OPTIMIZATION OF PROCESSING TO IMPROVE CRITICAL-CURRENT DENSITY OF AG BI-2223 TAPES/

Several recent developments in powder-in-tube (PIT) processing are pre sented. A cryogenic deformation process has been developed, involving rolling or pressing the wires and tapes in supercold conditions, such as in liquid nitrogen. Cryogenic deformation has been found to improve the density, grain alignment and Ag-oxide core interface and to incre ase dislocation density, thereby enhancing J(c) and flux pinning. By i ncorporating Pb into Bi-2212 phase the sintering temperature can be ra ised above 840 degrees C, resulting in a significant reduction of tota l sintering time from several hundred hours to 100 h. Recently, a new process to eliminate the decomposition and recovery of Bi-2223 during cooling and heating has been developed that further reduces the heat t reatment time for Ag/Bi-2223 tapes to 20-30 h, with J(c) and Bi-2223 v olume fraction in the tapes comparable with those in tapes treated for 120 h. A two-stage annealing procedure in the final thermal cycle has been used to eliminate residual amorphous phase and Bi-2201, which ha s been identified to be one of the major causes of weak links in PIT t apes. By incorporating hot deformation in a two-step process not only can Bi-2201 be eliminated, but also texture and density are improved, resulting in a very high J(c) = 56 800 A cm(-2) (I-c = 35.6 A) at 77 K in multifilamentary tapes.