Fe₃O₄ Nanoparticle/N-Doped Carbon Hierarchically Hollow Microspheres for Broadband and High-Performance Microwave Absorption at an Ultralow Filler Loading

Although the existing Fe3O4-based microwave absorbing materials (MAMs) have shown promising microwave absorbing (MA) capacity, it is highly desired but still remains a great challenge to achieve strong minimum reflection loss (RLmin) and broad effective frequency bandwidth (fe) at an ultralow filler loading. Herein, for the first time, by carbonizing hierarchical poly(urea-formaldehyde) microcapsules with Fe3O4 nanoparticle cores in a nitrogen atmosphere, Fe3O4 hybrid and N-doped hollow carbon microspheres (Fe3O4/CMs) with a hierarchical micro/nanostructure are prepared on a large scale and at a low cost to achieve extremely superior MA performances. Benefitting from their unique structure and diverse composition, which synergetically contribute to good impedance matching, strong dielectric/magnetic loss, and abundant multiscattering/reflection, Fe3O4/CM composites possessed a RLmin value reaching −60.3 dB and an fe of as broad as 6.4 GHz (7.2–13.6 GHz, covering the full X-band) at an ultralow filler loading of 10 wt % in paraffin wax, which are significantly superior to those of the previously reported state-of-the-art Fe3O4-based or hollow MAMs. Furthermore, the fe can be adjusted in the range of 4.5–18 GHz, covering 85% of the whole measured frequency range, via changing the thickness between 2.5 and 5.5 mm. This work offers new insights for developing advanced lightweight MAMs with strong absorption and a broad absorbing frequency range at a low filler loading.