Why is the electrocaloric effect so small in ferroelectrics?

TL;DR

This paper investigates why the electrocaloric effect in ferroelectrics is inherently small, attributing it to long-range dipole interactions, and proposes a strategy to enhance it by reducing polarization fluctuation correlations.

Contribution

It derives microscopic figures of merit linking ferroelectric interactions to electrocaloric performance and suggests a method to increase the effect by modifying correlation lengths.

Findings

Long-range dipole interactions limit the electrocaloric effect.

Shortening polarization fluctuation correlations can enhance the effect.

Theoretical figures of merit guide the search for better materials.

Abstract

Ferroelectrics are attractive candidate materials for environmentally friendly solid state refrigeration free of greenhouse gases. Their thermal response upon variations of external electric fields is largest in the vicinity of their phase transitions, which may occur near room temperature. The magnitude of the effect, however, is too small for useful cooling applications even when they are driven close to dielectric breakdown. Insight from microscopic theory is therefore needed to characterize materials and provide guiding principles to search for new ones with enhanced electrocaloric performance. Here, we derive from well-known microscopic models of ferroelectricity meaningful figures of merit which provide insight into the relation between the strength of the effect and the characteristic interactions of ferroelectrics such as dipole forces. We find that the long range nature of these interactions results in a small effect. A strategy is proposed to make it larger by shortening the correlation lengths of fluctuations of polarization.