The growth of melt-textured YBa2Cu3O7-δ superconductors

Sintered rods of stoichiometric YBa2Cu3O7-δ superconductor were transformed into quasi-single crystals by partial remelting in a temperature gradient. Each rod was positioned in the furnace so that one end remained solid while the rest was heated into the liquid plus Y2BaCuO5 two-phase region. The rods were then directionally solidified by pulling them slowly out of the furnace. Quenching experiments during growth showed that the growth front of the quasi-single crystals was at a temperature of only 940 deg. C, significantly below the pseudo-binary peritectic temperature of 1010 deg. C at which liquid, solid YBa2Cu3O7-δ and solid Y2BaCuO5 are in equilibrium with each other. It is proposed that the growth of YBa2Cu3O7-δ quasi-single crystals occurs primarily via the solidification reaction L → YBa2Cu3O7-δ followed by a variant eutectic reaction L → YBa2Cu3O7-δ + BaCuO2 rather than by the pseudo-binary peritectic reaction L + Y2BaCuO5 → YBa2Cu3O7-δ and that quasi-single-crystal growth breaks down when the invariant four-phase reaction L → YBa2Cu3O7-δ+CuO+BaCuO2 occurs. (author)