Sandwich-Structured PVDF-Based Composite Incorporated with

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 1500−1512

Sandwich-Structured PVDF-Based Composite Incorporated with Hybrid Fe3O4@BN Nanosheets for Excellent Dielectric Properties and Energy Storage Performance Yue Zhang,†,‡ Tiandong Zhang,†,§ Lizhu Liu,†,‡ Qingguo Chi,*,†,§,∥ Changhai Zhang,† Qingguo Chen,*,†,§ Yang Cui,†,§ Xuan Wang,†,§ and Qingquan Lei† †

Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, ‡School of Materials Science and Engineering, and §School of Electrical and Electronic Engineering, Harbin University of Science and Technology, 52 Xuefu Road, Harbin, Heilongjiang 150080, People’s Republic of China ∥ State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, 4 Second Section on Jianshe North Road, Chengdu, Sichuan 610054, People’s Republic of China ABSTRACT: High-performance energy storage materials are of essential importance in advanced electronics and pulsed power systems, and the polymer dielectrics have been considered as a promising energy storage material, because of its higher dielectric strength and more excellent flexibility compared with that of inorganic ceramic dielectrics. However, the energy storage capability of pristine polymer has been limited by its low intrinsic dielectric permittivity and ordinary ferroelectric performance. Herein, this work demonstrates a favorable method to achieve a sandwich-structured poly(vinylidene fluoride) (PVDF)-based composite by the electrospinning, solution casting, thermal quenching, and hot-pressing process. This innovative method combines with the 0.5Ba(Zr0.2Ti0.8)O30.5(Ba0.7Ca0.3)TiO3 nanofibers (BZT-BCT NFs), which possesses good ferroelectric hysteresis, and the hybrid particles of hexagonal boron nitride nanosheets (BNNSs) coated by ferroferric oxide (Fe3O4@BNNSs), which hold high breakdown strength (Eb). It is worth mentioning that the Fe3O4 particles disperse well on the surface of BNNSs to form the dipoles with the BNNSs at the interfacial region, resulting in an enhancement of the electric displacement and the dielectric permittivity of the composite. Furthermore, the influence of the volume fraction of filler particles on the electrical performance of composite was systematically investigated. Notably, the enhanced performance in terms of electric displacement (D), Eb, and discharged energy density (Ue) was achieved in the sandwiched BZT-BCT NFs-PVDF/Fe3O4@BNNSs-PVDF/BZT-BCT NFsPVDF composite. The Ue of ∼8.9 J/cm3 was achieved at 350 kV/mm, which was 740% higher than the Ue of biaxially oriented polypropylene (BOPP, Ue ≈ 1.2 J/cm3 at 640 kV/mm). Of particular note is that the hybrids Fe3O4@BNNSs play a critical role to enhance the D and Eb and suppress the remnant displacement (Dr) of sandwiched composite. This contribution proposes an efficient and scalable method to prepare polymer-based dielectric composite for the demanded applications.

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INTRODUCTION

for the single-component material; for example, the commercial polymer capacitor of BOPP always exhibits ultrahigh breakdown strength (Eb) accompanying with low εr, the typical ceramic capacitor of barium titanate (BT) possesses excellent dielectric permittivity and inferior breakdown strength, and both of them have a similar energy density.3,10−13 Hence, one sapiential strategy is combining the advantages of two kinds of materials, such as the polymer matrix and the ceramic fillers with different characteristics, to get a composite with outstanding comprehensive performance. Herein, the polymer material serves as the supporting matrix for the dielectric strength; meanwhile, the ceramic filler particles work as a candidate of polarized dipoles to enhance the polarization.

Currently, numerous experimental and theoretical efforts focus on the high-performance and lightweight polymer-based energy storage dielectrics with a low-cost and easy process for application in flexible and integrated electronic devices.1−6 The electrical energy can be stored and released in the dielectric capacitor with the inner dipoles polarized and depolarized under an applied electric fields. Accordingly, the energy storage density (U) of the dielectric capacitor is governed by the electric field (E) applied to the dielectric material and the electric displacement (D) of the internal dipole, which can be defined as U = ∫ E·dD.7,8 Meanwhile, the D is related the electric polarization (P) and the dielectric permittivity (εr), which is given by D = P + ε0εrE.9 Therefore, the value U of dielectric materials is ultimately determined by the electric polarization, the dielectric permittivity, and the dielectric strength. However, there are always some weak points © 2018 American Chemical Society

Received: November 3, 2017 Revised: January 3, 2018 Published: January 3, 2018 1500

DOI: 10.1021/acs.jpcc.7b10838 J. Phys. Chem. C 2018, 122, 1500−1512

Article

The Journal of Physical Chemistry C

eration is to surface modify inorganic particles, as mentioned in previous studies, which can improve the compatibility and alleviate the dielectric difference between the inorganic fillers and the organic matrix in the interfacial region.23,30−32 For instance, Huang et al. produced the core−shell structure BTPDA-Ag by dopamine (PDA) modification to improve the interface compatibility and to enhance the breakdown strength of dielectric composite.33 Also, Chi et al. used the PDA as modifier to improve the dispersibility of fillers in the matrix and to alleviate the dielectric difference between fillers and matrix.7 Moreover, in order to further improve the overall performance of the composite, the sandwich structure was widely used to produce polymer-based dielectrics by adjusting the content of fillers in different layers to obtain layers with different functions. On the basis of the aforementioned literature, we prepared the BZT-BCT NFs-PVDF/Fe3O4@BNNSs-PVDF/BZT-BCT NFs-PVDF composite with sandwich structure by the solutioncasting, thermal -quenching, and hot-pressing processes in this work. First, based on the reported literature, the Fe3O4 nanoparticle as an excellent filler was conducive to the polymer composite with a good dielectric constant.34−37 A small amount of the Fe3O4 nanoparticles used as the second fillers can remarkably improve the dielectric property of the composite, resulting from the greatly increased interfacial polarization by Fe3O4.34,36 Meanwhile, according to our previous research, the single layer of 3 vol % BZT-BCT NFs/PVDF composite had an excellent overall performance, for instance, a great electric displacement and a good breakdown strength, which were of great benefit to improve the performance of dielectric composite. Therefore, 3 vol % of BZT-BCT NFs was employed to prepare the outer layers in this work.7 We utilized the outer layers of the 3 vol % BZT-BCT NFs-PVDF composite (the BZT-BCT NFs dispersed in the PVDF matrix) to enhance the dielectric properties of composite. In addition, we employed the middle layer of Fe3O4@BNNSs-PVDF composite (the hybrid Fe3O4@BNNSs distributed in the PVDF matrix) to withstand the applied electric field. Herein, the BNNSs possess excellent electrical insulating properties including the high breakdown strength, low dielectric loss, and high electrical resistivity. In addition, the semiconductive Fe3O4 particles well distribute on the surface of BNNSs to form the dipoles at the interfacial region with the BNNSs, enhancing the electric displacement and the dielectric permittivity. The highlight of this work is that the constructed hybrid Fe3O4@BNNSs can not only reinforce the dielectric strength but also avoid the decrease of the dielectric permittivity of the middle layer, which results in an improvement of the dielectric properties of the composite film. Moreover, both the BZT-BCT NFs and the hybrid Fe3O4@BNNSs were coated by PDA to improve the compatibility and alleviate the dielectric difference. Furthermore, the influence of the volume fraction of hybrid Fe3O4@ BNNSs on the performance of composite was systematically investigated to obtain successful design of synergistic inorganic−organic composites, which may be applicable to the requirement of excellent energy storage properties.

Moreover, among the high-performance polymers currently explored as dielectric capacitor, the PVDF and its copolymersbased ferroelectric polymers are expected to become promising dielectric matrices due to their relatively high εr and large Eb.8,14 Because of the different conformations of highly polar C−F bonds in the crystalline domains of PVDF molecular chain there are in general four different polymorphs for PVDF, for instance, the paraelectric α-phase (trans−gauche conformation, nonpolar), the ferroelectric β-phase (all-trans conformation, polar), the γ-phase (trans−trans−trans−gauche conformation, weak polar), and the δ-phase (one polar α-phase). 15 Accordingly, the β-phase PVDF possesses the strongest polarity and the largest hysteresis loss, while the γ-phase PVDF has a weaker polarity and the smallest hysteresis loss in general polymorphs.16 Furthermore, Zhang et al. explored the influence of crystalline properties on the dielectric and energy storage performance of PVDF, which showed that the γ-phase PVDF could work normally under higher electric fields than α-phase PVDF and β-phase PVDF due to the absence of phase transition for α-phase PVDF and the early polarization saturation for β-phase PVDF.17 More importantly, the γphase can be obtained from β-phase transforming by a quenching process,15,18 which provides inspiration for preparing PVDF-based dielectrics. Besides, all sorts of ferroelectric oxides with high εr (up to hundreds or thousands) have been chosen to raise the value of εr.19−21 For example, Wang et al. induced barium titanate (BaTiO3, BT) nanoparticles with perovskite structure into the PVDF matrix to obtain the composite with a high dielectric polarization and a low remnant polarization under an electric field.10 Also, Zhai et al. fabricated the ceramic-polymer composite with SrTiO3 nanofibers (ST NF) and PVDF, which exhibited the enhanced dielectric constant and the reduced dielectric loss.22 Along this line, Zhai et al. studied the effect of one-dimensional (1D) 0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Zr0.2Ti0.8)O3 nanofibers (BCZT NFs) on the dielectric performance of PVDF-based composite, which proved that the BCZT NFs with a large aspect ratio significantly improved the dielectric and energy storage properties because of the 1D nanofibers with large dipole moments.23 Notably, for an improvement of the energy density, twodimensional (2D) materials with high breakdown strength were widely researched by previous works, because 2D fillers can be regarded as conduction barriers to limit the charge migration and hinder the expansion of electrical treeing during breakdown.6,14,24−27 For instance, Wang et al. induced the hexagonal boron nitride nanosheets (BNNSs) into PVDF matrix to improve the Eb and thermal conductivity of composite.6,28 Comparing with ferroelectric ceramics, the BNNSs possess excellent electrical insulating properties owing to the broad band gap of BN, including low dielectric loss and high electrical resistivity.26 However, the dielectric permittivity of BN is low (εr ≈ 4−7)28,29 relative to that of the perovskite-structured BCZT, which leads to the decreased dielectric permittivity of composite with increasing doping content of BN.6 Currently, Zhai et al. introduced the 2D NaNbO3 (NN) into the PVDF to enhance the dielectric performance of composite. Simultaneously, they prepared a trilayer architecture, which further reinforced the Eb and U.27 The results indicated that the 2D platelets with lower specific surface than zero-dimensional (0D) and 1D materials are propitious to alleviate surface energy and mitigate agglomeration of the inorganic fillers in matrix. Generally, another effective method of reducing the agglom-

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EXPERIMENTAL SECTION Materials. Poly(vinylidene fluoride) (PVDF) was produced by Shanghai 3F New Material Co., Ltd., China. Polyvinylpyrrolidone (PVP, Mw = 1 300 000) and zirconium acetyl acetone (C20H28ZrO8) were purchased from Alfa Aesar. Hexagonal boron nitride (h-BN), dopamine hydrochloride, and tris(hydroxymethyl)-aminomethane (Tris) were provided by 1501


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Figure 1. Schematic illustrations for the preparation and configurations of layer-by-layer casting for the sandwich nanocomposite film. (a) BZCT NFs were prepared by an electrospinning technique, (b) hybrid Fe3O4@BNNSs were fabricated by hydrothermal synthesis method, (c) monolayer PVDF-based composite film was prepared by solution casting, (d) trilayer PVDF-based composite film was produced by hot pressing, and (e) trilayer PVDF-based composite was undergone the thermal quenching. XRD patterns (f) and TEM graphs (g) for inorganic filling phase BNNSs (g1) and the hybrid particles of Fe3O4@BNNSs (g2).

Aladdin. Ferrous sulfate heptahydrate (FeSO4·7H2O) and ferric chloride hexahydrate (FeCl3·6H2O) were supplied by Tianjin TIANLI Chemical Reagents Ltd., China. Calcium hydroxide (Ca(OH)2), barium hydroxide octahydrate (Ba(OH)2·8H2O), tetrabutyltitanate (Ti(OCH2CH2CH2CH3)4), and acetyl acetone (C5H8O2) were offered by Shandong Sinocera Functional Material Co., Ltd., China. Dimethylformamide (DMF), sodium hydroxide (NaOH), ethanol, and acetic acid were supplied by Sinopharm Chemical Reagent Co., China. All chemicals were analytical grade and used as received without further purification. Preparation of PDA@BZT-BCT nanofibers. The BZTBCT nanofibers were prepared by the sol−gel and electrospinning method, which can be seen in Figure 1a.7,23 First, the Ba(OH)2·8H2O (2.17 g) powders were dissolved in a solution of ethanol (3.8 mL), acetic acid (9.1 mL), and acetyl acetone (1.5 mL) mixture and stirred for 30 min. Then the Ca(OH)2 (0.90 g) and C20H28ZrO8 (0.39 g) were added into the above mixture solution. After that Ti(OC4H9)4 (2.5 mL) was dissolved into the mixed solution and continuously stirred for another 30 min. Finally, the PVP powders (0.70 g) were added into the clear solution and intensely stirred for 24 h. Afterward, the BZT-BCT precursor solution with PVP was injected into the syringe, and the applied electric field was about 1 kV/cm. Finally, the obtained amorphous nanofibers with PVP were air calcined at 700 °C for 3 h to obtain the perovskite crystal structure of BZT-BCT NFs with different aspect ratios. The PDA was coated on the surface of nanofibers to achieve the BZT-BCT NFs with excellent dispersion and compati-

bility.31 Tris-HCl buffer agent was first dissolved in deionized water (pH = 8.5); then PDA was added into Tris-HCl aqueous solution and stirred for 30 min to obtain the stable PDA solution. Second, the BZT-BCT NFs were immersed into above the PDA solution and vigorously stirred at ambient temperature for 12 h. This was followed by the BZT-BCT NFs modified by PDA being centrifuged, washed, and dried in a vacuum oven at 80 °C for 12 h. Preparation of PDA@Fe3O4@BNNSs Hybrid Particles. The Fe3O4@BNNSs hybrid particles were synthesized via hydrothermal synthesis, which can be seen in Figure 1b.38,39 First, the two-dimensional nanosheets h-BN powders were produced by liquid exfoliation according previous work. Briefly, 2 g of h-BN powders was dispersed in 200 mL of deionized water and subjected to tip-type sonication (360 W, 1200 W × 30%) for 24 h. After sonication, the mixture was centrifuged at 3000 rpm for 10 min, and the collected supernatant was centrifuged at 9000 rpm for 30 min to precipitate BN nanosheets (BNNSs). Finally, the BNNSs were obtained by vacuum drying overnight at 80 °C. Precursor solution A was obtained as follows. First, FeSO4· 7H2O and FeCl3·6H2O were dissolved in deionized water and stirred for 1 h to get the homogeneous precursor solution A (Sol A). Second, the NaOH was dissolved in the deionized water and stirred to obtain solution B (Sol B), and the BNNSs were added into Sol B to form mixture C (Mix C). Third, the Sol A was added into Mix C and stirred for 30 min to achieve mixture D (Mix D). The homogeneous Mix D was slowly transferred into the stainless steel autoclave with the Teflon 1502


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hybrid particles were synthesized via the hydrothermal reaction. The crystalline structure of hybrid particles was characterized by XRD and is presented in Figure 1f. As shown in Figure 1f, the typical peaks of hexagonal BN could be observed at 2θ = 26.8°, 41.6°, 43.9°, 50.2°, 55.2°, 75.9°, and 82.2° assigned to the (002), (100), (101), (102), (004), (110), and (112) diffraction planes with reference data (PDF no. 00-034-0421), respectively. The semiconductive particles of Fe3O4 had been successfully introduced onto the surface of BNNSs by hydrothermal reaction, which was proved by the obvious characteristic peaks of (311) at 2θ = 35.5° (PDF no. 00-0110614) in Figure 1f. At the same time, Figure 1g presents the TEM graphs of inorganic filler BNNSs and the hybrid particles of Fe3O4@BNNSs. It was shown in Figure 1g that the diameter of BNNSs was about 200 nm, and the Fe3O4 particles with a diameter of about 20 nm uniformly grew and dispersed on the surface of the BNNSs particles, which was beneficial for improving the properties of dielectric composite used for capacitor application. The sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles), B-P/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P composites were fabricated by solution casting, hot pressing, and quenching treatment. Also, the XRD patterns of the sandwich-structure composites compounded with inorganic particles and polymer are presented in Figure 2. Figure 2a shows the XRD patterns of the sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites, which clearly indicated that the composites mainly consisted of the PVDF, the perovskite structure of BZT-BCT, the lamellar structure of BNNSs, and semiconductive particles of Fe3O4. Obviously, there were three major phases: the nonpolar γ and α and the polar β in the matrix of PVDF seen from the inset of Figure 2a1. It is notable that the diffraction peaks at 2θ = 18.5°, 20.1°, and 26.8° are assigned to γ (020), γ (110), and γ (022), respectively.9,15,17 The paraelectric γ-phase, which possessed the low remnant displacement (Dr), was induced by the quenching process.18,40 Meanwhile, the diffraction peak of nonpolar α(100) appeared at 2θ = 17.94°, and the diffraction peak of polar β(200) appears at 2θ = 20.7°. Moreover, the typical perovskite structure of BZT-BCT appears at 2θ = 22.1°, 31.5°, 38.7°, 45.1°, and 55.9°, which were associated with the crystal planes of (100), (110), (111), (200), and (211). Obviously, a typical peak of hexagonal BN was observed at 2θ = 26.8° assigned to the (002) diffraction plane. Accordingly, the characteristic peak of semiconductive Fe3O4 corresponds to the (311) diffraction plane at 2θ = 35.5°. Also, it was found that the characteristic peak positions of polymer matrix and inorganic fillers, which are shown in Figure 2b of the XRD patterns for the sandwichstructure B-P/5 vol % BN-P/B-P and P/5 vol % Fe3O4@BN-P/ P composites, were very similar to the peak positions demonstrated in Figure 2a. Notably, the perovskite structure of BZT-BCT nanofibers was not added into the outer layers of the sandwich-structure P/5 vol % Fe3O4@BN-P/P composite, so the diffraction peaks of BZT-BCT were not shown in the XRD patterns of P/5 vol % Fe3O4@BN-P/P composites. Meanwhile, the SEM images of the cross-sectional morphology for the sandwich-structured B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid Fe3O4@BNNSs) composites could be observed and analyzed in Figure 3. As can be seen from the SEM micrographs, the final thicknesses of the sandwich-structure composites were about

liner, and then the autoclave was kept at a temperature of 100 °C for 24 h. Then the reactor was cooled to ambient temperature; finally, the Fe3O4@BN hybrid particles were obtained by centrifugation, washing with distilled water to neutral pH, and drying under vacuum overnight at 80 °C. Further, the method of surface modification of Fe3O4@BN by PDA was similar to the preparation of PDA@BZT-BCT nanofibers. Therefore, the preparation method of PDA@ Fe3O4@BN hybrid particles is similar to the above preparation of PDA@BZT-BCT nanofibers. Preparation of PDA@BZT-BCT NFs/PVDF and PDA@ Fe3O4@BNNSs/PVDF Composites. In the conventional process, as shown in Figure 1c, the PDA@BZT-BCT NFs and PDA@Fe3O4@BNNSs were dispersed in DMF solution with different volume contents (1, 3, 5 vol % and 7 vol %) and stirred vigorously. This was followed by adding a certain amount of PVDF powders dissolved in the above suspension solution and continuously stirred for 24 h to form the homogeneous suspension mixture. Then the suspension mixture was slowly dropped onto the clean glass substrate using the scraping method, followed by drying in a vacuum oven at 60 °C for 10 h to remove the organic solvent. Finally, the monolayer composites were heated at 200 °C for 8 min and quenched in ice water immediately to get composites with a thickness of ∼10 μm. Preparation of Sandwich-Structured Composite. The sandwich-structured dielectric composite was fabricated by hot pressing with an outer layer of 3 vol % PDA@BZT-BCT NFs/ PVDF and middle layer of x vol % PDA@Fe3O4@BNNSs/ PVDF (x = 1, 3, 5, and 7 vol % of hybrid particles), which are presented in Figure 1d and 1e. Three layers were stacked by hot pressing at 170 °C for 10 min and immediately cooled to ambient temperature. Finally, the sandwich structure of 3 vol % PDA@BZT-BCT NFs-PVDF/x vol % PDA@Fe3O4@BNNSsPVDF/3 vol % PDA@BZT-BCT NFs-PVDF (x = 1, 3, 5, and 7 vol % of hybrid particles) (hereafter, B-P/x vol % Fe3O4@BNP/B-P in short) composite was fabricated with a thickness of ∼28 μm. Characterization. The X-ray diffraction (XRD) analyses were characterized by a PANalytical Empyrean, which is used to analyze the crystalline structures of BN, Fe3O4, and the PVDFbased composites. Field emission scanning electron microscope (FE-SEM) tests were performed with a Hitachi SU8020 Uhr by which the microstructure of the fillers and composites was examined. Transmission electron microscope (TEM) images were carried out through a FEI TECNAI2-12 to obtain the dispersion of Fe3O4 on the surface of BNNSs. A thin layer of aluminum electrodes (diameters of 3 and 25 mm) were evaporated on both sides of the films for measurements. The dielectric properties were collected in the frequency range from 100 Hz to 1 MHz by a broad-band impedance analyzer from GmbH Novocontrol Alpha-A. Dielectric breakdown strength tests were obtained with a precision power supply test equipment YDZ-560. Electric displacement−electric field (D− E) loops and current−voltage (I−V) tests were conducted using a Radiant Premier II Ferroelectric Test System at a frequency of 10 Hz. For each electrical performance of composites, several of the same samples were collected to ensure repeatability of the data.

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RESULTS AND DISCUSSION Crystalline Structure and Morphology of Inorganic Fillers and PVDF-Based Composite. The Fe3O4@BNNSs 1503


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BNNSs/PVDF) for these composites was about 1:2. Obviously, there were almost no macrostructural imperfections, such as chasms and holes, between the three layers. Meanwhile, the BZT-BCT NFs in the outer layers and the Fe3O4@BNNSs in the middle layers were homogeneously dispersed in PVDF and satisfactorily compatible with the polymer matrix due to the modifier of PDA, which was in favor of establishing a good performance for composites. In addition, the slight aggregations gradually appeared in the middle layer with increasing volume fractions of Fe3O4@BNNSs, which had an adverse influence on the properties of the PVDF-based composites. Dielectric Properties of the PVDF-Based Composites. For dielectric properties analysis at ambient temperature, the frequency dependence of the dielectric permittivity and dielectric loss for the sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites are illustrated in Figure 4a. Clearly, the dielectric permittivity of the B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composite increased first and then decreased with increasing content of hybrid particles Fe3O4@BNNSs over the whole frequency range from 100 Hz to 1 MHz (Figure 4a1), which was attributed to the hybrid particles of Fe3O4@BNNSs. This might be because the dielectric polarization of hybrid particles had been enhanced by introduction of a small amount semiconductive particles of Fe3O4, which increased the interfacial polarization between Fe3O4 and BNNS. Here, in this work, the interfacial polarization may arise from the diffusion layer of the semiconductor−insulator structure41,42 at the interfaces between the semiconductive Fe3O4 and the insulative BNNS, which is one of four main types of polarizations in the dielectric material (electronic, ionic, dipolar, and interfacial polarizations).43 Furthermore, according to the research results of Gupta et al. and Shen et al., the schematic illustrations of composites filled with Fe3O4@BNNSs hybrid are shown in Figure 5.43,44 The enhanced dielectric permittivity could not be attributed solely to the increase in the area of the interfaces. Besides, the confinement of interfaces within the PDA shell, as shown in Figure 5a, leads to percolated interfacial regions and gives rise to the increased polarization of the hybrid and hence much more moderate dielectric permittivity without reduction.44 Meanwhile, the dielectric loss (Figure 4a2) of composites was less than 0.23 over the whole frequency range. Also, the dielectric loss increased with increasing frequency due to the gradual generation of relaxation resulting from the fact that the dipole could not rapidly shift with the increasing rate of change of alternating electric field. The agglomerations and voids had been introduced into the matrix when the content of Fe3O4@ BNNSs reached 7 vol %; the smaller dielectric constant and accumulated space charge induced by agglomerations and voids lead to a decline of the dielectric constant and a slight increase of dielectric loss. Thus, the dielectric permittivity decreased from the maximum of ∼17.0 for B-P/5 vol % Fe3O4@BN-P/BP to a value of ∼16.3 for B-P/7 vol % Fe3O4@BN-P/B-P at 100 Hz. Furthermore, the frequency dependence of the dielectric permittivity and dielectric loss for B-P/5 vol % Fe3O4@BN-P/ B-P, B-P/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P composites was investigated in Figure 4b. It is obvious that the dielectric permittivity of B-P/5 vol % BN-P/B-P and P/5 vol % Fe3O4@BN-P/P composites was 13.6 and 14.7 at 100 Hz (Figure 4b1), respectively, both of which were lower than that

Figure 2. XRD patterns for the sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) (a), BP/5 vol % BN-P/B-P and P/5 vol % Fe3O4@BN-P/P (b) composites: (a1 and b1) 2θ = 10−90°, (a2 and b2) 2θ = 16−22°.

Figure 3. SEM images of cross-sectional morphology for the sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites: (a) x = 1, (b) x = 3, (c) x = 5, and (d) x = 7.

28 μm, and the thickness ratio of the outer layer (PDA@BZTBCT NFs/PVDF) and the middle layer (PDA@Fe3O4@ 1504


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Figure 4. Relationship between the dielectric properties and the frequency for the sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) (a) and B-P/5 vol % BN-P/B-P and P/5 vol % Fe3O4@BN-P/P (b) composites: (a1 and b1) dielectric permittivity and (a2 and b2) dielectric loss. Leakage current vs electric field for the sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) (c1), B-P/5 vol % BN-P/B-P and P/5 vol % Fe3O4@BN-P/P (c2) composites.

and permittivity of B-P/5 vol % Fe3O4@BN-P/B-P composite, which is consistent with the experimental results we reported previously.7 It was mainly because the BZT-BCT ceramic itself not only had a high dielectric polarization but also possessed the one-dimensional fiber structure to further enhance the directional polarization of the composite.23 Leakage Current of the PVDF-Based Composites. In order to demonstrate the influence of the inorganic filler on the dielectric loss of composites, the electric field dependence of the leakage current for the sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites is presented in Figure 4c1. As can be seen from Figure 4c1, the leakage current of B-P/x vol % Fe3O4@BN-P/ B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composite increased with the increasing content of Fe3O4@BNNSs in the middle layer. This result was consistent with the variation tendency of the dielectric loss with the content of Fe3O4@ BNNSs hybrid particles in the composite. In particular, the leakage current of the composite with a content of 7 vol % Fe3O4@BNNSs had the largest increase than other composites over the whole electric field range due to much more imperfections induced by the hybrid particles into the B-P/7 vol % Fe3O4@BN-P/B-P composite. The electric field dependence of the leakage current for the sandwich-structure B-P/5 vol % Fe3O4@BN-P/B-P, B-P/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P composites is shown in Figure 4c2, which demonstrated that there was a small difference in leakage current for these three composites. The results indicated that the effect of a small amount of BZT-BCT nanofibers (content of 3 vol %), which compounds with PVDF as the outer layer of the sandwich-structure composite, on the leakage current of the composite was not significant. Similarly, the influence of tiny amounts of Fe3O4 particles dispersed on the surface of BNNSs on the leakage current was almost negligible. It was also important that the PDA acted as a modifier and binder and mitigated the performance difference between the inorganic filler and the organic matrix. Accordingly, this modification method could avoid the accumulation of free electrons and space charges at the interface, resulting in the large leakage current. Breakdown Behavior of the PVDF-Based Composites. According to the formula of energy density expressed as U =

Figure 5. Schematic illustrations of composites filled with Fe3O4@ BNNSs hybrid. Distribution of dielectric permittivity was superimposed in different regions of the composite. Abrupt increase of dielectric permittivity indicated the enhanced interfacial polarization in the interfacial regions. Interfacial region in c is magnified to show the diffuse electronic layer of semiconductor−insulator structure.

of B-P/5 vol % Fe3O4@BN-P/B-P at the same frequency. In addition, as shown in Figure 4b2, the dielectric loss of three kinds of composites was less than 0.23, and there was almost no difference over the whole frequency range from 100 Hz to 1 MHz. Also, the dielectric loss also increased with increasing frequency due to the gradual generation of relaxation. Comparing with the dielectric properties of B-P/5 vol % BNP/B-P composite, it was found that introduction of a slightly semiconductive Fe3O4 onto the surface of BNNSs in the middle layer of composite enhanced the dielectric polarization and permittivity of B-P/5 vol % Fe3O4@BN-P/B-P composite, which explained that the dielectric property of the composite was improved by the hybrid particles of Fe3O4@BNNSs. Simultaneously, comparing with the dielectric properties of P/5 vol % Fe3O4@BN-P/P composite, it can be seen that introduction of 3 vol % BZT-BCT nanofibers in the outer layer of the composite could improve the dielectric polarization 1505


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The Journal of Physical Chemistry C ∫ E·dD for nonlinear dielectric material, we know that both the dielectric polarization (D) and the breakdown strength (E) are the key parameters to obtain the energy density for the dielectric material. At the same time, a series of performances of dielectric composites was limited by the low breakdown strength, so that the breakdown strength of the composites was obtained by a precision power supply test equipment and analyzed by the two-parameter Weibull distribution: P(E) =1 − exp(−(E/Eb)β), where P(E) is the cumulative probability of electric field failure, E is the experimental breakdown strength, Eb is a scale parameter associated with the breakdown strength when the cumulative failure probability was 63.5%, and β is a shape parameter referring to the linear regressive fit of this distribution. Both sides of the above formula underwent logarithmic operations; a new equivalence formula could be written as follows: ln(−ln(1 − P(E))) = β ln E − β ln Eb. ln(−ln(1 − P(E))) versus ln E was obtained and is plotted in Figure 6. The results of the linear fitting and Weibull parameters are shown in Table 1. Apparently, Figure 6a shows the results of Weibull distribution of Eb for the sandwichstructure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites. It is shown in Figure 6a that all of the Eb of composites were higher than 310 kV/mm, which

could be mainly attributed to the BNNSs with high insulativity and great tolerance for the electric field. In other aspects, the Eb of B-P/1 vol % Fe3O4@BN-P/B-P composite (∼400 kV/mm) was much greater than that of B-P/7 vol % Fe3O4@BN-P/B-P composite(∼310 kV/mm), which indicated that the E b monotonically decreased with increasing filler content. Also, the Eb obviously decreased by almost 60 kV/mm with the increase of content of Fe3O4@BNNSs from 5 to 7 vol %, which may indicate that the Eb of the composite can be deteriorated when the higher doping content of hybrid fillers of Fe3O4@ BNNSs is introduced into the matrix. Moreover, introduction of Fe3O4@BNNSs could induce a concentration of the local electric field in the interfacial region of the polymer matrix and the inorganic fillers due to the dielectric difference. The networks eventually formed after the concentrated local electric field gradually percolated, so the Eb of the composites monotonically decreased with the volume fraction of Fe3O4@ BNNSs increasing. Furthermore, the finite element simulation (COMSOL Multiphysics) was carried out on the B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites to understand the influence of the content of hybrid Fe3O4@BNNSs on the electric field strength as shown in Figure 6b. For the simulation the dielectric permittivity of Fe3O4, two-dimensional BNNSs, one-dimensional BZT-BCT NFs, and PVDF are around 20, 4, 3000, and 9, respectively. An electric field of ∼200 kV/mm was applied on the models from the top to the bottom. Obviously, the low electric field was distributed in the outer layers, and the high electric field was mainly concentrated in the middle layer, which always happened in the sandwich-structure composites because of the different functional properties of the introduction of BZTBCT NFs in the outer layers and the Fe3O4@BNNSs in the middle layer. In addition, the local electric field was near the ends of the hybrid Fe3O4@BNNSs due to the large dielectric difference between the hybrid particles and the PVDF polymer. Meanwhile, much more obviously the distortion of the local electric field appeared in the model when more hybrid Fe3O4@ BNNSs were introduced into the matrix. Finally, the conductive channel gradually formed, resulting from the particles being in contact with each other, which deteriorated the Eb of the composite. This evidenced that the Eb decreased as the volume fraction of Fe3O4@BNNSs increased. As seen from Table 1, the correlation coefficient R is the degree of obedience to the Weibull distribution. The closer to 1 R was, the better the data followed the Weibull distribution. For this series of breakdown strength measurements, the correlation coefficient R of all the composites is greater than 0.93, which represents good fitting results. Meanwhile, the standard error S was calculated by the least-squares method, and the lower S was, the smaller error was. Here, for the B-P/x vol % Fe3O4@ BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites, the range of S value was from 0.90 to 1.75, meaning a low error. It was remarkable that the value of β was higher than 12.30 for these composites due to the structural integrity and high quality of composites. Because the β value quantifies the scattering state of the experimental data, for example, the higher β, there is less scattering. In this work, a large value of β indicates that homogeneous composites were achieved by the compounding process of solution casting and hot pressing. Naturally, the Weibull distribution of the breakdown strength for the sandwich-structure B-P/5 vol % Fe3O4@BN-P/B-P, BP/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P

Figure 6. Weibull distribution of breakdown strength (a) and finite element simulation of electric field strength (b) for the sandwichstructure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites. 1506


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Table 1. Linear Fitting Results and Weibull Parameters of the Sandwich-Structure B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) Composites linear fitting results

Weibull parameters

samples

slope

ln(−ln(1 − P(E)) intercept

R

S

β

E0 (kV/mm)

1% 3% 5% 7%

19.69 17.76 15.35 12.30

−117.97 −105.85 −90.84 −70.56

0.93 0.93 0.97 0.95

1.64 1.75 0.96 0.90

19.69 17.76 15.35 12.30

399.96 387.62 371.64 310.00

Figure 7. Weibull distribution of breakdown strength (a) and finite element simulation of macroscopic equipotential line (b) for the sandwichstructure B-P/5 vol % Fe3O4@BN-P/B-P (b1), B-P/5 vol % BN-P/B-P (b2), and P/5 vol % Fe3O4@BN-P/P (b3) composites.

particles, which could be easier to capture charges. Also, the mobile charges were constrained by traps; thus, the conductive path was avoided forming during breakdown.47,48 Therefore, a small amount of semiconductive particles, which homogeneously grew on the surface of BNNSs and evenly dispersed in the matrix, could be of benefit for enhancing the breakdown. The finite element simulation of macroscopic equipotential line for the sandwich-structure B-P/5 vol % Fe3O4@BN-P/B-P, P/5 vol % Fe3O4@BN-P/P, and B-P/5 vol % BN-P/B-P composites is illustrated in Figure 7b. For the simulation, the dielectric permittivity of monolayer Fe3O4@BNNSs-PVDF (Fe3O4@BN-P), BNNSs-PVDF (BNP), BZT-BCT NFs-PVDF (B-P), and PVDF (P) is around 16, 7, 13, and 9, respectively. An electric voltage of ∼6000 V was applied on the models from the top to the bottom, which decreased gradually from 6000 to 0 V. Clearly, the electric field redistribution also happened in the sandwich structure composites. Simultaneously, the trilayer could be looked as three capacitors connected in series, so the local electric field of each layer could be obtained as follows9,49

composites are measured and analyzed in Figure 7. As shown in Figure 7a, the breakdown strength of P/5 vol % Fe3O4@BN-P/ P composite (∼315 kV/mm) was much lower than that of B-P/ 5 vol % Fe3O4@BN-P/B-P composite (∼372 kV/mm). The strikingly enhanced breakdown strength of B-P/5 vol % Fe3O4@BN-P/B-P composite might be ascribed to the following three reasons. First, to the best of our knowledge, introduction of BZT-BCT nanofibers in the outer layer of the composite could suppress formation of a conductive pathway and decrease the transfer of charge carriers (free electrons and space charges).7,23 More importantly, the polymer chain was bound in the interfacial region of the PVDF matrix and the BZT-BCT nanofibers linked by the modifier PDA, so the chain mobility was inhibited.45,46 Additionally, the BZT-BCT nanofibers hindered the expansion of the electrical treeing under the applied external electric field.7,23 These above factors gave rise to the improved breakdown strength of B-P/5 vol % Fe3O4@ BN-P/B-P composite. Afterward, comparing with the B-P/5 vol % BN-P/B-P composite, we found that the slightly improved breakdown strength of B-P/5 vol % Fe3O4@BN-P/B-P composite was ascribed to a little Fe3O4 grown on the surface of BNNSs. It turned out that more deep traps around the wide band-gap BNNSs were introduced by inorganic nanosemiconductive

E1 = 1507

V 2d1 +

ε1 d ε2 2

(1) DOI: 10.1021/acs.jpcc.7b10838 J. Phys. Chem. C 2018, 122, 1500−1512

Article

The Journal of Physical Chemistry C E2 =

V ε d 2 + 2 ε2 d1

electric field for the composites was measured and analyzed. Figure 8a−d shows the displacement−electric field (D−E)

(2)

1

where E1 and E2 are the local electric field of the outer layer and middle layer, respectively. Also, V = 6000 V is the applied voltage on the composite, and d1 = 7.0 μm and d2 = 14.0 μm are the thickness of the outer layer and middle layer, respectively. Also, ε1 and ε2 are the dielectric permittivity of the outer layer and middle layer. According to the calculation formula, the local electric field of the composite is shown in Table 2. Table 2. Local Electric Field of Every Layer Calculated by the Capacitive Voltage Divider composites

ε1

ε2

E1 (kV/mm)

E2 (kV/mm)

B-P/5 vol % Fe3O4@BN-P/B-P B-P/5 vol % BN-P/B-P P/5 vol % Fe3O4@BN-P/P

13 13 9

16 7 16

236 150 274

192 279 154

As shown in Figure 7 and Table 2, it was found that the local electric field of each layer was different due to the redistribution. For the B-P/5 vol % Fe3O4@BN-P/B-P composite, both the outer layer of B-P and the middle layer of Fe3O4@BN-P possessed a great polarization, and the dielectric difference between layers was small; moreover, the local electric field of layers was well distributed owing to a small quantity of conductive Fe3O4 nanoparticles, so that the breakdown strength of B-P/5 vol % Fe3O4@BN-P/B-P composite was relatively higher than other composites. For the B-P/5 vol % BN-P/B-P composite, the middle layer of BNP had a greater redistributed local electric field with sparser equipotential lines than outer layers resulting from the much larger dielectric difference, which decreased the dielectric strength of the sandwich-structured composite. Meanwhile, the outer layer of P, which worked as the supporting layer of the applied electric field, could not overcome the increasing redistributed local electric field; moreover, the high dielectric difference between the outer and the middle layers was harmful to the breakdown strength. These two reasons lead the breakdown strength of the P/5 vol % Fe3O4@BN-P/P composite which decreases rapidly. Table 3 gives the linear fitting results and Weibull parameters of the sandwich-structure B-P/5 vol % Fe3O4@BN-P/B-P, B-P/ 5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P composites. It is clearly shown that the results follow the Weibull distribution due to the R being closer to 1, and high-quality composites were obtained, which was inferred from a large value β and a small error S, referring to the analysis from Table 1. Polarization Behavior of the PVDF-Based Composites. Herein, the polarization behavior depending on the external

Figure 8. Displacement−electric field (D−E) hysteresis loops for the sandwich-structure B-P/x vol % Fe3O4@BN-P/B-P (a) x = 1, (b) x = 3, (c) x = 5, and (d) x = 7 vol % of hybrid particles, B-P/5 vol % BNP/B-P (e), and P/5 vol % Fe3O4@BN-P/P (f) composites.

hysteresis loops of the sandwich-structure B-P/x vol % Fe3O4@ BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites. According to Figure 8a−d, introduction of hybrid Fe3O4@BNNSs could enhance the electric displacement of composites along with the volume fraction of hybrid particles increasing. For instance, the electric displacement of B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites were ∼7.0, ∼7.6, ∼8.3, and ∼9.1 μC/cm2, respectively, corresponding to x = 1, 3, 5, and 7 vol % composites at the same electric field of 290 kV/mm. Obviously, the hybrid particles of Fe3O4@BNNSs could enhance the inner displacement of composites under the electric field, because a small amount of Fe3O4 particles grown on the surface of BNNSs contributed to improve the polarization of dipoles. Furthermore, the electric field also directly influenced the displacement of composite, because the capabilities of withstanding the electric field were different resulting from the much different breakdown strength of the composite. For example, the maximum displacement was ∼8.9, ∼9.7, and ∼10.4 μC/cm2, respectively, corresponding to x = 1, 3, and 5 vol % composites at 350 kV/mm and ∼9.1 μC/cm2 at 290 kV/

Table 3. Linear Fitting Results and Weibull Parameters of the Sandwich-Structure B-P/5 vol % Fe3O4@BN-P/B-P, B-P/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P Composites linear fitting results

Weibull parameters

samples

slope

ln(−ln(1 − P(E)) intercept

R

S

β

E0 (kV/mm)

B-P/5 vol % Fe3O4@BN-P/B-P B-P/5 vol % BN-P/B-P P/5 vol % Fe3O4@BN-P/P

15.35 14.89 12.91

−90.84 −87.41 −74.28

0.97 0.98 0.95

0.96 0.70 1.03

15.35 14.89 12.91

371.64 354.38 315.35

1508


Article

The Journal of Physical Chemistry C mm for B-P/7 vol % Fe3O4@BN-P/B-P composite. Notably, the electric field applied on the composite could not be over the 290 kV/mm when the content of Fe3O4@BNNSs was above 7 vol %, which was consistent with the measured results of the breakdown strength for the composite. This might be caused by the imperfections induced by the large amount of inorganic fillers in the middle layer of the composite. Interestingly, the remnant displacement (Dr) decreased slightly from ∼2.6 μC/cm2 for B-P/1 vol % Fe3O4@BN-P/B-P composite to 2.4 μC/cm2 for B-P/5 vol % Fe3O4@BN-P/B-P composite; hereafter, Dr increased to ∼2.7 μC/cm2 for B-P/7 vol % Fe3O4@BN-P/B-P composite with the further increase of Fe3O4@BNNSs content presumably, owing to the aggregation of hybrid particles. Referring to the previous literature, it was apparent that the appropriate Fe3O4@BNNSs could weaken the Dr mainly due to the BNNSs added.14 As is well known, the smaller the value Dr was the lower hysteresis loss. This is in favor of the enhancement of energy density and efficiency. Besides, the polarization behavior of sandwich-structure B-P/ 5 vol % Fe3O4@BN-P/B-P, B-P/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P composites is also analyzed in Figure 8c, 8e, and 8f. For instance, the electric displacement of composites were ∼8.3, ∼6.2, and ∼7.4 μC/cm2, respectively, corresponding to B-P/5 vol % Fe3O4@BN-P/B-P, B-P/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P composites at the same electric field of 290 kV/mm. Comparing with the B-P/5 vol % BN-P/ B-P composite, the electric displacement of B-P/5 vol % Fe3O4@BN-P/B-P composite was effectively improved by introduction of a small amount of semiconductive Fe3O4 onto the surface of BNNSs. Moreover, the much higher electric displacement of B-P/5 vol % Fe3O4@BN-P/B-P composite than that of P/5 vol % Fe3O4@BN-P/P composite stems from addition of BZT-BCT nanofibers with high polarization and dielectric permittivity in the outer layer. Similarly, the applied electric field was also an important reference. According to the previous section in the Weibull distribution of the breakdown strength for composites, it is of interest to note that introduction of BZT-BCT nanofibers in the outer layer and a small amount of semiconductive Fe3O4 particles in the middle layer collectively influenced the electric field resistance of materials and subsequently affected the electric displacement of composite. Energy Storage Performance of the PVDF-Based Composites. The energy storage density was originated from the D−E hysteresis loops and calculated by the formula of Ue = ∫ E·dD, where Ue was the discharged energy density here. Figure 9a delivers the results of the energy density and efficiency for the sandwich-structure B-P/x vol % Fe3O4@BNP/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites at 10 Hz under different applied electric fields. As can be seen from Figure 9a, as the volume fraction of Fe3O4@BNNSs increased, the Ue of the composites also increased at the same applied electric field. For instance, Ue reached up to ∼6.7, ∼7.6, and ∼8.9 J/cm3, respectively, corresponding to x = 1, 3, and 5 vol % at the same electric field of 350 kV/mm and ∼7.1 J/cm3 at 290 kV/mm for B-P/7 vol % Fe3O4@BN-P/B-P composite at a frequency of 10 Hz. Unfortunately, the great tolerance for the electric field was substantially compromised by the increasing content of inorganic fillers, which could have an adverse impact on the improvement of energy storage density. Consequently, the value of Ue was closely related to the displacement and the electric field. Fundamentally, the polarization could be

Figure 9. Relationship of the energy density and efficiency with the electric field for the sandwich-structure B-P/x vol % Fe3O4@BN-P/BP (x = 1, 3, 5, and 7 vol % of hybrid particles), B-P/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P composites.

strengthened by the hybrid particles of Fe3O4@BNNSs; meanwhile, the electric field was maintained below Eb with appropriate inorganic fillers. Thus, the B-P/5 vol % Fe3O4@ BN-P/B-P composite possessed excellent discharged energy density among these composites. Subsequently, the energy efficiency (η), calculated by η = Ue/ U × 100%, where U is the charged energy density, for B-P/x vol % Fe3O4@BN-P/B-P (x = 1, 3, 5, and 7 vol % of hybrid particles) composites was higher than 0.4 (η > 40%), and all of the η of composites monotonically decreased with increasing electric field. Interestingly, the value η gradually increased when the content of hybrid Fe3O4@BNNSs increased from 1 to 7 vol %, which might be due to the Dr being suppressed by hybrid Fe3O4@BNNSs. Furthermore, the relationships of Ue and η with the electric field for B-P/5 vol % Fe3O4@BN-P/B-P, B-P/5 vol % BN-P/BP, and P/5 vol % Fe3O4@BN-P/P composites are given in Figure 9b, which gives maximum Ue of ∼8.9 J/cm3 at 350 kV/ mm, ∼6.4 J/cm3 at 350 kV/mm, and ∼5.1 J/cm3 at 290 kV/ mm at 10 Hz, respectively, corresponding to B-P/5 vol % Fe3O4@BN-P/B-P, B-P/5 vol % BN-P/B-P, and P/5 vol % Fe3O4@BN-P/P composites. The variation of Ue with different fillers is consistent with the variation of the polarization behavior in the previous section. Thus, we came to the conclusion that the variation of Ue is primarily associated with a small amount of semiconductive Fe3O4 particles and the high dielectric performance of BZT-BCT nanofibers in the polymer matrix. Meanwhile, the difference of η for this series of composites was not obvious, which is nearly consistent with the variation of the intrinsic leakage current of these composites. 1509


Article


Table 4. Comparison of Dielectric Permittivity (εr), Electric Field Strength (E), and Discharged Energy Density (Ue) between This Work and Other Polymer-Based Dielectrics Published in Recent Years samples

εr (@100 Hz)

E (kV/mm)

Ue (J/cm3)

yearref

2.1 vol % BCZT@PATP NFs/PVDF 7.5 vol % F-TiO2/P(VDF-HFP) 2.5 vol % BaTiO3@SiO2 NF/PVDF 5 vol % BaTiO3@PMPCS/P(VDF-HFP) BT1%-PVDF-BT1% 5 vol % SrTiO3 NP/PVDF B-P/5 vol % Fe3O4@BN-P/B-P

12 12 13 20 10 10 17

380 160 330 300 390 270 350

8.2 1.4 6.3 7.5 7.0 5.1 8.9

201723 201730 201550 201751 201752 201753 this work

Films and Integrated Devices (KFJJ201601), the Youth Innovative Talents Training Plan of Ordinary Undergraduate Colleges in Heilongjiang (UNPYSCT-2016157), and Science Funds for the Young Innovative Talents of HUST (201102).

Table 4 shows a comparison of the dielectric permittivity (εr), electric field strength (E), and discharged energy density (Ue) between this work and other polymer-based dielectrics published in previous publications.23,30,50−53 The discharged energy density of previously reported composites could not reach 8.9 J/cm3 of the B-P/5 vol % Fe3O4@BN-P/B-P composite. In brief, the B-P/5 vol % Fe3O4@BN-P/B-P sandwichstructured composite had a good energy storage performance due to the high dipole displacement of inorganic fillers and excellent interface compatibility by the polydopamine modification and also because of the enhanced breakdown strength and the suppressed remnant displacement.

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(1) Chu, B.; Zhou, X.; Ren, K.; Neese, B.; Lin, M.; Wang, Q.; Bauer, F.; Zhang, Q. M. A Dielectric Polymer with High Electric Energy Density and Fast Discharge Speed. Science 2006, 313, 334−336. (2) Khanchaitit, P.; Han, K.; Gadinski, M. R.; Li, Q.; Wang, Q. Ferroelectric Polymer Networks with High Energy Density and Improved Discharged Efficiency for Dielectric Energy Storage. Nat. Commun. 2013, 4, 3845. (3) Niu, Y.; Bai, Y.; Yu, K.; Wang, Y.; Xiang, F.; Wang, H. Effect of the Modifier Structure on the Performance of Barium Titanate/ Poly(vinylidene fluoride) Nanocomposites for Energy Storage Applications. ACS Appl. Mater. Interfaces 2015, 7, 24168−24176. (4) Yao, L.; Wang, D.; Hu, P.; Han, B. Z.; Dang, Z. M. Synergetic Enhancement of Permittivity and Breakdown Strength in AllPolymeric Dielectrics toward Flexible Energy Storage Devices. Adv. Mater. Interfaces 2016, 3, 1600016. (5) Hu, P.; Wang, J.; Shen, Y.; Guan, Y.; Lin, Y.; Nan, C. W. Highly Enhanced Energy Density Induced by Hetero-Interface in SandwichStructured Polymer Nanocomposites. J. Mater. Chem. A 2013, 1, 12321−12326. (6) Li, Q.; Han, K.; Gadinski, M. R.; Zhang, G.; Wang, Q. High Energy and Power Density Capacitors from Solution-Processed Ternary Ferroelectric Polymer Nanocomposites. Adv. Mater. 2014, 26, 6244−6249. (7) Chi, Q.; Ma, T.; Zhang, Y.; Cui, Y.; Zhang, C.; Lin, J.; Wang, X.; Lei, Q. Significantly Enhanced Energy Storage Density for Poly(vinylidene fluoride) Composites by Induced PDA-Coated 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 Nanofibers. J. Mater. Chem. A 2017, 5, 16757−16766. (8) Jiang, J.; Zhang, X.; Dan, Z.; Ma, J.; Lin, Y.; Li, M.; Nan, C. W.; Shen, Y. Tuning Phase Composition of Polymer Nanocomposites towards High Energy Density and High Discharge Efficiency by NonEquilibrium Processing. ACS Appl. Mater. Interfaces 2017, 9, 29717− 29731. (9) Zhang, Y.; Chi, Q.; Liu, L.; Zhang, C.; Chen, C.; Wang, X.; Lei, Q. Enhanced Electric Polarization and Breakdown Strength in the AllOrganic Sandwich-Structured Poly(vinylidene fluoride)-Based Dielectric Film for High Energy Density Capacitor. APL Mater. 2017, 5, 076109. (10) Wang, Y.; Cui, J.; Yuan, Q.; Niu, Y.; Bai, Y.; Wang, H. Significantly Enhanced Breakdown Strength and Energy Density in Sandwich-Structured Barium Titanate/Poly(vinylidene fluoride) Nanocomposites. Adv. Mater. 2015, 27, 6658−6663. (11) Han, K.; Li, Q.; Chanthad, C.; Gadinski, M. R.; Zhang, G.; Wang, Q. A Hybrid Material Approach Toward Solution Processable Dielectrics Exhibiting Enhanced Breakdown Strength and High Energy Density. Adv. Funct. Mater. 2015, 25, 3505−3513. (12) Luo, S.; Shen, Y.; Yu, S.; Wan, Y.; Liao, W. H.; Sun, R.; Wong, C. P. Construction of a 3D-BaTiO3 Network Leading to Significantly

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CONCLUSION In summary, the sandwich-structured BZT-BCT NFs-PVDF/ Fe3O4@BNNSs-PVDF/BZT-BCT NFs-PVDF composites were prepared by solution-casting, thermal-quenching, and hot-pressing processes. The aim of BZT-BCT NFs in the outer layer was first to enhance the dielectric properties for composite due to the high electric displacement of BZT-BCT ceramic and second to strengthen the dielectric strength of the composite by the large aspect ratio of the nanofibers. Meanwhile, the purpose of hybrid Fe3O4@BNNSs in the middle layer was to reduce the hysteresis loss and further reinforce the breakdown strength of the composite, and interestingly, the small amount of semiconductive Fe3O4 particles homogeneously distributed onto the surface of BNNSs were conducive to enhancing the dielectric polarization and permittivity. Consequently, a large dielectric permittivity of ∼17 at 100 Hz coupled with the excellent discharged energy density of ∼8.9 J/cm3 at 350 kV/ mm for sandwich-structured PVDF-based composite was achieved. This simple and stable preparation process makes the polymer-based dielectrics promising candidates for flexible and compact energy storage devices with enhanced overall performance.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax/Tel.: +86 451 86391681. *E-mail: [email protected]. ORCID

Qingguo Chi: 0000-0001-6923-337X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the National Natural Science Foundation of China (61640019), the Open Foundation of State Key Laboratory of Electronic Thin 1510


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