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Transport critical current density (J c ) is a limiting factor in the practical application of (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O y (BSCCO-2223) high-temperature superconducting materials. The minimum requirement is for a J c of 10 5 A cm −2 in long wires and tapes, at 77 K and in a high magnetic field. The issue of current distribution inside Ag-sheathed tapes has been resolved by showing that the region of high critical current density is next to the silver. In this paper we report a J c value > 10 5 A cm −2 at 77 K in a self-field in Bi 1.8 Pb 0.4 Sr 2.0 Ca 2.2 Cu 3.0 O y superconducting tapes made by the Ag wire-in-tube method, in which a silver wire was introduced into the silver tube. The silver wire extended along the entire length of the tape and reduced the difference between the nominal cross section of the superconductor and its actual current-carrying cross section. At 77 K and self-field, a maximum J c of 2 × 10 5 A cm −2 (critical current I c = 22 A) was measured after three pressing cycles at 800 • C in a 7% O 2 atmosphere. The average thickness of the superconducting layer between the sides of silver layers was in the order of 1-2 µm and the width was 3.6 mm. This study confirms that the current-carrying region in BSCCO tapes is the layer adjacent to the Ag sheath. Previously it was shown that the superconductor core for thicker superconductor layers (∼ 70 µm) does not carry significant current.

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The transport current properties of Ag-sheathed (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O y BSCCO-2223 superconducting tapes were analysed as a function of cooling rate. Changing the cooling rate showed a pronounced effect on the transport current of the superconductor heat-treated at 810 • C in a 7% O 2 atmosphere. Results indicated that, during fast cooling, the thin layer of BSCCO adjacent to the Ag sheath is under compression. The compressive stress causes microcracking and affects the alignment and interconnectivity of 2223 grains in the thin layer of BSCCO next to the silver sheath. This changes the current path through the high-critical-current-density region in the superconductor. A comparison between furnace-cooled tapes and slowly cooled tapes (10 • C h −1 to 780 • C and then 1 • C min −1 to room temperature) showed that the latter attained two or three times higher I c values. Based on this observation, a cooling schedule that includes several intermediate cooling steps is suggested. The effect of the lattice parameter on T c was investigated. Behaviour analogous to that of the 2212 phase was found, T c was affected by changes in the c-axis of the 2223 phase. However, the effect was not as pronounced as it was in the 2212 phase.