MICROSTRUCTURAL CHANGES OF DIESEL SOOT DURING OXIDATION

Changes in the microstructure of diesel soot during oxidation in air were examined by means of an analytical electron microscope. The soot was sampled using a stainless-steel mesh placed in an engine exhaust pipe. The samples at 25, 50, and 75 wt% burnoff were obtained by controlling the holding time in an electric furnace at 500°C. In the early stage of oxidation, an occluded "SOF (soluble organic fraction)" within the soot particles is released and the soot particles take on a porous structure. Also, layer planes of crystallites within the soot particles grow successively and are arranged, a turbostratic structure. In the last stage of oxidation, the size of the layer planes of the crystallites in the soot particles does not become smaller, but crystallites at the outermost shell of the soot particles flake off instead, reducing the size of the particles. That is, the oxidative disappearance of the soot particles is concluded to involve a disintegration and stripping of the outer layer, induced by strain energy in the layer plane that accompanies the growth of the crystallite layer under high temperature. © 1991.