Low temperature resistivity of Au-YBa2Cu3O7-delta sintered materials: influence of porosity and secondary phases

The resistivity rho of sintered Au and YBa2Cu3O7 - delta (Au and YBCO) mixe d powders is measured at T = 60 K, as a function of the gold volume fractio n phi. The samples retain zero resistivity up to phi = phi(c) = 0.62. In th e range phi is an element of [phi(c), 1], the curve rho(phi) is bell-shaped , with a maximum at phi similar or equal to 0.75. Scanning electron microsc opy observations suggest that the increase of resistivity can be attributed both to porosity and to secondary phases inducing contact resistances betw een YBCO and Au grains. A random conductor network model is used to test th e relative influence of both parameters. The numerical data show that poros ity alone cannot account for the resistivity maximum. Conversely, a peak is indeed observed in the simulations when a substantial interface resistivit y is imposed.