【作者】 王雷；

【导师】 万怡灶；

【作者基本信息】 天津大学 ， 材料学， 2012， 硕士

【摘要】 随着电子设备的广泛应用带来的电磁污染问题和军事领域对隐身材料的需求,电磁波吸收材料得到了快速发展。近年来,碳材料在电磁波吸收方面的研究受到越来越多的关注。本文以连续碳纤维为基体,通过电镀和热氧化法在其表面沉积磁性镀层,对磁性碳纤维的形貌、成分和电磁性能进行系统研究,探讨其理论吸波性能。最后选取合适的吸波剂,制备硅橡胶基吸波片材复合材料,研究其实际吸波性能。采用传统电镀法在碳纤维表面制备FeCo合金镀层,在不同温度下获得了不同的合金镀层形貌,随温度升高依次为片状、点状和锥状。镀层合金晶体结构均为Co3Fe7,厚度为0.5-1.0μm。制备磁性碳纤维/石蜡复合材料,研究其2-18 GHz频率段内的电磁参数,进而计算理论吸波值。结果发现不同温度下的产物介电常数差异比较大,而磁导率相近,较高的介电常数使吸波性能明显下降。降低复合材料中磁性碳纤维的质量分数后,其介电常数降低,获得了良好的吸波性能。为了进一步提高磁性碳纤维的吸波性能,首先利用热氧化法在FeCo合金镀层中引入Fe和Co的氧化物,350℃氧化下的产物具有较高的磁导率,在高频范围内改善了阻抗匹配性能,使高频小于-10 dB的有效频宽达到5 GHz。其次结合电镀和热氧化法制备出CuO/FeCo合金/碳纤维双镀层磁性材料,镀层表面生成纳米柱状CuO,产物表现出铁磁性,在保持低的介电常数的同时,双磁性镀层增大了材料的磁导率,提高了阻抗匹配性能,在中高频表现出优异的理论电磁波吸收性能,最大值为-47.6 dB,厚度降低至1.6 mm。该吸波剂独特的微米级三层结构有利于电磁波的进入和内部多次反射,从而增大电磁波损耗。以甲基乙烯基硅橡胶为基体,羰基铁粉和短切FeCo合金/碳纤维磁性材料为复合吸波剂,制备出贴片型吸波复合材料。SEM结果表明吸波剂在复合材料中得到了良好的分散。在羰基铁粉含量为50%时,含量为2%和4%的磁性碳纤维即能大幅度提高复合材料的实际吸波性能,最大吸收值为-21.3 dB,有效频宽扩大至6.2 GHz。磁性碳纤维的加入降低了复合材料的体积电阻率和密度,提高了其拉伸强度,该复合材料有望成为一种吸波性能优良的轻质贴片型吸波材料。综上所述,利用电镀和热氧化法可以在碳纤维表面简单、高效的制备磁性合金和氧化物镀层,调节原始碳纤维的电磁参数,提高其理论吸波性能。短切磁性碳纤维作为吸波剂能有效提高硅橡胶基复合材料的实际吸波性能,在电磁波吸收领域具有良好的应用潜能。更多还原

【Abstract】 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.更多还原