Microstructure controls and their effects on the properties of Bi2Sr2Ca1Cu2Ox superconductors

This paper reports our recent progresses in the development of Bi₂Sr₂Ca₁Cu₂O _x /Ag tape conductors for the applications of magnetic field generation in liquid helium or around 20 K, using a refrigerator. We have carried out extensive work to optimize the processing parameters, investigating the relationship between the microstructure and transportJ _c. We have found that the partial melting in oxygen atmosphere is effective to have large transportJ _c with good reproducibility. The pre-annealing and intermediate rolling (PAIR) process has been successfully applied to the multilayer conductors to improve the grain alignment and transportJ _c. TheJ _c of 5×10⁵A/cm² at 4·2 K and 10 T has been achieved, which is the highest value reported so far. Two magnets fabricated by using different types of Bi-2212/Ag conductors were tested. One is a magnet designed as an insert magnet for a 18 T-class large bore Nb-Ti/Nb₃Sn superconducting magnet. The conductor of this magnet was multifilamentary tape processed by powder-in-tube method. TheI _c was 98 A in the backup field of 18 T, which generated the self field of 1·79 T. A large pancake coil was fabricated with multilayer conductor and tested under the operation of cryocooler system. The coil was stably operated up to theJ _c of the coil at the temperatures below 30 K.