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)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (Bi-2223) composites from different sources. In the present case, we have studied the 30-77 K irreversibility field and J/sub c/ performance of post annealed Bi-2223 samples. Post annealing in the temperature range 800750(degree)C significantly raises both J/sub c/ and the irreversibility field. At 77 K, the irreversibility field was characterized both by the decrement field, H/sub p/, defined by J/sub c/(H)Rexp(-H/H/sub p/) and by the glass transition field, H/sub 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/sub p/ and the Kramer function extrapolation field, H/sub K/ were used. H/sub p/ (77 K) ranges from 0.143-0.170 T and H/sub g/ (77 K) from 0.082-0.164 T. Higher post annealing temperatures produced larger values of both H/sub K/ and H/sub p/ at 30-50 K. H/sub K/ (30 K) varied from 8-10 T. Two tapes from leading manufacturers had H/sub 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.