Research Reveals the Atomic Structures of Diesel Particulates
Argonne's research on diesel particulate matter (PM) has probed beyond what is possible with commercially available instruments and provided unprecedented details about the morphology, fractal geometry, and atomic microstructure of PM. When Argonne researchers used a Raman scattering spectroscopy technique to explore diesel PM at the atomic level, they achieved — for the first time — a fundamental understanding of the formation and destruction mechanisms in diesel PM.
|At high engine speeds and load conditions, many diesel particles exhibited concentric ring patterns, as shown in high-resolution transmission electron microscopy (TEM) images. This type of atomic structure within diesel particles has long been suspected to be made of graphite.
Argonne researchers used high-resolution TEM with Raman scattering spectroscopy, followed by theoretical analysis, to verify previous qualitative observations of graphitic structures. Raman scattering spectroscopy is a highly sensitive optical analysis technique that can study atomic microstructures in carbonaceous material. It is sensitive to the structural change or rearrangement of carbon crystallites. It can also provide a detailed view of a particle sample in depth when an argon ion laser is used to excite the sample at a penetration depth of about 100 nanometers (nm).
The typical Raman scattering spectrum of diesel particulates displays two distinct peaks: a D peak around 1,350 cm-1 and a G peak around 1,580-1,600 cm-1. Another important feature of Raman scattering spectroscopy is that the degree of graphitic crystal structures can be evaluated by the full width at half maximum (FWHM) of peak curves in a Raman spectrum. The peak intensity ratio between the D and G bands is inversely proportional to the crystallite dimension of graphite. For example, an average crystal size was calculated to be 5.23 nm for diesel particulates sampled at a given engine operation condition in a light-duty engine. Graphitic structures in carbon materials are typically turbostratic, where the carbon atoms located in a single hexagon layer are connected to one another, and the carbon layers are stacked on top of each other in a random orientation.
The interatomic distance (Rc-c) between carbon atoms and the interplanar distance (d002) are two important parameters in determining the degree of graphitic crystallite structures. With the calculated crystallite dimensions (La) determined from the Raman scattering data, the d002 and Rc-cwere calculated to be 0.141 and 0.34 nm, respectively. The magnitude of these two parameters for typical carbon graphite is known to be d002 = 0.1422 and Rc-c = 0.3354 nm, respectively. Each parameter differs at about 1.0 % between the two materials. Therefore, the atomic microstructures of diesel particulates were quantitatively measured, and it was shown that their internal microstructures closely resemble those of typical graphite.
The characteristics of graphitic microstructures within diesel particles depend on engine operating conditions. Graphitic crystallite dimensions and the FWHM of Raman scattering spectrum peaks were calculated as a function of the exhaust temperature for light-duty diesel PM. Sizes of graphitic crystal structures increased with increasing temperature, and the FWHM decreased with temperature, which indicates that diesel particulates become more stable and organized in internal atomic structures as the engine load increases. On the basis of these results, any regeneration scheme for a diesel particulate filter system may need to be optimized according to engine operating conditions.