Important Considerations for Processing Bi-Based High-Temperature Superconducting Tapes and Films for Bulk Applications
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plex processing have limited conductor development. For the Bi-based system, the basic processing steps are becoming known, the grains are well connected, and the weak link problem can be controlled. This permits applications in the temperature range 4-77 K, depending on the field and current density requirements of the particular use. The highest Jc value reported for 2223 phase BSCCO of 53,700 A/cm2 (77 K, OT),8 was achieved in a short tape a few centimeters long. Fabricating long lengths with these high Jc values is not straightforward, but values of about 10,000 A/cm2 (77 K, 0T) are reported for 114 m long conductors.9 The materials science challenges of these complicated systems are to understand the interrelationships between the chemistry, the processing, and the resulting microstructure and electromagnetic properties so that the technology can advance from short to long lengths of high Jc conductor. At the outset, it is useful to consider a working model of the ideal hightemperature superconducting wire. The microstructure should be single-phase with highly aligned grains. However, as discussed below, the superconducting BSCCO phases cannot yet be made phase pure, so real tapes contain second-phase inclusions. In the best tapes, the inclusions
are small in size, occupy only a small volume fraction of the core, do not segregate to grain boundaries, and do not perturb the grain alignment too much. In addition, the dimensions of the superconducting core should be uniform, with no sausaging, and the core of such conductors should not contain cracks or pores that can block the current flow. Clearly, optimizing the processing implies understanding a very large number of variables. This is the challenge my article describes.
Overview of BSCCO Tapes and Films Three superconducting phases exist in the Bi-Sr-Ca-Cu-O (BSCCO) system. They are known by their ideal Bi:Sr:Ca:Cu stoichiometries as 2201 (Tc ~ 7-20 K), 2212 (Tc ~ 75-90 K), and 2223 (Tc ~ 110 K). Throughout this paper, 2201, 2212, and 2223 refer to the respective superconducting phase, not to the actual composition of the phase. All three are two-dimensional, pseudotetragonal, micalike materials (Figure 2) that cleave easily along the (001) planes due to weak bonding between the two adjacent Bi-O planes. The supercurrent is carried preferentially in the (001) CuO2 planes. Both 2212 and 2223 wires are being studied, with the major emphasis on 2223 because of its higher Tc. Both the 22121(H3 and 22231U2'""7 phases exist over a range of stoichiometries, and neither has been prepared absolutely phase pure. 2212 exists over a wider range of stoichiometries than 2223, and it forms more readily than 2223. 2223 is difficult to synthesize, and substituting Pb for part of the Bi makes it easier to synthesize. All of the wire work has been done on Pb-doped 2223. (In the remainder of this paper, 2223 means Pb-doped 2223.) 2223 is stable in a narrow temperature range between about 805 and 840°C in 0.075 atm O2.18 At higher temperatures it melts, and at lower temperature
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