Electrocaloric Response Research Articles

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.

Temperature response features of ferroelectric ceramics in electrocaloric effect study

Temperature response of a material to an external electric field is the main method for electrocaloric effect study in ferroelectrics. In this work, for 0.65PbFe2/3W1/3O3-0.35PbTiO3 solid solution as a model object, it is shown that with an increase in the electric field strength, current filamentation effect can occur. It leads to formation of local regions of increased conductivity in the sample. The associated thermal effect have short characteristic times, due to the small volume of the filament. They are comparable to the times of the electrocaloric response of the material, and can lead to significant errors in the detection of the electrocaloric effect. Keywords: electrocaloric effect, temperature response, ferroelectric ceramics, current filamentation effect.

Temperature response features of ferroelectric ceramics in electrocaloric effect study

Temperature response of a material to an external electric field is the main method for electrocaloric effect study in ferroelectrics. In this work, for 0.65PbFe2/3W1/3O3-0.35PbTiO3 solid solution as a model object, it is shown that with an increase in the electric field strength, current filamentation effect can occur. It leads to formation of local regions of increased conductivity in the sample. The associated thermal effect have short characteristic times, due to the small volume of the filament. They are comparable to the times of the electrocaloric response of the material, and can lead to significant errors in the detection of the electrocaloric effect. Keywords: electrocaloric effect, temperature response, ferroelectric ceramics, current filamentation effect.

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.

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.

Gate-tunable charge carrier electrocaloric effect in trilayer graphene

The electrocaloric (EC) effect is the change in temperature and entropy of a material driven by the application of an electric field. Our tight-binding calculations linked to Fermi statistics, show that the EC effect can be produced in trilayer graphene (TLG) structures connected to a heat source, triggered by changes in the electronic density of states (DOS) at the Fermi level when external gate fields are applied on the outer graphene layers. We demonstrate that entropy changes are sensitive to the stacking arrangement in TLG systems. The AAA-stacked TLG presents an inverse EC response (cooling) regardless of the temperature value and gate field potential strength, whereas the EC effect in ABC-stacked TLG remains direct (heating) above room temperature. We reveal otherwise the TLG with Bernal-ABA stacking generates both the direct and inverse EC response within the same sample, associated with gate-dependent electronic transitions of thermally excited charge carriers from the valence band to the conduction band in the band structure. The novel charge carrier electrocaloric effect we propose in quantum layered systems may bring a wide variety of prototype van der Waals materials that could be used as versatile platforms to controlling the thermal response in nanodevices.

Direct and indirect studies of the electrocaloric effect in single crystalline ferrielectric (NH4)2SO4

The influence of an electric field on the phase transition parameters and the electrocaloric response in ferrielectric (NH4)2SO4 was studied using a universal multifunctional adiabatic calorimeter. The very low sensitivity of the phase transition temperature to the electric field, dT0/dE ≈ 5·10-3 K/(kV/cm), was shown to be the reason for the low maximum value of the intensive electrocaloric effect, ΔTAD, at E = 11 kV/cm: direct and indirect measurements gave the values of 0.01 K and 0.05 K, respectively. However, the extensive electrocaloric response revealed by an analysis of the entropy–temperature–electric field phase diagram was relatively large, with a value of ΔSECE ≈ − 19 J/kg K, due to the significant contribution of a jump in the excess entropy to the total value, δS0/ΔS0 ~ 0.37. An additional irreversible increase in temperature under the electric field was associated with the Joule effect and indicated a significant electrical conductivity of ammonium sulphate.

A Metal-Free Molecular Antiferroelectric Material Showing High Phase Transition Temperatures and Large Electrocaloric Effects.

Antiferroelectric (AFE) materials, featuring an antiparallel alignment of electric dipoles in the adjacent sublattices, are keeping a great promise toward solid-state refrigeration applications on account of their electrocaloric (EC) effects. Although extensive studies have been performed on inorganic oxide counterparts (e.g., PbZrO3 and AgNbO3), metal-free molecular AFE alternatives with the above-room-temperature EC activities are quite scarce but urgently demanded in terms of environmental issues. Herein, we present a new metal-free molecular AFE, cyclohexylmethylammonium bromide (CMB), which exhibits the unusual antiferroelectric-ferroelectric-paraelectric phase transitions around 364 and 368 K upon heating. The phase transition temperatures are much higher than the majority of known molecular AFE materials. The practical utilization level of electric polarization (∼6 μC/cm2) is clearly evidenced by the typical double polarization-electric field hysteresis loops. Strikingly, large positive and negative EC responses with the temperature changes (ΔT) of 4.2 and -3 K are achieved under an electric field of 20 kV/cm. The origin of its antiferroelectricity and EC properties is elucidated by the antipolar reorientation of cations along with displacement of bromine anions, being distinct from the known mechanism of inorganic oxides. Such intriguing AFE behaviors, including large polarization and EC effects, reveal great potentials of CMB for the solid-state refrigeration. This study sheds light on further exploration of new AFE candidates toward environmentally friendly solid-state cooling devices.

Direct Visualization of Anti‐Ferroelectric Switching Dynamics via Electrocaloric Imaging

AbstractThe large electrocaloric coupling in PbZrO3 allows using high‐speed infrared imaging for visualizing anti‐ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single‐phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s−1. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus‐like electrocaloric response.

Effect of BaO–CaO–SiO2 addition on dielectric and electrocaloric properties of lead-free 0.2Ba(Ti0.9Sn0.1)O3-0.8Ba(Zr0.18Ti0.82)O3 bulk ceramics

In recent years, compared with traditional refrigeration technology, the electrocaloric refrigeration technology has been applied in variety of green cooling devices due to its high efficiency and environmentally friendly. Recent findings reveal that the ceramic system with additives (such as, LiBiO2, PbO) can create large ECE under low electric field. In this work, the ternary glass BaO–CaO–SiO2 (BCS) was used as a sintering aid to enhance the electrocaloric (EC) response of 0.2Ba(Ti0.9Sn0.1)O3-0.8Ba(Zr0.18Ti0.82)O3(BZSnT20) bulk ceramics. The EC properties of BZSnT20 could be also improved via controlling sintering temperature and the content of BCS sintering aid. It was found that BZSnT20 with 0.5 wt% BCS show the maximum EC response of 3.2 K under 7 MV/m with sintering temperature of 1262 °C. These results demonstrated that the ECE of BZSnT20 can be boosted by using appropriate additives, which provides an effective route to realize large ECE under low electric fields.

Critical behavior of relaxor [formula omitted]: Interplay between polar nano regions, electrocaloric and energy storage response

We have investigated the critical behavior of relaxor Pb0.91La0.09Zr0.65Ti0.35O3[(PLZT 9/65/35] to unveil the systematic effect of Polar Nano Regions (PNRs) on electrocaloric effect (ECE) and energy storage properties via dielectric spectroscopy and isothermal P (polarization) vs E (electric field) loops measurements. To capture the basic notion about PNR driven phase evolution and realize the relevance of the mean-field approach, we have followed the master curve scheme for ∆SE(E,T) profile. The observed ∆SE(T) curves of various E are well collapsed into a single universal curve with two reference temperatures, this approach usually applied to deal the multiphase magnetic systems. This suggests that the polar activity associated with PNRs is diverse in a certain manner from the typical displacive polar domains. Additionally, we have disclosed the energy storage aspect of PNRs in relaxors. The thermal profile of efficiency reveals that the PNRs play a conclusive role in the energy storage application.

Structural, dielectric, and ferroelectric properties of Na0.5(Bi1-xNdx)0.5TiO3 ceramics for energy storage and electrocaloric applications

Lead-free relaxor ferroelectric Na0·5(Bi1-xNdx)0.5TiO3 (xNdNBT, x = 0–30%) ceramics were synthesized using the conventional solid-state method. Detailed Rietveld refinement revealed transitions from rhombohedral (x = 0, 5%), to a coexistence of R3c and Pnma symmetries (x = 10, 15%), followed by a tetragonal P4/mmm phase for the high Nd-concentrations (x = 25, 30%). High-temperature dielectric stability was found over a large temperature range (for instance ±15% from -90 °C to 405 °C for x = 30%) with very low dielectric losses, especially at high temperatures. The substitution of Bi3+ by Nd3+ results in a perturbation of the ferroelectric order and favors short-range polar regions in the system. From the ferroelectric measurements, we demonstrated the stabilization of a coexistence of non-ergodic and ergodic relaxor state or ergodic relaxor state at specific doping concentrations. Besides, high-energy storage properties were obtained at high temperatures, with Wrec > 1.15 J/cm3 in the temperature range 140 °C–200 °C for the 5NdNBT and a moderate Wrec between 0.5 and 0.77 J/cm3 in a large temperature range from 25 °C to 200 °C and with high efficiency >70% for 10NdNBT composition. In addition, a high electrocaloric response (ΔT = 1.8 K at 95 kV/cm) was found using the indirect method for the 5NdNBT composition, with great temperature stability below 20% over a broad temperature range (80–140 °C) and with a high ΔT/ΔE > 0.19 K m/V. The obtained electrocaloric response was related to the thermal evolution of the polar regions in mixed ergodic and non-ergodic relaxor states under an applied electric field.

A unified perturbative approach to electrocaloric effects

The electrocaloric effect, that is, the temperature change experienced by an insulator upon application of an electric field, offers promising ecofriendly alternatives to refrigeration. However, the theoretical treatments of this response are mostly case specific and lack a unified picture revealing the similarities and differences among the various known effects. Here, we show that the electrocaloric effect lends itself to a straightforward interpretation when expressed as a Taylor series in the external field. Our formalism explains in a unified and simple way the most notable small-field effects reported in the literature, namely the so-called normal and inverse electrocaloric responses, corresponding to an increase or decrease of temperature under applied field, as usually found in ferroelectrics or antiferroelectrics, respectively. This helps us to clarify their physical interpretation. We then discuss in detail atomistic simulations for the prototype ferroelectric PbTiO3, explicitly evaluating subtle predictions of the theory, such as the occurrence of competing contributions to the electrocaloric response.

Effect of Mn/Nb heterovalent substitution on the electrocaloric response and energy storage performance of Ba(Sn, Ti)O3 relaxor-ferroelectrics

Effect of Mn/Nb heterovalent substitution on the electrocaloric response and energy storage performance of Ba(Sn, Ti)O3 relaxor-ferroelectrics

Electrocaloric effects in multiferroics

An atomistic effective Hamiltonian is used to compute electrocaloric (EC) effects in rare-earth substituted BiFeO$_{3}$ multiferroics. A phenomenological model is then developed to interpret these computations, with this model indicating that the EC coefficient is the sum of two terms, that involve electric quantities (polarization, dielectric response), the antiferromagnetic order parameter, and the coupling between polarization and antiferromagnetic order. The first one depends on the polarization and dielectric susceptibility, has the analytical form previously demonstrated for ferroelectrics, and is thus enhanced at the ferroelectric Curie temperature. The second one explicitly involves the dielectric response, the magnetic order parameter and a specific magnetoelectric coupling, and generates a peak of the EC response at the N\'eel temperature. These atomistic results and phenomenological model may be put in use to optimize EC coefficients.

Electric-field-driven coexistence of positive and negative electrocaloric effects near room temperature for high-efficiency two-stage cooling

The electrocaloric (EC) effect is promising for the next-generation solid-state refrigeration, yet it is difficult to improve the EC response near room temperature with either the positive electrocaloric (PEC) or negative electrocaloric (NEC) effect alone. In this work, we propose and demonstrate a strategy that achieves the coexistence of PEC and NEC effects near room temperature via electric-field-induced ferroelectric–ferroelectric phase transition in Ba(ZrxTi1−x)O3 solid solutions, while the PEC effect is generally found around paraelectric–ferroelectric transition and the NEC effect is around ferroelectric–antiferroelectric phase boundaries. Utilizing such a coexistence of PEC and NEC effects, we propose a prototype two-stage cooling device, wherein both the EC cooling temperature change and efficiency are higher than its counterpart with only the PEC or NEC effect, offering a new pathway for EC cooling in practical applications near room temperature.

Multifunctional Cantilevers as Working Elements in Solid-State Cooling Devices

Despite the challenges of practical implementation, electrocaloric (EC) cooling remains a promising technology because of its good scalability and high efficiency. Here, we investigate the feasibility of an EC cooling device that couples the EC and electromechanical (EM) responses of a highly functionally, efficient, lead magnesium niobate ceramic material. We fabricated multifunctional cantilevers from this material and characterized their electrical, EM and EC properties. Two active cantilevers were stacked in a cascade structure, forming a proof-of-concept device, which was then analyzed in detail. The cooling effect was lower than the EC effect of the material itself, mainly due to the poor solid-to-solid heat transfer. However, we show that the use of ethylene glycol in the thermal contact area can significantly reduce the contact resistance, thereby improving the heat transfer. Although this solution is most likely impractical from the design point of view, the results clearly show that in this and similar cooling devices, a non-destructive, surface-modification method, with the same effectiveness as that of ethylene glycol, will have to be developed to reduce the thermal contact resistance. We hope this study will motivate the further development of multifunctional cooling devices.

Origin of large negative electrocaloric effect in antiferroelectric PbZrO3

We have studied the electrocaloric response of the archetypal antiferroelectric $\mathrm{PbZr}{\mathrm{O}}_{3}$ as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of \ensuremath{-}3.5 K to an electroheating of $+5.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics.

Synthesis and characterizations of (Ba1-xCax)Ti4O9, 0 ≤ x ≤ 0.9 ceramics

Manufacturing of energy storage devices with good efficiency and fewer losses has been a distinct topic. A material with good dielectric properties and fewer energy losses is the basic demand of the electronic industry. Barium tetra titanate ceramic material (BaTi4O9) is a prominent dielectric material with average permittivity and low losses. However, we report a Ca2+ substituted Barium tetra titanate (BaTi4O9) with enhancement in dielectric properties and reduction in losses (Ba1-xCax) T4O9 was prepared using the conventional solid-state solution technique. X-ray diffraction analysis confirmed the formation of the orthorhombic phase (with space group = Pnmm). A drastic change in grain size was observed by increasing the Ca2+ substitution. Dielectric properties have been measured at 4 GHz and found maximum dielectric constant = 40 and loss = 0.00014. The optimum electro-caloric response rises from 0.56 to 0.69 k at ΔE = 1.34 Vμm−1. The effect of substitution Ca2+ contents and microstructure on dielectric properties were analyzed using a precision impedance analyzer in a vast frequency range i.e. 4 Hz to 20 MHz. Enhanced dielectric properties with low energy losses made it a prominent material for energy storage devices.

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.