Electrocaloric Materials

Electrocaloric materials convert electrical energy into thermal energy. They have the ability to change their entropy reversibly under the action of an electric field: the polarization induced by the electric field is responsible, in most cases, for a higher order within the material, associated with a decrease in entropy. Types of material include single crystals and ceramics (in bulk, thick-film or thin-film form), as well as polymers. In the absence of heat exchange (the adiabatic case), the temperature of the material can be varied by several degrees, or even tens of degrees, by the application of strong electric fields.

Research into electrocaloric materials developed strongly in the early 2000s, after a few achievements in the 1970s. As the effect measured at the time was too weak to be practically exploitable, it was the development of thin-film and polymer structures that enabled us to achieve high dielectric strength and significant electrocaloric effects.

For some materials, the entropy change is sufficient to move thermal energy from a cold source to a hot one. The electrocaloric material forms the core of a solid-state cooling device, a true thermal machine that can compete with other refrigeration devices (gas compression/expansion systems, evaporation, thermoelectric effect, magnetocaloric effect, etc.). The advantage of such devices is the absence of refrigerant, enabling better integration and a more compact layout. Compared with thermoelectric materials, whose efficiency in practice can hardly exceed 10%, electrocaloric materials theoretically offer reversible operation, leading to energy efficiency close to the ideal Carnot cycle.

At this stage, few experimental electrocaloric demonstrators have been presented, as research has so far focused on the material itself. However, the regenerative cycles developed for the application of the magnetocaloric effect can be transposed to the electrocaloric case.

This article presents a review of the electrocaloric properties of a number of existing materials, after defining a few theoretical elements for understanding the quantification of these effects, and experimental characterization techniques.