Xuemei Yi

Preparation of β-Si6-zAlzOzN8-z (z = 1-3) by combustion synthesis

β-Si6−zAlzOzN8−z (z = 1, 2, and 3) powders were combustion synthesized under a low nitrogen pressure of 1 MPa. Si, Al and SiO2 were used as the starting materials, and β-SiAlON (z = 1) was added as diluent. The CSed powders then were sintered by spark plasma sintering at 1600 °C for 12 min. The products were analyzed by X-Ray Diffraction (XRD) and FE-SEM. The Vickers hardness was measured by a Vickers microhardness tester at room temperature. The purity of the CSed products reached above 86% for z = 1, 2, and 3. Pure and dense β-SiAlONs (z = 1, 2 and 3) were obtained by SPS of CSed powders without any sintering additives, and the relative densities reached greater than 98% in theoretical. The Vickers hardness showed decreasing with the increase of z value, and the data ranges between 14.7 and 17.6 GPa due to different z values.

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.