| 1. |
- Ahlberg Helgee, Edit, 1986, et al.
(författare)
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Origin of Space Charge in Grain Boundaries of Proton-Conducting BaZrO3
- 2013
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Ingår i: Fuel Cells. - : Wiley. - 1615-6846 .- 1615-6854. ; 13:1, s. 19-28
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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.
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| 2. |
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| 3. |
- Björketun, Mårten, 1976, et al.
(författare)
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Effect of acceptor dopants on the proton mobility in BaZrO3: A density functional investigation
- 2007
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Ingår i: Phys. Rev. B. ; 76, s. 054307-
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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.
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| 4. |
- Björketun, Mårten, 1976, et al.
(författare)
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Protonic defects in pure and doped La2Zr2O7 pyrochlore oxide
- 2009
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 178:31-32, s. 1642-1647
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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.
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| 5. |
- Björketun, Mårten, 1976, et al.
(författare)
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Structure and thermodynamic stability of hydrogen interstitials in BaZrO3 perovskite oxide from density functional calculations
- 2007
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Ingår i: Faraday Discuss.. ; 134, s. 247-265
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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.
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| 6. |
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| 7. |
- Christensen, Mikael, 1973, et al.
(författare)
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Morphology of WC grains in WC-Co alloys: Theoretical determination of grain shape
- 2007
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Ingår i: Acta Mater.. ; 55, s. 1515-
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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.
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| 8. |
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| 9. |
- Christensen, Mikael, 1973, et al.
(författare)
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Strength and reinforcement of interfaces in cemented carbides
- 2006
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Ingår i: Int. J. Refract. Hard Mater.. ; 24, s. 80-
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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.
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| 10. |
- Eklöf-Österberg, Carin, 1987, et al.
(författare)
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The role of oxygen vacancies on the vibrational motions of hydride ions in the oxyhydride of barium titanate
- 2020
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 8:13, s. 6360-6371
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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.
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