Dielectric And Mechanical Properties Of SiO₂ Ceramic Matrix Composites

SiO₂ ceramic matrix composites have excellent dielectric properties,thermal shock resistance and high temperature ablation resistance,which are widely used as wave-transparent materials in radome or antenna window of aircraft.However,with the increase of aircraft Mach number,higher mechanical properties of radome materials are required,and the improvement of mechanical properties of SiO₂ ceramic matrix composites has become an urgent need in this field.Therefore,it is of great significance to develop new fiber-reinforced SiO₂ ceramic composites.In this thesis,the micro-composition,microstructure,thermal,mechanical and dielectric properties of ablative Al₂O₃ fiber reinforced SiO₂ ceramic composites（Al₂O₃/SiO₂）and SiO₂ fiber reinforced SiO₂ ceramic composites were studied.A design scheme of Al₂O₃/SiO₂ wave-transparent composites with enhanced mechanical properties was proposed.In order to reveal the effect of Al₂O₃ fiber and SiO₂ fiber on the thermal and mechanical properties of SiO₂ ceramic composites,the comparative study of two kinds of fiber reinforced SiO₂ ceramic composites was carried out.In the range of room temperature to 800℃,the thermal expansion coefficient of SiO₂ fiber reinforced SiO₂ ceramic composite is less than 1×10^-6/K,and the thermal expansion coefficient of Al₂O₃ fiber reinforced SiO₂ ceramic composite is still less than 3×10^-6/K,which meets the requirements of practical application.Due to the high modulus of Al₂O₃ fibers,the mechanical properties of Al₂O₃ fiber reinforced SiO₂ ceramic composites are significantly higher than those of SiO₂ fiber reinforced SiO₂ ceramic composites,and the compressive and flexural properties are as high as 63.2 MPa and 95.6 MPa,respectively.Based on the results of physical characterization and CT analysis of Al₂O₃/SiO₂ composites,the simulation analysis of the effect of porosity on mechanical properties of Al₂O₃ fiber composites was carried out,and the critical porosity and pore size for mechanical failure of specimens with tensile strength ranging from 20 MPa to 35 MPa were obtained.The mechanical failure theory model was established,and the exponent factor n related to the pore size was proposed.The larger the pore size is,the smaller the n value is,and the faster the tensile strength decreases with the increase of porosity.Through theoretical calculation and simulation analysis,the high temperature stability of Al₂O₃/SiO₂ composites is confirmed,and the cracks will not occur due to the difference of thermal expansion between the two components.The modulus of Al₂O₃ fiber and SiO₂ matrix were used to carry out the simulation calculation,and the influence of Al₂O₃ fiber content on the mechanical properties was obtained.The dielectric properties of SiO₂ ceramic matrix composites were studied.A new method for accurately measuring the dielectric properties of small size wave-transparent samples after ablation is proposed by exploring and analyzing the disturbance effects of the coupling hole of the cylindrical resonator and the dielectric constant and size of the sample on the resonant mode.Based on this method,the dielectric constant and loss tangent of SiO₂ ceramic composites at different ablation temperatures were measured and analyzed,and the dielectric properties of SiO₂ ceramic composites after ablation in different temperature regions were obtained.The relationship between the volume fraction of different Al₂O₃ fibers and the dielectric properties was studied,and the range of fiber volume fraction of Al₂O₃/SiO₂ composites as wave-transparent materials was proposed.The research results provide a theoretical basis for the design of load-bearing and wave-transmitting integrated materials.