ZnO composites with defects in ZnO for regulating the impedance matching

ZnO composites with defects in ZnO for regulating the impedance matching

Accepted Manuscript Excellent microwave absorption of FeCo/ZnO composites with defects in ZnO for regulating the impedance matching Xiukun Bao, Xiaole...

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Accepted Manuscript Excellent microwave absorption of FeCo/ZnO composites with defects in ZnO for regulating the impedance matching Xiukun Bao, Xiaolei Wang, Xinao Zhou, Guimei Shi, Ge Xu, Jin Yu, Yinyan Guan, Yajing Zhang, Da Li, Chuijin Choi PII:

S0925-8388(18)32910-4

DOI:

10.1016/j.jallcom.2018.08.036

Reference:

JALCOM 47124

To appear in:

Journal of Alloys and Compounds

Received Date: 9 June 2018 Revised Date:

3 August 2018

Accepted Date: 4 August 2018

Please cite this article as: X. Bao, X. Wang, X. Zhou, G. Shi, G. Xu, J. Yu, Y. Guan, Y. Zhang, D. Li, C. Choi, Excellent microwave absorption of FeCo/ZnO composites with defects in ZnO for regulating the impedance matching, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.08.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Excellent microwave absorption of FeCo/ZnO composites with defects in ZnO for regulating the impedance matching Xiukun Baoa , Xiaolei Wang∗a, Xinao Zhoua, Guimei Shia, Ge Xua, Jin Yua, Yinyan Guana, Yajing Zhangb, Da Lic, and ChuIjin Choid Department of Chemistry and Environment, School of Science, Shenyang University of

Technology, Shenyang 110870, PR China. b

College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang

110142, PR China.

Shenyang National Laboratory for Materials Science, Institute of Metal Research, and

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c

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a

International Centre for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, PR

d

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China.

Korea Institute of Materials Science, 797 Changwondaero, Seongsangu, Changwon, Gyeongnam,

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51508, Korea



Corresponding author: E-mail: [email protected] Fax:(+86) 24-25496502 Tel: (+86) 24-25496502

ACCEPTED MANUSCRIPT ABSTRACT: FeCo/ZnO composites have been successfully prepared through liquid-phase reduction process for the formation of FeCo polyhedrons and sequentially thermal decomposition of colloidal mixture of FeCo and Zn(Ac)2·2H2O under nitrogen atmospheres. ZnO nanoparticles are homogeneously deposited on the

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surface of FeCo polyhedrons and the level of oxygen-vacancy defects in ZnO can be elevated with the increase of ZnO content. By comparison with FeCo polyhedrons, FeCo/ZnO composites exhibit excellent microwave absorption. The optimal RL value can reach -34.8 dB at 14.8 GHz and effective bandwidth (RL< -10 dB) is 5.1 GHz in

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the frequency range of 12.4-17.5 GHz with a matching thickness of 1.5 mm. The integrated bandwidth with RL < -10 dB can reach 14.1 GHz covering 3.4-17.5 GHz.

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Theory analysis demonstrates the interfacial polarization, dipole polarization and high conductivity due to oxygen-vacancy defects in FeCo/ZnO composites contribute to enhancement of dielectric loss capacity, which is more favorable for impedance matching.

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absorption.

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Keywords: FeCo/ZnO; Defect; Dielectric loss; Impedance matching; Microwave

ACCEPTED MANUSCRIPT 1. Introduction With the widespread development of electronic communication devices in civil and military fields, various kinds of microwave absorption materials (MAMS) have been intensively exploited for solving electromagnetic pollution issues [1-2]. Traditional

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microwave absorbents are classified into two types of magnetic loss and dielectric loss, such as ferrite and carbon fibers. More recently, multiple-phase composites combination with advantage of each constituent have aroused more attention and magnetic-dielectric composites have been considered to be excellent microwave

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absorbents originating from synergistic interaction between magnetic loss and dielectric loss, which can mainly contribute to proper impedance matching and

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dissipative ability of electromagnetic wave [3-10]. As far as magnetic components are concerned, soft magnetic metal/alloys are commonly selected, which are superior to ferrite due to their relatively high complex permeability in gigahertz range in terms of Snoek's limit, such as [email protected] [8], Fe/TiO2 [9], Fe/[email protected] [10], [email protected] [11], and Co/CoO [12]. As is known, the microwave absorption properties are generally

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associated with the complex permittivity, the complex permeability and impedance matching of the microwave absorbents, which can be effectively regulated by the component, morphology, size and defects [13-15].

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ZnO nanomaterials have been broadly investigated in the paste decades due to their charming physicochemical properties and potential applications in sensor [16], catalyst [17], photoluminescence [18], microwave absorption [19], etc. As one kind of

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microwave absorbents, much attention has been paid to construct ZnO architectures to achieve enhanced microwave absorption properties. For example, ZnO hollow microspheres with urchin-like morphology composed of nanoflakes were prepared by Chen and coworkers and showed an optimal reflection loss of -20 dB at 14.3 GHz with a thickness of 3 mm, which could be attributed to the oxygen vacancy inducing interfacial polarization of ZnO nanosheet [20]. ZnO networks composed of crossed micro/nanorods were synthesized by Zhang and coworkers and the minimum reflection loss reached -37 dB at 6.2 GHz with a thickness of 4 mm due to unique morphology [21]. Porous ZnO nanosheets were fabricated by Ma and coworkers and

ACCEPTED MANUSCRIPT exhibited an optimal reflection loss of -34.5 dB at 10.7 GHz with a matching thickness of 1.5 mm, which could be mainly derived from the lamellar porous microstructures [22]. On the other hand, it has been reported that the level of defects in semiconductors has a remarkable effect on their photoluminescence and

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photocatalysis performances [23-26]. For example, photocatalysis properties of ZnO nanorods prepared by Zhang and coworkers were remarkably improved with high level of surface defects [26]. Although experimental results confirm that defects in ZnO can yield extra dipole polarization [20, 27], the correlation of the complex

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permittivity and defects with different level in ZnO are seldom reported, which is vital to the impedance matching and microwave absorption. Thus, combination of soft

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magnetic FeCo with a highest saturation magnetization of ca. 240 emu/g and dielectric ZnO with tunable defects to fabricate FeCo/ZnO composites can not only simultaneously possess magnetic loss and dielectric loss, but also have changeable impedance matching indirectly regulated by dielectric loss based on the different level of defects in ZnO, and should have excellent microwave absorption properties.

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In the present work, FeCO/ZnO composites have been successfully synthesized by the liquid phase synthesis of FeCo polyhedrons and subsequently the pyrolysis of the colloidal mixture of FeCo and Zn(Ac)2·2H2O in the nitrogen atmospheres. With

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the increase of usage of Zn(Ac)2·2H2O, the level of oxygen vacancies in FeCo/ZnO composites can be increased. By comparation with FeCo polyhedrons, FeCo/ZnO composites exhibit excellent microwave absorption performance. The optimal RL

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value can reach -34.8 dB at 14.8 GHz and effective bandwidth (RL< -10 dB) is 5.1 GHz in the frequency range of 12.4-17.5 GHz with a matching thickness of 1.5 mm. The integrated bandwidth with RL < -10 dB can reach 14.1 GHz covering 3.4-17.5 GHz. The enhanced microwave absorption properties can be mainly originated from oxygen-vacancy defects in ZnO modulating impedance matching. 2. Experimental section All chemical reagents in analysis grade were purchased from Sinopharm Chemical Reagent Co. Ltd., Shenyang, China and were used without further purification. 2.1 Preparation of FeCo polyhedrons

ACCEPTED MANUSCRIPT Typical process for preparation of FeCo polyhedrons is described as following, according to previous report [28]: 10 mmol of FeSO4·7H2O and 10 mmol of CoCl2·6H2O were dissolved in 180 mL distilled water under vigorous mechanical stirring for 0.5 h at room temperature under nitrogen atmosphere. Then, 40 mL of

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N2H4·H2O (80 wt.%) and 0.25 mol of NaOH was in turn added the above solution and maintained at 65 oC for 1 h. After that, the resultant samples were collected by external magnets and washed by distilled water and ethanol for several times. Finally, the as-prepared samples were dried at 40 oC under vacuum overnight.

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2.2 Preparation of FeCo/ZnO composites

0.1 g of the as-prepared FeCo polyhedrons were added into 30 mL of absolute ethanol

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containing a certain amount of Zn(Ac)2·2H2O solution with strong mechanical stirring for 0.5 h at room temperature. Afterward, the suspension was slowly heated up to 45 o

C for the volatilization of solvent and formation of colloidal mixture. Then, the

colloidal mixture was further thermal treated at 500 oC for 2 h under nitrogen atmospheres. The usage of Zn(Ac)2·2H2O in the present work is 100 mg, 200 mg, 300

and FZ3, respectively. 2.3 Characterization

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mg and 500 mg, corresponding to the FeCo/ZnO samples labeled as FZ, FZ1, FZ2

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The phase identification of the samples were performed by X-ray diffraction (XRD) methods on a D/Max 2200 diffractometer with Cu Kα radiation (λ = 0.15406 nm). The thermogravimetric analysis (TGA) was performed on a Mettler Toledo thermal

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analyzer in the temperature range of room temperature to 750 oC at a heating rate of 10 oC/min. The morphology of the samples was characterized by a MIRA LMN field-emission scanning electron microscopy (FESEM) operated under an acceleration voltage of 20.0 kV. Raman spectra were determined on a Confocal Raman Microscope using 532 nm laser. The Magnetic properties were obtained at room temperature by a vibrating sample magnetometer (VSM, Lakeshore 7400) under magnetic field up to 10 kOe. For the measurement of microwave absorption, the absorbents were comprised of the samples and paraffin with weight ratio of 3:2 and were then pressed into cylinder with an inner diameter of 3.04 mm, an outer diameter

ACCEPTED MANUSCRIPT of 7.00 mm, and a thickness of 2 mm. The electromagnetic parameters were determined in the frequency range of 1-18 GHz by a vector network analyzer (Agilent E5071C) and reflection loss (RL) was calculated according to the transmission line theory.

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3. Results and discussion 3.1 Physical characterization

The crystalline structure of the samples is shown in Fig. 1. All of the diffraction peaks in FeCo samples can be well assigned to face-centered cubic structures (JCPDS card

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No. 49-1567), which indicates that single phase of FeCo can be obtained without any additional impurity. Through the pyrolysis of Zn(Ac)2·2H2O, diffraction peaks of ZnO

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are detected besides FeCo phase, which can be ascribed to the hexagonal wurtzite phase (JCPDS card No. 36-1451). Moreover, the mass ratio of ZnO to FeCo in FeCo/ZnO composites are increased and can be evaluated to 1:2.8 and 1:1.4 for FZ1 and FZ2 on basis of comparison of integral area of main peak of ZnO and FeCo. This result demonstrates that the amount of ZnO in composites can be effective regulated

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by the usage of Zn(Ac)2·2H2O in the present method. The chemical reaction mechanism of ZnO deriving from the pyrolysis of Zn(Ac)2·2H2O under nitrogen atmosphere can be described as the following equation: (1)

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Nitrogen Zn(CH 3COO) 2   → ZnO + CH 3COCH 3 + CO2

The percentage of ZnO in the FeCo/ZnO composites was further examined by

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TGA method. As shown in Fig. 2(a), all of the samples show the weight increase from starting temperature of ca. 446 oC to termination temperature of ca. 650 oC under air atmospheres. Moreover, the weight increase is estimated to be 29 wt.%, 27.5 wt. %, 25 wt.% and 22 wt.% for FZ, FZ1, FZ2, and FZ3 samples, respectively, which is gradually declined with the increase of usage of Zn(Ac)2·2H2O. The typical XRD pattern of FeCo/ZnO composites thermal treated at 800 oC for 2 h under air atmospheres is shown in Fig. 2(b). Besides the main peaks of ZnO, the other phases of CoFe2O4 (JCPDS card No.22-1086) and Co3O4 (JCPDS card No.42-1467) can be found. Thus, the chemical reaction in this thermal treatment process can be depicted

ACCEPTED MANUSCRIPT as following: Air 6 FeCo + 8O2  → 3CoFe2O4 + Co3O4

(2)

And the percentage of ZnO in the FeCo/ZnO composites can be calculated by the following equations:

6M FeCo wt.% 8M O2

(3)

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y = 1-

Where, y is the percentage of ZnO in FeCo/ZnO composites, M is the relative molecular mass and wt.% is the weight increase. The content of ZnO is calculated to

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be 21 wt.%, 27 wt.%, 33 wt.%, and 41 wt.%, corresponding to the FZ, FZ1, FZ2, and FZ3 samples, respectively, which further confirm the results of XRD.

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In order to inspect the morphology of the samples, the FESEM images and corresponding element mapping are shown in Fig. 3. In Fig. 3(a)-(b), it is clearly observed that FeCo samples are composed of homogeneously dispersed polyhedrons with average particle size of ca. 1.73 µm, along with small particles of ca. 90 nm depositing on the surface. For FZ1 composites, abundant of ZnO particles of ca.62 nm

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are deposited on the surface of FeCo polyhedrons and connect with each others shown in Fig. 3(c)-(d). In Fig. 3(e)-(f), for FZ3 composites with further increase of ZnO content, ZnO particles of ca. 72 nm form continuous networks and more FeCo

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polyhedrons are immersed in ZnO matrix. A random FeCo/ZnO composites particles are chosen for element analysis, as shown in Fig. 3(g)-(k). The distribution of

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elements (Fe, Co, Zn, O) are uniform. Moreover, Fe and Co element are mainly located at center region, and other Zn and O elements are overlapped throughout the whole area, which verifies that the ZnO particles are evenly deposited on the surface of FeCo polyhedrons.

Static magnetic properties of the samples are measured by VSM at room temperature, as shown in Fig. 4. The M-H curves of the samples show a typical soft-ferromagnetic characteristic. The saturation magnetization (Ms) of FeCo is ca. 100 emu/g and is lower than that of bulk FeCo, which is attributed to the crystal defect and surface spin-disorder [29]. Moreover, the Ms of FeCo/ZnO composites is

ACCEPTED MANUSCRIPT gradually declined due to the increase of non-magnetic ZnO component. In addition, Coercivity (Hc) of FeCo/ZnO composites is smaller than that of FeCo polyhedron, which may be related to the decrease of shape anisotropy and enhancement of crystallinity of FeCo components to some extent induced by thermal treatment

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process [30]. The defects of ZnO in FeCo/ZnO composites can be generally detected by Raman spectra, as shown in Fig. 5. Both of the composites shows the characteristic peaks of 474 and 658 cm-1, which can be associated with interface or surface photon

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mode and multiple-photon scattering, respectively [31, 32]. The peak at 588 cm-1 of FZ1 samples is a contribution of the E1 (LO) mode of ZnO associated with oxygen

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deficiency [33]. The intensity of Raman peak at 588 cm-1 is enhanced and shifts to 566 cm-1 for FZ3 samples, indicating a high concentration of the oxygen vacancies in FZ2 samples [34]. Thus, it is reasonable that oxygen vacancies in ZnO can be created deriving from the decomposition of Zn(Ac)2·2H2O under anaerobic environment and the concentration of oxygen vacancies can be improved with the increase of ZnO

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content. 3.2 Microwave analysis

To investigate the microwave absorption of the as-prepared sample, the dependence of

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electromagnetic parameters (the complex permittivity of εr=ε′-jε′′ and the complex permeability of µr=µ′-jµ′′) of the samples/paraffin composites on the frequency are measured in the frequency range of 1-18 GHz, as shown in Fig. 6. The real part (ε′

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and µ′) and imaginary part (ε′′ and µ′′) symbolize the storage and loss capability for electromagnetic wave, respectively. In Fig. 6(a) and 6(b), ε′ and ε′′ of the as-prepared samples exhibit evident relaxation behaviors throughout the whole measured frequency range. Three obvious resonance peaks located at 6, 10 and 15 GHz can be observed in ε′′ versus f curves and the resonance peaks do not shift with the introduction of ZnO. Moreover, the values of ε′ and ε′′ of FeCo/ZnO composites are higher than those of FeCo polyhedrons and are gradually improved with the increase of ZnO content. FZ3 exhibits the maximum of the complex permittivity and the value of ε′ and ε′′ can reach 15-14.5 and 3-3.8 with visible fluctuation in 1-18 GHz range,

ACCEPTED MANUSCRIPT respectively. In the present system, oxygen vacancies in ZnO can act as polarization center and induce dipole polarization, which can enhance with the increase of ZnO content based on Raman analysis. Moreover, heterogenous interface between FeCo and ZnO due to different electronegativity can yield extra interfacial polarization.

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Thus, the improvement of ε′ with the increase of ZnO content can be attributed to the defect dipole polarization and interfacial polarization [22, 27, 30]. On the other hand, according to the free electron theory [35], ε′′ ≈ 1/2πε0ρf, where ρ is the resistivity. Higher ε′′ means higher conductivity. Recent researches confirm that the

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concentration of oxygen vacancies in ZnO can significantly affect its electrical conductivity and visible light absorption and its electrical conductivity can be

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heightened with the concentration increase of oxygen vacancies due to the narrowing of bandgap [24, 36]. It is logical that the level of oxygen vacancies elevated by the increase of ZnO content can induce the decrease of electron transition depth and yield extra free electron, which can contribute to high electrical conductivity. Furthermore, continuously conductive ZnO networks are prone to form based on the observation of

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Fig. 3(c)-(f), which can afford the routes for electron transport. Thus, ε′′ can be enhanced with the increase of ZnO content and the nonlinear resonance behaviors are mainly ascribed to high conductivity and defect-inducing dipole polarization. In Fig.

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6(c) and 6(d), both of µ′ and µ′′ are gradually declined with the increase of ZnO content due to its non-magnetic nature. Moreover, multiple magnetic-resonance peaks can be observed in µ′′ versus f curves. In general, the magnetic loss mechanism is

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commonly originated from the magnetic hysteresis, domain wall displacement, eddy current loss, natural resonance, and exchange resonance mode [10-12, 28]. Magnetic hysteresis occurs under high magnetic field and domain wall displacement appears in the low frequency of 1-100 MHz. If the magnetic loss are derived from the eddy current loss, the values of µ ′′( µ ′)−2 f −1 can be equal to a constant value of 2πµ0 d 2σ / 3 (d is the thickness of absorber, σ is the electrical conductivity, µ0 is the permeability of vacuum) and be independence of frequency [14, 28]. As shown in Fig. 7, the values of µ ′′( µ ′)−2 f −1 for all the samples are gradually decreased with the

ACCEPTED MANUSCRIPT increase of frequency, demonstrating that eddy current loss can be excluded. Thus, natural resonance and exchange resonance should be the main magnetic loss mechanism in our samples deriving from cooperative effect of small size, shape and/or surface anisotropy, and spin wave excitation, which maybe induced from the

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anisotropic characteristic of FeCo polyhedrons with small particles assembling on the surface [28, 30]. In order to further illuminate the changeable tendency of loss tangent with the increase of ZnO content, magnetic loss tangent (tanδµ=µ"/µ') and dielectric loss tangent (tanδε=ε"/ε') are calculated. As is shown in Fig. 8, for FeCo polyhedrons,

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tanδµ is much higher than tanδε in the whole measured frequency range, which is ascribed to remarkably multiple magnetic-resonance behaviors, and magnetic loss

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dominates the microwave absorption properties. For FeCo/ZnO composites, it is obviously seen that tanδε is gradually enhanced and tanδµ is gradually reduced to some extent with the increase of ZnO content. These results indicate that loss tangent can be effectively regulated by ZnO content.

According to the transmit-line theory, the reflection loss (RL) of the samples (60

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wt.%)/paraffin composites in the frequency range of 1-18 GHz with different thickness are calculated by the following equations [3-10]:

RL = 20 log ( Z in − Z 0 ) / ( Z in + Z 0 )

(5)

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Z in = Z 0 ( µ r / ε r )1/ 2 tanh[ j (2π fd / c)( µr ε r )1/2 ]

(4)

Where, Z0 is the impendence of free space, Zin is the input impendence, f is the

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microwave frequency, d is the thickness of the composites, c is the velocity of light. As shown in Fig. 9, FeCo polyhedrons exhibit an optimal RL value of -23.7 dB at 14.2 GHz with a thickness of 1.5 mm. With the increase of ZnO content, the minimum of RL values of FeCo/ZnO composites with a thickness of 1.5 mm can be enhanced to -25.5 dB, -33.7 dB, -34.8 dB and -29.3 dB corresponding to FZ, FZ1, FZ2and FZ3, respectively. By comparing with FeCo, FZ and FZ1, all of the RL values of FZ2 and FZ3 with different matching thickness are evidently improved and lower than -10 dB in the whole frequency range. The optimal RL value of FZ2 can reach -34.8 dB at 14.8 GHz and effective bandwidth (RL< -10 dB) is 5.1 GHz in the

ACCEPTED MANUSCRIPT frequency range of 12.4-17.5 GHz with a matching thickness of 1.5 mm. The integrated bandwidth of FZ2 with RL < -10 dB can reach 14.1 GHz covering 3.4-17.5 GHz. These results confirm that the microwave absorption of FeCo/ZnO composites with oxygen-vacancy in ZnO is superior to that of the FeCo polyhedron. In addition,

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RL dip shift to low frequency with the increase of thickness of microwave absorbents and double RL peaks with a strong RL peak together with a small RL peak appear when the thickness is large than 4 mm. These behaviors can be attributed to a quarter-wavelength model and have been reported in [email protected] nanoflake [11], and

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similar to that of multiple-layer microwave absorbers [37, 38]. By comparing with microwave absorption properties of other magnetic-dielectric absorbents, FeCo/ZnO

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composites not only show intensively microwave absorption with a thin matching thickness, but also possess a broad bandwidth, as shown in Table 1. Thus, FeCo/ZnO composites reported here have a potential application in microwave absorption field. On basis of electromagnetic theory, excellent microwave absorption properties should synchronously satisfy impedance matching and strong attenuation ability.

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Impedance matching can be evaluated by the absolute value of difference of magnetic loss tangent and dielectric loss tangent (△=∣tanδε-tanδµ∣) [39, 40]. The relative smaller △ value means that more incident wave can not reflect from the surface and

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transport into the absorbents. The attenuation ability can describe the microwave absorption of the absorbent in terms of various loss mechanisms, which can be written

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according to transmission line theory as following [41, 42]:

α=

2πf × ( µ ′′ε ′′ − µ ′ε ′) + ( µ ′′ε ′′ − µ ′ε ′) 2 + ( µ ′ε ′′ + µ ′′ε ′) 2 c

(3)

Where, f is the frequency of electromagnetic wave and c is the velocity of light. As shown in Fig. 10(a), the △ values of FeCo/ZnO composites are smaller than that of FeCo polyhedrons, demonstrating preferable impedance matching. Simultaneously, the attenuation constants of FeCo/ZnO composites are similar to that of FeCo polyhedrons except a little decrease in high frequency, shown in Fig. 10(b). The oxygen-vacancy defects of ZnO in FeCo/ZnO composites can yield dipole polarization and high conductivity, which can contribute to the enhancement of

ACCEPTED MANUSCRIPT dielectric loss tangent. Moreover, the level of oxygen-vacancy defects of ZnO in FeCo/ZnO composites can be heightened with the increase of ZnO content. That is to say, dielectric loss tangent can be effectively intensified due to oxygen-vacancy defects of ZnO indirectly regulated by content of ZnO in FeCo/ZnO composites. On

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the other hand, the magnetic loss tangent is inevitable gradually decreased with the increase of ZnO content. Thus, the cooperative effect of magnetic and dielectric loss tangent can contribute preferable impendence matching and FeCo/ZnO composites can possess improved microwave absorption performance.

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4. Conclusions

FeCo/ZnO composites with ZnO particles homogeneously depositing on the surface

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of FeCO polyhedrons have been prepared by a facile method through liquid reduction synthesis of FeCo polyhedrons and following the pyrolysis of Zn(Ac)2·2H2O in the nitrogen atmospheres. The dielectric loss tangent can be effectively improved with the increase of ZnO contents and contribute to favorable impedance matching, which is mainly attributed to the oxygen-vacancy defects in ZnO. The optimal RL value can

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reach -34.8 dB at 14.8 GHz and effective bandwidth (RL< -10 dB) is 5.1 GHz in the frequency range of 12.4-17.5 GHz with a matching thickness of 1.5 mm. The integrated bandwidth with RL < -10 dB can reach 14.1 GHz covering 3.4-17.5 GHz.

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FeCo/ZnO composites have potential application in microwave absorption field. Acknowledgements

This work has been supported by National Natural Science Foundation of China

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(51601120, 51301114, 51171185), Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3D1A1027800), Natural Science Foundation of Liaoning Province (20170540679) and Science Research Foundation of Education Department of Liaoning Province (LQ2017011). References [1] Y. Zhang, Y. Huang, T. Zhang, H. Chang, P. Xiao, H. Chen, Z. Huang, Y. Chen, Broadband and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam, Adv. Mater. 27 (2015) 2049-2053.

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Interfaces 7 (2015) 9776-9783.

ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of the as-prepared samples: (a) FeCo, (b) FZ1 and (c) FZ3. Fig. 2 (a) TGA curves of FeCo/ZnO composites and (b) XRD pattern of the FZ2 samples thermal treated at 800 oC for 2 h under air atmosphere.

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Fig. 3 FESEM images of the as-prepared samples: (a) and (b) FeCo, (c) and (d) FZ1, (e) and (f) FZ3; The element mapping of FZ3: (g)-(k).

Fig. 4 Room-temperature M-H loops of FeCo, FZ1 and FZ3 samples. Fig. 5 Raman spectra of FeCo/ZnO composites: (a) FZ1and (b) FZ3.

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Fig. 6 Frequency dependence of the complex permittivity and the complex permeability of the as-prepared samples/paraffin composites: (a) real part and

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(b) imaginary part of the complex permittivity; (c) real part and (d) imaginary part of the complex permeability.

Fig. 7 Changeable tendency of µ ′′( µ ′)−2 f −1 of the as-prepared samples/paraffin composites versus frequency.

Fig. 8 Frequency dependence of loss tangent of the as-prepared samples/paraffin

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composites: (a) dielectric loss tangent and (b) magnetic loss tangent Fig. 9 Microwave absorption properties of the as-prepared samples/paraffin composites: (a) FeCo, (b) FZ, (c) FZ1, (d) FZ2 and (e) FZ3; (f)

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comparison of optimal RL values. Fig. 10 As-prepared samples/paraffin composites: (a) impedance matching and (b)

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Comparison of microwave absorption properties of FeCo/ZnO composites with other microwave absorbents

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-40.2/2.5 -29.3/3.9 -48.2/2.1 -40.0/2 -40/1.5 -34.8/1.5

3.4-18 (14.6) 3.6-18 (14.4) 2.5-12 (9.5) 13.5 3.1-14.4 (11.3) 3.4-17.5 (14.1)

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1.5-5.0 1.5-4.0 1.5-4.0 1.5-5.0 1.5-4.5 1.5-5.0

[9] [10] [11] [13] [15] here

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FeCo/graphene Fe/[email protected] FeCo/C [email protected]/RGO Ni/SiO2 FZ2

Integrated thickness (mm)

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Optimal RL (dB) and Thickness (mm)

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ACCEPTED MANUSCRIPT Highlights ·The preferable impedance matching of FeCo/ZnO composites can be achieved by regulating the ZnO content. ·The optimal reflection loss (RL) can reach -34.8 dB with a matching thickness of 1.5 mm.

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·The effective bandwidth with RL < -10 dB achieve 14.1 GHz covering 3.4-17.5 GHz with

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