Thermal conductivity of the amorphous and nanocrystalline phases of the beech wood biocarbon nanocomposite

Natural composites (biocarbons) obtained by carbonization of beech wood at different carbonization temperatures $T_{\mathrm{carb}}$ in the range of 800–2400$^\circ$C have been studied using X-ray diffraction. The composites consist of an amorphous matrix and nanocrystallites of graphite and graphene. The volume fractions of the amorphous and nanocrystalline phases as functions of $T_{\mathrm{carb}}$ have been determined. Temperature dependences of the phonon thermal conductivity $\varkappa(T)$ of the biocarbons with different temperatures $T_{\mathrm{carb}}$ (1000 and 2400$^\circ$C) have been analyzed in the range of 5–300 K. It has been shown that the behavior of $\varkappa(T)$ of the biocarbon with $T_{\mathrm{carb}}$ = 1000$^\circ$C is controlled by the amorphous phase in the range of 5–50 K and by the nanocrystalline phase in the range of 100–300 K. The character of $\varkappa(T)$ of the biocarbon with $T_{\mathrm{carb}}$ = 2400$^\circ$C is determined by the heat transfer (scattering) in the nanocrystalline phase over the entire temperature range of 5–300 K.