Bioquímica, Facultad de Ciencias, Universidad de Valparaíso, Chile

    The measured energy spectra of proton beams irradiating multi-walled carbon nanotubes (MWCNT) at normal incidence have been reproduced through semi-classical simulations. The experimental setup consisted of MWCNTs dispersed on top of a holey amorphous carbon (a-C) thin foil. The proton energy loss distribution in the forward direction was measured by the transmission technique, showing two well differentiated peaks, as depicted in the figure for the case of a 10.1 keV incident proton beam. By using a semi-classical simulation of the proton trajectory through the nanotube, we have elucidated the origin of these peaks, concluding that the experimental low energy-loss peak corresponds to quasi-planar channelling of protons moving between the outer walls of the MWCNT, whereas the experimental high energy-loss peak is mainly due to protons traversing the supporting a-C substrate after moving through the MWCNTs in quasi-planar channeling.

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    12:10h – O65

    EFFECTS OF RADIATION ON THE MECHANICAL PROPERTIES OF NANOTUBES AND NANOWIRES

    K. Nordlund1, A. Ilinov1, W. Ren1, A. Kuronen1, M. W. Ullah1, F. Djurabekova1, A. V. Krasheninnikov1,2

    1PB 43, Department of Physics, University of Helsinki, Finland

    2Department of applied physics, Aalto University, Finland

    Although it is well known that radiation can be used to modify the mechanical properties of many kinds of bulk and thin film materials, the effects of radiation on nanomaterials are much less well understood. Using molecular dynamics computer simulations, we have examined the effects of ion and electron irradiation on the mechanical properties of nanowires and nanotubes. Generally we find that for single wires and tubes, radiation softens both kinds of materials [1,2,3]. On the other hand, in carbon nanotube bundles, multiwalled carbon nanotubes and macroscopic nanotube paper the strength of the material can strongly increase due to irradiation [4,5,6]. I will discuss the physico-chemical reasons behind how irradiation modified nanotubes and nanowires, and experimental results confirming the theoretical predictions.

    [1] E. Holmström, L. Toikka, A. V. Krasheninnikov, and K. Nordlund, Phys. Rev. B. 82, 045420 (2010).

    [2] W. Ren, A. Kuronen, and K. Nordlund, Phys. Rev. B 86, 104114 (2012).

    [3] M. Sammalkorpi, A. Krasheninnikov, A. Kuronen, K. Nordlund, and K. Kaski, Phys. Rev. B 70, 245416 (2004)

    [4] M. Huhtala, A. Krasheninnikov, J. Aittoniemi, S. J. Stuart, K. Nordlund, and K. Kaski, Phys. Rev. B 70, 045404 (2004).

    [5] M. Sammalkorpi, A. V. Krasheninnikov, A. Kuronen, K. Nordlund, and K. Kaski, Nucl. Instr. Meth. Phys. Res. B 228, 142 (2005).

    [6] J. A. Ãström, A. V. Krasheninnikov, and K. Nordlund, Phys. Rev. Lett. 93, 215503 (2004)

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    12:30h – O66

    EVOLUTION OF GOLD NANOPARTICLES UNDER IRRADIATION BY KINETIC MONTE CARLO SIMULATION

    Thi Hai Yen Vu, Marc Hayoun, Yaasiin Ramjauny and Giancarlo Rizza

    Laboratoire des Solides Irradiés, École Polytechnique, CEA, CNRS, F-91128 Palaiseau France

    When metallic nanoparticles (NPs) embedded in a silica matrix are irradiated with charged particles in the nuclear stopping regime two effects are active either in synergy or in competition [1-2]. Ballistic effects are destructive and lead to the dissolution of the NPs. Thermal effects favor the evolution toward a thermodynamically stable state. At the crossover between these two trends, thermal and irradiation effects are supposed to compensate one another. Kinetic Monte Carlo (KMC) simulation is used to probe the evolution of a Au-SiO2 nanocomposite under sustained irradiation with 4MeV Au++ ions at different temperatures. Simulations are then compared to the experimental results. Our KMC code, on a 3D rigid lattice, includes atomic diffusion (vacancy mechanism) and ballistic displacement events whose chosen frequency ratio is fulfilled through the Bortz-Kalos- Lebowitz algorithm [3]. The atomic jumps for diffusion are accepted by means of the Metropolis algorithm according to the thermal or radiation enhanced diffusion coefficient. Displacement cascades of the gold atoms are taken into account on average by using a decreasing exponential distribution of relocation distances, r: 1/ exp(-r/), [4]. First, the kinetic evolution of 4nm Au NPs has been completely characterized in term of size, density and size distribution for temperatures ranging from 300K to 1100K. Above 900K, the nanocomposite evolves in a classical coarsening regime. Below this temperature the system becomes resistant to the coarsening process. We found that 900K represents the crossover between thermal and ballistic effects, i.e. the temperature where the two effects compensate one another. Besides, we investigate the dependence of the dissolution law on the NP size. Indeed, experimental results suggest that two dissolution regimes exist. For Au NPs larger than about 10nm the diameter linearly decreases with the fluence, whereas for smaller NP sizes the scaling law becomes exponential. We discuss the existence these two dissolution regimes within a theoretical approach based on a modified Frost-Russel model [5], and KMC simulations. In particular, we show that the key parameter is the recoil generation rate.

    [1] G.C. Rizza, M. Strobel, K.H. Heinig, and H. Bernas, “Ion irradiation of gold inclusions in SiO2: Experimental evidence for inverse Ostwald ripening,” Nucl. Instr. and Meth. B 178, 78 (2001).

    [2] G.C. Rizza , H. Cheverry, T. Gacoin, A. Lamasson, and S. Henry, “Ion beam irradiation of embedded nanoparticles: Toward an in situ control of size and spatial distribution,” J. Appl. Phys. 101, 14321 (2007).

    [3] A.B. Bortz, M.H. Kalos, and J.L. Lebowitz, “A new algorithm for Monte Carlo simulation of Ising spin systems,” J. Comput. Phys. 17, 10 (1975).

    [4] P. Sigmund and A. Gras-Marti, “Theoretical Aspects of Atomic Mixing by Ion Beams,” Nucl. Instr. & Meth. 25, 182 (1981).

    [5] H.J. Frost and K.C. Russell, “Recoil resolution and particle stability under irradiation,” J. Nucl. Mat. 103&104, 1427 (1981).

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    12:50h – O67

    MODELING ION BEAM STIMULATED GROWTH OF GALLIUM FLUORIDE PILLARS ON GAN

    Alan Bahm1,2, Aurelien Botman1, Steven Randolph and Marcus Straw1, Milos Toth2

    1FEI Company, 5350 Northeast Dawson Creek Drive, Hillsboro, Oregon 97124, USA

    2School of Physics and Advanced Materials, University of Technology,

    Sydney PO 123, Broadway, New South Wales 2007, Australia

    A novel ion-beam stimulated self-ordering growth process was discovered which results in the formation of microscopic pillars [1], see figure 1. A Ga+ ion beam was used to irradiate GaN in the presence of a XeF2 precursor gas. Excess concentration of gallium on the surface caused Ga droplets to form and transform into growing, Ga-filled pillars with a fluoride sheath. This growth process was captured over time by electron imaging, allowing production of movies and data on the geometry as a function of time. The geometry of the growing pillar was modeled by accounting for excess Ga generation and mass-transport of the concentration over the substrate and pillar surfaces. The model produced constants for the yield of excess gallium per incident ion, and the density of the fluoride sheath. The model also predicted where new pillars were likely to nucleate.

    Figure 1 (left) SEM image of Ga droplets on GaN formed in XeF2 during ion beam irradiation in vacuum, growing to form pillars in XeF2. The scale bar represents 5 μm, and the arrow shows the direction of the ion beam. (right) The self-ordering cycle discovered to be responsible for the pillar growth.

    [1] Physical Review Letters 111(13) 135503 (2013), doi:10.1103/PhysRevLett.111.135503

    13:10h – Closing