Mechanical and dielectric properties of porous Si₃N₄-SiO₂ composite ceramics

In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol-gel infiltration-sintering was developed to fabricate a porous Si₃N₄-SiO₂ composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol-gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si₃N₄-SiO₂ composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m^1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10^-3 at a resonant frequency of 14 GHz.