Phase diagram studies in the Bi(Pb)-Sr-Ca-Cu-O system

Abstract

The title system (with 20% Pb for Bi) was studied by annealing and quenching mixtures along the Bi 2Sr 2CuO x-CaCuO 2 pseudo-binary line, which includes the nominal compositions of the known superconductors in the (Bi,Pb) 2Sr 2Ca n−1Cu nO x series. The phase equilibria were determined between n = 1 and 15 in the temperature range 828–890 °C. The n = 1 (Ca-free system) is complex due to the coexistence of n = 1 with another phase of very similar composition, identified as the “23.7Å-phase”. For 3 ≤ n ≤ 10, successive reactions with increasing temperatures are as follows (neglecting secondary phases): 2212 → 2223 → 2212 → 2201 → 2201 + “23.7Å-phase”. Regions of equilibrium with a liquid phase include either n = 1, n = 1 + 2, or n = 2 + 3. The n = 3 (2223) phase forms best for 4 ≤ n ≤ 6, in a narrow temperature range just below the solidus line. Its formation is much slower outside this composition range. A pseudo-eutectic is tentatively located slightly above n = 4. Other stable phases in this system include(Ca 2−xSr x)CuO 3 and Ca 2PbO4.