Hisashi Yoshino

Critical current density of wire type Y-Ba-Cu oxide superconductor

We have fabricated wire type Y-Ba-Cu oxide superconductor by powder metallurgy technique. A large critical current density Jc of 7.25 × 102 A/cm2 at 77 K and 0 T and the resistive Tc transition of 89 K to 87 K were obtained for the wire type conductor heat treated without Cu sheath. However, the Jc(77 K) drastically decreased by a factor of 200, applying a magnetic field of 1 T. On the contrary, Jc(4.2 K) little changed; 1.63 × 102 A/cm2 at 1 T and 1.33 × 102 A/cm2 at 12 T. Based on the magnetic field dependence of Jc, we estimate upper critical field Bc2 (i.e. corresponding to Bc2 defined by Tc (offset)) and GL coherence length ζ.

A new model of fault current limiter using YBCO thin film

Resistive fault current limiters which consist of a YBCO film and a normal conducting film were constructed. The YBCO film was connected by indium wires to the normal conducting film in parallel at intervals of about 10 mm to increase the allowed voltage across the fault current limiter during the current-limiting operation. Current limiting experiments show that an allowed voltage across the fault current limiter during current-limiting operation of over 13 V/cm was achieved. Moreover, by combining in parallel 2 sets of 3 serially connected fault current limiters fabricated using 100 mm/spl times/10 mm YBCO film, a rated power of over 20 kVA is obtained.

Phase diagram of the system YBa2Cu3O7−δ−BaCuO2·CuO

In order to grow a single crystal of YBa2Cu3O7−δ from a non-stoichiometric melt, a part of the pseudo-binary phase diagram of the system YBa2Cu3O7−δ−BaCuO2·CuO was investigated. The solvent was determined to be composed of the eutectic of the two constituent oxides, BaCuO2 and CuO. The composition of the eutectic was determined to be BaOCuO=12. On the basis of the phase diagram, single crystals were prepared which had a rectangular shape of up to 4×4×0.1 mm3.

Single-Crystal Growth of Bi2(Sr, Ca)3-dCu2Ox and its Superconductivity

A single crystal of Bi2.2(Sr, Ca)3-dCu2Ox was grown by a self-flux method using a solution containing excess Bi2O3 and CuO compared to the composition of Bi2(Sr, Ca)3-dCu2Ox in the Bi-Sr-Ca-Cu-O system. The dimensions of the single crystal obtained were up to 7×5×1 mm3. The critical temperature of the single crystal was 84 K which was determined by both magnetization and electrical resistivity measurements. The ratio of resistivities in the c-plane and along the c-axis was found to be approximately 35 at Tc onset.

Improvement of maximum working voltage of resistive fault current limiter using YBCO thin film and metal thin film

We have investigated the resistance of current limiting elements in order to improve the working voltage. The element consisted of YBCO thin film on sapphire substrate and metal film on AlN substrate. The working voltage was improved by optimizing the resistance of metal film, and the high value of 83 V/cm was obtained. Furthermore, a module composed of 12-connected elements in series was assembled. The current limiting properties of this module were measured using a capacity discharge circuit. The short circuit current of 1.74 kA was reduced to 460 A, the voltage drop of was 5.45 kV peak, and the dissipated energy was 15 kJ.

Enhanced flux pinning in Ag-sheathed (Bi, Pb)2Sr2Ca2Cu3O10+ delta tapes by post-annealing

A significant enhancement of Jc in a magnetic field for Ag-sheathed Bi-Pb-Sr-Ca-Cu-O (BPSCCO) tapes has been achieved by post-annealing at reduced temperature after processing. This post-annealing at reduced temperature introduced extra oxygen into the oxide superconductor. Weak links were eliminated and Jc has been increased in the post-annealed tapes by a factor of 1.5 to 3 at 0 T over the normally processed tapes without post-annealing. The optimized Jc values for the post-annealed tapes ranged from 30000 to 57000 A cm-2 at 77.3 K and 0 T. The irreversibility lines measured by a DC SQUID exhibited an improvement in flux pinning in the post-annealed tapes compared with that in normally processed tapes. The introduced oxygen ions are responsible for the enhanced Jc and may affect two dimensionality in superconductivity for the BPSCCO system.

Bulk and Wire Type Y–Ba–Cu Oxide Superconductor

We have fabricated the bulk Y–Ba–Cu oxide, various types of wire conductors and coils, and especially studied the critical current density Jc. Large Jcs were obtained at 77K and OT: 725A/cm2 for the non sheath wire, 560A/cm2 for the Ag sheathed wire. However, the Jc(77K) drastically decreased by a factor of hundred in a magnetic field lower than 1T while Jc(4.2K) little changed up to 12T. Based on the magnetic field dependence of Jc, for example, 2T of upper critical field Bc2(77K) and 47 to 80T of Bc2(4.2K) were obtained for the wire conductor.

Flux pinning by a‐axis grains in c‐axis‐oriented Y‐Ba‐Cu‐O films

We have investigated the relationship between the microstructures and pinning forces by measuring the magnetic‐field dependence and angular dependence of Jc in several kinds of YBCO thin films having different microstructures. A high‐Jc value was kept even when the magnetic field was applied perpendicular to the film plane in the case of a c‐axis‐oriented film which was studded with a‐axis‐oriented grains. The boundaries between the a‐axis‐oriented grain and the c‐axis‐oriented grain are considered to be effective as pinning centers.

Cross-substitution of Bi and Sr in Bi2Sr2CaCu2O8+δ

Abstract The continuously varying solid solution in the Bi2+xSr2−xCaCu2O8+δ system has been clarified within the range −0.1

Oxygen Deficiency and Cu Valence States of Superconducting Y-Ba-Cu Oxide

The oxygen deficiency, Cu valence states and superconducting properties were analized on Y0.4Ba0.6CuO3-x. The critical temperature and the normal-state resistivity varied depending on the change of CuIII/CuII ratio and/or the oxygen deficiency caused by the sintering atmospheres.