Well-Defined Core-Shell Fe₃O₄@Polypyrrole Composite Microspheres with Tunable Shell Thickness: Synthesis and Their Superior Microwave Absorption Performance in the Ku Band

Highly regulated core-shell Fe₃O₄@polypyrrole composite microspheres have been successfully prepared via chemical oxidative polymerization in the presence of poly(vinyl alcohol) and p-toluenesulfonic acid. The polypyrrole shell thickness can be adjusted from 20 to 80 nm with the variation of the pyrrole/Fe₃O₄ ratio. Investigations of the microwave absorbing properties indicate that the polypyrrole shell plays an important role, and the maximum reflection loss of composite microspheres can reach as much as -31.5 dB (>99.9% absorption) at 15.5 GHz with a matching layer thickness of 2.5 mm. Compared to the physically blended Fe₃O₄-PPy composites, Fe₃O₄@polypyrrole composite microspheres not only possess better reflection loss performance but also have a wider absorbing bandwidth of 5.2 GHz (12.8-18 GHz) in the Ku band, which may be attributed to the intensive synergistic effect of dielectric loss from polypyrrole shells and magnetic loss from Fe₃O₄ cores. Therefore, regulated core-shell Fe₃O₄@polypyrrole composite microspheres are postulated to be more promising in microwave absorption applications.