Naruaki Tomita

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.

Development of superconducting magnets using Bi-2212/Ag tapes

Several pancake type superconducting coils were fabricated using Bi-2212/Ag tapes prepared by the combination of continuous dip-coating process and melt-solidification and tested in various temperatures and bias fields. A small Bi-2212/Ag double stacked pancake coil(13 mm/spl phi/ (inner bore)/46.5 mm/spl phi/ (outer diameter)) was used as an insert magnet of a conventional superconducting magnet system. In the bias field of 20.9 T, the generated field of the Bi-2212/Ag coil was of the Bi-2212/Ag coil was 0.9 T, at the I/sub c/ of 310A (criterion 10/sup -13/ /spl Omega/m), in the saturated superfluid helium temperature (/spl sim/1.8 K). Thus, this superconducting magnet system achieved generation of magnetic field of 21.8 T in the full superconducting state. A larger Bi-2212/Ag double stacked pancake type magnet (20 mm/spl phi/ (inner bore)/spl times/94 mm/spl phi/ (outer)) was also fabricated. The generated fields of the magnet at 4.2 K in 0 and 20 T bias fields were 2.6 T (I/sub c/=385A(10/sup -13/ /spl Omega/m)) and 1.08 T (I/sub c/=160 A(2/spl times/10/sup /spl times/13/ /spl Omega/m)), respectively. At about 20 K, in helium gas, the generated field of the magnet was 1.53 T (I/sub c/=225A(10/sup -13/ /spl Omega/m)).<<ETX>>

Influence of Ag‐Au and Ag‐Cu alloys on Bi2Sr2CaCu2Ox superconductor

The influence of Ag‐Au and Ag‐Cu alloy substrates on the microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi‐2212) was studied. The critical current density (Jc) of Bi‐2212/(Ag‐Au) was not degraded by increasing the Au content of the Ag‐Au alloy, while the Jc of Bi‐2212/(Ag‐Cu) was drastically degraded by increasing the Cu content of the Ag‐Cu alloy. The microstructure of Bi‐2212 on the Ag‐Au alloy was uniform and no precipitates were observed. The Jc degradation of Bi‐2212 in contact with the Ag‐Cu alloy was understood by the fact that Cu absorption of Bi‐2212 from the Ag‐Cu alloys changed the composition of the oxide layer. Ag‐Au alloys are superior substrate materials for Bi‐2212 for power current leads for superconducting magnets, taking into account their low thermal conductivity.

Characteristics of strengthened Ag substrates for Bi2Sr2CaCu2Ox doctor‐bladed tapes

The characteristics of Ag‐(Mg,Ni) and Ag‐Mn alloys as substrate materials for Bi2Sr2CaCu2Ox (Bi‐2212) tapes and their influence on the superconducting properties were studied. Alloying with Mg+Ni or Mn drastically decreased the grain size of Ag after heat treatment. The additives in Ag‐(Mg,Ni) and Ag‐Mn alloys were oxidized during heat treatment in air. Both the fine grain structure and the internal oxidation of the additives increased the mechanical strength of the Ag alloys. Both alloys were 3–6 times stronger than pure Ag. The superconducting properties of Bi‐2212 prepared on these Ag alloys were slightly degraded; however the critical current densities at 4.2 K in 8 T were still well over 104 A/cm2.

Comparative study on 180 MeV Cu11+ irradiation effect on textured YBa2Cu3Ox and Bi2Sr2CaCu2Oy

Abstract 180 MeV Cu 11+ irradiation was carried out on textured Y-123 tapes and the effects of irradiation on the microstructure and superconducting properties were investigated. The results were compared to those of Bi-2212. The superconducting transition temperature T c of Y-123 is much less sensitive to irradiation than that of Bi-2212. The increase of J c by the irradiation, J c, after irrad. − J c, before irrad. , of Y-123 is comparable to that of Bi-2212 at low-temperature region. At high temperatures, on the other hand, defects introduced by the irradiation are much more effective as pinning centers in Y-123 than those in Bi-2212. Flux creep was suppressed by the irradiation for both oxides. However, essential features of flux creep were not changed by the irradiation for both oxides. These differences in pinning behaviors between Y-123 and Bi-2212 can be understood in terms of the difference in two-dimensionality of the oxides.

Development of Bi-oxide superconducting tapes and coils

Abstract We have developed Bi-2212 and 2223 tapes. For Bi-2212, two double stacked pancake type coils were fabricated using Bi -2212 Ag tapes prepared by a combination of the continuous dip-coating process and melt-solidification. A small coil (13 mm inner bore, 46.5 mm outer diameter) was inserted in a conventional superconducting magnet system. In a bias field of 20.9 T, the generated field of the coil was 0.9 T, at an I c of 310 A (criterion 10 −13 Ωm) at 1.8 K. Thus, the superconducting magnet system achieved the generation of a field of 21.8 T in the full superconducting state. A large coil (20 mm inner bore, 94 mm outer diameter) generated a field of 2.6 T ( I c = 385 A (10 −13 Ωm) ) at 4.2 K and 1.53 T ( I c = 225 A (10 −13 Ωm) ) at 20 K in self-field. For Bi-2223, tapes were prepared by the powder-in-tube technique using Ag-10% Cu- x %M ( x = 0–1.0, M = Ti, Zr, Hf or Au) alloy sheaths. The high J c values of 5–7 × 10 4 A cm −2 at 4.2 K and 14 T were obtained for the tapes doped with x = 0.03–0.1 at.% Ti, 0.1 at.% Zr, 0.1 at.% Hf or 0.3% Au. These tapes have a modified Bi-2223 grain structure at the sheath/core interface and also a dense and more aligned microstructure, resulting in higher J c values.

Properties of Bi2Sr2CaCu2Oy/Ag Composite Tapes and Coils Melt-Solidified with Bi2Al4O9

We have fabricated Bi2Sr2CaCu2Oy(Bi-2212)/Ag composite tapes and small scale pancake coils by applying melt-solidifying method. The Bi2Al4O9 compound was found to be an effective vapor source of bismuth oxide for suppressing its vaporization from Bi-2212 layer during the high-temperature heat treatment resulting in the improvement of the J c homogeneity of tapes and coils. Short Bi-2212/Ag tapes showed high J c values above 2×105A/cm2 at 4.2K in 10T. A double stacked pancake coil using Bi-2212/Ag long tapes generated 1.64T and 1.25T under bias fields of 0T and 6T, respectively, at 4.2K.

Properties of Bi2Sr2CaCu2Ox/Ag-Alloy Composite Tapes

Alloying an Ag substrate was performed in order to improve thermal and mechanical properties of Bi2Sr2CaCu2Ox(Bi-2212)/Ag composite tapes. Ag-Au alloy, Ag-Cu alloy, Ag-(Mg,Ni) alloy and Ag-Mn alloy were investigated. Alloying with Au drastically decreased electrical conductivity (thermal conductivity) of Ag substrate, but did not degrade the superconducting properties. Addition of (Mg,Ni) or Mn was effective to increase mechanical strength, which was 3–11 times higher than that of pure Ag. Critical current density (J c ) of the composite tapes was degraded with increasing the additive content of Ag-(Mg,Ni) and Ag-Mn alloys, but the J c value was still well over 104 A/cm2 of practical level for addition under lat%.

Development of Bi-2212 coils applying dip-coating and melt-solidification process

We report our recent results on the fabrications and performance tests of Bi-2212/Ag and Bi-2212/(Ag-alloy) coils prepared by the continuous dip-coating process and the melt-solidification method. A small double stacked Bi-2212/Ag pancake coil was used as an insert magnet of a conventional superconducting magnet system. Ic of the coil was 310A in the bias field of 20.9T at 1.8K. At this critical current, the generated magnetic field of Bi-2212/Ag coil was 0.9T. Thus, the total magnetic field generated by the superconducting magnets reached 21.8T. Ag-Mg-Zr alloys were used as the substrates of Bi-2212 to improve the mechanical properties of composite tapes. The Bi-2212/(Ag-(Mgg,Zr)) coil was tested at 4.2K under the bias field of 18T. The I c , and J c , were 302A and 46,000A/cm2, respectively. At this critical current, the generated magnetic field of Bi-2212/Ag coil was 0.83T.

Anomaly in J c -B Hysteresis for Bi-2212/Ag Tape around 19T

Transport JC was measured at 4.2K in high magnetic fields up to 23T for Bi-2212/Ag composite tape and we found an anomaly in JC-B hysteresis around 19T. Magnetic field was applied parallel to the tape surface by using a water-cooled resistive magnet. JC-B hysteresis below 18T is same as a typical one for oxide superconductors. Above 19T, an inversion in JC-B hysteresis was observed. Between 18.7T and 18.6T in field decreasing sequence, Je jumps 20% in its value. At a stable field above 19T after field increasing sequence, JC decreases with time.