The Formation Mechanism Of (Bi,Pb)-2223 Phase and a Pseudo-Binary Phase Diagram of (Bi,Pb)-Sr-Ca-Cu-O System

Abstract

We propose a pseudo-binary phase diagram of the (Bi,Pb)-Sr-Ca-Cu-O system along the line of Bi_1.6Pb_0.4Sr₂Ca_n-1Cu_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)CuO₂ (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)₂CuO₃ + (Sr,Ca)CuO₂ ⇔ 2212 and the eutectic reaction L ⇔ 2201 + 2212 is only about 3^°C. From this, for compositions 1.2<n, the peritectic reaction L + (Sr,Ca)₂CuO₃ + (Sr,Ca)CuO₂ ⇒ 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)₂CuO₃. At around 890^°C, (Sr,Ca)₂CuO₃ reacts with the liquid phase to produce (Sr,Ca)CuO₂. At around 865^°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^°C.