Preparation of Mullite Porous Ceramics With Low Thermal Conductivity by Control of Mullite Morphology and Pore Size

ABSTRACT Increasing porosity is currently the primary method for improving the thermal insulation properties of mullite ceramics. However, due to limitations in fabrication techniques (including forming methods, green body drying shrinkage, and ceramic sintering shrinkage), the porosity of mullite ceramics is constrained to approximately 90%, making it difficult to further improve their thermal insulation properties by increasing porosity. In this paper, we propose optimizing the thermal insulation properties of mullite ceramics by synergistically controlling the mullite whisker morphology and pore size, while ensuring a sufficiently high porosity. Using the hydratable alumina gel system with carbon fibers as a pore‐forming agent, a series of mullite ceramics with porosities of 86% to 87% were synthesized via gel casting. In the sintering system using alumina and kaolin as raw materials, mullite seeds are introduced via polycrystalline mullite fibers to control the growth and development of mullite crystals, thereby constructing the nano‐micrometer multi‐scale pore structure. With incorporated short‐columnar mullite whiskers and nanoscale pores in the sintered ceramic, the thermal conductivity of the porous mullite ceramic at room temperature decreased from 0.151 to 0.085 W/(m·K). The improvement in thermal insulation performance is primarily attributed to the increased number of grain boundaries that hinder phonon propagation and to the “trapping” of gas within the nanopores, which reduces the contributions of solid‐state and gaseous heat conduction to the overall thermal conductivity, respectively. The strategy of synergistically controlling mullite whisker morphology and pore size offers a new perspective for enhancing the thermal insulation properties of mullite porous ceramics.