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Synthesis, multi-nonlinear dielectric resonance and electromagnetic absorption properties of hcp-cobalt particles
Journal of Magnetism and Magnetic Materials 354 (2014) 7–11
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis, multi-nonlinear dielectric resonance and electromagnetic absorption properties of hcp-cobalt particles Shulai Wen n, Ying Liu n, Xiuchen Zhao, Jingwei Cheng, Hong Li School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2013 Received in revised form 29 September 2013 Available online 31 October 2013
Hcp-cobalt particles were successfully prepared by a liquid phase reduction method, and the microstructure, static magnetic properties, electromagnetic and microwave absorption properties of the cobalt particles with irregular shape were investigated in detail. The measured results indicate that the saturation magnetization was less than that of hcp-Co single crystals, and the coercivity was larger than that of bulk cobalt crystal. The permittivity presents multi-nonlinear dielectric resonance, which may result from the irregular shape containing parts of cutting angle of dodecahedron of cobalt particles. The real part of permeability decreases with the frequency, and the imaginary part has a wide resonant peak. The paraffin-based composite containing 70 wt% cobalt particles possessed strong absorption characteristics with a minimum RL of 38.97 dB at 10.81 GHz and an absorption band with RL under 10 dB from 8.72 to 13.26 GHz when the thickness is 1.8 mm, which exhibits excellent microwave absorption in middle and high frequency. The architectural design of material morphologies is important for improving microwave absorption properties toward future application. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Cobalt particle Magnetic property Dielectric loss Electromagnetic absorption
1. Introduction The electromagnetic microwave absorption materials have been attracting much attention as antielectromagnetic interference coating, self-concealing technology, and microwave darkroom, and to obtain excellent microwave absorbers, extensive studies have been carried out to the synthesis of electromagnetic microwave absorption materials in recent years [1–3]. For example, Cao et al. [4] prepared cagelike ZnO/SiO2 nanocomposites showed a minimum absorption of 10.68 dB at 12.79 GHz. Yang and her partners [5] synthesized electrospun magnetic carbon composite fibers, and its minimum RL value was about 23 dB at 11.5 GHz matching the thickness of 4 mm. Wang and his colleague [6] prepared graphene–Fe3O4 nanohybrids, and the minimum RL value was 40.36 dB at 7.04 GHz with the thickness of 5 mm. As reported by Tong et al. [7] the minimum values for urchin-like α-Fe2O3 and urchin-like Fe3O4 were 9.2 dB and 29.96 dB, respectively. Liu et al. [8] found that the (Fe0.995Ti0.005)85Si9.6Al5.4 composite showed the minimum RL value of 23.34 dB at 1.55 GHz. For the microwave absorbers, magnetic metallic particles are favorable, because their large saturation magnetization could lead to higher permeability according to the Snoek limit [9,10].
n
Corresponding authors E-mail addresses: [email protected] (S. Wen), [email protected] (Y. Liu).
As a typical magnetic material, cobalt possesses high saturation magnetization, multicrystal phases, and various morphologies [11–13]. Unfortunately, studies on the microwave absorption properties of cobalt particles are few. Thus, the development of simple and reliable routes remains challenging to researchers. In this work, a liquid phase reduction method is used to prepare hcp-cobalt particles, which exhibit excellent electromagnetic microwave absorption, and the possible mechanism is discussed in detail.
2. Experimental 2.1. Synthesis of cobalt particles All chemicals used in this work were of analytical grade and were used as received without further purification. The synthesis of cobalt particles was carried out in a 3-necked flask equipped with an ultrasound reactor. In a typical synthesis, 1.4 g CoSO4 7H2O, 8.4 g C4O6H4KNa, 4 g C16H33(CH3)3NBr (CTAB) and 1 g PEG4000 were dissolved in 100 mL deionized water under vigorous stirring for 40 min at 40 1C, and then 1 mL 80% N2H4 H2O was quickly added to the above solution, followed by 14 g NaOH. Afterwards, the reaction temperature was fixed at 90 1C, and the ultrasonic power and frequency were 1000 W and 40 kHz, respectively. Thirty seconds later, the solution was cooled to room temperature. The dark gray precipitates were separated, washed with deionized water and absolute ethanol several times, and dried under vacuum at 40 1C for 24 h to collect the cobalt particles.
0304-8853/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.10.030
8
S. Wen et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 7–11
2.2. Characterization of cobalt particles X-ray diffraction (XRD) pattern was recorded on a Bruker D8 Advance diffractometer in Mo Kα radiation (λ ¼0.7093 Å) operated at 50 kV and 30 mA. The morphologies of the as-synthesized samples were observed by a QUANTA600 scanning electron microscopy (SEM) operated at 25 kV. The magnetization measurement was performed by vibrating sample magnetometer (VSM). The complex dielectric permittivity (εr ¼ ε′ jε″) and magnetic permeability (μr ¼ μ′ jμ″) were obtained using a HP8722ESS vector network analyzer to measure the scattering parameters by the coaxial reflection/transmission technique in the frequency range of 2–18 GHz. The samples were prepared by randomly dispersing the cobalt particles into a paraffin matrix with a volume fraction of 70 wt% and pressing into coaxial rings with an outer diameter of 7.0 mm, an inner diameter of 3.0 mm and a thickness of 2.0 mm for electromagnetic wave measurements.
3. Results and discussion As well known that cobalt has two stable crystal structure elemental cobalt at ambient pressures (hcp below 425 1C and fcc at higher temperature) [14]. The phase structure and composition of as-synthesized samples were clearly characterized by XRD, as shown in Fig. 1. The strong and sharp diffraction peaks manifest the good crystallization of the samples. All the XRD peaks of samples at 2θ ¼ 18.8561, 20.1931, 21.4021, 32.9111, 35.9571, 38.8651, 39.5991, 52.4271 and 53.4151 can be well indexed to the (100), (002), (101), (110), (103), (112), (201), (211) and (114) planes of hcp-cobalt,
Fig. 1. XRD pattern of as-synthesized samples.
respectively. No characteristic peaks of fcc-cobalt are detected, which demonstrates that hcp-cobalt particles have been obtained. Fig. 2 shows typical SEM images of the cobalt particles. After measuring large quantities of the cobalt particles, it can be well seen that most of them are of irregular shape, which contains parts of cutting angle of dodecahedron, and the average size is 5–9 μm, as clearly shown in Fig. 2(a) and (b). Fig. 3 shows that the cobalt particles are ferromagnetic and the magnetization saturates below the magnetic field of 10,000 Oe. The saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) are 133.29 emu/g, 10.56 emu/g and 179.50 Oe at 300 K. The Ms of cobalt particles is smaller than the highest Ms reported for the bulk cobalt (E162 emu/g) [15]. As shown from the following equation [16]: 3=2 Nd T M s ðTÞ ¼ nef f μB ½1 0:118α A Tc where Tc is the Curie temperature, and T is the working temperature. The saturation magnetization Ms depends on the number of magnetic atoms N, density d, α (a constant relation to the crystal), relative atomic mass A, Bohr magneton μB, the effective Bohr magneton number neff, the atomic magnetic moment, and the temperature. This reduction of saturation magnetization Ms of the flower-like cobalt particles compared to hcp-Co single crystals may be attributed to [C4O6H4]2 or [C16H33(CH3)3N] fixing cobalt atomic magnetic moments, which reduces the effective Bohr mageton number neff. The coercivity Hc of cobalt particles is much larger than that of bulk cobalt (E10 Oe) [17,18]. This may be attributed to the increasing effective barrier for the hcp crystal structure which has larger
Fig. 3. Hysteresis loops of cobalt particles at room temperature. (The inset is magnified hysteresis loops from 250 to 250 Oe.)
Fig. 2. Typical SEM images of the cobalt particles (a) 2500 and (b) 5000 .
S. Wen et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 7–11
manetocrystalline anisotropy, and the stronger surface pinning effect due to the adsorption of [C4O6H4]2 or [C16H33(CH3)3N] þ on the surfaces of cobalt particles. Fig. 4 shows the complex permittivity and complex permeability versus frequency for the paraffin-based composites containing 70 wt% cobalt particles dispersed in a paraffin matrix. In addition, the complex permittivity and permeability of the paraffin-based composites are fundamental physical quantities in determining the microwave properties [19–23]. As shown in Fig. 4 (a), the real part (ε0 ) of complex permittivity gradually increases from 10.92 to 11.76 over the whole frequency range of 2–18 GHz, and exhibits two resonant peaks at 7.25 and 11.40 GHz, respectively. It can be also seen that the imaginary part (ε}) and the dielectric loss (tan δε ¼ ε″=ε′) increase from 0.11 to 0.57 and 0.010 to 0.049, respectively, and show five resonant peaks at 3.03, 5.76, 9.37, 14.83 and 17.74 GHz. These variations can be explained on the basis of space charge polarization model of Wagner [24] and Maxwell [25] and is in agreement with Koop's phenomenological theory. According to Koop's model, the increase of the real part (ε0 ) of complex permittivity at higher frequencies can be attributed to the interfacial space-charge polarization which arises from the heterogeneous structure of the samples. Interfacial polarization is always present in materials comprised of more than one phase like a metal paraffin composites. This kind of polarization arising at the interfaces is due to the migration of charge carriers through different phases of the composite material resulting in differential charge accumulation at the interfaces. When these charges are made to move by the application of an external electric field, the motion will be hindered at various points of the composite material differently, causing space charge to appear. The appearance of such space charge can distort the macroscopic field and appears as polarization to an external observer. Interfacial polarization is present in materials with considerable electrically homogeneous. Hence composite materials will exhibit large interfacial polarization within them under an external electric field. Metal particles embedded in an insulator matrix can act as charge centers and can contribute to the enhancement of dielectric permittivity because of interfacial polarization [26]. The appearance of multi-nonlinear resonance peaks may be attributed to the composition and irregular shape of the cobalt particles. As we known, the oxide shell maybe formed on the surface of cobalt particles easily at different levels via a liquid phase reduction method, as shown in Ref. [27], the different ratios of the shell thickness to the particle radius contribute to multiresonance peaks; moreover, the higher-order multipoles of cobalt particles and the interaction of inter-particle also introduced multi-resonance peaks. From Fig. 4(b), it can be seen that the real part ðμ′Þ of permeability decreases from 1.82 to 0.22 with frequency
9
increasing, which shows excellent frequency dispersion. The imaginary part ðμ″Þ has a resonance peak at 12.47 GHz,and the large resonance band is observed in the range of 2–18 GHz. Kittel [28] linked the resonance frequency to the magnetocrystalline anisotropy field: f r ¼ ðγ =2π ÞH a , so the resonance frequency present in the middle and high frequency in the range of 2–18 GHz due to the large magnetocrystalline anisotropy energy of hcp-cobalt particles as shown above, and the large resonance band may be interpreted as a consequence of size and morphology of the cobalt particles. On one hand, the micrometer-sized particles, owing to their large size with respect to a magnetic wall, are made up of several magnetic domains, and the large resonance band may be interpreted as a consequence of their magnetic polydomain configuration. Indeed, it is well known that, for partially magnetized samples, the internal demagnetizing fields resulting from the distribution of magnetic domains, contribute to the resonance broadening, on the other hand, the frequency band broadening is also related to the morphology of the particles because of the effect of the demagnetization fields which are related to the shapes of cobalt particles [29–32]. Magnetic loss (tanδμ ¼ μ}=μ0 ) increases from 0.14 to 1.97 with frequency in the 2–12 GHz frequency range, and then keeps more or less constant in the higher frequency range. As a typical magnetic material, the loss of cobalt particles mainly originates from the magnetic loss. In general, the microwave magnetic loss of magnetic materials is mostly associated with magnetic hysteresis, domain wall resonance, eddy current loss, natural resonance [33], and exchange resonance [34]. In the current study, the permeability of the as-obtained cobalt particles could be ascribed to eddy current losses, natural resonance, and exchange resonance, rather than magnetic hysteresis and domain wall resonance. This is because magnetic hysteresis stemming from irreversible magnetization occurs only in a highly applied
Fig. 5. Frequency dependence of μ}ðμ0 Þ 2 f
1
.
Fig. 4. Frequency dependence of permittivity (a), and permeability (b) for the paraffin-based composites containing 70 wt% cobalt particles.
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S. Wen et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 7–11
field, whereas domain wall resonance derived from multi-domain materials occurs only in the low frequency. Therefore, it can be deduced that the resonance peak below 3 GHz comes chiefly from domain wall resonance, which slightly shifted to higher frequencies owing to the decreased size. 1 To confirm the above results, the equation μ}ðμ0 Þ 2 f ¼ ð2=3Þ πμ0 d2 s, where μ0 is the vacuum permeability, was used to investigate the eddy current loss contribution to the imaginary part of permeability μ} relative to the thickness d and the electric conductivity s of the composites [35–37]. According to the equation, if the magnetic loss only stems from the eddy current loss, then the values of μ0 ðμ0 Þ 2 f
1
should be constant 1
when the frequency is changed. The value of μ0 ðμ0 Þ 2 f of the cobalt particles is shown in Fig. 5. A wide resonance peak in the range of 2–18 GHz was observed, which suggests that the magnetic loss of cobalt particles is chiefly caused not only by eddy current loss but also by natural resonance and exchange resonance. The reflection loss (RL) of an absorbing material backed on a conductor can be calculated using the relative complex permeability and permittivity at a given frequency and thickness according to the transmit-line theory [38,7]: RL ¼ 20 log jðZ in Z 0 Þ=ðZ in þ Z 0 Þj
interface. At some special frequency point, these two reflected waves are out of phase by 1801 and cancel each other for the thickness of absorber satisfying the quarter-wave thickness criteria, which means the peak frequency dependence of the RL for the ferromagnetic metal based composite complies with the quarter-wavelength (λ/4) matching model [39–42]: dm ¼
4f m
nc pffiffiffiffiffiffiffiffiffiffiffiffiðn ¼ 1; 3; …Þ jεr μr j
ð3Þ
where fm is the peak frequency of RL, dm is the thickness of the sample, εr and μr are the complex permittivity and permeability at fm respectively, and c is the velocity of light. According to this model, the peak frequency is inversely proportional to the thickness. In addition, when the samples thickness is thicker than the critical thickness (n ¼3), two peaks appear simultaneously. One at lower frequency is relative to the λ/4 condition, and the other at cal higher frequency from 3λ/4 condition. A comparison of the dm
ð1Þ
where Z0 is the impedance of free space, and Zin is the input characteristic impedance, which can be express as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi Z in ¼ Z 0 ðμr =εr Þ tanh jð2π f d=cÞ μr εr ð2Þ where c is the velocity of light and d is the thickness of an absorber. Fig. 6(a) shows the calculated reflection loss curves for the paraffin-based composites with 70 wt% cobalt particles. The composites possessed excellent absorption properties with a minimum RL of 38.97 dB at 10.81 GHz, and an absorption band with RL under 10 dB from 8.72 to 13.26 GHz, which corresponds to a matching thickness of 1.8 mm. Furthermore, absorption band with RL under 10 dB from 10.86 to 17.00 GHz and 9.37 to 14.41 GHz, when the thickness is 1.5 mm and 1.7 mm, respectively, which exhibits excellent microwave absorption in middle and high frequency. Fig. 6(b) shows the dependence of RL on the thickness d of the paraffin-based composites. Absorption below 10 dB can be achieved as long as the thickness is thinner than 3.75 mm. Moreover, an absorption exceeding 20 dB is obtained for the thickness of 1.29–2.41 mm. From Fig. 6(a), it can be seen that the peak frequency of RL shifts to lower frequency range with the thickness increase, and more than one peak of RL appears when the thickness is thicker than a critical value for the paraffin-based composites. To illustrate this phenomenon further, a model is established as shown in Fig. 7, when an electromagnetic wave is incident on an absorber sample backed by a metal plate, it is partially reflected from air to absorber interface and partially reflected from absorber to metal
Fig. 7. Schematic illustration of the single-layer absorber.
Fig. 8. Dependence of λ/4 and 3λ/4 thickness on frequency for the paraffin-based composite containing 70 wt% cobalt particles.
Fig. 6. Frequency (a) and thickness (b) dependence of the RL for the paraffin-based composite containing 70 wt% cobalt particles under different thicknesses.
S. Wen et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 7–11 sim
calculated from Eq. (3) (n ¼1 and 3) with dm simulated by Eq. (1) for each composite are shown in Fig. 8. From the figure, the sim simulate results dm agree very well with the calculated values cal dm , which implies that the microwave absorption mechanism of the paraffin-based composites can be explained by the λ/4 matching model. 4. Conclusions In summary, hcp-cobalt particles with irregular shape containing parts of cutting angle of dodecahedron are successfully prepared by a liquid phase reduction method. The paraffin-based composites containing 70 wt% cobalt particles possessed strong absorption characteristics with a minimum RL of 38.97 dB at 10.81 GHz and an absorption band with RL under 10 dB from 8.72 to 13.26 GHz. The excellent microwave absorption in middle and high frequency was ascribed to their unique morphologies, which result in larger electromagnetic parameters, multi-nonlinear dielectric resonance, and eddy current loss. The result gives insight into the absorption mechanism of these cobalt particles. Considering the low-cost and facile synthesis process of cobalt particles with peculiar morphologies, the present study offers promising materials for microwave absorption. References [1] [2] [3] [4]
F.S. Wen, W.L. Zuo, H.B. Yi, et al., Physica B 404 (2009) 3370–3567. L. Qiao, X.H. Han, B. Gao, et al., J. Appl. Phys. 105 (2009) 053911. L. Zhen, Y.X. Gong, J.T. Jiang, et al., J. Appl. Phys. 104 (2008) 034312. Maosheng Cao, Xiaoling Shi, Xiaoyong Fang, et al., Appl. Phys. Lett. 91 (2007) 203110. [5] Ying Yang, Zhen Guo, Huan Zhang, et al., J. Appl. Polym. Sci. (2013)4285–4288. [6] Tieshi Wang, Zhaohong Liu, Mingming Lu, et al., J. Appl. Phys. 113 (2013) 024314. [7] G.X. Tong, W.H. Wu, J.G. Guan, et al., J. Alloys Compd. 509 (2011) 4320–4326.
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[8] J.H. Liu, T.Y. Ma, H. Tong, et al., J. Magn. Magn. Mater. 322 (2010) 940–944. [9] J.L. Snoek, Physica (Amsterdam) 14 (1948) 207. [10] O. Acher, S. Dubourg, Phys. Rev. B 77 (2008) 104440. [11] J.C. Bao, Z. Xu, J.M. Hong, et al., Scr. Mater. 50 (2004) 19. [12] H. Karami, E. Mohammadzadeh, Int. J. Electrochem. Sci. 5 (2010) 1032. [13 H.Z. Wang, L. Zhang, J.J. Huang, et al., J. Nanopart. Res. 13 (2011) 1709. [14] Victor F. Puntes, Kannan Krishnan, A. Paul Alivisatos, Top. Catal. 19 (2002) 145–148. [15] Y.L. Li, J.Z. Zhao, X.D. Su, et al., J. Colloid Interface Sci. 336 (2009) 41–45. [16] G.X. Tong, W.H. Wu, Q. Hua, et al., J. Mater. Res. 26 (2011) 682. [17] S. Chikazumi, Physics of Magnetism, John Wiley & Sons, New York, 1964. [18] Yajing Zhang, Qi Yao, Ying Zhang, Tieyu Cui, Cryst. Growth Des. 8 (2008) 3206–3212. [19] C. Brosseau, F. Boulic, P. Queffelec, et al., J. Appl. Phys. 81 (1997) 882–891. [20] C. Brosseau, P. Molinie, F. Boulic, et al., J. Appl. Phys. 89 (2001) 8297–8310. [21] B. Francoise, B. Christian, M.Y. Le, et al., J. Phys. D: Appl. Phys. 31 (1998) 1904. [22] L. Zong, S. Zhou, R. Sun, et al., J. Polym. Sci. B: Polym. Phys. 42 (2004) 2871–2877. [23] S. Osaki, J. Polym. Sci. B: Polym. Phys. 33 (1995) 685–690. [24] K.W. Wagner, Ann. Phys. (Leipzig) 40 (1913) 817. [25] J.C. Maxwell, Electricity and Magnetism, vol. 1, Oxford University Press, Oxford (Section 328). [26] E. Muhammad, Abdul Jamal, P.A. Joy, Philip Kurian, M.R. Anantharaman, Mater. Sci. Eng. B 156 (2009) 24–31. [27] N. Harfield, J. Mater. Sci. 35 (2000) 5809–5816. [28] C. Kittel, Phys. Rev. 155 (1948) 73. [29] D. Polder, J. Smit, Rev. Mod. Phys. 89 (1953) 25. [30] G. Viau, F. Fiévet-Vincent, F. Fiévet, et al., J. Appl. Phys. 81 (1997) 2749–2754. [31] Philippe Toneguzzo, Guillaume Viau, Olivier Acher, et al., Adv. Mater. 13 (1998) 1032–1035. [32] Ph. Toneguzzo, G. Viau, O. Acher, et al., J. Mater. Sci. 35 (2000) 3767–3784. [33] G. Bertotti, J. Appl. Phys. 57 (1985) 2110–2117. [34] A. Aharoni, J. Appl. Phys. 39 (1991) 7762–7764. [35] F. Wen, H. Yi, L. Qiao, et al., Appl. Phys. Lett. 92 (2008) 042507–042513. [36] M. Wu, Y.D. Zhang, S. Hui, et al., Appl. Phys. Lett. 80 (2002) 4404–4406. [37] Yong Yang, Cailing Xu, Yongxin Xia, et al., J. Alloys Compd. 493 (2010) 549–552. [38] S.S. Kim, S.T. Kim, J.M. Ahn, et al., J. Magn. Magn. Mater. 271 (2004) 39. [39] B.C. Wang, J.L. Zhang, T. Wang, et al., J. Alloys Compd. 567 (2013) 21–25. [40] B.C. Wang, J.Q. Wei, Y. Yang, et al., J. Magn. Magn. Mater. 323 (2011) 1101. [41] B.C. Wang, J.Q. Wei, L. Qiao, et al., J. Magn. Magn. Mater. 324 (2012) 761. [42] P. Xu, X. Han, J. Jiang, X. Wang, A. Wen, J. Phys. Chem. C 111 (2007) 12603–12608.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis, multi-nonlinear dielectric resonance and electromagnetic absorption properties of hcp-cobalt particles Shulai Wen n, Ying Liu n, Xiuchen Zhao, Jingwei Cheng, Hong Li School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2013 Received in revised form 29 September 2013 Available online 31 October 2013
Hcp-cobalt particles were successfully prepared by a liquid phase reduction method, and the microstructure, static magnetic properties, electromagnetic and microwave absorption properties of the cobalt particles with irregular shape were investigated in detail. The measured results indicate that the saturation magnetization was less than that of hcp-Co single crystals, and the coercivity was larger than that of bulk cobalt crystal. The permittivity presents multi-nonlinear dielectric resonance, which may result from the irregular shape containing parts of cutting angle of dodecahedron of cobalt particles. The real part of permeability decreases with the frequency, and the imaginary part has a wide resonant peak. The paraffin-based composite containing 70 wt% cobalt particles possessed strong absorption characteristics with a minimum RL of 38.97 dB at 10.81 GHz and an absorption band with RL under 10 dB from 8.72 to 13.26 GHz when the thickness is 1.8 mm, which exhibits excellent microwave absorption in middle and high frequency. The architectural design of material morphologies is important for improving microwave absorption properties toward future application. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved.
Keywords: Cobalt particle Magnetic property Dielectric loss Electromagnetic absorption
1. Introduction The electromagnetic microwave absorption materials have been attracting much attention as antielectromagnetic interference coating, self-concealing technology, and microwave darkroom, and to obtain excellent microwave absorbers, extensive studies have been carried out to the synthesis of electromagnetic microwave absorption materials in recent years [1–3]. For example, Cao et al. [4] prepared cagelike ZnO/SiO2 nanocomposites showed a minimum absorption of 10.68 dB at 12.79 GHz. Yang and her partners [5] synthesized electrospun magnetic carbon composite fibers, and its minimum RL value was about 23 dB at 11.5 GHz matching the thickness of 4 mm. Wang and his colleague [6] prepared graphene–Fe3O4 nanohybrids, and the minimum RL value was 40.36 dB at 7.04 GHz with the thickness of 5 mm. As reported by Tong et al. [7] the minimum values for urchin-like α-Fe2O3 and urchin-like Fe3O4 were 9.2 dB and 29.96 dB, respectively. Liu et al. [8] found that the (Fe0.995Ti0.005)85Si9.6Al5.4 composite showed the minimum RL value of 23.34 dB at 1.55 GHz. For the microwave absorbers, magnetic metallic particles are favorable, because their large saturation magnetization could lead to higher permeability according to the Snoek limit [9,10].
n
Corresponding authors E-mail addresses: [email protected] (S. Wen), [email protected] (Y. Liu).
As a typical magnetic material, cobalt possesses high saturation magnetization, multicrystal phases, and various morphologies [11–13]. Unfortunately, studies on the microwave absorption properties of cobalt particles are few. Thus, the development of simple and reliable routes remains challenging to researchers. In this work, a liquid phase reduction method is used to prepare hcp-cobalt particles, which exhibit excellent electromagnetic microwave absorption, and the possible mechanism is discussed in detail.
2. Experimental 2.1. Synthesis of cobalt particles All chemicals used in this work were of analytical grade and were used as received without further purification. The synthesis of cobalt particles was carried out in a 3-necked flask equipped with an ultrasound reactor. In a typical synthesis, 1.4 g CoSO4 7H2O, 8.4 g C4O6H4KNa, 4 g C16H33(CH3)3NBr (CTAB) and 1 g PEG4000 were dissolved in 100 mL deionized water under vigorous stirring for 40 min at 40 1C, and then 1 mL 80% N2H4 H2O was quickly added to the above solution, followed by 14 g NaOH. Afterwards, the reaction temperature was fixed at 90 1C, and the ultrasonic power and frequency were 1000 W and 40 kHz, respectively. Thirty seconds later, the solution was cooled to room temperature. The dark gray precipitates were separated, washed with deionized water and absolute ethanol several times, and dried under vacuum at 40 1C for 24 h to collect the cobalt particles.
0304-8853/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2013.10.030
8
S. Wen et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 7–11
2.2. Characterization of cobalt particles X-ray diffraction (XRD) pattern was recorded on a Bruker D8 Advance diffractometer in Mo Kα radiation (λ ¼0.7093 Å) operated at 50 kV and 30 mA. The morphologies of the as-synthesized samples were observed by a QUANTA600 scanning electron microscopy (SEM) operated at 25 kV. The magnetization measurement was performed by vibrating sample magnetometer (VSM). The complex dielectric permittivity (εr ¼ ε′ jε″) and magnetic permeability (μr ¼ μ′ jμ″) were obtained using a HP8722ESS vector network analyzer to measure the scattering parameters by the coaxial reflection/transmission technique in the frequency range of 2–18 GHz. The samples were prepared by randomly dispersing the cobalt particles into a paraffin matrix with a volume fraction of 70 wt% and pressing into coaxial rings with an outer diameter of 7.0 mm, an inner diameter of 3.0 mm and a thickness of 2.0 mm for electromagnetic wave measurements.
3. Results and discussion As well known that cobalt has two stable crystal structure elemental cobalt at ambient pressures (hcp below 425 1C and fcc at higher temperature) [14]. The phase structure and composition of as-synthesized samples were clearly characterized by XRD, as shown in Fig. 1. The strong and sharp diffraction peaks manifest the good crystallization of the samples. All the XRD peaks of samples at 2θ ¼ 18.8561, 20.1931, 21.4021, 32.9111, 35.9571, 38.8651, 39.5991, 52.4271 and 53.4151 can be well indexed to the (100), (002), (101), (110), (103), (112), (201), (211) and (114) planes of hcp-cobalt,
Fig. 1. XRD pattern of as-synthesized samples.
respectively. No characteristic peaks of fcc-cobalt are detected, which demonstrates that hcp-cobalt particles have been obtained. Fig. 2 shows typical SEM images of the cobalt particles. After measuring large quantities of the cobalt particles, it can be well seen that most of them are of irregular shape, which contains parts of cutting angle of dodecahedron, and the average size is 5–9 μm, as clearly shown in Fig. 2(a) and (b). Fig. 3 shows that the cobalt particles are ferromagnetic and the magnetization saturates below the magnetic field of 10,000 Oe. The saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) are 133.29 emu/g, 10.56 emu/g and 179.50 Oe at 300 K. The Ms of cobalt particles is smaller than the highest Ms reported for the bulk cobalt (E162 emu/g) [15]. As shown from the following equation [16]: 3=2 Nd T M s ðTÞ ¼ nef f μB ½1 0:118α A Tc where Tc is the Curie temperature, and T is the working temperature. The saturation magnetization Ms depends on the number of magnetic atoms N, density d, α (a constant relation to the crystal), relative atomic mass A, Bohr magneton μB, the effective Bohr magneton number neff, the atomic magnetic moment, and the temperature. This reduction of saturation magnetization Ms of the flower-like cobalt particles compared to hcp-Co single crystals may be attributed to [C4O6H4]2 or [C16H33(CH3)3N] fixing cobalt atomic magnetic moments, which reduces the effective Bohr mageton number neff. The coercivity Hc of cobalt particles is much larger than that of bulk cobalt (E10 Oe) [17,18]. This may be attributed to the increasing effective barrier for the hcp crystal structure which has larger
Fig. 3. Hysteresis loops of cobalt particles at room temperature. (The inset is magnified hysteresis loops from 250 to 250 Oe.)
Fig. 2. Typical SEM images of the cobalt particles (a) 2500 and (b) 5000 .
S. Wen et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 7–11
manetocrystalline anisotropy, and the stronger surface pinning effect due to the adsorption of [C4O6H4]2 or [C16H33(CH3)3N] þ on the surfaces of cobalt particles. Fig. 4 shows the complex permittivity and complex permeability versus frequency for the paraffin-based composites containing 70 wt% cobalt particles dispersed in a paraffin matrix. In addition, the complex permittivity and permeability of the paraffin-based composites are fundamental physical quantities in determining the microwave properties [19–23]. As shown in Fig. 4 (a), the real part (ε0 ) of complex permittivity gradually increases from 10.92 to 11.76 over the whole frequency range of 2–18 GHz, and exhibits two resonant peaks at 7.25 and 11.40 GHz, respectively. It can be also seen that the imaginary part (ε}) and the dielectric loss (tan δε ¼ ε″=ε′) increase from 0.11 to 0.57 and 0.010 to 0.049, respectively, and show five resonant peaks at 3.03, 5.76, 9.37, 14.83 and 17.74 GHz. These variations can be explained on the basis of space charge polarization model of Wagner [24] and Maxwell [25] and is in agreement with Koop's phenomenological theory. According to Koop's model, the increase of the real part (ε0 ) of complex permittivity at higher frequencies can be attributed to the interfacial space-charge polarization which arises from the heterogeneous structure of the samples. Interfacial polarization is always present in materials comprised of more than one phase like a metal paraffin composites. This kind of polarization arising at the interfaces is due to the migration of charge carriers through different phases of the composite material resulting in differential charge accumulation at the interfaces. When these charges are made to move by the application of an external electric field, the motion will be hindered at various points of the composite material differently, causing space charge to appear. The appearance of such space charge can distort the macroscopic field and appears as polarization to an external observer. Interfacial polarization is present in materials with considerable electrically homogeneous. Hence composite materials will exhibit large interfacial polarization within them under an external electric field. Metal particles embedded in an insulator matrix can act as charge centers and can contribute to the enhancement of dielectric permittivity because of interfacial polarization [26]. The appearance of multi-nonlinear resonance peaks may be attributed to the composition and irregular shape of the cobalt particles. As we known, the oxide shell maybe formed on the surface of cobalt particles easily at different levels via a liquid phase reduction method, as shown in Ref. [27], the different ratios of the shell thickness to the particle radius contribute to multiresonance peaks; moreover, the higher-order multipoles of cobalt particles and the interaction of inter-particle also introduced multi-resonance peaks. From Fig. 4(b), it can be seen that the real part ðμ′Þ of permeability decreases from 1.82 to 0.22 with frequency
9
increasing, which shows excellent frequency dispersion. The imaginary part ðμ″Þ has a resonance peak at 12.47 GHz,and the large resonance band is observed in the range of 2–18 GHz. Kittel [28] linked the resonance frequency to the magnetocrystalline anisotropy field: f r ¼ ðγ =2π ÞH a , so the resonance frequency present in the middle and high frequency in the range of 2–18 GHz due to the large magnetocrystalline anisotropy energy of hcp-cobalt particles as shown above, and the large resonance band may be interpreted as a consequence of size and morphology of the cobalt particles. On one hand, the micrometer-sized particles, owing to their large size with respect to a magnetic wall, are made up of several magnetic domains, and the large resonance band may be interpreted as a consequence of their magnetic polydomain configuration. Indeed, it is well known that, for partially magnetized samples, the internal demagnetizing fields resulting from the distribution of magnetic domains, contribute to the resonance broadening, on the other hand, the frequency band broadening is also related to the morphology of the particles because of the effect of the demagnetization fields which are related to the shapes of cobalt particles [29–32]. Magnetic loss (tanδμ ¼ μ}=μ0 ) increases from 0.14 to 1.97 with frequency in the 2–12 GHz frequency range, and then keeps more or less constant in the higher frequency range. As a typical magnetic material, the loss of cobalt particles mainly originates from the magnetic loss. In general, the microwave magnetic loss of magnetic materials is mostly associated with magnetic hysteresis, domain wall resonance, eddy current loss, natural resonance [33], and exchange resonance [34]. In the current study, the permeability of the as-obtained cobalt particles could be ascribed to eddy current losses, natural resonance, and exchange resonance, rather than magnetic hysteresis and domain wall resonance. This is because magnetic hysteresis stemming from irreversible magnetization occurs only in a highly applied
Fig. 5. Frequency dependence of μ}ðμ0 Þ 2 f
1
.
Fig. 4. Frequency dependence of permittivity (a), and permeability (b) for the paraffin-based composites containing 70 wt% cobalt particles.
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S. Wen et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 7–11
field, whereas domain wall resonance derived from multi-domain materials occurs only in the low frequency. Therefore, it can be deduced that the resonance peak below 3 GHz comes chiefly from domain wall resonance, which slightly shifted to higher frequencies owing to the decreased size. 1 To confirm the above results, the equation μ}ðμ0 Þ 2 f ¼ ð2=3Þ πμ0 d2 s, where μ0 is the vacuum permeability, was used to investigate the eddy current loss contribution to the imaginary part of permeability μ} relative to the thickness d and the electric conductivity s of the composites [35–37]. According to the equation, if the magnetic loss only stems from the eddy current loss, then the values of μ0 ðμ0 Þ 2 f
1
should be constant 1
when the frequency is changed. The value of μ0 ðμ0 Þ 2 f of the cobalt particles is shown in Fig. 5. A wide resonance peak in the range of 2–18 GHz was observed, which suggests that the magnetic loss of cobalt particles is chiefly caused not only by eddy current loss but also by natural resonance and exchange resonance. The reflection loss (RL) of an absorbing material backed on a conductor can be calculated using the relative complex permeability and permittivity at a given frequency and thickness according to the transmit-line theory [38,7]: RL ¼ 20 log jðZ in Z 0 Þ=ðZ in þ Z 0 Þj
interface. At some special frequency point, these two reflected waves are out of phase by 1801 and cancel each other for the thickness of absorber satisfying the quarter-wave thickness criteria, which means the peak frequency dependence of the RL for the ferromagnetic metal based composite complies with the quarter-wavelength (λ/4) matching model [39–42]: dm ¼
4f m
nc pffiffiffiffiffiffiffiffiffiffiffiffiðn ¼ 1; 3; …Þ jεr μr j
ð3Þ
where fm is the peak frequency of RL, dm is the thickness of the sample, εr and μr are the complex permittivity and permeability at fm respectively, and c is the velocity of light. According to this model, the peak frequency is inversely proportional to the thickness. In addition, when the samples thickness is thicker than the critical thickness (n ¼3), two peaks appear simultaneously. One at lower frequency is relative to the λ/4 condition, and the other at cal higher frequency from 3λ/4 condition. A comparison of the dm
ð1Þ
where Z0 is the impedance of free space, and Zin is the input characteristic impedance, which can be express as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi Z in ¼ Z 0 ðμr =εr Þ tanh jð2π f d=cÞ μr εr ð2Þ where c is the velocity of light and d is the thickness of an absorber. Fig. 6(a) shows the calculated reflection loss curves for the paraffin-based composites with 70 wt% cobalt particles. The composites possessed excellent absorption properties with a minimum RL of 38.97 dB at 10.81 GHz, and an absorption band with RL under 10 dB from 8.72 to 13.26 GHz, which corresponds to a matching thickness of 1.8 mm. Furthermore, absorption band with RL under 10 dB from 10.86 to 17.00 GHz and 9.37 to 14.41 GHz, when the thickness is 1.5 mm and 1.7 mm, respectively, which exhibits excellent microwave absorption in middle and high frequency. Fig. 6(b) shows the dependence of RL on the thickness d of the paraffin-based composites. Absorption below 10 dB can be achieved as long as the thickness is thinner than 3.75 mm. Moreover, an absorption exceeding 20 dB is obtained for the thickness of 1.29–2.41 mm. From Fig. 6(a), it can be seen that the peak frequency of RL shifts to lower frequency range with the thickness increase, and more than one peak of RL appears when the thickness is thicker than a critical value for the paraffin-based composites. To illustrate this phenomenon further, a model is established as shown in Fig. 7, when an electromagnetic wave is incident on an absorber sample backed by a metal plate, it is partially reflected from air to absorber interface and partially reflected from absorber to metal
Fig. 7. Schematic illustration of the single-layer absorber.
Fig. 8. Dependence of λ/4 and 3λ/4 thickness on frequency for the paraffin-based composite containing 70 wt% cobalt particles.
Fig. 6. Frequency (a) and thickness (b) dependence of the RL for the paraffin-based composite containing 70 wt% cobalt particles under different thicknesses.
S. Wen et al. / Journal of Magnetism and Magnetic Materials 354 (2014) 7–11 sim
calculated from Eq. (3) (n ¼1 and 3) with dm simulated by Eq. (1) for each composite are shown in Fig. 8. From the figure, the sim simulate results dm agree very well with the calculated values cal dm , which implies that the microwave absorption mechanism of the paraffin-based composites can be explained by the λ/4 matching model. 4. Conclusions In summary, hcp-cobalt particles with irregular shape containing parts of cutting angle of dodecahedron are successfully prepared by a liquid phase reduction method. The paraffin-based composites containing 70 wt% cobalt particles possessed strong absorption characteristics with a minimum RL of 38.97 dB at 10.81 GHz and an absorption band with RL under 10 dB from 8.72 to 13.26 GHz. The excellent microwave absorption in middle and high frequency was ascribed to their unique morphologies, which result in larger electromagnetic parameters, multi-nonlinear dielectric resonance, and eddy current loss. The result gives insight into the absorption mechanism of these cobalt particles. Considering the low-cost and facile synthesis process of cobalt particles with peculiar morphologies, the present study offers promising materials for microwave absorption. References [1] [2] [3] [4]
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