Phonon entropy engineering for caloric cooling

Abstract

Electrocaloric cooling, with the advantages of zero global warming potential, high efficiency, smart size, etc., is regarded as a promising next-generation technology for green refrigeration. The exotic negative electrocaloric effect (ECE) in antiferroelectric materials forms the basis to improve the caloric cooling power density, but the underlying mechanism remains elusive. By using a fully first-principles method, we successfully simulate the electric field-triggered structural phase transition from antiferroelectric to ferroelectric in a prototypical antiferroelectric material PbZrO3 (PZO). Through tracking the phonon entropy evolution and measuring the temperature-dependent polarization along the transition path, we disclose that the negative ECE in PZO originates from the latent heat associated with phonon entropy rather than the previously recognized dipolar entropy. Accordingly, a new concept of phonon entropy engineering is proposed that engineering the density of states especially for low-frequency phonons can modulate the phonon entropy, which provides an effective route to enhance the cooling power density.

Antiferroelectric oxide thin-films: Fundamentals, properties, and applications

“…Fig. 12c-d summarize the ECE of different AFE material systems, which clearly indicate that with increasing thickness, ECE strength ΔT/ΔE enhances significantly [5,39,287,[362][363][364][365][366]367,369,. For example, bulk PLZT system has an ECE strength as large as 0.25 K cm/kV while the PLZT film system only has a value around 0.025 K cm/kV, almost one order of magnitude smaller.…”

“…The negative ECE is generally attributed to the non-collinear response of dipoles to an external electric field [5], which increases the entropy and decreases the temperature. While a recent study argued that the negative ECE actually originates from the latent heat related to phonon entropy of the AFE-FE phase transition, rather than the non-collinear response of dipoles [366]. The inconsistency in the origin of the negative ECE in AFEs indicates that there is still a lot of work to be done.…”

“…Reproduced with permission from[367]. The ECE strength ΔT/ΔE (c) and ΔE (d) dependent ΔT[5,39,164,287,[362][363][364][365][366]367,369,.…”