A flexible metamaterial based on liquid metal patterns embedded in magnetic medium for lightweight microwave absorber

Highlights

• The metamaterial absorber achieved maximum microwave absorption of −23.36 dB at 0.9 mm thickness and 1.91 g/cm³ density.

• The metamaterial absorber can maintain good absorption performance after multiple bending.

• Liquid metal replaces solid metal so that the absorber has good resistance to bending fatigue.

• This work provides a strategy for reducing weight and thickness by bonding magnetic materials and metamaterials.

Abstract

Lightweight electromagnetic absorbing materials with thinner thickness and low density are highly desired for practical applications. In this work, a thin and lightweight magnetic metamaterial absorber was proposed by embedding liquid metal resonant patterns in magnetic composites medium with a low filling rate. Due to the coupling of magnetic medium and metamateria as demonstrated by CST (Computer Simulation Technology) simulation, the strong electromagnetic resonant loss and multiple reflections were rationally introduced. The obtained metamaterial absorber endows the strong absorption performance of −23.63 dB, an effective absorption bandwidth (EAB, with RL less than −10 dB) of 2.46 GHz under 0.9 mm and with a 1.91 g/cm³ density merely. The introduction of resonance patterns in the magnetic composites medium is an efficient way to fabricate lightweight and thin thickness metamaterials for high-performance microwave absorbers.

Introduction

Nowadays, with the development of 5G technology and the trend of miniaturization of electronic equipment, microwave absorbing materials (MAMs) are widely used in stealth equipment [1], wireless equipment [2], and electronic devices [3]. In general, the weight and thickness of MAMs are two important indicators for lightweight. Besides, as flexible wearable devices getting more and more attentions, the issue of electromagnetic interference in wearable electronic devices is becoming prominent. Therefore, the MAMs for flexible wearable devices need excellent absorbing performance as well as high flexibility [4,5].

Magnetic materials were widely studied as MAMs due to its high permeability and andabundant magnetic loss mechanisms [[6], [7], [8], [9]]. However, high permeability requires a higher volume fraction of magnetic materials, which means the density of the absorber will be very large, according to the percolation theory [10]. To reduce the weight of absorbers, researchers have made a lot of attempts. For example, the carbon materials as a kind of lightweight material are expected to replace magnetic materials [[11], [12], [13], [14]]. Nevertheless, carbon materials present poor impedance matching characteristics due to high electric conductivity, and the absence of magnetism loss for MAMs. Therefore, the matching thickness of the pure carbon MAMs is larger than that of magnetic MAMs. Furthermore, the magnetic/dielectric composites should be an effective way to reduce the filling ratio of magnetic materials and total density which exhibit matched impedance and improve attenuation ability. He et al. combined Fe₃O₄ nanoparticles, graphene oxide, and conductive oligomer ternary by using a solvent-thermal route. The maximum RL of ternary composites reaches −45 dB at a matching thickness of 5 mm [15]. Zhang et al. prepared the carbon-coated Ni(C) nanocapsules by a modified arc-discharge method in methane atmosphere [16]. The experimental results show that the bandwidth with RL less than −10 dB is up to 3.2 GHz under 50 wt% loading. However, according to the quarter-wavelength theory [17,18], the thickness of the MAMs composite with low permeability will increase. The high permeability of the magnetic composite result from the increase of the filling ratio of the magnetic material. Therefore, to maintain good absorbing performance for MAMs, the reduction of density and thickness is often contradictory [[19], [20], [21], [22], [23], [24]].

It is difficult to solve the contradiction of simultaneously reducing the density and thickness of the absorber only by adjusting the electromagnetic parameters of the MAMs. Therefore, introducing more new loss mechanisms inside of absorbers is necessary. The perfect metamaterial absorber that is not limited by the quarter-wavelength theory was firstly proposed by Landy et al. [25]. The metamaterial absorber consists of an array of split-ring resonators (SRRs) spaced a distance above a ground plane and separated by a dielectric layer. This metamaterial absorber was demonstrated a simulated absorptivity of 99 % at 11.48 GHz, and its total thickness is just about 0.72 mm because it achieves perfect absorption through electromagnetic resonance. However, the effective band of metamaterial absorbers is less than 0.5 GHz because the high absorption is only achieved near the resonant frequency. To broaden the bandwidth, more complex structures and larger thickness are usually adopted [26]. Fei Ding et al. designed an ultra-broadband metamaterial absorber, which is composed of a periodic array of metal-dielectric multilayered quadrangular frustum pyramids. The experimental absorption at normal incidence is above 90 % over 7.8–14.7 GHz [27]. Its thickness is more than 5 mm and the complex structure is too difficult to fabricate. Currently, most of the reported metamaterial absorbers are based on dielectric substrates and its thicknesses are still too big [4,27,28]. Rozanov has pointed out that the absorbing bandwidth can be greatly improved by increasing the effective permeability of the absorber [9]. Zhang et al. successfully designed a magnetic metamaterial absorber with needlepoint-shape patterns, which expands the absorption bandwidth of the single magnetic absorber by more than 65 % [29]. Therefore, introducing the magnetic composites medium into the metamaterial absorber should be a feasible strategy to improve the absorption performance [[30], [31], [32]]. Furthermore, considering the requirements of the absorber on flexible electronic devices, liquid metal as a novel material that has both metallic and fluid properties is used for flexible metamaterial absorber [[33], [34], [35]].

Herein, we present a metamaterial absorber based on magnetic composites medium and liquid metal resonance patterns. It is desirable to realize the lightweight and thin thickness of the magnetic MAMs by embedding liquid metal resonance patterns with a low filling ratio magnetic medium. The experiment samples were made by casting and evaporation, and optimum structure sizes were determined by CST simulator. The simulation and measurement results show that the proposed absorber could have almost 2.5 GHz effective absorption bandwidth (EAB, with RL less than −10 dB) with a total thickness of 0.9 mm and 10 vol% magnetic composites medium. The mechanism of absorber is attributed to multiple reflections of electromagnetic waves inside the absorber and the electromagnetic resonant loss of magnetic metamaterials.