Metamaterial absorbers have been widely studied and continuously concerned owing to their excellent resonance features of ultra‐thin thickness, light‐weight, and high absorbance. Their applications, however, are typically restricted by the intrinsic dispersion of materials and strong resonant features of patterned arrays (mainly referring to narrow absorption bandwidth). It is, therefore essential to reassert the principles of building broadband metamaterial absorbers (BMAs). Herein, the research progress of BMAs from principles, design strategies, tunable properties to functional applications are comprehensively and deeply summarized. Physical principles behind broadband absorption are briefly discussed, typical design strategies in realizing broadband absorption are further emphasized, such as top‐down lithography, bottom‐up self‐assembly, and emerging 3D printing technology. Diversified active components choices, including optical response, temperature response, electrical response, magnetic response, mechanical response, and multi‐parameter responses, are reviewed in achieving dynamically tuned broadband absorption. Following this, the achievements of various interdisciplinary applications for BMAs in energy‐harvesting, photodetectors, radar‐IR dual stealth, bolometers, noise absorbing, imaging, and fabric wearable are summarized. Finally, the challenges and perspectives for future development of BMAs are discussed.

Review of Broadband Metamaterial Absorbers: From Principles, Design Strategies, and Tunable Properties to Functional Applications

1D structures have been gaining traction in the microwave absorption (MA) field benefiting from their electromagnetic (EM) anisotropy. However, there remain considerable challenges in adjusting EM properties by structural design. Herein, using the coaxial electrospinning and solvothermal method, the EM gradient has been achieved in TiO2@Co/C@Co/Ni multilayered microtubes. From the outer layer to the inner one, the impedance matching is gradually worsened, while the EM loss capacity is continuously enhanced, facilitating both the incidence and attenuation of microwave. Besides, 1D structural anisotropy simultaneously realizes multilevel magnetic interaction and 3D conductive double network. Therefore, the 1D EM-gradient hierarchical TiO2@Co/C@Co/Ni carbon microtube composite exhibits excellent MA performance. Its maximum reflection loss (RL) value reaches -53.99 dB at 2.0 mm and effective absorption bandwidth (EAB, RL ≤ -10 dB) is as wide as 6.0 GHz, covering most of the Ku band with only 15% filling. The unique design of 1D EM-gradient hierarchical composites promises great potential in the construction of advanced MA materials.

1D Electromagnetic-Gradient Hierarchical Carbon Microtube via Coaxial Electrospinning Design for Enhanced Microwave Absorption

Ultra-broadband electromagnetic absorption has become a critical issue competing with advanced broadband electromagnetic detection systems. In this study, an innovative strategy for the broadband absorption is proposed via combining periodic patterns and the hierarchical structure with gradient electromagnetic performance, which introduces an abnormal electromagnetic response for improved maneuverability in impedance matching and multiple electromagnetic loss mechanisms. The electromagnetic parameters for each layer were determined by the fixing content of multi-walled carbon nanotubes (MWCNTs) in gypsum, and the periodic stepped meta-structure was optimized via the finite element simulation. The optimized hierarchical MWCNTs/gypsum metamaterial with a thickness of 8.0 mm realized an average absorption intensity of −21.6 dB (>99.3% absorption) in the frequency band of 4–40 GHz. Particularly, broadband EM absorption for reflection loss less than −10 dB and −15 dB was achieved in the ranges of 4.2–40 GHz and 5.6–40 GHz, respectively. The ultra-broadband and strong electromagnetic absorption properties of the proposed metamaterial should be the synergic result of the excellent impedance matching and the multiple EM wave loss caused by the multi-scale integrated structure. Therefore, the proposed MWCNTs/gypsum metamaterial with the innovative strategy of combining the hierarchical structure and periodic patterns, could provide an insight for achieving high-performance absorption materials. 

Multi-scale integrated design and fabrication of ultra-broadband electromagnetic absorption utilizing multi-walled carbon nanotubes-based hierarchical metamaterial

Absorbers with gradient or stepwise gradient design have been demonstrated to be very beneficial for broadband radar absorption. However, it is still rather difficult to achieve strong absorption at low frequencies such as 2 GHz by a thin gradient medium absorber. In this study, metastructure design is introduced to a stepwise gradient nanocomposite containing flaky carbonyl iron power (FCIP) and graphene nanosheet (GNS) as fillers, in order to obtain a thin and ultra-broadband radar absorber. The matrix of the nanocomposite is hybrid of cyanate ester (CE) and Bisphenol-A based epoxy (E51). The absorption band of the fabricated 5 mm thick nanocomposite covers the full measured frequency band (2–18 GHz, 26.5–40 GHz) below −10 dB, except for the range of 5.7–7.6 GHz (below −9dB).

Gradient nanocomposite with metastructure design for broadband radar absorption

With the rapid development of communication technology in civil and military fields, the problem of electromagnetic radiation pollution caused by the electromagnetic wave becomes particularly prominent and brings great harm. It is urgent to explore efficient electromagnetic wave absorption materials to solve the problem of electromagnetic radiation pollution. Therefore, various absorbing materials have developed rapidly. Among them, iron (Fe) magnetic absorbent particle material with superior magnetic properties, high Snoek’s cut-off frequency, saturation magnetization and Curie temperature, which shows excellent electromagnetic wave loss ability, are kinds of promising absorbing material. However, ferromagnetic particles have the disadvantages of poor impedance matching, easy oxidation, high density, and strong skin effect. In general, the two strategies of morphological structure design and multi-component material composite are utilized to improve the microwave absorption performance of Fe-based magnetic absorbent. Therefore, Fe-based microwave absorbing materials have been widely studied in microwave absorption. In this review, through the summary of the reports on Fe-based electromagnetic absorbing materials in recent years, the research progress of Fe-based absorbing materials is reviewed, and the preparation methods, absorbing properties and absorbing mechanisms of iron-based absorbing materials are discussed in detail from the aspects of different morphologies of Fe and Fe-based composite absorbers. Meanwhile, the future development direction of Fe-based absorbing materials is also prospected, providing a reference for the research and development of efficient electromagnetic wave absorbing materials with strong absorption performance, frequency bandwidth, light weight and thin thickness.

/pdf/recent-progress-in-iron-based-microwave-absorbing-composites-2qd0p0qh.pdf

Recent Progress in Iron-Based Microwave Absorbing Composites: A Review and Prospective

Ultrawideband microwave absorbers have wide applications in civil industries and stealth technology. To achieve an ultrawideband absorption, flaky carbonyl iron particles (FCIPs) fabricated by a ball milling method were used as absorbents. The electromagnetic parameters of six FCIPs/wax absorbing composites with different weight ratios were measured in the frequency ranges of 2–18 GHz and 26.5–40 GHz. With these composites, particle swarm optimization was applied to design an ultrawideband triple-layered planar microwave absorber according to the absorbing performance calculated by the transmission-line theory. The reflection loss (RL) of the designed 6 mm thick absorber is below − 10 dB in 91% of the concerned bands. According to this design, a gradient triple-layered sample was prepared. Its measured RL is below − 10 dB in more than 90% of the concerned bands, although there are some shifts of the absorption peaks comparing with the simulated result. Benefiting from both the λ/4 resonance effect causing the standing wave patterns at the absorption peaks and the gradient structure enhancing the absorbance, this novel ultrawideband multilayer planar absorber offers a promising solution for practical applications.

Design and preparation of an ultrawideband gradient triple-layered planar microwave absorber using flaky carbonyl iron as absorbent

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Design and preparation of an ultrawideband gradient triple-layered planar microwave absorber using flaky carbonyl iron as absorbent

Biomimetic Material Simulating Solar Spectrum Reflection Characteristics of Yellow Leaf

Infrared suppression of a power-generation cabin with a cold plate system