THE FORMATION MECHANISM OF (Bi,Pb)-2223 PHASE AND A PSEUDO-BINARY PHASE DIAGRAM OF (Bi,Pb)-Sr-Ca-Cu-O SYSTEM

摘要

We propose a pseudo-binary phase diagram of the (Bi,Pb)-Sr-Ca-Cu-0 system along the line of Bi_(1.6)Pb_(0.4)Sr_2Ca_(n-1)Cu_nO_y in the temperature range 820-1000°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)CuO_2 (n→∝) phases. The 2201 phase is solid solution which is stable at 1 ≦ n≦1.2. The eutectic composition point is close to the maximum solid solution composition of the 2201 phase (n～1.2). The temperature interval between the peritectic reaction L + (Sr,Ca)_2CuO_3 + (Sr,Ca)CuO_<=> 2212 and the eutectic reaction L<=>2201 + 2212 is only about 3°C. From this, for compositions 1.2 2212 will be suppressed by rapid solidification, resulting in the formation of the 2201 phase directly through the eutectic reaction L => 2201 +2212. For composition n=3, it can be seen that at temperatures above 940°C, CaO and the liquid phase are stable. During cooling process, these two phases react peritectically to produce (Sr,Ca)_2CuO_3. At around 890°C, (Sr,Ca)_2CuO_3 reacts with the liquid phase to produce (Sr,Ca)CuO_2. At around 865°C, (Sr,Ca)_2CuO_3 and (Sr,Ca)CuO_2 reacts with the liquid phase to produce the 2212 phase. The 2223 phase was transformed by a pentectoid reaction of the 2212 phase and residual (Sr,Ca)_2CuO_3, (Sr,Ca)CuO_2 at around 855°C.