Negative Electrocaloric Effect Research Articles

Colossal negative electrocaloric effects in lead-free bismuth ferrite-based bulk ferroelectric perovskite for solid-state refrigeration

A colossal negative electrocaloric effect is demonstrated for the first time in versatile bismuth ferrite-based perovskite.

Giant negative electrocaloric effect in PbZrO3/0.88BaTiO3–0.12Bi(Mg1/2,Ti1/2)O3 multilayered composite ferroelectric thin film for solid-state refrigeration

A giant negative electrocaloric (EC) effect in a PbZrO3/(0.88BaTiO3–0.12 Bi(Mg1/2,Ti1/2)O3) (PZ/(BT-BMT)) multilayered composite ferroelectric (MCFE) thin film which is grown on Pt(111)/Ti/SiO2/Si(100) substrates by the sol-gel method is investigated in this work. The negative EC effect in the PZ/(BMT-BT) MCFE thin film is greatly higher than that in the PZ AFE thin film with an adiabatic temperature change (ATC) ΔT = 1.5 K. The ATC ΔT of the PZ/(BMT-BT) MCFE thin film is −32 K under the applied electric field change ΔE = 1151 kV/cm. The result is conducive to enhance the EC refrigeration efficiency greatly.

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.

Temperature induced phase transformations and negative electrocaloric effect in (Pb,La)(Zr,Sn,Ti)O3 antiferroelectric single crystal

Temperature induced phase transitions and electrocaloric effect (ECE) of (Pb,La)(Zr,Sn,Ti)O3 (PLZST) single crystals have been comprehensively studied. Based on the in situ evolution of domain structures and dielectric properties of the PLZST crystals, the phase transitions during heating are in the sequence of orthorhombic antiferroelectric → rhombohedral ferroelectric → cubic paraelectric. Coexistence of the negative and positive ECEs has been achieved in the PLZST single crystals. A negative ECE value of −1.26 °C and enhanced electrocaloric strength of −0.21 K mm/kV near the Curie temperature have been obtained. A modified Landau model gives a satisfactory description of the experimentally observed unusual ECE. Moreover, a temperature–electric field phase diagram is also established based on theoretical analysis. Our results will help people understand better the electrocaloric family, particularly on the negative and/or positive effect in antiferroelectrics and ferroelectrics.

Coexistence of negative and positive electrocaloric effect in lead-free 0.9(K0.5Na0.5)NbO3 - 0.1SrTiO3 nanocrystalline ceramics

The coexistence of negative and positive electrocaloric effect have been experimentally demonstrated in lead-free 0.9(K0.5Na0.5)NbO3 - 0.1SrTiO3 nanocrystalline ceramics using an indirect thermodynamic method. The electrocaloric response of −0.85K at 276K and 0.43K at 330K were calculated at 40kV/cm. The corresponding electrocaloric coefficient were estimated to be −0.24×10−7K·mV−1 and 0.7×10−7K·mV−1 at 273K and 333K, respectively. A large electrical storage energy density 1.46J/cm3 were achieved. The results indicate that materials possess a huge potential for electrocaloric refrigeration as a micro-cooler and energy storage capacity for electronic devices.

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.

Exceptionally High Negative Electro-Caloric Effects of Poly(VDF⁻co⁻TrFE) Based Nanocomposites Tuned by the Geometries of Barium Titanate Nanofillers.

Exceptionally high electro-caloric effects (ECEs) are observed in nanocomposites consisting of poly(vinylidene fluoride-co-trifluoroethylene) (VDF–co–TrFE) copolymer and barium titanate (BT) nanoparticles and nanowires. The poly(VDF–co–TrFE) matrix nanocomposites containing 5% volume fraction of BT nanowires are found to exhibit a negative ECE temperature change as large as 12 °C or a refrigeration effect of 8.3 J/g, which is much larger than those reported to date. The mechanisms of negative ECE and the enhanced negative ECE in the nanocomposites consisting of poly(VDF–co–TrFE) and BT nanowires are explained by the Kauzmann theory on glassy polar states and the interaction between BT nanofillers and the copolymer matrix. The effects of geometries of BT nanofillers on the negative ECEs are elucidated by P-E loop measurements, and dielectric and dynamical mechanical analyses. The nanocomposites, with their enhanced negative ECE tuned by the geometries of BT nanofillers, provide us with promising ECE refrigerants for practical application to small-sized and environmentally-friendly ECE coolers in the heat management of electronic devices.

Effect of acceptor doping on phase transitions of HfO2 thin films for energy-related applications

Fluorite structured HfO2 or ZrO2 thin films have been intensively studied for memory- and energy-related applications since their ferroelectricity was first reported in 2011. The phase transition between the nonpolar tetragonal and the polar orthorhombic phase in these new ferroelectric materials is believed to be promising for energy harvesting, energy storage and solid state cooling. The temperature dependent phase transition and resulting strong pyroelectric and electrocaloric effect have been reported for Si-doped HfO2 and (Hf,Zr)O2 thin films. In this study, the effect of acceptor (Al and Gd) doping into HfO2 thin films on their temperature dependent phase transition was systematically examined. The phase transitions in Al- and Gd-doped HfO2 thin films were much broader compared to Si-doped HfO2 and (Hf,Zr)O2 films. The maximum adiabatic temperature change (ΔT) values of Al- and Gd-doped HfO2 film were 5.7 and 3.1K, respectively. A giant negative electrocaloric effect with ΔT of −7.4K could be observed for Al-doped HfO2. The various factors which can potentially affect the phase transitions of HfO2 films, such as dopant size, grain size distribution, spatial dopant distribution, and oxygen vacancy distribution were carefully examined to understand the different phase transition behavior. From the various factors, the distribution of oxygen vacancies is suggested as the origin of the different phase transitions of HfO2 films doped with trivalent and tetravalent dopants.

(Invited) Phase Transition and Related Energy Applications of (Hf,Zr)O2 Films

A new ferroelectric phase (space group: Pca21) in (Hf,Zr)O2 thin film and doped/undoped HfO2 has been intensively studied since 2011.[1,2] The films could show large remanent polarizations (They are generally ~20 μC/cm2 and can be even larger than 40 μC/cm2 for La-doped HfO2) with extremely small film thickness (It is generally ~10 nm and can be even as thin as ~2.5 nm).[3,4] Moreover, they could endure more than 109 times of switching with the electric field of 2.5 MV/cm.[2] The Curie temperature of (Hf,Zr)O2 could be controlled by changing Zr:Hf ratio, and the (Hf,Zr)O2 films showed first-order like phase transition.[5] Interestingly, the two-step polarization switching could be observed in rather wide temperature range near room temperature for Hf0.4Zr0.6O2 film,[5] which could be observed in a very narrow temperature range for BaTiO3in a previous study.[6] The phase transition involved with the new ferroelectric phase is believed to be highly promising for various applications such as electrostatic supercapacitor, energy harvester, and electrocaloric cooler.[7-9] The Hf0.3Zr0.7O2 thin film could store electrostatic energy up to 46 J/cm3.[7] The temperature dependent change of polarization of Hf0.2Zr0.8O2 thin film could harvest electrical energy of 11.6 J/cm3 cycle from heat using Olsen cycle.[8] This also can be used for solid state cooling based on electrocaloric effect (The adiabatic temperature change was 13.4 K) when the inverse Olsen cycle is used.[8] It was suggested that a monolithic device with aforementioned various functions can be achieved on Si substrate.[8] In 2016, the giant negative electrocaloric effect (The adiabatic temperature change was -10.8 K.) in Hf0.5Zr0.5O2 film was also reported based on the abnormal pyroelectric properties.[9] It is expected that the combination of giant positive and negative electrocaloric effect in (Hf,Zr)O2films can make the solid state cooling device more efficient.[10] In this presentation, the fundamentals for phase transition in (Hf,Zr)O2films will be discussed based on the first-­order phase transition model involving the orthorhombic and tetragonal phases. The application of the phase transition behaviors for the various energy applications including energy storage/harvesting and solid state cooling will be presented in detail. [1] T. Boescke et al. Appl. Phys. Lett. 99, 102903 (2011). [2] M. H. Park et al. Adv. Mater. 27, 1811 (2015). [3] J. Meuller et al. IEEE International Electron Devices Meeting (IEDM) (2013). [4] A. Chernikova et al. ACS Appl. Mater. Interfaces 8, 7232 (2016). [5] M. H. Park and H. J. Kim et al. Nanoscale 8, 13898 (2016). [6] W. J. Merz, Phys. Rev. 91, 513–517 (1953). [7] M. H. Park et al. Adv. Electron. Mater. 4, 1400610 (2014). [8] M. H. Park et al. Nano Energy 12, 131 (2015). [9] M. H. Park et al. Adv. Mater. 28, 7956 (2016). [10] Li et al. Europhys. Lett. 102, 47004 (2013).

A stress-mediated negative/positive electrocaloric effect in Bi4Ti3O12 nanoparticles

A stress-mediated electrocaloric (EC) effect in Bi4Ti3O12 ferroelectric nanoparticle was predicted using a phase field simulation in combination with a thermodynamics analysis. The Bi4Ti3O12 nanoparticle exhibited both a positive and negative EC effect under the applied electric fields. The negative EC effect was transformed into the positive EC effect under compressive stress; however, the negative EC effect appeared across a wider temperature range than the positive effect and could even be enhanced under tensile stress. The results provide a new way to tune the sign of the EC effect and to enhance the EC coefficient in ferroelectric materials.

Influence of 90° charged domain walls on the electrocaloric effect in PbTiO3 ferroelectric thin films

The electrocaloric (EC) effect in PbTiO3 ferroelectric thin films (FETFs) with an array of partially compensated head-to-head (HH) and tail-to-tail (TT) 90° charged domain walls (CDWs) has been studied by using a phase field method. The calculation results indicate that the magnitude of the adiabatic temperature change ΔT increases with the diminution of the charge density (ρ*), in which ρ* is provided by charged defects or band bending. In particular, a negative EC effect caused by the direction difference between the applied electric field and the dipole is found near the HH and TT domain walls when ρ* > 2.0. The EC strength |ΔT|/|ΔE| is 0.020 K cm kV−1 with the positive ΔT = 2.03 K at the external dimensionless electric field Eb*=0.08 (ΔE = 104 kV cm−1). These results indicate that the CDWs can provide an efficient way to adjust the EC effect of FETFs for refrigeration applications at room temperature.

Coexistence of negative and positive electrocaloric effect in Sr and Nb co-doped Pb(Zr,Ti)O3 ferroelectric ceramics

The electrocaloric effect has been investigated in Sr and Nb co-doped Pb(Zr,Ti)O3 (PSNZT) ceramics synthesized by conventional solid state reaction. The coexistence of negative and positive electrocaloric effect was revealed in PSNZT ceramics. The electrocaloric response values of −0.38K at 80°C and 0.65K at 180°C were observed under an electric field of 1.5kV/mm. The corresponding electrocaloric coefficient values were calculated to be −0.25KmmkV−1 and 0.43KmmkV−1 respectively. The mechanism of the coexistence of negative and positive electrocaloric effect was discussed.

Positive and negative electrocaloric effect inBaTiO3in the presence of defect dipoles

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped ${\mathrm{BaTiO}}_{3}$ is studied by means of lattice-based Monte-Carlo simulations using a Ginzburg-Landau type effective Hamiltonian. Oxygen vacancy-acceptor associates are described by fixed local dipoles with orientation parallel or antiparallel to the external field. By a combination of canonical and microcanonical simulations the ECE is directly evaluated. Our results reveal that in the case of antiparallel defect dipoles the ECE can be positive or negative depending on the dipole density. Moreover, a transition from a negative to positive ECE can be observed when the external field increases. These transitions are due to the delicate interplay of internal and external fields and are explained by the domain structure evolution and related field-induced entropy changes. The results are in good qualitative agreement to those obtained by molecular dynamics simulations employing an ab initio based effective Hamiltonian. Finally, a modified electrocaloric cycle, which makes use of the negative ECE in the presence of defect dipoles, is proposed to enhance the cooling effect.

Direct and indirect measurements on electrocaloric effect: Recent developments and perspectives

It has been ten years since the discovery of the giant electrocaloric effect in ferroelectric materials showed that it is possible to employ this effect for substantial cooling applications. This last decade has been marked by increasing research interest, especially in characterizing and measuring the electrocaloric effect using both the so-called indirect and direct approaches. In this context, a comprehensive summary and careful reexamination of these approaches are very timely and of great importance to justify the assumptions used in different measurement techniques. This review is therefore dedicated to cover recent important and rapid advances from both the indirect and direct measurements and provides critical insights relevant for quantifying the electrocaloric effect. It involves electrocaloric materials from normal ferroelectrics, antiferroelectrics, and relaxors, and it fundamentally focuses on how the electrocaloric entropy changes in response to electric field in these typical electrocalorics. The article addresses recent developments, especially during the past three years, such as technical selection of proper polarization-electric field loops, negative electrocaloric effect in antiferroelectrics and relaxors, the controversial debate on the indirect method in relaxors, the important role of field dependence of specific heat, kinetic factors, and so on. Moreover, this review also is concerned with extracting reliable data by direct measurements. Four typical techniques and devices used recently, such as thermocouples, differential scanning calorimeters, specifically designed calorimeters, and scanning thermal microscopy, are briefly reviewed, while infrared cameras are emphasized. We hope that our review will not only provide a useful background to understand fundamentally the electrocaloric effect and what one really measures but also may act as a practical guide to exploit and develop electrocalorics towards the design of suitable devices.

Giant Negative Electrocaloric Effects of Hf0.5 Zr0.5 O2 Thin Films.

Hafnia (HfO2 )-zirconia (ZrO2 ) solid solution films show giant positive (ΔT = 13.4 K) and negative (ΔT = -10.8 K) electrocaloric effects that can be simply controlled by tuning the Hf/Zr ratio. It is expected that the combination of the electrocaloric effects with opposite signs in this lead-free, simple, binary oxide can significantly improve the efficiency of electrocaloric cooling.

Anomalous negative electrocaloric effect in a relaxor/normal ferroelectric polymer blend with controlled nano- and meso-dipolar couplings

In general, a dielectric material will eject (or absorb) heat when an electric field is applied and absorb (or eject) heat when the field is removed, under isothermal condition, which is known as the normal (or negative) electrocaloric (EC) effect. For some applications, it is highly desired that an EC material will absorb heat (cooling the surrounding) without subsequent heating under an electric pulse. Here, we show that such an EC material can be realized in a properly designed hybrid normal ferroelectric/relaxor ferroelectric polymer blend in which the normal ferroelectric component induces dipole ordering in the relaxor polymer in the poled state, which can be switched to a de-poled state by an external field. More importantly, the de-poled state can be maintained by the relaxor component when the de-poling field is removed. Consequently, the hybrid blend exhibits a large cooling (an isothermal entropy change ΔS = 11.5 J kg−1 K−1) without the subsequent heating upon the application of an electric pulse.

Effects of morphotropic phase boundary transitions and structural transformations on Olsen cycle assisted thermo-electrical energy conversion: A case study on Bi0.5Na0.5TiO3-xBaTiO3 system

This article reports the peculiarity in the thermo-electrical energy conversion (by Olsen cycle) capability of Bi0.5Na0.5TiO3-xBaTiO3 (BNT-BT) system caused by the coupled effect of structural transformations and phase transitions in vicinity of the morphotropic phase boundary (MPB). The calculated maximum energy density estimated for nearly identical ambient conditions for MPB (x = 0.06) lies in the range of 40–110 J/L (40–110 kJ/m3) while moving away from the MPB the same elevates dramatically on both the ends. The utmost energy density found in system is 1000 J/L (1000 kJ/m3) for BNT-0.3BT (temperature range: 42.6–108.7°C; applied electric field: 0–6.5 MV/m). This energy density is significantly higher than the reported thermo-electrical energy generation (680 kJ/m3) using negative electrocaloric effect in BNT-xBT systems. It is found that ferroelectric to antiferroelectric transition as well as transformation suppresses harvested energy density at MPB. Contemporary, rhombohedral to tetrahedral transitions contributes towards enhancement in the energy harnessing capability of the system. Consequently, it is revealed that in contrast to the rombohedral phase (a = b = c) the non-centro symmetric disturbances caused by thermal fluctuations in tetrahedral phase (a≠c) are higher and facilitate the domain wall switching leading to magnification of energy density.

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.

Giant pyroelectric energy harvesting and a negative electrocaloric effect in multilayered nanostructures

This work examines the potential of PbZr0.53Ti0.47O3/CoFe2O4 (PZT/CFO) multi-layered nanostructures (MLNs) to achieve a giant electrocaloric effect (ECE) and enhanced pyroelectric energy harvesting.