Study Of Structure And Properties Of REBCO Thick Films And Interlayers Fabricated By MOCVD

To improve the current carrying capacity of the superconducting film is an increasinglyurgent need for power applications of high-temperature superconductors. The value ofcritical current depends on the film thickness （d） and the critical current density （Jc）. It iswell known that the critical current density of pure YBa₂Cu₃O_7-δ（YBCO） films can becomevery high due to a number of growth-induced structural defects such as dislocations. Forpractical applications, however, the flux pinning still needs to be further improved toovercome the vortex motion at high magnetic fields. Therefore, to obtain a larger currentcarrying capacity needs to seek a breakthrough in the thickness of the superconducting filmand the pinning centers. In this paper, photo-assisted metal organic chemical vapordeposition （MOCVD） epitaxial system was employed to fabricate REBCO high quality thickfilms and interlayers of multilayer structure, and their structure and properties wereinvestigated.In this paper, high-quality YBCO epitaxial films were grown by a photo-assistedMOCVD technique, the films with single-crystal-like morphology, high crystalline andsuperconductive quality were characterized. The highest value of Jcis3.75MA cm^-2at77Kin self-field. The effects of substrate temperature and precursors’ proportion on morphology,crystallographic and superconductive quality were investigated. Base on the optimizedprocess conditions, high-quality YBCO thick films was fabricated by controlling thedeposition time. The reason why epitaxial YBCO thick films with high-qualitysingle-crystal-like morphology can be grown up to about2.1μm thick is illustrated from the point of view of photo activation. Meanwhile, the evaluation of film properties with variousthicknesses was systematically investigated. Therefore, the PhA-MOCVD technique mayprovide some advantages for high-rate growth of high-quality YBCO thick films over otherdeposition techniques for various important applications.As the preliminary preparation for the research of multilayer structure, GdBa₂Cu₃O₇-δ（GdBCO） was firstly used as the interlayer. Photo-assisted MOCVD was employed tofabricate pure GdBCO films for the first time. The effects of substrate temperature （Ts） andoxygen partial pressure （PO2） on microstructure, growth rate and superconducting criticalcurrent density （Jc） were investigated. The （001） preferred orientation of the GdBCO filmswas enhanced with increasing Ts. Film growth rate and degree of texture were found to beinfluenced by PO2. The highly c-axis oriented GdBCO film with Jcof2.5MA/cm²at77K inself-field was obtained at Tsof810°C and PO₂of4Torr, and the morphology of GdBCOfilm was characterized as single-crystal-like. A growth rate of0.107μm/min is realized bythe PhA-MOCVD technique.High-quality GdBCO films with thicknesses of0.14-1.62μm were fabricated byPhA-MOCVD technique. Evaluation of film microstructure and strain-relaxationphenomenon in the growing films, and their effects on the crystallographic andsuperconducting properties of the GdBCO films were investigated. A dense and no grainboundary visible, single-crystal-like cross-sectional morphology was observed. For a0.31μm GdBCO film sample, the full width at half-maximum were0.08°and0.47°forout-of-plane and in-plane orientations, respectively. To our knowledge, this is the best valuereported to date for GdBCO heteroepitaxial films, which demonstrate that the films possesswell biaxial texture. The c-axis lattice parameters of the GdBCO films grown with differentthicknesses were used to determine the out-of-plane strain. The result shows that the straincould be gradually relieved as film thickness increasing. As films were getting thicker, theirJcvalues decrease gradually. This is mainly attributed to a lack of pinning centers in thethicker films.We choose Y₂O₃nano thin film structure as another interlayer for the study of multilayer structure in future. Self-assembling of strain-induced Y₂O₃nano thin film structure wasfabricated on pattern vicinal substrate LAO （100） by photo-assisted MOCVD. Nucleationand growth processes of Y₂O₃nanostructure self-assembled as rows along the terraces ofpattern substrate were investigated with various growth parameters. It is found that densityand size of Y₂O₃nanostructures can be tuned and well controlled by varying substratetemperature （Ts） and oxygen partial pressure. With elevating Ts, the nanostructures graduallygrow larger and sparser. This phenomenon could be illustrated by two competitive kineticprocesses, i.e. surface diffusion of adatoms and yttrium desorption. Morphologies anddensity of these nanostructures were also influenced by variation of oxygen partial pressure.To further discuss the growth kinetics, a more clearly quasi-linear distribution was obtainedand the coarsening effect is modified by varying growth time. Solid cone shaped Y₂O₃nanostructures with relatively uniform size and density were prepared.