The formation mechanism of Bi,Pb(2223) outgrowths in multi-filamentary tapes

We present a micro-structural study of the formation of superconducting outgrowths in multi-filamentary Bi,Pb(2223)/Ag tapes. Such bridges between the filaments have been shown to be capable of carrying sizeable currents and thus can greatly increase AC losses in these conductors. By quenching a series of tapes at different reaction times, we were able to study how these outgrowths nucleate and develop. A statistical analysis of their number and length shows that they nucleate at predetermined sites and that their growth holds equal tread with the Bi,Pb(2223) formation within the filaments, both processes being governed by the availability of transient liquid. The orientation distribution of the outgrowths shows that they usually nucleate at 'dents' in the Ag/ceramic interface caused by the mechanical deformation of the inherently inhomogeneous precursor powder. The influence of precursor powder coarseness,-composition and annealing atmosphere are also discussed. The data suggest that outgrowths formation can be minimized by the use of homogeneous and fine-grained powders without hard particle agglomerates.