Formation of the high-Tc phase in lead-doped Bi-Sr-Ca-Cu-O superconductors

The characteristics of Bi-Sr-Ca-Cu-O superconductors have been investigated as a function of the amount of Pb addition. The volume fraction of the high-T_c phase increases with increasing Pb addition up to x=0.3 in (Bi_1–xPb_x)₂Sr₂Ca₂Cu₃O_y and above that it decreases. The grain size is increased and density is decreased with increasing Pb addition, due to two-dimensional grain growth which results from formation of the high-T_c phase. The specimen with x=0.3 has a transition temperature of 104 K and high magnetic susceptibility due to the fact that most of the volume fraction is of the high-T_c phase. Pb addition cause formation of Ca₂PbO₄ as a secondary phase and it produces a partially melted liquid phase below the sintering temperature, which acts as a flux and promotes formation of the high-T_c phase.