| 1. |
- Ahlberg Helgee, Edit, 1986, et al.
(författare)
-
Origin of Space Charge in Grain Boundaries of Proton-Conducting BaZrO3
- 2013
-
Ingår i: Fuel Cells. - : Wiley. - 1615-6846 .- 1615-6854. ; 13:1, s. 19-28
-
Tidskriftsartikel (refereegranskat)abstract
- Segregation energies of oxygen vacancies and protons near three symmetric tilt grain boundaries (GBs) in BaZrO3 are determined using density functional theory. Two of the GBs have the [-110] direction as tilt axis whith a (111) or (112) plane as GB plane, while the third has the [001] direction as tilt axis and a (210) plane as GB plane. Both defects are found to segregate to all three GBs, with vacancy segregation energies of -0.5 and -1.5 eV and proton segregation energies of about -0.8 eV. The effects of the calculated segregation energies on defect concentrations and electrostatic potential in the GB region are investigated using a thermodynamic space charge model. An increased concentration of defects is seen in all GBs, giving electrostatic potential barriers around 0.6 V at 400-900 K. Protons are found to give important contributions to the space charge in all three GBs.
|
|
| 2. |
|
|
| 3. |
- Björketun, Mårten, 1976, et al.
(författare)
-
Effect of acceptor dopants on the proton mobility in BaZrO3: A density functional investigation
- 2007
-
Ingår i: Phys. Rev. B. ; 76, s. 054307-
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, the stability and mobility of protonic defects in acceptor doped BaZrO3 are investigated using density functional theory in conjunction with kinetic modeling. The interaction between the mobile protons and Zr-site substituted trivalent dopants—Ga, Sc, In, Y, and Gd—is assessed by extensive mapping of migration pathways. In all cases, the interaction is attractive, which results in a stabilization, and hence a reduction of the mobility, of the proton. However, its strength and range vary substantially and are found to be strongly correlated to the ionic radius of the dopant. Further, it is shown that the observed correlation is related to a variation in strength of hydrogen bonds formed between protons and next nearest lattice oxygen ions in different environments. Finally, migration barriers and attempt frequencies obtained from first principles are used as input to a jump-diffusion model. The proton self-diffusion coefficient is evaluated for the different substituted phases, and the experimental trend in activation energies is reproduced.
|
|
| 4. |
- Björketun, Mårten, 1976, et al.
(författare)
-
Protonic defects in pure and doped La2Zr2O7 pyrochlore oxide
- 2009
-
Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 178:31-32, s. 1642-1647
-
Tidskriftsartikel (refereegranskat)abstract
- Densi ty- functional calculations have been used to study protonic defects in stoichiometric and substituted La2Zr2O7 pyrochlore oxide. The preferred proton sites have been pinpointed in the stoichiometric material and their relative stability determined. Based on these results a continuous migration pathway that enables long-range transport is proposed. In addition, the interaction energies between protons and divalent Ba, Sr, Ca, and Mg dopants located on the La- and Zr-sites have been calculated. All interactions are found to be attractive; they are strong in the case of Ba and Mg and quite weak for Sr and Ca suggesting that samples acceptor doped with the latter two ions should show the highest proton conductivity.
|
|
| 5. |
- Björketun, Mårten, 1976, et al.
(författare)
-
Structure and thermodynamic stability of hydrogen interstitials in BaZrO3 perovskite oxide from density functional calculations
- 2007
-
Ingår i: Faraday Discuss.. ; 134, s. 247-265
-
Tidskriftsartikel (refereegranskat)abstract
- Density functional calculations have been used to study the electronic structure, preferred sites in the lattice, formation energies and vibrational frequencies for hydrogen interstitials in different charge states in the cubic phase of perovskite-structured BaZrO3. By combining ab initio results with thermodynamic modeling, defect formation at finite temperature and pressure has been investigated. We demonstrate how the site selectivity and spatial distribution of dopant atoms in the lattice can be affected by changes in the environmental conditions (atomic chemical potentials, oxygen partial pressure and temperature) used during processing of the material. In addition, we have calculated the thermodynamic parameters of the water uptake reaction for an acceptor-doped BaZrO3 crystal in equilibrium with a humid atmosphere. The interaction energies between a protonic defect and the investigated Ga, Gd, In, Nd, Sc, and Y dopants were found to be attractive, and we show that a simple model of defect association may reproduce an experimentally observed trend in the hydration enthalpy.
|
|
| 6. |
|
|
| 7. |
- Christensen, Mikael, 1973, et al.
(författare)
-
Morphology of WC grains in WC-Co alloys: Theoretical determination of grain shape
- 2007
-
Ingår i: Acta Mater.. ; 55, s. 1515-
-
Tidskriftsartikel (refereegranskat)abstract
- A theoretical approach to predict grain morphology from interface energies computed using density-functional theory is presented and applied to the WC–Co system. The dependences of the WC grain shape on the geometrical misfit at the Co/WC metal–ceramic interface, and on the carbon chemical potential are investigated. The WC grains are predicted to be hexagonal when the interfaces are assumed to be coherent, and change to a truncated triangular shape where the long prism side lengths are five times longer than the short side lengths, when the interfaces are assumed to be incoherent.
|
|
| 8. |
|
|
| 9. |
- Christensen, Mikael, 1973, et al.
(författare)
-
Strength and reinforcement of interfaces in cemented carbides
- 2006
-
Ingår i: Int. J. Refract. Hard Mater.. ; 24, s. 80-
-
Tidskriftsartikel (refereegranskat)abstract
- The tendency for segregation of 3d-transition metals to WC/WC interfaces in cemented carbides, and resulting effect on grain boundary cohesion, have been assessed by density-functional theory calculations. The results show that the propensity for segregation increases along the 3d-series, both to free carbide surfaces and to carbide–carbide interfaces. The tendency for metal substitution in W-rich boundaries is very low while 3d atoms readily segregate to C-rich boundaries by carbon substitution. The strengthening effect on grain boundary cohesion in the latter case is largest for V, Cr, and Mn, but is substantial for all segregants in this study.
|
|
| 10. |
- Eklöf-Österberg, Carin, 1987, et al.
(författare)
-
The role of oxygen vacancies on the vibrational motions of hydride ions in the oxyhydride of barium titanate
- 2020
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 8:13, s. 6360-6371
-
Tidskriftsartikel (refereegranskat)abstract
- Perovskite-type oxyhydrides, BaTiO3-xHx, represent a novel class of hydride ion conducting materials of interest for several electrochemical applications, but fundamental questions surrounding the defect chemistry and hydride ion transport mechanism remain unclear. Here we report results from powder X-ray diffraction, thermal gravimetric analysis, nuclear magnetic resonance spectroscopy, inelastic neutron scattering (INS), and density functional theory (DFT) simulations on three metal hydride reduced BaTiO3 samples characterized by the simultaneous presence of hydride ions and oxygen vacancies. The INS spectra are characterized by two predominating bands at around 114 (omega(perpendicular to)) and 128 (omega(parallel to)) meV, assigned as fundamental Ti-H vibrational modes perpendicular and parallel to the Ti-H-Ti bond direction, respectively, and four additional, weaker, bands at around 99 (omega(1)), 110 (omega(2)), 137 (omega(3)) and 145 (omega(4)) meV that originate from a range of different local structures associated with different configurations of the hydride ions and oxygen vacancies in the materials. Crucially, the combined analyses of INS and DFT data confirm the presence of both nearest and next-nearest neighbouring oxygen vacancies to the hydride ions. This supports previous findings from quasielastic neutron scattering experiments, that the hydride ion transport is governed by jump diffusion dynamics between neighbouring and next-nearest neighbouring hydride ion-oxygen vacancy local structures. Furthermore, the investigation of the momentum transfer dependence of the INS spectrum is used to derive the mean square displacement of the hydride ions, which is shown to be in excellent agreement with the calculations. Analysis of the mean square displacement confirms that the hydrogen vibrational motions are localized in nature and only very weakly affected by the dynamics of the surrounding perovskite structure. This insight motivates efforts to identify alternative host lattices that allow for a less localization of the hydride ions as a route to higher hydride ion conductivities.
|
|
| 11. |
- Fors, Dan, 1982, et al.
(författare)
-
First-principles investigation of the stability of MN and CrMN precipitates under coherency strains in alpha-Fe (M = V, Nb, Ta)
- 2011
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:11
-
Tidskriftsartikel (refereegranskat)abstract
- We perform a systematic ab initio study of the interface energetics of thin coherent rocksalt (nacl) structured MN and tetragonal CrMN films in bcc Fe (M = V, Nb, Ta), motivated by the vital role of MN and CrMN precipitates for the long-term creep resistance in 9%-12%Cr steels. The similarities and differences in the work of separations and the elastic costs for the coherency strains are identified, and the possibility for formation of coherent films are discussed. Our findings provide valuable information of the interface energetics, which in continuation can be combined with thermodynamical modeling to obtain a better understanding of the initial nucleation stage of the MN and CrMN precipitates, and their influence on the long-term microstructural evolution in 9%-12%Cr steels.
|
|
| 12. |
|
|
| 13. |
- Fors, Dan, 1982, et al.
(författare)
-
Theoretical investigation of moderate misfit and interface energetics in the Fe/VN system
- 2010
-
Ingår i: Computational Materials Science. - : Elsevier BV. - 0927-0256. ; 50:2, s. 550-559
-
Tidskriftsartikel (refereegranskat)abstract
- In this study an ab initio based approach to determine the effect of moderate misfit on energies and structures for interfaces is presented and applied to the Fe(0 0 1)/VN(0 0 1) system. The interface energetics of the coherent and semicoherent structures of thin VN films in Fe is investigated in order to determine how the misfit is taken up. The coherent interface is directly treated with ab initio calculations, whereas the semicoherent interface energy is accessed by using a Peierls-Nabarro framework, in which ab initio data for chemical interactions across the interface is combined with a continuum description to account for the elastic distortions. The continuum treatment is here extended to thoroughly account for the anisotropy in the materials. Our approach shows that the elastic contribution to the total interface energy dominates for both the coherent and semicoherent structure and must therefore be accurately accounted for in the interface description. Further, the Peierls-Nabarro framework for the semicoherent interface is evaluated by comparing a full scale two-dimensional solution to one-dimensional approximations. We show that the one-dimensional treatment is sufficient in the present case for accurate results, and consequently interactions at dislocation intersections at the interface do not have to be considered. © 2010 Elsevier B.V. All rights reserved.
|
|
| 14. |
- Fors, Dan, 1982, et al.
(författare)
-
Theoretical study of interface structure and energetics in semicoherent Fe(001)/MX(001) systems (M=Sc, Ti, V, Cr, Zr, Nb, Hf, Ta; X=C or N)
- 2010
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 82:19, s. 195410-
-
Tidskriftsartikel (refereegranskat)abstract
- We perform a systematic ab initio study of the electronic and atomic structure of semicoherent interfaces between bcc Fe and NaCl MX (M=Sc, Ti, V, Cr, Zr, Nb, Hf, Ta; X=C or N). The interface energetics is accessed by using a Peierls-Nabarro framework, in which ab initio data for the chemical interactions across the interface are combined with a continuum description to account for the elastic distortions. The key factors to the trends in the interface energy are identified and discussed with respect to the size of the misfit and the electronic structure of the MX phase. Our approach shows that the inclusion of lattice misfit can have a significant contribution to the interface energy (up to 1.5 J/m2) and must therefore be thoroughly accounted for in the interface description. The results will have important bearings on our ability to understand and describe precipitate stability in steels.
|
|
| 15. |
- Fransson, Erik, 1990, et al.
(författare)
-
Complexions and grain growth retardation: First-principles modeling of phase boundaries in WC-Co cemented carbides at elevated temperatures
- 2021
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 216
-
Tidskriftsartikel (refereegranskat)abstract
- WC-Co cemented carbides combine superb hardness with high toughness making them ideal for usage in metal machining and in wear resistant tools. Controlling the WC grain size is important during sintering as grain size plays a crucial role for the mechanical properties of the material. Experimental studies have observed different growth rates and grain morphologies in W-rich and C-rich materials, but the mechanism behind this has not been clarified. Here, we consider the possibility of an interface-stabilized state, a complexion, at the WC/Co phase boundary in cemented carbides, namely thin WC films with cubic structure. An interfacial phase diagram is derived using ab-initio calculations and first-principles modeling. Cluster expansions are employed to model carbon vacancies and Monte Carlo simulations to sample the configurational entropy. Force-constant fitting is used to extract the harmonic free energy for ground-state structures and the effects from anharmonicity and electronic excitations are effectively incorporated from a companion study on WC bulk phases. We predict the stabilization of thin cubic WC films at liquid phase sintering temperatures but only at W-rich conditions. This is consistent with experimental findings where thin films with cubic stacking have been observed predominantly in W-rich materials. We use this knowledge to suggest an explanation for the observed different growth rates and grain morphologies in W-rich and C-rich cemented carbides.
|
|
| 16. |
- Fransson, Erik, 1990, et al.
(författare)
-
Dynasor - A Tool for Extracting Dynamical Structure Factors and Current Correlation Functions from Molecular Dynamics Simulations
- 2021
-
Ingår i: Advanced Theory and Simulations. - : Wiley. - 2513-0390. ; 4:2
-
Tidskriftsartikel (refereegranskat)abstract
- Perturbative treatments of the lattice dynamics are widely successful for many crystalline materials; however, their applicability is limited for strongly anharmonic systems, metastable crystal structures and liquids. The full dynamics of these systems can, however, be accessed via molecular dynamics (MD) simulations using correlation functions, which includes dynamical structure factors providing a direct bridge to experiment. To simplify the analysis of correlation functions, here the dynasor package is presented as a flexible and efficient tool that enables the calculation of static and dynamical structure factors, current correlation functions as well as their partial counterparts from MD trajectories. The dynasor code can handle input from several major open source MD packages and thanks to its C/Python structure can be readily extended to support additional codes. The utility of dynasor is demonstrated via examples for both solid and liquid single and multi-component systems. In particular, the possibility to extract the full temperature dependence of phonon frequencies and lifetimes is emphasized.
|
|
| 17. |
- Fransson, Erik, 1990, et al.
(författare)
-
First-principles modeling of complexions at the phase boundaries in Ti-doped WC-Co cemented carbides at finite temperatures
- 2021
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 5:9
-
Tidskriftsartikel (refereegranskat)abstract
- WC-Co cemented carbides have a unique combination of high hardness and good toughness, making them ideal as tool materials in applications such as metal machining or rock drilling. Dopants are commonly added to retard grain growth and thereby creating a harder material. Thin films with cubic structure have been observed experimentally at phase boundaries between hexagonal WC and fcc Co-rich binder when doping with, e.g., Ti, V, or Cr. These films are generally considered to play a crucial role in the grain growth inhibition effect. Therefore, the thermodynamics of these thin cubic films is important to understand. Here, we construct, using ab initio calculations and modeling, an interfacial phase diagram for thin cubic films in Ti-doped WC-Co. We consider C <-> vacancy and W <-> Ti substitutions by constructing alloy cluster expansions and use Monte Carlo simulations to calculate the configurational free energy. Furthermore, force-constant fitting is used to extract the harmonic free energy for the ground-state structures. Additionally, we use information from thermodynamic databases to couple our atomic-scale calculations to overall compositions of typical WC-Comaterials. We predict that Ti segregates to WC/Co phase boundaries to form thin cubic films of two metallic layer thickness, both at solid-state and liquid-phase sintering temperatures. Furthermore, we predict that these films are stable also for low doping concentrations when no Ti-containing carbide phase precipitates in the material. We show that Ti essentially only segregates to the inner layer of the thin cubic film leaving an almost pure W layer towards Co, an ordering which has been observed in recent experimental high-resolution transmission electron microscopy studies.
|
|
| 18. |
- Fransson, Erik, 1990, et al.
(författare)
-
Understanding Correlations in BaZrO 3 : Structure and Dynamics on the Nanoscale
- 2024
-
Ingår i: Chemistry of Materials. - 1520-5002 .- 0897-4756. ; 36:1, s. 514-523
-
Tidskriftsartikel (refereegranskat)abstract
- Barium zirconate (BaZrO3) is one of few perovskites that is claimed to retain an average cubic structure down to 0 K at ambient pressure while being energetically very close to a tetragonal phase obtained by condensation of a soft phonon mode at the R-point. Previous studies suggest, however, that the local structure of BaZrO3 may change at low temperature, forming nanodomains or a glass-like phase. Here, we investigate the global and local structure of BaZrO3 as a function of temperature and pressure via molecular dynamics simulations using machine-learned potential with near density functional theory (DFT) accuracy. We show that the softening of the octahedral tilt mode at the R-point gives rise to weak diffuse superlattice reflections at low temperatures and ambient pressure, which are also observed experimentally. However, we do not observe any static nanodomains but rather soft dynamic fluctuations of the ZrO6 octahedra with a correlation length of 2-3 nm over time scales of about 1 ps. This soft dynamic behavior is the precursor of a phase transition and explains the emergence of weak superlattice peaks in measurements. On the other hand, when increasing the pressure at 300 K, we find a phase transition from the cubic to the tetragonal phase at around 16 GPa, also in agreement with experimental studies.
|
|
| 19. |
- Gren, Martin, 1989, et al.
(författare)
-
A computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides
- 2020
-
Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 87
-
Tidskriftsartikel (refereegranskat)abstract
- Interfaces and surfaces often play a vital role for the properties of polycrystalline materials, such as cemented carbides, and the study of these planar defects is, therefore, of great importance. Cemented carbides (or hardmetals) is a unique class of materials where hard carbide (WC) grains, usually micrometer sized, are embedded in a more ductile metal binder phase (usually Co) in order to combine superb strength with high hardness, making them ideal as tool material in e.g. metal machining. In the manufacturing and industrial usage of cemented carbides temperatures reach high levels, especially in the former case where the material is sintered at temperatures where the binder phase is a liquid. This is a computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides upto and above the melting temperature of Co. We make use of an analytical bond order potential (ABOP) fitted to density functional theory (DFT) data in order to make the free energy calculations feasible. A variety of free energy methods are used: including quasi harmonic approximation, temperature and thermodynamic integration, and calculation of liquid surface tension and work of adhesion for phase boundaries. We present the temperature dependent interface and surface energies for some typical cases, data which should be useful as a supplement to other studies limited to 0 K.
|
|
| 20. |
- Gren, Martin, 1989, et al.
(författare)
-
Modeling of vibrational and configurational degrees of freedom in hexagonal and cubic tungsten carbide at high temperatures
- 2021
-
Ingår i: Physical Review Materials. - 2475-9953. ; 5:3
-
Tidskriftsartikel (refereegranskat)abstract
- Transition metal carbide is a class of materials characterized by high hardness, high melting points, and low chemical reactivity. It is widely used in industrial applications involving exposure to elevated temperatures, aggressive media, and heavy loads, and is thus of technological and industrial importance. In this paper the high-temperature thermodynamic properties of tungsten carbide, WC, is studied. At most temperatures below melting, WC assumes a hexagonal structure with essentially no vacancies (δ-WC). Only at very high temperatures (around 3000 K), WC crystallizes in the cubic rocksalt structure (γ-WC), which is more common for the transition metal carbides and in the case for WC can contain up to 40% carbon vacancies. At lower temperatures, γ-WC can, however, form as thin interfacial structures or nanoparticles. Hence, the thermodynamic properties of both γ-WC and δ-WC are of relevance. Here, we conduct a first-principles density-functional theory based computational investigation of the γ-WC and δ-WC phases, which requires modeling of high carbon vacancy concentrations at high temperatures. The configurational degrees of freedom are modeled with an alloy cluster expansion and sampled through Monte Carlo simulations. To account for the dynamic instability of the cubic γ-WC phase at low temperatures, the vibrational degrees of freedom are treated using effective harmonic models constructed from ab initio molecular dynamics simulations. Finally, we obtain a part of the W-C phase diagram in reasonably quantitative agreement with experimental data.
|
|
| 21. |
- Gren, Martin, 1989, et al.
(författare)
-
Wetting of surfaces and grain boundaries in cemented carbides and the effect from local chemistry
- 2019
-
Ingår i: Materialia. - : Elsevier BV. - 2589-1529. ; 8
-
Tidskriftsartikel (refereegranskat)abstract
- Wettability is an important factor in the sintering of cemented carbides. We study wetting of surfaces and grain boundaries in WC–Co och WC–Ni cemented carbides using density functional theory (DFT). Based on experimental observations of WC grain orientations in cemented carbides, relevant model interfaces are created. The local chemical composition at the interfaces is taken into account by substitution of interface atoms, and the effect of temperature is evaluated based on a companion study of temperature dependent interface energies in cemented carbides. The results indicate that the wettability of Ni and Co on WC surfaces are similar. Furthermore, the wettability of Co on the commonly occurring basal WC surface is better in W-rich materials compared to C-rich materials. At liquid phase sintering temperatures we get perfect wetting in W-rich materials, while only partial wetting in C-rich materials, which is in agreement with recent experiments on wetting in WC–Co cemented carbides. The segregation of binder phase atoms to WC/WC grain boundaries stabilize grain boundaries and make them more resistance against infiltration (wetting) by binder phase. We find that the amount of dissolved binder atoms in essentially all studied WC/WC grain boundaries are of half a monolayer proportion, which is in agreement with experimental studies. Further, in WC–Co there is a stronger resistance against grain boundary infiltration compared to WC–Ni. We find that the continuous skeleton of WC grains seen after sintering and which is crucial for the superior mechanical strength of the material exists already during liquid phase sintering.
|
|
| 22. |
- Jedvik Granhed, Erik, 1979, et al.
(författare)
-
Band: Vs. polaron: Vibrational motion and chemical expansion of hydride ions as signatures for the electronic character in oxyhydride barium titanate
- 2019
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 7:27, s. 16211-16221
-
Tidskriftsartikel (refereegranskat)abstract
- The oxyhydride phase of barium titanate, BaTiO3-xHx, is a mixed hydride ion and electron conductor. The substitution of oxygen with hydrogen to form a hydride ion is accompanied by donation of an electron to the initially empty titanium 3d conduction band. It is not clear, however, whether the electron forms a delocalized state where it is shared among all titanium ions forming a bandstate, or if it localizes on a titanium ion and forms a bound electron polaron. Here, we investigate polaron formation in this material using density-functional theory (DFT) calculations, where the self-interaction error has been corrected by the DFT + U method and the HSE hybrid functional. While calculated formation energies do not provide a conclusive description of the electronic state, a comparison of the results from first-principles phonon calculations with vibrational spectra measured with inelastic neutron scattering (INS) suggests that the electrons form bandstates in bulk BaTiO3-xHx. This is further supported by comparison of the computed chemical expansion of the involved defect species with experimental data of the lattice expansion in the oxyhydride formation. The oxyhydride phase of barium titanate, BaTiO3-xHx, should thus exhibit metallic-like conductivity.
|
|
| 23. |
- Jedvik Granhed, Erik, 1979, et al.
(författare)
-
BaZrO3 stability under pressure: The role of nonlocal exchange and correlation
- 2020
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 101:22
-
Tidskriftsartikel (refereegranskat)abstract
- The ground-stale structure of BaZrO3 is experimentally known to be cubic down to absolute zero. However, there exist several measured properties and experimental characterizations that earlier computational works have failed to accurately describe and explain within this cubic symmetry. Among these properties and observations are the dielectric constant and the parallel mean-squared relative displacement value that tracks the fluctuations in distance for Ba-O atom pairs. Previous density-functional theory (DFT) studies have resolved the issue by assuming that BaZrO(3 )undergoes a phase transition from cubic to tetragonal I4/mcm symmetry, possibly while forming a glasslike state that reflects cubic symmetry on average. In this paper, we show that the set of experimental results can indeed be satisfactorily explained by DFT entirely within the cubic symmetry. We find that past theory limitations arose from the choice of exchange-correlation-functional approximations and that the inclusion of Fock exchange in hybrids significantly improves the DFT performance. We also find that the inclusion of nonlocal correlation effects is beneficial. We conclude by making a prediction for the phase-transition pressure for the transition from cubic to tetragonal symmetry at zero kelvin.
|
|
| 24. |
- Jedvik Granhed, Erik, 1979, et al.
(författare)
-
Size and shape of oxygen vacancies and protons in acceptor-doped barium zirconate
- 2015
-
Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 275, s. 2-8
-
Tidskriftsartikel (refereegranskat)abstract
- The defect induced chemical expansion in acceptor-doped barium zirconate is investigated using density-functional theory (DFT) calculations. The two defect species involved in the hydration reaction, the +2 charged oxygen vacancy and the proton interstitial forming a hydroxide ion, are considered both as free defects and in association with the dopants Y, In, Sc and Ga. The defect induced strain tensor lambda is introduced, which provides a natural generalisation of the ordinary chemical expansion to three dimensions and to anisotropic distortions. Both the addition of a vacancy and a proton cause anisotropic distortions and a net contraction of the lattice, indicating that both the vacancy and the hydroxide ion are smaller than the oxygen ion. The contraction is considerably larger for the vacancy and the net effect in hydration, when a vacancy is filled and two protons are added, is an expansion, consistent with the experimental findings. The effect of the dopants on the chemical expansion in hydration is found to be quite small, even if it is assumed that both the vacancy and the proton are fully associated with a dopant atom in the lattice.
|
|
| 25. |
- Johansson, Sven, 1981, et al.
(författare)
-
A computational study of special grain boundaries in WC–Co cemented carbides
- 2015
-
Ingår i: Computational Materials Science. - : Elsevier BV. - 0927-0256. ; 98, s. 345-353
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, we model Sigma = 2 and Sigma = 1 {10 (1) over bar0}parallel to{10 (1) over bar0} WC/WC boundaries in WC-Co using density functional theory (DFT). In particular, the misfit structure of the Sigma = 2 twist boundary is modeled explicitly with a previously developed Peierls-Nabarro model for misfit dislocations. The grain boundary energy of the twist boundary is found to be 0.7 J/m(2), which is small in comparison with energies of general WC/WC boundaries. The misfit structure can be described as a square network of screw dislocations with Burgers vectors 1/6 . Our calculations show that Co will not segregate to the Sigma = 2 twist boundary, which contrasts with predictions for other WC/WC boundaries that typically give half a monolayer of segregated Co.
|
|
| 26. |
- Johansson, Sven, 1981, et al.
(författare)
-
A computational study of thin cubic carbide films in WC/Co interfaces
- 2011
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 59:1, s. 171-181
-
Tidskriftsartikel (refereegranskat)abstract
- We examine the propensity for the formation of thin films of cubic TiC, VC, CrC, ZrC, NbC, MoC, HfC, TaC and WC in the metal ceramic interface between simple hexagonal WC and Co using density functional theory Films of TiC, VC, CrC and MoC are predicted to be stable, of which VC and MoC show the strongest propensity for formation The results provide an explanation for the experimen tally observed WC grain growth inhibition effect and alteration of WC grain shape attained in VC-doped WC Co.
|
|
| 27. |
- Johansson, Sven A.E., et al.
(författare)
-
CCBuilder: a software that produces synthetic microstructures of WC-Co cemented carbides
- 2019
-
Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 78, s. 210-218
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, the software CCBuilder and its methods to generate synthetic three-dimensional (3D) microstructures of WC-Co cemented carbides are presented. Generating synthetic microstructures is critical, as it enables accurate virtual testing where influences from microstructural parameters can be investigated in a systematic and controlled way. The model of the microstructure is based on a voxel description of the material volume and the WC grains are represented as truncated triangular prisms. The grains are inserted into the simulation domain and to obtain a realistic microstructure a combination of packing and a Monte Carlo Potts simulation is used. The orientations of all individual WC grains are known and it is shown that the microstructure is built up of two interpenetrating networks, the WC grains and the Co phase. Numerical studies are presented to show how input parameters to CCBuilder, such as grain size, grain shape, packing strategy, voxel density, system size, and Monte-Carlo steps affect the resulting simulated microstructure. The obtained microstructures are geometrically realistic and the contiguity of the carbide phase shows very good agreement with published experimental data.
|
|
| 28. |
- Johansson, Sven, 1981, et al.
(författare)
-
First-principles derived complexion diagrams for phase boundaries in doped cemented carbides
- 2016
-
Ingår i: Current Opinion in Solid State and Materials Science. - : Elsevier BV. - 1359-0286. ; 20:5, s. 299-307
-
Forskningsöversikt (refereegranskat)abstract
- This article reviews a method for calculating an equilibrium interfacial phase diagram depicting regions of stability for different interface structures as function of temperature and chemical potentials. Density functional theory (DFT) is used for interfacial energies, Monte Carlo simulations together with cluster expansions based on DFT results for obtaining configurational free energies, and CALPHAD-type modeling for describing the thermodynamic properties of the adjoining bulk phases. An explicit case, vanadium doped cemented carbides, is chosen to illustrate the progress in the research field and the interfacial diagram, a complexion diagram, for the phase boundary WC(0001)/Co is constructed as function of temperature and chemical potentials.
|
|
| 29. |
- Johansson, Sven, 1981, et al.
(författare)
-
First-principles study of an interfacial phase diagram in the V-doped WC-Co system
- 2012
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 86:3, s. 035403-
-
Tidskriftsartikel (refereegranskat)abstract
- We present a method of determining an interfacial phase diagram using density functional theory calculations for interface energetics. Cluster expansions based on these calculations are used in Monte Carlo simulations to obtain configurational free energies. The method is applied to study the segregation of V to the WC(0001)/Co interface and to construct the corresponding interface diagram, a "complexion" phase diagram. By CALPHAD-type analysis for the adjoining bulk phases, a connection with real materials is made. We find that, in equilibrium, the interface contains a thin V-rich film for a wide range of temperatures and chemical potentials of V corresponding to V additions below the (V, W)C-x solubility limit. The results are compared with available experimental data, and implications of the strong segregation of V for the sintering process of V-doped WC-Co are discussed.
|
|
| 30. |
|
|
| 31. |
- Johansson, Sven, 1981, et al.
(författare)
-
Theory of ultrathin films at metal-ceramic interfaces
- 2010
-
Ingår i: Philosophical Magazine Letters. - : Informa UK Limited. - 0950-0839 .- 1362-3036. ; 90:8, s. 599-609
-
Tidskriftsartikel (refereegranskat)abstract
- A theoretical model for understanding the formation of interfacial thin films is presented, which combines density functional theory calculations for interface energies with thermodynamic modeling techniques for multicomponent bulk systems. The theory is applied to thin film formation in VC-doped WC-Co cemented carbides. It is predicted that ultrathin VC films may exist in WC/Co interfaces at the high temperature sintering conditions where most of the WC grain growth occurs, which provides an explanation of the grain growth inhibiting effect of VC additions in the WC-Co system.
|
|
| 32. |
|
|
| 33. |
|
|
| 34. |
|
|
| 35. |
- Karlberg, Gustav, 1975, et al.
(författare)
-
Quantum treatment of H adsorbed on a P(111) surface
- 2001
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 65:3, s. 033406 -
-
Tidskriftsartikel (refereegranskat)abstract
- A static potential energy surface for 1/4 of a monolayer of adsorbed hydrogen atoms on a Pt(111) surface has been calculated using first-principles density-functional theory calculations. The Schrodinger equation is solved for the hydrogen atoms in this potential. The results agree well with stable site, vibrational spectroscopy, and diffusion measurements, and resolve the conflicting interpretations of those experiments.
|
|
| 36. |
- Karlberg, Gustav, 1975, et al.
(författare)
-
Water desorption from an oxygen covered Pt(111) surface: multi-channel desorption
- 2006
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 124:20, s. 204712-
-
Tidskriftsartikel (refereegranskat)abstract
- Mixed OH/H2O structures, formed by the reaction of O and water on Pt(111), decompose near 200 K as water desorbs. With an apparent activation barrier that varies between 0.42 and 0.86 eV depending on the composition, coverage, and heating rate of the film, water desorption does not follow a simple kinetic form. The adsorbate is stabilized by the formation of a complete hydrogen bonding network between equivalent amounts of OH and H2O, island edges, and defects in the structure enhancing the decomposition rate. Monte Carlo simulations of water desorption were made using a model potential fitted to first-principles calculations. We find that desorption occurs via several distinct pathways, including direct or proton-transfer mediated desorption and OH recombination. Hence, no single rate determining step has been found. Desorption occurs preferentially from low coordination defect or edge sites, leading to complex kinetics which are sensitive to both the temperature, composition, and history of the sample.
|
|
| 37. |
- Karlsson, Maths, 1978, et al.
(författare)
-
Quasielastic neutron scattering of hydrated BaZr0.90A0.10O2.95 (A = Y and Sc)
- 2009
-
Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 180:1, s. 22-28
-
Tidskriftsartikel (refereegranskat)abstract
- Proton motions in hydrated proton conducting perovskites BaZr 0.90 A 0.10 O 2.95 (A = Y and Sc) have been investigated using quasielastic neutron scattering. The results reveal a localized motion on the ps time scale and with an activation energy of ~ 10-30 meV, in both materials. The temperature dependence of the total mean square displacement of the protons shows an onset of this motion at a temperature of about 300 K. The low activation energy, much lower than the activation energy for the macroscopic proton conductivity, suggests that this motion is not the rate-limiting process for the long-range proton diffusion, i.e. it is not linked to the two materials significantly different proton conductivities. In fact, a comparison of the QENS results with density functional theory calculations indicates that for both materials the observed motion may be ascribed to intra-octahedral proton transfers occurring close to a dopant atom. © 2008 Elsevier B.V. All rights reserved.
|
|
| 38. |
- Karlsson, Maths, 1978, et al.
(författare)
-
Using Neutron Spin-Echo To Investigate Proton Dynamics in Proton-Conducting Perovskites
- 2010
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 22:3, s. 740-742
-
Tidskriftsartikel (refereegranskat)abstract
- The applicability and potential of neutron spin-echo (NSE) to study proton dynamics in proton-conducting ceramics was studied. The experiment was performed on hydrated BaZr 0,90 Y 0.10 O 2.95 (10Y:BZO), a cubic perovskite with a relatively high proton conductivity. The NSE experiment was performed at the IN15 spectrometer at Institut Laue-Langevin (ILL) in Grenoble, France. The relaxation rate for two Q-values for the temperature 563 K was determined. The first-principles calculations were carried out within the framework of density functional theory (DFT). The calculated diffusion harriers far from Y-dopants are found to he 0.20 and 0.18 eV for the proton transfer and hydroxyl rotation motion, respectively. The binding energy to a Y-dopant is 0.16 eV, and the influence of the Y-dopant on the energetics for the proton is quite extended in space, including both the first and the second coordination shells.
|
|
| 39. |
- Karlsson, Maths, 1978, et al.
(författare)
-
Vibrational Properties of Protons in Hydrated BaInxZr1-xO3-x/2
- 2005
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 72, s. 1-7
-
Tidskriftsartikel (refereegranskat)abstract
- We investigate the local proton dynamics in the proton conducting hydrated perovskite system BaInxZr1-xO3-x/2 (x=0.25-0.75) using infrared spectroscopy and first-principles calculations. We show that oxygen vacancies and dopant atoms in the vicinity of the proton tilt the proton toward a neighboring oxygen creating strongly hydrogen-bonded configurations. This is manifested as a strong redshift of the O-H stretch band in the infrared absorption spectrum. We also find considerable fluctuations of the nearest and next-nearest oxygen-proton distances with time, resulting in additional spectral broadening. By comparing the frequencies of computed O-H stretch modes we can relate specific local configurations to different parts of the broad O-H stretch band. Even though hydrogen-bonded configurations favor proton transfer they hinder the long-range migration by decreasing the reorientational rate. Thus, in order to optimize the proton mobility it is important to avoid extreme configurations caused by either oxygen vacancies or dopant atoms in the perovskite structure.
|
|
| 40. |
- Lay, S., et al.
(författare)
-
Interface structure in a WC-Co alloy co-doped with VC and Cr3C2
- 2012
-
Ingår i: Journal of Materials Science. - : Springer Science and Business Media LLC. - 0022-2461 .- 1573-4803. ; 47:3, s. 1588-1593
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of carbide additions is investigated at the atom scale by studying the interface between the cubic carbide phase and the WC prismatic facet in a VC Cr 3 C 2 co-doped WC Co alloy using high-resolution transmission electron microscopy. The alloy was prepared from WC powder about 0.5 lm in size and sintered at 1400°C for 2 h. The orientation relationship between WC and the MC carbide at the prismatic facet of WC is the same orientation as the one found at a WC/(Cr, W)C interface in Cr 3 C 2 -doped alloys. the image of WC shows triangular patterns indicating the position of the C atomic columns in this crystal. In MC crystal, some elongation of the bright areas parallel to the interface is observed on the simulation. The presence of dislocations at interfaces can be related to the parametric mismatch between the two neighboring crystals, to the presence of different stable structural units, or to an angular deviation between the crystals.
|
|
| 41. |
|
|
| 42. |
|
|
| 43. |
- Lindman, Anders, 1985, et al.
(författare)
-
Comparison of Space-Charge Formation at Grain Boundaries in Proton-Conducting BaZrO3 and BaCeO3
- 2017
-
Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 29:18, s. 7931-7941
-
Tidskriftsartikel (refereegranskat)abstract
- Acceptor-doped BaZrO3 (BZO) and BaCeO3 (BCO) both exhibit considerable bulk proton conductivity, which makes them suitable as electrolytes in electrochemical devices. However, these materials display high grain-boundary (GB) resistance that severely limits the overall proton transport in polycrystalline samples. This effect has been attributed to the presence of space charges at the GBs, which form because of segregation of protons and charged oxygen vacancies. This blocking behavior is less prominent in BCO, but in contrast to BZO, BCO suffers from poor chemical stability. The aim with the present work is to elucidate why GBs in BZO are more resistive than GBs in BCO. We use density-functional theory (DFT) calculations to study proton and oxygen vacancy segregation to several GBs and find that the oxygen vacancy segregation energy is quite similar in both materials while the tendency for proton segregation is larger in BZO compared with that in BCO. This is not related to the GBs, which display similar proton formation energies in both materials, but because of different formation energies for protons in the bulk regions. This can be understood from a stronger hydrogen bond formation in bulk BCO compared with that in bulk BZO. Furthermore, segregation energies are evaluated in a space-charge model,. and the resulting space-charge potentials provide a consistent explanation of the experimentally observed difference in GB conductivity.
|
|
| 44. |
- Lindman, Anders, 1985, et al.
(författare)
-
Defect segregation to grain boundaries in BaZrO3 from first-principles free energy calculations
- 2017
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 5:26, s. 13421-13429
-
Tidskriftsartikel (refereegranskat)abstract
- Acceptor-doped BaZrO3 is currently the most promising ceramic proton conductor for application in electrolyzers and reactor membranes. Its overall proton conductivity is, however, limited by space charge formation due to defect segregation to grain boundaries (GBs). In this contribution, we determine the vibrational contribution to the free energy of GB formation and defect segregation in the symmetric Sigma 3 (112)[110] tilt GB in BaZrO3 in order to elaborate on the high temperature GB defect chemistry. We consider segregation of several different defect species: oxygen vacancies (v(O)(center dot center dot) ), protons (OHO center dot) and the four trivalent acceptor dopants Sc, In, Y and Gd (M '(Zr). The calculations reveal that the segregation free energy of v(O)(center dot center dot) decreases with temperature while it remains fairly constant for (OHO center dot ), which increase the relative stability of similar to with respect to (OHO center dot ) with increasing temperature. As a consequence the onset for hydration of the GB core is shifted towards lower temperatures. For the dopants we find that both the segregation energy and entropy correlate with the ionic radius, where they are positive for the smaller dopants (Sc and In) while negative for the larger (Y, Gd). In turn, this leads to similar segregation free energies for all species, which are close to zero, at high temperatures where dopants are mobile, implying that dopant segregation is almost entirely driven by segregated positively charged defects. Neglecting the phonon contribution erroneously leads to the conclusion that the thermodynamic driving force for dopant segregation increases with increasing dopant radii, and it is therefore important to consider free energies when predicting the high temperature defect chemistry of BaZrO3 GBs.
|
|
| 45. |
- Lindman, Anders, 1985, et al.
(författare)
-
Implications of the band gap problem on oxidation and hydration in acceptor-doped barium zirconate
- 2015
-
Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 91:24, s. Art. no. 245114-
-
Tidskriftsartikel (refereegranskat)abstract
- Charge carrier concentrations in acceptor-doped proton-conducting perovskites are to a large extent determined by the hydration and oxidation of oxygen vacancies, which introduce protons and holes, respectively. First-principles modeling of these reactions involves calculation of formation energies of charged defects, which requires an accurate description of the band gap and the position of the band edges. Since density-functional theory (DFT) with local and semilocal exchange-correlation functionals (LDA and GGA) systematically fails to predict these quantities this can have serious implications on the modeling of defect reactions. In this study we investigate how the description of band gap and band-edge positions affects the hydration and oxidation in acceptor-doped BaZrO3. First-principles calculations are performed in combination with thermodynamic modeling in order to obtain equilibrium charge carrier concentrations at different temperatures and partial pressures. Three different methods have been considered: DFT with both semilocal (PBE) and hybrid (PBE0) exchange-correlation functionals, and many-body perturbation theory within the G(0)W(0) approximation. All three methods yield similar results for the hydration reaction, which are consistent with experimental findings. For the oxidation reaction, on the other hand, there is a qualitative difference. PBE predicts the reaction to be exothermic, while the two others predict an endothermic behavior. Results from thermodynamic modeling are compared with available experimental data, such as enthalpies, concentrations, and conductivities, and only the results obtained with PBE0 and G(0)W(0), with an endothermic oxidation behavior, give a satisfactory agreement with experiments.
|
|
| 46. |
- Lindman, Anders, 1985, et al.
(författare)
-
Oxygen vacancy segregation in grain boundaries of BaZrO3 using interatomic potentials
- 2013
-
Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 230:C, s. 27-31
-
Tidskriftsartikel (refereegranskat)abstract
- We have used classical interatomic potentials to determine the structure, interface energy and oxygen vacancy segregation energies of eight different grain boundaries (GBs) in BaZrO3 with tilt axis [-110]. Two of these have been studied previously with density functional theory and the agreement is satisfactory. The results suggest that oxygen vacancies prefer to reside near the boundary interface for all these GBs. The minimum segregation energies range between −1.86 eV and −0.57 eV, and the typical core width is about 10 Å. The resulting depletion layers have been evaluated using a thermodynamic space-charge model. Space-charge potential barriers between 0.2 and 0.8 eV were obtained with dopant concentrations of 5% and 10%.
|
|
| 47. |
- Lindman, Anders, 1985, et al.
(författare)
-
Polaronic contributions to oxidation and hole conductivity in acceptor-doped BaZrO₃
- 2016
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 94:7
-
Tidskriftsartikel (refereegranskat)abstract
- Acceptor-doped perovskite oxides like BaZrO3 are showing great potential as materials for renewable energy technologies where hydrogen acts an energy carrier, such as solid oxide fuel cells and hydrogen separation membranes. While ionic transport in these materials has been investigated intensively, the electronic counterpart has received much less attention and further exploration in this field is required. Here, we use density functional theory (DFT) to study hole polarons and their impact on hole conductivity in Y-doped BaZrO3. Three different approaches have been used to remedy the self-interaction error of local and semilocal exchange-correlation functionals: DFT + U, pSIC-DFT, and hybrid functionals. Self-trapped holes are found to be energetically favorable by about -0.1 eV and the presence of yttrium results in further stabilization. Polaron migration is predicted to occur through intraoctahedral transfer and polaron rotational processes, which are associated with adiabatic barriers of about 0.1 eV. However, the rather small energies associated with polaron formation and migration suggest that the hole becomes delocalized and bandlike at elevated temperatures. These results together with an endothermic oxidation reaction [A. Lindman, P. Erhart, and G. Wahnstrom, Phys. Rev. B 91, 245114 (2015)] yield a picture that is consistent with experimental data for the hole conductivity. The results we present here provide new insight into hole transport in acceptor-doped BaZrO3 and similar materials, which will be of value in the future development of sustainable technologies.
|
|
| 48. |
- Lindman, Anders, 1985, et al.
(författare)
-
Theoretical modeling of defect segregation and space-charge formation in the BaZrO3 (210) 001 tilt grain boundary
- 2013
-
Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 252, s. 121-125
-
Tidskriftsartikel (refereegranskat)abstract
- Density-functional theory (DFT) has been used to determine the structure and interface energy of different rigid body translations (RBTs) of the (210)10011 grain boundary (GB) in BaZrO3. There exist several different stable structures with almost equally low interfacial energy. Segregation energies of protons and oxygen vacancies have been determined for the most stable (210)10011 grain boundary structure. The results suggest that both defect species favor segregation to the same site at the boundary interface with minimum segregation energies of - 1.45 eV and - 1.32 eV for vacancies and protons respectively. The segregation energies have been used in a thermodynamic space-charge model to obtain equilibrium defect concentrations and space-charge potentials at a 10% dopant concentration. Space-charge,potential barriers around 0.65 V were obtained at intermediate temperatures under hydrated conditions, where protons are the main contributor to the excess core charge. The potential is slightly lower under dry conditions. (C) 2013 Elsevier B.V. All rights reserved.
|
|
| 49. |
- Liu, Fang, 1975, et al.
(författare)
-
Effect of Boron on Carbide Coarsening at 873 K (600°C) in 9 to 12 pct Chromium Steels
- 2012
-
Ingår i: Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science. - : Springer Science and Business Media LLC. - 1073-5623. ; 43:11, s. 4053-4062
-
Tidskriftsartikel (refereegranskat)abstract
- The addition of small amounts of boron to 9 to 12 pct chromium steels has been found to decrease their creep rate at 823 K to 923 K (550 °C to 650 °C). In this article, the behavior of boron during austenitizing, tempering, and isothermal heat treatment at 873 K (600 °C) is studied using high-resolution microscopy and microanalysis as well as using atomistic modeling. It was found that increasing the boron content from 9 to 40 ppm decreased the coarsening constant of M23C6 by a factor of almost 2. Most of the added boron was incorporated in M23C6. Atomistic modeling showed that boron diffusion in ferrite is dominated by an interstitial mechanism at 873 K (600 °C). However, the generation of vacancies when carbide precipitates dissolve may promote a distribution with substitutional boron atoms. The absence of a fast mechanism for the transition from substitutional to interstitial occupancy will make the slow substitutional boron diffusion in the matrix rate controlling for the coarsening process.
|
|
| 50. |
|
|
