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A typical disordered carbon material, phenol-based activated carbon fibers, has been characterized by conductivity measurements and electron spin resonance (ESR) measurements. This carbon is well known as an excellent adsorbent with huge specific surface areas, however, another aspect i.e. electronic properties is focused in this paper. Since this material has a high ratio of surface atoms, interesting phenomena are expected to emerge from the surface states.The results show that this carbon is an unusual insulator of which conductivity at room temperature is rather high (about 10S/cm). Although this material belongs to so-called hard carbon, an insulator-metal transition is found to take place at rather low temperatures between 1300-1500K.Corresponding to the transition, a fast spin relaxation is observed. A conventional microstructure model for disordered carbons, i.e., congregation of ultra fine powders, can well account for the insulating properties.

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