xCoFe composites at multiple frequency bands

Journal of Magnetism and Magnetic Materials 493 (2020) 165699

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Research articles

Enhanced microwave absorption properties of (1−x)CoFe2O4/xCoFe composites at multiple frequency bands

T

Jun Zhoua, Xiangfeng Shua, Yueqin Wanga, Jialin Maa, Yin Liua, , Ruiwen Shub, Lingbing Kongc, ⁎

⁎

a

School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, Anhui, China School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, Anhui, China c College of New Materials and New Energies, Shenzhen Technology University, Shenzhen 518118, Guangdong, China b

ARTICLE INFO

ABSTRACT

Keywords: CoFe2O4 CoFe Natural resonance Impedance matching Microwave absorption

(1−x)CoFe2O4/xCoFe composites with x = 0.3, 0.4, 0.5, 0.6 were synthesized by using a hydrothermal method. The face-centered cubic structure CoFe2O4 adheres to the polyhedral CoFe surface due to strong magnetic interaction to form a composite material. The effects of compositions of the CoFe2O4/CoFe composites on their microwave absorption properties were studied. The sample with a composition of 0.4CoFe2O4/0.6Co4Fe6 showed optimal reflection loss (RL) −58.22 dB at 12.96 GHz, and the matching thickness was 1.45 mm. Meanwhile, it had an effective absorption frequency range of 11.12–15.28 GHz (RL < −10 dB), with a bandwidth of 4.16 GHz, covering the X-band and Ku-band. The enhanced microwave absorption properties of the composites, as compared with the two components, are attributed to the strengthened natural resonance and the increased impedance matching. Most importantly, by adjusting their composition, the CoFe2O4/CoFe composites can be used as microwave absorbers at C, X or Ku bands. Consequently, these CoFe2O4/CoFe composites are promising candidates that can be used to design effective microwave absorbers over wide frequency bands.

1. Introduction Electromagnetic pollution has been one of the most concerned pollution in the 21st century. It not only affects the routine utilization of electronic devices, but also potential threats human health. Therefore, it is highly demanded to develop high performance microwave absorbing materials [1–5]. Generally, microwave absorber materials are mainly composites based on ferrites, magnetic metal powders, or conductive polymers [6–9]. Ferrite-based composites have been the most promising candidates for microwave absorber applications, because ferrites have high magnetic permeability and electrical resistivity. Specifically, high electrical resistivity avoids the skin effect encountered for metallic conductors at high frequency electromagnetic fields, thus minimize the reflection of electromagnetic waves [10,11]. Meanwhile, magnetic metal, including Fe, Co, Ni and their alloys, have strong magnetic properties, with both free electron absorption and magnetic loss for electromagnetic wave absorption. Recently, it has been reported that manganese ions (Mn2+) in nano manganese ferrite (MnFe2O4) are replaced by different transition metal ions, including Zn2+, Fe2+, Ni2+ and Co2+. It has been found that appropriate substitution of M2+ ions can easily adjust magnetic properties including coercivity and blocking temperature (TB), and it is expected that an

⁎

effective microwave absorber can be applied [12]. It is well known that the microwave absorption properties of a material are dependent on its complex permittivity, complex permeability and impedance matching [4,6,13–15]. Various materials and strategies have been reported for microwave absorption applications. For example, Lv et al found that the optimal parameter for the synthesis of Fe50Co50 alloy powder was ultrasonication time of 3 h, with an optimized reflection loss (RL) of −22 dB [16]. A Ni-Co ferrite (Ni1−xCoxFe2O4) was prepared by sol–gel method. It was found that increasing the content of Co can effectively enhance the microwave absorption performance [17]. Two-dimensional (2D) VO2(M) was synthesized by using a hydrothermal method, with an optimal RL of −45.8 dB and an effective absorption bandwidth of 3.37 GHz (RL < −10 dB) [18]. Nano-sized Mn-Zr replaces Ni-Co spinel ferrite, and it has been found that proper adjustment of the ratio can effectively improve the microwave absorption performance [19]. Besides, it is widely accepted that conventional one-component absorbers do not meet new performance requirements, such as strong absorption, wide absorption bandwidth, small thickness and lightweight [20,21]. In this regard, core-shell structures for microwave absorbers have received considerable attention, due to their outstanding microwave absorption performances. The polarization at the interfaces

Corresponding authors. E-mail addresses: [email protected] (Y. Liu), [email protected] (L. Kong).

https://doi.org/10.1016/j.jmmm.2019.165699 Received 9 June 2019; Received in revised form 1 August 2019; Accepted 12 August 2019 Available online 14 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials 493 (2020) 165699

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Table 1 Compositions and synthetic conditions of the (1−x)CoFe2O4/xCoFe composites. Samples number

The molar ratio of CoFe2O4:CoFe

CoFe molar ratio of Co:Fe

Reaction temperature (℃)

S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9

0:1 1:0 7:3 6:4 5:5 4:6 4:6 4:6 4:6

7:3 – 7:3 7:3 7:3 7:3 6:4 5:5 4:6

160 110 160,110 160,110 160,110 160,110 160,110 160,110 160,110

Fig. 3. Hysteresis loops of the samples, with the insets showing enlarged view. Table 2 Magnetic parameters of the samples. Samples

S-1

S-2

S-6

S-7

S-8

S-9

Ms(emu/g) Mr(emu/g) Mr/Ms Hc(Oe)

209.97 30.03 0.14 190.64

78.89 42.98 0.54 1446.69

116.08 40.98 0.35 964.58

139.04 43.82 0.32 1010.77

125.98 36.38 0.29 861.49

122.86 38.17 0.31 947.41

−43.0 dB. The authors believed that the high absorption property was attributed to the void space of the core-shell structures that effectively attenuated the incident electromagnetic waves [27]. As a typical spinel ferrite, CoFe2O4 has been extensively studied, due to its large saturation magnetization and high Snoek's limit. Li et al fabricated LPA-SWCNT/ CoFe2O4 composites, which exhibited enhanced microwave absorption performance as compared with the single component materials, achieving optimal RL of −30.7 dB [28]. Graphene/BaFe12O19/CoFe2O4 nanocomposites have been developed, which displayed an optimum RL of −32.4 dB [29]. In addition, the magnetic properties have been significantly changed by replacing the NiCuZn spinel ferrite with Tm3+ [30]. Including saturation magnetization, coercivity and remanence ratio have a tendency to decline, which provides an effective way to develop a high performance microwave absorber. In present work, synthesized (1−x)CoFe2O4/xCoFe (x = 0.3, 0.4, 0.5 and 0.6) composites by using a hydrothermal method in order to combine the advantages of the CoFe2O4 and CoFe. It was found that the dielectric loss could be increased and the eddy current could be suppressed by optimizing the ratio of CoFe2O4 to CoFe. As a result, the impedance matching was optimized, thus leading to enhanced microwave absorption performances.

Fig. 1. XRD patterns of samples.

2. Experimental 2.1. Synthesis of CoFe alloy powder Typically, 1.8 mmol FeCl2·4H2O and 4.2 mmol CoCl2·6H2O were dissolved in deionized water. Then, 10 mL NaOH (5 mol/L) was mixed with 9.6 mL N2H4·H2O (50 wt%) to form a mixed solution, which was added dropwise to the above solution under vigorous stirring for 10 min. After that, the solution was transferred to an autoclave and heated at 160 °C for 110 h. The product was magnetically separated and thoroughly washed with deionized water and absolute ethanol (three times each). Finally, the product was dried overnight at 60 °C in a vacuum over, denoted as S-1.

Fig. 2. FESEM images of representative samples: (a) S-1, (b) S-2, (c) S-3, (c) S-4, (e) S-5 and (f) S-6.

of the core-shell structures and the scattering of the void space tended to consume the energy of electromagnetic waves [22-26]. For instance, a C@NiCo2O4@Fe3O4 composite was reported to exhibit optimal RL of 2

Journal of Magnetism and Magnetic Materials 493 (2020) 165699

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Fig. 4. Microwave properties of the samples: (a) real permittivity (ε′), (b) imaginary permittivity (ε″), (c) real permeability (μ') and (d) imaginary permeability (μ′′).

2.2. Synthesis of CoFe2O4 micropowder

field emission scanning electron microscopy (FESEM, Zeiss, Gemini Sigma 300/VP). Magnetization was measured by using a vibrating sample magnetometer (VSM, HH-2) at room temperature. Electromagnetic parameters including complex permittivity and complex permeability were recorded by using a vector network analyzer (VNA, AV3672B-S) over the frequency range of 2–18 GHz. To prepared the samples for microwave measurement, the powders were uniformly mixed with molten paraffin, with a paraffin-powder mass ratio of 1:4. After solidification, the mixtures were pressed into a cylindrical shape with a mold, with an inner diameter of 3.04 mm, an outer diameter of 7 mm and thicknesses of 2–3 mm.

To synthesize CoFe2O4 powder, 6 mmol FeCl2·4H2O and 3 mmol CoCl2·6H2O were dissolved in deionized water. The solution was heated at 110 °C for 3 h in a high pressure reactor. Then, the supernatant was mixed with 36 mL NaOH (0.792 mol/L) solution under stirring. The above mixture was transferred to an autoclave and heated at 110 °C for 10 h. After that, the product was washed with deionized water and absolute ethanol each for three times, followed by overnight drying at 60 °C in vacuum. The sample was denoted as S-2. 2.3. Synthesis of (1−x)CoFe2O4/xCoFe composites Representatively, to synthesize 0.7CoFe2O4/0.3CoFe composite, S-1 was dispersed in deionized water, while 5 mL PEG-400 was added, with the aid of ultrasonication for 30 min. Then, 14 mmol FeCl2·4H2O and 7 mmol CoCl2·6H2O were dissolved into the above solution with vigorous stirring. After that, 84 mL NaOH (1.848 mol/L) solution was added with stirring for 30 min. The mixture was transferred to an autoclave and heated at 110 °C for 10 h. The product was magnetically separated and washed thoroughly with deionized water and absolute ethanol. Finally, the sample was vacuum dried for 12 h at 60 °C, which was denoted as S-3. For clarity, detailed compositions and reaction conditions of the composite materials are listed in Table 1.

3. Results and discussion

2.4. Characterization

3.2. Morphological analysis

Phase composition of the samples was characterized by using X-ray diffraction (XRD, Shimadzu, LabX XRD-6000), with Cu Kα radiation (λ = 0.154 nm). Microstructure analysis was carried out by using a

FESEM images of the samples are presented in Fig. 2. As showed in Fig. 2 (a) and (b), the particles have only morphology in each case, confirming the presence of one-component CoFe and CoFe2O4,

3.1. Structural analysis Fig. 1 shows XRD patterns of the samples. The S-1 sample has three characteristic peaks, corresponding to (1 1 0), (2 0 0) and (2 1 1) planes of the body-centered cubic (bcc) CoFe alloy (JCPDS No. 49-1568). All the diffraction peaks of S-2 sample belong to CoFe2O4 (JCPDS No.221086). Also, the diffraction peaks of the composites S-3-S-9 indicate that the ferrite and the alloy phases coexist, confirming the formation of the (1−x)CoFe2O4/xCoFe composites.

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Fig. 5. Cole-Cole semicircles (ε″-ε′) of the samples in the frequency range of 2–18 GHz.

between CoFe and CoFe2O4 and the CoFe is magnetically stronger than CoFe2O4. 3.3. Magnetic properties Fig. 3 shows hysteresis loops of the samples. Saturation magnetization (Ms), remanence magnetization (Mr), remanence ratio (Mr/Ms) and coercive force (HC) are listed in Table 2. Comparatively, CoFe2O4 exhibits a hard magnetic behavior, while CoFe is magnetically soft. According to Skamsky's exchange coupling theory, as the value Mr/Ms is > 0.5, there is effective exchange coupling [31]. The values of all the composite samples are < 0.5. However, without the exchange coupling, the Ms of the composite is the linear sum of the two components [32]. As can be seen from Table 2, the Ms value of S-6 is 116.08 emu/g, which is not the linear sum of S-1 (209.97 emu/g) and S-2 (78.89 emu/g). Thence, this means that the dipole interaction between the particles is accompanied by weak exchange coupling behavior [31]. Usually, low values of coercivity and remanence magnetization mean high magnetic loss, thus leading to high microwave absorption performances [33]. It is worth noting that the sample S-9 has a relatively low Mr and Hc, implying that it would be a promising candidate as microwave absorbing materials.

Fig. 6. C0-f curves of the samples in the frequency range of 2–18 GHz.

respectively. In comparison, the composites have particles with two morphology, as shown in Fig. 2 (c–f). Also, it is found that the CoFe2O4 is prone to adhere to the surface of the CoFe particles, as the content of the alloy is increased. It is especially pronounced for the sample S-6, as demonstrated in Fig. 2 (f). The CoFe particles are nearly fully covered by CoFe2O4. This can be attributed to the strong magnetic interaction

3.4. Microwave absorption properties Microwave absorption properties of a material are closely related to its complex permittivity (εr = ε′ − jε″) and complex permeability 4

Journal of Magnetism and Magnetic Materials 493 (2020) 165699

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Fig. 7. Reflection loss (a) and impedance matching (b) curves of the experimental samples.

Fig. 8. Reflection loss curves of the samples at different thicknesses.

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particular, the μ″ value of S-2-S-9 fluctuates within the range of 2–18 GHz. This may be due to interface effects, size effects and natural resonance. The different interfaces provided by the composite, as well as the dimensional difference between the two components, cause a variety of scattering of incident electromagnetic waves in the material, and electromagnetic parameters have some fluctuations [27,47,48]. Generally, magnetic losses at microwave frequencies are mainly contributed by eddy current losses and natural resonance losses [13,49–52]. To further understand the effect of magnetic loss on microwave absorption performance, the following equation is cited [53]:

Table 3 Microwave absorption properties of the samples. Samples

S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9

EMA performances Matching thickness (mm)

Optimal RL Value (dB)

Optimal RL fm(GHz)

Bandwidth (RL < −10 dB)

Bandwidth (RL < −20 dB)

1.30 5.50 2.20 2.30 1.45 1.38 1.47 1.40 1.45

−38.27 −23.28 −44.73 −45.53 −47.34 −50.18 −52.14 −50.06 −58.22

14.48 15.52 8.40 8.00 12.48 14.48 13.20 14.72 12.96

10.56–18.00 14.16–16.32 6.32–9.20 6.48–8.80 10.88–14.32 12.48–17.12 11.28–15.68 12.56–17.60 11.12–15.28

13.20–15.84 15.28–15.68 8.08–8.64 7.08–8.24 12.00–13.04 13.92–15.12 12.56–13.84 14.16–15.60 12.32–13.60

C0 = D2

)2 + ( )2 = (

s

)2 ,

= µ (µ ) 2f

1,

(2)

When the magnetic loss is entirely eddy current loss, C0 is a constant that does not vary with the external electromagnetic field, since ρ (resistivity) and D (particle size) have been determined. Fig. 6 shows C0-f curves of the samples. Within 2–5 GHz, the C0 value of S-3-S-9 showed a significant decrease. However, within 5–18 GHz, the C0 value changes less. The eddy current loss has been effectively suppressed, owing to the high resistance and the effective dispersion of the powders in the insulating medium. Therefore, magnetic loss of the samples mainly comes from natural resonance loss rather than eddy current loss. Specially, natural resonance behavior of electromagnetic parameters leads to the presence of interfacial polarization, electric dipole polarization and conductance losses [47], which is the reason for the enhancement of potential microwave absorption performance. Microwave absorption performance is usually expressed as reflection loss (RL), which is given by [54,55]:

(μr = μ'− jμ′′). The storage and consumption of electric energy and magnetic energy are represented by the real (ε′ and μ') and imaginary (ε″ and μ′′) parts, respectively [34–38]. Fig. 4 shows electromagnetic properties of the sample. As seen in Fig. 4 (a), ε′ values of the samples are in the range of 6–18 over the frequency range of 2–18 GHz. Besides, ε′ values have shown significant fluctuations. Variation of ε′ with frequency is similar to those widely observed on the open literature [39,40]. It is worth noting that ε′ values of the composites are higher than those of S-1 and S-2, which could be attributed to the increased volume fraction of the interface in the composites. Therefore, composites would have stronger interfacial dipole polarization [41,42]. The ε″ values are in the range of 0.1–7, as shown in Fig. 4 (b). Similarly, ε″ values of the composites are higher than S-1 and S-2. According to the free electron theory: ε″ = 1/2ε0πfρ, where ε0 is the dielectric constant of vacuum and ρ is the resistivity (Ω) of the material. It can speculates that the high resistivity is higher for the composite material. Interestingly, the ε″ value exhibits a strong peak at 7–9 GHz, indicating that resonance behavior may occur when the sample has a high resistivity. Over the frequency range, there could be dielectric relaxation due to the electric dipole at the high-frequency electromagnetic field and the accumulated charge at the interface of the composites [6]. As the polarization charge cannot keep up with the variation of the external electromagnetic field, dielectric relaxation is present. Consequently, composite samples (S-3-S-9) have higher dielectric constant and also high dielectric loss, with which enhanced microwave absorption performances would be expected. Generally, Debye theory can be used to understand the mechanism of dielectric loss at microwave frequencies. The real and imaginary parts of the dielectric constant can be described by the following equation [43]:

(

1

RL = 20 log |(Zin

Z0)/(Zin + Z0 )|,

Zin = Z0 (µr / r )1/2 tanh [j (2 fd/ c )(µr r )1/2],

(3) (4)

where Zin is the input impedance of a single layer of absorbing material, Z0 is the free space impedance, εr is the complex permittivity, μr is the complex permeability, f is the test frequency and d is the thickness of the absorbing material. In addition, impedance matching is an important factor in determining microwave absorption performances. Ideally, the electromagnetic waves should not be reflected at the interface between the absorbing material and free space, so that electromagnetic wave entirely enters the absorber [56]. Impedance matching is described as the following equation [57]:

Z = |Zin/ Z0| = (µr / r )1/2 tanh [j (2 fd/ c )(µr r )1/2],

(5)

Fig. 7 shows RL curves and impedance matching curves of the samples. As seen in Fig. 7 (a), the S-5 and S-8 samples exhibit relatively high RL values. However, their Z values are not close to 1, as observed in Fig. 7 (b). This means that the electromagnetic waves are partially reflected at the interface of the test thickness. Therefore, the performance requirements of a large number of attenuating electromagnetic waves cannot be satisfied. To further evaluate microwave absorption performances of the composites, RL curves of all samples at different thicknesses are shown in Fig. 8. Detailed parameters are listed in Table 3. Obviously, at optimal thicknesses, the composite samples (S-3-S-9) have higher performances than of the two components (S-1 and S-2), as shown in Fig. 9. Among the composite samples, the sample S-6 (0.4CFO/0.6CF) shows the highest RL value (−50.18 dB). On the basis of S-6, the Co:Fe molar ratio of the CoFe alloy is varied. Interestingly, the sample S-9 (0.4CFO/0.6Co4Fe6) reaches a RL value of −58.22 dB, with an effective absorption bandwidth (RL < −10 dB) of 4.14 GHz (11.12–15.28 GHz), covering the X-band and Ku band. Fig. 10 shows a three-dimensional RL diagram of S-9. It can be clearly seen that the RL is mostly less than −20 dB in the region where the thicknesses are in the range of 1–2 mm. In addition, there is an optimum RL at 12.96 GHz. Impedance matching profiles of the samples

(1)

where ε∞ and εs are relative dielectric constant and static dielectric constant, respectively. The ε″ − ε′ curves would be a single semicircle, known as a Cole-Cole semicircle. Each semicircle represents a Debye process [43]. Fig. 5 shows ε″ − ε′ plots of the samples. Obviously, the curves have at least two semicircles, which are marked with arrows in Fig. 5. Importantly, they have certain degrees of distortion, which further indicates that multiple processes are responsible for the enhanced microwave absorption properties of the material, such as conductance losses [43,44]. Magnetic loss is another important factor affecting microwave absorption performances. μ′ and μ″ are shown in Fig. 4(c) and (d). It is found that S-1 has relatively high magnetic permeability, which is similar to those reported in the open literature [45,46]. The a value of the composite material (S-2-S-9) has approximately the same trend. In the range of 2–18 GHz, the value of μ′ decreases from 1.7 to about 1.2. In

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Fig. 9. Reflection loss curves and contour maps of the samples at different thicknesses: (a,b) S-6, (c,d) S-7, (e,f) S-8 and (g,h) S-9.

are shown in Fig. 11 (a). Most strikingly, the Z values of the samples S6–S-9 are close to 1. This means that electromagnetic waves can nearly entirely enter the absorber. In addition, Table 4 lists some literature data and our results as a comparison. Moreover, decay constant (α) is another parameter that can be used to evaluate the microwave absorption properties of absorbing materials, which is defined as [66]:

=

2 f × c

(µ

µ )+

(µ

µ ) 2 + (µ

+ µ )2 ,

Obviously, S-1 has higher α value, which could be linked to its high magnetic loss. Nevertheless, the microwave absorption properties of the materials should be a reflection of the synergistic effect of impedance matching and high attenuation constant [27,67]. Fig. 12 shows a schematic diagram describing possible absorption mechanisms of the CoFe2O4/CoFe composite materials. The incident electromagnetic waves are converted into thermal energy in various ways, including natural resonance, interfacial polarization, dielectric relaxation and dielectric/magnetic loss.

(6)

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Fig. 12. Schematic of microwave absorption mechanisms of the CoFe2O4/CoFe composites.

electromagnetic wave absorption capabilities, as compared with CoFe2O4 and CoFe, which are attributed to enhanced natural resonance and increased impedance matching. More importantly, the absorption performances can be optimized by adjusting the ratios of CoFe2O4:CoFe and Co:Fe. Specifically, the sample 4CoFe2O4/6Co4Fe6 has an optimum reflection loss of −58.22 dB when the matching thickness is 1.45 mm, with an effective absorption bandwidth (RL < −10 dB) of 4.16 GHz, covering the X-band and Ku-band. These (1−x)CoFe2O4/xCoFe could be used as promising microwave absorbers for various practical applications.

Fig. 10. Three-dimensional representation of reflection loss of S-9.

4. Conclusions (1−x)CoFe2O4/xCoFe composites have been synthesized by using a low-cost hydrothermal method. Due to the strong magnetic interaction, CoFe2O4 adheres tightly to the surface of the CoFe alloy to form a composite. The CoFe2O4/CoFe composites showed enhanced

Fig. 11. Impedance matching profiles (a) and attenuation constants (b) of S-6 ~ S-9.

Table 4 Comparison of microwave absorption properties of the (1−x)CoFe2O4/xCoFe composites with those of some recently reported materials. Samples

Fe-500 Co dendrites Co4Fe6 CoFe2O4 CoFe/C Co3Fe7/C CoFe2O4/RGO CoFe2O4/graphene Hollow CoFe2O4–Co3Fe7 S-6 S-7 S-8 S-9

EMA performances Matching thickness(mm)

Optimal RL Value(dB)

Optimal RL fm(GHz)

Bandwidth (RL < -10 dB)

Ref

3.00 3.00 1.70 2.50 1.70 2.00 1.60 2.00 2.00 1.38 1.47 1.40 1.45

−36.89 −35.60 −52.20 −34.10 −46.50 −35.30 −44.10 −42.00 −41.60 −50.18 −52.14 −50.06 −58.22

≈8 5.60 10.48 13.40 12.56 9.10 15.60 12.90 – 14.48 13.20 14.72 12.96

– – 7.60–13.60 12.30–14.00 10.88–14.80 6.80–13.10 13.30–18.00 11.20–15.79 7.40–10.40 12.48–17.12 11.28–15.68 12.56–17.60 11.12–15.28

[58] [59] [60] [61] [62] [63] [64] [65] [4] This work This work This work This work

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