Dielectric polymers with high dipole density have the potential to achieve very high energy density, which is required in many modern electronics and electric systems. We demonstrate that a very high energy density with fast discharge speed and low loss can be obtained in defect-modified poly(vinylidene fluoride) polymers. This is achieved by combining nonpolar and polar molecular structural changes of the polymer with the proper dielectric constants, to avoid the electric displacement saturation at electric fields well below the breakdown field. The results indicate that a very high dielectric constant may not be desirable to reach a very high energy density.

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 degrees C), where an isothermal entropy change of more than 55 joules per kilogram per kelvin degree and adiabatic temperature change of more than 12 degrees 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 storage plays a key role in mobile electronic devices, stationary power systems, and hybrid electric vehicles. [1][2][3] There is a great need for development of new materials with superior electrical energy density since current ceramics and polymers fall significantly short of rising demands in advanced applications. The introduction of inorganic nanoparticles into polymer matrices to form dielectric polymer nanocomposites represents one of the most promising and exciting avenues to this end. [4][5][6][7][8][9][10][11] This approach is motivated by the idea that the combination of ceramic materials of large permittivity with polymers of high breakdown strength could lead to a large energy storage capacity, as energy density is proportional to the product of permittivity and the square of the applied electric field. Moreover, large interfacial areas in the composites containing nanometer scale fillers promote the exchange coupling effect through a dipolar interface layer and result in higher polarization levels and dielectric responses. [12,13] Compared to conventional ceramic materials, polymer-based dielectric materials also offer processing advantages including mechanical flexibility and the ability to be molded into intricate configurations for electronic and electric devices with reduced volume and weight. While most of the current studies on dielectric nanocomposites are focused on the enhancement of dielectric permittivity, few examples have investigated dielectric properties and associated energy densities at high electric fields. [14][15][16] Ferroelectric metal oxides such as Pb(Zr,Ti)O 3 (PZT), Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMNT), and BaTiO 3 have been popular choices as filler materials in dielectric nanocomposites because of their high permittivities. However, from the energy storage point of view, inclusion of nanoparticles with permittivities on the order of hundreds and even thousands into polymers, which generally possess a permittivity less than 10, might not be desirable for an appreciable increase in energy density. As the filler has a much greater permittivity than the polymer matrix, most of the increase in effective dielectric permittivity comes though an increase in the average field in the polymer matrix with very little of the energy being stored in the high permittivity filler phase.[17] Furthermore, the presence of a large contrast in permittivity between two phases gives rise to a highly inhomogeneous electric field and thus a significantly reduced effective breakdown strength of the composite. [18] In this communication, we report high-energy-density polymer nanocomposites based on surface-functionalized TiO 2 nanocrystals as dopants in a ferroelectric poly(vinylidene fluoride-tertrifluoroethylene-ter-chlorotrifluoroethylene) (P(VDF-TrFE-CTFE)). In this approach, the polymer matrix and TiO 2 filler possess comparable dielectric permittivities of 42 and 47, respectively, measured using an inductance, capacitance, resistance (LCR) meter at room temperature and 1 kH...

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The high electric displacement (D>0.1 C/m 2 ) 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/cm 3 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 μs) 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.

The flexoelectricity of several thermoplastic and thermosetting polymers was investigated by testing the dielectric polarization response under bending deformation of polymer cantilevers. All the polymers studied showed a flexoelectric response with a flexoelectric coefficient of the order of the 10−9–10−8 C/m. Based on a comparison of the flexoelectric response of the different polymers studied, we discuss factors that may influence the generation of flexoelectricity in polymeric materials.

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 (<300 Hz) mechanical resonance leads to high effective d33>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.