Phase evolution and the mechanism of Jc increase during post-annealing in Ag/(Bi, Pb)2223 tapes

We have investigated the influence of post-annealing on the current transport and phase/microstructure evolution in Ag/(Bi, Pb)2223 tapes. After post-annealing, the critical current density was about 1.4 times higher. Also the superconducting transition was greatly sharpened and Tc became much higher, even when a magnetic field was applied. The Rietveld analysis suggested that, before post-annealing, a liquid phase remained in the tape, whose mass fraction was estimated as 9.3% from the sum of the phases that formed during post-annealing. Transmission electron microscopy observations revealed that amorphous layers or lattice disorder areas existed at a large number of the (Bi, Pb)2223 grain boundaries. These insulating layers block the superconducting current paths or produce a Josephson-coupled weak link boundary. Upon post-annealing, phase and structure changes took place at the grain boundaries. The insulating amorphous layer converted into metallic 2212 layers. In addition, a faceted structure was formed at the boundary interface such that the tilt boundary became a superconducting through-path. It is strongly suggested that these two factors are critical for increase of the Jc upon post-annealing.