Diffusion phenomena and crystallization path during the growth of LFZ Bi-Sr-Ca-Cu-O superconducting fibres

The laser floating zone technique is used to grow highly textured fibres in the Bi-Sr-Ca-Cu-O superconducting system. Composition profiles along the fibre axis show a strong segregation of Bi to the molten zone, this region being depleted of Ca. Maximum and minimum values of the effective distribution coefficients (k) of cations are determined, the extreme values of segregation being k_Ca = 1.64 and k_Bi = 0.60. The dependence of k on the pulling rate R allowed the calculation of the equilibrium distribution coefficients k₀ of the elements and of the average diffusion coefficients D_Bi = 1.2×10^-10 m² s^-1, D_Sr = 2.7×10^-10 m² s^-1 and D_Ca = 12.8×10^-10 m² s^-1 in the Cu-rich melt. The approximate length of the diffusion layer δ is 21 µm. The crystallization path is represented in the composition tetrahedron of Bi₂O₃-SrO-CaO-CuO. Liquid immiscibility is observed. The non-superconducting (Sr_0.3Ca_0.7)CuO₂ (`1/1') phase, with dendritic morphology, is the first phase to solidify. The superconducting matrix is the result of an early nucleation of `4413' intergrowth on the surface of `1/1' dendrites followed by the side-by-side crystallization of the `4413' and `2212' phases from a peritetic-type reaction.