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Electrocaloric effect

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Temperature change of a material under an electric field

The electrocaloric effect is a phenomenon in which a material shows a reversible temperature change under an applied electric field.

Introduction<br>[edit]

The electrocaloric effect (ECE) is a phenomenon observed in dielectric materials, where a reversible temperature/entropy change occurs due to the alignment and reordering of dipoles under an applied electric field. When an electric field is applied, the dipoles within the dielectric material align, leading to a decrease in dipolar entropy and the release of heat, resulting in a temperature rise.[1] Conversely, when the electric field is removed, the dipoles return to a more disordered state, causing the material to absorb heat from its surroundings, resulting in a temperature decrease. This effect is being explored for use in solid-state cooling applications, particularly in areas where traditional cooling methods may be less efficient or impractical, such as in portable devices, microelectronics, and distributed thermal management.[2]

The electrocaloric effect is often considered to be the physical inverse of the pyroelectric effect. The electrocaloric effect should not be confused with the Thermoelectric effect (specifically, the Peltier effect), in which a temperature difference occurs when a current is driven through an electric junction with two dissimilar conductors.

Historical Background<br>[edit]

Before 2006, the electrocaloric effect (ECE) observed was relatively small, typically producing a temperature change of about 2.5 K at temperatures above 200 °C, and about 2 K at room temperature. Lead scandium tantalate (PST)[3] was studied in 1989, and exhibited a temperature change of 2.5 K.

Breakthrough Discoveries<br>[edit]

In 2006, researchers discovered a giant electrocaloric effect in 350 nm thin-film PbZr0.95Ti0.05O3 (PZT), generating a notable 12 K temperature change near 220 °C.[4] The device structure consisted of a thin film (PZT) on top of a much thicker substrate, but the figure of 12 K represents the cooling of the thin film only. The net cooling of such a device would be lower than 12 K due to the heat capacity of the substrate to which it is attached. This effect, particularly strong near phase transitions like the Curie temperature, far exceeded previous results in bulk materials. The study highlighted thin films' potential for solid-state cooling and suggested further material improvements could enhance practical applications.

In 2008, researchers discovered a giant electrocaloric effect in ferroelectric polymers near room temperature.[5] The poly(vinylidene fluoride-trifluoroethylene) copolymer [P(VDF-TrFE)] exhibited an adiabatic temperature change of over 12 °C and an entropy change exceeding 55 J/kgK near the ferroelectric-paraelectric transition at ~70 °C. Incorporating chlorofluoroethylene (CFE) into the copolymer achieved the giant ECE at room temperature for the first time, and demonstrated the potential to apply the ECE in cooling applications of daily life. The electrocaloric P(VDF-TrFE-CFE) terpolymers have been commercialized and are available from Arkema. The large ECE of the commercial EC polymers has enabled world-wide R&D efforts in EC cooling technologies.

Recent Developments<br>[edit]

A 2019 study demonstrated significant electrocaloric effects in multilayer capacitors (MLCs) of lead scandium tantalate (PST) ceramics.[6] These materials achieved temperature changes of up to 5.5 K near room temperature. The research highlights PST MLCs' potential for efficient and compact cooling applications, offering an alternative to magnetocaloric systems.

For EC cooling devices, the applied electric fields to the EC materials in the devices should be much lower than the dielectric breakdown field for reliable EC device operation while generating a high ECE. In general, the applied field should be less than 25% of the dielectric breakdown. To address this challenge, in 2021, researchers developed a high-entropy polymer that achieved an EC temperature change of 7.5 K temperature change under a low electric field of 50 MV/m.[7] By modifying a P(VDF-TrFE-CFE) terpolymer with double bonds, they enhanced dipolar entropy and reduced the energy barrier for phase transitions. This class of polymers also demonstrated excellent durability, maintaining performance over one million cycles.

In 2023, researchers developed a new ferroelectric polymer with subnanometer-scale pores, created by introducing and evaporating dimethylhexynediol (DMHD).[8] This process significantly enhanced the electrocaloric effect (ECE), achieving a temperature change of over 20 K under a low electric field. The...