NiFe2O4, Fe3O4–FexNiy or FexNiy loaded porous activated carbon balls as lightweight microwave absorbents

Abstract

Porous activated carbon balls (PACB) with high specific surface area of 985.5 m² g⁻¹ and abundant pore structure were used as a matrix to synthesize a series of magnetic PACB composites through in situ impregnation of Fe³⁺ and Ni²⁺ ions as well as subsequent carbothermic reaction. The experiments involved the preparation of Fe³⁺–Ni²⁺ loaded PACB and the heat-treatment process in an Ar atmosphere. The morphology, from flakes to microspheres, coupled with the crystalline phase of the magnetic constituents in PACB, from Fe₂O₃–NiFe₂O₄ to Fe_0.64Ni_0.36–FeNi, can be well controlled by adjusting the reaction temperature. It was revealed that the framework structure of PACB was not destroyed during the carbothermic process, and the high surface area (713.2–808.2 m² g⁻¹) of PACB composites was still preserved, providing sufficient space to form an effective interface between the magnetic constituents and the PACB. The PACB composites exhibited excellent microwave absorption, a minimum reflection loss (RL) of −30 dB and effective bandwidth (RL below −10 dB) of 5.04 GHz were observed for the Fe_0.64Ni_0.36–FeNi/PACB composite when the coating thickness reached 2 and 1.7 mm respectively. Importantly, the absorption bandwidth covered the whole frequency range (2–18 GHz) with a thickness of 1.4–8 mm. The introduction of more alloy phase benefited the relative complex permittivity and permeability of the PACB composites, which in turn enhanced the microwave absorption. The microwave absorbability could be tuned via changing the coating thickness and composition of the absorbent, showing that the as-synthesized PACB composites were versatile in practical application. By considering the high surface area, low density (1.1–1.3 g cm⁻³) and remarkable absorption properties, we believe the PACB composites can be promising candidates as lightweight and effective microwave absorbents.