Graphene sheets segregated by barium titanate for polyvinylidene fluoride composites with high dielectric constant and ultralow loss tangent

Composites: Part A 78 (2015) 318–326

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Graphene sheets segregated by barium titanate for polyvinylidene fluoride composites with high dielectric constant and ultralow loss tangent Yuhan Li a, Yunjie Shi a, Fanyi Cai b, Jian Xue b, Feng Chen a,⇑, Qiang Fu a,⇑ a b

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, People’s Republic of China Nari Group Corporation State Grid Electric Power Research Institute, Nanjing 211000, China

a r t i c l e

i n f o

Article history: Received 25 June 2015 Received in revised form 12 August 2015 Accepted 26 August 2015 Available online 1 September 2015 Keywords: A. Polymer-matrix composites (PMCs) A. Hybrid B. Electrical properties E. Compression moulding

a b s t r a c t In this paper, we report a unique method to develop polyvinylidene fluoride (PVDF) composites with high dielectric constant and low loss tangent by loading relatively low content of graphene-encapsulated barium titanate (BT) hybrid fillers. BT particles encapsulated with graphene oxide (BT-GO) were prepared via electrostatic self-assembly and subsequent chemical reduction resulted in BT-RGO particles. SEM morphology revealed that RGO sheets were segregated by BT particles. The hybrid fillers have two advantages for tuning dielectric properties: loading extremely low content of RGO can be exactly controlled and individual RGO sheets segregated by BT particles would prevent leakage current. As a result, PVDF composites filled with BT-RGO displayed improved dielectric properties before percolative behavior occurred. Composites filled with 30 vol% BT-RGO have a dielectric constant and loss tangent (tan d) value of 67.5 and 0.060 (1 kHz), respectively. By contrast, dielectric constant and tan d of composites filled with 30 vol% BT-GO and BT were 57.7 and 38.3, 0.076 and 0.042 (1 kHz), respectively. The improvement of dielectric constant is attributable to the formation of microcapacitors by highly conductive RGO sheets segregated by BT particles. Meanwhile, the distance between adjacent RGO sheets is large enough to prevent leakage current from tunneling conductance, by which tan d is remarkably constrained. The composites could achieve excellent dielectric properties by loading relatively low amount of ceramic fillers, which indicates that this method can be used as guideline for reduce the usage amount of ceramic fillers. Ó 2015 Published by Elsevier Ltd.

1. Introduction Polymer-based dielectric composites possess advantages of flexibility and low-temperature processability. Researchers have devoted efforts to realizing its promising applications in the field of electronic industry, especially in terms of energy storage devices like embedded capacitors [1]. High dielectric constant materials reduce the use amount so that it meets the demands of miniaturization of electronic devices. Ultralow dielectric loss is also a significant factor that determines whether a composite could be put into practical use or not. Therefore, it is necessary to balance the relation between dielectric constant and tan d for the development of excellent polymer-based dielectric composites. Currently, there are two categories that aim to improve dielectric constant of polymer composites: (1) adopting percolative phenomenon of conductive fillers, through which the dielectric constant undergoes gigantic increase as the fillers form conductive ⇑ Corresponding authors. http://dx.doi.org/10.1016/j.compositesa.2015.08.031 1359-835X/Ó 2015 Published by Elsevier Ltd.

network within matrix; (2) loading with intrinsically high dielectric constant ceramics. The first category usually brings about extremely high dielectric constant and the excellent mechanical properties and processability of polymer matrix could be guaranteed by relatively low filling content. Nevertheless, this type of composites inevitably suffers from high tan d which impedes its industrial applications [2,3]. Since leakage current originated from conductive pathway primarily contributes to the tan d, many researchers have made attempts to create barrier layers on surface of conductive fillers in order to constrain the generation of leakage current [4–8]. For example, Wang et al. [7] coated graphene with poly (vinyl alcohol) to prevent graphene sheets from direct contact, resulting in a relatively high percolation threshold (2.20 vol%). Although dielectric constant was improved, tan d remains in a high level (
0.5, 1 kHz). Based on the same idea, Li et al. [6] prepared graphene decorated with insulating polyaniline (PANI). The authors found that dielectric constant and tan d of composites filled with graphene/PANI were 40 and 0.12 while values for composites filled with undecorated grapheme were measured to be 20 and

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1250 (1 kHz), respectively. Hence, it can be seen that incorporation of insulating barrier layer on conductive fillers is able to constrain the improvement of tan d. Referring to the second category, some researchers tried to take advantages of ceramic fillers with high dielectric constant and ultralow tan d such as BT [9–12], calcium copper titanate (CCTO) [13], BST [14] and BCZT [15] for developing polymeric dielectric materials. In spite of remarkable dielectric properties, ceramic fillers suffer from some drawbacks. It is necessary to load high volume content to obtain polymer composites with dielectric constant higher than 50. Large amount of ceramics would undermine the flexibility and processability of composites. Accordingly, it is of great importance to seek an appropriate way of lowering the filling content with excellent dielectric properties preserved. Recently, there arises a trend of preparing polymer-based dielectric composites that combines advantageous features of ceramic and conductive fillers. Some researchers prepared metal-ceramics or semiconductor-ceramics hybrids for improving dielectric constant [16–20]. Fang et al. [17] found that BT-Ag/PVDF composites has a tan d value of 0.1 with dielectric constant two time higher than composites loaded with unhybridized BT. The authors regarded the discrete Ag nanoparticles on surface of BT as the key point that contributes to high dielectric constant and low tan d. Wang et al. [21] attempted to simultaneously incorporated BT and insulating PANI coated graphene into PVDF which also exhibited percolation threshold. They obtained composites with 65 of dielectric constant and 0.35 of tan d (1 MHz) via rationally tuning the ratio of the two fillers. Shen et al. [22] found that BT particles served to segregate graphene sheets so that the percolation threshold increased and the dielectric constant improved as the content of graphene increased. Regarding the recent research, it is still of great challenge to pursue ultralow tan d (<0.1) and reduce the using amount of dielectric ceramics via simultaneously loading ceramic and conductive fillers. Herein, we adopted electrostatic self-assembly to prepare hybrid particles containing extremely low amount of RGO sheets which were individually segregated by BT particles. RGO sheets were found to be somewhat paralleled, which facilitated the formation of quasi-microcapacitors within the composites that accounts for the improvement of dielectric constant. The tan d was effectively suppressed because the discretely distributed RGO sheets have large enough adjacent distance to circumvent tunneling conductance that primarily contributes to tan d. These composites successfully lower the use amount of ceramic fillers while acquiring high dielectric constant and ultralow tan d.

2. Experimental sections 2.1. Materials BT powders (diameter
1 lm, 99.9%, density 6.02 g cm3), 3-a minopropyltriethoxysilanecoupling agent (APS, purity > 98%), hydrazine monohydrate (purity > 99%) were purchased from Chengdu Best-reagent company; Ethanol was bought from Chengdu Changzheng chemical reagent company; Ammonia solution (25%) was purchased from Chengdu Jinshan chemical reagent Co., Ltd.; Graphite powders were provided by Qingdao Black Dragon graphite Co., Ltd., China; Potassium permanganate (KMnO4), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2), sodium nitrate (NaNO3), sodiumhydroxide (NaOH), DMF were purchased from Kermel Chemical reagent plant (Tianjin, China); PVDF powders (Solef 6020, density 1.78 g cm3, melt mass-flow rate (MFR) <2.0 g/10 min tested by ASTM D1238) were purchased from Shanghai Alliedneon Co., Ltd., China.

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2.2. Preparation of BT-GO and BT-RGO GO was prepared by a modified Hummers method presented elsewhere [23]. Encapsulating BT particles with GO was carried out similar to the method used in literatures [24,25]. A typical procedure is presented in the following. Firstly, 10 g of BT powders was added into 300 mL of ethanol then vigorously stirred by a disperser (IKA, T25 digital ULTRA-TURRAXÒ) with a speed of 12,000 rpm for 10 min; 1.0 mL of APS was dropped into the mixture and brought to magnetic stirring at room temperature for 24 h followed by centrifugation and wash with deionized water for five times to remove impurities, and the as-prepared sample was denoted as BT-NH2. Secondly, 400 mL of aqueous GO dispersion with a concentration of 0.2 mg/mL was preliminarily prepared and then sonicated for 1 h to downsize GO sheets; 500 mL of BTNH2 aqueous suspension was obtained via vigorous stirring with disperser for 10 min (12,000 rpm). Finally, GO dispersion was decanted into BT-NH2 suspension and magnetically stirred for 1 h, during which the color of the mixture gradually turned from light brown to clear and transparent, accompanied with generation of light yellowish flocculates; the flocculates were repeatedly washed with deionized water for five times and air-dried in oven, the obtained powders were denoted as BT-GO. The procedure of BT encapsulated with graphene is basically identical with an additional chemical reduction of GO: 10 g of BT-GO powders was added into 400 mL of deionized water and brought to vigorous stirring for 10 min with a speed of 12,000 rpm; the mixture was then put into 80 °C water bath, 1 mL of ammonia was dropped to tune the pH to alkaline; 2 mL of hydrazine was added and magnetically stirred for 6 h to reduce GO; the obtained grey mixture was repeatedly washed with deionized water for five times and finally air-dried in oven, resulting in BT-RGO. 2.3. Preparation of PVDF composites filled with BT, BT-GO and BT-RGO BT, BT-GO and BT-RGO powders were grinded for 30 min before use. The content of fillers in composites is expressed with volume fraction, hence, the required mass fraction needs to be calculated in the following equation:

x¼

v qf qp þ v ðqf  qp Þ

where x, v , qf and qp is mass fraction, volume fraction, density of BT and density of PVDF, respectively. The volume fractions of BT and BT-GO were set to 10 vol%, 20 vol%, 30 vol%, 40 vol% and 50 vol% while the volume fractions of BT-RGO were set to 10 vol%, 20 vol%, 30 vol%, 35 vol%, 40 vol%, 45 vol% and 50 vol%. A desirable amount of PVDF and filler powders for each sample was carefully weighed by using scale; PVDF powders were dissolved in 60 mL of DMF in 80 °C water bath for 30 min, then the preliminarily weighed filler powders were added and followed by vigorous stirring with disperser for 10 min (12,000 rpm); at the moment stirring halted the mixture was decanted into 400 mL of ethanol for co-precipitation; the flocculates were centrifuged and air-dried in oven to remove residual solvent; samples with a thickness of 0.2 mm for dielectric tests were prepared through hot compression at 210 °C with 10 min plasticizing and 10 MPa of holding pressure. 2.4. Characterization Cryo-fractured morphology of composites and morphology of fillers were investigated on an inspect scanning electron microscope (SEM) instrument (FEI) with an acceleration voltage of 20 kV. Transmission electron microscope (TEM, FEI-Tecnai G2 F20

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S-TWIN type) was utilized to obtain detailed morphology of BT-GO. The weight percentage of wrapping GO or RGO on BT particles was studied by Thermal Gravimetric Analysis (TGA, TA instrument, Q500, USA) in air with a ramp rate of 10 °C/min from room temperature to 600 °C. Infrared spectroscopy (FTIR) using the Nicolet 6700 was carried out in transmission mode. Dielectric properties including dielectric constant and tan d were measured on Agilent HP4294A in a frequency range of 40 Hz–10 MHz.

3. Results and discussion 3.1. Morphologies of fillers Encapsulation procedure is displayed in Fig. 1. FTIR results (Fig. 2) demonstrated that the as-received BT powders have two peaks at 1630 cm1 and 3430 cm1 which are attributed to bending and stretching vibration of hydroxyl groups, respectively. This indicates that the as-received BT particles are readily to perform chemical bondage with APS coupling agent, resulting in a large amount of positively charged amino groups on the surface of BT particles [26]. The peaks at 1380 cm1 and 1560 cm1, attributing to bending vibration of C–N and stretching vibration of N–H (Fig. 2), confirmed the successful chemical bonding of APS coupling agent on BT particles. Subsequently, GO sheets bonded with negatively charged carboxylic groups carried out electrostatic attraction with the positively charged BT particles. In this case, GO sheets acting as soft cloth due to large specific area and flexibility tended to wrap the BT particles. Chemical reduction with hydrazine rendered GO convert to RGO. TGA curves provide quantitative analysis of the content of wrapping GO or RGO. The weight feed ratio of GO to BT particles was 0.008:1, so the calculated weight percentage of GO in BT-GO powder is 0.793 wt% if GO sheets were completely absorbed on BT particles. BT-GO curves shown in Fig. 3 could be used to estimate GO content since GO sheets can be thoroughly pyrolyzed into volatile gas above 450 °C in air. It can be deduced from the residual content that GO is about 0.8 wt% which is basically identical to the calculated value. This result implies that all of GO sheets performed complete encapsulation on BT particles via electrostatic self-assembly. The TGA curve of BT-GO underwent two abrupt weight loss. The first one occurred at around 200 °C which is likely caused by the fact that oxygen-containing groups are readily to

Fig. 2. FTIR spectrum of (a) BT, (b) BT-NH2, (c) BT-GO and (d) BT-RGO; the inset shows detailed peak signals. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. TGA curves of BT, BT-GO and BT-RGO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

detach near this temperature [27]. The second one started in the vicinity of 400 °C, attributing to the pyrolysis of carbon [28]. It can be observed that the TGA curve of BT-RGO behaved in a different way that the weight loss started from
350 °C till the residual

Fig. 1. Schematic illustration of preparation of BT-GO and BT-RGO via electrostatic self-assembly. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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content remained constant at
500 °C. This discrepancy lies in the fact that chemical reduction removed most of oxygen-containing groups so that it almost directly proceeded to the process of carbon pyrolysis. This feature of TGA curve also serves to reflect that GO was successfully converted to RGO via chemical reduction. It can be deduced from the weight loss curve that the RGO covers a percentage of
0.55% in BT-RGO. Morphologies of hybrid fillers were observed by using SEM and TEM. Fig. 4a reveals that BT particles are not perfect spherical grains but irregular pellets with obvious edges. BT particles have an average diameter of
1 lm and some individual pellets are prone to closely clustering together in a face-on-face manner. Fig. 4b reflects the difference between BT-GO and BT particles that surface of partial BT-GO particles turn out to be wrinkled which is indicative of encapsulation of the soft cloth-like GO sheets. It can be found that some BT particles have bare surface with no GO sheets wrapped, demonstrating that the feed ratio adopted enabled all GO sheets to encapsulate BT particles. There is no substantial difference between the morphologies of BT-RGO and BTGO particles (Fig. 4b and c). Theoretically, RGO sheets obtained by chemical reduction carry less negative charges so that it would generate diminished electrostatic attraction with BT particles which might bring about detachment of wrapping RGO sheets. TEM image of BT-GO shown in Fig. 4d clearly reflects that BT particles are veiled by GO sheets. This result confirms the successful encapsulation of BT particles with GO sheets. We intentionally controlled the feed ratio of GO to BT so that all GO sheets performed encapsulation but only partial BT particles were wrapped. In this way, GO sheets or RGO sheets were individually segregated and sparsely distributed among BT particles. Fig. 5 presents the cross-sectional fracture surfaces of composites loaded with 40 vol% of BT-GO and BT-RGO particles. It can be scrutinized in Fig. 5a and c that both BT-GO and BT-RGO particles are homogeneously dispersed within the PVDF matrix. Fig. 5b shows that PVDF threads are consecutively connected with each other and BT particles are closely packed and embedded within

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the three-dimensional meshes of PVDF threads. Wrinkled GO sheets marked by arrows can be clearly spotted among the densely compacted BT particles (Fig. 5b). By contrast, RGO sheets detected in 40 vol% BT-RGO/PVDF composite have somewhat restacking crinkles (Fig. 5d), which is probably induced by p–p stacking of individual RGO sheets. Both GO and RGO sheets spread out like cloth instead of closely crimpled aggregates meanwhile they are sparsely distributed within composites and segregated by BT particles. This unique result distinguishes our method with other reported literatures that designed a series of tedious process to prevent graphene from aggregation [21]. Moreover, overdose of graphene inevitably causes augment of leakage current that would produce largely increased tan d. Herein, the content of GO and RGO are calculated to be no more than 0.584 wt% and 0.4 wt%. Herein, ultralow loading of GO and RGO enables it to easily prevent overdose. 3.2. Dielectric properties Fig. 6 illustrates dependency of dielectric constant and tan d of BT/PVDF, BT-GO/PVDF and BT-RGO/PVDF composites on frequency at room temperature. Dielectric constant and tan d perform gradual increment as the filling content of BT increases (Fig. 6a and d). Specially, dielectric constant and tan d present a dropping trend as the frequency increases in the range of 40 –103 Hz, which is resulted from the interfacial polarization between BT particles and PVDF matrix [29]. In this range of frequency, nomadic charges induced by applied electric field accumulate at the interface between BT particles and PVDF matrix. The polarization induced by these space charges is able to generate prominent increment of dielectric constant and tan d. However, as the frequency increases it becomes gradually harder for the establishment of polarization to keep up the pace with the reversion vector of applied electric field, which in turn causes the decrease of dielectric constant and tan d. In the range of 103–105 Hz, it clearly demonstrates that dielectric constant and tan d have little dependency on frequency. This means the

Fig. 4. SEM images of (a) BT, (b) BT-GO, (c) BT-RGO and (d) TEM image of BT-GO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. SEM morphology images of (a) and (b) 40 vol% BT-GO/PVDF composites, and (c) and (d) 40 vol% BT-RGO/PVDF composites with different magnifications. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Frequency dependence on dielectric constant of PVDF composites loaded with (a) BT, (b) BT-GO and (c) BT-RGO; tan d of PVDF composites loaded with (d) BT, (e) BTGO and (f) BT-RGO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

intrinsic dielectric properties of BT particles dominate the overall dielectric properties of composites in this range of frequency. It is noteworthy that the dielectric constant of composites filled with 40 vol% and 50 vol% of BT particles is 60 and 70, respectively, while tan d keeps in a ultralow level below 0.075. In the case of frequency exceeding 105 Hz, the dielectric constant attenuates and tan d dra-

matically improves as the frequency increases. This phenomenon is due to relaxation of dipolar polarization of PVDF which mainly contributes to the dielectric properties of composites in this range of frequency. As the frequency gradually increases the relaxation time of PVDF dipoles mismatch with the direction change of applied electric field, leading to decrease of dielectric constant

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Fig. 7. Frequency dependence on AC electrical conductivity of composites filled with (a) BT-GO and (b) BT-RGO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1 Comparison of dielectric constant (Dk) and tan d of BT/graphene/PVDF composites loading with various content of fillers at 100 Hz, 1 kHz and 10 kHz.

a

PVDF composites

100 Hz Dk/tan d

1 kHz Dk/tan d

10 kHz Dk/tan d

10 vol% BT-RGOa 20 vol% BT-RGO 30 vol% BT-RGO 20 wt% BT + 0.25 wt% RGO [31] +0.5 wt% RGO +1.0 wt% RGO 30 vol% BT + 1.25 vol% fRGO [21] +1.40 vol% fRGO

19.6/0.120 35.8/0.127 74.2/0.110
13/
0.100
13/
0.120
20/
0.240
245/
0.350
155/
2.100

18.3/0.044 32.7/0.050 67.5/0.060
12/
0.055
12.5/
0.060
17/
0.120
175/
0.200
90/0.700

17.6/0.032 31.2/0.038 62.7/0.062
11/
0.048
12/
0.050
15/0.070
125/0.190
65/
0.45

Calculated mass fraction of BT and RGO for 10 vol%, 20 vol% and 30 vol% BT-RGO are 27.3 wt% and 0.15 wt%, 45.8 wt% and 0.25 wt%, 59.2 wt% and 0.32 wt%, respectively.

Fig. 8. Schematic illustration of microcapacitors of graphene segregated by BT particles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and improvement of tan d caused by the intermolecular friction during relaxation of dipoles [22]. The dielectric properties of BT-GO/PVDF and BT-RGO/PVDF composites behave in a different way which lies in the fact that both dielectric properties of BT particles and percolative behaviors mainly affect the overall dielectric properties of composites (Fig. 6b–f). As mentioned above, TGA results proved that a copious amount of oxygen-containing groups can be removed during hot compression which could bring about restoration of graphitic structure and improve electrical conductivity. Accordingly, both BT-GO/PVDF and BT-RGO/PVDF composites display typical

percolative behaviors with respect to dielectric constant as shown in Fig. 6b and c. As for BT-GO/PVDF composites (Fig. 6b and e), the dielectric constant shows little dependency on frequency and the tan d maintains in quite a low level when the filling content of BT-GO is lower than 30 vol%, owing to the fact that the content of thermally reduced GO is extremely minor so that dielectric properties of BT particles primarily contribute to the overall dielectric properties; percolative behavior imposes remarkable influence on dielectric properties, resulting in abrupt increment of dielectric constant and tan d when the loading content of BT-GO excesses 30 vol%; composites filled with 40 vol% and 50 vol% of BT-GO exhibit typical interfacial polarization which is caused by the nomadic charges originated from thermally reduced GO. Fig. 6c and f illustrate that the change of dielectric constant and tan d is more notable for BT-RGO/PVDF composites. The distinctions for such phenomenon are twofold: on the one hand, the improvement extent of dielectric constant is far beyond that of BT-GO/PVDF, for instance, composite loaded with 50 vol% BT-GO has a dielectric constant of 240 and a tan d of 0.3 (100 Hz) while its counterpart with BT-RGO has an amazing dielectric constant of 1750 and a tan d of
6 (100 Hz), on the other hand, the dependency of dielectric constant of BT-RGO/PVDF composites on frequency exhibits in a broader range as is demonstrated in Fig. 6c that the dielectric constant presents an attenuating trend in the range of 40–105 Hz. The discrepancy of dielectric behaviors of BT-GO/PVDF and BTRGO/PVDF composites could be revealed by the difference of electrical conductivity and distributing morphology of hybrid particles. As shown in Fig. 7a, AC conductivity of BT-GO/PVDF composites increases as the content of fillers increases. This is due to the incorporation of thermally reduced GO homogeneously dispersed in composites as illustrated in Fig. 5b. Fig. 7b reveals that the change

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Fig. 9. Variation of dielectric constant (a) and (b), and loss tangent (c) and (d) for composites of BT-GO/PVDF and BT-RGO/PVDF as the increase of volume fraction at four different frequencies. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of AC conductivity induced by RGO is more obvious. When BT-RGO reaches a content of 35 vol%, it occurs typical percolation because there emerges a plateau in the range of low frequency with filling content excessing 35 vol%, which is indicative of formation of conductive pathway. As discussed above, RGO sheets are not mutually contacted but segregated by BT particles. The distance between RGO sheets reduces as the content of BT-RGO increases to a certain value so that it is able to occur tunneling conductance through neighboring highly conductive RGO sheets [30]. Comparatively, AC conductivity of BT-GO/PVDF improves in a slow pace which is due to the relatively lower electrical conductivity of thermally reduced GO. It can be observed from Fig. 7a and b that AC

conductivity of composites with BT-RGO no more than 30 vol% exhibits lower values than that of BT-GO. The morphologies of GO and RGO in composites could serve as an explanation for that. GO sheets are individually dispersed while RGO sheets are restacked as planar sheets and embedded in composites (Fig. 5d). This means the density of effective conductive fillers contribute to formation of network is lower in comparison with thermally reduced GO. Nevertheless, the overall AC conductivity of BT-RGO/ PVDF composites surpasses that of BT-GO/PVDF composites as the filling content is higher than 30 vol%. This is owing to higher conductivity of RGO sheets. Since the content of RGO is extremely low and segregated by BT particles, the dielectric properties of

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composites could be tuned to be in a rational range by adjusting loading content of BT-RGO. Fig. 6c and f displays dependence of dielectric properties of BTRGO/PVDF composites with filling content no more than 35 vol% on frequency. It is obviously shown that dielectric constant and tan d perform a great leap upward increase. The dielectric constant at 100 Hz was measured to be around 240, a value 23 times of neat PVDF (10.4), while the tan d increases to about 0.3 which is much higher than that of neat PVDF (0.047). The improvement of dielectric constant could be attributable to interfacial polarization in that the highly conductive RGO sheets are able to donate a large number of delocalized p electrons that will be accumulated at interface. There are twofold reasons accounting for the increase of tan d: the first one can be ascribed to interfacial polarization; the other one is induced by leakage current stemmed from tunneling conductance that is implemented by closely neighboring RGO sheets. The difference of tan d between composites filled with 30 vol% and 35 vol% of BT-RGO has a great gap (Fig. 6f). This signals that leakage current plays a dominant role for tan d once the loading content excesses 30 vol%. On the contrary, the predominant factor contributing to tan d originates from interfacial polarization between fillers and PVDF matrix for composites with filling content no more than 30 vol% because the variation of tan d seem to be similar to that of BT/PVDF composites (Fig. 6d). It is noteworthy that the dielectric properties of composites with loading content of BT-RGO are particularly excellent. Compared with other work as shown is Table 1, our work display lower tan d. Composites loaded with 10 vol%, 20 vol% and 30 vol% BT-RGO do not exhibit abrupt increase of tan d while the dielectric constant has been improved. Results of literatures show obvious sharp change of tan d to an extent that goes far beyond 0.100. Although the dielectric constant has been tremendously improved, tan d of these samples is out of tolerance for practical use. By contrast, samples of our work remain relatively high dielectric constant and ultralow tan d (less than 0.100) at 1 kHz and 10 kHz. To elaborate it in detail, composite with 30 vol% of BT-RGO has a dielectric constant of 67.5 and an ultralow tan d value of 0.060 (1 kHz), at the meantime, it exhibits weak dependence of dielectric constant on frequency in the range of 40–105 Hz. In comparison, dielectric constant and tan d of composites with 30 vol% of BT and BT-GO are 38.3 and 0.043, 57.7 and 0.076, respectively (Fig. 6). This comparative study manifests that incorporation of BT-RGO with appropriate amount serves to efficiently improve dielectric constant and maintain ultralow tan d. A model is proposed to explain it as shown in Fig. 8. RGO sheets segregated by BT particles facilitate to form microcapacitors which effectively improve dielectric constant whereas the gap distance of these microcapacitors is not close enough to generate tunneling conductance which directly increases tan d to a large extent. Fig. 9 presents variation of dielectric properties of composites loaded with BT, BT-GO and BT-RGO at four different frequencies. Incorporation of thermally reduced GO and RGO substantially affects the overall dielectric properties of composites. Specifically, the dielectric constant of BT/PVDF composites displays a linear relationship referring to volume fraction of BT and the values at four frequencies remain similar (Fig. 9a). As shown in Fig. 9b, the tan d at 1 kHz and 10 kHz stably keeps in an extremely level below 0.05 because interfacial polarization and relaxation of PVDF dipoles hardly impose any influence. However, the tan d at 100 kHz increases to
0.075 due to relaxation of PVDF dipoles while the tan d at 100 Hz is found to be
0.100 due to the influence of interfacial polarization. Dielectric properties of BT-GO/PVDF and BT-RGO/PVDF composites behave in a nonlinear manner accompanying with discrepancy which is mainly derived from the different electrical conductivity of thermally reduced GO and RGO. BT-RGO/ PVDF composites exhibit typical percolative phenomenon due to highly conductive RGO. The dielectric constant and tan d slowly

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increase as the content of BT-RGO is no more than 30 vol% but it undergoes sharp increase when the content of BT-RGO excesses 30 vol% (Fig. 9e and f). Due to relatively lower electrical conductivity of thermally reduced GO, BT-GO/PVDF composites shows moderate percolative behavior. Fig. 9c shows that the increment of dielectric constant is more obvious than BT/PVDF composites but weaker than BT-RGO/PVDF. It can be concluded that composites filled with BT-RGO possess the optimal overall dielectric properties. Specifically, with the filling content no more than 35 vol% at 1 kHz, the dielectric constant of BT/PVDF composites increases from 18.5 to 38.3 while the tan d increases from 0.036 to 0.042; the dielectric constant increases from 19.2 and 18.5 to 57.7 and 67.5 while tan d increases from 0.042 and 0.043 to 0.076 and 0.060 for BT-GO/PVDF and BT-RGO/ PVDF composites, respectively. Hence, it is more effective to improve dielectric constant by incorporating BT-RGO in a level below percolation threshold and maintain ultralow tan d. This is of great significance for the practical applications, especially giving an impetus to industrialization of embedded capacitors because this method not merely cuts down the cost and circumvents the negative influence of ceramic fillers on flexibility and processability of composites by reducing the using amount of BT but also maintains relatively high dielectric constant and ultralow tan d. 4. Conclusions In summary, we adopted the principle of electrostatic selfassembly to partially encapsulate BT particles with GO and RGO sheets. BT-RGO/PVDF composites display optimal overall dielectric properties as the filling content does not excess percolation threshold. The dielectric constant and tan d of composites with 30 vol% of BT-RGO is 67.5 and 0.060 while the values for BT-GO/PVDF and BT/ PVDF composites are 57.7, 38.3, 0.076 and 0.042, respectively. The distribution state of RGO sheets is the critical factor that accounts for such excellent properties. The RGO sheets segregated by BT particles facilitate to form microcapacitors which substantially improve the dielectric constant. In the meantime, the gap distance of these microcapacitors is not close enough to generate tunneling conductance so that tan d maintains in an ultralow level because leakage current is effectively circumvented. This method is useful to prepare polymer-based composites filled with relative low content of BT particles which is able to obtain high dielectric constant and industrial level of tan d. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 51173112 and 21274095) and State Grid Corporation of China. References [1] Ortiz RP, Facchetti A, Marks TJ. High-k organic, inorganic, and hybrid dielectrics for low-voltage organic field-effect transistors. Chem Rev 2010;110(1):205–39. [2] Dang ZM, Wang L, Yin Y, Zhang Q, Lei QQ. Giant dielectric permittivities in functionalized carbon-nanotube/electroactive-polymer nanocomposites. Adv Mater 2007;19(6):852–7. [3] He F, Lau S, Chan HL, Fan J. High dielectric permittivity and low percolation threshold in nanocomposites based on poly(vinylidene fluoride) and exfoliated graphite nanoplates. Adv Mater 2009;21(6):710–5. [4] Yang C, Lin Y, Nan CW. Modified carbon nanotube composites with high dielectric constant, low dielectric loss and large energy density. Carbon 2009;47(4):1096–101. [5] Zhang Y, Huo P, Wang J, Liu X, Rong C, Wang G. Dielectric percolative composites with high dielectric constant and low dielectric loss based on sulfonated poly(aryl ether ketone) and a-MWCNTs coated with polyaniline. J Mater Chem C 2013;1(25):4035–41. [6] Li M, Huang X, Wu C, Xu H, Jiang P, Tanaka T. Fabrication of two-dimensional hybrid sheets by decorating insulating PANI on reduced graphene oxide for

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