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Abstract
Composite microspheres with carbonyl iron cores and graphite shells are prepared by a mechanical ball milling method. The microwave absorption properties of these microspheres are investigated in terms of complex permittivity and permeability, impedance matching property, and reflection loss. The morphology and conductivity of the graphite can be well tuned by controlling the milling time to achieve the improvement in its impedance matching characteristics. In addition, by integration of the composition and unique structure, the carbonyl iron/graphite microspheres possess better absorption properties than milling graphite. The microspheres show highly strong electromagnetic wave (EMW) absorption with a minimum reflection loss (RL) of −55.2 dB, which could be attributed to the good impedance matching and effective complementarities between magnetic and dielectric components. More importantly, the qualified frequency bandwidth of the absorber is up to 9.7GHz (8.3-18GHz) with a matching thickness of 2.1mm. The results indicate that carbonyl iron/graphite microspheres may be excellent candidate materials for EMW absorption applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the rapid development of information technology, electromagnetic interference (EMI) has caused more and more serious electromagnetic pollution [1–5]. Excellent EMW absorbing materials therefore have attracted wide attention over the past decade with the aim of eliminating the electromagnetic waves fundamentally by converting EM energy into thermal energy or dissipating through interference [6–12]. Nowadays, good EMW absorption characteristics in some cases have been successfully achieved in metallic magnets, ferrites, and their hybrids, while they usually suffer from large loading content, high density, and narrow absorbing bandwidth [13–16]. One of the effective ways to solve these problems is to integrate conventional magnetic materials with light dielectric materials, which using effective complementarities between magnetic and dielectric components [17–19]. Generally, carbon-based materials are attractive candidate materials because of their varieties of excellent properties, including high permittivity, high corrosion-resistance, low density, and versatile processing [20,21].
To fulfill preferable EMW absorption, considerable efforts have been made towards the development of EMW absorbing materials consisting of both magnetic loss and dielectric loss components [22–25]. For example, Li et al. prepared rod-like Fe/amorphous ceramics core/shell structured composite, and a strong reflection loss of −35.04 dB is observed at 10.96 GHz at a thickness of 2.4mm [26]. However, the effective bandwidth with RL exceeding −10 dB remains only 2.56 GHz. Similarly, Huang et al. fabricated the flaky graphite/cobalt zinc ferrite composites through the coprecipitation method. As a result, the reflection loss was less than −10 dB in frequency range of 10.3-13.5 GHz and the maximum reflection loss was −33.85 dB at 11.7 GHz when the coating thickness was 2.5 mm [27]. In addition, Sun et al. demonstrated that incorporating Fe3O4 into graphene could improve the impedance matching characteristic by a facile solvothermal route to synthesize laminated magnetic graphene. When the as-prepared composite was mixed with paraffin at a mass percentage of 40%, the absorption bandwidth (<-10 dB) at 10.4-13.2 GHz was achieved with a coating thickness of 2.0 mm [28]. Although significant improvements have been achieved in these dual-loss absorbers, there absorption bandwidths are not satisfying and preparation processes are too complex, high cost and small output, which can more or less hinder their practical applications. Therefore, it is highly desirable to exploit facile and efficient strategies to solve this problem.
As a typical and important dielectric material, flake graphite has been explored for potential applications in microwave absorption materials because of its high permittivity, low density and cost, and high temperature-resistance [29,30]. Secondly, as a traditional soft magnetic material applied in microwave absorber at a high-frequency band, carbonyl iron has attracted extensive concerns because of its large saturation magnetization, high Snoek’s limit, high Curie temperature, and low cost [31,32]. Finally, constructing core-shell particles based on the magnetic nanomaterials is expected to achieve strong and wide absorption have been reported by Liu et al. [33]. Thus, by combining the high complex permittivity of the graphite and high complex permeability of carbonyl iron particles, the high microwave absorption of the composites can be obtained.
The aim of this work is to synthesize carbonyl iron/graphite microspheres by ball milling and combine dielectric-magnetic characteristics. This synthetic strategy is facile, controllable, and low-cost. The microstructure and EM properties of the composites can be well tuned by changing the milling time and adjusting the ratios of carbonyl iron particles in the composites. Moreover, the EMW absorbing properties and mechanisms of these composite microspheres have been systematically investigated. Interestingly, the optimized carbonyl iron/graphite composite microspheres exhibited strong EM wave absorbing capability with RL values up to −55.2 dB and the effective absorption bandwidth (EAB) reaches up to 9.7 GHz at a thickness of 2 mm. Our results suggest that the carbonyl iron/graphite microspheres with wide absorption bandwidth and strong absorption characteristic can be applied as an ultra-broadband and highly efficient EMW absorbing material for practical applications.
2. Experiment
2.1. Chemicals
Graphite was obtained from Zhengzhou Heng Chang Metal Materials Factory (natural, 3000 mesh). Carbonyl iron powders were purchased from Jiangsu Tianyi Ultra-fine metal Powder Co., Ltd. All other solvents were supplied by Shanghai Aladdin Chemical Reagent Corporation.
2.2. Synthesis of ball-mill graphite
In a typical experiment, the pristine graphite was placed into a stainless steel capsule containing stainless steel balls of 5 mm in diameter. The capsules were then sealed and fixed in the planetary ball-mill machine, and agitated with 500 rpm for 6 h, 12 h, 24 h and 48 h, respectively. The pristine graphite is marked as H0 and the ball milled products are marked as H12, H24 and H48 in ascending time sequences.
2.3. Synthesis of carbonyl iron/graphite microspheres
Carbonyl iron powders and pristine graphite were mixed at different weight ratios of graphite alloy to carbonyl iron powders (6:6, 4:8, 3:9) and mechanically milled for 48h on the planetary ball-mill machine. The mechanical milling rod was kept at a rotation speed of 500 rpm. The ball-to-power mass ratio was 10:1. The composite materials at different weight fractions (6:6, 4:8, 3:9) were marked as C1, C2, and C3, respectively. For convenience, the sample H48 is also named C0 in the content comparison carbonyl iron powders.
2.4. Characterization
The crystal phase of the samples was obtained by X-ray diffractometer (Model D8 ANVANCE) with Cu-Ka radiation. SEM images were obtained by using a scanning electron microscope. The electrical conductivity of the graphite samples was measured by four-pin probe (SZ-82) (Suzhou Telecommunications Instrument, China). In order to measure the EM parameters, paraffin, which makes little contribution to EM loss, was selected as the matrix material. The mixtures mixing homogeneously the paraffin wax with 50 wt% products were pressed into toroidal-shaped samples with dimensions of 7.0 × 3.0 × 2.0 mm. The EM parameters of complex permittivity and complex magnetic permeability were measured using a network analyzer (CETC41 AV3629) in the frequency range from 2 to 18 GHz by the coaxial-line method.
3. Experimental results and discussions
The XRD patterns of the pristine graphite (H0/C0), ball-milling graphite (H12, H24, and H48) and carbonyl iron/graphite composite microspheres (C1, C2, and C3) are shown in Fig. 1. XRD diffraction patterns in Fig. 1(a) show a strong (002) peak at 26.5°for the pristine graphite. With the increase in milling time, the (002) peak gradually weakens until it nearly disappears, which suggests that the crystal structure of graphite is destroyed by ball milling. As shown in Fig. 1(b), all the carbonyl iron/ graphite microspheres (C1, C2, and C3) display two primary diffraction peaks at 44.8°, 65.2° which can be assigned to the (100), and (200) planes of α-Fe with cubic structure (JCPDS 06-0696), respectively. Raman spectra are used to characterize the structure of carbon in carbonyl iron/graphite microspheres. The Raman spectra in Fig. 1(c) show two typical peaks at about 1358 and 1622 cm−1 named the D band and the G band, respectively. The D band is related to disorder or defects of carbon and the G band arises from the vibrations of sp2 carbon atoms in-plane [34]. Compared to the Raman spectra of graphite, that of carbonyl iron/graphite composites present G peak. This phenomenon is because of defective graphite structure in carbonyl iron/graphite composites after ball milling.
Fig. 1 XRD patterns of as-prepared (a) ball-milling graphite and (b) carbonyl iron/graphite microspheres. (c) Raman spectra of different carbonyl iron/graphite composites. XPS survey spectra (d), C 1s (e), and Fe 2p (f) spectra for composite microspheres (C2).
To further investigate the surface chemical state of carbonyl iron/graphite, XPS measurement is used. The survey scans confirms the presence of Fe, O and C elements in carbonyl iron/graphite, as showed in Fig. 1(d). The C1s spectrum in Fig. 1(e) could be deconvoluted into three peaks in binding energy range of 280-290 eV. So there are three species of C in the carbonyl iron/graphite composites. The fitting peaks at 284.8, 285.6 and 288.1ev should be assigned to the C-C/C = C, C-O and C = O, respectively [16]. As showed in Fig. 1(f), the Fe 2p spectra shows two main peaks at 711.69 (Fe 2p3/2) 725.04 eV (Fe 2p1/2), as well as an obvious satellite peak. These characteristics correspond to the features of Fe3+ ions, which should result from the oxidation of Fe after exposure in air. The binding energy at 706.9 eV and 720.0 eV correspond to peaks of Fe 2p3/2 and Fe 2p1/2, respectively [22]. It's worth noting that iron oxide phase was not observed in the XRD results. This is because only a thin surface layer of Fe particles was oxidized in the carbonyl iron/graphite. XPS is a surface analysis technology used to investigate the surface composition and element valence less than ten nanometers [22].
The morphologies of the pure flaky graphite, carbonyl iron powders and carbonyl iron/graphite microspheres are measured by FE-SEM. As shown in Fig. 2(a), natural flake graphite has a distinct layered structure, flat level and an average transverse size of about 10um. With the increase in milling time, the graphite layer (Fig. 2(b)) appears curled but the lamellar structure does not break. After ball milling for 48 h, the layered structure of the graphite was broken and ground into particles of about 3um. Due to the large surface energy, small particles aggregate into large clumps. The increase of ball-milling time also induced a continuous decrease in the sample grain size until 48 h to reach a steady state. For the pure carbonyl iron microspheres, it can be found from Fig. 2(e) that the microspheres are spherical and remarkably uniform with an average size of about 1.41um. The graphite is assembled on the surface of spherical carbonyl iron, as shown in Figs. 2(f)-2(h). However, for the composite C1, few spherical-like shape composites appear. With the increasing carbonyl irons ratio, the graphite could be uniformly distributed on the carbonyl iron microspheres
Fig. 2 SEM images of milled graphite and carbonyl iron/graphite composite microspheres obtained by mechanical milling: (a) H0, (b) H12, (c) H24, (d) H48, (e) carbonyl iron, (f) C1, (g) C2, (h) C3.
For further research of microstructure of the carbonyl iron/graphite composites, as-prepared sample of C2 is investigated by TEM and the images are shown in Fig. 3. After milling with graphite, the composite microspheres possess typical core-shell structure. In Fig. 3(b) and 3(c), the carbonyl iron microsphere is spherical and uniform with the size about 500-1.5um. The carbonyl iron core is encapsulated by graphite layers forming a core-shell, which is demonstrated by Fig. 3(a). The thickness of graphite layers coating on carbonyl iron is about 200nm-2um. From the magnified images in Fig. 3(c) and 3(d), some nanometer flake and nanometer particle in graphite layers coating are observed.
According to the EMW absorption principle, the attenuation energy ratio and interfacial impedance gap are considered as the two critical factors determining the EMW absorbing properties of the material [35]. When it comes to a sole dielectric EMW absorbing material, such as graphene, graphite, and their hybrids, the incident wave propagation from free space into the material is mainly affected by its the dielectric permittivity and electrical conductivity [36]. For better understanding the EMW absorbing performance, we investigated electrical conductivities and complex permittivity in 2-18 GHz of various ball-milling graphite. The electrical conductivities for all the milling graphite are shown in Fig. 4. It’s worth noting that even for H48, the conductivity still stay above 600 S/m, which does not perfectly meet the requirement of the impedance matching condition and therefore appearing disadvantageous microwave reflection [37].
The real parts of complex permittivity represent the storage capability of the electric energy, and the imaginary parts of complex permittivity describe the energy dissipation or loss [38]. In addition, a smaller real permittivity narrows the interfacial impedance gap result in weakening reflection of microwave [39]. The higher imaginary permittivity, the more EMW energy gets attenuated [40,41]. Most importantly, the proper balance between the real and imaginary permittivity is the key to achieve an excellent EMA absorption behavior [38]. Figure 5 gives complex permittivity, the values of Zin/Z0 and the RL values for all ball-milling graphite. Both real permittivity (εʹ) and imaginary permittivity (εʺ) dramatically decrease with the increase of ball-milling time, which is attributable to the decrease of graphitization. The values of Zin/Z0 in ball-milling graphite composites are calculated and the results are illustrated in Fig. 5(c). It is indicated that the ball-milling graphite composites with 48 hour time exhibits the good electromagnetic impedance matching properties, and the values of Zin/Z0 are nearly 1 in the frequency range of 10-13 GHz. The RL curves for all the ball milling graphite samples in 2–18 GHz are shown in Fig. 5(d). The natural flake graphite displays the weakest EMW absorbing ability with the maximum absorption intensity of −9 dB at 6.5 GHz. With the ball milling time increasing from 6h to 48h, a maximum RL of −24.8 dB is achieved at 11.0 GHz. More significantly, the effective absorption bandwidth (<-10dB) is improved dramatically up to 3.2 GHz (9.7–12.9 GHz). The result is consistent with the impedance matching performance change, which indicates the importance of the impedance matching to a preferable microwave absorbing ability.
Fig. 5 Frequency dependences of real parts (a) and imaginary parts (b) of the complex permeability, (c) relative input impedance and (d) reflection losses with matching thickness of 2 mm for milling graphite paraffin wax composite.
In order to further improve the impedance matching performance and introduce magnetic loss, we combine graphite with carbonyl iron by the mechanically milling method. Figure 6 gives the complex permittivity (εr = εʹ-jεʺ) and permeability (μr = μʹ-jμʺ) values for all carbonyl iron/graphite microspheres. As shown in Fig. 6(a), the complex permittivity and permeability gradually decrease with increasing frequency and show frequency dispersion behavior in the whole frequency range (2–18 GHz). We also have fitted the relative dielectric constant in Fig. 7. The black plots are real and imaginary parts of complex permeability. The red plots are corresponding fitting results. As weight fraction increases from C0 to C3, the εʹ values of the composites vary in the range of 15.0–9.7, 12.1–7.4, 11.6–4.3 and 7.2–4.5, respectively. It’s clearly seen that from Figs. 6(a)-6(d) both εʹ and εʺ of the carbonyl iron/graphite microspheres decrease monotonically with either the carbonyl iron mass fraction increasing. It can be ascribed to the reduced electrical loss properties of flaky graphite. In addition, the εʺ curves of CI, C2, and C3 firstly decrease with increasing frequency and then appear several obvious relaxation peaks attributable to multiple polarization relaxation processes. These dielectric relaxation peaks in permittivity spectra originate from interface polarization and conduction loss. The interface polarization is caused by the fact that the components on both sides of the interface have different electrical conductivity or dielectric constant, and there is the accumulation of charges at the interface under the action of EM field [22]. Therefore, interfacial polarization may occur in interfaces among graphite/carbonyl iron, paraffin/graphite, graphite/air, and so on. These results show that the carbonyl iron/graphite microspheres possess a better dielectric loss.
Fig. 6 Frequency dependence of real and imaginary parts of complex permittivity and permeability of milled graphite and carbonyl iron/graphite microspheres: C0 (a), C1 (b), C2 (c), and C3 (d).
To analyze the diversified dielectric loss mechanisms in carbonyl iron/graphite composites, the curves of εʺ versus εʹ are plotted in Fig. (8), which would be a single semicircle (so-called Cole-Cole semicircle). Each semicircle represents one relaxation process according to the Debye theory [42]. It is very interesting that the plots of C1, C2, and C3 display more Cole-Cole semicircles than those of C0. Meanwhile, it is found that C1, C2, and C3 possess more distorted Cole-Cole semicircles. These phenomena should be attributed to interfacial polarization relaxation between carbonyl iron particles and graphite sheets [43].
Fig. 8 Plots of εʺ versus εʹ for milled graphite and carbonyl iron/graphite microspheres: (a) C0, (b) C1, (c) C2, and (d) C3 in the frequency range of 2−18 GHz.
On the other hand, magnetic loss is another crucial factor for determining the EM wave absorbing performance. The values of μʹ are in the range of 1.2, and the values of μʺ are around 0, as showed in Fig. 6(a). Therefore, it is suggested that the loss mechanism of the C0 consists of dielectric loss. Compared with other samples, the C2 has the largest μʺ value and appears a strong resonant peak at 16 GHz. It is obvious that the μʹ and μʺ increase with the mass ratio of CI increasing in all the measured frequency range of 2–18 GHz. In addition, CI is a traditional soft magnetic material with large saturation magnetization, high Snoek limit and high permeability. The relative complex permeability of the composites increased due to the increased fraction of magnetic CI. Therefore, the increased magnetic loss can be achieved in the carbonyl iron/graphite composites by adding CI.
The reflection loss (RL) values of samples were calculated using the relative complex permittivity and permeability at a given frequency and thickness layer according to the transmit line theory, which is summarized as the following equations [43]:
where Zin is the input impedance of the absorber, Z0 is the characteristic impedance of free space, μr and εr are the complex relative magnetic permeability and dielectric permittivity, respectively, f is the frequency of microwaves, d is the thickness of an absorber, c is the velocity of light.Figure 9 illustrates the reflection loss of milled graphite and carbonyl iron/graphite composite microspheres with layer thicknesses of 0.5−4.0 mm in the frequency range of 2−18 GHz. Apparently, all composites exhibit that the absorption peak shifts to a low frequency direction with increase in thickness. The phenomenon could be explained by the following equations [16]:
where tm is the matching thickness. When the EMW reaches absorbents, a reflected wave is generated, and the transmitted wave reaches the metal background generating another reflected wave. If the phase difference of two reflected waves is 180°, tm and fm satisfy the equation, result in an extinction of them. In this case, the reflection loss reaches the minimum value. The absorption peaks of these samples move to a higher frequency with the increment of carbonyl iron mass fraction increasing. The carbonyl iron weight ratio have important effects on the EM wave absorbing capability of the carbonyl iron/graphite microspheres, as shown in Figs. 9(a)-9(d). For C0, it can be seen that the minimum reflection loss reaches −28.8 dB at 15.1 GHz with a matching thickness of 1.5mm, while these values are −32.1 dB, 17.4GHz, 1.5mm for C1, −55.2 dB, 11.0 GHz, 2.0mm for C2, and −35.6dB, 7.9 GHz, 3.5mm for C3 respectively. When the carbonyl iron weight ratio increases to C2 in Fig. 9(c), the reflection loss was less than −10 dB in frequency range of 8.3-18 GHz and the optimum RL peak value of −46 dB can be found at 9.7 GHz when the coating thickness is 2.1 mm. This sample exhibited excellent and enhanced microwave absorption properties. However, the absorption bandwidth (<-10 dB) is obviously narrowed down when the carbonyl iron weight ratio increases to C3. The high reflection loss of the composite materials can be attributed to the good electromagnetic match and the coexistence of dielectric loss and magnetic loss.Fig. 9 Frequency dependent reflection loss (RL) curves of milled graphite and carbonyl iron/graphite composites with different carbonyl iron weight ratios ((a) C0, (b) C1, (c) C2, (d) C3).
In addition, compared with other materials based on magnetic materials with carbonaceous materials, carbonyl iron/graphite microspheres show superior comprehensive properties with strong absorption performance and broad frequency bandwidth, as shown in Table 1. The carbonyl iron/graphite microspheres also show more excellent EMW absorption performance than others carbonyl iron-based materials [31,44–47]. Compared with the density of commercial carbonyl iron/wax, that of the carbonyl iron/graphite/wax has decreased by 66%. Therefore, carbonyl iron/graphite microspheres with high microwave absorption is a promising candidate for microwave absorbing material in high frequency range.
Table 1. Comparison of microwave absorption properties of with other materials from previous studies
In general, the microwave absorption properties of materials should be determined by their EM impedance matching performance, EMW loss ability and dissipating through interference [48]. The remarkable microwave absorption ability of carbonyl iron/graphite microspheres should derive from the effective complementarities between carbonyl iron cores and graphite shells. As is well-known, EM impedance matching is the crucial parameter for the EMW absorption, which requires that the materials (Zin) and free space (Z0) should satisfy |Zin/Z0| equal or close to 1.0 [49]. The |Zin/Z0| values of the carbonyl iron/graphite microspheres at a thickness of 2.0 mm are calculated and the results are displayed in Fig. 10(a). It is indicated that C2 composites exhibits the optimal EM impedance matching properties, and the values of Zin/Z0 are nearly 1 in the frequency range of 9-18 GHz. Another crucial parameter, the attenuation constant (α), which determines the microwave attenuation ability, is calculated according to the Formula (4) [50].
Fig. 10 Frequency dependence of (a) relative input impedance and (b) attenuation constant of milled graphite and carbonyl iron/graphite microspheres.
The attenuation constants of pristine graphite (H0), ball-mill graphite (H48) and carbonyl iron/graphite microspheres(C1, C2 and C3) are showed in Fig. 8(b). Despite the pristine graphite with the largest α, its impedance matching is the worst. So the natural flake graphite displays the weakest EMW absorbing performance. Benefiting from the best EM impedance matching and large attenuation constant, the carbonyl iron/graphite microspheres for C2 displays the best microwave absorption properties.
4. Conclusion
In summary, we have demonstrated the controllable synthesis of composite microspheres with carbonyl iron cores and graphite shells by a mechanically milling method, which is a facile, controllable, low-cost, large-scale and eco-friendly ball-milling process without involving hazardous chemicals. By varying the ball-milling time and loss constituent, carbonyl iron/graphite composite microspheres with good impedance matching can be readily synthesized. Furthermore, the microwave absorption properties of these microspheres were investigated in terms of complex permittivity and permeability. With the good impedance matching and the synergistic effect between magnetic and dielectric components, the as-synthesized carbonyl iron/graphite microspheres possess a lower reflection loss and wider absorption frequency range than those of the pure graphite. The results display that the maximum RL value could reach −55.2 dB and the qualified frequency bandwidth of the absorber is up to 9.7GHz (8.3-18GHz). The excellent performance was attributed to the well-designed structure of the composite, and the synergistic effect between CI and graphite. Such results indicate that these carbonyl iron/graphite microspheres may be attractive candidate materials for microwave absorption applications.
Funding
National Natural Science Foundation of China (Grant Nos. 61378002 and 61571186) and the Key Research and Development Plan of Hunan Province (Grant No. 2017NK2121).
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