【作者】 邹田春；

【导师】 赵乃勤；

【作者基本信息】 天津大学 ， 材料学， 2007， 博士

【摘要】 作为隐身技术发展的关键之一,微波吸收材料的研究越来越受到世界各国的高度重视,成为国防科技领域的热点。在民用方面,随着科学技术和电子工业的发展,电磁辐射的负面效应已经影响到人类生活的各个方面,利用吸波材料来减弱或消除电磁波污染是一种有效的方法。微波吸收材料研究的关键在于设计、制备高效吸收剂(体),活性碳纤维(毡)由于其吸波频带宽、质量轻、吸收强、成本低、易于设计和物化性能稳定的特点,有望成为性能优异的新型吸波剂(体)。本文对粘胶基活性碳纤维及其复合材料的吸波性能、制备工艺和吸波机理进行系统的研究,在此基础上,设计了两种新型吸波体-活性碳毡电路屏和活性碳毡天线结构,详细研究了两种吸波体的吸波特性和吸波机理。主要研究内容如下:用粘胶基活性碳纤维作为吸收剂制备活性碳纤维/环氧树脂复合材料,研究了纤维含量、试样结构等因素对吸波性能的影响,并通过对比实际测量和理论计算得到的反射率,探索了材料的吸波机理。结果表明:活性碳纤维/环氧树脂复合材料对电磁波的主要衰减机制是介电损耗,复合材料上、下表面的干涉相消作用使得反射衰减曲线在中、高频段出现周期性波动。纤维的含量、分布方式以及吸波层厚度对复合材料的吸波性能有很大的影响。在吸波层中纤维含量相同的情况下,活性碳纤维梯度设计可以显著提高复合材料的吸波性能,吸波层中四个结构层纤维含量分别为0.24wt%、0.48wt%、0.72wt%和0.96wt%时,材料在4.46-18GHz频率范围内对电磁波有-10dB以下的吸收,7.12GHz时取得最大反射衰减-25.9dB。通过研究制备工艺对活性碳纤维微观结构和性能的影响规律,分析了纤维在微观层次上与电磁波的相互作用机制,探讨了其微观吸波机理。结果表明:制备工艺对活性碳纤维微观结构、电磁性能和吸波性能有较大的影响。活化时间20min、活化温度900℃、碳化时间60min、碳化温度425℃条件下制备的活性碳纤维,吸波性能最佳,纤维含量梯度分布复合材料的有效带宽达13.68GHz,7.0-18GHz频率范围内,反射衰减高于-20dB。活性碳纤维对电磁波的微观吸波机制包括π电子极化弛豫和界面极化,其中,π电子极化弛豫是纤维有效损耗电磁波的主要原因,界面极化机制在电磁衰减中起次要作用。研究了长度1~3mm短切活性碳纤维的介电性能,发现纤维具有频响效应,有利于展宽吸波频带。以短切活性碳纤维作为吸收剂,依据阻抗匹配原理,优化设计了厚度4mm的四层吸波材料,2~18GHz频率范围内反射率达到-10dB以下的频带宽度为8GHz,最大反射衰减-39.3dB。依据分块设计原理,优化设计含三个亚区域的吸波材料,三个区域的面积比S1:S2:S3=3:1:1时,8.2-13.5GHz频率范围内有低于-10dB的反射率,最大反射衰减-28.9dB。以活性碳毡为材质制备感性电路屏,研究了电路屏阵列单元的结构、尺寸参数对含电路屏复合材料吸波性能的影响,并依据实验结果对材料的吸波机理进行了探讨。结果表明:含感性活性碳毡电路屏复合材料对电磁波的主要吸收机制是电磁波在电路屏和反射板之间的多次反射、衰减。电路屏的谐振频率与含电路屏复合材料吸收频带的变化规律相一致,当电路屏中碳毡所占面积比适当时,入射波的衰减量与电路屏对它的透过率近似呈正比关系。具有多个谐振频率的分形电路屏是活性碳毡电路屏的发展方向之一,经合理设计,含Minkowski分形电路屏复合材料在5.3-18GHz频率范围内有-10dB以下的吸收,有效带宽达12.7GHz。将天线结构首次引入到吸波材料的设计中,研究了含天线结构复合材料吸波性能的影响因素及其吸波机理。结果表明:天线阵列/环氧树脂复合材料对电磁波的吸收机理为天线的电阻损耗。天线的尺寸参数、材质和两臂间电阻阻值与材料的吸波性能密切相关。在天线的起始半径、外半径、参数和两臂间电阻分别为4mm、42mm、23.86mm和150?的条件下,阿基米德平面螺旋天线阵列/环氧树脂复合材料的最大反射衰减-23dB,有效带宽达11GHz。含活性碳毡对称振子阵列复合材料对电磁波的吸收呈各向异性,振子与入射电场平行时,其吸波性能明显优于与入射电场垂直时的吸波性能,设计材料的有效带宽可达13GHz,最大吸收峰值-30.3dB。更多还原

【Abstract】 As one of the keys for stealthy technologies, microwave absorbing materials (MAMs) have been paid more and more attention all over the world, and they have become the hotspot in national defense fields. In domestic technologies, with the development of science and technology as well as the development of electronic industries, electromagnetic waves (EMWs) are bringing disastrous harm to human environment. The utilization of MAMs is an effective method to eliminate negative effects of EMWs. The primary goal of MAM research is to design and produce high-performance absorbents (absorbing structures).Activated carbon fibers (ACFs) have the characteristics of broad absorption band, light mass, strong absorption, low cost, easy design and stable physico-chemical performance, and it will become a kind of promising absorbent.In this paper, microwave absorbing property, preparation technology and absorbing mechanism for viscose-based ACFs and ACF composites have been systematically investigated. On the basis of above research, two kinds of new absorbing structures: inductive activated carbon-fiber felt screens (IACFFSs) and antenna structures are designed, and their absorbing property and mechanism have been studied. The main research contents are summarized as follows:Viscose-based ACFs / epoxy resin composites are prepared, and the influences of different fiber contents and composite structures on the absorbing property were studied. Through the comparison between actually measured and theoretically calculated reflection loss, the absorbing mechanism was studied. The results show that dielectric loss is the main mechanism for EMW attenuation of epoxy resin composites containing viscose-based ACFs, and interference counteraction of the composite top and bottom surfaces is the reason for periodic fluctuation of absorption curves in median and higher frequencies. The absorbing performances of composites are strongly dependent on ACF content, distribution and arrangement modes and the absorbing layer thickness. In the case that average ACF content in the absorbing layer is identical, ACF gradient design can significantly improve the absorbing property of composites. When ACF contents of four structure layers in the absorbing layer are 0.24wt%, 0.48wt%, 0.72wt% and 0.96wt%, the composite has a reflection loss below -10dB in the frequency range from 4.46GHz to 18GHz, and the maximum absorption reaches -25.9dB at 7.12GHz.Through analyzing the effects of preparation technologies on microstructures and performances of ACFs, the microscopic interaction between ACFs and EMWs and the absorbing mechanism were investigated. The results indicate that preparation processing parameters (carbonization temperature and activation time) have a great influence on microstructure and characteristics of ACFs. ACFs prepared at the conditions (activation time=20min, activation temperature=900℃, carbonization time=60min, carbonization temperature=425℃) show the optimum absorbing effect. The composite with gradient distributing ACFs provides a bandwidth below -10dB of 13.68GHz, and the reflection loss is below -20dB in the frequency range 7.0-18GHz. EMW microscopic absorbing mechanism of viscose-based ACFs consists ofπelectron and interfacial polar relaxation.πelectron polar relaxation is the leading reason for electromagnetic loss, and interfacial polar relaxation takes the secondary action in electromagnetic attenuation.Dielectric property of ACFs with average length of 1~3mm were investigated. The results show that ACFs have the frequency response effect, which is helpful for broadening the absorbing bands. According to ACF permittivity, the optimal design is carried out for four-layer microwave absorbing materials with thickness of 4mm based on the impedance matching design method. Following the optimization results the four-layer absorbing material was prepared, and it obtains a reflection loss below -10dB over 8GHz and the minimum value reaches -39.3dB in 2-18GHz. By applying the block design method, the composite was divided into three sub-regions, and it achieves a reflection loss below -10dB in 8.2-13.5GHz and the minimum value is -28.9dB when the area ratio of three sub-regions is S1:S2:S3=3:1:1.ACF felt is used as the material of inductive screens, and the effects of the lattice and element configurations of IACFFSs on the absorbing property of composites with IACFFSs embedded in them were investigated. On the basis of experimental data, the absorbing mechanism was studied. The results show that the main EMW attenuating mechanism of composites containing IACFFSs is EMW multiple reflection and attenuation between the screen and reflector plate. Resonant frequencies of IACFFSs have the same alteration rules as absorption bands of composites containing IACFFSs, and the decrement of the incident wave is direct proportion to the EMW transmission ratio of the screen when the ACF felt area ratio is proper. The fractal screen, which has multi-resonant frequencies, would be a kind of promising IACFFS for microwave absorption. If properly designed, the composite containing the Minkowski fractal screen can show a reflection loss blow -10dB in the frequency region 5.3-18GHz.For the first time, the antenna structure was used as the absorbing structure to design MAMs. The influence factors for the absorbing property and the absorbing mechanism were studied. The results show that microwave absorbing mechanism of antenna structure/epoxy resin composites is resistance loss. The absorptivity greatly depends on antenna kinds, dimension parameters and the magnitude of resistance connecting antenna arms. When the initial radius R0 , the outer radiusR1 , the constant R and the magnitude of resistance connecting two arms are 4mm, 42mm, 23.86mm and 510? respectively, the composite containing the Archimedean plane spiral antenna array exhibits a reflection loss below -10dB in the bandwidth 11GHz and the minimum value reaches -23dB. Microwave absorption of composites containing ACF felt dipole arrays presents anisotropy, and when dipoles are parallel to the incident electric field, composites show better absorbing performances. If properly designed, the composite containing the ACF felt dipole array obtains an effective bandwidth 13GHz and the maximum reflection loss -30.3dB.更多还原