Preparation of Al 2 O 3 ‐mullite thermal insulation materials with AlF 3 and SiC as aids by microwave sintering

Al 2 O 3 ‐mullite composites were prepared under the synergy effect of AlF 3 and SiC aids by microwave heating. The phase composition, microstructure, porosity, flexural strength, thermal shock resistance, and thermal conductivity were investigated. The XRD results revealed that the content of mullite phase steadily increased with the increasing of AlF 3 content. The microstructure showed that the lower content (≤1 wt%) of AlF 3 led to the formation of granular mullite and the higher content (≥3 wt%) of AlF 3 led to the formation of mullite whiskers, which could form an interlocking structure. In addition, the SiC hot spots can also promote the generation of mullite whiskers by microwave sintering. The thermal shock resistance was significantly improved by the interlocking structure of mullite whiskers. The residual rate of flexural strength of the composite with 3 wt% AlF 3 was 86%. The composite with 3 wt% AlF 3 additives got its optimized thermal conductivity from 30°C to 950°C, the value was between 0.819 and 1.021 W/(mK), which possess excellent thermal insulation performance.