In-situ high temperature optical microscopic observations of crystallization mechanism in NdBa{sub}2Cu{sub}3O{sub}x

摘要

The pseudo-binary Nd(Ba{sub}2)(Cu{sub}3)(O{sub}x)-(Ba{sub}3)(Cu{sub}10)(O{sub}13) phase diagrams and the crystallization of Nd(Ba{sub}2)(Cu{sub}3)(O{sub}x) have been in-situ observed using high-temperature optical microscopy in air and under three different reduced oxygen atmospheres namely, 1%, 0.1% and 0.0097% 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 Nd(Ba{sub}2)(Cu{sub}3)(O{sub}x) can be crystallized from both high-temperature solution and peritectic me under reduced oxygen atmospheres the crystallization of Nd(Ba{sub}2)(Cu{sub}3)(O{sub}x) is only possible from a peritectic melt. The in-situobservations of crystallization of Nd(Ba{sub}2)(Cu{sub}3)(O{sub}x) from high-temperature solutions revealed that the growth mechanism changes from 3D via 2D layer-by-layer to continuos dendritic growth with increasing cooling rate. On the other hand, theNd(Ba{sub}2)(Cu{sub}3)(O{sub}x) crystallization from a peritectic melt involved two distinct steps (i) conversion of (Nd{sub}4)(Ba{sub}2)(Cu{sub}2)(O{sub}x) into Nd(Ba{sub}2)(Cu{sub}3)(O{sub}x) and (ii) nucleation and growth ofNd(Ba{sub}2)(Cu{sub}3)(O{sub}x) The growth morphology in this case was independent of the cooling rate. This result and the direct observation of the dissolution of (Nd{sub}4)(Ba{sub}2)(Cu{sub}3)(O{sub}x) into the liquid suggested that the solutediffusion is the rate-limiting factor