Coated Conductor, Perovskite-type Buffer Layer Study

The research of coated conductors YBa₂Cu₃O_7-d(YBCO) is a great interest topic for high temperature superconductor duo to its exhibiting higher-in-field critical current densities at 77 K, compared with the first generation Bi-2223/Ag superconducting tape. The preparation of superconductor buffer layers is an important part for the development of YBCO coated conductors. The development trend in the buffer layer is transferring from multilayer to single layer and from multi-deposited route to single deposited route. For complex multilayer, the differences of physical and chemical properties lead to the combination of many preparation technologies for fabrication of the films. The mixed technology induces complex process, longer time and higher cost. An increase of number in buffer layer has some disadvantages in controlling the growth, microstructure and interface of the buffer layer. Therefore, to simplify buffer layer structure has been become a development direction. To date, the achievement of the materials choice and fabrication technology in USA and Japan bring a hope to single buffer layer for long tape fabrication. The perovskite type compound is an ideal buffer layer. In this thesis, we choose a simple perovskite oxide compound La_0.7S_t0.3MnO₃ (LSMO) and double-perovskite oxide compound La₂NiMnO₆ (LNMO) and Nd₂NiMnO₆ (NNMO) as for buffer layer. The bulk of buffer layer was made by solid state reaction and chemical solution deposition (CSD). The optimal process for the microstructure and morphology of bulk was investigated. The single buffer layers La₂NiMnO₆, Nd₂NiMnO₆ and La_0.7S_r0.3MnO₃ on Ytttia-stabilized cubic Zirconia (YSZ) single crystalline substrates were fabricated by metal organic decomposition (MOD). The new process was developed for preparing single buffer layer by MOD methods. This method is decided to prepare stable LSMO,LNMO and NNMO precursor solution.This thesis is composed of three parts. In first part, the properties and preparation technology and development of YBCO coated conductors were introduced. Moreover, the foundation and the scheme of research are presented. The second part summarizes the sample preparation method and principle and the analysis and detection methods of thin film. The third part is the research of simple perovskite buffer layer, complex perovskite buffer layer and the Conclusions section.1. The bulk of La₂NiMnO_6+δ was fabricated by solid state reaction and the phase microstructure was analyzed by X-ray Diffraction (XRD). The bulk of La₂NiMnO_6+δ phase was obtained at 1150℃, 1200℃and 1350℃, respectively. However, it was found that a few of La₂NiO₄ phase appeared in the La₂NiMnO_6+δ powders under any heat treatment conditions.2. The bulk of La₂NiMnO_6+δ was prepared by CSD, and the microstructure and the morphology was characterized by scanning electron microscopy (SEM) and XRD. The bulk of monoclinic La₂NiMnO_6+δ can be obtained by heat treatment under 800℃-1000℃for 3h when precursor solution was a mixture of La(NO₃)₃, Ni(NO₃)₂ and Mn(CH₃COO)₂ and the content of La in the mixture was lower. The distribution of crystal grain is uniformity and compact. The grain size is about 200 nm.3. The bulk of Nd₂NiMnO₆ was prepared by CSD and its microstructure and morphology were characterized by SEM and XRD. The bulk of monoclinic Nd₂NiMnO₆ was obtained by heat treatment under 1000℃- 1300℃for 3 h when precursor solution was a mixture of Nd(NO₃)₃, Ni(NO₃)₂ and Mn(CH₃COO)₂. The distribution of crystal grain is uniformity and compact and the grain size is about 200nm.4. The preparation of MOD for precursor solution fabricated La₂NiMnO₆, Nd₂NiMnO₆ and La_0.7S_r0.3MnO₃ buffer layer was achieved by using different precursors and solvents.5. The single buffer layer on YSZ single crystalline substrates was obtained by MOD using La₂NiMnO₆, Nd₂NiMnO₆ and La_0.7S_r0.3MnO₃, respectively. The crystal orientation and surface morphology was tested by XRD and atomic force micrograph. The process of buffer layer is optimize and improved.