Carbon Nanotubes And Their Polymer Composites Microwave Absorption Properties

Carbon nanotubes (CNTs) with both hollow layer structure and unique electromagnetic properties are ideal quasi-one-dimensional nano-materials. CNTs have specific electromagnetic properties because of special structure, such as high aspect ratio, high surface area, small size et al. Considerable advances have been made experimentally in exploring the electromagnetic properties(especially microwave absorbing properties) of CNTs materials.In recent years, some excellent microwave absorbing materials containing CNTs have been synthesized, having broad application prospects in both military stealth and microwave radiation protectionThe advances in researches for CNTs, such as synthesis and purification, physical and chemical properties, applications prospect, research fields, methods and achievements are reviewed entirely at first.Based on the experimental data for both the electromagnetic parameters and the microwave absorbing properties of CNTs, the microwave absorbing mechanisms of CNTs are investigated and revealed systematically. The following conclusions are obtained. (1) Under microwave irradiation, charge dipoles form on the surface of carbon nanotubes. The interaction of microwave electric field with charge dipoles causes crystal lattice libration, resulting in the losses of microwave energy through heating. (2)For helical CNTs, the cross-polarization of electromagnetic fields will give rise to the chiral microwave absorption. (3)The structural defects of CNTs can enhance charge polarization, which will cause more dielectric loss. (4)CNTs have a series of novel physical effects, such as small size effect, effect of large specific surface area, quanta-size effect and tunneling effect, contributing to microwave absorption. (5)The complex permittivity of CNTs is large, but their microwave permeability is comparatively small. Therefore, intrinsic CNTs display weak absorption to microwave.Based on both the material microstructure-properties correlation theory and an equivalent resistance-capacitance network, the complex conductivity, the relative complex permittivity and electromagnetic wave absorption coefficient of CNTs /polymer composite are theoretically deduced in microwave frequency range using the logarithmic rule. Then a microwave absorption model, which can predict microwave absorbing properties of CNTs/polymer composite, is developed. In this model, the size and shape of CNTs, the real and imaginary parts of the permittivities of CNTs and polymer are considered in detail. It is theoretically proved that the addition of CNTs to the polymer result in explicit increases of the complex permittivity and the microwave dielectric loss tangent absorption of the sample due to the RC network formation and introduction of conducting paths to the composites. And the value of imaginary permittivity of the composite increases more obvious than that of real permittivity with increasing CNTs content percentage in the sample.