[en] We have studied by in situ neutron diffraction the crystallization process of the Bi2212 phase sheathed in Ag-tapes. During the first heating of the tape, there is a grain reorientation of the silver tape, in such a way that the (1 1 1) planes are orientated perpendicularly to the tape. After melting of the 2212 phase at 880 deg. C and the peritectic decomposition, the phase is quickly reformed during the cooling between 880 deg. C and 750 deg. C. During the long annealing process at 890 deg. C, we have observed a small change in the c parameter, that can be attributed to an oxidation process

[en] Dense Bi2223 superconductors have been successfully formed by hot stacking-forging process (HSF). Neutron diffraction measurements were used to investigate the bulk textures of HSF-Bi2223 samples. Angular dependence of transport critical current density, J_c values were measured at various temperatures and different applied magnetic fields. Several textured pieces were hot-stacked. This procedure leads to an increase of both the sample thickness and the nominal engineering critical current (I_c), favourable hints for use of textured-Bi2223 in power generation supplies

[en] Polycrystalline sinters of YBa₂Cu₃O_7-δ (δ≤0.1) superconductor have been irradiated by 3.5 GeV Xe ions and investigated by high resolution electron microscopy (HREM). Discontinuous tracks due to the high energy density deposited into electronic excitation and ionization have been characterized. They exhibit a dotted microstructure, similarly to that previously described for magnetic insulators. Such tracks seem to act as centers of nucleation and growth of defects through the Cu and BaO layers, perpendicularly to the c axis, leading either to local contrast modulations with a doubling of the ''a'' parameter or to the amorphization of the slices when a critical density of defects is exceeded. The appearance of these amorphous layers should be responsible for the rapid T_c decrease for fluences higher than 3x10¹² Xe/cm². (orig.)

[en] The addition of silver to YBa₂Cu₃O_7-δ (Y123) is well known to decrease its decomposition temperature. This study demonstrates the success in welding large Y123 textured samples with Ag doped Y123 junctions. Attempts to weld two melt textured domains of Y123 by a 1 mm thick layer of Ag doped Y123 are reported. The crucial importance of the initial Y211 excess content in the spacer material is pointed out. Microstructural observations, trapped field measurements and transport critical current density measurements prove that for a given Y211 amount a very high joint quality can be obtained. A transport critical current density as high as 10 kA/cm² is registered through the junction at 77 K and 5 T when the applied field is parallel to the (a,b) planes

[en] Epilayers of yttrium iron garnet Y₃Fe₅O₁₂ on a (111)-Gd₃Ga₅O₁₂ substrate were irradiated with 50 MeV ³²S, 50 MeV ⁶³Cu, and 235 MeV ⁸⁴Kr ion beams. Film thicknesses were always smaller than the mean projected ranges of the incoming ions to avoid implantation effects in the layers. Dc-resistivity and saturation magnetization of ion-bombarded films were then measured at room temperature as a function of ion fluence ranging between 10¹¹ and 10¹⁶ ions cm^-2. Resistivity was found to decrease when fluence is increased above a threshold (≤10¹³ ions cm^-2). The threshold fluence increases as the ion energy loss is reduced. ⁸⁴Kr resistivity data level off above 3x10¹² ions cm^-2 after a two-order-of-magnitude decrease, whereas ⁶³Cu and ³²S data decrease by seven orders of magnitude according to a nearly parabolic law versus fluence without any saturation up to 10¹⁶ ions cm^-2. The saturation of damage observed with ⁸⁴Kr bombardment is interpreted by the overlap of paramagnetic disordered track cores, while in the case of ⁶³Cu and ³²S bombardment, such a picture is not valid. These two different behaviours might be interpreted by a change in the morphology of the extended defects produced by electronic excitations above a threshold of electronic stopping power. (orig.)

[en] The Grenoble High Magnetic Field Laboratory was built to meet the requirements of scientists for experiments implying the use of very high magnetic fields. Currently, the facility operates for over 5000 h a year and thanks to its most powerful magnets, it allows the production of steady magnetic fields up to 30 T. Scientists have access to the high magnetic field facility provided they have submitted a proposal for experiments in high magnetic fields and have obtained the Program Committee's approval to carry out their research project. As illustration of the use of our specific instrumentation, we present a study of the behavior of the critical currents of superconducting tapes, performed in collaboration between CNRS Grenoble, France and NIN Xi'an, China

[en] A superconducting fault current limiter using YBCO single domains requires long elements cut along the ab-planes or along the c-axis. These elements have to be assembled in series in order to produce a limited fault current after the quench to the resistive state under the network voltage. The superconductor total length is much smaller with c-axis elements because their high resistivity after transition contributes to a very high electric field during the current limitation. The critical current density has to be decreased in order to be sure that the Joule effect related to the limiting current does not increase too much the material temperature before its interruption by a current breaker. The inhomogeneously oxygenated YBCO elements limit the current even if the only surface layer is superconducting and produce an electric field up to 5000 V/m. A homogeneous oxygenation without macrocrack is obtained when the thickness along the ab-planes is thin. Several elements have been tested by applying a 20 ms duration fault current in a liquid nitrogen bath at T=77 K and above

[en] Since the discovery of high T_c superconductivity in ternary copper oxides many studies of irradiation damage in these materials have been performed. By contrast with electrons and fast neutrons which produce defect by elastic collisions, swift heavy ions (E>1 GeV) induce changes by inelastic collisions because of electronic stopping powers about 2000 times higher than nuclear stopping powers over 90% of the ion path. The purpose of this paper is to report on recent experimental results obtained for high T_c copper oxides irradiated by 2.9 GeV-Krypton ions and 3.5 GeV-Xenon ions. (author)

[en] We study the limitation possibilities of bulk Bi high T_c materials. For this we test these materials with AC or DC currents above their critical currents. We study particularly the evolution of the voltage with time or with current. The material, the value of the current and the time duration play important parts. For sintered Bi samples the voltage depends only on the current even for values much larger than the critical current. With textured samples the V(I) curves shows an hysteretic behaviour due to a warming up. The textured materials are more interesting than sintered ones in terms of required volume for the current limitation. In both cases the superconductors are in a dissipative state but not in the normal state. This state is nevertheless reached if the dissipated energy inside the sample is sufficient. We have tried to apply a magnetic field on the samples in order to trigger a more effective limitation. The voltage increases but with a limited effect for currents much higher (3-4 times) than the critical zero field current. We think that the dissipative state is due mainly to the grain boundaries which become resistive above the critical current. (orig.)

[en] A detailed analysis of the bismuth, lead and thallium cuprates is presented. It is shown that the 6s² lone pair of Bi(II), Pb(II) and Tl(I) plays a role in the incommensurability of the structure but is not linked to superconductivity. In the case of thallium cuprates it is observed that the mixed valence Tl(II) | Tl(I) induces a complex non stoichiometry and that Tl(I) destroys superconductivity