Lightweight Fe@C hollow microspheres with tunable cavity for broadband microwave absorption

Lightweight and high-performance microwave absorption materials with wide effective bandwidth are highly required due to the increasingly severe electromagnetic radiation. It is now still imperative to elaborately tune the constituents and microstructures of absorbers for improving microwave absorption capability over wide frequency ranges. Herein, sulfonated polystyrene@Fe(OH)(3)@polydopamine (SPS@Fe(OH)(3)@PDA) microspheres are firstly synthesized with ferric sulphate and dopamine as precursors of Fe(OH)(3) and amorphous carbon, and then thermally annealed at 600 degrees C to generate lightweight Fe@C hollow microspheres. The thermal annealing effectively decomposes SPS templates to gases, and carbonizes the PDA shell to amorphous carbon, which also reduces the ferric irons to magnetic Fe nanoparticles via the carbothermal reduction reaction. Interestingly, the microwave absorption performances of the Fe@C hollow microspheres are obviously improved by increasing the cavity sizes. The Fe@C hollow microspheres with the largest cavity of 780 nm exhibit the broadest effective absorption bandwidth of 7.5 GHz and the superior maximum reflection loss of -37.7 dB. These outstanding performances are attributed not only to the favourable synergistic effect of dielectric and magnetic losses, but also to the optimized hollow structure enhancing the attenuation capability and improving impedance matching. This work provides an efficient approach for designing and fabricating high-performance and broad-band microwave absorbers with tunable peak frequency.