Chemically deposited La2Zr2O7buffer layers for YBCO-coated conductors: film growth and microstructure

An adequate buffer layer architecture is of great importance for YBa 2 Cu 3 O 7−δ (YBCO)-coated conductor fabrication. We present a transmission electron microscopy (TEM) analysis of La 2 Zr 2 O 7 (LZO) buffer layers on biaxially textured Ni–5 at.%W substrates for YBCO-coated conductors prepared by chemical solution deposition (CSD). The LZO thin films were heat-treated at 900 and 1050 °C respectively. Electron diffraction patterns, and bright and dark-field images were used to determine the microstructure, texture and the nanoporosity of the films. By x-ray diffraction the films were found to be [100] oriented and strongly biaxially textured. Although x-ray diffraction suggests an epitaxial growth of LZO on Ni it was shown by TEM that this was not the case. The grain size of the films is between 100 and 300 nm and therefore much smaller than the Ni grain size of 40 µm. Appropriate acquisition conditions for scanning electron microscopy (SEM) and TEM imaging are given to identify the nanogranularity of the films. For the film annealed at 1050 °C high-resolution SEM images clearly show a polycrystalline LZO microstructure and the grain size can readily be determined. Electron diffraction rings are more pronounced than for the film annealed at 900 °C, indicating a higher level of polycrystallinity in the film. SEM images of the film annealed at 900 °C yield no evidence of a polycrystalline microstructure; only single misoriented LZO grains separated by 500 nm are observed. Nanovoids 10–40 nm in size were found in the LZO buffer layers with a high density. The voids had approximately cuboid shape, indicating an anisotropy of the surface energy in LZO. The surface planes of the voids were identified as {111} lattice planes. Despite the nanoporosity, which is a typical feature of CSD-grown buffer layers, the LZO buffer layers act as efficient Ni diffusion barriers. Energy dispersive x-ray microanalysis (EDX) in the transmission electron microscope yielded the composition of the films. In the LZO films no Ni-rich secondary phases were detected and significant C contamination occurred during spectrum acquisition.