Improvement of the microstructure and critical current densities of Bi-2212 round wires with a precisely controlled heat treatment

We succeeded in obtaining excellent Jc values of Bi2212 multifilamentary round wire with a precisely controlled heat treatment. The heat treatment, which consisted of melt solidification and a subsequent slow-cooling process, was applied to Bi-2212 superconductors. The Ic values of Bi-2212 wires heat treated with a maximum processing temperature Tmax below the optimum Tmax of Bi-2212 were zero, while high Ic values of 450 A, Jc = 280 kA cm-2 at 4.2 K and 10 T magnetic field, were obtained in a very narrow temperature range of the optimum Tmax. When the wires were heat treated with Tmax over the optimum Tmax, however, the Ic values were almost one-half of the highest Ic values. We found that the optimum Tmax window was very narrow for our multifilamentary wires. The well controlled heat treatment resulted in uniform filaments with few outgrowths from multifilaments and, hence, high Ic values for the wires