Electrocaloric Response Research Articles

Electrocaloric properties of ferroelectric-paraelectric superlattices controlled by the thickness of paraelectric layer in a wide temperature range

As functions of the paraelectric layer thickness, misfit strain and temperature, the electrocaloric properties of ferroelectric-paraelectric superlattices are investigated using a time-dependent Ginzburg-Landau thermodynamic model. Ferroelectric phase transition driven by the relative thickness of the superlattice is found to dramatically impact the electrocaloric response. Near the phase transition temperature, the magnitude of the electrocaloric effect is maximized and shifted to lower temperatures by increasing the relative thickness of paraelectric layer. Theoretical calculations also imply that the electrocaloric effect of the superlattices depends not only on the relative thickness of paraelectric layer but also on misfit strain. Furthermore, control of the relative thickness of paraelectric layer and the misfit strain can change availably both the magnitude and the temperature sensitivity of the electrocaloric effect, which suggests that ferroelectric-paraelectric superlattices may be promising candidates for use in cooling devices in a wide temperature range.

Enhanced electrocaloric effect in lead-free BaTi1−xSnxO3 ceramics near room temperature

The electrocaloric effect in lead-free BaTi1−xSnxO3 (BTSn, x = 0.08, 0.105, and 0.14) ferroelectric ceramics was studied by using an indirect method. It was found that the largest electrocaloric response could be achieved in BTSn with x = xQP = 0.105 near room temperature with an adiabatic temperature change ΔT of 0.61 K and an electrocaloric strength ΔT/ΔE of 0.31 K mm kV−1, under a modest electric field ΔE of 20 kV cm−1, which is comparable with the best values reported in lead-free materials. These enhanced values are attributed to the multiphase (four phases) coexistence at x = xQP corresponding to the quasi-quadruple point composition.

Enhanced electrocaloric and pyroelectric response from ferroelectric multilayers

Room temperature pyroelectric properties and adiabatic temperature change of (001)-textured ferroelectric multilayers on Si are computed by taking into account electrostatic interlayer interactions and thermal strains. We show that by adjusting internal electrical fields through changing relative thicknesses in a multilayer ferroelectric construct, electrothermal properties can be significantly enhanced. A quantitative analysis is provided for BaTiO3-PbZr0.2Ti0.8O3 (BTO-PZT) and SrTiO3-PbZr0.2Ti0.8O3 (STO-PZT) multilayers. For instance, 0.74 × BTO-0.26 × PZT and 0.35 × STO-0.65 × PZT bilayers show ∼120% and 65% increase in electrocaloric response, respectively, compared to PZT films on Si for ΔE = 500 kV/cm.

Experimentally validated finite element model of electrocaloric multilayer ceramic structures

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.

Large electrocaloric effect induced by the multi-domain to mono-domain transition in ferroelectrics

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.

Giant mechanically-mediated electrocaloric effect in ultrathin ferroelectric capacitors at room temperature

Using a phenomenological approach, we demonstrate that a giant mechanically-mediated electrocaloric effect can be obtained in ultrathin ferroelectric SrRuO3/BaTiO3/SrRuO3 capacitors at room temperature. Our results show that the electrocaloric properties of such capacitors can be systematically tuned by applying an external stress. The depolarizing field, whose effect is usually ignored in the literature, is found to be detrimental to the electrocaloric response, especially for the thinner films. Moreover, a remarkable enhancement and broadening of the electrocaloric response can be achieved in relatively thick films under compressively loaded conditions compared with the unloaded case.

Electrical and thermal properties of vinylidene fluoride–trifluoroethylene-based polymer system with coexisting ferroelectric and relaxor states

Dielectric, thermal, and electrocaloric investigations of poly(vinylidene fluoride–trifluoroethylene) P(VDF–TrFE) copolymer, irradiated with high-energy electrons, are reported. While the ferroelectric copolymer is transformed into a relaxor system at high irradiation doses, dielectric investigations, particularly nonlinear dielectric experiments, i.e., temperature dependences of the second and the third harmonic dielectric response, clearly evidence that at lower doses ferroelectric and relaxor states coexist in the P(VDF–TrFE). This is confirmed by the differential scanning calorimetry, which further reveals the influence of irradiation on the copolymer crystallinity and melting point. Finally, it is shown that large electrocaloric response of VDF–TrFE-based polymers is further enhanced in systems with coexisting relaxor and normal ferroelectric states.

Performance enhancement of cylindrical ferroelectric transducers

Abstract In this paper, the feasibility of using ferroelectric materials as a thermal transducer based on electrocaloric effect (ECE) has been studied. The electrocaloric response of the ferroelectric capacitor PMN-25PT which is a ceramic with the formula of 0.75Pb(Mg 1/3 Nb 2/3 )O 3 –0.25PbTiO 3 and the dynamics of temperature variations at the inner boundary of a cylindrical sample to an applied periodic electric field have been studied. Alternative switching of the electrocaloric element allow the generation of directed heat flux. At first the outer boundary of the sample is put in convection condition and then in constant temperature condition. Inner boundary is insulated in two cases. Results show that different boundary conditions affect the transducer performance.

Impact of critical point on piezoelectric and electrocaloric response in barium titanate

The prediction that the piezoelectric tensor is diverging at the critical point is verified for BaTiO${}_{3}$ (BTO), which is the oldest perovskite structured ferroelectric material with an extremely long and eventful research history. Here we investigate experimentally by dielectric and calorimetric measurements the existence and the position of the critical point in the electric-field-temperature ($E$-$T$) phase diagram of BTO in the vicinity of the paraelectric to ferroelectric phase transition. Measurements of the piezoelectric coefficient ${d}_{31}$ as a function of the temperature and the electric field applied along the [001] direction show a critical enhancement of the piezoelectric response in the vicinity of the critical point, in agreement with recent calculations by Porta et al. [J. Phys.: Condens. Matter 22, 345902 (2010)]. The electrocaloric responsivity is found to be enhanced due to the latent heat on the paraelectric to ferroelectric transition locus below the critical point.

Computational modeling the electrocaloric effect for solid-state refrigeration

ABSTRACTThe electrocaloric effect holds promise for possible application in refrigeration technologies. There is much interest in this subject and experimental studies have shown the possibility for creating materials with a modest sized electrocaloric response. However, theoretical studies lag behind the experimental effort due to the lack of computational methods to accurately study the finite temperature response. Here the freely distributed feram, an effective Hamiltonian molecular dynamics method, is demonstrated for predicting the electrocaloric response of BaTiO3.

Electrocaloric Response of Ferroelectric Material Applicable as Electrothermal Transducer

Electrocaloric response of the PMN-10PT is measured experimentally and compared with the numerical results. Based on the compatibility of the experimental and numerical results, feasibility of using ferroelectric materials as an electrothermal transducer has been investigated. In this study, electrocaloric response of three different ferroelectric capacitors (PMN-10PT, PMN-25PT, and PZN-4.5PT) under an applied periodic electric field have been investigated. Alternative switching of the electrocaloric elements with specific boundary conditions generates a directed heat flux. It can be concluded that each ferroelectric material can be used as a transducer in a special temperature range that in which it has good electrocaloric response.

Giant Electrocaloric Effect AroundTc

We use molecular dynamics with a first-principles-based shell model potential to study the electrocaloric effect (ECE) in lithium niobate, LiNbO(3), and find a giant electrocaloric effect along a line passing through the ferroelectric transition. With an applied electric field, a line of maximum ECE passes through the zero field ferroelectric transition, continuing along a Widom line at high temperatures with increasing fields, and along the instability that leads to homogeneous ferroelectric switching below T(c) with an applied field antiparallel to the spontaneous polarization. This line is defined as the minimum in the inverse capacitance under an applied electric field. We investigate the effects of pressure, temperature and an applied electric field on the ECE. The behavior we observe in LiNbO(3) should generally apply to ferroelectrics; we therefore suggest that the operating temperature for refrigeration and energy scavenging applications should be above the ferroelectric transition region to obtain a large electrocaloric response. The relationship between T(c), the Widom line, and homogeneous switching should be universal among ferroelectrics, relaxors, multiferroics, and the same behavior should be found under applied magnetic fields in ferromagnets.

Maximizing the number of coexisting phases near invariant critical points for giant electrocaloric and electromechanical responses in ferroelectrics

Ferroelectric materials directly convert electrical energy to mechanical or thermal work and are critical to applications such as sensors, transducers, actuators, and cooling devices. Numerous efforts have been undertaken to develop materials with high electrocaloric (EC) and electromechanical (EM) responses. Here, we present a theoretical analysis, based on thermodynamic fundamentals, for developing ferroelectric materials with high EC and EM responses, i.e., searching for and operating the material near an invariant critical point (ICP). We show that by tailoring the constraints to maximize the number of coexisting phases near ICPs, large EC and EM responses may be realized.

Electrocaloric properties of epitaxial strontium titanate films

The electrocaloric (EC) response of strontium titanate thin films is computed as a function of misfit strain, temperature, electric field strength, and electrode configuration using a nonlinear thermodynamic theory. For films in a capacitor configuration on compressive substrates, the transition between paraelectric and strain-induced ferroelectric tetragonal phases produces a large adiabatic temperature change, ΔT = 5 K, at room temperature for electric field changes ΔE = 1200 kV/cm. For films on tensile substrates, the transition between the paraelectric and strain-induced ferroelectric orthorhombic phases can also be accessed using inter-digitated electrodes (IDEs). The maximum EC response occurs for IDEs with a [110] orientation.

Bridging the Macroscopic and Atomistic Descriptions of the Electrocaloric Effect

First-principles-based simulations are used to simulate the electrocaloric effect (ECE) in Ba(0.5)Sr(0.5)TiO(3) alloys. In analogy with experimental studies we simulate the effect directly and indirectly (via the use of Maxwell thermodynamics). Both direct and indirect simulations utilize the same atomistic framework that allows us to compare them in a systematic way and with an atomistic precision for the very first time. Such precise comparison allows us to provide a bridge between the atomistic and macroscopic descriptions of the ECE and identify the factors that may critically compromise or even destroy their equivalence. Our computational data reveal the intrinsic features of ECE in ferroelectrics with multiple ferroelectric transitions and confirm the potential of these materials to exhibit giant electrocaloric response. The coexistence of negative and positive ECE in one material as well as an unusual field-driven transition between them is predicted, explained at an atomistic level, and proposed as a potential way to enhance the electrocaloric efficiency.

Tailoring electrically induced properties by stretching relaxor polymer films

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.

Analysis of the dynamics of electrocaloric response in ferroelectrics using a ferromagnetic resonator

A method is proposed for studying the dynamics of the electrocaloric response of a ferroelectric capacitor to the pulsed action of the electric field, and the results of measurements are reported. The temperature of the capacitor was measured with the help of a ferromagnetic film resonator. The sensitivity of temperature measurements was ∼10−4 K/kHz. The recalculated temperature dependence of the resonance frequency showed that the ferroelectric ceramic sample was heated by 0.065 K and was cooled by 0.041 K relative to the thermostat temperature under the action of high-voltage pulses. The relaxation time of the electrocaloric effect of the 0.87PMN-0.13PT relaxor ferroelectric ceramic is 4.5 μs.

Influence of the critical point on the electrocaloric response of relaxor ferroelectrics

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.

Effect of domain walls on the electrocaloric properties of Pb(Zr1−x,Tix)O3 thin films

The electrocaloric properties of polydomain epitaxial Pb(Zr1-x,Tix)O3 thin films are investigated using a Ginzburg-Landau-Devonshire thermodynamic model as a function of strain, temperature, and composition for 0.65 ≤ x ≤ 1. Polarization transitions driven by epitaxial strain and extrinsic contributions from domain wall displacements are found to dramatically impact the electrocaloric response. Careful choice of epitaxial misfit strain and composition allows one to harness the intrinsic and extrinsic contributions to obtain large adiabatic temperature changes much below the Curie temperature of the material.

Modeling of enhanced electrocaloric effect above the Curie temperature in relaxor ferroelectrics

The electrocaloric (EC) effect offers promise as a means to realize solid-state refrigeration, which requires EC materials possessing a pronounced pyroelectric effect over a broad temperature range. Pauli’s master equation is adopted to investigate the recently observed phenomenon of enhanced EC effect above the Curie temperature in relaxor ferroelectrics. The proposed approach allows the EC coefficient to be determined within the framework of classic Landau–Ginzburg–Devonshire thermodynamics and the Maxwell relation, taking into account both the depolarization effect and dielectric permittivity dispersion based on the concept of superparaelectricity and the nanopolar region. We analyze three contributions of the EC effect: temperature-dependent dielectric dispersion, intrinsic pyroelectric effect and enhanced dielectric stiffness. The maximum EC coefficient is determined through the derivatives of the three components with respect to temperature. The proposed approach, in which the evolution of polarization correlation length is accounted for, cannot only provide a microscopic explanation for the thermally driven enhancement of EC responses, but also improves upon the existing models for estimating the EC effect in paraelectric phase of relaxors. Finally, some potential approaches for engineering the enhancement of EC coefficient are also suggested.