Structure-mediated transition in the behavior of elastic and inelastic properties of beach tree bio-carbon

Microstructural characteristics and amplitude dependences of the Young modulus $E$ and of internal friction (logarithmic decrement $\delta$) of bio-carbon matrices prepared from beech tree wood at different carbonization temperatures $T_{\mathrm{carb}}$ ranging from 600 to 1600$^\circ$C have been studied. The dependences $E(T_{\mathrm{carb}})$ and $\delta(T_{\mathrm{carb}})$ thus obtained revealed two linear regions of increase of the Young modulus and of decrease of the decrement with increasing carbonization temperature, namely, $\Delta E\sim A\Delta T_{\mathrm{carb}}$ and $\Delta\delta\sim B\Delta T_{\mathrm{carb}}$, with $A\approx$ 13.4 MPa/K и $B\approx$ -2.2 $\times$ 10$^{-6}$ K$^{-1}$ for $T_{\mathrm{carb}}<$ 1000$^\circ$C and $A\approx$ 2.5 MPa/K and $B\approx$ -3.0 $\times$ 10$^{-7}$ K$^{-1}$ for $T_{\mathrm{carb}}>$ 1000$^\circ$C. The transition observed in the behavior of $E(T_{\mathrm{carb}})$ and $\delta(T_{\mathrm{carb}})$ at $T_{\mathrm{carb}}$ = 900–1000$^\circ$C can be assigned to a change of sample microstructure, more specifically, a change in the ratio of the fractions of the amorphous matrix and of the nanocrystalline phase. For $T_{\mathrm{carb}}<$ 1000$^\circ$C, the elastic properties are governed primarily by the amorphous matrix, whereas for $T_{\mathrm{carb}}>$ 1000$^\circ$C the nanocrystalline phase plays the dominant part. The structurally induced transition in the behavior of the elastic and microplastic characteristics at a temperature close to 1000$^\circ$C correlates with the variation of the physical properties, such as electrical conductivity, thermal conductivity, and thermopower, reported in the literature.