Percolation in ternary composites: threshold position and critical behaviour

The conductivity of ternary composites (carbon fibers, carbon spheres and polymer) is presented. A transition from an insultating to a conducting regime is observed above a critical volume fraction of conducting particles. Near the threshold a power law dependence of the conductivity is found. Both results are analysed in the frame of the percolation theory. In particular the influence of the relative amount of fibers and spheres on the percolation threshold and on the critical exponent is discussed and quantitatively explained using simple geometrical arguments Nous presentons la conductivite de composites ternaires (fibres de carbone, spheres de carbone, polymere). Une transition metal-isolant est observee au-dessus d'une concentration critique en particules conductrices. Pres du seuil nous observons une dependance en loi de puissance de la conductivite. Ces deux resultats sont analyses dans le cadre de la theorie de la percolation. En particulier nous discutons l'influence de la proportion fibres-spheres sur le seuil de percolation et sur l'exposant critique observe a l'aide d'arguments geometriques simples