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

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 N-2, 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.