Electron Holography of Yolk–Shell Fe3O4@mSiO2 Microspheres for Use in Microwave Absorption

Yolk–shell nanostructures are very promising alternatives as next-generation microwave absorption materials. However, it still remains a great challenge to understand structure–property relationships and develop highly efficient yolk–shell structured microwave absorbers. Herein, a series of dielectric-magnetic yolk–shell Fe 3 O 4 @mesoporous SiO 2 microspheres (denoted as Fe 3 O 4 @mSiO 2 ) were synthesized by combining a modified Stöber method and hydrothermal etching process. The microwave absorption performance of the Fe 3 O 4 @mSiO 2 yolk–shell microspheres could be optimized by tuning the void space and shell number easily. By increasing the average diameter of the void space from 380 to 490 nm, the maximum reflection loss (RL) value of −38.7 dB at 8 GHz was achieved with a composite thickness of 2 mm, and the effective bandwidth (RL < −10 dB) was beyond 4 GHz. Compared with the single-shelled counterparts, the double-shelled Fe 3 O 4 @mSiO 2 microspheres exhibited significantly enhanced absorption properties. Importantly, electron holography analysis confirmed that the enhanced microwave absorption could be ascribed to the unique architecture. Our research can give insights into understanding the structure–property relationships and provide an avenue for fabrication of high-performance and lightweight microwave absorbers based on yolk–shell structures.