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Double-layer microwave absorber based on CoFe2O4 ferrite and carbonyl iron composites
Journal of Alloys and Compounds 584 (2014) 249–253
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Double-layer microwave absorber based on CoFe2O4 ferrite and carbonyl iron composites Yuan Liu, Xiangxuan Liu ⇑, Xuanjun Wang No. 603 Faculty, Xi’an Research Institute of High Technology, Xi’an 710025, China
a r t i c l e
i n f o
Article history: Received 5 June 2013 Received in revised form 2 August 2013 Accepted 9 September 2013 Available online 19 September 2013 Keywords: Absorbing materials Impedance matching Double-layer Electromagnetic properties
a b s t r a c t The microwave absorption properties of carbonyl iron (CI) and CoFe2O4 ferrite with single-layer and double-layer composite absorbers were investigated based on the electromagnetic transmission line theory and the impedance matching principle in the frequency range from 2 GHz to 18 GHz. XRD and SEM were used to characterize the structure of the powdered composites. Coaxial method was used to measure the electromagnetic parameters of CI and CoFe2O4. The reflectances of the matching and absorbing layers of different thicknesses and orders were also calculated. The results show that using CoFe2O4 as matching layer and CI as absorbing layer can greatly expand the coating absorption bandwidth and decrease the reflectance peak. Double-layer absorbers have much better microwave absorption properties than single-layer absorbers. The microwave absorption properties of the double-layer structure are influenced by the coupling interactions between the absorbing and matching layers. The double-layer microwave absorbers with reflection loss less than 10 dB over the range from 8.6 GHz to 18 GHz (the absorption bandwidth is 9.4 GHz), which almost covers the X-band (8.2 –12.4 GHz) and the whole Ku-band (12.4– 18 GHz) were achieved when the thicknesses of CoFe2O4 and CI were 2.4 and 0.5 mm, respectively. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction With the escalating electromagnetic pollution and the development of stealth technology for military platforms, an increasing number of studies have focused on microwave absorbing materials [1–4]. Based on the principle of reducing the radar scattering cross section of the target [5,6], radar stealth technology is divided into two major categories, namely, shape stealth and radar-absorbing coating stealth. Shape stealth needs to change the shape to meet standards, but results in decreased performance. By contrast, radar-absorbing coating technology is often used in radar stealth technology because of its simple maintenance, high performance, low cost, and so on [7]. Carbonyl iron (CI) and ferrite have been extensively applied in absorbing materials because of their advantages, such as simple preparation process, low cost, large magnetic loss angle, and strong absorbing ability [8–17]. However, drawbacks, such as narrow resonance frequency band and large matching thickness, limit their further application in stealth fields. Numerous studies on absorbing coating that works effectively in certain frequency and absorption bandwidth have been conducted. In single-layer absorbing materials, the number of parameters is limited and absorption
⇑ Corresponding author. Tel.: +86 02983348158. E-mail address: [email protected] (X. Liu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.049
bandwidth is narrow. Single-layer absorbing materials cannot easily satisfy the requirements for broad-band absorption. Therefore, the development of multilayer absorbing materials becomes increasingly important to fully utilize the properties of different layers of absorbing agents absorbing at a wider bandwidth to achieve optimal absorption [18,19]. In this study, we designed a novel double-layer microwave absorber by combining the magnetic characteristics of CoFe2O4 ferrite and CI to achieve a unique kind of absorber with wide band absorption, simple production process, and low cost.
2. Theory of double-layer microwave absorption The geometry of a double-layer absorber is shown in Fig. 1. According to the transmission theory [20], the calculation formulas are as follows [21,22]:
Z in Z 0 RðdBÞ ¼ 20 log10 Z in þ Z 0 Z in ¼ Z 2
pffiffiffiffiffiffiffiffiffiffiffiffiffi Z in1 þ Z 2 tanh jð2pfd2 =cÞ l2 =e2 pffiffiffiffiffiffiffiffiffiffiffiffiffi Z 2 þ Z in1 tanh jð2pfd2 =cÞ l2 =e2
qffiffiffiffiffiffiffiffiffiffiffiffiffi Z in1 ¼ Z 1 tanh jð2pfd1 =cÞ l1 =e1
ð1Þ
ð2Þ
ð3Þ
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4. Results and discussion 4.1. Characteristics of CoFe2O4 and CI powder
Fig. 1. Geometry of a double-layer absorber.
where Zin is the input impedance at the free space and material interface; Zin1 is the input impedance at the first and second layers; Z0, Z1, and Z2 denote the characteristics impedance of a vacuum, the first layer, and the second layer, respectively; e is the relative complex permittivity; and l is the relative complex permeability. Let d, c, and f be the thickness of layer, velocity of light in free space, and the frequency of the electromagnetic (EM) wave in free space, respectively. 3. Experimental
The XRD spectra of CoFe2O4 and CI are illustrated in Fig. 2. Characterized XRD peaks in the CoFe2O4 spectrum (Fig. 2a) were compared with the characteristic XRD peaks of the sample spectrum with the powder diffraction card database (JCPDS 22-1068). CoFe2O4 anastomosis was very good, no peaks corresponding to impurities were found. Sharp XRD peaks were found for the crystal plane. The relative intensity and the crystalline form tend to be complete, indicating that pure CoFe2O4 powder was generated. The XRD spectrum of CI (Fig. 2b) is consistent with the standard pattern (JCPDS 06-0696). The obvious crystal phase structure and oxidefree diffraction peaks correspond to that of unoxidized CI. The scanning electron micrographs for the CoFe2O4 and CI samples are shown in Figs. 3a and b, respectively. CoFe2O4 powder particles (Fig. 3a) have sharp edges and corners, polyhedron in shape, with size of approximately 2 lm, and only a small amount of which are larger than 3 lm. CI particles (Fig. 3b) are spherical in shape with diameter ranging from 1 lm to 4 lm. They seem to gather in small groups.
3.1. Sample preparation The samples used for EM parameter measurement were prepared by mixing 50 wt.% CI power and 50 wt.% wax as well as 50 wt.% CoFe2O4 powder and 50 wt.% wax. The powder-wax composites were thoroughly stirred by mortar grinding. A proper amount of alcohol was added, and then composites were sheared in a high-speed emulsifying machine, evaporated to dryness at room temperature, and milled into powder. Lastly, the composites were cast into toroid-shaped specimens with 3 mm inner diameter, 7.0 mm outer diameter, and 2–5 mm long. The coating used for the reflection loss of the double-layer absorber measurement was prepared as follows. An aluminum substrate with a standard size (18 mm 18 mm 3 mm) was initially washed with pure water, then with acetone and dried for later use. CI powder (50 wt.%) that was used as absorbing layer was dispersed into an epoxy resin and polyamide by solvent addition under a high-energy ultrasonic treatment for 30 min. A hardener was then added into the mixtures, followed by stirring at 1000 rpm for 10 min. Finally, the composite materials were fabricated on an aluminum substrate and dried in an oven (45 °C). After the absorbing layer solidified, matching layers made from 50 wt.% CoFe2O4 powder and 50 wt.% of other materials (epoxy resins, polyamide, a hardener, and some solvent) were fabricated similarly as the absorbing layer. The thickness of the layer was controlled using different thickness models.
3.2. Measurement of the properties and structure The resulting crystalline phases were characterized using X-ray diffraction (XRD; D/max–IIB, Japan). Data were recorded using Cu Ka radiation at 40.0 kV and 100.0 mA in the 2h region from 15° to 70°, with a scanning speed of 15°/min. A VEGA II XMU INCA scanning electron microscope was employed for morphological analysis. The EM parameters (complex permeability and permittivity) were measured using a vector network analyzer (HP-8720ES) in the frequency range from 2 GHz to 18 GHz. The reflection loss of the prepared absorbers versus frequency was studied using an HP 8510B vector network analyzer and standard horn antennas in an anechoic chamber.
a
4.2. Complex permittivity and permeability of samples Complex permittivity and permeability represent the dielectric and the dynamic magnetic properties of materials. Complex permittivity and permeability of CI and CoFe2O4 powder are shown in Fig. 4. The variation in the real component e0 of complex permittivity for powdered CI and CoFe2O4 are shown in Fig. 4a, in which the real component e0 of the CI ranges from 21.9 to 19.7 as frequency increases (Fig. 4a). The real component e0 of CoFe2O4 remains constant at approximately 3.7 within the frequency from 2 GHz to 18 GHz. Two obvious peaks of e00 for CI are obtained at approximately 4.2 GHz and 12.9 GHz, and one broad peak of e00 for CoFe2O4 is obtained from approximately 13.5 GHz to 18 GHz (Fig. 4b). The dielectric loss presents different loss mechanisms as frequency increases [23]. When the frequency is relatively low, the loss is determined by the peak conductance, and the loss is independent of the frequency. As the frequency increases to the microwave frequency band, the mechanisms involved are relaxation polarization loss and electric conductance loss. The increase in the imaginary component of the permittivity as the frequency increases may be the consequence of the increased relaxation polarization loss and electric conductance loss [24–26]. As frequency increases, the l0 value of CI has a tendency to rapidly decrease within the range 2–18 GHz for the domain-wall motion and relaxation (Fig. 4c) [27]. The l0 value of CoFe2O4 is essentially unchanged between 2 GHz and 15 GHz, then declines
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Fig. 2. XRD patterns of CoFe2O4 ferrite (a) and CI (b) powders.
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Real Permittivity (ε′)
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Fig. 3. Scanning electron micrographs of CoFe2O4 ferrite (a) and CI (b) powders.
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Fig. 4. Complex permittivity (e) and permeability (l) of CI and CoFe2O4 powder.
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Fig. 5. Microwave reflection loss curves under different thicknesses of CI (a) and CoFe2O4 ferrite (b) Alpha.
slowly from 15 GHz to 16 GHz and becomes stable thereafter. The imaginary l00 component of permeability for CI and CoFe2O4 powder is illustrated in Fig. 4d. The l00 value of CI sharply declines at 2– 4 GHz and becomes stable from 4 GHz to 11 GHz because of the domain-wall resonance and relaxation. The imaginary l00 component of CoFe2O4 is almost constant zero at the frequency range
from 6 GHz to 14 GHz, slowly rises from 14 GHz to 16 GHz. A broad peak could be observed from 4 GHz to 6 GHz. Generally, the microwave magnetic loss of ferrite magnetic materials mainly originates from the domain wall resonance and natural ferromagnetic resonance [28]. Domain wall resonance usually occurs in the low-frequency region (<2 GHz). However, resonance caused by
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the spin rotational component occurs at high-frequency regions [1]. Therefore, the change in the permeability (l0 , l00 ) of CoFe2O4 powder from 2 GHz to 18 GHz is due to resonance. 4.3. Microwave absorption properties of single-layer and double-layer microwave absorbers Microwave reflection loss curves are shown in Fig. 5 with different thicknesses of CI and CoFe2O4. The reflection peak moves to the low-frequency region as the thickness of the absorbent CI coating increases with minimum reflectance peak of 14.6 dB at a thickness of 1 mm, at which the frequency has a width less than
10 dB and reaches 4 GHz (Fig. 5a). When the minimum reflectance is 25.0 dB with a thickness of 2 mm, the frequency width is less than 10 dB and reaches just 2 GHz. A better absorption effect of the CoFe2O4 absorbent will be achieved with a thickness of at least 7 mm (Fig. 5b). Thus, simply using an absorber with only CI or CoFe2O4 cannot satisfy the ‘‘thin and wide’’ requirements of an absorbing material. To increase the absorption bandwidth and reduce the reflectivity of the absorbent coating while keeping it as thin as possible, computer-aided calculations have been performed considering different thicknesses and order conditions. The reflection loss of double-layer absorbers, which are composed of a matching layer filled with CoFe2O4 and an absorbing layer filled with CI with various thicknesses, is depicted in Fig. 6. The minimum reflection loss increases with the thickness of the absorbing layer when the matching layer thickness is 0.5 mm. However, the minimum reflection loss first increases, when the matching layer thickness is 1.0 mm, and then decreases with increasing absorbing layer thickness. The reflection loss of double-layer absorber composed of the CIfilled matching layer and the CoFe2O4-filled absorbing layer with varying thicknesses is shown in Fig. 7. When the thickness of CI is 0.5 mm and CoFe2O4 is the absorbing layer, a reduced reflectance peak and an expanded bandwidth may be obtained. When the thickness of CI is 1.0 mm, adding the CoFe2O4 coating can reduce the reflectance peak, but the broadening of the bandwidth is not obvious. When the thickness of CI is 1.5 mm, the effect on the reflectivity peak and absorption band is not improved compared with just a single CI coating layer. When using CoFe2O4 as matching layer, the bandwidth of reflectivity below 10 dB is wider, whereas when CI is the absorbing layer, the peak of reflectivity is lower. In conclusion, double-layer absorbers using CoFe2O4 as matching layer and CI as absorbing layer could achieve better absorbing properties. When the thickness of CI is 0.5 mm, and CoFe2O4 is used as matching layer, the reflectance peak may be reduced and absorption bandwidth may be widened. The reflection loss curves of varying thickness of the doublelayer absorber are shown in Fig. 8. The minimum reflection loss increases with the thickness of the matching layer when CoFe2O4 is less than 2.4 mm thick. When the CoFe2O4 thickness is 2.4 mm and CI thickness is 0.5 mm, a set of excellent absorbing properties is obtained, with minimum reflectance of 38.2 dB, and at less than 10 dB, the bandwidth reaches 9.4 GHz. 4.4. Absorbing performance test A comparison of the measured reflection loss of the designed double-absorbent coating at thickness d = 2.9 mm and the
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Fig. 8. Microwave reflection loss of double-layer absorbers consisting of a matching layer (d1) filled with CoFe2O4 and an absorbing layer (d2) filled with CI from 2 GHz to 18 GHz.
Y. Liu et al. / Journal of Alloys and Compounds 584 (2014) 249–253
properties of CI and CoFe2O4 have been significantly improved, which can be potentially applied in engineering.
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calculated results is illustrated in Fig. 9. Although the measured results and calculated values slightly differ, both measured and theoretically calculated results have similar curve patterns and absolute values. The difference between the calculated and measured values can be attributed to the surface irregularity of the absorber samples, the differences in the actual thickness of the layers, etc. [1]. 5. Conclusions Using CoFe2O4 as matching layer and CI as absorbing layer helps expand the coating absorption bandwidth and reduce the reflectivity. When we chose corresponding thicknesses of 2.4 mm and 0.5 mm, the achieved minimum peak of reflection loss was 38.2 dB, and bandwidth at less than 10 dB reached 9.4 GHz. The results illustrate that using CoFe2O4 as matching layer can effectively expand the bandwidth. Through the comparison between the measured and calculated reflection losses for the designed double-absorbent coating, the accuracy of optimal results was verified. Compared with single-layer absorber, the absorbing
[1] Z.Y. Zhang, X.X. Liu, X.J. Wang, Y.P. Wu, R. Li, J. Alloys Comp. 525 (2012) 114– 119. [2] W.B. Yang, Y.Y. Fu, A. Xia, K. Zhang, Z. Wu, J. Alloys Comp. 518 (2012) 6–10. [3] Z.Y. Zhang, X.X. Liu, X.J. Wang, Y.P. Wu, Y. Liu, J. Magn. Magn. Mater. 324 (2012) 2177–2182. [4] G. Liu, L.Y. Wang, G.M. Chen, S.C. Hua, C.Q. Ge, H. Zhang, R.B. Wu, J. Alloys Comp. 514 (2012) 183–188. [5] H. Mosallaei, Y. Rahmat, IEEE Trans. Antenn Propag. 48 (2000) 1594–1606. [6] T. Giaimakopoulou, A. Oikonomou, G. Kordas, J. Magn. Magn. Mater. 271 (2004) 224–229. [7] J.M. Zhang, J. Sun, Ship Electron. Eng. 32 (2012) 18–21. [8] G.X. Tong, J.G. Guan, W.Y. Zhang, W. Zhang, W. Wang, D.M. Dong, Acta Metall. Sin. 44 (2008) 1001–1005. [9] Y.K. Liu, Y.J. Feng, X.W. Wu, X.G. Han, J. Alloys Comp. 472 (2009) 441–445. [10] X. Shen, Y.X. Wang, X. Yang, L.Q. Lu, L. Huang, J. Mater. Sci.: Mater. Electron. 21 (2010) 630–634. [11] P.C. Fannin, C.N. Marin, I. Malaescu, N. Stefu, P. Vlazan, S. Novaconi, P. Sfirloaga, S. Popescu, C. Couper, Mater. Des. 32 (2011) 1600–1607. [12] R.S. Meena, Sudeshna Bhattachrya, Ratnamala Chatterjee, Mater. Sci. Eng. B 171 (2010) 133–138. [13] S. Ashima, A. Sanghi, R. Agarwal, J. Alloys Comp. 513 (2012) 436–444. [14] X.X. Chen, Y. Ye, J.P. Cheng, J. Inorg. Mater. 26 (2011) 449–457. [15] C.Q. Yu, C.Z. Wan, L. Fa, M.Z. Dong, J. Magn. Magn. Mater. 321 (2009) 1591– 1593. [16] R. Han, J.Q. Wei, X.H. Han, H.B. Yi, T. Wang, F.S. Li, Chinese Sci. B 55 (2010) 2570–2575. [17] R.Z. Cui, B.Y. Wang, T. Wang, F.S.J. Li, Funct. Mater. 42 (2011) 218–221. [18] L.X. Wang, J. Zhang, X.G. Huang, G.Z. Wang, Q.T.J. Zhang, Mater. Sci. Eng. 29 (2011) 688–691. [19] Y.W. Chun, Q.S. Xiang, Z.S. Fu, W.Z. Yong, D.W. Ying, Mater. Des. 35 (2012) 363–368. [20] Y. Naito, K. Suetake, IEEE Trans. Microwave Theory Technol. 19 (1971) 65–72. [21] G.X. Zeng, H.Y. Zhang, Y. Ge, T.L. Chen, J. Mater. Eng. 5 (2009) 49–52. [22] M.R. Meshram, N.K. Agrawal, Bharoti Sinha, P.S. Misra, J. Magn. Magn. Mater. 271 (2004) 207–214. [23] Z.D. Guan, Z.T. Zhang, J.S. Jiao, Physical Property of Inorganic Materials, Beijing, 1992. [24] P.S. Neelakanta, J. Phys. Condens. Mat. 2 (1990) 4935–4947. [25] P.C.P. Watts, W.K. Hsu, A. Barnes, B. Chambers, Adv. Mater. 15 (2003) 600–603. [26] Y.B. Feng, T. Qiu, C.Y. Shen, J. Magn. Magn. Mater. 318 (2007) 8–13. [27] S.T. Jiang, W. Li, Condensed Magnetic Matter, Science Press, Beijing, 2003. [28] M.Z. Wu, Y.D. Zhang, S. Hui, T.D. Xiao, S.H. Ge, W.A. Hines, J.I. Budnick, G.W. Taylor, Appl. Phys. Lett. 23 (2002) 4404.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Double-layer microwave absorber based on CoFe2O4 ferrite and carbonyl iron composites Yuan Liu, Xiangxuan Liu ⇑, Xuanjun Wang No. 603 Faculty, Xi’an Research Institute of High Technology, Xi’an 710025, China
a r t i c l e
i n f o
Article history: Received 5 June 2013 Received in revised form 2 August 2013 Accepted 9 September 2013 Available online 19 September 2013 Keywords: Absorbing materials Impedance matching Double-layer Electromagnetic properties
a b s t r a c t The microwave absorption properties of carbonyl iron (CI) and CoFe2O4 ferrite with single-layer and double-layer composite absorbers were investigated based on the electromagnetic transmission line theory and the impedance matching principle in the frequency range from 2 GHz to 18 GHz. XRD and SEM were used to characterize the structure of the powdered composites. Coaxial method was used to measure the electromagnetic parameters of CI and CoFe2O4. The reflectances of the matching and absorbing layers of different thicknesses and orders were also calculated. The results show that using CoFe2O4 as matching layer and CI as absorbing layer can greatly expand the coating absorption bandwidth and decrease the reflectance peak. Double-layer absorbers have much better microwave absorption properties than single-layer absorbers. The microwave absorption properties of the double-layer structure are influenced by the coupling interactions between the absorbing and matching layers. The double-layer microwave absorbers with reflection loss less than 10 dB over the range from 8.6 GHz to 18 GHz (the absorption bandwidth is 9.4 GHz), which almost covers the X-band (8.2 –12.4 GHz) and the whole Ku-band (12.4– 18 GHz) were achieved when the thicknesses of CoFe2O4 and CI were 2.4 and 0.5 mm, respectively. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction With the escalating electromagnetic pollution and the development of stealth technology for military platforms, an increasing number of studies have focused on microwave absorbing materials [1–4]. Based on the principle of reducing the radar scattering cross section of the target [5,6], radar stealth technology is divided into two major categories, namely, shape stealth and radar-absorbing coating stealth. Shape stealth needs to change the shape to meet standards, but results in decreased performance. By contrast, radar-absorbing coating technology is often used in radar stealth technology because of its simple maintenance, high performance, low cost, and so on [7]. Carbonyl iron (CI) and ferrite have been extensively applied in absorbing materials because of their advantages, such as simple preparation process, low cost, large magnetic loss angle, and strong absorbing ability [8–17]. However, drawbacks, such as narrow resonance frequency band and large matching thickness, limit their further application in stealth fields. Numerous studies on absorbing coating that works effectively in certain frequency and absorption bandwidth have been conducted. In single-layer absorbing materials, the number of parameters is limited and absorption
⇑ Corresponding author. Tel.: +86 02983348158. E-mail address: [email protected] (X. Liu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.049
bandwidth is narrow. Single-layer absorbing materials cannot easily satisfy the requirements for broad-band absorption. Therefore, the development of multilayer absorbing materials becomes increasingly important to fully utilize the properties of different layers of absorbing agents absorbing at a wider bandwidth to achieve optimal absorption [18,19]. In this study, we designed a novel double-layer microwave absorber by combining the magnetic characteristics of CoFe2O4 ferrite and CI to achieve a unique kind of absorber with wide band absorption, simple production process, and low cost.
2. Theory of double-layer microwave absorption The geometry of a double-layer absorber is shown in Fig. 1. According to the transmission theory [20], the calculation formulas are as follows [21,22]:
Z in Z 0 RðdBÞ ¼ 20 log10 Z in þ Z 0 Z in ¼ Z 2
pffiffiffiffiffiffiffiffiffiffiffiffiffi Z in1 þ Z 2 tanh jð2pfd2 =cÞ l2 =e2 pffiffiffiffiffiffiffiffiffiffiffiffiffi Z 2 þ Z in1 tanh jð2pfd2 =cÞ l2 =e2
qffiffiffiffiffiffiffiffiffiffiffiffiffi Z in1 ¼ Z 1 tanh jð2pfd1 =cÞ l1 =e1
ð1Þ
ð2Þ
ð3Þ
250
Y. Liu et al. / Journal of Alloys and Compounds 584 (2014) 249–253
4. Results and discussion 4.1. Characteristics of CoFe2O4 and CI powder
Fig. 1. Geometry of a double-layer absorber.
where Zin is the input impedance at the free space and material interface; Zin1 is the input impedance at the first and second layers; Z0, Z1, and Z2 denote the characteristics impedance of a vacuum, the first layer, and the second layer, respectively; e is the relative complex permittivity; and l is the relative complex permeability. Let d, c, and f be the thickness of layer, velocity of light in free space, and the frequency of the electromagnetic (EM) wave in free space, respectively. 3. Experimental
The XRD spectra of CoFe2O4 and CI are illustrated in Fig. 2. Characterized XRD peaks in the CoFe2O4 spectrum (Fig. 2a) were compared with the characteristic XRD peaks of the sample spectrum with the powder diffraction card database (JCPDS 22-1068). CoFe2O4 anastomosis was very good, no peaks corresponding to impurities were found. Sharp XRD peaks were found for the crystal plane. The relative intensity and the crystalline form tend to be complete, indicating that pure CoFe2O4 powder was generated. The XRD spectrum of CI (Fig. 2b) is consistent with the standard pattern (JCPDS 06-0696). The obvious crystal phase structure and oxidefree diffraction peaks correspond to that of unoxidized CI. The scanning electron micrographs for the CoFe2O4 and CI samples are shown in Figs. 3a and b, respectively. CoFe2O4 powder particles (Fig. 3a) have sharp edges and corners, polyhedron in shape, with size of approximately 2 lm, and only a small amount of which are larger than 3 lm. CI particles (Fig. 3b) are spherical in shape with diameter ranging from 1 lm to 4 lm. They seem to gather in small groups.
3.1. Sample preparation The samples used for EM parameter measurement were prepared by mixing 50 wt.% CI power and 50 wt.% wax as well as 50 wt.% CoFe2O4 powder and 50 wt.% wax. The powder-wax composites were thoroughly stirred by mortar grinding. A proper amount of alcohol was added, and then composites were sheared in a high-speed emulsifying machine, evaporated to dryness at room temperature, and milled into powder. Lastly, the composites were cast into toroid-shaped specimens with 3 mm inner diameter, 7.0 mm outer diameter, and 2–5 mm long. The coating used for the reflection loss of the double-layer absorber measurement was prepared as follows. An aluminum substrate with a standard size (18 mm 18 mm 3 mm) was initially washed with pure water, then with acetone and dried for later use. CI powder (50 wt.%) that was used as absorbing layer was dispersed into an epoxy resin and polyamide by solvent addition under a high-energy ultrasonic treatment for 30 min. A hardener was then added into the mixtures, followed by stirring at 1000 rpm for 10 min. Finally, the composite materials were fabricated on an aluminum substrate and dried in an oven (45 °C). After the absorbing layer solidified, matching layers made from 50 wt.% CoFe2O4 powder and 50 wt.% of other materials (epoxy resins, polyamide, a hardener, and some solvent) were fabricated similarly as the absorbing layer. The thickness of the layer was controlled using different thickness models.
3.2. Measurement of the properties and structure The resulting crystalline phases were characterized using X-ray diffraction (XRD; D/max–IIB, Japan). Data were recorded using Cu Ka radiation at 40.0 kV and 100.0 mA in the 2h region from 15° to 70°, with a scanning speed of 15°/min. A VEGA II XMU INCA scanning electron microscope was employed for morphological analysis. The EM parameters (complex permeability and permittivity) were measured using a vector network analyzer (HP-8720ES) in the frequency range from 2 GHz to 18 GHz. The reflection loss of the prepared absorbers versus frequency was studied using an HP 8510B vector network analyzer and standard horn antennas in an anechoic chamber.
a
4.2. Complex permittivity and permeability of samples Complex permittivity and permeability represent the dielectric and the dynamic magnetic properties of materials. Complex permittivity and permeability of CI and CoFe2O4 powder are shown in Fig. 4. The variation in the real component e0 of complex permittivity for powdered CI and CoFe2O4 are shown in Fig. 4a, in which the real component e0 of the CI ranges from 21.9 to 19.7 as frequency increases (Fig. 4a). The real component e0 of CoFe2O4 remains constant at approximately 3.7 within the frequency from 2 GHz to 18 GHz. Two obvious peaks of e00 for CI are obtained at approximately 4.2 GHz and 12.9 GHz, and one broad peak of e00 for CoFe2O4 is obtained from approximately 13.5 GHz to 18 GHz (Fig. 4b). The dielectric loss presents different loss mechanisms as frequency increases [23]. When the frequency is relatively low, the loss is determined by the peak conductance, and the loss is independent of the frequency. As the frequency increases to the microwave frequency band, the mechanisms involved are relaxation polarization loss and electric conductance loss. The increase in the imaginary component of the permittivity as the frequency increases may be the consequence of the increased relaxation polarization loss and electric conductance loss [24–26]. As frequency increases, the l0 value of CI has a tendency to rapidly decrease within the range 2–18 GHz for the domain-wall motion and relaxation (Fig. 4c) [27]. The l0 value of CoFe2O4 is essentially unchanged between 2 GHz and 15 GHz, then declines
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Fig. 3. Scanning electron micrographs of CoFe2O4 ferrite (a) and CI (b) powders.
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Fig. 4. Complex permittivity (e) and permeability (l) of CI and CoFe2O4 powder.
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slowly from 15 GHz to 16 GHz and becomes stable thereafter. The imaginary l00 component of permeability for CI and CoFe2O4 powder is illustrated in Fig. 4d. The l00 value of CI sharply declines at 2– 4 GHz and becomes stable from 4 GHz to 11 GHz because of the domain-wall resonance and relaxation. The imaginary l00 component of CoFe2O4 is almost constant zero at the frequency range
from 6 GHz to 14 GHz, slowly rises from 14 GHz to 16 GHz. A broad peak could be observed from 4 GHz to 6 GHz. Generally, the microwave magnetic loss of ferrite magnetic materials mainly originates from the domain wall resonance and natural ferromagnetic resonance [28]. Domain wall resonance usually occurs in the low-frequency region (<2 GHz). However, resonance caused by
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the spin rotational component occurs at high-frequency regions [1]. Therefore, the change in the permeability (l0 , l00 ) of CoFe2O4 powder from 2 GHz to 18 GHz is due to resonance. 4.3. Microwave absorption properties of single-layer and double-layer microwave absorbers Microwave reflection loss curves are shown in Fig. 5 with different thicknesses of CI and CoFe2O4. The reflection peak moves to the low-frequency region as the thickness of the absorbent CI coating increases with minimum reflectance peak of 14.6 dB at a thickness of 1 mm, at which the frequency has a width less than
10 dB and reaches 4 GHz (Fig. 5a). When the minimum reflectance is 25.0 dB with a thickness of 2 mm, the frequency width is less than 10 dB and reaches just 2 GHz. A better absorption effect of the CoFe2O4 absorbent will be achieved with a thickness of at least 7 mm (Fig. 5b). Thus, simply using an absorber with only CI or CoFe2O4 cannot satisfy the ‘‘thin and wide’’ requirements of an absorbing material. To increase the absorption bandwidth and reduce the reflectivity of the absorbent coating while keeping it as thin as possible, computer-aided calculations have been performed considering different thicknesses and order conditions. The reflection loss of double-layer absorbers, which are composed of a matching layer filled with CoFe2O4 and an absorbing layer filled with CI with various thicknesses, is depicted in Fig. 6. The minimum reflection loss increases with the thickness of the absorbing layer when the matching layer thickness is 0.5 mm. However, the minimum reflection loss first increases, when the matching layer thickness is 1.0 mm, and then decreases with increasing absorbing layer thickness. The reflection loss of double-layer absorber composed of the CIfilled matching layer and the CoFe2O4-filled absorbing layer with varying thicknesses is shown in Fig. 7. When the thickness of CI is 0.5 mm and CoFe2O4 is the absorbing layer, a reduced reflectance peak and an expanded bandwidth may be obtained. When the thickness of CI is 1.0 mm, adding the CoFe2O4 coating can reduce the reflectance peak, but the broadening of the bandwidth is not obvious. When the thickness of CI is 1.5 mm, the effect on the reflectivity peak and absorption band is not improved compared with just a single CI coating layer. When using CoFe2O4 as matching layer, the bandwidth of reflectivity below 10 dB is wider, whereas when CI is the absorbing layer, the peak of reflectivity is lower. In conclusion, double-layer absorbers using CoFe2O4 as matching layer and CI as absorbing layer could achieve better absorbing properties. When the thickness of CI is 0.5 mm, and CoFe2O4 is used as matching layer, the reflectance peak may be reduced and absorption bandwidth may be widened. The reflection loss curves of varying thickness of the doublelayer absorber are shown in Fig. 8. The minimum reflection loss increases with the thickness of the matching layer when CoFe2O4 is less than 2.4 mm thick. When the CoFe2O4 thickness is 2.4 mm and CI thickness is 0.5 mm, a set of excellent absorbing properties is obtained, with minimum reflectance of 38.2 dB, and at less than 10 dB, the bandwidth reaches 9.4 GHz. 4.4. Absorbing performance test A comparison of the measured reflection loss of the designed double-absorbent coating at thickness d = 2.9 mm and the
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properties of CI and CoFe2O4 have been significantly improved, which can be potentially applied in engineering.
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calculated results is illustrated in Fig. 9. Although the measured results and calculated values slightly differ, both measured and theoretically calculated results have similar curve patterns and absolute values. The difference between the calculated and measured values can be attributed to the surface irregularity of the absorber samples, the differences in the actual thickness of the layers, etc. [1]. 5. Conclusions Using CoFe2O4 as matching layer and CI as absorbing layer helps expand the coating absorption bandwidth and reduce the reflectivity. When we chose corresponding thicknesses of 2.4 mm and 0.5 mm, the achieved minimum peak of reflection loss was 38.2 dB, and bandwidth at less than 10 dB reached 9.4 GHz. The results illustrate that using CoFe2O4 as matching layer can effectively expand the bandwidth. Through the comparison between the measured and calculated reflection losses for the designed double-absorbent coating, the accuracy of optimal results was verified. Compared with single-layer absorber, the absorbing
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