1/f or flicker noise in cellular percolation systems -: art. no. 024207

Flicker or 1/f noise is studied in a series of four composite discs, which consists of carbon black (ground and unground), graphite, and graphite/boron nitride, as the conducting components coating, and a common insulating matrix of talc wax. The measurements were done on the conducting side (phi>phi(c)) of the critical volume fraction (phi(c)), within a frequency range of 1.2-1001.2 Hz. The results are analyzed in terms of Hooge's empirical formula with frequency and voltage exponents gamma and m, respectively. Values of gamma obtained are in the range 0.97-1.2. Samples with larger (phi-phi(c)) have mapproximate to2, while those with smaller (phi-phi(c)) have m significantly lower than 2. The normalized noise at 10 Hz (S-v10 Hz/V-m) obey the well-established relationships S-v(f)/V(m)proportional to(phi-phi(c))(-k) and S-v(f)/V(m)proportional toR(w), where V is the voltage across the sample with resistance R, while m, k, and w are exponents. However, a change in the value of the exponent k and w was observed in the measured systems, with k taking the values k(1)similar to0.75-5.23 close to phi(c) and k(2)similar to2.23-5.54 further into the conducting region. Values of w(1) range from 0.36-1.37, while w(2)similar to0.99-1.59. The k(1)(w(1)) are observed when m<2. The nonuniversality of the k(1) and k(2) regimes are interpreted as due to the superposition of the behavior that results from the geometry (a random voidlike structure) and the behavior resulting from the presence of non-Ohmic, intergranular contacts between the conducting grains. These exponents are tested for consistency using w=k/t, and compared with predictions from recent theoretical models.