2223 PHASE FORMATION IN BI(PB)-SR-CA-CU-O .2. THE ROLE OF TEMPERATURE - REACTION-MECHANISM

The formation mechanism, the temperature range for the growth, and the thermal stability of the 2223 phase in Bi(Pb)-Sr-Ca-Cu-O have been investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPMA, four-probe resistance and ac susceptibility measurement. The formation of the 2223 phase was found to follow a dissolution-precipitation process. A 2212 phase first reacts with the liquid phase formed via an incongruent melting of the Ca2PbO4 phase, and a dissolution of CaO and CuO takes place. The 2201 phase, which has the largest negative free energy, is then precipitated from the melt; the nucleation and growth of the 2223 phase are subsequently developed by the reaction between the 2201 phase precipitates and ions of Ca2+ and Cu2+ present in the liquid phase. The 2223 phase is formed at temperatures in the range 827-degrees-C < T < 856-degrees-C. The optimum temperature T(f) for the formation of 2223 phase is 845-degrees +/- 5-degrees-C. The 2223 phase is thermodynamically unstable at temperatures above 856-degrees-C.