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Preparation and microwave absorption properties of Ni–Fe3O4 hollow spheres
Materials Science and Engineering B 164 (2009) 112–115
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Preparation and microwave absorption properties of Ni–Fe3 O4 hollow spheres Zhibin Li, Yida Deng, Bin Shen, Wenbin Hu ∗ State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, PR China
a r t i c l e
i n f o
Article history: Received 15 October 2008 Received in revised form 5 May 2009 Accepted 2 August 2009 Keywords: Hollow spheres Microwave absorption Magnetite
a b s t r a c t Nickel–magnetite hollow spheres with the diameter about 150 nm were prepared by an autocatalytic reduction method. The effect of Fe3 O4 content in hollow spheres on microwave absorption properties has been investigated. With an increase of magnetite content, permittivity of sphere–wax composite decreased, while permeability increased. For composite layers, a minimum reflection loss (RL, −38.4 dB) was predicted at 10.8 GHz with a thickness of 2.5 mm. In addition, the increasing content of magnetite resulted in the peak of RL moves to higher frequency and the minimal reflection loss increases obviously. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, electromagnetic interference (EMI) has become a more serious problem due to wide application of electromagnetic waves in gigahertz (GHz) range for mobile phone, local area network, radar systems and so on. Microwave absorbing materials are effective reducing EMI and therefore intensively attractive. A number of materials have been described in the prior research works [1–6], which are capable of absorbing electromagnetic radiation. However, the conventional absorptive materials such as metal powder and ferrites are quite heavy, which restricts their use in application requiring lightweight. As one of ways to overcome this problem, microwave absorptive materials with low density have been developed [7–10]. Magnetic hollow powder is one of materials with low density used as microwave absorber. Recently, applications of nickel hollow spheres or fiber on microwave absorption have been reported [11–13]. However, studies of hollow magnetic composites for the application in microwave absorbers were very limited. In the presented study, the microwave absorption behaviors of Ni–Fe3 O4 hollow spheres by an autocatalytic reduction method are reported. 2. Experimental procedure 2.1. Preparation Analytically pure nickel sulfate (NiSO4 ·6H2 O), ferrous sulfate (FeSO4 ·7H2 O), sodium hypophosphite (NaH2 PO2 ·H2 O) and sodium hydroxide (NaOH) were used as starting materials. During the
∗ Corresponding author. Tel.: +86 21 34202987; fax: +86 21 34202749. E-mail address: material [email protected] (W. Hu). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.08.004
preparation process, firstly, 21 g nickel sulfate and 5.6 g ferrous sulfate were dissolved in de-ionized water to get 200 ml mixture solution. At the same time, using 6.8 g sodium hydroxide and 21.2 g sodium hypophosphite, 200 ml alkali solution and reducing agent were obtained, respectively. All solutions were preheated at 90 ◦ C for 5 min. Then the mixture solution and alkali solution were mixed by violent stirring, followed by producing a viridescent colloid. And then the sodium hypophosphite solution was added to the asprepared colloid by evenly stirring. Finally, after the reaction had taken place for about 2 h, one dark-gray powder was obtained. The powder was repeatedly washed with dilute hydrochloric acid and de-ionized water. After being dried in vacuum furnace at 100 ◦ C, 5.2 g final products were obtained. By adjusting the mol ratio between nickel sulfate and ferrous sulfate in the mixture solution at the first step, hollow spheres with different content ratio between Ni and Fe3 O4 were obtained. In this work, four samples (1–4) were prepared and corresponding mol ratios in the mixture solution are 4:1, 3:2, 1:1 and 2:3, respectively. 2.2. Characterization The morphology of Ni–Fe3 O4 hollow spheres was examined using a FEI SIRION 200 field emission scanning electron microscope (FESEM) and a JEOL-2100 transmission electron microscope (TEM). Phase analysis of the particles was studied from 20◦ to 80◦ using a D/max 2550 V X-ray diffractometer at 40 kV and 100 mA (Cu K␣ radiation). The density of the powders was measured by using Archimedes’ principle. In the range of 2–18 GHz, the complex relative permittivity and permeability were measured on an Agilent E8362B network analyzer by using coaxial transmission/reflection (T/R) technology. Samples were prepared by dispersing the powders randomly in wax with volume fraction of 40%, and then toroid samples were fab-
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Fig. 3. XRD spectra of hollow spheres. Fig. 1. Typical FESEM (a), TEM (b) micrograph and ED pattern (c) of hollow spheres.
described as follows [14]: ricated with inner diameter of 3 mm and outer diameter of 7 mm. According the transmit-line theory, the reflection loss was calculated by using measured complex permittivity and permeability. 3. Results and discussion The morphology of Ni–Fe3 O4 hollow spheres was observed using FESEM and TEM. All samples show similar characters. The typical micrographs of hollow spheres are shown in Fig. 1. Samples show sphere morphology with the diameter of about 150 nm. According to the TEM image in Fig. 1b, hollow structure can be seen. The formation mechanism of hollow structure is similar to that of nickel hollow spheres [11]. During the preparation process, as shown in Fig. 2, the colloidal particles containing Ni(OH)2 and Fe(OH)2 were used as sacrificed template. When the sodium hypophosphite was added into the solution, the redox reaction was catalyzed around the colloidal particles. And then the grid comprising nickel and magnetite was generated quickly. At the same time, with the consumption of Ni2+ , Fe2+ and OH− , Ni(OH)2 and Fe(OH)2 continually decomposed in order to maintain equilibrium. The colloidal particle became smaller and smaller, and eventually disappeared. With the redox reactions and deposition process proceeding, the nickel and magnetite assembled on the previously produced grids and a shell was formed finally. In Fig. 1c, the electron diffraction (ED) pattern indicates that hollow spheres are made of nanocrystalline and the crystal structure is face-centered cubic. During preparation process, nickel was reduced by sodium hypophosphite, and the autocatalysis reduction reactions can be
Ni2+ + H2 PO2 − + H2 O → Ni + HPO3 2− + 3H+
(1)
The formation mechanism of magnetite was not the reduction reaction, but maybe a chemical control oxidation from Fe2+ in a reducing atmosphere of aqueous sodium hypophosphite [15]. The total reaction is related: 3Fe2+ + H2 PO2 − + O2 + 7OH− → Fe3 O4 + HPO3 2− + 4H2 O
(2)
The presence of nickel and magnetite in hollow spheres was confirmed by XRD pattern shown in Fig. 3. The patterns could be indexed to magnetite with inverse cubic spinel structure and face-centered cubic (fcc) nickel and the crystal structure is consistent with ED result in Fig. 1c. The broad peaks and strong noise background indicate that hollow spheres are the mixture of noncrystalline and nanocrystalline. According to the EDX results in Table 1, with an increase of concentration of ferrous sulfate in the mixture solution during the preparation process, content of magnetite in hollow spheres increased, which resulted in the enhancement of peaks of magnetite in XRD pattern. It is found that the P content in samples increases with an increase of Fe element. The phosphorus is a by-product during the preparation process and the reaction can be described as: 3H2 PO2 − → H2 PO3 − + H2 O + 2OH− + 2P
(3)
OH−
ion are produced at the It can be seen that phosphorus and same time. According to the reaction equation (2), the production of magnetite will consume OH− ion. With the increase of magnetite content in samples, the consumption of OH− ion increases and the deposition of P is accelerated. The density of hollow spheres is also listed in Table 1 and it can be found that, because of hollow structure, the value is smaller than those of both nickel (8.88 g/cm3 ) and magnetite (5.17 g/cm3 ). Table 1 EDX results and density of Ni–Fe3 O4 hollow spheres prepared with different mol rate of Ni2+ /Fe2+ .
Fig. 2. Scheme of procedure for preparing Ni–Fe3 O4 hollow spheres.
Sample
Ni (at.%)
Fe (at.%)
P (at.%)
Density (g/cm3 )
1 2 3 4
79 71 67 35
8 14 24 45
13 15 19 20
3.11 3.31 3.42 3.42
114
Z. Li et al. / Materials Science and Engineering B 164 (2009) 112–115
Fig. 4. The real (a) and imaginary (b) part of complex permittivity of samples 1–4.
The microwave absorption properties of Ni–Fe3 O4 hollow spheres were investigated at the range of 2–18 GHz. Fig. 4 illustrates complex permittivity (εr = εr − jεr ) of spheres–wax composites with volume ratio of 4:6. It can be seen that all samples show a similar variety trend and both εr and εr are sensitive to content of magnetite Fe3 O4 . The increase of Fe3 O4 content in hollow spheres results in the decrease of conductivity, which causes that the value of both real and imaginary parts of permittivity becomes smaller. With an increase of frequency, the real parts of permittivity decrease lightly, but imaginary parts increase. And resonance peaks present around 14.2 GHz. The resonance position is consistent with that of Fe3 O4 –coated hollow glass spheres [9]. In general, the permittivity originates from orientation polarization, atomic polarization and electronic polarization. Normally, the resonance caused by vacancy or pores usually dominates in the low-frequency regions. High-frequency resonance is attributed to atomic and electronic polarization [16]. So resonance peaks in the curves of both the real and imaginary parts of complex permittivity can be interpreted as the results of atomic and electronic polarization. Complex permeability (r = r − jr ) of powder–wax composites are shown in Fig. 5. As the Fe3 O4 content increases in hollow spheres, both real and imaginary parts of permeability increase. As shown in Fig. 5a, real parts r of complex permeability for all samples decrease with the increasing frequency, which is due to both eddy current loss and ferromagnetic resonance [17]. In Fig. 5b, with an increase of frequency, imaginary parts r of complex permeability decrease and a resonance peak appears around 5.6 GHz.
Generally, for ferrite magnetic materials, the microwave magnetic loss originates mainly from domain wall resonance and natural ferromagnetic resonance. Resonance due to domain wall movement normally occurs at low-frequency region (<2 GHz). However, resonance due to spin rotational component occurs at high-frequency region. So the resonance peak may attribute to the natural resonance. For the composite layer terminated by metal plate, the normalized input impedance Zin at the surface is given by [18]:
Zin =
2fd √
r tanh j εr
c
r εr
(4)
where f is the frequency of electromagnetic wave, d is the thickness of the composite layer and c is the velocity of electromagnetic wave in free space. The reflection loss of normal incident electromagnetic wave at the surface of composite layer can be calculated using the following equation:
Zin − 1 Z +1
RL = 20 log
(5)
in
By using the actual values of complex permittivity and permeability as shown in Figs. 4 and 5, the reflection loss of all samples were calculated. Fig. 6 shows the results for sample 4 at different thickness. The minimal reflection loss corresponds to the occurrence of minimal reflection of the microwave power for the
Fig. 5. The real (a) and imaginary (b) part of complex permeability of samples 1–4.
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former, which causes the enhancement of microwave absorption properties. 4. Conclusion Ni–Fe3 O4 hollow spheres with the diameter about 150 nm were prepared by an autocatalytic reduction method. The influence of Fe3 O4 content in hollow spheres on microwave absorption properties has been investigated. With an increase of magnetite content, permittivity of sphere–wax composite decreased, while the value of permeability increased. By using actual values of complex permittivity and permeability, reflection loss of composite layers was calculated. The increasing content of magnetite resulted in the peak of RL moves to higher frequency and the minimal reflection loss increases obviously. Additionally, a minimum reflection loss (−38.4 dB) was predicted at 10.8 GHz with a thickness of 2.5 mm. Fig. 6. Reflection loss of sample 4 with various thicknesses (1.5, 2.0, 2.5 and 3.0 mm).
Acknowledgments This work is supported by National Natural Science Foundation of China (Grant no. 50474004), Shanghai Science and Technology Committee Nano Special Fund (Grant no. 0552nm004), the “Dawn” Program of Shanghai Education Commission and the New-Century Training Program Foundation for Talents from the Ministry of Education of China. The authors would like to thank the Instrumental Analysis Center of Shanghai Jiao Tong University for the characterization of samples. References
Fig. 7. Reflection loss of samples 1–4 at the thickness of 2 mm.
particular thickness. As shown in Fig. 6, the reflection loss is dependent sensitively on the thickness of the absorber. As thickness of composite layer increases, the peak of RL is moving to lower frequency and becomes narrower. For Ni–Fe3 O4 hollow spheres, the maximum attenuation of the incident wave is observed for sample 4 with a thickness of 2.5 mm, and the peak value of reflection loss is −38.4 dB, which is better than that of hollow glass or ceramic spheres coated with magnetic films [7,9,10]. Fig. 7 shows the reflection loss for the composites consisting of various powders. All samples have a thickness of 2 mm. As shown in Fig. 6, with the increase of magnetite content, the peak of RL moves to higher frequency and the minimal reflection loss increases obviously, which indicates that increasing magnetite content in hollow spheres improve the microwave absorption properties. Compared with hollow polyaniline/Fe3 O4 microsphere [19], the influence of the magnetite content on microwave absorption properties of Ni–Fe3 O4 shows a similar character. Introducing the magnetite into hollow spheres will reduce the dielectric loss and enhance the magnetic loss. And the effect of the later is stronger than that of the
[1] A.N. Yusoff, M.H. Abdullah, S.H. Ahmad, S.F. Jusoh, A.A. Mansor, S.A.A. Hamid, Journal of Applied Physics 92 (2002) 876–882. [2] H. Guo, W. Xu, M.H. Cui, N.L. Yang, D.L. Akins, Chemical Communications 9 (2003) 1432–1433. [3] S. Sugimoto, K. Haga, T. Kagotani, K. Inomata, Journal of Magnetism and Magnetic Materials 290–291 (Part 2) (2005) 1188–1191. [4] Z. Zhou, L. Chu, S. Hu, Materials Science and Engineering B: Solid-State Materials for Advanced Technology 126 (2006) 93–96. [5] S. Sano, S. Takayama, Y. Takao, A. Tsuzuki, Y. Makino, ISIJ International 47 (2007) 588–591. [6] G. Alvareza, H. Montielb, M.A. Castellanosc, J. Heirasd, R. Valenzuelaa, Materials Science and Engineering B 150 (2008) 175–179. [7] Z.W. Liu, L.X. Phua, Y. Liu, C.K. Ong, Journal of Applied Physics 100 (2006), 093902-093901-093906. [8] G. Mu, N. Chen, X. Pan, K. Yang, M. Gu, Applied Physics Letters 91 (2007). [9] J. Wei, J. Liu, S. Li, Journal of Magnetism and Magnetic Materials 312 (2007) 414–417. [10] S.S. Kim, S.T. Kim, J.M. Ahn, K.H. Kim, Journal of Magnetism and Magnetic Materials 271 (2004) 39–45. [11] Y. Deng, X. Liu, B. Shen, L. Liu, W. Hu, Journal of Magnetism and Magnetic Materials 303 (2006) 181–184. [12] H. Zhang, Y. Liu, Q. Jia, H. Sun, S. Li, Powder Technology 178 (2007) 22–29. [13] H. Zhang, Y. Liu, Journal of Alloys and Compounds 458 (2008) 588– 594. [14] X. Jiang, W. Shen, The Fundamental and Practice of Electroless Plating, National Defense Industry Press, Beijing, 2000. [15] R. Fan, X.H. Chen, Z. Gui, L. Liu, Z.Y. Chen, Materials Research Bulletin 36 (2001) 497–502. [16] A. Verma, A.K. Saxena, D.C. Dube, Journal of Magnetism and Magnetic Materials 263 (2003) 228–234. [17] D. Wan, X. Ma, Physics of Magnetism, University of Electric Science and Technology of China, Chengdu, 1994, pp. 406–459. [18] S.M. Abbas, R.P.R.C. Aiyar, O. Prakash, Bulletin of Materials Science 21 (1998) 279–282. [19] C. Yang, H. Li, D. Xiong, Z. Cao, Reactive & Functional Polymers 69 (2009) 137–144.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Preparation and microwave absorption properties of Ni–Fe3 O4 hollow spheres Zhibin Li, Yida Deng, Bin Shen, Wenbin Hu ∗ State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, PR China
a r t i c l e
i n f o
Article history: Received 15 October 2008 Received in revised form 5 May 2009 Accepted 2 August 2009 Keywords: Hollow spheres Microwave absorption Magnetite
a b s t r a c t Nickel–magnetite hollow spheres with the diameter about 150 nm were prepared by an autocatalytic reduction method. The effect of Fe3 O4 content in hollow spheres on microwave absorption properties has been investigated. With an increase of magnetite content, permittivity of sphere–wax composite decreased, while permeability increased. For composite layers, a minimum reflection loss (RL, −38.4 dB) was predicted at 10.8 GHz with a thickness of 2.5 mm. In addition, the increasing content of magnetite resulted in the peak of RL moves to higher frequency and the minimal reflection loss increases obviously. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In recent years, electromagnetic interference (EMI) has become a more serious problem due to wide application of electromagnetic waves in gigahertz (GHz) range for mobile phone, local area network, radar systems and so on. Microwave absorbing materials are effective reducing EMI and therefore intensively attractive. A number of materials have been described in the prior research works [1–6], which are capable of absorbing electromagnetic radiation. However, the conventional absorptive materials such as metal powder and ferrites are quite heavy, which restricts their use in application requiring lightweight. As one of ways to overcome this problem, microwave absorptive materials with low density have been developed [7–10]. Magnetic hollow powder is one of materials with low density used as microwave absorber. Recently, applications of nickel hollow spheres or fiber on microwave absorption have been reported [11–13]. However, studies of hollow magnetic composites for the application in microwave absorbers were very limited. In the presented study, the microwave absorption behaviors of Ni–Fe3 O4 hollow spheres by an autocatalytic reduction method are reported. 2. Experimental procedure 2.1. Preparation Analytically pure nickel sulfate (NiSO4 ·6H2 O), ferrous sulfate (FeSO4 ·7H2 O), sodium hypophosphite (NaH2 PO2 ·H2 O) and sodium hydroxide (NaOH) were used as starting materials. During the
∗ Corresponding author. Tel.: +86 21 34202987; fax: +86 21 34202749. E-mail address: material [email protected] (W. Hu). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.08.004
preparation process, firstly, 21 g nickel sulfate and 5.6 g ferrous sulfate were dissolved in de-ionized water to get 200 ml mixture solution. At the same time, using 6.8 g sodium hydroxide and 21.2 g sodium hypophosphite, 200 ml alkali solution and reducing agent were obtained, respectively. All solutions were preheated at 90 ◦ C for 5 min. Then the mixture solution and alkali solution were mixed by violent stirring, followed by producing a viridescent colloid. And then the sodium hypophosphite solution was added to the asprepared colloid by evenly stirring. Finally, after the reaction had taken place for about 2 h, one dark-gray powder was obtained. The powder was repeatedly washed with dilute hydrochloric acid and de-ionized water. After being dried in vacuum furnace at 100 ◦ C, 5.2 g final products were obtained. By adjusting the mol ratio between nickel sulfate and ferrous sulfate in the mixture solution at the first step, hollow spheres with different content ratio between Ni and Fe3 O4 were obtained. In this work, four samples (1–4) were prepared and corresponding mol ratios in the mixture solution are 4:1, 3:2, 1:1 and 2:3, respectively. 2.2. Characterization The morphology of Ni–Fe3 O4 hollow spheres was examined using a FEI SIRION 200 field emission scanning electron microscope (FESEM) and a JEOL-2100 transmission electron microscope (TEM). Phase analysis of the particles was studied from 20◦ to 80◦ using a D/max 2550 V X-ray diffractometer at 40 kV and 100 mA (Cu K␣ radiation). The density of the powders was measured by using Archimedes’ principle. In the range of 2–18 GHz, the complex relative permittivity and permeability were measured on an Agilent E8362B network analyzer by using coaxial transmission/reflection (T/R) technology. Samples were prepared by dispersing the powders randomly in wax with volume fraction of 40%, and then toroid samples were fab-
Z. Li et al. / Materials Science and Engineering B 164 (2009) 112–115
113
Fig. 3. XRD spectra of hollow spheres. Fig. 1. Typical FESEM (a), TEM (b) micrograph and ED pattern (c) of hollow spheres.
described as follows [14]: ricated with inner diameter of 3 mm and outer diameter of 7 mm. According the transmit-line theory, the reflection loss was calculated by using measured complex permittivity and permeability. 3. Results and discussion The morphology of Ni–Fe3 O4 hollow spheres was observed using FESEM and TEM. All samples show similar characters. The typical micrographs of hollow spheres are shown in Fig. 1. Samples show sphere morphology with the diameter of about 150 nm. According to the TEM image in Fig. 1b, hollow structure can be seen. The formation mechanism of hollow structure is similar to that of nickel hollow spheres [11]. During the preparation process, as shown in Fig. 2, the colloidal particles containing Ni(OH)2 and Fe(OH)2 were used as sacrificed template. When the sodium hypophosphite was added into the solution, the redox reaction was catalyzed around the colloidal particles. And then the grid comprising nickel and magnetite was generated quickly. At the same time, with the consumption of Ni2+ , Fe2+ and OH− , Ni(OH)2 and Fe(OH)2 continually decomposed in order to maintain equilibrium. The colloidal particle became smaller and smaller, and eventually disappeared. With the redox reactions and deposition process proceeding, the nickel and magnetite assembled on the previously produced grids and a shell was formed finally. In Fig. 1c, the electron diffraction (ED) pattern indicates that hollow spheres are made of nanocrystalline and the crystal structure is face-centered cubic. During preparation process, nickel was reduced by sodium hypophosphite, and the autocatalysis reduction reactions can be
Ni2+ + H2 PO2 − + H2 O → Ni + HPO3 2− + 3H+
(1)
The formation mechanism of magnetite was not the reduction reaction, but maybe a chemical control oxidation from Fe2+ in a reducing atmosphere of aqueous sodium hypophosphite [15]. The total reaction is related: 3Fe2+ + H2 PO2 − + O2 + 7OH− → Fe3 O4 + HPO3 2− + 4H2 O
(2)
The presence of nickel and magnetite in hollow spheres was confirmed by XRD pattern shown in Fig. 3. The patterns could be indexed to magnetite with inverse cubic spinel structure and face-centered cubic (fcc) nickel and the crystal structure is consistent with ED result in Fig. 1c. The broad peaks and strong noise background indicate that hollow spheres are the mixture of noncrystalline and nanocrystalline. According to the EDX results in Table 1, with an increase of concentration of ferrous sulfate in the mixture solution during the preparation process, content of magnetite in hollow spheres increased, which resulted in the enhancement of peaks of magnetite in XRD pattern. It is found that the P content in samples increases with an increase of Fe element. The phosphorus is a by-product during the preparation process and the reaction can be described as: 3H2 PO2 − → H2 PO3 − + H2 O + 2OH− + 2P
(3)
OH−
ion are produced at the It can be seen that phosphorus and same time. According to the reaction equation (2), the production of magnetite will consume OH− ion. With the increase of magnetite content in samples, the consumption of OH− ion increases and the deposition of P is accelerated. The density of hollow spheres is also listed in Table 1 and it can be found that, because of hollow structure, the value is smaller than those of both nickel (8.88 g/cm3 ) and magnetite (5.17 g/cm3 ). Table 1 EDX results and density of Ni–Fe3 O4 hollow spheres prepared with different mol rate of Ni2+ /Fe2+ .
Fig. 2. Scheme of procedure for preparing Ni–Fe3 O4 hollow spheres.
Sample
Ni (at.%)
Fe (at.%)
P (at.%)
Density (g/cm3 )
1 2 3 4
79 71 67 35
8 14 24 45
13 15 19 20
3.11 3.31 3.42 3.42
114
Z. Li et al. / Materials Science and Engineering B 164 (2009) 112–115
Fig. 4. The real (a) and imaginary (b) part of complex permittivity of samples 1–4.
The microwave absorption properties of Ni–Fe3 O4 hollow spheres were investigated at the range of 2–18 GHz. Fig. 4 illustrates complex permittivity (εr = εr − jεr ) of spheres–wax composites with volume ratio of 4:6. It can be seen that all samples show a similar variety trend and both εr and εr are sensitive to content of magnetite Fe3 O4 . The increase of Fe3 O4 content in hollow spheres results in the decrease of conductivity, which causes that the value of both real and imaginary parts of permittivity becomes smaller. With an increase of frequency, the real parts of permittivity decrease lightly, but imaginary parts increase. And resonance peaks present around 14.2 GHz. The resonance position is consistent with that of Fe3 O4 –coated hollow glass spheres [9]. In general, the permittivity originates from orientation polarization, atomic polarization and electronic polarization. Normally, the resonance caused by vacancy or pores usually dominates in the low-frequency regions. High-frequency resonance is attributed to atomic and electronic polarization [16]. So resonance peaks in the curves of both the real and imaginary parts of complex permittivity can be interpreted as the results of atomic and electronic polarization. Complex permeability (r = r − jr ) of powder–wax composites are shown in Fig. 5. As the Fe3 O4 content increases in hollow spheres, both real and imaginary parts of permeability increase. As shown in Fig. 5a, real parts r of complex permeability for all samples decrease with the increasing frequency, which is due to both eddy current loss and ferromagnetic resonance [17]. In Fig. 5b, with an increase of frequency, imaginary parts r of complex permeability decrease and a resonance peak appears around 5.6 GHz.
Generally, for ferrite magnetic materials, the microwave magnetic loss originates mainly from domain wall resonance and natural ferromagnetic resonance. Resonance due to domain wall movement normally occurs at low-frequency region (<2 GHz). However, resonance due to spin rotational component occurs at high-frequency region. So the resonance peak may attribute to the natural resonance. For the composite layer terminated by metal plate, the normalized input impedance Zin at the surface is given by [18]:
Zin =
2fd √
r tanh j εr
c
r εr
(4)
where f is the frequency of electromagnetic wave, d is the thickness of the composite layer and c is the velocity of electromagnetic wave in free space. The reflection loss of normal incident electromagnetic wave at the surface of composite layer can be calculated using the following equation:
Zin − 1 Z +1
RL = 20 log
(5)
in
By using the actual values of complex permittivity and permeability as shown in Figs. 4 and 5, the reflection loss of all samples were calculated. Fig. 6 shows the results for sample 4 at different thickness. The minimal reflection loss corresponds to the occurrence of minimal reflection of the microwave power for the
Fig. 5. The real (a) and imaginary (b) part of complex permeability of samples 1–4.
Z. Li et al. / Materials Science and Engineering B 164 (2009) 112–115
115
former, which causes the enhancement of microwave absorption properties. 4. Conclusion Ni–Fe3 O4 hollow spheres with the diameter about 150 nm were prepared by an autocatalytic reduction method. The influence of Fe3 O4 content in hollow spheres on microwave absorption properties has been investigated. With an increase of magnetite content, permittivity of sphere–wax composite decreased, while the value of permeability increased. By using actual values of complex permittivity and permeability, reflection loss of composite layers was calculated. The increasing content of magnetite resulted in the peak of RL moves to higher frequency and the minimal reflection loss increases obviously. Additionally, a minimum reflection loss (−38.4 dB) was predicted at 10.8 GHz with a thickness of 2.5 mm. Fig. 6. Reflection loss of sample 4 with various thicknesses (1.5, 2.0, 2.5 and 3.0 mm).
Acknowledgments This work is supported by National Natural Science Foundation of China (Grant no. 50474004), Shanghai Science and Technology Committee Nano Special Fund (Grant no. 0552nm004), the “Dawn” Program of Shanghai Education Commission and the New-Century Training Program Foundation for Talents from the Ministry of Education of China. The authors would like to thank the Instrumental Analysis Center of Shanghai Jiao Tong University for the characterization of samples. References
Fig. 7. Reflection loss of samples 1–4 at the thickness of 2 mm.
particular thickness. As shown in Fig. 6, the reflection loss is dependent sensitively on the thickness of the absorber. As thickness of composite layer increases, the peak of RL is moving to lower frequency and becomes narrower. For Ni–Fe3 O4 hollow spheres, the maximum attenuation of the incident wave is observed for sample 4 with a thickness of 2.5 mm, and the peak value of reflection loss is −38.4 dB, which is better than that of hollow glass or ceramic spheres coated with magnetic films [7,9,10]. Fig. 7 shows the reflection loss for the composites consisting of various powders. All samples have a thickness of 2 mm. As shown in Fig. 6, with the increase of magnetite content, the peak of RL moves to higher frequency and the minimal reflection loss increases obviously, which indicates that increasing magnetite content in hollow spheres improve the microwave absorption properties. Compared with hollow polyaniline/Fe3 O4 microsphere [19], the influence of the magnetite content on microwave absorption properties of Ni–Fe3 O4 shows a similar character. Introducing the magnetite into hollow spheres will reduce the dielectric loss and enhance the magnetic loss. And the effect of the later is stronger than that of the
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