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

superconductor science and technology/superconductor science and technology

Evidence for length-dependent wire expansion, filament dedensification  and consequent degradation of critical current density in Ag-alloy sheathed  Bi-2212 wires