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

Abstract

An adequate buffer layer architecture is of great importance for YBa2Cu3O7−δ (YBCO)-coated conductor fabrication. We present a transmission electron microscopy (TEM) analysis of La2Zr2O7 (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.

Effect of the annealing process on the microstructure of La2Zr2O7 thin layers epitaxially grown on LaAlO3 by metalorganic decomposition

“…Alternative techniques to determine the crystalline quality of the surface are EBSD and RHEED, which need vacuum systems and can be only used on small samples [13]. Otherwise, crystalline quality can be probed by TEM [14]. However, this technique needs a hard and long preparation step and it can be only used for exploratory works.…”

High-resolution transmission electron microscopy observations of La2Zr2O7 thin layers on LaAlO3 obtained by chemical methods

“…An interesting paper was published by this group, 7 which gives evidence of the presence of voids. Tilt measurements are equivalent to our results.…”

La2Zr2O7single buffer layer for YBCO RABiTS coated conductors

“…According to recent searches [16][17], 80 nm of LZO is thick enough to ensure a good barrier against oxygen diffusion. The withdrawal speed was varied to obtain different film thicknesses.…”