Naoki Ayai

The Bi-2223 Superconducting Wires With 200A-Class Critical Current

Critical current of a silver-sheathed Bi-2223 tape using controlled-overpressure sintering has attained 201 A, which is equivalent to 493 A per 10 mm-width. The engineering current density is 21kA/cm2 , while the critical current density on the total filaments area is 56 . Critical currents are also improved noticeably at low temperatures and high magnetic fields normal to the tape plane. Typical critical currents have reached 500 A at 4.2 K in 5 T and 200 A at 20 K in 10 T. The highest critical currents of long wires currently have reached 195 A for 70 m and 191 A for 178 m. The wires with 200 A-class critical current are similar in mechanical properties to 150 A-class long wires commercially produced by Sumitomo Electric using the controlled-overpressure sintering. The microstructure study has predicted far higher potential of critical current, according to the confirmed remaining issues on the 200 A-class tapes; incomplete orientation order, aggregation of alkaline earth cuprates and inter-granular Bi-2212.

Development of Ag-sheathed Bi-2223 superconducting wires and their applications

Silver-sheathed multifilamentary BiPbSrCaCuO 2223 superconducting wires with long length and high Jc of over 3/spl times/10/sup 4/ A/cm (77.3 K, 0 T) were developed by using the powder-in-tube method. For future industrial applications, high Jc with high Ic wires, high strength wires and low AC loss wires were also developed for large scale magnet applications and electric apparatus for AC use. In the case of AC conductor, twist effect was investigated in order to reduce the wire AC loss, PVF coating technology was applied to high strength wire. PVF coating technology made it possible to obtain amperage parallel conductor with low AC loss, because each strand is electrically insulated. As a progress of technology, we could fabricate many application prototypes. In the high amperage conductor application, current leads and cable conductors were developed. In the magnet application, pancake magnets cooled by GM refrigerator and operated at around 20 K were developed, We have also developed the transformer as an AC application operated in the liquid nitrogen.

Electrical and Mechanical Properties of DI-BSCCO Type HT Reinforced With Metallic Sheathes

Electrical and mechanical characteristics of Type HT tape, which is standard 4.2 mm wide DI-BSCCO tape reinforced with metallic tapes, have been evaluated. Longitudinal distributions of critical current and n-index in kilometer long Type HT tape has proved uniform from end to end just like the original insert tape, which is Type H tape. Type HT-CA reinforced with 50 mum thick heat-resistant copper alloy is highly balanced tape with high mechanical properties and low splice resistance. Type HT-SS reinforced with 20 mum thick stainless steel has the best mechanical properties, which has been demonstrated under the actual environment in high field magnet, namely the hoop stress load test energizing a one-turn coil in external high magnetic field and liquid helium.

Mechanical behavior and strain dependence of the critical current of DI-BSCCO tapes

In order to clarify the mechanical properties and their influence on the critical current of DI-BSCCO tapes with and without lamination, mechanical tests and critical current measurements were performed, and the corresponding residual strain exerted on the BSCCO component was measured by a neutron diffraction technique. Also the residual strain analysis was calculated using the rule of mixtures. The measured modulus of elasticity, the second slope, and the stress corresponding to the force free strain were self-consistently explained on the basis of this analytical model. The calculated residual strain in the BSCCO component was found to be nearly identical with that determined by neutron diffraction for all DI-BSCCO tapes except for the case of the low strength insert tape. The difference between the strain corresponding to 95% Ic retention and the force free strain was explained by the fracture strain of BSCCO filaments. An approximate expression to evaluate the residual strain in the BSCCO filaments is also presented along with a procedure for estimating the initial temperature T0 at which the residual strain starts to accumulate during cooling after the fabrication heat treatment.

Development of Nb3Al superconductors for International Thermonuclear Experimental Reactor (ITER)

Abstract A fabrication technique of a Nb3Al strand using mass-production billets was developed for the framework of International Thermonuclear Experimental Reactor (ITER) Engineering Design Activity (EDA). The weight of a multifilament billet is about 70 kg, which corresponds to the total length of the wire of about 16 000 m. The critical current density is over 600 A mm−2 at 4.2 K, 12 T, and the hysteresis loss for the field parallel to the wire axis is lower than 600 mJ cm−3 at ±3 T. The residual resistivity of the chrome-plated wire is drastically improved to be lower than 1.6×10−10 Ωm with setting the large copper area at the centre, which reduces the chrome diffused into the copper. The yield of the superconducting strand has been improved, resulting in a yield of 70% for wire longer than 1500 m. The total volume of fabricated Nb3Al superconducting strand for ITER Nb3Al insert coil, which is fabricated in the ITER-Central Solenoid (CS) model coil program, is about 210 000 m, i.e. 1000 kg. This is the first experience of mass-production for Nb3Al superconductors in the world.

Status of Bi-2223 tapes performance and development

We are developing Bi-2223 superconducting tapes on a wide viewpoint from fundamental superconducting property to application usage. In basic characterization of superconductivity, the behavior of critical current density (J/sub c/) in terms of both temperature and magnetic field was studied in detail and a new scaling law which can estimate the value of J/sub c/ in multiple conditions is defined. There were significant improvements in the long length fabricating process. The 1 km order length tape having J/sub c/ at 77 K over 20 kA/cm/sup 2/ can be produced reproducibly. The distribution of J/sub c/ through the whole length is very uniform. These results in both basic characterization and production progress made it possible and easy to design a high field magnet. Then, we have successfully developed a magnet using a refrigerator.

Development of a Bi-2223 HTS Magnet for 3T MRI System for Human Brains

We are developing a cryogen-free high temperature superconducting (HTS) magnet system for a compact 3T MRI system for human brains. In the conceptual design, the magnet system consists of 5 main coils that are layer windings of Bi-2223 tapes. The magnet system will have 500 mm room temperature bore and be operated at 20 K using G-M cryocoolers. The target field is 3 T ±5 ppm for 250 mm (dia,) and 200 mm (length) volume. All the coils will be connected in series and operated in driven mode. Controlled overpressure (CT-OP) processed Bi-2223/Ag tapes which are reinforced with Cu-alloy laminations are to be used for these coils. We investigated Ic-B-T performance in detail for short samples of the tape. We fabricated and tested five small layer-wound coils using the tape (38.4 ~ 46.2 m piece for each coil). Each small coil could be energized up to its expected current that was calculated using the short sample performance and the coil parameters. The maximum electromagnetic force (hoop stress) reached 137 MPa, and caused no degradation in the coil performance. These results show that our layer-winding techniques and the conductor performance (Ic-B-T and homogeneity along length) can be applicable and suitable for our 3T MRI magnet.

Improvement of Reversible Strain Limit for Critical Current of DI-BSCCO Due to Lamination Technique

The DI (dynamically innovative)-BSCCO-Bi2223 tapes achieved high critical current as well as high modulus of elasticity. Further the reversible strain limit and the corresponding stress for critical current have been remarkably increased by means of lamination technique. During the course of development, their optimized architecture has been designed based on the principle of the rule of mixture for maximizing the force free strain exerted on the superconducting component. The reversible strain/stress limit (A rev/R rev) was defined as a strain, at which the critical current recovers to the level of 99% I co. Selecting several kinds of laminating materials and changing condition of the fabrication, the excellent Cu alloy-3ply tape with I co of 311 A/cm was realized of which A rev and R rev reached 0.42% and 300 MPa, respectively. Further during the theoretical analysis, the increase of reversible strain limit were made clear to be attributed to the increase of thermally induced residual strain as well as the compensation effect of laminated layers against a local fracture mode.

Total AC losses in twisted and untwisted multifilamentary Bi-2223 superconducting tapes carrying AC transport current in AC longitudinal magnetic field

In some electrical apparatuses, superconducting tapes are exposed to the longitudinal magnetic field. In this work, AC losses were measured in twisted and untwisted Bi-2223 tapes carrying AC transport current in the AC longitudinal magnetic field. In twisted tapes, the transport, magnetization and total losses depend on the relative direction of the longitudinal magnetic field to the direction of the transport current, while the field direction does not influence the AC loss characteristics in untwisted tapes. In the Z-twisted tapes, the total AC loss is larger in the longitudinal magnetic field that is anti-parallel to the transport current than in the longitudinal magnetic field of another direction. Numerical analysis shows that this field direction dependence of the total AC loss results from the change in the current distribution. In the longitudinal magnetic field that is anti-parallel to the transport current, the total AC loss in the Z-twisted tape is more than that in the untwisted tape. This dependence on the field direction is reversed in S-twisted tapes. It is to be noted that the twist increases the total AC loss in a longitudinal magnetic field of a certain direction, while it reduces the AC loss in the transverse magnetic field.

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.