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  • 1. Klarbring, Johan
    et al.
    Vekilova, Olga Yu
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala University, Sweden.
    Nilsson, Johan O.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala University, Sweden.
    Simak, Sergei I.
    Ionic conductivity in Sm-doped ceria from first-principles non-equilibrium molecular dynamics2016In: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 296, p. 47-53Article in journal (Refereed)
    Abstract [en]

    Sm-doped ceria is a prospective electrolyte material for intermediate-temperature solid-oxide fuel cells (IT-SOFC). Equi- librium ab initio molecular dynamics (AIMD) studies of oxygen ion diffusion in this material are currently impractical due to the rareness of diffusive events on the accessible timescale. To overcome this issue we have performed ab ini- tio non-equilibrium molecular dynamics calculations of Sm-doped ceria using the color-diffusion algorithm. Applying an external force field we have been able to increase the frequency of diffusive events over the simulation time, while keeping the physical mechanism of diffusion intact. We have investigated the temperature dependence of the maximum strength of the applied external field that could be used while maintaining the response of the system in a linear regime. This allows one to obtain the diffusivity at zero field. The bulk ionic conductivity has been calculated and found to match the experimental data well. We have also compared the description of the diffusion process by our method to previous findings and show that the migration mechanism and site preference of oxygen vacancies with respect to the Sm dopants is well reproduced. 

  • 2.
    Nilsson, Johan O.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    First-principles studies of kinetic effects in energy-related materials2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Quantum mechanical calculations based on first-principles (lat. ab initio) methods have over the past decades proved very successful for the study of many materials properties. Based solely on the fundamental constants of physics, the strength of these methods lies not only in describing existing materials, but also in predicting completely new ones. This thesis contains work both related to the quest for improved materials, and to the development of new methods.

    Equilibrium ab initio molecular dynamics methods are powerful for simulating diffusion in solids but are accompanied with high computational costs. This is related to the inherent slowness of the diffusion process in solids. To tackle this problem, we implement the color-diffusion algorithm into the Vienna ab initio simulation package to perform non-equilibrium ab initio molecular dynamics (NEMD) simulations. Ion diffusion in ceria doped with Gd and Sm is studied, and the calculated conductivities is found to agree well with experiment. However, although the NEMD method significantly lowers the computational cost, statistical quality in the calculated conductivity still comes expensive. Knowing the error resulting from limited statistics is therefore important.

    We derive an analytical expression for the error in calculated ion conductivity, which is verified numerically using the Kinetic Monte Carlo (KMC) method. Being developed particularly for the simulation of slow events, the great advantage of the KMC method over the NEMD method is that it is much less computationally expensive. This allows for long simulation times and large system sizes. The effect of dopant type and dopant distribution on the oxygen ion diffusivity is investigated with KMC simulations of rare-earth doped ceria. The full set of diffusion barriers in the simulation cell is calculated from first-principles within a density functional theory (DFT) framework.

    This Thesis also includes a study of processes involving water on a rutile TiO2(110) surface. The basic processes are: diffusion, dissociation, recombination, and clustering of water molecules. The barriers for these processes are calculated with DFT employing different exchange-correlation (XC) functionals. Using the barriers calculated from two XC functionals, we perform KMC simulations and find that the choice of XC functional radically alters the dynamics of the simulated water-titania system.

  • 3.
    Nilsson, Johan O.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Leetmaa, Mikael
    Vekilova, Olga Yu
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Oxygen diffusion in ceria doped with rare-earth elementsManuscript (preprint) (Other academic)
  • 4.
    Nilsson, Johan O.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Leetmaa, Mikael
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Vekilova, Olga Yu.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden.
    Oxygen diffusion in ceria doped with rare-earth elements2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, p. 13723-13730Article in journal (Refereed)
    Abstract [en]

    We examine the effects of the dopant type and the dopant distribution on the ion diffusion in ceria doped with rare-earth elements (Pr, Nd, Pm, Sm, Eu, and Gd). Diffusion is simulated by means of a Kinetic Monte Carlo method using input transition rates derived from diffusion barriers calculated in the framework of density functional theory (DFT). Based on diffusion simulations, we discuss the characteristics of the dopants in terms of the diffusion barriers, and study oxygen ion trajectories for different dopants and distributions. Our simulations show a trend of increasing ion diffusivity with increasing atomic number for all distributions.

  • 5.
    Nilsson, Johan O.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Leetmaa, Mikael
    Vekilova, Olga Yu
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala University, Sweden.
    Statistical error in simulations of Poisson processes: Example of diffusion in solids2016In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 94, no 8, article id 085206Article in journal (Refereed)
    Abstract [en]

    Simulations of diffusion in solids often produce poor statistics of diffusion events. We present an analytical expression for the statistical error in ion conductivity obtained in such simulations. The error expression is not restricted to any computational method in particular, but valid in the context of simulation of Poisson processes in general. This analytical error expression is verified numerically for the case of Gd-doped ceria by running a large number of kinetic Monte Carlo calculations. 

  • 6.
    Nilsson, Johan O.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Leetmaa, Mikael
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Wang, B.
    Zguns, Pjotrs
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala University, Sweden.
    Pašti, I.
    Sandell, A.
    Skorodumova, Natalia
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala University, Sweden.
    Modeling Kinetics of Water Adsorption on the Rutile TiO2 (110) Surface: Influence of Exchange-Correlation Functional2018In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 255, no 3, article id 1700344Article in journal (Refereed)
    Abstract [en]

    The accuracy of the theoretical description of materials properties in the framework of density functional theory (DFT) inherently depends on the exchange-correlation (XC) functional used in the calculations. Here we investigate the influence of the choice of a XC functional (PBE, RPBE, PW91, and PBE0) on the kinetics of the adsorption, diffusion and dissociation of water on the rutile TiO2(110) surface using a combined Kinetic Monte Carlo (KMC) – DFT approach, where the KMC simulations are based on the barriers for the aforementioned processes calculated with DFT. We also test how the adsorption energy of intact and dissociated water molecules changes when dispersion interactions are included into the calculations. We consider the beginning of the water layer formation varying coverage up to 0.2 monolayer (ML) at temperatures up to 180 K. We demonstrate that the dynamics of the simulated water–titania system is extremely sensitive to the choice of the XC functional.

  • 7.
    Nilsson, Johan O.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Leetmaa, Mikael
    Wang, Baochang
    Zguns, Pjotrs A.
    Sandell, Anders
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Kinetics of water adsorption on the rutile TiO2(110) surfaceManuscript (preprint) (Other academic)
  • 8.
    Nilsson, Johan O.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Vekilova, Olga Yu
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hellman, Olle
    Klarbring, Johan
    Simak, Sergei I.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ionic conductivity in Gd-doped CeO2: Ab initio color-diffusion nonequilibrium molecular dynamics study2016In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 2, article id 024102Article in journal (Refereed)
    Abstract [en]

    A first-principles nonequilibrium molecular dynamics (NEMD) study employing the color-diffusion algorithm has been conducted to obtain the bulk ionic conductivity and the diffusion constant of gadolinium-doped cerium oxide (GDC) in the 850-1150 K temperature range. Being a slow process, ionic diffusion in solids usually requires simulation times that are prohibitively long for ab initio equilibrium molecular dynamics. The use of the color-diffusion algorithm allowed us to substantially speed up the oxygen-ion diffusion. The key parameters of the method, such as field direction and strength as well as color-charge distribution, have been investigated and their optimized values for the considered system have been determined. The calculated ionic conductivity and diffusion constants are in good agreement with available experimental data.

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