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This study is focused on the spinel structure of metal oxides and sulfides, from which the ternary (NiCo2O4/NiCo2S4) and quaternary (Fe0.5Ni0.5Co2O4/Fe0.5Ni0.5Co2S4) samples with hollow sphere structures were prepared. Among these samples, Fe0.5Ni0.5Co2S4 was highly effective in its ability to attenuate electromagnetic waves, wherein a broader absorption bandwidth of 6.2 GHz could be achieved with a thinner coating layer of 1.3 mm. The cation distribution rule for Fe, Co and Ni ions in the spinel structure is given according to hybrid orbital theory to support the excellent electromagnetic absorption properties. Relying on the distribution of Fe, Co and Ni cations, the probable electron transmission and coupling between Fe3+/Co2+ and Fe3+/Fe3+ adjacent cation ion pairs could occur at the octahedral site (B site), which reflects the enhanced dielectric loss. [ABSTRACT FROM AUTHOR]

Hierarchical hollow carbon@Fe@Fe3O4 nanospheres were synthesized by a simple template method and another pyrolysis process. Interestingly, the thickness of hollow carbon spheres is tunable by a simple hydrothermal approach. The as-prepared carbon@Fe@Fe3O4 shows excellent microwave absorption properties. In detail, the maximum effective frequency is up to 5.2 GHz with an optimal reflection loss value of −40 dB while the coating thickness is just 1.5 mm. Meanwhile, such absorption properties can be maintained via controlling the thickness of the hollow carbon. For instance, in another coating layer of 2 mm, the effective frequency is still more than 5 GHz as the carbon thickness declines to 12 nm. As novel electromagnetic absorbers, the composites also present the lower density feature due to the hollow carbon sphere frame. The excellent electromagnetic absorption mechanism may be attributed to the obvious interface polarization, and strong magnetic loss ability resulting from the Fe and Fe3O4 shell. Besides, owing to the dielectric feature of carbon, the hollow carbon core is beneficial for the attenuation ability. [ABSTRACT FROM AUTHOR]

Impedance matching and the attenuation constant, α, are two key parameters in determining electromagnetic absorption properties. Although materials with single magnetic or dielectric loss properties have a high α value, they nonetheless suffer from poor impedance matching. The design of magnetic and dielectric composites might possibly be an effective method of solving this problem, but unfortunately the introduction of magnetic material may give a poor value of α. In order to obtain absorptive materials with high impedance matching and a high value of α, we have designed a novel ternary composite of MnO2@Fe–graphene. A 30 nm wide rod-like strip of MnO2 was first obtained by a simple liquid process. Liquid decomposition of Fe(CO)5 was then carried out to deposit iron on the surface of the rod-like structure, and the MnO2@Fe was finally loaded on graphene by a liquid deposition technique. The resulting ternary composite exhibited attractive electromagnetic absorption properties, in which the optimal reflection loss of up to −17.5 dB obtained with a thin coating thickness of 1.5 mm was able to satisfy the requirements of lightness of weight and a high degree of absorption. The effective bandwidth frequency of MnO2@Fe–GNS is broader than that of pure MnO2 or MnO2@Fe, possibly due to its moderate impedance matching and attenuation ability. The possible attenuation mechanism will also be discussed. [ABSTRACT FROM AUTHOR]