- Email: [email protected]
The magnetic and microwave absorbing properties of the as spun Nd-Fe-Co-B nanocomposites
Accepted Manuscript The Magnetic and Microwave absorbing properties of the as spun Nd-Fe-Co-B nanocomposites Li Jun, Xie Guozhi, Ji Peicheng, Qu Jie, Chen Jiangwei, Chen Jing PII: DOI: Reference:
S0304-8853(17)31379-3 http://dx.doi.org/10.1016/j.jmmm.2017.07.023 MAGMA 62954
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
6 May 2017 26 June 2017 6 July 2017
Please cite this article as: L. Jun, X. Guozhi, J. Peicheng, Q. Jie, C. Jiangwei, C. Jing, The Magnetic and Microwave absorbing properties of the as spun Nd-Fe-Co-B nanocomposites, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.07.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
The Magnetic and Microwave absorbing properties of the as spun Nd-Fe-Co-B nanocomposites Li Jun， Xie Guozhi∗， Ji Peicheng， Qu Jie， Chen Jiangwei， Chen Jing School of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing, 210023, P. R. China
Abstract: In this paper, microstructure, magnetic and microwave absorbing properties of Nd xFe85-xCo4B11 nanocomposites (x = 3, 5, 7, 9) flaky-shaped particles prepared by melt spun were investigated. The analyses of XRD spectra suggest that all samples only have the single α -Fe phase. With the increase of the Nd content, the complex permittivity-frequency and permeability-frequency were increased at first, and then decreased with higher Nd content. The resin composites of 70 wt% of these nanocomposite powders showed excellent electromagnetic wave absorption properties in GHz range. A minimum reflection loss of -23.1dB is obtained at 9.8GHz for composite Nd 9Fe76Co4B11. Key words: melt spun; microstructure; microwave absorbers 1. Introduction The utilization of microwave absorbing materials has become a research focus because the electromagnetic interference has been seriously emerged due to the rapid improvement of electronics industry [1, 2]. Traditional materials like spinel ferrites and hexagonal ferrites still have some inevitable weaknesses such as thick coat and high density. Moreover, spinel ferrites do not have a strong absorption in gigahertz range because the sharp decreases of the imaginary part of permeability in this range due to the Snoek’s limit [3].On the other hand, metallic magnetic material have a superior performance in the terms of absorbing because their Snoek’s limits exist at a high frequency range [4]. ∗
Corresponding author: School of Electronic Science and Engineering, Nanjing University of
Posts and Telecommunications, Nanjing 210023, P. R. China Tel: +86-13813875072 Fax: +86-25-85866131 Email address: [email protected]
Most applications of melt-spun NdFeB powder are in the form of isotropic polymer-bonded magnets [5]. They are extensively used in dc motors for automobile, HDDR, CD-ROM, DVD-ROM and other family electrical equipment and so on [6-8]. As a typical flaky metal alloy, melt spun NdFeB powders are expected to be excellent candidate for using in the electromagnetic wave absorbing at gigahertz owing to its great saturation magnetization and large anisotropy field [9]. However, the absorbing properties of melt-spun NdFeB were usually studied in high frequency（≥25GHz）due to its higher coercive force [10]. The absorbing properties of Nd-Fe-B nanocomposites with low Nd content had been reported in our previous work [11]. In this study, the cobalt element has been doped into the NdFeB-based alloys in order to improve absorbing properties due to its wonderful magnetic properties and good ductility [12]. The effect of rare earth Nd content on the microwave properties of NdFeCoB nanocomposites in the 2-18GHz range was investigated.
2. Experiment The NdxFe85-xCo4B11 (x = 3, 5, 7, 9) alloys were prepared using metals in purity of Nd 99.8 wt%, FeB alloy (B 24.99 wt %), Co 99.8wt% and Fe 99.5 wt% as raw materials, respectively. These ingots were initially alloyed by arc melting for three times to obtain a homogeneous composition, and were then melt spun in a molybdenum wheel under an argon atmosphere. Wheel speeds of 30 m/s were used. A vibrating cup milling time of 2 h was used to prepare the nanocomposite powder. Phase identification of the samples were carried out by X-ray diffraction (XRD) with Cu Kα radiation at a scan rate (2θ) of 0.05°/s. Magnetic hysteresis loops were collected using vibrating sample magnetometer (VSM) with an applied magnetic field up to ±1×104Oe at room temperature. Composite materials were prepared by dispersing the NdxFe85-xCo 4B11 (x = 3, 5, 7, 9) alloys powder in paraffin wax, and the weight fraction of powder is 70 wt%. The powder wax composites were die-pressed to form cylindrical toroidal specimens with 7.0mm outer diameter, 3.04mm inner diameter, and 3-6mm thickness. The measurements of complex permeability µ and
permittivity ε for the specimens were carried out using an Agilent E8363B vector network analyzer in the 2-18 GHz ranges, according to the ASTM D5568-2008.
3. Results and discuss Fig.1 shows the X-ray diffraction patterns of the NdxFe85-xCo4B11（x=3, 5, 7, 9） melt-spun ribbons. The diffraction peaks in all samples are associated with the (110) plane of the single α -Fe phase. Moreover, it can be observed the weak refection peaks for x=3 and x=5 samples which are the (2 0 0) planes of α-Fe phase, while that of x = 7 and x = 9 samples were almost disappeared. However, with the Nd contend increased, the width of (1 1 0) planes were relatively broad with the intensity gradually decreased and the (2 0 0) planes were gradually vanished.
(1 1 0)
Intensity(arb.units)
(2 0 0)
X=9
X=7
X=5
X=3
20
30
40
50 0 60 2-Theta( )
70
80
Fig.1 The XRD pattern of as-spun NdxFe85-xCo4B11（x=3, 5, 7, 9）alloys
Fig.2 shows the hysteresis loop of samples, which gives the saturation magnetization (Ms) and coercivity (Hc) of the Nd xFe85-xCo 4B11（x=3, 5, 7, 9） as spun ribbons (also in Table 1). In general, the values of Ms and Hc are very high compared with soft magnetic materials, and the magnetic hysteresis loops are quite smooth. The Ms is gradually decreased and the Hc is gradually increased with the increase of the Nd content. The similar result is reported by Y.-Q. Wu et al [13].
160
160 40
σ ( emu/g)
120
σ ( emu/g)
80 40
40
0
120 80
0
X=3 X=5 X=7 X=9 -40
-40 -600
-400
-200
0
200
400
40
600
H(Oe)
0
0
-40
-40
-80
-80
X=3 X=5 X=7 X=9
-120 -160
-10000
-5000
0
-120 -160
5000
H(Oe) Fig.2 Magnetic hysteresis loops for melt spun NdxFe85-xCo4 B11（x=3, 5, 7, 9）powders Table 1 saturation magnetization (Ms) and coercivity (Hc)of the melt spun NdxFe85-xCo4B11（x=3, 5, 7, 9）powders
Hc (Oe)
X=5
X=7
X=9
154
150
146
144
86.49
108.2
113.3
122.9
Real permeability(µ ')
2.6
x=3 x=5 x=7 x=9
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0
2
4
6
8
10
12
14
Frequency (GHz)
16
18
20
Imaginary permeability(µ'')
Ms (emu/g)
X=3
1.0
x=3 x=5 x=7 x=9
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0
2
4
6
8
10
12
14
16
18
Frequency (GHz)
Fig.3 The real permeability spectra and imaginary permeability spectra of as-spun NdxFe85-xCo4B11（x=3, 5, 7, 9）alloys
The frequency dependence of the complex permeability is shown in Fig.3. For
20
both samples, the value of µ ' arrives the largest value at 2 GHz and then smoothly
decreases with increases of frequency. However, the value of µ ' ' increases for both samples at low frequency arrive maximum value about 6 GHz, then decrease with increases of frequency. The jump of µ ' ' for samples may results from the multi-resonance [14].
Real permeability(ε ')
22 20 18 16 14 12 10
Imaginary permeability(ε '')
6.0 x=3 x=5 x=7 x=9
24
x=3 x=5 x=7 x=9
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
2
4
6
8
10
12
14
16
Frequency (GHz)
18
20
0
2
4
6
8
10
12
14
16
18
Frequency (GHz)
Fig. 4 The real permittivity spectra and imaginary permittivity spectra of NdxFe85-xCo4B11（x=3, 5, 7, 9）samples
Fig.4 shows the frequency dispersion of the complex permittivity. In general, it can be seen that for both samples, the value of ε ' and ε ' ' remain vibrating with the frequency. The value of ε ' varies from 24 for the Nd5Fe80Co4B11 sample to 9 for that of the Nd7Fe78Co4B11 sample at 2GHz. However, it is well known that lower ε ' and
ε ' ' are favored for the microwave absorbing properties. Obviously the appropriate content of Nd for melt spun NdFeB nanocomposites meets this demand. According to the transmission line theory, the reflection loss (RL) of normal incident electromagnetic wave at the absorber surface can be calculated from the relative permeability and permittivity at a given frequency and absorber thickness using the following equations [1]: μ
Z0 = ට ε0 , 0
μ
Zin = ටε tanh (j
2πfd ඥμ ∙ ε), c
ିబ
RL = 20lg ቚ
ାబ
and
ቚ.
Here, Z0 is the impedance of air, µ0, ε0 are the permeability and permittivity of air, respectively, f the frequency of the electromagnetic wave, d is the thickness of the absorber, c is the velocity of light, Zin is the input impedance of the absorber, and
20
µ, ε are the complex permeability and complex permittivity of absorber, respectively. 0
Reflection Loss(dB)
-3 -6 -9 -12 -15 -18
X=3 X=5 X=7 X=9
-21 -24 2
4
thickness=2mm 6
8
10
12
14
16
18
Frequency(GHz) Fig. 5 The reflection loss curve of as-spun NdxFe85-xCo4B11 （x=3, 5, 7, 9）powders
Fig. 5 shows the frequency dependence of the calculated reflection loss of the NdxFe85-xCo4B11（x=3, 5, 7, 9）powders with thickness of 2 mm. The optimum minimal reflection loss (RL) is realized for Nd9Fe76Co4B11 sample with RL value of -23.9 dB at 10.2 GHz. For Nd7Fe78Co4B11, Nd5Fe80Co4B11 and Nd3Fe82Co4B11 composites, the value of RL are -23.6 dB at 9.8 GHz, -13.8 dB at 5.3 GHz and -16.1 dB at 6.8 GHz, respectively, and lower than that of the Nd9Fe76Co4B11 sample. It should be noted that the width of the minimum RL peak for Nd9Fe76Co4B11 sample is about 12GHz (RL<-5dB). 4. Conclusions
The microwave properties of melt spun NdxFe85-xCo4B11（x=3, 5, 7, 9）alloys
were studied. The XRD spectra show that only α-Fe phase was observed for all samples. The results show that the complex permeability ( µ = µ '− jµ ' ' ), permittivity ( ε = ε '− jε ' ' ) and reflection loss of samples are heavily depend on the content of Nd. For Nd9Fe76Co4B11 sample, it has RL value of -23.9 dB (lower than 1%) at 10.2 GHz (the matching thickness is 2mm), which is favored to act as a good microwave absorbing materials. Acknowledgements:
The authors would acknowledge financial supports from the National Natural Science Foundation of China (NSFC) (11304159), the Natural Science Foundation of Jiangsu Province (BK20161512).
References:
[1] B.S. Zhang, G. Lu, Y. Feng, J. Xiong, H.X. Lu, Electromagnetic and microwave absorption properties of Alnico powder composites, J.Magn.Magn.Mater. 299 (2006) 205-210. [2] L.X. Lian, L.J. Deng, M. Han, Microwave electromagnetic and absorption properties of NdଶFeଵସ B/α − Fe nanocomposites in the 0.5–18 and 26.5–40GHz ranges, J.Appl. Phys. 101 (2007) 207. [3] G.Z. Xie, X.L. Song, B.S. Zhang, D.M. Tang, Q. Bian, H.X. Lu, Microstructure and electromagnetic properties of flake-like Nd-Fe-B nanocomposite powders with different milling times, Powder Technol. 210 (2011) 220-224. [4] M. Itoh , K. Nishiyama , F. Shogano , T. Murota , K. Yamamotob, M. Sasada , K.I. Machida, Recycle of rare earth sintered magnet powder scraps as electromagnetic wave absorbers in gigahertz range, J. Alloys Compounds 451 (2008) 507-509. [5] D.N. Brown, Z. Wu, F. He, D.J. Miller, J.W. Herchenroeder, Dysprosium-free melt-spun permanent Magnets, J. Phys.: Condens. Matter. 26 (2014) 064202. [6] J.K. Hyo, S.K. Chang, S.S. Pan, A New Anisotropic Bonded NdFeB Permanent Magnet and Its Application to a Small DC Motor, IEEE Trans. Magn. 46 (2010) 2314-2317. [7] W.Q. Liu, R.J. Hu, M. Yue, Y.X. Yin, D.T. Zhang, Preparation and properties of isotropic Nd-Fe-B bonded magnets with sodium silicate binder, J.Magn.Magn. Mater. 435(2017) 187-193. [8] W.Z. Qin, J. He, J.H. Meng, Research on Composite Powder and Magnet Properties of Bonded NdFeB Magnets, Adv. Mater. Res. 535-537(2012) 1314-1318. [9] J.L. Xiong, S.K. Pan, L.C. Cheng, X. Liu, P.H. Lin, Q.R. Yao, Preparation and microwave absorption properties of NdFeB alloys, J. Rare Earths 33(2015) 514. [10] L.X. Lian , L.J. Deng, M. Han, S.D. Feng, Effect of Nd content on natural resonance frequency and microwave permeability of Nd2Fe14B/α-Fe nanocomposites in 26.5–40 GHz frequency range, J. Alloys Compounds 441 (2007) 301-304 . [11] G.Z. Xie, P. Wang, B.S. Zhang, L.K. Yuan, Y. Shi, P.H. Lin, H.X. Lu,
Electromagnetic wave-absorption properties of rapidly quenched of Nd–Fe–B nanocomposites with low Nd content, J.Magn.Magn. Mater. 320 (2008) 1026-1029. [12] C.K. He，S.K. Pan, L.C. Cheng，X. Liu, Y.J. Wu, Effect of rare earths on microwave absorbing properties of RE-Co alloys, J. Rare Earths 33( 2015) 271. [13] Y.-Q. Wu, M.J. Kramer, Z. Chen, B.-M. Ma, D.H. Ping, K. Hono, Microstructural control of Nb addition in nanocrystalline hard magnets with different Nd content , IEEE Trans. Magn. 39 (2003) 2935. [14] L.J. Deng, P.H. Zhou, J.L. Xie, L. Zhang, Characterization and microwave resonance in nanocrystalline FeCoNi flake composite, J.Appl.Phys.101,103916 (2007).
Highlights 1. Effect of Nd contents on magnetic properties and electromagnetic parameters of quenched Co-doped NdFeB alloys were studied. 2. The quenched Co-doped NdFeB alloys exhibits a single α-Fe phase，and still remain soft magnetic properties. 3. Take advantage of the soft magnetic properties, Co-doped NdFeB nanocomposites have excellent absorbing properties at 2-18 GHz.
S0304-8853(17)31379-3 http://dx.doi.org/10.1016/j.jmmm.2017.07.023 MAGMA 62954
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
6 May 2017 26 June 2017 6 July 2017
Please cite this article as: L. Jun, X. Guozhi, J. Peicheng, Q. Jie, C. Jiangwei, C. Jing, The Magnetic and Microwave absorbing properties of the as spun Nd-Fe-Co-B nanocomposites, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.07.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
The Magnetic and Microwave absorbing properties of the as spun Nd-Fe-Co-B nanocomposites Li Jun， Xie Guozhi∗， Ji Peicheng， Qu Jie， Chen Jiangwei， Chen Jing School of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing, 210023, P. R. China
Abstract: In this paper, microstructure, magnetic and microwave absorbing properties of Nd xFe85-xCo4B11 nanocomposites (x = 3, 5, 7, 9) flaky-shaped particles prepared by melt spun were investigated. The analyses of XRD spectra suggest that all samples only have the single α -Fe phase. With the increase of the Nd content, the complex permittivity-frequency and permeability-frequency were increased at first, and then decreased with higher Nd content. The resin composites of 70 wt% of these nanocomposite powders showed excellent electromagnetic wave absorption properties in GHz range. A minimum reflection loss of -23.1dB is obtained at 9.8GHz for composite Nd 9Fe76Co4B11. Key words: melt spun; microstructure; microwave absorbers 1. Introduction The utilization of microwave absorbing materials has become a research focus because the electromagnetic interference has been seriously emerged due to the rapid improvement of electronics industry [1, 2]. Traditional materials like spinel ferrites and hexagonal ferrites still have some inevitable weaknesses such as thick coat and high density. Moreover, spinel ferrites do not have a strong absorption in gigahertz range because the sharp decreases of the imaginary part of permeability in this range due to the Snoek’s limit [3].On the other hand, metallic magnetic material have a superior performance in the terms of absorbing because their Snoek’s limits exist at a high frequency range [4]. ∗
Corresponding author: School of Electronic Science and Engineering, Nanjing University of
Posts and Telecommunications, Nanjing 210023, P. R. China Tel: +86-13813875072 Fax: +86-25-85866131 Email address: [email protected]
Most applications of melt-spun NdFeB powder are in the form of isotropic polymer-bonded magnets [5]. They are extensively used in dc motors for automobile, HDDR, CD-ROM, DVD-ROM and other family electrical equipment and so on [6-8]. As a typical flaky metal alloy, melt spun NdFeB powders are expected to be excellent candidate for using in the electromagnetic wave absorbing at gigahertz owing to its great saturation magnetization and large anisotropy field [9]. However, the absorbing properties of melt-spun NdFeB were usually studied in high frequency（≥25GHz）due to its higher coercive force [10]. The absorbing properties of Nd-Fe-B nanocomposites with low Nd content had been reported in our previous work [11]. In this study, the cobalt element has been doped into the NdFeB-based alloys in order to improve absorbing properties due to its wonderful magnetic properties and good ductility [12]. The effect of rare earth Nd content on the microwave properties of NdFeCoB nanocomposites in the 2-18GHz range was investigated.
2. Experiment The NdxFe85-xCo4B11 (x = 3, 5, 7, 9) alloys were prepared using metals in purity of Nd 99.8 wt%, FeB alloy (B 24.99 wt %), Co 99.8wt% and Fe 99.5 wt% as raw materials, respectively. These ingots were initially alloyed by arc melting for three times to obtain a homogeneous composition, and were then melt spun in a molybdenum wheel under an argon atmosphere. Wheel speeds of 30 m/s were used. A vibrating cup milling time of 2 h was used to prepare the nanocomposite powder. Phase identification of the samples were carried out by X-ray diffraction (XRD) with Cu Kα radiation at a scan rate (2θ) of 0.05°/s. Magnetic hysteresis loops were collected using vibrating sample magnetometer (VSM) with an applied magnetic field up to ±1×104Oe at room temperature. Composite materials were prepared by dispersing the NdxFe85-xCo 4B11 (x = 3, 5, 7, 9) alloys powder in paraffin wax, and the weight fraction of powder is 70 wt%. The powder wax composites were die-pressed to form cylindrical toroidal specimens with 7.0mm outer diameter, 3.04mm inner diameter, and 3-6mm thickness. The measurements of complex permeability µ and
permittivity ε for the specimens were carried out using an Agilent E8363B vector network analyzer in the 2-18 GHz ranges, according to the ASTM D5568-2008.
3. Results and discuss Fig.1 shows the X-ray diffraction patterns of the NdxFe85-xCo4B11（x=3, 5, 7, 9） melt-spun ribbons. The diffraction peaks in all samples are associated with the (110) plane of the single α -Fe phase. Moreover, it can be observed the weak refection peaks for x=3 and x=5 samples which are the (2 0 0) planes of α-Fe phase, while that of x = 7 and x = 9 samples were almost disappeared. However, with the Nd contend increased, the width of (1 1 0) planes were relatively broad with the intensity gradually decreased and the (2 0 0) planes were gradually vanished.
(1 1 0)
Intensity(arb.units)
(2 0 0)
X=9
X=7
X=5
X=3
20
30
40
50 0 60 2-Theta( )
70
80
Fig.1 The XRD pattern of as-spun NdxFe85-xCo4B11（x=3, 5, 7, 9）alloys
Fig.2 shows the hysteresis loop of samples, which gives the saturation magnetization (Ms) and coercivity (Hc) of the Nd xFe85-xCo 4B11（x=3, 5, 7, 9） as spun ribbons (also in Table 1). In general, the values of Ms and Hc are very high compared with soft magnetic materials, and the magnetic hysteresis loops are quite smooth. The Ms is gradually decreased and the Hc is gradually increased with the increase of the Nd content. The similar result is reported by Y.-Q. Wu et al [13].
160
160 40
σ ( emu/g)
120
σ ( emu/g)
80 40
40
0
120 80
0
X=3 X=5 X=7 X=9 -40
-40 -600
-400
-200
0
200
400
40
600
H(Oe)
0
0
-40
-40
-80
-80
X=3 X=5 X=7 X=9
-120 -160
-10000
-5000
0
-120 -160
5000
H(Oe) Fig.2 Magnetic hysteresis loops for melt spun NdxFe85-xCo4 B11（x=3, 5, 7, 9）powders Table 1 saturation magnetization (Ms) and coercivity (Hc)of the melt spun NdxFe85-xCo4B11（x=3, 5, 7, 9）powders
Hc (Oe)
X=5
X=7
X=9
154
150
146
144
86.49
108.2
113.3
122.9
Real permeability(µ ')
2.6
x=3 x=5 x=7 x=9
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0
2
4
6
8
10
12
14
Frequency (GHz)
16
18
20
Imaginary permeability(µ'')
Ms (emu/g)
X=3
1.0
x=3 x=5 x=7 x=9
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0
2
4
6
8
10
12
14
16
18
Frequency (GHz)
Fig.3 The real permeability spectra and imaginary permeability spectra of as-spun NdxFe85-xCo4B11（x=3, 5, 7, 9）alloys
The frequency dependence of the complex permeability is shown in Fig.3. For
20
both samples, the value of µ ' arrives the largest value at 2 GHz and then smoothly
decreases with increases of frequency. However, the value of µ ' ' increases for both samples at low frequency arrive maximum value about 6 GHz, then decrease with increases of frequency. The jump of µ ' ' for samples may results from the multi-resonance [14].
Real permeability(ε ')
22 20 18 16 14 12 10
Imaginary permeability(ε '')
6.0 x=3 x=5 x=7 x=9
24
x=3 x=5 x=7 x=9
5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
2
4
6
8
10
12
14
16
Frequency (GHz)
18
20
0
2
4
6
8
10
12
14
16
18
Frequency (GHz)
Fig. 4 The real permittivity spectra and imaginary permittivity spectra of NdxFe85-xCo4B11（x=3, 5, 7, 9）samples
Fig.4 shows the frequency dispersion of the complex permittivity. In general, it can be seen that for both samples, the value of ε ' and ε ' ' remain vibrating with the frequency. The value of ε ' varies from 24 for the Nd5Fe80Co4B11 sample to 9 for that of the Nd7Fe78Co4B11 sample at 2GHz. However, it is well known that lower ε ' and
ε ' ' are favored for the microwave absorbing properties. Obviously the appropriate content of Nd for melt spun NdFeB nanocomposites meets this demand. According to the transmission line theory, the reflection loss (RL) of normal incident electromagnetic wave at the absorber surface can be calculated from the relative permeability and permittivity at a given frequency and absorber thickness using the following equations [1]: μ
Z0 = ට ε0 , 0
μ
Zin = ටε tanh (j
2πfd ඥμ ∙ ε), c
ିబ
RL = 20lg ቚ
ାబ
and
ቚ.
Here, Z0 is the impedance of air, µ0, ε0 are the permeability and permittivity of air, respectively, f the frequency of the electromagnetic wave, d is the thickness of the absorber, c is the velocity of light, Zin is the input impedance of the absorber, and
20
µ, ε are the complex permeability and complex permittivity of absorber, respectively. 0
Reflection Loss(dB)
-3 -6 -9 -12 -15 -18
X=3 X=5 X=7 X=9
-21 -24 2
4
thickness=2mm 6
8
10
12
14
16
18
Frequency(GHz) Fig. 5 The reflection loss curve of as-spun NdxFe85-xCo4B11 （x=3, 5, 7, 9）powders
Fig. 5 shows the frequency dependence of the calculated reflection loss of the NdxFe85-xCo4B11（x=3, 5, 7, 9）powders with thickness of 2 mm. The optimum minimal reflection loss (RL) is realized for Nd9Fe76Co4B11 sample with RL value of -23.9 dB at 10.2 GHz. For Nd7Fe78Co4B11, Nd5Fe80Co4B11 and Nd3Fe82Co4B11 composites, the value of RL are -23.6 dB at 9.8 GHz, -13.8 dB at 5.3 GHz and -16.1 dB at 6.8 GHz, respectively, and lower than that of the Nd9Fe76Co4B11 sample. It should be noted that the width of the minimum RL peak for Nd9Fe76Co4B11 sample is about 12GHz (RL<-5dB). 4. Conclusions
The microwave properties of melt spun NdxFe85-xCo4B11（x=3, 5, 7, 9）alloys
were studied. The XRD spectra show that only α-Fe phase was observed for all samples. The results show that the complex permeability ( µ = µ '− jµ ' ' ), permittivity ( ε = ε '− jε ' ' ) and reflection loss of samples are heavily depend on the content of Nd. For Nd9Fe76Co4B11 sample, it has RL value of -23.9 dB (lower than 1%) at 10.2 GHz (the matching thickness is 2mm), which is favored to act as a good microwave absorbing materials. Acknowledgements:
The authors would acknowledge financial supports from the National Natural Science Foundation of China (NSFC) (11304159), the Natural Science Foundation of Jiangsu Province (BK20161512).
References:
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Highlights 1. Effect of Nd contents on magnetic properties and electromagnetic parameters of quenched Co-doped NdFeB alloys were studied. 2. The quenched Co-doped NdFeB alloys exhibits a single α-Fe phase，and still remain soft magnetic properties. 3. Take advantage of the soft magnetic properties, Co-doped NdFeB nanocomposites have excellent absorbing properties at 2-18 GHz.