Abstract
We demonstrated a characterization method of entropy‐change measurement for the study of the electrocaloric effect (ECE). After the specific heat capacity was measured under different ...applied fields using a differential scanning calorimeter (DSC) accompanied with DC power supply, the electrocaloric Δ
S
was calculated from the temperature integral of specific heat capacities based on the basic definition of entropy. The Δ
S–T
curve of BaTiO
3
single crystal showed a sharp peak around
T
c
, which increased gradually and shifted to higher temperature with the rise of applied field. A high ECE with Δ
S
= 1.9 J/kg K and Δ
T
= 1.6 K is achieved under a quite low field of 10 kV/cm. The results agreed with the thermodynamic calculation. It provides a direct, precise, and time‐efficient method for the electrocaloric studies.
Ceramics with the composition (0.94−x)Na0.5Bi0.5TiO3–0.06BaTiO3–xSrTiO3 (NBBSTx) where x=0.10, 0.15, 0.20, and 0.25 were synthesized by a conventional solid-state reaction method to investigate their ...electrocaloric effect (ECE) and pyroelectric energy harvesting (PEH) properties. The ferroelectric, dielectric, and pyroelectric properties of the prepared ceramics were measured and discussed. It is found that the strontium titanate (ST) content and bias field greatly affect the ferroelectric–relaxor transition. Increasing ST content lowers the depolarization temperature of the ceramics, and both the ECE and PEH behavior of the ceramics strongly depend on their ST content because of the composition-induced decrease of the ferroelectric–relaxor transition temperature. The present investigation demonstrates that the ECE and PEH properties of NBBSTx ceramics can be tuned by introducing ST. Furthermore, a high PEH density of 425kJ/m3 is obtained for NBBST0.20, which is much higher than those of conventional Pb-based ferroelectrics.
The environmentally friendly and low-cost ferroelectric refrigeration based on the electrocaloric effect (ECE) has attracted considerable attention to replace the conventional vapor compression ...cooling technology. The ECE has been experimentally demonstrated in lead-free 0.9(K0.5Na0.5)NbO30.1Sr(Sc0.5Nb0.5)O3 (KNN-0.1SSN) polycrystalline ferroelectric ceramics using an indirect thermodynamic method. A large electrocaloric effect (adiabatic temperature change) of 0.28 K was observed near the transition temperature of 357 K for electric field 25 kV/cm, and that corresponds to an electrocaloric responsivity of ΔT/ΔE = 0.27 × 10−7 K mV−1, significantly larger than the pristine KNN based ceramics. A concurrent increment in specific entropy change of the material was also observed. The results indicate that ferroelectric KNN-0.1SSN materials possess a huge potential for electrocaloric refrigeration as a micro-cooler for electronic devices.
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•First report on electrocaloric effect in 0.9(K0.5Na0.5)NbO30.1Sr(Sc0.5Nb0.5)O3.•Large ΔT values are achieved in a wide temperature range near room temperature.•Electrocaloric responsivity is larger than the pristine KNN based ceramics.•Huge potential for electrocaloric refrigeration as a micro-cooler for devices.
The pseudo-first-order phase transition in 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics leads to a sharp increase in temperature change (ΔT) in the vicinity of the ferroelectric-to-relaxor transition ...temperature TFR (~100 °C) Appl. Phys. Lett. 110 (2017) 182904. In this study, we add the 0.78Bi0.5Na0.5TiO3-0.06BaTiO3-0.16(Sr0.7Bi0.2)TiO3 relaxor phase to the 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ferroelectric matrix to tune its electrocaloric effect. The results show that the addition of the relaxor phase plays a vital role in phase and local-structure evolution. A transition occurs between the ferroelectric and ergodic relaxor phases when the mass fraction of the latter increases to 30% (x = 0.3), as verified by X-ray diffraction analysis, Raman spectroscopy, and polarization-electric field (P-E) hysteresis loops. Furthermore, addition of the relaxor phase reduces the TFR from 76 °C at x = 0.1–55 °C at x = 0.2; however, this transition disappears at x = 0.3 and 0.4 composite. In-situ piezo-force microscopy (PFM) images illustrate that domains can be written into x = 0.1 and 0.2 ceramics with a valley in the piezoresponse curves. Increasing the temperature agitates the domain arrangement and decreases the contrast for PFM images; this indicates a gradual phase transition in the composite. The temperature corresponding to maximum ΔT exhibits a downward shift (0.58 K at 80 °C for x = 0.1 and 0.5 K at 65 °C for x = 0.2), while the temperature-ΔT curves are flat when x = 0.3 and 0.4. Moreover, the maximum ΔT shows a decrease with an increase in the relaxor phase content; this is believed to be related to a decrease in the latent heat due to a pseudo-first-order to second-order transition. Thus, we suggest that the incorporation of a relaxor phase into ferroelectric matrices is an effective technique to tune their electrocaloric effect and improve the thermal stability of ceramic composites.
((Bi0.5Na0.5TiO3)0.88-(BaTiO3)0.12)(1-x)-(LiNbO3)x (x = 0.0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, and 0.07; abbreviated as LiNbO3-doped BNT-BT) ceramics possessing many excellent performances (large ...electrostrain, negative electrocaloric effect and energy storage density with high efficiency) was fabricated by the conventional solid-state reaction method. A large electrostrain (maximum ~ 0.34% at 100 kV/cm and room temperature) with high thermal stability over a broad temperature range (~80 K) is obtained at x = 0.03. A large energy storage density (maximum Wenergy ~ 0.665 J/cm3 at 100 kV/cm and room temperature) with a high efficiency (η ~ 49.3%) is achieved at x = 0.06. Moreover, a large negative electrocaloric (EC) effect (maximum ΔT ~ 1.71 K with ΔS ~ - 0.22 J/(K kg) at 70 kV/cm)) is also obtained at x = 0.04. Phase transition (from ferroelectric to antiferroelectric and then to relaxor) induced by increasing the doping amount of LiNbO3 plays a very key role on the optimization of these performances. These findings and breakthroughs make the LiNbO3-doped BNT-BT ceramics very promising candidates as multifunctional materials.
Ferroelectrics are promising candidate materials for electrocaloric refrigeration. Materials with a large electrocaloric effect (ECE) near room temperature and a broad working temperature range are ...getting closer to practical applications. However, the enhanced ECE is always achieved under high electric field, which limits their wide cooling applications. In this paper, the phase diagram of lead-free BaHfxTi1−xO3 (BHT) ferroelectric ceramics was established. A large ECE under relatively low electric field (ΔE = 10 kV/cm) is firstly reported in BHT ferroelectric ceramics. The direct temperature change (ΔT = 0.35 °C under 10 kV/cm) in BHT ceramics is comparable with those reported in the literature under high electric fields. Meanwhile, the electrocaloric efficiency (ΔT/ΔE = 0.35 K mm kV−1 under 10 kV/cm) is thirteen percent higher than the best value reported previously under high electric field (ΔE = 145 kV/cm). We demonstrate that the ECE can be greatly enhanced by tuning the composition of the lead-free BHT ceramics to its first-order phase transition (FPT), invariant critical point (ICP) or diffuse phase transition (DPT). It is shown that the enhancement in ECE is strongly dependent on the nature of structural phase transition and electric field coupling effect, which has been confirmed by both the indirect and direct ECE measurements. A phenomenological explanation based on Landau model was also proposed to understand this phenomenon. Our findings in this work may provide a better understanding and design methodology for developing more practically useful electrocaloric materials.
Fig. 1(a) displays the direct ECE measurements of the BHT (x = 0.17) ceramic under a low electric field (i.e., 10 kV/cm) at 50 °C. Both the exothermic and endothermic peaks are of the similar magnitude, which verifies the reversibility of the electrocaloric effect. Fig. 1(b) presents both the direct and indirect ECE around the DPT under low electric field (i.e., 10 kV/cm). It is found that although the thermal loss during measurements has been neglected, the direct data in this figure is still higher than the indirect data. Meanwhile, the direct ECE reaches the maximum (ΔT = 0.32 °C) and a giant electrocaloric efficiency ΔT/ΔE = 0.32 K mm kV−1 is obtained at 40 °C, which agrees with the ECE estimated from the indirect ECE measurements. The BHT ceramic near DPT shows composition fluctuations, which provides more polar or nonpolar metastable states and hence the system is more sensitive to electric field. Thus, phase structure and the electric field coupling effect are considered to be the main contributions for the enhancement of ECE near DPT. Display omitted
Electrocaloric effect (ECE) is considered well applicable in the field of next generation solid state refrigeration technology. In this article, the structural properties and ECE response were ...altered and optimized by tailoring the Zr/Ti molar ratio in (Pb0.94La0.04)(ZrxSn0.30Ti0.70-x)O3 (x = 0.54, 0.52, 0.50 and 0.48) polycrystalline ceramics. Multiple phase transition behaviors including antiferroelectric (AFE) state to ferroelectric (FE) state transition under applied electric field, depolarization phase change behavior and AFE state to paraelectric state (PE) transition were observed, and their effects on the ECE response were investigated in detail. Significant improvement of ECE adiabatic temperature change of 1.04 K at room temperature (30 °C) was obtained by Maxwell method in (Pb0.94La0.04)(Zr0.52Sn0.30Ti0.18)O3 bulk ceramic through AFE→FE phase transition, while the highest ΔT of 2.73 K with electrocaloric strength (ΔT·ΔE−1) of 0.039 K cm kV−1 were observed in (Pb0.94La0.04)(Zr0.50Sn0.30Ti0.20)O3 bulk ceramic at temperature close to its Curie peak. The high ECE temperature change and ECE strength make them to be candidates for using in next generation solid-state refrigeration device.
Ferroelectric ceramic with a large electrocaloric (EC) effect at a very low electric field is very attractive in the next solid state refrigeration technology. In this work, two ...Pb(Sc0.25In0.25Nb0.25Ta0.25)O3 (PSINT) medium-entropy ceramics were successfully synthesized by a spark plasma sintering (SPS) technology, including one-step-SPS processed and two-step-SPS processed samples. A large EC effect (△T ∼ 0.85 K) with a high EC strength (△T/△E ∼ 0.021 K cm/kV) around room temperature are obtained at a very low electric field (∼40 kV/cm) in the two-step-SPS processed sample. Moreover, the working temperature range is very broad (∼120 K), which can be responsible for the high relaxation degree of the dielectric peak. It can be believed that the PSINT medium-entropy ceramics can be promising candidates for application in the next-generation EC cooling devices.
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.
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•First report on electrocaloric effect in (1-x)(K0.5Na0.5)NbO3-xSrZrO3.•Large ΔT values are achieved in a wide temperature range near room temperature.•Electrocaloric responsivity is larger than the pristine KNN based ceramics.•Huge potential for electrocaloric refrigeration as a micro-cooler for devices.
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