Effect of annealing on structure and superconducting properties in Fe(Se,Te)

Abstract In this paper, the effect of post synthesis annealing treatments on a Fe(Se,Te) polycrystalline material is evaluated and discussed. The samples have been obtained via melting route. The material has been subjected to a high-temperature annealing treatment, carried out for 45 h at 680 °C. The role of the cooling step was investigated comparing samples obtained after a controlled cooling or after quenching in liquid nitrogen. From a morpho-structural point of view, the annealing treatment improves homogeneity, with respect to pristine samples, and influences secondary phase precipitate morphology. Regarding superconducting properties, a key role of the cooling procedure is assessed: controlled cooling leads in fact to a significant improvement of high field behaviour with respect to the melted material, while quenched samples are characterized by a worsening of the superconducting properties. Despite the overall worsening, however, the quenched samples show evidence of the presence of superconducting phases characterized by a remarkably high critical temperature (Tc > 18 K), observed for these materials only in films or under pressure

Effect of oxygen contamination on densification of Fe(Se,Te)

Abstract The optimization of sintering behaviour of iron chalcogenides superconducting materials is mandatory to enhance their critical current density, in order to pursuit their application in the production of superconducting wires. In this context it has been investigated here the effect of oxygen contamination on the material densification, considering the issues related to industrial oxygen-free isolated production lines. Our results show that the densification process is negatively affected by oxygen contamination. However, despite the difference in density, all sintered samples are characterized by similar structural and morphological features, and show comparable electrical and magnetic properties, with low critical current densities (Jc<103 A/cm2). These results suggest that densification is not the key limiting factor in these conditions, and that grain boundary or misorientation factors may play a greater role in limiting the performance of sintered iron chalcogenides superconductors

Nature of the 5f states in actinide metals

Actinide elements produce a plethora of interesting physical behaviors due to the 5f states. This review compiles and analyzes progress in understanding of the electronic and magnetic structure of the 5f states in actinide metals. Particular interest is given to electron energy-loss spectroscopy and many-electron atomic spectral calculations, since there is now an appreciable library of core d -> valence f transitions for Th, U, Np, Pu, Am, and Cm. These results are interwoven and discussed against published experimental data, such as x-ray photoemission and absorption spectroscopy, transport measurements, and electron, x-ray, and neutron diffraction, as well as theoretical results, such as density-functional theory and dynamical mean-field theory.Comment: 136 pages in Word format, 29 Figures; Accepted to Reviews of Modern Physic

Optical reflectivity measurements on thorium metal samples

L'étude systématique de la réflectivité optique a été entreprise sur des échantillons de thorium dont la surface a été préparée par polissage mécanique et électrolytique ou qui ont été utilisés tels quels après obtention par le procédé van Arkel. Les résultats des mesures de réflectivité optique sont comparés avec les résultats publiés. Une augmentation de la rugosité de la surface provoque une diminution de la réflectivitié et induit probablement des plasmons de surface mis en évidence par le minimum de réflectivité mesuré à 4 eV. Les meilleurs résultats n'ont été obtenus qu'avec des surfaces électropolies, cependant, dans des conditions bien déterminées, le procédé van Arkel conduit à de bons résultats.Thorium samples, the surfaces of which were mechanically polished, electropolished and as grown from the van Arkel process, were systematically studied by optical reflectivity measurements. These optical data were compared with published measurements showing significant discrepancies. Increasing surface roughness was found to lower the reflectivity considerably and probably to induce surface plasmons indicated by a deep minimum in reflectivity at 4 eV. Best reflectivity data were obtained only for electropolished surfaces. Well chosen conditions for the van Arkel process give also good results

Hydrogen Production by Using Manganese Ferrite: Evidences and Benefits of a Multi-Step Reaction Mechanism

Hydrogen production by water splitting with MnFe2O4/Na2CO3 system was studied at 973 K. An intermediate phase, resulting from decarbonatation of MnFe2O4/Na2CO3 mixture in inert atmosphere, proved to be effective in hydrogen reduction from water with stoichiometric yield. The presence of a highly reactive intermediate phase suggests the feasibility of a high efficiency, three-step, thermochemical cycle for hydrogen production. In fact, the possibility of obtaining CO2 separately from the gases mixture dramatically enhances process efficiency