Near-zero sintering shrinkage porous mullite ceramics with high porosity and low thermal conductivity

Porous mullite ceramics exhibit exceptional mechanical and thermal properties in high-temperature environments, making them suitable for applications in thermal insulation. However, a notable challenge associated with high porosity is the tendency for increased sintering shrinkage during the sintering process. The sintering shrinkage can lead to unpredictable cracking and deformation, negatively impacting the final properties of the porous ceramics. In this study, porous mullite ceramics were fabricated using aluminium powder, alumina powder, and silica sol through foam-gelcasting and freeze-drying methods. The oxidative expansion of the aluminium powder was employed to offset the sintering shrinkage. The influence of alumina powder content on the microstructure and properties of porous mullite ceramics was investigated. Particularly, at the alumina content of 50 wt%, the porous mullite ceramics exhibited near-zero sintering shrinkage (0.3%). These ceramics demonstrated high porosity ranging from 78.9% to 83.3%, accompanied with low thermal conductivity (0.24-0.29 W/(m·K)) and great mechanical properties (0.51-1.02 MPa). The temperature difference over 750 °C revealed porous ceramics get excellent thermal stability as ideal lightweight high-temperature thermal insulation materials. This innovative approach presents a novel idea for fabricating complex net-shape porous mullite ceramic components with enhancing yield and low cost.