Continuous solidification of YBa2Cu3O7−x by isothermal undercooling

Growth kinetics and microstructural development of single crystal YBa₂Cu₃O_7−x (Y123) superconductors, prepared by a melt texturing method under isothermal undercooling conditions, were investigated. At small undercooling, ΔT = 6 K, the initial growth of Y123 crystal was terminated at a size of ∼8 mm × 8 mm in the semisolid phase before the entire sample was fully solidified through the peritectic reaction. With incremental increase of ΔT, the growth of the crystal continued so that the whole sample (22 mm diameter and 8 mm thickness) solidified as a single crystal. Termination of the crystal growth at a given undercooling temperature was attributed to the coarsening process of Y211 particles in the melt associated with their surface energy contributions. Isothermal growth of large size Y123 single crystal requires a continuous supply of Y solute to the solidification interface which can be achieved by continuing dissolution of coarsening Y211 particles through further undercooling of the melt.