Fabrication And Properties Of Novel Porous Ceramics For Radome Materials

There are some key factors that restrict the application fields of radome materials,such as bulk density,dielectric properties,mechanical strength,thermal shock resistance,corrosion resistance,particle erosion resistance,etc.Fused silica ceramics possess low dielectric constant and dielectric loss tangent,good thermal shock resistance,which makes them important materials for low-Mach missle radomes.Silicon nitride-based ceramics have high mechanical strength,moderate dielectric properties,excellent high-temperature performance,thus they have been candidate materials for high-Mach missle radomes.Porous structure can effectively improve the wave-transparent properties of materials,and their application range in missle radomes could be widened.However,pores can weaken the mechanical strength in some extent.Therefore,it is essential to select appropriate fabrication process for preparing porous ceramics which have both relatively high mechanical strength and low dielectric constant and loss tangent.In order to solve the problems mentioned above,four novel methods for porous ceramics have been proposed in this study,which are listed as follows:Silica hollow microspheres were directly sintered in the inert atmosphere,and then porous silica ceramics with uniform pores,relatively high strength,and excellent dielectric properties have been fabricated via self-foaming process at elevated temperature.To enhance the mechanical strength and improve the sintering process,Al₂O₃-SiO₂ composite microspheres have been developed via regulating the compositions of microspheres.Alumina strengthened porous silica ceramics were prepared through directly sintering microspheres in the air.The mechanism of self-foaming process has been studied.The results show that the mechanical and dielectric properties of porous ceramics could be effectively controlled via changing the sintering temperature.Porous Si₃N₄ ceramics have been fabricated by choosing Si₃N₄ hollow microspheres as pore-forming agent,combining with gelcasting method and pressureless sintering process.The remarkable improvement in the properties of porous Si₃N₄ ceramics could be obtained via adjusting the solid loadings of Si₃N₄ slurry,Si₃N₄hollow microspheres content and sintering parameters.Porous SiAlON ceramics and porous Si₂N₂O ceramics with excellent performance have been fabricated by choosing ceramic hollow microspheres as pore-forming agent and reactant.Ceramic hollow microspheres could react with the ceramic matrix to form SiAlON phases or Si₂N₂O phase,meanwhile porous structure could be obtained during this process.Porous Si₃N₄-based ceramics could effectively overcome the relatively poor corrosion resistance,relatively high dielectric constant and dielectric loss of porous Si₃N₄ ceramics.For optimizing dielectric performance of obtained porous Si₃N₄ ceramics,ultralight Si₃N₄ ceramic foams have been fabricated via particle-stabilized foams method,which uses long-chain surfactant as the hydrophobic modification agent of Si₃N₄ particles for the first time.The prepared Si₃N₄ ceramic foams have high porosity of 87.2%⁹5.3%,compressive strength of 0.25⁷.97MPa,and their dielectric constant and loss tangent can be lowered to 1.35,1.70×10^-3 respectively.Their pores distribute uniformly with single-layer wall consisted of elongatedβ-Si₃N₄ grains interlocked with each other.The novel methods for porous ceramics developed in this study are all simple and convenient.Furthermore,the obtained porous ceramics have excellent performance,which could have promising applications in radome materials.