Transition characteristics of a carbonized wood cell wall investigated by scanning thermal microscopy (SThM)

The transition characteristics of Quercus rubra wood fiber cell walls during carbonization were investigated by means of scanning thermal microscopy (SThM). Wood samples were carbonized at temperatures from 200 to 600 °C. SThM investigations indicated that the secondary wall (S₂) and compound middle lamella (CML) of the wooden fiber cell wall (FCW) could easily be distinguished in the cross section because of the difference in thermal conductivity between them, but when the carbonization temperature was increased, the difference decreased and disappeared at carbonization temperatures above 325 °C. Confocal Raman microscopy studies indicated that the concentrations of cellulose and lignin differed significantly in the S₂ and CML, and when carbonized at temperatures higher than 325 °C, the chemical component of the FCW became homogenous. The changes of the dimensions of FCW were also investigated. The FCW became noticeably thinner during carbonization. Compared to native wood, the thickness decreased by 62% when carbonized at 600 °C and the loss of thickness occurred simultaneously with weight loss. The key temperature was 300 °C, with most shrinkage and weight loss occurring before 300 °C.