| Major current-limiting mechanisms (CLMs) restrict the critical current density Jc of Ag-sheathed (Bi,Pb)2Sr 2Ca2Cu3Ox (2223) composite conductors. Overpressure (OP) processing of 2223 has been developed to enhance the connectivity and thus the Jc by removing the CLMs of porosity, cracks, and Bi2Sr2Ca,Cu2Ox (2212) in 2223. OP processing improved the microstructure of 2223 tapes and increased their Jc by about 30%. Record Jc enhanced irreversibility field, and the lowest 2212 SQUID index have been achieved in OP processed 2223 tapes. OP processing improved 2223 connectivity and resulted in higher local Jc and a larger number of high Jc local regions compared to conventional 1 bar processing. OP processing has been applied on the thermal slide heat treated (TSHT) 2223 tapes. It was found that adding a rolling step to the TSHT samples prior to OP processing increased the Jc and irreversibility field. Quenching experiments have been carried out to gain better understanding of the role of residual liquid, which is another CLM, the formation of Pb-rich (Bi,Pb)3Sr2Ca2Cu1Ox (3221) phase in the post anneal process, and their correlations to the electromagnetic properties of 2223. Jc and irreversibility field were enhanced by the PA, in which the 3221 was formed. The improvement of Jc and irreversibility field was strongly correlated to the removal of residual liquid by crystallizing the liquid and forming 3221 and other phases. The 3221 was observed to begin forming in the heating step of the PA and significantly increased in the soaking step. Well-defined variations of the Jc irreversibility field, and Tc as a function of quenching temperature were observed during the 2223 processing. These variations were attributed to the removal/reforming of residual liquid and the changes of oxygen and Pb content in the 2223 and 2212. |
