Phase transformation kinetics during thermal annealing of LFZ Bi-Sr-Ca-Cu-O superconducting fibers in the range 800-870°C

Phase development of laser floating zone (LFZ) fibers of Bi2SrO2Ca2Cu4O11 (2224) nominal composition during isothermal annealing at 800-870 degrees C for 1.5-24 h is investigated. The as-grown fibers have axially aligned Sr0.3Ca0.7CuO2 ("1/1") dendrites of constant average thickness delta = 7.8 mu m, an interdendritic matrix with the Bi4Sr4CaCu3O14 ("4413") intergrowth, the Bi2Sr2CaCu2O8 ("2212") phase and CuO cubic grains. Primary "1/1" cuprate and "4413" constituent are metastable and soon react at T greater than or equal to 800 degrees C transforming into equilibrium phases, "2212", Sr0.3Ca1.7CuO3 ("2/1") and Sr7Ca7Cu24O41 ("14/24"). The growth kinetics of "2212", "2/1" and "14/24" fit a parabolic dependence on time, revealing a diffusion controlled mechanism of reaction in the solid state with activation energies in the 170-400 kJ mol(-1) range. For T greater than or equal to 835 degrees C, the Bi2Sr2Ca2Cu3O10 ("2223") phase develops. The reaction rates of all phases are dependent on the volume fraction of "1/1" in the as-grown fibers. Although the growth mechanism of "2223" resembles that of "2212", by enrichment of the "2212" plates in Ca and Cu from "2/1" and CuO phases, the overall reaction is a four-step reaction with an activation energy close to 1.5 MJ mol(-1). Present results confirm the "2212"-"2223"-"2/1"-"14/24" as the set of compatibility phases for the 2224 composition at 835-870 degrees C when the "2223" phase is thermodynamically stable in air. (C) 1999 Elsevier Science B.V. All rights reserved.