Preparation of Al₂O₃‐mullite thermal insulation materials with AlF₃ and SiC as aids by microwave sintering

Abstract

Al2O3‐mullite composites were prepared under the synergy effect of AlF3 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 AlF3 content. The microstructure showed that the lower content (≤1 wt%) of AlF3 led to the formation of granular mullite and the higher content (≥3 wt%) of AlF3 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% AlF3 was 86%. The composite with 3 wt% AlF3 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.

Preparation of lightweight corundum-mullite thermal insulation materials by microwave sintering

“…During the sintering process, the silicon carbide could become a hot spot under the action of microwave field and partly oxidized to SiO 2 , which could react with Al 2 O 3 and AlF 3 to form AlOF and SiF 4 vapours near 660°C. Finally, the gen- erated AlOF and SiF 4 vapours had a gas-phase reaction in the released direction and the mullite whiskers were formed [31]. The mullite whiskers with criss-crossing structure might be caused by the AlF 3 crystal nucleus, while the interlocking mullite whiskers might be formed by the synergistic effect of SiC crystal nucleus and gas phase provided by AlF 3 .…”

“…5). The reason is the fact that the influence of phase composition on thermal conductivity was weaker than that of porosity [31,35].…”

“…The microwave heating technique has been applied to prepare metal materials, refractories, ceramics and other materials [26][27][28][29][30]. Thus, Fan et al [31] prepared corundum-mullite lightweight insulation materials by microwave sintering at 1500°C using SiC and AlF 3 aids. The results showed that addition of 3 wt.% AlF 3 enables formation of mullite whiskers and an interlocking structure with greatly enhanced thermal shock resistance.…”

“…The results showed that addition of 3 wt.% AlF 3 enables formation of mullite whiskers and an interlocking structure with greatly enhanced thermal shock resistance. The most of the pores had a pore size less than 1 µm, which reduced the thermal conductivity which was in the range from 0.819 to 1.021 W/(m•K) at temperatures between 30 and 950°C [31].…”

Incorporation of SCBA in Red Ceramics and Sintering in Microwave Oven

“…Although microwave sintering has numerous advantages such as lower grain growth, higher densification, better mechanical properties, reduction in temperature, selective heating, rapid heating and energy saving [23,24] , some problems must be solved so that the heating occurs in an adequate way, such as the thermal instabilities, the low absorption of microwaves by transparent materials (low dielectric loss at low temperature) such as red ceramic, which allows the passage of microwaves through the material [19,25,26] .…”