Impedance matching is important for achieving high-efficiency microwave absorbers. The high conductivity of dielectric loss materials such as pure metals and carbon nanomaterials generally results in poor absorption owing to the low impedance matching between the absorbers and air. Carbon nanostructures are very promising candidates for high-efficiency absorption because of their attractive features including low density, high surface area, and good stability. Herein, a new strategy is proposed to improve the impedance matching of dielectric loss materials using electrospun carbon nanofibers as an example. The carbon nanofibers are coated with specifically designed gradient multilayer nanofilms with gradually increasing electroconductibility synthesized by doping ZnO with different Al₂O₃ content (AZO) by atomic layer deposition. The gradient nanofilms are composed of five layers of dielectric films, namely, pure Al₂O₃, AZO (5:1, the pulse cycle ratio of ZnO to Al₂O₃), pure ZnO, AZO (10:1), and AZO (20:1). The versatile gradient films serve as intermediate layers to tune the impedance matching between air and the carbon nanofiber surfaces. Therefore, the carbon nanofibers coated with gradient films of rationally selected thicknesses exhibit remarkably enhanced microwave absorption performance, and the optimal reflection loss reaches?58.5 dB at 16.2 GHz with a thickness of only 1.8 mm. This work can help further understand the contribution of impedance matching to microwave absorption. Our strategy is general and can be applied to improve the absorption properties of other dielectric loss materials and even for applications in other fields.

In this work, atomic layer deposition (ALD) was employed to fabricate coaxial multi-interface hollow Ni-Al₂O₃-ZnO nanowires. The morphology, microstructure, and ZnO shell thickness dependent electromagnetic and microwave absorbing properties of these Ni-Al₂O₃-ZnO nanowires were characterized. Excellent microwave absorbing properties with a minimum reflection loss (RL) of approximately –50 dB at 9.44 GHz were found for the Ni-Al₂O₃-100ZnO nanowires, which was 10 times of Ni-Al₂O₃ nanowires. The microwave absorption frequency could be effectively varied by simply adjusting the number of ZnO deposition cycles. The absorption peaks of Ni-Al₂O₃-100ZnO and Ni-Al₂O₃-150ZnO nanowires shifted of 5.5 and 6.8 GHz towards lower frequencies, respectively, occupying one third of the investigated frequency band. The enhanced microwave absorption arose from multiple loss mechanisms caused by the unique coaxial multi-interface structure, such as multi-interfacial polarization relaxation, natural and exchange resonances, as well as multiple internal reflections and scattering. These results demonstrate that the ALD method can be used to realize tailored nanoscale structures, making it a highly promising method for obtaining high- efficiency microwave absorbers, and opening a potentially novel route for frequency adjustment and microwave imaging fields.

An atomic layer deposition (ALD) method has been employed to synthesize Fe₃O₄/graphene and Ni/graphene composites. The structure and microwave absorbing properties of the as-prepared composites are investigated. The surfaces of graphene are densely covered by Fe₃O₄ or Ni nanoparticles with a narrow size distribution, and the magnetic nanoparticles are well distributed on each graphene sheet without significant conglomeration or large vacancies. The coated graphene materials exhibit remarkably improved electromagnetic (EM) absorption properties compared to the pristine graphene. The optimal reflection loss (RL) reaches -46.4 dB at 15.6 GHz with a thickness of only 1.4 mm for the Fe₃O₄/graphene composites obtained by applying 100 cycles of Fe₂O₃ deposition followed by a hydrogen reduction. The enhanced absorption ability arises from the effective impedance matching, multiple interfacial polarization and increased magnetic loss from the added magnetic constituents. Moreover, compared with other recently reported materials, the composites have a lower filling ratio and smaller coating thickness resulting in significantly increased EM absorption properties. This demonstrates that nanoscale surface modification of magnetic particles on graphene by ALD is a very promising way to design lightweight and high-efficiency microwave absorbers.