Research On Electromagnetic And Thermal Protection Mechanism Of Porous Silicon Carbide And Its Composites

Since space vehicles work in the complex space environment of ultra-high temperatures,frequent alternations of hot and cold,and severe electromagnetic radiation,thermal and electromagnetic protection systems are essential for the stable and long-term service of spacecraft,and space stations.It is significance for space vehicles to develop multi-functional integrated materials such as ultra-low density,high-temperature resistance,shock resistance,high-efficiency heat insulation,and electromagnetic shielding.Compared with traditional polymer-based,carbon-based,and metal-based materials,silicon carbide is more suitable for preparing new types of protection materials with load-bearing capacity due to its characteristics of low density,high strength,high-temperature resistance,high chemical stability,and low linear expansion coefficient.However,the low dielectric loss,high thermal conductivity,and brittleness caused by the high-strength covalent bond is the problem to be solved.This paper focuses on temperature resistance,lightweight,specific mechanical properties,heat insulation performance,electromagnetic absorption,and shielding performance as the guidance,a series of silicon carbide and silicon carbide-based porous composite materials were constructed combing the microstructure design and functional modification of silicon carbide materials.Then,the effects of structure and composition on the electromagnetic dissipation,heat conduction,and load transfer of porous silicon carbide are discussed and analyzed.The main research contents of this thesis are as follows.(1)Through the precursor conversion method combined with the template method prepared a sort of in situ silicon nitride nanowires silicon carbide foam.The heterogeneous interface composed of silicon carbide,silicon nitride,silicon oxide,and residual carbon causes more polarization loss to deplete electromagnetic waves,hence the composite foam silicon carbide foam with in-situ nanowires shows excellent wave absorption performance.The maximum reflection loss(RL)of silicon carbide foam with in-situ nanowires reaches-50.3 d B at 16.48 GHz with an effective bandwidth(RL<-10 d B)of 3.6 GHz.Owing to the silicon nitride nanowires delaying the crack growth in the foam under the compression process by bridging and migration,the toughness and compressive strength of composite silicon carbide foam is increased.The specific strength of Si C composite foam is 4.3 MPa·cm~3/g,and the toughness is 356%of that of non-nanowire foam.(2)Through the carbothermic reaction with silicon monoxide using carbon foam as a template prepared a lightweight silicon carbide foam.The reaction mechanism was discussed in detail,it is believed that the abundant pores and large specific surface area of the carbon skeleton are conducive to the silicon monoxide molecules entering the carbon skeleton to react with carbon,which makes the carbon thermal reaction more complete.The silicon carbide foam with 99%porosity possesses an extremely low thermal conductivity(0.024 W/m·K)due to the low heat transfer efficiency of the gas phase heat conduction mode.Owing to the continuous structure and bent high internal aspect ratio stem,the specific strength of silicon carbide foam reached 16.67 MPa/g·cm~3 and has about 4%resilience and certain fatigue resistance.(3)Using silicon carbide nanowires as raw materials,silicon carbide nanowires aerogel was prepared by hydrothermal method,freeze-drying method,and high-temperature oxidation fusion.The aerogel has a dense cellular porous structure,the pore wall is composed of interconnected silicon carbide nanowires,and the surface of the nanowires is covered with a silica coating.The ultra-low density(6 mg/cm~3 to 35 mg/cm~3),extended heat transfer path,and amorphous silica composition endow the silicon carbide nanowire aerogel with ultra-low thermal conductivity(～0.026W/m·K).The aerogel exhibits excellent mechanical strength(specific strength 15.1 MPa/cm~3·g)and 50%recoverable compression deformation due to the synergistic effect of elastic cellular pores and continuous one-dimensional base unit.(4)Using biomass cotton nonwoven fabric and silicon monoxide powder as raw materials,through a template conversion method based on carbon thermal reaction prepared a silicon carbide/carbon composite fabric with in situ growth of silicon carbide nanowires.The fibers inside the composite fabric are complete and continuous,and intertwined silicon carbide nanowires form a secondary network in the fiber network.The silicon carbide/carbon composite fabric has good flexibility and excellent mechanical strength.The composite fabric with a density of 40 mg/cm3exhibits a maximum tensile strength of 0.100 MPa and 5.3%fracture deformation.In addition,the thermal conductivity of the composite fabric increases from 0.048 W/m·K to 0.107 W/m·K with an increase of temperatures in the range of normal temperature to 500℃,which shows heat insulation at high temperatures.Due to the heterogeneous interface-induced polarization relaxation,enhanced electrical conductivity,and the dense network structure formed by nanowires and fibers,composite fabrics show electromagnetic shielding capability dominated by absorption shielding.In the frequency range of 8～12 GHz,the EMI efficiency of fabrics with a thickness of 1 mm is about 40 d B.87 Figures,5 Tables,and 216 References are listed in the dissertation.