  High engineering critical current density JE of >500 A/mm2 at 20 T and 4.2 K
can be regularly achieved in Ag-sheathed multifilamentary Bi2Sr2CaCu2Ox
(Bi-2212) round wire when the sample length is several centimeters. However,
JE(20 T) in Bi-2212 wires of several meters length, as well as longer pieces
wound in coils, rarely exceeds 200 A/mm2. Moreover, long-length wires often
exhibit signs of Bi-2212 leakage after melt processing that are rarely found in
short, open-end samples. We studied the length dependence of JE of
state-of-the-art powder-in-tube (PIT) Bi-2212 wires and gases released by them
during melt processing using mass spectroscopy, confirming that JE degradation
with length is due to wire swelling produced by high internal gas pressures at
elevated temperatures [1,2]. We further modeled the gas transport in Bi-2212
wires and examined the wire expansion at critical stages of the melt processing
of as-drawn PIT wires and the wires that received a degassing treatment or a
cold-densification treatment before melt processing. These investigations
showed that internal gas pressure in long-length wires drives creep of the Ag
sheath during the heat treatment, causing wire to expand, lowering the density
of Bi-2212 filaments, and therefore degrading the wire JE; the creep rupture of
silver sheath naturally leads to the leakage of Bi-2212 liquid. Our work shows
that proper control of such creep is the key to preventing Bi-2212 leakage and
achieving high JE in long-length Bi-2212 conductors and coils.
