Giant Negative Electrocaloric Effect over an Ultra-Wide Temperature Region in Relaxation Frozen State Ferroelectrics

Optimized Electrocaloric Refrigeration in Lead-Free NaNbO3-Based Ceramics via AFE ↔ FE Phase Transition Modulation

An outstanding challenge for eco-friendly ferroelectric (FE) refrigeration is to achieve a large adiabatic temperature change within a broad temperature range originating from the electrocaloric (EC) effect, which is expected to be realized in antiferroelectric (AFE) materials owing to the large entropy change during electric field and thermally induced phase transition. In this work, a large EC response over a wide response temperature range can be achieved slightly above room temperature via designing the phase transition of NaNbO3. An irreversible to reversible AFE-FE phase transition on heating induced by the introduction of CaZrO3 into NaNbO3 plays a key role in the optimized electrocaloric refrigeration. Accordingly, accompanying the local structure transformation corresponding to the B-site ions, the transition temperature between the square polarization-electric field (P-E) hysteresis loop (the irreversible AFE-FE phase transition induced by the electric field) and the repeatable double P-E hysteresis loop (the electric field induced reversible AFE-FE phase transition) was tailored to around room temperature, in favor of extending large entropy change to the wide temperature range. This work provides an efficient approach to designing lead-free EC materials with excellent EC performance, promoting the advancement of environmentally friendly solid-state cooling technology.

Enhanced Electrocaloric Effect of Lead Scandium Tantalate by Zirconium Doping

The electrocaloric effect (ECE) is a novel technology that offers high efficiency and environmental friendliness, making it suitable for solid-state refrigeration applications. Among the extensively studied ECE materials, lead scandium tantalate (PST) stands out for its excellent performance. However, its applications are restricted by its narrow working temperature range. To overcome this limitation, we explore the enhancement of the ECE through zirconium ion doping. We synthesized PbSc0.5-0.5xTa0.5-0.5xZrxO3 samples (x = 0, 0.025, 0.05, 0.075). The introduction of zirconium ions led to an increase in the Curie temperature from 28.9 °C (x = 0) to 55.5 °C (x = 0.075). Additionally, the relaxation factor γ of the ceramics increased from 1.40 (x = 0) to 1.59 (x = 0.075). The temperature span (Tspan) exhibited a rising trend with increasing x, reaching 10.9 K at x = 0.075. The maximum temperature change (ΔTmax) was observed at x = 0.025, with a value of 1.94 K. X-ray diffraction (XRD) patterns revealed that zirconium ion doping influenced the B-site ordering degree, thereby regulating the ECE. To further validate the results, we employed direct measurements and thermodynamic calculations. Overall, the regulation of ionic ordering through zirconium doping effectively enhances the ECE performance. These findings contribute to the development of advanced materials for solid-state refrigeration technologies.

Substantially Enhanced Electrocaloric Effect in Ba(Zr0.2Ti0.8)O3 Lead-Free Ferroelectric Ceramics via Lattice Stress Engineering

As an alternative to conventional vapor-compression refrigeration, cooling devices based on electrocaloric (EC) materials are environmentally friendly and highly efficient, which are promising in realizing solid-state cooling. Lead-free ferroelectric ceramics with competitive EC performance are urgently desirable for EC cooling devices. In the past few decades, constructing phase coexistence and high polarizability have been two crucial factors in optimizing the EC performance. Different from the external stress generated through heavy equipment and inner interface stress caused by complex interface structures, the internal lattice stress induced by ion substitution engineering is a relatively simple and efficient means to tune the phase structure and polarizability. In this work, we introduce low-radius Li⁺ into BaZr_0.2Ti_0.8O₃ (BZT) to form a particular A-site substituted cell structure, leading to a change of the internal lattice stress. With the increase of lattice stress, the fraction of the rhombohedral phase in the rhombohedral-cubic (R-C) coexisting system and ferroelectricity are all pronouncedly enhanced for the Li₂CO₃-doped sample, resulting in the significant enhancement of saturated polarization (P_s) as well as EC performance [e.g., adiabatic temperature change (ΔT) and isothermal entropy change (ΔS)]. Under the same conditions (i.e., 333 K and 70 kV cm^-1), the ΔT of 5.7 mol % Li₂CO₃-doped BZT is 1.37 K, which is larger than that of the pure BZT ceramics (0.61 K). Consequently, in cooperation with the great improvement of electric field breakdown strength (E_b) from 70 to 150 kV cm^-1, 5.7 mol % Li₂CO₃-doped BZT achieved a large ΔT of 2.26 K at a temperature of 333 K, which is a competitive performance in the field of electrocaloric effect (ECE). This work provides a simple but effective approach to designing high-performance electrocaloric materials for next-generation refrigeration.