The formation mechanism of (Bi,Pb)-2223 phase and a pseudo-binary phase diagram of (Bi,Pb)-Sr-Ca-Cu-O system

We propose a pseudo-binary phase diagram of the (Bi,Pb)-Sr-Ca-Cu-O system along the line of Bi1.6Pb0.4SnCan-1CunOy in the temperature range 820-1000 degrees C by combining the observation of high-temperature optical microscope with results of XRD, SEM, EDS, DTA and AC susceptibility measurements. This diagram shows three kinds of peritectic reactions, one eutectic reaction and one peritectoid reaction. The equilibrium solid phases in this diagram are the 2201 (n=1), 2212 (n=2), 2223 (n=3) and (Sr,Ca)CuO2 (n-->infinity) phases. The 2201 phase is solid solution which is stable at ii n less than or equal to 1.2. The eutectic composition point is close to the maximum solid solution composition of the 2201 phase (n similar to 1.2). The temperature interval between the peritectic reaction L + (Sr,Ca)2CuO3 + (Sr,Ca)CuO2 double left right arrow 2212 and the eutectic reaction L double left right arrow 2201 + 2212 is only about 3 degrees C. From this, for compositions 1.2<n, the peritectic reaction L + (Sr,Ca)2CuO3 + (Sr,Ca)CuO2 double right arrow 2212 will be suppressed by rapid solidification, resulting in the formation of the 2201 phase directly through the eutectic reaction L double right arrow 2201 + 2212. For composition n=3, it can be seen that at temperatures above 940 degrees C, CaO and the liquid phase are stable. During cooling process, these two phases react peritectically to produce (Sr,Ca)2CuO3. At around 890 degrees C, (Sr,Ca)2CuO3 reacts with the liquid phase to produce (Sr,Ca)CuO2. At around 865 degrees C, (Sr,Ca)2CuO3 and (Sr,Ca)CuO2 reacts with the liquid phase to produce the 2212 phase. The 2223 phase was transformed by a peritectoid reaction of the 2212 phase and residual (Sr,Ca)2CuO3, (Sr,Ca)CuO2 at around 855 degrees C.