Preparation Of YBa₂Cu₃O_7-δ Films By Low Fluorine-Content Sol-Gel Process

It is significant for device application to synthesize high-quality high temperature superconducting （HTS） thin films due to YBa₂Cu₃O_7-δ （YBCO） HTS thin films are the base of HTS electric device, while YBCO coated conductor （CC）, as the second generation high temperature superconducting tape （2G HTS tape）, which will substitute for Bi-system （BSCCO/Bi-2223） have great potentiality in research and application fields. Thus, it is always our goal to obtain high quality and property YBCO films with repeatable process. At present, Sol-Gel method contained fluorine was considered as a optimal process for scale-up owing to its low cost. But high fluorine-content could cause severe effect on the surface quality （such as compactability and smoothness） of YBCO films. Therefore, in this paper YBCO films with favorable property on LaAlO₃ （LAO） single crystal substrate have been prepared by a novel low fluorine-content Sol-Gel process, and growth process of the film has also been researched. In order to apply this low fluorine process to 2G HTS tape, we made several prior exploratory researches of textured RABiTS-Ni and buffer layer, which is significant to prepare YBCO coated conductor subsequently.In our works, stable YBCO precursor solution low-fluorine contained was obtained using yttrium, barium and cuprum acetates as raw material after they were dissolved in methanol using diethylenertiamine （DETA）, trifluoroacetic acid and α-methacrylic acid as chelating agent respectively. YBCO films on LAO substrate were synthesized by this low-fluorine process, which could decrease the volume of HF-gas generated in comparation with the conventional all-TFA method. Thus the heat-treatment period prepared a single YBCO specimen was shortened distinctly. The results of X-ray θ-2θ scanning showed that YBCO films grown preferentially along c-axis orientation on LAO substrate, （005）ω-scanning and （103）ψ-scanning indicated the films had good out-of-plane and in-plane texture. HRTEM and selected area electron diffraction （SAED） studies showed that the film is YBCO three-ply perovskite structure. Critical transition temperature T_c=90K was obtained using AC susceptibility measurement. Critical electric density J_c measured by J_c-scan Leipzig system is about 2 MA/cm².HRTEM images indicate that the excellent coherence and epitaxy relation （without any amorphous transition areas） were kept between YBCO films and LAO substrate, and YBCO grains mainly grown along c-axis orientation except for little a-axis grains nearby the surface of YBCO films. The projective model schematics of interface atom-structure indicate that YBCO film started to grow along c-axis orientation from yttrium atom-layer not barium atom-layer. According to the nucleation theory of the films, it is learned that the films should grow in the mode of two-dimensional plane at the beginning of nucleation and growth; however, it had to change the mode from two-dimensional plane to three-dimensional island growth as a result of lattice distortion during subsequent growth stage. The whole growth process consists of three stages which are thermolysis of gel films, formation of YBCO phase and nucleation growth of YBCO grains respectively.Strong （200） preferred orientation was obtained through optimal recrystallizing heat treatment of cold-rolled RABiTS-Ni for YBCO coated conductor. NiO seed layer with strong （200） preferred orientation can be formed after optimal chemical pretreatment and self-oxidation epitaxy treatment are performed on RABiTS-Ni with strong （200） preferred orientation. CeO₂ gel film was prepared on RABiTS-Ni with strong （200） preferred orientation by sol-gel method, and then CeO₂ buffer layer with （200） preferred orientation is obtained through optimal heat treatment. It is quite advantageous for growth of YBCO film to prepare high textured NiO seed and CeO₂ buffer layer.