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reduced graphene oxides for a high-performance EM wave absorber
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Synthesis of core-shell ZnFe2O4@SiO2 hollow microspheres/reduced graphene oxides for a high-performance EM wave absorber ⁎
Na Zhang, Ying Huang , Meng Zong, Xiao Ding, Suping Li, Mingyue Wang Department of Applied Chemistry and The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an, 710072 PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Composites Hollow microspheres RGO EM wave absorption properties
The core-shell ZnFe2O4@SiO2 hollow microspheres/reduced graphene oxides (RGO-ZnFe2O4@SiO2) composites were successfully synthesized by a rational process. SEM and TEM results illustrate that the ZnFe2O4@SiO2 hollow microspheres are of core-shell structure with an average diameter of about 1 µm mixed with graphene sheets. Compared with ZnFe2O4@SiO2 hollow microspheres and RGO-ZnFe2O4 composites, the as-prepared RGO-ZnFe2O4@SiO2 composites exhibit excellent electromagnetic (EM) wave absorption properties in terms of both the maximum reflection loss and the absorption bandwidth. The maximum reflection loss of RGO-ZnFe2O4@SiO2 composites is −45.8 dB at 7.6 GHz with the thickness of 3.7 mm and the absorption bandwidth with the reflection loss below −10 dB is up to 14.5 GHz (from 3.5 to 18.0 GHz) with a thickness in the range of 2.0–6.0 mm. It may also be stressed that the developed composites cover the whole X band (8.0– 12.0 GHz), which could be used for military radars and direct broadcast satellites (DBS).
1. Introduction The EM wave absorbing materials have attracted much attention owing to the expanded EM interference problems [1,2]. The ideal EM wave absorbers are required to have a wide absorption bandwidth, strong absorption characteristic, low density, good thermal stability and antioxidant capability [3,4]. In the past few decades, most research has been concerned with spinel ferrites (MFe2O4, M=Fe, Co, Ni, Zn, etc) [5–7] which exhibits interesting magnetic, magneto-resistive and magneto-optical properties. Among them, spinel ferrites hollow spheres can exhibit special physical and chemical properties different from solid particles due to their low density and large specific surface area. In recent years, a number of studies on spinel ferrites hollow sphere composites have been reported in the literatures [8–11]. Fu et al. [8] synthesized novel CoFe2O4 hollow spheres/graphene composites by a facile vapor diffusion method in combination with the calcination at 550 °C. The maximum reflection loss of −18.5 dB is observed at 12.9 GHz with a thickness of 2.0 mm and the effective absorption frequency range is from 11.3 to 15.0 GHz. Xu et al. [9] fabricated a novel kind of bowl-like Fe3O4 hollow spheres/RGO by a facile solvothermal method. The sample containing 30 wt% as-synthesized hollow Fe3O4/RGO with a thickness of 2.0 mm exhibits a maximum absorption of −24.0 dB at 12.9 GHz as well as a absorption bandwidth of 4.9 GHz (from 10.8 to 15.7 GHz) corresponding to the reflection loss below −10 dB. Li et al. [10] prepared nickel-Fe3O4 ⁎
hollow spheres with the diameter about 150 nm by an autocatalytic reduction method. For composite layers, a maximum reflection loss (−38.4 dB) is predicted at 10.8 GHz with a thickness of 2.5 mm. Yan et al. [11] selectively synthesized monodisperse ZnFe2O4 hollow nanospheres and nanosheets via a facile solvothermal method. The maximum reflection loss is up to −31.4 dB at 10.5 GHz, however, only one wide and shallow dip (the maximum reflection loss of −17.9 dB) shows at around 10.0 GHz for the sheet-like sample. Carbon-based materials, as a class of promising EM wave absorbing materials, exhibit several exceptional properties such as lightweight, wide absorption frequency, high thermal stability and high chemical stability [12–14]. Graphene, a new kind of carbon nanostructure material consisting of two-dimensional sp2 bonded sheets, has not only a stable structure but also a very high specific surface area, which can be used as a lightweight EM wave absorber [15,16]. However, the EM wave absorption property of pure graphene is very poor due to its attenuation toward the EM wave only dependent on the dielectric loss as well as its poor impedance matching characteristic [17]. Therefore, how to design and prepare good EM wave absorbing materials based on graphene still remain a challenge. Herein, we design and synthesize RGO-ZnFe2O4@SiO2 composites by a facile route. The crystalline structure, morphology and EM wave absorbing properties of the as-prepared composites were investigated. The composites exhibit excellent EM wave absorption performances in terms of both the maximum reflection loss and the absorption
Corresponding author. E-mail addresses: [email protected] (N. Zhang), [email protected] (Y. Huang).
http://dx.doi.org/10.1016/j.ceramint.2016.09.035 Received 16 August 2016; Received in revised form 28 August 2016; Accepted 5 September 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhang, N., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.035
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bandwidth. It is believed that such composites can find wide application in the EM wave absorbing area. 2. Experimental 2.1. Preparation of ZnFe2O4 and ZnFe2O4@SiO2 hollow microspheres ZnFe2O4 hollow microspheres were prepared using a facile template-free solvothermal strategy combined with the subsequent thermal treatment process with some modification [18]. The as-prepared ZnFe2O4 hollow microspheres were dispersed in a mixture of deionized water and absolute ethanol (1:9) by ultrasonication. Then, ammonia (1.5 mL) and TEOS (0.3 mL) were added dropwise, respectively. The solution was stirred for 8 h at room temperature. The product was washed with deionized water and absolute ethanol by magnetic decantation and fully dried at 60 °C under vacuum to obtain ZnFe2O4@SiO2 hollow microspheres. 2.2. Preparation of RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites
Fig. 1. XRD patterns of GO (a), RGO (b), ZnFe2O4 (c), ZnFe2O4@SiO2 (d), RGOZnFe2O4 (e) and RGO-ZnFe2O4@SiO2 composites (f).
Graphene Oxide (GO) was synthesized by using natural graphite flakes according to the Hummer’s method [19]. The composites were prepared as follows: 150 mg GO was dispersed in 150 mL of deionized water and ultrasonicated for 2 h. Then the obtained ZnFe2O4@SiO2 hollow microspheres were dispersed in above suspension solution. The solution was stirred for 7 h under ultrasonic to mix intensively with GO, then transferred into a 200 mL Teflon-lined stainless steel autoclave and kept in an oven at 180 °C for 15 h. The resulting product was washed with deionized water and absolute ethanol for several times, then dried at 60 °C under vacuum for 8 h to obtain RGOZnFe2O4@SiO2 composites. For comparison purposes, the RGOZnFe2O4 composites were also prepared by similar procedures with adding ZnFe2O4 hollow microspheres.
graphitic structure (002) in graphene sheets [21]. All of diffraction peaks at 2θ=18.1°, 30.0°, 35.4°, 42.9°, 53.4°, 57.0°, 62.4° correspond to (111), (220), (311), (400), (422), (511) and (440) planes of ZnFe2O4, and can be indexed in the cubic spinel crystal structure of ZnFe2O4 (Fig. 1c–f) (JCPDS Card No. 65-3111). For ZnFe2O4@SiO2 (Fig. 1d) and RGO-ZnFe2O4@SiO2 composites (Fig. 1f), no other peaks can be detected, indicating the amorphous structure of SiO2 in composites. And for RGO-ZnFe2O4 (Fig. 1e) and RGO-ZnFe2O4@SiO2 composites (Fig. 1f), no other diffraction peaks resulted from GO or graphene can be detected in the composites, suggesting that GO is effectively reduced into graphene and the direct restacking of graphene sheets may be refrained by homogeneously loading ZnFe2O4 and ZnFe2O4@SiO2 hollow microspheres. The morphology and microstructure of the as-prepared composites were further investigated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the hollow structure of ZnFe2O4 can be observed obviously with an approximately irregular size of 1 µm, which is assembled by a large scale of closely interconnected nanosized sheets as primary building blocks. The energy dispersive X-ray (EDX) results indicate the presence of Zn, Fe and O elements in the sample. Fig. 2b shows the SEM and TEM images of ZnFe2O4@SiO2 hollow microspheres. It is clear that SiO2 coats on the surface of ZnFe2O4 hollow microspheres irregularly, consisting with TEM image. In addition, the EDX mapping results further confirm the growth of SiO2 on the surface of ZnFe2O4 hollow microspheres and the atomic ratio of O, Si, Fe and Zn is 75.57%, 14.86%, 7.21% and 2.37%, respectively. Fig. 2c displays the typical SEM images of RGO-ZnFe2O4 composites. It is obvious that the large two-dimensional structures can be observed under a SEM microscope, and the graphene sheet shows a crumpled structure. The EDX image confirms the presence of Zn, Fe, C and O elements in the composites. The SEM images of RGO-ZnFe2O4@SiO2 composites is shown in Fig. 2d. It is also can be seen that the graphene sheet in the composites shows a crumpled structure. Moreover, the EDX mapping results confirm the presence of C, O, Si, Fe and Zn elements in the composites, and the atomic ratio of them is 10.29%, 69.18%, 12.37%, 6.31% and 1.85%, respectively. The surface composition of RGO-ZnFe2O4@SiO2 composites was characterized by X-ray photoelectron spectroscopy (XPS). The survey spectrum (Fig. 3a) shows that the RGO-ZnFe2O4@SiO2 composites consist of C, O, Zn, Fe and Si elements. Two main binding energy peaks at 1021.6 eV and 1044.9 eV can be assigned to Zn 2p3/2 and Zn 2p1/2, respectively. And the binding energy peaks at 711.1 and 724.9 eV are attributed to Fe 2p3/2 and Fe 2p1/2, respectively, suggesting the existence of ZnFe2O4 [18]. The Si 2p spectrum in which the peak
2.3. Characterizations The crystal structure properties were characterized by X-ray diffraction (XRD, Rigaku, Cu Kα). X-ray photoelectron spectroscopy measurements were determined by a spectrometer with Mg Kα radiation (XPS, ESCALAB 250 Thermofisher Co). The morphology characteristics were investigated by scanning electron microscope (SEM, Supra 55, German ZEISS) and transmission electron microscopy (TEM, Tecnai F30 G2, FEI, USA). Raman spectra were measured by using a Via Laser Raman spectrometer (Renishaw Co., England). The magnetic properties were obtained by vibrating sample magnetometer (VSM, Riken Denshi, BHV-525). EM parameters were analyzed by a vector network analyzer (NA, HP8720ES) in the range of 2–18 GHz. The synthesized samples were pressed into toroidal shaped samples with an outer diameter of 7.0 mm, inner diameter of 3.04 mm and height of about 3.0 mm according to the mass ratio 2:1 of paraffin and RGO-ZnFe2O4@SiO2 (or ZnFe2O4@SiO2 or RGO-ZnFe2O4) composites. 3. Results and discussions The phase and crystal structures of the synthesized samples were investigated by X-ray diffraction (XRD). Fig. 1 shows the XRD patterns of as-prepared GO, RGO, ZnFe2O4, ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites. The diffraction peak of GO (Fig. 1a) appears at 2θ=11.8°, corresponding to the d-spacing of 0.75 nm between layers of GO, which is attributed to the formation of oxygen functionalities groups [20]. For RGO (Fig. 1b), the intense peak at 2θ=11.8° disappears and a broader diffraction peak appears at 2θ=25.0° with the d-spacing of 0.36 nm, which is possibly due to the successful reduction of GO and the short-range order stacked of 2
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Fig. 2. SEM images and the corresponding EDS maps of ZnFe2O4 hollow microspheres (a), RGO-ZnFe2O4 (c) and RGO-ZnFe2O4@SiO2 composites (d); SEM and TEM images the corresponding EDS maps of ZnFe2O4@SiO2 hollow microspheres (b).
The value of ID/IG is 0.85 for GO. It can be observed that the ID/IG of RGO-ZnFe2O4 (ID/IG=0.90) and RGO-ZnFe2O4@SiO2 (ID/IG=0.99) composites are both higher than that of GO, which can be contributed to the highly disorder nature of graphene and the decrease in the average size of sp2 domains upon reduction of GO. The defects play an important role in EM wave absorbing and will further explain the mechanism of EM wave absorbing [26]. The field dependence of magnetization for ZnFe2O4, ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites was measured by VSM at room temperature with the magnetic field swept back and forth between −13.0 and +13.0 kOe, as shown in Fig. 5. The samples all exhibit typical soft magnetic behavior. The value of saturation magnetization (Ms) is 20.0 emu g−1 for ZnFe2O4 hollow microspheres, 11.5 emu g−1 for ZnFe2O4@SiO2 hollow microspheres, 9.5 emu g−1 for RGO-ZnFe2O4 composites and 7.8 emu g−1 for RGOZnFe2O4@SiO2 composites. This decrease in magnetism is mainly attributed to the decrease in the weight ratio of ZnFe2O4 in the composites. The frequency dependence of its EM parameters (the relative complex permittivity (εr=ε′−jε″), the relative complex permeability (μr=μ′−jμ″), the dielectric tangent loss (tan δE=ε′/ε″) and the magnetic
located at 102.3 eV is shown in Fig. 3d, which confirms the existence of SiO2 in the composites [22]. The C1s spectrum of RGO-ZnFe2O4@SiO2 composites (Fig. 3c) consists of four functional groups: C–C/C=C (284.6 eV), C–O (286.4 eV), C=O (287.8 eV) and O–C=O (289.3 eV) groups [23]. Compared with GO (Fig. 3b), the intensity of carbon binding to oxygen decreases rapidly, especially for C–O, suggesting the remarkable reduction of GO [24]. Such a transformation of RGOZnFe2O4@SiO2 composites implies a good electronic conductivity, which is favorable for enhancing EM wave absorbing. All of the above results indicate that the RGO-ZnFe2O4@SiO2 composites are formed. The Raman spectra of GO, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites are illustrated in Fig. 4, which further supports the preparation of RGO-ZnFe2O4@SiO2 composites. It can be clearly observed that the Raman spectra of GO, RGO-ZnFe2O4 and RGOZnFe2O4@SiO2 composites all show two peaks centered at 1340 cm−1 (D band) and 1590 cm−1 (G band). The D band is corresponded to disordered vibrations of C-sp3 atoms and the G band is assigned to inplane vibration modes of C-sp2 atoms in a 2-dimensional hexagonal lattice. The intensity ratio (ID/IG) is used to react the degree of defects in graphene or edges [25]. The structural changing from graphene oxides to the as-prepared composites could be observed by the ID/IG. 3
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Fig. 3. XPS spectra: wide scan of RGO-ZnFe2O4@SiO2 composites (a); C 1s spectrum of GO (b); C 1s spectrum (c) and S 2p spectrum (d) of RGO-ZnFe2O4@SiO2 composites.
Fig. 4. Raman spectra of GO, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites.
Fig. 5. Magnetization curves of ZnFe2O4, ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGOZnFe2O4@SiO2 composites.
tangent loss (tan δM=μ′/μ″)), is used for characterization of dielectric loss and magnetic loss properties of absorbers. The real parts of permittivity and permeability symbolize the storage ability of electric and magnetic energy, while the imaginary parts of permittivity and permeability are related to the dissipation of electric and magnetic energy [27]. Therefore, the paraffin wax combined with ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites
were fabricated to measure the relative complex permittivity and the relative complex permeability. Fig. 6 shows the real (ε′) and imaginary (ε″) parts of relative complex permittivity, the real (μ′) and imaginary (μ″) parts of relative complex permeability and the dielectric tangent loss (tan δE=ε′/ε″) and the magnetic tangent loss (tan δM=μ′/μ″) in the frequency range of 2– 18 GHz for ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 4
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Fig. 6. The relative complex permittivity real parts (a) and imaginary parts (b); The relative complex permeability real parts (c) and imaginary parts (d); the corresponding dielectric loss tangents (e) and magnetic loss tangents (f) of ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites.
ZnFe2O4@SiO2 composites are due to the fact that the addition of graphene may increase the electric polarization and electric conductivity, since εr is an expression of polarizability of a material, which consists of dipolar polarization and electric polarization. It is also observed that the RGO-ZnFe2O4@SiO2 composites show lower values of ε′ and ε″, compared with RGO-ZnFe2O4 composites, caused by the low electrical conductivity of SiO2 semiconductor. For RGOZnFe2O4@SiO2 composites, the μ′ values maintain around 1.0, whereas the μ″ values decrease gradually from 1.4 to −0.1 in the whole frequency rang, similar to those of ZnFe2O4@SiO2 and RGO-ZnFe2O4
composites, respectively. As shown in Fig. 6a, it can be observed that the ε′ values of ZnFe2O4@SiO2 hollow microspheres almost maintain in 3 over the frequency range of 2–18 GHz, while the ε′ values of RGOZnFe2O4 and RGO-ZnFe2O4@SiO2 composites decrease from 17.3 to 8.8 and 9.3–5.9 with several fluctuations, respectively, and are both higher than the ZnFe2O4@SiO2 hollow microspheres. Meanwhile, for RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites, the values of ε″ are in the range of 13.3–3.0 and 4.3–1.3, respectively, both higher than the ZnFe2O4@SiO2 hollow microspheres (in the range of 1.3–0), as presented in Fig. 6b. The higher ε′ and ε″ for RGO-ZnFe2O4 and RGO5
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Fig. 7. Calculated reflection loss of ZnFe2O4@SiO2 (a), RGO-ZnFe2O4 (b) and RGO-ZnFe2O4@SiO2 composites (c) with a thickness in the range of 2.0–6.0 mm and corresponding three dimensional presentation (d–f). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
ductivity, further improving the EM wave absorption performance. And compared with RGO-ZnFe2O4 composites, after the introduction of SiO2, the electrical conductivity of RGO-ZnFe2O4@SiO2 composites become weak, leading to a smaller gap between the dielectric tangent loss and the magnetic tangent loss of RGO-ZnFe2O4@SiO2 composites, which can improve impedance matching. By comparing tan δE and tan δM, we can see that the RGO-ZnFe2O4@SiO2 composites possess a far higher magnetic tangent loss in the low frequency (2.0–5.0 GHz) and a slightly higher dielectric tangent loss in the high frequency (5.0– 18.0 GHz), revealing that the magnetic loss plays the main role in the
composites, as shown in Fig. 6c and d. From Fig. 6e, the values of tan δE for RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites are in the range of 0.81–0.34 and 0.49–0.23, respectively, both higher than that of the ZnFe2O4@SiO2 hollow microspheres (about 0), indicating that the dielectric loss occurs over the whole frequency range for them. Meanwhile, for RGO-ZnFe2O4@SiO2 composites, the values of tan δM are from 1.5 to −0.1, similar to those of ZnFe2O4@SiO2 and RGOZnFe2O4 composites, as shown in Fig. 6f. Compared with ZnFe2O4@SiO2 hollow microspheres, the addition of graphene may increase the electric polarization and electric con6
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Fig. 8. Comparison of the reflection loss (a) and the absorption bandwidth with the reflection loss below −10 dB (b) of ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites.
frequency, d is the thickness of absorb layer, c is the velocity of EM wave in vacuum, and εr and μr are respectively the relative complex relative permittivity and permeability, respectively. Fig. 7a–f display the calculated reflection losses of ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites. It is clear that the thickness of absorbers has a great influence on the EM wave absorbing properties. Moreover, the absorption peaks for two samples shift toward the lower frequency as the absorber thickness increases. As shown in Fig. 7a and d, for ZnFe2O4@SiO2 hollow microspheres, the reflction loss is almost below −10 dB. For RGO-ZnFe2O4 composites (Fig. 7b and e), the maximum reflction loss reaches −14.4 dB at 11.9 GHz and the reflction loss below −10 dB (90% absorption) is up to 3.2 GHz (from 10.6 to 13.8 GHz) with a thickness of 2.0 mm. For RGO-ZnFe2O4@SiO2 composites (Fig. 7c and f), the maximum reflection loss is −45.8 dB at 7.6 GHz with the thickness of 3.7 mm and the bandwidth corresponding to reflction loss below −10 dB can reach 4.0 GHz (from 7.7 to 11.7 GHz) with the thickness of 3.0 mm. In addition, the absorption bandwidth with reflction loss below −10 dB is up to 14.5 GHz (from 3.5 to 18.0 GHz) with a thickness in the range of 2.0–6.0 mm, as indicated by a blue line. Moreover, the absorption bandwidth below −20 dB also covers a wide frequency range (from 6.2 to 11.8 GHz), as indicated by a olive line. Accordingly, the reflction loss of RGO-ZnFe2O4@SiO2 composites below −10 dB can be obtained in the range of 3.5– 18 GHz which covers in wide frequency bands from the S to Ku band in the absorber thickness from 2.0 to 6.0 mm, indicating the excellent EM wave absorption properties of RGO-ZnFe2O4@SiO2 composites. Fig. 8a shows the comparison of maximum reflection loss at different matching thicknesses for ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites. The RGO-ZnFe2O4@SiO2 composites exhibit well EM wave absorption properties at every thickness when compared to ZnFe2O4@SiO2 and RGO-ZnFe2O4 composites. Fig. 8b presents the absorption bandwidth of three samples below −10 dB. The absorption bandwidth indicates that the RGOZnFe2O4@SiO2 composites can be used in a wide frequency range. All the above results suggest that the RGO-ZnFe2O4@SiO2 composites exhibit more excellent EM wave absorption performance in terms of both the maximum reflction loss and the absorption bandwidth. The results demonstrate that the RGO-ZnFe2O4@SiO2 composites could be used as a kind of candidate absorber. To understand the physical phenomenon and the possible mechanism giving rise to the enhanced EM wave absorption properties of RGO-ZnFe2O4@SiO2 composites, the quarter-wave thickness criteria was used to analyze the experimental results [28]. In this criteria, the maximum reflection loss can be achieved at a certain EM wave frequency (fm) if the thickness (tm) of composites satisfies the matching
Fig. 9. Frequency dependence of the reflection loss with different thicknesses (a); simulations of the absorber thickness (tm) versus peak frequency (fm) under λ/4 conditions (b); the relationship between the impedance matching characteristics (Z =| Zin/Z0|) and the EM wave frequency (c) of RGO-ZnFe2O4@SiO2 composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
low frequency and the dielectric loss occurs in the high frequency. Meanwhile, the EM wave can be effectively attenuated by multiple reflections because of the hollow structure of ZnFe2O4. To further reveal the EM wave absorption properties, the reflection losses (RL) are calculated according to transmission line theory as the following equations [1,2]:
RL (dB) = 20 log
(Z
in
)(
− Zo / Z in + Z 0
(
Z in = Zo u r /εr tanh [ j 2πfd /c
)
u r εr ]
)
(1) (2)
where Zin is the input impedance of absorber, f is the EM wave 7
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equation [29]:
t m = nc /(4fm
μr εr )
(3)
where |εr| and |μr| are respectively the modulus of measured εr and μr at fm. According to this criteria, the peak frequency is inversely proportional to the thickness. According to equation [3], the tm can be simulated, which denotes as tmfit, as indicated by a red line in Fig. 9b. In the figure, all black dots (denoted as tmexp, and the thickness achieved from the maximum reflction loss in Fig. 9a) are almost located around the quarter-wave thickness criteria for RGO-ZnFe2O4@SiO2 composites. Thus, the absorber will exhibit excellent EM wave absorption property due to quarter-wavelength interference phenomenon. In addition, the reflction loss is highly related to the impedance matching characteristics (Z=|Zin/Z0|，the modulus of normalized impedance matching characteristics), expressing as the following equation [4] and it is an important parameter to reduce the reflection of EM wave at the air-absorber interface [30].
Z = Z in /Zo =
(
u r /εr tanh [ j 2πfd /c
)
u r εr ]
(4)
When Z is equal or close to 1, the impedance matching characteristic of absorber can easily incident in the absorber to be attenuated rather than reflected at the absorber surface and the EM wave will be converted to thermal energy or dissipated through interference. The frequency dependence of Z for RGO-ZnFe2O4@SiO2 composites is obtained as shown in Fig. 9c. It can be found that the Z values are close to 1 at the frequencies of 7.6 GHz at the absorber thickness of 3.7 mm on the quarter-wave thickness criteria, which has the maximum reflection loss (−45.8 dB), according with the above conclusions. Therefore, the well-matched characteristic impedance also contributes to the EM wave absorption property of RGO-ZnFe2O4@SiO2 composites. 4. Conclusion In summary, the RGO-ZnFe2O4@SiO2 composites which has an obviously enhanced EM wave absorption property were successfully synthesized by a rational process. When it was evaluated as an EM wave absorber, the composites exhibit enhanced EM wave absorption properties in terms of both the maximum reflection loss value and the absorption bandwidth. The results indicated that the maximum reflection loss of RGO-ZnFe2O4@SiO2 composites can reach −45.8 dB at 7.6 GHz with the thickness of 3.7 mm and the absorption bandwidth with the reflection loss below −10 dB can reach 14.5 GHz (from 3.5 to 18.0 GHz) with a thickness in the range of 2.0–6.0 mm. And for RGOZnFe2O4@SiO2 composites, the magnetic loss plays the main role in the low frequency and the dielectric loss occurs in the high frequency. It is believed that such composites will find their wide application in EM wave absorbing area. Acknowledgments This work was supported by the Spaceflight Foundation of China (Grant no. 2014-HT-XGD), the Spaceflight Innovation Foundation of China (Grant no. 2014KC11023). References [1] C. Wang, R.T. Lv, Z.H. Huang, F.Y. Kang, J.L. Gu, Synthesis and microwave absorbing properties of FeCo alloy particles/graphite nanoflake composites, J. Alloy. Compd. 509 (2011) 494–498. [2] Y.X. Huang, Y. Wang, Z.M. Li, Z. Yang, C.H. Shen, C.C. He, Effect of pore morphology on the dielectric properties of porous carbons for microwave absorp-
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Synthesis of core-shell ZnFe2O4@SiO2 hollow microspheres/reduced graphene oxides for a high-performance EM wave absorber ⁎
Na Zhang, Ying Huang , Meng Zong, Xiao Ding, Suping Li, Mingyue Wang Department of Applied Chemistry and The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, School of Science, Northwestern Polytechnical University, Xi’an, 710072 PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: Composites Hollow microspheres RGO EM wave absorption properties
The core-shell ZnFe2O4@SiO2 hollow microspheres/reduced graphene oxides (RGO-ZnFe2O4@SiO2) composites were successfully synthesized by a rational process. SEM and TEM results illustrate that the ZnFe2O4@SiO2 hollow microspheres are of core-shell structure with an average diameter of about 1 µm mixed with graphene sheets. Compared with ZnFe2O4@SiO2 hollow microspheres and RGO-ZnFe2O4 composites, the as-prepared RGO-ZnFe2O4@SiO2 composites exhibit excellent electromagnetic (EM) wave absorption properties in terms of both the maximum reflection loss and the absorption bandwidth. The maximum reflection loss of RGO-ZnFe2O4@SiO2 composites is −45.8 dB at 7.6 GHz with the thickness of 3.7 mm and the absorption bandwidth with the reflection loss below −10 dB is up to 14.5 GHz (from 3.5 to 18.0 GHz) with a thickness in the range of 2.0–6.0 mm. It may also be stressed that the developed composites cover the whole X band (8.0– 12.0 GHz), which could be used for military radars and direct broadcast satellites (DBS).
1. Introduction The EM wave absorbing materials have attracted much attention owing to the expanded EM interference problems [1,2]. The ideal EM wave absorbers are required to have a wide absorption bandwidth, strong absorption characteristic, low density, good thermal stability and antioxidant capability [3,4]. In the past few decades, most research has been concerned with spinel ferrites (MFe2O4, M=Fe, Co, Ni, Zn, etc) [5–7] which exhibits interesting magnetic, magneto-resistive and magneto-optical properties. Among them, spinel ferrites hollow spheres can exhibit special physical and chemical properties different from solid particles due to their low density and large specific surface area. In recent years, a number of studies on spinel ferrites hollow sphere composites have been reported in the literatures [8–11]. Fu et al. [8] synthesized novel CoFe2O4 hollow spheres/graphene composites by a facile vapor diffusion method in combination with the calcination at 550 °C. The maximum reflection loss of −18.5 dB is observed at 12.9 GHz with a thickness of 2.0 mm and the effective absorption frequency range is from 11.3 to 15.0 GHz. Xu et al. [9] fabricated a novel kind of bowl-like Fe3O4 hollow spheres/RGO by a facile solvothermal method. The sample containing 30 wt% as-synthesized hollow Fe3O4/RGO with a thickness of 2.0 mm exhibits a maximum absorption of −24.0 dB at 12.9 GHz as well as a absorption bandwidth of 4.9 GHz (from 10.8 to 15.7 GHz) corresponding to the reflection loss below −10 dB. Li et al. [10] prepared nickel-Fe3O4 ⁎
hollow spheres with the diameter about 150 nm by an autocatalytic reduction method. For composite layers, a maximum reflection loss (−38.4 dB) is predicted at 10.8 GHz with a thickness of 2.5 mm. Yan et al. [11] selectively synthesized monodisperse ZnFe2O4 hollow nanospheres and nanosheets via a facile solvothermal method. The maximum reflection loss is up to −31.4 dB at 10.5 GHz, however, only one wide and shallow dip (the maximum reflection loss of −17.9 dB) shows at around 10.0 GHz for the sheet-like sample. Carbon-based materials, as a class of promising EM wave absorbing materials, exhibit several exceptional properties such as lightweight, wide absorption frequency, high thermal stability and high chemical stability [12–14]. Graphene, a new kind of carbon nanostructure material consisting of two-dimensional sp2 bonded sheets, has not only a stable structure but also a very high specific surface area, which can be used as a lightweight EM wave absorber [15,16]. However, the EM wave absorption property of pure graphene is very poor due to its attenuation toward the EM wave only dependent on the dielectric loss as well as its poor impedance matching characteristic [17]. Therefore, how to design and prepare good EM wave absorbing materials based on graphene still remain a challenge. Herein, we design and synthesize RGO-ZnFe2O4@SiO2 composites by a facile route. The crystalline structure, morphology and EM wave absorbing properties of the as-prepared composites were investigated. The composites exhibit excellent EM wave absorption performances in terms of both the maximum reflection loss and the absorption
Corresponding author. E-mail addresses: [email protected] (N. Zhang), [email protected] (Y. Huang).
http://dx.doi.org/10.1016/j.ceramint.2016.09.035 Received 16 August 2016; Received in revised form 28 August 2016; Accepted 5 September 2016 Available online xxxx 0272-8842/ © 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zhang, N., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.09.035
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bandwidth. It is believed that such composites can find wide application in the EM wave absorbing area. 2. Experimental 2.1. Preparation of ZnFe2O4 and ZnFe2O4@SiO2 hollow microspheres ZnFe2O4 hollow microspheres were prepared using a facile template-free solvothermal strategy combined with the subsequent thermal treatment process with some modification [18]. The as-prepared ZnFe2O4 hollow microspheres were dispersed in a mixture of deionized water and absolute ethanol (1:9) by ultrasonication. Then, ammonia (1.5 mL) and TEOS (0.3 mL) were added dropwise, respectively. The solution was stirred for 8 h at room temperature. The product was washed with deionized water and absolute ethanol by magnetic decantation and fully dried at 60 °C under vacuum to obtain ZnFe2O4@SiO2 hollow microspheres. 2.2. Preparation of RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites
Fig. 1. XRD patterns of GO (a), RGO (b), ZnFe2O4 (c), ZnFe2O4@SiO2 (d), RGOZnFe2O4 (e) and RGO-ZnFe2O4@SiO2 composites (f).
Graphene Oxide (GO) was synthesized by using natural graphite flakes according to the Hummer’s method [19]. The composites were prepared as follows: 150 mg GO was dispersed in 150 mL of deionized water and ultrasonicated for 2 h. Then the obtained ZnFe2O4@SiO2 hollow microspheres were dispersed in above suspension solution. The solution was stirred for 7 h under ultrasonic to mix intensively with GO, then transferred into a 200 mL Teflon-lined stainless steel autoclave and kept in an oven at 180 °C for 15 h. The resulting product was washed with deionized water and absolute ethanol for several times, then dried at 60 °C under vacuum for 8 h to obtain RGOZnFe2O4@SiO2 composites. For comparison purposes, the RGOZnFe2O4 composites were also prepared by similar procedures with adding ZnFe2O4 hollow microspheres.
graphitic structure (002) in graphene sheets [21]. All of diffraction peaks at 2θ=18.1°, 30.0°, 35.4°, 42.9°, 53.4°, 57.0°, 62.4° correspond to (111), (220), (311), (400), (422), (511) and (440) planes of ZnFe2O4, and can be indexed in the cubic spinel crystal structure of ZnFe2O4 (Fig. 1c–f) (JCPDS Card No. 65-3111). For ZnFe2O4@SiO2 (Fig. 1d) and RGO-ZnFe2O4@SiO2 composites (Fig. 1f), no other peaks can be detected, indicating the amorphous structure of SiO2 in composites. And for RGO-ZnFe2O4 (Fig. 1e) and RGO-ZnFe2O4@SiO2 composites (Fig. 1f), no other diffraction peaks resulted from GO or graphene can be detected in the composites, suggesting that GO is effectively reduced into graphene and the direct restacking of graphene sheets may be refrained by homogeneously loading ZnFe2O4 and ZnFe2O4@SiO2 hollow microspheres. The morphology and microstructure of the as-prepared composites were further investigated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the hollow structure of ZnFe2O4 can be observed obviously with an approximately irregular size of 1 µm, which is assembled by a large scale of closely interconnected nanosized sheets as primary building blocks. The energy dispersive X-ray (EDX) results indicate the presence of Zn, Fe and O elements in the sample. Fig. 2b shows the SEM and TEM images of ZnFe2O4@SiO2 hollow microspheres. It is clear that SiO2 coats on the surface of ZnFe2O4 hollow microspheres irregularly, consisting with TEM image. In addition, the EDX mapping results further confirm the growth of SiO2 on the surface of ZnFe2O4 hollow microspheres and the atomic ratio of O, Si, Fe and Zn is 75.57%, 14.86%, 7.21% and 2.37%, respectively. Fig. 2c displays the typical SEM images of RGO-ZnFe2O4 composites. It is obvious that the large two-dimensional structures can be observed under a SEM microscope, and the graphene sheet shows a crumpled structure. The EDX image confirms the presence of Zn, Fe, C and O elements in the composites. The SEM images of RGO-ZnFe2O4@SiO2 composites is shown in Fig. 2d. It is also can be seen that the graphene sheet in the composites shows a crumpled structure. Moreover, the EDX mapping results confirm the presence of C, O, Si, Fe and Zn elements in the composites, and the atomic ratio of them is 10.29%, 69.18%, 12.37%, 6.31% and 1.85%, respectively. The surface composition of RGO-ZnFe2O4@SiO2 composites was characterized by X-ray photoelectron spectroscopy (XPS). The survey spectrum (Fig. 3a) shows that the RGO-ZnFe2O4@SiO2 composites consist of C, O, Zn, Fe and Si elements. Two main binding energy peaks at 1021.6 eV and 1044.9 eV can be assigned to Zn 2p3/2 and Zn 2p1/2, respectively. And the binding energy peaks at 711.1 and 724.9 eV are attributed to Fe 2p3/2 and Fe 2p1/2, respectively, suggesting the existence of ZnFe2O4 [18]. The Si 2p spectrum in which the peak
2.3. Characterizations The crystal structure properties were characterized by X-ray diffraction (XRD, Rigaku, Cu Kα). X-ray photoelectron spectroscopy measurements were determined by a spectrometer with Mg Kα radiation (XPS, ESCALAB 250 Thermofisher Co). The morphology characteristics were investigated by scanning electron microscope (SEM, Supra 55, German ZEISS) and transmission electron microscopy (TEM, Tecnai F30 G2, FEI, USA). Raman spectra were measured by using a Via Laser Raman spectrometer (Renishaw Co., England). The magnetic properties were obtained by vibrating sample magnetometer (VSM, Riken Denshi, BHV-525). EM parameters were analyzed by a vector network analyzer (NA, HP8720ES) in the range of 2–18 GHz. The synthesized samples were pressed into toroidal shaped samples with an outer diameter of 7.0 mm, inner diameter of 3.04 mm and height of about 3.0 mm according to the mass ratio 2:1 of paraffin and RGO-ZnFe2O4@SiO2 (or ZnFe2O4@SiO2 or RGO-ZnFe2O4) composites. 3. Results and discussions The phase and crystal structures of the synthesized samples were investigated by X-ray diffraction (XRD). Fig. 1 shows the XRD patterns of as-prepared GO, RGO, ZnFe2O4, ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites. The diffraction peak of GO (Fig. 1a) appears at 2θ=11.8°, corresponding to the d-spacing of 0.75 nm between layers of GO, which is attributed to the formation of oxygen functionalities groups [20]. For RGO (Fig. 1b), the intense peak at 2θ=11.8° disappears and a broader diffraction peak appears at 2θ=25.0° with the d-spacing of 0.36 nm, which is possibly due to the successful reduction of GO and the short-range order stacked of 2
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Fig. 2. SEM images and the corresponding EDS maps of ZnFe2O4 hollow microspheres (a), RGO-ZnFe2O4 (c) and RGO-ZnFe2O4@SiO2 composites (d); SEM and TEM images the corresponding EDS maps of ZnFe2O4@SiO2 hollow microspheres (b).
The value of ID/IG is 0.85 for GO. It can be observed that the ID/IG of RGO-ZnFe2O4 (ID/IG=0.90) and RGO-ZnFe2O4@SiO2 (ID/IG=0.99) composites are both higher than that of GO, which can be contributed to the highly disorder nature of graphene and the decrease in the average size of sp2 domains upon reduction of GO. The defects play an important role in EM wave absorbing and will further explain the mechanism of EM wave absorbing [26]. The field dependence of magnetization for ZnFe2O4, ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites was measured by VSM at room temperature with the magnetic field swept back and forth between −13.0 and +13.0 kOe, as shown in Fig. 5. The samples all exhibit typical soft magnetic behavior. The value of saturation magnetization (Ms) is 20.0 emu g−1 for ZnFe2O4 hollow microspheres, 11.5 emu g−1 for ZnFe2O4@SiO2 hollow microspheres, 9.5 emu g−1 for RGO-ZnFe2O4 composites and 7.8 emu g−1 for RGOZnFe2O4@SiO2 composites. This decrease in magnetism is mainly attributed to the decrease in the weight ratio of ZnFe2O4 in the composites. The frequency dependence of its EM parameters (the relative complex permittivity (εr=ε′−jε″), the relative complex permeability (μr=μ′−jμ″), the dielectric tangent loss (tan δE=ε′/ε″) and the magnetic
located at 102.3 eV is shown in Fig. 3d, which confirms the existence of SiO2 in the composites [22]. The C1s spectrum of RGO-ZnFe2O4@SiO2 composites (Fig. 3c) consists of four functional groups: C–C/C=C (284.6 eV), C–O (286.4 eV), C=O (287.8 eV) and O–C=O (289.3 eV) groups [23]. Compared with GO (Fig. 3b), the intensity of carbon binding to oxygen decreases rapidly, especially for C–O, suggesting the remarkable reduction of GO [24]. Such a transformation of RGOZnFe2O4@SiO2 composites implies a good electronic conductivity, which is favorable for enhancing EM wave absorbing. All of the above results indicate that the RGO-ZnFe2O4@SiO2 composites are formed. The Raman spectra of GO, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites are illustrated in Fig. 4, which further supports the preparation of RGO-ZnFe2O4@SiO2 composites. It can be clearly observed that the Raman spectra of GO, RGO-ZnFe2O4 and RGOZnFe2O4@SiO2 composites all show two peaks centered at 1340 cm−1 (D band) and 1590 cm−1 (G band). The D band is corresponded to disordered vibrations of C-sp3 atoms and the G band is assigned to inplane vibration modes of C-sp2 atoms in a 2-dimensional hexagonal lattice. The intensity ratio (ID/IG) is used to react the degree of defects in graphene or edges [25]. The structural changing from graphene oxides to the as-prepared composites could be observed by the ID/IG. 3
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Fig. 3. XPS spectra: wide scan of RGO-ZnFe2O4@SiO2 composites (a); C 1s spectrum of GO (b); C 1s spectrum (c) and S 2p spectrum (d) of RGO-ZnFe2O4@SiO2 composites.
Fig. 4. Raman spectra of GO, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites.
Fig. 5. Magnetization curves of ZnFe2O4, ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGOZnFe2O4@SiO2 composites.
tangent loss (tan δM=μ′/μ″)), is used for characterization of dielectric loss and magnetic loss properties of absorbers. The real parts of permittivity and permeability symbolize the storage ability of electric and magnetic energy, while the imaginary parts of permittivity and permeability are related to the dissipation of electric and magnetic energy [27]. Therefore, the paraffin wax combined with ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites
were fabricated to measure the relative complex permittivity and the relative complex permeability. Fig. 6 shows the real (ε′) and imaginary (ε″) parts of relative complex permittivity, the real (μ′) and imaginary (μ″) parts of relative complex permeability and the dielectric tangent loss (tan δE=ε′/ε″) and the magnetic tangent loss (tan δM=μ′/μ″) in the frequency range of 2– 18 GHz for ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 4
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Fig. 6. The relative complex permittivity real parts (a) and imaginary parts (b); The relative complex permeability real parts (c) and imaginary parts (d); the corresponding dielectric loss tangents (e) and magnetic loss tangents (f) of ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites.
ZnFe2O4@SiO2 composites are due to the fact that the addition of graphene may increase the electric polarization and electric conductivity, since εr is an expression of polarizability of a material, which consists of dipolar polarization and electric polarization. It is also observed that the RGO-ZnFe2O4@SiO2 composites show lower values of ε′ and ε″, compared with RGO-ZnFe2O4 composites, caused by the low electrical conductivity of SiO2 semiconductor. For RGOZnFe2O4@SiO2 composites, the μ′ values maintain around 1.0, whereas the μ″ values decrease gradually from 1.4 to −0.1 in the whole frequency rang, similar to those of ZnFe2O4@SiO2 and RGO-ZnFe2O4
composites, respectively. As shown in Fig. 6a, it can be observed that the ε′ values of ZnFe2O4@SiO2 hollow microspheres almost maintain in 3 over the frequency range of 2–18 GHz, while the ε′ values of RGOZnFe2O4 and RGO-ZnFe2O4@SiO2 composites decrease from 17.3 to 8.8 and 9.3–5.9 with several fluctuations, respectively, and are both higher than the ZnFe2O4@SiO2 hollow microspheres. Meanwhile, for RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites, the values of ε″ are in the range of 13.3–3.0 and 4.3–1.3, respectively, both higher than the ZnFe2O4@SiO2 hollow microspheres (in the range of 1.3–0), as presented in Fig. 6b. The higher ε′ and ε″ for RGO-ZnFe2O4 and RGO5
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Fig. 7. Calculated reflection loss of ZnFe2O4@SiO2 (a), RGO-ZnFe2O4 (b) and RGO-ZnFe2O4@SiO2 composites (c) with a thickness in the range of 2.0–6.0 mm and corresponding three dimensional presentation (d–f). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
ductivity, further improving the EM wave absorption performance. And compared with RGO-ZnFe2O4 composites, after the introduction of SiO2, the electrical conductivity of RGO-ZnFe2O4@SiO2 composites become weak, leading to a smaller gap between the dielectric tangent loss and the magnetic tangent loss of RGO-ZnFe2O4@SiO2 composites, which can improve impedance matching. By comparing tan δE and tan δM, we can see that the RGO-ZnFe2O4@SiO2 composites possess a far higher magnetic tangent loss in the low frequency (2.0–5.0 GHz) and a slightly higher dielectric tangent loss in the high frequency (5.0– 18.0 GHz), revealing that the magnetic loss plays the main role in the
composites, as shown in Fig. 6c and d. From Fig. 6e, the values of tan δE for RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites are in the range of 0.81–0.34 and 0.49–0.23, respectively, both higher than that of the ZnFe2O4@SiO2 hollow microspheres (about 0), indicating that the dielectric loss occurs over the whole frequency range for them. Meanwhile, for RGO-ZnFe2O4@SiO2 composites, the values of tan δM are from 1.5 to −0.1, similar to those of ZnFe2O4@SiO2 and RGOZnFe2O4 composites, as shown in Fig. 6f. Compared with ZnFe2O4@SiO2 hollow microspheres, the addition of graphene may increase the electric polarization and electric con6
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Fig. 8. Comparison of the reflection loss (a) and the absorption bandwidth with the reflection loss below −10 dB (b) of ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites.
frequency, d is the thickness of absorb layer, c is the velocity of EM wave in vacuum, and εr and μr are respectively the relative complex relative permittivity and permeability, respectively. Fig. 7a–f display the calculated reflection losses of ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites. It is clear that the thickness of absorbers has a great influence on the EM wave absorbing properties. Moreover, the absorption peaks for two samples shift toward the lower frequency as the absorber thickness increases. As shown in Fig. 7a and d, for ZnFe2O4@SiO2 hollow microspheres, the reflction loss is almost below −10 dB. For RGO-ZnFe2O4 composites (Fig. 7b and e), the maximum reflction loss reaches −14.4 dB at 11.9 GHz and the reflction loss below −10 dB (90% absorption) is up to 3.2 GHz (from 10.6 to 13.8 GHz) with a thickness of 2.0 mm. For RGO-ZnFe2O4@SiO2 composites (Fig. 7c and f), the maximum reflection loss is −45.8 dB at 7.6 GHz with the thickness of 3.7 mm and the bandwidth corresponding to reflction loss below −10 dB can reach 4.0 GHz (from 7.7 to 11.7 GHz) with the thickness of 3.0 mm. In addition, the absorption bandwidth with reflction loss below −10 dB is up to 14.5 GHz (from 3.5 to 18.0 GHz) with a thickness in the range of 2.0–6.0 mm, as indicated by a blue line. Moreover, the absorption bandwidth below −20 dB also covers a wide frequency range (from 6.2 to 11.8 GHz), as indicated by a olive line. Accordingly, the reflction loss of RGO-ZnFe2O4@SiO2 composites below −10 dB can be obtained in the range of 3.5– 18 GHz which covers in wide frequency bands from the S to Ku band in the absorber thickness from 2.0 to 6.0 mm, indicating the excellent EM wave absorption properties of RGO-ZnFe2O4@SiO2 composites. Fig. 8a shows the comparison of maximum reflection loss at different matching thicknesses for ZnFe2O4@SiO2, RGO-ZnFe2O4 and RGO-ZnFe2O4@SiO2 composites. The RGO-ZnFe2O4@SiO2 composites exhibit well EM wave absorption properties at every thickness when compared to ZnFe2O4@SiO2 and RGO-ZnFe2O4 composites. Fig. 8b presents the absorption bandwidth of three samples below −10 dB. The absorption bandwidth indicates that the RGOZnFe2O4@SiO2 composites can be used in a wide frequency range. All the above results suggest that the RGO-ZnFe2O4@SiO2 composites exhibit more excellent EM wave absorption performance in terms of both the maximum reflction loss and the absorption bandwidth. The results demonstrate that the RGO-ZnFe2O4@SiO2 composites could be used as a kind of candidate absorber. To understand the physical phenomenon and the possible mechanism giving rise to the enhanced EM wave absorption properties of RGO-ZnFe2O4@SiO2 composites, the quarter-wave thickness criteria was used to analyze the experimental results [28]. In this criteria, the maximum reflection loss can be achieved at a certain EM wave frequency (fm) if the thickness (tm) of composites satisfies the matching
Fig. 9. Frequency dependence of the reflection loss with different thicknesses (a); simulations of the absorber thickness (tm) versus peak frequency (fm) under λ/4 conditions (b); the relationship between the impedance matching characteristics (Z =| Zin/Z0|) and the EM wave frequency (c) of RGO-ZnFe2O4@SiO2 composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
low frequency and the dielectric loss occurs in the high frequency. Meanwhile, the EM wave can be effectively attenuated by multiple reflections because of the hollow structure of ZnFe2O4. To further reveal the EM wave absorption properties, the reflection losses (RL) are calculated according to transmission line theory as the following equations [1,2]:
RL (dB) = 20 log
(Z
in
)(
− Zo / Z in + Z 0
(
Z in = Zo u r /εr tanh [ j 2πfd /c
)
u r εr ]
)
(1) (2)
where Zin is the input impedance of absorber, f is the EM wave 7
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equation [29]:
t m = nc /(4fm
μr εr )
(3)
where |εr| and |μr| are respectively the modulus of measured εr and μr at fm. According to this criteria, the peak frequency is inversely proportional to the thickness. According to equation [3], the tm can be simulated, which denotes as tmfit, as indicated by a red line in Fig. 9b. In the figure, all black dots (denoted as tmexp, and the thickness achieved from the maximum reflction loss in Fig. 9a) are almost located around the quarter-wave thickness criteria for RGO-ZnFe2O4@SiO2 composites. Thus, the absorber will exhibit excellent EM wave absorption property due to quarter-wavelength interference phenomenon. In addition, the reflction loss is highly related to the impedance matching characteristics (Z=|Zin/Z0|，the modulus of normalized impedance matching characteristics), expressing as the following equation [4] and it is an important parameter to reduce the reflection of EM wave at the air-absorber interface [30].
Z = Z in /Zo =
(
u r /εr tanh [ j 2πfd /c
)
u r εr ]
(4)
When Z is equal or close to 1, the impedance matching characteristic of absorber can easily incident in the absorber to be attenuated rather than reflected at the absorber surface and the EM wave will be converted to thermal energy or dissipated through interference. The frequency dependence of Z for RGO-ZnFe2O4@SiO2 composites is obtained as shown in Fig. 9c. It can be found that the Z values are close to 1 at the frequencies of 7.6 GHz at the absorber thickness of 3.7 mm on the quarter-wave thickness criteria, which has the maximum reflection loss (−45.8 dB), according with the above conclusions. Therefore, the well-matched characteristic impedance also contributes to the EM wave absorption property of RGO-ZnFe2O4@SiO2 composites. 4. Conclusion In summary, the RGO-ZnFe2O4@SiO2 composites which has an obviously enhanced EM wave absorption property were successfully synthesized by a rational process. When it was evaluated as an EM wave absorber, the composites exhibit enhanced EM wave absorption properties in terms of both the maximum reflection loss value and the absorption bandwidth. The results indicated that the maximum reflection loss of RGO-ZnFe2O4@SiO2 composites can reach −45.8 dB at 7.6 GHz with the thickness of 3.7 mm and the absorption bandwidth with the reflection loss below −10 dB can reach 14.5 GHz (from 3.5 to 18.0 GHz) with a thickness in the range of 2.0–6.0 mm. And for RGOZnFe2O4@SiO2 composites, the magnetic loss plays the main role in the low frequency and the dielectric loss occurs in the high frequency. It is believed that such composites will find their wide application in EM wave absorbing area. Acknowledgments This work was supported by the Spaceflight Foundation of China (Grant no. 2014-HT-XGD), the Spaceflight Innovation Foundation of China (Grant no. 2014KC11023). References [1] C. Wang, R.T. Lv, Z.H. Huang, F.Y. Kang, J.L. Gu, Synthesis and microwave absorbing properties of FeCo alloy particles/graphite nanoflake composites, J. Alloy. Compd. 509 (2011) 494–498. [2] Y.X. Huang, Y. Wang, Z.M. Li, Z. Yang, C.H. Shen, C.C. He, Effect of pore morphology on the dielectric properties of porous carbons for microwave absorp-
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