Thermal Expansion and Thermal Shock Resistance of Cordierite Honeycombs Used in Catalytic Converters

Extruded ceramic honeycombs are used as substrates for catalytic converter in automotive emission control. The low thermal expansion and thus high thermal shock resistance(TSR) of cordierite honeycomb monoliths are important since monoliths have to withstand severe thermal stresses during the use. Besides thermal expansion, flexural strength and modulus of elasticity are the factors effecting TSR. To meet the demanding requirement of high conversion efficiency with low pressure drop and long durability, the honeycombs of variety of cell density and wall thickness are designed. Various Cordierite honeycombs are extruded and thermal properties are studied and correlated with calculated values and presented in this paper.

The thermal expansion is measured in two directions, using a Dilatometer. Anisotropy in thermal expansion is observed in all the samples. Thermal expansion is found to be low along the channels and high across the wall. This is mainly attributed to the preferential orientation of cordierite crystals in the honeycomb monolith. Measurement of thermal expansion for various configurations shows the variation of thermal expansion with honeycomb cell design.

Measurement of TSR is difficult because of complexity of shape. TSR is measred using Oxy-hydrogen flame for heating the honeycombs of various configurations and detecting the failure by acoustic emission technique. FEM is used to find out the critical temperature difference (ΔTc) of failure and for studying the stress distribution during thermal shock testing. Flexural strength required to calculate thermal stresses are measured by 3-point bending and elastic properties by sonic resonance method.