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Surface modified BaTiO3 nanoparticles by titanate coupling agent induce significantly enhanced breakdown strength and larger energy density in PVDF nanocomposite
Composites Science and Technology 156 (2018) 109e116
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Surface modified BaTiO3 nanoparticles by titanate coupling agent induce significantly enhanced breakdown strength and larger energy density in PVDF nanocomposite Penghao Hu*, Shengmin Gao, Yangyang Zhang, Liang Zhang, Chengchen Wang Institute for Advanced Materials & Technology, School of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing, 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2017 Received in revised form 29 November 2017 Accepted 23 December 2017 Available online 28 December 2017
Dielectric capacitors are promising in micro-electronics, portable equipment and hybrid electric vehicles due to their specific features of flexibility, ultrahigh operating voltage and fast charging-discharging rate. The dielectric properties of polymer-based nanocomposite are much related to the interface binding between fillers and matrix. In this work, a surface modification approach employed newfound titanate coupling agent was developed to improve the compatibility between BT nanoparticles and PVDF matrix. After treated by the modifier TC-2, a coating layer contained with active organic groups was formed on the surface of BT nanoparticles. Benefited from the improved dispersibility and compatibility of modified BT nanoparticles in PVDF matrix, the breakdown strength of the nanocomposites was much enhanced. The monodisperse mBT-2 nanoparticles treated with appropriate amount of modifier dramatically enlarged the breakdown strength from 397 kV/mm for neat PVDF to 517 kV/mm for 4 vol% mBT-2 loading nanocomposite. Compared with BT/PVDF, the improvements on the energy storage performance in mBT2/PVDF are significant. The maximum discharged energy density of 11.27 J/cm3 for 4 vol% loading mBT-2/ PVDF is nearly double of that for 4 vol% loading BT/PVDF, and the energy efficiency for mBT-2/PVDF is also increased. The modification method originally represented here has great potential in developing high energy density nanocomposites for advanced applications. © 2017 Elsevier Ltd. All rights reserved.
Keywords: A. polymer-matrix composites (PMCs) A. Nano particles A. Coupling agents B. Surface treatments B. Electrical properties
1. Introduction As one of the important energy storage devices, dielectric capacitor plays a crucial role in the fields of modern compact electronic components and power systems. High energy density capacitors are promising in micro-electronics, portable equipment and hybrid electric vehicles [1]. Although dielectric polymers are becoming the regnant materials for capacitor attributed to their specific features of flexibility, ultrahigh operating voltage and fast charging-discharging rate, the relatively low energy density much limits their application [2e4]. As expressed by the formula R Ue ¼ EdD and D ¼ εE, where the Ue, D, ε, E represents the electric energy density, electric displacement, dielectric permittivity and electric field, respectively. Thus, the energy density of a capacitor generally depends on the dielectric permittivity and the withstand
* Corresponding author. E-mail address: [email protected] (P. Hu). https://doi.org/10.1016/j.compscitech.2017.12.025 0266-3538/© 2017 Elsevier Ltd. All rights reserved.
voltage (breakdown strength, Eb) of its core dielectrics, and the low energy density of polymer is precisely due to their low dielectric permittivity (commonly 2e3). The ferroelectric polymer such as poly (vinylidene fluoride) (PVDF) and its co-polymers have been widely investigated due to their relatively high ε of about 10, which are the promising matrixes for the new generation of energy storage dielectrics [5]. To further increase the dielectric permittivity to achieve larger energy density, the polymer-based nanocomposites contained with high-ε nanofillers have been received extensive research [6e8]. The introducing of high-ε ceramic nanofillers (such as BaTiO3, TiO2, Ba1xSrxTiO3 and so on) into polymers was effective in achieving increased ε in nanocomposites, but it's hard to obtain higher Ue because they dramatically reduced the Eb of polymer matrix. It's found that the weakening of Eb is mainly attributed to the poor compatibility between the inorganic fillers and the organic matrixes. Both the aggregation of the fillers and the defects existing at the interfaces between ceramic nanofillers and polymer matrix facilitate charges passing and thus result in the Eb deterioration.
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It's necessary to improve the compatibility between fillers and matrix to maintain the breakdown strength of the polymer nanocomposites, and several approaches were developed during the past decade, including grafting functionalized polymer chains onto the surface of nanofillers [9e12], in situ polymerization of monomer on nanofillers [13e16], introducing hydroxyl onto the surface of nanofillers [17e19], and modifying nanofillers with coupling surfactant [20e24]. In these methods, the surface modification is facile and the most commonly adopted. The modifier contained functional groups was coated onto the surface of nanofiller and form a bonding shell layer. The active shell layer on one hand changes the surface energy of the nanofillers to reduce the aggregation and on the other hand enhances the combination between fillers and matrix by molecular bonding, which is beneficial for eliminating the negative effect on the breakdown strength by nanofillers [25]. For instance, Kim et al. modified BaTiO3 (BT) nanoparticles with a pentafluorobenzyl phosphonic acid to improve their compatibility with P(VDF-HFP) matrix, and achieved Eb of above 200 kV/mm even in 50 vol% loading composite [26]. Song et al. fabricated PVDF nanocomposites contained with functionalized BT nanofillers coating with polydopamine, the Eb of which was much increased compared with the uncoated ones [27]. In the work of Yu et al. the BT nanoparticles treated with polyvinylprrolidone (PVP) present lower current density and higher breakdown strength in PVDF composite films [28]. Zhang et al. enhanced the dielectric tunability and breakdown strength of Ba0$6Sr0$4TiO3/PVDF nanocomposite via interface modification by a silane coupling agent (KH550), and found 4 wt% as the optimal content of modifier [29]. Consequently, with the quadratic dependence on Eb, the Ue is largely increased in the nanocomposites with enhanced breakdown strength [30,31]. Up to date, benefited from comprehensive approach, the breakdown strength in some of PVDF-based nanocomposite films is close to and even beyond the value of the neat polymer, and the surface modification on nanofillers has been an essential process in the strategy to achieve high energy density [32e38]. In the present work, a titanate coupling agent (TC-2) is adopted as the surface modifier to improve the compatibility of BT nanoparticles (NPs) in polymer nanocomposite. The modified BT (mBT) NPs coated with TC-2 molecules are obtained via facile solution method and are introduced in PVDF matrix. Benefited from the decrease of both interfacial polarization and defects, the breakdown strength of mBT/PVDF is much enhanced compared with that of neat PVDF and BT/PVDF nanocomposite. The effect of additive amount of TC-2 on the dispersity of mBT in matrix is investigated in detail. Insufficient or excessive amount of modifier of TC-2 leads to different levels of aggregation of nanofillers in matrix which is inefficient for improving breakdown strength. Uniform dispersion is observed in nanocomposite contained with nanoparticles treated with appropriate amount of TC-2 which breakdown strength was highly enhanced. Besides the breakdown strength, the stored energy density, discharged energy density and energy efficiency are all increased in mBT/PVDF, especially the maximal discharged energy density of 11.27 J/cm3 is nearly double of that of BT/PVDF with the same content loading. The original surface modification process with TC-2 reported here are promising in fabricating high energy density polymer nanocomposites. 2. Experimental 2.1. Materials Barium titanate (BaTiO3, 99.5%) particles with an average size of 100 nm were purchased from Aladdin (China). Titanate coupling agent (Product NO. TC-2, more information can be found in
Ref. [39]) was purchased from Taicang Chemical Co., Ltd (Anhui, China). Poly (vinylidene fluoride) (PVDF) was supplied by ARKEMA (Shenzhen, China). Hydrogen peroxide (H2O2, 30%) and solvents such as ethanol (A.R.), isopropyl alcohol (A.R.) and N,N-dimethylformamide (DMF) were obtained from Sinopharm Chemical Reagent Co., Ltd (China). 2.2. Surface modification of BT nanoparticles The BT nanoparticles were surface coated by TC-2 molecule via a solution method, which is schematically exhibited in Fig. 1. Firstly, the BT NPs were treated by H2O2 for hydroxylation, and then the hydroxylated BT nanoparticles was added into isopropyl alcohol in beaker followed by an ultra-sonication and then stirred for 30 min at room temperature. Secondly, the TC-2 was dropwise added into the liquid mixture with stirring, after that the suspensions were continuously stirred at 70 C for 4 h. Thirdly, the precipitates were centrifuged and washed with deionized water and ethanol for several times to remove the solvent and unreacted reagents. After drying at 70 C for 24 h in oven, the modified BT nanoparticles were obtained. 2.3. Fabrication of nanocomposite films The pristine BT or mBT nanoparticles and PVDF powder with fillers contents of 2, 4, 6, 8, 10 vol% were proportionally dispersed in DMF respectively. All the mixtures were processed according to the similar procedure that ultrasonication for 1 h, followed by stirring for 12 h, to form a stable suspension. The suspension was then cast onto glass plate with a laboratory casting equipment. The as-cast films were dried at 45 C for 10 h for solvent volatilizing and were heated at 200 C for 5 min and then immediately quenched in ice water. The final composite films were dried at 70 C and peeled from the substrates, the thickness of which was about 15 mm. 2.4. Characterization The morphologies of the mBT nanoparticles and corresponding element analysis were performed on field emission transmission electron microscopy (FETEM, Tecnai G2 F20, FEI, USA). Fouriertransform infrared spectroscopy (FT-IR) was measured with a spectrometer (Nexus 670, Nicolet, USA) over the range of 500e4000 cm1. X-ray photoelectron spectroscopy (XPS) was characterized using an X-ray photoelectron spectrometer equipment (AXIS ULTRADLD, Kratos, USA) with Al Ka radiation. Contact angles between pressed pellet of pristine BT or mBT NPs and DMF solution of PVDF were measured on the instrument (JC2000DF, Powereach, China) under ambient conditions by a sessile solution drop method. Reported data are averages of five measurements at different places on the sample. The cross-section morphologies of the BT/PVDF and mBT/PVDF nanocomposite films were observed by scanning electron microscope (SEM, JEM-7500F, JEOL Ltd., Japan). For electric measurement, copper electrodes of about 60 nm in thickness were sputtered on both sides of the nanocomposite films using a mask with 3 mm diameter eyelets. The dielectric permittivity and the loss of the nanocomposite films were measured by a precision impedance analyzer (HP 4294A, Agilent Technologies Inc., USA) at room temperature. Electric breakdown strength was tested by Dielectric Withstand Voltage Test (HT-50, Guilin Electrical Equipment Scientific Research Institute, China) at a ramping rate of 200 V/s and a limit current of 5 mA. The electric displacementelectric field (D-E) loops were measured at 10 Hz by a ferroelectric tester (Model TF Analyzer 1000, aixACCT, Germany) with a limited current of 1 mA.
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Fig. 1. (a) Schematic drawing of the surface modification procedure of the modified BT nanoparticle; (b) High-resolution TEM image of mBT NP; (c) EDS elements analysis corresponds to the points in (b); (d) XPS spectra of pristine BT and mBT NPs, high-resolution XPS spectra of P 2p & 2s are shown in the inset; (e) FT-IR spectra of pristine BT NPs, TC-2 and mBT NPs.
3. Results and discussion The surface modification process is exhibited in the schematic drawing in Fig. 1a. The raw BT nanoparticles were firstly hydroxylated to facilitate the TC-2 molecule link with the oxhydryl on the surface. Then the TC-2 molecules connected with the oxhydryl and formed a coating layer on the surface of BT NPs. As shown in Fig. 1b, a distinct amorphous layer can be observed on the surface of modified BT nanoparticle in the high revolution TEM image. The Energy Dispersive Spectrometer (EDS) performed at the corresponding point in Fig. 1c represents Ba, Ti, C, O, P, which accords to all elements in BT NPs and TC-2. In addition, XPS and FT-IR measurements were employed to confirm the effective coating of TC-2 molecules on the surface of BT nanoparticle. XPS spectra of raw BT and mBT are comparatively exhibited in Fig. 1d. The additional peaks of P 2p and P 2s at the binding energy of 131 and 189 eV in the spectrum of mBT confirm the phosphorus-containing TC-2 had been adhered on the surface of BT NPs. Moreover, compared with untreated BT, the peak density of Ba is decreased in the spectrum of mBT, revealing the formation of organic coating layer that shield the BT NPs. As shown in infrared spectra in Fig. 1e, there is no obvious absorption peak except TieO at 550 cm1 in spectrum of raw BT, while obvious absorption peaks at 3500, 2900 and
940e1040 cm1 can be observed in spectrum of both TC-2 and mBT, which is corresponds to the characteristic of hydrogenbonded group (eOH), CeH stretching vibration and PeO stretching vibration, respectively. Based on the above performances, it is convinced that TC-2 had been effectively coated on the surface of BT NPs. The improved compatibility between mBT NPs and PVDF matrix can be identified by the wettability measurement [40]. The contact angle between pristine BT pressed pellet and PVDF solution in DMF slightly changes from 66 to 58 (from Fig. 2a1 to Fig. 2a2) in 1 min, while the contact angle between mBT pressed pellet and PVDF solution in DMF obviously changes from 57 to 29 (from Fig. 2b1 to Fig. 2b2) in the same duration. As comparatively shown in Fig. 2c, the contact angle between mBT NPs and PVDF solution is far less than that of pristine BT NPs at the beginning, and represents faster decreasing with the time. Good wettability is necessary for combination between nanoparticles and matrix in composite because “contact” is the prerequisite of compatibility. The surface energy difference between inorganic BT and organic PVDF prevents them from good combining, which is expressed in poor wettability between nanoparticles in solution and often results in interfacial defects in nanocomposite. After treated, the TC-2 molecules form a coating layer on the surface of BT NPs which is miscible with PVDF
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Fig. 3. TEM images of (a) mBT-1, (b) mBT-2, (c) mBT-3, (d) mBT-4.
Fig. 2. Contact angle photos of (a) pristine BT and (b) mBT pellet between PVDF solution respectively; (c) The contact angle evolution with time.
solution. The relatively small contact angle reveals the improved infiltrating ability of PVDF solution on the mBT nanoparticles, reflecting better compatibility between mBT and PVDF matrix from the side. The quickly decreasing of contact angle also indicates better combination between mBT and PVDF and predicts fewer interfacial defects between the in composite. The breakdown strength of mBT/PVDF nanocomposite will also benefited from the improved compatibility. To investigate the optimal additive amount of modifier TC-2 on the dielectric properties of mBT/PVDF nanocomposite films, different proportions of TC-2 were added into the liquid mixture in the second step of BT NPs surface modification process (See experimental section). For each 1 g raw BT NPs in 100 ml isopropyl alcohol, 0.1.0.2, 0.3, 0.4 ml of TC-2 were added respectively in the solvent and treated with the same processing, and the corresponding final products of modified BT NPs were named mBT-1, mBT-2, mBT-3, mBT-4. The TEM images of mBT NPs treated with different additive amount of TC-2 are shown in Fig. 3. The coating layers on the surface of BT NPs represent different morphologies with different additive amount of modifier. For mBT-1 in Fig. 3a which was treated with 0.1 ml of TC-2, the amorphous layer is obscure on the surface of nanoparticles. For mBT-2 in Fig. 3b which was treated with 0.2 ml of TC-2, the coating layer is distinct and the nanoparticles show excellent monodispersion. For mBT-3 in Fig. 3c which was treated with 0.3 ml of TC-2, the coating layer become thick and some nanoparticles adhere to each other. For mBT-4 in Fig. 3d which was treated with 0.4 ml of TC-2, the amorphous layer is contiguous over the nanoparticles which are stuck together. The
morphology of the coating layer on the surface of nanoparticles will affect the disperse state of mBT NPs in PVDF matrix. The breakdown strength values of BT/PVDF and mBT/PVDF nanocomposite films were achieved from Weibull distribution (see details in Fig. P2 & Table P2 in Ref. [39]) and exhibited in Fig. 4. The Eb of BT/PVDF which contained unmodified BT NPs decreases monotonously with the content rising, for instance the Eb of 10 vol% BT/PVDF is below 200 kV/mm, which is about half of that of neat PVDF. Compared with BT/PVDF, the Eb of mBT/PVDF which contained modified BT NPs is much increased. The most obvious increment is obtained in the composites contained with 2 & 4 vol% mBT, the Eb of which is even much higher that of neat PVDF. The nonmonotonic evolution of Eb as function of nanofillers content loading in nanocomposites is frequently observed in previous works, where the maximal Eb is often achieved in the composite with 3e5 vol% content loading [29e31,37]. The charges tend to accumulate in the interface region in polymer-based composite contained with nanofillers, because of contrast on the dielectric permittivity between fillers and matrix. In low content loading nanocomposite, where the filler particles are relatively away from each other, the charge carriers could be trapped in the interface regions to restrain charge migration, which has positive effect in
Fig. 4. Breakdown strength of BT/PVDF and mBT/PVDF as function of nanoparticles content loading.
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improving breakdown strength. In high content loading nanocomposite, where the filler particles are relatively close to each other, the charge carriers would transfer from an interface region to another to facilitate charge migration, which has negative effect in maintaining breakdown strength. In present case, the most effective content loading in enhancing Eb of mBT/PVDF nanocomposites is 2e4 vol%, as shown in the shadow area in Fig. 4. However, the mBT NPs treated with different amount of TC-2 bring different improvement to the Eb of nanocomposite films. With same fillers loading, the Eb for mBT-1/PVDF and mBT-4/PVDF are nearly equal which are lower than that of mBT-3/PVDF, and the Eb for mBT-2/PVDF is the largest. For example, the 2 vol% loading mBT-1/PVDF and mBT-4/PVDF represent similar Eb value of ~438 kV/mm, while the value is 461 kV/mm for mBT-3/PVDF and 508 kV/mm for mBT-2/PVDF. The maximal Eb of 517 kV/mm is achieved in mBT-2/PVDF contained with 4 vol% content loading, which is 18% higher than that of mBT-4/PVDF with the same loading. The dispersity of nanoparticles is crucial for breakdown strength in polymer-based nanocomposite. The SEM image of cross section of 4 vol% BT/PVDF nanocomposite is shown in Fig. 5a, lots of aggregations of BT NPs can be observed in the matrix. As mentioned above, the difference on surface energy leads to low compatibility between PVDF matrix and BT nanoparticles, generating heterogeneous dispersion. After treated with TC-2, the hydrogen in modifier molecules on surface of BT NPs could link to the fluorine in PVDF segmers with hydrogen bond (See Fig. P1-3 in Ref. [39]), as schematically exhibited in Fig. 5b, which is beneficial for increasing
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compatibility between mBT and PVDF. As seen in SEM image of cross section in Fig. 5cef, the aggregation is obviously decreased in the mBT/PVDF nanocomposites, however, is also discriminating related with the additive amount of TC-2. For mBT-1/PVDF in Fig. 5c, the nanoparticles generally disperse uniformly in matrix with little aggregation, for mBT-2/PVDF in Fig. 5d, a homogenous morphology of composite inside can be distinctly observed. For mBT-3/PVDF in Fig. 5e, some small aggregation of nanoparticles distributes in the matrix, and for mBT-4/PVDF in Fig. 5f, several big aggregations with massive nanoparticles can be observed. The additive amount of TC-2 affects the breakdown strength of mBT/PVDF nanocomposites via determining the dispersity and
Fig. 6. Schematic drawing of the effect of the additive amount of TC-2 on the coating layer morphology on the surface of BT NPs and the compatibility between mBT NPs with PVDF matrix.
Fig. 5. SEM images of the cross-section morphologies of nanocomposite films contained with 4 vol% nanoparticles of (a) BT/PVDF, (c) mBT-1/PVDF, (d) mBT-2/PVDF, (e) mBT-3/PVDF (f) mBT-4/PVDF; (b) Schematic drawing of the surface bonding between mBT NP with PVDF segmer.
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interfacial combination of mBT NPs in PVDF matrix. As schematically shown in Fig. 6, the BT NP treated with insufficient amount of modifier forms thin or partial coating layer containing a few active groups on surface of each nanoparticle, which can only create incomplete combination with matrix, leading to little aggregation of nanofillers in mBT-1/PVDF. The BT NP treated with appropriate amount of modifier forms moderate and whole coating layer, which will create complete combination between nanofillers and matrix, leading to uniform dispersion of nanoparticles in mBT-2/PVDF. Profited from the weakened charges accumulation, the interfacial polarization was reduced thus the breakdown strength was enhanced. The nanoparticles treated with excessive amount of modifier forms thick and contiguous coating layer over surface of several nanoparticles, leading to relatively large aggregation of nanofillers in mBT-3/PVDF and mBT-4/PVDF, which increases interfacial polarization. Consequently, the breakdown strength is discrepant in nanocomposites contained with mBT NPs treated with different additive amount of TC-2. The dielectric permittivity and loss at 1 kHz of the nanocomposite films are exhibited in Fig. 7 (See frequency-dependent dielectric permittivity and loss in Fig. P3 in Ref. [39]). The ε is increased with nanofillers loading rising, but continuously decreased in the nanocomposites contained with the same loading of mBT NPs treated with increasing additive amount of TC-2. For example, the ε is reduced from 12.3 for mBT-1/PVDF to 10.8 for mBT-4/PVDF with 4 vol% content loading. Certainly, the coating layer of organic modifier on nanofillers will restrain accumulation of charges in the interface region to reduce the interfacial polarization, which is an important source of dielectric polarization in nanocomposite. From mBT-1 to mBT-4, the coating layer of TC-2 with increasing thickness gradually all below 0.05 in the nanocomposites and slightly decrease from mBT-1/PVDF to mBT-4/ PVDF. As mentioned above, the energy density is dependent on dielectric permittivity and quadratically dependent on applied electric field. Compared with BT/PVDF, although the dielectric permittivity is a little decreased after surface modification, the highly increased breakdown strength will much enlarge the energy density of mBT/PVDF nanocomposite. The unipolar D-E loops of pristine PVDF and nanocomposite films contained with 4 vol% nanoparticles loading are shown in Fig. 8a. From the loops, the electric displacement of BT/PVDF is larger than that of mBT/PVDF at same electric field, and slightly decreases form mBT-1/PVDF to mBT-4/PVDF according to the evolution of dielectric permittivity. However, the mBT/PVDF with enhanced breakdown strength can
Fig. 8. (a) D-E loops of neat PVDF and nanocomposites contained with 4 vol% content loading of nanoparticles; (b) Electric field-dependence of Discharged energy density and energy efficiency of neat PVDF and nanocomposites contained with 4 vol% content loading of nanoparticles.
be applied at higher electric field to generate slender shape of polarization loop which leads to larger maximal energy density. The discharged energy density (Udis) and energy efficiency (h) as function of electric field of the nanocomposites calculated from D-E loops (See calculation details and results in Fig. P4-1, P4-2 & Table P4 in Ref. [39]) are exhibited in Fig. 8b. Benefited from the enhanced breakdown strength, the maximal Udis of mBT/PVDF are much larger than that of neat PVDF and BT/PVDF, and the maximum of 11.27 J/cm3 is achieved in mBT-2/PVDF at 520 kV/mm. The energy efficiency of mBT/PVDF is also improved, especially for mBT-2/PVDF, the high h of above 0.6 at Udis larger than 10 J/cm3 is beneficial for practical use in energy storage. 4. Conclusion
Fig. 7. The revolutions of dielectric permittivity and loss of the BT/PVDF and mBT/ PVDF nanocomposite films with the nanoparticles loading.
A surface modification approach was developed to enhance the breakdown strength of PVDF nanocomposites. After treated by modifier TC-2, a coating layer contained with active organic groups was formed on the surface of BT NPs which highly improves the compatibility between the nanofillers and matrix. The additive amount of TC-2 affects the dispersity of mBT nanoparticles in polymer matrix, which leads to different behaviors of dielectric properties in mBT/PVDF nanocomposite films. With the optimal amount of modifier, the monodisperse mBT-2 NPs largely enhance the breakdown strength from 397 kV/mm for neat PVDF to 517 kV/ mm for mBT-2/PVDF nanocomposite. Benefited from the enlarged breakdown strength, the discharged energy density reach maximum of 11.27 J/cm3. Compared with BT/PVDF, the improvements on the energy storage performance of mBT-2/PVDF are
P. Hu et al. / Composites Science and Technology 156 (2018) 109e116
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[14]
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Fig. 9. Normalized values of breakdown strength, stored energy density, discharged energy density and energy efficiency, taking values of PVDF as reference.
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significant. As shown in Fig. 9, taking values of PVDF as reference, the normalized values including Eb, Usto, Udis, and h of mBT-2/PVDF are much larger than those of BT/PVDF, especially the Udis is increased by double. The originally discovered surfactant TC-2 is much effective for BT nanoparticles in enhancing the breakdown strength and enlarging energy density in PVDF composite, and will probably useful in other inorganic-polymer systems. The facile modification process originally represented here has great potential in fabricating high energy density nanocomposites for advanced dielectric capacitors.
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Acknowledgments
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This work was supported by the National Basic Research Program of China (973-Program, Grant No. 2015CB654603) and National Science Foundation of China (Grant No. 51402015).
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titanate/poly(vinylidene fluoride) nanocomposites, Adv. Mater. 27 (2015) 6658e6663. [37] X. Zhang, Y. Shen, B. Xu, Q.H. Zhang, L. Gu, J.Y. Jiang, J. Ma, Y.H. Lin, C.W. Nan, Giant energy density and improved discharge efficiency of solution-processed polymer nanocomposites for dielectric energy storage, Adv. Mater. 28 (2016) 2055e2061. [38] P.H. Hu, Y. Shen, Y.H. Guan, X.H. Zhang, Y.H. Lin, Q.M. Zhang, C.W. Nan, Topological-structure modulated polymer nanocomposites exhibiting highly enhanced dielectric strength and energy density, Adv. Funct. Mater. 24 (2014)
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Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech
Surface modified BaTiO3 nanoparticles by titanate coupling agent induce significantly enhanced breakdown strength and larger energy density in PVDF nanocomposite Penghao Hu*, Shengmin Gao, Yangyang Zhang, Liang Zhang, Chengchen Wang Institute for Advanced Materials & Technology, School of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing, 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2017 Received in revised form 29 November 2017 Accepted 23 December 2017 Available online 28 December 2017
Dielectric capacitors are promising in micro-electronics, portable equipment and hybrid electric vehicles due to their specific features of flexibility, ultrahigh operating voltage and fast charging-discharging rate. The dielectric properties of polymer-based nanocomposite are much related to the interface binding between fillers and matrix. In this work, a surface modification approach employed newfound titanate coupling agent was developed to improve the compatibility between BT nanoparticles and PVDF matrix. After treated by the modifier TC-2, a coating layer contained with active organic groups was formed on the surface of BT nanoparticles. Benefited from the improved dispersibility and compatibility of modified BT nanoparticles in PVDF matrix, the breakdown strength of the nanocomposites was much enhanced. The monodisperse mBT-2 nanoparticles treated with appropriate amount of modifier dramatically enlarged the breakdown strength from 397 kV/mm for neat PVDF to 517 kV/mm for 4 vol% mBT-2 loading nanocomposite. Compared with BT/PVDF, the improvements on the energy storage performance in mBT2/PVDF are significant. The maximum discharged energy density of 11.27 J/cm3 for 4 vol% loading mBT-2/ PVDF is nearly double of that for 4 vol% loading BT/PVDF, and the energy efficiency for mBT-2/PVDF is also increased. The modification method originally represented here has great potential in developing high energy density nanocomposites for advanced applications. © 2017 Elsevier Ltd. All rights reserved.
Keywords: A. polymer-matrix composites (PMCs) A. Nano particles A. Coupling agents B. Surface treatments B. Electrical properties
1. Introduction As one of the important energy storage devices, dielectric capacitor plays a crucial role in the fields of modern compact electronic components and power systems. High energy density capacitors are promising in micro-electronics, portable equipment and hybrid electric vehicles [1]. Although dielectric polymers are becoming the regnant materials for capacitor attributed to their specific features of flexibility, ultrahigh operating voltage and fast charging-discharging rate, the relatively low energy density much limits their application [2e4]. As expressed by the formula R Ue ¼ EdD and D ¼ εE, where the Ue, D, ε, E represents the electric energy density, electric displacement, dielectric permittivity and electric field, respectively. Thus, the energy density of a capacitor generally depends on the dielectric permittivity and the withstand
* Corresponding author. E-mail address: [email protected] (P. Hu). https://doi.org/10.1016/j.compscitech.2017.12.025 0266-3538/© 2017 Elsevier Ltd. All rights reserved.
voltage (breakdown strength, Eb) of its core dielectrics, and the low energy density of polymer is precisely due to their low dielectric permittivity (commonly 2e3). The ferroelectric polymer such as poly (vinylidene fluoride) (PVDF) and its co-polymers have been widely investigated due to their relatively high ε of about 10, which are the promising matrixes for the new generation of energy storage dielectrics [5]. To further increase the dielectric permittivity to achieve larger energy density, the polymer-based nanocomposites contained with high-ε nanofillers have been received extensive research [6e8]. The introducing of high-ε ceramic nanofillers (such as BaTiO3, TiO2, Ba1xSrxTiO3 and so on) into polymers was effective in achieving increased ε in nanocomposites, but it's hard to obtain higher Ue because they dramatically reduced the Eb of polymer matrix. It's found that the weakening of Eb is mainly attributed to the poor compatibility between the inorganic fillers and the organic matrixes. Both the aggregation of the fillers and the defects existing at the interfaces between ceramic nanofillers and polymer matrix facilitate charges passing and thus result in the Eb deterioration.
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It's necessary to improve the compatibility between fillers and matrix to maintain the breakdown strength of the polymer nanocomposites, and several approaches were developed during the past decade, including grafting functionalized polymer chains onto the surface of nanofillers [9e12], in situ polymerization of monomer on nanofillers [13e16], introducing hydroxyl onto the surface of nanofillers [17e19], and modifying nanofillers with coupling surfactant [20e24]. In these methods, the surface modification is facile and the most commonly adopted. The modifier contained functional groups was coated onto the surface of nanofiller and form a bonding shell layer. The active shell layer on one hand changes the surface energy of the nanofillers to reduce the aggregation and on the other hand enhances the combination between fillers and matrix by molecular bonding, which is beneficial for eliminating the negative effect on the breakdown strength by nanofillers [25]. For instance, Kim et al. modified BaTiO3 (BT) nanoparticles with a pentafluorobenzyl phosphonic acid to improve their compatibility with P(VDF-HFP) matrix, and achieved Eb of above 200 kV/mm even in 50 vol% loading composite [26]. Song et al. fabricated PVDF nanocomposites contained with functionalized BT nanofillers coating with polydopamine, the Eb of which was much increased compared with the uncoated ones [27]. In the work of Yu et al. the BT nanoparticles treated with polyvinylprrolidone (PVP) present lower current density and higher breakdown strength in PVDF composite films [28]. Zhang et al. enhanced the dielectric tunability and breakdown strength of Ba0$6Sr0$4TiO3/PVDF nanocomposite via interface modification by a silane coupling agent (KH550), and found 4 wt% as the optimal content of modifier [29]. Consequently, with the quadratic dependence on Eb, the Ue is largely increased in the nanocomposites with enhanced breakdown strength [30,31]. Up to date, benefited from comprehensive approach, the breakdown strength in some of PVDF-based nanocomposite films is close to and even beyond the value of the neat polymer, and the surface modification on nanofillers has been an essential process in the strategy to achieve high energy density [32e38]. In the present work, a titanate coupling agent (TC-2) is adopted as the surface modifier to improve the compatibility of BT nanoparticles (NPs) in polymer nanocomposite. The modified BT (mBT) NPs coated with TC-2 molecules are obtained via facile solution method and are introduced in PVDF matrix. Benefited from the decrease of both interfacial polarization and defects, the breakdown strength of mBT/PVDF is much enhanced compared with that of neat PVDF and BT/PVDF nanocomposite. The effect of additive amount of TC-2 on the dispersity of mBT in matrix is investigated in detail. Insufficient or excessive amount of modifier of TC-2 leads to different levels of aggregation of nanofillers in matrix which is inefficient for improving breakdown strength. Uniform dispersion is observed in nanocomposite contained with nanoparticles treated with appropriate amount of TC-2 which breakdown strength was highly enhanced. Besides the breakdown strength, the stored energy density, discharged energy density and energy efficiency are all increased in mBT/PVDF, especially the maximal discharged energy density of 11.27 J/cm3 is nearly double of that of BT/PVDF with the same content loading. The original surface modification process with TC-2 reported here are promising in fabricating high energy density polymer nanocomposites. 2. Experimental 2.1. Materials Barium titanate (BaTiO3, 99.5%) particles with an average size of 100 nm were purchased from Aladdin (China). Titanate coupling agent (Product NO. TC-2, more information can be found in
Ref. [39]) was purchased from Taicang Chemical Co., Ltd (Anhui, China). Poly (vinylidene fluoride) (PVDF) was supplied by ARKEMA (Shenzhen, China). Hydrogen peroxide (H2O2, 30%) and solvents such as ethanol (A.R.), isopropyl alcohol (A.R.) and N,N-dimethylformamide (DMF) were obtained from Sinopharm Chemical Reagent Co., Ltd (China). 2.2. Surface modification of BT nanoparticles The BT nanoparticles were surface coated by TC-2 molecule via a solution method, which is schematically exhibited in Fig. 1. Firstly, the BT NPs were treated by H2O2 for hydroxylation, and then the hydroxylated BT nanoparticles was added into isopropyl alcohol in beaker followed by an ultra-sonication and then stirred for 30 min at room temperature. Secondly, the TC-2 was dropwise added into the liquid mixture with stirring, after that the suspensions were continuously stirred at 70 C for 4 h. Thirdly, the precipitates were centrifuged and washed with deionized water and ethanol for several times to remove the solvent and unreacted reagents. After drying at 70 C for 24 h in oven, the modified BT nanoparticles were obtained. 2.3. Fabrication of nanocomposite films The pristine BT or mBT nanoparticles and PVDF powder with fillers contents of 2, 4, 6, 8, 10 vol% were proportionally dispersed in DMF respectively. All the mixtures were processed according to the similar procedure that ultrasonication for 1 h, followed by stirring for 12 h, to form a stable suspension. The suspension was then cast onto glass plate with a laboratory casting equipment. The as-cast films were dried at 45 C for 10 h for solvent volatilizing and were heated at 200 C for 5 min and then immediately quenched in ice water. The final composite films were dried at 70 C and peeled from the substrates, the thickness of which was about 15 mm. 2.4. Characterization The morphologies of the mBT nanoparticles and corresponding element analysis were performed on field emission transmission electron microscopy (FETEM, Tecnai G2 F20, FEI, USA). Fouriertransform infrared spectroscopy (FT-IR) was measured with a spectrometer (Nexus 670, Nicolet, USA) over the range of 500e4000 cm1. X-ray photoelectron spectroscopy (XPS) was characterized using an X-ray photoelectron spectrometer equipment (AXIS ULTRADLD, Kratos, USA) with Al Ka radiation. Contact angles between pressed pellet of pristine BT or mBT NPs and DMF solution of PVDF were measured on the instrument (JC2000DF, Powereach, China) under ambient conditions by a sessile solution drop method. Reported data are averages of five measurements at different places on the sample. The cross-section morphologies of the BT/PVDF and mBT/PVDF nanocomposite films were observed by scanning electron microscope (SEM, JEM-7500F, JEOL Ltd., Japan). For electric measurement, copper electrodes of about 60 nm in thickness were sputtered on both sides of the nanocomposite films using a mask with 3 mm diameter eyelets. The dielectric permittivity and the loss of the nanocomposite films were measured by a precision impedance analyzer (HP 4294A, Agilent Technologies Inc., USA) at room temperature. Electric breakdown strength was tested by Dielectric Withstand Voltage Test (HT-50, Guilin Electrical Equipment Scientific Research Institute, China) at a ramping rate of 200 V/s and a limit current of 5 mA. The electric displacementelectric field (D-E) loops were measured at 10 Hz by a ferroelectric tester (Model TF Analyzer 1000, aixACCT, Germany) with a limited current of 1 mA.
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Fig. 1. (a) Schematic drawing of the surface modification procedure of the modified BT nanoparticle; (b) High-resolution TEM image of mBT NP; (c) EDS elements analysis corresponds to the points in (b); (d) XPS spectra of pristine BT and mBT NPs, high-resolution XPS spectra of P 2p & 2s are shown in the inset; (e) FT-IR spectra of pristine BT NPs, TC-2 and mBT NPs.
3. Results and discussion The surface modification process is exhibited in the schematic drawing in Fig. 1a. The raw BT nanoparticles were firstly hydroxylated to facilitate the TC-2 molecule link with the oxhydryl on the surface. Then the TC-2 molecules connected with the oxhydryl and formed a coating layer on the surface of BT NPs. As shown in Fig. 1b, a distinct amorphous layer can be observed on the surface of modified BT nanoparticle in the high revolution TEM image. The Energy Dispersive Spectrometer (EDS) performed at the corresponding point in Fig. 1c represents Ba, Ti, C, O, P, which accords to all elements in BT NPs and TC-2. In addition, XPS and FT-IR measurements were employed to confirm the effective coating of TC-2 molecules on the surface of BT nanoparticle. XPS spectra of raw BT and mBT are comparatively exhibited in Fig. 1d. The additional peaks of P 2p and P 2s at the binding energy of 131 and 189 eV in the spectrum of mBT confirm the phosphorus-containing TC-2 had been adhered on the surface of BT NPs. Moreover, compared with untreated BT, the peak density of Ba is decreased in the spectrum of mBT, revealing the formation of organic coating layer that shield the BT NPs. As shown in infrared spectra in Fig. 1e, there is no obvious absorption peak except TieO at 550 cm1 in spectrum of raw BT, while obvious absorption peaks at 3500, 2900 and
940e1040 cm1 can be observed in spectrum of both TC-2 and mBT, which is corresponds to the characteristic of hydrogenbonded group (eOH), CeH stretching vibration and PeO stretching vibration, respectively. Based on the above performances, it is convinced that TC-2 had been effectively coated on the surface of BT NPs. The improved compatibility between mBT NPs and PVDF matrix can be identified by the wettability measurement [40]. The contact angle between pristine BT pressed pellet and PVDF solution in DMF slightly changes from 66 to 58 (from Fig. 2a1 to Fig. 2a2) in 1 min, while the contact angle between mBT pressed pellet and PVDF solution in DMF obviously changes from 57 to 29 (from Fig. 2b1 to Fig. 2b2) in the same duration. As comparatively shown in Fig. 2c, the contact angle between mBT NPs and PVDF solution is far less than that of pristine BT NPs at the beginning, and represents faster decreasing with the time. Good wettability is necessary for combination between nanoparticles and matrix in composite because “contact” is the prerequisite of compatibility. The surface energy difference between inorganic BT and organic PVDF prevents them from good combining, which is expressed in poor wettability between nanoparticles in solution and often results in interfacial defects in nanocomposite. After treated, the TC-2 molecules form a coating layer on the surface of BT NPs which is miscible with PVDF
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Fig. 3. TEM images of (a) mBT-1, (b) mBT-2, (c) mBT-3, (d) mBT-4.
Fig. 2. Contact angle photos of (a) pristine BT and (b) mBT pellet between PVDF solution respectively; (c) The contact angle evolution with time.
solution. The relatively small contact angle reveals the improved infiltrating ability of PVDF solution on the mBT nanoparticles, reflecting better compatibility between mBT and PVDF matrix from the side. The quickly decreasing of contact angle also indicates better combination between mBT and PVDF and predicts fewer interfacial defects between the in composite. The breakdown strength of mBT/PVDF nanocomposite will also benefited from the improved compatibility. To investigate the optimal additive amount of modifier TC-2 on the dielectric properties of mBT/PVDF nanocomposite films, different proportions of TC-2 were added into the liquid mixture in the second step of BT NPs surface modification process (See experimental section). For each 1 g raw BT NPs in 100 ml isopropyl alcohol, 0.1.0.2, 0.3, 0.4 ml of TC-2 were added respectively in the solvent and treated with the same processing, and the corresponding final products of modified BT NPs were named mBT-1, mBT-2, mBT-3, mBT-4. The TEM images of mBT NPs treated with different additive amount of TC-2 are shown in Fig. 3. The coating layers on the surface of BT NPs represent different morphologies with different additive amount of modifier. For mBT-1 in Fig. 3a which was treated with 0.1 ml of TC-2, the amorphous layer is obscure on the surface of nanoparticles. For mBT-2 in Fig. 3b which was treated with 0.2 ml of TC-2, the coating layer is distinct and the nanoparticles show excellent monodispersion. For mBT-3 in Fig. 3c which was treated with 0.3 ml of TC-2, the coating layer become thick and some nanoparticles adhere to each other. For mBT-4 in Fig. 3d which was treated with 0.4 ml of TC-2, the amorphous layer is contiguous over the nanoparticles which are stuck together. The
morphology of the coating layer on the surface of nanoparticles will affect the disperse state of mBT NPs in PVDF matrix. The breakdown strength values of BT/PVDF and mBT/PVDF nanocomposite films were achieved from Weibull distribution (see details in Fig. P2 & Table P2 in Ref. [39]) and exhibited in Fig. 4. The Eb of BT/PVDF which contained unmodified BT NPs decreases monotonously with the content rising, for instance the Eb of 10 vol% BT/PVDF is below 200 kV/mm, which is about half of that of neat PVDF. Compared with BT/PVDF, the Eb of mBT/PVDF which contained modified BT NPs is much increased. The most obvious increment is obtained in the composites contained with 2 & 4 vol% mBT, the Eb of which is even much higher that of neat PVDF. The nonmonotonic evolution of Eb as function of nanofillers content loading in nanocomposites is frequently observed in previous works, where the maximal Eb is often achieved in the composite with 3e5 vol% content loading [29e31,37]. The charges tend to accumulate in the interface region in polymer-based composite contained with nanofillers, because of contrast on the dielectric permittivity between fillers and matrix. In low content loading nanocomposite, where the filler particles are relatively away from each other, the charge carriers could be trapped in the interface regions to restrain charge migration, which has positive effect in
Fig. 4. Breakdown strength of BT/PVDF and mBT/PVDF as function of nanoparticles content loading.
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improving breakdown strength. In high content loading nanocomposite, where the filler particles are relatively close to each other, the charge carriers would transfer from an interface region to another to facilitate charge migration, which has negative effect in maintaining breakdown strength. In present case, the most effective content loading in enhancing Eb of mBT/PVDF nanocomposites is 2e4 vol%, as shown in the shadow area in Fig. 4. However, the mBT NPs treated with different amount of TC-2 bring different improvement to the Eb of nanocomposite films. With same fillers loading, the Eb for mBT-1/PVDF and mBT-4/PVDF are nearly equal which are lower than that of mBT-3/PVDF, and the Eb for mBT-2/PVDF is the largest. For example, the 2 vol% loading mBT-1/PVDF and mBT-4/PVDF represent similar Eb value of ~438 kV/mm, while the value is 461 kV/mm for mBT-3/PVDF and 508 kV/mm for mBT-2/PVDF. The maximal Eb of 517 kV/mm is achieved in mBT-2/PVDF contained with 4 vol% content loading, which is 18% higher than that of mBT-4/PVDF with the same loading. The dispersity of nanoparticles is crucial for breakdown strength in polymer-based nanocomposite. The SEM image of cross section of 4 vol% BT/PVDF nanocomposite is shown in Fig. 5a, lots of aggregations of BT NPs can be observed in the matrix. As mentioned above, the difference on surface energy leads to low compatibility between PVDF matrix and BT nanoparticles, generating heterogeneous dispersion. After treated with TC-2, the hydrogen in modifier molecules on surface of BT NPs could link to the fluorine in PVDF segmers with hydrogen bond (See Fig. P1-3 in Ref. [39]), as schematically exhibited in Fig. 5b, which is beneficial for increasing
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compatibility between mBT and PVDF. As seen in SEM image of cross section in Fig. 5cef, the aggregation is obviously decreased in the mBT/PVDF nanocomposites, however, is also discriminating related with the additive amount of TC-2. For mBT-1/PVDF in Fig. 5c, the nanoparticles generally disperse uniformly in matrix with little aggregation, for mBT-2/PVDF in Fig. 5d, a homogenous morphology of composite inside can be distinctly observed. For mBT-3/PVDF in Fig. 5e, some small aggregation of nanoparticles distributes in the matrix, and for mBT-4/PVDF in Fig. 5f, several big aggregations with massive nanoparticles can be observed. The additive amount of TC-2 affects the breakdown strength of mBT/PVDF nanocomposites via determining the dispersity and
Fig. 6. Schematic drawing of the effect of the additive amount of TC-2 on the coating layer morphology on the surface of BT NPs and the compatibility between mBT NPs with PVDF matrix.
Fig. 5. SEM images of the cross-section morphologies of nanocomposite films contained with 4 vol% nanoparticles of (a) BT/PVDF, (c) mBT-1/PVDF, (d) mBT-2/PVDF, (e) mBT-3/PVDF (f) mBT-4/PVDF; (b) Schematic drawing of the surface bonding between mBT NP with PVDF segmer.
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interfacial combination of mBT NPs in PVDF matrix. As schematically shown in Fig. 6, the BT NP treated with insufficient amount of modifier forms thin or partial coating layer containing a few active groups on surface of each nanoparticle, which can only create incomplete combination with matrix, leading to little aggregation of nanofillers in mBT-1/PVDF. The BT NP treated with appropriate amount of modifier forms moderate and whole coating layer, which will create complete combination between nanofillers and matrix, leading to uniform dispersion of nanoparticles in mBT-2/PVDF. Profited from the weakened charges accumulation, the interfacial polarization was reduced thus the breakdown strength was enhanced. The nanoparticles treated with excessive amount of modifier forms thick and contiguous coating layer over surface of several nanoparticles, leading to relatively large aggregation of nanofillers in mBT-3/PVDF and mBT-4/PVDF, which increases interfacial polarization. Consequently, the breakdown strength is discrepant in nanocomposites contained with mBT NPs treated with different additive amount of TC-2. The dielectric permittivity and loss at 1 kHz of the nanocomposite films are exhibited in Fig. 7 (See frequency-dependent dielectric permittivity and loss in Fig. P3 in Ref. [39]). The ε is increased with nanofillers loading rising, but continuously decreased in the nanocomposites contained with the same loading of mBT NPs treated with increasing additive amount of TC-2. For example, the ε is reduced from 12.3 for mBT-1/PVDF to 10.8 for mBT-4/PVDF with 4 vol% content loading. Certainly, the coating layer of organic modifier on nanofillers will restrain accumulation of charges in the interface region to reduce the interfacial polarization, which is an important source of dielectric polarization in nanocomposite. From mBT-1 to mBT-4, the coating layer of TC-2 with increasing thickness gradually all below 0.05 in the nanocomposites and slightly decrease from mBT-1/PVDF to mBT-4/ PVDF. As mentioned above, the energy density is dependent on dielectric permittivity and quadratically dependent on applied electric field. Compared with BT/PVDF, although the dielectric permittivity is a little decreased after surface modification, the highly increased breakdown strength will much enlarge the energy density of mBT/PVDF nanocomposite. The unipolar D-E loops of pristine PVDF and nanocomposite films contained with 4 vol% nanoparticles loading are shown in Fig. 8a. From the loops, the electric displacement of BT/PVDF is larger than that of mBT/PVDF at same electric field, and slightly decreases form mBT-1/PVDF to mBT-4/PVDF according to the evolution of dielectric permittivity. However, the mBT/PVDF with enhanced breakdown strength can
Fig. 8. (a) D-E loops of neat PVDF and nanocomposites contained with 4 vol% content loading of nanoparticles; (b) Electric field-dependence of Discharged energy density and energy efficiency of neat PVDF and nanocomposites contained with 4 vol% content loading of nanoparticles.
be applied at higher electric field to generate slender shape of polarization loop which leads to larger maximal energy density. The discharged energy density (Udis) and energy efficiency (h) as function of electric field of the nanocomposites calculated from D-E loops (See calculation details and results in Fig. P4-1, P4-2 & Table P4 in Ref. [39]) are exhibited in Fig. 8b. Benefited from the enhanced breakdown strength, the maximal Udis of mBT/PVDF are much larger than that of neat PVDF and BT/PVDF, and the maximum of 11.27 J/cm3 is achieved in mBT-2/PVDF at 520 kV/mm. The energy efficiency of mBT/PVDF is also improved, especially for mBT-2/PVDF, the high h of above 0.6 at Udis larger than 10 J/cm3 is beneficial for practical use in energy storage. 4. Conclusion
Fig. 7. The revolutions of dielectric permittivity and loss of the BT/PVDF and mBT/ PVDF nanocomposite films with the nanoparticles loading.
A surface modification approach was developed to enhance the breakdown strength of PVDF nanocomposites. After treated by modifier TC-2, a coating layer contained with active organic groups was formed on the surface of BT NPs which highly improves the compatibility between the nanofillers and matrix. The additive amount of TC-2 affects the dispersity of mBT nanoparticles in polymer matrix, which leads to different behaviors of dielectric properties in mBT/PVDF nanocomposite films. With the optimal amount of modifier, the monodisperse mBT-2 NPs largely enhance the breakdown strength from 397 kV/mm for neat PVDF to 517 kV/ mm for mBT-2/PVDF nanocomposite. Benefited from the enlarged breakdown strength, the discharged energy density reach maximum of 11.27 J/cm3. Compared with BT/PVDF, the improvements on the energy storage performance of mBT-2/PVDF are
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Fig. 9. Normalized values of breakdown strength, stored energy density, discharged energy density and energy efficiency, taking values of PVDF as reference.
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significant. As shown in Fig. 9, taking values of PVDF as reference, the normalized values including Eb, Usto, Udis, and h of mBT-2/PVDF are much larger than those of BT/PVDF, especially the Udis is increased by double. The originally discovered surfactant TC-2 is much effective for BT nanoparticles in enhancing the breakdown strength and enlarging energy density in PVDF composite, and will probably useful in other inorganic-polymer systems. The facile modification process originally represented here has great potential in fabricating high energy density nanocomposites for advanced dielectric capacitors.
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Acknowledgments
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This work was supported by the National Basic Research Program of China (973-Program, Grant No. 2015CB654603) and National Science Foundation of China (Grant No. 51402015).
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