Effects of carbon nanotubes on the AC conductivity, permittivity, and electromagnetic shielding performance of polyurethane foams in the 15 Hz–26.5 GHz frequency range

This study investigates the impact of increasing carbon nanotube (CNT) concen- tration on the AC conductivity, dielectric, and electromagnetic shielding proper- ties of polyurethane (PU) foams. Four CNT-PU foam samples with different CNT contents (1 wt.%, 2.3 wt.%, 6 wt.%, and 11.2 wt.%) were fabricated using the dip coating method. Three different measurement techniques were employed to char- acterize the AC conductivity and complex permittivity of the CNT-PU composites across various frequency ranges (kHz, MHz, and GHz). COMSOL software was also used to calculate the scattering parameters and visualize the electric field distribution. The microstructure SEM images of CNT-PU samples confirmed that the CNTs formed interconnected and well-distributed networks on the PU surface skeleton, leading to conductive pathways that boost the absorption loss. Therefore, increasing CNT content increased the permittivity and conductivity of CNT-PU samples, and consequently, the total shielding effectiveness (SET). The incorporation of 11.2 wt.% CNT greatly enhanced the AC conductivity, increas- ing it from 3.6 × 10–6 S/cm (pure PU) to 0.0068 S/cm at 10 MHz. Furthermore, the corresponding values of complex permittivity also increased from 1.32—j 0.0059 to 8.65—j 3.7 at 1000 MHz. The complex permittivity of 11.2 wt.% CNT sample was 3.76—j 3.56 at 25 GHz. The samples with 6 wt.% CNT and 11.2 wt.% CNT met the standard requirements for the electromagnetic interference shielding applications because their total shielding effectiveness (SET) values surpassed the 20 dB threshold within the 18–26.5 GHz frequency range. The visualization of the electric field distribution further confirmed that the 11.2 wt.% CNT sample greatly attenuated the electric field propagation through the material. The contribution of the absorption loss mechanism to the SET of CNT-PU samples surpassed 94% in the 18–26.5 GHz frequency range, indicating that the absorption loss was the main shielding mechanism in our samples.