TRANSPORT-PROPERTIES NEAR THE METAL-INSULATOR-TRANSITION IN HEAT-TREATED ACTIVATED CARBON-FIBERS

The dc electrical conductivity (sigma) of activated carbon fibers (ACF 's) heat treated at temperatures (T-HT) ranging from 300 to 2500 degre es C has been measured from 4.2 K to room temperature, and the magneto resistance of these same fibers:has been measured for magnetic-field s trengths up to 0.8 T at 4.2 K. With heat treatment below similar to 12 00 degrees C, the heat-treated ACF's exhibit similar transport propert ies to those of the as-prepared ACF's with no heat treatment, in that the electrical conductivities for these ACF's all show the exp[-(T-0/T )(1/2)] temperature dependence, characteristic of granular metallic sy stems. A modified Coulomb-gap variable-range-hopping conduction model is proposed to explain this temperature dependence in ACF's. As T-HT r eaches similar to 1200 degrees C, an electronic transition takes place , as evidenced by an increase in the absolute value of a at 300 K by a lmost two orders of magnitude, a drastic change in the temperature dep endence of sigma, a reversal of the dependence of sigma on disorder, a sign change from positive to negative in the magnetoresistance, and t he emergence of anisotropy in the magnetoresistance for the heat-treat ed ACF's. Most Of these changes in the transport properties are consis tent with the metal-insulator transition observed in similarly disorde red systems, signifying that nanoporous materials, with their porosity controllable by heat treatment, can be used to study electronic behav ior in the strong localization regime as well as near the metal-insula tor transition.