THE GROWTH OF MELT-TEXTURED YBA2CU3O7-DELTA SUPERCONDUCTORS

Sintered rods of stoichiometric YBa2Cu3O7-delta superconductor were tr ansformed into quasi-single crystals by partial remelting in a tempera ture 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 pull ing them slowly out of the furnace. Quenching experiments during growt h showed that the growth front of the quasi-single crystals was at a t emperature of only 940 degrees C, significantly below the pseudo-binar y peritectic temperature of 1010 degrees C at which liquid, solid YBa2 Cu3O7-delta and solid Y2BaCuO5 are in equilibrium with each other. It is proposed that the growth of YBa2Cu3O7-delta quasi-single crystals o ccurs primarily via the solidification reaction L --> YBa2Cu3O7-delta followed by a variant eutectic reaction L --> YBa2Cu3O7-delta + BaCuO2 rather than by the pseudo-binary peritectic reaction L + Y2BaCuO5 --> YBa2Cu3O7-delta and that quasi-single-crystal growth breaks down when the invariant four-phase reaction L --> YBa2Cu3O7-delta + CuO + BaCuO 2 occurs.