Preparation And Microwave Absorption Properties Of The C/SiC/SiO₂Composites

C/C composite materials with low density, high intensity than modulus, high heatconductivity, low thermal expansion coefficient, fracture toughness, good resistanceto abrasion, antiablative etc. So it was widely used in aviation, aerospace, nuclear,chemical industry, medicine, and other fields. But the conductivity of C/C compositeis too better, so it can not be used for microwave absorbing materials. At present, themain methods to improve C/C composite microwave absorbing performance ispreparation surface absorbing coatings. C/C composite materials were composed ofcarbon fiber and matrix carbon composition, one of the preparation methods isimpregnated asphalt or resin into pores of carbon fiber, then treat withhigh-temperature, so that the pitch or resin carbonization filled in the pores, and theC/C composite up to a certain density by repeated impregnation and carbonizationprocess. If we added proper material in the asphalt or resin and make them react to thepitch-based or asphalt-based carbon to generate a substrate material with lowelectrical conductivity and microwave absorbing properties, it can reduce conductivityof C/C composites and improve the microwave absorbing performance.In this paper, absorption properties of five different carbon materials wereinvestigaeted by analyzing and comparing their dielectric constant, theoreticalreflection loss and actual reflection loss. The results show that the absorptionproperties of active carbon-based and pitch-based carbon are both very good as thefrequency bandwith of reflection loss less than-5dB is wide when the coatingthickness is1.5mm. They are suitable for high wave absorbing property of C/Ccomposites matrix carbon.On the results above, C/SiC/SiO₂composites based on active carbon or pitchcarbon were prepared by sol-gel and carbothermal reduction with active carbon andpitch as carbon resource and TEOS as silicon resource. During the preparation process,heat treatment temperature, ratio and species of raw material are the main factorsaffecting the component and the electromagnetic wave absorption properties ofC/SiC/SiO₂composites based on active carbon or pitch carbon. The influences of theheat treatment temperature, the ratio and the species of raw material on the component,structure, electromagnetic properties of C/SiC/SiO₂composites were investigatedprimarily. The results show that he absorbing performance of the activated carbon is greatly increased after it modified. When the activated carbon with low surface areamixed with40%TEOS and heated at1600℃, theoretical reflection loss of theprepared C/SiC/SiO₂composites increased from-6.6dB to-30.8dB and thebandwideth below-5dB increased from3.5GHz to6.1GHz; when the medium warmcoal pitch mixed with30%TEOS and heated at1600℃, theoretical reflection loss ofthe prepared C/SiC/SiO₂composites increased from-15.3dB to-26.4dB and thebandwideth below-5dB increased from4.1GHz to5GHz. The heat treatmenttemperature, ratio and coating thickness can affect the low frequency absorptione. Thelow frequency absorption will be improved and absorption peak move to lowfrequency while the heat treatment temperature decreased with ratio and the thicknessof the coating increased.Then, we prepared microwave absorption coat using appropriate preparedC/SiC/SiO₂composites. The actural reflection loss and influence of thickness of theactural reflection loss of the prepared C/SiC/SiO₂composites were investigated.Results show that the actural reflection loss of the C/SiC/SiO₂composites based onactive carbon is-27.7dB, and the actural reflection loss of the C/SiC/SiO₂compositesbased on pith carbon is-12.5dB. Both of them were less than the theoretical reflectionloss in3mm and were closed with the theoretical reflection loss in1.5mm.With these results， we prepared pitch-based C/C composites, which hasimpregnated SiC and SiO₂inside by pitch and TEOS as impregnating agent, andmeasured its absorption properties. The results show that absorption properties of thepitch-based C/C composites was similar to that of C/SiC/SiO₂composites based onactivated carbon, implying that this may be an idea way to desgn C/C composite withmicrowave absorbing.