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Electromagnetic and microwave absorbing properties of the composites containing flaky FeSiAl powders mixed with MnO2 in 1–18 GHz
Journal of Magnetism and Magnetic Materials 401 (2016) 567–571
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Electromagnetic and microwave absorbing properties of the composites containing flaky FeSiAl powders mixed with MnO2 in 1–18 GHz Haibing Xu a,b, Shaowei Bie a,b,n, Jianjun Jiang a,b, Wei Yuan a,b, Qian Chen a,b, Yongshun Xu a,b a b
School of Optical and Electronic Information, Huazhong University of Science & Technology, Hubei 430074, China Key Laboratory of Functional Materials for Electronic Information (B), MOE, Huazhong University of Science and Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 May 2015 Received in revised form 14 September 2015 Accepted 25 October 2015 Available online 26 October 2015
The flaky FeSiAl/ irregular shaped MnO2 composite with the different mass ratios were prepared by using a two-roll mixer and a vulcanizing machine. The morphologies of the composite absorbers were characterized by a scanning electron microscope. The microwave electromagnetic properties of the composites were measured using a vector network analyzer in the range of 1–18 GHz. The effect of the mass ratio of FeSiAl/MnO2 on the microwave loss properties of the composites was investigated. The results show that the reflection loss (RL) values exceeding 20 dB from 3.5 to 16.5 GHz can be obtained for the flaky FeSiAl/MnO2 mass ratio of 1:1 from 1.5 mm to 5 mm. In addition, the FeSiAl/MnO2 composite with the FeSiAl/MnO2 mass ratio of 7:3 has 10 dB bandwidth of 6.6 GHz (from 11.4–18 GHz) with a thickness of 1.5 mm. It is found that the flaky FeSiAl/MnO2 composites can be potential microwave absorption materials. & 2015 Elsevier B.V. All rights reserved.
Keywords: Composite Microwave absorption materials Electromagnetic properties
1. Introduction Recently, a series of investigations have concentrated on the radar absorbing materials (RAM) with the properties of wide band and strong absorption. RAM commonly can be classified into two categories: magnetic and dielectric absorbing materials. The microwave absorbing property of the RAM is determined by the complex permeability μr ¼ μ′ iμ″, permittivity εr ¼ ε′ iε″, the electromagnetic impedance matching, and the microstructure of the absorber [1]. Generally, the magnetic absorbers have excellent absorptivity in lower frequency range because of high complex permeability. The microstructures of the magnetic particles such as crystal structure, particle size which may be changed by ball milling affect the complex electromagnetic parameters. Anisotropic magnetic particles may have a higher resonance frequency above Snoek's limit [2] in the gigahertz frequency range due to their low eddy current loss coming from particle shape effects. On the other hand, the dielectric absorbers are used in the higher frequency range, and have good dielectric properties with large thicknesses [3,4]. Some reports show that mixed different n Corresponding author at: School of Optical and Electronic Information, Huazhong University of Science & Technology, Hubei 430074, China. Fax: þ86 27 87544472. E-mail address: [email protected] (S. Bie).
http://dx.doi.org/10.1016/j.jmmm.2015.10.093 0304-8853/& 2015 Elsevier B.V. All rights reserved.
absorbers comprising two fillers for both dielectric and magnetic characteristics can exhibit better microwave absorbing properties, such as CNTs/CoFe2O4 [5] and γ-Fe2O3/C nanocomposites [6]. The FeSiAl powders were investigated because of their microwave absorbing properties in the microwave frequency range as a type of typical magnetic materials [7–10], which achieve higher complex permittivity and complex permeability after ball milling. However, the higher permittivity may result in poor impedance matching and could not obtain a relatively wide frequency band [11]. To improve the poor electromagnetic impedance matching, the magnetic absorbents are mixed with dielectric materials because of their excellent cooperative effect [12,13]. MnO2 is a kind of classical dielectric absorbing material and possesses microwave attenuation properties [14,15]. Moreover, there are few reports about the electromagnetic and microwave absorption properties of FeSiAl mixed with MnO2. In this paper, we fabricated flaky FeSiAl/ MnO2 composite by using a two-roll mixer and a vulcanizing machine and investigated the effect of different mass ratios of FeSiAl/MnO2 on thier electromagnetic and microwave absorption properties. The anisotropy caused by the ball milling as well as combining FeSiAl as a magnetic material with MnO2 as a dielectric material can lead to microwave absorbers with both high magnetic and dielectric losses, which are controllable by adjusting the mass ratios of FeSiAl/MnO2 in the composites.
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H. Xu et al. / Journal of Magnetism and Magnetic Materials 401 (2016) 567–571
2. Experimental details and methods Commercially available water atomized irregular shape FeSiAl alloy powder, purchased from Advanced Technology & Materials Co., Ltd (AT&M) with a composition of 9 wt% silicone and 6 wt% aluminum and 85 wt% iron, is used as absorbing filler of composite RAMs. The raw FeSiAl, having average particle sizes between 10 μm and 40 μm, labeled as “a1” in Table 1. The flaky FeSiAl powders were obtained by ball milling the raw FeSiAl for 24 h, labeled as “a2” in Table 1. The ball-to-powder mass ratio was 10:1. Anhydrous alcohol mixed with a few drops of oleic acid was used as a process of controlling agent to avoid the aggregation and oxidation of particles. Commercially available MnO2 powders were purchased from Tianjin Tianli Chemical Reagent Co., Ltd, which were mixed into the FeSiAl flakes by mass ratios of flaky FeSiAl/ MnO2 ¼7/3 (labeled as “a3”), 1/1 (“a4”) and 3/7 (“a5”), as shown in Table 1. The morphology of microstructure of the samples were studied by scanning electron microscopy (ESEM, quanta 200, FEI). The raw FeSiAl, flaky FeSiAl, and flaky FeSiAl/MnO2 composite powders with a mass ratio of 60 wt%, were added into silicone rubber. Mixed and vulcanized by double roll rubber. mixing mill and flat vulcanizer respectively, they were processed into microwave absorbing sheets. The as-prepared composite powders were homogenously dispersed in silicone rubber with a total mass fraction of 60 wt% (powder) and processed into toroidal-shape specimens with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm and thickness of 2 mm for transmission/reflection measurements. The complex relative dielectric permittivity and magnetic permeability (εr ¼ ε′ iε″, μr ¼ μ′ iμ″) of toroidal specimens were obtained using a vector network analyzer (HP8720B) in the range of 1–18 GHz. The RL of electromagnetic waves of as-prepared samples backed by a perfect conductor can be calculated from the measured complex relative permittivity and permeability data for the given frequency and absorber thickness by using the following equations [16]:
Zin = Z 0 μ /ε tanh [j(2πfd/c ) εμ ]
(1)
R = 20 log |(Zin − Z 0)/(Zin + Z 0)|
(2)
Where Zin is the input impedance when the electromagnetic wave incidence is normal to the absorbing material, f is the frequency of the electromagnetic wave, d is the thickness of the absorber layer, c is the velocity of light, and Z0 is the impedance of free space.
3. Results and discussion The morphologies of the raw FeSiAl, the flaky FeSiAl and the flaky FeSiAl/MnO2 composite absorbers with different FeSiAl/ MnO2 mass ratios are shown in Fig. 1. The raw FeSiAl alloy powder in an irregular shape with particle size of 10–40 μm can be seen clearly in Fig. 1(a). The morphologies of the FeSiAl alloy powders become flaky by the effect of shearing action and knock-on effect during ball-milling process in Fig. 1(b). Thicknesses of the flaky Table 1 Sample number and their description. Sample number
Description
a1 a2 a3 a4 a5
The The The The The
raw FeSiAl flaky FeSiAl/ball milling 24 h flaky FeSiAl/MnO2 ¼7:3/ball milling 24 h flaky FeSiAl/MnO2 ¼1:1/ball milling 24 h flaky FeSiAl/MnO2 ¼3:7/ball milling 24 h
FeSiAl alloy powders are less than 2 μm. The eddy current effect is reduced with the particle shape changing from irregular shape to flaky one. Such morphology of flaky FeSiAl absorbers is beneficial to acquire relatively high resonance frequency. In Fig. 1(c), (d) and (e), we can observe that the sizes of the massive MnO2 are nearly between 20 μm and 40 μm and the FeSiAl flakes around them. In all, aspect ratios such as length/thickness of the milled FeSiAl are much higher than that of the raw ones. As is shown in Fig. 1(c), (d) and (e), we can observe that there also appear massive MnO2 particles besides flaky FeSiAl and the massive MnO2 particles content increase gradually as the mass ratio of FeSiAl/MnO2 varies from 7:3 to 3:7. In Fig. 2, it can be observed that the complex permittivity and permeability of the raw FeSiAl, the flaky FeSiAl, and the flaky FeSiAl/MnO2 absorbing materials with different mass ratios including 7:3, 1:1, and 3:7 filled in silicone rubber with the same total weight fraction of 60 wt%. It can be seen from Fig. 2 that the real and imaginary part of permittivity and permeability spectra for the flaky FeSiAl absorbing materials are all higher than the raw FeSiAl absorbing materials in the frequency range of 1–18 GHz, respectively. After ball-milling, the real part of relative permeability increased by a small degree in the frequency range of 1– 9 GHz, whereas the imaginary part increased by a great margin over the whole frequency range of 1–18 GHz, especially in the range of 1–7 GHz, contributing to a great magnetic loss. As is shown in Fig. 2(a), it can be seen that the ε′ decrease in turn from a2 to a5. More significantly, it is found that the values of ε″ is obviously increased as the mass content of MnO2 in the FeSiAl/MnO2 composite absorber increases from 0:10, 3:7, 1:1 and 7:3 during 1–9 GHz, but it decreases at the frequency range of 9– 18 GHz in Fig. 2(b). Sample a2 exhibits relatively high ε′ and ε″ due to the large polarization between the FeSiAl flakes with large surface area [11,17,18]. It can be seen that both the ε′ and ε″ of the sample a3–a5 substantially decrease and are inclined to be independent of frequency. This phenomenon is probably related to higher electrical resistivity and fewer flakes of the composite particles, which determine the interfacial polarization of free charge on the conductor/insulator interface for the composite RAM with conductive particles dispersed in an insulated matrix [19]. As is shown in Fig. 2. (c), the real part of complex permeability μ′ of declines from 3.12, 1.97, 1.69, 1.38 to around 0.9 with increasing of the frequency in the 1–18 GHz range for a2, a3, a4 and a5, respectively. The imaginary part of permeability μ″ of a2 initially increases with frequency increases. Meanwhile, the μ″ of a4 and a5 curves exhibit distinct peak in a broad frequency range due to natural resonance and exchange resonance coming from the large anisotropy field and wide size range of the composite particles. It can be found that a peak around 4 GHz appears in the curves of μ″ and the frequency where the peaks stay remains almost unvaried with the mass ratio of FeSiAl/MnO2 increases. The μ ′ and μ″ decrease when the mass ratio of MnO2 increases, which could be attributed to the reduction of the specific saturation magnetization and the increased fraction of nonmagnetic particles. It is pointed out that large value of μ′ and μ″ is essential for microwave absorbers to realize good absorbing properties as a result of good impedance matching and large magnetic loss. Fig. 3 shows the RL for the composites of 60 wt% as-prepared flaky FeSiAl (a) and flaky FeSiAl/MnO2 absorbers with mass ratio 7:3 (b), 1:1 (c) and 3:7 (d) at different thickness. Obviously, all samples exhibit that the absorption peak moves toward the low frequency direction with the increase of sample thickness. It can be seen from Fig. 3(a), an optimal RL of 34.3 dB is reached at 3.2 GHz for a layer of 3 mm thickness, while the absorption exceeding 20 dB obtained in the 2.3–4.1 GHz range in the thickness from 2.5 mm to 4 mm. Through analyzing the data, the sample a2 has the strong absorption peaks of sample a2 stay
H. Xu et al. / Journal of Magnetism and Magnetic Materials 401 (2016) 567–571
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Fig. 1. ESEM images of (a) raw FeSiAl, (b) flaky FeSiAl and flaky FeSiAl/MnO2 with FeSiAl: MnO2 mass ratios of 7:3 (c), 1:1 (d) and 3:7 (e).
Fig. 2. (a) Real part and (b) imaginary part of the complex permittivity, (c) the real part and (d) imaginary part of complex permeability of the raw FeSiAl, flaky FeSiAl, and the milled flake-shaped FeSiAl/MnO2 composites.
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Fig. 3. The simulated RL for the flaky FeSiAl (a), the flaky FeSiAl /MnO2 composites with mass ratio of (b) FeSiAl/MnO2 ¼ 7:3, (c) 1:1 and (d) 3:7 in different thickness from 1.5–5 mm.
Table 2 Electromagnetic wave absorption characteristics of raw FeSiAl, flaky FeSiAl and flaky FeSiAl/MnO2 composites with different mass ratios. Sample
RLmin (dB)
fmin (GHz)
f20 (GHz) (RL o 20 dB)
f10max (GHz) (RL o 10 dB, single layer)
d10 (mm) (RL o 10 dB)
a1 a2 a3 a4 a5
35.9 34.3 33.2 41.8 23.6
18 3.2 7.2 8.2 7.8
11.6–18 2.4–4 4.2–15.4 3.5–16.5 5.4–9.9
9.8–15.1 5.3–8 11.4–18 13–18 10.8–14.7
2 1.5 1.5 1.5 2
RLmin and fmin mean the minimum reflection loss and its corresponding frequency, respectively. f20 means the frequency range of exceeding 20 dB and f10 stands for the bandwidth of exceeding 10 dB with its corresponding the thickness (d10).
around the low frequency region with the thickness of 1.5–5 mm. As the mass ratio of the MnO2/FeSiAl absorber increases from 0:1 to 3:7, it can be observed that the resonance peaks of sample a3 in Fig. 3(b) move towards to the high frequency region. Moreover, the frequency range of the absorption exceeding 20 dB increases from the range of 2.3–4.1 GHz range to the range of 4.1–15.5 GHz range with the thickness range from 2.5–4 mm to 1.5–4 mm. Compared with the sample a3, the frequency range of the sample a4 exceeding 20 dB increases from the range of 4.1–15.5 GHz range to the range of 3.5 16.5 GHz range with the thickness range from 1.5–4 mm to 1.5–5 mm in Fig. 3(c). For these samples, the RL peak actually corresponds to a practical matching point on the present condition, hence, the lower value of ε′ and μ′, the higher frequency of RL peak appears [18]. These results are of significance since the absorption frequency ranges of the flaky FeSiAl/MnO2 composites can be adjusted easily by changing the thickness of the composite RAM. Moreover, the
absorption bandwidth with the RL exceeding 10 dB of sample a3 for the thickness of 1.5 mm can reach up to 6.6 GHz (from 11.4 to 18 GHz), which are all wider than the reported FeSiAl(Cr) [20] ( 10 dB bandwidth is 2 GHz) and the reported FeSiAl flakes with nylon [21] ( 10 dB bandwidth is 0 GHz) with the same thickness and the reported FeSiAl/graphite ( 10 dB bandwidth is 2.4 GHz (from 13.68 to 16.08 GHz)) [22]. In Fig. 3(d), the whole absorption peaks of sample 5 move towards the high frequency region a little. But the absorption intensity has weakened, compared with sample 4 in Fig. 3(c). Obviously the absorption values exceeding 20 dB were obtained only in the frequency range of 5.5–8.1 GHz for thickness of 3–4 mm. The electromagnetic wave characteristics of all the samples are summarized in Table 2. The absorption properties of the two samples above have distinct advantages compared with the milling FeSiAl [10], the FeSiAl/ nylon [21] and the FeSiAl/ graphite [22]. The RLs for melt-
H. Xu et al. / Journal of Magnetism and Magnetic Materials 401 (2016) 567–571
quenching FeSiAl [7], Fe–Si–Al/BaTiO3/Nd2Fe14B [23], FeSiAl and carbonous materials [8], annealing and milling FeSiAl [9] are relatively poor, with RL 20 dB in the 1–18 GHz range. In all, we can know that the FeSiAl/MnO2 composites possess the broader frequencies ranges (RL 20 dB) and the thinner absorber matching thickness (RL 20 dB). A RL value of 20 dB corresponds to 99% attenuation of the electromagnetic wave, which can be considered as potentially effective absorption material. Therefore, it is available that the FeSiAl/MnO2 composite RAMs can realize better absorbing performances over the whole frequency range due to their modified complex permittivity and permeability from the mutual contributions of magnetic and magnetic absorbers.
4. Conclusion In conclusion, flaky FeSiAl powders were obtained by ballmilling raw FeSiAl powders. The flaky FeSiAl powders and the flaky FeSiAl/MnO2 composite powders were used as the absorbent fillers based on silicone rubber matrix. The results demonstrated that the sample a2 has high magnetic loss in the low frequency region because of satisfactory shape of flake. The minimum RL of sample a4 reached up to 41.8 dB, at 8.2 GHz with a matching thickness of 2.5 mm. Moreover, its reflection loss values exceeding 20 dB (99% power absorption) can be obtained in 3.5–16.5 GHz with choosing an appropriate layer thickness between 1.5 mm and 5 mm. In addition, the absorption bandwidth with the reflection loss exceeding 10 dB of sample a3 for the thickness of 1.5 mm can reach up to the broadest range of 6.6 GHz (from 11.4 to 18 GHz), which covers half of the X-band and the whole Ku-band, indicating that the proposed flaky FeSiAl/MnO2 composites can be used as an attractive candidate for a thin microwave absorbing material.
Acknowledgments The authors gratefully acknowledge financial support from the Natural Science Foundation of Hubei Province, China (Grant number 2012FFB02210) and the National Natural Science Foundation of China (Grant no. 61172003)
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Electromagnetic and microwave absorbing properties of the composites containing flaky FeSiAl powders mixed with MnO2 in 1–18 GHz Haibing Xu a,b, Shaowei Bie a,b,n, Jianjun Jiang a,b, Wei Yuan a,b, Qian Chen a,b, Yongshun Xu a,b a b
School of Optical and Electronic Information, Huazhong University of Science & Technology, Hubei 430074, China Key Laboratory of Functional Materials for Electronic Information (B), MOE, Huazhong University of Science and Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 May 2015 Received in revised form 14 September 2015 Accepted 25 October 2015 Available online 26 October 2015
The flaky FeSiAl/ irregular shaped MnO2 composite with the different mass ratios were prepared by using a two-roll mixer and a vulcanizing machine. The morphologies of the composite absorbers were characterized by a scanning electron microscope. The microwave electromagnetic properties of the composites were measured using a vector network analyzer in the range of 1–18 GHz. The effect of the mass ratio of FeSiAl/MnO2 on the microwave loss properties of the composites was investigated. The results show that the reflection loss (RL) values exceeding 20 dB from 3.5 to 16.5 GHz can be obtained for the flaky FeSiAl/MnO2 mass ratio of 1:1 from 1.5 mm to 5 mm. In addition, the FeSiAl/MnO2 composite with the FeSiAl/MnO2 mass ratio of 7:3 has 10 dB bandwidth of 6.6 GHz (from 11.4–18 GHz) with a thickness of 1.5 mm. It is found that the flaky FeSiAl/MnO2 composites can be potential microwave absorption materials. & 2015 Elsevier B.V. All rights reserved.
Keywords: Composite Microwave absorption materials Electromagnetic properties
1. Introduction Recently, a series of investigations have concentrated on the radar absorbing materials (RAM) with the properties of wide band and strong absorption. RAM commonly can be classified into two categories: magnetic and dielectric absorbing materials. The microwave absorbing property of the RAM is determined by the complex permeability μr ¼ μ′ iμ″, permittivity εr ¼ ε′ iε″, the electromagnetic impedance matching, and the microstructure of the absorber [1]. Generally, the magnetic absorbers have excellent absorptivity in lower frequency range because of high complex permeability. The microstructures of the magnetic particles such as crystal structure, particle size which may be changed by ball milling affect the complex electromagnetic parameters. Anisotropic magnetic particles may have a higher resonance frequency above Snoek's limit [2] in the gigahertz frequency range due to their low eddy current loss coming from particle shape effects. On the other hand, the dielectric absorbers are used in the higher frequency range, and have good dielectric properties with large thicknesses [3,4]. Some reports show that mixed different n Corresponding author at: School of Optical and Electronic Information, Huazhong University of Science & Technology, Hubei 430074, China. Fax: þ86 27 87544472. E-mail address: [email protected] (S. Bie).
http://dx.doi.org/10.1016/j.jmmm.2015.10.093 0304-8853/& 2015 Elsevier B.V. All rights reserved.
absorbers comprising two fillers for both dielectric and magnetic characteristics can exhibit better microwave absorbing properties, such as CNTs/CoFe2O4 [5] and γ-Fe2O3/C nanocomposites [6]. The FeSiAl powders were investigated because of their microwave absorbing properties in the microwave frequency range as a type of typical magnetic materials [7–10], which achieve higher complex permittivity and complex permeability after ball milling. However, the higher permittivity may result in poor impedance matching and could not obtain a relatively wide frequency band [11]. To improve the poor electromagnetic impedance matching, the magnetic absorbents are mixed with dielectric materials because of their excellent cooperative effect [12,13]. MnO2 is a kind of classical dielectric absorbing material and possesses microwave attenuation properties [14,15]. Moreover, there are few reports about the electromagnetic and microwave absorption properties of FeSiAl mixed with MnO2. In this paper, we fabricated flaky FeSiAl/ MnO2 composite by using a two-roll mixer and a vulcanizing machine and investigated the effect of different mass ratios of FeSiAl/MnO2 on thier electromagnetic and microwave absorption properties. The anisotropy caused by the ball milling as well as combining FeSiAl as a magnetic material with MnO2 as a dielectric material can lead to microwave absorbers with both high magnetic and dielectric losses, which are controllable by adjusting the mass ratios of FeSiAl/MnO2 in the composites.
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H. Xu et al. / Journal of Magnetism and Magnetic Materials 401 (2016) 567–571
2. Experimental details and methods Commercially available water atomized irregular shape FeSiAl alloy powder, purchased from Advanced Technology & Materials Co., Ltd (AT&M) with a composition of 9 wt% silicone and 6 wt% aluminum and 85 wt% iron, is used as absorbing filler of composite RAMs. The raw FeSiAl, having average particle sizes between 10 μm and 40 μm, labeled as “a1” in Table 1. The flaky FeSiAl powders were obtained by ball milling the raw FeSiAl for 24 h, labeled as “a2” in Table 1. The ball-to-powder mass ratio was 10:1. Anhydrous alcohol mixed with a few drops of oleic acid was used as a process of controlling agent to avoid the aggregation and oxidation of particles. Commercially available MnO2 powders were purchased from Tianjin Tianli Chemical Reagent Co., Ltd, which were mixed into the FeSiAl flakes by mass ratios of flaky FeSiAl/ MnO2 ¼7/3 (labeled as “a3”), 1/1 (“a4”) and 3/7 (“a5”), as shown in Table 1. The morphology of microstructure of the samples were studied by scanning electron microscopy (ESEM, quanta 200, FEI). The raw FeSiAl, flaky FeSiAl, and flaky FeSiAl/MnO2 composite powders with a mass ratio of 60 wt%, were added into silicone rubber. Mixed and vulcanized by double roll rubber. mixing mill and flat vulcanizer respectively, they were processed into microwave absorbing sheets. The as-prepared composite powders were homogenously dispersed in silicone rubber with a total mass fraction of 60 wt% (powder) and processed into toroidal-shape specimens with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm and thickness of 2 mm for transmission/reflection measurements. The complex relative dielectric permittivity and magnetic permeability (εr ¼ ε′ iε″, μr ¼ μ′ iμ″) of toroidal specimens were obtained using a vector network analyzer (HP8720B) in the range of 1–18 GHz. The RL of electromagnetic waves of as-prepared samples backed by a perfect conductor can be calculated from the measured complex relative permittivity and permeability data for the given frequency and absorber thickness by using the following equations [16]:
Zin = Z 0 μ /ε tanh [j(2πfd/c ) εμ ]
(1)
R = 20 log |(Zin − Z 0)/(Zin + Z 0)|
(2)
Where Zin is the input impedance when the electromagnetic wave incidence is normal to the absorbing material, f is the frequency of the electromagnetic wave, d is the thickness of the absorber layer, c is the velocity of light, and Z0 is the impedance of free space.
3. Results and discussion The morphologies of the raw FeSiAl, the flaky FeSiAl and the flaky FeSiAl/MnO2 composite absorbers with different FeSiAl/ MnO2 mass ratios are shown in Fig. 1. The raw FeSiAl alloy powder in an irregular shape with particle size of 10–40 μm can be seen clearly in Fig. 1(a). The morphologies of the FeSiAl alloy powders become flaky by the effect of shearing action and knock-on effect during ball-milling process in Fig. 1(b). Thicknesses of the flaky Table 1 Sample number and their description. Sample number
Description
a1 a2 a3 a4 a5
The The The The The
raw FeSiAl flaky FeSiAl/ball milling 24 h flaky FeSiAl/MnO2 ¼7:3/ball milling 24 h flaky FeSiAl/MnO2 ¼1:1/ball milling 24 h flaky FeSiAl/MnO2 ¼3:7/ball milling 24 h
FeSiAl alloy powders are less than 2 μm. The eddy current effect is reduced with the particle shape changing from irregular shape to flaky one. Such morphology of flaky FeSiAl absorbers is beneficial to acquire relatively high resonance frequency. In Fig. 1(c), (d) and (e), we can observe that the sizes of the massive MnO2 are nearly between 20 μm and 40 μm and the FeSiAl flakes around them. In all, aspect ratios such as length/thickness of the milled FeSiAl are much higher than that of the raw ones. As is shown in Fig. 1(c), (d) and (e), we can observe that there also appear massive MnO2 particles besides flaky FeSiAl and the massive MnO2 particles content increase gradually as the mass ratio of FeSiAl/MnO2 varies from 7:3 to 3:7. In Fig. 2, it can be observed that the complex permittivity and permeability of the raw FeSiAl, the flaky FeSiAl, and the flaky FeSiAl/MnO2 absorbing materials with different mass ratios including 7:3, 1:1, and 3:7 filled in silicone rubber with the same total weight fraction of 60 wt%. It can be seen from Fig. 2 that the real and imaginary part of permittivity and permeability spectra for the flaky FeSiAl absorbing materials are all higher than the raw FeSiAl absorbing materials in the frequency range of 1–18 GHz, respectively. After ball-milling, the real part of relative permeability increased by a small degree in the frequency range of 1– 9 GHz, whereas the imaginary part increased by a great margin over the whole frequency range of 1–18 GHz, especially in the range of 1–7 GHz, contributing to a great magnetic loss. As is shown in Fig. 2(a), it can be seen that the ε′ decrease in turn from a2 to a5. More significantly, it is found that the values of ε″ is obviously increased as the mass content of MnO2 in the FeSiAl/MnO2 composite absorber increases from 0:10, 3:7, 1:1 and 7:3 during 1–9 GHz, but it decreases at the frequency range of 9– 18 GHz in Fig. 2(b). Sample a2 exhibits relatively high ε′ and ε″ due to the large polarization between the FeSiAl flakes with large surface area [11,17,18]. It can be seen that both the ε′ and ε″ of the sample a3–a5 substantially decrease and are inclined to be independent of frequency. This phenomenon is probably related to higher electrical resistivity and fewer flakes of the composite particles, which determine the interfacial polarization of free charge on the conductor/insulator interface for the composite RAM with conductive particles dispersed in an insulated matrix [19]. As is shown in Fig. 2. (c), the real part of complex permeability μ′ of declines from 3.12, 1.97, 1.69, 1.38 to around 0.9 with increasing of the frequency in the 1–18 GHz range for a2, a3, a4 and a5, respectively. The imaginary part of permeability μ″ of a2 initially increases with frequency increases. Meanwhile, the μ″ of a4 and a5 curves exhibit distinct peak in a broad frequency range due to natural resonance and exchange resonance coming from the large anisotropy field and wide size range of the composite particles. It can be found that a peak around 4 GHz appears in the curves of μ″ and the frequency where the peaks stay remains almost unvaried with the mass ratio of FeSiAl/MnO2 increases. The μ ′ and μ″ decrease when the mass ratio of MnO2 increases, which could be attributed to the reduction of the specific saturation magnetization and the increased fraction of nonmagnetic particles. It is pointed out that large value of μ′ and μ″ is essential for microwave absorbers to realize good absorbing properties as a result of good impedance matching and large magnetic loss. Fig. 3 shows the RL for the composites of 60 wt% as-prepared flaky FeSiAl (a) and flaky FeSiAl/MnO2 absorbers with mass ratio 7:3 (b), 1:1 (c) and 3:7 (d) at different thickness. Obviously, all samples exhibit that the absorption peak moves toward the low frequency direction with the increase of sample thickness. It can be seen from Fig. 3(a), an optimal RL of 34.3 dB is reached at 3.2 GHz for a layer of 3 mm thickness, while the absorption exceeding 20 dB obtained in the 2.3–4.1 GHz range in the thickness from 2.5 mm to 4 mm. Through analyzing the data, the sample a2 has the strong absorption peaks of sample a2 stay
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Fig. 1. ESEM images of (a) raw FeSiAl, (b) flaky FeSiAl and flaky FeSiAl/MnO2 with FeSiAl: MnO2 mass ratios of 7:3 (c), 1:1 (d) and 3:7 (e).
Fig. 2. (a) Real part and (b) imaginary part of the complex permittivity, (c) the real part and (d) imaginary part of complex permeability of the raw FeSiAl, flaky FeSiAl, and the milled flake-shaped FeSiAl/MnO2 composites.
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Fig. 3. The simulated RL for the flaky FeSiAl (a), the flaky FeSiAl /MnO2 composites with mass ratio of (b) FeSiAl/MnO2 ¼ 7:3, (c) 1:1 and (d) 3:7 in different thickness from 1.5–5 mm.
Table 2 Electromagnetic wave absorption characteristics of raw FeSiAl, flaky FeSiAl and flaky FeSiAl/MnO2 composites with different mass ratios. Sample
RLmin (dB)
fmin (GHz)
f20 (GHz) (RL o 20 dB)
f10max (GHz) (RL o 10 dB, single layer)
d10 (mm) (RL o 10 dB)
a1 a2 a3 a4 a5
35.9 34.3 33.2 41.8 23.6
18 3.2 7.2 8.2 7.8
11.6–18 2.4–4 4.2–15.4 3.5–16.5 5.4–9.9
9.8–15.1 5.3–8 11.4–18 13–18 10.8–14.7
2 1.5 1.5 1.5 2
RLmin and fmin mean the minimum reflection loss and its corresponding frequency, respectively. f20 means the frequency range of exceeding 20 dB and f10 stands for the bandwidth of exceeding 10 dB with its corresponding the thickness (d10).
around the low frequency region with the thickness of 1.5–5 mm. As the mass ratio of the MnO2/FeSiAl absorber increases from 0:1 to 3:7, it can be observed that the resonance peaks of sample a3 in Fig. 3(b) move towards to the high frequency region. Moreover, the frequency range of the absorption exceeding 20 dB increases from the range of 2.3–4.1 GHz range to the range of 4.1–15.5 GHz range with the thickness range from 2.5–4 mm to 1.5–4 mm. Compared with the sample a3, the frequency range of the sample a4 exceeding 20 dB increases from the range of 4.1–15.5 GHz range to the range of 3.5 16.5 GHz range with the thickness range from 1.5–4 mm to 1.5–5 mm in Fig. 3(c). For these samples, the RL peak actually corresponds to a practical matching point on the present condition, hence, the lower value of ε′ and μ′, the higher frequency of RL peak appears [18]. These results are of significance since the absorption frequency ranges of the flaky FeSiAl/MnO2 composites can be adjusted easily by changing the thickness of the composite RAM. Moreover, the
absorption bandwidth with the RL exceeding 10 dB of sample a3 for the thickness of 1.5 mm can reach up to 6.6 GHz (from 11.4 to 18 GHz), which are all wider than the reported FeSiAl(Cr) [20] ( 10 dB bandwidth is 2 GHz) and the reported FeSiAl flakes with nylon [21] ( 10 dB bandwidth is 0 GHz) with the same thickness and the reported FeSiAl/graphite ( 10 dB bandwidth is 2.4 GHz (from 13.68 to 16.08 GHz)) [22]. In Fig. 3(d), the whole absorption peaks of sample 5 move towards the high frequency region a little. But the absorption intensity has weakened, compared with sample 4 in Fig. 3(c). Obviously the absorption values exceeding 20 dB were obtained only in the frequency range of 5.5–8.1 GHz for thickness of 3–4 mm. The electromagnetic wave characteristics of all the samples are summarized in Table 2. The absorption properties of the two samples above have distinct advantages compared with the milling FeSiAl [10], the FeSiAl/ nylon [21] and the FeSiAl/ graphite [22]. The RLs for melt-
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quenching FeSiAl [7], Fe–Si–Al/BaTiO3/Nd2Fe14B [23], FeSiAl and carbonous materials [8], annealing and milling FeSiAl [9] are relatively poor, with RL 20 dB in the 1–18 GHz range. In all, we can know that the FeSiAl/MnO2 composites possess the broader frequencies ranges (RL 20 dB) and the thinner absorber matching thickness (RL 20 dB). A RL value of 20 dB corresponds to 99% attenuation of the electromagnetic wave, which can be considered as potentially effective absorption material. Therefore, it is available that the FeSiAl/MnO2 composite RAMs can realize better absorbing performances over the whole frequency range due to their modified complex permittivity and permeability from the mutual contributions of magnetic and magnetic absorbers.
4. Conclusion In conclusion, flaky FeSiAl powders were obtained by ballmilling raw FeSiAl powders. The flaky FeSiAl powders and the flaky FeSiAl/MnO2 composite powders were used as the absorbent fillers based on silicone rubber matrix. The results demonstrated that the sample a2 has high magnetic loss in the low frequency region because of satisfactory shape of flake. The minimum RL of sample a4 reached up to 41.8 dB, at 8.2 GHz with a matching thickness of 2.5 mm. Moreover, its reflection loss values exceeding 20 dB (99% power absorption) can be obtained in 3.5–16.5 GHz with choosing an appropriate layer thickness between 1.5 mm and 5 mm. In addition, the absorption bandwidth with the reflection loss exceeding 10 dB of sample a3 for the thickness of 1.5 mm can reach up to the broadest range of 6.6 GHz (from 11.4 to 18 GHz), which covers half of the X-band and the whole Ku-band, indicating that the proposed flaky FeSiAl/MnO2 composites can be used as an attractive candidate for a thin microwave absorbing material.
Acknowledgments The authors gratefully acknowledge financial support from the Natural Science Foundation of Hubei Province, China (Grant number 2012FFB02210) and the National Natural Science Foundation of China (Grant no. 61172003)
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