Boosting Impedance Matching by Depositing Gradiently Conductive Atomic Layers on Porous Polyimide for Lightweight, Flexible, Broadband, and Strong Microwave Absorption.

Gradient structures are effective for microwave absorbing but suffer from inadequate lightweight and poor flexibility, making them fall behind the comprehensive requirements of electromagnetic protection. Herein, we propose a hierarchical gradient structure by integration with porous and sandwich structures. Specifically, polyimide (PI) foams are used as a robust and flexible skeleton, in which the foam cell walls are sandwiched by TiCT, ZnO, and ZrO atomic layers in sequence. Owing to the decreasing conductivity of TiCT, ZnO, and ZrO, they form gradient impedance matching layers on both sides of the PI foam cell walls, significantly enhancing the absorbing intensity for microwaves. In addition, the porous and sandwich structures can synergistically facilitate multiple reflections, increasing the number of interactions between microwave and foam cell walls. Therefore, the resulting lightweight ZrO@ZnO@TiCT@PI (ZrZnTP) composite foams reach a minimum reflection loss of -68.4 dB with an effective absorbing bandwidth covering the whole X band (8.2-12.4 GHz). The ZrZnTP also exhibits outstanding flexibility even at an extremely low temperature of -196 °C (i.e., liquid nitrogen). This work offers a general approach to realizing hierarchically integrated structures of gradient, porousness, and sandwich structures for lightweight, flexible, broadband, and strong microwave absorbing materials.