Three-dimension soft X-ray tomographic reconstruction of particulates emitted from a diesel engine

The diesel particulate matter (PM) is typically agglomerates of soot coated with condensed organic compounds. Three-dimensional (3D) measurement of PM particle structures is currently not undertaken as established techniques usually damage the structure of PM particles by dissociating or evaporating the condensed organic compounds absorbed on the soot. In this work, PM particles emitted from a diesel engine were sampled by a micro-orifice uniform deposit impactor and their soot content was measured by a thermo-gravimetric analyzer. The morphology and microstructure of individual PM particle were non-destructively studied using synchrotron soft X-ray tomography (SXT). The 3D tomographic reconstruction from the aligned tilt-series images yielded useful information about PM volumetric characters. Fractal-like agglomerate of soot and the condensed organic compounds were identified. The results showed that the agglomerates are constituted by loosely attached aggregates which are composed of spherules holding tightly to each other. Aggregate size distributions exhibited no specific trend with respect to the engine load and PM size in this work, while soot volume fractions calculated based on the 3D images of PM particles, decreased with increasing load and PM size. The fractal dimensions of agglomerates and PM particles were for the first time presented and calculated by image analysis. The essential properties of PM particles, agglomerates and aggregates are characterized. The population-averaged fractal dimension of agglomerates was smaller and more sensitive to engine load and PM size than that of aggregates, indicating that agglomerates was less compact than aggregates. This work demonstrated the significant advantages of SXT in the description of actual PM morphologies with high accuracy.