In-situ Catalytic Growth Of Carbon Nanofibers On Graphite And Their Microwave Absorption Properties

Flake graphite has been used in the military application of electromagnetic waveabsorption since the World War II. But with the development of technologies of themilitary and civilian devices, conventional absorbers can no longer meet the increasingrequirements such as thinner thickness, lighter weight, broader bandwidth, higherabsorption and stronger temperature and chemical resistance. Graphite has highelectricity and permittivity. However, its significant anisotropy makes theelectromagnetic wave from certain direction reflect directly, thus failing to be absorbed.One dimensional nanocarbons such as carbon nanofibers (CNFs) and chiral carbonnanospirals (CNSs) can be used as an ideal fillers owning to their unique morphologiesand structures. Nevertheless, the nano-size and the tendency to aggregate make themdifficult in dispersion so that their superiority in microwave absorption can hardly bepresented. In addition, the chiral-CNSs possess various extraordinary physicalproperties, but their growth mechanism are currently uncertain, thus hindering theircontrollable preparation and pratical applications.In view of the above issues, based on the layered structure of graphite, weproposed to combine the catalysts with the graphite substrate and prepared two kindsof reaction precursors. One is graphite oxide (GO)/transition metal salts and the otheris metal chlorides-graphite intercalation compounds (GICs). Then we used theseprecursors to in-situ grow one dimensional nanocarbons such as CNFs and CNSs onthe graphite substrate by CVD method with acelytene as carbon source, to constructthree kinds of three dimensional carbon composites, graphite/carbon nanofibers(G/CNFs), graphite/carbon spiral nanofibers (G/CSNFs) and graphite/carbon spiralnanoribbons (G/CSNRs), aiming at adjusting the anisotropy of pure graphite andimproving the poor dispersion of one dimensional carbon nanomaterials to take fulladvantage of their microwave absorption properties. We examined the influences ofseveral primary processing parameters during the preparation on the microstructuresof the as-obatained products and explored the formation mechanism of G/CNSs,offering the references for the future controllable synthesis of chiral microwaveabsorbers. Additionally, we made studied G/CNFs, G/CNSFs and G/CNSRs intomicrowave absorbing materials, measured the relative electromagnetic parameters andcalculated the theoretical reflection loss at different addition content and thickness.Lastly, we evaluated the microwave absorption properties of those three-dimensional carbon composites. The main results are as following:(1) G/CNFs can be successfully prepared by CVD method and using GO/transitionmetals as precursors, acetylene as carbon source and hydrogen as reducing gas.Through the investigation the effects of the type and content of transition metal andtemperature, we found that the catalytic effect of Fe(NO3)3on the substrate of GO wassuperior to that of Cu(NO3)2at650℃, and when the content of Fe(NO3)3was increasedto0.3mol/L, the diameter of the as-grown CNFs were decreased. In addition, Fe(NO3)3had no catalytic function when the temperature was as low as580℃, but overheat canalso cause the weakening of catalytic properties.(2) Metal chlorides-GICs are excellent reaction precursors for the formation ofthree-dimensional carbon composites. Using FeCl3-CuCl2-GICs as precursors, we canobtain two kinds of G/CNSs, i.e., G/CSNFs and G/CSNRs. Temperatures, the contentsof acetylene, the compositions of atmosphere and the types of GICs all playedimportant roles in the microstructures of the products. CSNRs can be catalyzed andgrown by tenary FeCl3-CuCl2-GICs at720℃while CSNFs can at790℃. When thetemperature was set as720℃, CSNRs can be formed by FeCl3-CuCl2-GICs at33%ofacetylene and CSNFs can be synthesized at50%of acetylene. CNSs of larger size canalso be prepared in the atmosphere of acetylene and argon. The main products of binaryFeCl3-GICs were G/CNFs and none CNSs were observed.(3) During the formation process of the three-dimensional carbon structures,tenary FeCl3-CuCl2-GICs played the role of catalyst carriers and graphite substrates.Owning to their unique structures, intercalated metal chlorides can be deintercalatedtogether and formed active catalyst particles to react with acetylene or hydrogen. Lastly,due to the disparity of carbon solubility with different metals, carbon atomsprecipitated from the surface of catalyst particles inhomogeneously, thus forming spiralmorphology in the end. Meanwhile, the diameter and thickness of CNSs are dependenton the size of catalyst particles.(4) G/CNFs, G/CSNFs and G/CSNRs can show great microwave absorptionproperties at the thickness range of1.3-1.6mm. G/CNFs enjoy strong absorption in thelower frequency range. G/CSNFs possess relative broader absorption bandwidth whichcan cover the whole X or Ku band. G/CSNRs can not only have large reflection loss,but also keep the same bandwidth at a wide thickness range, allowing for a relative lowdemands for the machining accuracy. As a result, we can choose among them asmicrowave absorbers according to the practical requirements.