Catalyst substrate is crucial component influencing performance, robustness, and durability of catalytic converter systems. Design targets for ceramic catalyst substrates include High geometric surface area (GSA), Large open frontal area (OFA), Low thermal mass and heat capacity, High use temperature, Low coefficient of thermal expansion, Good coatability, Washcoat compatibility, Strength and oxidation resistance.
A large positive CTE means that the body expands significantly when heated, a negative CTE means that the body contracts when its temperature increase. When the material heating is not uniform, as may be caused by non-uniform gas velocity or catalyst exotherms, some parts of the catalyst support are expanding more than others, thus straining the material between the hotter and cooler regions. High CTE coefficient in combination with high temperature gradients will cause high internal strain. Therefore, it is desirable for catalyst support materials to have low thermal expassion coefficients. Cellular cordierite substrate typically have lower CTE values along the axial direction of the part. Higher CTE coefficients are measured along the tangential direction which makes the tangential direction less resistance to thermal shock than the axial direction.
One of the key durability requirements of ceramic catalyst supports is an adequate thermal shock resistance to survive temperature gradients. Due to heat losses from the catalytic converter, the center region of the substrate experiences higher temperatures than its periphery, which induces tensile stresses in the outer region (the center region may experience tensile stresses during cool-down). The magnitude of these stresses depends on the CTE, the E-modulus and the radial temperature gradient. These stresses must be kept well below the modulus of rupture of the support to prevent material failure. The thermal shock parameter defined as the ratio of mechanical strain tolerance to thermal strain imposed by the radial temperature gradient is often used as a relative measure of the material ability to withstand steep temperature gradients. The higher thermal shock, the better the thermal shock capability of the material. In gasoline applications, temperature of the center and peripheral region of the substrate are 825°C and 450°C respectively. Thermal shock may be computed for both of the axial and tangential directions.
Substrate strength is important for withstanding packing loads and subsequent use in the vehicle exhaust stream with the related exposure to engine vibrations, road shock and temperature gradients. High strength substrates are desirable for robust catalytic converter systems. The strength of the ceramic material itself is controlled by the type of intra and intercrystalline bonding, the porosity, pore size distribution and flaw population. The strength of the cellular structure of the substrate is further determined by its dimensions