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Electromagnetic Wave Absorption Performance on FeO Polycrystalline Synthesized by the Synergy Reduction of Ethylene Glycol and Diethylene Glycol 3

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Jin dou Ji, Yue Huang, Jinhua Yin, Xiuchen Zhao, Xingwang Cheng, Jun He, Jingyun Wang, Xiang Li, and Jiping Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11533 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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The Journal of Physical Chemistry

Electromagnetic Wave Absorption Performance on Fe3O4 Polycrystalline Synthesized by the Synergy Reduction of Ethylene Glycol and Diethylene Glycol Jindou Ji,†,‡ Yue Huang,† Jinhua Yin,‡ Xiuchen Zhao,† Xingwang Cheng,† Jun He,§ Jingyun Wang,ǀǀ Xiang Li*† and Jiping Liu† †

School of materials science and engineering, Beijing Institute of Technology, Beijing 100081, China ‡

§

Physic department, University of Science and Technology Beijing, Beijing 100083, China

Central Iron and Steel Research Institute, Division of Functional Materials, Beijing 100081, China

ǀǀ

Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking Univisity, Beijing 100871, China

ABSTRACT: Fe3O4 nanoparticles were synthesized by hydrothermal method with the synergy reduction of ethylene glycol (MEG) and diethylene glycol (DEG). The purity, grain size, magnetism and the microwave absorption performance of the samples can be controlled by the concentration of DEG in the precursor solution. Under the optimized condition of synthesis, the product is of highly crystallized cubic Fe3O4 and the crystallite size of Fe3O4 is about 44-60 nm

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with the saturation magnetization about 96 emu/g. The Fe3O4 paraffin composites exhibit excellent microwave absorption properties at the frequency range of 1-18 GHz, which are attributed to the electron transition resonance, natural resonance and polarization of Fe3O4. The minimum reflection loss of Fe3O4 synthesized under the MEG and DEG content of 3.58 mol/l and 2.10 mol/l, respectively, can reach -42 dB at a thickness of 2.1 mm. And the effective absorption bandwidth of the samples can reach 3.9 GHz (7.3~11.2 GHz) at the thickness of 2.5 mm. The result demonstrates that Fe3O4 nanoparticles synthesized with DEG as auxiliary reducing agent and surfactant have a good microwave absorption performance.

1. INTRODUCTION

Recently, the impact of high-frequency electromagnetic (EM) interference and EM pollution on the environment and human physical and mental health are causing more and more attention1 and the developing of novel advanced high performance EM radiation absorbers is of great significance.2,3 Magnetic oxide ferrite with high dielectric properties and good permeability is an effective microwave absorber, which has attracted the attention of the academia.4-7 In addition, the ferrites generally have a high resistivity (106~1010 Ω•m) which helps to avoid the skin effects as performed under high-frequency EM waves, making EM radiation enter the interior of the absorbers and well absorbed. Therefore, magnetic oxide ferrites have been extensively utilized in the fields of military stealth defense, civil microwave darkroom, EM communication interference and microwave absorption.8-12 Fe3O4 is a kind of ferrite with anti-spinel structure with the Curie temperature of 585 °C. Fe3O4 has an AB2O4 structure, where the A position (tetrahedral position) is Fe3+, and Fe2+ and Fe3+ are located at the B position (octahedral position) and both of them are basically disordered in B


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site,13,14 then electrons can move quickly between the two oxidation states of the iron, which has a significant effect on the dielectric loss of the iron oxide solids. At the same time, Fe2+ is easily polarized at high frequency, and this polarization can cause EM wave loss, which has a certain synergistic effect on the absorption of EM radiation.8 Studies on Fe3O4 showed that the real part of its complex permittivity is in range of 6~7, while its corresponding imaginary part is about 0.2; the real part of the magnetic permeability is about 1.74 and the imaginary part reaches 0.5 at the maximum.15 As is well known, the absorption characteristic of Fe3O4 strongly depends on its dielectric properties and magnetic properties. In fact, the enhancement of the imaginary part of the permeability of Fe3O4 is beneficial to the microwave absorption performance, by improving the impedance matching of the materials. It also meets the requirements of the advanced microwave absorber with the absorption of "bandwidth, multi-function, light weight, low thickness".16 Moreover, it has been demonstrated that the EM wave absorption ability of Fe3O4 strongly depends on its crystalline quality, grain size and morphology.17-20 Accordingly, extensive efforts have been devoted to synthesizing Fe3O4 by different methods and

processes,

including coprecipitation,21 solvothermal

method,22 sol-gel

method,23

electrospinnning,15 reflux heating,5 and microwave assisted synthesis.24 However, the crystallinity of the particles obtained by the coprecipitation method is relatively poor with its grains agglomerated, resulting in uneven particle size. The sol-gel method uses metal alkoxide as a precursor resulting in high cost, while the gelation often needs long cycle process. Fe3O4 prepared by the solvothermal method has the advantages of small and uniform particle size, controllable morphology and avoids calcination at high temperature.25,26 In addition, polyols are highly effective in solvothermal processes and the synthesized Fe3O4 nanoparticles have high


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yields.27 And the growth rate and the grain size of Fe3O4 can be regulated by the diethylene glycol (DEG) due to its high viscosity.28 In this paper, Fe3O4 nano-crystallines were prepared by hydrothermal method using ethylene glycol (MEG) as reducing agent and DEG as auxiliary reducing agent and surfactant. And then the EM radiation absorption properties of the produced samples were studied. The results demonstrated that Fe3O4, produced under the optimized solution (MEG 3.58 mol/l, DEG 2.10 mol/l), has the highest reflection loss peak at the frequency of 11.8 GHz, and the maximum reflection loss is -42 dB at the thickness of 2.1 mm. The microwave absorption band width is 3.2 GHz (9.5 GHz-12.7 GHz) corresponding the reflection loss peak less than -10 dB. When the thickness is 2.5 mm, at the frequency of 10 GHz, the center highest reflection loss peak is -19 dB, and the effective microwave absorption band width is 3.9 GHz (7.29 GHz-11.2 GHz). The results in this paper indicate that a new route has been developed to synthesize Fe3O4 polycrystallines with much improved microwave absorption characteristics. This strategy can also be applied to fabricate other ferrite and metal micro/nanocrystalline materials as well, and thus has extensive potential applications. 2. EXPERIMENTAL SECTION Synthesis of Fe3O4 Nanos-tructures. All chemicals used in this work are of analytical grade without any further purification. Ethylene glycol (MEG) is used as a reducing agent, diethylene glycol (DEG) is used as a surfactant and the reducing agent. FeCl3·6H2O (2.16 g, 8 mmol) and a small amount of sodium citrate dihydrate were dissolved in a certain amount of MEG, and the mixture was magnetic stirred at room temperature for 10 min to form a stable solution. Subsequently, a certain amount of DEG and sodium acetate trihydrate (4.0 g, 29 mmol) were added to the solution and the stirring was continued until the sodium acetate trihydrate was


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completely dissolved. Then deionized water was added to the above solution to form a homogeneous solution of 80 ml. The solution was then transferred into a Teflon-lined stainlesssteel autoclave (100 mL capacity), sealed, maintained at 200 °C for 16 hours, and then naturally cooled to room temperature. The final mixed product was washed several times by ethanol and deionized water and separated by magnetic decantation. And the obtained solids were dried in oven under nitrogen at 70 °C for 3 hours. Finally, the collected black powder was stored in a dry environment to prevent oxidation. Solvent thermal reaction process is as follows: Fe3+ + 3OH - = Fe(OH)3 (reddish brown colloid)

Fe(OH)3 → FeOOH + H 2O FeOOH + H 2O + e- → Fe(OH) 2 + OH − Fe(OH) 2 + 2FeOOH → Fe3O 4 + 2H 2O

The method of reducing ferric iron to ferrous iron by MEG is shown in Figure 1. Due to the strong oxidative properties of Fe3+, in the solution, MEG is easily converted to aldehydeor or acid by dehydrogenation or oxygenation of alcoholic hydroxyl groups, respectively. During this process, the electrons are transferred to Fe3+, the ferric ions are reduced to divalent ferrous ions and FeOOH is converted to Fe(OH)2. Characterization. The structure and phase of the product were identified by powder X-ray diffraction (XRD, Smartlab(3), Rigaku, Japan) with Cu Kα radiation diffractometer (λ= 0.15406nm, operating voltage 35 KV, current 40 mA) over the scan range 10 ° ~ 80 °, scanning speed 4 ° min-1, high-resolution transmission electron microscope (TEM, JEM-2100F, JEOL, Belgium) and X-ray photoelectron spectroscopy (XPS, PHI QUANTERA-II SXM, ULVAC-PHI, Inc. Japan). The macroscopic magnetic properties of the products were characterized by a sample vibrating magnetometer (VSM, Versolab, Quantum Design, USA) at room temperature with an


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applied magnetic field of -20 ~ 20 KOe. The complex permittivity and complex permeability of the samples were measured by vector network analyzer (VNA, N5230C, Agilent technologies, USA) in the frequency range of 1 ~ 18 GHz, and the measured samples were uniformly mixed with paraffin to form a cylindrical ring having an inner diameter of 3.0 mm, an outer diameter of 7.0 mm and a height of 2 mm. 3. RESULTS AND DISCUSSION Figure 2a shows the XRD patterns of the samples synthesized under the solutions with the MEG concentration of 3.58 mol/l and different DEG content (named as DEG0, DEG1.05, DEG2.10, DEG3.15 and DEG4.21). The peaks in XRD patterns of different samples are located at 2θ=18.30°, 30.08°, 35.43°, 37.06°, 43.07°, 53.47°, 56.97° and 62.53°, which can be assigned as (111), (220), (311), (222), (400), (422), (511) and (440) of cubic inverse spinel structure Fe3O4 (JCPDS No. 19-0629), respectively. However, there are weak peaks in XRD patterns at 2θ=24.14°, 33.15°, 40.85°, 49.48°, 54.09° and 63.99°, which correspond to the (012), (104), (113), (024), (116) and (300) (stars marked in the figure) crystal planes of Fe2O3 (JCPDS No. 330664). Figure 2a indicates that the structure of the sample DEG0 (the precursor solution without DEG) is constructed by Fe3O4 and a few parts of α-Fe2O3. Besides, as the concentration of DEG gradually increases, the content of α-Fe2O3 decreases monotonously. To carefully confirm the change of the α-Fe2O3 in the studied samples, the detailed peak of XRD patterns that comes from α-Fe2O3 was graphed in Figure 2b. Figure 2b indicates that as the concentration of DEG increases, the intensity of the corresponding XRD peaks located at 33.15° gradually reduces and disappears. These results suggest that the purity of Fe3O4 samples strongly depends on the concentration of DEG in the precursor solutions, namely, the purity of the synthesized product


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can be controlled by adjusting the concentration of DEG. Figure 2b demonstrates that high purity Fe3O4 sample DEG2.10 is produced when the concentrations of DEG is 2.10 mol/l in the solution. Figure 2c shows the peaks of the XRD patterns of Fe3O4 (311) plane of different samples. One can see from Figure 2c that as the concentration of DEG gradually increases, the plane of (311) shifts to low angles, which implies that the lattice space of this orientation increases little by little. Compared with Fe3+, Fe2+ has a large ionic radius. When Fe2+ replaces Fe3+ in the AB2O4 structures, the bond length of Fe3O4 will elongate and the cell volume increases as well.29 Therefore, the result in Figure 2c reveals the fact that more and more Fe3+ was replaced by Fe2+ in the octahedral structure of Fe3O4 samples, which causes an increase of the interplanar spacing. The average crystallite size of the samples has been calculated from the full width at half maximum (FWHM) of the intensity of XRD patterns peak by Debye-Scherrer formula:30 D = Kλ / β cos θ

(1)

Where D is the size of the crystallite, K is the curvature constant (0.89), λ is the wavelength of the X-ray (0.15406 nm), β is the FWHM, and θ is Bragg diffraction angle. The calculated particle size of Fe3O4 grains against the concentration of DEG in the precursor solutions is shown in Figure 2d. It is shown that with the increase of the concentration of DEG in the solutions, there is a peak in the particle size, and the particle size of the sample DEG2.10 is about 60 nm. This result shows that DEG not only improves the purity but also controls the grain size of the produced Fe3O4 samples. The change of particle size observed by XRD is in agreement with that observed by TEM (Figure S1, Supporting Information). Considering that DEG has the similar hydroxyl structure as that of MEG, it is also possible to reduce Fe3+ by dehydrogenation. However, the length of main chain of DEG is longer than that


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of MEG, which makes the reducibility of DEG weaker than that of MEG.28 Thus, DEG was selected as an auxiliary reducing agent to control the reaction and product growth when preparing a pure phase of Fe3O4 nanoparticles. DEG works as reductant to reduce Fe3+ as its concentration in the precursor solutions is low. In this case, a layer of DEG absorbs on the surface of the nuclei of Fe3O4 by means of its hydroxyl, promoting the formation of the product. Such effects will accelerate the growth of Fe3O4, leading to an increase in the grain size and the quality of crystalline of the produced sample.31 However, as the concentration of DEG is much high in the precursor solutions, DEG coats on the surface of the particles and blocks the exchange channel of ions between nucleated Fe3O4 and the solution, thus, decreases the growth rate of Fe3O4, and gives rise to the small grain size of the produced samples.31,32 All these analyses are consistent with the study of XRD shown in Figure 2d. XPS spectra of the samples is shown in the Figure 3. Figure 3a plots the full spectrums of the samples, which indicate that the peaks values at 724.2, 710.3, 529.7 and 284.2 eV can be indexed to binding energies of Fe 2p1/2, Fe 2p3/2, O1s and C1s, respectively. It confirms the existence of Fe, O and C (from the air) elements in the samples. In order to further determine the composition of the samples, the details of elements Fe is shown in Figure 3b. Figure 3b shows that the characteristic peaks of Fe 2p3/2 and Fe 2p1/2 oxidation states are located at 710.8 and 724.2 eV, respectively, which are closed to the XPS results published by Liu et al.20 As the concentration of DEG increases, the peaks shift to low binding energy, which is most likely due to the formation of Fe2+. The fitted Fe 2p XPS spectrum is shown in Figure 4. Figure 4 indicates that the fitted Fe 2p XPS spectrum in Fe 2p3/2 region is deconvoluted into several main peaks: the lowest binding energy peak at ~709.5eV corresponds to Fe2+, while the binding energy peak at ~710.8 eV and


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~712.6 eV are attributed to Fe3+ (octahedral species and tetrahedral species).33 As the concentration of DEG increases, the satellite peak situated at ~718.7 eV, which is the fingerprints of the electronic structures of Fe2O3,20,24,33-36 gradually decreases. It indicates that as the concentration of DEG increases, the purity of the samples increases, which is consistent with the result of XRD analysis. Besides, as the DEG increases, the characteristic peak area of Fe2O3 at ~724 eV decreases, which can also well explain the increase in product purity. From the XPS data, the composition of the samples DEG0, DEG1.05, DEG2.10, DEG3.15 and DEG4.21 are the following : 0.295Fe2O3-0.705Fe3O4, 0.056Fe2O3-0.944Fe3O4, Fe3O4, Fe2+0.976Fe3+2.024O4, and Fe2+0.955Fe3+2.045O4, respectively. When DEG is 2.10 mol/l, pury Fe3O4 is obtained. When DEG is more than 2.10 mol/l, it can be seen that the proportion of Fe3+ to Fe2+ increases in Fe3O4, which is most likely due to the surface oxidation of small particles. Figure 5 shows the room temperature magnetic hysteresis (M-H) loops of the studied samples. All the applied magnetic fields are in the range of -20 KOe ~ 20 KOe. The insert of Figure 5 is the magnified graphs of M-H under low applied magnetic field. From Figure 5, it can be seen that with the increase of the content of DEG in the precursor solutions, the saturation magnetization value and the coercivity of the corresponding samples observably increase. The highest saturation magnetization of our samples nearly reaches that of the bulk Fe3O4 (98 emu/g).37,38 As known, bulk α-Fe2O3 has a weak ferromagnetic at room temperature and its saturation magnetization value is less than 1 emu/g,39 the nanostructure α-Fe2O3 is paramagnetic characteristic owe to the mismatch of its surface spin and defects. In AB2O4 structure of Fe3O4, due to the increase of divalent ferrous ions, the unequal number of ions in the antiparallel spin arrangements result in an increase in the net spin magnetic moments. In this way, the saturation


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magnetization of samples increase. As indicated by the XRD study in Figure 2, with the increase of DEG, more and more Fe3+ ions in the precursor are reduced to Fe2+ ions, and the content of αFe2O3 gradually decreases, eventually getting high quality Fe3O4. The coercivity of magnetic materials effectively depends on its magnetocrystalline anisotropy, grain size and geometrical shape.40,41 Moreover, the lattice dislocation of crystalline, defects as well as the residual stress will result in a change in the effective anisotropy of magnetocrystalline, triggering the increase in coercivity.40,42 Thus, the increase of the coercivity of our samples could be explained by the decrease of the grain size and the increase of the magnetic anisotropic field,43 which is also confirmed by the XRD results shown in Figure 2. Consequentially, DEG is particular suitable reductant and surfactant to synthesize high quality and pure Fe3O4 polycrystalline with controlled grain scales. Table.1 shows the detailed saturation magnetization (Ms) and coercivity (Hc) of products that were synthesized under different concentration of diethylene glycol. Microwave absorption properties are highly associated with the complex permittivity (εr=ε'iε") and complex permeability (µr=µ'-iµ"). Figure 6 shows the frequency dependence of εr and µr of Fe3O4 prepared under the different concentration of DEG in the frequency range of 1~18 GHz. The real part ε' and µ' characterize the ability of the material to store the electromagnetic field energy, while the imaginary part ε" and µ" characterize the ability to the electromagnetic loss of the material. The larger ε" and µ" show the enhancement of the loss, thus, indicate the improvement of the microwave absorption performance of the material.19 As shown in Figure 6a,b, from the sample DEG0 to the sample DEG4.21, the complex permittivity of the produced sample gradually increases. In detail, in the range of 1~10 GHz, the real part of the permittivity (Figure 6a) increases from 8.1 to 14. While the imaginary part of permittivity of the


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corresponding samples increases from 0.2 to 0.5 (Figure 6b). When DEG content is 4.21 mol/l, the permittivity of the produced sample reaches the maximum (ε'=14, ε"=0.5). Previous studies reported that the real part the permittivity is in the range of 6~7 and the imaginary part of the permittivity is about 0.2,15 therefore, the complex permittivity is much enhanced in our samples, indicating that the products have high polarization ability and dielectric loss.44 According to free electronic theory,40,45

ε′′ = 1 / 2πε 0 ρf

(2)

where ε0 is the vacuum dielectric constant, ρ is the resistivity, and f is the microwave frequency. The imaginary part of the permittivity is inversely proportional to the resistivity of the absorber at a given frequency. With the increase of the concentration of DEG in solutions, the imaginary part of the permittivity of the produced samples gradually increases, indicating that the resistivity of the products decrease, which is beneficial to enhance the absorption ability of the products. In general, dielectric losses can be explained by electron polarization, dipole polarization, ion polarization, interfacial polarization, orientation polarization, and space charge polarization.40,46 Considering that ion polarization and electron polarization work above the THz band and our research involves 1~18 GHz frequency band, the ion polarization and electron polarization effects should not be considered in this work.40 Thus, the resonance of permittivity of these samples should arise from the space charge polarization, dipole polarization, and interfacial polarization. In combination with the dielectric loss tangent (tanδe=ε"/ε') shown in Figure 6c, it can be seen that the dielectric loss tangent of the sample DEG0 is prominently reduced under the frequency range of 2 GHz. However, such a behavior was much suppressed and finally disappeared in other samples. According to the results of the structure study shown in Figure 2, samples DEG0 and DEG1.05 have some Fe2O3 impurity phase, and as DEG increases, Fe2O3


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impurity gradually decreases and eventually disappears. Therefore, interfacial polarization between Fe3O4 and Fe2O3 could contribute to the dielectric loss prominently variety in frequency range below 2 GHz. As shown in Figure 6b,c, when the frequency is about 10 GHz, there is a dielectric loss peak, which could be attributed to the electron transition resonance between Fe2+ and Fe3+.47 The electron hopping effectively enhances the conductivity of the sample,48 which is conductive to improving the imaginary part of the permittivity (eq.(2)). In this way, the microwave absorption performance of the corresponding samples improves, which has been firmed by high dielectric loss (Figure 6b) and dielectric loss tangent (Figure 6c), consistent with previous reports.47,49 When the frequency is higher than 10 GHz, both the real part and the imaginary part of the permittivity increase with the increase of frequency, indicating that the dielectric loss enhanced drastically, and such loss is mainly due to the dipole orientation polarization of octahedral Fe2+ in Fe3O4.50 Considering that the Fe2+ is easily polarized,8 with the increase of DEG, the content of ferrous ions in the solution gradually increases, thus the polarization of the corresponding samples enhances.51 It is consistent with the report by Liu et al.20 Figure 6d,e and f show the dependence of the complex permeability and frequency of the studied samples, on the real part (Figure 6d), the imaginary part (Figure 6e) and the permeability loss tangent (Figure 6f). Figure 6d,e shows that the real part of the permeability is in range of 1.7~3.2, and the imaginary part is in range of 0.55~1.32, indicating that the permeability of sample is significantly increased compared with the previous study in which the reported real and imaginary parts of permeability are about 1.74 and 0.5, respectively.15 And the imaginary part of permeability increases about 2.6 times as compared with the reported ones, indicating the magnetic loss of the corresponding samples increases drastically.15 As shown in Figure 6d,e, the


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products containing Fe2O3 have small permeability (µ'=1.7, µ"=0.5), while the permeability gradually increases to the maximum (µ'=3.2, µ"=1.32) as the Fe2O3 impurity reduces or disappears in the studied samples. As the frequency increases in the range of 1~10 GHz, the real part of the permeability reduces from 3.2 to 0.6, and the imaginary part decreases from 1.32 to 0.2. When the frequency is more than 10 GHz, as the frequency increases gradually, the variation of the real part of the permeability is nearly ignored, while the imaginary part of the permeability linearly decreases. In general, the magnetic loss mainly comes from hysteresis, magnetic domain wall displacement, eddy current and natural resonance.44,52 However, the hysteresis loss is irreversible in the external weak magnetic field, usually negligible; domain wall resonance usually occurs in the 1~100 MHz frequency range. Considering the studied frequency band in this work, the magnetic loss mainly should come from the natural resonance and eddy current loss in our samples.53 The natural resonance frequency is related to the magnetic anisotropic properties of the material, according to ferromagnetic resonance theory, the relationship between the natural resonance frequency fr and the anisotropy energy Ha is:45,54 2πf r = γH a

(3)

H a = 4 K / 3µ 0 M s

(4)

K = µ0MsHc / 2

(5)

Where γ is the gyromagnetic ratio, K is the anisotropy coefficient, µ0 is the vacuum permeability (4π×10-7 N•A-2), Hc is the coercivity, and Ms is saturation magnetization. It can be seen from the equation that the high anisotropy energy is related to the high anisotropy coefficient, and the increase of the anisotropic energy causes an increase of the natural resonance frequency. The imaginary part of the permeability has a peak at the frequency of 1.8 GHz shown in Figure 6e, which is induced by the natural resonance loss of Fe3O4.16 On the basis of equation (3), the


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calculated natural resonance frequency (fr) of Fe3O4 is about 1.2 GHz, suggesting that the fr of our samples shifts to high frequency. Previous studies on Fe(C) nanocapsules55 and Co/C/PA composites44 demonstrated that the enhancement of the magnetocrystalline anisotropy induces a high frequency shift of fr. Therefore, it is reasonable to deduce that the shift of fr to high frequency in our samples could be attributed to the small size effect, namely, that the anisotropy energy of small size materials may be increased due to its surface anisotropic field.19 When the samples contain Fe2O3, the fr is located at 6.2 GHz (Figure 6e), which could be due to the increase of the anisotropy coefficient caused by the interfacial coupling between Fe3O4 and Fe2O3.55 Moreover, when the frequency is higher than 14 GHz, the imaginary part of the permeability is negative, similar with the reports of study on Fe3O4@SiO2 by Wang et al.26 and on Fe3O4@C by Li et al.56 in which the authors attributed the negative permeability to the conversion of magnetic energy from the composite material into electrical energy. In the light of the dielectric loss tangent and the permeability loss tangent shown in Figure 6c,f, it can be seen that at low frequencies (2~10 GHz), the microwave absorption comes from the magnetic loss, while the dielectric loss dominates at high frequencies. For ferromagnetic absorbers, the microwave absorption properties are also affected by the high frequency band vortex effect.49 The eddy current loss in our samples should be considered. As known, the eddy current loss(C) is related to the diameter d and the conductivity σ of the absorbers, and can be expressed as:46 C = µ ′′(µ ′) −2 f −1 = 2πµ 0 σd 2 / 3

(6)

where µ" and µ' are the imaginary and real parts of the permeability, respectively, µ0 is the vacuum permeability, and f is the microwave frequency. If the magnetic loss results from the eddy current loss, the values of C should be constant at different frequencies.26,50 Based on


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equation (6), the relationships between eddy current loss and frequency of our samples are shown in Figure 7. In Figure 7, there is a fatted peak in the 1~12 GHz range for each curve; while in range of 12~18 GHz, it can be observed that the value of the eddy current loss (C) decreases slightly with increasing frequency. The more details of the eddy current loss in the range of 12~18 GHz are shown in the inset of Figure 7. As the frequency increases from 12 to 18 GHz, the value of C of the sample DEG2.10 decreases from 0.03 to -0.02. Therefore, the eddy current loss could be excluded. In order to further understand the microwave absorption performance, the microwave reflection loss (RL) curves of the samples are calculated from the transmission line theory, through the complex permittivity and complex permeability at the given frequency and absorber thickness:8,46

RL = 20 lg

Z in − Z 0 Z in + Z 0

{

Z in = Z 0 (µ r / ε r )1 / 2 tanh j(2πfd / c)(µ r ε r )1 / 2

(7)

}

(8)

Where Z0=(µ0/ε0)1/2 is the free-space impedance, Zin is the input impedance, c is the speed of light in vacuum, f is the microwave frequency, d is the thickness of the absorber, εr and µr represent the relative complex permeability and the complex permittivity of the sample, respectively. According to equation (7) and (8), it can be seen that the material has great absorption properties only when the material satisfies the impedance matching conditions, that is, the input impedance (Zin) and the free space impedance (Z0) are equal (Zin=Z0). Therefore, the impedance matching is related to the thickness of the material, frequency, the complex permeability and the complex permittivity. As the thickness is determined for an applied EM wave material, the impedance matching depends mainly on its complex permittivity and complex


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permeability at a certain frequency. For our samples, they have great dielectric properties and magnetic properties, in which the value of permittivity is larger than the value of permeability. According to Equation (8), the material should have great impedance matching only if the values of complex permittivity and complex permeability are matched.57 Figure 8 shows the dependence of the microwave absorption RL on the frequency of samples synthesized at different DEG concentration solutions, and the RL performance of the corresponding samples is effective modified by its thickness and synthesis process. Figure 8 suggests that as the thickness of the samples increases, the microwave absorption peaks values gradually shift to low frequency, which can be explained from fm=c/2πµ"d, that is, the matching frequency (fm) is inversely proportional to the matching thickness.26 For each sample, there is a peak and matching frequency for the reflection loss at different thicknesses. Figure 8 indicates that with the increase of the thickness of samples, the RL loss has a maximum value. The maximum RL of samples decreases from -21.8 to -51.0 dB with the increase of DEG content in the solutions, while the matching thickness of the corresponding samples decreases from 4.0 to 2.1~2.4 mm. The observed largest bandwidth of RL less than -10 dB is about 3.91 GHz (8.23-12.14 GHz) of the sample DEG1.05, and the matching thickness is of 2.4 mm. In detail, the RL loss peak of this sample is constructed by two sub-peaks located at the C-band (4~8 GHz) and Ku-band (12~18 GHz).5 The more details about reflection loss data are shown in Table.2. Recently, the enhancement of microwave absorption performance caused by interfacial polarization has attracted wide attention. In the study of the carbon nanotubes (graphene) Fe3O4 (named as Fe3O4(C)) composites, the RL increased from -6 dB to -37.3 dB.58 While on Fe3O4graphene heterogeneous composites, remarkable enhanced microwave absorption performance realized and the bandwidth increased to 4.5 GHz (RL≤-10 dB).25 Moreover, in the study of Ni/C


16

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foam, Ni covered on the porous graphite carbon induced the microwave absorption properties of the material prominent enhanced with the RL increasing from 0 to -45 dB.59 The significant increase in the microwave absorption performance of the above materials were due to the contribution of material interface polarization. Based on the above reports, the largest RL bandwidth observed in our samples could be attributed to the strong electronic transition resonance15,20 and the interfacial polarization of α-Fe2O3 and Fe3O4. And when α-Fe2O3 gradually disappears, the interfacial polarization between α-Fe2O3 and Fe3O4 is weakened and disappeared, as a consequence, the bandwidth is narrow, which is in consistent with the results shown in Figure 6c-e. As shown in Figure 8, the RL peaks of samples synthesized under different DEG content are located at X-band (8~12 GHz) which could be due to the electron transition resonance, and it corresponds to a drastic increase of the imaginary part of the permittivity and dielectric loss at the 10 GHz loss (Figure 6b,c). In the range of 1~18 GHz, when the matching thickness of the samples is in range of 1.5~4.0 mm, the reflection loss less than -10 dB of the samples covers almost the whole frequency band: the whole C-band and X-band, and some of Ku-band (12 GHz~13.3 GHz, 16.9 GHz~17.8 GHz). Combined with the complex permittivity and complex permeability of the samples shown in Figure 8, the microwave loss in the C-band mainly comes from the natural resonance loss. And in Ku-band, the microwave loss is induced by Fe2+ polarization relaxation.50 However, the performance of EM absorption in Ku-band is weak, which could be due to the increase of complex permittivity and the reduction of complex permeability leading to impedance mismatch (eq.(8)). In X-band, the microwave absorption performance enhancement of Fe3O4 is derived from electron transition resonance. 4. CONCLUSIONS


17


Page 18 of 36

The Fe3O4 nanocrystals with great crystallinity are prepared by solvothermal method, and the size of Fe3O4 nanocrystals is about 44-60 nm. In a given concentration of MEG, with the increase of the concentration ratio of DEG and MEG, the intensity of the electron transition resonance at 10 GHz increases, which induces an enhancement of the microwave absorption performance. When the DEG is 4.21 mol/l (MEG 3.58 mol/l), the real part of permeability can reach to 3.2; and the imaginary part is 1.3, which is 1.6 times higher than that of reported one (µ"=0.5).15 The enhancement of permeability also makes a great contribution to the improving the microwave absorption performance. At the thickness of 2.5 mm, the effective microwave absorption width of pure Fe3O4 can reach 3.9 GHz (RL≤-10 dB). ASSOCIATED CONTENT

Supporting Information Available Additional information related to TEM images of the nanoparticles and the statistical analysis of the nanoparticles sizes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


18

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We thank the National Natural Science Foundation of China (Grand No.11474019 and No.51371055) for financial support. REFERENCES (1)

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Table.1. The Saturation Magnetization (Ms) and Coercivity (Hc) of Nanomaterials Fe3O4 Prepared with Different Concentrations of Diethylene Glycol. saturation magnetization (Ms) (emu/g)

coercivity (Hc) (Oe)

DEG0

50.8

95

DEG1.05

72

120

DEG2.10

88

123

DEG3.15

96

126

DEG4.12

91.3

123

precursor

Table 2. The Reflection Loss Data of the Samples Synthesized under Ethylene Glycol Concentration of 3.56 mol/l

precursor

dmma (mm)

fmb (GHz )

RLmax c (dB)

fwd (GHz )

DEG0

4

6.27

-21.8

2.64

DEG1.05

2.4

11.12

-48.8

3.91

DEG2.10

2.1

11.71

-38.4

3.23

DEG3.15

2.3

11.03

-51

2.97

DEG4.21

2.1

11.71

-40.6

3.06

a

matching thickness. bmatching frequency. maximum reflection loss. deffective bandwidth(RL

Polycrystalline Synthesized b - ACS Publications - American Chemical

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