EFFECT OF CARBON IMPURITIES ON BI,PB(2223) PHASE-FORMATION AND CRITICAL-CURRENT DENSITIES IN SILVER-SHEATHED BI,PB(2223) TAPES

An extensive study on the detrimental effects of C on the critical current density of silver-sheathed Bi,Pb(2223) rapes has been undertaken. Calcinations at 820 degrees C in air, short-time low-energy milling and calcinations in CO2 free atmospheres are effective in diminishing the amount of C in Bi,Pb(2223)/Ag tapes. We have studied the effect of C by preparing two types of Bi,Pb(2223)/Ag rapes under identical conditions except for the calcination atmosphere (one in air and another in air and 0.5% vol. of CO2). For the C poor tapes (with 0.036 wt.% C), the amount of Bi(2212) in the calcined powder is higher, the reaction to Bi,Pb(2223) is faster, the critical current densities j(c) and their field dependence are better compared to the C rich tapes (with 0.056 wt.% C). The excess C is located preferentially at the grain boundaries and favors the formation of secondary phases. This is reflected by a steeper initial dope of j(c)(B) for the C rich tapes.