Research On Broadband Microwave Absorption Of SiC Fiber Reinforced Resin Matrix Composites

The microwave absorbing SiC fiber reinforced composites have comprehensive advantages such as light weight,high strength,and adjustable microwave absorbing performance,which are advanced structure/function integrated materials and have been successfully applied to advanced stealth devices abroad.The structural design is the key to realize the broadband absorption of composites,but the systematic design and optimization of the absorbing structure of continuous fiber reinforced composites have not been carried out yet.In this work,continuous SiC fiber with adjustable resistivity was used as absorbent and reinforcement,and epoxy resin was used as matrix to prepare SiCf/Resin composites.Transmission line theory was used to explore the interaction between the structure and the absorbing property of SiCf/Resin composites,and the following conclusions are obtained:(1)Based on the transmission line theory and enumeration algorithm,the ideal permittivity of the microwave absorbing SiC fiber reinforced resin composites to achieve broadband microwave absorbing performance in 8.2～18 GHz was derived.The results show that,to achieve the absorption of-10 dB in the whole frequency band,the single-resistivity SiC fiber reinforced composites have strict requirements on the permittivity and frequency dispersion features,which is beyond the adjustable range of SiCf/Resin composites.(2)Based on the calculation results,typical high(H-SiCf,～105 Ω·cm)and low resistance(L-SiCf,～102 Ω·cm)SiC fibers were selected as the surface layer and lossy layer,respectively.The feasibility of matrix doping with carbon black to adjust permittivity was studied.The results show that the ε’ and ε" can be controlled in the range of 4～14 and 0.7～16 respectively by doping 0～10 wt%carbon black in LSiCf/Resin.When the doping amount is 2.5 wt%or 5 wt%,the permittivity of the composite can meet the ideal value of the lossy layer.After the optimization,2.8 mm H-SiCf/Resin and 2 mm L-SiCf/Resin doped with 5 wt%carbon black were selected as surface layer and lossy layer respectively.The simulated reflection loss of the laminated composite is better than-13.3 dB in 8.2～18 GHz,and the tested reflection loss basically corresponds to the simulated results.(3)Based on the calculation results,the influence of reducing the resistivity of SiC fiber on the permittivity of the lossy layer was studied.The results show that the permittivity of the composites increases with the decrease of the resistivity.When the fiber resistivity drops from～102 Ω·cm to～100Ω·cm(EL-SiCf),the average ε’ of the composites increases from 4.0 to 7.4,the average ε" increases from 0.7 to 9.1,and the values and frequency dispersion features are in the ideal permittivity range.Under the combination of H-SiCf/Resin,the simulated reflection loss of 5.1 mm laminated composite is better than-12.8 dB in 8.2～18 GHz,and the tested reflection loss basically corresponds to the simulated results.(4)The influence of surface modification on the absorbing properties of SiC fibers was studied.The results show that the impedance matching ability can be improved by preparing BN coating or SiO2+BN coating on the low resistance fiber(EL-SiCf),and the permittivities of the corresponding composites are still in the ideal range.The introduction of impedance matching coating broadens the range of thickness for SiCf/Resin composites with the same absorption level,which improves the allowable error range of thickness in preparation.This superiority enhance the tested absorption property.For example,the tested reflection loss of the laminated composite with SiO2+BN coating is better than-11.1 dB in 8.2～18 GHz.