Microstructure, elastic and inelastic properties of partially graphitized biomorphic carbons

The microstructural characteristics and amplitude dependences of the Young’s modulus $E$ and internal friction (logarithmic decrement $\delta$) of biocarbon matrices prepared by beech wood carbonization at temperatures $T_{\text{carb}}$ = 850–1600$^\circ$C in the presence of a nickel-containing catalyst have been studied. Using X-ray diffraction and electron microscopy, it has been shown that the use of a nickel catalyst during carbonization results in a partial graphitization of biocarbons at $T_{\text{carb}}\ge$ 1000$^\circ$C: the graphite phase is formed as 50- to 100-nm globules at $T_{\text{carb}}$ = 1000$^\circ$C and as 0.5- to 3.0-$\mu$m globules at $T_{\text{carb}}$ = 1600$^\circ$C. It has been found that the measured dependences $E(T_{\text{carb}})$ and $\delta(T_{\text{carb}})$ contain three characteristic ranges of variations in the Young’s modulus and logarithmic decrement with a change in the carbonization temperature: $E$ increases and $\delta$ decreases in the ranges $T_{\text{carb}}<$ 1000$^\circ$C and $T_{\text{carb}}>$ 1300$^\circ$C; in the range 1000 $<T_{\text{carb}}<$ 1300$^\circ$C, $E$ sharply decreases and $\delta$ increases. The observed behavior of $E(T_{\text{carb}})$ and $\delta(T_{\text{carb}})$ for biocarbons carbonized in the presence of nickel correlates with the evolution of their microstructure. The largest values of $E$ are obtained for samples with $T_{\text{carb}}$ = 1000 and 1600$^\circ$C. However, the samples with $T_{\text{carb}}$ = 1600$^\circ$C exhibit a higher susceptibility to microplasticity due to the presence of a globular graphite phase that is significantly larger in size and total volume.