Thermal conductivity of partially graphitized biocarbon obtained by carbonization of medium-density fiberboard in the presence of a Ni-based catalyst

The thermal conductivity $k$ and resistivity $\rho$ of biocarbon matrices, prepared by carbonizing medium-density fiberboard at $T_{\operatorname{carb}}$ = 850 and 1500$^\circ$C in the presence of a Ni-based catalyst (samples MDF-C(Ni)) and without a catalyst (samples MDF-C), have been measured for the first time in the temperature range of 5–300 K. X-ray diffraction analysis has revealed that the bulk graphite phase arises only at $T_{\operatorname{carb}}$ = 1500$^\circ$C. It has been shown that the temperature dependences of the thermal conductivity of samples MDFC- 850 and MDF-C-850(Ni) in the range of 80–300 K are to each other and follow the law of $k(T)\sim T^{1.65}$, but the use of the Ni-catalyst leads to an increase in the thermal conductivity by a factor of approximately 1.5, due to the formation of a greater fraction of the nanocrystalline phase in the presence of the Ni-catalyst at $T_{\operatorname{carb}}$ = 850$^\circ$C. In biocarbon MDF-C-1500 prepared without a catalyst, the dependence is $k(T)\sim T^{1.65}$, and it is controlled by the nanocrystalline phase. In MDF-C-1500(Ni), the bulk graphite phase formed increases the thermal conductivity by a factor of 1.5–2 compared to the thermal conductivity of MDF-C-1500 in the entire temperature range of 5–300 K; $k(T$ = 300 K) reaches the values of $\sim$10 W m$^{-1}$ K$^{-1}$, characteristic of biocarbon obtained without a catalyst only at high temperatures of $T_{\operatorname{carb}}$ = 2400$^\circ$C. It has been shown that MDF-C-1500(Ni) in the temperature range of 40–300 K is characterized by the dependence, $k(T)\sim T^{1.3}$, which can be described in terms of the model of partially graphitized biocarbon as a composite of an amorphous matrix with spherical inclusions of the graphite phase.