Microstructural evolution at the initial stages of annealing in a Bi-2223 multifilament tape

The microstructural evolution at the initial stage of annealing in a multifilament Bi-2223 (2223) tape is studied in quenched samples using XRD, SEM and EDS. The annealing was carried out at 830 degreesC under reduced oxygen partial pressure. Samples were quenched in air upon reaching 830 degreesC, as well as after 1, 2, 5, 10, 30 and 52 h at 830 degreesC. It was found that the initial liquid formation was associated with alkaline-earth cuprate (AEC) particles such as (Ca,Sr)(14)Cu24Oz and (Ca,Sr)(2)CuO3. The liquid is rich of Pb and the AEC particles dissolve and supply Cu and Ca into the liquid through direct contact. The 2223 kinetics are well correlated with the structural parameters of liquid and AEC particles. During the first hour the liquid activity is confined to the neighbourhood of AEC particles and the rate of 2223 formation is very low. Between 2 and 10 h the liquid amount increases and the size (measured in tape plane) is comparable with the particle spacing indicating a liquid activity on an overall scale and sufficient feeding of Cu and Ca. Consequently, the 2223 develops at a fast rate. After 10 li the liquid amount is decreased, the particle spacing is far larger than the liquid size and the rate of 2223 formation slows down. The early 2223 forms as precipitates in the structure. The possible formation mechanisms are discussed.