Positive Electrocaloric Effects Research Articles

Correlation between multi-factor phase diagrams and complex electrocaloric behaviors in PNZST antiferroelectric ceramic system

Ferroelectric (FE) phase transition with a large polarization change benefits to generate large electrocaloric (EC) effect for solid-sate and zero-carbon cooling application. However, most EC studies only focus on the single-physical factor associated phase transition. Herein, we initiated a comprehensive discussion on phase transition in Pb_0.99Nb_0.02[(Zr_0.6Sn_0.4)_1−<i>x</i>Ti_<i>x</i>]_0.98O₃ (PNZST100<i>x</i>) antiferroelectric (AFE) ceramic system under the joint action of multi-physical factors, including composition, temperature, and electric field. Due to low energy barrier and enhanced zero-field entropy, the multi-phase coexistence point (<i>x</i> = 0.12) in the composition–temperature phase diagram yields a large positive EC peak of maximum temperature change (Δ<i>T</i>_max) = 2.44 K (at 40 kV/cm). Moreover, the electric field–temperature phase diagrams for four representative ceramics provide a more explicit guidance for EC evolution behavior. Besides the positive EC peaks near various phase transition temperatures, giant positive EC effects are also brought out by the electric field-induced phase transition from tetragonal AFE (AFE_T) to low-temperature rhombohedral FE (FE_R), which is reflected by a positive-slope boundary in the electric field–temperature phase diagram, while significant negative EC responses are generated by the phase transition from AFE_T to high-temperature multi-cell cubic paraelectric (PE_MCC) with a negative-slope phase boundary. This work emphasizes the importance of phase diagram covering multi-physical factors for high-performance EC material design.

Coexistence of giant positive and large negative electrocaloric effects in lead-free ferroelectric thin film for continuous solid-state refrigeration

The solid-state cooling technique utilizing electrocaloric (EC) materials is an alternative approach to tackle the greenhouse effect caused by traditional vapor-compression refrigeration. However, such a promising technique is severely hampered by the lack of proper materials considering that most existing EC materials with a single positive/negative EC effect exhibit a limited cooling effect. Here, the coexistence of a positive and negative EC effects has been achieved in a lead-free Na0.5Bi0.5(Ti0.97W0.01Fe0.02)O3 ferroelectric film. A state-of-the-art positive adiabatic temperature change (ΔT) of ~56 K accompanied by an isothermal entropy change (ΔS) of ~64 J K−1 kg−1 at 143 °C and a large negative ΔT of ~ − 17 K with a ΔS of ~ − 24 J K−1 kg−1 at 55 °C are obtained under a strong electric field strength of 2692 kV cm−1. Oxygen vacancy-related defect dipoles play a critical role in the negative EC effect at lower temperatures, while the phase transition is responsible for the positive EC effect at higher temperatures. Meanwhile, the film exhibits a high EC strength with a maximum ΔT/ΔE of 0.021 K cm kV−1, together with a ΔS/ΔE of 0.024 J cm K−1 kg−1 kV−1. This work ensures a giant total temperature change by utilizing and combining both the negative and positive EC effects in a dual cooling process.

Room‐Temperature Symmetric Giant Positive and Negative Electrocaloric Effect in PbMg0.5W0.5O3 Antiferroelectric Ceramic

AbstractAs an emerging solid‐state refrigeration technology with zero‐emission and high energy conversion efficiency, there is a compelling need for ferroelectric materials with giant electrocaloric effects (ECEs) at room temperature suitable for refrigeration applications. The complex perovskite antiferroelectric (AFE), PbMg0.5W0.5O3, containing non‐equivalent B‐site ions with a symmetric giant positive and negative ECE near room temperature is presented. At the Curie temperature of 36 °C, the first‐order AFE–paraelectric phase transition gives rise to a large enthalpy change of 3.92 J g−1, more than four times that of BaTiO3. This leads to a significant ECE under the influence of an electric field. The direct electrocaloric characterization shows that the adiabatic temperature change, ΔT, exhibits symmetric peaks with a giant positive maximum of 1.79 K (ΔS = 1.68 J kg−1 K−1) at 36 °C and a negative maximum of −2.02 K (ΔS = −1.93 J kg−1 K−1) at 34 °C. The ultrahigh magnitude of ΔT near room temperature makes PbMg0.5W0.5O3 a superior electrocaloric material far beyond traditional PbZrO3‐based AFEs. The coexistence of symmetric giant positive and negative ΔT to further improve cooling efficiency is expected. In addition, the good reversibility and negligible leakage current should pave the way for practical applications.

Giant electrocaloric effect in BiFeO3 and La codoped PbZr0.7Ti0.3O3 epitaxial thin films in a broad temperature range

Ferroelectric thin/thick films with large electrocaloric (EC) effect are critical for solid state cooling technologies. Here, large positive EC effects with two EC peaks in a broad temperature range (∼100 K) were obtained in 0.95Pb0.92La0.08(Zr0.70Ti0.30)0.98O3-0.05BiFeO3 (BFOLa-codoped PZT) epitaxial thin films deposited on the (100), (110) and (111) oriented SrTiO3 (STO) substrates by a sol-gel method. The thin film deposited on the (111) oriented STO substrate exhibited a stronger EC effect (∼20.6 K at 1956 kV/cm) near room temperature. However, the thin films deposited on the (100) and (110) oriented STO substrates exhibited a stronger EC effect (∼18.8 K at 1852 kV/cm and ∼20.8 K at 1230 kV/cm, respectively) around the peak of the dielectric permittivity (Tm, ∼375 K). Particularly, as the direction of the applied electric field was switched (E < 0), the ΔT of the (100)-oriented thin films around Tm was enhanced significantly from 18.8 K to 38.1 K. The self-induced-poling during the preparing process maybe plays a key role on the magic phenomenon. It can be concluded that the BFOLa-codoped PZT epitaxial thin films are promising candidates for application in the next solid-state cooling devices.

Effect of electric field orientation on ferroelectric phase transition and electrocaloric effect

The influence of electric field orientation on phase transitions and electrocaloric effects (ECEs) in BaTiO3 single crystals is studied by performing molecular dynamics simulation of first-principles-based effective Hamiltonian. The ECE is directly characterized via the adiabatic temperature change (ΔT) under the microcanonical ensemble. The simulation results demonstrate that different orientation relationship between the electric field and the polarization direction of ferroelectric phase leads to abundant phase structures and ECE behaviors. The electric-field-induced phase transitions produce remarkable ECE peaks/valleys, whereas the polarization rotation and/or extension without phase transition induced by the electric field produces small positive ECEs, which are insensitive to temperature and electric field direction. For the tetragonal-cubic phase transition, ECEs are always positive regardless of the applied electric field direction, and the value and width of ECE peak both increase with the reduction of the angle between electric field and crystallographic orientation. For the orthorhombic-tetragonal and rhombohedral-orthorhombic phase transitions, the sign of ECE gradually changes from negative to positive during the field direction rotates in planes between crystallographic orientations, and the coexistence of positive and negative appears when the field along non-crystallographic directions. Our simulated results shed light on a comprehensive physical understanding of the electric-field-orientation dependent ECE and offer instruction for the design of the refrigeration cycle by combining positive and negative ECEs.

Coexistence of positive and negative electrocaloric effects in lead free perovskite structured ferroelectrics

Abstracts The coexistence of positive and negative electrocaloric effects (ECE) was observed in the sol-gel derived Pb-free perovskite structured (Bi1/2Na1/2)1-xBaxTiO3 (x = 0, 0.04, 0.05, 0.06, 0.07) (BNBT) ceramics. The maximum electrocaloric temperature change of 2.35 °C has been observed at 100 kV/cm for x = 0.06 at about 70 °C before the ferroelectric to antiferroelectric (FE-to-AFE) phase transition temperature. The coexistence of positive and negative ECE at different temperatures may be attributed to two different temperature dependent phase transitions in the ferroelectrics or antiferroelectrics. The sign reversal of ECE in the same material can be utilized to increase the efficiency of the refrigeration cycle and to develop efficient environmentally friendly cooling systems.

Giant Negative and Positive Electrocaloric Effects Coexisting in Lead‐Free Na0.5Bi4.5Ti4O15 Films Over a Broad Temperature Range

Giant negative and positive electrocaloric effects (ECEs) coexist in lead‐free Na0.5Bi4.5Ti4O15 (NBTO) ferroelectric films with layered perovskite structure over a broad temperature range, with ΔT peaks of −14.8 °C (at 273 K) in the negative electrocaloric (EC) region (273–328 K), of 3.45 °C (at 373 K) in the positive EC region (328–418 K) and of −6.76 °C (at 483 K) in another negative EC region (418–513 K), respectively, under the electric field of 900 kV cm−1. Such giant ECEs are attributed to both the high break‐down electric field and prominent ferroelectricity. And the coexistence originates in the competition of the dipole polarization between both Na0.5Bi0.5TiO3 (NBT) and Bi4Ti3O12 (BTO) cells of NBTO films over the whole temperature region. Furthermore, the outstanding anti‐fatigue features with temperature stability for NBTO films ensure the durability of EC refrigeration. This work will prompt a novel technology for next generation of environmental‐friendly solid‐state cooling devices.

Polarization rotation and the electrocaloric effect in barium titanate

We study the electrocaloric effect in the classic ferroelectric BaTiO3 through a series of phase transitions driven by applied electric field and temperature. We find both negative and positive electrocaloric effects, with the negative electrocaloric effect, where temperature decreases with applied field, in monoclinic phases. Macroscopic polarization rotation is evident through the monoclinic and orthorhombic phases under applied field, and is responsible for the negative electrocaloric effect.

Near room temperature giant negative and positive electrocaloric effects coexisting in lead–free BaZr0.2Ti0.8O3 relaxor ferroelectric films

BaZr0.2Ti0.8O3 (BZT) relaxor ferroelectric (FE) films with tetragonal structure were deposited on Pt/TiO2/SiO2/Si(100) substrates via a sol-gel technique. A giant electrocaloric effect (ECE) was observed in the compact films with ΔT = 43.6K and ΔS = 61.9J/Kkg at near room temperature of 366.5 K, which was attributed to the high breakdown electric field of E = 1010kV/cm. Meanwhile, large negative ECE (ΔT = − 5.2 K and ΔS = − 8.8J/Kkg at room temperature of 305K, ΔT = − 5.1K and ΔS = − 7.2J/Kkg at near room temperature of 375 K) were achieved. The abnormal negative ECE is originated from structural transformation in relaxor ferroelectrics. The coexisting of giant negative and positive electrocaloric effects makes it more effective to realize the refrigeration during the application or removal of an electric field. The maximum electrocaloric coefficient (ζmax = 0.043Kcm/kV) and refrigeration efficiency (COP = 35.18) of the films were obtained at near room temperature of 366.5K. The giant electroelectric effect makes BZT films promising as lead-free materials for application in environment-friendly refrigeration equipment at near room temperature.

Flexible control of positive and negative electrocaloric effects under multiple fields for a giant improvement of cooling capacity

This paper demonstrated the flexible control of positive and negative electrocaloric effects (ECEs) in ⟨001⟩-Pb(Mg1/3Nb2/3)0.7Ti0.3O3 single crystal, and a dual cooling cycle is proposed using their combination to improve cooling capacity. The ECE exhibits a complex evolution of positive-negative-positive within 20–140 °C, where the negative ECE originates from the electric field-induced transition from rhombohedral phase to high-symmetric tetragonal phase. Since the coexistence of different ECEs at some temperatures, the positive and negative ECEs alternately appear in neighboring cycles under proper applied fields, i.e., dual cooling. A significant improvement of ∼150% in cooling capacity is directly characterized by the isothermal heat flow measurement.

Coexistence of multiple positive and negative electrocaloric responses in (Pb, La)(Zr, Sn, Ti)O3 single crystal

The electrocaloric effect has been investigated in antiferroelectric (Pb, La)(Zr, Sn, Ti)O3 (PLZST) single crystals grown by the flux method. The measurements of polarization versus electric field loops on unpoled crystals revealed that at room temperature, a critical electric field of 1.8 kV/mm is needed to induce a ferroelectric phase from an antiferroelectric phase. The dielectric properties demonstrated that the induced ferroelectric phase recovers to antiferroelectric phase when temperature is above the depolarization temperature (70 °C–100 °C). Coexistence of the negative and positive electrocaloric effect has been achieved in ⟨001⟩-oriented PLZST single crystals. Multiple electrocaloric response values of −0.054 °C at room temperature, 0.17 °C near the depolarization temperature, −0.14 °C at 125 °C, and 0.75 °C around Curie temperature have been observed under an electric field of 3 kV/mm. The coexistence of multiple negative and positive electrocaloric effects in one material provides a possibility to design solid-state refrigerator technologies to enhance the electrocaloric efficiency.

Double hysteresis loops and large negative and positive electrocaloric effects in tetragonal ferroelectrics.

Phase field modelling and thermodynamic analysis are employed to investigate depolarization and compression induced large negative and positive electrocaloric effects (ECEs) in ferroelectric tetragonal crystalline nanoparticles. The results show that double-hysteresis loops of polarization versus electric field dominate at temperatures below the Curie temperature of the ferroelectric material, when the mechanical compression exceeds a critical value. In addition to the mechanism of pseudo-first-order phase transition (PFOPT), the double-hysteresis loops are also caused by the abrupt rise of macroscopic polarization from the abc phase to the c phase or the sudden fall of macroscopic polarization from the c phase to the abc phase when the temperature increases. This phenomenon is called the electric-field-induced-pseudo-phase transition (EFIPPT) in the present study. Similar to the two types of PFOPTs, the two types of EFIPPTs cause large negative and positive ECEs, respectively, and give the maximum absolute values of negative and positive adiabatic temperature change (ATC ΔT). The temperature associated with the maximum absolute value of negative ATC ΔT is lower than that associated with the maximum positive ATC ΔT. Both maximum absolute values of ATC ΔTs change with the variation in the magnitude of an applied electric field and depend greatly on the compression intensity.