T. Shen, A. Ghosh, L. Cooley, J. Jiang
High engineering current density JE of >500 A/mm2 at 20 T and 4.2 K can be regularly achieved in the 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 Bi-2212 wires often exhibit signs of leakage after melt processing that are rarely found in short, open-ended samples. Expanding on work by Malagoli et al. [1,2], combined with detecting gas releases during heat treating the state-of-the-art powder-in-tube (PIT) Bi-2212 wire using mass spectroscopy and modeling gas transport in Bi-2212 wire, we explicitly show that JE degradation with length is a direct consequence of Bi-2212 de-densification due to wire swelling produced by high internal gas pressures at elevated temperatures. We 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, further showing that wire swells because under the influence of internal gas pressure, silver creeps and ruptures, and that the creep rupture of silver sheath naturally leads to the leakage of Bi-2212 liquid. Identifying the mechanisms of JE degradation and leakage led us to conclude that it is feasible to develop a long-length, leak-free Bi-2212 conductor that carries a JE of >500 A/mm2 for very high field superconducting magnet technology through a combination of reducing or removing the source of internal gases with techniques such as applying an external gas pressure during heat treatment to inhibit de-densification of Bi-2212 or to even induce compressive Ag creep to further densify Bi-2212.
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http://arxiv.org/abs/1301.5341
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