OBSERVATIONS ON CRYSTAL-GROWTH MECHANISMS IN THE DIRECTIONALLY SOLIDIFIED HIGH-TEMPERATURE SUPERCONDUCTOR Y1BA2CU3O7-DELTA

The growth behaviour of Y1Ba2Cu3O7-delta(Y1-2-3) crystals has been stu died by horizontal directional solidification of samples heated into t he liquid (L) plus Y2Ba1Cu1O5(Y2-1-1) phase field. Cylindrical samples of initially stoichiometric Y1-2-3 composition were solidified at rat es ranging from 1 to 10 mm h(-1) using temperature gradients between 2 .5 and 5 K mm(-1). A novel method was employed to support the L+Y2-1-1 semi-solid and minimize reaction with the crucible. Selected samples were quenched during solidification so that growth mechanisms could be studied. Quasi-single crystals of Y1-2-3 formed, providing that the g rowth rate did not exceed 1 mm h-l and that the temperature gradient w as > 3.5 K mm(-1) The quasi-single crystals contained particles of Y2- 1-1 as well as Ba-Cu-rich bands as secondary phases and had a preferre d orientation of [001] at 45 degrees to the growth axis when grown fro m a polycrystalline seed crystal. No preferred orientation developed w hen samples were grown without a seed. In quenched samples, macroscopi c growth steps were observed on the (001) plane, and within the cylind rical sample the faces of the growing crystal were found to be mutuall y perpendicular planes. During solidification liquid was found to be l ost from the L+Y2-1-1 semi-solid region of the sample. This occurred b oth by wetting of the support bars and by liquid migration into the se ed crystal region.