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Electromagnetic and microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size
Ceramics International xxx (xxxx) xxx–xxx
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Electromagnetic and microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size Liang Zhoua,b,∗, Jiaojiao Yua,∗∗, Zhenjun Wanga,b, Hongbo Wanga,b,∗∗∗, Julong Huanga, Wenyu Mua, Hongqian Zhenga a b
School of Materials Science and Engineering, Chang'an University, Xi'an, 710064, China Engineering Research Center of Transportation Materials of Ministry of Education, Chang'an University, Xi'an, 710061, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Permittivity Permeability Microwave absorption FeSiAl Flaky graphite
The increasing electromagnetic interference problems have drawn much attention to microwave absorbing materials. To satisfy the needs of practical application, FeSiAl and flaky graphite filled Al2O3 composites were sintered by hot-pressing for microwave absorption application. The effect of FeSiAl particle size on the electromagnetic and microwave absorption properties was investigated in the X-band (8.2–12.4 GHz). The results show that the dielectric properties enhance significantly with increasing FeSiAl particle size, which is attributed to the increased interfacial polarization and conductance loss. As a result of the favorable impedance matching and appropriate electromagnetic attenuation, the reflection loss (RL) of the composites filled with 25–48 μm flaky FeSiAl achieves -15.2 dB at 10.6 GHz and the effective absorption bandwidth (RL < -10 dB) is 1.2 GHz in 10.0–11.2 GHz with a matching thickness of 1.0 mm. It indicates that FeSiAl and flaky graphite filled Al2O3 composites are potential candidates for thin-thickness microwave absorbing materials, and the microwave absorption properties can be enhanced by adjusting absorbent particle size.
1. Introduction With the extensive application of wireless communication devices in the military, commercial and civil fields, more and more electromagnetic interference problems have appeared, which poses a threat to human health and normal operation of the sensitive electronic devices [1,2]. Because of the capability of attenuating electromagnetic waves by interference and transforming electromagnetic energy into heat energy, the ideal microwave absorbing materials with light weight, broad width, thin thickness and strong absorption have gained more attention in the last few years [3]. Compared with microwave absorbing coatings, structure-type microwave absorbing composites integrate microwave absorption and load-bearing capacity, which have become a researching focus. In the past decades, many kinds of microwave absorbing composites such as RGO/silica textile/PF [4], Fe@CNT/SiC [5] and PVDF/Ni-chains [6] have been investigated to satisfy the practical application. Recently, microwave absorbing composites with magnetic loss absorbents have been widely exploited. Among the magnetic loss absorbents, FeSiAl alloys have got a lot of attention because of its low cost,
high magnetization and excellent temperature stability [7]. However, FeSiAl alloys usually bear a rapid reduction of the complex permeability in gigahertz (GHz) frequency band in virtue of the Snoek's limit. Fortunately, flaky FeSiAl possesses a much larger Snoek's limit due to its shape anisotropy. In addition, its high aspect ratio has been proven to help enhance the magnetic loss in the high frequency range, which is favorable to suppressing the eddy current effect and making it excellent candidate as microwave absorbing material [8]. Nevertheless, the practical application of the composites filled with single flaky FeSiAl absorbent is still limited owing to the relative high density and narrow effective absorption bandwidth. In order to meet the practical requirements, the most promising approach to solve the problem is constructing hybrid loss composites with other absorbents, which behave dielectric or/and conductance loss mechanisms. Therefore, dielectric or conductance loss absorbents (such as CNTs [9,10], flaky graphite [11], ZnO [12] and so on) have been synchronously introduced as the addictive to improve the microwave absorption properties. Among numerous absorbents, flaky graphite (FG) is proven to be a prospective candidate for the purpose of microwave absorption application owing to its physicochemical stability, large conductance loss, low-cost, and
∗
Corresponding author. School of Materials Science and Engineering, Chang'an University, Xi'an, 710064, China. Corresponding author. ∗∗∗ Corresponding author. School of Materials Science and Engineering, Chang'an University, Xi'an, 710064, China. E-mail addresses: [email protected] (L. Zhou), [email protected] (J. Yu), [email protected] (H. Wang). ∗∗
https://doi.org/10.1016/j.ceramint.2019.10.155 Received 27 August 2019; Received in revised form 15 October 2019; Accepted 16 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liang Zhou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.155
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Tescan, Vega3 SBH, Brno, Czech) using backscattered electron (BSE) mode, and the energy dispersive spectrometer (EDS) attached with SEM was used for the determination of the elemental distributions. The complex permittivity and complex permeability were tested in the Xband by the rectangle wave-guide method with a E8362B PNA vector network analyzer. The test samples were processed into standard rectangular blocks with 22.86 mm length and 10.16 mm width. On the basis of the transmission line theory, the formula for calculating the reflection loss (RL) of FeSiAl and flaky graphite filled Al2O3 composites is expressed as follows [16,17].
wide source [13]. In our previous work, FeSiAl/Al2O3 composites have been fabricated and the influence of FeSiAl content on the complex permittivity and microwave absorption properties were systematically investigated in the X-band [14]. It was found that the absorbent content plays a significant role in the complex permittivity values, and the composite filled with appropriate absorbent content exhibits the desirable microwave absorption properties. However, the regulation of microwave absorption properties from the aspect of absorbent content is limited. As is well known, interfacial polarization has a dramatic impact on the microwave absorption properties, which could be further enhanced by means of increasing the interfacial area between the absorbents and the insulating matrix [15]. It is worth noting that the decrease of absorbent size is beneficial to increasing the interfacial area. Furthermore, the existence of more interfaces will give rise to the multi-scattering of electromagnetic waves within the composites, which contributes to the enhanced electromagnetic attenuation. Based on the above considerations, the microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites can be also further improved by adjusting the absorbent particle size. Therefore, the present work is aimed at expanding the regulation scope of the microwave absorption properties for FeSiAl and flaky graphite filled Al2O3 composites by adjusting the absorbent particle size. The influence of FeSiAl particle size on the complex permittivity and complex permeability were investigated in the X-band. The microwave absorption properties of the composites with different FeSiAl particle size were studied and the possible mechanisms were also discussed. It is found that the investigated FeSiAl and flaky graphite filled Al2O3 composites with appropriate FeSiAl particle size are potential candidates as ideal microwave absorbing materials.
RLdB = 20lg (Z in − Z0)/(Z in + Z0)
(1)
Z in = Z 0 μr / εr tanh[j(2πfd/c) μr εr ]
(2)
where Z 0 represents the input impedance of the free space, Z in represents the input impedance of the interfaces between the absorber and the free space, εr and μr represent the relative permittivity and permeability of the absorber, f represents the frequency of incident electromagnetic waves, c represents the light velocity in the free space, and d represents the thickness of the absorber. 3. Results and discussion 3.1. Phase composition and microstructure The X-ray diffraction patterns of FeSiAl and flaky graphite filled Al2O3 composites are illustrated in Fig. 1. The diffraction peaks with 2θ at 44.996°, 65.530°, and 83.024° exhibit the characteristic reflections of α-Fe (Si, Al) (JCPDS No. 45-1206) phase with the bcc structure from (110), (200), and (211) planes. Furthermore, it can be found that the composites contain α-Fe (Si, Al), graphite (JCPDS No. 99-0057) and αAl2O3 (JCPDS No. 80-0786) crystalline phases without any other phases, indicating that no chemical reaction among the components occurs during hot-pressed sintering. It is worth mentioning that the well reservation of FeSiAl and FG absorbents in the process of hot-pressed sintering ensured that the electromagnetic properties of the composites were retained. Fig. 2 illustrates the SEM images and EDS mapping images of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size. According to the backscattered electron SEM image analysis shown in Fig. 2(a–c), the FeSiAl phase presents color bright, the Al2O3 matrix appears color gray, while the FG seems color black. As further observed in Fig. 2(d), the flake-shaped FeSiAl absorbent is randomly distributed in the Al2O3 matrix based on the Fe-Si and Al-O element mapping results. The flake-shaped black regions possess higher carbon content,
2. Experimental 2.1. Materials Fe-9.6Si-5.4Al alloy powders (purity > 99.0%) with flake shape were purchased from Xi'an Hongyuan Electronic Materials Co., Ltd. Flaky graphite (FG, purity > 99.9%) was < 45 μm in diameter and supplied by Qingdao Risheng Graphite Co., Ltd. α-Al2O3 (purity > 99.9%) with a mean particle size of 0.8 μm was used as matrix and provided by Tangshan Sipurui Functional Ceramic Materials Co., Ltd. 2.2. Preparation of the composites The mixtures containing 95 wt% Al2O3 powders, 1 wt% FG and 4 wt % FeSiAl powders with different FeSiAl particle size were milled for 8 h using a planetary ball mill with Al2O3 jars and balls in alcohol medium. The mass ratio of ball to powder was 5:1, and the ball-milling speed was 250 rpm. The ball-milling process was carried out at room temperature in air. After the mixtures were oven-dried at 80 °C for 3 h in air, they were screened by a screen of 100 mesh. Subsequently, the mixed powders were placed into a graphite grinder. Then, the hot-pressed sintering was carried out in vacuum with the sintering temperature of 1350 °C and a load of 30 MPa. The heating rates were 10 °C/min, 10 °C/ min and 5 °C/min at the temperature ranges of room temperature900 °C, 900–1200 °C and > 1200 °C, respectively. The samples were kept heating for 90 min at 1350 °C, and finally followed by furnace cooling. The surfaces of the hot-pressed sintered composites contaminated by graphite were removed by sand paper. 2.3. Characterization The crystalline phase was identified by X-ray diffraction (XRD, Rigaku, D/Max2500, Tokyo, Japan) with a Cu-Kα radiation at 40 kV and 40 mA operated in the 2θ range of 20o-90°. The morphology of the polished surface was detected by scanning electron microscope (SEM,
Fig. 1. XRD patterns of FeSiAl and flaky graphite filled Al2O3 composites. 2
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Fig. 2. SEM images of FeSiAl and flaky graphite filled Al2O3 composites with (a) 18–25 μm; (b) 25–48 μm; (c) 48–75 μm FeSiAl particle size, and (d) EDS element mappings.
values of the composites filled with 18–25 μm and 25–48 μm FeSiAl present an increasing tendency with increasing the frequency in 8.2–12.4 GHz. However, as the FeSiAl particle sizes are 25–48 μm and 48–75 μm, the ε′′ values show a downward trend in 8.2–10.6 GHz and then increase in 10.6–12.4 GHz with increasing the frequency. This phenomenon exhibits typically frequency dispersion effect, which is favorable to expanding the microwave absorption bandwidth of the composites. As we all known, the polarization mechanisms are divided into interfacial polarization, orientational polarization, electronic polarization, and ionic polarization [20]. For the investigated composites, interfacial polarization generally occurs in the interfaces among FeSiAl, FG and Al2O3 for the heterogeneous composites filled with conductive absorbents dispersed in the insulating Al2O3 matrix. Moreover, on the basis of the aforementioned discussions, the possibility of contact among adjacent absorbent particles for the composites with larger FeSiAl particles is prone to form longer electron shifting paths, which stimulates the further polarization. Thus, the above two polarization mechanisms contribute to the enhanced complex permittivity, and the formation of longer electron shifting paths plays a dominant role in the
indicating that FG is present in these regions or the polishing process with abrasive paste results in carbon remaining in these areas. Moreover, the interfaces among FeSiAl, FG and Al2O3 within the composites are beneficial to the generation of interfacial polarization. Meanwhile, the possibility of contact among FeSiAl or/and FG particles is prone to form, especially for the composites filled with larger absorbent particle size. 3.2. Electromagnetic properties The complex permittivity and complex permeability play a dominant role in determining the microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites. To indicate the possible mechanisms of microwave absorption properties, the real (ε′) and imaginary (ε ′′) parts of the complex permittivity for the composites with different FeSiAl particle size are exhibited in Fig. 3. Both the ε′ and ε′′ values increase evidently with increasing the FeSiAl particle size in the X-band. It is noteworthy that the similar variation tendency has also been observed in other reports [18,19], indicating that the complex permittivity relies heavily on the absorbent particle size. Besides, the ε′
Fig. 3. The (a) real (ε′) and (b) imaginary (ε ′′ ) parts of complex permittivity for FeSiAl and flaky graphite filled Al2O3 composites with varying FeSiAl particle size. 3
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Fig. 4. The (a) real ( μ′) and (b) imaginary ( μ′′ ) parts of complex permeability for FeSiAl and flaky graphite filled Al2O3 composites with varying FeSiAl particle size.
Fig. 5. Calculated RL of FeSiAl and flaky graphite filled Al2O3 composites with different (a) FeSiAl particle size (d = 1.0 mm) and (b) composite thickness (FeSiAl particle size: 25–48 μm).
addition, there is a resonance peak in the μ′′ curves, which is attributed to the natural resonance of ferromagnetic materials. As is well known, magnetic loss mainly consists of natural resonance, exchange resonance, domain-wall displacement, hysteresis loss and eddy current loss in the electromagnetic wave frequency region [23]. For FeSiAl and flaky graphite filled Al2O3 composites, flake-shaped FeSiAl effectively suppresses the eddy current effect, indicating that the magnetic loss in the investigated composites may be ascribed to the natural resonance and exchange resonance. And the increase of magnetic dipole moment caused by the increase of FeSiAl particle size leads to the enhancement of complex permeability. In addition, the μ′ values of the composites with larger FeSiAl particle size decrease with increasing the frequency, which is induced by the movement and relaxation of magnetic domain wall. In comparation, the microwave absorbing mechanisms of FeSiAl and flaky graphite filled Al2O3 composites mainly rely on conductance loss and dielectric relaxation loss due to their lower variation range of complex permeability.
ε′ values [21]. On the other hand, the interfacial polarization contributes to the ε′, and the relevant relaxation loss enhances the ε ′′ values. Generally, the dielectric loss is mainly associated with conductance loss and relaxation loss, which can be expressed according to the Debye theory as follows [22]:
′′ ε′′ = σ / ωε0 + εrelax
(3)
where σ represents the electrical conductivity, ω represents the angular ′ ′ refrequency, ε0 represents the permittivity of free space, and εrelax presents the relaxation loss. In comparison, the ε′′ values of the composites filled with smaller FeSiAl particles are mainly resulted from the ′ ′ component corresponding to polarization. For the composites with εrelax larger FeSiAl particles, the conductive networks are easily formed and the σ / ωε0 component is obviously improved, resulting in the enhancement of conductance loss and thus increasing the ε ′′ values. The real ( μ′) and imaginary ( μ′′) parts of complex permeability for the composites with different FeSiAl particle size are exhibited in Fig. 4. It can be observed from Fig. 4(a) that the μ′ values increase with the increase of FeSiAl particle size in the low frequency region, while they exhibit an opposite tendency in the high frequency range. Meanwhile, the μ′′ values increase with increasing flaky FeSiAl particle size. In
3.3. Microwave absorption properties In order to study the effects of the FeSiAl particle size and the thickness on the microwave absorption properties, the reflection loss 4
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presented in Fig. 6(a). It is easy to find that the input impedance of the composite filled with 18–25 μm FeSiAl exhibits an increasing tendency as the frequency increases in the X-band. Meanwhile, the composites with 25–48 μm and 48–75 μm FeSiAl particle size present similar input impedance, and their input impedance increases in low frequency and decreases in high frequency with increasing the frequency. In comparison, the composite filled with 18–25 μm FeSiAl exhibits the highest input impedance value, which is most close to 377 Ω and exhibits the superior impedance matching characteristic. In addition, the attenuation coefficient α represents the electromagnetic attenuation capacity of microwave absorbing materials, which is calculated from the following formula [26]:
(RL) of FeSiAl and flaky graphite filled Al2O3 composites was calculated and the RL curves were shown in Fig. 5. Generally, the RL value of -10dB corresponds to 90% absorption of incident electromagnetic waves, and hence the frequency ranges below -10dB are considered as effective absorption bandwidth. As observed in Fig. 5(a), the minimum reflection loss (RLmin) increases with increasing FeSiAl particle size. In comparison, the composite filled with 25–48 μm FeSiAl exhibits preferable microwave absorption properties with effective absorption bandwidth of 1.2 GHz in 10.0–11.2 GHz and RLmin of -15.2 dB at 10.6 GHz. The calculated RL values of the composites with varying thickness as the FeSiAl particle size is 25–48 μm are illustrated in Fig. 5(b). It is obvious that the RLmin values decrease firstly, and then increase with increasing the thickness from 0.8 to 1.3 mm. Furthermore, it is significantly observed from Fig. 5 that the RLmin values turn to low frequency region with increasing the FeSiAl particle size and the thickness. This phenomenon can be explained by quarter-wavelength theory. The dependence of the matching frequency (fm) with the thickness (dm) is expressed as follows [24]:
fm =
c × 4dm
α=
(μ′′ε ′′ − μ′ε′) +
2
(μ′′ε ′′ − μ′ε′) + (μ′′ε′ − μ′ε ′′)
2
(5)
The calculated attenuation coefficients of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size are exhibited in Fig. 6(b). It could be clearly found that the attenuation coefficients increase with the increase of FeSiAl particle size, and the composite filled with 48–75 μm FeSiAl possesses the maximal attenuation coefficient, indicating that the composites filled with larger sized FeSiAl particle can attenuate the electromagnetic waves much more efficiently. However, as exhibited in Fig. 5(a), the composite filled with 48–75 μm FeSiAl does not possess the most excellent microwave absorption properties, which is attributed to the fact that few incident electromagnetic waves enter its interior due to the poor impedance matching. In comparison, the composite filled with 25–48 μm FeSiAl exhibits the desirable microwave absorption properties due to its favorable impedance matching and appropriate electromagnetic wave attenuation abilities.
−1
tan2δμ ⎞ 1 ⎛ ⎜1 + ⎟ 8 ⎠ ε′μ′ ⎝
2 πf × c
(4)
where δμ represents the magnetic loss tangent. From Eq. (4), the matching frequency fm of RL peak is inversely proportional to the dm and ε′μ′ . On the basis of the above analysis, the ε′ values increase greatly while the μ′ values present a lower variation range with increasing FeSiAl particle size, and the obviously enhanced ε′ values play a dominant role in the increasing tendency of the overall ε′μ′ values with increasing FeSiAl particle size. Therefore, it is logical that fm decreases evidently with increasing the FeSiAl particle size and the thickness as shown in Fig. 5(a) and (b), respectively. On the basis of this phenomenon, the matching frequency of the composites can by adjusted by manipulating the FeSiAl particle size and the thickness according to the demands of the practical applications. Impedance matching and attenuation coefficient are important parameters to evaluate the microwave absorbing properties, which can help us to further illustrate the microwave absorbing mechanisms. The impedance matching Z (Z= Zin/ Zo ) determines the degree of incident electromagnetic waves entering the absorber. The closer Z value gets to 1, the more electromagnetic waves enter the absorber. In other words, the intrinsic impedance of the electromagnetic wave absorber should be as close as possible to the theoretic input impedance 377 Ω of the free space [25]. According to Eq. (2), the input impedance values of the composites with varying FeSiAl particle size were calculated and
4. Conclusions In this paper, FeSiAl and flaky graphite filled Al2O3 composites have been fabricated by a facile method for microwave absorption applications. The complex permittivity is efficiently improved with the increase of FeSiAl particle size, which contributes to the strong attenuation ability because of the increased interfacial polarization and conductance loss. Due to the favorable input impedance and appropriate microwave attenuation, the reflection loss of FeSiAl and flaky graphite filled Al2O3 composites with 25–48 μm FeSiAl particle size achieves -15.2 dB at 10.6 GHz when the matching thickness is 1.0 mm. In conclusion, FeSiAl and flaky graphite filled Al2O3 composites could obtain the effective microwave absorption in the X-band by means of
Fig. 6. (a) Input impedance and (b) attenuation coefficients of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size (d = 1.0 mm). 5
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adjusting the absorbent particle size, and the composites with outstanding comprehensive performance are desirable candidates for microwave absorption applications.
[10]
[11]
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[12]
[13]
Acknowledgements [14]
This work was financially supported by the National Natural Science Foundation of China (No. 51302018), the Fundamental Research Funds for the Central Universities from Chang'an University (No. 300102319309, 300102219509 and 300102319501), Key Research and Development Program in Shaanxi Province of China (No. 2019GY174), and the Chang'an Scholar Program of Chang'an University (No. 201807CQ014).
[15]
[16]
[17]
[18]
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Electromagnetic and microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size Liang Zhoua,b,∗, Jiaojiao Yua,∗∗, Zhenjun Wanga,b, Hongbo Wanga,b,∗∗∗, Julong Huanga, Wenyu Mua, Hongqian Zhenga a b
School of Materials Science and Engineering, Chang'an University, Xi'an, 710064, China Engineering Research Center of Transportation Materials of Ministry of Education, Chang'an University, Xi'an, 710061, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Permittivity Permeability Microwave absorption FeSiAl Flaky graphite
The increasing electromagnetic interference problems have drawn much attention to microwave absorbing materials. To satisfy the needs of practical application, FeSiAl and flaky graphite filled Al2O3 composites were sintered by hot-pressing for microwave absorption application. The effect of FeSiAl particle size on the electromagnetic and microwave absorption properties was investigated in the X-band (8.2–12.4 GHz). The results show that the dielectric properties enhance significantly with increasing FeSiAl particle size, which is attributed to the increased interfacial polarization and conductance loss. As a result of the favorable impedance matching and appropriate electromagnetic attenuation, the reflection loss (RL) of the composites filled with 25–48 μm flaky FeSiAl achieves -15.2 dB at 10.6 GHz and the effective absorption bandwidth (RL < -10 dB) is 1.2 GHz in 10.0–11.2 GHz with a matching thickness of 1.0 mm. It indicates that FeSiAl and flaky graphite filled Al2O3 composites are potential candidates for thin-thickness microwave absorbing materials, and the microwave absorption properties can be enhanced by adjusting absorbent particle size.
1. Introduction With the extensive application of wireless communication devices in the military, commercial and civil fields, more and more electromagnetic interference problems have appeared, which poses a threat to human health and normal operation of the sensitive electronic devices [1,2]. Because of the capability of attenuating electromagnetic waves by interference and transforming electromagnetic energy into heat energy, the ideal microwave absorbing materials with light weight, broad width, thin thickness and strong absorption have gained more attention in the last few years [3]. Compared with microwave absorbing coatings, structure-type microwave absorbing composites integrate microwave absorption and load-bearing capacity, which have become a researching focus. In the past decades, many kinds of microwave absorbing composites such as RGO/silica textile/PF [4], Fe@CNT/SiC [5] and PVDF/Ni-chains [6] have been investigated to satisfy the practical application. Recently, microwave absorbing composites with magnetic loss absorbents have been widely exploited. Among the magnetic loss absorbents, FeSiAl alloys have got a lot of attention because of its low cost,
high magnetization and excellent temperature stability [7]. However, FeSiAl alloys usually bear a rapid reduction of the complex permeability in gigahertz (GHz) frequency band in virtue of the Snoek's limit. Fortunately, flaky FeSiAl possesses a much larger Snoek's limit due to its shape anisotropy. In addition, its high aspect ratio has been proven to help enhance the magnetic loss in the high frequency range, which is favorable to suppressing the eddy current effect and making it excellent candidate as microwave absorbing material [8]. Nevertheless, the practical application of the composites filled with single flaky FeSiAl absorbent is still limited owing to the relative high density and narrow effective absorption bandwidth. In order to meet the practical requirements, the most promising approach to solve the problem is constructing hybrid loss composites with other absorbents, which behave dielectric or/and conductance loss mechanisms. Therefore, dielectric or conductance loss absorbents (such as CNTs [9,10], flaky graphite [11], ZnO [12] and so on) have been synchronously introduced as the addictive to improve the microwave absorption properties. Among numerous absorbents, flaky graphite (FG) is proven to be a prospective candidate for the purpose of microwave absorption application owing to its physicochemical stability, large conductance loss, low-cost, and
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Corresponding author. School of Materials Science and Engineering, Chang'an University, Xi'an, 710064, China. Corresponding author. ∗∗∗ Corresponding author. School of Materials Science and Engineering, Chang'an University, Xi'an, 710064, China. E-mail addresses: [email protected] (L. Zhou), [email protected] (J. Yu), [email protected] (H. Wang). ∗∗
https://doi.org/10.1016/j.ceramint.2019.10.155 Received 27 August 2019; Received in revised form 15 October 2019; Accepted 16 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Liang Zhou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.155
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Tescan, Vega3 SBH, Brno, Czech) using backscattered electron (BSE) mode, and the energy dispersive spectrometer (EDS) attached with SEM was used for the determination of the elemental distributions. The complex permittivity and complex permeability were tested in the Xband by the rectangle wave-guide method with a E8362B PNA vector network analyzer. The test samples were processed into standard rectangular blocks with 22.86 mm length and 10.16 mm width. On the basis of the transmission line theory, the formula for calculating the reflection loss (RL) of FeSiAl and flaky graphite filled Al2O3 composites is expressed as follows [16,17].
wide source [13]. In our previous work, FeSiAl/Al2O3 composites have been fabricated and the influence of FeSiAl content on the complex permittivity and microwave absorption properties were systematically investigated in the X-band [14]. It was found that the absorbent content plays a significant role in the complex permittivity values, and the composite filled with appropriate absorbent content exhibits the desirable microwave absorption properties. However, the regulation of microwave absorption properties from the aspect of absorbent content is limited. As is well known, interfacial polarization has a dramatic impact on the microwave absorption properties, which could be further enhanced by means of increasing the interfacial area between the absorbents and the insulating matrix [15]. It is worth noting that the decrease of absorbent size is beneficial to increasing the interfacial area. Furthermore, the existence of more interfaces will give rise to the multi-scattering of electromagnetic waves within the composites, which contributes to the enhanced electromagnetic attenuation. Based on the above considerations, the microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites can be also further improved by adjusting the absorbent particle size. Therefore, the present work is aimed at expanding the regulation scope of the microwave absorption properties for FeSiAl and flaky graphite filled Al2O3 composites by adjusting the absorbent particle size. The influence of FeSiAl particle size on the complex permittivity and complex permeability were investigated in the X-band. The microwave absorption properties of the composites with different FeSiAl particle size were studied and the possible mechanisms were also discussed. It is found that the investigated FeSiAl and flaky graphite filled Al2O3 composites with appropriate FeSiAl particle size are potential candidates as ideal microwave absorbing materials.
RLdB = 20lg (Z in − Z0)/(Z in + Z0)
(1)
Z in = Z 0 μr / εr tanh[j(2πfd/c) μr εr ]
(2)
where Z 0 represents the input impedance of the free space, Z in represents the input impedance of the interfaces between the absorber and the free space, εr and μr represent the relative permittivity and permeability of the absorber, f represents the frequency of incident electromagnetic waves, c represents the light velocity in the free space, and d represents the thickness of the absorber. 3. Results and discussion 3.1. Phase composition and microstructure The X-ray diffraction patterns of FeSiAl and flaky graphite filled Al2O3 composites are illustrated in Fig. 1. The diffraction peaks with 2θ at 44.996°, 65.530°, and 83.024° exhibit the characteristic reflections of α-Fe (Si, Al) (JCPDS No. 45-1206) phase with the bcc structure from (110), (200), and (211) planes. Furthermore, it can be found that the composites contain α-Fe (Si, Al), graphite (JCPDS No. 99-0057) and αAl2O3 (JCPDS No. 80-0786) crystalline phases without any other phases, indicating that no chemical reaction among the components occurs during hot-pressed sintering. It is worth mentioning that the well reservation of FeSiAl and FG absorbents in the process of hot-pressed sintering ensured that the electromagnetic properties of the composites were retained. Fig. 2 illustrates the SEM images and EDS mapping images of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size. According to the backscattered electron SEM image analysis shown in Fig. 2(a–c), the FeSiAl phase presents color bright, the Al2O3 matrix appears color gray, while the FG seems color black. As further observed in Fig. 2(d), the flake-shaped FeSiAl absorbent is randomly distributed in the Al2O3 matrix based on the Fe-Si and Al-O element mapping results. The flake-shaped black regions possess higher carbon content,
2. Experimental 2.1. Materials Fe-9.6Si-5.4Al alloy powders (purity > 99.0%) with flake shape were purchased from Xi'an Hongyuan Electronic Materials Co., Ltd. Flaky graphite (FG, purity > 99.9%) was < 45 μm in diameter and supplied by Qingdao Risheng Graphite Co., Ltd. α-Al2O3 (purity > 99.9%) with a mean particle size of 0.8 μm was used as matrix and provided by Tangshan Sipurui Functional Ceramic Materials Co., Ltd. 2.2. Preparation of the composites The mixtures containing 95 wt% Al2O3 powders, 1 wt% FG and 4 wt % FeSiAl powders with different FeSiAl particle size were milled for 8 h using a planetary ball mill with Al2O3 jars and balls in alcohol medium. The mass ratio of ball to powder was 5:1, and the ball-milling speed was 250 rpm. The ball-milling process was carried out at room temperature in air. After the mixtures were oven-dried at 80 °C for 3 h in air, they were screened by a screen of 100 mesh. Subsequently, the mixed powders were placed into a graphite grinder. Then, the hot-pressed sintering was carried out in vacuum with the sintering temperature of 1350 °C and a load of 30 MPa. The heating rates were 10 °C/min, 10 °C/ min and 5 °C/min at the temperature ranges of room temperature900 °C, 900–1200 °C and > 1200 °C, respectively. The samples were kept heating for 90 min at 1350 °C, and finally followed by furnace cooling. The surfaces of the hot-pressed sintered composites contaminated by graphite were removed by sand paper. 2.3. Characterization The crystalline phase was identified by X-ray diffraction (XRD, Rigaku, D/Max2500, Tokyo, Japan) with a Cu-Kα radiation at 40 kV and 40 mA operated in the 2θ range of 20o-90°. The morphology of the polished surface was detected by scanning electron microscope (SEM,
Fig. 1. XRD patterns of FeSiAl and flaky graphite filled Al2O3 composites. 2
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Fig. 2. SEM images of FeSiAl and flaky graphite filled Al2O3 composites with (a) 18–25 μm; (b) 25–48 μm; (c) 48–75 μm FeSiAl particle size, and (d) EDS element mappings.
values of the composites filled with 18–25 μm and 25–48 μm FeSiAl present an increasing tendency with increasing the frequency in 8.2–12.4 GHz. However, as the FeSiAl particle sizes are 25–48 μm and 48–75 μm, the ε′′ values show a downward trend in 8.2–10.6 GHz and then increase in 10.6–12.4 GHz with increasing the frequency. This phenomenon exhibits typically frequency dispersion effect, which is favorable to expanding the microwave absorption bandwidth of the composites. As we all known, the polarization mechanisms are divided into interfacial polarization, orientational polarization, electronic polarization, and ionic polarization [20]. For the investigated composites, interfacial polarization generally occurs in the interfaces among FeSiAl, FG and Al2O3 for the heterogeneous composites filled with conductive absorbents dispersed in the insulating Al2O3 matrix. Moreover, on the basis of the aforementioned discussions, the possibility of contact among adjacent absorbent particles for the composites with larger FeSiAl particles is prone to form longer electron shifting paths, which stimulates the further polarization. Thus, the above two polarization mechanisms contribute to the enhanced complex permittivity, and the formation of longer electron shifting paths plays a dominant role in the
indicating that FG is present in these regions or the polishing process with abrasive paste results in carbon remaining in these areas. Moreover, the interfaces among FeSiAl, FG and Al2O3 within the composites are beneficial to the generation of interfacial polarization. Meanwhile, the possibility of contact among FeSiAl or/and FG particles is prone to form, especially for the composites filled with larger absorbent particle size. 3.2. Electromagnetic properties The complex permittivity and complex permeability play a dominant role in determining the microwave absorption properties of FeSiAl and flaky graphite filled Al2O3 composites. To indicate the possible mechanisms of microwave absorption properties, the real (ε′) and imaginary (ε ′′) parts of the complex permittivity for the composites with different FeSiAl particle size are exhibited in Fig. 3. Both the ε′ and ε′′ values increase evidently with increasing the FeSiAl particle size in the X-band. It is noteworthy that the similar variation tendency has also been observed in other reports [18,19], indicating that the complex permittivity relies heavily on the absorbent particle size. Besides, the ε′
Fig. 3. The (a) real (ε′) and (b) imaginary (ε ′′ ) parts of complex permittivity for FeSiAl and flaky graphite filled Al2O3 composites with varying FeSiAl particle size. 3
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Fig. 4. The (a) real ( μ′) and (b) imaginary ( μ′′ ) parts of complex permeability for FeSiAl and flaky graphite filled Al2O3 composites with varying FeSiAl particle size.
Fig. 5. Calculated RL of FeSiAl and flaky graphite filled Al2O3 composites with different (a) FeSiAl particle size (d = 1.0 mm) and (b) composite thickness (FeSiAl particle size: 25–48 μm).
addition, there is a resonance peak in the μ′′ curves, which is attributed to the natural resonance of ferromagnetic materials. As is well known, magnetic loss mainly consists of natural resonance, exchange resonance, domain-wall displacement, hysteresis loss and eddy current loss in the electromagnetic wave frequency region [23]. For FeSiAl and flaky graphite filled Al2O3 composites, flake-shaped FeSiAl effectively suppresses the eddy current effect, indicating that the magnetic loss in the investigated composites may be ascribed to the natural resonance and exchange resonance. And the increase of magnetic dipole moment caused by the increase of FeSiAl particle size leads to the enhancement of complex permeability. In addition, the μ′ values of the composites with larger FeSiAl particle size decrease with increasing the frequency, which is induced by the movement and relaxation of magnetic domain wall. In comparation, the microwave absorbing mechanisms of FeSiAl and flaky graphite filled Al2O3 composites mainly rely on conductance loss and dielectric relaxation loss due to their lower variation range of complex permeability.
ε′ values [21]. On the other hand, the interfacial polarization contributes to the ε′, and the relevant relaxation loss enhances the ε ′′ values. Generally, the dielectric loss is mainly associated with conductance loss and relaxation loss, which can be expressed according to the Debye theory as follows [22]:
′′ ε′′ = σ / ωε0 + εrelax
(3)
where σ represents the electrical conductivity, ω represents the angular ′ ′ refrequency, ε0 represents the permittivity of free space, and εrelax presents the relaxation loss. In comparison, the ε′′ values of the composites filled with smaller FeSiAl particles are mainly resulted from the ′ ′ component corresponding to polarization. For the composites with εrelax larger FeSiAl particles, the conductive networks are easily formed and the σ / ωε0 component is obviously improved, resulting in the enhancement of conductance loss and thus increasing the ε ′′ values. The real ( μ′) and imaginary ( μ′′) parts of complex permeability for the composites with different FeSiAl particle size are exhibited in Fig. 4. It can be observed from Fig. 4(a) that the μ′ values increase with the increase of FeSiAl particle size in the low frequency region, while they exhibit an opposite tendency in the high frequency range. Meanwhile, the μ′′ values increase with increasing flaky FeSiAl particle size. In
3.3. Microwave absorption properties In order to study the effects of the FeSiAl particle size and the thickness on the microwave absorption properties, the reflection loss 4
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presented in Fig. 6(a). It is easy to find that the input impedance of the composite filled with 18–25 μm FeSiAl exhibits an increasing tendency as the frequency increases in the X-band. Meanwhile, the composites with 25–48 μm and 48–75 μm FeSiAl particle size present similar input impedance, and their input impedance increases in low frequency and decreases in high frequency with increasing the frequency. In comparison, the composite filled with 18–25 μm FeSiAl exhibits the highest input impedance value, which is most close to 377 Ω and exhibits the superior impedance matching characteristic. In addition, the attenuation coefficient α represents the electromagnetic attenuation capacity of microwave absorbing materials, which is calculated from the following formula [26]:
(RL) of FeSiAl and flaky graphite filled Al2O3 composites was calculated and the RL curves were shown in Fig. 5. Generally, the RL value of -10dB corresponds to 90% absorption of incident electromagnetic waves, and hence the frequency ranges below -10dB are considered as effective absorption bandwidth. As observed in Fig. 5(a), the minimum reflection loss (RLmin) increases with increasing FeSiAl particle size. In comparison, the composite filled with 25–48 μm FeSiAl exhibits preferable microwave absorption properties with effective absorption bandwidth of 1.2 GHz in 10.0–11.2 GHz and RLmin of -15.2 dB at 10.6 GHz. The calculated RL values of the composites with varying thickness as the FeSiAl particle size is 25–48 μm are illustrated in Fig. 5(b). It is obvious that the RLmin values decrease firstly, and then increase with increasing the thickness from 0.8 to 1.3 mm. Furthermore, it is significantly observed from Fig. 5 that the RLmin values turn to low frequency region with increasing the FeSiAl particle size and the thickness. This phenomenon can be explained by quarter-wavelength theory. The dependence of the matching frequency (fm) with the thickness (dm) is expressed as follows [24]:
fm =
c × 4dm
α=
(μ′′ε ′′ − μ′ε′) +
2
(μ′′ε ′′ − μ′ε′) + (μ′′ε′ − μ′ε ′′)
2
(5)
The calculated attenuation coefficients of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size are exhibited in Fig. 6(b). It could be clearly found that the attenuation coefficients increase with the increase of FeSiAl particle size, and the composite filled with 48–75 μm FeSiAl possesses the maximal attenuation coefficient, indicating that the composites filled with larger sized FeSiAl particle can attenuate the electromagnetic waves much more efficiently. However, as exhibited in Fig. 5(a), the composite filled with 48–75 μm FeSiAl does not possess the most excellent microwave absorption properties, which is attributed to the fact that few incident electromagnetic waves enter its interior due to the poor impedance matching. In comparison, the composite filled with 25–48 μm FeSiAl exhibits the desirable microwave absorption properties due to its favorable impedance matching and appropriate electromagnetic wave attenuation abilities.
−1
tan2δμ ⎞ 1 ⎛ ⎜1 + ⎟ 8 ⎠ ε′μ′ ⎝
2 πf × c
(4)
where δμ represents the magnetic loss tangent. From Eq. (4), the matching frequency fm of RL peak is inversely proportional to the dm and ε′μ′ . On the basis of the above analysis, the ε′ values increase greatly while the μ′ values present a lower variation range with increasing FeSiAl particle size, and the obviously enhanced ε′ values play a dominant role in the increasing tendency of the overall ε′μ′ values with increasing FeSiAl particle size. Therefore, it is logical that fm decreases evidently with increasing the FeSiAl particle size and the thickness as shown in Fig. 5(a) and (b), respectively. On the basis of this phenomenon, the matching frequency of the composites can by adjusted by manipulating the FeSiAl particle size and the thickness according to the demands of the practical applications. Impedance matching and attenuation coefficient are important parameters to evaluate the microwave absorbing properties, which can help us to further illustrate the microwave absorbing mechanisms. The impedance matching Z (Z= Zin/ Zo ) determines the degree of incident electromagnetic waves entering the absorber. The closer Z value gets to 1, the more electromagnetic waves enter the absorber. In other words, the intrinsic impedance of the electromagnetic wave absorber should be as close as possible to the theoretic input impedance 377 Ω of the free space [25]. According to Eq. (2), the input impedance values of the composites with varying FeSiAl particle size were calculated and
4. Conclusions In this paper, FeSiAl and flaky graphite filled Al2O3 composites have been fabricated by a facile method for microwave absorption applications. The complex permittivity is efficiently improved with the increase of FeSiAl particle size, which contributes to the strong attenuation ability because of the increased interfacial polarization and conductance loss. Due to the favorable input impedance and appropriate microwave attenuation, the reflection loss of FeSiAl and flaky graphite filled Al2O3 composites with 25–48 μm FeSiAl particle size achieves -15.2 dB at 10.6 GHz when the matching thickness is 1.0 mm. In conclusion, FeSiAl and flaky graphite filled Al2O3 composites could obtain the effective microwave absorption in the X-band by means of
Fig. 6. (a) Input impedance and (b) attenuation coefficients of FeSiAl and flaky graphite filled Al2O3 composites with different FeSiAl particle size (d = 1.0 mm). 5
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adjusting the absorbent particle size, and the composites with outstanding comprehensive performance are desirable candidates for microwave absorption applications.
[10]
[11]
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[12]
[13]
Acknowledgements [14]
This work was financially supported by the National Natural Science Foundation of China (No. 51302018), the Fundamental Research Funds for the Central Universities from Chang'an University (No. 300102319309, 300102219509 and 300102319501), Key Research and Development Program in Shaanxi Province of China (No. 2019GY174), and the Chang'an Scholar Program of Chang'an University (No. 201807CQ014).
[15]
[16]
[17]
[18]
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