Microporous Co_1−xFe_x@C nanoparticles: Strong wideband microwave absorbers for reflection loss less than −20 dB

For advanced microwave absorption materials, the bandwidth of reflection loss (RL) ≤ −20 dB is becoming more and more important due to their applications under some extreme occasions where most of the electromagnetic signals must be absorbed. Herein, in-situ carbon-coated Co-Fe-Al precursor nanoparticles (NPs) were prepared by hydrogen plasma metal reaction (HPMR), and then microporous Co_1−xFe_x@C NPs with an average particle size of about 50 nm were successfully prepared by chemical dealloying. They exhibit a distinct core-shell structure with an amorphous carbon shell of approximately 3 nm and a Co_1−xFe_x alloy core containing micropores of 1.1 nm. As the amount of Fe in the microporous Co_1−xFe_x@C NPs increases from 2.1 wt% to 12.0 wt%, the saturation magnetization gradually enlarges from 82.6 to 92.4 emu g⁻¹ whereas the coercive force only decreases slightly. The Co_0.98Fe_0.02@C sample shows the widest bandwidth (RL ≤ −10 dB) of 11.3 GHz, and surprisingly the bandwidth corresponding to the RL below −20 dB reaches 6.0 GHz, far superior to other Co-based absorbers. Meanwhile, its minimum RL (RL_min) value reaches −102.3 dB with a thickness of 1.8 mm. As the content of Fe increases to 12.0 wt%, the absorption bandwidth (RL ≤ −20 dB) of Co_0.88Fe_0.12@C sample decreases to 5.7 GHz while its RL_min value strengthens to −133.7 dB. Both the two samples added with Fe exhibit much wider absorption bandwidth (RL ≤ −20 dB) compared with Co@C NPs without Fe addition. The excellent absorption properties are mainly attributed to the facts that microporous morphology promotes the interfacial and dipole polarization, and Fe addition enhances the magnetic resonance. The Co_1−xFe_x@C NPs may pave a new way for designing effective broadband absorbers for RL ≤ −20 dB to meet the high-performance requirements.