An analysis of the role of non-governmental organizations working with refugees in Durban.

Thesis (M.A.)-University of KwaZulu-Natal, Durban, 2007.No abstract available

Modelling of major phases formation during solidification of Ferro-Silicon-Magnesium

Ductile cast iron, also known as nodular cast iron, is a graphite-rich cast iron with high impact and fatigue resistance, due to its nodular graphite inclusions. Ductile cast iron is produced by incorporating additives (often FeSi alloys) to the iron base metal at different production steps to obtain the desired graphite shape. A crucial step is the addition of Magnesium to promote the spheroidization of the graphite. The most common method is by adding crushed and sized Ferro-Silicon-Magnesium (FSM). The alloy composition, microstructure, and sizing are assumed to affect the key parameters of this reaction, namely, reactivity, recovery, and slag formation. Therefore, the study of the solidification of FSM is important to understand and predict its performance at the foundries. The present work aims at understanding and predicting numerically the formation of the major phases during the solidification process. Two approaches have been used: thermodynamic calculations through Thermo-Calc solver and phase field modelling using MICRESS. The models have been calibrated by comparison with advanced statistical characterization of the microstructure. The results indicate a competitive growth of the major phases and transformation of phases in solid state that can be emulated by the model.publishedVersio

«Man må regne med å dø når man er over 60 år». En KLoK-oppgave om utfylling av dødsattest og unaturlige dødsfall

Gruppen har valgt utfylling av Legeerklæring om dødsfall/Melding om unaturlig dødsfall (dødsattest) som tema i denne prosjektoppgaven, nærmere bestemt delen som omhandler registrering og melding av unaturlige dødsfall. Problemstillingen lyder som følger: Hvordan endre dødsattesten for å forbedre registrering ,og melding til politiet, av unaturlige dødsfall? Kunnskapsgrunnlaget for oppgaven baseres i stor grad på det norske lovverket, i tillegg også publiserte forskningsresultater fra Akershus Universitetssykehus (Ahus). Kvalitetsforbedringsprosjektet finner sted på Ahus og skal gjennomføres ved å gjøre endringer i dødsattestskjemaet i DIPS. Gruppen planlegger å benytte seg av en resultatindikator der man sammenligner andel unaturlige dødsfall som blir registrert og meldt til politiet før og etter prosjektperioden. Vi vurderer vårt tiltak til å være gjennomførbart. Hvis tiltaket fungerer på Ahus, bør det også innføres på nasjonalt plan

Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO₃ perovskites

We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size/tolerance factor, epitaxial strain, ferroelectricity, and magnetism in the perovskite manganite series, AMnO3 (A = Ca2+, Sr2+, Ba2+). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because Coulomb repulsion associated with cation off-centering is reduced. Interestingly, the presence of ferroelectric polarization favors ferromagnetic (FM) over antiferromagnetic (AFM) ordering due to suppressed AFM superexchange as the polar distortion bends the Mn–O–Mn bond angles away from the optimal 180°. Intriguingly, we find that polar distortions compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases; conversely the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for FM than AFM orderings of the same structure type. Our findings suggest a rich and complex phase diagram, in which defect chemistry, polarization, structure, and magnetism can be modified using chemical potential, stress or pressure, and electric or magnetic fields

A first-principles Study of epitaxial Interfaces between Graphene and GaAs

Epitaxial interfaces between graphene and GaAs(111) have been investigated through a first-principles study using density functional theory (DFT). The GaAs(111) surface at the interface is assumed to be 2x2 reconstructed with Ga vacancies. Three different relative phase orientations between the GaAs(111) surface and graphene have been studied; 0deg, 16.1deg and 30deg, respectively, where three translations of the 0deg configuration have been considered. Within the three considered epitaxial phase orientations, the GaAs phase is strained in the (111) plane to accommodate a lattice mismatch of 6.3%, 10% and -8.2% in the 0deg, 16.1deg and 30deg orientations, respectively. Biaxial straining of bulk GaAs has been studied, both in the zincblende (ZB) phase ((111) plane) and in the meta-stable wurtzite (WZ) phase ((0001) plane). The relative stabilities of the two phases are found to depend on the degree of strain. Biaxial straining of GaAs(111) surfaces have also been investigated. The surface reconstruction energy is highly dependent on the degree of strain for GaAs(111)-2x2 reconstructed surfaces. The interactions between GaAs and graphene at the interface have been studied by two different exchange-correlation functionals: The semi-local GGA functional PBEsol, and the van der Waals (vdW) functional optPBE. The latter is found to yield a significantly stronger interaction energy, which signifies a substantial vdW contribution to the interactions at the GaAs/graphene interface. The interaction energy between GaAs and graphene is estimated for the five configurations considered. The estimates vary in the range 0.24-0.31 J/m^2. The strongest interaction energy is found for the 30deg configuration

Percolation Transition in Hole-Conducting Acceptor-Doped Barium Zirconate

Acceptor-doped perovskite oxides are highly attractive materials for renewable energy applications such as protonic ceramic fuel cells. Cathode materials with mixed protonic and electronic conductivity are then highly desirable. Here, we consider acceptor-doped BaZrO3 as a potential mixed conductor using the density functional theory. We study the effect of the dopant atom on the formation of electronic hole polarons and establish that doping of BaZrO3 with Fe and Y, respectively, leads to completely different polaron configurations, in which the polaron is centered on the dopant when BaZrO3 is Fe-doped and on an oxygen ion when BaZrO3 is Y-doped. These differences are attributed to the difference in covalency between Fe-O and Y-O bonds, with the former having a much more covalent character. The stronger covalency also results in a stronger association of the polaron with Fe than Y. With a large Fe concentration of 25%, the acceptor state is significantly smeared in energy with a near closing of the electronic band gap. At this concentration level, Fe-doped BaZrO3 is thus on the verge of becoming metallic along the Fe-clusters, supporting the experimental finding of percolation conductivity in Fe-doped BaZrO3 [Kim et al., Solid State Ionics 2014, 262, 875]. In contrast, the effect of increased dopant concentration on the electronic structure in Y-doped BaZrO3 is only minor. Our results provide new atomistic insights into hole transport in acceptor-doped BaZrO3 and highlight the importance of the chemical nature of the dopant atom on the material\u27s electronic properties

Microscopic Link between Electron Localization and Chemical Expansion in AMnO3 and ATiO3 Perovskites (A = Ca, Sr, Ba)

The microscopic origin of chemical expansion in perovskite oxides, due to formation of oxygen vacancies accompanied by formal reduction of a 3d transition metal, is studied by first-principles calculations. We compare the II–IV manganite and titanate series, having Ca, Sr, or Ba on the A site. In particular, the effect of electron localization is elucidated by systematically varying the Hubbard U, and we find that the localization behavior is significantly different in the manganites and titanates. The chemical expansion is explicitly calculated for all compounds, and we demonstrate that increasing on-site repulsion (Hubbard U) on the B site in the lattice yields increased chemical expansion in the manganites and reduced chemical expansion in the titanates. The opposite behavior of the manganites and titanates arises from different electrostatic screenings of oxygen vacancies. We show that this can be attributed to differences in electronic energy levels, specifically that Mn–O bonds are more covalent than Ti–O bonds. Fundamental understanding of electronic and crystal chemical origins of the important phenomenon of chemical expansion is required for rational design of oxide materials for energy technology, sensors, and actuators. We hope our analysis will inspire further fundamental studies of other oxides for solid state ionics applications

First-principles study of the effect of (111) strain on octahedral rotations and structural phases of LaAlO3

The structural and electronic response of LaAlO3 to biaxial strain in the (111) plane is studied by density functional theory (DFT) and compared with strain in the (001) plane and isostatic strain. For (111) strain, in-plane rotations are stabilized by compressive strain and out-of-plane rotations by tensile strain. This is an opposite splitting of the modes compared with (001) strain. Furthermore, for compressive (111) strain, in-plane rotations are degenerate with respect to the rotation axis, giving rise to Goldstone-like modes. We rationalize these changes in octahedral rotations by analyzing the VA/VB polyhedral volume ratios. Finally, we investigate how strain affects the calculated band gap, and find a 28% difference between the strain planes under 4% tension. This effect is attributed to different A-site dodecahedral crystal field splitting for (001) and (111) strains

Strain-phonon coupling in (111)-oriented perovskite oxides

Strain-phonon coupling, in terms of the shift in phonon frequencies under biaxial strain, is studied by density functional theory calculations for 20 perovskite oxides strained in their (111) and (001) planes. While the strain-phonon coupling under (001) strain follows the established, intuitive trends, the response to (111) strain is more complex. Here we show that strain-phonon coupling under (111) strain can be rationalized in terms of the Goldschmidt tolerance factor and the formal cation oxidation states. The established trends for coupling between (111) strain and in-phase and out-of-phase octahedral rotational modes as well as polar modes provide guidelines for rational design of (111)-oriented perovskite thin films

Coupling and competition between ferroelectricity, magnetism, strain, and oxygen vacancies in AMnO3 perovskites

We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size/tolerance factor, epitaxial strain, ferroelectricity, and magnetism in the perovskite manganite series, AMnO3 (A = Ca2+, Sr2+, Ba2+). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because Coulomb repulsion associated with cation off-centering is reduced. Interestingly, the presence of ferroelectric polarization favors ferromagnetic (FM) over antiferromagnetic (AFM) ordering due to suppressed AFM superexchange as the polar distortion bends the Mn–O–Mn bond angles away from the optimal 180°. Intriguingly, we find that polar distortions compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases; conversely the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for FM than AFM orderings of the same structure type. Our findings suggest a rich and complex phase diagram, in which defect chemistry, polarization, structure, and magnetism can be modified using chemical potential, stress or pressure, and electric or magnetic fields