Electromagnetic wave-transparent porous silicon nitride ceramic prepared by gel-casting combined with in-situ nitridation reaction

To achieve the balance between mechanical properties and electromagnetic wave-transparent properties of porous silicon nitride (Si₃N₄), the key is to form an interlocking microstructure constituted by columnar β-Si₃N₄ crystals. This structure can be realized by liquid-phase sintering. However, grain boundaries which affect high temperature properties and volume shrinkage during sintering are inevitable. We proposed a strategy to realize this structure by gel-casting of β-Si₃N₄ whisker (Si₃N_4w) and Si powder followed by in-situ nitridation of Si. To achieve chemically-stable slurry containing micro-sized Si with low viscosity, a novel formulation was developed. Two key structural parameters of the interlocking Si₃N_4w network, i.e., density of the Si₃N_4w skeleton and inter-whisker bonding mode, were adjusted by composition of raw materials and nitridation temperature. The flexural strength, dielectric constant and loss of the porous ceramics are 44.9 MPa, 2.7 and 2 × 10⁻³, when the volume fraction of Si₃N_4w/Si is 5 and the nitriding temperature is 1400 °C.