Connectivity and limitation of critical current in Bi-Pb-Sr-Ca-Cu/Ag tapes

We have shown experimentally that the remanent magnetic moments of Y-Ba-Cu-O thin-film networks and of Bi-2223/Ag monofilamentary tapes show remarkable similarities, as their magnetic moments are both composed of intersquare (intergrain) and intrasquare (intragrain) magnetic moments. Starting from the geometrical definition of connectivity in a thin-film network, we show that connectivity in a Bi-2223/Ag monofilamentary tape is solely defined by the ratio of the average grain size to the width of the superconducting core and by the ratio of the intergrain to intragrain remanent magnetic moments at saturation. The measured upper limit for the connectivities ranged from $4.2\times {10}^{-3}$ in a tape with low critical current density $J_c$ to $2.9\times {10}^{-2}$ in a tape with $J_c(77\mathrm{}\mathrm{K},0\mathrm{}\mathrm{T})\simeq 2\times {10}^8{\mathrm{A}\mathrm{}\mathrm{m}}^{\mathrm{-}2}.$ Our study reveals that even good Bi-2223/Ag tapes suffer from very low grain connectivity. We estimate that in the case of perfect connectivity a $J_c(77\mathrm{}\mathrm{K},0\mathrm{}\mathrm{T})$ between $8\times {10}^9$ and $6\times {10}^{10}{\mathrm{A}\mathrm{}\mathrm{m}}^{\mathrm{-}2}$ would be achievable.