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Vol. 9. Issue 1.
Pages 762-772 (January - February 2020)
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Vol. 9. Issue 1.
Pages 762-772 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.11.016
Open Access
Three-dimensional graphene supported Fe3O4 coated by polypyrrole toward enhanced stability and microwave absorbing properties
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Jinhuan Lia,
Corresponding author
jinhuan_nj@nuaa.edu.cn

Corresponding author.
, Huanmin Jia, Yanfang Xub, Jiaojiao Zhanga, Yi Yana
a College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
b Nanjing Research Institue, Inner Mongolia North Heavy Industries Group Corp. Ltd, Yudao Street 29, Nanjing 210016, China
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Abstract

The composites GFPs were prepared through anchoring the microspheres of Fe3O4 cores coated with polypyrrole shells (Fe3O4@Ppy) on the three-dimensional (3D) graphene areogel (GA) with the one-step chemical reduction method. Good microwave absorption properties of the composites in 2−18 GHz can be obtained through tuning the content of Fe3O4@Ppy. For GFP1:3 the minimum reflection loss (RL) can reach −40.53 dB at 6.32 GHz and the effective bandwidth of the reflection loss peak with the thickness of 2.5 mm reaches 5.12 GHz. Especially, the stability of Fe3O4 can be improved due to the coating of Ppy shells. As expected, the enlarged dielectric properties derived from Ppy itself, the enhanced conductivity and abundant interfaces due to the introduction of Ppy as shells largely contribute to the improved microwave absorption capacity. The component synergy of 3D graphene, Ppy and Fe3O4 leading to the good impedance matching condition also plays a key role in achieving the enhanced microwave absorption performance. Most importantly, GFPs still maintain ultra-light nature. The contribution provides an effect avenue to prepare stable and light-weighted materials in a simple method for practical microwave absorption.

Keywords:
Three-dimensional graphene
Fe3O4
Polypyrrole
Fe3O4@Ppy
Microwave absorption
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1Introduction

Nowadays, electromagnetic (EM) radiations generated from various electronic devices and ongoing instrumentation leads to serious environmental pollution and potential health hazards. Also the interference of EM wave could disrupt functioning of many electronic devices and the EM radiation from military facilities could be detected by radars causing the danger of being attacked. Therefore, microwave absorption (MA) materials have attracted much attention for their crucial applications in the fields of military aircrafts, environment protection and communication equipment [1–3]. Plentiful microwave absorbers have been investigated in the past decade such as ferrite [4], carbonyl iron [5], carbon materials [6,7] and conducting polymers [8,9]. However, due to poor impedance and limited loss mechanism the microwave absorption materials with sole component cannot simultaneously meet the requirements of lightweight, wide bandwidth, strong absorption properties, anti-oxidative capability and flexible designability for ideal MA materials

To gain excellent MA property, developing various composites based on magnetic loss and dielectric loss components to explore the synergetic effect of these two loss mechanisms is definitely an effective approach [10–12]. Fe3O4 as a typical magnetic loss absorber has been most frequently investigated mainly due to its excellent magnetic properties, soft metallic nature, large magnetic anisotropy, good biocompatibility, and low toxicity [11]. According to the available literature survey about Fe3O4 based MA materials, the Fe3O4/C composites is a research hotspot owing to its tunable properties and high chemical stability of carbon materials as well as significantly synergetic or complementary behavior between Fe3O4 and carbon [11,13–15].

Graphene as a novel nano-carbon has been investigated for microwave absorption materials due to their low density, superior physical and chemical properties [1–3,11,14,16–18]. Especially, constructing three-dimensional (3D) graphene has attracted increasing research attentions because 3D graphene can not only maintain the intrinsic properties of graphene, but also demonstrate more advanced features in virtue of their structure [19]. Compared with 2D graphene sheets, 3D graphene can be easily tuned in conductivity and physical structure through adjusting the starting material and the reduction conditions, and also the severe aggregation problem of graphene layers is abandoned during the material preparation [20,21]. It is reported that the 3D graphene based materials designed with proper chemical composition and physical structure could provide the balance between excellent impedance matching and high microwave loss, and deliver an excellent MA property [22–25].

Because the excellent MA property might be obtained through constructing composites of 3D graphene and Fe3O4 in view of the contributions from loss mechanisms and from structure feature, the 3D graphene/Fe3O4 nanocomposites for microwave absorbing materials have been reported [26]. However, the minimum reflection loss (−27.0 dB) and the properties in view of effective bandwidth (below −10 dB) and thickness are all not impressive. Recently, some designed methods were put forward to improve the MA properties of the materials based on 3D graphene and Fe3O4 nanocomposites. For examples, a monolithic three-dimensional Fe3O4/graphene material [27], a three-dimensional graphene/Fe3O4/carbon microtube of sandwich-type architecture material [28], a ZnO/Fe3O4/graphene composite [29] and a three-dimensional SiO2@Fe3O4 core/shell nano-rod array/graphene architecture material [30] were prepared, and enhanced MA properties or excellent MA properties in a particular band were obtained. Generally, for these above examples, much delicate fabricating methods and morphologies are needed. However, the Fe3O4 usually has the fatal defect of frangibility to oxidation. It can be imagined that due to the high porosity and the large size of pores (˜30 μm order of magnitude) in 3D graphene, Fe3O4 nanoparticles decorated on the cell surface of graphene framework still behave like bare or partially bare Fe3O4 and would experience the oxidation. At present, the stability of Fe3O4 in MA materials has not received enough attention.

Basically, oxidation résistance of Fe3O4 nanoparticles can be achieved through coating anti-oxidative shells on its surface. What’s more, the coating shells leading to core-shell microstructure facilitates more multiple reflections or scatterings by endowing multi-interfaces because of the heterogeneity of composition and structure of the materials, all of which can promote polarization and the attenuation of incident microwave [13,31–33]. Polypyrrole (Ppy) is one of the most extensively studied conductive polymers, and it has attracted great interest in constructing microwave absorbers due to its light weight, high electrical conductivity and favorable physicochemical properties [8,34,35]. Also, it could be facilely prepared through in-situ polymerization. These merits render Ppy as a promising shell material with a lot of good performance expectations. Furthermore, we believe that the polymer shells can endow the better toughness of the material architecture than those inorganic ones such as C [13,15,36] and SiO2[30], leading to the good application properties.

As a part of an effort to develop applicable light-weighted MA materials based on three-dimensional graphene/Fe3O4 with improved MA properties in a simple method, we prepared Fe3O4@Ppy core-shell microspheres in which Fe3O4 nanoparticles were embedded by Ppy, and they were anchored on the reduced three-dimensional graphene areogel (GA) by one-step chemical reduction process. Benefiting from the suitable polarity of Ppy, Fe3O4@Ppy microspheres could form stable dispersion in GO solution and evenly distribute in the 3D graphene skeleton to form homogeneous 3D graphene/Fe3O4@Ppy composites (GFPs). The stability and MA properties were evaluated to verify the design and expectations.

2Materials and methods2.1Synthesis of Fe3O4@Ppy core-shell microspheres

Fe3O4 nanoclusters were prepared according to the previous report [37]. Fe3O4 (0.5 g) was dispersed in distilled water (100 mL) with strong mechanical stirring to form a dark brown suspension and FeCl3.6H2O (9.0 g) was dissolved in distilled water (50 mL) to form a yellow solution. The Fe3O4 suspension and the FeCl3.6H2O solution were then mixed with stirring for 3 h. Subsequently, to the mixture pyrrole monomer (0.3 mL) was injected drop-wise followed by fast adding 20 mL sodiumdodecylsulfate (SDS) solution (5.85 wt%) and the mechanical stirring was maintained for 12 h. The as-prepared black solution was washed with ethanol and distilled water in sequence assisted by magnet separation for three times. The final product named as Fe3O4@Ppy was freeze-dried for 12 h.

2.2Synthesis of 3D gaphene/Fe3O4@Ppy cmposites

Graphene oxide was prepared by the modified Hummers method [38]. A certain weight of Fe3O4@Ppy microspheres were added into 10 mL GO (4 mg/mL) assisted by ultrasound to form a homogeneous suspension. Then ethylenediamine (25 µL) was injected into the above suspension which was sealed in a 60 × 30 mm glass vial before the reaction at 95 °C for 6 h. The resulting aerogels were obtained after soaked with distilled water for three days and freeze-dried for 2 days. According to the weight ratio of GO to Fe3O4@Ppy, the final composites were named as GFP1:1, GFP1:2, GFP1:3 and GFP1:4. The pristine GA was prepared with the same method without adding Fe3O4@Ppy.

2.3Characterization

The evaluation of stability was performed as the following test. The aqueous solutions of pH 0.1, 0.3, 0.5 and 1.0 were prepared by adding a certain amount of concentrated hydrochloric acid. Solid powders (Fe3O4 and Fe3O4@Ppy) were weighed and added into above different acidic solutions of 20 mL, respectively, and maintained for 2 h with stirring. The solids separated by the magnetic separation were dried at 80 °C and weighted. The percents of weight loss were calculated and used to evaluate the stability of Fe3O4 and Fe3O4@Ppy under harsh condition.

Scanning electron microscopy (SEM) images were obtained by a Hitachi S-4800 field emission SEM at 15KV and transmission electron microscopy (TEM) was carried out on a Philips Tecnai 12 microscopy at 120KV. Fourier-transform infrared (FTIR) spectra were obtained with a Nicolet Nexus 670 infrared spectrometer. Raman spectra were measured with HoloLab series 5000 Raman spectroscopy system (514 nm excitation of the laser). The X-ray diffraction (XRD) patterns were recorded on a Bruker D8 ADVANCE X-ray diffractometer. The tests of magnetic properties of samples were carried out using Lake Shore’s Vibrating Sample Magnetometer VSM model 7307. The composites were soaked into molten paraffin and then sheared into rings with outer diameter about 7 mm and inner diameter about 3 mm. Then these rings were slipped into standard mold to make the measure size more precise. The complex permittivity and complex permeability were tested via a vector network analyzer (PAN-X N5244A) in the range of 2−18 GHz and the MA properties were calculated according to the transmission line theory.

3Results and discussions3.1Preparation and characteristics of 3D gaphene/Fe3O4@Ppy composites

Fe3O4@Ppy nanoparticles were prepared with the common ion absorption effect [39]. The positive charge (Fe3+) on the Fe3O4 nanoparticles can not only prevent the aggregation of Fe3O4 nanoparticles to some extent, but also initiate pyrrole monomer polymerization. As shown in Fig. 1(a), Fe3O4 nanoparticles were prepared with the mean diameter of 400 nm. And after in-situ polymerization of Ppy, the Fe3O4 cores as shown in Fig. 1(b, c, d) were wrapped up by a Ppy shell of 60−70 nm thickness and the Fe3O4@Ppy nanoparticles were obtained in strawberry structure.

Fig. 1.

SEM and TEM image (inset) of Fe3O4 (a), SEM (b) and TEM (c,d) images of Fe3O4@Ppy, water dispersion of Fe3O4 (e) and Fe3O4@PPy (f), and magnetic separation of Fe3O4@Ppy (g).

(0.22MB).

The suspension of Fe3O4@Ppy nanoparticles in water (Fig. 1(f)) can maintain stable after 2 days, while the pristine Fe3O4 suspension (Fig. 1(e)) settled down, showing that the dispersion stability of Fe3O4@Ppy nanoparticles was greatly enhanced by coating Ppy shell on the surface of Fe3O4. Suitable polarity of Ppy provides the good compatibility with water and subsequently improves dispersibility of Fe3O4. This good dispersion would guarantee the uniform distribution of Fe3O4@Ppy on the 3D graphene skeleton in the final materials. The Fe3O4@Ppy nanoparticles could be isolated from the water suspension using a magnetic separating method (see Fig. 1(g)). The results of the corrosion test were shown in Fig. 1(d). Fe3O4@Ppy nearly remains the initial weight, clearly showing that Ppy shells are effective to prevent Fe3O4 from corroding. However, Fe3O4 loses more and more weight with the acidity of the solution increasing, and when the solution reaches the pH of 0.1, Fe3O4 loses more than 10 wt% weight.

The preparation program of 3D graphene/Fe3O4@Ppy was exhibited in Fig. 2(a). From the appearance it can be noticed that the volume of the aerogels gradually increases with the addition of Fe3O4@Ppy (see Fig. 2(b)), but the volume reaches a platform (see Fig. 2(c)). For the largest sample GFP 1:4, the volume density is 37.5 mg/cm3, while the GFP1:3 only has the volume density of 28.6 mg/cm3, showing their ultra-light nature.

Fig. 2.

The preparation program (a), appearance pictures (b), the bulk volume and the volume density (c) and the weight losses of 3D graphene/Fe3O4@Ppy aerogels in different acidic solutions (d).

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Fig. 3 shows FTIR spectrum of several materials. Compared with Fe3O4, Fe3O4@Ppy shows the new peaks at 1560 cm−1 and 1454 cm−1 belonging to the symmetrical and unsymmetrical stretching vibration peaks of polypyrrole rings and 904 cm−1and 738 cm−1 belonging to the out-of-plane bending vibration of C–N in the pyrrole rings [39]. 1639 cm−1 and 1311 cm−1are assigned to the aromatic CC and −OH vibrations derived from the residual oxygen containing groups of GO [38]. It is worth noting that the characteristic peaks of Ppy at 1450 cm−1 and 904 cm−1 are strengthened to a degree due to the interaction between Ppy and graphene layer.

Fig. 3.

FTIR spectrum of several materials.

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Fe3O4 crystal patterns in Fe3O4@Ppy nanoparticles and GFPs can be detected through XRD as shown in Fig. 4. The main peaks of Fe3O4 at 2θ = 30.0°, 35.4°, 43.0°, 53.4°, 56.9° and 62.5° corresponding to 220, 311, 400, 440 and 511 lattice planes can be observed, confirming the existence of Fe3O4 in Fe3O4@Ppy and GFP1:3 [40]. According to Debye-Scherrer equation (D=kλ/βcosθ, λ is X-ray wavelength; k is the shape factor; D is the average diameter of the crystal in angstroms; θ is the Bragg angle in degrees and β is the line broadening measured by half-height in radians) [41], the average diameter of the crystal can be calculated to be 19.9 nm. This confirms that the present Fe3O4 is the assembly of smaller Fe3O4 nanoparticles with the size of 19.9 nm, which can be discerned from the appearance of Fe3O4 in Fig. 1(a).

Fig. 4.

XRD patterns of several materials.

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Fig. 5 shows Raman spectra of GFP1:3 together with those of GO and GA for comparison. D and G bands are located at about 1340 cm-1 and 1600 cm-1. The ID/IG intensity ratio increases from 0.87 of GO to 1.15 of GA, and to 1.01 of GFP1:3, showing more defects in GA and GFP1:3 [42]. Also, it can be detected that the peaks of GA and GFP 1:3 show obvious blue shifts compared with those of GO, which further confirms that more defects were formed during the reduction of GO and GA formation. Defects do be benefit for the attenuation of electromagnetic wave. It is noted that GFP1:3 possesses a little bit lower ID/IG (1.01) compared with that of GA probably because Fe3O4@Ppy introduction prevents the graphene layers from aggregation leading to the restoration of GFP structure defects to a degree.

Fig. 5.

Raman patterns of GFP1:3 and GA compared with that of GO.

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Cross-sectional SEM images in Fig. 6(a, b) were taken to investigate the structure and morphology of GFPs. It can be detected that the GFP exhibits three-dimensional graphene frameworks full of void spaces with the size of ˜50 µm. Fe3O4 as white dots evenly scatters on the graphene shell. The good scattering of Fe3O4 particles should be derived from the good dispersion of Fe3O4@Ppy nanoparticles in GO solution (see Fig. 1). Fig. 6(c) is the TEM image of the GFP1:3. It can be seen that the 3D graphene sheet in network walls is full of wrinkle and folding [43]. The Ppy shells can be also clearly detected and didn’t be ruined, showing that Ppy shells coated on Fe3O4 cores could maintain stable during the preparation of GFPs. This guarantees the stable properties of GFPs.

Fig. 6.

SEM (a,b) and TEM (c) images of GFP1:3.

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Fig. 7 shows the magnetic properties of as prepared Fe3O4, Fe3O4@Ppy and GFP1:3 measured at room temperature. Fe3O4@Ppy and GFP1:3 exhibited nearly the same saturated magnetization of around 18 emu/g. Compared with the pure Fe3O4 nanoparticles (82 emu/g), a reduction of saturated magnetization for Fe3O4@Ppy and GFP1:3 can be detected. It is reasonable that the components of nonmagnetic Ppy and graphene decrease the magnetism of GFPs.

Fig. 7.

Magnetic hysteresis loop of Fe3O4, Fe3O4@Ppy and GFP1:3.

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3.2Electromagnetic absorbing performance and mechanism

The RL values of the materials in the frequency range of 2−18 GHz are estimated by the complex permittivity and complex permeability varying with different frequencies and thicknesses according to the transmission line theory by the following Eq.s (1) and (2) [1,3]:

Where Zin is the input impedance of the composites, f is the microwave frequency, d is the thickness of the absorbed layer, and c is the velocity of electromagnetic wave in a vacuum. The relative complex permittivity (εr=ε'-jε'') and permeability (μr =μ'-jμ'') could be calculated from the complex permittivity (ε' and ε'') and complex permeability (μ' and μ'') of the composites measured with a vector network analyzer.

As shown in Fig. 8, for the pristine GA the minimum RL moves to low frequency with the increase of the thickness due to the electromagnetic wave dimensional resonance with the increase of coating thickness, which is consistent with the predication based on the equation (fm=c/2πμ''d, where fm is the frequency of the minimum reflection loss peak and d is the matching thickness) [23]. With the increase of Fe3O4@Ppy addition, the minimum RL of GFPs shifts from low to high frequency band. Specifically, for GFP1:1 the minimum RL is -20.4 dB at 6.0 GHz, for GFP1:2 it is −40.5 dB at 6.3 GHz, for GFP1:3 it is −39.2 dB at 9.0 GHz and for GFP1:4 it is −18.2 dB at 15.7 GHz compared with only −14.3 dB at 6.6 GHz for GA. The effective bandwidth (RL< −10 dB) of the reflection loss peak also tends to be wider and the thickness tends to be thinner for a higher content of Fe3O4@Ppy. For example, the minimum RL exhibits the effective bandwidth of 5.1 GHz (12.9–18.0 GHz) for the GFP1:4 with the thickness of 1.5 mm and 4.1 GHz (7.3–11.4 GHz) for the GFP1:3 with the thickness of 3.5 mm compared with 2.7 GHz (5.2–7.9 GHz) for the GFP1:2 with the thickness of 5.0 mm and 2.2 GHz (5.0–7.2 GHz) for the GFP1:1 with the thickness of 5.5 mm. The effective bandwidth of the minimum RL for GA is only 2.0 GHz (5.6–7.6 GHz) with a 5.5 mm thickness. Especially, the maximum effective bandwidth of GFP1:3 reaches 5.12 GHz (11.12–16.24 GHz) with the thickness of 2.5 mm.

Fig. 8.

RL curves of GA and GFPs (GFP1:1, GFP1:2, GFP1:3 and GFP1:4) with different compositions at various thickness.

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In general, the RL exceeding -10 dB in a wide frequency range of 4.5−18 GHz for GFP1:2 and of 4.4−18 GHz for GFP1:3 and especially for GFP1:3 the RL exceeding −20 dB in the frequency range of 5.8–18 GHz are obtained by changing the thickness. A typical RL value of −10 dB corresponding to 90 % absorption is suitable for practical application. This means that the good performance in different frequency range could be tuned by changing the content of Fe3O4@Ppy introduced.

The electromagnetic parameters of GA and GFPs with different compositions are shown in Fig. 9. For the GA and all the GFPs, the real part (ε') and the imaginary part of complex permittivity (ε'') decline over the 2−18 GHz range with a slight fluctuation at high frequency caused by charge polarization, which is the similar frequency dispersion behavior with that of the pristine carbon materials [44]. Furthermore, with the addition of Fe3O4@Ppy nanoparticles, ε' and ε'' values of GFP1:1, 1:2 and 1:3 mildly increase and the GFP1:3 exhibits the highest ε' and ε'' values among them. It is worth noting that for GPF1:4 the complex permittivity (ε' and ε'') dramatically increase and the decline trend of ε'' with frequency is sharp compared with that of GA and the other GFP samples. The dielectric loss tangent (tanδe=ε''/ε') was also calculated based on electromagnetic parameters and is shown in Fig. 9(c). Tanδe values of the materials keep the similar decreasing trend with ε'' with the frequency increasing. However, for GFP1:4 the tanδe value declines sharply from 1.7 at 2 GHz, and is below that of GFP1:3 at about 12.2 GHz and becomes the smallest among all the materials at above 16.3 GHz.

Fig. 9.

Frequency dependence of real parts(a), imaginary parts(b) and loss tangents(c) of complex permittivity, and frequency dependence of real parts(d), imaginary parts(e) and loss tangents(f) of complex permeability for GFPs with different compositions.

(0.29MB).

According to the free electron theory that ε' represents the storage capacity of electric energy and ε'' represents the energy loss capacity of the material, and higher ε'' indicates higher energy transfer [45]. Furthermore, ε' is an expression of polarization ability of a material which mainly arises from dipolar polarization and interfacial polarization at microwave frequency [46]. Thus, the increased ε' should be ascribed to the enhanced dipolar polarization provided by abundant surface functions, the defects of the 3D graphene and Ppy in GFPs, and the enhanced interfacial polarization due to the increased interfaces in core-shell Fe3O4@Ppy microspheres and the graphene network cell interfaces with Fe3O4@Ppy microspheres (Scheme 1). These various polarizations contribute to the dielectric loss according to Debye theory [46].

Scheme 1.

Schematic diagram of several mechanisms in GFPs.

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Another contribution to the dielectric loss is from conductivity loss. According to the free electron theory, higher ε'' indicates higher conductivity (ε''≈1/πε0ρf, where ρ is the resistivity) [44,45]. Ppy is a conducting polymer, with the addition of more Fe3O4@Ppy nanoparticles, more Ppy shells will contribute to the conduction of GFPs, which can be deduced from the increased ε''. In general, the permittivity can be represented by the Debye relaxation expression, the relationship of ε' and ε'' can be deduced as the equation (3) [47]:

Where, εs and ε are the stationary dielectric constant and relative dielectric constant at the high-frequency limit, respectively. The plot of ε' versus ε'' would be Cole-Cole semicircles. A semicircle means a dielectric relaxation progress corresponding to a Debye dipolar relaxation. Fig. 10 shows the ε'˜ε'' curves of GFPs. It is obvious that the GFPs display more semicircles compared with GA. For example, there are seven distinguishable semicircles for GFP1:4 and only four for GA, suggesting that the core-shell Fe3O4@Ppy endows GFPs multiple dielectric relaxation processes.

Fig. 10.

Cole-cole semicircles for GA, GFP1:1, GFP1:3 and GFP1:4 in the frequency of 2−18 GHz.

(0.13MB).

The complex permeability behaviors of GFPs are exhibited in Fig. 9(d–f). It can be seen from the figure that the GA and GFP1:1, 1:2 and 1:3 show the similar behavior and the complex permeability (the real part μ' and the imaginary part μ'') declines over 2−18 GHz range. In contrast to ε' and ε'', the variation of μ' and μ'' for different compositions is small and shows some fluctuations on different frequencies. The imaginary part of permeability for the GA, GFP1:1, 1:2 and 1:3 is lower than 0.2. The weak magnetic loss property is implied because the real and imaginary parts of the complex permeability are directly proportional to the energy density and magnetic loss power stored in the medium. From Fig. 9(f), the magnetic loss tangent (tanδm= μ''/μ') values of the GFPs except for GFP1:4 are smaller than the tanδe, further showing the relatively weak magnetic loss properties. However, it should be noted that GFP1:4 shows a much different behavior of complex permeability and magnetic loss tangent from GA and the other GFPs. It shows more evident fluctuation of complex permeability properties and much increased μ'' and tanδm over 2−18 GHz range.

The magnetic loss mainly originates from hysteresis, domain wall resonance, natural ferromagnetic resonance and eddy current effect [48,49]. The hysteresis loss is negligible in the weak field, and the domain wall resonance loss usually occurs at much lower frequency (MHz). Therefore, the natural ferromagnetic resonance exhibiting in 1−10 GHz and the eddy current in high frequency are the two main factors affecting the complex permeability (μ' and μ''). Unsteady values of C0 (Co=μ''(μ')-2f-1) for GFPs at 2−18 GHz suggest that the composites may not have eddy current loss [50]. It implies that the natural ferromagnetic resonance is the key mechanism of magnetic loss and contributes to the reflection loss in low frequency range.

As above exhibition, reflection losses of GA and GFP1:1, GFP1:2 and GFP1:3 mainly come from the dielectric loss, and they all have a weak magnetic loss, so the dielectric value cannot be too high to break the matching. This is also the reason why the Fe3O4@Ppy content has the upper limit. For GFP1:4 containing an excess of Fe3O4@Ppy the dielectric and magnetic properties are all greatly improved (see Fig. 9). It is hard for the sharply decreased dielectric loss from a high value to match with the increased magnetic loss from a low value. The impedance matching condition could be evaluated by |Zin/Z0| value (Zin=Z0(μr/εr)12), and the |Zin/Z0| closed to 1 leads to the good impedance matching and the possibility of excellent EM absorption performance of the absorber [51–54]. As shown in Fig. 11, for GFP1:4 the |Zin/Z0| value shows substantial deviation from 1, confirming the poor impedance matching condition. Therefore, for GFP1:4 the performance of microwave absorption degrades and most of incident microwave will be reflected off at the surface although GFP1:4 possesses the enhanced dielectric loss and magnetic loss. It also can be seen that compared with GA and GFP1:1, GFP1:2 and GFP1:3 possess the good impedance matching condition and exhibit the most significant enhancement of dielectric properties and thus show the best MA performance as the above exhibition.

Fig. 11.

The normalized input impedance |Zin/Z0| of GFPs with different compositions.

(0.14MB).

In a word, the present MA properties of GFPs may be explained by several observations. First, the introduction of Ppy as shell material plays an important role in enhancing the MA properties and stability. Enhanced dielectric loss due to Ppy introduction contributes to the further energy loss derived from the Debye relaxation process, interface polarization and other polarization mechanisms. Second, the 3D graphene aerogel endows porous structure for attaching Fe3O4@Ppy nanoparticles leading to the abundant interface and ultra-light weight of the MA material besides providing basic dielectric properties and conductivity. When the microwave penetrated the material it was trapped by the inner porous space and the cross planes between the graphene layers and Fe3O4@Ppy microspheres (scheme 1). Third, the Fe3O4 nano-component contributes the enhanced MA properties in low frequency range even though this contribution is not so large. The Fe3O4 in 3D graphene aerogel is also believed to be benefit for widening absorption bands and obtaining the better impedance matching condition [27–29,31]. Of course, better impedance matching derived from a suitable dosage of Fe3O4@Ppy in 3D graphene aerogels and the synergy effect of three components (Fe3O4, Ppy and 3D graphene) is premise for all the above mentioned factors to work.

4Conclusions

In order to enhance microwave absorption and achieve property stability core-shell Fe3O4@Ppy microspheres were prepared and anchored on the reduced three-dimensional graphene aerogel (GA) through a one-step chemical reduction and self-assembly method, and the composites GFPs with different Fe3O4@Ppy compositions were obtained. The minimum reflection loss of -40.53 dB at 6.32 GHz and the effective bandwidth of 5.12 GHz (11.12–16.24 GHz) with the thickness of 2.5 mm were achieved through tuning the addition of Fe3O4@Ppy. Excellent MA performance was mainly ascribed to enhanced dielectric loss due to the cooperation of Ppy shells. The suitable magnetic property mainly derived from natural ferromagnetic resonance of Fe3O4@Ppy is very important for improving MA properties in low frequency range and achieving the good impedance matching condition. Importantly, the enhanced stability and ultra-low density were confirmed. The strategy is instructive to design stable and light-weighted MA materials for practical application.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful for grants from the Aeronautical Science Foundation of China (2017ZF52065) and Nanjing University of Aeronautics and Astronautics Open Foundation (kfjj20170620).

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