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Scripta Materialia 67 (2012) 613–616 www.elsevier.com/locate/scriptamat
Carbon microtube/Fe3O4 nanocomposite with improved wave-absorbing performance X. Huang,a M. Lu,a X. Zhang,a G. Wen,a,b,⇑ Y. Zhoua and L. Feic a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China School of Materials Science and Engineering, Harbin Institute of Technology at Weihai, Weihai 264209, People’s Republic of China c Yunshan Carbon Industry Co. Ltd., Luobei, Heilongjiang Province 154200, People’s Republic of China Received 28 February 2012; revised 18 June 2012; accepted 18 June 2012 Available online 23 June 2012
We report a facile strategy to prepare a carbon microtube (CMT)/Fe3O4 nanocomposite with superior performance. The Fe3O4 nanoparticles obtained are about 400 nm in size and homogeneously anchor around the CMTs. The results show that the maximum reﬂection loss of the CMT/Fe3O4 nanocomposite is about 40 dB at 10.64 GHz and the eﬀective absorbing band width is about 4 GHz with a thickness of 2.0 mm, highlighting the importance of the CMTs with nanomagnetic particles for wave-absorbing applications. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Carbon microtubes (CMTs); Nanocomposite; Microstructure; Electromagnetic absorbing
Electromagnetic wave-absorbing (EMA) materials are continually in demand not only for popular consumer electronics but also for military needs. So far, materials such as graphitic/nongraphitic carbon , SiC , ferrite [3,4], ultraﬁne metal powder (Fe, Co and Ni [5,6]) and ferromagnetic alloys [7–9], etc., have been exploited as EMA materials. Among these, carbon nanotubes (CNTs) have attracted extensive interest due to a higher theoretical surface area (190 m2 g1), lower density, large number dangling bonds creating interface polarization, and the macroscopic quantum tunneling eﬀect—all properties required in high-performance electromagnetic materials . Recently, in order to determine the relationship of electromagnetism to both magnetic and dielectric loss, numerous studies have been carried out to introduce ferrite particles into CNTs to enhance the ferromagnetism of the nanocomposite. Several processes, including simple mechanical mixing [11,12], in situ reaction , sol–gel  etc., have been developed to synthesize nanoparticles, which have resulted in signiﬁcant improvements. However, the use of CNTs still has a number of various disadvantages, such as low purity, diﬃculty of dispersion in the composite, appropriate microstructural control, etc.
author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China; e-mail: [email protected]
To prepare materials with improved EMA performance by exploiting the graphitic structure of CNTs and playing on their advantages remains a considerable challenge. Carbon microtubes (CMTs) are a unique form of carbon with micrometer-scale internal diameters and thin walls comprising a few graphitic layers. CMTs are potential EMA material due to their having similar microstructures to CNTs, low density (apparent density 0.16–0.18 g cm3), low manufacturing cost, easy dispersion in the composite and simple large-scale synthesis . In addition, it is generally thought that large-diameter carbon tubes can lead to the enhancement of the polarization eﬀect with a higher rate of capacitance, resulting in a strong dielectric loss. Therefore, it is reasonable to believe that composites of CMTs with nanostructured ferrite particles can eﬃciently combine their respective advantages to obtain EMA materials with superior performance. Herein, we report a facile strategy to synthesize such a composite of Fe3O4 nanoparticles anchored on CMTs through a solvothermal-reduction reaction. The composite displays superior EMA performance with the improved permittivity and permeability, highlighting the importance of its application in the wave-absorbing ﬁeld. The CMTs used in the present study are prepared through a gas pressure enhanced chemical vapor
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.06.024
X. Huang et al. / Scripta Materialia 67 (2012) 613–616
deposition method similar to that described in Ref. . The CMTs have uniform internal diameters of about 1 lm, are several millimeters in length and have a wall thickness of about 5 nm. CMT/Fe3O4 nanocomposite particles were prepared through a solvothermal-reduction reaction between FeCl3 and ethylene glycol with CMT to yield CMT decorated with Fe3O4 nanoparticles. FeCl36H2O (0.5 g) was dissolved in ethylene glycol (15 ml) to form a clear solution, followed by the addition of 0.375 g polyethylene and CMT (0.04 g) to form a black solution with the help of an ultrasonic bath. Next, 1.35 g NaAc was slowly added to the solution with vigorous stirring for 30 min, and solution was then sealed in a Teﬂon-lined stainless-steel autoclave (25 ml capacity). The autoclave was heated to and maintained at 200 °C for 8 h, and allowed to cool to room temperature. The black products, about 30 wt.% CMT/Fe3O4 nanocomposite particles, were washed several times with ethanol and dried at 60 °C overnight, according to the procedure followed in Ref. . The phase composition was examined by X-ray diffraction (XRD; Rigaku D/max-rB, with Cu Ka) and the morphology of the CMT/Fe3O4 nanocomposite particles was observed by ﬁeld-emission scanning electron microscopy (FE-SEM; Hitachi S-4700); transmission electron microscopy (TEM) images were obtained using on a Tecnai F30 FEG operating at 100 kV accelerating voltage. The electromagnetic parameters of the composite were measured on a vector network analyzer (VNA; Agilent N5230A) in transmission/reﬂection mode in the 2–18 GHz band. The electromagnetic parameters of the specimens were derived from the S parameters through HP85071 software. The concentration of CMT/Fe3O4 nanocomposite particles in VNA specimens is 20 vol.%, and 80 vol.% of paraﬃn which was interfused
as the binder. The thoroughly mixed CMT/Fe3O4/parafﬁn powder with the above-mentioned composition was pressed into a mold to fabricate coaxial specimens. The specimens were coaxial toroidals with an outer diameter of 7 mm, inner diameter of 3 mm and a thickness of 3.5–4 mm. The VNA measurement was performed on four specimens with diﬀerent thicknesses to avoid discrepancies. For comparison, Fe3O4 nanoparticles and CMTs were also prepared in the same conditions. Figure 1 shows a typical XRD pattern of the as-prepared CMT/Fe3O4 nanocomposite. Compared to that of pure Fe3O4, an additional clear (0 0 2) diﬀraction peak appears at 2h of 26.5°, which can be indexed as the graphite (JCPDS No. 41-1487). All of the other diﬀraction peaks can be ascribed to the well-crystallized Fe3O4 with a body-centered orthorhombic (a = 0.591 nm, b = 5.945 nm, c = 8.388 nm) structure (JCPDS No. 751609). These results indicate that a composite consisting of well-developed graphitic CMTs and well-crystallized Fe3O4 was obtained. No other new phase was found therein. The density of the CMT/Fe3O4 nanocomposite particles is 3.71 g cm3. To determine the chemical composition of the CMT/ Fe3O4 nanocomposite, X-ray photoelectron spectroscopy (XPS) measurements were carried out in the range of 0–1200 eV (Fig. 1b–d). Figure 1b is the wide scan spectrum of the sample, in which the photoelectron lines at a binding energy of about 285, 530 and 711 eV are attributed to C1s, O1s and Fe2p, respectively. The Fe2p XPS spectra of the composite exhibit two peaks at 711 and 724.9 eV, corresponding to the Fe2p1/2 and Fe2p3/2, which conﬁrms that the oxide in the sample is Fe3O4 . The presence of Fe3O4 can be further conﬁrmed by the O1s XPS peak at 530.1 eV, which 5000
O1s Fe2p1/2 Fe2p3/2
Binding Energy (eV)
2Theta (Deg.) 6000
4500 4000 3500
3000 2500 2000
1500 1500 1000
Binding Energy (eV)
Binding Energy (eV)
Figure 1. (a) XRD patterns of Fe3O4 and the CMT/Fe3O4 nanocomposite particles (, Fe3O4; h, graphite), (b) XPS spectrum of the CMT/Fe3O4 composite, (c) Fe2p and (d) O1s XPS spectra of CMT/Fe3O4 composite.
X. Huang et al. / Scripta Materialia 67 (2012) 613–616
sented in Figure 3. It is obvious that the values of the complex permittivity (e0 , e00 ) of the CMT/Fe3O4 nanocomposite are all higher than those CMTs: the e0 of the CMT/Fe3O4 nanocomposite remains at 15–20 in the frequency range 2–14 GHz. The dielectric relaxations due to the interfacial polarization appear around 13 and 16 GHz. When the conductive phase is distributed in the insulating matrix to form composite materials, the free charge gathering will exist in the insulation/ conductor interface due to the diﬀerence in the twophase conductive performance. The polarization of these charges under the action of external electromagnetic ﬁelds becomes the main mechanism which determining the eﬀective dielectric properties of the composite [18,19]. In general, when the external ﬁeld frequency is low, the real part e0 is higher and the imaginary part e00 is lower for the dielectric polarization following the change in the external ﬁeld. With increasing frequency of the external ﬁeld, the dielectric loss occurs for the polarization of the free charge gradually and does not follow the ﬁeld change, corresponding to the imaginary part which gradually increased to a higher peak value, and a strong dielectric relaxation occurs, as shown in Figure 3. On the other hand, it is generally believed that the interfacial polarization level of the conductor/insulator composites and the width of the dielectric relaxation frequency band are aﬀected by the conductor’s electrical conductivity: the higher the conductivity, the higher the interfacial polarization and the wider the dielectric relaxation frequency band. In general, the improved electrical conductivity and proper dielectric relaxation are favorable for improving microwave absorption properties. [20,21]. In comparison, distinct characteristics of dielectric relaxation, i.e. a decreased e0 with an increase in frequency, and peaks in e00 , are observed at 13 and 16 GHz in the CMT/ Fe3O4 nanocomposite with a wide band of strong dielectric relaxation behavior covering almost the entire 8– 18 GHz band. As shown in Figure 3b, the real part (l0 ) of the permeability of the CMT/Fe3O4 nanocomposites decreases from 1.63 to 0.84. Additionally, the l00 of the CMT/ Fe3O4 nanocomposites is slightly higher than that of Fe3O4 with a decreasing trend in the 2–18 GHz band, and the value of the l00 is negative at 13–18 GHz, which probably results from the enhancement of the conductivity of the composite. The CMT/Fe3O4 nanocompos-
Figure 2. SEM (a) and TEM (b) images of CMT/Fe3O4 nanocomposite. The inset in (b) is the SAED pattern of Fe3O4 nanoparticles and CMTs with [1 1 0], [1 0 0] and [0 0 2] planes in the nanocomposite, respectively, indicative of the single-crystalline nature of Fe3O4 nanoparticles and the polycrystalline graphite structure of CMTs.
corresponds to the oxygen species in the Fe3O4 phase (Fig. 1d); the small O1s peak at 532.0 eV in Figure 1d indicates the presence of oxygen-containing groups (such as –OH and –COOH) which bonded with C atoms in the CMTs . Figure 2 shows SEM and TEM images of the as-prepared CMT/Fe3O4 nanocomposite. It can be seen from Figure 2a that the small Fe3O4 nanoparticles are anchored around the CMTs. The selected-area electron diﬀraction (SAED) pattern (inserted in Fig. 2b) clearly demonstrates the well-textured and single-crytalline nature of Fe3O4 and the graphitic structure of CMTs in the CMT/Fe3O4 nanocomposite, which is consistent with the XRD results. There are several distinctive characteristics apparent in Figure 2. First, it can be seen that the Fe3O4 nanoparticles have a size of about 300–400 nm and are homogeneously anchored on the outer wall of the CMTs, indicating that, compared to the carbon nanotube composite, it is easy to get a homogeneous dispersion with a higher content of about 30 wt.% CMTs in the composite. Secondly, it should be noted that, even after a long period of sonication during the preparation of the TEM specimen, there is strong interaction between the Fe3O4 nanoparticles and the CMTs. In addition, the strong anchoring and the homogeneous dispersion of the Fe3O4 nanoparticles and the CMTs can eﬃciently prevent nanoparticle aggregation, avoiding a decrease in the highly active surface area. Some Fe3O4 nanoparticles are possibly inside the CMTs, and further study is required to elucidate this point. The electromagnetic performances of the as-prepared CMT/Fe3O4 nanocomposite, CMTs and Fe3O4 nanoparticles in the frequency range 2–18 GHz are preε
// ε for Fe3O4/CMTs // ε for CMT
// μ for CMTs/ Fe3O4
// μ for Fe3O4
Figure 3. The complex permittivity (a) and complex permeability (b) plotted against frequency for the CMT/Fe3O4 nanocomposite particle/paraﬃn and CMT/paraﬃn composite specimens.
X. Huang et al. / Scripta Materialia 67 (2012) 613–616
CMT/Fe3O4 nanocomposite was successfully prepared by a solvothermal-reduction reaction between FeCl3 and ethylene glycol with CMTs. The Fe3O4 nanoparticles obtained are about 400 nm in size and homogeneously anchor around the CMTs. The improvement in permittivity is attributed to the occurrence of multiple dielectric relaxations. As a result, the CMT/Fe3O4 nanocomposite exhibits a strong RL (38.21 dB) with a Df of about 4 GHz, highlighting the advantages of the Fe3O4 (probably including other ferrite) anchored to the CMTs for improving the EMA properties for wave-absorbing applications.
Reflection loss (dB)
CMTs/Fe O d=2mm 3 4
Fe O d=2mm 3 4
CMTs/Fe O d=2.5mm 3 4
Fe O d=2.5mm 3 4
CMTs/Fe O d=3mm 3 4
Fe O d=3mm 3 4
0 -5 -10 -15 -20 -25 -30 -35 -40 2
Figure 4. Reﬂection loss vs. frequency of CMTs, CMT/Fe3O4 nanocomposites and Fe3O4 nanoparticles for various thicknesses.
ites form a conductive network based on the good conductivity of CMTs. When there is an alternating magnetic ﬁeld, an induced opposite magnetic ﬁeld is generated for the vortex due to the free electrons under the Lorenz force, and thus part of the electric ﬁeld which is transformed into the magnetic ﬁeld can be released, resulting in the e00 value peak and the l00 being negative. According to transmission line theory , the relationship between the reﬂection loss (RL) of the CMT/ Fe3O4 nanocomposite particles with assumed thickness ranging from 2 to 3 mm and frequency is shown in Figure 4. The RL of the nanocomposite/paraﬃn specimens at diﬀerent assumed coating thicknesses can be calculated using Eqs. (1) and (2), in which lr and er are the complex permeability and the complex permittivity, respectively. The f, d, c and Zin coeﬃcients are the frequency, the coating thickness, the velocity of light and the input impedance of absorber, respectively. jZin 1j ð1Þ RLðdBÞ ¼ 20 log jZin þ 1j Zin ¼ ðlr =er Þ
tan h½jð2pfd=cÞðlr er Þ
As indicated in Figure 4, the RL cannot be less than 20 dB for the single Fe3O4 with a thickness of 2–3 mm. For the CMT/Fe3O4 nanocomposite, an optimal maximun RL of about 38.21 dB at around 10.64 GHz is obtained with the thickness of 2 mm, which also has a wider eﬀective absorbing band width Df. The Df is generally deﬁned as the width of the band in which the RL exceeds 10 dB; Df is about 4 GHz for this composite. The enhanced RL results from the combination of both the magnetic loss and a strong dielectric loss when adding the CMTs into the Fe3O4 particles. It is noteworthy that the RL shift of the CMTs is nearly 23 dB with a thickness of 2 mm with a Df more than 6.5 GHz. The eﬃciency with which CMT absorb microwaves will mostly depend on the formation of an eﬀective conductive path; as a typical dielectric loss material, the CMTs can reduce eddy currents that drastically decrease highfrequency permeability. On the other hand, large-diameter CMTs have more structural defects compared with CNTs, leading to the enhancement of the polarization eﬀect, which results in strong dielectric loss with a higher rate of capacitance .
This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 5102 1002), Natural Scientiﬁc Research Innovation Foundation in Harbin Institute of Technology (HIT.NSRIF.2011109, HIT.NSRIF.2010121), the Fundamental Research Funds for the Central Universities (HIT.ICRST.2010009), and Yunshan Carbon Industry Co. Ltd.  W. Song, M. Cao, Z. Hou, J. Yuan, X. Fang, Scripta Mater. 61 (2009) 201.  X. Liu, Z. Zhang, Y. Wu, Composites part B 42 (2011) 326.  A. Drmota, J. Koselj, M. Drofenik, A. Znidarsic, J. Magn. Magn. Mater. 324 (2012) 1225.  M. Pardavi-Horvath, J. Magn. Magn. Mater. 215 (2000) 171.  X. Li, Y. Duang, Y. Zhao, L. Zhu, Prog. Nat. Sci.: Mater. Int. 21 (2011) 392.  F. Ma, Y. Qin, F. Wang, D. Xue, Scripta Mater. 63 (2010) 1145.  R. Che, L. Peng, X. Duan, Q. Chen, X. Liang, Adv. Mater. 5 (2004) 401.  L. Zhen, Y. Gong, J. Jiang, W. Shao, J. Appl. Phys. 104 (2008) 034312.  Y. Zhao, X. Zhang, J. Xiao, Adv. Mater. 17 (2005) 915.  J. Thomassin, I. Huynen, R. Jerome, Polymer 51 (2010) 115.  G. Liu, L. Wang, G. Chen, S. Hua, C. Ge, H. Zhang, R. Wu, J. Alloys Compd. 514 (2012) 183.  D. Micheli, R. Pastore, C. Apollo, M. Marchetti, G. Gradoni, V. Primiani, F. Moglie, IEEE Trans. Microwave Theory Tech. 59 (2011) 2633.  H. Zhu, Y. Bai, R. Liu, N. Lun, Y. Qi, F. Han, J. Bi, J. Mater. Chem. 21 (2011) 13581.  W. Wang, Q. Li, C. Chang, Synth. Met. 161 (2011) 44.  G. Wen, H. Yu, X. Huang, Carbon 49 (2011) 4059.  Y. Zhan, R. Zhao, Y. Lei, F. Meng, J. Zhong, X. Liu, J. Magn. Magn. Mater. 323 (2011) 1006.  T. Missana, C. Maﬃotte, M. Garcı´a-Gutie´rrez, J. Colloid Interface Sci. 261 (2003) 154.  P. Watts, W. Hsu, A. Barnes, Adv. Mater. 15 (2003) 600.  M. Mezeme, S. Lasquellec, C. Brosseaua, J. Appl. Phys. 109 (2011) 014302.  I.J. Youngs, N. Bowler, K.P. Lymer, S. Hussian, J. Phys. D: Appl. Phys. 38 (2005) 188.  Y. Yang, B.S. Zhang, W.D. Xu, Y.B. Shi, Z.S. Jiang, J. Magn. Magn. Mater. 256 (2003) 129.  Y. Natio, K. Suetake, IEEE Trans. Microwave Theory Tech. 19 (1971) 65.  M.H. Al-Saleh, U. Sundararaj, Carbon 47 (2009) 1738.