Takahiro Taneda

Development of in-plane aligned YBCO tapes fabricated by inclined substrate deposition

Abstract High temperature superconducting thin film tapes, which consist of a flexible metal substrate, buffer layer, and Y 1 Ba 2 Cu 3 O x (YBCO) layer have been developed for power application. Ni-alloy tape was used as the substrate. Both the YBCO layer and the buffer layer were formed by pulsed laser deposition (PLD) because of an advantageous of high deposition rate. Inclined substrate deposition (ISD) was developed to introduce in-plane alignment for the buffer layer on the non-textured metal substrate. YBCO films epitaxially deposited on the ISD buffer layer by normal PLD had high J c values over 10 5 A/cm 2 at 77 K. For verification of high rate productivity of the PLD/ISD method, we demonstrated high rate deposition of the yttria stabilized zirconia buffer layer and YBCO superconducting layer. We also performed continuous deposition of over 10 m YBCO thin film tapes with a reel-to-reel substrate transfer system combined with 200 W high power laser equipment.

Spiral Pitch Adjusted Multi-layer Conductor with Alloy-sheathed Wire

Current distribution of spiral pitch adjusted multi-layer conductor was investigated with rogowski coils. Condition of the spiral pitch to suppress the unbalanced current distribution was determined by circuit model regarded the multi-layer conductor as a circuit network with mutual and self inductances. The conductor was fabricated with alloy-sheathed tapes. It was confirmed unbalanced current at each layer was reduced and the conductor loss was reduced in spiral pitch adjusted conductor.

Angular dependence of critical current in YBCO tapes fabricated by inclined substrate deposition

Abstract In this study, the angular dependence of Jc is investigated in detail for YBa2Cu3Oy (YBCO) tape fabricated by the inclined substrate deposition method. The result shows that Jc has a notable peak when the magnetic field is perpendicular to the plane of the tape, which indicates that the tape has lower Jc at medium degrees of the angle of the field to the tapes plane. The peak gradually diminishes as the field increases. This study confirms that data of the perpendicular and the parallel field dependence of Jc, as such, are not sufficient for evaluation of practical performance of YBCO tape.

Reproducible superconducting gap on clean surfaces of BiSrCaCuO prepared by etching with a scanning tunneling microscope tip

Abstract Tunneling properties on the basal plane of a high- T c superconductor, Bi 2 Sr 2 CaCu 2 O y , have been investigated by cryogenic scanning tunneling microscopy/spectroscopy (STM/STS). The tunneling spectra obtained on the as-cleaved surfaces showed significant scattering, ranging from semiconducting to superconducting behavior, depending on the measurement position. On fresh surfaces prepared by tip apex etching procedure at 4.2 K, a clear superconducting gap structure was reproducibly observed with a flat background conductance curve. We have also experienced that during the tip scanning the background conductance suddenly changed from a flat shape to a V shape accompanied with discontinuity in an STM image, suggesting that the V-shaped tunneling conductance reflects the electronic nature of the tip apex.

Electrical properties of YBa2Cu3O7−x tapes fabricated by ISD method

Abstract We have been developing YBa 2 Cu 3 O y (YBCO) wire fabricated by the inclined substrate deposition method. For investigation of advantages of these wires, electrical properties were studied. The J c measured as a function of the temperature and the magnetic field reveals that the YBCO wire has good J c – B – T property. The current–voltage characteristics of a 1 m-long wire shows that the worst section tends to dominate the performance of the wire. Transport ac losses were measured for the 1 m-long wire and were found to be consistent with the values derived by Norriss ellipse model.

High-J c Property of Round Strand for HTS Transposed Cable Conductor

Ag-sheathed Bi-2223 round strands for high temperature superconducting (HTS) transposed cable conductors were developed. The critical current density (Jc) at 77K, OT reached 1.69×108A/m2 in short length sample and 1.0×108A/m2 in 50m-long sample. Several analyses were done to evaluate the basic properties of round strand, such as Bi-2223 volume fraction, crystal alignment, and magnetic field dependence of Jc. It was confirmed that the above properties were improved with increasing Jc.

Development of Long Length Round Strand for HTS Transposed Cable Conductor

Long length round strand of Ag-sheathed Bi-2223 for HTS transposed cable conductor was developed. The strand was produced by the powder-in-tube method and had high critical current density (Jc) value due to its structure. The Jc was 8,100A/cm2 in 12m at 77K, 0T. Mechanical properties of two kinds of round strands, sheath material of which were pure Ag and Ag-alloy, were also investigated. The Ag-alloy one was turned out to be stronger than pure Ag one.

Development of Bi-2223 Cable Conductors

Two kind of conductors, pitch adjusted multi-layered conductors and transposed conductors, were investigated by using (Bi,Pb)2Sr2Ca2Cu3Ox (Bi-2223) Ag-sheathed wires to reduce a.c. losses. It was found that a practical cable conductors with low a.c. losses was possible by equalizing the impedance of each layer by means of adjusting spiral pitch in the multi-layered conductor, or by realizing round shape strand with higher Jc over 10kA/cm2 at 77K in the transposed conductor. Based on these conductor designs, Bi-2223 wires, Ag-alloy sheathed tape and round shape strand were developed by the powder-in-tube method. lkA-class and 2kA-class transposed cable conductors assembling sub-cables that consist of stranded Bi-2223 round strands were fabricated and evaluated. It was demonstrated that a.c. loss of this conductor was one order lower than that of the non-transposed conductor that which has the multi-layer structure assembling Bi-2223 Ag sheathed tape shape strand. In the case of pitch adjusted multi-layered conductors, a lkA-class conductors were also fabricated. Low a.c. loss and uniform current distributions were confirmed in these conductors.

HTS Transposed Cable Conductor with Low AC Loss

Transposed cable conductor to reduce a.c. loss was investigated by using Bi-2223 Ag-sheathed round strand. Properties required for the conductor was discussed regarding with characteristics of the strand. Based on the conductor design, Bi-2223 Ag-sheathed round strand was developed by the powder-in-tube method. A 2kA-class transposed conductor assembling double stranded compact flat sub-cable was fabricated and evaluated. It was demonstrated that a.c. loss of the conductor was much lower than that of multi-layered conductor assembling Bi-2223 Ag-sheathed tape.

Local variation of electronic structure in BiSrCaCuO investigated by cryogenic STM/STS

Abstract Local variation of electronic structure on the cleaved surface of BiSrCaCuO has been investigated by cryogenic STM/STS under magnetic fields up to 6T. Most of the cleaved surface showed a semiconducting nature, possibly due to the loss of oxygen from the surface in an inert atmosphere. However, it was found that the surface degraded layers can be removed by the STM scanning with appropriate bias conditions. On the fresh surfaces as etched, a clear superconducting gap structure was reproducibly observed. At 6 T, the vortex lattice was not imaged by STM/STS, suggesting that the STM tip repels the vortex lines.