Melting relations of Bi_2O_3-SrO-CaO-CuO superconductors at various oxygen fugacities

摘要

In an effort to synthesize monophasic superconducting oxides in the quaternary system Bi_2O_3-SrO-CaO-CuO two studies have been conducted. The melting relations of superconducting oxides Bi_2Sr_2-Ca-Cu_2O_8 (Bi-2212) and Bi_2Sr_2-Ca_2Cu_3O_(10) (Bi-2223) have been experimentally investigated separately at various oxygen fugacities {f(O_2)} from 1 to 3.10~(-4) bar at 1 atm total pressure. Severalinteresting properties of the system have been found with respect to this portion of the quaternary; the modal proportions of liquid produced at the solidus depend on the oxygen fugacity. The amount of liquid produced at the Bi-2212 solidus reached a maximum of approximately 75percent at f(O_2) = 0.1 bar, Bi-2223's maximum proportion of liquid formed at the solidus continues to increase up to the lowest oxygen fugacity f(O_2) = 0.01 bar studied. CaO (with a small amount of dissolved SrO) is the liquidus phase over the entire range of f(O_2) for both compositions. (Sr,Ca)_2CuO_3 is the second phase to crystallize at high f(O_2) but, as f(O_2) decreases, its primary phase volume shifts for Bi-2212, and Bi_2(Ca,Sr)_3O_6 is the second phase to form. (Sr,Ca)_(14)Cu_(24)O_(41) exhibited liquidus relationship only in high oxygen fugacities. Only Bi-2212 melts to assemblages that include bismuth-bearing crystalline phases. Bi_9(Ca,Sr)_(16)O_x is stable only at high oxygen fugacities and Bi_2(Ca,Sr)_3O_6 is only stable at the lower values. At the lowest oxygen fugacities, Bi-2212 liquids crystallize (Sr,Ca)CuO_2 at a higher temperature than (Sr,Ca)_2CuO_3 and no (Sr,Ca)CuO_2 was found in Bi-2223 bulk compositions. These variations apply constraints on the positions of the univariant and invariant phase equilibria and these constraints are described.