Electromagnetic wave absorbing properties of SrFe12-2xCoxTixO19 hexaferrite–CNT–epoxy composites

Abstract

A wideband electromagnetic (EM) wave absorber for the X-band (8–12 GHz) is designed using M-type hexaferrite, SrFe12-2xCoxTixO19 (x = 1.1, 1.2, 1.25, and 1.3)–CNT (0–3 wt%)–epoxy (10 wt%) composites. As x increases from 1.1 to 1.3, the ferromagnetic resonance (FMR) of the hexaferrite powder gradually decreases from 15.0 GHz to 6.4 GHz due to the decrease of magnetocrytalline anisotropy. The EM absorption frequency range changes in accordance with changes in the FMR frequency. Among the series of samples, the x = 1.25 sample demonstrates EM absorption properties optimized for the X-band with the lowest reflection loss (RL) of −40 dB and satisfying RL <−10 dB in the 7–13 GHz range. When 1 wt% carbon nanotubes (CNTs) are added to the composite (x = 1.25), permittivities ε′ and ε″ increased significantly owing to the CNT particle’s high conductivity, and the intensive EM absorbing region expanded to a wide frequency–thickness range. A further increase in the CNT content (2–3 wt%) weakened the overall EM absorption due to an increase in reflection. Adding 1–2 wt% of CNT to hexaferrite is very effective because it not only increases the shielding effectiveness, but also yields better EM absorption at lower thicknesses of 1.5 mm or less.