Carbonized wood monoliths—Characterization Original Research Article

By using a combination of very accurately cut wood specimens and highly controlled pyrolysis conditions, we have for the first time obtained precise volumetric and linear shrinkage measurements for wood carbonization which we use to formulate a new theory of cellulose microfibril dominance. Characterization of these crack-free carbonized wood monoliths was performed by measuring density, dimensions, acoustic velocity, yield and crystallinity for heat treatment temperatures ranging from 400 to 2500 °C. Axial dimensions increased at the highest temperatures while radial and tangential dimensions continued to decrease. Acoustic velocity in the axial direction remained constant at 5 mm/μs above 1200 °C. Radial and tangential velocities went through a maximum before diminishing at the highest temperatures. Helium density of powdered carbonized monoliths was found to go through a maximum of 1.9 g/cm3 at 1000 °C before decreasing at the highest temperatures to 1.4 g/cm3, the value obtained for 400 °C char. The experimental results are used to support a model for the relationship between the cellulose molecular arrangement in the wood and the microstructure of the resulting carbon monoliths. It is proposed that cellulose microfibrils dominate the mechanism of dimensional change during pyrolysis and result in a preferred orientation of crystallites in the carbonized wood.