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polythiophene
Composites Science and Technology 117 (2015) 215e224
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
A novel ternary hybrid electromagnetic wave-absorbing composite based on BaFe11.92(LaNd)0.04O19-titanium dioxide/multiwalled carbon nanotubes/polythiophene Xiaowei Hong a, b, Yu Xie a, b, *, Xiaoying Wang c, Mingjun Li a, Zhanggao Le d, **, Yunhua Gao b, Yan Huang a, Yuancheng Qin a, Yun Ling a a
Department of Materials Chemistry, Nanchang Hangkong University, Nanchang, 330063, PR China Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, Chinese Academy of Sciences, Beijing, 100190, PR China Department of Applied Chemistry, South China University of Technology, Guangzhou, 510640, PR China d Department of Applied Chemistry, East China Institute of Technology, Nanchang, 330013, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2014 Received in revised form 18 June 2015 Accepted 28 June 2015 Available online 2 July 2015
We report a novel ternary magnetic-conductive hybrid material with high conductivity, good magnetism, high stability and good wave-absorption. The hybrid material is based on doped barium ferrite (BF)-titanium dioxide (TD) and multiwalled carbon nanotubes (MCNTs) coated by polythiophene (PTh) matrix. The BF-TD/MCNTs/PTh composites were synthesized through in-situ chemical polymerization of thiophene in the presence of the BF-TD composites and MCNTs. The resulting composites were characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), four-probe conductivity tester, vibrating sample magnetometer (VSM) and network analyzer. XRD and FT-IR indicated that the BF-TD/ MCNTs/PTh composites were successfully synthesized with strong interactions among constituents. DTA-TGA analysis suggested that the decomposition of BF-TD/MCNTs/PTh composites included three stages. The first two stages were in accordance with the decomposition of BF-TD composites. The third stage was attributed to the decomposition of polythiophene. SEM and TEM demonstrated that BF-TD and MCNTs were well coated by polythiophene. The electromagnetic parameters showed that polythiophene and MCNTs could significantly improve the conductivity and the wave-absorbing performance of the BFTD composites. When the MCNT content was 20 wt.% and the ratio of the barium ferrite to titanium dioxide was 4:5, BF-TD/MCNTs/PTh composites exhibited the best wave-absorbing performance. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Nano composites Functional composites Electrical properties Magnetic properties
1. Introduction At present, more and more wave-absorbing materials have been developed. However, single material can hardly meet the requirements of strong absorption and wide absorption frequency band, due to its weak absorption, narrow absorption frequency band, and heavy density and so on. Composite materials, which incorporate different functions and characteristics of individual material, may develop new functional materials and solve these problems. In recent years, magnetic-conductive composites with
* Corresponding author. Department of Materials Chemistry, Nanchang Hangkong University, No. 696 Fenghe Southern Avenue, Nanchang, 330063, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Xie), [email protected] (Z. Le). http://dx.doi.org/10.1016/j.compscitech.2015.06.022 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
excellent performance have attracted wide attention because of their adjustable electromagnetic performance, light in weight, easy preparation and so on. More and more attentions have been paid to the hybrid inorganic-organic composite materials for various applications [1e3]. Therefore, magnetic-conductive composites are promising materials with more electromagnetic loss, which provide well synergetic behavior between magnetic and conductive materials. Hexagonal BaFe12O19 ferrite is widely used due to its high Curie temperature, high coercivity, excellent magnetization and good magnetocrystalline anisotropy [4]. The properties of the barium ferrite can be adjusted by controlling its morphology and particle size [5]. On the other hand, during the past few decades, electrically conjugated conducting polymers were of great interest for exploring the enhanced functional materials due to their magnetic,
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electronic, and optical characteristics [6e10]. Among the conducting polymers, polythiophene is one of the most widely used conducting polymers. It possesses many promising applications in rechargeable battery, electromagnetic interference (EMI) shielding, corrosion devices, chemical sensor, and microwave absorbing materials due to its unique properties, such as easy preparation, excellent conductivity and light weight [11e17]. Rare earth ions such as Ce3þ and La3þ possess unpaired 4f electrons. These unique electron structures can effectively improve the electromagnetic properties of ferrite and widen the bandwidth through doping the Ba2þ and Fe3þ [18,19]. Some studies indicated that nonmagnetic oxides such as SnO2 and SiO2 could modify the electromagnetic properties of ferrites [20e24]. In this work, TiO2 is chosen to enhance the wave absorption properties of the barium ferrite. The discovery of carbon nanotubes (CNTs) has generated great interest for a variety of applications due to their excellent mechanical, thermal and electrical properties, as well as high aspect ratio and nanometer scale diameter [25e27]. MCNTs could improve the mechanical, thermal and electrical properties of the polymer composite by the interactions between the MCNTs and polymer composites [28e30]. Herein, we for the first time report the preparation of doped barium ferrite-titanium dioxide/multiwalled carbon nanotubes/ polythiophene composites by in-situ chemical polymerization method. Rare earth La3þ and Nd3þ ions and titanium dioxide were introduced to enhance the electromagnetic properties of the barium ferrites. Multiwalled carbon nanotubes were used to improve the electronic properties of the polythiophene composites. The electromagnetic parameters showed that multiwalled carbon nanotubes could significantly improve the conductivity of the polythiophene composites and the wave-absorbing performance of the BaFe11.92(LaNd)0.04O19- titanium dioxide (BF-TD) composites. When the MCNT content was 20 wt.% and the ratio of the barium ferrite to titanium dioxide was 4:5, the BF-TD/MCNTs/PTh composites exhibited the best wave-absorbing performance. 2. Experiment 2.1. Materials Thiophene (C4H4S), multiwalled carbon nanotubes (MCNTs), hydrochloride (HCl), ammonium persulfate ((NH4)2S2O8), barium nitrate (Ba(NO3)2), lanthanum nitrate (La(NO3)3$9H2O), neodymium nitrate (Nd(NO3)3$5H2O), ferric nitrate (Fe(NO3)3$9H2O), citric acid (C6H8O7$H2O), ethylene glycol (C2H6O2), tetrabutyl titanate (C16H36O4Ti) and ammonia (NH3$H2O) are all analytical reagent. 2.2. Purification of MCNTs MCNTs were added into concentrated nitric acid and were refluxed at 90 C for 5 h. Pure MCNTs were obtained by filtering and washing the suspension with 0.1 M HCl and deionized (DI) water, and dried under vacuum at 50 C for 24 h. 2.3. Preparation of BaFe11.92(LaNd)0.04O19/titanium dioxide (BF-TD) composites Stoichiometric amounts of La(NO3)3$9H2O, Nd(NO3)3$5H2O, Ba(NO3)2 and Fe(NO3)3$9H2O were dissolved in citric acid solution. The pH value of the system was adjusted to weak acidic with ammonia. The system stopped being stirred at 80 C until the barium ferrite wet gel was formed. Tetrabutyl titanate was dissolved into ethylene glycol in a ration and hydrolyzed at 120 C for 2 h. The titanium dioxide gel was obtained. The barium ferrite wet gel and the titanium dioxide gel were mixed with the mass ratios of
3:5, 4:5, 5:5, 6:5 and 7:5 respectively with ultrasonic dispersion for 0.5 h. The mixture gel was quickly inflated and loose and black powders were produced with lots of gas releasing in selfpropagating combustion reaction. Finally, the black powders were sintered at 1000 C for 3 h. Then the target products were obtained. 2.4. Fabrication of doped barium ferrite-titanium dioxide/carbon nanotubes/polythiophene composites Composites were prepared by in situ polymerization in dichloromethane solution. BF-TD composites and MCNTs were suspended in aqueous solution with ultrasonic dispersion for 1 h. Thiophene monomer and HCl solution were added into the suspension and the suspension was stirred for 0.5 h with ultrasonic dispersion for 3 h. The mass ratios of BF-TD composites, MCNTs and thiophene monomer were 0.1:1:0.2, 0.15:1:0.2, 0.2:1:0.2, 0.25:1:0.2, and 0.3:1:0.2, respectively. (NH4)2S2O8 in aqueous solution was slowly added dropwise into the suspension within 1 h. The suspension was stirred at 0 C for 10 h. The product was obtained by filtering and washing the suspension with 0.1 M HCl and DI water, and dried under vacuum at 60 C for 20 h. 2.5. Characterization and electromagnetic properties measurement The morphology and properties of samples were characterized as blow. The structure of the materials was performed by PANalytical system by using Cu Ka1 (l ¼ 0.154 nm) radiation. The FT-IR spectra were recorded in KBr by using FT-IR spectrometer Model Nicolet 5700. TG analysis was performed at a heating rate of 10 C in nitrogen on SDTQ 600. The morphology of composites was observed through SEM (SU1510, HITACHI) and TEM (JEM-2100F). The magnetic properties of samples were tested on VSM (Lakeshore 7404). The conductive of composites were obtained by four-probe conductivity tester (RTS-9). Vector Network Analyzer (HP8722ES) was used to obtain parameters for the samples of composites in the 1e18 GHz range at room temperature. The values of complex permittivity (ε) and permeability (m) of the composite materials were calculated from the measured values of Sparameters. 3. Results and discussion Phase investigation of the crystallized products is performed by XRD and the diffraction patterns are presented in Fig. 1. Fig. 1a shows the characteristic diffraction peaks of BF-TD composites, where the diffraction peaks at 2q ¼ 25.2 , 33.6 and 36.1 [31] are ascribed to the characteristic diffraction peaks of BF and the peaks at 2q ¼ 18.6 , 25.5 and 33.2 are attributed to the characteristic diffraction peaks of TD [32]. The typical XRD pattern of PTh (Fig. 1c) presents two broad diffraction peaks centered at 2q ¼ 17.2 and 26.3 , which can be ascribed to the intermolecular pep stacking which emerges slight shift compared to the literature [33]. Fig. 1b shows XRD patterns of BF-TD/MCNTs/PTh composites which include the characteristic diffraction peaks of BF-TD, MCNTs and PTh. The characteristic diffraction peak of MCNTs is at 2q ¼ 26.1 [34] which is close to that of PTh. In addition, it is obvious that the characteristic diffraction peaks of PTh in the composites are weak compared to those of pristine PTh, which can be produced by the interactions among BF-TD composites, MCNTs and PTh. Fig. 2 shows the FT-IR spectra of BF-TD composite, BF-TD/ MCNTs/PTh composite and PTh. In Fig. 2c, two peaks in the range of 2750e3000 cm1 are attributed to the characteristic CeH stretching vibrations, and the peak at 1640 cm1 is assigned to C]C characteristic peak. The peak at 784 cm1 is assigned to the CeH out-of-plane vibrations of the 2, 5-substituted thiophene ring
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Fig. 1. XRD patterns: (a) BF-TD composites, (b) BF-TD/MCNTs/PTh composites, (c) PTh.
Fig. 2. FT-IR spectra: (a) BF-TD composite, (b) BF-TD/MCNTs/PTh composite, (c) PTh.
created by the polymerization of thiophene monomers. The peak at around 692 cm1 denotes the CeS stretch in the thiophene ring [35e37]. The peaks at 596 cm1 and 440 cm1 are attributed to the characteristic FeeO and TieO stretching vibration band [32,38]. The IR spectra of BF-TD/MCNTs/PTh composite (Fig. 2b) is almost identical to that of PTh (Fig. 2a). But to some extent, there is a slight blue shift for the PTh in the spectra of composites. In addition, it is observed that the intensity of the peak at 696.5 cm1 becomes weak. The peaks at 1112.5 cm1 and 1627.4 cm1 are attributed to the characteristic absorption peaks of MCNTs and appear slightly blue shift compared to the literature [32]. It suggests that the BF-TD composites and MCNTs are well coated by PTh chains and there exists some interactions among them in the composites, which
decrease the electron density and reduce the atomic force constant [39]. Those results confirm that composites are composed of the PTh, BF-TD composites and MCNTs. TG analyses of PTh and BF-TD/MCNTs/PTh composite are shown in Fig. 3. The weight loss of the two samples can be divided into three stages. For the PTh, the first stage is assigned to the loss of water and other volatiles at lower temperature (lower than 110 C). The second stage above 197 C can be attributed to the thermal degradation of the PTh chains and volatilization of the oligomer [40]. The third stage of PTh is at 410 C. The TGA curve of composite indicates that the decomposition temperature of the composite is 230 C, higher than that of pure PTh. The third weight loss of composite starts at 520 C. It indicates that the stability of the
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Fig. 3. TG analysis: (a) BF-TD/MCNTs/PTh composite and (b) PTh.
composite is better than that of PTh. The improved stability can result from the interactions among the PTh, and BF-TD composites and MCNTs. In addition, BF-TD composites and MCNTs are well coated by the PTh chains. The morphology of composites, BF-TD composite and BF-TD/ MCNTs/PTh composite are shown in the Fig. 4 through SEM images. From Fig. 4a, rod-like BF particles and cubic TD particles can be clearly observed are clear seen. Fig. 4a shows that the particles size of TD is bigger than that of BF particles. From Fig. 4b and c, it can be obviously observed that lots of bending carbon nanotubes
coated by PTh have agglomerated densely because the introduction of hydrochloride can increase the polarity of PTh, resulting in the increase of intermolecular force. In addition, the density of MCNTs in Fig. 4d is higher than that in Fig. 4c. Fig. 5 shows the TEM images of BF-TD/MCNTS/PTh composites. In Fig. 5a, typical TEM images of MCNTs can be seen and MCNTs are coated by PTh. At the same time, the black core is coated by the PTh. Electronic diffraction pattern indicates that the black core is BF-TD composite, because BF-TD composite is the only crystal material in the BF-TD/MCNTs/PTh composite. So it can be concluded that BF-TD
Fig. 4. SEM analysis: (a) BF-TD composite; (b) PTh; (c) BF-TD/MCNTs/PTh composite (7.7 wt% MCNTs); (d) BF-TD/MCNTs/PTh composite (20 wt% MCNTs).
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Fig. 5. TEM analysis: (a) and (b) BF-TD/MCNTS/PTh composites.
composite and MCNTs are both coated by PTh. In Fig. 5b, it can be clearly seen that BF-TD composite is adsorbed over the surface of the MCNTs and BF-TD composite and MCNTs are well coated by the PTh. Those results confirm that the composites are composed of polycrystalline BF-TD composite, MCNTs and PTh. This result is in accordance with the former result of FIRT analysis. From the above analysis, a simulation scheme for the formation of novel hybrid materials is proposed to illustrate the preparation process of the BF-TD/MCNTs/PTh composites (Fig. 6). In the first step, MCNTs is refluxed in concentrated HNO3 solution at 120 C for 5 h for removing the residues of metal catalysts. In the second step, a small number of carboxylic groups on the surface of the purified MCNTs can adsorb BF-TD composites [41]. In the third step, Cl is adsorbed onto the surface of BF-TD composites. Then thiophene cation is attracted by Cl and uniformly distributed over the surface of BF-TD composites in the fourth step. Finally, the target products are obtained in the in-situ chemical polymerization of thiophene with the ammonium persulfate as an initiator. The magnetic properties of all samples are tested at room temperature and all samples exhibit a clear hysteresis behavior. The hysteretic loops are shown in the Figs. 7e9, and the magnetic parameters are summarized in Tables 1 and 2, respectively. The proportion of BF and TD is 3:5 in the five samples. The area of all the samples' hysteretic loops is close, and the five samples possess similar hysteretic loss ability. It can be concluded that the variation of the magnetic parameters are as follows: with the increase of the MCNTs content, the coercive force value (Hc) gradually increases and the magnetic saturation strength value (Ms) decreases gradually, as well as the magnetic residual strength value (Mr). It indicates that the MCNTs content affects the magnetization of composites for two reasons. One is that there are many delocalization electrons in the MCNTs structure, which enable the formation of pep conjugate structures between MCNTs and PTh [42]. The MCNTs may influence the BF-TD composite's magnetic properties through the PTh because PTh can adjust the magnetic properties of BF-TD composite [34,43]. Another reason is that MCNTs possess the obvious effects of small size, surface and interface. The content of MCNTs is 20.0 wt.% in the composites. Fig. 8 shows that the area of hysteretic loop of sample b is the biggest one and it indicates that sample possesses the best hysteretic loss ability. It can be seen that the Hc value varies with the ratio of BF and TD. The Hc value first increases and then decreases. A possible reason for this variation is the interactions among the BF-TD composites, MCNTs and PTh. The above analysis shows that when the content of MCNTs is 20.0 wt.% and the ratio of BF and TD is 3:5, the composite possesses the best magnetic properties.
Fig. 9 shows the magnetization hysteretic loop for BF-TD composite with the ratio of BF and TD is 3:5. The Hc, Ms and Mr of the BF-TD composite are 2496.3 Oe, 3.5685 emu g1 and 1.7564 emu g1, respectively. Comparing the Figs. 7 and 8 to Fig. 9, it can be easily concluded that the Hc, Ms and Mr magnetic parameters of the BF-TD/MCNTs/PTh composite are much smaller than that of the BF-TD composite. The reasons for the decrease of magnetic parameters are as follows. Firstly, the introduction of the nonmagnetic polythiophene and MCNTs can hinder the grain continuity and result in the demagnetizing field [44]. Secondly, according to the equation of MS ¼ 4mS [45], MS is related to the volume fraction of the particles (4) and the saturation moment of a single particle (mS). So the MS of BF-TD/MCNTs/PTh composite mainly depends on the volume fraction of the magnetic ferrite particles, due to the non-magnetic PTh coating layer contribution to the total magnetization, resulting in a decrease in the saturation magnetization. Thirdly, in the polymerization process, the ferrite surface defects, such as pores and cracks, are covered by depositing PTh on the ferrite surface and crystallite boundaries. In addition, surface spinning of magnetic moments at ferrite nanoparticle/ support interface may result in a decrease in magnetic surface anisotropy of ferrite particles [46]. Composites were prepared to cylindrical samples with F20 mm and 2 mm thickness. To discuss the influence of MCNTs content on the conductivity, the conductivity of samples was tested by fourprobe conductivity tester at room temperature. From Fig. 10, it is obviously observed that the conductivity of all the samples increases with the content of MCNTs. Because the structure of MCNTs is similar to the graphite and MCNTs possess good electrical properties. On the other hand, there are many delocalization electrons in the MCNTs structure, which benefits for the formation of pep conjugate structures between MCNTs and PTh, resulting in improvement in the electrical properties of PTh. Composites whose ratio of BF and TD is 4:5 possess the best electrical properties. It indicates that the optimum ratio of BF and TD in composites is 4:5. To evaluate the microwave electromagnetic properties of polythiophene matrix composites filled with BF-TD composites and MCNTs, the real part and imaginary part of the permittivity (ε0 , ε00 ) and permeability (m0 , m00 ), dielectric loss angle tangent (tgdε ¼ ε00 /ε0 ) and magnetic loss angle tangent (tgdm ¼ m00 /m0 ) of the composites are measured in the frequency range of 1e18 GHz. The MCNTs content of all the BF-TD/MCNTs/PTh composites is 20.0 wt%. The effect of the ratio of BF and TD from 3:5 to 7:5 on the microwave electromagnetic of composites is discussed. All the composites are mixed with the paraffin with a mass ratio of 3:7. All the electromagnetic parameters are shown from Figs. 11e13.
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Fig. 6. Scheme of preparation of the BF-TD/MCNTs/PTh composites.
Ref. [48] shows the BF-TD/MCNTS/PTh composites. As seen in Ref. [48], four peaks are found in the spectrum of ε0 and the four peaks are on the 1 GHz, 7 GHz, 13 GHz and 17.5 GHz, respectively. When the ratio of BF and TD is 4:5, the BF-TD composite obtains the
best ε0 values. The ε00 values also have four peaks as seen in the spectrum of ε00 . As shown in the Fig. 11, the permittivity (ε0 , ε00 ) of BFTD/MCNTs/PTh composites decrease with the frequency. When the ratio of BF and TD is 4:5, BF-TD/MCNTs/PTh composite possesses
Fig. 7. Magnetization hysteretic loops for BF-TD/MCNTs/PTh composites containing MCNTs: (a)7.7 wt%,(b)11.1 wt%,(c)14.3 wt%,(d)17.2 wt%,(e)20 wt%.
Fig. 8. Magnetization hysteretic loops for BF-TD/MCNTs/PTh composites containing the ratio BF and TD: (a)3:5, (b) 4:5,(c) 5:5, (d) 6:5, (e) 7:5.
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Fig. 9. Magnetization hysteretic loop for BF-TD composite.
Table 1 Effect of MCNTS content on the magnetization of composites. Content
Hc/Oe
Ms/emu g1
Mr/emu g1
7.7 wt% 11.1 wt% 14.3 wt% 17.2 wt% 20.0 wt%
235.48 238.17 233.80 240.04 245.88
0.62432 0.52412 0.59537 0.53261 0.57312
0.13645 0.12204 0.10125 0.12225 0.13225
Table 2 Effect of the BF and TD ratio on the magnetization of composites. Ratio
Hc/Oe
Ms/emu g1
Mr/emu g1
3:5 4:5 5:5 6:5 7:5
245.88 655.78 250.52 240.70 211.87
0.57321 0.78965 0.69442 0.85112 0.51420
0.32102 0.22231 0.17333 0.21717 0.04902
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the best permittivity. Comparing Fig. 11 to reference [48], it is easy to be observed that the permittivity values of BF-TD/MCNTs/PTh composites are much bigger than that of BF-TD composites. Therefore, MCNTs and PTh are effective to improve the permittivity of BF-TD composites and the dielectric loss. Moreover, the interaction between BF-TD composites and MCNTs and PTh are believed. The real part (ε0 ) is related with the amount of polarization in the material and the imaginary part (ε00 ) is mainly associated with the dissipation of energy. The dielectric performance of the material is determined by ionic, electronic, orientational, and space charge polarization. The heterogeneity of the material contributes to the space charge polarization appears. The presence of insulating BF-TD composites in the conducting matrix (PTH) leads to the formation of more interface and a heterogeneous system owing to some space charge accumulating at the interface that contributes toward the higher microwave absorption in the composites. The contribution to the orientational polarization is due to the presence of bound charges (dipoles). In conjugated polymers, two types of charged species are present: one polaron/bipolaron system that is mobile and free to move along the chain and others are bound charges (dipoles) which have only restricted mobility and account for strong polarization in the system [4]. With the frequency increasing, the dipoles present in the system cannot reorient themselves fast enough to respond to applied electric field, and therefore, dielectric constant decreases. Due to the difference in the dielectric constant and conductivity of BF-TD composites and PTH and MCNTS, charge carriers present in PTH and MCNTS were trapped, and as a result, space charge is developed at the interface of the BF-TD particles and the PTH. This also leads to the generation of space charge at the heterogeneous interface leading to field distortion. The tendency for the interfacial polarization is expected to be decreased resulting in a decrease in polarizability and loss factor with the increase of frequency. Therefore, with the increase in frequency, ε0 and ε00 decreases. The imaginary part of permeability (m00 ) starts increasing near the critical frequency and has a maxima at the resonance frequency, which presents the maximum power loss. The larger the static value of m, the lower is the frequency at which this decrease occurs as given by Snoek's law. In BF-TD particles, due to the large
Fig. 10. The conductivity of composites with the different content of MCNTs.
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Fig. 11. The ε0 and ε00 of BF-TD/MCNTs/PTh composites.
Fig. 12. The m00 and m00 of BF-TD/MCNTs/PTh composites.
Fig. 13. The tgdε and tgdm of BF-TD/MCNTs/PTh composites.
anisotropy field, the coupling of the magnetic dipole is strong. With the frequency of the applied field increasing, the magnetic dipole tries to rotate with the frequency but at higher frequency due to strong anisotropy, the induced magnetization (B) lags behind the applied field (H) which results in magnetic losses. There are three peaks in the m0 and two peaks m00 spectra of BF-TD composites in Ref. [48]. It is well known that there are three kinds of magnetization mechanism: domain wall motion, magnetization rotation and gyromagnetic spin rotation [47]. Two mechanisms are responsible for the two peaks in the m00 spectra: domain wall motion is at 8 GHz and spin rotation is at 14 GHz. Fig. 12 shows the permeability of BF-TD/MCNTs/PTh composites. It is worthy noting that the peak of domain wall motion vanishes. The reason for that is still further studied. Comparing reference [48] to Fig. 12, it is obvious to find that the m0 values of BF-TD composites are close to those of BF-TD/MCNTs/PTh composites. But the m00 is improved, especially in the high frequency, and increases from 6 GHz. It
indicates that MCNTs and PTh can effectively improve the permeability of BF-TD composites and the dielectric loss and widen the frequency band. The tgdm and tgdε can be used to evaluate the performance of microwave absorption of materials. Reference [48] shows the tgdm and tgdε of BF-TD composites. The curves of tgdm and tgdε are similar to those of m00 and ε00 . There are three dielectric loss peaks and the strongest dielectric loss peak is at 17 GHz. The tgdm decreases with the frequency and the maximum magnetic loss is at 1 GHz. Fig. 13 shows the tgdm and tgdε of BF-TD/MCNTs/PTh composites. The tgdε slightly increases and the tgdm first decreases and then increases with the frequency. Comparing reference [48] to Fig. 13, the maximum value of tgdm and tgdε are 0.06 and 0.12 in BF-TD composites. However, the maximum value of tgdm and tgdε are 0.5 and 0.95 in BF-TD/MCNTs/PTh composites, respectively. So it can be concluded that MCNTs and PTh improve the microwave absorption and widen the effective frequency band of BF-TD composites.
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Taking the dielectric loss and magnetic loss into account, when the ratio of the BF and TD is 4:5 and the MCNTs content is 20.0 wt%, the BF-TD/MCNTs/PTh composites have the best performance of microwave absorption. 4. Conclusion The BF-TD/MCNTs/PTh composites with good electromagnetic and microwave absorption properties were prepared by in situ polymerization of PTh in the presence of BF-TD composites and MCNTs. The electromagnetic and microwave absorption properties of the BF-TD/MCNTs/PTh composites were investigated as the function of the MCNTs content and the ratio of BF and TD. The MCNTs and PTh enhance the microwave absorption performance and widen the frequency of the BF-TD composites due to the interactions among the BF-TD composites and MCNTs and PTh. When the ratio of BF and TD is 4:5 and the MCNTs content is 20.0 wt.%, the BF-TD/MCNTs/PTh composites exhibit the strongest microwave absorption. This work indicates that BF-TD/MCNTs/PTh composite could be a promising microwave absorption material with a wide frequency band.
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Acknowledgment
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This work was financially supported by National Natural Science Foundation of China (Nos. 20904019, 21067004 and 51273089), Aviation Science Fund (Nos. 2011ZF56015, 2013ZF56025), Natural Science Foundation of Jiangxi Province (No. 20132BAB203018), Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CSA (Nos. PCOM201228, PCOM201401), Jiangxi Province Education Department of Science and Technology Project (No. GJJ13491), Jiangxi Province Youth Scientists Cultivating Object Program (No. 20112BCB23017), and Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University (No. ST201222007).
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
A novel ternary hybrid electromagnetic wave-absorbing composite based on BaFe11.92(LaNd)0.04O19-titanium dioxide/multiwalled carbon nanotubes/polythiophene Xiaowei Hong a, b, Yu Xie a, b, *, Xiaoying Wang c, Mingjun Li a, Zhanggao Le d, **, Yunhua Gao b, Yan Huang a, Yuancheng Qin a, Yun Ling a a
Department of Materials Chemistry, Nanchang Hangkong University, Nanchang, 330063, PR China Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, Chinese Academy of Sciences, Beijing, 100190, PR China Department of Applied Chemistry, South China University of Technology, Guangzhou, 510640, PR China d Department of Applied Chemistry, East China Institute of Technology, Nanchang, 330013, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 August 2014 Received in revised form 18 June 2015 Accepted 28 June 2015 Available online 2 July 2015
We report a novel ternary magnetic-conductive hybrid material with high conductivity, good magnetism, high stability and good wave-absorption. The hybrid material is based on doped barium ferrite (BF)-titanium dioxide (TD) and multiwalled carbon nanotubes (MCNTs) coated by polythiophene (PTh) matrix. The BF-TD/MCNTs/PTh composites were synthesized through in-situ chemical polymerization of thiophene in the presence of the BF-TD composites and MCNTs. The resulting composites were characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), four-probe conductivity tester, vibrating sample magnetometer (VSM) and network analyzer. XRD and FT-IR indicated that the BF-TD/ MCNTs/PTh composites were successfully synthesized with strong interactions among constituents. DTA-TGA analysis suggested that the decomposition of BF-TD/MCNTs/PTh composites included three stages. The first two stages were in accordance with the decomposition of BF-TD composites. The third stage was attributed to the decomposition of polythiophene. SEM and TEM demonstrated that BF-TD and MCNTs were well coated by polythiophene. The electromagnetic parameters showed that polythiophene and MCNTs could significantly improve the conductivity and the wave-absorbing performance of the BFTD composites. When the MCNT content was 20 wt.% and the ratio of the barium ferrite to titanium dioxide was 4:5, BF-TD/MCNTs/PTh composites exhibited the best wave-absorbing performance. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Polymer-matrix composites (PMCs) Nano composites Functional composites Electrical properties Magnetic properties
1. Introduction At present, more and more wave-absorbing materials have been developed. However, single material can hardly meet the requirements of strong absorption and wide absorption frequency band, due to its weak absorption, narrow absorption frequency band, and heavy density and so on. Composite materials, which incorporate different functions and characteristics of individual material, may develop new functional materials and solve these problems. In recent years, magnetic-conductive composites with
* Corresponding author. Department of Materials Chemistry, Nanchang Hangkong University, No. 696 Fenghe Southern Avenue, Nanchang, 330063, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Xie), [email protected] (Z. Le). http://dx.doi.org/10.1016/j.compscitech.2015.06.022 0266-3538/© 2015 Elsevier Ltd. All rights reserved.
excellent performance have attracted wide attention because of their adjustable electromagnetic performance, light in weight, easy preparation and so on. More and more attentions have been paid to the hybrid inorganic-organic composite materials for various applications [1e3]. Therefore, magnetic-conductive composites are promising materials with more electromagnetic loss, which provide well synergetic behavior between magnetic and conductive materials. Hexagonal BaFe12O19 ferrite is widely used due to its high Curie temperature, high coercivity, excellent magnetization and good magnetocrystalline anisotropy [4]. The properties of the barium ferrite can be adjusted by controlling its morphology and particle size [5]. On the other hand, during the past few decades, electrically conjugated conducting polymers were of great interest for exploring the enhanced functional materials due to their magnetic,
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electronic, and optical characteristics [6e10]. Among the conducting polymers, polythiophene is one of the most widely used conducting polymers. It possesses many promising applications in rechargeable battery, electromagnetic interference (EMI) shielding, corrosion devices, chemical sensor, and microwave absorbing materials due to its unique properties, such as easy preparation, excellent conductivity and light weight [11e17]. Rare earth ions such as Ce3þ and La3þ possess unpaired 4f electrons. These unique electron structures can effectively improve the electromagnetic properties of ferrite and widen the bandwidth through doping the Ba2þ and Fe3þ [18,19]. Some studies indicated that nonmagnetic oxides such as SnO2 and SiO2 could modify the electromagnetic properties of ferrites [20e24]. In this work, TiO2 is chosen to enhance the wave absorption properties of the barium ferrite. The discovery of carbon nanotubes (CNTs) has generated great interest for a variety of applications due to their excellent mechanical, thermal and electrical properties, as well as high aspect ratio and nanometer scale diameter [25e27]. MCNTs could improve the mechanical, thermal and electrical properties of the polymer composite by the interactions between the MCNTs and polymer composites [28e30]. Herein, we for the first time report the preparation of doped barium ferrite-titanium dioxide/multiwalled carbon nanotubes/ polythiophene composites by in-situ chemical polymerization method. Rare earth La3þ and Nd3þ ions and titanium dioxide were introduced to enhance the electromagnetic properties of the barium ferrites. Multiwalled carbon nanotubes were used to improve the electronic properties of the polythiophene composites. The electromagnetic parameters showed that multiwalled carbon nanotubes could significantly improve the conductivity of the polythiophene composites and the wave-absorbing performance of the BaFe11.92(LaNd)0.04O19- titanium dioxide (BF-TD) composites. When the MCNT content was 20 wt.% and the ratio of the barium ferrite to titanium dioxide was 4:5, the BF-TD/MCNTs/PTh composites exhibited the best wave-absorbing performance. 2. Experiment 2.1. Materials Thiophene (C4H4S), multiwalled carbon nanotubes (MCNTs), hydrochloride (HCl), ammonium persulfate ((NH4)2S2O8), barium nitrate (Ba(NO3)2), lanthanum nitrate (La(NO3)3$9H2O), neodymium nitrate (Nd(NO3)3$5H2O), ferric nitrate (Fe(NO3)3$9H2O), citric acid (C6H8O7$H2O), ethylene glycol (C2H6O2), tetrabutyl titanate (C16H36O4Ti) and ammonia (NH3$H2O) are all analytical reagent. 2.2. Purification of MCNTs MCNTs were added into concentrated nitric acid and were refluxed at 90 C for 5 h. Pure MCNTs were obtained by filtering and washing the suspension with 0.1 M HCl and deionized (DI) water, and dried under vacuum at 50 C for 24 h. 2.3. Preparation of BaFe11.92(LaNd)0.04O19/titanium dioxide (BF-TD) composites Stoichiometric amounts of La(NO3)3$9H2O, Nd(NO3)3$5H2O, Ba(NO3)2 and Fe(NO3)3$9H2O were dissolved in citric acid solution. The pH value of the system was adjusted to weak acidic with ammonia. The system stopped being stirred at 80 C until the barium ferrite wet gel was formed. Tetrabutyl titanate was dissolved into ethylene glycol in a ration and hydrolyzed at 120 C for 2 h. The titanium dioxide gel was obtained. The barium ferrite wet gel and the titanium dioxide gel were mixed with the mass ratios of
3:5, 4:5, 5:5, 6:5 and 7:5 respectively with ultrasonic dispersion for 0.5 h. The mixture gel was quickly inflated and loose and black powders were produced with lots of gas releasing in selfpropagating combustion reaction. Finally, the black powders were sintered at 1000 C for 3 h. Then the target products were obtained. 2.4. Fabrication of doped barium ferrite-titanium dioxide/carbon nanotubes/polythiophene composites Composites were prepared by in situ polymerization in dichloromethane solution. BF-TD composites and MCNTs were suspended in aqueous solution with ultrasonic dispersion for 1 h. Thiophene monomer and HCl solution were added into the suspension and the suspension was stirred for 0.5 h with ultrasonic dispersion for 3 h. The mass ratios of BF-TD composites, MCNTs and thiophene monomer were 0.1:1:0.2, 0.15:1:0.2, 0.2:1:0.2, 0.25:1:0.2, and 0.3:1:0.2, respectively. (NH4)2S2O8 in aqueous solution was slowly added dropwise into the suspension within 1 h. The suspension was stirred at 0 C for 10 h. The product was obtained by filtering and washing the suspension with 0.1 M HCl and DI water, and dried under vacuum at 60 C for 20 h. 2.5. Characterization and electromagnetic properties measurement The morphology and properties of samples were characterized as blow. The structure of the materials was performed by PANalytical system by using Cu Ka1 (l ¼ 0.154 nm) radiation. The FT-IR spectra were recorded in KBr by using FT-IR spectrometer Model Nicolet 5700. TG analysis was performed at a heating rate of 10 C in nitrogen on SDTQ 600. The morphology of composites was observed through SEM (SU1510, HITACHI) and TEM (JEM-2100F). The magnetic properties of samples were tested on VSM (Lakeshore 7404). The conductive of composites were obtained by four-probe conductivity tester (RTS-9). Vector Network Analyzer (HP8722ES) was used to obtain parameters for the samples of composites in the 1e18 GHz range at room temperature. The values of complex permittivity (ε) and permeability (m) of the composite materials were calculated from the measured values of Sparameters. 3. Results and discussion Phase investigation of the crystallized products is performed by XRD and the diffraction patterns are presented in Fig. 1. Fig. 1a shows the characteristic diffraction peaks of BF-TD composites, where the diffraction peaks at 2q ¼ 25.2 , 33.6 and 36.1 [31] are ascribed to the characteristic diffraction peaks of BF and the peaks at 2q ¼ 18.6 , 25.5 and 33.2 are attributed to the characteristic diffraction peaks of TD [32]. The typical XRD pattern of PTh (Fig. 1c) presents two broad diffraction peaks centered at 2q ¼ 17.2 and 26.3 , which can be ascribed to the intermolecular pep stacking which emerges slight shift compared to the literature [33]. Fig. 1b shows XRD patterns of BF-TD/MCNTs/PTh composites which include the characteristic diffraction peaks of BF-TD, MCNTs and PTh. The characteristic diffraction peak of MCNTs is at 2q ¼ 26.1 [34] which is close to that of PTh. In addition, it is obvious that the characteristic diffraction peaks of PTh in the composites are weak compared to those of pristine PTh, which can be produced by the interactions among BF-TD composites, MCNTs and PTh. Fig. 2 shows the FT-IR spectra of BF-TD composite, BF-TD/ MCNTs/PTh composite and PTh. In Fig. 2c, two peaks in the range of 2750e3000 cm1 are attributed to the characteristic CeH stretching vibrations, and the peak at 1640 cm1 is assigned to C]C characteristic peak. The peak at 784 cm1 is assigned to the CeH out-of-plane vibrations of the 2, 5-substituted thiophene ring
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Fig. 1. XRD patterns: (a) BF-TD composites, (b) BF-TD/MCNTs/PTh composites, (c) PTh.
Fig. 2. FT-IR spectra: (a) BF-TD composite, (b) BF-TD/MCNTs/PTh composite, (c) PTh.
created by the polymerization of thiophene monomers. The peak at around 692 cm1 denotes the CeS stretch in the thiophene ring [35e37]. The peaks at 596 cm1 and 440 cm1 are attributed to the characteristic FeeO and TieO stretching vibration band [32,38]. The IR spectra of BF-TD/MCNTs/PTh composite (Fig. 2b) is almost identical to that of PTh (Fig. 2a). But to some extent, there is a slight blue shift for the PTh in the spectra of composites. In addition, it is observed that the intensity of the peak at 696.5 cm1 becomes weak. The peaks at 1112.5 cm1 and 1627.4 cm1 are attributed to the characteristic absorption peaks of MCNTs and appear slightly blue shift compared to the literature [32]. It suggests that the BF-TD composites and MCNTs are well coated by PTh chains and there exists some interactions among them in the composites, which
decrease the electron density and reduce the atomic force constant [39]. Those results confirm that composites are composed of the PTh, BF-TD composites and MCNTs. TG analyses of PTh and BF-TD/MCNTs/PTh composite are shown in Fig. 3. The weight loss of the two samples can be divided into three stages. For the PTh, the first stage is assigned to the loss of water and other volatiles at lower temperature (lower than 110 C). The second stage above 197 C can be attributed to the thermal degradation of the PTh chains and volatilization of the oligomer [40]. The third stage of PTh is at 410 C. The TGA curve of composite indicates that the decomposition temperature of the composite is 230 C, higher than that of pure PTh. The third weight loss of composite starts at 520 C. It indicates that the stability of the
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Fig. 3. TG analysis: (a) BF-TD/MCNTs/PTh composite and (b) PTh.
composite is better than that of PTh. The improved stability can result from the interactions among the PTh, and BF-TD composites and MCNTs. In addition, BF-TD composites and MCNTs are well coated by the PTh chains. The morphology of composites, BF-TD composite and BF-TD/ MCNTs/PTh composite are shown in the Fig. 4 through SEM images. From Fig. 4a, rod-like BF particles and cubic TD particles can be clearly observed are clear seen. Fig. 4a shows that the particles size of TD is bigger than that of BF particles. From Fig. 4b and c, it can be obviously observed that lots of bending carbon nanotubes
coated by PTh have agglomerated densely because the introduction of hydrochloride can increase the polarity of PTh, resulting in the increase of intermolecular force. In addition, the density of MCNTs in Fig. 4d is higher than that in Fig. 4c. Fig. 5 shows the TEM images of BF-TD/MCNTS/PTh composites. In Fig. 5a, typical TEM images of MCNTs can be seen and MCNTs are coated by PTh. At the same time, the black core is coated by the PTh. Electronic diffraction pattern indicates that the black core is BF-TD composite, because BF-TD composite is the only crystal material in the BF-TD/MCNTs/PTh composite. So it can be concluded that BF-TD
Fig. 4. SEM analysis: (a) BF-TD composite; (b) PTh; (c) BF-TD/MCNTs/PTh composite (7.7 wt% MCNTs); (d) BF-TD/MCNTs/PTh composite (20 wt% MCNTs).
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Fig. 5. TEM analysis: (a) and (b) BF-TD/MCNTS/PTh composites.
composite and MCNTs are both coated by PTh. In Fig. 5b, it can be clearly seen that BF-TD composite is adsorbed over the surface of the MCNTs and BF-TD composite and MCNTs are well coated by the PTh. Those results confirm that the composites are composed of polycrystalline BF-TD composite, MCNTs and PTh. This result is in accordance with the former result of FIRT analysis. From the above analysis, a simulation scheme for the formation of novel hybrid materials is proposed to illustrate the preparation process of the BF-TD/MCNTs/PTh composites (Fig. 6). In the first step, MCNTs is refluxed in concentrated HNO3 solution at 120 C for 5 h for removing the residues of metal catalysts. In the second step, a small number of carboxylic groups on the surface of the purified MCNTs can adsorb BF-TD composites [41]. In the third step, Cl is adsorbed onto the surface of BF-TD composites. Then thiophene cation is attracted by Cl and uniformly distributed over the surface of BF-TD composites in the fourth step. Finally, the target products are obtained in the in-situ chemical polymerization of thiophene with the ammonium persulfate as an initiator. The magnetic properties of all samples are tested at room temperature and all samples exhibit a clear hysteresis behavior. The hysteretic loops are shown in the Figs. 7e9, and the magnetic parameters are summarized in Tables 1 and 2, respectively. The proportion of BF and TD is 3:5 in the five samples. The area of all the samples' hysteretic loops is close, and the five samples possess similar hysteretic loss ability. It can be concluded that the variation of the magnetic parameters are as follows: with the increase of the MCNTs content, the coercive force value (Hc) gradually increases and the magnetic saturation strength value (Ms) decreases gradually, as well as the magnetic residual strength value (Mr). It indicates that the MCNTs content affects the magnetization of composites for two reasons. One is that there are many delocalization electrons in the MCNTs structure, which enable the formation of pep conjugate structures between MCNTs and PTh [42]. The MCNTs may influence the BF-TD composite's magnetic properties through the PTh because PTh can adjust the magnetic properties of BF-TD composite [34,43]. Another reason is that MCNTs possess the obvious effects of small size, surface and interface. The content of MCNTs is 20.0 wt.% in the composites. Fig. 8 shows that the area of hysteretic loop of sample b is the biggest one and it indicates that sample possesses the best hysteretic loss ability. It can be seen that the Hc value varies with the ratio of BF and TD. The Hc value first increases and then decreases. A possible reason for this variation is the interactions among the BF-TD composites, MCNTs and PTh. The above analysis shows that when the content of MCNTs is 20.0 wt.% and the ratio of BF and TD is 3:5, the composite possesses the best magnetic properties.
Fig. 9 shows the magnetization hysteretic loop for BF-TD composite with the ratio of BF and TD is 3:5. The Hc, Ms and Mr of the BF-TD composite are 2496.3 Oe, 3.5685 emu g1 and 1.7564 emu g1, respectively. Comparing the Figs. 7 and 8 to Fig. 9, it can be easily concluded that the Hc, Ms and Mr magnetic parameters of the BF-TD/MCNTs/PTh composite are much smaller than that of the BF-TD composite. The reasons for the decrease of magnetic parameters are as follows. Firstly, the introduction of the nonmagnetic polythiophene and MCNTs can hinder the grain continuity and result in the demagnetizing field [44]. Secondly, according to the equation of MS ¼ 4mS [45], MS is related to the volume fraction of the particles (4) and the saturation moment of a single particle (mS). So the MS of BF-TD/MCNTs/PTh composite mainly depends on the volume fraction of the magnetic ferrite particles, due to the non-magnetic PTh coating layer contribution to the total magnetization, resulting in a decrease in the saturation magnetization. Thirdly, in the polymerization process, the ferrite surface defects, such as pores and cracks, are covered by depositing PTh on the ferrite surface and crystallite boundaries. In addition, surface spinning of magnetic moments at ferrite nanoparticle/ support interface may result in a decrease in magnetic surface anisotropy of ferrite particles [46]. Composites were prepared to cylindrical samples with F20 mm and 2 mm thickness. To discuss the influence of MCNTs content on the conductivity, the conductivity of samples was tested by fourprobe conductivity tester at room temperature. From Fig. 10, it is obviously observed that the conductivity of all the samples increases with the content of MCNTs. Because the structure of MCNTs is similar to the graphite and MCNTs possess good electrical properties. On the other hand, there are many delocalization electrons in the MCNTs structure, which benefits for the formation of pep conjugate structures between MCNTs and PTh, resulting in improvement in the electrical properties of PTh. Composites whose ratio of BF and TD is 4:5 possess the best electrical properties. It indicates that the optimum ratio of BF and TD in composites is 4:5. To evaluate the microwave electromagnetic properties of polythiophene matrix composites filled with BF-TD composites and MCNTs, the real part and imaginary part of the permittivity (ε0 , ε00 ) and permeability (m0 , m00 ), dielectric loss angle tangent (tgdε ¼ ε00 /ε0 ) and magnetic loss angle tangent (tgdm ¼ m00 /m0 ) of the composites are measured in the frequency range of 1e18 GHz. The MCNTs content of all the BF-TD/MCNTs/PTh composites is 20.0 wt%. The effect of the ratio of BF and TD from 3:5 to 7:5 on the microwave electromagnetic of composites is discussed. All the composites are mixed with the paraffin with a mass ratio of 3:7. All the electromagnetic parameters are shown from Figs. 11e13.
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Fig. 6. Scheme of preparation of the BF-TD/MCNTs/PTh composites.
Ref. [48] shows the BF-TD/MCNTS/PTh composites. As seen in Ref. [48], four peaks are found in the spectrum of ε0 and the four peaks are on the 1 GHz, 7 GHz, 13 GHz and 17.5 GHz, respectively. When the ratio of BF and TD is 4:5, the BF-TD composite obtains the
best ε0 values. The ε00 values also have four peaks as seen in the spectrum of ε00 . As shown in the Fig. 11, the permittivity (ε0 , ε00 ) of BFTD/MCNTs/PTh composites decrease with the frequency. When the ratio of BF and TD is 4:5, BF-TD/MCNTs/PTh composite possesses
Fig. 7. Magnetization hysteretic loops for BF-TD/MCNTs/PTh composites containing MCNTs: (a)7.7 wt%,(b)11.1 wt%,(c)14.3 wt%,(d)17.2 wt%,(e)20 wt%.
Fig. 8. Magnetization hysteretic loops for BF-TD/MCNTs/PTh composites containing the ratio BF and TD: (a)3:5, (b) 4:5,(c) 5:5, (d) 6:5, (e) 7:5.
X. Hong et al. / Composites Science and Technology 117 (2015) 215e224
Fig. 9. Magnetization hysteretic loop for BF-TD composite.
Table 1 Effect of MCNTS content on the magnetization of composites. Content
Hc/Oe
Ms/emu g1
Mr/emu g1
7.7 wt% 11.1 wt% 14.3 wt% 17.2 wt% 20.0 wt%
235.48 238.17 233.80 240.04 245.88
0.62432 0.52412 0.59537 0.53261 0.57312
0.13645 0.12204 0.10125 0.12225 0.13225
Table 2 Effect of the BF and TD ratio on the magnetization of composites. Ratio
Hc/Oe
Ms/emu g1
Mr/emu g1
3:5 4:5 5:5 6:5 7:5
245.88 655.78 250.52 240.70 211.87
0.57321 0.78965 0.69442 0.85112 0.51420
0.32102 0.22231 0.17333 0.21717 0.04902
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the best permittivity. Comparing Fig. 11 to reference [48], it is easy to be observed that the permittivity values of BF-TD/MCNTs/PTh composites are much bigger than that of BF-TD composites. Therefore, MCNTs and PTh are effective to improve the permittivity of BF-TD composites and the dielectric loss. Moreover, the interaction between BF-TD composites and MCNTs and PTh are believed. The real part (ε0 ) is related with the amount of polarization in the material and the imaginary part (ε00 ) is mainly associated with the dissipation of energy. The dielectric performance of the material is determined by ionic, electronic, orientational, and space charge polarization. The heterogeneity of the material contributes to the space charge polarization appears. The presence of insulating BF-TD composites in the conducting matrix (PTH) leads to the formation of more interface and a heterogeneous system owing to some space charge accumulating at the interface that contributes toward the higher microwave absorption in the composites. The contribution to the orientational polarization is due to the presence of bound charges (dipoles). In conjugated polymers, two types of charged species are present: one polaron/bipolaron system that is mobile and free to move along the chain and others are bound charges (dipoles) which have only restricted mobility and account for strong polarization in the system [4]. With the frequency increasing, the dipoles present in the system cannot reorient themselves fast enough to respond to applied electric field, and therefore, dielectric constant decreases. Due to the difference in the dielectric constant and conductivity of BF-TD composites and PTH and MCNTS, charge carriers present in PTH and MCNTS were trapped, and as a result, space charge is developed at the interface of the BF-TD particles and the PTH. This also leads to the generation of space charge at the heterogeneous interface leading to field distortion. The tendency for the interfacial polarization is expected to be decreased resulting in a decrease in polarizability and loss factor with the increase of frequency. Therefore, with the increase in frequency, ε0 and ε00 decreases. The imaginary part of permeability (m00 ) starts increasing near the critical frequency and has a maxima at the resonance frequency, which presents the maximum power loss. The larger the static value of m, the lower is the frequency at which this decrease occurs as given by Snoek's law. In BF-TD particles, due to the large
Fig. 10. The conductivity of composites with the different content of MCNTs.
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Fig. 11. The ε0 and ε00 of BF-TD/MCNTs/PTh composites.
Fig. 12. The m00 and m00 of BF-TD/MCNTs/PTh composites.
Fig. 13. The tgdε and tgdm of BF-TD/MCNTs/PTh composites.
anisotropy field, the coupling of the magnetic dipole is strong. With the frequency of the applied field increasing, the magnetic dipole tries to rotate with the frequency but at higher frequency due to strong anisotropy, the induced magnetization (B) lags behind the applied field (H) which results in magnetic losses. There are three peaks in the m0 and two peaks m00 spectra of BF-TD composites in Ref. [48]. It is well known that there are three kinds of magnetization mechanism: domain wall motion, magnetization rotation and gyromagnetic spin rotation [47]. Two mechanisms are responsible for the two peaks in the m00 spectra: domain wall motion is at 8 GHz and spin rotation is at 14 GHz. Fig. 12 shows the permeability of BF-TD/MCNTs/PTh composites. It is worthy noting that the peak of domain wall motion vanishes. The reason for that is still further studied. Comparing reference [48] to Fig. 12, it is obvious to find that the m0 values of BF-TD composites are close to those of BF-TD/MCNTs/PTh composites. But the m00 is improved, especially in the high frequency, and increases from 6 GHz. It
indicates that MCNTs and PTh can effectively improve the permeability of BF-TD composites and the dielectric loss and widen the frequency band. The tgdm and tgdε can be used to evaluate the performance of microwave absorption of materials. Reference [48] shows the tgdm and tgdε of BF-TD composites. The curves of tgdm and tgdε are similar to those of m00 and ε00 . There are three dielectric loss peaks and the strongest dielectric loss peak is at 17 GHz. The tgdm decreases with the frequency and the maximum magnetic loss is at 1 GHz. Fig. 13 shows the tgdm and tgdε of BF-TD/MCNTs/PTh composites. The tgdε slightly increases and the tgdm first decreases and then increases with the frequency. Comparing reference [48] to Fig. 13, the maximum value of tgdm and tgdε are 0.06 and 0.12 in BF-TD composites. However, the maximum value of tgdm and tgdε are 0.5 and 0.95 in BF-TD/MCNTs/PTh composites, respectively. So it can be concluded that MCNTs and PTh improve the microwave absorption and widen the effective frequency band of BF-TD composites.
X. Hong et al. / Composites Science and Technology 117 (2015) 215e224
Taking the dielectric loss and magnetic loss into account, when the ratio of the BF and TD is 4:5 and the MCNTs content is 20.0 wt%, the BF-TD/MCNTs/PTh composites have the best performance of microwave absorption. 4. Conclusion The BF-TD/MCNTs/PTh composites with good electromagnetic and microwave absorption properties were prepared by in situ polymerization of PTh in the presence of BF-TD composites and MCNTs. The electromagnetic and microwave absorption properties of the BF-TD/MCNTs/PTh composites were investigated as the function of the MCNTs content and the ratio of BF and TD. The MCNTs and PTh enhance the microwave absorption performance and widen the frequency of the BF-TD composites due to the interactions among the BF-TD composites and MCNTs and PTh. When the ratio of BF and TD is 4:5 and the MCNTs content is 20.0 wt.%, the BF-TD/MCNTs/PTh composites exhibit the strongest microwave absorption. This work indicates that BF-TD/MCNTs/PTh composite could be a promising microwave absorption material with a wide frequency band.
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[14]
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[18] [19]
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Acknowledgment
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This work was financially supported by National Natural Science Foundation of China (Nos. 20904019, 21067004 and 51273089), Aviation Science Fund (Nos. 2011ZF56015, 2013ZF56025), Natural Science Foundation of Jiangxi Province (No. 20132BAB203018), Key Laboratory of Photochemical Conversion and Optoelectronic Materials, TIPC, CSA (Nos. PCOM201228, PCOM201401), Jiangxi Province Education Department of Science and Technology Project (No. GJJ13491), Jiangxi Province Youth Scientists Cultivating Object Program (No. 20112BCB23017), and Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University (No. ST201222007).
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