Yoshimitsu Hishinuma

Study on the heat treatment condition to improve coupling of grains in Bi2−xPbxSr2CaCu2Oy/Ag tapes

Abstract We initially investigated the stable region of Ca 2 PbO 4 impurity phase in Bi 2− x Pb x Sr 2 CaCu 2 O y ((Bi,Pb)-2212)/Ag tapes, in the oxygen partial pressure ( P O 2 )–temperature ( T ) diagram using bulk (Bi,Pb)-2212 samples. With increasing temperature and decreasing P O 2 , the volume fraction of Ca 2 PbO 4 decreased and Pb in Ca 2 PbO 4 dissolved in Bi-2212 phase. These results indicate that rapid cooling from high temperature under low P O 2 is effective to avoid the segregation of Ca 2 PbO 4 . Taking this result into account, we prepared (Bi,Pb)-2212/Ag tapes applying rapid cooling. We found that rapid cooling is more effective to improve the coupling of grains and bring about high critical current density ( J c ) values than slow cooling in (Bi,Pb)-2212/Ag tapes of x =0.2. This is due to the reduction of the segregation of Ca 2 PbO 4 at the grain boundaries of (Bi,Pb)-2212 by the high cooling rate.

Preparation of Bi-2223 bulk composed with silver-alloy wire

Abstract We prepared Bi-2223 sintered bulk composed with silver alloy wires to improve simultaneously both the superconductivity and the mechanical properties. When 24 Ag alloy-wires of 0.4 mm in diameter were composed, the critical current density ( J c ) at 77.3 K in the self-field was obtained 700 A/cm 2 . Scanning electron microscope observation results showed that dense structure of Bi-2223 plate-like grains was formed around the interfacial region between the Bi-2223 oxide and metal Ag. The mechanical property was estimated by three point bending test. The bending stress of 80 MPa at the rupture was obtained for Bi-2223 AgMg alloy composite bulk irrespective of the number of the wires. In the AgZr alloy-wires composite bulks, the bending stress decreased to the half value of that in Bi-2223 bulk without the wires. After the maximum bending stress the sample fractured separately in Bi-2223 bulk without alloy-wires, while the composite did not fracture but only fine cracks were induced.

Microstructure and Jc property on the Bi-2223 bulk prepared by inserting Ag layers

Abstract The microstructure and critical current density ( J c ) property of (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O y (Bi-2223)/Ag multi-layered bulks were investigated. They were prepared by stacking Ag sheets alternately into the oxide, and then sintered at 840°C for 10–50 h. From the XRD and SEM results, high c -axis alignment and dense structure of Bi-2223 plate-like grains were observed near the region of the interface between Bi-2223 layer and Ag sheet, and the other regions in the oxide were random and porous structures. Further, we measured J c value using the four-probe method at 77 K and self-field. J c values of the multi-layered bulk were improved in proportion to increasing the number of Ag sheets and the maximum J c is estimated to be 320 A/cm 2 in the sample with four Ag sheets. We thought that improvement of J c performance was caused by highly c -axis aligned and densely structured region formed by Ag sheets.

Improvement of Jc and mechanical property in Bi-2223/Ag wires composite bulk

Abstract We studied on improvement of J c and mechanical properties of oxide bulk by composing Ag wires. As results, J c values of the Ag wires composite bulk were improved in proportional to the pieces of Ag wires. When 42 pieces of Ag wires were composed, the J c value of the composite bulk was estimated to be 800 A/cm 2 at 4.2 K and self-field. This J c value was about twice higher than that of no Ag wire bulk. We found that the magnetic field dependence of the J c value of the composite bulk was smaller than that of no Ag wire bulk. The good alignment and dense structure of (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) phase was observed near the interface between Bi-2223 oxide and Ag wires. It is mentioned that this structure brings about improvement of J c property and the magnetic field dependence of the J c value for the composite bulk. It is also found that mechanical property of Bi-2223 bulk is improved with increasing pieces of composed Ag wires from the results of a three point bending test.

J/sub c/ increase of BPSCCO-2223 bulk by composing with Ag wires

To improve the superconducting properties of the high-T/sub c/ superconductors, we prepared the (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ (BPSCCO-2223) bulk composite with embedded Ag wires. The composites were hand-made by stacking alternately Ag wires into the oxide powder in the thickness direction, and then the prepared samples sintered at 840/spl deg/C for 50 hours. SEM and EPMA observation showed that good c-axis oriented and densely structured BPSCCO-2223 plate-like grains could be formed around the interfacial region between BPSCCO-2223 oxide and metal Ag wire. We measured critical current density (J/sub c/) performance at 77.3 K and self-field and/or 4.2 K under several magnetic fields. As the J/sub c/ values of the BPSCCO-2223/Ag wires composite bulks were higher than that of BPSCCO-2223 bulk without Ag wires, J/sub c/ values were improved with increasing the number of Ag wires at several measurement conditions. The maximum J/sub c/ was obtained about 230 A/cm/sup 2/ at 77.3 K and self-field, 800 A/cm/sup 2/ at 4.2 K and self-field when forty-two Ag wires were composed. These J/sub c/ values corresponded to about twice that of BPSCCO-2223 oxide bulk without Ag wire. The improvement of J/sub c/ value was brought about by forming good c-axis oriented and densely structured BPSCCO-2223 plate-like grains.

Effect of CIP process on superconducting properties of Bi-2223/Ag wires composite bulk

Abstract In order to improve superconducting property of Bi-2223 superconductor bulk, Ag wires were composed in the bulk. The composite was made by stacking alternately Ag wires and the calcined powder. The sample was sintered at 840 °C for 50 h in air. After treatment with a cold isostatic press as an intermediate pressing, the sample was sintered again. The maximum J c at 4.2 K and self-field of the sample composed with 24 Ag wires of 0.4 mm in diameter was obtained to be 2000 A/cm 2 . Scanning electron microscope and electron probe microanalyzer observation results showed that highly c -axis oriented and densely structured Bi-2223 plate-like grains could be formed around the interfacial region between the superconducting oxide and the metal Ag.

Microstructure of Bi-2223 Phase in oxide/Ag Multi-Layered Bulk

Bi-2223 is suitable to be applied under low field and near liquid nitrogen temperature. Jc value of the bulk materials is much smaller than that of the wire and tape materials. This should limit application field of the bulk. Then, we tried to improve Jc properties of Bi-2223 bulk by inserting Ag sheets into oxide. It was found that Jc values of the multi-layered bulk were improved in proportional to the number of Ag sheets. We investigated the microstructure of the Bi-2223/Ag multi-layered bulk. As a result, good alignment and dense structure of the oxide phase was observed near the interface between Bi-2223 layer and Ag sheet.

Nd-422 content dependence of superconductivity in a large-sized single-domain Nd-system superconductor

We investigated the effect of the molar ratio of the Nd4Ba2Cu2O10 (Nd-422) phase in a NdBa2Cu3Ox (Nd-123) superconductor on the superconducting properties of a large-sized single-domain Nd-system melt-textured bulk fabricated using a seeding and temperature gradient method in 0.1% oxygen atmosphere. The single domain of Nd-system superconductors, about 18 mm in diameter and 5 mm in thickness, was fabricated reproducibly. The maximum Jc value was about 4.5 × 104 A cm−2 at 77.3 K and self-field, and the maximum trapped field was about 0.34 T when the molar ratio of Nd-123/Nd-422 was 1/0.25.

Improving superconductivity and mechanical properties of Bi-2223/Ag-wire composite bulk by cold isostatic pressing

To improve superconductivity and mechanical properties of Bi-2223 sintered bulk, silver (Ag) wires of 0.4 mm in diameter were added to the bulk. The sample was sintered at 840/spl deg/C for 50 hours in air. After treatment with cold isostatic pressing (CIP), the sample was re-sintered. When twenty-four Ag wires were composed, the critical current density (J/sub c/) at 4.2 K and self-field increased from 1,200 A/cm/sup 2/ to 2,000 A/cm/sup 2/ with the CIP process. SEM observation results showed that high alignment and dense structures of Bi-2223 plate-like grains were formed around the interfacial region between the superconducting oxide and the metal Ag. The mechanical properties were measured by a three point bending test. The maximum bending stress of 60 MPa was obtained in the bulk without Ag wire, which was fractured separately. With the CIP process the maximum bending stress increased to 90 MPa for the composite. The composite did not fracture but only fine cracks were induced after the maximum bending stress.

Superconductivity deterioration of single-domain Nd-system melt-textured bulk doped with Pt

Abstract We fabricated single-domain Nd-system superconductors by a seeding and temperature gradient method in 0.1% oxygen atmosphere. Pt doping effect on the superconductivity was investigated. In the sample doping with 1.0 wt.% of Pt, the T c (on-set) and the T c (off-set) were 93 and 87 K, respectively, and the T c (off-set) was several degrees lower than that of undoped sample, and the J c value in the magnetic field up to 1.3 T decreased to one-third of the undoped sample. It was suggested that the J c lowering came from a broad superconducting transition with the lowering T c (off-set).