Research On Preparation Process And Microwave Absorbing Properties Of Polyimide Composite

Integration of structure and function has been a main direction for the development of material science and technology,especially for the absorbing material.It is an inevitable trend of novel aerospace materials to realize the aim of the absorbing material characters including thin thickness,broadband,low density and strong absorbing property at the same time have good heat resistance.In this paper,high performance polyimide resin was used as matrix,SiC fiber(SiCf)was used as reinforcement,carbon black(CB),carbon nanotubes(CNT),graphene plates(GNP)and Ti3SiC2 were used as absorbent,then the absorbent/polyimide composites and SiCf reinforced polyimide composites were prepared by hot pressing method.The microstructure,electromagnetic and microwave absorption properties in the X band(8.2~12.4GHz)and mechanical properties of composites were characterized and studied.The mian research contents and results are summarized as follows:By analyzed the curing kinetic of the polyimide oligomer,the key temperature and curing kinetic model of polyimide were obtained.The curing activation energy of polyimide was 99.8KJ/mol and the reaction order was one order.Through the thermal-infrared technique,the decomposition step of polyimide at high temperature was studied.The polyimide absorbing composites were prepared by using CB,CNT,CB/CNT and Ti3SiC2 as absorbent and studied the different kinds of dielectric absorbent on the electromagnetic and microwave absorbing properties of absorbent/polyimide composites.For the CB/polyimide composites,the complex permittivity increases with increasing CB content,while the mechanical properties were first increase then decrease with increasing CB content.When the CB content was 6wt%,the composite has good absorbing property;CNT/Polyimide composites and CB/CNT/ polyimide composites,the complex permittivity increases with increasing absorbent content and temperature.When the content of CNT or CB/CNT was 5wt%,the composites have optimum absorbing properties.Using CB replaced CNT could enhance the complex permittivity of CB/CNT/polyimide composite and also have the same mechanical property as CNT/polyimide composite.The influences of the surface condition of SLFSiCf,the pressure,the temperature and the post-curing condition on the mechanical properties of SLFSiCf composites were studied.The results indicated that when the SLFSiCf was disizing,the pressure was 10 MPa,the temperature for applied pressure was 240?,and the post curing condition was 380?,8h,the SLFSiCf /polyimide composite has optimum mechanical properties.Two kinds of SiC fiber(KD-1SiCf and SLFSiCf)were characterized and analyzed,the results manifested that the morphology of KD-1SiCf and SLFSiCf was similar and both surface were smooth.KD-1SiCf and SLFSiCf were carbon excess,but the way of carbon excess was different.The excess carbon existed on the surface of KD-1SiCf,while uniformaly distributed in the SLFSiCf.The graphitization degree of KD-1SiCf was higher than that of SLFSiCf,therefore the electrical conductivity of KD-1SiCf was much higher than that of SLFSiCf The effect of two kinds of SiC fiber on the mechanical,electromagnetic and microwave absorbing properties of SiCf/polyimide were illustrated.The mechanical properties of SLFSiCf and KD-1SiCf composites were higher than 600 MPa.Though the complex permittivity of two kinds of SiCf composites was different,the absorbing properties of them were not good due to the fact that the impedance matching degree and attenuation characteristics cannot meet the requirement at the same time.The effect of heat treatment on KD-1SiCf and the hybrid woven of KD-1SiCf and SLFSiCf on the complex permittivity of the polyimide composites were characterized.The complex permittivity of KD-1SiCf composites decreases with increasing the time of KD-1SiCf heat treatment.When using hybrid woven SiC fiber to fabricate composites,the complex permittivity of composites increases with increasing the content of KD-1SiCf.By those methods,the complex permittivity of SiCf reinforced polyimide composites can be adjusted,but the absorbing properties were not good at the measured frequency range.CB,CNT and GNP were chose as absorbent to adjust the dielectric and microwave absorbing properties of SLFSiCf composites,the complex permittivity of SLFSiCf composites increases with increasing filler content and the mechanical properties of SLFSiCf composites decrease with increasing filler content.For the SLFSi Cf composites,introducing carbon based absorbent would improve the absorbing ability,but the bandwidth was narrow.Use matching layer and different lossy layer to simulate the absorbing properties of double layer absorbing composites.The results indicated that the absorbing properties of double layer composites were better than that of single layer absorbing composites at the same thickness.The influences of thermal oxidation and hygrothermal aging on the performances of SLFSiCf composites were investigated.It was found that with increasing thermal oxidation time,the weight,mechanical properties and decomposition temperature of polyimide composites decreased.However,the complex permittivity changed small and exhibited high heat stability.For the hygrothermal aging,the weight of composites first increased and then kept stability,the mechanical properties decreased with increasing the hygrothermal aging time.The dielectric constant increased with increasing time,when the time reached 200 h,the complex permittivity of SLFSiCf/polyimide composites kept almost constant.