Preparation Of Nano Carbon Fibers And Porous Carbon Fibers By Polymer Blend Technique And The Research On Their Radar Absorbing Properties

In modern warfare, the stealth technology has become an effective method for the increasing defense and offense performance of the military forces. For the radar absorbing material is the key of the stealth technology, it becomes the focal point of the research in this field. It has been recognized that the carbon material is one of the most excellent radar absorbing materials, especially carbon materials with varied morphologies. Therefore, it is significant to prepare nano and porous carbon fibers and study their radar absorbing properties..In this paper, carbon nanofibers (CNFs) and porous carbon fibers (PCFs) were prepared through polymer blend technique. These carbon fibers were employed as the radar absorbing fillers and the epoxy as the matrix to prepare single-layered radar absorbing composites, further more, their radar absorbing properties were discussed. The main content of this paper is as follow.Phenol-formaldehyde (PF) was used as carbon precursor polymer (CPP), polyethylene (PE), polypropylene (PP) and maleic anhydride grafted polypropylene (PP-g-MAH) as thermally decomposed polymer (TDP), respectively. The blend fibers were prepared by melt-spinning technique and carbonized at high temperature under a nitrogen atmosphere. After carbonization of the blend fibers, the TDP component was removed and the PF component left in the form of thin carbon fibers. The compatibility, micro-phase structures of blend systems and effects of the experimental parameters, such as the spinning speed, on the morphology of the thin carbon fibers were investigated via polarized light microscopy and SEM. Graphite structures of the obtained thin carbon fibers under different carbonization temperatures were investigated through Raman spectra and XRD. The results showed that the spinnability of PF/PE blend system was poor, and the PF/PP-g-MAH blend system was suitable to spin. The morphologies of PF/PP and PF/PP-g-MAH blend fibers were observed by SEM. It was found that the size of the PF dispersed phase in PP-g-MAH was smaller than that in PP. The diameter of dispersed phase and produced carbon fibers also decreased with increasing of spinning speed, and the diameter of prepared CNFs could be smaller than150nanometers. Raman spectra and XRD explained that the CNFs treated at800℃generated some graphite structure, and the graphitization degree of CNFs became higher with raising temperature of carbonization.PAN/PMMA blend solutions were prepared after blending them in N, N-dimethylformamide (DMF). The morphologies of PAN/PMMA blend films were observed by SEM. The results showed that PAN component was the dispersed phase when its content was30%; and it became matrix when its content was70%. As the molecular weight of PAN increased from5×104(PAN5) to8×104(PAN8), the size of the dispersed phase in the blend system decreased.The PAN/PMMA blend fibers with PAN/PMMA=30/70and70/30were prepared via wet spinning. After oxidization and carbonization of PAN/PMMA blend fibers, the CNFs and PCFs were prepared from the30/70and70/30blend fibers separately. The results of SEM informed that the molecular weight of PAN had an important influence on the diameter of the CNFs and the pore size of PCFs. The diameter of CNFs and pore size of PCFs prepared from PAN8was smaller than that from PAN5. Thus the molecular weight of PAN could be used to control the morphology of carbon fibers.The degree of the graphitization became higer with increasing temperature of carbonization. The conductivity of the carbon fibers also increased with increasing the temperature of carbonization. When the temperature was chosen as1400℃, the conductivity of resultant CNFs reached to4.97×102S/cm.The obtained carbon fibers were employed as the radar absorbing filler and the epoxy as the matrix to prepare the flat composites with2-6%wt addition of the filler. The complex dielectric constants at8-12GHz were measured. It was revealed that both the real part and the imaginary part of the complex dielectric constants rose up, moreover, the loss tangent also increased with the increase of the filler.Based on the values of dielectric constants, the reflection loss of corresponding composites were calculated, the results indicated that the value of reflection loss kept reduced with increasing of the filler, and the frequency corresponding to the microwave absorbing peak shifted to lower value as increasing the thickness of composites. The minimum reflection losses of the composites at8-12GHz with6%of CNFs and PCFs were-12.18dB and-30.85dB respectively, the band widths below-10dB were1.05GHz and3.15GHz. We measured the radar absorbing performance of single-layered flat composites in arch method. The tested results were similar with the simulated ones. Comparing the CNFs, PCFs5and PCFs8, PCFs8showed the best performance of radar absorbing. The composite containing PCFs8performed lowest reflection loss and the band range with reflection loss smaller than-10dB was wider than the others. It was considered that when the PCFs8was 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.