Critical current density normal to the tape plane and current-transfer length of (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 silver-sheathed tapes

Abstract

We have measured the critical current density parallel ( j c) and normal (j c n ) to the plane of monofilamentary (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 silver-sheathed tapes prepared on the laboratory scale ( l ≤ 10 m) by the powder-in-tube technique with rolling as the only tape-forming process. For the range of critical current densities along the tape j c( T = 77 K, B = 0 T) = 20…30 kA/cm 2 we obtain j c n ) = 2…3 kA/cm 2 normal to the tape plane. We have a directly measured the current-transfer length L c, i.e. the length along the tape that is necessary for the distribution of the current over the filament cross-section. As an upper limit we find L c ≤ 2 mm at T = 77 K for tapes with j c( T = 77 K, B = 0 T) > 20 kA/cm 2. Transmission electron microscopy of the polycrystalline (Bi,Pb) 2Sr 2Ca 2Cu 3O 10 filament confirms the dominant role of the frequent small-angle c-axis tilt grain boundaries (“railway switches”) for the current transport, and the observed low anisotropy ratio j c j c n ≈ 10 as well as the magnetic-field dependence j c n (B) agree well with the “railway-switch” model.