1 Introduction

The microwave absorption materials have been widely investigated to solve the electromagnetic interferences and pollution caused by the integration and microminiaturization of electron devices [1,2,3,4,5]. To increase the microwave absorption properties at gigahertz, the effective combination between the dielectric and the magnetic loss of the absorbent has attracted more attention [6,7,8]. Here, the dielectric and magnetic loss abilities, which are mainly associated with the complex permittivity (εr = ε′ + iε″) and permeability (μr = μ′ + iμ″), have been achieved by the combination between carbon-based materials and ferromagnetic materials [9,10,11,12], such as FeCo/carbon [12,13,14], metal/carbon [15,16,17,18,19,20], Fe3O4/carbon [21,22,23,24,25], and metal/oxide [26,27,28,29]. However, the complex and expensive synthesis approaches of the aforementioned materials could thus limit their applications.

Consequently, microscale magnetic iron powders can be regarded as promising absorbers because of their high magnetic permeability and saturation magnetization. However, the deficiency of dielectric loss mechanism could thus result in the mismatch of electromagnetic impedance and reduce their loss abilities at gigahertz [30,31,32,33,34,35]. To solve these holdbacks and achieve a proper impedance matching, the efficient approach is to composite them with other type absorbers, in which their absorption bandwidth and attenuation coefficient could be further optimized [35,36,37], for example, Fe/SiO2 [38], Fe/TiO2 [39], Fe/MnO2 [40], Fe/CNT [41], and Fe/SiC [42]. Among all these candidates, silicon carbide (SiC) has received more concentrations ascribed to its excellent chemical stability, dielectric loss capability, and microwave absorption efficiency [43, 44]. It has been reported that the composites prepared through Fe and SiC could achieve satisfactory microwave absorption performances at gigahertz regions [45,46,47], in which the magnetic elements introduced magnetic loss abilities while the introduction of SiC tailors the impedance matching and meanwhile increases the dielectric loss performance. Despite these achievements, the SiC/Fe composites were usually fabricated by the mechanical milling process, which still requires a long preparation time and high energy cost. Thus, a simple synthesis approach to such core/shell structural composites needs to be explored.

In this work, we papered a series of core/shell structural Fe/SiC composites by a heat-assisted surface adhesion process. Their magnetic/dielectric impedance matching performance could be optimized by the synergistic effect of heterogeneous components. By adjusting the ratio of Fe and SiC powders, the electromagnetic properties could be tuned, resulting in a broad absorption bandwidth, strong reflection loss, and high microwave attenuation performance. Additionally, ascribing to the cost-effective production approach of Fe/SiC composites, it promises great potential in engineering applications. In addition, the applications of these composites can be evaluated in the thermoplastics and thermosetting systems with other functions to figure out their electromagnetic wave absorption performances [48,49,50,51].

2 Experimental

2.1 Preparation

Commercial iron powders (~ 35 µm) and silicon carbide powders (~ 500 nm) were purchased from ZhongNuo Advanced Material Technology Co., Ltd., Beijing, China. Firstly, iron powders and silicon carbide powders with a molar mass ratio of 2:1 were poured into a three-necked flask. Secondly, the mixture was annealed for 2 h at 120 °C with a stirring speed of 500 rpm under a nitrogen atmosphere. Finally, the as-made composite denoted as Fe:SiC 2:1 could be collected for usage. Fe:SiC 1:1, Fe:SiC 1:2, and Fe:SiC 1:3 composites were obtained by adjusting the molar mass ratio of two powders as 1:1, 1:2, and 1:3, respectively.

2.2 Characterizations

The phase component analysis was performed by X-ray diffraction (XRD) with a Cu-Kα (λ = 0.15405 nm) irradiation at a voltage of 30 kV and a current of 30 mA at a scan step of 0.2°. The microstructure images were obtained by JEOL JSM-7001F field emission scanning electron microscope (FESEM) at an accelerating voltage of 15 kV. The magnetic hysteresis loop measurement was recorded on a Lakeshore vibrating sample magnetometer (VSM) with a magnetic field of ± 12 kOe.

2.3 Electromagnetic measurements

For measurement of the electromagnetic parameters, a Keysight N5222A vector network analyzer (VNA) was calibrated by the through-reflect-line (TRL) standard technique. The scattering parameters were tested by a K-band waveguide (Model: 42 WAL-100, A-INFOMW). The sample was produced by uniformly mixing the Fe/SiC composites with 30 wt% paraffin, in which the filling ratio is set as the maximum loading contents of the composited materials.

3 Results and discussion

The phase components were analyzed by XRD. Figure 1 shows the XRD patterns of the original Fe powders, SiC powders, and Fe/SiC composites synthesized at ratios of Fe to SiC of 2:1, 1:1, 1:2, and 1:3. The diffraction peaks at 44.7°, 65.0°, and 82.3° are aligned with the (110), (200), and (211) planes of the iron with body-centered cubic (BCC) structure (JCPDS Card 65–4899). After surface modification, five new peaks appear at 34.2°, 35.8°, 38.2°, 60.2°, and 71.0°, respectively, which can be assigned to the (101), (102), (103), (110), and (201) planes of β-SiC (JCPDS Card 73–1663). It should be noted that the Fe peaks remain intact after SiC coating without the formation of other peaks, indicating the high degree of purity of the Fe/SiC composites. As the contents of SiC are increased, its diffraction peaks become more distinct, suggesting the increased coverages of SiC on the surface of Fe powders.

Fig. 1
figure 1

The XRD patterns of Fe, SiC, and Fe/SiC composites

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Morphologies of the synthesized products are presented in Fig. 2. SEM images (Fig. 2a) of the initial Fe powders indicate spherical shapes with a diameter of 30–40 μm. Figure 2b exhibits SiC powders of a flaky shape with a diameter of 500 nm. After the heat-assisted surface adhesion process of 2 h, the microstructures of Fe/SiC composites are shown in Fig. 2c, d, and e. Compared with Fig. 2a, it is clearly observed that the surface of the particles is not smooth, evidencing that the SiC powders are completely coated on the surface of Fe powders. This is further affirmed by energy-dispersive spectrometer (EDS) results, illustrating the Si and Fe elemental distribution for an individual core/shell structural particle, as shown in Fig. 2f, g.

Fig. 2
figure 2

The SEM images and elemental mapping of samples. a Fe powders, b SiC powders, c–f Fe:SiC 1:3 composite with different magnifications, and g the Fe and Si element overlay mapping images

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Field-dependent magnetization curves and the diagram of saturation magnetization and coercivity of Fe and Fe/SiC composites with different ratios are presented in Fig. 3a, b. The magnetic hysteresis loops were measured at room temperature between the magnetic field range of ± 12 kOe. The Fe and Fe/SiC powders with ratios of 2:1, 1:1, 1:2, and 1:3 are corresponding to the saturation magnetizations (Ms) of 193 emu/g, 161 emu/g, 127 emu/g, 92 emu/g, and 67 emu/g, the coercive fields (Hc) of 26.2 Oe, 13.5 Oe, 11.7 Oe, 10.8 Oe, and 20.5 Oe, respectively. They all have a declining trend due to the increase of the non-magnetic SiC components. The residual magnetization of all the samples was low, indicating that the Fe/SiC composites were soft magnetic at room temperature. Figure 3b shows that the saturation magnetization and coercivity are gradually decreased along with the ratio of SiC powders increasing. The increasing SiC contents could accelerate the formation of SiC shells on the Fe powders; such additional non-magnetic materials could thus reduce the Ms values.

Fig. 3
figure 3

Magnetic performances. a The magnetic hysteresis loops of Fe and Fe/SiC composites with different ratios. b The diagram of saturation magnetization and coercivity as a function of ratios

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The relatively complex permeability (μr = μ′ − jμ″) and the relatively complex permittivity (εr = ε′ − jε″) were measured at the frequency range of 18–26.5 GHz (K-band). Figure 4a, b show the real part (μ′) and imaginary part (μ″) of complex permeability (μr) of Fe and Fe/SiC composites with different ratios. The μ′ of almost all the coated samples are higher than the original Fe powders. The μ″ of Fe/SiC composites at ratios of 1:1 and 1:3 are lower than Fe powders, while the others show opposite changes. The tangential magnetic loss (tanδm = μ″/μ′) and the tangential dielectric loss (tanδe = ε″/ε′) had a dominant influence on the EM absorption [52]. The frequency-dependent tangential magnetic loss of Fe/SiC composites is presented in Fig. 4c. The tanδm values of all powders are quite low, within the range of 0 to 0.2. The decreasing trend of tanδm versus frequency was almost the same for all samples, and meanwhile the magnetic loss capability has been decreased due to the corresponding increase of the SiC nanoparticles.

Fig. 4
figure 4

Electromagnetic properties of Fe/SiC composites with different ratios of 2:1, 1:1, 1:2, and 1:3. a and b Relatively complex permeability. c The magnetic tangential loss. d and e Relatively complex permittivity. f The dielectric tangential loss

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Figure 4d, e show the real part (ε′) and imaginary part (ε″) of complex permittivity (εr) of SiC and Fe/SiC composites with different ratios. Except for the original SiC powders, the ε′ and ε″ of Fe/SiC composite at the radio of 1:1 had the highest values, reaching 9.2 and 5.4, respectively. In Fig. 4f, all the samples show a similar rising trend in tanδe, which becomes more obvious after coating SiC particles in the frequency region between 18 and 26.5 GHz. The values of tanδe are above 0.5 for the samples with Fe to SiC of ratios of 1:1 and 1:3, and a clear peak is seen around 19 GHz. Especially, the values of tanδe for the sample at ratios of 1:3 are the highest to 0.9 among all the samples. The values are particularly elevated in the frequency regions of 18 to 20 GHz, indicating the increased microwave energy loss abilities. Commonly, the improvement of dielectric loss capacity in gigahertz is always related to the polarization and relaxation process. Therefore, the Cole–Cole curves have been calculated and exhibited in Fig. 5; there are multiple polarization and relaxation processes that can be recognized in the core/shell structural Fe/SiC composites. Moreover, it can be noticed that the increased permittivity value is together with the decreased permeability value, indicating that an energy transformation phenomenon has occurred between the permittivity and permeability, which can be usually recognized in the closed-packed particle systems [53, 54]. Thus, the energy transformation results in increased dielectric loss performance and meanwhile induces the negative values of complex permeability.

Fig. 5
figure 5

Cole–Cole curves of the core/shell structural Fe/SiC composites. The ratios of Fe and SiC are a 2:1, b 1:1, c 1:2, and d 1:3, respectively

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After coating SiC, there is an insulating shell on the surface of the Fe particles, forming a large amount of Fe/SiC heterogeneous interface. Owing to different electrical conductivities between Fe and SiC powders, when the electromagnetic wave acts, the electric charge in the SiC powders could be transferred to the Fe powders by overcoming the barrier of the interface [55]. Meanwhile, the electric charge is accumulated at the interface, causing the induced charge to lag. Such an interfacial polarization behavior leads to polarization relaxation, which depletes more electromagnetic wave energy [56, 57].

The EM absorption performance of Fe, SiC, and Fe/SiC composites was evaluated in the 18–26.5-GHz band by calculating the reflection loss (RL) according to transmission line theory [58]. The reflection loss RL (dB) could be described by the following equations [59]:

$$\mathrm{RL}\left(\mathrm{dB}\right)=20\mathrm{lg}\left|\frac{{Z}_{\mathrm{in}}-1}{{Z}_{\mathrm{in}}+1}\right|$$
(1)
$${Z}_{\mathrm{in}}=\sqrt{\frac{{\mu }_{r}}{{\varepsilon }_{r}}}\mathrm{tanh}i\frac{2\pi f}{c}\sqrt{{\mu }_{r}{\varepsilon }_{r}}d$$
(2)

As shown in Fig. 6a, b, Fe powders exhibited poor microwave absorption in the lower frequency range at 18–26.5 GHz. By incorporating dielectric SiC powders, the minimum reflection losses of Fe/SiC composites with Fe:SiC = 2:1, 1:1, 1:2, and 1:3 were −25 dB at 20.7 GHz, −43 dB at 20.9 GHz, −24 dB at 18 GHz, and −51 dB at 19.4 GHz, respectively, as shown in Fig. 6c–l. The mostly optimized Fe/SiC composite is at Fe to SiC ratio of 1:3, which is attributed to both improved dielectric loss and excellent impedance matching.

Fig. 6
figure 6

The three-dimensional reflection loss mapping and corresponding contour plots. a and b Fe powders, c and d SiC powders, and Fe/SiC composites synthesized at ratios of Fe to SiC of e and f 2:1, g and h 1:1, i and j 1:2, and k and l 1:3. The thicknesses are ranging from 0 to 5 mm with an interval of 0.01 mm

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Figure 7 provides the calculated reflection loss curves and bandwidth of Fe powders, SiC powders, and Fe/SiC composites. When the reflection loss is lower than −10 dB, it presents 90% absorption efficiency and the corresponding frequency band is called the effective absorption bandwidth [60]. Figure 7a, b illustrate that the effective absorption bandwidth of original Fe powders was 3.3 GHz in the thickness range from 1.23 to 1.31 mm. In comparison, Fig. 7c–l indicate the effective absorption bandwidth of the Fe/SiC composites (Fe:SiC = 2:1, 1:1, 1:2, 1:3) centered at 20.9 ~ 26.5 GHz, 18.4 ~ 24.4 GHz, 18 ~ 23.3 GHz, and 18 ~ 25.6 GHz at the absorber thicknesses of 1.2 ~ 2.3 mm, 1.3 ~ 2.4 mm, 1.17 ~ 2.12 mm, and 1.2 ~ 4.69 mm, respectively. It is obvious that the introduced SiC shell broadened the effective absorption bandwidth. Meanwhile, higher SiC content caused a remarkable improvement in microwave absorption properties.

Fig. 7
figure 7

Microwave absorbing performances of samples. a and b Fe powders, c and d SiC powders, and Fe/SiC composites synthesized at ratios of Fe to SiC of e and f 2:1, g and h 1:1, i and j 1:2, and k and l 1:3

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4 Conclusions

In conclusion, the Fe/SiC composites were synthesized by a heat-assisted surface adhesion process. By optimizing the component ratios of micro Fe and nano SiC powders, the relatively complex permittivity and permeability of composites could be effectively adjusted, in which the composites with a ratio of Fe to SiC of 1:3 present an improved electromagnetic impedance matching performance. For the best optimized Fe/SiC composite, the minimum reflection loss reached −51 dB at 19.4 GHz and the absorption bandwidth less than < −10 dB ranged from 18 to 25.6 GHz at the absorber thicknesses of 1.2–4.69 mm. This study provides a simple approach to synthesize the Fe/SiC composites for high-performance microwave absorption.