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 k 0 of the elements and of the average diffusion coefficients D Bi = 1.2×10 -10 m 2 s -1 , D Sr = 2.7×10 -10 m 2 s -1 and D Ca = 12.8×10 -10 m 2 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 Bi 2 O 3 -SrO-CaO-CuO. Liquid immiscibility is observed. The non-superconducting (Sr 0.3 Ca 0.7 )CuO 2 (`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.