Tomoichi Kamo

Flux pinning in single Tl-layer 1223 superconductors

Abstract The presence of pinning centers in high temperature superconductors (HTSCs) is a prerequisite for obtaining a high critical current density ( J c ) in a magnetic field. We have discovered that, perhaps more generally, HTSCs having only a single layer between the superconducting copper layers (for example the YBaCuO or TlSrCaCuO systems) can possess effective pinning centers that are absent in the double layer cases. Several samples prepared by a partial melt process, with the nominal composition (Tl 0.5 Pb 0.5 ) 1 (Sr 0.8 Ba 0.2 ) 2 Ca 2 Cu 3 O 9 , showed the high intragrain J c , by the magnetization method, greater than 2 × 10 4 A/cm 2 at 77 K and 1 T. This result suggests that more effective pinning centers exist in the (1223) phase of the modified TlSrCaCuO system (ref. [1]). We propose an explanation why both YBa 2 Cu 3 O 7 and (1223) of TSCCO contain useful pinning centers, whereas both Bi- and Tl-double-layer superconductors do not. Effective pinning, in this case greater for single layers rather than double layers, is related to the strength of the superconductive order-parameter in these layers intervening the CuO layers.

Superconductivity of Tl-Sr-Ca-Cu-O System in Relation to Tl-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O Systems

It was found that Tl-Sr-Ca-Cu-O system gives rise to at least two superconducting phases, a high-Tc phase at 100 K and a low-Tc phase at about 75 K. The two phases are isostructural to those of Bi-Sr-Ca-Cu-O system. Comparing the Tcs and crystal structures of the Tl and Bi systems, it was suggested that Tc is mainly determined by the combination of two alkaline earth elements, i.e., Sr/Ca or Ba/Ca, when the repetition unit in the layered perovskites is equal.

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.

Ag-sheathed Tl−Ba−Ca−Cu−O superconductor tape with Tc∼ 120 K

Ag-sheathed Tl2Ba2Ca2Cu3O10 tapes with a Tc of 120 K were fabricated by cold rolling. The tapes annealed at an optimized condition yielded Jc=6700 A/cm2 (77 K/0 T) and 100 A/cm2 (77 K/1 T). The Jcs of the TBCCO tapes were twice as high as those of YBCO tapes in the absence of external magnetic fields, and about ten times as high as those of YBCO in the presence of magnetic fields.

Rietveld Refinement of the Structure of TlSr2CaCu2O7 by X-Ray Powder Diffraction Data

Single phase TlSr2CaCu2O7 was synthesized, and the crystal structure was refined from X-ray powder diffraction data using Rietveld analysis. The structure has single Tl-O sheets and tetragonal symmetry (a=3.7859 A, c=12.104 A) with space group P4/mmm, and is similar to TlBa2CaCu2O7 and Tl0.5Pb0.5Sr2CaCu2O7 (1212 type). Possible substitution of about 14% of Ca-sites with Tl ions was suggested.

Correlation between irreversibility magnetic fields and the longest CuO layer spacing in high-Tc superconductors

Abstract Thin films of the (1223) phase of the TlSrCaCuO system (Tl-(1223), single TlO layer) and the (2223) phase of the TlBaCaCuO system (Tl-(2223), double TlO layers) were prepared by the laser ablation method. The irreversibility magnetic field (ifH∗) obtained from the resistivity change in magnetic fields were found to be 2.0 T and 14.4 T at T/Tc = 0.6 for Tl-(2223) and Tl-(1223), respectively. The H ∗ of high Tc-superconductors including YBaCuO and biSrCaCuO systems are shown to depend strongly on the longest CuO layer spacing (d), i.e. the H ∗ decreases in the order, Y-(1223)>Tl-(2223)> Tl-(2223)> Bi-(2212). The phenomenon may be the result of Josephson weak coupling between the superconducting CuO layers. It seems that the longest CuO interlayer spacing determines the H ∗ for a magnetic field applied perpendicular to the a−b plane.

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.

Comparison of transport properties between Tl-(1223) and Tl-(2223) phases of Tl-Ba-Ca-Cu-O systems

Thin films of the (1223) phase (Tl-(1223), single Tl-O layer) and the (2223) phase (Tl-(2223), double Tl-O layers) of the Tl-Ba-Ca-Cu-O system were prepared using the laser ablation method. Both films had a high transport critical current density (Jc) of 1.5×105 A/cm2 at 77 K in zero magnetic field. When the magnetic field was applied parallel to the a-b plane (a-b plane//B), transport properties in both films at different temperatures were similar. On the other hand, in fields perpendicular to the a-b plane (a-b plane ⊥B), Jc of the Tl-(1223) thin film was higher than that of the Tl-(2223) thin film at 40, 60 and 77 K. The irreversible magnetic fields (Bc2* values) obtained from the resistivity change in the magnetic fields were 8.0 and 2.0 T at T/Tc=0.6 for the Tl-(1223) and Tl-(2223) thin films, respectively. These results suggested that the thickness of the Tl-O layer, which is non superconductive, determines transport properties for fields applied perpendicular to the a-b plane.

Relationship between crystal structures and solid solution of Tl-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O superconductors

We report on the superconductivity and crystal structures in Tl-(Ba, Sr)-Ca-Cu-O (TBSCC) system. In the case of TBSCC system, the 2223 phase was obtained as a single phase more easily compared with the Tl-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems. As a result, we achieved a high superconducting volume fraction, 95% above 100 K. The crystallographic transformation was caused by changing the ratio of Ba to Sr. By the X-ray diffraction and TEM analysis, the crystal structure was determined as a 2223 structure consisting of Tl-O bilayers in the Ba-rich side, and a 1223 structure consisting of Tl-O monolayer in the Sr-rich side.