Kouhei Yamazaki

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.

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.

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.

Uniaxial strain dependence of the critical current of DI-BSCCO tapes

In order to explain the effect of uniaxial strain on the critical current of DI-BSCCO-Bi2223 tapes, we employed a springboard sample holder that can smoothly and continuously apply both tensile and compressive strains to tape samples. Over a narrow tensile strain region, the critical current in the tapes decreased linearly with increasing strain and returned reversibly with decreasing strain. When compressive strain was applied, the critical current first increased and then reached a weak maximum. Thereafter, it decreased monotonically with further increases in compressive strain. At room temperature, the local strain exerted on BSCCO filaments was measured by means of a quantum beam diffraction technique. Over the whole tensile strain region up to 0.2% and the small compressive strain range, the local strain changed linearly with applied strain. When the compressive strain was applied beyond the relaxation strain, the local strain (measured by diffraction) versus the applied strain (measured using a strain gauge) deviated from linearity, which is characteristic of strain relaxation and the onset of BSCCO filament fracture. Thus, the strain at the maximum critical current corresponds to a crossover point in strain, above which the critical current decreased linearly and reversibly with increasing applied strain, and below which the critical current decreased due to the BSCCO filament fracture. In this paper, we clearly characterize the reversible range terminated by both compressive and tensile strains, in which filaments do not fracture. Our analysis of the compressive regime beyond the relaxation strain suggests that although BSCCO filament fracture is the primary factor that leads to a decrease in critical current, the critical current in those regions of filaments that are not fractured increases linearly and reversibly with decreasing applied strain at compressive strains well beyond the reversible region for the tape.

DI-BSCCO wire with Ic over 200 A at 77 K

The highest Ic of a latest DI-BSCCO® tape, which is the high performance silver sheathed Bi2223 tapes produced with CT-OP® technique, has reached 210 A (500A/cm-width) at 77K, self-field. The wide-range measurement of Ic using temperature variable system also has proved the highest performance at each temperature and external magnetic field. The combination of CT-OP® and stainless steel lamination techniques has concurrently obtained Ic higher than 200 A, rated tensile stress more than 300 MPa and rated tensile strain more than 0.4 % at 77 K. A texture analysis using synchrotron radiation has revealed explicit correlation between Jc and c-axis misalignment angle in CT-OP® processed tapes.

The Current Transport Properties of a 200 A-Class Bi-2223 Superconducting Wire at Various Temperatures and Magnetic Fields

Silver-sheathed Bi-2223 multi-filamentary tapes using controlled-overpressure sintering have attained the highest critical current of 200 A-class at 77 K, self field. The 200 A-class Bi-2223 tape has been evaluated on the critical currents at various temperatures from 4.2 K to 90 K and magnetic fields up to 12 T applied perpendicularly to the tape plane. The critical currents are 500 A at 4.2 K, 5 T, 370 A at 20 K, 3 T and 200 A at 20 K, 10 T typically. This is an improvement by a factor of 1.3-1.5 compared to current commercial tapes produced using controlled-overpressure sintering.

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.

Possible Breakthrough for Higher Tc Discovered in Bi(Pb)2223 Practical Tapes

By systematic studies on the post-annealing effect in air for Bi(Pb)2223 practical tapes, Tc(onset) was found to be enhanced up to 115.2 K by annealing at 680°C for 250 h from 110.2 K for an as-synthesized tape. Annealing at ~700°C for a long time always increased the Tc(onset) of the tape above 114 K, at which the generation of the Pb3221 phase simultaneously occurred during annealing. The slightly increased c-axis length was confirmed for annealed samples. The compositional change of cations in Bi(Pb)2223 during post-annealing is believed to be responsible for the Tc enhancement of the present system.

High Strength/High Strain Tolerance DI-BSCCO Tapes by Means of Pre-Tensioned Lamination Technique

Excellent high tolerance on the yield stress and critical current of BSCCO-Bi2223 tapes was realized by the pretensioned lamination technique. The yield stress at room temperature and the stress to 95% Ic retention exceeded 430 and 510 MPa, respectively. In order to make clear the potential of this technique, the influence of thickness of lamination sheet and degree of pretension was investigated in detail. The observed yield stress at which the macroscopic yielding starts due to the fracture of BSCCO filaments has been successfully evaluated by calculation as a function of stainless steel sheet thickness and pretension, which is useful as engineering data. The relaxation stress at which the local strain starts to relax due to filament fracture was expressed as a unique function of the thermal residual strain exerted on BSCCO filaments together with the residual strain induced by the pretension treatment. The force free stress at which the local stress exerted BSCCO filaments becomes zero was proportional in magnitude to the residual strain exerted on BSCCO filaments. The stress to 95% Ic retention was elucidated to give a good indication, at which the critical current decreases abruptly due to the fracture of BSCCO filaments.

Recent Progress in High Performance Ag-Sheathed Bi2223 Wire (DI-BSCCO®)

Sumitomo Electric has been developing the silver-sheathed Bi2223 multi-filamentary wires since the discovery of Bi-based superconductors. DI-BSCCO is the high performance wires produced using the controlled-overpressure (CT-OP) sintering technique. The present commercial DI-BSCCO can provide the uniform high critical current, Ic, up to 180 A with length over 2000 m, and recently 200 A were succeeded to be obtained by the same kind of 1000 m length wires, resulting from the improvement and control of the microstructure in Bi2223 multi-filaments. The short trial wires of several meters have exhibited the highest Ic over 240 A at 77K in self-field (corresponding to 580 A per 1 cm-width). Besides, the optimization of carrier density after CT-OP led to further enhancement of Ic, reached 250 A. All the derivative products also have uniform critical current properties over entire length even after lamination with the reinforcing metals. The performances of DI-BSCCO can meet the growing needs for various application of high temperature superconductor like high in-field applications, such as magnets and motors. The recent progress in transport properties of commercial DI-BSCCO and R&D short trial wires is shown.