Fe-Doped MnO2 Nanostructures for Attenuation–Impedance Balance-Boosted Microwave Absorption

The attenuation–impedance property plays the key role in the microwave absorption process. In this work, the phenomenon of Fe inducing the gradient phase and morphology transition of MnO2 has been well explored. As the Fe doping content increases, the phase evolution from δ-MnO2 to α-MnO2 and the morphology transition from nanosheets to nanowires occur. Based on this, the multiphase MnO2 nanostructures composed by δ-MnO2 nanosheets and α-MnO2 nanowires are controllably synthesized, in which the former has a high impedance matching characteristic and the latter owns an excellent attenuation ability. By adjusting the Fe3+ doping content, the multiphase MnO2 can realize the balance between attenuation and impedance. When the doping ratio (Fe/Mn) is 7.14 mol %, in the frequency range of 2–18 GHz, the attenuation coefficient of the MnO2 composite is 14.77–194.35, the intrinsic impedance coefficient is 0.29–0.40, and the effective absorption bandwidth (reflection loss < −10 dB) can reach 5.44 GHz when the thickness of the absorbing layer is only 2 mm. This work provides an effective method for designing broadband microwave absorption materials.