Alginate-templated synthesis of CoFe/carbon fiber composite and the effect of hierarchically porous structure on electromagnetic wave absorption performance
Graphical abstract
Introduction
Due to the aggravation of electromagnetic (EM) pollution and interference issues, electromagnetic wave (EMW) absorbing materials capable of converting EM energy to thermal energy through dielectric loss or/and magnetic loss have attracted considerable attention for military, civilian and commercial applications [1,2]. Advanced EMW absorbers are required to possess the advantages of strong absorption, broad bandwidth, thin thickness and lightweight [3,4]. Composite absorber consisting of both dielectric and magnetic components attracts considerable attention, because the EMW absorption properties can theoretically benefit from the complementarities between the permittivity and permeability of materials. Among various magnetic materials, magnetic metals such as Fe, Co, Ni and related alloys are more suitable for high-frequency EMW absorption due to their higher saturation magnetization and Snoek's limit, compatible dielectric loss, as well as distinguishable permeability in gigahertz frequency [5]. In particular, bimetallic alloys recently stimulate considerable interest because the alloying process could induce electron transfer and enhance spin polarizability which contribute to better EMW absorption properties [6,7].
To achieve lightweight feature, carbon fibers (CFs) with the advantages of low density, excellent electrical conductivity, good thermal and chemical stabilities, and low cost, have been considered as one of the most potential candidates for dielectric components [8]. The one-dimensional (1D) structure with large aspect ratio enables CFs readily construct a continuous conductive network in matrix which is favorable to dissipate EMW via induced current. Compared with typical 1D carbon material, i.e. carbon nanotubes, CFs can be designed with applicable electrical conductivity, which facilitates the optimization of impedance matching [9]. So far, many magnetic particle/CF composite absorbers have been successfully fabricated using electrospinning and electrodeposition methods, such as FeNi/CF, Fe/CF and FeCo/CF [[9], [10], [11]]. The electrospinning usually needs organic solvents to dissolve polymer (carbon resource) during the synthetic process, which causes environmental burden and increases the manufacturing cost. Meanwhile, the slow production rate and low yield of electrospinning limit the large-scale fabrication. As to electrodeposition method, the deposited magnetic metal/alloy particles exclusively distribute on the CF surface. Thus, the generated heterointerfaces between magnetic particles and CFs are quite limited which restricts the interfacial polarization effect. So far, the magnetic particle/CF composites derived from electrospinning and electrodeposition methods seem to reach a ceiling of EMW absorption performance. Therefore, there is an urgent need to develop new synthetic methods that are green, fast, low cost, and scalable. Meanwhile, a deeper understanding on the structure-performance relationship is highly desirable for better guiding the design of CF-based absorbers.
Recent advances in carbon-based absorbers have revealed that the void structure in carbon not only benefits the reduction of weight density but also significantly changes the EMW absorption performance. Theoretically, the void could induce interfacial polarization, act as an “effective medium” to modulate dielectric property, and provide scattering centers for EMW multi-scattering that leads to more EM energy loss by increasing the propagation path of EMW [[12], [13], [14], [15]]. Compared with carbon materials containing monoscale void, the latest findings report that the well-designed multiscale voids may be more conductive to the improvement of EMW absorption performance. For instance, Xu et al. reported that the carbon microspheres containing mesoporous carbon shell and submicron cavity exhibited much better EMW absorption properties than hollow carbon microspheres and solid carbon microspheres [16]. Qiu et al. found that the EMW absorption properties of activated carbon with hierarchical pore structure outperformed other carbon materials with uniform mesopores [17]. Although the “multiscale-pore effect” has only been attempted in very limited carbon structures, it motivates us to fabricate hierarchically porous CFs composed of submicron/micron-sized and nanoscale pores. Relevant exploration may find the chance to greatly improve the EMW absorption performance of CF absorbers, and further deepen the understanding of multiscale void in modulating EMW absorption properties.
In this work, we report a green and scalable strategy for fabricating CoFe/hierarchically porous CF (HPCF) composite using alginate fiber as a template. The naturally rich and non-toxic alginate contains a large number of “egg-box” which could accommodate metal ions via coordination with negatively charged α−L−guluronate (G) blocks [18]. The type and amount of metal ions immobilized in the “egg-box” can be easily adjusted by changing the metal salt solution, thus enabling the modulation of composition and loading of the magnetic particles on HPCFs. Compared with the previously reported mesopores CF composite absorbers, the high proportion of submicron macropores in CoFe/HPCF leads to a considerable improvement of EMW absorption. Remarkably, the CoFe/HPCF achieves a significant reflection loss (RL) of −69.1 dB and a broad absorption bandwidth of 5.2 GHz with a filler loading as low as 15 wt% and a quite thin matching thickness of 1.6 mm. The comprehensive absorption properties of CoFe/HPCF are superior to other analogues. In addition, a series of highly efficient composite absorbers were prepared using this method, including CoNi/HPCF, Fe/HPCF and Co/HPCF. We believe that the disclosed multiscale pore effect and the proposed synthetic method can provide more ideas and guidance for high-performance absorber design.
Section snippets
Materials
Sodium alginate was purchased from the Xilong Scientific Co., Ltd. Calcium chloride anhydrous (CaCl2), cobalt chloride hexahydrate (CoCl2·6H2O), iron chloride hexahydrate (FeCl3·6H2O), nickel acetate tetrahydrate (C4H6O4Ni·4H2O) and ethanol were purchased from J&K Scientific Ltd and used without further purification. Hydrochloric acid (HCl) was purchased from Beijing Chemical Works.
Synthesis of Ca−alginate fiber
Calcium−alginate fiber (Ca−AF) was produced by wet spinning according to a previous report [19]. Briefly, sodium
Results and discussion
Fig. 1(a)−(c) illustrate the synthesis of CoFe alloy/carbon fiber composite using alginate fiber as a template. As shown in Fig. 1(a) and Fig. S3(a), the white-colored Ca−AF prepared by a wet-spinning method, is straight and smooth with a uniform diameter of ∼10 μm. After being protonated via ion exchange reaction between Ca2+ and H+, the obtained H−AF was immersed in a mixed solution containing Co (II) and Fe (III) salts. Thus, the CoFe−AF was produced through immobilizing the Co2+ and Fe3+
Conclusion
CoFe/HPCF composite absorber with hierarchically porous structure is designed and prepared for highly efficient EMW absorption. Natural alginate fibers were used as templates to immobilize Co2+ and Fe3+ ions in abundant “egg box” structure via coordination with negatively charged α−L−guluronate blocks. During carbonization process, the alginate fibers were converted to porous CFs; simultaneously metal ions aggregated and formed CoFe2O4 octahedral particles at low temperature, followed by
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (U1832138, 51731002 and 51671010), Beijing Municipal Natural Science Foundation (No. 2172031), Beijing Municipal Science and Technology Project (No. Z161100002116029), Aeronautical Science Foundation of China (No. 2016ZF51049), and Fundamental Research Funds for the Central Universities.
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