K. Fujino

DI-BSCCO wires by Controlled over pressure sintering

Sumitomo Electric successfully developed drastically innovative Bi-2223 (DIBSCCO), namely, commercially produced Bi2223 long length wires using the controlled over pressure sintering (CT-OP) with unique properties quite different from conventional silver sheathed BSCCO wires. CT-OP prevented pores occurring in BSCCO cores, so it reformed conventional Bi2223 wires to DI-BSCCO with excellent properties of higher critical currents, stronger mechanical strength and better durability against temperature rise in cryogen such as pressurized liquid nitrogen. It enhanced the critical current by 50 percent conventional wires sintered in normal atmospheres. Critical tensile stress was also improved by more than 150 percent. Any ballooning defects and degradation of critical current, one of the critical problems for the conventional BSCCO wires, were not found in full length of several km long DI-BSCCO tapes after 24 hours immersion into 1MPa liquid nitrogen.

Performance of all high-Tc superconducting magnets generating 4 T and 7 T at 20 K

Refrigerator-cooled all high-Tc superconducting magnets, i.e. coils and current leads, 4 T and 7 T types, have been developed. The 4 T type successfully produced a central magnetic field of 4.0 T over 30 min and 3.5 T for 48 h. High ramp rate excitation up to 0.35 T s-1 was achieved. The 7 T type magnet stably generated 7.1 T over 24 h and was excited at a rate of 7 T min-1 . These ramp rates were 20-100 times faster than that of the refrigerator-cooled magnets using metallic superconductors. A new cooling scheme and stability criterion for conduction-cooled magnets are proposed.

Controlled Overpressure Processed Bi2223 Wires for Power Applications

Progress in the performance of the controlled overpressure (CT‐OP) processed (Bi,Pb)2Sr2Ca2Cu3Ox (Bi2223) wire is reviewed. Optimization of the CT‐OP processing improved microstructure of Bi2223 wires and increased their critical current (Ic) by greater than 60% compared to normal pressure processing. The CT‐OP processing effectively removed pores and cracks. The SEM show CT‐OP wires had very dense, uniform, and well connected Bi2223 grain. Densification of the CT‐OP wires prevents liquid nitrogen penetration during long term exposure to liquid nitrogen of them for use in power cable applications. Ballooning caused by trapped nitrogen, that expands when warming up to room temperature, doesn’t occur in CT‐OP wires. These high performance levels in CT‐OP wires have enabled commercial level applications such as power cables, magnets and motors.

Fabrication of a high current superconductor in Y-system by excimer laser ablation

A hundred pieces of thin film Y1Ba2Cu3Ox(YBCO) superconducting tape, each of which was 1.1meter long, were fabricated using KrF excimer laser deposition. Almost all the tapes had Jc values more than 1.0×104 A/cm2. The best data showed the Jc of 4.5×104A/cm2 and the Ic of 4.3A at 77.3K for 1.1m voltage tap spacing. The sixty tapes were spirally wound in six layers on a stainless steel former to assemble a high current conductor. The Jc of 1.8×104A/cm2 and the Ic of 103A were achieved for 1.0m in length of the conductor. With the Jc improvement, process conditions of the YBCO deposition were refined. The Jc of 1.0×105A/cm2 is obtained for a short sample. Morphological features of the high Jc film are also discussed.

Characteristics of Resistance in High-Tc Superconductor Carrying Currents above Ic

When a superconductor is applied to a fault current limiter (FCL), the superconductor is exposed to currents somewhat larger than the critical current (Ic). Therefore, it is necessary to understand how the resistance behaves under the excess currents so as to evaluate temperature increase in the device and pressure rise in the vessel due to vaporization of the coolant. Resistance appearing under current above Ic was measured and investigated for high-T c superconducting wire. For the measurements, we prepared Bi-tape wire having Ag-Au alloy for its matrix, whose resistivity is ten times larger than that of silver. The measurements were conducted at 77K under various magnetic fields. Measured data show that the resistance approaches to its free flux flow value as the current increases when H>1T, and that it strongly depends on the current for lower fields.

Intergranular transport properties in c-axis oriented YBa2Cu3Oy polycrystalline films

The current–voltage (I–V) characteristics and the resistive transition under magnetic field were extensively investigated on c-axis oriented YBCO polycrystalline films with a different amount of an in-plane grain alignment. All the parameters deduced from the scaling analyses in the intergranular region along with the vortex-glass-to-liquid transition indicate that the intergranular current transport is not via weak links but is due to strongly coupled channels regardless of a degree of an in-plane grain alignment.

Inclined Substrate Deposition

The development of the ISD method has made it possible to deposit a film with in-plane alignment on a polycrystalline metal substrate and achieve a Jc value ten times higher than normal PLD tape. We also confirmed that the resulting mechanical properties and other practical properties are improved. Since the ISD method can be used with metal substrates, which are inexpensive and easily made into long tape material, it is believed that it will be a very powerful method for future mass production.

Current-Voltage Characteristics of c-Axis Oriented Polycrystalline YBCO Films

Current-voltage isothermal characteristics of c-axis oriented YBCO polycrystalline films were investigated under constant magnetic field. For each field, a boundary temperature, Tg, which separates I–V curves into two categories was identified. Above Tg, the I–V curves exhibited positive curvature with linear part at low currents whereas those with negative curvature were observed below Tg. Although this behavior is qualitatively consistent with the vortex glass-liquid transition, the scaling analysis revealed that the relevant critical exponents were quite different from those reported for crystalline films, with somewhat large dynamic exponents(7~9) and small static exponents(0.5~0.7). This result seems to pose a question as to the description of the data along with the vortex glass-liquid transition theory and rather suggest alternative models based on the conventional thermal depinning of vortices.

Fabrication of Superconducting Y-Ba-Cu-O Tapes by Excimer Laser Ablation

1meter long thin film Y-Ba-Cu-O superconducting tapes were successfully fabricated using a continuous laser deposition. Hastelloy tapes with an yittria-stabilized-zirconia(YSZ) buffer layer were employed as flexible substrates. The critical current density(Jc) of 2.6×104A/cm2 at 77.3K was obtained for 0.85meter voltage tap spacing. With a high rate tape deposition, the Jc of 2.4×104A/cm2 and the Ic of 2.3A(voltage tap spacing; 1meter) were achieved within 1hour of Y-Ba-Cu-O deposition. The best data obtained for short samples are Jc=6.2×104A/cm2 and Ic=4.7A. It was confirmed that the bending strain of 0.65%(radius; 12mm) does not degrade the Jc of the tape.

Superconducting Y-Ba-Cu-O Tapes Fabricated by Excimer Laser Deposition

Thin film Y-Ba-Cu-O superconducting tapes with the length up to 20cm were successfully prepared using a continuous laser deposition technique. Sintered yittria-stabilized zirconia tapes were used as flexible substrates. The as-grown Y-Ba-Cu-O films were highly c-axis oriented over the full length of the tape. The critical current density(Jc) at 77.3K of 3.4×104A/cm2 was obtained for 15cm voltage tap spacing. The highest value of local Jcs was 6.8×104A/cm2 (1cm spacing). The critical current(Ic) of 3.5A was achieved for 15cm voltage tap spacing. The crystal orientations and the morphological feature of the thin film tape are discussed in connection with the Jc values. The effect of bending strain on the Jc were also determined.