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Critical Currents and Grain Boundary Dislocations In YBa2Cu3O7-x Superconductors

M. F. Chisholm1 and S. J. Pennycook1

Nature 351, 47 (1991)

Full Article (PDF 432 KB)

A new electron microscopy method for forming high-resolution images with strong chemical sensitivity has been used to examine grain boundaries in the high-temperature oxide superconductors. The new method forms images using large-angle elastically scattered electrons. Thus, the image is not a reconstruction of the object from the diffreacted beams, as in phase contrast, but instead a map showing at atomic resloution the scattering power of the sample. The scattering power for high-angle scattering is strongly dependent on atomic number Z, giving the images chemical sensitivity. Using this technique, we have shown for the first time the true nature of the defects present at individual low-angle tilt grain boundaries that are responsible for the drastic reduction in the critical current density in this material. Such boundaries are seen to consist of an asymmetric array of dislocations. When the boundary tilt angle exceeds ~7.5°, the defects are amorphous and chemically identical to the adjacent YBa2Cu3O7-x grains (see Fig. 1).

This research demonstrates that the achievement of high critical current densities in the oxide superconductors depends strongly on the boundary geometry. Segretgation of impurities or the formationof itergranular glassy phases is not necessary to produce drastic reductions in critical currents. The disturbed region associated with the defects that are obstacles to curent flow is the result of intrinsic structural relaxation of a chemically cleean boundary. As the tilt angle increasesk, the number of defects increases further, restricting the superconducting path (see Fig. 2). This kind of information is needed for unraveling the most important problem of critical currents in the ceramic superconductors; also, it illustrates the usefulness of the high-resolution chemical sensitivity of Z-contrast imaging.

 

Ultradispersed Pt on g-alumina

 
  Figure 1. Z-contrast images of a 13° [100] tilt boundary. a, Both grains oriented for electron channeling along the (001) planes, showing an array of triangular amorphous zones at the boundary. b, Sample tilted so that the right-hand grain is not channelling, showing no detectable contast variations along the grain boundary.
   
 
  Figure 2. Calculated strain field for an infinite array of edge dislocations at a 2°, 5° and 10° tilt boundary, showing the contour for a strain (εxx, the largest strain component) ≥1%, plotted using reduced distances Y=y/D and X=x/D, where D is the dislocation spacing.
   
 
  1. Solid State Division, Oak Ridge National Laboratory

 Oak Ridge National Laboratory