Microstructural evolution during charcoal carbonization by X-ray diffraction analysis

X-ray diffraction (XRD) of monolithic pieces of carbonized medium-density fiberboard (c-MDF) provided new insights on the microstructural evolution occurring during charcoal carbonization. Classical XRD theory was used to correlate the {002} peak intensity to the amount of carbon in large turbostratic crystallites and to bulk dimensional changes. This new analytical technique could be used to study the microstructural evolution of other monolithic carbon materials (including soft carbons) or of specific processes (including chemical activation). The quasipercolation model, a new 'percolation-like' model, was created based on XRD analysis of monolithic c-MDF. As the carbonization temperature (T_carb) increased above 600 °C, the large turbostratic crystallites grew very little, but the graphene sheets grew substantially. Volumetric shrinkage suggested that turbostratic crystallites were drawn closer together as the low-density disordered carbon was converted into high-density graphene sheets. At approximately 900 °C, the large graphene sheets and the large turbostratic crystallites significantly impinged on each other. The increased impingement of conductive phases with increasing carbonization temperature would cause the commonly observed nonmetal-metal transition of hard carbon materials. The quasipercolation model also suggested the source of the nanoporosity that is critical in activated carbons.