| Electromagnetic wave absorption (EMA) materials are drawing extensive attention because of increasing electromagnetic pollution in daily life as well as military requirements for stealth weapon systems. Due to low density, high strength and excellent electrical property, carbon materials are suitable candidates for EMA materials. In this paper, electroplating and thermal oxidation were adopted to prepare magnetic coating on continuous carbon fibers. The structure, morphology and electromagnetic wave parameters of magnetic hybrid carbon fibers were studied, and the mechanism of electromagnetic wave absorption was also discussed. Finally, the proper absorber was chosen for the preparation of silicon rubber based composites, and the practical absorption ability was evaluated.FeCo alloy coated carbon fibers (MCF) with controlled surface morphology were prepared by the electroplating method at different temperatures. Three different morphologies of the coatings, including sheet-like, spot-like and pyramid-like structures were obtained. The crystal structure of the uniform coatings was Co3Fe7 with thickness of about 0.5-1.0μm. The complex electromagnetic parameters of magnetic carbon fibers mixed with paraffin were measured in the frequency range of 2-18 GHz. The results indicated considerable difference of complex permittivity and similar complex permeability, high complex permittivity led to weak EMA. The complex permittivity was lowered by the decrease of mass diffraction of MCF in composite, and excellent absorption ability was achieved.In order to improve the EMA properties, firstly, thermal oxidation was used to obtain oxidates of Fe and Co in alloy coating, the complex permeability became higher and the impedance match was optimized in high frequency. It was worth noting that the effective bandwidth was broadened to 5 GHz. Secondly, CuO/FeCo alloy/carbon hybrid fibers were fabricated by electroplating Cu on FeCo alloy/carbon hybrid fibers and followed by thermal oxidation, nanorod-like CuO was formed on the surface. The product exhibited soft ferromagnetism, the low complex permittivity was maintained and the complex permeability was enhanced. CuO/FeCo alloy/carbon hybrid fibers/paraffin composite displayed excellent absoption ability at middle and high frequency range, the strongest RL was -47.6 dB for a thinner layer of 1.6 mm. The special structure of micron-triple-layer enhanced the final absorbing property by inner multi-reflection and loss of lectromagnetic wave.For practical EMA test, methyl vinyl silicone rubber (MVQ) based composites were fabricated with carbonyl iron (CI) and chopped MCF as complex absorbers. SEM results showed that the absorbers were dispersed evenly in rubber matrix. When the content of CI was fixed to 50 wt%, the RL were decreased drmatically by the addition of MCF with mass difraction of 2 wt% and 4 wt%, and the optimism RL reached -21.3 dB with a broad effective bandwidth of 6.2 GHz. The density and volumn resistivity of the composites were reduced by the addition of MCF, and the tensile strength was improved. The composites could be used as light EMA pasters with wide and strong absroption ability.In conclusion, electroplating and thermal oxidation seemed efficient ways to prepare magnetic carbon fibers, adjust the electromagnetic parameters, and improve the absorpion ability of carbon fibers. As excellent absorber, chopped MCF could enchance the practical absorpion propery efficiently and have a great potential for application in EMA field. |
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