Microstructure of pulsed laser deposited YBa₂Cu₃O_7-δ films on yttria-stabilized zirconia/CeO₂ buffered biaxially textured Ni substrates

Transmission electron microscopy was used to study the microstructure of pulsed laser deposited YBa₂Cu₃O_7-δ (YBCO) layers on four different biaxially textured Ni-substrates with epitaxial oxide buffer layers. The oxide layer sequence, YBCO/YSZ/CeO₂/Ni (YSZ signifies yttria-stabilized zirconia) was the same in each sample, but different deposition techniques were used to deposit the buffer layers. The thicknesses of all three oxide layers varied from sample to sample. A columnar sub-grain structure composed of equiaxed sub-grains in the plane of the film was observed in the YBCO layer of each sample. The sub-grain shape and local mosaic spread (∼3°) appeared to be largely insensitive to the surface topography, density, or method of depositing the YSZ layer immediately below. The sub-grain size (0.2-1.2 μm) increased with film thickness (0.2-2.3 μm), but showed no obvious correlation with the microstructure of the underlying YSZ. The measured misorientation angles (θ) at individual sub-grain boundaries suggested that a large fraction of the boundaries in the sub-grain boundary network have θ in the range of 2-3°. For most boundaries, the measured θ was below the 5° "threshold" angle for weak link behavior. Thus, a representative picture of the sub-grain boundary network above a single grain of nickel in a film of sub-micron thickness might be a honeycomb shaped arrangement of 2.5° [0 0 1] tilt boundaries spaced 0.25 μm apart and containing dislocations at a spacing of 10 nm. This structure probably has consequences for the structure of grain boundaries in the YBCO that are replicated from the Ni template below. The superposition of the Ni grain boundary network with the grain boundary network due to mosaic spread almost certainly introduces substantial variations of θ along such boundaries on the sub-micron to micron length scale.