【作者】 王文涛；

【导师】 赵勇；

【作者基本信息】 西南交通大学 ， 材料学， 2010， 博士

【摘要】 YBa2Cu3O7-z(YBCO)涂层导体即第二代高温超导带材是当前超导材料领域的研究热点,在电力系统、交通、军事、医学等方面有着巨大的应用前景。目前,制约涂层导体大规模应用的主要因素是成本高昂的制备技术。本文正是以低成本、高性能、易推广的制备技术为研究目标开发出了一种新型无氟高分子辅助金属有机物沉积(PA-MOD)技术和一种新型的高温部分熔融热处理工艺以及与之相应的磁通钉扎中心引入技术,并围绕这一技术开发的基础科学问题展开了较为深入的研究,使该技术的可靠性和重复性达到了较好的水平。采用优化的工艺参数制备出的YBCO超导薄膜其超导转变温度Tc达到91 K,临界电流密度Jc(77 K,0 T)达到3.5 MA／cm2第1章主要结合高温超导材料的研究进展及应用要求介绍了本课题的研究背景。其次,以基础科研和实际应用为导向提出了课题的主要研究内容,并且概括了论文的篇章结构。第2章首先回顾了超导电性的基础知识并对典型的高温超导材料作了详细介绍。继而阐述了涂层导体的结构、制备技术、性能及其应用。从化学溶液法的分类和制备流程两方面重点介绍了溶胶凝胶法、金属有机物沉积法和混合法的特点。并着重介绍了现行的两种典型的化学溶液沉积法即三氟醋酸盐法(TFA-MOD)和三甲基乙酸盐法(TMAP)的制胶工艺流程、热处理工艺路线及成相过程的化学反应机制。最后阐述了磁通钉扎的基础知识并归纳了目前提高YBCO涂层导体临界电流密度的方法。第3章主要分为实验方案设计和样品表征两个方面。在实验方案设计中全面介绍了制备YBCO薄膜的整个工艺流程。其中重点介绍了课题组自主开发的无氟PA-MOD法的整个开发过程,并给出了最终的制胶流程。在样品表征方面,主要涉及到的测试手段包括热分析、晶体结构和织构、表面和断面微结构、超导磁性质和电输运性质。自主开发的新型无氟高分子辅助金属有机物沉积(PA-MOD)技术具有以下特点：精确控制原料的化学计量比、成本低廉、涂层溶液制备过程简单易操作、同时高分子化合物的引入有效地增强了涂层溶液的均匀性和稳定性。传统的无氟化学溶液法制备YBCO薄膜过程中容易生成杂相BaCO3,该杂相通常堆积在YBCO的晶界处从而阻碍超导电流的传输,使得采用该方法制备的YBCO薄膜性能低下。20多年来,这一技术一直得不到发展。我们通过对无氟化学溶液法沉积YBCO超导薄膜的分解过程、成相工艺、热处理参数的优化以及相应的机理进行系统的研究,开发出在低温热处理过程中利用水蒸气与BaCO3反应的机理,使得制备过程中形成的BaCO3被分解,从而有效地解决了无氟化学溶液法制备过程中的这一难题,显著地提高了YBCO薄膜的性能以及制备技术的重复性。采用这一技术制备出的薄膜其超导转变温度达到90 K,77 K白场下的临界电流密度达到1 MA／cm2。这一技术对开发出低成本制备高性能YBCO高温超导涂层导体有着重要的意义。为了制备出高性能YBCO薄膜,我们研究了新型无氟PA-MOD法中不同高分子添加剂对涂层溶液合成以及对YBCO薄膜制备的影响。通过研究不同高分子添加剂制备的YBCO薄膜的性能,得到了收缩率最小的、辅助效率最佳的高分子聚乙烯醇缩丁醛(PVB),使超导薄膜的微观形貌显著改善、裂纹减少。在此基础上,开发出高温部分熔融热处理工艺,即在YBCO前驱薄膜中生成少量液相。液相的生成有利于增强前驱膜与基片之间的浸润性并促进其外延生长,同时液相能够弥合分解过程中形成的部分微裂纹和微孔。此外,较高的熔融温度还分解掉残余的BaCO3。这一新型的制备技术进一步改善了YBCO薄膜的微观结构,显著提高了其致密度和平整性,使孔洞率下降、裂纹减少,并使得超导相的成分控制更为精确。这些性能的改善使得YBCO超导薄膜的载流性能出现大幅度上升,在77 K,0 T下的临界电流密度达到了3.5 MA／cm2。这一性能指标在无氟化学法制备的YBCO薄膜中处于国际最好水平之一。由于化学溶液法工艺条件对组分的敏感性,由固相法所得到的制备参数不能直接作为指导化学溶液法制备掺杂的YBCO薄膜。因此采用化学溶液法引入磁通钉扎中心历来是YBCO超导薄膜研究的一个难点。为此,我们分别研究了微量Co3+、Fe3+、Zn2+、Ni2+、Li+等杂质离子的Cu位掺杂对YBa2Cu307-z超导薄膜结构和临界电流密度的影响。结果表明,微量杂质离子掺杂的YBCO薄膜具有良好的双轴织构以及更加平整致密的表面形貌。而且适量杂质离子掺杂使得YBCO薄膜的有场临界电流密度Jc显著提高。此外,Co3+和Zn2+名义掺杂量为0.001的YBCO薄膜的不可逆场Hirr和磁通钉扎力Fp也明显提高,表明通过微量杂质离子掺杂可以有效地改善YBC0薄膜的磁通钉扎性能。由于GdBCO具有比YBCO更高的载流性能,为了进一步提高涂层导体用超导层的性能,我们将所获得的技术推广到GdBCO系统。主要研究了微量Co3+、Fe3+、Zn2+、Ni2+的Cu位掺杂对GdBa2Cu3O7_Z薄膜结构和临界电流密度的影响。通过优化掺杂GdBCO薄膜的成相温度和熔融温度,制得了高性能的GdBCO薄膜。与YBCO体系不同的是,GdBCO体系中由于Gd位替代Ba位会生成在高温高场下可能作为钉扎中心的弱超导的固溶相。因此,杂质离子掺杂造成的纳米级缺陷和弱超导的固溶相均对提高GdBCO薄膜在高温高场下的Jc值有所贡献。此外,我们还研究了稀土元素以及BaZrO3掺杂对Y(Gd)Ba2Cu307-z临界电流密度的影响。根据约化临界电流密度随磁场变化的研究结果,过量稀土元素掺杂、稀土元素Y位部分替代主要提高了YBCO薄膜在低场下的磁通钉扎性能。而Nd、Eu、Gd等量共掺主要提高了薄膜在高温高场下的磁通钉扎性能,2%BaZrO3掺杂可以显著提高GdBCO薄膜在高温低场下的磁通钉扎性能。我们初步分析了在Y(Gd)BCO薄膜中采用不同方式引入钉扎中心的磁通钉扎机制。最后,我们着重分析了影响YBCO薄膜外延生长的热力学和动力学机理,并对控制薄膜织构和表面微结构的部分因素进行了探讨。我们对前驱薄膜形成及YBCO薄膜成相的化学反应机制进行了初步的阐述,并在此基础上应用经典形核理论推导出了先驱薄膜在单晶基片上非均匀形核的临界晶核尺寸、形核驱动力和形核所需要克服的能量壁垒。研究结果表明,通过调控反应过程中的结晶温度、水分压和氧分压可以控制YBCO薄膜以界面形核为主,将均匀的体相形核降到较低的水平。同时也可以通过改变这些参数来增加反应的驱动力,增大非均匀形核的成核率。更多还原

【Abstract】 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.更多还原