  It is well known that longer Bi-2212 conductors have significantly lower
critical current density (Jc) than shorter ones, and recently it has become
clear that a major cause of this reduction is internal gas pressure generated
during heat treatment, which expands the wire diameter and dedensifies the
Bi-2212 filaments. Here we report on the length-dependent expansion of 5 to 240
cm lengths of state-of-the-art, commercial Ag alloy-sheathed Bi-2212 wire after
full and some partial heat treatments. Detailed image analysis along the wire
length shows that the wire diameter increases with distance from the ends,
longer samples often showing evident damage and leaks provoked by the internal
gas pressure. Comparison of heat treatments carried out just below the melting
point and with the usual melt process makes it clear that melting is crucial to
developing high internal pressure. The decay of Jc away from the ends is
directly correlated to the local wire diameter increase, which decreases the
local Bi-2212 filament mass density and lowers Jc, often by well over 50%. It
is clear that control of the internal gas pressure is crucial to attaining the
full Jc of these very promising round wires and that the very variable
properties of Bi-2212 wires are due to the fact that this internal gas pressure
has so far not been well controlled.
