Preparation And Electromagnetic Characters Of Coated Composite Short Carbon Fibers

In recent years short carbon fibers (SCF) have been received increasing attention, due to their high strength and modulus, good conductivity and low density which makes them suitable for light-weight shielding materials. However, there are two obvious weaknesses in the electromagnetic shielding coatings filled with SCF, the poor dispersion stability of SCF in coatings and the lower electromagnetic characteristic of SCF materials difficult to reach the application demands of new shielding materials. One of the effective methods to improve the performances is the material composite technology. In this dissertation, several coated models were proposed to fabricate composite materials to improve the dispersion stability and electromagnetic of modified SCF based on the electromagnetic, materials science theory and effective electromagnetic parameters theory of the composite materials. The effective electromagnetic equations were deduced based on the theory of the composite materials effective mediums. Multilayer electromagnetic shielding materials were prepared by the modified SCF. The research results afford theory and technology guidance, meanwhile it may give a new design method and experiment base for the same composite materials.From physics and materials science point of view, models of conducting polyaniline-coated SCF and conducting polyaniline and magnetic materials-coated SCF were proposed to improve the performance of SCF. The effective parameters equations about the fiber composites were educed. Theoretic base was afforded to this dissertation.According to single layer coated effective electromagnetic parameters theory and the conducting polymer coated SCF model, the conducting polyaniline-coated SCF (PAOSCF) was prepared. Conducting polyaniline was polymerized by in-situ polymerization and subsequent deprotonation on the surface of oxidized SCF. The effect of nAPS/AN, hydrochloric acid concentration, reaction temperature and reaction time on the structure and electric properties of polyaniline-coated SCF were studied. It was found that use of ultrasonic dispersion improved the rate of reaction and decreased the polymerization time. In the experiment condition of the dissertation, when the nAPS/AN 1.5, hydrochloric acid concentration 1.0mol/L, reaction temperature 10℃±3℃, and reaction time 12h,the PAOSCF with 0.084?˙cm surface resistance, good adhesion and compact coated layer were obtained. XPS element analysis of PAOSCF is shown that PANI bonded on the surface of oxidized SCF with–CO-N- chemical bonds, which is consistent with the design model of polyaniline-coated SCF. It was also found that polyaniline coated layer can enhance the complex permittivity of PAOSCF, propitious to increasing absorption loss. However, the complex permeability is 1-0j, because conducting polyaniline is electrical loss material.According to complex composite effective electromagnetic parameter theory, the conducting polyaniline and magnetic powders coated SCF were prepared. The mechanism of the conducting polyaniline and magnetic powder coating on oxidized SCF was studied by EDS and XPS. The results indicated that the Fe…O-C- chemical bonds were formed between polyaniline and magnetic powder, the polyaniline coated magnetic powder formed a conducting and magnetic layer on the surface of oxidized SCF with–CO-N- chemical bonds, which is consistent with the design model of conducting polyaniline and magnetic -coated SCF. The complex permittivity and complex permeability of conducting polyaniline and magnetic -coated SCF was studied. The results showed that magnetic powders can improve complex permeability, and improve magnetic loss of modified SCF, but the dielectric performance of modified SCF is decreased because magnetic powder may break the structure of conducting polyaniline and the gateway of electron.Non-conducting polyaniline coated SCF (NPASF) was studied and exploited to meet the especial demand of shielding materials for protecting the electronic equipments which do not form short circuit on the surface. The results showed that NPASCF with low magnetic loss took on electromagnetic wave attenuation characteristic.Finally, the analysis of the component, structure to CAD of shielding performance of the modified SCF system f<2GHz was completed in the dissertation. This CAD method may give a new method to evaluate the shielding effect of composite fillers applied in coating. The modified SCF coatings were prepared. The results of the shielding effectiveness showed that the modified SCF system can improve the absorption loss, according with the development direction of new shielding materials.