Mamoru Ishii

Superconducting Characteristics and Microstructure of Bi–Pb–Sr–Ca–Cu–O Ceramics

The influence of the processing on the superconducting characteristics was investigated for the new high-Tc Bi-Pb-Sr-Ca-Cu-O ceramics prepared under various sintering conditions. The critical temperature Tc,zero and the critical current density Jc at 77 K under zero magnetic field were strongly dependent on the sintering temperature. The maximum values of 105 K and 26 A/cm2 were obtained for the specimen sintered at 845°C. For the specimen sintered at 850°C, these values were lower although the volume fraction of the high-Tc phase was higher than that of the specimen sintered at 845°C. This difference was discussed in terms of the microstructure of the superconducting ceramics.

Crystalline phases in superconductor Ba-Y-Cu-O-with high Tc prepared by melting method

A superconducting Ba-Y-Cu-O oxide with high Tc was prepared by a melt quenching method, and the effect of melt quenching conditions on the value of Tc and crystalline phases in quenched and annealed samples was examined. It was found that the values of Tc,zero for annealed samples were 88 or 89 K, irrespective of the first melt quenching conditions. The crystalline phases in quenched samples were a mixture of Y2O3 and BaCuO2, and the main crystalline phase in annealed samples was orthorhombic Ba2YCu3O7-δ Superconducting Ba2YCu3O7-δ oxides were formed very easily by annealing the melt quenched samples.

Influence of cooling rate on superconducting characteristics of Bi-Pb-Sr-Ca-Cu-O ceramics

The influence of the cooling rate on superconducting characteristics was investigated for Bi-Pb-Sr-Ca-Cu-O ceramics. It was found that the critical temperature Tc,zero and the critical current density Jc at 77 K were strongly dependent on the cooling condition. The optimum cooling condition to maximize Tc,zero and Jc was in the range from about 50°C/h to 100°C/h.

Effect of Annealing on Superconductivity in Bi-Pb-Sr-Ca-Cu-O System

The effect of annealing on the superconductivity in the Bi-Pb-Sr-Ca-Cu-O system was investigated. The tailing phenomenon was observed in the R-T curves for all samples annealed at temperatures in the range of 600°C to 825°C though the as-sintered sample showed a superconducting transition above 100 K. This phenomenon was enhanced in the sample annealed at 700°C.

Formation of the High-Tc Phase of Bi1-xPbxSr0.90Ca1.10Cu1.50Oy (0≦x≦0.05)

A seeding process was applied to prepare the high-Tc phase of Bi-based superconducting oxides with small amount of PbO. The application of the seeding process enables us to prepare the high-Tc phase more easily. The lattice constants decreased with increasing PbO content. The temperature dependence of resistivity was also examined.

Influence of Intermediate Pressing on Superconducting Characteristics in Bi-Pb-Sr-Ca-Cu-O System

The influence of intermediate pressing after the initial sintering on the superconducting characteristics in the Bi-Pb-Sr-Ca-Cu-O system was investigated. Though the sample prepared by the conventional process had a porous structure with a Tc,zero of 105 K, an as-pressed sample prepared by intermediate pressing after the initial sintering had relatively dense structure and its superconducting transition became very broad. However, further sintering caused significant improvements in the superconducting characteristics.

Preparation of Superconducting Bi-Pb-Sr-Ca-Cu-O Glass Ceramics with Tc,zero=106 K

Superconducting Bi-Pb-Sr-Ca-Cu-O glass ceramics were prepared by the melt quenching method. The Tc,onset and Tc,zero of the sample annealed at 845°C for 24 h in air were 115 K and 106 K, respectively. From the results of X-ray diffraction and SEM observation, it was inferred that the c-axis was oriented in parallel with the surface of the annealed sample.

Magnetic Shielding Property of Bi(Pb)–Sr–Ca–Cu–O Superconducting Tube

The magnetic shielding property of a Bi(Pb)-Sr-Ca-Cu-O superconducting tube was examined. The measured internal magnetic flux density of the tube, as a function of the applied magnetic field, agreed with the critical state model calculated with the consideration of the hysteresis of the critical current density on the magnetic field.

Magnetic Detector Using Bi-Pb-Sr-Ca-Cu-O Superconductive Film

The Bi-Pb-Sr-Ca-Cu-O film prepared by rf magnetron sputtering was annealed at 780°C for 2 h and 828°-847° for 160 h (two-step annealing). The obtained film made use of a magnetic detector, based on the magnetoresistive effect in the oxide superconductor. Sensitivity to a magnetic field was measured for the obtained films. The 842°C annealed film showed an excellent property as a magnetic detector, and its magnetic field resolution estimated from the sensitivity to the magnetic field and the voltage noise was 1.82×10-6 G/√Hz at 100 Hz. This film was mainly composed of plate like crystals which were partially parallel and partially perpendicularly oriented to the surface of the MgO substrate. It is considered that the grain boundaries between the grains with different crystal axes act as weak links in this compound.

Fabrication of superconducting YBa2Cu3O7−δ films by a tape casting method

Films of the superconductor YBa2Cu3O7-δ(YBCO) were prepared by a tape casting method. The resistivity of the films was measured by the four-probe method and the superconduction was observed at Tc=96 K(onset) and 91 K(end).