Ultra-broadband microwave absorption of honeycomb-like three-dimensional carbon foams embedded with zero-dimensional magnetic quantum dots

The magnetic response behavior of microwave absorbing materials (MAMs) is beneficial to the expansion of the effective bandwidth in the high-frequency electromagnetic fields. However, the high density of magnetic components does not match with the lightweight requirement of MAMs. In response to this contradiction, the metal-organic framework (MOF) ZIF-67 material and melamine foam were used in the present work as the magnetic component and self-sacrificial template, respectively. A three-dimensional honeycomb carbon foam with zero-dimensional magnetic Co quantum dots (Co-QDs/CF) was prepared by a simple two-step method. The unique honeycomb structure significantly improved impedance matching and carrier mobility, providing more scattering paths for the electromagnetic wave conduction while reducing the overall density. In turn, the uniformly embedded magnetic Co-QDs endowed the 3D carbon foam with excellent high-frequency magnetic response while ensuring light weight. The ultra-small size (<10 nm) enhanced the exchange coupling between magnetic particles while suppressing the eddy current effect, and the magnetic loss capability was greatly improved. Interestingly, Co-QDs acting as numerous micro-resistors formed a large three-dimensional conductive network with the carbon skeleton, which demonstrated outstanding electron transport ability, so that the conductance loss was significantly enhanced. Surprisingly, the effective absorption bandwidth of Co-QDs/CF in the 2–18 GHz band was as high as 8.74 GHz with a matching thickness of 2.5 mm, while the corresponding filling rate was only 20 wt%. Thus, the proposed ingenious strategy to construct three-dimensional conductive networks using ultra-small magnetic quantum dots opens up new prospects for the development of lightweight broadband MAMs.