Preparation And Characterization Of Electromagnetic Wave Absorbing Materials Based On Carbon

Electromagnetic (EM) wave has been applied in more and more fields with the development of electronic technology. EM wave absorption materials have been drawing much attention of researchers for their importance in solving electromagnetic interference (EMI) and making stealth weapons. It is a focus of research to develop EM wave absorption materials with low density and wide absorbing bandwidth. Carbon based materials have been applied to microwave absorption for a long history and the improvement of their properties is still a focus of research. Novel microwave absorbing materials based on carbon have been developed by carbonizing polyacrylonitrile (PAN) films and PAN/iron particles composites at different temperatures.PAN films were carbonized in lower temperatures (600℃/650℃/700℃/750℃/800℃/850℃/900℃) compared with the normal practice (over1000℃) in order to obtain porous amorphous carbon EM wave absorption materials with relatively lower conductivity of electricity and lower permittivity. The influences of temperature on their permittivity and EM wave absorption capability were also investigated, indicating that real part of relative permittivity decreased with frequency increasing, both real part and imaginary part of permittivity increased with carbonizing temperature increasing and that permittivity of materials carbonized at800℃and above is much bigger that of materials carbonized at750℃and below. In general, the dielectric loss also had rising trend as carbonizing temperature increased. Materials carbonized at700℃,750℃and800℃exhibited relatively better dielectric properties with balance of real part and imaginary part of permittivity, so they were suitable to be used as the matrix of high-performance EM wave absorption materials based on carbon.Based on the research aforesaid, EM wave absorption composite materials with micro-sized magnetic particles dispersed in amorphous carbon were fabricated conveniently and economically by carbonizing PAN/carbonyl iron composites at700℃,750℃and800℃with pure nitrogen as protective atmosphere. The composite materials had good electromagnetic wave absorption properties with both electrical loss and magnetic loss. Effective reflection loss (RL<-10dB) was observed in a large frequency range of6.0GHz and the minimum absorption peak around-33.0dB appeared at approximately15GHz with the absorber thickness of2.0mm for the paraffin samples with composite powders carbonized at750℃. Carbonizing temperature had significant influence on the composition and properties of the composite materials. With carbonizing temperature increasing, both real part and imaginary part of permittivity became bigger, the absorption peaks got sharper and the EM loss in low frequency range became more efficient.Protective atmosphere may also influence the phase composition, so a series of materials composed of different magnetic particles from those carbonized in pure nitrogen were fabricated by carbonizing PAN/carbonyl iron composites at700℃,750℃and800℃with atmosphere with little oxygen instead of pure nitrogen as protective atmosphere. The composite materials also had relatively good electromagnetic wave absorption capability with electrical loss and magnetic loss. Effective reflection loss (RL<-10dB) was observed in a large frequency range of4GHz and the minimum absorption peak around-19dB appeared at approximately11.5GHz with the absorber thickness of2.0mm for the paraffin samples with composite powders carbonized at800℃.EM wave absorption composite materials with nano-sized magnetic part-icles dispersed in amorphous carbon were fabricated by carbonizing PAN/na-no-sized iron particles composites at750℃and800℃with pure nitrogen as protective atmosphere. The composite materials carbonized at800℃exhibited much better electromagnetic wave absorption capability than that at750℃Effective reflection loss (RL<-10dB) was observed in a large frequency range of5.0GHz and the minimum absorption peak around-38.0dB appeared at approximately13.0GHz with the absorber thickness of2.0mm for the paraffin samples with composite powders carbonized at800℃.Compared with the amorphous carbon absorption materials, the composite absorption materials based on carbon carbonized all exhibited much better absorption capability, indicating the feasibility of the modification method.The minimum RL peaks were found to move toward lower frequency region with increasing thickness, which is a common rule for all the materials aforesaid. It was obvious that the samples with composite powders carbonized at750℃and800℃had much more effective reflection loss than that carbonized at only700℃both in pure nitrogen and atmosphere with little oxygen. This is because that the dielectric loss is still the key loss mechanism for the lowness of permeability due to the relative lack of magnetic particles in all the composite materials and that dielectric loss increases with carbonizing temperature increasing.