Keywords

Introduction

Microwave absorption materials are widely used in the television, broadcast, radar, microwave anechoic chamber and electronic devices. [1]. Graphene , periodic honeycomb shaped new carbon material [2], becomes a potential nanoscale building block for new absorbing materials due to its outstanding properties. The two dimensional conjugate structure of graphene [3] contributes to its high dielectric loss, low density, special surface properties, etc. [4]. However, its remarkable dielectric loss ability and non-magnetic feature lead to poor impedance matching and thus a low attenuation property in microwave absorption [5,6,7]. In order to improve microwave absorption properties, one of the effective approaches is to load magnetic materials, typically ferrites, on graphene [8,9,10]. For example, when reduced graphene oxide (r-GO) was coated with Fe3O4 through a co-precipitation method, the reflection loss (RL) value of rGO/Fe3O4 composite (filling ratio = 8 wt%) reached −47.9 dB at 10.1 GHz with the effective absorption bandwidth (RL < −10 dB) ranging from 6.5 to 10.3 GHz (coating layer thickness of 2.0 mm) [8]. Other ferrites are also used for improving the magnetic performance of graphene matrix composite. A recent study showed that rugby-shaped CoFe2O4/graphene composites via a vapor diffusion method had the minimum RL of −39.0 dB (filling ratio = 60 wt%) at 10.9 GHz and corresponding effective absorption bandwidth was 4.7 GHz at 2.0 mm [9]. Another report demonstrated that NiFe2O4 nanorod/graphene composites of 2.0 mm thickness (filling ratio of 60 wt%), prepared by an ionic liquid assisted one-step hydrothermal approach, achieved minimum RL of −29.2 dB with the effective absorption bandwidth of 4.4 GHz [10]. The good microwave absorption performance of those composites was largely attributed to enhanced electron transmission when two or more ions replacing Fe cations within the spinel structure [11]. For this mechanism, it is anticipated that Co–Ni ferrites may be another promising candidate to be assembled onto graphene for further improvement of absorption performance

In this paper, a novel composite constituted by Co0.5Ni0.5Fe2O4 and graphene was synthesized via microwave hydrothermal method in just several minutes. The minimum RL of the composite of 3.0 mm thickness reached −13.1 dB at 17.2 GHz with an effective absorption bandwidth of 3.1 GHz.

Experimental

Materials

All chemical reagents used in this experiment, including natural graphite powder (<48 µm), sulphuric acid (H2SO4), potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrogen peroxide (H2O2), cobalt acetate (Co(OAc)2), nickel acetate (Ni(OAc)2), ferrous chloride (FeCl2), ammonia (NH3∙H2O), and ethanol were of analytical grade and used as received without further purification.

Preparation of Co0.5Ni0.5Fe2O4/Graphene Composite

For preparation of Co0.5Ni0.5Fe2O4/graphene composite, 2.0 mmol of FeCl2, 0.5 mmol of Co(OAc)2 and 0.5 mmol of Ni(OAc)2 were mixed and dissolved in 80 mL of distilled water under magnetic stirring. Subsequently, 50 mg of GO, which was preliminarily prepared by the Hummers method using the natural graphite powder as starting material [12], was added into the above solution under ultrasonic treatment to form a homogeneous solution. The mixture was stirred for 4 h at room temperature. The pH value of the suspension was adjusted to 10 by adding NH3∙H2O. The mixture was stirred for 1 h at room temperature. Then, the reaction mixture was heated at 160 °C in a microwave reactor at power of 500 W for 25 min. The resultant precipitate was filtered, washed with deionized water and ethanol, and finally dried in a vacuum oven at 70 °C for 12 h.

Characterization of Co0.5Ni0.5Fe2O4/Graphene Composite

The electromagnetic parameters (permittivity and permeability) of the composite were measured with an AV3629 vector network analyzer using the coaxial line method. Samples were first prepared by homogeneously mixing the nanocomposite with paraffin (the weight content of the as-prepared powder was about 20 wt%), and then the mixture was pressed into a toroid with an inner diameter of 3.0 mm, an outer diameter of 7.0 mm and a height of 2.0 mm. Subsequently, the samples were embedded into a copper holder and connected between the waveguide flanges of the instrument for measurement of the parameters.

To assess the microwave absorption performance of Co0.5Ni0.5Fe2O4/graphene composite, its reflection loss (RL) was calculated by the following equations [13, 14]:

$$ {\text{RL}}\left( {\text{dB}} \right) \, = \, 20 \times { \log }\left| {\frac{{{\text{Z}}_{in} - {\text{Z}}_{0} }}{{{\text{Z}}_{in} + {\text{Z}}_{0} }}} \right| $$
(1)
$$ Z_{\text{in}} = Z_{0} \times \sqrt {\frac{{\upmu_{r} }}{{\upvarepsilon_{r} }}} \times \text{Tan} {\text{h}}\left[ { {\text{j}} \times \frac{{2 \times\uppi \times {\text{f}} \times {\text{d}}}}{\text{c}} \times \sqrt {\upmu_{r} \times\upvarepsilon_{r} } } \right] $$
(2)
$$ Z_{0} = \sqrt {\frac{{\upmu_{0} }}{{\upvarepsilon_{0} }}} $$
(3)

where Zin is the input impedance of the absorber, Z0 is the input impedance of the free space, εr (εr = εr′ − jεr″) is the complex relative permittivity of the absorber, μr (μr = μr′ − jμr″) is the complex relative permeability of the absorber, c is the velocity of electromagnetic waves in free space, ƒ is the frequency and d is the absorber layer thickness.

Results and Discussion

Electromagnetic Absorption Properties

Figure 1a shows the real part (εr′) and imaginary part (εr″) of the complex relative permittivity of Co0.5Ni0.5Fe2O4/graphene composite. It is revealed that the εr′ value decreased gradually from 2.89 to 2.59 and the εr″ value increased from 0.003 to 0.479, respectively, in the frequency range of 2–18 GHz. Figure 1b shows the real part (μr′) and imaginary part (μr″) of the complex relative permeability of Co0.5Ni0.5Fe2O4/graphene composite. It reveals that the values of μr′ were in the range of 0.95–1.11 over 2–18 GHz and the μr″ values exhibited a major peak at 10.16 GHz and a minor peak at around 4.64 GHz. From Fig. 1c, it is found that most of the values of the dielectric loss tangent (tan δεr) were larger than the values of the magnetic loss tangent (tan δμr) from 2 to 18 GHz, except the frequency ranges of 4.48–6.40 and 9.60–10.88 GHz. Therefore, we can conclude that the electromagnetic wave attenuation mechanism of Co0.5Ni0.5Fe2O4/graphene composite was mainly ascribed to electrical loss in the frequency range of 2–18 GHz. The higher tan δμr values than the tan δεr values were associated with the two peaks of the μr″ value in Fig. 1b.

Fig. 1
figure 1

Complex relative permittivity (a), complex relative permeability (b), loss tangent (c) and reflection loss (d) of Co0.5Ni0.5Fe2O4/graphene composite

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Figure 1d shows the RL curves of Co0.5Ni0.5Fe2O4/graphene composite with different thicknesses. Its minimum RL was up to −13.1 dB at 17.2 GHz, and the bandwidth corresponding to the RL values below −10 dB (90% of electromagnetic wave absorption) was 3.1 GHz (from 14.9 to 18.0 GHz) with a thickness of 3.0 mm. In addition, the minimum RL values obviously decreased first and then increased with increasing the layer thickness.

To investigate the possible mechanism of the enhanced microwave absorption properties of Co0.5Ni0.5Fe2O4/graphene composite, the Cole-Cole semicircle curve and the values C0 [μr″ (μr′)−2ƒ−1] versus frequency are presented in Fig. 2. As for the Debye dipolar relaxation, the complex relative permittivity can be expressed by the following equation [15]:

Fig. 2
figure 2

Cole-Cole curves (a) and C0 versus frequency curve of Co0.5Ni0.5Fe2O4/graphene composite (b)

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$$ \upvarepsilon_{r} = \varepsilon_{\infty } + \frac{{\varepsilon_{s} - \varepsilon_{\infty } }}{1 + j2\pi f\tau } = {\varepsilon_{r}^{\prime}} - {j \varepsilon_{r}^{\prime\prime}} $$
(4)

where ƒ, εs, ε∞ and τ are the frequency, static permittivity, relative dielectric permittivity at the high-frequency limit, and polarization relaxation time, respectively. Thus, εr′ and εr″ can be described by the following equations:

$$ {\varepsilon_{r}^{\prime}} = \varepsilon_{\infty } + \frac{{\varepsilon_{s} - \varepsilon_{\infty } }}{{1 + \left( {2\pi f} \right)^{2} \tau^{2} }} $$
(5)
$$ {\varepsilon_{r}^{\prime\prime}} = \frac{{2\pi f\tau \left( {\varepsilon_{s} - \varepsilon_{\infty } } \right)}}{{1 + \left( {2\pi f} \right)^{2} \tau^{2} }} $$
(6)

According to Eqs. (5) and (6), the relationship between εr′ and εr″ can be deduced as follows:

$$ \left( {{\varepsilon_{r}^{\prime}} - {\varepsilon_{\infty }} } \right)^{2} + \left( {\varepsilon_{r}^{\prime\prime}} \right)^{2} = \left( {{{\varepsilon_{s} - {\varepsilon_{\infty } }}}} \right)^{2} $$
(7)

Thus, the curve of εr′ versus εr″ is a single semicircle, which is generally called the Cole-Cole semicircle. Each semicircle corresponds to one Debye relaxation process. Figure 2a shows the εr′–εr″ curve of Co0.5Ni0.5Fe2O4/graphene in the frequency range from 2 to 18 GHz. The curve of εr′ versus εr″ shows that Co0.5Ni0.5Fe2O4/graphene composite had a distinct segment of three semicircles, demonstrating that the composites had multi-dielectric relaxation processes. In other words, the Debye relaxation process played a key role in improving the dielectric properties of Co0.5Ni0.5Fe2O4/graphene composite. In general, the microwave magnetic loss of magnetic materials originates mainly from hysteresis, domain wall resonance, natural ferromagnetic resonance, and the eddy current effect [16]. The hysteresis loss comes from irreversible magnetization and is negligible in a weak applied field. The domain wall resonance occurs only in multidomain materials and usually in the 1–100 MHz range [17]. In this study, the complex relative permeability was measured in the frequency range of 2–18 GHz. Therefore, hysteresis and domain wall resonance were not the main factors that led to magnetic loss of the composite. Instead, only natural ferromagnetic resonance or the eddy current effect might contribute to the microwave magnetic loss. If the eddy current effect occurs at the absorber, the C0 (C0 = μr″ (μr′)−2ƒ−1 = 2πμ0σd2/3) will be a constant when the frequency varies. It is shown in Fig. 2b that these C0 values were not constant with the increasing frequency. Therefore, the magnetic loss of the composite was considered to be originated from natural ferromagnetic resonance.

The mechanism of excellent electromagnetic absorbing performance for Co0.5Ni0.5Fe2O4/graphene composite was investigated in detail (Fig. 3). The excellent wave absorption properties and wider absorption bandwidth of Co0.5Ni0.5Fe2O4/graphene composite can be explained as follows. Firstly, the remarkable dielectric loss ability of graphene was conducive to absorbing electromagnetic waves. Loading Co0.5Ni0.5Fe2O4 nanoparticles not only effectively reduced the excessive dielectric loss of graphene but also enhance good magnetic properties of the composite, contributing to impedance matching. More interfaces and defects could be induced after the introduction of Co0.5Ni0.5Fe2O4 nanoparticles, which caused more interface polarization [18]. Secondly, the multiple reflection could occur at defect of the graphene and Co0.5Ni0.5Fe2O4 nanoparticles, extending the propagative routes of electromagnetic wave. In addition, the natural resonances produced by Co0.5Ni0.5Fe2O4 could greatly increase magnetic losses and convert electromagnetic energy into thermal energy, thus absorbing electromagnetic waves.

Fig. 3
figure 3

Possible electromagnetic absorbing mechanism of the Co0.5Ni0.5Fe2O4/graphene composite

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Compared with other absorbers, the Co0.5Ni0.5Fe2O4/graphene composite exhibited significantly enhanced electromagnetic absorbing ability, as shown in Table 1. Furthermore, it was only 20 wt% of the filling ratio for the Co0.5Ni0.5Fe2O4/graphene composite. The above results demonstrated that the Co0.5Ni0.5Fe2O4/graphene composite had strong absorption, wide absorption bandwidth and light weight.

Table 1 Electromagnetic absorption performance of similar absorbers
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Conclusions

A Co0.5Ni0.5Fe2O4/graphene composite was synthesized via a simple and rapid microwave method. It was shown that the Co0.5Ni0.5Fe2O4/graphene composite had good dielectric and magnetic properties. The minimum RL of Co0.5Ni0.5Fe2O4/graphene composite was −13.1 dB at 17.2 GHz and the absorption bandwidth with the reflection loss below −10 dB was 3.1 GHz with a thickness of 3.0 mm. The good performance of the composite was attributed to the high dielectric loss of graphene in association with multi-dielectric relaxation processes and magnetic loss of the ferrite mainly originated from natural ferromagnetic resonance.