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The effect of transverse magnetic field treatment on wave-absorbing properties of FeNi alloy powders
Author’s Accepted Manuscript The effect of transverse magnetic field treatment on wave-absorbing properties of FeNi alloy powders Hui Zhao, Zhenghou Zhu, Chao Xiong, Xing Xu, Qianying Lin www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(16)31963-1 http://dx.doi.org/10.1016/j.jmmm.2016.08.087 MAGMA61772
To appear in: Journal of Magnetism and Magnetic Materials Received date: 17 December 2015 Revised date: 25 August 2016 Accepted date: 28 August 2016 Cite this article as: Hui Zhao, Zhenghou Zhu, Chao Xiong, Xing Xu and Qianying Lin, The effect of transverse magnetic field treatment on waveabsorbing properties of FeNi alloy powders, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.08.087 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 galley proof before it is published in its final citable 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 effect of transverse magnetic field treatment on wave-absorbing properties of FeNi alloy powders Hui Zhao, Zhenghou Zhu*, Chao Xiong, Xing Xu, Qianying Lin (College of Material Science and Engineering, Nanchang University, Nanchang, China) *Corresponding Author: Zhenghou Zhu, E-Mail: [email protected], Address: Honggutan New District Xuefu Avenue No. 999, Nanchang, Jiangxi, China.
Abstract: The Fe50Ni50 alloy powders were successfully synthesized with the liquid phase reduction
method and then treated under the transverse magnetic field of 200 kA/m. The influences of transverse magnetic field treatment on microstructures and radar absorbing properties of the powders were mainly investigated. Whether the powders were treated under the transverse magnetic field or not, the main phases of Fe50Ni50 alloy powders were FeNi3 and a small amount of Fe2O3. Results showed that the real part of complex permeability μ' of the Fe50Ni50 alloy powders in 1~5 GHz increased significantly, especially at the frequency of 1 GHz, the μ' increased from 2.2 to 2.8 after transverse magnetic field treatment. The magnetic loss tanδm of the Fe50Ni50 alloy powders after transverse magnetic field treatment was ≥ 0.3 in the frequency range of 1~13 GHz and 0.7~1.05 in the frequency range of 3.5~9.0 GHz. Compared with those of the untreated powders, the wave-absorbing properties of the powders after transverse magnetic field treatment were significantly improved. The Fe50Ni50 alloy powders coatings with thickness of 1.5 mm exhibited excellent wave-absorbing properties after transverse magnetic field treatment, and the qualified absorption band width reached nearly 3 GHz when the reflectivity |R| was ≥ 10 dB. Keywords: FeNi alloy powders; Magnetic field; Permeability; Wave-absorption properties
1. Introduction In recent researches, using new materials and multi-layer composite structures have become the key way to improve the absorbing effect and broaden the absorption band[1-5]. Magnetic wave-absorbing agents are the most widely used and mature material in a wide variety of wave-absorbing materials [6-10]. Magnetic metal powders have many advantages such as good temperature stability, high permeability and large electromagnetic loss which are conducive to the good impedance matching and the absorption band broadening. Soft magnetic FeNi alloy has attracted a large amount interest among the scientists and practitioners due to their high magnetic properties[11~14]. And many people including our group have paid much attention to the studies on the microwave absorbing properties of the FeNi alloy powders. Researches show that FeNi alloy powders can be used as a promising lightweight electromagnetic wave absorber. Fe100-xNix alloy nanopowders were fabricated with liquid phase reduction method and the wave-absorbing properties of the powders were systematically studied [15,16]. In the frequency range of 1~18GHz, the ingredient of Ni content has great influence on the absorbing properties of Fe100-xNix alloy nanopowders. And the Fe100-xNix nanopowders have excellent wave-absorbing properties including Fe20Ni80 alloy nanopowders and Fe50Ni50 alloy nanopowders. But there are still many problems with the FeNi powders when using as wave-absorbing agent, such as the bad wave-absorbing properties in the range of L band (1~2 GHz) and S band (2~4 GHz). And the main reason is the low permeability in the range of 1~4 GHz. Hence, to further improve the values of complex permeability in 1~4 GHz and electromagnetic loss is the research focus of FeNi alloy powders as the wave-absorbing agents[15,17]. 1
Appropriate heat treatment can improve the permeability-frequency characteristics of ferromagnetic materials[18-21]. Transverse magnetic field treatment is a special kind of heat treatments, which can use the external magnetic field to alter the soft magnetic properties through changing magnetic domains and internal stress of materials. And the directions of magnetic domains are consistent after the transverse magnetic field treatment, which are chaotic after conventional heat treatment. At present, transverse magnetic treatment has been widely used to improve magnetic properties of magnetic materials such as amorphous alloy strips, Fe-based nanocrystalline alloy strips, magnetic thin films etc [22-26]. And the permeability of the soft magnetic alloy strips in high frequency can be improved after the transverse magnetic field treatment. In this paper, based on above characteristics of the transverse magnetic field treatment and the waveabsorbing properties of Fe100-xNix alloy nanopowders, Fe50Ni50 alloy nanopowders were treated with transverse magnetic field, which aimed at elevating the complex permeability in high frequency especially in 1~4 GHz and improving the electromagnetic loss to ameliorate wave-absorbing properties.
2. Materials and characteristics 2.1 Materials Fe50Ni50 alloy powders were prepared by liquid phase reduction method. The content of Ni was modified to 50atom% in this study. NiSO4 and FeSO4 reacted with N2H4.H2O for about 30 minutes to generate Fe50Ni50 alloy powders with the solution pH of 14 and the temperature of 85 oC. The obtained black powders were separated from the solution by using magnetic separation method. Then Fe50Ni50 alloy powders were put in the mould, and pressed into annular cores with outer diameter of 20 mm and inner diameter of 10mm and thickness of 5 mm. And the annular cores were treated with the transverse magnetic field of 200 kA/m at the vacuum degree of -0.1 MPa and the temperature of 400 oC for 200 minutes in a vacuum tube type resistance furnace. After mechanical crushed and sieved, Fe50Ni50 alloy powders after the transverse magnetic field treatment were obtained. Fig.1 shows the geometric profile of the annular core and the transverse magnetic field direction.
Fig. 1 Geometric profile of annular core and transverse magnetic field direction
2.2 Characteristics The phase identification of the Fe50Ni50 alloy powders was performed by X-ray diffraction (XRD) on a Bruker-axe D8 ADVANCE X-ray diffractmeter with CuKα radiation. And the test conditions were that tube voltage was 40 KV, the current was 40 mA, and the step was 0.02o. The microstructures and morphologies were observed employing a high-resolution transmission electronic microscopy JEM-2100 (HRTEM). Then Fe50Ni50 alloy powders were mixed with paraffin in the mass ratio of 80:20. The mixtures were made into coaxial rings with thickness of 2.0 mm, of which the outside and inside diameters were 7 mm and 3.04 mm, 2
respectively. Then the electromagnetic parameters (including complex permittivity and complex permeability) of the mixtures were measured by coaxial-type measuring equipment and HP8722ES vector network analyser over a frequency range of 1~18 GHz. The inductance of the annular cores in 10~200 kHz was tested using TH2816B digital electric bridge. The effective permeability (μe) of the cores in 10~200 kHz was calculated according to the equal 1.
e
Ls Le 107 2 4 N Se
(1)
Where, μe is the effective permeability. Ls is the inductance. Le and Se is the effective path length and effective sectional area of the annular cores, respectively. N is the winding number.
3. Result and discussion TEM micrograph shows that the Fe50Ni50 alloy powders exhibits a spherical morphology and the size of the powders is in nanometer scale (Fig.2).
Fig. 2 Microphotography of Fe50Ni50 alloy powders (311)
180 160
(111)
200
Fe2O3 FeNi3
60
(220)
80
(440)
(220)
100
(200) (422) (511)
120
(221)
Intensity(a.u.)
140
40
2
20 0 20
1 30
40
50
60
2dgree
70
80
90
Fig. 3 XRD patterns of Fe50Ni50 alloy powders 1 untreated powders, 2 treated powders
Fig. 3 shows XRD patterns of the Fe50Ni50 alloy powders. Whether the powders were treated under the transverse magnetic field or not, the main phase of Fe50Ni50 alloy powders was FeNi3 and a small amount of Fe2O3. The effects of the transverse magnetic field treatment on the electromagnetic parameters of the Fe50Ni50 alloy powders are shown in Fig. 4. Transverse magnetic field treatment has few effects on shapes of the permittivity-frequency curves or permeability-frequency curves, except the values of the electromagnetic parameters in 1~18 GHz frequency band.
3
18.5
(a)
18.0
6
treated untreated
17.5
(b) treated untreated
5
17.0
4
16.5 16.0 15.5
3 2
15.0 1
14.5 14.0 0
2
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0
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4
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(c)
1.4
treated untreated
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Frequency/GHz
Frequency/GHz (d)
treated untreated
1.2 1.0
2.0
0.8 0.6
1.5
0.4 1.0 0.2 0.5
0
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8
10
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0.0
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0
2
4
Frequency/GHz
6
8
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Frequency/GHz
Fig. 4 Electromagnetic parameters of Fe50Ni50 alloy powders (a) real part of complex permittivity – frequency curves (b) imaginary part of complex permittivity – frequency curves (c) real part of complex permeability – frequency curves (d) imaginary part of complex permeability – frequency curves
The real part and imaginary part of the complex permittivity of the Fe50Ni50 alloy powders are slightly increased after the transverse magnetic field treatment (Fig. 4 (a) and (b)). The transverse magnetic field treatment could reduce the crystal defects and release internal stress of the powders which were favorable for electron loss, so the resistance of Fe50Ni50 alloy powders reduced and the conductivity of the powders increased. The imaginary part of the complex permittivity of the powders increased with the frequency increasing (Fig. 4(b)), because the electrical conductivity increases along with the increase of frequency, meanwhile, the coupling effects between particles are enhanced and the conductive network is formed due to the action of electromagnetic wave (Equal 2). (2) 2 f 0 In the equal (2), ε0 is permittivity of vacuum, f is the frequency of electromagnetic wave, and σ is electrical conductivity. The real part of complex permeability (μ') of Fe50Ni50 alloy powders is 2.8~1 in 1~6 GHz (Fig. 4(c)). After transverse magnetic field treatment, the imaginary part of complex permeability (μ") in 1~16GHz is increased (Fig. 4 (d)). At the same time, the μ' is increased significantly in 1~5GHz, mainly because that the enhanced magnetic structures and the decreased internal stress lead to the better magnetic properties of the Fe50Ni50 alloy powders after the thermal magnetic field treatment. To better evaluate the effect of the transverse magnetic field treatment on the permeability of Fe50Ni50 alloy powders, the effective permeability curves of Fe50Ni50 powders annular cores in the lower frequency band are drawn, as shown in fig. 5.
4
Effective permeability
11.0 10.8
untreated treated
10.6 10.4 10.2 10.0
0
30
60
90
120
150
180
210
Frequency/kHz Fig. 5 The effective permeability of Fe50Ni50 alloy powders annular cores in 10~200kHz
In 10~200 kHz frequency, the effective permeability of the Fe50Ni50 powders annular cores changes from 10~10.2 to 10.9~11.0 after transverse magnetic field treatment. And the variation of the effective permeability of the cores could be calculated according to the equal (3). At the frequency of 10kHz, the variation of the effective permeability of the cores is the largest, about 8.5%, while that is about 7.9% in 80~200 kHz frequency. After the transverse magnetic field treatment, the effective permeability of Fe50Ni50 powders cores are obviously improved, which means that the values of permeability of Fe50Ni50 alloy powders get better.
e e
e e0 100% e0
(3)
Where, μe is the effective permeability of the annular core after transverse magnetic field treatment, μe0 is the effective permeability of the annular core without transverse magnetic field treatment. 0.35
1.2
(a)
0.30
treated untreated
0.8
0.20 0.15
0.6 0.4
0.10
0.2
0.05 0.00
treated untreated
1.0
tanm
tane
0.25
(b)
0
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10
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18
Frequency/GHz
0
2
4
6
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Frequency/GHz
Fig. 6 Electromagnetic loss of Fe50Ni50 alloy powders (a) dielectric loss – frequency curves (b) magnetic loss – frequency curves
Fig. 6 shows the electromagnetic loss - frequency curves of the Fe50Ni50 alloy powders. According to the absorption mechanism of RAM, when the electromagnetic waves enter into the interior of the sample, the electromagnetic waves should be attenuated quickly, which requires large electromagnetic loss. That is to say, the electromagnetic loss tanδ need to be large and generally more than 0.3. Only in this way, the sample has excellent wave-absorbing properties. As shown in fig. 6, the values of dielectric loss tanδe and magnetic loss tanδm of the treated powders are larger than those of the untreated powders. After transverse magnetic field treatment, the tanδe values of the Fe50Ni50 alloy powders are increased by about 0.1 in the frequency range of 1~18 GHz. The loss mechanism of the Fe50Ni50 alloy powders is dominated by magnetic loss because of the larger value of tanδm in the frequency range of 1~15 GHz. But after transverse magnetic field treatment, the value of the tanδm is slightly 5
smaller than that of the tanδe in 15~18 GHz, hence, the loss mechanism should be dominated by magnetic loss and dielectric loss in 15~18 GHz. The tanδm of the untreated Fe50Ni50 alloy powders is ≥ 0.3 in the frequency range of 1.5~12 GHz, and 0.5~0.7( ≥0.5 ) in 3~9 GHz. Meanwhile, the tanδm of the treated Fe50Ni50 alloy powders is ≥ 0.3 in the frequency range of 1~13GHz, especially in the frequency range of 3.5~9.0 GHz, that is 0.7~1.05. Thus, the waveabsorbing properties of the Fe50Ni50 alloy powders after transverse magnetic field treatment are improved significantly. When the electromagnetic wave is vertical incidence, the reflectivity R of microwave absorber consisted of monolayer material can be calculated following the equal (4) according to the transmission line theory. 2 fd r r 1 c 2 fd r tanh j r r 1 r c
r
R 20 lg
r tanh j
(4)
R/dB
Where μr, εr and d is differential permeability, differential permittivity and thickness of a sample respectively. f is frequency. C is the speed of light. The values of μr and εr can be calculated according to the electromagnetic parameters shown in Fig. 4. The R of the monolayer Fe50Ni50 alloy powders coatings after transverse magnetic field treatment in the frequency range of 1~18GHz is shown in fig. 7 0 -3(b) -6 -9 -12 -15 -18 -21 -24 -27 -30 -33 0 2
1mm 1.5mm 2mm 3mm 4mm 5mm 4
6
8
10
12
14
16
18
Frequency/GHz Fig. 7 Reflectivity of Fe50Ni50 alloy powders coatings after transverse magnetic field treatment in 1~18GHz
In 1~18 GHz, the electromagnetic wave absorption peak (main absorption peak) appears as the thickness of the Fe50Ni50 alloy powders coatings reaches 1.5 mm and shifts towards the lower frequency with the thickness increasing. The secondary absorption peak appears as the thickness is up to 4 mm. Table 1 Wave-absorbing frequency bands of Fe50Ni50 alloy powders coatings after transverse magnetic field treatment Absorption band width (R≤-
Central frequency of main
10dB) /GHz
absorption peak /GHz
2.9
13.92
10.01-10.35
0.34
10.01
3
3.21-4.91
1.7
3.89
4
2.36-3.55
1.19
2.97
5
1.85-2.7
0.85
2.19
d/mm
Frequency band (R≤-10dB) /GHz
1.5
12.39-15.28
2
Absorbing characteristics with different wave-absorbing requirements can be designed through changing thicknesses of the wave-absorbing coatings (Table 1). When the thickness of Fe50Ni50 alloy powders coatings 6
after transverse magnetic field treatment is 1.5 mm, the absorption band width reaches 2.9 GHz when R is less than -10 dB in 1~18 GHz.
4. Conclusion (1) Whether the Fe50Ni50 alloy powders are treated under the transverse magnetic field or not, the main phases of the powders are FeNi3 and a small amount of Fe2O3. (2) The magnetic loss (tanδm) of the Fe50Ni50 alloy powders after transverse magnetic field treatment is ≥ 0.3 in the frequency range of 1~13 GHz, and 0.7~1.05 in the frequency range of 3.5~9.0 GHz. Compared with those of untreated powders (less than 0.7), the wave-absorbing properties are improved significantly. (3) Fe50Ni50 alloy powders coating after the transverse magnetic field treatment, with thickness of 1.5mm, has excellent Radar wave-absorbing properties, and the absorption band width when R is less than -10 dB in 1~18 GHz, reaches about 3 GHz.
Acknowledgements This manuscript was based upon work supported by National Natural Science Foundation of China (grant no. 61361008) and 973 project prophase research subject of China（grant no. 2010CB635112）
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Highlights: 1. FeNi alloy powders prepared by liquid phase reduction method, were treated with the transverse magnetic field of 200kA/m. 2. Influences of transverse magnetic field treatment on the wave-absorbing properties of magnetic powders were investigated. 3. μ' of FeNi alloy powders in 1~5GHz increased significantly after transverse magnetic field treatment. 4. FeNi alloy powders coating after transverse magnetic field treatment, with thickness of 1.5mm, has excellent wave-absorbing properties.
8
PII: DOI: Reference:
S0304-8853(16)31963-1 http://dx.doi.org/10.1016/j.jmmm.2016.08.087 MAGMA61772
To appear in: Journal of Magnetism and Magnetic Materials Received date: 17 December 2015 Revised date: 25 August 2016 Accepted date: 28 August 2016 Cite this article as: Hui Zhao, Zhenghou Zhu, Chao Xiong, Xing Xu and Qianying Lin, The effect of transverse magnetic field treatment on waveabsorbing properties of FeNi alloy powders, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.08.087 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 galley proof before it is published in its final citable 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 effect of transverse magnetic field treatment on wave-absorbing properties of FeNi alloy powders Hui Zhao, Zhenghou Zhu*, Chao Xiong, Xing Xu, Qianying Lin (College of Material Science and Engineering, Nanchang University, Nanchang, China) *Corresponding Author: Zhenghou Zhu, E-Mail: [email protected], Address: Honggutan New District Xuefu Avenue No. 999, Nanchang, Jiangxi, China.
Abstract: The Fe50Ni50 alloy powders were successfully synthesized with the liquid phase reduction
method and then treated under the transverse magnetic field of 200 kA/m. The influences of transverse magnetic field treatment on microstructures and radar absorbing properties of the powders were mainly investigated. Whether the powders were treated under the transverse magnetic field or not, the main phases of Fe50Ni50 alloy powders were FeNi3 and a small amount of Fe2O3. Results showed that the real part of complex permeability μ' of the Fe50Ni50 alloy powders in 1~5 GHz increased significantly, especially at the frequency of 1 GHz, the μ' increased from 2.2 to 2.8 after transverse magnetic field treatment. The magnetic loss tanδm of the Fe50Ni50 alloy powders after transverse magnetic field treatment was ≥ 0.3 in the frequency range of 1~13 GHz and 0.7~1.05 in the frequency range of 3.5~9.0 GHz. Compared with those of the untreated powders, the wave-absorbing properties of the powders after transverse magnetic field treatment were significantly improved. The Fe50Ni50 alloy powders coatings with thickness of 1.5 mm exhibited excellent wave-absorbing properties after transverse magnetic field treatment, and the qualified absorption band width reached nearly 3 GHz when the reflectivity |R| was ≥ 10 dB. Keywords: FeNi alloy powders; Magnetic field; Permeability; Wave-absorption properties
1. Introduction In recent researches, using new materials and multi-layer composite structures have become the key way to improve the absorbing effect and broaden the absorption band[1-5]. Magnetic wave-absorbing agents are the most widely used and mature material in a wide variety of wave-absorbing materials [6-10]. Magnetic metal powders have many advantages such as good temperature stability, high permeability and large electromagnetic loss which are conducive to the good impedance matching and the absorption band broadening. Soft magnetic FeNi alloy has attracted a large amount interest among the scientists and practitioners due to their high magnetic properties[11~14]. And many people including our group have paid much attention to the studies on the microwave absorbing properties of the FeNi alloy powders. Researches show that FeNi alloy powders can be used as a promising lightweight electromagnetic wave absorber. Fe100-xNix alloy nanopowders were fabricated with liquid phase reduction method and the wave-absorbing properties of the powders were systematically studied [15,16]. In the frequency range of 1~18GHz, the ingredient of Ni content has great influence on the absorbing properties of Fe100-xNix alloy nanopowders. And the Fe100-xNix nanopowders have excellent wave-absorbing properties including Fe20Ni80 alloy nanopowders and Fe50Ni50 alloy nanopowders. But there are still many problems with the FeNi powders when using as wave-absorbing agent, such as the bad wave-absorbing properties in the range of L band (1~2 GHz) and S band (2~4 GHz). And the main reason is the low permeability in the range of 1~4 GHz. Hence, to further improve the values of complex permeability in 1~4 GHz and electromagnetic loss is the research focus of FeNi alloy powders as the wave-absorbing agents[15,17]. 1
Appropriate heat treatment can improve the permeability-frequency characteristics of ferromagnetic materials[18-21]. Transverse magnetic field treatment is a special kind of heat treatments, which can use the external magnetic field to alter the soft magnetic properties through changing magnetic domains and internal stress of materials. And the directions of magnetic domains are consistent after the transverse magnetic field treatment, which are chaotic after conventional heat treatment. At present, transverse magnetic treatment has been widely used to improve magnetic properties of magnetic materials such as amorphous alloy strips, Fe-based nanocrystalline alloy strips, magnetic thin films etc [22-26]. And the permeability of the soft magnetic alloy strips in high frequency can be improved after the transverse magnetic field treatment. In this paper, based on above characteristics of the transverse magnetic field treatment and the waveabsorbing properties of Fe100-xNix alloy nanopowders, Fe50Ni50 alloy nanopowders were treated with transverse magnetic field, which aimed at elevating the complex permeability in high frequency especially in 1~4 GHz and improving the electromagnetic loss to ameliorate wave-absorbing properties.
2. Materials and characteristics 2.1 Materials Fe50Ni50 alloy powders were prepared by liquid phase reduction method. The content of Ni was modified to 50atom% in this study. NiSO4 and FeSO4 reacted with N2H4.H2O for about 30 minutes to generate Fe50Ni50 alloy powders with the solution pH of 14 and the temperature of 85 oC. The obtained black powders were separated from the solution by using magnetic separation method. Then Fe50Ni50 alloy powders were put in the mould, and pressed into annular cores with outer diameter of 20 mm and inner diameter of 10mm and thickness of 5 mm. And the annular cores were treated with the transverse magnetic field of 200 kA/m at the vacuum degree of -0.1 MPa and the temperature of 400 oC for 200 minutes in a vacuum tube type resistance furnace. After mechanical crushed and sieved, Fe50Ni50 alloy powders after the transverse magnetic field treatment were obtained. Fig.1 shows the geometric profile of the annular core and the transverse magnetic field direction.
Fig. 1 Geometric profile of annular core and transverse magnetic field direction
2.2 Characteristics The phase identification of the Fe50Ni50 alloy powders was performed by X-ray diffraction (XRD) on a Bruker-axe D8 ADVANCE X-ray diffractmeter with CuKα radiation. And the test conditions were that tube voltage was 40 KV, the current was 40 mA, and the step was 0.02o. The microstructures and morphologies were observed employing a high-resolution transmission electronic microscopy JEM-2100 (HRTEM). Then Fe50Ni50 alloy powders were mixed with paraffin in the mass ratio of 80:20. The mixtures were made into coaxial rings with thickness of 2.0 mm, of which the outside and inside diameters were 7 mm and 3.04 mm, 2
respectively. Then the electromagnetic parameters (including complex permittivity and complex permeability) of the mixtures were measured by coaxial-type measuring equipment and HP8722ES vector network analyser over a frequency range of 1~18 GHz. The inductance of the annular cores in 10~200 kHz was tested using TH2816B digital electric bridge. The effective permeability (μe) of the cores in 10~200 kHz was calculated according to the equal 1.
e
Ls Le 107 2 4 N Se
(1)
Where, μe is the effective permeability. Ls is the inductance. Le and Se is the effective path length and effective sectional area of the annular cores, respectively. N is the winding number.
3. Result and discussion TEM micrograph shows that the Fe50Ni50 alloy powders exhibits a spherical morphology and the size of the powders is in nanometer scale (Fig.2).
Fig. 2 Microphotography of Fe50Ni50 alloy powders (311)
180 160
(111)
200
Fe2O3 FeNi3
60
(220)
80
(440)
(220)
100
(200) (422) (511)
120
(221)
Intensity(a.u.)
140
40
2
20 0 20
1 30
40
50
60
2dgree
70
80
90
Fig. 3 XRD patterns of Fe50Ni50 alloy powders 1 untreated powders, 2 treated powders
Fig. 3 shows XRD patterns of the Fe50Ni50 alloy powders. Whether the powders were treated under the transverse magnetic field or not, the main phase of Fe50Ni50 alloy powders was FeNi3 and a small amount of Fe2O3. The effects of the transverse magnetic field treatment on the electromagnetic parameters of the Fe50Ni50 alloy powders are shown in Fig. 4. Transverse magnetic field treatment has few effects on shapes of the permittivity-frequency curves or permeability-frequency curves, except the values of the electromagnetic parameters in 1~18 GHz frequency band.
3
18.5
(a)
18.0
6
treated untreated
17.5
(b) treated untreated
5
17.0
4
16.5 16.0 15.5
3 2
15.0 1
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(c)
1.4
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Frequency/GHz
Frequency/GHz (d)
treated untreated
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2.0
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18
Frequency/GHz
Fig. 4 Electromagnetic parameters of Fe50Ni50 alloy powders (a) real part of complex permittivity – frequency curves (b) imaginary part of complex permittivity – frequency curves (c) real part of complex permeability – frequency curves (d) imaginary part of complex permeability – frequency curves
The real part and imaginary part of the complex permittivity of the Fe50Ni50 alloy powders are slightly increased after the transverse magnetic field treatment (Fig. 4 (a) and (b)). The transverse magnetic field treatment could reduce the crystal defects and release internal stress of the powders which were favorable for electron loss, so the resistance of Fe50Ni50 alloy powders reduced and the conductivity of the powders increased. The imaginary part of the complex permittivity of the powders increased with the frequency increasing (Fig. 4(b)), because the electrical conductivity increases along with the increase of frequency, meanwhile, the coupling effects between particles are enhanced and the conductive network is formed due to the action of electromagnetic wave (Equal 2). (2) 2 f 0 In the equal (2), ε0 is permittivity of vacuum, f is the frequency of electromagnetic wave, and σ is electrical conductivity. The real part of complex permeability (μ') of Fe50Ni50 alloy powders is 2.8~1 in 1~6 GHz (Fig. 4(c)). After transverse magnetic field treatment, the imaginary part of complex permeability (μ") in 1~16GHz is increased (Fig. 4 (d)). At the same time, the μ' is increased significantly in 1~5GHz, mainly because that the enhanced magnetic structures and the decreased internal stress lead to the better magnetic properties of the Fe50Ni50 alloy powders after the thermal magnetic field treatment. To better evaluate the effect of the transverse magnetic field treatment on the permeability of Fe50Ni50 alloy powders, the effective permeability curves of Fe50Ni50 powders annular cores in the lower frequency band are drawn, as shown in fig. 5.
4
Effective permeability
11.0 10.8
untreated treated
10.6 10.4 10.2 10.0
0
30
60
90
120
150
180
210
Frequency/kHz Fig. 5 The effective permeability of Fe50Ni50 alloy powders annular cores in 10~200kHz
In 10~200 kHz frequency, the effective permeability of the Fe50Ni50 powders annular cores changes from 10~10.2 to 10.9~11.0 after transverse magnetic field treatment. And the variation of the effective permeability of the cores could be calculated according to the equal (3). At the frequency of 10kHz, the variation of the effective permeability of the cores is the largest, about 8.5%, while that is about 7.9% in 80~200 kHz frequency. After the transverse magnetic field treatment, the effective permeability of Fe50Ni50 powders cores are obviously improved, which means that the values of permeability of Fe50Ni50 alloy powders get better.
e e
e e0 100% e0
(3)
Where, μe is the effective permeability of the annular core after transverse magnetic field treatment, μe0 is the effective permeability of the annular core without transverse magnetic field treatment. 0.35
1.2
(a)
0.30
treated untreated
0.8
0.20 0.15
0.6 0.4
0.10
0.2
0.05 0.00
treated untreated
1.0
tanm
tane
0.25
(b)
0
2
4
6
8
10
12
14
16
0.0
18
Frequency/GHz
0
2
4
6
8
10
12
14
16
18
Frequency/GHz
Fig. 6 Electromagnetic loss of Fe50Ni50 alloy powders (a) dielectric loss – frequency curves (b) magnetic loss – frequency curves
Fig. 6 shows the electromagnetic loss - frequency curves of the Fe50Ni50 alloy powders. According to the absorption mechanism of RAM, when the electromagnetic waves enter into the interior of the sample, the electromagnetic waves should be attenuated quickly, which requires large electromagnetic loss. That is to say, the electromagnetic loss tanδ need to be large and generally more than 0.3. Only in this way, the sample has excellent wave-absorbing properties. As shown in fig. 6, the values of dielectric loss tanδe and magnetic loss tanδm of the treated powders are larger than those of the untreated powders. After transverse magnetic field treatment, the tanδe values of the Fe50Ni50 alloy powders are increased by about 0.1 in the frequency range of 1~18 GHz. The loss mechanism of the Fe50Ni50 alloy powders is dominated by magnetic loss because of the larger value of tanδm in the frequency range of 1~15 GHz. But after transverse magnetic field treatment, the value of the tanδm is slightly 5
smaller than that of the tanδe in 15~18 GHz, hence, the loss mechanism should be dominated by magnetic loss and dielectric loss in 15~18 GHz. The tanδm of the untreated Fe50Ni50 alloy powders is ≥ 0.3 in the frequency range of 1.5~12 GHz, and 0.5~0.7( ≥0.5 ) in 3~9 GHz. Meanwhile, the tanδm of the treated Fe50Ni50 alloy powders is ≥ 0.3 in the frequency range of 1~13GHz, especially in the frequency range of 3.5~9.0 GHz, that is 0.7~1.05. Thus, the waveabsorbing properties of the Fe50Ni50 alloy powders after transverse magnetic field treatment are improved significantly. When the electromagnetic wave is vertical incidence, the reflectivity R of microwave absorber consisted of monolayer material can be calculated following the equal (4) according to the transmission line theory. 2 fd r r 1 c 2 fd r tanh j r r 1 r c
r
R 20 lg
r tanh j
(4)
R/dB
Where μr, εr and d is differential permeability, differential permittivity and thickness of a sample respectively. f is frequency. C is the speed of light. The values of μr and εr can be calculated according to the electromagnetic parameters shown in Fig. 4. The R of the monolayer Fe50Ni50 alloy powders coatings after transverse magnetic field treatment in the frequency range of 1~18GHz is shown in fig. 7 0 -3(b) -6 -9 -12 -15 -18 -21 -24 -27 -30 -33 0 2
1mm 1.5mm 2mm 3mm 4mm 5mm 4
6
8
10
12
14
16
18
Frequency/GHz Fig. 7 Reflectivity of Fe50Ni50 alloy powders coatings after transverse magnetic field treatment in 1~18GHz
In 1~18 GHz, the electromagnetic wave absorption peak (main absorption peak) appears as the thickness of the Fe50Ni50 alloy powders coatings reaches 1.5 mm and shifts towards the lower frequency with the thickness increasing. The secondary absorption peak appears as the thickness is up to 4 mm. Table 1 Wave-absorbing frequency bands of Fe50Ni50 alloy powders coatings after transverse magnetic field treatment Absorption band width (R≤-
Central frequency of main
10dB) /GHz
absorption peak /GHz
2.9
13.92
10.01-10.35
0.34
10.01
3
3.21-4.91
1.7
3.89
4
2.36-3.55
1.19
2.97
5
1.85-2.7
0.85
2.19
d/mm
Frequency band (R≤-10dB) /GHz
1.5
12.39-15.28
2
Absorbing characteristics with different wave-absorbing requirements can be designed through changing thicknesses of the wave-absorbing coatings (Table 1). When the thickness of Fe50Ni50 alloy powders coatings 6
after transverse magnetic field treatment is 1.5 mm, the absorption band width reaches 2.9 GHz when R is less than -10 dB in 1~18 GHz.
4. Conclusion (1) Whether the Fe50Ni50 alloy powders are treated under the transverse magnetic field or not, the main phases of the powders are FeNi3 and a small amount of Fe2O3. (2) The magnetic loss (tanδm) of the Fe50Ni50 alloy powders after transverse magnetic field treatment is ≥ 0.3 in the frequency range of 1~13 GHz, and 0.7~1.05 in the frequency range of 3.5~9.0 GHz. Compared with those of untreated powders (less than 0.7), the wave-absorbing properties are improved significantly. (3) Fe50Ni50 alloy powders coating after the transverse magnetic field treatment, with thickness of 1.5mm, has excellent Radar wave-absorbing properties, and the absorption band width when R is less than -10 dB in 1~18 GHz, reaches about 3 GHz.
Acknowledgements This manuscript was based upon work supported by National Natural Science Foundation of China (grant no. 61361008) and 973 project prophase research subject of China（grant no. 2010CB635112）
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Highlights: 1. FeNi alloy powders prepared by liquid phase reduction method, were treated with the transverse magnetic field of 200kA/m. 2. Influences of transverse magnetic field treatment on the wave-absorbing properties of magnetic powders were investigated. 3. μ' of FeNi alloy powders in 1~5GHz increased significantly after transverse magnetic field treatment. 4. FeNi alloy powders coating after transverse magnetic field treatment, with thickness of 1.5mm, has excellent wave-absorbing properties.
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