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 °C-650 °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.