Porous Fe3O4/C microspheres for efficient broadband electromagnetic wave absorption
Graphical abstract
Introduction
In recent years, electromagnetic (EM) pollution and interference originating from the rapidly growing information technology sector have emerged as threats to human and animal health [1]. Health concerns drive the development of high performance EM wave absorbers (EMWAs), which can dissipate EMWs efficiently by converting them into thermal energy [2], [3], [4], [5]. At present, considerable research effort is being directed towards the discovery and future commercialization of EMWA materials. Magnetic particles (including Co3O4, Fe3O4, CoFe2O4, iron carbonyl, Fe2O3, BaCo2Fe16O27 and BaFe12O19) [6], [7], [8], [9], [10] and various other nanoparticles (Ag, Ni, Fe, Co and ZnO) [11], [12], [13] display excellent EMW attenuation properties. However, the large thicknesses of these materials are usually required to achieve efficient EMWA, while their narrow absorbing bandwidth restricts their practical applications [14], [15]. To realize EMWAs with broad bandwidth absorption, magnetic nanomaterials are typically combined with lightweight dielectric materials (e.g., carbon materials or conductive polymers) [16], [17], resulting in synergistic effects that benefit EMWA [18], [19], [20].
Porous structures, due to their large pore volumes, high surface area and controllable architectures [21], [22], [23], [24] are efficient EMW absorbers due to their multi-reflection and interfacial polarization properties [25], [26]. Among lightweight porous materials used for broadband EMWA, porous carbon materials represent an ideal candidate because of their high dielectric loss, low cost, low density and easy preparation [16]. Zhao et al. studied the EMWA performance of ordered mesoporous carbons, and reported a maximum reflection loss (RLmax) of −10.0 dB at a thickness of 2 mm. However, the effective absorption bandwidth (EAB) (< −10 dB) was 0 GHz [27]. To achieve a wide absorption bandwidth, carbon absorbers must be combined with other absorbers to achieve better impedance matching [28], [29]. For example, Fang and co-workers prepared wormhole-like porous carbon/Co0.2Fe2.8O4. Broad band absorption in the range of 12.8–18 GHz (< −10 dB) with an RLmax of −29.2 dB could be achieved at a thickness of 2.0 mm [30]. Qiang et al. synthesized porous Fe/C structures using high-temperature pyrolysis of Prussian blue, and an RL value of −22.6 dB was achieved at a thickness of 2.0 mm with an EAB range of 13.7–18 GHz [31].
Inspired by the studies mentioned above, this work targets the development of improved EMWA materials that are based on porous carbon microspheres containing Fe3O4 nanoparticles. Compared with porous carbon [32], the porous Fe3O4/C microspheres were expected to exhibit superior EMWA properties due to synergetic effects of the dielectric loss (carbon) and magnetic loss (Fe3O4). Additionally, the 3D porous carbon network of the microspheres was expected to promote multiple reflections and scattering of EMWs, further improving the EMWA performance.
Section snippets
Materials
Iron (III) chloride hexahydrate (FeCl3·6H2O), sodium carbonate (Na2CO3), polyvinylpyrrolidone ((C6H9NO)n, k-30), trisodium citrate dihydrate (C6H5Na3O7·2H2O) and glucose (C6H12O6) were purchased from Sinopharm Chemical Reagent Co., Ltd, and used without further purification.
Synthesis of porous Fe3O4/C microspheres
As displayed in Scheme 1, the porous Fe3O4/C microspheres were prepared according to a hydrothermal process [33]. Briefly, FeCl3·6H2O (2.1 mmol), PVP (1.0 g), trisodium citrate (2.40 mmol) and Na2CO3 (3.60 mmol) were added
Morphologies of porous Fe3O4/C microspheres
The hydrothermal reaction time influenced the morphology of the Fe3O4/C microspheres. SEM images (Fig. 1) reveal that the product that formed after 18 h has an irregular morphology (Fig. 1a), comprising both spherical and non-spherical particles. However, when the reaction time is increased to 24, 30 or 48 h, Fe3O4/C microspheres with average diameters in the range of 6–10 µm are obtained (Fig. 1b-d, respectively). The microspheres contain numerous pores, with the surface of the microspheres
Conclusions
Hierarchically porous Fe3O4/C microspheres with an excellent performance as EMW absorbers were successfully fabricated. The microspheres contained both small (< 10 nm) and large mesopores (30–40 nm) and partially graphitic and homogeneously embedded Fe3O4 nanocrystals. Porous Fe3O4/C microspheres with a carbon content of 71.5 wt% had outstanding EMWA properties with a RLmax value of −31.75 and a wide effective EMWA bandwidth (RL < −10 dB) of 7.76–12.88 GHz for an absorber thickness of 3.0 mm.
Acknowledgements
This project was supported by the National Natural Science Foundation of China (No. 41476059) and China Postdoctoral Science Foundation (No. 2016M600557).
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