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Hydrothermal synthesis of carbonyl iron-carbon nanocomposite: Characterization and electromagnetic performance
Results in Physics 7 (2017) 1978–1986
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Hydrothermal synthesis of carbonyl iron-carbon nanocomposite: Characterization and electromagnetic performance Hakimeh Pourabdollahi, Ali Reza Zarei ⇑ Faculty of Chemistry and Chemical Engineering, Malek Ashtar University of Technology, P.O. Box 15875-1774, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 10 April 2017 Received in revised form 5 June 2017 Accepted 11 June 2017 Available online 16 June 2017 Keywords: Hydrothermal synthesis Carbonyl iron-carbon nanocomposite Microwave absorption Reflection loss
a b s t r a c t In this research, the electromagnetic absorption properties of the carbonyl iron-carbon (CI/C) nanocomposite prepared via hydrothermal reaction using glucose as carbon precursor was studied in the range of 8.2–12.4 GHz. In hydrothermal reaction, glucose solution containing CI particles, placed in autoclave for 4 h under 453 K. Using surface coating technology is a method that prevents Cl oxidation and improves CI electromagnetic absorption. The structure, morphology and magnetic performances of the prepared nanocomposites were characterized by X-ray diffraction (XRD), energy dispersive spectrometry (EDS), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The electromagnetic properties including complex permittivity (er), the permeability (mr), dielectric loss, magnetic loss, reflection loss, and attenuation constant were investigated using a vector network analyzer. For The CI/C nanocomposite, the bandwidth of 10 dB and 20 dB were obtained in the frequency range of 9.8–12.4 and 11.0–11.8 GHz, respectively. As well as, the reflection loss was 46.69 dB at the matching frequency of 11.5 GHz, when the matching thickness was 1.3 mm. While for CI particles the reflection loss for 4.4 mm thickness was 16.86 dB at the matching frequency of 12.3 GHz. The results indicate that the existence layer of carbon on carbonyl iron enhance the electromagnetic absorbing properties. Therefore, this nanocomposite can be suitable for in the radar absorbing coatings. Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Introduction Recently, due to many applications, attention has increased to electromagnetic (EM) wave technology. Although electrical equipments have made our lives more comfortable, continuous electromagnetic radiation causes stress to human society, because the electromagnetic pollution of the environment has many damages to humans. Hence, it is a necessary and fundamental assignment of development in science, engineering and technology for enhancing the research of EM protection materials, absorbers of EM waves and anti-EM innovations [1,2]. The absorber materials have absorption centers with the characteristics which can attenuate electromagnetic waves. These absorbing materials, in addition to the matching incident impedance with the inherent impedance of materials to penetrate into them, have electrical and magnetic absorption mechanisms for the loss of EM waves. Nevertheless, there are for the most part two sorts of absorbing materials including the dielectric loss and the magnetic loss according to the absorbing mechanisms. For the situation of a magnetic material,
⇑ Corresponding author.
attenuations are produced by changes in the arrangement and rotation of the magnetization spin. In the case of a dielectric material, dissipating is performed by dielectric particles involving ohmic losses [3,4]. Carbonyl iron (CI) as a kind of conventional magnetizable filler with the benefits of high saturation magnetization, high Curie temperature, low eddy-current loss due to the efficacy of particle shape, large amount of magnetic permeability and soft magnetic properties such as quite low hysteresis behavior had been concentrated on by numerous scientists. The saturation magnetization of the pure CI particles is about four times higher than that of iron oxides [5,6]. The conventional carbonous materials based on graphite had the EM absorption property due to the dielectric loss, their lower density and rich electric transport properties but their magnetic property is feeble [7]. Fabrication of nanocomposites having two fillers with dielectric and magnetic properties is much desirable and noteworthy. Because the dielectric–magnetic composite absorption materials is one of profitable approach to improve properties of electromagnetic absorption contrasted with single dielectric or magnetic material. Therefore, much attempt should be made to attain novel absorbent which dielectric-magnetic hybrid structure to adjust the
E-mail address: [email protected] (A.R. Zarei). http://dx.doi.org/10.1016/j.rinp.2017.06.012 2211-3797/Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
H. Pourabdollahi, A.R. Zarei / Results in Physics 7 (2017) 1978–1986
electromagnetic parameters for obtaining a desired reflection loss (RL) value. For example, some hybrid composites have been reported that show enhanced and improved electromagnetic absorption properties [8–10]. There are various procedures have been employed to prepare carbon coated magnetic particles and are synthesized these nanocomposites such as high temperature techniques including carbon arc discharge [11], flowing thermal plasma jet [12], magnetron and ion-beam co-sputtering [13], laser pyrolysis of organometallic compounds [14] and spraying methods [15], electron irradiation [16], modified arc deposition technique [17], multistep preparation methods by using carbon deposition (from the catalytic decomposition of gaseous carbon sources) onto alumina-supported metal particles and then removing alumina [18], high-temperature annealing of the mixtures of carbonbased materials and metal precursors, catalytic chemical vapor deposition, pyrolysis of organometallic compounds, catalytic decomposition of methane [19]. However, these techniques require complex equipment and are generally costly. In recent years, much endeavor was put to create strategies that high temperature reaction has been utilized to carbon coating on the magnetic particles in which variety of magnetic materials have been used [20]. In addition, the utilizing of CI is frequently limited by the oxidation sensitivity due to considerable oxidation of the iron particles during operation, which has declined the magnetic properties of CI particles. Thus, utilizing surface coating technology is a valuable way which can lead increment of the yield of electromagnetic absorption [21]. There is no report about the preparation of carbon coated carbonyl iron (CI) by hydrothermal reaction. In this work, hydrothermal reaction has been used to synthesis carbonyl iron/carbon nanocomposite and glucose was chosen as carbon source for formation of carbon layers on carbonyl iron. The structural and electromagnetic characteristics of the prepared nanocomposite were evaluated.
Experimental
1979
Preparation of carbon coated carbonyl iron particles The CI/C nanocomposite were synthesized by a simple hydrothermal reaction, In the typical reaction, the first, 0.40 g of carbonyl iron particles was sonicated and rinsed with deionized water and ethanol for several times in sequence, then dispersed into 80 mL glucose solution (0.5 M), and the mixture was sonicated for 20 min. The reactant mixture placed in a 50 ml teflon-sealed autoclave and was deoxygenated beforehand under the flowing nitrogen. The autoclave was heated for 4 h at 453 K. Finally, the products isolated from the mixture by the magnetic field were cleaned by three cycles of washing in water and in alcohol. Then the sample was dried in vacuum oven. Preparation of sample for electromagnetic wave analyzes and sample measurement For compare to electromagnetic parameters of samples, the specimens were prepared by mixing CI and CI/C nanocomposite with paraffin wax at a ratio of 3:1. The particles were homogeneously dispersed in paraffin and pressed into rectangle molds with the size of 10.16 mm 22.86 mm 2.5 mm for electromagnetic properties test. The scattering parameters of samples were measured in the 8.2–12.4 GHz range by using vector network analyzer and then permittivity, permeability and reflection loss (RL) of samples were calculated using Agilent 8510C materials measurement software. Determination of complex scattering parameters including the reflection coefficient (S11 or S22), and transmission coefficient (S21 orS12) enables the calculation of the complex permittivity (er) and the permeability (mr) could be deduced using the Nicolson–Ross– Weir algorithm [22]:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 k 1
ð1Þ
k ¼ ½ðs211 s221 Þ þ 1=ð2s11 Þ
ð2Þ
T ¼ ½S11 þ S21 C0 =½1 ðS11 þ S21 ÞC0
ð3Þ
C0 ¼ k
Reagents and solutions Carbonyl iron particles consisting of >97% of iron particles produced by BASF (Germany), glucose (C6H12O6) and ethanol with analytical grade were purchased from Merck (Darmstadt, Germany) and deionized water (DI) was used throughout the study.
lr ¼
1 þ C0 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Kð1 C0 Þ 1=k20 1=k2c
ð4Þ
er ¼
1=K2 þ 1=k2c Þk20 lr
ð5Þ
2
Instruments
K2 ¼ ½lnðTÞ=2pd
The phase of powders was estimated by X-ray diffraction (XRD) patterns were recorded by PAN analytical X’ Pert Pro. X-ray diffractometer using Cu Ka radiation (k = 1.54 Å). The energy dispersive spectrometry (EDS) analysis was studied by Oxford Instruments. The Characterization and particle size of the sample were studied using a transmission electron microscope (TEM) model of EM10C (Zeiss, Germany) operating at 100 kV accelerated voltage. The magnetic properties were measured by using a vibrating sample magnetometer (VSM, TM-XYZTB-SIH). The scattering parameters [the reflection coefficient (S11, S22 parameters) and transmission coefficient (S21, S12 parameters)] were measured by an Agilent 8510C network analyzer in the range of 8.2–12.4 GHz. The complex permittivity, complex permeability and reflection loss (RL) of carbonyl iron/carbon nanocomposite–paraffin wax composites were calculated from the S11 and S21 parameters by Agilent 8510C materials measurement software using the Nicolson–Ross model.
e0 1 ¼ ½V C =V S ½ðf C f S Þ=2fs
ð7Þ
e00 ¼ ½V C =4V S ½1=Q S 1=Q C
ð8Þ
l0 1 ¼ ½V C =kV S ½ðf C f S Þ=f S
ð9Þ
l00 ¼ ½V C =2kV S ½1=Q S 1=Q C
ð10Þ
ð6Þ
where C0 is the reflection coefficient of the material surface, T was the transition coefficient in the materials, k0 is the microwave wavelength in the air, and d is the thickness of the absorbing material. Also Vc and Vs are the volumes of the empty cavity and the sample, fc and fs are the resonance frequencies of the empty cavity and the sample, Qc and Qs are the quality factor for the empty cavity and the perturbed case, k is the geometrical parameter.
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Results and discussion Characterization of nanocomposite structure The typical XRD patterns of prepared sample were shown in Fig. 1. As canbe seen, three main diffraction peaks associated with the (1 1 0), (2 0 0) and (2 1 1) planes of cubic crystalline structure of iron are located at 2h = 44.6°, 64.9° and 82.3°. Also There was no found of diffraction peak of carbon (2h = 26.40°), which was shown that the carbon was amorphous at the condition using our method and the crystal phase of CI particles is not changed during coating process. The peaks broadening can be attributed to amorphous carbon. Besides, no other peaks of impurities or oxides are detected. The CI particles are almost without oxidation, due to the protection layers of carbon. The existence of carbon layer can be confirmed by a combination of EDS spectrum in Fig. 2. The EDS spectrum indicates the existence of carbon element on carbonyl iron particles surface after hydrothermal reaction, which proves the successful coating process. In the EDS spectrum of related sample, C and Fe elements are identified. Combining the results from XRD and EDS, the nanocomposite of CI/C, which did not have any other phases, was formed during hydrothermal reaction. Also, observed peaks at 1.5–2 keV and 2–2.5 keV are correspond to Si and Au elements, respectively. Fig. 3 gives the TEM images of the obtained nanocomposite, and some dark particles quasi- spherical in shape with the mean size of about 57 nm dispersed in the light carbon layer with the average thickness is about 34 nm that determined by statistical analysis of about 50 TEM images are observed. Such contrast is clearly demonstrated in Fig. 3.The present of bright-field around a darkspot in the TEM image indicate that the particles encapsulated in the carbon. In other hand, the carbon shell can be observed as a light area surrounding dark core of CI. There are some small particles with high contrast in the nanocomposite, suggesting that the CI nanoparticles coated with C layers and the growth of carbon composites have a role as the passivation shell. Based on the TEM characterization, most of the CI particles (metallic cores) were coated by carbon layers. According to the literature, the glucose is firstly polymerized into polysaccharides by the intermolecular dehydration and then carbonized into the formation [23,24]. If CI particles as the nuclei pre-exist in the solution, such process will prefer to occur on their surfaces, which results in the formation
of composite with the structure of CI/C nanocomposite. From Fig. 3, it was clearly observed that a carbon layer adhered around the surface of carbonyl iron particles, which shown that most of the carbonyl iron grains were coated with carbon particles. Moreover, the layer of C coated on CI could protect the CI from oxidation. The magnetic properties of nanocomposite The magnetic properties CI particles and the CI/C nanocomposite were analyzed by VSM. Fig. 4 shows the magnetization curves of the samples. It can be seen that after the hydrothermal reaction. The saturation magnetization of CI particles decreases from 225 to 131 emu/g for CI coated by carbon. As expected CI/C nanocomposite has lower Ms than CI particles which due to the presence of the non-magnetic carbon layers with diamagnetic properties around CI particles. The CI particles and the CI/C nanocomposite represent coercivities (Hc) of 59 and 166 (Oe), respectively. The coercive force of CI particles coated by carbon was larger. It indicated that the carbon was successfully coated on CI surface. Therefore, the hydrothermal reaction, and followed by, coating of most of the CI cores by carbon layers has been increasing coercive force. Complex permittivity and dielectric loss The complex permittivity of the CI and CI/C nanocomposite are presented in Fig. 5a. The variation in the real component (e0 ) of complex permittivity for CI particles and CI/C nanocomposite are shown in Fig. 5a and b in which the real component (e0 ) of the CI and CI/C nanocomposite ranges from 8.24 to 9.64 and 8.43 to 11.03, respectively. Also the imaginary component (e00 ) of the CI and CI/C nanocomposite ranges from 0.43 to 2.06 and 1.48 to 6.73, respectively. The carbon often had a good dielectric loss property, and the carbon might have the interaction effect with the CI, as a result, it could improve the permittivity. The dielectric loss presents different loss mechanisms. The higher complex permittivity of CI/C nanocomposite compared with CI is due to the hydrothermal reaction. The increase of the real part of permittivity (e0 ) is attributed to increase in the interface polarization of space charges, taking place at the interfaces of metals and carbon and as well as an insulator (paraffin), which occurs between components with large surface area would enhance the polarization process [25]. Furthermore, the increasing of the imaginary part of
Fig. 1. The XRD patterns of CI and the CI/C nanocomposite.
H. Pourabdollahi, A.R. Zarei / Results in Physics 7 (2017) 1978–1986
1981
Fig. 2. The EDS spectrum of the CI/C nanocomposite.
Fig. 3. The TEM image of the CI/C nanocomposite.
permittivity (e00 ) of CI/C nanocomposite compared with CI is due to increase in relaxation polarization loss and the conductance loss. According to the free electron theory [26], e00 = r/2pe0f, where r is the resistivity, thus the e00 increases with a decrease in the electrical resistivity. With the frequency changing, a strong vortex is generated due to that polarization can’t keep up with the change of electric field under the electromagnetic wave and the eddy currents will release a lot of heat in internal flow, so the vortex causes a portion of incident electromagnetic wave energy consumption [27]. At the same time, the complex permittivity of CI/C nanocomposite exhibits fluctuations compared with CI, which is ascribed to displacement current lag at the structure interface, which is similar to same experimental [26]. In order to further understand the dielectric response of the particles, the dielectric loss tangent analysis [28] and the Debye
equation analysis [29], were used. In Fig. 5b The dielectric loss factor (tan(de) = e00 /e0 ) which major contribution for electromagnetic absorption for the CI and CI/C nanocomposite are plotted as a function of frequency. There are more tan(de) peaks at for CI/C nanocomposite than tan(de) peaks of CI which due to the relaxation process. The average values of tan(de) of the nanocomposite is higher than that of the CI, ascribed to the dual dielectric resonance induced by carbon layers. This parameter shows greater ability of the absorber to convert the EM energy into heat. The interfacial electric polarization due to the multi-interfaces, the multi-interfaces between carbon layer and CI core, carbon layer and paraffin can be benefit for the electromagnetic wave absorption by improve the dielectric loss. The Debye equation was important and effective for the dielectric property. According to the Debye dipolar relaxation expression
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Fig. 4. The hysteresis loops of the CI/C nanocomposite, (a) coercive force of CI and the CI/C nanocomposite, (b) saturation magnetization of CI and the CI/C nanocomposite.
er ¼ e1 þ
es e1 ¼ e0 ðf Þ þ ie00 ðf Þ 1 þ ixs
ð11Þ
where x = 2pf in which f is the frequency and es and e1 are stationary and infinite frequency dielectric constants, x is the angular frequency, and s is the relaxation time of dipoles, respectively. The plot of e00 versus e0 would be a single semicircle, which is usually defined as the Cole-Cole semicircle [30]. The dielectric relaxation mechanisms are described by Cole–Cole plots. The nanocomposite containing CI/C shows two segments of semicircles in Fig. 5c, suggesting the existence of dual dielectric relaxation processes, while each semicircle related to a Debye dipolar relaxation. In general, during the alternation of the applied field (or the activation of an EM wave), a redistribution process of the charges occurs periodically between CI cores and C layers, Actually, the configuration and internal fractal structure is proportional to the quantity of charge stored on the surface [31,32]. Therefore, the dielectric relaxation of carbon shells, an additional interfacial relaxation between CI and C is also produced because of the presence of interfaces at structure [31]. From this point of view, the higher dielectric constant can obtain in the coatings with the carbons. There are actually several types of polarization with different relaxation times due to the dielectric response to some different degrees. The combined dual dielectric losses in CI/C nanocomposite are the origins of the enhanced EM absorption abilities. Complex permeability and magnetic loss The complex permeability of the CI and CI/C nanocomposite are shown in Fig. 6a. The real component (l0 ) of the CI and CI/C nanocomposite ranges from 1.95 to 2.10 and 2.33 to 2.57, respectively. Also, the imaginary component (l00 ) of the CI and CI/C nanocomposite ranges from 0.21 to 0.44 and 0.40 to 0.69, respec-
tively. The l0 and l00 values for CI are attributed the domain-wall motion and relaxation [33]. But the absolute values of l0 and l00 for CI/C nanocomposite are higher than that of CI nanoparticles which could be ascribed to the protection of carbon layers on the magnetic properties. The magnetic loss mainly consists of natural resonance, domain wall displacement, magnetic hysteresis and eddy current loss. Fig. 6b shows the magnetic loss factor (tan(dl) = l00 /l0 ) which major contribution for electromagnetic absorption. The tan(dl) of CI/C nanocomposite is bigger than that of CI particles at the frequency range of 8.2–12.4 GHz, due to the natural resonance. In fact, higher magnetism (or magnetic property) leads to the greater dissipating ability of EM waves to be converted to the heat. Meanwhile, as depicted in Fig. 6c, the values of l00 /(m0 )2/f decrease with increasing the frequency. Therefore, it means that the magnetic loss is mainly caused by the natural resonance [34]. If eddy current loss causes the magnetic loss, the values of l00 /(m0 )2/f should be constant when frequency is varied (in frequency variations) [35]. Measurement of electromagnetic wave absorption The reflection-loss (RL) of the absorber was calculated using the relative permeability lr and the relative permittivity er at a given frequency and absorber thickness by means of the following expressions [36]:
rffiffiffiffiffi
Z in ¼
lr j2pfd pffiffiffiffiffiffiffiffiffi lr er tanh c er
ð12Þ
rffiffiffiffiffiffi Z0 ¼
l0 e0
ð13Þ
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(a)
(a) 12
3
(ε′CI/C) (μ′CI/C )
10
2.5
(ε′CI) Complex permeability
Complex permittivity
(μ′CI)
8
6
4
(ε″CI/C)
2
(εε″CI)
2
1.5
1
(μ″CI/C) 0.5
(μ μ″CI) 0 8.2
0 8.2
(b)
8.9
9.6
10.3 11 Frequency (GHz)
11.7
8.9
9.6
12.4
10.3 11 Frequency (GHz)
11.7
12.4
(b) 0.5
1
CI/C CI/C
CI
CI
Magnetic loss factor
Dielectric loss factor
0.8
0.4
0.6
0.4
0.3
0.2
0.1
0.2
0
0 8.2
8.9
9.6
10.3
11
11.7
12.4
8.2
8.9
9.6
Frequency (GHz)
(c)
(c)
8
10.3 11 Frequency (GHz)
11.7
12.4
0.02
CI/C CI
CI 0.015
CI/C
μ″/(μ′) 2/f
Imaginary permittivity
6
4
0.01
2
0.005
0
0 8
9
10
11
12
8.2
8.9
9.6
10.3 11 Frequency (GHz)
11.7
12.4
Real permittivity Fig. 5. (a) Frequency dependence of complex permittivity of CI and the CI/C nanocomposite, (b) Frequency dependence of dielectric loss of CI and the CI/C nanocomposite, and (c) Frequency dependence of typical Cole–Cole plots of CI and the CI/C nanocomposite.
Fig. 6. (a) Frequency dependence of complex permeability of CI and the CI/C nanocomposite, (b) Frequency dependence of magnetic loss of CI and the CI/C nanocomposite, and (c) Frequency dependence of value l00 /(m0 )2/f of CI and the CI/C nanocomposite.
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Z in Z 0 RL ¼ 20log Z in þ Z 0
ð14Þ
where Zin is the input impedance of the absorber, Z0 is the impedance of free space, er and mr are the relative complex permeability and permittivity, respectively, f is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of light in free space. The thickness of the sample is one of major factors affecting both the intensity of the RL peak and the value of the frequency at the RL minimum. To further investigate the electromagnetic wave absorption properties, the calculated reflection loss (RL) of the CI and CI/C nanocomposite are as function of frequency were shown in Fig. 7. The bandwidth of RL below 10 dB, below 20 dB and their corresponding thickness were the most important parameters for evaluating the performance of electromagnetic absorber. Therefore, based on the different EM properties of the CI and CI/C nanocomposite absorber, the electromagnetic absorber with both wide bandwidth absorption (RL below 10 dB and 20 dB), and minimum thickness were designed and fabricated. It can be seen, for CI particles and CI/C nanocomposite, all the absorption peaks shift to low frequency direction with increasing the thickness in the 8.2–12.4 GHz range. The optimal reflection loss and the bandwidth below 5 dB, 10 dB (90% power absorption) and 20 dB (99% power absorption) are summarized in Table 1. As seen from the Table 1, for CI particles, the band width with RL more than 10 dB reached approximately more than 2 GHz at the various matching thicknesses. It can be seen that a maximum reflection loss value of 16.86 dB was observed at 12.3 GHz with a matching thickness of 4.4 mm and there was no RL values below 20 dB. Also for CI/ C nanocomposite the bandwidth in which 90% of the incident wave is absorbed (<10 dB) is maximum for 1.5 thickness which has the frequency range of 8.7–12.4 GHz. also the bandwidths of RL <10 are for thickness of 1.3 and 1.4 mm are in the frequency range of 9.8–12.4 GHz and 9.2–12.4 GHz respectively which reached approximately more than 3 GHz. The bandwidths of RL <20 equal to more than 99% absorption of the incident wave are for thickness
0 -5
8.2
8.7
9.2
9.7
of 1.3, 1.4, and 1.5 mm can be obtained in the frequency range of 11.0–11.8 GHz, 10.6–11.3 GHz and 10.2–10.7 GHz respectively. As shown in Fig. 7b for CI/C nanocomposite, the minimum reflection loss increases from 21.34 to 46.69 dB with decrease in the thickness from 1.5 mm to 1.3 mm. The minimum RL of the CI/C nanocomposite was mainly due to the matching thickness effect, while the matching thickness effect of the CI was much weaker. As discussed above, the coating with carbon layer had a positive effect on the absorbent and could increase the permittivity of the composites. It could enhance the microwave absorbing property of nanocomposites. As shown in Fig. 7, the effect of carbon on the electromagnetic wave absorption properties is clearly seen from the figure, for CI/ C nanocomposite, the maximum RL of about 46.69 dB is obtained in the matching frequency 11.5 GHz for 1.3 mm thickness. Thus, it can be concluded by integrating the two absorbent in the CI/C nanocomposite, the bandwidth absorption (RL below 10 dB and 20 dB) and optimal reflection loss both increase and thickness decrease. As we know, the design of EM waves absorbing materials with low reflection requires two important conditions [37]: impedance matching characteristic and attenuation characteristic. The impedance matching degree is determined by the combination of the six parameters, e00 , e0 , l00 , l0 , d, and w, only the optimal combination of the six parameters, the impedance matching between the surface of the electromagnetic absorbing materials and the air could be achieved to make the RL approach 0. The results indicate that the incorporation of layer of carbon into the structure of CI particles enhances the complex permittivity, permeability and loss tangents of the material and improves the microwave absorption properties in the 8.2 12.4 GHz range is attributed to the good impedance match characteristics and thus shows high electromagnetic absorption with thin thickness in the X band. In addition, EM-wave attenuation in the interior of absorber is one of key factors for an excellent absorber. The attenuation constant a, which determines the attenuation properties of materials, can be determined as:
10.2
10.7
11.2
Reflection loss (dB)
-10 -15 -20 -25 -30 -35 -40
CI/C 1.3mm CI/C 1.4mm CI/C 1.5mm CI 4.4mm CI 4.6mm CI 4.8mm
-45 -50
Frequency (HGz) Fig. 7. The calculated reflection loss (RL) of CI and the CI/C nanocomposite.
11.7
12.2
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H. Pourabdollahi, A.R. Zarei / Results in Physics 7 (2017) 1978–1986 Table 1 Optimal RL, matching frequency and effective absorption bandwidths for various thicknesses of CI particles and CI/C nanocomposite. Sample
Matching thickness (mm)
Matching frequency (GHz)
Optimal RL (dB)
Bandwidth (GHz) (RL <5 dB)
Bandwidth (GHz) (RL <10 dB)
Bandwidth (GHz) (RL <20 dB)
CI
4.4 4.6 4.8 1.3 1.4 1.5
12.3 11.8 11.1 11.5 11.0 10.3
16.86 16.08 16.05 46.69 25.21 21.34
8.8–12.4 8.6–12.4 8.4–12.4 8.7–12.4 8.2–12.4 8.2–12.4
10.8–12.4 10.3–12.2 9.7–10.8 9.8–12.4 9.2–12.4 8.7–12.4
– – – 11.0–11.8 10.6–11.3 10.2–10.7
CI/C
Table 2 Comparison of electromagnetic parameters of the proposed method with some of the reported in literature. Sample
Matching thickness (mm)
Matching frequency (GHz)
Optimal RL (dB)
Bandwidth (GHz) (RL <10 dB)
References
Barium ferrite/CNT Strontium ferrite/CNT Carbonyl iron-graphite Graphite-coated Fe SiO2-coated carbonyl iron/polyimide Carbonyl iron/epoxy silicone Co coated carbonyl iron Carbonyl iron/Tic BaCe0.2Fe11.8O19/CNTs nanocomposite
3 1.5 8 2.5 2.1 2 1.9 1.8 1.3
10.5 9.7 10.0 10.8 9.1 10.6 10.3 10.1 11.5
30.79 29 26.52 33.1 33 42.5 51 41.30 46.69
4 3 2.5 2 2 4 3 4 4
[39] [40] [41] [42] [43] [44] [45] [46] In this research
a¼
pffiffiffi 2p f c rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðl00 e00 l0 e0 Þ þ ðl00 e00 l0 e0 Þ2 þ ðe0 l00 þ e00 l0 Þ2
erties are attributed to the improved magnetic/dielectric loss and attenuation constant by carbon layer shells.
ð15Þ Conclusion
where f is the frequency of the EM-wave and c is the velocity of light [38]. Fig. 8 shows the relation between the attenuation constant and frequency. It is evident from the fig that the present investigated materials exhibit the maximum value of the attenuation constant at higher frequency. In addition, as seen in Fig. 8, the frequency dependence of a for the CI/C nanocomposite has bigger a at 8.2–12.4 GH, indicating the excellent attenuation or EM-wave absorption at 8.2–12.4 GHz. From above equation, higher values of l00 would result in higher a. Enhanced EM-wave absorption prop-
Electromagnetic wave absorbing nanocomposite composed of CI and C as absorbent was fabricated by hydrothermal reaction. The effect of carbon layers on the magnetic and EMcharacteristic of CI particles have been investigated in detail. It was revealed from VSM graphs that existing carbon in nanocomposite cause of reduction of saturation magnetization from 225 to 131emu/g due to diamagnetic properties of carbon. For CI particles, the bandwidth with RL more than 10 dB can be reached in the frequency range of 10.8–12.4 GHz. The optimal reflection loss for 4.4 mm matching thickness was 16.86 dB at the matching frequency of 12.3 GHz. For the CI/C nanocomposite, the optimal RL was 46.69 dB in the frequency of 11.5 GHz, when the thickness is 1.3 mm. Also the bandwidth in 90% power absorption and 99% power absorption can be obtained in the frequency range of 9.8– 12.4 GHz and 11.0–11.8 GHz respectively. The measured RL results show the CI/C nanocomposite can be increased bandwidth of the RL below 10 dB and 20 dB and optimal reflection loss in thin thickness. Due to the existence layer of C coated on CI, the absorbing properties were improved by integrating the dielectric absorbent and magnetic absorbent. The electromagnetic parameters of CI/C nanocomposite is better or comparable to some of the previously reported techniques. A comparison of the results is given in Table 2. As reported in table 2 (comparative table), in this work, electromagnetic parameters like bandwidth and thickness were improved with dielectric-magnetic hybrid nanostructure. Thus, it is helpful for enhancement of the magnetic/dielectric loss, optimal reflection loss and attenuation constant, which makes excesses EM-wave absorption properties in the range of 8.2–12.4 GHz.
References
Fig. 8. The attenuation constant of CI and CI/C nanocomposite.
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Results in Physics journal homepage: www.journals.elsevier.com/results-in-physics
Hydrothermal synthesis of carbonyl iron-carbon nanocomposite: Characterization and electromagnetic performance Hakimeh Pourabdollahi, Ali Reza Zarei ⇑ Faculty of Chemistry and Chemical Engineering, Malek Ashtar University of Technology, P.O. Box 15875-1774, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 10 April 2017 Received in revised form 5 June 2017 Accepted 11 June 2017 Available online 16 June 2017 Keywords: Hydrothermal synthesis Carbonyl iron-carbon nanocomposite Microwave absorption Reflection loss
a b s t r a c t In this research, the electromagnetic absorption properties of the carbonyl iron-carbon (CI/C) nanocomposite prepared via hydrothermal reaction using glucose as carbon precursor was studied in the range of 8.2–12.4 GHz. In hydrothermal reaction, glucose solution containing CI particles, placed in autoclave for 4 h under 453 K. Using surface coating technology is a method that prevents Cl oxidation and improves CI electromagnetic absorption. The structure, morphology and magnetic performances of the prepared nanocomposites were characterized by X-ray diffraction (XRD), energy dispersive spectrometry (EDS), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The electromagnetic properties including complex permittivity (er), the permeability (mr), dielectric loss, magnetic loss, reflection loss, and attenuation constant were investigated using a vector network analyzer. For The CI/C nanocomposite, the bandwidth of 10 dB and 20 dB were obtained in the frequency range of 9.8–12.4 and 11.0–11.8 GHz, respectively. As well as, the reflection loss was 46.69 dB at the matching frequency of 11.5 GHz, when the matching thickness was 1.3 mm. While for CI particles the reflection loss for 4.4 mm thickness was 16.86 dB at the matching frequency of 12.3 GHz. The results indicate that the existence layer of carbon on carbonyl iron enhance the electromagnetic absorbing properties. Therefore, this nanocomposite can be suitable for in the radar absorbing coatings. Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Introduction Recently, due to many applications, attention has increased to electromagnetic (EM) wave technology. Although electrical equipments have made our lives more comfortable, continuous electromagnetic radiation causes stress to human society, because the electromagnetic pollution of the environment has many damages to humans. Hence, it is a necessary and fundamental assignment of development in science, engineering and technology for enhancing the research of EM protection materials, absorbers of EM waves and anti-EM innovations [1,2]. The absorber materials have absorption centers with the characteristics which can attenuate electromagnetic waves. These absorbing materials, in addition to the matching incident impedance with the inherent impedance of materials to penetrate into them, have electrical and magnetic absorption mechanisms for the loss of EM waves. Nevertheless, there are for the most part two sorts of absorbing materials including the dielectric loss and the magnetic loss according to the absorbing mechanisms. For the situation of a magnetic material,
⇑ Corresponding author.
attenuations are produced by changes in the arrangement and rotation of the magnetization spin. In the case of a dielectric material, dissipating is performed by dielectric particles involving ohmic losses [3,4]. Carbonyl iron (CI) as a kind of conventional magnetizable filler with the benefits of high saturation magnetization, high Curie temperature, low eddy-current loss due to the efficacy of particle shape, large amount of magnetic permeability and soft magnetic properties such as quite low hysteresis behavior had been concentrated on by numerous scientists. The saturation magnetization of the pure CI particles is about four times higher than that of iron oxides [5,6]. The conventional carbonous materials based on graphite had the EM absorption property due to the dielectric loss, their lower density and rich electric transport properties but their magnetic property is feeble [7]. Fabrication of nanocomposites having two fillers with dielectric and magnetic properties is much desirable and noteworthy. Because the dielectric–magnetic composite absorption materials is one of profitable approach to improve properties of electromagnetic absorption contrasted with single dielectric or magnetic material. Therefore, much attempt should be made to attain novel absorbent which dielectric-magnetic hybrid structure to adjust the
E-mail address: [email protected] (A.R. Zarei). http://dx.doi.org/10.1016/j.rinp.2017.06.012 2211-3797/Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
H. Pourabdollahi, A.R. Zarei / Results in Physics 7 (2017) 1978–1986
electromagnetic parameters for obtaining a desired reflection loss (RL) value. For example, some hybrid composites have been reported that show enhanced and improved electromagnetic absorption properties [8–10]. There are various procedures have been employed to prepare carbon coated magnetic particles and are synthesized these nanocomposites such as high temperature techniques including carbon arc discharge [11], flowing thermal plasma jet [12], magnetron and ion-beam co-sputtering [13], laser pyrolysis of organometallic compounds [14] and spraying methods [15], electron irradiation [16], modified arc deposition technique [17], multistep preparation methods by using carbon deposition (from the catalytic decomposition of gaseous carbon sources) onto alumina-supported metal particles and then removing alumina [18], high-temperature annealing of the mixtures of carbonbased materials and metal precursors, catalytic chemical vapor deposition, pyrolysis of organometallic compounds, catalytic decomposition of methane [19]. However, these techniques require complex equipment and are generally costly. In recent years, much endeavor was put to create strategies that high temperature reaction has been utilized to carbon coating on the magnetic particles in which variety of magnetic materials have been used [20]. In addition, the utilizing of CI is frequently limited by the oxidation sensitivity due to considerable oxidation of the iron particles during operation, which has declined the magnetic properties of CI particles. Thus, utilizing surface coating technology is a valuable way which can lead increment of the yield of electromagnetic absorption [21]. There is no report about the preparation of carbon coated carbonyl iron (CI) by hydrothermal reaction. In this work, hydrothermal reaction has been used to synthesis carbonyl iron/carbon nanocomposite and glucose was chosen as carbon source for formation of carbon layers on carbonyl iron. The structural and electromagnetic characteristics of the prepared nanocomposite were evaluated.
Experimental
1979
Preparation of carbon coated carbonyl iron particles The CI/C nanocomposite were synthesized by a simple hydrothermal reaction, In the typical reaction, the first, 0.40 g of carbonyl iron particles was sonicated and rinsed with deionized water and ethanol for several times in sequence, then dispersed into 80 mL glucose solution (0.5 M), and the mixture was sonicated for 20 min. The reactant mixture placed in a 50 ml teflon-sealed autoclave and was deoxygenated beforehand under the flowing nitrogen. The autoclave was heated for 4 h at 453 K. Finally, the products isolated from the mixture by the magnetic field were cleaned by three cycles of washing in water and in alcohol. Then the sample was dried in vacuum oven. Preparation of sample for electromagnetic wave analyzes and sample measurement For compare to electromagnetic parameters of samples, the specimens were prepared by mixing CI and CI/C nanocomposite with paraffin wax at a ratio of 3:1. The particles were homogeneously dispersed in paraffin and pressed into rectangle molds with the size of 10.16 mm 22.86 mm 2.5 mm for electromagnetic properties test. The scattering parameters of samples were measured in the 8.2–12.4 GHz range by using vector network analyzer and then permittivity, permeability and reflection loss (RL) of samples were calculated using Agilent 8510C materials measurement software. Determination of complex scattering parameters including the reflection coefficient (S11 or S22), and transmission coefficient (S21 orS12) enables the calculation of the complex permittivity (er) and the permeability (mr) could be deduced using the Nicolson–Ross– Weir algorithm [22]:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 k 1
ð1Þ
k ¼ ½ðs211 s221 Þ þ 1=ð2s11 Þ
ð2Þ
T ¼ ½S11 þ S21 C0 =½1 ðS11 þ S21 ÞC0
ð3Þ
C0 ¼ k
Reagents and solutions Carbonyl iron particles consisting of >97% of iron particles produced by BASF (Germany), glucose (C6H12O6) and ethanol with analytical grade were purchased from Merck (Darmstadt, Germany) and deionized water (DI) was used throughout the study.
lr ¼
1 þ C0 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Kð1 C0 Þ 1=k20 1=k2c
ð4Þ
er ¼
1=K2 þ 1=k2c Þk20 lr
ð5Þ
2
Instruments
K2 ¼ ½lnðTÞ=2pd
The phase of powders was estimated by X-ray diffraction (XRD) patterns were recorded by PAN analytical X’ Pert Pro. X-ray diffractometer using Cu Ka radiation (k = 1.54 Å). The energy dispersive spectrometry (EDS) analysis was studied by Oxford Instruments. The Characterization and particle size of the sample were studied using a transmission electron microscope (TEM) model of EM10C (Zeiss, Germany) operating at 100 kV accelerated voltage. The magnetic properties were measured by using a vibrating sample magnetometer (VSM, TM-XYZTB-SIH). The scattering parameters [the reflection coefficient (S11, S22 parameters) and transmission coefficient (S21, S12 parameters)] were measured by an Agilent 8510C network analyzer in the range of 8.2–12.4 GHz. The complex permittivity, complex permeability and reflection loss (RL) of carbonyl iron/carbon nanocomposite–paraffin wax composites were calculated from the S11 and S21 parameters by Agilent 8510C materials measurement software using the Nicolson–Ross model.
e0 1 ¼ ½V C =V S ½ðf C f S Þ=2fs
ð7Þ
e00 ¼ ½V C =4V S ½1=Q S 1=Q C
ð8Þ
l0 1 ¼ ½V C =kV S ½ðf C f S Þ=f S
ð9Þ
l00 ¼ ½V C =2kV S ½1=Q S 1=Q C
ð10Þ
ð6Þ
where C0 is the reflection coefficient of the material surface, T was the transition coefficient in the materials, k0 is the microwave wavelength in the air, and d is the thickness of the absorbing material. Also Vc and Vs are the volumes of the empty cavity and the sample, fc and fs are the resonance frequencies of the empty cavity and the sample, Qc and Qs are the quality factor for the empty cavity and the perturbed case, k is the geometrical parameter.
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Results and discussion Characterization of nanocomposite structure The typical XRD patterns of prepared sample were shown in Fig. 1. As canbe seen, three main diffraction peaks associated with the (1 1 0), (2 0 0) and (2 1 1) planes of cubic crystalline structure of iron are located at 2h = 44.6°, 64.9° and 82.3°. Also There was no found of diffraction peak of carbon (2h = 26.40°), which was shown that the carbon was amorphous at the condition using our method and the crystal phase of CI particles is not changed during coating process. The peaks broadening can be attributed to amorphous carbon. Besides, no other peaks of impurities or oxides are detected. The CI particles are almost without oxidation, due to the protection layers of carbon. The existence of carbon layer can be confirmed by a combination of EDS spectrum in Fig. 2. The EDS spectrum indicates the existence of carbon element on carbonyl iron particles surface after hydrothermal reaction, which proves the successful coating process. In the EDS spectrum of related sample, C and Fe elements are identified. Combining the results from XRD and EDS, the nanocomposite of CI/C, which did not have any other phases, was formed during hydrothermal reaction. Also, observed peaks at 1.5–2 keV and 2–2.5 keV are correspond to Si and Au elements, respectively. Fig. 3 gives the TEM images of the obtained nanocomposite, and some dark particles quasi- spherical in shape with the mean size of about 57 nm dispersed in the light carbon layer with the average thickness is about 34 nm that determined by statistical analysis of about 50 TEM images are observed. Such contrast is clearly demonstrated in Fig. 3.The present of bright-field around a darkspot in the TEM image indicate that the particles encapsulated in the carbon. In other hand, the carbon shell can be observed as a light area surrounding dark core of CI. There are some small particles with high contrast in the nanocomposite, suggesting that the CI nanoparticles coated with C layers and the growth of carbon composites have a role as the passivation shell. Based on the TEM characterization, most of the CI particles (metallic cores) were coated by carbon layers. According to the literature, the glucose is firstly polymerized into polysaccharides by the intermolecular dehydration and then carbonized into the formation [23,24]. If CI particles as the nuclei pre-exist in the solution, such process will prefer to occur on their surfaces, which results in the formation
of composite with the structure of CI/C nanocomposite. From Fig. 3, it was clearly observed that a carbon layer adhered around the surface of carbonyl iron particles, which shown that most of the carbonyl iron grains were coated with carbon particles. Moreover, the layer of C coated on CI could protect the CI from oxidation. The magnetic properties of nanocomposite The magnetic properties CI particles and the CI/C nanocomposite were analyzed by VSM. Fig. 4 shows the magnetization curves of the samples. It can be seen that after the hydrothermal reaction. The saturation magnetization of CI particles decreases from 225 to 131 emu/g for CI coated by carbon. As expected CI/C nanocomposite has lower Ms than CI particles which due to the presence of the non-magnetic carbon layers with diamagnetic properties around CI particles. The CI particles and the CI/C nanocomposite represent coercivities (Hc) of 59 and 166 (Oe), respectively. The coercive force of CI particles coated by carbon was larger. It indicated that the carbon was successfully coated on CI surface. Therefore, the hydrothermal reaction, and followed by, coating of most of the CI cores by carbon layers has been increasing coercive force. Complex permittivity and dielectric loss The complex permittivity of the CI and CI/C nanocomposite are presented in Fig. 5a. The variation in the real component (e0 ) of complex permittivity for CI particles and CI/C nanocomposite are shown in Fig. 5a and b in which the real component (e0 ) of the CI and CI/C nanocomposite ranges from 8.24 to 9.64 and 8.43 to 11.03, respectively. Also the imaginary component (e00 ) of the CI and CI/C nanocomposite ranges from 0.43 to 2.06 and 1.48 to 6.73, respectively. The carbon often had a good dielectric loss property, and the carbon might have the interaction effect with the CI, as a result, it could improve the permittivity. The dielectric loss presents different loss mechanisms. The higher complex permittivity of CI/C nanocomposite compared with CI is due to the hydrothermal reaction. The increase of the real part of permittivity (e0 ) is attributed to increase in the interface polarization of space charges, taking place at the interfaces of metals and carbon and as well as an insulator (paraffin), which occurs between components with large surface area would enhance the polarization process [25]. Furthermore, the increasing of the imaginary part of
Fig. 1. The XRD patterns of CI and the CI/C nanocomposite.
H. Pourabdollahi, A.R. Zarei / Results in Physics 7 (2017) 1978–1986
1981
Fig. 2. The EDS spectrum of the CI/C nanocomposite.
Fig. 3. The TEM image of the CI/C nanocomposite.
permittivity (e00 ) of CI/C nanocomposite compared with CI is due to increase in relaxation polarization loss and the conductance loss. According to the free electron theory [26], e00 = r/2pe0f, where r is the resistivity, thus the e00 increases with a decrease in the electrical resistivity. With the frequency changing, a strong vortex is generated due to that polarization can’t keep up with the change of electric field under the electromagnetic wave and the eddy currents will release a lot of heat in internal flow, so the vortex causes a portion of incident electromagnetic wave energy consumption [27]. At the same time, the complex permittivity of CI/C nanocomposite exhibits fluctuations compared with CI, which is ascribed to displacement current lag at the structure interface, which is similar to same experimental [26]. In order to further understand the dielectric response of the particles, the dielectric loss tangent analysis [28] and the Debye
equation analysis [29], were used. In Fig. 5b The dielectric loss factor (tan(de) = e00 /e0 ) which major contribution for electromagnetic absorption for the CI and CI/C nanocomposite are plotted as a function of frequency. There are more tan(de) peaks at for CI/C nanocomposite than tan(de) peaks of CI which due to the relaxation process. The average values of tan(de) of the nanocomposite is higher than that of the CI, ascribed to the dual dielectric resonance induced by carbon layers. This parameter shows greater ability of the absorber to convert the EM energy into heat. The interfacial electric polarization due to the multi-interfaces, the multi-interfaces between carbon layer and CI core, carbon layer and paraffin can be benefit for the electromagnetic wave absorption by improve the dielectric loss. The Debye equation was important and effective for the dielectric property. According to the Debye dipolar relaxation expression
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Fig. 4. The hysteresis loops of the CI/C nanocomposite, (a) coercive force of CI and the CI/C nanocomposite, (b) saturation magnetization of CI and the CI/C nanocomposite.
er ¼ e1 þ
es e1 ¼ e0 ðf Þ þ ie00 ðf Þ 1 þ ixs
ð11Þ
where x = 2pf in which f is the frequency and es and e1 are stationary and infinite frequency dielectric constants, x is the angular frequency, and s is the relaxation time of dipoles, respectively. The plot of e00 versus e0 would be a single semicircle, which is usually defined as the Cole-Cole semicircle [30]. The dielectric relaxation mechanisms are described by Cole–Cole plots. The nanocomposite containing CI/C shows two segments of semicircles in Fig. 5c, suggesting the existence of dual dielectric relaxation processes, while each semicircle related to a Debye dipolar relaxation. In general, during the alternation of the applied field (or the activation of an EM wave), a redistribution process of the charges occurs periodically between CI cores and C layers, Actually, the configuration and internal fractal structure is proportional to the quantity of charge stored on the surface [31,32]. Therefore, the dielectric relaxation of carbon shells, an additional interfacial relaxation between CI and C is also produced because of the presence of interfaces at structure [31]. From this point of view, the higher dielectric constant can obtain in the coatings with the carbons. There are actually several types of polarization with different relaxation times due to the dielectric response to some different degrees. The combined dual dielectric losses in CI/C nanocomposite are the origins of the enhanced EM absorption abilities. Complex permeability and magnetic loss The complex permeability of the CI and CI/C nanocomposite are shown in Fig. 6a. The real component (l0 ) of the CI and CI/C nanocomposite ranges from 1.95 to 2.10 and 2.33 to 2.57, respectively. Also, the imaginary component (l00 ) of the CI and CI/C nanocomposite ranges from 0.21 to 0.44 and 0.40 to 0.69, respec-
tively. The l0 and l00 values for CI are attributed the domain-wall motion and relaxation [33]. But the absolute values of l0 and l00 for CI/C nanocomposite are higher than that of CI nanoparticles which could be ascribed to the protection of carbon layers on the magnetic properties. The magnetic loss mainly consists of natural resonance, domain wall displacement, magnetic hysteresis and eddy current loss. Fig. 6b shows the magnetic loss factor (tan(dl) = l00 /l0 ) which major contribution for electromagnetic absorption. The tan(dl) of CI/C nanocomposite is bigger than that of CI particles at the frequency range of 8.2–12.4 GHz, due to the natural resonance. In fact, higher magnetism (or magnetic property) leads to the greater dissipating ability of EM waves to be converted to the heat. Meanwhile, as depicted in Fig. 6c, the values of l00 /(m0 )2/f decrease with increasing the frequency. Therefore, it means that the magnetic loss is mainly caused by the natural resonance [34]. If eddy current loss causes the magnetic loss, the values of l00 /(m0 )2/f should be constant when frequency is varied (in frequency variations) [35]. Measurement of electromagnetic wave absorption The reflection-loss (RL) of the absorber was calculated using the relative permeability lr and the relative permittivity er at a given frequency and absorber thickness by means of the following expressions [36]:
rffiffiffiffiffi
Z in ¼
lr j2pfd pffiffiffiffiffiffiffiffiffi lr er tanh c er
ð12Þ
rffiffiffiffiffiffi Z0 ¼
l0 e0
ð13Þ
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(a)
(a) 12
3
(ε′CI/C) (μ′CI/C )
10
2.5
(ε′CI) Complex permeability
Complex permittivity
(μ′CI)
8
6
4
(ε″CI/C)
2
(εε″CI)
2
1.5
1
(μ″CI/C) 0.5
(μ μ″CI) 0 8.2
0 8.2
(b)
8.9
9.6
10.3 11 Frequency (GHz)
11.7
8.9
9.6
12.4
10.3 11 Frequency (GHz)
11.7
12.4
(b) 0.5
1
CI/C CI/C
CI
CI
Magnetic loss factor
Dielectric loss factor
0.8
0.4
0.6
0.4
0.3
0.2
0.1
0.2
0
0 8.2
8.9
9.6
10.3
11
11.7
12.4
8.2
8.9
9.6
Frequency (GHz)
(c)
(c)
8
10.3 11 Frequency (GHz)
11.7
12.4
0.02
CI/C CI
CI 0.015
CI/C
μ″/(μ′) 2/f
Imaginary permittivity
6
4
0.01
2
0.005
0
0 8
9
10
11
12
8.2
8.9
9.6
10.3 11 Frequency (GHz)
11.7
12.4
Real permittivity Fig. 5. (a) Frequency dependence of complex permittivity of CI and the CI/C nanocomposite, (b) Frequency dependence of dielectric loss of CI and the CI/C nanocomposite, and (c) Frequency dependence of typical Cole–Cole plots of CI and the CI/C nanocomposite.
Fig. 6. (a) Frequency dependence of complex permeability of CI and the CI/C nanocomposite, (b) Frequency dependence of magnetic loss of CI and the CI/C nanocomposite, and (c) Frequency dependence of value l00 /(m0 )2/f of CI and the CI/C nanocomposite.
1984
H. Pourabdollahi, A.R. Zarei / Results in Physics 7 (2017) 1978–1986
Z in Z 0 RL ¼ 20log Z in þ Z 0
ð14Þ
where Zin is the input impedance of the absorber, Z0 is the impedance of free space, er and mr are the relative complex permeability and permittivity, respectively, f is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of light in free space. The thickness of the sample is one of major factors affecting both the intensity of the RL peak and the value of the frequency at the RL minimum. To further investigate the electromagnetic wave absorption properties, the calculated reflection loss (RL) of the CI and CI/C nanocomposite are as function of frequency were shown in Fig. 7. The bandwidth of RL below 10 dB, below 20 dB and their corresponding thickness were the most important parameters for evaluating the performance of electromagnetic absorber. Therefore, based on the different EM properties of the CI and CI/C nanocomposite absorber, the electromagnetic absorber with both wide bandwidth absorption (RL below 10 dB and 20 dB), and minimum thickness were designed and fabricated. It can be seen, for CI particles and CI/C nanocomposite, all the absorption peaks shift to low frequency direction with increasing the thickness in the 8.2–12.4 GHz range. The optimal reflection loss and the bandwidth below 5 dB, 10 dB (90% power absorption) and 20 dB (99% power absorption) are summarized in Table 1. As seen from the Table 1, for CI particles, the band width with RL more than 10 dB reached approximately more than 2 GHz at the various matching thicknesses. It can be seen that a maximum reflection loss value of 16.86 dB was observed at 12.3 GHz with a matching thickness of 4.4 mm and there was no RL values below 20 dB. Also for CI/ C nanocomposite the bandwidth in which 90% of the incident wave is absorbed (<10 dB) is maximum for 1.5 thickness which has the frequency range of 8.7–12.4 GHz. also the bandwidths of RL <10 are for thickness of 1.3 and 1.4 mm are in the frequency range of 9.8–12.4 GHz and 9.2–12.4 GHz respectively which reached approximately more than 3 GHz. The bandwidths of RL <20 equal to more than 99% absorption of the incident wave are for thickness
0 -5
8.2
8.7
9.2
9.7
of 1.3, 1.4, and 1.5 mm can be obtained in the frequency range of 11.0–11.8 GHz, 10.6–11.3 GHz and 10.2–10.7 GHz respectively. As shown in Fig. 7b for CI/C nanocomposite, the minimum reflection loss increases from 21.34 to 46.69 dB with decrease in the thickness from 1.5 mm to 1.3 mm. The minimum RL of the CI/C nanocomposite was mainly due to the matching thickness effect, while the matching thickness effect of the CI was much weaker. As discussed above, the coating with carbon layer had a positive effect on the absorbent and could increase the permittivity of the composites. It could enhance the microwave absorbing property of nanocomposites. As shown in Fig. 7, the effect of carbon on the electromagnetic wave absorption properties is clearly seen from the figure, for CI/ C nanocomposite, the maximum RL of about 46.69 dB is obtained in the matching frequency 11.5 GHz for 1.3 mm thickness. Thus, it can be concluded by integrating the two absorbent in the CI/C nanocomposite, the bandwidth absorption (RL below 10 dB and 20 dB) and optimal reflection loss both increase and thickness decrease. As we know, the design of EM waves absorbing materials with low reflection requires two important conditions [37]: impedance matching characteristic and attenuation characteristic. The impedance matching degree is determined by the combination of the six parameters, e00 , e0 , l00 , l0 , d, and w, only the optimal combination of the six parameters, the impedance matching between the surface of the electromagnetic absorbing materials and the air could be achieved to make the RL approach 0. The results indicate that the incorporation of layer of carbon into the structure of CI particles enhances the complex permittivity, permeability and loss tangents of the material and improves the microwave absorption properties in the 8.2 12.4 GHz range is attributed to the good impedance match characteristics and thus shows high electromagnetic absorption with thin thickness in the X band. In addition, EM-wave attenuation in the interior of absorber is one of key factors for an excellent absorber. The attenuation constant a, which determines the attenuation properties of materials, can be determined as:
10.2
10.7
11.2
Reflection loss (dB)
-10 -15 -20 -25 -30 -35 -40
CI/C 1.3mm CI/C 1.4mm CI/C 1.5mm CI 4.4mm CI 4.6mm CI 4.8mm
-45 -50
Frequency (HGz) Fig. 7. The calculated reflection loss (RL) of CI and the CI/C nanocomposite.
11.7
12.2
1985
H. Pourabdollahi, A.R. Zarei / Results in Physics 7 (2017) 1978–1986 Table 1 Optimal RL, matching frequency and effective absorption bandwidths for various thicknesses of CI particles and CI/C nanocomposite. Sample
Matching thickness (mm)
Matching frequency (GHz)
Optimal RL (dB)
Bandwidth (GHz) (RL <5 dB)
Bandwidth (GHz) (RL <10 dB)
Bandwidth (GHz) (RL <20 dB)
CI
4.4 4.6 4.8 1.3 1.4 1.5
12.3 11.8 11.1 11.5 11.0 10.3
16.86 16.08 16.05 46.69 25.21 21.34
8.8–12.4 8.6–12.4 8.4–12.4 8.7–12.4 8.2–12.4 8.2–12.4
10.8–12.4 10.3–12.2 9.7–10.8 9.8–12.4 9.2–12.4 8.7–12.4
– – – 11.0–11.8 10.6–11.3 10.2–10.7
CI/C
Table 2 Comparison of electromagnetic parameters of the proposed method with some of the reported in literature. Sample
Matching thickness (mm)
Matching frequency (GHz)
Optimal RL (dB)
Bandwidth (GHz) (RL <10 dB)
References
Barium ferrite/CNT Strontium ferrite/CNT Carbonyl iron-graphite Graphite-coated Fe SiO2-coated carbonyl iron/polyimide Carbonyl iron/epoxy silicone Co coated carbonyl iron Carbonyl iron/Tic BaCe0.2Fe11.8O19/CNTs nanocomposite
3 1.5 8 2.5 2.1 2 1.9 1.8 1.3
10.5 9.7 10.0 10.8 9.1 10.6 10.3 10.1 11.5
30.79 29 26.52 33.1 33 42.5 51 41.30 46.69
4 3 2.5 2 2 4 3 4 4
[39] [40] [41] [42] [43] [44] [45] [46] In this research
a¼
pffiffiffi 2p f c rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðl00 e00 l0 e0 Þ þ ðl00 e00 l0 e0 Þ2 þ ðe0 l00 þ e00 l0 Þ2
erties are attributed to the improved magnetic/dielectric loss and attenuation constant by carbon layer shells.
ð15Þ Conclusion
where f is the frequency of the EM-wave and c is the velocity of light [38]. Fig. 8 shows the relation between the attenuation constant and frequency. It is evident from the fig that the present investigated materials exhibit the maximum value of the attenuation constant at higher frequency. In addition, as seen in Fig. 8, the frequency dependence of a for the CI/C nanocomposite has bigger a at 8.2–12.4 GH, indicating the excellent attenuation or EM-wave absorption at 8.2–12.4 GHz. From above equation, higher values of l00 would result in higher a. Enhanced EM-wave absorption prop-
Electromagnetic wave absorbing nanocomposite composed of CI and C as absorbent was fabricated by hydrothermal reaction. The effect of carbon layers on the magnetic and EMcharacteristic of CI particles have been investigated in detail. It was revealed from VSM graphs that existing carbon in nanocomposite cause of reduction of saturation magnetization from 225 to 131emu/g due to diamagnetic properties of carbon. For CI particles, the bandwidth with RL more than 10 dB can be reached in the frequency range of 10.8–12.4 GHz. The optimal reflection loss for 4.4 mm matching thickness was 16.86 dB at the matching frequency of 12.3 GHz. For the CI/C nanocomposite, the optimal RL was 46.69 dB in the frequency of 11.5 GHz, when the thickness is 1.3 mm. Also the bandwidth in 90% power absorption and 99% power absorption can be obtained in the frequency range of 9.8– 12.4 GHz and 11.0–11.8 GHz respectively. The measured RL results show the CI/C nanocomposite can be increased bandwidth of the RL below 10 dB and 20 dB and optimal reflection loss in thin thickness. Due to the existence layer of C coated on CI, the absorbing properties were improved by integrating the dielectric absorbent and magnetic absorbent. The electromagnetic parameters of CI/C nanocomposite is better or comparable to some of the previously reported techniques. A comparison of the results is given in Table 2. As reported in table 2 (comparative table), in this work, electromagnetic parameters like bandwidth and thickness were improved with dielectric-magnetic hybrid nanostructure. Thus, it is helpful for enhancement of the magnetic/dielectric loss, optimal reflection loss and attenuation constant, which makes excesses EM-wave absorption properties in the range of 8.2–12.4 GHz.
References
Fig. 8. The attenuation constant of CI and CI/C nanocomposite.
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