Enhanced high-field transport critical current density of superconducting bulk Y-Ba-Cu-O prepared by rapid solidification and directional annealing

Strongly textured high-Tc bulk Y-Ba-Cu-O was prepared with rapid solidification and directional annealing processes. Sintered YBa2Cu3O7 (1:2:3) samples, produced by conventional ceramic synthesis, were rapidly quenched with a twin rolling mill and directionally annealed by moving the sample at a speed of 0.2 cm/h through a temperature gradient of up to 60°C/cm from 1020 to 900°C. We have measured transport Jc of 2000 A/cm2 by pulse current and 1150 A/cm2 by dc at 77 K in a field of 15 T. It has been found that the Jc values measured by pulse and dc methods are approximately equal in high magnetic fields (8-15 T) and Jc measured by pulse current is larger than that measured by dc in magnetic fields below 8 T. The measured values of Jc decrease monotonically with increasing external magnetic field. Scanning and transmission-electron- microscopy studies showed that the 1:2:3 crystallites grow with their c axes perpendicular to the sample traveling direction. Crystallites of ∼2-4 μm of Y2BaCuO5 (2:1:1) phase were observed also with their c axes perpendicular to the sample traveling direction. Grains with twin crystals orientated at ∼90°from each other were observed. Precipitates of CuO were observed using energy-dispersive x-ray analysis. Dislocations with a density of ∼5×109/cm2 were found in the 1:2:3 crystals. These dislocations lie on the a-b plane. The enhancement of Jc probably is due to flux pinning by the dislocations and to the preferred orientation of the crystallites with the a-b plane parallel to the sample traveling direction.