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C and Core–Shell Fe–Fe3[email protected] composites as efficient microwave absorbents
Accepted Manuscript Mesoporous Fe/C and Core–Shell Fe-Fe3C@C Composites as Efficient Microwave Absorbents Guomin Li, Liancheng Wang, Wanxi Li, Yao Xu PII:
S1387-1811(15)00146-8
DOI:
10.1016/j.micromeso.2015.02.054
Reference:
MICMAT 7031
To appear in:
Microporous and Mesoporous Materials
Received Date: 28 December 2014 Accepted Date: 28 February 2015
Please cite this article as: G. Li, L. Wang, W. Li, Y. Xu, Mesoporous Fe/C and Core–Shell Fe-Fe3C@C Composites as Efficient Microwave Absorbents, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.02.054. 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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Graphical abstract
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Mesoporous Fe/C and Core–Shell Fe-Fe3C@C Composites as Efficient Microwave Absorbents Guomin Li a,b, Liancheng Wang a, Wanxi Li a,b, Yao Xu a,* a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
b
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of Sciences, Taiyuan 030001, China
University of Chinese Academy of Sciences, Beijing 100049, China
*Corresponding author. E-mail: [email protected], Tel: +86-0351-4049859.
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Abstract: Mesoporous Fe/C and core–shell Fe-Fe3C@C composites were successfully prepared through the in-situ polymerization of Fe3+/phenolic resin
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coupled with F127 and the subsequent high-temperature carbonization. The experiments involved the preparation of an iron-containing carbon precursor and the heat-treatment process. Two composites with different morphology and structure could be obtained by changing the content of Fe(NO3)3·9H2O in the precursor. The crystalline phase, structure and microwave absorption of the two composites were investigated. Fe particles were uniformly embedded into the mesoporous networks to
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form mesoporous Fe/C composite with high surface area of 467.3 m2/g and low density of 1.92 g/cm3. The Fe-Fe3C particles encapsulated by graphitized carbon layers formed the core–shell structure with surface area and density of 259.5 m2/g and
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2.67 g/cm3. Fe/C and Fe-Fe3C@C composites exhibited excellent electromagnetic absorbing ability, the effective absorption bandwidth reached 3.36 and 5.04 GHz with
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the matching thicknesses of 2 and 1.5 mm correspondingly. This originated mainly from the effective impedance match and multiple interfacial polarizations. Furthermore, the increase of Fe3+ not only promoted the graphitization degree of carbon shell, but also increased the complex permittivity and permeability of core– shell structure, thus improved the impedance matching. Owing to high surface area, low density and excellent microwave absorbability, the mesoporous Fe/C and core– shell Fe-Fe3C@C composites are promising candidates as lightweight and high-efficiency microwave absorbents. Keywords: mesoporous; core–shell structure; composites; lightweight; microwave
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1. Introduction In recent years, the microwave absorption materials (MAMs) have received considerable attention because of the increasingly prominent electromagnetic
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pollution deriving from the daily communication and entertainment. It has been more and more urgent to design and fabricate MAMs to protect environment as well as human health escaping from electromagnetic interference. Up to now, many efforts have been focused on the synthesis of various MAMs, such as traditional magnetic
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fillers (carbonyl-iron, Fe, Co, Ni, and ferrites), [1–5] carbon materials (carbon black, carbon fibers, mesoporous carbon, activated carbon, carbon nanocoil, carbon
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nanotubes and graphene, etc.), [6–10] conductive polymers, [11, 12] and composite absorbents. [13–15] Meanwhile, MAMs with numerous morphologies and structures have been reported to play an important role in the microwave absorption. [16, 17] Therefore, increasing interest has been given to the synthesis of MAMs with specific morphology currently. Xu et al. obtained porous flower-like Fe3O4 by decomposition
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of the iron alkoxide precursors and proved the porous Fe3O4 to have the potential as electromagnetic absorbing material. [18] Ordered mesoporous NiFe2O4 was synthesized by the nanocasting method and exhibited excellent microwave
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absorbability. [19]
As typical dielectric loss MAMs, carbon materials have received widely attention
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because of their abundant resource, easy preparation, relative low density, tunable properties, and low cost. [10] On the basis of theoretical analysis and experimental investigation, it is generally accepted that introduction of pores in the MAMs is an effective way to reduce the density and to modify the complex permittivity of the absorbents. This can benefit the optimized characteristic impedance and enhanced microwave absorption in return. [20, 21] Therefore, carbon-based MAMs with large surface area and high porosity appear as a class of multi-functional materials with promising application in the field of electromagnetic shielding. [22, 23] Recently, the core–shell composites, combining dielectric loss and magnetic loss materials
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loss characteristic and chemical stability. [27, 28]
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MAMs, carbon is one of the best candidates for the shell materials due to its dielectric
In this work, we report the synthesis of mesoporous Fe/C and core–shell Fe-Fe3C@C composites with high surface area under hydrothermal condition
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followed by carbothermic reaction. The morphology and structure of as-prepared products could be controlled via changing the content of Fe(NO3)3·9H2O in the
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system. Furthermore, the possible formation mechanism and transformation process during the reaction were also proposed and discussed. The mesoporous Fe/C and core–shell Fe-Fe3C@C composites exhibit excellent microwave absorption ability, showing great potential as high-efficiency and lightweight electromagnetic absorbing
2. Experimental
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materials.
2.1 Preparation of mesoporous Fe/C In
a
typical
synthesis,
Pluronic
F127
(EO106PO70EO106,
Mw=12,600),
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1,3,5-trimethylbenzene (TMB), Hexamethylenetetramine (HMT), and resorcin were in turn dissolved in 18 ml distilled water under vigorous stirring at room temperature.
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After the formation of a homogenous solution with the molar ratio of F127, TMB, HMT, and resorcin = 1: 21: 31.5: 63, and 0.15 M Fe(NO3)3·9H2O was slowly added and the solution turned into bluish violet. It was then transferred into a Teflon-lined stainless autoclave, heated at 100 oC for 12 h. Afterwards, the resulting chocolate brown product was collected by filtration, washed with water several times and dried at 60 oC. Finally, the as-made products were thermally treated at 900 oC for 3 h, at a heating rate of 1 oC/min in Ar atmosphere and then were ground in an agate mortar until no lumps were observed. The powder has an average grain size of ~ 1 µm. (see Fig. S1 in supporting information)
ACCEPTED MANUSCRIPT 2.2 Preparation of core–shell Fe-Fe3C@C The procedure of preparing core–shell Fe-Fe3C@C is the same as part 2.1, and the only difference is that the molar weight of Fe(NO3)3·9H2O was increased to 0.45 M. 2.3 Characterization
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The composition and phase purity of the as-synthesized samples were analyzed by a D8 Advance Bruker AXS diffractometer with Cu-Kα radiation (λ=1.5406 Å), in scan steps of 0.01° in a 2θ range from 10° to 90°. Raman spectra were recorded on a Horiba LabRAM HR800 spectrometer with an Ar+ laser. The N2 adsorption–
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desorption analysis was measured on a Micromeritics ASAP 2010 instrument. The pore volumes were determined using the adsorbed volume at a relative pressure of
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0.99 and the multipoint BET surface area was estimated in a relative pressure range from 0.05 to 0.25. The thermal stability of the composites was tested in a thermogravimetric differential analyser (Model Netzsch Sta 409PC, Germany) under air atmosphere at a heating rate of 10 oC/min. The microscopic morphology of the samples was observed on transmission electron microscope (TEM, JEOL JEM-1011)
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and field-emission scanning electron microscope (FESEM, JSM-7001F) equipped with an energy-dispersive X-ray analyser (EDX, QUANTAX200). The magnetic properties were measured on a vibrating sample magnetometer (VSM Lakeshore
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Model 7400) at room temperature.
2.4 Microwave absorption measurement The specimen for microwave absorption measurement using coaxial wire method
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were prepared by uniformly mixing the samples in a paraffin matrix and pressing the mixture into a cylindrical shaped compact (Φouter =7.0 mm and Φinner =3.0 mm). Here, paraffin serves as the binder and it is transparent to electromagnetic waves. The relative permittivity and permeability values of the specimens with 12.5 and 33 wt% of Fe/C and Fe-Fe3C@C were measured in the 2–18 GHz with a vector network analyzer (Agilent N5230) at room temperature. Based on the measured electromagnetic parameters, the curve of reflection loss (RL) versus frequency at different coating thickness can be calculated according to transmission line theory.
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3. Results and discussion As illustrated in Scheme 1, the synthesis procedure consists of three main steps. First of all, after reactants (F127, Resorcinol, TMB and Hexamethylenetetramine) all dissolved in water, the resorcinol interacted intensely with the PEO blocks of F127
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via hydrogen bonds. The hydrophobic TMB, which improves the mesoscopic ordering of the products, [29] could couple with the polypropylene oxide (PPO) blocks. Next, Fe(NO3)3·9H2O was added into the above homogeneous solution, and Fe3+ would react with the phenolic hydroxyl through complexation and combine with the PEO
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blocks to form the complex, [30–32] which enables the binding force between Fe3+ and resorcinol-F127 system. HMT would gradually decompose to yield formaldehyde
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and ammonia under hydrothermal conditions. The released formaldehyde molecules were in turn condensate with the resorcinol molecules associated with the hydrophilic segments of triblock copolymers under the acid condition. [33] The gradual polymerization reactions induced the formation and growth of resol-F127 micelles, in which the Fe3+ was firmly fixed. Subsequently, the packing of the micelles and the
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further polymerization of resol promoted the formation of ordered mesostructure. At the final stage, F127 decomposed and Fe3+ was transformed into zero-valent iron via the carbothermal reaction step by step with the increasing temperature, and the Fe
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particles occupied the mesopores to form mesoporous Fe/C. It is important to point out that when the content of Fe(NO3)3·9H2O is increased to 0.45 M, the critical
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micelle concentration (CMC) of F127 will increase as nitrate ions are salting-in ions that are able to enhance the solubility of the PPO blocks. [34] Accordingly, F127 will not form integrated micelles in the solution with relatively higher concentration of ferric nitrate and serve primarily as the carbon source as well as reducing agent, just like the role played by resol in the carbonization process, during which the carbon atoms adsorbed onto and then moved along the Fe surface, finally led to the formation of carbon-encapsulated metallic particles on the basis of dissolution-precipitation mechanism. [35] 3.1 Crystalline phase and composition analysis
ACCEPTED MANUSCRIPT Fig. 1 presents the X-ray diffraction (XRD) patterns of the as-prepared composites. As can be seen, when the Fe3+ content is low, there exist three main diffraction peaks at 44.67°, 65.02°, and 82.33° corresponded to the crystalline planes of (110), (200) and (211), indicating the presence of Fe (JCPDS No. 06-0696). The broad peak
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between 20° and 30° is ascribed to the (002) lattice planes of the disordered carbon phase. [36] While for the sample of Fe-Fe3C@C, apart from the characteristic peaks of Fe, there are also several weak diffraction peaks that can be indexed as the crystalline planes of Fe3C species (JCPDS No. 35-0772), revealing the co-existence of
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Fe and Fe3C with well-crystallized structure in the composite when the molar weight of Fe3+ was increased. It should be mentioned that in this case, the broad peak
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disappears and is replaced by a slight diffraction peak at 2θ ≈26.5°, which can be indexed to (002) plane of graphite. [37, 38] This confirms that the high concentration of Fe3+ is beneficial for the formation of Fe3C species and the improvement of carbon crystallinity. That is, the additional Fe not only reacts with the carbon to form Fe3C but also catalyzes the amorphous carbon to transform into graphitized carbon.
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Meanwhile, the absence of Fe3C in Fe/C may be due to the content of Fe3+ is too small and the amount of Fe3C is too low to be detected. Moreover, the structural characterization of carbon framework was further confirmed by Raman spectroscopy. As shown in the inset in Fig. 1, the spectra were collected within the range between
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800 and 2000 cm–1, in which two characteristic bands appeared. That is the so-called D peak at around 1350 cm–1 and G peak at about 1590 cm–1, which represent the
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disordered carbon structure and graphite carbon, respectively. It is evident that the graphitization degree represented by the relative intensity ratio (ID/IG) is enhanced with the increase of Fe3+ content, which is well consistent with the XRD results. Fig. 2 shows the TG curves of the Fe/C and Fe-Fe3C@C composites. TG analysis
shows that distinct weight-increasing phenomenon is visible for both samples at 332 o
C and 413 oC, respectively, which is independent of the structure of the composites,
indicating that the weight increase from oxidation of Fe/Fe3C exceeds the weight loss from oxidation of carbon. Furthermore, with the increase of Fe3+ content, the weight increase is enhanced. The contents of carbon and iron in the samples can also be
ACCEPTED MANUSCRIPT calculated from TG curves. The total weight loss of 78.6 and 43.4 wt% for the samples during the oxidation process suggests that the weight contents of Fe in Fe/C and Fe-Fe3C@C composites are approximately 15 and 40 wt%, assuming that all Fe has transferred to Fe2O3 and all C has been burnt out under air atmosphere.
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3.2 Morphology and microstructure 3.2.1 Mesoporous Fe/C
To observe the detailed microscopic structures, TEM images provide some insights into the morphology and structure of the samples, and the representative images are
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shown in Fig. 3 and Fig. 5. As shown in Fig. 3a and 3b, parallel channels with a d spacing of 5.4 nm are clearly observed on the Fe/C, and the black iron nanocrystal
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particles are uniformly dispersed in the mesoporous carbon.
The N2 adsorption-desorption isotherms for Fe/C and Fe-Fe3C@C composites are given in Fig. 4. In both cases, the obtained isotherms can be classified as typically type IV with very large hysteresis loops between the adsorption and desorption isotherms in the P/P0 range from 0.4 to 1.0, indicating the characteristic of
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mesoporous materials according to the IUPAC classification. [39] This is quite similar to those observed previously for many other mesoporous transition mental/mental oxides synthesized by one-pot synthesis method. [40, 41] Meanwhile, from the textural properties summarized in Table 1, we see that the Fe/C composite exhibits
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higher surface area of 467.3 m2/g and larger pore volume of 0.6 m3/g than that (259.5 m2/g and 0.4 m3/g) of Fe-Fe3C@C. The densities of the Fe/C and Fe-Fe3C@C
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composites are 1.92 and 2.67 g/cm3, which are much lower than that (7.86 and 7.69 g/cm3) of bulk Fe and Fe3C, implying that mesoporous Fe/C and core–shell Fe-Fe3C@C are lightweight. 3.2.2 Core–shell Fe-Fe3C@C For the Fe-Fe3C@C, it can be clearly seen from Fig. 5a and 5b that the sample is mainly composed of well-dispersed spherical or ellipsoidal particles with size ranging from 50 to 200 nm. The black metallic particles are completely encapsulated in carbon matrix. In addition, the higher resolution TEM image of a typically isolated particle and the corresponding selected-area electron diffraction (SAED) pattern are
ACCEPTED MANUSCRIPT shown in Fig. 5c and 5d. From Fig. 5c, we can see that the graphite-like carbon shell of about 10 nm can be easily identified along the outer surface of the metallic core with an interlayer spacing of 0.34 nm. Besides, two classes of lattice fringe are identified, and their corresponding inter-plane distances are 0.25 and 0.14 nm, which
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match the inter-plane distances of (100) and (0-21) planes. The SAED pattern in Fig. 5d noticeably indicates that the diffraction spots belong to the (100), (1-21) and (0-21) planes with an incident electron beam along the [012] direction, showing the composition of the selected area of the particle is Fe3C.
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From the above observation, we know that the content of Fe(NO3)3·9H2O is crucial to the structure of the final products. When the content of Fe(NO3)3·9H2O is low, the
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CMC of F127 is not changed basically and the as-synthesized product exhibits the mesoporous structure, which is similar to the previous report. [42] Whereas the situation is completely opposite in the presence of more Fe(NO3)3·9H2O, producing the core–shell Fe-Fe3C@C composite, which obeys the dissolution-precipitation mechanism. [43, 44] In this case, during the carbothermal reduction process, the
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carbon atoms will continually dissolve and diffuse into the Fe surface at high temperature to form a metal-carbon solid solution, and precipitate steadily from the solid solution once to be supersaturated, forming ordered carbon structure around the
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metal. Among them, some carbon remained in the solution may form iron carbides (Fe3C) phase. During the cooling process, some dissolved carbon precipitate outward in the form of graphitic layers which closely encapsulated the magnetic Fe-Fe3C
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particles to form the core–shell structure. As is well known, transition metals such as Fe, Co and Ni are active catalysts for the formation of graphite from pitches, hydrocarbons, carbon monoxide and so on. [45] Consequently, in our case, the extra Fe3+ can also enhance the graphitization of the carbon shell besides forming the Fe3C. In addition to the above-mentioned core–shell structures, it can be observed in Fig. 5a that the Fe-Fe3C@C particles are surrounded by limited amount of amorphous carbon that can obstruct the adherence of the magnetic encapsulated particles. [46] Additionally, hollow carbon spheres can also be found in the composite (see the arrows), which is synthesized by the exudation and agglomeration of magnetic cores
ACCEPTED MANUSCRIPT during the above dissolution-precipitation process, leaving only the hollow carbon shell. 3.3 Magnetic and Microwave absorption The magnetic properties of the as-prepared composites are investigated by VSM at
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300 K. Fig. S2 (in supporting information) shows the magnetization curves of Fe/C and Fe-Fe3C@C composites, and their magnetic parameters are listed in Table 1. As shown in Fig. S2, these composites can be endued with magnetism after introducing magnetic Fe and Fe-Fe3C particles and the magnetization curves exhibit
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ferromagnetic characteristic. It can be seen that the values of Ms and Mr increase from 16.3 and 4.3 emu/g to 48.3 and 7.7 emu/g, respectively. This is mainly resulted
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from different content of magnetic constituents and particle size in the composites. To further study the microwave absorption of as-prepared composites, their theoretical RL was obtained from the measured complex permittivity (εr=ε′–jε″) and complex permeability (µr=µ′–jµ″) at a given frequency and the thickness of microwave absorbing materials according to the transmit line theory, which can be
= tanh
#$% &'
RL = 20lg "
"
#$% ('
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defined with the following equations: [8, 47]
√
(1) (2)
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where d is the thickness of the absorbent, f is the frequency of microwave, c is the velocity of light, and Zin is the input impedance of absorbent. The coating thickness is
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a crucial parameter affecting RL intensity and the frequency position of minimum absorption dip. [19] Therefore, we calculated the RL at different thicknesses of 1 mm, 1.5 mm, 2 mm, 2.5 mm, and 3 mm. Fig. 6 shows the RL curves in the frequency range of 2–18 GHz for the product/paraffin composites, it can be seen that the microwave frequency corresponding to the minimum RL moves to low frequency range with the increase of coating thickness, and the relationship between frequency and thickness can be interpreted in terms of the equation f =c/2πdµ″, [48] where µ″ is the imaginary part of relative permeability. Consequently, the microwave absorption can be modulated simply by adjusting the coating thickness of the prepared composites for
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and only the absorbent with RL less than –10 dB can be considered in practical application. Hence, the effective bandwidth (the bandwidth corresponding to the RL < –10 dB) can reach 3.36 GHz when the thickness is 2 mm for Fe/C. As for Fe-Fe3C@C, the effective bandwidth is 5.04 GHz with the matching thickness of 1.5 mm, nearly
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covers the whole Ku-band (12–18 GHz). Additionally, we compare the Fe/C and Fe-Fe3C@C composites with other recently reported absorbents in microwave
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absorbing property, [8, 9, 13, 19, 49, 50] as shown in Table 2. It is clear that the Fe/C and Fe-Fe3C@C composites have a lower filling rate and thinner coating thickness, and still exhibit relatively wider bandwidth. These results further reveal that mesoporous Fe/C and core-shell Fe-Fe3C@C composites are lightweight and efficient microwave absorbents.
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3.4 Microwave absorption mechanism
In our case, the excellent microwave absorption can be explained by the following factors. First, absorption property of an absorbent are highly related to its complex
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permittivity and complex permeability, which are very important parameters for designing microwave absorbents with excellent property. The real parts of complex permittivity (ε′) and complex permeability (µ′) stand for the storage capability of
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electric and magnetic energy, and the imaginary parts (ε″ and µ″) represent the dissipation ability. [51, 52] The complex permittivity spectra of the Fe/C and Fe-Fe3C@C composites are shown in Fig. 7a, from which it can be seen that the values of ε′ and ε″ trend to decrease with increasing frequency and are enhanced gradually with increasing the content of magnetic constituents, showing frequency dependence in the whole frequency range (2–18 GHz). This is the typical frequency dispersion behaviour, existing widely in pristine carbon materials [10, 53] and carbon-based materials, [54, 55] which benefits the enhancement of microwave absorbing ability. [13] In contrast, the ε′ and ε″ of Fe-Fe3C@C show fluctuations at
ACCEPTED MANUSCRIPT about 5.5 and 14 GHz, this is reasonable as both real permittivity (polarization) and imaginary permittivity (electric loss) are correlated, [56] similar phenomenon has also been reported by other group. [57] While for Fe/C, the variation of complex permittivity (ε′ and ε″) is relatively inconspicuous. The difference is partly attributed
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to their microstructure. As mentioned above, Fe/C has more abundant pore structure. It has been proved that rich pore structure can decrease the complex permittivity, [10, 58] and this is also consistent with the Maxwell Garnet theory. [59] Meanwhile, the reasonably large surface area and rich porosity can provide more active sites for the
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reflection and scattering of electromagnetic wave. [25, 60]
Fig. 7b shows µ′ and µ′′ of the Fe/C and Fe-Fe3C@C composites. It can be seen
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that there is an obvious enhancement of µ′ and µ′′ for Fe-Fe3C@C compared with Fe/C, which further leads to the improved absorbing property for Fe-Fe3C@C composites. It is widely acknowledged that the magnetic loss of magnetic absorbent originates mainly from eddy current effect, natural resonance, and exchange resonance. [7] The role of eddy current loss to the µ′′ is associated with the diameter d
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of the nanoparticle, electric conductivity σ, and the permeability µ0 of vacuum, which can be described by µ″≈2πµ0(µ′)2σd2f/3. If the magnetic loss only comes from the eddy current loss, the equation µ″(µ′)–2f–1=2πµ0σd2/3 should keep constant with changing frequency. [61] Fig. 8 shows the variation of µ″(µ′)–2f–1 versus f of Fe/C and
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Fe-Fe3C@C composites. The value of µ″(µ′)–2f–1 remains relatively constant in the 4– 18 GHz range for Fe/C, whereas the situation is compeletely opposite for Fe-Fe3C@C
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as its µ″(µ′)–2f–1 value exhibits evident fluctuations, implying that the eddy current loss makes main contribution to the magnetic loss for Fe/C but not for Fe-Fe3C@C. Based on the Aharoni’s theory, [62] the resonance peaks at high frequency in the curve of µ″ for Fe-Fe3C@C are related to the exchange resonance, and the peaks located at lower frequency are attributed to the natural ferromagnetic resonance. [63] Second, the Fe particles are embedded into the mesoporous carbon and the Fe-Fe3C particles are encapsulated by graphitized carbon layer for Fe/C and Fe-Fe3C@C composites (Fig. 3 and Fig. 5). As a result, there exists effective interface between the magnetic particles and the carbon matrix, with the charge transfer between carbon and
ACCEPTED MANUSCRIPT Fe/Fe-Fe3C particles. It is well accepted that the interfacial polarization and associated relaxation will enhance the absorption property, which can be validated by the Cole– Cole semicircle. According to the Debye dipolar relaxation, [64] the relative complex permittivity (εr) can be written as / &0
(3)
'(1234
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) = * + , - = . +
where τ0、εs and ε∞ are the relaxation time, the static dielectric constant, and the dielectric constant at infinite frequency. From equation 3, it can be deduced that &
/ 0 * = . + '(523 67
234 5/ &0 6 '(5234 67
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- =
(4)
4
(5)
and ε″ is as follows * −
/9 0
+ 5 - 6 =
/ &0
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based on equation 4 and 5, it can be further deduced that the relationship between ε′
(6)
thus the plot of ε′ versus ε″ would be a single semicircle, which is usually defined as the Cole–Cole semicircle, and each semicircle corresponds to one Debye relaxation
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process. [65] Plots of ε′ versus ε″ for Fe/C and Fe-Fe3C@C composites are shown in Fig. 9, from which three and five superimposed Cole–Cole semicircles as well as a linear curve are found for the two samples, suggesting that there are ternary and
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quinary dielectric relaxation processes. It is obvious that the number of semicircles for Fe-Fe3C@C is more than that of Fe/C, which indicates that the interface polarization
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effect of Fe-Fe3C@C is more apparent because of its distinctive core–shell structure. Third, another vital parameter relating to RL is the concept of matched
characteristic impedance, where the characteristic impedance of the absorbing materials should be nearly equal to that of the free space to achieve zero reflection at the front surface of the materials. [13] This means that if the magnetic loss (tanδm=µ″/µ′) and the dielectric loss (tanδe=ε″/ε′) is out of balance, most of microwave will be reflected off at the surface of absorbents, resulting in poor microwave absorption. Fig. 10 shows the frequency dependence of the loss tangent for Fe/C and Fe-Fe3C@C composites. The curves of dielectric and magnetic loss tangents coexist
ACCEPTED MANUSCRIPT symmetrically for all samples, and the difference between the complex permittivity and complex permeability is reasonable. If we adjust the content of Fe/C and Fe-Fe3C@C mixed in paraffin, the microwave absorption will be inevitably influenced. This is also the reason why the filling rate of the Fe/C and Fe-Fe3C@C is
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fixed at 12.5 and 33 wt%, respectively, under which the composites maintain the optimum microwave absorbability. From Fig. 10, it can also be seen that dielectric loss tangent is larger than the magnetic loss tangent, which further indicates that dielectric loss dominates the microwave absorption enhancement of the composites.
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Moreover, the dielectric loss and magnetic loss of Fe-Fe3C@C are enhanced simultaneously compared with Fe/C, inducing improved microwave absorption for
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Fe-Fe3C@C.
4. Conclusions
Mesoporous Fe/C and core–shell Fe-Fe3C@C composites were controllably prepared through in-situ polymerization of Fe3+–containing carbon precursor and subsequent high-temperature treatment. The as-prepared products with high surface
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area exhibited excellent microwave absorbing ability, which might arise from their characteristic structure as well as better impedance match and multiple interfacial polarizations. This study showed that the Fe/C and Fe-Fe3C@C composites are
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promising lightweight and efficient microwave absorbents. Furthermore, this facile synthesis strategy may open up a new avenue for microwave absorption applications.
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Acknowledgements
This work was supported by the National Science Foundation of ShanXi (No.
2012011005-1).
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Table 1 Textual and magnetic properties * of mesoporous Fe/C and core–shell Fe-Fe3C@C composites Surface area (m2/g)
Pore volume (m3/g)
Pore size (nm)
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
ρ (g/cm3)
Fe/C Fe-Fe3C@C
467.3 259.5
0.6 0.4
5.1 5.7
16.3 48.3
4.3 7.7
560.2 366.6
1.92 2.67
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Sample
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* Specific surface area was according to BET, the pore-size distribution was derived from the adsorption branches of the isotherms using BJH method, pore volume was estimated from the amount absorbed at a relative pressure of P/P0 = 0.99, saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) were measured at room temperature. Density (ρ) was measured using a pycnometer. Table 2 Microwave absorption of some reported absorbents
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Sample
Mesoporous NiFe2O4 Fe3O4/Al2O3/CNCs h-Ni/GN α-Co/GN Fe3O4@C RGO-SCI Fe/C Fe-Fe3C@C
Filling rate (wt%)
Effective bandwidth (GHz)
Coating thickness (mm)
Ref.
25 25 60 60 50 60 12.5 33.3
1.5 3.5 4 5.5 3 4.19 3.36 5.04
3 2 3 2 2 3 2 1.5
[19] [8] [49] [9] [13] [50] This work
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Scheme 1. Schematic illustration of the synthesis procedure for the mesoporous Fe/C and core–
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shell Fe-Fe3C@C composites.
Fig. 1. X-ray diffraction patterns and Normal Raman spectra (inset) of mesoporous Fe/C and core–shell Fe-Fe3C@C composites.
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Fig. 2. TG curves of mesoporous Fe/C and core–shell Fe-Fe3C@C composites.
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Fig. 3. TEM images of mesoporous Fe/C recorded along [110] (a) and [001] (b) directions.
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composites.
Fig. 5. TEM images of core–shell Fe-Fe3C@C (a, b). HRTEM image (c) and the SAED pattern (d)
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Fig. 6. Microwave RL curves of (a) mesoporous Fe/C and (b) core–shell Fe-Fe3C@C with
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different coating thickness.
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Fig. 7. Frequency dependence of (a) relative complex permittivity and (b) permeability of
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mesoporous Fe/C and core–shell Fe-Fe3C@C composites.
Fig. 8. Values of µ″(µ′)–2f–1 of mesoporous Fe/C and core–shell Fe-Fe3C@C composites versus frequency.
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Fig. 9. Typical Cole–Cole semicircles (ε′ versus ε″) for (a) mesoporous Fe/C and (b) core–shell Fe-Fe3C@C in the frequency range of 2–18 GHz.
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Fig. 10. Frequency dependence of the loss tangent of mesoporous Fe/C and core–shell
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Fe-Fe3C@C composites.
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Highlights Mesoporous Fe/C and core–shell Fe-Fe3C@C absorbents were designed and synthesized.
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Fe/C and Fe-Fe3C@C absorbents process high surface area of 467.3 and 259.5 m2/g. The bandwidth reaches 3.36 and 5.04 GHz when the matching thickness is 2 and 1.5
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mm.
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Supporting information
Mesoporous Fe/C and Core–Shell Fe-Fe3C@C Composites as Efficient Microwave Absorbents
a
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Guomin Li a,b, Liancheng Wang a, Wanxi Li a,b, Yao Xu a,*
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
of Sciences, Taiyuan 030001, China
University of Chinese Academy of Sciences, Beijing 100049, China
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b
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*Corresponding author. E-mail: [email protected], Tel: +86-0351-4049859.
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Fig. S1. SEM images of (a) mesoporous Fe/C, (d) core–shell Fe/Fe3C@C and their corresponding
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elemental mapping images of (b, e) C, (c, f) Fe.
Fig. S1 shows the SEM images of the grounded Fe/C and Fe-Fe3C@C
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composites after annealing and their corresponding elemental mapping images. It is notable that the as-obtained powder has an average grain size of ~ 1 µm. The EDX elemental maps of C and Fe demonstrate their distribution, and it is evident that Fe element exhibits uniform distribution within the C matrix, which is consistent with the
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TEM results.
Fig. S2. Magnetic hysteresis loop of mesoporous Fe/C and core–shell Fe-Fe3C@C composites at 300 K.
S1387-1811(15)00146-8
DOI:
10.1016/j.micromeso.2015.02.054
Reference:
MICMAT 7031
To appear in:
Microporous and Mesoporous Materials
Received Date: 28 December 2014 Accepted Date: 28 February 2015
Please cite this article as: G. Li, L. Wang, W. Li, Y. Xu, Mesoporous Fe/C and Core–Shell Fe-Fe3C@C Composites as Efficient Microwave Absorbents, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.02.054. 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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Graphical abstract
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Mesoporous Fe/C and Core–Shell Fe-Fe3C@C Composites as Efficient Microwave Absorbents Guomin Li a,b, Liancheng Wang a, Wanxi Li a,b, Yao Xu a,* a
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
b
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of Sciences, Taiyuan 030001, China
University of Chinese Academy of Sciences, Beijing 100049, China
*Corresponding author. E-mail: [email protected], Tel: +86-0351-4049859.
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Abstract: Mesoporous Fe/C and core–shell Fe-Fe3C@C composites were successfully prepared through the in-situ polymerization of Fe3+/phenolic resin
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coupled with F127 and the subsequent high-temperature carbonization. The experiments involved the preparation of an iron-containing carbon precursor and the heat-treatment process. Two composites with different morphology and structure could be obtained by changing the content of Fe(NO3)3·9H2O in the precursor. The crystalline phase, structure and microwave absorption of the two composites were investigated. Fe particles were uniformly embedded into the mesoporous networks to
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form mesoporous Fe/C composite with high surface area of 467.3 m2/g and low density of 1.92 g/cm3. The Fe-Fe3C particles encapsulated by graphitized carbon layers formed the core–shell structure with surface area and density of 259.5 m2/g and
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2.67 g/cm3. Fe/C and Fe-Fe3C@C composites exhibited excellent electromagnetic absorbing ability, the effective absorption bandwidth reached 3.36 and 5.04 GHz with
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the matching thicknesses of 2 and 1.5 mm correspondingly. This originated mainly from the effective impedance match and multiple interfacial polarizations. Furthermore, the increase of Fe3+ not only promoted the graphitization degree of carbon shell, but also increased the complex permittivity and permeability of core– shell structure, thus improved the impedance matching. Owing to high surface area, low density and excellent microwave absorbability, the mesoporous Fe/C and core– shell Fe-Fe3C@C composites are promising candidates as lightweight and high-efficiency microwave absorbents. Keywords: mesoporous; core–shell structure; composites; lightweight; microwave
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1. Introduction In recent years, the microwave absorption materials (MAMs) have received considerable attention because of the increasingly prominent electromagnetic
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pollution deriving from the daily communication and entertainment. It has been more and more urgent to design and fabricate MAMs to protect environment as well as human health escaping from electromagnetic interference. Up to now, many efforts have been focused on the synthesis of various MAMs, such as traditional magnetic
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fillers (carbonyl-iron, Fe, Co, Ni, and ferrites), [1–5] carbon materials (carbon black, carbon fibers, mesoporous carbon, activated carbon, carbon nanocoil, carbon
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nanotubes and graphene, etc.), [6–10] conductive polymers, [11, 12] and composite absorbents. [13–15] Meanwhile, MAMs with numerous morphologies and structures have been reported to play an important role in the microwave absorption. [16, 17] Therefore, increasing interest has been given to the synthesis of MAMs with specific morphology currently. Xu et al. obtained porous flower-like Fe3O4 by decomposition
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of the iron alkoxide precursors and proved the porous Fe3O4 to have the potential as electromagnetic absorbing material. [18] Ordered mesoporous NiFe2O4 was synthesized by the nanocasting method and exhibited excellent microwave
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absorbability. [19]
As typical dielectric loss MAMs, carbon materials have received widely attention
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because of their abundant resource, easy preparation, relative low density, tunable properties, and low cost. [10] On the basis of theoretical analysis and experimental investigation, it is generally accepted that introduction of pores in the MAMs is an effective way to reduce the density and to modify the complex permittivity of the absorbents. This can benefit the optimized characteristic impedance and enhanced microwave absorption in return. [20, 21] Therefore, carbon-based MAMs with large surface area and high porosity appear as a class of multi-functional materials with promising application in the field of electromagnetic shielding. [22, 23] Recently, the core–shell composites, combining dielectric loss and magnetic loss materials
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loss characteristic and chemical stability. [27, 28]
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MAMs, carbon is one of the best candidates for the shell materials due to its dielectric
In this work, we report the synthesis of mesoporous Fe/C and core–shell Fe-Fe3C@C composites with high surface area under hydrothermal condition
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followed by carbothermic reaction. The morphology and structure of as-prepared products could be controlled via changing the content of Fe(NO3)3·9H2O in the
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system. Furthermore, the possible formation mechanism and transformation process during the reaction were also proposed and discussed. The mesoporous Fe/C and core–shell Fe-Fe3C@C composites exhibit excellent microwave absorption ability, showing great potential as high-efficiency and lightweight electromagnetic absorbing
2. Experimental
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materials.
2.1 Preparation of mesoporous Fe/C In
a
typical
synthesis,
Pluronic
F127
(EO106PO70EO106,
Mw=12,600),
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1,3,5-trimethylbenzene (TMB), Hexamethylenetetramine (HMT), and resorcin were in turn dissolved in 18 ml distilled water under vigorous stirring at room temperature.
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After the formation of a homogenous solution with the molar ratio of F127, TMB, HMT, and resorcin = 1: 21: 31.5: 63, and 0.15 M Fe(NO3)3·9H2O was slowly added and the solution turned into bluish violet. It was then transferred into a Teflon-lined stainless autoclave, heated at 100 oC for 12 h. Afterwards, the resulting chocolate brown product was collected by filtration, washed with water several times and dried at 60 oC. Finally, the as-made products were thermally treated at 900 oC for 3 h, at a heating rate of 1 oC/min in Ar atmosphere and then were ground in an agate mortar until no lumps were observed. The powder has an average grain size of ~ 1 µm. (see Fig. S1 in supporting information)
ACCEPTED MANUSCRIPT 2.2 Preparation of core–shell Fe-Fe3C@C The procedure of preparing core–shell Fe-Fe3C@C is the same as part 2.1, and the only difference is that the molar weight of Fe(NO3)3·9H2O was increased to 0.45 M. 2.3 Characterization
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The composition and phase purity of the as-synthesized samples were analyzed by a D8 Advance Bruker AXS diffractometer with Cu-Kα radiation (λ=1.5406 Å), in scan steps of 0.01° in a 2θ range from 10° to 90°. Raman spectra were recorded on a Horiba LabRAM HR800 spectrometer with an Ar+ laser. The N2 adsorption–
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desorption analysis was measured on a Micromeritics ASAP 2010 instrument. The pore volumes were determined using the adsorbed volume at a relative pressure of
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0.99 and the multipoint BET surface area was estimated in a relative pressure range from 0.05 to 0.25. The thermal stability of the composites was tested in a thermogravimetric differential analyser (Model Netzsch Sta 409PC, Germany) under air atmosphere at a heating rate of 10 oC/min. The microscopic morphology of the samples was observed on transmission electron microscope (TEM, JEOL JEM-1011)
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and field-emission scanning electron microscope (FESEM, JSM-7001F) equipped with an energy-dispersive X-ray analyser (EDX, QUANTAX200). The magnetic properties were measured on a vibrating sample magnetometer (VSM Lakeshore
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Model 7400) at room temperature.
2.4 Microwave absorption measurement The specimen for microwave absorption measurement using coaxial wire method
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were prepared by uniformly mixing the samples in a paraffin matrix and pressing the mixture into a cylindrical shaped compact (Φouter =7.0 mm and Φinner =3.0 mm). Here, paraffin serves as the binder and it is transparent to electromagnetic waves. The relative permittivity and permeability values of the specimens with 12.5 and 33 wt% of Fe/C and Fe-Fe3C@C were measured in the 2–18 GHz with a vector network analyzer (Agilent N5230) at room temperature. Based on the measured electromagnetic parameters, the curve of reflection loss (RL) versus frequency at different coating thickness can be calculated according to transmission line theory.
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3. Results and discussion As illustrated in Scheme 1, the synthesis procedure consists of three main steps. First of all, after reactants (F127, Resorcinol, TMB and Hexamethylenetetramine) all dissolved in water, the resorcinol interacted intensely with the PEO blocks of F127
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via hydrogen bonds. The hydrophobic TMB, which improves the mesoscopic ordering of the products, [29] could couple with the polypropylene oxide (PPO) blocks. Next, Fe(NO3)3·9H2O was added into the above homogeneous solution, and Fe3+ would react with the phenolic hydroxyl through complexation and combine with the PEO
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blocks to form the complex, [30–32] which enables the binding force between Fe3+ and resorcinol-F127 system. HMT would gradually decompose to yield formaldehyde
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and ammonia under hydrothermal conditions. The released formaldehyde molecules were in turn condensate with the resorcinol molecules associated with the hydrophilic segments of triblock copolymers under the acid condition. [33] The gradual polymerization reactions induced the formation and growth of resol-F127 micelles, in which the Fe3+ was firmly fixed. Subsequently, the packing of the micelles and the
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further polymerization of resol promoted the formation of ordered mesostructure. At the final stage, F127 decomposed and Fe3+ was transformed into zero-valent iron via the carbothermal reaction step by step with the increasing temperature, and the Fe
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particles occupied the mesopores to form mesoporous Fe/C. It is important to point out that when the content of Fe(NO3)3·9H2O is increased to 0.45 M, the critical
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micelle concentration (CMC) of F127 will increase as nitrate ions are salting-in ions that are able to enhance the solubility of the PPO blocks. [34] Accordingly, F127 will not form integrated micelles in the solution with relatively higher concentration of ferric nitrate and serve primarily as the carbon source as well as reducing agent, just like the role played by resol in the carbonization process, during which the carbon atoms adsorbed onto and then moved along the Fe surface, finally led to the formation of carbon-encapsulated metallic particles on the basis of dissolution-precipitation mechanism. [35] 3.1 Crystalline phase and composition analysis
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between 20° and 30° is ascribed to the (002) lattice planes of the disordered carbon phase. [36] While for the sample of Fe-Fe3C@C, apart from the characteristic peaks of Fe, there are also several weak diffraction peaks that can be indexed as the crystalline planes of Fe3C species (JCPDS No. 35-0772), revealing the co-existence of
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Fe and Fe3C with well-crystallized structure in the composite when the molar weight of Fe3+ was increased. It should be mentioned that in this case, the broad peak
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disappears and is replaced by a slight diffraction peak at 2θ ≈26.5°, which can be indexed to (002) plane of graphite. [37, 38] This confirms that the high concentration of Fe3+ is beneficial for the formation of Fe3C species and the improvement of carbon crystallinity. That is, the additional Fe not only reacts with the carbon to form Fe3C but also catalyzes the amorphous carbon to transform into graphitized carbon.
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Meanwhile, the absence of Fe3C in Fe/C may be due to the content of Fe3+ is too small and the amount of Fe3C is too low to be detected. Moreover, the structural characterization of carbon framework was further confirmed by Raman spectroscopy. As shown in the inset in Fig. 1, the spectra were collected within the range between
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800 and 2000 cm–1, in which two characteristic bands appeared. That is the so-called D peak at around 1350 cm–1 and G peak at about 1590 cm–1, which represent the
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disordered carbon structure and graphite carbon, respectively. It is evident that the graphitization degree represented by the relative intensity ratio (ID/IG) is enhanced with the increase of Fe3+ content, which is well consistent with the XRD results. Fig. 2 shows the TG curves of the Fe/C and Fe-Fe3C@C composites. TG analysis
shows that distinct weight-increasing phenomenon is visible for both samples at 332 o
C and 413 oC, respectively, which is independent of the structure of the composites,
indicating that the weight increase from oxidation of Fe/Fe3C exceeds the weight loss from oxidation of carbon. Furthermore, with the increase of Fe3+ content, the weight increase is enhanced. The contents of carbon and iron in the samples can also be
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3.2 Morphology and microstructure 3.2.1 Mesoporous Fe/C
To observe the detailed microscopic structures, TEM images provide some insights into the morphology and structure of the samples, and the representative images are
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shown in Fig. 3 and Fig. 5. As shown in Fig. 3a and 3b, parallel channels with a d spacing of 5.4 nm are clearly observed on the Fe/C, and the black iron nanocrystal
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particles are uniformly dispersed in the mesoporous carbon.
The N2 adsorption-desorption isotherms for Fe/C and Fe-Fe3C@C composites are given in Fig. 4. In both cases, the obtained isotherms can be classified as typically type IV with very large hysteresis loops between the adsorption and desorption isotherms in the P/P0 range from 0.4 to 1.0, indicating the characteristic of
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mesoporous materials according to the IUPAC classification. [39] This is quite similar to those observed previously for many other mesoporous transition mental/mental oxides synthesized by one-pot synthesis method. [40, 41] Meanwhile, from the textural properties summarized in Table 1, we see that the Fe/C composite exhibits
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higher surface area of 467.3 m2/g and larger pore volume of 0.6 m3/g than that (259.5 m2/g and 0.4 m3/g) of Fe-Fe3C@C. The densities of the Fe/C and Fe-Fe3C@C
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composites are 1.92 and 2.67 g/cm3, which are much lower than that (7.86 and 7.69 g/cm3) of bulk Fe and Fe3C, implying that mesoporous Fe/C and core–shell Fe-Fe3C@C are lightweight. 3.2.2 Core–shell Fe-Fe3C@C For the Fe-Fe3C@C, it can be clearly seen from Fig. 5a and 5b that the sample is mainly composed of well-dispersed spherical or ellipsoidal particles with size ranging from 50 to 200 nm. The black metallic particles are completely encapsulated in carbon matrix. In addition, the higher resolution TEM image of a typically isolated particle and the corresponding selected-area electron diffraction (SAED) pattern are
ACCEPTED MANUSCRIPT shown in Fig. 5c and 5d. From Fig. 5c, we can see that the graphite-like carbon shell of about 10 nm can be easily identified along the outer surface of the metallic core with an interlayer spacing of 0.34 nm. Besides, two classes of lattice fringe are identified, and their corresponding inter-plane distances are 0.25 and 0.14 nm, which
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match the inter-plane distances of (100) and (0-21) planes. The SAED pattern in Fig. 5d noticeably indicates that the diffraction spots belong to the (100), (1-21) and (0-21) planes with an incident electron beam along the [012] direction, showing the composition of the selected area of the particle is Fe3C.
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From the above observation, we know that the content of Fe(NO3)3·9H2O is crucial to the structure of the final products. When the content of Fe(NO3)3·9H2O is low, the
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CMC of F127 is not changed basically and the as-synthesized product exhibits the mesoporous structure, which is similar to the previous report. [42] Whereas the situation is completely opposite in the presence of more Fe(NO3)3·9H2O, producing the core–shell Fe-Fe3C@C composite, which obeys the dissolution-precipitation mechanism. [43, 44] In this case, during the carbothermal reduction process, the
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carbon atoms will continually dissolve and diffuse into the Fe surface at high temperature to form a metal-carbon solid solution, and precipitate steadily from the solid solution once to be supersaturated, forming ordered carbon structure around the
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metal. Among them, some carbon remained in the solution may form iron carbides (Fe3C) phase. During the cooling process, some dissolved carbon precipitate outward in the form of graphitic layers which closely encapsulated the magnetic Fe-Fe3C
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particles to form the core–shell structure. As is well known, transition metals such as Fe, Co and Ni are active catalysts for the formation of graphite from pitches, hydrocarbons, carbon monoxide and so on. [45] Consequently, in our case, the extra Fe3+ can also enhance the graphitization of the carbon shell besides forming the Fe3C. In addition to the above-mentioned core–shell structures, it can be observed in Fig. 5a that the Fe-Fe3C@C particles are surrounded by limited amount of amorphous carbon that can obstruct the adherence of the magnetic encapsulated particles. [46] Additionally, hollow carbon spheres can also be found in the composite (see the arrows), which is synthesized by the exudation and agglomeration of magnetic cores
ACCEPTED MANUSCRIPT during the above dissolution-precipitation process, leaving only the hollow carbon shell. 3.3 Magnetic and Microwave absorption The magnetic properties of the as-prepared composites are investigated by VSM at
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300 K. Fig. S2 (in supporting information) shows the magnetization curves of Fe/C and Fe-Fe3C@C composites, and their magnetic parameters are listed in Table 1. As shown in Fig. S2, these composites can be endued with magnetism after introducing magnetic Fe and Fe-Fe3C particles and the magnetization curves exhibit
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ferromagnetic characteristic. It can be seen that the values of Ms and Mr increase from 16.3 and 4.3 emu/g to 48.3 and 7.7 emu/g, respectively. This is mainly resulted
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from different content of magnetic constituents and particle size in the composites. To further study the microwave absorption of as-prepared composites, their theoretical RL was obtained from the measured complex permittivity (εr=ε′–jε″) and complex permeability (µr=µ′–jµ″) at a given frequency and the thickness of microwave absorbing materials according to the transmit line theory, which can be
= tanh
#$% &'
RL = 20lg "
"
#$% ('
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defined with the following equations: [8, 47]
√
(1) (2)
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where d is the thickness of the absorbent, f is the frequency of microwave, c is the velocity of light, and Zin is the input impedance of absorbent. The coating thickness is
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a crucial parameter affecting RL intensity and the frequency position of minimum absorption dip. [19] Therefore, we calculated the RL at different thicknesses of 1 mm, 1.5 mm, 2 mm, 2.5 mm, and 3 mm. Fig. 6 shows the RL curves in the frequency range of 2–18 GHz for the product/paraffin composites, it can be seen that the microwave frequency corresponding to the minimum RL moves to low frequency range with the increase of coating thickness, and the relationship between frequency and thickness can be interpreted in terms of the equation f =c/2πdµ″, [48] where µ″ is the imaginary part of relative permeability. Consequently, the microwave absorption can be modulated simply by adjusting the coating thickness of the prepared composites for
ACCEPTED MANUSCRIPT application in different frequency bands. Besides, the minimum RL of Fe/C composite is –19.4 dB at 9.8 GHz with thickness of 2.5 mm (Fig. 6a), while the minimum RL reaches –18.1 dB at 10.1 GHz when the thickness is 2 mm for the sample of Fe-Fe3C@C (Fig. 6b). In general, RL of –10 dB means 90% microwave absorption
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and only the absorbent with RL less than –10 dB can be considered in practical application. Hence, the effective bandwidth (the bandwidth corresponding to the RL < –10 dB) can reach 3.36 GHz when the thickness is 2 mm for Fe/C. As for Fe-Fe3C@C, the effective bandwidth is 5.04 GHz with the matching thickness of 1.5 mm, nearly
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covers the whole Ku-band (12–18 GHz). Additionally, we compare the Fe/C and Fe-Fe3C@C composites with other recently reported absorbents in microwave
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absorbing property, [8, 9, 13, 19, 49, 50] as shown in Table 2. It is clear that the Fe/C and Fe-Fe3C@C composites have a lower filling rate and thinner coating thickness, and still exhibit relatively wider bandwidth. These results further reveal that mesoporous Fe/C and core-shell Fe-Fe3C@C composites are lightweight and efficient microwave absorbents.
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3.4 Microwave absorption mechanism
In our case, the excellent microwave absorption can be explained by the following factors. First, absorption property of an absorbent are highly related to its complex
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permittivity and complex permeability, which are very important parameters for designing microwave absorbents with excellent property. The real parts of complex permittivity (ε′) and complex permeability (µ′) stand for the storage capability of
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electric and magnetic energy, and the imaginary parts (ε″ and µ″) represent the dissipation ability. [51, 52] The complex permittivity spectra of the Fe/C and Fe-Fe3C@C composites are shown in Fig. 7a, from which it can be seen that the values of ε′ and ε″ trend to decrease with increasing frequency and are enhanced gradually with increasing the content of magnetic constituents, showing frequency dependence in the whole frequency range (2–18 GHz). This is the typical frequency dispersion behaviour, existing widely in pristine carbon materials [10, 53] and carbon-based materials, [54, 55] which benefits the enhancement of microwave absorbing ability. [13] In contrast, the ε′ and ε″ of Fe-Fe3C@C show fluctuations at
ACCEPTED MANUSCRIPT about 5.5 and 14 GHz, this is reasonable as both real permittivity (polarization) and imaginary permittivity (electric loss) are correlated, [56] similar phenomenon has also been reported by other group. [57] While for Fe/C, the variation of complex permittivity (ε′ and ε″) is relatively inconspicuous. The difference is partly attributed
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to their microstructure. As mentioned above, Fe/C has more abundant pore structure. It has been proved that rich pore structure can decrease the complex permittivity, [10, 58] and this is also consistent with the Maxwell Garnet theory. [59] Meanwhile, the reasonably large surface area and rich porosity can provide more active sites for the
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reflection and scattering of electromagnetic wave. [25, 60]
Fig. 7b shows µ′ and µ′′ of the Fe/C and Fe-Fe3C@C composites. It can be seen
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that there is an obvious enhancement of µ′ and µ′′ for Fe-Fe3C@C compared with Fe/C, which further leads to the improved absorbing property for Fe-Fe3C@C composites. It is widely acknowledged that the magnetic loss of magnetic absorbent originates mainly from eddy current effect, natural resonance, and exchange resonance. [7] The role of eddy current loss to the µ′′ is associated with the diameter d
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of the nanoparticle, electric conductivity σ, and the permeability µ0 of vacuum, which can be described by µ″≈2πµ0(µ′)2σd2f/3. If the magnetic loss only comes from the eddy current loss, the equation µ″(µ′)–2f–1=2πµ0σd2/3 should keep constant with changing frequency. [61] Fig. 8 shows the variation of µ″(µ′)–2f–1 versus f of Fe/C and
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Fe-Fe3C@C composites. The value of µ″(µ′)–2f–1 remains relatively constant in the 4– 18 GHz range for Fe/C, whereas the situation is compeletely opposite for Fe-Fe3C@C
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as its µ″(µ′)–2f–1 value exhibits evident fluctuations, implying that the eddy current loss makes main contribution to the magnetic loss for Fe/C but not for Fe-Fe3C@C. Based on the Aharoni’s theory, [62] the resonance peaks at high frequency in the curve of µ″ for Fe-Fe3C@C are related to the exchange resonance, and the peaks located at lower frequency are attributed to the natural ferromagnetic resonance. [63] Second, the Fe particles are embedded into the mesoporous carbon and the Fe-Fe3C particles are encapsulated by graphitized carbon layer for Fe/C and Fe-Fe3C@C composites (Fig. 3 and Fig. 5). As a result, there exists effective interface between the magnetic particles and the carbon matrix, with the charge transfer between carbon and
ACCEPTED MANUSCRIPT Fe/Fe-Fe3C particles. It is well accepted that the interfacial polarization and associated relaxation will enhance the absorption property, which can be validated by the Cole– Cole semicircle. According to the Debye dipolar relaxation, [64] the relative complex permittivity (εr) can be written as / &0
(3)
'(1234
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) = * + , - = . +
where τ0、εs and ε∞ are the relaxation time, the static dielectric constant, and the dielectric constant at infinite frequency. From equation 3, it can be deduced that &
/ 0 * = . + '(523 67
234 5/ &0 6 '(5234 67
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- =
(4)
4
(5)
and ε″ is as follows * −
/9 0
+ 5 - 6 =
/ &0
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based on equation 4 and 5, it can be further deduced that the relationship between ε′
(6)
thus the plot of ε′ versus ε″ would be a single semicircle, which is usually defined as the Cole–Cole semicircle, and each semicircle corresponds to one Debye relaxation
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process. [65] Plots of ε′ versus ε″ for Fe/C and Fe-Fe3C@C composites are shown in Fig. 9, from which three and five superimposed Cole–Cole semicircles as well as a linear curve are found for the two samples, suggesting that there are ternary and
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quinary dielectric relaxation processes. It is obvious that the number of semicircles for Fe-Fe3C@C is more than that of Fe/C, which indicates that the interface polarization
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effect of Fe-Fe3C@C is more apparent because of its distinctive core–shell structure. Third, another vital parameter relating to RL is the concept of matched
characteristic impedance, where the characteristic impedance of the absorbing materials should be nearly equal to that of the free space to achieve zero reflection at the front surface of the materials. [13] This means that if the magnetic loss (tanδm=µ″/µ′) and the dielectric loss (tanδe=ε″/ε′) is out of balance, most of microwave will be reflected off at the surface of absorbents, resulting in poor microwave absorption. Fig. 10 shows the frequency dependence of the loss tangent for Fe/C and Fe-Fe3C@C composites. The curves of dielectric and magnetic loss tangents coexist
ACCEPTED MANUSCRIPT symmetrically for all samples, and the difference between the complex permittivity and complex permeability is reasonable. If we adjust the content of Fe/C and Fe-Fe3C@C mixed in paraffin, the microwave absorption will be inevitably influenced. This is also the reason why the filling rate of the Fe/C and Fe-Fe3C@C is
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fixed at 12.5 and 33 wt%, respectively, under which the composites maintain the optimum microwave absorbability. From Fig. 10, it can also be seen that dielectric loss tangent is larger than the magnetic loss tangent, which further indicates that dielectric loss dominates the microwave absorption enhancement of the composites.
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Moreover, the dielectric loss and magnetic loss of Fe-Fe3C@C are enhanced simultaneously compared with Fe/C, inducing improved microwave absorption for
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Fe-Fe3C@C.
4. Conclusions
Mesoporous Fe/C and core–shell Fe-Fe3C@C composites were controllably prepared through in-situ polymerization of Fe3+–containing carbon precursor and subsequent high-temperature treatment. The as-prepared products with high surface
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area exhibited excellent microwave absorbing ability, which might arise from their characteristic structure as well as better impedance match and multiple interfacial polarizations. This study showed that the Fe/C and Fe-Fe3C@C composites are
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promising lightweight and efficient microwave absorbents. Furthermore, this facile synthesis strategy may open up a new avenue for microwave absorption applications.
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Acknowledgements
This work was supported by the National Science Foundation of ShanXi (No.
2012011005-1).
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Table 1 Textual and magnetic properties * of mesoporous Fe/C and core–shell Fe-Fe3C@C composites Surface area (m2/g)
Pore volume (m3/g)
Pore size (nm)
Ms (emu/g)
Mr (emu/g)
Hc (Oe)
ρ (g/cm3)
Fe/C Fe-Fe3C@C
467.3 259.5
0.6 0.4
5.1 5.7
16.3 48.3
4.3 7.7
560.2 366.6
1.92 2.67
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Sample
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* Specific surface area was according to BET, the pore-size distribution was derived from the adsorption branches of the isotherms using BJH method, pore volume was estimated from the amount absorbed at a relative pressure of P/P0 = 0.99, saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) were measured at room temperature. Density (ρ) was measured using a pycnometer. Table 2 Microwave absorption of some reported absorbents
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Sample
Mesoporous NiFe2O4 Fe3O4/Al2O3/CNCs h-Ni/GN α-Co/GN Fe3O4@C RGO-SCI Fe/C Fe-Fe3C@C
Filling rate (wt%)
Effective bandwidth (GHz)
Coating thickness (mm)
Ref.
25 25 60 60 50 60 12.5 33.3
1.5 3.5 4 5.5 3 4.19 3.36 5.04
3 2 3 2 2 3 2 1.5
[19] [8] [49] [9] [13] [50] This work
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Scheme 1. Schematic illustration of the synthesis procedure for the mesoporous Fe/C and core–
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shell Fe-Fe3C@C composites.
Fig. 1. X-ray diffraction patterns and Normal Raman spectra (inset) of mesoporous Fe/C and core–shell Fe-Fe3C@C composites.
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Fig. 2. TG curves of mesoporous Fe/C and core–shell Fe-Fe3C@C composites.
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Fig. 3. TEM images of mesoporous Fe/C recorded along [110] (a) and [001] (b) directions.
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composites.
Fig. 5. TEM images of core–shell Fe-Fe3C@C (a, b). HRTEM image (c) and the SAED pattern (d)
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of the particle in Fig. 5b.
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Fig. 6. Microwave RL curves of (a) mesoporous Fe/C and (b) core–shell Fe-Fe3C@C with
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different coating thickness.
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Fig. 7. Frequency dependence of (a) relative complex permittivity and (b) permeability of
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mesoporous Fe/C and core–shell Fe-Fe3C@C composites.
Fig. 8. Values of µ″(µ′)–2f–1 of mesoporous Fe/C and core–shell Fe-Fe3C@C composites versus frequency.
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Fig. 9. Typical Cole–Cole semicircles (ε′ versus ε″) for (a) mesoporous Fe/C and (b) core–shell Fe-Fe3C@C in the frequency range of 2–18 GHz.
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Fig. 10. Frequency dependence of the loss tangent of mesoporous Fe/C and core–shell
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Fe-Fe3C@C composites.
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Highlights Mesoporous Fe/C and core–shell Fe-Fe3C@C absorbents were designed and synthesized.
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Fe/C and Fe-Fe3C@C absorbents process high surface area of 467.3 and 259.5 m2/g. The bandwidth reaches 3.36 and 5.04 GHz when the matching thickness is 2 and 1.5
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mm.
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Supporting information
Mesoporous Fe/C and Core–Shell Fe-Fe3C@C Composites as Efficient Microwave Absorbents
a
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Guomin Li a,b, Liancheng Wang a, Wanxi Li a,b, Yao Xu a,*
Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
of Sciences, Taiyuan 030001, China
University of Chinese Academy of Sciences, Beijing 100049, China
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*Corresponding author. E-mail: [email protected], Tel: +86-0351-4049859.
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Fig. S1. SEM images of (a) mesoporous Fe/C, (d) core–shell Fe/Fe3C@C and their corresponding
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elemental mapping images of (b, e) C, (c, f) Fe.
Fig. S1 shows the SEM images of the grounded Fe/C and Fe-Fe3C@C
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composites after annealing and their corresponding elemental mapping images. It is notable that the as-obtained powder has an average grain size of ~ 1 µm. The EDX elemental maps of C and Fe demonstrate their distribution, and it is evident that Fe element exhibits uniform distribution within the C matrix, which is consistent with the
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TEM results.
Fig. S2. Magnetic hysteresis loop of mesoporous Fe/C and core–shell Fe-Fe3C@C composites at 300 K.