The Significance of Aspect Ratio in the Complex Permittivity of Single and Multi-walled Carbon Nanotube Composites

As 5G technology becomes ubiquitous, the threat posed by radio frequency interference coupling electromagnetic (EM) energy into susceptible electronic circuits grows in tandem. Mitigating this threat requires compact, lightweight, frequency tuned EM absorbers to protect sensitive electronics. Carbon nanotubes (CNTs) are effective additives for tuning the complex permittivity and achieving large losses in thin polymer composites; CNT aspect ratio, the ratio of length to diameter, plays a pivotal role in determined both permittivity and loss. We experimentally investigated the relationship between aspect ratio, volume loading, complex permittivity, and frequency dependent EM absorption in single and multi-walled CNTs. We discuss our results linking larger aspect ratios to increased real permittivity, independent of frequency, in the context of the effective medium theory for conducting "sticks" proposed by Lagarkov and Sarychev. We further consider various relaxation mechanisms to explain the more complex, frequency dependent, relationship between imaginary permittivity and aspect ratio up to 6 GHz.