Improvements of dielectric properties and energy storage performances in BaTiO 3 /PVDF nanocomposites by employing a thermal treatment process

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1 JOURNAL OF ADVANCED DIELECTRICS Vol. 8, No. 6 (2018) (10 pages) The Author(s) DOI: /S X Improvements of dielectric properties and energy storage performances in BaTiO 3 /PVDF nanocomposites by employing a thermal treatment process Yue-Mao Dang*, Ming-Sheng Zheng y,z and Jun-Wei Zha y *Tsinghua University High School Beijing , P. R. China y School of Biological and Chemical Engineering University of Science and Technology Beijing Beijing , P. R. China z zhengms@mail.tsinghua.edu.cn Received 12 October 2018; Revised 23 November 2018; Accepted 27 November 2018; Published 31 December 2018 The influence of thermal treatment on the dielectric properties and energy storage performances of a classical dielectric nanocomposite system (barium titanate/polyvinylidene fluoride PVDF) was discussed systematically. The results demonstrated that the permittivity of thermal treated nanocomposites increased and dielectric loss decreased compared with the untreated system. In addition, the energy density was also greatly improved due to the inclined residual polarization. For example, the energy density of the treated nanocomposite with 50 vol.% nanofillers was 3.14 times higher than the untreated nanocomposite at 50 MV/m. Moreover, the charge discharge efficiency was also promoted from 6.36% to 56.89%. According to the viewpoint of microstructure, the improvement of the dielectric and energy storage properties would be ascribed to the suppression on void defects in the interphase of dielectric nanocomposite by employing the thermal treatment process. Finally, thermal treatment turns out to be a simple and an effective method to improve the dielectric performances and energy storage properties in the dielectric nanocomposites. Keywords: Thermal treatment; BaTiO 3 /PVDF nanocomposites; dielectric properties; energy storage. 1. Introduction Film capacitors are the indispensable components in many electronic devices, vehicles and high-voltage electric systems. 1 3 However, the low permittivity of dielectric film hinders the further development of film capacitors. In recent years, adding high-permittivity (high-kþ nanofillers such as TiO 2, BaTiO 3, CaCu 3 Ti 4 O 12 etc. into the polymer matrix turns out to be an effective way to improve the dielectric properties. 4 8 In addition, it should be noted that we need not only high permittivity, but also require low dielectric loss and high breakdown strength for composites in the practical application. 9 For example, polypropylene has been the most popular polymer-based dielectric material for many years because of its extremely low dielectric loss ð 0:0004Þ and high breakdown strength about 600 MV/m. The low dielectric loss leads to the little energy loss during the charging and discharging procedure of power, which is very important for capacitors. But the energy density is limited due to the low permittivity ð 2:2Þ of polypropylene. Thus lots of efforts have been devoted to fabricate nanocomposites with high permittivity. 10 Polyvinylidene fluoride (PVDF) is often applied as the polymer matrix due to its relative high permittivity of about 10, and BaTiO 3 (BT) is the most important high-k nanofillers, thus the typical nanocomposite system (BT/PVDF) has been researched for many years However, because of the poor compatibility between BT and PVDF, void defects often tend to generate in the interphase, which will play a negative impact on the dielectric properties and decrease the breakdown strength. Therefore, how to decrease the void defects in the interphase of dielectric nanocomposite is a crucial issue. It has been proved that modification on the surface of BT nanoparticles can effectively improve the compatibility and the dispersion state in the polymer matrix, which will significantly improve the dielectric properties of the composites. 14 For example, Dang s group used H 2 O 2 to modify the surfaces of BT nanoparticles 15 and the hydroxylated BT (HO- BT) exhibited better compatibility with PVDF as compared to the unmodified BT because of the easier formation of hydrogen bonds with PVDF occurred through the strongly electronegative hydroxyls on the surface of BT nanoparticle. In addition, the modification also improved the dispersion state of nanofillers. Huang and Jiang believe that the polymer-based dielectric materials with excellent dielectric and energy storage properties can be prepared by using core shell This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 4.0 (CC-BY) License. Further distribution of this work is permitted, provided the original work is properly cited

2 nanoparticles, because the interphase between nanofillers and matrix is able to be manual optimized by fabricating different shell layers on the surface of nanoparticles, which will affect the microstructure and electrical characteristics of the composites. 14 For instance, they coated a layer of fluorinated acrylate polymers onto the surface of BT nanoparticles, and then added the core shell nanoparticles into the poly (vinylidene fluoride-hexafluoropropylene (P(VDF-HFP)). 16 The results showed that the core shell structure of BT nanoparticles effectively decreased the dielectric loss, promoted the breakdown strength, and improved the energy storage density of composites. In this work, a simple method (thermal treatment) to improve the compatibility as well as properties of BT/PVDF nanocomposites was introduced. It was demonstrated that thermal treatment effectively eliminated the void defects between BT nanoparticles and the PVDF matrix so that the dielectric and energy storage characteristics of the BT/PVDF nanocomposites were significantly improved. In fact, thermal treatment has been applied in many works to improve the performances of dielectric nanocomposites on purpose. 17,18 But there is no systematic investigation about the effect of thermal treatment on the dielectric and energy storage properties of dielectric nanocomposites, and the importance of thermal treatment has not been discussed. For example, Wang and her coworkers fabricated sandwich-structured barium titanate/poly(vinylidene fluoride-cohexafluoropropylene) (BT/P(VDF-HFP)) composite films through layer-by-layer tape casting procedure, then the flexible freestanding films were thermal treated at 200 C for 10 min followed by quenching in ice water immediately. 17 The results revealed that both the breakdown strength and permittivity of BT/P(VDF-HFP) were improved. Nan and Shen prepared 1D-BT@TiO 2 multi-level structural nanofibers successfully, and introduced them into PVDF and P(VDF TrFE CFE), respectively. The thermal treated topological-structure composite films exhibited high breakdown strength and electric polarization, which improved the energy density. 18 But there is no systematic investigation about the effect of thermal treatment on the dielectric and energy storage properties of dielectric nanocomposites, and the importance of thermal treatment has not been discussed. In this work, the influence of thermal treatment on BT/PVDF nanocomposites was reported in detail Fabrication of BT/PVDF composite films BT nanoparticles were dispersed into DMF by ultrasonication (400 W) for 1 h to realize the uniform dispersion. And then, certain amounts of PVDF powders were put into the solution with mechanical stirring and ultrasonication (600 W) for 6 h till PVDF was dissolved thoroughly. After eliminating the air in a vacuum oven for 0.5 h in the mixed solution at room temperature, the solution was cast onto a piece of glass using a scraper, and finally, it was fried in the oven at 70 C for 12 h to remove the solvent absolutely. For simplification, it is named the composites films with different volume fractions of BT (10%, 20%, 30%, 40% and 50%) as 10 BT/PVDF50 BT/PVDF Fabrication of thermal treated BT/PVDF composite films BT/PVDF composite films were peeled from the glass substrates. The temperature of thermal treatment was confirmed after a series of experiments. It was found that the lower temperature showed little promotion of composite films, and the higher temperature resulted in the destruction of films due to the higher mobility of PVDF. The results also proved the longer time of thermal treatment showed no further improvement. Thus, the thermal treatment was carried out in an oven at 210 C for 5 min and then cooled down at room temperature, as shown in Fig. 1, a clean beaker was placed into the oven for 5 min firstly, and then the film was put on the top of the beaker. The heated beaker hindered the shrunk of film at high temperature. For simplification, we named the composite films after thermal treatment with different volume fractions of BT (10%, 20%, 30%, 40% and 50%) as T-10 BT/PVDFT-50 BT/PVDF, respectively. 2. Experimental 2.1. Materials BT nanoparticles (< 100 nm) was purchased from Aladdin Industrial Corporation, and PVDF powder (FR904) was supplied by Shanghai 3F New Materials Technology Co., LTD. N,N-dimethylformamide (DMF) was provided from Beijing Chemical Works. All the reagents were used as received without further purification. Fig. 1. Preparation process and thermal treatment pictures of BT/ PVDF composite film

3 2.4. Characterization The cross-section structure of the composites films before and after thermal treatment was observed by Scanning Electron Microscopy (SEM, Hitachi 8010). Differential Scanning Calorimetry (DSC, Shimadzu, DSC-60) was carried out with a rate of 2 C/min. Copper electrodes with diameter of 3 mm were sputtered on both sides of the films by the Resistance Evaporation Coating Equipment (ZHD- 300M2, Beijing Technol Science co., LTD) for the tests of dielectric and energy storage properties measurements. Permittivity, dielectric loss and AC conductivity were measured with a precision impedance analyzer (4294A, Agilent Technologies, Inc.) at room temperature with a frequency range from 10 2 to 10 7 Hz. The breakdown strength of composite films was obtained through Dielectric Strength Tester (CS2674A, 0 20 KV, Nanjing Changsheng instrument co., LTD) using two spherical copper electrodes. The electric Displacement Electric Field (D E) loops at 10 Hz of the composites were tested by Precision Materials Analyzer (Radiant Technologies, Inc). 3. Results and Discussions 3.1. Morphology and characterization of thermal treated BT/PVDF Scanning Electron Microscopy (SEM) was applied to reveal the morphological variations of BT/PVDF system before and after thermal treatment, as shown in Figs. 2 and 3. The crosssection images in Fig. 2 exhibited that the incorporation of BT into the matrix destroyed the dense structure of PVDF because of the poor compatibility between the inorganic filler and organic matrix so that the void defects (also named as interspace) are formed in the nanocomposites. This is an important problem for the composite films produced by solution-casting method. 19,20 The interspaces between nanofillers and medium play a negative role on the properties of composites. Improving the compatibility with inorganic filler and polymer matrix turned out to be an important target for many recent works, because the improved compatibility could reduce the interspaces, which will affect the dielectric properties of composite greatly. 21,22 As shown in Fig. 3, the images proved that the morphology of BT/PVDF has been changed remarkably after thermal treatment. It was assumed that the viscoelastic state of PVDF at 210 C adhered onto the surface of BT nanoparticles strongly, and this could improve the compatibility even after cooling down to room temperature because BT bonded with PVDF tightly. It was also observed that the void defects between them were diminished significantly. The interphases between BT and PVDF increased with thermal treatment, which would enhance the interfacial polarization of composites and improve the permittivity. 23 In addition, the diminished void defects in the matrix were beneficial for the promotion of breakdown strength. Differential Scanning Calorimeter (DSC) with a rate of 2 C/min was used to investigate the microstructural variations of composites, as shown in Fig. 4 and Table 1, and the results revealed that the melting points of BT/PVDF (before thermal treatment) barely changed comparing with PVDF. 24 The quick cooling down procedure resulted in imperfect PVDF crystals, which slightly decreased the melting points of T-PVDF and T-BT/PVDF (after thermal treatment). The weak melting process of T-10 BT/PVDF and T-30 BT/PVDF at around 155 C near main melting was possibly caused by the much higher thermal conductivity of BT than PVDF, which Fig. 2. Cross-sectional SEM images of pure PVDF, 10 BT/PVDF, (c) 20 BT/PVDF, (d) 30 BT/PVDF, (e) 40 BT/PVDF and (f) 50 BT/ PVDF composite films before thermal treatment

4 Fig. 3. SEM images of pure PVDF, 10 BT/PVDF, (c) 20 BT/PVDF, (d) 30 BT/PVDF, (e) 40 BT/PVDF and (f) 50 BT/PVDF composite films after thermal treatment. Fig. 4. DSC curves of untreated and thermal treated pure PVDF, 10 BT/PVDF and 30 BT/PVDF at a rate of 2 C/min. led to quicker heat transport around the BT nanoparticles. The crystals of PVDF near BT melted earlier than others, and this resulted in a lower Tm of T-BT/PVDF. In addition, no weak melting process was found in 10 BT/PVDF and 30 BT/ Table 1. DSC data of untreated and thermal treated BT/PVDF system (2 C/ min). Name PVDF T-PVDF 10 BT/ PVDF T-10 BT/ PVDF 30 BT/ PVDF T-30 BT/ PVDF ΔH (J/g) Xc (%) Tm ( CÞ PVDF due to the gaps between the fillers and matrix, and the comparison of BT/PVDF and T-BT/PVDF also revealed the change of microstructure after thermal treatment. It was noteworthy that the crystallinities of thermal treated films were lower than that of untreated films, which also proved the enhanced interaction between the nanofillers and polymer matrix. As shown in Fig. 5, X-ray diffraction (XRD) was applied to further detect the microstructural variations of composites. It was demonstrated that T-PVDF showed significant change of crystal forms comparing with PVDF. The characteristic peaks of 26.7, 20.0 and 18.4 proved that thermal treatment effectively enhanced the γ-phase of PVDF. 25,26 Both 10 BT/ PVDF and T-10 BT/PVDF showed strong characteristic peaks of BT. In addition, T-10 BT/PVDF exhibited more obvious peaks of PVDF crystals. Thus, it was assumed that thermal treatment could induce the formation of PVDF crystals, which also confirmed the enhanced interaction between nanofillers and matrix Dielectric and energy storage performances of thermal treated BT/PVDF As shown in Fig. 6, the dielectric properties of thermal treated BT/PVDF composites were significantly improved compared with the untreated BT/PVDF, especially at the low frequency zone. Figure 6 demonstrated that the permittivity of T-PVDF was higher than PVDF at low frequency zone, while the dielectric loss at high-frequency zone was larger. The changes of dielectric properties after thermal treatment were attributed to the enhanced γ-phase, 27 as shown in Fig. 5. Moreover, the dielectric loss stability of T-BT/PVDF at low-frequency was also been improved, as shown in Figs. 6(c) 6(g)

5 To exhibit the impact of thermal treatment on the dielectric properties, the permittivity, dielectric loss and AC conductivity at 100 Hz with different BT loading were investigated, as shown in Fig. 7. It was demonstrated that the permittivity after thermal treatment had a better increase as a function of BT content. For instance, the permittivity of T-PVDF was improved from 9.8 to 10.6 at 100 Hz, while T-10 BT/PVDF increased from 13.8 to And the permittivity of T-50 BT/PVDF was as high as 81.1, which was 12.1 higher than that of 50 BT/PVDF. The increment of permittivity for T-BT/PVDF was attributed to the enhanced interfacial polarization, 28 which came from the enlarged interphases between nanofillers and matrix, as shown in the Fig. 5. XRD curves of PVDF and 10 BT/PVDF before and after thermal treatment. comparison of Figs. 2 and 3. In addition, the dielectric loss was also decreased remarkably by thermal treatment. As shown in Fig. 7, the dielectric loss of T-10 BT/PVDF decreased from to comparing with 10 BT/PVDF at 100 Hz, and T-50 BT/PVDF declined from to It was worth noting that the dielectric loss of pure PVDF increased with thermal treatment. So, it can be assumed that the reductive dielectric loss of composites after thermal treatment comes from the state change of BT instead of PVDF. Moreover, the AC conductivity of T-BT/PVDF composites was higher than that of untreated BT/PVDF composites. It is well known that high AC conductivity usually results in high leakage current, which plays a negative impact Fig. 6. Permittivity comparison of untreated and thermal treated PVDF, 10 BT/PVDF, (c) 30 BT/PVDF, (d) 50 BT/PVDF composite films ( Hz); While (a 1 Þ,(b 1 Þ,(c 1 Þ and (d 1 Þ are the corresponding comparison of dielectric loss. (a1)

6 (c) (b1) (c1) (d) (d1) Fig. 6. (Continued)

7 on the dielectric loss. Thus the improved dielectric loss of T-BT/PVDF composites was not decided by AC conductivity. The restricted movements of both the PVDF molecular chains and BT nanoparticles due to the enhanced compatibility probably resulted in the decreased dielectric loss and improved the energy storage performances. The breakdown strengths of BT/PVDF (Fig. 8) and T-BT/PVDF (Fig. 8) were calculated using Weibull distribution and summarized as Fig. 7(d). 29 It was shown that the breakdown strengths of both the treated and untreated BT/ PVDF composites decreased as a function of filler loading. However, the voltage resistance exhibited significant improvement after thermal treatment. For example, T-10 BT/ PVDF was promoted from 259 MV/m to 298 MV/m and T-50 BT/PVDF increased from 66 MV/m to 85 MV/m. Electrical displacement Electric field (D E) loops of T-BT/PVDF and BT/PVDF composites were investigated to (c) Fig. 7. Comparison of permittivity, dielectric loss, (c) AC conductivity at 100 Hz and (d) breakdown strength for untreated and thermal treated BT/PVDF composite films with increased filler content. (d) reveal the changes of energy storage performances after thermal treatment, as shown in Fig. 9. At the same electric field intensity (50 MV/m), the larger residual polarization of T-PVDF resulted in worse energy storage performances. 30 In contrast, the residual polarization of BT/PVDF system decreased remarkably after thermal treatment. For instance, the value of T-50 BT/PVDF was only 1.21 uc/cm 2, while 50 BT/ PVDF was as high as 8.99 uc/cm 2. The energy storage density of PVDF was enhanced slightly after thermal treatment due to the improved permittivity (Fig. 6), but the increased residual polarization led to poorer charge discharge efficiency. On the contrary, T-BT/PVDF system exhibited significantly decreased residual polarization, which resulted in higher energy storage density and charge discharge efficiency. In order to describe the tendency of characteristics more intuitively, the maximum energy density and corresponding

8 Fig. 8. Breakdown strength of untreated and thermal treated BT/PVDF composite films at different filler loading (calculated by Weibull distribution). (c) Fig. 9. D E loops of untreated and thermal treated pure PVDF, 10 BT/PVDF, (c) 30 BT/PVDF and (d) 50 BT/PVDF at 50 MV/m. (d)

9 Fig. 10. Curves of energy storage density and charge discharge efficiency for untreated and thermal treated composite films with the increased BT content at breakdown strength and 50 MV/m. efficiency with different BT content were listed in Fig. 10. It was revealed that the energy storage density of both the treated and untreated composites reduced with the increasing BT loading. However, it was worth noting that the maximum energy density was improved after thermal treatment. For example, the value of T-30 BT/PVDF reached 2.69 J/cm 3, which was 3.45 times higher than that of 30 BT/PVDF. In addition, the charge discharge efficiency was also promoted from 3.90% to 46.2%. To explain the positive influence of thermal treatment on BT/PVDF system, the energy storage performances at 50 MV/m was shown in Fig. 10, and the results demonstrated that the energy storage density of T-BT/ PVDF increased as a function of filler concentration, and the energy density at 50 vol% BT was up to 0.69 J/cm 3, which was 3.14 times higher than 50 BT/PVDF and 4.93 times higher than pure PVDF. What s more, the efficiency was also improved from 6.36% to 56.9%. Thus, it can be concluded that the thermal treatment is an effective strategy to realize better energy storage properties. 4. Conclusions In summary, thermal treatment can promote the dielectric and energy storage properties of BT/PVDF composite films. To investigate these phenomena, SEM and DSC were applied to detect the microstructural variations of composites before and after thermal treatment. It was revealed that the compatibility of BT and PVDF was improved significantly, and this resulted in higher interfacial polarization, which enhanced the permittivity of T-BT/PVDF. The reduced void defects played a positive impact on the breakdown strength. Moreover, the movements of both the PVDF molecular chains and BT nanoparticles were restricted after thermal treatment, which possibly improved the dielectric loss and energy storage performances. Thus, thermal treatment turns out be an effective strategy to further promote the properties of dielectric composite films. Acknowledgments This work was financially supported by National Nature Science Foundation of China (Grant No ), State Grid Corporation Technology Project ( UK) and the Fundamental Research Funds for the Central Universities (No. FRF-TP C1). References 1 M. S. Zheng, J. W. Zha, Y. Yang, P. Han, C. H. Hu and Z. M. Dang, Enhanced breakdown strength of poly (vinylidene fluoride) utilizing rubber nanoparticles for energy storage application, Appl. Phys. Lett. 109, (2016). 2 B. Fan, F. Liu, G. Yang, H. Li, G. Zhang, S. Jiang and Q. Wang, Dielectric materials for high-temperature capacitors, IET Nanodielectr. 1, 32 (2018). 3 S. L. Zhong, Z. M. Dang, W. Y. Zhou and H. W. Cai, Past and future on nanodielectrics, IET Nanodielectr. 1, 41 (2018). 4 Z. M. Dang, J. K. Yuan, J. W. Zha, T. Zhou, S. T. Li and G. H. Hu, Fundamentals, processes and applications of high-permittivity polymer matrix composites, Prog. Mater. Sci. 57, 660 (2012). 5 G. Wang, X. Huang and P. Jiang, Bio-inspired fluoro-polydopamine meets barium titanate nanowires: A perfect combination to enhance energy storage capability of polymer nanocomposites, ACS Appl. Mater. Inter. 9, 7547 (2017). 6 P. Wu, L. Zhang and X. Shan, Microstructure and dielectric response of BaSrTiO 3 /P(VDF-CTFE) nanocomposites, Mater. Lett. 159, 72 (2015). 7 Y. Hao, X. Wang, K. Bi, J. Zhang, Y. Huang, L. Wu, P. Zhao, K. Xu, M. Lei and L. Li, Significantly enhanced energy storage

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