Fabrication and properties of porous silicon nitride wave-transparent ceramics via gel-casting and pressureless sintering

In order to fabricate a radome material with small volume shrinkage and not only good mechanical properties but also excellent dielectric properties, gel-casting and pressureless sintering were adopted to prepare porous silicon nitride (Si3N4) ceramics. By adjusting the solid loading and the additives' (Y2O3/Al2O3) content, the drying and sintering shrinkage of ceramics could be controlled under 5% and 15%, respectively. Through decreasing temperature and prolonging holding time properly, the flexural strength of porous Si3N4 with few sintering aids could get close to that of ceramics with more additives. Both the flexural strength and the logarithmic dielectric constant showed a linear relation with the porosity of ceramics. Typically, the porous Si3N4 sintered at 1650 °C for 3 h with only 3 wt% sintering additives displayed relatively good performance, attaining a porosity of 39.28%, a flexural strength of 146.0 MPa, a fracture toughness of 1.44 MPa m1/2 and a calculated dielectric constant of 4.03 at a resonant frequency of 14 GHz.