MoO 3 and AlF 3 effect on the properties of porous mullite ceramics with needle‐like microstructure

Porous mullite ceramics were synthesized via ceramic processing using a local industrial kaolin, alumina, and with AlF3 and MoO3 precursor additives to catalyze the formation of pores and acicular mullite grains. The textural properties, crystalline phases, microstructures, and mechanical properties of the obtained samples were comprehensively analyzed and compared with a reference mullite formulated without additives. The obtained ceramics have a well-defined porous microstructure, with a porosity of about 50%, and a pore size distribution in the 0.1–2 µm range. Ceramics that involve the addition of MoO3 present well-defined acicular mullite grains, with slightly higher porosity, larger pores, and better mechanical resistance compared to those processed using AlF3. To correlate the ceramic porosity with its mechanical properties, a simple model of spherical pores was proposed to assess the flexural strength of the dense ceramic. It was found that the inclusion of additives promoting needle-like microstructures increases the mechanical resistance up to four times the value determined for the ceramic without additives (flexural strength up to 390 MPa for zero-porosity extrapolation). These results, together with the refractoriness of mullite, allow inferring the potential applications of the developed materials as structural ceramics with relatively low density and for filtering applications.