Irreversibility fields of Bi-2223 at 30-77 K

Significant differences exist, of order a factor of two, in irreversibility field at 77 K of Ag-clad (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) composites from different sources. In the present case, we have studied the 30-77 K irreversibility field and J_c performance of post annealed Bi-2223 samples. Post annealing in the temperature range 800-750°C significantly raises both J_c and the irreversibility field. At 77 K, the irreversibility field was characterized both by the decrement field, H_p, defined by J_c(H)∝exp(-H/H_p) and by the glass transition field, H_g, defined by the change in sign of the curvature of the voltage-current characteristics. At lower temperatures, where only magnetization measurements were made, H_p and the Kramer function extrapolation field, H_K were used. H_p (77 K) ranges from 0.143-0.170 T and H_g (77 K) from 0.082-0.164 T. Higher post annealing temperatures produced larger values of both H_K and H_p at 30-50 K. H_K (30 K) varied from 8-10 T. Two tapes from leading manufacturers had H_K (30 K) values of 7.3 and 10 T, bracketing the results obtained in our post-annealed tapes. We conclude that sample-to-sample variations of irreversibility field at 77 K carry over to lower temperatures too and that optimizing the irreversibility field properties at 77 K is very valuable for lower temperature Bi-2223 performance too.