Understanding the Critical Current Density Limits of BiSrCaCuO High Temperature Superconductors
Final rept. 7 Aug 1996-7 Jul 2000
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The causes of bubbles that form when heat treating Ag-sheathed Bi2Sr2CaCu2O8 2212 tapes and wires, and methods to eliminate them were studied. H2O and CO2, which form bubbles, are present as condensed phases in the precursor oxide powder, and O2 is released from 2212 as it melts. Infrared spectroscopy of gases that evolved from the tape during heating showed the H2O and CO2 content in the tape could be significantly reduced by heating the tape at 700 deg C in vacuum for 24h then at 835 deg C for 48h in 1 atm O2 before melt processing the tape. No bubbles formed after the H2O and CO2 were removed, and the tape was heated at 1 deg Cm through the 2212 melting temperature. Overpressure processing hydrostatic pressing with a mixture of Ar and O2 up to 10 atm total pressure could prevent bubbling but it did not remove H2O and CO2 from the tape. Overpressure processing could also densify the 2212 core by compressing the Ag-sheath tape during the heat treatment, eliminating the packing porosity present in the as-rolled green tape.
- Electricity and Magnetism