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Epoxy-silicone filled with multi-walled carbon nanotubes and carbonyl iron particles as a microwave absorber
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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Epoxy-silicone filled with multi-walled carbon nanotubes and carbonyl iron particles as a microwave absorber Yuchang Qing *, Wancheng Zhou, Fa Luo, Dongmei Zhu State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi 0 an 710072, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Microwave absorbing composites with epoxy-silicone as matrix and both multi-walled car-
Received 22 May 2010
bon nanotubes (MWCNTs) and carbonyl iron (CI) particles as absorbers were prepared, and
Accepted 12 July 2010
their electromagnetic and microwave absorbing properties were investigated in the fre-
Available online 16 July 2010
quency range of 2–18 GHz. The microstructures of the composites show a uniform dispersion of the MWCNTs and CI particles in the matrix. The complex permittivity of the composites increased with increasing MWCNT content. A double resonance behavior of the complex permeability has been observed. One is due to the domain wall motion at about 7.5 GHz and the other is due to spin rotation at about 13.5 GHz. Reflection loss values exceeding 5 dB can be obtained in the frequency range of 10.4–18, 4.4–18 and 2–18 GHz, when the composite thickness is 0.5, 1 and 1.5 mm, respectively. A minimum reflection loss of 16.9 dB at 10.5 GHz and a bandwidth over the frequency range of 3.4–18 GHz with reflection loss below 10 dB can obtained for a composite filled with 0.5 vol% MWCNT and 50 vol% CI particles. Ó 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, many researchers have shown that carbon nanotubes (CNTs) have unique mechanical, electrical, magnetic, optical, and thermal properties [1–3]. Therefore, Multiwalled carbon nanotube (MWCNT)-filled polymeric composites have attracted many researchers’ attention in the manufacture of a variety of components for practical applications, such as electromagnetic interference shielding, microwave absorption, electronic packaging, and self-regulator heater [4–6]. It is well known that the complex permittivity (e* = e 0 je00 ) and permeability (l* = l 0 jl00 ) of the absorbers are main factors dominating the microwave absorption properties of a material. It is also speculated that magnetic material filled CNTs can be good microwave absorption material [7]. For this reason, the structure and microwave electromag-
netic properties of the magnetic material filled CNTs have been widely researched, for example, iron nanowires encased in MWCNTs [8], CoFe2O4/CNTs [9] and Fe/CNTs [10,11]. Che et al. [11] prepared Fe/CNTs filled composites as high-loss absorber to fabricated microwave absorption materials. Their results showed that the enhancement of microwave absorption of the Fe/CNTs filled composites was mainly contributed by magnetic loss which was caused by the encapsulated Fe in the CNTs. The objectives of this work are to fabricate microwave absorbing composites by using commercially available MWCNTs and carbonyl iron (CI) particles. The microwave electromagnetic properties of the composites are reported and the possible mechanisms are discussed. We investigate the microwave electromagnetic properties of epoxy-silicone composites filled with MWCNTs and CI particles for the
* Corresponding author: Fax: +86 29 8849 4574. E-mail address: [email protected] (Y. Qing). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.07.014
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following reasons. Firstly, recent studies have shown that CNT-filled polymer composites possess high real permittivity (polarization, e 0 ) as well as imaginary permittivity (absorption or electric loss, e00 ). As a result, CNT-filled polymer composites have been explored for potential applications in electromagnetic shielding or absorbing materials [12–19]. Secondly, CI particles have a large saturation magnetization and high Snoek’s limit which result in high complex permeability values at a wide frequency range. This factor makes CI particles are useful for microwave absorber at a high-frequency band [20,21]. Finally, the values of complex permittivity increased dramatically with small additions of MWCNTs to the polymer have been reported by Grimes et al. [22]. Thus, by combine the high complex permittivity of the MWCNTs and high complex permeability of CI particles, the high microwave absorption of the composites filled with MWCNTs and CI particles can be obtained.
2.
Experimental
2.1.
Materials
The matrix used in this work is an epoxy-silicone resin and the cure agent is polyamide resin, which were supplied by XI AN Leeo Technological Co. Raw commercial CI particles were purchased from Xinghua chemical Co. Ltd, Shanxi province, China. The MWCNTs were fabricated by the catalytic decomposition of CH4, supplied by the Shenzhen Nanotech port Co. Ltd., China. The diameter of MWCNTs was ranging from 60 to 100 nm, and length was 5–15 lm, and the purity was 95%. The sodium dodecyl benzene sulfonate (SDS) (Sinopharm Chemical Reagent Co. Ltd. China) as a surfactant was employed to obtain better dispersion of the MWCNTs.
2.2.
Sample preparation
The samples were fabricated by incorporating 50 vol% CI particles and different amounts of MWCNTs (the content of MWCNTs is 0.5, 1 and 1.5 vol%) into epoxy-silicone resin. The MWCNTs and SDS were dispersed in ethanol by an ultrasonic bath at room temperature for 1 h. After adding the CI particles into the ethanol based solution, the mixtures were uniformly mixed by stirring at 2000 rpm for 10 min and placed in oven at 80 °C to evaporate the ethanol completely. Then the resin and hardener was added into the mixtures, followed by stirring at 2000 rpm for 10 min. The mixtures were pre-cured at 90 °C for 30 min and then post-cured at 120 °C for 2 h.
2.3.
Measurements
The morphology of the composites was observed by scanning electron microscopy (SEM) (Model JSM-6360, JEOL, Tokyo, Japan). The complex permittivity e(f) and permeability l(f) of the composites were measured using the T/R coaxial line method in the frequency range of 2–18 GHz by a network analyzer (Agilent technologies E8362B: 10 MHz–20 GHz). The testing specimens had a cylindrical toroidal specimen: outer diameter of 7.0 mm, inner diameter of 3.03 mm and thickness of 2 mm.
3.
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Results and discussion
3.1. The microstructures of epoxy-silicone composites filled with MWCNTs and CI particles
resin
Fig. 1 shows the SEM images of the (a) MWCNTs and (b) CI particles. The CI particles are thin flakes of 2–5 lm in diameter and below 1 lm in thickness. Fig. 1c represents the backscatter electron images of the composites filled with MWCNTs and CI particles. The dispersion state and morphology of the epoxy-silicone resin composites filled with MWCNTs and CI particles were examined in detail by the fractured surface morphologies. As seen from Fig. 1d and e, the MWCNTs and CI particles were well dispersed in the matrix. The pristine MWCNTs exhibit a strong tendency to form entanglement and agglomerate which caused by the intermolecular van der Waals force between the MWCNTs. It can be said that the shear forces induced by the ultrasonic treatment and intense stirring mixing of the mixture overcoming the intermolecular van der Waals force, lead to a dramatic improvement of the dispersion of MWCNTs in the matrix [23,24]. Furthermore, the SDS as a surfactant and CI particles as a dispersing aid also improved the overall dispersion of MWCNTs in the epoxy-silicone resin matrix. The SEM images show that complex microstructures of the composites with multiple MWCNT bridging adjacent CI particles (as indicated by the arrow) can be obtained. Three dimensional network structures were generated through the MWCNT/CI particles/epoxy-silicone resin interfacial interactions, as shown in Fig. 1f.
3.2. Effects of the volume content of MWCNTs on the complex permittivity and permeability of the epoxy-silicone composites filled with MWCNTs and CI particles In order to evaluate the microwave electromagnetic of epoxysilicone resin composites filled with MWCNTs and CI particles, we measured the complex permittivity and permeability of the composites in the frequency range of 2–18 GHz. Fig. 2 shows the complex permittivity spectra of the composites containing MWCNTs and CI particles. With the content of MWCNTs increased from 0.5 to 1.5 vol%, both the real (e 0 ) and imaginary (e00 ) part of complex permittivity of the composites increased. The highest values of the e 0 and e00 of the composite filled with 1.5 vol% MWCNT and 50 vol% CI particles can reached to 46 and 37, respectively. The complex permittivity also trended to decrease with increasing frequency and showed frequency dependence in the whole frequency range (2–18 GHz). In general, the dielectric properties of conductive particle filled insulation resin composites depends on the characteristics of the matrix, the property and volume fraction of the filler, the configuration and internal structure of the composites, and the frequency of the electromagnetic waves [25–27]. Obviously, both the MWCNTs and CI particles were included in an insulating resin matrix to constitute heterogeneous composites. Interfacial polarization is an important polarization process and associated relaxation will give rise to dielectric loss mechanism of such composites [28]. Moreover, orientation polarization is another polarization mechanism and the associated relaxation phenomena also constitute the dielectric loss mechanisms of such composites.
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Fig. 1 – SEM images of the (a) MWCNTs. (b) CI particles. (c) Backscatter electron images of the composite filled with MWCNTs and CI particles. (d and e) SEM images of the fracture surface of the epoxy-silicone composite filled with MWCNTs and CI particles (the red arrows for the MWCNTs). (f) The schematic image to show the three dimensional network structure generated by the MWCNTs, CI particles and epoxy-silicone resin matrix.
Thus, the two main contributors for the complex permittivity of such composites are expected to be interfacial polarization and orientation polarization. It is reasonable that the higher complex permittivity (both the e 0 and e00 ) can be obtained when the composites filled with higher content of MWCNTs, due to the higher interfacial polarization at the MWCNT/CI particles/resin interfaces and orientation polarization of MWCNTs and CI particles. Furthermore, because the interfacial polarization can be more easily induced at lower frequency and the production of displacement current significantly lags behind the build-up potential as the frequency increased. Thus, it is reasonable that both e 0 and e00 decreased with increasing frequency and exhibited a visible frequency-dependent dielectric response [29,30]. The real permittivity value showed fluctuation at about 3 and 10 GHz
while the imaginary permittivity showed fluctuation about 4 and 11 GHz. This phenomenon is understandable because both real permittivity (polarization) and imaginary permittivity (electric loss) are correlated. Similar results have been reported in the MWCNT filled composites by Watts et al. [31,32]. The complex permeability spectra of the composites containing MWCNTs and CI particles are presented in Fig. 3a. The complex permeability dispersion curve of MWCNT filled epoxy-silicone resin is shown as the inset in Fig. 3a. The observed complex permeability of MWCNTs is mainly due to metal catalysts like Fe and Ni resulting from its synthesis process. As shown in Fig. 3a, two peaks are found in the spectrum of l00 –f for the composites containing MWCNTs and CI particles. For these two peaks, one is about 7.5 GHz and the other is about 13.5 GHz. In general, the magnetization
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Real part of permittivity
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polarized charge transfer has been observed when CNTs are contacting with magnetic materials [42]. Therefore, the double resonance behaviors of microwave magnetic permeability are believed due to the interactions between the MWCNTs and CI particles in the epoxy-silicone resin matrix. The interactions maybe occur between the nanosized residual magnetic metals (Fe and Ni) and MWCNTs, or between the MWCNTs and CI particles, as shown in the Fig. 1 d–f. However, the detailed effects of the MWCNTs on the complex permeability of the epoxy-silicone resin composites filled with MWCNTs and CI particles are still unclear and deeper investigation about the mechanism is still needed.
0.5vol% MWCNTs+50vol% CI 1vol% MWCNTs+50vol% CI 1.5vol% MWCNTs+50vol% CI
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The theoretical reflection loss values of the composites containing MWCNTs and CI particles can be obtained according to the transmission line theory. The reflection loss for a single-layer absorber is given by following relation:
0.5vol% MWCNTs+50vol% CI 1vol% MWCNTs+50vol% CI 1.5vol% MWCNTs+50vol% CI
40
3.3. The microwave absorption of epoxy-silicone composites containing MWCNTs and CI particles
RLðdBÞ ¼ 20log jðZin Z0 Þ=ðZin þ Z0 Þj
30
ð1Þ
where the input impedance of the absorber Zin is given
20 Zin ¼ Z0
rffiffiffiffiffi lr 2p pffiffiffiffiffiffiffiffi lr er fd tan h j c er
ð2Þ
10 0 2
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Frequency (GHz) Fig. 2 – Complex permittivity of the epoxy-silicone resin composites filled with MWCNTs and CI particles as a function of frequency.
changes of any ferromagnetic in radio frequency band are due to several types of magnetization mechanisms: domain wall motion, magnetization rotation, and gyromagnetic spin rotation (a natural resonance) [33–35]. It is well accepted that there are two mechanisms responsible for the permeability dispersion of CI particles: domain wall motion at lower frequencies and spin rotation at higher frequencies. We evaluated the frequency dependence of l–f by the frequency dispersion formula introduced by Tsutaoka [36], which describes a magnetic spectrum as a superposition of two resonance magnetization mechanisms: a resonance of domain wall motion and a natural resonance (gyromagnetic spin rotation). The fitting results are shown in Fig. 3b–d. It can be seen that the dominant role of complex permeability is contributed to the domain wall motion at lower frequencies while the spin rotation is at higher frequencies. The double resonance behaviors of microwave magnetic permeability also have been observed in NiCoZn ferrite/MWCNT/wax composites [37] and MWCNT filled composites containing Ni catalyst [38]. The investigations of CNTs filled with magnetic nanoparticles (Fe, Ni) showed that the interactions between the CNTs and magnetic nanoparticles have significantly effect on the magnetic properties of such composites [39–42]. For example, a measurable induced magnetic moment due to the spin
where Z0 is the impendency of the free space; lr and er are the relative permeability and permittivity of the absorber; f is the frequency of the electromagnetic wave; d is the thickness of the absorber; c is the velocity of light in free space. Thus, the reflectance of an absorber on metal surface is determined by the relative permittivity and permeability at a given frequency as well as the thickness of microwave absorbing materials. Fig. 4 shows the reflection loss of the composite containing 0.5 vol% MWCNT and 50 vol% CI particles with different composite thickness. The minimum reflection loss was found to move toward to the low frequency region (from 18 to 10.5 GHz) with increasing composite thickness. Also, the reflection loss values below 5 dB (over 70% microwave absorption) can be obtained in the frequency range of 10.4– 18, 4.4–18 and 2–18 GHz, when the composite thickness was 0.5, 1 and 1.5 mm, respectively. Especially, a minimum reflection loss value of 16.9 dB was obtained at 10.5 GHz and the 10 dB (over 90% microwave absorption) bandwidth was also obtained in the frequency range of 3.4–18 GHz for the composite thickness was 1.5 mm. The frequency band of reflection loss below 10 dB of the composites were broader than those previously reported CNT/CoFe2O4 spinel composites [9], (Fe, Ni)/C nanocapsules filled composites[16], CNTs filled with ferromagnetic alloy nanowires composites [43], Fe/ Fe3C–MWCNT composites [44] and Co–CNT filled composites [45]. It can be noticed that there are two possible contributions for microwave absorption, namely, dielectric loss (tan (de) = e00 / e 0 ) and magnetic loss (tan (dl) = l00 / l 0 ). In order to understand the actual contribution for the microwave absorption of the composites, both the dielectric loss and magnetic loss of the composite filled with 0.5 vol% MWCNT and 50 vol% CI particles were calculated based on the measured complex perme-
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0.5vol% 1vol% 1.5vol%
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(b) The complex permeability
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Fig. 3 – (a) Complex permeability of the epoxy-silicone resin composites filled with MWCNTs and CI particles as a function of the volume content of MWCNTs. The insert in (a) denoted the complex permeability of the epoxy-silicone resin composite filled with MWCNTs. (b–d) The calculation curves for domain wall rotation and spin rotation components of the complex permeability based on the experimental results: (b) is for the composite containing 0.5 vol% MWCNT and 50 vol% CI particles; (c) is for the composite containing 1 vol% MWCNT and 50 vol% CI particles; (d) is for the composite containing 1.5 vol% MWCNT and 50 vol% CI particles.
-5
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Dielectric loss
Reflection loss [dB]
0
sults from the contribution of both the CI particles and MWCNT/CI particles/resin three-dimension complex microstructures. Furthermore, a proper dielectric loss can also be obtained by adding a suitable content of MWCNTs into the CI particles filled epoxy-silicone resin composites (the values of dielectric loss changed from 0.27 to 1.42 in the frequency range of 2–18 GHz). Therefore, microwave absorption in these
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Frequency (GHz) Fig. 4 – Frequency dependent of the reflection loss of the composite filled with 0.5 vol% MWCNT and 50 vol% CI particles.
Magnetic loss
ability and permittivity, as shown in Fig. 5. The values of the magnetic loss increased from 0.76 to 3.11 in 2–8.5 GHz, later decreased to 1.95 in 8.5–10.3 GHz, and then increased with the frequency increasing (a maximum value of 3.81 can be obtained in 18 GHz). Compared with the CI particles filled epoxysilicone resin composites [46], the higher magnetic loss of the composites containing MWCNTs and CI particles mainly re-
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Frequency (GHz) Fig. 5 – The dielectric and magnetic loss of the epoxysilicone resin composite containing 0.5 vol% MWCNT and 50 vol% CI particles as a function of frequency.
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materials originated from the combination of magnetic loss of the CI particles and dielectric loss caused by the MWCNTs and the MWCNT/CI particles/resin interfaces. In generally, the excellent microwave absorbing properties of the absorber required the proper incorporated the following two keys (i) the impedance matching characteristic and (ii) the attenuation characteristic of the composites. The failure of either of the two keys will result in the decreased of microwave absorption. The MWCNT and CI particles filled composites with wider bandwidth microwave absorption was contributed to the match between the higher magnetic loss and suitable dielectric loss which can fulfill the impedance matching characteristic and attenuation characteristic. As discussed above, by simply change the content of MWCNTs and/or CI particles in the matrix, the dielectric loss and magnetic loss of the composites filled with MWCNTs and CI particles can be tuned easily to obtain a good match between each other, which resulted in these composites exhibited wider bandwidth absorbing property and thinner matching thickness.
4.
Conclusions
The electromagnetic and microwave absorbing properties of epoxy-silicone resin filled with MWCNTs and CI particles were studied as a function of frequency and MWCNT volume content. The complex permittivity of the composites decreased with increasing frequency and enhanced gradually with increasing the content of MWCNT in the frequency range of 2–18 GHz. The complex permeability exhibits two separated resonance peaks: one is about 7.5 GHz and the other is about 13.5 GHz. The double resonance behavior of the complex permeability of the epoxy-silicone composites filled with MWCNT and CI particles was mainly contributed to the domain wall motion at lower frequencies and the spin rotation at higher frequencies. For the composite filled with 0.5 vol% MWCNT and 50 vol% CI particles, the reflection loss values exceeding 10 dB are achieved in the frequency range of 3.4–18 GHz for the composite thickness is 1.5 mm. This work suggests that the MWCNTs and CI particles can be used to fabricate microwave absorber with wider bandwidth microwave absorption and thinner matching thickness.
Acknowledgment This work was supported by the fund of the State Key Laboratory of Solidification Processing in NWPU, No. KP200901.
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4 8 ( 2 0 1 0 ) 4 0 7 4 –4 0 8 0
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Epoxy-silicone filled with multi-walled carbon nanotubes and carbonyl iron particles as a microwave absorber Yuchang Qing *, Wancheng Zhou, Fa Luo, Dongmei Zhu State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi 0 an 710072, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Microwave absorbing composites with epoxy-silicone as matrix and both multi-walled car-
Received 22 May 2010
bon nanotubes (MWCNTs) and carbonyl iron (CI) particles as absorbers were prepared, and
Accepted 12 July 2010
their electromagnetic and microwave absorbing properties were investigated in the fre-
Available online 16 July 2010
quency range of 2–18 GHz. The microstructures of the composites show a uniform dispersion of the MWCNTs and CI particles in the matrix. The complex permittivity of the composites increased with increasing MWCNT content. A double resonance behavior of the complex permeability has been observed. One is due to the domain wall motion at about 7.5 GHz and the other is due to spin rotation at about 13.5 GHz. Reflection loss values exceeding 5 dB can be obtained in the frequency range of 10.4–18, 4.4–18 and 2–18 GHz, when the composite thickness is 0.5, 1 and 1.5 mm, respectively. A minimum reflection loss of 16.9 dB at 10.5 GHz and a bandwidth over the frequency range of 3.4–18 GHz with reflection loss below 10 dB can obtained for a composite filled with 0.5 vol% MWCNT and 50 vol% CI particles. Ó 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, many researchers have shown that carbon nanotubes (CNTs) have unique mechanical, electrical, magnetic, optical, and thermal properties [1–3]. Therefore, Multiwalled carbon nanotube (MWCNT)-filled polymeric composites have attracted many researchers’ attention in the manufacture of a variety of components for practical applications, such as electromagnetic interference shielding, microwave absorption, electronic packaging, and self-regulator heater [4–6]. It is well known that the complex permittivity (e* = e 0 je00 ) and permeability (l* = l 0 jl00 ) of the absorbers are main factors dominating the microwave absorption properties of a material. It is also speculated that magnetic material filled CNTs can be good microwave absorption material [7]. For this reason, the structure and microwave electromag-
netic properties of the magnetic material filled CNTs have been widely researched, for example, iron nanowires encased in MWCNTs [8], CoFe2O4/CNTs [9] and Fe/CNTs [10,11]. Che et al. [11] prepared Fe/CNTs filled composites as high-loss absorber to fabricated microwave absorption materials. Their results showed that the enhancement of microwave absorption of the Fe/CNTs filled composites was mainly contributed by magnetic loss which was caused by the encapsulated Fe in the CNTs. The objectives of this work are to fabricate microwave absorbing composites by using commercially available MWCNTs and carbonyl iron (CI) particles. The microwave electromagnetic properties of the composites are reported and the possible mechanisms are discussed. We investigate the microwave electromagnetic properties of epoxy-silicone composites filled with MWCNTs and CI particles for the
* Corresponding author: Fax: +86 29 8849 4574. E-mail address: [email protected] (Y. Qing). 0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2010.07.014
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following reasons. Firstly, recent studies have shown that CNT-filled polymer composites possess high real permittivity (polarization, e 0 ) as well as imaginary permittivity (absorption or electric loss, e00 ). As a result, CNT-filled polymer composites have been explored for potential applications in electromagnetic shielding or absorbing materials [12–19]. Secondly, CI particles have a large saturation magnetization and high Snoek’s limit which result in high complex permeability values at a wide frequency range. This factor makes CI particles are useful for microwave absorber at a high-frequency band [20,21]. Finally, the values of complex permittivity increased dramatically with small additions of MWCNTs to the polymer have been reported by Grimes et al. [22]. Thus, by combine the high complex permittivity of the MWCNTs and high complex permeability of CI particles, the high microwave absorption of the composites filled with MWCNTs and CI particles can be obtained.
2.
Experimental
2.1.
Materials
The matrix used in this work is an epoxy-silicone resin and the cure agent is polyamide resin, which were supplied by XI AN Leeo Technological Co. Raw commercial CI particles were purchased from Xinghua chemical Co. Ltd, Shanxi province, China. The MWCNTs were fabricated by the catalytic decomposition of CH4, supplied by the Shenzhen Nanotech port Co. Ltd., China. The diameter of MWCNTs was ranging from 60 to 100 nm, and length was 5–15 lm, and the purity was 95%. The sodium dodecyl benzene sulfonate (SDS) (Sinopharm Chemical Reagent Co. Ltd. China) as a surfactant was employed to obtain better dispersion of the MWCNTs.
2.2.
Sample preparation
The samples were fabricated by incorporating 50 vol% CI particles and different amounts of MWCNTs (the content of MWCNTs is 0.5, 1 and 1.5 vol%) into epoxy-silicone resin. The MWCNTs and SDS were dispersed in ethanol by an ultrasonic bath at room temperature for 1 h. After adding the CI particles into the ethanol based solution, the mixtures were uniformly mixed by stirring at 2000 rpm for 10 min and placed in oven at 80 °C to evaporate the ethanol completely. Then the resin and hardener was added into the mixtures, followed by stirring at 2000 rpm for 10 min. The mixtures were pre-cured at 90 °C for 30 min and then post-cured at 120 °C for 2 h.
2.3.
Measurements
The morphology of the composites was observed by scanning electron microscopy (SEM) (Model JSM-6360, JEOL, Tokyo, Japan). The complex permittivity e(f) and permeability l(f) of the composites were measured using the T/R coaxial line method in the frequency range of 2–18 GHz by a network analyzer (Agilent technologies E8362B: 10 MHz–20 GHz). The testing specimens had a cylindrical toroidal specimen: outer diameter of 7.0 mm, inner diameter of 3.03 mm and thickness of 2 mm.
3.
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Results and discussion
3.1. The microstructures of epoxy-silicone composites filled with MWCNTs and CI particles
resin
Fig. 1 shows the SEM images of the (a) MWCNTs and (b) CI particles. The CI particles are thin flakes of 2–5 lm in diameter and below 1 lm in thickness. Fig. 1c represents the backscatter electron images of the composites filled with MWCNTs and CI particles. The dispersion state and morphology of the epoxy-silicone resin composites filled with MWCNTs and CI particles were examined in detail by the fractured surface morphologies. As seen from Fig. 1d and e, the MWCNTs and CI particles were well dispersed in the matrix. The pristine MWCNTs exhibit a strong tendency to form entanglement and agglomerate which caused by the intermolecular van der Waals force between the MWCNTs. It can be said that the shear forces induced by the ultrasonic treatment and intense stirring mixing of the mixture overcoming the intermolecular van der Waals force, lead to a dramatic improvement of the dispersion of MWCNTs in the matrix [23,24]. Furthermore, the SDS as a surfactant and CI particles as a dispersing aid also improved the overall dispersion of MWCNTs in the epoxy-silicone resin matrix. The SEM images show that complex microstructures of the composites with multiple MWCNT bridging adjacent CI particles (as indicated by the arrow) can be obtained. Three dimensional network structures were generated through the MWCNT/CI particles/epoxy-silicone resin interfacial interactions, as shown in Fig. 1f.
3.2. Effects of the volume content of MWCNTs on the complex permittivity and permeability of the epoxy-silicone composites filled with MWCNTs and CI particles In order to evaluate the microwave electromagnetic of epoxysilicone resin composites filled with MWCNTs and CI particles, we measured the complex permittivity and permeability of the composites in the frequency range of 2–18 GHz. Fig. 2 shows the complex permittivity spectra of the composites containing MWCNTs and CI particles. With the content of MWCNTs increased from 0.5 to 1.5 vol%, both the real (e 0 ) and imaginary (e00 ) part of complex permittivity of the composites increased. The highest values of the e 0 and e00 of the composite filled with 1.5 vol% MWCNT and 50 vol% CI particles can reached to 46 and 37, respectively. The complex permittivity also trended to decrease with increasing frequency and showed frequency dependence in the whole frequency range (2–18 GHz). In general, the dielectric properties of conductive particle filled insulation resin composites depends on the characteristics of the matrix, the property and volume fraction of the filler, the configuration and internal structure of the composites, and the frequency of the electromagnetic waves [25–27]. Obviously, both the MWCNTs and CI particles were included in an insulating resin matrix to constitute heterogeneous composites. Interfacial polarization is an important polarization process and associated relaxation will give rise to dielectric loss mechanism of such composites [28]. Moreover, orientation polarization is another polarization mechanism and the associated relaxation phenomena also constitute the dielectric loss mechanisms of such composites.
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Fig. 1 – SEM images of the (a) MWCNTs. (b) CI particles. (c) Backscatter electron images of the composite filled with MWCNTs and CI particles. (d and e) SEM images of the fracture surface of the epoxy-silicone composite filled with MWCNTs and CI particles (the red arrows for the MWCNTs). (f) The schematic image to show the three dimensional network structure generated by the MWCNTs, CI particles and epoxy-silicone resin matrix.
Thus, the two main contributors for the complex permittivity of such composites are expected to be interfacial polarization and orientation polarization. It is reasonable that the higher complex permittivity (both the e 0 and e00 ) can be obtained when the composites filled with higher content of MWCNTs, due to the higher interfacial polarization at the MWCNT/CI particles/resin interfaces and orientation polarization of MWCNTs and CI particles. Furthermore, because the interfacial polarization can be more easily induced at lower frequency and the production of displacement current significantly lags behind the build-up potential as the frequency increased. Thus, it is reasonable that both e 0 and e00 decreased with increasing frequency and exhibited a visible frequency-dependent dielectric response [29,30]. The real permittivity value showed fluctuation at about 3 and 10 GHz
while the imaginary permittivity showed fluctuation about 4 and 11 GHz. This phenomenon is understandable because both real permittivity (polarization) and imaginary permittivity (electric loss) are correlated. Similar results have been reported in the MWCNT filled composites by Watts et al. [31,32]. The complex permeability spectra of the composites containing MWCNTs and CI particles are presented in Fig. 3a. The complex permeability dispersion curve of MWCNT filled epoxy-silicone resin is shown as the inset in Fig. 3a. The observed complex permeability of MWCNTs is mainly due to metal catalysts like Fe and Ni resulting from its synthesis process. As shown in Fig. 3a, two peaks are found in the spectrum of l00 –f for the composites containing MWCNTs and CI particles. For these two peaks, one is about 7.5 GHz and the other is about 13.5 GHz. In general, the magnetization
CARBON
Real part of permittivity
50
4 8 ( 20 1 0 ) 4 0 7 4–40 8 0
polarized charge transfer has been observed when CNTs are contacting with magnetic materials [42]. Therefore, the double resonance behaviors of microwave magnetic permeability are believed due to the interactions between the MWCNTs and CI particles in the epoxy-silicone resin matrix. The interactions maybe occur between the nanosized residual magnetic metals (Fe and Ni) and MWCNTs, or between the MWCNTs and CI particles, as shown in the Fig. 1 d–f. However, the detailed effects of the MWCNTs on the complex permeability of the epoxy-silicone resin composites filled with MWCNTs and CI particles are still unclear and deeper investigation about the mechanism is still needed.
0.5vol% MWCNTs+50vol% CI 1vol% MWCNTs+50vol% CI 1.5vol% MWCNTs+50vol% CI
40
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30 20 10 0 2
4
6
8
10
12
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16
18
Frequency (GHz)
Imaginary part of permittivity
50
The theoretical reflection loss values of the composites containing MWCNTs and CI particles can be obtained according to the transmission line theory. The reflection loss for a single-layer absorber is given by following relation:
0.5vol% MWCNTs+50vol% CI 1vol% MWCNTs+50vol% CI 1.5vol% MWCNTs+50vol% CI
40
3.3. The microwave absorption of epoxy-silicone composites containing MWCNTs and CI particles
RLðdBÞ ¼ 20log jðZin Z0 Þ=ðZin þ Z0 Þj
30
ð1Þ
where the input impedance of the absorber Zin is given
20 Zin ¼ Z0
rffiffiffiffiffi lr 2p pffiffiffiffiffiffiffiffi lr er fd tan h j c er
ð2Þ
10 0 2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 2 – Complex permittivity of the epoxy-silicone resin composites filled with MWCNTs and CI particles as a function of frequency.
changes of any ferromagnetic in radio frequency band are due to several types of magnetization mechanisms: domain wall motion, magnetization rotation, and gyromagnetic spin rotation (a natural resonance) [33–35]. It is well accepted that there are two mechanisms responsible for the permeability dispersion of CI particles: domain wall motion at lower frequencies and spin rotation at higher frequencies. We evaluated the frequency dependence of l–f by the frequency dispersion formula introduced by Tsutaoka [36], which describes a magnetic spectrum as a superposition of two resonance magnetization mechanisms: a resonance of domain wall motion and a natural resonance (gyromagnetic spin rotation). The fitting results are shown in Fig. 3b–d. It can be seen that the dominant role of complex permeability is contributed to the domain wall motion at lower frequencies while the spin rotation is at higher frequencies. The double resonance behaviors of microwave magnetic permeability also have been observed in NiCoZn ferrite/MWCNT/wax composites [37] and MWCNT filled composites containing Ni catalyst [38]. The investigations of CNTs filled with magnetic nanoparticles (Fe, Ni) showed that the interactions between the CNTs and magnetic nanoparticles have significantly effect on the magnetic properties of such composites [39–42]. For example, a measurable induced magnetic moment due to the spin
where Z0 is the impendency of the free space; lr and er are the relative permeability and permittivity of the absorber; f is the frequency of the electromagnetic wave; d is the thickness of the absorber; c is the velocity of light in free space. Thus, the reflectance of an absorber on metal surface is determined by the relative permittivity and permeability at a given frequency as well as the thickness of microwave absorbing materials. Fig. 4 shows the reflection loss of the composite containing 0.5 vol% MWCNT and 50 vol% CI particles with different composite thickness. The minimum reflection loss was found to move toward to the low frequency region (from 18 to 10.5 GHz) with increasing composite thickness. Also, the reflection loss values below 5 dB (over 70% microwave absorption) can be obtained in the frequency range of 10.4– 18, 4.4–18 and 2–18 GHz, when the composite thickness was 0.5, 1 and 1.5 mm, respectively. Especially, a minimum reflection loss value of 16.9 dB was obtained at 10.5 GHz and the 10 dB (over 90% microwave absorption) bandwidth was also obtained in the frequency range of 3.4–18 GHz for the composite thickness was 1.5 mm. The frequency band of reflection loss below 10 dB of the composites were broader than those previously reported CNT/CoFe2O4 spinel composites [9], (Fe, Ni)/C nanocapsules filled composites[16], CNTs filled with ferromagnetic alloy nanowires composites [43], Fe/ Fe3C–MWCNT composites [44] and Co–CNT filled composites [45]. It can be noticed that there are two possible contributions for microwave absorption, namely, dielectric loss (tan (de) = e00 / e 0 ) and magnetic loss (tan (dl) = l00 / l 0 ). In order to understand the actual contribution for the microwave absorption of the composites, both the dielectric loss and magnetic loss of the composite filled with 0.5 vol% MWCNT and 50 vol% CI particles were calculated based on the measured complex perme-
4 8 ( 2 0 1 0 ) 4 0 7 4 –4 0 8 0
0.5vol% 1vol% 1.5vol%
3
1.5
5
µ' µ''
1.0
0.5
0.0 2
4
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14
16
18
4
Frequency(GHz)
2 3
1 0 2
4
6
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18
(b) The complex permeability
Real part of permeability
(a) 4
Complex permeability
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Imaginary part of permeability
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Domain wall
3.0
2.0 1.5
Spin
1.0 0.5
Domain wall
0.0
2
4
6
Domain wall
3.0
µ' µ''
Spin
2.5 2.0 1.5
Spin
1.0
Domain wall
0.5
8
10
12
14
16
18
Frequency (GHz) The complex permeability
The complex permeability
(c)
3.5
Spin
2.5
Frequency (GHz) 4.0
µ' µ''
3.5
(d) 4.0
µ' µ''
Domain wall
3.5 3.0
Spin 2.5 2.0
Spin 1.5
Domain wall
1.0
0.0 2
4
6
8
10
12
14
16
2
18
4
6
8
10
12
14
16
18
Frequency (GHz)
Frequency (GHz)
Fig. 3 – (a) Complex permeability of the epoxy-silicone resin composites filled with MWCNTs and CI particles as a function of the volume content of MWCNTs. The insert in (a) denoted the complex permeability of the epoxy-silicone resin composite filled with MWCNTs. (b–d) The calculation curves for domain wall rotation and spin rotation components of the complex permeability based on the experimental results: (b) is for the composite containing 0.5 vol% MWCNT and 50 vol% CI particles; (c) is for the composite containing 1 vol% MWCNT and 50 vol% CI particles; (d) is for the composite containing 1.5 vol% MWCNT and 50 vol% CI particles.
-5
-10
4
1.50
0.5mm 1mm 1.5mm
1.25
Dielectric loss
Reflection loss [dB]
0
sults from the contribution of both the CI particles and MWCNT/CI particles/resin three-dimension complex microstructures. Furthermore, a proper dielectric loss can also be obtained by adding a suitable content of MWCNTs into the CI particles filled epoxy-silicone resin composites (the values of dielectric loss changed from 0.27 to 1.42 in the frequency range of 2–18 GHz). Therefore, microwave absorption in these
3
1.00 2 0.75 1
0.50
-15
0.25 2
4
6
8
10
12
14
16
18
Frequency (GHz) Fig. 4 – Frequency dependent of the reflection loss of the composite filled with 0.5 vol% MWCNT and 50 vol% CI particles.
Magnetic loss
ability and permittivity, as shown in Fig. 5. The values of the magnetic loss increased from 0.76 to 3.11 in 2–8.5 GHz, later decreased to 1.95 in 8.5–10.3 GHz, and then increased with the frequency increasing (a maximum value of 3.81 can be obtained in 18 GHz). Compared with the CI particles filled epoxysilicone resin composites [46], the higher magnetic loss of the composites containing MWCNTs and CI particles mainly re-
2
4
6
8
10
12
14
16
0 18
Frequency (GHz) Fig. 5 – The dielectric and magnetic loss of the epoxysilicone resin composite containing 0.5 vol% MWCNT and 50 vol% CI particles as a function of frequency.
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materials originated from the combination of magnetic loss of the CI particles and dielectric loss caused by the MWCNTs and the MWCNT/CI particles/resin interfaces. In generally, the excellent microwave absorbing properties of the absorber required the proper incorporated the following two keys (i) the impedance matching characteristic and (ii) the attenuation characteristic of the composites. The failure of either of the two keys will result in the decreased of microwave absorption. The MWCNT and CI particles filled composites with wider bandwidth microwave absorption was contributed to the match between the higher magnetic loss and suitable dielectric loss which can fulfill the impedance matching characteristic and attenuation characteristic. As discussed above, by simply change the content of MWCNTs and/or CI particles in the matrix, the dielectric loss and magnetic loss of the composites filled with MWCNTs and CI particles can be tuned easily to obtain a good match between each other, which resulted in these composites exhibited wider bandwidth absorbing property and thinner matching thickness.
4.
Conclusions
The electromagnetic and microwave absorbing properties of epoxy-silicone resin filled with MWCNTs and CI particles were studied as a function of frequency and MWCNT volume content. The complex permittivity of the composites decreased with increasing frequency and enhanced gradually with increasing the content of MWCNT in the frequency range of 2–18 GHz. The complex permeability exhibits two separated resonance peaks: one is about 7.5 GHz and the other is about 13.5 GHz. The double resonance behavior of the complex permeability of the epoxy-silicone composites filled with MWCNT and CI particles was mainly contributed to the domain wall motion at lower frequencies and the spin rotation at higher frequencies. For the composite filled with 0.5 vol% MWCNT and 50 vol% CI particles, the reflection loss values exceeding 10 dB are achieved in the frequency range of 3.4–18 GHz for the composite thickness is 1.5 mm. This work suggests that the MWCNTs and CI particles can be used to fabricate microwave absorber with wider bandwidth microwave absorption and thinner matching thickness.
Acknowledgment This work was supported by the fund of the State Key Laboratory of Solidification Processing in NWPU, No. KP200901.
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