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.