One-dimensional carbon nanotube@barium titanate@polyaniline multiheterostructures for microwave absorbing application
© Ni et al.; licensee Springer. 2015
Received: 10 December 2014
Accepted: 21 March 2015
Published: 11 April 2015
Multiple-phase nanocomposites filled with carbon nanotubes (CNTs) have been developed for their significant potential in microwave attenuation. The introduction of other phases onto the CNTs to achieve CNT-based heterostructures has been proposed to obtain absorbing materials with enhanced microwave absorption properties and broadband frequency due to their different loss mechanisms. The existence of polyaniline (PANI) as a coating with controllable electrical conductivity can lead to well-matched impedance. In this work, a one-dimensional CNT@BaTiO3@PANI heterostructure composite was fabricated. The fabrication processes involved coating of an acid-modified CNT with BaTiO3 (CNT@BaTiO3) through a sol–gel technique followed by combustion and the formation of CNT@BaTiO3@PANI nanohybrids by in situ polymerization of an aniline monomer in the presence of CNT@BaTiO3, using ammonium persulfate as an oxidant and HCl as a dopant. The as-synthesized CNT@BaTiO3@PANI composites with heterostructures were confirmed by various morphological and structural characterization techniques, as well as conductivity and microwave absorption properties. The measured electromagnetic parameters showed that the CNT@BaTiO3@PANI composites exhibited excellent microwave absorption properties. The minimum reflection loss of the CNT@BaTiO3@PANI composites with 20 wt % loadings in paraffin wax reached −28.9 dB (approximately 99.87% absorption) at 10.7 GHz with a thickness of 3 mm, and a frequency bandwidth less than −20 dB was achieved from 10 to 15 GHz. This work demonstrated that the CNT@BaTiO3@PANI heterostructure composite can be potentially useful in electromagnetic stealth materials, sensors, and electronic devices.
KeywordsBaTiO3 Polyaniline Carbon nanotube Heterostructure Microwave absorption
Serious electromagnetic interference (EMI) pollution arising from the drastic development of telecommunication and gigahertz (GHz) electronic systems has aroused great interest in electromagnetic-absorber technology to solve the problem [1-5]. An ideal electromagnetic wave absorbing material should exhibit light weight, be thin, and have a strong wave absorption and wide frequency range response [6,7].
Carbon materials, with dielectric loss features, have been used as a very important component in microwave absorbers [8-10]. Brosseau et al. [9,11,12] investigated the dielectric properties of a series of carbon black-filled polymers, and the complex effective permittivity for carbon black-filled polymeric systems was well discussed. It is well known that neat carbon materials possess a very high conductivity, resulting in mismatch impedance, thereby inducing very limited microwave absorption [13,14]. Therefore, carbon nanotubes (CNTs) have been used as filler to prepare CNT/polymer composite microwave absorbers . Recently, heterostructure-based CNT electromagnetic (EM) absorbers with good impedance matching have been fabricated, indicating that it could be an effective way to optimize the EM parameters and enhance the absorption performances [16,17]. Song et al.  fabricated ZnO-coated CNTs and dramatically improved their microwave absorption. Wang et al.  synthesized magnetite-decorated CNTs, which exhibited considerable EM absorbing ability. This was due to synergetic interactions between the magnetic nanocrystals and the CNTs. It has been observed that the interfaces of the composite materials also play an important role in EM absorption .
In order to meet further requirements of broadband microwave absorbers, multiple-phase heterostructure materials with new or enhanced EM absorption properties have been developed due to the interfacial polarization and confinement effect [13,20-22]. Polyaniline (PANI) is one of the most important conducting polymers with excellent environmental stability and tunable conductivity, and it has been considered as an ideal matrix or as a second phase incorporated with inorganic nanomaterials to achieve unique EM wave absorption [7,23-26]. However, the formation of multiple-phase composites with PANI encapsulated on the side surface of one-dimensional inorganic nanocomposites has been rarely reported.
The effect of absorbing materials is not only a strong absorption but also a wide absorption bandwidth that can be obtained by integrating the advantages of CNT, BaTiO3, and PANI with different loss mechanisms. PANI encapsulated on the surface of the composite may lead to an increase in matching impedance. In this report, we have coupled a sol–gel method and an in situ polymerization to successfully prepare a one-dimensional CNT@BaTiO3@PANI multiphase heterostructure composite. The structures, morphology, and conductive properties of the composite were fully characterized. Because of their special structural characteristics, well-matched characteristic impedances, and interfacial polarization induced by multiple interfaces in the composites, the multiple-phase heterostructure composites exhibit excellent EM absorption performances. Thus, one-dimensional CNT@BaTiO3@PANI heterostructures are very promising as a lightweight EM absorbing material.
Multiwalled CNTs (MWCNTs) with diameter ranging from 40 to 70 nm were obtained from Wako Pure Chemical Reagent Co., Ltd., Chuo-ku, Japan. Oxidation of MWCNTs was carried out in hot, concentrated nitric acid. Barium acetate (Ba(CH3COO)2), tetraisopropyl titanate (Ti(OC3H7)4), acetic acid, hydrochloric acid, ammonium peroxodisulfate (APS), sodium dodecyl benzene sulfonate (SDBS), and ethanol were supplied by Wuxi Zhanwang Chemical Reagent Co., Ltd., Yixing, China. Aniline (99%) was supplied by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All chemicals were used without further purification. Deionized water was used in all experiments.
Preparation of oxidized CNTs
Typically, MWCNTs (approximately 0.5 g) were acidified by concentrated nitric acid with vigorous stirring at 115°C for 6 h, to obtain oxidized CNTs with a large number of oxygen-containing reactive groups on the ends and sidewall. The acid-oxidized CNTs were then collected by filtration and washed with deionized water until a neutral pH value was obtained in the washing solution.
Synthesis of the BaTiO3 precursor
The BaTiO3 precursor was produced via a sol–gel method. The Ba and Ti cations were controlled at a 1:1 molar ratio. In a typical process, barium acetate (5 mmol) was dissolved into a mixture of acetic acid (5 mL) and anhydrous ethanol (20 mL) with vigorous stirring at 60°C water bath for 30 min, designated as solution A. Titanium isopropoxide (5 mmol) was dissolved into anhydrous ethanol (10 mL) containing deionized water (1 mL) with stirring for 15 min to make it homogeneous. That solution was then added to the mixed solution A slowly under continuous stirring conditions and maintaining a temperature of 60°C for 2 h and then aged at room temperature for 24 h.
In a typical fabrication experiment, the as-treated MWCNTs (100 mg) were dispersed in BaTiO3 sol solution by ultrasonication for 30 min, and then vigorous stirring was applied to the mixture suspension for reaction at 40°C for 4 h. The resulting suspension was filtrated, and the powder was dried and then calcined in a tube furnace at 700°C for 2 h under high-purity Ar atmosphere. CNT@BaTiO3 heterostructures were obtained.
For fabrication of CNT@BaTiO3@PANI, a portion of the as-prepared CNT@BaTiO3 heterostructures (50 mg) were mixed with SDBS (20 mg) dispersed in 28 mL deionized water and then ultrasonicated for 2 h to obtain a uniform suspension. HCl (5 mL, 1 mol/L) and aniline (5 mmol) were subsequently added under stirring condition. Until homogeneous suspension was achieved, APS aqueous solution (5 mmol of APS in 10 mL of deionized water) was dropwise added to the suspension. The polymerization process was applied in an ice bath for 24 h under stirring. The resulting precipitations were washed with deionized water and ethanol, followed by drying in an oven (50°C).
The morphology and microstructure of the products were characterized using field emission scanning electron microscopy (FE-SEM; Hitachi S-4800, Hitachi, Ltd., Chiyoda-ku, Japan) and transmission electron microscopy (TEM; JEOL JEM-2100 F, JEOL Ltd., Akishima-shi, Japan) with an accelerating voltage of 200 kV. The crystal structure of the prepared powders was analyzed with an X-ray diffractometer (D8-Discover, Bruker AXS, Billerica, MA, USA), using Cu Kα radiation. Fourier transform infrared spectroscopy (FT-IR) was performed using a Nicolet 5700 FTIR spectrometer (Thermo Electron Corp, Waltham, MA, USA) with KBr pellets. Thermogravimetric analysis (TGA) was carried out in nitrogen atmosphere from room temperature to 700°C at a heating rate of 5°C/min using a SDTA851e analyzer (Mettler-Toledo, Greifensee, Switzerland).
The composite samples used for electromagnetic measurements were prepared by loading the products in paraffin wax. The powder-wax compound was then pressed into toroidally shaped samples (φ out = 7 mm, φ inner = 3 mm, H thickness = 2 mm) for complex relative permittivity ε (ε = ε′ − jε″) and magnetic permeability μ (μ = μ′ − jμ″) measurements with a vector network analyzer (37247D, Anritsu Co., Ltd., Atsugi-shi, Japan) in the 0.5 to 15 GHz range.
Results and discussion
Morphology and structure analysis
Conductivity of the CNT@BaTiO3@PANI composites
The sample was pressed into circular sheets (thickness = 0.5 mm, diameter = 1 cm). The values of conductivity by a four-probe measurement for the CNT@BaTiO3 composites and CNT@BaTiO3@PANI composites were 0.39 and 1.51 S/cm, respectively. For the as-prepared CNT@BaTiO3 composites, the conductivity deceases significantly compared to the high conductivity of the raw CNTs. This was mainly caused by the insulating behavior of the BaTiO3 coating on the surface of the CNTs which hinders the charge transfer. Notably, the CNT@BaTiO3@PANI composites showed an evident increase in conductivity compared with the CNT@BaTiO3 composites. It can be attributed intrinsically to the conducting polymer due to the presence of a conjugated π electron system in their structure. The conductivity of the composites was in the magnitude of 10−1 to 100 S/cm, which can be considered as an appropriate conductivity level for a material used as a microwave absorber.
Electromagnetic wave absorption properties of the CNT@BaTiO3@PANI composites
Microwave absorption properties of samples
Sample (thickness of CNT@BaTiO 3 @PANI)
Microwave absorption properties of samples
RL m (dB)
f m (GHz)
Frequency range (GHz) (RL < −10 dB)
Frequency range (GHz) (RL < −20 dB)
12.1 ~ 15
7.9 ~ 15
10 ~ 15
5 ~ 15
6.4 ~ 6.9/7.5 ~ 8
4.0 ~ 6.5/11.4 ~ 15
3.1 ~ 4.6/11.4 ~ 15
3.5 ~ 3.9
In conclusion, a one-dimensional CNT@BaTiO3@PANI multiphase heterostructure composite was successfully fabricated via coupled sol–gel method and in situ polymerization. The structures, morphology, conductive properties, and microwave absorption performance of the composites have been characterized. The CNT@BaTiO3@PANI composites showed the best reflection loss of −28.9 dB at 10.7 GHz with a thickness of 3 mm, and a frequency bandwidth less than −20 dB was observed from 10 to 15 GHz. The excellent microwave absorption property was due to their special structural characteristics, well-matched characteristic impedances, and interfacial polarization induced by multiple interfaces in the composites. Furthermore, the content of the CNT@BaTiO3@PANI composites in the paraffin matrix was only 20 wt %, much lower than that of others recently reported. We believe that the one-dimensional CNT@BaTiO3@PANI heterostructures are very promising as effective lightweight fillers in highly effective electromagnetic attenuation.
This research was supported by Zhejiang Provincial Natural Science Foundation of China (No. LQ14E030010); Zhejiang Top Priority Discipline of Textile Science and Engineering (No. 2013YBZX04); The Young Researchers Foundation of Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University (No. 2013QN07); and Science Foundation of Zhejiang Sci-Tech University (ZSTU) (Nos. 13012147-Y and 13012062-Y).
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