Study On The Regulation, Structure And Properties Of High-temperature Resistant Carbon-based Microwave Absorbers

Abstract:According to the high-temperature structural microwave absorbing materials requirements of the new high-speed aircraft and other weapons, the aim of this study is to improve the microwave absorbing performance of the C/C composites. The control technique, structure and properties of high temperature resistant carbon-based (carbon fibers and pyrolytic carbon) microwave absorbers were investigated. On the basis of initial proven the electromagnetic response characteristics of the carbon fiber and pyrolytic carbon, we introduced BN, SiO2and SiC high-temperature resistant ceramic phase to modify carbon fibers and pyrolytic carbon. The microstructure, oxidation resistance and dielectric properties of the modified carbon fibers and pyrolytic carbon were studied systematically, and the microwave absorbing properties of the modified carbon fibers and pyrolytic carbon were also validated. Based on the above, the simulation design, preparation and microwave absorbing properties of the modified C/C composites were studied. The modified C/C composites with excellent microwave absorbing properties were prepared successfully. The main research content and conclusions are as follows:1) The electromagnetic response characteristics of the carbon fiber and pyrolytic carbon are preliminary proved. The permittivity of carbon fibers correlates with their shapes and arrangements. The polarization orientation has a significant impact on the microwave absorbing properties of the directional arranged long carbon fibers composites. The permittivity of pyrolytic carbon from the pyrolysis of propylene is largest, followed by the pyrolytic carbon from the pyrolysis of mesophase, and the pyrolytic carbon from the pyrolysis of furan resin is lowest. With the heat treatment temperature increase, the conductivity of pyrolytic carbon is significantly enhanced, leading to the significant increasing of the complex permittivity.2) The effect of surface coatings and in situ grown SiC nanofibers toward the microstructure, strength, oxidation resistance and dielectric properties of the carbon fiber were investigated, respectively. The modification technologies of the carbon fiber with SiO2coating, BN coating, SiC coating with interface phase and in situ grown SiC nanofibers were developed. Employing these technologies could significantly reduce the permittivity of the carbon fiber, regulate the dielectric properties of carbon fiber well, improve the oxidation resistance of carbon fiber simultaneously, and make the carbon fibers meet the requirements of high-temperature resistant microwave absorbers.3) The microstructure, chemical composition, oxidation resistance and dielectric properties of the pyrolytic carbon modificated by BN and SiO2were investigated, respectively. The modification technologies of the pyrolytic carbon with SiO2and BN high-temperature insulating ceramics were developed. Applying these technologies could regulate the dielectric properties of pyrolytic carbon well, improve the impedance matching and oxidation resistance of pyrolytic carbon, and obtain high-temperature resistant microve absorbing carbon matrix with controllable dielectric properties.4) The effect of BN/SiC coating modified carbon fibers and SiO2doping modified pyrolytic carbon toward the microwave absorbing properties of the composites were investigated, respectively. For the chopped short carbon fiber composites with sandwich structure, the microwave absorbing property of these composites can be improved obviously by employing the BN/SiC coating modified carbon fibers. For the directional arranged long carbon fibers composites with sandwich structure, under the condition of horizontal polarization test, to some extent, the microwave absorbing property of these composites can be improved both at high and low frequency by employing the BN/SiC coating modified carbon fibers. However, under the condition of vertical polarization test, the microwave absorbing properties of these composites were different. They were degraded at high frequency, but improved at low frequency. The microwave absorbing properties of the pyrolytic carbon composites can be significantly improved by employing the SiO2doping modified pyrolytic carbon, while the microwave absorbing properties degrade with the increase of the content of SiO2. 5) Based on the simulation design of the modified C/C composites with sandwich structure, the modified C/C composite with excellent microwave absorbing properties at room temperature and1050℃high temperature (under the vertical polarization testing) were prepared succesfully. The reflectivity of the composites are all less than-8dB in the8-18GHz band at room temperature, and the bandwidth of reflectivity less than-10dB was5.2GHz. Moreover, the reflectivity of composites ranged are all less than-6dB in the8-18GHz band at1050℃, and the bandwidth of reflectivity less than-10dB was4.6GHz. These investigations laid the foundation for developing high temperature structural microwave absorbing C/C composites which could meet the requirements of the practical application.