Microdiffraction measurements of the effects of grain alignment on critical current in high-temperature superconductors

While single crystals and epitaxial thin films of high temperature superconductors can carry large current densities, devices useful for applications such as power transmission and magnets cannot be produced because polycrystalline material cannot carry sufficient current densities. Efforts are underway to produce polycrystalline material in which grains are aligned to carry high current densities. We report x-ray and electron microdiffraction measurements of local grain alignment and models of how this grain alignment affects the critical current densities. TlCa ₂Sr ₂Cu ₃O _x samples can be grown on polycrystalline substrates with good c axis alignment but no overall a axis alignment. In TlCa ₂Sr ₂Cu ₃O _x, high critical current occurs in regions in which there is local a axis alignment. X-ray microdiffraction measurements of grain orientation were made with a monochromatic, 100 micrometer diameter beam produced by inserting a pinhole at the focus of an NSLS bending magnet beamline. Local grain orientation was measured by observing Bragg reflection as the sample was rotated. While x-ray data was taken at this low resolution over large areas, the orientation of individual grains was measured over small regions by measuring the Kikuchi pattern produced by inelastic scattering from a 100 nm electron beam. In both cases, the sample position was scanned to map grain orientation. With advanced x-ray optics currently under development, high-resolution maps of grain orientation will be available without the elaborate surface preparation required for electron diffraction. This will facilitate study of samples prepared in a wider variety of forms.