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.     

Phase evolution and origin of the high piezoelectric properties in lead-free BiFeO3–BaTiO3 ceramics

La-substitution effects for Bi³⁺-site in 0.7Bi_1.03(1-x)La_xFeO₃-0.3BaTiO₃ (abbreviated as BF30BT-100xLF with x = 0.00, 0.01, 0.035, 0.06, 0.07 and 0.10) ceramics were investigated systematically. All ceramics were synthesized by a conventional solid-state reaction method and quenched in water from its sintering temperature. The crystal structure Rietveld refinement shows that undoped BF30BT ceramic exhibited dominant rhombohedral (R) symmetry and gradually changed to the tetragonal (T) phase with La-doping. However, for x ≥ 0.07 compositions the lattice distortions (c_T/a_T and 90°−α_R) significantly decrease as a result crystal structures become close to the cubic-like (CL) phase. Hence, two different morphotropic phase boundaries (MPBs) were reported for BF30BT-100xLa ceramic system; one MPB-I between the R and T phases and the other MPB-II between T and CL phases. The largest direct piezoelectric coefficient (d₃₃ = 274 pC/N) with a high Curie temperature (T_C = 532 °C) for BF30BT-1LF composition was obtained due to the typical MPB-I between R and T phases. However, a maximum electric field-induced strain (S_max = 0.27%) with a high converse piezoelectric coefficient (d₃₃* = 500 pm/V) for BF30BT-7LF ceramic was mainly attributed to the MPB-II of T + CL phases and soft ferroelectric switching properties.     

High and temperature-insensitive piezoelectric performance in the lead-free Sm-doped BiFeO3–BaTiO3 ceramics with high Curie temperature

Large sensor piezoelectric constant (d₃₃ = 334 pC/N) and superior actuator piezoelectric constant (d₃₃* = 552 pm/V) as well as a high Curie temperature (T_C = 454 °C) were obtained simultaneously in the lead-free 0.67Bi_1.03FeO₃-0.33Ba_1-xSm_xTiO₃ ceramics. Such an excellent and temperature-insensitive piezoelectric performance with only 10% temperature variation of piezoelectric strain at the range of 25–125 °C is highly desirable for real applications. The structural origin of the enhanced piezoelectric performance is mainly attributed to the morphotropic phase boundary and the highest known tetragonality (c_T/a_T = 1.02) in such materials. Transmission electron microscopy and electro-mechanical phenomenological theory demonstrate that the superior d₃₃ and d₃₃* are associated with the hybrids nanodomains (60–90 nm) and flattened thermodynamic energy profile owing to the local structure heterogeneity. These results are superior as the piezoelectric properties are temperature-independent and the material has large d₃₃*, and high T_C compared to other lead-free piezoelectric ceramics.     

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 electric-energy storage in BiFeO3–BaTiO3 ceramics via tailoring microstructure and nanocluster

This study demonstrates the high energy-storage performance using 0.1 wt% MnO₂–added 0.7(Bi_1?xSm_xFeO₃)? 0.3(BaTiO₃) (x = 0–0.3) ceramics through tailoring microstructures and polar order. Sequential structure transitions were identified from a co-occurrence of nonpolar pseudo-cubic Pm-3m and ferroelectric rhombohedral R3c symmetries to antipolar orthorhombic Pbam and nonpolar orthorhombic Pnma symmetries as Sm substitution increases. Recoverable energy densities (W_rec) of 4.5 J/cm³ and 4.1 J/cm³ with efficiencies (η) of 62.1% and 78.1% were achieved respectively for x = 0.15 and 0.2 at a field of 220 kV/cm. The improved energy storage is associated with microstructure modification and complex grain matrix, consisting of grain boundaries, nanocluster/nanomosaic structures, core-shell structures, and polar nanoregions. The nanocluster/nanomosaic structures may act as barriers to suppress polar order and enhance dielectric breakdown strength. This work provides an efficient route to utilize binary BiFeO₃-BaTiO₃ ceramics for electrical energy storage.     

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.     

Enhanced dielectric energy storage performance of A-site Ca2+-doped Na0.5Bi0.5TiO3–BaTiO3–BiFeO3 Pb-free ceramics

(1-x) (0.98Na_0.5Bi_0.5TiO₃–0.01BaTiO₃–0.01BiFeO₃)–xCaTiO₃ (NBB-xCT) ceramics were produced using traditional solid-state synthesis methods. The surface morphology, domain structure, and electrical properties of the ceramic samples were systematically studied. In addition, the temperature and frequency stabilities of the NBB-15CT sample at 200 kV/cm were tested. Generally, NBB-xCT ceramics exhibit a typical single perovskite phase structure. The results indicate that the NBB-15CT ceramics showed a high energy density of 3.14 J/cm³ at 250 kV/cm. The piezoresponse force microscopy (PFM) results showed that the addition of CT broke the macrodomains of the 0.98Na_0.5Bi_0.5TiO₃-0.01BaTiO₃-0.01BiFeO₃ ceramic and helped to form nanodomains, leading to an improved energy storage performance. The above performance indicates that the specimens possess very good temperature-and frequency-dependent energy storage performances at 30–150 °C and 1–100 Hz. Moreover, the electric energy storage and release in the NBB-15CT ceramic indicated that the power density could reach 55.30 MV/cm³ at 180 kV/cm. Therefore, the NBB-15CT ceramic is a promising material for electrical capacitors.     

Coexistence of three ferroelectric phases and enhanced piezoelectric properties in BaTiO3–CaHfO3 lead-free ceramics

Perovskite ferroelectrics possess the fascinating piezoelectric properties near a morphotropic phase boundary, attributing to a low energy barrier that the results in structural instability and easy polarization rotation. In this work, a new lead-free system of (1-x)BaTiO₃-xCaHfO₃ was designed, and characterized by a coexistence of ferroelectric rhombohedral-orthorhombic-tetragonal (R-O-T) phases. With the increase amount of CaHfO₃ (x), a stable coexistence region of three ferroelectric phases (R-O-T) exists at 0.06 ≤ x ≤ 0.08. Both large piezoelectric coefficient (d₃₃～400 pC/N), inverse piezoelectric coefficient (d₃₃^*～547 pm/V) and planar electromechanical coupling factor (k_p～58.2%) can be achieved for the composition with x = 0.08 near the coexistence of three ferroelectric phases. Our results show that the materials with the composition located at a region where the three ferroelectric R-O-T phases coexist would have the lowest energy barrier and thus greatly promote the polarization rotation, resulting in a strong piezoelectric response.     

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.     

Microstructure,ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 ? x)BiFeO₃–0.25BaTiO₃–xBi_0.5K_0.5TiO₃ + 1 mol% MnO₂ ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi_0.5K_0.5TiO₃ retards the grain growth and induces two dielectric anomalies at high temperatures (T₁ ～ 450–550 °C and T₂ ～ 700 °C, respectively). After the addition of 2.5 mol% Bi_0.5K_0.5TiO₃, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.     

Dielectric behavior and impedance spectroscopy in lead-free BNT–BT–NBN perovskite ceramics for energy storage

The dielectric behavior, impedance spectroscopy and energy-storage properties of 0.85[(1−x)Bi_0.5Na_0.5TiO₃–xBaTiO₃]–0.15Na_0.73Bi_0.09NbO₃ [(BNT–xBT)–NBN] ternary ceramics were investigated. Temperature dependent permittivity curves displayed two depressed anomalies, resulting in significantly improved dielectric temperature stability. (BNT–9BT)–NBN showed a permittivity of 1680 at 150 °C with Δε/ε_150 °C varying no more than ±10% up to 340 °C. From the complex impedance analysis, grain and grain boundary shared the same time constant. The high temperature resistivity followed the Arrhenius law with E_a=1.7–2.0 eV, suggesting intrinsic band-type electronic conduction. The maximum energy-storage density of all the samples reached 1.1–1.4 J/cm³, accompanied with good temperature stability in the range of 25–140 °C. These results indicate that (BNT–xBT)–NBN system should be a promising lead-free material for energy-storage capacitor applications.     

Enhanced piezoelectric performance of donor La3+-doped BiFeO3–BaTiO3 lead-free piezoceramics

Lead-free 0.70Bi_1.03FeO₃-0.30Ba_(1-x)La_xTiO₃ piezoelectric ceramics (with x = 0.000, 0.005, 0.010, 0.015 and 0.035 abbreviated as 0.0BaLa, 0.5BaLa, 1.0BaLa, 1.5BaLa and 3.5BaLa) were prepared through the conventional solid-state reaction route followed by water quenching process. The X-ray diffraction profile shows that the substitution of Ba²⁺ = 1.61 Å with a donor ion, La³⁺ = 1.36 Å, has a profound impact on the crystal symmetry as results the crystal structure transform from dominant rhombohedral (R) to tetragonal (T) phase. A large remnant polarization (P_r = 30.3 μC/cm²) and an enhanced direct piezoelectric coefficient, (d₃₃ = 297 pC/N) together with a high Curie temperature (T_C = 530 °C) was obtained near to the morphotropic phase boundary (MPB) of the R and T phases. A maximum strain of (S_max = 0.188%) with corresponding converse piezoelectric coefficient (d₃₃* = 340 pm/V) and low strain hysteresis (≈20%) was found for 1.5BaLa ceramic. Additionally, the water quenching effect was more prominent for 1.0BaLa ceramic as observed from the high thermal hysteresis in heating/cooling results of the dielectric constant (ε_r), leading to the enhancement of ferroelectric switching behavior. Hence, a small amount of La³⁺-substitution for Ba²⁺-site is more effective for the enhancement of ferroelectric and piezoelectric properties, similarly to the donor La³⁺-doped Pb(Zr,Ti)O₃ ceramic.     

Structure–property relationships in BaTiO3–BiFeO3–BiYbO3 ceramics

Ba_1?xBi_xTi_1?xYb_x/2Fe_x/2O₃ ceramics were fabricated by the solid state reaction method. X-ray diffraction analyses show 0 ≤ x ≤ 0.04 ceramics to have an average crystal structure described by the non-centrosymmetric tetragonal P4 mm space group, whereas x ≥ 0.08 ceramics are consistent with a centrosymmetric cubic perovskite (space group Pm-3 m). Coexistence of both tetragonal and cubic symmetries is observed for x = 0.06. Raman spectroscopy analysis corroborate a change in average structure with increasing x, but also show the local crystal symmetry for x ≥ 0.08 ceramics to deviate from the idealized cubic perovskite structure. Dielectric data show a ferroelectric-to-relaxor crossover, which occurs in conjunction with the change in both the average and local crystal symmetry as indicated by X-ray and Raman data. For x ≥ 0.08, ceramics exhibit relaxor behavior, which is also accompanied by a shift of the permittivity maxima towards higher temperatures with increasing x.     

Grain size effect on electrical properties of Mn-modified 0.67BiFeO3–0.33BaTiO3 lead-free piezoelectric ceramics

To investigate how grain size affects the dielectric, ferroelectric, and piezoelectric properties of Mn-modified 0.67BiFeO₃–0.33BaTiO₃ ceramics, we prepared samples with a wide variety of grain sizes from 4.1 μm to 0.59 μm via a conventional solid-state process that use the normal and the two-step sintering methods. Small-signal dielectric measurements show that all the samples exhibit a relaxor-like behavior and that grain size has little influence on the room-temperature dielectric permittivity. For grain sizes below 2 μm, the remanent polarization P_r and piezoelectric coefficient d₃₃ decrease with the grain size, whereas they remain almost constant near P_r = 27 μC/cm² and d₃₃ = 70 pC/N in samples with grain sizes exceeding 2 μm. The mechanism underlying the observed grain size effect is discussed in terms of the electric-field-induced formation of macroscopic ferroelectric domains.     

Properties of spark plasma sintered pseudocubic BiFeO3–BaTiO3 ceramics

Perovskite solid solution materials, namely, 0.67BiFeO₃-0.33BaTiO₃, were synthesized by spark plasma sintering method. The effects of the spark plasma sintering temperature on phase purity, microstructure, and electric properties of the as-prepared materials were investigated. The materials could be referred as pseudocubic phases based on the X-ray diffraction patterns. The bulk density first increased and then decreased. The 880 °C-sintered-ceramics had the maximal density and a compact microstructure with grain size of 0.77 ± 0.34 μm. The dielectric constant as a function of temperature exhibited a broad peak. At the optimal spark-plasma-sintering temperature, enhanced ferroelectric properties were observed with a value of P_r ～ 21 μC/cm². This investigation on the spark plasma sintering process confirms it as an efficient approach to prepare outstanding performance BiFeO₃–BaTiO₃ ceramics.     

Dielectric properties and related ferroelectric domain configurations in multiferroic BiFeO3–BaTiO3 solid solutions

Dielectric properties and ferroelectric domain configurations of multiferroic xBaTiO₃–(1 ? x)BiFeO₃ (x = 0.10–0.33) solid solutions synthesized by conventional solid-state reaction, were reported. A structural transition from rhombohedral to pseudo-cubic structures appeared around x = 0.33, and the formation of impurity phase of Bi₂Fe₄O₉ was effectively depressed by doping BaTiO₃. Dielectric constants of xBaTiO₃–(1 ? x)BiFeO₃ solid solutions decreased with increasing the frequency, and the degree of decrease was related to the doping content of BaTiO₃. Transmission electron microscopy images revealed that the ferroelectric domain configurations in the multiferroic BiFeO₃–BaTiO₃ solid solutions with rhombohedral symmetry, exhibited a wavy character whereas a predominant intricate domain structure with fluctuating mottled contrast was observed in the multiferroic BiFeO₃–BaTiO₃ solid solution with pseudo-cubic phase structure. The presence of 1/2{1 1 1} superlattice spots in the selected area electron diffraction patterns taken from the multiferroic BiFeO₃–BaTiO₃ solid solutions with rhombohedral symmetry indicated that the ordered regions have a doubled perovskite unit cell.     

Non–ferroelectric intercalation structure based on Aurivillius phase Bi4Ti3O12: A research arena to achieve high energy storage performance

High density energy capacity is the benchmark of electronic industry, and its improvement is on the way. Here, the BaCaBi₄Ti₅O₁₈ films present high efficient energy storage trait, which is originated from ferroelectric/non–ferroelectric intercalation structure. The emergent of large gradient elastic energy in ferroelectric/non–ferroelectric intercalation structure not only crushes the ferroelectric domains, but also modulates space charges accumulation. Both factors contribute to the high polarization and intense breakdown strength. The contrast experiments were conducted by single ferroelectric intercalated structure Bi₅Ti₃FeO₁₅ films, which show almost non ferroelectric energy storage behavior. The finite element simulations are consistent with the experimental evidences. The present results may facilitate a way for structural design of Aurivillius family, which is capable of acting as remarkable energy storage tasks.     

Structural modification and evaluation of dielectric and ferromagnetic properties of Ce-modified BiFeO3–BaTiO3 ceramics

An investigation on Rare earth constituent Ce incorporated BiFeO₃–BaTiO₃ ceramics has been focused in the present study. The ceramic samples of (Bi₀.₇Ba_0.3)_1-xCe_x(Fe₀.₇Ti_0.3)O₃ (x = 0–0.12) were formulated adopting the cost-effective solid-state sintering method. The influence of aliovalent Ce ions on the structural, microstructural, dielectric, ferromagnetic, and optical properties of BiFeO₃–BaTiO₃ was evaluated in this paper. The coexistence of the Tetragonal and the Rhombohedral phases was established by the Rietveld refinement process. The refined crystallographic parameters showed maximum cell volume (V_cell) and the highest percentage of the Rhombohedral phase for x = 0.06; and consequently, the ceramic exhibited the topmost dielectric constant of 946 at x = 0.06. The scanning electron microscopy of the samples revealed the manifestation of polygonal grain morphology. Besides, remarkably improved ferromagnetic properties were evinced for Ce doped ceramics. The magnitude of saturation (M_s) and remnant (M_r) magnetizations were boosted from 0 emu/g and 0.0019 emu/g to 0.9186 emu/g and 0.3745 emu/g respectively with increasing x from 0 to 0.12. Additionally, the optical band gaps of all the samples were evaluated and found to be in the range of 2.941–3.077 eV.     

Piezoelectric properties and temperature stabilities of Mn- and Cu-modified BiFeO3–BaTiO3 high temperature ceramics

The structures, microstructures, electrical properties and the thermal stability have been investigated for the MnO₂-doped (1 ? x)BF–xBT system and the MnO₂ and CuO-doped (1 ? x)BF–xBT system, where x ranges from 0.25 to 0.35. The XRD analysis shows that the two systems have a single perovskite phase, and the MnO₂ and CuO-doped (1 ? x)BF–xBT system has a morphotropic phase boundary (MPB) with the coexistence of rhombohedral and pseudo-cubic phases in the system about x = 0.325. The addition of small amount of CuO was quite effective to lower the sintering temperature. The diffusive phase transition characteristics were observed in the MnO₂-doped (1 ? x)BF–xBT system and a normal ferroelectric phase transition characteristics were observed in the MnO₂ and CuO doped (1 ? x)BF–xBT system. Compared with the MnO₂ doped (1 ? x)BF–xBT system, the ?_m, Curie temperature (T_c), depoling temperature (T_d), and piezoelectrical properties were improved evidently with the MnO₂ and CuO doping.     

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.