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

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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. ***