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Now showing items 17 - 32 of 39

  • Temperature dependence of underdense nanostructure formation in tungsten under helium irradiation

    G. Valles   I. Martin-Bragado   K. Nordlund   A. Lasa   C. Björkas   E. Safi   J.M. Perlado   A. Rivera  

    Abstract Recently, tungsten has been found to form a highly underdense nanostructured morphology (“W fuzz”) when bombarded by an intense flux of He ions, but only in the temperature window 900–2000 K. Using object kinetic Monte Carlo simulations (pseudo-3D simulations) parameterized from first principles, we show that this temperature dependence can be understood based on He and point defect clustering, cluster growth, and detrapping reactions. At low temperatures (<900 K), fuzz does not grow because almost all He is trapped in very small He-vacancy clusters. At high temperatures (>2300 K), all He is detrapped from clusters, preventing the formation of the large clusters that lead to fuzz growth in the intermediate temperature range. Highlights • OKMC simulation of temperature window for fuzz formation. • Stable He-V clusters prevent fuzz formation at low temperatures. • Dissociation of He-V clusters prevent fuzz formation at high temperatures. • Fuzz formation rate increases with increasing temperature. • An incubation fluence observed in the simulations, similar to experimental observations.
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  • The relationship between gross and net erosion of beryllium at elevated temperature

    R.P. Doerner   I. Jepu   D. Nishijima   E. Safi   L. Bukonte   A. Lasa   K. Nordlund   T. Schwarz-Selinger  

    Abstract Surface temperature is a critical variable governing plasma–material interactions. PISCES-B injects controllable amounts of Be impurities into the plasma to balance, or exceed, the erosion rate of beryllium from samples in un-seeded plasma exposures. At low temperature, an order of magnitude more beryllium, than the beryllium mass loss measured in un-seeded discharges, needs to be seeded into the plasma to achieve no mass loss from a sample. At elevated temperature, no mass loss is achieved when the beryllium-seeding rate equals the mass loss rate in un-seeded discharges. Molecular dynamics simulations show that below 500 K, Be adatoms have difficulty surmounting the Ehrlich–Schwoebel barrier at the edge of a terrace. Above this temperature, an Arrhenius behavior is observed with an activation energy of 0.32 eV. Qualitatively, this indicates that at low surface temperature the deposited atoms may be more easily re-eroded, accounting for the increased seeding needed to balance the erosion.
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  • Simulation of redistributive and erosive effects in a-Si under Ar<sup>+</sup> irradiation

    A. Lopez-Cazalilla   A. Ilinov   L. Bukonte   K. Nordlund   F. Djurabekova   S. Norris   J.C. Perkinson  

    Abstract Ion beams are frequently used in industry for composition control of semiconducting materials as well as for surface processing and thin films deposition. Under certain conditions, low- and medium energy ions at high fluences can produce nanoripples and quantum dots on the irradiated surfaces. In the present work, we focus our attention on the study of irradiation of amorphous silicon (a-Si) target with 250 eV and 1 keV Ar + ions under different angles, taking into special consideration angles close to the grazing incidence. We use the molecular dynamics (MD) method to investigate how much the cumulative displacement of atoms due to the simulated ion bombardment contribute to the patterning effect. The MD results are subsequently analysed using a numerical module Pycraters that allows the prediction of the rippling effect. Ripple wavelengths estimated with Pycraters are then compared with the experimental observations, as well as with the results obtained by using the binary collisions approximation (BCA) method. The wavelength estimation based on the MD results demonstrates a better agreement with the experimental values. In the framework of the utilized analytical model, it can be mainly attributed to the fact that the BCA ignores low energy atomic interactions, which, however, provide an important contribution to the displacement of atoms following an ion impact.
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  • Cluster analysis of Dairy Herd Improvement data to discover trends in performance characteristics in large Upper Midwest dairy herds

    R.L. Brotzman   N.B. Cook   K. Nordlund   T.B. Bennett   A. Gomez Rivas   D. Döpfer  

    Abstract Principal component analysis (PCA) is a variable reduction method used on over-parameterized data sets with a vast number of variables and a limited number of observations, such as Dairy Herd Improvement (DHI) data, to select subsets of variables that describe the largest amount of variance. Cluster analysis (CA) segregates objects, in this case dairy herds, into groups based upon similarity in multiple characteristics simultaneously. This project aimed to apply PCA to discover the subset of most meaningful DHI variables and to discover groupings of dairy herds with similar performance characteristics. Year 2011 DHI data was obtained for 557 Upper Midwest herds with test-day mean ≥200 cows (assumed mostly freestall housed), that remained on test for the entire year. The PCA reduced an initial list of 22 variables to 16. The average distance method of CA grouped farms based on best goodness of fit determined by the minimum cophenetic distance. Six groupings provided the optimal fitting number of clusters. Descriptive statistics for the 16 variables were computed per group. On observations of means, groups 1, 2, and 6 demonstrated the best performances in most variables, including energy-corrected milk, linear somatic cell score (log of somatic cell count), dry period intramammary infection cure rate, new intramammary infection risk, risk of subclinical intramammary infection at first test, age at first calving, days in milk, and Transition Cow Index. Groups 3, 4, and 5 demonstrated the worst mean performances in most the PCA-selected variables, including DIM, age at first calving, risk of subclinical intramammary infection at first test, and dry period intramammary infection cure rate. Groups 4 and 5 also had the worst mean herd performances in energy-corrected milk, Transition Cow Index, linear somatic cell score, and new intramammary infection risk. Further investigation will be conducted to reveal patterns of management associated with herd categorization. The PCA and CA should be used when describing the multivariate performance of dairy herds and whenever working with over-parameterized data sets, such as DHI databases.
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  • Non-equilibrium properties of interatomic potentials in cascade simulations in tungsten

    A.E. Sand   J. Dequeker   C.S. Becquart   C. Domain   K. Nordlund  

    Abstract The reliability of atomistic simulations of primary radiation damage hinges on the quality of the interatomic potential. However, irradiation induced collision cascades involve strongly non-equilibrium processes, and thus depend on properties of potentials not usually included in the potential fitting. Here, we compare the predictions of five interatomic potentials for tungsten in cascade simulations with primary knock-on energies ranging from threshold energies for defect production, up to 200 keV. The highest energies represent the energetic recoils induced by the 14 MeV fusion neutron irradiation. We further compare properties related to dynamic collisions predicted by the different potentials to DFT calculations, to assess the accuracy of these predictions. We also present two hardened versions of a recent EAM-type potential, and demonstrate explicitly the importance of carefully adjusting the range of the potential at interaction distances smaller than those included in the fitting of potentials to equilibrium properties.
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  • Angular and velocity distributions of tungsten sputtered by low energy argon ions

    E. Marenkov   K. Nordlund   I. Sorokin   A. Eksaeva   K. Gutorov   J. Jussila   F. Granberg   D. Borodin  

    Abstract Sputtering by ions with low near-threshold energies is investigated. Experiments and simulations are conducted for tungsten sputtering by low-energy, 85–200 eV Ar atoms. The angular distributions of sputtered particles are measured. A new method for molecular dynamics simulation of sputtering taking into account random crystallographic surface orientation is developed, and applied for the case under consideration. The simulations approximate experimental results well. At low energies the distributions acquire “butterfly-like” shape with lower sputtering yields for close to normal angles comparing to the cosine distribution. The energy distributions of sputtered particles were simulated. The Thompson distribution remains valid down to near-threshold 85 eV case. Highlights • Angular and energy distributions of sputtered particles are considered for low-energy sputtering. • A new algorithm for molecular dynamics simulations of polycrystalline target is proposed. • Results of the simulations agree with experimental findings both obtained by us and found in literature.
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  • Atomistic simulation of ion irradiation of semiconductor heterostructures

    C. Fridlund   J. Laakso   K. Nordlund   F. Djurabekova  

    Abstract Recently the possibility to use ion beam mixing combined with suitable annealing has been suggested as a possible means to synthesize individual silicon quantum dots in a silica layer, with the possibility to function as single-electron transistors. For this to work, it is necessary to have a careful control of the ion beam mixing in Si/SiO 2 /Si heterostructures, as well as understand the nature of not only the composition, but also the chemical modification of the SiO 2 layer by the mixing with Si. We describe here a procedure to synthesize Si/SiO 2 /Si heterostructures in molecular dynamics, with an energy minimization scheme to create strong and stable interfaces. The created heterostructures are irradiated at energies and fluences matching corresponding experiments. The results show a considerable degree of interface mixing, as expected. They also show some densification of the silica layer due to recoil implantation, and formation of a considerable number of coordination defects. Due to the strong covalent bonding in silicon and silica, the densification is not fully elastically relaxed even in the presence of a nearby surface.
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  • Corrigendum to “Creating nanoporous graphene with swift heavy ions” [Carbon 114 (April 2017) 511–518]

    H. Vázquez   E.H. Åhlgren   O. Ochedowski   A.A. Leino   R. Mirzayev   R. Kozubek   H. Lebius   M. Karlušic   M. Jakšic   A.V. Krasheninnikov   J. Kotakoski   M. Schleberger   K. Nordlund   F. Djurabekova  

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  • Comparison of repulsive interatomic potentials calculated with an all-electron DFT approach with experimental data

    A.N. Zinoviev   K. Nordlund  

    Abstract The interatomic potential determines the nuclear stopping power in materials. Most ion irradiation simulation models are based on the universal Ziegler-Biersack-Littmark (ZBL) potential (Ziegler et al., 1983), which, however, is an average and hence may not describe the stopping of all ion-material combinations well. Here we consider pair-specific interatomic potentials determined experimentally and by density-functional theory simulations with DMol approach (DMol software, 1997) to choose basic wave functions. The interatomic potentials calculated using the DMol approach demonstrate an unexpectedly good agreement with experimental data. Differences are mainly observed for heavy atom systems, which suggests they can be improved by extending a basis set and more accurately considering the relativistic effects. Experimental data prove that the approach of determining interatomic potentials from quasielastic scattering can be successfully used for modeling collision cascades in ion-solids collisions. The data obtained clearly indicate that the use of any universal potential is limited to internuclear distances R < 7 af (af is the Firsov length).
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  • Vibration load with poker vibration : K. Freden, B. Johansson, L. Nordlund, D. Caple, J.-E. Hansson and S. Kihlberg 1981 Arbetarskyddsstyrelsen, Undersöknigsrapport 1981:17. (24 pages, 6 figures, 5 tables, 6 references) (in Swedish)

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  • Defect clustering in irradiation of GaN by single and molecular ions

    M.W. Ullah   A. Kuronen   F. Djurabekova   K. Nordlund   A.I. Titov   P.A. Karaseov  

    Abstract Atomistic simulations were used to study irradiation effects on photoluminescence (PL) decay time of GaN. Irradiations were done by single (F, P) and molecular ions (PF 4 ). Equal energy per mass (0.6 keV/amu) was used for all projectiles. Irradiation by the molecular ion shows faster PL decay time in comparison with the single ion. The simulation results show that single ions produce isolated point defects, whereas molecular ions produce big clusters of points defects. The total amount of defects produced by a PF 4 projectile and five individual cascades started by one P and four F single ions (P + 4 × F) were very close and their defect depth profile follows the same pattern. These findings suggest that defect clusters are one of the important reasons for fast PL decay. Highlights • We irradiate GaN with single and molecular ion. • We use molecular dynamics simulation for the study. • We study the effect on optical properties after irradiation. • Photoluminescence decay time shows non-linear behavior during ion irradiation. • This behavior can be explained by stable defect cluster formation by molecular ion.
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  • Atomistic simulations of the effect of reactor-relevant parameters on be sputtering

    E. Safi   C. Björkas   A. Lasa   K. Nordlund   I. Sukuba   M. Probst  

    Abstract Beryllium (Be) is the main plasma-facing material in the present day fusion reactor JET as well as in the upcoming ITER. Thus, the Be erosion plays a key role in predicting the life-time and viability of the reactors. In this work, Be surface erosion and morphology changes due to deuterium (D) irradiation are studied by using molecular dynamics simulations, varying key parameters such as particle flux, surface temperature and impact energy. At low temperatures, the main molecular species among the sputtered particles is BeD due to a low D surface concentration, as the incoming D projectiles cluster beneath the surface. At higher temperatures, the D surface concentration increases and larger species (BeD 2 , BeD 3 ) dominate the molecular erosion, lowering the BeD to Be ratio. When approaching the Be melting point, D desorbs from the surface, increasing the fraction of Be eroded as BeD. The larger molecules will dissociate as soon as entering the edge plasma, with only a minor contribution to the BeD formation. These findings correlate well with observations at JET. The effect of the incoming D flux on the results is negligible.
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  • Effects of the short-range repulsive potential on cascade damage in iron

    J. Byggmästar   F. Granberg   K. Nordlund  

    Abstract Recent work has shown that the repulsive part of the interatomic potential at intermediate atomic separations strongly affects the extent and morphology of the damage produced by collision cascades in molecular dynamics simulations. Here, we modify an existing embedded atom method interatomic potential for iron to more accurately reproduce the threshold displacement energy surface as well as the many-body repulsion at intermediate and short interatomic distances. Using the modified potential, we explore the effects of an improved repulsive potential on the primary damage production and the cumulative damage accumulation in iron. We find that the extent of the damage produced by single cascades, in terms of surviving Frenkel pairs, directly correlates with the change in threshold displacement energies. On the other hand, the damage evolution at higher doses is more dependent on the formation and stability of different defect clusters, defined by the near-equilibrium part of the interatomic potential.
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  • Defect structures and statistics in overlapping cascade damage in fusion-relevant bcc metals

    A.E. Sand   J. Byggmästar   A. Zitting   K. Nordlund  

    Abstract Most experimental work on radiation damage is performed to fairly high doses, where cascade overlap effects come into play, yet atomistic simulations of the primary radiation damage have mainly been performed in initially perfect lattice. Here, we investigate the primary damage produced by energetic ion or neutron impacts in bcc Fe and W. We model irradiation effects at high fluence through atomistic simulations of cascades in pre-damaged systems. The effects of overlap provide new insights into the processes governing the formation under irradiation of extended defects. We find that cascade overlap leads to an increase in the numbers of large clusters in Fe, while in W such an effect is not seen. A significant shift in the morphology of the primary damage is also observed, including the formation of complex defect structures that have not been previously reported in the literature. These defects are highly self-immobilized, shifting the damage away from the predominance of mobile 1 / 2 〈 111 〉 loops towards more immobile initial configurations. In Fe, where cascade collapse is extremely rare in molecular dynamics simulations of individual cascades, we observe the formation of vacancy-type dislocation loops from cascade collapse as a result of cascade overlap.
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  • Radiation damage production in massive cascades initiated by fusion neutrons in tungsten

    A.E. Sand   K. Nordlund   S.L. Dudarev  

    Abstract Neutrons in fusion reactors produce primary radiation damage from displacement cascades in tungsten (W) at an average PKA energy of 150 keV. We find, using molecular dynamics simulations, that cascades at this energy do not break up into subcascades. The massive amount of energy concentrated in the liquid-like heat spike facilitates a fairly high rate of formation of large dislocation loops and other defect structures, of sizes readily visible in today’s electron microscopes. We investigate the structures and distribution of the cascade debris in W predicted by different interatomic potentials. In particular, our simulations show the formation of 〈 1 0 0 〉 -type dislocation loops, in agreement with recent experiments and in contradiction to the earlier held view that only 1 / 2 〈 1 1 1 〉 -type loops occur in W.
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  • Effect of random surface orientation on W sputtering yields

    J. Jussila   F. Granberg   K. Nordlund  

    Highlights • The effect of random surface orientations on the sputtering yields was studied. • Low index surfaces and random orientations of the surface was investigated. • The sputtering yield depended heavily on the surface orientation. Abstract In this study, we investigate the sputtering yield of tungsten surfaces by energetic particles, focusing on the effect of surface orientation and the incoming irradiation angle, by means of molecular dynamics. We develop a simulation approach to simulate sputtering from completely random surface orientations. This allows obtaining the sputtering yields averaged over a sufficiently large number of orientations, to obtain statistically significant yields representative of a polycrystalline sample with random grain orientations. We find that the total sputtering yield is dependent on the surface orientation, and that the results for random surfaces are clearly different from that of any of the low-index ones or their average. The different low index surfaces and the random surfaces also showed that the sputtering yield is dependent on the incoming angle of the ion. The outgoing angle of the sputtered tungsten atoms was observed to be very sensitive to the surface orientation. Different features on the tungsten surface were observed to drastically affect the sputtering yield at certain angles.
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