Endre søk
Begrens søket
1 - 42 of 42
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf
Treff pr side
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sortering
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
  • Standard (Relevans)
  • Forfatter A-Ø
  • Forfatter Ø-A
  • Tittel A-Ø
  • Tittel Ø-A
  • Type publikasjon A-Ø
  • Type publikasjon Ø-A
  • Eldste først
  • Nyeste først
  • Skapad (Eldste først)
  • Skapad (Nyeste først)
  • Senast uppdaterad (Eldste først)
  • Senast uppdaterad (Nyeste først)
  • Disputationsdatum (tidligste først)
  • Disputationsdatum (siste først)
Merk
Maxantalet träffar du kan exportera från sökgränssnittet är 250. Vid större uttag använd dig av utsökningar.
  • 1. Carling, K. M.
    et al.
    Wahnstrom, G.
    Mattsson, T. R.
    Sandberg, Nils
    KTH, Tidigare Institutioner                               , Fysik.
    Grimvall, G.
    Vacancy concentration in Al from combined first-principles and model potential calculations2003Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 67, nr 5Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a comprehensive study of vacancy formation enthalpies and entropies in aluminum. The calculations are done in the framework of the local-density and generalized-gradient approximations in the density-functional formalism. To assess anharmonic contributions to the formation free energies, we use an interatomic potential with parameters determined from density-functional-theory calculations. We find that the binding energy for the nearest-neighbor divacancy is negative, i.e., it is energetically unstable. The entropy contributions slightly stabilize the divacancy but also the binding free energy at the melting temperature is found to be negative. We show that the anharmonic atomic vibrations explain the non-Arrhenius temperature dependence of the vacancy concentration in contrast to the commonly accepted interpretation of the experimental data in terms of the monovacancy-divacancy model.

  • 2. Carling, Karin
    et al.
    Wahnström, Göran
    Mattsson, Thomas R
    Mattsson, Ann E
    Sandberg, Nils
    KTH, Tidigare Institutioner, Fysik.
    Grimvall, Göran
    Vacancies in metals: From first-principles calculations to experimental data2000Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 85, nr 18, s. 3862-3865Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have revealed, and resolved, an apparent inability of density functional theory, within the local density and generalized gradient approximations, to describe vacancies in Al accurately and consistently. The shortcoming is due to electron correlation effects near electronic edges and we show how to correct for them. We find that the divacancy in Al is energetically unstable and we show that anharmonic atomic vibrations explain the non-Arrhenius temperature dependence of the vacancy concentration.

  • 3.
    Chang, Zhongwen
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Messina, Luca
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Yousfi, Amine
    Toijer, Elin
    Thuvander, Mattias
    Boizot, Bruno
    Brysbaert, Gauthier
    Metayer, Vincent
    Gorse-Pomonti, Dominique
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Electron irradiation accelerated Cu precipitation in cast iron and an FeCu model alloyManuskript (preprint) (Annet vitenskapelig)
  • 4.
    Chang, Zhongwen
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Terentyev, Dmitry
    Interaction Energy Calculations of Edge Dislocation with Point Defects in FCC Cu2013Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In order to improve the dislocation bias (DB) model of swelling under irradiation, a large scale of atomistic simulation of the interaction in face centered cubic (FCC) Cu model lattice between an edge dislocation (ED) and point defects such as a vacancy, a self-interstital atom (SIA) have been performed for various configurations. It is found dislocation core splits into partial cores after energy relaxation. Interactions with any SIA conficurations is one order of magnitute larger than with a vacancy. The reason that SIA creats a larger dilatation volumn than the vacancy is directly observed from calculation. Furthurmore, within the interaction range, an octahedron position rather than dumbbell in <100> direction is observed in the stable state after relaxation in interactions between a edge dislocation and a dumbbell SIA. Comparision of interaction energy in analytical and atomistic calculation shows that analytical one has a stronger interaction in vacancy-ED systems, suggesting that the bias factor (BF) from analytical calculation is larger than from atomistic calculation.

  • 5.
    Chang, Zhongwen
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Terentyev, Dmitry
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Dislocation bias factors in fcc copper derived from atomistic calculations2013Inngår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 441, nr 1-3, s. 357-363Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Atomistic calculations were employed in order to calculate the interaction energy of an edge dislocation with different point defects. The bias factor was calculated by applying a finite element method on the interaction energy landscapes obtained from the atomistic calculations. A comparison of the calculated bias factor with a model based on elasticity theory reveals around 30% discrepancy under conditions representative for electron irradiation at 600 degrees C. Possible reasons are discussed. The bias factor dependence on dislocation density and ambient temperature is presented and discussed.

  • 6.
    Chang, Zhongwen
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik. Swedish Radiation Safety Authority, Sweden.
    Dmitry, Terentyev
    Samuelsson, Karl
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Bonny, Giovanni
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Assessment of the dislocation bias in fcc metals and extrapolation to austenitic steels2015Inngår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 465Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A systematic study of dislocation bias has been performed using a method that combines atomistic and elastic dislocation-point defect interaction models with a numerical solution of the diffusion equation with a drift term. Copper, nickel and aluminium model lattices are used in this study, covering a wide range of shear moduli and stacking fault energies. It is found that the dominant parameter for the dislocation bias in fcc metals is the width of the stacking fault ribbon. The variation in elastic constants does not strongly impact the dislocation bias value. As a result of this analysis and its extrapolation, the dislocation bias of the widely applied austenitic stainless steels of 316 type is predicted to be about 0.1 at temperature close to the swelling peak (815 K) and typical dislocation density of 1014 m-2. This is in line with the bias calculated using the elastic interaction model, which implies that the prediction method can be used readily in other fcc systems even without EAM potentials. By comparing the bias values obtained using atomistic- and elastic interaction energies, about 20% discrepancy is found, therefore a more realistic bias value for the 316 type alloy is 0.08 in these conditions.

  • 7.
    Chang, Zhongwen
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Terentyev, Dmitry
    Nuclear Materials Science Institute, Belgium.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik. Swedish Radiation Safety Authority, Sweden.
    Samuelsson, Karl
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Anomalous bias factors of dislocations in bcc iron2015Inngår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 461, s. 221-229Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dislocation bias factors in bcc Fe have been calculated based on atomistic interaction energy maps on three kinds of dislocations, namely the a0/2〈1 1 1〉{1 1 0} screw, a0/2〈1 1 1〉{1 1 0} and a0〈1 0 0〉{0 0 1} edge dislocations. The results show that the dislocation bias is higher for the a0/2〈1 1 1〉 edge dislocation than for the a0〈1 0 0〉 edge dislocation, even though the latter possesses a larger Burgers vector. This indicates the importance of the dislocation core contribution. For the a0/2〈1 1 1〉{1 1 0} screw dislocation, a negative dislocation bias has been obtained, which implies a more efficient absorption of vacancies than of SIAs. The effect of coexistence of both edge- and screw dislocations are assessed by a total bias. A possible complementary mechanism for explaining the long swelling incubation time in bcc metals is suggested and discussed.

  • 8.
    Forsblom, Mattias
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Sandberg, Nils
    KTH, Tidigare Institutioner, Fysik.
    Grimvall, Göran
    KTH, Tidigare Institutioner, Fysik.
    Anharmonic effects in the heat capacity of Al2004Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, nr 16, s. 165106-1-165106-6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The vibrational heat capacity of aluminum at fixed volume is studied in molecular dynamics and Monte Carlo simulations, using effective interactions due to Ercolessi and Adams, Mishin , and Mei and Davenport. When experimental data are reduced to represent the classical vibrational heat capacity at fixed volume, the result is within about +/-2 % of 3k(B)/atom up to the melting temperature of Al, thus suggesting small anharmonic effects. Our simulations of the heat capacity are in good agreement with experiments, but also show that anharmonic effects are in fact large, with a cancellation between the low-order linear term in the temperature T and higher-order terms.

  • 9. Forsblom, Mattias
    et al.
    Sandberg, Nils
    KTH, Tidigare Institutioner, Fysik.
    Grimvall, Göran
    Energies and vibrational entrpoies of stacking faults and dislocation in Al and CuArtikkel i tidsskrift (Annet vitenskapelig)
  • 10.
    Forsblom, Mattias
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Sandberg, Nils
    KTH, Tidigare Institutioner                               , Fysik.
    Grimvall, Göran
    KTH, Tidigare Institutioner                               , Fysik.
    Vibrational entropy of dislocations in Al2004Inngår i: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 84, nr 6, s. 521-532Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The region nearest to a lattice defect must be described by an atomistic model, while a continuum model suffices further away from the defect. We study such a separation into two regions for an edge dislocation. In particular we focus on the excess defect energy and vibrational entropy, when the dislocation core is described by a cluster of about 500-100 atoms, embedded in a large discrete and relaxed, but static, lattice. The interaction between the atoms is given by a potential of the embedded-atom model type referring to Al. The dynamic matrix of the vibrations in the cluster is fully diagonalized. The excess entropy DeltaS near the core has positive and negative contributions, depending on the sign of the local strain. Typically, DeltaS/k(B) approximate to 2 per atomic repeat length along the dislocation core in fcc Al. In the elastic continuum region far from the dislocation core the excess entropy shows the same logarithmic divergence as the elastic energy. Although the work refers to a specific material and defect type, the results are of a generic nature.

  • 11.
    Henriksson, Krister O. E.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnkraftssäkerhet.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Wallenius, Janne
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Carbides in stainless steels: Results from ab initio investigations2008Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 93, nr 19Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The useful properties of steels are due to a complicated microstructure containing iron and chromium carbides. Only some basic physical properties of these carbides are known with high precision, although the carbides may have a vital impact on the performance and longevity of the steel. To improve on this situation, we have performed extensive density-functional theory calculations of several carbides. The quantitative results are in perfect agreement with the relative empirical stability of the carbides. Also, in contradiction with experimental data, we find that Cr23C6 responsible for the hardness of stainless steels is not the most stable chromium-dominated carbide.

  • 12. Magnusson, Helena
    et al.
    Sandberg, Nils
    KTH, Tidigare Institutioner, Fysik.
    Statistical analysis of grain boundary misorientation distributionsArtikkel i tidsskrift (Annet vitenskapelig)
  • 13. Malerba, L.
    et al.
    Terentyev, D. A.
    Bonny, G.
    Barashev, A. V.
    Bjorkas, C.
    Juslin, N.
    Nordlund, K.
    Domain, C.
    Olsson, P.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Wallenius, Janne
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Modelling of radiation damage in Fe-Cr alloys2007Inngår i: Journal of ASTM International, ISSN 1546-962X, E-ISSN 1546-962X, Vol. 4, nr 6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    High-Cr ferritic/martensitic steels are being considered as structural materials for a large number of future nuclear applications, from fusion to accelerator-driven systems and GenIV reactors. Fe-Cr alloys can be used as model materials to investigate some of the mechanisms governing their microstructure evolution under irradiation and its correlation to changes in their macroscopic properties. Focusing on these alloys, we show an example of how the integration of computer simulation and theoretical models can provide keys for the interpretation of a host of relevant experimental observations. In particular we show that proper accounting for two basic features of these alloys, namely, the existence of a fairly strong attractive interaction between self-interstitials and Cr atoms and of a mixing enthalpy that changes sign from negative to positive around 8 to 10% Cr, is a necessary and, to a certain extent, sufficient condition to rationalize and understand their behavior under irradiation. These features have been revealed by ab initio calculations, are supported by experimental evidence, and have been adequately transferred into advanced empirical interatomic potentials, which have been and are being used for the simulation of damage production, defect behavior, and phase transformation in these alloys. The results of the simulations have been and are being used to parameterize models capable of extending the description of radiation effects to scales beyond the reach of molecular dynamics. The present paper intends to highlight the most important achievements and results of this research activity.

  • 14. Malerba, L.
    et al.
    Terentyev, D. A.
    Bonny, G.
    Barashev, A. V.
    Bjrkas, C.
    Juslin, N.
    Nordlund, K.
    Domain, C.
    Olsson, Pär
    EDF-R and D Les Renardieres.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Wallenius, Janne
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Modelling of Radiation Damage in Fe-Cr Alloys2008Inngår i: EFFECTS OF RADIATION ON MATERIALS: 23RD INTERNATIONAL SYMPOSIUM / [ed] Lott, RG; Busby, JT, 2008, Vol. 1492, s. 159-176Konferansepaper (Fagfellevurdert)
    Abstract [en]

    High-Cr ferritic/martensitic steels are being considered as structural materials for a large number of future nuclear applications, from fusion to accelerator-driven systems and GenIV reactors. Fe-Cr alloys can be used as model materials to investigate some of the mechanisms governing their microstructure evolution under irradiation and its correlation to changes in their macroscopic properties. Focusing on these alloys, we show an example of how the integration of computer simulation and theoretical models can provide keys for the interpretation of a host of relevant experimental observations. In particular we show that proper accounting for two basic features of these alloys, namely, the existence of a fairly strong attractive interaction between self-interstitials and Cr atoms and of a mixing enthalpy that changes sign from negative to positive around 8 to 10 % Cr, is a necessary and, to a certain extent, sufficient condition to rationalize and understand their behavior under irradiation. These features have been revealed by ab initio calculations, are supported by experimental evidence, and have been adequately transferred into advanced empirical interatomic potentials, which have been and are being used for the simulation of damage production, defect behavior, and phase transformation in these alloys. The results of the simulations have been and are being used to parameterize models capable of extending the description of radiation effects to scales beyond the reach of molecular dynamics. The present paper intends to highlight the most important achievements and results of this research activity.

  • 15. Mattsson, T. R.
    et al.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Armiento, R.
    Mattsson, A. E.
    Quantifying the anomalous self-diffusion in molybdenum with first-principles simulations2009Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, nr 22Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    First-principles molecular-dynamics simulations based on a recently developed exchange-correlation functional show that self-diffusion in the refractory metal molybdenum is associated with strongly temperature-dependent activation energies for vacancy formation and migration. While static calculations of self-diffusion rates based on transition-state theory deviate systematically from experiments, with up to two orders of magnitude, the current results are accurate to within a mean deviation of 4 over the experimental range in temperature.

  • 16.
    Messina, Luca
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Nastar, Maylise
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik. Swedish Radiation Safety Authority, Sweden.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Systematic electronic-structure investigation of substitutional impurity diffusion and flux coupling in bcc iron2016Inngår i: Physical Review B, ISSN 2469-9950, Vol. 93, nr 18, artikkel-id 184302Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The diffusion properties of a wide range of impurities (transition metals and Al, Si, and P) in ferritic alloys are here investigated by means of a combined ab initio-atomic diffusion theory approach. The flux-coupling mechanisms and the solute-diffusion coefficients are inferred from electronic-structure calculations of solute-defect interactions and microscopic jump frequencies. All properties except the second-nearest-neighbor binding energy are found to have a characteristic bell shape as a function of the d-band filling for the 4d and 5d series, and an M shape for the 3d row because of the out-of-trend behavior of Mn. The solute jump frequencies are governed by compressibility, which makes diffusion of large solutes faster, although this effect is partially compensated for by lower attempt frequencies and larger correlations with the vacancy. Diffusion coefficients are predicted in a wide temperature range, far below the experimentally accessible temperatures. In accordance with experiments, Co is found to be a slow diffuser in iron, and the same behavior is predicted for Re, Os, and Ir impurities. Finally, flux-coupling phenomena depend on the iron jump frequencies next to a solute atom, which are mainly controlled by similar electronic interactions to those determining the binding energies. Vacancy drag and solute enrichment at sinks systematically arise below a solute-dependent temperature threshold, directly correlated with the electronic-level interactions at the equilibrium and the saddle-point states. Early transition metals with repulsive second-nearest-neighbor interactions also diffuse via vacancy drag, although they show a lower temperature threshold than the late metals. This confirms that drag is the most common solute-vacancy coupling mechanism in iron at low temperatures, and this is likely to be confirmed as well for impurity diffusion in other transition metals.

  • 17.
    Messina, Luca
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Nastar, Maylise
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Systematic electronic-structure investigation of substitutional impurity diffusion and flux coupling in bcc ironManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    The diffusion properties of a wide range of impurities (transition metals and Al, Si, and P) in ferritic alloys are here investigated by means of a combined ab initio-atomic diffusion theory approach. The flux-coupling mechanisms and the solute diffusion coefficients are inferred from electronic-structure calculations of solute-defect interactions and microscopic jump frequencies. All properties except the second nearest-neighbor binding energy are found to have a characteristic bell shape as a function of the d-band filling for the 4d and 5d series, and an M-shape for the 3d row because of the out-of-trend behavior of Mn. The solute jump frequencies are governed by compressibility, which makes diffusion of large solutes faster, although this effect is partially compensated for by lower attempt frequencies and larger correlations with the vacancy. Diffusion coefficients are predicted in a wide temperature range, far below the experimentally-accessible temperatures. In accordance with experiments, Co is found to be a slow diffuser in iron, and the same behavior is predicted for Re, Os, and Ir impurities. Finally, flux-coupling phenomena depend on the iron jump frequencies next to a solute atom, which are mainly controlled by similar electronic interactions to those determining the binding energies. Vacancy drag and solute enrichment at sinks systematically arise below a solute-dependent temperature threshold, directly correlated with the electronic-level interactions at the equilibrium and the saddle-point states. Early transition metals with repulsive second nearest-neighbor interactions also diffuse via vacancy drag, although they show a lower temperature threshold than the late metals. This confirms that drag is the most common solute-vacancy coupling mechanism in iron at low temperatures, and this is likely to be confirmed as well for impurity diffusion in other transition metals. 

  • 18.
    Pukari, Merja
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    He, Kr and Xe diffusion in ZrN: An atomic scale study2013Inngår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 438, nr 1/3, s. 7-14Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The atomic scale diffusion mechanisms for He, Kr and Xe in the nitride fuel component ZrN are developed from first principles. The vacancy formation energies reveal a prevalent N vacancy concentration in the material. However, a high N self-diffusion barrier hinders vacancy-aided Kr and Xe diffusion. High, attractive binding energies of interstitial Xe and Kr to a N vacancy effectively eliminate interstitial diffusion mechanism for these gases. In comparison, He exhibits considerable degrees of freedom, as it is weekly bound to a N vacancy, enhances N-vacancy aided diffusion, has the lowest interstitial migration barrier, and has the capacity to be reintroduced into the ZrN lattice as an interstitial. N self-diffusion barriers are lowered if the diffusing N is in close proximity to a substitutional atom. The obtained results suggest a high release of He, while the majority of Kr and Xe is retained, in agreement with experiments.

  • 19.
    Pukari, Merja
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Runevall, Odd
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Wallenius, Jan
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Vacancy formation and solid solubility in the U-Zr-N system2010Inngår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 406, nr 3, s. 351-355Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    For the purpose of developing a nuclear fuel with enhanced thermophysical properties and better irradiation performance density functional theory calculations are used to explore UN, ZrN and (U, Zr)N. Negative deviation of ground state energy from the ideal solution model as well as energetically favourable maximal distance between substitutional metal atoms in respective nitrides indicate mutual solubility of UN and ZrN at all temperatures. Nitrogen vacancy formation energies in UN (1.81 eV) and ZrN (1.40 eV) are considerably lower than metal vacancy formation energies. A substitutional Zr atom in UN has little effect on nitrogen vacancy formation energies (similar to 1.79 eV), while U in ZrN decreases the value by similar to 0.1 eV (similar to 1.30 eV) due to elastic stress and charge density redistribution in the material. The relative distance between a substitutional metal atom and a vacancy in UN has little influence over the radially declining displacement pattern induced by the substitutional atom, while in ZrN the relaxation of atoms is governed by the position of the vacancy. The calculated vacancy formation energies indicate a lower surface energy of ZrN in comparison with UN.

  • 20.
    Runevall, Odd
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Helium cluster dissolution in molybdenum2009Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 21, nr 33, s. 335401-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Helium retention and diffusion in molybdenum is studied on an atomistic scale with ab initio methods. The thermal stability of helium–vacancy clusters is quantified within the framework of density functional theory. Calculated helium emission rates are used to derive a desorption spectrum which is compared with experimental results. The agreement between the current calculations and available experiments is satisfactory except in the high temperature end of the spectrum. The current results indicate that above 1100 K He migration is assisted by lattice defects such as vacancies, rather than through interstitial diffusion.

  • 21.
    Runevall, Odd
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Helium induced void and bubble formation in MgO2012Inngår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 60, s. 53-58Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Trapping of helium in magnesium oxide vacancies and vacancy clusters is studied using empirical potential calculations. Based on the results, a rate-theory model is formulated in order to simulate the kinetics and long time dynamics of helium trapping. The simulations are in good agreement with helium annealing experiments in the literature. In particular, the tendency for helium to stabilise vacancy clusters is found to be strongly dependent on He concentration and temperature.

  • 22.
    Runevall, Odd
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Modeling gas diffusion on a microscopic scale2010Inngår i: Transactions of the American Nuclear Society, American Nuclear Society, 2010, s. -845Konferansepaper (Fagfellevurdert)
  • 23.
    Runevall, Odd
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Self-diffusion in MgO-a density functional study2011Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, nr 34, s. 345402-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Density functional theory calculations have been performed to study self-diffusion in magnesium oxide, a model material for a wide range of ionic compounds. Formation energies and entropies of Schottky defects and divacancies were obtained by means of total energy and phonon calculations in supercell configurations. Transition state theory was used to estimate defect migration rates, with migration energies taken from static calculations, and the corresponding frequency factors estimated from the phonon spectrum. In all static calculations we corrected for image effects using either a multipole expansion or an extrapolation to the low concentration limit. It is shown that both methods give similar results. The results for self-diffusion of Mg and O confirm the previously established picture, namely that in materials of nominal purity, Mg diffuses extrinsically by a single vacancy mechanism, while O diffuses intrinsically by a divacancy mechanism. Quantitatively, the current results are in very good agreement with experiments concerning O diffusion, while for Mg the absolute diffusion rate is generally underestimated by a factor of 5-10. The reason for this discrepancy is discussed.

  • 24.
    Runevall, Odd
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    The Impact of Helium on Void Nucleation in MolybdenumManuskript (preprint) (Annet vitenskapelig)
  • 25.
    Runevall, Odd
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Wallenius, Janne
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Helium transport in a CERMET fuel matrix2009Inngår i: Proceedings of Actinide and Fission Product Partitioning and Transmutation - Tenth Information Exchange Meeting, 2009, 2009Konferansepaper (Fagfellevurdert)
  • 26.
    Sandberg, Nils
    KTH, Tidigare Institutioner                               , Fysik.
    Atomistic simulation of crystal defects in metals2002Doktoravhandling, med artikler (Annet vitenskapelig)
  • 27.
    Sandberg, Nils
    KTH, Tidigare Institutioner                               , Fysik.
    Atomistic simulations of defects2000Licentiatavhandling, med artikler (Annet vitenskapelig)
  • 28.
    Sandberg, Nils
    KTH, Tidigare Institutioner                               , Fysik.
    Jump dynamics of point defects in Al and Cu: short-time corrections to the transition-state theory rateArtikkel i tidsskrift (Annet vitenskapelig)
  • 29.
    Sandberg, Nils
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik. Swedish Radiat Safety Author, Sweden.
    Chang, Z.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Messina, Luca
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Korzhavyi, Pavel
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap.
    Modeling of the magnetic free energy of self-diffusion in bcc Fe2015Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, nr 18, artikkel-id 184102Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A first-principles based approach to calculating self-diffusion rates in bcc Fe is discussed with particular focus on the magnetic free energy associated with diffusion activation. First, the enthalpies and entropies of vacancy formation and migration in ferromagnetic bcc Fe are calculated from standard density functional theory methods in combination with transition state theory. Next, the shift in diffusion activation energy when going from the ferromagnetic to the paramagnetic state is estimated by averaging over random spin states. Classical and quantum mechanical Monte Carlo simulations within the Heisenberg model are used to study the effect of spin disordering on the vacancy formation and migration free energy. Finally, a quasiempirical model of the magnetic contribution to the diffusion activation free energy is applied in order to connect the current first-principles results to experimental data. The importance of the zero-point magnon energy in modeling of diffusion in bcc Fe is stressed.

  • 30.
    Sandberg, Nils
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Chang, Zhongwen
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, Pär
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Korzhavyi, Pavel
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialteknologi.
    Modeling of the magnetic free energy of self-diusion in bcc FeManuskript (preprint) (Annet vitenskapelig)
  • 31.
    Sandberg, Nils
    et al.
    KTH, Tidigare Institutioner, Fysik.
    Grimvall, Göran
    Anharmonic contribution to the vacancy formation in Cu2001Inngår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 63, nr 18Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Experimental data on the vacancy concentration in metals, when shown in an Arrhenius plot, give an approximate straight line as a function of 1/T but often with a slight upward curvature close to the melting point T-m. This feature has usually been attributed to a divacancy contribution. We have performed molecular dynamics simulations on copper, using a slightly modified model potential due to Cleri and Rosato (Ref. 7). Our results show that divacancy effects are negligible, but that there is an important contribution from anharmonicity in the lattice vibrations, giving rise to the curvature in the Arrhenius plot. As a consequence, one must apply significant corrections to the vacancy formation enthalpy and entropy before they are compared with theoretical values that usually refer to zero kelvin and harmonic vibrations, respectively.

  • 32.
    Sandberg, Nils
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Grimwall, Göran
    Driving forces for recrystallization: a molecular dynamics approach1999Inngår i: Proceedings of ReX '99 (JIMIS-10), the Fourth International Conference on Recrystallization and Related Phenomena / [ed] T Sakai; H G Suzuki, Sendai: JIM, Japan Inst. of Metals , 1999Konferansepaper (Annet vitenskapelig)
  • 33.
    Sandberg, Nils
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Henriksson, Krister O. E.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnkraftssäkerhet.
    Wallenius, Janne
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Carbon impurity dissolution and migration in bcc Fe-Cr: First-principles calculations2008Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, nr 9Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    First-principles density-functional theory calculations for C solution enthalpies, H-sol, and diffusion activation enthalpies, H-diff, in body-centered-cubic Fe and Cr are presented. The results for C in Fe compare well with experiments, provided that the effect of magnetic disordering is accounted for. Likewise, in Cr, the calculated Hsol and Hdiff agree well with available experiments. In both materials, the deviation between calculated enthalpies and critically assessed experimental enthalpies are less than 0.05 eV. Further, first-principles calculations for the interaction energies between a solute (e.g., a Cr atom in bcc Fe) and an interstitial C atom are presented. The results are in conflict with those inferred from internal friction (IF) experiments in disordered Fe-Cr-C alloys. A simple model of C relaxation in disordered Fe-Cr is used to compare theoretical and experimental IF curves directly. The results suggest that a more extensive study of the energetic, thermodynamic, and kinetic aspects of C migration in Fe-Cr is needed.

  • 34. Sandberg, Nils
    et al.
    Holmestad, R.
    First-principles calculations of impurity diffusion activation energies in Al2006Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, nr 1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present first-principles density functional theory calculations of the diffusion activation energies of Mg, Si, and the 3d transition metals Sc-Zn in Al. In general, the calculated activation energies are underestimated with respect to experiments by 5-25%. The trend seen in experiments, namely, that impurities around Ti, V, and Cr have high diffusion activation energies leading to "anomalously" slow diffusion, is well reproduced in the present calculations. We provide an explanation in terms of electrostatic screening effects.

  • 35.
    Sandberg, Nils
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Korzhavyi, Pavel
    KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Tillämpad materialfysik.
    Theoretical study of irradiation induced hardening and embrittlement in spent nuclear fuel holders, relevant for the Swedish long-term storage2009Rapport (Fagfellevurdert)
  • 36.
    Sandberg, Nils
    et al.
    KTH, Tidigare Institutioner                               , Fysik.
    Magyari-Kope, Blanka
    Mattsson, Thomas R
    Self-diffusion rates in Al from combined first-principles and model-potential calculations2002Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 89, nr 6Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Monovacancy diffusion alone dominates over diffusion due to divacancies and interstitials in Al for all temperatures up to the melting point. Deviations from a single Arrhenius dependence are due to anharmonicity. The conclusion is based on a combination of theoretical methods, from density functional theory to thermodynamic integration, without fitting to experimental data. The calculated diffusion rate agrees with experimental data over 11 orders of magnitude.

  • 37.
    Sandberg, Nils
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Slabanja, M.
    Holmestad, Randi
    Ab initio simulations of clustering and precipitation in Al-Mg-Si alloys2007Inngår i: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 40, nr 3, s. 309-318Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A class of proposed coherent precipitate structures (Guinier-Preston zones) in the Al-Mg-Si alloy are investigated using first-prin- ciples density functional theory methods. The cluster expansion method is used to extract effective interaction parameters, providing the means for large scale energy calculations of alloy structures. The Mg1Si1 L1(0) structure and structures related to the Mg5Si6 beta '' phase are studied in more detail, and e.g., precipitate/matrix interface energies are presented. Using direct first-principles calculations we show that the former phase is dynamically unstable and thus must be stabilized by the surrounding Al matrix. Monte Carlo simulations and free-energy techniques are used to study the Al rich side of the phase diagram with the current CE parameters, and kinetic Monte Carlo simulations are used to study clustering in the disordered phase. The implications of our findings are discussed in the framework of classical nucleation theories, and we outline possible nucleation mechanisms.

  • 38. Terentyev, D.
    et al.
    Juslin, N.
    Nordlund, K.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Fast three dimensional migration of He clusters in bcc Fe and Fe-Cr alloys2009Inngår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 105, nr 10Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this work, we perform atomistic molecular dynamics simulations to assess the properties of small helium vacancy (He-V) and pure He clusters in body-centered cubic Fe and in Fe-90-Cr-10 (Fe-10Cr) random alloy. The following two goals are pursued: determining diffusion mechanisms of He-V clusters occurring in dynamic simulations and revealing a possible influence of Cr on the mobility/stability of He-V clusters in the Fe-10Cr alloy. We also present a newly developed set of interatomic potentials for the Fe-Cr-He system, fitted to a set of specially performed density functional theory calculations. The obtained results show that the dissociation energies of the studied He-V clusters, as well as the migration energy of He interstitial, are not significantly affected in the alloy compared to pure Fe. It was found that small pure He clusters with sizes up to four atoms, that were assumed to be immobile in many previous studies devoted to He-release/accumulation kinetics, in fact, exhibit fast three dimensional motion with a migration energy of tens of meV. The presence of 10% Cr in the Fe matrix, however, retards their mobility. We discuss possible reasons for the decreased diffusivity of these He clusters in the Fe-Cr alloy.

  • 39.
    Wallenius, Janne
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Abrikosov, I. A.
    Chakarova, Roumiana
    KTH, Tidigare Institutioner, Fysik.
    Lagerstedt, C.
    Malerba, L.
    Olsson, P.
    Pontikis, V.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Terentyev, D.
    Development of an EAM potential for simulation of radiation damage in Fe-Cr alloys2004Inngår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 329-33, s. 1175-1179Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have developed a set of EAM potentials for simulation of Fe-Cr alloys. By relaxing the requirement of reproducing the pressure-volume relation at short distances and by fitting to the thermal expansion coefficients of Fe and Cr, stability of theself-interstitial could be obtained. For Cr, properties of the paramagnetic state were applied, providing a positive Cauchy pressure. Mixed Fe-Cr pair potentials were fitted to the calculated mixing enthalpy of ferromagnetic Fe-Cr. Simulation of thermal ageing in Fe-Cr alloys using the Fe-20Cr potential exhibited pronounced Cr-precipitation for temperatures below 900 K, a feature not observed at any temperature using a potential fitted to the mixing enthalpy of Fe-5Cr.

  • 40.
    Wallenius, Janne
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Henriksson, Krister O. E.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnkraftssäkerhet.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, P.
    The role of magnetism in modelling of fuels and structural materials2008Inngår i: Transactions of the American Nuclear Society, 2008Konferansepaper (Fagfellevurdert)
  • 41.
    Wallenius, Janne
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Olsson, Pär
    Lagerstedt, Christina
    KTH, Tidigare Institutioner, Fysik.
    Sandberg, Nils
    KTH, Tidigare Institutioner, Fysik.
    Chakarova, Roumiana
    KTH, Tidigare Institutioner, Fysik.
    Pontikis, Vassilis
    KTH, Tidigare Institutioner, Fysik.
    Modeling of chromium precipitation in Fe-Cr alloys2004Inngår i: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 69, s. 094103-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have implemented a set of Embedded Atom Method (EAM) potentials for simulation of Fe-Cr alloys. The functions for the pure elements were fitted to the respective elastic constants, vacancy formation energy, and thermal expansion coefficients. For Cr, properties of the paramagnetic state were applied, providing a positive Cauchy pressure and hence applicability of the EAM. By relaxing the requirement of reproducing the pressure-volume relation at short interaction distances, stability of the <110> self-interstitial could be obtained. Our Fe-potential gives E-<110>(f)-E-<111>(f)=-0.23 eV. Mixed Fe-Cr pair potentials were fitted to the calculated mixing enthalpy of ferromagnetic Fe-Cr, which is negative for Cr concentrations below 6%. Simulation of thermal aging in Fe-Cr alloys using a potential fitted to the mixing enthalpy of Fe-20Cr exhibited pronounced Cr-precipitation for temperatures below 900 K, in agreement with the phase diagram. No such ordering was observed at any temperature using a potential fitted to the mixing enthalpy of Fe-5Cr. Applied to recoil cascade simulations the new potentials predict a smaller number of surviving defects than potentials found in the literature. We obtain a cascade efficiency of 0.135 NRT for damage energies inbetween 10 and 20 keV. An enhanced probability for Cr atoms to end up in defect structures is observed.

  • 42.
    Wallenius, Janne
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Sandberg, Nils
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Henriksson, Krister
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Reaktorfysik.
    Atomistic modelling of the Fe-Cr-C system2011Inngår i: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 415, nr 3, s. 316-319Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    For the purpose of modelling the impact of carbon on radiation damage phenomena in steels, we have performed an extensive set of first principle calculations on the Fe-Cr-C system. The calculated solution and diffusion enthalpies of carbon in iron and in chromium agree well with experimental data, as do the relative formation energies of mono-carbides, cementite. Flagg and M(23)C(6) carbides. Our data further indicate that interstitial carbon is attracted to a solute iron atom in bcc chromium, while the reaction between carbon and a solute chromium atom in bcc iron is repulsive. An empirical potential fitted to data for iron carbides is capable of reproducing melting behaviour of cementite, while the predicted interaction with point defects agrees less well with DFT data than a potential recently published by Hepburn and Ackland.

1 - 42 of 42
RefereraExporteraLink til resultatlisten
Permanent link
Referera
Referensformat
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Annet format
Fler format
Språk
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Annet språk
Fler språk
Utmatningsformat
  • html
  • text
  • asciidoc
  • rtf