Lightweight porous Co3O4 and Co/CoO nanofibers with tunable impedance match and configuration-dependent microwave absorption properties

Abstract

In this study, we fabricated one-dimensional porous Co₃O₄ and Co/CoO nanofibers by calcination of cobalt(II) oxalate dehydrate precursors in an environment filled with air and N₂, respectively. The porous configurations of Co₃O₄ and Co/CoO nanofibers are determined by the calcination temperatures, which can effectively tune complex permittivity and impedance match. The minimal reflection loss of porous Co₃O₄ nanofibers is −23.8 dB at 11.4 GHz with a thickness of 2.0 mm, which results from good impedance match, dielectric loss (dipole polarization) and one-dimensional shape effect (point discharge as an antenna receiver, and long microwave travel distance due to multiple reflections and scattering). For the porous Co/CoO nanofibers, in comparison with Co₃O₄, due to the existence of magnetic Co, they exhibit better microwave absorption properties. The minimal RL value is −48.4 dB at 16.1 GHz, and the effective absorption (RL below −10 dB) can reach 4.2 GHz (13.8–18 GHz) with a thickness of only 1.5 mm. Besides, in terms of the above mentioned absorption mechanisms of porous Co₃O₄, the magnetic loss (natural and exchange resonance) and interfacial polarization between Co and CoO also contribute to the microwave absorption. These one-dimensional porous Co₃O₄ and Co/CoO nanofibers are proved to be efficient and lightweight absorbers with promising applications.