Dielectric,ferroelectric, and energy storage properties of Ba(Zn1/3Nb2/3)O3-modfied BiFeO3–BaTiO3 Pb-Free relaxor ferroelectric ceramics

Pb-free bulk ceramics (1-x)[0.65BiFeO₃-0.35BaTiO₃]-xBa(Zn_1/3Nb_2/3)O₃ were produced by traditional solid-state reaction route. In this experiment, Ba(Zn_1/3Nb_2/3)O₃ (BZN) was introduced to destroy long-range order domains in order to obtain higher energy storage performance. Impedance and XPS analysis indicate that oxygen vacancies exist and participate in relaxation processes at high temperatures. With the increase of BZN content, the dielectric relaxation behavior is improved, the hysteresis loop becomes thinner, remnant polarization decreases, and the breakdown electric field increases to 180 kV/cm in 15BZN. A maximum W_rec (1.62 J/cm³) is eventually reached in 7BZN with great temperature stability. The highest efficiency is 91% in 15BZN with W_rec of 1.28 J/cm³. Charge-discharge tests show that ceramics have a quick discharge time of t_0.9 < 0.1 μs, which makes BZN-doped ceramics a potential candidate for energy storage devices.     

Novel NaNbO3–Sr0.7Bi0·2TiO3 lead-free dielectric ceramics with excellent energy storage properties

NaNbO₃ (NN) is considered to be one of the most prospective lead-free antiferroelectric energy storage materials due to the merits of low cost, nontoxicity, and low density. Nevertheless, the electric field-induced ferroelectric phase remains dominant after the removal of the electric field, resulting in large residual polarization, which prevents NN ceramics from obtaining superior energy storage performance. In this work, the relaxor ferroelectric Sr_0·7Bi_0·2TiO₃ (SBT) was chosen to partially replace the NN ceramics, and the introduction of the nanodomain of the relaxor ferroelectric hinders the generation of field-induced ferroelectric phases, allowing the material to combine the large polarization strength of the relaxor ferroelectric with the near-zero residual polarization of the antiferroelectric. Large recoverable energy storage density (4.5 J cm^?3) and ultra-high energy storage efficiency (90.3%) were gained in NN-20SBT under an electric field of 288 kV cm^?1. Furthermore, superior temperature (25–120 °C) and frequency (1–500 Hz) stabilities were acquired. These performances demonstrate that NN-20SBT ceramics are potential candidates as dielectric materials for high energy storage density pulsed power capacitors.     

Improved dielectric energy storage performance of Na0.5Bi0.5TiO3–Sr0.7Nd0.2TiO3 lead-free ceramics by adding an appropriate amount of AgNbO3

Based on the significant advantages of dielectric ceramics in high power energy storage, (1-x) (0.55Bi_0.5Na_0.5TiO₃-0.45Sr_0.7Nd_0.2TiO₃)-xAgNbO₃ (NBSNT-xAN) ceramics were prepared by traditional solid phase method. The introduction of AN in NBSNT ceramics not only increased the degree of relaxation, but also refined the grain size, enhanced the BDS, and finally improved the energy storage performance. It is found that the NBSNT-0.5AN ceramics obtained an effective energy storage density as high as 3.08 J/cm³ and an efficiency of 79.94%. In addition, good thermal stability and temperature stability were exhibited in the range of 30–120 °C and 10–350 Hz, and at the same time, it performed very well in the pulsed test at room temperature and variable temperatures. This provides a design idea for the miniaturization and integration of energy storage ceramic materials.     

Phase structure,piezoelectric, ferroelectric,and electric-field-induced strain properties of Nb-modified 0.8Bi0.5Na0.5TiO3−0.2Sr0.85Bi0.1TiO3 ceramics

0.8Bi_0.5Na_0.5Ti_(1-x)Nb_xO₃−0.2Sr_0.85Bi_0.1TiO₃ (BNT-SBT-xNb, x = 0.00, 0.01, 0.02, and 0.03) piezoelectric ceramics were prepared by traditional solid state reaction and the influence of Nb substitution on the phase structure, ferroelectric, piezoelectric, and electric-field-induced strain properties in BNT-SBT ceramics were studied. XRD results exhibited that Nb⁵⁺ ions could fully diffuse into BNT-SBT structure to form a solid solution when x = 0.01. P-E loops and S-E curves suggested that the ferroelectric phase transformed to ergodic relaxor state (FE-to-ER) with the increasing the amount of Nb additive, indicating the ferroelectric long-ranged order was disturbed by the excess of Nb. With increasing Nb doping, phase transition temperature from normal ferroelectric to ergodic relaxor (short for T_F-R) could be reduced from 120 °C to 40 °C. Furthermore, for sample with x = 0.01, the normalized strain d₃₃^* got a maximum value ~571 pm/V due to the phase transition from ergodic relaxor to ferroelectric (ER-to-FE) under electric field.     

Phase transition behavior and electrical properties of lead-free (Bi0.5K0.5)TiO3–LiNbO3 relaxor ferroelectric ceramics

(1?x)(Bi_0.5K_0.5)TiO₃–xLiNbO₃ ((1?x)BKT–xLN) lead-free relaxor ferroelectric ceramics were prepared by a conventional solid-state route and their phase transition behavior and the corresponding electrical properties were investigated. A morphotropic phase boundary separating rhombohedral and tetragonal phases was identified in the composition range of 0.015<x<0.03, where the improved electrical properties of piezoelectric constant d₃₃=75 pC/N and electromechanical coupling factor k_p=0.18 were obtained. Moreover, all samples show typical relaxor behavior characterized by the presence of diffuse phase transition and frequency dispersion. It was found that the dielectric relaxation behavior of BKT ceramics can be obviously enhanced with the addition of LN. In addition, the effect of the LN addition on the ferroelectric properties was also investigated by measuring polarization versus electric field hysteresis loops.     

Dielectric,ferroelectric, and depolarization properties of B-site manganese-doped 0.925(Bi0.5Na0.5)TiO3–0.075BaTiO3 solid solutions

(Bi_0.5Na_0.5)_0.925Ba_0.075(Ti_1−xMn_x)O₃ (x=0, 0.2, 1.0, and 2.0 mol%) ceramics were prepared by solid-state-reaction method to study dielectric, ferroelectric, and depolarization properties. The manganese (Mn) doping can suppress dielectric permittivity and increase relaxor behavior. Coercive field (E_c) increases, while remanent polarization (P_r) decreases as the Mn content increases. P_r exhibits discontinuous anomalies as a function of temperature in all compositions, implying a polarization reorganization of local domains. The depolarization temperature (T_d) reaches the highest value (~152 °C) in 0.2%Mn, and decreases as MnO₂ content increases. The increased T_d in 0.2%Mn is due to two-phase coexistence and structural thermal stability induced by Mn ions. This work suggests that the moderate Mn doping can enhance T_d in lead-free piezoceramics for applications at elevated temperatures.     

Energy storage performance of BiFeO3–SrTiO3–BaTiO3 relaxor ferroelectric ceramics

Dielectric capacitors reveal great potential in the application of high power and/or pulsed power electronic devices owing to their ultrafast charge–discharge rate and ultrahigh power density. Among various dielectric capacitors, the environment-friendly lead-free dielectric ceramics have drawn extensive investigations in recent years. Nevertheless, the relatively small recoverable energy storage density (W_rec) is still an obstacle for their application. Herein, the (0.55−x)BiFeO₃–0.45SrTiO₃–xBaTiO₃ ternary ceramics with 0.1 wt% MnO₂ were prepared by the solid-state reaction, and achieved enhanced relaxor behavior as well as breakdown strength E_b. As a result, the x = 0.12 ceramic exhibited superior comprehensive energy storage performance of large E_b (50.4 kV/mm), ultrahigh W_rec (7.3 J/cm³), high efficiency η (86.3%), relatively fast charge–discharge speed (t_0.9 = 6.1 μs) and outstanding reliability under different frequency, fatigue, and temperature, indicating that the BiFeO₃-based relaxor ferroelectric ceramics are prospective alternatives for electrostatic energy storage.     

Bi0·5Na0·5TiO3–Sr0.85Bi0·1TiO3 ceramics with high energy storage properties and extremely fast discharge speed via regulating relaxation temperature

Bi_0·5Na_0·5TiO₃-based relaxor ferroelectric ceramics have recently gained increasing attention due to their outstanding energy storage properties. However, the trade-off between the recoverable energy storage density/efficiency and discharge rate resulted from the hysteresis of domain switching process, severely limits their applications. Herein, a strategy realizing synergistic excellent energy storage properties and fast discharge rate is proposed through regulating relaxation temperature. The relaxation temperature was decreased to below room temperature by introducing Sr_0·85Bi_0·1TiO₃ into Bi_0·5Na_0·5TiO₃ [(1-x)Bi_0·5Na_0·5TiO₃-xSr_0.85Bi_0·1TiO₃, x = 0.5–0.7)], enabling the small size and weak correlation polar nanoregions (PNRs) with relatively high polarization. The trade-off was overcome by reducing the hysteresis of electrical switching of weak correlation PNRs. Thus, large recoverable energy storage density of 2.32 J/cm³ and high efficiency of 80.1% (250 kV/cm) were achieved simultaneously for x = 0.7 ceramics. Meanwhile, extremely rapid discharge rate (<30 ns) and remarkable power density of 63.7 MW/cm³, which were superior to the previously reported lead-free ceramics were realized. Besides, the 0.3BNT-0.7SBT ceramics also possess good thermal stability over 25 °C–115 °C at 100 kV/cm and good frequency stability (5–100 Hz). These properties make the 0.3BNT-0.7SBT ceramic an ideal candidate for energy storage applications.     

Enhanced energy storage properties of BiAlO3 modified Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 lead-free antiferroelectric ceramics

Lead-free (1−x)(0.75Bi_0.5Na_0.5TiO₃–0.25Bi_0.5K_0.5TiO₃)–xBiAlO₃ (BNT–BKT–100xBA, x=0–0.10) ceramics were prepared by two-step sintering method and their phase structure, micro morphology and electrical properties were systematically investigated. X-ray diffraction analysis indicates a pure perovskite phase for x≤0.06 as well as a structural evolution from a tetragonal toward a pseudocubic phase. Transmission electron microscopy study of the x=0.04 composition reveals the existence of antiferroelectric phase with a⁰a⁰c⁺ oxygen octahedron tilting which is in the form of nano-domains. Polarization-electric field and current-electric field hysteresis loops demonstrate that the increase of BA concentration destroys the ferroelectric order and strengthens antiferroelectric order. A much enhanced energy storage density of 1.15 J/cm³ and efficiency of 73.2% is achieved under 105 kV/cm at x=0.06. In addition, its energy storage property is found to depend weakly on temperature within the measurement range of 25–150 °C.     

High energy-storage performance of lead-free Ba0.4Sr0.6TiO3–Sr0.7Bi0.2TiO3 relaxor-ferroelectric ceramics with ultrafine grain size

Relaxor-ferroelectric (RFE) ceramics possess slender ferroelectric hysteresis loop and low remnant polarization (P_r). They have great potential to provide excellent energy-storage performance as dielectric energy-storage materials. Herein, a lead-free 0.8Ba_0.4Sr_0.6TiO₃–0.2Sr_0.7Bi_0.2TiO₃ (0.8BST–0.2SBT) RFE ceramic with high energy-storage performance has been realized successfully. The addition of Bi³⁺ and increase in Sr²⁺content at the A site of the BST can effectively inhibit the growth of grains for high breakdown strength (E_b). As a result, an ultrafine average grain size of 0.7 μm was obtained in 0.8BST–0.2SBT RFE ceramic, affording a high E_b of 300 kV/cm. Further investigation revealed that the mutual conversion of short-range polar nanoregions and long-range-ordered ferroelectric domains upon application and withdrawal of a 300 kV/cm applied electric field resulted in a high maximum polarization (P_max) of 31 μC/cm² and a low P_r of 2.5 μC/cm². Hence, the 0.8BST–0.2SBT RFE ceramic simultaneously exhibited a high recoverable energy-storage density of 3.3 J/cm³ and a high energy-storage efficiency of 85% at 300 kV/cm. Additionally, a good energy-storage performance was reported over a temperature range of 50°C-120 °C and frequency from 10 to 1000 Hz, making the 0.8BST-0.2SBT RFE ceramic a potential lead-free dielectric energy-storage material.     

Enhancing the dielectric and energy storage properties of lead-free Na0.5Bi0.5TiO3–BaTiO3 ceramics by adding K0.5Na0.5NbO3 ferroelectric

A novel strategy of enhancing the dielectric and energy storage properties of Na_0.5Bi_0.5TiO₃–BaTiO₃ (NBT–BT) ceramics by introducing a K_0.5Na_0.5NbO₃ (KNN) ferroelectric phase is proposed herein, and its underlying mechanism is elucidated. The lead-free KNN ceramic decreases the residual polarisation and increases the electric breakdown strength of the NBT–BT matrix through the simultaneous modification of its A-sites and B-sites. The obtained NBT?BT?x?KNN ceramics have a perovskite structure with unifying grains. A bulk 0.9NBT–BT–0.1KNN ceramic sample with a thickness of 0.2 mm possesses a high energy storage density of 2.81 J/cm³ at an applied electric field of 180 kV/cm. Moreover, it exhibits good insulation properties and undergoes rapid charge and discharge processes. Therefore, the obtained 0.9NBT–BT–0.1KNN ceramic can be potentially used in high-power applications because of its high energy density, good insulation properties, and large discharge rate.     

Dielectric and ferroelectric hysteresis properties of 1–3 lead magnesium niobate–lead titanate ceramic/Portland cement composites

In this work, lead magnesium niobate–lead titanate (PMN–PT) ceramic was cut and filled with Portland cement (PC) to produce 1–3 connectivity PMN–PT/PC composites. Dielectric and ferroelectric hysteresis properties of these composites with PMN–PT ceramic volume content of 60% were investigated. Room temperature dielectric constant (?_r) at 1 kHz of the PMN–PT/PC composite was found to be ≈1500. At higher frequency (20 kHz), the dielectric constant was reduced to the value of ≈1300. Ferroelectric (polarization–electric field) hysteresis loops at 10–90 Hz and varying electric field were measured. The “instantaneous” remnant polarization (P_ir) at 50 Hz and at the electric field of 7 kV/cm of the PMN–PT/PC composite was found to be ≈10 μC/cm². These values of 1–3 composites therefore are promising when compared to previous results of composites at similar conditions.     

Dielectric and piezoelectric properties of (1−x)(Bi,Na)TiO3–x(Bi,K)TiO3 lead-free ceramics for piezoelectric energy harvesters

The piezoelectric and dielectric properties of the (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ (x=0.12, 0.14, 0.18, 0.20 and 0.30) lead-free ceramics were investigated. Specimens were prepared by the conventional mixed oxide method and sintered at 1170 °C in air. Scanning electron microscopy indicated that increasing x from 0.12 to 0.30 causes a decrease in the grain size. The (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ ceramics shows a homogeneous microstructure and excellent dielectric and piezoelectric properties. Specimens with optimum composition showed a piezoelectric charge constant d₃₃ of 166 pC/N, an electromechanical coupling factor k_p of 0.5, a dielectric constant ε_r of 1591.32 at 1 kHz and generated power output of 37.49 nW/cm².     

Improving the energy storage performance of 0.88(Bi0.4Ba0.2Na0.2K0.2)TiO3–0.12Sr(Mg1/3Nb2/3)O3 high-entropy relaxor ceramics by AlN doping

In this study, we report the effects of AlN additives on the microstructure and energy performance of 0.88(Bi_0.4Ba_0.2Na_0.2K_0.2)TiO₃-0.12Sr(Mg_1/3Nb_2/3)O₃ high-entropy relaxor ceramics. We show that AlN partially reacts with/dissolves into the matrix and only forms a secondary phase when its concentration is above a threshold. AlN doping can only affect the shape of the P-E loop and improve the breakdown strength when it formed the secondary phase. The breakdown strength first increased and then decreased with increasing AlN concentration. When the AlN concentration is 6 mol%, the ceramic has the optimal energy storage performance with a breakdown electric field of 340 kV/cm, a recoverable energy density of 3.85 J/cm³, and an efficiency of 85.8%. The study suggests that the addition of AlN is an effective way to improve the energy storage performance of high-entropy ceramics.     

High energy storage efficiency and fast discharge property of temperature stabilized Ba0.4Sr0.6TiO3–Bi(Mg0.5Ti0.5)O3 ceramics

(1?x)Ba_0.4Sr_0.6TiO₃-xBi(Mg_0.5Ti_0.5)O₃ ((1?x)BST-xBMT) relaxor ferroelectric ceramics were prepared by a conventional solid-state method. In this work, the microstructure, dielectric properties, and pulsed charge–discharge properties were investigated. The doping of BMT caused a decrease in the surface energy and grain boundary energy, and contributed to the formation of polar nanoregions (PNRs). The existence of PNRs was confirmed by piezoresponse force microscopy measurements (PFM) on the 0.8BST-0.2BMT ceramic. The PNRs responded quickly under an AC voltage, thus the pulsed charge–discharge time was short (<80 ns), aiming to realize time compression to improve the power density (P_D). The PNRs were not closely connected to each other and adverse to the formation of leakage current and pinning, thus inhibiting charge transfer at the grain boundaries and contributing to the high energy storage efficiency (η ～ 93%). In addition, the 0.8BST-0.2BMT ceramic also displayed excellent temperature stability. The capacitance-temperature dependence satisfied the requirement of X8R (?55–150 °C, ΔC/C_{25 °C} ≤ ±15%), and η had no obvious fluctuation in the temperature range from 25 °C to 150 °C. This study could provide a successful method to achieve a temperature stable and high η, and a fast charge–discharge process.     

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.     

High-efficiency energy storage in Nd-doped (1-x)BiFeO3–xBaTiO3 relaxor ferroelectric ceramics

Dielectric capacitors as energy storage electronics have drawn much attention due to ultrahigh power densities with quick charging and discharging rates. In this report, A-site Nd-doped (1-x)BiFeO₃-xBaTiO₃ (x = 0.2–0.45) relaxor ferroelectric ceramics with superior storage efficiencies were prepared with 0.1 wt% MnO₂ additive. Energy-storage efficiency (η) increases from 63.7% to 89% at 190 kV/cm as BaTiO₃ increases accompanied by recoverable energy densities (W_rec) in the range of 2.5–2.7 J/cm³. The energy-storage performance persists thermally stable up to 125 °C. The superior storage efficiency is associated with growth of the cubic Pm-3m symmetry and the core-shell structure with increasing BaTiO₃ content. The formation of nanocluster/nanomosaic structure also plays an essential role as a barrier in suppressing the long-range polarization order. This work provides a design of binary rare-earth doped BiFeO₃–BaTiO₃ dielectric ceramics for thermally stable and high-efficiency electrical energy storage.     

Optimized piezoelectric and structural properties of (Bi,Na)TiO3–(Bi,K)TiO3 ceramics for energy harvester applications

We investigated (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ (x=0, 0.14, 0.16, 0.18, 0.20, and 0.22) compositions of lead-free piezoelectric ceramics for potential energy harvester applications. Composition and sintering temperature of (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ were varied to extract the optimized processing temperature with each composition. We compared and analyzed sintering temperature-dependent surface morphologies and electrical properties. Maximum piezoelectric charge constant of 180 pC/N were obtained from the 0.8(Bi,Na)TiO₃–0.2(Bi,K)TiO₃composition at the sintering temperature of 1180 °C. Temperature dependent dielectric permittivity was measured to know the phase transition. We corresponded two different anomaly peaks, observed at 84 and 290 °C, as the rhombohedral-tetragonal and tetragonal-cubic phase transitions, respectively. Due to these phase transitions, different shapes of polarization-electric field loops (P-E loops) were measured and compared. Finally, output power of 42.39 nW/cm² were obtained for the (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ lead free piezoelectric ceramics.     

Dielectric and Magnetic properties of (1 − x)BiFeO3–xBa0.8Sr0.2TiO3 ceramics

The polycrystalline samples of (1 ? x)BiFeO₃–xBa_0.8Sr_0.2TiO₃ (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4 and x = 1) were prepared by the conventional solid state reaction method. The effect of substitution in BiFeO₃ by Ba_0.8Sr_0.2TiO₃ on the structural, dielectric and magnetic properties was investigated. X-ray diffraction study showed that these compounds crystallized at room temperature in the rhombohedral distorted perovskite structure for x ≤ 0.3 and in cubic one for x = 0.4. As Ba_0.8Sr_0.2TiO₃ content increases, the dielectric permittivity increases. This work suggests also that the Ba_0.8Sr_0.2TiO₃ substitution can enhance the magnetic response at room temperature. A remanent magnetization M_r and a coercive magnetic field H_C of about 0.971 emu/g and 2.616 kOe, respectively were obtained in specimen with composition x = 0.1 at room temperature.     

Morphotropic phase boundary and electrical properties of lead-free (K0.5Bi0.5)TiO3–Bi(Ni0.5Ti0.5)O3 relaxor ferroelectric ceramics

Lead-free relaxor ferroelectric ceramics (1?x)(K_0.5Bi_0.5)TiO₃–xBi(Ni_0.5Ti_0.5)O₃ were prepared by a conventional solid-state route, the phase transition behavior and corresponding electrical properties were investigated. A typical morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric phases was identified to be in the range of 0.05<x<0.07 where the optimum piezoelectric and electromechanical properties of d₃₃=126 pC/N and k_P=18% were achieved. Most importantly, a high Curie temperature ~320 °C, around which the material shows a typical relaxor ferroelectric behavior characterized by the presence of diffuse phase transition and frequency dispersion, was obtained in MPB compositions, significantly higher than those of some existing MPB lead-free titanate systems. These results demonstrate a tremendous potential of the studied system for device applications.