Baojin Chu

Large Electrocaloric Effect in Ferroelectric Polymers Near Room Temperature

Applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the electrocaloric effect (ECE), it was determined that a large ECE can be realized in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer at temperatures above the ferroelectric-paraelectric transition (above 70°C), where an isothermal entropy change of more than 55 joules per kilogram per kelvin degree and adiabatic temperature change of more than 12°C were observed. We further showed that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene).

Electrical Energy Density and Discharge Characteristics of a Poly(vinylidene fluoride-chlorotrifluoroethylene)Copolymer

The high electric displacement (D>0.1 C/m2) and breakdown field (600 MV/m) in polyvinylidene fluoride based polymers suggest high electrical energy density in this class of polymers. By defect modifications which reduce or eliminate the remnant polarization in the polymer, a high electrical energy density can indeed be obtained. This paper shows that in properly prepared P(VDF-CTFE) copolymer film capacitors, an electrical energy density ~25 J/cm3 can be obtained with a breakdown field higher than 600 MV/m. The dielectric and discharge behavior of the polymer films were investigated. The results reveal that there are strong frequency dispersions in both the dielectric and discharge behavior. The dielectric constant decreases with frequency and the discharged energy density is also reduced at shorted discharge time (~1 mus) due to increased ESR for fast discharge. The results indicate the potential of this class of polymers for high energy density capacitors and suggest the need for further tuning of the polymer compositions to reduce the frequency dispersion.

Recent development of high energy density polymers for dielectric capacitors

High energy density dielectric materials are desirable for capacitors and other energy storage systems. Two approaches were developed to achieve high electric energy density: explore high dielectric constant (K) materials and improve high operation electric field. Relaxor ferroelectric polyvinylidene fluoride (PVDF) based copolymers P(VDF-HFP), P(VDF-CTFE) and terpolymer P(VDF-TrFE-CFE) have been proven to possess high electric energy density. An energy density of over 25 J/cm3 has been achieved in PVDF-based polymers, which represents the state of art in high energy density polymers. Aromatic polyurea thin films were developed through vapor phase deposition, exhibiting relatively high dielectric constant, low loss, high breakdown field (>800 MV/m) and consequently high energy density (>12 J/cm3). Its thermal stability up to 200°C and high charge-discharge efficiency (>90%) make it attractive for high temperature capacitors. Investigation through SEM, AFM and other experiments indicated unbalanced aromatic polyurea could exhibit apparent high-K (~15) due to the non-uniformity of film thickness and surface morphology. This article reviews the recent development of these high performance polymers.

Relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer for high energy density storage capacitors

This paper investigates the relaxor ferroelectric polymer-poly(vinylidene fluoride/trifluoroethylene/chlorofluoroethylene) terpolymer for energy storage capacitors. It is found that the high dielectric constant (>50 at 1 kHz) and high reversible polarization in the terpolymer lead to a high electric energy density ~ 10 J/cm3 , achieved under an electric field of more than 350 MV/m. The high dielectric constant also causes the polarization saturation at fields much below the breakdown field and whereby the discharged energy density increases nearly linearly with applied field, distinctively different from the low dielectric constant linear dielectric polymers whose energy density rises with square of the applied field. The strong frequency dispersion and nonlinear polarization response (polarization saturation) of the relaxor terpolymer result in a low effective capacitance at the beginning of the discharge and the effective capacitance increases with time during the discharge. Furthermore, due to the frequency dispersion and nonlinear effect, the discharged energy density of the terpolymer to a resistor load RL increases with RL. A large R L will lead to high discharge efficiency in the terpolymer capacitor

Electrocaloric effect of the relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer

We investigate the temperature dependence of the electrocaloric effect in a relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer and show that the isothermal entropy change ΔS is proportional to the square of the electric displacement D (ΔS=−1/2βΔD2) and the coefficient β increases with temperature. This temperature dependent behavior of β is caused by the ferroelectric relaxor nature of the polymer in which the polarization response from the nanopolar regions does not generate much entropy change. Consequently, the electrocaloric effect in the relaxor ferroelectric terpolymer is smaller than that in the normal ferroelectric copolymer.

Large enhancement in polarization response and energy density of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) by interface effect in nanocomposites

The authors report the finding of large enhancement in the electric energy density and electric displacement level in nanocomposites of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor ferroelectric polymer/ZrO2 nanoparticles. Through the interface effect, the presence of the nanoparticles (with only 1.6vol% of ZrO2 nanoparticles in the composite) raises the maximum electric displacement D from 0.085C∕m2 under 400MV∕m in the neat terpolymer to more than 0.11C∕m2 under 300MV∕m in the nanocomposites. Consequently, a high energy density of 10.5J∕cm3 can be achieved under a lower field (300MV∕m) in the nanocomposites compared with the neat terpolymer.

Flexure mode flexoelectric piezoelectric composites

We present a flexure mode composite design to generate steep transverse strain gradient to exploit large flexoelectric coefficient μ1122 of (Ba,Sr)TiO3 (BST) ceramics. Very strong direct piezoelectric effect was observed in composites due to the flexoelectricity. In a single unit composite, sharp low frequency ( 2000 pC/N as the result of enhancement of strain gradient at resonance. Giant nonresonance d33 well beyond piezoelectric single crystal about 4350 pC/N was measured at a temperature around Curie temperature of BST ceramic in a six unit three layer composite.

Effect of metal-polymer interface on the breakdown electric field of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer

The authors investigate the effect of electrode material and charge injection on the breakdownelectric field of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer. The results indicate that the breakdown is mainly initiated at electrode-polymer interfaces, the properties of which are controlled by metal materials and deposition conditions. Interfaces with lower leakage currents display higher breakdown fields. Terpolymer films with Al and Cr electrodes have lower leakage current and higher breakdown field compared to those with Au and Ag electrodes. The difference in charge injection and breakdown field is attributed to both the existence of interfacial blocking layers and the difference in Schottky barrier height.

Enhancement of dielectric energy density in the poly(vinylidene fluoride)-based terpolymer/copolymer blends

This paper investigates dielectric properties of the blends of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer and poly(vinylidene fluoride-chlorotrifluoroethylene) copolymer. The high field (>50 MV/m) polarization response of the blends with a small amount of copolymer (5 and 10 wt % copolymer) is higher than that of the neat terpolymer due to the interface contribution. At the same time the breakdown field was also greatly improved by mixing a small amount of the copolymer with the terpolymer. Consequently, a higher energy density of about 11.5 J/cm3 is obtained in these blends in contrast to about 9 J/cm3 in the terpolymer.

Microstructure and electromechanical responses in semicrystalline ferroelectric relaxor polymer blends

Ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-1,1-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer with VDF/TrFE/CFE composition of 59.2∕34.6∕7.2 (mole %) exhibits a high dielectric constant (∼50) around room temperature and strong electromechanical response. It can form miscible blends with poly(methyl methacrylate) (PMMA). This paper reports the results of a systematic investigation of the influence of PMMA on the microstructure and electromechanical responses of the terpolymer blends with PMMA content up to 10wt%. It was found that the crystallinity of the blends decreases nearly linearly with increased PMMA content. Since the dielectric constant and polarization response of P(VDF-TrFE-CFE) terpolymer are mainly from the crystalline region, these properties accordingly exhibit proportional reduction with increased PMMA concentration. Nevertheless, a small amount (∼5wt%, for instance) of PMMA can raise the elastic modulus of the blend quite markedly while the field-induced strain le...