Synthesis and characterization of nano Si₃N₄ powder via sol foaming and carbothermal reduction and nitridation

Nano Si₃N₄ powder was successfully synthesized using a combination of sol foaming and carbothermal reduction and nitridation techniques. The process utilized fumed silica (amorphous nano SiO₂ powder) as the silicon source, sucrose (C₁₂H₂₂O₁₁) as the carbon source, ammonium chloride (NH₄Cl) as the foaming agent, and nitric acid (HNO₃) as the oxidant, to prepare precursor of the Si₃N₄ powder. The objective of this research was to investigate the influence of various factors on the size of the Si₃N₄ particles. Specifically, the molar ratio of NH₄Cl to SiO₂ (ε), the foaming temperature, the sintering temperature, and the holding time were analyzed in detail. The results revealed that under the optimized conditions of ε = 1, a SiO₂ : C₁₂H₂₂O₁₁ = 1 : 1.5, and a foaming temperature of 350 °C, a fluffy precursor was obtained. This precursor featured amorphous SiO₂ evenly coated with organic carbon, which was crucial for the subsequent synthesis process. Subsequently, the precursor was subjected to a nitridation process at 1500 °C for 2 h. This resulted in the formation of nano α-Si₃N₄ powder. The powder displayed a specific surface area of about 52 m²/g, an average particle size of approximately 20 nm, and a high degree of sphericity.