THE INFLUENCE OF CARBON ON MELT PROCESSING AG SHEATHED BI2SR2CACU2O8 TAPE

We have investigated the influence of carbon on the melt processing of Ag sheathed Bi2Sr2CaCu2O8 (2212) tapes. With increasing carbon content the melting and freezing temperatures of the 2212 phase decrease, varying 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 tape. With increasing carbon content, the amount of Bi-2(Sr,Ca)(4)O-x in the melt decreases, and the amount of (Sr,Ca)(2)CuO3 increases. SrCO3 forms at high carbon contents. Methods to reduce the carbon content in 2212 powder have been studied. It can be removed by heating in flowing O-2 at 800 degrees C or in vacuum at 600 degrees C. The critical current 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 microstructure.