Yutaka Iwai

Influence of the oxygen partial pressure on the solubility of PbO in the (Bi, Pb)2Sr2CaCu2O8+y superconducting oxides

Abstract A systematic study of the solubility of PbO in the ( Bi 1− x Pb x ) 2 Sr 2 CaCu 2 O 8+ y compound at various oxygen partial pressures has been carried out. The solubility was found to increase as the oxygen partial pressure decreased. The change in lattice parameter with PbO concentration supported the solubility data. By annealing the 2212 phase with a large amount of PbO under a high oxygen partial pressure, a secondary phase having an XRD pattern similar to that of Ca 2 PbO 4 was precipitated. Simultaneously, the 2212 phase transformed from a monoclinic (nearly orthorhombic) to a tetragonal structure in the 2212 phase with a small amount of PbO. The annealing effect in the (Bi, Pb)-based superconductor can be explained by the variation of the solubility of PbO due to the oxidation of the Pb 2+ ions, when the secondary phase is assumed to be isostructural with Ca 2 PbO 4 .

Liquid quenched superconductor Ba-Y-Cu-O with Tc,zero=88 K and AC Josephson effect at 77 K

A superconducting Ba-Y-Cu-O with Tc,zero=88 K was prepared by a liquid quenching method, in which the melt of nominal composition Ba2YCu3O7-δ was first quenched and then the sample was annealed at 900°C for 48 h in air. AC Josephson effect was observed at 77 K in point-contacts made of the present samples. This new method is applicable to the preparation of superconducting thin films, wires, tapes and superconduciting cavities.

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 YBa2Cu4O8 by a seeding method at ambient pressure

Abstract A seeding method was applied to the processing method of YBa 2 Cu 4 O 8 superconducting oxide (124 phase). Seed of the 124 phase was prepared by the HIP process. Seed with 25 wt.% was mixed with the matrix oxide prepared by decomposition of nitric salt. They were pressed and fired at 850°C under oxygen gas flow (1 atm of oxygen partial pressure). After repeating the firing, the 124 single phase was obtained. The sample of this 124 phase exhibited superconductivity at onset temperature of 82 K and zero resistive point at 76 K. From the matrix oxide (unseeded), however, the 124 phase could not be formed under the same conditions.

Superconductivity in non-modulated (rmBi, Pb)2Sr2Ca1−xYxCu2Oy(x=0–1.0)

Abstract Modulation free (Bi, Pb) 2 Sr 2 Ca 1−x Y x Cu 2 O y were prepared in the composition range of 0 c of 96K was observed in the sample of x =0.3. Unit cell of (Bi, Pb) 2 Sr 2 Ca 1−x Y x Cu 2 O y showed the monoclinic feature where x is nearly 0. The lattice parameters a, b, and c increased with increasing x.

Influence of oxygen partial pressure on the solubility of Y2O3 in the high TC phase of Bi-Pb-Sr-Ca(Y)-Cu-O superconducting oxide

The solubility of Y2O3 in the high TC phase (2223 phase) of (Bi, Pb)-based superconducting oxides was investigated to examine the possibility of substitution of Y3+ for Ca2+. Under PO2 =0.21 atm, approximately x=0.02 (x=Y/Ca+Y) of Y2O3 was found to be soluble. The solubility was measured as a function of the oxygen partial pressure (PO2 ) at 790°C. Under lower PO2 , the Y-substituted 2223 phase was unstable and decomposed partially into the 2212 phase. The stable area of the 2223 phase was narrowed and the solubility of Y2O3 in the 2223 phase was decreased with decreasinff PO2 . Electrical resistivity was increased and TC,zero was decreased by doping with Y2O3.

Critical Current Density of Bi-Pb-Sr-Ca-Cu-O Superconducting Ceramics

The critical current densities of Bi-Pb-Sr-Ca-Cu-O ceramics, which have a zero resistance state above 100 K, were studied. The critical current densities at 77 K under a zero magnetic field showed the low values (10?20 A/cm2) in spite of Tc,zero=105 K and decreased drastically by applying the magnetic field.

Observation of GdBa2Cu3O7-δ Ceramic Microstructure

The microstructures of high-Tc superconductor GdBa2Cu3O7-δ prepared under various sintering conditions were observed by using SEM and EPMA. The second phase was found on the grain boundaries of the samples sintered at temperatures higher than 970°C. The phase was confirmed to consist of more Ba and less Gd in comparison with the composition of the grain.

Formation of Y2Ba4Cu7Oy at Ambient Pressure

Preparation of Y2Ba4Cu7Oy (247 phase) was examined at 1 atm of oxygen partial pressure under the following conditions; (l)using NaNO3 as a reaction accelerator, (2)without reaction accelerator and without seed and (3)using the seeding method, respectively. They were pressed and fired at 850–900°C under oxygen gas flow. The 247 phase was obtained in the temperature range from 860°C to 890°C using NaNO3. Under the condition(2), however, the 247 phase could not be formed in the same temperature range. But, the seeding method was effective; the 247 phase formed at the temperature range from 870°C to 890°C. The sample of the 247 phase prepared at 870°C under the condition(1) exhibited superconductivity with zero resistive point at 93 K.