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[en] The goal of this theoretical study is to illustrate the potential of three different MgB_2 tapes, developed by Columbus Superconductors, for application in cylindrical coils. First, the distribution of critical currents and electric fields of individual turns is compared when the winding of the model coil is made with tapes having different I_c(B) and anisotropy values. Second, the influence of the winding geometry on basic parameters of cylindrical coils which consist of a set of pancake coils, such as critical current I_c_m_i_n, central magnetic field B_0 and stored energy E, is analysed. The winding geometry of the coils, i.e. the outer winding radius and the coil length, with the same inner winding radius, was changed from a disc shape to a long thin solenoid in such a way that the overall tape length was held constant, and considered as a parameter. Finally, the winding cross-section of the coil is optimized with respect to the constant tape length in order to reach the maximum central field. The results of calculations show that for a given overall tape length and inner winding radius there exists only one winding geometry which generates the maximum central field. The overall tape length, as a parameter, is changed in a broad range from 500 m to 10 km. All calculations were performed using the experimental data measured at 20 K while the effect of the anisotropy in the I_c(B) characteristic of the short samples is taken into account. (paper)
[en] In this paper we carry out a direct comparison between transport and superconducting properties-namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field-of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAsO1-xFx with the largest Tc and the largest anisotropy and Fe1+yTe1-xSex, with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAsO1-xFx series, we find that a single-band description allows us to extract an approximate estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of the Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data for the Fe1+y(Te1-x, Sex) series exhibit a strongly compensated behaviour, which can be described only within a multiband model. In the Fe1+y(Te1-, Sex) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: on one hand it dopes electrons in the system and on the other hand it introduces localized magnetic moments, responsible for Kondo like scattering and likely pairbreaking of Cooper pairs. Hence, Fe excess also plays a crucial role in determining superconducting properties such as the Tc and the upper critical field Hc2. The huge Hc2 values of the Fe1+yTe1-xSex samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO1-xFx samples. Hence, magnetic scattering by excess Fe seems to drive the system in the dirty regime, but its detrimental pairbreaking role seems not to be as severe as predicted by theory. This issue has yet to be clarified, addressing the more fundamental issue of the interplay between magnetism and superconductivity.
[en] Superconducting epitaxial FeSe0.5Te0.5 thin films are prepared on SrTiO3(001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy are studied with different experimental techniques: x-rays diffraction, reflection high energy electron diffraction, scanning tunneling microscopy and atomic force microscopy. The substrate temperature during the deposition is found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate, without evidence of any other orientation. Moreover, such films exhibit a metallic behavior over the whole measured temperature range and the critical temperature is above 17 K, which is higher than the target value.
[en] A substantially different behaviour was observed between MgB2 wires produced either by in situ or ex situ processing after applying the recently developed technique of cold high pressure densification (or CHPD). In contrast to in situ wires, where densification at 1.5 GPa on binary and ternary alloyed in situ MgB2 wires causes an enhancement of mass density and a strong enhancement of Jc, ex situ wires up to 2 GPa show only a negligible enhancement of the MgB2 mass density, while a considerable enhancement of Jc is still observed. In both cases, this reflects an enhancement of grain connectivity, however in ex situ wires, the enhancement of Jc is connected to the partial disruption of the oxide layer around each MgB2 powder particle, in contrast to the enhancement of Jc in situ wires, which is correlated to a smaller void fraction and a larger contact area between neighbouring grains. It is well known that Jc of ex situ wires decreases after longer exposition times when exposed to air prior to annealing; after several months Jc falls to values ≤50% of the original value. After cold pressing, we have found that Jc of the same wire exceeded the original values, even after exposing the unreacted wire for >1 year to air. A lower electrical resistivity is measured on pressed ex situ MgB2 wires, which confirms the improvement of grain connectivity due to the breakage of the oxide layers. Thus, the application of high pressure at room temperature allows us to recover the values of Jc for ex situ wires even after they have been degraded either by low quality MgB2 precursors or by prolonged ageing of the already formed wire prior to the final heat treatment. The limits of the Jc enhancement in ex situ wires are discussed. (paper)
[en] GdSr2RuCu2O8 oriented films have been deposited on SrTiO3(100) substrate by d.c. sputtering. Films deposited in pure oxygen atmosphere have shown c-axis oriented GdSr2RuCu2O8 phase after an appropriate ex-situ thermal treatment in oxygen atmosphere. X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry have been used to check the crystal structure, the morphology and the composition of the samples. The investigations have been performed before and after the oxygen annealing pointing out an increased structural order.The samples have been characterized magnetically in the range from 1.9 to 400 K. A selected sample has exhibited a possible ferromagnetic transition with a Curie temperature of about 150 K. All the samples measured by transport measurements show a semiconducting behaviour down to 4.2 K. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
[en] Full text: Magnetic nanoparticles embedded in multi walled carbon nanotubes display nowadays a continuous increasing technological interest due to their possibility to be used in magnetic storage devices, medical imaging technologies, quantum computing and other industrial applications as magnetic sensors and permanent magnets. We present here the synthesis of multi walled carbon nanotubes (MWCNT) grown from magnetic nanoparticles used as catalyst. The starting magnetic nanoparticles were synthesized according to the method developed by Hou and Gao  and the MWCNT obtained in a catalytic micro reactor through the complete decomposition of methane as CH4(g) → C (nano) + 2H2(gas) Extended physical characterisation was performed on MWCNT with nickel nanoparticles. TEM analysis indicated a wide size distribution of particles centered around two mean values of about 4 and 9 nanometers. DC magnetic measurements gave evidence of superparamagnetic behaviour of the system above the blocking temperature TB , i.e. the scaling of the magnetisation vs. H/T and the fit by a double Langevin equation confirming thus the bimodal size distribution already observed by TEM. Below TB = 68 K , obtained by the temperature dependence of the coercive field Hc, the system has the hysteretic behaviour typical of a ferromagnet, confirmed also by the field cooled (FC) and zero field cooled (ZFC) measurements at different applied magnetic fields. Preliminary results on MWCNT embedded with Sm2Co17 nanoparticles will be also presented and compared. References:  Y. Hou, S.Gao, J. Mater.Chem. 13 (2003) 1510  F.C.Fonseca, G.F.Goya, R.F.Jardim, R.Muccillo, N.L.V. Carreno, E.Longo, E.R.Leite, Phys.rev.B 66 (2002) 1044
[en] We study the effect of synthesis temperature on the phase formation in nano(n)-SiC added bulk MgB2 superconductor. In particular we study: lattice parameters, amount of carbon (C) substitution, microstructure, critical temperature (Tc), irreversibility field (Hirr), critical current density (Jc), upper critical field (Hc2) and flux pinning. Samples of MgB2 + (n-SiC)x with x = 0.0, 0.05 and 0.10 were prepared at four different synthesis temperatures, i.e. 850, 800, 750 and 700 deg. C with the same heating rate and holding time of 10 deg. C min-1 and 2.5 h, respectively. We found 750 deg. C to be the optimal synthesis temperature for n-SiC doping in bulk MgB2 in order to get the best superconducting performance in terms of Jc, Hc2 and Hirr. Carbon substitution enhances the Hc2 while the low temperature synthesis is responsible for the improvement in Jc due to the smaller grain size, defects and nano-inclusion induced by incorporation of C into the MgB2 matrix, which is corroborated by HRTEM (high-resolution transmission electron microscopy) results.
[en] SmFeAsO and the isostructural superconducting SmFeAs(O0.93F0.07) phase were prepared and characterized by means of Rietveld refinement of x-ray powder diffraction data, scanning electron microscope observation, transmission electron microscope analysis, and resistivity and magnetization measurements. Sintering treatment strongly improves the grain connectivity, but, on the other hand, induces a competition between the thermodynamic stability of the oxy-pnictide and Sm2O3, thus affecting the sample purity. In the pristine sample both magnetization and resistivity measurements clearly indicate that two different sources of magnetism are present: the former related to Fe ordering at T∼140 K and the latter due to the Sm ions that order antiferromagnetically at low temperature. The feature at 140 K disappears in the F-substituted sample, and a superconducting transition appears, at low temperatures. The magnetoresistivity curves of the F-substituted sample probably indicate very high critical field values
[en] The offshore wind market demands a higher power rate and more reliable turbines in order to optimize capital and operational costs. The state-of-the-art shows that both geared and direct-drive conventional generators are difficult to scale up to 10 MW and beyond due to their huge size and weight. Superconducting direct-drive wind generators are considered a promising solution to achieve lighter weight machines. This work presents an innovative 10 MW 8.1 rpm direct-drive partial superconducting generator using MgB_2 wire for the field coils. It has a warm iron rotor configuration with the superconducting coils working at 20 K while the rotor core and the armature are at ambient temperature. A cooling system based on cryocoolers installed in the rotor extracts the heat from the superconducting coils by conduction. The generator’s main parameters are compared against a permanent magnet reference machine, showing a significant weight and size reduction. The 10 MW superconducting generator concept will be experimentally validated with a small-scale magnetic machine, which has innovative components such as superconducting coils, modular cryostats and cooling systems, and will have similar size and characteristics as the 10 MW generator. (paper)
[en] We studied the effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO by means of point-contact Andreev-reflection spectroscopy. The results show that the SmFe1−xRuxAsO0.85F0.15 system keeps a multigap character at least up to x = 0.50, and that the gap amplitudes Δ1 and Δ2 scale almost linearly with the local critical temperature TcA. The gap ratios 2Δi/kBTc remain approximately constant only as long as Tc ≥ 30 K, and increase dramatically when Tc decreases further. This trend seems to be common to many Fe-based superconductors, irrespective of their family. Based on first-principle calculations of the bandstructure and of the density of states projected on the different bands, we show that this trend, as well as the Tc dependence of the gaps and the reduction of Tc upon Ru doping, can be explained within an effective three-band Eliashberg model as being due to a suppression of the superfluid density at finite temperature that, in turn, modifies the temperature dependence of the characteristic spin-fluctuation energy. (paper)