Effect of Po2 and Ag on the Phase Formation of the Pb-2223 Superconductor
Winnie K. Wong-Ng, Lawrence P. Cook, W Greenwood
The chemical reactions and the compositional characteristics of liquids which lead to the phase formation of the Pb-2223 ((Bi,Pb):Sr:Ca:Cu:O) superconductor have been studied for a precursor composition of Bi1.8Pbo.4Sr2Ca2.2Cu3Ox. The combined techniques of quenching, powder X-ray diffraction, differential thermal analysis (DTA), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) were used for this study. Samples were annealed under pure air (CO2- and moisture-scrubbed) and under a mixture of 7.5 volume % O2/ 92.5 volume % Ar. Under pure air, the precursor composition was within the five-phase subsolidus volume: 2212-CuO-(ca,Sr)2CuO3-(Ca,Sr)2PbO4-(Sr,Ca)14Cu24O41, whereas under 7.5 volume % O2 the five-phase subsolidus volume prior to melting was: 2212-CuO-3221-(Ca,Sr)2CuO3 -(Ca,Sr)2PbO4. In both cases, the formation of a metastable liquid was found to assist in the formation of the 2223 phase from the precursor solids. Under pure air, the reaction which produced metastable liquid was: CuO + 2212 + (Sr,Ca)14Cu24O41 + (Ca,Sr)2PbO4 > (Ca,Sr)2CuO3 + L. Under 7.5 volume % O2, the reaction was: CuO + 2212 + 3221 + (Ca,Sr)2PbO4 > (Ca,Sr)2CuO3 + L. Long-term annealing of precursor materials above the temperature of metastable melting resulted in a stable equilibrium assemblage containing 2223. The temperature of melting of this assemblage was consistently higher than that of the precursor materials. Depending on the temperature of the long-term annealing, both metastable and stable melts assisted in the formation of the 2223 phase. The presence of Ag in both the pure air and the 7.5 volume % O 2 experiments lowered the temperature of the melting events. Ag partially dissolved in the liquid, and its presence altered the subsolidus equilibria when the sample was annealed under 7.5 volume % O 2. These results are discussed with respect to their processing implications.