2001 Volume 42 Issue 2 Pages 220-226
Since the micro/macro-structures affect the critical current density (Jc) and the mechanical properties of superconductive YBCO oxides, the following numerical and analytical studies of solidification process of faceted 123 (YBa2Cu3O7−X) crystals from liquid+211 (Y2BaCuO5) phases are essential to clarify the solidification mechanism and improve the properties of YBCO . To clarify the effects of growth mode and conditions on the microstructures of 123 crystals, two-dimensional numerical simulation of faceted peritectic growth of 123 crystal was performed by considering (a) growth of 123 crystal, (b) melting of 211 particles in the liquid, and (c) solute diffusion in the liquid. The growth rate (R) of 123 crystal was approximated by: R=ag·ΔTk2, where ag was kinetic growth constant, and ΔTk was kinetic undercooling of faceted interface. The kinetic melting constant (am) and superheating (ΔTm) was also used for evaluation of melting rate of 211 phase. Solute distributions in the liquid during the 123 growth were calculated by FDM, and the distributions of residual 211 particles and liquid pools in the faceted 123 crystals were evaluated from the experimentally obtained log-normal distributions of 211 particles in the liquid of YBCO . The calculated results agreed well with the experimental ones. Transition of macrostructures from columnar to equiaxed 123 crystals in unidirectionally solidified YBCO was also studied experimentally and analytically. Critical transition conditions (:relations of growth rate (R) and temperature gradient (G)) were calculated by equations obtained from nucleation and growth theories, and compared with experimental results.