The microstructures and critical current densities of Bi-2212 multi-filamentary round wires were investigated. The microstructures and Jc properties of Bi-2212 composite wires are very sensitive to heat treatment. We studied the dependence of Jc on maximum processing temperature Tmax precisely. The highest Ic values of 450 A (Jc=2.8×109 A/m2) were obtained at very narrow temperature range of Tmax, around 1155 K. The Ic values of wires heat-treated with Tmax below this optimum processing temperature Top were zero. When the wires were heat-treated over Top, however, Ic values were almost half values of the highest Ic values. Scanning electron microscope observation indicated that Bi-2212 filaments were not melted for Tmax below Top. Above Top, on the other hand, many bridges between filaments were observed, which were due to the outgrowth of Bi-2212. The wires heat treated at optimum temperature had fewer bridging of filaments than the wires heat treated above Top. The well-controlled heat-treatment resulted in a few outgrowths from multifilament and high Ic values of wires.
