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Japanese cedar (Cryptomeria japonica) preheated at 700°C was subsequently heated to 1800°C and characterized by electron microscopy, X-ray diffraction, and micro-Raman spectroscopy. The degree of disorder of carbon crystallites and the amount of amorphous phase decreased considerably with an increase in heat treatment temperature to 1400°C, while carbon crystallites clearly developed above this temperature, showing that the microstructure of carbonized wood undergoes drastic changes around 1400°C. Besides showing the bands for sp 2 -bonded carbon, the Raman spectra showed a shoulder near 1100 cm −1 assigned to sp 3 -bonded carbon. With an increase of heat treatment temperature, the peak position of the Raman sp 3 band shifted to a lower frequency from 1190 to 1120 cm −1 , which is due to the transformation of sp 3 -bonded carbon from an amorphous phase to a nanocrystalline phase. These data showed that the microstructure of carbonized wood from 700° to 1800°C consisted of the combination of sp 2 -and sp 3 -bonded carbon, which is probably due to the disordered microstructure of carbonized wood. It is suggested that the sp 3 -bonded carbon is transformed from an amorphous structure to a nanocrystalline structure with the growth of polyaromatic stacks at temperatures above 1400°C.

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