Electrical energy density and dielectric properties of poly(vinylidene fluoride-chlorotrifluoroethylene)/BaSrTiO3 nanocomposites

doi:10.1016/j.ceramint.2011.08.033

Ceramics International

Volume 38, Issue 2, March 2012, Pages 1071-1075

https://doi.org/10.1016/j.ceramint.2011.08.033 Get rights and content

Abstract

A series of poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE))/barium strontium titanate (BST) nanocomposites were fabricated by solution casting method. The addition of BST nanoparticles could enhance both the dielectric constant and the displacement of the resultant composite significantly. The surface activation of BST nonaparticles with KH550 was confirmed as an effective way to improve the breakdown strength of the composite. The high electric displacement (D > 15 μC/cm²), breakdown field (>200 MV/m) and low dielectric loss in P(VDF-CTFE)/BST nanocomposites suggest that the high electrical energy density may be desirable. That indicates the potential application of this class of copolymer/ceramics nanocomposites for high energy storage components.

Introduction

Poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) have attracted considerable research interests in past decades due to their great potential in advanced applications such as actuators, memories, transducers and sensors [1], [2], [3]. It has been reported that the ferroelectric PVDF or P(VDF-TrFE) modified by incorporating certain content of chlorotrifluoride ethylene (CTFE) or chlorodifluoride ethylene (CDFE or CFE) as defects exhibits large polarization and electrostrictive strain response [4], [5]. More recently, it has been found that the addition of comonomer defects may also turn their crystal phase from high polar β-phase into α- or γ-phase with lower polarity [6]. As a result, the polarization saturation electric field of these modified copolymers was significantly enhanced while the remnant polarization was dramatically reduced. Therefore, the ferroelectric polymers have been turned into ferroelectric elastomers characterized with a much slimmer displacement–electric field (D–E) hysteresis loops. That means the electric energy stored in the dielectrics could be released freely as the dielectric field removed and they are highly expected to be used in high pulse capacitors for energy storage purpose. For example, the saturation electric field of P(VDF-TrFE-CFE) was reported to be extended to over 500 MV/m with a discharged energy density of 13 J/cm³ [7]. Moreover, the stretched P(VDF-CTFE) films in low polar α crystal phase were reported to be applied under electric field over 650 MV/m with an energy density of 25 J/cm³ [8]. That means the P(VDF-CTFE) is more desirable for energy storage purpose compared to P(VDF-TrFE-CTFE) terpolymer.

Ceramic particles, such as barium titanate (BaTiO₃), lead zirconate titanate (PZT) and copper calcium titanate (CCTO), have been widely applied as fillers to prepare polymer/ceramic composite with enhanced dielectric constant as well as high energy density as indicated in the formula of U_e = 1/2ɛ₀ɛ_rE² [9], [10], [11]. However, the addition of large quantity of ceramics into the polymer matrix usually resulted into the dramatic decrease of the breakdown electric filed (E_b) of the composites although it was rarely mentioned. For example, J. Robertson's group reported that the acrylic/BaTiO₃ composites exhibit a relatively high dielectric constants (ɛ_r > 40) but a rather low breakdown electric field (E_b < 8 MV/m) [12]. Therefore, the energy density of the resultant composite was not significantly improved as expected. In addition, the energy density of composites calculated from above formula may be overestimated since the D–E loops of many composites are not in linear against applied electric field, especially for the composites containing ferroelectric matrix or fillers characterized with a rather fat hysteresis loops as indicated in the literature [7]. Therefore, the discharged energy density of poly(vinylidene-hextrafluoropropylene)/TiO₂ composite should be much lower than the one (7 J/cm³) calculated with the formula of U_e = 1/2ɛ₀ɛ_rE² in the literature [13]. It has been widely accepted that the low energy density of composites is mainly attributed to their low breakdown strength, which is closely related to the weak interface between the organic polymer and inorganic compounds induced by their poor compatibilities.

In the present study, the dielectric and energy storage properties of P(VDF-CTFE)/BST composites were carefully studied. The high-dielectric-constant barium strontium titanate (BST) nanoparticles with high surface activity were synthesized via a wet-chemical route followed by surface modification with a silane coupling agent (KH550) to improve the compatibility between ceramics and polymer [14], [15]. P(VDF-CTFE) (94/6 in molar ratio) and surface modified BST nanoparticles were utilized to fabricate the composite films by a solution casting method. Thermal treatment (quenching the composite films from 200 °C into ice-water bath) was employed to improve E_b of the composites [16].

Section snippets

Experimental

BST ((Ba_0.7Sr_0.3)TiO₃) nanoparticles were synthesized by a wet-chemical method as follows. An aqueous solution of citric acid (0.1 mol) and titanium (IV) isopropoxide (0.1 mol) was prepared under vigorous stirring. Meanwhile, another aqueous solution of citric acid (0.1 mol) in the mixture of barium acetate (0.07 mol) and strontium acetate (0.03 mol) was prepared and kept in another container. Immediately, these two solutions were mixed and followed by continuous stirring for 3 h. Ethyl glycol was

Results and discussion

The XRD results of BST nanoparticles are presented in Fig. 1, and the main peaks at about 31.8°, 39.2°, 45.5° and 56.6° correspond to the diffraction of plane (1 0 0), (1 1 1), (2 0 1) and (2 1 1), respectively. These peaks may confirm the tetragonal structure of B_0.7Sr_0.3TiO₃. According to results reported in the literature, BST particles in this crystal structure possess a higher dielectric constant than that in the other crystal forms [14].

The morphology of BST nanoparticles was characterized with

Conclusions

BST ceramic nanoparticles were synthesized by wet-chemical route and surface modified with silane coupling agent. A more favorable interface between the copolymer and surface activated nanoparticles was obtained than that filled with neat BST nanoparticles. Therefore, a large polarization and high breakdown electric field in the P(VDF-CTFE)/BST composites had been observed. High discharged energy density of 6.5 J/cm³ at a breakdown field of 250 MV/m was obtained in the composite film with 10 wt%

Acknowledgements

This work was supported by National Basic Research Program of China (973 Program) No. 2009CB623306 and the National Nature Science foundation of China-NSAF (Grant Nos. 10976022 and 50903065), SRF for ROCS, SEM, Program for New Century Excellent Talents in University, and the Fundamental Research Funds for the Central Universities.

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View full text

Electrical energy density and dielectric properties of poly(vinylidene fluoride-chlorotrifluoroethylene)/BaSrTiO3 nanocomposites

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Polymer

Thin Solid Films

Ferroelectric polymers

Science

Giant electrostriction and relaxor ferroelectric behavior in electron-irradiated poly(vinylidenefluoride-trifluoroethylene) copolymer

Science

The future of nanodielectrics in the electrical power industry

IEEE Trans. Dielect. Elect. Insul.

Ferroelectric and electromechanical properties of poly(vinylidene-fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymer

Appl. Phys. Lett.

Electromechanical properties of poly(vinylidene-fluoridechlorotrifluoroethylene) copolymer

Appl. Phys. Lett.

Ferroelectric poly(vinylidenefluoride-trifluoroethylene-chlorotrifluoroethylene)s: effect of molecular weight on dielectric property

Macromol. Symp.

Electrical energy density and discharge characteristics of a poly(vinylidene fluoride-chlorotrifluoroethylene) copolymer

IEEE Trans. Dielect. Elect. Insul.

Electrical energy density and dielectric properties of poly(vinylidene fluoride-chlorotrifluoroethylene)/BaSrTiO₃ nanocomposites