INTERGRAIN BOUNDARY ASYMMETRY AND THE CRITICAL-CURRENT OF HTSC CERAMICS

The critical current of individual intergrain contacts in HTSC ceramic s defining its transport critical current depends on the ''constructio n'' of these contacts and, in particular, on their geometry. We analyz e the current-carrying properties of intercrystallite tilt boundaries in terms of the dislocation model of such boundaries, proceeding from the assumption of substantial suppression of the order parameter by lo cal mechanical stresses arising in the vicinity of the boundary. The s patial distribution of stresses depends on the misorientation angle th eta of adjacent grains and the orientation angle phi of the boundary. Calculations reveal mechanical stresses on the asymmetrical boundary ( phi not-equal 0) to be on the average weaker than on the symmetrical o ne (phi = 0). It must lead to the increase of the current-carrying cap acity of asymmetrical boundaries. The results obtained with regard to this circumstance describe much better the experimental dependences of the tilt boundary critical current on the misorientation angle theta. Another peculiarity of the asymmetrical boundary, that is a ''random' ' character of the spatial distribution of stresses, leads to a random distribution of the critical current local density over the contact a rea. For such (Josephson) contacts the magnetic field dependence of th e critical current is a double-step one which allows one to explain ex perimentally observable double-step magnetic field dependences of the transport critical current of a good number of HTSC ceramics.