Near-net-size porous mullite ceramics with tuneable sintering shrinkage for thermal insulation applications

The microstructure and properties of porous ceramics are severely affected by volume shrinkage during the sintering process. Therefore, reducing sintering shrinkage while maintaining the properties of porous ceramics is a serious challenge. Herein, near-net-size porous mullite ceramics with tuneable sintering shrinkage were prepared by the foam-gelcasting method using kyanite as raw material and aluminum fluoride and aluminum powders as additives through the volume expansion generated by the sintering reaction. The effects of Al powder content on the sintering shrinkage, porosity, compressive strength and thermal conductivity of porous mullite ceramics were investigated. The results show that the sintering shrinkage gradually decreases with increasing Al powder content, and expansion is even produced. When the Al powder content was 5 wt%, the sintering shrinkage of porous mullite ceramics was −0.21 %, with porosity of 84.2 %, compressive strength of 2.45 ± 0.15 MPa, and thermal conductivity of 0.19 W/(m·K) at room temperature. The results of the high-temperature fire resistance test showed that the temperature difference between the front and back sides of the sample under the flame gun reached 800 °C, indicating good thermal insulation performance. The near-net-size sintering process of porous ceramics can effectively reduce the subsequent processing cost, open new paths for their production, and broaden the application fields of porous mullite ceramics.