Ferroelectric refrigeration

Ferroelectric refrigeration. Just as a ferromagnet has a spontaneous magnetic moment that can be controlled by an applied magnetic field, a ferroelectric has a spontaneous electric polarization that can be controlled by an applied electric field. And in what’s known as the electrocaloric effect, applying or removing the electric field induces a reversible temperature change in a ferroelectric. If, as part of a thermal cycle, the electric field is adiabatically raised to heat the material and lowered to cool the material, a ferroelectric can function as a heat pump for heating or cooling. Such materials thus hold promise for compact, small-scale, solid-state refrigeration. Though early work achieved temperature changes of only a few kelvin, researchers in 2006 were able to cool a 350-nm-thick ferroelectric film by 12 K. Recent experiments by Yang Bai (University of Science and Technology Beijing) and colleagues have shown that for barium titanate, the cooling effect extends to temperatures above the ferroelectric transition and increases as the field increases. At the Carnegie Institution of Washington, staff scientist Ronald Cohen and summer intern Maimon Rose have now further explored the behavior. Through first-principles atomic-scale molecular dynamics simulations on lithium niobate, the pair confirmed that cooling indeed occurs both below and above the ferroelectric transition and found that it peaks at temperatures that, for a given applied field, maximize the material’s dielectric susceptibility, which relates the polarization and the field strength. Generalizing to other ferroelectrics, the two concluded that the operating temperature for refrigeration and other energy-scavenging applications should be well above the transition temperature. Moreover, a large electrocaloric effect should be observable in any insulator with a large, temperature-dependent dielectric susceptibility. (M. C. Rose, R. E. Cohen, Phys. Rev. Lett. 109, 187604, 2012.)