Influence of the Oxygen Partial Pressure on the Phase Evolution During Bi-2212 Wire Melt Processing

We have studied the influence of the oxygen partial pressure pO2up to 5.5 bar on the phase changes that occur during the melt processing of a state-of-the-art Bi-2212 multifilamentary wire. Phase changes have been monitored in situ by high energy synchrotron X-ray diffraction. We found that the stability of the Bi-2212 phase is reduced with increasing pO2. For pO2> 1 bar, a significant amount of the Bi-2212 phase decomposes upon heating in the range of 400 °C-650 °C. The extent of decomposition strongly increases with increasing pO2, and at pO2= 5.5 bar Bi-2212 completely decomposes in the solid state. Textured Bi-2212 can be formed during solidification when pO2is reduced to 0.45 bar when the precursor is molten. Since the formation of current-limiting second phases is very sensitive to pO2when it exceeds 1 bar, we recommend to reduce the oxygen partial pressure below the commonly used pO2= 1 bar, in order to increase the pO2margins and to make the overpressure process more robust.