The Investigation On Rapid Preparation And Mechanism Of YBCO Coated Conductors By CSD Methods

The development of YBa₂Cu₃O_7-δ （YBCO） coated conductors has advanced the application of high temperature superconducting （HTS） materials. However, the elimination of weak links produced by a weak coupling between the superconducting grains is requirement for the preparation of YBCO superconductors with high critical current densities.Thus, one way to solve this problem is depositing the YBCO superconductive layer onto a textured substrate with a strong （001）[100] cube texture orientation. Presently, there are two main techniques for textured template: rolling assisted biaxially textured substrates （RABiTS）and ion beam assisted deposition （IBAD）.In addition, buffer layers are added between the textured substrate and superconducting layer, which serve as a barrier to inter-diffusion of Cu or Ni, and a template for subsequent growth of superconducting layer. The buffer layer on the metal substrate tapes could prevent ion diffusion of metallic elements from the metal tape into the YBCO superconductive layer, which provide a template to transfer the texture from the substrate to the superconductive layer.Although 2G superconductor tapes based on such multi-layer buffer architectures are commercial, the low cost and rapid preparation of coated conductors are the prerequisite and guarantee for the industrial production of conductors. Two effective approaches of the cost reduction for YBCO coated conductors are simplifying the architecture of buffer layers such as a single buffer layer and shortening the preparation time for each layer. Among the various approaches of preparing buffer layer and YBCO films, chemical solution deposition（CSD） method has emerged as a very cost-effective and competitive approach for industrial production of coated conductors because of their accuracy in composition control and low costs of the preparation without high-vacuum apparatus, which has become the future direction of development.However, the higher heat-treatment temperature would result in the uncontrolled oxidation of the metal substrate under strong oxygen atmosphere during YBCO（OOL）processing, which degrade the superconducting performance seriously. So, it is necessary to oxidize NiO（LOO） layer formed on nickel tapes by surface-oxidation epitaxy （SOE） method,the biaxial orientation of NiO functions as a barrier layer of nickel diffusion and chemical reaction between YBCO and nickel tape, which is considered to be a simple and efficient way for YBCO industrial production. Although the previous oxidation of pure Ni to form NiO has been studied well in air, the higher oxidation temperature above 1100℃ reduces the mechanical strength of the baseband, which is not conducive to the roll-to-roll preparation.So, the SOE process in this thesis including medium temperature oxidation process （790 ℃,20mins） in argon atmosphere and annealing process （400 ℃, 60h） at low temperature in air has been introduced to obtain the NiO（L00） thin film and reduce the oxidation temperature.In the previous preparation of coated conductors by CSD method, the processes of solvent volatilization and precursor decomposition were always carried out simultaneously to guarantee the epitaxial film formation with a slow speed of calescence, and the whole heat treatment process required more than 10h. In this thesis, the rapid preparation of buffer layers and the superconducting layer by CSD method are obtained by keeping the solvent volatilization below the the solvent boiling point from the precursors decomposition in solid state.First of all, by adjusting the composition of the precursor solution, the new SmBiO₃（SBO） and Ce_0.8Sm_0.2O_2-Δ（CSO） precursor solution have been obtained with the lactic acid and nitrate precursors （Samarium nitrate and Bismuth nitrate, Samarium nitrate and Cerium nitrate）, an dense, smooth and high-textured SBO or CSO film has been obtained by adjusting technology parameters, and the whole heat treatment process of buffer layer was shorted for less than 2h.In addition, the quality of buffer layer directly affects the property of the subsequent YBCO superconducting layer. The influence of the growth temperature, the growth time, the oxygen partial pressure and the lattice mismatch between SBO buffer layer and substrate have been discussed in detail, only the smooth and high-textured buffer layer can guarantee the epitaxial growth of YBCO superconducting layer with the high performance. So, the detail analysis of the main factors during the epitaxial process in this thesis is of great significance for the industrialization of coated conductors. At the same time, it is useful to clear the basic questions of epitaxial growth, and provide guidance for epitaxial growth of other materials.By the similar method for rapid deposition of YBCO（OOL） superconducting layer, the rapid preparation technologies for YBCO superconducting layer deposited on LaAlO₃ （LAO）single crystal substrate have been explored firstly. And then, the technologies have been used for depositing YBCO superconducting layer onto SBO/YSZ, SBO/NiO/Ni-W and CSO/YSZ substrates, respectively. The whole heat treatment process for YBCO superconducting layer has been shortened within 4h. These research results in this thesis can be provided for the rapid preparation of YBCO coated conductors.