Broad Phase Transition of Fluorite‐Structured Ferroelectrics for Large Electrocaloric Effect

Abstract

Field‐induced ferroelectricity in (doped) hafnia and zirconia has attracted increasing interest in energy‐related applications, including energy harvesting and solid‐state cooling. It shows a larger isothermal entropy change in a much wider temperature range compared with those of other promising candidates. The field‐induced phase transition occurs in an extremely wide temperature range, which contributes to the giant electrocaloric effect. This article examines the possible origins of a large isothermal entropy change, which can be related to the extremely broad phase transitions in fluorite‐structured ferroelectrics. While the materials possess a high entropy change associated with the polar–nonpolar phase transition, which can contribute to the high energy performance, the higher breakdown field compared with perovskites practically determines the available temperature range.

Large electrocaloric effect with high thermal and electric field cycling stability in solution-processed Y:HfO2 thin films

“…32 Park et al reported that the practical value of Δ S could be expected between 171 and 342 mJ cm −3 K −1 . 33 This large entropy difference is the possible cause of large ECE entropy change around the polar to the non-polar phase transition. However, before this study, ECE Δ S (or Δ T ) of HfO 2 -based materials was not high enough to be compared with perovskite or polymer-based materials.…”

“…In 2019, Park et al suggested that it would be possible to achieve the theoretically estimated ECE Δ S and Δ T in HfO 2 by suitable doping and process optimization. 33 In our present work, this has been achieved through Y-doping and optimized thin film preparation methods (multi-step deposition and step-wise annealing 24 ).…”

Emerging Fluorite-Structured Antiferroelectrics and Their Semiconductor Applications

“…As mentioned previously, fluorite-structured AFEs usually undergo a reversible field-induced phase transition between the nonpolar t-phase and polar o-phase, which represents a firstorder phase transition in FEs that occurs within a certain temperature region. 57 Unlike the second-order phase transition (also called a "continuous phase transition"), during which the second derivatives of the free energy obtained for particular thermodynamic variables change abruptly at the Curie temperature (T c ), the first-order phase transition is described by the discontinuity in the first derivative with respect to a thermodynamic variable. 2 Rabe et al suggested that the firstorder phase transition under an E ext must be accompanied by a transition from a nonpolar phase to a polar phase, which satisfies the antiferroelectricity criteria.…”

Novel Fluorite‐Structured Materials for Solid‐State Refrigeration

“…[65] A practical ΔS value for a HfO 2 -based material may be between 171 and 342 mJ K −1 cm −3 . [154] However, the GGA method overestimates the entropy and free-energy values, whereas the LDA method underestimates them. [65,133] This may reflect the effect of smaller differences in calculated energy values between LDA and GGA.…”