PERCOLATION EFFECTS IN THE V-I-CHARACTERISTICS OF GRANULAR YBA2CU3O7-DELTA

V - I characteristics of granular YBa2Cu3O7-delta samples with very different grain size and thickness have been studied in the temperature range 65 - 85K and magnetic fields up to 35 Oe. The results present clear evidence of percolation effects in a 3D network of weakly linked superconducting grains. The onset of dissipation follows a V is-proportional-to (I - I(c))n power law n assumes values between 2 and 2.6 and does not depend on microstructure or thickness. It shows a very weak dependence, if any, on the external magnetic field, H, and temperature, T. This suggests that the shape of the low current side of the distribution of critical currents of the links is also quite insensitive to H and T. It also shows that the transition to the coherent state takes place at T > 85K. Peculiar percolation effects have been observed when the thickness of the sample approaches the grain size. V is-proportional-to (I - I(c))n onsets of disspation with exponent n practically the same as in other granular superconductorcs (both conventional and high temperature ones) present clear evidence for the percolation effects in 3D network of intergrain links.