Katsumi Nomura

Fabrication conditions and superconducting properties of Ag‐sheathed Bi‐Sr‐Ca‐Cu‐O tapes prepared by partial melting and slow cooling process

Ag‐sheathed Bi2Sr2Ca1Cu2Ox(Bi‐2212) tapes with various oxide superconductor core (SC‐core) thicknesses, SC‐core ratios (SC‐core area to overall cross‐sectional area), and tape configurations were prepared by a partial melting and slow cooling process. For single‐core tapes the critical current density, Jc(core), for SC‐core, increases with decreasing SC‐core thickness, due to the highly aligned Bi‐2212 grains, and the highest Jc(core) of 1.4×105 A/cm2 at 4.2 K in 10 T is achieved at a core thickness of 15 μm. Furthermore, the Jc(core) increases with increasing SC‐core ratio (i.e., with decreasing Ag‐sheath thickness). From the practical point of view, high Jc(overall) as well as high critical current (Ic) is more important than Jc(core). This requirement is attained by the double‐tube method, which shows the highest Jc(overall) value of 3.0×104 A/cm2 at 4.2 K in 10 T. In the tape fabrication process, the slow heating rate just below the partial melting becomes important, because oxygen released from the S...

Low thermal conductive Bi-2223 tapes sheathed with Ag-Au alloys

With the view of applying to power current leads for superconducting magnet systems and for other cryogenic power handling systems, low thermal conductive Bi-2223 superconducting tapes sheathed with Ag-Au alloy were fabricated and their thermal conductivities were measured from 12 to 260 K. The critical current density (overall-J/sub c/) was about 1700 A/cm/sup 2/ at 77 K, 0 T and remained nearly constant irrespective of Au concentration up to 11 at.%. The tape sheathed with Ag+11 at.%Au alloy, of which the superconductor cross-section ratio f/sub sc/ was 0.65, had a thermal conductivity value about 0.2 W/cmK at 77 K this value is as low as that of Cu-Zn. It was found that the thermal conductivity of the tape was close to the calculated one based on f/sub sc/ and the independently measured thermal conductivities of the Ag-Au alloy acid the Bi-2223 superconductor. The superconducting tapes sheathed with the alloy were confirmed to be suitable for the application as power current leads. >

Generation of 21.5 T by a superconducting magnet system using a Bi2Sr2CaCu2Ox/Ag coil as an insert magnet

We fabricated a Bi2Sr2CaCu2Ox/Ag pancake coil by applying a dip‐coating process, which was used as an insert coil of conventional superconducting magnet system. The critical current of the insert coil at saturated superfluid helium temperature (1.8 K) in the bias field of 20.8 T, was 275 A, and the field generated at this current was 0.7 T. As a result, we could achieve the field generation of 21.5 T at the center of the magnet system which was the highest field obtained by the full superconducting state.

Processing and fabrication of Bi2Sr2CaCu2Ov/Ag tapes and small scale coils

Abstract Optimipation of heat treatment conditions for a Bi2Sr2CaCu2Op(Bi-2212)/Ag melt-solidified tape was carried out. Effects of the maximum temperature for partial melting, cooling rate in solidifying process, cooling rate after the melt-solidification and partial pressure of oxygen on Jc were examined. A tape prepared under optimized conditions showed a high Jc value of 2.4 × 105 A/cm2 at 4.2 K in 12 T. Small pancake coils were fabricated using Bi-2212/Ag long tapes prepared by a continuous dip-coating method. The generated field of a coil was higher than 1 T at 4.2 K in bias fields less than 4 T and its critical current remained greater than 200 A even at 20 K under zero bias field. This means that the Bi-2212/Ag coil has sufficient potential for practical use as a superconductng magnet at 20 K.

CHARACTERISTICS OF AG-AU ALLOY SHEATHED BI-PB-SR-CA-CU-O SUPERCONDUCTING TAPES FOR CURRENT LEADS

Oxide superconductor tape with Ag‐Au sheath was studied for current leads. A 100‐cm‐long current lead of 1000 A with 0.2 W/kA heat leakage was designed using Ag‐11 at. % Au alloy sheath with low thermal conductivity. The overall current density of the superconducting tape is required to be 350–770 A/cm2 at 77 K when the superconductor cross‐section ratio in the tape is 0.35–0.7. Bi‐Pb‐Sr‐Ca‐Cu‐O superconducting tape with Ag‐11 at. % Au alloy sheath was fabricated and the critical current density Jc was measured. The overall‐Jc was 1700 A/cm2 at 77 K under 0 T. The possibility of the current lead of 1000 A under 0.28 T with 0.2 W/kA was made clear.

Improvement of reproducibility of high transport Jc for Bi2Sr2CaCu2Oy/Ag tapes by controlling Bi content

A new method to obtain high Jc Bi-2212/Ag tapes with high reproducibility was developed. Melt-solidifying with Bi2Al4O9 is found to be an effective method for suppressing the vaporization of bismuth oxide during the high-temperature heat treatment. By application of this method, the homogeneity of the microstructure of the Bi-2212 layer was improved considerably due to a decrease of the Bi-poor phase, (Sr, Ca)-Cu-O. The reproducibility of Jc values was also improved corresponding to the microstructure, and Jc maintained more than 1×105 A/cm2 at 4.2 K in 7 T even for a 1 m tape.

Fabrication of a Bi-2212/Ag Pancake Coil Generating 6000 G at 4.2 K in 12 T

A Bi2Sr2CaCu2Oy (Bi-2212)/Ag single pancake coil with dimensions of 45 mm(outer)/13 mm(inner)×20 mm was fabricated by the wind, react and wind (W & R & W) method. The critical current of the coil was 230 A at 4.2 K in 12 T assuming the criterion as 10-13 Ωm. At this moment, a generated field of 6000 G was detected at the center position of the coil by the Hall probe measurement.

Growth mechanism of high peritectic temperature Nd1+xBa2−xCu3O7−δ thick film on low peritectic temperature YBa2Cu3O7−δ seed film by liquid phase epitaxy

Abstract YBa 2 Cu 3 O 7− δ (YBCO) thin films were used as seed films for growing high peritectic temperature ( T P ) Nd 1+ x Ba 2− x Cu 3 O 7− δ (NdBCO) thick films by liquid phase epitaxy. On heating to a temperature above T P , polycrystalline YBCO grains agglomerate and then partially decompose. The metastable YBCO phase takes the lowest energy configuration, an enlarged size and a shape with an equilibrium curvature, which prohibits them from immediate decomposition and dissolution. As a result, the retained YBCO grains act as heteroeptaxial-seeds for the growing NdBCO grains in the Nd–Ba–Cu–O solution. Yttrium was detected in the NdBCO grains grown in the initial stage by EPMA, strongly supporting a model of the YBCO-seeded NdBCO growth.

Thermal and electrical properties of AgAu and AgCu alloy tapes for metal stabilizers of oxide superconductors

Abstract In order to develop new metal stabilizers with a low heat transport capacity for Bi-based oxide superconducting power current leads, the thermal conductivity and electrical resistivity of AgAu and AgCu alloy tapes heat treated in air were investigated from 15 to 260 K. The thermal conductivity of AgAu alloy tapes decreased drastically at low temperatures with increasing Au content after the heat treatment necessary to realize the superconducting wire. By contrast, the thermal conductivity of AgCu alloy tapes hardly decreased because of oxidation of Cu during the heat treatment.

Preferential growth mechanism of YBCO film on MgO substrate in initial stage of LPE growth

The mechanism of improving the in-plane alignment of LPE-grown YBCO films on MgO substrate was investigated by the experiments and model analysis. The preferential dissolution and growth of the islands in seed film due to the difference in the in-plane alignment relationship between the island and substrate were observed even when the size of the seed islands were similar. The phenomena can be explained by the coarsening mechanism by considering the difference in the interface energies between YBCO grains and MgO substrate. The preferential growth model was developed based on the above idea. The results of the calculation by the model are in good agreement with the experimental results.