Core-Shell Co3Fe7@C Composite as Efficient Microwave Absorbent

LI Guo-Min ZHU Bao-Shun LIANG Li-Ping TIAN Yu-Ming,* Lü Bao-Liang WANG Lian-Cheng,*

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Core-Shell Co3Fe7@C Composite as Efficient Microwave Absorbent

LI Guo-Min1ZHU Bao-Shun1LIANG Li-Ping1TIAN Yu-Ming1,*Lü Bao-Liang2WANG Lian-Cheng2,*

(1;2)

To reduce the density of the absorbent Co3Fe7, a core-shell Co3Fe7@C microwave absorbent was synthesized by preparing an iron/cobalt-containing carbon precursor followed by high-temperature carbonization. According to the X-ray diffraction (XRD) and transmission electron microscopy (TEM) results, Co3Fe7particles were coated with graphitized carbon layers to form a core–shell structure. Furthermore, the Co3Fe7@C composite with a surface area and density of 358.5 m2·g?1and 2.25 g·cm?3, respectively, exhibited excellent microwave absorbability. A minimum reflection loss (RL) of ?43.5 dB and an effective bandwidth (RL below ?10 dB) of 4.1 GHz were obtained at the coating thickness of 2 mm, which could be mainly attributed to the effective impedance match and multiple interfacial polarizations. Owing to the low density and remarkable microwave absorption, we believe that the Co3Fe7@C composite can be a promising candidate for use as a lightweight and efficient microwave absorbent.

Core-shell structure; Composite; Lightweight; Microwave absorption

1 Introduction

In recent years, much effort has been focused on microwave absorption materials (MAMs) not only for the sake of solving serious electromagnetic pollution arising from the daily communication and entertainmentbut also for the stealth of military equipment1,2. In general, MAMs can be categorized into dielectric loss and magnetic loss absorbents. Properties including thin thinness, low density, strong absorption and wide bandwidth are generally expected in MAMs3,4. While the traditional MAMs, such as magnetic absorbents composed of Fe, Co, Ni element and dielectric absorbents represented by TiO2, MnO2, BaTiO3are facing challenges because of their single loss mechanism. Meanwhile, the high specific gravity and low specific surface area restrict their practical applications. To solve the problem,great efforts have been focused on the magnetic-dielectric coupled MAMs5?7.

As is well known, carbon materials including carbon black, graphene, carbon fibers, carbon nanotubes and carbon nanocoil have shown potential as novel carrier materials due to their low cost, large surface area and low density8,9.In addition, the carbon materials are typical dielectric absorbents, which attractwidespread attention. As a result, it is wise to incorporate carbon materials with magnetic constituents by different methods. And the synergetic effect between magnetic materials and carbon materials for the enhancement in microwave absorption has also been proved10,11. Liu.12have prepared the (Fe, Ni)/C nanocapsules, and the core-shell structure was proved to enhance the microwave absorption. The Fe3O4/graphene and Ni/graphene composites were synthesized by employing atomic layer deposition strategy and the as-prepared composites showed significantly improved microwave absorbility compared to the pristine graphene13. More recently, Li.14fabricated the CoFe2O4/GO and FeCo/G hybridsa facile one-pot polyol route combined withreduction, results showed that FeCo/G exhibited improved microwave absorption than CoFe2O4/GO for the effective impedance matching between graphene and magnetic particles.

In view of the above-mentioned facts, we report the synthesis of core-shell Co3Fe7@C composite as efficient and lightweight absorbent. The Co3Fe7@C composite possesses high surface area of 358.5 m2·g?1, with alloy particles entirely coated in carbon matrix. The microwave absorption properties of as-prepared samples were studied based on the complex permittivity and permeability. The results show that the composite exhibits excellent microwave absorption, benefiting from the effective impedance match and multiple interfacial polarizations.

2 Experimental

2.1 Material preparation

All the reagents were A.R. grade and were used in preparation without further purification. In a typical synthesis, Pluronic F127 (EO106PO70EO106,w= 12600, Sigma), 1,3,5-trimethylbenzene (TMB), hexamethylenetetramine (HMT), and resorcin were in turn dissolved in 18 mL distilled water under vigorous stirring at room temperature. After the formation of a homogenous solution with the molar ratio of F127, TMB, HMT and resorcin = 1 : 21 : 31.5 : 63, 0.2 mol·L?1Fe(NO3)3·9H2O and 0.1 mol·L?1Co(NO3)2·6H2O (other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.) were slowly added with the solution turning into bluish violet. It was then transferred into a Teflon-lined stainless autoclave, heated at 100 °C for 12 h. Afterwards, the resulting chocolate brown product was collected by filtration, washed with water several times and dried at 70 °C. Finally, the as-made sample was thermally treated at 900 °C for 3 h, at a heating rate of 1 °C·min?1in Ar atmosphere and then was ground in an agate mortar.

2.2 Material characterization

To reveal the crystallization structures of the products, X-ray diffraction (XRD) measurement was carried out with a D8 Advance Bruker AXS diffractometer with Cu-Kradiation (= 0.15406 nm), in scan steps of 0.02° in a 2range of 10°?90°. The microscopic morphology of the samples was observed on transmission electron microscope (TEM, JEOL JEM-1011). The N2adsorption-desorption analysis was measured on a Micromeritics ASAP 2010 instrument. Raman spectra were recorded on a Horiba LabRAM HR800 spectrometer with an Ar+laser. The thermal stability of the composites was tested in a thermogravimetric differential analyser (METTLER TOLEDO 3+) under air atmosphere at a heating rate of 10 °C·min?1. The magnetic properties were measured on a vibrating sample magnetometer (VSM Lakeshore Model 7400) at room temperature.

2.3 Electromagnetic parameter measurement

The specimen for microwave absorption measurement were prepared by uniformly mixing the as-prepared composites in a paraffin matrix which is used as the binder and pressing the mixture into a cylindrical shaped compact (outer= 7.00 mm andinner= 3.04 mm). The relative complex permittivity and permeability values of the specimen with 15%() of the composite were measured in 2?18 GHz with a vector network analyzer (Agilent N5230). Based on the measured electromagnetic parameters, the RL coefficient of the electromagnetic wave (normal incidence) at the surface of a single-layer absorbent backed by a perfect conductor at certain frequency and layer thickness can be calculated according to transmission line theory.

3 Results and discussion

3.1 Phase, structure and morphology characterization

As previously illustrated15, the content of metal ions plays an important role in the composition and microstructure of the composites during the synthesis procedures. Therefore, in this work the relative high concentration of multicomponent metal ion (Fe3+, Co2+) was adjusted to prevent the F127 forming integrated micelles in the solution. Accordingly, the F127 serve mainly as the carbon source as well as reducing agent in the carbonization process, during which the carbon atoms adsorbed onto and then moved along the Co3Fe7particles, leading to the formation of carbon-coated alloy particles on the basis of dissolution-precipitation mechanism16.

Fig.1 shows the X-ray diffraction (XRD) patterns of the Co3Fe7@C composite. As can be seen, there are three main diffraction peaks at 44.67°, 65.02°, and 82.33°, which can be indexed as the crystalline planes of (110), (200) and (211), indicating the presence of Co3Fe7(JCPDS No. 48-1816). It should be mentioned that, based on the stoichiometric ratio, there should be Co in the XRD patterns of the composite, the absence of Co may be due to the amount of Co is too low to be detected. The same phenomenon was also reported in Co3Fe7/C microspheres17. Besides, a slight diffraction peak at 2≈ 26.5° is also observed, which corresponds to (002) plane of graphite.18This can be further confirmed by Raman spectroscopy. As shown in Fig.2, the spectra were collected within the range between 500 and 2000 cm?1, in which two characteristic bands appeared. That is the so-calledpeak at around 1350 cm-1and G peak at about 1590 cm?1, which represent the disordered carbon structure and graphite carbon, respectively. It is obvious that the high temperature and the existence of Co3Fe7particles promote the graphitization degree of Co3Fe7@C composite during the carbonization process. Additionally, the TG curve of the Co3Fe7@C composite is shown in Fig.S1 (in Supporting Information), from which the weight content of Co3Fe7is about 35% ().

To explore the detailed microscopic structure and morphology of Co3Fe7@C, TEM is used to provide more detailed information, and the representative images are shown in Fig.3. As shown in Fig.3(a), the sample is mainly composed of well-dispersed ellipsoidal or spherical particles with size ranging from 20 to 200 nm. And the alloy particles are totally encapsulated in carbon matrix. In addition, the TEM image of typically isolated particles is shown in Fig.3(b), it is found that there exists a carbon shell of ~10 nm coated on the Co3Fe7particle, forming the core-shell structure. Meanwhile, the Co3Fe7@C composite exhibits high surface area of 358.5 m2·g?1, large pore volume of 0.5 m3·g?1and low apparent density of 2.25 g·cm?3, which implies that core-shell Co3Fe7@C composite is lightweight material.

3.2 Magnetic properties and microwave absorption analysis

The magnetic hysteresis loop of the Co3Fe7@C composite at room temperature is shown in Fig.S2 (see Supporting Information). As shown in Fig.S2, the values ofsandr are 32.0 and 3.3 emu·g?1, respectively, demonstrating that the Co3Fe7@C is ferromagnetic. To reveal the microwave absorption of Co3Fe7@C, the theoretical RL was calculated through its measured complex permittivity (r=′ ? j″) and complex permeability (r=′ ? j″) at various coating thicknesses and frequencies, based on the following equations3,19:

in= (r/r)1/2tanh[j(2π/)(rr)1/2] (1)

RL(dB) = 20log|(in?0)/(in+0)|(2)

whereinis the input impedance of the absorbent,is the frequency of microwave,is the velocity of light, andis the thickness of the absorbent. It is known that the coating thickness is one of the crucial parameters which affect RL intensity and the frequency position of minimum absorption dip20.Consequently, we calculated the RL at representative thicknesses of 1, 1.5, 2, 2.5, and 3 mm. Fig.4(a) shows the RL curves for the Co3Fe7@C composite. As can be seen, it is interesting to find that microwave frequency of the absorption dips shifts negatively with the increase of coating thickness. It can deduce that the coating thickness and the corresponding frequency of an absorption dips obey theequation=/2π″21, where″ is the imaginary part of complex permeability. That is, the microwave absorbing ability can be adjusted directly by changing the coating thickness of the absorbents for application in different frequency bands. Fig.4(b) shows the RL curves of pristine carbon material prepared by the same experimental process without adding iron nitrate and cobalt nitrate. The microwave absorption of pristine carbon mainly stems from dielectric loss, with a minimum RL value of ?9.1 dB. The RL properties of Co3Fe7@C composite are enhanced substantially in comparison with pristine carbon.

Fig.1 X-ray diffraction patterns of core-shell Co3Fe7@C composite.

Fig.2 Normal Raman spectra of core-shell Co3Fe7@C composite.

Fig.3 TEM images of core-shell Co3Fe7@C composite.

Moreover, the minimum RL of Co3Fe7@C composite is ?43.5 dB at 12.9 GHz with thickness of 2 mm. Generally, ?10 dB means 90% microwave absorption, which is the minimum requirement for absorbent in practical application. Besides, the effective bandwidth corresponding to the RL < ?10 dB can reach 4.1 GHz when the matching thickness is 2 mm for Co3Fe7@C composite. Compared with other composite absorbents11,22?27, the Co3Fe7@C exhibits lower filling content and thinner matching thickness with relatively wider effective bandwidth, as shown in Table 1. This further demonstrates thatthe core-shell Co3Fe7@C composite is lightweight and high-efficiency microwave absorbent.

Fig.4 Microwave RL curves of (a) core-shell Co3Fe7@C composite and (b) pristine carbon.

Table 1 Microwave absorption of some reported absorbents.

3.3 Electromagnetic characteristics

To disclose the intrinsic mechanism of microwave absorption of the Co3Fe7@C composite, the complex permittivity and complex permeability must be taken into consideration firstly. The complex permittivity spectra of the Co3Fe7@C composite is shown in Fig.5(a), from which it can be seen that the values ofand″ trend to decrease with increasing frequency and keep the coincident tendency in the whole frequency range (2?18 GHz). This shows frequency dependence and is the typical frequency dispersion behaviour28?30, leading to the enhancement of microwave absorption. Fig.5(b) shows′ and′′ of the Co3Fe7@C composite. As can be seen, the′ value keeps steady in the frequency of 2?10 GHz, followed by a fluctuation during high frequency region, varing in the range of 1.03?0.84. While the corresponding″ value increases slowly at low frequency region and then decreases, and there is a tiny resonance peak around 14 GHz which may be due to the exchange resonance according to Aharoni’s theory31,32, proven in ferromagnetic nanoparticles. Furthermore, the exist of eddy current loss can be decided by″ ≈ 2π0(′)22/3, where0is the permeability in vacuum,is electric conductivity andis the diameter of the nanoparticle. It is generally agreed that if the magnetic loss only originates from the eddy current loss, equation″(′)?2?1= 2π02/3 should be constant33. The plot of″(′)?2?1was shown in Fig.6. It is obvious that the value of″(′)?2?1exhibits evident fluctuations, implying that the eddy current loss is not the main contribution to the magnetic loss for Co3Fe7@C.

For the other hand, the Co3Fe7particles are coated by graphitized carbon layer and encapsulated in carbon matrix for Co3Fe7@C composite (see Fig.3). As a result, there exists effective interface between the magnetic particles and the carbon matrix, with the charge transfer between carbon and Co3Fe7particles. Plot of″ for Co3Fe7@C composite is shown in Fig.S3 (in Supporting Information), from which two Cole-Cole semicircles as well as a linear curve are found. This may suggest that there exist dual relaxation processes and interface polarization effect in Co3Fe7@C composite32.

Fig.6 Values of μ″(μ′)?2f?1 of core-shell Co3Fe7@C composite versus frequency.

Fig.7 Frequency dependence of the loss tangent of core-shell Co3Fe7@C composite.

Lastly, there are other contributions for microwave absorption. That is dielectric loss (tane=″/′), magnetic loss (tanm=″/′) and matched characteristic impedance. Fig.7 shows the frequency dependence of the loss tangent for Co3Fe7@C composite. It can be clearly seen that the curves of dielectric and magnetic loss tangent coexist symmetrically and there are intersection points at specific frequencies. Therefore, in our case, the microwave absorption is ascribed to dielectric loss, magnetic loss and their better match. Also,it can be expected that the large surface area of Co3Fe7@C may offer additional pathway for the transmission of electromagnetic waves, benefiting to multiple reflections and enhancement of microwave absorption34,35.

4 Conclusions

The core-shell Co3Fe7@C absorbent was successfully obtained by polymerization of Fe3+-Co2+-containing carbon precursor and following high-temperature treatment. The composite possessed high specific surface area and exhibited outstanding microwave absorption, which resulted from multiple interfacial polarizations as well as the better match between magnetic and dielectric loss. This study showed that the Co3Fe7@C composite was promising lightweight and efficient microwave absorbent.

Supporting Information:available free of chargethe internet at http://www.whxb.pku.edu.cn.

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基于核殼結構Co3Fe7@C的高效微波吸收材料

力國民1朱保順1梁麗萍1田玉明1,*呂寶亮2王連成2,*

(1太原科技大學材料科學與工程學院，太原 030024；2中國科學院山西煤炭化學研究所，炭材料重點實驗室，太原 030001)

為了降低吸波劑Co3Fe7的密度，本文采用原位聚合Fe3+-Co2+/碳前驅體及高溫碳化制備得到Co3Fe7@C復合微波吸收材料。X射線衍射(XRD)和掃描電子顯微鏡(SEM)測試結果表明Co3Fe7顆粒被石墨碳層包覆形成核殼結構，復合物的比表面積和表觀密度分別為358.5 m2·g?1、2.25 g·cm?3。核殼結構Co3Fe7@C復合物顯示出優(yōu)異的微波吸收性能，當涂層厚度為2 mm時，其最低反射損耗(RL)達到最低值?43.5 dB，對應的有效帶寬為4.1 GHz，歸因于復合物有效的阻抗匹配特性及多重界面極化效應。由于低密度及優(yōu)異的微波吸收性能，Co3Fe7@C復合物有望作為一種潛在的輕質、高效微波吸收材料。

核殼結構；復合物；輕質；微波吸收

O64；TQ050.4+3

10.3866/PKU.WHXB201704174

December 6, 2016;

April 3, 2017;

April 17, 2017.

Corresponding authors.TIAN Yu-Ming, Email: tym1654@126.com; Tel: +86-0351-2161130. WANG Lian-Cheng, Email: wanglc@sxicc.ac.cn; Tel: +86-351- 4063121.

The project was supported by the Doctoral Scientific Research Foundation of Taiyuan University of Science and Technology, China (20152030).

太原科技大學博士科研啟動基金(20152030)資助項目