Frequency dependence of electrical conduction of composites below the percolation threshold

The percolation threshold p_c of carbon black (CB)-high density polyethylene (HDPE) composites is discussed based on the critical exponent of conductivity and the change of relative permittivity with CB loading. It is concluded that p_c is the second transition where a sharp increase in conductivity for CB loading becomes gradual. There are two components in ac loss current. One is a current which is independent of frequency and becomes prevalent especially in a region just below p_c. This current shows a non-ohmic behavior. The other component is due to a dielectric loss current which may take place in tunneling gaps. These results support the conclusion that the electrical conduction mechanism below p_c is tunneling