Study of Grain Boundaries in Superconducting YBCO Thin Films: Correlation Between Critical Current Densities and Boundary Geometry/Structure/Chemistry

本项目将对高温超导体YBa2Cu307- x （YBCO）的晶界进行全面研究。PI将研究在织化Ni/Cu带上生长的织化薄膜中经常出现的001倾斜边界，以及单晶衬底上YBCO薄膜中的某些特殊边界。一些YBCO倾斜边界将通过使用高度特定的双晶、三晶、n晶作为薄膜生长的模板来制备。低指数平面作为边界平面将被设计在这些模板基板上，以避免在YBCO薄膜中出现面状。个别边界上的临界电流密度将通过边界约瑟夫森涡成像来测量（这些涡的宽度与局部临界电流密度直接相关）。磁通量子成像采用扫描超导量子干涉装置显微镜（SSM）进行。通过扫描电子显微镜（SEM）研究相同边界的后向散射通道菊池模式，以确定每个边界的5个几何参数。边界的临界电流密度将绘制为几何参数的函数。任何方向错误的面外分量或方向错误轴不是100的分量都将被记录。在SSM和SEM中观察到的单个边界将与样本上的基准点相匹配。重要边界的特殊区域将被制备成平面视图样品，用于透射电子显微镜下的衍射对比显微镜、电子衍射显微镜、高分辨率电子显微镜、能量色散x射线分析的边界成分作图和电子能量损失谱的化学作图。将检测到边界偏析和晶间膜。除了完美的化学计量目标，稍微偏离化学计量的目标将被用来制作YBCO薄膜。这些非化学计量膜可以用来评估对不同边界的偏析的敏感性程度。显微镜下这001个倾斜边界的几何规格将与它们的结构、成分映射、局部约瑟夫森渗透长度、临界电流密度和整体薄膜成分有关。材料中的晶界通常控制着材料的性能。在高温超导体中尤其如此，这限制了由这些材料制成的设备在商业和军事应用中的实用性。了解晶粒边界效应对超导性能的影响将使器件工程师能够利用这些材料开发出更好的器件。＊＊＊

9803212 Chan This project will perform a comprehensive study of the grain boundaries in the high-temperature superconductor YBa2Cu307- x (YBCO). The PI will study 001 tilt boundaries that often occur in textured films grown on textured Ni/Cu tapes as well as certain special boundaries in YBCO films on single crystal substrates. Some YBCO tilt boundaries will be prepared by using highly specific bicrystals, tricrystals, n-crystals as templates for film growth. Low indices planes as boundary planes will be engineered in these template substrates to avoid faceting in the YBCO films. Critical current densities on individual boundaries will be measured by imaging the boundary Josephson vortices (the widths of these vortices are directly related to the local critical current density). The magnetic flux quantum imaging is performed by scanning superconducting quantum interference device microscopy (SSM). The same boundaries will be studied by scanning electron microscopy (SEM) with back-scattering channeling Kikuchi patterns to identify the 5 geometric parameters of each boundary. Critical current densities of boundaries will be plotted as a function of the geometric parameters. Any out of plane component of misorientation or with a misorientation axis other than 100 will be noted and recorded. Individual boundaries observed in SSM and SEM will be matched by fiducial points on the samples. Special areas of important boundaries will be prepared into planar view samples for observation in transmission electron microscopes employing diffraction contrast microscopy, electron diffraction, high resolution electron microscopy, boundary composition mapping with energy dispersive x-ray analysis, and chemical mapping with electron energy-loss spectroscopy. Boundary segregation and intergranular films will be detected. In addition to perfectly stoichiometric targets, slightly off-stoichiometric targets will be used to make YBCO films. These off stoichiometric films can be used to assess the degree of susceptibility to segregation of different boundaries. The geometric specifications of these 001 tilt boundaries from microscopy will be related to their structure, their composition mapping, their local Josephson penetration lengths, their critical current densities, and the overall film compositions. %%% Grain boundaries in materials often control the properties of that material. This is especially true in high- temperature superconductors, and limits the usefulness of devices made from these materials in commercial and military applications. An understanding of the grain boundary effect on the superconducting properties will enable device engineers to develop better devices from these materials. ***