Formation of Different Si₃N₄ Nanostructures by Salt-Assisted Nitridation

Silicon nitride (Si₃N₄) products with different nanostructure morphologies and different phases for Si₃N₄ ceramic with high thermal conductivity were synthesized by a direct nitriding method. NaCl and NH₄Cl were added to raw Si powders, and the reaction was carried out under a nitrogen gas flow of 100 mL/min. The phase composition and morphologies of the products were systemically characterized by X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy. At 1450 °C, the NaCl content was 30 wt %, the NH₄Cl content was 3 wt %, and the maximum α-Si₃N₄ content was 96 wt %. The process of Si nitridation can be divided into three stages by analyzing the reaction schemes: in the first stage (25-900 °C), NH₄Cl decomposition and the generation of stacked amorphous Si₃N₄ occurs; in the second stage (900-1450 °C), NaCl melts and Si₃N₄ generates; and in the third stage (>1450 °C), α-Si₃N₄ → β-Si₃N₄ phase change and the evaporation of NaCl occurs. The products are made of two layers: a thin upper layer of nanowires containing different nanostructures and a lower layer mainly comprising fluffy, blocky, and short needlelike products. The introduction of NaCl and NH₄Cl facilitated the evaporation of Si powders and the decomposition of Al₂O₃ from porcelain boat and furnace tube, which resulted in the mixing of N₂, O₂, Al₂O, and Si vapors and generated Al _xSi _yO _z nanowires with rough surfaces and lead to thin Si₃N₄ nanowires, nanobranches by the vapor-solid (VS), vapor-liquid-solid (VLS), and the double-stage VLS base and VS tip growth mechanisms.