Poly(Vinyldene Fluoride-Trifluoroethylene-Chlorofluoroethylene) Terpolymer as High-energy-density Capacitor Materials
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Poly(Vinyldene Fluoride-Trifluoroethylene-Chlorofluoroethylene) Terpolymer as High-energy-density Capacitor Materials Baojin Chu, Xin Zhou, Bret Neese, and Q. M. Zhang* Electrical Engineering Department and Materials Research Institute The Pennsylvania State University, University Park, PA 16802 Francois Bauer Institut Franco-Allemand de Recherches de Saint-Losis, France
ABSTRACT Recently, PVDF-TrFE-CFE terpolymer was developed in Penn State University. The polymer exhibits relaxor ferroelectric behavior. At room temperature, the low-field dielectric constant can be as high as 50-60, more than ten times larger than other dielectric polymer materials, such as Polypropylene, the most widely-used polymer materials for capacitor applications. Due to the large change of electric field induced polarization and high breakdown field, energy density of the terpolymer can reach ~10 J/cm3, much larger than other polymer materials. In this paper, experimental results on energy density and non-linear dielectric behavior of the terpolymer will be reported. INTRODUCTION Poly(Vinylidene fluoride (PVDF) and its copolymers with trifluoroethylene (PVDF-TrFE) are a group of important polymer ferroelectric materials. PVDF-TrFE with 50%-80% mol% VDF exhibits ferroelectric-paraelectric phase transition below their melting temperature Tm. It was found that by certain dose of high energy electron irradiation, the long range ordering of dipoles was broken and PVDF-TrFE copolymer can be converted into relaxor ferroelectric materials. The room temperature dielectric properties were greatly improved [1]. The electromechanical response is also enhanced. It was further found that introduction of the third bulky monomer Chlorofluoroethylene (CFE) into polymer chain had the same effect [2]. Poly(Vinyldene Fluoride-Trifluoroethylene-Chlorofluoroethylene) (PVDF-TrFE-CFE) terpolymers have dielectric constant of around 50-60 at room temperature. It can be expected that energy density of this group of terpolymer is much higher than other polymer dielectrics. In this paper, the high energy density of PVDF-TrFE-CFE terpolymers was reported. The nonlinear dielectric behavior was further studied. EXPERIMENTAL DETAILS PVDF-TrFE-CFE terpolymer film was prepared by solution casting method. The polymer powder was first dissolved in DMF and cast on the glass slides. After drying, the polymer film was put into the oven and annealed at 110oC-120oC for 5-10h to increase the crystallinity of film. Aluminum or gold was used as electrodes to measure the electrical properties of materials. Change of dielectric properties with temperature was measured by an automated system composed of HP4284 LCR meter, furnace and computer. The energy density was measured and
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calculated by polarization-electric field (P-E) loop. By applying a half triangular electric field, the change of polarization was measured by integrating the current flowing through the sample. The discharged energy density can be calculated from the formula: Energy density D= ∫ E
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