Search

Ni–Cr alloys have been focused on as a new promising material for the reinforcement of DI-BSCCO wire. In addition to Ni–Cr alloy lamination, the thinner Type H wire and pretension have been employed. The fabricated Type H wires laminated with 35- $\mu\text{m} $ -thick Ni–Cr alloys have shown no degradation in $I_c$ properties and surpassed all the Type HT wires laminated with the thicker stainless-steel tapes in the tensile strength and double bending diameter. It was demonstrated that the wire could tolerate the practical extent of tensile load up to 10 000 cycles. The in-field $I_c$ performance has hardly been compromised with the moderate pretension. These results, along with the smaller cross-sectional size, have led to the compatibility between high $J_e$ and high mechanical strength. The Type HT wire with the thinner Type H wire and 30- $\mu\text{m}$ -thick Ni–Cr tapes are currently developed.

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.

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.

By applying the high isostatic pressure in the final heat treatment, we have raised the critical current (I/sub c/) of multifilamentary (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (Bi2223) long length wires by greater than 30%. Mechanical performance has reached levels over 250 MPa critical tensile stress at 77 K. The density of filaments in Bi2223 wire was estimated from mass density measurement and found to be increased to filament density. The controlled overpressure process has been found to advantageously heal cracks and porosities of filaments. Densification of the filaments prevents liquid nitrogen penetration during long term exposure to liquid nitrogen of Bi2223 wire for use in power cable applications. Ballooning caused by trapped nitrogen, that expands when warming up to room temperature, doesn't occur in the controlled overpressure processed wire. These high performance levels in long length Bi2223 wires have enabled commercial level products and projects such as power cables, magnets and many others.

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.

A remarkable improvement in the strain dependence of critical current and the mechanical properties of DI-BSCCO-Bi2223 tapes was achieved by means of the pretensioned lamination technique. In order to elucidate the origin of this high performance, the local strain exerted on BSCCO filaments was investigated by means of the synchrotron radiation technique. The relaxation strain Ar was expressed by the sum of thermal strain and tensile strain in the fracture of BSCCO filaments. It was concluded that the central parameter when considering the improvement of strain induced properties is the force free strain exerted on BSCCO filaments together with their own strength.