This review aims to summarise the progress in some materials and structures for electromagnetic applications, such as microwave absorption, electric shielding and antenna designs, which have been developed in recent years. Composites with spherical powders for microwave absorption focus mainly on those based on ferrites (especially hexagonal), carbonyl iron and related alloys and various newly emerged nanosized materials. Composites with long conductive fibres as fillers will be summarised, with speical attentions to prediction, measurment and evaluation of their performances. Metamaterials include structures for microwave absorbing applications, tunable materials or structures with reflection or transmission coefficients that are tunable by external magnetic or electric fields, and specially designed structures for microwave absorbing applications, with thickness much smaller than that of conventional composite materials and performances that can be optimised by the physical properties of substrates, and new metamaterials constructed with ferrite cores wound by metallic wire coils that exhibited unique magnetic properties, with extremely high real and imaginary permeability, which are adjustable or tunable by varying their configurations. Magnetodielectric materials, with matching permeability and permittivity, together with sufficiently low magnetic and dielectric loss tangents, with potential applications in antenna miniaturisation, will be discussed.

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Developing an ultimate electromagnetic (EM)-absorbing material that can not only dissipate EM energy but also convert the generated heat into electricity is highly desired but remains a significant challenge. Here, we report a hybrid Sn@C composite with a biological cell-like splitting ability to address this challenge. The composite consisting of Sn nanoparticles embedded within porous carbon would split under a cycled annealing treatment, leading to more dispersed nanoparticles with an ultrasmall size. Benefiting from an electron-transmitting but a phonon-blocking structure created by the splitting behavior, an EM wave-electricity device constructed by the optimum Sn@C composite could achieve an efficiency of EM to heat at widely used frequency region and a maximum thermoelectric figure of merit of 0.62 at 473 K, as well as a constant output voltage and power under the condition of microwave radiation. This work provides a promising solution for solving EM interference with self-powered EM devices.

In this study, we report a porous-carbon-based Mo2C nanocomposite (NCs) as the microwave absorber via typical carbothermal reduction using metal-organic frameworks (MOFs) and polyoxometalates (POMs) as the precursors, which have been rarely applied in electromagnetic (EM) wave absorption areas. The elaborately designed NCs not only bring about good impedance matching, but also possess strong dissipation ability due to the large surface areas and porous features. Thanks to the material characteristics as well as structural advantages, the as-prepared Mo2C/C NCs with 20 wt% sample loading exhibit remarkable microwave absorbing performance. The minimum RL value reaches -49.19 dB at matching thickness of 2.6 mm, and the best effective bandwidth (RL < -10 dB) of 4.56 GHz at 1.70 mm was also achieved. Moreover, the NCs overcome the intrinsic drawback of traditional carbon materials, that is, centralized effective absorption always occurred at high frequency (>10 GHz) and the minimum RL value of the NCs shifted to 9.04 GHz. Clearly, in this study, we not only developed the Mo2C NC as the new light absorber, but also paved the way to synthesize other available transition metal carbides using MOFs and POMs.

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It is still a formidable challenge to ameliorate the low-frequency electromagnetic property of conventional microwave-absorbing materials, which may be conquered by the coexistence of both strong dielectric and magnetic loss ability in low-frequency range and the perfect balance between complex permittivity and permeability with the help of structural design. Herein, by virtue of appropriate composition and structure of Co[HCOO]·dimethylformamide parallelepipeds, one-dimensional spongelike metallic Co can be directly synthesized for the first time with strong magnetic loss in the low-frequency range. Furthermore, attenuation ability and impedance matching condition have been improved through the construction of interfacial structures between inner cobalt and surface carbon. With the structure of carbon changed from fragments to vertically aligned nanoflakes and eventually to a thick layer with extra fragments, the dielectric loss would be continuously strengthened, while the magnetic loss maintains well, followed by a remarkable decline. A perfect balance between dielectric and magnetic loss has been achieved by sample S-Co/C-0.3 with minimum reflection loss value around -20 dB and effective absorption frequency range about 3.84 GHz in the C band. Excellent microwave absorption performance can also be realized in X and Ku bands. In addition, as-prepared Co and Co/C composites can also be potentially applied in electromagnetic shielding. The findings may pave the way for the manufacture of metal-based metal-organic framework derivatives and the design of lightweight low-frequency electromagnetic materials.

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