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Why is cordierite used for honeycomb catalytic converter supports?

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advanced structural ceramics Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.

Cordierite, a magnesium aluminosilicate ( $M g_{2} A l_{4} S i_{5} O_{18}$ ), is the material of choice for honeycomb catalytic converter supports primarily due to its exceptionally low thermal expansion.1

In the context of automotive ceramics, this property is critical because it provides high resistance to thermal shock. As catalytic converters are subjected to rapid and extreme temperature fluctuations during the operation of an internal combustion engine, a material with low thermal expansion is necessary to prevent the fragile, thin-walled honeycomb structure from cracking or failing under thermal stress.21

While the extruded cordierite monolith serves as the structural substrate, it is typically coated with a wash of alumina to provide the high surface area required to support the active platinum-group metal catalysts.1