High-k Materials with Low Dielectric Loss Based on Two Superposed

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High‑k Materials with Low Dielectric Loss Based on Two Superposed Gradient Carbon Nanotube/Cyanate Ester Composites Binghao Wang, Dake Qin, Guozheng Liang,* Aijuan Gu,* Limei Liu, and Li Yuan Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Materials Science and Engineering College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu, 215123, People’s Republic of China ABSTRACT: Higher dielectric constant, lower dielectric loss, and good processing characteristics have been the goal for developing high-k materials in actual cutting-edge applications. The distribution, motion, and the rearrangement of space charges are the key factors in determining the dielectric properties of a material; however, few reports focus on this subject. Here, materials with high dielectric constant and low dielectric loss, consisting of two superposed gradient multiwall carbon nanotube (MWCNT)/cyanate ester (CE) composites, coded as [g-MWCNT/CE]2, were uniquely prepared. When the loading of MWCNTs is 0.5 wt %, either MWCNT0.5/CE or [g-MWCNT0.5/CE]2 composite shows the largest dielectric constant of its corresponding type of composites, the value is 136 or 306 (at 1 Hz); meanwhile, the dielectric loss tangent at 1 Hz of [g-MWCNT0.5/CE]2 material is 0.21, only about 2.9 × 10−5 times the value of the MWCNT0.5/CE composite. By investigating the distribution of space charges, the unique dielectric behavior of the [gMWCNT/CE]2 materials is found to result from the reinforced space charge polarization and subdued leakage current induced by the presence of the “conductor fault” between the two superposed gradient composites. These interesting data suggest that the special structure of [g-MWCNT0.5/CE]2 materials is beneficial to fabricate high-k composites with low dielectric loss. distribution along the direction of the thickness.16 Compared with traditional MWCNT/CE composites of which MWCNTs are uniformly distributed in all directions of the composite, the g-MWCNT/CE composite has extremely low dielectric loss, but the highest dielectric constant of the g-MWCNT/CE composite is only 64 at 1 Hz, far lower than the value of the uniform MWCNT/CE composites.17,18 The other is a doublelayer material based on MWCNT/CE composite and a polyethylene (PE) film.19 With the same content of MWCNTs, the PE-MWCNT/CE material shows a higher dielectric constant and much lower dielectric loss than the MWCNT/ CE composite. However, PE has low melting point (Tm ≈ 110 °C),20 and the interfacial adhesion between PE and the MWCNT/CE composite is poor. In addition, PE has a very low dielectric constant,21 which has a negative influence on increasing the dielectric constant of the PE-MWCNT/CE material; in other words, the contribution of PE on the dielectric constant results from the configuration of the materials, but not from the inherent property of PE. Therefore, it is interesting to fabricate high-k composites that overcome the above disadvantages. This interesting research has confirmed that different dielectric properties can be obtained by changing the macrostructure of the composites. However, the corresponding works have their disadvantages, as described above. Besides, as we

1. INTRODUCTION Owing to the functions of storing energy and balancing the distribution of electric field of cable terminal,1−3 high-k (or dielectric constant) composites have been regarded as the key base materials of many cutting-edge fields including electronic information, electrical, mechanical, and biochemical engineering.4−6 Electric conductor/polymer composite has been proven to be one important type of high-k composites, as it usually has a small loading of conductors, so the composite has good processing characteristic as the polymer does.7,8 However, the dielectric loss at low frequencies of this type of composite is generally very high,9,10 which makes the composite unsuitable for actual applications (especially for those with harsh requirements) due to the huge energy waste and large risk of service. Therefore, preparing high-k electric conductor/polymer composites with low dielectric loss is a premise for actual applications of the electric conductor/polymer composites. Many works have been performed to address this target, and they focused on fabricating composites based on new conductors and/or polymer.11−13 However, as we have known, in a given conductor/polymer composite, the dielectric constant and loss at low frequencies are mainly related to the space charge polarization and conduction loss, respectively.14,15 Therefore, the dielectric properties are greatly dependent on the spatial structure of conductors in the composites. Recently, we prepared two types of composites with special structures. One is a permittivity gradient multiwall carbon nanotube (MWCNT)/cyanate ester (CE) composite, coded as g-MWCNT/CE, of which MWCNTs were in gradient © 2013 American Chemical Society

Received: March 20, 2013 Revised: July 5, 2013 Published: July 23, 2013 15487

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Figure 1. The SEM images of the top (a), middle (b), and bottom (c, d, e) parts of the cross-section along the thickness direction of the [gMWCNT0.7/CE]2 material.

cast into a mold and degassed under vacuum at 130 °C for 10 min. After that, the mold was put into an oven for curing and postcuring via the procedures of 150 °C/2 h + 180 °C/2 h + 200 °C/2 h + 220 °C/2 h, and 240 °C/4 h, successively, to get a cured composite, designed as MWCNTn/CE, where n is the weight fraction of MWCNTs in the composite. CE resin was also prepared using above procedure for nMWCNT/CE, except that no MWCNTs were added. 2.3. Fabrication of [g-MWCNT/CE]2 Materials. Appropriate amounts of CE and MWCNTs were blended at 80 °C for 1 h with a vigorous stirring under ultrasound, and then prepolymerized at 135 °C for 50−80 min with stirring to obtain a prepolymer. The prepolymer was divided into two equal parts. One was cast into a mold and degassed under vacuum at 130 °C for 20 min, followed by curing at temperatures of 150 °C for 1 h and 180 °C for 0.5 h. Subsequently, the resultant composite was cooled naturally. Another part of the prepolymer was cast on the above composite, and then degassed at 130 °C for 20 min. After that the whole composite was cured and postcured via the procedures of 150 °C/2 h + 180 °C/2 h + 200 °C/2 h + 220 °C/2 h, and 240 °C/4 h, successively. The resultant product is denoted as [g-MWCNTn/CE]2. 2.4. Characterizations. A scanning electron microscope (Hitachi S-4800, Tokyo, Japan) was employed to observe the morphologies of the composites. All of the samples should be dried at 60 °C for 1 h before testing. Raman spectra were recorded using an Almega dispersive Raman spectrometer (Thermo Nicolet, Madison, WI) with an Ar+ laser (514.5 nm) at room temperature.

have known, the dielectric properties of the electric conductor/ polymer composites are interface controlled,22 so the distribution, motion, and rearrangement of space charges are the key factors in determining the dielectric properties of the composite;23 however, few reports focus on this subject. Therefore, it is of great interest to develop new high-k composites with low dielectric loss by designing and fabricating composites with unique macro-structure based on utilizing common conductors and polymers; and then discuss the configuration-space charge-dielectric property relationship. In this work, two g-MWCNT/CE composites were superposed to form a two-layer material labeled as [g-MWCNT/ CE]2. The electric and dielectric properties of the [gMWCNT/CE]2 materials and traditional MWCNT/CE composites were systematically investigated. Some interesting and unexpected phenomena were discovered, the origins of these phenomena were explored by investigating the difference in the distribution and the rearrangement of space charges between [g-MWCNT/CE]2 and MWCNT/CE composites.

2. EXPERIMENTAL SECTION 2.1. Materials. CE used was 2, 2′-bis(4-cyanatophenyl) isopropylidene, which was purchased from Jiangsu Wudu Resin Factory, China. MWCNTs with a purity of 95% were bought from Shenzhen Nonotech Port Company (China), their average outer diameter was less than 10 nm, and the length was 5−15 μm. 2.2. Preparation of MWCNT/CE Composites and CE Resin. Appropriate amounts of CE and MWCNTs were blended at 80 °C for 1 h with a vigorous stirring under ultrasound, and then prepolymerized at 135 °C for 100−160 min with stirring to obtain a prepolymer. The prepolymer was 15488

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was cast on the lower composite layer, the resin of the lower layer only had a low curing degree and thus would continuously cure with the prepolymer of the upper layer. Therefore, the two-step procedure used for preparing [g-MWCNT/CE]2 composites is effective. Figure 3 shows the Raman spectra of CE resin, MWCNTs, the top and bottom surfaces of the [g-MWCNT0.7/CE]2

Thermogravimetric (TG) analysis was performed on a Perkin-Elmer TGA-7 instrument (Wellesley, MA) at a heating rate of 10 °C/min in a nitrogen atmosphere from 40 to 800 °C. Electric and dielectric properties were tested using a broadband dielectric spectrometer (Novocontrol Concept 80, Hundsangen, Germany) at room temperature over a wide frequency range (from 1 to 107 Hz). The dimensions of each sample were (25 ± 0.2) × (25 ± 0.2) × (1 ± 0.1) mm3. The space charge distribution was measured by the pulsed electro-acoustic method (PEA-P3, Shanghai Jiao Tong University, China). The cathode material was an aluminum electrode covered by a semiconductor, and the anode material was aluminum. An electric field (10 kV mm−1) was exerted on the sample at 25 °C for 30 min. After that, the sample was placed in a short circuit for another 30 min. The space charge accumulation and dissipation were measured during the polarization and depolarization process. The resolution of the measuring apparatus was 20 μm. The breakdown strengths of CE resin and composites were measured using a dc dielectric strength tester (ZIJJ-2, Shanghai Sute Electric Co., LTD, China) in a voltage ramp rate of 500 V/s and a current intensity of 10 mA. Eight independent breakdown measurements were performed for each type of materials (about 1 mm).

Figure 3. Raman spectra of CE resin, MWCNTs, the top and bottom surfaces of the [g-MWCNT0.7/CE]2 material.

3. RESULTS AND DISCUSSION 3.1. Microstructure of the [g-MWCNT/CE]2 Materials. The [g-MWCNT0.7/CE]2 material was cut along the direction of its thickness to get a cross-sectional sample, which can be divided into top, middle, and bottom parts. The morphologies of the three parts were observed using the SEM technique, and the corresponding pictures are shown in Figure 1. Few MWCNTs can be seen in the top part (Figure 1a), while a large number of MWCNTs are observed in the bottom part (Figure 1b). Interestingly, the middle part (Figure 1c) can be divided into two areas that have obviously different morphologies and numbers of MWCNTs, the boundary of the two areas is believed to be the interface of two composites. To get a clearer observation, two pictures with larger magnification (Figure 1d,e) of the two areas (circled by squares) are provided. The above results shown in the SEM images clearly demonstrate that the two g-MWCNT0.7/CE composites have been merged into a whole, and MWCNTs are gradient distributed in each layer of the [g-MWCNT0.7/CE]2 material. Briefly, the structure of the [g-MWCNT/CE]2 material can be schematically depicted in Figure 2, which results from the two-step process. Specifically, the prepolymers have low degree of polymerization, so the MWCNTs in the prepolymers have enough time to reach sedimentation due to the gravity; however, before the prepolymer of the upper layer

material. The Raman spectra of both surfaces of the [gMWCNT0.7/CE]2 material seem to be the combination of those of CE resin and MWCNTs. However, interestingly, the spectrum of the top surface is different from that of the bottom surface, reflected by the different shifts at which G-band and Dband appear,24 and the different relative intensity ratio of the Gband (or D-band) for MWCNTs to the typical peaks (ca. 1603 cm−1) of the CE resin. As the top surface shows weaker D- and G-bands than the bottom surface, so the loading of MWCNTs in the top surface is smaller than that in the bottom surface.25 Through the TG curves (Figure 4) in an air atmosphere,26 the loadings of MWCNTs in the top and bottom surfaces of [gMWCNT0.7/CE]2 composite were calculated to be 0.26 wt % and 0.99 wt %, respectively. 3.2. Electric Properties. 3.2.1. Electrical Conductivities of MWCNT/CE and [g-MWCNT/CE]2 Materials. Figure 5 gives the

Figure 4. TG curves of components of the [g-MWCNT0.7/CE]2 material.

Figure 2. Schematic structure of [g-MWCNTn/CE]2 materials. 15489

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Figure 5. Frequency dependence of the AC conductivity of the MWCNT/CE and [g-MWCNT/CE]2 materials with different loadings of MWCNTs.

Figure 6. Dependence of the conductivity at 1 Hz on the content of MWCNTs in MWCNT/CE and [g-MWCNT/CE]2 materials. The inset shows the log−log plot of the conductivity vs (p − pc) for MWCNT/CE composites.

dependence of AC conductivity on the frequency of the MWCNT/CE and [g-MWCNT/CE]2 materials with different loading of MWCNTs (p). It can be observed that the AC conductivities of MWCNT/CE and [g-MWCNT/CE] 2 materials have different dependence on either the p value or the frequency. For the MWCNT/CE composite, when p < 0.4 wt %, its AC conductivity linearly increases as the frequency increases, exhibiting the nonconductive feature.27 However, when p > 0.4 wt %, the AC conductivity at the low frequencies ( pc

Figure 7. 3D plots of the dielectric constant-frequency-the content of MWCNTs for MWCNT/CE and [g-MWCNT/CE]2 materials.

for MWCNT/CE and [g-MWCNT/CE]2 materials. Specifically, when p < 0.3 wt %, the dielectric constant of either MWCNT/CE or [g-MWCNT/CE]2 material is stable and small over the whole frequency range, which gradually increases as the content of MWCNTs increases. However, when p > 0.3 wt %, the dielectric constant of each type of composite is greatly dependent on the frequency, and the value at 1 Hz for either MWCNT/CE or [g-MWCNT/CE]2 material shows a sudden increase with the increase of the p value. As the p value continuously increases, the dielectric constants of both materials reduce. Moreover, with the same loading of MWCNTs, the MWCNT/CE composite has lower dielectric constant than the [g-MWCNT/CE]2 material, this trend reaches the

(1)

where σ is the conductivity of the MWCNT/CE composite, pc is the percolation threshold, and t is critical conductivity exponent. Using a least-squares fit for repeated experiments as shown in the inset plot of Figure 6, the pc value of the MWCNT/CE composite was calculated to be 0.37 wt %. However, for the [gMWCNT/CE]2 material, its electric conductivity shows a 15490

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maximum degree when the loading of MWCNTs is 0.5 wt %. In detail, the dielectric constant of [g-MWCNT0.5/CE]2 materials at 1 Hz is 306, about 2.6 and 4.8 times the values of the MWCNT0.5/CE and g-MWCNT/CE composites, respectively.16 The dielectric loss of each type of composites also shows a great dependence on the frequency and the content of MWCNTs, as shown in Figure 8. Similar to other

Figure 8. 3D plots of the dielectric loss tangent-frequency-the content of MWCNTs for MWCNT/CE and [g-MWCNT/CE]2 materials.

conductor/polymer composites, the dielectric loss decreases with the increase of frequency, but the dielectric loss at low frequencies enlarges as the loading of MWCNTs increases, and has a remarkable jump when the loading of MWCNTs is 0.3 wt %. With the increase of the loading of MWCNTs, the dielectric loss of the MWCNT/CE composite continuously jumps; however, interestingly, the dielectric loss of the [g-MWCNT/ CE]2 composite does not increase any longer. As a result, the dielectric losses at 1 Hz of [g-MWCNT0.5/CE]2 and [gMWCNT0.7/CE]2 materials are 0.21 and 0.01, only about 2.9 × 10−5 and 1.25 × 10−6 times the values of MWCNT0.5/CE and MWCNT0.7/CE composites, respectively. On the basis of above discussion, it can be stated that the [gMWCNT0.7/CE]2 materials have a much higher dielectric constant and lower dielectric loss than traditional MWCNT/ CE composites. 3.2.3. Origin of Dielectric Property. In order to declare the origin of above attractive dielectric properties of the [gMWCNT/CE]2 materials, the distributions of space charges in both MWCNT/CE and [g-MWCNT/CE]2 materials were detected. Figure 9 shows the relationship between space charge density and the thicknesses of CE resin, MWCNT/CE and [gMWCNT/CE]2 materials at 10 kV/mm with different times. For CE resin (Figure 9a), there is an accumulation of positive charges (about 2 C/m3) at a distance of 50 μm from the cathode electrode. These charges are called heterocharges.30,31 These heterocharges originate from the directional movement of impure charges in CE resin under the electric field. Because heterocharges must appear in pairs, however, the heterocharges do not appear inside the CE resin that near the anode, so it can

Figure 9. The distributions of charges under the 10 kV mm−1 for CE resin (a), MWCNT0.5/CE (b), and [g-MWCNT0.5/CE]2 (c) materials.

be deduced that a direct charge injection took place at the anode under the 10 kV/mm electric field,32 and the positive and negative charges were neutralized. Similar distribution of charges is also found in the MWCNT0.5/CE composite (Figure 9b), while the positive charge density at a distance of 50 μm from the cathode is 9 C/ m3, indicating that there are more charges in the MWCNT0.5/ CE composite. These charges have directional movement in the electric field, however, due to the existence of a large amount of the MWCNT-CE interfaces, they cannot across these traps, and thus bring fluctuation in the curve of MWCNT0.5/CE composite due to the aggregation of MWCNTs.33 The aggregation of these positive and negative charges would 15491

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form space charge polarization in microscopic space, and thereby increases the dielectric constant.34 Interestingly, the [g-MWCNT0.5/CE]2 material has significantly different space charge distribution (Figure 9c) compared with the CE resin and MWCNT0.5/CE composite. Specifically, there is a peak with a positive charge density of 6.0 C/m3 at the distance of 50 μm away from the cathode, while a negative charge density of about 7.0 C/m3 appears at a distance of 600 μm away from the cathode. This can be attributed to the electroneutrality. Simultaneously, there is a positive charge density peak (with a maximum density of 9.5 C/m3) at a distance of 600 μm away from the cathode, demonstrating that there exists a place where the trap energy is so high that the space charges cannot go through.35 Considering the equal distance between the cathode and anode as well as the structure of the [g-MWCNT0.5/CE]2 material, the place is believed to be the interface between the two composites. As the conductor fault appears in the interface, where positive and negative charges aggregate, leading to a significantly improved dielectric constant. Figure 10 shows the charge density distributions of CE resin, MWCNT0.5/CE, and [g-MWCNT0.5/CE]2 materials under a short circuit condition with different times. Obviously, CE resin and MWCNT0.5/CE composite have similar curves (Figure 10a,b), which are completely different from the plot of the [gMWCNT0.5/CE]2 material (Figure 10c). Specifically, after the CE resin and MWCNT0.5/CE composite remained in a shortcircuit condition for 600 s, the space charge densities of both materials disappear due to the charge neutralization. However, the charge density of the [g-MWCNT0.5/CE]2 material does not reduce at the interface, and even the [g-MWCNT0.5/CE]2 material remained in a short-circuit for 1800 s, further demonstrating that the interface between the two gMWCNT0.5/CE composites is a deep trap.36 The above investigation and discussion of space charge polarization confirm that there is an interface between the two g-MWCNT0.5/CE composites of the [g-MWCNT0.5/CE]2 material, which is in good agreement with the morphologies of the materials shown in Figure 1. The interface indeed brings difference in the space charge polarity and thus dielectric property. To clearly depict the origin behind the unique dielectric property of the [g-MWCNT0.5/CE]2 material, Figure 11 schematically gives the spatial distributions of MWCNTs and space polarized charges in the MWCNT/CE (Figure 11a) and [g-MWCNT/CE]2 (Figure 11b) materials. As the [g-MWCNT0.5/CE]2 material consists of two gMWCNT0.5/CE composites, and the two composites are similar, so MWCNTs have similar distribution in the two composites. Different from the traditional composites based on MWCNTs, there is a conductor fault in the interface between the two composites, so the electric charges cannot transfer through the conductive routes, and then positive and negative charges accumulate in the interface, leading to the space charge polarization (SCP).37 For the conductor/polymer composite, the positive charges would gather at the places within the matrix where contact with the electric conductors, and the negative charges would accumulate on the surface of the electric conductors.38 However, CE resin contains a very low concentration of free charge carriers compared with electric conductors,39 so the intensity of SCP is mainly dependent on the number of carriers in CE resin, or the concentration of CE resin. When the p value is low, almost each MWCNT can form

Figure 10. The distributions of charges under the short circuit condition for CE resin (a), MWCNT0.5/CE (b), and [gMWCNT0.5/CE]2 (c) materials.

SCP with high intensity; along the thickness, the loading of MWCNTs in the [g-MWCNT/CE]2 material increases, all of the free charge carriers in CE resin are assigned to form SCP, but the intensity of SCP around each MWCNT is relatively low. Note that the loading of MWCNTs in the bottom of the upper g-MWCNT0.5/CE composite is larger than pc, but the conductor fault breaks the conductive route, so the electric charges still run across between CE resin and the MWCNTs. However, the interface between two composites is a resin-rich area, and the top surface of the lower composite has a larger amount of resin, so it is possible for the large amount of MWCNTs in the bottom of the upper g-MWCNT0.5/CE composite form SCP with high intensity. However, the SCP 15492

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Figure 11. Schematic diagrams of the distributions of MWCNTs and space polarized charges in the MWCNT/CE (a) and [g-MWCNT/CE]2 (b) materials.

By comparing the conductivities of the MWCNT/CE composite, it can be found that as the content of MWCNTs increases, the conductivity of MWCNT/CE composite increases by 4 orders of magnitude, which is much larger than the change of the dielectric constant. Therefore, the tan δ in the low frequency region is primarily determined by the AC conductivity, that is, the existence of a conductor fault in the [gMWCNT/CE]2 material significantly decreases the conductivity, and thereby dramatically reduces the tan δ. 3.2.4. Breakdown Strength. Figure 12 shows the dc breakdown strengths of CE resin, MWCNT0.5/CE, g-

formed in the bottom of the lower composite has lower intensity due to the lack of CE resin. In the case of g-MWCNT/CE composite, there is no conductor fault, so conductive paths will be formed; in addition, the large amount of MWCNTs in the bottom of the g-MWCNT/CE composite can not form strong SCP due to the less amount of the CE resin. As a result, the total intensity of SCP in the g-MWCNT/CE composite is not as high as that in the [g-MWCNT/CE]2 material. For the dielectric loss tangent of a conductor/polymer composite, the content of its electric conductors and the frequency are the main concerns. When the p value is low, the tan δ is mainly originated from the polarization loss of space charges (dominating at 103−106 Hz).40 Compared with the MWCNT/CE composite, the [g-MWCNT/CE]2 material has the conductor fault that produces extra polarization loss of space charges. Therefore, with the same content of MWCNTs, the [g-MWCNT/CE]2 material shows a higher tan δ value over the whole frequency than the MWCNT/CE composite. However, when p > pc, the conductive paths have been formed, so the conduction loss plays a leading role at low frequency region; moreover, tan δ has a relation with the conductivity (σ′) and the real dielectric constant (ε′) as shown in eq 2:41

tan δ ≈

σ′ ωε0ε′

(2)

where ω is the angular frequency, and ε0 is the dielectric constant in vacuum. Obviously, for a given ω, the tan δ is proportional to σ′/ε′. Observing Figures 5 and 7, the AC conductivity of [gMWCNT0.5/CE]2 material at 1 Hz is 3.0 and 1.4 times that of [g-MWCNT0.3/CE]2 and [g-MWCNT0.4/CE]2 materials, respectively, while the dielectric constant of the former respectively increases by 5.5 and 3.5 times that of [gMWCNT0.3/CE]2 and [g-MWCNT0.4/CE]2 materials, so compared with the [g-0.5MWCNT/CE]2 material, [g0.3MWCNT/CE]2 and [g-0.4MWCNT/CE]2 materials have relatively higher σ′/ε′, and thus higher tanδ.

Figure 12. The dc breakdown strengths of CE resin, MWCNT0.5/CE, g-MWCNT0.5/CE, and [g-MWCNT0.5/CE]2 materials.

MWCNT0.5/CE, and [g-MWCNT0.5/CE]2 materials. The average breakdown strength (Ebr) of the CE resin is 26 kV/mm, which is larger than those of MWCNT0.5/CE, g-MWCNT0.5/ CE and [g-MWCNT0.5/CE]2 materials because of the addition of conductive MWCNTs.42 Interestingly, the [g-MWCNT0.5/ CE]2 composite has higher Ebr than both g-MWCNT0.5/CE and MWCNT0.5/CE composites, meaning that the structure of the composite also plays an important role in the breakdown 15493

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strength. This is attributed to the fact that the gradient distribution of MWCNTs and the presence of conductor fault increase the charge scattering and hinder the directional movement in high voltage to the same extent,43 and thus increasing the breakdown strength.

4. CONCLUSIONS Using a two-step procedure, [g-MWCNT/CE]2 materials, consisting of two superposed gradient multiwall carbon nanotube (MWCNT)/cyanate ester (CE) composites, with very high dielectric constant and low dielectric loss were prepared. Compared with the MWCNT/CE composite, the [gMWCNT/CE]2 material with the same loading of MWCNTs shows lower conductivity, and no percolation phenomenon is observed. No macro-interface is found between two g-MWCNT/CE composites, but a conductor fault exists between the two composites. The existence of the conductor fault in [gMWCNT/CE]2 materials forms deep traps, this not only leads to the accumulation of a large amount of positive and negative charges, but also form a mass of space charge polarization, and as a result, significantly increases the dielectric constant. In the low frequency region, the tan δ of a material is dependent on the ratio of conductivity to dielectric constant. When the content of MWCNTs is larger than 0.4 wt %, the [gMWCNT/CE]2 materials exhibit much lower tan δ than MWCNT/CE composites. In addition, the [g-MWCNT0.5/ CE]2 material has relatively high breakdown strength, which is suitable for actual applications.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 512 65880967; fax: +86 512 65880089; e-mail: [email protected] (G.L.), [email protected] (A.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Science Foundation of China (51173123), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Major Program of Natural Science Fundamental Research Project of Jiangsu Colleges and Universities (11KJA430001), and Suzhou Applied Basic Research Program (SYG201141) for financially supporting this project.

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