The influence of cation stoichiometry on the critical currents and the phase formation in Ag-sheathed Bi(2223) tapes

We investigated the effect of different variations of the cation stoichiometry on the critical currents and on the phase formation of 2223 phase in Ag-sheathed Bi(2223) tapes. Our studies in this context were focused on the influence of variations of the copper, lead and bismuth contents on the critical currents of the tapes. Varying the copper content in the range 3.0<x(Cu)<3.4 in Bi1.95Pb0.30Sr2.0Ca2.1CuxOy, we observed a maximum critical current for x(Cu) values around x(Cu) 3.2. For the variation of the Pb content we found a plateau-like region between x(Pb) 0.25 and x(Pb) = 0.35, for a variation of x(Pb) in the range of 0.2 < x(Pb) < 0.4 in Bi1.95PbxSr2.0Ca2.1Cu3.0Oy. A simultaneous variation of Bi and Pb content showed nearly no effect on the final critical currents. Finally, results of a variation of the Bi content between x(Bi) = 1.65 and x(Bi) 1.95 indicated a decrease of critical currents towards low Bi contents and an optimum x(Bi) at around 1.85. For the Cu and the Pb contents the dependence of the critical currents on the reaction temperature was investigated. For both systems we found that the results were only valid for a limited temperature region. The result for varied Cu content was confirmed for temperatures lower than 822 deg. C. For higher temperatures samples with lower Cu contents achieved higher critical currents. In addition, the dependence on the Pb content turns out to be strongly temperature dependent and was modified for temperatures below 812 deg. C and for temperatures above 821 deg. C. Furthermore, the dependence of the phase formation on the copper and lead contents has been studied by x-ray analysis. Stoichiometry variations strongly affect the kinetics of phase formation and thus the achieved critical currents in the tapes. (author)