- Email: [email protected]
polyimide composites
Journal of Magnetism and Magnetic Materials 491 (2019) 165643
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Anisotropic particle geometry effect on magnetism and microwave absorption of carbonyl iron/polyimide composites
T
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Jie Dong , Wancheng Zhou, Chunhai Wang, Linlin Lu, Fa Luo, Dongmei Zhu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbonyl iron Permittivity Permeability Reflection loss
Two carbonyl iron (CI)/polyimide (PI) composites (65:35 wt) with different CI particle geometry (flake like carbonyl iron FCI against spherical carbonyl iron SCI) were prepared. Due to the difference of demagnetization factor from the geometry anisotropy for SCI and FCI, the saturation magnetization Ms suppresses from 126 emu/ g to 116 emu/g and the coercivity Hc from 25 Oe to 4.6 Oe. The geometry anisotropy of CI particles from spherical one to flake like one enhances the real part of permittivity and permeability loss of the CI/PI composites. The microwave absorption peak is shifted to lower frequency by 5.2 GHz for FCI/PI composite comparing to SCI/PI. A whole X and Ku bands (from 8.2 to 18 GHz) reflection loss (RL) < −5 dB absorption bandwidth is achieved in FCI/PI composite with thickness ≥1.2 mm and the minimum RL value of −19.28 dB appears at 11.62 GHz at thickness of 1.5 mm. The composites show excellent thermal stability with change of complex permittivity and permeability less than 7% after heated at 230 °C for 10 h.
1. Introduction In recent years, with the rapid development of science and technology, the application of electromagnetic waves in the military and commercial fields becomes more and more widely. Especially, in the military, electromagnetic stealth has received more and more attention [1–6]. Because of the advantage of simplified process, high efficiency and good absorbing properties under the condition of thin thickness, the coating composite materials are more and more widely used as microwave absorbing materials [7–9]. As a commonly used magnetic loss absorbent, carbonyl iron (CI) powder has the advantages of low density, high Curie temperature, good thermal stability, large magnetic permeability and dielectric constant [10,11]. Many researches had been done on improving the microwave absorbing properties of CI powder by mixing them with dielectric loss absorbents [2,3,12–16], such as, reduced graphene oxide (rGO) [12], BaTiO3 [2], graphene nanosheets [3,14], carbon black [13], BaFe12O19 [15], Si/C/N nano-powder [16], etc. Through the addition of these dielectric absorbing agents, the prepared absorbing material combines both the dielectric loss and the magnetic loss, which achieves higher absorbing efficiency. For example, Chen et al. [12] added rGO to FCI/epoxy composite, the minimum reflection loss (RL) reached −32.3 dB at 11.0 GHz for 2 mm thick 50% wt rGO/FCI/epoxy absorber, which shows a higher absorption than that of FCI/epoxy without rGO
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absorbent showing the minimum RL about −23 dB at 12.2 GHz. The microstructure of CI powder seriously affects the microscopic behavior of the composites [8,11,16–20]. For example, Wang et al. [18] found that the grain size, internal strain and aspect ratio of CI powder dominates the intergranular exchange coupling effect and initial permeability at high frequency and the absorbing properties of composite. Wu et al. [11] found that the effective magnetic permeability strongly depends on the particle size of magnetic spheres in the composite. Generally, the spherical CI particle can be broken to flake ones by a ball milling process [10,19,21,22]. Abshinova and Li [21] studied the effect of milling time of CI on permeability of CI/silicone (40%−55% vol. CI). They found the flake like CI shows larger coercivity (Hc) and smaller saturation magnetization (Ms) than the spherical CI powder. Wen et al. [22] studied the particle microstructure of CI and microwave absorption of CI/epoxy (40:60 wt) composite. They found the composite with CI ball-milled by 4 h shows highest microwave absorption (RL ~ −10 dB at ~8 GHz). Yang and Liang [10] studied the microwave properties of CI/epoxy composite (40%−50% wt CI) and found a higher microwave absorption in the composite with flake like CI than that of spherical one. However, none of them studied the anisotropic effects of CI particles on both the magnetic and microwave absorption properties. The microstructure/morphology of CI particles in different investigations are obviously different, which makes it hard for a cross referring. To our knowledge, it is important to study the anisotropic
Corresponding author. E-mail address: [email protected] (J. Dong).
https://doi.org/10.1016/j.jmmm.2019.165643 Received 2 February 2019; Received in revised form 21 June 2019; Accepted 25 July 2019 Available online 26 July 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 491 (2019) 165643
J. Dong, et al.
effects of CI particles on both the magnetism and microwave absorption of CI based composite. In this paper, we selected CI powders with significant particle geometry difference (flake versus sphere) as filler and prepared CI/ polyimide (PI) composites to study the anisotropic effect from CI particles. The morphology, magnetic properties and microwave absorption were carefully investigated. 2. Experimental section 2.1. Material and sample preparation The flaky carbonyl iron (FCI) and spherical carbonyl iron (SCI) powders were from Shannxi Xinghua Chemical Co.,Ltd. The planar diameter of the FCI particle is 2–8 μm and the thickness is less than 1 μm. The particle size of the SCI is 1–5 μm. The fine polyimide powder resin synthetized from bismaleimide was from Wuhan Zhisheng Science & Technology Co. Ltd, China, with the martin heat resistance temperature of 260 ℃. The 65:35 wt (23:77 vol) of CI powder and PI powder were mixed and ball-milled for 24 h with ethanol as medium. After milling, the mixed slurries were poured into a beaker, to allow the alcohol naturally volatilized in a fume hood. To drive off the alcohol completely, the beaker was then placed at 60 °C for 12 h in an oven. The dried mixture were ground and sieved by a 200 mesh sieve to obtain a uniform powder. The powder was placed in a mold and cured at 230 °C for 1 h under a pressure of 5 MPa. Finally, the CI/PI bulks with the size of 100 mm × 100 mm × 2 mm were obtained. 2.2. Characterization The micro-morphology of CI powder and the composites were characterized by scanning electron microscope (SEM, Tescan Vega3 BH, Brno, Czech Republic). The magnetization curves of composites were tested at room temperature using a vibrating sample mag-netometer (VSM, Riken Denshi, BHV-525). The complex permittivity and permeability of the composite were recorded using wave guide method by a network analyzer (Agilent technologies E8362B). The sample measured in X and Ku bands were 22.86 mm × 10.16 mm × 2 mm and 15.89 mm × 7.89 mm × 2 mm, respectively. The RL of the composites was calculated according to transmission line theory [23]. 3. Results and discussion 3.1. The morphology The morphology of two CI powders and composites are shown in Fig. 1. The FCI particles show planar shapes. The diameters of FCI are between 2 and 8 μm and the proportion of 6–8 μm is > 90%. In FCI/PI composite see Fig. 1(b), the FCI powders are randomly embedded in the resin matrix and in the resin and clear flake CI geometry can be recognized with the thickness < 500 nm. Therefore, for flake CI powder, the axial ratio n is about 10. In Fig. 1(c) and (d), the particle size of the SCI powder is 1–5 μm with the 90% vol. of SCI powder in a size range 2–3 μm, and the axial ratio n is 1. SCI powders are also randomly embedded in the resin in SCI/PI composite. Thus, both FCI/PI and SCI/PI composites are in excellent homogeneity. 3.2. The magnetic properties of composites The room temperature hysteresis loops (magnetization versus field strength H, M-H) of two FCI and SCI powders incorporated PI composites are shown in Fig. 2. The hysteresis curves of two composites are similar to S-shaped indicates a superparamagnetic like behavior as the microscopic particles of CI are close to the magnetic domain (microscopic in size). For flake CI powder, the critical particle size for single domain structure is about ~60 nm, while the spherical CI powder is
Fig. 1. SEM images of FCI and SCI powders and the composite. (a) FCI powder, (b) FCI/PI composite, (c) SCI powder, (d) SCI/PI composite.
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Journal of Magnetism and Magnetic Materials 491 (2019) 165643
J. Dong, et al.
Fig. 2. Hysteresis loops of FCI/PI and SCI/PI composites.
~20 nm [24–27]. Therefore, each grain (smaller than or equals to the particle size) is composed of tens of magnetic domains. Moreover, the number of magnetic domains of flake CI powder is less than that of spherical CI powder, and it is more similar to superparamagnetic behavior. The saturation magnetization Ms of FCI/PI composite is smaller than that of SCI/PI, which is 116 emu/g and 126 emu/g, respectively. A smaller Ms of FCI than that of SCI (195 emu/g against 209 emu/g) is also observed by Abshinova and Li [21], which is consistent with our investigation. A zoomed view of the M-H curve is shown as inset of Fig. 2. Small coercivity is observed in both samples, which is also observed by Abshinova and Li [21]. The coercivity of FCI/PI and SCI/PI are 4.6 Oe and 25 Oe, respectively, and the FCI/PI shows a higher slope (∂M/∂H) than that of SCI/PI and a lower demagnetization factor, which means the FCI/PI is more easily to be magnetized. Thus, there is an obvious particle geometry anisotropic effect on the magnetic behavior for the CI and composites filled from it, which suppresses both the saturation magnetization Ms and coercivity Hc of the material. It should be noted this disagrees with the results of Abshinova and Li [21]. Fig. 3. Complex permittivity and permeability of FCI/PI and SCI/PI composites. (a) complex permittivity, (b) complex permeability.
3.3. The complex permittivity and permeability properties
composite. However, for the imaginary part, the FCI/PI composite is larger than that of SCI/PI. Therefore, the magnetic loss of FCI/PI composite is higher than that of SCI/PI. For magnetic absorber incorporated resin composite materials, hysteresis loss, eddy current loss and residual loss mainly occurred when electromagnetic waves are incident on the surface of the material [30]. And residual loss is mainly caused by dimensional resonance, ferromagnetic resonance, natural resonance and domain wall resonance [31]. Each of them shows different response frequency ranges. In 8.2–18 GHz, the magnetic loss is dominated by eddy current loss and natural resonance [8]. When the applied field frequency matches the processing frequency of magnetic spin in magnetic material, the maximum electromagnetic loss occurs. As could be seen from Fig. 2, although the saturation magnetization of SCI powder is greater than that of flake powder, the FCI/PI shows a higher slope (∂M/∂H) than that of SCI/PI and lower demagnetization factor indicating the easy magnetization direction in the sheet plane. Therefore, as the magnetization of CI/PI composites shows an obvious dependence on the particle geometry of CI filled, the permeability of FCI/PI and SCI/PI are obviously different and at 8.2 GHz the magnetic loss of FCI/PI composite is almost 1.6 times higher than that of SCI/PI.
Fig. 3(a) and (b) show the complex permittivity and permeability curves of FCI/PI and SCI/PI composites. The dielectric property can be written as ε = ε′-jε′′, where the real part ε′ represents the ability of a substance to store charges and the imaginary part ε′′ represents the dielectric loss of the material. The magnetic permeability of the material can be described by the formula of μ = μ′-jμ′′. In which μ′ means the magnetism of the material and μ′′ means the hysteresis loss and loss caused by domain resonance [28]. In Fig. 3(a), with the frequency increase, the real part (ε′) and imaginary part (ε′′) of complex permittivity of two kinds of CI/PI are almost unchanged. The ε′ of FCI/PI and SCI/PI at 8.2 GHz are 12 and 6.5, while the imaginary parts are 0.15 and 0.09, respectively. In the whole 8.2–18 GHz wavebands, the real part of complex permittivity of FCI/PI is much higher than that of SCI/PI composite. This is because the area of the parallel plates formed by the FCI powder is much larger than the area of the SCI powder, so the ability to form a capacitance to store charges is much larger than the capacity of the spherical ones. The imaginary part of permittivity of material at different angular frequency ω can be written as ε′′ = 1/ρωε0, where ρ is the resistivity [29]. The imaginary parts of two CI/PI composites are similar as the resistivity of the components is similar. Hence, the dielectric loss tanδE = ε ''/ ε ' of composite SCI/PI is almost twice higher than that of FCI/PI composite. The complex permeability of FCI/PI and SCI/PI composites decreases with the increase of frequency. In 8.2–18 GHz, the real part of permeability of SCI/PI composite is higher than that of FCI/PI
3.4. The microwave absorbing property According to transmission line theory, the RL of single-layer backed perfect metal plate can be calculated by the follow formula [32,33]: 3
Journal of Magnetism and Magnetic Materials 491 (2019) 165643
J. Dong, et al.
Fig. 5. Normalized input impedance of CI/PI composites at 1.5 mm. (a) real part of impedance, (b) imaginary part of impedance.
Fig. 4. RL of two kinds of CI/PI composites. (a) FCI/PI composite. (b) SCI/PI composites.
Zin-Z0 RL(dB) =20 log Zin+Z0
−17 dB at 16.8 GHz. Therefore, as expected, a strong geometry anisotropic effect from the CI particles is observed on the microwave absorption of CI/PI composite. The microwave absorption peak of composites moves to low frequency by 5.2 GHz from CI filler anisotropic particle geometry, which makes a wider bandwidth and stronger absorption in X and Ku bands for the composite made from it. However, it can be seen that the intensity of the RL peak value does not change much. According to Eq. (2), the real part and imaginary part of normalized input impedance of composites at 1.5 mm are calculated and show in Fig. 5. A perfect matching needs an impedance with real part and imaginary part equals to 1 and 0, respectively [36]. For FCI/PI composite, in the bands of 9–13 GHz the real part of normalized impedance is 0.7–0.8 and the imaginary part of composite is closed to −0.2 ~ 0.2 which makes a good impedance matching and attenuation and a higher absorption performance is observed. For SCI/PI composite, similar impedance matching locates in 14–18 GHz, and the reflectivity of SCI/ PI composite in these bands is low.
(1)
in which, RL is the reflectivity of material with the unit of dB. Zin is input impedance of the absorber, and Z0 is the impedance of free space. Zin can be written as the following equation:
Zin = Z0
μr εr
2πft tanh ⎛j μr εr ⎞ ⎝ c ⎠
(2)
where μr and εr are the relative complex permittivity and permeability of absorber, f is the frequency of incident microwave, t is the thickness of composite and c is the velocity of free space. The curves of reflectivity with frequency and thickness of the two composites are shown in Fig. 4. The frequency of RL peak appeared shifts to low frequency with the increase of composites thickness which agrees with the λ/4 relation between thickness and frequency [34]. The bandwidth of RL ≤ −5 dB represents that the absorption of electromagnetic waves is more than 68%, and the RL ≤ −10 dB represents that the absorption of electromagnetic waves is more than 90% [35]. The whole X and Ku bands (from 8.2 to 18 GHz) RL < −5 dB absorption bandwidth is achieved in FCI/PI composite with thickness ≥1.2 mm and the minimum RL value of −19 dB appears at 11.6 GHz when the thickness of composite is 1.5 mm. And at the thickness of 1.5 mm, the bandwidth of RL less than −10 dB is from 8.7 to 15.7 GHz. As a comparison, for SCI/PI composite with the same thickness, the bandwidth with RL less than −10 dB ranges from 13.5–18 GHz with the minimum RL value of
3.5. Thermal stability Fig. 6(a) and (b) show the complex permittivity and permeability of composites after heat treatment at 230 °C of 10 h. After heat treatment at 230 °C for 10 h, the complex permittivity and permeability of both FCI/PI and SCI/PI composites almost keep unchanged. Fig. 6(c1, c2) and (d1, d2) show the ratio of complex permittivity and permeability of 4
Journal of Magnetism and Magnetic Materials 491 (2019) 165643
J. Dong, et al.
Fig. 6. Contrast curves of complex permittivity and permeability of FCI/PI and SCI/PI composites after heat treatment. (a) complex permittivity, (b) complex permeability, (c1, c2) ratio of real/imaginary part of complex permittivity after and before heat treatment, (d1, d2) ratio of real/imaginary part of complex permeability after and before heat treatment.
composites after and before heat treatment. The ε′ of FCI/PI and SCI/PI composites increase by 6.8% and 5.9% compared with the composites before heat treatment. In terms of ratio, the imaginary part of the complex permittivity increases by 2–3 times. Since the imaginary part of the complex permittivity is small, the increase in the imaginary part may be caused by measurement errors. Compared with before heat treatment, the μ′ and μ′′ of FCI/PI and SCI/PI composites both increase or decrease within 6.5%. Thus, the resin covers the iron powder very well and prevents oxidation of the iron powder in air. Therefore, both the FCI/PI and SCI/PI composite shows excellent thermal stability at temperature up to 230 °C. 4. Conclusion Two CI/PI composites with different CI particle geometry were prepared with the weight ratio of CI powder to resin 65:35. The planar diameter of the FCI particle is 2–8 μm and the thickness is less than 500 nm whilst the particle size of the SCI is 1–5 μm. All CI powders are randomly embedded in the resin matrix and homogenous composites obtained. Obvious particle geometry anisotropic effect on the magnetic behavior, complex permittivity, complex permeability and microwave absorption observed for the CI and composites filled from the two CI powders. The saturation magnetization of SCI/PI composite is greater than that of FCI/PI. The geometry anisotropy of CI particles from spherical to flake like enhances the real permittivity and permeability loss of the CI/PI composites. The microwave absorption peak is shifted to lower frequency for the FCI/PI composite comparing to the SCI/PI one. A whole X and Ku bands (from 8.2 to 18 GHz) RL < −5 dB absorption bandwidth is achieved in FCI/PI composite with thickness ≥1.2 mm and minimum RL value of −19 dB appears at 11.6 GHz when the thickness of composite is 1.5 mm. For SCI/PI composite with the thickness of 1.5 mm, the minimum RL value reaches −17 dB at 16.8 GHz. Therefore, as expected, a strong geometry anisotropic effect from the CI particles is observed on the microwave absorption of CI/PI composite. The microwave absorption peak of composites moves to low frequency from CI filler anisotropic particle geometry, which makes a wider bandwidth and stronger absorption in X and Ku bands for the composite. The composites show excellent thermal stability with change of complex permittivity and permeability after heated at 230 °C for 10 h less than 7%. Acknowledgements This work was supported by Fundamental Research Funds for the Central Universities, China (No. 3102017ZY050) and the State Key Laboratory of the Solidification Processing in NWPU, China (No. KP201604). References [1] J.H. Oh, K.S. Oh, C.G. Kim, C.S. Hong, Design of radar absorbing structures using glass/epoxy composite containing carbon black in X-band frequency ranges, Compos. Part B 35 (2004) 49–56. [2] Y. Qing, W. Zhou, F. Luo, D. Zhu, Optimization of electromagnetic matching of carbonyl iron/BaTiO3 composites for microwave absorption, J. Magn. Magn. Mater 323 (2011) 600–606. [3] Y. Qing, D. Min, Y. Zhou, F. Luo, W. Zhou, Graphene nanosheet- and flake carbonyl iron particle-filled epoxy-silicone composites as thin-thickness and wide-bandwidth microwave absorber, Carbon 86 (2015) 98–107. [4] Y. Duan, W. Liu, L. Song, T. Wang, A discrete structure: FeSiAl/carbon black composite absorption coatings, Mater. Res. Bull. 88 (2017) 41–48. [5] E. Acıkalın, K. Coban, A. Sayıntı, Nanosized hybrid electromagnetic wave absorbing
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Anisotropic particle geometry effect on magnetism and microwave absorption of carbonyl iron/polyimide composites
T
⁎
Jie Dong , Wancheng Zhou, Chunhai Wang, Linlin Lu, Fa Luo, Dongmei Zhu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, Shaanxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbonyl iron Permittivity Permeability Reflection loss
Two carbonyl iron (CI)/polyimide (PI) composites (65:35 wt) with different CI particle geometry (flake like carbonyl iron FCI against spherical carbonyl iron SCI) were prepared. Due to the difference of demagnetization factor from the geometry anisotropy for SCI and FCI, the saturation magnetization Ms suppresses from 126 emu/ g to 116 emu/g and the coercivity Hc from 25 Oe to 4.6 Oe. The geometry anisotropy of CI particles from spherical one to flake like one enhances the real part of permittivity and permeability loss of the CI/PI composites. The microwave absorption peak is shifted to lower frequency by 5.2 GHz for FCI/PI composite comparing to SCI/PI. A whole X and Ku bands (from 8.2 to 18 GHz) reflection loss (RL) < −5 dB absorption bandwidth is achieved in FCI/PI composite with thickness ≥1.2 mm and the minimum RL value of −19.28 dB appears at 11.62 GHz at thickness of 1.5 mm. The composites show excellent thermal stability with change of complex permittivity and permeability less than 7% after heated at 230 °C for 10 h.
1. Introduction In recent years, with the rapid development of science and technology, the application of electromagnetic waves in the military and commercial fields becomes more and more widely. Especially, in the military, electromagnetic stealth has received more and more attention [1–6]. Because of the advantage of simplified process, high efficiency and good absorbing properties under the condition of thin thickness, the coating composite materials are more and more widely used as microwave absorbing materials [7–9]. As a commonly used magnetic loss absorbent, carbonyl iron (CI) powder has the advantages of low density, high Curie temperature, good thermal stability, large magnetic permeability and dielectric constant [10,11]. Many researches had been done on improving the microwave absorbing properties of CI powder by mixing them with dielectric loss absorbents [2,3,12–16], such as, reduced graphene oxide (rGO) [12], BaTiO3 [2], graphene nanosheets [3,14], carbon black [13], BaFe12O19 [15], Si/C/N nano-powder [16], etc. Through the addition of these dielectric absorbing agents, the prepared absorbing material combines both the dielectric loss and the magnetic loss, which achieves higher absorbing efficiency. For example, Chen et al. [12] added rGO to FCI/epoxy composite, the minimum reflection loss (RL) reached −32.3 dB at 11.0 GHz for 2 mm thick 50% wt rGO/FCI/epoxy absorber, which shows a higher absorption than that of FCI/epoxy without rGO
⁎
absorbent showing the minimum RL about −23 dB at 12.2 GHz. The microstructure of CI powder seriously affects the microscopic behavior of the composites [8,11,16–20]. For example, Wang et al. [18] found that the grain size, internal strain and aspect ratio of CI powder dominates the intergranular exchange coupling effect and initial permeability at high frequency and the absorbing properties of composite. Wu et al. [11] found that the effective magnetic permeability strongly depends on the particle size of magnetic spheres in the composite. Generally, the spherical CI particle can be broken to flake ones by a ball milling process [10,19,21,22]. Abshinova and Li [21] studied the effect of milling time of CI on permeability of CI/silicone (40%−55% vol. CI). They found the flake like CI shows larger coercivity (Hc) and smaller saturation magnetization (Ms) than the spherical CI powder. Wen et al. [22] studied the particle microstructure of CI and microwave absorption of CI/epoxy (40:60 wt) composite. They found the composite with CI ball-milled by 4 h shows highest microwave absorption (RL ~ −10 dB at ~8 GHz). Yang and Liang [10] studied the microwave properties of CI/epoxy composite (40%−50% wt CI) and found a higher microwave absorption in the composite with flake like CI than that of spherical one. However, none of them studied the anisotropic effects of CI particles on both the magnetic and microwave absorption properties. The microstructure/morphology of CI particles in different investigations are obviously different, which makes it hard for a cross referring. To our knowledge, it is important to study the anisotropic
Corresponding author. E-mail address: [email protected] (J. Dong).
https://doi.org/10.1016/j.jmmm.2019.165643 Received 2 February 2019; Received in revised form 21 June 2019; Accepted 25 July 2019 Available online 26 July 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 491 (2019) 165643
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effects of CI particles on both the magnetism and microwave absorption of CI based composite. In this paper, we selected CI powders with significant particle geometry difference (flake versus sphere) as filler and prepared CI/ polyimide (PI) composites to study the anisotropic effect from CI particles. The morphology, magnetic properties and microwave absorption were carefully investigated. 2. Experimental section 2.1. Material and sample preparation The flaky carbonyl iron (FCI) and spherical carbonyl iron (SCI) powders were from Shannxi Xinghua Chemical Co.,Ltd. The planar diameter of the FCI particle is 2–8 μm and the thickness is less than 1 μm. The particle size of the SCI is 1–5 μm. The fine polyimide powder resin synthetized from bismaleimide was from Wuhan Zhisheng Science & Technology Co. Ltd, China, with the martin heat resistance temperature of 260 ℃. The 65:35 wt (23:77 vol) of CI powder and PI powder were mixed and ball-milled for 24 h with ethanol as medium. After milling, the mixed slurries were poured into a beaker, to allow the alcohol naturally volatilized in a fume hood. To drive off the alcohol completely, the beaker was then placed at 60 °C for 12 h in an oven. The dried mixture were ground and sieved by a 200 mesh sieve to obtain a uniform powder. The powder was placed in a mold and cured at 230 °C for 1 h under a pressure of 5 MPa. Finally, the CI/PI bulks with the size of 100 mm × 100 mm × 2 mm were obtained. 2.2. Characterization The micro-morphology of CI powder and the composites were characterized by scanning electron microscope (SEM, Tescan Vega3 BH, Brno, Czech Republic). The magnetization curves of composites were tested at room temperature using a vibrating sample mag-netometer (VSM, Riken Denshi, BHV-525). The complex permittivity and permeability of the composite were recorded using wave guide method by a network analyzer (Agilent technologies E8362B). The sample measured in X and Ku bands were 22.86 mm × 10.16 mm × 2 mm and 15.89 mm × 7.89 mm × 2 mm, respectively. The RL of the composites was calculated according to transmission line theory [23]. 3. Results and discussion 3.1. The morphology The morphology of two CI powders and composites are shown in Fig. 1. The FCI particles show planar shapes. The diameters of FCI are between 2 and 8 μm and the proportion of 6–8 μm is > 90%. In FCI/PI composite see Fig. 1(b), the FCI powders are randomly embedded in the resin matrix and in the resin and clear flake CI geometry can be recognized with the thickness < 500 nm. Therefore, for flake CI powder, the axial ratio n is about 10. In Fig. 1(c) and (d), the particle size of the SCI powder is 1–5 μm with the 90% vol. of SCI powder in a size range 2–3 μm, and the axial ratio n is 1. SCI powders are also randomly embedded in the resin in SCI/PI composite. Thus, both FCI/PI and SCI/PI composites are in excellent homogeneity. 3.2. The magnetic properties of composites The room temperature hysteresis loops (magnetization versus field strength H, M-H) of two FCI and SCI powders incorporated PI composites are shown in Fig. 2. The hysteresis curves of two composites are similar to S-shaped indicates a superparamagnetic like behavior as the microscopic particles of CI are close to the magnetic domain (microscopic in size). For flake CI powder, the critical particle size for single domain structure is about ~60 nm, while the spherical CI powder is
Fig. 1. SEM images of FCI and SCI powders and the composite. (a) FCI powder, (b) FCI/PI composite, (c) SCI powder, (d) SCI/PI composite.
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Fig. 2. Hysteresis loops of FCI/PI and SCI/PI composites.
~20 nm [24–27]. Therefore, each grain (smaller than or equals to the particle size) is composed of tens of magnetic domains. Moreover, the number of magnetic domains of flake CI powder is less than that of spherical CI powder, and it is more similar to superparamagnetic behavior. The saturation magnetization Ms of FCI/PI composite is smaller than that of SCI/PI, which is 116 emu/g and 126 emu/g, respectively. A smaller Ms of FCI than that of SCI (195 emu/g against 209 emu/g) is also observed by Abshinova and Li [21], which is consistent with our investigation. A zoomed view of the M-H curve is shown as inset of Fig. 2. Small coercivity is observed in both samples, which is also observed by Abshinova and Li [21]. The coercivity of FCI/PI and SCI/PI are 4.6 Oe and 25 Oe, respectively, and the FCI/PI shows a higher slope (∂M/∂H) than that of SCI/PI and a lower demagnetization factor, which means the FCI/PI is more easily to be magnetized. Thus, there is an obvious particle geometry anisotropic effect on the magnetic behavior for the CI and composites filled from it, which suppresses both the saturation magnetization Ms and coercivity Hc of the material. It should be noted this disagrees with the results of Abshinova and Li [21]. Fig. 3. Complex permittivity and permeability of FCI/PI and SCI/PI composites. (a) complex permittivity, (b) complex permeability.
3.3. The complex permittivity and permeability properties
composite. However, for the imaginary part, the FCI/PI composite is larger than that of SCI/PI. Therefore, the magnetic loss of FCI/PI composite is higher than that of SCI/PI. For magnetic absorber incorporated resin composite materials, hysteresis loss, eddy current loss and residual loss mainly occurred when electromagnetic waves are incident on the surface of the material [30]. And residual loss is mainly caused by dimensional resonance, ferromagnetic resonance, natural resonance and domain wall resonance [31]. Each of them shows different response frequency ranges. In 8.2–18 GHz, the magnetic loss is dominated by eddy current loss and natural resonance [8]. When the applied field frequency matches the processing frequency of magnetic spin in magnetic material, the maximum electromagnetic loss occurs. As could be seen from Fig. 2, although the saturation magnetization of SCI powder is greater than that of flake powder, the FCI/PI shows a higher slope (∂M/∂H) than that of SCI/PI and lower demagnetization factor indicating the easy magnetization direction in the sheet plane. Therefore, as the magnetization of CI/PI composites shows an obvious dependence on the particle geometry of CI filled, the permeability of FCI/PI and SCI/PI are obviously different and at 8.2 GHz the magnetic loss of FCI/PI composite is almost 1.6 times higher than that of SCI/PI.
Fig. 3(a) and (b) show the complex permittivity and permeability curves of FCI/PI and SCI/PI composites. The dielectric property can be written as ε = ε′-jε′′, where the real part ε′ represents the ability of a substance to store charges and the imaginary part ε′′ represents the dielectric loss of the material. The magnetic permeability of the material can be described by the formula of μ = μ′-jμ′′. In which μ′ means the magnetism of the material and μ′′ means the hysteresis loss and loss caused by domain resonance [28]. In Fig. 3(a), with the frequency increase, the real part (ε′) and imaginary part (ε′′) of complex permittivity of two kinds of CI/PI are almost unchanged. The ε′ of FCI/PI and SCI/PI at 8.2 GHz are 12 and 6.5, while the imaginary parts are 0.15 and 0.09, respectively. In the whole 8.2–18 GHz wavebands, the real part of complex permittivity of FCI/PI is much higher than that of SCI/PI composite. This is because the area of the parallel plates formed by the FCI powder is much larger than the area of the SCI powder, so the ability to form a capacitance to store charges is much larger than the capacity of the spherical ones. The imaginary part of permittivity of material at different angular frequency ω can be written as ε′′ = 1/ρωε0, where ρ is the resistivity [29]. The imaginary parts of two CI/PI composites are similar as the resistivity of the components is similar. Hence, the dielectric loss tanδE = ε ''/ ε ' of composite SCI/PI is almost twice higher than that of FCI/PI composite. The complex permeability of FCI/PI and SCI/PI composites decreases with the increase of frequency. In 8.2–18 GHz, the real part of permeability of SCI/PI composite is higher than that of FCI/PI
3.4. The microwave absorbing property According to transmission line theory, the RL of single-layer backed perfect metal plate can be calculated by the follow formula [32,33]: 3
Journal of Magnetism and Magnetic Materials 491 (2019) 165643
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Fig. 5. Normalized input impedance of CI/PI composites at 1.5 mm. (a) real part of impedance, (b) imaginary part of impedance.
Fig. 4. RL of two kinds of CI/PI composites. (a) FCI/PI composite. (b) SCI/PI composites.
Zin-Z0 RL(dB) =20 log Zin+Z0
−17 dB at 16.8 GHz. Therefore, as expected, a strong geometry anisotropic effect from the CI particles is observed on the microwave absorption of CI/PI composite. The microwave absorption peak of composites moves to low frequency by 5.2 GHz from CI filler anisotropic particle geometry, which makes a wider bandwidth and stronger absorption in X and Ku bands for the composite made from it. However, it can be seen that the intensity of the RL peak value does not change much. According to Eq. (2), the real part and imaginary part of normalized input impedance of composites at 1.5 mm are calculated and show in Fig. 5. A perfect matching needs an impedance with real part and imaginary part equals to 1 and 0, respectively [36]. For FCI/PI composite, in the bands of 9–13 GHz the real part of normalized impedance is 0.7–0.8 and the imaginary part of composite is closed to −0.2 ~ 0.2 which makes a good impedance matching and attenuation and a higher absorption performance is observed. For SCI/PI composite, similar impedance matching locates in 14–18 GHz, and the reflectivity of SCI/ PI composite in these bands is low.
(1)
in which, RL is the reflectivity of material with the unit of dB. Zin is input impedance of the absorber, and Z0 is the impedance of free space. Zin can be written as the following equation:
Zin = Z0
μr εr
2πft tanh ⎛j μr εr ⎞ ⎝ c ⎠
(2)
where μr and εr are the relative complex permittivity and permeability of absorber, f is the frequency of incident microwave, t is the thickness of composite and c is the velocity of free space. The curves of reflectivity with frequency and thickness of the two composites are shown in Fig. 4. The frequency of RL peak appeared shifts to low frequency with the increase of composites thickness which agrees with the λ/4 relation between thickness and frequency [34]. The bandwidth of RL ≤ −5 dB represents that the absorption of electromagnetic waves is more than 68%, and the RL ≤ −10 dB represents that the absorption of electromagnetic waves is more than 90% [35]. The whole X and Ku bands (from 8.2 to 18 GHz) RL < −5 dB absorption bandwidth is achieved in FCI/PI composite with thickness ≥1.2 mm and the minimum RL value of −19 dB appears at 11.6 GHz when the thickness of composite is 1.5 mm. And at the thickness of 1.5 mm, the bandwidth of RL less than −10 dB is from 8.7 to 15.7 GHz. As a comparison, for SCI/PI composite with the same thickness, the bandwidth with RL less than −10 dB ranges from 13.5–18 GHz with the minimum RL value of
3.5. Thermal stability Fig. 6(a) and (b) show the complex permittivity and permeability of composites after heat treatment at 230 °C of 10 h. After heat treatment at 230 °C for 10 h, the complex permittivity and permeability of both FCI/PI and SCI/PI composites almost keep unchanged. Fig. 6(c1, c2) and (d1, d2) show the ratio of complex permittivity and permeability of 4
Journal of Magnetism and Magnetic Materials 491 (2019) 165643
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Fig. 6. Contrast curves of complex permittivity and permeability of FCI/PI and SCI/PI composites after heat treatment. (a) complex permittivity, (b) complex permeability, (c1, c2) ratio of real/imaginary part of complex permittivity after and before heat treatment, (d1, d2) ratio of real/imaginary part of complex permeability after and before heat treatment.
composites after and before heat treatment. The ε′ of FCI/PI and SCI/PI composites increase by 6.8% and 5.9% compared with the composites before heat treatment. In terms of ratio, the imaginary part of the complex permittivity increases by 2–3 times. Since the imaginary part of the complex permittivity is small, the increase in the imaginary part may be caused by measurement errors. Compared with before heat treatment, the μ′ and μ′′ of FCI/PI and SCI/PI composites both increase or decrease within 6.5%. Thus, the resin covers the iron powder very well and prevents oxidation of the iron powder in air. Therefore, both the FCI/PI and SCI/PI composite shows excellent thermal stability at temperature up to 230 °C. 4. Conclusion Two CI/PI composites with different CI particle geometry were prepared with the weight ratio of CI powder to resin 65:35. The planar diameter of the FCI particle is 2–8 μm and the thickness is less than 500 nm whilst the particle size of the SCI is 1–5 μm. All CI powders are randomly embedded in the resin matrix and homogenous composites obtained. Obvious particle geometry anisotropic effect on the magnetic behavior, complex permittivity, complex permeability and microwave absorption observed for the CI and composites filled from the two CI powders. The saturation magnetization of SCI/PI composite is greater than that of FCI/PI. The geometry anisotropy of CI particles from spherical to flake like enhances the real permittivity and permeability loss of the CI/PI composites. The microwave absorption peak is shifted to lower frequency for the FCI/PI composite comparing to the SCI/PI one. A whole X and Ku bands (from 8.2 to 18 GHz) RL < −5 dB absorption bandwidth is achieved in FCI/PI composite with thickness ≥1.2 mm and minimum RL value of −19 dB appears at 11.6 GHz when the thickness of composite is 1.5 mm. For SCI/PI composite with the thickness of 1.5 mm, the minimum RL value reaches −17 dB at 16.8 GHz. Therefore, as expected, a strong geometry anisotropic effect from the CI particles is observed on the microwave absorption of CI/PI composite. The microwave absorption peak of composites moves to low frequency from CI filler anisotropic particle geometry, which makes a wider bandwidth and stronger absorption in X and Ku bands for the composite. The composites show excellent thermal stability with change of complex permittivity and permeability after heated at 230 °C for 10 h less than 7%. Acknowledgements This work was supported by Fundamental Research Funds for the Central Universities, China (No. 3102017ZY050) and the State Key Laboratory of the Solidification Processing in NWPU, China (No. KP201604). References [1] J.H. Oh, K.S. Oh, C.G. Kim, C.S. Hong, Design of radar absorbing structures using glass/epoxy composite containing carbon black in X-band frequency ranges, Compos. Part B 35 (2004) 49–56. [2] Y. Qing, W. Zhou, F. Luo, D. Zhu, Optimization of electromagnetic matching of carbonyl iron/BaTiO3 composites for microwave absorption, J. Magn. Magn. Mater 323 (2011) 600–606. [3] Y. Qing, D. Min, Y. Zhou, F. Luo, W. Zhou, Graphene nanosheet- and flake carbonyl iron particle-filled epoxy-silicone composites as thin-thickness and wide-bandwidth microwave absorber, Carbon 86 (2015) 98–107. [4] Y. Duan, W. Liu, L. Song, T. Wang, A discrete structure: FeSiAl/carbon black composite absorption coatings, Mater. Res. Bull. 88 (2017) 41–48. [5] E. Acıkalın, K. Coban, A. Sayıntı, Nanosized hybrid electromagnetic wave absorbing
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