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I. Hossain1 2 J. Jiang1 G. Bradford1 3 David Larbalestier1 2 4 Eric Hellstrom1 2 4

1, Florida State University, Tallahassee, Florida, United States
2, Florida State University, Tallahassee, Florida, United States
3, Florida State University, Tallahassee, Florida, United States
4, Florida State University, Tallahassee, Florida, United States

Achieving high Jc in Bi2Sr2CaCu2Ox (Bi-2212) round wire requires overpressure heat treatment (OPHT) to eliminate current-limiting bubbles. The OPHT has a maximum temperature of 890 °C and is done with 50 bar overpressure with 1 bar oxygen partial pressure (PO2). During OPHT, the diameter of the Bi-2212 wire shrinks about 4%, with about 80% of this densification occurring during a 2 h hold at 820 °C before the Bi-2212 powder melts. As part of his PhD studies, Maxime Matras from our group investigated predensifying Bi-2212 round wire for 2 h at 820 °C at 50 bar overpressure with different PO2, followed by the full OPHT with 1 bar PO2. A surprising result of his study was that Jc of Ag-0.2 wt% Mg sheathed Bi-2212 round wire increased by more than 20% when the 820 °C predensification was done in 5 bar PO2, compared to predensification using 1 bar PO2. We are investigating what causes this significant increase in Jc and the processing parameters (t, T, PO2) to optimize this increase in Jc by predensification. This presentation will report on results of on-going predensification studies at temperatures ranging from 500 °C to 850 °C, at different PO2, for different times. Jc, irreversibility field, and critical temperature are being measured to determine the optimum heat treatment parameters. Scanning electron microscopy of fully-processed Bi-2212 wire is being used to understand the microstructure-property relationships.

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