Dual-Gradient Structure Design in Polyimide Foam for Efficient and Ultra-Broadband Microwave Absorption

As pivotal lightweight materials for electromagnetic protection architectures, polymer composite foams mainly rely on multilayered stacking with electromagnetic-property gradients to attain broadband microwave absorption (MA). Although such structural designs enable simultaneous enhancement of electromagnetic attenuation and impedance matching, yielding satisfactory MA performance, inherent limitations, including prohibitive thickness and susceptibility to interlayer delamination, substantially compromise structural integrity and protective efficacy. Herein, magnetic field induction and component regulation are combined to manipulate foaming kinetics of the in situ chemical foaming strategy, achieving dual-gradient structure with coordinated distributions of functional particles and pore size in polyimide foam. This monolithic architecture can avoid the interface bonding defects of multi-layer foams while maximizing the reduction of impedance mismatch caused by the electromagnetic property discrepancy between layers, thereby further broadening the effective absorption bandwidth (EAB) of 14.1 GHz. The intrinsic mechanism of dual-gradient structure on the MA performance is revealed via multiscale simulation from micro and macro levels. Consequently, the dual-gradient MA foam shows exceptional compression resistance, thermal insulation, and flame retardancy, demonstrating broad applicability in extreme environments with high temperature and compression.