Observations on crystal growth mechanisms in the directionally solidified high temperature superconductor Y₁Ba₂Cu₃O_7−δ

Abstract

The growth behaviour of Y₁Ba₂Cu₃O_7−δ(Y1-2-3) crystals has been studied by horizontal directional solidification of samples heated into the liquid (L) plus Y₂Ba₁Cu₁O₅(Y2-1-1) phase field. Cylindrical samples of initially stoichiometric Y1-2-3 composition were solidified at rates 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 growth rate did not exceed 1 mm h⁻¹ and that the temperature gradient was > 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 preferred orientation of [001] at 45° to the growth axis when grown from a polycrystalline seed crystal. No preferred orientation developed when samples were grown without a seed. In quenched samples, macroscopic growth steps were observed on the (001) plane, and within the cylindrical sample the faces of the growing crystal were found to be mutually perpendicular planes. During solidification liquid was found to be lost from the L+Y2-1-1 semi-solid region of the sample. This occurred both by wetting of the support bars and by liquid migration into the seed crystal region.