Formation of Different Si 3 N 4 Nanostructures by Salt-Assisted Nitridation.

Silicon nitride (Si3 N4 ) products with different nanostructure morphologies and different phases for Si3 N4 ceramic with high thermal conductivity were synthesized by a direct nitriding method. NaCl and NH4 Cl 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 NH4 Cl content was 3 wt %, and the maximum α-Si3 N4 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), NH4 Cl decomposition and the generation of stacked amorphous Si3 N4 occurs; in the second stage (900-1450 °C), NaCl melts and Si3 N4 generates; and in the third stage (>1450 °C), α-Si3 N4 → β-Si3 N4 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 NH4 Cl facilitated the evaporation of Si powders and the decomposition of Al2 O3 from porcelain boat and furnace tube, which resulted in the mixing of N2 , O2 , Al2 O, and Si vapors and generated Al x Si y O z nanowires with rough surfaces and lead to thin Si3 N4 nanowires, nanobranches by the vapor-solid (VS), vapor-liquid-solid (VLS), and the double-stage VLS base and VS tip growth mechanisms.