Enhanced flux pinning in (Bi, Pb)-2223/Ag tapes by slightly over-doped Pb

Bi_2.2-xPb_xSr₂Ca_2.2Cu₃O_y silver sheathed ((Bi, Pb)-2223/Ag) tapes with different Pb doping amounts, x = 0.4, 0.5, 0.6, are prepared by the powder-in-tube method. X-ray diffraction analyses manifest that the tape with x = 0.5 has more Pb-rich phase than that of a tape with x = 0.4. For the tape with x = 0.6, there is a large amount of Ca₂PbO₄ phase and a small amount of Pb-rich phase. Scanning electron microscopy and transmission electron microscopy observations show that the Pb-rich phase with nanometre order of magnitude is embedded in the (Bi, Pb)-2223 grain matrix of samples. The magnetic field dependence of the critical current density of the tape with x = 0.5 is noticeably improved at 77 K, and its irreversibility line is also shifted to higher temperatures and magnetic fields in comparison with the tape with x = 0.4, which is usually considered as the standard Pb doping level in preparing (Bi, Pb)-2223/Ag tapes. The resistivity measurement of the normal state reveals that the anisotropy of the tape with x = 0.5 is also decreased compared with the tape with x = 0.4. The enhanced flux pinning seems to mainly originate from the fine Pb-rich phase particles themselves and the extra defects induced by the larger Pb-rich phase particles.