Bret Neese

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.

Enhanced electrocaloric effect in ferroelectric poly(vinylidene-fluoride/ trifluoroethylene) 55/45 mol % copolymer at ferroelectric-paraelectric transition

The electrocaloric effect (ECE) of the ferroelectric poly(vinylidene-fluoride/trifluoroethylene) 55/45 mol % copolymer was directly measured over a broad temperature range using a specially designed calorimetry method. The data reveal a large ECE occurring at the ferroelectric-paraelectric (FE–PE) phase transition where an adiabatic temperature change ∼12 °C was induced under a field of 120 MV/m, which is much higher than that previously observed at above the FE–PE transition. The directly measured ECE also confirms the earlier results deduced from the indirect method. The experimental data also show that there are secondary effects contributing to the observed ECE in the polymer.

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.

Direct Measurements of the Giant Electrocaloric Effect in Soft and Solid Ferroelectric Materials

The giant electrocaloric (EC) effect is of great importance for application in cooling or heating devices of new generation, which would be friendlier for environment. Recent predictions of the existence of the giant EC effect in polymeric and inorganic ferroelectric relaxors are based solely on the indirect measurements of the electric polarization and not on a direct measurement of the EC effect itself. Here a method and analysis of direct measurements of the giant EC effect in various soft and solid ferroelectric materials in the form of thick and thin films is presented. The field dependence of the EC effect is shown for PbMg1/3Nb2/3O3 (PMN) ceramics and P(VDF-TrFE) (68/32 mol%) copolymers.

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...

Piezoelectric responses in poly(vinylidene fluoride/hexafluoropropylene) copolymers

The authors show that a high transverse piezoelectric response with both high piezoelectric d31 (d31=43.1pm∕V) and electromechanical coupling k31 coefficients (k31=0.187), much higher than those in the piezoelectric poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) copolymers, can be obtained in poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] 10wt% copolymers under quasistatic condition. Furthermore, the copolymers also display a higher d31 coefficient compared to the d33 coefficient, which seems to be unusual compared with most other piezopolymers. The experimental data suggest that the origin of the unusual piezoelectric response in these P(VDF-HFP) copolymers originates from a reversible change between a poled α-like structure and β-like structure. The phase change nature also results in a large frequency dispersion of the piezoelectric response and a smaller d31 (=20.5pm∕V) at 50kHz.

Interfaces in poly(vinylidene fluoride) terpolymer/ZrO2 nanocomposites and their effect on dielectric properties

Enhancement of weak and high field dielectric responses was observed in poly(vinylidene fluoride) terpolymer/ZrO2 nanocomposites. A frequency-dependent interfacial polarization mechanism (related to the interfacial area between polymer and nanoparticles) is responsible for the enhancement of the dielectric constant at the low field. Due to the improvement of chain mobility and stabilization of the polar phase in the polymer/nanoparticle interface regions, the high field dielectric response of nanocomposites was also enhanced in the certain particle loading. Based on the high-field polarization saturation phenomenon, the dimension of interface regions in the nanocomposites was estimated as 20–40 nm.