High dielectric constant and low loss of polymeric dielectric composites filled by carbon nanotubes adhering BaTiO₃ hybrid particles

Abstract

We reported the preparation of carbon nanotubes adhering BaTiO₃ nanoparticles (BT@CNTs) via chemical vapor deposition (CVD). Scanning electron microscopy, transmission electron microscope and Raman spectroscopy were carried out in order to confirm the successful adhesion. Dielectric properties of the PVDF-HFP composites filled by BT@CNTs were also studied. With the content of BT@CNTs increasing, the dielectric constant of composites remarkably increased, but the loss tangent gradually decreased. At 10 vol%, the dielectric constant of the PVDF-HFP/BT@CNTs composite was significantly increased to 27.7 at 50 Hz, which was 3 times and 65% higher than that of the pure PVDF-HFP and PVDF-HFP/BT composite, respectively. The loss tangent of the PVDF-HFP/BT@CNTs composite was lower even than that of pure PVDF-HFP. These attractive features of the PVDF-HFP/BT@CNTs composites suggested that the method proposed herein was a new approach to develop high performance composites, especially those with high dielectric constant and low dielectric loss.

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Introduction

In recent years, polymer composites with high dielectric constant have been paid much attention for their promising applications in capacitors, gate dielectrics actuators, energy storage and high-density electronic packaging technology, etc.^1–6 Specifically, capacitors with high dielectric constant and low dielectric loss are highly desired to meet the miniaturization requirement of the electronic devices and ensure good performance. To achieve better performance in these areas, many works have been focused on improving the dielectric properties of the materials by introducing various fillers into the polymer matrix.^7–9 One of the traditional ways is to develop percolative dielectric materials with conductive particles as fillers. For the percolative polymer composites, various nano-sized metal particles including Ni, Al, and Ag have been employed as conductive fillers.^10–15 According to the percolation theory, the effective dielectric constant of the composites can be dramatically increased when the concentration of conductive fillers approaches percolation threshold. However, the high loss tangent, and high conductivity make it difficult to acquire desired electrical property of the polymer–conductor composites. To reduce the conductivity and loss tangent of the percolative composites, researchers have performed an insulating interlayer or shell on the surface of conductive fillers to prevent the conductive particles from contacting with each other, or introducing ceramics to the conductor–polymer systems.^2,16,17 However, it is still hard to achieve a balance between high permittivity and low loss. Another way to improve the dielectric constant is to disperse some high-k ceramic powders into polymers to form 0–3 type composites. Ferroelectric ceramic powders with high dielectric constant, such as barium titanate (BT) and calcium copper titanate (CCTO), have also been chosen as fillers in order to acquire relatively high dielectric constant.^18–20 However, the largest disadvantage of ceramic/polymer based composites is that the high volume fraction (>50 vol%) of the filler is necessary to fabricate high-k composites, which may significantly deteriorate the flexibility of the polymer matrix. Because perfect interface between polymer and ceramic powder can enhance effectively interfacial polarization, many studies have been focused on the modification of the ceramics nanoparticles filler to increase the compatibility between ceramics and polymer matrix. The surface of ceramics is usually modified by surfactant adsorptions, polymer coatings or conductive materials coatings, and a physical compatibility along with more novel interface can be formed to guarantee good dispersion of the ceramics particles.^21–24 Recently, many researchers had reported that the nano-sized Ag particles discretely deposited on the BT surface can efficiently enhance the dielectric permittivity of the BT-Ag/PVDF composite. Theoretical calculations suggested that the enhanced permittivity of BT-Ag/PVDF composites should arise from the ultrahigh permittivity of BT-Ag fillers, which was over 10⁴.²⁵ Lots of previous studies demonstrated that conductive materials coatings on the ceramic particles could result in enhanced dielectric properties in the polymer based composites.

In the present study, the fabrication of carbon nanotubes (CNTs) adhering BaTiO₃ nanoparticles and PVDF-HFP/BT@CNTs composites were investigated. The CNTs adhering BaTiO₃ nanoparticles were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM) and Raman spectra, and the composites were also studied by scanning electron microscope (SEM) and X-ray diffraction (XRD). In addition, the dielectric properties of the composites filled by the BT@CNTs and correlation with the structure and morphology of the composites are also discussed. The BT@CNTs in the composites was found to have significant and interesting influences on the dielectric properties of the composite system.

Experimental

Materials

PVDF-HFP was purchased from Sigma-Aldrich Co. Ltd. BaTiO₃ nanoparticles (BT) purchased from Aladdin Industrial Corporation with the size of 100 nm were sieved out prior to use. Carbon nanotubes adhering BaTiO₃ hybrid particles (BT@CNTs) were fabricated by CVD, and the preparation process was shown in the Scheme 1. Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and methanol were purchased from Shanghai Chemical Corp. All chemicals are used without further purification. All other chemicals were obtained as analytical grade products and used without further purification.

Scheme 1 Synthesis of BT@CNTs.

Preparation of BT@CNTs

In this process, 3 g Ni(NO₃)₂·6H₂O was dissolved into methyl alcohol. A 7 g BT was then added with fast stirring. The mixture was stirred vigorously for 60 min and then heated at 100 °C for 4 h to remove methanol, which was subsequently cooled to room temperature. The products were BaTiO₃@Ni(NO₃)₂·6H₂O. In order to adhere CNTs to the BaTiO₃ nanoparticles, the catalyst-coated BaTiO₃ nanoparticles were placed at the centre position of a quartz tube reactor heated to about 800 °C under flowing argon–hydrogen (10 : 1 molar ratio) gas mixture. After reaching to the desired temperature (800 °C), a flow of methane (CH₄) gas was introduced into the reactor with a rate of 20 sccm for about 30 min. A black coating covered the surface of the nanoparticles is CNTs. Then quartz tube reactor was subsequently cooled to room temperature. The black powder is BT@CNTs. The technology parameters in the fabrication of BT@CNTs are listed in the Table 1.

Table 1 Technology parameters of BT@CNTs

	Pressure	Catalyst	Temperature	Gas	Time
CNTs	Ordinary pressure	Ni	800 °C	CH4/H2–Ar 20/80	30 min

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Preparation of PVDF-HFP composites

The PVDF-HFP/BT@CNTs composites were prepared by employing solution blend. The BT@CNTs were ultrasonically dispersed in N,N-dimethylformamide (DMF) for 0.5 h. Then PVDF-HFP was added into the system with mechanical stir for 2 h at 70 °C. Then the mixture was casted on clean glass plates and dried at 120 °C for 2 h in an oven. The prepared composites filled with various filler concentration and thickness of 25–35 μm were collected for test.

Characterization

The scanning electron microscope (SEM) observation was performed on a Helios Nanolab 600i. The transmission electron microscope observation was performed on a Tecnai F30. The wide-angle X-ray diffraction (XRD) experiment was performed on a Philips X'Pert diffractometer with CuKα radiation generated at 40 kV and 40 mA. Raman analysis was carried out using a micro-Raman Renishaw spectrometer. The laser line (514 nm) was used to excite the sample. The broadband dielectric spectroscopy was carried out using a Agilent 4294. Prior to dielectric measurements, the relative humidity (≈50%) of the measurement environment remain constant throughout the investigation.

Results and discussions

Characterization of BT@CNTs and composites

Raman spectra can be used to reveal the graphitic nature of the fabricated CNTs. Fig. 1 shows the Raman spectra of the BT@CNTs powder. The peak at 1574 cm⁻¹ is a so-called G-band, and those at 1321 cm⁻¹ are D-band. The G-band comes from the stretching of conjugated double bonds corresponding to sp² hybridization during the formation of the arylnanotube bond, and the D-band is associated with vibrations of carbon atoms with dangling bonds in plane terminations of the disordered graphite or glassy carbons. The intensity ratio of I_D/I_G is known to depend on the structural characteristics of CNTs, and is a usual measurement of the graphitic ordering. The I_D/I_G value of the spectrum suggests a defective structure or a lower degree of graphitization in CNTs structure. In the Fig. 1, the I_D/I_G value of the spectrum is about 0.8, which demonstrates that higher quantity CNTs are adhered on BT nanoparticles.

Fig. 1 Raman spectra of BT@CNTs.

The SEM and TEM images of as-synthesized BT@CNTs hybrid particles are shown in Fig. 2. As shown in Fig. 2(a) and (b), it is clear that the CNTs with a diameter of 30–70 nm are dispersed on the BT surface. The Fig. 2(b) displays a image of BT@CNTs with higher magnification, which clearly shows that one side of the CNTs is in connect with the BT particles. In order to further confirm the adhesion, TEM image of BT@CNTs and high resolution TEM image of interfacial microstructures are displayed in the Fig. 2(c) and (d), respectively. The observation in Fig. 2(c) is consistent with that in Fig. 2(b), which indicates that the CNTs are adhered on the surface of BT. Fig. 2(d) confirms that the combination between BT and CNTs is strong, and no phase separation exists.

Fig. 2 SEM images (a and b) and TEM images (c and d) of BT@CNTs, (d) is the high resolution TEM image in the dashed line square of (c).

Fig. 3 shows the XRD patterns of pure PVDF-HFP and PVDF-HFP/BT@CNTs composite. The peaks at 18.4°, 20.8° and 26.6° correspond to the PVDF α-phase, β-phase and γ-phase diffraction, respectively. For the composite, the characteristic diffraction peaks of BaTiO₃ appear at about 2θ = 22°, 31°, 38°, which are associated with typical structure of perovskite BaTiO₃ in crystal planes of (100), (110) and (111). Because the characteristic of CNTs in XRD pattern is not obvious, there is no peak corresponding to CNTs. After BT@CNTs were incorporated into the PVDF-HFP matrix, the XRD pattern of the composite shows both BaTiO₃ and PVDF-HFP diffraction peaks, which clearly demonstrates that BT@CNTs are filled in the polymer matrix.

Fig. 3 XRD patterns of PVDF-HFP and PVDF-HFP/BT@CNTs composite.

In order to research the effect of hybrid filler on the dielectric properties of composites, we fabricated two kinds of composites containing BT@CNTs and BT filler, respectively. Fig. 4 displays cross section SEM images of PVDF-HFP/BT and PVDF-HFP/BT@CNTs composites. BT particles with sphere disperse homogeneously into the PVDF-HFP matrix. Moreover, the dispersion of BT@CNTs in the matrix is excellent. There are some wire-like fillers found (dashed lines) in the matrix, which should be the CNTs.

Fig. 4 Cross section SEM images of (a) PVDF-HFP/10 vol% BT, (b) PVDF-HFP/10 vol% BT@CNTs composite.

Dielectric properties of composites

The dependence of the dielectric constant of composites on the frequency is shown in Fig. 5. With the frequency increasing, the dielectric constants of two composites both decrease, because the different types of polarizations will not keep up with the variation of the AC frequency and gradually vanish one by one. With BT@CNTs content increasing, the dielectric constant of the composites increases. As the ceramic fillers have a much larger permittivity, most of the increase in effective dielectric constant of the composites comes from the increase in the average field in the polymer matrix. Compared with the composites filled by BT, it is clearly demonstrated that the dielectric constant of the composite can be significantly improved by filling BT@CNTs.

Fig. 5 Dependence of dielectric constant of PVDF-HFP/BT@CNTs composites on the frequency at room temperature.

The Fig. 6 shows loss tangent as a function of frequency for PVDF-HFP/BT@CNTs composites. With the frequency increasing, the loss tangent of composites decreases firstly, then increases. The loss tangent of PVDF matrix is around 0.065 at 50 Hz. The values of PVDF-HFP/1 vol% BT@CNTs, PVDF-HFP/5 vol% BT@CNTs and PVDF-HFP/10 vol% BT@CNTs are 0.035, 0.05 and 0.043, respectively. In addition, the corresponding loss tangent of composite with 10 vol% BT is 0.072, higher than that of PVDF-HFP and PVDF-HFP/BT@CNTs composites. The results demonstrate that the BT@CNTs rather than BT can not only enhance the dielectric constant, but also reduce the loss tangent.

Fig. 6 Dependence of dielectric loss of PVDF-HFP/BT and PVDF-HFP/BT@CNTs composites with different filler contents on the frequency at room temperature.

Fig. 7 shows dielectric constant and loss tangent of the composites measured at 50 Hz as a function of volume fraction BT@CNTs. It should be noted that the dielectric constant of the composites increases with content of BT@CNTs increasing. Interestingly, the loss tangent has a downtrend when the BT@CNTs are incorporated into matrix. Evidently, the trend is not consistent with percolation theory, which is always used to explain many dielectric phenomena in the composites filled by CNTs.

Fig. 7 Dielectric constant and loss tangent of the composites measured at 50 Hz as a function of volume fraction BT@CNTs.

The Fig. 8 shows the dielectric constant and loss tangent of PVDF-HFP, PVDF-HFP/10 vol% BT@CNTs and PVDF-HFP/10 vol% BT composites. Among samples above, the dielectric properties of PVDF-HFP/BT@CNTs composite are outstanding because of its highest dielectric constant and lowest loss tangent. For the PVDF-HFP/10 vol% BT@CNTs composite, the dielectric constant is about 27.7 at 50 Hz. And yet, the values of PVDF-HFP and PVDF-HFP/10 vol% BT composite are only about 9.6 and 16.7, respectively. It should be noted that the dielectric constant of the composite with 10 vol% BT@CNTs is 65% higher than the sample with 10 vol% BT and 3 times larger than the polymer matrix. In the light of these dielectric features, it can be concluded that the increment in dielectric constant of PVDF-HFP/BT@CNTs composite is attributed to the combination of BT and CNTs. The CNTs, as a kind of conductive filler own high aspect ratio and have a function to enhance dielectric constant effectively. Besides, the interfacial polarization, which normally exists between polymer and conductive fillers give rise to a much increased dielectric constant. However, the downtrend of loss tangent does not agree with some classical theory, for example Maxwell–Wagner and percolation theory. The phenomenon may be resulted from the following reasons. First of all, the content of CNTs in the BT@CNTs is much low, which is far from percolation threshold of CNTs in the polymer. Secondly, one side of CNTs is connected with insulting BT particles, which efficiently prevents the continuous contact among CNTs in the matrix and suppresses the formation of the conducting path in the composite. With the CNTs adhering to the BT, the miscibility of filler and polymer matrix should be improved. In addition, the length and diameter of CNTs is small, in the range of 0.5–1 μm and 30–70 nm, respectively. A type of conductive filler with such small size possibly induces Coulomb blockade effect into polymer matrix. The size of CNTs conforms to requirement of “Coulomb Island”.^7,26 The tunnelling electron creates an additional barrier due to the charging energy e²/2C (where e is the electron charge unit, and C is the capacitance of the Coulomb island) to inhibit the further transfer of electrons. When the charging energy exceeds the thermal fluctuation energy k_BT (where k_B is the Boltzman constant, and T is the absolute temperature), the Coulomb blockade will occur, which inhibits the charge transfer through the small island below a certain voltage threshold and leads to an increase in resistance. Thus, the loss tangent of PVDF-HFP/BT@CNTs is lower than that of PVDF-HFP and PVDF-HFP/BT composite.

Fig. 8 Comparasion of dielectric constant and loss tangent at 50 Hz between PVDF-HFP, PVDF-HFP/BT@CNTs and PVDF-HFP/BT composites.

Conclusions

1. The BT@CNTs were fabricated by adhering CNTs to the surface of BT nanoparticles via CVD.

2. The high dielectric constant PVDF-HFP/BT@CNTs composites were prepared via casting method. For the PVDF-HFP/10 vol% BT@CNTs composite, the dielectric constant is about 27.7 at 50 Hz, which is 65% higher than the composite with 10 vol% BT and 3 times larger than the polymer matrix.

3. Compared with PVDF-HFP and PVDF-HFP/BT composite, the loss tangent of PVDF-HFP/BT@CNTs is lowest, which may be due to the Coulomb blockade effect of CNTs on the surface of BT nanoparticles.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (Grant no. 11272102).

Notes and references

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