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High energy storage density of poly(vinylidene fiuoride) bulk nanocomposites at low electric field induced by giant dielectric constant ceramic nanopowders
Author’s Accepted Manuscript High energy storage density of poly(vinylidene fiuoride) bulk nanocomposites at low electric field induced by giant dielectric constant ceramic nanopowders Zhuo Wang, Tian Wang, Yujia Xiao, Wenwen Nian, Haonan Chen www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)32179-5 https://doi.org/10.1016/j.ceramint.2018.08.119 CERI19154
To appear in: Ceramics International Cite this article as: Zhuo Wang, Tian Wang, Yujia Xiao, Wenwen Nian and Haonan Chen, High energy storage density of poly(vinylidene fiuoride) bulk nanocomposites at low electric field induced by giant dielectric constant ceramic n a n o p o w d e r s , Ceramics International, https://doi.org/10.1016/j.ceramint.2018.08.119 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 galley proof before it is published in its final citable 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.
Ceramics International draft (Supplement Issues for AMEC-11)
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High energy storage density of poly(vinylidene fiuoride) bulk nanocomposites at low electric field induced by giant dielectric constant ceramic nanopowders Zhuo Wang, Tian Wang*, Yujia Xiao, Wenwen Nian, Haonan Chen School of Materials Science and Engineering, Shaanxi University of Science and Technology – XuefuZhonglu 3, Xi’an 710021, PR China Available online date provided by the publisher __________________________________________________________________________________________________________
Abstract Ba(Fe0.5Ta0.5)O3/poly(vinylidene fiuoride) (BFT/PVDF) bulk nanocomposites are fabricated via a physical mixing method followed by heat treatment. The BFT particles in nanocomposites exhibit excellent dispersion and compatibility. The enhanced dielectric constant and loss have been obtained, which is attributed to the giant dielectric constant of BFT and the enhanced interfacial polarization. An energy storage density up to 2.18 J/cm3 is achieved in the nanocomposites with BFT filler content of 2 vol % at a low electric field of 500 kV/cm. This value of energy storage density at low electric field is much higher than other polymer-based bulk nanocomposites. It is believed that the study may provide a new direction to enhance the energy storage density of ceramic/polymer nanocomposites. Keywords: Polymeric composites; Low electric field; Energy storage density
________________________________________________________________________________________________ 1. Introduction With the rapid development of electric industry, there are great needs for advanced electric energy storage systems, and it is necessary to create the innovative materials with high energy storage density [1]. By integrating two or more materials with complementary properties, nanocomposites offer the potential to have performance far beyond those of the constituent materials [2]. In this situation, ceramic/polymer nanocompositess have emerged as time has required. Utilizing the properties of both, polymers exhibit high breakdown strength, while the ceramics have a high dielectric constant. Poly(vinylidene fiuoride) (PVDF) has received much attention due to its dipoles, which give rise to polar behavior [3]. And it has a dielectric constant of up to 10. Meanwhile, Ba(Fe0.5Ta0.5)O3 (BFT) ceramics have not only a giant dielectric constant but also a low loss tangent and excellent frequency stability [4]. Thus, the research about BFT/PVDF nanocomposites is valuable for future energy storage devices [5]. Up to now, the research of ceramic/polymer nanocomposites can be divided into two types: bulk materials for high dielectric constant with high loading ceramic and film materials for high energy storage density with low loading ceramic. Little research focuses on bulk materials for high energy storage density with low loading ceramic, which is due to the reason that it is difficult to be breakdown using a normal withstand voltage test system. However, the value of breakdown strength can be tested with giant dielectric ceramic loading. It is valuable to achieve a high energy storage density at low electric field. The advantages of the low electric field lie in two reasons. On the one hand, the probability of failure will significantly decrease. On the other hand, the applications of electrostatic capacitors may be broadened at the low electric field, making them viable choices for a number of electronic devices operated under low voltage [6]. In this paper, BFT/PVDF bulk composites are prepared and investigated, which exhibit excellent energy storage density at low electric field. We hope to open the doors to the improvement of applications using giant dielectric ceramic nanopowders as a filler. 2. Experimental 2.1 Materials. *
Corresponding author. Tel.: +8615114845870; Fax.:+02986168801 E-mail address: [email protected] (Tian Wang) Pages provided by publisher
Z. Wang et al. Oxalic acid dehydrate ((COOH)2•2H2O), iron nitrate nonahydrate (Fe(NO3)3•9H2O), barium nitrate (Ba(NO3)2), ammonium hydroxide (NH4OH), were supplied from Sinopharm Chemical Reagent Co., Ltd. Tantalum chloride (TaCl5) were purchased from Alfa Aesar Co., Ltd. Poly(vinylidene fluoride) (PVDF, Mw=534,000) was obtained from Sigmaaldrich Co., LLC. 2.2 Preparation of the BFT /PVDF Nanocomposites. BFT nanopowders were prepared by an oxalate-coprecipitation route. TaCl5 was dissolved in ethanol, and (COOH)2·2H2O, Fe(NO3)3·9H2O and Ba(NO3)2 were dissolved in deionized water respectively. The above solutions were completely mixed via stirring. PH value of the mixed solution was adjusted to 10.0 by NH4OH. The nanopowders, after centrifugation and drying, were calcined at 1000 oC for 2 h. BFT/PVDF nanocomposites were prepared via a physical mixing method followed by heat treatment. Firstly, BFT nanopowders and PVDF were weighed accurately and mixed by vibro-milling for 1 h. Next, the mixture was drying for 24 h at 80 oC. Then, the nanocomposites were pressed into disks by uniaxial compression at 80 MPa. Finally the disks were heat treated in a vacuum furnace at 170 oC for 2 h. 2.3 Characterization. XRD patterns of samples were collected on an X-ray diffractometer (Rigaku D/max-2200PC). The cross-sectional morphology of samples was obtained by using SEM (Hitachi S-4800). The dielectric properties of samples were measured using an impedance analyzer (Agilent E4980A). P-E hysteresis loops were obtained using ferroelectric test system (Radiant Premier II). Prior to measuring dielectric properties and P-E loops, the disks were polished to a thickness of 0.15 (±0.02) mm and sputtered gold electrodes on both sides. 3. Results and discussion XRD patterns of BFT powders, PVDF powders, PVDF disks and BFT/PVDF nanocomposites are shown
Fig.1. XRD patterns of BFT powders, PVDF powders, PVDF disks and BFT/PVDF nanocomposites
Fig.2. Cross-sectional SEM images of BFT/PVDF nanocomposites: (a) PVDF, (b) 2 vol % BFT, (c) 4 vol % BFT, (d) 6 vol % BFT, (e) 8 vol % BFT, (f) 10 vol % BFT.
in Fig. 1. BFT powders exhibit a typical perovskite structure. PVDF powders and disks show the single phase of α. For BFT/PVDF nanocomposites, the addition of BFT powders has a strong effect on the characteristic diffraction peaks of PVDF in intensity, which can be ascribed to the shielding effect for high intensity of diffraction peaks in BFT[7]. Fig. 2(a)-(f) show the fractured cross section morphologies of nanocomposites filled with 0, 2, 4, 6, 8 and 10 vol % of BFT powders, respectively. It is observed that nano-sized BFT powders are homogeneously dispersed in the PVDF. And the nanocomposites are dense without holes, which lays the foundation for excellent dielectric properties of nanocomposites. Frequency dependences of dielectric constant and dielectric loss tangent for BFT/PVDF nanocomposites
Fig.3. Frequency dependences of (a) dielectric constant and (b) dielectric loss tangent for BFT/PVDF nanocomposites
Fig.4. (a) Frequency dependences of AC conductivity and (b) AC conductivity at 102 Hz for BFT/PVDF nanocomposites
are shown in Fig. 3. It is observed that both the dielectric constant and dielectric loss tangent of nanocomposites increase with increasing concentrations of BFT fillers, which is attributed to the giant dielectric constant of BFT and the enhanced
Z. Wang et al. interfacial polarization. For the nanocomposite with 10 vol% BFT, the dielectric constant at 100 Hz is 1.7 times higher than that of pure PVDF. The high dielectric constant has significantly effects on polarization. Furthermore, the dielectric constant and dielectric loss tangent of all of nanocomposites slowly decreases with the increase of frequency from 20-104 Hz. It is believed that the decrease of the dielectric constant is mainly caused by the reduction in Maxwell-Wagner-Sillars interfacial polarization[8]. It is originated from the regions of accumulated trap charges induced by defects in nanocomposites. However, at the frequency range above 104 Hz, a distinct decrease in dielectric constant and a rapid increase in dielectric loss tangent with the increase of frequency is observed, which is due to the reduction in dipolar orientation polarization. It is related to the micro-Brownian motion of the whole chain, known as α relaxation[9]. The alternating current conductivity of BFT/PVDF nanocomposites is shown in Fig. 4. The conductivity of the BFT/PVDF nanocomposites gradually increases with the increasing of frequency and remains less than 10-8 S/cm at 100 Hz. Nevertheless, a rapid increase is found when the volume fractions of BFT nanopowdes are from 2 vol % to 4 vol %. It can be explained that lots of conductive paths between BFT and PVDF is formed with high filler content. BFT is semiconducting, which induces that the soft percolation regime is beginning to take effect to form conductive paths at low filler content[10]. When the BFT concentration is less than 4 vol %, BFT nanoparticles are far away from each others, and the conductive paths at interfaces are difficult to formation. On the contrary, the conductive paths are easily formed, thus resulting the increment of conductivity of the nanocomposites with filler content. The Weibull distribution and breakdown strength of BFT/PVDF nanocomposites are shown in Fig. 5(a) and (b). The Weibull distribution is calculated as P( E) 1 exp( ( E / Eb ) m ) , where P is the cumulative failure probability, the characteristic breakdown strength (Eb) is obtained when the cumulative probability of electric failure equals at 63.2 %, and m is the shape parameter associated with linear regressive fit of the date distribution. A higher value of m indicates a higher level of reliability. It is worth noting that the breakdown strength of BFT/PVDF nanocomposites first increases and then decreases with the increase of BFT volume fraction, which may be ascribed to the more serious structural imperfection phenomena caused by the micro voids between ceramic and polymer when more fillers are employed in the nanocomposites[11]. The nanocomposites filled with 2 vol % BFT have a high breakdown strength of about 493 kV/cm, which is higher than that of pure PVDF (469 kV/cm). The energy storage density (U) of polymer-based composites is always dominated by the breakdown characteristics[12]. As a result, the nanocomposite filled with 2 vol% BFT nanopowders exhibits an excellent energy storage density of 2.18 J/cm3 under a low electric field of 500 kV/cm (Fig. 5(c) and (d)). For comparative analysis, the energy storage density of some other polymer-based
Fig.5. (a) Weibull distribution, (b) breakdown strength, (c) P-E loops and (d) energy storage density of BFT/PVDF nanocomposites
Table 1. Energy storage density and breakdown strength of PVDF-based bulk nanocomposites. Compositions 0.16BaTiO3/0.84PVDF 0.40PbZr0.52Ti0.48O2/0.60PVDF 0.10Na1/3Ca1/3Bi1/3Cu3Ti4O12/ 0.05Ba(Fe0.5Nb0.5)O3/0.85PVDF 0.02Ba(Fe0.5Ta0.5)O3/0.98PVDF
U (J/cm3) 0.008 1.28
Eb (kV/cm) 489 750
1.45
600
[15]
2.1
500
In this work
References [13] [14]
bulk nanocomposites is listed in Table 1[13–15]. It can be seen that the energy storage density of BFT/PVDF nanocomposites under a low electric field is significantly higher than other bulk nanocomposites. Most importantly, the filler content of BFT is only 2 vol %, which is lower than the filler-content requirement in other works. It is believed that the superior energy storage properties is mainly caused by the giant dielectric effect of BFT nanopowders, which induces a strongly enhanced dielectric constant loading to high polarization[16]. This advantage can be reflected by the competitive price in the future industrial applications. 4. Conclusions BFT/PVDF nanocomposites are fabricated via a physical mixing method followed by heat treatment. The dense bulk nanocomposites with well dispersed particles are examined by SEM. The enhanced dielectric constant and loss have been obtained, which is attributed to the giant dielectric constant of BFT and the enhanced interfacial polarization. An energy storage density up to 2.18 J/cm3 is achieved in the nanocomposites with BFT filler content of 2 vol % at a low electric field of 500 kV/cm. This value of energy storage density at low electric field is much higher than other polymer-based bulk nanocomposites. It is believed that the functional BFT/PVDF nanocomposites with good energy storage performance can be applied in future high-technology fields. Acknowledgements The present work was supported by National Natural Science Foundation of China (51572160), the Natural Science Foundation of Shaanxi Province (2016JQ5083, 2017KJXX-44), China Postdoctoral Science Foundation (2015M572516, 2016T90881) and Graduate Innovation Fund of Shaanxi University of Science and Technology. References [1] E. Karden, B. Fricke, T. Miller, K. Snyder, Energy storage devices for future hybrid electric vehicles, J. Power Sources. 168 (2007) 2-11. [2] Y. Bai, Z. Cheng, V. Bharti, H. Xu, Q. Zhang, High-dielectric-constant ceramic-powder polymer composites, Appl. Phys. Lett. 76 (2000) 3804-3806. [3] L. Zhu, Q. Wang, Novel ferroelectric polymers for high energy density and low loss dielectrics, Macromolecules. 45 (2012) 29372954. [4] Z. Wang, X.M. Chen, L. Ni, Y.Y. Liu, X.Q. Liu, Dielectric relaxations in Ba(Fe1/2Ta1/2)O3 giant dielectric constant ceramics, Appl. Phys. Lett. 90 (2007) 102905. [5] Z. Wang, T. Wang, C. Wang, Y. Xiao, P. Jing, Y. Cui, Y. Pu, Poly(vinylidene fluoride) flexible nanocomposite films with dopamine-coated giant dielectric ceramic nanopowders, Ba(Fe0.5Ta0.5)O3, for high energy-storage density at low electric field, ACS Appl. Mater. Interfaces. 9 (2017) 29130-29139. [6] Y. Shen, D. Shen, X. Zhang, J. Jiang, Z. Dan, Y. Song, Y. Lin, M. Li, C.-W. Nan, High energy density of polymer nanocomposites at low electric field induced by modulation of topological-structure, J. Mater. Chem. A. 4 (2016) 8359-8365. [7] J. Fu, Y. Hou, M. Zheng, Q. Wei, M. Zhu, H. Yan, Improving dielectric properties of PVDF composites by employing surface modified strong polarized BaTiO3 particles derived by molten salt method, ACS Appl. Mater. Interfaces. 7 (2015) 24480-24491. [8] E. Tuncer, Y. V. Serdyuk, S.M. Gubanski, Dielectric mixtures: Electrical properties and modeling, IEEE Trans. Dielectr. Electr. Insul. 9 (2002) 809-828. [9] Prateek, V.K. Thakur, R.K. Gupta, Recent progress on ferroelectric polymer-based nanocomposites for high energy density capacitors: synthesis, dielectric properties, and future aspects, Chem. Rev. 116 (2016) 4260-4317.
Z. Wang et al. [10] Y. Yang, B. Zhu, Z. Lu, Z. Wang, C. Fei, D. Yin, R. Xiong, J. Shi, Q. Chi, Q. Lei, Polyimide/nanosized CaCu3Ti4O12 functional hybrid films with high dielectric permittivity, Appl. Phys. Lett. 101 (2013) 042904. [11] Z. Pan, L. Yao, J. Zhai, B. Shen, S. Liu, Excellent energy density of polymer nanocomposites containing BaTiO 3@Al2O3 nanofibers induced by moderate interfacial area, J. Mater. Chem. A. 4 (2016) 13259-13264. [12] Y. Wang, J. Cui, Q. Yuan, Y. Niu, Y. Bai, H. Wang, Significantly enhanced breakdown strength and energy density in sandwichstructured barium titanate/poly(vinylidene fluoride) nanocomposites, Adv. Mater. 27 (2015) 6658-6663. [13] S. Luo, Y. Shen, S. Yu, Y. Wan, W.-H. Liao, R. Sun, W. Ching-Ping, Construction of a 3D-BaTiO3 network leading to significantly enhanced dielectric permittivity and energy storage density of polymer composites, Energy Environ. Sci. 10 (2017) 137-144. [14] H. Tang, Y. Lin, H.A. Sodano, Enhanced energy storage in nanocomposite capacitors through aligned PZT nanowires by uniaxial strain assembly, Adv. Energy Mater. 2 (2012) 469-476. [15] T. Wang, Z. Wang, C. Wang, Y. Xiao, The size-matching effect in 0.1Na1/3Ca1/3Bi1/3Cu3Ti4O12-xBa(Fe0.5Nb0.5)O3-(0.9-x)PVDF composites, Ceram. Int. 43 (2017) S239-S243. [16] Z. Wang, T. Wang, C. Wang, Y. Xiao, Mechanism of enhanced dielectric performance in Ba(Fe0.5Ta0.5)O3/ poly(vinylidene fluoride) nanocomposites, Ceram. Int. 43 (2017) S244-S248.
PII: DOI: Reference:
S0272-8842(18)32179-5 https://doi.org/10.1016/j.ceramint.2018.08.119 CERI19154
To appear in: Ceramics International Cite this article as: Zhuo Wang, Tian Wang, Yujia Xiao, Wenwen Nian and Haonan Chen, High energy storage density of poly(vinylidene fiuoride) bulk nanocomposites at low electric field induced by giant dielectric constant ceramic n a n o p o w d e r s , Ceramics International, https://doi.org/10.1016/j.ceramint.2018.08.119 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 galley proof before it is published in its final citable 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.
Ceramics International draft (Supplement Issues for AMEC-11)
Header will be provided by the publisher
High energy storage density of poly(vinylidene fiuoride) bulk nanocomposites at low electric field induced by giant dielectric constant ceramic nanopowders Zhuo Wang, Tian Wang*, Yujia Xiao, Wenwen Nian, Haonan Chen School of Materials Science and Engineering, Shaanxi University of Science and Technology – XuefuZhonglu 3, Xi’an 710021, PR China Available online date provided by the publisher __________________________________________________________________________________________________________
Abstract Ba(Fe0.5Ta0.5)O3/poly(vinylidene fiuoride) (BFT/PVDF) bulk nanocomposites are fabricated via a physical mixing method followed by heat treatment. The BFT particles in nanocomposites exhibit excellent dispersion and compatibility. The enhanced dielectric constant and loss have been obtained, which is attributed to the giant dielectric constant of BFT and the enhanced interfacial polarization. An energy storage density up to 2.18 J/cm3 is achieved in the nanocomposites with BFT filler content of 2 vol % at a low electric field of 500 kV/cm. This value of energy storage density at low electric field is much higher than other polymer-based bulk nanocomposites. It is believed that the study may provide a new direction to enhance the energy storage density of ceramic/polymer nanocomposites. Keywords: Polymeric composites; Low electric field; Energy storage density
________________________________________________________________________________________________ 1. Introduction With the rapid development of electric industry, there are great needs for advanced electric energy storage systems, and it is necessary to create the innovative materials with high energy storage density [1]. By integrating two or more materials with complementary properties, nanocomposites offer the potential to have performance far beyond those of the constituent materials [2]. In this situation, ceramic/polymer nanocompositess have emerged as time has required. Utilizing the properties of both, polymers exhibit high breakdown strength, while the ceramics have a high dielectric constant. Poly(vinylidene fiuoride) (PVDF) has received much attention due to its dipoles, which give rise to polar behavior [3]. And it has a dielectric constant of up to 10. Meanwhile, Ba(Fe0.5Ta0.5)O3 (BFT) ceramics have not only a giant dielectric constant but also a low loss tangent and excellent frequency stability [4]. Thus, the research about BFT/PVDF nanocomposites is valuable for future energy storage devices [5]. Up to now, the research of ceramic/polymer nanocomposites can be divided into two types: bulk materials for high dielectric constant with high loading ceramic and film materials for high energy storage density with low loading ceramic. Little research focuses on bulk materials for high energy storage density with low loading ceramic, which is due to the reason that it is difficult to be breakdown using a normal withstand voltage test system. However, the value of breakdown strength can be tested with giant dielectric ceramic loading. It is valuable to achieve a high energy storage density at low electric field. The advantages of the low electric field lie in two reasons. On the one hand, the probability of failure will significantly decrease. On the other hand, the applications of electrostatic capacitors may be broadened at the low electric field, making them viable choices for a number of electronic devices operated under low voltage [6]. In this paper, BFT/PVDF bulk composites are prepared and investigated, which exhibit excellent energy storage density at low electric field. We hope to open the doors to the improvement of applications using giant dielectric ceramic nanopowders as a filler. 2. Experimental 2.1 Materials. *
Corresponding author. Tel.: +8615114845870; Fax.:+02986168801 E-mail address: [email protected] (Tian Wang) Pages provided by publisher
Z. Wang et al. Oxalic acid dehydrate ((COOH)2•2H2O), iron nitrate nonahydrate (Fe(NO3)3•9H2O), barium nitrate (Ba(NO3)2), ammonium hydroxide (NH4OH), were supplied from Sinopharm Chemical Reagent Co., Ltd. Tantalum chloride (TaCl5) were purchased from Alfa Aesar Co., Ltd. Poly(vinylidene fluoride) (PVDF, Mw=534,000) was obtained from Sigmaaldrich Co., LLC. 2.2 Preparation of the BFT /PVDF Nanocomposites. BFT nanopowders were prepared by an oxalate-coprecipitation route. TaCl5 was dissolved in ethanol, and (COOH)2·2H2O, Fe(NO3)3·9H2O and Ba(NO3)2 were dissolved in deionized water respectively. The above solutions were completely mixed via stirring. PH value of the mixed solution was adjusted to 10.0 by NH4OH. The nanopowders, after centrifugation and drying, were calcined at 1000 oC for 2 h. BFT/PVDF nanocomposites were prepared via a physical mixing method followed by heat treatment. Firstly, BFT nanopowders and PVDF were weighed accurately and mixed by vibro-milling for 1 h. Next, the mixture was drying for 24 h at 80 oC. Then, the nanocomposites were pressed into disks by uniaxial compression at 80 MPa. Finally the disks were heat treated in a vacuum furnace at 170 oC for 2 h. 2.3 Characterization. XRD patterns of samples were collected on an X-ray diffractometer (Rigaku D/max-2200PC). The cross-sectional morphology of samples was obtained by using SEM (Hitachi S-4800). The dielectric properties of samples were measured using an impedance analyzer (Agilent E4980A). P-E hysteresis loops were obtained using ferroelectric test system (Radiant Premier II). Prior to measuring dielectric properties and P-E loops, the disks were polished to a thickness of 0.15 (±0.02) mm and sputtered gold electrodes on both sides. 3. Results and discussion XRD patterns of BFT powders, PVDF powders, PVDF disks and BFT/PVDF nanocomposites are shown
Fig.1. XRD patterns of BFT powders, PVDF powders, PVDF disks and BFT/PVDF nanocomposites
Fig.2. Cross-sectional SEM images of BFT/PVDF nanocomposites: (a) PVDF, (b) 2 vol % BFT, (c) 4 vol % BFT, (d) 6 vol % BFT, (e) 8 vol % BFT, (f) 10 vol % BFT.
in Fig. 1. BFT powders exhibit a typical perovskite structure. PVDF powders and disks show the single phase of α. For BFT/PVDF nanocomposites, the addition of BFT powders has a strong effect on the characteristic diffraction peaks of PVDF in intensity, which can be ascribed to the shielding effect for high intensity of diffraction peaks in BFT[7]. Fig. 2(a)-(f) show the fractured cross section morphologies of nanocomposites filled with 0, 2, 4, 6, 8 and 10 vol % of BFT powders, respectively. It is observed that nano-sized BFT powders are homogeneously dispersed in the PVDF. And the nanocomposites are dense without holes, which lays the foundation for excellent dielectric properties of nanocomposites. Frequency dependences of dielectric constant and dielectric loss tangent for BFT/PVDF nanocomposites
Fig.3. Frequency dependences of (a) dielectric constant and (b) dielectric loss tangent for BFT/PVDF nanocomposites
Fig.4. (a) Frequency dependences of AC conductivity and (b) AC conductivity at 102 Hz for BFT/PVDF nanocomposites
are shown in Fig. 3. It is observed that both the dielectric constant and dielectric loss tangent of nanocomposites increase with increasing concentrations of BFT fillers, which is attributed to the giant dielectric constant of BFT and the enhanced
Z. Wang et al. interfacial polarization. For the nanocomposite with 10 vol% BFT, the dielectric constant at 100 Hz is 1.7 times higher than that of pure PVDF. The high dielectric constant has significantly effects on polarization. Furthermore, the dielectric constant and dielectric loss tangent of all of nanocomposites slowly decreases with the increase of frequency from 20-104 Hz. It is believed that the decrease of the dielectric constant is mainly caused by the reduction in Maxwell-Wagner-Sillars interfacial polarization[8]. It is originated from the regions of accumulated trap charges induced by defects in nanocomposites. However, at the frequency range above 104 Hz, a distinct decrease in dielectric constant and a rapid increase in dielectric loss tangent with the increase of frequency is observed, which is due to the reduction in dipolar orientation polarization. It is related to the micro-Brownian motion of the whole chain, known as α relaxation[9]. The alternating current conductivity of BFT/PVDF nanocomposites is shown in Fig. 4. The conductivity of the BFT/PVDF nanocomposites gradually increases with the increasing of frequency and remains less than 10-8 S/cm at 100 Hz. Nevertheless, a rapid increase is found when the volume fractions of BFT nanopowdes are from 2 vol % to 4 vol %. It can be explained that lots of conductive paths between BFT and PVDF is formed with high filler content. BFT is semiconducting, which induces that the soft percolation regime is beginning to take effect to form conductive paths at low filler content[10]. When the BFT concentration is less than 4 vol %, BFT nanoparticles are far away from each others, and the conductive paths at interfaces are difficult to formation. On the contrary, the conductive paths are easily formed, thus resulting the increment of conductivity of the nanocomposites with filler content. The Weibull distribution and breakdown strength of BFT/PVDF nanocomposites are shown in Fig. 5(a) and (b). The Weibull distribution is calculated as P( E) 1 exp( ( E / Eb ) m ) , where P is the cumulative failure probability, the characteristic breakdown strength (Eb) is obtained when the cumulative probability of electric failure equals at 63.2 %, and m is the shape parameter associated with linear regressive fit of the date distribution. A higher value of m indicates a higher level of reliability. It is worth noting that the breakdown strength of BFT/PVDF nanocomposites first increases and then decreases with the increase of BFT volume fraction, which may be ascribed to the more serious structural imperfection phenomena caused by the micro voids between ceramic and polymer when more fillers are employed in the nanocomposites[11]. The nanocomposites filled with 2 vol % BFT have a high breakdown strength of about 493 kV/cm, which is higher than that of pure PVDF (469 kV/cm). The energy storage density (U) of polymer-based composites is always dominated by the breakdown characteristics[12]. As a result, the nanocomposite filled with 2 vol% BFT nanopowders exhibits an excellent energy storage density of 2.18 J/cm3 under a low electric field of 500 kV/cm (Fig. 5(c) and (d)). For comparative analysis, the energy storage density of some other polymer-based
Fig.5. (a) Weibull distribution, (b) breakdown strength, (c) P-E loops and (d) energy storage density of BFT/PVDF nanocomposites
Table 1. Energy storage density and breakdown strength of PVDF-based bulk nanocomposites. Compositions 0.16BaTiO3/0.84PVDF 0.40PbZr0.52Ti0.48O2/0.60PVDF 0.10Na1/3Ca1/3Bi1/3Cu3Ti4O12/ 0.05Ba(Fe0.5Nb0.5)O3/0.85PVDF 0.02Ba(Fe0.5Ta0.5)O3/0.98PVDF
U (J/cm3) 0.008 1.28
Eb (kV/cm) 489 750
1.45
600
[15]
2.1
500
In this work
References [13] [14]
bulk nanocomposites is listed in Table 1[13–15]. It can be seen that the energy storage density of BFT/PVDF nanocomposites under a low electric field is significantly higher than other bulk nanocomposites. Most importantly, the filler content of BFT is only 2 vol %, which is lower than the filler-content requirement in other works. It is believed that the superior energy storage properties is mainly caused by the giant dielectric effect of BFT nanopowders, which induces a strongly enhanced dielectric constant loading to high polarization[16]. This advantage can be reflected by the competitive price in the future industrial applications. 4. Conclusions BFT/PVDF nanocomposites are fabricated via a physical mixing method followed by heat treatment. The dense bulk nanocomposites with well dispersed particles are examined by SEM. The enhanced dielectric constant and loss have been obtained, which is attributed to the giant dielectric constant of BFT and the enhanced interfacial polarization. An energy storage density up to 2.18 J/cm3 is achieved in the nanocomposites with BFT filler content of 2 vol % at a low electric field of 500 kV/cm. This value of energy storage density at low electric field is much higher than other polymer-based bulk nanocomposites. It is believed that the functional BFT/PVDF nanocomposites with good energy storage performance can be applied in future high-technology fields. Acknowledgements The present work was supported by National Natural Science Foundation of China (51572160), the Natural Science Foundation of Shaanxi Province (2016JQ5083, 2017KJXX-44), China Postdoctoral Science Foundation (2015M572516, 2016T90881) and Graduate Innovation Fund of Shaanxi University of Science and Technology. References [1] E. Karden, B. Fricke, T. Miller, K. Snyder, Energy storage devices for future hybrid electric vehicles, J. Power Sources. 168 (2007) 2-11. [2] Y. Bai, Z. Cheng, V. Bharti, H. Xu, Q. Zhang, High-dielectric-constant ceramic-powder polymer composites, Appl. Phys. Lett. 76 (2000) 3804-3806. [3] L. Zhu, Q. Wang, Novel ferroelectric polymers for high energy density and low loss dielectrics, Macromolecules. 45 (2012) 29372954. [4] Z. Wang, X.M. Chen, L. Ni, Y.Y. Liu, X.Q. Liu, Dielectric relaxations in Ba(Fe1/2Ta1/2)O3 giant dielectric constant ceramics, Appl. Phys. Lett. 90 (2007) 102905. [5] Z. Wang, T. Wang, C. Wang, Y. Xiao, P. Jing, Y. Cui, Y. Pu, Poly(vinylidene fluoride) flexible nanocomposite films with dopamine-coated giant dielectric ceramic nanopowders, Ba(Fe0.5Ta0.5)O3, for high energy-storage density at low electric field, ACS Appl. Mater. Interfaces. 9 (2017) 29130-29139. [6] Y. Shen, D. Shen, X. Zhang, J. Jiang, Z. Dan, Y. Song, Y. Lin, M. Li, C.-W. Nan, High energy density of polymer nanocomposites at low electric field induced by modulation of topological-structure, J. Mater. Chem. A. 4 (2016) 8359-8365. [7] J. Fu, Y. Hou, M. Zheng, Q. Wei, M. Zhu, H. Yan, Improving dielectric properties of PVDF composites by employing surface modified strong polarized BaTiO3 particles derived by molten salt method, ACS Appl. Mater. Interfaces. 7 (2015) 24480-24491. [8] E. Tuncer, Y. V. Serdyuk, S.M. Gubanski, Dielectric mixtures: Electrical properties and modeling, IEEE Trans. Dielectr. Electr. Insul. 9 (2002) 809-828. [9] Prateek, V.K. Thakur, R.K. Gupta, Recent progress on ferroelectric polymer-based nanocomposites for high energy density capacitors: synthesis, dielectric properties, and future aspects, Chem. Rev. 116 (2016) 4260-4317.
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