EFFECT OF UNDERCOOLING TEMPERATURE ON THE SOLIDIFICATION KINETICS ANDMORPHOLOGY OF Y-BA-CU-O DURING MELT TEXTURING

The phase content of a series of YBa2Cu3O7-delta (123) bars was determ ined as a function of the holding time during the post-melting solidif ication process at different undercooling temperatures. Quantitative X RD analysis of these samples shows that 123 recrystallization occurs i n three distinct stages after cooling from the melting temperature. Th e first stage is an incubation period during which no 123 forms, follo wed by a rapid liquid-assisted reaction stage, then finally a slow sol id-state sintering stage. Nucleation of 123 from the partial melt whic h contains 211 + BaCuO2 + liquid is a low-probability process which re sults in only a limited number of nuclei forming during the texturing process. The undercooling temperature is a critical parameter which co ntrols the final morphology of 123 in the case of non-directional soli dification. Large amounts of undercooling results in a microstructure characterized by randomly-oriented dendritic needles or plates, while a low degree of undercooling yields large, blocky domains of oriented 123.