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Enhanced microwave absorption properties of barium ferrites by Zr4+-Ni2+ doping and oxygen-deficient sintering
Journal of Magnetism and Magnetic Materials 494 (2020) 165828
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Research articles
Enhanced microwave absorption properties of barium ferrites by Zr4+-Ni2+ doping and oxygen-deficient sintering ⁎
Yujing Zhanga,c, Chuyang Liub,c, , Xinrui Zhaob, Minhao Yaoa, Xuefei Miaoa, Feng Xua,
T
⁎
a
School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China School of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China c State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Barium ferrites Microwave absorption Natural resonance Oxygen-deficient sintering
To achieve excellent microwave absorption properties within the frequency range of 2–18 GHz, Zr4+-Ni2+ codoped barium ferrites BaFe12-2x(ZrNi)xO19 have been synthesized through a sol–gel process and subsequent sintering. Broadened bandwidth of ∼6.9 GHz and strong microwave absorption of −59.3 dB is attained for the ferrite with x = 0.6 and sintering in Ar. The excellent microwave absorption properties are attributed to both the effective shifting of resonance peaks by Zr4+-Ni2+ substitution and the enhanced dielectric loss by generating lattice vacancies through the oxygen-deficient sintering process. This work would be helpful to understand the nature of manipulating the resonance behaviors and further improving the microwave absorption efficiency of traditional ferrite absorbers.
1. Introduction Microwave detections and communications are playing essential roles in both the military and civil industries, resulting in the arising interests on exploiting high-efficient electromagnetic (EM) wave absorbers for stealthy and shielding technologies [1–5]. Excellent EM wave absorption materials should be qualified with high absorption efficiency, broad absorption bandwidth, low density and low cost as well [6–8]. Previous studies have proved the materials such as metal particles [9–11], metallic oxides [12–16], ferrites [17–19], carbon materials [20–23] and conductive polymers [24] to be potential EM wave absorbers due to their outstanding absorption efficiency and unique material features. While, achieving a wide-band absorption in the low radar frequency region (2–18 GHz in particular of fire-controlled radar for military use) is still a very challenging work. Among them, M-type barium ferrite (BaFe12O19) is favorable in practical applications due to its easy preparation, nontoxicity and extremely low cost. The ferrite works as a microwave absorber because of its remarkable magnetic loss that occurs from the natural resonance [25,26]. However, the natural resonance frequency of BaFe12O19 is reported to be as high as ∼43.5 GHz [27]. Therefore, to achieve absorption properties within 2–18 GHz, the natural resonance frequency should be modulated to the lower range. It is known that the natural resonance frequency of ferrites is directly proportional to the magnetoanisotropy (HA), which is determined by the exchange-coupling of the ⁎
Fe3+ ions in the lattice [28,29]. HA is supposed to decrease if the Fe3+ ions are replaced by non-magnetic or weaker magnetic ions. The more the Fe3+ ions are replaced, the lower the HA might be. Therefore, the reflection loss (RL) peaks could shift to a lower frequency range following the natural resonance with the Fe3+ ions being replaced. On the other aspect, improving the complex permittivity might further enhance the microwave absorption efficiency for the increased attenuation ability and improved quarter-wavelength cancellation. By decreasing the oxygen partial pressure during the sintering process, the polarization relaxation and conductivity will be enhanced in the ferrites under alternated field as more oxygen vacancies could be generated in the crystal lattice [30]. Consequently, the increased dielectric loss would contribute to the higher dissipation of the EM waves. Herein, aiming at both shifting the natural resonance frequency and improving the EM wave absorption ability, Zr4+-Ni2+ ions co-doped ferrites BaFe12-2x(ZrNi)xO19 were pre-synthesized and further sintered in Ar. The modified ferrites exhibit strongest absorption of −59.3 dB and wide bandwidth of ∼6.9 GHz within the frequency range of 2–18 GHz. The correlations between Zr4+-Ni2+ ions doping content and resonance behavior were also systematically studied and discussed. This work suggests a feasible way to prepare high-performance ferrite absorbers though a simple ironic replacing and oxygen-deficient sintering process.
Corresponding authors. E-mail addresses: [email protected] (C. Liu), [email protected] (F. Xu).
https://doi.org/10.1016/j.jmmm.2019.165828 Received 27 June 2019; Received in revised form 22 August 2019; Accepted 9 September 2019 Available online 11 September 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 494 (2020) 165828
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2. Experimental
belong to the BaFe12O19 (space group P63/mmc). There are minor phases with weak diffraction peaks and indexed as BaFe2O4, BaFeO3 and γ-Fe2O3 in all the Zr4+-Ni2+ doped samples. Considering the amount of impurities is too little, they might not significantly affect the microwave absorption properties of the ferrites. Meanwhile, it should be noted that sintering the samples in Ar atmosphere will not destroy the hexagonal structure of the BaFe12O19 phase. Fig. 2 illustrated the SEM images of the BaFe12-x(ZrNi)xO19 powders with x = 0.6 and 0.9 by sintering in air and Ar respectively. The powder samples show typically plate-like grains. When the doping content of Zr4+-Ni2+ is low (x = 0.6), the ferrites show grain sizes of 1–2 μm in length and ∼0.2 μm in height for both samples sintered in air and Ar (Fig. 2a and b). With increasing the substitution amount of Zr4+-Ni2+ from x = 0.6 to x = 0.9, abnormal grain growth occurs (Fig. 2c and d). The grain growth can be attributed to the induced lattice distortions with Zr4+-Ni2+ ions doping that decreasing the critical formation energy and increasing the diffusion rate in the non-stoichiometric system [31,32]. Fig. 3 displays the room temperature hysteresis loops of the BaFe12x(ZrNi)xO19 ferrites (x = 0.6, 0.7, 0.8 and 0.9). It is obvious that the samples are approaching saturation with the external field being increased up to 1.5 kOe. The saturation magnetization (Ms) decreases from ∼50 emu/g to ∼35 emu/g for the ferrites sintered in air (Fig. 3a) and from ∼55 emu/g to ∼28 emu/g for the ones sintered in Ar (Fig. 3b) with x increasing from 0.6 to 0.9. Meanwhile, the coercivity (Hc) also decreases from ∼0.19 kOe to ∼0.13 kOe and from ∼0.20 kOe to ∼0.07 kOe for the ferrites sintered in air and Ar, respectively. The weakened magnetic properties can be ascribed to the substitution of Fe3+ by the non-magnetic ions (Zr4+) and the weaker magnetic ions (Ni2+) that deteriorates the net magnetism and magnetic anisotropy (HA). HA of all the ferrites can be obtained by the law of approach to saturation (LAS), which is expressed as Eq. (1) [18,33].
Zr4+-Ni2+ co-doped barium ferrites (BaFe12-x(ZrNi)xO19) with x ranging from 0.6 to 0.9 were synthesized by sol–gel method. Barium nitrate Ba(NO3)2·5H2O, Ferric nitrate Fe(NO3)3·9H2O, Zirconium nitrate Zr(NO3)4·5H2O, and Nickel nitrate Ni(NO3)2·4H2O were selected as Ba, Fe, Zr and Ni sources. Citric acid C6H807·H2O was chosen as complexing agent and combustion improver. All of the chemicals belong to analytical grade and were used without further purification. The sol precursors were obtained by dissolving Ba(NO3)2·5H2O, Fe(NO3)3·9H2O, Zr (NO3)4·5H2O, Ni(NO3)2·4H2O and C6H8O7·H2O in the deionized water with the molar ratio of 1: 12-2x : x : x : 19 (x = 0.6 ∼ 0.9) and magnetic stirring for 3 h. Afterwards, adjusting the pH value of the solutions to be 7.0 with NH3H2O and evaporating the solution at 120 ℃ for 72 h to attain the dried gels. Finally, BaFe12-x(ZrNi)xO19 powders with different Zr4+-Ni2+ substitutions content were obtained by further heating the gels at 450 ℃ and sintering at 1400 ℃ for 3 h in Ar atmosphere. For comparison, the samples sintered in air with the same chemical composition were also prepared. The phase constitutes of the ferrites were identified by X-ray diffraction (XRD, PANalytical B V Empyrean 200895, Cu Kα radiation) and micromorphology of the sintered powders were observed by scanning electron microscopy (SEM, Hitachi SU-70 FESEM). The magnetic properties were measured with a Vibrating Sample Magnetometer (VSM, Lakeshore 4710). Before the electromagnetic (EM) property measurement, the ferrite powders were mixed with paraffin at the weight ratio of 4:1 and then the mixture was cast into a ring mold with thickness of 2.0 mm, inner diameter of 3 mm, and outer diameter of 7 mm. The complex permittivity and permeability of the ring samples were then obtained by using an Agilent vector network analyzer (E8363C PNA).
A B M = Ms ⎛1 − − 2 ⎞ + χP H H H ⎠ ⎝
3. Results and discussion To investigate the phase constitutes of the sintered barium ferrites, the X-ray diffraction patterns of the samples with different Zr4+-Ni2+ substitution contents and sintering atmospheres are recorded. As can be seen in Fig. 1, all the samples exhibit the same diffraction peaks that
(1)
where A is the inhomogeneity parameter, B is the anisotropy parameter and χp is the high field differential susceptibility. B of hexagonal symmetry can be expressed as Eq. (2).
B = HA2 /15
(2)
According to the calculation and linear fitting, HA changes from 7.10 kOe to 5.04 kOe for the ferrites sintered in air and from 6.96 kOe to 4.87 kOe for the ferrites sintered in Ar. One can see that the HA decreases significantly with the substitution of Zr4+-Ni2+ ions, though the barium ferrite (BaFe12O19) is a typical hard magnetic material with HA ∼ 15.4 kOe [18]. It suggests that the barium ferrite can be transformed from a hard magnetic material to a potential soft magnetic material by doping with Zr4+-Ni2+ ions. More importantly, the soft magnetism for the doped barium ferrite might further give risen to the complex permeability. The effective EM wave absorption properties are strongly depended on the complex permittivity (εr = ε' − jε'') and the complex permeability (μr = μ′ − jμ'')of the absorbers. The real part of complex permittivity (ε') and complex permeability (μ′) represent the storage of electric energy and magnetic energy. The imaginary part of complex permittivity (ε″) and complex permeability (µ″) represent the loss of electric energy and magnetic energy, respectively. Fig. 4 summarizes the EM parameters (ε', ε'', μ' and μ'') of the BaFe12-x(ZrNi)xO19 with x ranging from 0.6 to 0.9 over 2–18 GHz in different sintering atmospheres. Seen from Fig. 4a, μ' values are around 1 for all the samples and follow an overall decreasing tendency when the frequency increases. The natural resonance frequency of BaFe12O19 is reported to be ∼43.5 GHz which is beyond the measured frequency range. However, one can observe the asymmetrical resonance peaks in μ'' (Fig. 4b) and the positive resonance peaks indicate the successful shifting of natural resonance to the desired frequency range of 2–18 GHz. Meanwhile, the
Fig. 1. X-ray diffraction patterns of the barium ferrites BaFe12-x(ZrNi)xO19 (x = 0.6, 0.7, 0.8 and 0.9) sintered in air (a) and Ar (b), respectively. 2
Journal of Magnetism and Magnetic Materials 494 (2020) 165828
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Fig. 2. Scanning electron microscopy images of the barium ferrites BaFe12-x(ZrNi)xO19 with x = 0.6 (a) and 0.9 (c) sintered in air, and x = 0.6 (b) and 0.9 (d) sintered in Ar.
frequency in BaFe12O19 by Zr4+-Ni2+ doping. Meanwhile, higher Zr4+Ni2+ concentrations lead to the decreased resonance peak intensities of the doped sample, which would possibly result in the weakened EM wave absorptions. From Fig. 4c and d, it is obvious that the complex permittivity (ε', ε'') decreases with the frequency increasing. However, the permittivity exhibits relatively higher values in the samples sintered in Ar than that in air over the test frequency. ε' increases from ∼5 to ∼7.5 and ε'' increases from ∼1 to ∼2 by comparing the samples sintered in air to that in Ar. The increased complex permittivity of the ferrites sintered in Ar is mainly attributed to the formation of oxygen vacancies that induces additional polarization relaxation and conductivity [30], which enhances the complex permittivity of the BaFe12-x(ZrNi)xO19 ferrites. Based on model of metal backplane, the RL values of BaFe124+ -Ni2+ doping content over 2–18 GHz x(ZrNi)xO19 with different Zr were calculated by Eqs. (3) and (4):
RL = 20 log
Zin = Z0 Fig. 3. Room temperature hysteresis loops of the of the barium ferrites BaFe12(x = 0.6, 0.7, 0.8 and 0.9) sintered in air (a) and Ar (b), respectively.
Zin − Z0 Zin + Z0
μr 2πfd tanh ⎡j μr εr ⎤ εr ⎣ c ⎦
(3)
(4)
where Zin is the input impedance of the absorber, Z0 is the characteristic impedance of free space, f is the frequency and c is the velocity of light [34,35]. Fig. 5 exhibits the RL values of BaFe12-x(ZrNi)xO19 sintered in air over 2–18 GHz with thickness varying from 0 to 5 mm. The blue area at the bottom surface represents the region that RL is below −10 dB (RL < −10 dB ensures the absorptivity larger than 90% and is considered to be suitable for applications). As is seen, the samples of x = 0.6 and 0.7 possess RL values < -10 dB (Fig. 5a and b) within the frequency range of 2–18 GHz. The minimum RL values with x from 0.6 to 0.9 is −19.6 dB at 13.2 GHz, −15.1 dB at 12.3 GHz, −9.6 dB at 11.8 GHz and −9.2 dB at 11.9 GHz, respectively. The RL values are exactly consistent with the resonance peak intensities that decreases with increased Zr4+-Ni2+ content (Fig. 4b). Fig. 6 sums up the RL
x(ZrNi)xO19
doped samples are all displaying broadened resonance peaks that from 6–14 GHz and 8–18 GHz, which are beneficial for increasing the bandwidth of the EM wave absorptions. The broadened peaks are ascribed to the Zr4+-Ni2+ ions substitution that generates multiple HA and leads to the multiple magnetic natural resonance behavior of the ferrites [19,25]. The peak frequency reduces when x changes from 0.6 to 0.7, and then shows no obvious reduction with x further increasing to 0.9. It indicates x = 0.7 to be the up limit for modulating the resonance
3
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Fig. 4. Electromagnetic parameters of (a) μ', (b) μ'', (c) ε' and (d) ε'' of the barium ferrites BaFe12-x(ZrNi)xO19 (x = 0.6, 0.7, 0.8 and 0.9) by sintering in air and Ar, respectively.
3.8 GHz (10.3–14.1 GHz) at a thicker 2.85 mm. While, the other two samples with higher Zr4+-Ni2+ content of x = 0.8 and 0.9 shows no effective absorptions frequency regions that below −10 dB. Fig. 7 shows the RL values of the BaFe12-x(ZrNi)xO19 ferrites sintered
values of the samples with the minimum RL peaks under the matching thicknesses. It is found that the sample sintered in air with x = 0.6 shows largest bandwidth (RL < 10 dB) of 6.3 GHz (10.9–17.2 GHz) at 2.5 mm. And the sample with x = 0.7 possesses narrow bandwidth of
Fig. 5. The frequency and thickness-dependent reflection loss (RL) of the barium ferrites BaFe12-x(ZrNi)xO19 sintered in air with different Zr4+-Ni2+ contents. (a) x = 0.6, (b) x = 0.7 (c) x = 0.8 and (d) x = 0.9. 4
Journal of Magnetism and Magnetic Materials 494 (2020) 165828
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Fig. 6. Reflection loss (RL) spectra of the barium ferrites BaFe12-x(ZrNi)xO19 sintered in air with strongest absorption peak under matching thickness. (a) x = 0.6, (b) x = 0.7 (c) x = 0.8 and (d) x = 0.9.
Fig. 7. The frequency and thickness-dependent reflection loss (RL) of the barium ferrites BaFe12-x(ZrNi)xO19 sintered in Ar with different Zr4+-Ni2+ contents: (a) x = 0.6, (b) x = 0.7 (c) x = 0.8 and (d) x = 0.9.
in Ar over 2–18 GHz with thickness varying from 0 to 5 mm. By sintering the BaFe12-x(ZrNi)xO19 ferrites in Ar, greatly enhanced EM wave absorption efficiency was achieved. As is seen, all the samples possess absorption frequency regions that below −10 dB. Fig. 8 displays the RL values of the samples sintered in Ar with the strongest RL peak under matching thickness. The sample with x = 0.6 shows the highest absorption efficiency with RLmin of −53.9 dB at the frequency of 11.0 GHz
and thickness of 2.5 mm, which is nearly 3 times of the one sintered in air. Moreover, the corresponding bandwidth is also broadened to 6.9 GHz (from 8.36 to 15.28 GHz). The minimum RL values of the rest samples sintered in Ar are −36.1 dB at 14.0 GHz, −24.5 dB at 7.6 GHz and −16.2 dB at 12.2 GHz with x from 0.7 to 0.9, showing much more competitive EM wave absorption properties than the samples sintered in air. The above results demonstrate the effectiveness of improving the 5
Journal of Magnetism and Magnetic Materials 494 (2020) 165828
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Fig. 8. Reflection loss (RL) spectra of the barium ferrites BaFe12-x(ZrNi)xO19 sintered in Ar with strongest absorption peak under matching thickness. (a) x = 0.6, (b) x = 0.7 (c) x = 0.8 and (d) x = 0.9.
absorption performance of barium ferrites by sintering in oxygen-deficient conditions. To further clarify the reasons of enhanced absorption properties for the Ar-sintered ferrites, the impedance matching is taken into consideration. The impedance matching between the absorber and free space is an important factor that guarantees EM waves to enter the absorbers instead of being reflected directly at the air-absorber interface [36,37]. In fact, perfectly matched materials with Zin = Z0 are rare to find in the natural world. Thus in the practical occasions, the |Zin/Z0| value is required be close to 1 for small. To obtain the impedance matching condition of the absorber, the thickness is assumed to be satisfied with the quarter-wavelength law and is given as [25]:
d=
cn 4f |εr |·|μr |
α=
μr ⎡ πn μr εr ⎤ tanh ⎢j ⎥ εr ⎣ 2 |εr|·|μr | ⎦
(μ''ε'' − μ'ε') +
(μ''ε'' − μ'ε')2 + (μ'ε'' + μ''ε')2
(7)
where f is the frequency and c is the velocity of light. From this equation, one can deduce that the higher values of µ″ and ε″ are, the larger the attenuation constant α is. As shown in Fig. 9c and d, the attenuation constants follow a similar vibration to the imaginary part of complex permeability (µ″) (Fig. 4b), indicating the dominant role of µ″ in the EM wave dissipation process for the barium ferrites. Meanwhile, it is noted that the attenuation constants of the samples sintered in Ar possess higher values than the samples sintered in air, demonstrating the superior microwave attenuation abilities compared to those sintered in air. As a result, the Ar-sintered samples show significantly enhanced EM wave absorption properties. For the Ar-sintered ferrites in the present case, manipulated natural resonance behaviors contributes the high magnetic loss in the frequency range of 2–18 GHz. The induced oxygen vacancies by sintering the ferrites in oxygen-deficient conditions increase the permittivity of samples. Moreover, the impedance matching and attenuation constant of ferrites sintered are improved with the synergetic effects of the enhanced magnetic loss and dielectric loss. Consequently, the samples sintered in Ar show more efficient EM wave absorptions than the ones with the same chemical compositions sintered in air.
(5)
where n is odd integer. By merging Eqs. (4) and (5), |Zin/Z0| is obtained as following:
|Zin/ Z0| =
2 πf × c
(6)
The calculated |Zin/Z0| of the samples by sintering in air and Ar are shown in Fig. 9a and b. The |Zin/Z0| value ranges from 1.6 to 2.1 for the samples sintered in air and from 1.4 to 1.9 for the samples sintered in Ar. The slightly decreased |Zin/Z0| values for the samples sintered in Ar mainly result from the increased complex permittivity. Considering the fact that the samples contain the same BaFe12-x(ZrNi)xO19 phase structure (seen in Fig. 1), the impedance matching does not exhibit great changes for the samples sintered in different atmospheres. In addition to the good impedance matching that minimizes the EM wave reflection, the EM wave dissipation in the absorber is another key factor. Here, the attenuation constant (α) is applied to evaluate the interior attenuation abilities of the ferrites [38]. The value of the constant (α) can be calculated by the following expression:
4. Conclusions In summary, high-performanced ferrite absorbers were successfully obtained through a Zr4+-Ni2+ co-doping and subsequent oxygen-deficient sintering process. The Ar-sintered BaFe12-x(ZrNi)xO19 ferrite with x = 0.6 shows the excellent reflection loss of −53.9 dB at the frequency of 11.0 GHz, which is nearly 3 times of the one sintered in air. The effective EM wave absorption frequency of BaFexTiO19 was regulated to 2–18 GHz and a broad bandwidth of ∼ 6.9 GHz was achieved at the same time by regulating the natural resonance behaviors of the BaFe12x(ZrNi)xO19 ferrites. This work provides inspirations and suggests a convenient procedure for developing competitive ferrite absorbers. 6
Journal of Magnetism and Magnetic Materials 494 (2020) 165828
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Fig. 9. The calculated impedance matching (|Zin/Z0|) and the attenuation constant (α) for the barium ferrites BaFe12-x(ZrNi)xO19 sintered in air (a), (c), and sintered in Ar (b), (d), respectively.
Acknowledgements [12]
This work was supported by the National Natural Science Foundation of China (Nos. 51801103, 51802155, 51801102, U1832191), the Natural Science Foundation of Jiangsu Province (No.BK20180443), the Aeronautical Science Foundation of China (No.2018ZF52078), the visiting scholar fund of state key laboratory of silicon materials (No. SKL2019-09).
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276–282. [32] H.C. Fang, Z. Yang, C.K. Ong, Y. Li, C.S. Wang, Preparation and magnetic properties of (Zn-Sn) substituted barium hexaferrite nanoparticles for magnetic recording, J. Magn. Magn. Mater. 187 (1998) 129–135. [33] Z.J. Li, Z.L. Hou, W.L. Song, X.D. Liu, W.Q. Cao, X.H. Shao, M.S. Cao, Unusual continuous dual absorption peaks in Ca-doped BiFeO3 nanostructures for broadened microwave absorption, Nanoscale 8 (2016) 10415–10424. [34] K.L. Zhang, J.Y. Zhang, Z.L. Hou, S. Bi, Q.L. Zhao, Multifunctional broadband microwave absorption of flexible graphene composites, Carbon 141 (2019) 608–617. [35] X. Wang, Q.F. Li, Z.J. Su, W. Gong, R.Z. Gong, Y.J. Chen, V.G. Harris, Enhanced microwave absorption of multiferroic Co2Z hexaferrite-BaTiO3 composites with tunable impedance matching, J. Alloys Compd. 643 (2015) 111–115. [36] X.G. Huang, J. Zhang, M. Lai, T.Y. Sang, Preparation and microwave absorption mechanisms of the NiZn ferrite nanofibers, J. Alloys Compd. 627 (2015) 367–373. [37] J.Y. Yu, F.L. Chi, Y.P. Sun, J.J. Guo, X.G. Liu, Assembled porous Fe3O4@g-C3N4 hybrid nanocomposites with multiple interface polarization for stable microwave absorption, Ceram. Int. 44 (2018) 19207–19216.
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Research articles
Enhanced microwave absorption properties of barium ferrites by Zr4+-Ni2+ doping and oxygen-deficient sintering ⁎
Yujing Zhanga,c, Chuyang Liub,c, , Xinrui Zhaob, Minhao Yaoa, Xuefei Miaoa, Feng Xua,
T
⁎
a
School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China School of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China c State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Barium ferrites Microwave absorption Natural resonance Oxygen-deficient sintering
To achieve excellent microwave absorption properties within the frequency range of 2–18 GHz, Zr4+-Ni2+ codoped barium ferrites BaFe12-2x(ZrNi)xO19 have been synthesized through a sol–gel process and subsequent sintering. Broadened bandwidth of ∼6.9 GHz and strong microwave absorption of −59.3 dB is attained for the ferrite with x = 0.6 and sintering in Ar. The excellent microwave absorption properties are attributed to both the effective shifting of resonance peaks by Zr4+-Ni2+ substitution and the enhanced dielectric loss by generating lattice vacancies through the oxygen-deficient sintering process. This work would be helpful to understand the nature of manipulating the resonance behaviors and further improving the microwave absorption efficiency of traditional ferrite absorbers.
1. Introduction Microwave detections and communications are playing essential roles in both the military and civil industries, resulting in the arising interests on exploiting high-efficient electromagnetic (EM) wave absorbers for stealthy and shielding technologies [1–5]. Excellent EM wave absorption materials should be qualified with high absorption efficiency, broad absorption bandwidth, low density and low cost as well [6–8]. Previous studies have proved the materials such as metal particles [9–11], metallic oxides [12–16], ferrites [17–19], carbon materials [20–23] and conductive polymers [24] to be potential EM wave absorbers due to their outstanding absorption efficiency and unique material features. While, achieving a wide-band absorption in the low radar frequency region (2–18 GHz in particular of fire-controlled radar for military use) is still a very challenging work. Among them, M-type barium ferrite (BaFe12O19) is favorable in practical applications due to its easy preparation, nontoxicity and extremely low cost. The ferrite works as a microwave absorber because of its remarkable magnetic loss that occurs from the natural resonance [25,26]. However, the natural resonance frequency of BaFe12O19 is reported to be as high as ∼43.5 GHz [27]. Therefore, to achieve absorption properties within 2–18 GHz, the natural resonance frequency should be modulated to the lower range. It is known that the natural resonance frequency of ferrites is directly proportional to the magnetoanisotropy (HA), which is determined by the exchange-coupling of the ⁎
Fe3+ ions in the lattice [28,29]. HA is supposed to decrease if the Fe3+ ions are replaced by non-magnetic or weaker magnetic ions. The more the Fe3+ ions are replaced, the lower the HA might be. Therefore, the reflection loss (RL) peaks could shift to a lower frequency range following the natural resonance with the Fe3+ ions being replaced. On the other aspect, improving the complex permittivity might further enhance the microwave absorption efficiency for the increased attenuation ability and improved quarter-wavelength cancellation. By decreasing the oxygen partial pressure during the sintering process, the polarization relaxation and conductivity will be enhanced in the ferrites under alternated field as more oxygen vacancies could be generated in the crystal lattice [30]. Consequently, the increased dielectric loss would contribute to the higher dissipation of the EM waves. Herein, aiming at both shifting the natural resonance frequency and improving the EM wave absorption ability, Zr4+-Ni2+ ions co-doped ferrites BaFe12-2x(ZrNi)xO19 were pre-synthesized and further sintered in Ar. The modified ferrites exhibit strongest absorption of −59.3 dB and wide bandwidth of ∼6.9 GHz within the frequency range of 2–18 GHz. The correlations between Zr4+-Ni2+ ions doping content and resonance behavior were also systematically studied and discussed. This work suggests a feasible way to prepare high-performance ferrite absorbers though a simple ironic replacing and oxygen-deficient sintering process.
Corresponding authors. E-mail addresses: [email protected] (C. Liu), [email protected] (F. Xu).
https://doi.org/10.1016/j.jmmm.2019.165828 Received 27 June 2019; Received in revised form 22 August 2019; Accepted 9 September 2019 Available online 11 September 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 494 (2020) 165828
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2. Experimental
belong to the BaFe12O19 (space group P63/mmc). There are minor phases with weak diffraction peaks and indexed as BaFe2O4, BaFeO3 and γ-Fe2O3 in all the Zr4+-Ni2+ doped samples. Considering the amount of impurities is too little, they might not significantly affect the microwave absorption properties of the ferrites. Meanwhile, it should be noted that sintering the samples in Ar atmosphere will not destroy the hexagonal structure of the BaFe12O19 phase. Fig. 2 illustrated the SEM images of the BaFe12-x(ZrNi)xO19 powders with x = 0.6 and 0.9 by sintering in air and Ar respectively. The powder samples show typically plate-like grains. When the doping content of Zr4+-Ni2+ is low (x = 0.6), the ferrites show grain sizes of 1–2 μm in length and ∼0.2 μm in height for both samples sintered in air and Ar (Fig. 2a and b). With increasing the substitution amount of Zr4+-Ni2+ from x = 0.6 to x = 0.9, abnormal grain growth occurs (Fig. 2c and d). The grain growth can be attributed to the induced lattice distortions with Zr4+-Ni2+ ions doping that decreasing the critical formation energy and increasing the diffusion rate in the non-stoichiometric system [31,32]. Fig. 3 displays the room temperature hysteresis loops of the BaFe12x(ZrNi)xO19 ferrites (x = 0.6, 0.7, 0.8 and 0.9). It is obvious that the samples are approaching saturation with the external field being increased up to 1.5 kOe. The saturation magnetization (Ms) decreases from ∼50 emu/g to ∼35 emu/g for the ferrites sintered in air (Fig. 3a) and from ∼55 emu/g to ∼28 emu/g for the ones sintered in Ar (Fig. 3b) with x increasing from 0.6 to 0.9. Meanwhile, the coercivity (Hc) also decreases from ∼0.19 kOe to ∼0.13 kOe and from ∼0.20 kOe to ∼0.07 kOe for the ferrites sintered in air and Ar, respectively. The weakened magnetic properties can be ascribed to the substitution of Fe3+ by the non-magnetic ions (Zr4+) and the weaker magnetic ions (Ni2+) that deteriorates the net magnetism and magnetic anisotropy (HA). HA of all the ferrites can be obtained by the law of approach to saturation (LAS), which is expressed as Eq. (1) [18,33].
Zr4+-Ni2+ co-doped barium ferrites (BaFe12-x(ZrNi)xO19) with x ranging from 0.6 to 0.9 were synthesized by sol–gel method. Barium nitrate Ba(NO3)2·5H2O, Ferric nitrate Fe(NO3)3·9H2O, Zirconium nitrate Zr(NO3)4·5H2O, and Nickel nitrate Ni(NO3)2·4H2O were selected as Ba, Fe, Zr and Ni sources. Citric acid C6H807·H2O was chosen as complexing agent and combustion improver. All of the chemicals belong to analytical grade and were used without further purification. The sol precursors were obtained by dissolving Ba(NO3)2·5H2O, Fe(NO3)3·9H2O, Zr (NO3)4·5H2O, Ni(NO3)2·4H2O and C6H8O7·H2O in the deionized water with the molar ratio of 1: 12-2x : x : x : 19 (x = 0.6 ∼ 0.9) and magnetic stirring for 3 h. Afterwards, adjusting the pH value of the solutions to be 7.0 with NH3H2O and evaporating the solution at 120 ℃ for 72 h to attain the dried gels. Finally, BaFe12-x(ZrNi)xO19 powders with different Zr4+-Ni2+ substitutions content were obtained by further heating the gels at 450 ℃ and sintering at 1400 ℃ for 3 h in Ar atmosphere. For comparison, the samples sintered in air with the same chemical composition were also prepared. The phase constitutes of the ferrites were identified by X-ray diffraction (XRD, PANalytical B V Empyrean 200895, Cu Kα radiation) and micromorphology of the sintered powders were observed by scanning electron microscopy (SEM, Hitachi SU-70 FESEM). The magnetic properties were measured with a Vibrating Sample Magnetometer (VSM, Lakeshore 4710). Before the electromagnetic (EM) property measurement, the ferrite powders were mixed with paraffin at the weight ratio of 4:1 and then the mixture was cast into a ring mold with thickness of 2.0 mm, inner diameter of 3 mm, and outer diameter of 7 mm. The complex permittivity and permeability of the ring samples were then obtained by using an Agilent vector network analyzer (E8363C PNA).
A B M = Ms ⎛1 − − 2 ⎞ + χP H H H ⎠ ⎝
3. Results and discussion To investigate the phase constitutes of the sintered barium ferrites, the X-ray diffraction patterns of the samples with different Zr4+-Ni2+ substitution contents and sintering atmospheres are recorded. As can be seen in Fig. 1, all the samples exhibit the same diffraction peaks that
(1)
where A is the inhomogeneity parameter, B is the anisotropy parameter and χp is the high field differential susceptibility. B of hexagonal symmetry can be expressed as Eq. (2).
B = HA2 /15
(2)
According to the calculation and linear fitting, HA changes from 7.10 kOe to 5.04 kOe for the ferrites sintered in air and from 6.96 kOe to 4.87 kOe for the ferrites sintered in Ar. One can see that the HA decreases significantly with the substitution of Zr4+-Ni2+ ions, though the barium ferrite (BaFe12O19) is a typical hard magnetic material with HA ∼ 15.4 kOe [18]. It suggests that the barium ferrite can be transformed from a hard magnetic material to a potential soft magnetic material by doping with Zr4+-Ni2+ ions. More importantly, the soft magnetism for the doped barium ferrite might further give risen to the complex permeability. The effective EM wave absorption properties are strongly depended on the complex permittivity (εr = ε' − jε'') and the complex permeability (μr = μ′ − jμ'')of the absorbers. The real part of complex permittivity (ε') and complex permeability (μ′) represent the storage of electric energy and magnetic energy. The imaginary part of complex permittivity (ε″) and complex permeability (µ″) represent the loss of electric energy and magnetic energy, respectively. Fig. 4 summarizes the EM parameters (ε', ε'', μ' and μ'') of the BaFe12-x(ZrNi)xO19 with x ranging from 0.6 to 0.9 over 2–18 GHz in different sintering atmospheres. Seen from Fig. 4a, μ' values are around 1 for all the samples and follow an overall decreasing tendency when the frequency increases. The natural resonance frequency of BaFe12O19 is reported to be ∼43.5 GHz which is beyond the measured frequency range. However, one can observe the asymmetrical resonance peaks in μ'' (Fig. 4b) and the positive resonance peaks indicate the successful shifting of natural resonance to the desired frequency range of 2–18 GHz. Meanwhile, the
Fig. 1. X-ray diffraction patterns of the barium ferrites BaFe12-x(ZrNi)xO19 (x = 0.6, 0.7, 0.8 and 0.9) sintered in air (a) and Ar (b), respectively. 2
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Fig. 2. Scanning electron microscopy images of the barium ferrites BaFe12-x(ZrNi)xO19 with x = 0.6 (a) and 0.9 (c) sintered in air, and x = 0.6 (b) and 0.9 (d) sintered in Ar.
frequency in BaFe12O19 by Zr4+-Ni2+ doping. Meanwhile, higher Zr4+Ni2+ concentrations lead to the decreased resonance peak intensities of the doped sample, which would possibly result in the weakened EM wave absorptions. From Fig. 4c and d, it is obvious that the complex permittivity (ε', ε'') decreases with the frequency increasing. However, the permittivity exhibits relatively higher values in the samples sintered in Ar than that in air over the test frequency. ε' increases from ∼5 to ∼7.5 and ε'' increases from ∼1 to ∼2 by comparing the samples sintered in air to that in Ar. The increased complex permittivity of the ferrites sintered in Ar is mainly attributed to the formation of oxygen vacancies that induces additional polarization relaxation and conductivity [30], which enhances the complex permittivity of the BaFe12-x(ZrNi)xO19 ferrites. Based on model of metal backplane, the RL values of BaFe124+ -Ni2+ doping content over 2–18 GHz x(ZrNi)xO19 with different Zr were calculated by Eqs. (3) and (4):
RL = 20 log
Zin = Z0 Fig. 3. Room temperature hysteresis loops of the of the barium ferrites BaFe12(x = 0.6, 0.7, 0.8 and 0.9) sintered in air (a) and Ar (b), respectively.
Zin − Z0 Zin + Z0
μr 2πfd tanh ⎡j μr εr ⎤ εr ⎣ c ⎦
(3)
(4)
where Zin is the input impedance of the absorber, Z0 is the characteristic impedance of free space, f is the frequency and c is the velocity of light [34,35]. Fig. 5 exhibits the RL values of BaFe12-x(ZrNi)xO19 sintered in air over 2–18 GHz with thickness varying from 0 to 5 mm. The blue area at the bottom surface represents the region that RL is below −10 dB (RL < −10 dB ensures the absorptivity larger than 90% and is considered to be suitable for applications). As is seen, the samples of x = 0.6 and 0.7 possess RL values < -10 dB (Fig. 5a and b) within the frequency range of 2–18 GHz. The minimum RL values with x from 0.6 to 0.9 is −19.6 dB at 13.2 GHz, −15.1 dB at 12.3 GHz, −9.6 dB at 11.8 GHz and −9.2 dB at 11.9 GHz, respectively. The RL values are exactly consistent with the resonance peak intensities that decreases with increased Zr4+-Ni2+ content (Fig. 4b). Fig. 6 sums up the RL
x(ZrNi)xO19
doped samples are all displaying broadened resonance peaks that from 6–14 GHz and 8–18 GHz, which are beneficial for increasing the bandwidth of the EM wave absorptions. The broadened peaks are ascribed to the Zr4+-Ni2+ ions substitution that generates multiple HA and leads to the multiple magnetic natural resonance behavior of the ferrites [19,25]. The peak frequency reduces when x changes from 0.6 to 0.7, and then shows no obvious reduction with x further increasing to 0.9. It indicates x = 0.7 to be the up limit for modulating the resonance
3
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Fig. 4. Electromagnetic parameters of (a) μ', (b) μ'', (c) ε' and (d) ε'' of the barium ferrites BaFe12-x(ZrNi)xO19 (x = 0.6, 0.7, 0.8 and 0.9) by sintering in air and Ar, respectively.
3.8 GHz (10.3–14.1 GHz) at a thicker 2.85 mm. While, the other two samples with higher Zr4+-Ni2+ content of x = 0.8 and 0.9 shows no effective absorptions frequency regions that below −10 dB. Fig. 7 shows the RL values of the BaFe12-x(ZrNi)xO19 ferrites sintered
values of the samples with the minimum RL peaks under the matching thicknesses. It is found that the sample sintered in air with x = 0.6 shows largest bandwidth (RL < 10 dB) of 6.3 GHz (10.9–17.2 GHz) at 2.5 mm. And the sample with x = 0.7 possesses narrow bandwidth of
Fig. 5. The frequency and thickness-dependent reflection loss (RL) of the barium ferrites BaFe12-x(ZrNi)xO19 sintered in air with different Zr4+-Ni2+ contents. (a) x = 0.6, (b) x = 0.7 (c) x = 0.8 and (d) x = 0.9. 4
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Fig. 6. Reflection loss (RL) spectra of the barium ferrites BaFe12-x(ZrNi)xO19 sintered in air with strongest absorption peak under matching thickness. (a) x = 0.6, (b) x = 0.7 (c) x = 0.8 and (d) x = 0.9.
Fig. 7. The frequency and thickness-dependent reflection loss (RL) of the barium ferrites BaFe12-x(ZrNi)xO19 sintered in Ar with different Zr4+-Ni2+ contents: (a) x = 0.6, (b) x = 0.7 (c) x = 0.8 and (d) x = 0.9.
in Ar over 2–18 GHz with thickness varying from 0 to 5 mm. By sintering the BaFe12-x(ZrNi)xO19 ferrites in Ar, greatly enhanced EM wave absorption efficiency was achieved. As is seen, all the samples possess absorption frequency regions that below −10 dB. Fig. 8 displays the RL values of the samples sintered in Ar with the strongest RL peak under matching thickness. The sample with x = 0.6 shows the highest absorption efficiency with RLmin of −53.9 dB at the frequency of 11.0 GHz
and thickness of 2.5 mm, which is nearly 3 times of the one sintered in air. Moreover, the corresponding bandwidth is also broadened to 6.9 GHz (from 8.36 to 15.28 GHz). The minimum RL values of the rest samples sintered in Ar are −36.1 dB at 14.0 GHz, −24.5 dB at 7.6 GHz and −16.2 dB at 12.2 GHz with x from 0.7 to 0.9, showing much more competitive EM wave absorption properties than the samples sintered in air. The above results demonstrate the effectiveness of improving the 5
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Fig. 8. Reflection loss (RL) spectra of the barium ferrites BaFe12-x(ZrNi)xO19 sintered in Ar with strongest absorption peak under matching thickness. (a) x = 0.6, (b) x = 0.7 (c) x = 0.8 and (d) x = 0.9.
absorption performance of barium ferrites by sintering in oxygen-deficient conditions. To further clarify the reasons of enhanced absorption properties for the Ar-sintered ferrites, the impedance matching is taken into consideration. The impedance matching between the absorber and free space is an important factor that guarantees EM waves to enter the absorbers instead of being reflected directly at the air-absorber interface [36,37]. In fact, perfectly matched materials with Zin = Z0 are rare to find in the natural world. Thus in the practical occasions, the |Zin/Z0| value is required be close to 1 for small. To obtain the impedance matching condition of the absorber, the thickness is assumed to be satisfied with the quarter-wavelength law and is given as [25]:
d=
cn 4f |εr |·|μr |
α=
μr ⎡ πn μr εr ⎤ tanh ⎢j ⎥ εr ⎣ 2 |εr|·|μr | ⎦
(μ''ε'' − μ'ε') +
(μ''ε'' − μ'ε')2 + (μ'ε'' + μ''ε')2
(7)
where f is the frequency and c is the velocity of light. From this equation, one can deduce that the higher values of µ″ and ε″ are, the larger the attenuation constant α is. As shown in Fig. 9c and d, the attenuation constants follow a similar vibration to the imaginary part of complex permeability (µ″) (Fig. 4b), indicating the dominant role of µ″ in the EM wave dissipation process for the barium ferrites. Meanwhile, it is noted that the attenuation constants of the samples sintered in Ar possess higher values than the samples sintered in air, demonstrating the superior microwave attenuation abilities compared to those sintered in air. As a result, the Ar-sintered samples show significantly enhanced EM wave absorption properties. For the Ar-sintered ferrites in the present case, manipulated natural resonance behaviors contributes the high magnetic loss in the frequency range of 2–18 GHz. The induced oxygen vacancies by sintering the ferrites in oxygen-deficient conditions increase the permittivity of samples. Moreover, the impedance matching and attenuation constant of ferrites sintered are improved with the synergetic effects of the enhanced magnetic loss and dielectric loss. Consequently, the samples sintered in Ar show more efficient EM wave absorptions than the ones with the same chemical compositions sintered in air.
(5)
where n is odd integer. By merging Eqs. (4) and (5), |Zin/Z0| is obtained as following:
|Zin/ Z0| =
2 πf × c
(6)
The calculated |Zin/Z0| of the samples by sintering in air and Ar are shown in Fig. 9a and b. The |Zin/Z0| value ranges from 1.6 to 2.1 for the samples sintered in air and from 1.4 to 1.9 for the samples sintered in Ar. The slightly decreased |Zin/Z0| values for the samples sintered in Ar mainly result from the increased complex permittivity. Considering the fact that the samples contain the same BaFe12-x(ZrNi)xO19 phase structure (seen in Fig. 1), the impedance matching does not exhibit great changes for the samples sintered in different atmospheres. In addition to the good impedance matching that minimizes the EM wave reflection, the EM wave dissipation in the absorber is another key factor. Here, the attenuation constant (α) is applied to evaluate the interior attenuation abilities of the ferrites [38]. The value of the constant (α) can be calculated by the following expression:
4. Conclusions In summary, high-performanced ferrite absorbers were successfully obtained through a Zr4+-Ni2+ co-doping and subsequent oxygen-deficient sintering process. The Ar-sintered BaFe12-x(ZrNi)xO19 ferrite with x = 0.6 shows the excellent reflection loss of −53.9 dB at the frequency of 11.0 GHz, which is nearly 3 times of the one sintered in air. The effective EM wave absorption frequency of BaFexTiO19 was regulated to 2–18 GHz and a broad bandwidth of ∼ 6.9 GHz was achieved at the same time by regulating the natural resonance behaviors of the BaFe12x(ZrNi)xO19 ferrites. This work provides inspirations and suggests a convenient procedure for developing competitive ferrite absorbers. 6
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Fig. 9. The calculated impedance matching (|Zin/Z0|) and the attenuation constant (α) for the barium ferrites BaFe12-x(ZrNi)xO19 sintered in air (a), (c), and sintered in Ar (b), (d), respectively.
Acknowledgements [12]
This work was supported by the National Natural Science Foundation of China (Nos. 51801103, 51802155, 51801102, U1832191), the Natural Science Foundation of Jiangsu Province (No.BK20180443), the Aeronautical Science Foundation of China (No.2018ZF52078), the visiting scholar fund of state key laboratory of silicon materials (No. SKL2019-09).
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