Low-temperature synthesis of highly porous whisker-structured mullite ceramic from kaolin

In this study, the mullitization temperature was drastically reduced to 900 °C through adding boehmite sol to the kaolin/α-Al₂O₃ reaction couple. The low mullitization temperature was probably ascribed to: (1) high solubility of γ-Al₂O₃ in the SiO₂-rich liquid during sintering, (2) adhesive effect of boehmite sol and (3) high reactivity of calcined kaolin. Also, starch particles and polystyrene (PS) micro-spheres were utilized as porogens to prepare high-porosity mullite. Compared to starch particles, the PS micro-spheres were more suitable for fabricating high-porosity mullite. The effects of PS content and sintering temperature on the mullitization reaction and properties of porous mullite were studied. The result demonstrated that single-phase mullite could only be achieved at 1300 °C for samples with porogen. Excessive addition of PS micro-spheres (>32 wt%) would inhibit the porosity increase due to the sample shrinkage during sintering. As the amount of PS increased from 22% to 32%, porosity of the samples increased from 73.29% to 80.10%, compressive strength decreased from 15.55 ± 0.52 to 9.89 ± 0.12 MPa and thermal conductivity decreased from 0.26 to 0.16 W/m/K.