Directional solidification and microstructural studies of the peritectic Y 2 BaCuO 5 phase
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Directional solidification using a Bridgman furnace was performed to produce textured Y2BaCuO5 (211) rods of both stoichiometric and off-stoichiometric compositions and to investigate microstructure formation at various solidification rates. The solidification morphology of the samples changed from planar to cellular and eventually to equiaxed blocky grains with increasing solidification rate. The microstructure of the stoichiometric 211 sample revealed elongated, aligned YBa2Cu3Oy (123) phase residual in the 211 matrix. The 211 samples rich in Y2O3 phase showed no trace of residual 123 but did show trapped Y2O3 particles. The morphology of the Y2O3 particles varied from spherical to a rodlike morphology as well along the length of a specific sample as also with decreasing growth rates in different samples. The Y2O3 particles in samples exposed for longer time to the liquid phase at high temperatures exhibited coarsening and unidirectional coalescence into a rodlike morphology and retained their morphology even in the 211 matrix after solidification.
I. INTRODUCTION
Microstructure evolution of the superconducting YBa2Cu3Oy (123) phase by a peritectic reaction has been the subject of many studies so as to improve the performance of the superconducting material for practical applications.1–13 Based on the fundamental peritectic process, where solid Y2BaCuO5 (211) particles react with liquid to form 123, a variety of solidification processes such as MTG, QMG, SLMG, IG, etc. have been developed to yield a favorable microstructure for good superconducting properties.14–19 Basically, in all these processes an intimate mixture of the properitectic 211 phase and liquid phases (BaCuO2 + CuO) are either directionally or isothermally solidified through the peritectic temperature (Tp) of 123. The resulting microstructure consists of millimeter-sized domains in which interconnected platelets of 123 are mutually aligned. Due to the incomplete peritectic reaction and also when starting from a composition with excess of 211, the properitectic 211 phase appears as randomly distributed small-sized inclusions within these domains. The multiple domain formation is suppressed when the solidification is performed in the presence of an oriented seed crystal or in steep temperature gradients, like in zone melting or Bridgman-type furnaces leading to domain sizes of a few centimeters.1,20 The 211 is a properitectic phase (Fig. 1) during the formation of the peritectic 123 phase and plays an important role in determining the microstructure and the superconducting properties of the final 123 phase. J. Mater. Res., Vol. 16, No. 4, Apr 2001
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The 211 phase itself also forms by a peritectic reaction between the Y2O3 and liquid phases in the Y–Ba–Cu–O system, the respective peritectic temperature for this reaction being around 1260 °C, approximately 150 °C above the Tp for the formation of the 123 phase. Although, the 211 phase is an important component in the Y–Ba–Cu–O system, there are v
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