Fabrication And Properties Of Porous ?-SiAlON Dielectric Ceramis

The radome is a critical component of the high-speed aircraft,that is,one of the structural component of the aircraft which is also an important part of the radar guidance system.With the increase of flight speed,the working environment of the radome is more severe,which requires the radome material with high temperature and ablation resistance,excellent high temperature mechanical properties and dielectric properties.As one member of the Si-N based ceramics,?-Sialon has high chemical stability and oxidation resistance and low thermal conductivity.In recent years,the study of the elongated morphology has greatly improved the strength of?-Sialon ceramics.The pores that introduced into ceramic body can reduce the density of ceramics and further lower the dielectric constant.Therefore,?-Sialon is a promising radome material.In this paper,porous?-Sialon ceramics were prepared by pressureless sintering.The manufacturing factors including raw material composition,sintering processing,molding method and green body porosity that effects on the properties of porous ceramics were studied systematically.In the design of the raw material composition of?-SiAlON ceramics chapter,our results show that?-SiAlON using Y and Mg as stabilizers formed a large portion of second phases after pressureless sintering;Y-composition sample takes severe sintering shrinkage;the Mg-and Li-compositions formed an equiaxed crystal morphology after sintering.These three stabilizer elements system cannot meet the requirements for preparing a high-strength,high-porosity and single-phase porous ceramics.The Ca-composition is able to form both elongated grains and a mediocre porosity,which is suitable for our porous?-SiAlON research.It is found that the high liquid phase is an important factor for the formation of elongated grains.The use of pure?-Si₃N₄ as raw material can effectively improve the porosity of the sample and make the elongated morphology more uniform.The investigation on the preparation of?-SiAlON porous ceramics by dry pressing and pressureless sintering shows that the porosity of the sample is affected by the pressing pressure and sintering temperature,while the mechanical properties,thermal conductivity and dielectric constant of the material are affected by the porosity.As the porosity increases,the values of properties above decreases.The sintering temperature affects the porosity,which further changes the grain morphology and dielectric loss of ceramics.Whether?-SiAlON can grow along c axis depends on the kinetic conditions during grain growth.Increasing the temperature can significantly promote elongated morphology.Sample pressed under 2MPa and sintered at 1810°C produced a uniform elongated morphology with high bending strength?185 MPa?,40%porosity,dielectric constant of 5.5,dielectric loss of 3.96×10-3 and a low thermal conductivity of 1.601 W/m·K,these comprehensive properties is able to initially meet the application requirements of high-speed radome materials.The?-SiAlON porous ceramics with high porosity is prepared by freeze-drying process and foaming process:the porosity of ceramics prepared by freeze-drying method mainly depends on the solid content of the slurry,which determines the mechanical properties,thermal conductivity and dielectric constant,as the porosity increases,the values of properties above decreases.The 80.8%porosity sample has a low dielectric constant of 2.56 and a low thermal conductivity of 0.324 W/m·K.The sintering temperature affects the porosity,grain morphology and dielectric loss of the ceramic.The sample sintered at 1810°C produced a uniform fine elongated morphology with a low dielectric loss of 2.9×10^-3.There are three possible reasons for anisotropic grain growth:Oswald ripening,peritectic transformation,and reduced number of crystal nuclei.In the foaming method,the porosity of the sintered body gradually increases as the solid content of the slurry decreases,but when the solid content is less than 17%,the green body collapses during the sintering process,and the porosity decreases.The porosity of the green body has a significant impact on the grain morphology.When the porosity of the green body increases,the grain morphology gradually changes into anisotropic.