Controlled heterogeneous nucleation of melt-textured YBa 2 Cu 3 O 6+x by addition of Al 2 O 3 particles
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The reaction between alumina and yttrium barium cuprate subjected to a melt-texturing heat treatment was studied. Microstructural examination of quenched, partially transformed samples revealed that at —1050 °C (which is above the incongruent melting temperature of YBa2Cu3O6+;,;) a reaction layer forms at the alumina interface. The reaction products were identified as Ba 6 Y2Al 4 0 15 and a copper-rich liquid phase. On cooling below the peritectic temperature, aligned domains of YBa 2 Cu 3 06« (123) were observed to nucleate preferentially at the reaction layer. For samples of melt-textured 123 deliberately seeded with alumina particles, it was found that nucleation and growth of 123 occurred exclusively at the particles. A reaction sequence for the formation of the BagY2Al4Oi5 is put forward, together with a discussion of the possible nucleation mechanisms for the 123.
I. INTRODUCTION Melt-texturing or directional solidification of YBa2Cu 3 O 6+x (123) has been regarded as the most viable method of increasing the critical current density (Jc) of bulk 123 material. Previous studies have shown that aligned 123 can be formed via a peritectic reaction in the temperature range (950 °C-1000 °C) during melttexturing.1"3 Workers at Lehigh postulated that the peritectic transformation involves dissolution of Y2BaCuOs (211) particles, and subsequent precipitation of 123 from the melt.4 This model has since been confirmed by other workers.5"7 The significance of the Lehigh solution/precipitation model is that (unlike for the conventional peritectic reaction) the 211 particles are not the preferred nucleation sites. It should be feasible, therefore, to control nucleation via the incorporation of heterogeneous sites. Further, according to the model, the growth rate of 123 will be determined by the slowest of the following processes: dissolution of 211, diffusional transport through the liquid, and precipitation of 123. Since the number and the size of 123 domains will be determined by the relative rates of nucleation and growth of 123, the potential exists to optimize the microstructure through appropriate control of the above parameters. In the course of preliminary studies on the melttexturing of 123, pressed pellets of 123 were placed on alumina substrates, heated above the peritectic temperature, annealed at 970 °C for different times, and then quenched to room temperature. In this manner it was possible to study the materials in the partially transformed state. It was observed that the 123 domains always originated from the bottom surface of the samples. At first, this was attributed to heterogeneous nucleation at the substrate surface, but closer examination revealed a 2128
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J. Mater. Res., Vol. 8, No. 9, Sep 1993
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distinct reaction layer between the 123 domains and the alumina. It appears, therefore, that preferential nucleation of the 123 occurred at the interface of the aforementioned reaction layer. It should be noted that although chemical reactions between 12
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