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 Ba_1-xCe_xTi_0.99Mn_0.01O₃ 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 (ΔT_max?=?1.22?K, ΔT/ΔE?=?0.41?K mm/kV) is found in the Ba_0.995Ce_0.005Ti_0.99Mn_0.01O₃ 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 Ba_0.970Ce_0.030Ti_0.99Mn_0.01O₃ 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.     

Phase-transition induced optimization of electrostrain,electrocaloric refrigeration and energy storage of LiNbO3 doped BNT-BT ceramics

((Bi_0.5Na_0.5TiO₃)_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 W_energy ~ 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.     

Large electrocaloric effect in two-step-SPS processed Pb(Sc0.25In0.25Nb0.25Ta0.25)O3 medium-entropy ceramics

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(Sc_0.25In_0.25Nb_0.25Ta_0.25)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.     

Long term stability of electrocaloric response in barium zirconate titanate

The stability of the electrocaloric effect under electric field cycling is an important consideration in the development of solid-state cooling devices. Here we report measurements carried out on Ba(Zr_0.2Ti_0.8)O₃ ceramics which reveal that the adiabatic temperature change, polarization-electric field hysteresis loops and dielectric permittivity/loss show stable behavior up to 10⁵ cycles. We further demonstrate that the loss in electrocaloric response observed after 10⁵ cycles is associated with the migration of oxygen vacancies. As a result, the electrical properties of the material are changed leading to an increase in leakage current and Joule heating. Reversing the polarity of the electric field after every 10⁵ cycles changes the migration direction of oxygen vacancies, thereby preventing charge accumulation at grain boundaries and electrodes. By doing so, the electrocaloric stability is improved and the adiabatic temperature remains constant even after 10⁶ cycles, much higher than achieved in commercially available barium titanate ceramics.     

Direct measurement of electrocaloric effect in lead-free (Na0.5Bi0.5)TiO3-based multilayer ceramic capacitors

Multilayer ceramic capacitors (MLCCs) of lead-free NBT-based ceramics are produced and their electrocaloric effect (ECE) is characterised for the first time. Dense MLCCs with 97μm-thick active layers are successfully produced by tape casting. Dielectric permittivity measurements reveal the MLCCs to have properties similar with that reported for the corresponding bulk ceramics, including T_d∼50°C and T_S∼100°C. Direct ECE measurements also reveal agreement and confirm the previously reported tendency of the high-field ECE peak to shift towards T_S. The highest value of ECE, ΔT_max∼1.7K is measured at 90°C under 90kV/cm. A low breakdown strength of 93kV/cm needs to be solved to realise stronger electric fields and achieve commercially viable ECE values.     

Pyroelectric and electrocaloric effect of 〈1 1 1〉-oriented 0.9PMN-0.1PT single crystal

In this paper, the polarization vs. electric field hysteresis loops of 〈1 1 1〉-oriented 0.9PbMg_1/3Nb_2/3O₃-0.1PbTiO₃ (0.9PMN-0.1PT) single crystal at different temperatures (20-110 °C) were measured. The adiabatic temperature change ΔT of 〈1 1 1〉-oriented 0.9PMN-0.1PT single crystal due to the application or withdraw of electric field were calculated through the thermodynamic relation. The largest temperature change ΔT achieves ∼1 K with only a change of 40 kV/cm electric field, the mechanism of the electrocaloric effect (ECE) is discussed for 0.9PMN-0.1PT crystal. The pyroelectric coefficient of 0.9PMN-0.1PT under bias field was calculated according to the data of hysteresis loop. The result shows that 0.9PMN-0.1PT have large pyroelectric coefficient under bias field, the largest (∂P/∂T)_E value achieves −0.5 μC/cm² K.     

Phase–composition and temperature dependence of electrocaloric effect in lead–free Bi0.5Na0.5TiO3–BaTiO3–(Sr0.7Bi0.2□0.1)TiO3 ceramics

The (0.94–x)Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–x(Sr_0.7Bi_0.2□_0.1)TiO₃ (BNT–BT–xSBT, 0 ≤ x ≤ 0.24) solid solution ceramics were synthesized via a conventional solid–state reaction method and the correlation of phase structure, piezoelectric, ferroelectric properties and electrocaloric effect (ECE) was investigated in detail. The ECE in lead–free BNT–BT–xSBT ceramics was measured directly using a home–made adiabatic calorimeter with maximum adiabatic temperature change ΔT = 0.4 K with x = 0.08 under the electric field E = 6 kV/mm at room temperature. The position of maximum ECE was found in the vicinity of nonergodic and ergodic phase boundary, where the maximum change in entropy occurs as a result of the field–induced phase transformation between the ergodic and long–range ferroelectric phase. Besides, the mechanism for the shift of ECE peak is discussed in detail. Finally, the temperature dependence of ECE for BNT–BT–xSBT (x = 0, 0.04 and 0.08) was also investigated. This work may present a guideline for designing BNT–based ferroelectric relaxor ceramics for EC cooling technologies.     

Near room temperature giant negative and positive electrocaloric effects coexisting in lead–free BaZr0.2Ti0.8O3 relaxor ferroelectric films

BaZr_0.2Ti_0.8O₃ (BZT) relaxor ferroelectric (FE) films with tetragonal structure were deposited on Pt/TiO₂/SiO₂/Si(100) substrates via a sol-gel technique. A giant electrocaloric effect (ECE) was observed in the compact films with ΔT = 43.6 K and ΔS = 61.9 J/K kg at near room temperature of 366.5 K, which was attributed to the high breakdown electric field of E = 1010 kV/cm. Meanwhile, large negative ECE (ΔT = ? 5.2 K and ΔS = ? 8.8 J/K kg at room temperature of 305 K, ΔT = ? 5.1 K and ΔS = ? 7.2 J/K kg at near room temperature of 375 K) were achieved. The abnormal negative ECE is originated from structural transformation in relaxor ferroelectrics. The coexisting of giant negative and positive electrocaloric effects makes it more effective to realize the refrigeration during the application or removal of an electric field. The maximum electrocaloric coefficient (ζ_max = 0.043 K cm/kV) and refrigeration efficiency (COP = 35.18) of the films were obtained at near room temperature of 366.5 K. The giant electroelectric effect makes BZT films promising as lead-free materials for application in environment-friendly refrigeration equipment at near room temperature.     

Large electrocaloric effect in lead-free Ba1-xCaxTi1-yZryO3 ceramics under strong electric field at room-temperature

In this study, lead-free Ba_1-xCa_xZr_yTi_1-yO₃ (BCTZ(x, y)) ceramics were prepared by means of the classic solid-state reaction method. The morphotropic phase transition (MPB) from the orthorhombic to the tetragonal phase (O-T) was identified in this composition. Besides, the identification of those two structures at room temperature (RT) was made possible thanks to an X-ray diffraction (XRD) study. In order to determine the phase transitions dielectric measurements were conducted. Based on Maxwell equation, the electrocaloric (EC) effect in the studied ceramics was performed via the indirect method. The compositions gave maximum EC temperature changes (ΔT) at above their T_C on application of a 3?kV/mm electric field. These temperature changes are ΔT?=?0.565?K at T_EC=?392?K, ΔT?=?0.548?K at T_EC=?365?K and ΔT?=?0.235?K at T_EC=?307?K for BCZT(10%,5%), BCZT(13%,10%) and BCZT(20%,15%), respectively. At RT, these compositions provided a very interesting EC coefficient (ξ?=?ΔT/ΔE) compared to the pure BaTiO₃ (BT). These values, lying between 0.105 Kmm/kV and 0.188Kmm/kV for ΔE?=?3?kV/mm, are also greater than those related to some lead-based ferroelectric.