Katsuzo Aihara

Introduction of pinning centers into Tl‐(1223) phase of Tl–Sr–Ca–Cu–O systems

Pinning centers were successfully introduced into the (1223) phase of Tl–Sr–Ca–Cu–O system using its derivative (Tl/Pb)–(Sr/Ba)–Ca–Cu–O with the nominal composition of (0.5/0.5)−(1.6/0.4)–2–3. The bulk sample showed a strong flux pinning characteristics. The critical current density, Jc, was obtained from the magnetization measurement. The intragrain Jc was ∼43,000 A/cm2 at 77 K and 1 T. The transport Jc of a tape with the (1223) material in core was ∼500 A/cm2 at 77 K and 5 T.

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.

Fabrication of Ag-Sheathed Ba-Y-Cu Oxide Superconductor Tape

Ba2YCu3O7-δ tapes were fabricated by cold rolling. The critical current density Jc apparently depended on the thickness of the tape and increased markedly with decreasing tape thickness. The maximum Jc of 3330 A/cm2 at 77 K in a zero magnetic field was achieved in an Ag-sheathed BYCO tape, 0.06 mm in thickness. The presence of a 20 mT magnetic field reduced the Jc value by 1/100.

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.

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.

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.

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.

Properties of Tl2Ba2Ca2Cu3Ox thin films with a critical temperature of 122 K prepared by excimer laser ablation

Superconducting thin films with a single Tl2Ba2Ca2Cu3Ox(2223) phase were fabricated on MgO(100) substrates using excimer laser ablation and post annealing. The films had a highly c-axis-oriented structure and showed zero resistance at 122 K. Diamagnetism also appeared at 122 K and a 95% transition was achieved at 110 K. The critical current density of the film at 77 K was 1.5×105 A/cm2 at 0 T.

Neutron Diffraction Study on Preferred Orientation of Ag-Sheathed Y-Ba-Cu-O Superconductor Tape with Jc=1000–3000 A/cm2

Ag-sheathed tapes using tetragonal phase of YBa2Cu3Ox were fabricated by cold rolling. Neutron diffraction study was undertaken to investigate the orientation of YBCO grains in the Ag-sheathed tape. It was clearly observed that the c-plane of crystal was aligned parallel to the cold rolling surface with a standard deviation of 26.7 degree. The preferred orientation was enhanced by using the tetragonal phase.

Phase Transition in YBa2Cu3O7-x through Hydrogen Ion Implantation

Phase transition in YBa2Cu3O7-x through hydrogen ion implantation was investigated by means of X-ray diffraction. It was found that the orthorhombic-to-tetragonal phase transition was induced by the implantations in the ion fluences of 7.6×1016 H+/cm2 and above. At the 2.3×1017 H+/cm2 implantation, the extra phases Y2BaCuO5 and CuO were also formed. These phenomena suggested that the implanted hydrogen atoms deoxidized the YBa2Cu3O7-x to produce H2O which reacted with YBa2Cu3O7-x.