Improvement of grain connectivity in Bi2223/Ag tapes by reducing Bi2201 phase at grain boundaries

Abstract

The critical current of Bi2223/Ag tapes with different secondary phase content was measured in an applied magnetic field at 77 K. The phase content was altered using different cooling rates after the final sintering, or by using an additional sintering stage at a lower temperature. The difference in strong link grain connectivity in the tapes was obtained by extracting strong link critical current ( I c0 s) from the I c vs. H curves. Flux pinning was monitored via the field of the peak of pinning force density. Secondary phase content was assessed using X-ray diffraction. SEM analysis was used to study the secondary phase distribution and microstructure in the tapes. Normally prepared tapes showed an initial increase of normalized I c0 s with J c0. For J c0>20 kA/cm 2, however, normalized I c0 s saturated at about 35% [J. Horvat, W.G. Wang, R. Bhasale, Y.C. Guo, H.K. Liu, S.X. Dou, Physica C 275 (1997) 327], while tapes with low Bi2201 content did not show this saturation of normalized I c0 s. This resulted in a substantial improvement of field dependence of I c over conventional PIT tapes. For the field direction parallel to the tape plane, 28% of I c0 was retained at 1 T. SEM analysis revealed that the Bi2201 was located between the grains. It is suggested that by removing Bi2201 from low angle tilt boundaries, a higher number of strong links was formed in the tapes.