Formation of Different SiN Nanostructures by Salt-Assisted Nitridation.

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