【作者】 颜士斌；

【导师】 姚忻；

【作者基本信息】 上海交通大学 ， 凝聚态物理， 2012， 博士

【摘要】 作为工业应用和科学研究的重要基础，人们对薄膜热稳定性的研究已经持续了数十年。自YBCO薄膜的过热性质在YSNG（YBCO seeded NdBCOgrowth）生长中被发现以来，稀土钡铜氧薄膜的高热稳定性先后被报道。反常于一般纳米薄膜的热稳定性，稀土钡铜氧薄膜的这种奇异特性对于其在块体材料熔融织构生长中的应用以及对薄膜的过热机制研究有着极其重要的意义。目前，REBCO薄膜高热稳定性最直接的应用就是在REBCO晶体、液相外延厚膜以及块体材料的生长中作为籽晶诱导生长。近年来，铁基超导材料在全球范围内受到了普遍的关注。液相外延生长技术能够制备高结晶性，组分均匀、微结构易控制的厚膜材料，由于其快速沉积和低成本的特点，其在器件应用方面具有非常诱人的前景。作为铁基超导器件应用的前提，铁基液相外延厚膜的制备对超导物性研究以及器件应用研究方面都具有重要的科学意义。本论文的科学意义在于研究了熔化过程中熔化生长阶段对于REBCO薄膜热稳定性的影响，提出了包晶反应的熔化模型，对于薄膜熔化过程中熔化生长的差异导致的热稳定性不同给出了理论解释。另外，本文研究了大尺寸块体材料生长中的薄膜籽晶应用、温度程序以及生长速度改进的问题。最后，本文使用液相外延生长技术在世界上首次制备了FeTe_1-xSe_x超导厚膜，为铁基超导器件应用提供了研究基础。具体内容如下：1. REBCO薄膜热稳定性及熔融织构生长中薄膜热稳定性的影响因素之前对REBCO薄膜的热稳定性研究主要集中在不同微结构对于薄膜热稳定的影响。而不同气氛、不同REBCO系统下薄膜热稳定性的差异由薄膜本身的属性所决定，一定程度反映了REBCO本质对热稳定性的影响。本文在不同氧分压下比较了NdBCO薄膜热稳定性以及空气气氛下YBCO与NdBCO薄膜的热稳定性不同。结果表明拥有同样微结构薄膜的热稳定性差异主要由熔化过程中的熔化生长过程决定。而薄膜的熔化生长速度与RE211相的生长速度以及熔化前沿溶质浓度差和生长前沿溶质浓度差的比值成正比。由于包晶反应中熔化过程的相似性，该理论可以更广泛的适用于发生包晶反应的RE123材料体系的熔化过程。在REBCO薄膜的应用方面，本文发现前驱体对薄膜上杂质的吸附作用是REBCO薄膜在熔融织构生长中过热因素之一。在研究微结构对薄膜热稳定性影响时，发现薄膜中缓冲层的引入可以改善薄膜的面内取向，提高其热稳定性。另外，本文还研究了不同面内取向晶粒在MgO基板上的粗化竞争机制，发现45°晶粒在粗化竞争中占据优势。这是使用8对称薄膜籽晶诱导生长块体材料时45°取向优先生长的原因。2. REBCO薄膜籽晶应用及大尺寸REBCO块体材料生长高温超导REBCO块体材料在诸如磁悬浮，超导储能等方面具有非常广泛的应用前景。而块体材料的尺寸是制约其应用的关键因素之一。为了抑制块体材料生长降温过程中产生的过大溶质过饱和，本文研究了等温温度及保温时间对块体材料的影响，发现合理等温缓冲区间的使用可以减小降温阶段产生的过大溶质过饱和，抑制自发形核的产生。本文还介绍了使用等温缓冲区间在低氧分压下诱导生长Ag掺杂NdBCO块体材料的工作。所得样品直径为38mm，为目前国际上尺寸最大的NdBCO块体材料。为了提高YBCO块体材料的生长速度，本文使用纯氧气氛进行YBCO熔融织构生长。实验发现，纯氧气氛可以将块体生长速度提高约1.5倍以上。分析认为，导致生长速度提高的主要因素为纯氧气氛下Y元素在熔体中溶解度及扩散系数的提高。该技术可有效提高YBCO块体材料生长，用于大尺寸高性能块体材料生长。3. FeTe_1-xSe_x超导晶体生长及液相外延厚膜制备目前为止，在FeTe_1-xSe_x晶体生长与薄膜制备方面已经有许多研究组做了大量的工作。但是，FeTe_1-xSe_x体系的液相外延厚膜材料制备仍未见报道。本文使用熔体法制备了FeTe_1-xSe_x晶体，并对晶体进行了XRD成相分析，确认了相形成。PPMS测试显示，晶体的起始超导转变温度Tconset约为12.5K。另外，本文使用液相外延生长技术在铝酸镧（LAO）基板上首次得到了FeTe_1-xSe_x厚膜材料。光学显微镜及扫描电子显微镜图像显示厚膜的晶粒为方形且与基板成一定规则排列。膜的厚度约为20um。XRD确认了该薄膜为FeTe_1-xSe_x相且成（00l）取向。EDX数据显示厚膜的不同区域拥有相似的组分，且实际组分与名义组分相差很小。通过本论文的工作，希望能对REBCO超导薄膜的热稳定性的影响因素以及熔化机制给出一定的指导，期望在将来的工作中能够探索控制薄膜热稳定性的途径并寻找具有更高热稳定性的薄膜材料以适应其在材料生长中的应用。另外，通过摸索铁基超导材料液相外延生长技术，我们期望在以后的工作中可以通过微结构控制改善其物理性能，以满足铁基超导材料理论研究及实际应用的需要。更多还原

【Abstract】 The thermal stability of thin films has been studied for decades as a basisfor industrial applications and for fundamental scientific research. Since thesuperheating phenomenon of YBa2Cu3O7-x（Y-123） was observed in the liquidphase epitaxial growth of Nd1+xBa2-xCu3O7-x（Nd-123） thick film, many workshave been reported on high thermal stabilities of RE1+xBa2-xCu3O7-x（RE-123）thin films. The study on this novel property of REBCO films is important forapplications and for obtaining a fundamental understanding of melting. Assuitable seeds, RE-123thin films are widely employed in the liquid phaseepitaxial （LPE） growth of RE-123thick films, growth of single crystals andmelt-textured （MT） bulks.On the other hand, iron based superconductors have gained a large amountof interest recently. Understanding these new superconductors requireshigh-quality epitaxial films to explore intrinsic electromagnetic properties andevaluate device applications. So far, extensive researches have been carried outto prepare iron-based crystals and thin films. However, liquid phase epitaxialfilm of these materials is rarely reported. By the liquid phase epitaxial growth（LPE）, films with high crystalline quality, uniform composition and controllableorientation can be achieved. Due to the fast deposition rate as well as low cost ofLPE growth, it is a promising technique to prepare iron-based epitaxial films. In this thesis, the difference of thermal stability between REBCO thin filmswas investigated. Under an assumption of quasi-equilibrium, a simplified modelfor the peritectic melting of RE-123was suggested, which clarifies themechanism of different thermal stabilities between REBCO thin films. Inaddition, we have prepared FeTe_1-xSe_xthick film by LPE growth for the firsttime. The main progresses are listed below:1. The thermal stability of REBCO thin films and its superheating in MTgrowthSo far, many studies on the thermal stability of RE-123films have beenperformed in connection with the quality and orientation of the films.Differences in thermal stabilities between REBCO thin films under variousatmospheres may directly correlated to the nature of the film. We investigatedthe thermal stabilities of NdBCO thin films under varied atmospheres and thedifference of thermal stabilities between NdBCO and YBCO thin films. Wefound that the diversity in the melting growth process of thin films dominatedthe thermal stability of these films. The melting rate of RE-123is proportional tothe growth rate of the RE-211phase and the ratio of the concentration differencebetween RE-211growth front and RE-123melting border. Owing to thesimilarity among RE-123materials, we suppose this model might be universallyapplicable for the peritectic melting of thin films beyond superconductors.During the melt textured growth process, the adsorption of Ba-Cu-O liquid bythe pellet was believed a reasonable factor for the high thermal stability forREBCO thin films. The study on the influence of thin film microstructure on itsthermal stability showed that the employment of YBCO buffer layer improvesthe in-plane alignment of NdBCO thin film and eventually enhances its thermalstability in MT process. In addition, the mechanism of epitaxial coarsening of YBCO grains with different in-plane alignments on MgO substrate was studied.It was found that the coarsening process dominated by45°grain in the MTprocess is the main reason for the difference of the preferrd in-plane orientationbetween the LPE and MT growth.2. Application of REBCO thin film in MT growth and large sized bulkgrowthREBCO superconductors are promising materials for wide applications. Thesize of the bulk material is the main restraining factor for these applications. Toovercome the problem of the over high supersaturation, a Tmax2procedure wasintroduced in the MT growth. It was found that a reasonable temperature and anappropriate holding time of Tmax2decrease the supersaturation and thereforeavoid the homogeneous nucleation. Additonally, an Ag doped NdBCO bulkmaterial with a diameter of38mm, which is a world-record size for NdBCObulk superconductor, was achieved by Oxygen Control Melt growth （OCMG）technique using an Tmax2temperature profile. The low growth rate and narrowgrowth window are main limits for large sized REBCO bulk growth. By anemployment of pure oxygen pressure as the growth atmosphere, we reported asuccess in enhanced growth rate of the melt-textured （MT） YBCO bulk. Theenhanced growth rate can be interpreted by both increased solubility anddiffusion coefficient of yttrium in the Ba–Cu–O liquid under high oxygen partialpressure. The most important advantage of this technique is that the enhancedgrowth rate of Y123gives less opportunity for self-nucleation in the liquid,which takes place beyond the growth window and the embryonic period.3. FeTe_1-xSe_xcrystal and epitaxial film growthWe successfully prepared FeTe_1-xSe_xcrystal by melt growth method. Thephase formation of the crystal is determined by X-Ray differaction （XRD） measurement. The temperature and field dependence of the magnetization showthat the Tc onset value of the FeTe_1-xSe_xcrystal is about12.5K. Using LPEgrowth technique, we fabricated FeTe_1-xSe_xepitaxial thick film on （00l）-orientedLAO substrate. Optical micrographs and SEM images show that FeTe_1-xSe_xgrains on the substrate are in a quadrate shape and align along the same direction.The out-plane orientation of the film confirmed by XRD measurement is （00l）FeTe_1-xSe_x//（00l）LAO. The actual composition of the film was determined byenergy dispersive x-ray detector （EDX）, the result shows that the film is uniformin the composition, and the actual composition is very close to the nominal one.The work presented here enables us a comprehensive understanding of thethermal stability and the melting process of REBCO thin films, further to realizethe thermal stability control to fulfill the requirement of the bulk growth. Inaddition, with the pioneering work of LPE growth of FeTe_1-xSe_xfilm, a preciselycontrol of film orientation and microstructure can be expected, which will meetthe needs of practical application and fundamental research.更多还原