GROWTH OF LARGE YBA2CU3O7-X CRYSTALS IN THE PRESENCE OF A TEMPERATURE-GRADIENT

While attempting to reproduce growth procedures for YBa2Cu3O7-x described earlier in the literature, we obtained small crystals in the flux cavities, but in a very non-reproducible manner. We improved significantly the "harvesting" of crystals, in size and number, by applying a temperature gradient of a few degrees per centimeter across the crucible. The crystals did not grow in cavities but at the surface of the flux, which had retired to the colder part of the crucible and solidified. Some crystals were quenched from 830-degrees-C while some were slowly cooled to room temperature. After the same oxygenation time (one month at 450-degrees-C, flowing high purity O2 at atmospheric pressure), quenched crystals had in general a lower T(c). We associate this disparity with the dimensions of the twinning domains and the oxygen diffusion anisotropy, in addition to the Al contamination.