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Effects of Multi-walled Carbon Nanotubes on the Electromagnetic Absorbing Characteristics of Composites Filled with Carbonyl Iron Particles
J. Mater. Sci. Technol., 2012, 28(1), 34–40.
Effects of Multi-walled Carbon Nanotubes on the Electromagnetic Absorbing Characteristics of Composites Filled with Carbonyl Iron Particles Yonggang Xu† , Deyuan Zhang, Jun Cai, Liming Yuan and Wenqiang Zhang Bionic and Micro/Nano/Bio Manufacturing Technology Research Center, School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China [Manuscript received March 24, 2011, in revised form August 20, 2011]
The electromagnetic (EM) wave absorbing property of silicone rubber filled with carbonyl iron particles (CIPs) and multi-walled carbon nanotubes (MWCNTs) was examined. Absorbents including MWCNTs and spherical/ flaky CIPs were added to silicone rubber using a two-roll mixer. The complex permittivity and complex permeability were measured over the frequency range of 1–18 GHz. The two EM parameters were verified and the uniform dispersion of MWCNTs and CIPs was confirmed by comparing the measured reflection loss (RL) with the calculated one. As the MWCNT weight percent increased, the RL of the spherical CIPs/silicone rubber composites changed insignificantly. It was attributed to the random distribution of spherical CIPs and less content of MWCNTs. On the contrary, for composites filled with flaky CIPs the absorption bandwidth increased at thickness 0.5 mm (RL value lower than –5 dB in 8–18 GHz) and the absorption ratio increased at lower frequency (minimum –35 dB at 3.5 GHz). This effect was attributed to the oriented distribution of flaky CIPs caused by interactions between the two absorbents. Therefore, mixing MWCNTs and flaky CIPs could achieve wider-band and higher-absorption ratio absorbing materials. KEY WORDS: Absorbing materials; Carbon nanotubes; Carbonyl iron particles; Composites; Reflection loss
1. Introduction The electromagnetic (EM) waves have been widely used in both military and civil applications under the development of communication technology[1,2] . The caused electromagnetic interference (EMI) and electromagnetic compatibility (EMC) problems have gradually attracted people s attention. Fabricating absorbing materials provides an effective way to overcome these problems due to the dielectric loss or magnetic loss. Recently many researches have been carried out on the absorbents and agents to improve absorbing properties of the composites. It is demonstrated that a good absorbing material should satisfy two important conditions: 1) the intrinsic impedance † Corresponding author. Ph.D.; Tel.: +86 10 82339604; Fax: +86 10 82316603; E-mail address: [email protected] (Y.G. Xu).
of the absorbing materials is made to be equal to that of the free space; 2) the incident EM wave must enter and be rapidly attenuated through the material layer. Therefore, adjusting the complex permittivity and complex permeability of composites plays a key role in achieving excellent absorbing properties. At present, the promising microwave absorbing materials should be wide-band, light-weight, thinthickness and hard-strength. Carbonyl iron particles (CIPs) are widely used in absorbing plates or absorbing coatings as an effective absorbent, they possess a good EM microwave absorption property due to the large values of saturation magnetization and large Snoke s limit at gigahertz (GHz) frequency. For example, the minimum RL value reached –12.2 dB with 60 vol.% CIPs added to the matrix at a thickness of 1 mm[3] , –42.5 dB with 55 wt% carbonyl iron added at 10.6 GHz[4] . Moreover, many researchers gradually
Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
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pay more attention to the unique absorbent carbon nanotubes (CNTs) for their special electromagnetic, mechanical or thermal property as a multifunctional materials and structures[5,6] , especially hybrid materials such as, CNTs/CoFe2 O4 [7] , raw MWCNTs/Fe catalyst[8] , Fe/CNTs[9] etc. However, the wide use of these hybrid composites is restricted by the complicated technological conditions, preparation processes and rare production. Mixing CNTs and CIPs is an effective way to prepare absorbing composites of good performance[10,11,12] . Unfortunately, the enhancement mechanism of CNTs on the absorbing property is still unclear and needs further research. The purpose of this work is to analyze the MWCNTs’ influence on the microwave EM absorption properties of silicone rubber composites filled with spherical/flake CIPs. The mainly focus is on the effects of weight content of MWCNTs and shape of the CIPs on the reflection loss of the absorbing composites. 2. Experimental 2.1 Materials preparation The matrix used in this work was Methyl vinyl silicone rubber with 2, 5-dimethyl hexane used as vulcanized assistants, both were supplied by Laizhou Jintai Silicon Industry Co. Ltd, China. Raw commercial spherical CIPs were supplied by Xinghua chemical Co., Ltd, China, and raw commercial flaky CIPs, supplied by Shenyang Hangda Technology Co., Ltd, China, were fabricated by a mechanical milling process using the spherical CIPs. The MWCNTs were purchased from Anhui Gold Sun Nano Techonology Co., Ltd, China. The average diameter of spherical CIPs was 3 μm, the diameter of flaky CIPs was about 5 μm and the thickness was about 0.5 μm, diameters of the MWCNTs were 10–30 nm with the length of 1– 2 μm. The morphology of the three particles is shown in Fig. 1. The 45% volume fraction of the spherical CIPs or flaky CIPs was added into the silicone rubber, and then different amount MWCNTs (the weight ratio was 0.5 wt%, 1 wt% and 1.5 wt% relative to the silicone rubber weight) were mixed in each composite. The established six samples with different proportions are shown in Table 1. The silicone rubber and absorbents were mixed in a two-roll mixer including two procedures. Firstly the MWCNTs were added to the silicone rubber to guaranty a better dispersion, because the roll-mixer applied a shear force on the mixture and could overcome the intermolecular Van Der Waals force[13,14] . Secondly the vulcanized assistants and spherical/flaky CIPs were added to the previous compounds. The testing samples of EM parameters measurement were modeled to a toroidal shape with an outer diameter of 7.0 mm, an inner diameter of 3.04 mm and a thickness of 2 mm. Testing samples of reflection loss measurement were modeled to a square shape with length 180 mm and thickness 0.5 mm. All
Fig. 1 SEM images of (a) spherical CIPs, (b) flaky CIPs and (c) the MWCNTs Table 1 Mixture proportions of each composites No. M1 M2 M3 M4 M5 M6
CIPs CIPs Silicone rubber MWCNTs type /g /g /g spherical 114 20 0.1 spherical 114 20 0.2 spherical 114 20 0.3 flaky 114 20 0.1 flaky 114 20 0.2 flaky 114 20 0.3
of them were vulcanized into pieces at 180 ◦ C under a pressure of 10 MPa for 5 min on a vulcanizing machine. 2.2 Testing The morphology and microstructure of the composites were observed by scanning electron microscopy (SEM JEOL JSM-5800) to evaluate the dispersion state of MWCNTs and CIPs in each composite. The RL of composites was measured by arch method in a frequency range from 1 to 12 GHz using
Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
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Fig. 2 SEM images of the microstructure of (a) spherical CIPs/MWCNTs and (b) flaky CIPs/MWCNTs, distribution state of (c) spherical CIPs and (d) flaky CIPs
the square shape samples. The effective complex dielectric permittivity and magnetic permeability of the absorbing composites were measured using the transmission method with an AV3627 vector network analyzer and coaxial transmission line in the frequency range of 1–18 GHz. Then the calculated RL could be established, for a single-layer absorbing material, the RL of normal incident EM wave at the absorber surface is given by Zin − Z0 (1) R = 20 lg Zin + Z0 μr μ0 (2) η= εr ε0 γ=j
2π √ μr εr λ0
Zin = η tanh(γd)
(3) (4)
where Zin is the normalized input impedance of the microwave absorbing composites, Zin =120π is the intrinsic impedance of free space, η is intrinsic impedance of absorbing composites, γ denotes the propagation constant, εr , μr and ε0 , μ0 are complex permittivity and complex permeability of absorption materials and free space, respectively, λ0 is the wavelength of microwave, and d is the thickness of the absorbing composite.
3. Results and Discussion 3.1 Verification on the dispersion of MWCNTs and CIPs in silicone rubber composites Fig. 2 shows the dispersion state and morphology of the absorbing material filled with MWCNTs and spherical/flaky CIPs. It was necessary to verify the measured complex permittivity and complex permeability due to the MWCNTs strong tendency to agglomerate, which might lead to significantly anisotropic and non-uniform EM properties of the toroidal shape samples. SEM images and comparison of calculated RL results with the measured ones support an effective solution to verify the uniform dispersion and EM parameters. Taking Sample M2 for example (1 wt% MWCNTs added to the composites filled with 45 vol.% spherical CIPs), as shown in Fig. 3, the calculated RL results matched well with the measured ones, the maximum deviation was 0.48 dB, and Pearson correlations coefficient was 0.996. It indicated that the MWCNTs and CIPs were uniformly dispersed in the composites, and the EM parameters could be used to compute the EM absorption property of the absorbing composites. 3.2 Effects of the weight content of MWCNTs on the EM parameters Fig. 4 shows the complex permittivity and com-
Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
Fig. 3 Verification on the RL between the calculated results and the measured ones
plex permeability of each composite dependent on the frequency. It could be observed that silicon rubber composites filled with flaky CIPs had a larger real part of permittivity (ε ) and imaginary part of permeability (μ ) than those of composites filled with spherical ones. Each μ curve of composites filled with flaky CIPs had a peak range from 5 to 7 GHz. It was owing to the shape effect of CIPs, for the flaky ones had
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a larger value of saturation magnetization than that of spherical ones. Similar results had been reported in the composites filled with CIPs by Zhang et al.[3] and Sheng et al.[15] . As the weight content of MWCNTs increased, the real part of permeability (μ ) of absorbing composites filled with spherical CIPs (Sample M1, M2 and M3) changed insignificantly, to the contrary of the imaginary part of permittivity (ε ) and imaginary part of permeability (μ ), the two parameters were increased. Meanwhile, the influence of MWCNTs weight on permittivity (ε) of composites filled with spherical CIPs was complicated, yet the values did not change conspicuously. The enhancement of CNTs on the absorbing property of composites added CIPs was confirmed for their dielectric loss as the weight content was 5% large[10] . While according to the results of Tong et al.[16] , the enhanced effects on absorbing properties of composites filled with 35 vol.% spherical CIPs were due to MWCNTs high electrical conductivity, permittivity and dielectric loss. And Qing et al.[12] showed that the significant effect was due to the interactions between the MWCNTs and flaky CIPs, yet the interactions were still unclear. However, the EM results in this paper were in good accordance with the later
Fig. 4 EM parameters of the composites filled with MWCNTs and spherical/flaky CIPs as a function of frequency: (a) the real part of permittivity, (b) the imaginary part of permittivity, (c) the real part of permeability, (d) the imaginary part of permeability
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Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
Fig. 5 RL of composites as a function of frequency at different thickness: (a) 0.5 mm, (b) 1 mm, (c) 1.5 mm, (d) 2 mm
reference, but not consistent with the previous two. In terms of the reported equations on the effective permeability of mixing absorbents[17] : ∗ = f tan δmt + (1 − f ) tan δms tan δmeff
(5)
where f is the volume content of the filler in the matrix, tanδ ∗meff is the magnetic loss tangents of the mixture, tanδ ms and tanδ mt denote that of absorbents and the matrix respectively, while in this paper the silicone rubber and MWCNTs composite could be seen as the matrix for the complex permeability and magnetic loss tangents are nearly the same as those of the silicone rubber matrix (μ ≈1, μ ≈0, δ ms ≈0)[18–20] . It could be apparently seen in Fig. 4 that, the magnetic loss tangents (μ /μ ) of composites filled with spherical CIPs changed insignificantly, to the contrary of that filled with flaky ones. Therefore, the measured results seemed to be contradictory to Eq. (5). Because the more MWCNTs were added into the absorbing composites, the less volume fraction of flaky CIPs would be, μ and tanδ ∗meff of the composites filled with 1.5 wt% MWCNTs and flaky CIPs should have been less than those of the other two samples. However, according to the morphology of the mi-
crostructure and distribution of spherical or flaky CIPs (Fig. 2(c) and (d)), it could be clearly obtained that the distribution of spherical CIPs was random and that of flaky ones was apparently oriented, the differences were mainly caused by the shape of CIPs. The insignificant effect of MWCNTs on the absorbers might be owing to the distribution and adding content of the absorbents. In this work about 45 vol.% spherical CIPs was added to the absorbing composites, 10 vol.% larger than the content in the previous research[16] , while the content of MWCNTs added was much less (maximum 0.22% relative to the composites) than that in the above two researches. The random distribution of the spherical CIPs with larger volume content mainly determined the EM property of the composites and made the influence of MWCNTs on EM characteristics insignificant. While the oriented distribution of flaky ones with an anisotropy electromagnetic property made the effects conspicuous. As given by Han et al.[21] , the average thickness of flaky CIPs (0.5 μm) was lower than the skin depth of the particles, and the eddy current effects were effectively suppressed. As the flaky CIPs was oriented, the magnetic moments lay preferentially in
Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
the plane of the flaky due to demagnetization effects. The parallel arrangement of easy planes of the particles resulted in the higher permeability[22] and the composite filled with a better orientation flaky CIPs would have a larger μ than that with the less oriented ones added. Therefore, with the weight content of MWCNTs increased to 1.5%, the distribution of composites filled with spherical CIPs would be still random, while those filled with flaky ones had a more oriented distribution. However, the detailed effects of the optimal oriented direction of the flaky CIPs as MWCNTs added are unclear and still need deeper investigation. 3.3 Effects of MWCNTs on the reflection loss of the composites Fig. 5 shows the RL of all the six absorption composites with a thickness from 0.5 mm to 2 mm, in which the influence of MWCNTs on the EM wave absorption property would be revealed. As the thickness increased to 2 mm, the matching frequency of each sample shifted to the lower, the RL value decreased to a minimum value and then increased, except the composites filled with spherical CIPs/MWCNTs with a thickness of 0.5 mm, for which the matching frequency was larger than 18 GHz. As the fillers were flaky CIPs and MWCNTs, each RL curve of the composites with a thickness of 0.5 mm had two peaks. Microwave absorption of these composites was originated from the combination of magnetic loss of the CIPs and dielectric loss of MWCNTs[12] , and when the thickness increased, the dielectric loss could be much less than the magnetic loss, resulting in only one RL peak in each curve. With the same thickness and weight content of absorbents, the matching frequency of composites filled with flaky CIPs/MWCNTs was much lower than that filled with spherical CIPs/MWCNTs. It was mainly attributed to the shape of the CIPs, because the flaky ones had a larger complex permeability and real part of permittivity than those of the spherical ones. The relationship between the matching thickness (dm ) and the matching frequency (fm ) could be shown in the equation: fm = 4dcm · √ 1 1 + 18 tan2 δμ , where c |μ ε |
denotes the velocity of the light, tanδ u denotes the magnetic loss tangents. Composites with a larger |μ ε | would have a less fm . As the weight content of MWCNTs increased, the RL value of each composite filled with spherical CIPs changed insignificantly, which is owing to the nearly same EM parameters. To the contrary, with more MWCNTs added to the composite filled with flaky CIPs the absorbing band of RL value lower than –5 dB was broadened (widest band 8–18 GHz) as the thickness is 0.5 mm, about 2 GHz wider than the previous research[12] , which was attributed to the lager imaginary part of permeability and complex permittivity. While the thickness increased to 2 mm, the minimum RL was decreased,
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and deviation of them became larger (1.3 dB at 1 mm, 3.8 dB at 1.5 mm, 9.7 dB at 2 mm). With more MWCNTs added to the silicone rubber composites, the absorbing band of RL value lower than –10 dB gradually shifted to the lower, yet the bandwidth was decreased (5.3–8.2 GHz at 1 mm, 3.4–6.44 GHz at 1.5 mm, 2.7– 4.6 GHz at 2 mm). It indicated that at a thin thickness of 0.5 mm, the added MWCNTs to silicone rubber/flaky CIPs composites achieved better impedance matching characteristic and attenuation characteristic. While increasing the thickness of composites filled with flaky CIPs, adding more MWCNTs could enhance the absorption ratio at lower frequency. The above results and analysis confirmed that mixing the MWCNTs and flaky CIPs as absorbents could achieve an excellent absorbing material with wider band at a thin thickness, higher absorption ratio at a lower frequency. 4. Conclusions (1) The MWCNTs had different effects on the EM absorption property between composites filled with spherical CIPs and those filled with flaky CIPs. (2) The EM absorption property changed insignificantly mainly due to the random distribution of larger content spherical CIPs and less content MWCNTs, whereas the enhancement of MWCNTs on absorption property of the composites added flaky CIPs were mainly attributed to the oriented distribution of flaky particles resulted by the interactions between the two absorbents. (3) Enhanced effects of MWCNTs on the composites added flaky CIPs had a contribution to fabricating wider band absorbers with thin thickness and higher absorption ratio absorbers at lower frequency.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 50805005), the National “863” Project of China (Grant No. 2009AA043804), and the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (Grant No. 2007B32). REFERENCES [1 ] Y.C. Qing, W.C. Zhou, F. Luo and D.M. Zhu: Mater. Rev., 2009, 23, 1. [2 ] Y.B. Feng, T. Qiu, X.Y. Li and C.Y. Shen: Mater. Sci. Technol., 2008, 16, 589. [3 ] B.S. Zhang, Y. Feng, J. Xiong, Y. Yang and H.X. Lu: IEEE Trans. Magn., 2006, 42, 1778. [4 ] Y.C. Qing, W.C. Zhou, F. Luo and D.M. Zhu: J. Magn. Magn. Mater., 2009, 321, 25. [5 ] H.Y. Zhang, G.X. Zeng, Y. Ge, T.L. Chen and L.C. Hu: J. Appl. Phys., 2009, 105, 054314. [6 ] Z.F. Liu, G. Bai, Y. Huang, Y.F. Ma, F. Du, F.F. Li, T.Y. Guo and Y.S. Chen: Carbon, 2007, 45, 821. [7 ] R.C. Che, C.Y. Zhi, C.Y. Liang and X.G. Zhou: Appl.
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Phys. Lett., 2006, 88, 033105. [8 ] A.P. Shpak, S.P. Kolesnik, G.S. Mogilny, Y.N. Petrov, V.P. Sokhatsky, L.N. Trophimova, B.D. Shanina and V.G. Gavriljuk: Acta Mater., 2007, 55, 1769. [9 ] R.C. Che, L.M. Peng, X.F. Duan, Q. Chen and X.L. Liang: Adv. Mater., 2004, 16, 401. [10] D.L. Zhao and Z.M. Shen: J. Inorg. Mater., 2005, 20, 608. [11] Y. Li, C.X. Chen, X.Y. Pan, Y.W. Ni, S. Zhang and J. Huang: Physica B, 2009, 404, 1343. [12] Y.C. Qing, W.C. Zhou, F. Luo and D.M. Zhu: Carbon, 2010, 48, 4074. [13] D.R. Iosif and V.H. Suong: Carbon, 2009, 47, 1958. [14] R.K. Gaurav, P. Sven, G. Andreas, P. Petra and H. Gert: Polymer, 2010, 51, 2708. [15] F.S. Wen, W.L. Zuo, H.B. Yi, N. Wang, L. Qiao and
F.S. Li: Physica B, 2009, 404, 3567. [16] G.X. Tong, W.H. Wu, Q. Hua, Y.Q. Miao, J.G. Guan and H.S. Qian: J. Alloy. Compd., 2011, 509, 451. [17] K.C. Pitman, M.W. Lindley, D. Simkin and J.F. Cooper: IEEE Proc. on Radar and Signal Processing: Radar Signal Process, 1991, 138, 223. [18] L.J. Deng and M.G. Han: Appl. Phys. Lett., 2007, 91, 023119. [19] S.E. Lee, J.H. Kang and C.G. Kim: Comp. Struct., 2006, 76, 397. [20] J.B. Kim, S.K. Lee and C.G. Kim: Comp. Sci. Technol., 2008, 68, 2909. [21] R. Han, L. Qian, T. Wang and F.S. Li: J. Alloy. Compd., 2010, 509, 2734. [22] Z.W. Li, Z.H. Yang and L.B. Kong: Appl. Phys. Lett., 2010, 96, 092507.
Effects of Multi-walled Carbon Nanotubes on the Electromagnetic Absorbing Characteristics of Composites Filled with Carbonyl Iron Particles Yonggang Xu† , Deyuan Zhang, Jun Cai, Liming Yuan and Wenqiang Zhang Bionic and Micro/Nano/Bio Manufacturing Technology Research Center, School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China [Manuscript received March 24, 2011, in revised form August 20, 2011]
The electromagnetic (EM) wave absorbing property of silicone rubber filled with carbonyl iron particles (CIPs) and multi-walled carbon nanotubes (MWCNTs) was examined. Absorbents including MWCNTs and spherical/ flaky CIPs were added to silicone rubber using a two-roll mixer. The complex permittivity and complex permeability were measured over the frequency range of 1–18 GHz. The two EM parameters were verified and the uniform dispersion of MWCNTs and CIPs was confirmed by comparing the measured reflection loss (RL) with the calculated one. As the MWCNT weight percent increased, the RL of the spherical CIPs/silicone rubber composites changed insignificantly. It was attributed to the random distribution of spherical CIPs and less content of MWCNTs. On the contrary, for composites filled with flaky CIPs the absorption bandwidth increased at thickness 0.5 mm (RL value lower than –5 dB in 8–18 GHz) and the absorption ratio increased at lower frequency (minimum –35 dB at 3.5 GHz). This effect was attributed to the oriented distribution of flaky CIPs caused by interactions between the two absorbents. Therefore, mixing MWCNTs and flaky CIPs could achieve wider-band and higher-absorption ratio absorbing materials. KEY WORDS: Absorbing materials; Carbon nanotubes; Carbonyl iron particles; Composites; Reflection loss
1. Introduction The electromagnetic (EM) waves have been widely used in both military and civil applications under the development of communication technology[1,2] . The caused electromagnetic interference (EMI) and electromagnetic compatibility (EMC) problems have gradually attracted people s attention. Fabricating absorbing materials provides an effective way to overcome these problems due to the dielectric loss or magnetic loss. Recently many researches have been carried out on the absorbents and agents to improve absorbing properties of the composites. It is demonstrated that a good absorbing material should satisfy two important conditions: 1) the intrinsic impedance † Corresponding author. Ph.D.; Tel.: +86 10 82339604; Fax: +86 10 82316603; E-mail address: [email protected] (Y.G. Xu).
of the absorbing materials is made to be equal to that of the free space; 2) the incident EM wave must enter and be rapidly attenuated through the material layer. Therefore, adjusting the complex permittivity and complex permeability of composites plays a key role in achieving excellent absorbing properties. At present, the promising microwave absorbing materials should be wide-band, light-weight, thinthickness and hard-strength. Carbonyl iron particles (CIPs) are widely used in absorbing plates or absorbing coatings as an effective absorbent, they possess a good EM microwave absorption property due to the large values of saturation magnetization and large Snoke s limit at gigahertz (GHz) frequency. For example, the minimum RL value reached –12.2 dB with 60 vol.% CIPs added to the matrix at a thickness of 1 mm[3] , –42.5 dB with 55 wt% carbonyl iron added at 10.6 GHz[4] . Moreover, many researchers gradually
Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
35
pay more attention to the unique absorbent carbon nanotubes (CNTs) for their special electromagnetic, mechanical or thermal property as a multifunctional materials and structures[5,6] , especially hybrid materials such as, CNTs/CoFe2 O4 [7] , raw MWCNTs/Fe catalyst[8] , Fe/CNTs[9] etc. However, the wide use of these hybrid composites is restricted by the complicated technological conditions, preparation processes and rare production. Mixing CNTs and CIPs is an effective way to prepare absorbing composites of good performance[10,11,12] . Unfortunately, the enhancement mechanism of CNTs on the absorbing property is still unclear and needs further research. The purpose of this work is to analyze the MWCNTs’ influence on the microwave EM absorption properties of silicone rubber composites filled with spherical/flake CIPs. The mainly focus is on the effects of weight content of MWCNTs and shape of the CIPs on the reflection loss of the absorbing composites. 2. Experimental 2.1 Materials preparation The matrix used in this work was Methyl vinyl silicone rubber with 2, 5-dimethyl hexane used as vulcanized assistants, both were supplied by Laizhou Jintai Silicon Industry Co. Ltd, China. Raw commercial spherical CIPs were supplied by Xinghua chemical Co., Ltd, China, and raw commercial flaky CIPs, supplied by Shenyang Hangda Technology Co., Ltd, China, were fabricated by a mechanical milling process using the spherical CIPs. The MWCNTs were purchased from Anhui Gold Sun Nano Techonology Co., Ltd, China. The average diameter of spherical CIPs was 3 μm, the diameter of flaky CIPs was about 5 μm and the thickness was about 0.5 μm, diameters of the MWCNTs were 10–30 nm with the length of 1– 2 μm. The morphology of the three particles is shown in Fig. 1. The 45% volume fraction of the spherical CIPs or flaky CIPs was added into the silicone rubber, and then different amount MWCNTs (the weight ratio was 0.5 wt%, 1 wt% and 1.5 wt% relative to the silicone rubber weight) were mixed in each composite. The established six samples with different proportions are shown in Table 1. The silicone rubber and absorbents were mixed in a two-roll mixer including two procedures. Firstly the MWCNTs were added to the silicone rubber to guaranty a better dispersion, because the roll-mixer applied a shear force on the mixture and could overcome the intermolecular Van Der Waals force[13,14] . Secondly the vulcanized assistants and spherical/flaky CIPs were added to the previous compounds. The testing samples of EM parameters measurement were modeled to a toroidal shape with an outer diameter of 7.0 mm, an inner diameter of 3.04 mm and a thickness of 2 mm. Testing samples of reflection loss measurement were modeled to a square shape with length 180 mm and thickness 0.5 mm. All
Fig. 1 SEM images of (a) spherical CIPs, (b) flaky CIPs and (c) the MWCNTs Table 1 Mixture proportions of each composites No. M1 M2 M3 M4 M5 M6
CIPs CIPs Silicone rubber MWCNTs type /g /g /g spherical 114 20 0.1 spherical 114 20 0.2 spherical 114 20 0.3 flaky 114 20 0.1 flaky 114 20 0.2 flaky 114 20 0.3
of them were vulcanized into pieces at 180 ◦ C under a pressure of 10 MPa for 5 min on a vulcanizing machine. 2.2 Testing The morphology and microstructure of the composites were observed by scanning electron microscopy (SEM JEOL JSM-5800) to evaluate the dispersion state of MWCNTs and CIPs in each composite. The RL of composites was measured by arch method in a frequency range from 1 to 12 GHz using
Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
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Fig. 2 SEM images of the microstructure of (a) spherical CIPs/MWCNTs and (b) flaky CIPs/MWCNTs, distribution state of (c) spherical CIPs and (d) flaky CIPs
the square shape samples. The effective complex dielectric permittivity and magnetic permeability of the absorbing composites were measured using the transmission method with an AV3627 vector network analyzer and coaxial transmission line in the frequency range of 1–18 GHz. Then the calculated RL could be established, for a single-layer absorbing material, the RL of normal incident EM wave at the absorber surface is given by Zin − Z0 (1) R = 20 lg Zin + Z0 μr μ0 (2) η= εr ε0 γ=j
2π √ μr εr λ0
Zin = η tanh(γd)
(3) (4)
where Zin is the normalized input impedance of the microwave absorbing composites, Zin =120π is the intrinsic impedance of free space, η is intrinsic impedance of absorbing composites, γ denotes the propagation constant, εr , μr and ε0 , μ0 are complex permittivity and complex permeability of absorption materials and free space, respectively, λ0 is the wavelength of microwave, and d is the thickness of the absorbing composite.
3. Results and Discussion 3.1 Verification on the dispersion of MWCNTs and CIPs in silicone rubber composites Fig. 2 shows the dispersion state and morphology of the absorbing material filled with MWCNTs and spherical/flaky CIPs. It was necessary to verify the measured complex permittivity and complex permeability due to the MWCNTs strong tendency to agglomerate, which might lead to significantly anisotropic and non-uniform EM properties of the toroidal shape samples. SEM images and comparison of calculated RL results with the measured ones support an effective solution to verify the uniform dispersion and EM parameters. Taking Sample M2 for example (1 wt% MWCNTs added to the composites filled with 45 vol.% spherical CIPs), as shown in Fig. 3, the calculated RL results matched well with the measured ones, the maximum deviation was 0.48 dB, and Pearson correlations coefficient was 0.996. It indicated that the MWCNTs and CIPs were uniformly dispersed in the composites, and the EM parameters could be used to compute the EM absorption property of the absorbing composites. 3.2 Effects of the weight content of MWCNTs on the EM parameters Fig. 4 shows the complex permittivity and com-
Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
Fig. 3 Verification on the RL between the calculated results and the measured ones
plex permeability of each composite dependent on the frequency. It could be observed that silicon rubber composites filled with flaky CIPs had a larger real part of permittivity (ε ) and imaginary part of permeability (μ ) than those of composites filled with spherical ones. Each μ curve of composites filled with flaky CIPs had a peak range from 5 to 7 GHz. It was owing to the shape effect of CIPs, for the flaky ones had
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a larger value of saturation magnetization than that of spherical ones. Similar results had been reported in the composites filled with CIPs by Zhang et al.[3] and Sheng et al.[15] . As the weight content of MWCNTs increased, the real part of permeability (μ ) of absorbing composites filled with spherical CIPs (Sample M1, M2 and M3) changed insignificantly, to the contrary of the imaginary part of permittivity (ε ) and imaginary part of permeability (μ ), the two parameters were increased. Meanwhile, the influence of MWCNTs weight on permittivity (ε) of composites filled with spherical CIPs was complicated, yet the values did not change conspicuously. The enhancement of CNTs on the absorbing property of composites added CIPs was confirmed for their dielectric loss as the weight content was 5% large[10] . While according to the results of Tong et al.[16] , the enhanced effects on absorbing properties of composites filled with 35 vol.% spherical CIPs were due to MWCNTs high electrical conductivity, permittivity and dielectric loss. And Qing et al.[12] showed that the significant effect was due to the interactions between the MWCNTs and flaky CIPs, yet the interactions were still unclear. However, the EM results in this paper were in good accordance with the later
Fig. 4 EM parameters of the composites filled with MWCNTs and spherical/flaky CIPs as a function of frequency: (a) the real part of permittivity, (b) the imaginary part of permittivity, (c) the real part of permeability, (d) the imaginary part of permeability
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Fig. 5 RL of composites as a function of frequency at different thickness: (a) 0.5 mm, (b) 1 mm, (c) 1.5 mm, (d) 2 mm
reference, but not consistent with the previous two. In terms of the reported equations on the effective permeability of mixing absorbents[17] : ∗ = f tan δmt + (1 − f ) tan δms tan δmeff
(5)
where f is the volume content of the filler in the matrix, tanδ ∗meff is the magnetic loss tangents of the mixture, tanδ ms and tanδ mt denote that of absorbents and the matrix respectively, while in this paper the silicone rubber and MWCNTs composite could be seen as the matrix for the complex permeability and magnetic loss tangents are nearly the same as those of the silicone rubber matrix (μ ≈1, μ ≈0, δ ms ≈0)[18–20] . It could be apparently seen in Fig. 4 that, the magnetic loss tangents (μ /μ ) of composites filled with spherical CIPs changed insignificantly, to the contrary of that filled with flaky ones. Therefore, the measured results seemed to be contradictory to Eq. (5). Because the more MWCNTs were added into the absorbing composites, the less volume fraction of flaky CIPs would be, μ and tanδ ∗meff of the composites filled with 1.5 wt% MWCNTs and flaky CIPs should have been less than those of the other two samples. However, according to the morphology of the mi-
crostructure and distribution of spherical or flaky CIPs (Fig. 2(c) and (d)), it could be clearly obtained that the distribution of spherical CIPs was random and that of flaky ones was apparently oriented, the differences were mainly caused by the shape of CIPs. The insignificant effect of MWCNTs on the absorbers might be owing to the distribution and adding content of the absorbents. In this work about 45 vol.% spherical CIPs was added to the absorbing composites, 10 vol.% larger than the content in the previous research[16] , while the content of MWCNTs added was much less (maximum 0.22% relative to the composites) than that in the above two researches. The random distribution of the spherical CIPs with larger volume content mainly determined the EM property of the composites and made the influence of MWCNTs on EM characteristics insignificant. While the oriented distribution of flaky ones with an anisotropy electromagnetic property made the effects conspicuous. As given by Han et al.[21] , the average thickness of flaky CIPs (0.5 μm) was lower than the skin depth of the particles, and the eddy current effects were effectively suppressed. As the flaky CIPs was oriented, the magnetic moments lay preferentially in
Y.G. Xu et al.: J. Mater. Sci. Technol., 2012, 28(1), 34–40.
the plane of the flaky due to demagnetization effects. The parallel arrangement of easy planes of the particles resulted in the higher permeability[22] and the composite filled with a better orientation flaky CIPs would have a larger μ than that with the less oriented ones added. Therefore, with the weight content of MWCNTs increased to 1.5%, the distribution of composites filled with spherical CIPs would be still random, while those filled with flaky ones had a more oriented distribution. However, the detailed effects of the optimal oriented direction of the flaky CIPs as MWCNTs added are unclear and still need deeper investigation. 3.3 Effects of MWCNTs on the reflection loss of the composites Fig. 5 shows the RL of all the six absorption composites with a thickness from 0.5 mm to 2 mm, in which the influence of MWCNTs on the EM wave absorption property would be revealed. As the thickness increased to 2 mm, the matching frequency of each sample shifted to the lower, the RL value decreased to a minimum value and then increased, except the composites filled with spherical CIPs/MWCNTs with a thickness of 0.5 mm, for which the matching frequency was larger than 18 GHz. As the fillers were flaky CIPs and MWCNTs, each RL curve of the composites with a thickness of 0.5 mm had two peaks. Microwave absorption of these composites was originated from the combination of magnetic loss of the CIPs and dielectric loss of MWCNTs[12] , and when the thickness increased, the dielectric loss could be much less than the magnetic loss, resulting in only one RL peak in each curve. With the same thickness and weight content of absorbents, the matching frequency of composites filled with flaky CIPs/MWCNTs was much lower than that filled with spherical CIPs/MWCNTs. It was mainly attributed to the shape of the CIPs, because the flaky ones had a larger complex permeability and real part of permittivity than those of the spherical ones. The relationship between the matching thickness (dm ) and the matching frequency (fm ) could be shown in the equation: fm = 4dcm · √ 1 1 + 18 tan2 δμ , where c |μ ε |
denotes the velocity of the light, tanδ u denotes the magnetic loss tangents. Composites with a larger |μ ε | would have a less fm . As the weight content of MWCNTs increased, the RL value of each composite filled with spherical CIPs changed insignificantly, which is owing to the nearly same EM parameters. To the contrary, with more MWCNTs added to the composite filled with flaky CIPs the absorbing band of RL value lower than –5 dB was broadened (widest band 8–18 GHz) as the thickness is 0.5 mm, about 2 GHz wider than the previous research[12] , which was attributed to the lager imaginary part of permeability and complex permittivity. While the thickness increased to 2 mm, the minimum RL was decreased,
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and deviation of them became larger (1.3 dB at 1 mm, 3.8 dB at 1.5 mm, 9.7 dB at 2 mm). With more MWCNTs added to the silicone rubber composites, the absorbing band of RL value lower than –10 dB gradually shifted to the lower, yet the bandwidth was decreased (5.3–8.2 GHz at 1 mm, 3.4–6.44 GHz at 1.5 mm, 2.7– 4.6 GHz at 2 mm). It indicated that at a thin thickness of 0.5 mm, the added MWCNTs to silicone rubber/flaky CIPs composites achieved better impedance matching characteristic and attenuation characteristic. While increasing the thickness of composites filled with flaky CIPs, adding more MWCNTs could enhance the absorption ratio at lower frequency. The above results and analysis confirmed that mixing the MWCNTs and flaky CIPs as absorbents could achieve an excellent absorbing material with wider band at a thin thickness, higher absorption ratio at a lower frequency. 4. Conclusions (1) The MWCNTs had different effects on the EM absorption property between composites filled with spherical CIPs and those filled with flaky CIPs. (2) The EM absorption property changed insignificantly mainly due to the random distribution of larger content spherical CIPs and less content MWCNTs, whereas the enhancement of MWCNTs on absorption property of the composites added flaky CIPs were mainly attributed to the oriented distribution of flaky particles resulted by the interactions between the two absorbents. (3) Enhanced effects of MWCNTs on the composites added flaky CIPs had a contribution to fabricating wider band absorbers with thin thickness and higher absorption ratio absorbers at lower frequency.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 50805005), the National “863” Project of China (Grant No. 2009AA043804), and the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (Grant No. 2007B32). REFERENCES [1 ] Y.C. Qing, W.C. Zhou, F. Luo and D.M. Zhu: Mater. Rev., 2009, 23, 1. [2 ] Y.B. Feng, T. Qiu, X.Y. Li and C.Y. Shen: Mater. Sci. Technol., 2008, 16, 589. [3 ] B.S. Zhang, Y. Feng, J. Xiong, Y. Yang and H.X. Lu: IEEE Trans. Magn., 2006, 42, 1778. [4 ] Y.C. Qing, W.C. Zhou, F. Luo and D.M. Zhu: J. Magn. Magn. Mater., 2009, 321, 25. [5 ] H.Y. Zhang, G.X. Zeng, Y. Ge, T.L. Chen and L.C. Hu: J. Appl. Phys., 2009, 105, 054314. [6 ] Z.F. Liu, G. Bai, Y. Huang, Y.F. Ma, F. Du, F.F. Li, T.Y. Guo and Y.S. Chen: Carbon, 2007, 45, 821. [7 ] R.C. Che, C.Y. Zhi, C.Y. Liang and X.G. Zhou: Appl.
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