Research On Superconducting Properties Of Ybco Coated Conductor Prepared By Chemical Solution Deposition

The superconducting films of high temperature superconductor coatedconductors （CC） have pseudo single crystal structures based on the biaxiallytextured buffer layer templates. Coated conductors have high critical currentdensity at liquid nitrogen temperature, and show great potential for practicalapplications. Generally, coated conductors are constructed by four components,which are flexible metal substrate, buffer layer, YBa2Cu3Oy（YBCO）superconducting layer and protective layer. Several technologies have beencombined to obtain biaxially textured templates and high performance YBCOsuperconducting layers, including Rolling Assisted Biaxially Tectured Substrates（RABiTS）, Ion Beam Assited Deposition （IBAD）, Pulsed Laser Deposition（PLD）, Sputtering, Metal Organic Chemical Vapor Deposition （MOCVD）,Chemical Solution Deposition （CSD）. So far there are two main techniques forthe deposition of YBCO superconducting layer, PLD based on vacuumtechnology is an important method for high performance superconducting layer,and CSD based on non-vacuum technology is a low cost method forindustrialization of coated conductors. CSD method has several advantages, suchas low cost, easily precise control of film composition and reel-to-reel.Accordingly, it has attracted more and more attentions. In order to increase theperformance of YBCO superconducting layer, we should investigate the phaseformation and crystallization mechanism of YBCO film prepared by CSDprocess. We also should pay more attention to the important problems, such asthe special properties of traditional all-TFA solution for slow pyrolysis, theformation of cracks in precursor film, nucleation and growth of YBCO, theformation of a-axis grain, and enhancement of flux pinning properties.In order to understand the mechanism of decomposition and crystallizationof YBCO, optimize the parameters and improve the superconducting propertiesof films, this paper is devoted to study YBCO films prepared by CSD methodusing the traditional all-TFA solution and low fluorine solution. The changes ofbands and decomposition behaviors of YBCO gels are analyzed by Thermal Analysis and Fourier Transforms Infrared Spectroscopy. The key parametersinvolved in the decomposition, growth, and the flux pinning of YBCO films areinvestigated by using various characterization techniques, including X-raydiffraction, optical microscopy, atomic force microscopy, scanning electronmicroscopy and Jc-B measurement. The researches of thesis include theoptimization of traditional TFA-MOD, development of new low fluorine solutionand enhancement of flux pinning properties by chemical doping.1. The process of TFA-MOD includes precursor solution, decomposition andcrystallization. During the preparation of precursor solution, coppertrifluoroacetate has great influence on the properties of YBCO-TFA gel for slowpyrolysis process. During the pyrolysis process, the relative humidity is criticalto achieve an YBCO film with crack-free surface. The heating rate has greatinfluence on the formation of buckling surface. During the firing process, thestress-induced spontaneous dewetting of YBCO film can be observed at higherfiring temperatures. The low oxygen partial pressure is beneficial to thesuppression of a-axis grains.2. The effect of single crystal substrate with different lattice mismatch andthe effect of La2Zr2O7buffer layer’s roughness on the growth of YBCO filmshave been investigated. The effect of MOD process on the microstructure ofbuffer layers is also discussed. The smooth surface of LZO layer is veryimportant for the highly texture and good performance of YBCO film. Duringthe CSD process, the interfacial reaction and the substrate oxidation are observed.BaCeO3is formed by the reaction between Ba（O,F）2and CeO2. The YBCO filmshave been deposited by CSD process with optimized parameters on differentsubstrates. Sample （YBCO/LAO） shows an optimized performance of Jc=².7MA/cm2at77K self field. The critical current of another sample（YBCO/CeO2/YSZ/CeO2/NiW） is about60A/cm-w at77K self field.3. In order to lower the sensitivity of traditional all-TFA solution, reduce thefluorine content and increase the efficiency, a new fluorine solution has beendeveloped by using copper benzoate as precursor. The decomposition time forthe low fluorine solution can be greatly reduced, and the precursor film showscrack-free and homogeneous surface in the whole heating rate range of1-10K/min. The decomposition time （1-2.5h） of the low fluorine solution is shortened to1/4-1/5of that for all-TFA solution. The low fluorine approach canbe useful for the fabrication of YBCO films with fast calcination process. Duringthe crystallization, the compositions of intermediated phase change with theannealing temperature, which demonstrate that the growth mechanism of the lowfluorine solution is still “ex-situ BaF₂” process. The water vapor pressure can becontrolled to realize the uniform nucleation and fast growth of YBCO grains.The YBCO film prepared by low fluorine solution on buffered NiW substrate hasalso demonstrated high performance.4. Two kinds of YBCO films with different pining types （YGd_xBa₂Cu₃O_y andYBCO+xBaZrO₃） are prepared by chemical solution deposition. The effects ofdoping content on the properties of YBCO films have been investigated. The fluxpinning properties of YGdBCO film can be enhanced by the substitution of Yions by Gd ions and the precipitation of RE₂O₃, which can both work as effectivepinning centers. The length of c-axis in YGdBCO film increases with the contentof Gd ions. When the content of Gd is at10%, YGd_0.1Ba₂Cu₃O_yfilm has goodperformance in magnetic field （J_c=0.18MA/cm²at77K,1T, F_p=¹.7GN/m³）,which is almost twice of that for the pure YBCO film. On the other hand, thesecond phase BaZrO₃can be formed in YBCO film by addition of Zr compoundin the precursor solution. The appearances of second phase make the film denseand increase the flux pinning. YBCO with5%BaZrO₃doping shows higherperformance in magnetic field with J2c=4.9MA/cm@77K, self field, and J_c=0.42MA/cm²at77K,1T, F_p=⁴GN/m³.