Lightweight Si₃N₄@SiO₂ ceramic foam for thermal insulation and electromagnetic wave transparency

Increasing porosity is one of the most direct ways to improve the thermal insulation and dielectric properties of materials. Until now, many wet methods for preparing Si₃N₄ ceramic foams usually face the problems of complex rheology, long period, and expensive cost, and the reported pore sizes of Si₃N₄ ceramic foams are typically micron-grade, resulting in a lack of competitiveness in thermal insulation and wave-transparent applications. In this paper, the Si₃N₄@SiO₂ ceramic foams were prepared using an efficient dry-method, which combined three processes of low temperature chemical vapor deposition (LTCVD), template, and isostatic pressing. The method has the advantages of simple operation and short preparation period, and can realize near-net size molding and mass production. In addition, the evolution mechanisms of honeycomb microstructure and composition of Si₃N₄@SiO₂ ceramic foam during sintering were studied by chemical reaction thermodynamics. The as-prepared Si₃N₄@SiO₂ ceramic foam possesses low density (0.377 g·cm⁻³), high compressive strength (7.5 MPa), low thermal conductivity (0.0808 W·m⁻¹·K⁻¹), and excellent dielectric properties (ε < 1.32, tan δ < 0.009) in the frequency range of 8–18 GHz, and its maximum working temperature in air can reach up to 1100 °C. It will be recommended to be applied in the interlayer of Si₃N₄ ceramic radome to improve its thermal insulation and electromagnetic wave transparency performances.