Anin situ study of the annealing behaviour of BiSCCO/Agtapes

Abstract

The phase transformations and structural changes occurring during initial heating and annealing of an Ag-clad high-Tc superconducting tape of the (Bi, Pb)2Sr2Ca2Cu3Ox type are investigated. The annealing takes place in air at an operating temperature of 835 °C. Using x-ray diffraction with 100 keV photons from a synchrotron source the concentration, stoichiometry and texture of the dominant phases are monitored in situ during the transformation of BiSCCO from (Bi, Pb)2Sr2CaCu2Ox (2212) to (Bi, Pb)2Sr2Ca2Cu3Ox (2223). In addition, information on grain size and residual strain is obtained. During heating the (Ca, Sr)2PbO4 additive decomposes between 700 °C and 820 °C. Simultaneously, the residual strain in the 2212 grains is relieved and the c-axis alignment of the grains is substantially improved. Moreover, the Pb content of the 2212 structure increases continuously. We interpret these results as being related to a temperature-dependent solubility limit of Pb in 2212, leading to a substantial grain growth of the phase. Above 812 °C 2212 partly decomposes to form (Ca, Sr)2CuO3 and a liquid. At the operating temperature 2212 and (Ca, Sr)2CuO3 react with the liquid to form 2223. During the conversion the 2212 lattice expands, indicating that the remaining 2212 grains contain less and less Pb. The final 2212 and 2223 textures are approximately identical, and Avrami plots of the transformation kinetics give exponents m in the range 1 < m < 2. During the annealing the 2212 linewidth is constant, implying that there is neither strain nor finite-size broadening of the 2212 peaks during the transformation. This points to a transformation mechanism where only a few 2212 grains transform at a given time. Implications of these findings are discussed in relation to intercalation and nucleation-and-growth models.