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Poly(vinylidene fluoride) nanocomposites with a small loading of core-shell structured BaTiO3@Al2O3 nanofibers exhibiting high discharged energy density and efficiency
Accepted Manuscript Poly(vinylidene fluoride) nanocomposites with a small loading of core-shell structured BaTiO3@Al2O3 nanofibers exhibiting high discharged energy density and efficiency Shaohui Liu, Jiao Wang, Bo Shen, Jiwei Zhai, Haoshan Hao, Limin Zhao PII:
S0925-8388(16)33667-2
DOI:
10.1016/j.jallcom.2016.11.186
Reference:
JALCOM 39683
To appear in:
Journal of Alloys and Compounds
Received Date: 23 August 2016 Revised Date:
10 November 2016
Accepted Date: 13 November 2016
Please cite this article as: S. Liu, J. Wang, B. Shen, J. Zhai, H. Hao, L. Zhao, Poly(vinylidene fluoride) nanocomposites with a small loading of core-shell structured BaTiO3@Al2O3 nanofibers exhibiting high discharged energy density and efficiency, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.11.186. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Poly(Vinylidene Fluoride) nanocomposites with a small loading of
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core-shell structured BaTiO3@Al2O3 nanofibers exhibiting high
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discharged energy density and efficiency
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Shaohui Liu1, 2, Jiao Wang1, Bo Shen2, Jiwei Zhai2∗, Haoshan Hao1∗, Limin Zhao1 1
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School of Science, Henan Institute of Engineering, Zhengzhou 451191, China
Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials
Research Laboratory, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai
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201804, China
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Abstract: BaTiO3@Al2O3 core-shell nanofibers (BT@Al2O3 NF) have been
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successfully synthesized. Homogeneous nanocomposites consisting of BT@Al2O3 NF
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and a Poly(vinylidene fluoride) (PVDF) polymer matrix have been prepared by the
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solution casting method. A systematic study was investigated on the effect of
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BT@Al2O3 NF filler introduction on the discharged energy density performance of
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the nanocomposite. Al2O3 shell dramatically reduces the leakage current by prevent
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the contact between BT NF fillers in nanocomposites and minimize the
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Maxwell-Wagner-Sillars interfacial polarization, which results in the enhancement of
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the breakdown strength of nanocomposites films. Simultaneously, the nanocomposites
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have higher maximum polarization and lower the remnant polarization than that of
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PVDF films under the same electric field. The maximum discharged energy density of
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the nanocomposites with 2.5 vol.% BT@Al2O3 NF reaches 7.1J/cm3 at 3800kV/cm
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∗ ∗
Corresponding author E-mail address: [email protected]. Corresponding author E-mail address: [email protected]. 1
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with an efficiency of above 65.1%. This work may provide an effective solution for
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enhancing the discharged energy density of nanocomposites films.
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Keywords: discharged energy density; poly(vinylidene fluoride) nanocomposites;
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interfacial polarization; BaTiO3@Al2O3 nanofibers
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Introduction
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High discharged energy density nanocomposite dielectric capacitors have been the
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major enabler for a number of potential applications in advanced electronic and
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electric power applications, such as hybrid electric vehicles, electronic components
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and pulsed power sources[1-5]. Compared to batteries and super capacitors, dielectric
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capacitors have some advantages such as fast charge/discharge (<1 µs), high power
7
density and wide operating temperature range. Currently, the commercial applications
8
for energy storage are neat polymers, such as biaxial oriented polypropylene (BOPP),
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due to their excellent breakdown strength, ease formability and low manufacturing
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cost. Nevertheless, polymer film capacitors suffer from either a low dielectric
11
constant (<10) or an unacceptably low efficiency limiting their applications[6]. In
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addition, high discharged energy density and efficiency capacitors are highly
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desirable and have been motivated by reducing the size and cost of dielectric
14
capacitors.
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In the last decade, poly(vinylidene fluoride) (PVDF) have attracted much interest due
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to their excellent breakdown strength and relatively high dielectric constant[7]. To
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enhance the discharged energy density of the PVDF, an effective way is introducing
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high dielectric constant ceramic fillers, such as Pb(Zr,Ti)O3, Ba1-xSrxTiO3 and BaTiO3,
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into the polymer matrix. Tremendous achievements have been made in recent years
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on ehancement the energy storage capacity of PVDF [8, 9]. PVDF nanocomposites
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exhibit high discharged energy density. However, low discharged energy efficiency
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and high losses have prevented the large scale commercialization of capacitors based
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ACCEPTED MANUSCRIPT on PVDF nanocomposites. One major challenge in designing dielectric capacitors is
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to satisfy high discharged energy density while maintaining high discharging
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efficiency, even at very high applied electric fields.
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Low discharging efficiency of nanocomposites originates from interfacial polarization
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[10, 11]. Generally, introduction of the ceramic fillers with high dielectric constant
6
into the polymer matrix results in Maxwell−Wagner−Sillars (MWS) interfacial
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polarization due to the large difference of the dielectric constant between ceramic
8
fillers and polymer matrix in composite[8]. The long relaxation time is of MWS
9
polarization is due to the low bulk conductivity in each phase. These polarization
10
charges are trapped in the discharge process due to the long relaxation process of
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interfacial polarization. The key to improve the discharged energy density and
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discharged efficiency at high electric fields is suppression the interfacial polarization
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[12].
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To reduce the MWS interfacial polarization, filler with core-shell structure was
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introduced to overcome the problem. Su et al reported that enhanced energy density in
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PVDF nanocomposites are obtained by introduction BaTiO3@TiO2 core-shell fillers.
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Yu et al reported that core-shell structured BaTiO3/SiO2 nanoparticles /PVDF
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nanocomposites have been synthesized [13]. The results showed that the energy
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density of nanocomposites can be significantly improved by the incorporation of the
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SiO2 shell layer to reduce of energy loss. The reason of the greatly enhancement is
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that SiO2 layer coating on BT surface can suppress the leakage current and reduce the
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interfacial polarization [13]. Wu et al. reported that core-shell structure TiO2/carbon
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ACCEPTED MANUSCRIPT nanotubes composites were prepared and significantly increased dielectric constant
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and low loss were achieved [14]. These results show that the greatly enhancement of
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the energy density is attributed to the designed formation of core-shell structured
4
nanofillers[11]. The insulating Al2O3 with moderate dielectric constant (≈10) is
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introduced as shell layer on surface of BT nanofibers to form moderate interfacial area
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to reduce the MWS interfacial polarization, which leads to the gradient verities of the
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dielectric constant between the fillers and polymer matrix.
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Experimental
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one-dimensional (1D) materials exhibit superiority in enhanment discharged energy
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density performance of nanocomposites at lower volume fractions of fillers in
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comparison to their spherical counterparts[15-19]. Little effort has been focused on
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the enhancement of discharged energy storage density and discharged efficiency of
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the composites using 1D core-shell structure nanofibers. In this work, BT@Al2O3 NF
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have been successfully synthesized through the electrospinning and sol-gel method.
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Homogeneous nanocomposites consisting of BT@Al2O3 NF and a Poly(vinylidene
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fluoride) (PVDF) have been prepared by the solution casting method. A systematic
17
study was investigated on the effect of BT@Al2O3 NF filler introduction on the
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discharged energy density performance of the nanocomposite.
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Experimental section
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BT NF were synthesized via electrospinning method[20, 21]. The Al2O3 layer on the
21
surface of the BT NF was prepared through sol-gel method. Al(NO3)3 was dispersed
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in glacial acetic acid with stirring and heating for 3h. Then, a small amount of
theoretical
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calculation
have
demonstrated
that
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ACCEPTED MANUSCRIPT polyvinyl alcohol was added into the solution. The mixture was then stirred for 6 h.
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The BT NF were dispersed in alumina precursor sol solution with stirring for 10 h.
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And subsequently the solution was then centrifuged at 3000rpm for 10min and then
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washed with distilled water and ethanol and dried at 80 °C for 12 h and then annealed
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at 700 oC for 3 h in air to obtain the core-shell structure BT@Al2O3 NF.
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To improve the compatibility between the filler and matrix, the fillers have been
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functionalized with 3-aminopropyltriethoxysilane (APS). The nanocomposite films
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were prepared by dispersing the BT@Al2O3 NF (or BT NF) fillers into PVDF in
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N,N-dimethylformamide solution under vigorous stirring at 40 oC for 10 h to make it
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stable and homogeneous, then cast onto an indium tin oxide (as the bottom electrode)
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and dried at 60 oC for 24 h. Moreover, the films were heated at 200 oC for 5 min, then
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immediately quenched in ice-water and dired at 100 oC again. The thickness of the
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films was maintained between 10 and 15µm.
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Characterization
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The morphology of the samples was analyzed by Scanning electron microscopy
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(XL30-FEG, Philips, Netherlands). The 1D core-shell structure of the BT@Al2O3 NF
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was characterized by Transmission electron microscopy (TEM) (CM200FEG, Philips,
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Netherlands). The breakdown strength was analyzed by a withstand voltage test
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(ENTAI, Nanjing, China) in a silicone oil bath at room temperature. The voltage type
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in breakdown test is DC with a rate of rise of 200 V s-1. A set of 10 samples was
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employed for each condition. Leakage current density is obtained at 1000 kV cm-1 by
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Keithley 2400 source meter. The polarization–electric field loops (P-E) were
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ACCEPTED MANUSCRIPT measured at 100Hz in silicone oil using a Premier II ferroelectric test system. The
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dielectric properties are obtained by E4980A LCR meter (Agilent, Palo Alto, USA) in
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the frequency range from 100 Hz to 2 MHz at various temperatures. The complex
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dielectric constant is calculated as follows16, 17:
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ε∗ = ε′ − iε′′
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The complex electric modulus formalism is defined by the following equation [22]:
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Μ′ =
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(1)
ε′ ε′ + ε′′2 ε′′ Μ ′′ = 2 ε′ + ε′′2
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Results and discussion
(2)
(3)
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Fig 1 shows the TEM image of the BT@Al2O3 NF fillers. The core-shell structure has
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been obviously observed from the nanofibers, in which the thickness of the inner core
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layer and outer shell layer is approximately 150 nm and 6 nm, respectively. To
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determine further the composition of the BT@Al2O3 NF, energy dispersive
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spectrometry (EDS) studies are conducted, as shown in Fig. 1(b). EDS analysis
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identified the elemental distribution, as exhibited in Fig. 1(b). The A point in the inner
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shell is dominated by Ba, Ti and O elements while the B point in the outer rod is
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mainly composed by Al and O elements, further indicating the successful synthesis of
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core-shell structure BT@Al2O3 NF. We assume that the density of BT NF is 6 g/cm3,
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the density of Al2O3 layer is 3.5g/cm3 and the average diameter of BT NF is 150 nm
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and lengths is 10µm, the thickness of the Al2O3 layer is about 6 nm, then we can
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estimate that the mass proportion of Al2O3 is about 2.3 wt % in the BT@Al2O3 NF.
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Fig. 1 (a) TEM image of BT@Al2O3 NF at high magnifications. BT@Al2O3 NF at low magnifications is shown in the inset.(b) EDS analysis corresponding to BT@Al2O3 NF
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Fig. 2(a)-(b) show the surface SEM and cross-section SEM of 5% BT@Al2O3
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NF/PVDF nanocomposites, as shown in Fig. 2(a) and (b). Surface modified
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BT@Al2O3 NF are distributed homogeneously in the matrix of nanocomposites,
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indicating an excellent dispersion and strong interfacial interaction between PVDF
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and BT@Al2O3 NF fillers. Furthermore, Fig. 2(a) and (b) also reveals that the
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nanofibers tend to orient in the in-plane direction of the nanocomposite films.
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Fig. 2 (a) Surface SEM (b) cross-section SEM of 5% BT@Al2O3 NF/PVDF nanocomposites
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To investigate the influence of BT@Al2O3 NF fillers on composite performance,
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leakage current density for the nanocomposites film of BT@Al2O3 NF and those of
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BT NF has been measured, as shown in Fig. 3. The leakage current density of both
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nanocomposites gradually increases with the content of fillers. The leakage current 8
ACCEPTED MANUSCRIPT density is about 1.5×10-5, 3.5×10-5 and 5×10-5A/cm2 at 1000kV/cm for the films of
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BT@Al2O3 NF with a concentration of 2.5 vol.%, 5 vol.% and 7.5 vol.%, respectively.
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The increase of the leakage current is attributed to increased numbers of air voids,
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defects and agglomeration introduced by fillers [23]. However, the nanocomposites
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film of BT@Al2O3 NF consistently show a lower leakage current density compared to
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those of BT NF. The reduction of the leakage current is attributed to coating Al2O3
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insulation layers on the surface of BT NF, which can prevent the contact between
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ceramic fillers in nanocomposites[24].
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Fig. 3 Summary of the leakage current density comparison for BT NF/PVDF and BT@Al2O3 NF /PVDF nanocomposites at 1000 kV/cm
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The breakdown strength is an important parameter in determining the energy storage
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density of nanocomposites in practical application. The characteristic breakdown
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strength of the PVDF nanocomposites is analyzed using Weibull statistics, which are
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given by Equation (4) and Equation (5),
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X i = ln( Ei )
(4)
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Yi = ln(− ln(1 − i / (n + 1)))
(5)
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where X i and Yi are two parameters in Weibull distribution function, Ei is the
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resultant breakdown voltage of the samples, n is the sum of samples in the 9
ACCEPTED MANUSCRIPT experiments, and i is the serial number of samples. The samples are arranged in
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ascending order of breakdown strength values so that E1 ≤ E2 ≤ ⋅⋅⋅Ei ⋅⋅⋅ ≤ En .
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According to the Weibull distribution, X i and Yi have a linear relationship. β is the
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shape parameter, which is related to the slope of the data. The intercept on the x-axis
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is lnE, where E represents the breakdown strength at the cumulative failure
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probability of 63.2%. Weibull distributions for breakdown strength of the
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nanocomposites loaded with various concentrations of the BT NF and BT@Al2O3
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were carried out and the results are shown in Fig. 4(a) and 4(b). As seen from Fig. 4,
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all the plots show a relatively good linearity and the values of shape parameter β for
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the samples are higher than eight, indicating an excellent reliability in practical
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application.
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Fig. 4 Weibull plots of the breakdown strength for BT NF/PVDF and BT@Al2O3 NF /PVDF nanocomposites loaded with various concentrations of fillers.
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The characteristic breakdown strength of each nanocomposites was calculated and
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shown in Fig. 5. The results illustrate that the breakdown strength of the
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nanocomposites of BT@Al2O3 NF is higher than that of BT NF at the same volume
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fraction of fillers. For example, at 7.5 vol.%, the breakdown strength of the
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nanocomposite with the BT@Al2O3 is 2800 kV/cm, about 1.2 times that of the BT NF 10
ACCEPTED MANUSCRIPT (2300 kV/cm). The improvement of the breakdown strength can be attributed to two
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reasons. On the one hand, Al2O3 layer on the surface of BT NF reduces the dielectric
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differences between the two phases in the nanocomposites and further results in the
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reduction space charges accumulation in PVDF composites, which can improve
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electric field in the nanocomposites and enhance the breakdown strength. On the other
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hand, the Al2O3 insulated layer can effectively enhance the insulation of BT NF and
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prevent the contact between fillers in polymer matrix to reduce the leakage current of
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nanocomposites.
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Fig. 5 Breakdown strength comparison for BT NF/PVDF and BT@Al2O3 NF /PVDF nanocomposites
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The frequency dependence of the imaginary electric modulus (M″) of the composites
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at high temperatures is shown in Fig. 6(a). It is obviously observed that the peaks of
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interfacial polarization relaxation shifts to a high frequency with increase of
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measuring temperature. This phenomenon is correlated with the dipolar relaxation and
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MWS interfacial polarization.
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the function between the relaxation frequency and measuring temperature through the
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The values of Ea can be calculated from the slope of
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ACCEPTED MANUSCRIPT −Ea ) , Where f, Ea, kB and T is the peak maximum frequency of kBT
equation f = f0 exp(
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M″, the activation energy, the Boltzmann constant and absolute temperature,
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respectively[22]. Fig 6(b) show Arrhenius plots of the ln(fmax) vs. 1000/T for the 2.5
4
vol.% BT NF/PVDF and BT@Al2O3 NF/PVDF nanocomposite films. The values of
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Ea for the 2.5 vol.% BT NF/PVDF, 2.5, 5 and 7.5 vol.% BT@Al2O3 NF/PVDF are
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0.84eV, 0.68 eV, 0.72 eV, 0.80eV, respectively.The Ea of MWS refers to the
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activation energy of conduction in each phase. The Ea value of nanocomposites with
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BT@Al2O3 NF is lower compared with that of BT NF/PVDF, which indicates that it
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is less sensitive to temperature increase.
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Fig. 6 (a) Frequency dependent M″ for the2.5 vol.% BT@Al2O3 NF/PVDF nanocomposites films at different temperatures. (b) Activation energy Ea for the 2.5 vol.% BT NF/PVDF and different contents BT@Al2O3 NF nanocomposite films 12
ACCEPTED MANUSCRIPT In order to investigate the discharged energy density of the nanocomposites at high
2
electric fields, P–E loops of pure PVDF and the nanocomposite with various
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concentrations of fillers were measured at 100 Hz and room temperature as shown in
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Fig 7(a). The incorporation of the BT@Al2O3 NF fillers into the PVDF polymers
5
obviously increases the maximum polarization of the nanocomposites. Most notably,
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very minor increase in remnant polarizations with increasing concentration of the
7
fillers is observed. This result indicated that the use of BT@Al2O3 NF fillers provide a
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relatively high maximum polarization while eliminating the remnant polarization and
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ultimately the discharged energy performances of the nanocomposites. Moreover, the
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nanocomposite exhibits narrow P-E loop at high electric field, which is favorable to
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enhance discharged energy density and discharged efficiency. Fig 7(b) illustrates the
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P-E loops of the nanocomposite of 2.5 vol. % BT@Al2O3 NF and that of 2.5 vol. %
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BT NF. The former exhibits narrow P-E loop, lower maximum polarization and most
14
importantly, remarkably reduced remnant polarization, compared to the latter. The
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discharged
16
concentrations of BT@Al2O3 NF is presented in Fig.7(c). The maximum discharged
17
energy density of the nanocomposites films with 2.5 vol.% BT@Al2O3 NF reaches
18
7.1J/cm3 at 3800 kV/cm, while that of pure PVDF is only 2.8 J/cm3 at 4000 kV/cm.
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The results clearly indicates that the introduction of core-shell structure BT@Al2O3
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NF ceramic fillers into PVDF can significantly improve the discharged energy density
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performance.
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For the practice application, a high energy storage density and high discharge
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efficiency (η) are desired simultaneously. The discharge efficiency could is defined
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according to the formula: η =
3
discharged energy, and energy loss, respectively. The energy loss density ( Ul ) is
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calculated by the numerical integration of closed area of the hysteresis loops.
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Fig. 7(d) gives the discharged efficiency of the nanocomposite films. A high
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discharged efficiency calculated for the nanocomposite films filled with 2.5 vol.%
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BT@Al2O3 NF is 82% at 1000 kV/cm and can still be maintained 65.1% even at 3800
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kV/cm, whereas the discharged efficiency calculated for the PVDF films exhibits
9
considerable reduction from 71.7% at 1000 kV/cm to 58.2% at 3300kV/cm. The
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results indicates that the introduction of BT@Al2O3 NF fillers can enhance the energy
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density while at the same time maintaining a high discharged efficiency
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Fig. 7. (a) P-E loops for nanocomposites filled at room temperature with different volume fraction
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of BT@Al2O3 NF at 100Hz (b) P–E loops for 2.5 vol.% BT@Al2O3 NF nanocomposites and 2.5 vol.%
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Ud U = 1 − l , where U s , Ud , and Ul are the stored, Us Us
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BT NF /PVDF nanocomposites (c) Discharge energy density of nanocomposites (c) Efficiency of
nanocomposites films
The improvement
of
discharge energy density in
BT@Al2O3
NF/PVDF
4
nanocomposites is mainly contributed to the Al2O3 layers on the surface of BT NF to
5
form moderate interfacial area, which can reduce the dielectric differences between
6
the two phases and improve the MWS interfacial polarization in the nanocomposites.
7
To illustrate the MWS interfacial polarization in the nanocomposites, the frequency
8
dependence of dielectric constant and loss of the nanocomposite of 2.5 vol. %
9
BT@Al2O3 NF and that of 2.5 vol. % BT NF is presented Fig. 8. It can be seen that
10
the dielectric constant of the nanocomposites of BT NF is higher than that of BT@Al2O3
11
NF. Dielectric constant of all composites decreases with frequency because the dipoles
12
of filler and polymer cannot shift their orientation sufficiently fast when the frequency
13
of the applied electric field exceeds the relaxation frequency. In addition, loss of the
14
nanocomposites remains low with little variation at low frequency, followed by rapid
15
increases at high frequency as a result of α a relaxation which is associated with the
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glass transition of pure PVDF polymer.
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Fig. 8. Frequency dependencies of the dielectric constant and loss tangent of the dielectric 15
ACCEPTED MANUSCRIPT constant and loss of the nanocomposite of 2.5 vol. % BT@Al2O3 NF and that of 2.5 vol. % BT NF
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Dielectric relaxation are observed from dielectric spectra of nanocomposites. To study
3
relaxation, frequency dependence of real part (eps′) and the imaginary part (eps″) of
4
of BT NF composites and BT@Al2O3 NF composites is shown in Fig. 9. relaxation
5
intensity or relaxation time can be described by a superposition of independent Debye
6
functions.
7
ε ′(ω ) = ε ∞ + ∑
8
ε ∞ is the dielectric constant at the high frequency limit, n was taken as 15 to
9
accurately capture the shape of the curve, ∆ε i is the relaxation intensity of each term
10
with ω and τ i being the angular frequency and relaxation time, M represents the
11
shift in intensity, while S represents the shift in relaxation time, respectively. By
12
matching the simulation results to the experimental data (Fig 9), M and S are fitted to
13
be 0.6 and 0.9. This results show that relaxation intensity and relaxation time of the
14
nanocomposites of BT NF is higher than that of BT@Al2O3 NF, implying a reduced
15
relaxation intensity and an recreased relaxation time of the interface region[25-29].
16
Because the Al2O3 layer on the surface of BT NF can reduce the dielectric differences
17
between the two phases and improve the MWS interfacial polarization in the
18
nanocomposites[13]. Suppression the MWS interfacial polarization is beneficial for
19
the improvement of discharged energy density and efficiency. Less polarization
20
charges are trapped in the discharge process and the majority of charges can be
21
released by reducing MWS interfacial polarization, which means high energy storage
22
efficiency. Therefore, the Al2O3 layer on the surface of BT NF significantly enhances
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Μ∆ε i 2 i =1 1 + ( Sωτ i )
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ACCEPTED MANUSCRIPT discharged energy density of the nanocomposites through the reduction of the MWS
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interfacial polarization of the nanocomposites.
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Fig. 9. Frequency dependence of real part (eps′) and the imaginary part (eps″) of the nanocomposite of 2.5 vol. % BT@Al2O3 NF and that of 2.5 vol. % BT NF
Conclusions
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Homogeneous nanocomposites, using the 1D core-shell structure BT@Al2O3 NF as
8
fillers and PVDF as matrix, has been prepared through a solution casting method.
9
TEM results showed that Al2O3 was uniformly coated on the surface of BT NF with
10
an average thickness of 6 nm. SEM results showed that the BT@Al2O3 NF were
11
dispersed homogeneously in the PVDF. The nanocomposites of BT@Al2O3 NF
12
exhibit enhanced breakdown strength, reduced energy loss and enhanced discharged
13
energy density under high electric field compared to those of BT NF. The maximum
14
discharged energy density of 7.1 J/cm3 was obtained in the nanocomposite film with
15
2.5 vol.% BT@Al2O3 NF, which is 154% higher than that of the pure PVDF. The
16
efficiency of the nanocomposites films with 2.5 vol.% BT@Al2O3 NF is higher than
17
82% at electric fields below 1000 kV/cm and can still be maintained at 65.1% even at
18
3800 kV/cm. Such significant enhancement is closely related to the combined effect
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Acknowledgments
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This research was supported by the Ministry of Sciences and Technology of China
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through 973-project under Grant (2015CB654601), Natural Science Foundation of
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Henan Department of Education (17A430013), Doctoral Program of Henan Insititute
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of Engineering (D2016015, D2016016) and the Programs for Science and Technology
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Development, Henan Province, China (132102210035, 142300410439).
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References:
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[1] X. Huang, P. Jiang, Core–Shell Structured High‐k Polymer Nanocomposites for Energy Storage and Dielectric Applications, Adv Mater, 27 (2015) 546-554.
[2] L. Wu, X. Wang, H. Gong, Y. Hao, Z. Shen, L. Li, Core-satellite BaTiO 3@ SrTiO 3 assemblies for a local compositionally graded relaxor ferroelectric capacitor with enhanced energy storage density and high energy efficiency, Journal of Materials Chemistry C, 3 (2015) 750-758. [3] S.H. Liu, S.X. Xue, W.Q. Zhang, J.W. Zhai, Enhanced dielectric and energy storage density induced by surface-modified BaTiO3 nanofibers in poly(vinylidene fluoride) nanocomposites, Ceram Int, 40 (2014)
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15633-15640.
[4] L. Wu, X. Wang, L. Li, Core-shell BaTiO 3@ BiScO 3 particles for local graded dielectric ceramics with enhanced temperature stability and energy storage capability, J Alloy Compd, 688 (2016) 113-121.
[5] S.X. Xue, S.H. Liu, W.Q. Zhang, J.W. Wang, L.J. Tang, B. Shen, J.W. Zhai, Dielectric properties and
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charge-discharge behaviors in niobate glass ceramics for energy-storage applications, J Alloy Compd, 617 (2014) 418-422.
[6] X. Zhang, Y. Shen, Q. Zhang, L. Gu, Y. Hu, J. Du, Y. Lin, C.W. Nan, Ultrahigh Energy Density of Polymer Nanocomposites Containing BaTiO3@ TiO2 Nanofibers by Atomic‐Scale Interface Engineering, Adv
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9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
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1
Mater, 27 (2015) 819-824.
[7] B.J. Chu, X. Zhou, K.L. Ren, B. Neese, M.R. Lin, Q. Wang, F. Bauer, Q.M. Zhang, A dielectric polymer with high electric energy density and fast discharge speed, Science, 313 (2006) 334-336. [8] L.Y. Xie, X.Y. Huang, Y.H. Huang, K. Yang, P.K. Jiang, Core@Double-Shell Structured BaTiO3-Polymer Nanocomposites with High Dielectric Constant and Low Dielectric Loss for Energy Storage Application, J Phys Chem C, 117 (2013) 22525-22537. [9] Z.M. Dang, J.K. Yuan, S.H. Yao, R.J. Liao, Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage, Adv Mater, 25 (2013) 6334-6365. [10]
X.Y.
Huang,
C.Y.
Zhi,
P.K.
Jiang,
D.
Golberg,
Y.
Bando,
T.
Tanaka,
Polyhedral
Oligosilsesquioxane-Modified Boron Nitride Nanotube Based Epoxy Nanocomposites: An Ideal Dielectric Material with High Thermal Conductivity, Adv Funct Mater, 23 (2013) 1824-1831. [11] L.Y. Xie, X.Y. Huang, C. Wu, P.K. Jiang, Core-shell structured poly(methyl methacrylate)/BaTiO3 18
ACCEPTED MANUSCRIPT nanocomposites prepared by in situ atom transfer radical polymerization: a route to high dielectric constant materials with the inherent low loss of the base polymer, J Mater Chem, 21 (2011) 5897-5906. [12] H.X. Tang, H.A. Sodano, High energy density nanocomposite capacitors using non-ferroelectric nanowires, Appl Phys Lett, 102 (2013) 063901. [13] K. Yu, Y.J. Niu, Y.Y. Bai, Y.C. Zhou, H. Wang, Poly(vinylidene fluoride) polymer based BaTiO3/SiO2 nanoparticles, Appl Phys Lett, 102 (2013) 102903.
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nanocomposites with significantly reduced energy loss by filling with core-shell structured [14] C. Wu, X.Y. Huang, X.F. Wu, J.H. Yu, L.Y. Xie, P.K. Jiang, TiO2-nanorod decorated carbon nanotubes for high-permittivity and low-dielectric-loss polystyrene composites, Compos Sci Technol, 72 (2012) 521-527.
[15] S. Liu, S. Xiao, S. Xiu, B. Shen, J. Zhai, Z. An, Poly (vinylidene fluoride) nanocomposite capacitors Ba 0.6 Sr 0.4 TiO 3 nanofibers, RSC Adv, 5 (2015) 40692-40699.
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with a significantly enhanced dielectric constant and energy density by filling with surface-fluorinated [16] S.H. Liu, J.W. Zhai, A small loading of surface-modified Ba0.6Sr0.4TiO3 nanofiber-filled nanocomposites with enhanced dielectric constant and energy density, RSC Adv, 4 (2014)
M AN U
40973-40979.
[17] Y. Song, Y. Shen, H.Y. Liu, Y.H. Lin, M. Li, C.W. Nan, Improving the dielectric constants and breakdown strength of polymer composites: effects of the shape of the BaTiO3 nanoinclusions, surface modification and polymer matrix, J Mater Chem, 22 (2012) 16491-16498. [18] Y. Song, Y. Shen, P.H. Hu, Y.H. Lin, M. Li, C.W. Nan, Significant enhancement in energy density of polymer composites induced by dopamine-modified Ba0.6Sr0.4TiO3 nanofibers, Appl Phys Lett, 101 (2012) 152904.
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[19] G. Wang, X. Huang, P. Jiang, Tailoring Dielectric Properties and Energy Density of Ferroelectric Polymer Nanocomposites by High-k Nanowires, Acs Appl Mater Interfaces, 7 (2015) 18017-18027. [20] D.Y. Lee, K.H. Lee, M.H. Lee, N.I. Cho, B.Y. Kim, Synthesis of electrospun BaSrTiO3/PVP nanofibers, J Sol-gel Sci Techn, 53 (2010) 43-49.
[21] H.P. Li, H. Wu, D.D. Lin, W. Pan, High T-c in Electrospun BaTiO3 Nanofibers, J Am Ceram Soc, 92
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(2009) 2162-2164.
[22] W. Wu, X.Y. Huang, S.T. Li, P.K. Jiang, T. Toshikatsu, Novel Three-Dimensional Zinc Oxide Superstructures for High Dielectric Constant Polymer Composites Capable of Withstanding High Electric Field, J Phys Chem C, 116 (2012) 24887-24895.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
[23] K. Yu, Y.J. Niu, Y.C. Zhou, Y.Y. Bai, H. Wang, Nanocomposites of Surface-Modified BaTiO3 Nanoparticles Filled Ferroelectric Polymer with Enhanced Energy Density, J Am Ceram Soc, 96 (2013) 2519-2524.
[24] Z. Li, L.A. Fredin, P. Tewari, S.A. DiBenedetto, M.T. Lanagan, M.A. Ratner, T.J. Marks, In Situ Catalytic Encapsulation of Core-Shell Nanoparticles Having Variable Shell Thickness: Dielectric and Energy Storage Properties of High-Permittivity Metal Oxide Nanocomposites, Chem Mater, 22 (2010) 5154-5164. [25] Y. Huang, T.M. Krentz, J.K. Nelson, L.S. Schadler, Y. Li, H. Zhao, L.C. Brinson, M. Bell, B. Benicewicz, K. Wu, Prediction of interface dielectric relaxations in bimodal brush functionalized epoxy nanodielectrics by finite element analysis method, in:
2014 IEEE Conference on Electrical Insulation
and Dielectric Phenomena (CEIDP), IEEE, 2014, pp. 748-751. [26] Y. Huang, X. Huang, L.S. Schadler, J. He, P. Jiang, Core@ double-shell Structured Nanocomposites: 19
ACCEPTED MANUSCRIPT a Route to High Dielectric Constant and Low Loss Material, Acs Appl Mater Interfaces, (2016). [27] Y. Huang, L.S. Schadler, Understanding the Strain-Dependent Dielectric Behavior of Carbon Black Reinforced Natural Rubber: An interfacial or bulk phenomenon?, arXiv preprint arXiv:1609.09434, (2016). [28] R. Bartnikas, R. Eichhorn, A.S.f. Testing, Materials, A.C.D.-o.E.I. Materials, I.d.r. d'Hydro-Québec, U.C. Corporation, Electrical properties of solid insulating materials: molecular structure and electrical behavior, 1983. [29] X. Huang, C. Zhi, Polymer Nanocomposites: Electrical and Thermal Properties, Springer, 2016.
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Highlights 1. BaTiO3@Al2O3 nanofibers/PVDF nanocomposite films were prepared. 2. The discharged energy density performance of the nanocomposite films were
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investigated.
2. The discharged energy density of the nanocomposite films are enhanced compared
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with PVDF.
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3. The enhancement mechanism of the nanocomposite films was proposed.
S0925-8388(16)33667-2
DOI:
10.1016/j.jallcom.2016.11.186
Reference:
JALCOM 39683
To appear in:
Journal of Alloys and Compounds
Received Date: 23 August 2016 Revised Date:
10 November 2016
Accepted Date: 13 November 2016
Please cite this article as: S. Liu, J. Wang, B. Shen, J. Zhai, H. Hao, L. Zhao, Poly(vinylidene fluoride) nanocomposites with a small loading of core-shell structured BaTiO3@Al2O3 nanofibers exhibiting high discharged energy density and efficiency, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.11.186. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Poly(Vinylidene Fluoride) nanocomposites with a small loading of
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core-shell structured BaTiO3@Al2O3 nanofibers exhibiting high
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discharged energy density and efficiency
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Shaohui Liu1, 2, Jiao Wang1, Bo Shen2, Jiwei Zhai2∗, Haoshan Hao1∗, Limin Zhao1 1
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School of Science, Henan Institute of Engineering, Zhengzhou 451191, China
Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, Functional Materials
Research Laboratory, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai
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201804, China
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Abstract: BaTiO3@Al2O3 core-shell nanofibers (BT@Al2O3 NF) have been
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successfully synthesized. Homogeneous nanocomposites consisting of BT@Al2O3 NF
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and a Poly(vinylidene fluoride) (PVDF) polymer matrix have been prepared by the
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solution casting method. A systematic study was investigated on the effect of
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BT@Al2O3 NF filler introduction on the discharged energy density performance of
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the nanocomposite. Al2O3 shell dramatically reduces the leakage current by prevent
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the contact between BT NF fillers in nanocomposites and minimize the
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Maxwell-Wagner-Sillars interfacial polarization, which results in the enhancement of
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the breakdown strength of nanocomposites films. Simultaneously, the nanocomposites
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have higher maximum polarization and lower the remnant polarization than that of
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PVDF films under the same electric field. The maximum discharged energy density of
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the nanocomposites with 2.5 vol.% BT@Al2O3 NF reaches 7.1J/cm3 at 3800kV/cm
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∗ ∗
Corresponding author E-mail address: [email protected]. Corresponding author E-mail address: [email protected]. 1
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with an efficiency of above 65.1%. This work may provide an effective solution for
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enhancing the discharged energy density of nanocomposites films.
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Keywords: discharged energy density; poly(vinylidene fluoride) nanocomposites;
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interfacial polarization; BaTiO3@Al2O3 nanofibers
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Introduction
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High discharged energy density nanocomposite dielectric capacitors have been the
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major enabler for a number of potential applications in advanced electronic and
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electric power applications, such as hybrid electric vehicles, electronic components
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and pulsed power sources[1-5]. Compared to batteries and super capacitors, dielectric
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capacitors have some advantages such as fast charge/discharge (<1 µs), high power
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density and wide operating temperature range. Currently, the commercial applications
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for energy storage are neat polymers, such as biaxial oriented polypropylene (BOPP),
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due to their excellent breakdown strength, ease formability and low manufacturing
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cost. Nevertheless, polymer film capacitors suffer from either a low dielectric
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constant (<10) or an unacceptably low efficiency limiting their applications[6]. In
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addition, high discharged energy density and efficiency capacitors are highly
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desirable and have been motivated by reducing the size and cost of dielectric
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capacitors.
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In the last decade, poly(vinylidene fluoride) (PVDF) have attracted much interest due
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to their excellent breakdown strength and relatively high dielectric constant[7]. To
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enhance the discharged energy density of the PVDF, an effective way is introducing
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high dielectric constant ceramic fillers, such as Pb(Zr,Ti)O3, Ba1-xSrxTiO3 and BaTiO3,
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into the polymer matrix. Tremendous achievements have been made in recent years
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on ehancement the energy storage capacity of PVDF [8, 9]. PVDF nanocomposites
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exhibit high discharged energy density. However, low discharged energy efficiency
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and high losses have prevented the large scale commercialization of capacitors based
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to satisfy high discharged energy density while maintaining high discharging
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efficiency, even at very high applied electric fields.
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Low discharging efficiency of nanocomposites originates from interfacial polarization
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[10, 11]. Generally, introduction of the ceramic fillers with high dielectric constant
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into the polymer matrix results in Maxwell−Wagner−Sillars (MWS) interfacial
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polarization due to the large difference of the dielectric constant between ceramic
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fillers and polymer matrix in composite[8]. The long relaxation time is of MWS
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polarization is due to the low bulk conductivity in each phase. These polarization
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charges are trapped in the discharge process due to the long relaxation process of
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interfacial polarization. The key to improve the discharged energy density and
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discharged efficiency at high electric fields is suppression the interfacial polarization
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[12].
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To reduce the MWS interfacial polarization, filler with core-shell structure was
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introduced to overcome the problem. Su et al reported that enhanced energy density in
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PVDF nanocomposites are obtained by introduction BaTiO3@TiO2 core-shell fillers.
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Yu et al reported that core-shell structured BaTiO3/SiO2 nanoparticles /PVDF
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nanocomposites have been synthesized [13]. The results showed that the energy
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density of nanocomposites can be significantly improved by the incorporation of the
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SiO2 shell layer to reduce of energy loss. The reason of the greatly enhancement is
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that SiO2 layer coating on BT surface can suppress the leakage current and reduce the
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interfacial polarization [13]. Wu et al. reported that core-shell structure TiO2/carbon
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ACCEPTED MANUSCRIPT nanotubes composites were prepared and significantly increased dielectric constant
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and low loss were achieved [14]. These results show that the greatly enhancement of
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the energy density is attributed to the designed formation of core-shell structured
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nanofillers[11]. The insulating Al2O3 with moderate dielectric constant (≈10) is
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introduced as shell layer on surface of BT nanofibers to form moderate interfacial area
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to reduce the MWS interfacial polarization, which leads to the gradient verities of the
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dielectric constant between the fillers and polymer matrix.
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Experimental
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one-dimensional (1D) materials exhibit superiority in enhanment discharged energy
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density performance of nanocomposites at lower volume fractions of fillers in
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comparison to their spherical counterparts[15-19]. Little effort has been focused on
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the enhancement of discharged energy storage density and discharged efficiency of
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the composites using 1D core-shell structure nanofibers. In this work, BT@Al2O3 NF
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have been successfully synthesized through the electrospinning and sol-gel method.
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Homogeneous nanocomposites consisting of BT@Al2O3 NF and a Poly(vinylidene
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fluoride) (PVDF) have been prepared by the solution casting method. A systematic
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study was investigated on the effect of BT@Al2O3 NF filler introduction on the
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discharged energy density performance of the nanocomposite.
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Experimental section
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BT NF were synthesized via electrospinning method[20, 21]. The Al2O3 layer on the
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surface of the BT NF was prepared through sol-gel method. Al(NO3)3 was dispersed
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in glacial acetic acid with stirring and heating for 3h. Then, a small amount of
theoretical
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demonstrated
that
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ACCEPTED MANUSCRIPT polyvinyl alcohol was added into the solution. The mixture was then stirred for 6 h.
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The BT NF were dispersed in alumina precursor sol solution with stirring for 10 h.
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And subsequently the solution was then centrifuged at 3000rpm for 10min and then
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washed with distilled water and ethanol and dried at 80 °C for 12 h and then annealed
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at 700 oC for 3 h in air to obtain the core-shell structure BT@Al2O3 NF.
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To improve the compatibility between the filler and matrix, the fillers have been
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functionalized with 3-aminopropyltriethoxysilane (APS). The nanocomposite films
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were prepared by dispersing the BT@Al2O3 NF (or BT NF) fillers into PVDF in
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N,N-dimethylformamide solution under vigorous stirring at 40 oC for 10 h to make it
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stable and homogeneous, then cast onto an indium tin oxide (as the bottom electrode)
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and dried at 60 oC for 24 h. Moreover, the films were heated at 200 oC for 5 min, then
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immediately quenched in ice-water and dired at 100 oC again. The thickness of the
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films was maintained between 10 and 15µm.
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Characterization
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The morphology of the samples was analyzed by Scanning electron microscopy
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(XL30-FEG, Philips, Netherlands). The 1D core-shell structure of the BT@Al2O3 NF
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was characterized by Transmission electron microscopy (TEM) (CM200FEG, Philips,
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Netherlands). The breakdown strength was analyzed by a withstand voltage test
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(ENTAI, Nanjing, China) in a silicone oil bath at room temperature. The voltage type
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in breakdown test is DC with a rate of rise of 200 V s-1. A set of 10 samples was
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employed for each condition. Leakage current density is obtained at 1000 kV cm-1 by
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Keithley 2400 source meter. The polarization–electric field loops (P-E) were
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dielectric properties are obtained by E4980A LCR meter (Agilent, Palo Alto, USA) in
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the frequency range from 100 Hz to 2 MHz at various temperatures. The complex
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dielectric constant is calculated as follows16, 17:
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ε∗ = ε′ − iε′′
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The complex electric modulus formalism is defined by the following equation [22]:
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Μ′ =
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(1)
ε′ ε′ + ε′′2 ε′′ Μ ′′ = 2 ε′ + ε′′2
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Results and discussion
(2)
(3)
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Fig 1 shows the TEM image of the BT@Al2O3 NF fillers. The core-shell structure has
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been obviously observed from the nanofibers, in which the thickness of the inner core
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layer and outer shell layer is approximately 150 nm and 6 nm, respectively. To
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determine further the composition of the BT@Al2O3 NF, energy dispersive
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spectrometry (EDS) studies are conducted, as shown in Fig. 1(b). EDS analysis
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identified the elemental distribution, as exhibited in Fig. 1(b). The A point in the inner
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shell is dominated by Ba, Ti and O elements while the B point in the outer rod is
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mainly composed by Al and O elements, further indicating the successful synthesis of
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core-shell structure BT@Al2O3 NF. We assume that the density of BT NF is 6 g/cm3,
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the density of Al2O3 layer is 3.5g/cm3 and the average diameter of BT NF is 150 nm
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and lengths is 10µm, the thickness of the Al2O3 layer is about 6 nm, then we can
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estimate that the mass proportion of Al2O3 is about 2.3 wt % in the BT@Al2O3 NF.
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Fig. 1 (a) TEM image of BT@Al2O3 NF at high magnifications. BT@Al2O3 NF at low magnifications is shown in the inset.(b) EDS analysis corresponding to BT@Al2O3 NF
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Fig. 2(a)-(b) show the surface SEM and cross-section SEM of 5% BT@Al2O3
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NF/PVDF nanocomposites, as shown in Fig. 2(a) and (b). Surface modified
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BT@Al2O3 NF are distributed homogeneously in the matrix of nanocomposites,
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indicating an excellent dispersion and strong interfacial interaction between PVDF
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and BT@Al2O3 NF fillers. Furthermore, Fig. 2(a) and (b) also reveals that the
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nanofibers tend to orient in the in-plane direction of the nanocomposite films.
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Fig. 2 (a) Surface SEM (b) cross-section SEM of 5% BT@Al2O3 NF/PVDF nanocomposites
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To investigate the influence of BT@Al2O3 NF fillers on composite performance,
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leakage current density for the nanocomposites film of BT@Al2O3 NF and those of
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BT NF has been measured, as shown in Fig. 3. The leakage current density of both
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nanocomposites gradually increases with the content of fillers. The leakage current 8
ACCEPTED MANUSCRIPT density is about 1.5×10-5, 3.5×10-5 and 5×10-5A/cm2 at 1000kV/cm for the films of
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BT@Al2O3 NF with a concentration of 2.5 vol.%, 5 vol.% and 7.5 vol.%, respectively.
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The increase of the leakage current is attributed to increased numbers of air voids,
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defects and agglomeration introduced by fillers [23]. However, the nanocomposites
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film of BT@Al2O3 NF consistently show a lower leakage current density compared to
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those of BT NF. The reduction of the leakage current is attributed to coating Al2O3
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insulation layers on the surface of BT NF, which can prevent the contact between
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ceramic fillers in nanocomposites[24].
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Fig. 3 Summary of the leakage current density comparison for BT NF/PVDF and BT@Al2O3 NF /PVDF nanocomposites at 1000 kV/cm
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The breakdown strength is an important parameter in determining the energy storage
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density of nanocomposites in practical application. The characteristic breakdown
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strength of the PVDF nanocomposites is analyzed using Weibull statistics, which are
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given by Equation (4) and Equation (5),
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X i = ln( Ei )
(4)
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Yi = ln(− ln(1 − i / (n + 1)))
(5)
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where X i and Yi are two parameters in Weibull distribution function, Ei is the
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resultant breakdown voltage of the samples, n is the sum of samples in the 9
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ascending order of breakdown strength values so that E1 ≤ E2 ≤ ⋅⋅⋅Ei ⋅⋅⋅ ≤ En .
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According to the Weibull distribution, X i and Yi have a linear relationship. β is the
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shape parameter, which is related to the slope of the data. The intercept on the x-axis
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is lnE, where E represents the breakdown strength at the cumulative failure
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probability of 63.2%. Weibull distributions for breakdown strength of the
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nanocomposites loaded with various concentrations of the BT NF and BT@Al2O3
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were carried out and the results are shown in Fig. 4(a) and 4(b). As seen from Fig. 4,
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all the plots show a relatively good linearity and the values of shape parameter β for
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the samples are higher than eight, indicating an excellent reliability in practical
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application.
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Fig. 4 Weibull plots of the breakdown strength for BT NF/PVDF and BT@Al2O3 NF /PVDF nanocomposites loaded with various concentrations of fillers.
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The characteristic breakdown strength of each nanocomposites was calculated and
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shown in Fig. 5. The results illustrate that the breakdown strength of the
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nanocomposites of BT@Al2O3 NF is higher than that of BT NF at the same volume
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fraction of fillers. For example, at 7.5 vol.%, the breakdown strength of the
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nanocomposite with the BT@Al2O3 is 2800 kV/cm, about 1.2 times that of the BT NF 10
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reasons. On the one hand, Al2O3 layer on the surface of BT NF reduces the dielectric
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differences between the two phases in the nanocomposites and further results in the
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reduction space charges accumulation in PVDF composites, which can improve
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electric field in the nanocomposites and enhance the breakdown strength. On the other
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hand, the Al2O3 insulated layer can effectively enhance the insulation of BT NF and
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prevent the contact between fillers in polymer matrix to reduce the leakage current of
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nanocomposites.
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Fig. 5 Breakdown strength comparison for BT NF/PVDF and BT@Al2O3 NF /PVDF nanocomposites
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The frequency dependence of the imaginary electric modulus (M″) of the composites
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at high temperatures is shown in Fig. 6(a). It is obviously observed that the peaks of
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interfacial polarization relaxation shifts to a high frequency with increase of
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measuring temperature. This phenomenon is correlated with the dipolar relaxation and
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MWS interfacial polarization.
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the function between the relaxation frequency and measuring temperature through the
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The values of Ea can be calculated from the slope of
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equation f = f0 exp(
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M″, the activation energy, the Boltzmann constant and absolute temperature,
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respectively[22]. Fig 6(b) show Arrhenius plots of the ln(fmax) vs. 1000/T for the 2.5
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vol.% BT NF/PVDF and BT@Al2O3 NF/PVDF nanocomposite films. The values of
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Ea for the 2.5 vol.% BT NF/PVDF, 2.5, 5 and 7.5 vol.% BT@Al2O3 NF/PVDF are
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0.84eV, 0.68 eV, 0.72 eV, 0.80eV, respectively.The Ea of MWS refers to the
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activation energy of conduction in each phase. The Ea value of nanocomposites with
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BT@Al2O3 NF is lower compared with that of BT NF/PVDF, which indicates that it
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is less sensitive to temperature increase.
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Fig. 6 (a) Frequency dependent M″ for the2.5 vol.% BT@Al2O3 NF/PVDF nanocomposites films at different temperatures. (b) Activation energy Ea for the 2.5 vol.% BT NF/PVDF and different contents BT@Al2O3 NF nanocomposite films 12
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electric fields, P–E loops of pure PVDF and the nanocomposite with various
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concentrations of fillers were measured at 100 Hz and room temperature as shown in
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Fig 7(a). The incorporation of the BT@Al2O3 NF fillers into the PVDF polymers
5
obviously increases the maximum polarization of the nanocomposites. Most notably,
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very minor increase in remnant polarizations with increasing concentration of the
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fillers is observed. This result indicated that the use of BT@Al2O3 NF fillers provide a
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relatively high maximum polarization while eliminating the remnant polarization and
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ultimately the discharged energy performances of the nanocomposites. Moreover, the
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nanocomposite exhibits narrow P-E loop at high electric field, which is favorable to
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enhance discharged energy density and discharged efficiency. Fig 7(b) illustrates the
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P-E loops of the nanocomposite of 2.5 vol. % BT@Al2O3 NF and that of 2.5 vol. %
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BT NF. The former exhibits narrow P-E loop, lower maximum polarization and most
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importantly, remarkably reduced remnant polarization, compared to the latter. The
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discharged
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concentrations of BT@Al2O3 NF is presented in Fig.7(c). The maximum discharged
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energy density of the nanocomposites films with 2.5 vol.% BT@Al2O3 NF reaches
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7.1J/cm3 at 3800 kV/cm, while that of pure PVDF is only 2.8 J/cm3 at 4000 kV/cm.
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The results clearly indicates that the introduction of core-shell structure BT@Al2O3
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NF ceramic fillers into PVDF can significantly improve the discharged energy density
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performance.
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For the practice application, a high energy storage density and high discharge
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efficiency (η) are desired simultaneously. The discharge efficiency could is defined
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according to the formula: η =
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discharged energy, and energy loss, respectively. The energy loss density ( Ul ) is
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calculated by the numerical integration of closed area of the hysteresis loops.
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Fig. 7(d) gives the discharged efficiency of the nanocomposite films. A high
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discharged efficiency calculated for the nanocomposite films filled with 2.5 vol.%
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BT@Al2O3 NF is 82% at 1000 kV/cm and can still be maintained 65.1% even at 3800
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kV/cm, whereas the discharged efficiency calculated for the PVDF films exhibits
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considerable reduction from 71.7% at 1000 kV/cm to 58.2% at 3300kV/cm. The
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results indicates that the introduction of BT@Al2O3 NF fillers can enhance the energy
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density while at the same time maintaining a high discharged efficiency
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Fig. 7. (a) P-E loops for nanocomposites filled at room temperature with different volume fraction
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of BT@Al2O3 NF at 100Hz (b) P–E loops for 2.5 vol.% BT@Al2O3 NF nanocomposites and 2.5 vol.%
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Ud U = 1 − l , where U s , Ud , and Ul are the stored, Us Us
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BT NF /PVDF nanocomposites (c) Discharge energy density of nanocomposites (c) Efficiency of
nanocomposites films
The improvement
of
discharge energy density in
BT@Al2O3
NF/PVDF
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nanocomposites is mainly contributed to the Al2O3 layers on the surface of BT NF to
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form moderate interfacial area, which can reduce the dielectric differences between
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the two phases and improve the MWS interfacial polarization in the nanocomposites.
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To illustrate the MWS interfacial polarization in the nanocomposites, the frequency
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dependence of dielectric constant and loss of the nanocomposite of 2.5 vol. %
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BT@Al2O3 NF and that of 2.5 vol. % BT NF is presented Fig. 8. It can be seen that
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the dielectric constant of the nanocomposites of BT NF is higher than that of BT@Al2O3
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NF. Dielectric constant of all composites decreases with frequency because the dipoles
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of filler and polymer cannot shift their orientation sufficiently fast when the frequency
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of the applied electric field exceeds the relaxation frequency. In addition, loss of the
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nanocomposites remains low with little variation at low frequency, followed by rapid
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increases at high frequency as a result of α a relaxation which is associated with the
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glass transition of pure PVDF polymer.
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Fig. 8. Frequency dependencies of the dielectric constant and loss tangent of the dielectric 15
ACCEPTED MANUSCRIPT constant and loss of the nanocomposite of 2.5 vol. % BT@Al2O3 NF and that of 2.5 vol. % BT NF
2
Dielectric relaxation are observed from dielectric spectra of nanocomposites. To study
3
relaxation, frequency dependence of real part (eps′) and the imaginary part (eps″) of
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of BT NF composites and BT@Al2O3 NF composites is shown in Fig. 9. relaxation
5
intensity or relaxation time can be described by a superposition of independent Debye
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functions.
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ε ′(ω ) = ε ∞ + ∑
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ε ∞ is the dielectric constant at the high frequency limit, n was taken as 15 to
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accurately capture the shape of the curve, ∆ε i is the relaxation intensity of each term
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with ω and τ i being the angular frequency and relaxation time, M represents the
11
shift in intensity, while S represents the shift in relaxation time, respectively. By
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matching the simulation results to the experimental data (Fig 9), M and S are fitted to
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be 0.6 and 0.9. This results show that relaxation intensity and relaxation time of the
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nanocomposites of BT NF is higher than that of BT@Al2O3 NF, implying a reduced
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relaxation intensity and an recreased relaxation time of the interface region[25-29].
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Because the Al2O3 layer on the surface of BT NF can reduce the dielectric differences
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between the two phases and improve the MWS interfacial polarization in the
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nanocomposites[13]. Suppression the MWS interfacial polarization is beneficial for
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the improvement of discharged energy density and efficiency. Less polarization
20
charges are trapped in the discharge process and the majority of charges can be
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released by reducing MWS interfacial polarization, which means high energy storage
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efficiency. Therefore, the Al2O3 layer on the surface of BT NF significantly enhances
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Μ∆ε i 2 i =1 1 + ( Sωτ i )
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n
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ACCEPTED MANUSCRIPT discharged energy density of the nanocomposites through the reduction of the MWS
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interfacial polarization of the nanocomposites.
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Fig. 9. Frequency dependence of real part (eps′) and the imaginary part (eps″) of the nanocomposite of 2.5 vol. % BT@Al2O3 NF and that of 2.5 vol. % BT NF
Conclusions
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Homogeneous nanocomposites, using the 1D core-shell structure BT@Al2O3 NF as
8
fillers and PVDF as matrix, has been prepared through a solution casting method.
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TEM results showed that Al2O3 was uniformly coated on the surface of BT NF with
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an average thickness of 6 nm. SEM results showed that the BT@Al2O3 NF were
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dispersed homogeneously in the PVDF. The nanocomposites of BT@Al2O3 NF
12
exhibit enhanced breakdown strength, reduced energy loss and enhanced discharged
13
energy density under high electric field compared to those of BT NF. The maximum
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discharged energy density of 7.1 J/cm3 was obtained in the nanocomposite film with
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2.5 vol.% BT@Al2O3 NF, which is 154% higher than that of the pure PVDF. The
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efficiency of the nanocomposites films with 2.5 vol.% BT@Al2O3 NF is higher than
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82% at electric fields below 1000 kV/cm and can still be maintained at 65.1% even at
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3800 kV/cm. Such significant enhancement is closely related to the combined effect
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Acknowledgments
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This research was supported by the Ministry of Sciences and Technology of China
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through 973-project under Grant (2015CB654601), Natural Science Foundation of
5
Henan Department of Education (17A430013), Doctoral Program of Henan Insititute
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of Engineering (D2016015, D2016016) and the Programs for Science and Technology
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Development, Henan Province, China (132102210035, 142300410439).
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References:
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[1] X. Huang, P. Jiang, Core–Shell Structured High‐k Polymer Nanocomposites for Energy Storage and Dielectric Applications, Adv Mater, 27 (2015) 546-554.
[2] L. Wu, X. Wang, H. Gong, Y. Hao, Z. Shen, L. Li, Core-satellite BaTiO 3@ SrTiO 3 assemblies for a local compositionally graded relaxor ferroelectric capacitor with enhanced energy storage density and high energy efficiency, Journal of Materials Chemistry C, 3 (2015) 750-758. [3] S.H. Liu, S.X. Xue, W.Q. Zhang, J.W. Zhai, Enhanced dielectric and energy storage density induced by surface-modified BaTiO3 nanofibers in poly(vinylidene fluoride) nanocomposites, Ceram Int, 40 (2014)
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15633-15640.
[4] L. Wu, X. Wang, L. Li, Core-shell BaTiO 3@ BiScO 3 particles for local graded dielectric ceramics with enhanced temperature stability and energy storage capability, J Alloy Compd, 688 (2016) 113-121.
[5] S.X. Xue, S.H. Liu, W.Q. Zhang, J.W. Wang, L.J. Tang, B. Shen, J.W. Zhai, Dielectric properties and
EP
charge-discharge behaviors in niobate glass ceramics for energy-storage applications, J Alloy Compd, 617 (2014) 418-422.
[6] X. Zhang, Y. Shen, Q. Zhang, L. Gu, Y. Hu, J. Du, Y. Lin, C.W. Nan, Ultrahigh Energy Density of Polymer Nanocomposites Containing BaTiO3@ TiO2 Nanofibers by Atomic‐Scale Interface Engineering, Adv
AC C
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
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1
Mater, 27 (2015) 819-824.
[7] B.J. Chu, X. Zhou, K.L. Ren, B. Neese, M.R. Lin, Q. Wang, F. Bauer, Q.M. Zhang, A dielectric polymer with high electric energy density and fast discharge speed, Science, 313 (2006) 334-336. [8] L.Y. Xie, X.Y. Huang, Y.H. Huang, K. Yang, P.K. Jiang, Core@Double-Shell Structured BaTiO3-Polymer Nanocomposites with High Dielectric Constant and Low Dielectric Loss for Energy Storage Application, J Phys Chem C, 117 (2013) 22525-22537. [9] Z.M. Dang, J.K. Yuan, S.H. Yao, R.J. Liao, Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage, Adv Mater, 25 (2013) 6334-6365. [10]
X.Y.
Huang,
C.Y.
Zhi,
P.K.
Jiang,
D.
Golberg,
Y.
Bando,
T.
Tanaka,
Polyhedral
Oligosilsesquioxane-Modified Boron Nitride Nanotube Based Epoxy Nanocomposites: An Ideal Dielectric Material with High Thermal Conductivity, Adv Funct Mater, 23 (2013) 1824-1831. [11] L.Y. Xie, X.Y. Huang, C. Wu, P.K. Jiang, Core-shell structured poly(methyl methacrylate)/BaTiO3 18
ACCEPTED MANUSCRIPT nanocomposites prepared by in situ atom transfer radical polymerization: a route to high dielectric constant materials with the inherent low loss of the base polymer, J Mater Chem, 21 (2011) 5897-5906. [12] H.X. Tang, H.A. Sodano, High energy density nanocomposite capacitors using non-ferroelectric nanowires, Appl Phys Lett, 102 (2013) 063901. [13] K. Yu, Y.J. Niu, Y.Y. Bai, Y.C. Zhou, H. Wang, Poly(vinylidene fluoride) polymer based BaTiO3/SiO2 nanoparticles, Appl Phys Lett, 102 (2013) 102903.
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nanocomposites with significantly reduced energy loss by filling with core-shell structured [14] C. Wu, X.Y. Huang, X.F. Wu, J.H. Yu, L.Y. Xie, P.K. Jiang, TiO2-nanorod decorated carbon nanotubes for high-permittivity and low-dielectric-loss polystyrene composites, Compos Sci Technol, 72 (2012) 521-527.
[15] S. Liu, S. Xiao, S. Xiu, B. Shen, J. Zhai, Z. An, Poly (vinylidene fluoride) nanocomposite capacitors Ba 0.6 Sr 0.4 TiO 3 nanofibers, RSC Adv, 5 (2015) 40692-40699.
SC
with a significantly enhanced dielectric constant and energy density by filling with surface-fluorinated [16] S.H. Liu, J.W. Zhai, A small loading of surface-modified Ba0.6Sr0.4TiO3 nanofiber-filled nanocomposites with enhanced dielectric constant and energy density, RSC Adv, 4 (2014)
M AN U
40973-40979.
[17] Y. Song, Y. Shen, H.Y. Liu, Y.H. Lin, M. Li, C.W. Nan, Improving the dielectric constants and breakdown strength of polymer composites: effects of the shape of the BaTiO3 nanoinclusions, surface modification and polymer matrix, J Mater Chem, 22 (2012) 16491-16498. [18] Y. Song, Y. Shen, P.H. Hu, Y.H. Lin, M. Li, C.W. Nan, Significant enhancement in energy density of polymer composites induced by dopamine-modified Ba0.6Sr0.4TiO3 nanofibers, Appl Phys Lett, 101 (2012) 152904.
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[19] G. Wang, X. Huang, P. Jiang, Tailoring Dielectric Properties and Energy Density of Ferroelectric Polymer Nanocomposites by High-k Nanowires, Acs Appl Mater Interfaces, 7 (2015) 18017-18027. [20] D.Y. Lee, K.H. Lee, M.H. Lee, N.I. Cho, B.Y. Kim, Synthesis of electrospun BaSrTiO3/PVP nanofibers, J Sol-gel Sci Techn, 53 (2010) 43-49.
[21] H.P. Li, H. Wu, D.D. Lin, W. Pan, High T-c in Electrospun BaTiO3 Nanofibers, J Am Ceram Soc, 92
EP
(2009) 2162-2164.
[22] W. Wu, X.Y. Huang, S.T. Li, P.K. Jiang, T. Toshikatsu, Novel Three-Dimensional Zinc Oxide Superstructures for High Dielectric Constant Polymer Composites Capable of Withstanding High Electric Field, J Phys Chem C, 116 (2012) 24887-24895.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44
[23] K. Yu, Y.J. Niu, Y.C. Zhou, Y.Y. Bai, H. Wang, Nanocomposites of Surface-Modified BaTiO3 Nanoparticles Filled Ferroelectric Polymer with Enhanced Energy Density, J Am Ceram Soc, 96 (2013) 2519-2524.
[24] Z. Li, L.A. Fredin, P. Tewari, S.A. DiBenedetto, M.T. Lanagan, M.A. Ratner, T.J. Marks, In Situ Catalytic Encapsulation of Core-Shell Nanoparticles Having Variable Shell Thickness: Dielectric and Energy Storage Properties of High-Permittivity Metal Oxide Nanocomposites, Chem Mater, 22 (2010) 5154-5164. [25] Y. Huang, T.M. Krentz, J.K. Nelson, L.S. Schadler, Y. Li, H. Zhao, L.C. Brinson, M. Bell, B. Benicewicz, K. Wu, Prediction of interface dielectric relaxations in bimodal brush functionalized epoxy nanodielectrics by finite element analysis method, in:
2014 IEEE Conference on Electrical Insulation
and Dielectric Phenomena (CEIDP), IEEE, 2014, pp. 748-751. [26] Y. Huang, X. Huang, L.S. Schadler, J. He, P. Jiang, Core@ double-shell Structured Nanocomposites: 19
ACCEPTED MANUSCRIPT a Route to High Dielectric Constant and Low Loss Material, Acs Appl Mater Interfaces, (2016). [27] Y. Huang, L.S. Schadler, Understanding the Strain-Dependent Dielectric Behavior of Carbon Black Reinforced Natural Rubber: An interfacial or bulk phenomenon?, arXiv preprint arXiv:1609.09434, (2016). [28] R. Bartnikas, R. Eichhorn, A.S.f. Testing, Materials, A.C.D.-o.E.I. Materials, I.d.r. d'Hydro-Québec, U.C. Corporation, Electrical properties of solid insulating materials: molecular structure and electrical behavior, 1983. [29] X. Huang, C. Zhi, Polymer Nanocomposites: Electrical and Thermal Properties, Springer, 2016.
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Highlights 1. BaTiO3@Al2O3 nanofibers/PVDF nanocomposite films were prepared. 2. The discharged energy density performance of the nanocomposite films were
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2. The discharged energy density of the nanocomposite films are enhanced compared
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3. The enhancement mechanism of the nanocomposite films was proposed.