In situ study of equilibrium phenomena and kinetics in a BiSCCO Ag tape

Structural changes during the annealing of a mono-filament BiSCCO/Ag tape in air are monitored in situ by synchrotron X-ray diffraction. Starting at 750 degrees C, a very slow ramp rate is used, followed by high temperature cycling to study equilibrium phenomena and kinetics. For the first time, the concentration of the partial liquid is directly monitored.

A 3321 phase dissolves below 790 degrees C. Between 760 degrees C and 820 degrees C, the 2212 lattice parameters contract, indicating incorporation of ph and/or Ca. At the same time, grain growth takes place, relieving strains, Between 820 degrees C and 840 degrees C (Ca,Sr)(2)PbO4 dissolves incongruously, while the amount of liquid increases.

Conversion of 2212 to 2223 takes place at all temperatures above 820 degrees C. Above 833 degrees C, (Ca,Sr)(2)CuO3 appears. Cycling between 845 degrees C and 860 degrees C-where 2212 has almost disappeared-reveals fast, nearly reversible changes, indicative of several eutectics involving the 2212, 2223, (Ca,Sr)(2)CuO3 and liquid phases.

By cycling, the incorporated Ca and/or Pb is lost irreversibly. Cooling data are consistent with precipitation of 2212 and 2223 by layer-on-layer growth on the existing grains. Some comments on growth models and a comparison to similar annealing experiments with constant operation temperature are given. (C) 1999 Elsevier Science B.V.

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