Fabrication Of Porous Silicon Carbide Based Materials And Study On Their Electromagnetic Wave Absorption Properties

The development and application of radar systems and precision-guided weapons threaten the large spacecrafts and military equipments.The electromagnetic wave absorption materials can effectively reduce the signal of target,and is crucial for improving the stealth and defense capabilities of the target.Traditional magnetic materials have certain drawbacks,such as high weight density,poor thermal stability,poor processing capability,and narrow absorbing bandwidth,which limit their practical applications.In this paper,the controllable preparation of SiC-based porous materials with different morphology and porosity were studied.The formation processes of porous SiC-based materials were studied.The influence of morphology,composition,porous structure and temperature on the electromagnetic parameters and electromagnetic properties were systematically discussed.Biomass-derived SiC-based aerogels were synthesized through freeze-drying,direct carbonization and carbothermal reduction using eggplants as carbon precursors.The obtained biomass-derived SiC-based aerogels possess highly porous,interconnected,and three-dimensional structures inherited from eggplant.Their density was in the range of 0.050-0.076 g/cm3.The biomass-derived SiC-based aerogel prepared at 1500 ? consisted of an interconnected porous structure and onedimensional SiC nanowires in situ grown from the walls.The formation process of SiC walls and SiC nanowires were analyzed in the perspective of thermodynamics and kinetics.Low density biomass-derived SiC-based aerogel showed good compressive properties.Its maximun compressive stress was 2.12 MPa,which is nearly 50-fold higher than that of porous carbon material derived from eggplant.The effects of preparation condition,porous structure and morphology on the electromagnetic wave absorption properties were systematically studied.For biomass-derived SiC-based aerogel prepared at 1500 ?,the minimum reflection loss reached up to-43 dB at 13.0 GHz and their effective absorption bandwidth was 4.0 GHz at 2.0 mm thickness.In order to obtain a porous elastic SiC-based material by carbothermal reduction method,a carbon fiber felt with large volume of interconnected pores and excellent elasticity was used as the carbon template.Our method can be used to fabricate elastic SiC-based felt with adjustable size and the method is suitable for large-scale up.The elastic SiC-based felt retained the fibrous morphology of the carbon fiber felt,and the sample density was in the range of 0.079-0.17 g/cm3.When the reaction temperature was 1500 ?,The as-obtained sample showed hierarchical structure consisted of SiC fibers and SiC nanowires.Notably,the SiC fibers and SiC nanowires were tangled with each other.The formation process of SiC fibers and SiC nanowires were analyzed.Due to the unique hierarchical structure,the elastic SiC-based felt showed good elasticity,which could recover without fracture when released at the compressive strain of 20%.The influence of reaction condition,microstructure and morphology on the electromagnetic wave absorption and compression properties of elastic SiC-based felt were studied.The influence of high-temperature on the electromagnetic parameters and electromagnetic wave absorption properties of elastic SiC-based felt was also disccussed.When the thickness was 2.8 mm,the minimum reflection loss of the elastic SiC-based felt at room temperature was-60 dB.As the temperature was increased to 600 ?,the elastic SiC-based felt also showed excellent electromagnetic wave absorption properties.Its minimum reflection loss was-14 dB,and the effective absorption bandwidth was 3.9 GHz.To overcome the oxidation of traditional dielectric-magnetic composite materials,Co-doped SiC nanowires with with different Co contents were fabricated by a facile carbothermal reduction approach.The Co dopants in the lattice of SiC could maintain stable even at high temperature.The effects of defect structure,electronic structure and magnetic properties on the electromagnetic parameters and electromagnetic wave absorption properties of Co-doped SiC nanowires were studied.The optimal Co-doped SiC nanowires were selected as a building block to fabricate porous SiC-based nanoceramic by pore-forming method.The porous SiC-based nanoceramics with porosity in the range of 44.2%-65.6% was obtained by changing the amout of pore-forming reagent.The porous SiC-based nanoceramics showed excellent compressive properties with maximum compressive stresses of 9.2-15.4 MPa.The effects of composition and porosity on the electromagnetic wave absorption properties were analyzed.The effective dielectric constants of the sample could be tuned to optimize the impedance matching conditions by adjusting the porosity.The magnetic Co dopants could improve the dielectric loss and magnetic loss of Co-doped SiC nanowires,which enhance the electromagnetic wave absorption properties of the porous SiC-based nanoceramic.When the porosity was 53.6%,the SiC-based porous nanoceramic showed excellent electromagnetic wave absorption performance: the minimum reflection loss at 11.9 GHz was-45 dB at 1.27 mm thickness,and the effective absorption band width was 2.1 GHz.