Transport properties near the metal-insulator transition in heat-treated activatedrotect carbon fibers

The dc electrical conductivity (σ) of activated carbon fibers (ACF’s) heat treated at temperatures (${\mathit{T}}_{\mathrm{HT}}$) ranging from 300 to 2500 °C has been measured from 4.2 K to room temperature, and the magnetoresistance of these same fibers has been measured for magnetic-field strengths up to 0.8 T at 4.2 K. With heat treatment below ∼1200 °C, the heat-treated ACF’s exhibit similar transport properties 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[-(${\mathit{T}}_0$/T${)}^{1/2}$] temperature dependence, characteristic of granular metallic systems. A modified Coulomb-gap variable-range-hopping conduction model is proposed to explain this temperature dependence in ACF’s. As ${\mathit{T}}_{\mathrm{HT}}$ reaches ∼1200 °C, an electronic transition takes place, as evidenced by an increase in the absolute value of σ at 300 K by almost two orders of magnitude, a drastic change in the temperature dependence of σ, a reversal of the dependence of σ on disorder, a sign change from positive to negative in the magnetoresistance, and the emergence of anisotropy in the magnetoresistance for the heat-treated ACF’s. Most of these changes in the transport properties are consistent with the metal-insulator transition observed in similarly disordered systems, signifying that nanoporous materials, with their porosity controllable by heat treatment, can be used to study electronic behavior in the strong localization regime as well as near the metal-insulator transition.