Controlled grain orientation at the surface of Bi2Sr2CaCu2Ox superconducting glass-ceramics

Bi₂Sr₂CaCu₂O_x glass with Cu⁺/(Cu⁺ + Cu²⁺) = 0.76 was prepared by using a conventional melt-quenching method, and crystalline phases and grain orientations at the surface of superconducting glass-ceramics obtained by annealing at various atmospheres were examined. The grain orientation of the Bi₂Sr₂CaCu₂O_x phase (low-T_c phase) at the surface was severely affected by oxygen partial pressure in annealing. The favourable grain orientation, in which the plate-like grains of the low-T_c phase are oriented parallel to the surface plane, was first established in the samples obtained through a newly developed two-step annealing method: first annealing at 780°C in oxygen and second annealing at above 750°C in nitrogen. It was concluded that the favourable grain orientation of the low-T_c phase at the surface occurred due to the formation of a liquid phase in nitrogen.