The phase equilibria near (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10+␦ ((Bi,Pb)-2223) was studied in the temperature range of 750°-820°C in air and in reducing atmospheres. In air, (Bi,Pb)-2223 was in equilibrium with one or more of the following phases: (Bi,Pb) 2 Sr 2 CaCu 2 O 8+␦ ((Bi,Pb)-2212), (Sr,Ca) 14 Cu 24 O 41 (14:24), (Ca,Sr) 2 CuO 3 , (Bi,Pb) 4 (Sr,Ca) 5 -CuO x (451), Ca 2 PbO 4 , and CuO. Ca 2 PbO 4 and the 451 phase were not in equilibrium with (Bi,Pb)-2223 at an oxygen partial pressure ( p O 2 ) of 10 −3 atm, which is indicative of higher lead solubility in (Bi,Pb)-2223 under more-reducing conditions. In a second set of experiments, the lead content in both (Bi,Pb)-2212 single crystals and (Bi,Pb)-2223 powders was investigated as a function of p O 2 at a constant temperature of 750°C. Lead solubility in (Bi,Pb)-2212 and (Bi,Pb)-2223 increased as p O 2 decreased.
Reaction couples of either ((Bi,Pb)-2212) single crystals with `' powder or (Bi-2212) single crystals with `' powder were used to study the formation reaction of (Bi,Pb)-2223. The samples were annealed between and , in air, for 65 h. Microstructural observations were made of the different geometrical interfaces. No (Bi,Pb)-2223 platelets were formed below . Above , the (Bi,Pb)-2212 and (Bi-2212) crystals were found to break up near the interfaces with the powders, and (Bi,Pb)-2223 platelets were formed adjoining the (Bi,Pb)-2212 and (Bi-2212) crystallites. No evidence for large quantitites of liquids was found, although small, rounded particles having a liquid droplet appearance of a Bi/Pb rich phase of stoichiometry near the phase were observed on many of the (Bi,Pb)-2223 platelet surfaces. The experiments suggest that the formation of (Bi,Pb)-2223 occurs simultaneously with decomposition of (Bi,Pb)-2212 and the presence of liquid droplets, providing further support for the dissolution - reprecipitation model for phase formation.
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