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 (Si 3 N 4 ), the key is to form an interlocking microstructure constituted by columnar β -Si 3 N 4 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 -Si 3 N 4 whisker (Si 3 N 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 Si 3 N 4w network, i.e., density of the Si 3 N 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 Si 3 N 4w /Si is 5 and the nitriding temperature is 1400 °C.