Nanostructured Barium Titanate / Carbon Nanotubes Incorporated Polyaniline as Synergistic Electromagnetic Wave Absorbers

The three-dimensional (3D) conductive network structures formed by barium titanate/carbon nanotubes incorporated polyaniline were favorable for strengthening electromagnetic absorption capability. Herein, an easy and flexible method consisting of sol-gel technique, in situ polymerization, and subsequent mechanical method have been developed to prepare the barium titanate/carbon nanotubes incorporated polyaniline (CNTs/BaTiO 3 /PANI or CBP) ternary composites. The dielectric properties and microwave absorption properties of CNTs/BaTiO 3 /PANI composites were investigated in the frequency range of 2–18GHz by vector network analyzer. Interestingly, it is found that the CNTs/BaTiO 3 /PANI composites with 3D conductive network structures presented outstanding electromagnetic absorption properties, which may be attributed to the high impedance matching behavior and improved dielectric loss ability and novel synergistic effect. Additionally, it also can be supposed that the “geometrical effect” of composite was more beneficial to absorbing the incident electromagnetic wave.The CNTs/BaTiO 3 /PANI composite (the mass ratio of CNTs/BaTiO 3 to PANI is 2 : 3) exhibits the best microwave absorption properties, of which the minimum reflection loss value can reach −30.9 dB at 8GHz and the absorption bandwidth with a reflection loss blew −10 dB ranges from 7.5 to 10.2 GHz.


Introduction
Arising from the rapid development of information technology, the military stealth technique has attracted increasing attention owning its application in the modern warfare.Also, electromagnetic interference has greatly threatened human health and disturbed all contemporary electrical and electronic systems from daily life to space exploration [1,2].Thus, it is urgent demand for a high-performance microwave absorbing material with low density, tiny thickness, strong wave absorption, and broad bandwidth [3].
In this context, carbon nanotubes (CNTs) can be a promising material for reduction electromagnetic radiations due to the combined light weight and remarkable mechanical and electronic properties [4][5][6].However, the well conductivity of carbon nanotubes used as microwave absorbing material has the shortcoming of poor impedance matching.In this regard, magnetic materials and dielectric materials may be favorable to improving the impedance matching of CNTs [7].For example, the reflection loss of PP magnetic polymer nanocomposites with MWNTs is achieved −20 dB at 20.0 GHz [2].Besides, carbon nanotubes combination with a dielectric material also can improve the impedance matching and induce losses like capacitor effect, antenna effect, polarization effect, and so forth.Barium titanate (BaTiO 3 ) [8] represents one of the most studied dielectric materials due to its high dielectric constant, positive temperature coefficient, and nonlinear optical properties.For example, Huang et al. [9] synthesized BaTiO 3 /MWCNT nanocomposites with the mass ratio BaTiO 3 : MWCNT = 25 : 1 which exhibited excellent absorption properties.Zhu et al. [10] found that MWCNT covered with BaTiO 3 possessed better microwave absorbing properties than the pure MWCNTs in the frequency range 11-15 GHz.In addition, conducting polymers have attracted a great deal of attention as microwave absorber owing to their distinct features.Among the different conducting polymers, polyaniline (PANI) has been the material of choice due to various reasons including lightweight, corrosion resistance, facile synthesis, good environmental stability, controllable electrical conductivity, and dielectric loss ability [11,12].Guo et al. [13] fabricated a PANI/epoxy nanocomposite with high quality dispersion of the PANI in epoxy matrix, and the uniformly dispersed structure endowed nanocomposite with good mechanical property and outstanding electrical conductivity (3.8017 × 10 10 Ω cm).Besides, PANI has the potential application in the field of energy storage, energy saving, and anticorrosion [14].In particular, it also has become an ideal choice as a component of microwave absorber to achieve a perfect impedance matching.
In present work, to attain an impedance match to achieve greater microwave absorption properties, nanostructured CNTs/BaTiO 3 and PANI have been incorporated together by a facile approach to form a stable, effective, light weight, and environmentally friendly microwave absorber of CNTs/BaTiO 3 /PANI composite.It is anticipated that the CNTs/BaTiO 3 /PANI composite shows excellent microwave absorbing properties, owing to high impedance matching, enhanced synergistic effect, and improved dielectric loss.Meanwhile, it is also worth mentioning that the 3D conductive network structures of CNTs/BaTiO 3 /PANI composite with greater microwave absorption properties may be affected by the "geometrical effect."

Experimental
2.1.Materials.Aniline (An) and butyl titanate (Ti(OC 4 H 9 ) 4 ) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd.Barium acetate (Ba(CH 3 COO) 2 ) and other chemicals were purchased from Aladdin Industrial Corporation.Carbon nanotubes (diameter, 40-70 nm) were provided by Wako Pure Chemical Industries, Ltd.BaTiO 3 Sol.1.7 g of Ti(OC 4 H 9 ) 4 was dissolved in the mixed solution of 1 mL of CH 3 COOH and 10 mL of CH 3 CH 2 OH at room temperature to form titanium sol.Then 1.275 g of Ba(CH 3 COO) 2 dissolved with the mixed solution of 1 mL of distilled water and 4 mL of CH 3 COOH was added dropwise into the titanium sol.After a reaction at 40 ∘ C for 2 h under constant stirring, a homogeneous transparent BaTiO 3 sol was formed.

Synthesis of the
CNTs/BaTiO 3 Composites.0.05 g of the CNTs was oxidized by HNO 3 and then added into BaTiO 3 sol system under sonicated for 1 h.The CNTs/BaTiO 3 xerogel was obtained by the sol system aged for 24 h at room temperature and then dried for 24 h at 60 ∘ C. Finally, the CNTs/BaTiO 3 xerogel transferred to a tube furnace and annealed at 700 ∘ C for 1 h under nitrogen atmosphere to obtain CNTs/BaTiO 3 (CB).

Preparation of CNTs
/BaTiO 3 /PANI Composites. 2 mmol of aniline was added into 10 mL of HCl (1 mol/L) under stirring condition.Until homogenous suspension was achieved, APS aqueous solution (2 mmol of APS in 10 mL of deionized water) was dropwise added to the suspension.The CNTs/BaTiO 3 : PANI (mass ratio) = 1 : 4 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) to obtain PANI.CNTs/BaTiO 3 /PANI (CBP) composites were obtained by using mechanical method as follows: different mass ratios of the as-prepared CNT/BaTiO 3 and PANI were loaded in a stainless jar and ball-milled by planetary ball mill (Retsch PM 100) at a rotation speed of 250 rpm for 2 h.The specific parameters were shown in Table 1.

2.5.
Characterization.The morphology of samples was observed by scanning electron microcopy (FE-SEM; Hitachi S-4800), and the crystal structure of the prepared powders was analyzed with an X-ray diffractometer (Bruker AXS, D8-Discover), using Cu K radiation.Fourier transform infrared spectroscopy (FT-IR) was performed using a Nicolet 5700 FT-IR spectrometer (Thermo Electron Corp., USA) with KBr pellets.Conductivity was measured by four-point probe (SZT-2B).The composite samples used for electromagnetic measurements were prepared by loading the products in paraffin wax (30 wt% CB or CBP composites were mixed with wax).The powder-wax compound was then pressed into toroidally shaped samples ( out = 7 mm and  inner = 3 mm) for complex permittivity  ( =   −   ) and permeability  ( =   −   ) measurements with a vector network analyzer (N5224A, Agilent) in the 2-18 GHz range.

Morphology and Structure Analysis.
Representative FE-SEM images of PANI, CNTs, and CB are shown in Figures 1(a)-1(c).The morphology of PANI is displayed in Figure 1(a) as short rod-like, and the average diameter is about 55.8 nm (Figure 1(d)).For the CNTs, seen in Figure 1(b), the nanotubes with smooth surface and diameter in the rage of 20-90 nm can be clearly observed (Figure 1(e)).The morphology of CB presented in Figure 1(c) indicates that the BaTiO 3 is mounted onto the surface of the CNTs uniformly, and the surface is no longer smooth.
The morphology of CBP 1 , CBP 2 , CBP 3 , and CBP 4 presented in Figures 2(a)-2(d) shows that homogeneous composites of CBP have formed.In attention, it can be seen that a 3D network structure has been built up between PANI and CB.Meanwhile, significant effect of synergic would form from the network, which could distinctly enhance the wave absorption.
Functional groups of samples are also detected by FT-IR measurement, as shown in Figure 4.For PANI, the characteristic peaks at 1560 cm −1 and 1475 cm −1 are attributed to the C=C stretching vibration of the quinoid (Q) ring and the benzenoid (N) ring, respectively, indicating the presence of the emeraldine salt of PANI.The peak at 1300 cm −1 is due to the C-N stretching vibration in PANI.The peak at 1241 cm −1 is assigned to the stretching vibration of the CN •+ in the polaron structure of PANI [20][21][22].The characteristic peak at 1106 cm −1 can be attributed to the stretching of C=N(N=Q=N).For the CB, the absorption bands centered at 518 cm −1 and 430 cm −1 are due to Ti-O stretching vibrations and characteristic of BaTiO 3 [23,24].For the CBP, the peaks of CB almost cannot be detected due to the strong absorption peaks of PANI.Meanwhile, neither blueshift nor redshift happened in the spectrum of CBP (dash lines are shown in Figure 4), indicating that the structure of CB and PANI was not changed, which is consistent with the results of XRD.
The conductivity property of samples was measured, as shown in Figure 5.For CNTs (treated by HNO 3 ), it can be seen that the electrical conductivity is about 1.1202 S/cm.Also, it is clear that the conductivity of CBP increased gradually with increasing the content of PANI in the system.Particularly, the conductivity of pure PANI reached 3.7425 S/cm.Here it should be pointed out that conductivity of these samples falls in the range from 10 −2 S/cm to 10 1 S/cm desired for exhibiting good microwave absorb responses [25].Therefore, conductivity of CBP may be beneficial to the enhancement of microwave absorption.

Electromagnetic Wave Absorption Properties of CBP
Composites.It is believed that the complex permittivity and permeability of the absorber determine the microwave absorption properties.In order to evaluate the microwave absorption properties of CB and CBP, the complex permittivity and permeability of the composites were measured in frequency range of 2-18 GHz.As shown in Figure 6  CB, leading to improvement of dielectric constant.As shown in Figure 6(b), for all samples, the variation tendency of the imaginary parts   of the complex permeability was similar to the real parts   .Particularly, several relaxation peaks on   curves were observed.It can be attributed to typical characteristics of nonlinear resonant behaviors [26].As mentioned above, the value of complex permittivity can be improved effectively with the appropriate content of PANI.
The complex permeability spectra of the CB and CBP composites are presented in Figure 7.It is obviously exhibited that the real part   and imaginary part   of samples remain almost constant in the whole frequency range with the value being about 1 and 0, respectively.It can be concluded that the main contribution to the microwave absorption of samples results from the dielectric loss rather than the magnetic loss.
To further study the microwave absorption properties, the reflection loss (RL) properties of samples were calculated according to transmission line theory as follows: The normalized input impedance ( in ) is given by the formula: where  is the microwave frequency in Hz,  is the thickness of the absorber in m,  is the velocity of light in free space in m/s, and   and   are the complex permittivity and permeability, respectively.Based on the electromagnetic parameters (the complex values of permittivity and permeability), the RL can be calculated for the given frequency with various thicknesses according to (1) and ( 2).The calculated reflection loss curves of composites in the range of 2-18 GHz are shown in Figure 8. Reflection loss of CB and CBP samples at a thickness of 4 mm is shown in Figure 8(a); it can be observed that the sample of CBP 3 exhibits high-performance microwave absorption compared with CB and others.It shows that the minimum RL value is up to −30.9 dB at 8 GHz and bandwidth corresponding to the reflection loss lower than −10 dB is from 7.5 GHz to 10.2 GHz.The excellent microwave absorption of the CBP 3 can be accounted by the fact that the prominent interfacial polarization and synergistic action are formed due to the interaction between conductive network of PANI and electric field.Furthermore, impedance matching has been improved with the appropriate content of PANI.Nevertheless, the microwave absorption of CBP 4 is sharply deteriorated with the PANI content of composites further increasing.The reason is that a high value of permittivity and conductivity  will result in strong reflection and poor impedance matching.The reflection loss curves of CBP 3 with different thicknesses are shown in Figure 8(b).It is clear that the minimum reflection loss peak shifted to the low frequency with the increasing thickness, which is corresponding to the theory of quarter-wave principle [27,28].Based on above analysis, it concludes that the high-performance microwave absorption of CBP can be improved significantly by tunable components.
The main microwave absorption mechanism of CBP is proposed in Figure 9.It is well known that the microwave energy attenuation by CNTs and CB is relying on electronic polarization and interfacial polarization.Therefore, microwave absorption of composites can be enhanced through dielectric polarization relaxation for the CNTs decorated with BaTiO 3 nanoparticles.In addition, it is also noted that network formed between PANI and CB not only reinforcing synergistic action but also strengthening interfacial polarization.Furthermore, the impedance matching properties of materials have been improved significantly by PANI.Thus, compared with CB, the microwave absorption of CBP is improved significantly.

Conclusions
Barium titanate/carbon nanotubes incorporated polyaniline composites with 3D conductive network structure have been successfully prepared by an easy and flexible method, which exhibit excellent microwave absorbing properties.The good dielectric properties and high impedance matching of CBP composites are due to presence of CB and PANI, as well as the synergistic effect and geometrical effect due to the microstructure which collectively contributes towards the outstanding performance.The CBP with the mass ratio of CB : PANI = 2 : 3 (CBP 3 ) shows the best microwave absorption properties.The maximum reflection loss of CBP 3 is −30.9 dB at 8 GHz with a 4 mm thick sample layer and filler loading of 30 wt% paraffin wax, and the bandwidth with a reflection loss less than −10 dB covers wide frequency range from 7.5 to 10.2 GHz.Therefore, it is reasonable to believe their potential for making futuristic microwave absorbers.

Figure 6 :
Figure 6: Complex permittivity of samples: (a) real part and (b) imaginary part.

Figure 7 :
Figure 7: Complex permeability of samples: (a) real part and (b) imaginary part.

Figure 8 :
Figure 8: (a) Reflection loss of samples with the thickness of 4 mm and (b) reflection loss of CBP 3 with different thickness.

Table 1 :
Composite absorbing material of CBP with different mass ratio.