Barrier efficiency of sponge-like La₂Zr₂O₇buffer layers for YBCO-coated conductors

Abstract

Solution derived La 2 Zr 2 O 7 films have drawn much attention for potential applications as thermal barriers or low-cost buffer layers for coated conductor technology. Annealing and coating parameters strongly affect the microstructure of La 2 Zr 2 O 7 , but different film processing methods can yield similar microstructural features such as nanovoids and nanometer-sized La 2 Zr 2 O 7 grains. Nanoporosity is a typical feature found in such films and the implications for the functionality of the films is investigated by a combination of scanning transmission electron microscopy, electron energy-loss spectroscopy and quantitative electron tomography. Chemical solution based La 2 Zr 2 O 7 films deposited on flexible Ni-5at.%W substrates with a {100}<001> biaxial texture were prepared for an in-depth characterization. A sponge-like structure composed of nanometer sized voids is revealed by high-angle annular dark-field scanning transmission electron microscopy in combination with electron tomography. A three-dimensional quantification of nanovoids in the La 2 Zr 2 O 7 film is obtained on a local scale. Mostly non-interconnected highly facetted nanovoids compromise more than one-fifth of the investigated sample volume. The diffusion barrier efficiency of a 170 nm thick La 2 Zr 2 O 7 film is investigated by STEM-EELS yielding a 1.8 ± 0.2 nm oxide layer beyond which no significant nickel diffusion can be detected and intermixing is observed. This is of particular significance for the functionality of YBa 2 Cu 3 O 7-δ coated conductor architectures based on solution derived La 2 Zr 2 O 7 films as diffusion barriers.

Assumption-free morphological quantification of single anisotropic nanoparticles and aggregates

“…Materials that are not prone to electron beam damage can profit from dual axis tomography 71,72 which reduces the missing wedge to a missing pyramid. On-axis tomography of needle-shaped samples can be tilted 360°, 73 effectively eliminating all problems caused by random orientation and the missing wedge. 74 Additionally, the sample geometry may allow certain a priori information to be considered and incorporated in model-based reconstruction algorithms such as DART.…”

Roughness and Texture of Epitaxial LZO Thin Films Grown on RABiTS Ni5W Substrates

“…Due to this property, many papers showed that LZO reproduced exactly the grain morphology of the substrate obtained by the RABiTS process [15]. The microstructure of LZO was made of small rounded crystallites (20-50 nm) with nanovoids [7,14] originating from the released gases during the CSD process, but the crystallization process made these voids cubic [8]. The grain boundaries of the Ni substrate were transferred to the LZO microstructure by the epitaxial process giving rise to the typical grain morphology mentioned above.…”

“…LZO acts as a barrier against diffusion of the Ni toward YBCO [7,8] and must limit the oxidation during the different steps of the synthesis process of the coated conductor [9,10]. This buffer layer is used as a template to grow epitaxially the superconducting layer made of YBa 2 Cu 3 O 7 (YBCO) and for example deposited by MOCVD [11][12].…”

Surface engineering of biaxial Gd2Zr2O7 thin films deposited on Ni–5at%W substrates by a chemical solution method

“…The impurities that form during the reaction may consume some amount of the GZO precursors or films, leading to weaker reflection intensities in the well crystallized sample D. Similar reactions at the film-substrate interface were observed in other oxide films under reducing atmosphere, even using single crystals as substrates. 19,20,32,33 Proposal of a two-step annealing procedure for GZO films grown on Ni5W substrates…”