| 1. |
- Ahlberg Helgee, Edit, 1986, et al.
(författare)
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Adsorption of metal atoms at a buckled graphene grain boundary using model potentials
- 2016
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226 .- 2158-3226. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Two model potentials have been evaluated with regard to their ability to model adsorption of single metal atoms on a buckled graphene grain boundary. One of the potentials is a Lennard-Jones potential parametrized for gold and carbon, while the other is a bond-order potential parametrized for the interaction between carbon and platinum. Metals are expected to adsorb more strongly to grain boundaries than to pristine graphene due to their enhanced adsorption at point defects resembling those that constitute the grain boundary. Of the two potentials considered here, only the bond-order potential reproduces this behavior and predicts the energy of the adsorbate to be about 0.8 eV lower at the grain boundary than on pristine graphene. The Lennard-Jones potential predicts no significant difference in energy between adsorbates at the boundary and on pristine graphene. These results indicate that the Lennard-Jones potential is not suitable for studies of metal adsorption on defects in graphene, and that bond-order potentials are preferable.
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| 2. |
- Ahlberg Helgee, Edit, 1986
(författare)
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Atomistic Simulation of Interfaces: Proton transport across BaZrO3 grain boundaries
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to the negative environmental effects of fossil fuels it is necessary to develop technology that may reduce or eliminate the need for oil and coal. Fuel cells are highly important in this context as they provide an efficient way of converting chemical energy into electrical energy. However, the development is hampered by a lack of electrolyte materials able to function at temperatures high enough to enable use of hydrocarbon fuels, yet low enough to avoid the wear on component materials caused by high operating temperatures. Solid oxide proton conductors are found to have several of the characteristics of a good electrolyte material in this temperature range, but increasing the conductivity to the level needed in practical applications remains a challenge. The aim of this thesis is to elucidate microscale phenomena that affect the performance of proton-conducting oxides. The material under investigation is BaZrO3, which is regarded as a promising electrolyte material due to its chemical stability and high grain interior conductivity. However, the grain boundaries in the material are highly resistive and lower the total conductivity. The cause of this high grain boundary resistivity has been investigated using atomistic simulations and thermodynamic modelling. Particular attention is devoted to the role of defect segregregation to the grain boundaries.From atomistic simulations it has been found that positively charged defects such as oxygen vacancies and protons segregate to the grain boundaires of BaZrO3. The accumulation of positive charge in the grain boundaries creates a potential barrier and leads to depletion of positive mobile defects from the surrounding region, impeding transport across the boundary. Thermodynamic models have been used to determine the height of the potential barrier resulting from segregation of positive defects, and the results compare well with experimental findings.
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| 3. |
- Ahlberg Helgee, Edit, 1986, et al.
(författare)
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Diffraction and near-zero transmission of flexural phonons at graphene grain boundaries
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 91:20
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Tidskriftsartikel (refereegranskat)abstract
- Graphene grain boundaries are known to affect phonon transport and thermal conductivity, suggesting that they may be used to engineer the phononic properties of graphene. Here, the effect of two buckled grain boundaries on long-wavelength flexural acoustic phonons has been investigated as a function of angle of incidence using molecular dynamics. The flexural acoustic mode has been chosen due to its importance to thermal transport. It is found that the transmission through the boundaries is strongly suppressed for incidence angles close to 35 degrees. Also, the grain boundaries are found to act as diffraction gratings for the phonons.
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| 4. |
- 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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| 5. |
- Ahlberg Helgee, Edit, 1986, et al.
(författare)
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Scattering of flexural acoustic phonons at grain boundaries in graphene
- 2014
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 90:4
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the scattering of long-wavelength flexural phonons against grain boundaries in graphene using molecular dynamics simulations. Three symmetric tilt grain boundaries are considered: one with a misorientation angle of 17.9 degrees displaying an out-of-plane buckling 1.5 nm high and 5 nm wide, one with a misorientation angle of 9.4 degrees and an out-of-plane buckling 0.6 nm high and 1.7 nm wide, and one with a misorientation angle of 32.2 degrees and no out-of-plane buckling. At the flat grain boundary, the phonon transmission exceeds 95% for wavelengths above 1 nm. The buckled boundaries have a substantially lower transmission in this wavelength range, with a minimum transmission of 20% for the 17.9 degrees boundary and 40% for the 9.4 degrees boundary. At the buckled boundaries, coupling between flexural and longitudinal phonon modes is also observed. The results indicate that scattering of long-wavelength flexural phonons at grain boundaries in graphene is mainly due to out-of-plane buckling. A continuum mechanical model of the scattering process has been developed, providing a deeper understanding of the scattering process as well as a way to calculate the effect of a grain boundary on long-wavelength flexural phonons based on the buckling size.
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| 6. |
- Ahlberg Helgee, Edit, 1986
(författare)
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Significance of grain boundaries for transport phenomena in graphene and proton-conducting barium zirconate
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Grain boundaries can have a significant influence on the properties of polycrystalline materials. When determining the type and extent of this influence it is frequently useful to employ computational methods such as density functional theory and molecular dynamics, which can provide models of the grain boundary structure at the atomistic level. This work investigates the influence of grain boundaries in two different materials, barium zirconate and graphene, using atomistic simulations.Barium zirconate is a proton conducting material with a potential application as a fuel cell electrolyte. However, the presence of grain boundaries has been found to lower the proton conductivity. Here, density functional theory has been used to investigate the segregation of positively charged defects, such as oxygen vacancies and protons, to the grain boundaries. It has been found that both defect types segregate strongly to the grain boundaries, which gives rise to an electrostatic potential that depletes the surrounding region of protons and impedes transport across the grain boundary. A thermodynamical space-charge model has been employed to relate the theoretical results to experimentally measurable quantities.The carbon allotrope graphene has many potential applications in for example electronics, sensors and catalysis. It has also been mentioned as a possible materialfor phononics and heat management applications due to its unique vibrational properties, which give it a high thermal conductivity. Grain boundaries have been found to decrease the thermal conductivity, but they may also provide a method for manipulating the vibrational properties. The work included in this thesis investigates the scattering of long-wavelength flexural phonons, i.e. phonons with polarization vectors pointing out of the graphene plane, at grain boundaries. Grain boundaries in graphene frequently cause out-of-plane deformation, buckling, of the graphenesheet, and it is found that this buckling is the main cause of scattering of longwavelength flexural phonons. Based on this result a continuum mechanical model of the scattering has been constructed, with a view to facilitating the study of systems too large to be modelled by molecular dynamics.
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| 7. |
- Lindman, Anders, 1985, et al.
(författare)
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Comparison of Space-Charge Formation at Grain Boundaries in Proton-Conducting BaZrO3 and BaCeO3
- 2017
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 29:18, s. 7931-7941
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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.
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| 8. |
- Lindman, Anders, 1985, et al.
(författare)
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Oxygen vacancy segregation in grain boundaries of BaZrO3 using interatomic potentials
- 2013
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 230:C, s. 27-31
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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%.
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| 9. |
- Lindman, Anders, 1985, et al.
(författare)
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Theoretical modeling of defect segregation and space-charge formation in the BaZrO3 (210) 001 tilt grain boundary
- 2013
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 252, s. 121-125
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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.
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| 10. |
- Nyman, Joakim, 1975, et al.
(författare)
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Oxygen vacancy segregation and space-charge effects in grain boundaries of dry and hydrated BaZrO3
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:6, s. 061903-
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Tidskriftsartikel (refereegranskat)abstract
- A space-charge model is applied to describe the equilibrium effects of segregation of double-donor oxygen vacancies to grain boundaries in dry and wet acceptor-doped samples of the perovskite oxide BaZrO3. The grain boundary (GB) core vacancy concentrations and electrostatic potential barriers resulting from different vacancy segregation energies were evaluated. Density-functional calculations on vacancy segregation to the mirror-symmetric Σ3 (112) [-110] tilt grain boundary are also presented. Our results indicate that oxygen vacancy segregation can be responsible for the low grain boundary proton conductivity in BaZrO3 reported in the literature.
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