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Synthesis and characterisation of hexaferrites with magnetoplumbite structure for application to novel hybrid structural fibresThompson, Simon January 2004 (has links)
No description available.
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Stable strontium isotope fractionation in marine and terrestrial environmentsStevenson, Emily Isabel January 2012 (has links)
The work reported in this thesis applies a new isotope tracer, stable strontium isotopes (δ<sup>88/86</sup>Sr), to address questions concerning changes in global climate that occur in response to continental weathering processes, and to constrain the modern marine geochemical Sr cycle. Stable Sr isotopes are a relatively new geochemical proxy, and as such their behavior needs to be understood in differing forms of marine calcium carbonate, the archives from which records of past stable Sr variability in the oceans can be constructed. Foraminifera, coccoliths and corals (both aragonite and high Mg calcite) acquire δ<sup>88/86</sup>Sr values lighter than that of modern day seawater, (approximately 0.11, 0.05, 0.2 and 0.19 ‰ lighter than seawater at ~25°C respectively) providing a measureable offset which can be used to constrain the modern Sr outputs from the ocean and provide a better understanding of the modern Sr cycle. Using foraminifera as a sedimentary archive the first marine δ<sup>88/86</sup>Sr record of seawater over the last two glacial cycles has been constructed, and used to investigate changing carbonate input and output over this 145 kyr period. Modelling of the large excursion of δ<sup>88/86</sup>Sr to heavier values during Marine Isotope Stage (MIS) 3, reveals that this is more likely to be due to local changes in seawater or post-depositional alteration, rather then whole ocean changes. In the terrestrial environment δδ88/86<sup>Sr</sup> has been measured in the dissolved load of rivers from the Himalaya. It is found that, in general, rivers draining carbonate catchments possess lighter isotopic δ<sup>88/86</sup>Sr values than those from rivers draining silicates. Covariations of either δ88/86Sr vs. δ30Si or δ<sup>88/86</sup>Sr vs. 1/[Sr] can be used to distinguish between rivers draining different catchment areas.
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Synthesis, characterization and investigation on the magnetic and electronic structure of strontium iron oxides / Synthèse, caractérisation et étude des propriétés magnétiques et de la structure électronique d’oxydes de fer et de strontiumLiu, Qiang 21 March 2013 (has links)
Les diverses perovskites de strontium-fer présentent de très fortes corrélations entre la structurecristalline et les phénomènes d’ordre de lacunes d’oxygènes, de charge, de spin et d’orbitales. Danscette thèse, nous avons réalisé une étude systématique des relations entre les ordres de charges etles ordres de spins selon les différents environnements cristallographiques rencontrés pour lescations Fe3+ et Fe4+ dans la phase Sr4Fe4O11, pour le cation Fe3+ dans les phases Sr3Fe2O6 et Sr2Fe2O10et pour le cation Fe2+ dans la phase SrFeO2. Les synthèses des phases polycristallines furent réaliséesvia des voies « solide » ou « sol-gel » en complément de la synthèse de cristaux préparés à l’aide d’unfour à image à fusion de zone verticale pour les phases Sr4Fe4O11 et Sr2Fe2O10. La qualité cristalline etchimique des oxydes fut contrôlée par diffraction de rayons X sur poudre et spectroscopieMössbauer alors que l’étude de la structure électronique de chaque phase a été réalisée à l’aide despectroscopie d’absorption de rayons X. Finalement, les structures magnétiques des phases Sr3Fe2O6et Sr4Fe4O11 sont aussi présentées. / The relationship of the crystallographic, magnetic, and electronic structure have long been of highinterest in research. Strontium iron oxides have structural, charge, spin, and orbital degrees offreedom, and thus give rich information to study for the condensed matter scientists. In this thesis,we have systematically studied the strontium iron oxides based on the freedom of the iron charge:Fe3+ and Fe4+ mixed valence compound SrFeO2,75 , Fe3+ compound Sr3Fe2O6 and SrFeO2,5 with differentcoordination around Fe3+ and Fe2+ compound SrFeO2. The synthesis of the polycrystalline compoundsare through either solid state reaction or sol-gel method. Single crystals of SrFeO2,75 and SrFeO2,5have been prepared by floating zone furnace. The purity of all the compounds is checked by lab x-raydiffraction and Mössbauer spectroscopy. Electronic structures have been studied by x-ray absorptionspectroscopy for all these compounds. Special efforts have been used to investigate the magneticstructure of SrFeO2,75 and Sr3Fe2O6 .
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