Conduction mechanisms in carbon-loaded composites

Results on the conductivity as a function of temperature of carbon black-polymer composites with a wide range of loadings are presented and discussed. Inter-particle transfers dominate the resistivity in the low-loading composites, and the conductivity is described quantitatively by the theory of fluctuation-induced tunnelling. This mechanism together with orbit shrinkage accounts for the observed positive magnetoresistance. In the highly loaded composites the conduction mechanism within the carbon black becomes important. This is found to be well described by theories of two-dimensional conduction in a disordered metal. The two-dimensional nature results from the graphite layered structure of carbon black. The magnetoresistance of the highly loaded composites is also found to be positive and quantitatively consistent with resulting from interaction effects. We are unable to explain the apparent suppression of the weak-localization contribution to their magnetoresistance.