CONTROLLED HETEROGENEOUS NUCLEATION OF MELT-TEXTURED YBA2CU3O6+X BY ADDITION OF AL2O3 PARTICLES

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 approximately 1050-degrees-C (which is above the incongruent melting temperature of YBa2Cu3O6+x) a reaction layer forms at the alumina interface. The reaction products were identified as Ba6Y2Al4O15 and a copper-rich liquid phase. On cooling below the peritectic temperature, aligned domains of YBa2Cu3O6+x (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 Ba6Y2Al4O15 is put forward, together with a discussion of the possible nucleation mechanisms for the 123.