W. Zhang et Ee. Hellstrom, THE INFLUENCE OF CARBON ON MELT PROCESSING AG SHEATHED BI2SR2CACU2O8 TAPE, Physica. C, Superconductivity, 234(1-2), 1994, pp. 137-145
We have investigated the influence of carbon on the melt processing of
Ag sheathed Bi2Sr2CaCu2O8 (2212) tapes. With increasing carbon conten
t the melting and freezing temperatures of the 2212 phase decrease, va
rying from similar to 865 to 850 degrees C for carbon contents ranging
from 0.02 to 0.16 wt.%, respectively, in the 2212 powder. The carbon
content affects the phase assemblage in the melt and fully-processed t
ape. With increasing carbon content, the amount of Bi-2(Sr,Ca)(4)O-x i
n the melt decreases, and the amount of (Sr,Ca)(2)CuO3 increases. SrCO
3 forms at high carbon contents. Methods to reduce the carbon content
in 2212 powder have been studied. It can be removed by heating in flow
ing O-2 at 800 degrees C or in vacuum at 600 degrees C. The critical c
urrent density in tape with high carbon content is low (1 to 1.5 kA/cm
(2)) due to large nonsuperconducting phases and porosity in the micros
tructure.