Yasuko Torii

Preparation of Ag-Sheathed (Tl, Bi)–Ca–Sr–Cu–O Superconducting Wire

In the Tl-Ca-Ba-Cu-O system, Tl-O single layered and double layered structures with general formulas Tl1Can-1Ba2CunO2n+3(n=1~6) and Tl2Can-1Ba2 CunO2n+4 (n=1 ~4) have been obtained. We fabricated Ag-sheathed Tl-Ca-Ba- Cu-O superconducting wires using powders of Pb-added Tl1Ca 3Ba2Cu4+O11 and of Tl2Ca2Ba2Cu3O10 phases whose critical temperatures are the highest among the respective structures. The production process of the wires was cold- working and heat treatment. The measurement of transport critical current was carried out in the range of magnetic fields from zero to 2.5T. The critical current density Jc at 77K for the wire of (Tl,Pb)1Ca 3Ba2Cu4O11 is 10800A/cm2, and that value for Tl2Ca2Ba2Cu3O10 wire is 5500A/cm2. There is a tendency for the wire specimens with homogeneous structure of superconducting phases to have high Jc values. The Jc of the (Tl,Pb)1Ca3 Ba2Cu4O11 wire decreases rapidly by 0.1T, while the decrease of Jc above 1T is not so remarkable. The magnetic field dependency of Jc of the Tl2Ca2Ba2Cu3O10 wire shows a two-step decrease in Jc versus B curves; the first step is from zero to 0.1T, and the second step is beyond 1T. The direction of the magnetic field against the plane of the wire does not considerably influence the Jc values of either of the wires, because the anisotropic property of the wire is small.

Superconductivity in the Tl-Bi-Ca-Ba-Cu-O and Tl-Bi-Ca-Sr-Cu-O Systems

The influence of the substitution of Bi for Tl in forming the superconducting phases in Tl-Bi-Ca-Ba-Cu-O and Tl-Bi-Ca-Sr-Cu-O systems was investigated. In the Tl-Bi-Ca-Ba-Cu-O system, the Tl Ca2Ba2Cu3O9 phase forms from a 10% to 40% substitution of Bi for Tl in the nominal composition. With a further increaseing Bi, the superconducting phase disappears because of the formation of BaBiO3. On the other hand, the superconducting phase of (Tl, Bi)CaSr2Cu2O7 forms from a 10% to 90% substitution of Bi for Tl in the Tl-Bi-Ca-Sr-Cu-O system. It is recognized that the substitution of Bi for Tl stabilizes and accelerates the formation of the (Tl, Bi)CaSr2Cu2Ox phase.

Superconductivity in Tl-Bi-Ca-Sr-Cu-O and Tl-Bi-Pb-Ca-Sr-Cu-O Systems

The influence of the substitution of Bi for Tl on the formation of the superconducting phases in Tl-Bi-Ca-Sr-Cu-O and Tl-Bi-Pb-Ca-Sr-Cu-O systems was investigated. The superconducting (Tl, Bi)Ca2Sr2Cu3O9 phase with critical temperature of 113 K showing both zero resistivity and diamagnetism is prepared in a Tl-Bi-Ca-Sr-Cu-O system. The crystal structure of this phase is tetragonally symmetric with lattice constants of a=3.813(3) A and c=15.266(5) A. The superconducting tetragonal phase of (Tl, Bi, Pb)Ca2Sr2Cu3O9 is also formed in a Tl-Bi-Pb-Ca-Sr-Cu-O system.

Resistive transition in TlBiSrCaCuO thin films in magnetic fields

Abstract The resistive transition for a c -axis oriented (Tl, Bi)Sr 2 Ca 2 Cu 3 O χ thin film with a critical temperature of 106 K was investigated in magnetic fields up to 9 T. The magnetic field-induced resistive transition shows a considerable anisotropic broadening behavior; a much broader transition in the field perpendicular to the ab -plane than in a parallel field. This behavior is strongly influenced by the magnetic field component perpendicular to the ab -plane. The resistivity in magnetic fields approaches zero exponentially versus temperature. The activation energy U 0 (0) deduced from the slope of the Arrhenius curve is expressed by the power law of U 0 (0)∝ B -1 . The resistive transition in magnetic fields for (Tl, Bi)Sr 2 Ca 2 Cu 3 O χ is attributed to thermally activated flux creep.

Magnetic-Field Angle Dependence of The Critical Current Density in Bi-2223 Superconducting Tape

Critical current densities (Jc) have been investigated in Ag — sheathed Bi-2223 superconducting tapes which were prepared by powder-in-tube technique. The Jc was measured in terms of both magnitude (B) and angle (Θ) to the tape surface of the magnetic field. In the high angle range, that is, the magnetic field component perpendicular to the tape surface (BsinΘ) was large, Jc values were able to be expressed by one universal function independently of the magnitude of the magnetic field. That function was proportional to the logarithm of BsinΘ (i.e., Jc ∝ log(BsinΘ)). Jc values were out of that function when the angle was decreased and saturated in the range near Θ = 0. The angles at which Jc began to part from the above function hardly depended on the magnitude of magnetic field. It was considered that the Jc of the sample was basically dominated by the magnetic field component perpendicular to the tape surface, that is, parallel to the c-axis of the crystal structure. The suppression of Jc in the low angle range may be due to misaligned grains. The misaligned angle was investigated by SEM. It was estimated that the mean misalignment angle was about 10°.

Preparation of the Tl-Bi-Sr-Ca-Cu-O Superconducting Thin Film

Thin films of (Tl, Bi) Sr2CaCu2Ox (1212 phase) and (Tl, Bi) Sr2Ca2Cu3Ox (1223 phase) with single Tl-O layered structures were prepared using a two-step process which involved a deposition of film followed by annealing under a thallium oxide vapor. When the films were annealed at 800–860°C, crystallization from the amorphous to 1212 phase occurred, and at 920°C, the 1223 phase was mainly formed. The highest zero resistivity temperature of the 1223 film was 109K. According to the TEM observation, films of both the 1212 and 1223 phase were almost c-axis oriented and some intergrowth of the 1212 phase was seen in the 1223 phase.