Sandwich-Structured PVDF-Based Composite Incorporated with

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Sandwich-Structured PVDF-Based Composite Incorporated with Hybrid FeO@BN Nanosheets for Excellent Dielectric Properties and Energy Storage Performance 3

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Yue Zhang, Tiandong Zhang, Lizhu Liu, Qingguo Chi, Changhai Zhang, Qingguo Chen, Yang Cui, Xuan Wang, and Qingquan Lei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10838 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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The Journal of Physical Chemistry

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, Harbin University of Science and Technology, 52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China ‡

School of Materials Science and Engineering, Harbin University of Science and

Technology, 52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. China §

School of Electrical and Electronic Engineering, Harbin University of Science and

Technology, 52 Xuefu Road, Harbin, Heilongjiang 150080, P. R. 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, P. R. 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 the 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 1

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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)O3-0.5(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 the 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, 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. 

INTRODUCTION

Currently, numerous experimental and theoretical efforts are focus on the high2


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performance and light-weight polymer-based energy storage dielectrics with low-cost and easy process for the 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 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 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, 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 the composite with an outstanding comprehensive performance. Herein, the polymer material serves as the supporting matrix for the dielectric strength; meanwhile, the ceramic filler particles work as the candidate of polarized dipoles to enhance the polarization. Moreover, among the high-performance polymers currently explored as dielectric capacitor, the PVDF and its co-polymers based ferroelectric polymers are expected to 3



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 general four different polymorphs for PVDF, for instance, the paraelectric α-phase (trans-gauche conformation, non-polar), the ferroelectric β-phase (all-trans conformation, polar), the γ-phase (trans-trans-transgauche 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 owns 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 γphase PVDF could work normally under the 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 γ phase can be obtained from β phase transforming by quenching process,15,18 which lights an inspiration for preparing PVDF-based dielectrics. Besides, all sorts of ferroelectric oxides with the high εr (up to hundreds or thousands) have been chosen to raise the value of εr.19-21 Such as, Wang et al. induced the barium titanate (BaTiO3, BT) nanoparticles with perovskite-structure into the PVDF matrix to gain the composite with a high dielectric polarization and a low remnant polarization under an electric field.10 And 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 one4


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dimensional (1D) 0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Zr0.2Ti0.8)O3 nanofibers (BCZT NFs) on the dielectric performances of PVDF-based composite, which proved that the BCZT NFs with large aspect ratio significantly improved the dielectric and energy storage properties because of the 1D nanofibers with large dipole moments.23 Notably, for the improvement of the energy density, the two-dimensional (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 But 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 the 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 the 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 agglomeration is to surface-modify inorganic particles, as mentioned in previous literatures, which can improve the compatibility and 5



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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 BT-PDA-Ag by dopamine (PDA) modification to improve the interface compatibility and to enhance the breakdown strength of dielectric composite.33 And, Chi et al. used the PDA as modifier to improve the dispersibility of fillers in 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. Based on the aforementioned literatures, we prepared the BZT-BCT NFsPVDF/Fe3O4@BNNSs-PVDF/BZT-BCT NFs-PVDF composite with

sandwich

structure by solution-casting, thermal quenching and hot-pressing process in this work. At first, based on the reported literatures, the Fe3O4 nanoparticle as an excellent filler was conducive to the polymer composite with a good dielectric constant.34-37 Because, a small amount of the Fe3O4 nanoparticles used as the second fillers can remarkably improve the dielectric property of 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 performances of dielectric composite. Therefore, the 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 6


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(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 semi-conductive Fe3O4 particles well distributes 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 the constructed hybrid Fe3O4@BNNSs not only can reinforce the dielectric strength but avoid the decrease of the dielectric permittivity of middle layer, which results in the 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 gain a successful design of synergistic inorganic-organic composites, which may be applicable to the requirement of excellent energy storage properties. 

EXPERIMENTAL 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 7



provided by 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. And all the chemicals were analytical grade and used as received without further purification. Preparation of PDA@BZT-BCT nanofibers. The BZT-BCT nanofibers were prepared by the sol-gel and electrospinning method, which can be seen in the Figure 1(a).7,23 First of all, the Ba(OH)2·8H2O (2.17 g) powders were dissolved in the liquid 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 above mixture solution. After that, Ti(OC4H9)4 (2.5 ml) was dissolved to 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. Afterwards, 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 aircalcined at 700 °C for 3 h to gain the perovskite crystal structure of BZT-BCT NFs with different aspect ratio. The PDA was coated on the surface of nanofibers to achieve the BZT-BCT NFs with the excellent dispersion and compatibility.31 Tris-HCl buffer agent was firstly dissolved 8


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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. Secondly, the BZT-BCT NFs were immersed into above PDA solution and vigorously stirred at ambient temperature for 12 h. Followed by, the BZT-BCT NFs modified by PDA was gained by 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 the hydrothermal synthesis, which can be referred to in the Figure 1(b).38,39 First of all, the two-dimensional nanosheets h-BN powders were produced by liquid exfoliation according previous work. Briefly, 2 g h-BN powders were dispersed in 200 mL deionized water and subjected to tip-type sonication (360 W, 1200 W×30%) for 24 h. After the 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. Followed by the precursor solution A was gained as follows. First, the 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 gain solution B (Sol B), and the BNNSs was 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-liner, and then the autoclave was kept at the temperature of 100 °C for 24 h. Then the reactor was cooled to ambient 9



temperature, finally, the Fe3O4@BN hybrid particles were obtained by centrifugation and washing with distilled water to the neutral pH, and drying under vacuum overnight at 80 °C. Further, the method of surface modification of Fe3O4@BN by PDA was similar with the preparation of PDA@BZT-BCT nanofibers. Therefore, the preparation method of PDA@Fe3O4@BN hybrid particles can refer to above preparation of PDA@BZT-BCT nanofibers.

Figure 1. The Schematic illustrations for the preparation and configurations of layer-by-layer casting for the sandwich nano-composite 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. The XRD patterns (f) and TEM graphs (g) for inorganic filling phase BNNSs (g1) and the hybrid particles of Fe3O4@BNNSs (g2). 10


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Preparation of PDA@BZT-BCT NFs/PVDF and PDA@Fe3O4@BNNSs/PVDF composites. In the conventional process, as shown in the Figure 1(c), the PDA@BZTBCT NFs and PDA@Fe3O4@BNNSs were dispersed in the DMF solution with different volume contents (1 vol.%, 3 vol.%, 5 vol.% and 7 vol.%) and stirred vigorously, respectively. Followed by a certain amount of PVDF powders dissolved in above suspension solution and continuously stirred for 24 h to form the homogeneous suspension mixture. After that, the suspension mixture was slowly dropped onto the clean glass substrate using 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 the composites with the thickness of ~10 μm. Preparation of sandwich-structured composite. The sandwich-structured dielectric composite was fabricated by hot-pressing with 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 the Figure 1(d) and (e). 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@BNNSs-PVDF/3 vol.% PDA@BZT-BCT NFs-PVDF (x=1, 3, 5 and 7 vol.% of hybrid particles) (hereafter, B-P/x vol.% Fe3O4@BN-P/B-P in short) composite was fabricated with the thickness of ~ 28 μm. Characterization. The X-ray diffraction (XRD) analysis were characterized by a 11



PANalytical Empyrean, which is used to analyze the crystalline structures of BN, Fe3O4 and the PVDF-based composites. Field emission scanning electron microscope (FESEM) tests were performed with Hitachi SU8020 Uhr by which the microstructure of fillers and composites was examined. Transmission electron microscope (TEM) images were carried out through a FEI TECNAI2-12 to gain the dispersion of Fe3O4 on the surface of BNNSs. A thin layer of Aluminum electrodes (diameter of 3 mm and 25 mm) were evaporated on both sides of films for measurements. Dielectric properties were collected in the frequency range of 100 Hz-1 MHz by a broadband impedance analyzer of GmbH Novocontrol Alpha-A. Dielectric breakdown strength tests were obtained under a precision power supply test equipment YDZ-560. Electric displacementelectric field (D-E) loops and current-voltage (I-V) tests were conducted by using a Radiant Premier II Ferroelectric Test System at frequency of 10 Hz. For each electrical performance of composites, several same samples were collected to ensure the repeatability of the data. 

RESULTS AND DISCUSSION Crystalline structure and morphology of inorganic fillers and PVDF-based

composite. The Fe3O4@BNNSs hybrid particles were synthesized via the hydrothermal reaction. The crystalline structure of hybrid particles were characterized by XRD and presented in Figure 1(f). As shown in Figure 1(f), 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°assigning to the (002), (100), (101), (102), (004), (110) and (112) diffraction planes with reference data (PDF Number: 00-034-0421), respectively. The semi-conductive particles of Fe3O4 12


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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 Number: 00-011-0614) in Figure 1(f). At the same time, the Figure 1(g) presented the TEM graphs of inorganic filler BNNSs and the hybrid particles of Fe3O4@BNNSs. It was shown in Figure 1(g) that the diameter of BNNSs was about 200 nm, and the Fe3O4 particles with the diameter of about 20 nm uniformly grew and dispersed on the surface of the BNNSs particles, which was beneficial for improving 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. And the XRD patterns of the sandwich-structure composites compounded with inorganic particles and polymer were presented in Figure 2. The Figure 2(a) showed 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 semi-conductive particles of Fe3O4. Obviously, there were three major phases: the non-polar γ and α, and the polar β in the matrix of PVDF seen from the inset of Figure 2(a1). It was notable that the diffraction peaks at 2θ=18.5°, 20.1° and 26.8° 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 13



Figure 2. The 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), B-P/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 °.

Meanwhile, the diffraction peak of non-polar α(100) appeared at 2θ=17.94°, and diffraction peak of polar β(200) appears at 2θ=20.7°. Moreover, typical perovskite structure of BZT-BCT appeared 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, typical peak of hexagonal BN was observed at 2θ=26.8° assigning to (002) diffraction plane. Accordingly, the characteristic peak of semi-conductive Fe3O4 corresponds to (311) diffraction plane at 2θ=35.5°. And it was found that the characteristic peak positions of polymer matrix and inorganic fillers, which were shown in Figure 2(b) of

14


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XRD patterns for the sandwich-structure B-P/5 vol.% BN-P/B-P and P/5 vol.% Fe3O4@BN-P/P composites, were very similar with peak positions demonstrated in Figure 2(a). Notably, the perovskite structure of BZT-BCT nanofibers were 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 XRD patterns of P/5 vol.% Fe3O4@BN-P/P composites.

Figure 3. The 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, (d) x=7.

Meanwhile, the SEM images of cross-sectional morphology for the sandwichstructured 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 28 μm, and the thickness ratio of the outer layer (PDA@BZTBCT NFs/PVDF) and the middle layer (PDA@Fe3O4@BNNSs/PVDF) for these composites was about 1:2. Obviously, there were almost no macro-structural

15



imperfections, such as chasms and holes, between the three layers. Meanwhile, the BZT-BCT NFs in outer layers and the Fe3O4@BNNSs in middle layers were homogeneously dispersed in PVDF and were satisfactorily compatible with the polymer matrix due to the modifier of PDA, which was in favor of establishing a good performance for composites. In additional, the slight aggregations gradually appeared in the middle layer with the increasing volume fractions of Fe3O4@BNNSs, which had an adverse influence on properties of the PVDF-based composites. Dielectric properties of the PVDF-based composites. For dielectric properties analysis at ambient temperature, the frequency dependence of 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 were illustrated in Figure 4(a).

Figure 4. The relationship between dielectric properties and the frequency for the sandwichstructure B-P/x vol.% Fe3O4@BN-P/B-P (x=1, 3, 5 and 7 vol.% of hybrid particles) (a), B-P/5 vol.% BN-P/B-P and P/5 vol.% Fe3O4@BN-P/P (b) composites (a1) and (b1) dielectric permittivity, (a2) and (b2) dielectric loss. The 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.

Clearly, the dielectric permittivity of the B-P/x vol.% Fe3O4@BN-P/B-P (x=1, 3, 5 16


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and 7 vol.% of hybrid particles) composite increased first and then decreased with the increasing content of hybrid particles Fe3O4@BNNSs over the whole frequency range from 100 Hz to 1 MHz (Figure 4(a1)), which was attributed to the hybrid particles of Fe3O4@BNNSs. That might be because the dielectric polarization of hybrid particles had been enhanced by the introduction of a small amount semi-conductive 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 semiconductor-insulator structure 41-42 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 were shown in Figure 5.43,44 The enhanced dielectric permittivity could not be attributed solely to the increase in the area of interfaces. Besides, the confinement of interfaces within the PDA shell, as shown in Figure 5(a), 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

17



Figure 5. Schematic illustrations of composites filled with Fe3O4@BNNSs hybrid. The distribution of dielectric permittivity was superimposed in different regions of the composite. The abrupt increase of dielectric permittivity indicated the enhanced interfacial polarization in the interfacial regions. The interfacial region in (c) is magnified to show the diffuse electronic layer of semiconductor-insulator structure.

Meanwhile, the dielectric loss (Figure 4(a2)) of composites was less than 0.23 over the whole frequency range. And the dielectric loss increased with the increasing frequency due to the gradual generation of relaxation resulting from that 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 to 7 vol.%, the smaller dielectric constant and accumulated space charge induced by agglomerations and voids leaded to a decline of dielectric constant and a slightly increase of dielectric loss. So the dielectric permittivity decreased from the maximum of ~17.0 for B-P/5 vol.% Fe3O4@BN-P/B-P to the value of ~16.3 for B-P/7 vol.% Fe3O4@BN-P/B-P at 100 Hz.

18


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Furthermore, the frequency dependence of 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@BNP/P composites was investigated in Figure 4(b). 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 were 13.6 and 14.7 at 100 Hz (Figure 4(b1)), respectively, both of which were lower than that of B-P/5 vol.% Fe3O4@BN-P/B-P at the same frequency. In addition, as shown in Figure 4(b2), 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. And the dielectric loss also increased with the 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 the introduction of a little bit semi-conductive 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 dielectric property of 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 the introduction of 3 vol.% BZT-BCT nanofibers in the outer layer of composite could improve the dielectric polarization and permittivity of B-P/5 vol.% Fe3O4@BN-P/B-P composite, which was consistent with the experimental results we have reported previously.7 It was mainly because the BZTBCT ceramic itself not only had a high dielectric polarization, but also possessed the one-dimensional fiber structure to further enhance the directional polarization of composite.23 19



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 leakage current for the sandwich-structure B-P/x vol.% Fe3O4@BNP/B-P (x=1, 3, 5 and 7 vol.% of hybrid particles) composites was presented in Figure 4(c1). As can be seen from Figure 4(c1), the leakage current of B-P/x vol.% Fe3O4@BNP/B-P (x=1, 3, 5 and 7 vol.% of hybrid particles) composite increased with the increasing content of Fe3O4@BNNSs in 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. Especially, the leakage current of composite with the 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 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 was shown in Figure 4(c2), which demonstrated that there was a small difference in leakage current for these three composites. This results indicated that the effect of a small amount of BZT-BCT nanofibers (content of 3 vol.%), which compound with PVDF as the outer layer of sandwich structure composite, on the leakage current of the composite was not significant. Similarly, the influence of tiny amounts of Fe3O4 particles dispersed on surface of BNNSs on the leakage current was almost negligible. It was also important that the PDA acted as modifiers and binders mitigated the performance difference between the inorganic filler and organic matrix. Accordingly, 20


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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=ʃ E·dD for nonlinear dielectric material, we know that both the dielectric polarization (D) and the breakdown strength (E) are the key parameters to gain the energy density for the dielectric material. At the same time, a series of performances of dielectric composites were limited by the low breakdown strength, so that the breakdown strength of composites were 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) was cumulative probability of electric field failure, E was the experimental breakdown strength, and Eb was a scale parameter associated with breakdown strength when the cumulative failure probability was 63.5%, and β was a shape parameter referring to the linear regressive fit of this distribution. Both sides of the above formula were undergone the logarithmic operations, a new equivalence formula could be written as followed: ln(-ln(1-P(E)))=βlnE-βlnEb, then the ln(-ln(1P(E))) versus lnE was obtained and plotted in Figure 6. The results of liner fitting and Weibull parameters were shown in Table 1. Apparently, the Figure 6(a) exhibited the results of Weibull distribution of Eb for the sandwich-structure B-P/x vol.% Fe3O4@BN-P/B-P (x=1, 3, 5 and 7 vol.% of hybrid particles) composites. It was shown in Figure 6(a) that all 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 21



(~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 Eb monotonically decreased with the increasing fillers content. And the Eb obviously decreased by almost 60 kV/mm with the increase of content of Fe3O4@BNNSs from 5 vol.% to 7 vol.%, which may indicate that the Eb of composite can be deteriorated when the higher doping content of hybrid fillers of Fe3O4@BNNSs introduced into the matrix. Moreover, the introduction of Fe3O4@BNNSs could induce the concentration of local electric field in interfacial region of polymer matrix and the inorganic fillers owing to the dielectric difference. The networks eventually formed after the concentrated local electric field gradually percolated, so the Eb of 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 content of hybrid Fe3O4@BNNSs on electric field strength shown in Figure 6(b). For the simulation, the dielectric permittivity of Fe3O4, two-dimensional BNNSs, one-dimensional BZT-BCT NFs and PVDF is around 20, 4, 3000 and 9, respectively. The electric field of ~200 kV/mm was applied on the models from the top to bottom. Obviously, the low electric field was distributed in outer layers and the high electric field was mainly concentrated in middle layer, which always happened in the sandwich structure composites because of the different functional properties of the introduction of BZT-BCT NFs in outer layers and the Fe3O4@BNNSs in middle layer. In addition, the local electric field was near the ends of the hybrid Fe3O4@BNNSs attributed to the large dielectric difference 22


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between the hybrid particles and the PVDF polymer. Meanwhile, much more obviously the distortion of 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 in contact with each other, which deteriorated the Eb of composite. This evidenced that the Eb decreased with the volume fraction of Fe3O4@BNNSs increasing.

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

Seen from the Table 1, the correlation coefficient R means the degree of obedience

23



Page 24 of 53

to the Weibull distribution. The closer to 1 the 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 the good fitting results. Meanwhile, the standard error S was calculated by least square 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, the less scattering. In this work, a great value β indicated that the homogeneous composites were achieved by the compounding process of solution-casting and hot-pressing. 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 Samples 1% 3% 5%

19.69 17.76 15.35

ln(-ln(1-P(E)) intercept -117.97 -105.85 -90.84

7%

12.30

-70.56

slope

Weibull parameters S

0.93 0.93 0.97

1.64 1.75 0.96

19.69 17.76 15.35

E0 (kV/mm) 399.96 387.62 371.64

0.95

0.90

12.30

310.00

R

β

Naturally, the Weibull distribution of breakdown strength 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 were measured and analyzed in Figure 7. As shown in Figure 7(a), 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). 24


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Figure 7. Weibull distribution of breakdown strength (a) and finite element simulation of macroscopic equipotential line (b) for the sandwich-structure 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.

The strikingly enhanced breakdown strength of B-P/5 vol.% Fe3O4@BN-P/B-P composite might be ascribed to the following three reasons. Firstly, to the best of our knowledge, the introduction of BZT-BCT nanofibers in outer layer of composite could suppress the formation of 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 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. Afterwards, comparing with the B-P/5 vol.% 25



Page 26 of 53

BN-P/B-P composite, we found that the slightly improved breakdown strength of BP/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 bandgap BNNSs were introduced by inorganic nano-semi-conductive particles, which could be easier to capture charges. And the mobile charges were constrained by traps, thus the conductive path was avoided forming during breakdown.47,48 Therefore, a small amount of semi-conductive particles, which homogeneously grew on the surface of BNNSs and evenly dispersed in the matrix, could be benefit for enhancing the breakdown. Besides, 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 was illustrated in Figure 7(b). For the simulation, the dielectric permittivity of monolayer Fe3O4@BNNSs-PVDF (Fe3O4@BN-P), BNNSs-PVDF (BN-P), BZT-BCT NFs-PVDF (B-P) and PVDF (P) is around 16, 7, 13 and 9, respectively. The electric voltage of ~6000 V was applied on the models from the top to 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 capacitor connected in series, so the local electric field of each layer could be obtained by the formula as follow:9,49 E1 

E2 

V

(1)

 2 d1  1 d 2 2 V

(2)

 d 2  2 2 d1 1 26


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E1 and E2 were the local electric field of outer layer and middle layer, respectively. And V=6000 V was the applied voltage on the composite, and d1=7.0 μm and d2=14.0 μm were the thickness of outer layer and middle layer, respectively. And the ε1 and ε2 were the dielectric permittivity of outer layer and middle layer. According to the calculation formula, the local electric field of composite was shown in Table 2. Table 2. Local electric field of every layer calculated by the capacitive voltage divider Composites

ε1

ε2

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

E1 (kV/mm) 236 150 274

E2 (kV/mm) 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 nano-particles, so that the breakdown strength of B-P/5 vol.% Fe3O4@BN-P/B-P composite was relatively higher than other composites. For the BP/5 vol.% BN-P/B-P composite, the middle layer of BN-P had a greater redistributed local electric field with sparser equipotential lines than outer layers resulting from the much more dielectric difference which decreased the dielectric strength of 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 outer and middle layers was harmful to the breakdown strength. These two above reasons lead the breakdown 27



Page 28 of 53

strength of the P/5 vol.% Fe3O4@BN-P/P composite decreases rapidly. At the same time, the Table 3 gave 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 was clearly shown that the results well followed the Weibull distribution due to the R closer to 1, and the high quality composites were obtained, which was inferred from a great value β and a small error S, referring to the analyzation about Table 1. 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 Samples

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

15.35 14.89

ln(-ln(1-P(E)) intercept -90.84 -87.41

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

12.91

-74.28

slope

S

Weibull parameters

0.97 0.98

0.96 0.70

15.35 14.89

E0 (kV/mm) 371.64 354.38

0.95

1.03

12.91

315.35

β

R

Polarization behavior of the PVDF-based composites. Herein, the polarization behavior depending on the external electric field for the composites was measured and analyzed. Figure 8(a-d) exhibited the displacement-electric field (D-E) 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 the Figure 8(a-d), the 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 μC/cm2, ~7.6 μC/cm2, ~8.3 μC/cm2 and ~9.1 μC/cm2, respectively, corresponding to x=1, 3, 5 and 7 vol.% composites at the same electric field of 290 28


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kV/mm. Obviously, the hybrid particles of Fe3O4@BNNSs could enhance the inner displacement of composites under the electric field, because a little bit of Fe3O4 particles grown on the surface of BNNSs contributed to improve the polarization of dipoles.

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.

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 more different breakdown strength of composite. Such as, the maximum displacement was ~8.9 μC/cm2, ~9.7 μC/cm2 and ~10.4 μC/cm2, respectively, 29



corresponding to x=1, 3 and 5 vol.% composites at 350 kV/mm and ~9.1 μC/cm2 at 290 kV/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 content of Fe3O4@BNNSs was above 7 vol.%, which was consistent with the measured results of breakdown strength for composite. This might be caused by the imperfections induced by the large amount of inorganic fillers in the middle layer of 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 adding.14 As well known, the smaller value Dr was, the lower hysteresis loss was. That was in favor for the enhancement of energy density and efficiency. Besides, the polarization behavior of sandwich-structure B-P/5 vol.% Fe3O4@BNP/B-P, B-P/5 vol.% BN-P/B-P and P/5 vol.% Fe3O4@BN-P/P composites were also analyzed in Figure 8(c)(e)(f). For instance, the electric displacement of composites were ~8.3 μC/cm2, ~6.2 μC/cm2 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 the introduction of a little bit semi-conductive Fe3O4 onto the 30


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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 was stemming from the adding BZT-BCT nanofibers with high polarization and dielectric permittivity in outer layer. Similarly, the applied electric field was also an important reference. According to the previous section in the Weibull distribution of breakdown strength for composites, it was of interest to note that the introduction of BZT-BCT nanofibers in outer layer and a small amount of semi-conductive Fe3O4 particles in middle layer collectively influenced the electric field resistance of materials, and subsequently affected the electric displacement of composite. Energy storage performances of the PVDF-based composites. The energy storage density was originated from the D-E hysteresis loops and calculated by the formula of Ue=ʃ E·dD, where Ue was the discharged energy density here. Brightly, Figure 9a delivered the results of the energy density and efficiency for the sandwich-structure BP/x vol.% Fe3O4@BN-P/B-P (x=1, 3, 5 and 7 vol.% of hybrid particles) composites at 10 Hz under the different applied electric field. As can be seen from the Figure 9(a), as the volume fraction of Fe3O4@BNNSs increased, the Ue of composites also increased at the same applied electric field. For instance, the Ue reached up to ~6.7 J/cm3, ~7.6 J/cm3 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 the 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 31



adverse impact on the improvement of energy storage density. Consequently, the value Ue was closely related to the displacement and the electric field. Fundamentally, the polarization could be strengthened by the hybrid particles of Fe3O4@BNNSs; meanwhile, the electric field was maintained below the Eb with appropriate inorganic fillers. So the B-P/5 vol.% Fe3O4@BN-P/B-P composite possessed the excellent discharged energy density among these composites.

Figure 9. The relationship of the energy density and efficiency with the 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), BP/5 vol.% BN-P/B-P and P/5 vol.% Fe3O4@BN-P/P composites.

Subsequently, the energy efficiency (η), which was calculated by η=Ue/U×100% where U meant 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 the η of composites monotonically decreased with the electric field increasing. Interestingly, 32


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the value η gradually increased when the content of hybrid Fe3O4@BNNSs increased from 1 vol.% to 7 vol.%, which might be due to the Dr suppressed by hybrid Fe3O4@BNNSs. Furthermore, the relationships of the Ue and η with the electric field 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 were drawn in Figure 9(b), which gave that the maximum Ue was ~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 was consistent with the variation of polarization behavior in previous section. Thus, we came to this conclusion that the variation of Ue was primarily associated with a small amount of semi-conductive Fe3O4 particles and the high dielectric performance of BZT-BCT nanofibers in polymer matrix. Meanwhile, the difference of η for this series of composites was not obvious, which was nearly consistent with the variation of intrinsic leakage current of these composites. 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 εr (@100Hz)

E (kV/mm)

Ue (J/cm3)

2.1 vol % BCZT@PATP NFs/PVDF

12

380

8.2

201723

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 13 20 10 10 17

160 330 300 390 270 350

1.4 6.3 7.5 7.0 5.1 8.9

201730 201550 201751 201752 201753 this work

Samples

Ref.

Table 4 showed the comparison of dielectric permittivity (εr), electric field strength 33



(E) and the 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 up to the 8.9 J/cm3 of B-P/5 vol.% Fe3O4@BN-P/B-P composite. In brief, the B-P/5 vol.% Fe3O4@BN-P/B-P sandwich-structured composite had a good energy storage performance due to the high dipole displacement of inorganic fillers and the excellent interface compatibility by the polydopamine modification, and also because of the enhanced breakdown strength and the suppressed remnant displacement. 

CONCLUSION

In summary, the sandwich-structured BZT-BCT NFs-PVDF/Fe3O4@BNNSsPVDF/BZT-BCT NFs-PVDF composites were prepared by solution-casting, thermal quenching and hot-pressing process. The aim of BZT-BCT NFs in the outer layer were firstly to enhance the dielectric properties for composite due to the high electric displacement of BZT-BCT ceramic, and secondly to strengthen the dielectric strength of composite by the large aspect ratio of nanofibers. Meanwhile, the purpose of hybrid Fe3O4@BNNSs in the middle layer were to reduce the hysteresis loss and further reinforce the breakdown strength of composite, and interestingly, the little bit semiconductive Fe3O4 particles homogeneously distributed onto the surface of BNNSs were conducive to enhancing the dielectric polarization and permittivity. Consequently, the great 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 34


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composite were achieved in this contribution. This simple and stable preparation process makes the polymer-based dielectrics promising candidates for flexible and compact energy storage devices with enhanced overall performance. 

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. Fax/Tel.: +86 451 86391681. *E-mail: [email protected]. ORCID Yue Zhang: 0000-0002-9325-3343 Qingguo Chi: 0000-0001-6923-337X Notes The authors declare no competing financial interest. 

ACKNOWLEDGEMENTS

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

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43



TOC Graphic

44


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Figure 1 107x95mm (300 x 300 DPI)



Figure 2 121x174mm (300 x 300 DPI)


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Figure 3 70x58mm (300 x 300 DPI)





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Figure 6 137x221mm (300 x 300 DPI)


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Figure 8 92x101mm (300 x 300 DPI)


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Figure 9 107x136mm (300 x 300 DPI)


Sandwich-Structured PVDF-Based Composite Incorporated with

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