Chemically deposited La2Zr2O7 buffer layers for YBCO-coated conductors: film growth and microstructure

Abstract

An adequate buffer layer architecture is of great importance forYBa2Cu3O7−δ (YBCO)-coated conductor fabrication. We present a transmission electron microscopy (TEM) analysisof La2Zr2O7 (LZO) buffer layers on biaxially textured Ni–5 at.%W substrates for YBCO-coated conductorsprepared by chemical solution deposition (CSD). The LZO thin films were heat-treated at 900 and1050 °C respectively. Electron diffraction patterns, and bright and dark-field images were usedto determine the microstructure, texture and the nanoporosity of the films. Byx-ray diffraction the films were found to be [100] oriented and strongly biaxiallytextured. Although x-ray diffraction suggests an epitaxial growth of LZO on Ni itwas shown by TEM that this was not the case. The grain size of the films isbetween 100 and 300 nm and therefore much smaller than the Ni grain size of40 µm. Appropriate acquisition conditions for scanning electron microscopy (SEM) and TEMimaging are given to identify the nanogranularity of the films. For the film annealed at1050 °C high-resolution SEM images clearly show a polycrystalline LZO microstructure and the grain sizecan readily be determined. Electron diffraction rings are more pronounced than for the film annealedat 900 °C, indicating a higher level of polycrystallinity in the film. SEM images of the film annealed at900 °C yield no evidence of a polycrystalline microstructure; only single misoriented LZO grainsseparated by 500 nm are observed. Nanovoids 10–40 nm in size were found in the LZO bufferlayers with a high density. The voids had approximately cuboid shape, indicating ananisotropy 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-grownbuffer layers, the LZO buffer layers act as efficient Ni diffusion barriers. Energydispersive x-ray microanalysis (EDX) in the transmission electron microscope yieldedthe composition of the films. In the LZO films no Ni-rich secondary phases weredetected and significant C contamination occurred during spectrum acquisition.