Porous closed-cell Si₃N₄ ceramics for thermal insulation, electromagnetic wave transmission, and load-bearing: A novel preparation route to control closed-pore structure

Porous closed-cell silicon nitride (Si₃N₄) ceramics (PCC-Si₃N₄), characterized by low thermal conductivity, low dielectric constant, and low dielectric loss, are promising candidates for applications in thermal insulation and high-temperature electromagnetic wave-transparent radomes. This study presents a novel preparation route for PCC-Si₃N₄, using Si₃N₄ hollow microspheres (Si₃N_4hm) as pore-forming agents. Initially, Si₃N_4hm–Si green bodies were fabricated through gel-casting, followed by in situ nitridation of Si powders to pre-strengthen them into Si₃N_4hm–Si₃N₄ preforms. Subsequently, the pre-strengthened Si₃N_4hm–Si₃N₄ preforms were densified via the precursor infiltration and pyrolysis (PIP) process to produce PCC-Si₃N₄. By varying the volume ratio of Si₃N_4hm to Si in the initial slurry, a series of PCC-Si₃N₄ with different phase compositions and porosities were synthesized, resulting in distinct trends for flexural, dielectric properties, and thermal conductivity. Specifically, the flexural strength, real part of the complex permittivity (ε′), and thermal conductivity of PCC-Si₃N₄ gradually decrease with increasing total porosity, whereas the dielectric loss (tanδ) progressively increases. These findings allow for the tuning of PCC-Si₃N₄ properties for specific applications through the rational control of total porosity.