The influence of carbon on the critical current densities of melt processed AgCu-sheathed Bi-2212 wires

Influences of oxygen and carbon dioxide gases released during the melting process on the homogeneity of the critical current density have been studied in AgCu-sheathed Bi-2212 round wires in long length. The variation of the pressure and mass of these gases within the wires have been calculated during the course of heat treatment. Numerical results show that processing in air results in insufficient oxygen in the vicinity of the solidification temperatures, suggesting that the oxygen gas is less responsible than the carbon dioxide gas for the gas porosity in the oxide cores. As for the carbon dioxide gas, effects of heating rate to the maximum heating temperature, pre-annealing at 850 °C in air and sealing ends of wires on the critical current densities have been investigated. Pre-annealing has increased the critical current densities of the short wires, while increasing heating rate and sealing ends of wires have caused degradation and large scatter in critical current densities. These results indicate that carbon contained in the oxide core of the wires before the melting process causes much more deleterious effects on critical current densities of long length AgCu-sheathed Bi-2212 wires than those of short length wires.