Tsuneyuki Kanai

Dopant Effects on the Superconductivity in the Bi-Sr-Ca-Cu-O System

The effects of 34 different dopants on the BiSrCaCu2Oy ceramics were investigated. The dopants can be classified into four categories depending on their solubilities and substitutional properties in the Bi-Sr-Ca-Cu-O structure. Dopants in the first group, i.e. Fe, Co, Ni and Zn, dissolve and substitute into the copper site and can significantly reduce the transition temperatures (Tcs) of the high- and low-Tc phases. The second group of dopants, i.e. rare-earth elements, substitute into the Ca sites and cause the disappearance of the high-Tc phase and increase the Tcs in the low-Tc phase. The dopants in the third group, i.e. alkaline metals and phosphorus, have a strong tendency to decompose the superconducting phase. The remainder of the dopants investigated were classified as the fourth group. Superconductivity of the high- and low-Tc phase of the fourth group remains almost uneffected.

Crystalline structures and superconducting properties of rapidly quenched BiSrCaCu2Ox ceramics

Phase changes and superconducting properties of rapidly quenched BiSrCaCu2Ox ceramics have been studied. The rapidly quenched sample consists of an amorphous state which begins to crystallize around 510°C. The crystal system was found to be tetragonal with a=5.39 A and c=24.5 A. By annealing above 700°C, a tetragonal system with a=5.41 A and c=30.8 A was produced. The former crystal was a semiconductor and the latter was a superconductor with Tc=75 K. A superconducting phase with Tc=105 K was observed in a sample annealed at 840°C. The superconducting volume ratio in the samples had a maximum at 803–840°C. The decrease of the volume ratio at the higher temperatures is considered to result from a decomposition of the superconducting phase (75 K) into a nonsuperconducting phase.

Control of oxygen release from Bi-2212 phase in a partial melt process

A serious problem for Ag-sheathed Bi-2212 wires produced by a partial melt process is oxygen release which causes effluence of the oxide materials out of the Ag sheath. Released oxygen content for the Bi2Sr2Ca1Cu2AgxOy (x=0, 0.2 and 0.4) was evaluated by the TGA weight change during the partial melt process. It was found that (i) the powders released less oxygen when they are calcined at temperatures closer to the melting point, despite the calcining atmosphere and (ii) the addition of 0.2 mol Ag and higher oxygen partial pressure in the partial melt process are effective for reducing oxygen release. The reduction in released oxygen content was attributed to the doped Ag becoming liquid at the partial melting temperature of the Bi-2212 phase and the Ag liquid accommodating the released oxygen.

Degradation by Mechanical Grinding and Recovery by Annealing in the Superconducting Phases of the Bi–Pb–Sr–Ca–Cu–O System

It has been found that mechanical grinding may degrade phases from supercounducting to non-superconducting; i.e., a sample with 73vol% of the high-Tc phase (105K phase), 18vol% of the low-Tc phase (75K phase) and 8vol% of a non-superconducting phase resulted in a sample with 98vol% of non- superconducting phase after grinding for 62min. X-ray analysis showed that cleavage fracture of the high-Tc phase in the {001} planes occurred more easily than that of the low-Tc phase by mechanical grinding. The {001} fracture gradually changed to a more isotropic fracture on prolonged grinding. It was also found that the degraded sample did not recover easily to the initial conditions compared with the amorphous samples initially free of the high-Tc phase. A process which includes crystal grinding, causing the degradation of the superconducting phases, may not therefore be appropriate for fabricating practical superconducting wires or sheets.

Properties of Bi(Pb)-Sr-Ca-Cu-O superconductor prepared by a novel oxidizing solidification process

A novel oxidizing solidification process has been proposed to obtain a high-Tc phase through melt process in the Bi-Sr-Ca-Cu-O system. Calcined powder of a nominal composition of Bi1.84Pb0.34Sr2Ca2Cu3Oy was put into Au vessels and the partial pressure of oxygen was increased from 0 to 1 atm at selected constant temperatures. Resistive measurements yielded a value of Tczero=110 K in the sample processed at 855 degrees C for 100 h. On the other hand, the Tczero was 85 K for a conventional solidification process. X-ray and susceptibility analyses showed that the oxidizing solidification process allowed the formation of the high-Tc phase more easily than a conventional cooling solidification process. The distributed phases in the samples differed in the composition shape and size of a (Sr,Ca)-Cu-O phase and in the amount of the 2223 phase.

Dependence of critical current density on microstructures in superconducting Bi(Pb)-Sr-Ca-Cu-O wires

The dependence of the critical current density Jc on microstructures in superconducting Ag-sheathed Bi(Pb)-SrCa-Cu-O wires has been investigated. A mixture of the oxide powders with a composition of Bi1.84Pb0.34Sr1.9Ca2.0Cu3.1Ox was calcined and the resulting powder was placed in silver tubes. The tubes were processed by drawing and rolling to give 0.3 nm thick tapes. The tapes were annealed at 825-843 degrees C for 300 h with intermediate pressing. Different annealing temperatures gave three types of sample with different microstructures. The critical current density of the samples depended on the kind of foreign phases, such as the Cu-free (Bi,Pb)-(Sr,Ca)-O phase or 2201 phase, as well as the orientation of the a-b plane. The weak link structure of the samples was an S-N-S junction regardless of the annealing conditions. The flux pinning force density Fp followed the power law (1-(T/Tc)2)m; the exponent m was much larger than that for metal superconductors and was different in the foreign phases.

Properties of TL-2223 Superconducting Thick Films Formed by Plasma Spraying

We report on the fabrication and properties of superconducting thick films of Tl2(Ba1.6Sr0.4)Ca2Cu3Ox(Tl-2223)(1) produced by the plasma spray method on Ag substrates. Various post-annealing conditions were investigated. Scanning electron microscope (SEM) and energy dispersive X-ray analysis (EDX) showed the annealed films to be composed primarily of the Tl-2223 phase with a small amount of Ca-Cu-O. The Jc of these films reached 3800A/cm2 at 77K and 0T.

Limiting Factors of Critical Current for Bi-Pb-Sr-Ca-Cu-O Oxide Superconductor

Critical current limiting factors for Bi-Pb-Sr-Ca-Cu-O oxide superconductor were investigated. A mixture of the oxide powders with a composition of Bi1.84Pb0.34Sr1.9Ca2.0Cu3.1Ox was calcined and the powders were placed in silver tubes. The tubes were processed by drawing and rolling to give resulted in 0.3mm thick tapes. The tapes were annealed at 825–843°C for 300h with intermediate pressing. Different annealing temperatures gave three types of samples with different microstructures. The critical current density Jc of the samples depended on the kind of foreign phases such as the Cu free (Bi,Pb)-(Sr,Ca)-O phase or 2201 phase as well as the a-b plane orientation. The weak link structure of the samples was an S-N-S junction regardless of the annealing conditions.

Properties of a Fluorine-Doped Y-Ba-Cu-Oxide Superconductor Prepared by Hot-Pressing

Fluorine doped Y-Ba-Cu-oxide samples prepared by hot-pressing showed superconductivity without a subsequent heat treatment in oxygen, in contrast to the semiconducting behavior of undoped samples. CuF2 powder was used as the fluorine source. Powders with the nominal composition YBa2Cu3OyF0.2 were hot-pressed in oxygen atmosphere at 900–950°C; a density of 5.72–5.83 g/cm3 was obtained. The Meissner effect was observed at 93 K and 75 K in the samples hot-pressed at 900°C and 950°C, respectively. The differing superconductivity between the undoped and F-doped samples may be due to changes in the average copper valence in the Y-Ba-Cu-O system.