Measurements of universal and non-universal percolation exponents in macroscopically similar systems

Abstract

Conductivity results and 1/ f noise ( S v) measurements for percolation systems with a cellular structure (composites in which small conductor particles are embedded on the surface of larger regular shaped insulator particles) are given. The usual DC percolation parameters ( t, s and φ c) were obtained from fitting the results to the percolation equations. φ c values for the systems have been found to lie in the range 0.01–0.07, while both non-universal and close to universal values have been measured for the exponents s and t. Flicker (1/f ) noise results on the systems give an additional exponent w from the relationship S v/ V 2= KR w . For the systems measured so far, the exponent w is observed to take different values w 1 close to and w 2 further away from the conductor–insulator transition, for φ> φ c. The very different values ( s, t and w), obtained for the various conducting powders, in the same macroscopic structure, indicates that the way the powders distribute themselves on the insulating particles, and the nature of the interparticle (cluster) contacts, is a major factor in determining the critical exponents. The results are compared with those obtained from random void models of percolation systems.