While Diesel vehicles are increasingly ousted, the only sustainable way to still use them is to issue more stringent regulations, allowing less and less emissions. Diesel Particulate Filters (DPFs) allow exhaust gas from cars and trucks to be ‘cleaned’ from soots and ashes, before it is emitted in the atmosphere. They therefore are the sentinels of our environment, at least for the next few years.
BAM divisions Micro NDT and Structure Analysis have worked on understanding how the arrangement of pores in such filters impacts their mechanical (strength and thermal shock resistance) and thermal properties (thermal expansion). In fact, state-of-the-art characterization techniques and know-how on such materials are available at BAM. In particular, X-ray Computed Tomography (XCT) and X-ray refraction radiography (XRRR), both implemented in the lab and at the Synchrotron BESSY II, HZB, Berlin, are particularly powerful tools to investigate such complicated materials.
The porous structure of a DPF is visible in microscopy images (cross-sections), but the quantification of the properties of such porous structure (interconnection, orientation, size distribution) can only be made through three-dimensional XCT data and suitable X-ray refraction techniques. XRRR has been developed and perfectioned at BAM Division Micro NDT. It allows detecting and quantifying the amount of tiny defects (with size above 1 nm), which cannot otherwise be observed in a non-destructive fashion, but only through tedious scanning electron microscopy work. In the case of DPFs XRRR allowed quantifying the orientation of the porous structure in a few DPF materials, pointing out analogies and differences among them.
Such orientation is of extremely high importance for the functionality of the materials, since it determines not only the mechanical properties, but also the filtration efficiency of the DPF: if pores have the right size distribution they filter well; if they have the right orientation, they ensure that the solid material properly dissipates heat (such filters work at 800°C). Advanced analysis of X-ray diffraction data disclosed the orientation of the crystals composing the solid material, and electron microscopy work corroborated the findings. The combination of all these techniques allowed a breakthrough piece of knowledge: the crystal orientation of the solid material in DPFs is intimately linked to the orientation of the porous space in the filter (and to the production route). The correlation is, however, different for each material, depending on its composition. Therefore, optimizing the functional and physical properties of each type of DPFs will become easier.
The correlation between porosity characteristics and the crystallographic texture in extruded stabilized aluminium titanate for diesel particulate filter applications
Cong Chen, Bernd R. Müller, Carsten Prinz, Julia Stroh, Ines Feldmann, Giovanni Bruno
published in Journal of the European Ceramic Society, Vol. 40, pages 1592-1601
BAM, Division Structure Analysis, Division Micro NDT