Sheng-Guo Lu

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).

Organic and inorganic relaxor ferroelectrics with giant electrocaloric effect

The electrocaloric effect (ECE) in inorganic thin film and organic relaxor ferroelectrics is investigated by directly measuring the ECE around room temperature. The results reveal that giant ECEs can be obtained in the high energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) relaxor copolymer and in the La-doped Pb(ZrTi)O3 relaxor ceramic thin films, which are much larger than that from the normal ferroelectric counterparts. The large ECE observed, compared with normal ferroelectrics, is likely caused by the large number of disordered fluctuating polarization entities in relaxor ferroelectrics which can lead to extra entropy contributions and larger ECE.

Comparison of directly and indirectly measured electrocaloric effect in relaxor ferroelectric polymers

We report the directly measured electrocaloric effect (ECE) (the adiabatic temperature change ΔT) of relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer and its blend with poly(vinylidene fluoride-chlorotrifluoroethylene). The results show that the directly measured ΔT in the relaxor terpolymer is much larger than that deduced from Maxwell relation and that the relaxor terpolymer possesses a giant ECE at room temperature. The large difference between the directly measured ΔT and that deduced indicates that the Maxwell relation, which is derived for ergodic systems, is not suitable for deducing ECE in the relaxor ferroelectric polymers, which are nonergodic (polar-glass) material systems.

Pyroelectric and electrocaloric materials

Both the pyroelectric and electrocaloric effects originate from the cross-coupling between polarization and temperature in insulating dielectrics. Although both effects have been studied for many decades for various applications and large pyroelectric effect has been observed in many polar-dielectrics, it is only very recently that a large electrocaloric effect (ECE) was obtained in ferroelectric ceramic thin films and polymers, which revives the interest in the ECE. This review will summarize typical properties of pyroelectric and electrocaloric materials, present figures of merit for both phenomena, examine the relationship between the pyroelectric and electrocaloric effect. Moreover, we will also present theoretical works, experimental results, and material modifications to achieve large responses in electrocaloric materials.

Enhancing the magnetoelectric response of Metglas/polyvinylidene fluoride laminates by exploiting the flux concentration effect

Magnetic flux concentration effect of Metglas as a function of its sheet aspect ratio was investigated for Metglas/polyvinylidene fluoride laminates. Taking advantage of this effect, the magnetoelectric voltage coefficient of 21.46 V/cm∙Oe for a laminate with 1 mm wide and 30 mm long Metglas sheet (25 μm thick) is achieved, which is much higher than those reported earlier in similar laminates without making use of the flux concentration effect. The results demonstrate an effective means to significantly enhance the sensitivity of magnetostrictive/piezoelectric composite laminates as weak magnetic field sensors.

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.

Tunable temperature dependence of electrocaloric effect in ferroelectric relaxor poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene terpolymer

The electrocaloric effect (ECE) was directly measured in a relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer. It was observed that the temperature dependence of ECE of the terpolymer films depends critically on the film preparation conditions. While the uniaxially stretched terpolymer films show pronounced temperature dependence of ECE, the non-stretched films exhibit nearly temperature independent ECE from 5 °C to 45 °C. Such a difference is likely caused by the changes in possible polar states and polar-correlation range by film stretching. Besides, large ECE (T > 15 °C) can be induced in both films at 30 °C and 150 MV/m.

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.

Giant electrocaloric effect in ferroelectric poly(vinylidenefluoride-trifluoroethylene) copolymers near a first-order ferroelectric transition

We present directly measured electrocaloric effect (ECE) from the poly(vinylidenefluoride-trifluoroethylene) 65/35 mol. % copolymer. The data reveal a large difference in the ECE between that measured in applying and in removing the electric field. The difference is significantly reduced by modifying the copolymer with 20 Mrad of high energy electron irradiation. Moreover, an isothermal entropy change ΔSint = 160 J kg−1 K−1 and an adiabatic temperature change ΔTint = 35 °C can be induced in the irradiated copolymer. These results demonstrate the promise of achieving a significant ECE in ferroelectric polymers near first-order ferroelectric-paraelectric transition where multiple intermediate phases can exist.

Polar-fluoropolymer blends with tailored nanostructures for high energy density low loss capacitor applications

A polar-fluoropolymer blend consisting of a high energy density poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF-CTFE)) and a low dielectric loss poly(ethylene-chlorotrifluoroethylene) (ECTFE) was developed. Both the blend and crosslinked blend films exhibit a dielectric constant of 7 and low loss (∼1%), as expected from the classical composite theory. Moreover, introducing crosslinking in the blends can lead to a marked reduction of losses in blend films at high fields while maintaining a high energy density. At 250 MV/m, a loss of 3% can be achieved in the crosslinked blend compared with 7% loss in pure blend, which is already much below that of neat P(VDF-CTFE) (∼35%).