Influence of growth parameters on the microstructure of directionally solidified Bi_2Sr_2CaCu_2O_y

Laser-heated float zone growth was used to study the directional solidification behavior of Bi–Sr–Ca–Cu–O superconductors. The phases that solidify from the melt, their morphology, and their composition are altered by growth rate. Highly textured microstructures are achieved by directional solidification at all growth rates. The superconducting phase is found always to have the composition Bi_2.5Sr_2CaCu_2.2O_y when grown from boules with composition 2:2:1:2 (BiO_1.5:SrO:CaO:CuO). Planar growth fronts of Bi_2.5Sr_2CaCu_2.2O_y are observed when the temperature gradient divided by the growth rate ( G/R ) is larger than 3 ⊠ 10^11 K-s/m^2 in 2.75 atm oxygen. Thus, the 2212 compound was observed to solidify directly from the melt at the slowest growth rates used in this study. Measurement of the steady-state liquid zone composition indicates that it becomes bismuth-rich as the growth rate decreases. Dendrites of the primary solidification phase, (Sr_ 1−x Ca_x)_14Cu_24O_y, form in a matrix of Bi_2.5Sr_2CaCu_2.2O_y when G/R is somewhat less than 3 ⊠ 10^11 K-s/m_2. Observed microstructures are consistent with a peritectic relationship among Bi_2.5Sr_2CaCu_2.2O_y, (Sr_ 1−x Ca_x)_14Cu_24O_y ( x = 0.4), and a liquid rich in bismuth at elevated oxygen pressure. At lower values of G/R , Sr_3Ca_2Cu_5O_y is the primary solidification phase and negligible Bi_2.5Sr_2CaCu_2.2O_y forms in the matrix.

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