Microstructure of diesel soot particles probed by electron microscopy: First observation of inner core and outer shell

It is well known that soot particles emitted from a diesel engine are usually observed as chain-like aggregates (secondary particles) composed of several tens to hundreds of primary spherical particles. The mature aggregates are not easily eliminated by in-cylinder oxidation. Thus, lowering particulate emission from a diesel engine is difficult. In order to determine whether or not there are effective solutions for reducing soot, the early stage of the formation process before the aggregation needs to be clarified. The authors carried out an ultra-high-resolution electron microscopic observation to analyze the microstructure of diesel soot in further detail. In the present study, the authors adopted two types of TEM techniques to make the microstructure of diesel soot clear: one is a phase-contrast method, and another is a hollow-cone beam method. It was revealed that a primary particle of diesel soot has two distinct parts, each with different structures. The details of these structures are discussed in the Results.