| Coated conductors (CC) is based on the oxide films of buffer layer and superconductor lay with bi-axial texture fabricated on the Ni-W substrate or single crystal substrate. Generally coated conductors are multi-layer composite materials composed of flexible metal substrate, buffer layer, protective layer and YBa2Cu3O7-δ (YBCO) superconducting layer with high critical current density at liquid nitrogen temperature. There are two kinds of techniques by which buffer layers and superconductor layers were obtained, that is, chemical solution deposition (CSD) and physics vapor deposition (PVD). PVD based on vacuum technology is an important method for high performance superconducting layer, and CSD based on non-vacuum technology is a low cost method for industrialization of coated conductors. Metal-organic deposition (MOD) is one of CSD methods, has emerged as commercially viable non-vacuum method for producing both buffer layer and YBa2Cu30y (YBCO) superconducting layer because of its many desirable aspects, including the precise control of stoichiometry and composition of metal oxide precursor, ease of the formation of epitaxial oxide films, relatively easy scale-up of films and cost reduction. In this manner, the fabrication of buffer layers and conductor layers by MOD method is still overwhelming. In order to deeply understand the mechanism about buffer layers and conductor layers, we would investigate the mechanism of texture transfer and prevention diffusing in buffer layers, the influence between buffer lay and superconductor layer. All aspects above mentioned benefits the structure and properties of buffer layer and superconducting layer, and will promote the industrialization of coated conductors.In the thesis, buffer layer including LZO and CeO2-δ were fabricated in the traditional Ar-4%H2 and oxidizing atmosphere containing CO2. YBCO films were prepared by CSD method using low fluorine solution. All films were characterized by XRD for crystallinity and epitaxial growth of films. Optical microscopy (OM), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to check the surface morphologies. Surface analysis was performed by X-ray photoelectron spectrometer (XPS). The research contents are as followings.CeO2-δ is one of the most promising buffer layer for coated conductors due to its high stability at elevated temperature, small lattice mismatch and similar thermal expansion coefficient with YBa2Cu307-δ (YBCO) layer. However, some cracks and large surface roughness are formed easily on the surface of CeO2-δ film prepared by CSD technique when the thickness of CeO2-δ film is beyond a critical thickness. So far the development of CeO2-δ on RABiTS-NiW substrate has been of certain level limited. CeO2-δ buffer layer with a thickness of 250nm was prepared in oxidizing atmosphere containing CO2. The crack on the surface is inhibited after residue carbon reacts with CO2 which is in proper concentration. The FWHM values of the omega-scan and the phi-scan are 9.3° and 6.3° respectively, which imply good texture in c-axis and a-b plane. Meanwhile, according to the surface chemical composition analysis, the reasons of the formation of the cracks are discussed in this paper. Based on the analysis, cracks came into being with tensile stress, and thus agraphitic carbon and a few carbonate distribute along the cracks. The tensile stress was related to the oxygen vacancy in the film. With the conversion of Ce from the tetravalent to trivalent state, the density of oxygen vacancy in the film increases and the inner tensile stress cumulates, which finally bring about the formation of cracks on the surface.Ce0.8Mo.202_s films (M= Ti, Zr and Hf) with (00l) preferred orientation have been prepared on biaxially textured Ni-W substrates by metal organic decomposition (MOD) method. The influence of ionic radii of doping cations on the epitaxial growth process of Ce0.8Mo.202-δ (M= Ti, Zr and Hf) films and oxygen diffusion in the buffer layer were explored. Ce0.9Mo.102-δ films (M= Ba, La and Zr) have been fabricated too. The influence of valence of doping cations on the epitaxial growth process of Ce0.9Mo.1O2-δ(M= Ba, La and Zr) films and oxygen diffusion in the buffer layer were investigated. Our results show that dopants with different radius change lattice parameter of CeO2-δ film and mismatch and compressive strain. The dopants with different ionic radii may affect the growth mode of CeO2-δ film. The oxygen diffusion behavior is related to the migration energy, vacancy formation energy and binding energy of dopants and vacancies. For Ti, Zr and Hf doped CeO2-δ, the rate of oxygen diffusion decreases in sequence with the increase of migration energy, which may be the main influencing factor. Meanwhile, Valences of doping cations is also related to the decreases of vacancy formation energies and doping energies. Higher valence of doping cations would results in the increase of vacancy formation energy but leads to the decrease of vacancy concentration at the same time. What’s more, larger ionic radius would bring about the decrease of the migration energy of oxygen ions. So Ti and Ba doped film show severe oxygen diffusion due to steric effects. These results are significant to improve the performance of buffer layers for coated conductors.We studied the effect of mismatch between substrate and CeO2-δ buffer layer on the epitaxial growth process of CeO2-δ films. CeO2-δ thin films were prepared on LaA103 (LAO), YSZ and Ni-W substrates by MOD technique. It shows that compressive strain in CeO2-δ thin films increases as increasing the lattice mismatch, which results in the increase of the concentration of grain boundaries and the decrease of the growth rate for CeO2-δ grains. A small lattice mismatch between substrate and buffer layer is beneficial for the increase of texture degree of CeO2-δ thin film. A dependence of the evolution of the surface morphology and roughness for CeO2-δ thin films on the lattice mismatch between substrate and CeO2-δ epitaxial film is not confirmed.We fabricated buffer layers with different surface morphologies under different atmosphere, which are used for research on the influence of the buffer layer on the superconductor layer. All results indicate that the buffer layer with an atomically flat area is helpful to the growth of superconductor phase, and the buffer layer with spherical grains decreases the concentration of nucleation centers. The surface of the buffer layer is distributed with rough crystal grains. As more a-axial grains in the YBCO layer could weaken its superconductivity. The texture of the buffer layer has influence on the growth mode of the superconductor. It shows that the substrate texture of the mental is easy to be transmitted to the superconductor layer, as there is high degree of out-of-plan and in-plan texture on the thicker buffer layer.We studied the influence of YBCO thickness on the barry efficiency of LZO buffer layer for oxygen diffusion. In the multi-layer of CC, the buffer layer is thinner compared with that of the superconductor layer, which is not prone to preventing diffusion. Thicker YBCO layer is beneficial not only to carrying high current but also to suppressing the diffusion in buffer layer. What’s more, thicker layer is also helpful to avoiding the oxidation of substrate and to maintaining the high texture degree of buffer layer. Also we studied the influence of some technological factors, for example, temperature and oxygen partial pressure, on the texture degree of buffer layer. We consider that high oxygen partial pressure facilitates the texture degree and suppresses the oxygen diffusion, while the influence tendency of temperature is reverse to that of oxygen partial pressure. For the ways of RABITS to CC, the decrease of the texture degree is mainly due to the oxidation of substrate and the oxygen diffusion in buffer layers. |
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