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Electrocaloric refrigeration shows potential as a viable alternative to vapor-compression and/or thermo-electric refrigeration. One of the main challenges that need to be addressed in electrocaloric technology is the fatigue behavior of electrocaloric materials, in terms of both structural and functional aspects. Here, a comprehensive evaluation of the fatigue behavior of the 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 (PMN-10PT) bulk relaxor ferroelectric (or shortly relaxor) ceramic at room temperature is performed. First, the temperature-change dependence on the slew rate was studied. It is shown that the adiabatic conditions are well approached at the slew rate above 1 kV s−1, at which the adiabatic temperature change of 1.3 K was measured at the electric field change of 90 kV cm−1. Then, the durability limits (i.e., the fatigue life) of ten PMN-10PT samples were investigated during unipolar electric field cycling. The results showed that the material could withstand up to 106 cycles at the electric field change of 90 kV cm−1 with only minor degradation of the functional properties (less than 5% of the maximum adiabatic temperature change). Hence, PMN-10PT can be considered as a promising material for use in an electrocaloric cooling device, but some critical issues that caused premature failure of several samples would need to be further addressed and improved.

We report on numerical investigation of the electrocaloric effect, an important physical phenomenon in compositionally graded ferroelectric (CGFE) nanowires made of Pb ( 1 − x ) Sr x TiO 3 whose material compositions spatially vary along the longitudinal direction of nanowires. In this study, all numerical evidences are demonstrated through the phase field model based on the Ginzburg–Landau theory. Here, the change of polarization with temperature can be tailored by controlling the gradient of material compositions in CGFE nanowires. The isothermal entropy and adiabatic temperature changes are enhanced with an increasing compositional gradient in CGFE nanowires. The electrocaloric effect is found to have high magnitude near room temperature in CGFE nanowires of PbTiO 3 (top surface)/ SrTiO 3 (bottom surface), which is much larger than that in homogeneous ferroelectric nanowire with the same average material composition. In addition, the high electrocaloric effect in CGFE nanowires of PbTiO 3 / SrTiO 3 is achieved in a wide range near room temperature. We also discuss the potential of CGFE nanowires for electrocaloric cooling applications, and more importantly, this work, in essence, opens a new route to enhance the electrocaloric properties of ferroelectric nanowires near room temperature by controlling the gradient of material compositions.

The correlation between dielectric permittivity and electrocaloric (EC) temperature change (ΔTEC) has been investigated in (1 − x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 (PMN–100xPT, with x = 0, 0.05, and 0.10) relaxor ferroelectric ceramics. At a given electric field, both peak temperatures, including the temperature of the permittivity peak (Tm) and the temperature of the maximum of the ΔTEC (TEC max) increase with increasing PT content. The peak of the dielectric permittivity is, regardless of the applied electric field, always at a higher temperature than is the TEC max, and the temperature gap between both maxima progressively increases with increasing applied DC bias. This is particularly true above the threshold field, which induces the long-range ordered ferroelectric state. The results, which are explained in terms of the electric field–temperature phase diagram of relaxor systems, thus reveal that Tm can only roughly mark the temperature of the upper boundary of the temperature–electric field window, where the EC responsivity (ΔTEC/ΔE) is the highest.

The compositional manipulation of the multiphase coexistence in relaxor–ferroelectrics has been reported to be an effective approach to generate the giant electrocaloric effect. In this work, we systemically investigate the temperature-dependent electrocaloric effect in BaZr0.18Ti0.82O3–BaSn0.11Ti0.89O3 relaxors, where Zr and Sn are introduced into BaTiO3 to develop the multiphase coexistence at the ferroelectric–paraelectric transition. Through the direct measurement by the heat flux sensor, a large temperature change around 4 K under an electric field of 10 MV m−1, combined with a broad temperature span (20 °C–60 °C), has been observed. For comparison, the electrocaloric properties are also indirectly deduced based on Maxwell equations and Landau–Ginzburg–Devonshire phenomenological theory, and the validity of the indirect methods in this relaxor–ferroelectric system has been discussed. This work would shed light on developing giant electrocaloric materials with a wide operating temperature range.

The electrocaloric effect (ECE) in lead-free (Bi0.5Na0.5)TiO3-SrTiO3 (BNT-ST) ceramics was investigated by direct and indirect measurements. Polarization-electric field hysteresis loops of the BNT-ST ceramics changed from well-saturated shapes at room temperature to constricted shapes around the depolarization temperature due to an electric field-induced phase transition. A large adiabatic temperature change due to ECE (ΔTEC) was obtained around the phase transition temperature (TFR) by indirect measurements, which was confirmed by direct measurements. The 0.85BNT-0.15ST sample showed a large negative ΔTEC in the indirect measurements below TFR where the polarization increases, while the directly measured ΔTEC displayed positive values. It suggests that the indirect method using the Maxwell relation is not valid in the temperature range where the polarization increases with temperature. The X-ray diffraction structural refinements showed that the poled 0.85BNT-0.15ST sample has mixed phases of a rhombohedral R3C and a tetragonal P4bm in the temperature range where the polarization increases below TFR.

The incorporation of bulky defects into ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymers can result in a relaxor behavior, accompanied by outstanding electrocaloric properties near room temperature. However, it remains elusive whether ferroelectric transition occurs in relaxor terpolymers, which is critical for the design of electrocaloric cooling devices. In this work, we study the electrocaloric fatigue in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (61.8/30.4/7.8 mol. %) terpolymer, in which we revisit its controversial issue of the phase transition. We observe that the electrocaloric response of the terpolymers remains nearly constant (ΔS = 25.5 J K−1 kg−1, ΔT = 5.1 K at 100 MV m−1) within 50 cycles at different temperatures around room temperature. Moreover, we conduct temperature-dependent X-ray diffraction and Fourier-transform infrared spectroscopy measurements on the terpolymer to provide insights into the evolution in intermolecular lattice spacing and intramolecular conformation. We find no clear evidence of the ferroelectric-to-paraelectric phase transition near the temperature range, where differential scanning calorimeter measurement displays an endothermic peak.

A nonlinear thermodynamic analysis is adopted to study the electrocaloric effect of B a x S r 1 − x Ti O 3 (BST) solid solutions. The results reveal that the location of maximum adiabatic temperature change Δ T can be shifted from a high temperature to room temperature via tuning chemical composition x. Comparing the electrocaloric properties of BST with those values reported in other electrocaloric materials, it is found that BST with x = 0.7 shows a large electrocaloric response over a broad temperature range near room temperature and possesses a large electrocaloric strength Δ T / Δ E ∼ 1.978 × 10 − 6 K m/V due to a significant change in polarization with respect to temperature. This suggests that BST with high electrocaloric performance has potential for practical cooling applications.

Negative environmental consequences of traditional cooling technologies have spurred the development of newly emerged green cooling devices rooting in electrocaloric effect (ECE). The relaxor ferroelectrics represented by BaZrTiO3 ceramics are being actively explored as a promising candidate for constructing ECE cooling devices. However, the reported electrocaloric (EC) response of BaZrTiO3 is still limited, particularly under low and moderate electric fields, which greatly inhibits its practical application. In this work, we propose an interfacial coupling modulation approach to enhance the EC response of BaZrTiO3 by introducing composition homogeneous and composition gradient multilayered structures. A systematic work was performed to study the dielectric, ferroelectric, and EC performances of BaZrTiO3 multilayered ceramics consisting of BaZr0.20Ti0.80O3 and/or BaZr0.17Ti0.83O3. It was found that the optimized BaZr0.17Ti0.83O3/BaZr0.20Ti0.80O3 bilayered films displayed the maximum EC response of 4.1 K under 10 MV m−1 at ambient conditions, corresponding to a high level of the coefficient of performance up to 18.33. The results demonstrate the efficiency of interfacial coupling modulation in boosting the EC response of relaxor ferroelectric ceramics, opening up an avenue to explore advanced electrocaloric systems with a giant electrocaloric effect.Negative environmental consequences of traditional cooling technologies have spurred the development of newly emerged green cooling devices rooting in electrocaloric effect (ECE). The relaxor ferroelectrics represented by BaZrTiO3 ceramics are being actively explored as a promising candidate for constructing ECE cooling devices. However, the reported electrocaloric (EC) response of BaZrTiO3 is still limited, particularly under low and moderate electric fields, which greatly inhibits its practical application. In this work, we propose an interfacial coupling modulation approach to enhance the EC response of BaZrTiO3 by introducing composition homogeneous and composition gradient multilayered structures. A systematic work was performed to study the dielectric, ferroelectric, and EC performances of BaZrTiO3 multilayered ceramics consisting of BaZr0.20Ti0.80O3 and/or BaZr0.17Ti0.83O3. It was found that the optimized BaZr0.17Ti0.83O3/BaZr0.20Ti0.80O3 bilayered films displayed the maximum EC response of 4.1 K u...

The electrocaloric effect (ECE) in lead-free relaxor ferroelectrics has gained significant interest over the past decades. However, certain aspects of the ECE in relaxor ferroelectrics, such as Bi0.5Na0.5TiO3, remain poorly understood. In this work, we investigate the ECE by considering Bi0.5Na0.5TiO3-SrTiO3 (BNT-ST) as an example. The results show that, for BNT-0.25ST ceramics, the directly-measured ECE is optimal when the freezing temperature is tailored to be about room temperature. For this material, ΔT = 0.51 K under an electric field of 6 kV/mm and the ECE has excellent thermal stability (the instability η ≤ 20% in the range 30–120 °C). The addition of strontium increases the cubic-phase fraction, enhances the local random field, and changes the local structure, as clarified by in situ Raman spectroscopy and piezoelectric force morphology. In addition, we discuss in detail the correlation between the ECE and local structure. This work thus improves our understanding of the ECE in BNT-based materials for EC cooling technologies.The electrocaloric effect (ECE) in lead-free relaxor ferroelectrics has gained significant interest over the past decades. However, certain aspects of the ECE in relaxor ferroelectrics, such as Bi0.5Na0.5TiO3, remain poorly understood. In this work, we investigate the ECE by considering Bi0.5Na0.5TiO3-SrTiO3 (BNT-ST) as an example. The results show that, for BNT-0.25ST ceramics, the directly-measured ECE is optimal when the freezing temperature is tailored to be about room temperature. For this material, ΔT = 0.51 K under an electric field of 6 kV/mm and the ECE has excellent thermal stability (the instability η ≤ 20% in the range 30–120 °C). The addition of strontium increases the cubic-phase fraction, enhances the local random field, and changes the local structure, as clarified by in situ Raman spectroscopy and piezoelectric force morphology. In addition, we discuss in detail the correlation between the ECE and local structure. This work thus improves our understanding of the ECE in BNT-based materials...

A device concept of an electrocaloric solid-state refrigeration is presented in this paper. The core component of the device is flexural mode composite actuators, each of which is comprised of an electrocaloric layer and an elastic substrate layer bonded together. The composite actuators have an electric field induced temperature change due to the electrocaloric effect and also have large electric field induced flexural deformation due to the converse piezoelectric effect, which enables the device to bend to contact with the heat source (or heat sink) for transferring heat. An analytical model is derived by considering multi-physical couplings for an edge-clamped circular composite device, which can accurately predict the temperature change of the device as compared with the indirect approach derived from the Maxwell relation. The model shows that the temperature change is a combinatorial result from the couplings of thermal, electric, and mechanical field in the device. Moreover, the model sheds light on exploring the optimization of the solid-state refrigeration device and indicates that different thickness ratios and radius ratios of the composite actuators have a large influence on the cooling performance of the refrigerator.

The electrocaloric effect (ECE) is the temperature change in a material induced by electrical field variation under adiabatic condition. Considering an external electric load applied on a cracked ferroelectric solid, a non-uniform electric field would be induced at the crack tip, and thus, incompatible strain field and local stress concentration would be generated around it. Furthermore, the enormous strain energy and the electrostatic energy would affect the polarization switching of the ferroelectric solid, important for the electrocaloric response. In this paper, the large negative and positive ECEs in a ferroelectric sheet with a conducting crack are investigated by the phase field method with the consideration of time-dependent Ginzburg-Landau equation. The numerical calculations indicated that the polarization field generates a sharp rise during the domain transition from polydomain to monodomain under a certain electric load. Large negative ECEs, about –10.21 K and –7.55 K, are obtained at 135 °C and 85 °C, respectively. The domain transition temperature is much lower than the Curie temperature, which enlarges the existence scope of the large ECE in ferroelectrics. The results also imply that the domain transition from a multi-domain state to a single domain takes place with the minimization of total free energy, which involves the courses of the electric field, stress field, temperature, and polarization interaction. Therefore, the non-uniform distributions of the stress-electric fields induced by the crack play an important role in ECE.The electrocaloric effect (ECE) is the temperature change in a material induced by electrical field variation under adiabatic condition. Considering an external electric load applied on a cracked ferroelectric solid, a non-uniform electric field would be induced at the crack tip, and thus, incompatible strain field and local stress concentration would be generated around it. Furthermore, the enormous strain energy and the electrostatic energy would affect the polarization switching of the ferroelectric solid, important for the electrocaloric response. In this paper, the large negative and positive ECEs in a ferroelectric sheet with a conducting crack are investigated by the phase field method with the consideration of time-dependent Ginzburg-Landau equation. The numerical calculations indicated that the polarization field generates a sharp rise during the domain transition from polydomain to monodomain unde...

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.

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.

In ferroelectric ultrathin films, built-in electric fields are often present due to the flexoelectric effect and the difference of work functions at asymmetric electrodes, which may change the properties of the materials. In this paper, the influence of build-in electric fields induced by flexoelectric effect and/or work function difference on the misfit strain-temperature phase diagrams, and the electrocaloric properties of epitaxial BaTiO3 ultrathin films are investigated by using an extended nonlinear thermodynamic theory. It is found that the flexoelectric effect, i.e., the coupling of polarization and strain gradient, changes the misfit strain-temperature phase diagrams notably, in which the phases with out-of-plane polarizations increase due to the presence of a built-in field. The electrocaloric properties are remarkably enhanced when the built-in fields induced by both the flexoelectric effect and work function difference are considered. In particular, a giant adiabatic temperature change of 7.89 K in ultrathin Pt/BaTiO3/SrRuO3 capacitors at 460 K is predicted. Moreover, it is demonstrated that the peak of adiabatic temperature change versus working temperature is shifted from a high temperature to room temperature, suggesting that ferroelectric ultrathin films with asymmetric electrodes and strain gradient are promising candidates for room temperature refrigeration.

In multiferroic materials, adiabatic temperature changes can be obtained by the combined application of electric, stress, and magnetic fields. These external stimuli provide additional channels of entropy variations resulting in a multi-caloric response. In ferroelectric (FE) materials, caloric responses can be obtained with the application of electric and mechanical fields. Here, we compute the intrinsic elastocaloric and stress–mediated electrocaloric behavior of prototypical FE materials using the Landau–Devonshire theory of phase transformations with appropriate electrical and electro–mechanical boundary conditions. We show that an elastocaloric adiabatic temperature variation of 12.7 °C can be obtained in PbTiO3 with the application of uniaxial tensile stress of 500 MPa near its Curie point. This is 59% higher than its pure intrinsic electrocaloric response for an electric field difference of 100 kV/cm. Moreover, external stresses allow the maximum electro–elastocaloric response to be tuned towards r...

Direct and indirect electrocaloric measurements were performed on the new Tellurium (Te) doped Ba0.8Ca0.2TiO3 (BCT) ceramics. The effects of Te addition on structural, electrical, and electrocaloric properties of BCT ceramics were investigated. The incorporation of the Te element in the BCT induced the decrease of the Curie temperature and the enhancement of the electrocaloric effect. The significant electrocaloric temperature change ΔT = 1.237 K determined by the direct method was obtained at the relatively moderate field of ∼25 kV/cm in Ba0.8Ca0.2Ti(1−x)TexO3 with x = 0.02. The corresponding electrocaloric responsivity ΔT/ΔE = 0.495 × 10−6 K m V−1 is higher than that observed in pure BCT ceramics and is one of the highest reported so far in lead-free ferroelectric materials. The material's coefficient of performance was determined at the phase transition with a maximal value of 14.7.

A detailed formula for the equivalent pyroelectric coefficient of a bimorph structure acting as a pyroelectric detector was deduced. It is shown that electric fields occur in the structure when it is subjected to a temperature variation, due to the unequal charges generated by the two components. The field dependence of the ferroelectric polarization has to be considered in this case and the (∂P/∂E) derivative appears explicitly in the deduced formula. The effect is that the equivalent pyroelectric coefficient has different values for heating and cooling. This could lead to a redressing effect on the pyroelectric signal if the temperature difference varies periodically. The equivalent pyroelectric coefficient is, also, thickness dependent. This fact offers the possibility of designing structures with high values of pyroelectric coefficient compared with the values of the component phases.

The temperature dependences of heat expansion, elastocaloric effect, magnetocaloric effect, and the shift in the critical temperature of the transition due to tensile stress have been measured using samples of the same concentration Fe49Rh51 in the antiferromagnetic‐ferromagnetic (AF‐F) transition range. Using data on specific heat and magnetocaloric effects, entropy‐temperature diagrams for the alloy were made for various magnetic fields. The ratios ΔS(TcH)/TcH, where ΔS(TcH) is the entropy change during transition and TcH is the transition’s critical temperature, were found to be ∼3.98×102 erg/g K2 which is close to the value of the change in the electronic specific heat coefficient Δγ obtained by other researchers. It has been concluded that the change in the electronic part of entropy is the main mechanism of the transition. The phenomenological model is proposed, taking into account the electronic entropy change during the transition. Calculations using the model give values for the main thermodynamic parameters of the transition (free energy change ΔF=1.91×106 erg/g, the critical temperature shift ∂Tc/∂P=3.08×10−9 K cm2/Dyn and ∂Tc/∂H=0.788 K/kOe due to hydrostatic pressure and magnetic field respectively), which are in agreement with experimental data.

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.

We explore the intrinsic feature of electrocaloric effect (ECE) accompanied by ferroelectric (FE)-paraelectric (PE) transition for displacive-type organic ferroelectrics using Green's function theory. It is demonstrated that decreasing elastic constant K or increasing spin-lattice coupling λ can enhance the ECE, as well as polarization P and transition temperature TC. Indeed, one expects that the optimal operating temperature for solid-state refrigeration is around room temperature, at which the ECE achieves its maximum. As TC is tuned to ∼310 K, it presents larger ECE response and remanent polarization with lower coercive field for smaller K value, suggesting that well flexible displacive-type organic ferroelectrics are excellent candidates both for electric cooling and data storage in the design of nonvolatile FE random-access memories. Furthermore, in an electric field, it provides a bridge between a Widom line that denotes FE-PE crossover above TC and a metaelectric transition line below TC that demon...

Ferroelectrics form an important class of materials and are employed for a variety of applications. However, specific applications dictate the need of tailored ferroelectric response. This creates a requirement to obtain ferroelectric materials with tunable properties. Generally, chemical modifications or domain engineering are employed to this effect. This study attempts to shed light on the use of compressive pre-stresses for tuning and enhancing the ferroelectric properties. For the purpose, polarization versus electric field hysteresis data for 68Pb(Mn1/3Nb2/3)O3-32PbTiO3 (PMN-PT) single crystals were obtained as a function of uniaxial compressive stresses and operating temperatures. These data were utilized to investigate the effects of mechanical confinement for four individual case studies of electrocaloric effect, electrical energy storage, pyroelectric, and piezoelectric effect. A significant improvement was obtained for all case studies. The adiabatic temperature change was improved by ≈80% (28 ...

We report thermal to electrical energy conversion from a 150 nm thick BaTiO3 film using pyroelectric cycles at 1 kHz. A microfabricated platform enables temperature and electric field control with temporal resolution near 1 μs. The rapid electric field changes as high as 11 × 105 kV/cm-s, and temperature change rates as high as 6 × 105 K/s allow exploration of pyroelectric cycles in a previously unexplored operating regime. We investigated the effect of phase difference between electric field and temperature cycles, and electric field and temperature change rates on the electrical energy generated from thermal-electrical cycles based on the pyroelectric Ericsson cycle. Complete thermodynamic cycles are possible up to the highest cycle rates tested here, and the energy density varies significantly with phase shifts between temperature and electric field waveforms. This work could facilitate the design and operation of pyroelectric cycles at high cycle rates, and aid in the design of new pyroelectric systems.

One of the induced multiferroic materials, CuO has the magnetic as well as ferroelectric phase transition at same temperature (TN1 ∼ 213 K, TC and TN2 ∼ 230 K). These type of materials can show two types of entropy; magnetic field induced entropy (Magnetocaloric Effect) as well as electeric field induced entropy (Electrocaloric Effect). The presence of both type of entropy may be called “Multicaloric Entropy” (M. Vopson, Solid State Commun. 152, 2067 (2012) and Meng et al., Phys. Lett. A 377, 567 (2013)). We observed that “Multicaloric Entropy” in the induced multiferroic materials also depends on the magnetoelectric interaction (γ). Therofore, this numerical attempt to calculate the entropy of CuO may be useful for the future “electro-magnetic” based refrigerator technology.

A novel finite element model to simulate the electrocaloric response of a multilayer ceramic capacitor (MLCC) under real environment and operational conditions has been developed. The two-dimensional transient conductive heat transfer model presented includes the electrocaloric effect as a source term, as well as accounting for radiative and convective effects. The model has been validated with experimental data obtained from the direct imaging of MLCC transient temperature variation under application of an electric field. The good agreement between simulated and experimental data, suggests that the novel experimental direct measurement methodology and the finite element model could be used to support the design of optimised electrocaloric units and operating conditions.

The electrocaloric properties of multi-domain ferroelectrics are investigated using a phase field model. The simulation results show that the extrinsic contribution from the multi-domain to mono-domain transition driven by temperature significantly enhances the electrocaloric response. Due to the abrupt decrease of polarization in the direction of electric field during the domain transition, a large adiabatic temperature change is achieved for the ferroelectrics subjected to a tensile strain. Furthermore, the domain transition temperature can be tuned by external strains as the phase transition temperature. A compressive strain decreases the domain transition temperature while a tensile strain increases it. The large temperature change associated with the domain transition provides guidance to engineer domain structures by strain to optimize the electrocaloric properties of ferroelectric materials below the Curie temperature.

We report the influence of blending the poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) terpolymer [P(VDF–TrFE–CFE), a member of the relaxor polymer family that exhibits fast response speeds, giant electrostriction, high electric energy density, and large electrocaloric effect] with the ferroelectric poly(vinylidene fluoride–trifluoroethylene) copolymer [P(VDF–TrFE)] on its dielectric response. Although both components form separate crystalline phases, at low copolymer content, the P(VDF–TrFE–CFE)/P(VDF–TrFE) blends entirely exhibit a relaxorlike linear dielectric response, since the interfacial couplings to the bulky defects in the terpolymer convert the normal ferroelectric copolymer into a relaxor. On the other hand, the linear and particularly nonlinear dielectric experiments, i.e., temperature dependences of the second and the third harmonic dielectric response, clearly evidence that in blends with 20–50 wt. % of P(VDF–TrFE), the ferroelectric and relaxor states coexist. The nonlinear dielectric response further reveals the onset of ferroelectric behavior also in blends with low copolymer amount, due to a high VDF content in the terpolymer, which increases the ferroelectric interactions: While in relaxor polymers with lower VDF content, the third order nonlinear dielectric constant, in accordance with the theoretical predictions, exhibits solely positive values, here it changes sign even in the pure P(VDF–TrFE–CFE).

The microscopic mechanisms of the negative electrocaloric effect (ECE) of the single-phase (1−x)(0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-xBa1/2Sr1/2TiO3 (BNT-BT-BST) perovskite solid solutions fabricated via the sol-gel technique are explored in this study. Dielectric and mechanical relaxation analyses are employed to investigate the ferroelectric and structural transitions of the samples. The electrocaloric properties of the samples were measured by thermodynamics Maxwell relations. The difference between the depolarization temperature (Td) and the maximum dielectric constant temperature (Tm) was found to decrease with increasing BST content. Doping with BST stabilized the ferroelectric phase along with unifying the EC temperature changes (ΔT) to only negative values. The origin of the uniform negative ECE of BNT-BT-BST is discussed.

The electrocaloric effect (ECE) in the Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) solid solution system was investigated by means of detailed direct temperature measurements as a function of temperature, composition, and electric field. The (1−x)PMN-xPT ceramics of compositions 0 ≤ x ≤ 0.3 were fabricated by the columbite route. In opposite to conventional ferroelectrics, the maximum of electrocaloric effect was found to shift from the proximity of depolarization/Curie temperature to higher temperatures above a certain composition-dependent electric field strengths. Especially, the compositions with low PT content showed a broadened temperature range of electrocaloric effect. With increasing PbTiO3 concentration, the magnitude of ΔT increased, and the temperature dependence of the maximum ECE response gradually developed towards a more pronounced anomaly typical for conventional ferroelectrics. The arising high temperature electrocaloric effect in the ergodic relaxor phase was attributed to the contribution from po...

The influence of intrinsic surface tension on the electrocaloric effect (ECE) in the ferroelectric nanomaterial with vortex domain structures is studied by using the phase field method. The calculation results show that a giant adiabatic temperature change (ΔT = 5.8 K) related to the toroidal moment change appears in the PbTiO3 (PTO) ferroelectric nanoparticle with the surface tension coefficient μ = 5 N/m under the vorticity vector of curled electric field (Q1 = 0 mV/A2, ΔQ1 = 0.9 mV/A2) at room temperature. The magnitude of the adiabatic temperature change decreases with the increase in surface tension. The decrease in size is found to enhance the ECE of PTO nanoparticle with vortex domain structures when the surface tension is not considered.

A direct electrocaloric effect (ECE) measurement system, based on a modified-differential scanning calorimeter (DSC), allowing the acquisition of both thermal (ECE, heat capacity) and electrical (P-E loops, leakage current) information simultaneously, was used to analyze 〈001〉-oriented PbMg1/3Nb2/3O3-30PbTiO3 single crystals. Different electric-field-induced phase transitions were identified on direct ECE measurements and confirmed by dielectric measurements. The strongest ECE (ΔTEC = 0.65 K) was measured for an applied electric field E = 10 kV/cm just above the temperature of depolarization. The direct ECE measurements were compared with indirect measurements obtained from dielectric polarization measurements versus electric field and temperature and a very good agreement was found. A region with negative ΔTEC was identified by both direct and indirect measurements. This phenomenon was attributed to the formation of a reversible field-induced phase transition towards a state with a different polar direction.

Electrically induced behavior was investigated and compared in the non-stretched and uniaxially stretched poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE) terpolymer—a member of the P(VDF-TrFE)-based relaxor polymers family that exhibits fast response speeds, giant electrostriction, high electric energy density, and large electrocaloric effect. Although the temperature dependence of the low-field complex dielectric constant is almost identical in the non-stretched and stretched samples, the dc bias electric field via higher nonlinear contribution more heavily alters the dielectric response of the non-stretched terpolymer. The polarization response and, particularly, the induced electrostrictive strain are, on the other hand, much higher in the more-oriented stretched films. The changes in polar-correlation range induced by film stretching also strongly influence the directly measured electrocaloric response, which shows more pronounced temperature dependence in the stretched terpolymer. These results suggest that electrically induced properties of relaxor polymer films can be tailored by controlling the preparation conditions.

The electrocaloric effect (ECE) of BaTiO3multilayer thick film was investigated by direct calorimetric measurement. The ECE increases monotonically with the enhancement of applied field. The maximum ECE occurs above T c and shifts to higher temperature with increasing applied field. Under an ultrahigh field of 800 kV/cm, it exhibits a giant ECE of ΔT = 7.1 K and ΔS = 10.1 J/kg · K at 80 °C. The ECE heat follows a general power-law relation with the varying rate of applied field within a certain range. A high cooling power (∼50 W/kg) is achieved based on the net-cooling resulting from the different varying rates of rising and falling fields.

The electrocaloric effect (ECE), i.e., the conversion of electric energy into heat, is of great importance for application in new generation cooling or heating devices that would be friendlier to the environment. Here, utilizing direct measurements of the ECE change of the temperature ΔT via a high resolution calorimeter, we study the ECE as a function of the magnitude of the electric-field step E in the vicinity of the critical point in several bulk relaxor ferroelectric ceramic systems. Relatively large ΔT of ∼2 to 3 K were obtained at modest fields of 90 kV/cm, even in the case of ceramic materials. The effective responsivity ΔT/E as a function of the electric field shows a characteristic peak near the critical point, which demonstrates the importance of proximity to the critical point for the enhancement of the electrocaloric effect. Experimental results are in good agreement with the theoretical calculations based on the spherical random-bond random-field model.

(100)-oriented Pb0.97La0.02(Zr0.95Ti0.05)O3 (PLZT) antiferroelectric films with a thickness of about 1.7 μm were deposited on Pt-buffered silicon substrates via a sol-gel process. The room-temperature capacitance density of the films was above 195 nF/cm2 over the frequency range from 1 kHz to 1 MHz. The recoverable energy density was enlarged with increasing measurement field, and a maximum value of 12.4 J/cm3 at 1120 kV/cm was obtained at room temperature. From the temperature-dependent electric-field-induced polarization hysteresis loops, a maximum reversible adiabatic temperature change, ΔT = 8.5 °C, was obtained near the phase-transition temperature.

The effects of misfit strain on the electrocaloric (EC) properties of BaTiO3 thin films arestudied by using the Landau-Devonshire thermodynamic theory. The “misfit-strain temperature” phase diagrams for different electric fields are constructed. It was found that the EC effect of a BaTiO3 thin film is strongly dependent on misfit strain. More interestingly, we found that BaTiO3 thin films with tensile misfit strain exhibit a sizable EC effect in a wider temperature range in comparison with those with compressive misfit strain.

The modified transverse Ising model taking into account the four-spin exchange interaction and quantum fluctuation, as well as the mechanical constraint of the substrate, is constructed and applied to investigate the electrocaloric effect (ECE) in BaTiO3 thin films. It is found that the temperature dependence of ECE strongly depends on both the four-spin exchange interaction and quantum fluctuation. Most importantly, we achieve the adiabatic temperature change ΔT=11.78 K at Tm=490 K, which can be comparable with that observed experimentally in PbZr0.95Ti0.05O3 thin films and ferroelectric polymers. Furthermore, the internal stresses resulting from the clamping effect of the substrate play a crucial role in the ECE of ferroelectric films. Control of the misfit stress by appropriate choice of substrate provides an effective means to improve the adiabatic temperature change for use in cooling or thermodielectric power conversion devices.

The electrocaloric effect and dielectric tunability of BaTiO3 ferroelectric nanoshells on Si and MgO cores are investigated using the modified Landau–Ginzburg–Devonshire theory, in which the surface tension and thermal strain are taken into account. The numerical results exhibit a peak of electrocaloric coefficient near the critical nanoshell thickness accompanied with the size-driven phase transition. In addition to the enhanced adiabatic temperature difference, the compressive thermal strain also significantly improves the dielectric tunability. More importantly, the ferroelectric nanoshell displays pronounced electrocaloric effect: ΔT(Tm)=2.09 K for the nanoshell on Si core and ΔT(Tm)=2.33 K on MgO core, respectively. Essentially, the ferroelectric nanoshell provides an effective means to acquire good electrocaloric effect and high dielectric tunability by adjusting the wall thickness, core radius, annealing temperature, and various core materials, which may effectively contribute to the stress level i...

A modified Landau–Devonshire thermodynamic theory is employed to investigate the effect of grain size on the electrocaloric effect of dense BaTiO3 nanoceramics. Theoretical analysis indicates that both the magnitude of the electrocaloric effect and the temperature corresponding to the maximum of electrocaloric effect depend on not only the grain size but also the electric field. Most importantly, BaTiO3 nanoceramics have the giant electrocaloric coefficient and adiabatic temperature change. Moreover, it is worth noting that the decrease of grain size reduces the temperature to a great extent where the electrocaloric effect is maximized. Therefore, control of the grain size may be used to adjust the electrocaloric properties of dense BaTiO3 nanoceramics.

A phenomenological Landau–Devonshire thermodynamic theory is employed to investigate the effect of the misfit strain on the electrocaloric effect of single domain epitaxial Pb(Zr1−xTix)O3 thin films grown on thick cubic substrates. Theoretical analysis indicates that the epitaxial Pb(Zr1−xTix)O3 thin films have the large electrocaloric coefficient at room temperature and the electrocaloric properties largely depend on the misfit strain and the applied electric field. Furthermore, the applied field decreases the electrocaloric coefficient and reduces its misfit strain dependence.

Based on the phenomenological Landau–Devonshire thermodynamic theory, the film thickness dependence of electrocaloric effect in epitaxial Ba0.6Sr0.4TiO3 thin films is investigated. Theoretical analysis shows that the Ba0.6Sr0.4TiO3 thin films grown on LaAlO3 and MgO substrates have the giant electrocaloric effect at room temperature. Our theoretical calculations also indicate that the entropy and electrocaloric effect strongly depend on film thickness.

A thermodynamic analysis of the electrocaloric (EC) effect in BaTiO3 ferroelectric thin films has been carried out under differing mechanical boundary conditions. It is shown that both the magnitude of the electrocaloric effect and temperature at which it is maximized depend not only on the extent of the applied field change but also on the value of the initial field. For initial fields smaller than a critical value the EC effect is largest at the phase transition temperature but the effect is a strong function of temperature. For external electrical fields larger than this value, conversely, the EC effect is the largest at a higher temperature and is a weak function of temperature. Perfect lateral clamping transforms the first-order phase transition into a second-order transition, lowering the magnitude of the electrocaloric effect and dependence on temperature. Compressive and tensile misfit strains also alter the nature of the phase transition and affect the electrocaloric properties in an analogous way. A compressive misfit strain shifts the maximum in the EC effect to higher temperatures, reduces its magnitude, and reduces its dependence on temperature, while tensile misfit strain results in the opposite effects. Control of the misfit strain by appropriate choice of substrate provides potential means to vary both the magnitude and the temperature sensitivity of the EC effect for use in cooling or thermodielectric power conversion devices.

Pyroelectric and electrocaloric characterization has been determined for 0.75Pb(Mg1∕3Nb2∕3)O3–0.25PbTiO3 relaxor based single crystal and ceramic. Differential scanning calorimetry was used for measuring the electrocaloric response for different electric fields in the vicinity of the Curie temperature. For both ceramic and crystals the maximum activity is found to be around the transition temperature. On the other hand hysteresis loops for different temperatures were used to predict the electrocaloric effect with very good qualitative agreements with direct measurements. Pyroelectric coefficient is found to be much larger for ⟨111⟩ single crystals reaching 1300×10−6Cm−2K−1 whereas the ceramic reaches only 750×10−6Cm−2K−1. Higher pyroelectric coefficient and lower dielectric permittivity lead to outstanding figures of merits for sensors and energy harvesting, with a gain of 260% for voltage responsivity and more than 500% for energy harvesting. Although having a much larger pyroelectric activity, the elect...

Variations of sample temperature arising from the electrocaloric effect, antiferroelectric to ferroelectric transitions, and hysteresis‐loss heating have been recorded together with the hysteresis loops for four different ceramic ferroelectrics near room temperature. Both the electric‐field dependence and the temperature dependence of the electrocaloric effect were accurately predicted by the usual thermodynamic expression for the effect. Values for the heat capacity and the pyroelectric constant were also obtained from the data. The sample temperature of one composition which becomes antiferroelectric under zero applied field near 40°C was monitored at various temperatures as antiferroelectric to ferroelectric transitions were forced by an electric field. Above 55°C, the temperature change at the transition could be fitted with a theory originally developed for the field‐enforced antiferroelectric to ferroelectric transition in PbZrO3.