AC and DC conductivity, magnetoresistance, and scaling in cellular percolation systems

Percolation phenomena, which include the ac and dc conductivity, dielectric constant, and magnetoresistance, are studied in a series of seven cellular composites, consisting of small conductor particles embedded on the surface of larger insulator particles. Carbon black (ground and unground), graphite, graphite–boron-nitride, niobium carbide, nickel, and magnetite (formula presented) powders were the conducting components with talc-wax powder as the common insulating component. The dc conductivity results were fitted to the standard percolation equations and to a two-exponent phenomenological equation, which yields the percolation parameters (formula presented) (formula presented) s, t, and (formula presented) in the ideal limits. Both universal and nonuniversal values of s and t are measured in the systems. Close to the percolation threshold (formula presented) the ac conductivity (formula presented) and the dielectric constant (formula presented) are found to scale as (formula presented) and (formula presented) All these exponents are examined using the most recent theories and compared with previous studies. The dielectric constant exponent (formula presented) from (formula presented) is shown to be frequency dependent. The exponents (formula presented) (magnetoresistance) and (formula presented) (from magnetoconductivity) in composites are not yet clearly understood but these and previous results show that (formula presented) dc scaling is shown in a real composite comprising (formula presented) and talc wax. © 2003 The American Physical Society.