Lightweight and highly efficient microwave absorbers based on commercial multiwalled carbon nanotubes

The transmission of information using wireless technology has simplified people’s everyday lives; however, it has resulted in electromagnetic (EM) pollution, which endangers human health and equipment operation. Furthermore, the continuous progress in radar detection technology requires lightweight microwave absorbing materials coated on the surfaces of modern weapons and military equipment, especially aircraft; the objective is to reduce the enemy defense penetration and improve the battlefield survivability of military systems.

Microwave absorbing materials (MAMs) are highly important in protecting human health and ensuring the normal operation of electronic equipment, as well as improving the EM stealth capability of weapons and military equipment. MAMs absorb or attenuate the incident EM waves by converting their EM energy into thermal energy. Therefore, the development of MAMs that can cope with complex EM environments and exhibit broad bandwidth, light weight, small thickness, and strong absorption capability has become the research focus in related fields. In recent years, carbonaceous materials, such as carbon nanotubes (CNTs), carbon nanofibers, reduced graphene oxides, hollow carbon spheres, porous carbon, and onion-like carbon, have been extensively used as dielectric lossy fillers in MAMs because of their low density, good electrical conductivity, high dielectric loss, as well as excellent thermal stability and corrosion resistance. Notably, CNTs with a one-dimensional hollow nanostructure and tunable permittivity have attracted significant attention in the field of EM functional materials. Despite the recent progress in the development of CNT-based MAMs, designing and manufacturing lightweight and highly efficient CNT-based microwave absorbers remains a challenge. The performance of a microwave absorber mainly depends on its relative complex permittivity and complex permeability. Loading a microwave absorber with dielectric fillers has been widely regarded as a simple and effective technique to control its EM properties. Herein, commercial multiwalled carbon nanotubes (MWCNTs) are directly employed as fillers to develop lightweight and highly efficient microwave absorbers by simply adjusting their filling content.

Using an MWCNT filling ratio of only 3 wt% in the paraffin matrix, an excellent absorption performance can be achieved; this is attributed to the improved balance between impedance matching and EM attenuation. For example, an absorber with a 2.5-mm thickness achieves a minimum return loss (RL) of −58.6 dB at 14.3 GHz, and an absorber with a 2.6 mm thickness achieves a maximum effective absorption bandwidth of 6.8 GHz with RL < −10 dB, thus covering the entire Ku band. This absorption performance is significantly better than that achieved in many previously reported carbon-based microwave absorbers with a much higher filling ratio. The simulation results of a radar cross-section (RCS) showed that an absorber with a 3 wt% MWCNT filling ratio achieves a maximum RCS reduction of −38.94 dB·m² and sufficient radar stealth capability, effectively decreasing the possibility of target detection by a radar.

Overall, commercial MWCNTs with excellent microwave absorption and radar stealth performance could be used in the field of EM functional materials, covering a wide range of applications, from EM interference shielding for civilian electronics to radar stealth for advanced military equipment and systems.