Enhanced microwave absorption properties and mechanism of core/shell structured magnetic nanoparticles/carbon-based nanohybrids
Graphical abstract
In the article, core/shell Fe3O4/C, Fe/helical carbon nanotubes were synthesized selectively. The results indicated that the optimum reflection loss (RL) could reach −47.1 dB at 17.39 GHz with a matching thickness of 1.39 mm. The absorption bandwidth with the RL below −20 dB was up to 11.59 GHz. Moreover, based on the obtained results, the possibly enhanced microwave absorption mechanisms were also discussed in detail.
Introduction
Microwave absorbing materials (MAMs), which can dissipate electromagnetic (EM) wave by converting EM energy into thermal energy, have attracted more and more attention due to serious EM pollution caused by the rapidly extensive application of wireless equipment, radar systems and local area networks, etc [1], [2], [3]. For MAMs, the reflection and attenuation properties are mainly determined by the balance between the complex permittivity and permeability . It is well known that the reflection loss (RL), return coefficient () and attenuation constant () are defined by the following equations [4], [5]:where is the frequency of EM wave, is the thickness of an absorber, is the velocity of light and is the input impedance of absorber. It is well known that the return coefficient of an ideal absorber should be zero, which means that it must fulfill the optimal impedance matching condition: . However, it is difficult to meet this condition on the single dielectric or magnetic loss materials due to their mismatch in the values of and [6], [7], [8], [9], [10], [11], [12]. Therefore, considerable attention has been paid to develop high efficiency MAMs with light weight, low thickness, wide absorption frequency range, strong absorption capability, and good antioxidation properties in the past years [13], [14], [15], [16], [17], [18].
Among these MAMs, core/shell structured magnetic nanoparticles/carbon-based nanohybrids have received increasing attention owing to their unique interfacial and synergistic effects [19], [20], [21], [22], [23], [24]. For example, Kim and Che et al. synthesized core/shell Fe/carbon nanotubes (CNTs), and proved their excellent microwave absorption capabilities, respectively [25], [26]. Terada et al. reported that the as-synthesized Fe3C/C had an excellent EM wave absorption capability in the range of 0.9–9.0 GHz [27]. Narayanan et al. found that Ni-filled CNTs could exhibit an enhanced microwave absorption property in the S band [28]. And other nanohybrids such as Ni/C [29], Co/C [30], Fe3O4/Fe/C [31], Fe-carbon nanofibers [32], Li0.32Zn0.26Cu0.1Fe2.32O4/CNTs [33], Fe3O4 or Ni/carbon nanocoils [34], Fe/CNTs, Co/CNTs, and Ni/CNTs [35], [36], [37], [38], [39], [40], [41] were also demonstrated to have excellent microwave absorption performances. However, these core/shell structured carbon-based nanohybrids still suffer from some problems such as low efficiency of encapsulation, large thickness of absorber, optimal RL usually above −40 dB, and so on. Moreover, their enhanced microwave absorbing mechanisms were seldom discussed.
The aim of this work was to synthesize high encapsulation efficiency of magnetic nanoparticle/carbon-based nanohybrids and investigate their microwave absorption properties and mechanisms. Herein, we proposed a simple and efficient scheme to synthesize different categories of core/shell structured magnetic nanoparticle/carbon-based nanohybrids. Moreover, based on the measured complex permittivity and permeability, the microwave absorbing properties and mechanisms of the obtained nanohybrids were discussed in detail.
Section snippets
Synthesis of Fe2O3 nanotubes and magnetic nanoparticles/carbon-based nanohybrids
Same as the method reported previously [42], [43], Fe2O3 nanotubes were synthesized by hydrothermal method firstly. Then, the as-obtained Fe2O3 nanotubes were used as catalyst for catalytic decomposition of acetylene at 400, 500 and 600 °C, respectively. After cooling to room temperature (RT), different categories of magnetic nanoparticles/carbon-based nanohybrids could be obtained. For easy description, the products generated at 400, 500 and 600 °C were denoted hereinafter as C-400, C-500 and
Phases, microstructures and possible formation mechanism of core/shell structured nanohybrids
Fig. 1 shows XRD patterns of the obtained catalyst and as-synthesized samples. As shown in Fig. 1a, the diffraction peaks located at 24.1, 33.2, 35.6, 40.9, 49.5, 54.1 and 57.6° can be assigned to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6) and (0 1 8) crystal planes of Fe2O3 (JCPDS No. 86-0550), and no impurities such as FeOOH or Fe3O4 are detected. The result indicates that the used catalyst is single phase of Fe2O3. Fig. 1b presents the XRD pattern of C-400. As indicated by the symbols in Fig. 1b,
Conclusions
In summary, we proposed an efficient scheme to synthesize core/shell structured magnetic nanoparticles/carbon-based nanohybrids through the decomposition of acetylene directly over Fe2O3 nanotubes. And different categories of core/shell structured nanohybrids such as Fe3O4/C, Fe/HCNTs could be synthesized selectively by increasing the pyrolysis temperature from 400 to 600 °C. The investigations indicated that the microwave absorption capabilities of the obtained nanohybrids enhanced gradually
Acknowledgments
This work was supported by the International Cooperation Project of Guizhou Province (2012-7002), the Excellent Talents of Guizhou Province (2014-239), the National Science Foundation of Guizhou Province (2014-2059), the Postdoctoral Science Foundation of China (2015M570427), the Science and Technology Innovation Team of Guizhou Province (2015-4017), the National Science Foundation of China (Grant Nos. 11364005 and 11174132), and the Foundation of the National Key Project for Basic Research (
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