Enhanced mechanical and thermal insulation properties of mullite-based thermal insulation materials by heat treatment in reducing atmosphere

To improve the properties of mullite-based thermal insulation materials typically prepared by foam-casting method, samples were heat treated under carbon monoxide atmosphere created by a carbon bed at 600 °C and 800 °C, respectively. The effects of reduction process on microstructure, pore size distribution, phase evolution, cold crushing strength (CCS), and thermal conductivity of the resultant materials were systematically studied. The results showed that the formation of high aspect ratio mullite whiskers and small amounts of glassy phases among whiskers during the reduction process are demonstrated as the key factors in improving the properties of the resultant materials. The mechanical properties are determined to be significantly enhanced for the binding effect of glassy phase among whiskers with soaking duration gradually increased to 48 h. And the lower thermal conductivity of samples treated for 24–48 h are mainly due to the high connectivity and lowered size of pores from the accumulation of high aspect ratio whiskers. Compared with samples treated at 800 °C, both the highest CCS (4.2 MPa) and minimum thermal conductivity (0.15 W/m K) were obtained in the sample heat treated at 600 °C for 48 h. Hence, it was demonstrated that the use of low-temperature reduction process could enhance the mechanical and thermal insulation properties of mullite-based thermal insulation materials.