Abstract
An electrical conduction model for highly-disordered carbon is developed to account for the fractional temperature dependence of the conductivity. It is assumed that the conduction consists of many independent Arrhenius-type processes with various activation energies and the density of these Arrhenius processes has a A-shape distribution as a function of activation energy. Then the proposed model can reproduce the conductivity behavior which has been widely observed for disordered systems.
Phenol-based activated carbon fibers, as a representative highly-disordered system, are used for conductivity measurements to check the validity of the proposed model. The activated carbon fibers are heat treated in Ar to control the degree of structural disorder. As an index for the degree of disorder, the activation energy for conduction is estimated by the model. The results show that the activation energy, which was -25meV before heat treatment, decreases by heat treatment and an insulator→metal transition takes place between HTT=1000-1200°C Of importance is that the fractional temperature dependence, which has been ascribed to Mott's variable range hopping or the Coulomb interaction model, can also be derived by the presented mechanism.
