Observation of the formation and growth of secondary phases in Bi-2223/Ag tapes

Formation and growth of secondary phases in Bi-2223/Ag tapes have been investigated. Samples were heat treated at several temperatures for different periods under reduced oxygen partial pressure. The composition and microstructural evolution of secondary phases during the sintering process were characterized by SEM, XRD and EDS. Using high resolution SEM, the (Ca,Sr)2CuO3 (2:1), CuO and (Ca,Sr)14Cu24O41 (14:24) phases could be clearly identified, and details of the Bi-2212, Bi-2223 structure, as well as the quenched liquid region could be also observed. Experiments showed the liquid region of quenched samples contained small Cu-rich precipitates, and the majority of the liquid was rich in Pb, Bi and Ca. The increase of the liquid region area fraction and the disappearance of the 2212 phase both take place quickly, in contrast to the slower development of the total secondary phase area fraction. The amount of the large secondary phase particles was lowest for samples sintered at 830 deg. C in 8.5% O2. At higher temperatures, the amount of the 2:1 phase increased and the particle size of the 14:24 became larger, while at lower temperatures, the CuO particles appeared at very beginning and tended to grow rather fast