CoFe2O4 and/or Co3Fe7 loaded porous activated carbon balls as a lightweight microwave absorbent

In order to prepare a lightweight and efficient microwave absorbent, porous activated carbon balls (PACB) were used to load Fe3+ and Co2+ ions, because the PACB carrier has a high specific surface area of 800 m2 g−1 and abundant pores, including micropores and macropores. The loaded Fe3+ and Co2+ ions in the PACB composite were transformed into magnetic CoFe2O4 and/or Co3Fe7 particles during subsequent heat-treatment under an Ar atmosphere. According to the XRD and SEM results, the magnetic particles were embedded in the PACB macropores and showed different crystalline phases and morphologies after heat-treatment. CoFe2O4 flakes with spinel structure were obtained at approximately 450 °C, and were then transformed into loose quasi-spheres between 500 °C and 600 °C, where CoFe2O4 and Co3Fe7 coexisted because of the partial reduction of CoFe2O4. Co3Fe7 microspheres appeared above 700 °C. The density of the magnetic PACB composites was in the range of 2.2–2.3 g cm−3. The as-synthesized PACB composites exhibited excellent microwave absorbability, which was mainly attributed to the magnetism of CoFe2O4 and Co3Fe7, as well as the presence of graphitized carbon. The minimum reflection loss value of the CoFe2O4–Co3Fe7–PACB composite reached −32 dB at 15.6 GHz, and the frequency of microwave absorption obeyed the quarter-wavelength matching model, showing a good match between dielectric loss and magnetic loss. The microwave reflection loss (RL) value could be modulated by adjusting the composition and thickness of the PACB composite absorbent. PACB composites with CoFe2O4–Co3Fe7 are a promising candidate for lightweight microwave absorption materials.