Two percolation thresholds due to geometrical effects:: experimental and simulated results

The electrical properties of a mixture of ethylene-propylene-diene monomer rubber and silicon carbide (SiC) have been measured as a function of filler concentration. It was found that mixtures containing angular SiC grains have a conductivity that displays not one, but two percolation thresholds. Different types of contacts between the conducting particles, being represented by edge and face connections, respectively, can explain the phenomenon. The two percolation thresholds are obtained at volume fractions of about 0.25 and 0.40, respectively. These values are higher than those predicted by theory, which can be explained by dispersion effects with only one phase being granular and the other being continuous. The value of the conductivity at the central plateau was found to be close to the geometric mean of the limiting conductivities at low and high concentrations. This is in good agreement with theory. With rounded SiC grains only one threshold is obtained, which is consistent with only one type of contact. The concentration dependence of the conductivity was simulated using a three-dimensional impedance network model that incorporates both edge and face contacts. The double-threshold behaviour also appears in the calculations. By dispersing the conducting particles more evenly than random, the thresholds are shifted towards higher concentrations as observed in the experiments.