Abstract:
This paper reports on measurements of the thermopower $S$ of high-porosity samples of beech wood biocarbon with micron-sized sap pores aligned with the tree growth direction. The measurements have been performed in the temperature range 5–300 K. The samples have been fabricated by pyrolysis of beech wood in an argon flow at different carbonization temperatures $(T_{\mathrm{carb}})$. The thermopower $S$ has been measured both along and across the sap pores, thus offering a possibility of assessing its anisotropy. The curves $S(T_{\mathrm{carb}})$ have revealed a noticeable increase of $S$ for $T_{\mathrm{carb}}<$ 1000$^\circ$C for all the measurement temperatures. This finding fits to the published data obtained for other physical parameters, including the electrical conductivity of these biocarbons, which suggests that for $T_{\mathrm{carb}}\sim$ 1000$^\circ$C they undergo a phase transition of the insulator-(at $T_{\mathrm{carb}}<$ 1000$^\circ$C)-metal-(at $T_{\mathrm{carb}}>$ 1000$^\circ$C) type. The existence of this transition is attested also by the character of the temperature dependences $S(T)$ of beech wood biocarbon samples prepared at $T_{\mathrm{carb}}$ above and below 1000$^\circ$C.
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