Percolation exponents and thresholds obtained from the nearly ideal continuum percolation system graphite-boron nitride

Compressed disks made from graphite and, its mechanical but not electrical isomorph, boron nitride as well as graphite-boron nitride powders, undergoing compression, are nearly ideal continuum percolation systems, as the ratio of their conductivities is nearly 10(-18) and the scatter of the experimental points near the critical volume fraction phi(c) is very small. The following measurements, with the characteristic exponent(s) in brackets, are made on some or all of the samples in (axial) and at right angles (radial) to the direction of compression, as a function of the volume fraction of graphite (phi); dc conductivity (s and t), dielectric constant (s), magnetoresistivity t(perpendicular to)), and noise power (K). The noise power is also measured as function of resistance (w) and volume (b'). The phi(c)'s obtained for all measurements are consistent and explicable. The results for the exponents are less well understood but, where possible, these results are compared with theoretical predictions and previous experiments. The reasons for the nonuniversality of t are clarified.