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Enhanced energy density in P(VDF-HFP) nanocomposites with gradient dielectric fillers and interfacial polarization
Journal of Alloys and Compounds 696 (2017) 1220e1227
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Enhanced energy density in P(VDF-HFP) nanocomposites with gradient dielectric fillers and interfacial polarization Qiao Huang a, Hang Luo a, b, **, Chao Chen a, Xuefan Zhou a, Kechao Zhou a, Dou Zhang a, * a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2016 Received in revised form 14 November 2016 Accepted 9 December 2016 Available online 10 December 2016
Dielectric capacitors are of urgently demand in modern micro-electric industry. The surface modified inorganic filler introduced to polymer matrix represents a promising avenue for the dielectric material's enhancement of energy storage density. To ease the electric field concentration of the composite induced by permittivity difference between ceramic fillers and polymer matrix, the specific dielectric fillers of TiO2 nanowires modified BT particles were synthesized in this study. Gradient dielectric composite consisting of poly(vinylidene fluoride-co-hexafluoropylene) [P(VDF-HFP)] matrix (εr~10), TiO2 shell (εr~40), and BaTiO3 (BT) core (εr~1000) were focused to investigate the electric field contribution and interface polarization. The results revealed that the permittivity of the composites increased as a result of large interfacial polarization induced by the large specific surface area of the nanowires as compared to the composite with randomly mixed TiO2/BT fillers. The breakdown strength of the composite was slightly improved with 20 vol% fillers, attributed to the fact that the electric field intensification was weakened by the BT@TiO2/Dop gradient dielectric fillers. The composite could endure up to 106 times of field cycling at the applied cycling field and the leakage current density was rather low. The composites with 20 vol% fillers exhibited a discharged energy density of 2.8 J/cm3 at a low electric field, which was much higher than that of the neat P(VDF-HFP). The findings of this research introduced a new inorganic particle with large specific surface area and gradient dielectric permittivity as filler in composite for energy storage application. © 2016 Elsevier B.V. All rights reserved.
Keywords: Gradient dielectric Interfacial polarization Nanocomposites P(VDF-HFP) Energy density
1. Introduction Dielectric capacitors with high energy density have attracted much attention in recent years as a result of the growing requirements for compact, low-cost electronic and power systems, dielectric materials with high dielectric permittivity, low loss and sizable energy capacity that can effectively storage and release electric energy, thus playing a key role in improving the energy density of the devices [1e3]. Generally, the energy density (Ue) is determined by the applied electric field (E) and the electric displacement (D) of the dielectric:
* Corresponding author. ** Corresponding author. State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China. E-mail addresses: [email protected] (H. Luo), [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.12.117 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Z Ue ¼
0 EdD Dmax
(1)
where Dmax is the electric displacement at the highest field, E is the applied electric field. For linear dielectric materials such as polymers and their composites, Equation (1) can be expressed as
Ue ¼
1 1 DE ¼ εr ε0 E2 2 2
(2)
where εr is the relative permittivity and ε0 is the vacuum permittivity (8.85 1012 F/m) [2,4]. The dielectric permittivity and breakdown strength are of significant importance to the increase of energy density (Ue), and Ue is largely determined by the breakdown strength (EB). The current use of commercial dielectric material biaxially oriented polypropylene (BOPP) is due to its large breakdown strength. However, its energy density is limited by the low dielectric permittivity (εr < 2). Therefore, the BOPP can hardly meet the ever increasing demand of the electronic factory [5e8].
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Generally, ceramics and polymers are the two major types of dielectric materials used for dielectric capacitor technologies. The majority of current research on the dielectric materials has focused on the composites containing high-dielectric-permittivity ceramic fillers embedded in the polymer matrix [9e12]. Ferroelectric ceramics like BaTiO3 (BT) or Pb(Zr,Ti)O3 (PZT) are the most commonly used fillers because of their high dielectric permittivity (εr BT > 1000, εr PZT > 2500) [13,14]. While the fillers with high dielectric permittivity can effectively enhance the average dielectric permittivity of the composite, the large difference of permittivity between the fillers and polymer matrix can lead to highly inhomogeneous electric field distribution, and generally give rise to significant reduction of effective breakdown strength in dielectric composites [15e17]. Moreover, in many cases, the high dielectric permittivity of the composites achieved by adding high volume fraction of inorganic particles could undoubtedly result in a sharp decrease of breakdown strength, as the density of cracks and void defects go upward originate from the agglomeration of inorganic particles and phase separation [18,19]. Therefore, to increase the dielectric permittivity and maintain breakdown strength of the composite, many experimental and theoretical studies aiming at enhancing the dielectric compatibility have been carried out. Choosing an appropriate interlay is a critical approach to optimize the dielectric compatibility. Lu et al. reported that a large breakdown strength (E > 340 kV/mm) can be obtained by using ultrathin TiO2 shell layer encapsulated BT, and a high energy density 12.2 J/ m3 can be achieved [20]. Li et al. utilized aluminum oxide encapsulated BT as the fillers, resulting in a low dielectric loss and the dielectric permittivity equaling to 6.2 can be achieved [21]. Li et al. synthesized oxide sublayers by atomic layer deposition, and a giant dielectric permittivity was obtained [22]. However, all the breakdown strength of the above mentioned works decreased as a function of the volume fraction of fillers. Recent studies show that the composites with enhanced dielectric permittivity can be obtained by enlarging the interfacial areas as a result of promotion of polarization in the fillers and the polymer matrix interface [16,20,23e25]. Based on the previous research, the fillers with large specific surface area are promising candidates because of their positive effects on the permittivity optimization. Sun et al. studied the size influence of the dielectric properties of nano-sized and micro-sized silica particles embedded in epoxy matrix. The results indicated that the dielectric permittivity and the loss factor were higher in nanocomposites than those for microcomposites at low frequencies [26]. Tang et al. reported that a high energy density can be achieved by incorporating high aspect ratio of TiO2 nanowires in composite. The nanocomposite exhibited energy density as high as 12 J/m3 at 450 kV/mm [27]. Tang et al. utilized high aspect ratio of PZT nanowires with a mean diameter of 160 nm and a mean length of 2.2 mm as filler, the energy density was 77.8% increase over composite with PZT nanowires with lower aspect ratio [6]. Thus, design more interfacial areas in the dielectric composite is an effective way to enhance the permittivity with no expense of electric strength. In this work, the TiO2 nanowires modified BT particles were synthesized by a solvothermal method and investigated systematically. The effect of TiO2 nanowire layers are as follows: 1) They act as a buffer layer with intermediate dielectric permittivity between the high dielectric permittivity filler BT and the low dielectric permittivity matrix poly(vinylidene fluoride-cohexafluoropylene) [P(VDF-HFP)] (εr < 10), the BT@TiO2/Dop/ P(VDF-HFP) composite with gradient dielectric permittivity is expected to release electric field intensification and improve the breakdown strength. 2) Our previous works reveal that by modifying the surface of the filler using chemical method, the dispersion of the fillers could be optimized and the quality of composite could
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be improved [19,28]. Therefore, the dopamine-modified TiO2 nanowires have strong interaction with the P(VDF-HFP) matrix, which can enhance the dispersion of the BT@TiO2/Dop particles and thus effectively hinder electric breakdown. 3) The TiO2 nanowires can boost the interfacial areas between the fillers and P(VDFHFP) matrix, hence promoting the interfacial polarization and the dielectric permittivity are expected to be enhanced. The results show that the dielectric permittivity of composite was largely enhanced with TiO2 nanowires modified BT particles as filler when compared to composites with disorder mixed BT/TiO2 particles as filler. The energy density of 2.8 J/m3 was achieved at 240 kV/mm. This work combined the superiority of enlarged interfacial areas and gradient dielectric fillers. Therefore, propose a new view on surface modification. 2. Experimental procedures 2.1. Chemicals and materials Tetrabutyl titanate (TBT), N,N-dimethylformamide (DMF), barium titanate (BaTiO3), and P(VDF-HFP) polymer (pellets with less than 15% HFP) were all purchased from Aladdin Industrial Corporation, China. Glycerol and ethyl alcohol were supplied by guoyao company, China. Dopamine hydrochloride was purchased from Alfa Company. All chemicals were used as received without further purification. The BT JCPDS No.75-1606 and TiO2 JCPDS No.21-1272 were used as references for the XRD analysis. 2.2. Synthesis of the TiO2 nanowires modified BT particles The TiO2 nanowires modified BT particles were synthesized using a template-free solvothermal approach [29]. In a typical synthesis procedure, 7.31 mmol BT powder with diameter of about 2 mm and 5.85 mmol TBT were mixed with 30 ml ethyl alcohol and 10 ml glycerol. After stirring for about 5 min at room temperature, the mixture was transferred into a 100 ml Teflon-lined autoclave maintained at 180 C for 24 h. After cooling down to room temperature, the white precipitates were collected by centrifugation at 7000 rpm for 5 min, washed with ethanol, and dried at 60 C for 12 h. The products were calcined in air at 550 C for 2 h and harvested. The obtained white precipitate were added into 100 ml H2O2 at 106 C for 10 h, and then the products were collected and washed with deionized water. After dried under vaccum at 80 C for 12 h, they were dispersed in 0.01 M dopamine hydrochloride aqueous solution and stirred at 60 C for 10 h, and then centrifuged from the solution and washed with deionized water. The modified products were dried at 65 C overnight under vacuum. For comparison, 7.31 mmol BT particles and 5.85 mmol TiO2 nanoparticles were randomly mixed, stired in 0.01 M dopamine hydrochloride aqueous solution and the resultant products were finally obtained by a similar way. 2.3. Preparation of BT@TiO2/Dop/P(VDF-HFP) composites The TiO2 nanowires modified BT particles were ball-milled in DMF for 48 h and mixed with P(VDF-HFP) for another 48 h by ballmilling. The resultant suspension was immediately cast onto a glass and dried at 80 C for 12 h under vacuum condition. The dried composite sheets were compressed into films at 200 C under a pressure of about 15 MPa. Gold electrodes were sputtered on both sides of the film using a mask with 2 mm diameter eyelets. The fabrication process of TiO2 nanowires modified BT particles was shown in Fig. 1. The surface of BT particles were modified by TiO2 nanowires precursors after the solvothermal process. After calcine, the morphologies of the solvothermal products were well
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Fig. 1. Schematic of preparation procedures for BT@TiO2/Dop/P(VDF-HFP) composites.
preserved and the TiO2 precursors become anatase TiO2 crystal. Then the products were modified by dopamine with the expectation of forming strong chemical bond between the TiO2/P(VDFHFP) interface [30e32]. The modified BT@TiO2/Dop particles were then dispersed in P(VDF-HFP) solution by ball milling. After that, the BT@TiO2/Dop/P(VDF-HFP) film were obtained via solution casting. The films were hot pressed before testing. 2.4. Characterization The crystalline phases and morphology of the products were examined using a Rigaku D-Max/2550VBþ X-ray diffractometer (XRD) (Cu-Ka radiation) and a JSM-6390 field emission scanning electron microscopy. The dopamine coating was characterized by Fourier transform infrared spectroscopy (FT-IR). Frequencydependent dielectric permittivity and dielectric loss were measured using an Agilent 4294A LCR meter with the frequency ranging from 1 kHz to 10 MHz. Polarization-electric field (P-E) hysteresis loops, and leakage current density were studied by a ferroelectric testing unit (Precision Premier ІІ) at room temperature and 10 Hz. Fatigue endurance was measured by ferroelectric tester at room temperature. 3. Results and discussion 3.1. Structural and morphology properties The XRD patterns of the synthesized particles before and after calcination and the FT-IR of TiO2 nanowires modified BT particles before and after dopamine modification are shown in Fig. 2. As can be seen from Fig. 2a, before calcination, the peaks of BT particles can fully indexed to JCPDS No.75-1606, while the peaks of TiO2 were weak which means the products were not highly crystallined. After calcination, the intensity of TiO2 peaks became intense. All the diffraction peaks of the products calcined at 550 C could be fully indexed to anatase TiO2 (JCPDS No.21-1272) and BT (JCPDS No.751606). For example, the characteristic diffraction peaks of BT appeared at about 22 , 31, and 38 , which were associated with typical structures of perovskite BT in the crystal planes of (100), (110), and (111). No additional phases were detected, indicating that the TiO2 precursors were all converted to pure anatase TiO2. The FT-IR results, as shown in Fig. 2b, demonstrated that no visible absorption signal of the uncoated BT/TiO2 particles was observed between 4000 and 1000. After the dopamine modification, the N-H bending vibrations appeared at 1634 cm1, the C-C bending
vibrations appeared at 1498 cm1 and the C-N bending vibrations appeared and 1284 cm1, which means the dopamine was successfully coated on the surface of BT/TiO2 particles. The FT-IR results were corresponding to the results reported by Song et al. [32]. Fig. 3 shows the morphology of the products after calcination. As can be seen from Fig. 3a, the obtained products exhibited uniform shape and size with an average diameter of about 2 mm. Fig. 3b shows an image of single BT particle with partial TiO2 modified surface. It is can be seen that the TiO2 nanowires distributed homogeneously on the BT surface. The insert in Fig. 3b shows us the TiO2 nanowires were with a mean diameter of 60 nm and a mean length of 300 nm. The “sea urchin” like structure of the filler would not collapse into scattered nanopetals after prolonged calcination or grinding, indicating that on the one hand, the TiO2 nanowires were not randomly aggregations but ordered self-assemblies [29,33], on the other hand, the TiO2 nanowires grown on the BT surface were not randomly pasted but nucleated on it as the anatase TiO2 and tetragonal BT have small lattice mismatch [34,35]. The TiO2 nanowires were expected to act as a dielectric buffer layer to weak the electric intensification, and on the other hand to enlarge the interface between the fillers and the polymer matrix so as to make contribution to the enhancement of dielectric permittivity. 3.2. Dielectric properties To discuss the effect of the TiO2 modified BT fillers on the dielectric properties of composite, the frequency dependence of the dielectric properties are shown in Fig. 4 with the frequency ranging from 100 Hz to 100 MHz. The dielectric properties of the composites with randomly mixed TiO2/BT fillers are also shown as a comparison. As can be seen, the composites with TiO2 nanowires modified BT fillers had improved dielectric permittivity compared to the composites with randomly mixed TiO2/BT fillers. The maximum dielectric permittivity of the composite with the TiO2 nanowires modified BT fillers increased from 9.8 to 14.7 (Fig. 4a), whereas that of the composite with the randomly mixed TiO2/BT fillers was 7.2e10.1 (Fig. 4d). This can be attributed to the enlarged interface by the TiO2 nanowires compared to the nano TiO2 particles [9,14,20]. The dielectric loss of the composites with TiO2 nanowires modified BT fillers was as low as 0.037 at 1000 Hz, the dielectric loss decreased as the loading of TiO2 nanowires modified BT fillers increased in the high frequency range (Fig. 4b). The dielectric loss of the composites with randomly mixed TiO2/BT fillers also increased with different loading of fillers, but they did not show clear regularity as a function of volume fraction (Fig. 4e), this could be owing to the heterogeneous distribution of randomly mixed microparticles and nanoparticles, the high density of voids and defects in the film could result in dielectric properties fluctuation. The conductivity of the composites are shown in Fig. 4c and f. The conductivity of the composites with TiO2 nanowires modified BT fillers were relatively low (Fig. 4c), even the loading of the fillers was up to 20%, the conductivity increased as the loading increased over the frequency range. While the conductivity of the composites with randomly mixed TiO2/BT fillers did not show clear regularity as a function of volume fraction, as shown in Fig. 4f. 3.3. D-E loops and the breakdown strength of the obtained composites Typical electric displacement-electric field (D-E) loops of the composites measured at various volume fractions with the TiO2 nanowires modified BT fillers or TiO2/BT randomly mixed fillers are shown in Fig. 5, the electric field was 80 kV/mm. As can be seen from Fig. 5a, the concentration of TiO2 nanowires modified BT filler
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Fig. 2. (a) XRD pattern of the products before and after calcination. (b) The FT-IR of TiO2 nanowires modified BT particles before and after dopamine modification.
Fig. 3. SEM image of the TiO2 nanowires modified BT particles: (a) Agglomerated particles. (b) A single particle with part of BT particle surface exposed outside (the insert revealed the average aspect ratio of the TiO2 nanowires).
Fig. 4. Frequency dependence of (a) dielectric permittivity, (b) dielectric loss, (c) conductivity of the BT@TiO2/Dop/P(VDF-HFP) composites, (d) dielectric permittivity, (e) dielectric loss, and (f) conductivity of the composites with randomly mixed TiO2/BT particles.
notably raised the electric displacement, attributed to the high dielectric permittivity of the fillers. The results were consistent with the results in Fig. 4a. It is also shown that the composites with TiO2 nanowires modified BT fillers had large electric displacement than that of the composites with randomly mixed TiO2/BT fillers, as
a results of higher dielectric permittivity of the samples. For example, under the electric field of 80 kV/mm and 20 vol% loading, the composites with TiO2 nanowires modified BT fillers achieved an electric displacement of 1.84 mC/cm2, while the composites with TiO2/BT randomly mixed fillers only had an electric displacement of
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Fig. 5. Electric displacement-electric field (D-E) loops of (a) composites with BT@TiO2/Dop fillers and (b) composites with TiO2/BT randomly mixed fillers at 80 kV/mm.
1.23 mC/cm2 (Fig. 5b). Fig. 6 shows the electric breakdown strength of the composites with different volume fraction of TiO2 nanowires modified BT fillers, the Weibull analysis was used to calculate the characteristic breakdown strength of the composites with TiO2 nanowires modified BT fillers and the composites with randomly mixed fillers of TiO2 and BT particles. The characteristic breakdown strength was analyzed with the function as:
PðEÞ ¼ 1 exp ðE=EBÞb
(3)
where P(E) is the cumulative probability of electric failure, E is the experimental breakdown strength, EB is a scale parameter refers to the breakdown strength at the cumulative failure probability of 63.2% that is also regarded as the characteristic breakdown strength, and b is the Weibull modulus associated with the linear regressive fit of the distribution [36e38]. All the samples maintained a relatively high breakdown strength over 200 kV/mm due to tightly bonding between the fillers and polymer matrix, as shown in Fig. 6a. Specially, the EB of composites with 5 vol% TiO2 nanowires modified BT fillers was 277.08 kV/mm, this figure reduces to 232.28 kV/mm with 10 vol% TiO2 nanowires modified BT fillers but then increased to 254.80 kV/mm as the volume fraction of loading increased to 20%. Generally, the breakdown strength would decrease with the increase of volume fraction of loading (<20 vol%), as a results of the agglomeration and voids would increase with the volume fraction of loading increase. Therefore, the decrease of breakdown strength occurred [29,38e41]. The particular phenomenon in the work may be attributed to following reason. Dielectric gradient fillers BT@TiO2/Dop in the composites play a positive effects on the release of electric field intensification,
which could ease the reduction of breakdown strength caused by voids, cracks and other defects. The results indicated that the effect of weaken electric field intensification by the gradient filler was overweight the effect of higher defects on the breakdown strength of the composites. In addition, it can be seen from Fig. 6b that the breakdown strength of the composites with gradient dielectric fillers increased from 232.86 kV/mm to 254.85 kV/mm (>9% promotion), while the breakdown strength of the composites with randomly mixed TiO2/BT fillers increased from 270.11 kV/mm to 271.71 kV/mm (<1% promotion). This phenomenon could be attributed to the different average particle size and the different morphology of the fillers. 3.4. The fatigue endurance of the composites A high fatigue endurance is required for long-term stability during the charge-discharge cycling process of the dielectric capacitor. Therefore, the polarization fatigue behavior of the composite was studied as a function of the charge-discharge cycles up to 106 using a 1 kHz pulse signal under a 40 kV/mm electric field. Fig. 7 shows the variation of saturate polarization (Pmax) and remnant polarization (Pr) during fatigue testing. As can be seen, all the samples with different volume fraction could endure up to 106 times of field cycling at the applied cycling field, indicating that the composites exhibit high fatigue endurance up to 106 switching cycles at room temperature. The high fatigue endurance of the composites may be due to the following reasons: (i) High quality composite film possess high breakdown strength, the ceramic fillers was modified by dopamine before being added to the P(VDFHFP) matrix, which can greatly improve the dispersibility of the ceramic particles in the composites and the compatibility between
Fig. 6. Weibull plots of the breakdown strength of (a) the composites with BT@TiO2/Dop as fillers and (b) Characteristic breakdown strength comparison of the composites with BT@TiO2/Dop fillers and composites with TiO2/BT randomly mixed fillers.
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3.6. Energy density and energy efficiency of the obtained composites
Fig. 7. Fatigue endurance of the composites with BT@TiO2/Dop fillers at room temperature.
The energy density and energy efficiency are shown in Fig. 9. It demonstrates that the discharged energy density of the composites was strongly dependent on the volume fraction of the loading. The maximum discharged energy density was 2.8 J/cm3 at 240 kV/mm with the composite containing 20 vol% TiO2 nanowires modified BT fillers (Fig. 9a). The maximum energy density of each sample are shown in Fig. 9b, as can be seen, the maximum energy density of composite with 20 vol% TiO2 nanowires modified BT fillers reached 3.92 J/cm3. All the energy efficiency showed a downward trend as the applied electric goes up. The efficiency of the composites with 10 vol% and 20 vol% fillers were lower than 40%, indicating that the dielectric loss was relatively higher than many ferroelectric materials under a high electric field, the huge dielectric loss might be resulted from the micro-sized fillers used in the composites.
ceramic fillers and matrix. (ii) The fatigue endurance measurements of the composite were in an electric field of 40 kV/mm, which was much lower than the breakdown strength of the composites. For example, the breakdown strength of the composite with 5 vol% TiO2 nanowires modified BT fillers was as high as 277.08 kV/mm, as shown in Fig. 6b. Thus, the composites can keep working after 106 times of field cycling. (iii) The polarization of the composites keep a relatively stable value, especially when the content of the fillers is lower than 10 vol%, as shown in Fig. 7, which indicate that the composites exhibited long-term stability of the switchable polarization under electric field cycling.
3.5. The leakage current densities of the composites To investigate the influence of the enlarged interface of the composites, the leakage current densities of the composites with different volume fraction of filler were tested. As can be seen from Fig. 8, the leakage current densities of the samples were all laid in the range of 109 to 106 A/cm2. The leakage current followed a reasonable increase trend with the raise of applied electric field. The composite with higher loading of fillers exhibited larger leakage current density under a fixed electric field, attributed to the defects and the space charge introduced by BT@TiO2/Dop fillers [20,42].
Fig. 8. Leakage current density-electric field curves of the obtained composites.
Fig. 9. The (a) discharged energy density, (b) maximum energy density and (c) energy efficiency of the composites with different volume fraction of BT@TiO2/Dop fillers.
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Although the dielectric permittivity and breakdown strength were optimized and a considerable improvement of energy density was achieved by using the TiO2 nanowires modified BT particles as filler, there was substantial big room for improving the energy density by reducing energy loss. 4. Conclusion The TiO2 nanowires modified BT particles were synthesized and introduced to the P(VDF-HFP) matrix; thus, the composite with BT@TiO2/Dop/P(VDF-HFP) gradient dielectric distribution was obtained. The composite with TiO2 nanowires enlarged the interfacial polarization and leaded to a clear enhancement of dielectric permittivity compared to the composite with randomly mixed TiO2/BT particles as fillers. The effect of weakening the electric field intensification were remarkable when introducing 20 vol% fillers into the polymer matrix. The enhancement of breakdown strength was clear when compared the composite with BT@TiO2/Dop gradient dielectric fillers to the composite with randomly mixed TiO2/BT particles, attributed to the size and morphology differences of the fillers. The fatigue endurance test revealed that the composite could endure up to 106 times of field cycling at the applied cycling field and the leakage current density is rather low. The maximum discharged energy density of the composite reached 2.8 J/cm3 with 20 vol% loading of fillers at 260 kV/mm. The results obtained in this study would be helpful in exploring suitable filler candidates for the improvement of dielectric material's energy storage density. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51672311), Science and Technology Project of Hunan Province, China (no. 2016WK2022), Postdoctoral Research Foundation of Central South University, and State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. References [1] Baojin Chu, Xin Zhou, Kailiang Ren, Bret Neese, Minren Lin, Qing Wang, F. Bauer, Q.M. Zhang, A dielectric polymer with high electric energy density and fast discharge speed, Science 313 (2006) 334e336. [2] 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) 6334e6365. [3] L.A. Fredin, Z. Li, M.A. Ratner, M.T. Lanagan, T.J. Marks, Enhanced energy storage and suppressed dielectric loss in oxide core-shell-polyolefin nanocomposites by moderating internal surface area and increasing shell thickness, Adv. Mater. 24 (2012) 5946e5953. [4] X. Zhang, Y. Shen, Q.H. Zhang, L. Gu, Y.H. Hu, J.W. Du, Y.H. Lin, C.W. Nan, Ultrahigh energy density of polymer nanocomposites containing BaTiO3@TiO2 nanofibers by atomic-scale interface engineering, Adv. Mater. 27 (2015) 819e824. [5] J. Ho, R. Ramprasad, S. Boggs, Effect of alteration of antioxidant by UV treatment on the dielectric strength of BOPP capacitor film, IEEE Trans. Dielectr. Electr. Insul. 14 (2007) 1295e1301. [6] H. Tang, H.A. Sodano, Ultra high energy density nanocomposite capacitors with fast discharge using Ba0.2Sr0.8TiO3 nanowires, Nano Lett. 13 (2013) 1373e1379. [7] T.C. Mike Chung, Functionalization of polypropylene with high dielectric properties: applications in electric energy storage, Green. Sustain. Chem. 02 (2012) 29e37. [8] M. Rabuffi, G. Picci, Status quo and future prospects for metallized polypropylene energy storage capacitors, IEEE Trans. Plasma Sci. 30 (2002) 1939e1942. [9] H. Tang, Z. Zhou, H.A. Sodano, Relationship between BaTiO3 nanowire aspect ratio and the dielectric permittivity of nanocomposites, ACS Appl. Mater. Interfaces 6 (2014) 5450e5455. [10] X. Zhang, Y. Shen, B. Xu, Q. Zhang, L. Gu, J. Jiang, J. Ma, Y. Lin, C.W. Nan, Giant energy density and improved discharge efficiency of solution-processed polymer nanocomposites for dielectric energy storage, Adv. Mater. 28
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Huang, An all-organic composite actuator material with a high dielectric constant, Nature 419 (2002) 284e287. [25] J.Y. Li, Exchange coupling in P(VDF-TrFE) copolymer based all-organic composites with giant electrostriction, Phys. Rev. Lett. 90 (2003). [26] Y.Y. Sun, Z.Q. Zhang, C.P. Wong, Influence of interphase and moisture on the dielectric spectroscopy of epoxy/silica composites, Polymer 46 (2005) 2297e2305. [27] H. Tang, H.A. Sodano, High energy density nanocomposite capacitors using non-ferroelectric nanowires, Appl. Phys. Lett. 102 (2013) 063901. [28] H. Luo, C. Chen, K. Zhou, X. Zhou, Z. Wu, D. Zhang, Enhancement of dielectric properties and energy storage density in poly(vinylidene fluoride-cohexafluoropropylene) by relaxor ferroelectric ceramics, RSC Adv. 5 (2015) 68515e68522. [29] N. Xu, L. Hu, Q. Zhang, X. Xiao, H. Yang, E. Yu, Significantly enhanced dielectric performance of poly(vinylidene fluoride-co-hexafluoropylene)-based composites filled with hierarchical flower-like TiO2 particles, ACS Appl. Mater. Interfaces 7 (2015) 27373e27381. [30] Q. Yue, M.H. Wang, Z.K. Sun, C. Wang, C. Wang, Y.H. Deng, D.Y. Zhao, A versatile ethanol-mediated polymerization of dopamine for efficient surface modification and the construction of functional core-shell nanostructures, J. Mater. Chem. B 1 (2013) 6085e6093. [31] C. Chao, J.D. Liu, J.T. Wang, Y.W. Zhang, B. Zhang, Y.T. Zhang, X. Xiang, R.F. Chen, Surface modification of halloysite nanotubes with dopamine for enzyme immobilization, ACS Appl. Mater. Interfaces 5 (2013) 10559e10564. [32] Y. Song, Y. Shen, H.Y. Liu, Y.H. Lin, M. Li, C.W. Nan, Enhanced dielectric and ferroelectric properties induced by dopamine-modified BaTiO3 nanofibers in flexible poly(vinylidene fluoride-trifluoroethylene) nanocomposites, J. Mater. Chem. 22 (2012) 8063e8068. [33] X.J. Sun, J.W. Wang, Y. Xing, Y. Zhao, X.C. Liu, B. Liu, S.Y. Hou, Surfactantassisted hydrothermal synthesis and electrochemical properties of nanoplateassembled 3D flower-like Cu3V2O7(OH)2$2H2O microstructures, Crystengcomm 13 (2011) 367e370. [34] J.G. Lisoni, C.H. Lei, T. Hoffmann, V.M. Fuenzalida, Hydrothermal growth of BaTiO3 on TiO2 single crystals, Surf. Sci. 515 (2002) 431e440. [35] Z. Zhou, Y. Lin, H. Tang, H.A. Sodano, Hydrothermal growth of highly textured BaTiO(3) films composed of nanowires, Nanotechnology 24 (2013) 095602. [36] V. Tomer, E. Manias, C.A. Randall, High field properties and energy storage in nanocomposite dielectrics of poly(vinylidene fluoride-hexafluoropropylene), J. Appl. Phys. 110 (2011). [37] 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)
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced energy density in P(VDF-HFP) nanocomposites with gradient dielectric fillers and interfacial polarization Qiao Huang a, Hang Luo a, b, **, Chao Chen a, Xuefan Zhou a, Kechao Zhou a, Dou Zhang a, * a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2016 Received in revised form 14 November 2016 Accepted 9 December 2016 Available online 10 December 2016
Dielectric capacitors are of urgently demand in modern micro-electric industry. The surface modified inorganic filler introduced to polymer matrix represents a promising avenue for the dielectric material's enhancement of energy storage density. To ease the electric field concentration of the composite induced by permittivity difference between ceramic fillers and polymer matrix, the specific dielectric fillers of TiO2 nanowires modified BT particles were synthesized in this study. Gradient dielectric composite consisting of poly(vinylidene fluoride-co-hexafluoropylene) [P(VDF-HFP)] matrix (εr~10), TiO2 shell (εr~40), and BaTiO3 (BT) core (εr~1000) were focused to investigate the electric field contribution and interface polarization. The results revealed that the permittivity of the composites increased as a result of large interfacial polarization induced by the large specific surface area of the nanowires as compared to the composite with randomly mixed TiO2/BT fillers. The breakdown strength of the composite was slightly improved with 20 vol% fillers, attributed to the fact that the electric field intensification was weakened by the BT@TiO2/Dop gradient dielectric fillers. The composite could endure up to 106 times of field cycling at the applied cycling field and the leakage current density was rather low. The composites with 20 vol% fillers exhibited a discharged energy density of 2.8 J/cm3 at a low electric field, which was much higher than that of the neat P(VDF-HFP). The findings of this research introduced a new inorganic particle with large specific surface area and gradient dielectric permittivity as filler in composite for energy storage application. © 2016 Elsevier B.V. All rights reserved.
Keywords: Gradient dielectric Interfacial polarization Nanocomposites P(VDF-HFP) Energy density
1. Introduction Dielectric capacitors with high energy density have attracted much attention in recent years as a result of the growing requirements for compact, low-cost electronic and power systems, dielectric materials with high dielectric permittivity, low loss and sizable energy capacity that can effectively storage and release electric energy, thus playing a key role in improving the energy density of the devices [1e3]. Generally, the energy density (Ue) is determined by the applied electric field (E) and the electric displacement (D) of the dielectric:
* Corresponding author. ** Corresponding author. State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China. E-mail addresses: [email protected] (H. Luo), [email protected] (D. Zhang). http://dx.doi.org/10.1016/j.jallcom.2016.12.117 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Z Ue ¼
0 EdD Dmax
(1)
where Dmax is the electric displacement at the highest field, E is the applied electric field. For linear dielectric materials such as polymers and their composites, Equation (1) can be expressed as
Ue ¼
1 1 DE ¼ εr ε0 E2 2 2
(2)
where εr is the relative permittivity and ε0 is the vacuum permittivity (8.85 1012 F/m) [2,4]. The dielectric permittivity and breakdown strength are of significant importance to the increase of energy density (Ue), and Ue is largely determined by the breakdown strength (EB). The current use of commercial dielectric material biaxially oriented polypropylene (BOPP) is due to its large breakdown strength. However, its energy density is limited by the low dielectric permittivity (εr < 2). Therefore, the BOPP can hardly meet the ever increasing demand of the electronic factory [5e8].
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Generally, ceramics and polymers are the two major types of dielectric materials used for dielectric capacitor technologies. The majority of current research on the dielectric materials has focused on the composites containing high-dielectric-permittivity ceramic fillers embedded in the polymer matrix [9e12]. Ferroelectric ceramics like BaTiO3 (BT) or Pb(Zr,Ti)O3 (PZT) are the most commonly used fillers because of their high dielectric permittivity (εr BT > 1000, εr PZT > 2500) [13,14]. While the fillers with high dielectric permittivity can effectively enhance the average dielectric permittivity of the composite, the large difference of permittivity between the fillers and polymer matrix can lead to highly inhomogeneous electric field distribution, and generally give rise to significant reduction of effective breakdown strength in dielectric composites [15e17]. Moreover, in many cases, the high dielectric permittivity of the composites achieved by adding high volume fraction of inorganic particles could undoubtedly result in a sharp decrease of breakdown strength, as the density of cracks and void defects go upward originate from the agglomeration of inorganic particles and phase separation [18,19]. Therefore, to increase the dielectric permittivity and maintain breakdown strength of the composite, many experimental and theoretical studies aiming at enhancing the dielectric compatibility have been carried out. Choosing an appropriate interlay is a critical approach to optimize the dielectric compatibility. Lu et al. reported that a large breakdown strength (E > 340 kV/mm) can be obtained by using ultrathin TiO2 shell layer encapsulated BT, and a high energy density 12.2 J/ m3 can be achieved [20]. Li et al. utilized aluminum oxide encapsulated BT as the fillers, resulting in a low dielectric loss and the dielectric permittivity equaling to 6.2 can be achieved [21]. Li et al. synthesized oxide sublayers by atomic layer deposition, and a giant dielectric permittivity was obtained [22]. However, all the breakdown strength of the above mentioned works decreased as a function of the volume fraction of fillers. Recent studies show that the composites with enhanced dielectric permittivity can be obtained by enlarging the interfacial areas as a result of promotion of polarization in the fillers and the polymer matrix interface [16,20,23e25]. Based on the previous research, the fillers with large specific surface area are promising candidates because of their positive effects on the permittivity optimization. Sun et al. studied the size influence of the dielectric properties of nano-sized and micro-sized silica particles embedded in epoxy matrix. The results indicated that the dielectric permittivity and the loss factor were higher in nanocomposites than those for microcomposites at low frequencies [26]. Tang et al. reported that a high energy density can be achieved by incorporating high aspect ratio of TiO2 nanowires in composite. The nanocomposite exhibited energy density as high as 12 J/m3 at 450 kV/mm [27]. Tang et al. utilized high aspect ratio of PZT nanowires with a mean diameter of 160 nm and a mean length of 2.2 mm as filler, the energy density was 77.8% increase over composite with PZT nanowires with lower aspect ratio [6]. Thus, design more interfacial areas in the dielectric composite is an effective way to enhance the permittivity with no expense of electric strength. In this work, the TiO2 nanowires modified BT particles were synthesized by a solvothermal method and investigated systematically. The effect of TiO2 nanowire layers are as follows: 1) They act as a buffer layer with intermediate dielectric permittivity between the high dielectric permittivity filler BT and the low dielectric permittivity matrix poly(vinylidene fluoride-cohexafluoropylene) [P(VDF-HFP)] (εr < 10), the BT@TiO2/Dop/ P(VDF-HFP) composite with gradient dielectric permittivity is expected to release electric field intensification and improve the breakdown strength. 2) Our previous works reveal that by modifying the surface of the filler using chemical method, the dispersion of the fillers could be optimized and the quality of composite could
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be improved [19,28]. Therefore, the dopamine-modified TiO2 nanowires have strong interaction with the P(VDF-HFP) matrix, which can enhance the dispersion of the BT@TiO2/Dop particles and thus effectively hinder electric breakdown. 3) The TiO2 nanowires can boost the interfacial areas between the fillers and P(VDFHFP) matrix, hence promoting the interfacial polarization and the dielectric permittivity are expected to be enhanced. The results show that the dielectric permittivity of composite was largely enhanced with TiO2 nanowires modified BT particles as filler when compared to composites with disorder mixed BT/TiO2 particles as filler. The energy density of 2.8 J/m3 was achieved at 240 kV/mm. This work combined the superiority of enlarged interfacial areas and gradient dielectric fillers. Therefore, propose a new view on surface modification. 2. Experimental procedures 2.1. Chemicals and materials Tetrabutyl titanate (TBT), N,N-dimethylformamide (DMF), barium titanate (BaTiO3), and P(VDF-HFP) polymer (pellets with less than 15% HFP) were all purchased from Aladdin Industrial Corporation, China. Glycerol and ethyl alcohol were supplied by guoyao company, China. Dopamine hydrochloride was purchased from Alfa Company. All chemicals were used as received without further purification. The BT JCPDS No.75-1606 and TiO2 JCPDS No.21-1272 were used as references for the XRD analysis. 2.2. Synthesis of the TiO2 nanowires modified BT particles The TiO2 nanowires modified BT particles were synthesized using a template-free solvothermal approach [29]. In a typical synthesis procedure, 7.31 mmol BT powder with diameter of about 2 mm and 5.85 mmol TBT were mixed with 30 ml ethyl alcohol and 10 ml glycerol. After stirring for about 5 min at room temperature, the mixture was transferred into a 100 ml Teflon-lined autoclave maintained at 180 C for 24 h. After cooling down to room temperature, the white precipitates were collected by centrifugation at 7000 rpm for 5 min, washed with ethanol, and dried at 60 C for 12 h. The products were calcined in air at 550 C for 2 h and harvested. The obtained white precipitate were added into 100 ml H2O2 at 106 C for 10 h, and then the products were collected and washed with deionized water. After dried under vaccum at 80 C for 12 h, they were dispersed in 0.01 M dopamine hydrochloride aqueous solution and stirred at 60 C for 10 h, and then centrifuged from the solution and washed with deionized water. The modified products were dried at 65 C overnight under vacuum. For comparison, 7.31 mmol BT particles and 5.85 mmol TiO2 nanoparticles were randomly mixed, stired in 0.01 M dopamine hydrochloride aqueous solution and the resultant products were finally obtained by a similar way. 2.3. Preparation of BT@TiO2/Dop/P(VDF-HFP) composites The TiO2 nanowires modified BT particles were ball-milled in DMF for 48 h and mixed with P(VDF-HFP) for another 48 h by ballmilling. The resultant suspension was immediately cast onto a glass and dried at 80 C for 12 h under vacuum condition. The dried composite sheets were compressed into films at 200 C under a pressure of about 15 MPa. Gold electrodes were sputtered on both sides of the film using a mask with 2 mm diameter eyelets. The fabrication process of TiO2 nanowires modified BT particles was shown in Fig. 1. The surface of BT particles were modified by TiO2 nanowires precursors after the solvothermal process. After calcine, the morphologies of the solvothermal products were well
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Fig. 1. Schematic of preparation procedures for BT@TiO2/Dop/P(VDF-HFP) composites.
preserved and the TiO2 precursors become anatase TiO2 crystal. Then the products were modified by dopamine with the expectation of forming strong chemical bond between the TiO2/P(VDFHFP) interface [30e32]. The modified BT@TiO2/Dop particles were then dispersed in P(VDF-HFP) solution by ball milling. After that, the BT@TiO2/Dop/P(VDF-HFP) film were obtained via solution casting. The films were hot pressed before testing. 2.4. Characterization The crystalline phases and morphology of the products were examined using a Rigaku D-Max/2550VBþ X-ray diffractometer (XRD) (Cu-Ka radiation) and a JSM-6390 field emission scanning electron microscopy. The dopamine coating was characterized by Fourier transform infrared spectroscopy (FT-IR). Frequencydependent dielectric permittivity and dielectric loss were measured using an Agilent 4294A LCR meter with the frequency ranging from 1 kHz to 10 MHz. Polarization-electric field (P-E) hysteresis loops, and leakage current density were studied by a ferroelectric testing unit (Precision Premier ІІ) at room temperature and 10 Hz. Fatigue endurance was measured by ferroelectric tester at room temperature. 3. Results and discussion 3.1. Structural and morphology properties The XRD patterns of the synthesized particles before and after calcination and the FT-IR of TiO2 nanowires modified BT particles before and after dopamine modification are shown in Fig. 2. As can be seen from Fig. 2a, before calcination, the peaks of BT particles can fully indexed to JCPDS No.75-1606, while the peaks of TiO2 were weak which means the products were not highly crystallined. After calcination, the intensity of TiO2 peaks became intense. All the diffraction peaks of the products calcined at 550 C could be fully indexed to anatase TiO2 (JCPDS No.21-1272) and BT (JCPDS No.751606). For example, the characteristic diffraction peaks of BT appeared at about 22 , 31, and 38 , which were associated with typical structures of perovskite BT in the crystal planes of (100), (110), and (111). No additional phases were detected, indicating that the TiO2 precursors were all converted to pure anatase TiO2. The FT-IR results, as shown in Fig. 2b, demonstrated that no visible absorption signal of the uncoated BT/TiO2 particles was observed between 4000 and 1000. After the dopamine modification, the N-H bending vibrations appeared at 1634 cm1, the C-C bending
vibrations appeared at 1498 cm1 and the C-N bending vibrations appeared and 1284 cm1, which means the dopamine was successfully coated on the surface of BT/TiO2 particles. The FT-IR results were corresponding to the results reported by Song et al. [32]. Fig. 3 shows the morphology of the products after calcination. As can be seen from Fig. 3a, the obtained products exhibited uniform shape and size with an average diameter of about 2 mm. Fig. 3b shows an image of single BT particle with partial TiO2 modified surface. It is can be seen that the TiO2 nanowires distributed homogeneously on the BT surface. The insert in Fig. 3b shows us the TiO2 nanowires were with a mean diameter of 60 nm and a mean length of 300 nm. The “sea urchin” like structure of the filler would not collapse into scattered nanopetals after prolonged calcination or grinding, indicating that on the one hand, the TiO2 nanowires were not randomly aggregations but ordered self-assemblies [29,33], on the other hand, the TiO2 nanowires grown on the BT surface were not randomly pasted but nucleated on it as the anatase TiO2 and tetragonal BT have small lattice mismatch [34,35]. The TiO2 nanowires were expected to act as a dielectric buffer layer to weak the electric intensification, and on the other hand to enlarge the interface between the fillers and the polymer matrix so as to make contribution to the enhancement of dielectric permittivity. 3.2. Dielectric properties To discuss the effect of the TiO2 modified BT fillers on the dielectric properties of composite, the frequency dependence of the dielectric properties are shown in Fig. 4 with the frequency ranging from 100 Hz to 100 MHz. The dielectric properties of the composites with randomly mixed TiO2/BT fillers are also shown as a comparison. As can be seen, the composites with TiO2 nanowires modified BT fillers had improved dielectric permittivity compared to the composites with randomly mixed TiO2/BT fillers. The maximum dielectric permittivity of the composite with the TiO2 nanowires modified BT fillers increased from 9.8 to 14.7 (Fig. 4a), whereas that of the composite with the randomly mixed TiO2/BT fillers was 7.2e10.1 (Fig. 4d). This can be attributed to the enlarged interface by the TiO2 nanowires compared to the nano TiO2 particles [9,14,20]. The dielectric loss of the composites with TiO2 nanowires modified BT fillers was as low as 0.037 at 1000 Hz, the dielectric loss decreased as the loading of TiO2 nanowires modified BT fillers increased in the high frequency range (Fig. 4b). The dielectric loss of the composites with randomly mixed TiO2/BT fillers also increased with different loading of fillers, but they did not show clear regularity as a function of volume fraction (Fig. 4e), this could be owing to the heterogeneous distribution of randomly mixed microparticles and nanoparticles, the high density of voids and defects in the film could result in dielectric properties fluctuation. The conductivity of the composites are shown in Fig. 4c and f. The conductivity of the composites with TiO2 nanowires modified BT fillers were relatively low (Fig. 4c), even the loading of the fillers was up to 20%, the conductivity increased as the loading increased over the frequency range. While the conductivity of the composites with randomly mixed TiO2/BT fillers did not show clear regularity as a function of volume fraction, as shown in Fig. 4f. 3.3. D-E loops and the breakdown strength of the obtained composites Typical electric displacement-electric field (D-E) loops of the composites measured at various volume fractions with the TiO2 nanowires modified BT fillers or TiO2/BT randomly mixed fillers are shown in Fig. 5, the electric field was 80 kV/mm. As can be seen from Fig. 5a, the concentration of TiO2 nanowires modified BT filler
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Fig. 2. (a) XRD pattern of the products before and after calcination. (b) The FT-IR of TiO2 nanowires modified BT particles before and after dopamine modification.
Fig. 3. SEM image of the TiO2 nanowires modified BT particles: (a) Agglomerated particles. (b) A single particle with part of BT particle surface exposed outside (the insert revealed the average aspect ratio of the TiO2 nanowires).
Fig. 4. Frequency dependence of (a) dielectric permittivity, (b) dielectric loss, (c) conductivity of the BT@TiO2/Dop/P(VDF-HFP) composites, (d) dielectric permittivity, (e) dielectric loss, and (f) conductivity of the composites with randomly mixed TiO2/BT particles.
notably raised the electric displacement, attributed to the high dielectric permittivity of the fillers. The results were consistent with the results in Fig. 4a. It is also shown that the composites with TiO2 nanowires modified BT fillers had large electric displacement than that of the composites with randomly mixed TiO2/BT fillers, as
a results of higher dielectric permittivity of the samples. For example, under the electric field of 80 kV/mm and 20 vol% loading, the composites with TiO2 nanowires modified BT fillers achieved an electric displacement of 1.84 mC/cm2, while the composites with TiO2/BT randomly mixed fillers only had an electric displacement of
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Fig. 5. Electric displacement-electric field (D-E) loops of (a) composites with BT@TiO2/Dop fillers and (b) composites with TiO2/BT randomly mixed fillers at 80 kV/mm.
1.23 mC/cm2 (Fig. 5b). Fig. 6 shows the electric breakdown strength of the composites with different volume fraction of TiO2 nanowires modified BT fillers, the Weibull analysis was used to calculate the characteristic breakdown strength of the composites with TiO2 nanowires modified BT fillers and the composites with randomly mixed fillers of TiO2 and BT particles. The characteristic breakdown strength was analyzed with the function as:
PðEÞ ¼ 1 exp ðE=EBÞb
(3)
where P(E) is the cumulative probability of electric failure, E is the experimental breakdown strength, EB is a scale parameter refers to the breakdown strength at the cumulative failure probability of 63.2% that is also regarded as the characteristic breakdown strength, and b is the Weibull modulus associated with the linear regressive fit of the distribution [36e38]. All the samples maintained a relatively high breakdown strength over 200 kV/mm due to tightly bonding between the fillers and polymer matrix, as shown in Fig. 6a. Specially, the EB of composites with 5 vol% TiO2 nanowires modified BT fillers was 277.08 kV/mm, this figure reduces to 232.28 kV/mm with 10 vol% TiO2 nanowires modified BT fillers but then increased to 254.80 kV/mm as the volume fraction of loading increased to 20%. Generally, the breakdown strength would decrease with the increase of volume fraction of loading (<20 vol%), as a results of the agglomeration and voids would increase with the volume fraction of loading increase. Therefore, the decrease of breakdown strength occurred [29,38e41]. The particular phenomenon in the work may be attributed to following reason. Dielectric gradient fillers BT@TiO2/Dop in the composites play a positive effects on the release of electric field intensification,
which could ease the reduction of breakdown strength caused by voids, cracks and other defects. The results indicated that the effect of weaken electric field intensification by the gradient filler was overweight the effect of higher defects on the breakdown strength of the composites. In addition, it can be seen from Fig. 6b that the breakdown strength of the composites with gradient dielectric fillers increased from 232.86 kV/mm to 254.85 kV/mm (>9% promotion), while the breakdown strength of the composites with randomly mixed TiO2/BT fillers increased from 270.11 kV/mm to 271.71 kV/mm (<1% promotion). This phenomenon could be attributed to the different average particle size and the different morphology of the fillers. 3.4. The fatigue endurance of the composites A high fatigue endurance is required for long-term stability during the charge-discharge cycling process of the dielectric capacitor. Therefore, the polarization fatigue behavior of the composite was studied as a function of the charge-discharge cycles up to 106 using a 1 kHz pulse signal under a 40 kV/mm electric field. Fig. 7 shows the variation of saturate polarization (Pmax) and remnant polarization (Pr) during fatigue testing. As can be seen, all the samples with different volume fraction could endure up to 106 times of field cycling at the applied cycling field, indicating that the composites exhibit high fatigue endurance up to 106 switching cycles at room temperature. The high fatigue endurance of the composites may be due to the following reasons: (i) High quality composite film possess high breakdown strength, the ceramic fillers was modified by dopamine before being added to the P(VDFHFP) matrix, which can greatly improve the dispersibility of the ceramic particles in the composites and the compatibility between
Fig. 6. Weibull plots of the breakdown strength of (a) the composites with BT@TiO2/Dop as fillers and (b) Characteristic breakdown strength comparison of the composites with BT@TiO2/Dop fillers and composites with TiO2/BT randomly mixed fillers.
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3.6. Energy density and energy efficiency of the obtained composites
Fig. 7. Fatigue endurance of the composites with BT@TiO2/Dop fillers at room temperature.
The energy density and energy efficiency are shown in Fig. 9. It demonstrates that the discharged energy density of the composites was strongly dependent on the volume fraction of the loading. The maximum discharged energy density was 2.8 J/cm3 at 240 kV/mm with the composite containing 20 vol% TiO2 nanowires modified BT fillers (Fig. 9a). The maximum energy density of each sample are shown in Fig. 9b, as can be seen, the maximum energy density of composite with 20 vol% TiO2 nanowires modified BT fillers reached 3.92 J/cm3. All the energy efficiency showed a downward trend as the applied electric goes up. The efficiency of the composites with 10 vol% and 20 vol% fillers were lower than 40%, indicating that the dielectric loss was relatively higher than many ferroelectric materials under a high electric field, the huge dielectric loss might be resulted from the micro-sized fillers used in the composites.
ceramic fillers and matrix. (ii) The fatigue endurance measurements of the composite were in an electric field of 40 kV/mm, which was much lower than the breakdown strength of the composites. For example, the breakdown strength of the composite with 5 vol% TiO2 nanowires modified BT fillers was as high as 277.08 kV/mm, as shown in Fig. 6b. Thus, the composites can keep working after 106 times of field cycling. (iii) The polarization of the composites keep a relatively stable value, especially when the content of the fillers is lower than 10 vol%, as shown in Fig. 7, which indicate that the composites exhibited long-term stability of the switchable polarization under electric field cycling.
3.5. The leakage current densities of the composites To investigate the influence of the enlarged interface of the composites, the leakage current densities of the composites with different volume fraction of filler were tested. As can be seen from Fig. 8, the leakage current densities of the samples were all laid in the range of 109 to 106 A/cm2. The leakage current followed a reasonable increase trend with the raise of applied electric field. The composite with higher loading of fillers exhibited larger leakage current density under a fixed electric field, attributed to the defects and the space charge introduced by BT@TiO2/Dop fillers [20,42].
Fig. 8. Leakage current density-electric field curves of the obtained composites.
Fig. 9. The (a) discharged energy density, (b) maximum energy density and (c) energy efficiency of the composites with different volume fraction of BT@TiO2/Dop fillers.
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Although the dielectric permittivity and breakdown strength were optimized and a considerable improvement of energy density was achieved by using the TiO2 nanowires modified BT particles as filler, there was substantial big room for improving the energy density by reducing energy loss. 4. Conclusion The TiO2 nanowires modified BT particles were synthesized and introduced to the P(VDF-HFP) matrix; thus, the composite with BT@TiO2/Dop/P(VDF-HFP) gradient dielectric distribution was obtained. The composite with TiO2 nanowires enlarged the interfacial polarization and leaded to a clear enhancement of dielectric permittivity compared to the composite with randomly mixed TiO2/BT particles as fillers. The effect of weakening the electric field intensification were remarkable when introducing 20 vol% fillers into the polymer matrix. The enhancement of breakdown strength was clear when compared the composite with BT@TiO2/Dop gradient dielectric fillers to the composite with randomly mixed TiO2/BT particles, attributed to the size and morphology differences of the fillers. The fatigue endurance test revealed that the composite could endure up to 106 times of field cycling at the applied cycling field and the leakage current density is rather low. The maximum discharged energy density of the composite reached 2.8 J/cm3 with 20 vol% loading of fillers at 260 kV/mm. The results obtained in this study would be helpful in exploring suitable filler candidates for the improvement of dielectric material's energy storage density. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51672311), Science and Technology Project of Hunan Province, China (no. 2016WK2022), Postdoctoral Research Foundation of Central South University, and State Key Laboratory of Powder Metallurgy, Central South University, Changsha, China. References [1] Baojin Chu, Xin Zhou, Kailiang Ren, Bret Neese, Minren Lin, Qing Wang, F. Bauer, Q.M. Zhang, A dielectric polymer with high electric energy density and fast discharge speed, Science 313 (2006) 334e336. [2] 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) 6334e6365. [3] L.A. Fredin, Z. Li, M.A. Ratner, M.T. Lanagan, T.J. Marks, Enhanced energy storage and suppressed dielectric loss in oxide core-shell-polyolefin nanocomposites by moderating internal surface area and increasing shell thickness, Adv. Mater. 24 (2012) 5946e5953. [4] X. Zhang, Y. Shen, Q.H. Zhang, L. Gu, Y.H. Hu, J.W. Du, Y.H. Lin, C.W. Nan, Ultrahigh energy density of polymer nanocomposites containing BaTiO3@TiO2 nanofibers by atomic-scale interface engineering, Adv. Mater. 27 (2015) 819e824. [5] J. Ho, R. Ramprasad, S. Boggs, Effect of alteration of antioxidant by UV treatment on the dielectric strength of BOPP capacitor film, IEEE Trans. Dielectr. Electr. Insul. 14 (2007) 1295e1301. [6] H. Tang, H.A. Sodano, Ultra high energy density nanocomposite capacitors with fast discharge using Ba0.2Sr0.8TiO3 nanowires, Nano Lett. 13 (2013) 1373e1379. [7] T.C. Mike Chung, Functionalization of polypropylene with high dielectric properties: applications in electric energy storage, Green. Sustain. Chem. 02 (2012) 29e37. [8] M. Rabuffi, G. Picci, Status quo and future prospects for metallized polypropylene energy storage capacitors, IEEE Trans. Plasma Sci. 30 (2002) 1939e1942. [9] H. Tang, Z. Zhou, H.A. Sodano, Relationship between BaTiO3 nanowire aspect ratio and the dielectric permittivity of nanocomposites, ACS Appl. Mater. Interfaces 6 (2014) 5450e5455. [10] X. Zhang, Y. Shen, B. Xu, Q. Zhang, L. Gu, J. Jiang, J. Ma, Y. Lin, C.W. Nan, Giant energy density and improved discharge efficiency of solution-processed polymer nanocomposites for dielectric energy storage, Adv. Mater. 28
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