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SiO2 composite particles and their superior electromagnetic properties in microwave band
Materials Letters 64 (2010) 57–60
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis of Fe/SiO2 composite particles and their superior electromagnetic properties in microwave band X.J. Wei, J.T. Jiang, L. Zhen ⁎, Y.X. Gong, W.Z. Shao, C.Y. Xu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 4 June 2009 Accepted 6 October 2009 Available online 9 October 2009 Keywords: Fe/SiO2 Sol–gel preparation Nanocomposite Microstructure Electromagnetic properties EMA performance
a b s t r a c t Fe/SiO2 composite particles were synthesized by hydrogen reduction of Fe2O3/SiO2 precursor, which was prepared by sol–gel method. A reduction temperature higher than 600 °C is required for the complete conversion of Fe2O3 to Fe. Fe/SiO2 composite particles exhibit superior complex permittivity and permeability in the microwave band. A reflection loss higher than −70 dB as well as a broad absorption band can be simultaneously obtained for Fe/SiO2-based coatings about 2 mm in thickness, suggesting that the Fe/SiO2 composite particles are a promising candidate for high performance electromagnetic absorption materials. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, ferromagnetic/dielectric composite particles on nanometer or sub-micron scale have attracted intense attention because of their potential technological importance as electromagnetic wave absorbing (EMA) materials [1–3]. The unique electromagnetic properties of fine ferromagnetic/dielectric composite particles are attributed to the special characteristics in compositions as well as in microstructures. First, ferromagnetic metal structures in composite particles possess high saturation magnetization (Ms) and, in turn, high permeability (μ) and high ferromagnetic losses. Secondly, the introduction of dielectric component is proved to be an effective way to protect fine metal structures from aggregation and oxidation, which greatly improved the serve stability. Furthermore, the multipolarization on ferromagnetic/dielectric interfaces may conduce to high EMA efficiency [4]. Inspired by the potential merits of this category of materials, studies on ferromagnetic/dielectric composite structures were extensively conducted in the past few years. Zhong et al. [5] prepared Fe/ SiO2 and FeNi3/SiO2 composite particles, in which α-Fe or FeNi3 particles were coated with thin SiO2 shells about 0.5 nm in thickness. The prepared Fe/SiO2 composite particles possess high μ′ in the band lower than 1 GHz, but the electromagnetic properties in the microwave band were not investigated. Very recently, Zhang et al. prepared Fe/ZnO nano-capsules through a discharge method [4], and excellent EMA performance was observed in a coating using Fe/ZnO ⁎ Corresponding author. P.O. Box 433, Harbin Institute of Technology, Harbin 150001, China. Tel.: +86 451 8641 2133; fax: +86 451 8641 3922. E-mail address: [email protected] (L. Zhen). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.10.005
nano-capsules as fillers. It was believed that the multi-polarization on Fe/ZnO interfaces contributes to the high EMA performance. In this work, we prepared Fe/SiO2 composite particles through a process similar to that described in Ref. [5]. Microstructure and electromagnetic properties of the prepared composite particles were investigated, and the EMA performances of the coatings containing Fe/SiO2 composite particles as fillers were evaluated. 2. Materials and methods Fe/SiO2 composite particles were prepared by a sol–gel method combined with a hydrogen reduction process similar to previous report [5]. In a typical process, 0.06 mol FeCl2·4H2O and 0.09 mol citric were dissolved in 600 ml ethanol absolute and stirred at 60 °C for 3 h. Then, 1 mL tetraethyl silicate (TEOS) was added into the solution and then the molar ratio of Fe:Si in this final sol is about 13:1. After dried at 80 °C, the obtained xerogel was heat-treated at 450 °C for 3 h in air. The obtained precursory powders were reduced in a H2 atmosphere for 2 h at temperatures of 300, 600, 800 and 900 °C, respectively. The prepared composite particles were named in turn as R300, R600, R800 and R900, respectively. The phase identification and structure analysis of the composite particles were performed by XRD method (Regaku D/max-rB) by using CuKα radiation with a wave length of 1.5418 Å. The morphology of the particle was observed on a scanning electron microscope (SEM, FEI-Sirion). The complex electromagnetic parameters were measured on a vector network analyzer (VNA, Agilent, N5230A) in 2–18 GHz band. The VNA specimens were fabricated by pressing a mixture of Fe/ SiO2 particles and paraffin into a steel mode. The content of Fe/SiO2 was set as 15 vol.% for all VNA specimens. According to the transmit-
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X.J. Wei et al. / Materials Letters 64 (2010) 57–60
Fig. 1. XRD patterns of particles prepared at different temperatures.
line theory [4], the reflection loss (RL) was calculated from the electromagnetic parameters to evaluate the EMA performance of coating using the composite particles as filler.
3. Results and discussion XRD patterns of the composite particles reduced at different temperatures are shown in Fig. 1. It is shown that the particles prepared at 300 °C can be indexed to be Fe2O3, which indicates that the precursory oxide can not be reduced at 300 °C. For the particles prepared at 600, 800 and 900 °C, the diffraction peaks at 44.65, 65.02 and 82.34° could be all indexed to the α-Fe, which reveals a complete
reduction of the precursor Fe2O3. No peak of SiO2 is observed in all these patterns, implying that the SiO2 is of amorphous state. SEM images of the composite particles prepared at different temperatures are shown in Fig. 2. The SEM image of sample R300 demonstrates that the unreduced Fe2O3 are dollops with lots of microvoids. With the increase of reduction temperature, some particles of nanometer scale appear on the dollops, as shown in the SEM image of R600 (Fig. 2(b)). These particles are believed to be Fe particles considering the XRD patterns of R600. The particle size is ulteriorly determined to be less than 100 nm by conducting the Scherrer formula on peaks of α-Fe in the XRD pattern of R600, which confirmed in turn the phase identification. When the reduction temperature increases, these nano-particles grow larger and agglomerate together to form large particles. Specifically, the particles are about 1 μm and 3 μm in diameter in sample R800 and sample R900, respectively. SiO2 can not be distinguished in the SEM images but it is believed that the SiO2 is on the surface of Fe particles as postulated [5]. The content of Fe is about 78.2 vol.% for all the prepared Fe/SiO2 particles the Fe:Si ratio is preset to be 13:1 in all the sol. The complex permittivity and the complex permeability specified are shown in Fig. 3. These measurement results reveal that the electromagnetic properties of the precursor Fe2O3 particles are evidently lower than that of the Fe/SiO2 composite particles. The difference in electromagnetic properties is attributed to the difference in the electrical and magnetic properties of the fillers. Free charges polarization can occur on Fe/SiO2 or Fe/paraffin interfaces because of the conductivity disparity between interfaces, which then leads to a high permittivity. Similar behaviors were previously observed in various kinds of composites with insulate matrix and conductive fillers [6]. Moreover, the polarization in interfaces may lead to high ε″ because of the phase lag, which may benefit the electromagnetic
Fig. 2. SEM images of the products reduced from the precursory Fe2O3/SiO2 at different temperatures, (a) 300 °C, (b) 600 °C, (c) 800 °C and (d) 900 °C.
X.J. Wei et al. / Materials Letters 64 (2010) 57–60
59
Fig. 3. The complex permittivity (a) and complex permeability (b) of the specimens containing particles reduced from the precursory Fe2O3/SiO2 at different temperatures in 2–18 GHz band.
losses. What's more, Fe possesses much higher saturation magnetization (Ms) as compared with Fe2O3, which contributes to the higher permeability according to the Gilbert equation. For specimens containing R600, R800 and R900 as fillers, the complex permittivity as well as the complex permeability decreases with the increase of reduction temperature. This decrease in electromagnetic parameter is due to the increase of the particle size. As discussed above, the permittivity of insulator/conductor composite relays on the collective contributions of the interface polarization in the bulk specimens. When Fe/SiO2 particles grow larger, the total area of insulator/conductor interfaces decreases and thus the permittivity decreases. On the other hand, since Fe particles in simple R800 and simple R900 are good conductors with diameter of a few microns, the induced eddy current will evidently deteriorate the permeability in the microwave band [7]. The reflection loss (RL) of coatings containing Fe/SiO2 composite particles as fillers is demonstrated in Fig. 4. R300 based coatings possess RL no higher than −2.5 dB all through the 2–18 GHz band as shown in Fig. 4(a), suggesting a fairly low EMA efficiency. In contrast, coatings containing Fe/SiO2 composite particles as fillers exhibit much better RL, which infer high EMA efficiency. For instance, R800 based coatings possess high RL when various thicknesses are applied. The typical matching thicknesses for these coatings are 1.5–2.5 mm, which is much thinner than that of EMA coatings using ferrite powders as fillers [8]. R800 based coatings possess RL higher than −70 dB at a matching thickness of 2.138 mm or 1.608 mm; What's
more, the location of absorption peaks can be adjusted conveniently in a wide frequency range by changing the coating thickness. High electromagnetic parameters, specially the dielectric loss induced by the intense dielectric polarization on conductor/insulator interfaces, are believed to contribute to the high EMA performance. Zhang [4] obtained high EMA performance in Fe/ZnO based coatings and believed that the multi-polarization on insulator/conductor interfaces is a critical loss mechanism. Another merit of R800 based coatings is that they present broad absorption band. For instance, the coating of about 2 mm can provide RL higher than 10 dB in a band range 5 GHz in width, which reveals an excellent thickness-matching characteristic, which we think are due to the declivitous characteristic of the electromagnetic parameters. As has been studied, the electromagnetic parameters both μ′ and ε′ should be higher in low-frequency band than that in high-frequency band to obtain good thickness-matching characteristic [9,10]. 4. Conclusions Fe/SiO2 composite particles were prepared through hydrogenthermal reduction of precursory oxide. The precursory Fe2O3 can be reduced to Fe at temperatures higher than 600 °C in a H2 atmosphere and the resulted Fe/SiO2 particles grow larger as the temperature increases. The prepared Fe/SiO2 composite particles are found to possess high permittivity and high permeability because of the intense polarization on conductor/insulator interfaces and the weak
Fig. 4. Frequency dependence of the reflection losses of coatings containing reduced particles as fillers (a) R300, and (b) R800.
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X.J. Wei et al. / Materials Letters 64 (2010) 57–60
influence of eddy current. The coating containing Fe/SiO2 particles exhibit excellent EMA performance including thin thickness, high EMA efficiency and wide absorbing band. It can be concluded that the Fe/SiO2 composite particles are an excellent candidate for high performance EMA fillers. Acknowledgements This work was supported financially by the Science Foundation for Outstanding Youth of Heilongjiang Province, Nature Science Foundation of China (NSFC50671031), and Program of Excellent Team at Harbin Institute of Technology. References [1] Che RC, Peng LM, Duan XF, Chen Q, Liang XL. Microwave absorption enhancement and complex permittivity and complex permeability of Fe encapsulated in carbon nanotubes. Adv Mater 2004;16:401–5.
[2] Liu JR, Itoh M, Machida K. Electromagnetic wave absorption properties of α-Fe/ Fe3B/Y2O3 nanocomposites in gigahertz range. Appl Phys Lett 2003;83:4017–9. [3] Zhang XF, Dong XL, Huang H, Liu YY, Wang WN, Zhu XG, Lv B, Lei JP. Microwave absorption properties of the carbon-coated nickel nanocapsules. Appl Phys Lett 2006;89(053115):1–3. [4] Liu XG, Geng DY, Meng H, Shang PJ, Zhang ZD. Microwave-absorption properties of ZnO-coated iron nanocapsules. Appl Phys Lett 2008;92:173117–1–13. [5] Tang NJ, Zhong W, Jiang HY, Han ZD, Zou WQ, Du YW. core-shell nanoparticles. Solid State Commun 2004;132:71–4. [6] Bowler N. Designing dielectric loss at microwave frequencies using muti-layered filler particles in a composite. IEEE Trans Dielectr Electr Insul 2006;13:703–10. [7] Wu LZ, Ding J, Jiang HB, Chen LF, Ong CK. Particle size influence to the microwave properties of iron based magnetic particular composite. J Magn Magn Mater 2005;285:233–9. [8] Sugimoto S, Haga K, Kagotani T, Inomata K. Microwave absorption properties of Ba M-type ferrite prepared by a modified coprecipitation method. J Magn Magn Mater 2005;290–291:1188–91. [9] Wallace JL. Broadband magnetic microwave absorbers: fundamental limitations. IEEE Trans Magn 1993;29:4209–14. [10] Zhao HF, Qin B, Hu DN, Qin RH. The design rules for single-layer radar absorbents. J Harbin Inst Techn 1993;25:104–10 (in Chinese).
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Synthesis of Fe/SiO2 composite particles and their superior electromagnetic properties in microwave band X.J. Wei, J.T. Jiang, L. Zhen ⁎, Y.X. Gong, W.Z. Shao, C.Y. Xu School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 4 June 2009 Accepted 6 October 2009 Available online 9 October 2009 Keywords: Fe/SiO2 Sol–gel preparation Nanocomposite Microstructure Electromagnetic properties EMA performance
a b s t r a c t Fe/SiO2 composite particles were synthesized by hydrogen reduction of Fe2O3/SiO2 precursor, which was prepared by sol–gel method. A reduction temperature higher than 600 °C is required for the complete conversion of Fe2O3 to Fe. Fe/SiO2 composite particles exhibit superior complex permittivity and permeability in the microwave band. A reflection loss higher than −70 dB as well as a broad absorption band can be simultaneously obtained for Fe/SiO2-based coatings about 2 mm in thickness, suggesting that the Fe/SiO2 composite particles are a promising candidate for high performance electromagnetic absorption materials. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, ferromagnetic/dielectric composite particles on nanometer or sub-micron scale have attracted intense attention because of their potential technological importance as electromagnetic wave absorbing (EMA) materials [1–3]. The unique electromagnetic properties of fine ferromagnetic/dielectric composite particles are attributed to the special characteristics in compositions as well as in microstructures. First, ferromagnetic metal structures in composite particles possess high saturation magnetization (Ms) and, in turn, high permeability (μ) and high ferromagnetic losses. Secondly, the introduction of dielectric component is proved to be an effective way to protect fine metal structures from aggregation and oxidation, which greatly improved the serve stability. Furthermore, the multipolarization on ferromagnetic/dielectric interfaces may conduce to high EMA efficiency [4]. Inspired by the potential merits of this category of materials, studies on ferromagnetic/dielectric composite structures were extensively conducted in the past few years. Zhong et al. [5] prepared Fe/ SiO2 and FeNi3/SiO2 composite particles, in which α-Fe or FeNi3 particles were coated with thin SiO2 shells about 0.5 nm in thickness. The prepared Fe/SiO2 composite particles possess high μ′ in the band lower than 1 GHz, but the electromagnetic properties in the microwave band were not investigated. Very recently, Zhang et al. prepared Fe/ZnO nano-capsules through a discharge method [4], and excellent EMA performance was observed in a coating using Fe/ZnO ⁎ Corresponding author. P.O. Box 433, Harbin Institute of Technology, Harbin 150001, China. Tel.: +86 451 8641 2133; fax: +86 451 8641 3922. E-mail address: [email protected] (L. Zhen). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.10.005
nano-capsules as fillers. It was believed that the multi-polarization on Fe/ZnO interfaces contributes to the high EMA performance. In this work, we prepared Fe/SiO2 composite particles through a process similar to that described in Ref. [5]. Microstructure and electromagnetic properties of the prepared composite particles were investigated, and the EMA performances of the coatings containing Fe/SiO2 composite particles as fillers were evaluated. 2. Materials and methods Fe/SiO2 composite particles were prepared by a sol–gel method combined with a hydrogen reduction process similar to previous report [5]. In a typical process, 0.06 mol FeCl2·4H2O and 0.09 mol citric were dissolved in 600 ml ethanol absolute and stirred at 60 °C for 3 h. Then, 1 mL tetraethyl silicate (TEOS) was added into the solution and then the molar ratio of Fe:Si in this final sol is about 13:1. After dried at 80 °C, the obtained xerogel was heat-treated at 450 °C for 3 h in air. The obtained precursory powders were reduced in a H2 atmosphere for 2 h at temperatures of 300, 600, 800 and 900 °C, respectively. The prepared composite particles were named in turn as R300, R600, R800 and R900, respectively. The phase identification and structure analysis of the composite particles were performed by XRD method (Regaku D/max-rB) by using CuKα radiation with a wave length of 1.5418 Å. The morphology of the particle was observed on a scanning electron microscope (SEM, FEI-Sirion). The complex electromagnetic parameters were measured on a vector network analyzer (VNA, Agilent, N5230A) in 2–18 GHz band. The VNA specimens were fabricated by pressing a mixture of Fe/ SiO2 particles and paraffin into a steel mode. The content of Fe/SiO2 was set as 15 vol.% for all VNA specimens. According to the transmit-
58
X.J. Wei et al. / Materials Letters 64 (2010) 57–60
Fig. 1. XRD patterns of particles prepared at different temperatures.
line theory [4], the reflection loss (RL) was calculated from the electromagnetic parameters to evaluate the EMA performance of coating using the composite particles as filler.
3. Results and discussion XRD patterns of the composite particles reduced at different temperatures are shown in Fig. 1. It is shown that the particles prepared at 300 °C can be indexed to be Fe2O3, which indicates that the precursory oxide can not be reduced at 300 °C. For the particles prepared at 600, 800 and 900 °C, the diffraction peaks at 44.65, 65.02 and 82.34° could be all indexed to the α-Fe, which reveals a complete
reduction of the precursor Fe2O3. No peak of SiO2 is observed in all these patterns, implying that the SiO2 is of amorphous state. SEM images of the composite particles prepared at different temperatures are shown in Fig. 2. The SEM image of sample R300 demonstrates that the unreduced Fe2O3 are dollops with lots of microvoids. With the increase of reduction temperature, some particles of nanometer scale appear on the dollops, as shown in the SEM image of R600 (Fig. 2(b)). These particles are believed to be Fe particles considering the XRD patterns of R600. The particle size is ulteriorly determined to be less than 100 nm by conducting the Scherrer formula on peaks of α-Fe in the XRD pattern of R600, which confirmed in turn the phase identification. When the reduction temperature increases, these nano-particles grow larger and agglomerate together to form large particles. Specifically, the particles are about 1 μm and 3 μm in diameter in sample R800 and sample R900, respectively. SiO2 can not be distinguished in the SEM images but it is believed that the SiO2 is on the surface of Fe particles as postulated [5]. The content of Fe is about 78.2 vol.% for all the prepared Fe/SiO2 particles the Fe:Si ratio is preset to be 13:1 in all the sol. The complex permittivity and the complex permeability specified are shown in Fig. 3. These measurement results reveal that the electromagnetic properties of the precursor Fe2O3 particles are evidently lower than that of the Fe/SiO2 composite particles. The difference in electromagnetic properties is attributed to the difference in the electrical and magnetic properties of the fillers. Free charges polarization can occur on Fe/SiO2 or Fe/paraffin interfaces because of the conductivity disparity between interfaces, which then leads to a high permittivity. Similar behaviors were previously observed in various kinds of composites with insulate matrix and conductive fillers [6]. Moreover, the polarization in interfaces may lead to high ε″ because of the phase lag, which may benefit the electromagnetic
Fig. 2. SEM images of the products reduced from the precursory Fe2O3/SiO2 at different temperatures, (a) 300 °C, (b) 600 °C, (c) 800 °C and (d) 900 °C.
X.J. Wei et al. / Materials Letters 64 (2010) 57–60
59
Fig. 3. The complex permittivity (a) and complex permeability (b) of the specimens containing particles reduced from the precursory Fe2O3/SiO2 at different temperatures in 2–18 GHz band.
losses. What's more, Fe possesses much higher saturation magnetization (Ms) as compared with Fe2O3, which contributes to the higher permeability according to the Gilbert equation. For specimens containing R600, R800 and R900 as fillers, the complex permittivity as well as the complex permeability decreases with the increase of reduction temperature. This decrease in electromagnetic parameter is due to the increase of the particle size. As discussed above, the permittivity of insulator/conductor composite relays on the collective contributions of the interface polarization in the bulk specimens. When Fe/SiO2 particles grow larger, the total area of insulator/conductor interfaces decreases and thus the permittivity decreases. On the other hand, since Fe particles in simple R800 and simple R900 are good conductors with diameter of a few microns, the induced eddy current will evidently deteriorate the permeability in the microwave band [7]. The reflection loss (RL) of coatings containing Fe/SiO2 composite particles as fillers is demonstrated in Fig. 4. R300 based coatings possess RL no higher than −2.5 dB all through the 2–18 GHz band as shown in Fig. 4(a), suggesting a fairly low EMA efficiency. In contrast, coatings containing Fe/SiO2 composite particles as fillers exhibit much better RL, which infer high EMA efficiency. For instance, R800 based coatings possess high RL when various thicknesses are applied. The typical matching thicknesses for these coatings are 1.5–2.5 mm, which is much thinner than that of EMA coatings using ferrite powders as fillers [8]. R800 based coatings possess RL higher than −70 dB at a matching thickness of 2.138 mm or 1.608 mm; What's
more, the location of absorption peaks can be adjusted conveniently in a wide frequency range by changing the coating thickness. High electromagnetic parameters, specially the dielectric loss induced by the intense dielectric polarization on conductor/insulator interfaces, are believed to contribute to the high EMA performance. Zhang [4] obtained high EMA performance in Fe/ZnO based coatings and believed that the multi-polarization on insulator/conductor interfaces is a critical loss mechanism. Another merit of R800 based coatings is that they present broad absorption band. For instance, the coating of about 2 mm can provide RL higher than 10 dB in a band range 5 GHz in width, which reveals an excellent thickness-matching characteristic, which we think are due to the declivitous characteristic of the electromagnetic parameters. As has been studied, the electromagnetic parameters both μ′ and ε′ should be higher in low-frequency band than that in high-frequency band to obtain good thickness-matching characteristic [9,10]. 4. Conclusions Fe/SiO2 composite particles were prepared through hydrogenthermal reduction of precursory oxide. The precursory Fe2O3 can be reduced to Fe at temperatures higher than 600 °C in a H2 atmosphere and the resulted Fe/SiO2 particles grow larger as the temperature increases. The prepared Fe/SiO2 composite particles are found to possess high permittivity and high permeability because of the intense polarization on conductor/insulator interfaces and the weak
Fig. 4. Frequency dependence of the reflection losses of coatings containing reduced particles as fillers (a) R300, and (b) R800.
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X.J. Wei et al. / Materials Letters 64 (2010) 57–60
influence of eddy current. The coating containing Fe/SiO2 particles exhibit excellent EMA performance including thin thickness, high EMA efficiency and wide absorbing band. It can be concluded that the Fe/SiO2 composite particles are an excellent candidate for high performance EMA fillers. Acknowledgements This work was supported financially by the Science Foundation for Outstanding Youth of Heilongjiang Province, Nature Science Foundation of China (NSFC50671031), and Program of Excellent Team at Harbin Institute of Technology. References [1] Che RC, Peng LM, Duan XF, Chen Q, Liang XL. Microwave absorption enhancement and complex permittivity and complex permeability of Fe encapsulated in carbon nanotubes. Adv Mater 2004;16:401–5.
[2] Liu JR, Itoh M, Machida K. Electromagnetic wave absorption properties of α-Fe/ Fe3B/Y2O3 nanocomposites in gigahertz range. Appl Phys Lett 2003;83:4017–9. [3] Zhang XF, Dong XL, Huang H, Liu YY, Wang WN, Zhu XG, Lv B, Lei JP. Microwave absorption properties of the carbon-coated nickel nanocapsules. Appl Phys Lett 2006;89(053115):1–3. [4] Liu XG, Geng DY, Meng H, Shang PJ, Zhang ZD. Microwave-absorption properties of ZnO-coated iron nanocapsules. Appl Phys Lett 2008;92:173117–1–13. [5] Tang NJ, Zhong W, Jiang HY, Han ZD, Zou WQ, Du YW. core-shell nanoparticles. Solid State Commun 2004;132:71–4. [6] Bowler N. Designing dielectric loss at microwave frequencies using muti-layered filler particles in a composite. IEEE Trans Dielectr Electr Insul 2006;13:703–10. [7] Wu LZ, Ding J, Jiang HB, Chen LF, Ong CK. Particle size influence to the microwave properties of iron based magnetic particular composite. J Magn Magn Mater 2005;285:233–9. [8] Sugimoto S, Haga K, Kagotani T, Inomata K. Microwave absorption properties of Ba M-type ferrite prepared by a modified coprecipitation method. J Magn Magn Mater 2005;290–291:1188–91. [9] Wallace JL. Broadband magnetic microwave absorbers: fundamental limitations. IEEE Trans Magn 1993;29:4209–14. [10] Zhao HF, Qin B, Hu DN, Qin RH. The design rules for single-layer radar absorbents. J Harbin Inst Techn 1993;25:104–10 (in Chinese).