Microstructural evolution and performance factors of Si-O-N ceramics with dielectric and mechanical properties

The Si-O-N system ceramics, which include silicon nitride (Si₃N₄), silica, and silicon oxynitride (Si₂N₂O), not only enhance thermal stability and wave-transparent performance simultaneously but also exhibit excellent designability in terms of composition and microstructure. They hold significant potential to replace conventional wave-transparent materials, such as Si₃N₄. Herein, we investigated the preparation of Si-O-N system ceramics using gas pressure sintering method with silicon nitride and silicon dioxide as raw materials. The main focus is on the effects of sintering temperature and holding time on the microstructure, mechanical properties, and dielectric properties (both at room temperature and high temperature) of Si-O-N system ceramics. It was found that at a sintering temperature of 1700 °C, the Si-O-N ceramics predominantly exhibited the Si₂N₂O phase. However, with increasing sintering time, the Si₂N₂O transformed into Si₃N₄. The mechanical properties of the Si-O-N ceramics sintered at 1700 °C for 3 h showed that the density, porosity, flexural strength, and fracture toughness were 2.54 ± 0.01 g·cm⁻³, 4.32 %, 278 ± 17 MPa, and 7.45 MPa·m^1/2, respectively. Their wave-transparent performance is exceptional, characterized by a dielectric constant with a real part of 5.27 and an imaginary part of 0.02 at 10 GHz, along with a remarkably low dielectric loss of 0.0038 at the same frequency.