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  • 1. Airiskallio, E.
    et al.
    Nurmi, E.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, M. P. J.
    Johansson, Bengt
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Magnetic origin of the chemical balance in alloyed Fe-Cr stainless steels: First-principles and Ising model study2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 92, p. 135-140Article in journal (Refereed)
    Abstract [en]

    Iron-chromium is the base material for most of the stainless steel grades. Recently, new insights into the origins of fundamental physical and chemical characteristics of Fe-Cr based alloys have been achieved. Some of the new results are quite unexpected and call for further investigations. The present study focuses on the magnetic contribution in the atomic driving forces related to the chemical composition in Fe-Cr when alloyed with Al, Ti, V, Mn, Co, Ni, and Mo. Using the ab initio exact muffin-tin orbitals method combined with an Ising-type spin model, we demonstrate that the magnetic moment of the solute atoms with the induced changes in the magnetic moments of the host atoms form the main factor in determining the mixing energy and chemical potentials of low-Cr Fe-Cr based alloys. The results obtained in the present work are related to the designing and tuning of the microstructure and corrosion protection of low-Cr steels. (C) 2014 Elsevier B. V. All rights reserved.

  • 2. Airiskallio, E.
    et al.
    Nurmi, E.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, M. P. J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Magnetic origin of the chemical balance in alloyed Fe-Cr stainless steels: First-principles and Ising model study2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 92, p. 135-140Article in journal (Refereed)
    Abstract [en]

    Iron-chromium is the base material for most of the stainless steel grades. Recently, new insights into the origins of fundamental physical and chemical characteristics of Fe-Cr based alloys have been achieved. Some of the new results are quite unexpected and call for further investigations. The present study focuses on the magnetic contribution in the atomic driving forces related to the chemical composition in Fe-Cr when alloyed with Al, Ti, V, Mn, Co, Ni, and Mo. Using the ab initio exact muffin-tin orbitals method combined with an Ising-type spin model, we demonstrate that the magnetic moment of the solute atoms with the induced changes in the magnetic moments of the host atoms form the main factor in determining the mixing energy and chemical potentials of low-Cr Fe-Cr based alloys. The results obtained in the present work are related to the designing and tuning of the microstructure and corrosion protection of low-Cr steels.

  • 3.
    Almroth, Per
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Management and Engineering, Solid Mechanics . Linköping University, The Institute of Technology.
    Hasselqvist, M.
    Demag Delaval Indust. Turbomach. AB, Finspång.
    Simonsson, Kjell
    Linköping University, The Institute of Technology. Linköping University, Department of Management and Engineering, Solid Mechanics .
    Sjöström, Sören
    Linköping University, Department of Management and Engineering, Solid Mechanics . Linköping University, The Institute of Technology.
    Viscoplastic-plastic modelling of IN7922004In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 29, no 4, p. 437-445Article in journal (Refereed)
    Abstract [en]

    At high temperatures metallic materials behave in a viscous manner exemplified by strain rate dependence, stress relaxation and creep deformation. At low temperatures however, these effects are extremely small, and the behaviour is strain rate independent and shows no or very small relaxation effects. Finally there exists an intermediate region, in which the material behaviour is close to strain rate independent for high strain rates but at the same time shows time dependent inelastic effects, such as stress relaxation and creep. For IN792 this occurs at temperatures around 650 °C. The article describes the extension of a power-law viscoplastic model describing the behaviour of IN792 at 850 °C, also to describe the behaviour at 650 °C, by bounding the elastic-viscoplastic stress-space by a plastic yield surface. The model parameters have been estimated using data from creep test and tailored step relaxation tests, and the model fits well to both the step relaxation data aimed at resembling relevant component conditions and long term creep data. © 2003 Published by Elsevier B.V.

  • 4.
    Almroth, Per
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Mechanical Engineering, Solid Mechanics.
    Hasselqvist, Magnus
    Sjöström, Karl Henning Sören
    Linköping University, The Institute of Technology. Linköping University, Department of Mechanical Engineering.
    Simonsson, Kjell
    Linköping University, The Institute of Technology. Linköping University, Department of Mechanical Engineering, Solid Mechanics.
    Modeling of the high temperature behaviour of IN792 in gas turbine hot parts2002In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 25, no 3, p. 305-315Article in journal (Other academic)
    Abstract [en]

    The material parameters for two isothermal viscoplastic models with deliberately limited sets of material parameters have been estimated. The models are to describe the behaviour of the nickel based superalloy IN792 in a gas turbine hot part application. The models are based on a power law flow equation and the state variable used is backstress. The model calibration is done by least-squares optimization using non-standard constitutive tests that are aimed at describing relevant component conditions. The constitutive tests give information about the kinematic hardening effects for the backstress evolution equations, while secondary creep data provides stress versus inelastic strain rate information for the flow equation. All tests are uniaxial and isothermal. With the estimated parameter sets the models give relatively good fits to the data. The results suggest that the models can be used to describe the high temperature behaviour of IN792. ⌐ 2002 Elsevier Science B.V. All rights reserved.

  • 5. Al-Zoubi, N.
    et al.
    Punkkinen, M. P. J.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of magnesium on hydrogenated ScAl(1-x)Mg(x) alloys: A theoretical study2011In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 50, no 10, p. 2848-2853Article in journal (Refereed)
    Abstract [en]

    Ab initio total energy calculations, based on the projector augmented wave method and the exact muffin-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 ScAl(1-x)Mg(x) (0 <= x <= 0.3) alloys. According to the experiments, ScAl(1-x)Mg(x) adopts the CsCl structure, and upon hydrogen absorption it decomposes into ScH(2) with CaF(2) 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.

  • 6.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Influence of Magnesium on hydrogenated ScAl1-xMgx alloys: a theoretical study2011In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 50, no 10, p. 2848-2853Article in journal (Refereed)
    Abstract [en]

    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.

  • 7.
    Al-Zoubi, Noura
    et al.
    Tafila Tech Univ, Dept Appl Phys, Tafila, Jordan.
    Schonecker, Stephan
    KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Li, Xiaoqing
    KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Li, Wei
    KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Johansson, Börje
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden; Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary.
    Elastic properties of 4d transition metal alloys: Values and trends2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 159, p. 273-280Article in journal (Refereed)
    Abstract [en]

    Using the Exact Muffin-Tin Orbitals method within the Perdew-Burke-Ernzerhof exchange-correlation approximation for solids and solid surfaces (PBEso1), we study the single crystal elastic constants of 4d transition metals (atomic number Z between 39 and 47) and their binary alloys in the body centered cubic (bcc) and face centered cubic (fcc) structures. Alloys between the first neighbors Z(Z + 1) and between the second neighbors Z(Z + 2) are considered. The lattice constants, bulk moduli and elastic constants are found in good agreement with the available experimental and theoretical data. It is shown that the correlation between the relative tetragonal shear elastic constant C-fcc'-2C(bcc)' and the structural energy difference between the fcc and bcc lattices Delta E is superior to the previously considered models. For a given crystal structure, the equiatomic Z(Z + 2) alloys turn out to have similar structural and elastic properties as the pure elements with atomic number (Z + 1). Furthermore, alloys with composition Z(1-x)(Z + 2)(x) possess similar properties as Z(1-2x)(Z + 1)(2x). The present theoretical data on the structural and the elastic properties of 4d transition metal alloys provides consistent input for coarse scale modeling of material properties.

  • 8.
    Al-Zoubi, Noura
    et al.
    Tafila Tech Univ, Dept Appl Phys, Tafila, Jordan..
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic properties of 4d transition metal alloys: Values and trends2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 159, p. 273-280Article in journal (Refereed)
    Abstract [en]

    Using the Exact Muffin-Tin Orbitals method within the Perdew-Burke-Ernzerhof exchange-correlation approximation for solids and solid surfaces (PBEso1), we study the single crystal elastic constants of 4d transition metals (atomic number Z between 39 and 47) and their binary alloys in the body centered cubic (bcc) and face centered cubic (fcc) structures. Alloys between the first neighbors Z(Z + 1) and between the second neighbors Z(Z + 2) are considered. The lattice constants, bulk moduli and elastic constants are found in good agreement with the available experimental and theoretical data. It is shown that the correlation between the relative tetragonal shear elastic constant C-fcc'-2C(bcc)' and the structural energy difference between the fcc and bcc lattices Delta E is superior to the previously considered models. For a given crystal structure, the equiatomic Z(Z + 2) alloys turn out to have similar structural and elastic properties as the pure elements with atomic number (Z + 1). Furthermore, alloys with composition Z(1-x)(Z + 2)(x) possess similar properties as Z(1-2x)(Z + 1)(2x). The present theoretical data on the structural and the elastic properties of 4d transition metal alloys provides consistent input for coarse scale modeling of material properties.

  • 9. Ans, Muhammad
    et al.
    Iqbal, Javed
    Eliasson, Bertil
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Saif, Muhammad Jawwad
    Ayub, Khurshid
    Opto-electronic properties of non-fullerene fused-undecacyclic electron acceptors for organic solar cells2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 159, p. 150-159Article in journal (Refereed)
    Abstract [en]

    Due to limitations of fullerene based acceptor molecules for solar cell applications, research is recently diverted to explore non-fullerene acceptor molecules. In this regard, four new A-D-A type fused ring electron acceptor molecules (M1, M2, M3 and M4) are evaluated for their opto-electronic properties for transparent organic solar cells. These molecules contain strong electron donor undecacyclic linked with four different acceptor moieties, 2-(3-ethly-5-methylene-4-oxothiazolidin-2-yluidene)malononitrile (M1), 2-(5,6-dicyano-2-methylene-3-oxo -2,3-dihydroindene-1-ylidene)malononitrile (M2), 2-(5-methylene-6-oxo-tetrahydro-1H-cyclopenta-thiophene-4(5H)-ylidene)malononitrile (M3), and 3-ethyl-5-methylene-2-thioxothiazolidin-4-one (M4). The electronic and optical properties of these molecules are compared with the reference molecule R, which is recently reported as excellent non-fullerene based acceptor molecule. Among all molecules, M2 exhibits the maximum red shift where absorption appears 893.5 nm with B3LYP/6-31 + + G(d,p) level of theory due to highly extended conjugation between electron withdrawing end-capped acceptor moieties. The calculated Open circuit voltage (V-oc) of reference molecule R is 1.78 eV with donor polymer PTB7-Th while molecule M2 exhibits the V-oc value of 1.86 eV.

  • 10.
    Barkar, Thomas
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Höglund, Lars
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Odqvist, Joakim
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Effect of concentration dependent gradient energy coefficient on spinodal decomposition in the Fe-Cr system2018In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 143, p. 446-453Article in journal (Refereed)
    Abstract [en]

    The Cahn–Hilliard equation is solved with thermodynamic and kinetic input, using the Thermo-Calc and DICTRA software packages rather than simpler models e.g. regular solution. A concentration dependent expression for the gradient energy coefficient is introduced and its effect on simulated decomposition is discussed. Simulations were carried out in 2D and 3D using the FiPy software package modified for non-linear problems.

  • 11.
    Berns, H.
    et al.
    Institut für Werkstofle, Ruhr-Universität Bochum, Bochum, Germany .
    Melander, A.
    lnstitutet för Metallforskning, Stockholm, Sweden .
    Weichert, D.
    Laboratoire de Mécanique de Lille, Lille, France .
    Asnafi, Nader
    lnstitutet för Metallforskning, Stockholm, Sweden .
    Broeckmann, C.
    Institut für Werkstofle, Ruhr-Universität Bochum, Bochum, Germany .
    Gross-Weege, A.
    Laboratoire de Mécanique de Lille, Lille, France .
    A new material for cold forging tools1998In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 11, no 3, p. 166-180Article in journal (Refereed)
    Abstract [en]

    A new tool material for cold forging applications was developed using numerical simulation techniques (FEM) for the design and a powder metallurgical route (HIP) for the production. The basic idea was to find an optimized microstructure of the two phase material by simulating different distributions of hard particles within the metal matrix. On the micro-scale, loading was applied by a field of deformations which was obtained by a macroscopical simulation of a particular cold forming process in bolt making. A new double dispersion microstructure was found to show the best resistance against crack propagation. Specimens were produced by hot isostatic pressing. Afterwards the new material was tested in the laboratory. Wear resistance and bending strength of the double dispersive material are comparable to a standard dispersion material with the same volume fraction of particles, but fracture toughness is increased by about 30%. Testing the new material in bolt making showed that the life time of the tool is increased by a factor of 8.

  • 12.
    Bohlén, Martin
    et al.
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Molecular Dynamics Studies of the Influence of Single Wall Carbon Nanotubes on the Mechanical Properties of Poly(vinylidene fluoride)2013In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 68, p. 73-80Article in journal (Refereed)
    Abstract [en]

    Molecular dynamics simulations and geometry optimizations based on the Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force field were performed to understand the effect of Single Wall Carbon Nanotubes (SWCNTs) on the mechanical properties of Poly(vinylidene fluoride) (PVDF). In particular, the Young’s modulus, bulk and shear modulus, pullout energy, pullout force, interfacial shear stress and interfacial bonding energy were calculated. The presence of the SWCNTs can increase the Young’s modulus of the systems studied here by 1 GPa in the direction of the SWCNT axis, although this depends on the distance between neighboring SWCNTs. The calculated interfacial shear stress was between 100 and 129 MPa, which is in agreement with results obtained for other SWCNT-polymer systems. The results, and in particular those obtained for the bulk and shear modulus, show that SWCNTs do not have a significant effect on the bulk mechanical properties. Functionalizing the SWCNTs may yield stronger adhesion between the nanotube and the polymer, thereby achieving improved mechanical properties. ⺠Computational studies using molecular dynamics and molecular mechanics. ⺠Effect of single wall carbon nanotubes on the mechanical properties of Poly(vinylidene fluoride). ⺠Alignment of the nanotubes plays a crucial role for the reinforcing effect. ⺠When aligned, an increase in Young’s modulus of approximately 1 GPa could be observed. ⺠The interfacial shear stress was calculated to be in the range of 100–129 MPa.

  • 13. Bondarchuk, S. V.
    et al.
    Minaev, Boris F.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Two-dimensional honeycomb (A7) and zigzag sheet (ZS) type nitrogen monolayers. A first principles study of structural, electronic, spectral, and mechanical properties2017In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 133, p. 122-129Article in journal (Refereed)
    Abstract [en]

    Two single-bonded 2D nitrogen allotropes of the honeycomb (A7) and zigzag sheet (ZS) topology have been calculated using density functional theory (DFT). The optical (vibrational, absorption, nuclear magnetic resonance), thermodynamic and elastic properties of the A7 and ZS sheets have been calculated for the first time. The band structure calculation have revealed a semiconducting nature of the ZS sheet with a direct gap of 1.246 eV, while the A7 monolayer behaves as an insulator with an indirect gap of 3.842 eV. Phonon dispersion calculations have justified these structures as vibrationally stable 2D materials. The IR spectroscopy completely failed in the characterization of the studied materials, while the Raman spectroscopy can be effectively applied for the experimental spectral identification. The absorption spectra demonstrate complete opacity of the A7 and ZS monolayers to the UV irradiation only above ca. 9 and 6 eV, respectively. Thus, the studied materials are expected to be transparent to the visible light. The electron arrangement of the nitrogen nuclei in the studied polynitrogen sheets is denser compared to the N2 molecule which follows from the calculation of the values of magnetic shielding tensors. The elastic constants reveal a robust mechanical stability of the studied 2D nitrogen allotropes. The Young moduli values are only twice as lower than that of the graphene molecule.

  • 14. Bonny, G.
    et al.
    Domain, C.
    Castin, N.
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Engineering.
    Malerba, L.
    The impact of alloying elements on the precipitation stability and kinetics in iron based alloys: An atomistic study2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 161, p. 309-320Article in journal (Refereed)
    Abstract [en]

    Iron based industrial steels typically contain a large number of alloying elements, even so-called low alloyed steels. Under irradiation, these alloying elements form clusters that have a detrimental impact of the mechanical properties of the steel. The stability and formation mechanisms of such clusters are presently not fully understood. Therefore, in this work, we study the thermal stability and formation kinetics of small solute clusters in the bcc Fe matrix. We use density functional theory (DFT) to characterize the binding energy of vacancy/solute clusters containing Cr, Mn, Ni, Cu, Si and P, thereby exploring >700 different configurations. The constructed DFT data base is used to fit a cluster expansion (CE) for the vacancy-FeCrMnNiCuSiP system. In turn, the obtained CE is applied in atomistic kinetic Monte Carlo simulations to study the effect of Mn, Ni, Cr, Si and P on the precipitation formation in the FeCu alloy. We conclude that the addition of Mn and Ni delay the precipitation kinetics by an order of magnitude. The additional alloying with traces of P/Si further delays the kinetics by an additional order of magnitude. We found that Si plays an essential role in the formation of spatially mixed MnNiCuSi cluster

  • 15.
    Borodulina, Svetlana
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Kulachenko, Artem
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Tjahjanto, Denny
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Constitutive modeling of a paper fiber in cyclic loading applications2015In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 110, p. 227-240Article in journal (Refereed)
    Abstract [en]

    The tensile response of dense fiber-based materials like paper or paperboard is mainly dependent of the properties of the fibers, which store most of the elastic energy. In this paper, we investigate the influence of geometrical and material parameters on the mechanical response of the pulp fibers used in paper manufacturing. We developed a three-dimensional finite element model of the fiber, which accounts for microfibril orientation of cellulose fibril, and the presence of lignin in the secondary cell wall. The results showed that the change in the microfibril orientation upon axial straining is mainly a geometrical effect, and is independent of the material properties of the fiber, as long as the deformations are elastic. Plastic strain accelerates the change in microfibril orientation and thus makes it material-dependent. The results also showed that the elastic modulus of the fiber has a non-linear dependency on microfibril angle, with elastic modulus being more sensitive to the change of microfibril angle around small initial values of microfibril angles. Based on numerical results acquired from a 3D fiber model supported by available experimental evidence, we propose an anisotropic-kinematic hardening plasticity model for a fiber within a beam framework. The proposed fiber model is capable of reproducing the main features of the cyclic tensile response of a pulp fiber, such as stiffening due to changing microfibril angle. The constitutive model of the fiber was implemented in a finite-element model of the fiber network. By using the fiber network model, we estimated the level of strain that fiber segments accumulate before the typical failure strain of the entire network is reached.

  • 16. Breidi, A.
    et al.
    Fries, S. G.
    Palumbo, M.
    Ruban, Andrei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Materials Center Leoben Forschung GmbH, Leoben, Austria.
    First-principles modeling of energetic and mechanical properties of Ni-Cr, Ni-Re and Cr-Re random alloys2016In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 117, p. 45-53Article in journal (Refereed)
    Abstract [en]

    We apply the exact-muffin-tin-orbitals (EMTO) method to investigate structural properties, formation enthalpies, mechanical stability and polycrystalline moduli in Ni-Re, Ni-Cr and Cr-Re disordered fcc, bcc and hcp phases. Substitutional disorder is treated by using the coherent potential approximation (CPA). We predict the alloy lattice parameters in good agreement with the experiment. We find a continuous softening, as a function of Cr composition, of the tetragonal shear modulus C' in fcc Ni-Cr phase indicating mechanical instability in Cr-rich Ni-Cr alloys. On the other hand, we show that the mechanical stability of fcc Ni-Re alloys persists through the whole composition range. We observe an intriguing behaviour of the Young's modulus vs. the intrinsic ductility curve in Ni-rich Ni-Re fcc phase.

  • 17.
    Castin, N.
    et al.
    Studie Ctr Kerneenergie, Ctr Etud Energie Nucl SCK CEN, NMS Unit, Boeretang 200, B-2400 Mol, Belgium..
    Pascuet, M. I.
    Consejo Nacl Invest Cient & Tecn CONICET, Godoy Cruz 2290 C1425FQB CABA, Buenos Aires, DF, Argentina..
    Messina, Luca
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Domain, C.
    EDF R&D, Dept Mat & Mecan Composants, F-77250 Moret Sur Loing, France..
    Olsson, Pär
    KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
    Pasianot, R. C.
    Consejo Nacl Invest Cient & Tecn CONICET, Godoy Cruz 2290 C1425FQB CABA, Buenos Aires, DF, Argentina.;Comis Nacl Energia Atom CNEA, Gcia Mat, Av Gral Paz 1499, RA-1650 San Martin, Argentina..
    Malerba, L.
    Studie Ctr Kerneenergie, Ctr Etud Energie Nucl SCK CEN, NMS Unit, Boeretang 200, B-2400 Mol, Belgium..
    Advanced atomistic models for radiation damage in Fe-based alloys: Contributions and future perspectives from artificial neural networks2018In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 148, p. 116-130Article in journal (Refereed)
    Abstract [en]

    Machine learning, and more specifically artificial neural networks (ANN), are powerful and flexible numerical tools that can lead to significant improvements in many materials modelling techniques. This paper provides a review of the efforts made so far to describe the effects of irradiation in Fe-based and W-based alloys, in a multiscale modelling framework. ANN were successfully used as innovative parametrization tools in these models, thereby greatly enhancing their physical accuracy and capability to accomplish increasingly challenging goals. In the provided examples, the main goal of ANN is to predict how the chemical complexity of local atomic configurations, and/or specific strain fields, influence the activation energy of selected thermally-activated events. This is most often a more efficient approach with respect to previous computationally heavy methods. In a future perspective, similar schemes can be potentially used to calculate other quantities than activation energies. They can thus transfer atomic-scale properties to higher-scale simulations, providing a proper bridging across scales, and hence contributing to the achievement of accurate and reliable multiscale models.

  • 18. Cimpoesu, Fanica
    et al.
    Frecus, Bogdan
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Oprea, Corneliu I.
    Panait, Petre
    Girtu, Mihai A.
    Disorder, exchange and magnetic anisotropy in the room-temperature molecular magnet V[TCNE](x) - A theoretical study2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 91, p. 320-328Article in journal (Refereed)
    Abstract [en]

    We report quantum chemical calculations to address yet unresolved and puzzling questions regarding the structural and magnetic disorder of V[TCNE](x) (TCNE = tetracyanoethylene, x similar to 2), the first room-temperature molecule-based magnet. Starting from an ideal lattice model, containing TCNE ligands either tetra- or bi-connected to vanadium(II) ions, we identify the key sources of structural disorder, explaining the amorphousness and non-stoichiometric nature of V[TCNE](x). The proposed model is prone to static disorder in terms of the bulk distribution of the tetra-connected TCNE species and to dynamic effects due to the relative rotational freedom of the bi-connected TCNE moieties. Density functional theory (DFT) calculations of the model system with rotated TCNE molecules show a rough energy landscape, consistent with the presence of magnetic irreversibilities in the system. The broken symmetry DFT approach evidences ferrimagnetic spin orientation for all TCNE configurations, ruling out the spin glass model. Multiconfigurational calculations with additional spin-orbit interaction allow for the account of the single-ion-anisotropy of the V(II) ions in different environments. We determine a small uniform zero-field-splitting (D-c = -0.03 K) of the bulk as well as a sizeable random anisotropy (D-r = 0.56 K) due to TCNE vacancies. We clarify the interplay of ferrimagnetism and random magnetic anisotropy in this system, which favours correlated sperimagnetic and not spin glass behaviour, in agreement with puzzling experimental data. Our approach goes beyond the material of interest here, as it can be applied to other disordered molecular magnets by correlating the sources of disorder with their effects on the magnetic properties.

  • 19.
    Edin, Emil
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Luo, Wei
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kaplan, Bartek
    Sandvik Coromant R&D, Stockholm, Sweden.
    Blomqvist, Andreas
    Sandvik Coromant R&D, Stockholm, Sweden.
    First principles study of C diffusion in WC/W interfaces observed in WC/Co tools after Ti-alloy machining2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 161, p. 236-243Article in journal (Refereed)
    Abstract [en]

    Ti-alloys have many qualities making them ideal for use in aerospace applications, medical implants and chemical industries such as high strength to weight ratio, good high temperature strength and chemical stability. One downside to Ti-alloys is, however, that they are considered difficult to machine. Several investigations have been made in order to understand the wear mechanisms present in machining of Ti-alloys and the most common understanding is a combination of attrition and dissolution-diffusion. Observations by Odelros et al. [1] have shown that there exists a small layer of pure bcc W on top of the outermost WC grains after turning of Ti-6Al-4V. In order for such a layer to form C has to diffuse away from the WC leaving behind only W. In this work Density Functional Theory (DFT) is used together with Harmonic Transition State Theory (HTST) to investigate the prefactors and barriers for C diffusion into and within two different WC/W interfaces, [0001]/[111] and [10 (1) over bar0]/[100]. The diffusion into the interfaces show that the barrier for the [0001]/[111] interface is more than twice as high as the barrier for the [10 (1) over bar0]/[100] interface. Diffusion within the interfaces show, on average, slightly higher barriers for the [0001]/[111] interface.

  • 20.
    Eitelberger, Johannes
    et al.
    Vienna University of Technology, Austria.
    Bader, Thomas K.
    Vienna University of Technology, Austria.
    de Borst, Karin
    Vienna University of Technology, Austria.
    Jäger, Andreas
    Vienna University of Technology, Austria.
    Multiscale prediction of viscoelastic properties of softwood under constant climatic conditions2012In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 55, p. 303-312Article in journal (Refereed)
    Abstract [en]

    This paper covers the development and validation of a multiscale homogenization model for linear viscoelastic properties of wood. Starting point is the intrinsic structural hierarchy of wood, which is accounted for by several homogenization steps. Using the correspondence principle, an existing homogenization model for the prediction of elastic properties of wood is adapted herein for upscaling of viscoelastic characteristics. Accordingly, self-consistent, Mori–Tanaka, and unit-cell-based techniques are employed, leading to pointwise defined tensorial creep and relaxation functions in the Laplace-Carson domain. Subsequently, these functions are back-transformed into the time domain by means of the Gaver-Stehfest algorithm. With this procedure the orthotropic macroscopic creep behavior of wood can be derived from the isotropic shear behavior of the lignin-hemicellulose phase. A comparison of model predictions for viscoelastic properties of softwood with corresponding experimentally derived values yields very promising results and confirms the suitability of the model.

  • 21. Ericsson, A.
    et al.
    Fisk, Martin
    Malmö University, Faculty of Technology and Society (TS), Department of Materials Science and Applied Mathematics (MTM).
    Hallberg, H.
    Modeling of nucleation and growth in glass-forming alloys using a combination of classical and phase-field theory2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 165, p. 167-179Article in journal (Refereed)
    Abstract [en]

    For metallic glasses, it is of vital importance to understand the glass formation properties and to be able to predict the crystallization process in the supercooled liquid. In the present work, we model the process of nucleation and growth using a combination of classical nucleation and phase-field theory. A diffusion coupled phase-field model is used to evaluate the work of formation and the growth behavior of the critical nucleus. The results are combined with classical nucleation and JMAK theory in order to estimate the glass forming ability of the compositions Cu64Zr36, Cu10Zr7 and CuZr2 in terms of TTT-diagrams and critical cooling rates. It is found that the work of formation of the critical nucleus from the phase-field theory agrees with the classical theory when the critical size is larger than the width of the solid-liquid interface. At smaller critical sizes, the work of formation deviates approximately linearly between the two theories. Furthermore, it is shown that the growth behavior from the phase-field simulations agree with analytical expressions of the growth rate from the classical theory.

  • 22.
    Fisk, Martin
    et al.
    Malmö högskola, School of Technology.
    Ion, John C.
    Lindgren, Lars-Erik
    Flow stress model for IN718 accounting for evolution of strengthening precipitates during thermal treatment2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 82, no 1, p. 531-539Article in journal (Refereed)
    Abstract [en]

    A flow stress model describing precipitate hardening in the nickel based alloy InconelÒ 718 following thermal treatment is presented. The interactions between precipitates and dislocations are included in a dislocation density based material model. Compression tests have been performed using solution annealed, fully-aged and half-aged material. Models were calibrated using data for solution annealed and fully-aged material, and validated using data from half-aged material. Agreement between experimental data and model predictions is good.

  • 23.
    Fisk, Martin
    et al.
    Materials Science and Applied Mathematics, Faculty of Technology and Society, Malmö University.
    Ion, John
    Division of Materials Science, Malmö University.
    Lindgren, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Flow stress model for IN718 accounting for evolution of strengthening precipitates during thermal treatment2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 82, p. 531-539Article in journal (Refereed)
    Abstract [en]

    A flow stress model describing precipitate hardening in the nickel based alloy Inconel® 718 following thermal treatment is presented. The interactions between precipitates and dislocations are included in a dislocation density based material model. Compression tests have been performed using solution annealed, fully-aged and half-aged material. Models were calibrated using data for solution annealed and fully-aged material, and validated using data from half-aged material. Agreement between experimental data and model predictions is good.

  • 24.
    Grånäs, Oscar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Thunström, Patrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nordström, Lars
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Björkman, T.
    Wills, J. M.
    Charge self-consistent dynamical mean-field theory based on the full-potential linear muffin-tin orbital method: Methodology and applications2012In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 55, p. 295-302Article in journal (Refereed)
    Abstract [en]

    Full charge self-consistence (CSC) over the electron density has been implemented into the local density approximation plus dynamical mean-field theory (LDA + DMFT) scheme based on a full-potential linear muffin-tin orbital method (FP-LMTO). Computational details on the construction of the electron density from the density matrix are provided. The method is tested on the prototypical charge-transfer insulator NiO using a simple static Hartree-Fock approximation as impurity solver. The spectral and ground state properties of bcc Fe are then addressed, by means of the spin-polarized T-matrix fluctuation exchange solver (SPTF). Finally the permanent magnet SmCo5 is studied using multiple impurity solvers, SPTF and Hubbard I, as the strength of the local Coulomb interaction on the Sm and Co sites are drastically different. The developed CSC-DMFT method is shown to in general improve on materials properties like magnetic moments, electronic structure and the materials density.

  • 25.
    Haghighatpanah, Shayesteh
    et al.
    University of Borås, School of Engineering.
    Bolton, Kim
    University of Borås, School of Engineering.
    Molecular-level computational studies of single wall carbon nanotube: polyethylene composites2013In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 69, p. 443-454Article in journal (Refereed)
    Abstract [en]

    Minimum energy structures of short (3,3) single wall carbon nanotube (SWCNT)–polyethylene (PE) structures, as well as the binding energy between the SWCNT and PE, were obtained from three commonly used molecular mechanics force fields and first principles methods. The molecular force fields were the Dreiding, Universal and Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force fields and the first principles methods included the B3LYP density functional and MP2 post-Hartree Fock methods with, typically, 6-311G, 6-311G(d,p) and 6-311G(2d,2p) basis sets. These calculations show that the results obtained from all force fields are in qualitative agreement with the first principles results, and that PE prefers to be aligned with a non-zero angle along the SWCNT axis, where the angle depends on the force field or first principles method used. This indicates that longer PE chains may wrap around SWCNTs. This was studied using the COMPASS force field with longer (5,5) SWCNTs interacting with a PE chain and, in agreement with the minimum energy calculations, the PE wrapped around the SWCNT thereby increasing the radius of gyration of the PE. This force field was also used to assess the effect of (5,5) SWCNTs on the mechanical properties of PE nanocomposites. The calculated interfacial shear stress and interfacial bonding energy of SWCNT–PE structures was 141.09 MPa and 0.14 N/m. The simulations show that using short SWCNTs as reinforcement does not increase the Young’s modulus for the systems studied here, whereas longer, aligned SWCNTs increased the Young’s modulus in the SWCNT axial direction.

  • 26.
    Jensen, Mads Mønster
    et al.
    Technical University of Denmark, Denmark.
    De Weerdt, K.
    Norwegian University of science and Technology, Norway.
    Johannesson, Björn
    Technical University of Denmark, Denmark.
    Geiker, M. R.
    Norwegian University of science and Technology, Norway.
    Use of a multi-species reactive transport model to simulate chloride ingress in mortar exposed to NaCl solution or sea-water2015In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 105, p. 75-82Article in journal (Refereed)
    Abstract [en]

    Simulations of ion ingress in Portland cement mortar using a multi-species reactive mass transport model are compared with experimental test results. The model is an extended version of the Poisson–Nernst–Planck equations, accounting for chemical equilibrium. Saturated mortar samples were exposed after 8 days of hydration to a NaCl-solution or natural sea-water in a laboratory controlled environment for the experimental part. The samples were analyzed for the total chloride content after 21, 90 and 180 days of exposure, at varying depths from the exposed surface. The applicability of the reactive mass transport model is shown by comparing the simulation and experimental results. The tortuosity factor used in the simulations is adjusted to obtain the best reproduction of the experimental results. The model predicts the total chloride content satisfactorily, despite assumptions in the simulation like fixed hydration degree over time. Improvements and suggestions for further development of the model are discussed, e.g. extended hydration description, improved overall chemical description and a more strict use of the tortuosity factor.

  • 27.
    Jensen, Mads Mønster
    et al.
    Technical University of Denmark, Denmark.
    Johannesson, Björn
    Technical University of Denmark, Denmark.
    Geiker, Mette Rica
    Norwegian University of Science and Technology, Norway.
    Framework for reactive mass transport: phase change modeling of concrete by a coupled mass transport and chemical equilibrium model2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 92, p. 213-223Article in journal (Refereed)
    Abstract [en]

    Reactive transport modeling is applicable for a range of porous materials. Here the modeling framework is focused on cement-based materials, where ion diffusion and migration are described by the Poisson–Nernst–Planck equation system. A two phase vapor/liquid flow model, with a sorption hysteresis description is coupled to the system. The mass transport is solved by using the finite element method where the chemical equilibrium is solved explicitly by an operator splitting method. The iphreeqc library is used as chemical equilibrium solver. The equation system, solved by iphreeqc, is explained for aqueous, pure phase and solid solution reactions. Numerical examples, with cement-based materials, are constructed to demonstrate transient phase change modeling. A simulation of pure multi-species leaching from the material, showing deterioration of the solid phases is described and calculated. A second simulation, showing multi-species ingress with formation of new solid phases in the domain is described and calculated. It is shown that the numerical solution method is capable of solving the reactive mass transport system for the examples considered.

  • 28.
    Jernkvist, Lars Olof
    Malmö högskola, School of Technology.
    Multi-field modelling of hydride forming metals Part II: application to fracture2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 85, p. 383-401Article in journal (Refereed)
    Abstract [en]

    In Part I of the present article, we formulated a continuum-based computational model for stress- and temperature-directed diffusion of hydrogen in metals that form brittle binary hydrides, such as Zr and Ti alloys. Among the space–time dependent parameters calculated by the model are the volume fraction and the mean orientation of hydride precipitates. These parameters are of importance for quantifying the embrittlement of hydrided materials. In this second part of the work, we use measured data for the strength and toughness of hydrided Zr alloys to correlate the local fracture properties of the two-phase (metal + hydride) material to the aforementioned parameters. The local fracture properties are used as space–time dependent input to a cohesive zone type submodel for fracture, which is fully integrated with the hydrogen transport model from Part I. The complete model is validated against fracture tests on hydrogen-charged Zr–2.5%Nb, a material used in nuclear reactor pressure tubes. More precisely, we study local embrittlement and crack initiation at a blunt and moderately stressed notch, resulting from gradual accumulation of hydrides at the notch during temperature cycling. We also simulate tests on crack initiation and growth by delayed hydride cracking, a subcritical crack growth mechanism with a complex temperature dependence. From the results of the simulations, we conclude that the model reproduces many observed features related to initiation and propagation of hydride induced cracks in the Zr–2.5%Nb material. In particular, it has the capacity to reproduce effects of the material’s temperature history on the fracture behaviour, which is important for many practical applications.

  • 29. Jernkvist, Lars Olof
    et al.
    Massih, Ali
    Malmö högskola, School of Technology.
    Multi-field modelling of hydride forming metals: part I: model formulation and validation2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 85, p. 363-382Article in journal (Refereed)
    Abstract [en]

    A computational model for hydrogen transport, hydrogen induced deformation and fracture in metals that form binary hydrides, such as Zr and Ti alloys, is presented. The model uses a continuum description of the two-phase (metal + hydride) material, and solves the multi-field partial differential equations for temperature and stress-directed hydrogen diffusion together with mechanical equilibrium in a three-dimensional finite element setting. Point-kinetics models are used for metal-hydride phase transformation and stress-directed orientation of hydride precipitates, while a cohesive zone fracture model caters for initiation and propagation of cracks. The local fracture properties of the hydrided material are correlated to the calculated local concentration and orientation of the hydride precipitates, which have a strong embrittling effect on the material. In Part I of this two-part paper, we present sub-models applied for the aforementioned phenomena together with a detailed description of their numerical implementation. The applicability of the model is then demonstrated by simulating five independent experiments on hydrogen transport, metal-hydride phase transformation and stress-directed hydride orientation in zirconium alloys. Based on the results, we conclude that the model captures these phenomena over a wide range of thermo-mechanical loading conditions, including thermal cycling. Part II of the paper is focussed on fracture, and includes details on the fracture model and its validation against tests and experiments on initiation and propagation of hydride induced cracks.

  • 30. Jha, Prakash Chandra
    et al.
    Seal, Prasenjit
    Sen, Sabyasachi
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Chakrabarti, Swapan
    Static and dynamic polarizabilities of (CdSe)(n) (n=1-16) clusters2008In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 44, no 2, p. 728-732Article in journal (Refereed)
    Abstract [en]

    Density functional theoretical calculations have been performed on small and medium-sized (CdSe)(n) (n = 1-16) clusters in order to evaluate the variation in electric polarizability and anisotropy in polarizability with increase in the cluster size at both static and Nd:YAG laser frequencies. The electric polarizability values at static frequency show a rapid decrease up to heptamer only with the exception observed for (CdSe)(5) clusters where a sudden increase in the polarizability value is observed. An even-odd oscillating behavior is observed in the anisotropy values between the dimer and heptamer CdSe clusters. Apart from polarizability and anisotropy calculations, the variation in the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital and second difference of energies with the cluster size have also been calculated. just like the anisotropy values, an even-odd oscillating behavior is also observed in the second difference value for the smaller CdSe clusters.

  • 31.
    Jin, Lai -Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Non-stationary creep simulation with a modified Armstrong-Frederick relation applied to copper2009In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 46, no 2, p. 339-346Article in journal (Refereed)
    Abstract [en]

    A previously formulated model using back stress to handle non-stationary creep during power-law breakdown is further developed. In particular, the way to integrate the back stress is modified. Usually the Armstrong-Frederick relation has been applied, but it can give unphysical results in the sense that the back stress exceeds the tensile strength of the material. Such a problem can be solved by replacing the back stress term in this relation with the back stress deviator. The creep model is applied to copper canister in waste packages intended for encapsulating spent nuclear fuel. These waste packages will be placed in the bedrock at a depth of about 500 m as a final stage of disposal. During storage, radioactivity-induced thermal evolution raises temperature in repositories and water-saturation generates pressure directly on the copper canister. The thermally activated creep in copper canister occurs readily. To estimate the amount of creep deformation, a finite element model is set up to compute the evolution of creep deformation in copper canister. The creep model takes both stationary and non-stationary creep into account The computed maximum creep strain is shown to be 7.8% over 10 years, which should not cause failure since measured creep elongations are in the range of 15-40%.

  • 32.
    Jin, Lai-Zhe
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Creep of copper canisters in power-law breakdown2008In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 43, no 3, p. 403-416Article in journal (Refereed)
    Abstract [en]

    According to the Swedish KBS-3 concept the spent nuclear fuel will be placed in copper canisters 500 m down in the bedrock. In thestorage, the canister will creep under conditions that are well inside the power-law breakdown regime. To prevent creep rupture fromoccurring that could cause leakage of nuclides, finite element models are set up to study the evolution of creep deformation in the coppercanisters. In this paper, two finite element models for the secondary creep are formulated. The first one is based on a fundamental climb–glide creep law valid over a wide range of temperatures. The second one is on the basis of a generalised Norton equation fitted to secondarycreep data of phosphorus doped pure copper. The creep deformation is shown to be much larger in the lid and the bottom of the canistersthan in the cylindrical wall. In the latter a stationary creep state is reached only after very long time (30000 years). Since the deformation inthe copper canister is restricted by a cast iron insert and stress concentrations are reduced with time, the total creep strain is limited.

  • 33. Jivkov, Andrey
    et al.
    Stevens, N.P.C.
    Marrow, T.J.
    A three-dimensional computational model for intergranular cracking2006In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 38, no 3, p. 442-453Article in journal (Refereed)
    Abstract [en]

    A three-dimensional mechanical model for intergranular crack propagation is presented. The model follows the spirit of existing percolation-like models but offers the inclusion of mechanical effects. This is necessary in order to account more accurately for the crack driving force and the effect of crack bridging ligaments, observed experimentally to be formed by fracture resistant boundaries. The model uses a regular representation of the material's microstructure and a categorisation of grain boundaries as beneficial and detrimental to fracture. This categorisation makes the model applicable to assessing material's resistance to intergranular stress corrosion cracking. The model mechanical behaviour is consistent with experimental observations and demonstrates its capability of simulating the development of bridges in the crack wake as well as crack coalescence. Results show that increasing the fraction of resistant boundaries increases the degree of crack tip shielding developed. This is expected to increase the resistance to stress corrosion crack propagation. The model offers a significant reduction of the computational resources usually needed to simulate intergranular propagation.

  • 34. Kaewmaraya, T.
    et al.
    Ramzan, M.
    Sun, WeiWei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Sagynbaeva, M.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Atomistic study of promising catalyst and electrode material for memory capacitors: Platinum oxides2013In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 79, p. 804-810Article in journal (Refereed)
    Abstract [en]

    Platinum oxides have the technological importance as evidenced by numerous studies concentrating on their crystal structures to attain the clear atomistic understanding but the controversy exists between the experimental and theoretical studies. In our present study, we report the electronic and optical properties of crystalline PtO and PtO2 on the basis of Heyd-Scuseria-Ernzerhof (HSE06) functional within the framework of the density functional theory (DFT). We present the structural parameters, electronic and optical properties of several proposed structures of PtO and PtO2. We find that PtS-type structure of PtO and CaCl2-type structure of PtO2 are the most stable structures of these materials on the basis of hybrid functional and they appear to be semiconductors with band gap values of 0.87 eV and 1.85 eV, respectively. The mechanical stability of these structures is also confirmed by calculating the phonon band structures. The corresponding structural parameters are found in good agreement with experimental values. Furthermore, we present the bader charge analysis and optical properties of these phases.

  • 35.
    Kaewmaraya, Thanayut
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    De Sarkar, Abir
    Sa, Baisheng
    Sun, Z.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Strain-induced tunability of optical and photocatalytic properties of ZnO mono-layer nanosheet2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 91, p. 38-42Article in journal (Refereed)
    Abstract [en]

    Strain-induced tunability of several properties of ZnO monolayer nanosheet has been systematically studied using density functional theory. The band gap of the sheet varies almost linearly with uniaxial strain, while it shows a parabola-like behavior under homogeneous biaxial strain. Tensile strain reduces ionicity of Zn-O bonds, while compressive strain increases it. This provides ample implications for the photocatalytic dissociation of water molecules and the scission of polar molecules on ZnO nanosheet. The dynamical stability of the sheet assessed by the calculation of its vibrational frequencies has shown the sheet to be unstable for 10% and 7.5% compressive biaxial homogeneous strain. 

  • 36. Kaewmaraya, Thanayut
    et al.
    De Sarkar, Abir
    Sa, Baisheng
    Sun, Z.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Department of Physics and Astronomy, Uppsala University.
    Strain-induced tunability of optical and photocatalytic properties of ZnO mono-layer nanosheet2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 91, p. 38-42Article in journal (Refereed)
    Abstract [en]

    Strain-induced tunability of several properties of ZnO monolayer nanosheet has been systematically studied using density functional theory. The band gap of the sheet varies almost linearly with uniaxial strain, while it shows a parabola-like behavior under homogeneous biaxial strain. Tensile strain reduces ionicity of Zn-O bonds, while compressive strain increases it. This provides ample implications for the photocatalytic dissociation of water molecules and the scission of polar molecules on ZnO nanosheet. The dynamical stability of the sheet assessed by the calculation of its vibrational frequencies has shown the sheet to be unstable for 10% and 7.5% compressive biaxial homogeneous strain.

  • 37.
    Kaewmaraya, Thanayut
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ramzan, Muhammad
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sun, W.
    Sagynbaeva, Myskal
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Applied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH), SE-100 44 Stockholm, Sweden.
    Atomistic study of promising catalyst and electrode material for memory capacitors: Platinum Oxides2013In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 79, p. 804-810Article in journal (Refereed)
    Abstract [en]

    Platinum oxides have the technological importance as evidenced by numerous studies concentrating on their crystal structures to attain the clear atomistic understanding but the controversy exists between the experimental and theoretical studies. In our present study, we report the electronic and optical properties of crystalline PtO and PtO2 on the basis of Heyd-Scuseria-Ernzerhof (HSE06) functional within the framework of the density functional theory (DFT). We present the structural parameters, electronic and optical properties of several proposed structures of PtO and PtO2. We find that PtS-type structure of PtO and CaCl2-type structure of PtO2 are the most stable structures of these materials on the basis of hybrid functional and they appear to be semiconductors with band gap values of 0.87 eV and 1.85 eV, respectively. The mechanical stability of these structures is also confirmed by calculating the phonon band structures. The corresponding structural parameters are found in good agreement with experimental values. Furthermore, we present the bader charge analysis and optical properties of these phases. 

  • 38. Karkina, L. E.
    et al.
    Karkin, I. N.
    Kuznetsov, A. R.
    Razumov, I. K.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. RAS, Inst Met Phys, Russia.
    Gornostyrev, Yu. N.
    Solute-grain boundary interaction and segregation formation in Al: First principles calculations and molecular dynamics modeling2016In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 112, p. 18-26Article in journal (Refereed)
    Abstract [en]

    The interaction between solute atoms (Mg, Si, Ti) and grain boundaries (GBs) of different types in Al are investigated using two approaches: first principles total energy calculations and large scale atomistic simulations. We have found that both deformation (size effect) and electronic (charge transfer) mechanisms play an important role in solute-GB interaction. The deformation and electronic contributions to GB segregation energy for the considered solutes have been analyzed in dependence on the impurity and the GB type. Mg and Si atoms are calculated to segregate to GBs, while Ti atoms to repel from, GBs in Al. For the case of a symmetric special-type GB the interaction is found to be short-ranged. For a general-type GB the range of GB-solute interaction is found to be considerably longer. A method to estimate the segregation capacity of a GB has been proposed, which takes into account the solute-solute interactions, and shown to be able to correctly describe the GB enrichment in alloying elements. The features of the segregation formation in fine-grained materials produced by severe plastic deformation are discussed.

  • 39. Karlström, Gunnar
    et al.
    Lindh, Roland
    Department of Theoretical Chemistry, Lund University.
    Malmqvist, Per-Åke
    Roos, Björn O
    Ryde, Ulf
    Veryazov, Valera
    Widmark, Per-Olof
    Cossi, Maurizio
    Schimmelpfennig, Bernd
    Neogrady, Pavel
    Seijo, Luis
    MOLCAS: a program package for computational chemistry2003In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 28, no 2, p. 222-239Article in journal (Refereed)
    Abstract [en]

    The program system MOLCAS is a package for calculations of electronic and structural properties of molecular systems in gas, liquid, or solid phase. It contains a number of modern quantum chemical methods for studies of the electronic structure in ground and excited electronic states. A macromolecular environment can be modeled by a combination of quantum chemistry and molecular mechanics. It is further possible to describe a crystalline material using model potentials. Solvent effects can be treated using continuum models or by combining quantum chemical calculations with molecular dynamics or Monte-Carlo simulations. MOLCAS is especially adapted to treat systems with a complex electronic structure, where the simplest quantum chemical models do not work. These features together with the inclusion of relativistic effects makes it possible to treat with good accuracy systems including atoms from the entire periodic system. MOLCAS has effective methods for geometry optimization of equilibria, transition states, conical intersections, etc. This facilitates studies of excited state energy surfaces, spectroscopy, and photochemical processes.

  • 40. Kissavos, A. E.
    et al.
    Simak, S. I.
    Olsson, Pär
    Department of Neutron Research, Ångström Laboratory, Uppsala University.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Abrikosov, I. A.
    Total energy calculations for systems with magnetic and chemical disorder2006In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 35, no 1, p. 1-5Article in journal (Refereed)
    Abstract [en]

    The accuracy of the exact muffin-tin orbitals method combined with the coherent potential approximation (EMTO-CPA) for total energy calculations for systems with magnetic and chemical disorder, which is present simultaneously, is investigated. The mixing enthalpy of ordered, as well as disordered FeCo, FeNi, and FeCu equiatomic ferromagnetic alloys is calculated with the EMTO-CPA method and with the full-potential projector augmented wave (PAW) method. The results are compared and found to be in excellent agreement with each other. The EMTO-CPA method, in combination with disordered local moment model, is then applied to calculate the mixing enthalpy of the random paramagnetic face-centered cubic (fcc) FeCo alloy, as well as body-centered cubic (bcc) FeCr and FeV alloys over the whole concentration range. The results are compared with experimental data and a very good agreement is found again.

  • 41. Kissavos, A. E.
    et al.
    Simak, S. I.
    Olsson, Pär
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Neutron Research.
    Vitos, Levente
    Abrikosov, Igor A.
    Total energy calculations for systems with magnetic and chemical disorder2006In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 35, no 1, p. 1-5Article in journal (Refereed)
    Abstract [en]

    The accuracy of the exact muffin-tin orbitals method combined with the coherent potential approximation (EMTO-CPA) for totalenergycalculations for systems with magnetic and chemicaldisorder, which is present simultaneously, is investigated. The mixing enthalpy of ordered, as well as disordered FeCo, FeNi, and FeCu equiatomic ferromagnetic alloys is calculated with the EMTO-CPA method and with the full-potential projector augmented wave (PAW) method. The results are compared and found to be in excellent agreement with each other. The EMTO-CPA method, in combination with disordered local moment model, is then applied to calculate the mixing enthalpy of the random paramagnetic face-centered cubic (fcc) FeCo alloy, as well as body-centered cubic (bcc) FeCr and FeV alloys over the whole concentration range. The results are compared with experimental data and a very good agreement is found again.

  • 42.
    Kissavos, Andreas E.
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Simak, Sergey I.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Olsson, P.
    Department of Neutron Research, Ångström Laboratory, Uppsala University, Uppsala, Sweden.
    Vitos, L.
    Applied Material Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden.
    Abrikosov, Igor A.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Total energy calculations for systems with magnetic and chemical disorder2006In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 35, no 1, p. 1-5Article in journal (Refereed)
    Abstract [en]

    The accuracy of the exact muffin-tin orbitals method combined with the coherent potential approximation (EMTO-CPA) for total energy calculations for systems with magnetic and chemical disorder, which is present simultaneously, is investigated. The mixing enthalpy of ordered, as well as disordered FeCo, FeNi, and FeCu equiatomic ferromagnetic alloys is calculated with the EMTO-CPA method and with the full-potential projector augmented wave (PAW) method. The results are compared and found to be in excellent agreement with each other. The EMTO-CPA method, in combination with disordered local moment model, is then applied to calculate the mixing enthalpy of the random paramagnetic face-centered cubic (fcc) FeCo alloy, as well as body-centered cubic (bcc) FeCr and FeV alloys over the whole concentration range. The results are compared with experimental data and a very good agreement is found again.

  • 43.
    Klintenberg, Mattias
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Possible high-temperature superconductors predicted from electronic structure and data-filtering algorithms2013In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 67, p. 282-286Article in journal (Refereed)
    Abstract [en]

    We report here the completion of the electronic structure of the majority of the known stoichiometric inorganic compounds, as listed in the Inorganic Crystal Structure Data-base (ICSD). We make a detailed comparison of the electronic structure, crystal geometry and chemical bonding of cuprate high temperature superconductors, with the calculated over 60,000 electronic structures. Based on compelling similarities of the electronic structures in the normal state and a data-filtering technique, we propose that high temperature superconductivity is possible for electron-or hole-doping in a much larger group of materials than previously considered. The identified materials are composed of over 100 layered compounds, most which hitherto are untested with respect to their super conducting properties. Of particular interest are the following materials; Ca2CuBr2O2, K2CoF4, and Sr2MoO4, which are discussed in detail.

  • 44. Koci, L.
    et al.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    The impact of system restriction in molecular dynamics applied to the melting of Ne at high pressure2008In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 44, no 2, p. 605-610Article in journal (Refereed)
    Abstract [en]

    There are two major ways to perform molecular dynamics (MD) calculations, namely classical and ab initio MD. As ab initio techniques require considerably longer calculation times, it is of interest to compare the results of the two methods. Furthermore, when melting is studied with MD, the use of coexistent solid and liquid structures (two-phase) in the calculations, instead of only a solid structure (one-phase), can have a substantial impact on the results obtained. In this work, comparisons have been made between classical and ab initio methods applied to one- and two-phase systems for the melting of Ne at high pressure. The temperatures needed to melt the classical one-phase system are somewhat higher compared to the two-phase temperatures, evaluated at the same pressure. Furthermore, there is a significant discrepancy comparing the one-phase ab initio curve to previously reported classical predictions. At 150 GPa, the calculations in this work show a melting temperature approximately 1000 K above the estimate based on an exponential-6 potential. However, there is a close match between the one-phase ab initio curve and the classical one-phase results in this work. This suggests a possible agreement between a two-phase ab initio and classical two-phase melting curve.

  • 45.
    Korzhavyi, Pavel A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Monovacancy in copper: Trapping efficiency for hydrogen and oxygen impurities2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 84, p. 122-128Article in journal (Refereed)
    Abstract [en]

    The structure and binding energy of vacancy-impurity complexes in copper are studied using first-principles calculations based on density functional theory. A single vacancy is found to be able to trap up to six hydrogen atoms which tend to be situated inside the vacancy at off-center positions (related to the octahedral interstitial positions of the ideal fcc lattice). The binding energy of an H atom dissolved in the Cu lattice (octahedral interstitial position) to a vacancy is calculated to be about 0.24 eV, practically independent of the number of H atoms already trapped by the vacancy, up to the saturation with 6 hydrogens. For an oxygen impurity in Cu, a monovacancy is shown to be a deep trap (with a binding energy of 0.95 eV). The position of a trapped O atom inside a vacancy is off-center, almost a half-way from the nearest octahedral interstitial to the vacancy center. Such a vacancy-O cluster is shown to be a deep trap for dissolved hydrogen (the calculated binding energy is 1.23 eV). The trapping results in the formation of an OH-group, where the H atom is situated near the vacancy center, and the O atom is displaced from the center along a 〈100〉 direction towards a nearby octahedral interstitial position. Further hydrogenation of the monovacancy-OH cluster is calculated to be energetically unfavourable. McNabb-Forster's equations are generalised to describe the competition between a deep hydrogen trap and a shallow one. It is demonstrated that the deep trap is almost fully filled, which explains why some of hydrogen is strongly bound and cannot be removed without vacuum treatment at elevated temperatures.

  • 46.
    Koči, L.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory.
    Ahuja, R.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Belonoshko, A. B.
    The impact of system restriction in molecular dynamics applied to the melting of Ne at high pressure2008In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 44, no 2, p. 605-610Article in journal (Refereed)
    Abstract [en]

    There are two major ways to perform molecular dynamics (MD) calculations, namely classical and ab initio MD. As ab initio techniques require considerably longer calculation times, it is of interest to compare the results of the two methods. Furthermore, when melting is studied with MD, the use of coexistent solid and liquid structures (two-phase) in the calculations, instead of only a solid structure (one-phase), can have a substantial impact on the results   obtained. In this work, comparisons have been made between classical and ab initio methods applied to one- and two-phase systems for the melting of Ne at high pressure. The temperatures needed to melt the classical one-phase system are somewhat higher compared to the two-phase temperatures, evaluated at the same pressure. Furthermore,   there is a significant discrepancy comparing the one-phase ab initio curve to previously reported classical predictions. At 150 GPa, the calculations in this work show a melting temperature approximately 1000 K above the estimate based on an exponential-6 potential. However, there is a close match between the one-phase ab initio curve and the classical one-phase results in this work. This suggests a possible agreement between a two-phase ab initio and classical two-phase melting curve.

  • 47.
    Krasilnikov, O M
    et al.
    National University of Science and Technology MISIS, Russia .
    Belov, M P
    National University of Science and Technology MISIS, Russia .
    Lugovskoy, A V
    National University of Science and Technology MISIS, Russia .
    Yu Mosyagin, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology. National University of Science and Technology MISIS, Russia .
    Kh Vekilov, Yu
    National University of Science and Technology MISIS, Russia .
    Elastic properties, lattice dynamics and structural transitions in molybdenum at high pressures2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 81, p. 313-318Article in journal (Refereed)
    Abstract [en]

    The structural phase transitions in molybdenum under pressures are investigated on the basis of first principle analysis of elastic constants behavior and phonon dispersions. The definition of the effective elastic constants of nth order ( nP2), governing the elastic properties of a loaded crystal, is given. The effective elastic constants of second and third order and the phonon dispersions are calculated by DFT methods in the pressure range of P = 0 - 1400 GPa, T = 0 K. The calculation results at P = 0 are in good agreement with the available experimental data. On the basis of the obtained results the stability of the bcc phase of molybdenum under pressure and the possibility of the phase transition are investigated. It is shown that the effective elastic constant eC0 which corresponds to the tetragonal uniform strain of a loaded crystal undergoes significant softening at P andgt; 400 GPa. In the same pressure range the frequencies of the transverse branch T-[110](-) [zeta zeta 0] also begin to soften and already at P approximate to 1000 GPa they become imaginary near the wave vector [1/4 1/4 0]. The bcc -andgt; dhcp phase transition associated with the softening of eC0 and the soft mode T-[110](-)[1/4 1/4 0] is discussed.

  • 48.
    Larsson, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Ågren, John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
    Combined probability distributions of random-walks: A new method to simulate diffusion processes2005In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 34, no 3, p. 254-263Article in journal (Refereed)
    Abstract [en]

    Two related methods to simulate diffusion processes are presented. Both are based on conceiving diffusion as a random-walk process. Several example simulations are presented: single phase diffusion couples, diffusion controlled growth and prediction of Kirkendall porosity. Comparisons with experimental results and simulation software based on established technique (DICTRA) show good agreement with results obtained from the presented models.

  • 49.
    Larsson, Peter
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahuja, Rajeev
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science.
    Liivat, Anti
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
    Thomas, John O.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry.
    Structural and electrochemical aspects of Mn substitution into Li2FeSiO4 from DFT calculations2010In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 47, no 3, p. 678-684Article in journal (Refereed)
    Abstract [en]

    DFT calculations are presented which probe the effect of low-concentration Mn substitution of the Fe-sites in Li2FeSiO4: the promising new and potentially cheap cathode material for upscaled Li-ion battery applications. The LixFe0.875Mn0.125SiO4 System investigated could be achieved by replacing 12.5% of the Fe-sites in 2 x 2 x 1 and 2 x 2 x 2 supercells by Mn ions. The evolution of Bader charges and partial densities of states (DOS) have been followed under a stepwise delithiation process. A clear structural distortion is seen to occur at the Mn-site on delithiation, suggesting possible structural instability. Oxidation of Mn beyond 3+ is calculated to occur at potentials in excess of 4.7 V, implying that oxidation of well separated (>10 angstrom) low-concentration Mn ions to Mn4+ is energetically unfavourable in the LixFe0.875Mn0.125SiO4 structure. This, together with previous DFT results for higher levels of Mn substitution into Li2FeSiO4, indicates that capacity increase in Li2Fe1 (-) yMnySiO4 through a > 1 electron redox reaction may not be so readily attainable in practice, either for high or low Mn concentrations.

  • 50.
    Lebègue, Sébastien
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics.
    Arnaud, B.
    Alouani, M.
    Molecular dynamics simulation and chemical bonding analysis of MgB2C22006In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 37, no 3, p. 220-225Article in journal (Refereed)
    Abstract [en]

    The possible superconducting material MgB2C2 is studied by means of the Projector Augmented Plane wave method. The experimental lattice parameters and internal coordinates are reproduced with accuracy by means of a molecular dynamics calculation. A chemical bonding analysis is performed and it is found that the charge transfer from the Mg atoms to the BC network is not complete, meaning that a significant part of covalent bonding is present in this bond. In addition, a crystal orbital overlap population calculation reveals the degree of covalent bonding between atoms. This analysis is of particular importance and helps improve our general understanding of the superconductivity in these type of materials.

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