Simulation And Optimization On Microwave Absorbing Properties Of Carbon Nanotubes Composite Materials

Microwave absorbing coating are widely used in the national defense of military and civilian anti-microwave radiation technology due to its high effective absorption for a certain band of electromagnetic radiation and better performance of reducing electromagnetic pollution than that of electromagnetic shielding. Composite materials containing carbon nanotubes have a wide spectrum range and high intensity of absorption, which makes them to be one of the hot fields of materials research. So the study of containing carbon nanotubes composite absorbing coatings has a great prospect in military and civilian fields. In this thesis, we mainly focus on simulation of equivalent electromagnetic parameters of composite materials with carbon nanotubes and microwave absorbing properties of carbon nanotubes/epoxy composite absorbents, design for array carbon nanotubes/epoxy composite absorbing coating, optimization of microwave absorption and preparation of carbon nanotubes/magnetic oxide composites.Absorbing properties and impedance of materials depend on its electromagnetic parameters. Different morphology and models of carbon nanotubes would exhibit different absorbing properties, the main reasons are due to different electromagnetic parameters of carbon nanotubes. Based on resistor-capacitor network model of composites (cuming equation) and equivalent circuit model of carbon nanotubes, we studied the expression of equivalent electromagnetic parameters of composite absorbents such as carbon nanotubes/epoxy (carbon nanotubes/paraffin). The results showed that, in the composite material with smaller mass fraction of carbon nanotubes, when carbon nanotube diameter increases from 10nm to 50nm, the real and imaginary parts of complex permittivity are increasing; when the length of carbon nanotubes increases from 5μm to 65μm, the real and imaginary parts of complex are also increasing, The simulation values of complex permittivity with different diameter and tube length are consistent with the experimental results reported in some papers. From the established expression of electromagnetic parameters, we simulate the carbon nanotubes/nickel ferrite composites. The results suggest the values of simulation are consistent with the experimental values.CNT/non-conductive polymer are one of the most simple composite absorbents, so the simulation of absorption for microwave of these composite is simple as well as respectative. Based on electromagnetic wave propagation and the absorption law in loss dielectric and the equivalent transmission line model, reflection ratio expression of arbitrary angle incident microwave was discussed. Based on the calculation model of equivalent electromagnetic parameters, the effect on absorbing properties of carbon nanotubes and epoxy coating among the coating parameters (such as tube length, pipe diameter, mass fraction and coating thickness) were investigated. The result suggests that the simulation microwave reflection ratio of coating is consistent with the experimental value with variation of mass fraction of carbon nanotubes. According to the four dimensional slices among the parameters of mass fraction of carbon nanotubes, coating thickness, frequency and the reflection ratio of microwave vertical incidecing, the design of absorbing coating of carbon and epoxy composites was studied.In array carbon nanotubes/epoxy composite materials, the arrangement and orientation of carbon nanotubes is regular and the mass fraction of carbon nanotubes is big, so the mutual effect of carbon nanotubes is obvious. Therefore, the mutual effect between carbon nanotubes cannot be ignored when equivalent electromagnetic parameters of the array carbon nanotubes are calculated. Based on the electric circuit model of carbon nanotubes, the mutual resistance calculation model of parallel current carrying wires and the theory of equivalent transmission lines, the absorbing properties of the array of carbon nanotubes/epoxy composite coatings and its design of coating were studied at vertical incidence of microwave into the coating. The results show that the law of variation of microwave reflection ratio with coating thickness is close to the experimental results reported, when the distance between carbon nanotubes is 200nm and the tube length is 1mm.Carbon nanotubes have strong dielectric loss ability, but real and imaginary parts of complex permittivity of carbon nanotubes are both very high, so the impedance between carbon nanotubes and air cannot match with each other at all, which leads to the poor absorbing properties of pure carbon nanotubes. Adding magnetic oxides into the carbon nanotubes such as cobalt ferrite or nickel ferrite can reduce the complex permittivity of carbon nanotubes and but increase permeability of materials at the same time. So it can increase the impedance matching ability between carbon nanotubes composite absorbents and air. Using the sol-gel method, cobalt ferrite and nickel ferrite were prepared. Then carbon nanotubes and cobalt ferrite(nickel ferrite) and paraffin were mixed, the four samples were obtained with mass fraction of carbon nanotubes at 10%,20%,30%,40%. The electromagnetic parameters were measured by microwave vector network analyzer. According to electromagnetic parameters of the mixture, the reflection ratio, optimization and design of carbon nanotubes/magnetic oxide/paraffin were studied. The results show that in carbon nanotubes/cobalt ferrite/paraffin system, the microwave reflection ratio is optimum when the mass fraction of carbon nanotubes is 20% and the coating thickness is lmm; while in carbon nanotubes/nickel ferrite/paraffin system, the composite material has wide range absorbing properties when the mass fraction of carbon nanotubes is 10%.