Positive Electrocaloric Effect Research Articles

Achieving large negative electrocaloric effect in AgNbO3-based antiferroelectric ceramics based on dipole disordering

The electrocaloric effect (ECE) is considered as an environmentally friendly refrigeration technology, which can be used to replace vapor compression refrigeration. Lead-free antiferroelectric (AFE) ceramics are promising electrocaloric materials because of large saturation polarization and polarization change with temperature, which can generate large positive or negative ECE. In this work, the lead-free Ag(Nb1-0.8xHfx)O3 (x = 0, 0.001 and 0.003) AFE ceramics are fabricated through a conventional solid-state reaction method which shows a decreased the phase transition temperature of M1 - M2 toward room temperature (RT), and a large negative ECE (ΔT = −2.3 K under 130 kV cm−1) is obtained near the ambient temperature by using an indirect measurement in ANH0.001 AFE ceramic. The large negative ECE behavior is ascribed to dipole disordering (i.e. the anti-parallel dipoles are broken and gradually becomes disordered under an external electric field) resulting from the field-induced phase transition under a modest electric field. This work shows that AgNbO3-based AFE ceramics could be used in the refrigeration devices as an electrocaloric material.

Field-Induced Antiferroelectric-Ferroelectric Transformation in Organometallic Perovskite Displaying Giant Negative Electrocaloric Effect.

Antiferroelectric materials with an electrocaloric effect (ECE) have been developed as promising candidates for solid-state refrigeration. Despite the great advances in positive ECE, reports on negative ECE remain quite scarce because of its elusive physical mechanism. Here, a giant negative ECE (maximum ΔS ∼ -33.3 J kg-1 K-1 with ΔT ∼ -11.7 K) is demonstrated near room temperature in organometallic perovskite, iBA2EA2Pb3I10 (1, where iBA = isobutylammonium and EA = ethylammonium), which is comparable to the greatest ECE effects reported so far. Moreover, the ECE efficiency ΔS/ΔE (∼1.85 J cm kg-1 K-1 kV-1) and ΔT/ΔE (∼0.65 K cm kV-1) are almost 2 orders of magnitude higher than those of classical inorganic ceramic ferroelectrics and organic polymers, such as BaTiO3, SrBi2Ta2O9, Hf1/2Zr1/2O2, and P(VDF-TrFE). As far as we know, this is the first report on negative ECE in organometallic hybrid perovskite ferroelectric. Our experimental measurement combined with the first-principles calculations reveals that electric field-induced antipolar to polar structural transformation results in a large change in dipolar ordering (from 6.5 to 45 μC/cm2 under the ΔE of 18 kV/cm) that is closely related to the entropy change, which plays a key role in generating such giant negative ECE. This discovery of field-induced negative ECE is unprecedented in organometallic perovskite, which sheds light on the exploration of next-generation refrigeration devices with high cooling efficiency.

Electrocaloric effects of Ba1-xCaxTi0.9Zr0.1O3 through both direct and indirect measurements

The electrocaloric effect (ECE) of lead-free BaTiO3 based materials has drawn considerable attention for application in next-generation refrigeration, which has been widely investigated through indirect measurement methods; however, this method is sometimes ineffective because negative and positive ECEs coexist in one material. The paradox of ECE phenomena should be clarified; therefore, in this work, the ECEs of modified BaTiO3 were investigated through both direct and indirect measurement methods. The investigated BaTiO3 compositions herein are (Ba1-xCax)(Ti0.9Zr0.1)O3 (0.05 ≤ x ≤ 0.20), abbreviated as BZT-100xCa. Through indirect measurements, the ECE reached its maximum value around the Curie temperature TC, and the coexistence of negative and positive ECEs was observed in BZT-5Ca, BZT-10Ca, and BZT-15Ca. A laboratory-built ECE direct measurement system utilizing a platinum resistor was used to measure the ECE of BZT-100xCa. Through direct measurement, the maximum ECE appears several Kelvins above the TC, which coincides with the ECE through the indirect measurement, but no negative ECE is observed. Meanwhile, the ECE measured through direct measurement was smaller than that measured through indirect measurement. Through direct measurements, BZT-15Ca exhibited the largest ΔT of 0.291 K and ΔT/ΔE of 0.182 × 10−6 K m/V under 16 kV/cm, indicating that BZT-15Ca has a better ECE performance than previously reported modified BaTiO3 ceramics. By comparison, it can be concluded that the ECE through the indirect measurement method can reflect the approximate variation tendency of the actual ECE, and the negative ECE measured indirectly should be certified through direct measurement method, which doesn't influence the reliability of ECE through the indirect measurement.

Impact of annealing process on the properties of 0.85Pb(Sc0.5Ta0.5)O3-0.15PbTiO3 ceramics

We have synthesized a ceramic sample 0.85Pb(Sc0.5Ta0.5)O3-0.15PbTiO3 [0.85PST-0.15PT] of perovskite structure by using a conventional solid-state reaction method. Systematic impact of annealing time (AT) is studied methodically for comprehending the ferroelectric and dielectric properties, as well as electrocaloric effect and energy harvesting performance. After 40 h of AT and using the electric field of 40 kV/cm on 0.85PST-0.15PT, we have achieved maximum ferroelectric polarization (P = 46.9 μC/cm2), energy capture performance (W = 0.542 J/cm3), and large positive electrocaloric effect (ΔT ∼ 1.63 K and ΔS ∼ 1.23 J/(K.kg)). Additionally, we also achieved high dielectric constant (ε = 23,350) and low dielectric loss (tanδ = 0.039) at the frequency of 1 Hz. As compared to an unannealed situation, the results on ceramic sample with 40 h of AT have revealed increase in P, ε, ΔT, ΔS, and W by 36%, 27%, 37%, 12% and 14%, respectively. Optimal values of AT differed radically for achieving the highest performance in each category. The best performance of energy storage (Wenergy ∼ 0.638 J/cm3) and energy loss (Wloss ∼ 0.498 J/cm3) densities are achieved with the AT of 30 h. The performance of 0.85PST-0.15PT sample increased gradually with the increase of AT until it attained the highest value and then decreased by further increasing AT – exhibiting a peak distribution. Our investigations have provided valuable insights of optimizing the performance of 0.85PST-0.15PT ceramic system through annealing.

Phase‐structure evolution and electrocaloric effect in Sr2NaNb4.3Ta0.7O15‐based filled tungsten bronze ceramics

AbstractTo investigate the effect of A‐site substitution on phase‐structure evolution and electrocaloric effect (ECE) in filled tungsten bronze ceramics, Sr2–3x/2LaxNaNb4.3Ta0.7O15 (x = 0, 0.02, 0.04, and 0.06) ceramics are prepared through a two‐step solid‐state reaction method. Rietveld refinement shows that La2O3 addition achieves a partial transition from orthorhombic (Im2a) to tetragonal phase (P4bm). A decrease in phase transition temperature, enhancement in distortion degree, and relaxation behavior are observed with increasing La2O3 content, as demonstrated by dielectric, in situ Raman spectra, and ferroelectric properties. The indirectly measured ECE shows a non‐monotonic variation, with room‐temperature ECE ranging from 0.19, 0.15, 0.23 to finally 0.19 K for x = 0, 0.02, 0.04, and 0.06 compositions, respectively. This evolution trend is explained by an interplay of relaxation and multiphase coexistence. Directly measured ECE is also performed and a positive ECE character with a good temperature stability is observed. This work not only establishes relationships between phase‐structure evolution and ECE in filled Sr2NaNb4.3Ta0.7O15 tungsten bronze ceramics, but also presents its possibility in applications of thermally stable EC devices.

Performance Study on an Electrocaloric Heat Pump Based on Ga-Based Liquid Metal

A solid-state heat pump using the electrocaloric effect (ECE) provides a new idea for the future development of heat pumps. However, most of the electrocaloric (EC) heat pumps presented in the literature are low in efficiency and use at least one moving part, which significantly reduces the reliability of the heat pump and adds to its complexities. In this context, combining the positive and negative ECEs, we proposed a plate-laminar non-mobile EC heat pump adopting Gallium-based liquid metal as an intermediate medium to guarantee highly efficient heat transfer. Numerical simulation in COMSOL Multiphysics has been performed to investigate the correlation between different operating parameters and the performance of the EC heat pump. Changing the temperature span only, a COP of 8.13 and a UVHP of 746.1 W·dm−3 were obtained at a temperature span of 7 K. It was also found that the UVHP increased by 28.45% and COP increased by 25.46% after adding one layer of EC material. The electric-induced quantity of heat and cooling capacity was found to significantly affect the heating performance. The biggest heating power of 7132.7 W·dm−3 was obtained under 200 MV·m−1, and the biggest COP of 14.84 was obtained under 150 MV·m−1 at a cyclic period of 8 s. This study provides a highly efficient, non-mobile EC heat pump that employs fluid-thermal conjugated heat transfer, and exploration of the parameters makes the optimization of the heat pump possible by fine-tuning the operation parameters.

Large negative electrocaloric response induced by nanoscale phase transition in (Bi, Na)TiO3-based thin films

Electrocaloric effect (ECE) offers an efficient and environmentally friendly route for solid-state cooling. Either positive or negative ECE could exhibit a large adiabatic temperature change (ΔT). Compared to the positive electrocaloric response, the investigation of negative ECE is lagging behind, largely due to the fact that its origin is still elusive. In this work, the negative ECE behavior of conventional ferroelectric thin films, namely 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 (BNBT), was studied. A remarkable ΔT of −26.1 K was acquired near 160 °C under a moderate electric field of 875 kV/cm, attributing to the ferroelectric phase transition in the polar nanoregions from rhombohedral (R3c) to tetragonal (P4bm), as confirmed by temperature-dependent dielectric permittivity, Raman spectra, and x-ray reciprocal space mapping. The BNBT thin film presents a high electrocaloric coefficient (ΔT/ΔE) of 0.0298 K cm kV−1, transcending that of the most reported negative electrocaloric response of thin films.

Positive and Negative Electrocaloric Effect in Lead-Free Silver Niobate Antiferroelectric Ceramic Depending on Affluent Phase Transition

We prepared a dense AgNbO3 ceramic using a conventional solid-state reaction method. The phase structure, electrical properties and electrocaloric effect (ECE) were systematically investigated. Large negative and positive ECEs (−4.38 °C at 65 °C and 2.3 °C at 210 °C) under an external electric field of 180 kV·cm−1 were obtained in the eco-friendly AgNbO3 antiferroelectric (AFE) ceramic due to affluent phase transition and a high electric field. The large positive and negative ECEs originated from the phase transition between ferrielectric (FIE) phases (the orthorhombic space group (Pmc21) and AFE phases (Pbcm) tuned by an applied external field. Additionally, a probable mechanistic model was proposed to illustrate the generation of positive and negative ECEs. This study may provide guidelines for the design of high-efficiency solid-state cooling devices.

Thickness dependence of PbZr0.52Ti0.48O3 thin film ferroelectric parameters

Thickness is an important parameter of ferroelectric thin films, which could have a strong influence on device performance based on them. In this paper, we demonstrate the mechanism of the thickness dependence of the ferroelectric performance of a Pb(Zr0.52Ti0.48)O3 (PZT52/48) film prepared with a sol-gel method. It is observed that the spontaneous polarization (Ps) is enhanced, while the dielectric permittivity (ɛ), the coercivity field (Ec), residual polarization (Pr), efficiency (η) and energy density (Wenergy) are reduced with thicker film. In the meantime, the positive electrocaloric (EC) effect gradually transits into a negative regime. Changes in electrical properties are attributed to the increase in the tetragonal phase and the decrease in rhombohedral phase structure, due to the variation of in-plane tensile stresses with thickness which is confirmed by the atomic image of high-resolution transmission Electron microscopy. These findings provide guidance for study in modern high performance ferro-electronic device and material.

Giant negative electrocaloric effect over a wide temperature region (263–373 K) in lead-free BiFeO3/B4Ti3O12 composite films

A giant negative electrocaloric effect is found in lead-free BiFeO3/Bi4Ti3O12 composite films with ΔT = −5.717 K under a relatively suitable working applied field of ΔE = 305 kV/cm. More importantly, the negative electrocaloric effect is achieved over a wide temperature range (263–373 K). The strong electrocaloric effect is due to the BiFeO3 layer and Bi4Ti3O12 layer combined action to decrease thermal entropy in relatively modest electric fields. The negative electrocaloric effects allow a new type of solid-state refrigeration equipment to gradually become possible, which can make more effective use of energy and gain hope to enhance the cooling efficiency by combining with the ideal positive electrocaloric effect.

Effects of thermal and electrical hysteresis on phase transitions and electrocaloric effect in ferroelectrics: A computational study

Hysteresis, including thermal and electrical hysteresis, is a basic feature of ferroelectrics with a first-order phase transition and has consequences on properties, including the electrocaloric effect (ECE). Here we systematically study the influence of thermal and electrical hysteresis on the ECE by monitoring the history dependence of zero-field, and field heating and cooling, for three typical phase transitions in BaTiO3 single crystals. We employ molecular dynamics simulations using an effective Hamiltonian based on first-principles calculations. We find that within the hysteresis regions of first-order phase transitions, where large ECEs occurs, the widest temperature span appears under zero field and this shrinks with increasing electric field. Moreover, the ECE is irreversible when one of the initial or final states with varying electric fields is located inside the hysteresis region. However, it is reversible when two states fully cross the hysteresis region. In continuous electric field cycles, the irreversible ECE has a large thermal response when the electric field is changed at first, but becomes smaller with subsequent changes in the field. The electrical hysteresis loss results in an increase of temperature under continuous field cycles regardless of positive or negative ECEs. Our work provides a basis for the efficient design of reversible electrocaloric cycles for refrigeration applications.

The structural, dielectric, electrocaloric, and energy storage properties of lead-free Ba0·90Ca0·10Zr0·15Ti0·85O3

Lead-free ferroelectric materials with Electrocaloric Effect (ECE) and recoverable energy (W rec ) represent a major breakthrough in the technological race for manufacturing low-cost and eco-friendly smart multifunctional devices. Here, guided by the benefits resulting from successive ferroelectric-ferroelectric (F–F) and ferroelectric-pseudo cubic (F-PC) transitions including broad operational temperature span, lead-free Ba 0·90 Ca 0·10 Zr 0·15 Ti 0·85 O 3 (BCZT) ferroelectric ceramic located near the phase convergence region was prepared using the conventional solid-state solution. In this study, the systematic investigation of structural and microstructural properties via X-Ray Powder Diffraction (XRPD), scanning electron microscopy (SEM), temperature-dependence Raman spectroscopy and dielectric properties confirm the formation of a pure perovskite material which undergoes consecutive R 3 c - R 3 m , R 3 m - Amm 2, Amm 2- P 4 mm (F–F) phase transitions followed by (F-PC) transition in a wide temperature range centered around room temperature (RT). Polarization-Electric field (P-E) measurements, revealed the coexistence of Positive Electro-Caloric Effect (PECE) and Negative Electro-Caloric Effect (NECE) in a wide temperature span of [-40 °C, 95 °C]. More interestingly, we reached a large absolute temperature change (ΔT) and EC coefficient (ξ) values of (5.505 °C, 1.847 KmmkV −1 ), (0.861 °C, 0.289 KmmkV −1 ) and (1.897 °C, 0.637 Kmm kV −1 ) registered respectively at about −5, 42.5 and 74.5 °C under an electric field of 29.80 kVcm −1 . This work will promote further studies on lead-free BCZT ceramics towards multifunctional environmentally-friendly material promising for solid-state cooling technology.

Coexistence of giant positive and large negative electrocaloric effects in lead-free ferroelectric thin film for continuous solid-state refrigeration

The solid-state cooling technique utilizing electrocaloric (EC) materials is an alternative approach to tackle the greenhouse effect caused by traditional vapor-compression refrigeration. However, such a promising technique is severely hampered by the lack of proper materials considering that most existing EC materials with a single positive/negative EC effect exhibit a limited cooling effect. Here, the coexistence of a positive and negative EC effects has been achieved in a lead-free Na0.5Bi0.5(Ti0.97W0.01Fe0.02)O3 ferroelectric film. A state-of-the-art positive adiabatic temperature change (ΔT) of ~56 K accompanied by an isothermal entropy change (ΔS) of ~64 J K−1 kg−1 at 143 °C and a large negative ΔT of ~ − 17 K with a ΔS of ~ − 24 J K−1 kg−1 at 55 °C are obtained under a strong electric field strength of 2692 kV cm−1. Oxygen vacancy-related defect dipoles play a critical role in the negative EC effect at lower temperatures, while the phase transition is responsible for the positive EC effect at higher temperatures. Meanwhile, the film exhibits a high EC strength with a maximum ΔT/ΔE of 0.021 K cm kV−1, together with a ΔS/ΔE of 0.024 J cm K−1 kg−1 kV−1. This work ensures a giant total temperature change by utilizing and combining both the negative and positive EC effects in a dual cooling process.

Giant negative electrocaloric effect in antiferroelectric La-modified PbZrO3 thin films within wide temperature range

Antiferroelectric thin films have exhibited outstanding negative electrocaloric effect (ECE), which are potential candidate regarding to the solid-state refrigeration technology. In this work, Pb0.97La0.02ZrO3 (PLZ) antiferroelectric thin films were prepared on LaNiO3(LNO)/Pt electrode and LNO electrode by sol-gel method. Giant negative electrocaloric effect (ΔT ~ −13.2 K) can observed in wide temperature range (213–493 K) that deposited on composite electrode, which can be attributed to better electric conductivity of LNO/Pt composite electrode than LNO electrode. In addition to outstanding negative ECE, positive ECE also can be obtained near curie temperature, which is caused by phase transition from ferroelectric (FE) to paraelectric (PE) state. The optimized ECE property contributed to the phase transition and introduction of composite electrode, which pave the giant prospect considering high-efficiency solid-state refrigeration devices through combination of positive and negative ECE.

Large Room Temperature Negative Electrocaloric Effect in Novel Antiferroelectric PbHfO3 Films.

Extremely high temperature in a chip will severely affect the normal operation of electronic equipment; however, the traditional air conditioning cooling technology is unsuitable for integrated circuit cooling. It is necessary to develop convenient and high-efficiency cooling techniques. In this paper, PbHfO3 antiferroelectric (PHO AFE) film was fabricated by a sol-gel method and was first found to be a promising electrocaloric (EC) material with high temperature change (ΔT ∼ -7.7 K) and acceptable EC strength (|ΔT/ΔE| ∼ 0.023 K cm kV-1) at room temperature. In addition to the negative EC effect (ECE), a large positive ECE can be observed at high temperature. The outstanding ECEs and their combination will make the PHO film one of the potential candidates for next-generation solid-state refrigeration. To understand the underlying physical mechanism for positive and negative ECEs in the PHO AFE film, a modified Ginzburg-Landau-Devonshire free-energy theory is adopted.

P-GaN-substrate sprouted giant pure negative electrocaloric effect in Mn-doped Pb(Zr0.3Ti0.7)O3 thin film with a super-broad operational temperature range

Ferroelectric thin films simultaneously possessing large positive and negative electrocaloric (EC) effects are attractive to the solid-state temperature-controlled devices which can be used in many fields, such as modern electronics, communications, medical and military, etc. Here, it is demonstrated that the giant positive EC effect (△Tmax ~ 44.5 K and △Smax ~ - 42.8 JK−1 kg−1 at ~ 313 K) of the Mn-doped Pb(Zr0.3Ti0.7)O3 (PZT-Mn) thin film deposited by a sol-gel method on the Pt/TiOx/SiO2/Si can be tailored into a pure negative EC effect (△Tmax ~ - 23.5 K and △Smax ~ 16.3 JK−1 kg−1) with a recorded super-broad operational temperature range (~ 150 K) by directly depositing on the p-GaN-substrate that has a high carrier concentration (n = 4 × 1017). Under the sprouting of the p-GaN-substrate, an electric-field induced structural phase transition (nano-scaled tetragonal phase to rhombohedral phase) plays a key role in obtaining the recorded pure negative EC effect. It is concluded that direct deposition of ferroelectric thin film on the p-GaN-substrate can be used as a universal-simple-effective strategy to generate a pure negative EC effect in a broad operational temperature range (≥ 100 K).

The electrocaloric effect in BaTiO3:Eu ceramics determined by an indirect method

ABSTRACT The electrocaloric effect (ECE) was investigated in BaTiO3 ceramics, pure and doped with 2% of Eu3+ ions. The temperature and electric field dependences of ECE were investigated by the indirect method. Based on Maxwell relations, the ECE was characterized via P–T curves under different electric fields. The ECE experiments have been carried out from room temperature through the phase transition region and in the paraelectric phase. A normal positive ECE was found around the phase transition point and is of the order of 0.6 K for pure BaTiO3 and 0.15 K for the BT doped with Eu ions. The abnormal negative ECE was detected for the BaTiO3 ceramic doped with Eu ions above the temperature of the phase transition. This phenomenon was related to the existence of defects that originates from the fixed local polarization of defect dipoles.

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.

Thermodynamic Analysis of Stress‐Mediated Barocaloric Effect, Electrocaloric Effect, and Energy Storage of PbZrO3 Antiferroelectric Film

A thermodynamic model is established to investigate the effects of internal stress in PbZrO3 (PZ) antiferroelectric (AFE) film on the barocaloric effect (BCE), electrocaloric effect (ECE), and energy storage properties. It is found that the internal stress can change the phase transition behaviors and free energy barriers among AFE, ferroelectric (FE), and paraelectric (PE) phases. For the BCE, it occurs only in the FE phase induced by stress or temperature. Under a certain temperature, the optimized negative and positive ECE can be obtained by controlling the internal stress and electric field. In addition, the switching electric field of the AFE→FE phase transition increases and causes a larger recoverable energy density with decreasing tensile stress. These results are analyzed and explained by tricritical points (3 cp) with different stresses, temperatures, and electric fields. Therefore, this thermodynamic model provides an effective route to optimize the BCE, ECE, and energy storage properties of PZ films.

The emergence of tunable negative electrocaloric effect in ferroelectric/paraelectric superlattices

The electrocaloric effect in ferroelectrics provides a promising solid-state refrigeration technology to meet the urgent demand for heat management in the integrated circuits. Usually, ferroelectrics show a positive electrocaloric effect, i.e. the application of an electric field causes an increase in temperature. In contrast, the negative electrocaloric effect is also possible but much less explored. These two effects, if occur in the same system, could be combined to improve the cooling performance. Here we report the occurrence of both positive and negative electrocaloric effects in ferroelectric/paraelectric PbTiO3/SrTiO3 superlattice based on phase-field simulations. The superlattice is found to stabilize a variety of vortex dipole states (with the different number of vortex) when the PbTiO3 and SrTiO3 layers have suitable thicknesses. The unique response of these topological states to the external electric field gives rise to both positive and negative electrocaloric effect, and the latter is associated with the field-dependent vortex-to-polar state transformation. The multiplicity of the vortex dipole states brings tuning freedom for the optimization of electrocaloric properties. Moreover, the electrocaloric effect is also sensitive to the strength of the screening effect determined by the thickness of the SrTiO3 layer. We thus demonstrate a novel mechanism of negative electrocaloric effect in ferroelectrics with topological dipole states and indicates the screening effect as an effective way to engineer the electrocaloric performance.