Limits to the critical transport current in superconducting (Bi,Pb${)}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ silver-sheathed tapes: The railway-switch model

We have proposed the ‘‘railway-switch’’ model to describe the superconducting transport current in (Bi,Pb${)}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$ silver-sheathed tapes. The model assumes that in the textured polycrystalline filament the frequent small-angle c-axis tilt grain boundaries (‘‘railway switches’’) constitute strong links for the supercurrent. With the objective to identify the mechanisms that limit the critical-current density in the tapes we present measurements of the transport critical current normal to the tape plane and of the current-transfer length along the tape axis. From I-V curves we obtain the longitudinal critical-current distribution and compare it to the thickness variation of the filament. The experiments have been performed on monofilamentary powder-in-tube samples prepared in long lengths by rolling as the only tape-forming process. For all investigated samples the critical-current density at T=77 K in self-field was in the range ${\mathit{j}}_{\mathit{c}}$=20–30 kA/${\mathrm{cm}}^2$. We conclude that the dominant limitation for the transport critical current in the tapes is imposed by the low intragrain critical-current density ${\mathit{j}}_{\mathit{c}}^{\mathit{c}}$ along the c axis (as compared to the in-plane critical-current density ${\mathit{j}}_{\mathit{c}}^{\mathit{a}\mathit{b}}$) and by the even lower critical-current density ${\mathit{j}}_{\mathit{c}}^{\mathit{t}}$ across twist boundaries or intergrowths. Possibilities for an improvement of the performance of the (Bi,Pb${)}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$ silver-sheathed tapes are discussed.