In-situ high temperature optical microscopic observations of crystalization mechanism in NdBa2Cu_3O_x

摘要

The pseudo-binary NdBa_2Cu_3O_x-Ba_3Cu_(10)O_(13) phase diagrams and the crystallization of NdBa_2Cu_3O_x have been in-situ observed using high-temperature optical microscopy in air and under three different reduced oxygen atmospheres namely, 1percent, 0.1percent and 0.0097percent oxygen in argon. The liquidus line was found to become narrower both in composition and temperature with reducing oxygen content in the atmosphere. These results suggested that while in air NdBa_2Cu_3O_x can be crystallized from both high-temperature solution and peritectic me under reduced oxygen atmospheres the crystallization of NdBa_2Cu_3O_x is only possible from a peritectic melt. The in-situ observations of crystallization of NdBa_2Cu_3O_x from high-temperature solutions revealed that the growth mechanism changes from 3D via 2D layer-by-layer to continuous dendritic growth with increasing cooling rate. On the other hand, the NdBa_2Cu_3O_x crystallization from a peritectic melt involved two distinct steps (i) conversion of Nd_4Ba_xCu_2O_x into NdBa_2Cu_3O_x and (ii) nucleation and growth of NdBa_2Cu_3O_x. The growth morphology in this case was independent of the cooling rate. This result and the direct observation of the dissolution of Nd_4Ba_2Cu_2O_x into the liquid suggested that the solute diffusion is the rate-limiting factor.