Professor J. Bernholc, North Carolina State University, Raleigh, NC, USA, and Oak Ridge National Laboratories, Oak Ridge, TN; USA presents:
The usual means of storing electrical energy are either batteries, where the current induces chemical reactions, or capacitors, where especially chosen dielectrics enhance the stored energy. Since capacitors can be discharged far quicker than batteries and fuel cells, they have much higher power densities. At present, highly insulating polymers with large breakdown fields, such as polypropylene, are the dielectrics of choice. Nevertheless, their energy densities are quite low because of small dielectric constants. Ferroelectric polymers from the polyvinylidene fluoride (PVDF) family have significantly larger dielectric constants, yet their energy densities are still rather low. However, an admixture of a small amount of another polymer results in a dramatic (up to sevenfold) increase in the stored energy. We discuss the origin of this highly non-linear effect as due to a cooperative phase transition induced by the electric field, and describe the atomic transformations involved in this process. Recently, weakly coupled strongly dipolar polymers were shown to also have high power densities as well as low loss, even at high temperatures. We have uncovered the mechanisms underlying their high performance through large-scale molecular dynamics simulations, and found potential avenues for further enhancement.