DPF Materials

Clearly, a DPF needs to remove the carbon particulate, without significantly impeding the gas flow.  However, there are several further requirements.  A major one is stability when subjected to high temperatures, high thermal gradients and thermal shock.  This is particularly important during regeneration, which does not occur uniformly and can raise local temperatures to >1000˚C.  Even if the phases remain thermodynamically stable, thermal stresses may be high and can cause cracking, ending the life of the DPF (and releasing large amounts of particulate into the atmosphere before it is replaced.)  A figure of merit commonly used for thermal shock resistance (TSR) is given by

\[M = \frac{{{G_c}(1 - 2\nu )K}}{{E\alpha }}\]

where G_c is the fracture energy (toughness), n the Poisson ratio, K the thermal conductivity, E the Young’s modulus and α the thermal expansivity.  (M has units of power, with a high value reflecting a capacity to withstand high local rates of energy release without cracking.)  This equation can be used to identify candidate materials with good TSR and the M values shown in the table below are for materials commonly used for DPFs.  It can be seen, for example, that SiC has a high value of M (mainly because of its high thermal conductivity), while Al₂TiO₅ is also attractive (mainly because of its low expansivity and low stiffness).  However, the table also highlights an important point, concerning the effect of porosity, illustrated via the example of cordierite (the cheapest of the three materials).  The stiffness (E) is often reduced substantially by high levels of porosity, which DPFs require in any event.