Hollow Core–Shell Fe₃O₄@PPy Nanocomposites with Tunable Microwave Absorption Frequencies and High Reflection Loss

Core–shell structures are commonly employed in microwave absorption (MA) materials due to their distinctive interfacial architecture. In this study, ferroferric oxide@polypyrrole (Fe 3 O 4 @PPy) nanocomposites with hollow core–shell structures were successfully synthesized for MA using a solvothermal process, followed by in situ polymerization. The minimum reflection loss (R L min ) frequency of the nanocomposites can be precisely controlled by adjusting the thickness of the PPy shell. The thickening of the PPy shell enhances the conduction loss of the nanocomposites, thus, enhancing the dielectric loss of the nanocomposites. The fabricated FP-170, FP-180 and FP-190 nanocomposites exhibit remarkable MA properties at low frequencies, with their R L min reaching −63.82 dB, −84.92 dB, and −71.25 dB at 4.55 mm, 3.87 mm, and 2.64 mm, respectively. The maximum effective absorption bandwidth (EA B max ) of FP-170 extends to 3.48 GHz at 2.39 mm, FP-180 extends to 4.20 GHz at 2.38 mm, and FP-190 extends to 4.96 GHz at 2.16 mm. The FP-200 nanocomposite, which has PPy shell thickness of 15 nm, exhibits exceptional radar stealth performance, with Radar Cross-Section (RCS) values below −17.68 dB·m 2 within the range of −90° to 90°. Both simulation and experimental results prove that the hollow core–shell structured Fe 3 O 4 @PPy nanocomposite is an excellent candidate for MA applications.