Preparation Of Nano Carbon Fibers And Porous Carbon Fibers And The Research On Their Radar Absorbing Properties

The stealth technology is an effective method for increasing the defense and offense performance of the military forces in the modern war. As the key of the stealth technology, the radar absorbing material becomes the hot point of the research in this field. In this trend, the carbon material is recognized as the most excellent radar absorbing material because of its good radar absorbing and mechanical properties. Therefore, it is significant to study the carbon materials preparation and their radar absorbing properties both theoretically and practically.In this paper, polyacrylonitrile (PAN) was used as the carbon precursor polymer (CPP) and polymethylmethacrylate (PMMA) as thermally decomposed polymer (TDP) respectively. After blending them in N, N-dimethylformamide (DMF), PAN/PMMA blend solutions were prepared. The PAN/PMMA blend fibers were produced by wet spinning process. After oxidization and carbonization of the PAN/PMMA blend fibers, the porous carbon fibers and nano carbon fibers were produced. Furthermore, these two kinds of carbon fibers were employed as the radar absorbing fillers and the epoxy as the matrix to prepare single-layered radar absorbing composites, and their radar absorbing properties were investigated. The main content of this paper is as follow:The rheological behaviors of the PAN/PMMA blend solutions were measured by the dynamic rheometer, it was found that the molecular weight of the PAN influenced the relationship between log G' and logωof blend solutions. In the PAN5/PMMA blend system, when the angular frequencyωwas very low, logG' deviated from the relationship logG'∝21ogωobviously; while in PAN8/PMMA blend system, the relation logG'∝2logωcould keep on even in lowωregion. This indicates that there is non-homogeneous phase structure in the PAN5/PMMA blend solution. The result could also be approved by the solution stability observation, i.e. the phases separation occurred obviously in the PAN5/PMMA blend solution after 12-24 hours of place; whereas the PAN8/PMMA blend solution kept stable. The morphologies of PAN5/PMMA and PAN8/PMMA were observed by SEM. The results explained that the PAN and PMMA formed their own phase in the blend films. Generally, the component with high weight ratio in blend system easily became the matrix. As the increasing of the molecular weight of PAN, the size of the dispersed phase in the blend system decreased.The PAN/PMMA blend fibers of two different ratios (PAN/PMMA= 3/7 and 7/3) were produced after wet spinning. The phase structure in the PAN8/PMMA blend fibers was similar with the one in the blend film; but in the PAN5/PMMA blend fibers, the PAN easily became the matrix. Of course, the spinning speed and the drawing ratio could be used to control the size of the disperse phase in the blend fibers.After oxidization and carbonization of PAN/PMMA blend fibers, we made the carbon fibers materials. When the PAN was the matrix in the blend fibers, the porous carbon fibers was received; when the PAN was the dispersed phase, the nano carbon fibers was obtained.The degree of the graphitization of the carbon fibers materials could be controlled by the temperature of carbonization. The conductivity of the carbon fibers materials became high with increase of the temperature.The obtained carbon fibers were employed as the radar absorbing fillers and the epoxy as the matrix to prepare the flat composites with 2-8%wt addition of the filler. The complex dielectric constants at 8-12GHz were measured. It was revealed that with the increasing of the radar absorbing filler, both the real part and the imaginary part of the complex dielectric constants rose up and the loss tangent also increased. Based on the values of dielectric constants, the reflection loss of corresponding flat composites were calculated by the software programmed by us, and the calculated results were used to help us design and manufacture the single-layered radar flat absorbing composites.By using network analyzer (HP8722ES, produced by Agilent), we measured the radar absorbing performance of the single-layered radar absorbing flat composites in arch method. The results indicated that the reflection losses of the composites decreased as increasing of the radar absorbing materials, and the frequency corresponding to the reflection loss peak moved to the low frequency. The tested results were almost the same with the simulated ones. Comparing the porous carbon fibers with the nano carbon fibers, at the same weight percentage addition and the same thickness of the composites, the composite containing porous carbon fibers performs lower reflection loss and the range that the reflection loss smaller than -10dB was wider than that containing nano carbon fibers. For example, with 8% wt incorporation of the filler and the thickness of 3 mm, the lowest reflection loss of the flat composite containing the porous carbon fibers was -20dB, and its reflection loss was almost lower than -10dB in 8-12GHz; whereas the lowest reflection loss of the composite containing the nano carbon fibers was -16dB and the range that the reflection loss smaller than -10dB was only 2GHz wide.By making a preliminary research on the radar absorbing mechanism, we considered the attenuation of radar wave may be mainly attributed to the absorption of the conductive fillers and interference loss due to multiple reflection of radar wave in the composites. When the porous carbon fibers were used as the radar absorbing material, the paths of the electromagnetic wave became multi-ply and tortuous, so the lower reflection loss could be expected.The work made in this dissertation could enrich the design and manufacture of the composites where carbon fibers materials are fillers for radar absorbing purpose.