Preparation And Properties Of Porous Ceramics Radome Materials Using Hollow Microspheres

There are some key factors that restrict the application fields of wave-transparent materials,such as mechanical strength,dielectric properties,bulk density,corrosion resistance,thermal shock resistance,particle erosion resistance.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,the selection of process is very important for fabrication of porous ceramics which have both relatively high mechanical strength and low dielectric constant and loss tangent.In this study,two novel methods for fabrication of porous ceramics have been proposed,which are listed as follows:Porous Si2N2O ceramics with excellent performance have been fabricated by choosing ceramic hollow microspheres as pore-forming agent and reactant.Ceramic hollow microsphere could reacted with the ceramic matrix to form phases Si2N2O phase,while porous structure could be obtained during this process.Porous Si3N4-based ceramics could effectively overcome the relatively poor corrosion resistance and relatively high dielectric constant and dielectric loss of porous Si₃N₄ ceramics.Silica hollow microspheres are directly sintered in the inert atmosphere,and then porous silica ceramics with uniform pores,relatively high strength,and good dielectric properties have been fabricated via self-foaming process at elevated temperature.To enhance the mechanical strength and improve the fabrication conditions,Al₂O₃-SiO₂composite microspheres have been developed via regulating the compositions of spheres,and porous mullite ceramics were prepared through directly sintering them in the air.The mechanism of self-foaming process has been studied.The results show that the mechanical and dielectric properties of porous mullite ceramics could be well controlled via changing the compositions of spheres,holding time and sintering temperature.The porous ceramics processing methods developed in this study are all simple and convenient.Furthermore,the porous structure could be effectively regulated,and the porous ceramics have excellent performance,which could have promising applications in wave-transparent materials.