The effect of undercooling and Nd422 phase content on the nucleation of large Nd-Ba-Cu-O grains fabricated by top-seeded melt processing

The nucleation and growth of NdBa2Cu3O7-delta (Nd 123)-Nd4Ba2Cu2O10 (Nd422) single-grain composites in a controlled 1% O-2 in N-2 atmosphere were investigated in detail as a function of solidification temperature and Nd422 phase content using a top-seeded melt growth technique. A schematic process phase diagram in the peritectic solidification region of Nd-Ba-Cu-O (NdBCO) was constructed primarily from constant isothermal growth experiments at various temperatures for several compositions and used to fabricate large single-grain material by both isothermal and continuous slow cooling over a limited temperature range. The nucleation at the seed surface and subsequent growth of uniform grains was observed to depend critically on the controlled rate of grain growth and the temperature range over which solidification occurred.