Research On Preparation And Dielectric Properties Of Carbon Fiber Reinforced Structural Composites

Carbon fiber has excellent properties such as high strength,high modulus,low density and good electrical conductivity,so that its structural composites can be widely used in the aerospace and defense and military fields.It has the role of load-bearing and lightweight,but the dielectric constant of carbon fiber is high,and the impedance mismatch produces strong reflection of electromagnetic waves,so it needs to be modified and its dielectric properties researched.In this paper,SiO₂ and SiC modified coatings were prepared by sol-gel method and chemical vapor deposition method respectively,and C/C composites were prepared by resin impregnation curing carbonization method.Due to the high conductivity of the carbon matrix,high reflection of electromagnetic waves was caused.Therefore,carbon fiber reinforced resin-based composites were prepared by semi-curing process,and at the same time,using this as a guide to prepare high-temperature resistant structural composites,C_f/Si₃N₄ composites were prepared by hot pressing and sintering.XRD,SEM,EPMA and three-dimensional CT were used to characterize the phase and micro morphology of the materials.The complex permittivity of the samples in the X band was tested by a vector network analyzer,and the reflection characteristics of the composites to microwaves were analyzed.The SiO₂ coating prepared by the sol-gel method is complete but thin,and the bonding ability is not strong.Low dielectric SiO₂ coating can effectively reduce the dielectric constant of carbon fiber.The impedance of the modified C/C composite material does not match,because Carbon felt is a highly conductive carbon materials and the SiO₂ doped carbon matrix are still unable to block the conductive network to form a conductive current showing strong reflection.The SiC modified coating prepared by the CVD method is easy to control and has a uniform thickness of about 1.5?m.Carbon fibers in an array will form a conductive network.Carbon fibers axis parallel to the electric field will generate directional current,and the vertical electric field will easily transmit microwaves.Single-layer structure is difficult to meet the impedance matching,and the fiber spacing of 1.7mm is slightly better than the 3.6mm for carbon fibers reinforce resin matrix composites,and the loss peak moves to the high frequency with the increase of the spacing.The results of the dielectric constant of the double-layer structure composites with different fiber intervals are similar,and the complex permittivity is significantly lower than that of the single-layer.During the process of preparing C_f/Si₃N₄ composites at high temperature by hot-pressing sintering method,the matrix will partially decompose and diffuse inward to react with the bonding layer Si to make the structure loose.In addition,a part of Si melts and penetrates into the carbon fibers to enhance the bonding ability.Similar to the resin matrix,when carbon fibers axis is parallel to the electric field,the dielectric constant of the composites is higher than when it is perpendicular.The complex permittivity of the double-layer structure is significantly lower than that of the single layer.The smaller the fiber spacing,the smaller the imaginary part,the smaller the loss,but the reflectivity is still higher.First,Si bonding layer allows current conduction to occur when the carbon fiber is axially perpendicular to the electric field;second,the reflection characteristics of the double-layer carbon fiber cross-laminated layer are complementary,and the reflected electric field component synthesizes an elliptical polarization field,so the anisotropy is not obvious,and the reflection mainly depends on the parallel electric field.