Electrocaloric Response Research Articles

Understanding the Role of Ferroelastic Domains on the Pyroelectric and Electrocaloric Effects in Ferroelectric Thin Films.

Temperature- and electric-field-induced structural transitions in a polydomain ferroelectric can have profound effects on its electrothermal susceptibilities. Here, the role of such ferroelastic domains on the pyroelectric and electrocaloric response is experimentally investigated in thin films of the tetragonal ferroelectric PbZr0.2 Ti0.8 O3 . By utilizing epitaxial strain, a rich set of ferroelastic polydomain states spanning a broad thermodynamic phase space are stabilized. Using temperature-dependent scanning-probe microscopy, X-ray diffraction, and high-frequency phase-sensitive pyroelectric measurements, the propensity of domains to reconfigure under a temperature perturbation is quantitatively studied. In turn, the "extrinsic" contributions to pyroelectricity exclusively due to changes between the ferroelastic domain population is elucidated as a function of epitaxial strain. Further, using highly sensitive thin-film resistive thermometry, direct electrocaloric temperature changes are measured on these polydomain thin films for the first time. The results demonstrate that temperature- and electric-field-driven domain interconversion under compressive strain diminish both the pyroelectric and the electrocaloric effects, while both these susceptibilities are enhanced due to the exact-opposite effect from the extrinsic contributions under tensile strain.

Strongly enhanced electrocaloric effects in doped BaTiO3 with reduced grain size

Ferroelectrics are promising candidates in the field of green solid state refrigeration technology due to showing a thermal response with the application/removal of an electric field. However, thermal responses of ferroelectrics are too low; at present, this approach is not able to compete with the conventional refrigeration approach. Guzmán-Verri et al (2016 APL Materials 4, 064106) theoretically predicted that the thermal response of ferroelectrics can be elevated up to a considerable amount via modulation of the correlation length of the fluctuation of polarization. In this letter, we report a giant electrocaloric response () in , via manipulation of micro-structural configuration. Addition of during the sintering process is responsible for the growth of grains of small size, which alters the fluctuation of polarization and reduction in the domain wall, thereby increasing the thermal response. The dielectric properties of added BaTiO3 remain similar to the bare BaTiO3.

Effect of an electric field on the electrocaloric response of ferroelectrics**Project supported by the National Natural Science Foundation of China (Grant No. 11704242) and the Natural Science Foundation of Shanghai, China (Grant No. 17ZR1447200).

The electrocaloric effect of the model ferroelectric BaTiO3 was investigated using phenomenological theory. The results indicate that the applied electric field strength is a key factor for the induced electrocaloric response and there are two distinguishing electrocaloric responses. When a moderate electric field is applied, the electrocaloric temperature variation is small but the electrocaloric strength is high. In contrast, the electrocaloric temperature variation is large but electrocaloric strength is low when a very high electric field is applied. These results are consistent with the experimental observations on BaTiO3 based bulk and thin film ferroelectric materials.

Giant direct and inverse electrocaloric effects in multiferroic thin films

Refrigeration systems based on compression of greenhouse gases are environmentally threatening and cannot be scaled down to on-chip dimensions. In the vicinity of a phase transition caloric materials present large thermal responses to external fields, which makes them promising for developing alternative solid-state cooling devices. Electrocaloric effects are particularly well-suited for portable refrigeration applications; however, most electrocaloric materials operate best at non-ambient temperatures or require the application of large electric fields. Here, we predict that modest electric fields can yield giant room-temperature electrocaloric effects in multiferroic BiCoO$_{3}$ (BCO) thin films. Depending on the orientation of the applied field the resulting electrocaloric effect is either direct (heating) or inverse (cooling), which may enable the design of enhanced refrigeration cycles. We show that spin-phonon couplings and phase competition are the underlying causes of the disclosed caloric phenomena. The dual electrocaloric response of BCO thin films can be effectively tuned by means of epitaxial strain and we anticipate that other control strategies like chemical substitution are also possible.

Enhancing the electrocaloric effect in a relaxor polymer by including minor normal ferroelectric phase

Generating a large caloric effect under a low applied field is critical for realizing high performance solid state caloric cooling. Here, we report that a large enhancement in the electrocaloric response at a moderate electric field level over a relatively broad temperature range can be achieved in the relaxor poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] polymers by including the minor normal ferroelectric phase. We show that the terpolymer of P(VDF-TrFE-CFE) 69.7/30.3/6 mol. % which contains a minor normal ferroelectric phase, under a moderate electric field of 90 MV/m, can reach an adiabatic temperature change ΔT = 11 K, compared with ΔT = 6 K of the terpolymers with a pure relaxor phase under the same field. The results reveal that the presence of the minor ferroelectric phase facilitates the polarization switch in a temperature range near the ferroelectric transition while the relaxor phase maintains a large electrocaloric effect (ECE) over a broad temperature range, resulting in a large ECE over a relatively broad temperature range in the material.

Towards electrocaloric heat pump—A relaxor ferroelectric polymer exhibiting large electrocaloric response at low electric field

A polymer that generates large cooling under applied voltage is attractive for many applications. The past decade has witnessed the discovery and advancement in electrocaloric (EC) polymers, which display large electric field induced temperature and entropy changes. However, in contrast to the burgeoning literature on large electrocaloric effects (ECEs) in various ferroelectric materials, there are no EC devices employing these EC polymers, demonstrating a meaningful cooling power. Here, we show that it is the dielectric breakdown, the weakest link problem, in EC materials which poses a critical barrier for transitioning these advanced EC materials to practical EC coolers. Hence, high quality EC films and high performance of EC materials, exhibiting large ECE at low electric fields, are required to overcome this barrier. Here, we show that by expanding the compositions in EC polymers, a relaxor tetrapolymer exhibits a critical end point at low electric fields, leading to large ECE induced at low electric fields.

BaZrxTi1−xO3 Epitaxial Thin Films for Electrocaloric Investigations

AbstractEpitaxial BaZrxTi1−xO3 (BZT) thin films with a Zr content between x=0 and x=0.3 are grown by pulsed laser deposition on (001)‐oriented SrTiO3 single crystals utilizing a conducting SrRuO3 buffer layer and Pt top electrodes. An undisturbed epitaxial growth with a smooth and homogeneous surface structure is obtained for all Zr containing films, whereas some preferentially growing grains and twins are observed for pure BaTiO3 films. Temperature‐dependent measurements of the relative permittivity suggest a diffuse phase transition. The temperature of the maximum permittivity as well as the electrical polarizability at room temperature decrease with increasing Zr content. The observed frequency dependence suggests relaxor ferroelectric properties for all Zr‐containing films. The indirectly determined electrocaloric responses are considerably lower compared to thick films, which is primarily attributed to the diffuse character of the phase transition. A maximum adiabatic temperature change of about 0.3 K for an electric field change of 170 kV cm−1 is found for BaZr0.2Ti0.8O3.

Impact of Polarization Dynamics and Charged Defects on the Electrocaloric Response of Ferroelectric Pb(Zr,Ti)O3 Ceramics

AbstractThe impact of charged point defects on the electrocaloric response of morphotropic Pb(Zr,Ti)O3 (PZT) ceramics is investigated using direct electrocaloric and polarization hysteresis measurements. The electrocaloric response determined for undoped, hard‐, and soft‐doped PZT is rationalized by considering the dipolar entropy change over polarization change and hysteresis losses. The highest electrocaloric effect is observed in poled hard PZT with the field applied along the poling direction, which is related to the reduced hysteresis losses caused by internal electric fields associated with defect complexes. The weak hysteretic dependency of polarization as function of the electric field in hard PZT also allows inducing an inverse electrocaloric effect. The experimental results are compared with model calculations that provide a physical interpretation. The results reveal how the electrocaloric response of ferroelectrics can be optimized by defect engineering.

Enhanced electrocaloric response and high energy-storage properties in lead-free (1-x) (K0.5Na0.5)NbO3 - xSrZrO3 nanocrystalline ceramics

Recently electrocaloric effect in lead-free environmental friendly ferroelectrics is in its peak research. The feasible and existed electrocaloric response in lead-free (1-x) (K0.5Na0.5)NbO3 - xSrZrO3 (KNN-xSZ) nanocrystalline ceramics is demonstrated using Maxwell's thermodynamic approach. The maximum electrocaloric response of 1.19 K at 367 K was observed under the electric field of 35 kV/cm, and the corresponding electrocaloric responsivity value was found to be 3.40 × 10−7 K m/V at 367 K for x = 0.10. A significant coefficient of performance (COP) and electrical energy storage density (Wrec) were calculated to be 0.62 and 0.20 J/cm3, respectively. The atomistic mechanism of the electrocaloric effect is discussed. The results indicate that KNN-xSZ is a favourable material in the cooling system applications and energy-storage for electronic devices. The material allows electrocaloric cooling mode with versatile properties for the cooling operation by an external electric field and operating temperature.

Tunable electrocaloric and energy storage behavior in the Ce, Mn hybrid doped BaTiO3 ceramics

The electrocaloric effect and energy storage property are tuned in the Ba1-xCexTi0.99Mn0.01O3 ceramics prepared by the solid state reaction method. The ceramics with lower Ce content (x = 0.005, 0.015) show a better ΔT and ΔT/ΔE response. The ceramics with higher Ce content (x = 0.030, 0.040, 0.045) represent the broader ΔT peaks (50 K–60 K), and the higher energy storage density and efficiency. The largest electrocaloric response (ΔTmax = 1.22 K, ΔT/ΔE = 0.41 K mm/kV) is found in the Ba0.995Ce0.005Ti0.99Mn0.01O3 ceramics, which is comparable or even higher than that of the most isovalent substituting BaTiO3-based ceramics reported before. The maximum energy storage density 0.11 J/cm3 (E = 30 kV/cm) is obtained for the Ba0.970Ce0.030Ti0.99Mn0.01O3 ceramics, with high efficiency of 65–88% over a wide temperature range of 72 K. This work may open more opportunities to design high electrocalaric and energy storage performance lead-free systems from the viewpoint of the heterovalent and size mismatch substitution.

Tuning the electrocaloric effect by varying Sr concentration in ferroelectric Ba1−xSrxTiO3

The electrocaloric effect is investigated systematically in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{TiO}}_{3}$ ferroelectrics using a semiclassical direct computational approach. The data are reported for the technologically important range of Sr concentrations of 0.0--0.6, electric fields up to 1000 kV/cm, and temperatures ranging from 5 to 600 K. A detailed comparison of computational data with experimental data from the literature reveals semiquantitative agreement and suggests the origin of discrepancies. The electrocaloric change in temperature $\mathrm{\ensuremath{\Delta}}T$ shows strong dependence on Sr concentration which offers a way to tune electrocaloric response. In particular, the maximum electrocaloric $\mathrm{\ensuremath{\Delta}}T$ is found to decrease with the increase in Sr concentration, whereas the location of the maximum shifts towards lower temperatures following the Curie point of the ferroelectric. Surprisingly, the width of the peak in the dependence of $\mathrm{\ensuremath{\Delta}}T$ on the initial temperature is independent of the Sr concentration but shows a strong dependence on the applied electric field. Computational data are used to propose a compositionally graded ferroelectric ${\mathrm{Ba}}_{0.70}{\mathrm{Sr}}_{0.30}{\mathrm{TiO}}_{3}/{\mathrm{Ba}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{TiO}}_{3}/{\mathrm{Ba}}_{0.50}{\mathrm{Sr}}_{0.50}{\mathrm{TiO}}_{3}/{\mathrm{Ba}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{TiO}}_{3}$ whose $\mathrm{\ensuremath{\Delta}}T$ shows almost no temperature dependence in the technologically important range of temperatures and electric fields. Such a desirable feature could potentially lead to the enhancement of relative cooling power.

Enhanced electrocaloric effect in lead-free organic and inorganic relaxor ferroelectric composites near room temperature

We study the electrocaloric effect in the composites composed of the ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer matrix and lead-free BaZr0.2Ti0.8O3 (BZT) fillers. The P(VDF-TrFE-CFE)/BZT composite with 5 vol. % BZT exhibits a high adiabatic temperature change of ΔT = 7.4 K and a large isothermal entropy change of ΔS = 30 J kg−1 K−1 under an electric field of 90 MV m−1 at room temperature. The electrocaloric response in the composites is significantly enhanced when compared to those of the constituent components, which has been attributed to the polymer-filler interfacial coupling effect. In addition, the composites display a wide operational temperature window as a result of their ferroelectric relaxor nature.

Correlation Between Electrocaloric Response and Polarization Behavior: Slim‐Like and Square‐Like Hysteresis Loop

Electrocaloric response in slim‐like (Ba0.9Ca0.1)(Zr0.2Ti0.8)O3 and square‐like Pb(Zr0.95Ti0.05)O3 hysteresis loops are observed by cycling samples between positive and negative values of electric field, respectively. The slim‐like (Ba0.9Ca0.1)(Zr0.2Ti0.8)O3 ceramics represent a typical electrocaloric response, while the square‐like Pb(Zr0.95Ti0.05)O3 ceramics depict an unusual electrocaloric response. It is worth noting that the unusual electrocaloric response (inverse electrocaloric effect) can be induced by inverse electric field. Furthermore, the highest value of inverse electrocaloric effect is not obtained at negative coercive field but at values that are half of it. With the increase in inverse electric field, the value of induced unusual electrocaloric effect becomes weaker and only positive electrocaloric response is present at higher magnitudes of inverse field. In the present work, the relationship between electrocaloric response and polarization behavior of these two kinds of samples are discussed.

Giant electrocaloric effect in a cracked ferroelectrics

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.

Giant electrocaloric response in the prototypical Pb(Mg,Nb) O3 relaxor ferroelectric from atomistic simulations

An atomistic effective Hamiltonian is used to investigate electrocaloric (EC) effects of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) relaxor ferroelectrics in its ergodic regime, and subject to electric fields applied along the pseudocubic [111] direction. Such Hamiltonian qualitatively reproduces (i) the electric field-versus-temperature phase diagram, including the existence of a critical point where first-order and second-order transitions meet each other; and (ii) a giant EC response near such critical point. It also reveals that such giant response around this critical point is microscopically induced by field-induced percolation of polar nanoregions. Moreover, it is also found that, for any temperature above the critical point, the EC coefficient-versus-electric field curve adopts a maximum (and thus larger electrocaloric response too), that can be well described by the general Landau-like model proposed in [Jiang et al, Phys. Rev. B 96, 014114 (2017)] and that is further correlated with specific microscopic features related to dipoles lying along different rhombohedral directions. Furthermore, for temperatures being at least 40 K higher than the critical temperature, the (electric field, temperature) line associated with this maximal EC coefficient is below both the Widom line and the line representing percolation of polar nanoregions.

Interplay of conventional with inverse electrocaloric response in (Pb,Nb)(Zr,Sn,Ti)O3 antiferroelectric materials

The electrocaloric effect in ferroics is considered a powerful solid-state cooling technology. Its potential is enhanced by correlation to the inverse electrocaloric effect and leads into mechanisms of decreasing or increasing dipolar entropy under applied electric field. Nevertheless, the mechanism underlying the increase of the dipolar entropy with applied electric field remains unclear and controversial. This study investigates the electrocaloric response of the antiferroelectric Pb0.99Nb0.02[(Zr0.58Sn0.43)0.92 Ti0.08]0.98O3 in which the critical electric field is low enough to induce the ferroelectric phase over a broad temperature range. Utilizing temperature- and electric-field-dependent dielectric measurements, direct electrocaloric measurements, and in situ transmission electron microscopy, a crossover from conventional to inverse electrocaloric response is demonstrated. The origin of the inverse electrocaloric effect is rationalized by investigating the field-induced phase transition between antiferroelectric and ferroelectric phases. The disappearance of the latent heat at field-induced transition coincides with the crossover of the electrocaloric effect and demonstrates that the overall electrocaloric response is an interplay of different entropy contributions. This opens new opportunities for highly efficient, environmentally friendly cooling devices based on ferroic materials.

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.

Flexocaloric response of epitaxial ferroelectric films

The flexoelectric effect in dielectric materials generates an electric polarization as a result of strain gradient. Here, we show that the flexoelectric response also produces a flexocaloric adiabatic temperature variation in heteroepitaxial ferroelectric films that are either partially or completely relaxed. The flexocaloric temperature change of (001) BaTiO3 films on (001) SrTiO3 substrates is computed as a function of film thickness and temperature. Our calculations predict that a built-in flexocaloric temperature change of 0.61 °C can be realized in 20 nm thick epitaxial BaTiO3 films when compared with the intrinsic electrocaloric response of 0.75 °C for bulk, single-crystal BaTiO3 at 25 °C and applied electric field of 200 kV/cm. This demonstrates that the flexocaloric response can supplement electrocaloric temperature changes in thin film ferroelectrics and may play an important role in potential applications in on-chip solid-state cooling.

Enhancement of electrocaloric response through quantum effects

A semiclassical approach that incorporates quantum mechanical behavior of heat capacity in direct caloric effect simulations is proposed. Application of this methodology to study electrocaloric effect in prototypical ferroelectrics ${ \rm PbTiO_3 }$, and ${ \rm BaTiO_3 }$, reveals severe underestimation of electrocaloric response at lowest temperatures by classical simulations. The discrepancy between semiclassical and classical results are found to be largest in ferroics with Debye temperature exceeding the Curie point. A route to enhance electrocaloric effect by tuning the Debye temperature in composite materials is proposed.

The Impact of Hysteresis on the Electrocaloric Effect at First‐Order Phase Transitions

We study the impact of thermal hysteresis at the first‐order structural/ferroelectric phase transitions on the electrocaloric response in bulk BaTiO3 by performing molecular dynamics simulations for a first‐principles‐based effective Hamiltonian. We demonstrate that the electrocaloric response can conceptually be separated in two contributions: a discontinuous transitional part, stemming from the jump in entropy at the first order phase transition, and a continuous part, due to the change of polarization and entropy within each phase. This latter part increases with the strength of the applied field, but for small fields it is very small. In contrast, we find a large temperature change of ≈1 K resulting from the transition entropy, which is essentially independent of the field strength. However, due to the coexistence region close to the first order phase transition, this large electrocaloric response depends on the thermal history of the sample and is generally not reversible. We show that this irreversibility can be overcome by using larger fields.