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Sökning: swepub > Johansson Börje > Refereegranskat > (2010-2011) > Engelska

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1.
  • Olovsson, Weine, et al. (författare)
  • First principle calculations of core-level binding energy and Auger kinetic energy shifts in metallic solids
  • 2010
  • Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - Elsevier Science B.V., Amsterdam.. - 0368-2048. ; 178:Sp. Iss. SI, s. 88-99
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a brief overview of recent theoretical studies of the core-level binding energy shift (CLS) in solid metallic materials. The focus is on first principles calculations using the complete screening picture, which incorporates the initial (ground state) and final (core-ionized) state contributions of the electron photoemission process in X-ray photoelectron spectroscopy (XPS), all within density functional theory (DFT). Considering substitutionally disordered binary alloys, we demonstrate that on the one hand CLS depend on average conditions, such as volume and overall composition, while on the other hand they are sensitive to the specific local atomic environment. The possibility of employing layer resolved shifts as a tool for characterizing interface quality in fully embedded thin films is also discussed, with examples for CuNi systems. An extension of the complete screening picture to core-core-core Auger transitions is given, and new results for the influence of local environment effects on Auger kinetic energy shifts in fcc AgPd are presented.
2.
  • Airiskallio, E., et al. (författare)
  • High temperature oxidation of Fe-Al and Fe-Cr-Al alloys : The role of Cr as a chemically active element
  • 2010
  • Ingår i: Corrosion Science. - 0010-938X. ; 52:10, s. 3394-3404
  • Tidskriftsartikel (refereegranskat)abstract
    • Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the alpha-Al2O3 film which is formed on the surface provided temperature is above 900 degrees C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales. (C) 2010 Elsevier Ltd. All rights reserved.
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3.
  • Airiskallio, E., et al. (författare)
  • Third element effect in the surface zone of Fe-Cr-Al alloys
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 81:3
  • Tidskriftsartikel (refereegranskat)abstract
    • The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.
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4.
  • Al-Zoubi, Noura, et al. (författare)
  • Completeness of the exact muffin-tin orbitals : application to hydrogenated alloys
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 81:4
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the basis set convergence of the exact muffin-tin orbitals by monitoring the equation of state for Al, Cu, and Rh calculated in the conventional face-centered-cubic lattice (str-I) and in a face-centered-cubic lattice with one atomic and three empty sites per primitive cell (str-II). We demonstrate that three (spd) muffin-tin orbitals are sufficient to describe Al in both structures, but for str-II Cu and Rh at least five (spdfg) orbitals are needed to get converged equilibrium Wigner-Seitz radius (within <= 0.8%) and bulk modulus (<= 3.3%). We ascribe this slow convergence to the nearly spherical densities localized around the Cu and Rh atoms, which create strongly asymmetric charge distributions within the nearest cells around the empty sites. The potential sphere radius dependence of the theoretical results for structure str-II is discussed. It is shown that a properly optimized overlapping muffin-tin potential in combination with the spdfg basis yields acceptable errors in the equilibrium bulk properties. The basis set convergence is also shown on hydrogenated Sc and Sc-based alloys.
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5.
  • Al-Zoubi, Noura, et al. (författare)
  • Influence of Magnesium on hydrogenated ScAl1-xMgx alloys : a theoretical study
  • 2011
  • Ingår i: Computational materials science. - 0927-0256. ; 50:10, s. 2848-2853
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio total energy calculations, based on the projector augmented wave method and the exact mu±n-tin orbitals method in combination with the coherent-potential approximation, are used to examine the effect of magnesium on hydrogen absorption/desorption temperature and phase stability of hydrogenated ScAl1-xMgx (0 ≤ x ≤ 0:3) alloys. According to the  experiments, ScAl1-xMgx adopts the CsCl structure, and upon hydrogen absorption it decomposes into ScH2 with CaF2 structure and Al-Mg with face centered cubic structure. Here we demonstrate that the stability field of the hydrogenated alloys depends sensitively on Mg content and on the microstructure of the decomposed system. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with Mg concentration.
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6.
  • Al-Zoubi, Noura, et al. (författare)
  • The bain path of paramagnetic Fe-Cr based alloys
  • 2011
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 110:1, s. 013708
  • Tidskriftsartikel (refereegranskat)abstract
    • Employing the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, we calculated the total energy and local magnetic moments of paramagnetic Fe-Cr-M (M = Cr, Mn, Fe, Co, Ni) alloys along the tetragonal distortion (Bain) path connecting the body centered cubic (bcc) and the face centered cubic (fcc) structures. The paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all alloys, the local magnetic moments on Fe atoms decrease from the maximum value corresponding to the bcc phase toward the minimum value realized for the fcc phase. Cobalt atoms have non-vanishing local magnetic moments only for tetragonal lattices with c/a < 1.30, whereas the local magnetic moments of Mn show weak crystal structure dependence. We find that Cr stabilizes the bcc lattice and increases the energy barrier as going from the bcc toward the fcc phase. Both Co and Ni favor the fcc lattice and decrease the energy barrier relative to the bcc phase. On the other hand, the tetragonal distortion around the fcc phase is facilitated by Cr and to a somewhat lesser extent also by Ni, but strongly impeded by Co. Manganese has negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. Our findings on the alloying induced softening or hardening of Fe-Cr based alloys against tetragonal distortions are important for understanding the interstitial driven martensitic transformations in alloy steels.
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7.
  • Araujo, C. Moysés, et al. (författare)
  • Room temperature ferromagnetism in pristine MgO thin films
  • 2010
  • Ingår i: Applied Physics Letters. - 0003-6951. ; 96:23
  • Tidskriftsartikel (refereegranskat)abstract
    • Robust ferromagnetic ordering at, and well above room temperature is observed in pure transparent MgO thin films (<170 nm thick) deposited by three different techniques. Careful study of the wide scan x-ray photoelectron spectroscopy rule out the possible presence of any magnetic contaminants. In the magnetron sputtered films, we observe magnetic phase transitions as a function of film thickness. The maximum saturation magnetization of 5.7 emu/cm(3) is measured on a 170 nm thick film. The films above 500 nm are found to be diamagnetic. Ab initio calculations suggest that the ferromagnetism is mediated by cation vacancies.
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8.
  • Davis, Sergio, et al. (författare)
  • Model for diffusion at the microcanonical superheating limit from atomistic computer simulations
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 84:6, s. 064102
  • Tidskriftsartikel (refereegranskat)abstract
    • The diffusion statistics of atoms in a crystal close to the critical superheating temperature was studied in detail using molecular dynamics and Monte Carlo simulations. We present a continuous random-walk model for diffusion of atoms hopping through thermal vacancies. The results obtained from our model suggest that the limit of superheating is precisely the temperature for which dynamic percolation happens at the time scale of a single individual jump. A possible connection between the critical superheating limit and the maximization of the Shannon entropy associated with the distribution of jumps is suggested. As a practical application of our results, we show that an extrapolation of the critical superheating temperature (and therefore an estimation of the melting point) can be performed using only the dynamical properties of the solid state.
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9.
  • Davis, Sergio, et al. (författare)
  • Molecular dynamics simulation of zirconia melting
  • 2010
  • Ingår i: Central European Journal of Physics. - 1644-3608. ; 8:5, s. 789-797
  • Tidskriftsartikel (refereegranskat)abstract
    • The melting point for the tetragonal and cubic phases of zirconia (ZrO2) was computed using Z-method microcanonical molecular dynamics simulations for two different interaction models: the empirical Lewis-Catlow potential versus the relatively new reactive force field (ReaxFF) model. While both models reproduce the stability of the cubic phase over the tetragonal phase at high temperatures, ReaxFF also gives approximately the correct melting point, around 2900 K, whereas the Lewis-Catlow estimate is above 6000 K.
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10.
  • Davis, Sergio, et al. (författare)
  • SearchFill : A stochastic optimization code for detecting atomic vacancies in crystalline and non-crystalline systems
  • 2011
  • Ingår i: Computer Physics Communications. - 0010-4655. ; 182:5, s. 1105-1110
  • Tidskriftsartikel (refereegranskat)abstract
    • We present an implementation of a stochastic optimization algorithm applied to location of atomic vacancies. Our method labels an empty point in space as a vacancy site, if the total spatial overlap of a "virtual sphere", centered around the point, with the surrounding atoms (and other vacancies) falls below a tolerance parameter. A Metropolis-like algorithm displaces the vacancies randomly, using an "overlap temperature" parameter to allow for acceptance of moves into regions with higher overlap, thus avoiding local minima. Once the algorithm has targeted a point with low overlap, the overlap temperature is decreased, and the method works as a steepest descent optimization.Our method, with only two free parameters, is able to detect the correct number and coordinates of vacancies in a wide spectrum of condensed-matter systems, from crystals to amorphous solids, in fact in any given set of atomic coordinates, without any need of comparison with a reference initial structure.
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