Synthesis And Microwave Absorption Properties Of Multi-scale C/SiC Composites

The microwave absorbing materials(MAMs)can effectively convert the energy of electromagnetic waves into other forms of energy,such as thermal energy or mechanical energy.Nowadays,the traditional MAMs cannot meet the requirements of various applications.The C/SiC composite is a kind of potential lightweight MAMs with the advantages of suitable impedance matching performance and chemical stability.However,the formation mechanism of multi?scale C/SiC composite and the influence of their microstructure on dielectric properties,microwave absorbing properties,and oxidation resistance remain to be systematically explored.Herein,the carbon template method was used to prepare multi-scale C/SiC composite,including spherical C@SiC core-shell material,sphere-like C/SiC hollow material,one-dimensional coaxial SiC/CF material,and hollow SiC microtube.The formation mechanism of multi-scale C/SiC composite was summarized.The effect of raw materials and other parameters that influenced the phase composition,microstructure,and morphology were systematically studied.The resistance,dielectric,impedance,and attenuation properties of the composites were deeply investigated.Furthermore,the high-temperature resistance and hydrophilic properties of the composite were also evaluated.First,the spherical carbon template with different sizes was prepared by the hydrothermal method.Then the spherical C@SiC core-shell material was synthesized by a high-temperature solid-phase reaction using Si particles as raw materials.The influence of the size difference between the carbon template and generated SiC particles on the microstructure of the C/SiC composite was summarized.The results also revealed that the size of the core-shell material influenced the microwave absorbing performance of C@SiC composites.The smaller C@SiC composites,which had a larger specific surface area,enhanced the scattering and reflection of electromagnetic waves and improved the material's impedance matching and attenuation performance.Also,the smaller size of particles promoted the formation of the conductive network in the paraffin composite,which enhanced the conductivity loss of the material.The effective absorption bandwidth(EAB)at a thickness of 2.5 mm was 3.5 GHz(8.4-11.9 GHz),and the minimum reflection loss(RL)was-32.4 dB@10.7 GHz.To further enhance the reflection and scattering of microwaves,the aspergillus oryzae spore with a hollow structure was used as a carbon template.After carbonization,the carbon template reacted with Si powder to prepare a sphere-like C/SiC hollow material.The results revealed that the carbon template's surface morphology influenced the form of SiC in the C/SiC composite.The uneven surface of the carbon template caused the formation of isolated particles or short whiskers due to the confinement effect.The sphere-like C/SiC hollow material exhibited better microwave absorption properties in that EAB was 3.7 GHz(14.3-18 GHz)and the minimum RL was-33.8 dB@15.5 GHz at a thickness of 1.5 mm because of its hollow structure.Then the one-dimensional coaxial SiC/CF material was prepared to construct a three-dimensional conductive network that could enhance the conduction loss of the C/SiC composite.Based on the variation of the thickness of the SiC shell,the dielectric parameters,resistance,and microwave absorbing properties of the composite materials were investigated.As the SiC shell thickness decreased,the complex dielectric parameters and electrical conductivity gradually increased,and the relaxation time of interfacial polarization of SiC/CF was prolonged.The SiC/CF coaxial structure material showed excellent microwave absorption properties that the minimum RL was-31.6 dB@16.6 GHz and the EAB was 4.5 GHz(13-17.5 GHz)at the thickness of 1.1 mm.The formation of the conductive network effectively improved its conductance loss.The SiC shell enhanced the interfacial polarization loss and impedance matching characteristics of coaxial material.Besides,the SiC shell improved the oxidation resistance and wettability of the composite.To be applied in a higher-temperature environment,hollow SiC microtubes were obtained by oxidization of coaxial SiC/CF materials.The hollow SiC microtubes were composed of SiC,C,and amorphous SiO2 layers.The carbon particles uniformly embedded in the SiC particles,and the amorphous SiO2 covered the outside of the SiC microtube.The dielectric properties of the hollow SiC microtube could be optimized by controlling its wall thickness.The hollow SiC microtube showed a broader electromagnetic wave absorption bandwidth compared to that of other SiC materials.The minimum RL of hollow SiC microtube was-25.7 dB@14.9 GHz,and the EAB was 5 GHz(12.9-17.9 GHz)at a sample thickness of 2 mm.The multi-attenuated mechanism of hollow SiC microtube caused by embedded C,one-dimensional tubular structure,and SiO2 layer improved its microwave absorbing performance.