Electrocaloric Performance Research Articles

Electrocaloric effects in multilayer capacitors for cooling applications

Abstract

Giant electrocaloric and energy storage performance of [(K0.5Na0.5)NbO3](1\u2212x)-[LiSbO3]x nanocrystalline ceramics

Electrocaloric (EC) refrigeration, an EC effect based technology has been accepted as an auspicious way in the development of next generation refrigeration due to high efficiency and compact size. Here, we report the results of our experimental investigations on electrocaloric response and electrical energy storage properties in lead-free nanocrystalline (1 − x)K0.5Na0.5NbO3-xLiSbO3 (KNN-xLS) ceramics in the range of 0.015 ≤ x ≤ 0.06 by the indirect EC measurements. Doping of LiSbO3 has lowered both the transitions (TC and TO–T) of KNN to the room temperature side effectively. A maximal value of EC temperature change, ΔT = 3.33 K was obtained for the composition with x = 0.03 at 345 K under an external electric field of 40 kV/cm. The higher value of EC responsivity, ζ = 8.32 × 10−7 K.m/V is found with COP of 8.14 and recoverable energy storage of 0.128 J/cm3 with 46% efficiency for the composition of x = 0.03. Our investigations show that this material is a very promising candidate for electrocaloric refrigeration and energy storage near room temperature.

Electrocaloric behavior and temperature dependent scaling of dynamic hysteresis of BaxSr1-xTiO3 (x = 0.7, 0.8 and 0.9) bulk ceramics

This article presents electrocaloric effect in BaxSr1-xTiO3 ferroelectric ceramics using an indirect approach based on Maxwell’s relations. The peak electrocaloric performance is found to be an adiabatic temperature change of 0.67, 0.83, and 0.61 K and isothermal entropy change of 0.9, 1, and 0.7 J/kg.K for x = 0.7, 0.8, and 0.9, respectively, under an electric field of 33 kV/cm. The electrocaloric coefficient of performance (COP) of materials were also estimated as 17, 5.47, and 5.68 when x = 0.7, 0.8, and 0.9, respectively, in BaxSr1-xTiO3 at 33 kV/cm (303 K). Further, the ferroelectric hysteresis parameters scaling relations of coercive field (E c) and remnant polarization (P r) as a function of temperature have been estimated. The back switching polarization (P bc) as a function of temperature (T) was estimated by the Arrhenius law in order to understand the domain dynamics, and the average activation energy was evaluated.

Direct measurement of electrocaloric effect in lead-free Ba(SnxTi1-x)O3 ceramics

In this study, we report on investigation of the electrocaloric (EC) effect in lead-free Ba(SnxTi1-x)O3 (BSnT) ceramics with compositions in the range of 0.08 ≤ x ≤ 0.15 by the direct measurement method using a differential scanning calorimeter. The maximum EC temperature change, ΔTEC-max = 0.63 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.11 at ∼44 °C around the multiphase coexistence region. We observed that the EC effect also peaks at transitions between ferroelectric phases of different symmetries. Comparison with the results of indirect EC measurements from our previous work shows that the indirect approach provides reasonable estimations of the magnitude of the largest EC temperature changes and EC strength. However, it fails to describe correctly temperature dependences of the EC effect for the compositions showing relaxor-like behaviour (x = 0.14 and 0.15) because of their non-ergodic nature. Our study provides strong evidence supporting that looking for multiphase ferroelectric materials can be very useful to optimize EC performance.

Enhanced electrocaloric, pyroelectric and energy storage performance of BaCexTi1−xO3 ceramics

BaCexTi1−xO3 (BCT) ceramics with compositions x=0, 0.1, 0.12 and 0.15 were synthesized using conventional solid state reaction route. Systematic exploration of enhancing electrocaloric effect (ECE) in BaTiO3 by rare earth dopant Ce is presented. BaCe0.12Ti0.88O3 exhibited an electrocaloric strength of ∼0.35Km/MV at 351K, which caters the need for a series of high-level ECE material. Further, the temperature dependence of pyroelectric coefficient is established for all compositions. The pyroelectric figure of merits (FOMs) for current responsivity (Fi), voltage responsivity (Fv), detectivity (Fd) and energy harvesting (Fe and Fe*) are calculated and the results reveal that x=0.1 could be a technologically superior candidate for pyroelectric devices. Further, BaCe0.15Ti0.85O3 exhibited highest electrical energy storage performance of 115kJ/m3 compared with 71kJ/m3 in BaTiO3. Our findings in this work may provide a better understanding for developing high ECE materials combined with pyroelectric and energy storage performance of Ce substituted BaTiO3 ceramics.

Amplitudon and phason modes of electrocaloric energy interconversion

AbstractSolid-state electrothermal energy interconversion utilising the electrocaloric effect is currently being considered as a viable source of applications alternative to contemporary cooling and heating technologies. Electrocaloric performance of a dielectric system is critically dependent on the number of uncorrelated polar states, or ‘entropy channels’ present within the system phase space. Exact physical origins of these states are currently unclear and practical methodologies for controlling their number and creating additional ones are not firmly established. Here we employ a multiscale computational approach to investigate the electrocaloric response of an artificial layered-oxide material that exhibits Goldstone-like polar excitations. We demonstrate that in the low-electric-field poling regime, the number of independent polar states in this system is proportional to the number of grown layers, and that the resulting electrocaloric properties are tuneable in the whole range of temperatures below TC by application of electric fields and elastic strain.

Large‐Temperature‐Invariant and Electrocaloric Performance of Modified Barium Titanate for Solid‐State Refrigeration

AbstractEnormous advances have been made to enhance the electrocaloric responses of ferroelectric materials, and new advanced composites transcend the 50 K mark. However, such progress has ignored the need for temperature‐invariant performance, which is essential for continuous cooling. In this regard, we propose the use of Sr‐modified bulk ceramics with the nominal composition Ba0.85Ca0.15−xSrxTi0.9Zr0.1O3 (0<x<0.15). The material with x=0.075 displayed electrocaloric cooling of (0.7±0.05) K over a wide temperature range of 60 K (303–363 K) at 33 kV cm−1.

Why is the electrocaloric effect so small in ferroelectrics?

Ferroelectrics are attractive candidate materials for environmentally friendly solid state refrigeration free of greenhouse gases. Their thermal response upon variations of external electric fields is largest in the vicinity of their phase transitions, which may occur near room temperature. The magnitude of the effect, however, is too small for useful cooling applications even when they are driven close to dielectric breakdown. Insight from microscopic theory is therefore needed to characterize materials and provide guiding principles to search for new ones with enhanced electrocaloric performance. Here, we derive from well-known microscopic models of ferroelectricity meaningful figures of merit for a wide class of ferroelectric materials. Such figures of merit provide insight into the relation between the strength of the effect and the characteristic interactions of ferroelectrics such as dipolar forces. We find that the long range nature of these interactions results in a small effect. A strategy is proposed to make it larger by shortening the correlation lengths of fluctuations of polarization. In addition, we bring into question other widely used but empirical figures of merit and facilitate understanding of the recently observed secondary broad peak in the electrocalorics of relaxor ferroelectrics.

Enhanced electrocaloric effect in Ba0.85Ca0.15Zr0.1Ti0.9–xSnxO3 ferroelectric ceramics

ABSTRACTThe present work deals with the study of effect of Sn addition (0%–6% by mole) on the electrocaloric (EC) performance of Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) ceramics. Maxwell's relations in conjunction with data from experimental observations were used for the indirect caloric measurements. Small percentage change with Sn addition is found to increase the EC performance of BCZT ceramics. A peak adiabatic temperature change of 0.84 K (353 K) was predicted for 0–32 kV/cm electric field, significantly higher than pristine BCZT ceramics. A simultaneous increase in specific entropy change was also observed.

Composition dependent electrocaloric behavior of (SrxBa1-x)Nb2O6 ceramics

ABSTRACTThis article presents electrocaloric effect in (SrxBa1-x)Nb2O6 ceramics (where x = 0.25, 0.50 and 0.75) using an indirect approach based on Maxwell's relations. Here, we have calculated various parameters of electrocaloric effect like temperature change (ΔT), entropy change (ΔS) and heat carrying capacity (ΔQ) of material due to the change in polarization under two different electric fields of 30 kV/cm and 20 kV/cm. (SrxBa1-x)Nb2O6 ceramics is well known pyroelectric material, where by increasing Sr/Ba ratio the ferroelectric behavior turns towards relaxor behavior. While in terms of electrocaloric effect performance (temperature change ΔT) is increase as the Sr/Ba ratio increases. Additionally, maximum ΔT (0.30 K) was found for (SrxBa1-x)Nb2O6 ceramic having x=0.75 molar concentration under the electric fields of 30 kV/cm.

Large room temperature electrocaloric strength in bulk ferroelectric ceramics: an optimum solution

Development of a practical electrocaloric (EC) refrigeration system is hindered by four main drawbacks: (a) low adiabatic temperature change near room temperature, (b) requirement of large electric field to achieve desirable temperature change, (c) low isothermal entropy change and (d) low thermal capacity. Here, we demonstrate that replacing Ca by Sr in the bulk ceramics of nominal composition Ba0.85Ca0.15-xSrxTi0.9Zr0.1O3, can address these shortcomings. Evidently, a large of 2.4 K can be obtained near room temperature (303 K) at relatively low electric field (37 kV.cm−1). These numbers indicate an EC performance that exceeds most bulk materials, surpassing even those reported for advanced thin-film composites. Further, a comparative analysis of the EC strength against reveals that the Sr modified composition is the optimum material, for EC cooling applications.

Giant electrocaloric effect in lead-free Ba0.94Ca0.06Ti1−xSnxO3 ceramics with tunable Curie temperature

Electrocaloric effect in lead-free Ba0.94Ca0.06Ti1−xSnxO3 ceramics is studied using an indirect method. The Ba0.94Ca0.06Ti0.875Sn0.125O3 ceramic located near a multi-phase point shows best electrocaloric performance, which provides further experimental evidence for optimizing electrocaloric properties through constructing multiphase coexistence. Giant electrocaloric efficiency (∼0.4 K mm/kV) is achieved in this ceramic at about room temperature at a low electric field of 6 kV/cm. While large electrocaloric temperature (∼0.63 K) is obtained by further enhancing electric field (20 kV/cm), a decrease in electrocaloric efficiency (0.32 K mm/kV) is simultaneously observed, which is attributed to phase transition from first-order to more diffusive second-order under higher electric field.

Enhanced electrocaloric effect in Fe-doped (Ba0.85Ca0.15Zr0.1Ti0.9)O3 ferroelectric ceramics

Solid-state refrigeration systems employing bulk ferroelectric materials suffer from two main drawbacks of low temperature change and high depolarization temperature which prevent rapid commercialization. In this regard, the beneficial effects of Fe-doping on the electrocaloric performance of bulk ceramics with nominal composition Ba0.85Ca0.15Zr0.1Ti0.9−xFexO3 (BCZTO-Fe) have been investigated. Indirect predictions were made using Maxwell's relations in conjunction with data from experimental observations. It was revealed that 1% Fe doping can greatly improve the performance of BCZTO based compositions. A peak adiabatic temperature change of 0.86K (345K) was predicted for 0–37kV/cm electric field, significantly higher (>100%) than pristine BCZTO ceramic. While a simultaneous increase in specific entropy change was also observed. The values indicate a huge improvement of the caloric capacity at reduced temperature indicating the benefits of such doping.

Electrocaloric Behavior and Temperature‐Dependent Scaling of Dynamic Hysteresis of Ba0.85Ca0.15Ti0.9Zr0.1O3 Ceramics

This article presents electrocaloric effect in Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCTZO) using an indirect approach based on Maxwell's relations. The peak electrocaloric performance is found to be an adiabatic temperature change of 0.41 K with electrocaloric strength of 19 mK cm/kV and a heat carrying capacity of ~0.17 J/g under an electric field of 0–21.5 kV/cm. The ferroelectric hysteresis scaling relations for coercive field (EC), remnant polarization (Pr), and hysteresis area (<A>) as a function of temperature (T) are also systematically investigated. The power‐law temperature exponents are obtained for all the hysteresis parameters. The scaling relations are established as Ec ∝ T−0.6584, Pr ∝ T−1.59, and <A> ∝ T−1.01623. The presented scaling relations are compared with those reported in the literature for other ferroelectric materials.

Large electrocaloric strength in the (100)-oriented relaxor ferroelectric Pb[(Ni1/3Nb2/3)0.6Ti0.4]O3 single crystal at near morphotropic phase boundary

Relaxor ferroelectric Pb[(Ni1/3Nb2/3)0.6Ti0.4]O3 (PNNT) single crystal with composition close to the morphotropic phase boundary (MPB) was successfully prepared using the molten salt method. The analysis results of XRD and TEM indicated that the as-grown crystal is of almost pure perovskite phase with coexistence of tetragonal and rhombohedral. Typical relaxor behavior was observed when the crystal was aged. Large electrocaloric strength (ΔT/ΔE) of 44.97mKcm/kV was obtained near the phase transition temperature of paraelectric to ferroelectric, indicating that PNNT is a promising material for application in cooling system. Moreover, it is found that the aging of point defects may make an important contribution to the electrocaloric performance of the single crystal, in addition to the contribution of the transition of phases induced by the electric field.

Electrocaloric cooling based on relaxor ferroelectrics

In this work, we consider the electrocaloric effect in relaxor ferroelectrics above the temperature of the dielectric constant maximum. For the purpose of modelling, a modified Curie--Weiss law was adapted to approximate the complex physics of relaxors. The requirements to electrocaloric materials derived on this basis are fulfilled by relaxor single crystals, ceramics and high-quality thin films. Relaxor ferroelectrics exhibit a suitable electrocaloric performance starting at applied electric fields of about 25 V/μm.

Characterization of the electrocaloric effect and hysteresis loss in relaxor ferroelectric thin films under alternating current bias fields

We report characterization and analysis of the frequency-dependent temperature responses in thin films exhibiting the electrocaloric (EC) effect under AC bias fields using a high-precision lock-in technique. The temperature response detected by an embedded thin-film resistance thermometer is analyzed using the steady-periodic solutions of a 3D heat conduction model to extract the equivalent volumetric heat sources/sinks, which represent the combined effects of electrocaloric cooling/heating and hysteresis loss. The dependence of the measured heat source strengths on the bias field frequency and amplitude is consistent with our model prediction and independently measured dielectric properties. The volumetric heating rate due to hysteresis loss is estimated to be as much as 15% of the EC heating/cooling rates for solution-cast relaxor ferroelectric polymer films studied here. Our experimental approach enables a systematic study of the electrocaloric performance of thin films and deleterious impact of hysteresis loss.

The effect of out-of-plane strain on the electrocaloric performances of P(VDF-TrFE) vertical heteroepitaxial film

Considering the boundary conditions of 1-3-type P(VDF-TrFE) composite ferroelectric film, the effect of out-of-plane strain on the electrocaloric performances of vertical heteroepitaxial film is calculated by the nonlinear thermodynamic theory. The results indicate that the out-of-plane strain can effectively regulate the ferroelectric and electrocaloric performances including the polarization, electrocaloric coefficient and adiabatic temperature change under the action of vertical electric field. In a wide temperature range, the vertical heteroepitaxial film can present higher adiabatic temperature change than the pure P(VDF-TrFE) film by controlling the out-of-plane strain. This indicates that the vertical composite heteroepitaxial film with excellent electrocaloric performances will have potential applications in the microelectronic devices such as micropowers, optical communication diodes and infrared sensors.