| YBa2Cu3O7-z (YBCO) coated conductors (CCS), i.e. the second generation wires (tapes) is a focus in the present research field of superconducting materials. It has an enormous application prospect in electric power system, transportation, military and medical field. Now, the expensive preparation technology limits the large-scale applications of YBCO CCS. In this thesis, a fluorine-free polymer-assisted metal organic deposition (PA-MOD) technology and high temperature partial melting process as well as the corresponding flux pinning centers introduction method were newly developed with the objective of low-cost, high-performance and easy-scalability preparation technology. Moreover, several basic scientific issues with respect of this new preparation technology were deeply studied, and the reliability and reproducibility of this new technology has achieved a good level. YBCO film by the optimal processing parameters has the superconducting transition temperature Tc of 91 K and critical current density (77 K,0T)Jc up to 3.5 MA/cm2In chapter 1, research background of the dissertation has been introduced by combining the research development with application requests of high temperature superconducting materials. Moreover, the main study contents have been proposed oriented in the basic research and real applications and the architecture of the dissertation was summarized in detail.In chapter 2, the basic knowledge of superconductivity has been firstly reviewed and typical high temperature superconducting materials have been introduced in detail. Then the structure, preparation technology, properties and applications of CCS have been elaborated. And several chemical solution methods (CSD) such as sol gel,metal organic deposition and hybrid methods have been mainly introduced from the aspects of classification and procedures of the CSD methods. Additionally, two typical CSD methods including TFA-MOD and TMAP have been elaborated from the aspects of flow chart of solution synthesis, heat treatment profile and chemical reaction mechanism. At last, basic knowledge about flux pinning has been explained, and the present approaches of improving the Jc values of YBCO CCS have been summarized.In chapter 3, it includes the experimental design and characterization methods. The whole treating process of YBCO film fabrication has been introduced in the experimental design. The development process of self-developed non-fluorine PA-MOD method has been emphasized, and the final flow chart of solution synthesis has also been given in this part. Moreover, the major characterization approaches are thermal analysis, crystal structure and texture, surface and cross-sectional microstructure, superconducting magnetic property and electric transport properties.The newly developed PA-MOD approach has several advantages such as precise control of stoichiometry, low cost, simple operation process of coating solution preparation, and enhanced homogeneity and stability of coating solution through addition of polymers. BaCO3 impurity is easy to be formed in YBCO film by the traditional non-fluorine method, which precipitates in the YBCO grain boundaries thus prevent the current transportation. And this resulted in a low-performance YBCO film. The fluorine-free method has not been effectively developed over the past 20 years. In this thesis, we developed a heat treatment with reaction of BaCO3 and water gas in low temperature pyrolysis process through the systematic research of pyrolysis process, firing treatment and other treating parameters, which can decompose the formed BaCO3 impurity and significantly enhance the properties of YBCO film and improve the reproducibility of preparation technology. This treating process can effectively solve the difficult problem for YBCO film preparation by fluorine-free approach. YBCO film by this technology has Tc of 90 K and Jc (77 K,0 T) over 1 MA/cm2. Thus this technology has great significance to develop the YBCO coated conductors with low cost and high performance.Influence of different polymer additives in newly developed PA-MOD method on the coting solution synthesis and YBCO film preparation has been investigated in order to prepare high performance YBCO film. The polymer PVB has been identified as the smallest shrinkage and the best assistant efficiency by the study of properties of YBCO films by different polymers, which obviously improve the microstructure and decrease the microcracks of the film. On this basis, high temperature partial-melting process was developed i.e. littile liquid phase formation in precursor film. The formation of liquid phase is favorable to enhance the wettability between the precursor film and substrate and promote the epitaxial growth of the precursor film. Also, little liquid phase can make up part of the micro-holes amd microcracks formed in pyrolysis process. In addition, the residule BaCO3 can be decomposed at this high temperature. This newly developed fabrication technology can further improve the microstructure and enhance the density and smoothness the film, which decrease the holes and microcracks as well as control the stoichiometry of superconducting phase more precisely. These improved properties resulted in a great promotion of YBCO film, which has the Jc (77 K,0 T) up to 3.5 MA/cm2. This high Jc is one of the best results in the world for YBCO film by non-fluorine method.Because of the sensitivity of chemical solution deposition to the composition of starting materials, the treating parameters obtained by solid phase method can not directly take as the supervision for the doped YBCO film by chemical solution deposition. Therefore, it is ever a difficulty of introducing flux pinning centers by chemical solution deposition. Thus, the effect of dilute Co3+ Fe3+,Zn2+,Ni2+,Li+doping on the copper site of YBCO on the structure and critical current density of YBa2Cu3O7-z film has been studied. The results show that YBCO film with dilute impurity doping displays good biaxial texture and smoother as well denser surface morphology. And Jc value of the film with optimal doping level has been distinctly increased. In addition, the irreversibility field Hirr and flux pinning force Fp of YBCO film with Co3+ and Zn2+ doping level of 0.001 have been obviously improved. It is illustrated that dilute impurity doping can effectively improve the flux pinning properties of YBCO film.For the higher current carrying capabilities of GdBCO than YBCO, the obtained technology was extended in GdBCO system in order to further improve the properties of super layer for YBCO coated conductors. The effect of dilute Co3+,Fe3+,Zn2+,Ni2+ doping on the copper site on the structure and critical current density of GdBa2Cu3O7-z film has been mainly investigated. High performance GdBCO film has been prepared through optimizing the firing temperature and partial-melting temperature. The Gd/Ba substitution phase with weak superconductivity in the GdBCO system can be the potential pinning centers at higher temperature and fields, which is different with the YBCO system. Therefore, the nanoscale defects by dilute impurity doping and Gd/Ba substitution phase with weak superconductivity may jointly improve the Jc value of GdBCO film at higher temperature and fields.In addition, the influence of rare earth (RE) element and BaZrO3 doping on the Jc values of Y(Gd)BCO film has been investigated. According to the normalized Jc/Jc(0)-H curve, the flux pinning properties of YBCO film under the lower field can be improved by extra RE doping and RE partial substitution for Y site. While Nd, Eu, Gd substitution of Y site with the same doping level can improve the high-field flux pinning properties of the film, and 2%BaZrO3 doping can distinctly enhance the flux pinning properties at higher temperature and lower fields of GdBCO film. And the flux pinning mechanisms of Y(Gd)BCO films by different doping manners have been analysed.At last, the thermodynamic and dynamic conditions influencing the epitaxial growth of YBCO film have been analysed, and partial factors concerned on the control of texture and surface microstructure of the film has been discussed. The chemical reaction mechanism during the preparation of precursor film and phase formation of YBCO film has been speculated. On this basis, the critical nuclei size, nucleation driving force and nucleation energy barrier of heterogeneous nucleation of YBCO film on single crystal substrate have been derivated using classical nucleation theory. The results show that YBCO film can mainly nucleate by the interface nucleation through controlling the crystallized temperature, water partial pressure and oxygen partial pressure. Meanwhile, it can decrease the homogeneous bulk nucleation to a relatively lower level. And the driving force of chemical reaction and the heterogeneous nucleation rate can also be increased by adjusting these parameters. |
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