Title: Enhanced dielectric properties and energy storage of the sandwich‐structured poly(vinylidene fluoride‐ <i>co</i> ‐hexafluoropropylene) composite films with functional BaTiO <sub>3</sub> @Al <sub>2</sub> O <sub>3</sub> nanofibres
Abstract: IET NanodielectricsVolume 2, Issue 3 p. 103-108 Research ArticleOpen Access Enhanced dielectric properties and energy storage of the sandwich-structured poly(vinylidene fluoride-co -hexafluoropropylene) composite films with functional BaTiO3 @Al2 O3 nanofibres Jun-Wei Zha, Corresponding Author Jun-Wei Zha [email protected] State Key Laboratory of Electrical System, Department of Electrical Engineering, Tsinghua University, Beijing, 100084 People's Republic of China School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083 People's Republic of ChinaSearch for more papers by this authorShi-Cong Yao, Shi-Cong Yao School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083 People's Republic of ChinaSearch for more papers by this authorYan Qiu, Yan Qiu School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083 People's Republic of ChinaSearch for more papers by this authorMing-Sheng Zheng, Ming-Sheng Zheng State Key Laboratory of Electrical System, Department of Electrical Engineering, Tsinghua University, Beijing, 100084 People's Republic of ChinaSearch for more papers by this authorZhi-Min Dang, Zhi-Min Dang State Key Laboratory of Electrical System, Department of Electrical Engineering, Tsinghua University, Beijing, 100084 People's Republic of ChinaSearch for more papers by this author Jun-Wei Zha, Corresponding Author Jun-Wei Zha [email protected] State Key Laboratory of Electrical System, Department of Electrical Engineering, Tsinghua University, Beijing, 100084 People's Republic of China School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083 People's Republic of ChinaSearch for more papers by this authorShi-Cong Yao, Shi-Cong Yao School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083 People's Republic of ChinaSearch for more papers by this authorYan Qiu, Yan Qiu School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083 People's Republic of ChinaSearch for more papers by this authorMing-Sheng Zheng, Ming-Sheng Zheng State Key Laboratory of Electrical System, Department of Electrical Engineering, Tsinghua University, Beijing, 100084 People's Republic of ChinaSearch for more papers by this authorZhi-Min Dang, Zhi-Min Dang State Key Laboratory of Electrical System, Department of Electrical Engineering, Tsinghua University, Beijing, 100084 People's Republic of ChinaSearch for more papers by this author First published: 21 June 2019 https://doi.org/10.1049/iet-nde.2019.0010Citations: 17AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Polymer-based composites with ceramic fillers could combine the advantages of both, which can be potentially used in electrical and electronic technology. In this work, the barium titanate (BaTiO3) nanofibres and the core–shell structured BaTiO3 @Al2 O3 nanofibres with Al2 O3 insulation layer coated on the BaTiO3 surface were both prepared via the electrospinning method. The appropriate incorporation of the ceramic nanofibres effectively improves the dielectric properties and energy density of the polymer. Moreover, the poly(vinylidene fluoride-co -hexafluoropropylene)-based composite films with the three-layer sandwich structure were fabricated to further promote the dielectric properties. The results show that the outer two layers with a higher content of BaTiO3 nanofibres can make more contribution to the improved permittivity of the composites. In addition, the introduction of the interlayer with low loading of BaTiO3 @Al2 O3 nanofibres promotes the breakdown strength. This work gives rise to the potential in high energy storage applications. 1 Introduction Dielectric materials with large permittivity, high breakdown strength, low dielectric loss, and high energy storage density have attracted a lot of attention in the field of electronics. Furthermore, due to the excellent processing properties, flexibility and low cost of polymer-based dielectric films, the film capacitor shows remarkable potential in practical applications. The preparation of polymer-based composite dielectrics with high permittivity (high-k) could not only achieve satisfactory dielectric properties but also greatly enhance the energy density [1-3]. At present, high-k composites with improved energy density have been widely reported [4-7]. Barium titanate as an ABO3 perovskite has high permittivity, low dielectric loss and the n-type semiconductor characteristic, which has been used to fabricate the ceramic-based capacitors, sensors, and energy storage devices [8-10]. Besides, the BaTiO3 (BT) nanofillers also have been chosen as the ideal additives to improve the dielectric properties of polymer-based composites [11-14]. Generally, the BT nanofillers could be synthesised by the various methods, such as the solid phase reaction, sol precipitation, sol–gel process, electrospinning etc. [15-18]. A high concentration of 0D BT nanoparticles is usually needed for the pursuit of improved permittivity, which will lead to poor flexibility of composite films and limit its practical application. For example, in our previous work, BT nanoparticles were added into poly(vinylidene fluoride) (PVDF) to fabricate high-k composite films. The results exhibited that the permittivity of BT/PVDF with 50 vol% filler loading was only about 40, while the dielectric loss reached 0.12 [19]. Therefore, 1D nanofibres have been applied in the polymer matrix to improve the dielectric properties, because the high aspect ratio makes it easier to reach the percolation threshold, in which the permittivity of composite will show a significant improvement [20]. Recently, the synthesis of nanofibres with uniform diameter and higher aspect ratio has attracted more attention. Also, the electrospinning technique is considered to be one available approach to prepare the nanofibres [21]. This work reports the excellent dielectric properties of polyimide (PI) embedded with CaCu3 Ti4 O12 (CCTO) nanofibres. The dielectric behaviours were investigated over a frequency of 100 Hz to 1 MHz. It is shown that embedding CCTO nanofibres with a high aspect ratio (67) is an effective means to enhance the dielectric permittivity and reduce the percolation threshold. The dielectric permittivity of PI/CCTO nanofibre composites is 85 with 1.5 vol.% loading of filler, also the dielectric loss is only 0.015 at 100 Hz. Monte–Carlo simulation was used to investigate the percolation threshold of CCTO nanofibres reinforced PI matrix by using excluded volume theory and soft, hard-core models. The results are in good agreement with the percolation theory and the hard-core model can well explain the percolation phenomena in PI/CCTO nanofibre composites. The dielectric properties of the composites will meet the practical requirements for the application in high dielectric constant capacitors and high energy density materials [22]. In addition, the large difference of the permittivity between the BT and polymer matrix results in the uneven distribution of electric field and decreases the breakdown strength [23, 24]. Thus the core–shell structured nanofibres are also needed, for the shell layer could modulate the interfaces between the fillers and matrix, ameliorate the dispersion state of fillers and improve the distribution of electric field in the matrix, all of which will be helpful for improving the dielectric properties and breakdown strength [25-28]. The recent results on the dielectric properties have been shown in Table 1. Table 1. Recent researches on dielectric materials Materials Dielectric constant (100 Hz) Dielectric loss (100 Hz) Breakdown strength (kV/mm) Energy density (J/cm3) Efficiency (%) BT-8/MXene-2/PVDF [29] 77 0.15 220 ∼7.0 ∼40 BT/PVDF–CTFE [30] 53 0.04 200 4.9 ∼27 PVDF–CTFE/BT@HBP@PDA–Ag [31] 22 0.1 200 7 ∼60 BT@SiO2 /PVDF [32] 11.6 0.1 340 6.28 51.6 BT@Al2 O3 /PVDF [33] 10.56 0.05 360 6.19 — BZT–BCT@Fe3 O4 /PI [34] 5 0.18 265 2.27 86 sandwiched BT@Fe3 O4 /PVDF [35] 12 0.11 210 2.24 61 sandwiched BT@Al2 O3 /PVDF(this work) 20 0.05 162 1.86 42 In general, the energy density of dielectric materials can be calculated from the formula. Thus, the energy density is primarily dependent on the breakdown strength and electrical displacement of the material [24, 36]. However, due to the different permittivity between the ceramic and matrix, the doping of ceramic fillers into the polymer matrix will increase the local electric field in the matrix, resulting in the decreased breakdown strength [37]. It has been known that an insulating layer on the surface of ceramic fillers could effectively restrict the movement of free electrons and suppress the leakage current [38]. To further restraint the breakdown pathways in the matrix, multilayer composite films were fabricated in this work to realise better dielectric properties as well as higher energy density [39, 40]. In this work, PVDF-hexafluoropropylene (PVDF-HFP) was used as the matrix due to its exceptional dielectric properties. BT, as a high-k ceramic, was applied as the fillers to improve the permittivity of PVDF-HFP. The permittivity of alumina (Al2 O3) is similar to PVDF-HFP, and Al2 O3 also has some advantages such as high thermal conductivity, excellent insulating properties, easy processing and low cost, which make it a potential candidate of shell layer. The BT nanofibres and the core–shell structure BT@Al2 O3 nanofibres were both synthesised through the electrospinning process. The two nanofibres were incorporated into the PVDF-HFP copolymer, respectively. To improve the dielectric properties further, the sandwich structure PVDF–HFP composite films were prepared using a casting process followed by a hot-pressing method. The BT@Al2 O3 /PVDF–HFP as the interlayer was mainly used to enhance the breakdown strength of the composite films. Also, the PVDF–HFP with higher levels of BT nanofibres serves as the outer two layers which played a role in enhancing the dielectric properties of the composites. 2 Experimental 2.1 Materials Barium acetate (99%, Xilong chemical Co. Ltd), tetrabutyl titanate (TBOT) (98.5%, Beijing Xingjin Chemical Corp.), acetylacetone (99%, Sinopharm Chemical Reagent Co., Ltd) were used to prepare the BT nanofibres. Ethanol and acetic acid were provided by Beijing Chemical Works. N, N -dimethyl formamide (DMF) and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinylpyrrolidone (PVP, K90) with a molecular weight of 1.3 × 106 and aluminium triacetylacetone (98%) were purchased from Shanghai Aladdin Reagent Co., Ltd. PVDF–HFP was provided by Sigma-Aldrich Co., Ltd. All chemical reagents were of analytical reagent and used in the experiment without any further purification. 2.2 Preparation of the BT nanofibres via electrospinning To make the barium titanate precursor solution, barium acetate and TBOT were firstly dissolved in acetic acid, and then acetylacetone was added into the mixture. The molar ratio of barium acetate to the TBOT was set as 1:1.2, containing a 1:2 volume ratio of acetylacetone and TBOT. After mechanical stirring, the mixture was mixed with the ethanol solution of PVP previously arranged and stirred for 3 h to obtain the stable barium titanate precursor solution. The precursor solution prepared was added to the syringe. The precursor fibres were obtained by the electrospinning process as follows. The spinning voltage was 19 kV and the advancing speed of the syringe was 0.04 mm/min. The distance between the needle and the receiver was fixed at 13.0 cm. The resulting precursor fibres were collected on an aluminium foil and subsequently removed at 800°C for calcination to obtain the BT nanofibres. 2.3 Preparation of the BT@Al2 O3 nanofibres via coaxial electrospinning The molar ratio of barium titanate to alumina was set as 2:1. The precursor solution of alumina was acetone solution of aluminium acetylacetonate at a concentration of 0.274 mol/l. PVP solution (15 wt%) was made by dissolving the PVP powder in reagent grade ethanol under vigorous stirring, and then it was mixed with the acetone solution. The barium titanate and the alumina solution were charged into two syringes, respectively, and they were connected to the needle for coaxial electrospinning. The spinning voltage was 21.5 kV and the advancing speed was 0.009 mm/min. The distance between the needle and the collector is fixed at 13.0 cm. The mat of fibre was peeled off from the aluminium foil and cut into small pieces. The BT@Al2 O3 nanofibres could be successfully synthesised after calcination at 700°C in the air atmosphere. 2.4 Fabrication of the composite films The BT/PVDF–HFP and BT@Al2 O3 /PVDF–HFP single layer films were fabricated by casting technique. The nanofibres were evenly dispersed in DMF by ultrasonication for 2 h. Then the PVDF–HFP pellets were added into the solution above, followed by mechanical agitation for 12 h to get homogeneous solution with the pristine and intact microstructure of the nanofibres. The mixture solution was casted onto a glass plate and dried under 70°C for 12 h to remove the solvent completely. Finally, the single layer composite films with a thickness of 10 μm could be obtained. The sandwich-structured films were prepared through hot pressing. The outer two layers were BT/PVDF–HFP films and the inner layer was BT@Al2 O3 /PVDF–HFP film. The three layer films were put into the vulcanising press in order and firstly preheated at 160°C for 15 min, following by hot-pressing at 200°C under 10 MPa for 5 min to prepare sandwich-structured composite films. The whole fabrication process of the films is illustrated in Fig. 1. Fig. 1Open in figure viewerPowerPoint Schematic illustration of the preparation process of the sandwich-structured composite films 2.5 Characterisation Crystal phases of both BT and BT@Al2 O3 nanofibres were characterised by an X-ray diffraction (XRD) instrument (Japan Rigaku D/max-RC) over the 2θ ranging from 10° to 100°. The morphology of the sandwich-structured films was characterised by scanning electron microscopy (SEM, HITACHI SU-8010). The dielectric properties of the composites were tested using an impedance analyser (Agilent 4294A) in the frequency ranging from 102 to 106 Hz. The breakdown voltage of the composites was measured via a withstanding voltage tester (CS2674A, Nanjing Changsheng Instrument Co., Ltd) equipped with two spherical electrodes at room temperature. The limit represented breakdown current is set as 5 mA. The breakdown strength was calculated through Weibull distribution. The D–E loops of the films were measured by one ferroelectric tester (TF Analyser 2000 FE-Module ferroelectric tester, Germany). 3 Results and discussion The prepared precursors of BT nanofibres are shown in Fig. 2. The results demonstrated that the continuous and relatively uniform fibres with a diameter of about 500 nm were synthesised successfully under the given spinning condition. The calcination process ensured the formation of BT nanofibres. Also, it was discovered that the calcination temperature was an important parameter which mainly influences the morphologies of the resulted nanofibres. Fig. 2Open in figure viewerPowerPoint SEM images of the precursor of BT nanofibres Fig. 3 shows the microstructure of BT nanofibres fabricated by different calcination temperatures. The scale of most nanofibres can reach to several tens of microns. It was found that the diameter of BT nanofibres gets smaller and their surface becomes rougher with increased calcination temperature. The nanofibres with the relatively smooth surfaces were obtained at 700 and 800°C. When the temperature was up to 900°C, the granules or even some defects began to appear on the fibre surface. Fig. 3Open in figure viewerPowerPoint SEM images of the BT nanofibres calcined at different temperatures (a), (b) 700°C, (c), (d) 800°C, (e), (f) 900°C In order to analysis their crystalline forms, Fig. 4 shows the XRD patterns of BT nanofibres with different calcination temperatures. It is found that the peaks of all the nanofibres nearly appear at 2θ = 22.1° (1 0 0), 31.5° (1 1 0), 38.9° (1 1 1), 45.1° (2 0 0), 50.8° (2 1 0), 56.1° (2 1 1), 65.8° (2 2 0), 70.3° (2 2 1), 74.7° (3 1 0), 79.2° (3 1 1) and 83.4° (2 2 2), which are attributed to the cubic crystal of BT. Under the lower calcination temperature, the products have less impure peaks, implying their smooth surface. In this situation, the BT nanofibres with a higher aspect ratio and smooth surface after calcination at 800°C are well used to prepare the composites. Fig. 4Open in figure viewerPowerPoint XRD patterns of the obtained BT nanofibres Fig. 5 shows the XRD patterns of the BT@Al2 O3 nanofibres with a molar ratio of BT /Al2 O3 = 2/1 after different calcination temperatures. The results reveal that their main peak positions are almost the same as that of pure BT, which exhibit sharp and large peaks. With the increase of the calcination temperature, the characteristic peaks at 2θ = 25.5° corresponding to α -Al2 O3 appear and the peak intensity become larger, suggesting that it is more conducive for the formation of α -Al2 O3 at a higher temperature. Besides, the strong peaks at 2θ = 28.2° corresponding to Ba2 Al2 O4 are found to decrease with the increasing calcination temperature. This also indicates that it is a chemical bond rather than simple physical coating between Al2 O3 and BT. It facilitates the transformation of Ba2 Al2 O4 to α-Al2 O3 at a higher temperature. However, the concentration of BT decreases after the higher calcination temperature, which results in more interfaces between the Al2 O3 and BT and the increase of interface polarisation. It is easy for the mobility of the carriers along the surface of nanofibres, which will bring positive effects on the improvement of breakdown strength. In this work, the BT@Al2 O3 nanofibres with the molar ratio of BT/Al2 O3 = 2/1 calcined at 700°C were well used for preparing the composite films. Fig. 5Open in figure viewerPowerPoint XRD patterns of the BT@Al2 O3 nanofibres with different fractions of BT and Al2 O3 at different calcination temperatures Fig. 6 shows the SEM and transmission electron microscopy (TEM) images of the precursors of BT@Al2 O3 nanofibres prepared by the coaxial electrospinning method. We can see that the nanofibres have a relatively uniform diameter of about 100–300 nm and a clear core–shell structure, as shown in Fig. 6 b. Fig. 6Open in figure viewerPowerPoint Precursor of BT@Al2 O3 nanofibres (a) SEM images, (b) TEM images The BT@Al2 O3 nanofibres as shown in Fig. 7 a were obtained by calcining the precursors at 700°C for 3 h. It can be found that the whole surface of the nanofibres becomes rough, implying the deposition of the alumina layer on the surface of barium titanate. Fig. 7 b shows the TEM image of the BT@Al2 O3 nanofibres after calcination. It is observed that there is a layer of alumina particles surrounding BT nanofibres, indicating that the core–shell structured BT@Al2 O3 nanofibres are well prepared. Fig. 7Open in figure viewerPowerPoint BT@Al2 O3 nanofibres calcined at 700°C (a) SEM images, (b) TEM images The PVDF–HFP-based sandwich-structured composites were prepared via a hot pressing method. The PVDF–HFP films with 4 vol% BT@Al2 O3 nanofibres were selected as the intermediate layer to improve the breakdown performance of the composites for the highest breakdown strength, as shown in Table 2. Also, the PVDF–HFP films with different fractions of BT nanofibres served as the upper and lower layers to improve the permittivities of the composites. The sandwich composites are named based on the volume fractions of BT@Al2 O3 and BT nanofibres in the three layers. For example, 5–4–5 means the outer layers contain 5 vol% BT nanofibres, while the inner layer has 4 vol% BT@Al2 O3 nanofibres. SEM images of the cross-section of the sandwich structured composite films are shown in Fig. 8. The interfaces between the inner and outer layers as marked by the white lines in Fig. 8 could be observed. It was demonstrated that the nanofibres were dispersed uniformly in each layer and there were no interface separations or defects between the three layers. Table 2. Weibull parameters of the BT/PVDF–HFP, BT@Al2 O3 /PVDF-HFP, and the prepared sandwich-structured composite films BT/PVDF–HFP BT@Al2 O3 /PVDF–HFP Sandwich-structured composite Content (vol%) E 0 (kV/mm) β Thickness (μm) Content (vol%) E 0 (kV/mm) β Thickness (μm) E 0 (kV/mm) β Thickness (μm) 0 287 6.41 12 0 287 6.41 11 — — — — 2% 249 5.84 11 2% 318 6.96 12 — — — — 5% 190 6.06 11 4% 350 6.24 10 5-4-5 201 8.51 25 10% 171 6.62 10 6% 290 5.76 11 10-4-10 194 8.32 27 15% 134 7.45 12 8% 283 7.13 11 15-4-15 185 8.06 27 20% 122 7.23 12 10% 239 6.68 10 20-4-20 162 8.94 28 Fig. 8Open in figure viewerPowerPoint SEM images of a cross section of (a) 5–4–5, (b) 10–4–10, (c) 15–4–15, (d) 20–4–20 sandwich-structured composite films Weibull statistical distribution is widely used to analyse the breakdown probability of materials at a certain electrical field, which can be described as follows: (1) where P is the cumulative probability of the breakdown strength, E is the tested breakdown strength, β is the shape factor, and E 0 is the characteristic breakdown strength at the cumulative probability of 63.2%. Fifteen samples were tested for each kind of composite. The calculated Weibull parameters E 0 and β of the BT/PVDF–HFP, BT@Al2 O3 /PVDF–HFP and the prepared sandwich-structured composite films are listed in Table 2. For the BT/PVDF–HFP system, the E 0 decreases with the increase of BT content. For the BT@Al2 O3 /PVDF–HFP, the value of E 0 firstly increases then decreases. With the same concentration of fibres added, it is found that E 0 of the BT@Al2 O3 /PVDF–HFP composites is larger than that of BT/PVDF-HFP. In addition, although the E 0 of the sandwich-structured films is lower than that of the single layer with 4% BT@Al2 O3, it is larger than that of the single layer with BT. It is assumed that the inner layer as the barrier layer could restrain the migration of carriers in the electrical field. Moreover, the hot-pressing process could reduce the interior defects and the related trapping centres of carriers, resulting in larger breakdown strength of the sandwich-structured composite films. Fig. 9 shows the frequency dependence of permittivity and dielectric loss for the BT/PVDF–HFP, BT@Al2 O3 /PVDF–HFP and the sandwich structured composite films. It can be seen from Fig. 9 a that the dielectric constant of the composite films increases as a function of BT contents in the outer layers. With the addition of 20 vol% BT, the permittivity at 102 Hz is up to 20, which is much larger than that of the single layer film with 4 vol% BT@Al2 O3. Therefore, the large permittivities of the outer layers with higher contents of BT nanofibres make a remarkable contribution to the improved permittivity of the whole composite films. Also, the inner layer with core–shell structured BT@Al2 O3 fibres has a relatively low dielectric loss, which plays an important role in hindering the migration of carriers between the adjacent layers and restraining the increment of dielectric loss to a certain extent. Fig. 9Open in figure viewerPowerPoint Frequency dependence of permittivity and dielectric loss for the BaTiO3 /PVDF–HFP, BaTiO3 @Al2 O3 /PVDF–HFP and sandwich structured composite films (a) Permittivity, (b) Dielectric loss for the BT/PVDF and BT@Al2 O3 /PVDF–HFP, (c) Permittivity, (d) Dielectric loss for the sandwich structured composite films with different BT nanofibres content in outer layers It is well known that the energy storage characteristic of the dielectric materials is usually dependent on their permittivities and breakdown strengths. According to the electromagnetic theory, the electric displacement (D) of dielectrics will produce variation (dD) under certain electric field (E), which will give rise to the variation of storage energy density (EdD). Also, the energy density (Ud) can be expressed as follows: (2) As ferroelectric materials, the relationship between their electric field intensity and electric displacement is considered to be non-linear. Owing to the remnant polarisation and hysteresis effect existing in the materials, it is found that the charging curves will not overlap with the discharging ones. This is because some energy could not be released. Fig. 10 displays the D –E hysteresis loops of the sandwich structured composite films under various electrical fields. As shown in Figs. 10 a –c, it is demonstrated that the electric displacement of the composite films increases as a function of BT contents in the outer two layers under the same applied electrical field. In this case, the carriers will be easily captured by the interface, which makes them difficult to migrate. Owing to the lower breakdown strength of the 20–4–20 composite films, the smaller electric displacement is observed. Fig. 10Open in figure viewerPowerPoint Electric displacement–electric field (D−E) loops of the sandwich structured composite films under different applied electrical fields (a) 5–4–5, (b) 10–4–10, (c) 15–4–15, (d) 20–4–20 4 Conclusion In summary, the BT and the core–shell structured BT@Al2 O3 nanofibres were successfully fabricated through the electrospinning method and then were introduced into the PVDF–HFP matrix to prepare the sandwich-structured composite films, aiming at improving the dielectric properties. The BT/PVDF–HFP and BT@Al2 O3 /PVDF–HFP served as the outer and inner layer of the composite films, respectively. The results revealed that the outer layers with a higher concentration of BT showed larger permittivity and decreased breakdown strength. While the inner layer with 4 vol% BT@Al2 O3 fibres had relatively higher breakdown strength (∼350 kV/mm). It was found that the dielectric properties of the composite films were mainly affected by those of the outer layers. Compared to the BT filled composites, the addition of BT@Al2 O3 into the inner layer effectively restrained the decrease of the breakdown strength. Besides, the energy storage density of the films is strongly dependent on their dielectric constant and breakdown strength. For the 15–4–15 films, the dielectric permittivity at 100 Hz was up to 18.3 and the breakdown strength reached 185 kV/mm. 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