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Preparation and Properties of Carbon Based Composites as Microwave Absorbing Materials

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Tutor: WangChengGuo
School: Shandong University
Course: Materials Science
Keywords: PAN,magnetic modified materials,carbon based composite,electromagnetic parameter
Type: PhD thesis
Year:  2012
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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¡æ, 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.
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