Monolithic silicon nitride-based aerogels with large specific surface area and low thermal conductivity

C/silica (RF/SiO₂) aerogel (RFSA) was synthesized via a one-step sol-gel process and supercritical fluid drying. Then, monolithic silicon nitride (Si₃N₄) aerogel (SNA) was prepared via carbothermal nitridation of the RFSA in flowing N₂. The effects of the RFSA density and carbothermal temperature on the formation of the SNA were investigated. The evolution of the physical properties, chemical structure, morphology, pore structure, and thermal performance of the SNA was examined. Si₃N₄ nanocrystals were formed from the RFSA at a carbothermal temperature of 1500 °C. The as-prepared SNA had a low density (0.127 g/cm³), large specific surface area (445 m²/g) and low thermal conductivity (0.04909 W/(m·K)), which were far better than those of its state-of-art counterparts. Because of its good thermal stability, the SNA can be used as a thermal insulator and support at high temperatures.