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Based on an equivalent resistance–capacitance (RC) network, we investigate theoretically the complex permittivity and microwave absorption properties of carbon nanotubes (CNTs)/polymer composite in the frequency range of 50 MHz–3 GHz using the logarithmic mixing rule. Both the real and imaginary parts of the permittivities of CNTs and polymer are considered in detail. The simulated results show that the real and imaginary permittivities of the composite increase explicitly with increasing volume fraction of CNTs, and the latter is more sensitive. The calculated complex permittivity spectra of the composite are in good agreement with the available experimental data. In addition, a good linear relationship between microwave absorbance and frequency is found.

The microwave absorbing mechanisms of carbon nanotubes (CNTs), which were grown by iron-catalyzed high-pressure disproportionation (HiPco), are investigated. The influence of Fe nanoparticles on the microwave absorbing properties of CNTs is analyzed. The complex permittivity of hydrogen plasma in HiPco CNTs is deduced using double-fluid theory and phenomenological model. The microwave absorption coefficient of hydrogen plasma in HiPco CNTs is calculated in the range of 0.30–30 GHz under different conditions. The results indicate that strong microwave absorption of unpurified HiPco CNTs results partly from the interaction of the residual Fe nanoparticles with microwave radiation in CNTs, and microwave energy loss (around 2.45 GHz) is predominantly caused by the collisional absorption of the charge particle in hydrogen plasma in HiPco CNTs. The experimental phenomenon of strong microwave absorption (around 2.45 GHz) by HiPco CNTs is well explained by our theory.