摘   要
      以YBCO为代表的高温超导实用材料不仅具有液氮区的工作温度， 且上临界场也高达100 T以上，为其在强电应用领域奠定了优良的基础，具有广阔的前景。基于高温超导体REBa2Cu3O7-x(简称REBCO, RE = Y、Sm、Gd等稀土元素)的第二代高温超导带材兼备了REBCO超导体优良的基础物理特性和涂层导体的双轴织构优势，因而在液氮温区表现出优异的超导性能(高的临界电流密度和不可逆场)以及优异的机械强度等，因而在超导强电应用领域比第一代高温超导带材更具前景。为了解决带材在实际应用中超导线材制备的长度限制问题，特别是为了实现基于第二代高温超导带材的高温超导磁体的持续电流运行模式，第二代高温超导带材的超导接头技术受到人们广泛的关注和研究。
       本论文的具体工作围绕着实现第二代高温超导涂层导体超导接头的物理化学工艺探索所展开，从超导接头的工艺探索、接头制备、物理性能等方面开展了较为系统的研究。论文的主要工作包括如下内容：
       针对第二代高温超导REBa2Cu3O7-x(REBCO)带材超导接头制备的需求，系统研究了REBCO带材金属稳定层的剥离工艺。开发出了一种廉价、快速的化学方法用以剥离Y0.5Gd0.5Ba2Cu3O7-x带材的金属Cu/Ag稳定层，并深入研究了剥离过程的化学反应机制及其引起带材Ic衰减的具体原因。采用该方法剥离的带材，其超导层的结构和超导性能几乎没有受到影响，为后续的超导接头的制备奠定了可靠的基础。
       激光打孔技术作为解决第二代高温超导带材超导接头区提供渗氧通道的途径，需要弄清带材表面加工的微孔对其自身超导性能的影响。通过系统的电磁性能测量并结合有限元仿真手段，研究了YGdBCO带材微孔阵列结构(微孔的直径、分布间距及密度等)对带材超导性能(Ic、Tc、最大抗磁信号、低场下Ic的各向异性)及交流损耗的影响，发现优化后的微孔结构可提高带材在低场下的磁通钉扎特性且对带材自身的交流损耗有显著影响。
       研究了熔融原子扩散技术在制备第二代高温超导YGdBCO带材超导接头过程中的相关特性和机理，并应用纯氧气氛优化了接头的超导性能，成功制备出了第二代高温超导YGdBCO带材的超导接头。研究中发现超导层表面粗糙度是影响超导接头表面微观结构和超导电流输运特性的关键因素之一，提出了超导接头处超导电流输运特性主要由超导弱连接特性所支配的理论解释和相关模型。采用分子动力学模拟对接头区域原子扩散行为进行了模拟，取得了与实验结果相一致的结果。
       探索了制备第二代高温超导YGdBCO带材超导接头的新途径—“液相辅助烧结和织构融合”接头技术。与熔融原子扩散技术制备第二代高温超导带材接头不同，这项新技术是通过YGdBCO熔融分解产生的液相来提高接头界面处的结合强度，并通过这些液相的再结晶生长形成织构界面，因而该技术具有对YGdBCO层表面粗糙度的不敏感等特点。初步研究显示，采用该新技术可以制备出具有超导性能的第二代高温超导YGdBCO带材的超导接头，并展示出有较大的优化潜力。

                                                 Abstract
    Based on the high temperature superconductor REBa2Cu3O7-x (referred to, RE=Y, Sm, Gd and other rare earth elements), the second generation high temperature superconducting possess both the excellent basic physical properties of REBCO superconductors and the biaxial texture advantages of coated conductors. Due to this fact, the outstanding superconducting properties of REBCO tapes are exhibited in the liquid nitrogen temperature, which including high critical current density and irreversible field, excellent mechanical strength etc. Therefore, the REBCO tapes are more promising than the first generation high temperature superconducting tapes in the field of superconducting high current applications. In order to solve the problem of length limitation for the preparation of superconducting tapes in practical applications and realize the persistent current operation mode of the REBCO superconducting magnet especially, the superconducting joint of REBCO tapes is paid extensive attention and research.
    The specific content of this thesis focuses on the physical and chemical process exploration of the realization of the second-generation high temperature superconducting coated conductor superconducting joint. The systematic research has been carried out from the aspects of process exploration, preparation and physical properties of superconducting joint. The main work of the paper is as follows:
    Aiming at the requirements for the preparation of superconducting joints of the second-generation high temperature superconducting REBa2Cu3O7-x (REBCO) tapes, the eliminating process of the metal stabilizer of REBCO tapes was systematically studied. Developed a cheap and fast chemical method to etch the metal Cu/Ag stabilizer of Y0.5Gd0.5Ba2Cu3O7-x tapes, and deeply studied the chemical reaction mechanism of the etching process and the specific reason of the Ic attenuation of the YGdBCO tapes. The structure of the superconducting layer and the superconducting properties of the YGdBCO tapes etched by this method are hardly affected, which lays a solid foundation for the subsequent preparation of superconducting joints.
    Laser drilling technology is an effective method to provide oxygen diffusion channels for the superconducting joint of tapes. In this paper, the effect of microholes processed on the surface of YGdBCO tapes on its own superconducting performance was systematically studied. The systemic electromagnetic performance measurement and the finite element simulation methods were used to study the influence of the microholes array structure (the diameter of microholes, distribution distance and density) on the superconducting performance (Ic, Tc, maximum diamagnetic signal, Ic anisotropy under low field) and AC loss of the YGdBCO tapes. It was found that the optimized microholes structure can improve the magnetic flux pinning characteristics of the YGdBCO tapes under low field and have a significant impact on the AC loss of the YGdBCO tapes.
     In the research of preparing the superconducting joint of YGdBCO tapes by the fusion atom diffusion technology, the superconducting performance of the joints was optimized by using pure oxygen atmosphere, and the superconducting joint of YGdBCO tapes were successfully prepared. It was found that the surface roughness of the superconducting layer was one of the key factors affecting the surface microstructure and supercurrent transport characteristics of the superconducting joint. The dominant theoretical explanations and related models was proposed that the supercurrent transport characteristics at the superconducting joint are mainly caused by the superconducting weak connection characteristics. The molecular dynamics simulation was used to simulate the atomic diffusion behavior at joint region, and the results were consistent with experimental results.
     A novel method for preparing the superconducting joints of YGdBCO tapes was explored which called “liquid-phase-assisted sintering and texture fusion” technology. Unlike the fusion atom diffusion technology, this new technology uses the liquid phase generated by the melted YGdBCO to improve the bonding strength at the joint interface, and then form a textured interface by recrystallizing and growing these liquid phases. Therefore, this technology has the characteristics of insensitivity to the surface roughness of YGdBCO layer. Preliminary research shows that the new technology can be used to prepare the superconducting joints of YGdBCO tapes with superconducting properties, and has demonstrated great optimization potential.