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Magnetic and microwave absorbing properties of Ce-Co-based alloy powders driven with lanthanum content
Journal Pre-proof Magnetic and microwave absorbing properties of Ce-Co-based alloy powders driven with lanthanum content Yu He, Shunkang Pan, Jingjing Yu PII:
S1002-0721(19)30284-4
DOI:
https://doi.org/10.1016/j.jre.2019.10.006
Reference:
JRE 632
To appear in:
Journal of Rare Earths
Received Date: 11 April 2019 Revised Date:
11 September 2019
Accepted Date: 24 October 2019
Please cite this article as: He Y, Pan S, Yu J, Magnetic and microwave absorbing properties of Ce-Cobased alloy powders driven with lanthanum content, Journal of Rare Earths, https://doi.org/10.1016/ j.jre.2019.10.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Magnetic and microwave absorbing properties of Ce-Co-based alloy powders driven with lanthanum content Yu He, Shunkang Pan, Jingjing Yu School of material science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China Abstract: The composites of Ce-Co-based alloys doped with La content were fabricated via a vacuum arc melting method. The influences of La addition on microstructure, electromagnetic parameters, magnetic property and microwave absorbing property were measured by the corresponding equipment. The morphology characteristics manifest that all samples display sheet structure, and the average particle size of alloy powders increases with the increasing La content. The saturation magnetization (Ms) decreases with the increasing La addition as a whole. The minimum reflection loss (RL) of La0.4Ce1.6Co17 alloy powder can be obtained about –42.29 dB at 7.84 GHz with the matching thickness of 1.8 mm, and the corresponding effective bandwidth can achieve about 2.24 GHz. In addition, the minimum RL frequency moves towards a lower frequency region as the La content increased. The minimum RL of La0.3Ce1.7Co17 alloy powder is less than –20 dB ranging from 1.2 to 2.4 mm in the whole 4–16 GHz. The maximum bandwidth can reach about 4.88 GHz at the given thickness of 1.2mm. In general, these all indicate the La addition is beneficial to improving the microwave absorbing performance both in effective bandwidth and absorption intensity. Keywords: Ce-Co-based alloys; Magnetic property; microwave absorbing property; Reflection loss; rare earths 1.Introduction Abundant electromagnetic radiation are becoming more and more serious with the extensive use of electronic equipment such as mobile phone, computer, which not only greatly threatens human health but also creates information leakage to defense security.1-6 With the purpose of diminishing these detriments, the microwave absorbing materials have aroused intensive interest from researchers in recent decades. In general, outstanding microwave absorbing property can be determined by complex permittivity and permeability. To date, plenty microwave absorption materials, including dielectric loss materials and magnetic loss materials, have widely studied.3 Among them, absorbers with magnetic loss have been widely studied due to the high saturation magnetization and susceptibility. As typical magnetic loss materials, Co-based alloys can exhibit extraordinary potential for the application of absorbing electrical microwave, due to the ultrahigh saturation magnetization.7-9 Recently, there are a lot of literature can testify that Co-based powders have high magnetic loss and excellent microwave absorbing performances such as FeCo alloy,10-11 Cobalt nanoflakes,12 FeCoB powder composites.13 Foundation items: Project supported by the National Natural Science Foundation of China (51361007) and 2017 Director Fund of Guangxi Key Laboratory of Wireless Wideband Communication and Signal Processing (GXKL06170107). Corresponding Author: E-mail Address: [email protected]. (Shunkang Pan)
Currently, rare-earth with the unfilled 4f shell electrons is conducive to improve the microwave absorbing property causing by existence of the magnetic moment, which has become hot topic. 14-16 In addition, the light rare earth elements have high magnetic crystal anisotropy, dominated by low crystal symmetry caused by double hexagonal lattice or superhexagonal lattice.16 On the basis of Lou’s study,17 the magnetic and absorbing properties of HCMs can be tailored by La3+-doped effectively, and the absorption bandwidth of Ba1–xLaxFe12O19 (x=0.2, 0.4, 0.6) HCMs can reach 10, 8.1 and 8 GHz in the range of 1.5–3 mm. In addition, the study of Shang Tao indicates that Ms of OCLFO decreases after La3+ doping, while the specific surface area, coercivity value, ε" and µ" of O-CLFO increase.18 The minimum RL of O-CLFO reaches –46.47 dB with a thickness of 3.0 mm, and the effective absorption frequency bandwidth reaches 4.9 GHz. These all prove that the light rare earth La can improve the microwave absorbing property. Thus, the light rare earth La was chose to be doped in the Ce-Co-based alloy powders with the purpose of optimizing the microwave absorbing property and effective bandwidth in this work. 2.Experimental The Ce2–xLaxCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy ingots were fabricated by vacuum arc melting and high energy ball milling using La (>99.99% in purity), Ce (>99.99% in purity), Co (99.999% in purity) under protection of Ar atmosphere. The achieved ingots were sealed using vacuum sealing machine and heat-treated in a KSL-1200X chamber electric furnace for two weeks at 800 °C, and then quenched into ice-water mixture. The coarse powders, which were coarse breaking into 100 µm, were milled at the speed of 300 r/min for 20 h with the powder to ball weight ratio of 1:20 using a planet ball mill. The phase composition was measured by X-ray diffraction (XRD, Empyrean PIXcel 3D) with Cu Kα radiation. With the purpose to observe the influence on particle size of La dopant, the scanning electron microscope was used. The complex permittivity and permeability of samples were measured by the Agilent 8722ES vector network analyzer (VNA) using the toroidal samples, which were fabricated by pressing the mixture (mass ratio of paraffin and powders was 1:4) into a mold. In addition, the thickness of sample was controlled at about 3.5 mm. The reflection loss (RL) of single layer absorbing material can be calculated by the given thickness using the tested electromagnetic parameter on the basis of transmission line theory. The calculated equations of RL are as follows. where c is the light velocity in vacuum, f is the microwave frequency, d is the thickness of the absorber, and εr and µr are the relative permittivity and permeability of the sample, obtained from the experimental data.19
RL = −20 lg
Z in − Z 0 Z in + Z 0
(1)
Here, the normalized input impedance (Zin) and free space wave impedance (Z0) of microwave absorption layer can be expressed as
Z0 =
µ0 ε0
(2)
Z in = Z 0
µr 2π tanh j µ r ε r fd εr c
(3)
3.Results and discussions 3.1 XRD and SEM analysis of LaxCe2–xCo17 alloy Fig. 1 shows the scanning electron microscope (SEM) images of LaxCe2-xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders. The average particle size of LaxCe2–xCo17 alloy powders are 2.65, 3.74, 4.84, 5.22 and 5.68 µm, which are measured by Nano Measurer 1.2. That means the average particle size trends to increase with the rise of La content. On the side, it can be seen that all samples exist flaky morphology, which can be beneficial to the improvement of microwave absorbing property due to the multiple scattering caused by larger scattering area.20
Fig. 1 Scanning electron microscope (SEM) images of LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders The X-ray diffraction patterns (XRD) of the LaxCe2-xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders are depicted in Fig. 2. The main phase of all alloy samples is Ce2Co17 phase, and the corresponding space group is P63/mmc (No.194). Besides, the diffraction peak has a tendency to shift to a lower angle with increasing La content.
♥−−Ce2Co17
Intensity (a.u.)
42.5
43.0
43.5
♥
44.0
♥
44.5
45.0
x=0.5
♥♥ ♥ ♥♥ ♥♥
♥♥ ♥♥
♥
♥♥
x=0.4 x=0.3 x=0.2 x=0 20
30
40
50
60
70
80
2θ / (°)
Fig. 2 X-ray diffraction patterns (XRD) of the LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders 3.2 Magnetic property of LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders Fig. 3 shows the magnetic hysteresis loops of LaxCe2–xCo17 alloy powders, which measured between –20000 and 20000 Oe at 25 °C. Saturation magnetization (Ms) can be used to characterize the magnetism of ferromagnetic particles. It can be seen that the saturation magnetization decreases from 0.0797 to 0.0727 emu/mg with the La content increases from 0 to 0.5. What’s more, the materials exhibit low coercivity. 0.10
M / (emu/mg)
0.05 x=0 x=0.2 x=0.3 x=0.4 x=0.5
0.00
-0.05
-0.10
-20000 -10000
0
10000
20000
H / Oe
Fig. 3 Magnetic hysteresis loops of the LaxCe2-xCo17 alloy powders 3.3 Electromagnetic parameters of LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders Electromagnetic properties can be described by complex permittivity and complex permeability, which represent the storage capacity and lossy ability of absorbers, respectively.20-21 As can be seen from Fig. 4 (a, b), both the real part (ε') and imaginary part (ε") of complex permittivity trend to increase with the rise of La content, are ascribed to the combined action of relaxation polarization and conduction loss.22-23 On the basis of Fig. 1, the relaxation polarization loss can be improved due to the tendency of specific surface area with the increasing La content. In addition, the increased flake particles caused the convenience of forming the conductive networks, which is beneficial to the conduction loss.23
The frequency dependence of the complex permeability and magnetic loss of LaxCe2–xCo17 alloy powders are depicted in Fig. 4 (c, d). It can be seen that the µ' values of all samples has a downward trend on the whole with the increase of frequency, which may be ascribed to the limited speed of spin and domain-wall motion (displacement/rotation). The µ' and µ" all possess obvious resonance peaks in 4–16 GHz, which may related to the natural resonance and exchange resonance.2-3 Furthermore, the resonance peak frequency of µ" shifts to a lower frequency region. x=0 x=0.2 x=0.3 x=0.4 x=0.5
(a)
28 24
x=0 x=0.2 x=0.3 x=0.4 x=0.5
(b) 8.25
ε′′
ε′
5.50 20 16
2.75
12 0.00 4
6
8
10
12
14
16
4
6
8
10
f / GHz
0.8
(c)
2.5
12
14
16
f / GHz
x=0 x=0.2 x=0.3 x=0.4 x=0.5
0.6 µ′′
µ′
2.0
x=0 x=0.2 x=0.3 x=0.4 x=0.5
(d)
0.4
1.5 0.2 1.0 4
6
8
10
12
14
f / GHz
16
0.0
4
6
8
10
12
14
16
f / GHz
Fig. 4. Electromagnetic parameters of LaxCe2–xCo17 alloy powders (a): real part of complex permittivity (ε'); (b): imaginary part of complex permittivity (ε"); (c): real part of complex permeability (µ'); (d): imaginary part of complex permeability (µ") 3.4 Microwave absorbing performance of LaxCe2-xCo17 alloy powders The microwave reflection loss (RL) for LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders was calculated by above Eq. (1-3) with the measured electromagnetic parameters at the given thickness of 1.8 mm is shown in Fig. 5. The frequency of minimum reflection loss (RL) shifts to a lower frequency region, which may be attributed to the increase in the complex permittivity in Fig.4. The relationship between matching frequency and thickness can be expressed by Eq. (4).24 Where fm is the matching frequency, dm is the matching thickness, and δM is the magnetic loss. According to the formula, the value of ε' increased significantly and the value of µ' changed little at a given thickness. Therefore, the frequency of minimum reflection loss (RL) shifts to a lower frequency region caused by the increase of ε'µ' value.
fm =
c 4d m
1 1 2 1 + tan δ M ε'µ' 8
−1
(4)
In addition, the value of minimum RL increases at the beginning and then decrease with the increasing La content. The optimum microwave absorption performance was obtained for La0.4Ce1.6Co17 alloy powder, and a minimum reflection loss (RL) of –42.29 dB (99.99% absorption rate) was achieved at 7.84 GHz for a thickness of 1.8 mm. Moreover, the La0.3Ce1.7Co17 alloy powder can achieve the effective bandwidth (frequency width of RL<–10 dB) of 2.72 GHz, expressing that La0.3Ce1.7Co17 alloy powder is provided with outstanding broadband effect. These consequences manifest that the properly addition of light rare earth La can optimize the absorbing performance of Ce2Co17 based alloy powders. 0 (a)
RL / dB
-10 -20
x=0 x=0.2 x=0.3 x=0.4 x=0.5
-30 -40 4
6
8
10
12
14
16
f / GHz
Fig. 5 (a)Reflection loss of LaxCe2–xCo17 alloy powders; (b) 3D histogram of LaxCe2–xCo17 alloy powders between effective bandwidth and corresponding frequency, La addition For further improvement of microwave absorbing performance of LaxCe2–xCo17 alloy powder, the microwave absorption performance of La0.4Ce1.6Co17 and La0.3Ce1.7Co17 alloy with different thickness was studied, as shown in Figs. 6 and 7. As we can be seen from Fig. 6, the value of minimum RL can obtain about –42.29 dB at 1.8 mm, and the value has a increase tendency in the first and then trends to decrease with the increasing thickness. The frequency of RL moves towards a lower frequency, which can be explained by the following equation.25 in which λ stands for the wavelength of EM wave, |µr|, |εr| are the modulus of µr, εr with matching frequency fm, respectively. What’s more, the maximum effective bandwidth can be up to 4.32 GHz and the corresponding thickness is only 1.2 mm.
tm =
nλ nc (n = 1,3,5......) = 4 4 f m µr ε r
(5)
0 -10 RL / dB
(a) -20
1.2mm 1.4mm 1.6mm 1.8mm 2.0mm 2.2mm 2.4mm
-30 -40 -50 4
6
8
10
12
14
16
f / GHz
Effective bandwidth / GHz
5
(c) 4.32
4
3.52 2.88
3
2.24 1.84
2
1.52
1.28
1 0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
f / GHz
Fig.6 (a)Reflectance curves of La0.4Ce1.6Co17 powders with different thicknesses; (b)Three-dimensional relation diagram of reflection loss value, frequency and thickness; (c)Columnar relationship between effective bandwidth and thickness It is found that all reflection loss value is less than –20 dB ranging from 1.2 to 2.4 mm in Fig. 7. Compared with Fig. 6(c), the effective bandwidth of La0.3Ce1.7Co17 powder was larger than La0.4Ce1.6Co17 powder at the same thickness, indicating that La0.3Ce1.7Co17 powder possesses the better broadband effect. The minimum reflection value can reach about –39.16 dB at 10 GHz with the matching thickness of 1.6 mm, and the frequency of minimum reflection value also shifts to a lower frequency.
0
RL / dB
-10 (a) -20 1.2mm 1.4mm 1.6mm 1.8mm 2.0mm 2.2mm 2.4mm
-30 -40 -50 4
6
8
10
12
14
16
Effective bandwidth / GHz
f / GHz
(c) 4.88 5 4
3.76 3.2
3
2.72 2.08
2
1.76
1.52
1 0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
d / mm
Fig.7 (a) Reflectance curves of La0.3Ce1.7Co17 powders with different thicknesses; (b) Three-dimensional relation diagram of reflection loss value, frequency and thickness; (c) Columnar relationship between effective bandwidth and thickness 4. Conclusions In this paper, with the purpose to improve microwave absorbing performance of Ce-Co-based alloy powder, the flaky LaxCe2–xCo17 alloy powders were prepared successfully using related equipment. According to these consequences, we can see that all samples mainly consist of Ce2Co17 phase. The optimum microwave absorption strength was achieved for La0.4Ce1.6Co17 alloy powder, and a minimum reflection loss (RL) of –42.29 dB (99.99% absorption rate) was obtained at 7.84 GHz for a matching thickness of 1.8 mm. The La0.3Ce1.7Co17 alloy powder has the highest effective bandwidth of 4.88 GHz with the thickness of 1.2 mm, which is about three times compared to the thickness of 2.4 mm. These manifest that the LaxCe2–xCo17 alloy powders can be provided with the potential to be excellent microwave absorption materials in 4–16 GHz. References 1. Yang ZH, Li ZW, Yang YH, Xu ZC J. Optimization of ZnxFe3–xO4 Hollow Spheres for Enhanced Microwave Attenuation. ACS Appl Mater Interfaces. 2014;6:21911.
2. Wang XX, Ma T, Shu JC, Cao MS. Confinedly tailoring Fe3O4 clusters-NG to tune electromagnetic parameters and microwave absorption with broadened bandwidth. Chem Eng J. 2018;332:321. 3. Ma JR, Wang XX, Cao WQ, Han C, Yang HJ, Yuan J, et al. A facile fabrication and highly tunable microwave absorption of 3D flowerlike Co3O4-rGO hybrid-architectures. Chem Eng J. 2018;339:487. 4. Chuai D, Liu XF, Yu RH, Ye JR, Shi YQ. Enhanced microwave absorption properties of flake-shaped FePCB metallic glass/graphene composites. Compos Part A. 2016;89:33. 5. Wang HY, Zhu DM, Zhou WC, Luo F. Electromagnetic property of SiO2-coated carbonyl iron/polyimide composites as heat resistant microwave absorbing materials. J Magn Magn Mater. 2015;375:111. 6. Ren XH, Xu GL. Electromagnetic and microwave absorbing properties of NiCoZn-ferrites doped with La3+. J Magn Magn Mater. 2014;354:44. 7. Kong LB, Li ZW, Liu L, Huang R, Abshinova M, Yang ZH, et al. Recent progress in some composite materials and structures for specific electromagnetic applications. Int Mater Rev. 8.
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2013;58(4):203. Khani O, Shoushtari MZ, Ackland K, Stamenov P. The structural, magnetic and microwave properties of spherical and flake shaped carbonyl iron particles as thin multilayer microwave absorbers. J Magn Magn Mater. 2017;428:28. Zheng XL, Feng J, Pu FZ, Lan YY, Zang Y, Li XH, et al. Amorphous FexB100-x nanostructures: Facile synthesis, magnetic properties and their applications as enhanced microwave absorbers at S-and C-bands. Adv Powder Technol. 2016;27:704. Yang Y, Xu CL and Xia YX, Wang T, Li FS. Synthesis and microwave absorption properties of FeCo nanoplates. J Alloys Compd. 2010;493(1-2):549. Yan Y, Huang Y, Li PB, Wei C. Large-scale controlled synthesis of magnetic FeCo alloy with different morphologies and their high performance of electromagnetic wave absorption. J Mater Sci: Mater Electron. 2017;28(4):3159.
12. Ma F, Qin Y, Li YZ. Enhanced microwave performance of cobalt nanoflakes with strong shape anisotropy. Appl Phys Lett. 2010;96(20):202507. 13. Zhang SG, Zhu HX, Tian JJ, Pan DA, Liu B, Kang YT. Electromagnetic and microwave absorbing properties of FeCoB powder composites. Rare Met. 2013;32(4): 402. 14. Huang XG, Chen J, Wang LX, Zhang QT. Electromagnetic and microwave absorbing properties of W-type barium ferrite doped with Gd3+. Rare Met. 2011;30(1):44. 15. Luo JL, Pan SK, Qiao ZQ, Cheng LC, Wang ZZ, Lin PH. Electromagnetic and microwave absorption properties of flaky Nd-Ho-Fe particles. J Mater Sci: Mater Electron. 2017;28(21):16366. 16. Kang Q. New Microwave Absorbing Materials. Beijing: Science Press; 2005. 17. Lou HF, Lu XL, Pan YL, Wang XJ, Wang JJ, Sun SY, et al. Ba1-xLaxFe12O19 (x=0.0, 0.2, 0.4, 0.6) hollow microspheres: material parameters, magnetic and microwave absorbing properties. J Mater Sci: Mater Electron. 2016;27(11):11231.
18. Shang T, Lu QS, Chao LM, Qin YL, Yun YH, Yun GH. Effects of ordered mesoporous structure and La-doping on the microwave absorbing properties of CoFe2O4. Appl Surf Sci. 2018;434:234. 19. Zhang YL, Wang XX, Cao MS. Confinedly implanted NiFe2O4-rGO: Cluster tailoring and highly tunable electromagnetic properties for selective-frequency microwave absorption. Nano Res. 2018;11(3):1426. 20. Fang XY, Cao MS, Shi XL, Hou ZL, Song WL, Yuan J. Microwave responses and general model of nanotetraneedle ZnO: Integration of interface scattering, microcurrent, dielectric relaxation, and microantenna. J Appl Phys. 2010;107: 054304. 21. Yang WY, Hu QW, Qiao GY, Zha L, Liu SQ, Han JZ, et al. Tuning effect of silicon substitution on magnetic and high frequency electromagnetic properties of R2Fe17 and their composites. J Rare Earths. 2019; 37:1102. 22. Liu Y, Li YY, Su XL, Su XL, Xu J, Wang JB, et al. Electromagnetic and microwave absorption properties of flaky FeCrAl particles. J Mater Sci: Mater Electron. 2017;28(9):6619. 23. Liao SB. Ferromagnetic: Part II. Beijing, Beijing: Science Press; 1998. 24. Sun J, Xu HL, Yang Shen, Bi H, Liang WF, Yang RB. Enhanced microwave absorption properties of the milled flake-shaped FeSiAl/graphite composites. J Alloys Compd. 2013;548:18. 25. Lai YR, Wang SY, Qian DL, Zhang ST, Wang YP, Han SJ, et al. Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide. Ceram Int. 2017;43(15):12904. Graphical Abstract The reflection loss of La-Ce-Co alloy moved towards lower frequency with the increasing La addition. The optimum microwave absorption performance was obtained for La0.4Ce1.6Co17 alloy powder, and a minimum reflection loss (RL) of –42.29 dB was achieved at 7.84 GHz for a thickness of 1.8 mm. 0
RL / dB
-10 -20 x=0 x=0.2 x=0.3 x=0.4 x=0.5
-30 -40 4
6
8
10 f / GHz
12
14
16
Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
We have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S1002-0721(19)30284-4
DOI:
https://doi.org/10.1016/j.jre.2019.10.006
Reference:
JRE 632
To appear in:
Journal of Rare Earths
Received Date: 11 April 2019 Revised Date:
11 September 2019
Accepted Date: 24 October 2019
Please cite this article as: He Y, Pan S, Yu J, Magnetic and microwave absorbing properties of Ce-Cobased alloy powders driven with lanthanum content, Journal of Rare Earths, https://doi.org/10.1016/ j.jre.2019.10.006. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Magnetic and microwave absorbing properties of Ce-Co-based alloy powders driven with lanthanum content Yu He, Shunkang Pan, Jingjing Yu School of material science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China Abstract: The composites of Ce-Co-based alloys doped with La content were fabricated via a vacuum arc melting method. The influences of La addition on microstructure, electromagnetic parameters, magnetic property and microwave absorbing property were measured by the corresponding equipment. The morphology characteristics manifest that all samples display sheet structure, and the average particle size of alloy powders increases with the increasing La content. The saturation magnetization (Ms) decreases with the increasing La addition as a whole. The minimum reflection loss (RL) of La0.4Ce1.6Co17 alloy powder can be obtained about –42.29 dB at 7.84 GHz with the matching thickness of 1.8 mm, and the corresponding effective bandwidth can achieve about 2.24 GHz. In addition, the minimum RL frequency moves towards a lower frequency region as the La content increased. The minimum RL of La0.3Ce1.7Co17 alloy powder is less than –20 dB ranging from 1.2 to 2.4 mm in the whole 4–16 GHz. The maximum bandwidth can reach about 4.88 GHz at the given thickness of 1.2mm. In general, these all indicate the La addition is beneficial to improving the microwave absorbing performance both in effective bandwidth and absorption intensity. Keywords: Ce-Co-based alloys; Magnetic property; microwave absorbing property; Reflection loss; rare earths 1.Introduction Abundant electromagnetic radiation are becoming more and more serious with the extensive use of electronic equipment such as mobile phone, computer, which not only greatly threatens human health but also creates information leakage to defense security.1-6 With the purpose of diminishing these detriments, the microwave absorbing materials have aroused intensive interest from researchers in recent decades. In general, outstanding microwave absorbing property can be determined by complex permittivity and permeability. To date, plenty microwave absorption materials, including dielectric loss materials and magnetic loss materials, have widely studied.3 Among them, absorbers with magnetic loss have been widely studied due to the high saturation magnetization and susceptibility. As typical magnetic loss materials, Co-based alloys can exhibit extraordinary potential for the application of absorbing electrical microwave, due to the ultrahigh saturation magnetization.7-9 Recently, there are a lot of literature can testify that Co-based powders have high magnetic loss and excellent microwave absorbing performances such as FeCo alloy,10-11 Cobalt nanoflakes,12 FeCoB powder composites.13 Foundation items: Project supported by the National Natural Science Foundation of China (51361007) and 2017 Director Fund of Guangxi Key Laboratory of Wireless Wideband Communication and Signal Processing (GXKL06170107). Corresponding Author: E-mail Address: [email protected]. (Shunkang Pan)
Currently, rare-earth with the unfilled 4f shell electrons is conducive to improve the microwave absorbing property causing by existence of the magnetic moment, which has become hot topic. 14-16 In addition, the light rare earth elements have high magnetic crystal anisotropy, dominated by low crystal symmetry caused by double hexagonal lattice or superhexagonal lattice.16 On the basis of Lou’s study,17 the magnetic and absorbing properties of HCMs can be tailored by La3+-doped effectively, and the absorption bandwidth of Ba1–xLaxFe12O19 (x=0.2, 0.4, 0.6) HCMs can reach 10, 8.1 and 8 GHz in the range of 1.5–3 mm. In addition, the study of Shang Tao indicates that Ms of OCLFO decreases after La3+ doping, while the specific surface area, coercivity value, ε" and µ" of O-CLFO increase.18 The minimum RL of O-CLFO reaches –46.47 dB with a thickness of 3.0 mm, and the effective absorption frequency bandwidth reaches 4.9 GHz. These all prove that the light rare earth La can improve the microwave absorbing property. Thus, the light rare earth La was chose to be doped in the Ce-Co-based alloy powders with the purpose of optimizing the microwave absorbing property and effective bandwidth in this work. 2.Experimental The Ce2–xLaxCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy ingots were fabricated by vacuum arc melting and high energy ball milling using La (>99.99% in purity), Ce (>99.99% in purity), Co (99.999% in purity) under protection of Ar atmosphere. The achieved ingots were sealed using vacuum sealing machine and heat-treated in a KSL-1200X chamber electric furnace for two weeks at 800 °C, and then quenched into ice-water mixture. The coarse powders, which were coarse breaking into 100 µm, were milled at the speed of 300 r/min for 20 h with the powder to ball weight ratio of 1:20 using a planet ball mill. The phase composition was measured by X-ray diffraction (XRD, Empyrean PIXcel 3D) with Cu Kα radiation. With the purpose to observe the influence on particle size of La dopant, the scanning electron microscope was used. The complex permittivity and permeability of samples were measured by the Agilent 8722ES vector network analyzer (VNA) using the toroidal samples, which were fabricated by pressing the mixture (mass ratio of paraffin and powders was 1:4) into a mold. In addition, the thickness of sample was controlled at about 3.5 mm. The reflection loss (RL) of single layer absorbing material can be calculated by the given thickness using the tested electromagnetic parameter on the basis of transmission line theory. The calculated equations of RL are as follows. where c is the light velocity in vacuum, f is the microwave frequency, d is the thickness of the absorber, and εr and µr are the relative permittivity and permeability of the sample, obtained from the experimental data.19
RL = −20 lg
Z in − Z 0 Z in + Z 0
(1)
Here, the normalized input impedance (Zin) and free space wave impedance (Z0) of microwave absorption layer can be expressed as
Z0 =
µ0 ε0
(2)
Z in = Z 0
µr 2π tanh j µ r ε r fd εr c
(3)
3.Results and discussions 3.1 XRD and SEM analysis of LaxCe2–xCo17 alloy Fig. 1 shows the scanning electron microscope (SEM) images of LaxCe2-xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders. The average particle size of LaxCe2–xCo17 alloy powders are 2.65, 3.74, 4.84, 5.22 and 5.68 µm, which are measured by Nano Measurer 1.2. That means the average particle size trends to increase with the rise of La content. On the side, it can be seen that all samples exist flaky morphology, which can be beneficial to the improvement of microwave absorbing property due to the multiple scattering caused by larger scattering area.20
Fig. 1 Scanning electron microscope (SEM) images of LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders The X-ray diffraction patterns (XRD) of the LaxCe2-xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders are depicted in Fig. 2. The main phase of all alloy samples is Ce2Co17 phase, and the corresponding space group is P63/mmc (No.194). Besides, the diffraction peak has a tendency to shift to a lower angle with increasing La content.
♥−−Ce2Co17
Intensity (a.u.)
42.5
43.0
43.5
♥
44.0
♥
44.5
45.0
x=0.5
♥♥ ♥ ♥♥ ♥♥
♥♥ ♥♥
♥
♥♥
x=0.4 x=0.3 x=0.2 x=0 20
30
40
50
60
70
80
2θ / (°)
Fig. 2 X-ray diffraction patterns (XRD) of the LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders 3.2 Magnetic property of LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders Fig. 3 shows the magnetic hysteresis loops of LaxCe2–xCo17 alloy powders, which measured between –20000 and 20000 Oe at 25 °C. Saturation magnetization (Ms) can be used to characterize the magnetism of ferromagnetic particles. It can be seen that the saturation magnetization decreases from 0.0797 to 0.0727 emu/mg with the La content increases from 0 to 0.5. What’s more, the materials exhibit low coercivity. 0.10
M / (emu/mg)
0.05 x=0 x=0.2 x=0.3 x=0.4 x=0.5
0.00
-0.05
-0.10
-20000 -10000
0
10000
20000
H / Oe
Fig. 3 Magnetic hysteresis loops of the LaxCe2-xCo17 alloy powders 3.3 Electromagnetic parameters of LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders Electromagnetic properties can be described by complex permittivity and complex permeability, which represent the storage capacity and lossy ability of absorbers, respectively.20-21 As can be seen from Fig. 4 (a, b), both the real part (ε') and imaginary part (ε") of complex permittivity trend to increase with the rise of La content, are ascribed to the combined action of relaxation polarization and conduction loss.22-23 On the basis of Fig. 1, the relaxation polarization loss can be improved due to the tendency of specific surface area with the increasing La content. In addition, the increased flake particles caused the convenience of forming the conductive networks, which is beneficial to the conduction loss.23
The frequency dependence of the complex permeability and magnetic loss of LaxCe2–xCo17 alloy powders are depicted in Fig. 4 (c, d). It can be seen that the µ' values of all samples has a downward trend on the whole with the increase of frequency, which may be ascribed to the limited speed of spin and domain-wall motion (displacement/rotation). The µ' and µ" all possess obvious resonance peaks in 4–16 GHz, which may related to the natural resonance and exchange resonance.2-3 Furthermore, the resonance peak frequency of µ" shifts to a lower frequency region. x=0 x=0.2 x=0.3 x=0.4 x=0.5
(a)
28 24
x=0 x=0.2 x=0.3 x=0.4 x=0.5
(b) 8.25
ε′′
ε′
5.50 20 16
2.75
12 0.00 4
6
8
10
12
14
16
4
6
8
10
f / GHz
0.8
(c)
2.5
12
14
16
f / GHz
x=0 x=0.2 x=0.3 x=0.4 x=0.5
0.6 µ′′
µ′
2.0
x=0 x=0.2 x=0.3 x=0.4 x=0.5
(d)
0.4
1.5 0.2 1.0 4
6
8
10
12
14
f / GHz
16
0.0
4
6
8
10
12
14
16
f / GHz
Fig. 4. Electromagnetic parameters of LaxCe2–xCo17 alloy powders (a): real part of complex permittivity (ε'); (b): imaginary part of complex permittivity (ε"); (c): real part of complex permeability (µ'); (d): imaginary part of complex permeability (µ") 3.4 Microwave absorbing performance of LaxCe2-xCo17 alloy powders The microwave reflection loss (RL) for LaxCe2–xCo17 (x=0, 0.2, 0.3, 0.4, 0.5) alloy powders was calculated by above Eq. (1-3) with the measured electromagnetic parameters at the given thickness of 1.8 mm is shown in Fig. 5. The frequency of minimum reflection loss (RL) shifts to a lower frequency region, which may be attributed to the increase in the complex permittivity in Fig.4. The relationship between matching frequency and thickness can be expressed by Eq. (4).24 Where fm is the matching frequency, dm is the matching thickness, and δM is the magnetic loss. According to the formula, the value of ε' increased significantly and the value of µ' changed little at a given thickness. Therefore, the frequency of minimum reflection loss (RL) shifts to a lower frequency region caused by the increase of ε'µ' value.
fm =
c 4d m
1 1 2 1 + tan δ M ε'µ' 8
−1
(4)
In addition, the value of minimum RL increases at the beginning and then decrease with the increasing La content. The optimum microwave absorption performance was obtained for La0.4Ce1.6Co17 alloy powder, and a minimum reflection loss (RL) of –42.29 dB (99.99% absorption rate) was achieved at 7.84 GHz for a thickness of 1.8 mm. Moreover, the La0.3Ce1.7Co17 alloy powder can achieve the effective bandwidth (frequency width of RL<–10 dB) of 2.72 GHz, expressing that La0.3Ce1.7Co17 alloy powder is provided with outstanding broadband effect. These consequences manifest that the properly addition of light rare earth La can optimize the absorbing performance of Ce2Co17 based alloy powders. 0 (a)
RL / dB
-10 -20
x=0 x=0.2 x=0.3 x=0.4 x=0.5
-30 -40 4
6
8
10
12
14
16
f / GHz
Fig. 5 (a)Reflection loss of LaxCe2–xCo17 alloy powders; (b) 3D histogram of LaxCe2–xCo17 alloy powders between effective bandwidth and corresponding frequency, La addition For further improvement of microwave absorbing performance of LaxCe2–xCo17 alloy powder, the microwave absorption performance of La0.4Ce1.6Co17 and La0.3Ce1.7Co17 alloy with different thickness was studied, as shown in Figs. 6 and 7. As we can be seen from Fig. 6, the value of minimum RL can obtain about –42.29 dB at 1.8 mm, and the value has a increase tendency in the first and then trends to decrease with the increasing thickness. The frequency of RL moves towards a lower frequency, which can be explained by the following equation.25 in which λ stands for the wavelength of EM wave, |µr|, |εr| are the modulus of µr, εr with matching frequency fm, respectively. What’s more, the maximum effective bandwidth can be up to 4.32 GHz and the corresponding thickness is only 1.2 mm.
tm =
nλ nc (n = 1,3,5......) = 4 4 f m µr ε r
(5)
0 -10 RL / dB
(a) -20
1.2mm 1.4mm 1.6mm 1.8mm 2.0mm 2.2mm 2.4mm
-30 -40 -50 4
6
8
10
12
14
16
f / GHz
Effective bandwidth / GHz
5
(c) 4.32
4
3.52 2.88
3
2.24 1.84
2
1.52
1.28
1 0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
f / GHz
Fig.6 (a)Reflectance curves of La0.4Ce1.6Co17 powders with different thicknesses; (b)Three-dimensional relation diagram of reflection loss value, frequency and thickness; (c)Columnar relationship between effective bandwidth and thickness It is found that all reflection loss value is less than –20 dB ranging from 1.2 to 2.4 mm in Fig. 7. Compared with Fig. 6(c), the effective bandwidth of La0.3Ce1.7Co17 powder was larger than La0.4Ce1.6Co17 powder at the same thickness, indicating that La0.3Ce1.7Co17 powder possesses the better broadband effect. The minimum reflection value can reach about –39.16 dB at 10 GHz with the matching thickness of 1.6 mm, and the frequency of minimum reflection value also shifts to a lower frequency.
0
RL / dB
-10 (a) -20 1.2mm 1.4mm 1.6mm 1.8mm 2.0mm 2.2mm 2.4mm
-30 -40 -50 4
6
8
10
12
14
16
Effective bandwidth / GHz
f / GHz
(c) 4.88 5 4
3.76 3.2
3
2.72 2.08
2
1.76
1.52
1 0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
d / mm
Fig.7 (a) Reflectance curves of La0.3Ce1.7Co17 powders with different thicknesses; (b) Three-dimensional relation diagram of reflection loss value, frequency and thickness; (c) Columnar relationship between effective bandwidth and thickness 4. Conclusions In this paper, with the purpose to improve microwave absorbing performance of Ce-Co-based alloy powder, the flaky LaxCe2–xCo17 alloy powders were prepared successfully using related equipment. According to these consequences, we can see that all samples mainly consist of Ce2Co17 phase. The optimum microwave absorption strength was achieved for La0.4Ce1.6Co17 alloy powder, and a minimum reflection loss (RL) of –42.29 dB (99.99% absorption rate) was obtained at 7.84 GHz for a matching thickness of 1.8 mm. The La0.3Ce1.7Co17 alloy powder has the highest effective bandwidth of 4.88 GHz with the thickness of 1.2 mm, which is about three times compared to the thickness of 2.4 mm. These manifest that the LaxCe2–xCo17 alloy powders can be provided with the potential to be excellent microwave absorption materials in 4–16 GHz. References 1. Yang ZH, Li ZW, Yang YH, Xu ZC J. Optimization of ZnxFe3–xO4 Hollow Spheres for Enhanced Microwave Attenuation. ACS Appl Mater Interfaces. 2014;6:21911.
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RL / dB
-10 -20 x=0 x=0.2 x=0.3 x=0.4 x=0.5
-30 -40 4
6
8
10 f / GHz
12
14
16
Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
We have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.