Electrocaloric Response Research Articles

Interplay of c- and a-domains on the anomalous electrocaloric effect in BaTiO3 (001) single crystals

Electrocaloric effects of adiabatic temperature change via the application of external electric fields are explored for energy-efficient solid-state refrigeration. These effects are typically estimated from the thermodynamic analyses of polarization and field in electrocaloric materials, which implies that higher field application gives larger temperature changes. However, this may not be always true. Here, using both indirect and direct methods, we report an anomalous effect where larger thermal changes occur by applications of lower fields in a multi-domain BaTiO3 (001) single crystal. A large temperature change of 1.9 K under a low field change of 8 kV/cm at 404 K is observed in a multi-domain BaTiO3 (001) single crystal in comparison to that of 1.4 K at a high field change of 30 kV/cm. We attribute this counterintuitive effect to the interplay of the c- and a-domains in the BaTiO3 (001) single crystal under the influence of temperature and field changes. This work provides a fundamental understanding of the complex role of domains in governing the electrocaloric response of ferroelectric materials which is often overlooked but critical for their practical applications.

Ferroelectric terpolymer films with enhanced cooling efficiency: An integrated approach considering electrocaloric response and dielectric losses

Ferroelectric terpolymer films with enhanced cooling efficiency: An integrated approach considering electrocaloric response and dielectric losses

Electronic cooling and energy harvesting using ferroelectric polymer composites

Thermal management emerges as a grand challenge of next-generation electronics. Efforts to develop compact, solid-state cooling devices have led to the exploration of the electrocaloric effect of ferroelectric polymers. Despite recent advances, the applications of electrocaloric polymers on electronics operating at elevated temperatures remain essentially unexplored. Here, we report that the ferroelectric polymer composite composed of highly-polarized barium strontium titanate nanofibers and electron-accepting [6,6] phenyl-C61-butyric acid methyl ester retains fast electrocaloric responses and stable cyclability at elevated temperatures. We demonstrate the effectiveness of electrocaloric cooling in a polymer composite for a pyroelectric energy harvesting device. The device utilizes a simulated central processing unit (CPU) as the heat source. Our results show that the device remains operational even when the CPU is overheated. Furthermore, we show that the composite functions simultaneously as a pyroelectric energy converter to harvest thermal energy from an overheated chip into electricity in the electrocaloric process. This work suggests a distinct approach for overheating protection and recycling waste heat of microelectronics.

Revealing the mechanisms of electrocaloric effects by simultaneously direct measuring local electrocaloric and electrostrain under ambient conditions

Electrocaloric solid-state refrigeration is a primary candidate for the next-generation cooling method for microelectronic systems. However, the mechanisms of electrocaloric effect remain unclear due to the lack of direct correlation evidence, especially for novel negative electrocaloric (NEC) effect. In this work, we develop a high-accuracy direct measurement method for simultaneously measuring local electrocaloric and electrostrain responses under ambient conditions, providing the accompanying electrostrain evidence during electrocaloric effect. Combined with experiments, thermodynamic analyses and finite element simulations, it is found that the electrostrain is very large when NEC and large positive electrocaloric (PEC) responses occur, and NEC effect is caused by ferroelectric-ferroelectric phase transition, while large PEC response originates from paraelectric-ferroelectric phase transition. Results also indicate that NEC temperature change does not increase with electric field, and shows the maximum temperature change at phase transition critical electric field, different from PEC temperature change increasing with electric field. The developed method provides an alternative and sensitive pathway to investigating electrocaloric effect.

Influence of chemical strains on the electrocaloric response, polarization morphology, tetragonality, and negative-capacitance effect of ferroelectric core-shell nanorods and nanowires

Using Landau-Ginzburg-Devonshire (LGD) approach, we proposed the analytical description of the influence of chemical strains on spontaneous polarization and the electrocaloric response in ferroelectric core-shell nanorods. We postulate that the nanorod core presents a defect-free single-crystalline ferroelectric material, and elastic defects are accumulated in the ultrathin shell, where they can induce tensile or compressive chemical strains. Finite-element modeling (FEM) based on the LGD approach reveals transitions of domain-structure morphology induced by chemical strains in the BaTiO3 nanorods. Namely, tensile chemical strains induce and support the single-domain state in the central part of the nanorod, while the curled domain structures appear near the unscreened or partially screened ends of the rod. The vortexlike domains propagate toward the central part of the rod and fill it entirely, when the rod is covered by a shell with compressive chemical strains above some critical value. The critical value depends on the nanorod sizes, aspect ratio, and screening conditions at its ends. Both analytical theory and FEM predict that the tensile chemical strains in the shell increase the nanorod polarization, lattice tetragonality, and electrocaloric response well above the values corresponding to the bulk material. The physical reason for the increase is strong electrostriction coupling between the mismatch-type elastic strains induced in the core by chemical strains in the shell. Comparison with earlier XRD data confirmed an increase of the tetragonality ratio in tensile BaTiO3 nanorods compared to the bulk material. Obtained analytical expressions, which are suitable for the description of strain-induced changes in a wide range of multiaxial ferroelectric core-shell nanorods and nanowires, can be useful for strain engineering of advanced ferroelectric nanomaterials for energy storage, harvesting, electrocaloric applications, and negative capacitance elements. Published by the American Physical Society 2024

Achieving Ultrahigh Electrocaloric Response in (Bi0.5Na0.5)TiO3-Based Ceramics through B-Site Defect Engineering.

Lead-free electrocaloric (EC) ferroelectrics are considered ideal for the next generation of environmentally friendly solid-state refrigeration materials. However, their inferior performance compared to lead-based materials significantly restricts their potential application. According to phase-field simulations, it is predicted that the pinning effect of a moderate number of defects can effectively enhance the reversible polarization response associated with the entropy change. Herein, sodium-bismuth titanate (BNT) ceramics with high spontaneous polarization are selected to construct B-site defects by introducing Li+ and Nb5+. Under an electric field of 6 kV mm-1, ultrahigh EC temperature changes of ΔTpos = 1.77 and ΔTneg = 1.49 K are achieved at 65 °C by direct measurement (ΔTneg > 1 K over 55-120 °C). Furthermore, ΔTneg remains above 0.70 K in the temperature range from 25 to 130 °C, exhibiting immense potential for practical applications. This study offers a promising direction for optimizing the EC response in defect systems.

Electrocaloric response enhancement over a broad temperature range in lead-free BT-based ceramics

The electrocaloric effect (ECE) capabilities of environmentally sustainable BaTiO3 systems have garnered extensive scrutiny for their potential applications, owing to their intricate phase transition characteristics, facile miniaturization, heightened efficacy, and low cost. Nevertheless, persistent challenges loom in the quest for a substantial trade-off between significant temperature change (ΔT) and a wide operating temperature range (Tspan). In this study, we synthesized the Ba(1–1.5x)LaxTi0.9Zr0.1O3 (abbreviated as BLZT100x, x = 0, 0.005, 0.01, and 0.015) ferroelectric system and conducted thorough characterizations encompassing phase structure, electrical attributes, and electrocaloric properties. The results reveal a reduction in the phase transition temperature to lower regimes and an augmentation in the diffuse phase transition characteristics upon the incorporation of La3+ into the Ba(Zr, Ti)O3 matrix. This enhancement significantly bolsters the electrocaloric performance while expanding the viable operating temperature ranges. Notably, in the BLZT0.5 sample, an exceptional ΔT of 1.22 K under a moderate electric field of 50 kV/cm is observed, accompanied by an extraordinarily broad Tspan of 80 °C spanning from 60 °C to 140 °C, as determined through direct measurement method. These findings impeccably align with the prerequisites of practical refrigeration applications.

Phenomenological analysis of positive and negative electrocaloric effects in Rochelle salt

Ferroelectrics exhibiting both positive and negative electrocaloric effects stand out as promising materials for achieving high-efficiency solid-state refrigeration. The phase transition is acknowledged as a crucial factor in designing such materials. This study investigates the electrocaloric behavior and its correlation with phase transitions in Rochelle salt single crystal using Landau thermodynamic theory. Analysis of free energy density, ferroelectric, and dielectric data uncovers a complex temperature-dependent phase transition sequence of paraelectric–ferroelectric–paraelectric. This imparts Rochelle salt single crystal with intriguing field-induced phase transition behaviors and electrocaloric responses. Pronounced positive and negative electrocaloric effects are, respectively, observed near the high-temperature and low-temperature Curie point. The opposite sign of entropy differences between the ferroelectric and the two paraelectric phases is identified as the origin of the difference in electrocaloric response. These findings not only enhance our understanding of the electrocaloric effect but also provide a design solution for materials with the coexistence of positive and negative electrocaloric effects.

Enhanced electrocaloric response and energy storage in [(Bi0.5Na0.5)0.94Ba0.06]0.975Sr0.025TiO3 ceramic close to room temperature

Lead-free [(Bi0.5Na0.5)0.94Ba0.06]0.975Sr0.025TiO3 (BNBTS25), with morphotropic phase boundary, has attracted considerable attention due to its depolarization temperature, Td (Ferroelectric (FE)- Antiferroelectric (AFE) transition), which is in close proximity to room temperature (322K). Therefore, the research on electrocaloric and energy storage was limited to temperatures ranging from 223K to 363K. BNBTS25 presents firstly an intensive interests as promising candidate for environmental-friendly energy storage products. This material provides a good recoverable energy density up to Wrec = 577 mJ/cm3 with an energy-storage efficiency of η = 68% in the vicinity of Td. The origin of enhanced energy storage performance is discussed from a scientific point of view. Secondly, BNBTS25 exhibits a high electrocaloric effect (ECE), including an adiabatic temperature change of ΔT = 1.24 K and an electrocaloric responsivity of ξ = 0.301K.mm/kV was observed under an electric field of E = 40 kV/cm at a temperature near 290K. This high ECE performance exceeds that of several reported lead-free ceramics and even performs better than some lead based ceramics. The obtained results suggest that BNBTS25 ceramic is a promising candidate for both electrocaloric and energy storage applications in operating temperature such as cooling in microelectronic devices.

Large room-temperature electrocaloric response achieved by tailoring the first-order orthorhombic-tetragonal phase transition in BaTiO3-based ferroelectric ceramics

The development of refrigeration technology has burgeoned in response to the challenges posed by global warming. Notably, electrocaloric materials have garnered significant attention for their potential in miniaturization and high efficiency. However, investigations into the electrocaloric response in proximity to orthorhombic-tetragonal phase transition in BaTiO3-based ferroelectric ceramics remain scarce. In this study, we employ a lead-free x(Ba0.7Sr0.3)TiO3-(1-x)Ba(Zr0.2Ti0.8)O3 polycrystalline ceramics fabricated through the solid-state reaction method. The comprehensive analysis includes elucidation of the phase structure, surface morphologies, dielectric properties, domain switching behavior, and the electrocaloric effect. Integrating these findings with the phase diagram unveils optimal adiabatic temperature change (ΔT) achieved within the quadruple point coexistence region. That is, the highest ΔT of 0.53 K is achieved for x = 0.4 ceramic, which is attributed to the increased entropy resulting from multiple polar phases. Remarkably, the highest ΔT of 0.3 K is attained in x = 0.6 ceramic under a low driving electric field of 30 kV/cm at room temperature, precisely within the orthorhombic-tetragonal transition temperature. The investigation underscores the efficacy of regulating the first-order orthorhombic-tetragonal phase transition to achieve exceptional electrocaloric responses at room temperature.

Size-induced high electrocaloric response of dense ferroelectric nanocomposites

Analytical results obtained within Landau-Ginzburg-Devonshire approach and effective media models, predict that the synergy of Vegard stresses and size effects can significantly enhance the electrocaloric cooling (up to 7 times) of the BaTiO3 nanoparticles in comparison with a bulk BaTiO3. To compare with the considered effective media models, we measured the capacitance-voltage and current-voltage characteristics of the dense nanocomposites consisting of (28–35) vol% BaTiO3 nanoparticles incorporated in organic polymers and determined experimentally the effective dielectric permittivity and losses of the composites. Generalizing obtained analytical results, various ferroelectric nanoparticles spontaneously stressed by elastic defects, such as oxygen vacancies or any other elastic dipoles, which create a strong chemical pressure, can cause the giant electrocaloric response of dense ferroelectric nanocomposites. We have shown that the advantages of the studied lead-free dense nanocomposites are the good tunability of electrocaloric cooling temperature due to the strain and size effects in ferroelectric nanoparticles and the easy control of the high electrocaloric cooling by electric fields. This makes the dense ferroelectric nanocomposites promising for cooling of conventional and innovative electronic elements, such as FETs with high-temperature superconductor channels.

Strong electrocaloric response with ultrawide working temperature span in lead-free BaTiO3-based ceramics by composition design

Solid-state refrigeration technology based on electrocaloric effect provides a cost-effective and convenient approach for microelectronic device cooling. However, the working temperature range of electrocaloric materials is often narrow and incommensurate with that required by many microelectronic devices including computer chips and CPUs. In this work, the working temperature span of electrocaloric response in BaTiO3 ceramics were tuned by the simultaneous introduction of Sr and Hf (abbreviated as BSHT). Through rational variation of Sr and Hf content, not only the Curie temperature is reduced to ∼80 °C, but also the room-temperature phase structure of the ceramics can be effectively tuned from a single rhombohedral phase to coexistence of rhombohedral + orthorhombic or orthorhombic + tetragonal phases. Relaxor behavior of BSHT ceramics is evident by the frequency-dispersion of dielectric permittivity and the increased relaxor diffuseness factor γ. Temperature-dependent Raman spectra confirm that Ba0.85Sr0.15Hf0.06Ti0.94O3 (0.6BSHT) sustain a two-phase coexistence state over a broad temperature range. 0.6BSHT ceramics render a large adiabatic temperature change ΔT of 0.91 K under 40 kV cm−1 with a broad temperature span of ∼57 °C, covering the working temperature window of most microelectronic devices. Moreover, the electrocaloric response of 0.6BSHT shows excellent fatigue endurance, i.e., the adiabatic temperature change shows a trivial degradation of <4.4 % after 106 cycles. This work demonstrates that BaTiO3-based ceramics remain one of the most promising electrocaloric candidates and compositional modification is an effective route to tune the electrocaloric response for fulfilling the requirements of practical applications.

Stabilization of the first-order phase transition character and enhancement of the electrocaloric effect by Na0.5Bi0.5TiO3 substitution in BaTiO3 ceramics

First-order character of the ferroelectric phase transition in BaTiO3 (BT) is maintained by Na0.5Bi0.5TiO3 (NBT) substitution, which in turn causes large electrocaloric response.

Broadening the operating temperature span of the electrocaloric effect in lead-free ceramics via creating multi-stage phase transitions

The strategy of this study achieves a large electrocaloric response of ΔT &gt; 1 K within a temperature span of 66 °C. The cooling performance exceeds 90% and 80% of the maximum for temperature spans of 44 °C and 67 °C, respectively.

Dysprosium doping induced effects on structural, dielectric, energy storage density, and electro-caloric response of lead-free ferroelectric barium titanate ceramics

Dysprosium doping induced effects on structural, dielectric, energy storage density, and electro-caloric response of lead-free ferroelectric barium titanate ceramics

Optimized Electrocaloric Refrigeration in Lead-Free NaNbO3-Based Ceramics via AFE ↔ FE Phase Transition Modulation.

An outstanding challenge for eco-friendly ferroelectric (FE) refrigeration is to achieve a large adiabatic temperature change within a broad temperature range originating from the electrocaloric (EC) effect, which is expected to be realized in antiferroelectric (AFE) materials owing to the large entropy change during electric field and thermally induced phase transition. In this work, a large EC response over a wide response temperature range can be achieved slightly above room temperature via designing the phase transition of NaNbO3. An irreversible to reversible AFE-FE phase transition on heating induced by the introduction of CaZrO3 into NaNbO3 plays a key role in the optimized electrocaloric refrigeration. Accordingly, accompanying the local structure transformation corresponding to the B-site ions, the transition temperature between the square polarization-electric field (P-E) hysteresis loop (the irreversible AFE-FE phase transition induced by the electric field) and the repeatable double P-E hysteresis loop (the electric field induced reversible AFE-FE phase transition) was tailored to around room temperature, in favor of extending large entropy change to the wide temperature range. This work provides an efficient approach to designing lead-free EC materials with excellent EC performance, promoting the advancement of environmentally friendly solid-state cooling technology.

The strain-induced transitions of the piezoelectric, pyroelectric, and electrocaloric properties of the CuInP2S6 films

Low-dimensional ferroelectrics, ferrielectrics, and antiferroelectrics are of urgent scientific interest due to their unusual polar, piezoelectric, electrocaloric, and pyroelectric properties. The strain engineering and strain control of the ferroelectric properties of layered two-dimensional van der Waals materials, such as CuInP2(S,Se)6 monolayers, thin films, and nanoflakes, are of fundamental interest and especially promising for their advanced applications in nanoscale nonvolatile memories, energy conversion and storage, nano-coolers, and sensors. Here, we study the polar, piezoelectric, electrocaloric, and pyroelectric properties of thin strained films of a ferrielectric CuInP2S6 covered by semiconducting electrodes and reveal an unusually strong effect of a mismatch strain on these properties. In particular, the sign of the mismatch strain and its magnitude determine the complicated behavior of piezoelectric, electrocaloric, and pyroelectric responses. The strain effect on these properties is opposite, i.e., “anomalous,” in comparison with many other ferroelectric films, for which the out-of-plane remanent polarization, piezoelectric, electrocaloric, and pyroelectric responses increase strongly for tensile strains and decrease or vanish for compressive strains.

Finite-temperature properties of PbTiO3 by molecular dynamics simulations

PbTiO 3 is a prototypical ferroelectric perovskite that is known to undergo a temperature driven ferroelectric tetragonal to paraelectric cubic phase transition, but the understanding of some key phenomena and associated mechanisms underlying this transition remains unclear. Here, using molecular dynamics simulations based on first-principles effective Hamiltonian, we show the behaviors of the phase transition temperature Tc and adiabatic temperature change ΔT of PbTiO3 under an external electric field and tensile stress along the [001] direction. Our results show that the electric field E induces rising Tc via a linear relationTc∝ 0.3083E, rendering the phase transition to go from first-order with thermal hysteresis to second-order without thermal hysteresis above ∼200 kV/cm; meanwhile, a maximum electrocaloric response ΔTmax∼34 K is obtained under E=500 kV/cm. Moreover, external stress (σz) causes rising Tc via a linear relationTc∝160σz and improves the electrocaloric response ΔTmax when combined with the electric field. The present results offer insights into the physical processes and mechanisms that dictate finite-temperature properties of ferroelectric perovskite oxides, laying a foundation for further exploration of this intriguing class of materials.

Achieving improved electrocaloric effect and broad operation temperature by tailoring phase fraction in BaTiO3-based ceramics

Ferroelectric electrocaloric (EC) materials are a pivotal part of sustainable micro-cooling, which complements conventional cooling methods in miniaturized devices. However, the application of EC materials is largely limited by the unsatisfactory room-temperature EC response and narrow operation temperature window. In this work, with careful tuning the composition of BaTiO3 (BT)-based ceramics through the conventional doping method, enhanced EC properties were achieved in Ba0.8Sr0.2Zrx(Ti0.999Mn0.001)1-xO3 (BSZMT) (x = 0.05, 0.10, 0.15, and 0.20). We especially found that the electrocaloric temperature change (ΔT) of > 0.50 K was achieved in a broaden temperature ranging from 20 °C to 95 °C and a peak ΔT value of 1.08 K was achieved at temperature of 70 °C in our BSZMT with x = 0.15. To ascertain the physical mechanism, we employed hybrid characterization methods including XRD, Raman, and dielectric analysis together with Density Functional Theory (DFT) calculation. Our results clearly show that the doped Zr and Mn at B site of BSZMT effectively modulate the microstructure leading to coexistence of different ferroelectric phases and broken long-range ferroelectric order, which subsequently results in the enhanced EC performance and broad operating temperature window.

Electrocaloric behaviour of tape cast and grain oriented NBT-KBT ceramics

We have investigated the effects of grain orientation and tape casting process on the electrocaloric properties of 0.82Na0.5Bi0.5TiO3-0.18 K0.5Bi0.5TiO3 (0.82NBT-0.18KBT) ceramics at the Morphotropic Phase Boundary (MPB), using direct and indirect measurements. We observe a larger electrocaloric response for the template-free ceramics compared to 7 and 10 wt% template containing ones, suggesting that grain orientation along rhombohedral < 100 > does not improve the electrocaloric response. Indirect measurements yielded a large adiabatic temperature change of around 3 K under an electric field of 50 kV/cm, which is significantly higher than 0.9 K reached at a lower electric field of 40 kV/cm using the direct measurement.