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Co-substituted LiZnTiBi ferrite with equivalent permeability and permittivity for high-frequency miniaturized antenna application
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Co-substituted LiZnTiBi ferrite with equivalent permeability and permittivity for high-frequency miniaturized antenna application Fei Xiea,∗, Yang Chena, Minyu Baia, Pinglu Wangb a b
School of Optoelectronic Engineering, Xi'an Technological University, Xi'an, 710021, China Information and Navigation College, Air Force Engineering University, Xi'an, 710051, China
A R T I C LE I N FO
A B S T R A C T
Keywords: LiZnTiBi ferrite LTCC Co2O3 Equivalent permeability and permittivity Miniaturized antenna
In the present work, a Co-substituted LiZnTiBi (Li0.43Zn0.27CoxTi0.13Fe2.167-xBi0.003O4) ferrite with equivalent permeability and permittivity was prepared by a low-temperature sintering method. In detail, the LiZnTiBi ferrite with high permeability was obtained by a solid phase method. Next, different mass of Co2O3 (x = 0.100, 0.125, 0.150, 0.175 and 0.200) were added to reduce permeability (μr’) and enhance cut-off frequency (fr) of the ferrite. Results indicated that the Co-substituted LiZnTiBi ferrite with equivalent permeability and permittivity (μr’ = εr’ = ∼12.3) was obtained when additive amount of Co2O3 (x) is around 0.150. Meanwhile, the cut-off frequency (fr) of the ferrite is more than 500 MHz. Also, the influences of Co2O3 additive on the microstructure and phase structure of the LiZnTiBi ferrite have been investigated and discussed. SEM images indicated that the Co-substituted LiZnTiBi ferrites possess uniform and compact grains (average grain size is ∼1 μm). XRD patterns demonstrated that the LiZnTiBi ferrite material has a pure spinel structure. Rietveld refinement of XRD proved that Fe3+ is substituted by Co3+. And energy dispersive spectrometer (EDS) result indicated that the Co element is existed in ferrite grains. Such a low-temperature sintering ferrite substrate material with equivalent permeability and permittivity at high frequency (@500 MHz) should also be an alternative material for microwave devices, especially for miniature antenna.
1. Introduction Ferrite, an alternative material for microwave device applications with high resistivity, low dielectric loss and good magnetic properties, has been widely studied in the last few decades [1–5]. Recently, the ferrites with high cut-off frequency and permeability have been continually reported, which are available for microwave devices, such as chip inductors, filler, RF antenna and so on [3,6]. In general, the size of RF antenna depends on permittivity (εr’) of substrate material. In other word, the wavelength of antenna (λ) is inversely proportional to (εr’)1/ 2 . However, when magnetic ferrite is selected as substrate material, the size of antenna depends on miniaturization factor (n= (μr’ εr’)1/2). Meanwhile, the impedance matching (Z) can be expressed as: Z = η0 (μr’/εr’)1/2
(1)
where η0 is impedance of free space. Based on the two expressions, the ferrite with high and equivalent permeability and permittivity is ideal substrate for miniaturized antenna application [7–14]. To reduce permeability of ferrite ceramics, non-magnetic additives or high k-dielectric ceramics, such as TiO2, CaO, and so on, are often added ∗
[12,15]. The method is simple but effective for realizing equivalent permeability and permittivity of ferrite. However, for these composite materials, the influence of the dielectric materials on cut-off frequency of magnetic ferrite is negligible. To enhance cut-off frequency of ferrite, a suitable additive is vital. In the previous literature, Co2Z hexaferrite was proved to have high permeability at high frequency [16–18]. More importantly, the adding more Co2O3 can further enhance cut-off frequency of the ferrite. In the previous literature, NiCuZn ferrites, a class of spinel ferrite, have been wide studied due to their high permeability and low magnetic loss [19–22]. Recent, due to appropriate permeability (μr’ = ∼120) and high cut-off frequency (fr = ∼10 MHz), the LiZnTi ferrite have been proved to be a selectable material for miniaturized antenna [14,23]. More importantly, the LiZnTi ferrites possess low sintering temperature when Bi2O3 was selected as additive, which is an advantage for LTCC application. Thus, in the present work, Co2O3 additive was used for reducing permeability and increasing cut-off frequency of magnetic ferrite material. Also, Bi2O3-substituted LiZnTi (LiZnTiBi) ferrite was selected as substrate materials, which possess high permeability, low magnetic loss and low sintering temperature [24,25]. To reduce permeability and
Corresponding author. E-mail address: xiefl[email protected] (F. Xie).
https://doi.org/10.1016/j.ceramint.2019.06.008 Received 19 March 2019; Received in revised form 1 June 2019; Accepted 2 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Fei Xie, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.06.008
Ceramics International xxx (xxxx) xxx–xxx
F. Xie, et al.
and 0.200) were synthesized by solid phase method. In detail, raw materials, including Li2CO3, ZnO, TiO2, Bi2O3, Co2O3 and Fe2O3 (> 99%, Aladdin Reagent Co., Ltd.,) were weighted and mixed. Next, these mixtures were pre-sintered at 800 °C for 2 h. After grinding, the ferrite powders with spinel structure were obtained. Next, the hybrid powders were granulated using PVA binder. And then, these particles were pressed into toroidal samples at ∼8 MPa. Finally, the samples were sintered at 900 °C for 4 h at air atmosphere. 2.2. Characterization analysis Diffraction peaks of the ferrites were obtained by using XRD (Cu Kα radiation, Rigaku, Japan). Volume density of all samples was measured by the Archimedes method in distilled water. SEM images of the Cosubstituted LiZnTiBi ferrites were investigated using a scanning electron microscope (SEM, S–3400 N, and HITACHI). Magnetic and dielectric properties, including permeability and permittivity, of the ferrites were measured by Impedance Analyzer (E4991B, Agilent). All of the tests were carried out at room temperature.
Fig. 1. Rietveld refinement of Co-substituted Li0.43Zn0.27CoxTi0.13Fe2.167(x = 0.150) ferrite sintered at 900 °C. Difference lines (measured value [black points] - calculated value [red lines]) are displayed. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
xBi0.003O4
enhance cut-off frequency of the LiZnTiBi ferrite, the different mass of Co2O3 powders (x = 0.100, 0.125, 0.150, 0.175 and 0.200), as a substituted powder of the ferrite, were added. Finally, a class of Co-substituted LiZnTiBi ferrite with equivalent permeability and permittivity was obtained when moderate Co2O3 (x = 0.150) were added. Also, the phase structure, microstructure, magnetic-dielectric properties of the ferrites have been analyzed in detailed.
3. Results and discussions The XRD patterns of polycrystalline LiZnTiBi ferrite with various Co2O3 additives (x = 0.100, 0.125, 0.150, 0.175 and 0.200) sintered at 900 °C were shown in Fig. S1. These diffraction peaks of the samples are match up with Li0.435Zn0.195Fe2.37O4 ferrite in the JCPDS cards (i.e., JCPDS 37–1471). It was indicated that there are no other peaks that can be clearly detected although Co2O3 additives were added. To further analyze influences of Co2O3 on crystal structure of the LiZnTiBi ferrite, Rietveld refinement of the XRD pattern of the ferrite (x = 0.150) was carried out. The XRD refinement figure was shown in Fig. 1. Moreover, the important parameters, including occupancy and coordinates of
2. Experimental sections 2.1. Preparation of Co-substituted LiZnTiBi ferrite ceramics The LiZnTiBi (Li0.43Zn0.27Ti0.13CoxFe2.167-xBi0.003O4) ferrite ceramics with different Co2O3 additives (x = 0.100, 0.125, 0.150, 0.175
Fig. 2. (a) SEM image of the Co-substituted LiZnTiBi ferrite (x = 0.150) sintered at 900 °C; (b) histograms for grain size of the sample in Fig. 2(a); (c) enlarged area of the sample in Fig. 2(a); (d) energy dispersive spectrometer (EDS) result of the ferrite marked in Fig. 2(c)). 2
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Fig. 3. (a) Frequency dependence of permeability (μr’) and (b) magnetic loss (tan δμ) of Co-substituted LiZnTiBi ferrite sintered at 900 °C.
Fig. 4. Dielectric properties of LiZnTiBi ferrite with different Co2O3 additives (x = 0.100, x = 0.125, x = 0.150, x = 0.175 and x = 0.200): (a) permittivity and (b) dielectric loss.
data. In a word, a Co-substituted LiZnTiBi ferrite with pure spinel structure was obtained at 900 °C. Fig. 2a shows SEM image of the ferrite sample (x = 0.150) sintered at 900 °C. For other ferrite samples with various Co2O3 additives, the SEM images were showed in Fig. S2. Compared with grain size of LiZnTiBi ferrite without Co2O3 additives reported in previous work [14], all ferrite samples sintered at 900 °C possess similar grain size due to equal Bi2O3 sintering aids. The results also indicated that the Co2O3 has a little influence on grain growth at the low sintering temperature. To analyze homogeneity of the grains, we calculated the grain size of the sample (x = 0.150). The histogram for grain size of the Co-substituted LiZnTiBi ferrite sample was displayed in Fig. 2b. The figure indicated more than 85% grains have small grain size (∼1.0 μm). Fig. 2c shows enlarged area (red circle in Fig. 2a) of the sample. It was indicated that the Co-substituted LiZnTiBi ferrite sintered at 900 °C possess uniform and compact grains. The Energy dispersive X-ray spectroscopy of the sample was showed in Fig. 2d. All elements, including Fe, Ti, Zn, Bi, Co and O, are detected in the ferrite grain (marked in Fig. 2c). The proportions of elements are displayed in Table S2. Fig. 3 shows magnetic properties of the Co-substituted LiZnTiBi ferrite sintered at 900 °C. In previous work, we have reported that the LiZnTiBi ferrite without Co2O3 additive, the permeability is about 142 (fr less than 10 MHz) [14]. However, in Fig. 3a, the permeability (μr’) of the samples continually decreased when Co2O3 additives were added. For the sample (x = 0.200), the μr’ decreased from ∼26 to ∼7.6 at 10 MHz. This is due to reduction of saturation magnetization when Co2O3 was added. Thus, the permeability of the sample obviously decreased. According to Snoek's law [26], the permeability of magnetic material is inversely proportional to resonance frequency (cut-off frequency). Therefore, the cut-off frequency of the sample increased due to reduction of permeability. Also, as shown in Fig. 3b, the magnetic loss
Fig. 5. Density of the LiZnTiBi ferrite with different Co2O3 additives. Table 1 Important parameters of Co-substituted LiZnTiBi ferrite sintered at 900 °C (x = 0.100, 0.125, 0.150, 0.175 and 0.200) at 0.5 GHz. Samples
μ′
tanδμ
ε′
tanδε
(μ’/ε′)1/2
(μ’ε’)1/2
x = 0.100 x = 0.125 x = 0.150 x = 0.175 x = 0.200
17.19 13.68 12.47 10.05 8.66
1.71 1.05 0.83 0.59 0.48
13.27 13.13 11.96 11.44 10.94
9.06e-2 7.75e-2 6.00e-2 1.68e-2 1.55e-2
1.14 1.02 1.02 0.94 0.89
15.10 13.40 12.21 10.72 9.73
metal ions, of the ferrite sample were listed in Table S1. It was demonstrated that Fe3+ ions in the A-site were substituted by the Co3+. Also, positions of Bragg scattering are nearly the same as experimental 3
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Fig. 6. (a) Frequency dependence of the normalized impedance factor and miniaturization factor (b) Magnetic and dielectric loss of the Co-substituted LiZnTiBi ferrite (x = 0.150) sintered at 900 °C.
changed from 11.75 to 12.21 at wide frequency range (10 MHz∼1 GHz). In addition, the normalized impedance factor has stable value from 10 MHz to 500 MHz (1.02–1.08). However, the value abruptly reduced when frequency more than 500 MHz (i.e., cut-off frequency of the ferrite). Thus, as shown in Fig. 6b, the losses of the ferrite, including magnetic and dielectric loss, started to abruptly increase when frequency more than 500 MHz. In a word, adding moderate Co2O3 into LiZnTiBi ferrite can obtain a good substrate material with high miniaturization factor (∼12) and equivalent permeability and permittivity at a wide frequency range (10 MHz–500 MHz). And the magnetic ferrite material is promising for miniaturized antenna application.
of the Co-substituted LiZnTiBi ferrite obviously decreased when Co2O3 additives were introduced, especially in high frequency range. This can be explained by that the Co3+ ions in LiZnTiBi ferrite induce local uniaxial anisotropy under the influence of a local Weiss field or under an external field. As Su et al. described, the existence of the local uniaxial anisotropy also hinders the irreversible domain wall motion of the ferrite, which leads to the initial decrease of magnetic loss [27]. In a word, Co2O3 additive not only can reduce permeability and improve cut-off frequency, but also reduce magnetic loss of the ferrite at high frequency, which proved that the Co2O3 is a good candidate. Moreover, the dielectric properties of the samples were shown in Fig. 4. In Fig. 4a, the permittivity (εr’) of the samples has a stable value at wide frequency range (from 10 MHz to 1 GHz). For the all samples, the permittivity only changed from 11 to 13. Considering the measurement error, the change of εr’ values are small. Results indicated that the influences of Co2O3 on permittivity are weak. Fig. 4b shows dielectric loss of these ferrite samples. Similarly, adding Co2O3 can obviously reduce dielectric loss of these samples, especially in high frequency range. Fig. 5 shows the bulk density of the samples sintered at 900 °C. Due to higher density of Co2O3 (6.45 g/cm3), the density of Co-substituted LiZnTiBi ferrite continually increased. However, in Fig. 5, the sample (x = 0.150) possess maximum value. This is because that effect of pores on density is dominating. As shown in Fig. S2, when more Co2O3 were added, the samples possessed more pores between grains. In a word, for the Li0.43Zn0.27Ti0.13Fe2.167Bi0.003O4 ferrite, the influences of Co2O3 on magnetic property are more significant than dielectric property. Thus, adding moderate Co2O3 is feasible method to obtain ferrite material with equivalent permeability and permittivity at high frequency. More importantly, the Co2O3 can reduce magnetic and dielectric loss of the ferrite. For the substrate material applied to miniaturized antenna, normalized impedance ((μr’/εr’)1/2) and miniaturization factor ((μr’εr’)1/2) are two important parameters. Fig. S3 reveals the changes of the two parameters. In Fig. S3a, (μr’εr’)1/2 constantly decreased when more Co2O3 additives were introduced. This can be explained by reduction of permeability. Fig. S3b shows (μr’/εr’)1/2 of the samples with various Co2O3 additives. The value of (μr’/εr’)1/2 gradually approached to 1.0 when moderate Co2O3 additive were added. However, the additive amount (x) of Co2O3 more than 0.150, the value started to keep away from the best value (1.0). And the values of these important parameters were listed in Table 1. In Table 1, values of the two parameters are smaller at higher frequency. Interestingly, in the first figure, these broken lines of miniaturization factor have a point of intersection at x = 0.150. Thus, we calculated more values about the two parameters at different frequencies. In Fig. 6a, the miniaturization factor of the ferrite (x = 0.150)
4. Conclusion In this work, a Co-substituted LiZnTiBi ferrite substrate material with low sintering temperature, good miniaturization factor and normalized impedance factor were prepared. First of all, a low temperature sintering Li0.43Zn0.27Ti0.13Fe2.167Bi0.003O4 ferrite with high permeability and low magnetic loss was selected. In order to reduce permeability and enhance cut-off frequency of the LiZnTiBi ferrite, a certain amount of Co2O3 was added. Although Co2O3 additive cannot promote grain growth of the ferrite, it obviously decreases permeability (from ∼140 to ∼12). Meanwhile, the Co2O3 can reduce the losses, including magnetic and dielectric loss, of the ferrite. Finally, when the additive amount of Co2O3 (x) is 0.150, the ferrite sample possessed equivalent permeability and permittivity (μr’ = εr’ = ∼12.3) at wide frequency range. When working frequency is less than 500 MHz, the ferrite has high miniaturization factor (∼12) and good normalized impedance factor (∼1.0). Such a ferrite substrate material should also be an alternative material for microwave devices, such as miniature antenna. Acknowledgments This work was financially supported by Key Laboratory of Shaanxi Provincial Department of Education (Grant No. 17JS052). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.06.008. References [1] V.G. Harris, Modern microwave ferrites, IEEE Trans. Magn. 48 (2012) 1075–1104. [2] G. Catalan, J.F. Scott, Physics and applications of bismuth ferrite, Adv. Mater. 21 (2009) 2463–2485.
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[15] Z.L. Zheng, Y.W. Li, T. Liu, Q.Y. Feng, Novel high-frequency magneto-dielectric properties of CaO-SiO2 Co-doped NiZnCo spinel ferrites for RF and microwave device applications, IEEE Trans. Magn. 54 (2018) 4. [16] T. Nakamura, E. Hankui, Control of high-frequency permeability in polycrystalline (Ba,Co)-Z-type hexagonal ferrite, J. Magn. Magn. Mater. 257 (2003) 158–164. [17] Z.W. Li, Y.P. Wu, G.Q. Lin, Doping effect on complex permeability and permittivity for W-type barium ferrite composites, J. Appl. Phys. 102 (2007) 8. [18] K. Huang, X.S. Liu, S.J. Feng, Z.J. Zhang, J.Y. Yu, X.F. Niu, F.R. Lv, X. Huang, Structural and magnetic properties of Ca-substituted barium W-type hexagonal hexaferrites, J. Magn. Magn. Mater. 379 (2015) 16–21. [19] H. Su, X.L. Tang, H.W. Zhang, Y.L. Jing, F.M. Bai, Low-loss magneto-dielectric materials: approaches and developments, J. Electron. Mater. 43 (2014) 299–307. [20] Y. Yang, H.W. Zhang, J. Li, F. Xu, G.W. Gan, D.D. Wen, Effects of Bi2O3-Nb2O5 additives on microstructure and magnetic properties of low-temperature-fired NiCuZn ferrite ceramics, Ceram. Int. 44 (2018) 10545–10550. [21] F. Xu, D. Zhang, Y. Liao, H. Zhang, Microstructure, magnetic-dielectric properties of flexible composite film for high frequency applications, Ceram. Int. 45 (2019) 6350–6355. [22] F. Xu, D. Zhang, G. Wang, H. Zhang, Y. Yang, Y. Liao, L. Jin, Y. Rao, J. Li, F. Xie, G. Gan, Densification and magnetic properties of NiCuZn low-sintering temperature ferrites with Bi2O3-Nb2O5 composite additives, J. Alloy. Comp. 776 (2019) 954–959. [23] F. Xu, H.W. Zhang, F. Xie, Y.L. Liao, Y.X. Li, J. Li, L.C. Jin, Y. Yang, G.W. Gan, G. Wang, Q. Zhao, Investigation of grain boundary diffusion and grain growth of lithium zinc ferrites with low activation energy, J. Am. Ceram. Soc. 101 (2018) 5037–5045. [24] F. Xu, Y.L. Liao, D.N. Zhang, T.C. Zhou, J. Li, G.W. Gan, H.W. Zhang, Synthesis of highly uniform and compact lithium zinc ferrite ceramics via an efficient low temperature approach, Inorg. Chem. 56 (2017) 4512–4520. [25] L.J. Jia, Y.P. Zhao, F. Xie, Q. Li, Y.X. Li, C. Liu, H.W. Zhang, Composition, microstructures and ferrimagnetic properties of Bi-modified LiZnTiMn ferrites for LTCC application, AIP Adv. 6 (2016) 6. [26] A.N. Lagarkov, K.N. Rozanov, High-frequency behavior of magnetic composites, J. Magn. Magn. Mater. 321 (2009) 2082–2092. [27] H. Su, H.W. Zhang, X.L. Tang, B.Y. Liu, Y.K. An, High Q-factor NiCuZn ferrite with nanocrystalline ferrite particles and Co2O3 additives, Phys. Status Solidi A-Appl. Mat. 204 (2007) 576–580.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Co-substituted LiZnTiBi ferrite with equivalent permeability and permittivity for high-frequency miniaturized antenna application Fei Xiea,∗, Yang Chena, Minyu Baia, Pinglu Wangb a b
School of Optoelectronic Engineering, Xi'an Technological University, Xi'an, 710021, China Information and Navigation College, Air Force Engineering University, Xi'an, 710051, China
A R T I C LE I N FO
A B S T R A C T
Keywords: LiZnTiBi ferrite LTCC Co2O3 Equivalent permeability and permittivity Miniaturized antenna
In the present work, a Co-substituted LiZnTiBi (Li0.43Zn0.27CoxTi0.13Fe2.167-xBi0.003O4) ferrite with equivalent permeability and permittivity was prepared by a low-temperature sintering method. In detail, the LiZnTiBi ferrite with high permeability was obtained by a solid phase method. Next, different mass of Co2O3 (x = 0.100, 0.125, 0.150, 0.175 and 0.200) were added to reduce permeability (μr’) and enhance cut-off frequency (fr) of the ferrite. Results indicated that the Co-substituted LiZnTiBi ferrite with equivalent permeability and permittivity (μr’ = εr’ = ∼12.3) was obtained when additive amount of Co2O3 (x) is around 0.150. Meanwhile, the cut-off frequency (fr) of the ferrite is more than 500 MHz. Also, the influences of Co2O3 additive on the microstructure and phase structure of the LiZnTiBi ferrite have been investigated and discussed. SEM images indicated that the Co-substituted LiZnTiBi ferrites possess uniform and compact grains (average grain size is ∼1 μm). XRD patterns demonstrated that the LiZnTiBi ferrite material has a pure spinel structure. Rietveld refinement of XRD proved that Fe3+ is substituted by Co3+. And energy dispersive spectrometer (EDS) result indicated that the Co element is existed in ferrite grains. Such a low-temperature sintering ferrite substrate material with equivalent permeability and permittivity at high frequency (@500 MHz) should also be an alternative material for microwave devices, especially for miniature antenna.
1. Introduction Ferrite, an alternative material for microwave device applications with high resistivity, low dielectric loss and good magnetic properties, has been widely studied in the last few decades [1–5]. Recently, the ferrites with high cut-off frequency and permeability have been continually reported, which are available for microwave devices, such as chip inductors, filler, RF antenna and so on [3,6]. In general, the size of RF antenna depends on permittivity (εr’) of substrate material. In other word, the wavelength of antenna (λ) is inversely proportional to (εr’)1/ 2 . However, when magnetic ferrite is selected as substrate material, the size of antenna depends on miniaturization factor (n= (μr’ εr’)1/2). Meanwhile, the impedance matching (Z) can be expressed as: Z = η0 (μr’/εr’)1/2
(1)
where η0 is impedance of free space. Based on the two expressions, the ferrite with high and equivalent permeability and permittivity is ideal substrate for miniaturized antenna application [7–14]. To reduce permeability of ferrite ceramics, non-magnetic additives or high k-dielectric ceramics, such as TiO2, CaO, and so on, are often added ∗
[12,15]. The method is simple but effective for realizing equivalent permeability and permittivity of ferrite. However, for these composite materials, the influence of the dielectric materials on cut-off frequency of magnetic ferrite is negligible. To enhance cut-off frequency of ferrite, a suitable additive is vital. In the previous literature, Co2Z hexaferrite was proved to have high permeability at high frequency [16–18]. More importantly, the adding more Co2O3 can further enhance cut-off frequency of the ferrite. In the previous literature, NiCuZn ferrites, a class of spinel ferrite, have been wide studied due to their high permeability and low magnetic loss [19–22]. Recent, due to appropriate permeability (μr’ = ∼120) and high cut-off frequency (fr = ∼10 MHz), the LiZnTi ferrite have been proved to be a selectable material for miniaturized antenna [14,23]. More importantly, the LiZnTi ferrites possess low sintering temperature when Bi2O3 was selected as additive, which is an advantage for LTCC application. Thus, in the present work, Co2O3 additive was used for reducing permeability and increasing cut-off frequency of magnetic ferrite material. Also, Bi2O3-substituted LiZnTi (LiZnTiBi) ferrite was selected as substrate materials, which possess high permeability, low magnetic loss and low sintering temperature [24,25]. To reduce permeability and
Corresponding author. E-mail address: xiefl[email protected] (F. Xie).
https://doi.org/10.1016/j.ceramint.2019.06.008 Received 19 March 2019; Received in revised form 1 June 2019; Accepted 2 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Fei Xie, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.06.008
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and 0.200) were synthesized by solid phase method. In detail, raw materials, including Li2CO3, ZnO, TiO2, Bi2O3, Co2O3 and Fe2O3 (> 99%, Aladdin Reagent Co., Ltd.,) were weighted and mixed. Next, these mixtures were pre-sintered at 800 °C for 2 h. After grinding, the ferrite powders with spinel structure were obtained. Next, the hybrid powders were granulated using PVA binder. And then, these particles were pressed into toroidal samples at ∼8 MPa. Finally, the samples were sintered at 900 °C for 4 h at air atmosphere. 2.2. Characterization analysis Diffraction peaks of the ferrites were obtained by using XRD (Cu Kα radiation, Rigaku, Japan). Volume density of all samples was measured by the Archimedes method in distilled water. SEM images of the Cosubstituted LiZnTiBi ferrites were investigated using a scanning electron microscope (SEM, S–3400 N, and HITACHI). Magnetic and dielectric properties, including permeability and permittivity, of the ferrites were measured by Impedance Analyzer (E4991B, Agilent). All of the tests were carried out at room temperature.
Fig. 1. Rietveld refinement of Co-substituted Li0.43Zn0.27CoxTi0.13Fe2.167(x = 0.150) ferrite sintered at 900 °C. Difference lines (measured value [black points] - calculated value [red lines]) are displayed. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
xBi0.003O4
enhance cut-off frequency of the LiZnTiBi ferrite, the different mass of Co2O3 powders (x = 0.100, 0.125, 0.150, 0.175 and 0.200), as a substituted powder of the ferrite, were added. Finally, a class of Co-substituted LiZnTiBi ferrite with equivalent permeability and permittivity was obtained when moderate Co2O3 (x = 0.150) were added. Also, the phase structure, microstructure, magnetic-dielectric properties of the ferrites have been analyzed in detailed.
3. Results and discussions The XRD patterns of polycrystalline LiZnTiBi ferrite with various Co2O3 additives (x = 0.100, 0.125, 0.150, 0.175 and 0.200) sintered at 900 °C were shown in Fig. S1. These diffraction peaks of the samples are match up with Li0.435Zn0.195Fe2.37O4 ferrite in the JCPDS cards (i.e., JCPDS 37–1471). It was indicated that there are no other peaks that can be clearly detected although Co2O3 additives were added. To further analyze influences of Co2O3 on crystal structure of the LiZnTiBi ferrite, Rietveld refinement of the XRD pattern of the ferrite (x = 0.150) was carried out. The XRD refinement figure was shown in Fig. 1. Moreover, the important parameters, including occupancy and coordinates of
2. Experimental sections 2.1. Preparation of Co-substituted LiZnTiBi ferrite ceramics The LiZnTiBi (Li0.43Zn0.27Ti0.13CoxFe2.167-xBi0.003O4) ferrite ceramics with different Co2O3 additives (x = 0.100, 0.125, 0.150, 0.175
Fig. 2. (a) SEM image of the Co-substituted LiZnTiBi ferrite (x = 0.150) sintered at 900 °C; (b) histograms for grain size of the sample in Fig. 2(a); (c) enlarged area of the sample in Fig. 2(a); (d) energy dispersive spectrometer (EDS) result of the ferrite marked in Fig. 2(c)). 2
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Fig. 3. (a) Frequency dependence of permeability (μr’) and (b) magnetic loss (tan δμ) of Co-substituted LiZnTiBi ferrite sintered at 900 °C.
Fig. 4. Dielectric properties of LiZnTiBi ferrite with different Co2O3 additives (x = 0.100, x = 0.125, x = 0.150, x = 0.175 and x = 0.200): (a) permittivity and (b) dielectric loss.
data. In a word, a Co-substituted LiZnTiBi ferrite with pure spinel structure was obtained at 900 °C. Fig. 2a shows SEM image of the ferrite sample (x = 0.150) sintered at 900 °C. For other ferrite samples with various Co2O3 additives, the SEM images were showed in Fig. S2. Compared with grain size of LiZnTiBi ferrite without Co2O3 additives reported in previous work [14], all ferrite samples sintered at 900 °C possess similar grain size due to equal Bi2O3 sintering aids. The results also indicated that the Co2O3 has a little influence on grain growth at the low sintering temperature. To analyze homogeneity of the grains, we calculated the grain size of the sample (x = 0.150). The histogram for grain size of the Co-substituted LiZnTiBi ferrite sample was displayed in Fig. 2b. The figure indicated more than 85% grains have small grain size (∼1.0 μm). Fig. 2c shows enlarged area (red circle in Fig. 2a) of the sample. It was indicated that the Co-substituted LiZnTiBi ferrite sintered at 900 °C possess uniform and compact grains. The Energy dispersive X-ray spectroscopy of the sample was showed in Fig. 2d. All elements, including Fe, Ti, Zn, Bi, Co and O, are detected in the ferrite grain (marked in Fig. 2c). The proportions of elements are displayed in Table S2. Fig. 3 shows magnetic properties of the Co-substituted LiZnTiBi ferrite sintered at 900 °C. In previous work, we have reported that the LiZnTiBi ferrite without Co2O3 additive, the permeability is about 142 (fr less than 10 MHz) [14]. However, in Fig. 3a, the permeability (μr’) of the samples continually decreased when Co2O3 additives were added. For the sample (x = 0.200), the μr’ decreased from ∼26 to ∼7.6 at 10 MHz. This is due to reduction of saturation magnetization when Co2O3 was added. Thus, the permeability of the sample obviously decreased. According to Snoek's law [26], the permeability of magnetic material is inversely proportional to resonance frequency (cut-off frequency). Therefore, the cut-off frequency of the sample increased due to reduction of permeability. Also, as shown in Fig. 3b, the magnetic loss
Fig. 5. Density of the LiZnTiBi ferrite with different Co2O3 additives. Table 1 Important parameters of Co-substituted LiZnTiBi ferrite sintered at 900 °C (x = 0.100, 0.125, 0.150, 0.175 and 0.200) at 0.5 GHz. Samples
μ′
tanδμ
ε′
tanδε
(μ’/ε′)1/2
(μ’ε’)1/2
x = 0.100 x = 0.125 x = 0.150 x = 0.175 x = 0.200
17.19 13.68 12.47 10.05 8.66
1.71 1.05 0.83 0.59 0.48
13.27 13.13 11.96 11.44 10.94
9.06e-2 7.75e-2 6.00e-2 1.68e-2 1.55e-2
1.14 1.02 1.02 0.94 0.89
15.10 13.40 12.21 10.72 9.73
metal ions, of the ferrite sample were listed in Table S1. It was demonstrated that Fe3+ ions in the A-site were substituted by the Co3+. Also, positions of Bragg scattering are nearly the same as experimental 3
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Fig. 6. (a) Frequency dependence of the normalized impedance factor and miniaturization factor (b) Magnetic and dielectric loss of the Co-substituted LiZnTiBi ferrite (x = 0.150) sintered at 900 °C.
changed from 11.75 to 12.21 at wide frequency range (10 MHz∼1 GHz). In addition, the normalized impedance factor has stable value from 10 MHz to 500 MHz (1.02–1.08). However, the value abruptly reduced when frequency more than 500 MHz (i.e., cut-off frequency of the ferrite). Thus, as shown in Fig. 6b, the losses of the ferrite, including magnetic and dielectric loss, started to abruptly increase when frequency more than 500 MHz. In a word, adding moderate Co2O3 into LiZnTiBi ferrite can obtain a good substrate material with high miniaturization factor (∼12) and equivalent permeability and permittivity at a wide frequency range (10 MHz–500 MHz). And the magnetic ferrite material is promising for miniaturized antenna application.
of the Co-substituted LiZnTiBi ferrite obviously decreased when Co2O3 additives were introduced, especially in high frequency range. This can be explained by that the Co3+ ions in LiZnTiBi ferrite induce local uniaxial anisotropy under the influence of a local Weiss field or under an external field. As Su et al. described, the existence of the local uniaxial anisotropy also hinders the irreversible domain wall motion of the ferrite, which leads to the initial decrease of magnetic loss [27]. In a word, Co2O3 additive not only can reduce permeability and improve cut-off frequency, but also reduce magnetic loss of the ferrite at high frequency, which proved that the Co2O3 is a good candidate. Moreover, the dielectric properties of the samples were shown in Fig. 4. In Fig. 4a, the permittivity (εr’) of the samples has a stable value at wide frequency range (from 10 MHz to 1 GHz). For the all samples, the permittivity only changed from 11 to 13. Considering the measurement error, the change of εr’ values are small. Results indicated that the influences of Co2O3 on permittivity are weak. Fig. 4b shows dielectric loss of these ferrite samples. Similarly, adding Co2O3 can obviously reduce dielectric loss of these samples, especially in high frequency range. Fig. 5 shows the bulk density of the samples sintered at 900 °C. Due to higher density of Co2O3 (6.45 g/cm3), the density of Co-substituted LiZnTiBi ferrite continually increased. However, in Fig. 5, the sample (x = 0.150) possess maximum value. This is because that effect of pores on density is dominating. As shown in Fig. S2, when more Co2O3 were added, the samples possessed more pores between grains. In a word, for the Li0.43Zn0.27Ti0.13Fe2.167Bi0.003O4 ferrite, the influences of Co2O3 on magnetic property are more significant than dielectric property. Thus, adding moderate Co2O3 is feasible method to obtain ferrite material with equivalent permeability and permittivity at high frequency. More importantly, the Co2O3 can reduce magnetic and dielectric loss of the ferrite. For the substrate material applied to miniaturized antenna, normalized impedance ((μr’/εr’)1/2) and miniaturization factor ((μr’εr’)1/2) are two important parameters. Fig. S3 reveals the changes of the two parameters. In Fig. S3a, (μr’εr’)1/2 constantly decreased when more Co2O3 additives were introduced. This can be explained by reduction of permeability. Fig. S3b shows (μr’/εr’)1/2 of the samples with various Co2O3 additives. The value of (μr’/εr’)1/2 gradually approached to 1.0 when moderate Co2O3 additive were added. However, the additive amount (x) of Co2O3 more than 0.150, the value started to keep away from the best value (1.0). And the values of these important parameters were listed in Table 1. In Table 1, values of the two parameters are smaller at higher frequency. Interestingly, in the first figure, these broken lines of miniaturization factor have a point of intersection at x = 0.150. Thus, we calculated more values about the two parameters at different frequencies. In Fig. 6a, the miniaturization factor of the ferrite (x = 0.150)
4. Conclusion In this work, a Co-substituted LiZnTiBi ferrite substrate material with low sintering temperature, good miniaturization factor and normalized impedance factor were prepared. First of all, a low temperature sintering Li0.43Zn0.27Ti0.13Fe2.167Bi0.003O4 ferrite with high permeability and low magnetic loss was selected. In order to reduce permeability and enhance cut-off frequency of the LiZnTiBi ferrite, a certain amount of Co2O3 was added. Although Co2O3 additive cannot promote grain growth of the ferrite, it obviously decreases permeability (from ∼140 to ∼12). Meanwhile, the Co2O3 can reduce the losses, including magnetic and dielectric loss, of the ferrite. Finally, when the additive amount of Co2O3 (x) is 0.150, the ferrite sample possessed equivalent permeability and permittivity (μr’ = εr’ = ∼12.3) at wide frequency range. When working frequency is less than 500 MHz, the ferrite has high miniaturization factor (∼12) and good normalized impedance factor (∼1.0). Such a ferrite substrate material should also be an alternative material for microwave devices, such as miniature antenna. Acknowledgments This work was financially supported by Key Laboratory of Shaanxi Provincial Department of Education (Grant No. 17JS052). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.06.008. References [1] V.G. Harris, Modern microwave ferrites, IEEE Trans. Magn. 48 (2012) 1075–1104. [2] G. Catalan, J.F. Scott, Physics and applications of bismuth ferrite, Adv. Mater. 21 (2009) 2463–2485.
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