Fabrication and properties of Si2N2O-Si3N4 ceramics via direct ink writing and low-temperature sintering

Abstract

Direct ink writing (DIW) and low-temperature sintering methods were applied to prepare Si2N2O-Si3N4 ceramics for radome materials. Lattices of Si-SiO2 green body were printed by DIW with 78 wt % solid portion of water-based Si-SiO2 slurry, in which silicon particles and silica fume were used as the solid portion and Methylcellulose (HPMC) was used as the dispersant. Effects of HPMC addition on stability and silica fume content on rheological properties of the slurry were studied, respectively. The pseudoplastic mechanism of the slurry was analyzed. The Si-SiO2 green bodies were sintered at 1250 °C–1400 °C in nitrogen. The effect of temperature on phase composition, microstructure, mechanical and dielectric properties of samples was investigated. With the HPMC addition of 0.12 wt% and the silica fume proportion of 30 wt% in solid portion, a stable and pseudoplastic slurry with the yield stress of 110.9 Pa was obtained, which is suitable for DIW. With the decrease of initial holding temperature, more N2 enters the sample and reacts with silicon and silica fume, promoting the generation of Si2N2O and Si3N4. The optimal condition yields Si2N2O-Si3N4 ceramics with apparent porosity of 42.73%, compressive strength of 24.7 MPa, dielectric constant of 4.89 and loss tangent of 0.0054. It is found that columnar Si3N4 comes from a direct reaction between silicon and N2, and fibrous Si2N2O is mainly generated by the reaction between silicon, SiO(g), and N2 through the chemical vapor deposition mechanism. Good dielectric properties are achieved due to high porosity, high proportion of Si2N2O phase and no residual silicon.