Activated carbon fibers (ACFs) possess nanocrystalllites made up of graphene sheets that create conducting particles, nonuniformly distributed in space. The nanocrystallites are about 2.5 nm. Existence of voids makes ACFs a porous system, with specific surface area of 3000 m²/g. In such porous material with conductive particles it is necessary that the grains are linked structurally in order to hold sample together. But the linkage is not always electrical.
We report results of the temperature properties of electric resistivity of ACFs, where the model of granular metallic systems [1] is proposed for the electric conductivity of ACFs. For this model, dc electrical resistivity vs temperature is described as ρ(T) = ρ₀exp[(T₀/T)^1/p], where p = 2, ρ₀ is a proportionality constant and T₀ varies between 10²K and 10⁴K depending on metallic volume fraction.
EPR measurements of ACFs show localization of paramagnetic centers on nanocrystallites below 100K. It confirms the granular metal model for ACFs.

[1] B. Abeles et al. adv. Phys. 24 (1975) 407

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1. Confinement effects by means of dielectric methods