MnFe2O4-coated carbon nanotubes with enhanced microwave absorption: Effect of CNT content and hydrothermal reaction time

Carbon nanotubes (CNT) coated with magnetic nanoparticles are promising microwave absorbing materials with high performance. Despite several previous investigations, the relationship between intrinsic composition and preparation conditions of this composite material to its microwave loss mechanisms and absorbing properties have not yet been fully understood. We present a convenient and cost-effective hydrothermal method to deposit crystalline manganese iron oxide (MnFe2O4) over CNT. Adjusting the hydrothermal reaction time and CNT content allows control over the microstructure of the resulting CNT-MnFe2O4 nanocomposite and its microwave absorbing properties, resulting in a better microwave absorber than pure MnFe2O4. Composites synthesized at 150 °C for 10 h with 10 wt% CNT reveal a reflection loss as high as −38 dB at 5.7 GHz for an absorber thickness of 5 mm and frequency bandwidth (RL < −10 dB) up to 3.59 GHz. As the hydrothermal reaction time is reduced to 1 h, the minimum reflection loss increases significantly to −41 dB at 11.8 GHz for a 2 mm coating thickness. Defects and functional groups on the surfaces of CNT act as nucleation sites for MnFe2O4 and help reduce agglomeration, which decreases overall particle size. Microwave absorbing mechanisms are also discussed. Coating CNTs with manganese iron oxide increases microwave dissipation by enhancing conduction loss, interface polarization, relaxation polarization, and magnetic loss, including the contributions of eddy current and natural resonance, which are all related to the reaction time and CNT content. Consequently, the synthesis-property relationship could potentially be used to design CNT-based nanomaterials that have more effective microwave absorption.