Electromagnetic wave-absorbing properties of cellulose aerogels with integrated impedance continuous gradient structure

An impedance gradient structure represents an effective design strategy for broadening the frequency band of electromagnetic wave-absorbing materials. However, electromagnetic wave-absorbing materials with impedance gradient are typically prepared using a layer-by-layer composite method, which presents several challenges. These include the potential for easy separation and impedance mismatch between layers, which can lead to limitations in their application and reduced electromagnetic wave-absorbing strength. Consequently, polypyrrole (PPy) was synthesized via gas-phase polymerization along the thickness direction of cellulose nanofiber (CNF) aerogel, resulting in the preparation of CNF/PPy aerogel material with green and pollution-free cellulose CNF aerogel as the substrate. The CNF/PPy aerogel material prepared by this method exhibited an integrated impedance continuous gradient structure. Upon initiation of the polymerization reaction by 0.1 mol/L FeCl₃ for 60 min, the attenuation coefficient of the 7-mm-thick CNF/PPy aerogel material developed in this study was observed to be 2.71, with a characteristic impedance of 383.9 Ω on the electromagnetic wave incident surface. The attenuation coefficient of the electromagnetic wave transmission surface was 131.7, with a characteristic impedance of 132.5 Ω. The dielectric constant and impedance match of the intermediate layer fall between those of the other layers. This effectively realized impedance matching between the material surface and the air, and it also improved the problems of interlayer separation and reflection of the layer structure impedance gradient material. Furthermore, the effective wave-absorbing bandwidth (EAB) covered the entire X-band, and the minimum reflection loss (RLmin) could reach − 16.4 dB. An integrated impedance gradient structure, comprising a CNF/Ti₃C₂T_x/PPy aerogel material, was prepared with a CNF/Ti₃C₂T_x aerogel as the substrate under the same conditions. When the mass fraction of Ti₃C₂T_x is 10%, the EAB encompassed the entire X-band at 5.9 mm, and the RL_min is − 40.9 dB, which greatly improved the wave absorption strength.