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 (Fe₃O₄) 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 σᵢ, σc, s, t, and φc in the ideal limits. Both universal and nonuniversal values of s and t are measured in the systems. Close to the percolation threshold (φc), the ac conductivity (σₘᵣ) and the dielectric constant (εₘᵣ) are found to scale as σₘᵣ∝ωᵘ and εₘᵣ∝ω^-v. All these exponents are examined using the most recent theories and compared with previous studies. The dielectric constant exponent (s^'), from εₘᵣ∝(φc-φ)^-s^', is shown to be frequency dependent. The exponents gc (magnetoresistance) and tₘ (from magnetoconductivity) in composites are not yet clearly understood but these and previous results show that tₘ>t. dc scaling is shown in a real composite comprising Fe₃O₄ and talc wax.
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Chiteme et al. (2003) studied this question.