In-Situ High Temperature Optical Microscopic Observations of Crystallization Mechanism in NdBa 2 Cu 3 O x
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ABSTRACT The pseudo-binary NdBa 2Cu 3O-Ba 3CuloO13 phase diagrams and the crystallization of NdBa 2Cu 3Ox 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 NdBa 2Cu 3Ox can be crystallized from both high-temperature solution and peritectic melt; under reduced oxygen atmospheres the crystallization of NdBa 2Cu 3Ox is only possible from a peritectic melt. The in-situ observations of crystallization of NdBa 2Cu 3 Ox 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, the NdBa 2Cu 3Ox crystallization from a peritectic melt involved two distinct steps (i) conversion of Nd 4Ba 2Cu 2Ox into NdBa 2Cu 3 0× and (ii) nucleation and growth of NdBa2Cu 3Ox. 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 20× into the liquid suggested that the solute diffusion is the rate-limiting factor. INTRODUCTION An understanding of the crystallization mechanism is important for identifying the optimum growth conditions so that large and high-quality single crystals can be grown. The hightemperature superconductor NdBa 2Cu3 Ox (Nd123) is a promising material for practical applications because of its higher superconducting transition temperature (Tc = 96K) and larger critical current density in high magnetic fields [1]. However, Nd123 when synthesized in air forms a solid solution, Nd,+yBa 2.yCu 3Ox, because Nd 3+ can substitute Ba 2+ on its lattice sites. The Tc was observed to decrease nonlinearly with increasing y and a change to semiconducting behavior occurs at around y = 0.4 [1,21. One way to minimize the partial substitution of Ba 2+ by Nd3+ is the processing of Nd 123 under reduced oxygen atmosphere [1,3]. In this paper, we present the in-situ high-temperature optical microscopic observations of pseudo-binary phase diagrams between Nd123 and Ba3Cuj oO 0 3, and the crystallization of Nd123 in both air and under reduced oxygen atmospheres. EXPERIMENT NdBa2 Cu 3Ox and Ba 3Cu1 0Ol 3 samples prepared using solid state sintering method were mixed using acetone in the various compositions with 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80 and 100 mol% of (l/6)NdBa 2 Cu 3O,. The in-situ observations were made using a hightemperature optical microscope (OLYMPUS BH2-UMA). The details of the experimental setup are described elsewhere [4]. The NdBa 2Cu 3Ox-Ba 3CuoO13 phase diagrams were generated by in133
Mat. Res. Soc. Symp. Proc. Vol. 569 ©1999 Materials Research Society
situ observing the melting and dissolution of the all the prepared samples. For this the temperature of the samples was increased to 1200'C at a rate
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