Comparative study on the crack formations in the CeO₂ buffer layers for YBCO films on textured Ni tapes and Pt tapes

The crack formations in the CeO₂ buffer layers on textured Ni tapes and Pt tapes, which were used for the substrates of the high T_c superconducting YBCO films, were comparatively studied. Since these two metallic tapes, which were textured by rolling and recrystallization, have very different properties of thermal expansion coefficients, lattice matching conditions, and chemical reactivities with the buffer layers, the comparisons of their buffer layer growth features were suggestive. Both Ni and Pt showed no salient reactive diffusion in the interface with the buffer layers. Numerous cracks were formed in the CeO₂ buffer layer on Ni, while very few cracks were formed in the buffer layer on Pt. These results indicated it was unlikely that the effect of the thermal expansion coefficient differences of the films and the substrate caused the cracks. The CeO₂ film deposited by the sputtering method on the thin template CeO₂ film predeposited on Ni by e-beam evaporation, showed very few cracks. Detailed SEM observations indicated a possible relation between the cause of the crack formations and the lattice mismatching which seemed to distort the lattices and to produce the lattice defects.