Preparation of Si2N2O wave-transparent and thermal insulation materials with Na2CO3 and BN as aids by pressureless sintering

A hexagonal boron nitride (h-BN)-bonded porous silicon oxynitride (Si2N2O) composite with a low dielectric constant, low thermal diffusivity, and high thermal shock resistance was fabricated by pressureless sintering, and the phase, microstructure, oxidation resistance, and mechanical and dielectric properties of the composites were investigated. The results demonstrated that Na2CO3 facilitated http://www.youdao.com/w/eng/facilitated/javascript:void(0) ; the generation of Si2N2O. With the subsequent volatilisation of Na2O at high temperatures, near-pure porous Si2N2O ceramics was obtained. However, in the presence of excess Na2CO3 (above 6 wt%), Si2N2O decomposed into Si3N4. The hardness of the Si2N2O/h-BN composite decreased from 1.4 ± 0.15 GPa to 0.4 ± 0.13 GPa with the addition of h-BN, which significantly improved the workability of the material. Moreover, the material exhibited a stable thermal expansion coefficient (3.36 × 10 −6 K−1 at 200 °C and 4.08 × 10 −6 K−1 at 1300 °C), low dielectric constant, and low dielectric loss (e = 3.26, tan δ = 3.21 × 10−3 at a h-BN content of 9 wt%). Thus, it is a potential wave-transparent and thermal insulation material for microwave sintering furnaces.