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 degrees 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/mu s above 1200 degrees 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/cm(3) at 1000 degrees C before decreasing at the highest temperatures to 1.4 g/cm(3), the value obtained for 400 degrees 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. (C) 1997 Elsevier Science Ltd. All rights reserved. References: 17
展开▼
