ABSTRACT
The Bi2Sr2CaCu208 (2212) conductors* with the highest critical current density (JC) are made using melt processing^ An underlying assumption in most of the work on 2212 conductors is that a homogeneous, highly aligned microstructure is needed to attain a high Jc. The notable exception is the group at Vacuumschmelze, which first reported melt processing 2212 wire [1]. Their Ag-sheathed wire does not contain aligned 2212, but they want the conductor to have a homogeneous microstructure. Figure 1 shows a generic melt processing schedule for tape, wires, and films. The principle involved in melt processing is quite simple: the 2212 conductor is heated until it melts, then the 2212 phase reforms in situ in the conductor during cooling. However, the 2212 phase melts incongruently, generating a melt consisting of liquid and nonsuperconducting phases from which the superconducting 2212 phase must form during cooling. The incongruent melting causes severe problems when reforming the 2212 phase. In particular, the nonsuperconducting phases in the melt are not fully consumed during cooling, so they become remnant phases in the fully processed conductor. They block the current path, disturb the local 2212 alignment, and prevent full conversion back to 2212. These remnant nonsuperconducting phases need to be eliminated to achieve the ideal microstructure.
