Lightweight Radar Absorbing Materials Based On Hollow-porous Carbon Fibers

Radar absorbing materials(RAMs) have been widely used in stealth technology and electromagnetic compatibility technology, and that is why they are a researching hotspot all over the world. With the development of science and technology, lightweight RAMs have become one of the most important aspects of stealth technology. This dissertation will present thorough investigation on fibrous absorbers and their composites to meet the imminent demanding of lightweight RAMs.The methods for lightening RAMs were discussed according to their compositions and structures. Based on the analysis of the shapes and compositions of absorbers, polyacrylonitrile(PAN)-based hollow-porous carbon fibers(HPCFs) were firstly chosen as the main absorbers in RAMs.The influence of preoxidation and carbonization process on the appearance and micropores of HPCFs were investigated. The results show that the temperature and time have little influence on the appearance and micropores of HPCFs. It is found that the heating rate influences the appearance and micropores of HPCFs greatly. The intergradation layers of dual-layer were ruptured and the micropores reduced when the heating rate is too high.The changes of HPCFs compositions after the preoxidation and carbonization process were studied. The 0 content of hollow-porous cured fibers increased with the increase of curing temperature and curing time. The C content of HPCFs decreased as the curing temperature and curing time increased. The C content of fibers increased with the increase of carbonization temperature and carbonization time while the N content decreased.The volume conductivity of HPCFs and the complex permittivity of the resultant HPCFs/paraffin composites affected by the heating treatment were investigated. The preoxidation temperature, time and heating rate have great influence on the volume conductivity and the complex permittivity. The volume conductivity decreases with the increase of preoxidation temperature, time and heating rate. The real and imaginary parts of the complex permittivity (ε' andε") decrease with the increase of preoxidation temperature, time and heating rate. Carbonization temperature is the main factor in adjusting the volume conductivity and complex permittivity of HPCFs. The volume conductivity can be adjusted in range of 10^-3-10³Î©^-1·m^-1 when carbonization temperature changes from 550℃to 950℃. The carbonization time leads to a slight increase in the volume conductivity and complex permittivity while the heating rate has little influence. The volume conductivity of HPCFs and the complex permittivity of the HPCFs/paraffin composites can be adjusted by controlling the heating treatment process. The results of the preoxidation process indicate that the preoxidation temperature, time and heating rate can effectively adjust the composition, volume conductivity and complex permittivity of the HPCFs as well as their composites. Carbonization temperature is the main reason for the above change and carbonization time and heating rate are the secondary factors.The relationship among composition, structure and dielectric properties of HPCFs indicates that the changes of complex permittivity of HPCF composites depend on their volume conductivity. The C content and micropores both caused the change of the volume conductivity, among which the C content is the first factor.The preoxidation temperature has great influence on the apparent density of the cured fibers. The apparent density of HPCFs can be adjusted in range of 0.55-0.79g/cm³ when carbonization temperature changes from 550°C to 950°C. The apparent density of HPCFs can be adjusted by heating treatment conditions.In order to get lightweight RAMs, the HPCF composites were prepared by the mould-pressing technique based on the analysis and experiments. Effect of the volume and the l/a ratio of HPCFs, and the thickness of RAMs on the radar absorbing properties of HPCF composites were studied. Theε' andε" of HPCF composites increase with the increase of the volume fraction and the l/a ratio of HPCFs firstly, then change slowly. The reflectivity apex moves to the lower frequency with the volume fraction and the l/a ratio of HPCFs and the thickness of materials increasing.The radar absorbing properties of hollow-porous and solid carbon fibers as radar absorbents were comparatively investigated. The results show that the C content of HPCFs is lower than that of solid fibers obtained at the same condition. The volume conductivity of single solid carbon fiber is nearly 10 times that of the HPCF. The -10dB bandwidths of solid carbon fiber composites carbonized at 850°C and 950°C are both 0GHz, while those of the corresponding HPCF composites are up to 3.05GHz and 2.62GHz, respectively. Results indicate that the radar absorbing properties of the HPCF composites are better than those of solid carbon fiber composites.The wideband RAMs based on HPCFs were investigated according to multi-layer structure design. In order to get wideband RAMs, the reflectivity was simulated using RAMCAD software based on resistance matching principles and the multi-layer structure. The optimal reflectivity of three-layered radar absorbing composites of HPCFs and paraffin was -15.33dB at 7.60GHz and the bandwidth under -5dB was about 12.36GHz. The radar absorbing properties of three-layered HPCF composites are better than those of single layer composites.Two-layered radar absorbing materials with the carbon black and the short carbon fibers were prepared based on HPCF composites. The obtained materials have the bandwidths of 10.60 GHz and 10.26 GHz under -8dB with the carbon black and the short carbon fibers as the matching layer, provided that the thickness is 3.04mm. The bandwidth under -8dB of HPCF composites has only 2.15GHz and the lowest reflectivity is -9.25dB at the same time.A series of HPCF composites embedded with frequency selective surface (FSS) were investigated. The results indicate that the radar absorbing properties of HPCF composites with FSS can be improved. The results also show that both the size of the element and locations of the FSS in the composites are critical for the reflection property of the HPCF composites. The HPCF composites embedded FSS has a bandwidth of 11.18GHz under -10dB in the range of 2-18GHz, provided that the thickness and the density are 3.0mm and 1.0g/cm³, respectively.HPCFs are proved to be light conductive absorbers in view of their hollow structure and radar absorption properties. The results in this dissertation indicate that the lightweight and wideband composites based on HPCFs can be gained with suitable structure design.