Precipitate size refinement by CeO 2 and Y 2 BaCuO 5 additions in directionally solidified YBa 2 Cu 3 O 7
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Precipitate size refinement by CeO2 and Y2 BaCuO5 additions in directionally solidified YBa2 Cu3 O7 N. Vilalta, F. Sandiumenge, S. Pi˜nol, and X. Obradors Institut de Ci`encia de Materials de Barcelona, C.S.I.C., Campus de la UAB, 08193 Bellaterra, Spain (Received 4 August 1995; accepted 30 April 1996)
Directional solidification of YBa2 Cu3 O7 has been carried out through a Bridgman technique, and the influence of Y2 BaCuO5 and CeO2 additives on the size of Y2 BaCuO5 precipitates has been investigated. It is demonstrated in this work that the most efficient procedure to reduce the size of the Y2 BaCuO5 precipitates is to increase the concentration of nucleation centers present in the peritectic decomposition of YBa2 Cu3 O72x . A small concentration (0.3–1 wt. %) of CeO2 has a strong influence on the solidification process and on the size of Y2 BaCuO5 precipitates. It is shown that when CeO2 is added, further refinement of the size of precipitates results from the formation of nanometric Y2 O3 particles which further enhance the multinucleation effect. We have also observed that coarsening effects are avoided with CeO2 additives. I. INTRODUCTION
It is currently accepted that a narrow distribution of small Y2 BaCuO5 (211) (< 1mm) precipitates in aligned YBa2 Cu3 O7 (123) samples is crucial for the achievement of high critical current densities Jc . First Murakami et al.1 argued from magnetization measurements that Jc at 77 K scales with the interface area of the 211 precipitates embedded in the aligned 123 matrix. Later on, we confirmed the validity of the interface pinning mechanism for the whole temperature range2 and determined the temperature-magnetic field region where it dominates the Jc behavior.3 The processing and the effect of several additions to the ceramic precursors has been the subject of numerous investigations (see Refs. 4, 5, and 6 for comprehensive reviews). Among various additives, CeO2 has been shown to improve microstructural, mechanical, and superconducting parameters of the composite. Kim et al.7,8 indicated the formation of BaCeO3 and a refinement of 211 precipitates. We reported inductive Jc values above 105 Aycm2 at 77 K and zero field in CeO2 added samples.9 Previous microstructural studies suggested a modified growth mechanism by which the 211 precipitates nucleate heterogeneously on Y2 O3 nanometric particles previously generated by the subsolidus reaction between CeO2 and 211.9,10 Here we present a detailed analysis of the effects of properitectic 211 and CeO2 additions on the size distributions and solidification path of our directionally solidified 123y211 composites. Extensive reports on the microstructure and aging effects on the defect structures and Jc (H) of these samples are to be found in Refs. 11 and 12, respectively. II. EXPERIMENTAL
Superconducting 123 bars are prepared as described elsewhere.13 As precursors, we use 123 powder prepared 38
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J. Mater. Res., Vol. 12, No. 1, Jan 1997
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