Phase formation and critical current density in Bi,Pb(2223) tapes

The various fabrication and reaction processes leading to multifilamentary Bi,Pb(2223) tapes with high transport jc are described. In air, the Bi,Pb(2223) phase in pressed powders is found to be formed by nucleation, following the complete decomposition of the Bi,Pb(2212) intermediate phase. In this work, the precise formation conditions of the Pb-free phase Bi(2223) are described for the first time. It is shown that the presence of Pb leads to a lowering of the melting temperature from 879 to 873 deg. C and of that of the 'transient liquid' (ΔT ≅ 15 deg. C). The latter is formed after decomposition of the 2-layer compound and is necessary to the formation of the 3-layer compound. Thus Pb is found to have no influence on the formation mechanism of the 3-layer compound, which always starts by a nucleation process. Nucleation processes are also found to occur in the first phase of reaction inside Ag sheathed tapes, intercalation being possible at later stages. The highest values of the critical current density in Bi,Pb(2223) tapes reported at the present day are compared, taking into account the analysed tape lengths. The variation of jc in mono- and multifilamentary Bi,Pb(2223) tapes as a function of the applied magnetic field at 77 and at 4.2 K is described. A new process including deformation by a four-roll machine is presented, and the distribution of jc inside various tape configurations with similar critical current densities is analysed: for monofilamentary tapes, a maximum of 53,000 A cm-2 is found at the borders, the ratio Ic(max)/Ic(min) being 2.4, while for conventional multifilamentary tapes the maximum is found at the tape centre (35,000 A cm-2, ratio 1.6). For the new four-rolled tapes, a ratio of only 1.1 was measured, indicating a more homogeneous current distribution. In order to lower the AC losses due to the highly conductive Ag matrix, the new concept of 'oxide barriers' is introduced. A barrier of BaZrO3 surrounds each filament, thus leading to an enhanced radial resistivity, which in turn reduces AC losses. Finally, the current transport mechanism inside Bi,Pb(2223) tapes and the various current limiting mechanisms are discussed. (author)