Study On The Microwave Absorbing Performance Of Multi-walled Carbon Nanotubes/bismaleimide Composites

Structural radar absorbing materials (SRAMs) can not only absorb microwave but also bear loading, which are usually made up of matrix resin and microwave-absorber. SRAMs are the development trend of RAMs because they have the advantages such as broadening the absorbing band of microwave without increasing the weight of composites, and taking full advantage of design ability of composites to fabricate materials with integrated mechanical and electrical properties. Carbon nanotubes (CNTs) have been considered as ideal reinforcements and functional fillers to prepare high performance nanocomposites due to their excellent physical properties such as extremely high mechanical strength, good electronic and thermal conductivity. Allyl compounds modified bismaleimde (BMI-BA) is a typical resin of advanced polymeric composites (APC). In this paper, CNTs/BMI-BA composites were prepared and their absorbing performances were investigated in first time.Two methods, covalent or non-covalent, were used to functionalize CNTs. As-received multi-walled carbon nanotubes (r-MWCNTs) were purified by refluxing in a mixture of sulfuric acid/nitric acid (3:1 by volume), then acylated with acyl chloride followed by amidating with ethylene diamine to obtain amino-terminated MWCNTs (a-MWCNTs). FTIR was utilized to detect the presence of the grafted molecules. The graft ratio of diamines was quantitatively assessed using thermogravimetric analysis (TGA). Raman spectra of r- MWCNTs and a-MWCNTs indicated that covalent functionalization didn't disrupt the MWCNTs wall integrity. Field emission scanning electron microscope (FESEM) was used to characterize the dispersion of CNTs in BMI-BA matrix. Bisphenol A (BA) functionalized MWCNTs were obtained through 71-71 stacking interaction between MWCNTs and BA. FTIR and UV-vis spectroscopy were used to study the stacking interaction.The final properties of thermosetting composites have close relations with their curing process. Because a-MWCNTs can greatly increase the apparent viscosity of the matrix and react with the matrix resin, it is necessary to investigate curing kinetics of a-MWCNTs/BMI-BA system. The influence of the a-MWCNTs loading content onthe cure reaction and apparent viscosity of the system were studied by using FTIR and ARES, respectively. Isothermal DSC analyses suggested that .the addition of a-MWCNTs changed the cure reaction mechanism of the system and decreased the apparent cure activation energy of the system.Complex permittivity of r-MWCNTs and a-MWCNTs/BMI-BA in the range between 7.5 and 12GHz were characterized using network vector analyzer, and the results show that the absorbing performance of a-MWCNTs is worse than that of r-MWCNTs. While, the changing trend of tenuation factor suggests that a-MWCNTs has good absorbing performance in the higher bands than X-band. To improve the permeability of MWCNTs, capillary method was used to encapsulate magnetic iron oxide into MWCNTs. TEM pictures show that magnetic iron oxide was successfully encapsulated into MWCNTs.