GROWTH-MECHANISM OF 123 COLUMNAR CRYSTAL IN SUPERCONDUCTING YBCO OXIDE

The growth mechanism of 123 crystals is investigated to explain how the 123 (YBa2Cu3O7-x) columnar crystal grows with a facet interface from liquid + 211 (Y2BaCuO5) through a peritectic reaction with solute diffusion in liquid. The following relation is conducted between 123 growth rate (R) and undercooling (DELTATpc = Ti-Tp, Ti: interface temperature, Tp: peritectic temperature) by the analysis of kinetics for 123 crystal growth, which occurs by sequential three primitive processes: (1) growth of the 123 phase, (2) melting of the 211 phase, (3) diffusion of solute in liquid. R = (Ca/eta) . {(ML . DELTATpc + DELTACrc)/(Tp/m(L123) + Cf . Tp/m(L211))}2, where Ca is a constant, eta is the viscosity, ML = 1/m(L123) - 1/m(L211), m(L123) and m(L211) are the gradients of liquidus of 123 and 211, DELTACrc is the change in equilibrium solute concentration caused by surface energy of 211 particles at the 123 freezing front, Cf is a constant. This relation agrees well with the experimental results. The following approximate relation is also obtained between the maximum radius (r) of 211 particles which melts in the diffusion layer and the distance (Xd) from the 123 freezing front. r = (2GAMMA/m(L211))/[{Cb.Xd/(Tp-DELTATpc)}.{(ML . DELTATpc + DELTACrc)/(Tp/m(L123) + Cf.Tp/m(L211))}2 + DELTACrc], where Cb is a constant, GAMMA is the Gibbs-Thomson coefficient. The transition of 123 crystals from the columnar to exquiaxed structure is successfully simulated by using the above R-DELTATpc relation and the experimental result on undercooling (DELTATp) for the nucleation of 123 crystals.