In-situ reaction synthesis of porous Si2N2O-Si3N4 multiphase ceramics with low dielectric constant via silica poly-hollow microspheres

In the present work, a novel and facile process has been proposed to fabricate porous Si2N2O-Si3N4 multiphase ceramics with low dielectric constant (εr＜4.0). Since silica poly-hollow microspheres could serve as the source of SiO2 and the pore-forming agent, they have been introduced into Si3N4 slurry through the gelcasting technique. This process is benefited from the liquid phase sintering reaction between SiO2 and Si3N4 with the aid of sintering additives, leading to in-situ synthesis of Si2N2O phase and porous structure. The content of silica poly-hollow microspheres has great influence on the properties of the final products. It indicates that Si2N2O phase would become the major phase when the content of silica poly-hollow microspheres was above 25 wt%. Furthermore, the micromorphology results reveal that the content of pores with many smaller aggregate microspheres increases as microspheres amount rises. As a result, along with the addition of silica poly-hollow microspheres, the bulk density decreases to 1.32±0.01 g/cm3, and open porosity ranges from 28.4±0.4% to 52.0±0.5%. Porous Si2N2O-Si3N4 multiphase ceramics prepared with 25 wt% silica poly-hollow microspheres addition possess flexural strength of 42.3±3.8 MPa, low dielectric constant of 3.31 and loss tangent of 1.93×10−3. It turns out to be an effective method to fabricate porous Si2N2O-Si3N4 composites with excellent mechanical and dielectric properties, which could be applied to radome materials.