| 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. |
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