Hydrothermal self-assembled Fe₃O₄/CA core-shell composites for broadband microwave absorption

Highlights

• Fe₃O₄ and CA can easily complete self-assembly under hydrothermal conditions.

• The Fe₃O₄/CA composites have good impedance matching and rich loss preformance .

• The core–shell structure greatly enhances the interfacial polarization.

Abstract

Electromagnetic wave has emerged as a new source of environmental pollution in the daily life. Therefore, the prominent microwave absorbents are urgently needed. In this work, the carbon aerogel and Fe₃O₄ nanoparticle (Fe₃O₄/CA) composites with core–shell structure were facilely prepared by a self-assembly process between carbon aerogels and Fe ion under the hydrothermal condition. The microwave absorption performance was significantly enhanced due to the perfect impedance matching and effective interface polarization. Compared with Fe₃O₄ nanoparticles and carbon aerogels, Fe₃O₄/CA composites show the advantages of strong microwave absorption and broad absorption bandwidth. The maximum RL value of Fe₃O₄/CA composites is up to −50.96 dB at 4.80 GHz. And also, the effective absorption bandwidth reaches 3.82 GHz with the thickness of 2 mm.

Introduction

With the increasing use of the electromagnetic devices, the radiation problem of electromagnetic wave has become the focus of attention. Electromagnetic interference (EMI) not only becomes a catastrophic problem of electronic devices, but also affects human health and causes serious disease such as brain tumors [1] and leukemia [2]. Therefore, there is an urgent need for high-performance electromagnetic wave absorbents. Microwave absorbing materials have a wide range of applications, including communication instruments, space exploration, medical electronic equipment and so on [3]. Meanwhile, they are also widely used in military field [4]. For instance, radar absorbing materials are often used to reduce the detectability of targets by eliminating the reflection of radar signal on their surfaces [5], [6]. To date, an ideal microwave absorber should have the characteristics of lightweight, broad bandwidth, strong absorption and so on. Common electromagnetic absorption materials can be divided into two categories: magnetic absorption materials and dielectric absorption materials. Magnetic absorbers have excellent absorption properties, but their density is usually very high. Compared with the magnetic absorbers, the dielectric absorbers have lighter weight, whereas their impedance matching performance is poor and the absorption bandwidth is narrow. Carbon materials and their composites include grapheme [7], [8], [9], [10], [11], [12], carbon nanotubes [13], [14], [15], [16], [17], nanoporous carbon [18], [19] have been widely concerned in the field of microwave absorption due to their superior electrical properties, low cost, environmental friendliness, easy processability and high chemical stability [20]. In recent years, graphene and carbon nanotubes have been developed and shown excellent application in EMI [21]. Unfortunately, the synthesis of these materials requires expensive raw materials and the preparation process is extremely complex (such as chemical vapor deposition (CVD), Hummer method, arc discharge, etc.), which severely hinder their practical application. Therefore, it is urgent to develop suitable carbon materials with facile synthesis process.

It is reported that porous carbon materials (such as foamed carbon) have lower dielectric constant and better impedance matching, indicating that the modification of the porous structure is an effective way to design and prepare good electromagnetic absorbers. Carbon aerogel (CA) is a kind of nanoporous carbon material with light weight, adjustable pore size, high specific surface area and moderate conductivity [22]. More importantly, carbon aerogels show strong optical absorption in a wide optical band. Sun et al. studied the optical property of carbon aerogels with different 3D network nanostructure and found that the reflectivity of the carbon aerogels was strongly dependent on their micropores [23]. However, carbon aerogels have only dielectric loss, and their absorbing capability is limited. At the same time, Fe₃O₄ based composites have attracted great attention due to their magnetic property, soft metal property, huge magnetic anisotropy, good biocompatibility and low toxicity. However, Fe₃O₄ also has some problems such as high density, poor corrosion resistance and low impedance matching. The composition of the dielectric loss type carbon materials and the magnetic loss type Fe₃O₄ can effectively overcome the above problems, which gives full play to their respective advantages and improves the absorbing performance. Ternary core–shell Fe₃O₄@C@PANI composites synthesized by Manna et al. showed minimum reflection loss of −33 dB in the frequency range (2–8 GHz). Such high value of shielding efficiency was ascribed to the presence of dual interfaces and dielectric-magnetic integration in Fe₃O₄@C@PANI [24]. Wu at al. prepared Fe₃O₄/C composite flowers through a facile route including a solvothermal approach and a carbon reduction process, which displayed an optimal reflection loss value of −54.6 dB at 5.7 GHz when the thickness was 4.27 nm [25]. Fe₃O₄ (core) /C (shell) composites are considered to be an efficient absorption material because of the combination of chemical uniformity, magnetic loss, dielectric loss, structural loss, impedance matching and several key factors affecting microwave absorption. Therefore, Fe₃O₄/C composites with high dielectric loss and magnetic loss is a good choice for microwave absorption [26].

In this work, carbon aerogels Fe₃O₄/CA composites were synthesized by hydrothermal method. In the preparation process, the thickness of carbon layer was adjusted by changing the amount of CA. After successfully preparation of Fe₃O₄/CA composites, the influence of carbon aerogels on the Fe₃O₄ particles under hydrothermal environment was studied. Then the microwave absorption performance of the composite material loaded with different content of CA was further explored.