【作者】 王雯；

【导师】 王成国；

【作者基本信息】 山东大学 ， 材料学， 2012， 博士

【摘要】 吸波材料作为现代飞行器、武器装备的基础材料,是现代隐身技术的重要支撑。随着电子化和信息化技术的迅猛发展,各种电子和通信设备给人们带来极大便利的同时也对人们生存环境和健康造成危害。吸波材料不仅应用于军事隐形、对抗和反对抗,而且应用于人体安全防护、通讯及导航系统的电磁干扰、微波暗室的消除、安全信息保密等许多方面。因此高性能吸波材料的开发已成为研究的主要方向。目前,通过碳材料改性方法制备复合吸波材料的研究已经取得一定的研究进展,但是制备的复合材料仍然存在着吸收频带窄,强度不高,或工程应用实施困难等问题。在此基础上,本文结合实验室聚丙烯腈(PAN)基碳纤维的制备工艺,对碳纤维前驱体前驱体PAN聚合液进行改性处理,利用添加的磁性原材料与PAN中活性元素反应生成磁性能优异的铁磁性化合物改善复合材料的磁导率,研制兼具电损耗和磁损耗的新型碳基复合吸波材料。探索制备过程中热稳定气氛、碳化温度、磁性原材料含量等各工艺参数对碳基复合吸波材料性能的影响规律。并根据电磁波传输理论,结合碳材料和磁性材料的特性,利用计算机辅助优化设计碳基复合材料的电磁参数,选择相互匹配的介电常数和磁导率在磁性原材料选择方面,本文选用还原铁粉、纳米铁粉和草酸亚铁作为改性前驱体添加到PAN聚合溶液中,凝固成形后在N2氛围中进行热处理得到碳基复合材料。复合材料中Fe元素主要以Fe304的形式存在。其中加入还原铁粉和纳米铁粉制备的碳基复合材料表现出了优异的吸波性能,当涂层厚度分别为1.9mm和2.2mm时,在12.7～18GHz频段内,两种复合材料的反射率都小于-10dB。当涂层厚度增加至2.5mm时,最小反射损失值分别为-46dB和-29.8dB。由此可知,选择还原铁粉作为PAN基碳材料的改性前驱体制备碳基复合吸波材料的方法是切实可行的。还原铁粉加入PAN聚合液中,一部分Fe可以和PAN分子链上由IA引入的羧基反应,把氢置换出来,进入PAN分子链,另一部分主要以单质铁的形式存在,并且在热稳定化过程中稳定存在。在热处理过程中,Fe结合PAN大分子链上的活性N、O元素生成磁性化合物,提高碳基体的磁导率。多相磁性物质的存在有利于增加界面极化对介电常数的贡献,同样有利于磁导率的多个共振波峰的出现,拓宽吸波频带。通过对不同热稳定气氛和碳化温度的碳基复合材料的碳收率、电磁参数和吸波性能的研究表明：碳基复合材料的碳收率随着碳化温度的升高而减小；空气中预氧化后再碳化得到的碳基复合材料的碳收率较大,其次是氮气中热处理；热处理至700℃,氩气中得到的复合材料的碳收率最低。通过分析不同温度下的碳基复合材料的电磁参数可知,随着碳化温度的升高,碳基复合材料的介电常数增加,介电损耗增强。比较空气、氮气和氩气三种气氛中热稳定化的碳基复合材料的吸波性能可知,在空气中预氧化后再在N2氛围中热处理得到的复合材料的吸波性能较好。随着还原铁粉含量的增加,PAN线形大分子链的有序性遭到破坏,结晶度降低。当含量增加到一定值,复合材料中磁性物质会阻碍基体导电网络的形成。因此,复合材料的介电常数实部先增大后降低。然而,磁导率的大小不仅与磁性物质含量有关系,也与物相形态和各成分相对含量有关。分析吸波效果可知,在相同的厚度下,随着铁含量的增加吸收峰向低频移动,当含量达到一定值时,吸收峰返回高频段,复合材料的整体吸波性能下降。结合湿法纺丝工艺,将含有还原铁粉的PAN聚合液纺制成纤维,制备碳基复合纤维材料。通过性能分析可知,还原铁粉作为PAN基纤维的改性磁材料的前驱体,其颗粒尺寸较大,在纤维中分散不均匀,不利于与基体之间的结合和反应。因此在制备过程中应选用具有纳米级尺寸且结构疏松的改性磁材料,有利于改善复合材料的磁导率。而且在PAN复合原丝的纺制过程中,要避免还原铁粉的氧化。通过对碳基复合材料的各工艺参数进一步优化,制备了一种高性能的碳基复合薄膜材料。结果显示,材料涂层厚度为1.3mm时,低于-10dB的有效频段为6-13.2GHz和13.9-18GHz,在12.1GHz反射损失值最小为-21.1dB。当涂层厚度增加至1.6mm,高频段反射率损失减小,低频段反射率损失增大,2-10GHz和16.8-18GHz内反射率都低于-10dB,反射损失值在6.3GHz达到最小为-21.1dB。研究表明,碳基体和磁性物质的结构形态对复合材料的吸波性能有很大影响。多孔中空结构的碳基体可以多重散射进入材料内部的电磁波,从而可以有效达到衰减电磁波的目的。纳米级片状结构磁性材料可以有效的改善碳基复合材料的磁导率,提高复合材料整体的吸波性能。利用matlab程序软件模拟与碳基体介电常数相匹配的磁导率,通过分析数据可知,在低频段,吸波材料必须有高的介电常数和磁导率才能对电磁波有效吸收,且随着频率的增加,磁导率可以相应降低。然而,基体材料的介电常数值很小,则很难在低频段实现好的吸收效果；在相对高的频段内,碳基体对磁导率值的要求降低,而且较易实现与磁性材料的复合,从而达到好的吸波性能。更多还原

【Abstract】 As basic material of modern aircrafts, weapons and electronic equipment, microwave absorbing material is an important support for the modern stealth technology. With the rapid development of electronic and information technology, a lots of electronic and information equipment introduce a great deal of harm to the health and living environment of human beings while bringing people with great convenience. Thus, microwave absorbing material is not only used in military stealth, confrontation and anti-confrontation, but also applied in safe guarding for human body, elimination of electromagnetic interference and microwave dark-room produced by communication and navigation systems, information safety and so on. Therefore, the development of high-performance absorbing materials has become the main research direction recently. At present, some studies have made progress in the preparation of composites as absorbers by modification carbon materials. However, the absorbers still have some problems, such as narrow absorption band, weak electromagnetic wave absorption, or difficulty in engineering application. Based on the preparation technology of carbon fiber in our laboratory, the novel carbon based composites with dielectric and magnetic losses were prepared using polyacrylonitrile(PAN) and magnetic material as precursors in the paper, which reacted with active element from PAN to form ferromagnetic compounds and improved the permeability of carbon based composites. In the process of preparation, technological parameters of polymerization, forming, thermal stabilization and carbonization were studied, and various process parameters that effected on the absorbing properties were investigated, such as thermal stabilization atmospheres, carbonization temperatures, contents of modified magnetic materials and so on. According to the theory of electromagnetic wave transmission, combining with the characteristics of carbon materials and magnetic materials, the carbon based composites were designed, and the matched electromagnetic parameters were optimized.In the choice of modified magnetic material aspect, Fe, nano-Fe and FeC2O4·2H2O as the precursors were added in PAN solution. After heat treatment in N2atmosphere, carbon-based composites were obtained. Iron element exists in the form of Fe3O4in composite materials. The reflectivity was below-10dB in the frequency range of12.7-18GHz with1.9mm and2.2mm in thicknesses when the composites used Fe and nano-Fe as precursors. When the thickness increased to2.5mm, the minimum reflection loss value was-46dB and-29.8dB respectively. It indicates that the fabrication of carbon based composites using Fe as precursor is feasible.When Fe powders were added in PAN solution, a part of Fe reacted with carboxyl group, which was introduced by itaconic acid(IA) of PAN molecular chain, and displaced hydrogen. The other part of Fe was not involved in chemical reaction even in the process of thermal stabilization. During the heat treatment, Fe combined with active N and O to produce magnetic compounds, which would improve the complex permeability of carbon matrix. The interfacial polarization of multi-phase magnetic materials increased the contribution to the complex permittivity, and benefited to the appearance of permeability multi-resonance peaks and broadening of wave absorbing band.The carbon yield, electromagnetic parameters and wave absorption properties of the carbon based composites obtained at different thermal stabilization atmosphere and carbonization temperature were studied. The results showed that the carbon yield decreased with the increase of carbonization temperature, and compared with that of the composite heat-treated in N2atmosphere, the carbon yield of the composite stabilized in air before carbonization was bigger. For the composites thermal stabilized in N2, air and Ar atmospheres and carbonized at700℃, the carbon yield of that in Ar was lowest. Through analyzing the electromagnetic parameters of the composites carbonized at different temperatures, it showed that the permittivity increased with the increase of temperature. The composite, which was oxidative stabilized in air and carbonized in N2, displayed a better absorbing performance.With the content of Fe increase, the order of PAN linear macromolecular chain from PAN was destroyed, and the crystallinity was decreased. When the content increased to a certain value, the magnetic materials in the composite prevented the formation of conductive network. Therefore, the real part of permittivity increased firstly and then decreased. However, the complex permeability was also influenced by the phase, relative content and morphology of the magnetic substances. The absorption peak moved to the low frequency band with the increase of iron content. However, when the content increased to a certain value, it returned to the high frequency, meanwhile the overall microwave absorption property of the composite declined.PAN composite fiber was prepared using wet spinning method, followed by heat treatment. The performance analysis showed that the particle size of the iron as modified precursor was too large to disperse uniformly, and it was not conducive to the combination and reaction with the carbon matrix. Therefore, in order to improve the permeability of carbon fiber composites, it was necessary to select magnetic particles with nanometer size and loose structure as modification material, and avoid to be oxidized in the spinning process. In addition, through optimizing the process parameters of the carbon based composite, one high-performance composite was prepared. An optimal reflection loss of-21.1dB was obtained at12.1GHz with the-10dB bandwidth over the frequency ranges of6-13.2GHz and13.9～18GHz for the absorber thickness of1.3mm. When the thickness increased to1.6mm, the minimum reflection loss value of-19.9dB was observed at6.3GHz and the reflection loss values exceeding-10dB were obtained in the frequency ranges of2～10GHz and16.8-18GHz. The results showed that the structure of the carbon matrix and magnetic materials had a significant impact on the microwave absorption property of the composite. The carbon matrix with porous and hollow structure can scatter electromagnetic wave multiply, and effectively achieved the purpose of attenuating the electromagnetic wave. The flaky nanoscale magnetic materials can effectively improve the permeability and absorbing properties of the carbon-based composites.Complex permeability matched with the complex permittivity of the carbon matrix was simulated using Matlab software. The simulation data showed that the absorber, which had high permittivity and permeability, could effectively absorb electromagnetic wave of low frequency, and the value of permeability decreased with the increase of frequency. However, because of the low permittivity, it is difficult to achieve excellent absorption for the composite. During the relatively high frequency band of2～18GHz, lower permeability could meet the request of carbon matrix, thus, it was easier to composite with magnetic materials and had good absorption performance.更多还原