Microstructure, elastic, and inelastic properties of biomorphic carbons carbonized using a Fe-containing catalyst

The microstructure and amplitude dependences of the Young’s modulus $E$ and internal friction (logarithmic decrement $\delta$), and microplastic properties of biocarbon matrices BE-C(Fe) obtained by beech tree carbonization at temperatures $T_{\operatorname{carb}}$ = 850–1600$^\circ$C in the presence of an iron-containing catalyst are studied. By X-ray diffraction analysis and transmission electron microscopy, it is shown that the use of Fe-catalyst during carbonization with $T_{\operatorname{carb}}\ge$ 1000$^\circ$C leads to the appearance of a bulk graphite phase in the form of nanoscale bulk graphite inclusions in a quasi-amorphous matrix, whose volume fraction and size increase with $T_{\operatorname{carb}}$. The correlation of the obtained dependences $E(T_{\operatorname{carb}})$ and $\delta(T_{\operatorname{carb}})$ with microstructure evolution with increasing $T_{\operatorname{carb}}$ is revealed. It is found that $E$ is mainly defined by a crystalline phase fraction in the amorphous matrix, i.e., a nanocrystalline phase at $T_{\operatorname{carb}}<$ 1150$^\circ$C and a bulk graphite phase at $T_{\operatorname{carb}}>$ 1300$^\circ$C. Maximum values $E$ = 10–12 GPa are achieved for samples with $T_{\operatorname{carb}}\approx$ 1150 and 1600$^\circ$C. It is shown that the microplasticity manifest itself only in biocarbons with $T_{\operatorname{carb}}\ge$ 1300$^\circ$ C (upon reaching a significant volume of the graphite phase); in this case, the conditional microyield stress decreases with increasing total volume of introduced mesoporosity (free surface area).