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 pO(2) up 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 pO(2). For pO(2) > 1 bar, a significant amount of the Bi-2212 phase decomposes upon heating in the range of 400 degrees C-650 degrees C. The extent of decomposition strongly increases with increasing pO(2), and at pO(2) = 5.5 bar Bi-2212 completely decomposes in the solid state. Textured Bi-2212 can be formed during solidification when pO(2) is reduced to 0.45 bar when the precursor is molten. Since the formation of current-limiting second phases is very sensitive to pO(2) when it exceeds 1 bar, we recommend to reduce the oxygen partial pressure below the commonly used pO(2) = 1 bar, in order to increase the pO(2) margins and to make the overpressure process more robust.