Synthesis and textural evolution of mesoporous Si₃N₄ aerogel with high specific surface area and excellent thermal insulation property via the urea assisted sol-gel technique

Si₃N₄ aerogel, which is one of the most promising materials for application as high-temperature thermal insulation, is synthesized by the urea-assisted sol-gel technique, followed by the supercritical drying, carbonization, carbothermal reduction and carbon combustion processes. The effects of heat treatment temperatures on the physicochemical and textural evolution of mesoporous Si₃N₄ aerogel are investigated. Si₃N₄ aerogel is formed when the heat treatment temperature increases to 1500 °C, whereas it transforms into SiC phase with a further higher temperature of 1600 °C. The SEM analysis confirms the existence of Si₃N₄ nanoparticles, and the particle size of Si₃N₄ nanoparticles is around 20–40 nm, with pore size around 20–40 nm. The XPS measurement reveals that Si₃N₄ aerogel is composed of 74.4% Si₃N₄ phase, with lower content of 25.6% SiO₂. Si₃N₄ aerogel possesses a large BET specific surface area of 519.6 m²/g, which results from the mesoporous pores left after carbon combustion. This value is much larger than that of the conventional Si₃N₄ porous ceramics and Si₃N₄ nano materials ever reported. The BJH adsorption average pore diameter of Si₃N₄ aerogel is around 11.8 nm, with a large pore volume of 3.5 cm³/g. Si₃N₄ oxidation with forming amorphous SiO₂ layer on the surface is observed by the TG measurement. Moreover, the resulting Si₃N₄ aerogel shows low bulk density of 0.075 g/cm³, as well as low thermal conductivity of 0.045 W/(m·K) at room temperature. The mechanism of Si₃N₄ formation is based on the VS growth between C, SiO₂ and N₂. The structures evolution and formation mechanism of Si₃N₄ aerogel are also investigated in this study.