Thermal conductivity of high-porosity biocarbon preforms of beech wood

This paper reports on measurements performed in the temperature range 5–300 K for the thermal conductivity $\varkappa$ and electrical resistivity $\rho$ of high-porosity (cellular pores) biocarbon preforms prepared by pyrolysis (carbonization) of beech wood in an argon flow at carbonization temperatures of 1000 and 2400$^\circ$C. X-ray structure analysis of the samples has been performed at 300 K. The samples have revealed the presence of nanocrystallites making up the carbon matrices of these biocarbon preforms. Their size has been determined. For samples prepared at $T_{\mathrm{carb}}$ = 1000 and 2400$^\circ$C, the nanocrystallite sizes are found to be in the ranges 12–25 and 28–60 $\varkappa(T)$ are determined for the samples cut along and across the tree growth direction. The thermal conductivity $\varkappa$ increases with increasing carbonization temperature and nanocrystallite size in the carbon matrix of the sample. Thermal conductivity measurements conducted on samples of both types have revealed an unusual temperature dependence of the phonon thermal conductivity for amorphous materials. As the temperature increases from 5 to 300 K, it first increases in proportion to $T$, to transfer subsequently to $\sim T^{1.5}$ scaling. The results obtained are analyzed.