Ceramic Diesel Particulate Filters -
Compositional and Microstructural Challenges
to Meet Demanding Environments
Diesel engines are attractive alternatives to gasoline engines due to their fuel efficiency and high torque. However, the carbon soot produced as a combustion product requires after-treatment to remove these particles from the exhaust. Ceramic honeycomb “wall-flow” diesel particulate filters (DPFs) have emerged as a popular means for filtering out the carbon soot, as well as ash comprised of various complex oxides formed from oil and fuel additives and engine wear particles. During operation, the accumulated soot must be periodically removed by in situ combustion in order to avoid an unacceptably high back pressure against the engine. This process, known as regeneration, produces an exotherm that is capable of rapidly raising the temperature of the interior of the filter to over 1100oC in certain uncontrolled situations.
This application places numerous, and often conflicting, demands on the properties of a diesel particulate filter, including a high melting point in the presence of ash, high heat capacity, high thermal shock resistance, stability under both oxidizing and reducing conditions, chemical durability in the presence of acidic condensates, chemical compatibility with applied catalyst systems, a pore microstructure that can accommodate catalyst within the walls while allowing for easy flow of gas through the walls and still filtering out the particulates, and adequate mechanical strength. The manner by which these requirements have been addressed through materials research will be illustrated by several case studies of ceramic compositions based upon magnesium aluminum silicate (cordierite), aluminum titanate, and other oxide ceramics.