- Email: [email protected]
Microwave absorption properties of Ce-substituted M-type barium ferrite
Journal of Magnetism and Magnetic Materials 324 (2012) 802–805
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Microwave absorption properties of Ce-substituted M-type barium ferrite Sun Chang a,b,c,n, Sun Kangning a,b, Chui Pengfei a,b a b c
Engineering Ceramics Key Laboratory of Shandong Province, Shandong University, Jinan 250061, China Key Laboratory for Liquid–Solid Structure Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China Shandong Supervision and Inspection Institute for Product Quality, Shandabeilu Road 81, Jinan 250100, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2011 Received in revised form 5 September 2011 Available online 29 September 2011
Ce-substituted barium ferrite with chemical composition BaCe0.05Fe11.95O19 has been prepared by the citrate sol–gel method. The phase composition of BaCe0.05Fe11.95O19 was characterized by X-ray powder diffraction analysis (XRD). The complex permittivity and complex permeability, microwave absorption properties of the resulting powder were measured by the transmission/reflection coaxial line method in the range of 8–13 GHz. The results show that the resulting powder has a minimum reflection loss value of – 37.4 dB at 12.8 GHz with a matching thickness of 3.5 mm. & 2011 Elsevier B.V. All rights reserved.
Keywords: Ce-substituted barium ferrite Complex permittivity Complex permeability Microwave
1. Introduction In the recent years, much attention has been paid to microwave absorbers, because of the obvious increase in the use of aircraft, ship, microwave darkroom, anti-electromagnetic interference coating, microwave thermal seed materials, etc. [1–3]. Barium ferrites, which possess large saturation magnetization and high natural resonance frequency, have been well known as special kinds of absorbing materials [4,5]. It is well known that the dielectric and magnetic properties can be modulated by substitution for the Fe3 þ and Ba2 þ with other ions [6–9]. Due to typical relaxation characterization, rare earth elements (RE) may magically affect the electromagnetic properties of ferrites [10–13]. Recent investigations have shown that saturation magnetization, coercivity and anisotropy are improved after Ba2 þ or Fe3 þ is substituted by RE, which results in the change of the magnetic interactions [10,14]. Furthermore, in addition to lower complex permittivity, RE-substituted ferrites, present lesser matching thickness, larger bandwidth and obvious relaxation that contributes to improving impedance matching, compared to those without substitution [10]. The element cerium—Ce is known as a dopant applied in the field of absorbers, luminescent materials, magnetic materials, ceramics, catalysts, etc. [15]. However, the microwave properties of Ce-doped barium ferrites have been seldom reported. In the present work, an attempt was made to study microwave
n Corresponding author at: Engineering Ceramics Key Laboratory of Shandong Province, Shandong University, Jinan 250061, China. Tel./fax: þ86 531 88392439. E-mail addresses: [email protected] (S. Chang), [email protected] (S. Kangning).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.09.023
absorption properties of Ce-substituted M-type barium ferrites. The formation of the BaCe0.05Fe11.95O19 phase and crystalline properties of the ferrites have been discussed using XRD. In order to study the electromagnetic parameters and absorbing properties of the resulting powder, the BaCe0.05Fe11.95O19–paraffin wax composite was prepared. Paraffin wax is an insulating and nonmagnetic material. In addition, with zero value of the imaginary part of the complex permeability and permittivity, paraffin wax is transparent for electromagnetic wave. In this paper, paraffin wax was thus chosen as a matrix and binder to prepare toroidally shaped samples (inner diameter of 3.04 mm, outer diameter of 7 mm and thickness of about 3 mm), which were used to measure the complex permittivity (e ¼ e0 je00 ) and complex permeability (m ¼ m0 jm00 ) of BaCe0.05Fe11.95O19. 2. Material and methods In order to obtain homogeneous morphology and stoichiometry of the Ce-substituted barium ferrite, BaCe0.05Fe11.95O19 was synthesized by the sol–gel method. In this paper, all the reagents used were of analytical purity and used without further purification. Ba(NO3)2 –6H2O, Fe(NO3)3 9H2O and Ce(NO3)3 6H2O were used to incorporate metal ions. Citric acid was used as a chelating agent. A stoichiometric amount of Ba(NO3)2 6H2O, Fe(NO3)3 9H2O and Ce(NO3)3 6H2O was dissolved in a citric acid aqueous solution under stirring. The molar ratio of nitrates to citric acid was 1:1. Then, an appropriate amount of ammonia hydroxide solution was dropped into the solution under continuous stirring to adjust the pH value to about 7. The precursor mixture was heated by a water bath at 80 1C under stirring for 3 h. Then it was
S. Chang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 802–805
dried in a drying box at 120 1C until the gel formed. The dried gel was calcined at 210 1C in a silicon carbide furnace in air so as to remove the organic substance. Finally, the BaCe0.05Fe11.95O19 powder was obtained, after the sample was treated at 800 1C for 0.5 h. According to the above preparation process, BaFe12O19 was fabricated. Phase analysis of the synthesized BaCe0.05Fe11.95O19 was conducted primarily by X-ray diffraction using an X-ray powder diffractometer (RIGAKUD/Max-A) and Cu Ka radiation (l ¼1.5405). X-ray powder diffractometer was operated at 60 kV and 40 mA in a 2y range of 15–851 employing a step size of 0.02 and a speed of 121/min. An AV3618 network analyzer was employed to measure the complex permittivity (e ¼ e0 je00 ) and complex permeability (m ¼ m0 jm00 ) in the frequency range of 8.0–13.0 GHz, according to the transmission/reflection coaxial line method. The specimens were prepared by mixing BaCe0.05Fe11.95O19 with paraffin wax at a ratio of 70 mass% (ferrite powder). The homogeneous mixtures were pressed into a cylindrical shaped mold to form into toroidally shaped samples.
3. Results and discussion
peaks according to the following equation: ! 2 2 2 2 1 4 h þk þ l l ¼ þ 2 2 2 3 a c dh k l
0:89l bi cos y
ð1Þ
where l is the incident wavelength of Cu Ka, bi is the FWHM and y is the considered diffraction angle. The lattice parameters a and c were calculated from the value of dhkl corresponding to (107)
Fig. 2. The permittivity spectra of BaCe0.05Fe11.95O19 ferrite.
Fig. 1. XRD pattern of doped ferrites: (a) BaFe12O19 and (b) BaCe0.05Fe11.95O19.
ð2Þ
The lattice constants of BaCe0.05Fe11.95O19 were a ¼5.8947 A˚ ˚ which are slightly larger those that of and c ¼23.2943 A, BaFe12O19. These slight changes in the lattice constant may have been caused by the difference between the ionic radius of Ce3 þ ˚ and Fe3 þ (0.645 A). ˚ The XRD analysis revealed that the (1.034 A) doping of Ce did not change the hexagonal structure of BaFe12O19, and Ce was substituted into the crystal lattice. In this study, BaFe12O19 and BaCe0.05Fe11.95O19 were prepared by the sol–gel process at 800 1C, which was lower than that in the traditional preparation method. This is because the diffusion of the components in sol–gel system is in nanometer scales, which make the reaction easier than in the traditional preparation process. Fig. 2 shows the frequency dependence of the complex permeability (e0 and e00 ) for the BaCe0.05Fe11.95O19–paraffin wax in the range of 8–13 GHz. It can be observed that the real part of permittivity increased from 1.0 to 3.0 with the increases of frequency, whereas the e00 values showed a broad peak in the range of 8–13 GHz, and had a peak value of 1.73 at 12.0 GHz. Fig. 3 shows the frequency dependence of complex permeability
The XRD patterns of the as-prepared non-substituted (a) and Ce-substituted barium ferrite samples (b) are shown in Fig. 1. From the patterns, the two samples showed M-type hexagonal to be the major crystalline phase. Compared to pattern (a) there is no new phase appearing in pattern (b) after Ce was substituted into BaFe12O19. The results suggest that the Ce can completely substitute into the lattice of BaFe12O19 in this study. In addition, the average grain size of as-prepared powder is about 30.1nm, which was calculated from the XRD line broadening of the (114) peak according to Scherrer’s equation: Dh k l ¼
803
Fig. 3. The permeability spectra of BaCe0.05Fe11.95O19 ferrite.
804
S. Chang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 802–805
(m0 and m00 ) of the BaCe0.05Fe11.95O19–paraffin wax composite. It can be observed that the imaginary part of permeability increases with increases of frequency, presents a peak with a value of 0.969 at 9.9 GHz, and then decreases, whereas the real part of complex permeability has an obviously increasing trend in the range from 8.0 GHz to 13.0 GHz. As the ionic radius of Ce3 þ is greater than that of Fe3 þ , the substitution of Ce3 þ for Fe3 þ induces lattice defects, which result in increasing magnetic and dielectric loss. On the other hand, the magnetocrystalline anisotropy field increased with the Ce3 þ doping, thereby increasing magnetic hysteresis loss in the electric and magnetic fields. The m00 of BaCe0.05Fe11.95O19 is higher than that of BaFe12O19 reported previously. The m00 of barium ferrite is expressed as follows:
m00 ¼
Ms 2Ha a
ð3Þ
where Ms is the saturation magnetization, Ha is the magnetocrystalline anisotropy field and a is the extinction coefficient. So Ha can be described by the following equation: Ha ¼
Ms 2am00
Fig. 4. The calculated reflection loss (RL) of BaCe0.05Fe11.95O19 ferrite.
ð4Þ
According to the ferromagnetic resonance theory, the zero field ferromagnetic resonance frequency is expressed by the equation
gH a ð5Þ 2p where g is the gyromagnetic ratio and Ha is the magnetocrystal-
f¼
line anisotropy field. Therefore, the f can be also expressed as follows: f¼ ¼
gMs 4pam00
ð6Þ
So f is inversely related to m00 . On the basis of the above analysis, f can be shifted to a lower value after substituting Fe3 þ with Ce3 þ . Based on the above analysis, Ce-substituted BaFe12O19 exhibits excellent electromagnetic properties. Moreover, according to the transmission line theory [16], for a metal-backed microwave absorbing layer, the normalized input impedance (Zin) at the absorber surface is given by pffiffiffiffiffiffiffiffi pffiffiffiffiffiffi Z in ¼ Z o m=e tanhfjð2pf d=cÞ meg ð7Þ where e and m are, respectively, the complex permittivity (e ¼ e0 e00 ) and the complex permeability (m ¼ m0 –m00 ), Z0 is the impedance of air, c is the velocity of electromagnetic waves in free space, f is the frequency and d is the thickness of the absorber. The reflection loss (RL) is a function of the normalized input impedance (Zin), which can be expressed as shown below: RL ¼ 20 log9ðZ in Z o Þ=ðZ in þ Z o Þ9
ð8Þ
The reflection loss for BaCe0.05Fe11.95O19–paraffin wax composite layers can be calculated using Eqs. (7) and (8). Fig. 4 shows the frequency dependence of the calculated reflection loss of the BaCe0.05Fe11.95O19–paraffin wax composite at the layer thickness of 2.9, 3.5, 5.0 and 6.5 mm. The bandwidth with RL less than 10 dB reached more than 3 GHz at a matching thickness of 6.5 mm. We can see that when the layer thickness was reduced to 2.9 mm, the minimum reflection loss was 15.27 dB. In particular, a minimum reflection loss value of 37.4 dB was observed at 12.8 GHz with a matching thickness of 3.5 mm. These results suggested that the doped-ferrite has an attractive potential microwave application. Compared with pure M-type barium ferrite, usually with a high natural ferrite ferrimagnetic resonance frequency, the microwave absorbing property of the doped one is improved in the frequency band of 8–13 GHz[17].
4. Conclusions BaCe0.05Fe11.95O19 ferrite was synthesized by the sol–gel process. XRD analysis reveals that Ce-substituted barium ferrite crystallizes in a hexagonal structure and its average crystallite size is about 30.1 nm. The complex permittivity and permeability spectra and microwave absorbing properties of the doped ferrites were investigated in the frequency range 8.0–13.0 GHz. It is found that this doped ferrite has a minimum reflection loss value of – 37.4 dB at 12.8 GHz with a matching thickness of 3.5 mm. The results show that the microwave absorbing property was significantly improved when Ce was doped in barium ferrites. Our work suggests that BaCe0.05Fe11.95O19 can be used as a good microwave absorption material in the range of 8.0–13.0 GHz.
Acknowledgments This work was jointly supported by the Natural Science Foundation of the People’s Republic of China (Grant no. 81171463 and 30870610), China Postdoctoral Science Foundation (no. 200804401138) and Shandong Province Postdoctoral Innovation Foundation (no. 200902030).
References [1] X. Huang, J. Zhang, H. Wang, S. Yan, L. Wang, Q. Zhang, Er3 þ substituted W-type barium ferrite: preparation and electromagnetic properties, Journal of Rare Earths 28 (2010) 940–943. [2] A. Ghasemi, A. Hossienpour, A. Morisako, X. Liu, A. Ashrafizadeh, Investigation of the microwave absorptive behavior of doped barium ferrites, Materials and Design 29 (2008) 112–117. [3] V.M. Petrov, V.V. Gagulin, Microwave absorbing materials, Inorganic Materials 37 (2001) 93–98. [4] D. Lisjak, K. Bobzin, K. Richardt, M. Be´gard, G. Bolelli, L. Lusvarghi, A. Hujanen, ¨ P. Lintunen, M. Pasquale, E. Olivetti, M. Drofenik, T. Schlafer, Preparation of barium hexaferrite coatings using atmospheric plasma spraying, Journal of the European Ceramic Society 29 (2009) 2333–2341. [5] M. Pardavi-Horvath, Microwave applications of soft ferrites, Journal of Magnetism and Magnetic Materials 215–216 (2000) 171–183. [6] S.P. Gairola, V. Verma, A. Singh, L.P. Purohit, R.K. Kotnala, Modified composition of barium ferrite to act as a microwave absorber in X-band frequencies, Solid State Communications 150 (2010) 147–151. [7] G.B. Teh, N. Saravanan, D.A. Jefferson, A study of magnetoplumbite-type (M-type) cobalt–titanium-substituted barium ferrite, BaCoxTixFe12 2xO19 (x¼ 1–6), Materials Chemistry and Physics 105 (2007) 253–259. [8] Z. Yang, C.S. Wang, X.H. Li, H.X. Zeng, (Zn, Ni, Ti) substituted barium ferrite particles with improved temperature coefficient of coercivity, Materials Science and Engineering: B 90 (2002) 142–145.
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[9] Y. Liu, M.G.B. Drew, J. Wang, M. Zhang, Y. Liu, Efficiency and purity control in the preparation of pure and/or aluminum-doped barium ferrites by hydrothermal methods using ferrous ions as reactants, Journal of Magnetism and Magnetic Materials 322 (2010) 366–374. [10] M.A. Ahmed, N. Okasha, R.M. Kershi, Influence of rare-earth ions on the structure and magnetic properties of barium W-type hexaferrite, Journal of Magnetism and Magnetic Materials 320 (2008) 1146–1150. [11] W. Jing, Z. Hong, B. Shuxin, C. Ke, Z. Changrui, Microwave absorbing properties of rare-earth elements substituted W-type barium ferrite, ournal of Magnetism and Magnetic Materials 312 (2007) 310–313. [12] L. Lechevallier, J.M. Le Breton, J.F. Wang, I.R. Harris, Structural analysis of hydrothermally synthesized Sr1 xSmxFe12O19 hexagonal ferrites, ournal of Magnetism and Magnetic Materials 269 (2004) 192–196. [13] V.G. Harris, A. Geiler, Y. Chen, S.D. Yoon, M. Wu, A. Yang, Z. Chen, P. He, P.V. Parimi, X. Zuo, C.E. Patton, M. Abe, O. Acher, C. Vittoria, Recent advances
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in processing and applications of microwave ferrites, Journal of Magnetism and Magnetic Materials 321 (2009) 2035–2047. C.A. Stergiou, I. Manolakis, T.V. Yioultsis, G. Litsardakis, Dielectric and magnetic properties of new rare-earth substituted Ba-hexaferrites in the 2–18GHz frequency range, Journal of Magnetism and Magnetic Materials. 322, 1532–1535. C.K. Loong, M. Ozawa, The role of rare earth dopants in nanophase zirconia catalysts for automotive emission control, Journal of Alloys and Compounds 303–304 (2000) 60–65. Y. Naito, K. Suetake, Application of ferrite to electromagnetic wave absorber and its characteristics, IEEE Trans, IEEE Transactions on Microwave Theory and Techniques 19 (1971) 65–72. A. Ghasemi, A. Hossienpour, A. Morisako, A. Saatchi, M. Salehi, Electromagnetic properties and microwave absorbing characteristics of doped barium hexaferrite, Journal of Magnetism and Magnetic Materials 302 (2006) 429–435.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Microwave absorption properties of Ce-substituted M-type barium ferrite Sun Chang a,b,c,n, Sun Kangning a,b, Chui Pengfei a,b a b c
Engineering Ceramics Key Laboratory of Shandong Province, Shandong University, Jinan 250061, China Key Laboratory for Liquid–Solid Structure Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China Shandong Supervision and Inspection Institute for Product Quality, Shandabeilu Road 81, Jinan 250100, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 January 2011 Received in revised form 5 September 2011 Available online 29 September 2011
Ce-substituted barium ferrite with chemical composition BaCe0.05Fe11.95O19 has been prepared by the citrate sol–gel method. The phase composition of BaCe0.05Fe11.95O19 was characterized by X-ray powder diffraction analysis (XRD). The complex permittivity and complex permeability, microwave absorption properties of the resulting powder were measured by the transmission/reflection coaxial line method in the range of 8–13 GHz. The results show that the resulting powder has a minimum reflection loss value of – 37.4 dB at 12.8 GHz with a matching thickness of 3.5 mm. & 2011 Elsevier B.V. All rights reserved.
Keywords: Ce-substituted barium ferrite Complex permittivity Complex permeability Microwave
1. Introduction In the recent years, much attention has been paid to microwave absorbers, because of the obvious increase in the use of aircraft, ship, microwave darkroom, anti-electromagnetic interference coating, microwave thermal seed materials, etc. [1–3]. Barium ferrites, which possess large saturation magnetization and high natural resonance frequency, have been well known as special kinds of absorbing materials [4,5]. It is well known that the dielectric and magnetic properties can be modulated by substitution for the Fe3 þ and Ba2 þ with other ions [6–9]. Due to typical relaxation characterization, rare earth elements (RE) may magically affect the electromagnetic properties of ferrites [10–13]. Recent investigations have shown that saturation magnetization, coercivity and anisotropy are improved after Ba2 þ or Fe3 þ is substituted by RE, which results in the change of the magnetic interactions [10,14]. Furthermore, in addition to lower complex permittivity, RE-substituted ferrites, present lesser matching thickness, larger bandwidth and obvious relaxation that contributes to improving impedance matching, compared to those without substitution [10]. The element cerium—Ce is known as a dopant applied in the field of absorbers, luminescent materials, magnetic materials, ceramics, catalysts, etc. [15]. However, the microwave properties of Ce-doped barium ferrites have been seldom reported. In the present work, an attempt was made to study microwave
n Corresponding author at: Engineering Ceramics Key Laboratory of Shandong Province, Shandong University, Jinan 250061, China. Tel./fax: þ86 531 88392439. E-mail addresses: [email protected] (S. Chang), [email protected] (S. Kangning).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.09.023
absorption properties of Ce-substituted M-type barium ferrites. The formation of the BaCe0.05Fe11.95O19 phase and crystalline properties of the ferrites have been discussed using XRD. In order to study the electromagnetic parameters and absorbing properties of the resulting powder, the BaCe0.05Fe11.95O19–paraffin wax composite was prepared. Paraffin wax is an insulating and nonmagnetic material. In addition, with zero value of the imaginary part of the complex permeability and permittivity, paraffin wax is transparent for electromagnetic wave. In this paper, paraffin wax was thus chosen as a matrix and binder to prepare toroidally shaped samples (inner diameter of 3.04 mm, outer diameter of 7 mm and thickness of about 3 mm), which were used to measure the complex permittivity (e ¼ e0 je00 ) and complex permeability (m ¼ m0 jm00 ) of BaCe0.05Fe11.95O19. 2. Material and methods In order to obtain homogeneous morphology and stoichiometry of the Ce-substituted barium ferrite, BaCe0.05Fe11.95O19 was synthesized by the sol–gel method. In this paper, all the reagents used were of analytical purity and used without further purification. Ba(NO3)2 –6H2O, Fe(NO3)3 9H2O and Ce(NO3)3 6H2O were used to incorporate metal ions. Citric acid was used as a chelating agent. A stoichiometric amount of Ba(NO3)2 6H2O, Fe(NO3)3 9H2O and Ce(NO3)3 6H2O was dissolved in a citric acid aqueous solution under stirring. The molar ratio of nitrates to citric acid was 1:1. Then, an appropriate amount of ammonia hydroxide solution was dropped into the solution under continuous stirring to adjust the pH value to about 7. The precursor mixture was heated by a water bath at 80 1C under stirring for 3 h. Then it was
S. Chang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 802–805
dried in a drying box at 120 1C until the gel formed. The dried gel was calcined at 210 1C in a silicon carbide furnace in air so as to remove the organic substance. Finally, the BaCe0.05Fe11.95O19 powder was obtained, after the sample was treated at 800 1C for 0.5 h. According to the above preparation process, BaFe12O19 was fabricated. Phase analysis of the synthesized BaCe0.05Fe11.95O19 was conducted primarily by X-ray diffraction using an X-ray powder diffractometer (RIGAKUD/Max-A) and Cu Ka radiation (l ¼1.5405). X-ray powder diffractometer was operated at 60 kV and 40 mA in a 2y range of 15–851 employing a step size of 0.02 and a speed of 121/min. An AV3618 network analyzer was employed to measure the complex permittivity (e ¼ e0 je00 ) and complex permeability (m ¼ m0 jm00 ) in the frequency range of 8.0–13.0 GHz, according to the transmission/reflection coaxial line method. The specimens were prepared by mixing BaCe0.05Fe11.95O19 with paraffin wax at a ratio of 70 mass% (ferrite powder). The homogeneous mixtures were pressed into a cylindrical shaped mold to form into toroidally shaped samples.
3. Results and discussion
peaks according to the following equation: ! 2 2 2 2 1 4 h þk þ l l ¼ þ 2 2 2 3 a c dh k l
0:89l bi cos y
ð1Þ
where l is the incident wavelength of Cu Ka, bi is the FWHM and y is the considered diffraction angle. The lattice parameters a and c were calculated from the value of dhkl corresponding to (107)
Fig. 2. The permittivity spectra of BaCe0.05Fe11.95O19 ferrite.
Fig. 1. XRD pattern of doped ferrites: (a) BaFe12O19 and (b) BaCe0.05Fe11.95O19.
ð2Þ
The lattice constants of BaCe0.05Fe11.95O19 were a ¼5.8947 A˚ ˚ which are slightly larger those that of and c ¼23.2943 A, BaFe12O19. These slight changes in the lattice constant may have been caused by the difference between the ionic radius of Ce3 þ ˚ and Fe3 þ (0.645 A). ˚ The XRD analysis revealed that the (1.034 A) doping of Ce did not change the hexagonal structure of BaFe12O19, and Ce was substituted into the crystal lattice. In this study, BaFe12O19 and BaCe0.05Fe11.95O19 were prepared by the sol–gel process at 800 1C, which was lower than that in the traditional preparation method. This is because the diffusion of the components in sol–gel system is in nanometer scales, which make the reaction easier than in the traditional preparation process. Fig. 2 shows the frequency dependence of the complex permeability (e0 and e00 ) for the BaCe0.05Fe11.95O19–paraffin wax in the range of 8–13 GHz. It can be observed that the real part of permittivity increased from 1.0 to 3.0 with the increases of frequency, whereas the e00 values showed a broad peak in the range of 8–13 GHz, and had a peak value of 1.73 at 12.0 GHz. Fig. 3 shows the frequency dependence of complex permeability
The XRD patterns of the as-prepared non-substituted (a) and Ce-substituted barium ferrite samples (b) are shown in Fig. 1. From the patterns, the two samples showed M-type hexagonal to be the major crystalline phase. Compared to pattern (a) there is no new phase appearing in pattern (b) after Ce was substituted into BaFe12O19. The results suggest that the Ce can completely substitute into the lattice of BaFe12O19 in this study. In addition, the average grain size of as-prepared powder is about 30.1nm, which was calculated from the XRD line broadening of the (114) peak according to Scherrer’s equation: Dh k l ¼
803
Fig. 3. The permeability spectra of BaCe0.05Fe11.95O19 ferrite.
804
S. Chang et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 802–805
(m0 and m00 ) of the BaCe0.05Fe11.95O19–paraffin wax composite. It can be observed that the imaginary part of permeability increases with increases of frequency, presents a peak with a value of 0.969 at 9.9 GHz, and then decreases, whereas the real part of complex permeability has an obviously increasing trend in the range from 8.0 GHz to 13.0 GHz. As the ionic radius of Ce3 þ is greater than that of Fe3 þ , the substitution of Ce3 þ for Fe3 þ induces lattice defects, which result in increasing magnetic and dielectric loss. On the other hand, the magnetocrystalline anisotropy field increased with the Ce3 þ doping, thereby increasing magnetic hysteresis loss in the electric and magnetic fields. The m00 of BaCe0.05Fe11.95O19 is higher than that of BaFe12O19 reported previously. The m00 of barium ferrite is expressed as follows:
m00 ¼
Ms 2Ha a
ð3Þ
where Ms is the saturation magnetization, Ha is the magnetocrystalline anisotropy field and a is the extinction coefficient. So Ha can be described by the following equation: Ha ¼
Ms 2am00
Fig. 4. The calculated reflection loss (RL) of BaCe0.05Fe11.95O19 ferrite.
ð4Þ
According to the ferromagnetic resonance theory, the zero field ferromagnetic resonance frequency is expressed by the equation
gH a ð5Þ 2p where g is the gyromagnetic ratio and Ha is the magnetocrystal-
f¼
line anisotropy field. Therefore, the f can be also expressed as follows: f¼ ¼
gMs 4pam00
ð6Þ
So f is inversely related to m00 . On the basis of the above analysis, f can be shifted to a lower value after substituting Fe3 þ with Ce3 þ . Based on the above analysis, Ce-substituted BaFe12O19 exhibits excellent electromagnetic properties. Moreover, according to the transmission line theory [16], for a metal-backed microwave absorbing layer, the normalized input impedance (Zin) at the absorber surface is given by pffiffiffiffiffiffiffiffi pffiffiffiffiffiffi Z in ¼ Z o m=e tanhfjð2pf d=cÞ meg ð7Þ where e and m are, respectively, the complex permittivity (e ¼ e0 e00 ) and the complex permeability (m ¼ m0 –m00 ), Z0 is the impedance of air, c is the velocity of electromagnetic waves in free space, f is the frequency and d is the thickness of the absorber. The reflection loss (RL) is a function of the normalized input impedance (Zin), which can be expressed as shown below: RL ¼ 20 log9ðZ in Z o Þ=ðZ in þ Z o Þ9
ð8Þ
The reflection loss for BaCe0.05Fe11.95O19–paraffin wax composite layers can be calculated using Eqs. (7) and (8). Fig. 4 shows the frequency dependence of the calculated reflection loss of the BaCe0.05Fe11.95O19–paraffin wax composite at the layer thickness of 2.9, 3.5, 5.0 and 6.5 mm. The bandwidth with RL less than 10 dB reached more than 3 GHz at a matching thickness of 6.5 mm. We can see that when the layer thickness was reduced to 2.9 mm, the minimum reflection loss was 15.27 dB. In particular, a minimum reflection loss value of 37.4 dB was observed at 12.8 GHz with a matching thickness of 3.5 mm. These results suggested that the doped-ferrite has an attractive potential microwave application. Compared with pure M-type barium ferrite, usually with a high natural ferrite ferrimagnetic resonance frequency, the microwave absorbing property of the doped one is improved in the frequency band of 8–13 GHz[17].
4. Conclusions BaCe0.05Fe11.95O19 ferrite was synthesized by the sol–gel process. XRD analysis reveals that Ce-substituted barium ferrite crystallizes in a hexagonal structure and its average crystallite size is about 30.1 nm. The complex permittivity and permeability spectra and microwave absorbing properties of the doped ferrites were investigated in the frequency range 8.0–13.0 GHz. It is found that this doped ferrite has a minimum reflection loss value of – 37.4 dB at 12.8 GHz with a matching thickness of 3.5 mm. The results show that the microwave absorbing property was significantly improved when Ce was doped in barium ferrites. Our work suggests that BaCe0.05Fe11.95O19 can be used as a good microwave absorption material in the range of 8.0–13.0 GHz.
Acknowledgments This work was jointly supported by the Natural Science Foundation of the People’s Republic of China (Grant no. 81171463 and 30870610), China Postdoctoral Science Foundation (no. 200804401138) and Shandong Province Postdoctoral Innovation Foundation (no. 200902030).
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