Analysis on Electrocaloric Effect in (Ba, Sr)TiO3 Ferroelectrics Near Phase Transition Temperature
Authors: Tanaka Yukiya , Teranishi Takashi , Iguchi Ryo , Kondo Shinya , Kishimoto Akira 
Published: December 2024

In recent years, there has been attention towards cooling systems utilizing the electrocaloric effect (ECE) as micro-cooling elements in small electronic devices such as smartphones. ECE is the phenomenon where the internal polarization structure of a material changes due to the application or removal of an electric field, resulting in a change in entropy and thus delivering absorption or generation of heat. As ECE is driven via utilizing the external electric field, it exhibits high compatibility with electronic devices owing to lower power consumption rather than the electric current driven applications.

There are various dielectric materials that can be utilized for ECE. Commonly used ferroelectrics include macro-domain types, where domain walls with aligned spontaneous polarization generate dielectric properties through their vibrations, and relaxor types, where Polar Nanoregions (PNRs) with alignment of polarization in several unit cells exhibit dielectric properties through oscillations induced by an external electric field. The entropy variation in these mentioned polarizations under an external electric field delivers the temperature change, owing to the Maxwell's relation.

ECE in ferroelectrics is known to be maximized near the phase transition temperature (Curie temperature, T
c) and the dielectric maximum temperature (T
m) [1]. Considering the Maxwell's relation,ΔT due to the electrocaloric effect (ECE), the contributions to ΔT through the polarization P can be interpreted as the combination of two contributions; the growingpolarization represented by dielectric permittivity and the spontaneous polarization (P
bias). The former is expected to reflect the effect of the polarization structure. However, these contributions have not been quantified separately. In order to elucidate the mechanism, i.e., the reason for the ECE anomaly at the T
c and the T
m, the mentioned ΔP contributions need to be separately analyzed.

In this study, we performed the direct and indirect ECE measurements and analyzed the two contributions. For the directmeasurement, we utilized the lock-in thermography technique which enables the accurate temperature change under a ac field [2]. The indirect measurement was performed by analyzing the temperature-dependences of the dielectric permittivity and the spontaneous polarization acquired from the electric field-polarization measurement simultaneously performed with the direct measurement.

For the evaluation in this study, we employed the (Ba,Sr)TiO3 based ferroelectric materials exhibiting a relatively high ECE. The directly evaluatedΔ
T of the BST exhibited its maximum near the T
c as expected. The analysis showed that in a low electric field region, the factor determining the Δ
T is the temperature dependence of the spontaneous polarization, which can be found fromthe peak temperature of differentiation P
bias. In contrast, with increasing the electric field, the contribution of dielectric permittivity to the Δ
T is increased, which shifts the peak temperature of ECE to a higher temperature.

This analysis can be utilized to a variety of the ECE materials, yielding to the quantitative understanding of the each contribution (P
bias and dielectric constant) to the apparentΔ
T under a dc field.

Acknowledgement

The authors are grateful to Prof. Ken-ichi Uchida for providing a series of the facility and the equipment related to the direct ECE measurement.

[1] J.Li et al. J. Phys. Energy
5
, 2023, 024017

[2] Ryo Iguchi et al. Appl. Phys. Lett. 122, 2023, 082903