Electrocaloric Temperature Change Research Articles

High Energy Storage Efficiency and Electrocaloric Effect in Pb-Free Ba0.85Ca0.15Hf0.10Ti0.90O3 Ceramics for Sustainable Green Energy Applications

In this work, Pb-free Ba0.85Ca0.15Hf0.10Ti0.90O3 (BCHT) ferroelectric ceramic was synthesized via the sol-gel method and its structural, dielectric, ferroelectric, electrocaloric, and energy storage characteristics have been investigated under a low electric field of 20 kV/cm. The Rietveld refinement of XRD patterns shows good agreement with the co-existence of two phases tetragonal (P4mm) and cubic (Pm3m), with each phase contribution of 58 and 42% respectively. A volumetric porosity of around 11.5% is observed. The electrocaloric temperature change ΔT and entropy change ΔS have been estimated by an indirect approach using Maxwell relations. The room temperature value of – ΔS and ΔT are 0.207 J/Kg.K and 0.157 K respectively and a wide temperature span of 50K was observed. The room temperature observed enhanced total energy density, recoverable energy density, and energy storage efficiency were 84.43 mJ/cm3, 64.38 mJ/cm3, and 76.25 % respectively. The maximum energy storage efficiency of 85.03% was observed at 80oC. The Pb-free BCHT ceramic is a promising candidate for eco-friendly cooling and energy storage technology.

Improvement of the electrocaloric effect and energy storage performances in Pb-free ferroelectric Ba0.9Sr0.1Ti0.9Sn0.1O3 ceramic near room temperature

Perovskite-type Ba0.9Sr0.1Ti0.9Sn0.1O3(BSTSn) ceramic was synthesized by the sol-gel method. The P-E hysteresis loops were recorded at different temperatures to investigate the ferroelectric and energy storage properties of BSTSn ceramic. Enhanced recoverable energy density and high energy storage efficiency were found to be 58.08 ​mJ/cm3 and 84.36%, respectively at room temperature (RT) under a moderate applied electric field of 20 ​kV/cm. The electrocaloric effect (ECE) in the BSTSn ceramic was explored using two different approaches based on P–E hysteresis loops, and pyroelectric current measurements. The largest electrocaloric (EC) temperature change, ΔTmax ​≈ ​0.63 ​K was determined using the Maxwell relationship obtained near RT under 20 ​kV/cm. The corresponding EC responsivity (ξmax) value of 0.31 ​K ​mm/kV is one of the highest reported values in lead-free ferroelectrics near RT. This study demonstrates that the BSTSn ceramic is a promising candidate for solid-state cooling and energy storage applications.

Pressure-induced room temperature electrocaloric effect in BiFeO3-PbTiO3 solid solution based on Landau-Devonshire theory

The electrocaloric effect cooling is a promising next-generation cooling technology due to its high energy efficiency. In this work, the phase diagrams of BiFeO3-PbTiO3 solid solution have been investigated via Landau-Devonshire theory. Here, in the vicinity of morphotropic phase boundary, a relatively low electric field of 5 MV/m induces a giant electrocaloric temperature change (ΔT = 23.56 K) in a 0.81BiFeO3-0.19PbTiO3 solid solution at 630 K. It is found that the range of temperature change of ECE is broader with the increase of BiFeO3 mole fraction. In addition, a broader ECE working temperature range can be obtained near the MPB region by applying hydrostatic pressure of 1 GPa. These results suggest that this system is a potential candidate for practical electrocaloric materials.

Intelligent self-actuating lead-free cooling ceramics based on A-site defect engineering

Current interests in electrocaloric (EC) and electromechanical (EM) effects have been stimulated by the discovery that self-actuating electrocaloric materials designed by using the electrostrain and entropy change (ΔS) characteristics of relaxor ferroelectrics are particularly appealing for applications in intelligent refrigeration devices. Herein, the lead-free Bi0.5Na0.5TiO3-based (BNT) ceramics designed by A-site defect engineering (ADE) can realize the collaborative EM and EC responses, e.g. obtaining electrostrain ∼ 0.47%, electrostrictive coefficient ∼ 0.031 m4 C−2 and EC temperature change ∼ -0.512 K at room temperature. The temperature-dependent polarization electric current behaviors and local structures of ADE system show slowly-evolving nonergodic-ergodic transitions, corresponding to the EC and EM behaviors with a wide operating temperature range (i.e. electrostrain > 0.3% and electrostrictive coefficient > 0.030 m4 C−2 within 105 °C, EC temperature change > 0.400 K within 50–125 °C). Landau theory shows that ADE method can reduce the barrier of high temperature disturbance. Furthermore, the uniform distributions of sodium and oxygen vacancies are conducive to the electric field response and orientation of the local dipoles. Most importantly, this work will further facilitate the development of self-actuating refrigeration materials/devices and provide an alternative concept to combine EM and EC effects by introducing A-site defects.

Electrocaloric effect and high energy storage efficiency in lead-free Ba0.95Ca0.05Ti0.89Sn0.11O3 ceramic elaborated by sol–gel method

Structural, dielectric, ferroelectric, energy storage properties, and electrocaloric effect were studied in lead-free ceramic Ba0.95Ca0.05Ti0.89Sn0.11O3 (BCTSn) elaborated by the sol–gel method. Phase purity structure was confirmed from X-ray data using the Rietveld refinement analysis which revealed the coexistence of tetragonal (P4mm) and orthorhombic (Amm2) symmetries at room temperature. Phase transitions were detected by dielectric and differential scanning calorimetry measurements. The energy storage properties were determined from P-E hysteresis, and the electrocaloric properties were calculated indirectly via the Maxwell approach. The large value of electrocaloric temperature change of ΔT = 0.807 K obtained at a relatively small electric field of 30 kV cm−1, and the high energy storage efficiency can make BCTSn ceramic a promising candidate for environmentally friendly refrigeration and energy storage applications.

Electrocaloric and barocaloric effects in CsH2PO4 ferroelectric

To investigate the caloric effects in the CsH2PO4 ferroelectric, a modified pseudospin model of this crystal is used, which takes into account the dependence of the parameters of interaction between pseudospins on lattice strains. The model also takes into account the dependence of the effective dipole moment of a pseudospin on the order parameter. In the two-particle cluster approximation, the influence of the longitudinal electric field and hydrostatic pressure on the molar entropy of the crystal was studied. The electrocaloric and barocaloric effects were studied. The calculated electrocaloric temperature change is about 1 K; it can change its sign under the influence of hydrostatic pressure. Barocaloric temperature change is about -0.5 K; lattice anharmonicities were not taken into account in its calculations.

Large electrocaloric effect with high thermal and electric field cycling stability in solution-processed Y:HfO2 thin films

Si/Y-HfO2/Pt shows a large electrocaloric temperature change (ΔT) of 24.84 K with high reliability on thermal and electrical field cycling.

Temperature Dependent Electrical and Electrocaloric Properties of Textured 0.72PMN - 0.28PT Ceramics*

Lead magnesium niobate (PMN) - lead titanate (PT) solid solution ceramics in the ratio of 0.72PMN-0.28PT was produced by a combination of tape-casting in ⟨001⟩pc textured and random forms. The Lotgering factor, f, of textured ceramics was approximately calculated as 80%. Modified Curie-Weiss analysis indicated relaxor dominant behavior for both the random and textured ceramics. Development of texture led to an enhancement in the electromechanical properties with converse piezoelectric charge coefficient (d33*) under 20 kV/cm electric field reaching 545 pm/V for the textured ceramic. Electrocaloric (EC) behavior of random and textured ceramics were obtained from indirect measurements using temperature dependent polarization vs. electric field hysteresis loops. An EC temperature change (ΔTEC) of ∼0.5 K was calculated from the PMN-28PT ceramics at around 80 °C under an electric field of 60 kV/cm. Development of texture was demonstrated to have led to an anisotropy in the EC response.

High-entropy polymer produces a giant electrocaloric effect at low fields.

More than a decade of research on the electrocaloric (EC) effect has resulted in EC materials and EC multilayer chips that satisfy a minimum EC temperature change of 5 K required for caloric heat pumps1-3. However, these EC temperature changes are generated through the application of high electric fields4-8 (close to their dielectric breakdown strengths), which result in rapid degradation and fatigue of EC performance. Here we report a class of EC polymer that exhibits an EC entropy change of 37.5 J kg-1 K-1 and a temperature change of 7.5 K under 50 MV m-1, a 275% enhancement over the state-of-the-art EC polymers under the same field strength. We show that converting a small number of the chlorofluoroethylene groups in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer into covalent double bonds markedly increases the number of the polar entities and enhances the polar-nonpolar interfacial areas of the polymer. The polar phases in the polymer adopt a loosely correlated, high-entropy state with a low energy barrier for electric-field-induced switching. The polymer maintains performance for more than one million cycles at the low fields necessary for practical EC cooling applications, suggesting that this strategy may yield materials suitable for use in caloric heat pumps.

Enhancement of the electrocaloric effect in PbZr0.7Ti0.3O3 ceramics via La doping: Driven by phase co-existence or defect effects?

Lattice defects and their effects have been pivotal in studies of phase transitions in a wide range of materials. Introduction of such defects into a ferroelectric material through doping of secondary elements can be tailored towards specific applications but the mechanism through which the bulk properties change is seldom scrutinized. Here we study the effect of systematic La substitution into PbZr0.7Ti0.3O3 (PZT 70/30) ceramics whereby we analyzed the temperature dependent properties and estimated the temperature changes that could be induced upon application of an external electric field, namely the electrocaloric effect (ECE). Expecting the entropic changes to be maximal under an applied field, the suitability of the La doped PZT 70/30 system for EC applications had been a motivation to undertake the current task as this composition reportedly can host a rich variety of phases depending on La content including relaxor and antiferroelectric (AFE) states. An electrocaloric (EC) temperature change of 1.15 °C in a wide range of temperatures for 8% La doping at 45 kV/cm applied field was estimated from experimental data, the possible origins of which is discussed. We were able to explain the experimental results by adopting a Landau-Ginzburg based computational approach coupled with elasticity and electrostatics whereby La sites are treated as point defects in a PZT 70/30 lattice. The gradual slanting of the hystereses and reduction of the transition temperature in the samples with increasing La content is claimed to be a direct consequence of the electrical fields due to formation of dipolar defect complexes as backed by our simulations. The ECE is discussed in the light of the simulations and recent results for AFE ceramics.

Structure, dielectric, electrostrictive and electrocaloric properties of environmentally friendly Bi-substituted BCZT ferroelectric ceramics

In this study, environmentally friendly Bi-substituted [(Ba0.85Ca0.15)1-3x/2Bix](Zr0.1Ti0.9)O3 (BCZT-xBi) ferroelectric ceramics with x = 0–0.08 were prepared using a solid-state sintering method. The structures, dielectric, electrostrictive and electrocaloric properties of the Bi-substituted BCZT ceramics were thoroughly investigated. With an increase in the Bi3+ content, the temperature corresponding to the maximum permittivity (Tm) decreased monotonously. Meanwhile, the broadening of the dielectric peaks and the increase in the relaxation coefficient of the ceramics transformed their typical ferroelectric-to-paraelectric phase transition to diffuse phase transition (DPT). This was further confirmed by the trends shown by the ferroelectric properties of the ceramics. The current peak intensity in current-electric field (I-E) curves decreased with an increase in x and finally became constant, indicating that domain reversal disappeared gradually. High electrostrictive coefficient Q33 values of 0.0307, 0.0299 and 0.0223 m4/C2 were obtained for the ceramics with x = 0.02, 0.04 and 0.06 respectively. The Q33 values of the ceramics indicated their temperature-insensitive nature over the temperature range of 30–120 °C. Although the maximum value of the electrocaloric adiabatic temperature change (ΔT) (0.91 K) was achieved at x = 0.02, the thermally stability of ΔT is improved with an increase in x from 0.02 to 0.06. The results indicated that x = 0.06 improved the thermal stability of the electrostrictive and electrocaloric performance. By increasing the driving field strength, better electrostrictive and electrocaloric responses could be achieved.

Enhanced room-temperature electrocaloric effect in B and Mn modified Ba0.7Sr0.3TiO3 ceramics

Ba0.7Sr0.3TiO3 (BST) ceramic and Ba0.7Sr0.3TiO3 with 1 mol% B3+ ions and 0.5 mol% Mn2+ ions (BSTBM) ceramic are synthesized using citrate combustion technique. An enhanced electrocaloric effect has been reported in BSTBM ceramic. The addition of B3+ ions and Mn2+ ions lowers the sintering temperature of BST ceramic from 1275 °C to 1200 °C. The low temperature sintered BSTBM ceramics exhibit enhanced electrocaloric temperature change (△T = 1.92 K) accompany with large electrocaloric efficiency (△T/△E = 0.38 K m MV−1) under 50 kV/cm. The △T of BSTBM ceramic is eighty five percent higher than the value of BST ceramic.

Tailoring electrocaloric properties of Ba1-xSrxSnyTi1-yO3 ceramics by compositional modification

This paper presents compositional modifications on lead-free material system based on barium titanate to optimize its electrocaloric (EC) properties. By addition of strontium and tin we prepared solid solutions of Ba1-xSrxSnyTi1-yO3 (BSSnT-100∙x-100∙y) which allow for shifting of maximum EC effect to room temperature as well as broadening the temperature range of high EC temperature changes ΔTEC. Bulk ceramic samples of BSSnT were prepared by solid-state reaction route and the influence of Sn:Sr ratio on dielectric and ferroelectric properties was investigated. The EC temperature change was measured directly using resistance welded thermocouple wires. Most promising results could be achieved for BSSnT-18−6.5, which showed a maximum ΔTEC of 0.49 K around 30 °C with ΔTEC > 0.3 K in the temperature range from 10 °C to 50 °C under a comparatively low electric field change of 2 V μm−1.

Electrocaloric Effect of (K1-xNax)NbO₃ Single Crystal.

The electrocaloric effect of (K1-xNax)NbO₃ single crystal is calculated based on Landau-Devonshire theory. The electrocaloric coefficient and adiabatic temperature change are calculated with the variation of temperature in rhombohedral, orthorhombic and tetragonal phase. A maximum of electrocaloric coefficient is obtained in each ferroelectric phase. Moreover, the tetragonal phase presents the larger electrocaloric coefficient and adiabatic temperature change than that rhombohedral and orthorhombic phase. However, the large electrocaloric effect is also obtained at room temperature with the adiabatic temperature change of 0.5 K, which tallies with the experimental findings.

Longitudinal and Transverse Electrocaloric Effects in Glycinium Phosphite Ferroelectric

A modified proton ordering model of glycinium phosphite ferroelectric, which involves the piezoelectric coupling of the proton and lattice subsystems, is used for the investigation of the electrocaloric effect. The model also accounts for the dependence of the effective dipole moment on a hydrogen bond on an order parameter, as well as a splitting of parameters of the interaction between pseudospins in the presence of shear stresses. In the two-particle cluster approximation, the influence of longitudinal and transverse electric fields on components of the polarization vector and the dielectric permittivity tensor, as well as on thermal characteristics of the crystal, is calculated. Longitudinal and transverse electrocaloric effects are studied. The calculated electrocaloric temperature change is quite small, about 1K; however, it can change its sign under the influence of a transverse field.

Investigation of the electrocaloric effect in BaTiO3 multilayers by pASC calorimetry

The electrocaloric heat production in \(\hbox {BaTiO}_3\)-based multilayer ceramic capacitors with Y5V specification was measured in a direct way by means of an adiabatic calorimeter setup. Applying an electric field of 30 \(\hbox {MV}\,\hbox {m}^{-1}\) is found to result in a heat release of 0.94 \(\hbox {J}\,\hbox {g}^{-1}\) and an electrocaloric temperature change of 0.46 K, in good agreement with direct results in previous studies.

Influence of Synthesis-Related Microstructural Features on the Electrocaloric Effect for 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 Ceramics

Despite having a very similar electrocaloric (EC) coefficient, i.e., the EC temperature change divided by the applied electric field, the 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 (PMN-10PT) ceramic prepared by mechanochemical synthesis exhibits a much higher EC temperature change than the columbite-derived version, i.e., 2.37 °C at 107 °C and 115 kV/cm. The difference is due to the almost two-times-higher breakdown field of the former material, 115 kV/cm, as opposed to 57 kV/cm in the latter. While both ceramic materials have similarly high relative densities and grain sizes (>96%, ≈5 μm) and an almost correct perovskite stoichiometry, the mechanochemical synthesis contributes to a lower level of compositional deviation. The peak permittivity and saturated polarization are slightly higher and the domain structure is finer in the mechanochemically derived ceramic. The secondary phases that result from each synthesis are identified and related to different interactions of the individual materials with the electric field: an intergranular lead-silicate-based phase in the columbite-derived PMN-10PT and MgO inclusions in the mechanochemically derived ceramic.

Highly Tunable Multifunctional BaTiO3-Based Ferroelectrics via Site Selective Doping Strategy

Electric-field-driven polarization in ferroelectrics delivers extraordinary electro-caloric, electro-optic, and electromechanical properties, etc. However, the controllable multifunctional ferroelectric switching between different electro-related performances still faces great challenges and deserves study. Herein, multifunctional BaTiO3 (BT)-based ferroelectrics are conceived tuning electrocaloric and electrostrictive properties by controlling doping site of dopants, namely site selective doping strategy. The well-concerned Ce is selected for its multivalent trait (Ce3+/Ce4+) and effective adjustment of Curie temperature (TC) in BT. Guided by the distinguished dielectric characteristics via doping Ce3+ in Ba site or Ce4+ in Ti site, BT-based ferroelectrics can direct at different applications. For Ba-site doping, in conjunction with ion-size mismatch effect and defect engineering, manipulating TC to room temperature (RT) and driving second-order-like phase transition are obtained simultaneously. Then, large electrocaloric temperature change (∆Tmax=1.3 K) over a broad temperature span (∆T>1 K, 37~70 ℃) is achieved, which is crucial in developing zero-global-warming-potential refrigeration near RT. For Ti-site doping, unique diffuse phase transition is observed without sacrificing TC, leading to multiphase structures and the formation of nanosize domains, hence facilitating polarization rotation. Therefore, an enhanced electrostrictive strain (S>0.17%) with ultra-low hysteresis (Hhyst<5%) among a wide temperature range is obtained, which challenges toxic lead-contained counterparts in actuators.

Mechanical control of electrocaloric response in epitaxial ferroelectric thin films

The electrocaloric (EC) effect in dielectrics has shown great potential in the next-generation solid-state refrigeration; however, most dielectrics cannot satisfy the requirement for refrigeration at various temperatures. How to control large EC response to various temperatures is a critical problem for practical cooling applications. In this work, based on entropy analysis, a nonlinear thermodynamic approach considering mechanical loading has been generalized to demonstrate mechanical control of EC response in epitaxial ferroelectric BaxSr1−xTiO3 (BST) thin films. The effects of chemical composition x, in-plane misfit strain um, and out-of-plane external uniaxial stress σ3 on the phase diagrams, phase transition temperatures, and EC response of BST thin films have been studied. The results reveal that a large EC temperature change ΔT appears in the vicinity of c-PE and r-aa phase boundaries, because the polarization component P3 becomes zero across both phase boundaries, leading to a large change of order in dipoles. It is found that the phase structures and the transition temperatures are sensitive to misfit strain and stress. Large EC response in BST thin films can be controlled and shifted to various temperatures over a wide temperature range for practical applications. It also indicates that the peak of ΔT can be shifted to higher temperature under in-plane compressive strain um, while it is shifted to lower temperature under tensile strain um or compressive stress σ3. Furthermore, the optimal combination of misfit strain and stress for the EC temperature change are identified, which further enhances EC response. These insights offer an alternative pathway to control and implement EC refrigeration over a wide range of temperature.

Electrocaloric cooling over high device temperature span

Summary Electrocaloric cooling demonstrates direct electricity utilization and high energy efficiency, and is often proposed as an environmentally benign, solid-state alternative to the conventional vapor compression cooling technology. As a result, the fabrication of effective electrocaloric materials and engineering of electrocaloric cooling devices have attracted increasing attention in the cooling community. However, the limited material adiabatic temperature change under safe operation field poses a major challenge to practical electrocaloric cooling device development, since real-world applications usually require device temperature span over 20 K. This perspective presents recent efforts devoted to design electrocaloric cooling devices based on active heat regeneration and cascading approaches; both strategies have achieved significant device temperature lift ~10 K with adiabatic temperature change of 2~3 K. The strategies discussed in this work are broadly applicable to the design of efficient and compact cooling systems.