1/for flicker noise in cellular percolation systems

Abstract

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 (Φ> Φ c ) of the critical volume fraction (Φ 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 y and m, respectively. Values of y obtained are in the range 0.97-1.2. Samples with larger (Φ-Φ c ) have m2, while those with smaller (Φ-Φ c ) have m significantly lower than 2. The normalized noise at 10 Hz (S v 1 0 H z /V m ) obey the well-established relationships S v ( f ) /V m (Φ-Φ c ) - k and S v ( f ) /V m R 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 ∼0.75-5.23 close to Φ c and k 2 ∼2.23-5.54 further into the conducting region. Values of w 1 range from 0.36-1.37, while w 2 ∼0.99-1.59. 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.