Formation, conversion, and elimination of intermediate spinel phase in MgO/kaolin mixture firing for cordierite

Cordierite ceramic is usually used for diesel particulate filter owing to its excellent low thermal expansion coefficient and high thermal shock resistance properties. However, the co-exited intermediate spinel phase can deteriorate the thermal and mechanical performances of cordierite ceramic product, because the spinel phase has much higher thermal expansion coefficient comparing to that of cordierite. In this study, two methods are utilized to reduce the spinel impurity in the cordierite ceramic. On the one hand, rational reaction resources were introduced to decrease spinel production. The formation of intermediate spinel phase is systematically researched by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman characterizations and the results clarified the preference of path “Enstatite + Mullite → Cordierite” for less spinel production in comparison to path “Enstatite + Al 2 O 3 → Cordierite + Spinel.” Additionally, MgO was introduced as fluxing agent to promote liquid-phase sintering, thus facilitating the conversion of spinel. On the other hand, the sintering schedule was improved by introducing a holding temperature gradient to promote the diffusion of Si 4+ and further promote the conversion of spinel into cordierite. With these methods, the residual spinel phase is minimized, the resulting high-purity cordierite has a 47% reduction in the thermal expansion coefficient from 3.07 × 10 −6 /K to 1.63 × 10 −6 /K compared to the original cordierite sample.