| With the rapid advancement of aerospace technology,thermal insulation materials for aircraft must possess low thermal conductivity,low density,high-temperature stability,as well as excellent electromagnetic wave absorption performance.Carbon fiber needle felts(CF)exhibit low thermal conductivity,high porosity,excellent high-temperature stability,mechanical properties and good electrical conductivity.Therefore,it can be used as a structure-electromagnetic wave(EMW)absorbing material.However,studies have shown that single carbon fiber as a reinforcement is difficult to bear the load with insufficient density,and owing to its high dielectric loss,it is not ideal either as a structural material or as an EMW absorbing material.By combining CF with precursor ceramics,ceramic matrix composite material was obtained,which could improve its load-electromagnetic wave absorbing performance effectively by improving its density,impedance matching and optimizing its dielectric constant.At present,the research on the preparation of ceramic matrix composites by precursor transformation method mainly focuses on its structural properties,while the research on its load-electromagnetic wave absorbing properties is rarely reported.In this paper,carbon fiber reinforced SiOC ceramic matrix composites were prepared by the precursor transformation method.By adjusting the microstructure of SiOC ceramics and in-situ growing SiC nanowire(SiCnw)to construct multi-scale reinforced SiOC ceramic matrix composites,which makes them possess load-electromagnetic wave absorbing properties,and the EMW absorbing mechanism is also studied.First,SiOC ceramics were prepared with methyl trimethoxy-silane and dimethyl dimethoxy-silane as precursors,and the microstructure evolution at different pyrolysis temperatures was also studied.The SiOC ceramic precursors transformed into ceramics at840℃,and their structure was amorphous when the pyrolysis temperature was from 840℃ to1300℃.When the pyrolysis temperature exceeded 1300℃,SiC nanocrystals and turbostratic carbon began to precipitate in the SiOC ceramic.In addition,SiOC ceramics obtained from the pyrolysis temperature of 1400℃ showed weak electromagnetic wave absorption performance,with a minimum reflection loss of-16.4 d B,and an effective absorption bandwidth of 1.52 GHz for a sample thickness of 2.5 mm.Si Fe OC ceramics were obtained by adding ferric nitrate and iron acetylacetone into SiOC ceramics.The results showed that the introduction of Fe decreased the crystallization temperature of SiC and increased the degree of graphitization of free carbon.The ceramic yield,high-temperature stability and dielectric properties of SiOC ceramics were also significantly improved.When iron nitrate was used as the Fe source,the addition amount was 3 wt.%,SiOC ceramics had the best electromagnetic wave absorption performance.When the sample thickness was 1.7 mm,the minimum reflection loss reached-38.1 d B,and the effective absorption bandwidth was 1.31 GHz.When iron acetylacetone was used as the Fe source,the addition amount of iron acetylacetone was only 1 wt.%,and the SiOC ceramics had the best electromagnetic wave absorption performance.The minimum reflection loss of-58.4d B occurred when the sample thickness was 1.7mm,and the effective absorption bandwidth reached 4.10 GHz.Second,in order to improve the interface binding strength between SiOC ceramic and carbon fiber needled felt,SiCnws were grown in situ inside carbon fiber needled felt by the chemical vapor infiltration method to obtain the SiCnws/CF composite reinforcement.The crystal phase composition,microstructure,electromagnetic wave absorption performance of SiCnws/CF composite reinforcement as well as growth mechanism of SiCnw were investigated.The results showed that the pristine three-dimensional CF had poor impedance matching characteristics,resulting in a minimum reflection loss of-6.7 d B.The introduction of SiCnws improved the impedance matching characteristics of carbon fiber,and enhanced the multireflection or scattering loss,interfacial polarization loss,conductance loss,and defect polarization loss owing to the porous conductive network formed by SiC nanowires with a large number of stacking displacements,which effectively improved the electromagnetic wave absorption performance of the SiCnws/CF composites.The minimum reflection loss of SiCnws/CF composite obtained at 1500℃ was-35.2 d B,and the effective absorption bandwidth was up to 5.50 GHz.Further,amorphous SiOC ceramics were introduced into CF and SiCnws/CF reinforcement by the precursor impregnation and pyrolysis(PIP)process.The load-electromagnetic wave absorbing properties of the CF/SiOC and SiCnws/CF/SiOC composites were investigated.The results showed that the compressive strengths of SiCnws/CF/SiOC composites in the x/y and z directions were 9.61±2.22 and 22.01±3.29 MPa,respectively,after two PIP processes.The compressive strengths of CF/SiOC composites in the x/y and z directions were 8.60±2.25 and 17.61±2.60 MPa,respectively.However,although the mechanical properties of SiCnws/CF/SiOC composites are improved,the minimum reflection loss of SiCnws/CF/SiOC composites decreases from-48.2 d B to-20 d B compared with CF/SiOC composites.Finally,SiCnws/CF/SiOC composites were prepared by one step PIP process.The effects of catalyst type and pyrolysis temperature on the morphology of SiC nanowires grown on the surface of carbon fiber were studied.SiC nanowires were grown uniformly along the radial direction of CF with diameters of 250-400 nm and lengths of 30-50μm at the pyrolysis temperature of 1500℃ using nickel nitrate as the catalyst.After two PIP processes,the density of the composite was 0.80 g/cm~3,and the compressive strengths of SiCnws/CF/SiOC composites were 8.69±0.79 and 16.41±3.54 MPa in the x/y and z directions,respectively.Bending strength in x/y direction was increased by 231%from 3.95±1.21 MPa to 12.53±2.46MPa,and in z direction by 128%from 3.29±0.91 MPa to 7.49±2.51 MPa.As the PIP time increased from one to two times,the thermal conductivity of the CF/SiOC composite increased from 0.492 W/(m·K)to 0.831 W/(m·K).After the introduction of SiC nanowires,the thermal conductivity of the SiCnws/CF/SiOC composite with the same density and preparation process,increased from 0.763 to 1.134 W/(m·K).SiCnws/CF/SiOC composites prepared by the one-step method were fabricated with amorphous SiOC ceramics as impedance regulating phase,and the SiC nanowires are not only used as the nano-mechanical reinforcing phase,but also as the excellent electromagnetic wave absorbing medium.Therefore,the composite had a minimum reflection loss of-46 d B,a corresponding sample thickness of 1.7 mm,and an effective absorption bandwidth of 4.30 GHz. |
PDF Full Text Request