Interface Strategy To Achieve Tunable High Frequency Attenuation

Feb 26, 2016 - ABSTRACT: Among all polarizations, the interface polar- ization effect is the most effective, especially at high frequency. The design ...
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Interface Strategy To Achieve Tunable High Frequency Attenuation Hualiang Lv,†,‡ Haiqian Zhang,† Guangbin Ji,*,† and Zhichuan J. Xu*,‡ †

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P.R, China School of Materials Sciences and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

‡

S Supporting Information *

ABSTRACT: Among all polarizations, the interface polarization effect is the most effective, especially at high frequency. The design of various ferrite/iron interfaces can significantly enhance the materials’ dielectric loss ability at high frequency. This paper presents a simple method to generate ferrite/iron interfaces to enhance the microwave attenuation at high frequency. The ferrites were coated onto carbonyl iron and could be varied to ZnFe2O4, CoFe2O4, Fe3O4, and NiFe2O4. Due to the ferrite/iron interface inducing a stronger dielectric loss effect, all of these materials achieved broad effective frequency width at a coating layer as thin as 1.5 mm. In particular, an effective frequency width of 6.2 GHz could be gained from the Fe@NiFe2O4 composite. KEYWORDS: carbonyl iron, ferrite, coating, dielectric loss, polarization, microwave absorption

1. INTRODUCTION Magnetic materials have been widely applied in various fields, including cancer therapy,1 magnetic resonance imaging,2 chemical separation,3 drug delivery,4 sensing,5 and even energy storage devices.6,7 Magnetic materials can also be used as electromagnetic absorbers for electromagnetic wave attenuation (EMA). It is known that the fast development of wireless techniques on electronic devices has caused electromagnetic wave interference (EMI) pollution, which disturbs the operation of electronic equipment and also threatens the health of living beings. The ideal EMA materials should have a broad effective frequency ( f E), in which the reflection loss (RLmin) should be lower than −10 dB. In addition, a very thin coating thickness (d) is also desired. Magnetic materials show great potential to solve EMI pollution problems, owing to their ideal impedance matching properties as compared to dielectric materials. It is known that magnetic materials offer improvement of the complex permeability parameters (μr). A higher μr value will result in a larger impedance matching value, and thus, an electromagnetic wave can easily incidence into an absorber, according to the following equations8 Z = Z1/Z0

(1)

Z1 = (μr /εr)1/2 Zo

(2)

μr = μ′ − jμ″

(3)

εr = ε′ − jε″

(4)

ε = (ε′2 + ε″2 )1/2

(5)

2

2 1/2

μ = (μ′ + μ″ )

where Z1 represents the impedance value of absorbing materials and Z0 is the impedance of the free space. A higher μr can lead to a larger Z. Meanwhile, the presence of interfaces in magnetic composite materials can lead to strong interface polarization, and it benefits the conversion of the incidence electromagnetic wave into thermal energy. Generally, high-performance magnetic absorbers are designed through three strategies. The first strategy is to make core/shell structures, in which magnetic materials are made as the core and the dielectric materials are the shell. Reported examples include Fe3O4@ZrO2,9 Fe3O4@ C, 10 Fe@SnO 2 , 11 and FeNi@C, 12 etc. The moderate impedance matching between core and shell materials and the double attenuation mechanism (dielectric and magnetic loss) results in an enhancement of microwave absorption properties. However, this strategy needs carefully designed synthetic methods, and the scale-up synthesis is always a problem. The second strategy is to simply mix magnetic materials with high dielectric property materials. For example, the mixture containing ZnO rods and hexagonal cone-like FeCo exhibited an optimal reflection loss value of −31 dB.13 Mixing low dielectric FeCo with high dielectric ZnO resulted in the low ε′ and ε″ values of the overall mixture material. This benefits the impedance matching. Similar absorbers, such as Fe3O4/ppy/CNT and Fe3O4/PANI, have been synthesized through this design, and both were able to achieve the RLmin value less than −10 dB.14,15 This strategy is quite easy to be conducted, since the synthesis of the two materials can be conducted separately and the synthesis can be scaled up readily. Received: December 25, 2015 Accepted: February 26, 2016 Published: February 26, 2016

(6) © 2016 American Chemical Society

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2.3. Characterizations. The powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 ADVANCE X-ray diffractometer (using Cu Kα radiation (λ = 0.154178 nm with 40 kV scanning voltage, 40 mA scanning current, and scanning range from 20° to 80°). A Hitachi S4800 type scanning electron microscope (operated at an acceleration voltage of 3.0 kV was used to observe the morphology features of as-synthesized materials. The magnetic properties were recorded on a Lakeshore 7400 series vibrating sample magnetometer (VSM) at room temperature. The Fourier Transform Infrared (FT-IR) spectra were recorded with a Perkin−Elmer IR spectrometer. The X-ray photoelectron spectroscopy (XPS) study was conducted on a PHI 5000 VersaProbe system with an Al Kα X-ray source. The elemental ratio was determined by the Inductively Coupled Plasm (ICP) approach (Optima 5300DV). The S parameters, including S11, S12, S21, and S22, were obtained by an Agilent PNA N5224A vector network analyzer using the coaxial-line method. The samples were prepared by homogeneously mixing the paraffin wax and composite (mass ratio = 7:3) and then pressed into toroidal-shaped samples (Φout: 7.0 mm, Φin: 3.04 mm). Then, the Agilent PNA software was used to calculate the ε′, ε″, μ′, μ″ values. Finally, the reflection loss curves with changed thickness (d) were drawn by the following formulas.22

However, subsequently the effort is needed to ensure that the two materials are mixed homogeneously as much as possible in the composite matrix. Otherwise, the impedance matching might not be achieved as desired. The third strategy is to assemble magnetic nanomaterials onto 2D nanomaterials such as graphene to form a hybrid structure. Graphene is a high dielectric material, while it cannot be used alone due to the limitation in impedance matching with the absorber matrix. Thus, it is often combined with magnetic materials. Popular examples include Fe 3 O 4 /graphene, 16 Fe/graphene, 17,18 Fe3O4@ZnO/graphene,19 and Fe3O4@NiO@SiO2/graphene.20 Due to the very large surface area of graphene and the super high porosity of wrapped graphene, the magnetic nanomaterials can be easily loaded onto graphene to form such a hybrid structure. This strategy has been found simple and effective with regard to the synthetic approaches. However, the graphene itself is not magnetic, and combing magnetic materials with graphene may lead to a decrease of the overall magnetic moment, which is a factor lowering the impedance matching. On the other hand, it should be noticed that, unlike metallic magnets, magnetic ferrites are a class of materials with relatively high saturation moment (among 80−100 emμ/g) as well as strong dielectric loss ability. Therefore, combining magnetic ferrites with high magnetic moment materials such as metallic iron may not only remain a high impedance matching property, but also increase integral dielectric loss ability. In this article, we report an approach to generate ferrite/iron interfaces through growing ferrites MFe2O4 (M = Zn, Fe, Co, Ni) onto flake-like iron (treated carbonyl iron). Micrometer scale Fe flakes were produced from carbonyl iron by a simple mechanical ball-milling method. The ferrites were then produced and assembled onto iron flakes at the same time through a solvothermal method. The type of ferrites is varied by simply varying the precursor type. Such an approach is able to produce ferrite covered carbonyl iron flakes in a large scale, and so-produced hybrids with ferrite/iron interfaces exhibited excellent microwave absorption properties.

Z in = Zo(μr /εr)1/2 tanh[j(2πf d(με )1/2 /c)] r r

(7)

RL(dB) = 20 log|(Z in − Zo)/(Z in + Zo)|

(8)

where Zin means the input impedance of the absorber, f is the frequency of electromagnetic wave, d is the coating thickness of the absorber, and c is the velocity of electromagnetic wave in free space. εr (εr = ε′ − jε″) and μr (μr = μ′ − jμ″) are the complex permittivity and permeability of the absorber.

3. RESULTS AND DISCUSSION Figure 1a shows the SEM image of the carbonyl iron as received. The carbonyl iron is sphere-like, and the diameter is

2. EXPERIMENTAL SECTION 2.1. Materials. Carbonyl iron (Fe) prepared by pyrolysis Fe(CO)5 was purchased from Beijing Rongxing Tech Co. Precursor metal salts (Zn(Ac) 2 , FeCl 2 , Co(Ac) 2 , Ni(Ac) 2 ), ammonium hydroxide (NH4OH), acetone, and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co. All of the reagents used in this experiment were analytically pure and used without further purification. 2.2. Synthesis Fe/MFe2O4 (M = Zn, Fe, Co, Ni) flake. Carbonyl iron flakes were prepared by a wet ball-milling approach based on the previous literature.21 The sphere-like carbonyl iron acted as the raw material. Typically, 10 g of sphere-like carbonyl iron was mixed with 100 g of steel ball (diameter ∼4 mm) in 100 mL steel cans. Afterward, 40 mL of acetone was added as solvent. The ball-milling time was set to 10 h with the speed of 500 rpm. Flake carbonyl iron power was obtained after drying at 30 °C. Subsequently, another solvethermal process was utilized to load the MFe2O4 on the carbonyl iron flakes. In the first stage, 0.25 g of carbonyl iron flake and 50 mg of precursor salts were added in a mixture solution which contained 75 mL of distilled water (DI) and 75 mL of EG. After mechanical stirring for 10 min, 5 mL of ammonium hydroxide was dropwise added into the mixture solution for another 10 min. Finally, the mixture was transferred to a 250 mL autoclave and heated at 150 °C for 12 h. The as-prepared Fe/MFe2O4 composites were separated by magnetic separation. The composites prepared with Zn(Ac)2, FeCl3, Co(Ac)2, and 150 mg of Ni(Ac)2 were denoted as S1, S2, S3, and S4, respectively.

Figure 1. SEM images of carbonyl iron before (a) and after (b) 10 h ball milling. (c) The XRD patterns of carbonyl iron before and after ball milling for 10 h.

about 1−4 μm. It is known that the sphere-shaped carbonyl iron is not an ideal morphology for microwave absorption due to the Snoek effect.23 It is often optimized into a flake shape to overcome the Snoek effect.24 Ball milling is an effective approach to produce carbonyl iron flakes in large scale. Figure 1b shows the carbonyl iron flakes obtained after ball milling for 10 h. The cross area of these flakes is mainly in the microscale, and the corresponding thickness is less than its skin depth (∼1 μm). Figure 1c shows the XRD patterns of carbonyl iron before and after ball milling. The two diffraction peaks located at 44.6° and 65.0° are matched well with (110) and (220) crystal planes 6530

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ACS Applied Materials & Interfaces of α-Fe (JCPDS No: 06-0696). It can be seen that the ball milling does not change a sample’s crystal characteristic, indicating that the ball milling only changed the morphology of carbonyl iron from sphere-like to flake-like. Carbonyl iron flakes were then coated with a layer of ferrites through the solvothermal process, as stated in the Experimental Section. Zn(Ac)2, FeCl3, Co(Ac)2, and Ni(Ac)2 were used as precursors for growing ZnFe2O4, Fe3O4, CoFe2O4, and NiFe2O4 over the iron flakes. Figure 2 shows the FT-IR spectra of as-synthesized

each pattern. The peaks of (200), (311), (222), (511), and (440) of ZnFe2O4 (JCPDS No: 01-1108) can be found in the XRD pattern of ZnFe2O4 coated iron flakes. The Fe3O4 coated iron flakes give Fe3O4 diffraction peaks of (220), (311), (400), (511), and (440) (JCPDS No: 01-1111). CoFe2O4 coated iron flakes show the CoFe2O4 diffraction peaks of (220), (311), (400), (511), (440), and (531) (JCPDS No.: 22-1086), and NiFe2O4 coated iron flakes show the NiFe2O4 diffraction peaks of (220), (311), (400), (511), (440), and (531) (JCPDS No.: 10-0325). Note that there is no diffraction peak of ZnO, NiO, and CoO, indicating that the iron flakes are indeed coated with ferrites. Figure 4 shows SEM images of ferrites coated carbonyl iron flakes. In general, the surface of iron flakes after the coating process became much rougher as compared with the iron flakes without ferrite coating (Figure 1b). This should be ascribed to the presence of ferrites on the iron surface. Figure 4a−c shows images of ZnFe2O4 coated iron flakes. The close view in Figure 4c clearly shows ZnFe2O4 particles grown on the carbonyl iron flakes. Interestingly, most of these ZnFe2O4 particles are in octagonal cone feature with 50−200 nm in edge. Figure 4d and e show that the Fe3O4 coating layer consists of Fe3O4 nanoparticles in the size range 50−100 nm. Figure 4f and g are images of the CoFe2O4 coating layer. It can be seen that the CoFe2O4 is in flake shape with a thickness of ∼10 nm. These flakes closely coat on carbonyl iron flakes. Figure 4h and i show the coating layer of NiFe2O4, which consists of nanoparticles in the size range 20−150 nm. EDX mapping is conducted to observe the elemental distribution (Figure S1). It was found that Zn, Co, and Ni ions are evenly distributed in each ferrite. The atomic ratios of M:Fe were determined by the ICP method. The atomic ratios of M:Fe are 10.7, 8.2, and 6.2% for Fe@ZnFe2O4, Fe@CoFe2O4, and Fe@NiFe2O4, respectively. The ratio of Fe3O4 to Fe (Fe@Fe3O4) cannot be determined by ICP. Therefore, it is determined through the calculation based on the measured magnetization value (Figure S2). The saturation magnetization (Ms) of carbonyl iron is 188 emμ/g, while that of Fe3O4 is 92 emμ/g. The saturation magnetization of Fe@Fe3O4 is 136 emμ/g. Thus, the weight ratio of Fe3O4 to Fe can be determined as 54:46. It is known that one of criteria of excellent electromagnetic absorbers is the broad effective frequency (f E), in which the reflection loss (RLmin) should be lower than −10 dB. Furthermore, being able to achieve these with the very thin coating thickness (usually less than 2 mm) is also desired.31 Figure 5 shows the calculated reflection loss of as-synthesized Fe@MFe2O4 composites at the thickness of 1.5 mm. It can be seen that all the composites exhibit excellent absorption properties. The lowest RLmin value given by Fe@ZnFe2O4 composite is nearly −39 dB, and the Fe@NiFe2O4 composite exhibited the lowest value, close to −27 dB. At the thickness of 1.5 mm, all Fe@MFe2O4 composites gave their f E values more than 4 GHz, which is quite impressive. In particular, the maximum f E value can be achieved as high as 6.2 GHz by Fe@ NiFe2O4 composite (11.2−18 GHz). This is quite larger than other similar absorbers as listed in Table 1.32−37 As the coating thickness increases to 2 mm (Figure 6), the Fe@Fe3O4 composite exhibited a lowest RLmin value of −56 dB and others gave their lowest RLmin in the range of −20 ∼ −23 dB. The f E values of these composites are still more than 4 GHz. For example, Fe@ZnFe2O4 showed the biggest f E value of ∼6.2 GHz (between 9.2−15.4 GHz). As compared to carbonyl iron flakes without ferrite coating (Figure S3), it can be seen that the addition of ferrite coating

Figure 2. FT-IR spectra of ferrites coated carbonyl iron flakes.

ferrites coated iron flakes. In general, the ferrite with spinel structure (MFe2O4) exhibits two absorption peaks: one is located at ∼450 cm−1 and the other is around 550 cm−1, which refer to the M−O and Fe−O bonds, respectively.25 In Figure 2, all ferrites coated iron flakes show similar spectra, where the above two characteristic absorption peaks of ferrites can be observed. For example, the absorption peaks around 410−440 cm−1 for these samples refer to Zn−O, Fe−O, Co−O, and Ni− O bonds.26−29 The absorption peak around 550 cm−1 in each sample should be the characteristic peak of the Fe−O bond. Figure 3 shows the XRD patterns of as-synthesized ferrites

Figure 3. XRD patterns of ferrites coated carbonyl iron flakes.

coated iron flakes. For each pattern, the strongest diffraction peak at 44.6° can be assigned to the diffraction of (110) of metallic iron (JCPDS No: 06-0696). However, the Fe diffraction peaks exhibit a slight shift, which should be due to the influence of ferrite coating layers.30 The other weak diffraction peaks can be assigned to the corresponding ferrite in 6531

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Figure 4. FE-SEM images of Fe@MFe2O4 composites: (a−c) Fe@ZnFe2O4, (d−e) Fe@Fe3O4, (f−g) Fe@CoFe2O4, and (h−i) Fe@NiFe2O4.

Figure 5. Reflection loss curves of Fe@MFe2O4 composites tested at 1.5 mm: (a) Fe@ZnFe2O4, (b) Fe@Fe3O4, (c) Fe@CoFe2O4, and (d) Fe@ NiFe2O4.

Table 1. Reflection Loss Information of Similar Absorbers Sample Fe@SiO2 Graphene/Fe CoxFe3‑xO4 NiFe2O4 Fe/NiFe2O4 ZnxFe3‑xO4 Fe@NiFe2O4

fE ∼1.5 ∼0.2 0 0 5.0 ∼5 6.2

GHz GHz GHz GHz GHz GHz GHz

RLmin ∼15 ∼ −11 < −10 < −10 −18 ∼ −45 −27

dB dB dB dB dB dB dB

Thickness 3.0 1.5 2.0 1.5 1.8 3.0 1.5

mm mm mm mm mm mm mm

α=

ref

2 πf (μ″ε″ − μ′ε′) c

{

1/2 1/2

+ ⎡⎣(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2 ⎤⎦

30 31 32 33 34 35 this work

}

(9)

It can be seen that a large impedance matching ratio and a strong attenuation constant are needed to reach highperformance microwave absorption. Figure 7a shows the impedance matching ratios of these composites. It shows that most of them gave their impedance match ratios larger than 0.3, revealing the good microwave matching feature. It should be noticed that there is a rare magnetic material that may reach such a high impedance matching value at high frequency. For example, CNTs@Fe showed the matching ratio 0.2−0.28 in the frequency range 14−18 GHz 40 and carbon@Fe@Fe 3 O 4 exhibited the matching ratio 0.22−0.27 in the range 14−18 GHz.41 Considering the need for a small coating thickness, it is

layers promoted the electromagnetic absorption properties. The high-performance electromagnetic absorption relies on the impedance matching and attenuation constant α, as expressed in eq 9:38,39 6532

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Figure 6. Reflection loss curves of Fe@MFe2O4 composites tested at 2.0 mm: (a) Fe@ZnFe2O4, (b) Fe@Fe3O4, (c) Fe@CoFe2O4, and (d) Fe@ NiFe2O4.

Figure 7. Impedance matching ratios (a) and the attenuation constant curves (b) of Fe@MFe2O4 composites (gray curve for Fe@ZnFe2O4, red for Fe@Fe3O4, orange for Fe@CoFe2O4, and green for Fe@NiFe2O4).

Figure 8. Electromagnetic parameters of Fe@MFe2O4: the real (a) and the imaginary (b) parts of permittivity; the real (c) and the imaginary (d) parts of permeability.

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originated from dielectric loss. High magnetization properties are favorable for impedance matching between the absorber and free space. Generally, when the frequency is beyond microwave frequency, dielectric loss mainly comes from polarization, including Debye dipolar relaxation, interface polarization, and electron polarization.48 At the first stage, in order to demonstrate the likely Debye dipolar effect, the relationship between ε′ and ε″ can be drawn as follows:49

meaningless to get high attenuation values in the lower frequency region, while it is quite important to have strong attenuation at high frequency. Figure 7b shows the attenuation constant of Fe@MFe2O4 composites. At 14−18 GHz, all these composites exhibit better attenuation ability (>400). This value is higher than that by NiCu alloy (200−240) and carbon-Fe/ Co/Ni (180−250).42,43 A high-performance absorber should have a moderate ε′ value (10−20). This value should not be too big, neither too small. For instance, graphene is not ideal since its ε′ value is higher than 100.44 Fe2O3 is also not ideal due to the low ε′ value of 3−5.45 Figure 8a shows the ε′ values of these composites. The values are moderate, in the range of 8−13. In addition, a sharp increase at 14−18 GHz can be observed for these composites, indicating the improvement of electron storage energy. The ε″ value represents dielectric loss ability. As for ferromagnetic absorbers, their ε″ values usually remain stable without large fluctuations at microwave frequency. It is interesting that these composites have an obvious increase after 15 GHz. Such an increase may possess a huge contribution on a lightweight microwave absorber, which can be well explained by the 1/4 wavelength equation:46 t m = nc/4f m(εrμr )1/2

(ε′ − (εs + ε∞)/2)2 + (ε″)2 = ((εs − ε∞)/2)2

The plot of ε′ versus ε″ may be a single semicircle which is denoted as the Cole−Cole semicircle. In Figure 10, three

(10)

It can be read that increasing ε″ at high frequency will lead to a smaller coating thickness (tm) according to eq 10. Figure 8c and d show their magnetic property features. Similar to other magnetic materials, both μ′ and μ″ values exhibit a decreasing tendency at 2−18 GHz. Due to the high magnetization values, high starting points for the real parts of the permittivity curves (μ0) are observed for these Fe@MFe2O4, which are crucial for maintainance of the large μ′ values and impedance matching properties at high frequency. However, Fe@MFe2O4 composites do not show excellent magnetic loss ability at 14−18 GHz because of smaller μ″ values (0−0.3). Additionally, most magnetic materials can also play a negative role on attenuation due to the eddy current effect, which can be calculated through the equation below:47 C0 = μ″(μ′)−2 f −1 = 2πμ0 d2δ

(12)

Figure 10. Debye relaxation curves of Fe@MFe2O4: gray for Fe@ ZnFe2O4, red for Fe@Fe3O4, orange for Fe@CoFe2O4, and green for Fe@NiFe2O4.

semicircles can be clearly found in each sample, suggesting the probable Debye dipolar relaxation effect. Besides the relaxation effect, interface polarization between Fe and ferrite is also another crucial factor to consume an electromagnetic wave. XPS was applied to further reveal the probable interface and electron polarization effect according to the reported findings.50,51 It is well-known that XPS can effectively measure the surface chemical environment. The probable shifting binding energy value is a vital evidence to reveal polarization. Theoretically, the standard binding value of Fe is 709.4 eV (3/ 2p) and the binding energy values of Fe in ferrite are 710.6 (ZnFe2O4), 711.4 eV (Fe3O4), 710.8 eV (CoFe2O4), and 710.5 eV (NiFe2O4), respectively.52−57 If interface polarization happens at the interface of Fe and ferrites, their corresponding binding energy values should present slight shifting, owning to the difference in electronegativity. Figure 11 shows the XPS results of these ferrites coated iron flakes. Apparently, both metallic Fe and cationic Fe in ferrite exhibit their binding energy values different from the standard ones. For all samples, the binding energy values of the metallic Fe are slightly bigger than the standard value of 709.4 eV. It may be originated from the interface polarization. It is well-known that Fe has an electronegativity value of 1.83, much smaller than its ionic state or other cations (Co2+, Ni2+, Zn2+). Therefore, the outermost layer electrons can be easily attracted by Fe3+ and other cations in each ferrite, which resulted in the decrease of Fe electron density. Hence, the attraction force by the Fe element nucleus atomic is stronger and reflected by the increase of the binding energy. In principle, if only the interface polarization exists, the binding energy of Fe3+ should become smaller. However, the binding energy of Fe3+ in Fe3O4 is larger than its standard value, indicating the possible electron polarization. As we know, ferrite is in the spinel structure, in which most of the special cations occupy A sites and Fe3+ stays in B sites.58 Electron polarization always happens between Fe3+ and special cations.

(11)

Co is the eddy current effect. When the absorber presents an eddy current effect, Co will be a constant. From Figure 9, we can get information that these values are changed for Fe@ MFe2O4 at 2−18 GHz, indicating the nonexistence of the eddy current effect. Thus, we can conclude that the novel electromagnetic loss ability of these composites is mainly

Figure 9. Eddy current data of Fe@MFe2O4 (gray for Fe@ZnFe2O4, red for Fe@Fe3O4, orange for Fe@CoFe2O4, and green for Fe@ NiFe2O4). 6534

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Figure 11. XPS spectra of Fe@MFe2O4 composites: (a-b) Fe@ZnFe2O4, (c) Fe@Fe3O4, (d-e) Fe@CoFe2O4, and (f-g) Fe@NiFe2O4.

the larger size of these flakes, the incidence electromagnetic will be multiple scattering among flakes.59,60 Note that the surface of the flakes is covered by monodispersed ferrite particles. These particles may change the direction of the incidence electromagnetic wave into disorder. Such a process may further scattering intensity. The electromagnetic wave may be attenuated by the opposite electromagnetic wave, as illustrated in Figure 12.

The outer layer electrons of Fe3+ will transfer to special cations under electron or magnetic field. Such a process may increase the binding energy of Fe3+ and lead to the smaller values of special cations. The increase of Fe3+ binding energy implies special cations influenced the electron density of Fe3+. Similarly, the binding energy values of special cations are 1019.8, 779.1, and 849.8 eV for Zn2+, Co2+, and Ni2+; smaller than their standard values of 1021.4, 777.9, and 855.4 eV, respectively, further indicating the possible electron polarization. In addition, the binding energy values of O 1s did not show an obvious shift. This matches well with those O 1s in ZnFe2O4, Fe3O4, CoFe2O4, and NiFe2O4, respectively (Figures S4−S7). In addition, we propose that there is a small part of the electromagnetic wave consumed by a scattering effect. Due to

4. CONCLUSION In summary, we have designed a simple strategy to grow ferrites (MFe2O4, M = Zn, Fe, Co, Ni) on carbonyl iron flakes. The asprepared Fe@MFe2O4 composites showed remarkable dielectric features at high frequency. The excellent electromagnetic 6535

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Research Article

ACS Applied Materials & Interfaces

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Figure 12. Schematic illustration for the possible scattering map.

absorption properties have been achieved by these ferrites coated carbonyl iron flakes. At a composite coating thickness of 1.5 mm, their fE values ranged 4.0−6.2 GHz, and all the RLmin values are below −20 dB. The dielectric absorption mechanism may be attributed to interface polarization and electron polarization effects.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12662. Element distribution, M−H curves, reflection loss, and XPS spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No.: 11575085), the Aeronautics Science Foundation of China (No.: 2014ZF52072), the Funding for Outstanding Doctoral Dissertation in NUAA (No.: BCXJ1509), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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