| 1. |
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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
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| 7. |
- 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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| 8. |
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| 9. |
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| 10. |
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| 11. |
- Karlberg, Gustav, 1975, et al.
(författare)
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Water desorption from an oxygen covered Pt(111) surface: multi-channel desorption
- 2006
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 124:20, s. 204712-
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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.
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| 12. |
- Karlsson, Maths, 1978, et al.
(författare)
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Quasielastic neutron scattering of hydrated BaZr0.90A0.10O2.95 (A = Y and Sc)
- 2009
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 180:1, s. 22-28
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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.
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| 13. |
- Karlsson, Maths, 1978, et al.
(författare)
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Vibrational Properties of Protons in Hydrated BaInxZr1-xO3-x/2
- 2005
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 72, s. 1-7
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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.
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| 14. |
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| 15. |
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| 16. |
- Schaller, Vincent, 1979, et al.
(författare)
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Effective magnetic moment of magnetic multicore nanoparticles
- 2009
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 80:9, s. 092406-
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Tidskriftsartikel (refereegranskat)abstract
- We carry out Monte Carlo simulations to study the effective magnetic moment mu(eff) in the low-field region of magnetic multicore nanoparticles. Transmission electron microscopy and scanning electron microscopy images show that these particles contain a number of magnetic nanocrystals (MNCs) randomly packed in a single cluster of total volume V(tot). We illustrate how the initial magnetic susceptibility chi(0) of magnetic multicore nanoparticles can be straightforward derived from mu(eff) computed at zero magnetic field. We observe that dipolar interactions between MNCs and polydispersity of the MNCs contribute to increase and to decrease mu(eff)/V(tot), respectively, while magnetic anisotropy of the MNCs does not show any effect. In all three cases, mu(eff)/V(tot) can be described by a linear relation to (mu B/k(B)T)(2) that we analytically derived for low applied fields.
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| 17. |
- Schaller, Vincent, 1979, et al.
(författare)
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Monte Carlo simulation of magnetic multi-core nanoparticles
- 2009
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 321:10, s. 1400-1403
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, a Monte Carlo simulation is carried out to evaluate the equilibrium magnetization of magnetic multi-core nanoparticles in a liquid and subjected to a static magnetic field. The particles contain a magnetic multi-core consisting of a cluster of magnetic single-domains of magnetite. We show that the magnetization of multi-core nanoparticles cannot be fully described by a Langevin model. Inter-domain dipolar interactions and domain magnetic anisotropy contribute to decrease the magnetization of the particles, whereas the single-domain size distribution yields an increase in magnetization. Also, we show that the interactions affect the effective magnetic moment of the multicore nanoparticles.
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| 18. |
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| 19. |
- Schaller, Vincent, 1979, et al.
(författare)
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Motion of nanometer sized magnetic particles in a magnetic field gradient
- 2008
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 104:9, s. no:093918-
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Tidskriftsartikel (refereegranskat)abstract
- Using magnetic particles with sizes in the nanometer range in biomedical magnetic separation has gained much interest recently due to their higher surface area to particle volume and lower sedimentation rates. In this paper, we report our both theoretical and experimental investigation of the motion of magnetic particles in a magnetic field gradient with particle sizes from 425 nm down to 50 nm. In the experimental measurements, we monitor the absorbance change of the sample volume as the particle concentration varies over time. We also implement a Brownian dynamics algorithm to investigate the influence of particle interactions during the separation and compare it to the experimental results for validation. The simulation agrees well with the measurements for particle sizes around 425 nm. Some discrepancies remain for smaller particle sizes, which may indicate that additional factors also influence the separation for the smaller size range. We observe that the separation process includes the formation of chainlike particle aggregates due to the magnetic dipole-dipole interactions between particles when subjected to an external magnetic field. We can also see that the hydrodynamic interaction between these chains contributes to reducing the separation time. In conclusion, we show that the formation of these particle aggregates, and to a less extent the hydrodynamic interactions between them contributes to significantly enhancing the particle separation process. © 2008 American Institute of Physics.
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| 20. |
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| 21. |
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| 22. |
- Slabanja, Mattias, 1976, et al.
(författare)
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Early stages of phase separation using three-dimensional atom probe and atomistic modelling
- 2007
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Ingår i: Surface and Interface Analysis. ; 39, s. 178-
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Tidskriftsartikel (refereegranskat)abstract
- The early stages of clustering in Al-14.8 Mg alloy have been studied using atomistic modelling and 3DAP analysis. Atomistic modelling was based on first-principles quantum mechanical calculations and Monte Carlo (MC) based techniques. A good agreement between the experimental results and simulated data was obtained showing the appropriateness of the theoretical approach used. It has been shown that after 720 h at 35 °C (308 K) clustering of Mg was clearly detectable in the experimental alloy. The simulated data after 200 MC steps corresponded well to the experimental results obtained after ageing for 720 h.
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| 23. |
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| 24. |
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| 25. |
- Sundell, Per G., 1977, et al.
(författare)
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Density-functional calculations of prefactors and activation energies for H diffusion in BaZrO3
- 2007
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Ingår i: Phys. Rev. B. ; 76, s. 094301-
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Tidskriftsartikel (refereegranskat)abstract
- Density-functional calculations are used to investigate hydrogen diffusion in the solid-state proton conductor BaZrO3. Activation energies and prefactors for the rate of proton transfer and reorientation are evaluated for a defect-free region of this simple cubic perovskite-structured oxide. Both semiclassical over-barrier jumps and phonon-assisted tunneling transitions between sites are considered. It is found that the classical barriers for the elementary transfer and reorientation steps are both of the order of 0.2 eV. The quantum-mechanical zero-point motion effects are found to be sizable, to effectively reduce the barrier heights, and to make the prefactors similar for the transfer and reorientation steps. The Flynn-Stoneham model [Phys. Rev. B 1, 3966 (1970)] of phonon-assisted tunneling yields an activation energy of around 0.2 eV and a very small prefactor for proton transfer, whereas the corresponding adiabatic model gives a similar activation energy but a much larger prefactor. It is suggested that the effect of other defects such as dopants has to be included for a proper description of hydrogen diffusion in this material.
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| 26. |
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| 27. |
- Sundell, Per G., 1977, et al.
(författare)
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Thermodynamics of doping and vacancy formation in BaZrO3 perovskite oxide from density functional calculations
- 2006
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Ingår i: Phys. Rev. B. ; 73, s. 104112-
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Tidskriftsartikel (refereegranskat)abstract
- Density functional calculations combined with thermodynamical modeling have been used to investigate defect formation in a perovskite-structured oxide in equilibrium with an oxygen containing atmosphere. We have calculated the electronic structure and formation energies for anion and cation vacancies and for Ga, Gd, In, Nd, Sc, and Y dopants incorporated on different lattice sites in BaZrO3. On energetic grounds, it is found that most of the investigated dopants (with the possible exception of Nd and Gd) preferably substitute for Zr atoms in the lattice. The interaction between dopants was found to be repulsive and correlated with the ionic radius of the dopant, while the dopant-oxygen vacancy interaction was found attractive. We also show that oxygen vacancies are not thermodynamically stable at low temperatures, but will form at the high temperatures and low oxygen partial pressures typically used during synthesis of the material.
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| 28. |
- Svensson, Henrik, 1982, et al.
(författare)
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Influence of Pt on the metal-oxide interface during high temperature oxidation of NiAl bulk materials
- 2009
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 51:3, s. 539-546
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of the investigation was to study the mechanisms improving oxide scale adherence in Pt-free and Pt-rich beta-NiAl materials during high temperature oxidation. Theoretical density-functional theory (DFT) calculations were used to calculate the work of separation of the beta-NiAl(111)/alpha-Al(2)O(3)(0001) metal-oxide interfaces, in pure NiAl and Pt-rich NiAl materials. The experimental work was focused on the studies of the metal-oxide interface, its development and morphology. Based on the theoretical and experimental results a complementary picture is presented for a better understanding of the Pt effect on the oxidation and oxide adhesion. It is shown that the interfacial bonding is decreased with addition of Pt to beta-NiAl. The beneficial effect of Pt on the adhesion energy is attributed to the enhancement of contact areas between the oxide and the metal. The influence of Pt on the diffusion of Al and the formation of interfacial voids is also discussed.
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| 29. |
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| 30. |
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| 31. |
- Wahnström, Göran, 1955
(författare)
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Diffusion in Solids
- 2005
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Ingår i: Handbook of Materials Modeling, Fundamental Models and Methods (Springer).
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 32. |
- Zhang, Qianfan, et al.
(författare)
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Path integral treatment of proton transport processes in BaZrO3.
- 2008
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Ingår i: Physical review letters. - 0031-9007 .- 1079-7114. ; 101:21
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Tidskriftsartikel (refereegranskat)abstract
- Nuclear quantum effects on proton transfer and reorientation in BaZrO3 is investigated theoretically using the ab initio path-integral molecular-dynamics simulation technique. The result demonstrates that adding quantum fluctuations has a large effect on, in particular, the transfer barrier. The corresponding rates and diffusion coefficient are evaluated using the path-centroid transition state theory. In contrast with what is found assuming classical mechanics for the nuclear motion, the reorientation step becomes rate limiting below 600 K.
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| 33. |
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| 34. |
- Östberg, Gustaf, 1971, et al.
(författare)
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Effect of Sigma2 grain boundaries on plastic deformation of WC-Co cemented carbides
- 2006
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Ingår i: Mat. Sci. and Eng. A.. ; 416, s. 119-
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Tidskriftsartikel (refereegranskat)abstract
- Cutting inserts of WC–6 wt.% Co were investigated before and after plastic deformation. The deformation tests were performed with a turning operation under realistic, yet controlled, conditions. SEM studies showed that after deformation the WC skeleton structure had broken up and thin lamellae of binder phase had formed in less than 10% of the grain boundaries. Ab initio calculations showed that ∑2 twist WC/WC boundaries have a high work of separation and the interface energy is not lowered by forming two Co/WC boundaries. Electron backscattered diffraction (EBSD) measurements showed an apparent increase in the relative occurrence of ∑2 boundaries due to poor indexing of the deformed material. The accumulation of dislocations in general boundaries is held responsible for the difficulty in identifying them compared with ∑2 boundaries. At the ∑2 tilt boundary the grains have the glide planes {1 0 -1 0} in common and at the ∑2 twist boundary there are three glide planes intersecting, which facilitate dislocation movements across the ∑2 boundaries.
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| 35. |
- Östberg, Gustaf, 1971, et al.
(författare)
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Effect of TaC on plastic deformation of WC-Co and Ti(C,N)-WC-Co
- 2006
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Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368. ; 24:1-2, s. 145-154
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Tidskriftsartikel (refereegranskat)abstract
- The plastic deformation resistance of metal cutting inserts made from a WC–Co cemented carbide, a Ti(C, N)–WC–Co cermet and corresponding materials with additions of TaC has been studied. The cermets were produced with both high and low carbon activity, resulting in a total of six materials. Ab initio calculations of some WC/WC grain boundary geometries suggest that both Co and Ta segregate substitutionally to the boundary and improve the grain boundary strength when substituting carbon. However, only Co segregation was found experimentally, probably due to (Ta, W)C formation. Plastic deformation tests were performed with a turning operation under controlled conditions. For the WC–Co, the addition of Ta had a positive effect for lower cutting speeds but at higher speeds the effect was negative. Three-point bending tests indicated a beneficial effect of Ta in WC–Co, which was also confirmed by internal friction (IF) measurements. However, after thermal cycling, the effect of Ta could be smaller, or even negative. The Ta cermet produced with low carbon activity exhibited a better plastic deformation resistance during cutting but no apparent effects of Ta could be seen either in IF measurements or in three-point bending tests of the cermets. However, a correlation was found between plastic deformation during turning and IF spectra. In the cermet materials, binder phase lamella formation promotes grain boundary sliding at high temperatures.
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| 36. |
- Östberg, Gustaf, 1971, et al.
(författare)
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Mechanisms of plastic deformation of WC-Co and Ti(C,N)-WC-Co
- 2006
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Ingår i: Int. J. Refract. Hard Mater.. - : Elsevier BV. - 0263-4368. ; 24:1-2, s. 135-144
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Tidskriftsartikel (refereegranskat)abstract
- The deformation of the cutting edge of inserts made from WC–Co and Ti(C, N)–19%WC–Co upon radial turning was measured as a function of the cutting speed. Ab initio calculations of some grain boundary geometries in WC–Co indicated that Co segregates to WC/WC grain boundaries in submonolayer proportions and increases their strength and resistance to Co infiltration. This was also confirmed with TEM-EDX. SEM studies showed that during plastic deformation the hard phase skeletons in both materials were partly broken up and infiltrated by binder phase. TEM observations showed a considerable deformation of the WC grains and some infiltrated grain boundaries were facetted along low energy planes. In the cermet, no deformation of the hard phase grains could be seen. The plastic deformation at high temperatures observed by three-point bending corresponds to a number of relaxation processes, detected by internal friction, related to grain boundary sliding. Both materials deform by grain boundary sliding, accommodated in the cermet by Co diffusion and in the WC–Co by deformation of the hard phase and diffusion of Co.
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