缓冲层外延生长及其与超导层的相互影响机制研究

发布时间：2018-04-18 05:07

本文选题：涂层导体 + 超导层　；参考：《陕西科技大学》2016年博士论文

【摘要】：涂层导体是基于双轴织构金属衬底上沉积缓冲层和超导层的类单晶氧化物涂层,在77K下具有高临界电流密度,因此是液氮温区强磁场应用的关键材料。涂层导体是由金属衬底／(单层、复合多层)缓冲层／超导层／保护层组成的多层膜结构复合材料。目前,缓冲层和超导层的制备技术主要分为两大类,其中,以真空技术为基础的脉冲激光沉积和磁控溅射是目前制备高性能涂层导体的主要技术途径；而以非真空技术为基础的化学溶液沉积是工业化低成本涂层导体的重要发展方向。由于化学溶液沉积技术具有成本低、薄膜组份易于控制以及易于实现连续制备等特点,所以采用化学溶液沉积技术制备缓冲层和超导层,是实现涂层导体大规模制备的关键技术途径。为了获得高性能的涂层导体带材,就有必要对化学溶液沉积过程中缓冲层的外延生长、织构传递以及阻隔扩散机制、缓冲层与超导层相互作用机制进行系统的研究,而且对缓冲层外延生长及其与超导层相互作用机制的研究也有利于缓冲层和超导层结构与性能的优化,并推动涂层导体的产业化进程。本论文采用传统Ar-H2还原性气氛和混有低浓度C02的混合气作为保护气体制备了不同形貌的氧化铈缓冲层,利用低氟前驱液采用快速制备法沉积了YBCO超导层。采用金相显微镜(OM)、原子力显微镜(AFM)、扫描电子显微镜(SEM)观察了缓冲层和超导层的宏观和微观形貌,并分析了它们的粗糙度；采用X射线衍射(XRD)技术对缓冲层和超导层的相组成、结晶度以及织构度进行了分析；采用X射线光电子能谱(XPS)对CeO2-δ缓冲层的表面以及内部化学成分进行分析,系统地研究了缓冲层的外延生长、织构传递和阻隔扩散机制、还进一步研究了缓冲层与超导层的相互作用机制,详细研究内容包括：(1)本文利用低浓度C02在合适的温区内与金属有机沉积法所沉积缓冲层中残留碳反应来达到明显的除碳效果,同时抑制薄膜表面裂纹的形成,制备的缓冲层厚膜,且其面内和面外扫描半高宽值分别为9.3°和6.30。同时,应用表面化学成分分析的方法,对裂纹形成的原因进行了详细讨论。分析认为裂纹是由于受到表面张应力而产生的,沿裂纹分布着无定型碳和少量碳酸盐的大颗粒。张应力的产生源于膜中氧空位的形成,随着部分Ce元素化合价的降低,氧空位浓度增加,薄膜内部张应力不断累积,最终在表面形成裂纹。(2)制备了Ce0.8Mo.202-δ (M= Ti、Zr和Hf)与未掺杂的CeO2-δ缓冲层薄膜,研究了掺杂元素的离子半径对缓冲层外延生长和阻隔扩散行为的影响；还制备了Ce0.9M0.1O2-δ (M= Ba、La和Zr)与未掺杂CeO2-δ缓冲层薄膜,研究了掺杂元素化合价对缓冲层阻隔扩散行为的影响,发现掺杂元素离子半径可以通过改变晶格常数来调节缓冲层与衬底之间的压应力,而薄膜内部的压应力有利于缓冲层发生(00l)取向生长；同时发现掺杂元素的离子半径对缓冲层薄膜的面外织构较其面内织构影响更大。掺杂离子半径对缓冲层的表面粗糙度也有较大影响,掺杂元素离子半径与Ce离子半径越接近,缓冲层的表面粗糙度越小。掺杂元素的离子半径和化合价会影响氧空位形成能、掺杂能、氧离子迁移能以及掺杂元素-氧缺陷的缔合能,其中离子半径对氧离子迁移能的影响最大,而化合价却对氧空位浓度和掺杂能的影响较大。(3)采用金属有机沉积(MOD)技术分别在LaAlO3 (LAO)、YSZ (Y稳定的Zr02)和Ni-W衬底上沉积了CeO2-δ薄膜,研究了缓冲层与衬底晶格失配对缓冲层结构、织构以及表面形貌的影响。随着晶格失配度增加,薄膜内部的压应变应力增加,晶界浓度增加,晶粒生长速率较小；晶格适配度小有利于薄膜的高织构度。(4)通过在不同气氛下制备表面形貌不同的缓冲层薄膜,研究缓冲层形貌对超导层的影响。结果表明,缓冲层原子级平坦的区域面积越大,有利于超导相的形成。在缓冲层表面粗糙的圆形晶粒区域,形核密度减小,YBCO层中a轴晶则较多,导致YBCO层超导性能降低。还研究了缓冲层的膜厚对超导层性能的影响,结果表明,较厚的缓冲层不仅有利于阻隔氧扩散、简化缓冲层结构,而且其面外和面内织构度都较高,也容易将金属衬底的织构传递到超导层。(5)通过MOD技术连续制备缓冲层和超导层,研究超导层厚度对缓冲层阻隔扩散性能的影响以及超导层制备工艺中氧分压和晶化温度对缓冲织构的影响。结果表明,在涂层导体的多层膜结构中,膜厚较小的缓冲层很难有效阻隔元素的扩散,而超导层厚膜不仅有利于承载较高电流,同时使缓冲层保持较大的织构度,并且有效抑制金属基带的氧化。在超导层晶化过程中,氧化性气氛有利于缓冲层织构的锐化,高温下由于局域元素扩散加速,导致缓冲层织构度的降低,而在RABITS路线中,缓冲层织构度的降低主要是由于金属基带氧化、Ni元素扩散造成的。
[Abstract]:The coating conductor is a kind of single crystal oxide coating based on the deposited buffer layer and the superconducting layer on the double - axis textured metal substrate . It has a high critical current density at 77K . The coating conductor is a multi - layer film structure composite composed of metal substrate / ( single - layer , composite multi - layer ) buffer layer / superconducting layer / protective layer . At present , the preparation technology of the buffer layer and the superconducting layer is divided into two main types , in which the pulse laser deposition and magnetron sputtering based on the vacuum technology are the main technical means for preparing the high - performance coating conductor .
In order to obtain a high - performance coating conductor strip , it is necessary to study the epitaxial growth , texture transfer and barrier diffusion mechanism of the buffer layer and the interaction mechanism between the buffer layer and the superconducting layer . In order to obtain the high - performance coating conductor strip , the macroscopic and microscopic appearance of the buffer layer and the superconducting layer are observed by using the traditional Ar - H2 reducing atmosphere and the mixed gas with low concentration CO2 as the protective gas , and the roughness of the coating conductor is analyzed .
The phase composition , crystallinity and texture of the buffer layer and the superconducting layer were analyzed by X - ray diffraction ( XRD ) .
The surface of CeO2 - 未 buffer layer and its internal chemical composition were analyzed by X - ray photoelectron spectroscopy ( XPS ) . The mechanism of epitaxial growth , texture transfer and barrier diffusion of buffer layer were studied systematically .
Ce0.9M0 . 1O2 - 未 ( M = Ba , La and Zr ) and undoped CeO2 - 未 buffer layer films were also prepared . The effect of doping element valence on the barrier diffusion behavior of buffer layer was studied . It was found that the ionic radius of the doped element can adjust the compressive stress between the buffer layer and the substrate by changing the lattice constant , and the compressive stress inside the film is beneficial to the growth of the buffer layer ( 00l ) ;
The effect of doping ion radius on the surface roughness of the buffer layer is much larger than that of the buffer layer . The ion radius and valence of the doped element can influence the oxygen vacancy formation energy , the doping energy , the oxygen ion migration energy and the association energy of the doped element - oxygen deficiency .
The results show that the thicker buffer layer is beneficial to the formation of the superconducting phase . The results show that the thicker buffer layer is beneficial to the formation of the superconducting phase . The results show that the thicker buffer layer is beneficial to the formation of the superconducting phase .

【学位授予单位】：陕西科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TM26

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