G Grasso

Fabrication and properties of monofilamentary MgB2 superconducting tapes

Nickel-sheathed MgB2 tapes were fabricated by means of the powder-in-tube method with a monocore configuration of the superconductor. The metallic tubes were filled with commercial reacted powders and cold-worked, respectively, by groove rolling, drawing, and rolling to flat tapes of about 4 mm in width and 0.35 mm in thickness. Portions of the manufactured conductors were heat-treated in an argon atmosphere at temperatures up to 940 °C. Conductors were characterized by transport measurements to determine the magnetic field dependence of the critical current density, the irreversibility line, as well as to establish an initial relationship between the thermo-mechanical treatment and the current carrying capacity of the manufactured MgB2 samples. Our optimized samples for the low field operation carry a critical current density of at least 4 × 105 A cm−2 at 4.2 K, 1 T. The main outcome of this study, however, is that the optimal heat treatment condition changes if the MgB2 conductor has to be employed respectively for a low- or high-field operation. Finally, the first successful demonstration of the current carrying capability of longer MgB2 tapes is also reported in this paper.

Transport properties of powder-in-tube processed MgB2 tapes

Abstract Nickel-sheathed magnesium diboride superconducting tapes have been fabricated by the powder-in-tube method from reacted commercially available powders. At 4.2 K and self field, a transport current density value j c of 10 5 A/cm 2 has been reached over long portions of unsintered monofilamentary tapes. The irreversibility lines both for a magnetic field oriented parallel and perpendicular to the MgB 2 superconducting tape have been determined by magnetoresistance measurements, showing some degree of anisotropy. These data are consistent with the X-ray diffraction observation of a slight texturing process of the MgB 2 grains occurring during the cold working procedure.

As-grown magnesium diboride superconducting thin films deposited by pulsed laser deposition

As-grown superconducting MgB2 thin films were deposited by pulsed laser deposition on magnesium oxide and sapphire substrates. Starting from a non-stoichiometric, Mg and B mixed-powder target, we were able to grow the superconducting phase during the film deposition, without any further annealing process. So far, samples grown in the temperature range of 400-450??C, and at an argon buffer pressure of the order of 10-2?mbar turned out to be superconducting with an onset temperature of the resistive transition at about 25?K. Even if the deposition process still needs to be fully optimized, we have demonstrated that this method allows us to achieve in?situ deposition of as-grown superconducting thin films. This procedure could therefore be promising for the deposition of high-quality epitaxial MgB2 thin films.

Radio frequency response of Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3O10+x superconducting tapes

The response of long (Bi,Pb)2Sr2Ca2Cu3O10 conductors fabricated by the oxide-powder-in-tube method to a radio frequency excitation was investigated while employed as the inductive part of large L-C resonating circuits. After removal of the outer silver sheath, superconducting devices cooled down to 77xa0K showed superior properties compared to equivalent non-superconducting circuits: Bi-based resonators, conceived for a working frequency in the range between 5 and 17xa0MHz, presented an improvement of the quality factor by a factor of 20. This result opens new perspectives for the application of Bi-based superconducting materials in the detection of a weak radio frequency signal, as in magnetic resonance imaging.

Neutron scattering studies of superconducting MgB2 tapes

The capability of manufacturing long superconducting MgB2 wires with already remarkable critical currents makes this material a very promising candidate for future applications. Tapes are prepared by the powder-in-tube technique. After the cold working procedure typically carried out by wire drawing and cold rolling, it has been found that a final sintering step carried out in argon atmosphere is a key process for further improving the superconducting properties of the conductors. To study the effect of the deformation and heat treatment processes, we performed neutron scattering experiment. Due to the high penetration depth of neutron inside matter, it was possible to analyse the MgB2 phase still wrapped in the Ni sheath. Our studies were carried out by a full spectra refinement by the Rietveld method. In the starting superconducting powder a large Mg deficiency was observed. In the tapes we found that the large forces applied during the cold working induced a large MgB2 lattice deformation, and that it is partly relaxed during the final sintering process. An important correlation of the residual stress with the critical temperature and the pinning properties was pointed out. We also observed the appearance of detrimental secondary phases during the sintering process. In particular, the MgB2 phase reacted with the nickel sheath and MgB2Ni2.5 was formed at temperatures higher than 850 °C. These results are of basic importance for a further optimization of the transport properties at moderate fields where applications of MgB2 tapes are already envisageable.

Fabrication and superconducting properties of powder-in-tube processed MgB2 tapes

Abstract The powder-in-tube process has been employed to fabricate long nickel-sheathed MgB 2 superconducting tapes from reacted commercially available powders. We have found that reacted MgB 2 powders reach a high degree of density inside the nickel sheath during the cold working procedure and, thanks to the large superconducting coherence length, a considerable current flow is therefore achievable even without sintering. At 4.2 K and self-field, a transport current density value j c of 10 5 A/cm 2 has been indeed reached over long portions of unsintered monofilamentary tapes. The irreversibility lines both for a magnetic field oriented parallel and perpendicular to the MgB 2 superconducting tape have been determined by magnetoresistance measurements. These data are consistent with a slight texturing process of the MgB 2 grains occurring during the cold working procedure.

Dissipation behaviour of YBa2Cu3Ox single crystals studied through magnetization and magnetization decay measurements

Hysteresis loop and magnetization decay measurements performed on a YBCO single crystal in the low temperature range (T<or=45 K) are presented. The hysteresis loops show the typical behaviour observed in such kind of sample with the so called fishtail effect. The magnetization decays are analysed in terms of the collective creep theory and, in the explored temperature range, they indicate a very different behaviour of the sample at two relatively near magnetic field values ( mu 0 H=1.6 T and mu 0 H=3.4 T) where the measurements were performed. The results support the hypothesis of a structural variation of the density and kind of pinning centres induced by field and temperature. This hypothesis could also explain the behaviour of the electric field E versus the current density J measured by flux creep annealing.

Radiofrequency response of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+x superconducting tapes

Abstract Superconducting (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 tapes fabricated by the oxide-powder-in-tube method have been employed to assemble and test L – C resonating circuits operating in the radiofrequency domain. The quality factor of these superconducting devices has been evaluated in a liquid nitrogen bath for resonating frequencies between 5 and 27 MHz. Both frequency and temperature dependence of the quality factor are in agreement with the Norris model for an elliptical current penetration inside the superconducting tape. The present results have been evaluated to take into account a possible application of such a device as an MRI detector.

Determination of the critical current density of Bi(2223)/Ag tapes from magnetic measurements: peculiarities and problems in the interpretation

Multifilamentary Bi(2223)/Ag tapes, prepared by the powder-in-tube technique, have been characterized by SQUID magnetometry at temperatures between 4.5 K and 85 K with the magnetic field applied both parallel and perpendicular to the conductor. By analysing measurements for H|| tape we evaluate the current density versus temperature and we compare it with that obtained by transport measurements. In such a way we clearly specify a temperature domain where the current density is limited by the grain boundaries and, on the contrary, where the current density is limited by the intrinsic properties of the material. The temperature dependence of the intragrain current density of the Bi(2223) phase has also been estimated. By analysing measurements for H|| tape the current density anisotropy and problems in the interpretation of the measurements related to the intrinsic grain misalignment are discussed.

Texture development studies in Ag-sheathed Bi-2223 tapes

A novel magnetic procedure has been introduced for the determination of the degree of texturing of the superconducting grains in Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox tapes. Our new method is based on the observation that, in the BSCCO family of superconductors, a relevant anisotropy is present between the in-plane and out-of-plane critical current densities of individual crystallites. By neglecting the magnetic moment of the single grain due to out-of-plane circulating currents, and by comparing magnetic hysteresis loops measured within different field orientations with respect to the tape direction, it has been possible to determine the average degree of texture of an assembly of BSCCO superconducting grains. As a result of our simple approach, we have measured the texture evolution of the precursor grains during the cold deformation process of unreacted multifilamentary (Bi, Pb)2Sr2Ca2Cu3Ox tapes; moreover, we have been also able to distinguish between longitudinal and transverse texture of the grains with respect to the tape direction. Finally, the usefulness of our method has been verified on tapes prepared with various precursor powders and cold deformation techniques.