The formation mechanism, the temperature range for the growth, and the thermal stability of the 2223 phase in Bi(Pb)-Sr-Ca-Cu-0 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 CazPbO4 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 Ca*+ and Cu2+ present in the liquid phase. The 2223 phase is formed at temperatures in the range 827°C < T < 856°C. The optimum temperature T, for the formation of 2223 phase is 845" * 5°C. The 2223 phase is thermodynamically unstable at temperatures above 856°C. [
The formation of superconducting phases in the Bi(Pb)-Sr-Ca-Cu-0 system has been systematically investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPIMA, ICP-AES, fourprobe dc resistance, and ac susceptibility. Starting compositions, firing temperature, and the duratilon of heat treatment, together with the atmosphere, were found1 to be critical in determining the preferred formation of the 2223 phase. This paper reports the effect of the initial chemical composition, emphasizing the importance of compositional control in the synthesis of the single 2223 phase. It has been shown that, with a correct starting composition and predetermined synthesis conditions, single 2223 phase can be obtained without intergrowth by the 2212 and other impurity crystalline phases. The optimum starting composition for the preferred growth of the 2223 phase was identified as being Bi,,7Pbo.3+ySr*CazCu30~ (y = O . l ) , with excess Pb added in order to compensate for its loss at high temperatures. The effect of Pb doping and excess Cu on the phase formation in the Bi oxide based superconducting system is discussed.[
The formation of second phases during the preparation of the 2223 phase and their stability in the Bi system under various annealing temperatures and atmospheres have been studied. The 2201 precipitates developed at -830"C, and their conversion to the 2223 phase can be completed in the temperature range 810°C 5 T < 830°C. A Ca2Pb04-like phase can precipitate from the liquid phase at -830°C during cooling. A (Sr,Ca)14C~24041 phase is usually found accompanying the synthesis of the 2223 phase. This secondary phase is stable in an oxidizing atmosphere but can be eliminated by annealing under a low oxygen atmosphere or by choosing a suitable starting composition and set of sintering conditions. The precipitation of Ca2Pb04-like phase can be avoided by using a relatively fast cooling rate. Unlike the YBa2Cu30, superconductor, the 2223 phase can be stable under a wide range of atmospheres, such as argon, air, and oxygen. [
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