Multi-microstructure Design,Fabrication,and Properties Of Silicon Nitride Materials Integrating Thermal Insulation And Wave Transmission

The radome of high-speed aircrafts often faces extreme environments such as aerodynamic loading,high temperature,sandstorm,and rain,so it must integrate the functions of bearing,wave transmission,heat insulation and weather resistance.Porous silicon nitride（Si3N4）ceramics is considered the best choice for wave-transparent radomes,but the applications of it in high-speed aircrafts are limited by its poor thermal insulation performance.Increasing the porosity to improve the thermal insulation performance of porous Si3N4 ceramic will inevitably weaken its mechanical properties.In addition,porous surface is also difficult to cope with the complex aerospace environments.Structure determines performance.It is difficult to achieve the multi-functional integration of Si3N4 ceramics depending on a single microstructure,but it is easier to endow Si3N4 ceramics with various microstructures to meet the requirements of different functions of radome.In order to improve the thermal insulation performance and environmental adaptability of Si3N4 radome,the microstructure of Si3N4 ceramic was designed from three aspects in this paper.In the first two chapters,the researches on the thermal insulation materials of Si3N4@SiOx nanofibrous aerogel and Si3N4@SiO2 foamed ceramic were carried out.In the last chapter,Si3N4/barium feldspar composite sealing coating was prepared and studied.The synergy of the three materials can solve the problem that the Si3N4 ceramic radome is limited in the application of high-speed aircrafts.In order to solve the problems of poor durability and insufficient high temperature oxidation resistance of Si3N4 nanofibrous aerogel,low temperature chemical vapor deposition（LTCVD）process was used to in situ build SiOx coating and SiOx nanowires in the Si3N4 nanofibrous aerogel prepared by directional freeze molding method,and the enhanced Si3N4@SiOx nanofibrous aerogel was obtained.The LTCVD process significantly improves the microstructure uniformity of the aerogel and produces three beneficial effects at the same time:（1）the intersections of Si3N4 nanofibers are anchored by SiOx coating,which enhances the skeleton of the aerogel and improves the oxidation resistance of Si3N4 nanofibers in high temperature environment;（2）SiOx nanowires entangle and repel each other,which improve the anti-deformation ability of nanofibrous aerogel;（3）the SiOx nanostructures enhance the hydrophobicity of nanofibrous aerogel.The results show that Si3N4@SiOx nanofibrous aerogel has excellent compression-resilience property（which can be repeatedly compressed in-196～1200 ^oC）,dielectric properties（in the frequency range of 8～18 GHz,the dielectric constant is around 1.01,and the loss tangent is around 0.005）and heat insulation performance(thermal conductivity 0.0157 W·m^-1·K^-1).As the flexible lining of the Si3N4 radome,Si3N4@SiOx nanofibrous aerogel can significantly improve the heat insulation performance of the radome without affecting its dielectric properties.In view of the disadvantages of complex operation,high cost and long preparation cycle of the currently used wet forming technologies,a new method for preparing high-porosity ceramics with near net size was developed by combining isostatic pressing technology,hard template method and LTCVD process,and was employed to prepare the Si3N4@SiO2 foamed ceramics.The microstructure and composition evolution mechanism of Si3N4@SiO2 foamed ceramics during sintering were studied by chemical reaction thermodynamics method.The SiO2 layers on the outside of the adjacent pore walls were fused together,which built a strong skeleton for the Si3N4@SiO2 foamed ceramic and improved its high temperature oxidation resistance.The results show that the foamed ceramic has high porosity（90%）,low thermal conductivity(0.08 W·m^-1·K^-1)and excellent dielectric properties（in the frequency range of 8～18 GHz,the dielectric constant is around 1.32,and the loss tangent is less than 0.009）,and its compressive strength（7.5 MPa）is higher than most of its counterparts.The Si3N4@SiO2 foamed ceramic acts as a thermal insulation interlayer of Si3N4 radome,and cooperates with the flexible Si3N4@SiOx nanofibrous aerogel to further improve the thermal insulation capability of the radome.It is aimed at the problem of poor environmental adaptability caused by high porosity of materials such as Si3N4@SiO2 foamed ceramic and porous Si3N4 ceramic,the Si3N4/barium feldspar composite coating was successfully prepared for the first time.Two aspects of research were carried out:（1）aiming at the problem of difficult synthesis of monoclinic barium feldspar at present,a new method for efficient synthesis of it was developed,and further a new idea for regulating the thermal expansion coefficient of Si3N4/barium feldspar composite material was proposed;（2）the material composition and spraying structure of the sealing coating were reasonably designed,the effects of sintering temperature and time on the microstructure of the coating were studied,and the microstructure evolution mechanism of coating during sintering was revealed.The results show that the coating possesses excellent pressuretightness at1100 ^oC,high water resistance（97%）,high tensile strength（15±1 MPa）and great thermal shock resistance.After 10 cycles thermal shock tests at△T=1170 ^oC,the waterproof rate and tensile strength of coating remain 85%and 67%,respectively.The cracks generated during the thermal shock test are healed by SiO2,thus prolonging the service life of the coating.The coating can significantly improve the environmental adaptability of porous substrate with little impact on its dielectric properties.