Thermal conductivity at the amorphous-nanocrystalline phase transition in beech wood biocarbon

High-porosity samples of beech wood biocarbon (BE-C) were prepared by pyrolysis at carbonization temperatures $T_{\mathrm{carb}}$ = 650, 1300, and 1600$^\circ$C, and their resistivity $\rho$ and thermal conductivity $\varkappa$ were studied in the 5–300 and 80–300 K temperature intervals. The experimental results obtained were evaluated by invoking X-ray diffraction data and information on the temperature dependences $\rho(T)$ and $\varkappa(T)$ for BE-C samples prepared at $T_{\mathrm{carb}}$ = 800, 1000, and 2400$^\circ$C, which were collected by the authors earlier. An analysis of the $\varkappa(T_{\mathrm{carb}})$ behavior led to the conclusion that the samples under study undergo an amorphous-nanocrystalline phase transition in the interval 800$^\circ$C $<T_{\mathrm{carb}}<$ 1000$^\circ$C. Evaluation of the electronic component of the thermal conductivity revealed that the Lorentz number of the sample prepared at $T_{\mathrm{carb}}$ = 2400$^\circ$C exceeds by far the classical Sommerfeld value, which is characteristic of metals and highly degenerate semiconductors.