Limits to the critical transport current in superconducting (Bi,Pb)2Sr2<…

We have proposed the ``railway-switch`` model to describe the superconducting transport current in (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10} silver-sheathed tapes. The model assumes that in the textured polycrystalline filament the frequent small-angle {ital 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 {ital I}-{ital 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 {ital T}=77 K in self-field was in the range {ital j}{sub {ital c}}=20--30 kA/cm{sup 2}. We conclude that the dominant limitation for the transport critical current in the tapes is imposed by the low intragrain critical-current density {ital j}{sub {ital c}}{sup {ital c}} along the {ital c} axis (as compared to the in-plane critical-current density {ital j}{sub {ital c}}{sup {ital a}{ital b}}) and by the even lower critical-currentmore » density {ital j}{sub {ital c}}{sup {ital t}} across twist boundaries or intergrowths. Possibilities for an improvement of the performance of the (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10} silver-sheathed tapes are discussed.« less