Specific features of electrical properties of porous biocarbons prepared from beech wood and wood artificial fiberboards

This paper reports on comparative investigations of the structural and electrical properties of biomorphic carbons prepared from natural beech wood, as well as medium-density and high-density fiberboards, by means of carbonization at different temperatures T _carb in the range 650–1000°C. It has been demonstrated using X-ray diffraction analysis that biocarbons prepared from medium-density and high-density fiberboards at all temperatures T _carb contain a nanocrystalline graphite component, namely, three-dimensional crystallites 11–14 Å in size. An increase in the carbonization temperature T _carb to 1000°C leads to the appearance of a noticeable fraction of two-dimensional graphene particles with the same sizes. The temperature dependences of the electrical resistivity ρ of the biomorphic carbons have been measured and analyzed in the temperature range 1.8–300 K. For all types of carbons under investigation, an increase in the carbonization temperature T _carb from 600 to 900°C leads to a change in the electrical resistivity at T = 300 K by five or six orders of magnitude. The dependences ρ(T) for these materials are adequately described by the Mott law for the variable-range hopping conduction. It has been revealed that the temperature dependence of the electrical resistivity exhibits a hysteresis, which has been attributed to thermomechanical stresses in an inhomogeneous structure of the biocarbon prepared at a low carbonization temperature T _carb. The crossover to the conductivity characteristic of disordered metal systems is observed at T _carb ? 1000°C.