Takumi Ban

Phase relation in Y211–Y123–Ag system and morphology of silver in Y123 crystal

Abstract Phase relation in the Y 2 BaCuO 5 (Y211)– YBa 2 Cu 3 O 7− x (Y123)–Ag system, which is important for the fabrication of Y123/silver composite, and the morphology of silver particles dispersed in the Y123 matrix were investigated. The formation temperature of Y123 monotonously decreased to about 970°C as increasing Ag content up to a pseudo-monotectic composition and kept constant at the temperature with further Ag addition. The Y123/silver composite could be obtained at the temperatures lower than 970°C. These results suggest that Y123 phase is solidified by monotecto-peritectic reaction; Y211+L1 (Ba–Cu rich liquid)=Y123+L2 (Ag rich liquid). The morphology of silver particles in Y123 was altered by the addition of excess Y211. In the samples without excess Y211, silver particles dispersed in Y123 were disk-like shaped. On the other hand, those became spherical in the samples with excess Y211. The capturing mechanism of L2 liquid is considered to be the Uhlmann, Chalmers, and Jackson (UCJ) type pushing one. The amount of residual Y211 particles at the growth interface is considered to cause the morphological difference of captured silver between samples with and without excess Y211.

Magnetic Shielding of High-Tc Superconducting Plates

Magnetic shielding properties were measured at 77 K in YBaCuO superconducting plates prepared by melt processes. Melt processed YBaCuO has a high Jc value, hence the material is promising for the application to magnetic shielding in a relatively high magnetic field. The shielding plate with 1 mm in thickness reduces an external magnetic field of 7.1 mT to a minimum value of less than 0.1 mT. A plate which consists of small samples in a tile shape is considered to be realistic to design a large shielding plate, and a laminated plate can enhance the shielding properties. The touching part in one layer does not extend from the bottom to the top of the shielding plate in three layers. The laminated plate in three layers reduces an applied magnetic field of 14.4 mT to an average value of about 8 mT.

Magnetic Shielding by Oxide Superconducting Plates

Since High-Tc superconductors such as Y-system and Bi-system show perfect diamagnetism at above 90 K, it is expected that a magnetic field will be shielded by superconductors cooled with liquid nitrogen. In this study, magnetic shielding effect of superconducting YBaCuO disks prepared by melt processes was measured at 77K. The results indicate that disk samples with ~16 mm in diameter and ~ 1 mm in thickness can reduce an external magnetic field in intermediate field range, but that the magnetic flux comes in through grainboundaries and cracks of the sample.

Magnetic properties of melt processed YBaCuO by micro Hall probe

Magnetic properties of a melt-processed YBaCuO superconductor were studied by using a microsize Hall probe. A bulk of YBaCuO superconductor, 30 mm/spl times/30 mm/spl times/1.9 mm, with a high critical current density at 77 K was prepared by the melt process. Two-dimensional distributions of the local magnetic induction, B/sub t/ over the surface of the YBaCuO superconducting plate, were measured at 77 K, when a magnetic field up to 100 mT was applied perpendicular to the sample surface. The distribution of the local magnetic induction indicates the magnetic shielding effect when the external magnetic field is applied. By using the microsize Hall probe with an active area of 50 /spl mu/m/spl times/50 /spl mu/m, the inhomogeneous distribution of the local magnetic induction due to the existence of a weak link region was observed. External magnetic field dependence of B/sub t/ at several positions was also measured in perpendicular magnetic fields of up to 1.4 T. The magnetization process on these positions of the YBaCuO superconductor clearly indicated the behavior of the critical state and the edge effect.

Trapped Flux Density Distribution on the Surface of Oxide Superconducting Plates

Two-dimensional distribution of flux density over the surface of a BiPbSrCaCuO superconducting plate was measured at 77 K, when a magnetic field of up to 100 mT was applied perpendicular to the sample surface. The distribution of flux density indicates a magnetic shielding effect when an external magnetic field is being applied. Simulation curves of a distribution of flux density over a sample surface were also obtained. The distribution of flux density over the sample surface changing the gap length between the sample surface and the Hall probe agreed with that of simulation qualitatively.

Magnetic shielding properties in melt processed YBaCuO superconductor

Abstract Magnetic shielding effect of YBaCuO disks prepared by a melt process was measured at 77K in a magnetic field of up to 100 mT. Disk samples 16 mm in diameter and 1 mm in thickness reduced an external magnetic field, but the magnetic field came in through grain boundaries of the sample. The effect of the grain size of samples on the trapped magnetic flux density over the sample surface was investigated in this study.

Melt Growth of YBaCuO and Magnetic Shielding Effect

YBaCuO superconductors were prepared by a melt process, using a tunnel type unidirectional temperature gradient furnace which consists of multi heating zones. The effect of heat treatment and Ag addition was investigated. Melt processed YBaCuO superconductors have a high critical current density at 77 K and a high magnetic field. Thus, the sample can shield a relatively high magnetic field. Two-dimensional distribution of flux density over the surface of the melt processed YBaCuO superconducting plate was measured at 77 K by using a Hall probe, when a magnetic field of up to 100 mT was applied perpendicular to the sample surface. Magnetic shielding effect of the YBaCuO sample is relatively uniform in spite of a polycrystal which consists of several domains.

Fabrication and Properties of Screen-Printed Ag-Bi2223 Tapes

Using a screen printing method, we fabricated Ag-Bi2223 monofilament and multifilament tapes (~11mm wide and -30 mm long) with filament number 1 < N < 5. As N increases in the sample, Ic at 77 K and 0 T increases monotonically until it reaches 54 A when N = 5, while Jc is around (1.6–1.9) x 104 A/cm2 (77K, 0T) showing a tendency for a decrease with N. The bend strain tolerance for monofilament tapes is 2–3 % strain, which is much superior to that for Ag-sheathed Bi2223 monofilament tapes (-0.2 % strain). Although the strain tolerance for multifilament tapes gradually decreases to — 1 % at N =3 and -0.4 % at N = 5, it is still superior to that for Ag-sheathed Bi2223 mofofilament tapes. Magnetic shielding properties of the samples in transverse magnetic fields are measured by a scanning Hall microprobe technique and the results are presented. We succeeded in fabricating wider monofilament tapes (~60mm x -60mm) with Ic=30.4 A corresponding to Jc — 5 x 103 A/cm2 (77K, 0T).

Flux Density Distribution on YBCO by Micro Hall Probe

Two-dimensional distributions of flux density over the surface of a YBaCuO superconducting plate were measured at 77 K by using a microsize Hall probe, when a magnetic field up to 88.1 mT was applied perpendicular to the sample surface. The microsize Hall probe has an active area of 50 µm × 50 µm. Therefore, inhomogeneous distributions of flux density over the sample surface, or inhomogeneous superconducting properties of the sample were detected. The distribution of flux density indicates a magnetic shielding effect when the external magnetic field is applied, and also a remnant magnetic field after the external magnetic field is removed. A bulk of YBaCuO superconductor with a high critical current density at 77 K and relatively high magnetic field was prepared by a melt process.

Melt Growth of YBa 2 Cu 3 O x Phase by Unidirectional Temperature Gradients Method

A tunnel type unidirectional temperature gradient furnace which consists of multi heating zones was used for preparing Y-system superconductors. Two-dimensional distribution of the flux density over the surface of the melt-processed YBaCuO superconducting plate was measured at 77 K, when a magnetic field of up to 100 mT was applied perpendicular to the sample surface.