Effect of the surface orientation on the microstructure of bulk Y-Ba-Cu-O joint

Microstructural studies were performed at the joining area between two blocks of Y-Ba-Cu-O bulk superconductors using sintered ErBa₂Cu₃O_y (Er123) welding solder when the joined surfaces were parallel to the (1 0 0) and (1 1 0) crystallographic planes. The residual liquid phase and the segregation of second phase particles were always observed in the joint with (1 0 0)/(1 0 0) surfaces, while no such defects were observed in the joint with (1 1 0)/(1 1 0) surfaces. Microstructural observations of the joint at several stages of the joining process showed that the growth front was always parallel to the (1 0 0) joined surfaces for the (1 0 0)/(1 0 0) joint, leading to the segregation of liquid phase and second phase particles. In contrast, many triangular crystals were observed at the (1 1 0)/(1 1 0) joint in that the (1 0 0) facet was tilted about 45° from the (1 1 0) joined surfaces, and thus allowing the complete reaction of the Er123 phase at the joint. Such growth morphology was the source of joint free from residual liquid phase and segregation of second phase that reduce the critical current in the (1 1 0)/(1 1 0) joint.