Experimental investigation of the Y 2 BaCuO 5 surface free energy during peritectic solidification of YBCO
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Experimental investigation of the Y2 BaCuO5 surface free energy during peritectic solidification of YBCO Wai Lo, D. A. Cardwell, J. C. L. Chow, and H-T. Leung IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge CB3 0HE, United Kingdom (Received 9 August 1996; accepted 20 November 1997)
The characteristic inhomogeneous distribution of nonsuperconducting Y2 BaCuO5 (211) inclusions in melt-processed YBa2 Cu3 O72d (123) grains has generally been attributed to 211 particle pushing by 123 growth fronts during peritectic solidification on the basis of reduced total surface free energy. Analysis of the morphology of the interfaces at the 211-211-123 and 211-211-liquid triple points in seeded melt-processed samples invalidates this assumption for the pure YBCO system and has implications of the mechanism of 211 particle segregation.
A fundamental requirement of bulk high temperature superconductors (HTS) for high field and high current engineering applications such as fault current limiters, magnetic bearings, and flywheels for energy storage1–4 is that they exhibit a high, field stable critical current density (Jc ) at 77 K. Large grain, melt-processed YBa2 Cu3 O72d (YBCO) exhibits a higher Jc of 50 3 104 A cm22 at 77 K in 1 T,5 compared with polycrystalline material (typically ,1000 A cm22 under the same conditions) and has emerged over recent years as the most likely bulk HTS material to fulfill this potential. The improvement in Jc in melt-processed YBCO is due partly to the elimination of weak links within the sample microstructure, which exist at grain boundaries in polycrystalline materials, and partly to the presence of nonsuperconducting Y2 BaCuO5 (211) phase inclusions in the superconducting YBa2 Cu3 O72d (123) phase matrix.6,7 The distribution, size, number density, and shape of 211 particles have been observed to correlate closely with Jc 8–10 and, as a result, have been the subject of extensive microstructural studies. In particular, an observed inhomogeneity in the 211 inclusion distribution in the 123 phase matrix has recently attracted significant interest.11–14 The inhomogeneity of 211 inclusions in YBCO is apparently similar to the distribution of second phase particles in metal matrix composites, which results from the pushing of the particles by the solidifying front of the matrix material. Based on this analogy, attempts have been made to apply conventional “particle-pushing” theory to explain the observed 211 distribution in the 123 YBCO matrix.13,14 There are, however, significant chemical and physical differences between the mechaJ. Mater. Res., Vol. 13, No. 8, Aug 1998
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nisms involved in the two systems. For example, 211 particle must necessarily dissolve in the peritectic liquid prior to solidification of the YBCO 123 phase,14–17 a process which depends fundamentally on chemical interactions between individual species at the growth front. Particle inclusions in metal matri
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