DIRECTIONAL PHASE-FORMATION ON MELTING VIA PERITECTIC REACTION

Directional melting and solidification of Ba2YCu3O6+delta in low oxygen partial pressures were investigated by the laser-heated floating zone (LHFZ) technique. The following reaction was observed on melting at P(O2) > 8 Pa: Ba2YCu3O6+delta --> BaY2CuO5 + L where BaY2CuO5 phase comprised equiaxed particles. At 8 Pa < P(O2) < 10(3) Pa, the preceding reaction occurred, and at higher temperatures, the following reaction also occurred to produce a rodlike BaY2O4 phase, oriented in the growth direction: BaY2CuO5 --> BaY2O4 + L. At pressures below about 8 Pa, the low temperature reaction observed was Ba2YCu3O6+delta --> BaY2O4 + L. In this case, the oriented BaY2O4 phase formed directly from Ba2YCu3O6+delta and was platelike in morphology. Directional growth of the BaY2O4 phase from Bay2CuO5 was modeled in a simple way. The BaY2CuO5 particles ahead of the Bay2O4 interface dissolve in the superheated liquid, and solute transport for BaY2O4 growth is assumed to be limited by diffusion along the growth direction over distances equal to half the rod or plate spacing. A relation between superheating at the growth interface and growth rate was derived from the present model. Some possibilities for making use of such aligned structures in the growth of the superconducting compound Ba2YCu3O6+delta are discussed.