Thermal anisotropy in sintered cordierite monoliths

Cordierite honeycombs with oxide compositions of 49.5–51 wt.% SiO2, 35.5–36 wt.% Al2O3 and 14–14.5 wt.% MgO were prepared from clay, talc, and alumina using conventional extrusion process. Effects of sintering parameters such as heating rate, soaking time, cooling rate as well as amount of calcined clay in the raw material compositions on the overall coefficient of thermal expansion (CTE) as well as thermal anisotropy in terms of CTE difference between the extrusion direction and across the extrusion direction of the cordierite honeycombs were studied. For this purpose, extruded honeycombs were sintered with various heating and cooling rates varying from 80 to 180°C h−1 up to a peak temperature of 1420°C for various soaking times ranging from 4 to 8 h. All the sintered cordierite honeycombs were characterized with respect to phase composition (XRD), morphology (SEM), bulk density, water absorption, apparent porosity, and CTE (dilatometric studies). XRD studies reveal that all the investigated sintered honeycombs contain >90% cordierite phase with smaller quantities of mullite, spinel and α-Al2O3. Raw materials compositions containing 60% calcined clay of total clay yielded crack free honeycombs with lower thermal anisotropy. Among the various parameters studied, honeycombs sintered with heating rate of 80°C h−1, soaking time of 8 h and cooling rate of 180°C h−1 showed minimum thermal anisotropy.